#
APS March Meeting 2014

## Volume 59, Number 1

##
Monday–Friday, March 3–7, 2014;
Denver, Colorado

### Session M48: Invited Session: Advances in Correlated Electron Systems

11:15 AM–2:15 PM,
Wednesday, March 5, 2014

Room: Mile High Ballroom 1A-1B

Sponsoring
Unit:
DCMP

Abstract ID: BAPS.2014.MAR.M48.1

### Abstract: M48.00001 : Evidence of f-electron localization at a heavy-fermion quantum critical point

11:15 AM–11:51 AM

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Abstract

####
Author:

Frank Steglich

(Max Planck Institute for Chemical Physics of Solids, 01187 Dresden, Germany)

The prototypical heavy-fermion compound YbRh$_{2}$Si$_{2}$ exhibits a
magnetic-field ($B)$ induced antiferromagnetic quantum critical point (QCP) at
$B_{c}$ ($\bot $c) $\approx $ 60 mT. As inferred from transport and
thermodynamic measurements a quantum-critical energy scale,
k$_{B}T$*($B)$, indicating a crossover of the Fermi surface, has been
established for this system [1]. Upon extrapolating finite-temperature ($T)$
data to $T =$ 0, one concludes (i) a vanishing of $T$*($B)$ [2] and (ii) an abrupt
drop in the (normal) Hall coefficient $R_{H}(B)$ [2, 3] at $B = B_{c}$,
verifying the proposal of a Kondo destroying QCP [4,5].
The dynamical processes underlying this apparent break-up of the Kondo
singlets have been explored [6-8] by studying the Lorenz ratio $L$/L$_{0}$ as a
function of $T $and $B$. Here, $L = \rho /w$ is the ratio of the electrical ($\rho)$ and thermal ($w =$ L$_{0}T$/$\kappa )$ resistivities, with $\kappa $ being
the thermal conductivity and L$_{0} =$ ($\pi $k$_{B})^{2}$/3e$^{2}$
Sommerfeld's constant. By properly taking care of bosonic
(magnon/paramagnon) contributions to the heat current which exist at finite
temperature only, extrapolation of the measured data to $T =$ 0 yields a
purely electronic Lorenz ratio $L$/L$_{0} =$ 1 at $B \ne B_{c}$. At $B =$
$B_{c}$, we extrapolate $L$/L$_{0} \approx $ 0.9. Therefore, the Wiedemann
Franz (WF) law holds at any value of the control parameter $B$, except for the
field-induced QCP [6], as is also illustrated by a pronounced heating of the
sample when measuring the low -- $T$ electrical resistivity in the vicinity of
the critical magnetic field [8]. This violation of the WF law is ascribed to
scatterings of the electronic heat carriers from \textit{fermionic} quantum-critical
fluctuations, namely those of the Fermi surface.
Work done in collaboration with H. Pfau, S. Lausberg, P. Sun, U. Stockert, M. Brando, S. Friedemann, C. Krellner, C. Geibel, S. Wirth, S. Kirchner, E. Abrahams and Q. Si.
\\[4pt]
[1] P. Gegenwart et al., Science \underline {315}, 969 (2007).\\[0pt]
[2] S. Friedemann et al., Proc. Natl. Acad. Sci. USA \underline {107}, 14547 (2010).\\[0pt]
[3] S. Paschen et al., Nature \underline {432}, 881 (2004).\\[0pt]
[4] Q. Si et al., Nature \underline {413}, 804 (2001).\\[0pt]
[5] P. Coleman et al., J. Phys.: Condens. Matter \underline {13}, R 723 (2001).\\[0pt]
[6] H. Pfau et al., Nature \underline {484}, 493 (2012).\\[0pt]
[7] H. Pfau et al., Phys. Rev. Lett. \underline {110}, 256403 (2013).\\[0pt]
[8] F. Steglich et al., arXiv: 1309.7260.

To cite this abstract, use the following reference: http://meetings.aps.org/link/BAPS.2014.MAR.M48.1