BCS temperature-dependent superconducting energy gap in domain boundaries of melt-textured YBa2Cu3O7

Attenuation and velocity measurements in low magnetic fields in the superconducting state of a melt-textured sample of YBa₂Cu₃O₇ are reported. Changes in both attenuation and relative velocitydv/v are observed at the penetration fieldH _c1 of its domain boundaries. The change indv/v atH _c1 appears to be proportional to the repulsive force between vortices. The change in atH _c1 may be produced by a relaxation process involving superconducting quasiparticles in the vicinity of the normal core of the vortices. Analysis of the relaxation time associated with this process yields a temperature-dependent energy gap which follows the BCS dependence. The relaxation time associated with the process is about 10^–11 sec.     

Nuclear relaxation in the superconducting state of (MDT-TTF)2AuI2

We report the results of our attempt to measure the proton nuclear relaxation rate, 1/T ₁, in the superconducting state of the title material. The relaxation rate in the superconducting state at a field of 1 T was found much longer than that in the normal state, but it became clear that the dominant contribution came from the normal core region. The nuclear relaxation at zero field was examined by using the field cycling technique. An ln(t) term in the relaxation curve was observed at low temperatures, suggesting the contribution of the creeping motion of vortices. We discuss the possibility to determine the intrinsic temperature dependence of 1/T ₁ in the superconducting state.     

The nature of the change in the conditions for surmounting pinning centers by dislocation in lead at the superconducting transition

The effect of the presence of Sb on the amplitude dependence of ultrasonic attenuation α in the concentration range 1.7×10 ^?3 to 4.25×10 ^?1 at % has been studied systematically on Pb single crystals at 1.4 and 4.2 K (superconducting and normal states) and 77 K. The dependence curve shifts toward greater amplitudesU with increasing concentrationC and to smallerU whenT goes from 1.4 to 4.2 K. The shift toward greaterU at the transition to the normal state is observed for all concentrations. The amplitude-independent attenuation α _i varies with concentration nonmonotonically: IncreasingC causes α _i first to diminish (the minimum atC=0.034 at %) and then to grow. AtC≥0.034at %, α _i does not change at the n-s transition. The studies suggest that the damping factor for dislocations in impure crystals practically does not change at the superconducting transition. Here the damping is essentially due to the dynamic dislocation-impurity interaction. The change in the conditions for a dislocation surmounting the pinning centers at the superconducting transition is determined by the quasistatic mechanism of lowering barriers.     

On the coupling current losses in a multifilamentary superconducting composite

The coupling current losses in a superconducting multifilamentary composite exposed to trapezoidally varying external magnetic field and carrying a small transport current are investigated for the volume pinning force density described with the dependence: F_v = α B^1?γ. Such a model allows estimation of the deviations from the solution based on Bean's (γ = 0) critical state model. Results indicate that there exists a region of small magnetic field amplitudes for which discrepancies are largest. They are also very sensitive to the rate of magnetic field change.     

Role of edge superconducting states in trapping of multi-quanta vortices by microholes. Application of the bitter decoration technique

The Bitter decoration technique is used to study the trapping of single and multiple quanta vortices by a lattice of circular microholes. By keeping a thin superconducting layer (the bottom) inside each hole we are able to visualise the trapped vortices. From this we determine, for the first time, the filling factor FF, i.e. the number of vortices captured inside a hole. In all cases the sample is cooled at a constant field before making the decoration. Two qualitatively different states of the vortex crystal are observed: (i) In case when the interhole distance is much larger than the coherence length, the filling factor averaged over many identical holes (〈(FF〉)) is a stepwise function of the magnetic flux (of the external field) through the hole, because each hole captures the same number of vortices. The density of fluxoids inside the openings is higher than in the uniform film, but much lower than it should be in the state of equilibrium. We claim that the number of trapped vortices is determined by the edge superconducting states which appear around each hole at the modified third critical fieldH _c3 ^* >H _c2 . BelowH _c2 such states produce a surface barrier of a new type. This barrier for the vortex entrance and exit is due to the strong increase of the order parameter near the hole edge. It keeps constant the number of captured vortices during the cooling at a fixed field, (ii) An increase of the hole density or of the hole radius initiates a sharp redistribution of fluxoids: all of them drop inside holes. This first order transition leads to a localization of all vortices and consequently to a qualitative change of the transport properties (TAFF in our case). In the resulting new state the filling factor is not any more the same for neighbouring holes and its averaged value is equal to the frustration of the hole network.     

Doping Dependent Quasiparticle Relaxation Dynamics in?SmFeAsO1?x F x Single Crystals: Comparison of?Spin-Density Wave and Superconducting States

We use optical femtosecond spectroscopy to investigate the quasiparticle relaxation and low-energy electronic structure in undoped and near-optimally doped SmFeAsO_1?x F _x iron-pnictide superconductor single crystals (SC). In the undoped SC, a single relaxation process with a divergent-like relaxation time at the spin density wave (SDW) transition is observed. From the relaxation time in the normal state, significantly above T _SDW, the second moment of the Eliashberg function is determined to be ?????? ²????135?meV². Below T _SDW, the temperature dependence of the photoexcited reflectivity transients indicates the appearance of a bottleneck due to opening of a SDW gap with a BCS-like temperature dependence and the characteristic magnitude, 2??_SDW/k _B T _SDW=7??3, at?4.2?K. In the superconducting SC, multiple relaxation processes are present. In addition to the relaxation processes observed in the normal state, a distinct superconducting state relaxation component is observed consistent with the presence of BCS T-dependent gaps.     

The influence of Cu2+ substitution on the magnetic properties of Fe-Zn and Ni-Zn ferrites

The influence of a Jahn-Teller ion on magnetization,g-values and linewidth in Zn _x Fe_3?x O₄ and Ni_1?x Zn _x Fe₂O₄ spinel ferrites has been investigated by introducing small quantities of Cu²⁺ ions. The magnetization of Cu substitution in both the systems but the curie temperature is affected only in FeZn ferrites. The lattice constant in NiZn is significantly reduced while in FeZn there is only a slight increase. The change ing _eff on copper substitution,_Δ g _eff, is very different in the two systems. In NiZn,_Δ g _eff has a uniform value, ～ 0·2, for all values ofx while in FeZn it has a peak atx=0·3 and vanishes atx=0 andx?0·5. The ferromagnetic linewidth_ΔH does not change significantly on copper substitution in both the systems. An explanation based on the perturbation produced due to the local lattice distortion by the Jahn-Teller mechanism explains the observed results satisfactorily.     

Effect of Ca doping on the structural and superconducting properties of EuBaSrCu3O7−δ

The effect of Ca doping in EuBaSrCu₃O_7?δ has been investigated by X-ray diffraction, ac susceptibility, and electrical resistivity measurements. X-Ray diffraction analyses of the samples showed that up to 30 at.% Ca can be substituted for Sr in EuBaSrCu₃O_7?δ, which has orthorhombic symmetry. The orthorhombicity decreases with increase inx, in EuBaSr_1?xCa_x Cu₃O_7?δ, the material becoming tetragonal atx=0.2. Ac susceptibility and dc electrical resistivity measurements as function of temperature from 10 to 300 K showed that the superconducting transition temperatureT _c decreases monotonically from 84 to 75K as the Ca concentrationx increases from 0.0 to 0.3 The decrease inT _c can be attributed to the structural change taking place in EuBaSrCu₃O₇ as a consequence of replacement of Sr by Ca.     

Nuclear relaxation in the superconducting state of (MDT-TTF)2AuI2

We report the results of our attempt to measure the proton nuclear relaxation rate, 1/T ₁, in the superconducting state of the title material. The relaxation rate in the superconducting state at a field of 1 T was found much longer than that in the normal state, but it became clear that the dominant contribution came from the normal core region. The nuclear relaxation at zero field was examined by using the field cycling technique. An ln(t) term in the relaxation curve was observed at low temperatures, suggesting the contribution of the creeping motion of vortices. We discuss the possibility to determine the intrinsic temperature dependence of 1/T ₁ in the superconducting state.     

Heat conduction in Ba1−xKxBiO3

We have investigated heat conduction of single crystal Ba_1?xK_xBiO₃ in the temperature range of 2–300 K and in a magnetic field of up to 6 Tesla. Temperature dependence of thermal conductivityκ(T) reveals the participation of both electrons and phonons with their relative contributions that depend critically on the potassium doping concentration. Crystals underdoped with potassium (samples with higherT _c) exhibit a strong suppression ofκ and a glass-like temperature dependence. In contrast, those with a higher potassium content (lowerT _c) show an increase as temperature decreases with a peak near 23 K. Field dependence ofκ(H) is also very sensitive to the level of potassium doping. Crystals exhibiting a large phonon contribution show an initial drop inκ(H) at low fields followed by a minimum and then a slow rise to saturation as the field increases. The initial drop is due to the additional phonon scattering by magnetic vortices as the sample enters a mixed state. The high field behavior ofκ(H), arising from a continuous break-up of Cooper pairs, exhibits scaling which suggests the presence of an unconventional superconducting gap structure in this material.     

Viscous Motion of Vortices in Superconducting Niobium Films

Motion of vortices at high velocity in niobium thin films is presented for the first time and treated as a viscous flux flow for both dirty and clean samples. The critical velocity v*, at which the abrupt transition occurs, yields a temperature determination of the inelastic relaxation time _in of quasiparticules outside the core, which turns to be temperature independent for the dirty film.     

Non-linear Flux Flow Resistance of Type-II Superconducting Films

We analyze the flux-flow (FF) regime in type-II superconducting films exhibiting quite strong pinning. By driving the vortex lattice (VL) up to high dissipative states, the moving VL undergoes an instability, leading to an abrupt change from the FF to the normal state, which is displayed in the current-voltage characteristics as a voltage jump at a critical vortex velocity v ^?. The temperature and magnetic field dependence of v ^? is investigated in different materials, and an unpredicted low field behavior of v ^?(B) is found. Moreover, for velocities lower than v ^?, a non-linear FF resistance is observed, with a ??peak?? in the current dependence of the dynamic resistance. This is a remarkable feature of a dynamic transition from disordered to ordered VL occurring in the FF state. We suggest that both unusual effects observed in v ^?(B) and R _FF(I) can be accounted for intrinsic pinning.     

Superconducting gap in Pr x Y1−x Ba2Cu4O8 and YBa2−y Sr y Cu4O8 probed by infrared phonon self-energies

We report a reflectivity study of thez-polarized TO-phonons of Pr _x Y_1?x Ba₂Cu₄O₈ and YBa_2?y Sr _y Cu₄O₈ alloys in the temperature range 10–300 K. Anomalies of the frequency and linewidth of the plane-oxygen vibration atω～300 cm^?1 due to the opening of the superconducting gap are found to occur upon crossing the superconducting transition temperatureT _c . Phonon self-energy effects are strongly dependent onT _c , providing evidence for a relative shift of the gap with respect to the energy of phonon.     

Effect of superconducting fluctuations on the nmr relaxation rate of high-T c superconductors

The effect of superconducting order parameter fluctuations on the nuclear-spin relaxation rate, 1/T ₁, is studied for clean two-dimensional systems by calculating the three Maki-Thomson-type diagrams which represent the lowest-order fluctuation contributions to the transverse susceptibility. For Gaussian fluctuations and for temperatures near the mean field transition temperature,T _c0, we employ a weak-coupling theory in which the pair-fluctuation propagator can also include pair-breaking effects. We also go beyond the Gaussian theory and take into account the interactions between Cooper-pair fluctuations corresponding to the fourth-order Ginzburg-Landau fluctuation terms. We compare our results with previous results in the dirty limit and in 3D. We obtain a pronounced peak in 1/T₁ atT _c and briefly discuss possible reasons why this peak is not observed.     

Heterogeneity of spiral wear patterns produced by local heating on amorphous polymers

We report on spiral wear patterns produced at constant angular velocity by hot tip atomic force microscopy (HT-AFM) on surfaces of two common amorphous polymers: polystyrene (PS) and polymethylmethacrylate (PMMA). Topography of these patterns is obtained with regular AFM cantilevers. Topography cross-sections taken from a center of each spiral at a given azimuthal angle Θ relate changes of surface corrugation h_corr with tangential velocity v of a thermal cantilever. Polymer wear is characterized by a power law h_corr(v) = α(v/v_max)^−β, which yields a pre-factor α and an exponent β. Below the glass transition temperature T_g, α is polymer specific and β varies weakly between similar conditions and samples. Variations of β are hypothesized to reflect polymer relaxation processes, which are expected to vary only weakly between amorphous polymers. At and above T_g, α approaches initial thermal tip indentation depth within a polymer, β plummets, and a power law relation of h_corr with v diverges. These results are explained by heterogeneous wear around T_g due to a local nature of glass transition. At all studied temperatures, additional wear heterogeneities are found as due to position on the polymer and Θ. Variations of α and β with position on the polymer are found to be only marginally larger then uncertainties of the thermal tip–polymer interface temperature. Variations of α and β with Θ are found to be largely influenced by buckling of thermal cantilevers traveling in a spiral pattern.     

Phase diagram for the helical texture in 3He-A

The stability region of the helical textures in³He-A in thev _s-H phase diagram is theoretically determined. Herev _sis the superflow velocity andH (v _s) is the external magnetic field. The pitch of the helixk ^–1 that minimizes the Gibbs free energy and the corresponding inclination angles ofl andd are determined numerically as functions ofv _sandH. The NMR frequencies (two longitudinal and four transverse frequencies) associated with the helical texture are obtained.Work supported by the National Science Foundation under Grant No. DMR 76-21032.     

Electronic specific heat of Tl2Ba2CuO6+δ from 2 K to 300 K for 0≤δ≤0.1

Using a high-precision differential technique with a resolution of 1∶10⁴, we have measured the heat capacity of Tl₂Ba₂CuO_6+δ over a temperature range 2–300 K for 0≤δ≤0.1. Anomalies atT _c are seen for all superconducting compositions measured, and the results are consistent with a temperature- andδ-independent normal-state electronic termγ _n～0.6 mJ/g-at. K². The samples with the largerT _c 's exhibit strong fluctuations in their specific heat, typical of a highly anisotropic 2D superconductor, but there is some evidence that these fluctuations become weaker atT _c falls-consistent with an increase in the coherence length on hole doping. At temperatures below 5 K an upturn in the data is observed which appears to increase in magnitude asT _c falls, correlating with the increase in the Curie term of published susceptibility data.     

Thermal conductivity of niobium in the purely superconducting and normal states

The thermal conductivity λ of four niobium samples has been measured between 1 and 10 K, both in the superconducting and normal states. The specimens differed in their crystal defect structures due to annealing at different temperatures (dislocations, grain boundaries) and, in one case, to subsequent fast neutron irradiation (dislocation loops). A procedure has been developed with which the electron and phonon contributions to the thermal conductivity can be separated with an accuracy not hitherto obtainable. All the samples proved to have the same energy gap at 0K:δ(0)=(1.95±0.02)kT _c . The phonon conductivity in the superconducting stateλ _p ^s has been compared with the formula of Bardeen, Rickayzen, and Tewordt extended for scattering mechanisms other than phonon-electron interaction. For the unirradiated samples at \({\text{T}} \lesssim 0.15T_{\text{c}} \) , λ _p ^s is proportional toT ², showing that dislocations are mainly responsible for the phonon scattering. The results are qualitatively in agreement with the theory of Klemens, giving a rough indication that the grain boundaries may be considered as arrays of line dislocations. Dislocation loops introduced by the neutron irradiation turn out to behave like clusters of point defects. A second consequence of the irradiation is an enhancement of the original dislocation scattering term.     

Influence of long-range and strong correlation effects on the electron-phonon coupling in high-T c oxides

Calculations of the Eliashberg functionα ²F and the corresponding transport functionα _tr ² F for high-T _c oxides are presented using a screened ionic model (rigidly shifted ionic potentials screened by charge carriers in the CuO₂ planes within the RPA) for the electron-phonon coupling. It is shown that this model yields a large difference between the transport and the superconducting electron-phonon interaction due to imperfect screening and contributions beyond nearest-neighbor interactions. Using these results, the electron and the lattice heat conductivities are calculated both in the normal and the superconducting state and compared with experiment. Finally, effects due to a strong on-site electron-electron repulsion are included in leading order in an 1/N expansion, whereN is the number of spin degrees of freedom. In particular, it is shown for the infiniteU, one-band Hubbard model that correlations tend to suppressα _tr ² F strongly andα ² F somewhat.     

Surface Effects on the Dynamic Behavior of Vortices in Type II Superconducting Strips with Periodic and Conformal Pinning Arrays

Using molecular dynamics techniques, we simulate the vortex behavior in a type II superconducting strip in the presence of triangular and two types of conformal pinning arrays, one with a pinning gradient perpendicular to the driving force (C1) and the other parallel (C2), at zero temperature. A transport force is applied in the infinite direction of the strip, and the magnetic field is increased until the rate between the density of vortices (n _v ) and pinning (n _p ) reaches n _v /n _p =?1.5. Our results show a monotonic increase, by steps, of the vortex density with the applied magnetic field. Besides, each pinning lattice presents a different vortex penetration delay. For the triangular pinning array, different than the case of infinite films, here the system exhibits only one pronounced depinning force peak at n _v /n _p =?1. However, the depinning force peak is present for only one value of field, in the range of fields where n _v /n _p =?1 is stable. For the case of conformal pinning arrays, in analogy to what is observed in infinite films, no commensurability depinning force peaks were found. In the present case, the C1 array is more efficient at low fields, all arrays are equivalent in the intermediate fields, and the C2 array is more efficient for high fields. We also show that for the C1 array at high fields, vortices depin following the conformal arches, from the edge to the center. For the C2 array, the depinning force is higher when compared to that of C1, because this particular conformal structure prevents the formation of easy vortex flow channels.