Electronic structure of UTe$_2$ under pressure

TL;DR

This work provides a detailed first-principles assessment of how UTe$_2$'s electronic structure evolves under hydrostatic and uniaxial pressures, focusing on the role of itinerant U 5f electrons via GGA+$U$. While most low-energy bands and Fermi-surface features show robustness to pressure for a broad range of $U$, a notable sensitivity emerges near $U=1.0$ eV, where pressure enhances $N(E_F)$ through band flattening, potentially affecting magnetic tendencies and superconducting instability. The study also reveals anisotropic responses under uniaxial stress, with direction-dependent DOS changes and Lifshitz transitions that depend on both pressure and the chosen $U$, suggesting possible routes for strain engineering of superconductivity and magnetic order. By comparing relaxed versus fixed-geometry calculations, the authors highlight the critical influence of atomic positions on FS topology and possible topological transitions in the superconducting state. Overall, the work connects microscopic electronic structures under pressure to the macroscopic phase diagram, offering a framework to interpret and guide experiments probing pressure- and strain-induced phenomena in UTe$_2$.

Abstract

A heavy-fermion paramagnet UTe$_2$ has been a strong candidate for a spin-triplet superconductor. Experiments on UTe$_2$ under pressure have been vigorously conducted, and rich phase diagrams have been suggested. Multiple superconducting phases exist in the pressure region of $0 \leq P < 1.8 \mathrm{\;GPa}$, and an antiferromagnetic ordered state is observed in the high pressure region $P > 1.8 \mathrm{\;GPa}$. However, under pressure, the underlying electronic structure in the normal state has not been clarified, although knowledge of electronic structures is essential for studying magnetic and superconducting states. As an indispensable step toward understanding the phase diagram of UTe$_2$, we study the electronic structure under hydrostatic and uniaxial stresses based on the density functional theory with and without employing structural optimization. It is shown that the low-energy band structure and Fermi surfaces are not sensitive to pressure for parameters where itinerant $f$-electrons are not essential. However, we find a significant pressure dependence for a certain Coulomb interaction $U$ of the GGA+$U$ calculation, where the large weight of $f$-electrons appears at the Fermi level. An increase in the density of states at the Fermi level is observed under pressure, which is attributed to compressive stress along the [010] crystallographic axis.

Figures (15)

• Figure 1: (Color online) (a) First Brillouin zone and $k$-path of UTe2. Fermi surfaces calculated by the GGA+$U$ method with (b) $U = 0.8\mathrm{\; eV}$, (c) $U = 1.1\mathrm{\; eV}$, and (d) $U = 2.0\mathrm{\; eV}$.
• Figure 2: (Color online) Pressure dependence of fractional coordinates, $z$ of U $4i$, $y$ of Te $4h$, and $z$ of Te $4j$ Wyckoff positions under hydrostatic pressure. The ionic relaxation is performed for various $U$ with using the interplated lattice constants in Fig. \ref{['fig:hydrostaticpressure_latticeconstants']}.
• Figure 3: (Color online) Band structures (left) and DOS (right) under hydrostatic pressure obtained by the relativistic GGA+$U$ method with (a, b) $U = 0$, (c, d) $U = 1.0\mathrm{\; eV}$, (e, f) $U = 1.25\mathrm{\; eV}$, and (g, h) $U = 2.0\mathrm{\; eV}$.
• Figure 4: (Color online) Side views of the electron sheet from the $k_x$ direction (left panels) and side views of the hole sheet from the $k_y$ direction (right panels) at $P=0$ (top panels) and $P=4\mathrm{\; GPa}$ (bottom panels) obtained by GGA+$U$ calculations for $U = 1.0\mathrm{\; eV}$.
• Figure 5: (Color online) Fractional coordinates $z$ of U $4i$, $y$ of Te $4h$, and $z$ of Te $4j$ Wyckoff positions under uniaxial stress, $\sigma_{100}$ (red), $\sigma_{010}$ (cyan), and $\sigma_{001}$ (green). These results are obtained by the GGA+$U$ method with $U=1.0\mathrm{\; eV}$.