Thermal Transport Properties and Quantum Criticality of Heavy Fermion YbRh₂Si₂

Abstract

We present the result of the thermal conductivity κ and the thermopower S measurements of heavy fermion compound YbRh 2 Si 2 down to 40 mK and under magnetic fields up to 5 T. In zero field, κ(T )/T shows an upturn at the antiferromagnetic (AF) transition temperature T N = 70 mK. The lorentz ratio L/L 0 shows about 0.8 down to T N and approaches to 1 as T → 0 K, indicating the dominant contribution of the electrons to the thermal conductivity. The upturn of κ(T )/T , therefore, may account for the enhancement of the mean free path of the electrons due to the AF ordering. The suppression of L/L 0 above T N suggests the presence of the inelastic scattering. On the other hand, the absolute value of S (T )/T steeply decreases at low temperatures near the quantum critical point (QCP), whereas S (T )/T becomes almost temperature-independent down to lowest temperature in the Fermi-liquid (FL) region. Moreover, the ratio of the Seebeck coefficient to the specific heat is suppressed strongly in the vicinity of the QCP while the ratio shows a constant value of the order of unity for FL region. Similar suppression of the ratio is also observed near the AF QCP in CeCoIn 5 .