Oxide Superconductors Research Articles

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

Correlation between optical phonon softening and superconducting Tc in YBa2Cu3Ox within d-wave Eliashberg theory

Abstract We provide a mathematical description, based on d-wave Eliashberg theory, of the strong correlation between the experimentally observed softening of Raman modes associated with in-plane oxygen motions and the corresponding superconducting critical temperature T c , as a function of oxygen doping x, in YBa2Cu3O x . The theoretical model provides a direct link between physical trends of soft optical Ag (in-plane) oxygen modes, the level of oxygen doping x, and the superconducting T c . Different regimes observed in the trend of T c vs doping can be related to corresponding regimes of optical phonon softening in the Raman spectra. These results provide further evidence related to the physical origin of high-temperature superconductivity in rare-earth cuprate oxides and to the significant role of electron–phonon coupling therein.

Irradiation effects on copper oxide superconductors including high-entropy REBCO(HE-REBCO)

Polycrystalline specimens of copper oxide superconductors YBa2Cu3O7-δ(YBCO), Y0.39Sm0.30Eu0.31Ba2Cu3O7-δ(ME-REBCO), Y0.18La0.24Nd0.14Sm0.14Eu0.15Gd0.15Ba2Cu3O7-δ (HE-REBCO) were irradiated with 1 MeV-He ions at room temperature. Magnetization measurement using superconducting quantum interference device (SQUID) and microstructure observation using transmission electron microscope (TEM) were conducted before and after irradiation. The improvement of properties in HE-REBCO and degradations in YBCO, ME-REBCO were suggested by SQUID measurement. HE-REBCO had smaller irradiation defects and volume fractions although disappearance of twin boundaries due to tetragonal transition occurred in peak-damage area as well as YBCO. These defects work as optimal flux pinning centers leading to improve properties in HEREBCO.

Winding angle optimization and testing of small-scale, non-planar, high-temperature superconducting stellarator coils

We designed and constructed two non-planar coils with high-temperature superconductors (HTS) based on shapes from the Wendelstein 7-X stellarator. Tape track orientation of the HTS was optimized to reduce the coil size as much as possible while staying within the strain limits of the gadolinium barium copper oxide (GdBCO) superconductor. This resulted in average coil radii of 0.23 m and 0.48 m at strain limits of up 0.45% to for the coil shapes that were chosen. The coils were produced by winding the GdBCO tapes onto 3D-printed plastic frames. We confirmed the integrity of the superconducting layer after winding by spatially resolved measurement of the critical current and by energizing the coils in liquid nitrogen. Coil 1 showed a resistance of 1.75μΩ and did not have any critical current degradation, while coil 5 had a resistance of 195μΩ and showed only one dropout, attributable to a handling error. We measured the magnetic field of the coil with a three-axis Hall probe system and found good agreement with predictions. This work demonstrates the manufacturing of small-scale, non-planar magnetic coils from commercially available HTS.

Observation of Superconductivity Up to 8.7 K in Reduced Potassium Tantalate

AbstractThe observation of superconductivity with a transition temperature (Tc) up to 8.7 K in KTaO3 single crystals annealed with CaH2 at 900–1000 °C is reported. The superconductivity is confirmed by both resistance and magnetization measurements and is 3D in nature. Characterizations of X‐ray photoelectron spectroscopy, X‐ray diffraction, and scanning transmission electron microscopy reveal that it locates in a 1‐µm‐order‐thick polycrystalline surface layer that shows a rock‐salt type structure, with a lattice constant of 0.454 nm, and can be chemically identified as KxTaOy (0.04 ≤ x ≤ 0.08, 1.24 ≤ y ≤ 1.35), depending on annealing conditions. Within the experimental ranges, the Tc is peaked at x ≈0.05, and increases with decreasing y, and the highest Tc is observed in K0.05TaO1.24. The Tc observed here is much higher than that of KTaO3, Ta, and pure TaO, and is also one of the highest among of all the known oxide superconductors with the same rock‐salt structure. The rather high Tc and its close connection with KTaO3 and Ta, both of which are promising materials for quantum computing, make KxTaOy potentially interesting as a building block in constructing future superconducting quantum devices.

Strain-induced long-range charge-density wave order in the optimally doped Bi2Sr2−xLaxCuO6 superconductor

The mechanism of high-temperature superconductivity in copper oxides (cuprate) remains elusive, with the pseudogap phase considered a potential factor. Recent attention has focused on a long-range symmetry-broken charge-density wave (CDW) order in the underdoped regime, induced by strong magnetic fields. Here by 63,65Cu-nuclear magnetic resonance, we report the discovery of a long-range CDW order in the optimally doped Bi2Sr2−xLaxCuO6 superconductor, induced by in-plane strain exceeding ∣ε∣ = 0.15 %, which deliberately breaks the crystal symmetry of the CuO2 plane. We find that compressive/tensile strains reduce superconductivity but enhance CDW, leaving superconductivity to coexist with CDW. The findings show that a long-range CDW order is an underlying hidden order in the pseudogap state, not limited to the underdoped regime, becoming apparent under strain. Our result sheds light on the intertwining of various orders in the cuprates.

Charge ordering as the driving mechanism for superconductivity in rare-earth nickel oxides

Charge ordering as the driving mechanism for superconductivity in rare-earth nickel oxides

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.

Toward Reliable Synthesis of Superconducting Infinite Layer Nickelate Thin Films by Topochemical Reduction.

Infinite layer (IL) nickelates provide a new route beyond copper oxides to address outstanding questions in the field of unconventional superconductivity. However, their synthesis poses considerable challenges, largely hindering experimental research on this new class of oxide superconductors. That synthesis is achieved in a two-step process that yields the most thermodynamically stable perovskite phase first, then the IL phase by topotactic reduction, the quality of the starting phase playing a crucial role. Here, a reliable synthesis of superconducting IL nickelate films is reported after successive topochemical reductions of a parent perovskite phase with nearly optimal stoichiometry. Careful analysis of the transport properties of the incompletely reduced films reveals an improvement in the strange metal behavior of their normal state resistivity over subsequent topochemical reductions, offering insight into the reduction process.

Mott Gap Filling by Doping Electrons through Depositing One Sub-Monolayer Thin Film of Rb on Ca2CuO2Cl2

Understanding the doping evolution from a Mott insulator to a superconductor probably holds the key to resolve the mystery of unconventional superconductivity in copper oxides. To elucidate the evolution of the electronic state starting from the Mott insulator, we dose the surface of the parent phase Ca2CuO2Cl2 by depositing Rb atoms, which are supposed to donate electrons to the CuO2 planes underneath. We successfully achieved the Rb sub-monolayer thin films in forming the square lattice. The scanning tunneling microscopy or spectroscopy measurements on the surface show that the Fermi energy is pinned within the Mott gap but close to the edge of the charge transfer band. In addition, an in-gap state appears at the bottom of the upper Hubbard band (UHB), and the Mott gap will be significantly diminished. Combined with the Cl defect and the Rb adatom/cluster results, the electron doping is likely to increase the spectra weight of the UHB for the double occupancy. Our results provide information to understand the electron doping to the parent compound of cuprates.

In-Situ Atomic-Scale Observation of Brownmillerite to Ruddlesden–Popper Phase Transition Tuned by Epitaxial Strain in Cobaltites

Phase transitions involving oxygen ion extraction within the framework of the crystallographic relevance have been widely exploited for sake of superconductivity, ferromagnetism, and ion conductivity in perovskite-related oxides. However, atomic-scale pathways of phase transitions and ion extraction threshold are inadequately understood. Here we investigate the atomic structure evolution of LaCoO3 films upon oxygen extraction and subsequent Co migration, focusing on the key role of epitaxial strain. The brownmillerite to Ruddlesden–Popper phase transitions are discovered to stabilize at distinct crystal orientations in compressive- and tensile-strained cobaltites, which could be attributed to in-plane and out-of-plane Ruddlesden–Popper stacking faults, respectively. A two-stage process from exterior to interior phase transition is evidenced in compressive-strained LaCoO2.5, while a single-step nucleation process leaving bottom layer unchanged in tensile-strained situation. Strain analyses reveal that the former process is initiated by an expansion in Co layer at boundary, whereas the latter one is associated with an edge dislocation combined with antiphase boundary. These findings provide a chemo-mechanical perspective on the structure regulation of perovskite oxides and enrich insights into strain-dependent phase diagram in epitaxial oxides films.

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.

Paramagnon excitations' theory for nuclear magnetic resonance and resonant inelastic X-ray scattering studies in doped plane copper oxide superconductors

Paramagnon excitations' theory for nuclear magnetic resonance and resonant inelastic X-ray scattering studies in doped plane copper oxide superconductors

Observation of superconductivity and enhanced upper critical field of η-carbide-type oxide Zr4Pd2O

We report the first observation of bulk superconductivity of a η-carbide-type oxide Zr4Pd2O. The crystal structure and the superconducting properties were studied through synchrotron X-ray diffraction, magnetization, electrical resistivity, and specific heat measurement. The superconducting transition was observed at Tc = 2.73 K. Our measurement revealed that the η-carbide-type oxide superconductor Zr4Pd2O shows an enhanced upper critical field μ0Hc2(0) = 6.72 T, which violates the Pauli-Clogston limit μ0HP = 5.29 T. On the other hand, we found that the enhanced upper critical field is absent in a Rh analogue Zr4Rh2O. The large μ0Hc2(0) of Zr4Pd2O would be raised from strong spin–orbit coupling with Pd-4d electrons. The discovery of new superconducting properties for Zr4Pd2O would shed light on the further development of η-carbide-type oxide superconductors.

Unexpected large spin currents in the normal state of an oxide superconductor

Unexpected large spin currents in the normal state of an oxide superconductor

Spin–orbit coupling enhanced electron–phonon superconductivity in infinite-layer BaBiO2

The recent discovery of infinite-layer nickel oxide superconductors has highlighted the importance of first-principles simulations. We predict an infinite-layer bismuth oxide superconductor BaBiO2, which is isostructural to NdNiO2. In this work, electronic structure, lattice dynamics, and electron–phonon interaction are studied, with special attention paid to the influence of spin–orbit coupling (SOC) on the above-mentioned quantities. Our calculations show that the structure will be dynamically stable under pressure and induce superconductivity, whether SOC is considered or not. In addition, SOC will significantly enhance the electron–phonon coupling (EPC), resulting in an increase in EPC constant λ from 0.43 to 0.73. We further find that the Fermi surface nesting is partially responsible for its superconductivity. A strong SOC changes the Fermi surface and enhances the nesting, and the EPC becomes stronger. Our results propose a bismuth-based superconductor, demonstrating the importance of SOC for its superconductivity and providing clues for further experimental synthesis.

Pressure-induced superconducting-insulating phase transition in copper oxide superconductors

Pressure-induced superconducting-insulating phase transition in copper oxide superconductors

Thoughts on the influence of Alex Mueller on high magnetic field technology

I briefly summarize some of the impacts that Alex Mueller's drive to discover superconductivity in low carrier density oxides has had on high magnetic field technology, especially on the desire to use the cuprate superconductors to generate magnetic fields now almost twice those possible with the Nb-based low temperature superconductors. There were some important lessons for applications of any new exotic temperature superconductor that had to be learned before the Bednorz and Mueller discovery could really impact applications. Above all is the lesson that a high transition temperature is no guarantee of applications unless conductors with high current density can be manufactured. Recent reports and hopes for an ambient pressure room temperature superconductor remind us that the hopes for pervasive superconducting technologies germinated with the discovery of superconductivity in LaBaCuO and went truly ballistic following the discovery of YBa2Cu3O7-δ (YBCO) with a 92 K transition temperature. Now that magnets made out of rare earth alloys of YBCO (REBCO) have entered routine service in NMR labs and prototype magnets made with REBCO appear ready to enable compact fusion reactors, we can finally say that Bednorz and Mueller's magnificent scientific achievement is having real world applications impact too.

Material aspects of oxygen isotope effect studies in high-temperature superconductors

The article discusses issues related to the material aspects of the study of the oxygen isotope effect in High-temperature superconductors. These studies were initiated by Prof. K.A. Müller shortly after he and Dr. G. Bednorz discovered high-temperature superconductivity. Research on the isotope effect was carried out in the years 1991–1995, and then continued for various classes of superconductors and other strongly correlated electron systems. The paper presents methods of synthesis of complex oxide superconductors doped with isotopic oxygen-18. Methods for the determination of oxygen content and isotopic enrichment are also discussed.