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

TL;DR

This study investigates how magnetic fluctuations and their anisotropy in UTe2 evolve with pressure, using a microscopic framework that combines DFT+U and RPA on a 72-orbital model. By comparing two representative U values, it shows that a quasi-two-dimensional Fermi surface (U = 2 eV) yields fluctuations along the a* direction that are suppressed under pressure, while a more three-dimensional Fermi surface (U = 1 eV) produces antiferromagnetic fluctuations along the b* axis that are enhanced with pressure and align better with experiments. The results highlight the crucial role of the momentum-space distribution of U-5f states and the DOS at EF in governing magnetism and its link to superconductivity, offering a microscopic basis for interpreting pressure-tuned magnetism in UTe2 and guiding future beyond-RPA treatments. The work suggests that incorporating dynamical correlations via DFT+DMFT and exploring superconducting pairing interactions could clarify the symmetry of the superconducting state and the potential for odd-parity pairing under pressure.

Abstract

Pressure-induced changes in the magnetic and superconducting properties of a spin-triplet superconductor candidate UTe2 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 eV, the Fermi surfaces have a quasi-two-dimensional shape, antiferromagnetic fluctuations develop with the wave vector along the 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 eV, where Fermi surfaces are more three-dimensional, antiferromagnetic fluctuations with Q2 = 0.22 b* appear, accompanied by anisotropy $χ^a > χ^c > χ^b$, consistent with experiments. Under pressure, antiferromagnetic fluctuations around Q2 are enhanced, the magnetic wave vector tilts slightly toward the a* direction due to Fermi-surface distortion, and the magnetic anisotropy is suppressed. These results demonstrate that the pressure evolution of magnetism in UTe2 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 UTe2.

Figures (16)

• Figure 1: Fermi surface for $U = 2~\mathrm{eV}$. (a) Three-dimensional view at ambient pressure ($P = 0~\mathrm{GPa}$). Yellow (purple) color denotes large (small) weight of U $5f$ orbitals. (b)--(d) Cross-sectional cuts at $P = 0$ and $4~\mathrm{GPa}$ taken at (b) $k_z = 0$, (c) $k_y = 0$, and (d) $k_x = 0$.
• Figure 2: Magnetic susceptibilities $\chi^a(\bm{q})$ (top), $\chi^b(\bm{q})$ (middle), and $\chi^c(\bm{q})$ (bottom) at ambient pressure (left) and high pressure $P=4\mathrm{\;GPa}$ (right) for $U = 2\mathrm{\;eV}$. Calculations are performed by the RPA for $(\tilde{U}, \tilde{V}, \tilde{J}, \tilde{J}') = (100, 40, 20, 20)\mathrm{\;meV}$ and $T = 50\mathrm{\;K}$.
• Figure 3: Pressure dependence of static uniform susceptibilities, $\chi^a(\bm{0})$ (red), $\chi^b(\bm{0})$ (blue), and $\chi^c(\bm{0})$ (green) for $U = 2\mathrm{\;eV}$. The anisotropy ratio (AR) is also shown (yellow).
• Figure 4: The spectral weight of U $5f$ states projected to $j=5/2$ space for $U = 2\mathrm{\;eV}$, which is shown in the $k_z = 0$ plane for (a) $P = 0$ and (b) $P = 4\mathrm{\;GPa}$. Circles and triangles denote the $\Sigma_0$ and $X$ points, respectively. The white arrow illustrates the nesting vector $\bm{Q}$.
• Figure 5: Fermi surface for $U = 1~\mathrm{eV}$. (a) Three-dimensional view at ambient pressure ($P = 0~\mathrm{GPa}$). Yellow (purple) color denotes large (small) weight of U $5f$ orbitals. (b)--(d) Cross-sections at $P = 0$ and $4~\mathrm{GPa}$ for (b) $k_z=0$, (c) $k_y=0$, and (d) $k_x=0$.