Evidence of s-wave superconductivity in the noncentrosymmetric La7Ir3.

Superconductivity in noncentrosymmetric compounds has attracted sustained interest in the last decades. Here we present a detailed study on the transport, thermodynamic properties and the band structure of the noncentrosymmetric superconductor La 7 Ir 3 (T c ~ 2.3 K) that was recently proposed to break the time-reversal symmetry. It is found that La7Ir3 displays a moderately large electronic heat capacity (Sommerfeld coefficient γ n ~ 53.1 mJ/mol K2) and a significantly enhanced Kadowaki-Woods ratio (KWR ~32 µΩ cm mol2 K2 J-2) that is greater than the typical value (~10 µΩ cm mol2 K2 J-2) for strongly correlated electron systems. The upper critical field Hc2 was seen to be nicely described by the single-band Werthamer-Helfand-Hohenberg model down to very low temperatures. The hydrostatic pressure effects on the superconductivity were also investigated. The heat capacity below T c reveals a dominant s-wave gap with the magnitude close to the BCS value. The first-principles calculations yield the electron-phonon coupling constant λ = 0.81 and the logarithmically averaged frequency ω ln = 78.5 K, resulting in a theoretical T c = 2.5 K, close to the experimental value. Our calculations suggest that the enhanced electronic heat capacity is more likely due to electron-phonon coupling, rather than the electron-electron correlation effects. Collectively, these results place severe constraints on any theory of exotic superconductivity in this system.

Raman scattering investigation of the quasi-one-dimensional superconductor Ta4Pd3Te16.

We have performed polarized Raman scattering measurements on the newly discovered superconductor Ta4Pd3Te16 (T(c) = 4.6 K). We observe 28 out of 33 Raman active modes, with frequencies in good accordance with first-principles calculations. Although most of the phonons observed vary only slightly with temperature and do not exhibit any asymmetric profile that would suggest strong electron-phonon coupling, the linewidth of the A(g) phonon mode at 89.9 cm(-1) shows an unconventional increase with temperature decreasing, which is possibly due to a charge-density-wave transition or the emergence of charge-density-wave fluctuations below a temperature estimated to fall in the 140-200 K range.

Quasi-linear magnetoresistance and the violation of Kohler's rule in the quasi-one-dimensional Ta4Pd3Te16 superconductor.

We report on the quasi-linear in field intrachain magnetoresistance in the normal state of a quasi-one-dimensional superconductor Ta4Pd3Te16 (Tc ~ 4.6 K). Both the longitudinal and transverse in-chain magnetoresistance shows a power-law dependence, Δρ∝B(α) with the exponent α close to 1 over a wide temperature and field range. The magnetoresistance shows no sign of saturation up to 50 T studied. The linear magnetoresistance observed in Ta4Pd3Te16 is found to be overall inconsistent with the interpretations based on the Dirac fermions in the quantum limit, charge conductivity fluctuations as well as quantum electron-electron interference. Moreover, it is observed that the Kohler's rule, regardless of the field orientations, is violated in its normal state. This result suggests the loss of charge carriers in the normal state of this chain-containing compound, due presumably to the charge-density-wave fluctuations.

Electronic structure of Eu(Fe0.79Ru0.21)2As2 studied by angle-resolved photoemission spectroscopy.

Eu(Fe(0.79)Ru(0.21))2As2 is suggested to be a nodeless superconductor based on the empirical correlation between pnictogen height (hPn) and superconducting gap behavior, in contrast to BaFe2(As(0.7)P(0.3))2 and Ba(Fe(0.65)Ru(0.35))2As2. We studied the low-lying electronic structure of Eu(Fe(0.79)Ru(0.21))2As2 with angle-resolved photoemission spectroscopy (ARPES). By photon energy dependence and polarization dependence measurements, we resolved the band structure in the three-dimensional momentum space and determined the orbital character of each band. In particular, we found that the dz2 -originated ζ band does not contribute spectral weight to the Fermi surface around Z, unlike BaFe2(As(0.7)P(0.3))2 and Ba(Fe(0.65)Ru(0.35))2As2. Since BaFe2(As(0.7)P(0.3))2 and Ba(Fe(0.65)Ru(0.35))2As2 are nodal superconductors and their hPn's are less than 1.33 Å, while the hPn of Eu(Fe(0.79)Ru(0.21))2As2 is larger than 1.33 Å, our results provide more evidence for a direct relationship between nodes, dz2 orbital character and hPn. Our results help to provide an understanding of the nodal superconductivity in iron-based superconductors.