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UTe$$_2$$; A Nearly insulating half-filled $$j = frac{5}{2}$$ 5$$f^3$$ heavy-fermion metal

Shick, A. B.*; Fujimori, Shinichi  ; Pickett, W. E.*

Correlated band theory implemented as a combination of density functional theory with exact diagonalization [DFT+$$U$$(ED)] of the Anderson impurity term with Coulomb repulsion U in the open 14-orbital 5f shell is applied to UTe$$_2$$. The small gap for $$U$$ =0, evidence of the half-filled $$j = frac{5}{2}$$ subshell of 5$$f^3$$ uranium, is converted for $$U$$ = 3 eV to a flat band semimetal with small heavy-carrier Fermi surfaces that will make properties sensitive to pressure, magnetic field, and off stoichiometry, as observed experimentally. Two means of identification from the Green's function give a mass enhancement of the order of 12 for already heavy (flat) bands, consistent with the common heavy-fermion characterization of UTe$$_2$$. The predicted Kondo temperature around 100 K matches the experimental values from resistivity. The electric field gradients for the two Te sites are calculated by DFT+$$U$$(ED) to differ by a factor of seven, indicating a strong site distinction, while the anisotropy factor $$eta$$ = 0.18 is similar for all three sites. The calculated uranium moment $$< M^2 >^{1/2}$$ of 3.5 $$mu_B$$ is roughly consistent with the published experimental Curie-Weiss values of 2.8 $$mu_B$$ and 3.3 $$mu_B$$, and the calculated separate spin and orbital moments are remarkably similar to Hund's rule values for an $$f^3$$ ion. The $$U$$ =3 eV spectral density is compared with angle-integrated and angle-resolved photoemission spectra, with agreement that there is strong 5f character at, and for several hundred meV below, the Fermi energy. Our results support the picture that the underlying ground state of UTe$$_2$$ is that of a half-filled $$j = frac{5}{2}$$ subshell with two half-filled $$m_j = pm frac{1}{2}$$ orbitals forming a narrow gap by hybridization, then driven to a conducting state by configuration mixing (spin-charge fluctuations). UTe$$_2$$ displays similarities to UPt$$_3$$ with its 5f dominated Fermi surfaces rather than a strongly localized Kondo lattice system.

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Category:Materials Science, Multidisciplinary

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