Field-driven phase transitions in a quasi-two-dimensional quantum antiferromagnet

TL;DR

This study investigates the magnetic phases of a quasi-two-dimensional quantum antiferromagnet, revealing multiple phase transitions including a magnon Bose-Einstein condensate and a fully polarized state, through various experimental techniques.

Contribution

It provides detailed experimental characterization of field-driven phase transitions in PHCC, highlighting the quantum paramagnet, antiferromagnetic order, and Bose-Einstein condensation phenomena.

Findings

Identification of four distinct magnetic phases.

Observation of a magnon Bose-Einstein condensate at 7.5 Tesla.

Detection of a fully polarized ferromagnetic state at 37 Tesla.

Abstract

We report magnetic susceptibility, specific heat, and neutron scattering measurements as a function of applied magnetic field and temperature to characterize the $S=1/2$ quasi-two-dimensional frustrated magnet piperazinium hexachlorodicuprate (PHCC). The experiments reveal four distinct phases. At low temperatures and fields the material forms a quantum paramagnet with a 1 meV singlet triplet gap and a magnon bandwidth of 1.7 meV. The singlet state involves multiple spin pairs some of which have negative ground state bond energies. Increasing the field at low temperatures induces three dimensional long range antiferromagnetic order at 7.5 Tesla through a continuous phase transition that can be described as magnon Bose-Einstein condensation. The phase transition to a fully polarized ferromagnetic state occurs at 37 Tesla. The ordered antiferromagnetic phase is surrounded by a renormalized classical regime. The crossover to this phase from the quantum paramagnet is marked by a distinct anomaly in the magnetic susceptibility which coincides with closure of the finite temperature singlet-triplet pseudo gap. The phase boundary between the quantum paramagnet and the Bose-Einstein condensate features a finite temperature minimum at $T=0.2$ K, which may be associated with coupling to nuclear spin or lattice degrees of freedom close to quantum criticality.