Showing papers on "High-temperature superconductivity published in 1995"
TL;DR: In this article, the phase of the order parameter is not important for determining the value of the transition temperature Tc and the change of many physical properties brought about by the transition, and the phase fluctuations, both classical and quantum, may have a significant influence on low-temperature properties.
Abstract: THE superconducting state of a metal is characterized by a complex order parameter with an amplitude and a phase In the BCS-Eliashberg mean-field theory1, which is a very good approximation for conventional metals, the phase of the order parameter is un-important for determining the value of the transition temperature Tc and the change of many physical properties brought about by the transition Here we argue that superconductors with low super-conducting carrier density (such as the organic and high-Tc oxide superconductors) are characterized by a relatively small phase 'stiffness9 and poor screening, both of which imply a significantly larger role for phase fluctuations As a consequence, in these mat-erials the transition to the superconducting state may not display typical mean-field behaviour, and phase fluctuations, both classical and quantum, may have a significant influence on low-temperature properties For some quasi-two-dimensional materials, notably underdoped high-temperature superconductors, the onset of long-range phase order controls the gross value of Tc as well as its systematic variation from one material to another
1,533 citations
01 Jan 1995
TL;DR: In this article, the authors presented papers presented at the international seminar on high temperature superconductivity, including the following papers: Oxygen disorder, superstructure and the dependence of T{sub c} on structure in oxide superconductors.
Abstract: This book contains papers presented at the international seminar on High Temperature Superconductivity. Included are the following papers: Oxygen disorder, superstructure and the dependence of T{sub c} on structure in oxide superconductors, structural instability and high temperature superconductivity, Theoretical aspects of radiation effects on high-T{sub c} superconductors.
752 citations
TL;DR: It is argued that many synthetic metals, including high temperature superconductors, are “bad matais”, with such a poor conductivity that the usual mean-field theory of superconductivity breaks down because of anomalously large classical and quantum fluctuations of the phase of the superconducting order parameter.
Abstract: It is argued that many synthetic metals, including high temperature superconductors are “bad matais”, with such a poor conductivity that the usual mean-field theory of superconductivity breaks down because of anomalously large classical and quantum fluctuations of the phase of the superconducting order parameter. Some consequences for high temperature superconductors are described.
483 citations
TL;DR: In this paper, the authors present a review of the state-of-the-art in the field of high-temperature superconductivity in transition-metal oxides, focusing on the low energy excitations, the topology of the Fermi surface in the normal state, and the superconducting gap.
362 citations
TL;DR: It is argued that in the mixed state of a type II superconductor, because of the difference of the chemical potential in a superconducting versus normal state, the vortex cores may become charged.
Abstract: It is argued that in the mixed state of a type II superconductor, because of the difference of the chemical potential in a superconducting versus normal state, the vortex cores may become charged. The extra electron density is estimated. The extra charge contributes to the dynamics of the vortices; in particular, it can explain in certain cases the change of the sign of the Hall coefficient below ${T}_{c}$ frequently observed in the high temperature superconductors.
131 citations
TL;DR: In this paper, the authors examined physical properties for the time-reversal-breaking state ( s ± i d ) near the (1, 1, 0) surface of d x 2 -y 2 -wave superconductors.
Abstract: In a previous paper a possibility of coexistence of two superconducting order parameters was discussed for the (1, 1, 0) surface of d x 2 - y 2 -wave superconductors. In the present report physical properties are examined particularly for the time-reversal-breaking state ( s ± i d ) near the (1, 1, 0) surface. The nature of coexisting order parameters shows up in the local density of states. It is also shown that the superconducting current flows along the surface due to the time reversal breaking and induces the magnetic field. The induced magnetic field can be used to detect the time-reversal-breaking states experimentally.
128 citations
TL;DR: A review of theoretical and experimental data on spontaneous phase separation in nonsuperconducting degenerate magnetic semiconductors and related cuprate high-temperature superconductors is given in this article.
Abstract: A review is given of theoretical and experimental data on spontaneous phase separation in nonsuperconducting degenerate magnetic semiconductors and related cuprate high-temperature superconductors. The following phenomena are considered: (1) The electronic phase separation occuring at frozen impurity positions as a result of charge carrier concentration in regions with a changed magnetic state; (2) impurity (chemical) phase separation when a nonuniform impurity distribution over a crystal is driven simultaneously by interaction between impurity atoms and by their tendency to concentrate inside regions with a changed magnetic ordering.
104 citations
IBM1
TL;DR: In this article, high-quality c-axis-oriented epitaxial films of the compound (Hg-1212) can support large in-plane current densities at temperatures higher than has been achieved for other superconductors.
Abstract: THE recent discovery1,2 of a family of mercury-based copper oxide superconductors having transition temperatures1–3 above 130 K is of considerable technological interest. But the viability of high-temperature superconductors for many applications will ultimately depend on the size of the current density, Jc, that they are able to support, not only at high temperatures, but also in high magnetic fields. For the cuprate superconductors, and in particular for Hg-based materials, the combination of high transition temperature1–3 and large mass anisotropy implies that the transport properties will be intrinsically limited by large thermal fluctuations and short superconducting coherence lengths4. Here we report that high-quality c-axis-oriented epitaxial films of the compound (Hg-1212; ref. 5) can support large in-plane current densities at temperatures higher than has been achieved for other superconductors. In low magnetic fields oriented normal to the film surface, we find Jc≳107 A cm−2 at 5 K and Jc∼ 105 A cm−2 at 110 K, at least an order of magnitude larger than for Bi- or Tl-based films6–11. For in-plane magnetic fields, the critical current (∼108 A cm−2) is close to the theoretical limit even at high fields, indicative of strong intrinsic pinning in this compound.
104 citations
TL;DR: Experimental studies of the time decay of the nonequilibrium magnetization in high-temperature superconductors, a phenomenon known as magnetic relaxation, are reviewed.
Abstract: We review experimental studies of the time decay of the nonequilibrium magnetization in high-temperature superconductors, a phenomenon known as magnetic relaxation. This effect has its origin in motion of flux lines out of their pinning sites due to thermal activation or quantum tunneling. The combination of relatively weak flux pinning and high temperatures leads to rich properties that are unconventional in the context of low temperature superconductivity and that have been the subject to intense studies. The results are assessed from a purely experimental perspective and discussed in the context of present phenomenological theories. [S0034-6861(96)00403-5]
75 citations
TL;DR: In this paper, the authors distinguish between the BCS model and the more general BCS theory, and carefully distinguish between BCS and the general theory of superconductivity, which is described well by the Bardeen-Cooper-Schrieffer model.
Abstract: Investigations of rare earth, Aactinide, organic and oxide compounds have yielded several new classes of exotic superconductors. These include magnetically ordered superconductors, A15 superconductors, buckyball superconductors, heavy‐electron superconductors, organic superconductors and high‐Tc oxide superconductors. These materials have properties significantly different from those of conventional superconductors such as Al and Zn, which are described well by the Bardeen‐Cooper‐Schrieffer model of superconductivity. We carefully distinguish between the BCS model and the more general BCS theory. In the BCS theory superconductivity arises, loosely speaking, from electron pairs that behave essentially as bosons and undergo macroscopic condensation to the lowest energy state at the critical temperature Tc The BCS model, presented in 1957, further specifies that the pairing is mediated by exchange of quantized lattice vibrations (phonons) between the electrons, yielding pairs with zero spin S (spin singlet) ...
67 citations
TL;DR: In this article, a macroscopic phenomenological model for the microwave properties of superconductors is presented based on the idea that there are two kinds of current carriers, and instead of the first London's equation a new equation is derived.
Abstract: In this paper a macroscopic phenomenological model for the microwave properties of superconductors is presented. The model is based on the idea that there are two kinds of current carriers, and instead of the first London's equation a new equation is derived. This model can be applied to both low- and high-temperature superconductors. Using this model, an expression for the microwave surface resistance is derived and the surface resistance versus frequency is calculated. The results show that the relation between resistance and frequency is not R/sub 3//spl sim//spl omega//sup 2/ as indicated by both BCS theory and London model, but R/sub s//spl sim//spl omega//sup a/, where a is between 1 and 2 (e.g. a=1.35) for thin film high-T/sub c/ superconductors YBa/sub 2/Cu/sub 3/O/sub 7-/spl delta//. The temperature dependence of R/sub s/ is simulated using the given model. These relations and the values of the surface resistance agree well with experimental results. A residual resistance may be interpreted from this model. >
TL;DR: In this article, the magnetic field and temperature dependence of the critical current density Jc and the voltage-current characteristics of a Bi-2223 tape in a magnetic field parallel to the c-axis are studied.
TL;DR: In this paper, the infrared reflectivity of La1.85Sr0.15CuO4 single crystals with E parallel to the c-axis was analyzed, and it was shown that the single-particle charge transport along the caxis is strongly incoherent in the normal state, and remains so in the superconducting state.
Abstract: We analyze the infrared reflectivity of La1.85Sr0.15CuO4 single crystals with E parallel to the c-axis. The plasma edge at around 6 meV (50 cm−1), which occurs only for T 0.1 eV) if we take into account that the single-particle charge transport along the c-axis is strongly incoherent in the normal state, and remains so in the superconducting state. From a comparison of the optical conductivity with model calculations based on s- and d-wave weak-coupling theory in the dirty limit, we find no evidence for a reduction of the c-axis quasi-particle scattering rate below Tc. The c-axis scattering rate, the normal-state c-axis plasma frequency, and Tc obey hγ > hνp ⪢ 3.5κBTc, which is exactly opposite to the clean limit.
01 Jan 1995
TL;DR: In this article, the authors make measurements of properties of high-temperature superconductors, for the purpose of improving material quality or for a particular application, and they make use of grain boundaries in these new materials.
Abstract: Publisher Summary This chapter intends to make measurements of properties of high-temperature superconductors, for the purpose of improving material quality or for a particular application. The properties of high-temperature superconductors have caused several changes to the techniques of superconducting measurements. The high transition temperatures and large anisotropies in these materials have provided many new insights into the dynamics of magnetic vortices. Dissipation at grain boundaries or other weak links in materials, in conventional superconductors, is correctly thought of as extrinsic. Conventional superconductors typically have larger volumes of superconducting coherence and, as a result, small imperfections in the material microstructure are nondetrimental. In high-temperature superconductors, this is not the case. Any defect in a unit cell can modify the structure over at least the range of coherence, causing a region of reduced superconductivity. For this reason, grain boundaries in these new superconductors are of great importance since very few applications exist for the materials in single crystalline form.
TL;DR: A review of the literature on different experimental techniques used to determine the irreversibility line and the critical current density in high-temperature superconductors can be found in this paper.
Abstract: We review the literature on different experimental techniques used to determine the irreversibility line and the critical current density in high-temperature superconductors. Some of our own recent results pertinent to this subject are presented. In particular, SQUID and a.c. susceptibility measurements performed on a large single crystal and a melt-processed sample are compared.
TL;DR: In this article, the authors analyzed the transport current loss in high-T c superconductors with the theory of Norris for elliptical conductors, which predicts different loss voltages according to where the voltage contacts are placed on the conductor which has been confirmed experimentally.
TL;DR: In this paper, a voltage tunable oscillator, a phase shifter, and a voltage-tunable phased array antenna system were proposed for high temperature superconductors and ferroelectric materials.
Abstract: The group at the University of Colorado has been working on the applications of BaSrTiO3 and high temperature superconductors to microwave circuits and systems with support from Superconducting Core Technologies, NASA and ONR. This work has included the development of a voltage tunable oscillator, a phase shifter, and a voltage tunable phased array antenna system. An important aspect of this program is the lattice match between the high temperature superconductor and the ferroelectric materials which enables us to grow thin films of either material on each other. By choosing the appropriate barium stronium ratio we are able to obtain changes in the dielectric constant of 5 to 1 in bulk capacitors at 77 K, the liquid nitrogen boiling temperature. Phase shifts of a little less than 126° at 10 Ghz have been obtained by applying 5 volts on a one centimeter coplanar line. The proposed phased array systems should enable us to point the beam radiated from the antenna in an arbitrary direction using two ...
TL;DR: In this paper, a magnetically levitated transport system in vacuum using the strong pinning force of high-T c superconductors was developed. But the system was not tested in a real application.
Abstract: We have developed a magnetically levitated transport system in vacuum using the strong pinning force of high-T c superconductors. This transport system consists of superconductors prepared by the quench-and-melt-growth (QMG) method and permanent magnets. The superconductors move within the cooling jacket inserted into the transport chamber. A transfer arm moves in the transport chamber without any contact or sliding, because stable levitation is brought about by magnetic interaction between the magnet attached to the arm and the superconductor. In this paper, structure and performance test results are described.
Patent•
18 Apr 1995TL;DR: In this paper, a mismatch of less than 7 percent with the a and b lattice parameters of the new compositions of garnets exhibit a mismatch with the HTSC parameters.
Abstract: c-axis oriented high temperature superconductors are deposited onto new compositions of garnets using pulsed laser deposition (PLD) with conditions of 85 mTorr of oxygen partial pressure; a block temperature of 730° C., a substrate surface temperature of 790° C. and a laser fluence of 1 to 2 Joules/cm2 at the target, a laser repetition rate of 10 Hz and a target to substrate distance of 7 cm and in which the a and b lattice parameters of the new compositions of garnets exhibit a mismatch of less than 7 percent with the a and b lattice parameters of the HTSC.
TL;DR: In this paper, a theoretical approach to identify Fermi surface contributions in high-resolution Compton scattering data is presented, which opens up new possibilities especially in Fermiology studies of high temperature superconductors.
Abstract: We present a novel theoretical approach to identify Fermi surface contributions in high resolution Compton scattering data. For FeAl Compton profiles we show that the Fermi-surface-related features (occupation of states) can be separated from those originating from the spatial extent of the wave functions. The first high resolution Compton scattering experiment done at the European Synchrotron Radiation Facility confirms these findings. This technique opens up new possibilities especially in Fermiology studies of high temperature superconductors.
TL;DR: In this article, the temperature dependence of the lattice parameters of single crystal superconductors Bi2Sr1.8CaCu2Ox was investigated using a low-temperature stage X-ray diffractometer.
Abstract: Using a low-temperature stage X-ray diffractometer, we have investigated the temperature dependence of the lattice parameters of single crystal superconductors Bi2Sr1.8CaCu2Ox. The experimental results show that (i) the lattice constants increase linearly with increasing temperature above about 100 K; (ii) thec-axis lattice parameter shows a kink in the superconducting transition region, while thea, b-axis parameters do not show any anomalous behavior in this region; (iii) botha, b, andc show negative thermal expansion coefficients below 40–50 K, which may be related to the characteristics of the Cu-O bond.
TL;DR: In this paper, a review of recent progress in the fundamental understanding of the high-temperature superconductors is reviewed with an eye toward its practical implications and a research agenda for the future, needed in support of the applications of these new materials is identified.
Abstract: Recent progress in the fundamental understanding of the high-temperature superconductors is reviewed with an eye toward its practical implications. An attempt is also made to identify a research agenda for the future, needed in support of the applications of these new materials. >
TL;DR: In this paper, the spin-Hamiltonian parameters of the EPR spectra have been reported for high-T c superconductors and in an oxygen-deficient 123-superconductor.
01 Dec 1995
TL;DR: In this article, the reasons to expect the formation of ordered or fluctuating charge stripes and antiphase spin domains in high temperature superconductors are surveyed, and some of the consequences for the physical properties of high temperature Superconductors and the mechanism of superconductivity are presented.
Abstract: The reasons to expect the formation of ordered or fluctuating charge stripes and antiphase spin domains in high temperature superconductors are surveyed. Evidence for such behavior is described, and some of the consequences for the physical properties of high temperature superconductors and the mechanism of superconductivity are presented.
TL;DR: In this article, the Schwinger-Dyson equations and free energy were derived in the approximation of weak coupling interaction for antiferromagnetic spin fluctuations in high temperature superconductors.
Abstract: The Schwinger–Dyson equations and equation for free energy are derived in the approximation of weak coupling interaction for antiferromagnetic spin fluctuations in high temperature superconductors. For the calculations the method of functional integrals was used. The thermodynamic of the system is calculated near Tc and the evaluations of the parameter ∆C/γTc are given.
TL;DR: In this article, the authors measured the normal state Fermi surface of a Pb 0.42 Bi 1.73 Sr 1.94 Ca 1.3 Cu 1.92 O 8+x (001) single crystal using angle resolved photoemission.
CNET1
TL;DR: In this article, the authors reported phase-locking of more than 1000 junctions at 93.7 GHz in Bi/sub 1.8/Pb/sub 0.2/Sr/sub 2/CaCu/Sub 2/O/sub 8+y/
Abstract: We report on microwave emission experiments of Bi/sub 2/Sr/sub 2/CaCu/sub 2/O/sub 8+y/-single crystals at frequencies between 12 GHz and 95 GHz. Due to the intrinsic Josephson effect these samples form series connections of typically 2000 contacts. For the first time we observed phase-locking of more than 1000 junctions at 93.7 GHz in Bi/sub 1.8/Pb/sub 0.2/Sr/sub 2/CaCu/sub 2/O/sub 8+y/ single crystals. Concerning the development of high frequency sources we report on first experiments on vertically structured thin films of Tl/sub 2/Ba/sub 2/Ca/sub 2/Cu/sub 3/O/sub 10/. >
TL;DR: In this paper, the authors examined the possible and practical use of temperatures below 77 K for power engineering applications, and the types of refrigeration to be employed, whether using wet cooling, gas cooling or dry cooling via thermal contact.