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Register a new userThermal conductivity through the quantum critical point in YbRh2Si2 at very low temperature
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The thermal conductivity of YbRh2Si2 has been measured down to very low temperatures under field in the basal plane. An additional channel for heat transport appears below 30 mK, both in the antiferromagnetic and paramagnetic states, respectively below and above the critical field suppressing the magnetic order. This excludes antiferromagnetic magnons as the origin of this additional contribution to thermal conductivity. Moreover, this low temperature contribution prevails a definite conclusion on the validity or violation of the Wiedemann-Franz law at the field-induced quantum critical point. At high temperature in the paramagnetic state, the thermal conductivity is sensitive to ferromagnetic fluctuations, previously observed by NMR or neutron scattering and required for the occurrence of the sharp electronic spin resonance fracture.
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The thermal conductivity measurements have been performed on the heavy-fermion compound YbRh2Si2 down to 0.04 K and under magnetic fields through a quantum critical point (QCP) at Bc = 0.66 T || c-axis. In the limit as T -> 0, we find that the Wiedemann-Franz law is satisfied within experimental error at the QCP despite the destruction of the standard signature of Fermi liquid. Our results place strong constraints on models that attempt to describe the nature of unconventional quantum criticality of YbRh2Si2.
The heavy-fermion metal YbRh$_{2}$Si$_{2}$ is a weak antiferromagnet below $T_{N} = 0.07$ K. Application of a low magnetic field $B_{c} = 0.06$ T ($perp c$) is sufficient to continuously suppress the antiferromagnetic (AF) order. Below $T approx 10$ K, the Sommerfeld coefficient of the electronic specific heat $gamma(T)$ exhibits a logarithmic divergence. At $T < 0.3$ K, $gamma(T) sim T^{-epsilon}$ ($epsilon: 0.3 - 0.4$), while the electrical resistivity $rho(T) = rho_{0} + aT$ ($rho_{0}$: residual resistivity). Upon extrapolating finite-$T$ data of transport and thermodynamic quantities to $T = 0$, one observes (i) a vanishing of the Fermi surface crossover scale $T^{*}(B)$, (ii) an abrupt jump of the initial Hall coefficient $R_{H}(B)$ and (iii) a violation of the Wiedemann Franz law at $B = B_{c}$, the field-induced quantum critical point (QCP). These observations are interpreted as evidence of a critical destruction of the heavy quasiparticles, i.e., propagating Kondo singlets, at the QCP of this material.
The in-plane thermal conductivity kappa(T) and electrical resistivity rho(T) of the heavy-fermion metal YbRh2Si2 were measured down to 50 mK for magnetic fields H parallel and perpendicular to the tetragonal c axis, through the field-tuned quantum critical point, Hc, at which antiferromagnetic order ends. The thermal and electrical resistivities, w(T) and rho(T), show a linear temperature dependence below 1 K, typical of the non-Fermi liquid behavior found near antiferromagnetic quantum critical points, but this dependence does not persist down to T = 0. Below a characteristic temperature T* ~ 0.35 K, which depends weakly on H, w(T) and rho(T) both deviate downward and converge in the T = 0 limit. We propose that T* marks the onset of short-range magnetic correlations, persisting beyond Hc. By comparing samples of different purity, we conclude that the Wiedemann-Franz law holds in YbRh2Si2, even at Hc, implying that no fundamental breakdown of quasiparticle behavior occurs in this material. The overall phenomenology of heat and charge transport in YbRh2Si2 is similar to that observed in the heavy-fermion metal CeCoIn5, near its own field-tuned quantum critical point.
We present a comprehensive experimental and theoretical investigation of the thermodynamic properties: specific heat, magnetization and thermal expansion in the vicinity of the field-induced quantum critical point (QCP) around the lower critical field $H_{c1} approx 2$,T in DTN . A $T^{3/2}$ behavior in the specific heat and magnetization is observed at very low temperatures at $H=H_{c1}$ that is consistent with the universality class of Bose-Einstein condensation of magnons. The temperature dependence of the thermal expansion coefficient at $H_{c1}$ shows minor deviations from the expected $T^{1/2}$ behavior. Our experimental study is complemented by analytical calculations and Quantum Monte Carlo simulations, which reproduce nicely the measured quantities. We analyze the thermal and the magnetic Gr{u}neisen parameters that are ideal quantities to identify QCPs. Both parameters diverge at $H_{c1}$ with the expected $T^{-1}$ power law. By using the Ehrenfest relations at the second order phase transition, we are able to estimate the pressure dependencies of the characteristic temperature and field scales.
We present the field and temperature behavior of the narrow Electron Spin Resonance (ESR) response in YbRh2Si2 well below the single ion Kondo temperature. The ESR g factor reflects a Kondo-like field and temperature evolution of the Yb3+ magnetism. Measurements towards low temperatures (>0.5K) have shown distinct crossover anomalies of the ESR parameters upon approaching the regime of a well defined heavy Fermi liquid. Comparison with the field dependence of specific heat and electrical resistivity reveal that the ESR parameters can be related to quasiparticle mass and cross section and, hence, contain inherent heavy electron properties.
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