Nuclear magnetic resonance study of the magnetic-field-induced ordered phase in the NiCl2-4SC(NH2)2 compound

TL;DR

This study uses nuclear magnetic resonance to investigate the magnetic-field-induced Bose-Einstein condensed phase in NiCl2-4SC(NH2)2, providing precise phase boundary measurements and comparing experimental results with numerical and analytical models.

Contribution

It offers detailed experimental data on the BEC phase in a quasi-1D antiferromagnetic compound and evaluates the accuracy of theoretical models against these measurements.

Findings

Precise phase boundary Tc(H) determined down to 40 mK.

Critical boson density n_c(Tc) measured and compared with models.

Mean-field approximation valid only for low boson densities (<4%).

Abstract

Nuclear magnetic resonance (NMR) study of the high magnetic field (H) part of the Bose-Einstein condensed (BEC) phase of the quasi-onedimensional (quasi-1D) antiferromagnetic quantum spin-chain compound NiCl2-4SC(NH2)2 (DTN) was performed. We precisely determined the phase boundary, Tc(H), down to 40 mK; the critical boson density, n_c(Tc); and the absolute value of the BEC order parameter S_perp at very low temperature (T = 0.12 K). All results are accurately reproduced by numerical quantum Monte Carlo simulations of a realistic three-dimensional (3D) model Hamiltonian. Approximate analytical predictions based on the 1D Tomonaga-Luttinger liquid description are found to be precise for Tc(H), but less so for S_perp(H), which is more sensitive to the strength of 3D couplings, in particular close to the critical field. A mean-field treatment, based on the Hartree-Fock-Popov description, is found to be valid only up to n_c = 4% (T < 0.3 K), while for higher n_c boson interactions appear to modify the density of states.