State Of High-temperature Superconductors Research Articles

Anomalous ac dielectric response and carrier state of c-axis oriented La 2− xSr xCuO 4 films

In order to study the “fractional Hall effect model” of the carrier state of high temperature superconductors, we measured the x dependence of the capacitance C t of multilayer structure Pd/La 2− x Sr x CuO 4/(100)SrTiO 3/Pd. A strong correspondence is found between the x- C t relationship and the filling factor ν dependence of the system energy in the fractional quantum Hall effect.

An equation of state of high-temperature superconductor

In the present paper the expression of cohesive energy and the bulk modulus as a function of volume are formulated for high-T c copper oxide superconductors. The model employed consists of long-range electrostatic Coulomb interaction and short-range overlap repulsion. The short-range overlap potential is considered in the Born-Lande inverse power form. The model, parameters of the Born-Lande model are calculated from the equilibrium condition and data of bulk modulus at room temperature. The computed values of pressure derivatives of bulk modulus atP=0 and the values of bulk modulus are found to be in very close agreement with experimental values for high-T c copper oxide and their nonsuperconducting parent compounds. It is also found that the quantity ΔU/U(V o) of these compounds increases with increasing hydrostistic pressure.

Correlation length of flux lines in the vicinity of the vortex glass-liquid transition in high-temperature superconductors

The correlation length of flux lines responsible for the scaling of current-voltage curves in the transition region between the glass and liquid states in high temperature superconductors is argued based on experimental results on various specimens of Bi-based superconductors. From the correspondence between the temperature dependence of the pinning correlation length and the static critical index, v, it is concluded that the responsible correlation length is the pinning correlation length. This insists that the pinning interaction governs the flux lines in the glass state. This hypothesis is supported by the fact that the transition line depends on the flux pinning strength and the dynamic critical index, z, is appreciably influenced by the width of the distribution of flux pinning strength. It is concluded that the so-called vortex glas-liquid transition is a kind of transition of flux lines between the glass state governed by the pinning and the depinned melt state due to the thermal activation.

Twin-boundary effect on the Hall conductivity in high-Tcsuperconducting thin films

The pinning influence on the Hall conductivity in the mixed state of high-temperature superconductors is still experimentally and theoretically controversial. In this work, the effect of twin-boundary pinning on the Hall conductivity is studied, with particular emphasis on the high-field results. The longitudinal and Hall resistivities were measured in YBa{sub 2}Cu{sub 3}O{sub 7{minus}{delta}} and YbBa{sub 2}Cu{sub 3}O{sub 7{minus}{delta}} thin films, up to 18 T: a signature due to the pinning of the vortices in the twin boundaries is clearly visible in the longitudinal resistivity {rho}{sub xx} and in the Hall resistivity {rho}{sub xy}, but disappears when the Hall conductivity {sigma}{sub xy}{congruent}{rho}{sub xy}/{rho}{sub xx}{sup 2} is computed. However, at lower temperatures, a minimum of d{sigma}{sub xy}/dT is proved to be a weak signature of the twin-boundary pinning. These results are confirmed by similar measurements done on one sample rotated 16{degree} away from the magnetic field direction. {copyright} {ital 1997} {ital The American Physical Society}

Evidence for polarons and itinerant carriers in superconductivity from time-resolved optical measurements on YBa2Cu3O7-△

Time-resolved optical relaxation measurements on timescales from 10 -13 to 10 8 s as a function of energy (0.8 eV - 3 eV) and temperature give evidence for the existence of two different carrier relaxation processes, indicating the presence of two distinct electronic subsystems in the normal state in high-temperature superconductors, one exhibiting band-like relaxation properties and the other those of localised, polaronic states. Both components of the electrodynamic response exhibit significant changes at T c, indicating that both carrier types may be involved in superconductivity. The experiments also show that both polarons and band-like carriers exist predominantly in the CuO planes, with an out-of-plane component of the order of 10-20% which is close to the experimentally observed occupancy ratio of holes on O ions of the CuO5 pyramids. Spectrally, the superconducting condensate is also found to be coupled to two distinct components in frequency (for both carriers): one below 0.1 eV and one in the range 1.5-2 eV, but with different anisotropies. A natural explanation of these data is that the two types of carrier correspond to the two phases suggested by the structural data and as such present the first direct evidence of two-component superconductivity. The under-doped sample data can be interpreted as evidence of pre-formed pairs above Tc.

Vortex movements parallel and perpendicular to twin boundaries seen through the Nernst and Seebeck effects of YBa 2Cu 3O 7−δ

Twin boundaries are the cause of important intrinsic pinning effects in the mixed state of high temperature superconductors. However, adjacent twin planes form channels which can guide the vortices with a nevertheless restricted motion. These phenomena have been studied in detail with a specially prepared sample of YBa 2Cu 3O 7−δ (YBCO). The latter is a heavily twinned single crystal containing a single domain i.e. the twin boundaries are parallel throughout the sample. Thus with a temperature gradient applied parallel and then perpendicular to the twin boundaries, one can observe vortex motion in these two directions by measuring the Seebeck and Nernst effects as a function of temperature for both magnetic field polarities. Field inversion is necessary for the separation of the odd and even components of the measured signals to exploit a hitherto unexplored property of the Seebeck and Nernst effects.

Temperature dependence of the mid-infrared dielectric function of YBCO in the ab plane

We have excited mid-infrared surface plasmons in two YBCO thin films of contrasting properties using attenuated total reflection of light and found that the imaginary part of the dielectric function decreases linearly with reduction in temperature. This result is in contrast with the commonly reported conclusion of infrared normal reflectance studies. If sustained it may clarify the problem of understanding the normal state properties of YBCO and the other cuprates. The dielectric function of the films, ε = ε 1 + i ε 2, was determined between room temperature and 80K: ε 1 was found to be only slightly temperature dependent but somewhat sample dependent, probably as a result of surface and grain boundary contamination. The imaginary part, ε 2, (and the real part of the conductivity, σ 1,) decreased linearly with reduction in temperature in both films. Results obtained were: for film 1: ε 1 = − 14.05 − 0.0024 T and ε 2 = 4.11 + 0.086 T and for film 2: ε 1 = −24.09 + 0.0013 T and ε 2 = 7.66 + 0.067 T where T is the temperature in Kelvin. An understanding of the results is offered in terms of temperature-dependent intrinsic intragrain inelastic scattering and temperature-independent contributions: elastic and inelastic grain boundary scattering and optical interband (or localised charge) absorption. The relative contribution of each is estimated. A key conclusion is that the interband (or localised charge) absorption is only ∼ 10%. Most importantly, the intrinsic scattering rate, 1/τ, decreases linearly with fall in temperature, T, in a regime where current theory predicts dependence on frequency, ω, to dominate. The coupling constant, λ, between the charge carriers and the thermal excitations has a value of 1.7, some fivefold greater than the far infrared value. These results imply a need to restate the phenomenology of the normal state of high temperature superconductors and seek a corresponding theoretical understanding.

Experimental investigation of the pairing state of high-temperature superconductors.

To address the issue of the pairing state in high-temperature superconductors, four different experimental investigations have been carried out on a single high-quality, untwinned single crystal of ${\mathrm{LuBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7\mathrm{\ensuremath{-}}\mathrm{\ensuremath{\delta}}}$. These include measurements of the specific heat in magnetic fields, both near the transition temperature and at low temperatures, investigations of the angular dependence of the transverse magnetic moment in the nonlinear Meissner regime, and the measurement of microwave properties leading to the temperature dependences of the penetration depth and the surface resistance at low temperatures. Some of the results raise questions relating to the existence of a pairing state with line nodes in the energy gap, whereas others appear to be consistent with this picture. The results taken together suggest that studies of the pairing state be examined critically for possible alternative explanations. \textcopyright{} 1996 The American Physical Society.

Josephson plasma resonance in the vortex state of high temperature superconductors

We study the microwave response perpendicular to the superconducting layers of high temperature superconductors in a wide range of temperatures and fields. A novel microwave response,i.e. Josephson plasma resonance, shows up in the superconducting state of Bi2Sr2CaCu2O8+δ. This resonance enable us to measure the magnitude of interlayer phase coherence quantitatively. Josephson plasmon is shown to interact strongly with the vortices. Therefore it provides a new powerful tool to probe the vortex state. The vortex state of high temperature superconductors is discussed in the light of the interlayer phase coherence.

Josephson plasma resonance in the vortex state of high temperature superconductors

Josephson plasma resonance in the vortex state of high temperature superconductors

The Pairing State of High Temperature Superconductors: Magnetic, Thermal and Microwave Measurements on a Single Crystal

The pairing state has been studied through measurements of the transvers magnetic moment in the nonlinear Meissner regime. This investigation, which probes properties over a volume determine by the penetration depth, and is thus a bulk measurement, does not support a pure \em{d}-wave picture of the pairing state. The crystal on which much of the data has been obtained was subject to other experimental investigations which reveal it to be of a quality comparable to those of the best cuprate crystals in the YBCO family. These include measurements of the specific heat in magnetic fields, both near the transition temperature and at low temperatures, the measurement of microwave properties leading to the temperature dependences of the penetration depth and the surface resistance at low temperatures. Only the microwave measurements were unequivocally in agreement with the findings of others which have been interpreted as being supportive of $\em{d}$-wave pairing. This suggests that it is necessary to seek alternative explanations for some data interpreted within a \em{d}-wave framework.

NMR Studies of the Normal State of High Temperature Superconductors

We review NMR and NQR studies in the normal state of high-T c superconducting cuprates. Emphasis is given on the three major contributions brought by these techniques: the presence of antiferromagnetic fluctuations, the possible justification of a single spin fluid model, and the opening of a pseudogap in the spin excitations for underdoped compounds. Some recent developments are addressed and compared to theoretical models. In particular, quantitative analysis of the nuclear relaxation are detailed for 63 Cu, 89 Y and 17 O nuclei, enabling a comparison with inelastic neutron scattering data, and finally pointing out to a serious difficulty with the one-band approach.

Antivortex motion in the mixed state of 2-D superconductors

The sign reversal of the Hall voltage in the mixed state of high temperature superconductors is explained by the motion of antivortices in bundles of the vortex lattice. In the frame of the Kosterlitz-Thouless mechanism of vortex-antivortex pair generation in 2-D superconductors, it is shown that the antivortex moves with a much larger Hall angle than other vortices.

Galvanomagnetic and thermomagnetic phenomena in the mixed state of high-temperature superconductors

The thermomagnetic phenomena in the mixed state of high-{Tc} superconductors are investigated in terms of the phenomenological three-fluid (superconductive and no rmal quasiparticles, and Abrikosov vortices) model. General relations are found between the various transport coefficients which are valid for the entire transition superconductive state {yields} mixed state {yields} normal state.

Tailored defects in HTSC — What can we learn from swift heavy ion irradiation?

Since the discovery of superconductivity above liquid nitrogen temperature in copper oxide based compounds many experiments have been performed on the one hand to study their mixed state properties and on the other hand to check their technological applicability. Due to their well defined cylindrical shape latent particle tracks (also known as columnar defects) are a powerful tool to investigate the mixed state in high temperature superconductors (HTSC) as they provide optimized pinning centers. Therefore in this paper we will present experiments performed with HTSC samples (epitaxial thin films on SrTiO 3 substrates, single crystals and technical tapes) containing columnar defects. We will outline the influence of these linear defects on the resistive phase transition and on transport I–V curves of YBa 2Cu 3O 7−δ (123) thin films. All HTSC known up to now exhibit a pronounced structure-related anisotropy. Hence we present magnetic and transport measurements concerning the angular dependence of the critical current in various irradiated HTSC compounds. Furthermore experiments with technical tapes of (BiPb) 2Sr 2Ca 2Cu 3O 10 (Bi2223) will be shown giving an overview of problems concerning technological applications. Finally in irradiated and nonirradiated Bi 2Sr 2CaCu 2O 8 (Bi2212) single crystals the flux line lattice is visualized via the high resolution Bitter decoration technique.

Diffusion and dynamical randomness of the flux flow state in high temperature superconductors

In strongly layered superconductors, the dynamical system of vortex lattice is a disordered state with large energy of the vortex bundle for small driving currents. We provide an evidence that broadening of the resistive transitions in HTSC measured under magnetic field is due to random diffusion and fast relaxational hopping of flux lines between different metastable configurations. A model is presented in which the dynamics of the two-dimensional vortex lattice are characterized by a diffusive transfer rather than distinct movements of the vortices. The predictions of the model are consistent with large scale numerical simulation on two-dimensional vortex lattice using molecular dynamics' methods. In addition, excellent agreements were established between the model calculations and the measured resistive transitions in twined (YBaCuO) single crystal high temperature superconductors (HTSC).

Phase coherence in YBCO-Pb SQUIDs: Direct experimental determination of the symmetry of the pairing state in high temperature superconductors

We describe an experiment designed to determine the symmetry of the pairing state in the cuprate superconductors. By studying the magnetic flux modulation of a dc SQUID formed by connecting tunnel junctions on the orthogonal faces of a YBCO single crystal with the conventional superconductor Pb, we can measure the spatial anisotropy of the phase of the order parameter. Preliminary results are complicated by SQUID asymmetries, but show evidence for a phase shift of π that is predicted for the d x2 −y2 pairing state.

Anomalous behavior of nickel-gallium substitutions in YBa 2Cu 3O 7−δ

Substitution of Ni and Ga together for Cu in YBa 2Cu 3O 7−δ is found to produce markedly different changes in superconducting transition temperatures than would be expected from the individual dopants. These elements were selected for their physical similarity to copper, so that any anomalous behavior might be interpreted as due primarily to the differing valences and the nickel magnetic moment. Substitutions of Zn and Ga were also studied, and the smaller deviation from the individual dopant behavior suggests that the magnetic nature of Ni may be significant perturbation to the high-temperature superconducting state. These results are consistent with a transition temperature decrease from the Ni magnetic moment competing with an increase from the NiCu valence difference.

Resistive dissipation in the flux liquid state in high-temperature superconductors

Using a simple model for single vortex motion, we calculate the field and temperature dependence of the flux liquid state resistivity, incorporating both the pinning and viscous drag forces. The calculated ϱ-H curves differ from the Bardeen-Stephen model. Our calculation results show that the resistivity in the whole transition region can be understood within the framework of flux motion. A continuous crossover from the thermally assisted flux flow dominated region to the flux flow dominated region is found.

Thermoelectric and Thermomagnetic Effects in High-Temperature Superconductors

ABSTRACTIn the mixed state of high-temperature superconductors the dominant part of the Seebeck and Nernst effect is due to the thermal diffusion of quasiparticles and vortices, respectively. Our understanding of the Seebeck effect is based on the two-fluid counterflow model of Ginzburg and its extension to the mixed state with the presence of vortices. From the Nernst effect the transport entropy of the vortices is obtained. We summarize the recent thermoelectric and thermomagnetic experiments, paying also attention to the role of the Magnus force (Hall effect) and to the thermal fluctuation effects near TC.