Superconductors

The prediction of superconductivity in a new material is generally limited to finding compounds with characteristics similar to known superconductors. Known superconducting Heusler compounds are Pd₂YSn and other Pd₂RESn compounds. By searching for electronic instabilities we have found five new superconductors in this material class: Ni₂ZrGa, Pd₂ZrGa, Pd₂HfGa, Pd₂ZrIn and Pd₂HfIn, Especially Nickel containing compounds are of interest for the study of competition between superconductivity and magnetism. In MgCNi₃ and LnNi₂B₂C, the magnetic character is suppressed, and superconductivity occurs.
FeSe is the simplest of the recently discovered superconductors and can serve therefore as a model system to gain insight into the mechanism of superconductivity and the structure to property relation of the iron pnictides and related compounds. FeSe is superconducting in the parent phase, no doping is necessary; even more the superconductivity depends strongly on the exact stoichiometry. Different from the arsenides a tetra to ortho transition is connected with the superconducting phase. Superconductivity disappears under pressure, when FeSe transforms reversible into a NiAs structure type. No static magnetism is observed over the whole pressure vs. temperature phase diagram. However Se-NMR spectroscopy gives evidence for short range magnetic fluctuations.

 

Key papers and collaborators:
The Structural Phase Transition in Fe_1+δSe
T. M. McQueen, A. J. Williams, P. W. Stephens, J. Tao, Y. Zhu, V. Ksenofontov, F.Casper, C. Felser, and R. J. Cava, Phys. Rev. Lett. 103 (2009) 057002
Electronic and Magnetic Phase Diagram of β-Fe_1.01Se with Superconductivity at 36.7 K under Pressure, Sergey Medvedev, T. M. McQueen, , Taras Palasyuk, M. Eremets, R. J. Cava, Ivan Trojan, S. Naghavi, F. Casper, V. Ksenofontov, G. Wortmann and C. Felser, Nature Materials 8 (2009) 630
Metal – Insulator transition in Fe_1.01-xCuxSe, A. J. Williams, T. M. McQueen, V. Ksenofontov, C. Felser, and R. J. Cava, J. Phys. Cond. Mat. 21 305701 (2009)
A Ni-based superconductor: the Heusler compound ZrNi₂Ga
Structure and superconductivity in LnNi₂B₂C: comparison of calculation and experiment S. M. Loureiro, C. Kealhofer, C. Felser, R. J. Cava, Sol. State Com. 119 (2001) 675.
Jürgen Winterlik, Gerhard H. Fecher, Claudia Felser, Martin Jourdan, Kai Grube Frederic Hardy and Hilbert von Löhneysen, Katherine Holman and R. J. Cava, Phys. Rev. B 78 (2008) 184506..

 

Collaborations:
Roser Valenti (Frankfurt), Jörg (Fink Berlin), Bernd Büchner (Dresden), Gerhard Wortmann (Paderborn), Jürgen Kübler (Darmstadt)

 

International Collaborations:
R.J . Cava (Princeton), David Singh (Oak Ridge)