Fermi-surface collapse and dynamical scaling near a quantum critical point

TL;DR

This study investigates quantum criticality in YbRh2Si2 through Hall effect measurements, revealing a Fermi-surface collapse and E/T scaling that challenge conventional theories and suggest a critical Kondo breakdown.

Contribution

It provides experimental evidence for quantum-dynamical scaling and Fermi-surface collapse, advancing understanding of quantum critical excitations in heavy-fermion systems.

Findings

Critical Hall crossover linked to quantum-critical point

Width of crossover scales with temperature, indicating E/T scaling

Evidence for simultaneous quantum-dynamical scaling and Kondo breakdown

Abstract

Quantum criticality arises when a macroscopic phase of matter undergoes a continuous transformation at zero temperature. While the collective fluctuations at quantum-critical points are being increasingly recognized as playing an important role in a wide range of quantum materials, the nature of the underlying quantum-critical excitations remains poorly understood. Here we report in-depth measurements of the Hall effect in the heavy-fermion metal YbRh2Si2, a prototypical system for quantum criticality. We isolate a rapid crossover of the isothermal Hall coefficient clearly connected to the quantum-critical point from a smooth background contribution; the latter exists away from the quantum-critical point and is detectable through our studies only over a wide range of magnetic field. Importantly, the width of the critical crossover is proportional to temperature, which violates the predictions of conventional theory and is instead consistent with an energy over temperature, E/T, scaling of the quantum-critical single-electron fluctuation spectrum. Our results provide evidence that the quantum-dynamical scaling and a critical Kondo breakdown simultaneously operate in the same material. Correspondingly, we infer that macroscopic scale-invariant fluctuations emerge from the microscopic many-body excitations associated with a collapsing Fermi-surface. This insight is expected to be relevant to the unconventional finite-temperature behavior in a broad range of strongly correlated quantum systems.