Session 23bC

The heavy fermion quantum critical point.

Catherine Pépin^a, Piers Coleman^b

^aSPhT, L'Orme des Merisiers, CEA-Saclay,91191 Gif-sur-Yvette, France

^bCenter for Materials Theory, Department of Physics and Astronomy, Rutgers University, Piscataway, NJ 08854, USA

We discuss non-Fermi liquid and quantum critical behavior in heavy fermion materials, focussing on the mechanism by which the electron mass appears to diverge at the quantum critical point. We ask whether the basic mechanism for the transformation involves electron diffraction off a quantum critical spin density wave, or whether a break-down in the composite nature of the heavy electron takes place at the quantum critical point. We show that the Hall constant changes continously in the first scenario, but may ``jump'' discontinuously at a quantum critical point where the composite character of the electron quasiparticles changes.

Breakdown of the Fermi surface at the quantum critical point in YbRh₂Si₂

P. Gegenwart^a, J. Custers^a, K. Neumaier^b, H. Wilhelm^a, C. Geibel^a, O. Trovarelli^a, F. Steglich^a

^aMax-Planck Institute for Chemical Physics of Solids, D-01187 Dresden, Germany

^bWalther Meissner Institute, D-85748 Garching, Germany

In YbRh₂Si₂ pronounced Non-Fermi liquid (NFL) effects are observed in thermodynamic, magnetic and transport properties above a weak antiferromagnetic (AF) phase transition at T_N=70 mK. The AF order is suppressed to T_N®0 either by i) the application of small critical magnetic fields B_c0 or ii) a slight expansion of the crystal lattice by substituting 5% of the Si atoms by Ge in YbRh₂(Si_1-xGe_x)₂. In both cases the NFL behavior extends to lowest T. For B > B_c0 (B_c0=0 for x=0.05) we observe a weakly polarized Landau FL at lowest T which fulfills the Kadowaki-Woods relation A/g₀²=const between the coefficients A of the resistivity and g₀ of the specific heat. The 1/(B-B_c0) divergence of A(B) indicates that the heavy quasiparticles diverge at the quantum critical point.

Inhomogeneous Magnetism and Hidden Order in URu₂Si₂

Hiroshi Amitsuka

Graduate School of Science, Hokkaido University, Sapporo 060-0810, Japan

Recent microscopic studies on URu₂Si₂ have proven the presence of some nonmagnetic ``hidden order" to be responsible for sharp bulk anomalies observed at 17.5 K ( º T_o) in this system, showing the puzzling tiny moments detected by neutron scattering to be ascribed to unusual coexistence ( ~ 1 %) of a normal moment ( ~ 0.25 m_B/U) antiferromagnetic phase. We present the neutron scattering and mSR measurements performed under hydrostatic pressure and uniaxial stress, and discuss the relationship between these two different ordered phases and possible interpretation of the hidden order parameter.