Comparison of Two Different Synthesis Methods of Single Crystals of Superconducting Uranium Ditelluride

Single crystal specimens of the actinide compound uranium ditelluride, UTe2, are of great importance to the study and characterization of its dramatic ...

University of Maryland

National Institute of Standards and Technology

Spin-triplet superconductors potentially host topological excitations that are of interest for quantum information processing. We report the discovery of spin-triplet superconductivity in UTe, featuring a transition temperature of 1.6 kelvin and a very large and anisotropic upper critical field exceeding 40 teslas. This superconducting phase stability suggests that UTe is related to ferromagnetic superconductors such as UGe, URhGe, and UCoGe. However, the lack of magnetic order and the observation of quantum critical scaling place UTe at the paramagnetic end of this ferromagnetic superconductor series. A large intrinsic zero-temperature reservoir of ungapped fermions indicates a highly unconventional type of superconducting pairing.

Nearly ferromagnetic spin-triplet superconductivity.

Chiral superconductors have been proposed as one pathway to realize Majorana normal fluid at its boundary. However, the long-sought 2D and 3D chiral superconductors with edge and surface Majorana normal fluid are yet to be conclusively found. Here, we report evidence for a chiral spin-triplet pairing state of UTe with surface normal fluid response. The microwave surface impedance of the UTe crystal was measured and converted to complex conductivity, which is sensitive to both normal and superfluid responses. The anomalous residual normal fluid conductivity supports the presence of a significant normal fluid response. The superfluid conductivity follows the temperature behavior predicted for an axial spin-triplet state, which is further narrowed down to a chiral spin-triplet state with evidence of broken time-reversal symmetry. Further analysis excludes trivial origins for the observed normal fluid response. Our findings suggest that UTe can be a new platform to study exotic topological excitations in higher dimension.

Anomalous normal fluid response in a chiral superconductor UTe.

Applied magnetic fields underlie exotic quantum states, such as the fractional quantum Hall effect and Bose-Einstein condensation of spin excitations. Superconductivity, however, is inherently antagonistic towards magnetic fields. Only in rare cases can these effects be mitigated over limited fields, leading to re-entrant superconductivity. Here, we report the coexistence of multiple high-field re-entrant superconducting phases in the spin-triplet superconductor UTe (ref. ). We observe superconductivity in the highest magnetic field range identified for any re-entrant superconductor, beyond 65 T. Although the stability of superconductivity in these high magnetic fields challenges current theoretical models, these extreme properties seem to reflect a new kind of exotic superconductivity rooted in magnetic fluctuations and boosted by a quantum dimensional crossover.

Extreme magnetic field-boosted superconductivity.

Spin triplet superconductivity in the Kondo lattice UTe appears to be associated with spin fluctuations originating from incipient ferromagnetic order. Here we show clear evidence of twofold enhancement of superconductivity under pressure, which discontinuously transitions to magnetic order, likely of ferromagnetic nature, at higher pressures. The application of a magnetic field tunes the system back across a first-order phase boundary. Straddling this phase boundary, we find another example of reentrant superconductivity in UTe. As the superconductivity and magnetism exist on two opposite sides of the first-order phase boundary, our results indicate other microscopic mechanisms may be playing a role in stabilizing spin triplet superconductivity in addition to spin fluctuations associated with magnetism.

Enhancement and reentrance of spin triplet superconductivity in UTe under pressure.

Low-temperature electrical and thermal transport, magnetic penetration depth, and heat capacity measurements were performed on single crystals of the actinide superconductor UTe to determine the structure of the superconducting energy gap. Heat transport measurements performed with currents directed along both crystallographic  and  axes reveal a vanishingly small residual fermionic component of the thermal conductivity. The magnetic field dependence of the residual term follows a rapid, quasilinear increase consistent with the presence of nodal quasiparticles, rising toward the -axis upper critical field where the Wiedemann-Franz law is recovered. Together with a quadratic temperature dependence of the magnetic penetration depth up to /  = 0.3, these measurements provide evidence for an unconventional spin-triplet superconducting order parameter with point nodes. Millikelvin specific heat measurements performed on the same crystals used for thermal transport reveal an upturn below 300 mK that is well described by a divergent quantum-critical contribution to the density of states (DOS). Modeling this contribution with a   power law allows restoration of the full entropy balance in the superconducting state and a resultant cubic power law for the electronic DOS below   , consistent with the point-node gap structure determined by thermal conductivity and penetration depth measurements.

I-Lin Liu

Maryland Quantum Materials Center,
Department of Physics,
Maryland Quantum Materials Center, Department of Physics

University of Maryland

Comparison of Two Different Synthesis Methods of Single Crystals of Superconducting Uranium Ditelluride

I-Lin Liu

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University of Maryland

Comparison of Two Different Synthesis Methods of Single Crystals of Superconducting Uranium Ditelluride

Maryland Quantum Materials Center,
Department of Physics,
Maryland Quantum Materials Center, Department of Physics