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Register a new userAnomalous c-axis Transport Response of UTe$_{2}$
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We study the temperature dependence of electrical resistivity for currents directed along all crystallographic axes of the spin-triplet superconductor UTe$_{2}$. We focus particularly on an accurate determination of the resistivity along the $c$-axis ($rho_c$) by using transport geometries that allow extraction of two resistivities along with the primary axes directions. Measurement of the absolute values of resistivities in all current directions reveals a surprisingly (given the anticipated highly anisotropic bandstructure) nearly isotropic transport behavior at temperatures above Kondo coherence, with $rho_c sim rho_b sim 2rho_a$, but with a qualitatively distinct behavior at lower temperatures. The temperature dependence of $rho_c$ exhibits a Kondo-like maximum at much lower temperatures compared to that of $rho_a$ and $rho_b$, providing important insight into the underlying electronic structure necessary for building a microscopic model of UTe$_{2}$.
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We present the results of out-of-plane electrical transport measurements on the heavy fermion superconductor CeCoIn$_{5}$ at temperatures from 40 mK to 400 K and in magnetic field up to 9 T. For $T <$ 10 K transport measurements show that the zero-field resistivity $rho_{c}$ changes linearly with temperature and extrapolates nearly to zero at 0 K, indicative of non-Fermi-liquid (nFL) behavior associated with a quantum critical point (QCP). The longitudinal magnetoresistance (LMR) of CeCoIn$_{5}$ for fields applied parallel to the c-axis is negative and scales as $B/(T+T^{*})$ between 50 and 100 K, revealing the presence of a single-impurity Kondo energy scale $T^{*} sim 2$ K. Beginning at 16 K a small positive LMR feature is evident for fields less than 3 tesla that grows in magnitude with decreasing temperature. For higher fields the LMR is negative and increases in magnitude with decreasing temperature. This sizable negative magnetoresistance scales as $B{^2}/T$ from 2.6 K to roughly 8 K, and it arises from an extrapolated residual resistivity that becomes negative and grows quadratically with field in the nFL temperature regime. Applying a magnetic field along the c-axis with B $>$ B$_{c2}$ restores Fermi-liquid behavior in $rho_{c}(T)$ at $T$ less than 130 mK. Analysis of the $T{^2}$ resistivity coefficients field-dependence suggests that the QCP in CeCoIn$_{5}$ is located emph{below} the upper critical field, inside the superconducting phase. These data indicate that while high-$T$ c-axis transport of CeCoIn$_{5}$ exhibits features typical for a heavy fermion system, low-$T$ transport is governed both by spin fluctuations associated with the QCP and Kondo interactions that are influenced by the underlying complex electronic structure intrinsic to the anisotropic CeCoIn$_{5}$ crystal structure.
We investigate the spin dynamics in the superconducting phase of UTe$_{2}$ by triple-axis inelastic neutron scattering on a single crystal sample. At the wave-vector $bf{k_1}$=(0, 0.57, 0), where the normal state antiferromagnetic correlations are peaked, a modification of the excitation spectrum is evidenced, on crossing the superconducting transition, with a reduction of the relaxation rate together with the development of an inelastic peak at $Omega$ $approx$ 1 meV. The low dimensional nature and the the $a$-axis polarization of the fluctuations, that characterise the normal state, are essentially maintained below $T_{sc}$. The high ratio $Omega/k_{B}T_{sc}$ $approx$ 7.2 contrasts with the most common behaviour in heavy fermion superconductors.
We present magnetoresistivity measurements on the heavy-fermion superconductor UTe$_{2}$ in pulsed magnetic fields $mu_0H$ up to 68~T and temperatures $T$ from 1.4 to 80~K. Magnetic fields applied along the three crystallographic directions $mathbf{a}$ (easy magnetic axis), $mathbf{b}$, and $mathbf{c}$ (hard magnetic axes), are found to induce different phenomena - depending on the field direction - beyond the low-field suppression of the superconducting state. For $mathbf{H}parallelmathbf{a}$, a broad anomaly in the resistivity is observed at $mu_0H^*simeq10$~T and $T = 1.4$~K. For $mathbf{H}parallelmathbf{c}$, no magnetic transition nor crossover are observed. For $mathbf{H}parallelmathbf{b}$, a sharp first-order-like step in the resistivity indicates a metamagnetic transition at the field $mu_0H_m simeq 35$~T. When the temperature is raised signature of first-order metamagnetism is observed up to a critical endpoint at $T_{CEP}simeq7$~K. At higher temperatures a crossover persists up to 28~K, i.e., below the temperature $T_chi^{max} = 35$~K where the magnetic susceptibility is maximal. A sharp maximum in the Fermi-liquid quadratic coefficient $A$ of the low-temperature resistivity is found at $H_m$. It indicates an enhanced effective mass associated with critical magnetic fluctuations, possibly coupled with a Fermi surface instability. Similarly to the URhGe case, we show that UTe$_{2}$ is a candidate for field-induced reentrant superconductivity in the proximity of $H_m$.
We examine static spin susceptibilities $chi_{alphabeta}({bf q})$ of spin components $S_{alpha}$ and $S_{beta}$ in the non-centrosymmetric tetragonal system. These show anomalous momentum dependences like $chi_{xx}({bf q})-chi_{yy}({bf q})sim q_x^2-q_y^2$ and $chi_{xy}({bf q})+chi_{yx}({bf q})sim q_x q_y$, which vanish in centrosymmetric systems. The magnitudes of the anomalous spin susceptibilities are enhanced by the on-site Coulomb interaction, especially, around an ordering wave vector. The significant and anomalous momentum dependences of these susceptibilities are explained by a group theoretical analysis. As the direct probe of the anomalous spin susceptibility, we propose a polarized neutron scattering experiment.
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 $5f^3$ 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. 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$mu_B$ (which are field-direction dependent), and the calculated separate spin and orbital moments are remarkably similar to Hunds 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 $m_j=pmfrac{1}{2}$ 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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