Fourfold symmetry in the ab-plane of the upper critical field for the high-Tc cuprates

We have found clear anisotropy in the ab plane with fourfold symmetry of the resistive superconducting transition under magnetic fields for single-crystal Pb₂Sr₂Y_0.62Ca_0,38Cu₃O₈, which is regarded as anisotropy of the upper critical field H_c2. This is not only qualitatively but also quantitatively similar to that formerly observed in La_1.86Sr_0.14CuO₄. The observed fourfold symmetry is explained as being mainly due to the anisotropy of the superconducting energy gap owing to pairing. H_c2, one of the bulk properties, supports pairing in the high-T_c superconductivity.     

Superconducting critical currents in Nb-Cu-Nb sandwiches

A preparation method is presented for Nb-Cu-Nb sandwiches for proximity-effect measurements with a large junction area of about 150×150 µm and intermediate Cu layers of thicknesses between 0.2 and 1.5 µm. The thickness of the Nb layers is about 150 µm. These samples are used to study the dependence of the superconducting critical currentI _c through the Cu layer as a function of a magnetic field applied parallel and perpendicular to the layers. Furthermore, the effects of flux lines trapped in the Nb layers on the critical currentsI _c andI _c are investigated. The results are used to discuss the influence of proximity effects in microcomposite superconductors.     

AC long-range phase-coherent effects of S-N-S junctions in the dirty limit

The conductance of a normal metal film (N) in contact with two superconductors (S) is calculated in the presence of a constant and oscillating V_ωcosωt bias voltage between N and S. It is shown that the conductance as a function of has a Shapiro-like step at = ħω/2e. This exists in the case when the spacing between two superconductors largely exceeds the coherence length. It is also shown that the Shapiro step height decreases rapidly when the junction size exceeds the phase breaking length.     

Hole Density in Charge Stripes and Effect of Next-Nearest-Neighbor Transfer

By applying a variational Monte Carlo method to the two-dimensional t-J model, a couple of properties of striped states are studied. In the charge domain walls vertical (or horizontal) to the lattice (VDW), the hole density is favorable in energy similarly for 0.5 , whereas in the diagonal domain walls (DDW) the stable hole density is limited to = 1. Negative next-nearest-neighbor transfer (t/t < 0) further stabilizes VDW with = 0.5 and DDW with = 1 against the -wave superconducting state.     

Josephson Current Between d-Wave Superconductors Through an Interface with Finite Transmission

We consider an interface with finite transmission between two superconductors. We show that in some cases the general non-linear boundary condition for the quasiclassical Green’s function derived independently by Kieselmann and Zaitsev can be factorized into a linear effective boundary condition. This result is then applied to the josephson current-phase relationships for junctions between two super conductors. In particular we study the phase difference χ_m across a junction where its energy is at a minimum as a function of the temperature and orientations of the superconductors. Experimental implications are discussed.     

Tunneling with very long relaxation times in glasses,organic materials,and Nb-Ti-H (D)

The long-time (t=10–200 h) heat release from glasses, from organic materials, and from Nb-Ti-H (D) was measured at 30T70 mK. For Suprasil W glass, Dimethyl-Siloxan, Stycast 1266, Stycast 2850 FT, Vespel, and for Nb-Ti-H (D) with various Ti and D concentrations, we found . Typical values are = 0.05 nW/g for the organic materials and for Nb-Ti-H (D) and = 0.005 nW/g for the glass att=100 h after cooldown from room temperature. For charging temperaturesT _i <5 K, we find the predicted dependence (investigated for Suprasil W glass and for Nb-Ti-D). The observed time and temperature dependences agree with predictions of the conventional two-level tunneling model for amorphous materials even at these very long times. No heat release was observed for Teflon, graphite, and Al₂O₃.     

Electronic Raman scattering in CuO2 superconductors

Experimental results for the electronic Raman effect in differently doped cuprate superconductors will be presented. We show that the B_2g-symmetry data are generally closely related to ordinary transport and are therefore most likely originating from the carriers, while the response at B_1g symmetry cannot be assigned to a specific type of excitations. In the superconducting state the B_2g pair breaking peaks scale with the transition temperature over a wide doping range. All results consistently suggest a strong anisotropy of the gap and can be modeled by assuming symmetry for the order parameter.     

Bose Condensation and the Static Pair Correlation Function in 4He

An investigation is made of the implications that the presence of a Bose condensate (BC) has for static pair correlation function, . The scattering of incident radiation from many particle states which contribute to the BC, is compared with that from states which do not. It is shown that in the former the scattering density of each atom is delocalised in position space, wheras in the latter it is localised, i.e. quite different physical scattering processes occur from the two types of state. A simple model of a dilute gas, based on variational wavefunctions of the ground state, predicts that in the ground state depends directly on the condensate fractionf. The paper provides support for the intuitive idea that localisation of atoms in momentum space is directly associated with a delocalisation in position space and consequent loss of spatial correlations.     

Stability analysis of a low energy vortex configuration

Recent work using the London theory of anisotropic superconductivity has shown that, for small values ofB and large values of the anisotropy, a lattice of vortex lines parallel to is not necessarily the minimum energy configuration. When is not parallel to a symmetry axis, the “double lattice”—a lattice of lines parallel to plus a lattice of lines parallel to —can have a lower energy. The question arises, however, of whether some other configuration can have a yet lower energy. To investigate this question, we perform a stability analysis of the double lattice. We find that its energy can indeed be lowered by certain deformations of the vortex lines.     

The model ofd-pairing in CuO2 planes of HTSC and the collective modes

We develop here for the first time a 2D path integral model of d-pairing in HTSC, which is analogous to the 3D model created earlier by Brusov and Brusova. Within this model we calculate the entire collective mode spectrum for all superconductive states of CuO₂ planes arisen from their symmetry classification. There are four collective modes in each phase. We found two high frequency modes in each phase while two remains modes seem to be Goldstone or quasi-Goldstone ones (having vanishing energy at zero momentum). It turns out that the collective mode spectrum in the phases and is the same.     

Two-lifetime model calculations of the quantum interference dominated transverse magnetoresistance of magnesium

The magnetoresistance of extremely pure, strain-free magnesium is dominated by the transport properties of a narrow slab of coupled orbits in the geometry in whichJ [11 0] andH lies in the (11 0) plane. In samples whose quantum state lifetime _s exceeds 10^–10 sec the conductivity of the coupled orbit system is controlled by _s as well as the large-angle scattering lifetime ₁. The magnetoresistance then exhibits complicated behavior, oscillating with |H| and varying rapidly with field orientation. The behavior withH aligned to within about 0.001° of [10 0] is due to manifold multiply connected open orbits, whereas forH tilted away from [10 0] by more than this the behavior is due to extended, multiply connected closed orbits. Detailed models for both these regimes are developed, taking into account the interference effects resulting from the feedback paths. Within these models the transport properties are calculated exactly, taking into account to all orders the amplitude propagation of wave packets within the network. Results are compared to experimental data and previous calculations, and the limitations of the model are discussed.Work supported in part by NSF grant DMR 7910533.     

Thermal conductivity and gap structure of the superconducting phases of UPt3

We present new measurements of the thermal conductivity (κ) of UPt₃ down to very low temperatures (16 mK) and under magnetic fields (up to 4 T) which cover all the superconducting phases of UPt₃. The measurements in zero field are compared with recent theoretical predictions for the thermal conductivity, which is dominated by impurity states at the lowest temperatures studied. The measurements under magnetic field at low temperatures are surprising since they don’t show the expected low field square root dependence, . The discontinuity ofdκ/dT atT _c changes drastically when passing from the high field low temperatureC phase to the low field high temperatureA phase: this is related to the change of the symmetry of the superconducting order parameter when crossing theA→C phase transition.     

The fluctuation-imposed limit for temperature measurement

In experimental sciences, random processes often place a fundamental limit on the achievable measurement resolution. A well known example is the Johnson noise voltage across a resistor. In this paper, we describe observations of the spontaneous transfer of heat in two equilibrium systems: one consisting of a thermometer linked to a reservoir, the other consisting of two thermometers connected to each other and linked to a reservoir for the purpose of temperature stabilization. In the second system, we find anti-correlations between the temperature fluctuations of the two thermometers at intermediate frequencies. We also find that the low frequency temperature noise density of the thermometers, in units of , is given by , whereR is the thermal resistance of the link between the thermometer and the reservoir. This implies that for noise reduction purposes,R is the only available engineering parameter to adjust. In a recent thermometer design, we have reduced R to achieve a low frequency temperature noise density of 5×10⁻¹¹ at 2.18 K.     

Bi2Sr2CaCu2O8+δ c-Axis Twist Josephson Junctions: A New Phase-Sensitive Test of Order Parameter Symmetry

Li et al. found that the critical current density across atomically clean c-axis twist junctions of Bi ₂ Sr ₂ CaCu ₂ O ₈₊ is the same as that of the constituent single crystal, , independent of the twist angle ₀ , even at and below T _c . We investigated theoretically if a -wave order parameter might twist by mixing in d _xy -wave components, but found that such mixing cannot possibly explain the data near to T _c . Combined with group theoretical arguments, we then conclude that the order parameter contains at least a substantial s-wave component, but does not contain any purported -wave component, except possibly below a second, unobserved phase transition. By studying tunneling models, we further conclude that the intrinsic c-axis Josephson tunneling in Bi ₂ Sr ₂ CaCu ₂ O ₈₊ is likely to be mostly incoherent.     

Textures in3He-A: Magnetic deformation in a narrow slab

A variational technique is used to analyze the deformed texture of superfluid³He-A in a narrow slab subject to a perpendicular magnetic field. Specific predictions are made for the anisotropy parameters and , averaged across the width.     

Détermination des contraintes maximale et minimale en valeur algébrique résultant d'un effort normal et d'un moment de flexion dans une section quelconque d'un profil, à partir de trois mesures de déformations superficielles

Résumé Les contraintes maximale et minimale en valeur algébrique engendrées dans une section quelconque d'un profil, par un effort normal et un moment de flexion quelconque , peuvent être déterminées à partir de trois mesures de déformations superficielles effectuées au niveau de cette section. De plus, il est possible de calculer l'effort normal et le moment de flexion ainsi que ses composantesM _GY ,M _GZ sur les axes principaux d'inertie , et de définir la position de l'axe neutre de flexion ainsi que celle de l'axe neutre de flexion et d'effort normal.

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Summary The determination of the maximum and minimum stresses in algebraic value generated in an arbitrary section of a profile by a normal force and an arbitrary bending moment is an important problem that frequently arises. Before showing that 3 measurements of surface deformations effected on the level of the section studied suffice completely to solve this problem some theoretical considerations are recalled. These concern the establishment of the formula giving the bending stress. For this we can attach to the section either arbitrary central axes or the main central inertia axes; this latter possibility leading to a simpler formula. The formulae needed for the determination of the maximum and minimum stresses due to and are then established, for arbitrary coordinates of measurement points. Simpler formulae may be obtained when it is possible practically to choose simple ratios between the coordinates. Next it is shown that one can compute not only but and its componentsM _GY ,M _GZ , on the main intertia axes , and determine the position of the neutral bending axis as well as that of the neutral bending and normal force axis. Finally, the practical utilization of the method is indicated and a concrete case treated.

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Flux line density gradient in the mixed state of type II superconductors

The value of the critical current for a type II (1) superconducting slab placed in a magnetic fieldH _e (H _c1H _e H _c2) parallel to the sample surface and with a currentI flowing perpendicular to the magnetic field is obtained. This critical current depends on the characteristic length of the mixed state . The critical state equation for a simple system with pinning is given. The equation relating the length with the Campbell pinning length is obtained in the special case . A formula for the surface impedance of the mixed state is given.     

Constraints on the pairing state of the cuprate superconductors

The pairing state of the cuprate superconductors is a key issue in understanding experiments and in clarifying possible mechanisms. A wide range of experiments now show that the gap function is predominantly in character. Here we discuss the relevant experiments, especially focusing on those which provide constraints on possible mixed pairing states, such as s+d and s+id.     

Upper critical field anisotropy in the Chevrel phase compound PbMo6S8

The anisotropy of the upper critical field,H _c2, was measured on deoxygenated single crystals of the Chevrel phase compound PbMo₆S₈. The superconducting transition temperature,T _c, was determined with the applied field pointing in various directions in the hexagonal plane. At a fixed value of the external field,T _c is an oscillating function of direction. Since theH _c2 versusT curve is linear in the field region we probed, the shape of the anisotropy inT _c at constantH is the same as the anisotropy ofH _c2 at constantT. The data reflect the low-temperature symmetry of the crystal structure. Two-fold symmetry was observed in the plane. The data were fitted to the effective mass model with uniaxial symmetry. The anisotropy ofH _c2 is more complex than that previously observed by us in Cu_1.8Mo₆S₈. We relate this observation to the structural differences of these compounds.