D. M. Paul

Superconductivity in two-dimensional NbSe2 field effect transistors

We describe investigations of superconductivity in few molecular layer NbSe2 field effect transistors. While devices fabricated from NbSe2 flakes less than eight molecular layers thick did not conduct, thicker flakes were superconducting with an onset Tc that was only slightly depressed from the bulk value for 2H-NbSe2 (7.2 K). The resistance typically showed a small, sharp high temperature transition followed by one or more broader transitions which usually ended in a wide tail to zero resistance at low temperatures. We speculate that these multiple resistive transitions are related to disorder in the layer stacking. The behavior of several flakes has been characterized as a function of temperature, applied field and back-gate voltage. We find that the conductance in the normal state and transition temperature depend weakly on the gate voltage, with both conductivity and Tc decreasing as the electron concentration is increased. The application of a perpendicular magnetic field allows the evolution of different resistive transitions to be tracked and values of the zero temperature upper critical field, Hc2(0), and coherence length, ξ(0), to be independently estimated. Our results are analyzed in terms of available theories for these phenomena.

Detection of time-reversal symmetry breaking in the noncentrosymmetric superconductor Re6Zr using muon-spin spectroscopy

We have investigated the superconducting state of the non-centrosymmetric compound Re6Zr using magnetization, heat capacity, and muon-spin relaxation/rotation (muSR) measurements. Re6Zr has a superconducting transition temperature, Tc = 6.75 K. Transverse-field muSR experiments, used to probe the superfluid density, suggest an s-wave character for the superconducting gap. However, zero and longitudinal-field muSR 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.

Damage Evolution on Sm and O Sublattices in Au-Implanted Samarium Titanate Pyrochlore

Damage evolution on the Sm and O sublattices in Sm2Ti2O7 single crystals irradiated with 1 MeV Au2+ ions at 170, 300, and 700 K was studied by Rutherford backscattering spectroscopy and 16O(d,p)17O nuclear reaction analysis along the 〈001〉 direction. The damage accumulation behavior at each irradiation temperature indicates that the relative disorder on the O sublattice is higher than that on the Sm sublattice, and the relative disorder, determined by ion channeling, on each sublattice follows a nonlinear dependence on dose that is well described by a disorder accumulation model. While there is little difference in damage accumulation behavior on the Sm sublattice at 170 and 300 K irradiation, the rate of damage accumulation decreases dramatically at 700 K due to dynamic recovery processes. The critical dose for amorphization at 170 and 300 K is 0.14 displacements per atom (dpa), and a higher dose of 0.22 dpa is observed under irradiation at 700 K. During thermal annealing in an 18O environment, a signifi...

Reconstruction from Small-Angle Neutron Scattering Measurements of the Real Space Magnetic Field Distribution in the Mixed State of Sr2RuO4

We have measured the diffracted neutron scattering intensities from the square magnetic flux lattice in the perovskite superconductor Sr2RuO4, which is thought to exhibit p-wave pairing with a two-component order parameter. The relative intensities of different flux lattice Bragg reflections over a wide range of field and temperature have been shown to be inconsistent with a single component Ginzburg-Landau theory but qualitatively agree with a two-component p-wave Ginzburg-Landau theory.

17O NMR characterisation of the oxygen sites in the Bi2Sr2Can−1CunO4+2n (n = 1, 2, 3) high temperature superconductors

Abstract 17 O NMR measurements have been performed on the Bi 2 Sr 2 Ca n −1 Cu n O 4+2 n ( n = 1, 2, 3) phases allowing the different oxygen sites to be identified. Detailed study of the n =3 phase in different applied magnetic fields has allowed extraction of the shift and quadrupole interaction parameters at the oxygen sites. For this phase the two distinct sets of copper planes have markedly different oxygen shifts, 1590 ppm and 1285 ppm, shift anisotropies, 1300 ppm and 2 plane having the smaller shift and anisotropy. This implies that the local susceptibility is lower in this plane and that the 2p occupancy is less. There is only a small shift difference for the CuO 2 plane oxygens between the superconducting n =2 phase and the nonsuperconducting (for our samples) n =1 phase. Only one resonance, at 265 ppm, is observed in all three phases for the SrO, BiO planes. We suggest that this is from the SrO plane and that the electric field gradient at the oxygen site produced by the incommensurate modulation of the BiO distances broadens the line from the BiO plane beyond detection.

Neutron scattering and muon spin relaxation measurements of the noncentrosymmetric antiferromagnet CeCoGe3

The magnetic states of the noncentrosymmetric pressure-induced superconductor CeCoGe3 have been studied with magnetic susceptibility, muon spin relaxation (?SR), single-crystal neutron diffraction, and inelastic neutron scattering (INS). CeCoGe3 exhibits three magnetic phase transitions at TN1=21,TN2=12, and TN3=8K. The presence of long-range magnetic order below TN1 is revealed by the observation of oscillations of the asymmetry in the ?SR spectra between 13 and 20 K and a sharp increase in the muon depolarization rate. Single-crystal neutron-diffraction measurements reveal magnetic Bragg peaks consistent with propagation vectors of k=(0,0,23) between TN1 and TN2,k=(0,0,58) between TN2 and TN3 and k=(0,0,12) below TN3. An increase in intensity of the (110) reflection between TN1 and TN3 also indicates a ferromagnetic component in these phases. These measurements are consistent with an equal moment two-up two-down magnetic structure below TN3 with a magnetic moment of 0.405(5)?B/Ce. Above TN2, the results are consistent with an equal moment two-up one-down structure with a moment of 0.360(6)?B/Ce. INS studies reveal two crystal-electric-field (CEF) excitations at ?19 and ?27meV. From an analysis with a CEF model, the wave functions of the J=52 multiplet are evaluated along with a prediction for the magnitude and direction of the ground-state magnetic moment. Our model correctly predicts that the moments order along the c axis, but the observed magnetic moment of 0.405(5)?B is reduced compared to the predicted moment of 1.0?B. This is ascribed to hybridization between the localized Ce3+ f electrons and the conduction band. This suggests that CeCoGe3 has a degree of hybridization between that of CeRhGe3 and the noncentrosymmetric superconductor CeRhSi3

Investigations of the superconducting states of noncentrosymmetric LaPdSi3 and LaPtSi3

The noncentrosymmetric superconductors LaPdSi3 and LaPtSi3 have been studied with magnetization, specific-heat, resistivity, and μSR measurements. These crystallize in the tetragonal BaNiSn3 structure and superconductivity is observed at Tc=2.65(5) K for LaPdSi3 and Tc=1.52(6) K for LaPtSi3. The results are consistent with both compounds being weakly coupled, fully gapped superconductors but μSR measurements reveal that LaPdSi3 is a bulk type-I superconductor while LaPtSi3 is a type-II material with a Ginzburg-Landau parameter of κ=2.49(4). This is further supported by specific-heat measurements, where the transition in an applied field is first order in LaPdSi3 but second order in LaPtSi3. The electronic specific heat in the superconducting state was analyzed using an isotropic s-wave model that gave Δ0/kBTc=1.757(4) for LaPdSi3 and 1.735(5) for LaPtSi3. The temperature dependence of the effective penetration depth [λeff(T)] of LaPtSi3 was extracted from μSR measurements and was fitted giving Δ0/kBTc=1.60(8) and λeff(0)=239(3) nm. A critical field of Bc(0) = 182.7 G was obtained for LaPdSi3 from μSR measurements, which is in good agreement with the calculated thermodynamic critical field.

Muon-spin-spectroscopy study of the penetration depth of FeTe0.5Se0.5

Muon-spin-spectroscopy measurements have been used to study the superconducting state of FeTe0.5Se0.5. The temperature dependence of the in-plane magnetic penetration depth, lambda(ab)(T), is found to be compatible with either a two-gap s+s-wave or an anisotropic s-wave model. The value for lambda(ab)(T) at T = 0 K is estimated to be lambda(ab)(0) = 534(2) nm.

Large, high quality single-crystals of the new Topological Kondo Insulator, SmB6

SmB6 has recently been predicted to be a Topological Kondo Insulator, the first strongly correlated heavy fermion material to exhibit topological surface states. High quality crystals are necessary to investigate the topological properties of this material. Single crystal growth of the rare earth hexaboride, SmB6, has been carried out by the floating zone technique using a high power xenon arc lamp image furnace. Large, high quality single-crystals are obtained by this technique. The crystals produced by the floating zone technique are free of contamination from flux materials and have been characterised by resistivity and magnetisation measurements. These crystals are ideally suited for the investigation of both the surface and bulk properties of SmB6.

FOURFOLD BASAL PLANE ANISOTROPY OF THE NONLOCAL MAGNETIZATION OF YNI2B2C

Studies of single crystal YNi{sub 2}B {sub 2}C have revealed a fourfold anisotropy of the equilibrium magnetization in the square crystallographic basal plane. This {pi}/2 periodicity occurs deep in the superconductive mixed state. In this crystal symmetry, an ordinary superconductive mass anisotropy (as in usual London theory) allows only a constant, isotropic response. In contrast, the experimental results are well described by generalized London theory incorporating nonlocal electrodynamics, as needed for this clean, intermediate-{kappa} superconductor. {copyright} {ital 1999} {ital The American Physical Society }