Quantum degradation of a second-order phase transition

Abstract

The specific heat, magnetization, and thermal expansion of single crystals of the antiferromagnetic insulator EuTe, measured at temperatures down to 2 K and in magnetic fields up to 90 kOe, demonstrate nontrivial properties. The N\'eel temperature, being $\ensuremath{\sim}9.8$ at $H=0$, decreases with magnetic field and tends to zero at $\ensuremath{\sim}76 \mathrm{kOe}$, therefore forming a quantum critical point. The heat capacity and thermal expansion coefficient reveal $\ensuremath{\lambda}$-type anomalies at the second order magnetic phase transition at low magnetic fields, evolving into simple jumps at high magnetic fields and low temperatures; these are well described in a fluctuation-free mean-field theory. The experimental data and the corresponding analysis favor the quantum concept of an effective increase in spatial dimensionality at low temperatures that suppresses a fluctuation-driven divergence at a second-order phase transition.