# Bose-Einstein condensation of triplons in the S=1 tetramer antiferromagnet K2Ni2(MoO4)3: A compound close to quantum critical point

## TL;DR

This study reports the observation of Bose-Einstein condensation of triplons in the S=1 tetramer antiferromagnet K2Ni2(MoO4)3, revealing its proximity to a quantum critical point through magnetic and thermodynamic measurements.

## Contribution

It demonstrates BEC of triplons in a new compound near a quantum critical point, with phase boundary behavior consistent with theoretical predictions.

## Key findings

- Observation of magnetic long-range order below 1.13 K
- Induction of BEC-like behavior by small magnetic fields
- Phase boundary follows a power-law with exponent close to 2/3

## Abstract

The structure of K2Ni2(MoO4)3 consists of S=1 tetramers formed by Ni^{2+} ions. The magnetic susceptibility chi(T) and specific heat Cp(T) data on a single crystal show a broad maximum due to the low-dimensionality of the system with short-range spin correlations. A sharp peak is seen in chi(T) and Cp(T) at about 1.13 K, well below the broad maximum. This is an indication of magnetic long-range order i.e., the absence of spin-gap in the ground state. Interestingly, the application of a small magnetic field (H>0.1 T) induces magnetic behavior akin to Bose-Einstein condensation (BEC) of triplon excitations observed in some spin-gap materials. Our results demonstrate that the temperature-field (T-H) phase boundary follows a power-law (T-T_{N})propotional to H^(1/alpha) with the exponent 1/alpha close to 2/3, as predicted for BEC scenario. The observation of BEC of triplon excitations in small H infers that K2Ni2(MoO4)3 is located in the proximity of a quantum critical point, which separates the magnetically ordered and spin-gap regions of the phase diagram.