title: Thermal and electrical transport across a magnetic quantum critical point.
abstract: 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 YbRh(2)Si(2) (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.
authors.affiliation: Max Planck Institute for Chemical Physics of Solids, Nöthnitzer Strasse 40, 01187 Dresden, Germany.
authors.forename: Heike
authors.initials: H
authors.lastname: Pfau
authors.forename: Stefanie
authors.initials: S
authors.lastname: Hartmann
authors.forename: Ulrike
authors.initials: U
authors.lastname: Stockert
authors.forename: Peijie
authors.initials: P
authors.lastname: Sun
authors.forename: Stefan
authors.initials: S
authors.lastname: Lausberg
authors.forename: Manuel
authors.initials: M
authors.lastname: Brando
authors.forename: Sven
authors.initials: S
authors.lastname: Friedemann
authors.forename: Cornelius
authors.initials: C
authors.lastname: Krellner
authors.forename: Christoph
authors.initials: C
authors.lastname: Geibel
authors.forename: Steffen
authors.initials: S
authors.lastname: Wirth
authors.forename: Stefan
authors.initials: S
authors.lastname: Kirchner
authors.forename: Elihu
authors.initials: E
authors.lastname: Abrahams
authors.forename: Qimiao
authors.initials: Q
authors.lastname: Si
authors.forename: Frank
authors.initials: F
authors.lastname: Steglich