Superconducting and normal-state properties of the noncentrosymmetric superconductor Re6Zr

We systematically investigate the normal and superconducting properties of non-centrosymmetric Re$_{6}$Zr using magnetization, heat capacity, and electrical resistivity measurements. Resistivity measurements indicate Re$_{6}$Zr has poor metallic behavior and is dominated by disorder. Re$_6$Zr undergoes a superconducting transition at $T_{\mathrm{c}} = \left(6.75\pm0.05\right)$ K. Magnetization measurements give a lower critical field, $\mu_{0}H_{\mathrm{c1}} = \left(10.3 \pm 0.1\right)$ mT. The Werthamer-Helfand-Hohenberg model is used to approximate the upper critical field $\mu_{0}H_{\mathrm{c2}} = \left(11.2 \pm 0.2\right)$ T which is close to the Pauli limiting field of 12.35 T and which could indicate singlet-triplet mixing. However, low-temperature specific-heat data suggest that Re$_{6}$Zr is an isotropic, fully gapped s-wave superconductor with enhanced electron-phonon coupling. Unusual flux pinning resulting in a peak effect is observed in the magnetization data, indicating an unconventional vortex state.Comment: 11 pages, 7 figures, 2 table

Superconductivity and the upper critical field in the chiral noncentrosymmetric superconductor NbRh2B2

NbRh2B2 crystallises in a chiral noncentrosymmetric structure and exhibits bulk type-II superconductivity below 7.46(5) K. Here we show that the temperature dependence of the upper critical field deviates from the behaviour expected for both Werthamer-Helfand-Hohenberg and the Ginzburg-Landau models and that μ0Hc2 (0) ~ 18 T exceeds the Pauli paramagnetic limit, μ0HP = 13.9 T. We explore the reasons for this enhancement. Transverse-field muon spectroscopy measurements suggest that the superconducting gap is either s-wave or (s + s)-wave, a the pressure dependence of Tc reveals the superconducting gap is primarily s- wave in character. The magnetic penetration depth lambda(0) = 595(5) nm. Heat capacity measurements reveal the presence of a multigap (s + s)-wave superconducting order parameter and moderate electron-phonon coupling

Shape-resonant superconductivity in nanofilms: from weak to strong coupling

Ultrathin superconductors of different materials are becoming a powerful platform to find mechanisms for enhancement of superconductivity, exploiting shape resonances in different superconducting properties. Here we evaluate the superconducting gap and its spatial profile, the multiple gap components, and the chemical potential, of generic superconducting nanofilms, considering the pairing attraction and its energy scale as tunable parameters, from weak to strong coupling, at fixed electron density. Superconducting properties are evaluated at mean field level as a function of the thickness of the nanofilm, in order to characterize the shape resonances in the superconducting gap. We find that the most pronounced shape resonances are generated for weakly coupled superconductors, while approaching the strong coupling regime the shape resonances are rounded by a mixing of the subbands due to the large energy gaps extending over large energy scales. Finally, we find that the spatial profile, transverse to the nanofilm, of the superconducting gap acquires a flat behavior in the shape resonance region, indicating that a robust and uniform multigap superconducting state can arise at resonance.Comment: 7 pages, 4 figures. Submitted to the Proceedings of the Superstripes 2016 conferenc

Valence-state mixing and reduced magnetic moment in Fe 3 − δ GeTe 2 single crystals with varying Fe content probed by x-ray spectroscopy

We present a spectroscopic study of the magnetic properties of Fe3−δGeTe2 single crystals with varying Fe content, achieved by tuning the stoichiometry of the crystals. We carried out x-ray absorption spectroscopy and analyzed the x-ray circular magnetic dichroism spectra using the sum rules, to determine the orbital and spin magnetic moments of the materials. We find a clear reduction of the spin and orbital magnetic moment with increasing Fe deficiency. Magnetic susceptibility measurements show that the reduction in magnetization is accompanied by a reduced Curie temperature. Multiplet calculations reveal that the Fe2+ state increasingly mixes with a higher valence state when the Fe deficiency is increased. This effect is correlated with the weakening of the magnetic moment. As single crystals are the base material for exfoliation processes, our results are relevant for the assembly of 2D magnetic heterostructures

Evidence for the coexistence of time-reversal symmetry breaking and Bardeen-Cooper-Schrieffer-like superconductivity in La7Pd3

Time-reversal symmetry breaking (TRSB) with a Bardeen-Cooper-Schrieffer (BCS) -like superconductivity occurs in a small, but growing number of noncentrosymmetric (NCS) materials, although the mechanism is poorly understood. We present heat capacity, magnetization, resistivity, and muon spin resonance/relaxation (μSR) measurements on polycrystalline samples of NCS La7Pd3. Transverse-field μSR and heat capacity data show La7Pd3 is a type-II superconductor with a BCS-like gap structure, while zero-field μSR results provide evidence of TRSB. We discuss the implications of these results for both the La7X3 (where X = Ni, Pd, Rh, Ir) group of superconductors and other CS and NCS superconductors for which TRSB has been observed

Dynamics of Nonequilibrium Dicke Models

Motivated by experiments observing self-organization of cold atoms in optical cavities we investigate the collective dynamics of the associated nonequilibrium Dicke model. The model displays a rich semiclassical phase diagram of long time attractors including distinct superradiant fixed points, bistable and multistable coexistence phases and regimes of persistent oscillations. We explore the intrinsic timescales for reaching these asymptotic states and discuss the implications for finite duration experiments. On the basis of a semiclassical analysis of the effective Dicke model we find that sweep measurements over 200ms may be required in order to access the asymptotic regime. We briefly comment on the corrections that may arise due to quantum fluctuations and states outside of the effective two-level Dicke model description.Comment: 27 pages, 20 figure

Anisotropic superconductivity and unusually robust electronic critical field in single crystal La7Ir3

Polycrystalline La$_{7}$Ir$_{3}$ is reported to show superconductivity breaking time-reversal symmetry while also having an isotropic $s$-wave gap. Single crystals of this noncentrosymmetric superconductor are highly desirable to understand the nature of the electron pairing mechanism in this system. Here we report the growth of high-quality single crystals of La$_{7}$Ir$_{3}$ by the Czochralski method. The structural and superconducting properties of these large crystals have been investigated using x-rays, magnetization, resistivity and heat capacity measurements. We observe a clear anisotropy in the lower and upper critical fields for magnetic fields applied parallel and perpendicular to the hexagonal $c$ axis. We also report the presence of a robust electronic critical field, that diverges from the upper critical field derived from heat capacity, which is the hallmark of surface superconductivity.Comment: 20 + 5 pages, 6 + 4 figures, 1 + 1 tables. Accepted for publication into Physical Review Material

Quantum muon diffusion and the preservation of time-reversal symmetry in the superconducting state of type-I rhenium

Elemental rhenium exhibiting type-II superconductivity has been previously reported to break time-reversal symmetry in the superconducting state. We have investigated an arc-melted sample of rhenium exhibiting type-I superconductivity. Low-temperature zero-field muon-spin relaxation measurements indicate that time-reversal symmetry is preserved in the superconducting state. Muon diffusion is observed, which is due to quantum mechanical tunneling between interstitial sites. The normal state behavior is characterized by the conduction electrons screening the muons and thermal broadening, and is typical for a metal. Energy asymmetries between muon trapping sites and the superconducting energy gap also characterize the superconducting state behavior

Magnetic structure investigation of the intercalated transition metal dichalcogenide V1/3NbS2

We investigate the temperature evolution of the magnetic structure of V1/3NbS2 using neutron diffraction techniques. We find that V1/3NbS2 has two propagation vectors: k0 = (0, 0, 0) and k1 = (0, 0, 1/3 ). The k0 vector can be associated with an antiferromagnetic ordering of in-plane moments with a refined value of 0.90(5)μB, and k1 can be associated with moments along the c axis in an up-down-down configuration with refined values of 1.21(12)μB and 0.61(6)μB. Both k0 and k1 magnetic components couple with an out-of-plane ferromagnetic moment consistent with magnetization data. Furthermore, single-crystal neutron diffraction shows evidence of diffuse magnetic scattering between the (010) and (01±1/3 ) Bragg peaks. We also characterize the field and temperature evolution of the magnetic structure in V1/3NbS2 by magnetic susceptibility and heat capacity measurements. The dc susceptibility measurements give an antiferromagnetic transition temperature of TN = 50 K, and the field scans reveal that the moment does not saturate at magnetic fields up to 100 kOe