State Of High-temperature Superconductors Research Articles

Superconducting spin reorientation in spin-triplet multiple superconducting phases of UTe2.

Superconducting (SC) state has spin and orbital degrees of freedom, and spin-triplet superconductivity shows multiple SC phases because of the presence of these degrees of freedom. However, the observation of spin-direction rotation occurring inside the SC state (SC spin rotation) has hardly been reported. Uranium ditelluride, a recently found topological superconductor, exhibits various SC phases under pressure: SC state at ambient pressure (SC1), high-temperature SC state above 0.5 gigapascal (SC2), and low-temperature SC state above 0.5 gigapascal (SC3). We performed nuclear magnetic resonance (NMR) and ac susceptibility measurements on a single-crystal uranium ditelluride. The b axis spin susceptibility remains unchanged in SC2, unlike in SC1, and decreases below the SC2-SC3 transition with spin modulation. These unique properties in SC3 arise from the coexistence of two SC order parameters. Our NMR results confirm spin-triplet superconductivity with SC spin parallel to b axis in SC2 and unveil the remaining of spin degrees of freedom in SC uranium ditelluride.

Role of anharmonicity in the phonon density of states of high temperature superconductors

Owing to the quantum dynamical approach of double time temperature dependent phonon-Green’s functions, the expression for phonon density of states (PDOS) has been developed via an almost complete Hamiltonian which consists of the contributions due to the unperturbed phonons and electrons, anharmonic phonon fields, localized phonon fields and that of electron–phonon interactions. It has been observed that the anhrmonicity play an vital role in the PDOS through energy line width. Enhancement in the PDOS due to electron–phonon interactions has been mainly discussed in this work, which, in turns, will be capable to explain a large number of dynamical properties of crystals even that of superconductors. The temperature dependence of PDOS has been found as a unique feature of the theory, which certainly becomes the outcome of the anharmonic interactions. The presence of electron–phonon interaction parameters in each term is an additional and new feature of the theory.

Topological Model of the Pseudogap State: Experimental Signatures

We summarize the basic ideas of our topological model of the pseudogap state of high temperature superconductors (HTS) as a condensate of charged magnetic monopoles, with a focus on new experimental signatures. These include the surface quantum Hall effect, the generation of electric fields when applying magnetic fields by the oblique Meissner effect, and the generation of circular electric fields surrounding electric currents by the oblique Ampère law.

Nano-based Josephson Tunneling Networks and High Temperature Superconductivity

The search for superconductors with high values of the critical temperature has started shortly after the first observation of the phenomenon in 1911. At present, there are several paths towards reaching this goal. Here we focus on the direction based on observing high temperature superconducting state in some metallic nanoclusters. The macroscopic high Tc superconducting current can be provided by the Josephson tunneling between such clusters.

Temperature scaling behavior of the linear magnetoresistance observed in high-temperature superconductors

An analytical model invoking variations in the charge-carrier density is used to generate magnetoresistance curves that are almost indistinguishable from those produced by sophisticated numerical models. This demonstrates that, though disorder is pivotal in causing linear magnetoresistance, the form of the magnetoresistance thus generated is insensitive to details of the disorder. Taken in conjunction with the temperature ($T$) dependence of the zero-field resistivity, realistic levels of disorder are shown to be sufficient to explain the linear magnetoresistance and field-$T$ resistance scaling observed in high-temperature pnictide and cuprate superconductors. Hence, though the $T$-linear zero-field resistance is a definite signature of the "strange metal" state of high-temperature superconductors, their linear magnetoresistance and its scaling is unlikely to be so.

Dynamic Vortex States in High-Temperature Superconductors under Pulsed Magnetization

A vortex system of the Bi-2212 superconductor in an external magnetic field linearly increasing at different rates has been numerically simulated using the Monte Carlo method. The effect of the concentration and distribution of pinning centers on the penetration of the magnetic flux has been studied. Features of the flux penetration have been observed and described in terms of the vortex system dynamics and the conditions for their occurrence have been determined. The increasing dependences of the saturation field on the defect concentration in homogeneous and inhomogeneous samples have been obtained.

Competing stripe and magnetic phases in the cuprates from first principles

Realistic description of competing phases in complex quantum materials has proven extremely challenging. For example, much of the existing density-functional-theory-based first-principles framework fails in the cuprate superconductors. Various many-body approaches involve generic model Hamiltonians and do not account for the interplay between the spin, charge, and lattice degrees of freedom. Here, by deploying the recently constructed strongly constrained and appropriately normed (SCAN) density functional, we show how the landscape of competing stripe and magnetic phases can be addressed on a first-principles basis both in the parent insulator YBa2Cu3O6 and the near-optimally doped YBa2Cu3O7 as archetype cuprate compounds. In YBa2Cu3O7, we find many stripe phases that are nearly degenerate with the ground state and may give rise to the pseudogap state from which the high-temperature superconducting state emerges. We invoke no free parameters such as the Hubbard U, which has been the basis of much of the existing cuprate literature. Lattice degrees of freedom are found to be crucially important in stabilizing the various phases.

Asymptotically Exact Method for Calculating the Density of States of High-temperature Superconductors

A method based on a combination of Monte Carlo and gradient descent algorithms is presented for calculating the spectral density from the Green’s function. The method does not contain systematic errors and, in principle, can be used to parameterize any spectral function with a given accuracy. The application of the method is illustrated by the restoration of the spectral density of iron-based superconductors. The method can be used to study properties of metal nanoclusters and other systems.

Performance analysis of cryocoolers based on reverse Brayton cycle and its modifications for cooling HTS devices

To maintain the superconducting state of high temperature superconductor (HTS) used for power application, higher refrigeration capacity cryocooler (>2 kW @ 70 K) is required. A reverse Brayton cycle based cryocooler with long-life and high reliability is one of the suitable candidates for cooling of HTS. The basic reverse Brayton cryocooler can be modified of basic configuration in order to achieve higher FOM. An analytical study focused on modified configurations of reverse Brayton cycle for HTS has been carried out. Neon is selected as working fluid since it has higher heat capacity than helium around 70 K. Six reverse Brayton based modified cycles are standard cycle, 2-stage compression, 2-stage series expansion, 2-stage parallel expansion (same pressure ratio), 2-stage parallel expansion (different pressure ratio), and modified cycle with intermediate cooling system for recuperator. The performances of the modified reverse Brayton cryocoolers are compared and reasons for performance variations are identified.

Fabrication and Experiment of Racetrack HTS Magnet for Stator Field-Excitation HTS Machine

This paper proposes a new kind of racetrack-shaped high-temperature superconducting (HTS) magnet for a stator field-excitation HTS machine, for the purpose of implementing stationary seal at the cooling Dewar. The key is to introduce a vacuum sandwich-structure into the proposed racetrack-shaped Dewars so that the operation temperature of field-excitation windings can be regulated at 63 K. The method of reducing pressure by vacuuming is applied accordingly to decrease the temperature of cryogenic liquid nitrogen to ensure the cooling effect. The test results show that all the fabricated racetrack HTS magnets can operate at HTS state when the excitation current reaches 40 A. Hence, the proposed racetrack HTS magnet is experimentally verified.

Spin-freezing perspective on cuprates

The high-temperature superconducting state in cuprates appears if charge carriers are doped into a Mott insulating parent compound. An unresolved puzzle is the unconventional nature of the normal state above the superconducting dome, and its connection to the superconducting instability. At weak hole-doping, a "pseudo-gap" metal state with signatures of time-reversal symmetry breaking is observed, which near optimal doping changes into a "strange metal" with non-Fermi liquid properties. Qualitatively similar phase diagrams are found in multi-orbital systems, such as pnictides, where the unconventional metal states arise from a Hund coupling induced spin-freezing. Here, we show that the relevant model for cuprates, the single-orbital Hubbard model on the square lattice, can be mapped onto an effective multi-orbital problem with strong ferromagnetic Hund coupling. The spin-freezing physics of this multi-orbital system explains the phenomenology of cuprates, including the pseudo-gap, the strange metal, and the d-wave superconducting instability. Our analysis suggests that spin-freezing is the universal mechanism which controls the properties of unconventional superconductors.

Superconductivity in FeSe Thin Films Driven by the Interplay between Nematic Fluctuations and Spin-Orbit Coupling.

The origin of the high-temperature superconducting state observed in FeSe thin films, whose phase diagram displays no sign of magnetic order, remains a hotly debated topic. Here we investigate whether fluctuations arising due to the proximity to a nematic phase, which is observed in the phase diagram of this material, can promote superconductivity. We find that nematic fluctuations alone promote a highly degenerate pairing state, in which both s-wave and d-wave symmetries are equally favored, and T_{c} is consequently suppressed. However, the presence of a sizable spin-orbit coupling or inversion symmetry breaking at the film interface lifts this harmful degeneracy and selects the s-wave state, in agreement with recent experimental proposals. The resulting gap function displays a weak anisotropy, which agrees with experiments in monolayer FeSe and intercalated Li_{1-x}(OH)_{x}FeSe.

Superconductivity well above room temperature in compressed MgH6

It has been suggested that hydrogen-rich systems at high pressure may exhibit notably high super-conducting transition temperatures. One of the more interesting theoretical predictions was that hydrogen sulfide can be metallized and the high-temperature superconducting state can be induced. A record critical temperature (203 K) was later confirmed for H3S in an experiment. In this paper, we investigated, within the framework of the Eliashberg formalism, the properties of compressed MgH6, which is expected to be a very good candidate for room-temperature superconductivity. This applies particularly to the pressure range from 300 to 400 GPa, where the transition temperature is close to 400 K. Moreover, the estimated thermodynamic properties and the resulting dimensionless ratios exceed the predictions of the Bardeen–Cooper–Schrieffer theory. This behavior is attributed to the strong electron–phonon coupling and retardation effects existing in hydrogen-dominated materials under high pressure.

Transition to the Normal State Induced by High Current Densities in High-Tc Superconductor Microbridges Under Applied Magnetic Fields

We report some of our experimental results about the transition to the normal state of high-temperature superconductors subjected to high current densities and, simultaneously, under external magnetic fields up to 1 T. Our data analysis, which is based on a recently published instability model, show that the quenching may be explained in terms of thermal instabilities due to self-heating favored by the nonlinear nature of the current–voltage characteristics of high- $T_{c}$ superconductors (HTS). In fact, we predict the density current $J^{\ast}$ at which the samples quench, i.e., jump abrupt to the normal state, with an accuracy around 1%. Beyond its interest from a fundamental point of view, this thermal model opens a practical way to estimate the quenching point thus avoiding the damaging or even burning up of devices in high power applications of HTS.

Spectroscopy of metal “superatom” nanoclusters and high-Tcsuperconducting pairing

A unique property of metal nanoclusters is the "superatom" shell structure of their delocalized electrons. The electronic shell levels are highly degenerate and therefore represent sharp peaks in the density of states. This can enable exceptionally strong electron pairing in certain clusters composed of tens to hundreds of atoms. In a finite system, such as a free nanocluster or a nucleus, pairing is observed most clearly via its effect on the energy spectrum of the constituent fermions. Accordingly, we performed a photoionization spectroscopy study of size-resolved aluminum nanoclusters and observed a rapid rise of the near-threshold density of states of several clusters ($Al_{37,44,66,68}$) with decreasing temperature. The characteristics of this behavior are consistent with compression of the density of states by a pairing transition into a high-temperature superconducting state with $T_c$>~100 K. This value exceeds that of bulk aluminum by two orders of magnitude. These results highlight the potential of novel pairing effects in size-quantized systems and the possibility to attain even higher critical temperatures by optimizing the particles' size and composition. As a new class of high-temperature superconductors, such metal nanocluster particles are promising building blocks for high-$T_c$ materials, devices, and networks.

A Novel Feature in Aluminum Cluster Photoionization Spectra and Possibility of Electron Pairing at T ≳ 100 K

A unique property of size-resolved metal nanocluster particles is their "superatom"-like electronic shell structure. The shell levels are highly degenerate, and it has been predicted that this can enable exceptionally strong superconducting-type electron pair correlations in certain clusters composed of just tens to hundreds of atoms. Here we report on the observation of a possible spectroscopic signature of such an effect. A bulge-like feature appears in the photoionization yield curve of a free cold aluminum cluster and shows a rapid rise as the temperature approaches ≈100 K. This is an unusual effect, not previously reported for clusters. Its characteristics are consistent with an increase in the effective density of states accompanying a pairing transition, which suggests a high-temperature superconducting state with Tc ≳ 100 K. Our results highlight the promise of metal nanoclusters as high-Tc building blocks for materials and networks.

Analytical microscopy of iron-based superconducting materials

AbstractInhomogeneity and phase separation in unconventional superconducting materials is a topic of increasing interest, as the competition between phases with different types of ordering is thought to play an important role in the emergence of the high temperature superconducting state. A wide range of experimental techniques are available for investigating the crystallography of these complex materials, however the majority are bulk probes. Here we briefly review some spatially resolved techniques that are being used to investigate phase separation in ironbased superconductors, with a focus on analytic Scanning Electron Microscopy techniques and synchrotron-based Photoelectron Microscopy.

Final state of thermal evolution of Jupiter-type planet

Inside the planet consisting mostly of hydrogen (the Jupiter-type planet), in the last stage of the thermal evolution there should be induced the high-temperature superconducting state. In particular, for the effective temperature of the planet (TS equal to about 5K), the superconducting phase of hydrogen will appear near the value of the pressures p1≡500GPa and p2≡2000GPa. Together with the further lowering of the temperature, the superconducting state will be induced in the hydrogen layers with the pressure successively distant from p1 and p2. The last superconducting layer will be created for p≃400GPa, at the moment, when TS drops to about 1.3K. It has been also shown that the rotating planet, in which the superconducting state was induced, is the source of the very weak magnetic field of the induction equal to about 10-11Gs. The results have been obtained on the basis of the analysis of the thermodynamic properties of the superconducting state in hydrogen subjected to the influence of the extremely high pressure at 3500GPa (close to the core of the hypothetical planet) and on the basis of the literature describing the superconducting phase in hydrogen at the lower pressures.

Renormalization effects and phonon density of states in high temperature superconductors

Using the versatile double time thermodynamic Green's function approach based on many body theory the renormalized frequencies, phonon energy line widths, shifts and phonon density of states (PDOS) are investigated via a newly formulated Hamiltonian (does not include BCS type Hamiltonian) that includes the effects of electron-phonon, anharmonicities and that of isotopic impurities. The automatic appearance of pairons, temperature, impurity and electron-phonon coupling of renormalized frequencies, widths, shifts and PDOS emerges as a characteristic feature of present theory. The numerical investigations on PDOS for the YBa2Cu3O7 − δ crystal predicts several new feature of high temperature superconductors (HTS) and agreements with experimental observations.

Echoes in correlated neural systems

Correlations are employed in modern physics to explain microscopic and macroscopic phenomena, like the fractional quantum Hall effect and the Mott insulator state in high temperature superconductors and ultracold atoms. Simultaneously probed neurons in the intact brain reveal correlations between their activity, an important measure to study information processing in the brain that also influences the macroscopic signals of neural activity, like the electroencephalogram (EEG). Networks of spiking neurons differ from most physical systems: the interaction between elements is directed, time delayed, mediated by short pulses and each neuron receives events from thousands of neurons. Even the stationary state of the network cannot be described by equilibrium statistical mechanics. Here we develop a quantitative theory of pairwise correlations in finite-sized random networks of spiking neurons. We derive explicit analytic expressions for the population-averaged cross correlation functions. Our theory explains why the intuitive mean field description fails, how the echo of single action potentials causes an apparent lag of inhibition with respect to excitation and how the size of the network can be scaled while maintaining its dynamical state. Finally, we derive a new criterion for the emergence of collective oscillations from the spectrum of the time-evolution propagator.