Specific heat of the $S = 1$ spin-dimer antiferromagnet Ba$_3$Mn$_2$O$_8$ in high magnetic fields

TL;DR

This study investigates the specific heat of the $S=1$ spin-dimer antiferromagnet Ba$_3$Mn$_2$O$_8$ under high magnetic fields, revealing multiple ordered phases, deviations from theoretical models, and insights into spin excitations and localization.

Contribution

The paper provides detailed experimental data on specific heat in high magnetic fields, identifying three ordered phases and analyzing spin excitations in Ba$_3$Mn$_2$O$_8$, extending understanding of spin-dimer systems.

Findings

Identified three ordered phases between magnetization plateau and low-field region.

Observed phase boundary exponents smaller than Bose-Einstein condensation predictions.

Revealed gapped behaviors and localization of triplet excitations at zero field and high field.

Abstract

We have measured the specific heat of the coupled spin-dimer antiferromagnet Ba$_3$Mn$_2$O$_8$ to 50 mK in temperature and to 29 T in the magnetic field. The experiment extends to the midpoint of the field region (25.9 T $\leq H \leq$ 32.3 T) of the magnetization plateau at 1/2 of the saturation magnetization, and reveals the presence of three ordered phases in the field region between that of the magnetization plateau and the low-field spin-liquid region. The exponent of the phase boundary with the thermally disordered region is smaller than the theoretical value based on the Bose-Einstein condensation of spin triplets. At zero field and 29 T, the specific-heat data show gapped behaviors characteristic of spin liquids. The zero-field data indicate that the gapped triplet excitations form two levels whose energies differ by nearly a factor of two. At least the lower level is well localized. The data at 29 T reveal that the low-lying excitations at the magnetization plateau are weakly delocalized.