**4. Field-induced superconductor** λ**–(BETS)2FeCl4**

The field-induced superconductor λ–(BETS)2FeCl4 is a quasi-two dimensional (2D) triclinic salt (space group P ) incorporating large magnetic 3d-Fe3+ ions (spin *S*d = 5/2) with the BETS-molecules inside which have highly correlated conduction electrons (π-electrons, spin *S*π = 1/2) from the Se-ions, where BETS is bis(ethylenedithio)tetraselenafulvalene (C10S4Se4H8). It was first synthesized in 1993 by Kobayashi *et al*. [4, 8, 9, 19].

λ–(BETS)2FeCl4 is one of the most attractive materials in the last two decades for the observation of interplay of superconductivity and magnetism and for the synthesis of magnetic conductors and superconductors.

We expect it to show strong competition between the antiferromagnetic (AF) order of the Fe3+ magnetic moments and the superconductivity of the material, where the properties of the conduction electrons are significantly tunable by the external magnetic field, together with the internal magnetic field generated by the local magnetic moments from the Fe3+ ions as well. Thus it has been of considerable interest in condensed matter and materials physics.

This interplay originates from the role of the magnetic 3d-Fe3+ ions moments including the effect of their strong interaction with the conduction π-electrons. Because of this interplay, λ–(BETS)2FeCl4 has an unusual phase diagram [Fig. 5 (c)], including an antiferromagnetic insulating (AFI) phase, a paramagnetic metallic (PM) phase, and a field-induced superconducting (FISC) phase [4, 8].

The crystal structure of λ–(BETS)2FeCl4 in a unit cell is shown in Fig. 6 (a) [20]. In each unit cell, there are four BETS molecules and two Fe3+ ions. The BETS molecules are stacked along the *a* and *c* axes to form a quasi-stacking fourfold structure.

**Figure 6.** (a) Crystal structure of λ–(BETS)2FeCl4 in a unit cell. (b) BETS molecule [20]. (c) Phase diagram of λ–(BETS)2FeCl4 [8].

Noticeably, the conducting layers comprised of BETS are sandwiched along the *b* axis by the insulating layers of FeCl4− anions. The least conducting axis is *b*, the conducting plane is *ac*, and the easy axis of the antiferromagnetic spin structure is ~30° away from the *c* axis (parallel to the needle axis of the crystal) [21, 22].

At the room temperature (298 K), the lattice constants are: *a* = 16.164(3), *b* = 18.538(3), *c* = 6.592(4) Angstrom (Ao), *α* = 98.40(1)o, β = 96.69(1)o, and γ = 112.52(1)o. The shortest distance between Fe3+ ions is 10.1 Ao within a unit cell, which is along the *a*-direction, and the nearest distance of Fe3+ ions between neighboring unit cells is 8.8 Ao [21].

## **5. NMR studies of** λ**–(BETS)2FeCl4**

10 Superconductors – Materials, Properties and Applications

This model proposes that there is a charge-imbalance length (or relaxation time) associated with the normal metal - superconductor boundaries of phase-slip centers [20]. Applying magnetic field reduces the charge-imbalance length (or relaxation time), resulting in a negative magnetoresistance at high currents and near *T*c. Thus the superconductivity in the

The impurity model deals with the superconductivity for nanoscale systems that have impurity magnetic moments with localized spins as magnetic superconductors [14], in which there is a strong Zeeman effect. According to this model, superconductivity is enhanced with the quenching of pair-breaking magnetic spin fluctuations by the applied

These are major theoretical models for the explanation of the anti-proximity effect in

The field-induced superconductor λ–(BETS)2FeCl4 is a quasi-two dimensional (2D) triclinic salt (space group P ) incorporating large magnetic 3d-Fe3+ ions (spin *S*d = 5/2) with the BETS-molecules inside which have highly correlated conduction electrons (π-electrons, spin *S*π = 1/2) from the Se-ions, where BETS is bis(ethylenedithio)tetraselenafulvalene

λ–(BETS)2FeCl4 is one of the most attractive materials in the last two decades for the observation of interplay of superconductivity and magnetism and for the synthesis of

We expect it to show strong competition between the antiferromagnetic (AF) order of the Fe3+ magnetic moments and the superconductivity of the material, where the properties of the conduction electrons are significantly tunable by the external magnetic field, together with the internal magnetic field generated by the local magnetic moments from the Fe3+ ions as well. Thus it has been of considerable interest in condensed matter and materials

This interplay originates from the role of the magnetic 3d-Fe3+ ions moments including the effect of their strong interaction with the conduction π-electrons. Because of this interplay, λ–(BETS)2FeCl4 has an unusual phase diagram [Fig. 5 (c)], including an antiferromagnetic insulating (AFI) phase, a paramagnetic metallic (PM) phase, and a field-induced

The crystal structure of λ–(BETS)2FeCl4 in a unit cell is shown in Fig. 6 (a) [20]. In each unit cell, there are four BETS molecules and two Fe3+ ions. The BETS molecules are stacked along

nanoscale systems. Their validity needs more experimental evidence.

(C10S4Se4H8). It was first synthesized in 1993 by Kobayashi *et al*. [4, 8, 9, 19].

**4. Field-induced superconductor** λ**–(BETS)2FeCl4** 

magnetic conductors and superconductors.

superconducting (FISC) phase [4, 8].

the *a* and *c* axes to form a quasi-stacking fourfold structure.

**c. Charge imbalance length model** 

nanowires is enhanced.

**d. Impurity model** 

magnetic field.

physics.

In order to study the mechanism of the superconductivity in λ–(BETS)2FeCl4 and to test the validity of the Jaccarino-Peter effect, as well as to understand the multi-phase properties of the material as show in the unusual phase diagram [Fig. 6 (c)], we successfully conducted a series of nuclear magnetic resonance (NMR) experiments.

These include both 77Se-NMR measurements and proton (1H) NMR measurements, as a function of temperature, magnetic field and angle of alignment of the magnetic field [20, 23, 24].
