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TT: Tiefe Temperaturen

TT 22: Quantenphasenüberg
änge

TT 22.3: Talk

Thursday, March 29, 2001, 12:15–12:30, J

Quantum Melting and Superconductivity in Barium-Germanium Clathrates — •M. Grosche, S. Paschen, H. Q. Yuan, W. Carrillo-Cabrera, C. Langhammer, G. Sparn, M. Baenitz, Y. Grin, and F. Steglich — MPI-CPfS, Nöthnitzer Str. 40, 01187 Dresden

The melting of magnetic order at low temperatures in transition metal or rare earth compounds by tuning the lattice density under hydrostatic pressure is serving as a successful guiding principle in the search for anomalous metallic states and unconventional superconductivity. Can a similar approach be applied to other types of electronic order? The newly-discovered cage-compound (Clathrate) Ba₆Ge₂₅ and its relatives consist of a rigid Germanium skeleton, into which Barium or other metal atoms are coordinated. These guest atoms can “rattle” freely at high temperatures, but lock into split positions below about 200 K. This structural phase transition is accompanied by significant changes in the electronic properties of the material, possibly indicating a charge density wave transition or even the formation of bipolaronic bands. At low temperatures <0.3K the resulting bad metal (ρ₀ ≃ 1.5 mΩ cm) undergoes a superconducting transition with a critical field that appears surprisingly high when compared to typical metallic superconductors and translates to a short coherence length ≃ 280 Å. We study the evolution of these anomalous ordered states, as the high temperature transitions are suppressed by hydrostatic pressure, and discuss the implications of the resulting phase diagram – which shows a maximum T_c close to 4K at the critical pressure – for related superconducting Clathrate compounds.