Enhancement and Discontinuity of Effective Mass through the First-Order Metamagnetic Transition in UTe2

In this study, the metamagnetic transitions in the novel spin-triplet superconductor UTe2 are investigated based on the recently developed simultaneous measurements of magnetization and sample temperature for the applied field along the orthorhombic b-axis and close to the [011] direction, where reentrant superconductivity (RSC) is detected below and above the first-order metamagnetic transition field H-m. Combining Maxwell's relation and the Clausius-Clapeyron equation, we obtain the field dependence of the Sommerfeld coefficient gamma through the first-order metamagnetic transition. A significant enhancement in the effective mass toward Hm is detected in both field directions. In contrast, above Hm, the effective mass discontinuously decreases for H parallel to b, while it discontinuously increases for H parallel to similar to[011], which plays a crucial role in the RSC.