**7. References**


[14] Rogachev, A., Wei, T.-C., Pekker, D., Bollinger, A. T., Goldbart, P. M. and Bezryadin, A., Magnetic-Field Enhancement of Superconductivity in Ultranarrow Wires, Phys. Rev. Lett. 97, 137001 (2006).

18 Superconductors – Materials, Properties and Applications

for our detailed 77Se-NMR work published in ref. [23].

superconductors. Appl. Phys. Lett. 1, 7 (1962).

Superconductivity, Phys. Rev. Lett. 53, 497 (1984).

supercurrent conversion, Phys. Rev. B 25, 4515 (1982).

superconducting loops and wires, Phys. Rev. B 40, 11392 (1989).

compounds. Phys. Rev. B 18, 4688 (1978).

*Department of Physics and Astronomy, University of California, Los Angeles, USA* 

We are grateful to Prof. S. E. Brown at UCLA for helpful discussions and help with the NMR experiments. We also thank other coauthors J. S. Brooks, A. Kobayashi, and H. Kobayashi

[1] Clogston, A. M., Upper limit for the critical field in hard superconductors, Phys. Rev.

[2] Chandrasekhar, B. S., A note on the maximum critical field of high-field

[4] Uji, S., Shinagawa, S., Terashima, T., Yakabe, T., Terai, Y., Tokumoto, M., Kobayashi, A., Tanaka, H., Kobayashi, H., Magnetic-field-induced superconductivity in a two-

[5] Meul, H. W., Rossel, C., Decroux, M., et al., Observation of Magnetic-Field-Induced

[6] Jaccarino, V. and Peter, M., Ultra-High-Field Superconductivity, Phys. Rev. Lett. 9, 290

[7] Maekawa, S. and Tachiki, M., Superconductivity phase transition in rare-earth

[8] Uji, S., Kobayashi, H., Balicas, L. and Brooks, J. S., Superconductivity in an Organic Conductor Stabilized by a High Magnetic Field, Advanced Materials 14, 243

[9] Kobayashi, H., Kobayashi, A., Cassoux, P., BETS as a source of molecular magnetic superconductors (BETS = bis(ethylenedithio)tetraselenafulvalene), Chem. Soc. Rev. 29,

[10] Blonder, G. E., Tinkham, M. and K.lapwijk T. M., Transition from metallic to tunneling regimes in superconducting microconstrictions: Excess current, charge imbalance, and

[11] Tian, M. L., Kumar, N., Xu, S. Y., Wang, J. G., Kurtz, J. S. and Chan, M. H. W., Phys.

[12] Santhanam, P., Umbach, C. P. and Chi, C. C., Negative magnetoresistance in small

[13] Xiong, P., Herzog, A. V. and Dynes, R. C., Negative Magnetoresistance in Homogeneous Amorphous Superconducting Pb Wires, Phys. Rev. Lett. 78, 927

[3] Tinkham, M., *Introduction to Superconductivity* (McGraw-Hill, New York, 1975).

dimensional organic conductor, Nature (London) 410, 908 (2001).

W. Gilbert Clark

**7. References** 

(1962).

(2002).

(1997).

325 (2000).

Rev. Lett. 95, 076802 (2005).

**Acknowledgement** 

Lett. 15, 266 (1962).

	- [29] Mori, T. and Katsuhara, Estimation of πd-Interactions in Organic Conductors Including Magnetic Anions, M., J. Phys. Soc. Jpn. 71, 826 (2002).

**Chapter 2** 

© 2012 Chen and Dong, licensee InTech. This is an open access chapter distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

© 2012 Chen and Dong, licensee InTech. This is a paper distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

**X-Ray Spectroscopy** 

Chi Liang Chen and Chung-Li Dong

http://dx.doi.org/10.5772/48611

**1. Introduction** 

Additional information is available at the end of the chapter

respect, spectroscopic measurements are still limited.

A recent study that identified high temperature superconductivity in Fe-based quatenary oxypnictides has generated a considerable amount of activity closely resembling the cuprate superconductivity discovered in the 1980s (Kamihare et al., 2008; Takahashi et al., 2008; Ren et al., 2008). This system is the first in which Fe plays an essential role in the occurrence of superconductivity. Fe generally has magnetic moments, tending to form an ordered magnetic state. Neutron-scattering experiments have demonstrated that mediated superconducting pairing may originate from magnetic fluctuations, similar to our understanding of that in high-*Tc* cuprates (de la Cruz et al., 2008; Xu et al., 2008). Binary superconductor FeSex is another example of a Fe-based superconductor with a less toxic property, leading to the discovery of several superconducting compounds (Hsu et al., 2008). The *Tc* value of FeSe is ~8 K in bulk form and exhibits a compositional dependence such that *Tc* decreases for over-doping or under-doping of compounds (McQueen et al., 2009; Wu et al., 2009), as does that of high-*Tc* cuprates. FeSe has received a significant amount of attention owing to its simple tetragonal symmetry P4/nmm crystalline structure, comprising a stack of layers of edge-sharing FeSe4 tetrahedron. The phase of FeSe heavily depends on Se deficiency and annealing temperature. While 400 °C annealing reduces the nonsuperconducting NiAs-type hexagonal phase and increases the PbO-type tetragonal superconducting phase (Hsu et al., McQueen et al., 2009; Wu et al., 2009; Mok et al., 2009), the role of Se deficiency remains unclear. Notably, this binary system is isostructural with the FeAs layer in quaternary iron arsenide. Also, band-structure calculations indicate that FeSe- and FeAs-based compounds have similar Fermi-surface structures (Ma et al., 2009), implying that this simple binary compound may significantly contribute to efforts to elucidate the origin of high-temperature superconductivity in these emerging Fe-based compounds. Therefore, although the electronic structure is of great importance in this

**Studies of Iron Chalcogenides** 
