Nematic superconductivity in doped Bi2Se3 topological superconductors

Nematic superconductivity is a novel class of superconductivity characterized by spontaneous rotational-symmetry breaking in the superconducting gap amplitude and/or Cooper-pair spins with respect to the underlying lattice symmetry. Doped Bi2Se3 superconductors, such as CuxBi2Se3, SrxBi2Se3, and NbxBi2Se3, are considered as candidates for nematic superconductors, in addition to the anticipated topological superconductivity. Recently, various bulk probes, such as nuclear magnetic resonance, specific heat, magnetotransport, magnetic torque, and magnetization, have consistently revealed two-fold symmetric behavior in their in-plane magnetic-field-direction dependence, although the underlying crystal lattice possesses three-fold rotational symmetry. More recently, nematic superconductivity is directly visualized using scanning tunneling microscopy and spectroscopy. In this short review, we summarize the current researches on the nematic behavior in superconducting doped Bi2Se3 systems, and discuss issues and perspectives.Comment: 20 pages (incl. 5 pages of reference list), 4 figures; Submitted for the proceedings of Erice Workshop 2018 "Majorana Fermions and Topological Materials Science". Small revisions are made on 14th Dec. Comments are welcom

Non-linear Temperature Dependence of Resistivity in Single Crystalline Ag5_5Pb2_2O6_6

We measured electrical resistivity, specific heat and magnetic susceptibility of single crystals of highly conductive oxide Ag_5Pb_2O_6, which has a layered structure containing a Kagome lattice. Both the out-of-plane and in-plane resistivity show T^2 dependence in an unusually wide range of temperatures up to room temperature. This behavior cannot be accounted for either by electron correlation or by electron-phonon scattering with high frequency optic phonons. In addition, a phase transition with a large diamagnetic signal was found in the ac susceptibility, which strongly suggests the existence of a superconducting phase below 48 mK.Comment: 5 pages, 5 figures; accepted for publication in Physcal Review B; Revised version: small correction in the caption of Fig.

Transport properties of Ag5Pb2O6: a three-dimensional electron-gas-like system with low-carrier-density

We report normal-state transport properties of the single-crystalline samples of the silver-lead oxide superconductor Ag5Pb2O6, including the electrical resistivity, magnetoresistance, and Hall coefficient. From the Hall coefficient measurement, we confirmed that the carrier density of this oxide is as low as 5x10^{21} cm^{-3}, one order of magnitude smaller than those for ordinary alkali metals and noble metals. The magnetoresistance behavior is well characterized by the axial symmetry of the Fermi surface and by a single relaxation time. The T^2 term of the resistivity is scaled with the specific heat coefficient, based on the recent theory for the electron-electron scattering. The present results provide evidence that Ag5Pb2O6 is a low-carrier-density three-dimensional electron-gas-like system with enhanced electron-electron scatterings.Comment: 7 pages, 4 figures. Accepted for publication in Phys. Rev.

Novel superconducting phenomena in quasi-one-dimensional Bechgaard salts

It is the saturation of the transition temperature Tc in the range of 24 K for known materials in the late sixties which triggered the search for additional materials offering new coupling mechanisms leading in turn to higher Tc's. As a result of this stimulation, superconductivity in organic matter was discovered in tetramethyl-tetraselenafulvalene-hexafluorophosphate, (TMTSF)2PF6, in 1979, in the laboratory founded at Orsay by Professor Friedel and his colleagues in 1962. Although this conductor is a prototype example for low-dimensional physics, we mostly focus in this article on the superconducting phase of the ambient-pressure superconductor (TMTSF)2ClO4, in which the superconducting phase has been studied most intensively among the TMTSF salts. We shall present a series of experimental results supporting nodal d-wave symmetry for the superconducting gap in these prototypical quasi-one-dimensional conductors.Comment: Review article with 35 pages and 19 figures. Title, text, figures, and references are modified. To be published in Compte Rendu de Physique. Comments are welcom

First-Order Superconducting Transition of Sr2RuO4

By means of the magnetocaloric effect, we examine the nature of the superconducting-normal (S-N) transition of Sr2RuO4, a most promising candidate for a spin-triplet superconductor. We provide thermodynamic evidence that the S-N transition of this oxide is of first order below approximately 0.8 K and only for magnetic field directions very close to the conducting plane, in clear contrast to the ordinary type-II superconductors exhibiting second-order S-N transitions. The entropy release across the transition at 0.2 K is 10% of the normal-state entropy. Our result urges an introduction of a new mechanism to break superconductivity by magnetic field.Comment: 13 pages (Main text: 5 pages; Supplemental Material: 8 pages). To be published in Physical Review Letter

Compact AC Susceptometer for Fast Sample Characterization down to 0.1 K

We report a new design of an AC magnetic susceptometer compatible with the Physical Properties Measurement System (PPMS) by Quantum Design, as well as with its adiabatic demagnetization refrigerator option. With the elaborate compact design, the susceptometer allows simple and quick sample mounting process. The high performance of the susceptometer down to 0.1 K is demonstrated using several superconducting and magnetic materials. This susceptometer provides a method to quickly investigate qualities of a large number of samples in the wide temperature range between 0.1 and 300 K.Comment: 6 pages (4 pages + 2-page appendix), 10 figure