Oxide Superconductors Research Articles

Direct connection between Mott insulators and d -wave high-temperature superconductors revealed by continuous evolution of self-energy poles

The high-temperature superconductivity in copper oxides emerges when carriers are doped into the parent Mott insulator. This well-established fact has, however, eluded a microscopic explanation. Here we show that the missing link is the self-energy pole in the energy-momentum space. Its continuous evolution with doping directly connects the Mott insulator and high-temperature superconductivity. We show this by numerically studying the extremely small doping region close to the Mott insulating phase in a standard model for cuprates, the two-dimensional Hubbard model. We first identify two relevant self-energy structures in the Mott insulator; the pole generating the Mott gap and a relatively broad peak generating the so-called waterfall structure, which is another consequence of strong correlations present in the Mott insulator. We next reveal that either the Mott-gap pole or the waterfall structure (the feature at the energy closer to the Fermi level) directly transforms itself into another self-energy pole at the same energy and momentum when the system is doped with carriers. The anomalous self-energy yielding the superconductivity is simultaneously born exactly at this energy-momentum point. Thus created self-energy pole, interpreted as arising from a hidden fermionic excitation, continuously evolves upon further doping and considerably enhances the superconductivity. Above the critical temperature, the same self-energy pole generates a pseudogap in the normal state. We thus elucidate a unified Mott-physics mechanism, where the self-energy structure inherent to the Mott insulator directly gives birth to both the high-temperature superconductivity and pseudogap.

Three-dimensional collective charge excitations in electron-doped copper oxide superconductors.

High-temperature copper oxide superconductors consist of stacked CuO2 planes, with electronic band structures and magnetic excitations that are primarily two-dimensional1,2, but with superconducting coherence that is three-dimensional. This dichotomy highlights the importance of out-of-plane charge dynamics, which has been found to be incoherent in the normal state3,4 within the limited range of momenta accessible by optics. Here we use resonant inelastic X-ray scattering to explore the charge dynamics across all three dimensions of the Brillouin zone. Polarization analysis of recently discovered collective excitations (modes) in electron-doped copper oxides5-7 reveals their charge origin, that is, without mixing with magnetic components5-7. The excitations disperse along both the in-plane and out-of-plane directions, revealing its three-dimensional nature. The periodicity of the out-of-plane dispersion corresponds to the distance between neighbouring CuO2 planesrather than to the crystallographic c-axis lattice constant, suggesting that the interplane Coulomb interaction is responsible for the coherent out-of-plane charge dynamics. The observed properties are hallmarks of the long-sought 'acoustic plasmon', which is a branch of distinct charge collective modes predicted for layered systems8-12 and argued to play a substantial part in mediating high-temperature superconductivity10-12.

Ultrathin Bismuth Film on High-Temperature Cuprate Superconductor Bi2Sr2CaCu2O8+δ as a Candidate of a Topological Superconductor.

One of the key challenges in condensed-matter physics is to establish a topological superconductor that hosts exotic Majorana fermions. Although various heterostructures consisting of conventional BCS (Bardeen-Cooper-Schrieffer) superconductors as well as doped topological insulators were intensively investigated, no conclusive evidence for Majorana fermions has been provided. This is mainly because of their very low superconducting transition temperatures ( Tc) and small superconducting-gap magnitude. Here, we report a possible realization of topological superconductivity at very high temperatures in a hybrid of Bi(110) ultrathin film and copper oxide superconductor Bi2Sr2CaCu2O8+δ (Bi2212). Using angle-resolved photoemission spectroscopy and scanning tunneling microscopy, we found that three-bilayer-thick Bi(110) on Bi2212 exhibits a proximity-effect-induced s-wave energy gap as large as 7.5 meV which persists up to Tc of Bi2212 (85 K). The small Fermi energy and strong spin-orbit coupling of Bi(110), together with the large pairing gap and high Tc, make this system a prime candidate for exploring stable Majorana fermions at very high temperatures.

Signatures of charge-order correlations in transport properties of electron-doped cuprate superconductors

The high-temperature superconductivity in copper oxides emerges under strong influence of spin correlations in doped Mott insulators. Recent discoveries of charge-order (CO) correlations in Y-based hole-doped cuprates as well as in electron-doped cuprates suggest that charge correlations should also play an important role on the electronic states of cuprates, although those correlations have been so far detected mainly by x-ray scattering measurements. Here we show signatures of CO correlations in transport properties of electron-doped cuprates as anomalous enhancement of the metal-to-insulator crossover temperature (Tmin) appears in the limited doping range near the onset of superconductivity, while it decreases exactly when superconductivity sets in. We explain this non-monotonous peak-like behavior of Tmin in terms of the evolution of the electronic states through development of CO correlations and appearance of the hole pockets in the folded Fermi surface, which impact on transport properties consecutively at different locations in the momentum space.

Conspiring interactions in a cuprate

Superconductivity More than 30 years after the discovery of high-temperature superconductivity in copper oxides, its mechanism remains a mystery. Electron pairing mediated solely by lattice vibrations—phonons—is thought to be insufficient to account for the high transition temperatures. He et al. found a rapid and correlated increase of the superconducting gap and electron-phonon interactions as the chemical composition of their bismuth-based cuprate samples was varied across a critical doping concentration. The interplay of electron-phonon with electron-electron interactions may lead to enhanced transition temperatures. Science , this issue p. [62][1] [1]: /lookup/doi/10.1126/science.aar3394

Toward a topological scenario for high-temperature superconductivity of copper oxides

The structure of the joint phase diagram demonstrating high-Tc superconductivity of copper oxides is studied on the basis of the theory of interaction-induced flat bands. Prerequisites for an associated topological rearrangement of the Landau state are established, and related non-Fermi-liquid (NFL) behavior of the normal states of cuprates is investigated. We focus on manifestations of this behavior in the electrical resistivity ρ(T), especially the observed gradual crossover from normal-state T-linear behavior ρ(T,x)=A1(x)T at doping x below the critical value xch of hole doping for termination of superconductivity, to T-quadratic behavior at x>xch, which is incompatible with predictions of the conventional quantum-critical-point scenario. It is demonstrated that the slope of the coefficient A1 is universal, being the same on both boundaries of the joint phase diagram of cuprates, in agreement with available experimental data.

Normal-state properties of the antiperovskite oxide Sr3−xSnO revealed by Sn119 -NMR

We have performed $^{119}$Sn-NMR measurements on the antiperovskite oxide superconductor Sr$_{3-x}$SnO to investigate how its normal state changes with the Sr deficiency. A two-peak structure was observed in the NMR spectra of all the measured samples. This suggests that the phase separation tends to occur between the nearly stoichiometric and heavily Sr-deficient Sr$_{3-x}$SnO phases. The measurement of the nuclear spin-lattice relaxation rate $1/T_1$ indicates that the Sr-deficient phase shows a conventional metallic behavior due to the heavy hole doping. In contrast, the nearly stoichiometric phase exhibits unusual temperature dependence of $1/T_1$, attributable to the presence of a Dirac-electron band.

Analysis of optical data using extended Drude model and generalized Allen’s formulas

Extended Drude model formalism has been successfully utilized for analyzing optical spectra of strongly correlated electron systems including heavy-fermion systems and high-Tc superconducting iron pnictides and cuprates. Furthermore, generalized Allen’s formulas have been developed and applied to extract the electron-boson spectral density function from measured optical data of high temperature superconductors including cuprates in various material phases. Here we used a reverse process to obtain various optical quantities starting from two typical electron-boson spectral density model functions for three intriguing (normal, pseudogap, and d-wave superconducting) material phases in cuprates. We also assigned the calculated optical results to designated regions in the phase diagram of hole-doped cuprates and compared them with the corresponding measured optical spectra of Bi2Sr2CaCu2 (Bi-2212). This comparison suggested that this way of optical data analysis can be a convincing method to study correlated electrons in the copper oxide superconductors and other superconducting systems as well.

Growth of YBa2Cu3O7 superconductor thin films using ethanolamine-based solutions via simple spin coating

Abstract A simple spin coating technique was used to deposit YBa2Cu3O7 (YBCO) thin films on glass substrates using an ethanolamine-based solvent, which is a relatively safe, nontoxic, fluorine-free, and inexpensive solvent used in the production process of yttrium barium copper oxide (YBCO) superconductors. In this study, high quality YBCO thin films fabricated via spin coating under varying deposited gel film annealing temperatures are proposed. Moreover, YBCO thin films were obtained at a low annealing temperature of at least 400 °C. The X-ray diffraction analysis spectra of the fabricated YBCO thin films revealed various unique YBCO peaks at 23°, 32°, and 40°, respectively. Additionally, we observed that the diffraction intensity of each YBCO peak increased as the annealing temperature increased. As the annealing time became longer at 800 °C, the c-axis of the YBCO oriented peak became stronger. Consequently, the higher annealing temperature was assumed to have resulted in the enhancement of the YBCO crystal growth. In contrast, a longer annealing time enhanced the re-growth of the YBCO crystal to the c-axis orientation. In secondary electronic spectroscopy (SEM) images of the YBCO thin film and substrate cross sections, the obtained YBCO thin film thickness was approximately 3 µm for both high and low annealing temperature samples. The obtained cross section of the YBCO thin film images was flat and clear. This study established that annealing temperature and time are very important factors for obtaining high quality YBCO thin film crystals. Additionally, we assumed that annealing temperature and time are important parameters for obtaining highly oriented YBCO thin film crystals.

Study on Cold-Drawing in Powder-in-Tube Process in Fabricating Silver-Sheathed (Bi,Pb)-Sr-Ca-Cu-O Tapes

Since Cu oxide superconductors have been discovered, the application for this type of superconductor especially in the form of superconducting tape to be used as electric cable to transport the electricity. The aim to this study is to analyse the manufacture of superconducting wire with superconducting pellets as its precursor. A pellet disc of High Temperature Superconducting (HTS) Bi,Pb-Sr-Ca-Cu-O has been made by using solid state method. This pellet has been prepared by mixing each of oxide powders together, and then sintered at 865°C for 90 hours. We have analysed the samples using X-Ray Diffractometer (XRD), Energy Dispersive x-ray Spectroscopy (EDS), Scanning Electron Microscopy (SEM) and also resistivity measurement to confirm its superconductivity. After confirmed the superconducting properties of the pellet, it was grinded into powder and inserted into silver tube with outer diameter of 10 mm and thickness of 1 mm. And then, drawn to make monofilament wire. It were then cut and inserted into another silver tube, and then re-drawing and rolled into multifilament tapes using Powder-in-Tube (PIT) method. The multifilament tapes was heated at 850°C for 12 hours. We confirmed its phase crystalline and superconductivity at pellet disc. However, when the pellet disc was made into wire and sintered at 850°C, it became amorphous and conductivity behaviour was decline. According to SEM analysis and resistivity measurement result, silver also penetrated inside the sample region of multifilament wire.

Deposition of YBCO nanoparticles on graphene nanosheets by using matrix-assisted pulsed laser evaporation

Matrix-assisted pulsed laser evaporation (MAPLE) process demonstrates a versatile and contamination free deposition. Compared to conventional laser ablation deposition, MAPLE technique can realize the retention of the properties and structures of deposited biomaterials and hybrid nanocomposites without damaging the backbone of organic macromolecules. Here, MAPLE process is applied to deposit yttrium barium copper oxide (YBCO) superconductor particles on graphene sheets. The microstructures and elemental composition of YBCO nanoparticle deposited on graphene sheet by the MAPLE process were studied as a function of the irradiation time (t). The amount of YBCO nanoparticles deposited on graphene is increased with increasing t. The average particle size of the spherical YBCO nanoparticles deposited on graphene sheets is decreased from 70 ± 25 nm to 50 ± 15 nm when t increases from 0.5 h to 2.0 h. This study demonstrates that MAPLE is a suitable process for depositing superconductor nanoparticles on graphene sheets.

Polymeric Copper Oxide: Preparation and Investigation of Its Structure and Optical Properties

A complex of polyvinyl alcohol (PVA) with copper hydroxide was used as a precursor to obtain polymeric copper oxide through thermal decomposition. The absence of Cu(OH)2 crystalline phase was observed for the component ratio up to 1 Cu(OH)2 molecular unit to 3 PVA residuals. The formation of crystalline copper oxide was not observed after the dehydration of this material. UV–VIS and IR spectroscopy, and computational modeling were used to study the structure and properties of the obtained materials. A comparison with other similar materials was drawn. It was found that experimental data are in general accordance with the computations based on the polymeric model for copper hydroxide/oxide as a component of hybrid interpolymeric complex with PVA. A distinctive feature observed for polymeric copper oxide is strong broadening of the optical absorption band at 400 nm. It is suggested that this effect is caused by strong electron–phonon interaction, which is also responsible for superconductivity of copper oxide based systems.

Perovskites: a class of materials with multiple functionalities and applications

First discovered in 1839 by Gustav Rose and named after the Russian mineralogist L. A. Perovksi, the perovskites have been extensively studied. These materials have a wide range of properties and many potential applications. The discovery of high temperature superconductivity in layered copper oxides in the mid-eighties and of efficient photovoltaic properties in hybrid organic-inorganic perovskite solar cells less than 10 years ago, have boosted the research efforts on these materials as well as the number of yearly publications.

Oxygen-hole pairs drive bond order and superconductivity in bismuth oxides

Oxygen-hole pairs drive bond order and superconductivity in bismuth oxides

Controlled synthesis of the antiperovskite oxide superconductor Sr3−xSnO

A large variety of perovskite oxide superconductors are known, including some of the most prominent high-temperature and unconventional superconductors. However, superconductivity among the oxidation state inverted material class, the antiperovskite oxides, was recently reported for the first time. In this superconductor, Sr3−xSnO, the unconventional ionic state Sn4− is realized and possible unconventional superconductivity due to a band inversion has been discussed. Here, we discuss an improved facile synthesis method, making it possible to control the strontium deficiency in Sr3−xSnO. Additionally, a synthesis method above the melting point of Sr3SnO is presented. We show temperature dependence of magnetization and electrical resistivity for superconducting strontium deficient Sr3−xSnO (Tc ∼ 5 K) and for Sr3SnO without a superconducting transition in alternating current susceptibility down to 0.15 K. Further, we reveal a significant effect of strontium raw material purity on the superconductivity and achieve substantially increased M/MMeissner (∼1) compared to the highest value reported so far. More detailed characterizations utilizing powder x-ray diffraction and energy-dispersive x-ray spectroscopy show that a minor cubic phase, previously suggested to be another Sr3−xSnO phase with a slightly larger lattice parameter, is SrO. The improved characterization and controlled synthesis reported herein enable detailed investigations on the superconducting nature and its dependency on the strontium deficiency in Sr3−xSnO.

Bending–Peeling Method to Measure Interface Strength of YBCO Tape

Modern yttrium barium copper oxide (YBCO) superconductor tape can carry a high critical current under a rather high magnetic field with good strain tolerance. However, the interface between the laminas is quite weak, and the interfacial strength has thus attracted considerable research interests. We designed a new method to analyze the interfacial strength of YBCO tape. A piece of tape was first embedded in a rectangular-section beam of epoxy resin, with the tape plane parallel to the middle cross section of the beam and the tape edge at the beam surface. The epoxy beam height was greater than the tape width. Three-point bending was applied to the beam, with one point on the opposite side of the tape across the beam and the other two points at the two ends. The interface was thus tensed during loading. In this way, the interfacial strength could be directly determined, and the interfacial energy could be calculated from the force–displacement curve. The crack interface was examined using scanning electron microscopy and analyzed using energy-dispersive X-ray spectroscopy. The average interface strength was 5.16 MPa. The experimental data were used to further simulate the crack behavior during the cooling process.

2p-YG-10 酸化物超伝導体Bi-2212の劈開面に現れる転位に伴う微細構造の微分干渉顕微鏡による観察

2p-YG-10 酸化物超伝導体Bi-2212の劈開面に現れる転位に伴う微細構造の微分干渉顕微鏡による観察

Gate-Induced Interfacial Superconductivity in 1T-SnSe2.

Layered metal chalcogenide materials provide a versatile platform to investigate emergent phenomena and two-dimensional (2D) superconductivity at/near the atomically thin limit. In particular, gate-induced interfacial superconductivity realized by the use of an electric-double-layer transistor (EDLT) has greatly extended the capability to electrically induce superconductivity in oxides, nitrides, and transition metal chalcogenides and enable one to explore new physics, such as the Ising pairing mechanism. Exploiting gate-induced superconductivity in various materials can provide us with additional platforms to understand emergent interfacial superconductivity. Here, we report the discovery of gate-induced 2D superconductivity in layered 1T-SnSe2, a typical member of the main-group metal dichalcogenide (MDC) family, using an EDLT gating geometry. A superconducting transition temperature Tc ≈ 3.9 K was demonstrated at the EDL interface. The 2D nature of the superconductivity therein was further confirmed based on (1) a 2D Tinkham description of the angle-dependent upper critical field Bc2, (2) the existence of a quantum creep state as well as a large ratio of the coherence length to the thickness of superconductivity. Interestingly, the in-plane Bc2 approaching zero temperature was found to be 2-3 times higher than the Pauli limit, which might be related to an electric field-modulated spin-orbit interaction. Such results provide a new perspective to expand the material matrix available for gate-induced 2D superconductivity and the fundamental understanding of interfacial superconductivity.

First complex oxide superconductor by atomic layer deposition.

We here report on atomic layer deposition (ALD) of the superconducting complex oxide La2-xSrxCuO4-y and provide details of the structural and electrical properties of such films. This is the first report on a complex oxide thin film with superconducting properties that has been deposited by atomic layer deposition.

Angle-resolved photoemission studies on iron based high temperature superconductors

Copper oxide superconductors and iron-based superconductors are two important families of high temperature superconductors. Their high-temperature superconductivity mechanism is a long-standing issue and still in hot debate in the field of condensed matter physics. The extensive and in-depth exploration of iron-based superconductors and their comparative study with copper oxide high-temperature superconductors are of great significance for the development of new quantum theory, the solution of high-temperature superconducting mechanism, the exploration of new superconductors and practical applications of superconductors. The macroscopic properties of materials are determined by their microscopic electronic structure. Revealing the microscopic electronic structure of high temperature superconductors is fundamental for understanding high temperature superconductivity. Angle-resolved photoelectron spectroscopy, due to its unique simultaneous energy, momentum and even spin resolving ability, has become the most direct and powerful experimental tool for detecting the microscopic electronic structure of materials, and has played an important role in the study of iron-based high-temperature superconductors. The revealing and discovery of the Fermi surface topology, superconducting energy gap and its symmetry, three-dimensionality, orbital selectivity, and electronic coupling mode in different iron-based superconductor systems provide an important basis for identifying and proposing new theory of iron-based superconductivity to solve high temperature superconductivity mechanism.