Magnetic fluctuations and anisotropy in UTe2: a multi-orbital study based on GGA+U and RPA

Abstract

Pressure-induced changes in the magnetic and superconducting properties of a spin-triplet superconductor candidate UTe$_2$ have attracted considerable interest, underscoring the need for microscopic theoretical insight. In this paper, we investigate magnetic fluctuations and their anisotropy at ambient pressure and under pressure using density functional theory (DFT) combined with the random phase approximation (RPA). For each pressure, we perform DFT+$U$ calculations for several values of the Coulomb interaction $U$, construct a 72-orbital periodic Anderson model, and calculate magnetic susceptibilities with use of the RPA. For $U = 2\mathrm{\;eV}$, the Fermi surface has a quasi-two-dimensional shape, antiferromagnetic fluctuations develop with the wave vector along the $\boldsymbol{a}^*$ axis, and the magnetic anisotropy follows $χ^b > χ^a > χ^c$. The antiferromagnetic fluctuations are suppressed under pressure because of a reduced density of states at the Fermi level, while the magnetic anisotropy is weakened. In contrast, for $U = 1\mathrm{\;eV}$, where the Fermi surface is more three-dimensional, antiferromagnetic fluctuations with $\boldsymbol{Q}_2 = 0.22\,\boldsymbol{b}^*$ appear, accompanied by anisotropy $χ^a > χ^c > χ^b$, consistent with experiments. Under pressure, antiferromagnetic fluctuations around $\boldsymbol{Q}_2$ are enhanced, the magnetic wave vector tilts slightly toward the $\boldsymbol{a}^*$ direction due to Fermi-surface distortion, and the magnetic anisotropy is suppressed. These results demonstrate that the pressure evolution of magnetism in UTe$_2$ is governed by the momentum-space distribution of U $5f$ states and the density of states at the Fermi level, providing a microscopic basis for understanding the magnetic and superconducting properties of UTe$_2$.