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

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.

8 pages, 6 figures, accepted in Physical Review B

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