Magnetovolume Effect on the First-Order Metamagnetic Transition in UTe$_2$

TL;DR

This study investigates the magnetovolume effects associated with the metamagnetic transition in UTe$_2$, revealing significant anisotropic magnetostriction and volume changes that suggest a link to valence instability and lattice instability.

Contribution

The paper provides the first thermodynamic analysis of magnetostriction across the metamagnetic transition in UTe$_2$, highlighting anisotropic effects and their implications for magnetic and valence instabilities.

Findings

Discontinuous magnetostriction at the metamagnetic transition around 35 T.

Significant anisotropic magnetostriction with opposite signs along different axes.

Volume shrinkage at the transition, indicating valence instability.

Abstract

The link between the metamagnetic transition and novel spin-triplet superconductivity of UTe$_2$ was discussed thermodynamically through magnetostriction measurements in a pulsed-magnetic field. We revealed a discontinuous magnetostriction across the metamagnetic transition at $\mu_0H_{\rm m}\approx 35$~T for the applied magnetic fields along the crystallographic $b$ axis in the orthorhombic structure. The resultanting volume magnetostriction of $\Delta V/V \approx-5.9\times 10^{-4}$ gives the initial pressure dependence of $H_{\rm m}$ by employing the Clausius-Clapeyron's equation, which agrees with previous pressure experiments. Further, significant anisotropic magnetostriction (AMS), derived by subtracting the averaged linear magnetostriction, was revealed. Contrary to the weakly field-dependent AMS along the $a$ axis, those along the $b$ and $c$ axes show strong field dependences with a similar magnitude but with opposite signs, indicating its lattice instability. The relationship between characteristic energy scales of magnetic fields and temperatures was discussed in terms of the Gr\"uneisen parameters compared to the other $f$-electron systems. The volume shrinkage in UTe$_2$ at $H_{\rm m}$, contrary to the volume expansion in typical heavy fermion metamagnets, pushes to invoke the link with the valence instability related to the itinerant-localized dual nature of the U magnetism.