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We report the first observation of the de Haas-van Alphen (dHvA) effect in the novel spin-triplet superconductor UTe2 using high quality single crystals with a high residual resistivity ratio (RRR) over 200. The dHvA frequencies, named α and β, are detected for the field directions between c- and a-axes. The frequency of branch β increases rapidly with the field angle tilted from c- to a-axis, while branch α splits, owing to the maximal and minimal cross-sectional areas from the same Fermi surface. Both dHvA branches, α and β reveal two kinds of cylindrical Fermi surfaces with a strong corrugation at least for branch α. The angular dependence of the dHvA frequencies is in very good agreement with that calculated by the generalized gradient approximation (GGA) method taking into account the on-site Coulomb repulsion of U = 2 eV. It indicates that the main Fermi surfaces are experimentally detected. The observed cyclotron effective masses are large in the range from 32 to 57m0. They are approximately 10-20 times lager than the corresponding band masses, consistent with the mass enhancement obtained from the Sommerfeld coefficient, γ and the calculated density of states at the Fermi level. The local density approximation (LDA) calculations of ThTe2 assuming U4+ with the 5f² localized model are in less agreement with our experimental results, in spite of the prediction for two cylindrical Fermi surfaces, suggesting a mixed valence states of U4+ and U3+ in UTe2. [ABSTRACT FROM AUTHOR]

Quantum spin liquid (QSL) is an exotic quantum phase of matter whose ground state is quantum-mechanically entangled without any magnetic ordering. A central issue concerns emergent excitations that characterize QSLs, which are hypothetically associated with quasiparticle fractionalization and topological order. Here we report highly unusual heat conduction generated by the spin degrees of freedom in a QSL state of the pyrochlore magnet Pr2Zr2O7, which hosts spin-ice correlations with strong quantum fluctuations. The thermal conductivity in high temperature regime exhibits a two-gap behavior, which is consistent with the gapped excitations of magnetic (M-) and electric monopoles (E-particles). At very low temperatures below 200 mK, the thermal conductivity unexpectedly shows a dramatic enhancement, which well exceeds purely phononic conductivity, demonstrating the presence of highly mobile spin excitations. This new type of excitations can be attributed to emergent photons (ν-particle), coherent gapless spin excitations in a spin-ice manifold. [ABSTRACT FROM AUTHOR]

The emergence of the nematic electronic state that breaks rotational symmetry is one of the most fascinating properties of the iron-based superconductors, and has relevance to cuprates as well. FeSe has a unique ground state in which superconductivity coexists with a nematic order without long-range magnetic ordering, providing a significant opportunity to investigate the role of nematicity in the superconducting pairing interaction. Here, to reveal how the superconducting gap evolves with nematicity, we measure the thermal conductivity and specific heat of FeSe1-xSx, in which the nematicity is suppressed by isoelectronic sulfur substitution and a nematic critical point (NCP) appears at xc≈ 0:17. We find that, in the whole nematic regime (0⩽x ⩽0:17), the field dependence of two quantities consistently shows two-gap behavior; one gap is small but highly anisotropic with deep minima or line nodes, and the other is larger and more isotropic. In stark contrast, in the tetragonal regime (x = 0:20), the larger gap becomes strongly anisotropic, demonstrating an abrupt change in the superconducting gap structure at the NCP. Near the NCP, charge fluctuations of dxz and dyz orbitals are enhanced equally in the tetragonal side, whereas they develop differently in the orthorhombic side. Our observation therefore directly implies that the orbital-dependent nature of the nematic fluctuations has a strong impact on the superconducting gap structure and hence on the pairing interaction. [ABSTRACT FROM AUTHOR]

To study the superconducting gap structure of BiS2-based layered compound NdO0.71F0.29BiS2 (Tc = 5 K), we measured the thermal conductivity κ, which is a sensitive probe of low-energy quasiparticle spectrum. In the absence of a magnetic field, residual linear term in the thermal conductivity κ0=T at T → 0 is vanishingly small, indicating that the residual normal fluid, which is expected for nodal superconductors, is absent. Moreover, the applied magnetic field hardly affects thermal conductivity in wide range of the vortex state, indicating the absence of Doppler shifted quasiparticles. These results provide evidence that NdO0.71F0.29BiS2 is a fully gapped superconductor. The obtained gap structure, along with the robustness of the superconductivity against the impurity, suggest a conventional s-wave superconducting state in NdO0.71F0.29BiS2. [ABSTRACT FROM AUTHOR]