Layered Superconductors Research Articles

Goos-Hänchen shifts around Fano resonances in superconducting photonic crystals embedded with graphene

Lateral Goos-Hänchen (GH) shifts at the Fano resonances are theoretically investigated in superconducting photonic crystals. The photonic system consists of superconductor layer pairs with embedded graphene and a dielectric layer. With an oblique incident light, multiple Fano resonances are realized in the transmission spectrum because of the embedded graphene. Large GH shifts of the reflected light, including positive (Δp) and negative (Δn) GH shifts, are obtained around the Fano resonances. The GH shifts almost keep stable for different chemical potentials of graphene. By decreasing hydrostatic pressure or incident angle, larger Δp and |Δn| are achieved and the corresponding frequency has a redshift. By increasing the ambient temperature, Δp and |Δn| first increase smoothly and then decrease sharply. Overall, Δp is more sensitive than Δn, while the reflectance corresponding to Δn is much larger than that corresponding to Δp. Large and tunable GH shifts in our study may have great potential for optical detectors and transducers.

Layered superconductors as nonlinear lenses for laser beams tunable by dc magnetic field

We develop a theory of focusing of a THz Gaussian laser beam by a layered superconducting slab of finite thickness in the presence of an external dc magnetic field in a nonlinear regime. We show that focusing abilities of the slab, which originate from the specific nonlinearity of the medium, can be flexibly tuned by the external magnetic field providing a way to control THz waves. We analytically study the main characteristics of the Gaussian beam transmitted through such a nonlinear lens, namely, a beam waist and a focusing distance, as functions of radiation frequency, beam intensity, and dc field magnitude. The results of analytic calculations are supported by the numerical simulation of the electromagnetic field distribution in the beam propagating through the superconducting slab. Published by the American Physical Society 2024

Practical magneto-optical imaging of the current density of coated conductors within liquid nitrogen.

Magneto-optical imaging (MOI) is widely used for magnetic studies of superconducting materials due to its advantages of full-field, real-time operation and high resolution. However, a traditional MOI system requires vacuum pumping, thermal shielding, and cooling by thermal conducting, thereby making the system very complex and expensive and increasing the time required to complete a set of experiments. In this study, a novel (to our knowledge) and practical approach for MOI within liquid nitrogen (LN) is proposed in which thermal conducting, thermal shielding, and vacuum pumping are no longer necessary. The key technique is realized through a semi-immersed polymethyl methacrylate (PMMA) bar in LN, and its size is optimized to ensure a stable temperature difference and polarized optical visualization within LN. With the improvised method, a defect in a superconducting layer of length approximately 250 µm in the coated conductor (CC) sample was detected. Additionally, the current density reduced by approximately 50% in magnitude compared to its neighbor region, thus demonstrating the effectiveness of the new approach. It is expected that this technique can further enhance the application of MOI as an efficient tool for industrial inspection of superconducting CCs.

Revealing the Microscopic Mechanism of Elementary Vortex Pinning in Superconductors

Vortex pinning is a crucial factor that determines the critical current of practical superconductors and enables their diverse applications. However, the underlying mechanism of vortex pinning has long been elusive, lacking a clear microscopic explanation. Here, using high-resolution scanning tunneling microscopy, we studied single vortex pinning induced by point defect in layered FeSe-based superconductors. We found the defect-vortex interaction drives low-energy vortex bound states away from EF, creating a “mini” gap that effectively lowers the system energy and enhances pinning. By measuring the local density of states, we directly obtained the elementary pinning energy and estimated the pinning force via the spatial gradient of pinning energy. The results are consistent with bulk critical current measurement. Furthermore, we showed that a general microscopic quantum model incorporating defect-vortex interaction can naturally capture our observation. It suggests that the local pairing near pinned vortex core is actually enhanced compared to unpinned vortex, which is beyond the traditional understanding that nonsuperconducting regions pin vortices. Our study thus unveils a general microscopic mechanism of vortex pinning in superconductors and provides insights for enhancing the critical current of practical superconductors. Published by the American Physical Society 2024

Temperature evolution and thermoelastic behavior of epoxy-impregnated REBCO HTS composite coils studied using a heat-force bi-directional coupling quench model

Quench and mechanical behavior are the key issues for the safe operation of epoxy-impregnated (Re)Ba2Cu3Ox (REBCO) pancake coil. The contribution of strain energy to the temperature field during heat propagation and the effect of temperature changes on the mechanical behaviour in the force field are considered. In the paper, the bi-directional coupling differential equations of heat conduction and elastic equilibrium are proposed, and their bi-directional coupling solution is realized using finite element method. The model is not based on homogenization assumption considering separately the anisotropic characteristics of each layer in a REBCO composite tape. In addition, the REBCO tape after quench is defined as a parallel circuit model in the finite element model. During the quench, the current is distributed between the superconducting (REBCO) and metal (copper, silver and Hastelloy) layers according to their resistance. The results show that for the self-field conditions, the strain-energy contribution that is taken into account in the heat-force bi-directional coupling model causes a 3 % smaller overall temperature of the coil, which is still necessary to consider for the quench behavior of highly sensitive superconducting coils. Another important result is that thermal stresses are the major contribution to the internal stress state of the coil in self-field, whereas under the background magnetic field of 5 T, the radial stress inside the coil has reached the strength range of peeling delamination.

The Identification of Microstructural Changes in High-Temperature Superconducting Tapes for Superconducting Fault Current Limiters

HTS 2G tapes used in Superconducting Fault Current Limiters (SFCLs) have properties that allow for the effective limitation of short-circuit currents; however, due to the specificity of the device operation, they should be characterized by the high stability of the parameters when repeatedly leaving the superconducting state. During the operation of SFCLs, a situation may occur in which the parameters of the HTS tapes used will change several times as a result of the action of short-circuit currents that exceed the critical current IC of the superconductor of the tape used. This paper presents the results of microstructural tests of 2G HTS tapes intended for SFCLs, subjected to surge currents corresponding to prospective short-circuit currents with values higher than their critical currents IC and for which IC changes were observed. The HTS tapes were examined using a JEOL 7600F field emission scanning electron microscope (SEM), and their chemical composition was analyzed using Energy-Dispersive X-ray Spectroscopy (EDS). The test results indicate the possibility of micro-damage in the form of cracks in the superconductor layer, as well as the interruption of the buffer layers and the oxidation of the silver layers. The analysis of the chemical composition of the HTS tape layers may indicate the occurrence of diffusion processes.

Critical Temperature and Order Parameter in Superconductor/Inhomogeneous Ferromagnet Heterostructures

A method for solving a self-consistent boundary value problem for the linearized Usadel equation is proposed. The method makes it possible to find the sample-normalized distribution of the superconducting order parameter and the critical temperature as functions of the problem parameters and to solve relatively complex spatially inhomogeneous problems, e.g., superconducting heterostructures containing inhomogeneous magnetic layers. Within this approach, layered structures containing superconducting and domain-split ferromagnetic layers are considered. The theory is compared with experiment for the Fe 20 Å/V 340 Å/Fe 8 Å/Cr/Fe 8 Å system.

Principles of two-dimensional terahertz spectroscopy of collective excitations: The case of Josephson plasmons in layered superconductors

Principles of two-dimensional terahertz spectroscopy of collective excitations: The case of Josephson plasmons in layered superconductors

Comparison of Levitation Properties between Bulk High-Temperature Superconductor Blocks and High-Temperature Superconductor Tape Stacks Prepared from Commercial Coated Conductors.

Bulk high-temperature superconductors (HTSs) such as REBa2Cu3O7-x (REBCO, RE = Y, Gd) are commonly used in rotationally symmetric superconducting magnetic bearings. However, such bulks have several disadvantages such as brittleness, limited availability and high costs due to the time-consuming and energy-intensive fabrication process. Alternatively, tape stacks of HTS-coated conductors might be used for these devices promising an improved bearing efficiency due to a simplification of manufacturing processes for the required shapes, higher mechanical strength, improved thermal performance, higher availability and therefore potentially reduced costs. Hence, tape stacks with a base area of (12 × 12) mm2 and a height of up to 12 mm were prepared and compared to commercial bulks of the same size. The trapped field measurements at 77 K showed slightly higher values for the tape stacks if compared to bulks with the same size. Afterwards, the maximum levitation forces in zero field (ZFC) and field cooling (FC) modes were measured while approaching a permanent magnet, which allows the stiffness in the vertical and lateral directions to be determined. Similar levitation forces were measured in the vertical direction for bulk samples and tape stacks in ZFC and FC modes, whereas the lateral forces were almost zero for stacks with the REBCO films parallel to the magnet. A 90° rotation of the tape stacks with respect to the magnet results in the opposite behavior, i.e., a high lateral but negligible vertical stiffness. This anisotropy originates from the arrangement of decoupled superconducting layers in the tape stacks. Therefore, a combination of stacks with vertical and lateral alignment is required for stable levitation in a bearing.

Prediction of novel layered indium halide superconductors

Abstract We design two new layered indium halide compounds LaOInF2 and LaOInCl2 by means of first-principles calculations and evolutionary crystal structure prediction. We find both compounds crystallize in a tetragonal structure with P4/nmm space group and have indirect band gaps of 2.58 eV and 3.21 eV, respectively. By substituting O with F, both of them become metallic and superconducting at low temperature. The F-doping leads to strong electron–phonon coupling in the low-energy acoustic phonon modes which is mainly responsible for the induced superconductivity. The total electron–phonon coupling strength are 1.86 and 1.48, while the superconducting transition temperature (T c) are about 7.2 K and 6.5 K with 10% and 5% F doping for LaOInF2 and LaOInCl2, respectively.

Superconducting diode effect under time-reversal symmetry.

In noncentrosymmetric superconductors, superconducting and normal conductions can interchange on the basis of the current flow direction. This effect is termed a superconducting diode effect (SDE), which is a focal point of recent research. The broken inversion and time-reversal symmetry is believed to be the requirements of SDE, but their intrinsic role has remained elusive. Here, we report strain-controlled SDEs in a layered trigonal superconductor, PbTaSe2. The SDE was found exclusively in a strained device with its absence in an unstrained device despite that it is allowed in unstrained trigonal structure. Moreover, the zero-field or magnetic field-even (magnetic field-odd) SDE is observed when the strain and current are along armchair (zigzag) direction The results unambiguously demonstrate the intrinsic SDE under time-reversal symmetry and the critical role of strain-induced electric polarization.

Ferroelectric proximity effects in two-dimensional FeSeTe

Recent studies have shown that proximity effects are able to substantially modulate the superconducting properties of various quasi-two-dimensional layered materials such as FeSe, FeSeTe, NbSe2, and NbS2. Due to their high surface charge concentration and high dielectric constants, ferroelectric materials provide an interesting avenue for inducing proximity effects in layered superconductors. In this study, we explore the interactions between FeSeTe and the two-dimensional ferroelectrics CuInP2S6 and CuInP2Se6. We found that contrary to the normal behavior of FeSeTe, FeSeTe/CuInP2S6, and FeSeTe/CuInP2Se6 heterostructures display a peculiar two-step superconducting transition. Further testing revealed a hysteresis loop in the IV curves of these samples when measured below the critical temperature indicating the presence of disorder and domains within FeSeTe. We conclude that these domains are responsible for the two-step transition in FeSeTe and hypothesize that they are induced by the domain structure of the aforementioned ferroelectric materials.

Optical Absorption in Tilted Geometries as an Indirect Measurement of Longitudinal Plasma Waves in Layered Cuprates.

Electromagnetic waves propagating in a layered superconductor with arbitrary momentum, with respect to the main crystallographic directions, exhibit an unavoidable mixing between longitudinal and transverse degrees of freedom. Here we show that this basic physical mechanism explains the emergence of a well-defined absorption peak in the in-plane optical conductivity when light propagates at small tilting angles relative to the stacking direction in layered cuprates. More specifically, we show that this peak, often interpreted as a spurious leakage of the c-axis Josephson plasmon, is instead a signature of the true longitudinal plasma mode occurring at larger momenta. By combining a classical approach based on Maxwell's equations with a full quantum derivation of the plasma modes based on modeling the superconducting phase degrees of freedom, we provide an analytical expression for the absorption peak as a function of the tilting angle and light polarization. We suggest that an all-optical measurement in tilted geometry can be used as an alternative way to access plasma-wave dispersion, usually measured by means of large-momenta scattering techniques like resonant inelastic X-ray scattering (RIXS) or electron energy loss spectroscopy (EELS).

Resilient Growth of a Highly Crystalline Topological Insulator-Superconductor Heterostructure Enabled by an Ex Situ Nitride Film.

Highly crystalline and easily feasible topological insulator-superconductor (TI-SC) heterostructures are crucial for the development of practical topological qubit devices. The optimal superconducting layer for TI-SC heterostructures should be highly resilient against external contamination and structurally compatible with TIs. In this study, we provide a solution to this challenge by showcasing the growth of a highly crystalline TI-SC heterostructure using refractory TiN (111) as the superconducting layer. This approach can eliminate the need for in situ cleavage or growth. More importantly, the TiN surface shows high resilience against contaminations during air exposure, as demonstrated by the successful recyclable growth of Bi2Se3. Our findings indicate that TI-SC heterostructures based on nitride films are compatible with device fabrication techniques, paving the way to the realization of practical topological qubit devices in the future.

A study of layered holographic superconductor

We have investigated a holographic model of a multi-layered superconductor in (2+1)-dimensions using the AdS/CFT correspondence. This correspondence allows us to study strongly interacting condensed matter systems through a weakly interacting gravitational theory. Our study focused on the effects of a finite system size on the superconductor’s properties. We observed significant variations in the critical temperature and critical magnetic field depending on the number of layers and the spacing between them. Notably, our results capture the transition from a two-dimensional (2D) to a three-dimensional (3D) behavior in the limit of a large number of closely spaced layers. This transition signifies a change in how the superconductivity operates within real material.

Ultrafast Terahertz Superconductor Van der Waals Metamaterial Photonic Switch

The high‐temperature layered superconductor (HTS) BSCCO is one of the key quantum material platforms in THz science and technology. Compact, stable, and reliable BSCCO THz photonic integrated circuit components can be developed to effectively and efficiently control and manipulate THz wave radiation, especially for future communication systems and network applications. Herein, the design, simulation, and modeling of ultrafast THz metamaterial photonic integrated circuits are reported on a few nanometer‐thick HTS BSCCO van der Waals (vdWs), capable of the active modulation of phase with constant transmission coefficient over a narrow‐frequency range. Meanwhile, the metamaterial circuit works as an amplitude modulator without significantly changing the phase in a different frequency band. Under the application of ultrashort optical pulses, the transient modulation dynamics of the THz metamaterial offer a fast‐switching timescale of 50 ps. The dynamics of picosecond light–matter interaction, Cooper pairs breaking, photoinduced quasiparticles generation and recombination, phonon bottleneck effect, and emission and relaxation of bosons in BSCCO vdW metamaterial arrays are discussed for the potential application of multifunctional superconducting photonic integrated circuits in communication and quantum technologies.

A novel joint-less second-generation high-temperature superconducting toroidal coil: Promise for fabricating compact toroidal magnetic fields

This paper presents a novel joint-less toroidal magnet made of second-generation high-temperature superconducting (2G-HTS) tapes. This approach effectively resolves the closed-loop issue for 2G-HTS magnets and has the potential to provide higher and more stable magnetic fields. Compared with traditional 2G-HTS toroidal magnets, while the current loops of the joint-less magnets have no resistance, a decrease in magnetic field still occurs, especially in coils with insulation. Therefore, this work focuses on the decrease of the magnetic field of this novel magnet using experimental and microanalytical methods. For the first time, it was verified that, by winding two coil groups, insulated and no-insulated, parallel charging does not cause interference between them. Furthermore, the magnetic field area was expanded by finite element analysis, and simulations showed that the magnetic field converged with increasing number of coils. Besides, we found that the decrease of the magnetic field was related to the damage of the tape during slitting and winding, where insulated coils were more susceptible to damage during winding. The damage usually occurred at the starting point of the tape slitting, because the copper layer was separated due to adhesion during the winding of insulated coils, which further caused the superconducting layer to detach, resulting in a decrease in the critical current. From our perspective, benefiting from the high critical field of the 2G-HTS tapes, this novel toroidal coil structure has significant implications for the construction of compact toroidal magnets.

Enhanced controllable triplet proximity effect in superconducting spin–orbit coupled spin valves with modified superconductor/ferromagnet interfaces

In a superconductor/ferromagnet hybrid, a magnetically controlled singlet-to-triplet Cooper pair conversion can modulate the superconducting critical temperature. In these triplet superconducting spin valves, such control usually requires inhomogeneous magnetism. However, in the presence of spin–orbit coupling from an interfacial heavy metal layer, the singlet/triplet conversion rate and, thus, critical temperature can be controlled via the magnetization direction of a single homogeneous ferromagnet. Here, we report significantly enhanced controllable pair conversion to a triplet state in a Nb/Pt/Co/Pt superconducting spin valve in which Pt/Co/Pt is homogenously magnetized and proximity-coupled to a superconducting layer of Nb. The Co/Pt interface furthest away from Nb is modified by a sub-nanometer-thick layer of Cu or Au. We argue that the enhancement is most likely associated from an improvement of the Co/Pt interface due to the insertion of Cu and Au layers. Additionally, the higher normalized orbital moments in Au measured using x-ray magnetic circular dichroism shows that increasing spin–orbit coupling enhances the triplet proximity effect—an observation supported by our theoretical calculations. Our results provide a pathway to enhancing triplet pair creation by interface engineering for device development in superspintronics.

The Effect of the Interfacial Resistance of the Superconducting-stabilizer Film on the Typical Sector Diffusion Pace for 2G HTS Tapes

High temperature superconducting (HTS) tapes of the second generation have been widely used as energy storage materials, such as in superconducting magnetic energy storage (SMES) devices. In order to enhance the current-carrying characteristics, these systems are typically run close to the critical currents of the coated conductors; as a result, hot spots may develop, which could cause the superconductor to become quenched. In order to prevent the start of hot spots and to reduce the amount of faulting, efforts have been made to raise the normal zone propagation velocity (NZPV) in this manuscript. The interfacial resistance between the superconductor and stabilizer layer, which can act as a current flow diverter during fault circumstances, has been shown to be the key to producing massive NZPV. The tape's architecture has been slightly modified by the addition of a high resistive layer between the superconducting and stabiliser layers, where various interfacial resistances have been utilised to forecast the temperature distribution between 10 cm lengths of HTS tape. A 2D numerical model was created using COMSOL to assess the NZPV and temperature distribution of the 2G superconducting tape. It has been concluded that larger NZPVs can be achieved by using substantial interfacial resistances to prevent superconducting tape quenching.

Unveiling intrinsic material and extrinsic pinning dimensionality in superconductors: Why Fe(Se,Te) is able to mimic YBCO

Discovery of iron-based superconductors paved the way to a competitor of high-temperature superconductors, easier to produce, better performing in high fields, and promising to be less expensive. Critical parameters are investigated by resistivity measurements as a function of temperature, field, and angle R(T,H,θ). This work presents a deep analysis of H-θ phase diagram of PLD-processed Fe(Se,Te) superconducting films, thus revealing material and pinning anisotropy at once. By selecting different thresholds along the R(T,H,θ) curves, all possible regimes emerge. Surprisingly, anisotropy arises moving from the upper critical field toward the irreversibility line: gradually a non-monotonous transition from 3D to 2D, and backward to 3D occurs. Although Fe(Se,Te) appears as a 3D superconductor, its anisotropic pinning landscape shows up similarities with an intrinsic layered superconductor and Fe(Se,Te) definitively mimics YBCO. We propose a general method to disentangle, in any other superconductor, material dimensionality and pinning anisotropy that are key constraints for applications.