Showing papers in "Journal of Superconductivity in 2000"

Spintronics, Electronics for the Next Millenium?

Abstract: Spintronics, in which the spin degree of freedom of the electron will play an important role in addition to or in place of the charge degree of freedom in mainstream electronics will be important as we start the new millenium. The prospects for this new electronics will be described in the article below.

Localized High-Tc Superconductivity on the Surface of Na-Doped WO3

Abstract: WO3 crystals were doped with Na on the surface to ∼7% nominal atomic concentration. Scanning tunneling microscopy and spectroscopy were employed, together with magnetization measurements, in the study of these crystals. Tunneling experiments reveal superconducting islands, 20–150 nm in size, covering about 10% of the surface, the rest of which is insulating. Magnetization measurements show that the superconducting phase formed at the surface has a critical temperature of 91 K, while tunneling spectroscopy yields superconducting gaps having values up to 2Δ/kT c ∼ 4. Presumably most of the sodium is concentrated in these islands and therefore they are metallic in nature, above 30% in atomic concentration. This material is therefore a noncuprate superconductor with a high critical temperature.

Polarons, Bipolarons, and Possible High-Tc Superconductivity in Metal-Ammonia Solutions

Abstract: We examine the nature of fluid metal-ammonia solutions with a special emphasis on the electronic structure and dynamics of polaronic and bipolaronic charge carriers. Importantly, we find that close to the compositionally-induced Nonmetal-to-Metal Transition in the fluid at low temperatures (ca. 240K), the vast majority (ca. 85% or above) of current carriers are highly mobile, diagmagnetic (S = 0) bipolarons. This raises the intriguing possibility, first proposed by R. A. Ogg in 1946, of a Bose–Einstein Condensation (BEC) of trapped electron pairs in vitreous, quenched metal-ammonia solutions. From a “modern” (2000) perspective we believe that there are important similarities to the situation in the crystalline layered cuprates, where we have argued elsewhere that High-T c superconductivity derives from the BEC of bipolarons in the electronically active CuO2 planes [A. S. Alexandrov and P. P. Edwards, Physica C 331, 97 (2000)]. We now propose that the search begins for high temperature superconductivity in quenched metal-ammonia and related solutions.

Photodoping Effects in High Critical Temperature Superconducting Films and Josephson Junctions

Abstract: Persistent photoconductivity is an interesting and unusual property of high temperature superconductors. Illumination of these compounds can lead to a change in doping that is persistent at low temperature and relaxes back to its initial value at elevated temperatures. This photodoping can give rise to an improvement of superconducting properties, which is contrary to the illumination effects in conventional superconductors. Furthermore, these photo-induced effects can also be used to probe and modify the properties of grain boundaries in grain boundary Josephson junctions. This paper reviews the experimental data and proposed theoretical models related to these photoinduced effects.

Optical Spectroscopy of Perovskite-Type Manganites

Abstract: Review is given on late optical studies on colossal magnetoresistance (CMR) manganites with perovskite or related structures. In La1−x Sr x MnO3 which undergoes a ferromagnetic transition, optical conductivity spectrum σ(ω) shows a change from a gap-like feature to metallic but highly diffussive one with increasing spin-polarization. Such a remarkable spectral weight transfer is also observed in a layered manganite, La2−2x Sr1+2x Mn2O7 (x = 0.4), with a mid-infrared peak structure due to inter-orbital $$(d_{x^2 - y^2 } \to d_{3z^2 - r^2 } )$$ transition. In Pr1−x Ca x MnO3 (x = 0.4) which undergoes the charge ordering phase transition, σ(ω) shows an anisotropic feature reflecting charge/orbital ordering pattern at 10 K. The charge-ordered state is transformed into a ferromagnetic metallic state by a magnetic field of 6.5 T at 30 K, which is manifested in a huge change of optical spectra over a wide photon-energy region (0.05 eV–3 eV).

NMR Evidence for Spatial Modulations in the Cuprates

Abstract: Nuclear magnetic resonance (NMR) data on Cu, apical and planar O in La1.85Sr0.15CuO4 are presented. Spin echo double resonance shows that the large Cu magnetic shift distribution is of short-length scale. Analysis of the O data reveals static modulations of the spin susceptibility with a spin–spin correlation function near zero. The Cu shift distribution is found to be of orbital origin. The full planar oxygen spectra show a correlated modulation of the electric field gradient with the spin susceptibility. Similar results on other cuprates indicate universality of these phenomena.

Levitation Force Between a Point Magnetic Dipole and Superconducting Sphere

Abstract: The magnetic interaction between a point dipole and a superconducting sphere in the Meissner state is investigated in detail. It is demonstrated that the special case of perfect flux exclusion can be solved by the method of images when the dipole is vertically magnetized. The incorporation of a nonzero penetration depth is then made by developing the solution within London theory. The analytic solution, given in terms of an infinite series representation, is refined by exactly summing the perfectly diamagnetic response. Throughout, special cases of physical interest are discussed and applications to magnetic force microscopy and levitation are described.

Electrical and thermal transport in perovskite manganites

Abstract: Experimental data on the temperature, magnetic field, and pressure dependencies of charge and heat transport in perovskite manganites, A 1−x A′ x MnO3, are reviewed. The transport mechanisms in the paramagnetic, ferromagnetic and charge-ordered states are discussed and the role of double-exchange interactions and lattice-distortions emphasized.

Phase Separation in a Two-Component Model for Cuprates

Abstract: In a brief review the role of interactions in a two-component model for electronic spectrum in cuprates is discussed. Interactions in the model result in a tendency to phase separation. It is speculated that the latter determines the scale of the “pseudogap” temperature, T*, while hybridization between localized and itinerant components moderates this tendency to the first-order transition and brings about isotope mass dependence through polaronic effects.

Tabulations and Correlations of Transition Temperatures of Classical Superconductors

Abstract: Superconducting transition temperatures Tc are tabulated for about 800 classical compounds classified by their structure type. Some general trends in the data such as the correlation of Tc values with the number of valence electrons are commented upon.

HTS Surface Wave Resonators

Abstract: A novel class of surface wave high-temperature superconductivity (HTS) resonators was investigated. The resonator consists of HTS film and one or two dielectric plates. One of the dielectric plates may be the HTS film substrate. Theoretical analysis of the resonator structure was carried out using the partial region method, with the 20 lowest waves being considered in every region. The resonant frequencies, quality of oscillations, and the field and current distributions were calculated. It was shown that there is a high-density and homogeneous microwave current flowing on the film surfaces. Experimental measurements carried out in the 3-cm wavelength range demonstrate good agreement with simulated data.

Effects of Precursors with Fine BaZrO3 Inclusions on the Growth and Microstructure of Textured YBCO

Abstract: We prepared precursors with fine BaZrO3 inclusions for melt-texture growth of high-temperature superconductors with nominal composition of Y1.5Ba2Cu3Oy + x BaZrO3 (x = 0, 0.04, 0.08, and 0.1), by the process of liquid-phase powder melt-texture growth (LPMG). We then investigated the effects of the precursors on the texture growth and the microstructure of composite YBa2Cu3O7-δ/Y2BaCuO5 (YBCO). The BaZrO3 particles were about 100 nm in size in the textured YBa2Cu3O7-δ (Y123), observed by the scanning electron microscopy (SEM). Owing to the pushing effects of growing fronts, fine particles were pushed out of and thus gathered in front of the growing fronts or between Y123 grains. The gathered particles in turn blocked the further growth of the Y123 grains. However, BaZrO3 particles seeded Y123 grains, giving rise to multigrain growths. These negative effects of BaZrO3 fine particles exclude themselves as effective flux pinning centers in the textured YBCO.

Lattice Effects on Charge Localization in Cuprates

Abstract: Recent experimental results on cuprates and manganites, including those of elastic and inelastic neutron scattering measurements, suggest that charges are not homogeneously distributed even in the metallic state in these compounds. Charge inhomogeneity results from spin/lattice charge constriction. In cuprates the LO phonons strongly reflect the temperature- and composition-dependent charge inhomogeneity and may possibly be involved in causing it. Unlike the static stripes that compete against superconductivity, the charge inhomogeneity seen by the LO phonons is markedly increased in the superconducting phase. A new mechanism of high-temperature superconductivity involving lattice/spin charge constriction is proposed.

Spin Dynamics in the Ferromagnetic Phase of Colossal Magnetoresistive Oxides

Abstract: A review is given that focuses on the spin dynamics in the ferromagnetic regime of the magnetoresistive oxides. At small wave vectors the quadratic dispersion relation is remarkably isotropic throughout the composition range, with a gap that is too small to measure with conventional neutron scattering techniques. At larger wave vectors the spin waves are strongly damped in the ground state, in contrast to expectations based on a simple half-metallic band structure, while the temperature dependence of the damping and renormalization of the spin waves is anomalous. An unusual spin-diffusion component to the fluctuation spectrum develops as the Curie temperature is approached, which appears to be related to the formation of polarons in the system and the consequent charge localization. Near and above TC the diffuse scattering from polarons is directly observed, as well as the correlations between the polarons.

Role of calcium in the evolution of superconductivity in a (La2-xRx)Ba2(CayCu4+y)Oz (R = Y, Er, Gd) system

Abstract: It has been reported that, by adding equal amounts of CaO and CuO to non superconducting La3Ba3Cu6O z (La-336), a series of superconductors with nominal compositions La3Ca y Ba3Cu6+yO z were prepared with maximum Tonc ∼ 80K. Similar studies on addition of CaO and CuO in nonsuperconducting LaBaCu2O z (La-112) resulted into superconducting LaCaBaCu3O z (La-1113). To date no attempt has been made to synthesize La2CaBa2Cu5O z (La-2125) superconducting phase by addition of CaO and CuO to non superconducting La2Ba2Cu4O z (La-224) system. Also no reports are published to study the effect of replacing larger La3+-ions (1.01A) by smaller rare earth ions viz Y3+(0.89A), Er3+(0.91A), Gd3+(0.91A) on the structural and superconducting properties of (La2−xR x )Ba2(Ca y Cu4+y)O z (LRBCaC); 0.0 ≤ x ≤ 0.5; y=2x system. In this paper, we report the method of synthesis, structural and superconducting property characterization using X-ray diffraction, oxygen content measurements using iodometry, resistivity measurements using d.c. four probe technique and a.c. susceptibility measurements in the temperature range RT to 15K. Also a comparative study, on the evolution of superconducting phase with Ca-concentration for different rare earth substitutions for LRBCaC system in the context of hole doping mechanism, is carried out.

Semiclassical Theory of Spin-Dependent Transport in Magnetic Multilayers

Abstract: The semiclassical Boltzmann equation is applied to spin-dependent transport in magnetic multilayers. The origin of the giant magnetoresistance effect is explained for both the case in which the current flows parallel to the layers and the case in which it is perpendicular to the layers. The approach is first-principles based and includes effects of vertex corrections and local fields.

Paramagnetic Meissner Effect

Abstract: The explanation for the paramagnetic Meissner effect (PME) in high-temperature superconductors (HTSC) and niobium disks, distinct from previous discussions, is proposed. It is based on the impurity mechanism of high-temperature superconductivity and the general localization approach to the superconductivity problem. Turns of the local magnetic moments of the two-level systems are considered to be the cause of this effect in HTSC. The estimations of the magnetic moment magnitude and the magnetic moment concentration are given. In niobium disks, this effect is also explained by turns of the local magnetic moments, but of another nature. The role of the layered structure is demonstrated. It is shown that the proposed models can explain all available experimental facts. The connection between this effect and the electric field effect in HTSC is discussed.

Structural Effects in Magnetoresistive Manganites and Mechanism of Metal-Insulator Transition

Abstract: Colossal magnetoresistivity (CMR) is caused by a magnetic field-induced insulator-to-metal transition. It is arguably one of the most dramatic phenomena that occur in solids due to competing forces in a complex system. In the CMR oxides the electron-phonon coupling and spin correlations that favor charge localization are competing against the electron kinetic energy and lattice elasticity that prefer charge delocalization. The CMR phenomenon occurs at the crossover point of charge-localized (insulating) and charge-delocalized (metallic) states, where the system is particularly susceptible to external stimuli such as a magnetic field. The competitions among these forces are usually considered globally as volume averages. However, since the system is complex and the phenomenon is non-linear, local fluctuations dominate the behavior of the system near the critical point. Therefore the system would appear spatially inhomogeneous if the measurement is made with certain time and length scales. To characterize and understand such a system it is required to deploy local approaches in which the competing interactions are evaluated locally. Experimentally, the pulsed neutron pair-density function (PDF) analysis is one of the methods of local structural study. Using the PDF technique it is suggested that the phase transition occurs via local and percolative processes. Also through the analysis of the local structure it is shown that the ionic size effect on the CMR phenomenon is not caused by a change in the band width as is usually assumed, but is due to local structural changes that affect polaron stability. The critical ionic size determined by this approach is in excellent agreement with experimental observations. The PDF results also indicate that the local structure of layered manganites is close to that of perovskite, suggesting that they share common elements of the CMR mechanisms.

A Critical Examination of the Spin Dynamics in High-Tc Cuprates

Abstract: A critical examination of the spin dynamics in high-T c cuprates is made in the light of recent inelastic neutron scattering results obtained by different groups. The neutron data show that incommensurate magnetic peaks in YBCO belong to the same excitation as the resonance peak observed at (π/a, π/a). Being observed only in the superconducting state, the incommensurability is rather difficult to reconcile with a stripe picture. We also discuss the link between the resonance peak spectral weight and the superconducting condensation energy.

Finite-Wave-Vector Phonon Coupling to Degenerate Electronic States in La2−xSrxCuO4

Abstract: We consider finite-k electron–phonon coupling appropriate for a superconductor with a very short coherence length and local intersite pairs From group theoretical analysis we find that k ≠ 0 phonons can couple to degenerate electronic states in La2− x Sr x CuO4 and a Jahn-Teller-like distortion is found to be operative for E g- and E u-symmetry O states Experimentally observed anomalies in inelastic neutron scattering, ESR, and EXAFS are found to be consistent with such an interaction of the τ1 symmetry Σ-point mode with the planar O P x and P y states The proposed interaction is found to naturally facilitate symmetry breaking associated with pairing and/or stripe formation

Photoconductivity in Manganites

Abstract: We report two types of photoconductivity effects observed by illumination of oxygen deficient manganites thin films with UV or visible light. One is the persistent photoconductivity effect observed at low temperature (T < 30 K) when the thin film is in the metallic state. This effect is analog to the persistent photoconductivity observed in the high T c superconductors particularly in oxygen deficient YBaCuO thin films. The other type of effect is a photo-induced transition to metallic state at temperatures T < 100 K observed in Pr2/3Sr1/3MnO3 thin film which stays always in the semiconducting state in the darkness. This photo induced metallic transition from a semiconducting state leads to a non persistent colossal photoconductivity (several orders of magnitude of decrease of the resistivity).

Microscopic Charge Inhomogeneities in Underdoped La2−xSrxCuO4: Local Structural Evidence

Abstract: We present local structural evidence for the existence of charge inhomogeneities at low temperature in underdoped and optimally doped La2−x Sr x CuO4. The inhomogeneities disappear for x ≥ 0.2. The evidence for the charge inhomogeneities comes from an anomalous increase in the inplane Cu–O bond length distribution in the underdoped samples as well as evidence for CuO6 octahedral tilt inhomogeneities in the intermediate range structure. Preliminary analysis of the temperature dependence of this phenomenon indicates that the inhomogeneities set in at temperatures in the range 60 K < T co < 130 K, which depends on doping.

Doping and Isotope Dependence of the Pseudogap in High-Tc Cuprates Observed by Neutron Crystal-Field Spectroscopy: A Fast Local Probe

Abstract: The temperature dependence of the relaxation rate of crystal-field excitations in the high-Tc superconductors HoBa2Cu4O8 and HoBa2Cu3Ox (6.4 Tc in the underdoped regime as well as for slightly overdoped samples. For HoBa2Cu4O8T* increases from 170 to 220 K upon oxygen isotope substitution (16O vs 18O). This huge isotope shift (which is absent in NMR and NQR experiments) suggests that the mechanism leading to an isotope effect on the pseudogap has to involve a time scale in the range 10−8 ≫ τ > 10−13 s.

Correlation Between Tc and Crystal Chemical Parameters of Cation Sublattice An+1(Cu)n in Perovskite Layer of High-Tc Cuprate Superconductors

Abstract: It was found the empirical dependence of T c of high-temperature superconducting cuprates (HTSC) from the ratio (J) of such parameters as the distances between Cu atoms in CuO2 plane and the distances from CuO2 plane to adjacent ones of A cations (A − Ca, Sr, Ba, Y, La, and so on), also from the size and charge of A cations and doping atoms with effect on T c is proved experimentally. All HTSC phases are divided only on two groups with an intrinsic dependence T c (J): the phases formed by single CuO2 plane and by several CuO2 planes. The closer approximation of that dependence gives the equation of polynomial of third degree.

Limits on Superconductivity in Nanoparticles and Nanowires

Abstract: Recent experimental results are consistent with the prediction that superconductivity is suppressed in aluminum nanoparticles which are so small (∼10 nm diameter) that the electronic energy level spacing exceeds the superconducting energy gap in bulk material. Very recent experiments on nanowires (diameter ∼5–10 nm, length ∼150 nm) of MoGe indicate that a dissipative phase transition between superconducting and normal behavior occurs when the normal resistance of the wire equals the superconducting quantum resistance h/4e2 ≈ 6.5 kΩ.

Competing and Cooperative Effects of Spin, Charge, and Phonon Degrees of Freedom in HTSC

Abstract: The phase diagram of high-Tc superconducting copper oxides (HTSC) exhibits an unusual complexity which is, at present, beyond any microscopic modeling. Motivated by very recent experimental findings—(i) a giant isotope effect on T* and (ii) doping-dependent phonon anomalies and “extra” modes—the role played by the lattice is reinvestigated and the consequences arising for the striped phase are discussed. By including nonlinear higher-order phonon–phonon and density–density interactions in the Hamiltonian, an isotope effect on T* is obtained, as well as substantial q-dependent phonon anomalies, where both effects are strongly doping dependent.

Analysis of the Pairing Interaction in the Cuprates

Abstract: Thermal difference reflectance (TDR) spectra taken on a large number of superconducting cuprates has enabled us to determine the energy dependence and strength of the pairing interaction in each. All show strong contributions from the phonons and a smaller, but significant contribution from an electronic transition near 1.7 eV. Recent improvements in the signal-to-noise ratio have revealed that the electronic excitation is accompanied in all cases by a weaker companion about 0.5 eV lower in energy. No other contributions are found. We identify these transitions as the d z2 to d x2–y2 excitations of the Cu ion in the 3d 9 and 3d 8 states, respectively, and have calculated the coupling strength mediated by each. The calculated values agree in order of magnitude with the observed strengths. We find that, in addition to the direct electronic coupling that corresponds to an electron–phonon-like term, the overlap between the oxygen and the copper orbitals leads to an exchange term. The direct term couples only to an s-wave gap, as for phonons, while the exchange term couples s-wave to d-wave, and vice versa. We discuss the consequences of this result.

Observation of Extremely Large, Field-Dependent Diamagnetism at 300 K in Certain Disordered Organic Materials

Abstract: Samples of some nonconjugated polymers exhibiting unusual conducting properties reveal huge, field-dependent diamagnetism at 300 K if concentration of charge carriers exceeds certain threshold about 2′1018 cm−3. Data obtained are consistent with theory of local electronic nanostructures that combine ferromagnetic ground state with some features of superconductors.

Metal-Insulator Transition in Doped Systems as a Percolation Phenomenon

Abstract: The percolative picture for the metal-insulator transition in doped materials is discussed. We focus on various properties of manganites. The approach has now a strong experimental support. The percolative phenomenon implies the presence of intrinsic inhomogeneity, but, nevertheless, the picture is different from that of the electronic phase separation.

Where Are We in HTCS

Abstract: Address given at the closing of the conference on “Major Trends in Superconductivity in the New Millennium,” MTSC 2000 and the opening of the “Symposium on Itinerant and Localized States in HTCS” SILS in HTCS.