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Register a new userThermoelectric response near a quantum critical point of beta-YbAlB4 and YbRh2Si2: A comparative study
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The thermoelectric coefficients have been measured on the Yb-based heavy fermion compounds beta-YbAlB4 and YbRh2Si2 down to a very low temperature. We observe a striking difference in the behavior of the Seebeck coefficient, S in the vicinity of the Quantum Critical Point (QCP) in the two systems. As the critical field is approached, S/T enhances in beta-YbAlB4 but is drastically reduced in YbRh2Si2. While in the former system, the ratio of thermopower-to-specific heat remains constant, it drastically drops near the QCP in YbRh2Si2. In both systems, on the other hand, the Nernst coefficient shows a diverging behavior near the QCP. The results provide a new window to the way various energy scales of the system behave and eventually vanish near a QCP.
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We present a study of thermoelectric coefficients in CeCoIn_5 down to 0.1 K and up to 16 T in order to probe the thermoelectric signatures of quantum criticality. In the vicinity of the field-induced quantum critical point, the Nernst coefficient nu exhibits a dramatic enhancement without saturation down to lowest measured temperature. The dimensionless ratio of Seebeck coefficient to electronic specific heat shows a minimum at a temperature close to threshold of the quasiparticle formation. Close to T_c(H), in the vortex-liquid state, the Nernst coefficient behaves anomalously in puzzling contrast with other superconductors and standard vortex dynamics.
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.
We analyze the magnetic and electronic properties of the quantum critical heavy fermion superconductor beta-YbAlB4, calculating the Fermi surface and the angular dependence of the extremal orbits relevant to the de Haas--van Alphen measurements. Using a combination of the realistic materials modeling and single-ion crystal field analysis, we are led to propose a layered Kondo lattice model for this system, in which two dimensional boron layers are Kondo coupled via interlayer Yb moments in a $J_{z}=pm 5/2$ state. This model fits the measured single ion magnetic susceptibility and predicts a substantial change in the electronic anisotropy as the system is pressure-tuned through the quantum critical point.
In metals near a quantum critical point, the electrical resistance is thought to be determined by the lifetime of the carriers of current, rather than the scattering from defects. The observation of $T$-linear resistivity suggests that the lifetime only depends on temperature, implying the vanishing of an intrinsic energy scale and the presence of a quantum critical point. Our data suggest that this concept extends to the magnetic field dependence of the resistivity in the unconventional superconductor BaFe$_2$(As$_{1-x}$P$_{x}$)$_2$ near its quantum critical point. We find that the lifetime depends on magnetic field in the same way as it depends on temperature, scaled by the ratio of two fundamental constants $mu_B/k_B$. These measurements imply that high magnetic fields probe the same quantum dynamics that give rise to the $T$-linear resistivity, revealing a novel kind of magnetoresistance that does not depend on details of the Fermi surface, but rather on the balance of thermal and magnetic energy scales. This opens new opportunities for the investigation of transport near a quantum critical point by using magnetic fields to couple selectively to charge, spin and spatial anisotropies.
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.
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