Reinforcement of superconductivity by quantum critical fluctuations of metamagnetism in UTe₂

The normal-conducting state of the superconductor UTe₂ is studied by entropy analysis for magnetic fields along the b-axis, obtained from magnetization using the relation (∂ M/∂ T)B=(∂ S/∂ B)T. We observe a strong increase in entropy with magnetic field due to metamagnetic fluctuations (spatially uniform, $Q=0$). The field dependence is well described by the Hertz-Millis-Moriya theory for quantum criticality of itinerant metamagnetism. Notably, the lower bound of the quantum-critical region coincides well with the position of the minimum in the superconducting transition temperature Tc(B). Hence, our results suggest that $Q=0$ fluctuations reinforce the superconductivity.