Intrinsic topological insulators are realized by alloying Bi(2)Te(3) with Bi(2)Se(3). Angle-resolved photoemission and bulk transport measurements reveal that the Fermi level is readily tuned into the bulk bandgap. First-principles calculations of the native defect landscape highlight the key role of anti-site defects for achieving this, and predict optimal growth conditions to realize maximally resistive topological insulators.

We have investigated the superconducting state of the noncentrosymmetric compound Re6Zr using magnetization, heat capacity, and muon-spin relaxation or rotation (μSR) measurements. Re6Zr has a superconducting transition temperature, Tc=6.75±0.05  K. Transverse-field μSR experiments, used to probe the superfluid density, suggest an s-wave character for the superconducting gap. However, zero and longitudinal-field μSR data reveal the presence of spontaneous static magnetic fields below Tc indicating that time-reversal symmetry is broken in the superconducting state and an unconventional pairing mechanism. An analysis of the pairing symmetries identifies the ground states compatible with time-reversal symmetry breaking.

Anomalous magnetic properties of the double perovskite ruthenate compound Sr 2 YRuO 6 are reported here. Magnetization measurements as a function of temperature in low magnetic fields show clear evidence for two components of magnetic order ͑T M1 ϳ 32 K and T M2 ϳ 27 K͒ aligned opposite to each other with respect to the magnetic-field direction even though only Ru 5+ moments can order magnetically in this compound. The second component of the magnetic order at T M2 ϳ 27 K results only in a magnetization reversal, and not in the negative magnetization when the magnetization is measured in the field-cooled ͑FC͒ mode. Isothermal magnetization ͑M-H͒ measurements show hysteresis with maximum coercivity ͑H c ͒ and remnant magnetization ͑M r ͒ at T ϳ 27 K, corroborating the presence of the two oppositely aligned magnetic moments, each with a ferromagnetic component. The two components of magnetic ordering are further confirmed by the double peak structure in the heat-capacity measurements. These anomalous properties have significance to some of the earlier results obtained for the Cu-substituted superconducting Sr 2 YRu 1−x Cu x O 6 compounds.

To trace the origin of time-reversal symmetry breaking (TRSB) in Re-based superconductors, we performed comparative muon-spin rotation/relaxation (µSR) studies of superconducting noncentrosymmetric Re 0.82 Nb 0.18 (T c = 8.8 K) and centrosymmetric Re (T c = 2.7 K). In Re 0.82 Nb 0.18 , the low-temperature superfluid density and the electronic specific heat evidence a fully-gapped superconducting state, whose enhanced gap magnitude and specific-heat discontinuity suggest a moderately strong electron-phonon coupling. In both Re 0.82 Nb 0.18 and pure Re, the spontaneous magnetic fields revealed by zero-field µSR below T c indicate time-reversal symmetry breaking and thus unconventional superconductivity. The concomitant occurrence of TRSB in centrosymmetric Re and noncentrosymmetric ReT (T = transition metal), yet its preservation in the isostructural noncentrosymmetric superconductors Mg 10 Ir 19 B 16 and Nb 0.5 Os 0.5 , strongly suggests that the local electronic structure of Re is crucial for understanding the TRSB superconducting state in Re and ReT . We discuss the superconducting order parameter symmetries that are compatible with the observations. Time reversal and spatial inversion are two key symmetries which influence at a fundamental level the electron pairing in the superconducting state: on the one hand, a number of unconventional superconductors exhibit spontaneous time-reversal symmetry breaking (TRSB) on entering the superconducting state; on the other hand, the absence of inversion symmetry above T c leads to an antisymmetric spin-orbit coupling (SOC), lifting the degeneracy of the conduction-band electrons and potentially giving rise to a mixed-parity superconducting state [1,2]. Some noncentrosymmetric superconductors (NCSC), such as CePt 3 Si [3], CeIrSi 3 [4], Li 2 Pt 3 B [5, 6], and K 2 Cr 3 As 3 [7, 8], exhibit line nodes in the gap, while others such as LaNiC 2 [9] and (La,Y) 2 C 3 [10], show multiple nodeless superconducting gaps. In addition, due to the strong influence of SOC, their upper critical field can greatly exceed the Pauli limit, as has been found in CePt 3 Si [11] and very recently in (Ta,Nb)Rh 2 B 2 [12]. In general, TRSB below T c and a lack of spatial-inversion symmetry of the crystal structure are independent events. Yet, in a few cases, such as in LaNiC 2 [13], La 7 Ir 3 [14], and, in particular, in the Re-based compounds Re 6 Zr [15], Re 6 Hf [16], Re 6 Ti [17], and Re 24 Ti 5 [18], TRSB below T c is concomitant with an existing lack of crystal inversion symmetry. Such an unusually frequent occurrence of TRSB among the superconducting ReT binary alloys (T = transition metal) is rather puzzling. Its persistence independent of the particular transition metal, points to a key role played by Re. To test such a hypothesis, and to ascertain the possible relevance of the noncentrosymmetric structure to TRSB in Re-based NCSC, we proceeded with a twofold study. On one hand we synthesized and investigated an-other Re-based NCSC, Re 0.82 Nb 0.18 . On the other hand, we considered the ...

Copies of full items can be used for personal research or study, educational, or not-for-profit purposes without prior permission or charge. Provided that the authors, title and full bibliographic details are credited, a hyperlink and/or URL is given for the original metadata page and the content is not changed in any way. The superconductivity of the noncentrosymmetric compound La7Ir3 has been investigated using muon spin rotation and relaxation (µSR). Zero-field measurements reveal the presence of spontaneous static or quasi-static magnetic fields below the superconducting transition temperature Tc = 2.25 K -a clear indication that the superconducting state breaks time-reversal symmetry. Furthermore, transverse-field rotation measurements suggest that the superconducting gap is isotropic, and that the pairing symmetry of the superconducting electrons is predominantly s-wave with an enhanced binding strength. The results indicate that the superconductivity in La7Ir3 may be unconventional, and paves the way for further studies of this family of materials. Publisher

The discovery of new families of unconventional superconductors is important both experimentally and theoretically, especially if it challenges current models and thinking. By using muon spin relaxation in zero-field, time-reversal symmetry breaking has been observed in Re6Hf. Moreover, the temperature dependence of the superfluid density exhibits s-wave superconductivity with an enhanced electron-phonon coupling. This, coupled with the results from isostructural Re6Zr, shows that the Re6X family are indeed a new and important group of unconventional superconductors.

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The noncentrosymmetric superconductor AuBe have been investigated using the magnetization, resistivity, specific heat, and muon-spin relaxation/rotation measurements. AuBe crystallizes in the cubic FeSi-type B20 structure with superconducting transition temperature observed at Tc = 3.2 ± 0.1 K. The low-temperature specific heat data, C el (T), indicate a weakly-coupled fully gapped BCS superconductivity with an isotropic energy gap 2∆(0)/kBTc = 3.76, which is close to the BCS value of 3.52. Interestingly, type-I superconductivity is inferred from the µSR measurements, which is in contrast with the earlier reports of type-II superconductivity in AuBe. The Ginzburg-Landau parameter is κGL = 0.4 < 1/ √ 2. The transverse-field µSR data transformed in the maximum entropy spectra depicting the internal magnetic field probability distribution, P(H), also confirms the absence of the mixed state in AuBe. The thermodynamic critical field, Hc, calculated to be around 259 Oe. The zero-field µSR results indicate that time-reversal symmetry is preserved and supports a spin-singlet pairing in the superconducting ground state. arXiv:1901.06492v1 [cond-mat.supr-con]