  We theoretically investigate the temperature effect in a Bose-Bose mixture
with attractive inter-species interactions, in the regime where a self-bound
ultradilute quantum droplet forms due to the subtle balance between the
attractive mean-field force and the repulsive force provided by Lee-Huang-Yang
quantum fluctuations. We find that in contrast to quantum fluctuations, thermal
fluctuations destabilize the droplet state and completely destroy it above a
threshold temperature. We show that the threshold temperature is determined by
the intra-species interaction energy. For a three-dimensional Bose-Bose
mixture, the threshold temperature is less than one-tenth of the Bose-Einstein
condensation temperature under the typical experimental conditions. With
increasing temperature, the droplet's equilibrium density gradually decreases
and can be reduced by several tens of percent upon reaching the threshold
temperature. We also consider a one-dimensional quantum droplet and find a
similar destabilization effect due to thermal fluctuations. The threshold
temperature in one dimension is roughly set by the binding energy of the
inter-species dimer. The pronounced thermal instability of a self-bound quantum
droplet predicted in our work could be examined in future experiments, by
measuring the temperature dependence of its central density and observing its
sudden disappearance at the threshold temperature.
