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Author: Müller, K.-H. ×

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    Electronic structure and aspects of unconventional superconductivity in NaxCoO2.yH2O
    (São Carlos : Universidade Federal de São Carlos, 2003) Rosner, H.; Drechsler, S.-L.; Fuchs, G.; Handstein, A.; Wälte, A.; Müller, K.-H.
    We examine the electronic structure of NaxCoO2.yH2O within the local density approximation. The parametrization of the band which forms the largest hole-Fermi surface centered at G shows significant deviations from what is frequently assumed in recent sophisticated theoretical studies. In particular, the commonly used nearest neighbor approaches in the framework of single band pictures are found to be unrealistic. The special role of H2O in screening the disorder in the charge reservoir is briefly discussed and compared with the case of Y1–xCaxCu3O6+d.
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    High-field phase diagram of the heavy-fermion metal YbRh2Si2
    (Milton Park : Taylor & Francis, 2006) Gegenwart, P.; Tokiwa, Y.; Westerkamp, T.; Weickert, F.; Custers, J.; Ferstl, J.; Krellner, C.; Geibel, C.; Kerschl, P.; Müller, K.-H.; Steglich, F.
    The tetragonal heavy-fermion (HF) metal YbRh2Si2 (Kondo temperature TK≈ 25 K) exhibits a magnetic field-induced quantum critical point related to the suppression of very weak antiferromagnetic (AF) ordering (TN = 70 mK) at a critical field of Bc = 0.06 T (B⊥ c). To understand the influence of magnetic fields on quantum criticality and the Kondo effect, we study the evolution of various thermodynamic and magnetic properties upon tuning the system by magnetic field. At B > Bc, the AF component of the quantum critical fluctuations becomes suppressed, and FM fluctuations dominate. Their polarization with magnetic field gives rise to a large increase of the magnetization. At B* = 10 T, the Zeeman energy becomes comparable to kB TK, and a steplike decrease of the quasi-particle mass deduced from the specific-heat coefficient indicates the suppression of HF behaviour. The magnetization M(B) shows a pronounced decrease in slope at B* without any signature of metamagnetism. The field dependence of the linear magnetostriction coefficient suggests an increase of the Yb-valency with field, reaching 3+ at high fields. A negative hydrostatic pressure dependence of B* is found, similar to that of the Kondo temperature. We also compare the magnetization behaviour in pulsed fields up to 50 T with that of the isoelectronic HF system YbIr2Si2, which, due to a larger unit-cell volume, has an enhanced TK of about 40 K.