Thermodynamic properties of Ba$_2$CoSi$_2$O$_6$Cl$_2$ in strong magnetic field: Realization of flat-band physics in a highly frustrated quantum magnet

TL;DR

This paper investigates the thermodynamic properties of Ba$_2$CoSi$_2$O$_6$Cl$_2$, demonstrating its potential as a flat-band quantum magnet and proposing experimental detection of magnon ordering phenomena.

Contribution

The study develops a localized-magnon theory tailored to Ba$_2$CoSi$_2$O$_6$Cl$_2$, linking experimental observations with flat-band physics and suggesting new experiments for phase transition detection.

Findings

Theory describes existing experimental data

Proposes field-driven phase transition involving localized magnons

Highlights Ba$_2$CoSi$_2$O$_6$Cl$_2$ as a flat-band quantum magnet

Abstract

The search for flat-band solid-state realizations is a crucial issue to verify or to challenge theoretical predictions for quantum many-body flat-band systems. For frustrated quantum magnets flat bands lead to various unconventional properties related to the existence of localized many-magnon states. The recently synthesized magnetic compound Ba$_2$CoSi$_2$O$_6$Cl$_2$ seems to be an almost perfect candidate to observe these features in experiments. We develop a theory for Ba$_2$CoSi$_2$O$_6$Cl$_2$ by adapting the localized-magnon concept to this compound. We first show that our theory describes the known experimental facts and then we propose new experimental studies to detect a field-driven phase transition related to a Wigner-crystal-like ordering of localized magnons at low temperatures.