High-temperature Superconducting Phase Research Articles

Molecular hydrogen in the N-doped LuH3 system as a possible path to superconductivity

The discovery of ambient superconductivity would mark an epochal breakthrough long-awaited for over a century, potentially ushering in unprecedented scientific and technological advancements. The recent findings on high-temperature superconducting phases in various hydrides under high pressure have ignited optimism, suggesting that the realization of near-ambient superconductivity might be on the horizon. However, the preparation of hydride samples tends to promote the emergence of various metastable phases, marked by a low level of experimental reproducibility. Identifying these phases through theoretical and computational methods entails formidable challenges, often resulting in controversial outcomes. In this paper, we consider N-doped LuH3 as a prototypical complex hydride: By means of machine-learning-accelerated force-field molecular dynamics, we have identified the formation of H2 molecules stabilized at ambient pressure by nitrogen impurities. Importantly, we demonstrate that this molecular phase plays a pivotal role in the emergence of a dynamically stable, low-temperature, experimental-ambient-pressure superconductivity. The potential to stabilize hydrogen in molecular form through chemical doping opens up a novel avenue for investigating disordered phases in hydrides and their transport properties under near-ambient conditions.

Europium-Subsituted Bismuth-Based Superconductors: Structural and Thermal Analysis via Chemical Sol-Gel Synthesis

The synthesis and characterization of Bi1.7-xPb0.3EuxSr2Ca2Cu3O10+δ superconductors with Eu substitution at varying ratios were investigated to determine their structural, electrical, and thermal properties. A sol-gel technique followed by calcination was employed to prepare samples with different substitution ratios. X-ray diffraction (XRD) was used for structural analysis, electrical properties with aid of nitrogen fluid measured by electrical resistivity, thermogravimetric analysis (TGA) conducted for thermal properties , and differential scanning calorimetry (DSC) were utilized for characterization" to improve clarity. XRD analysis revealed that the highest proportion of the high-temperature superconducting (HTS) phase occurred at a substitution factor of 0.2, corresponding to the highest critical temperature (Tc) value of 112 K. Raman spectra were conducting and showed a certain shifting with different substitution levels of Eu element. The thermal analysis highlighted the impact of substitution factor x on thermal stability, with the sample at x=0.2 exhibiting the highest thermal stability. TGA curves showed mass loss behavior for different x values, with distinct regions indicating the presence of residuals. The successful synthesis and characterization of these superconductors hold promise for practical applications.

Improvement in electrical properties of Bi-2212 superconducting materials substituted with large-scale nano-sized tin

In the current work, the effect of nano-sized Sn (50 nm)/Sr partial replacement on the superconducting properties such as crystal structure, quality of intra- and intergrain boundary coupling, dc electrical resistivity, and dc magnetization in the Bi-2212 ceramic superconductors were investigated. Ceramic superconductors with nominal composition of Bi2Sr2-x(Sn)xCa1Cu1.75Na0.25Oy where x = 0.25, 0.30, 0.35 and 0.40 were prepared by solid-state reaction method and characterized by powder X-ray diffraction (XRD), dc electrical resistivity, scanning electron microscopy (SEM) and magnetic hysteresis (M–H) measurements. Phase examination of by XRD indicated that the doping of x = 0.25 ratio nano-sized SnO2 to the strontium sites improved the formation of the Bi-2212 high-temperature superconducting phase. SEM micrographs showed that the morphological structure of all samples consisted of plate-like grains, which were separated from each other by grain boundaries, indicating the Bi-2212 superconducting phase. The highest superconductivity transition temperature among the samples was measured as the Tconset = 86 K at x = 0.25 in Bi2Sr2-x(Sn)xCa1Cu1.75Na0.25Oy. M–H loops of the Bi2Sr2-x(Sn)xCa1Cu1.75Na0.25Oy sample at x = 0.25 ratio is larger compared to other examples, indicating improvement intergrain connectivity as well as enhanced flux pinning centers. In addition, the critical current (Jc) values of the samples were calculated from M–H measurement using Bean’s critical current model. The best Jc values were obtained as 570 A/cm2 at 15 K, which is a relatively high value for BSCCO superconductors with polycrystalline structure.

Comparative Study of Structural, Electrical, and Mechanical Properties of (Tl, Hg)-1223 High Temperature Superconducting Phase Substituted by Lead Oxide and Lead Dioxide

Comparative Study of Structural, Electrical, and Mechanical Properties of (Tl, Hg)-1223 High Temperature Superconducting Phase Substituted by Lead Oxide and Lead Dioxide

Colloquium : Hydrodynamics and holography of charge density wave phases

Hydrodynamics is an old example of an effective description of complex matter, which describes the system's behavior at large length and timescales and lumps microscopic details into transport coefficients. A combination of hydrodynamics and the gauge-gravity duality, which was first explored in the context of string theory, has proven promising for a description of strongly correlated electron fluids. This Colloquium explains how to apply these techniques to strongly correlated materials where the electron fluid crystallizes, and in particular to the strange metal phase of high-temperature superconductors.

Spin fluctuations and superconductivity in Y5Rh6Sn18 doped with Pd and Co: Evidence of peak effect in the superconducting mixed state

We performed a systematic study of the nonmagnetic skutterudite-related ${\mathrm{Y}}_{5}{\mathrm{Rh}}_{6}{\mathrm{Sn}}_{18}$ superconductor, where the lattice disorder on the coherence length scale $\ensuremath{\xi}$ additionally generates a nonhomogeneous, high-temperature superconducting phase with disorder-enhanced critical temperature ${T}_{c}^{★}$. We have previously discussed various possibilities of local atomic disorder; one of the possibilities is doping. Our present studies focus on the series of ${\mathrm{Y}}_{5\ensuremath{-}\ensuremath{\delta}}({\mathrm{Rh}}_{5.5}{M}_{0.5}){\mathrm{Sn}}_{18}$ compounds ($\ensuremath{\delta}\ensuremath{\ll}1$), where the dopants $M=$ Co, Ir, Ru, and Pd, when they are smaller (Co) or larger (Pd) than Rh, generate the peak effect in fields smaller than the critical field ${H}_{c2}$. This phenomenon manifests itself as a weak peak in the real and imaginary parts of ac susceptibility and is more distinct in magnetoresistance. Using a simple theoretical model, we demonstrate that the effectiveness of this mechanism depends not only on the magnitude of the difference between the size of the dopant and the host atom, but also on whether the dopant is smaller or larger. The agreement between this prediction and our experimental data strongly supports the impurity-based scenario of the observed peak effect. Magnetoresistance isotherms of the remaining samples ($M=$ Ir and Ru) with the radius of $M$ being very similar to that of Rh show, however, a weak peak-effect-like behavior, which is mainly due to vacancies $\ensuremath{\delta}$ at Y sites, while the corresponding ac susceptibility isotherms exhibit a distinct peak at $H\ensuremath{\sim}{H}_{c2}$. This field-dependent ${\ensuremath{\chi}}_{\mathrm{ac}}$ anomaly appears to be similar in nature to the peak effect; however, it cannot be attributed to pinning and appears to be an equilibrium property of the system. We also report the coexistence of spin fluctuations and superconductivity for ${\mathrm{Y}}_{5}{\mathrm{Rh}}_{6}{\mathrm{Sn}}_{18}$ doped with Pd and Co.

Advances in the Synthesis and Superconductivity of Lanthanide Polyhydrides Under High Pressure

Room-temperature superconductors have long been the ultimate goal of scientists. Pressure-stabilized hydrides are a new rapidly growing class of high-temperature superconductors and are believed to be a new superconducting system, undoubtedly leading to a surge in the discovery of new hydrogen-rich materials. They are the forefront of physics and material science. Lanthanide polyhydrides formed under pressure are promising conventional superconductors. Especially, both the theoretical and experimental reports on lanthanum superhydrides under pressure, exhibiting superconductivity at temperatures as high as 250 K, have further stimulated an intense search for room-temperature superconductors in hydrides. This review focuses on the recent advances of crystal structures, stabilities, and superconductivity of lanthanide polyhydrides at high pressures, including the experimental results from our group. By using in situ four-probe electrical measurements and the synchrotron X-ray diffraction technique, we have identified several high-temperature superconducting phases: a lanthanum superhydride and two cerium superhydrides. The present work indicates that superconductivity declines along the La–Ce–Pr–Nd series, while magnetism becomes more and more pronounced. These discoveries have enriched the binary system of clathrate superhydrides and provided more hints for studying the role of rare earth metal elements having high-temperature superconductivity.

Pseudogap Isotope Effect as a Probe of Bipolaron Mechanism in High Temperature Superconductors.

A theory of a pseudogap phase of high-temperature superconductors where current carriers are translation invariant bipolarons is developed. A temperature of a transition from a pseudogap phase to a normal one is calculated. For the temperature of a transition to the pseudogap phase, the isotope coefficient is found. It is shown that the results obtained, in particular, the possibility of negative values of the isotope coefficient, are consistent with the experiment. New experiments on the influence of the magnetic field on the isotope coefficient are proposed.

Comparison of highly-compressed C2/m-SnH12 superhydride with conventional superconductors

Satterthwaite and Toepke (1970 Phys. Rev. Lett. 25 741) predicted high-temperature superconductivity in hydrogen-rich metallic alloys, based on an idea that these compounds should exhibit high Debye frequency of the proton lattice, which boosts the superconducting transition temperature, T c. The idea has got full confirmation more than four decades later when Drozdov et al (2015 Nature 525 73) experimentally discovered near-room-temperature superconductivity in highly-compressed sulphur superhydride, H3S. To date, more than a dozen of high-temperature hydrogen-rich superconducting phases in Ba–H, Pr–H, P–H, Pt–H, Ce–H, Th–H, S–H, Y–H, La–H, and (La, Y)–H systems have been synthesized and, recently, Hong et al (2021 arXiv:2101.02846) reported on the discovery of C2/m-SnH12 phase with superconducting transition temperature of T c ∼ 70 K. Here we analyse the magnetoresistance data, R(T, B), of C2/m-SnH12 phase and report that this superhydride exhibits the ground state superconducting gap of Δ(0) = 9.2 ± 0.5 meV, the ratio of 2Δ(0)/k B T c = 3.3 ± 0.2, and 0.010 < T c/T F < 0.014 (where T F is the Fermi temperature) and, thus, C2/m-SnH12 falls into unconventional superconductors band in the Uemura plot.

Improved control of atomic layering in perovskite-related homologous series

Homologous series are layered phases that can have a range of stoichiometries depending on an index n. Examples of perovskite-related homologous series include (ABO3)nAO Ruddlesden–Popper phases and (Bi2O2) (An−1BnO3n+1) Aurivillius phases. It is challenging to precisely control n because other members of the homologous series have similar stoichiometry and a phase with the desired n is degenerate in energy with syntactic intergrowths among similar n values; this challenge is amplified as n increases. To improve the ability to synthesize a targeted phase with precise control of the atomic layering, we apply the x-ray diffraction (XRD) approach developed for superlattices of III–V semiconductors to measure minute deviations from the ideal structure so that they can be quantitatively eradicated in subsequent films. We demonstrate the precision of this approach by improving the growth of known Ruddlesden–Popper phases and ultimately, by synthesizing an unprecedented n = 20 Ruddlesden–Popper phase, (ATiO3)20AO where the A-site occupancy is Ba0.6Sr0.4. We demonstrate the generality of this method by applying it to Aurivillius phases and the Bi2Sr2Can–1CunO2n+4 series of high-temperature superconducting phases.

The First Measurement for Mass Attenuation Coefficients, Effective Atomic Numbers, and Electron Densities of Y123 and Y358 High-Temperature Superconductors (HTS) for the Energies from 46.6 to 1332 keV

Y123 and Y358 high-temperature superconductor phases have experimental mass attenuation coefficients (MAC) determined at 46.5, 81, 122, 302, 356, 383, 662, 1173, and 1332 keV photon energies. The experimental values have been compared with theoretical values acquired employing the XCOM software and database. It has been observed that the theoretical and the empirical results were in perfect agreement. Also, it has been seen that the comparison of the MACs for all specimens examined with the corresponding photon energies complies with the exponential absorption law. The effective atomic numbers (Zeff) and electron densities (Nel) of Y123 and Y358 superconducting phases in different energy ranges have determined experimentally and theoretically. The Y358 superconductor specimen is a relatively better absorber than the Y123 between 40 and 100 keV.

Enhancing Superconductivity of the Nonmagnetic Quasiskutterudites by Atomic Disorder.

We investigated the effect of enhancement of superconducting transition temperature by nonmagnetic atom disorder in the series of filled skutterudite-related compounds (LaSn, CaRhSn, YRhSn, LuRhSn; Co, Ru, Rh), where the atomic disorder is generated by various defects or doping. We have shown that the disorder on the coherence length scale in these nonmagnetic quasiskutterudite superconductors additionally generates a non-homogeneous, high-temperature superconducting phase with (dilute disorder scenario), while the strong fluctuations of stoichiometry due to increasing doping can rapidly increase the superconducting transition temperature of the sample even to the value of (dense disorder leading to strong inhomogeneity). This phenomenon seems to be characteristic of high-temperature superconductors and superconducting heavy fermions, and recently have received renewed attention. We experimentally documented the stronger lattice stiffening of the inhomogeneous superconducting phase in respect to the bulk one and proposed a model that explains the behavior in the series of nonmagnetic skutterudite-related compounds.

Stoichiometry and defect superstructures in epitaxial FeSe films on SrTiO3

Cryogenic scanning tunneling microscopy is employed to investigate the stoichiometry and defects of epitaxial FeSe thin films on ${\mathrm{SrTiO}}_{3}$(001) substrates under various postgrowth annealing conditions. Low-temperature annealing with an excess supply of Se leads to the formation of Fe vacancies and superstructures, accompanied by a superconductivity (metal)-to-insulator transition in FeSe films. By contrast, high-temperature annealing could eliminate the Fe vacancies and superstructures, and thus recover the high-temperature superconducting phase of monolayer FeSe films. We also observe multilayer FeSe during low-temperature annealing, which is revealed to link with Fe vacancy formation and adatom migration. Our results document the very special roles of film stoichiometry and help unravel several controversies in the properties of monolayer FeSe films.

Probing the holographic Fermi arc with scalar field: numerical and analytical study

Fermi arcs are disconnected contour of Fermi surface, which can be observed in pseudo-gap phase of high temperature superconductors. Aiming to understand this pseudo-gap phenomena, we study a holographic Fermionic system coupled with a massive scalar field in an AdS black hole background. Depending on the boundary condition on the scalar field mode, we discuss two possible scenarios. When the scalar condenses below a critical temperature Tc, Fermi surface undergoes a transition from normal phase to pseudo-gap phase. Hence Tc can be the reminiscent of well known cross over temperature T* in cuprate superconductor, below which pseudo-gap appears at constant doping. In the second scenario, the bulk scalar develops a non-normalizable profile at arbitrary temperature for non-zero source at the boundary. Therefore, we can tune the Fermi spectrum by tuning a dual source at the boundary. The dual source for this case can be the reminiscent of hole doping in the real cuprate superconductor. For both the cases we have studied Fermi spectrum and observed anisotropic gap in the spectral function depending on the model parameter and studied the properties of Fermi arcs across different phases.

Mechanism of High-Temperature Superconductivity in Correlated-Electron Systems

It is very important to elucidate the mechanism of superconductivity for achieving room temperature superconductivity. In the first half of this paper, we give a brief review on mechanisms of superconductivity in many-electron systems. We believe that high-temperature superconductivity may occur in a system with interaction of large-energy scale. Empirically, this is true for superconductors that have been found so far. In the second half of this paper, we discuss cuprate high-temperature superconductors. We argue that superconductivity of high temperature cuprates is induced by the strong on-site Coulomb interaction, that is, the origin of high-temperature superconductivity is the strong electron correlation. We show the results on the ground state of electronic models for high temperature cuprates on the basis of the optimization variational Monte Carlo method. A high-temperature superconducting phase will exist in the strongly correlated region.

Chaos in balanced and unbalanced holographic s+p superconductors

In this work, we propose a toy model for a mixture of superconductors with competitive s and p modes using gauge/gravity duality. We demonstrate that the model undergoes phase transitions with the proper choice of different values for chemical potentials. We consider both balanced and unbalanced cases. We propose that the condensate field in the bulk toy model for a mixing s+p phase of high temperature superconductors undergoes a chaotic phase space scenario. Using a suitable measure function for chaos, we demonstrate the existence of chaotic dynamics via condensate fields. As an alternative method, we also investigate the phase portrait in the extended phase space for the condensates.

Observation of Persistent Currents in Finely Dispersed Pyrolytic Graphite

The trapped magnetic flux in the finely ground pyrolytic graphite sample annealed at 670 K in air has been observed. Flux trapping occurs on cooling of the sample from room temperature to 10 K in a magnetic field of 1 T. The magnitude and sign of the induced trapped moment remain unchanged when the applied magnetic field is varied within ±1 T at T K. The trapped magnetic flux is manifested in the displacement of the magnetization curve relative to that of the sample cooled in zero field. Displacement magnitude gradually decreases with the temperature increase up to 350 K, not reaching zero. The set of experimental observations probably reflects the presence in the sample of a granular high-temperature superconducting phase.

Hyperscaling violating black hole solutions and magneto-thermoelectric DC conductivities in holography

We derive new black hole solutions in Einstein-Maxwell-Axion-Dilaton theory with a hyperscaling violation exponent. We then examine the corresponding anomalous transport exhibited by cuprate strange metals in the normal phase of high-temperature superconductors via gauge/gravity duality. Linear temperature dependence resistivity and quadratic temperature dependence inverse Hall angle can be achieved. In the high temperature regime, the heat conductivity and Hall Lorenz ratio are proportional to the temperature. The Nernst signal first increases as temperature goes up but it then decreases with increasing temperature in the high temperature regime.

High-temperature superconducting phase of HBr under pressure predicted by first-principles calculations

The high pressure phases of HBr are explored with an ab initio crystal structure search. By taking into account the contribution of zero-point energy (ZPE), we find that the P4/nmm phase of HBr is thermodynamically stable in the pressure range from 150 to 200 GPa. The superconducting critical temperature (Tc) of P4/nmm HBr is evaluated to be around 73 K at 170 GPa, which is the highest record so far among binary halogen hydrides. Its Tc can be further raised to around 95K under 170 GPa if half of the bromine atoms in the P4/nmm HBr are substituted by the lighter chlorine atoms. Our study shows that, in addition to lower mass, higher coordination number, shorter bonds, and more highly symmetric environment for the hydrogen atoms are important factors to enhance the superconductivity in hydrides.

Compressed tetragonal phase in XFe2As2 ( X=Na , K, Rb, Cs) and in the alloy Na0.5K0.5Fe2As2

Motivated by the recent discovery of a high-temperature superconducting phase in ${\mathrm{KFe}}_{2}{\mathrm{As}}_{2}$ at 16 GPa accompanied by an isostructural phase transition from the tetragonal to the compressed tetragonal phase, we extend the study of pressure effects to the whole $X{\mathrm{Fe}}_{2}{\mathrm{As}}_{2}$ family ($X=\mathrm{Na}$, K, Rb, Cs). We demonstrate that the ionic radius of the $X$ atom determines the transition to the compressed phase which induces relevant changes in electronic properties and the Fermi surface which can enhance the superconducting pairing. Based on these results and in analogy with ${\mathrm{KFe}}_{2}{\mathrm{As}}_{2}$, we theoretically propose ${\mathrm{Na}}_{0.5}{\mathrm{K}}_{0.5}{\mathrm{Fe}}_{2}{\mathrm{As}}_{2}$ as a possible new superconductor material in its compressed tetragonal phase which we predict to happen above 6 GPa.