Divergence of the Heavy Quasiparticle Mass at the Antiferromagnetic Quantum Critical Point in YbRh_2Si_2

TL;DR

This study investigates the divergence of heavy quasiparticle mass at the antiferromagnetic quantum critical point in YbRh_2Si_2 through low-temperature thermodynamic and transport measurements, revealing non-Fermi liquid behavior and critical scaling.

Contribution

It provides experimental evidence of quasiparticle mass divergence at the quantum critical point in YbRh_2Si_2, highlighting non-Fermi liquid behavior and critical scaling near the AF transition.

Findings

Divergence of quasiparticle mass at the critical field B_{c0}

Non-Fermi liquid behavior observed in specific heat and resistivity

Scaling laws indicating singular scattering across the Fermi surface

Abstract

We report low temperature specific heat, C, magnetization, M, susceptibility, chi, and electrical resistivity, rho, measurements on high-quality single crystals of the heavy-fermion system YbRh_2(Si_{1-x} Ge_x)_2 (x=0 and 0.05). The undoped compound shows weak antiferromagnetic (AF) order at T_N=70 mK which is suppressed to below 10 mK by a tiny volume expansion in the x=0.05 system. In the latter pronounced deviations from Landau Fermi liquid (LFL) behavior occur, e.g. Delta rho ~ T over three decades in T. Both thermodynamic and magnetic properties show a crossover at about 0.3 K: At 0.3 K <= T <= 10 K we observe C/T ~ log(T_0/T) and a "non-Curie" behavior chi^{-1} ~ T^alpha with alpha<1 similar to what was found for the prototypical system CeCu_{5.9} Au_{0.1}. Below 0.3 K, chi turns into a Curie-Weiss dependence chi^{-1} ~ (T-Theta) indicating large unscreened Yb^{3+} moments whereas in C(T)/T a pronounced upturn occurs. In the undoped compound the AF order is suppressed continuously by critical fields B_{c0} ~= 0.06 T and 0.7 T applied perpendicular and parallel to the c-axis, respectively. For B>B_{c0} a LFL state with Delta rho = A(B)T^2 and C(T)/T = gamma_0(B) is induced, that fulfills the Kadowaki-Woods scaling A ~ gamma_0^2. Upon reducing the magnetic field to B_{c0} a 1/(B-B_{c0}) dependence of A(B) and gamma_0^2(B) indicates singular scattering at the whole Fermi surface and a divergence of the heavy quasiparticle mass.