Non-conventional Superconductivity and Anisotropic Spin Fluctuation in Heavy Fermion Systems PDF Imprimir Correo electrónico

Jueves 17 de mayo 2012

15h00-16h30 "Non-conventional superconductivity and Anisotropic Spin Fluctuation in Heavy-Fermion Systems"

Prof: Sinsaku KAMBE (Adanced Science Research Center, Japan Atomic Energy Agency; SPSMS/INAC CEA Grenoble).

Resumen: 

In unconventional superconductors, suprconducting. Cooper pairs are believed to be formed via magnetic fluctuations. In Fact, the superconducting transition temperature Tc roughly correales with the antiferromagnetic spin fluctuation energy in d-wave unconventional superconductors including high Tc-cuprates. In addition to this correlation, the suprconducting pairing symmetry and the magnetic anisotropy of the normal satate are found empirically to be strongly correlated in f-electron unconventional superconductors. Unconventional superconductivity appears with singlet (d-wave) pairing for cases of XY anisotropy, incontrast, in ferromagnetic systems, unconventional superconductivity with triplet (e.g. p-wave) pairing appears for cases of Ising anisotropy.  Furthermore Tc  is found to correlate with the strength of XY anisotropy determined by NMR in d-wave cases. In this presentation, the origin of XY anisotropy is discussed in terms of the orbital character, i.e. the angular momentum character jz  of j=5/2 multiplet for each piece of Fermi surfaces. Exceptional cases e.g. URu2Si2 from this correlation between magnetic anisotropy and unconventional superconductivity seem to have an interesting reason for the exception, wich wil be also addressed.

 
 
Tunneling Spectroscopy of Fluctuating and Localized Preformed Cooper Pairs in Highly ...... PDF Imprimir Correo electrónico

Jueves 13 de abril 2012

10h:30-11h:30 "Tunneling Spectroscopy of Fluctuating and Localized Preformed Cooper Pairs in Highly Disordered Superconducting Films

Prof: Claude Chapelier  (SPSMS, UMR-E9001,CEA-INAC/UJF, Grenoble-France)

Resumen:

We have performed tunneling spectroscopy on superconducting titanium nitride and indium oxide films in the vicinity of the disorder-driven superconductor-insulator transition (SIT). Tunnelling spectroscopy highlights a rather unusual superconducting state with a pseudogap regime above the critical temperature Tc [1]. We demonstrated that this pseudogap is the signature of short lived Cooper pairs that are preformed above Tc. It evolves at low temperature into an inhomogeneous superconducting system due to spatial fluctuations of the disorder at the mesoscopic scale in both materials [2,3]. However, the SIT in TiN and InO films display different characters. Ultrathin TiN films remain bad metals with dominating two-dimensional thermodynamic fluctuations when disorder is increased. In this case Tc goes to zero at the critical disorder of the SIT, whereas in InO, Tc remains above 1 K on the superconducting side of the SIT. In this latter situation, localization takes over and the preformed Cooper pairs above Tc can locally remain localized at zero temperature.

References
1. B. Sacépé, C. Chapelier, T.I. Baturina, V.M. Vinokur, M.R. Baklanov, and M. Sanquer, Nature Commun. 1 :140 doi : 10.1038/ncommns1140 (2010).
2. B. . Sacépé, C. Chapelier T.I. Baturina, V.M. Vinokur, M.R. Baklanov, and M. Sanquer, Phys. Rev. Lett. 101, 157006 (2008).
3. B. Sacépé, Th. Dubouchet, C.Chapelier,  M. Sanquer, M. Ovadia, Dan Shahar, M. Feigel'man, and L.B. Ioffe, Nature Physics 7, 239 (2011).

 

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