Multiple Magnetic Bilayers and Unconventional Criticality without Frustration in BaCuSi$_2$O$_6$

TL;DR

This study reveals that BaCuSi$_2$O$_6$ exhibits unconventional critical behavior due to multiple inequivalent bilayers with distinct interactions, challenging previous assumptions of frustration-driven dimensional reduction.

Contribution

The paper demonstrates that the effective intra-bilayer interactions are ferromagnetic, not frustrated, and introduces a model with three inequivalent bilayers explaining the non-universal scaling near the QCP.

Findings

Intra-bilayer interactions are ferromagnetic, excluding frustration.

Presence of three inequivalent bilayers with ratios 3:2:1.

Quantum Monte Carlo simulations match experimental data.

Abstract

The dimerized quantum magnet BaCuSi$_2$O$_6$ was proposed as an example of "dimensional reduction" arising near the magnetic-field-induced quantum critical point (QCP) due to perfect geometrical frustration of its inter-bilayer interactions. We demonstrate by high-resolution neutron spectroscopy experiments that the effective intra-bilayer interactions are ferromagnetic, thereby excluding frustration. We explain the apparent dimensional reduction by establishing the presence of three magnetically inequivalent bilayers, with ratios 3:2:1, whose differing interaction parameters create an extra field-temperature scaling regime near the QCP with a non-trivial but non-universal exponent. We demonstrate by detailed quantum Monte Carlo simulations that the magnetic interaction parameters we deduce can account for all the measured properties of BaCuSi$_2$O$_6$, opening the way to a quantitative understanding of non-universal scaling in any modulated layered system.