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Crystal growth, characterization, and point-contact Andreev-reflection spectroscopy of the noncentrosymmetric superconductor Mo 3 Al 2 C
No full textWe report on the first successful growth of single crystals of the noncentrosymmetric superconductor Mo3Al2C obtained by means of a cubic-anvil, high-pressure, and high-temperature technique. Composition, structure, and normal-state transport properties of the crystals were studied by means of x-ray diffraction, energy-dispersive x-ray spectroscopy, magnetic susceptibility, and resistivity measurements as a function of temperature. Variations in critical temperature (Tc) between 8.6 and 9.3 K were observed, probably due to the slightly different carbon stoichiometry of the samples. Single-crystal x-ray refinement confirmed the high structural perfection of the grown crystals. Remarkably, the refined Flack parameter values for all the measured crystals using a P4132 space-group model were consistently close to either 0 or 1, hence indicating that the considered crystals belong to two enantiomorphic space groups, P4132 and P4332. An anomaly in the resistivity is observed at 130 K, most likely associated with the onset of a charge-density-wave phase. The superconducting properties (and in particular the symmetry, the amplitude, and the temperature dependence of the superconducting gap) were studied by using point contact Andreev-reflection spectroscopy. The results confirm that Mo3Al2C is a moderately strongly-coupled superconductor with 2Delta/kBTc = 4 and unambiguously prove that the order parameter has an s-wave symmetry despite the asymmetric spin-orbit coupling arising from the lack of inversion symmetry
Energy dependence of the normal density of states and of the electron-phonon spectral function in the quasiparticle tunneling of high-Tc superconductors
No full textWe discuss the effects of quasiparticle energy dependence of the normal density of states on the Eliashberg equations in the case of high-Tc superconductors and compare the numerical results with our recent tunneling experimental data in Bi2Sr2CaCu2O8+x with Tc = 93 K. We also consider the possibility of quasiparticle energy dependence of the electron-phonon spectral function α2(Ω)F(Ω) and calculate its effects on the superconducting density of states
Probing the current-phase relation in Josephson point-contact junctions between Pb 0.6 In 0.4 and Ba 0.6 K 0.4 (FeAs) 2 superconductors
Full text linkThe Josephson effect in point contacts between an “ordinary” superconductor Pb 0.6In0.4 (Tc≈6.6K) and single crystals of the Fe-based superconductor Ba0.6K0.4(FeAs)2 (Tc≈38.5K), was investigated. In order to shed light on the order parameter symmetry of Ba0.6K0.4(FeAs)2, the dependence of the Josephson supercurrent Is on the temperature and on sin (dφ) with d= 1 , 2 was studied. The dependencies of the critical current on temperature Ic(T) and of the amplitudes of the first current steps of the current–voltage characteristic inexp(P)(n= 0 , 1 , 2) on the power of microwave radiation with frequency f=(1.5÷8)GHz were measured. It is shown that the dependencies Ic(T) are close to the well-known Ambegaokar–Baratoff (AB) dependence for tunnel contacts between “ordinary” superconductors and to the dependence calculated by Burmistrova et al. (Phys Rev B 91, 214501 (2015)) for microshorts between an “ordinary” superconductor and a two-band superconductor with s± order parameter symmetry at certain values of the transparency of boundaries and thickness of the transition layer. It is found that the dependencies inexp(P) cannot be approximated within the resistively shunted model using the normalized microwave frequencies Ω = 2 πf/ (2 eVc/ ħ) with characteristic voltages Vc= IcRN, (RN—normal resistance of the contact) found from the low-voltage parts of the current–voltage characteristics. The reasons for this failure are discussed and a method is proposed for accurately determining the value of Ω , which takes into account all the features of the point contact affecting the period of the dependence inexp(P). An analysis of the Ic(T) and inexp(P) dependencies shows that the superconducting current of the Josephson contacts under investigation is proportional to the sin of the phase difference φ, Is= Icsin(φ). The implications of these results on the symmetry of the order parameter are also discussed
Eliashberg equations, pair symmetry and tunnel density of states in the optimally-doped high-Tc superconductor Bi2Sr2CaCu2O8+x
No full textIn this work, in order to compare the theoretical density of states (DOS) to the tunneling data in the optimally-doped high-Tc superconductor (HTS) Bi2Sr2CaCu2O8+x, we calculate the real-axis direct solution of the d-wave equations for the retarded electron-boson interaction in presence of half-filling and impurities. The use of a spectral function containing an isotropic part αs2F(Ω) and an anisotropic one αd2F(Ω) and the simple assumption: αd2F(Ω) = gdαs2F(Ω) where gd is a constant leads, for appropriate values of the isotropic coupling constant λs and the anisotropic one λd, to solutions with only d-wave symmetry for the order parameter Δ(ω) and only s-wave one for the renormalization function Z(ω). The results properly fit both the critical temperature and the DOS in the whole energy range, as recently determined in our break-junction tunneling experiments, but appear unable to do the same in the case of recent STM data
Real-axis solution of Eliashberg equations in various order-parameter symmetries and the tunneling conductance of optimally-doped HTCS
No full textIn the present work we calculate the theoretical tunneling conductance curves of SIN junctions involving high-Tc superconductors, for different possible symmetries of the order parameter (s, d, s + id, s + d, anisotropics and extendeds). To do so, we solve the real-axis Eliashberg equations in the case of an half-filled infinite band. We show that some of the peculiar characteristics of HTSC tunneling curves (dip and hump at eV > Δ, broadening of the gap peak, zero bias and so on) can be explained in the framework of the Migdal-Eliashberg theory. The theoretical dI/dV curves calculated for the different symmetries at T=4 K are then compared to various experimental tunneling data obtained in optimally-doped BSCCO, TBCO, HBCO, LSCO and YBCO single crystals. To best fit the experimental data, the scattering by non-magnetic impurities has to be taken into account, thus limiting the sensitivity of this procedure in determining the exact gap symmetry of these materials. Finally, the effect of the temperature on the theoretical tunneling conductance is also discussed and the curves obtained at T = 2 K are compared to those given by the analytical continuation of the imaginary-axis solution
Structural and morphological properties of evaporated SiOx films Journal: Philosophical Magazine B-Physics of Condensed Matter Statistical Mechanics Electronic Optical and Magnetic Properties
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Synthesis and characterization of MgB2 thick films obtained by electrophoresis technique
No full textElectron-phonon spectral function and two-band model in tunneling measurement on MgB2
No full textIn the present work we calculate for the first time the density of states of MgB2 for different tunneling directions by directly solving the two-band Eliashberg equations in the real-axis formulation starting from the first-principle calculation of the interband and intraband electron-phonon spectral functions. This complicated numeric procedure allows preserving the fine structures of the DOS in the phonon energy range. We show that the numeric inversion of the standard single-band Eliashberg equations when applied to the densities of states obtained by the two-band model may lead to artifacts in the extracted electron-phonon spectral function α2 F(ω). We also suggest that the small DOS structures produced by the two-band inter-action at energies between 20 and 100 meV can be observed only at very low temperature in junctions in perfect clean limit
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