We report on optical reflectivity experiments performed on Cd3As2 over a broad range of photon energies and magnetic fields. The observed response clearly indicates the presence of 3D massless charge carriers. The specific cyclotron resonance absorption in the quantum limit implies that we are probing massless Kane electrons rather than symmetry-protected 3D Dirac particles. The latter may appear at a smaller energy scale and are not directly observed in our infrared experiments.

We report upper critical field B c2 (T ) data for LaO 0.9 F 0.1 FeAs 1−δ in a wide temperature and field range up to 60 T. The large slope of B c2 ≈ -5.4 to -6.6T/K near an improved T c ≈ 28.5 K of the in-plane B c2 (T ) contrasts with a flattening starting near 23 K above 30 T we regard as the onset of Pauli-limited behavior (PLB) with B c2 (0) ≈ 63 to 68 T. We interpret a similar hitherto unexplained flattening of the B c2 (T ) curves reported for at least three other disordered closely related systems as the Co-doped BaFe 2 As 2 , the (Ba,K)Fe 2 As 2 , or the NdO 0.7 F 0.3 FeAs (all single crystals) for applied fields H (a, b) also as a manifestation of PLB. Their Maki parameters have been estimated analyzing their B c2 (T ) data within the Werthamer-Helfand-Hohenberg approach. The pronounced PLB of (Ba,K)Fe 2 As 2 single crystals obtained from a Sn-flux is attributed also to a significant As deficiency detected by wave length dispersive x-ray spectroscopy as reported by Ni N et al. 2008 Phys. Rev. B 78 014507. Consequences of our results are discussed in terms of disorder effects within conventional (CSC) and unconventional superconductivity (USC). USC scenarios with nodes on individual Fermi surface sheets (FSS), e.g. p-and d-wave SC, can be discarded for our samples. The increase of dB c2 /dT | Tc by sizeable disorder provides evidence for an important intraband (intra-FSS) contribution to the orbital upper critical field. We suggest that it can be ascribed either to an impurity driven transition from s ± USC to CSC of an extended s ++ -wave state or to a stabilized s ± -state provided As-vacancies cause predominantly strong intraband scattering in the unitary limit. We compare our results with B c2 data from the literature which show often no PLB for fields below 60 to 70 T probed so far. A novel disorder related scenario of a complex interplay of SC with two different competing magnetic instabilities is suggested.

No abstract

No abstract

The polarized Raman spectra of the Cu2ZnSnSe4 and Cu2ZnGeSe4 single crystals were measured for various in-plane rotation angles on the basal (1 1 2) crystal facet. The position of up to 15 (for Cu2ZnSnSe4) and 16 (for Cu2ZnGeSe4) Raman peaks was determined in the spectral region 50–300 cm−1. From the analysis of the experimental dependence of the intensity of the Raman peaks with the rotation angle, a symmetry assignment for most of the detected modes and an estimation of numerical values of Raman tensor elements were derived. The kesterite type structure of Cu2ZnSnSe4 and Cu2ZnGeSe4 single crystals was established by the quantity of the observed nonpolar A and polar B (TO+LO) symmetry modes.

Understanding the doping mechanisms in the simplest superconducting copper oxide-the infinite-layer compound ACuO2 (where A is an alkaline earth metal)-is an excellent way of investigating the pairing mechanism in high-transition-temperature (high-Tc) superconductors more generally. Gate-induced modulation of the carrier concentration to obtain superconductivity is a powerful means of achieving such understanding: it minimizes the effects of potential scattering by impurities, and of structural modifications arising from chemical dopants. Here we report the transport properties of thin films of the infinite-layer compound CaCuO2 using field-effect doping. At high hole- and electron-doping levels, superconductivity is induced in the nominally insulating material. Maximum values of Tc of 89 K and 34 K are observed respectively for hole- and electron-type doping of around 0.15 charge carriers per CuO2. We can explore the whole doping diagram of the CuO2 plane while changing only a single electric parameter, the gate voltage.

A non-destructive Raman spectroscopy has been widely used as a complimentary method to X-ray diffraction characterization of Cu2ZnSnS4 (CZTS) thin films, yet our knowledge of the Raman active fundamental modes in this material is far from complete. Focusing on polarized Raman spectroscopy provides important information about the relationship between Raman modes and CZTS crystal structure. In this framework the zone–center optical phonons of CZTS, which is most usually examined in active layers of the CZTS based solar cells, are studied by polarized resonant and non-resonant Raman spectroscopy in the range from 60 to 500 cm−1 on an oriented single crystal. The phonon mode symmetry of 20 modes from the 27 possible vibrational modes of the kesterite structure is experimentally determined. From in-plane angular dependences of the phonon modes intensities Raman tensor elements are also derived. Whereas a strong intensity enhancement of the polar E and B symmetry modes is induced under resonance conditions, no mode intensity dependence on the incident and scattered light polarization configurations was found in these conditions. Finally, Lyddane-Sachs-Teller relations are applied to estimate the ratios of the static to high-frequency optic dielectric constants parallel and perpendicular to c-optical axis.

No abstract