Revealing three-dimensional quantum criticality by Sr-substitution in Han Purple

TL;DR

This study demonstrates that substituting Sr in Han Purple restores observable three-dimensional quantum critical scaling, revealing new insights into quantum phase transitions in quasi-2D materials.

Contribution

The paper shows that Sr-substitution simplifies the structure of Han Purple, enabling the observation of 3D quantum criticality through combined experimental and computational methods.

Findings

Restoration of 3D quantum critical scaling in Sr-substituted Han Purple.

Identification of magnetic Hamiltonian via neutron spectroscopy.

Confirmation of critical behavior through Bayesian analysis and Quantum Monte Carlo simulations.

Abstract

Classical and quantum phase transitions (QPTs), with their accompanying concepts of criticality and universality, are a cornerstone of statistical thermodynamics. An exemplary controlled QPT is the field-induced magnetic ordering of a gapped quantum magnet. Although numerous "quasi-one-dimensional" coupled spin-chain and -ladder materials are known whose ordering transition is three-dimensional (3D), quasi-2D systems are special for several physical reasons. Motivated by the ancient pigment Han Purple (BaCuSi$_{2}$O$_{6}$), a quasi-2D material displaying anomalous critical properties, we present a complete analysis of Ba$_{0.9}$Sr$_{0.1}$CuSi$_{2}$O$_{6}$. We measure the zero-field magnetic excitations by neutron spectroscopy and deduce the magnetic Hamiltonian. We probe the field-induced transition by combining magnetization, specific-heat, torque and magnetocalorimetric measurements with low-temperature nuclear magnetic resonance studies near the QPT. By a Bayesian statistical analysis and large-scale Quantum Monte Carlo simulations, we demonstrate unambiguously that observable 3D quantum critical scaling is restored by the structural simplification arising from light Sr-substitution in Han Purple.