Quantum critical scaling at a Bose-glass/superfluid transition: theory and experiment on a model quantum magnet

TL;DR

This study combines theory, simulation, and experiment to analyze the quantum critical behavior at the Bose-glass to superfluid transition in a model quantum magnet, revealing critical exponents and scaling laws.

Contribution

It provides the first comprehensive analysis of quantum critical scaling at this transition, including experimental verification and a critique of existing scaling theories.

Findings

Critical exponent z = d confirmed by simulations.

Correlation length exponent nu ≈ 0.75.

Power-law behavior of magnetization and specific heat observed.

Abstract

In this paper we investigate the quantum phase transition from magnetic Bose glass to magnetic Bose-Einstein condensation induced by a magnetic field in NiCl2.4SC(NH2)2 (dichloro-tetrakis-thiourea-Nickel, or DTN), doped with Br (Br-DTN) or site diluted. Quantum Monte Carlo simulations for the quantum phase transition of the model Hamiltonian for Br-DTN, as well as for site-diluted DTN, are consistent with conventional scaling at the quantum critical point and with a critical exponent z verifying the prediction z=d; moreover the correlation length exponent is found to be nu = 0.75(10) and the order parameter exponent to be beta = 0.95(10). We investigate the low-temperature thermodynamics at the quantum critical field of Br-DTN both numerically and experimentally, and extract the power-law behavior of the magnetization and of the specific heat. Our results for the exponents of the power laws, as well as previous results for the scaling of the critical temperature to magnetic ordering with the applied field, are incompatible with the conventional crossover-scaling Ansatz proposed by Fisher et al., [Phys. Rev. B 40, 546 (1989)], but they can all be reconciled within a phenomenological Ansatz in the presence of a dangerously irrelevant operator.