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 description1, 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 holds4,5,6. 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.

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Figure 1: Phase diagram and thermal conductivity of YbRh2Si2.
Figure 2: Thermal and electrical resistivity curves at low temperatures.
Figure 3: Violation and validity of the Wiedemann–Franz law at B ≈ B c and B > B c, respectively.
Figure 4: The evolution of the quasiparticle weights across a Kondo-breakdown quantum critical point.

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Acknowledgements

We thank P. Coleman, R. Daou, P. Gegenwart, N. E. Hussey, K. Ingersent, G. Kotliar, A. P. Mackenzie, H. von Löhneysen, J. Schmalian, A. J. Schofield, T. Senthil, S. Shastry and Z. Teśanovic for discussions. The work was in part supported by the DFG Research Unit 960 ‘Quantum Phase Transitions’, NSF grant DMR-1006985 and the Robert A. Welch Foundation grant C-1411. E.A., S.K., Q.S. and F.S. acknowledge support in part by the NSF under grant 1066293 and the hospitality of the Aspen Center for Physics.

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Correspondence to Frank Steglich.

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Pfau, H., Hartmann, S., Stockert, U. et al. Thermal and electrical transport across a magnetic quantum critical point. Nature 484, 493–497 (2012). https://doi.org/10.1038/nature11072

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