High-temperature Copper Oxide Superconductors Research Articles

Electron pockets in the Fermi surface of hole-doped high-Tc superconductors

High-temperature superconductivity in copper oxides occurs when the materials are chemically tuned to have a carrier concentration intermediate between their metallic state at high doping and their insulating state at zero doping. The underlying evolution of the electron system in the absence of superconductivity is still unclear, and a question of central importance is whether it involves any intermediate phase with broken symmetry. The Fermi surface of the electronic states in the underdoped 'YBCO' materials YBa2Cu3O(y) and YBa2Cu4O8 was recently shown to include small pockets, in contrast with the large cylinder that characterizes the overdoped regime, pointing to a topological change in the Fermi surface. Here we report the observation of a negative Hall resistance in the magnetic-field-induced normal state of YBa2Cu3O(y) and YBa2Cu4O8, which reveals that these pockets are electron-like rather than hole-like. We propose that these electron pockets most probably arise from a reconstruction of the Fermi surface caused by the onset of a density-wave phase, as is thought to occur in the electron-doped copper oxides near the onset of antiferromagnetic order. Comparison with materials of the La2CuO4 family that exhibit spin/charge density-wave order suggests that a Fermi surface reconstruction also occurs in those materials, pointing to a generic property of high-transition-temperature (T(c)) superconductors.

Charge-order-maximized momentum-dependent superconductivity

Charge ordering and superconductivity are observed in the phase diagrams of a variety of materials such as NbSe3, layered transition-metal dichalcogenides and high-temperature copper oxide superconductors, low-dimensional organics, Ba1−xKxBiO3 and so forth. Although both conventional charge-density-wave (CDW) and superconducting transitions show an energy gap in the single-particle density of states at the Fermi level (EF), their physical properties are poles apart: insulating behaviour for the CDW and zero resistivity in superconductors. Consequently, these two ground states are believed to compete with each other. Here we provide evidence for maximized superconductivity at points in momentum (k) space that are directly connected by the CDW ordering vector. Temperature-dependent angle-resolved photoemission spectroscopy of 2H-NbSe2 across the CDW and superconducting transitions (TCDW∼33 K and Tc=7.2 K, respectively) shows CDW-induced spectral-weight depletion at the same Fermi-surface-crossing k points, which evolve into the largest superconducting gaps. These k points also show the highest electron–phonon coupling and lowest Fermi velocities. Our results demonstrate that charge order can boost superconductivity in an electron–phonon coupled system, in direct contrast to the prevailing view that it only competes with superconductivity.

Cu0.5Tl0.5Ba2Ca3Cu4−y Zn y O12−δ (y=0, 1.0, 2.0, 3.0, 3.5): Superconductor with Four ZnO2 Planes

A new Cu0.5Tl0.5Ba2Ca3Cu4−yZnyO12−δ (y=0, 1.0, 2.0, 3.0, 3.5) superconductor with four ZnO2 planes is reported. The structure of the material remains tetragonal for all Zn doping concentration. The substitution of Zn at CuO2 planar site was carried out following Cu0.5Tl0.5Ba2Ca3Cu4−yZnyO12−δ (y=0, 1.0, 2.0, 3.0, 3.5) formula. Contrary to all previous studies of Zn doping in all copper oxide high temperature superconductors, the zero resistivity critical temperature Tc(R=0), critical current density and quantity of diamagnetism increase with increased Zn concentration. The onset temperature of superconductivity in these samples was observed at 128 K and Tc(R=0) at 122 K for y=3.5. The volume of the unit cell observed through X-ray diffraction scan is found to decrease with increase Zn doping; promoting an increase in Fermi vector KF and effective density of states which results in enhanced superconductivity parameters. The synthesis of Cu0.5Tl0.5Ba2Ca3Cu4−yZnyO12−δ material by this method is highly reproducible.

A temperature modulation photonic crystal Mach-Zehnder interferometer composed of copper oxide high-temperature superconductor

A tunable Mach-Zehnder interferometer with a two-dimensional photonic crystal structure using copper oxide high-temperature superconductor is proposed. This photonic crystal is composed of rods of which axes are perpendicular to the two-dimensional anisotropic copper oxide plane. By tuning the temperature of the superconductor, the refractive index of the superconductor as well as the photonic band gap can be changed. The photonic band structures of two-dimensional photonic crystals composed of the superconductor are calculated by using the plane-wave expansion method, and interference properties are investigated by using the finite-difference time-domain method. For our designed photonic crystal Mach-Zehnder interferometer, the simulation results show that the light transmission can be modulated from 92.7% to 1.4% with different temperature distributions.

Competing ferromagnetism in high-temperature copper oxide superconductors.

The extreme variability of observables across the phase diagram of the cuprate high-temperature superconductors has remained a profound mystery, with no convincing explanation for the superconducting dome. Although much attention has been paid to the underdoped regime of the hole-doped cuprates because of its proximity to a complex Mott insulating phase, little attention has been paid to the overdoped regime. Experiments are beginning to reveal that the phenomenology of the overdoped regime is just as puzzling. For example, the electrons appear to form a Landau Fermi liquid, but this interpretation is problematic; any trace of Mott phenomena, as signified by incommensurate antiferromagnetic fluctuations, is absent, and the uniform spin susceptibility shows a ferromagnetic upturn. Here, we show and justify that many of these puzzles can be resolved if we assume that competing ferromagnetic fluctuations are simultaneously present with superconductivity, and the termination of the superconducting dome in the overdoped regime marks a quantum critical point beyond which there should be a genuine ferromagnetic phase at zero temperature. We propose experiments and make predictions to test our theory and suggest that an effort must be mounted to elucidate the nature of the overdoped regime, if the problem of high-temperature superconductivity is to be solved. Our approach places competing order as the root of the complexity of the cuprate phase diagram.

Intrinsic heterogeneity and polaron formation in high temperature superconducting copper oxides

A variety of unconventional isotope and lattice effects have been observed in high temperature superconducting copper oxides (HTSC), which do not find an explanation within models based on strong correlations only. It is shown here, that polaron formation is the origin of these experimental findings. The polaron formation is, however, not homogeneous, but restricted to limited spatial regimes thus leaving the antiferromagnetic background partially intact and allowing for the coexistence of different ground states.

Multicomponent superconductivity: the complex interplay of spin charge and lattice

High temperature superconducting copper oxides (HTSC) have been shown to be intrinsically heterogeneous. Consequently multiple ground states coexist, which are either in competition with or of support to superconductivity. Here it is shown that spin, charge and lattice are intrinsically coupled to form a composite of locally charge carrying distorted areas in an antiferromagnetic matrix on the regular lattice. The dynamical interplay between both induces level shifts together with band narrowing of the spin and charge related bands, by means of which interband interactions are possible which lead to a substantial increase in the superconducting transition temperature T c . Further consequences of this multicomponent approach as, for example, isotope effects are discussed.

Spiral Spin State in High-Temperature Copper-Oxide Superconductors: Evidence from Neutron Scattering Measurements

An effective spiral spin phase ground state provides a new paradigm for the high-temperature superconducting cuprates. It accounts for the recent neutron scattering observations of spin excitations regarding both the energy dispersion and the intensities, including the "universal" rotation by 45 degrees around the resonance energy . The intensity has a 2D character even in a single twin crystal. The value of is related to the nesting properties of the Fermi surface. The excitations above are shown to be due to in-plane spin fluctuations, a testable difference from the stripe model. The form of the exchange interaction function reveals the effects of the Fermi surface, and the unique shape predicts large quantum spin fluctuations in the ground state.

Nodal quasiparticle in pseudogapped colossal magnetoresistive manganites

A characteristic feature of the copper oxide high-temperature superconductors is the dichotomy between the electronic excitations along the nodal (diagonal) and antinodal (parallel to the Cu-O bonds) directions in momentum space, generally assumed to be linked to the 'd-wave' symmetry of the superconducting state. Angle-resolved photoemission measurements in the superconducting state have revealed a quasiparticle spectrum with a d-wave gap structure that exhibits a maximum along the antinodal direction and vanishes along the nodal direction. Subsequent measurements have shown that, at low doping levels, this gap structure persists even in the high-temperature metallic state, although the nodal points of the superconducting state spread out in finite 'Fermi arcs'. This is the so-called pseudogap phase, and it has been assumed that it is closely linked to the superconducting state, either by assigning it to fluctuating superconductivity or by invoking orders which are natural competitors of d-wave superconductors. Here we report experimental evidence that a very similar pseudogap state with a nodal-antinodal dichotomous character exists in a system that is markedly different from a superconductor: the ferromagnetic metallic groundstate of the colossal magnetoresistive bilayer manganite La1.2Sr1.8Mn2O7. Our findings therefore cast doubt on the assumption that the pseudogap state in the copper oxides and the nodal-antinodal dichotomy are hallmarks of the superconductivity state.

Breakdown of One-Parameter Scaling in Quantum Critical Scenarios for High-Temperature Copper-Oxide Superconductors

We show that if the excitations which become gapless at a quantum critical point also carry the electrical current, then a resistivity linear in temperature, as is observed in the copper-oxide high-temperature superconductors, obtains only if the dynamical exponent z satisfies the unphysical constraint, z < 0. At fault here is the universal scaling hypothesis that, at a continuous phase transition, the only relevant length scale is the correlation length. Consequently, either the electrical current in the normal state of the cuprates is carried by degrees of freedom which do not undergo a quantum phase transition, or quantum critical scenarios must forgo this basic scaling hypothesis and demand that more than a single-correlation length scale is necessary to model transport in the cuprates.

High Tc superconductor re-entrant cavity filter structures

Microwave cavity filters are widely used in high frequency communication systems for high quality factor signal filtering. However, their large size makes it inconvenient for them to be used at lower operating frequencies especially in the 1–4 GHz frequency range of most of the present day commercial wireless applications. In order to improve their performance and to reduce their overall size, superconducting cavity filters were designed and simulated. Cylindrical and re-entrant cavity filters were fabricated, using seeded melt grown yttrium barium copper oxide high temperature superconductors. The resonant characteristics of these filters were measured, verifying the concept that small, high Q, cavity filters can be made for wireless applications.

Biaxially Aligned Yttria Stabilized Zirconia and Titanium Nitride Layers Deposited by Unbalanced Magnetron Sputtering

Control of the texture and the biaxial alignment of sputter deposited films has provoked a great deal of interest due to its technological importance. Indeed, many physical properties of thin films are influenced by the biaxial alignment. In this context, extensive research has been established to understand the growth mechanism of biaxially aligned Yttria Stabilized Zirconia (YSZ) as a buffer layer for high temperature superconducting copper oxides. In this work, the growth mechanism in general and the mechanism responsible of the biaxial alignment in detail were investigated for thin films of YSZ and TiN deposited by unbalanced magnetron sputtering using non-aligned polycrystalline stainless steel substrates. The mechanism responsible for the preferential out-of-plane alignment has been investigated by performing depositions on a non-tilted substrate. However, to study the in-plane alignment a tilted substrate was used. The microstructure of the deposited layers was characterised by Scanning Electron Microscopy (SEM) and Transmission Electron Microscopy (TEM). The crystallographic alignment has been investigated by X-ray diffraction (XRD) (angular scans and pole figures) and by Selective Area Diffraction (SAD). It was observed that the deposited layers show a zone T or zone II structure and the layers with a zone T structure consist of faceted grains. There seems to be a correlation between the crystal habit of these faceted grains and the measured biaxial alignment. A model for the preferential out-ofplane orientation, the in-plane alignment and the correlation between the microstructure and the biaxial alignment is proposed.

Growth Mechanism of Biaxially Aligned Magnesium Oxide Deposited by Unbalanced Magnetron Sputtering

For many years magnesium oxide (MgO) has been a topic of research as buffer layer for high-temperature superconducting copper oxides and as protective layer in plasma display panels. Since epitaxial growth of MgO is expensive, time consuming and size restricted, other techniques have been developed to grow highly oriented MgO layers for industrial processes. MgO thin films were deposited on a tilted polycrystalline substrate by reactive sputtering using an unbalanced magnetron. By varying different deposition parameters, it is possible to grow biaxially aligned MgO layers, i.e. layers with both out-of-plane and in-plane alignment. XRD measurements were performed to examine the crystallographic structure of the thin film. The preferential out-of-plane orientation is analysed by angular scans using the peak intensity of different reflections while the in-plane orientation is determined by (002) pole figures. Fully [111] out-of-plane oriented layers were grown with a strong in-plane alignment. SEM and TEM measurements were performed to reveal the topographical and cross-sectional microstructure and to investigate the texture evolution of the MgO layers. Evolutionary columnar growth and a roof-tile surface have been observed.

Technical Reports: Electron-Phonon Coupling in High-Temperature Cuprate Superconductors as Revealed by Angle-Resolved Photoemission Spectroscopy

Copper-oxide (cuprate) high-temperature superconductors are doped Mott insulators. The undoped parent compounds are antiferromagnetic insulators, and superconductivity occurs only when an appropriate number of charge carriers (electrons or holes) are introduced by doping. All cuprate materials contain CuO2 planes (Figure 1a) in their crystal structure; the doped carriers are believed to go into these CuO2 planes, which are responsible for high-temperature superconductivity. High-temperature superconductors are characterized by their unusual physical properties, both in the superconducting state (below the superconducting transition temperature Tc) and in the normal state (above Tc). Since the discovery of high-temperature superconductivity in 1986 [1], these unusual physical properties and the mechanism of superconductivity have been prominent issues in condensed matter physics [2].

Polaron formation as origin of unconventional isotope effects in cuprate superconductors

Various unconventional isotope effects have been reported in high-temperature superconducting copper oxides which are beyond the scheme of BCS theory. Their origin is investigated within polaron theory which leads to a renormalization of the single particle energies and introduces a level shift here. It is found that the exponential squeezing of the second nearest neighbour hopping integral carries the correct isotope effect on the superconducting transition temperature Tc, as well as the one on the penetration depth. The average superconducting gap is predicted to have an isotope effect comparable to the one on the penetration depth. The results imply that the coupling of the electronic degrees of freedom to the Jahn-Teller Q2-type mode is the origin of these isotope effects.

Pairing interactions and pairing mechanism in high-temperature copper oxide superconductors

The polaron binding energy ${E}_{p}$ in undoped parent cuprates has been determined to be about $1.0\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ from the unconventional oxygen-isotope effect on the antiferromagnetic ordering temperature. The deduced value of ${E}_{p}$ is in quantitative agreement with that estimated from independent optical data and that estimated theoretically from the measured dielectric constants. The substantial oxygen-isotope effect on the in-plane supercarrier mass observed in optimally doped cuprates suggests that polarons are bound into the Cooper pairs. We also identify the phonon modes that are strongly coupled to conduction electrons from the angle-resolved photoemission spectroscopy, tunneling spectra, and optical data. We consistently show that there is a very strong electron-phonon coupling feature at a phonon energy of about $20\phantom{\rule{0.3em}{0ex}}\mathrm{meV}$ along the antinodal direction and that this coupling becomes weaker towards the diagonal direction. We further show that high-temperature superconductivity in cuprates is caused by strong electron-phonon coupling, polaronic effect, and significant coupling with $2\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ $\mathrm{Cu}\ensuremath{-}\mathrm{O}$ charge transfer fluctuation.

Theory of the Knight Shift and Spin-Lattice Relaxation Rate in Pu-115

AbstractWe calculated the Knight shift and spin-lattice relaxation rates of Pu-115 compounds assuming d-wave superconductivity in the presence of strong impurity scattering. We discuss the implications for recent measurements of the spin-lattice relaxation rate in the Pu-115 compound PuRhGa5 by Sakai and coworkers [J. Phys. Soc. Jpn. 74, 1710 (2005)] and present a prediction for the corresponding Knight shift. In addition, we noticed a significant round-off of the spin-lattice relaxation rate 1/T1 just above the superconducting transition temperature that is not observed in the sister compound PuCoGa5. It appears that in PuRhGa5 superconductivity is mediated by spin fluctuations, too. This provides additional support to the scenario of superconducting pairing mediated by spin fluctuations in the Pu-115 compounds similar to the Ce-115 compounds and the high-temperature copper-oxide superconductors.

Influence of the deposition parameters on the biaxial alignment of MgO grown by unbalanced magnetron sputtering

For many years magnesium oxide (MgO) played a key role in the research of buffer layers for high-temperature superconducting copper oxides. For buffer layers, not only the out-of-plane alignment has to be taken into account, but also the in-plane orientation, which is important. The goal of our experiments is to grow biaxially aligned layers, i.e. layers with both out-of-plane alignment and in-plane alignment. The biaxially aligned MgO layers are deposited by unbalanced magnetron sputtering on a non-textured substrate. By varying different deposition parameters (e.g. inclination angle, pressure, reactive gas flow, distance between target and substrate, etc.) it is possible to improve the biaxial alignment. MgO layers with a preferential [1 1 1] out-of-plane orientation and an in-plane alignment with a FWHM of 19.6° in φ and 7.8° in χ are observed.

Polaronic origin of the isotope effect on the London penetration depth in high-temperature superconducting oxides

Direct measurements of the in-plane London penetration depth λ L have recently been performed on high-temperature superconducting copper oxides by a new low-energy muon spin rotation technique. The results show that λ L is isotope dependent, evidencing unconventional electron–phonon interactions as its source. The data are interpreted here in terms of polaronic effects on the single-particle energies, which leads to level shifts and exponential band narrowing. Good agreement with the experimental data is obtained.

On the break in the single-particle energy dispersions and the ‘universal’ nodal Fermi velocity in the high-temperature copper oxide superconductors

Recent data from angle-resolved photoemission experiments published by Zhou et al. [Nature, 423, 398 (2003)] concerning a number of hole-doped copper-oxide-based high-temperature superconductors reveal that in the nodal directions of the underlying square Brillouin zones (i.e. the directions along which the d-wave superconducting gap is vanishing) the Fermi velocities for some finite range of k inside the Fermi sea and away from the nodal Fermi wave vector k F are to within an experimental uncertainty of approximately 20% the same both in all the compounds investigated and over a wide range of doping concentrations and that, in line with earlier experimental observations, at some characteristic wave vector k ☆ away from k F ( typically amounts to approximately 5% of ) the Fermi velocities undergo a sudden change, with this change (roughly speaking, an increase for ) being the greatest (smallest) in the case of underdoped (overdoped) compounds. We demonstrate that these observations establish four essential facts: firstly, the ground-state momentum distribution function must be discontinuous at k = k ☆; with and denoting the measured velocities close to and , we obtain in which is the ‘Fermi’ velocity corresponding to the case in which (for two-body interaction potentials of shorter range than the Coulomb potential, whereby ); secondly, the single-particle spectral function must at k = k ☆ possess a coherent contribution corresponding to a well-defined quasiparticle excitation at an energy of approximately 70 meV below the Fermi energy; thirdly, the amount of discontinuity in at the nodal Fermi points must be small and ideally vanishing; fourthly, the long range of the two-body Coulomb potential is of vital importance for realization of a certain aspect of the observed behaviour in the Fermi velocities (specifi- cally) in the underdoped regime. The condition conforms with the observation through an earlier angle-resolved photoemission experiment by Valla et al. [Science, 285, 2110 (1999)], on the optimally doped compound Bi2Sr2CaCu2O8 + δ, which shows that the imaginary part of the self-energy along the nodal directions of the Brillouin zone and in the vicinity of the nodal Fermi points satisfies the scaling behaviour characteristic of marginal Fermi-liquid metallic states, for which is indeed vanishing. We present arguments advocating the viewpoint that the observed ‘kink’ in the measured energy dispersions cannot be a direct consequence of electron-phonon interaction, although a finite may possibly arise from this interaction. In other words, even though possibly vital, the role played by phonons in bringing about the latter ‘kink’ must be indirect. Our approach further provides a consistent interpretation of the observed sudden decrease in the width of the so-called ‘momentum distribution curve’ on ‖k| increasing above .