The break up of heavy electrons at a quantum critical point

TL;DR

This paper investigates the behavior of heavy electrons near a quantum critical point in a specific material, revealing that electron motion ceases at the critical point, indicating a new class of electronic behavior.

Contribution

It demonstrates that the electron effective mass and scattering diverge near a field-tuned quantum critical point in YbRh2Si2, showing universal behavior and electron decay into collective motions.

Findings

Electron effective mass diverges at the QCP.

Electron kinetic energy is proportional to applied magnetic field.

Ballistic electron motion vanishes at the QCP.

Abstract

The point at absolute zero where matter becomes unstable to new forms of order is called a quantum critical point (QCP). The quantum fluctuations between order and disorder that develop at this point induce profound transformations in the finite temperature electronic properties of the material. Magnetic fields are ideal for tuning a material as close as possible to a QCP, where the most intense effects of criticality can be studied. A previous study on theheavy-electron material $YbRh_2Si_2$ found that near a field-induced quantum critical point electrons move ever more slowly and scatter off one-another with ever increasing probability, as indicated by a divergence to infinity of the electron effective mass and cross-section. These studies could not shed light on whether these properties were an artifact of the applied field, or a more general feature of field-free QCPs. Here we report that when Germanium-doped $YbRh_2Si_2$ is tuned away from a chemically induced quantum critical point by magnetic fields there is a universal behavior in the temperature dependence of the specific heat and resistivity: the characteristic kinetic energy of electrons is directly proportional to the strength of the applied field. We infer that all ballistic motion of electrons vanishes at a QCP, forming a new class of conductor in which individual electrons decay into collective current carrying motions of the electron fluid.