Specific heat of aluminium-doped superconducting silicon carbide

The discoveries of superconductivity in heavily boron-doped diamond, silicon and silicon carbide renewed the interest in the ground states of charge-carrier doped wide-gap semiconductors. Recently, aluminium doping in silicon carbide successfully yielded a metallic phase from which at high aluminium concentrations superconductivity emerges. Here, we present a specific-heat study on superconducting aluminium-doped silicon carbide. We observe a clear jump anomaly at the superconducting transition temperature 1.5 K indicating that aluminium-doped silicon carbide is a bulk superconductor. An analysis of the jump anomaly suggests BCS-like phonon-mediated superconductivity in this system.Comment: 4 pages, 2 figure

Evidence for Multiple Phase Transitions in La_1-xCa_xCoO_3

We report thermal-expansion and specific-heat data of the series La_1-xCa_xCoO_3 for 0 <= x <= 0.3. For x = 0 the thermal-expansion coefficient alpha(T) features a pronounced maximum around T = 50 K caused by a temperature-dependent spin-state transition from a low-spin state (S=0) at low temperatures towards a higher spin state of the Co^3+ ions. The partial substitution of the La^3+ ions by divalent Ca^2+ ions causes drastic changes in the macroscopic properties of LaCoO_3. Around x ~ 0.125 the large maximum in alpha(T) has completely vanished. With further increasing x three different anomalies develop

Bulk superconducting phase with a full energy gap in the doped topological insulator Cu_xBi_2Se_3

The superconductivity recently found in the doped topological insulator Cu_xBi_2Se_3 offers a great opportunity to search for a topological superconductor. We have successfully prepared a single-crystal sample with a large shielding fraction and measured the specific-heat anomaly associated with the superconductivity. The temperature dependence of the specific heat suggests a fully-gapped, strong-coupling superconducting state, but the BCS theory is not in full agreement with the data, which hints at a possible unconventional pairing in Cu_xBi_2Se_3. Also, the evaluated effective mass of 2.6m_e (m_e is the free electron mass) points to a large mass enhancement in this material.Comment: 4 pages, 3 figure

Superconductivity in heavily boron-doped silicon carbide

The discoveries of superconductivity in heavily boron-doped diamond (C:B) in 2004 and silicon (Si:B) in 2006 renew the interest in the superconducting state of semiconductors. Charge-carrier doping of wide-gap semiconductors leads to a metallic phase from which upon further doping superconductivity can emerge. Recently, we discovered superconductivity in a closely related system: heavily-boron doped silicon carbide (SiC:B). The sample used for that study consists of cubic and hexagonal SiC phase fractions and hence this lead to the question which of them participates in the superconductivity. Here we focus on a sample which mainly consists of hexagonal SiC without any indication for the cubic modification by means of x-ray diffraction, resistivity, and ac susceptibility.Comment: 9 pages, 5 figure

Direct Measurement of the Out-of-Plane Spin Texture in the Dirac Cone Surface State of a Topological Insulator

We have performed spin- and angle-resolved photoemission spectroscopy of Bi2Te3 and present the first direct evidence for the existence of the out-of-plane spin component on the surface state of a topological insulator. We found that the magnitude of the out-of-plane spin polarization on a hexagonally deformed Fermi surface (FS) of Bi2Te3 reaches maximally 25% of the in-plane counterpart while such a sizable out-of-plane spin component does not exist in the more circular FS of TlBiSe2, indicating that the hexagonal deformation of the FS is responsible for the deviation from the ideal helical spin texture. The observed out-of-plane polarization is much smaller than that expected from existing theory, suggesting that an additional ingredient is necessary for correctly understanding the surface spin polarization in Bi2Te3.Comment: 4 pages, 3 figure