Vibrating reed studies on high-T c superconductors

We present vibrating reed (VR) measurements on single crystal and ceramic “1-2-3” and melt-processed polycrystalline Bi-based compounds in a wide range of temperature (4·2–100 K) and magnetic field (B=0–4 T). The “depinning line” (DL) determined by the VR technique is equivalent to the “irreversibility line” determined by magnetization and susceptibility measurements. A comparison of the results on single crystal and polycrystalline 1-2-3 compounds indicates that the VR technique is sensitive to the intragranular properties of the polycrystalline reed. It is found that the DL for 1-2-3 compounds is much steeper than that for Bi-based compounds, reflecting an intrinsically different pinning in both the materials, in agreement with the measured elastic coupling (Labusch constant α(B, T)).     

Asymmetry between flux penetration and flux expulsion in Tl-2212 superconductors

dc magnetic hysteresis as well as flux penetration and flux expulsion were investigated in Tl_2–y Ba₂CaCu₂O_8–x polycrystals and monocrystals. All measurements were performed at 35 K and in the 0–5 T field range. Hysteresis measurements revealed an irreversibility field of about 2 T. Existing models predict identical field-cooled (fc) and zero-field-cooled (zfc) magnetizations and vanishing time dependence above this field. Although the identical fc and zfc magnetizations are in fact observed, the time dependence vanishes only for flux penetration after zero-field cooling; a remanence is preserved after field cooling and decays with a finite relaxation rate. Activation energies calculated on the basis of the thermal activation model display a pronounced field dependence, and arelower for flux penetration than for flux expulsion in high fields (H3 T) for all orientations. This behavior of extreme layered superconductors contradicts classical theoretical models and questions the original definition of the irreversibility line as well. All of our results are consistent with the recent theory of lock-in transition, and can be well interpreted by using those principles.     

c-axis phonons and the enhancement of pairing correlations in high-temperature superconductors

We consider the two-dimensional Hubbard model including electron-phonon interaction. Strong local correlations (U limit) are taken into account within the mean-field approximation for auxiliary boson fields. Phonon-assisted transitions between intraand interlayer states are introduced as the source of coupling between two-dimensional CuO₂ layers. This type of processes effectively leads to the nonlinear (quadratic) interaction of intralayer electrons withc-axis phonons. We construct the Eliashberg equations for the resulting Hamiltonian and evaluate the superconducting transition temperatureT _c. Our model calculation demonstrates that a pronounced enhancement ofT _c in thed-wave channel is possible. The largest enhancement ofT _c tends to take place for small hole concentrations. This means that the coupling toc-axis phonons could compete with two-dimensional correlations responsible for the onset of antiferromagnetic order. It is remarkable that the two-dimensional features in the normal state are hardly affected by this specific interlayer interaction. Therefore,c-axis two-phonon-mediated interlayer coupling can cooperate with interlayer pair tunneling and substantially contribute to an increased pairing.     

Intergranular fluxons in high-Tcsuperconductors

The granular high-T _c superconductors can carry very low transport current, in comparison to that found in the bulk of the material. Magnetization and critical current measurements at very low field indicate that this low transport current behaves as expected from a critical state model. The presence of weak links between the grains in granular aggregates is firmly established, together with the Josephson character of such links. The existence of some kind of magnetic particles, and of a mechanism of pinning for them, is required to explain the critical state regime. In this paper we examine the flux structures which can be present in the granular systems, which can be described by an array of SQUIDs, and we show their similarities to fluxons in a continuous medium and to fluxons in uniform Josephson junctions. A simple model, based on a two-dimensional network of pointlike Josepson junctions, is adequate to demonstrate the existance of the IF (intergranular fluxon). Its characteristics depend on a single parameter, which gives the coupling strength between grains. The discreteness of the system is the cause of an intrinsic pinning of the intergranular fluxons.     

The flux-line lattice in high-Tc superconductors

The flux-line lattice in type-II superconductors has unusual nonlocal elastic properties which make it soft for short wavelengths of distortion. This softening is particularly pronounced in the highly anisotropic high-T _c superconductors (HTSC) where it leads to large thermal fluctuations and to thermally activated depinning of the Abrikosov vortices. Numerous transitions are predicted for these layered HTSC when the temperatureT, magnetic inductionB, or current densityJ are changed. In particular, the flux lines are now chains of two-dimensional (2D) pancake vortices which may evaporate by thermal fluctuations or may depin individually. At sufficiently highT, ohmic resistivity(T, B) is observed down toJ 0. This indicates that the flux lines are in a liquid state with no shear stiffness and with small depinning energy or that the 2D vortices can move independently. At lowerT, (T, B, J) is nonlinear since the pinning energy of an elastic vortex lattice or vortex glass increases with decreasingJ as predicted by theories of collective pinning and by vortex glass scaling.     

Superconductivity at higher temperatures in the Hg-Ba-Ca-Cu-O compound system

Our recent systematic examination of the pressure effect on high-temperature superconductors revealed that the highest achievable superconducting transition temperature (T _c) in the layered cuprates may lie between 150 and 180 K. We propose that the newly discovered Hg-Ba-Ca-Cu-O (HBCCO) compound system may be one of the most promising candidates for such a high-T _c superconductor, because of the possible large range of modulation doping associated with the linear oxygen coordination of divalent Hg in HBCCO. A record-high magnetically determined transition at 135 K with a zero resistivity at 134 K has been obtained by us in HgBa₂Ca₂Cu₃O₈₊ . TheT _c of HgBa₂Ca₂Cu₃O₈₊ was found to increase continuously with pressure at an increasing rate up to 17 kbar without any sign of saturation. The thermo-power shows an underdoped characteristic. These observations suggest that theT _c of HBCCO can be further enhanced with proper modulation doping without inducing any structural instabilities. The results together with the synthesis steps of HBCCO are summarized and discussed.     

Effects of flux flow, flux pinning, and flux creep upon the rf surface impedance of type-II superconductors

The presence of vortices is known to have an important effect upon the rf surface impedanceZ _s of type-II superconductors. In this paper we summarize a new theoretical approach that permits the influence of flux flow, flux pinning, and flux creep uponZ _s =R _s -iX _s to be calculated over a wide range of angular frequencies, magnetic inductionsB, and temperaturesT. The normal-fluid excitations and the nonlocality of vortex-generated fields are also accounted for. The physics is conveniently described in terms of a complex penetration depth(, B, T), to which the surface impedance is related viaZ _s = –i ₀(,B,T). We limit our attention to the linear response of the superconductor in the low-rf-power regime for which the surface impedance is power-independent.     

Photoemission spectroscopy of high-T c superconductors: Proofs in favor of the impurity mechanism

A new interpretation of photoemission spectroscopy data is proposed. It is based on the impurity mechanism of high-temperature superconductivity proposed earlier. The band observed near the Fermi level in this interpretation is the impurity band, and the observed large “Fermi surface” is the surface of the mobility edge, different in different directions. Many other details are also explained naturally.     

Synthesis and Physical Properties of the Hg-Based Superconductors with the 1212 Structure: Hg0.7 V0.3Sr2Ca1-xYxCu2O6+gd

A partial substitution of vanadium at the Hg site helps to stabilize the Sr analogue of the Hg-1212 phase. Simultaneous replacement of Ca by a rare earth ion is found necessary to form single-phase materials. Crystalline and nearly single-phase compounds were synthesized for x≥0.4 in the system Hg_0.7V_0.3Sr₂Ca{i−x}Y _x Cu₂O_6+δ. The lattice parameters show an increase in thea axis and a significant decrease in thec axis with increase inx. A maximumT _c of 85K was obtained for x= 0.4.T _c decreases with increase in Y concentration. However, all the compositions in the single-phase region show a broad superconducting transition. Annealing in vacuum and re-annealing in oxygen helps to improve the diamagnetic volume fraction, but the nature of the transition remained unaffected. Studies on the magnetizationhysteresis loops suggest a poor flux pinning nature of the compounds. Although the compounds have a large decrease in anisotropy along thec axis, partial occupancy of Y³⁺ in the Ca²⁺ layer adjacent to the CuO₂ conduction plane is detrimental to the flux pinning behavior. This study suggests that the presenceof defects in the proximity of the conduction plane in the high-T _c copper oxide system has an adverse effect on the superconducting properties.     

Low-temperature magnetic and electromagnetic studies of Y-Ba-Cu-O: Agx Ceramic superconductor

Magnetization and magnetic relaxation measurements have been studied at 4.2 K on YBa₂Cu₃O_7–:Ag _x (x=0 to 1.2) superconducting samples prepared by a coprecipritation technique. Remanent magnetization and hysteresis loops were found to be increased on Ag addition. Transport current density (J _c) was enhanced with the addition of Ag concentration. Flux creep rate was also increased with increase in Ag addition resulting in stronger pinning potential. Microwave-induced d.c. voltage measurements show a reduction of weak links with increase in Ag concentration. The enhancement of transportT _c is attributed to the stronger Josephson current paths due to the reduction of the total number of weak links after Ag addition.     

Magnetic properties of high-temperature superconductors: Analysis within the framework of the impurity mechanism

The impurity mechanism proposed earlier is used to interpret the magnetic properties of hightemperature superconductors. The appearance of superconductive granules and the existence of weak bonds in grains of ceramics and in single crystals is explained. The irreversibility line is obtained and its new interpretation as the line of thresholds of appearance of infinite nonsuperconductive cluster is given. The relaxation time of the magnetic susceptibility in alternating magnetic fields is calculated. The decrease of the Meissner fraction with increase in magnetic field and decrease in doping, and other magnetic properties are explained.     

Self-Consistent Calculation of the Self-Trapping Barrier for a Hole in the CuO2 Plane

We study the behavior of an extra hole added to the ground state of the antiferromagnetically ordered CuO₂ plane. Both the Cu--O hybridization and the O--O transfer are taken into account. As has been shown previously (V. Hizhnyakov and E. Sigmund, Physica C 156, 655 [1988]), the energetically most favorable situation is given when a localized state is formed in which the antiferromagnetic order is locally destroyed and a small ferromagnetic cluster is built up. We show that due to the increase of the magnetic energy induced by the spin–flip process (E _s0.15 eV), the localized and the metastable free-hole states are separated by a barrier of energy (E _b0.05 eV), which can reveal itself in various kinetic phenomena.     

Vortex Self-Organization in the Presence of Magnetic Pinning Arrays

Lateral nanostructuring is an efficient tool to control vortex confinement in superconductors. This will be illustrated by studying pinning phenomena in type-II superconducting Pb films with a lattice of submicron magnetic dots. We consider rectangular Co dots with in-plane magnetization and circular Co/Pt dots with out-of-plane magnetization. The domain structure of the Co dots can be changed from multi- to single-domain, resulting in an enhancement of their stray field. After covering this Co dot array with a Pb film, we demonstrate the influence of the local magnetic stray field of the dots on their flux pinning efficiency. The Co/Pt dots have a single-domain structure with their magnetic moment out of plane. Depending on the magnetic history, the magnetic moment of all dots can be aligned in positive or negative direction, or a random distribution of positive and negative magnetic moments of the dots can be achieved. For a Pb film covering this Co/Pt dot array, we observe an asymmetric magnetization loop due to the magnetic interactions between the vortices and the magnetic dots.     

s-Wave vs.d -Wave Superconductivity in the Two-Dimensional Hubbard Model

The anisotropic superconductivity has been considered in the two-dimensional Hubbard model. We allow for on and off-site intralayer Cooper pairs. Interlayer momentum-conserving Josephson tunneling which couples adjacent layers has been taken into account. The meanfield solution for the superconducting transition temperature demonstrates the competition between local Coulomb repulsion and interlayer tunneling in thes-wave channel. This competition leads to the mixedsd superconducting state at low doping. We also demonstrate thatc-phonon-assisted transitions between intra and interlayer states can effectively give rise to momentum-conserving Josephson tunneling.     

SQUID behavior at liquid nitrogen temperature in high-T c superconductors of the type Y-Ba-Cu-O

Quantum interference at liquid nitrogen temperature in a bulk sample of high-T _c superconductor YBa₂Cu₃O_9–y is described. The SQUID quantization loop is formed at random inside the sintered sample as an internal percolation loop with a Josephson weak link. The estimated values of this multiply connected quantization loop are: the inductance of the loopL _SQ4×10^–10 H, the critical currentI _c1×10^–6 A, the area of the loopA1×10^–8 m², and the hysteresis parameter _L 1. In the flux-locked loop regime the field sensitivity of this liquid nitrogen SQUID with a random loop is 10^–9 T/Hz^1/2 and the resolution is 5×10^–3 ₀/Hz^1/2.     

Estimation of intragrain magnetic flux motion viscosity in high-temperature superconductors

A simple method for evaluation of the lower limit of intragrain magnetic flux motion viscosity is presented. We have also shown that in our experiment the energy loss in an alternating magnetic field in a granular YBa₂Cu₃O_7–x superconductor prepared by the standard sintering method is determined by thermally assisted magnetic flux flow into superconducting grains and does not depend on the intergrain space.     

Study of the impact of HgO addition and low-field magnetic relaxation behaviour in granular high-T c superconductors

A series of high-T _c superconductors have been prepared with the HgO addition/substitution. Significant improvement in theT _conset as well asT _c0 was observed in all the cases. Substitution of Hg at the Sr site and Ba site in the case of (Bi, Pb)-Sr-Ca-Cu-O and Y-Ba-Cu-O systems, respectively over a range 0·01–0·6 at% helps in constructing an entire spectrum: improvement ofT _c0 up to 0·4 at % in the case of Bi-system and up to 0·03 at % in the case of Y123 system and slight drop inT _c thereafter. Such improvement is the result of abundant supply of highly reactive nascent oxygen all through the bulk. HgO decomposes and provides oxygen which helps in maintaining proper oxygen stoichiometry throughout the bulk. No Hg or Hg-based impure phases were observed in the X-ray diffraction spectra. Low-field (10–100 Oe) magnetic relaxation studies reveal faster relaxation of the intergranular critical state in the case of silver added samples as the grain boundary coupling energyE _J becomes quite uniform across the entire bulk which leads to smaller flux pinning energy. The distribution of the pinning energy is evaluated from the observed relaxation pattern and is found to be narrower in the case of silver added samples. It was also observed that the transportJ _c ∼ exp[−ΔT _c/T _c0] and the flux pinning energyU ∼ ΔT _c, where ΔT _c is the transition width and is a measure of the inhomogeneity within the sample. Such relationships may help in devising a strategy for achieving highJ _c, highU yet low ΔT _c. Silver addition turns out to be an effective tool in tailoring the sample properties depending on requirement.     

Bose condensation of the localized pairs in two-band superconductors

Superconductor with two singularities inherent in high-T_c materials, low carrier density and overlapping of the energy bands on the Fermi surface, is considered. Provided T=0, the order parameter ‡_n and chemical potential Μ are determined in the mean-field approximation for the state with Bose condensation of the localized pairs (Μ < 0). The equation for the energy of the bound state ε_b is also obtained and the relationship ε_b = 2Μ is established. The method of functional integration concerning the two-band model is developed. On the basis of this method, the crossover between the Fermi picture and the Bose one of the elementary excitations is demonstrated for the system in the presence of the two-particle bound state. The expression for the temperature of Bose condensationT _k is obtained and the contribution of the residual interaction between bosons for the systems with arbitrary dimension is determined.     

Microwave surface resistance of epitaxial YBa2Cu3O7 thin films at 18.7 GHz measured by a dielectric resonator technique

We used a dielectric resonator technique for highly sensitive measurements of the temperature dependence of the microwave surface resistanceR _s of 1×1 cm² superconducting films at 18.7 GHz. It consists of a sapphire disc positioned on the film under investigation within a copper cavity which is acting as a radiation shield. In the TE₀₁ oscillation mode the highly reproducible quality factor of about 10⁵ results in a sensitivity of ±50 forR _s measurements. The temperature dependence ofR _s can be measured up to values as high as 1 . We have investigated several YBa₂Cu₃O₇ thin films prepared by high oxygen pressure d.c. sputtering on LaAlO₃ and NdGaO₃. Our best films exhibit a pronounced nonlinear behavior of the d.c. resistivity(T) with(300K)/(100K) values of about 3.7. Those films show, besides the initial fall-off just belowT _c , a further strong decrease ofR _s at low temperatures. This was observed both at 18.7 GHz and 87 GHz, as measured by a conventional cavity end plate replacement technique. ForTT_c/2 these films exhibit an exp (–T _c/T) dependence ofR _s with-values around 0.4. These observations may be explained by a superconducting energy gap with 2/kT _c0.8 for charge carriers localized in the CuO chains for YBa₂Cu₃O₇.