Hall-effect evolution across a heavy-fermion quantum critical point

TL;DR

This study investigates the Fermi surface transformation at a heavy-fermion quantum critical point in YbRh2Si2, revealing a sudden change in the Hall coefficient indicative of an abrupt Fermi surface collapse at zero temperature.

Contribution

It provides experimental evidence for a sudden Fermi surface change at the QCP, supporting the local quantum criticality scenario over the spin density wave model.

Findings

Hall coefficient shows a rapid change near the QCP at low temperatures

Extrapolation suggests a sudden Fermi surface collapse at zero temperature

Results support a collapse of the heavy-fermion state at the QCP

Abstract

A quantum critical point (QCP) develops in a material at absolute zero when a new form of order smoothly emerges in its ground state. QCPs are of great current interest because of their singular ability to influence the finite temperature properties of materials. Recently, heavy-fermion metals have played a key role in the study of antiferromagnetic QCPs. To accommodate the heavy electrons, the Fermi surface of the heavy-fermion paramagnet is larger than that of an antiferromagnet. An important unsolved question concerns whether the Fermi surface transformation at the QCP develops gradually, as expected if the magnetism is of spin density wave (SDW) type, or suddenly as expected if the heavy electrons are abruptly localized by magnetism. Here we report measurements of the low-temperature Hall coefficient ($R_H$) - a measure of the Fermi surface volume - in the heavy-fermion metal YbRh2Si2 upon field-tuning it from an antiferromagnetic to a paramagnetic state. $R_H$ undergoes an increasingly rapid change near the QCP as the temperature is lowered, extrapolating to a sudden jump in the zero temperature limit. We interpret these results in terms of a collapse of the large Fermi surface and of the heavy-fermion state itself precisely at the QCP.