$z=2$ Quantum Critical Dynamics in a Spin Ladder

TL;DR

This study uses neutron scattering to explore the finite-temperature quantum critical dynamics of a spin ladder compound near a critical point, confirming theoretical predictions and revealing complex fluctuation behavior.

Contribution

It provides experimental validation of universal scaling laws at a quantum critical point with dynamical exponent z=2 in a spin ladder system.

Findings

Universal finite-temperature scaling of the transverse dynamic structure factor

Observation of strong non-universal longitudinal fluctuations at low temperatures

Striking scaling behavior in specific heat measurements

Abstract

By means of inelastic neutron scattering we investigate finite temperature dynamics in the quantum spin ladder compound (C$_5$H$_{12}$N)$_2$CuBr$_4$ (BPCB) near the magnetic field induced quantum critical point with dynamical exponent $z=2$. We observe universal finite-temperature scaling of the transverse local dynamic structure factor in spectacular quantitative agreement with long-standing theoretical predictions. At the same time, already at rather low temperatures, we observe strong non-universal longitudinal fluctuations. To separate the two, we make use of an intrinsic leg-exchange symmetry of the spin ladder. Complementary measurements of specific heat also reveal striking scaling behavior near the quantum critical point.