High-temperature Oxide Superconductors Research Articles

Normal state of the copper oxide high-temperature superconductors

Superconductivity in the copper oxide ceramics stands as a grand challenge problem as its solution is fundamentally rooted in the physics of strong coupling. In such problems, traditional calculational schemes based on the properties of single particles fail. Rather, the physics of strong coupling

Emergence of particle-hole symmetry near optimal doping in high-temperature copper oxide superconductors

High-temperature copper oxide superconductors (cuprates) display unconventional physics when they are lightly doped whereas the standard theory of metals prevails in the opposite regime. For example, the thermoelectric power, that is the voltage that develops across a sample in response to a temperature gradient, changes sign abruptly near optimal doping in a wide class of cuprates, a stark departure from the standard theory of metals in which the thermopower vanishes only when one electron exists per site. We show that this effect arises from proximity to a state in which particle-hole symmetry is dynamically generated. The operative mechanism is dynamical spectral weight transfer from states that lie at least 2 eV away from the chemical potential. We show that the sign change is reproduced quantitatively within the Hubbard model for moderate values of the on-site repulsion, $U$. For sufficiently large values of on-site repulsion, for example, $U=20t$ ($t$ the hopping matrix element), dynamical spectral weight transfer attenuates and our calculated results for the thermopower are in prefect agreement with exact atomic limit. The emergent particle-hole symmetry close to optimal doping points to pairing in the cuprates being driven by high-energy electronic states.

Enhanced Superconductivity in (Cu0.5Tl0.25M0.25)Ba2Ca2Cu3O10−δ Samples

The elements (M = Li, Na, K) of group I-A of the periodic table having lower electronegativity as compared to that of Tl have been partially doped at Tl sites in the (Cu0.5Tl0.25M0.25)Ba2O4−δ charge reservoir layer of (Cu0.5Tl0.25M0.25)Ba2Ca2Cu3O10−δ superconductor. The objective was to investigate the role of electron–phonon interactions and mobile charge carriers in the oxide high temperature superconductors. It has been observed from these studies that the superconductivity parameters were improved after the doping of lower electronegative elements in the charge reservoir layer. The substitution of lower electronegative elements retains smaller amount of oxygen in (Cu0.5Tl0.25M0.25)Ba2O4−δ charge reservoir layer, which in turn facilitates the transfer of mobile charge carriers to the conducting CuO2 planes.

ChemInform Abstract: Thermoanalytical Observations on Synthesis of Oxide Superconductors

Abstract This paper reviews applications of thermal analysis to the study of the synthesis of high-temperature oxide superconductors. TG and TG-DTA have been most widely used to determine heat treatment conditions for all types of oxide superconductors. Kinetic analysis has been applied successfully and reaction mechanisms have been determined for several synthetic routes of Ba2RCu3O7−x (R ≡ rare earth elements).

ChemInform Abstract: Determination of the Local Structure and Electronic States of High-Tc Superconductors by Scanning Tunneling Microscopy

To date, scanning tunneling microscopy (STM) has significantly advanced the understanding of the local structure, electronic properties, and reactivity of semiconductor surfaces. Less recognized is the fact that STM can also elucidate the local structural and electronic properties of low-dimensional materials. Herein, the authors demonstrate that STM can provide new and essential insight into the physical complexities of high-temperature copper oxide superconductors that is unavailable from conventional studies. After reviewing the basic theoretical concepts needed to evaluate STM data, the authors discuss several specific cases that illustrate the unique information that STM provides. These examples include (1) the atomic level nature of structural disorder in the BiO and TlO layers of Bi{sub 2}Sr{sub 2}CaCu{sub 2}O{sub 8} and Tl{sub 2}Ba{sub 2}CaCu{sub 2}O{sub 8} and the low-energy electronic states associated with these structural features, (2) the local structure and electronic consequences of metal substitution and oxygen doping in Bi{sub 2}Sr{sub 2}CaCu{sub 2}O{sub 8}, and (3) low-temperature tunneling spectroscopy measurements of the superconducting energy gap. Lastly, the authors summarize exciting future avenues to pursue in STM studies of these and other materials. 57 refs., 15 figs.

Critical Current Problems in HTc Superconductors

New results of the investigations critical current problems in the high temperature oxide superconductors are presented, basing on the flux pinning analysis. Model of the pinning interaction of the nanosized centers with pancake vortices is proposed, while comparison of the received results concerning the dependence of the potential barrier versus transport current with other pinning forces approaches is presented. Special attention is devoted to the investigations of the influence the initial position of the vortex captured at pinning center on the current–voltage characteristics and critical current.

Enhancement of superconductivity by pressure-driven competition in electronic order

Finding ways to achieve higher values of the transition temperature, T(c), in superconductors remains a great challenge. The superconducting phase is often one of several competing types of electronic order, including antiferromagnetism and charge density waves. An emerging trend documented in heavy-fermion and organic conductors is that the maximum T(c) for superconductivity occurs under external conditions that cause the critical temperature for a competing order to go to zero. Recently, such competition has been found in multilayer copper oxide high-temperature superconductors (HTSCs) that possess two crystallographically inequivalent CuO(2) planes in the unit cell. However, whether the competing electronic state can be suppressed to enhance T(c) in HTSCs remains an open question. Here we show that pressure-driven phase competition leads to an unusual two-step enhancement of T(c) in optimally doped trilayer Bi(2)Sr(2)Ca(2)Cu(3)O(10+delta) (Bi2223). We find that T(c) first increases with pressure and then decreases after passing through a maximum. Unexpectedly, T(c) increases again when the pressure is further raised above a critical value of around 24 GPa, surpassing the first maximum. The presence of this critical pressure is a manifestation of the crossover from the competing order to superconductivity in the inner of the three CuO(2) planes. We suggest that the increase at higher pressures occurs as a result of competition between pairing and phase ordering in different CuO(2) planes.

Zero-doping state and electron–hole asymmetry in an ambipolar cuprate

High-temperature copper oxide superconductors are usually doped with holes or electrons, not both. The discovery of a material that can be doped with both electrons and holes finally clarifies the region of low doping and the effect of the added dopants. A Mott insulator is a material that is insulating because of strong Coulomb repulsions between electrons. Doping charge carriers, electrons or holes into a Mott insulator can induce high-temperature superconductivity. Thus, what exactly happens when a charge carrier is doped into a Mott insulator is a key question in many-body physics1,2,3,4. To address this issue, ideally one should start from a zero-doping state5,6,7 and be able to introduce both holes and electrons in the dilute limit. However, such an idealized experiment has been impossible because of the lack of suitable materials. Here we show that a new ‘ambipolar’ cuprate makes it possible for the first time to cross the zero-doping state in the same material, which in turn allows us to address the physics of the extremely low-doping region. Surprisingly, we found that the antiferromagnetic ground state sharply changes between electron- and hole-doped sides, and this change is dictated by the existence of only 0.1 ppm of charge carriers. Moreover, we observed that the Neel temperature TN shows an unexpected reduction in a narrow range centred at the zero-doping state, across which the system exhibits asymmetric behaviours in transport measurements. Our findings reveal the inherently different nature of electron and hole doping in the dilute limit of a Mott-insulating cuprate.

ChemInform Abstract: The Role of Thermal Analysis in the Study of Oxide Superconductors

Since the discovery of high temperature oxide superconductors in 1986, synthesis became an important area in the direction of getting new materials with still higher J{sub c} and J{sub c} values. The various synthesis procedures have played a significant role in the nature of products and in turn on their physical properties. An optimization of the synthesis procedure is always desired and there is a need to establish new synthetic routes or to modify the existing ones. Thermal techniques, e.g., TG, DGAA, DSC, dilametry and so forth, in conjunction with other techniques, like X-ray diffraction, gas chromatography, and mass-spectrometry, play a significant role in the synthetic solid-state chemistry. Modern thermoanalytical techniques, like emanation thermal analysis (ETA), or thermopiezic techniques are of immense importance to study the various aspects of solids. The effect of oxygen partial pressure on the final product is also discussed in this review article. Other techniques, like high temperature XRD, thermodilatometry, and temperature programmed reduction (TPR), are also covered to elucidate several microscopic properties of the oxide superconductors. An emphasis is also made on thermoelectrometry and thermomagnetometry techniques.

Optimization of carriers by self-doping in Cu0.5Tl0.5Ba2Ca2Cu3−yMyO10−δ superconductor

The elements of fourth group (IV-A) of the periodic table have been doped in Cu0.5Tl0.5Ba2Ca2Cu3−yMyO10−δ (M=Si, Ge, Sn, and y=0, 1) samples to explore any possible electron-phonon interaction mechanism in oxide high temperature superconductors. We have observed that doped atoms with masses significantly different from that of Cu atom suppress the superconducting properties of the final compound. The postannealing experiments in flowing oxygen were also carried out to optimize the carriers density in the unit cell. After postannealing in oxygen the superconducting properties of all the samples are improved, which is most likely due to the optimization of carriers density in the material. It is observed from Fourier transform infrared absorption measurements that the CuO2 planar mode is hardened with the doping of Si, whereas, it is softened with the incorporation of Ge and Sn in the unit cell. The apical oxygen modes are softened with Si doping but these modes are hardened with the incorporation of Ge and Sn. It is most likely that the density of phonons is significantly altered due to increased anharmonic oscillations produced by heavier and lighter atoms in MO2/CuO2 (i.e., M=Sn,Si) planes, which suppress the phonon population from a desired level and hence the superconductivity. The shift in oxygen phonon modes after postannealing is most probably linked to the change in oxidation state of thallium.

Mobile or not?

Whether the magnetic response of the copper oxide high-temperature superconductors is governed by itinerant quasiparticles or localized electrons is a hotly debated question. Evidence for the latter suggests an intricate connection to the parent Mott insulator.

The chaotic points and XRD analysis of Hg-based superconductors

In this article, high Tc mercury based cuprate superconductors with different oxygen doping rates have been examined by means of magnetic susceptibility (magnetization) versus temperature data and X-ray diffraction pattern analysis. The under, optimally and over oxygen doping procedures have been defined from the magnetic susceptibility versus temperature data of the superconducting sample by extracting the Meissner critical transition temperature, Tc and the paramagnetic Meissner temperature, TPME, so called as the critical quantum chaos points. Moreover, the optimally oxygen doped samples have been investigated under both a.c. and d.c. magnetic fields. The related a.c. data for virgin(uncut) and cut samples with optimal doping have been obtained under a.c. magnetic field of 1 Gauss. For the cut sample with the rectangular shape, the chaotic points have been found to occur at 122 and 140 K, respectively. The Meissner critical temperature of 140 K is the new world record for the high temperature oxide superconductors under normal atmospheric pressure. Moreover, the crystallographic lattice parameters of superconducting samples have a crucial importance in calculating Josephson penetration depth determined by the XRD patterns. From the XRD data obtained for under and optimally doped samples, the crystal symmetries have been found in tetragonal structure.

Critical current enhancement by Lorentz force reduction in superconductor–ferromagnetnanocomposites

Ferromagnetic pinning centres in superconductors form much deeper potential wells thanequivalent insulating or metallic non-superconducting inclusions. However, the resultantpinning forces arising from magnetic inclusions are low because the magnetic interactiontakes place over the length scale of the magnetic penetration depth which is large intechnological superconductors. Nonetheless, we show that a magnetic inclusion can alsoreduce the Lorentz force on a vortex, yielding a substantially enhanced critical currentdensity for a given pinning force. We calculate this enhancement for a single vortex pinnedby a paramagnetic cylinder as well as a vortex lattice interacting with magnetic inclusions,and find that the inclusion of ferromagnetic particles or rods offers a practical meansof enhancing the critical currents in oxide high temperature superconductors.

HTC oxides: a collusion of spin, charge and lattice

We briefly summarize a perspective on high-temperature oxide superconductors (HTC) as systems of intrinsic, functional, and connected spatial scales, with an associated set of temporal scales. We emphasize the importance of local bonding constraints, leading to a framework of coexisting short- and long-range fields, as the source of multiscale systems in HTC and many other strongly correlated materials.

Inhomogeneous vortex distribution and magnetic coupling in oxide superconductor–ferromagnet hybrids

Hybrid systems of thin films of oxide ferromagnets and high-temperature superconductors have been investigated by scanning Hall probe microscopy (SHPM) to analyze the local magnetic flux density distribution at low temperatures. In addition to the intrinsic properties of the films themselves, such structures exhibit novel phenomena due to complex interactions arising at the interface between them. The latter can be divided into processes originating from either electronic or magnetic coupling, respectively. As a direct consequence, the distribution of vortices in the superconductor is strongly influenced by the magnetic background arising from the ferromagnet. The local magnetic information obtained from SHPM images provides clear evidence for the presence of a magnetic dipolar interaction between the magnetic domains of the ferromagnetic component and the vortex ensemble in the superconductor.

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.

The reactive liquid Mg infiltration process to produce large superconducting bulk MgB 2 manufacts

An alternative and simple manufacturing process is presented to produce high density bulk MgB 2 superconducting objects of large dimensions. The process avoids the use of high pressure apparatus and consists in the reactive infiltration of liquid Mg in B powders preforms. With an appropriately designed stainless steel container for the reactants, several manufacts of different shape have been obtained, including tubes, cylinders, rings or disks, of dimensions of the order of the tens of centimeters. The MgB 2 material, unlike high temperature oxide superconductors, allows an easy percolation of the supercurrents across the grains boundaries, even if it is in the polycrystalline form. Due to this property, the large MgB 2 manufacts obtained by the reactive liquid infiltration present high superconducting characteristics, as demonstrated by transport and magnetic measurements up to 35 K. In particular the magnetic levitation and the magnetic shielding capability appear as the most promising applicative fields in which there is a need for large and homogeneously superconducting bulk pieces. For space applications, the use of MgB 2 superconductors may enable a substantial improvement in the compactness and weight of cryogenic systems, with respect to the actual systems based on the liquid He temperatures. Furthermore, with respect to other high temperature oxide superconductors, MgB 2, besides the disadvantages of the need of a 20–30 K cryogenic system, presents the advantages of a relative lower density (2.4 g/cm 3), higher mechanical strength and easier processability, like that here described.