J. Ferstl

High-field phase diagram of the heavy-fermion metal YbRh2Si2

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.

Field-Induced Suppression of the Heavy-Fermion State in YbRh2Si2

We present calculations of the magnetic-field-induced changes of the heavy quasiparticles in YbRh2Si2 which are reflected in thermodynamic and transport properties. The quasiparticles are determined by means of the renormalized band method. The progressive de-renormalization of the quasiparticles in the magnetic field is accounted for using field-dependent quasiparticle parameters deduced from numerical renormalization group studies. Consequences for the interpretation of experimental data are discussed.

Low-energy spin fluctuations in the non-Fermi-liquid compound YbRh2Si2

Abstract We report on inelastic neutron scattering experiments on YbRh2Si2 powder to study the low-energy spin dynamics at temperatures between T = 0.8 and 22 K. The low-energy magnetic response is quasielastic. However, it exhibits an unusual form not modelled by a simple relaxation rate yielding a Lorentzian lineshape, but can satisfactorily be described by a phenomenological model involving a distribution of relaxation rates. The lower bound of the relaxation rates varies roughly linear with temperature indicating a pronounced slowing down of the critical modes above the antiferromagnetic ordering temperature TN ≈ 70 mK.