Bose-Einstein Condensation of S = 1 Ni spin degrees of freedom in NiCl2-4SC(NH2)2

TL;DR

This study provides experimental evidence for Bose-Einstein Condensation of Ni spin degrees of freedom in the organic compound DTN, confirming theoretical predictions through specific heat and magnetocaloric measurements near the quantum critical point.

Contribution

The paper demonstrates the occurrence of BEC in a S=1 Ni spin system in DTN, confirming theoretical models with experimental phase diagram data.

Findings

Quantum phase transition line follows a power-law with exponent ~1.47

Evidence supports BEC of Ni spins in DTN at low temperatures

Results align with BEC theory prediction of exponent 1.5

Abstract

It has recently been suggested that the organic compound NiCl$_2$-4SC(NH$_2$)$_2$ (DTN) exhibits Bose-Einstein Condensation (BEC) of the Ni spin degrees of freedom for fields applied along the tetragonal c-axis. The Ni spins exhibit 3D XY-type antiferromagnetic order above a field-induced quantum critical point at $H_{c1} \sim 2$ T. The Ni spin fluid can be characterized as a system of effective bosons with a hard-core repulsive interaction in which the antiferromagnetic state corresponds to a Bose-Einstein condensate (BEC) of the phase coherent $S = 1$ Ni spin system. We have investigated the the high-field phase diagram and the occurrence of BEC in DTN by means of specific heat and magnetocaloric effect measurements to dilution refrigerator temperatures. Our results indicate that a key prediction of BEC is satisfied; the magnetic field-temperature quantum phase transition line $H_c(T)-H_{c1} \propto T^\alpha$ approaches a power-law at low temperatures, with an exponent $\alpha = 1.47 \pm 0.06$ at the quantum critical point, consistent with the BEC theory prediction of $\alpha = 1.5$.