High-temperature Oxide Superconductors Research Articles

Advances in the physics of high-temperature superconductivity

The high-temperature copper oxide superconductors are of fundamental and enduring interest. They not only manifest superconducting transition temperatures inconceivable 15 years ago, but also exhibit many other properties apparently incompatible with conventional metal physics. The materials expand our notions of what is possible, and compel us to develop new experimental techniques and theoretical concepts. This article provides a perspective on recent developments and their implications for our understanding of interacting electrons in metals.

Frontier Physics with Correlated Electrons

I n the mid-1970s, many solid-state physicists, including ourselves, felt that our field was reaching maturity. The independent electron approximation worked marvelously well for most semiconductors and metals; the phase transition problem was solved; and the fundamentals of magnetism, ferroelectricity, and superconductivity appeared to be well in hand. There were some niggling concerns; for example, we still did not know why nickel oxide was an insulator rather than a metal. However, many of us suspected that this was a detail that would be worked out in due course. As this issue of Science dramatically exemplifies, our self-satisfaction was completely misplaced. A remarkable variety of new (and in some cases, old) materials have been discovered that cannot be understood at all with traditional ideas. These have in common the dominant role played by electron-electron interaction effects, and hence such systems are collected together under the general rubric of “highly correlated electron systems.” Examples are transition metal oxides, including, most spectacularly, the copper oxide high-temperature superconductors; heavy fermion metals, which typically involve rare earths or actinides; organic charge transfer compounds; and one- and two-dimensional electron gas systems. The possibilities for technological applications of some of these compounds are tantalizing, but they also present some of the deepest intellectual challenges in physics. These materials are typically characterized by the coexistence of different kinds of order, including charge and orbital and spin density waves, together with superconducting and magnetic order. Indeed, these different kinds of ordering, which are conventionally thought to compete with each other, instead often appear to be synergistic in these materials. Such systems may also exhibit quantum fluid ground states and exotic transitions from ordered to quantum disordered states at absolute zero temperature. The mathematical apparatus required to describe the physics of highly correlated electronic systems is still under development. The most elegant results have been obtained by mapping the solid-state problem onto an equivalent field theoretical model developed by particle theorists and then using the mathematical apparatus of field theory to determine the ground state and the low-lying excitations. Deep insights have also been gained from numerical calculations taking advantage of modern powerful computers and new, very efficient algorithms. Clearly, highly correlated electron systems present us with profound new problems that almost certainly will represent deep and formidable challenges well into this new century. To date, this field has been largely driven by experimental discoveries. We particularly admire the research done in Japan. Interestingly, one of the reasons why Japanese scientists have played a leadership role in this field is that physicists who specialize in both synthesizing new materials and growing large single crystals have always held revered positions in Japanese universities. Historically, this has not been the case in U.S. universities. U.S. scientists also envy the very large investments that other societies, especially Western Europe and Japan, are making in this field. In the United States, support for university research in solid-state physics has simply not kept pace with the progress and interest in the fundamental scientific problems posed by correlated-electron materials. In addition, neutron scattering is an absolutely indispensible tool for studying the exotic magnetic and charge ordering exhibited by highly correlated electronic systems. Not surprisingly, therefore, the recent permanent closing of the High Flux Beam Reactor at Brookhaven National Laboratory has sent shock waves through the U.S. solid-state physics community. Given the importance of this field from both the fundamental and, prospectively, practical points of view, one might hope that these funding and facility handicaps would be addressed as soon as possible.

Imaging the effects of individual zinc impurity atoms on superconductivity in Bi2Sr2CaCu2O8+delta

Although the crystal structures of the copper oxide high-temperature superconductors are complex and diverse, they all contain some crystal planes consisting of only copper and oxygen atoms in a square lattice: superconductivity is believed to originate from strongly interacting electrons in these CuO2 planes. Substituting a single impurity atom for a copper atom strongly perturbs the surrounding electronic environment and can therefore be used to probe high-temperature superconductivity at the atomic scale. This has provided the motivation for several experimental and theoretical studies. Scanning tunnelling microscopy (STM) is an ideal technique for the study of such effects at the atomic scale, as it has been used very successfully to probe individual impurity atoms in several other systems. Here we use STM to investigate the effects of individual zinc impurity atoms in the high-temperature superconductor Bi2Sr2CaCu2O8+delta. We find intense quasiparticle scattering resonances at the Zn sites, coincident with strong suppression of superconductivity within approximately 15 A of the scattering sites. Imaging of the spatial dependence of the quasiparticle density of states in the vicinity of the impurity atoms reveals the long-sought four-fold symmetric quasiparticle 'cloud' aligned with the nodes of the d-wave superconducting gap which is believed to characterize superconductivity in these materials.

Magnéli phases of anion-deficient rutile as lubricious oxides. Part I. Tribological behavior of single-crystal and polycrystalline rutile (Ti n O2n−1)

In part I of this paper series, wide temperature range SEM-tribometric results generated in vacuum and various partial pressures of oxygen are combined with relevant literature data to examine a hypothesis correlating the oxygen stoichiometry of the Ti n O2n−1 Magneli phases of the rutile polymorph of titania with their tribological behavior. Single-crystal and polycrystalline rutile specimens of narrow stoichiometry ranges were sliding against α-SiC and themselves. The surface shear strength changes were determined as a function of the thermal–atmospheric test environment, and the shear strength values were estimated by the coefficients of friction, the real area of contact and the published yield strength of rutile. The data appear to be sufficient and sufficiently reliable to confirm the accuracy of the hypothesis. The tendency of the rutile stoichiometry (ergo the friction) to shift as a function of temperature and partial pressure of oxygen causes this material to be thermo-oxidatively unstable for tribological applications in extreme environments. In part II, a study is described to formulate oxidatively more stable Magneli phases by Cu-doping, and test the new materials by SEM tribometry using a procedure used for pure rutile here in part I. By employing this doping methodology similar to creating high-temperature superconductive oxides in part II, some feasibility of producing oxidatively stable, lubricious oxides with acceptably low wear rates is indicated.

Coupling strength of charge carriers to spin fluctuations in high-temperature superconductors

In conventional superconductors, the most direct evidence of the mechanism responsible for superconductivity comes from tunnelling experiments, which provide a clear picture of the underlying electron-phonon interactions. As the coherence length in conventional superconductors is large, the tunnelling process probes several atomic layers into the bulk of the material; the observed structure in the current-voltage characteristics at the phonon energies gives, through inversion of the Eliashberg equations, the electron-phonon spectral density alpha2F(omega). The situation is different for the high-temperature copper oxide superconductors, where the coherence length (particularly for c-axis tunnelling) can be very short. Because of this, methods such as optical spectroscopy and neutron scattering provide a better route for investigating the underlying mechanism, as they probe bulk properties. Accurate reflection measurements at infrared wavelengths and precise polarized neutron-scattering data are now available for a variety of the copper oxides, and here we show that the conducting carriers (probed by infrared spectroscopy) are strongly coupled to a resonance structure in the spectrum of spin fluctuations (measured by neutron scattering). The coupling strength inferred from those results is sufficient to account for the high transition temperatures of the copper oxides, highlighting a prominent role for spin fluctuations in driving superconductivity in these materials.

Direct observation of d-orbital holes and Cu–Cu bonding in Cu2O

A striking feature of metal oxide chemistry is the unusual electronic and chemical behaviour of Cu(I) and Ag(I): a case in point is that detailed understanding of Cu–O bonding is essential to the theory of high-temperature copper oxide superconductors. Both cations are usually coordinated in a linear fashion to two oxygens, particularly for Cu(I). In many compounds, the Cu(I) and Ag(I) cations also adopt close-packed (and related) configurations with short metal–metal distances that are strongly suggestive of the occurrence of metal–metal bonding1,2 despite their formal nd10 configuration. Such observations have been explained3,4 by invoking the participation in bonding of electronic orbitals of higher principal quantum number—that is, (n + 1)s and (n + 1)p—accompanied by the creation of d-orbital holes on the metal ion. To test this hypothesis, we have used a recently developed method of quantitative convergent-beam electron diffraction5 combined with X-ray diffraction to map the charge-density distribution in the simple oxide Cu2O, the results of which we then compare with electronic-structure calculations. We are able to image directly the d holes on the copper atoms, and also demonstrate the existence of Cu–Cu bonding in this compound.

Charge fluctuations in YBa2Cu3O7-x high-temperature superconductors

Understanding the behaviour of the electrons in the high-temperature copper oxide superconductors remains a challenging problem. An important class of models1,2,3,4,5 argues that the distribution of electronic charge and spin is not homogeneous: rather, spin and charge adopt a dynamic arrangement in which the spins on the copper form antiferromagnetic stripes, separated by domain walls containing the charge1,2,3,4,5. The dynamic behaviour of the spins has been extensively studied by neutron scattering, and recent results6 have shown that the low-frequency fluctuations for different classes of materials display a universal spatial behaviour that is consistent with the stripe picture. But arguments for the existence of the stripe phases are difficult to sustain without a demonstration that charge is distributed in the domain walls. Here we report phonon measurements for the YBa2Cu3O7-x high-temperature superconductors, which reveal the presence of charge fluctuations. The inferred periodicity is that expected if the charge is located in the domain walls separating the spin stripes. Our results therefore provide strong support for the existence of a dynamic stripe phase in the high-temperature superconductors.

Search for efficient pinning centers in high-temperature cupric oxide superconductors

Our recent efforts to search for efficient flux pinning centers which are indispensable to attain a high critical current density (Jc) in high-temperature cupric oxide superconductors (HTSCs) are reviewed. A series of studies have been carried out through careful investigations of relationships between physical properties and microstructures of HTSCs. It is demonstrated that specific chemical substitutions result in novel microstructures which are generally associated with chemical instability for a phase separation. Two topics are presented: characteristic two-phase microstructures found in heavily Pb-substituted Bi2Sr2CaCu2O8+δ with a superconducting critical temperature (Tc) of about 85K, are suggested to be remarkably efficient correlated defects that can suppress a thermally activated flux flow and thus dramatically improve Jc at high temperatures and high magnetic fields. Interesting segregation phenomena in (Hg,Re)Ba2Can−1CunOy with Tc=132K, which occur on heating by a weak electron beam are also reported. The manipulation of these microstructures could be a key for future large-scale applications of HTSCs.

Experimental evidence of pairing fractional quantum Hall effect state in high temperature oxide superconductors caused by ‖ c zero-point oscillation in CuO 2 layer

Besides the anomalous `negative' capacitance of constant thickness La 2− x Sr x CuO 4 film at x≈1/2 n ( n, integer), we find the thickness dependence of the anomalous effect of ∝1/ N ( N, integer) character, where c-axis-oriented La 2− x Sr x CuO 4 film with localization is used. Based on the fractional quantum Hall effect (FQHE) model, we can explain the thickness dependence as the geometrical resonance of series of quasiparticle Laughlin states.

Superstructural ordering and phase transitions in oxide HTSC

From the experimental data analysis it follows that in oxide high-temperature superconductor (HTSC) systems there is the superstructural ordering which includes both spin and charge density waves. As a result the lattice period doubles and electron spectrum reconstructs with the appearing of dielectric state. At low doping (underdoped samples) for such reconstructed spectrum the Fermi surface arises and dielectric to metal transition takes place. At the higher doping level (optimal doping) the nearly square Fermi surface in the k x – k y plane appears around points (± π, ± π). The changes of the Fermi surface with doping coincided with the experimental photospectroscopy data (ARPES). These findings explain the anomalous thermal, magnetic and other properties of the oxide HTSC.

Neutron scattering from magnetic excitations in Bi2Sr2CaCu2O8+δ

Many of the physical properties of the copper oxide high-temperature superconductors appear to defy the conventional ‘one-electron’ theory of metals. The development of alternative theories incorporating strong electron correlations is currently at the forefront of research in condensed-matter physics. In this context, inelastic neutron scattering can provide valuable insight into collective magnetic excitations in the copper oxide superconductors and so guide these theoretical efforts. Such measurements require large single crystals, and have hitherto been restricted to just two families of high-temperature superconductors—La2−xSrxCuO4 (ref. 1) and YBa2Cu3O6+x (ref. 2). Although the magnetic spectra of these two materials bear certain similarities, there are also important differences. In particular, a sharpresonant spin excitation dominates the spectrum in the superconducting state of YBa2Cu3O6+x (3–10), but is not observed in La2−xSrxCuO4 (ref. 1). Here we report inelastic neutron scattering measurements of a different copper oxide system—Bi2Sr2CaCu2O8+δ—in which we observe a magnetic resonance peak in the superconducting state. This result implies that this excitation phenomenon is a general feature of the copper oxide superconductors, so extending the empirical basis for its theoretical description.

An analogue of superfluid 3He

A series of papers provides a description of the behaviour of the unconventional superconductor strontium ruthenate: a superconducting analogue of superfluid3He. Together they uphold established theories about how superconductivity driven by electron-electron interactions should work. This neat and tidy picture contrasts with the puzzling story of high-temperature copper oxide superconductors, with which ruthenates share a similar crystal structure.

Magnetically mediated superconductivity in heavy fermion compounds

In a conventional superconductor, the binding of electrons into the paired states that collectively carry the supercurrent is mediated by phonons — vibrations of the crystal lattice. Here we argue that, in the case of the heavy fermion superconductors CePd2Si2 and CeIn3, the charge carriers are bound together in pairs by magnetic spin–spin interactions. The existence of magnetically mediated superconductivity in these compounds could help shed light on the question of whether magnetic interactions are relevant for describing the superconducting and normal-state properties of other strongly correlated electron systems, perhaps including the high-temperature copper oxide superconductors.

Towards 1.0 Å resolution: taking advantage of dynamical scattering, and the benefits for structure retrieval

This paper is an exploration of the behaviour of high‐resolution transmission electron microscope (HRTEM) images at up to 1 Å resolution. The ultimate limits to HRTEM (structure) resolution and the manner in which strong scattering may lead to weak diffraction in heavy fcc metals are discussed. A resolution of 1.0 Å is somewhat better than the ultimate resolution presently achievable in a 400‐kV electron microscope. In heavy metals, such as platinum, it is found that the lattice fringe contrast is very low in the [011] projection, but that fringe contrast may be improved by imaging in the [111] projection. For atomic resolution imaging of the heavy metals in the [111] projection a resolution of 1.2 Å is required. For the study of oxygen position in high‐temperature superconducting (HTS) oxides a resolution of between 1.2 and 1.4 Å is required. At better than 1.2 Å resolution the thick crystal images in HTS oxides remain simple and are easily interpreted. At such resolution all atomic columns are separated for the HTS [010] projection and the dynamical diffraction effects improve the contrast of oxygen atoms relative to the metal atoms.

Evidence of an atomistic universal structural entity for high-temperature superconductivity

Evidence is presented that the spatial regions responsible for superconductivity in NdBa2Cu3O7, Nd2−zCezCuO4, and Nd2−zCezSr2Cu2NbO10 are different in the three materials, based on the locations of the superconducting condensates as extracted from: pair-breaking data, the locations of the charge reservoirs, the effects of Ce doping, crystal-field splitting, and the different charges on the Nd2−zCezCuO4 in “free” bulk Nd2−zCezCuO4 and in “superlattice” Nd2−zCezCuO4/SrO/NbO2/SrO/CuO2/. Analyses show that the universal entity responsible for superconductivity is not a spatially extended or planar structure, but is atomistic charge-reservoir oxygen. This suggests that Ba1−aKaPb1−bBibCu3 should be reclassified as a high-temperature oxide superconductor, with Tc≈32 K for b→1. Predictions of the critical temperatures of Nd2−zCezSr2Cu2NbO10 and Nd2−zCezSr2Cu2TaO10, based on this picture, are successful.

Superconductivity enhanced by Hg fission

For the successful application of high-temperature copper oxide superconductors, the problem of the ease of motion of magnetic vortices (quantized flux lines) within the material must be solved. The motion results in finite electrical resistance which prevents the desired loss-free conduction of current1. We demonstrate a solution to this problem by anchoring the vortices with crystallographic defects induced by fission of mercury atoms in a mercury/copper oxide superconductor.

Are all high-temperature superconductors p-type?

Arguments are presented that all high-temperature oxide superconductors are necessarily p-type. Tests are suggested.

Superconductor electronics, 1986-1996

This paper reviews progress in superconductor electronics over the decade since the discovery of the high temperature oxide superconductors (HTS). Almost simultaneously with that discovery was the development of a new process for low temperature superconductor (LTS) devices based on the Nb/AlOx/Mb whole wafer tunnel junction. Both of these developments facilitated a strong growth in applications. We review first the applications based on thin films with no Josephson junctions. This is mostly using HTS, which permits more convenient cooling thus making applications mere commercially viable. With their need for only one or two Josephson junctions, SQUID magnetometers have succeeded to the market place using HTS materials. Finally, we review progress in the volt standard and digital circuits, both of which require large numbers of junctions and at present can only be realized with LTS materials.

High Critical-Current Density in the Heavily Pb-Doped Bi2Sr2CaCu2O8+delta Superconductor: Generation of Efficient Pinning Centers

Critical-current density (Jc) is a parameter of primary importance for potential applications of high-temperature copper oxide superconductors. It is limited principally by the breakdown of zero-resistive current due to thermally activated flux flow at high temperatures and high magnetic fields. One promising method to overcome this limitation is to introduce efficient pinning centers into crystals that can suppress the flux flow. A marked increase in Jc was observed in Bi2Sr2CaCu2O8+delta (Bi-2212) single crystals doped with a large amount of Pb. By electron microscopy, characteristic microstructures were revealed that probably underlie the observed enhancement in Jc: thin (10 to 50 nanometers), platelike domains having a modulation-free structure appeared with spacings of 50 to 100 nanometers along the b axis.

Quasi-binary phase diagram of Nd 4Ba 2Cu 2O 10Ba 3Cu 5O 8 system

Nd 1Ba 2Cu 3O 7− y (Nd123) melt processed in a reduced oxygen atmosphere exhibits good superconducting characteristics including high T c and high J c. Hence, its single crystal is required for evaluation of fundamental superconducting properties as well as for future devices of high temperature superconductive oxides. Information of the phase diagram especially in the range of obtaining Nd123 single crystals from solution is indispensable to fabricate Nd123 single crystals by the Top-Seeded Solution Growth (TSSG), which has not been fully investigated yet. In this work, the temperature dependence of Nd solubility in the BaCuO solution with a Ba to Cu ratio of 3 to 5 was investigated by analyzing the composition of the solution stuck to the thermally equilibrated MgO single crystal which was dipped into the equilibrated solution at different temperatures. Transformation temperatures including the peritectic temperature and stable phases were measured by X-ray diffraction (XRD) by quenching samples held isothermally around the transformation temperature estimated in advance by differential thermal analysis (TG-DTA). From these experiments the phase diagram of quasi-binary system between Nd 4Ba 2Cu 2O 10 and 3BaCuO 2 + 2CuO(Ba 3Cu 5O 8) has been constructed.