High Temperature Superconductor Cuprate Research Articles

Influence of various scattering mechanisms on thermal conductivity of doped Pr1.3−xLa0.7CexCuO4 high-temperature superconductor

A theoretical analysis of thermal conductivity of high-temperature superconductor cuprates, Pr1.3−xLa0.7CexCuO4 (PLCCO), has been successfully investigated. In this formulation, the idea of relaxation times and other scattering processes has been developed from frequency (energy) line widths using a quantum dynamical method. The major emphasis of this study is on the influence of phonon-dopant atom scattering on the thermal conductivity of Ce-doped PLCCO, as well as other prominent scattering processes such as electron-phonon, cubic and quartic anharmonic phonon, cubic and quartic phonon interference, and so on. The proposed theory has achieved a satisfactory agreement between theory and experiment.

Synthesis of La2−xSrxCuO4 films via atomic layer-by-layer molecular beam epitaxy

Atomic layer-by-layer molecular beam epitaxy (ALL-MBE) is a sophisticated technique to synthesize high-temperature superconductor (HTS) materials. ALL-MBE produces single-crystal HTS films with atomically smooth surfaces and interfaces, as well as precise multilayer heterostructures engineered down to a single atomic layer level. This enables the fabrication of tunnel junctions, nanowires, nanorings, and other HTS devices of interest. Our group has focused on ALL-MBE synthesis and materials science of La2−xSrxCuO4 (LSCO), a representative HTS cuprate. In the past two decades, we have synthesized over three thousand LSCO thin films and characterized them by a range of analytical techniques. Here, we present in full detail a systematic process for the synthesis and engineering of atomically perfect LSCO films. The procedure includes the preparation of substrates, calibration of the elemental sources, the recipe for ALL growth of LSCO films without any secondary-phase precipitates, post-growth annealing of the films, and ex situ film characterization. This report should aid replication and dissemination of this technique of synthesizing single-crystal LSCO films for basic research as well as for HTS electronic applications.

Quantum Dynamical Approach to Various Scattering Mechanisms and Their Influences on Thermal Conductivity of Sr- and Zn-Doped La2CuO4 High-Temperature Superconductor Cuprate

Quantum Dynamical Approach to Various Scattering Mechanisms and Their Influences on Thermal Conductivity of Sr- and Zn-Doped La2CuO4 High-Temperature Superconductor Cuprate

BCS State with Additional Interaction

Formerly the idea of a superconducting state on the basis of the BCS state in a nonuniform structure was proposed to explain the superconductivity mechanism in high-temperature superconductors—cuprates. The idea consists in that the probability of electron states filling described by the function \(\text {\textit {v}}_{k}^{2}\) can be due to an additional pair electron-electron interaction. The value of the function \({\text {\textit {v}}_{k}^{2}}\, \approx \) 1/2 near the Fermi level should extend to the Debye energy. It was shown earlier that this can bring about a large magnification of the superconducting gap. It was supposed that the additional interaction can be realized through the interaction of matrix charge carriers and the Wigner crystal electron state islands. The Wigner crystal gap is assumed to correspond to the pseudogap e pg. Naturally, it would be of interest to clear up what conditions such a structure must satisfy for the supposed state to take place in it. This report discusses the requirements the structure and additional interaction should meet.

High current superconductivity in FeSe0.5Te0.5-coated conductors at 30 tesla

Although high-temperature superconductor cuprates have been discovered for more than 25 years, superconductors for high-field application are still based on low-temperature superconductors, such as Nb(3)Sn. The high anisotropies, brittle textures and high manufacturing costs limit the applicability of the cuprates. Here we demonstrate that the iron superconductors, without most of the drawbacks of the cuprates, have a superior high-field performance over low-temperature superconductors at 4.2 K. With a CeO(2) buffer, critical current densities >10(6) A cm(-2) were observed in iron-chalcogenide FeSe(0.5)Te(0.5) films grown on single-crystalline and coated conductor substrates. These films are capable of carrying critical current densities exceeding 10(5) A cm(-2) under 30 tesla magnetic fields, which are much higher than those of low-temperature superconductors. High critical current densities, low magnetic field anisotropies and relatively strong grain coupling make iron-chalcogenide-coated conductors particularly attractive for high-field applications at liquid helium temperatures.

Magnetic excitations in systems with a nonmagnetic ground state and valence fluctuations

In this work, we report and analyze the results of a cycle of the neutron investigations of a number of rare� earth compounds based on SmB6 , SmS, and YbB 12 . These systems are usually classified as Kondo insula� tors (1) (for the case of SmS, at a pressure of higher than 0.6 GPa) with various valence instability degrees of a rareearth ion. This degree is sufficiently high for SmB6 and its derivatives, varies in a wide range for "chemically compressed" Sm(Y)S (simulating the intermediatevalence state of SmS under compres� sion), and is slightly different from an integer for YbB12, as in heavyfermion systems. All of these com� pounds are characterized by the presence of a gap (or pseudogap) in the electron density of states near the Fermi energy and the nonmagnetic ground state, which are formed due to strong electron correlations at temperatures below a certain characteristic value of about several tens of Kelvin. In the diverse neutron experiments, the energy structure of the magneticexcitation spectrum, the characteristic features of the momentum dependence of the energy (dispersion relation), and the intensities of these excitations have been determined. The tem� perature dependence of the spectral function and the effect of the introduction of the defects into the mag� netic sublattice, i.e., the role of the cooperative effects on the formation of the ground state have been ana� lyzed. Specific excitations have been revealed in the energy range of 10-20 meV; the main properties of these excitations are similar to those of the socalled "resonance" mode, which is studied and actively dis� cussed in works devoted to the hightemperature superconductor cuprates (see (2, 3) and references therein).

Magnetic Coupling in La<sub>0.7</sub>Ca<sub>0.3</sub>MnO<sub>3</sub>/YBa<sub>2</sub>Cu<sub>3</sub>O<sub>7</sub>/La<sub>0.7</sub>Ca<sub>0.3</sub>MnO<sub>3</sub> Trilayers

We report on the interplay between ferromagnetism and superconductivity in trilayers La0.7Ca0.3MnO3/YBa2Cu3O7/La0.7Ca0.3MnO3 made of half metallic manganite and high temperature superconductor cuprate. Samples with a fully oxygenated cuprate show a magnetic field interval where the magnetizations of the manganite are aligned antiparallel. A considerable magnetoresistance accompanies the switching between magnetization configurations (parallel vs. antiparallel) of the manganite moments. Suppression of the free carrier density of the cuprate which occurs upon oxygen depletion, results in deep modifications in the shape of the normal state hysteresis loops indicating that there may be a magnetic coupling mediated by free carrier density of the cuprate. This result outlines the importance of quasiparticle transmission in the interplay between ferromagnetism and superconductivity in this kind of samples.

Small anisotropy, weak thermal fluctuations, and high field superconductivity in Co-doped iron pnictide Ba(Fe1−xCox)2As2

We performed high-field magnetotransport and magnetization measurements on a single crystal of the 122-phase iron pnictide Ba(Fe1−xCox)2As2. Unlike the high-temperature superconductor cuprates and 1111-phase oxypnictides, Ba(Fe1−xCox)2As2 showed practically no broadening of the resistive transitions under magnetic fields up to 45 T. We report the temperature dependencies of the upper critical field Hc2 both parallel and perpendicular to the c-axis, the irreversibility field Hirrc(T), and a rather unusual symmetric volume pinning force curve Fp(H) suggestive of a strong pinning nanostructure. The anisotropy parameter γ=Hc2ab/Hc2c deduced from the slopes of dHc2ab/dT=4.9 T/K and dHc2c/dT=2.5 T/K decreases from ∼2 near Tc, to ∼1.5 at lower temperatures, much smaller than γ for 1111pnictides and high-Tc cuprates.

Evolution with hole doping of the electronic excitation spectrum in the cupratesuperconductors

The recent scanning tunnelling results of Alldredge and co-workerson Bi-2212 and of Hanaguri and co-workers on Na–CCOC(Ca2−xNaxCuO2Cl2) are examined from the perspective of the Bardeen–Cooper–Schrieffer (BCS)/Bose–Einsteincondensation boson–fermion resonant crossover model for the mixed-valence hightemperature superconductor (HTSC) cuprates. The model specifies the two energy scalescontrolling the development of HTSC behaviour and the dichotomy often now alluded tobetween nodal and antinodal phenomena in the HTSC cuprates. An indication is extractedfrom the data as to how the choice of the particular HTSC system sees these two basicenergy scales (, the local pair binding energy, andΔsc, the nodal BCS-like gap parameter) evolve with doping and change in the degree ofmetallization of the structurally and electronically perturbed mixed-valent environment.

Preface: phys. stat. sol. (b) 242/1

AbstractWe have accepted with great pleasure the suggestion of the Guest Editor Miodrag Kulić to write a short preface to the special issue of this journal, which is devoted to the role played by electron–phonon interaction (EPI) in high‐temperature superconductors (HTSC). From the very beginning, it was absolutely clear to us that there is no metal in which the EPI could be ignored, and high‐temperature superconducting compounds cannot be an exception in this respect. We expressed this opinion, in particular, in our early Review Article [1] and in the talk [2] given at the Grenoble M2S HTSC Conference in 1994. We would like to emphasize that we were not in isolation. There have been many other researchers, some authors of this issue among them, who have also considered the EPI as an essential part of the physics of high‐temperature superconductors. However, a large part of researchers in the field, including a few famous scientists, have considered the EPI to be irrelevant to high‐temperature superconductivity. Up to now, we do not understand the scientific basis for such an opinion. Moreover, that point of view has never been shared by some other famous scientists; in this respect mention should be made of J. Friedel and A. A. Abrikosov.Turning back to physics, we would like to point out some features of high‐temperature superconducting cuprates, which should lead to the existence of a strong EPI in these materials. First of all, it is the proximity of these compounds, even in the optimally doped case, to the layered ionic crystals. This fact has been emphasized in our early publications as well as in many papers by other authors, and it is discussed in detail in the Review Article by C. Falter published in this issue. There are other approaches to the HTSC compounds, which allow to consider that a strong EPI exists. They are also based on some peculiarities in the crystalline and chemical structure of these compounds, in particular, on their multiphase nanoscale structure. This point is discussed by J. Phillips in this issue.There are also many experimental indications in favor of the existence of a strong EPI in the HTSC cuprates. For example, the behavior of the electron relaxation, the peculiarities of the phonon spectra, the interaction of the Josephson current with phonons, and the electron mass renormalization. All these phenomena have been discussed in the recent Review Articles [3, 4]. Currently, additional evidence was provided which has thrown new light on the role played by the EPI in HTSC systems. These are the ARPES experiments conducted by the Stanford group, which have given an unambiguous proof of the electron mass renormalization due to the EPI. A Review Article of this group by T. Cuk et al. is also presented in this issue. We should also mention the contribution of L. Pintschovius who presented new interesting results on the electron–phonon coupling effects observed by means of inelastic neutron scattering.A comprehensive discussion of a major part of the electron–phonon coupling effects presented in the Review Articles [1, 3] has been based on the traditional approach of the Eliashberg type. Up to now, we consider this approach to be quite suitable for pursuing a number of goals, mainly for describing properties of the normal state. Nevertheless, we do not disclaim the importance of more detailed investigations of the EPI, which take into account the strong anisotropy, the interplay between electron–phonon and electron–electron interaction, and the non‐adiabatic effects. Four Review Articles in this issue, by Schneider, by Rösch, Han, Gunnarsson and Crespi, by Kulić and Dolgov, and by Cappelluti and Pietronero are devoted to different aspects of these problems.To conclude, we would like to emphasize that the main problem related to the mechanism of superconductivity in the HTSC cuprates is the interplay between the strong EPI and the electron exchange and correlation. Unfortunately, previous work did not crack this problem and much effort should be made in the future. We hope that the publication of this issue will aid to attract the attention of many researchers to the investigation of unsolved problems of the EPI in HTSC systems.

Crystal Chemical Concept of Arrangement and Function of Layered Superconducting Materials

The crystal chemical concept of arrangement and function of layered superconducting materials is supposed. The concept is based on the results of our investigation of crystal chemistry of high-temperature superconductors (HTSC) cuprates, diborides AB2, and borocarbides of nickel RNi2B2C. According to these results (1) the main role in appearance of superconductivity played by the structural fragments—sandwiches A2(CuO2) in HTSC cuprates, A2(B2) in diborides, and RB(Ni) in nickel borocarbides but not the separate planes of CuO2, B2, or Ni; (2) correlations between T c and crystal chemical parameters of these sandwiches have similar character in all three classes of compounds, despite distinction of a nature of their superconductivity. The central idea of the concept consists of following: in contrast to metallic conduction, for which it is enough to provide only concentration and mobility of charge carriers, for occurrence of a superconductivity it is necessary to create in addition a space (channels) for stream of charge carriers, compression of stream of carriers, and focusing to direction on path of motion.

Influence of lattice parameter scaling on local electronic and magnetic properties in La2CuO4

A large amount of static and dynamic NMR experiments have been carried out to determine the spin properties in high temperature superconductors cuprates (HTSC). In the majority of cuprates the magnetic shift at the planar copper nucleus is independent of the temperature when the field is applied perpendicular to the CuO2 planes and does not exhibit the expected reduction of the spin susceptibility in the superconducting state. This has been explained by a coincidental cancellation of on-site and transferred hyperfine fields at the copper nucleus, which would lead to a vanishing spin shift. To examine this explanation we determine the hyperfine fields by ab initio cluster calculations for La2CuO4 within the framework of spin-polarized density functional theory. In particular we investigate the sensitivity of the hyperfine fields on the lattice constants.

Thermally activated phase-slip in high-temperature cuprates

Gd1-x-zPrxCazBa2Cu3O7-δ (GdPrCa-123)high-temperature superconductor cuprate (HTSC) samples with 0.0⩽x⩽0.3and 0.0⩽z⩽0.35 were prepared by standard solid-state reaction andcharacterized by XRD technique. The magnetoresistance of the samples weremeasured and analysed. The superconducting transition region broadened by theapplication of a magnetic field. The resistive transition showed two distinctparts. (1) A steep part near the onset of superconductivity, where the onsettransition temperature remained unchanged. For the Gd0.85Ca0.15-123sample, the steep part was more sensitive to the application of magneticfields. (2) A transition tail part. The experimental data in this region werefitted with an Ambegakor and Halperin (AH) phase-slip model. The AHparameter, γ(H), was used to estimate the critical current density atzero temperature of samples at various magnetic fields. It was observed thatthe critical current density decreased with Pr substitution and increased withCa substitution in Gd-123. An optimal value of the Ca doping concentration forincreasing the critical current density was determined. We suggest that the Prion probably acts to enhance the weak link and the Ca ion acts as a flux pinningcentre in the GdPrCa-123 system. Moreover, we observed that the AH parameter,γ, and activation energy, U0, depend not only on temperature andmagnetic field, but also on the Pr and Ca ion concentrations.Part of this paper has been reported at the MSM-99 conference [26].

Quenched decorated Ising model in three dimensions

The three-dimensional (3d) quenched decorated Ising model with competitive interactions is treated to simulate the 3d antiferromagnetic phase stability limit of the high-temperature superconductor cuprate compounds (e.g., La–Ba–Cu–O and Y–Ba–Cu–O). The model consists of planes in which the nodal spins interact antiferromagnetically ( J) with its nearest neighbors and ferromagnetically ( K) with a decorated spin on the bond between them, which is quenched and randomly distributed over the lattice. By using the framework of an effective-field theory, we discuss the variation of the critical concentration p c where the Néel temperature tends to zero for various values of α= J/ K. We obtain for α < 1 6 the critical concentration p c=0.0297 in excellent accordance with experimental result p c=0.0250 of the La 2− x Ba x CuO 4 ( x=2 p) compound (2-1-4). Whereas for the YBa 2Cu 3O 6+ x compound (1-2-3) we have obtained a better value p c=0.1588 ( p c exp=0.20) for α= 1 2 . Comparison with the experimental result of the phase diagram also was presented, and it is qualitatively in accordance only for the 1-2-3 compound.

Patterning polymer metal precursor for high temperature superconductor films

There are various applications of polymers in the field of high temperature superconductors (HTSCs). They are used as thickening agents for precursor solutions [1], in composites [2] or as complexing agents for metal ions [3]. In comparison with the preparation of HTSC cuprates from inorganic salts and oxides, the production from polymer complex compounds is more rapid, and it is possible to get bulk materials, fibres and thick or thin films [4]. We prepared precursors from polymethacrylic acid and novolaks. These polymer precursors are characterized by good film quality, and the precursor films are sensitive to electron beam radiation [5]. Thermoanalytical characterization of the precursor materials was used for optimizing heating conditions to get HTSC phases [6, 7]. We show the results of our investigation on