Critical Temperature Superconductors Research Articles

Symmetry-Shaped Singularities in High-Temperature Superconductor H3S.

The superconducting critical temperature of H3S ranks among the highest measured, at 203 K. This impressive value stems from a singularity in the electronic density-of-states, induced by a flat-band region that consists of saddle points. The peak sits right at the Fermi level, so that it gives rise to a giant electron-phonon coupling constant. In this work, we show how atomic orbital interactions and space group symmetry work in concert to shape the singularity. The body-centered cubic Brillouin Zone offers a unique 2D hypersurface in reciprocal space: fully connecting squares with two different high-symmetry points at the corners, Γ and H, and a third one in the center, N. Orbital mixing leads to the collapse of fully connected 1D saddle point lines around the square centers, due to a symmetry-enforced s-p energy inversion between Γ and H. The saddle-point states are invariably nonbonding, which explains the unconventionally weak response of the superconductor's critical temperature to pressure. Although H3S appears to be a unique case, the theory shows how it is possible to engineer flat bands and singularities in 3D lattices through symmetry considerations.

Van Hove Singularity and Enhanced Superconductivity in Ca-Intercalated Bilayer Graphene Induced by Confinement Epitaxy.

We demonstrate the impact of high-density calcium introduction into Ca-intercalated bilayer graphene on a SiC substrate, wherein a metallic layer of Ca has been identified at the interface. We have discerned that the additional Ca layer engenders a free-electron-like band, which subsequently hybridizes with a Dirac band, leading to the emergence of a van Hove singularity. Coinciding with this, there is an increase in the critical temperature for superconductivity. These findings allude to the manifestation of Ca-driven confinement epitaxy, augmenting superconductivity through the enhancement of attractive interactions in a pair of electron and hole bands with flat dispersion around the Fermi level.

КВАНТОВЫЙ РАЗМЕРНЫЙ ЭФФЕКТ В ЧИСТЫХ ПЛЕНКАХ АЛЮМИНИЯ

In the modern world, there's a notable trend towards the active miniaturization of electronic devices. With technological advances enabling the manipulation of nanostructures, there's an increasing focus on exploring quantum effects pivotal to such designs. One distinguishing feature of nanostructures is the quantum nature of the electron's energy spectrum. This spectrum becomes discrete in directions where electrons move. Depending on the direction of this confinement, structures can be categorized as nanoplates, quantum wires, or quantum dots. The properties of such structures can significantly differ from those observed in large-scale systems. When discussing superconductivity, particular emphasis is placed on its macroscopic quantum properties.The influence on electronic wave functions is reflected in the characteristics of the superconducting state on broader scales. The Bardeen-Cooper-Schrieffer (BCS) theory is frequently utilized to analyze these nanostructures. The Gor’kov equations method serves as a potent tool for tasks related to the BCS theory. For instance, it can determine the parameters of the superconducting state, critical temperature, and current. Components of these equations, like Green's functions, are associated with various system properties. Research in the early stages of superconductivity studies revealed that the critical temperature (Tc ) – the temperature at which a material transitions to a superconducting state – can differ significantly between thin films and bulk materials. Intriguingly, reducing the film's thickness can both decrease (e.g., in niobium) and increase (e.g., in aluminum) the Tc value. This study delves into the quantum size effect in thin aluminum films, paving the way for materials with higher transition temperatures. Such advancements can simplify and make the maintenance of superconducting systems more cost-effective. In this study, a theoretical relationship between the critical temperature of a thin aluminum film and its thickness was derived. The Green's function method was chosen, which hadn't been previously employed for this computation. This approach offers greater potential compared to other superconductivity theory methods, presenting extensive avenues for theoretical exploration in this domain. The authors are confident that this work will contribute to further research on quantum dimensional effects in low-dimensional superconducting structures.

Crystal varizonicity: fundamental and applied aspects.

A set of ideas about the homogeneous variable zone structure in the near-surface region of ionic crystals is presented. Its origin is shown using generalizations of the Seitz–Madelung scheme. A number of important consequences have been demonstrated: the peculiarity of local surface levels different from Tamm levels; increase in the critical temperature of superconductivity in a sandwich structure; a significant anti-degradation role of these levels in the processes of energy exchange at the interfaces. It is concluded that such homo varizonality is an excellent example of the implementation of the “Bonds and Bands” concept.

THE APPLICATION OF ANALYSIS OF VARIANCE TO DIFFERENT IRON‑BASED SUPERCONDUCTOR CRITICAL TEMPERATURE MODELING

Recently, Iron-based superconductors have shown promising properties of high critical temperature and high upper critical fields, which are prerequisites for applications in highfield magnets. Extensive research has been conducted on a modeling approach that contributes to predicting doped Iron-based superconductor critical temperature from structural and topological parameters. Statistical significance of differences in modeling approaches requires studies that can reliably distinguish between systematic approach effects and errors resulting from modeling approach variation. In this work, we introduce analysis of variance (ANOVA) to assess the statistical significance of differences in modeling approach variation. Comparisons of obtained results with Iron-based modeling approach variation data of support vector machine (SVM) and linear regression with natural logarithm transformation (LRNLT) were presented.

Film Thickness Dependence of the Critical Temperature of Superconductivity of LSCO Films: A Bipolaron Model Approach

Film Thickness Dependence of the Critical Temperature of Superconductivity of LSCO Films: A Bipolaron Model Approach

Ultrathin superconducting TaCxN1−x films prepared by plasma-enhanced atomic layer deposition with ion-energy control

This work demonstrates that plasma-enhanced atomic layer deposition (PEALD) with substrate biasing enables the preparation of ultrathin superconducting TaCxN1−x films. By comparing with films grown without substrate biasing, the enhanced ion energies yield a hundredfold reduction in room-temperature resistivity: a comparably low value of 217 μΩ cm is obtained for a 40 nm film. The ion-energy control enables tuning of the composition, counteracts oxygen impurity incorporation, and promotes a larger grain size. Correspondingly, the critical temperature of superconductivity (Tc) displays clear ion-energy dependence. With optimized ion energies, a consistently high Tc around 7 K is measured down to 11 nm film thickness. These results demonstrate the high ultrathin-film quality achievable through PEALD combined with substrate biasing. This process is particularly promising for the fabrication of low-loss superconducting quantum devices.

Superconductivity and strongly correlated materials in Bariloche

In June 1986, the news on the discovery of high critical temperature superconductivity at IBM, Switzerland, by the physicists Bednorz and Müller, reached the laboratories at the Bariloche Atomic Center in the Argentine Patagonia. The expertise in experimental techniques at low temperatures led by Paco de la Cruz and his team and also in crystal growth led by Daniel Esparza and his group, were important to reproduce the results in a question of a few months. This discovery produced enormous enthusiasm in the community and incentivized research in these materials in other laboratories and theoretical groups at the Bariloche Atomic Center, which had already a vast experience and trajectory in materials science. In this paper I will delve on how this striking discovery fostered theoretical and experimental research in Bariloche which lasts until today.

Coexistence of antiferromagnetism and unconventional superconductivity in a quasi-one-dimensional flat-band system: Creutz lattice

We study the coexistence of antiferromagnetism and unconventional superconductivity on the Creutz lattice which shows strictly flat bands in the noninteracting regime. The famous renormalized mean-field theory is used to deal with strong electron–electron repulsive Hubbard interaction in the effective low-energy t–J model, the superfluid weight of the unconventional superconducting state has been calculated via the linear response theory. An unconventional superconducting state with both spin-singlet and staggered spin-triplet pairs emerges beyond a critical antiferromagnetic coupling interaction, while antiferromagnetism accompanies this state. The superconducting state with only spin-singlet pairs is dominant with paramagnetic phase. The A phase is analogous to the pseudogap phase, which shows that electrons go to form pairs but do not cause a supercurrent. We also show the superfluid behavior of the unconventional superconducting state and its critical temperature. It is proven directly that the flat band can effectively raise the critical temperature of superconductivity. It is implementable to simulate and control strongly-correlated electrons’ behavior on the Creutz lattice in the ultracold atoms experiment or other artificial structures. Our results may help the understanding of the interplay between unconventional superconductivity and magnetism.

The Measurement and Analysis of the Hysteresis Loss in a Multifilament MgB2 Superconducting Wire

The article presents the results obtained from measuring the magnetization of and hysteresis loss in a multifilament MgB2 superconducting wire (produced by ASG Superconductors) for magnetic field applied in two directions: in parallel and perpendicular to the conductor axis. The measurement temperatures were varied in the range of 5–45 K. The MgB2 superconducting fraction magnetization curves were obtained by subtracting the magnetization of the wire ferromagnetic matrix measured at temperature T = 45 K, which is above the superconductor critical temperature Tc , from the magnetization value of the entire specimen. From these magnetization curves, the dependences of the hysteresis energy loss on the external magnetic field strength Qh(Н) were obtained. It is shown that, with the field oriented perpendicular to the MgB2 wire axis and at the external field characteristic values, the hysteresis loss is twice as large. A method for calculating the hysteresis loss in a superconducting wire and methods for reducing it are proposed. The obtained results can be used to optimize NMR and MRI facilities, superconducting motors and generators made using MgB2 superconducting wire produced by ASG Superconductors.

Magnetic flux trapping in hydrogen-rich high-temperature superconductors

Recent discoveries of superconductivity in various hydrides at high pressures have shown that a critical temperature of superconductivity can reach near-room-temperature values. However, experimental studies are limited by high-pressure conditions, and electrical transport measurements have been the primary technique for detecting superconductivity in hydrides. Here we implement a non-conventional protocol for the magnetic measurements of superconductors in a SQUID magnetometer and probe the trapped magnetic flux in two near-room-temperature superconductors H3S and LaH10 at high pressures. Contrary to traditional magnetic susceptibility measurements, the magnetic response from the trapped flux is almost unaffected by the background signal of the diamond anvil cell due to the absence of external magnetic fields. The behaviour of the trapped flux generated under zero-field-cooled and field-cooled conditions proves the existence of superconductivity in these materials. We reveal that the absence of a pronounced Meissner effect is associated with the very strong pinning of vortices inside the samples. This approach can also be a tool for studying multiphase samples or samples that have a low superconducting fraction at ambient pressure.

Electron-phonon coupling superconductivity and tunable topological state in carbon-rich selenide monolayers

The coexistence of Dirac cones and Van Hove singularities (VHSs), as the notable feature on prominent electronic band structure in recently hot-topic Lieb lattice and twisted graphene superlattice materials, has recently drawn tremendous attention since it offers an ideal platform for realizing correlation-driven electronic states (e.g., superconductivity and topological state). Here, we have identified two two-dimensional (2D) crystals, namely ${\mathrm{C}}_{4}\mathrm{Se}$ and ${\mathrm{C}}_{5}\mathrm{Se}$, which exhibit the coexistence of Dirac cones and VHSs. Based on ab initio calculations and the Bardeen-Cooper-Schrieffer theory, we investigated the electron-phonon coupling and possible superconductivity in both structures. The results indicate that ${\mathrm{C}}_{4}\mathrm{Se}$ possesses intrinsic superconducting states, whereas ${\mathrm{C}}_{5}\mathrm{Se}$ exhibits tunable superconductivity when doped. Their superconducting critical temperature (${T}_{c}$) can reach up to 11.6 and 11.2 K, respectively, surpassing the majority of 2D superconductors. Besides, we uncovered an approximate Dirac cone in ${\mathrm{C}}_{6}\mathrm{Se}$ with a small band gap of 0.17 eV. Via the application of a biaxial compressive strain, remarkably, the ${\mathrm{C}}_{6}\mathrm{Se}$ can be transformed into a topological insulator. These findings highlight the potential of carbon-rich C-Se 2D crystals as a promising platform for investigating fascinating band structures and physical states, thus advancing our comprehension of 2D crystals.

A Structure and the Superconductivity of LiBC

MgB2 has been found to be superconductor with a critical temperature of 39K by Akimitsu and coworkers in 2000. Then, extensive search has been made for searching similar superconductor among related intermetallic compounds. Lithium borocarbide (LiBC) is an analogue with similar structure to MgB2. In the structure of LiBC (P63/mmc), hexagonal sheets of B-C is in the place of B-B and Li is in the place of Mg contrast to MgB2. These result in the material being an insulator. LiBC is expected to be a new phonon-mediated Bardeen-Cooper-Schrieffer (BCS) superconductor. But to be this kind of superconductor, the material should be a conductor, so hole-doped method is used to make it a conductor. In the paper, a new stable structure of LiBC is discovered. Using structure prediction software CALYPSO and first principles calculation software, a new type structure of LiBC under ordinary pressure is discovered, and it has a crystal structure of <img width="30" height="15" src="http://article.sciencepublishinggroup.com/journal/606/6710005/image001.png" />. Quantum Espresso software is used to study the superconductivity of the structure, when the μ^*=0.10, election-phonon coupling parameter λ=0.4112, the critical temperature of superconductivity is 2.77 K, and the bands structure and the density of states of LiBC are obtained by the software. The structure and the superconductivity are also studied under some high pressure.

Ion-beam assisted sputtering of titanium nitride thin films

Titanium nitride is a material of interest for many superconducting devices such as nanowire microwave resonators and photon detectors. Thus, controlling the growth of TiN thin films with desirable properties is of high importance. This work aims to explore effects in ion beam-assisted sputtering (IBAS), were an observed increase in nominal critical temperature and upper critical fields are in tandem with previous work on Niobium nitride (NbN). We grow thin films of titanium nitride by both, the conventional method of DC reactive magnetron sputtering and the IBAS method, to compare their superconducting critical temperatures T_{c} as functions of thickness, sheet resistance, and nitrogen flow rate. We perform electrical and structural characterizations by electric transport and x-ray diffraction measurements. Compared to the conventional method of reactive sputtering, the IBAS technique has demonstrated a 10% increase in nominal critical temperature without noticeable variation in the lattice structure. Additionally, we explore the behavior of superconducting T_c in ultra-thin films. Trends in films grown at high nitrogen concentrations follow predictions of mean-field theory in disordered films and show suppression of superconducting T_c due to geometric effects, while nitride films grown at low nitrogen concentrations strongly deviate from the theoretical models.

Magnetically mediated hole pairing in fermionic ladders of ultracold atoms

Conventional superconductivity emerges from pairing of charge carriers—electrons or holes—mediated by phonons1. In many unconventional superconductors, the pairing mechanism is conjectured to be mediated by magnetic correlations2, as captured by models of mobile charges in doped antiferromagnets3. However, a precise understanding of the underlying mechanism in real materials is still lacking and has been driving experimental and theoretical research for the past 40 years. Early theoretical studies predicted magnetic-mediated pairing of dopants in ladder systems4–8, in which idealized theoretical toy models explained how pairing can emerge despite repulsive interactions9. Here we experimentally observe this long-standing theoretical prediction, reporting hole pairing due to magnetic correlations in a quantum gas of ultracold atoms. By engineering doped antiferromagnetic ladders with mixed-dimensional couplings10, we suppress Pauli blocking of holes at short length scales. This results in a marked increase in binding energy and decrease in pair size, enabling us to observe pairs of holes predominantly occupying the same rung of the ladder. We find a hole–hole binding energy of the order of the superexchange energy and, upon increased doping, we observe spatial structures in the pair distribution, indicating repulsion between bound hole pairs. By engineering a configuration in which binding is strongly enhanced, we delineate a strategy to increase the critical temperature for superconductivity.

Design and dimensioning of a test bench for investigating the relationship between critical temperature and axial mechanical stress of GdBaCuO tape conductors

PurposeSuperconductors offer several advantages compared with conventional conductors. However, it is not clear at this stage whether these types of conductors provide the same durability. For this reason, tape conductors under mechanical forces need to be studied in detail. The purpose of this paper is to investigate the relationship between critical temperature and axial mechanical stress of GdBaCuO tape conductors.Design/methodology/approachThe paper investigates the influence of axial mechanical stresses on the critical temperature of superconductors. For these investigations, a multi-physical test rig was developed, which makes it possible to perform these types of investigations. With the presented measurement methodology, the influence of mechanical stresses on the tape conductor can be determined.FindingsThe investigations show a correlation between the critical temperature and the acting mechanical stresses. The analytically presented approach to describe the transition temperature is valid for the investigated samples. In addition, it is determined that the effects are not reversible, and therefore, permanent damage to the tape conductor is observed.Originality/valueThe presented investigations make it possible to create more accurate models of GdBaCuO tape conductors. This enables to extend the superconducting state space, which so far depends on three critical quantities, by the quantity of the axial stress.

How pressure enhances the critical temperature of superconductivity in YBa2Cu3O6+y

High-temperature superconducting cuprates respond to doping with a dome-like dependence of their critical temperature (Tc). But the family-specific maximum Tc can be surpassed by application of pressure, a compelling observation known for decades. We investigate the phenomenon with high-pressure anvil cell NMR and measure the charge content at planar Cu and O, and with it the doping of the ubiquitous CuO2 plane with atomic-scale resolution. We find that pressure increases the overall hole doping, as widely assumed, but when it enhances Tc above what can be achieved by doping, pressure leads to a hole redistribution favoring planar O. This is similar to the observation that the family-specific maximum Tc is higher for materials where the hole content at planar O is higher at the expense of that at planar Cu. The latter reflects dependence of the maximum Tc on the Cu-O bond covalence and the charge-transfer gap. The results presented here indicate that the pressure-induced enhancement of the maximum Tc points to the same mechanism.

On the uniaxial strain (pressure) derivatives of the critical temperature of superconductivity of La[formula omitted]Sr[formula omitted]CuO4

The uniaxial strain (and pressure) derivatives of the critical temperature of superconductivity (Tc) of La2−xSrxCuO4 (LSCO) cuprate is studied within the extended Holstein–Hubbard model and bipolaronic mechanism of high-Tc superconductivity. The Tc of LSCO cuprate is associated with the Bose–Einstein condensation temperature, TBEC, of an ideal gas of the intersite bipolarons. Working on a modified chain model lattice of cuprates we expressed TBEC as a function of LSCO’s parameters (apical oxygen ion’s mass and its vibration frequency, the periods and the strains of the lattice). Using the experimental values of LSCO parameters the uniaxial strain derivatives and the uniaxial pressure derivatives of Tc i.e. ∂TBEC/∂ɛi and ∂TBEC/∂pi (i=a,c) of LSCO cuprate are calculated as a function of doping level x. The obtained results are compared with the available experimental data. Qualitative and quantitative similarities as well as differences in the doping dependencies of between our ∂TBEC/∂ɛi (∂TBEC/∂pi) and the experimental ∂Tc/∂ɛi (∂Tc/∂pi) (i=a,c) are discussed. Upon carried analysis we concluded that the uniaxial strain (pressure) derivatives of Tc of LSCO cuprate can be interpreted within the bipolaronic mechanism of superconductivity.

Towards in silico mining for superconductors – Cutting the Gordian knot

Utilizing the random forest model, feasibility of training machine learning regressor models to predict critical temperatures of superconductivity from Density Functional Theory (DFT) based electronic band structures is explored. This complementarity between experiment and theory draws inspiration from the merging of Kohn-Sham and Bogoliubov-De Gennes equations [W. Kohn, W, EKU Gross, and LN Oliveira, Int. J. of Quant. Chem.,36(23), 611–615 (1989)]. Features in the Kohn-Sham Density Functional Theory band structure away from EF becoming decisive for the superconducting gap demonstrates this divide-and-conquer physical understanding. Not committing to any microscopic mechanism for the SC at this stage, it implies that in different classes of materials, different electronic features are responsible for the superconductivity. However, training on known members of a class, the performance of new members may be predicted. The method is validated for the A15 materials, including both binary A3X and ternary A6XY intermetallics, A = V, Nb, demonstrating that the two do indeed belong to the same class of superconductors.

A first-principles study of theoretical superconductivity on RbH by doping without applied pressure

The structural, electronic, lattice dynamics, electron–phonon (el–ph) coupling, and superconducting (SC) properties of the alkali-metal hydride RbH, metallized through electron-doping by the construction of the solid-solution RbSr x H, are systematically analyzed as a function of Sr-content within the framework of density functional perturbation and Migdal-Eliashberg theories, taking into account the effect of zero-point energy contribution by the quasi-harmonic approximation. For the entire studied range of Sr-content, steady increments of the el–ph coupling constant and the SC critical temperature are found with progressive alkaline-earth metal content through electron-doping, reaching the values of λ = 1.92 and K with = 0.1(0). The steady rise of such quantities as a function of Sr-content is consequence of the metallization of the hydride as an increase of density of states at the Fermi level is observed, as well as the softening of the phonon spectrum, mainly coming from H-optical modes. Our results indicate that electron-doping on metal-hydrides is an encouraging alternative to look for superconductivity without applied pressure.