Quantum Criticality in an Organic Magnet

TL;DR

This study investigates quantum critical points in an organic magnet, revealing a phase diagram with two critical fields and identifying a magnon Bose-Einstein condensate phase with complex low-temperature behavior.

Contribution

It provides the first detailed phase diagram of a frustrated bilayer organic magnet, highlighting two quantum critical points and the nature of the ordered phase as a magnon BEC.

Findings

Two quantum critical points at 7.5 T and 37 T

Identification of a magnon Bose-Einstein condensate phase

Observation of a low temperature anomaly near Hc1

Abstract

Exchange interactions between $S=\frac{1}{2}$ sites in piperazinium hexachlorodicuprate produce a frustrated bilayer magnet with a singlet ground state. We have determined the field-temperature phase diagram by high field magnetization and neutron scattering experiments. There are two quantum critical points: $H_{c1}=7.5$ T separates a quantum paramagnet phase from a three dimensional, antiferromagnetically-ordered state while $H_{c2}=37$ T marks the onset of a fully polarized state. The ordered phase, which we describe as a magnon Bose-Einstein condensate (BEC), is embedded in a quantum critical regime with short range correlations. A low temperature anomaly in the BEC phase boundary indicates that additional low energy features of the material become important near $H_{c1}$.