**Experiment**

**Chapter 1** 

© 2012 Wu and Clark, 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 Wu and Clark, 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.

**Field-Induced Superconductors:** 

**NMR Studies of** λ**–(BETS)2FeCl4**

Additional information is available at the end of the chapter

scenario. There are two reasons responsible for this.

spins aligned (along the field) in a *p*-wave state.

It has been widely known in condensed matter and materials physics that the application of magnetic field to a superconductor will generally destroy the superconductivity as a usual

One is the Zeeman effect [1-2], where the alignment of the electron spins by the applied magnetic field can break apart the electron pairs for a spin-singlet (but not a spin-triplet) state. In this case, the electron spin pairs have opposite spins (such as the *s*-wave spins typical in type I superconductors). The applied magnetic field attempts to align the spins of both electrons along the field, thus breaking them apart if strong enough. In terms of energy, one electron gains energy while the other (as a pair) loses energy. If the energy difference is larger than the amount of energy holding the electrons together, then they fly apart and thus the superconductivity disappears. But this does not apply to the spin triplet superconductivity (*p*-wave superconductors) where the electron pairs already have their

The other is the orbital effect [3], which is a manifestation of the Lorentz force from the applied magnetic field since the electrons (as a pair) have opposite linear momenta, one electron rotating around the other in their orbitals. The Lorentz force on them acts in opposite directions and is perpendicular to the applied magnetic field, thus always pulling the pair apart. This does not matter with their spin pairing symmetries (*s*-wave, *p*-wave or *d*wave). In type-II superconductors, the Meissner screening currents associated with the vortex penetration in the applied magnetic field can also increase the electron kinetic energy (and momentum). Once this energy becomes greater than the energy that unites the two electrons, the electron pairs break apart and thus superconductivity is suppressed. Therefore, the orbital effect could be even more important in type-II superconductors.

Guoqing Wu and W. Gilbert Clark

http://dx.doi.org/10.5772/48361

**1. Introduction** 
