Temperature Superconductors Research Articles

Spin-fluctuation glue disfavors high-critical temperature of superconductivity?

Antiferromagnetic fluctuations are believed to be a promising glue to drive high-temperature superconductivity especially in cuprates. Here, we perform a close inspection of the superconducting mechanism from spin fluctuations in the Eliashberg framework by employing a typical one-band model on a square lattice. While spin fluctuations can eventually drive superconductivity as is well established, we find that the superconducting tendency is suppressed substantially by a seemingly negligible contribution from a small momentum transfer far away from . This suppression comes from phase frustration of the pairing gap and is expected to be a general feature due to the repulsive pairing interaction of spin fluctuations. Furthermore, we find that the momentum dependence of the pairing gap largely deviates from the functional form of , although this form is well established in cuprate superconductors. We argue that an instantaneous magnetic interaction plays the important role to understand high-critical temperature of superconductivity as well as the momentum dependence of the pairing gap.

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.

Effect of Material-Dependent Boundaries on the Interference Induced Enhancement of the Surface Superconductivity Temperature.

Using the tight-binding Bogoliubov-de Gennes formalism, we describe the influence of the surface potential on the superconducting critical temperature at the surface. Surface details are taken into account within the framework of the self-consistent Lang-Kohn effective potential. The regimes of strong and weak coupling of superconducting correlations are considered. Our study reveals that, although the enhancement of the surface critical temperature, originating from the enhancement of the localized correlation due to the constructive interference between quasiparticle bulk orbits, can be sufficiently affected by the surface potential, this influence, nonetheless, strongly depends on the bulk material parameters, such as the effective electron density parameter and Fermi energy, and is likely to be negligible for some materials, in particular for narrow-band metals. Thus, superconducting properties of a surface can be controlled by the surface/interface potential properties, which offer an additional tuning knob for the superconducting state at the surface/interface.

Study of the Influence of the Reluctance of a High- Temperature Superconductor on an Eddy-Current Axial Flux Superconducting Coupling

This work presents the influence of the reluctance of a novel hybrid superconducting coupling (NHSC) with axial flux. The coupling is made of the PMs in the primary and the copper and split superconductor with variable reluctance in the secondary. The study is made when the superconducting is in the vortex zone. The distribution of torque and magnetic field properties are examined and computed using a magnetic equivalent circuit (MEC) model. Since the permeability and the conductivity vary according to the temperature and the magnetic field around the superconductor, the graph of the evolution of the eddy current density is derived. By employing the curve fitting technique, we obtained the polynomial equation of the eddy current density, then this equation allowed us to figure out the formula of the eddy current density. By knowing the density of the eddy current, we were able to deduce the expressions of the bulk conductivity, then the London penetration depth, and the relative permeability of the superconductor. These expressions allowed us to determine how the superconductor's reluctance varies depending on the temperature. The force and output torque are calculated using a combination of Kirchhoff's law and Ampere's loop law, respectively, and also by considering the variation of the reluctance according to the temperature. In order to expand the coupling's operating margin, the performance is examined by utilizing a high-temperature superconductor with a temperature range of 77 K to 130 K. The strong agreement between the torque simulation results and the outcomes of the proposed model's calculations demonstrates the accuracy of both results.

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.

Diamagnetic response and phase stiffness for interacting isolated narrow bands

Superconductivity is a macroscopic manifestation of a quantum phenomenon where pairs of electrons delocalize and develop phase coherence over a long distance. A long-standing quest has been to address the underlying microscopic mechanisms that fundamentally limit the superconducting transition temperature, Tc. A platform which serves as an ideal playground for realizing "high"-temperature superconductors are materials where the electrons' kinetic energy is quenched and interactions provide the only energy scale in the problem. However, when the noninteracting bandwidth for a set of isolated bands is small compared to the interactions, the problem is inherently nonperturbative. In two spatial dimensions, Tc is controlled by superconducting phase stiffness. Here, we present a theoretical framework for computing the electromagnetic response for generic model Hamiltonians, which controls the maximum possible superconducting phase stiffness and thereby Tc, without resorting to any mean-field approximation. Our explicit computations demonstrate that the contribution to the phase stiffness arises from i) "integrating out" the remote bands that couple to the microscopic current operator and ii) the density-density interactions projected on to the isolated narrow bands. Our framework can be used to obtain an upper bound on the phase stiffness and relatedly Tc for a range of physically inspired models involving both topological and nontopological narrow bands with density-density interactions. We discuss a number of salient aspects of this formalism by applying it to a specific model of interacting flat bands and compare the upper bound against the known Tc from independent numerically exact computations.

Some finite temperature and ground state properties of high-Tc superconductivity within the planar t-J-U model

In high-Tc materials, considering pair hopping and on-site pairing of electrons, presence of two electrons at a site following Pauli exclusion principle (a t-J-U model) is relevant. Considering this novelty, within a two-dimensional (2D) t-J-U Hamiltonian, some thermodynamic and ground state properties of high-Tc cuprates at various hole doping concentrations (〈h〉) have been investigated. Any theory based on perturbation approximation is inapplicable in strongly correlated electron systems, therefore, Lanczos method of exact diagonalization on a square cluster (tilted) of 8-sites is followed. Results show that double occupancy (δ) is maximum for hole concentration 〈h〉 = 0.25 and increases at a particular temperature T = T(U). Local moment shows the opposite behavior. Spin-spin correlation is negative between nearest-neighbor sites, confirming antiferromagnetic nature of the system. It appears that on-site Coulomb repulsion (U/t ≥ 6.0) destabilizes the superconducting ground state. In the system, antiferromagnetism (AF) and superconductivity are seen to coexist. A uniform distribution of charges is seen with increasing on-site Coulomb repulsion (U), which results in constant electron–electron correlation.

Effect of electromagnetic permeability on transition temperature of superconductivity

The description of superconductivity at high-temperature is a problem that has recently been addressed. Transition temperature of superconductivity, [Formula: see text], depends on the lattice structure type, size and room pressure. In super-lattices and low-dimensional layered nanostructures, [Formula: see text] is increased by increasing the complexity of the structure and internal pressures in solid lattice. In this paper, we investigate the relation between physical parameters ([Formula: see text]) of matter and superconductivity properties as well as transition temperature ([Formula: see text]), and explain the superconductivity at high-temperature. In this study, a semi-classical electromagnetic description along with vortex topologic theory and quantum dynamic models with experimental data is considered to justify the relation between superconductivity phenomena and magnetic monopole properties. We find that the electromagnetic energy of magnetic monopole is in agreement with vortex energy in topological theory and it can get close to thermal energy at high-temperature. These models suggest that the superconductivity is related to the mobile monopole or vortices. We show that the electrical permittivity ([Formula: see text]) and magnetic permeability ([Formula: see text]) of matter have a key role in the superconductive properties.

Interference-induced surface superconductivity: Enhancement by tuning the Debye energy

In the usual perception, surface superconductivity is associated with the surface nucleation of a superconducting condensate above the upper critical field in type-II superconductors or with a rearrangement of phonon properties and the electron-phonon coupling near surfaces/interfaces. Recently, it has been found that there is another example when the surface superconducting temperature is increased up to $20--25%$ as compared to the bulk one due to constructive interference of superconducting pair states. In the present work, we demonstrate that in fact, such an interference-induced enhancement can be much more pronounced, up to nearly $70%$. Furthermore, here it is shown that such an interference enhancement persists over a wide range of microscopic parameters.

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.

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.

Anomalous enhancement of dilepton production due to soft modes in dense quark matter

We explore how the dilepton production rate is modified near the critical temperature of color superconductivity and QCD critical point by the soft modes inherently associated with the phase transitions of second-order. It is shown that the soft modes affect the photon self-energy significantly through so called the Aslamasov-Larkin, Maki-Thompson and density of states terms, which are known responsible for the paraconductivity in the metalic superconductivity, and cause an anomalous enhancement of the production rate in the low energy/momemtum region.

Coexistence of superconductivity and charge density wave instability in A15-Nb3Sn.

A15-type compound Nb3Sn has attracted much attention due to its relatively high critical temperature and critical field of superconductivity, making it a leading material for superconducting applications. In this study, we investigate the structural instability and superconductivity of Nb3Sn under hydrostatic pressure using first-principles calculations. We determine the electronic properties, phonon dispersion, electron-phonon coupling and the superconducting gap for Nb3Sn at pressures ranging from ambient to 9 GPa. Our results show that a significant electron density is present near the Fermi level due to the van Hove singularity, indicating the strong electron-phonon coupling. The phonon dispersion of Nb3Sn exhibits Kohn anomalies at three different wave vectors at a lower temperature. Moreover, above a pressure of 6 GPa, the charge density wave (CDW) instability disappeared, suggesting that pressure inhibits the CDW phase. The superconducting temperature is predicted to be TC = 18.62 K under ambient conditions, which is well consistent with the experimental results. We find that both the CDW and superconducting orders respond to pressure, with their transition temperatures decreasing as the pressure increases below 6 GPa. Above 6 GPa, the superconducting transition temperature increases slowly with pressure. Our results suggest that the instability in Nb3Sn is driven by the softening of the phonon modes due to the CDW caused by strong electron-phonon coupling. Therefore, the CDW phase and superconducting phase of Nb3Sn coexist at low pressure.

On synthesis, stability and superconductivity of ThNb2H12under pressure: ab-inito calculations

Systematic study employing first-principle electronic structure method along with the evolutionary structure search algorithm as implemented in the universal structure predictor evolutionary Xtallography (USPEX) code has been carried out to look for the possibility of synthesising the ternary hydride (ThxNbyHz) from constituent's thorium, niobium and hydrogen. Our study suggests that the ternary hydride with formula ThNb2H12 will be globally stable above 12.5 GPa indicating the possibility of its formation from constituents Th, Nb, H and binary compounds of Th-H and Nb-H. Further, though the global stability of this compound is predicted to be beyond 12.5 GPa, it is metastable at 0 GPa. We have theoretically explored different formation routes to synthesize this compound. The zero-pressure monoclinic structure (Cm) of metastable ThNb2H12 undergoes a transformation to hexagonal structure (P6/mmm) at 3.5 GPa and this hexagonal structure (P6/mmm) transforms to other hexagonal type structure (P6/m) at 50 GPa. The high pressure structural sequence so predicted through static lattice calculations has been further substantiated by confirming the elastic and lattice dynamic stability of each structure in the pressure regime of their stability. Employing Allen–Dynes modified McMillan [P. B. Allen and R. C. Dynes, Phys. Rev. B 12, 905 (1975)] formula along with the fixed Coulomb pseudopotential of 0.1 and calculated electron-phonon coupling parameter, the superconductive temperature has also been calculated for this compound. Our analysis indicates that this material will behave as superconductor with transition temperature (Tc) ∼ 25 K at 15 GPa, which, will reduce to 4 K upon further compression to 100 GPa. Our analysis on lattice thermal conductivity of this compound in P6/mmm phase, at 15 GPa, shows a significantly large anisotropy in conductivity tensor, especially at lower temperatures.

Superconductivity in Th–H and Pu–H Compounds under High‐Pressure Conditions: A First‐Principles Study

Hydrogen‐rich materials synthesized at megabar pressures have revolutionized the field of high‐temperature superconductivity. Among hydrogen‐rich materials, clathrate hydrides which show near‐room‐temperature superconductivity have been discovered in recent years. Herein, clathrate actinide hydrides are constructed, and their electronic properties and electron–phonon interaction as well as superconductivity under high pressures are theoretically investigated with density functional theory tools. The calculations adopt projected‐augmented‐wave (PAW) approach. The Perdew–Burke–Ernzerhof (PBE)‐type generalized gradient approximation (GGA) is used for the exchange–correlation potentials. Specifically, Th–H and Pu–H compounds are presented, of which the stability is demonstrated via calculated phonon dispersions, and the superconducting transition temperatures Tc under 100, 200, and 400 GPa are estimated. ThH10 among Th–H compounds has the highest transition temperature of superconductivity, and Tc of ThH10 can reach 151.8 K under 200 GPa. PuH10 is also better than other investigated Pu–H compounds in superconductivity, but the highest Tc of PuH10 is only 31.4 K under 400 GPa, much lower than that of ThH10.

Tip-induced superconductivity enhancement in single-crystalline PdSb by point-contact spectroscopy

We report our mechanical point-contact spectroscopy (MPCS) study on single-crystalline superconductor PdSb with ${T}_{c}\ensuremath{\sim}$ 1.32 K and ${\ensuremath{\mu}}_{0}{H}_{c2}(0)\ensuremath{\sim}$ 900 Oe, whose ${T}_{c}$ changes very slightly against hydrostatic pressure below 2.5 GPa. For MPCS with a soft Au tip, the Andreev reflection signals suggest the same superconducting temperature ${T}_{c}$ as in the pristine PdSb crystal and the conductance curves show a fully gapped behavior, yielding a superconducting gap value of ${\mathrm{\ensuremath{\Delta}}}_{0}=$ 0.23 meV with $2{\mathrm{\ensuremath{\Delta}}}_{0}/{k}_{\mathrm{B}}{T}_{c}\ensuremath{\sim}$ 4.25 in the strong-coupling regime. However, for MPCS with a hard W tip, an enhanced superconducting ${T}_{c}$ can be observed with a concomitant increase in the superconducting gap $\mathrm{\ensuremath{\Delta}}$ in a linear relation of 0.109 meV/kelvin, whereas $2{\mathrm{\ensuremath{\Delta}}}_{0}/{k}_{\mathrm{B}}{T}_{c}$ decreases from 4.25 to 3.5 in a negative correlation with ${T}_{c}$. From first-principles calculations, we observe a Lifshitz transition related to Pd-$4{d}_{{z}^{2}}$ orbitals under uniaxial pressure, which is absent under hydrostatic pressure. Such Lifshitz transition results in an enhanced density of states near the Fermi level $n({E}_{F})$, which is in line with the enhanced ${T}_{c}$. In addition, a soft phonon mode at $H$ is hardened under uniaxial pressure, suggesting a reduced coupling strength in this case. Such local strain effect provides a natural explanation for our MPCS results on PdSb.

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.