high-Tc Copper Oxide Superconductors Research Articles

Collective spin and charge excitations in the t-J-U model of high-[formula omitted] cuprates

The t-J-U model of high-Tc copper-oxide superconductors incorporates both the on-site Coulomb repulsion and kinetic exchange interaction and yields a semi-quantitative description of the static properties of those materials. We extend this analysis to dynamic quantities and address collective spin- and charge excitations in the correlated metallic state of the t-J-U model. We employ VWF+1/Nf approach that combines the variational wave function (VWF) approach with the expansion in the inverse number of fermionic flavors (1/Nf). It is shown that the resonant (paramagnon) contribution to the dynamic magnetic susceptibility remains robust as one interpolates between the Hubbard- and t-J-model limits, whereas the incoherent continuum undergoes substantial renormalization. Energy of the collective charge mode diminishes as the strong-coupling limit is approached. We also introduce the concept of effective kinetic exchange interaction that allows for a unified interpretation of magnetic dynamics in the Hubbard, t-J, and t-J-U models. The results are discussed in the context of recent resonant inelastic x-ray scattering experiments for the high-Tc cuprates.

Spectroscopic fingerprint of charge order melting driven by quantum fluctuations in a cuprate

Copper oxide high-TC superconductors possess a number of exotic orders that coexist with or are proximal to superconductivity. Quantum fluctuations associated with these orders may account for the unusual characteristics of the normal state, and possibly affect the superconductivity1–4. Yet, spectroscopic evidence for such quantum fluctuations remains elusive. Here, we use resonant inelastic X-ray scattering to reveal spectroscopic evidence of fluctuations associated with a charge order5–14 in nearly optimally doped Bi2Sr2CaCu2O8+δ. In the superconducting state, while the quasielastic charge order signal decreases with temperature, the interplay between charge order fluctuations and bond-stretching phonons in the form of a Fano-like interference increases, an observation that is incompatible with expectations for competing orders. Invoking general principles, we argue that this behaviour reflects the properties of a dissipative system near an order–disorder quantum critical point, where the dissipation varies with the opening of the pseudogap and superconducting gap at low temperatures, leading to the proliferation of quantum critical fluctuations, which melt charge order. X-ray scattering experiments show that the quantum fluctuations associated with charge order take a form that is incompatible with the idea of competition between charge order and superconductivity.

Thermoelectric power of (Cu0.5Tl0.5)-1223 superconducting phase added with BaSnO3 nanoparticles

In this study, we report the thermoelectric power (TEP) measurements of Cu0.5Tl0.5Ba2Ca2Cu3O10-δ added with BaSnO3 nanoparticles. BaSnO3 nanoparticles were prepared by chemical co-precipitation method, while (BaSnO3)x/(CuT1)-1223 superconducting samples with 0.00 ≤ x ≤ 1.50 wt% were prepared using the solid-state reaction method. The standard four-probe technique was applied to measure DC electrical resistivity in the temperature range from 300 to 77 K. Superconducting transition temperature (Tc) increases up to 117.5 for x = 0.25 wt.% and then it decreases with further x addition. The TEP coefficient was measured as a function of temperature in a wide temperature range from ∼77 K (liquid nitrogen temperature) up to 280 K using a standard differential technique. The behavior of the obtained TEP coefficient is suit with high-Tc copper-oxide superconductors. The results were investigated according to two-band model with an extra linear term. Several parameters such as the pseudo-gap temperature (T*), Fermi energy (EF) and Fermi temperature (TF) values were calculated and discussed in terms of nanoparticles BaSnO3 addition.

A brief analysis of annealing process for electron-doped cuprate superconductors

The high-Tc copper-oxide superconductors (cuprates) break the limit of superconducting transition temperature predicted by the BCS theory based on electron-phonon coupling, and thus it opens a new chapter in the superconductivity field. According to the valence of substitutents, the cuprates could be categorized into electron-and hole-doped types. So far, an enormous number of high-Tc cuprate superconductors have been intensively studied, most of them are hole-doped. In comparison with the hole-doped cuprates, the advantages of electron-doped cuprates (e.g. lower upper critical field, less-debated origin of “pseudogap”, etc.) make this family of compounds more suitable for unveiling the ground states. However, the difficulties in sample syntheses prevent a profound research in last several decades, in which the role of annealing process during sample preparation has been a big challenge. In this review article, a brief comparison between the electron-doped cuprates and the hole-doped counterparts is made from the aspect of electronic phase diagram, so as to point out the necessity of intensive work on the electron-doped cuprates. Since the electronic properties are highly sensitive to the oxygen content of the sample, the annealing process in sample preparation, which varies the oxygen content, turns out to be a key issue in constructing the phase diagram. Meanwhile, the distinction between electron-and hole-doped cuprates is also manifested in their lattice structures. It has been approved that the stability of the superconducting phase of electron-doped cuprates depends on the tolerance factor t (affected by dopants) doping concentration, temperature, and oxygen position. Yet it is known that the annealing process can vary the oxygen content as well as its position, the details how to adjust oxygen remain unclear. Recently, the experiment on Pr2-xCexCuO4-δ suggests that the oxygen position can be tuned by pressure. And, our new results on [La1.9Ce0.1CuO4-δ/SrCoO3-δ]N superlattices indicate that more factors, like strain, should be taken into account. In addition, the superconductivity in the parent compounds of electron-doped cuprates has emerged by employing a so-called “protective annealing” process. Compared to the traditional one-step annealing process, this new procedure contains an extra annealing step at higher temperature at partial oxygen pressure. In consideration of the new discoveries, as well as the Tc enhancement observed in multilayered structures of electron-doped cuprates by traditional annealing, a promising explanation based on the idea of repairing the oxygen defects in copper oxide planes is proposed for the superconductivity in parent compounds. Finally, we expect a comprehensive understanding of the annealing process, especially the factors such as atmosphere, temperature, and strain, which are not only related to the sample quality, but also to a precise phase diagram of the electron-doped cuprates.

Tunneling spectra of break junctions involving Nb3Sn

The electronic gap structure of Nb3Sn was measured by the break-junction (BJ) tunneling technique. The superconducting gap values are estimated to be in the range 2Δ = 4–5.5 meV at T = 4.2 K as follows from the observed distinct conductance peaks. In addition to the superconducting gap structure, we observed reproducible hump-like structures at the biases of about ±20 and ±50 mV. Such a coexistence of gap and hump structures resembles the situation found in the high-Tc copper-oxide superconductors. Above the superconducting critical temperature Tc ∼ 18 K, the humps appear as the only gap-like structures. Their possible origin is discussed in connection to the structural phase transition occurring in Nb3Sn.

Characteristics of the Oxygen Function in High Tc Copper Oxide Superconductors

Characteristics of the Oxygen Function in High Tc Copper Oxide Superconductors

Introduction to the Special Issue—Selected Contributions to the 4th International Conference on Fundamental Problems of High Temperature Superconductivity (FPS’11)

The problem of High Temperature Superconductivity remains highly topical: quite regularly novel HTS materials come on stage (copper oxide high-Tc superconductors in 1986, magnesium diboride in 2001, iron pnictide and iron chalcogenide compounds in 2008). Achieving progressively higher superconducting transition temperatures remains an encouraging motivation for researchers in the field. Up to now, Tc is the highest in copper-oxide perovskite materials with values higher than 150 K achieved in the 1980s and 1990s. Nevertheless, a commonly accepted approach to the problem of high temperature superconductivity is still missing. Discovery of superconductivity in iron pnictides with Tc’s up to 56 K introduced a new family of high-Tc superconductors. Their study and comparison with copper oxides provides an excellent chance to reveal the mechanism of high-Tc superconductivity in general, and in the two families, in particular. Cuprate superconductors are strongly correlated materials. The correlations lead to the anomalous properties of cuprates in the normal state and to the inapplicability of the Fermi-liquid model to their normal properties. In contrast, the iron-based pnictides in the superconducting state show signatures of weak rather than strong coupling. In the normal state, they seem to behave in accord with the Fermi liquid model. Contrary to cuprates, where an effective one-band electronic conduction model is sufficient to describe the low-energy electronic excitations, in the Fe-based compounds at least four bands contribute to superconduc-

Response of a Doped Mott Insulator to a Local Charge: Implications for Muon Spin Resonance

A charged impurity (such as a muon) in a metal induces a charge inhomogeneity which screens the electric field of the impurity and may also cause the spin fluctuation spectrum near the impurity to differ from that of thebulk. Doped Mott insulators (such as the high Tc copper-oxide superconductors are believed to be) present a particularly delicate case: the Mott correlations suppress the charge response, but also increasethe sensitivityof magnetic propertiesto changesinthe charge density. Here we summarize the implications of recent dynamical mean field calculations for this physics.

Electronic inhomogeneity in a Kondo lattice

Inhomogeneous electronic states resulting from entangled spin, charge, and lattice degrees of freedom are hallmarks of strongly correlated electron materials; such behavior has been observed in many classes of d -electron materials, including the high- T c copper-oxide superconductors, manganites, and most recently the iron–pnictide superconductors. The complexity generated by competing phases in these materials constitutes a considerable theoretical challenge—one that still defies a complete description. Here, we report a manifestation of electronic inhomogeneity in a strongly correlated f -electron system, using CeCoIn 5 as an example. A thermodynamic analysis of its superconductivity, combined with nuclear quadrupole resonance measurements, shows that nonmagnetic impurities (Y, La, Yb, Th, Hg, and Sn) locally suppress unconventional superconductivity, generating an inhomogeneous electronic “Swiss cheese” due to disrupted periodicity of the Kondo lattice. Our analysis may be generalized to include related systems, suggesting that electronic inhomogeneity should be considered broadly in Kondo lattice materials.

Publisher's Note: Loss of antinodal coherence with a singled-wave superconducting gap leads to two energy scales for underdoped cuprate superconductors [Phys. Rev. B82, 144516 (2010)

Received 19 November 2010DOI:https://doi.org/10.1103/PhysRevB.82.189903©2010 American Physical Society

Coexistence of Fermi arcs and Fermi pockets in a high-Tc copper oxide superconductor

In the pseudogap state of the high-transition-temperature (high-T(c)) copper oxide superconductors, angle-resolved photoemission (ARPES) measurements have seen Fermi arcs-that is, open-ended gapless sections in the large Fermi surface-rather than a closed loop expected of an ordinary metal. This is all the more puzzling because Fermi pockets (small closed Fermi surface features) have been suggested by recent quantum oscillation measurements. The Fermi arcs cannot be understood in terms of existing theories, although there is a solution in the form of conventional Fermi surface pockets associated with competing order, but with a back side that is for detailed reasons invisible to photoemission probes. Here we report ARPES measurements of Bi(2)Sr(2-x)La(x)CuO(6+delta) (La-Bi2201) that reveal Fermi pockets. The charge carriers in the pockets are holes, and the pockets show an unusual dependence on doping: they exist in underdoped but not overdoped samples. A surprise is that these Fermi pockets appear to coexist with the Fermi arcs. This coexistence has not been expected theoretically.

Competition between the pseudogap and superconductivity in the high-Tc copper oxides

In a classical Bardeen-Cooper-Schrieffer superconductor, pairing and coherence of electrons are established simultaneously below the critical transition temperature (T(c)), giving rise to a gap in the electronic energy spectrum. In the high-T(c) copper oxide superconductors, however, a pseudogap extends above T(c). The relationship between the pseudogap and superconductivity is one of the central issues in this field. Spectral gaps arising from pairing precursors are qualitatively similar to those caused by competing electronic states, rendering a standard approach to their analysis inconclusive. The issue can be settled, however, by studying the correlation between the weights associated with the pseudogap and superconductivity spectral features. Here we report a study of two spectral weights using angle-resolved photoemission spectroscopy. The weight of the superconducting coherent peak increases away from the node following the trend of the superconducting gap, but starts to decrease in the antinodal region. This striking non-monotonicity reveals the presence of a competing state. We demonstrate a direct correlation, for different values of momenta and doping, between the loss in the low-energy spectral weight arising from the opening of the pseudogap and a decrease in the spectral weight associated with superconductivity. We therefore conclude that the pseudogap competes with the superconductivity by depleting the spectral weight available for pairing.

The Progress on Study of Isotope Effects in High T<sub>C</sub> Copper Oxide Superconductors

Through the review of the correlation on the isotope effects research and superconductivity mechanism research as well as the progress on the study of the isotope effects in the high Tc copper oxide superconductors, we have found the fundamental factors that cause the different changes of the isotope effect value in different dope region in high Tc cupper oxide superconductors. We point the high-temperature superconductivity mechanism possible origin phonons mediation electrons.

FUNCTIONS OF OVERSTEP CHEMISTRY CALCULATION OXYGEN IN HIGH TcCOPPER OXIDE SUPERCONDUCTORS

This paper studies the functions of overstep chemistry calculation oxygen δ in high Tccopper oxide superconductors and analyzes the relative experiments' results. It is believed that the function of overstep chemistry calculation oxygen in Y series high Tccopper oxide superconductors is to adjust the change of effective electric charge P on the [Cu-O]plink or [Cu-O]Pplane and this affects the bonding intensity on the CuO2plane, which in turn, adjusts the Y series high oxide superconductor's Tc.

CHARACTERISTICS OF ISOTOPE EFFECT AND DOUBLE-BODY STRUCTURE MODEL

The characteristics of the oxygen-isotope effect (16 O /18 O ) are studied with the double-body structure physical model in high-Tc copper oxide superconductors. With the double-body structure physical model, some reasonable explanations are given to the phenomena of high-Tc copper oxide superconductor's isotope effect, weak isotope effect, and abnormal isotope effect.

Point-defects-induced vortex phase diagram in high-Tc superconductors: Monte Carlo simulation study

We investigated the vortex phase diagram of high-Tc copper-oxide superconductors in the presence of dense point defects (PDs), by using a Monte Carlo simulation based on the Lawrence–Doniach model. We found a temperature-driven depinning line within the Bragg and vortex glass phases, well below the melting line in the case of high anisotropy. We also found a field-driven transition line from the Bragg-to-vortex glass at low temperatures, accompanied by an abrupt reduction in the interlayer vortex correlation. The complex phase boundaries are drastically controlled by the pinning force of PDs, defect density, and anisotropy.

Microscopic electronic inhomogeneity in the high-Tc superconductor Bi2Sr2CaCu2O8+x.

The parent compounds of the copper oxide high-transition-temperature (high-Tc) superconductors are unusual insulators (so-called Mott insulators). Superconductivity arises when they are 'doped' away from stoichiometry. For the compound Bi2Sr2CaCu2O8+x, doping is achieved by adding extra oxygen atoms, which introduce positive charge carriers ('holes') into the CuO2 planes where the superconductivity is believed to originate. Aside from providing the charge carriers, the role of the oxygen dopants is not well understood, nor is it clear how the charge carriers are distributed on the planes. Many models of high-Tc superconductivity accordingly assume that the introduced carriers are distributed uniformly, leading to an electronically homogeneous system as in ordinary metals. Here we report the presence of an electronic inhomogeneity in Bi2Sr2CaCu2O8+x, on the basis of observations using scanning tunnelling microscopy and spectroscopy. The inhomogeneity is manifested as spatial variations in both the local density of states spectrum and the superconducting energy gap. These variations are correlated spatially and vary on the surprisingly short length scale of approximately 14 A. Our analysis suggests that this inhomogeneity is a consequence of proximity to a Mott insulator resulting in poor screening of the charge potentials associated with the oxygen ions left in the BiO plane after doping, and is indicative of the local nature of the superconducting state.

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.

An Equation of State for High-Temperature Superconductors

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 the pressure derivatives of the bulk modulus atP = 0 and the values of the 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 quantityAU/U( V 0 ) of these compounds increases with increasing hydrostatic pressure.

Polar Jahn-Teller Centers and isotope effect in copper oxide high-T c-superconductors

The CuO 4-cluster based copper oxides are considered as generalized quantum lattice bose-gas or a system of the local singlet bosons moving in a lattice of the hole Jahn-Teller centers [CuO 4 5−] JT. The model is illustrated by the qualitative and quantitative description of the various peculiarities of an isotope shift (IS-) effect in a series of 123-like oxides.