Superconducting State Research Articles

Enhanced weak superconductivity in trigonal γ-PtBi2

Electrical resistivity experiments show superconductivity at Tc=1.1 K in a high-quality single crystal of trigonal γ-PtBi2, with an enhanced critical magnetic field μ0Hc2(0)≳1.5 Tesla and a low critical current-density Jc(0)≈40 A cm−2 at H = 0. Both Tc and Hc2(0) are the highest reported values for stoichiometric bulk samples at ambient pressure. We found a weak Hc2 anisotropy with Γ=Hc2ab/Hc2c<1 , which is unusual among superconductors. Under a magnetic field, the superconducting transition becomes broader and asymmetric. Along with the low critical currents, this observation suggests an inhomogeneous superconducting state. In fact, no trace of superconductivity is observed through field-cooling–zero-field-cooling magnetization experiments.

Investigating Superconducting State Parameters in (TaNb)X(TiZrHf)1-X Compounds: A First Principle Approach Employing MS Excel as Toolkit

Investigating Superconducting State Parameters in (TaNb)_X(TiZrHf)_1-X Compounds: A First Principle Approach Employing MS Excel as Toolkit

Rapid optical switching of latched electrical resistance in a high-T c superconducting tape

We report microsecond timescale switching between the superconducting and normal states of commercially manufactured high-Tc superconducting wires using optical radiation to heat the conductor above Tc. The achieved voltages and switching times have significant implications for a new class of thermal switches. The report contains experimental data for photo-induced microsecond scale voltage transients as well as numerical analysis of heat propagation through the material in response to optical radiation. The microbridges are etched into commercially manufactured coated conductors and submerged in liquid nitrogen. By varying the magnitude of the transport current, two different optical responses are identified. At low transport currents, short-lived voltage transients occur, transitioning to persistent latching once the current is increased above a threshold value. The microbridge, therefore, behaves as a fast opening and closing switch when carrying low currents or an optically latched thyristor at high currents. This is understood to occur due to critical current suppression as a result of heating from the optical pulse. Optically induced heating reduces the superconducting volume fraction, and Joule heating due to the transport current interaction with the high temperature superconducting occurs, which either causes short-lived or stable self-heating normal regions dependent on the magnitude of the transport current. This study is concluded with an estimate of the energy necessary to drive the microbridge into the resistive state. The observed behavior can potentially be utilized as a switching element in superconducting transformer rectifiers where high frequency switch operation is required.

Leggett modes in a Dirac semimetal

AbstractExperiments have shown that several materials, including MgB2, iron-based superconductors and monolayer NbSe2, are multiband superconductors. Superconducting pairing in multiple bands can give rise to phenomena not available in a single band, including Leggett modes. A Leggett mode is the collective periodic oscillation of the relative phase between the phases of the superconducting condensates formed in the different bands. The experimental observation of Leggett modes is challenging because multiband superconductors are rare and because these modes describe charge fluctuations between bands and therefore are hard to probe directly. Also, the excitation energy of a Leggett mode is often larger than the superconducting gaps, and therefore they are strongly overdamped via relaxation processes into the quasiparticle continuum. Here, we show that Leggett modes and their frequency can be detected in a.c. driven superconducting quantum interference devices. We then use the results to analyse the measurements of such a quantum device, one based on a Dirac semimetal Cd3As2, in which superconductivity is induced by proximity to superconducting Al. These results show the theoretically predicted signatures of Leggett modes, and therefore we conclude that a Leggett mode is present in the two-band superconducting state of Cd3As2.

Static and dynamic magnetic behavior of YBCO/Co/IrMn heterostructures

The effect of the YBCO superconducting (SC) state on the magnetic properties of as-grown YBCO/Co/IrMn heterostructures has been systematically studied using magnetometry and ferromagnetic resonance. The obtained data showed that the superconductivity of the YBCO substrate strongly affects the ferromagnetic properties of the deposited Co layer deeper (up to 50 nm) than the coherence length of the YBCO (≃4nm) by an exchange interaction between the Co magnetic moments and the superconducting pairs at the YBCO/Co interface. The interfacial exchange interaction, switched on while the YBCO enters the SC state, pins Co spins and yields an enhancement of the Co magnetic properties.

Entanglement entropy of the maximum geminal of the BCS ground state

From the viewpoint of the Bose–Einstein condensation (BEC) of the fermion system, the maximum geminal of the second-order reduced density matrix of the superconducting state exactly corresponds to the Cooper pair. In this paper the entanglement entropy (EE) for the maximum geminal of the BCS ground state is evaluated. The EE behaves logarithmically with respect to the number of the maximum geminal. Furthermore, the disappearance point of superconductivity is defined on the basis of the fermion BEC. In the superconducting ground state, almost all electrons in the energy width of the gap parameter near the Fermi level are condensed as a maximum geminal. They suddenly change to normal electrons with a finite gap of the EE at the disappearance point like a first-order phase transition.

Fate of the superconducting state in floating islands of hybrid nanowire devices

Fate of the superconducting state in floating islands of hybrid nanowire devices

Ab-initio insights into the physical properties of XIr3 (X = La, Th) superconductors: A comparative analysis

Here we report the structural, elastic, bonding, thermo-mechanical, optoelectronic and superconducting state properties of recently discovered XIr3 (X = La, Th) superconductors utilizing the density functional theory (DFT) using two different exchange-correlations functionals. The elastic, bonding, thermal and optical properties of these compounds are investigated for the first time. The calculated lattice and superconducting state parameters are in reasonable agreement to those found in the literature. In the ground state, both the compounds are mechanically stable and possess highly ductile character, high machinability, low Debye temperature, low bond hardness and significantly high melting point. The thermal conductivities of the compounds are found to be very low which suggests that they can be used for thermal insulation purpose. The population analysis and charge density distribution map confirm the presence of both ionic and covalent bonds in the compounds with ionic bond playing dominant roles. The calculated band structure and DOS profiles indicate metallic character of XIr3 compounds. It is also found from the DOS profile that the Ir 5d-states dominate near the Fermi level. Unlike the significant anisotropy observed in elastic and thermal properties, all the optical constants of XIr3 compounds exhibit almost isotropic behavior. The optical parameters’ profiles correspond very well with the electronic band structure and DOS features. We have estimated the superconducting transition temperature (Tc) of XIr3 compounds in this work. The calculated values of Tc for LaIr3 and ThIr3 compounds are 4.91 K and 5.01 K, respectively. We have compared and contrasted all the physical properties of XIr3 (X = La, Th) compounds in this study.

&lt;i&gt;In Situ&lt;/i&gt; Measurement of Twist Propagation and Yarn Tension with Superconducting Magnetic Bearing Twisting Element for Ring Spinning Process

The twisting of yarn in one of the most widely used conventional ring spinning processes is based on the ring/traveler system. The existing limitation of productivity in the ring spinning process lies on the prevailing force relation between ring, traveler and yarn as well as the frictional heat between traveler and ring, principally in the ring traveler system. Recently, a frictional free twisting element based on a superconducting magnetic bearing (SMB) was developed to increase the productivity of ring spinning drastically, which was patented by ITM and Leibniz IFW Dresden. This SMB consists of a circular superconductor and a permanent magnet (PM) ring. In the superconducting state, the PM ring levitates and can freely rotate even up to an angular speed of 50.000 rpm. It is connected with the spindle via the yarn through a guide attached to the PM ring to impart yarn twist. Through the rotation of the PM ring, the twist propagates to the nip point of the delivery rollers. The twist propagation rate depends on different parameters, e.g., spindle speed, balloon geometry, and friction between yarn and yarn guides. A change in the level of twist affects the process, yarn tension, yarn breakage rate, and yarn properties. Hence, it is important to investigate the twist distribution to derive effective measures for improving the twist propagation especially at higher angular spindle speeds and thus eliminate weak points to increase process stability and yarn quality. The aim of these measurements is to analyze the twist distribution along the yarn path to understand the causes of yarn breakages. In this study, the yarn path was traced at different regions by in-situ measuring the helix angle of twisted yarn using a high-speed camera. From the recorded images, the number of twists per unit length was determined using the image processing software ‘Image J’. Thus, the measurement method allows a new insight into the problem of yarn breakages originating from twist propagation in order to take optimized measures at higher angular spindle speeds.

Quantum-Geometry-Induced Anomalous Hall Effect in Nonunitary Superconductors and Application to Sr_{2}RuO_{4}.

The polar Kerr effect and the closely related anomalous charge Hall effect are among the most distinguishing signatures of the superconducting state in Sr_{2}RuO_{4}, as well as in several other compounds. These effects are often thought to be derived from chiral superconducting pairing, and different mechanisms have been invoked for the explanation. However, the intrinsic mechanisms proposed previously often involve unrealistically strong interband Cooper pairing. We show in this Letter that, even without interband pairing, nonunitary superconducting states can support the intrinsic anomalous charge Hall effect, thanks to the quantum geometric properties of the Bloch electrons. The key here is to have a normal-state spin Hall effect, for which a nonzero spin-orbit coupling is essential. A finite charge Hall effect then naturally arises at the onset of a spin-polarized nonunitary superconducting pairing. It depends on both the spin polarization and the normal-state electron Berry curvature, the latter of which is the imaginary part of the quantum geometric tensor of the Bloch states. Applying our results to the weakly paired Sr_{2}RuO_{4} we conclude that, if the reported Kerr effect is of intrinsic origin, the superconducting state is most likely nonunitary and has odd parity. Our theory may be generalized to other superconductors that exhibit the polar Kerr effect.

Recent Progress of JT-60SA Project toward Plasma Operation

Abstract Superconducting tokamak JT-60SA plays an essential role in fusion research and development by supporting and complementing ITER project, providing directions to the DEMO design activity and fostering next generation scientists and engineers. Since the incident of the Equilibrium Field coil #1 during the Integrated Commissioning (IC) in March 2021, both EU and JA Implementing agencies (IAs) have examined how to ensure safety operation of JT-60SA by mitigating the risk of possible discharge occurrence inside the cryostat. Based on the experience of the Global Paschen tests, the IAs have established a strategy of risk mitigation measures, which is a combination of (i) reinforcement of insulation, (ii) avoiding unnecessary voltage application to the coil systems and (iii) immediate de-energization of the coils when deteriorated vacuum condition is detected. Thanks to the considerable efforts of the Integrated Project Team (IPT) members, the IC restarted in May 2023. After the confirmation of superconducting state of coil systems (TF, EF and CS), the coil energization test and the plasma operation (OP-1) starts. The first plasma was successfully achieved on 23 October 2023 with a limited value of applied voltage and current to the coils. The plasma configuration control will be also confirmed with low plasma current and low auxiliary heating power conditions. Based on the IO-F4E-QST collaboration, activities of JT-60SA have been shared with the IO and provided an important lesson learned for ITER assembly and commissioning, and will provide an outstanding contribution to fusion research at large. After OP-1, Maintenance &amp; Enhancement phase 1 (M/E-1) starts from January 2024, in which in-vessel components are installed, and heating system and diagnostic system are extensively upgraded to allow high power heating experiment planned in OP-2. In order to make the best use of JT-60SA, newly organized JT-60SA experiment team will refine the research plan in the future high heating power operation phase.

Nonreciprocal transport in the superconducting state of the chiral crystal NbGe2

Due to the lack of inversion, mirror or other roto-inversion symmetries, chiral crystals possess a well-defined handedness which, when combined with time-reversal symmetry breaking from the application of magnetic fields, can give rise to directional dichroism of the electrical transport phenomena via the magnetochiral anisotropy. In this study, we investigate the nonreciprocal magneto-transport in microdevices of NbGe2, a superconductor with structural chirality. A giant nonreciprocal signal from vortex motions is observed during the superconducting transition, with the ratio of nonreciprocal resistance to the normal resistance γ reaching 6×105 T−1⋅A−1. Interestingly, the intensity can be adjusted and even sign-reversed by varying the current, the temperature, and the crystalline orientation. Our findings illustrate intricate vortex dynamics and offer ways of manipulation on the rectification effect in superconductors with structural chirality.

Improving the recovery performance of superconducting fault current limiters using nitrogen/tetrafluoromethane mixed coolants

The superconducting fault current limiter (SFCL) improves the power grid safety by restricting the drastically increased current when the circuit fault occurs. However, the SFCL suffers from a long recovery time and can hardly recover to the superconducting state before the breaker attempts to reclose, which hinders its practical application. This paper presents the implementation of the rapid-recovery SFCL with efficient heat dissipation even if encountering a large fault current. This is enabled by thermal performance regulation of the working coolant through adding tetrafluoromethane (CF4) into the liquid nitrogen (LN2). The proposed mixed coolant features a high critical heat flux that suppresses the onset of film boiling. Controlled experiments were performed with the fabricated current-limiting coil immersed in the N2/CF4 mixed coolants containing 0, 20 and 40 mol% of CF4. Measured results regarding both quenching and recovery processes are reported, demonstrating an 85.4% reduction in recovery time for the 40 mol% composition compared to the conventionally used pure LN2. Analyses of the electrical parameters of the coil verify its effective quenching performance in the proposed immersion cooling system with an overall current limiting rate of over 55%. These findings lay a foundation for designing the rapid-recovery-type SFCL and facilitating its application in power systems.

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.

Emergence of Larkin-Ovchinnikov-type superconducting state in a voltage-driven superconductor

Emergence of Larkin-Ovchinnikov-type superconducting state in a voltage-driven superconductor

Tuning the Flat Band in Bi2O2Se by Pressure to Induce Superconductivity.

The discovery of superconductivity in twisted bilayer graphene has reignited enthusiasm in the field of flat-band superconductivity. However, important challenges remain, such as constructing a flat-band structure and inducing a superconducting state in materials. Here, we successfully achieved superconductivity in Bi2O2Se by pressure-tuning the flat-band electronic structure. Experimental measurements combined with theoretical calculations reveal that the occurrence of pressure-induced superconductivity at 30 GPa is associated with a flat-band electronic structure near the Fermi level. Moreover, in Bi2O2Se, a van Hove singularity is observed at the Fermi level alongside pronounced Fermi surface nesting. These remarkable features play a crucial role in promoting strong electron-phonon interactions, thus potentially enhancing the superconducting properties of the material. These findings demonstrate that pressure offers a potential experimental strategy for precisely tuning the flat band and achieving superconductivity.

Unconventional superconductivity in Cr-based compound Pr3Cr10−xN11

We report results of specific heat and muon spin relaxation (μSR) measurements on a polycrystalline sample of Pr3Cr10−xN11, which shows superconducting state below Tc = 5.25 K, a large upper critical field Hc2 ~ 20 T and a residual Sommerfeld coefficient γ0. The field dependence of γ0(H) resembles γ of the U-based superconductors UTe2 and URhGe at low temperatures. The temperature-dependent superfluid density measured by transverse-field μSR experiments is consistent with a p-wave pairing symmetry. ZF-μSR experiment suggests a time-reversal symmetry broken superconducting transition, and temperature-independent spin fluctuations at low temperatures are revealed by LF-μSR experiments. These results indicate that Pr3Cr10−xN11 is a candidate of p-wave superconductor which breaks time-reversal symmetry.

Interfacial magnon-mediated superconductivity in twisted bilayer graphene

The interfacial coupling between electrons and magnons in adjacent layers can mediate an attractive electron–electron interaction and induce superconductivity. We consider magic-angle twisted bilayer graphene sandwiched between two ferromagnetic insulators to optimize this effect. As a result, magnons induce an interlayer superconducting state characterized by p-wave symmetry. We investigate two candidate ferromagnets. The van der Waals ferromagnet CrI3 stands out because it allows compression to tune the superconducting state with an exponential sensitivity. This control adds a new dimension to the tunability of twisted bilayer graphene. Our results open a new path for exploring magnon-induced superconductivity.

Study on Extreme Quantum Matter and Functionality

Novel quantum-functional compounds that surpass the limits of traditional materials have been proposed. A prime example is the chiral superconductor that is applicable to quantum computing applications, realizing the ‘chiral superconducting state’ where Majorana quasi-particles are implemented. Another example is Kondo-Weyl semimetal, where correlated Kondo physics is intertwined with topological electronic band structure. The emergence of novel quantum-functional materials, which breaks the boundaries of traditional research fields and fosters convergence among diverse disciplines, stands as a key issue in condensed matter research. In this article, we introduce recent progress and future research direction in the study on extreme quantum matter and functionality.

Dissipationless Counterflow Currents above T_{c} in Bilayer Superconductors.

We report the existence of dissipationless currents in bilayer superconductors above the critical temperature T_{c}, assuming that the superconducting phase transition is dominated by phase fluctuations. Using a semiclassical U(1) lattice gauge theory, we show that thermal fluctuations cause a transition from the superconducting state at low temperature to a resistive state above T_{c}, accompanied by the proliferation of unbound vortices. Remarkably, while the proliferation of vortex excitations causes dissipation of homogeneous in-plane currents, we find that counterflow currents, flowing in the opposite direction within a bilayer, remain dissipationless. The presence of a dissipationless current channel above T_{c} is attributed to the inhibition of vortex motion by local superconducting coherence within a single bilayer, in the presence of counterflow currents. Our theory presents a possible scenario for the pseudogap phase in bilayer cuprates.