**3. Structures and properties**

108 Superconductors – Materials, Properties and Applications

Ni electrode), naphthalene, and azulene.

**(e)**

diffusion method is seen. (b) Single crystals of

**Scheme 2.**

soluble and insoluble materials. For example, single crystals of

**(a) (b) (c (d)**

prepared using 1) CuSCN + KSCN + 18-crown-6-ether, 2) TBA•SCN + CuSCN, or 3) Cu(NCS)2•K(18-crown-6-ether). Low solubility of the components of the electrolyte in the specific solvent usually retarded single crystal growth. Ionic liquids such as 1-ethyl-3 methylimidazolium (EMI, Scheme 2) salts of X were found to afford single crystals of high quality, recently. Regarding the surfaces of electrodes each research group has special treatments such as burning (but not melting) or polishing with very fine powder. The electrode surface can be treated by applying a current to switch the polarity in a 1 M H2SO4 solution. When the radical species are unstable in solution, CT solids can be grown by applying a high current at very low temperatures; *e.g.*, salts of fluoranthene (–30 °C, 2 mA,

**Figure 2.** (a) Galvanostatic electrocrystallization using 20 ml cells on the desk. Under the desk the

showing two-dimensional conducting plane (*bc*). (d) Single crystal of (TMTSF)2ClO4 with four gold

Besides the electrocrystallization, superconducting single crystals of good quality were prepared by direct chemical oxidation of ET with iodine in gas or with TBA•I3 or TBA•IBr<sup>2</sup> in solution. Better-quality single crystals of (TTF)[Ni(dmit)2]2 were obtained by the diffusion method of the metathesis reaction rather than electrocrystallization. No single crystals of the electron acceptor based superconductors were obtained except for the M(dmit)2 system.

κ

wires connected by gold paste. (e) Typical glass cells for electrocrystallization.

κ



### **3.1. Superconductors based on electron donors**

### *3.1.1. One-dimensional superconductors (TMTSF and TMTTF families)*

TMTSF [9–14] has provided eight quasi-one-dimensional superconductors; (TMTSF)2X with highest *Tc* ~ 3 K [11] (Table 1). Most of them were prepared by electrocrystallization using TBA•X as electrolyte except the NbF6 salt which can be only prepared by using ionic liquid EMI∙NbF6 [12]. They are isostructural to each other and the crystal structure of (TMTSF)2NbF6 is depicted in Fig. 3, where TMTSF molecules form a zigzag dimer that forms a segregated column along the face-to-face direction (*a*-axis) with no short Se∙∙∙Se atomic contacts (Fig. 3a,3b). Along the side-by-side direction (*b*-axis), rather short Se∙∙∙Se atomic contacts were seen (Fig. 3c), however, those less than the sum of the van der Waals radii (3.80 Å) are present only for X = ClO4 and FSO3. For the PF6 salt, *t*a and *t*b were estimated to be 0.25–0.30 eV and 0.031 eV, respectively. Consequently, the Fermi surface of (TMTSF)2X is not closed, but open with fair warping due to the lack of adequate side-by-side transfer interactions (Fig. 1a).

**Figure 3.** Crystal structure of (TMTSF)2NbF6 [12]. (a) Segregated column of TMTSF molecules. The numbers indicate the overlap integrals in 10–3 units. (b) Zigzag stacking of the TMTSF column. (c) Se∙∙∙Se atomic contacts (*d*7, *d*9) along the side-by-side direction. (d) Temperature dependence of resistivity under pressure.

Salts with octahedral anions exhibited MI transition at 11–17 K at AP due to SDW, and a superconductivity appeared with on-set *T*c of ca. 1 K at 0.6–1.2 GPa (Fig. 3d). Salts with (pseudo)tetrahedral anions, on the other hand, exhibited order-disorder (OD) transition of anion molecules when the superlattice created was coincide with the nesting vector of the warped Fermi surface (2*a* × 2*b*, Fig. 1a).

Development and Present Status of Organic Superconductors 111

Characteristicsc)

SDW (17 K) SDW (12 K) SDW (11 K)

(γ = 10.5, β= 11.4,

1.4-1.8 2.8 1.38 1.0

(eq. 2) which are related with *T*c by eq. 3 for the BCS-type superconductors.

γ and β SDW (12 K), FISDW SDW (12 K, *J* = 604 K)

OD (24 K, *a* × 2*b* × 2*c*), FISDW,

OD (40 K), SDW and SC (coexist)

are important quantities experimentally

SDW (5 K) by rapid cool OD (177 K, *a* × 2*b* × 2*c*) OD (88 K, *a* × 2*b* × 2*c*)

spin-Peierls (15 K) charge-order, AF (8 K)

F)/3 (1)

3 (2)

F)) (3)

AF (15 K)

Θ= 213)

X Symmetry

**TMTSF system** 

octahedral octahedral octahedral octahedral octahedral tetrahedral

tetrahedral

octahedral octahedral tetrahedral spherical

**TMTTF system** 

determined to obtain *D*(

: mJ mol–1 K–4

: Debye temperature, K

*3.1.2.1. BO superconductors*

pseudo-tetrahedral

ε

: Sommerfeld coefficient, mJ mol–1 K–2

 *T*c ∝

F): density of states at Fermi level per one spin.

PF6 AsF6 SbF6 NbF6 TaF6 ClO4

ReO4 FSO3

PF6 SbF6 BF4 Br

γ

β

Θ

*D*(ε σRT / S cm–1

Θ

**Table 1.** Organic superconductors of (TMTSF)2X and (TMTTF)2X

of anion and newly formed superlattice. FISDW: field-induced SDW.

F) (eq. 1) and

*T*maxa) / K

12-15 12-15 12-17 12 15 – 5 ~182 ~88

> γ= π2*k*B2*D*(

β

Θ

*3.1.2. Two-dimensional superconductors (BO, ET, and BETS families)* 

= 48π*Nk*B/5

exp(−1/*V*el-ph*D*(

*k*B: Boltzmann constant, *g*: coupling constant, *V*el-ph: electron-phonon coupling potential,

TTF derivatives with "peripheral addition of alkylchalcogeno groups" were found to be effective to increase dimensionality of CT solids and suppress the Peierls-type MI transition for many BO [15] and ET [16–20] conductors. The robust intermolecular interactions in the BO complexes have provided a metallic state even in the strongly disordered systems. The

*P*c / GPa

 0.65 0.95 1.05 1.2 1.1 0 – 0.95 0.5

5.2-5.4 5.4-9 3.35-3.75 2.6

ε

Θ

ε

a) *T*max: temperature at maximum conductivity. b) *T*c: on-set. c) SDW: spin density wave, OD: order-disorder transition

*T*c b) / K

1.1 1.1 0.38 1.26 1.35 1.4 – 1.2 3

The isomorphous (TMTTF)2X (TMTTF: Scheme 3) salts displayed superconductivity under high pressure of 2.6–9 GPa with *T*c less than 3 K for X = Br, BF4, PF6, and SbF6 [22–25]. Table 1 summarizes superconductors of (TMTSF)2X and (TMTTF)2X showing σRT, temperature at which conductivity shows maximum (*T*max) due to an MI transition, critical pressure to induce superconductivity (*P*c), *T*c, phenomena which cause the MI transition (transition temperature), and superlattice after the ordering of the anion molecules.

(TMTSF)2ClO4 is the only AP superconductor among them and shows no Hebel-Slichter coherence peak [26], which should be observed just below *T*c for a normal BCS-type superconductor having an isotropic gap [27], in the early measurements of relaxation rate of 1H NMR absorption. Later, the thermal conductivity suggested a fully-gapped order parameter [28]. The superconducting coherent lengths are 710 (//*a*), 340 (//*b*), and 20 Å (//*c*), indicating a quasi-one-dimensional character. The application of magnetic field breaks the superconducting state and induces a sequence of SDW (field-induced SDW: FISDW) states above 3 T. Upper critical magnetic field *H*c2 of the PF6 salt (*H*c2 = 6 (//*b*'), 4 (//*a*) T at 0.1 K) is far beyond the Pauli limit (*H*Pauli) for the BCS-type superconductor with weak coupling [10]. A generalized phase diagram including (TMTSF)2X and (TMTTF)2X indicates that the superconducting phase neighbors the magnetic SDW phase (Fig. 4)[29].

**Figure 4.** Generalized phase diagram for the (TMTSF)2X and (TMTTF)2X [29]. CL, SP, and SC refer to charge-localized (which corresponds to charge-ordered state), spin-Peierls, and superconducting states, respectively. The salts **a**–**d** at AP locates in the generalized diagram. **a**: (TMTTF)2PF6, **b**: (TMTTF)2Br, **c**: (TMTSF)2PF6, **d**: (TMTSF)2ClO4.

**Scheme 3.**



a) *T*max: temperature at maximum conductivity. b) *T*c: on-set. c) SDW: spin density wave, OD: order-disorder transition of anion and newly formed superlattice. FISDW: field-induced SDW. γ and β are important quantities experimentally determined to obtain *D*(εF) (eq. 1) and Θ(eq. 2) which are related with *T*c by eq. 3 for the BCS-type superconductors.

**Table 1.** Organic superconductors of (TMTSF)2X and (TMTTF)2X

γ: Sommerfeld coefficient, mJ mol–1 K–2

$$\mathcal{Y} = \pi^2 k \alpha^2 D(\mathfrak{e}\mathfrak{r})/\mathfrak{Z} \tag{1}$$

β: mJ mol–1 K–4

110 Superconductors – Materials, Properties and Applications

warped Fermi surface (2*a* × 2*b*, Fig. 1a).

(TMTSF)2PF6, **d**: (TMTSF)2ClO4.

**Scheme 3.**

Salts with octahedral anions exhibited MI transition at 11–17 K at AP due to SDW, and a superconductivity appeared with on-set *T*c of ca. 1 K at 0.6–1.2 GPa (Fig. 3d). Salts with (pseudo)tetrahedral anions, on the other hand, exhibited order-disorder (OD) transition of anion molecules when the superlattice created was coincide with the nesting vector of the

The isomorphous (TMTTF)2X (TMTTF: Scheme 3) salts displayed superconductivity under high pressure of 2.6–9 GPa with *T*c less than 3 K for X = Br, BF4, PF6, and SbF6 [22–25]. Table 1

which conductivity shows maximum (*T*max) due to an MI transition, critical pressure to induce superconductivity (*P*c), *T*c, phenomena which cause the MI transition (transition

(TMTSF)2ClO4 is the only AP superconductor among them and shows no Hebel-Slichter coherence peak [26], which should be observed just below *T*c for a normal BCS-type superconductor having an isotropic gap [27], in the early measurements of relaxation rate of 1H NMR absorption. Later, the thermal conductivity suggested a fully-gapped order parameter [28]. The superconducting coherent lengths are 710 (//*a*), 340 (//*b*), and 20 Å (//*c*), indicating a quasi-one-dimensional character. The application of magnetic field breaks the superconducting state and induces a sequence of SDW (field-induced SDW: FISDW) states above 3 T. Upper critical magnetic field *H*c2 of the PF6 salt (*H*c2 = 6 (//*b*'), 4 (//*a*) T at 0.1 K) is far beyond the Pauli limit (*H*Pauli) for the BCS-type superconductor with weak coupling [10]. A generalized phase diagram including (TMTSF)2X and (TMTTF)2X indicates that the

**Figure 4.** Generalized phase diagram for the (TMTSF)2X and (TMTTF)2X [29]. CL, SP, and SC refer to charge-localized (which corresponds to charge-ordered state), spin-Peierls, and superconducting states, respectively. The salts **a**–**d** at AP locates in the generalized diagram. **a**: (TMTTF)2PF6, **b**: (TMTTF)2Br, **c**:

σ

RT, temperature at

summarizes superconductors of (TMTSF)2X and (TMTTF)2X showing

temperature), and superlattice after the ordering of the anion molecules.

superconducting phase neighbors the magnetic SDW phase (Fig. 4)[29].

$$
\beta = 48\pi \text{Nks/} 5 \,\Theta^3\tag{2}
$$

Θ: Debye temperature, K

$$T\_{\mathfrak{c}\curvearrowright} \Theta \exp(-1/V\_{\mathfrak{e}\uph}D(\mathfrak{e}\mathfrak{e})) \tag{3}$$

*k*B: Boltzmann constant, *g*: coupling constant, *V*el-ph: electron-phonon coupling potential, *D*(εF): density of states at Fermi level per one spin.

### *3.1.2. Two-dimensional superconductors (BO, ET, and BETS families)*

### *3.1.2.1. BO superconductors*

TTF derivatives with "peripheral addition of alkylchalcogeno groups" were found to be effective to increase dimensionality of CT solids and suppress the Peierls-type MI transition for many BO [15] and ET [16–20] conductors. The robust intermolecular interactions in the BO complexes have provided a metallic state even in the strongly disordered systems. The

strong two-dimensionality in the BO complexes hardly exhibited any phase transition including the superconductivity (only two superconductors with *T*<sup>c</sup> ≤ 1.5 K were found) [30,31].

Development and Present Status of Organic Superconductors 113




, S⋅⋅⋅S) and ET⋅⋅⋅anion (hydrogen bonds) intermolecular

β

κ

ε

κ


κ


g = 24 meV)-semiconductor



θ

β-

α-

α-

> κ-


κ-type

to those for TMTSF, especially for salts with small and discrete anions such as I3, ClO4, PF6, etc., where polymorphic isomers are frequently afforded. In the salts with discrete linear anions such as I3 and I2Br, the component molecules have great freedom of motion and the donor packing pattern can be changed by thermal or pressure treatment [35,36]. Scanning tunneling microscope (STM) measurements [37–39] revealed that the surface structure of

(ET)2I3 crystals contains many defects, voids, and reconstruction of donor packing attributed to the unstable structure of the anion layers, while the surface structures of salts with

seen in Fig. 5b. Two ET molecules form a dimer unit which fits into each opening. In more accurate description, the hydrogen atom of one ethylene group of ET molecule fits into the core created by anion molecules, like a key-keyhole relation. The position of such an ethylene hydrogen atom projected onto the anion cores produces unique patterns; called

(about 31 superconductors) [40]. It means that the ET molecules arrange according to the

interactions, large conformational freedom of ethylene groups, flexible molecular framework, fairly narrow bandwidth (*W*), and strong electron correlations, which are represented by on-site Coulomb repulsion energy *U*, gave a rich variety of complexes with different crystal and electronic structures ranging from insulators to superconductors (corresponding references are cited in Ref. 16): Mott insulators (including spin-Peierls systems, antiferromagnets, spin-ladder systems, and quantum spin liquid), one-dimensional metals with CDW transition, two-dimensional metals with CDW transition, twodimensional metals with FISDW transition, charge-ordered insulators, monotropic complex

About 60 ET superconductors have so far been known. Table 2 summarizes selected ET superconductors and related salts. They are classified into three classes based on the transport behavior at AP: 1) Salts in Class I are metallic down to rather low *T*c. 2) Salts in Class II are close to a Mott insulator and a poor metal showing *T*c > 10 K. 3) Salts in Class III are insulators (Mott, CDW, or charge order). Figure 6 compares the temperature

showed a monotonous decrease of resistivity with upper curvature down to *T*c.

[51] (**2**, Class II) except a metallic regime near RT in 2. They have a semiconductive region

g = 104 meV) transition at ca. 42 K due to an AF fluctuation resulting in a weak ferromagnet below 27 K (Néel temperature *T*N = 27 K). Under a low pressure, it showed a

κ


κ


κ

(ET)2Cu[N(CN)2]Br [55] (**3**, Class II) exhibited similar behavior to that of

down to 70–80 K followed by a metallic behavior down to *T*c.

(**4**, Class III) ia a Mott insulator and showed a semiconductor (


κ

(ET)2MHg(SCN)4 (M = NH4 and K) are stable with no defects.

κ

β

anion core or opening pattern created by polymerized anions.

ππ

isomers, and two-dimensional metals down to low temperatures.

polymerized anions such as

The polymerized anions in

type (5 superconductors),

Different kinds of ET⋅⋅⋅ET (

dependence of resistivity for several

similar temperature dependence to that of

(ε

exhibited metallic behavior down to *T*c at 4.9 K [48].

### *3.1.2.2. ET superconductors*

The first ET two-dimensional organic metal down to low temperatures is (ET)2(ClO4)(TCE) [32]. Since then, hundreds of ET solids have been prepared. ET molecules tend to pile up one after the other with sliding to each other so as to minimize the steric hindrance caused by the terminal ethylene groups. A neutral ET molecule is non-planar and becomes almost flat on formation of the partial CT complex except the terminal ethylene groups which are thermally disordered at high temperatures. Segregated packing of such molecules leaves cavities along the molecular long axis, where counter anions and sometimes solvent molecules occupy. It was pointed out that the ethylene conformation is one of the key parameters determining the physical and structural properties including the superconductivity [33]. ET molecule also has a strong tendency to form proximate intermolecular S∙∙∙S contacts along the side-by-side direction leading to an increment of the side-by-side transfer integrals *t*⊥ (Fig. 5). The ET conductors are composed of alternating structures of two-dimensional conducting layer and insulating anion layer. Significant donor∙∙∙anion interactions arise from the short atomic contacts between the ethylene hydrogen atoms of ET and anion atoms around the anion openings in the anion layer as schematically shown in Fig. 5b [34].

**Figure 5.** (a) Schematic figure of an example of S∙∙∙S atomic contacts observed in ET salts. Thick dotted lines: Sin∙∙∙Sout. Thin dotted lines: Sout∙∙∙Sout. (b). Schematic view of κ-(ET)2Cu(NCS)2 indicating anion openings and transfer interactions (*t*//, *t*⊥, and *t*'⊥) [34]. ET dimers are nearly orthogonally aligned (κ-type packing). For κ-type salt, *t*// ~ *t*<sup>⊥</sup> >> *t*'⊥.

The steric hindrance exerted by bulky six-membered rings of ET molecules prevents the formation of intermolecular Sin∙∙∙Sin contacts (Sin: sulfur atom in the TTF skeleton, Fig. 5a). No particular patterns of intermolecular Sin∙∙∙Sout (Sout: sulfur atom in the six-membered ring) are favorable as well. As a consequence various kinds of S∙∙∙S contacts are produced depending on the donor packing patterns (α-, β-, θ-, κ-phases, and so forth, see Section 3-1-2-5), and are comparable to the other intermolecular interactions; *i.e.*, face-to-face (ππ) and donor∙∙∙anion interactions. Any interactions could not solely determine the donor packing picture. It is thus much more difficult to predict the donor packing pattern for the ET system compared to those for TMTSF, especially for salts with small and discrete anions such as I3, ClO4, PF6, etc., where polymorphic isomers are frequently afforded. In the salts with discrete linear anions such as I3 and I2Br, the component molecules have great freedom of motion and the donor packing pattern can be changed by thermal or pressure treatment [35,36]. Scanning tunneling microscope (STM) measurements [37–39] revealed that the surface structure of β- (ET)2I3 crystals contains many defects, voids, and reconstruction of donor packing attributed to the unstable structure of the anion layers, while the surface structures of salts with polymerized anions such as κ-(ET)2Cu(NCS)2 (Fig. 7b in Section 3-1-2-3) and α- (ET)2MHg(SCN)4 (M = NH4 and K) are stable with no defects.

112 Superconductors – Materials, Properties and Applications

[30,31].

*3.1.2.2. ET superconductors* 

schematically shown in Fig. 5b [34].

packing). For

κ

on the donor packing patterns (

strong two-dimensionality in the BO complexes hardly exhibited any phase transition including the superconductivity (only two superconductors with *T*<sup>c</sup> ≤ 1.5 K were found)

The first ET two-dimensional organic metal down to low temperatures is (ET)2(ClO4)(TCE) [32]. Since then, hundreds of ET solids have been prepared. ET molecules tend to pile up one after the other with sliding to each other so as to minimize the steric hindrance caused by the terminal ethylene groups. A neutral ET molecule is non-planar and becomes almost flat on formation of the partial CT complex except the terminal ethylene groups which are thermally disordered at high temperatures. Segregated packing of such molecules leaves cavities along the molecular long axis, where counter anions and sometimes solvent molecules occupy. It was pointed out that the ethylene conformation is one of the key parameters determining the physical and structural properties including the superconductivity [33]. ET molecule also has a strong tendency to form proximate intermolecular S∙∙∙S contacts along the side-by-side direction leading to an increment of the side-by-side transfer integrals *t*⊥ (Fig. 5). The ET conductors are composed of alternating structures of two-dimensional conducting layer and insulating anion layer. Significant donor∙∙∙anion interactions arise from the short atomic contacts between the ethylene hydrogen atoms of ET and anion atoms around the anion openings in the anion layer as

**Figure 5.** (a) Schematic figure of an example of S∙∙∙S atomic contacts observed in ET salts. Thick dotted

The steric hindrance exerted by bulky six-membered rings of ET molecules prevents the formation of intermolecular Sin∙∙∙Sin contacts (Sin: sulfur atom in the TTF skeleton, Fig. 5a). No particular patterns of intermolecular Sin∙∙∙Sout (Sout: sulfur atom in the six-membered ring) are favorable as well. As a consequence various kinds of S∙∙∙S contacts are produced depending

interactions. Any interactions could not solely determine the donor packing picture. It is thus much more difficult to predict the donor packing pattern for the ET system compared

openings and transfer interactions (*t*//, *t*⊥, and *t*'⊥) [34]. ET dimers are nearly orthogonally aligned (

κ



ππ κ-type

) and donor∙∙∙anion

lines: Sin∙∙∙Sout. Thin dotted lines: Sout∙∙∙Sout. (b). Schematic view of

α-, β-, θ-, κ

comparable to the other intermolecular interactions; *i.e.*, face-to-face (


The polymerized anions in κ-(ET)2Cu(NCS)2 form the insulating layer having openings as seen in Fig. 5b. Two ET molecules form a dimer unit which fits into each opening. In more accurate description, the hydrogen atom of one ethylene group of ET molecule fits into the core created by anion molecules, like a key-keyhole relation. The position of such an ethylene hydrogen atom projected onto the anion cores produces unique patterns; called αtype (5 superconductors), β-type (6 superconductors), θ-type (1 superconductor), and κ-type (about 31 superconductors) [40]. It means that the ET molecules arrange according to the anion core or opening pattern created by polymerized anions.

Different kinds of ET⋅⋅⋅ET (ππ, S⋅⋅⋅S) and ET⋅⋅⋅anion (hydrogen bonds) intermolecular interactions, large conformational freedom of ethylene groups, flexible molecular framework, fairly narrow bandwidth (*W*), and strong electron correlations, which are represented by on-site Coulomb repulsion energy *U*, gave a rich variety of complexes with different crystal and electronic structures ranging from insulators to superconductors (corresponding references are cited in Ref. 16): Mott insulators (including spin-Peierls systems, antiferromagnets, spin-ladder systems, and quantum spin liquid), one-dimensional metals with CDW transition, two-dimensional metals with CDW transition, twodimensional metals with FISDW transition, charge-ordered insulators, monotropic complex isomers, and two-dimensional metals down to low temperatures.

About 60 ET superconductors have so far been known. Table 2 summarizes selected ET superconductors and related salts. They are classified into three classes based on the transport behavior at AP: 1) Salts in Class I are metallic down to rather low *T*c. 2) Salts in Class II are close to a Mott insulator and a poor metal showing *T*c > 10 K. 3) Salts in Class III are insulators (Mott, CDW, or charge order). Figure 6 compares the temperature dependence of resistivity for several κ-(ET)2X with that of β-(ET)2AuI2 (**6**, Class I) which exhibited metallic behavior down to *T*c at 4.9 K [48]. κ-(ET)2Cu(CN)[N(CN)2] [49] (**1**, Class II) showed a monotonous decrease of resistivity with upper curvature down to *T*c. κ- (ET)2Cu[N(CN)2]Br [55] (**3**, Class II) exhibited similar behavior to that of κ-(ET)2Cu(NCS)2 [51] (**2**, Class II) except a metallic regime near RT in 2. They have a semiconductive region down to 70–80 K followed by a metallic behavior down to *T*c. κ-(ET)2Cu[N(CN)2]Cl [57–60] (**4**, Class III) ia a Mott insulator and showed a semiconductor (εg = 24 meV)-semiconductor (εg = 104 meV) transition at ca. 42 K due to an AF fluctuation resulting in a weak ferromagnet below 27 K (Néel temperature *T*N = 27 K). Under a low pressure, it showed a similar temperature dependence to that of κ-(ET)2Cu[N(CN)2]Br. κ-(ET)2Cu2(CN)3 [61–65] (**5**,

Class III) is semiconductive (a Mott insulator) and under pressure it also behaves similarly to **4** (semiconductor-metal-superconductor).

Development and Present Status of Organic Superconductors 115

Table 2 summarizes *T*c of H- and D-salts (salt using h8-ET and d8-ET, respectively; Scheme 4). The calculated *U*/*W* values close to unity suggest that those salts have strong electron

K is estimated) and D-salt of **4** (*T*c = 13.1 K at 0.03 GPa) [58] show the highest *T*c under pressure, while both are Mott insulators at AP. At AP, D-salt of **1** shows the highest *T*c of 12.3 K [50] followed by H-salt of **3** (*T*c = 11.8 K) [55,56]. These salts are electronically clean metals as evidenced by the observation of quantum oscillations (Shubnikov-de Haas (SdH), de Haas-van Alphen (dHvA)), and geometrical oscillations: angular dependent magnetoresistance oscillation (AMRO), which afford topological information for the Fermi

and physical properties [49–65]. Figure 7 shows the crystal structure of the prototype H-salt of **2**, anion structures, donor packing pattern, and calculated Fermi surface [51–54]. Table 2 summarizes the two kinds of ligand (L1, L2) in a salt and ratio *t*'/*t* for triangle geometry of ET dimers. These salts have polymerized anions in which ligand L1 forms infinite chain by coordinating to Cu1+ and ligand L2 coordinates to Cu1+ as pendant. ET molecules form a dimer and the ET dimers are arranged nearly orthogonally to each other forming twodimensional conducting ET layer in the *bc*-plane which is sandwiched by the insulating anion layers along the *a*-axis (Fig. 7a). Cu1+ and SCN form Cu⋅⋅⋅SCN⋅⋅⋅Cu⋅⋅⋅SCN⋅⋅⋅ zigzag infinite chain along the *b*-axis and other ligand SCN coordinates to Cu1+ by N atom to make an open space (indicated by ellipsoid in Fig. 7b, also see Fig. 5) to which an ET dimer fits. An ET dimer has one spin, and the dimers form triangle (Fig. 7c) whose shape is represented by

H-salt of **4** showed a complicated *T*-*P* phase diagram (Fig. 8a) [72–76]. Thoroughgoing studies under pressure showed a firm evidence of the coexistence of superconducting (**I-SC-2** phase: **I-SC** = incomplete superconducting) and AF phases [72–78], where the radical electrons of ET molecules played both roles of localized and itinerant ones. Under a pressure of ca. 20–30 MPa another incomplete superconducting phase (**I-SC-1**) appeared and the complete superconducting (**C-SC**) phase neighbored to this phase at higher pressures. Below these superconducting phases, reentrant nonmetallic (**RN**) phase was observed.

κ


κ

κ

Similar *T*-*P* phase diagrams were obtained for

*'*-(h8-ET)2ICl2 (on-set *T*c = 14.2 K at 8.2 GPa [68], mid-point *T*c of 13.4



**Scheme 4.**

correlation. Currently

surface [19,20,70].

the ratio *t*'/*t* (Fig. 7d).

*3.1.2.3.* κ

The κ

Next, we will focus mainly on the

*-type ET conductors* 


β

**Figure 6.** Temperature dependences of resistivity of 10 K class superconductors κ- (ET)2Cu(CN)[N(CN)2] (**1**), κ-(ET)2Cu(NCS)2 (**2**), κ-(ET)2Cu[N(CN)2]Br (**3**), and κ-(ET)2Cu[N(CN)2]Cl (**4**), which are compared with that of a good metal with low *T*<sup>c</sup> β-(ET)2AuI2 (**6**) and a Mott insulator κ- (ET)2Cu2(CN)3 (**5**) at AP.


a) \*: mid-point. \*\*on-set under uniaxial strain (see Fig. 9a). Others are the on-set values under hydrostatic pressure. b) SC: superconductor, SL: spin liquid.

**Table 2.** Selected ET conductors and superconductors. Except ET•TCNQ, the compound is represented by *Greek alphabet*-(ET)2X (Greek alphabet: type of donor stacking, L1, L2: ligand). a) Class **I** : good metal with low *T*c, **II**: 10 K class AP superconductor, **III**: Mott insulator. **1**–**6** are the numbers in Fig. 6.

### **Scheme 4.**

114 Superconductors – Materials, Properties and Applications

to **4** (semiconductor-metal-superconductor).

κ

L1, L2


N(CN)2

NCS

Br

Cl

*'*-(ET)2ICl2 3×10–2 14.2/8.2

*'*-(ET)2BrICl ~10–2 7.2/8.0

CN/NC

which are compared with that of a good metal with low *T*<sup>c</sup>

σRT/ S cm–1 *T*c a) / K H-salt D-salt

(ET)2Cu(CN)[N(CN)2] (**1**),

(ET)2Cu2(CN)3 (**5**) at AP.

 **I** κ

> β

> **6** β

**2** κ

**3** κ

**5** κ

 β

 β

 β

**III 4** κ

**II 1** κ

Class, Salt CuL1L2






SC: superconductor, SL: spin liquid.

Class III) is semiconductive (a Mott insulator) and under pressure it also behaves similarly

κ-

Characteristics, Quantum oscillations

= 218, SdH, dHvA

215, SdH, dHvA, AMRO

210, SdH, AMRO

AF *T*<sup>N</sup> = 19.5 K [69]


Θ

Θ=

Θ=

κ-

Ref

[41]

[51–54]

[55,56]

[57–60]

65]

κ


state at AP b)

− 0.55 SC SdH, dHvA, AMRO [42–47]

**Figure 6.** Temperature dependences of resistivity of 10 K class superconductors

8.1

2 12.8/0.0 3 GPa

GPa

GPa

low *T*c, **II**: 10 K class AP superconductor, **III**: Mott insulator. **1**–**6** are the numbers in Fig. 6.

2–7 6.8– 7.3\*\* κ



ET•TCNQ 10 − AF *T*<sup>N</sup> = 3 K [66]

*'*-(ET)2AuCl2 ~10–1 − AF *T*<sup>N</sup> = 28 K [68]

a) \*: mid-point. \*\*on-set under uniaxial strain (see Fig. 9a). Others are the on-set values under hydrostatic pressure. b)

**Table 2.** Selected ET conductors and superconductors. Except ET•TCNQ, the compound is represented by *Greek alphabet*-(ET)2X (Greek alphabet: type of donor stacking, L1, L2: ligand). a) Class **I** : good metal with


*t*'/*t U/W* Ground

5–50 11.2\* 12.3\* 0.66–0.71 0.87 SC AMRO [49,50]

13.1 0.75 0.90 AF SdH, AMRO, *T*N = 27

K

1.06 0.9 SL SdH, AMRO [49,61–

− AF *T*<sup>N</sup> = 22 K [67,68]

5–40 10.4\* 11.2\* 0.82–0.86 0.94 SC *γ* = 25, *β* = 11.2,

5–50 11.8\* 11.2\* 0.68 0.92 SC *γ* = 22, *β* = 12.8,

β


Table 2 summarizes *T*c of H- and D-salts (salt using h8-ET and d8-ET, respectively; Scheme 4). The calculated *U*/*W* values close to unity suggest that those salts have strong electron correlation. Currently β*'*-(h8-ET)2ICl2 (on-set *T*c = 14.2 K at 8.2 GPa [68], mid-point *T*c of 13.4 K is estimated) and D-salt of **4** (*T*c = 13.1 K at 0.03 GPa) [58] show the highest *T*c under pressure, while both are Mott insulators at AP. At AP, D-salt of **1** shows the highest *T*c of 12.3 K [50] followed by H-salt of **3** (*T*c = 11.8 K) [55,56]. These salts are electronically clean metals as evidenced by the observation of quantum oscillations (Shubnikov-de Haas (SdH), de Haas-van Alphen (dHvA)), and geometrical oscillations: angular dependent magnetoresistance oscillation (AMRO), which afford topological information for the Fermi surface [19,20,70].

Next, we will focus mainly on the κ-type superconductors with polymerized anions.

#### *3.1.2.3.* κ*-type ET conductors*

The κ-type superconductors κ-(ET)2CuL1L2 (**1**–**5** in Table 2) share some common structural and physical properties [49–65]. Figure 7 shows the crystal structure of the prototype H-salt of **2**, anion structures, donor packing pattern, and calculated Fermi surface [51–54]. Table 2 summarizes the two kinds of ligand (L1, L2) in a salt and ratio *t*'/*t* for triangle geometry of ET dimers. These salts have polymerized anions in which ligand L1 forms infinite chain by coordinating to Cu1+ and ligand L2 coordinates to Cu1+ as pendant. ET molecules form a dimer and the ET dimers are arranged nearly orthogonally to each other forming twodimensional conducting ET layer in the *bc*-plane which is sandwiched by the insulating anion layers along the *a*-axis (Fig. 7a). Cu1+ and SCN form Cu⋅⋅⋅SCN⋅⋅⋅Cu⋅⋅⋅SCN⋅⋅⋅ zigzag infinite chain along the *b*-axis and other ligand SCN coordinates to Cu1+ by N atom to make an open space (indicated by ellipsoid in Fig. 7b, also see Fig. 5) to which an ET dimer fits. An ET dimer has one spin, and the dimers form triangle (Fig. 7c) whose shape is represented by the ratio *t*'/*t* (Fig. 7d).

H-salt of **4** showed a complicated *T*-*P* phase diagram (Fig. 8a) [72–76]. Thoroughgoing studies under pressure showed a firm evidence of the coexistence of superconducting (**I-SC-2** phase: **I-SC** = incomplete superconducting) and AF phases [72–78], where the radical electrons of ET molecules played both roles of localized and itinerant ones. Under a pressure of ca. 20–30 MPa another incomplete superconducting phase (**I-SC-1**) appeared and the complete superconducting (**C-SC**) phase neighbored to this phase at higher pressures. Below these superconducting phases, reentrant nonmetallic (**RN**) phase was observed. Similar *T*-*P* phase diagrams were obtained for κ-(d8-ET)2X (X = Cu[N(CN)2]Cl and

Cu[N(CN)2]Br) with a parallel shift of pressure. They occur at the higher and lower pressure sides of the κ-(h8-ET)2Cu[N(CN)2]Cl for the Br and Cl salts, respectively. Contrary to the Hsalt, κ-(d8-ET)2Cu[N(CN)2]Cl exhibited no coexistence of the superconducting and AF phases. At AP, κ-(ET)2Cu[N(CN)2]I is a semiconductive and becomes superconducting under hydrostatic pressure above 0.12 GPa with *T*c of 7.7 K (on-set *T*c = 8.2 K), though the magnetic ordering was not clarified at AP [79].

Development and Present Status of Organic Superconductors 117

2) phase, respectively [81,82]. Since

l-(ET)2Cu(CF3)4(TCE) (*T*c = 4.0 K). If this explanation is

κ

*'*-(ET)2X (X = ICl2 [67,68] and BrICl [69]) having high *T*c in the ET

κ

: The superconducting coherent lengths are 29 and 3.1 Å for **2** at 0.5 K

ξ

//) and perpendicular to the plane (



ξ⊥). The

<sup>⊥</sup> is much smaller than the lattice

ξ// is

κ


α'κ-

> α*'*-

Alternating mixed donor packing motifs were observed in two phases of

α' + κ1 + α' + κ

κ

correct, this is an example of interface superconductivity [83,84].

*T*c is expected for the salt having a larger anion spacing. Such a

layered (

*D*(ε α' + κ

also for other types, *e.g.*,

observed data.

2. Coherent length

these results [93].

family are Mott insulators at AP.

these systems in comparison with

) and four-layered (

β

The followings are the superconducting characteristics of

larger than the lattice constants, however, the

effects is not fully understood yet consistently.

ξ

along the two-dimensional plane (

of which differ from those of the conventional BCS superconductors.

(ET)2Ag(CF3)4(TCE), *T*c (on-set) of which are 9.5 K and 11.0 K for the phase having two-

packing generally imparts semiconducting state, both systems have nano-scale hetero junction of semiconductive/superconductive interface, which is thought to give higher *T*c in

With increasing the distance between the ET dimers in Fig. 7a–d, the transfer interactions between ET dimers decrease; this may correspond to the decrease of *W* and to increase of

Topological structures of their Fermi surfaces studied by SdH, dHvA, and AMRO (Table 2) [19,20,53,70,71,85–87] show that the area of the closed Fermi surface relative to the first Brillouin zone and cyclotron mass calculated from SdH oscillations are 15.7% (α-orbit in Fig. 7e, 3.5*m*e) and 105% (β-orbit in Fig. 7e, 6.5*m*e) for **2** at AP, 4.4% (0.95*m*e) at 0.9 GPa and *ca.* 100% (6.7*m*e) at AP for **3**, and 15.5% (1.7*m*e) and 102% (3.5*m*e) at 0.6 GPa for **4**. Fermi surface of **2** (Fig. 7e) calculated based on the crystal structure is in good agreement with these

1. Upper critical magnetic field *Hc*2: **2** gave higher *H*c2 values for the magnetic field parallel to the two-dimensional plane than *H*Pauli based on a simple BCS model [88,89].

constant indicating the conducting layers along this direction is Josephson coupled. 3. Symmetry of superconducting state: No Hebel-Slichter coherence peak was observed in both **2** and **3** in 1H NMR measurements, ruling out the BCS *s*-wave state. The symmetry of the superconducting state of **2** had been controversially described as normal BCStype or non-BCS type, however, STM spectroscopy showed the *d*-wave symmetry with line nodes along the direction near π/4 from the *k*a- and *k*c-axes (*d*x2–y2) [90], and thermal conductivity measurements were consistent with that [91]. STM on **3** also showed the same symmetry [92]. A recent specific heat measuremen on **2** and **3** was consistent with

4. Inverse isotope effect: Inverse isotope effect has so far been observed for **1** [50], **2** [54], and **4** [58], while normal isotope effect for **3** [56]. The reason of the observed isotope

ξ

near the border between poor metals and Mott insulators. It is true not only for

F), and consequently *T*c is expected to increase. According to this line of thought, higher

**Figure 7.** κ-(ET)2Cu(NCS)2: (a) Crystal structure. (b) Anion layer viewed along the *a*-axis has anion openings (indicated by ellipsoid) to which an ET dimer fits. Picture is the dextrorotatory form. (c) Packing pattern (κ-type) of ET dimers along the *a*-axis. (d) Schematic view of triangular lattice of ET dimers which has one spin. White and black circles represent ET molecule and ET dimer, respectively. The *t*'/*t* represents the shape of the triangle. (e) Calculated Fermi surfaces of the *P*21 salts (κ- (ET)2Cu(NCS)2, κ-(ET)2Cu(CN)[N(CN)2]) showed the certain energy gap between a one-dimensional electron like Fermi surface (//*k*c) and a two-dimensional cylindrical hole-like one (α-orbit), while such a gap is absent in the *Pnma* salts (κ-(ET)2Cu[N(CN)2]Br, κ-(ET)2Cu[N(CN)2]Cl). For κ-(ET)2Cu(NCS)2, electrons move along the closed ellipsoid (α-orbit) to exhibit SdH oscillations [53], and at higher magnetic field (> 20 T) electrons hop from the ellipsoid to open Fermi surface to show circular trajectory (β-orbit, magnetic breakdown oscillations) [71].

**Figure 8.** (a) Phase diagram of κ-(h8-ET)2Cu[N(CN)2]Cl determined from electrical conductivity and magnetic measurements [72–76]. **N1**–**N4**: non-metallic phases, **M**: metallic phase, **RN**: reentrant nonmetallic phase, **I-SC-I** and **I-SC-II**: incomplete superconducting phases. **N3** shows growth of threedimensional AF ordered phase. **N4** is a weak ferromagnetic phase. (b) Proposed simplified phase diagram [80]. **a**: β-(ET)2I3, **b**: κ-(ET)2Cu(NCS)2, **c**: H-salt of κ-(ET)2Cu[N(CN)2]Br, **d**: D-salt of κ- (ET)2[N(CN)2]Br, **e**: κ -(ET)2[N(CN)2]Cl, **f**: β'-(ET)2ICl2.

Alternating mixed donor packing motifs were observed in two phases of α'κ- (ET)2Ag(CF3)4(TCE), *T*c (on-set) of which are 9.5 K and 11.0 K for the phase having twolayered (α' + κ) and four-layered (α' + κ1 + α' + κ2) phase, respectively [81,82]. Since α*'* packing generally imparts semiconducting state, both systems have nano-scale hetero junction of semiconductive/superconductive interface, which is thought to give higher *T*c in these systems in comparison with κl-(ET)2Cu(CF3)4(TCE) (*T*c = 4.0 K). If this explanation is correct, this is an example of interface superconductivity [83,84].

116 Superconductors – Materials, Properties and Applications

magnetic ordering was not clarified at AP [79].

sides of the

phases. At AP,

salt, κ

**Figure 7.**

κ

κ

κ

gap is absent in the *Pnma* salts (

**Figure 8.** (a) Phase diagram of

β


κ

diagram [80]. **a**:

(ET)2[N(CN)2]Br, **e**:

(β-orbit, magnetic breakdown oscillations) [71].

Packing pattern (

(ET)2Cu(NCS)2,

κ

κ

Cu[N(CN)2]Br) with a parallel shift of pressure. They occur at the higher and lower pressure

under hydrostatic pressure above 0.12 GPa with *T*c of 7.7 K (on-set *T*c = 8.2 K), though the










κ-


κ-

κ

openings (indicated by ellipsoid) to which an ET dimer fits. Picture is the dextrorotatory form. (c)

The *t*'/*t* represents the shape of the triangle. (e) Calculated Fermi surfaces of the *P*21 salts (


κ

κ

κ


dimers which has one spin. White and black circles represent ET molecule and ET dimer, respectively.

electron like Fermi surface (//*k*c) and a two-dimensional cylindrical hole-like one (α-orbit), while such a

electrons move along the closed ellipsoid (α-orbit) to exhibit SdH oscillations [53], and at higher magnetic field (> 20 T) electrons hop from the ellipsoid to open Fermi surface to show circular trajectory

**(a) (b) a bc d e f** 

magnetic measurements [72–76]. **N1**–**N4**: non-metallic phases, **M**: metallic phase, **RN**: reentrant nonmetallic phase, **I-SC-I** and **I-SC-II**: incomplete superconducting phases. **N3** shows growth of threedimensional AF ordered phase. **N4** is a weak ferromagnetic phase. (b) Proposed simplified phase

'-(ET)2ICl2.

κ


β

κ

With increasing the distance between the ET dimers in Fig. 7a–d, the transfer interactions between ET dimers decrease; this may correspond to the decrease of *W* and to increase of *D*(εF), and consequently *T*c is expected to increase. According to this line of thought, higher *T*c is expected for the salt having a larger anion spacing. Such a κ-type salt may be found near the border between poor metals and Mott insulators. It is true not only for κ-type but also for other types, *e.g.*, β*'*-(ET)2X (X = ICl2 [67,68] and BrICl [69]) having high *T*c in the ET family are Mott insulators at AP.

Topological structures of their Fermi surfaces studied by SdH, dHvA, and AMRO (Table 2) [19,20,53,70,71,85–87] show that the area of the closed Fermi surface relative to the first Brillouin zone and cyclotron mass calculated from SdH oscillations are 15.7% (α-orbit in Fig. 7e, 3.5*m*e) and 105% (β-orbit in Fig. 7e, 6.5*m*e) for **2** at AP, 4.4% (0.95*m*e) at 0.9 GPa and *ca.* 100% (6.7*m*e) at AP for **3**, and 15.5% (1.7*m*e) and 102% (3.5*m*e) at 0.6 GPa for **4**. Fermi surface of **2** (Fig. 7e) calculated based on the crystal structure is in good agreement with these observed data.

The followings are the superconducting characteristics of κ-type ET superconductors, some of which differ from those of the conventional BCS superconductors.


5. A very simplified *T*-*P* phase diagram for κ-(ET)2X was proposed (Fig. 8b), where only the parameter *U*/*W* is taken into account. Fig. 8b includes the salts **2**, **3**, **4**, β-(ET)2I3, and β'-(ET)2ICl2 [80], however, the metallic behavior of **2** above 270 K and that of **1**, whole behavior of **5**, and low-temperature reentrant behavior of **3** and 4 (Fig. 8a) cannot be allocated in this diagram. The β-(ET)2I3 in Fig. 8b should be βH-phase (*T*c ~ 8 K, see Section 3-1-2-5) and other two β-(ET)2I3 salts of *T*c ~ 1.5 K and ~2 K phases cannot be allocated though they should have the same *U*/*W* values. *T*c of **4** is higher than that of β'- (ET)2ICl2 in Fig. 8b contrary to the experimentally observed *T*c results. This phase diagram and "geometrical isotope effect" [94] point out that *T*c's of β-(ET)2I3, **2**, and **3** decrease with increasing pressure if only the parameter *U*/*W* or *D*(εF) is taken into account. This tendency has been observed under hydrostatic pressure but not under uniaxial pressure (see Sections 3-1-2-4, 3-1-2-5, 3-1-3). Thus the phase diagram in Fig. 8b remains incomplete, despite it is frequently used to explain the general trends for these salts.

Development and Present Status of Organic Superconductors 119

κ-type

κ-

L-salt was converted to

H-salt returned to

L-salt is characterized by having a

α-,

βL-salt

epoxy-method [62,65]. In both cases, a superconducting state readily appeared nearly above 0.1 GPa since the *t*'/*t* deviates from unity; *i.e.*, strong spin frustration was released. It is very noteworthy that the spin liquid phase is neighboring to the superconducting state and its *T*<sup>c</sup> is fairly anisotropic as shown in Fig. 9b. A plot of *T*c vs. *T*IM, which is a Mott insulator-metal transition temperature, indicates that in comparison with the hydrostatic pressure results, the uniaxial method afforded: 1) a much higher *T*c value, 2) an increase of *T*c at the initial pressure region, 3) an anisotropic pressure dependence, and 4) superconducting phase

remains at higher pressure. The uniaxial strain experiments including other

**Figure 9.** a) Temperature-uniaxial pressure phase diagram in the low temperature region of

**(a) (b)** 

1.5, 8.1, 2.5, 3.6, and 3.6 K, respectively [36,41,42,44,45,117–120]. The

depressurizing while keeping the sample below 125 K [45,46]. The

when the salt was kept above 125 K at AP. The

(ET)2Cu2(CN)3 [62,65]. The strain along the *c*-axis corresponds to a decrease of *t*'/*t* (left side), while the strain along the *b*-axis increases *t*'/*t* (right side). b) Pressure dependence of on-set *T*c by the uniaxial strain and hydrostatic pressure methods. *T*IM: a Mott insulator-metal transition temperature [65].

One of the most intriguing ET superconductors is the salt with I3 anion, which afforded

H-salt by pressurizing (hydrostatic pressure) above 0.04 GPa and then by

β



β

β

the *t*'/*t* departs from unity [116].

*3.1.2.5. Other ET superconductors* 

α-, αt-, βL-, βH-, γ-, θ-, and κ

**Scheme 5.**

αt-, βL-, βH-, δ-, ε-, γ-, θ-, and κ

the β

Among them,

superconductors clearly revealed that the *T*c increased as the *U*/*W* approaches unity and as

#### *3.1.2.4. Quantum spin liquid state in* κ*-(ET)2Cu2(CN)3 and neighboring superconductivity*

As mentioned, κ-type packing is characterized by the triangular spin-lattice (Fig. 7c,7d) where an ET dimer is a unit with *S* = 1/2 spin [95,96]. The line shape of 1H NMR absorption of κ-(ET)2Cu[N(CN)2]Cl [60] exhibited a drastic change below 27 K owing to the formation of three-dimensional AF ordering. On the other hand, the absorption band of κ- (ET)2Cu2(CN)3 remains almost invariant down to 32 mK indicating non-spin-ordered state: quantum spin liquid state [63]. The appearance of spin liquid state in κ-(ET)2Cu2(CN)3 is the consequence of significant spin frustration in this salt (*t*'/*t* = 1.06) in comparison with the less frustrated AF state in κ-(ET)2Cu[N(CN)2]Cl (*t*'/*t* = 0.75).

It has long been predicted that the geometrical spin frustration of antiferromagnets caused by the spin correlation in particular spin geometry (triangle, tetrahedral, Kagome (Scheme 5), etc.) prevents the permanent ordering of spins. So the spins of Ising system with AF interaction in the equilateral triangle spin lattice will not show any long-range order even at 0 K, and hence the phase, namely quantum spin liquid phase, has high degeneracy [97]. Such spin liquid state has only been predicted theoretically [98], and a variety of ideal materials have been designed and examined for long [99–102]. Since the discovery of the spin liquid state in κ-(ET)2Cu2(CN)3, several materials have been reported to have such exotic spin state: EtMe3Sb[Pd(dmit)2]2 [103], ZnCu3(OH)6Cl2 [104,105], Na4Ir3O8 [106], and BaCu3V2O8(OH)2 [107,108]. Some inorganic materials reported as spin liquid candidates were eliminated owing to the spin ordering at extremely low temperatures, etc [109–113]. Na4Ir3O8 and two organic compounds (κ-(ET)2Cu2(CN)3, EtMe3Sb[Pd(dmit)2]2) may be recognized as "soft" Mott insulators and have metallic state under pressure. Only κ-(ET)2Cu2(CN)3 has the superconducting phase next to spin-liquid state so far as described below.

The phase diagrams of TMTSF (Fig. 4), ET (Fig. 8), C60 [114] families and also electroncorrelated cuprate and iron pnictide high *T*c systems [115] indicate that a magnetic ordered state (SDW, AF) is allocated next to the superconducting state. Figure 9a shows the *T*-*P* phase diagram of κ-(ET)2Cu2(CN)3 at low temperature region by applying uniaxial strain along *c-* (*t*'/*t* decreases in this direction) and *b*- (*t*'/*t* increases in this direction) axes with epoxy-method [62,65]. In both cases, a superconducting state readily appeared nearly above 0.1 GPa since the *t*'/*t* deviates from unity; *i.e.*, strong spin frustration was released. It is very noteworthy that the spin liquid phase is neighboring to the superconducting state and its *T*<sup>c</sup> is fairly anisotropic as shown in Fig. 9b. A plot of *T*c vs. *T*IM, which is a Mott insulator-metal transition temperature, indicates that in comparison with the hydrostatic pressure results, the uniaxial method afforded: 1) a much higher *T*c value, 2) an increase of *T*c at the initial pressure region, 3) an anisotropic pressure dependence, and 4) superconducting phase remains at higher pressure. The uniaxial strain experiments including other κ-type superconductors clearly revealed that the *T*c increased as the *U*/*W* approaches unity and as the *t*'/*t* departs from unity [116].

**Figure 9.** a) Temperature-uniaxial pressure phase diagram in the low temperature region of κ- (ET)2Cu2(CN)3 [62,65]. The strain along the *c*-axis corresponds to a decrease of *t*'/*t* (left side), while the strain along the *b*-axis increases *t*'/*t* (right side). b) Pressure dependence of on-set *T*c by the uniaxial strain and hydrostatic pressure methods. *T*IM: a Mott insulator-metal transition temperature [65].

### *3.1.2.5. Other ET superconductors*

**Scheme 5.**

118 Superconductors – Materials, Properties and Applications

allocated in this diagram. The

Section 3-1-2-5) and other two

*3.1.2.4. Quantum spin liquid state in* 

κ

κ

As mentioned,

frustrated AF state in

of κ

state in

κ

compounds (

phase diagram of

κ

κ

β

5. A very simplified *T*-*P* phase diagram for

κ

allocated though they should have the same *U*/*W* values. *T*c of **4** is higher than that of

(ET)2ICl2 in Fig. 8b contrary to the experimentally observed *T*c results. This phase diagram

tendency has been observed under hydrostatic pressure but not under uniaxial pressure (see Sections 3-1-2-4, 3-1-2-5, 3-1-3). Thus the phase diagram in Fig. 8b remains incomplete, despite it is frequently used to explain the general trends for these salts.

where an ET dimer is a unit with *S* = 1/2 spin [95,96]. The line shape of 1H NMR absorption

(ET)2Cu2(CN)3 remains almost invariant down to 32 mK indicating non-spin-ordered state:

consequence of significant spin frustration in this salt (*t*'/*t* = 1.06) in comparison with the less

It has long been predicted that the geometrical spin frustration of antiferromagnets caused by the spin correlation in particular spin geometry (triangle, tetrahedral, Kagome (Scheme 5), etc.) prevents the permanent ordering of spins. So the spins of Ising system with AF interaction in the equilateral triangle spin lattice will not show any long-range order even at 0 K, and hence the phase, namely quantum spin liquid phase, has high degeneracy [97]. Such spin liquid state has only been predicted theoretically [98], and a variety of ideal materials have been designed and examined for long [99–102]. Since the discovery of the spin liquid

EtMe3Sb[Pd(dmit)2]2 [103], ZnCu3(OH)6Cl2 [104,105], Na4Ir3O8 [106], and BaCu3V2O8(OH)2 [107,108]. Some inorganic materials reported as spin liquid candidates were eliminated owing to the spin ordering at extremely low temperatures, etc [109–113]. Na4Ir3O8 and two organic

The phase diagrams of TMTSF (Fig. 4), ET (Fig. 8), C60 [114] families and also electroncorrelated cuprate and iron pnictide high *T*c systems [115] indicate that a magnetic ordered state (SDW, AF) is allocated next to the superconducting state. Figure 9a shows the *T*-*P*

along *c-* (*t*'/*t* decreases in this direction) and *b*- (*t*'/*t* increases in this direction) axes with




of three-dimensional AF ordering. On the other hand, the absorption band of


'-(ET)2ICl2 [80], however, the metallic behavior of **2** above 270 K and that of **1**, whole behavior of **5**, and low-temperature reentrant behavior of **3** and 4 (Fig. 8a) cannot be


the parameter *U*/*W* is taken into account. Fig. 8b includes the salts **2**, **3**, **4**,

β

β

and "geometrical isotope effect" [94] point out that *T*c's of

increasing pressure if only the parameter *U*/*W* or *D*(

κ

quantum spin liquid state [63]. The appearance of spin liquid state in

insulators and have metallic state under pressure. Only

superconducting phase next to spin-liquid state so far as described below.



β

κ

κ


β

ε

*-(ET)2Cu2(CN)3 and neighboring superconductivity*


β

H-phase (*T*c ~ 8 K, see


F) is taken into account. This


β'-

> κ-



One of the most intriguing ET superconductors is the salt with I3 anion, which afforded α-, αt-, βL-, βH-, δ-, ε-, γ-, θ-, and κ-type salts with different crystal and electronic structures. Among them, α-, αt-, βL-, βH-, γ-, θ-, and κ-type salts are superconductors with *T*c = 7.2, ~8, 1.5, 8.1, 2.5, 3.6, and 3.6 K, respectively [36,41,42,44,45,117–120]. The βL-salt was converted to the βH-salt by pressurizing (hydrostatic pressure) above 0.04 GPa and then by depressurizing while keeping the sample below 125 K [45,46]. The βH-salt returned to βL-salt when the salt was kept above 125 K at AP. The βL-salt is characterized by having a

superlattice appearing at 175 K with incommensurate modulations of ET and I3 to each other [121]. The formation of the superlattice was suppressed by the pressure above 0.04 GPa. Then the two ethylene groups in an ET molecule were fixed in the eclipsed conformation to give rise to more than 5 times higher *T*c in βH-salt. The *T*c of βH-salt decreased with increasing hydrostatic pressure monotonously, however, under the uniaxial stress the further *T*c increase taking a maximum at a piston pressure of 0.3–0.4 GPa is observed for both directions parallel and perpendicular to the donor stack [120]. The superconducting coherent lengths are ξ// = 630 (//*a*) – 610 Å (//*b*') and ξ⊥ = 29 Å (//*c*\*) for the βL-salt, and ξ// = 130 Å and ξ⊥ = 10 Å for the βH-salt.

Development and Present Status of Organic Superconductors 121


λ


κ

λ



superconductivity [130–136]. The

superconducting character (

**Scheme 6.**

λ

applied exactly parallel to the conducting layers [132]. The

ξ

*3.1.3. Superconductors of other electron donor molecules* 

through a superconducting to insulating transition on cooling [135]. The

[137] and heavy-fermion system [138]. Recently it has been reported that

ξ

// = 125 Å,

8.3 K. For the FeCl4 salt, a relaxor ferroelectric behavior in the metallic state below 70 K [133] and a firm nonlinear electrical transport associated with the negative resistance effect in the magnetic ordered state have been observed [135]. Moreover it has been found that the FeCl4 salt shows the field-induced superconducting transition under a magnetic field of 18–41 T

(X = Cl, Br) are AF superconductors (*T*N = 2.5 K, *T*c = 1.1 K for X = Br: *T*N = 0.45 K, *T*c = 0.17 K for X = Cl) [136]. Similar phenomena, namely AF, ferromagnetic, or field-induced superconductivity, have been observed in several inorganic solids such as Chevrel phase

= 8 K(on-set), 5.5 K(mid-point)) exhibited superconductivity in the minute size of four pairs of (BETS)2GaCl4 based on the STM study [139]. This salt has two-dimensional

Besides TMTSF, TMTTF, BO, ET, and BETS superconductors, there are other superconductors (Scheme 7, numbers in bracket are the total members of each superconducting family and the highest *T*c) of CT salts based on symmetric (BEDSe-TTF [140] and BDA-TTP [141–144]) and asymmetric donors (ESET-TTF [145], *S*,*S*-DMBEDT-TTF [146], *meso*-DMBEDT-TTF [147,148], DMET [149], DODHT [150], TMET-STF [151], DMET-TSF [152], DIETS [153], EDT-TTF [154], MDT-TTF [155,156], MDT-ST [157,158], MDT-TS [159], MDT-TSF [160–163], MDSe-TSF [164], DTEDT [165], and DMEDO-TSeF [166,167]).

**Scheme 7.** Donor molecules for organic superconductors except TMTSF, TMTTF, BO, ET, and BETS systems. Numbers in bracket are the total members of each superconductor and the highest *T*c.

<sup>⊥</sup> = 16 Å).

The α-(ET)2I3 exhibited nearly temperature independent resistivity down to 135 K, at which charge-ordered MI transition occurred [122,123]. It has been claimed that α-(ET)2I3 has a zero-gap state with a Dirac cone type energy dispersion like graphene [124,125]. Under hydrostatic pressure it became two-dimensional metal down to low temperatures (2 GPa), however, it became superconductor under the uniaxial pressure along the *a*-axis (0.2 GPa, on-set *T*c = 7.2 K), though along the *b*-axis it remained metal down to low temperature (0.3– 0.5 GPa) [117]. α-(ET)2I3 was able to be converted to mosaic polycrystal with *T*c ~ 8 K by tempering at 70–100 °C for more than 3 days, giving αt-salt which exhibited similar NMR pattern to that of the βH-salt [36]. Other α-type superconductors (α-(ET)2MHg(SCN)4: M = K, NH4, Rb, Tl) seem to have charge-ordered state near or next to the superconducting state with low *T*c (highest *T*c = 1.7 K for M = NH4). Uniaxial strain increased *T*c anisotropically (6 K at 0.5 GPa //*c*, 4.5 K at 1 GPa //*b*\* for M = NH4) [126]. The salts α-(ET)2MHg(SCN)4 were confirmed to retain their donor packing patterns under pressure at low temperature by the SdH observation. However, for α-(ET)2I3 under pressure at low temperature, no exact information is reported for the donor packing in the superconducting state.

Only one θ-type superconductor, θ-(ET)2I3, is known, however, one third of the obtained crystals are superconducting and others remain metallic [127]. Several θ-type salts are arranged by their inter-column transfer interactions, and the dihedral angle between columns in a phase diagram showing superconducting θ-(ET)2I3 is next to both the charge ordered state of θ-(ET)2MM'(SCN)4 (M = Tl, Rb, Cs, M' = Zn, Co) and metallic phase of θ- (ET)2Ag(CN)2 [128]. It is not clear which point the non-superconducting θ-(ET)2I3 occupies in this phase diagram. Tempering (70 °C, 2 h) all crystals of θ-(ET)2I3 induced superconductivity with higher *T*c (named θ<sup>Τ</sup>-(ET)2I3: sharp drop of resistivity at 7 K and dull drop at ~5 K) [129]. The tempering changed the ethylene conformation and position of I3 from disordered state in θ-phase to ordered state in θ<sup>Γ</sup>-phase. Therefore, the phase diagram of the θ-type salts needs further parameters concerning with the structure of the salts (ethylene conformation and position or disorder of anions).

### *3.1.2.6. BETS superconductors*

The most intriguing phenomenon among the 8 BETS (Scheme 6) superconductors (highest *T*<sup>c</sup> = 5.5 K) is the reentrant superconductor-insulator-superconductor transition under magnetic field for (BETS)2FeCl4. The λ- or κ-type BETS salts formed with tetrahedral anions FeX4 (X: Cl and Br) were studied in terms of the competition of magnetic ordering and superconductivity [130–136]. The λ-(BETS)2FeCl4 exhibited coupled AF and MI transitions at 8.3 K. For the FeCl4 salt, a relaxor ferroelectric behavior in the metallic state below 70 K [133] and a firm nonlinear electrical transport associated with the negative resistance effect in the magnetic ordered state have been observed [135]. Moreover it has been found that the FeCl4 salt shows the field-induced superconducting transition under a magnetic field of 18–41 T applied exactly parallel to the conducting layers [132]. The λ-(BETS)2Fe*x*Ga1–*x*X4 passes through a superconducting to insulating transition on cooling [135]. The κ-(BETS)2FeX4 (X = Cl, Br) are AF superconductors (*T*N = 2.5 K, *T*c = 1.1 K for X = Br: *T*N = 0.45 K, *T*c = 0.17 K for X = Cl) [136]. Similar phenomena, namely AF, ferromagnetic, or field-induced superconductivity, have been observed in several inorganic solids such as Chevrel phase [137] and heavy-fermion system [138]. Recently it has been reported that λ-(BETS)2GaCl4 (*T*<sup>c</sup> = 8 K(on-set), 5.5 K(mid-point)) exhibited superconductivity in the minute size of four pairs of (BETS)2GaCl4 based on the STM study [139]. This salt has two-dimensional superconducting character (ξ// = 125 Å, ξ<sup>⊥</sup> = 16 Å).

### **Scheme 6.**

120 Superconductors – Materials, Properties and Applications

give rise to more than 5 times higher *T*c in

βH-salt.

tempering at 70–100 °C for more than 3 days, giving

H-salt [36]. Other

at 0.5 GPa //*c*, 4.5 K at 1 GPa //*b*\* for M = NH4) [126]. The salts

α

θ

(ET)2Ag(CN)2 [128]. It is not clear which point the non-superconducting

this phase diagram. Tempering (70 °C, 2 h) all crystals of

information is reported for the donor packing in the superconducting state.

crystals are superconducting and others remain metallic [127]. Several

β


columns in a phase diagram showing superconducting

ξ

coherent lengths are

0.5 GPa) [117].

Only one

of the

pattern to that of the

θ

from disordered state in

*3.1.2.6. BETS superconductors* 

field for (BETS)2FeCl4. The

ordered state of

θ

⊥ = 10 Å for the

α

SdH observation. However, for

θ

superconductivity with higher *T*c (named

θ

(ethylene conformation and position or disorder of anions).

λ- or κ

Å and ξ

The α

superlattice appearing at 175 K with incommensurate modulations of ET and I3 to each other [121]. The formation of the superlattice was suppressed by the pressure above 0.04 GPa. Then the two ethylene groups in an ET molecule were fixed in the eclipsed conformation to

increasing hydrostatic pressure monotonously, however, under the uniaxial stress the further *T*c increase taking a maximum at a piston pressure of 0.3–0.4 GPa is observed for both directions parallel and perpendicular to the donor stack [120]. The superconducting

// = 630 (//*a*) – 610 Å (//*b*') and

charge-ordered MI transition occurred [122,123]. It has been claimed that

α

β

ξ



α


zero-gap state with a Dirac cone type energy dispersion like graphene [124,125]. Under hydrostatic pressure it became two-dimensional metal down to low temperatures (2 GPa), however, it became superconductor under the uniaxial pressure along the *a*-axis (0.2 GPa, on-set *T*c = 7.2 K), though along the *b*-axis it remained metal down to low temperature (0.3–

NH4, Rb, Tl) seem to have charge-ordered state near or next to the superconducting state with low *T*c (highest *T*c = 1.7 K for M = NH4). Uniaxial strain increased *T*c anisotropically (6 K

confirmed to retain their donor packing patterns under pressure at low temperature by the

arranged by their inter-column transfer interactions, and the dihedral angle between

θ


drop at ~5 K) [129]. The tempering changed the ethylene conformation and position of I3

The most intriguing phenomenon among the 8 BETS (Scheme 6) superconductors (highest *T*<sup>c</sup> = 5.5 K) is the reentrant superconductor-insulator-superconductor transition under magnetic

Cl and Br) were studied in terms of the competition of magnetic ordering and

H-salt. The *T*c of

⊥ = 29 Å (//*c*\*) for the

β

β

t-salt which exhibited similar NMR






θ-

θ


θ

<sup>Γ</sup>-phase. Therefore, the phase diagram

<sup>Τ</sup>-(ET)2I3: sharp drop of resistivity at 7 K and dull


θ

α



θ


θ


α

H-salt decreased with

L-salt, and

α

ξ// = 130


### *3.1.3. Superconductors of other electron donor molecules*

Besides TMTSF, TMTTF, BO, ET, and BETS superconductors, there are other superconductors (Scheme 7, numbers in bracket are the total members of each superconducting family and the highest *T*c) of CT salts based on symmetric (BEDSe-TTF [140] and BDA-TTP [141–144]) and asymmetric donors (ESET-TTF [145], *S*,*S*-DMBEDT-TTF [146], *meso*-DMBEDT-TTF [147,148], DMET [149], DODHT [150], TMET-STF [151], DMET-TSF [152], DIETS [153], EDT-TTF [154], MDT-TTF [155,156], MDT-ST [157,158], MDT-TS [159], MDT-TSF [160–163], MDSe-TSF [164], DTEDT [165], and DMEDO-TSeF [166,167]).

**Scheme 7.** Donor molecules for organic superconductors except TMTSF, TMTTF, BO, ET, and BETS systems. Numbers in bracket are the total members of each superconductor and the highest *T*c.

κ-(MDT-TTF)2AuI2 (*T*c = 3.5 K) exhibited a Hebel-Slichter coherent peak just below *T*c, indicating a BCS-type gap with *s*-symmetry [156]. On the other hand, *d*-wave like superconductivity has been suggested for β-(BDA-TTP)2SbF6 [143,144]. β-(BDA-TTP)2X (X = SbF6, AsF6) exhibited a slight *T*c increase at the initial stage of uniaxial strain parallel to the donor stack and interlayer direction while *T*c decreased perpendicular to the donor stack [142]. θ-(DIETS)2[Au(CN)4] exhibited superconductivity under uniaxial strain parallel to the *c*-axis (*T*c = 8.6 K at 1 GPa), though under hydrostatic pressure a sharp MI transition remained even at 1.8 GPa [153]. MDT-ST, MDT-TS, and MDT-TSF superconductors [157– 163] have non-integer ratio of donor and anion molecules such as (MDT-TS)(AuI2)0.441 making the Fermi level different from the conventional 3/4 filled band for TMTSF and ET 2:1 salts. The Fermi surface topology of (MDT-TSF)X (X = (AuI2)0.436, (I3)0.422) and (MDT-ST)(I3)0.417 has been studied by SdH and AMRO [158,161–163]. DMEDO-TSeF afforded eight superconductors. Six of them are κ-(DMEDO-TSeF)2[Au(CN)2](solvent) and the *T*c's of them (1.7–5.3 K) are tuned by the use of cyclic ethers as solvent of crystallization [167]. The superconducting coherent lengths indicate that β-(BDA-TTP)2SbF6 has two-dimensional character (ξ// = 105 Å, ξ<sup>⊥</sup> = 26 Å) while (DMET-TSeF)2AuI2 has quasi-one-dimensional character (1000, 400, and 20 Å).

Development and Present Status of Organic Superconductors 123

the fcc structure for *x* < 2.65. Such a band-filling control has been realized for Na2Cs*x*C60 (0 ≤ *x* ≤ 1) [179] and Li*x*CsC60 (2 ≤ *x* ≤ 6) [180], and shows that the *T*c decreases sharply as the

**Figure 10.** *T*c as a function of volume occupied per C603– in cubic M3C60 (M: alkali metal). **a**: K3C60, **b**: Rb3C60, **c**: Rb2CsC60, **d**: RbCs2C60, **e**: fcc Cs3C60 at 0.7 GPa, **f**: A15 Cs3C60 at 0.7 GPa, **g**: fcc Cs3C60 at AP, **h**:

In 2008, A15 or body-centered cubic (bcc) Cs3C60 phase, which shows the bulk superconductivity under an applied hydrostatic pressure, was obtained together with a small amount of by-products of body-centered orthorhombic (bco) and fcc phases, by a solution process in liquid methylamine [181]. Interestingly, the lattice contraction with respect to pressure results in the increase in *T*c up to around 0.7 GPa, above which *T*<sup>c</sup> gradually decreases. The trend in the initial pressure range is not explicable within the simple BCS theory. At AP, on the other hand, the A15 Cs3C60 shows an AF ordering below 46 K, verified by means of 133Cs NMR and μSR [182]. Very recently, it has been found that the fcc phase also shows an AF ordering at 2.2 K at AP and superconducting transition at 35 K under an applied hydrostatic pressure of about 0.7 GPa [183]. We note that *T*c of the both phases follows the universal relationship for M3C60 superconductors in the vicinity of the Mott boundary, as seen in Fig. 10. So far about 40 superconductors were prepared with the highest *T*c of 33 K (RbCs2C60) at AP and 38 K (A15 Cs3C60) under pressure (0.7

Doping of alkali metals in picene (Scheme 8), that has a wider band gap (3.3 eV) than 1.8 eV for pentacene, introduced superconductivity [184]. The bulk superconducting phase was observed below 7 K and 18 K for K3picene, and 7 K for Rb3picene, which are comparable to that of K3C60 (*T*c = 19 K). Since the energy difference between LUMO and LUMO+1 is very small (< 0.1 eV), the three electrons reside in the almost two-fold degenerate LUMO.

**4. Polyaromatic hydrocarbon superconductors** 

valence state on C60 deviates from –3.

A15 Cs3C60 at AP.

GPa).

### **3.2. Superconductors based on electron acceptors**

Icosahedral C60 molecule with *I*h symmetry has triply degenerate LUMO and LUMO+1 orbitals with *t*1u and *t*1g symmetries, respectively. In 1991, superconducting phase was observed below 19 K for the potassium-doped compounds prepared by a vapor-solid reaction [168], immediately after the isolation of macroscopic quantities of C60 solid [169]. Powder X-ray diffraction profile revealed that the composition of the superconducting phase is K3C60 and the diffraction pattern can be indexed to be a face-centered cubic (fcc) structure [170]. The lattice constant (*a* = 14.24 Å) is apparently expanded relative to the undoped cubic C60 (*a* = 14.17 Å). The superconductivity has been observed for many M3C60 (M: alkali metal), *e.g.*, Rb3C60 (*T*c = 29 K [171]), Rb2CsC60 (*T*c = 31 K [172]), and RbCs2C60 (*T*c = 33 K [172]), and their structures determined to be analogous to that of K3C60 with varying lattice constants. The *T*c varies monotonously with lattice constant, independently of the type of the alkali dopant (Fig. 10) [172,173]. The observation of a Hebel-Slichter peak in relaxation rate just below *T*c in NMR [174] and μSR [175] indicate the BCS-type isotopic gap. The decrease in *T*c due to the isotopic substitution [176] also supports the phonon-mediated pairing in M3C60, where the α value in *T*<sup>c</sup> ∝ (mass)–α (ideal value of α predicted by the BCS model is 0.5) is estimated to be 0.30(6) for K313C60 and 0.30(5) for Rb313C60.

Keeping the C60 valence invariant (–3), the intercalation of NH3 molecules (*e.g.*, (NH3)K3C60) results in the lattice distortion from cubic to orthorhombic accompanied by the appearance of AF ordering instead of superconductivity [177]. Changing the valence in cubic system also has a pronounced effect on *T*c. For example, *T*c in Rb3–*x*Cs*x*C60 prepared in liquid ammonia gradually increases as the mixing ratio approaches to *x* = 2 [178]. Further increasing the nominal ratio of Cs leads to a sizable decrease of *T*c, despite the lattice keeps the fcc structure for *x* < 2.65. Such a band-filling control has been realized for Na2Cs*x*C60 (0 ≤ *x* ≤ 1) [179] and Li*x*CsC60 (2 ≤ *x* ≤ 6) [180], and shows that the *T*c decreases sharply as the valence state on C60 deviates from –3.

122 Superconductors – Materials, Properties and Applications

superconductivity has been suggested for

superconductors. Six of them are

// = 105 Å,

character (1000, 400, and 20 Å).

pairing in M3C60, where the

superconducting coherent lengths indicate that

ξ

**3.2. Superconductors based on electron acceptors** 

α

value in *T*<sup>c</sup> ∝ (mass)–

Keeping the C60 valence invariant (–3), the intercalation of NH3 molecules (*e.g.*, (NH3)K3C60) results in the lattice distortion from cubic to orthorhombic accompanied by the appearance of AF ordering instead of superconductivity [177]. Changing the valence in cubic system also has a pronounced effect on *T*c. For example, *T*c in Rb3–*x*Cs*x*C60 prepared in liquid ammonia gradually increases as the mixing ratio approaches to *x* = 2 [178]. Further increasing the nominal ratio of Cs leads to a sizable decrease of *T*c, despite the lattice keeps

model is 0.5) is estimated to be 0.30(6) for K313C60 and 0.30(5) for Rb313C60.

α

(ideal value of

α

predicted by the BCS


SbF6, AsF6) exhibited a slight *T*c increase at the initial stage of uniaxial strain parallel to the donor stack and interlayer direction while *T*c decreased perpendicular to the donor stack

*c*-axis (*T*c = 8.6 K at 1 GPa), though under hydrostatic pressure a sharp MI transition remained even at 1.8 GPa [153]. MDT-ST, MDT-TS, and MDT-TSF superconductors [157– 163] have non-integer ratio of donor and anion molecules such as (MDT-TS)(AuI2)0.441 making the Fermi level different from the conventional 3/4 filled band for TMTSF and ET 2:1 salts. The Fermi surface topology of (MDT-TSF)X (X = (AuI2)0.436, (I3)0.422) and (MDT-ST)(I3)0.417 has been studied by SdH and AMRO [158,161–163]. DMEDO-TSeF afforded eight

(1.7–5.3 K) are tuned by the use of cyclic ethers as solvent of crystallization [167]. The

Icosahedral C60 molecule with *I*h symmetry has triply degenerate LUMO and LUMO+1 orbitals with *t*1u and *t*1g symmetries, respectively. In 1991, superconducting phase was observed below 19 K for the potassium-doped compounds prepared by a vapor-solid reaction [168], immediately after the isolation of macroscopic quantities of C60 solid [169]. Powder X-ray diffraction profile revealed that the composition of the superconducting phase is K3C60 and the diffraction pattern can be indexed to be a face-centered cubic (fcc) structure [170]. The lattice constant (*a* = 14.24 Å) is apparently expanded relative to the undoped cubic C60 (*a* = 14.17 Å). The superconductivity has been observed for many M3C60 (M: alkali metal), *e.g.*, Rb3C60 (*T*c = 29 K [171]), Rb2CsC60 (*T*c = 31 K [172]), and RbCs2C60 (*T*c = 33 K [172]), and their structures determined to be analogous to that of K3C60 with varying lattice constants. The *T*c varies monotonously with lattice constant, independently of the type of the alkali dopant (Fig. 10) [172,173]. The observation of a Hebel-Slichter peak in relaxation rate just below *T*c in NMR [174] and μSR [175] indicate the BCS-type isotopic gap. The decrease in *T*c due to the isotopic substitution [176] also supports the phonon-mediated

β




<sup>⊥</sup> = 26 Å) while (DMET-TSeF)2AuI2 has quasi-one-dimensional

β



β

κ

κ

[142]. θ

character (

ξ

**Figure 10.** *T*c as a function of volume occupied per C603– in cubic M3C60 (M: alkali metal). **a**: K3C60, **b**: Rb3C60, **c**: Rb2CsC60, **d**: RbCs2C60, **e**: fcc Cs3C60 at 0.7 GPa, **f**: A15 Cs3C60 at 0.7 GPa, **g**: fcc Cs3C60 at AP, **h**: A15 Cs3C60 at AP.

In 2008, A15 or body-centered cubic (bcc) Cs3C60 phase, which shows the bulk superconductivity under an applied hydrostatic pressure, was obtained together with a small amount of by-products of body-centered orthorhombic (bco) and fcc phases, by a solution process in liquid methylamine [181]. Interestingly, the lattice contraction with respect to pressure results in the increase in *T*c up to around 0.7 GPa, above which *T*<sup>c</sup> gradually decreases. The trend in the initial pressure range is not explicable within the simple BCS theory. At AP, on the other hand, the A15 Cs3C60 shows an AF ordering below 46 K, verified by means of 133Cs NMR and μSR [182]. Very recently, it has been found that the fcc phase also shows an AF ordering at 2.2 K at AP and superconducting transition at 35 K under an applied hydrostatic pressure of about 0.7 GPa [183]. We note that *T*c of the both phases follows the universal relationship for M3C60 superconductors in the vicinity of the Mott boundary, as seen in Fig. 10. So far about 40 superconductors were prepared with the highest *T*c of 33 K (RbCs2C60) at AP and 38 K (A15 Cs3C60) under pressure (0.7 GPa).

### **4. Polyaromatic hydrocarbon superconductors**

Doping of alkali metals in picene (Scheme 8), that has a wider band gap (3.3 eV) than 1.8 eV for pentacene, introduced superconductivity [184]. The bulk superconducting phase was observed below 7 K and 18 K for K3picene, and 7 K for Rb3picene, which are comparable to that of K3C60 (*T*c = 19 K). Since the energy difference between LUMO and LUMO+1 is very small (< 0.1 eV), the three electrons reside in the almost two-fold degenerate LUMO.

Considering the fact that Ca1.5picene also shows a superconducting phase below 7 K [185], three-fold charge transfer from dopants to one picene molecule would be responsible for emergence of the superconductivity. At present, although the crystal structures of the doped compounds are unclear, the refined lattice parameters are indicative of the deformation of the herringbone structure of pristine picene and the intercalation of dopants within the twodimensional picene layers. After the discovery of the picene-based superconductors, several superconductors have been found for alkali metal (*T*c = 7 K) [186] and alkaline-earth metal (*T*c ~ 5.5 K) [187] doped phenanthrene, potassium-doped 1,2:8,9-dibenzopentacene (*T*c = 33 K; partially decomposed) [188], and potassium-doped coronene (*T*c < 15 K) [185]. Among them, phenanthrene-based superconductors shows an enhancement of *T*c with increasing pressure, which is indicative of the non-BCS behavior.

Development and Present Status of Organic Superconductors 125

were known to superconduct with *T*c = 0.15 K for KC8 [194]. In 1980s and 1990s, further efforts were poured to synthesize GICs with higher *T*c, such as LiC2 with *T*c = 1.9 K [195]. In 2005, these efforts culminated in the discovery of CaC6 with *T*c as high as 11.5 K at AP [196], which goes up to 15.1 K under pressures up to 7.5 GPa [197]. In other alkaline-earth metal doped GICs, the superconducting phase was observed below 1.65 K for SrC6 and 6.5 K for YbC6 [198]. The apparent reduction of *T*c strongly suggests that the interlayer states of graphite have an impact on the electronic state of GIC, which was supported by theoretical calculations [199]. The conventional phonon mechanism in the framework of conventional BCS theory is generally accepted, due mainly to the observation of the Ca isotope effect with

A typical sp3 covalent system, diamond, is an electrical insulator with a wide band gap of 5.5 eV, and is well known for its hardness as well as its unique electronic and thermal properties. Superconductivity in diamond was achieved through heavy p-type doping by boron in 2004 (*T*c = 4 K), which was performed under high pressure (8–9 GPa) and high temperatures (2500–2800 K) [201]. Enhanced *T*c in homoepitaxial CVD films has been achieved as high as 11 K [202]. Doped boron introduces an acceptor level with a hole binding energy of 0.37 eV and results in a metallic state above a critical boron concentration in the range of a few atoms per thousand. The *T*c varies between 1 and 10 K with the doping level [203]. Superconducting gap estimated from STM [204] and isotopic substitution of

Accordingly, all of the carbon polymorphs, namely zero-dimensional C60 (sp2/sp3 character), one-dimensional carbon nanotube (sp2 character), two-dimensional graphite (sp2 character), and three-dimensional diamond (sp3 character) could provide superconductors, despite their covalent character being different. The superconductors with sp2/sp3 or sp2 carbons were realized either in themselves or by doping of metal atoms, while those with sp3

Since the discovery of the first metallic CT solid, TTF•TCNQ, in 1973 [8], much attention for organic (super)conductors has been devoted to plural component CT solids. Besides numerous studies on multi-component CT solids, several single-component organic conductors have been developed. Even though pentacene is known to be the first organic metal (semimetal) showing a decrease of resistivity down to ca. 200 K at 21.3 GPa [206], no superconductivity was reported so far on the solids composed of aromatic hydrocarbon solely. Electric conductivity increases by the enhancement of intermolecular interactions by appropriate use of hetero-atomic contacts. There are two single-component superconductors

σ

GPa, and superconductor at *T*c ~ 2 K at 52 GPa) [207,208] and hexaiodobenzene (*T*c = 0.6–0.7 K at around 33 GPa and ca. 2.3 K at 58 GPa) [209]. Both have peripheral chalcogen atoms, iodine, which may cause the increased electronic dimensionality of the solid under pressure

RT = 1 × 10–12 S cm–1 at AP,

σ

RT = 2 × 10 S cm–1 at 25

boron and carbon [205] follow the BCS picture, as MgB2 (*T*c = 39 K).

carbons were realized by substitution of boron for carbon.

**6. Single component superconductors** 

under extremely high pressure, *p*-iodanil (

owing to intermolecular iodine∙∙∙iodine contacts.

its exponent

α

= 0.5 [200].

### **Scheme 8.**

Contrary to the electron-doped system above described, it has been found that a cation radical salt (perylene)2Au(mnt)2, in which each perylene has an average charge of +0.5 and form segregated columns, show a superconductivity with *T*c = 0.3 K when the hydrostatic pressure above 0.5 GPa was applied to suppress CDW phase [189]. So far 8 aromatic hydrocarbon superconductors were prepared with the highest *T*c of 33 K at AP.
