high-Tc Oxide Research Articles

Novel Phase YBa2Cu3O5 derived from the High Tc Superconductor YBa2Cu3O7-x

The orthorhombic unit cell of high Tc oxide superconductor YBa2Cu3O7–x is indicated by two-dimensional CuO2 planes spanned in a-b directions. Partial removal of oxygen, down to x > 1, creates an insulating state and randomly distributed oxygen vacancies. An attempt has been made to increase the concentration of vacancies up to some critical value when their ordering could take place, while balance with copper cations will still maintain the original tetragonal structure. Removal of two oxygen atoms per unit cell results in the YBa2Cu3O5 compound and formation of linear Cu1+– O chains. The novel phase is an insulator, and doping in a low (50 –200 mbar) oxygen atmosphere at T < 800 K converts it to a conducting state.

Ambient pressure synthesis and structure and magnetic properties of a new A- and B-site ordered multinary quadruple perovskite.

Quadruple perovskites with high magnetic transition temperatures are an interesting class of compounds but are synthesized typically under high pressure. Ambient pressure synthesis of new multinary quadruple perovskites having a high global instability index (GII) and transition temperature can be interesting for future exploration of high-TC oxides. A new A- and B-site ordered multinary quadruple perovskite, LaCu3Fe2RuSbO12, is synthesized by conventional solid-state reactions at ambient pressure. Rietveld structure refinement revealed that the compound crystallizes in the Pn3̄ space group with a lattice parameter of 7.4556(4) Å. The compound showed complete 1 : 3 ordering of La and Cu at the A-site and 1 : 1 rock-salt ordering of Fe with Ru/Sb at the B-site. The compound is also probed with scanning and transmission electron microscopy and XPS to investigate the chemical composition, microstructure, lattice and oxidation states of the elements. Magnetic studies revealed antiferromagnetic (AFM) correlations with magnetic ordering transitions at ∼170 and 40 K. Furthermore, the M-H hysteretic behavior at 100 and 5 K indicated ferrimagnetism due to short-range AFM interactions among Fe3+(3d5) and Ru4+(4d4) spins involving Cu2+(↑)-Fe3+(↓)-Ru4+(↑) triads. The specific heat data reaffirmed the magnetic signatures while electrical transport showed semiconducting behavior with variable range hopping. The details of synthesis and structural and compositional studies along with the magnetic and electrical transport properties of LaCu3Fe2RuSbO12 are reported in this paper.

In Pursuit of the Exceptional: Research Directions for Machine Learning in Chemical and Materials Science.

Exceptional molecules and materials with one or more extraordinary properties are both technologically valuable and fundamentally interesting, because they often involve new physical phenomena or new compositions that defy expectations. Historically, exceptionality has been achieved through serendipity, but recently, machine learning (ML) and automated experimentation have been widely proposed to accelerate target identification and synthesis planning. In this Perspective, we argue that the data-driven methods commonly used today are well-suited for optimization but not for the realization of new exceptional materials or molecules. Finding such outliers should be possible using ML, but only by shifting away from using traditional ML approaches that tweak the composition, crystal structure, or reaction pathway. We highlight case studies of high-Tc oxide superconductors and superhard materials to demonstrate the challenges of ML-guided discovery and discuss the limitations of automation for this task. We then provide six recommendations for the development of ML methods capable of exceptional materials discovery: (i) Avoid the tyranny of the middle and focus on extrema; (ii) When data are limited, qualitative predictions that provide direction are more valuable than interpolative accuracy; (iii) Sample what can be made and how to make it and defer optimization; (iv) Create room (and look) for the unexpected while pursuing your goal; (v) Try to fill-in-the-blanks of input and output space; (vi) Do not confuse human understanding with model interpretability. We conclude with a description of how these recommendations can be integrated into automated discovery workflows, which should enable the discovery of exceptional molecules and materials.

Coexistence of superconductivity and charge-density waves: evidence from tunneling studies (Review article)

The review summarizes certain theoretical and experimental studies of the quasiparticle, J, and Josephson, IJ, currents between superconductors partially dielectrized with charge-density waves (CDWs). A relation between the Cooper and electron-hole pairings and their manifestations in high-Tc oxides have been analyzed. It was shown that the CDW fingerprints can be found in a lot of already observed dependences. All described results are considered in the framework of the historical context.

Local inversion-symmetry breaking in a bismuthate high-Tc superconductor

The doped perovskite BaBiO3 exhibits a maximum superconducting transition temperature (Tc) of 34 K and was the first high-Tc oxide to be discovered, yet pivotal questions regarding the nature of both the metallic and superconducting states remain unresolved. Although it is generally thought that superconductivity in the bismuthates is of the conventional s-wave type, the pairing mechanism is still debated, with strong electron-phonon coupling and bismuth valence or bond disproportionation possibly playing a role. Here we use diffuse x-ray scattering and Monte Carlo modeling to study the local structure of Ba1-xKxBiO3 across its insulator-metal boundary. We find no evidence for either long- or short-range disproportionation, which resolves a major conundrum, as disproportionation and the related polaronic effects are likely not relevant for the metallic and superconducting states. Instead, we uncover nanoscale structural correlations that break inversion symmetry, with far-reaching implications for the electronic physics. This unexpected finding furthermore establishes that the bismuthates belong to the broader classes of materials with hidden spin-orbit coupling and a tendency towards inversion-breaking displacements.

Spectroscopic fingerprint of charge order melting driven by quantum fluctuations in a cuprate

Copper oxide high-TC superconductors possess a number of exotic orders that coexist with or are proximal to superconductivity. Quantum fluctuations associated with these orders may account for the unusual characteristics of the normal state, and possibly affect the superconductivity1–4. Yet, spectroscopic evidence for such quantum fluctuations remains elusive. Here, we use resonant inelastic X-ray scattering to reveal spectroscopic evidence of fluctuations associated with a charge order5–14 in nearly optimally doped Bi2Sr2CaCu2O8+δ. In the superconducting state, while the quasielastic charge order signal decreases with temperature, the interplay between charge order fluctuations and bond-stretching phonons in the form of a Fano-like interference increases, an observation that is incompatible with expectations for competing orders. Invoking general principles, we argue that this behaviour reflects the properties of a dissipative system near an order–disorder quantum critical point, where the dissipation varies with the opening of the pseudogap and superconducting gap at low temperatures, leading to the proliferation of quantum critical fluctuations, which melt charge order. X-ray scattering experiments show that the quantum fluctuations associated with charge order take a form that is incompatible with the idea of competition between charge order and superconductivity.

Break-junction tunneling spectra of Bi2212 superconducting ceramics: Influence of inhomogeneous d-wave-Cooper-pairing and charge-density-wave order parameters

Conductance-voltage characteristics (CVCs) of tunnel break junctions made of Bi2Sr2CaCu2O8+δ crystals were measured. It was demonstrated that the CVCs have a V-shaped inner gap region, similar to those typical of CVCs for tunnel junctions between d-wave superconductors. The CVCs have different forms for different junctions, but all of them reveal weak dip-hump structures outside the inner gap region. Calculations of the tunnel current in the ab plane of the break junctions were carried out in the model of the inhomogeneous d-wave superconductor partially gapped by charge density waves (CDWs). The averaging of the tunnel current over the statistical distributions of both the superconducting and CDW order parameters was carried out. The theoretical results qualitatively reproduce the behavior of experimental curves. A conclusion was made that tunnel directionality and the statistical distributions of both order parameters are crucial factors governing the observed CVC shapes for break junctions made of high-Tc oxides.

Experimental evidence of a very thin superconducting layer in epitaxial indium nitride

Indium nitride is one of the very few semiconductors which is known to have a superconducting phase at temperatures of Tc > 1 K. Superconductivity occurs in a window of carrier densities of approximately 1018–1020 cm−3. This is a very low density when compared to other superconductors (i.e. metals, alloys, high Tc oxides) and thus raises interesting fundamental questions as well as technological possibilities. In this paper we address one key question about the dimensionality of the superconducting state of InN by using angle dependent critical field measurements. Our samples were grown by two different growth techniques (chemical vapour deposition and plasma-assisted molecular beam epitaxy) on c-oriented sapphire, with and without a GaN buffer layer. In both cases we find that for film thicknesses much larger than the coherence length d ≫ ξ, the angle dependence of the critical field (down to T < 280 mK) with respect to the c-axis continues to be clearly two-dimensional, demonstrating a characteristic cusp when the angle crosses 90° with respect to the c-axis. This indicates that the superconducting electrons are most likely confined to a layer much thinner than the thickness of the InN film. Further we find the magnitude of the gap to be 2Δ(0)/kBTc = 3.6, very close to the BCS prediction.

A Superconducting Dual-Channel Photonic Switch.

The mechanism of Cooper pair formation and its underlying physics has long occupied the investigation into high temperature (high-Tc ) cuprate superconductors. One of the ways to unravel this is to observe the ultrafast response present in the charge carrier dynamics of a photoexcited specimen. This results in an interesting approach to exploit the dissipation-less dynamic features of superconductors to be utilized for designing high-performance active subwavelength photonic devices with extremely low-loss operation. Here, dual-channel, ultrafast, all-optical switching and modulation between the resistive and the superconducting quantum mechanical phase is experimentally demonstrated. The ultrafast phase switching is demonstrated via modulation of sharp Fano resonance of a high-Tc yttrium barium copper oxide (YBCO) superconducting metamaterial device. Upon photoexcitation by femtosecond light pulses, the ultrasensitive cuprate superconductor undergoes dual dissociation-relaxation dynamics, with restoration of superconductivity within a cycle, and thereby establishes the existence of dual switching windows within a timescale of 80 ps. Pathways are explored to engineer the secondary dissociation channel which provides unprecedented control over the switching speed. Most importantly, the results envision new ways to accomplish low-loss, ultrafast, and ultrasensitive dual-channel switching applications that are inaccessible through conventional metallic and dielectric based metamaterials.

Tail-like regime and BCS-BEC crossover due to hybridization in a two-band superconductor

Superconductivity in strongly correlated systems is a remarkable phenomenon that attracts huge interest. The study of this problem is relevant for materials such as the high Tc oxides, pnictides and heavy fermions. These systems also have in common the existence of electrons of several orbitals that coexist at a common Fermi surface. In this paper we study the effect of pressure, chemical or applied on multi-band superconductivity. Pressure varies the atomic distances and consequently the overlap of the wave-functions in the crystal. This rearranges the electronic structure that we model including a pressure dependent hybridization between the bands. We consider the case of two-dimensional systems in a square lattice with inverted bands. We study the conditions for obtaining a pressure induced superconductor quantum critical point and show that hybridization, i.e. pressure can induce a Bardeen–Cooper–Schrieffer-Bose–Einstein condensation crossover in multi-band systems even for moderate interactions. We found a tail-like superconductor regime and briefly discuss the influence of the symmetry of the order parameter in the results.

Spin susceptibility of charge-ordered YBa2Cu3Oy across the upper critical field

The value of the upper critical field Hc2, a fundamental characteristic of the superconducting state, has been subject to strong controversy in high-Tc copper oxides. Since the issue has been tackled almost exclusively by macroscopic techniques so far, there is a clear need for local-probe measurements. Here, we use 17O NMR to measure the spin susceptibility [Formula: see text] of the CuO2 planes at low temperature in charge-ordered YBa2Cu3O y We find that [Formula: see text] increases (most likely linearly) with magnetic field H and saturates above field values ranging from 20 T to 40 T. This result is consistent with the lowest Hc2 values claimed previously and with the interpretation that the charge density wave (CDW) reduces Hc2 in underdoped YBa2Cu3O y Furthermore, the absence of marked deviation in [Formula: see text] at the onset of long-range CDW order indicates that this [Formula: see text] reduction and the Fermi-surface reconstruction are primarily rooted in the short-range CDW order already present in zero field, not in the field-induced long-range CDW order. Above [Formula: see text], the relatively low values of [Formula: see text] at [Formula: see text] K show that the pseudogap is a ground-state property, independent of the superconducting gap.

Quasiparticle conductance-voltage characteristics for break junctions involving d-wave superconductors: charge-density-wave effects

Quasiparticle tunnel conductance-voltage characteristics (CVCs), , were calculated for break junctions (BJs) made up of layered d-wave superconductors partially gapped by charge-density waves (CDWs). The current is assumed to flow in the ab-plane of electrodes. The influence of CDWs is analyzed by comparing the resulting CVCs with CVCs calculated for BJs made up of pure d-wave superconductors with relevant parameters. The main CDW-effects were found to be the appearance of new CVC peculiarities and the loss of CVC symmetry with respect to the V-sign. Tunnel directionality was shown to be one of the key factors in the formation of dependences. In particular, the orientation of electrodes with respect to the current channel becomes very important. As a result, can acquire a large variety of forms similar to those for tunnel junctions between superconductors with s-wave, d-wave, and mixed symmetry of their order parameters. The diversity of peculiarities is especially striking at finite temperatures. In the case of BJs made up of pure d-wave superconductors, the resulting CVC can include a two-peak gap-driven structure. The results were compared with the experimental BJ data for a number of high-Tc oxides. It was shown that the large variety of the observed current–voltage characteristics can be interpreted in the framework of our approach. Thus, quasiparticle tunnel currents in the ab-plane can be used as an additional mean to detect CDWs competing with superconductivity in cuprates or other layered superconductors.

Influence of the spatially inhomogeneous gap distribution on the quasiparticle current in c-axis junctions involving d-wave superconductors with charge density waves

The quasiparticle tunnel current J(V) between the superconducting ab-planes along the c-axis and the corresponding conductance were calculated for symmetric junctions composed of disordered d-wave layered superconductors partially gapped by charge density waves (CDWs). Here, V is the voltage. Both the checkerboard and unidirectional CDWs were considered. It was shown that the spatial spread of the CDW-pairing strength substantially smears the peculiarities of G(V) appropriate to uniform superconductors. The resulting curves G(V) become very similar to those observed for a number of cuprates in intrinsic junctions, e.g. mesas. In particular, the influence of CDWs may explain the peak-dip-hump structures frequently found for high-Tc oxides.

Formula omitted]- and [formula omitted]-wave superconductivity in a two-band model

Superconductivity in strongly correlated systems is a remarkable phenomenon that attracts a huge interest. The study of this problem is relevant for materials such as the high Tc oxides, pnictides and heavy fermions. In this work we study a realistic model that includes the relevant physics of superconductivity in the presence of strong Coulomb correlations. We consider a two-band model, since most of these correlated systems have electrons from at least two different atomic orbitals coexisting at their Fermi surface. The Coulomb repulsion is taken into account through a local repulsive interaction. Pairing is considered among quasi-particles in neighboring sites and we allow for different symmetries of the order parameter. In order to deal with the strong local correlations, we use the well known slave boson approach that has proved very successful for this problem. Here we are interested in obtaining the zero temperature properties of the model, specifically its phase diagram and the existence and nature of superconducting quantum critical points. We show that these can arise by increasing the mixing between the two bands. Since this can be controlled by external pressure or doping, our results have a direct relation with experiments. We show that the superconductor-to-normal transition can be either to a metal, a correlated metal or to an insulator. Also we compare the relative stability of s and d-wave paired states for different regions of parameter space and investigate the BCS–BEC crossover in the two-band lattice model as function of the strength of the pairing interaction.

Normal and Superconducting Properties of the High-Tc Oxides. The Van Hove Scenario

Normal and Superconducting Properties of the High-Tc Oxides. The Van Hove Scenario

Quasiparticle self-consistent GW study of cuprates: electronic structure, model parameters and the two-band theory for Tc

Despite decades of progress, an understanding of unconventional superconductivity still remains elusive. An important open question is about the material dependence of the superconducting properties. Using the quasiparticle self-consistent GW method, we re-examine the electronic structure of copper oxide high-Tc materials. We show that QSGW captures several important features, distinctive from the conventional LDA results. The energy level splitting between and is significantly enlarged and the van Hove singularity point is lowered. The calculated results compare better than LDA with recent experimental results from resonant inelastic xray scattering and angle resolved photoemission experiments. This agreement with the experiments supports the previously suggested two-band theory for the material dependence of the superconducting transition temperature, Tc.

Paramagnon excitations’ theory for resonant inelastic X-ray scattering in doped plane copper oxide superconductors

A relaxation function theory with paramagnon excitations for doped S=1/2 two-dimensional Heisenberg antiferromagnetic system in the paramagnetic state is given in view of magnetic response of high-Tc copper oxide superconductors as obtained by resonant inelastic X-ray scattering (RIXS). The results of the theory on Nd(La)–Ba(Sr)–Cu–O and Y–Ba–Cu–O family compounds give fair agreement without especially adjusted parameters to RIXS data. It is shown that RIXS data analysis depends on paramagnon damping and thus affected by approximations made for dynamic spin susceptibility.

Magnetic, electrical and structural properties of the Re-doped ruthenocuprate Ru1−xRexSr2GdCu2Oy

Despite the discovery of new superconductors classes, high-Tc oxides continue to be a current topic, because of their complex phase diagrams and doping-dependant effects (allowing one to investigate the interaction between orbitals), as well as structural properties such as lattice distortion and charge ordering, among many others. Ruthenocuprates are magnetic superconductors in which the magnetic transition temperature is much higher than the critical superconducting temperature, making them unique compounds. With the aim of investigating the dilution of the magnetic Ru sub-lattice, we proposed the synthesis of the Ru1−xRexSr2GdCu2Oy ruthenocuprate-type family, adapting the known two-step process (double perovskite+CuO) by directly doping the double perovskite, thus obtaining the perovskite compound Sr2GdRu1−xRexOy, which represents a new synthesis process to the best of our knowledge. Our samples were structurally characterized through X-ray diffraction, and the patterns were analysed via Rietveld refinement. A complete magnetic characterization as a function of temperature and applied field, as well as transport measurements were carried out. We discuss our results in the light of the two-lattice model for ruthenocuprates, and a relation between RuO2 (magnetic) and CuO2 (superconductor) sub-lattices can clearly be observed.

Superstripes and Percolating Nanoscale-Striped Puddles in Heterostructures at Atomic Limit

Lattice and electronic nanoscale phase separation in strongly correlated multiband systems confined in heterostructure at atomic limit called superstripes has been an object of the scientific debate at the international conference Superstripes 2013 focusing on “Quantum in Complex Matter: Superconductivity, Magnetism and Ferroelectricity” held in Ischia, Italy (May 27–June 1, 2013). The focus was on lattice granularity due to defects self-organization, lattice modulations at a critical misfit strain, and electronic phase separation in multiband Hubbard models near a 2.5 Lifshitz transition. The emerging superstripes scenario is a particular case of percolation superconductivity in networks of superconducting multicondensates superconducting puddles and their competition with phase-separated networks of nanoscale-striped magnetic puddles. This new emerging paradigm for high-Tc superconductor-layered oxides opens new perspectives for quantum electronics by controlling the complexity in functional oxides.

Theory of Antiferromagnetic Order in High-Tc Oxides: An Approach Based on Ginzburg–Landau Expansion

The mean-field phase diagram of antiferromagnetic order in t-J model has been examined, using the free energy obtained by Ginzburg-Landau (GL) expansion. We extended the usual GL theory in two ways: First, we have included higher order terms with respect to the spatial derivative (or wave number) to incorporate the incommensurate antiferromagnetic order. Second, we have also included higher order terms with respect to the order parameter amplitude, in order to treat the first order phase transition between paramagnetic and antiferromagnetic phase, which appears at some doping rates. We found the possibility of tricritical point and critical endpoint in the magnetic phase diagram of the high-Tc oxides associated with the commensurate and incommensurate antiferromagnetic order. The possible effects of thermal fluctuations and randomness (spin glass) are also discussed qualitatively based on the GL free energy.