Heike Pfau

Thermal and electrical transport across a magnetic quantum critical point

A quantum critical point (QCP) arises when a continuous transition between competing phases occurs at zero temperature. Collective excitations at magnetic QCPs give rise to metallic properties that strongly deviate from the expectations of Landau’s Fermi-liquid description, which is the standard theory of electron correlations in metals. Central to this theory is the notion of quasiparticles, electronic excitations that possess the quantum numbers of the non-interacting electrons. Here we report measurements of thermal and electrical transport across the field-induced magnetic QCP in the heavy-fermion compound YbRh2Si2 (refs 2, 3). We show that the ratio of the thermal to electrical conductivities at the zero-temperature limit obeys the Wiedemann–Franz law for magnetic fields above the critical field at which the QCP is attained. This is also expected for magnetic fields below the critical field, where weak antiferromagnetic order and a Fermi-liquid phase form below 0.07 K (at zero field). At the critical field, however, the low-temperature electrical conductivity exceeds the thermal conductivity by about 10 per cent, suggestive of a non-Fermi-liquid ground state. This apparent violation of the Wiedemann–Franz law provides evidence for an unconventional type of QCP at which the fundamental concept of Landau quasiparticles no longer holds. These results imply that Landau quasiparticles break up, and that the origin of this disintegration is inelastic scattering associated with electronic quantum critical fluctuations—these insights could be relevant to understanding other deviations from Fermi-liquid behaviour frequently observed in various classes of correlated materials.

Interplay between Kondo suppression and Lifshitz transitions in YbRh2Si2 at high magnetic fields.

We investigate the magnetic field dependent thermopower, thermal conductivity, resistivity, and Hall effect in the heavy fermion metal YbRh2Si2. In contrast to reports on thermodynamic measurements, we find in total three transitions at high fields, rather than a single one at 10 T. Using the Mott formula together with renormalized band calculations, we identify Lifshitz transitions as their origin. The predictions of the calculations show that all experimental results rely on an interplay of a smooth suppression of the Kondo effect and the spin splitting of the flat hybridized bands.

Superconducting gap structure of the skutterudite LaPt4Ge12 probed by specific heat and thermal transport

We investigated the superconducting order parameter of the filled skutterudite

Kondo Lattices in Magnetic Field

{\mathrm{LaPt}}_{4}{\mathrm{Ge}}_{12}

Introduction and Theoretical Background

, with a transition temperature of

Experimental Techniques for Transport Measurements

{T}_{c}=8.3

The Superconducting Order Parameter of LaPt4Ge12

K. To this end, we performed temperature and magnetic-field dependent specific-heat and thermal-conductivity measurements. All data are compatible with a single superconducting

The Wiedemann-Franz Law in YbRh2Si2

s

Ferromagnetic quantum criticality in the quasi-one-dimensional heavy fermion metal YbNi4P2

-wave gap. However, a multiband scenario cannot be ruled out. The results are discussed in the context of previous studies on the substitution series

Evidence of a Kondo Destroying Quantum Critical Point in YbRh2Si2

{\mathrm{Pr}}_{1\ensuremath{-}x}{\mathrm{La}}_{x}{\mathrm{Pt}}_{4}{\mathrm{Ge}}_{12}