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Singular charge fluctuations at a magnetic quantum critical point

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Science  17 Jan 2020:
Vol. 367, Issue 6475, pp. 285-288
DOI: 10.1126/science.aag1595

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Spin-charge entanglement

Many physical properties follow characteristic scaling laws near quantum critical points, which are associated with phase transitions at absolute zero temperature. The material YbRh2Si2 has an antiferromagnetic quantum critical point, where spin-related properties are expected to follow such a scaling. Unexpectedly, Prochaska et al. found that charge fluctuations follow a critical scaling as well. The researchers fabricated high-quality thin films of YbRh2Si2 and used transmission spectroscopy to measure the optical conductivity of the film and infer the scaling. Their findings point to a highly entangled state of charge and spin, which may also be responsible for the strangemetal phase in this material.

Science, this issue p. 285

Abstract

Strange metal behavior is ubiquitous in correlated materials, ranging from cuprate superconductors to bilayer graphene, and may arise from physics beyond the quantum fluctuations of a Landau order parameter. In quantum-critical heavy-fermion antiferromagnets, such physics may be realized as critical Kondo entanglement of spin and charge and probed with optical conductivity. We present terahertz time-domain transmission spectroscopy on molecular beam epitaxy–grown thin films of YbRh2Si2, a model strange-metal compound. We observed frequency over temperature scaling of the optical conductivity as a hallmark of beyond-Landau quantum criticality. Our discovery suggests that critical charge fluctuations play a central role in the strange metal behavior, elucidating one of the long-standing mysteries of correlated quantum matter.

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