High-temperature Oxide Superconductors Research Articles

Wavelength dependence of linear birefringence and linear dichroism of Bi2−xPbxSr2CaCu2O8+δ single crystals

Copper oxide high-temperature superconductors, such as Bi2Sr2CaCu2O8+δ (Bi2212), have garnered extensive research interest due to their high critical temperatures (Tc) surpassing the Bardeen–Cooper–Schrieffer (BCS) limit. The two-dimensional CuO₂ plane is widely regarded as the most crucial element of high-Tc cuprate superconductors. The anisotropy of this CuO₂ layer remains a topic of ongoing interest. Although a few experimental results have reported strong optical anisotropy in both ab and ac-planes of Bi2212 through optical “reflectivity” measurements, there is a lack of studies focusing on the optical anisotropy of these materials using optical “transmittance” measurements by utilizing ultraviolet and visible light. Using a generalized high-accuracy universal polarimeter, we observed significant linear birefringence and linear dichroism peaks in the UV region at room temperature. To investigate the origin of the significant optical anisotropy, single crystals of Bi2−xPbxSr2CaCu2O8+δ with different lead contents (x = 0, 0.4, and 0.6) were grown using the floating zone method, and the wavelength dependencies of linear birefringence and linear dichroism along the c axis were measured. The insights gained into the optical anisotropy of Bi2−xPbxSr2CaCu2O8+δ from this study are significant for discussing its origin of the mechanism of high-Tc superconductivity.

Investigation of Theoretical Considerations of High Temperature Oxide Superconductors

Investigation of Theoretical Considerations of High Temperature Oxide Superconductors

Exploring the Structural and Superconducting Properties of High Temperature Oxide Superconductors

Exploring the Structural and Superconducting Properties of High Temperature Oxide Superconductors

Multilayer neural network models for critical temperature of cuprate superconductors

A multi-layer neural network is used to extract the value of a superconductor’s Tc of cuprate from two models. The first model extracts Tc from the structure’s chemical composition whereas the second model extracts Tc from the structure’s chemical composition and the lattice parameters. The back-propagation algorithm is used to find the empirical equation of Tc. It can calculate error signals and redistribute backward propagation signals. This paper studies four systems of cuprate superconductors: YBaCuO, BiSrCaCuO, TlBaCaCuO, and HgBaCaCuO. In the first model, the Tc of four high-temperature oxide superconductors are calculated as a function of eight parameters and one output, which is Tc. Although the same output is produced in the second model, it is produced as a function of eleven parameters. The eight parameters are superconductor type number (Bi2212, Bi2223, Hg1201, Hg1212, Hg1223, Y123, Y124, Y247, Tl1223, Tl2212, Tl2223), first component composition, second component composition, third component composition, fourth component composition, atomic number of doping type, doping composition, and oxygen composition of the first model. The previous parameters with the three lattice parameters a, b and c are used in the second model. The trained deep learning models have shown a high degree of performance in matching the trained distributions. After analysing the results, we deduce electronegativity plays an important role in increasing Tc of cuprate superconductors. Using the obtained Tc prediction model, the scope is expanded to include the eleven unexplored multi-element materials. Candidates for superconductors with a higher Tc that can be synthesized are proposed. By comparison with other models of machine learning, the suggested models in this paper give the highest Tc for predicting new cuprate superconductors.

The strange metal state of the high-Tc cuprates

This paper is dedicated to the memory of Professor K. Alex Müller. I present a few remarks about my interactions with Alex over the years. Then I present a very brief summary of recent transport studies of the strange metal normal-state in the high-temperature copper oxide superconductors, a subject of great interest to Alex.

Tunable Photonic Band Gaps in Two-Dimensional Bravais–Moiré Photonic Crystal Composed of High-Tc Superconductors

In this study, we perform a theoretical study of light propagation properties in two-dimensional square photonic crystals (PCs) following Bravais–Moiré (BM) patterns composed of copper oxide high-temperature superconductors (HTSCs). The BM PCs are made of cylindrical cores formed from the combination of two square Bravais lattices. The Moiré pattern forms due to a commensurable rotation of one of these lattices with respect to the other. The dielectric function of the superconducting material is modeled by the two-fluid Gorter–Casimir theory. We report on the corresponding gap, the mapping as a function of the radius of dielectric cores, as well as the dispersion relations of TM modes for BM PCs and for the waveguide system built of defect lines within such a crystal. The BM PCs were composed of copper oxide HTSCs, which exhibit large tunability in terms of temperature.

Pairing correlation of the three-band Hubbard model with the nearest attraction

To gain a deeper understanding of the high-temperature superconductivity in doped cuprates, the constrained-path quantum Monte Carlo method was used to explore the superconducting pairing correlation function and effective pairing interaction in the three-band Hubbard model. Our main observation was that in the low doping region, compared to other symmetries, [Formula: see text]-wave superconducting pairing dominates. The Coulomb interaction [Formula: see text] suppresses [Formula: see text]-wave pairing correlation, while the effective pairing interaction is both enhanced by the increasing [Formula: see text] and the nearest attraction. These results are used to understand the superconducting state in copper oxide high-temperature superconductors.

Strain-Induced Ferromagnetic to Antiferromagnetic Crossover in d9-Ion (Cu2+ and Ag2+)-Layered Perovskites

A characteristic aspect of undoped high-temperature layered copper oxide superconductors is their strong in-plane antiferromagnetic coupling. This state is markedly different from that found in other chemically similar copper- or silver-layered fluorides, which display a ferromagnetic ground state. The latter has been connected in the literature with the presence of an orthorhombic deformation of the lattice that shifts the intermediate ligand between two metal ions to be closer to one and further from the other. This distortion is completely absent in the oxides, which are essentially tetragonal. However, no quantitative information exists about how this distortion influences the antiferromagnetic state and its relative stability with respect to the ferromagnetic state. Here, we carry out first-principles simulations to show that the fluorides in the parent tetragonal phase are also antiferromagnetic and that the antiferromagnetic-to-ferromagnetic transition is only triggered for a large enough distortion, with a typical ligand shift of 0.1 Å. Moreover, we employ a valence-bond model and second-principles simulations to show that the factor in superexchange that favors the antiferromagnetic state reduces as the ligand moves away from the symmetric metal–metal position. Importantly, we find that this distortion is sensitive to the application of an epitaxial strain which, in turn, allows controlling the difference of energy between ferromagnetic and antiferromagnetic states and thus the Curie or Néel temperatures. In fact, for compressive strains larger than 5.1%, this piezomagnetic effect makes K2CuF4 and Cs2AgF4 antiferromagnetic, making these two lattices close chemical analogs of oxide superconductors.

Unveiling the Degradation Mechanism of High-Temperature Superconductor Bi2Sr2CaCu2O8+δ in Water-Bearing Environments.

The physical properties of copper oxide high-temperature superconductors have been studied extensively, such as the band structure and doping effects of Bi2Sr2CaCu2O8+δ (Bi-2212). However, some chemical-related properties of these superconductors are rarely reported, such as their stability in water-bearing environments. Herein, we report experiments combined with ab initio calculations that address the effects of water in contact with Bi-2212. The evolution of Bi-2212 flakes with exposure to water for different time intervals was tested and characterized by optical microscopy (OM), atomic force microscopy (AFM), Raman spectroscopy, transmission electron microscopy (TEM), and electrical measurements. The thickness of Bi-2212 flakes is gradually decreased in water, and some thin flakes can be completely etched away after a few days. The stability of Bi-2212 in other solvents is also evaluated, including alcohol, acetone, HCl, and KOH. The morphology of Bi-2212 flakes is relatively stable in organic solvents. However, the flakes are etched relatively quick in HCl and KOH, especially in an acidic environment. Our results imply that hydrogen ions are primarily responsible for the deterioration of their properties. Both TEM and calculation results demonstrate that the atoms in the Bi-O plane are relatively stable when compared to the inner atoms in Sr-O, Ca-O, and Cu-O planes. This work contributes toward understanding the chemical stability of a Bi-2212 superconducting device in environmental medium, which is important for both fundamental studies and practical applications of copper oxide high-temperature superconductors.

Simulation of switching to voltage state in an intrinsic Josephson junction

When a current applied to an intrinsic Josephson junction (IJJ) in a copper oxide high-temperature superconductor is increased, the IJJ enters a voltage state and then the voltage increases stepwise. After these voltage states, when the current is lowered, THz oscillation occurs. The voltage state is described as a running state of a particle in a washboard potential, using the RCSJ model of the Josephson junction (JJ). In this study, we investigate these running states in the IJJ, using a coupled layered JJ model with the finite element method. Especially, we focus on time evolution of phase differences in all JJs in the IJJ under weak external current.

Disorder-robust phase crystal in high-temperature superconductors stabilized by strong correlations

The simultaneous interplay of strong electron–electron correlations, topological zero-energy states, and disorder is yet an unexplored territory but of immense interest due to their inevitable presence in many materials. Copper oxide high-temperature superconductors (cuprates) with pair breaking edges host a flat band of topological zero-energy states, making them an ideal playground where strong correlations, topology, and disorder are strongly intertwined. Here we show that this interplay in cuprates generates a fully gapped ‘phase crystal’ state that breaks both translational and time-reversal invariance, characterized by a modulation of the d-wave superconducting phase co-existing with a modulating extended s-wave superconducting order. In contrast to conventional wisdom, we find that this phase crystal state is remarkably robust to omnipresent disorder, but only in the presence of strong correlations, thus giving a clear route to its experimental realization.

Editors’ Choice—A Fruitful Transition of John B. Goodenough from Oxford to the University of Texas at Austin

It is a privilege for me to write this brief perspective on the transition of John B. Goodenough from Oxford to the University of Texas at Austin (UT Austin). While most people will tend to retire, Goodenough transitioned to UT Austin at the age of 64 in 1986 and has been making phenomenal contributions during the past 35 years, with a genuine passion for science and profound impact on the society at large. To highlight a few, the contributions at UT Austin include the following: chemistry and physics of high-temperature copper oxide superconductors; battery electrodes and electrolytes, including polyanion family of oxide cathodes, niobium titanium oxide anode, and Prussian Blue cathode; alkali-metal plating/stripping; solid-state batteries; flow batteries; oxygen reduction/evolution reaction catalysts; and solid oxide fuel cell electrodes and electrolytes. Finally, what distinguishes Goodenough more than anything else is his unique personal attributes.

Critical Current Density and Meissner Effect of Smart Meta-Superconductor MgB2 and Bi(Pb)SrCaCuO.

The smart meta-superconductor MgB2 and Bi(Pb)SrCaCuO increase the superconducting transition temperature (TC), but the changes in the transport critical current density (JC) and Meissner effect are still unknown. Here, we investigated the JC and Meissner effect of smart meta-superconductor MgB2 and Bi(Pb)SrCaCuO. The use of the standard four-probe method shows that Y2O3:Eu3+ and Y2O3:Eu3++Ag inhomogeneous phase significantly increase the JC, and JC decreases to a minimum value at a higher temperature. The Meissner effect was measured by direct current magnetization. The doping of Y2O3:Eu3+ and Y2O3:Eu3++Ag luminescent inhomogeneous phase causes a Meissner effect of MgB2 and Bi(Pb)SrCaCuO at a higher temperature, while the non-luminescent dopant reduces the temperature at which samples have Meissner effect. The introduction of luminescent inhomogeneous phase in conventional MgB2 and copper oxide high-temperature Bi(Pb)SrCaCuO superconductor increases the TC and JC, and Meissner effect is exerted at higher temperature. Therefore, smart meta-superconductivity is suitable for conventional and copper oxide high-temperature superconductors.

Interplay between Vortex Dynamics and Superconducting Gap Structure in Layered Organic Superconductors

Layered organic superconductors motivate intense investigations because they provide various unexpected issues associated with their low dimensionality and the strong electron correlation. Since layered organic superconductors possess simple Fermi surface geometry and they often share similarities to the high temperature oxide superconductors and heavy fermion compounds, research on layered organic superconductors is suitable for understanding the essence and nature of strongly correlated electron systems. In strongly correlated electron systems, one of the central problems concerning the superconducting (SC) state is the symmetry of the SC gap, which is closely related to the paring mechanism. Thus, experimental determination of the SC gap structure is of essential importance. In this review, we present the experimental results for the in-plane angular variation of the flux-flow resistance in layered organic superconductors k-(ET)2Cu(NCS)2, β″-(ET)2SF5CH2CF2SO3, and λ-(BETS)2GaCl4. The interplay between the vortex dynamics and nodal structures is discussed for these superconductors.

Overview of the SPARC tokamak

The SPARC tokamak is a critical next step towards commercial fusion energy. SPARC is designed as a high-field ($B_0 = 12.2$T), compact ($R_0 = 1.85$m,$a = 0.57$m), superconducting, D-T tokamak with the goal of producing fusion gain$Q>2$from a magnetically confined fusion plasma for the first time. Currently under design, SPARC will continue the high-field path of the Alcator series of tokamaks, utilizing new magnets based on rare earth barium copper oxide high-temperature superconductors to achieve high performance in a compact device. The goal of$Q>2$is achievable with conservative physics assumptions ($H_{98,y2} = 0.7$) and, with the nominal assumption of$H_{98,y2} = 1$, SPARC is projected to attain$Q \approx 11$and$P_{\textrm {fusion}} \approx 140$MW. SPARC will therefore constitute a unique platform for burning plasma physics research with high density ($\langle n_{e} \rangle \approx 3 \times 10^{20}\ \textrm {m}^{-3}$), high temperature ($\langle T_e \rangle \approx 7$keV) and high power density ($P_{\textrm {fusion}}/V_{\textrm {plasma}} \approx 7\ \textrm {MW}\,\textrm {m}^{-3}$) relevant to fusion power plants. SPARC's place in the path to commercial fusion energy, its parameters and the current status of SPARC design work are presented. This work also describes the basis for global performance projections and summarizes some of the physics analysis that is presented in greater detail in the companion articles of this collection.

The Strange Metal State of the Electron-Doped Cuprates

An understanding of the high-temperature copper oxide (cuprate) superconductors has eluded the physics community for over thirty years and represents one of the greatest unsolved problems in condensed matter physics. Particularly enigmatic is the normal state from which superconductivity emerges, so much so that this phase has been dubbed a “strange metal.” In this article, we review recent research into this strange metallic state as realized in the electron-doped cuprates with a focus on their transport properties. The electron-doped compounds differ in several ways from their more thoroughly studied hole-doped counterparts, and understanding these asymmetries of the phase diagram may prove crucial to developing a final theory of the cuprates. Most of the experimental results discussed in this review have yet to be explained and remain an outstanding challenge for theory.

Measurement and Analysis of Magnetic Properties of YBa2Cu3O7-y Bulk Superconductor

YBa2Cu3O7-y single crystal bulk high-temperature superconductor has high critical current density with large magnetic levitation and trapping magnetic forces. Despite its distinctive electromagnetic properties, the longtime heat treatment process of oxide high-temperature superconductor, a ceramic material, is widely considered as a factor that can degrade industrial applicability of superconductors seriously. For applications in power systems, metal-ceramic composites need to be fabricated to enhance magnetic performance. In this study, CeO2 was utilized as an additive to control Y211 particles and improve superconducting magnetic properties. The study achieved meaningful outcomes, given that it reduced the processing time by half (1/2) with the interior seeding technique, realized the effect of top seeding by positioning one additional seed layer on the top surface, and obtained a magnetic levitation force of 67.015 N. In order to improve the physical properties of superconducting materials, interior seeding technique was applied, and the effect of top seeding was obtained by placing one seed layer on the top surface. Therefore, when two seed layers were used to fabricate a specimen, it could improve physical properties of superconductors to the extent that both the top and bottom pellets could be used.

High-temperature superconductivity in monolayer Bi2Sr2CaCu2O8+δ.

Although copper oxide high-temperature superconductors constitute a complex and diverse material family, they all share a layered lattice structure. This curious fact prompts the question of whether high-temperature superconductivity can exist in an isolated monolayer of copper oxide, and if so, whether the two-dimensional superconductivity and various related phenomena differ from those of their three-dimensional counterparts. The answers may provide insights into the role of dimensionality in high-temperature superconductivity. Here we develop a fabrication process that obtains intrinsic monolayer crystals of the high-temperature superconductor Bi2Sr2CaCu2O8+δ (Bi-2212; here, a monolayer refers to a half unit cell that contains two CuO2 planes). The highest superconducting transition temperature of the monolayer is as high as that of optimally doped bulk. The lack of dimensionality effect on the transition temperature defies expectations from the Mermin-Wagner theorem, in contrast to the much-reduced transition temperature in conventional two-dimensional superconductorssuch as NbSe2. The properties of monolayer Bi-2212 become extremely tunable; our survey of superconductivity, the pseudogap, charge order and the Mott state at various doping concentrations reveals that the phases are indistinguishable from those in the bulk. Monolayer Bi-2212 therefore displays all the fundamental physics of high-temperature superconductivity. Our results establish monolayer copper oxides as a platform for studying high-temperature superconductivity and other strongly correlated phenomena in two dimensions.

Evidence for Intra-Unit Cell Magnetism in Superconducting Cuprates: a Technical Assessment

Intra unit cell (IUC) magnetic order observed by polarized neutron diffraction (PND) is one of the hallmarks of the pseudogap state of high-temperature copper oxide superconductors. This experimental observation, usually interpreted as a result of loop currents, has been recently challenged based on lower statistics data. We here address the crucial issue of polarization inhomogeneities in the neutron beams showing that the original data had a much better reproducibilty. Within these technical limitations, we here propose a self-consistent analysis that potentially solves the controversy. We show that all the reported PND experiments in superconducting cuprates are actually compatible with the existence of an IUC magnetism.

Tuning from failed superconductor to failed insulator with magnetic field.

Do charge modulations compete with electron pairing in high-temperature copper oxide superconductors? We investigated this question by suppressing superconductivity in a stripe-ordered cuprate compound at low temperature with high magnetic fields. With increasing field, loss of three-dimensional superconducting order is followed by reentrant two-dimensional superconductivity and then an ultraquantum metal phase. Circumstantial evidence suggests that the latter state is bosonic and associated with the charge stripes. These results provide experimental support to the theoretical perspective that local segregation of doped holes and antiferromagnetic spin correlations underlies the electron-pairing mechanism in cuprates.