# Fermi-Surface Selective Determination of the $\mathbf{g}$-Factor Anisotropy in URu$_2$Si$_2$

## TL;DR

This study investigates the anisotropy of the $g$-factor in URu$_2$Si$_2$'s hidden order state using quantum oscillations and critical field measurements, revealing strong directional dependence and insights into quasiparticle behavior.

## Contribution

It provides the first detailed analysis of $g$-factor anisotropy for multiple Fermi-surface pockets in URu$_2$Si$_2$, linking quantum oscillation data with superconducting properties.

## Key findings

- Strong $g$-factor anisotropy between $c$ axis and basal plane.
- Quantum oscillation anisotropy agrees with critical field anisotropy.
- Initial slope of $H_{c2}$ near $T_c$ not explained by effective mass anisotropy.

## Abstract

The $g$-factor anisotropy of the heavy quasiparticles in the hidden order state of URu$_2$Si$_2$ has been determined from the superconducting upper critical field and microscopically from Shubnikov-de Haas (SdH) oscillations. We present a detailed analysis of the $g$-factor for the $\alpha$, $\beta$ and $\gamma$ Fermi-surface pockets. Our results suggest a strong $g$-factor anisotropy between the $c$ axis and the basal plane for all observed Fermi surface pockets. The observed anisotropy of the $g$-factor from the quantum oscillations is in good agreement with the anisotropy of the superconducting upper critical field at low temperatures, which is strongly limited by the paramagnetic pair breaking along the easy magnetization axis $c$. However, the anisotropy of the initial slope of the upper critical field near $T_c$ cannot be explained by the anisotropy of the effective masses and Fermi velocities derived from quantum oscillations.