type-II Superconductors Research Articles

Boron based new high entropy alloy superconductor Mo0.11W0.11V0.11Re0.34B0.33

Superconducting high entropy alloys (HEAs) are new members of disordered superconductors. We report the synthesis and investigation of a new superconducting high entropy alloy Mo0.11W0.11V0.11Re0.34B0.33 (MWVRB). It crystallized in the tetragonal CuAl2 crystal structure with space group (I4/mcm). Comprehensive transport, magnetization and heat capacity measurements confirmed bulk type-II superconductivity having transition temperature T C = 4.0 K. The low temperature electronic specific heat suggests a fully gapped superconducting state in the weak coupling limit.

Superconducting properties of the spin Hall candidate Ta3Sb with eightfold degeneracy

We report the synthesis and characterization of phase pure Ta3Sb, a material predicted to be topological with eightfold degenerate fermionic states [Science 353, aaf5037 (2016)] and to exhibit a large spin Hall effect [Sci. Adv. 5, eaav8575 (2019]. We observe superconductivity in Ta3Sb with Tc~ 0.67 K in both electrical resistivity \r{ho}(T) and specific heat C(T) measurements. Field dependent measurements yield the superconducting phase diagram with an upper critical field of Hc2(0) ~ 0.95 T, corresponding to a superconducting coherence length of {\xi} ~18.6 nm. The gap ratio deduced from specific heat anomaly, 2{\Delta}0/kBTc is 3.46, a value close to the Bardeen-Cooper-Schrieffer (BCS) value of 3.53. From a detailed analysis of both the transport and thermodynamic data within the Ginsburg-Landau (GL) framework, a GL parameter of \k{appa} ~90 is obtained identifying Ta3Sb as an extreme type-II superconductor. The observation of superconductivity in an eightfold degenerate fermionic compound with topological surface states and predicted large spin Hall conductance positions Ta3Sb as an appealing platform to further explore exotic quantum states in multifold degenerate systems.

Experimental and first-principles studies of superconductivity in topological nodal line semimetal SnTaS2

We report a detailed study of superconductivity in polycrystalline SnTaS2 using electrical transport, magnetization and heat capacity measurements. SnTaS2 crystallizes in centrosymmetric hexagonal structure with space group . Electrical resistivity, magnetization and specific heat data suggest SnTaS2 to be a weakly coupled, type-II superconductor with K. First-principles calculations show signature for nodal line topology in the electronic band structure, protected by the spatial-inversion and time-reversal symmetries, that strongly gapped out by the inclusion of spin–orbit coupling. Superconductivity in layered SnTaS2 with nodal line topological state makes it a strong candidate to be considered for a 3D topological superconductor.

Hessian characterization of the pinning landscape in a type-II superconductor

Supercurrent transport requires vortices in superconductors to be pinned by material defects. Vortices are trapped in minima of the pinning potential landscape, and one has to determine what fraction of this landscape comes with a positive curvature. Such a curvature analysis is done by studying the Hessian matrix of the landscape. The authors find the stable area fraction to be unexpectedly low (yellow color in the figure). In particular, for Gaussian random landscapes it is $(3\ensuremath{-}\sqrt{3})/6\ensuremath{\approx}21%$. The results offer new hints for optimizing pinning landscapes. Their transdisciplinary application to natural landscapes exhibits a surprising universality.

Superconducting Phases in Neutron Star Cores

Using a phenomenological Ginzburg–Landau model that includes entrainment, we identify the possible ground states for the neutron and proton condensates in the core of a neutron star, as a function of magnetic field strength. Combining analytical and numerical techniques, we find that much of the outer core is likely to be a “type-1.5” superconductor (instead of a type-II superconductor as often assumed), in which magnetic flux is distributed inhomogeneously, with bundles of magnetic fluxtubes separated by flux-free Meissner regions. We provide an approximate criterion to determine the transition between this type-1.5 phase and the type-I region in the inner core. We also show that bundles of fluxtubes can coexist with non-superconducting regions, but only in a small part of the parameter space.

Anisotropic superconductivity in ZrB12 near the critical Bogomolnyi point

The superconductors with the Ginzburg-Landau (G-L) parameter ($\kappa$) $\sim1/\sqrt{2}$ exist near a critical Bogolomonyi (B) point where they show inter-type domains between type-I and type-II superconductivity. While such physics is well understood for isotropic superconductors, the experimental investigation of the physics of anisotropic superconductors near a critical B-point remains an unattained goal mainly due to the unavailability of model material systems. Theoretically, such superconductors are expected to show type-I or type-II behaviour for definite directions of an applied magnetic field. Here, from directional point-contact Andreev reflection spectroscopy and field-angle dependent ac magnetic susceptibility measurements, we show that ZrB$_{12}$ is an anisotropic superconductor, and it exhibits field-direction dependent type-I and type-II behaviour. These observations match remarkably well with the theoretical expectations for an anisotropic superconductor near a critical B-point. Therefore, our results project ZrB$_{12}$ as a model material system where the physics of inter-type anisotropic superconductivity can be explored experimentally.

Active rheology and anticommensuration effects for driven probe particles on two-dimensional periodic pinning substrates

For an assembly of particles interacting with a two-dimensional periodic substrate, a series of commensuration effects can arise when the number of particles is an integer multiple of the number of substrate minima. Such commensuration effects can appear for vortices in type-II superconductors with periodic pinning or for colloidal particles on optical landscapes. Under bulk external driving, the pinning or drag on the particles is strongly enhanced at commensuration. Here we consider the active rheology of a single particle driven through an assembly of particles coupled to a periodic substrate at different commensurate conditions. For increasing density at fixed driving force, we observe nonmonotonic drag along with what we call an anticommensuration effect where the drag or pinning effectiveness is reduced in commensurate states, opposite from the behavior typically observed under bulk driving. The velocity enhancement or drag reduction appears when the background particles form a crystalline state that is coupled more strongly to the substrate than to the driven particle, while under incommensurate conditions, the background particles are disordered and produce enhanced drag on the probe particle. The velocity noise of the driven particle has a narrow band signature at commensuration and a broad band signature away from commensuration. We map out the regions in which viscous flow, periodic flow, and a pinned phase appear. We show that the effects we observe are robust on both square and triangular substrate arrays and for both vortices in type-II superconductors and colloidal particles on optical landscapes.

Superconductivity in Heusler compound ScAu2Al

We report superconducting state properties and electronic structure of a full Heusler material ScAu2Al. The resistivity measurement indicates a zero-field (at nominal Earth’s field) superconducting transition temperature, T c = 5.12 K (in contrary to the previously reported value of 4.4 K), which falls in the highest T c -regime among the Heusler superconductors. The magnetization data shows that ScAu2Al is a moderate type-II superconductor, where the critical field values can be estimated from the Ginzburg–Landau–Abrikosov–Gorkov theory. The field-dependent magnetization response further shows signatures of flux jump in ScAu2Al. A sharp jump in the temperature dependent specific heat (C p ) data confirms bulk superconductivity. We report that the electron–phonon coupling constant, λ e–ph = 0.77, suggesting a moderate electron–phonon coupling in ScAu2Al. Further, we show that the observed λ e–ph value in ScAu2Al is the highest amongst the reported Heusler superconductors, indicating strong correlation between T c and λ e–ph values and significant role of electron–phonon coupling in mediating superconductivity in Heusler superconductors. Finally, we discuss the electronic properties and reveal the existence of van Hove singularity near the Fermi level in ScAu2Al.

Unusual superconductivity in the topological nodal-line semimetal candidate Sn x NbSe2-δ

We report the superconductivity of the topological nodal-line semimetal candidate Sn x NbSe2-δ with a noncentrosymmetric crystal structure. The superconducting transition temperature T c of Sn x NbSe2-δ drastically varies with the Sn concentration x and the Se deficiency δ, and reaches 12 K, relatively higher than those of known topological superconductors. The upper critical field of this compound shows unusual temperature dependence, inconsistent with the WHH theory for conventional type-II superconductors. In a low-Tc sample, the zero-temperature limit of the upper critical field parallel to the ab plane exceeds the Pauli paramagnetic limit estimated from the simple BCS weak coupling model by a factor of ∼ 2, suggestive of unusual superconductivity stabilized in Sn x NbSe2-δ . Together with the robust superconductivity against disorder, these observations indicate that Sn x NbSe2-δ is a promising candidate to explore topological superconductivity.

Investigations on superconductivity in an equi-atomic disordered Hf-Nb-Ta-Ti-V high entropy alloy

High entropy alloys (HEA) are expected to show unusual superconducting behavior due to chemical disorder along with highly distorted lattice. Herein, we report the synthesis, structure, and detailed superconducting properties of a new and nearly equi-atomic single phase disordered superconducting high entropy alloy with nominal composition Hf20Nb20Ta20Ti20V20. Rietveld refinement of the Synchrotron X-ray diffraction (SXRD) confirms the DFT predicted single-phase BCC structure in the HEA with lattice parameter a = 3.32 Å. Reverse monte carlo (RMC) modelling of the diffraction data evince the local atomic disorder and a considerable lattice distortion overriding the crystal structure due to significant variations in bond length and co-ordination numbers. Microstructural characterization using energy-dispersive X-ray spectroscopy (XEDS) equipped SEM shows dendritic morphology of the as solidified alloy with incomplete partition of the constituent elements. Transport, magnetization and low temperature specific heat measurement established that the current HEA is a moderately coupled type-II superconductor with a Tc of 5.0 K, lower critical field Hc1(0)=19mT and upper critical field Hc2(0) = 6.63T.

Creep effects on the Campbell response in type-II superconductors

Applying the strong pinning formalism to the mixed state of a type II superconductor, we study the effect of thermal fluctuations (or creep) on the penetration of an ac magnetic field as quantified by the so-called Campbell length $\lambda_\textrm{C}$. Within strong pinning theory, vortices get pinned by individual defects, with the jumps in the pinning energy ($\Delta e_\mathrm{pin}$) and force ($\Delta f_\mathrm{pin}$) between bistable pinned and free states quantifying the pinning process. We find that the evolution of the Campbell length $\lambda_{\rm C}(t)$ as a function of time $t$ is the result of two competing effects, the change in the force jumps $\Delta f_\mathrm{pin}(t)$ and a change in the trapping area $S_\mathrm{trap}(t)$ of vortices; the latter describes the area around the defect where a nearby vortex gets and remains trapped. Contrary to naive expectation, we find that during the decay of the critical state in a zero-field cooled (ZFC) experiment, the Campbell length $\lambda_{\rm C}(t)$ is usually nonmonotonic, first decreasing with time $t$ and then increasing for long waiting times. Field cooled (FC) experiments exhibit hysteretic effects in $\lambda_\textrm{C}$; relaxation then turns out to be predominantly monotonic, but its magnitude and direction depends on the specific phase of the cooling--heating cycle. Furthermore, when approaching equilibrium, the Campbell length relaxes to a finite value, different from the persistent current which vanishes at long waiting times $t$, e.g., above the irreversibility line. Finally, measuring the Campbell length $\lambda_\textrm{C}(t)$ for different states, zero-field cooled, field cooled, and relaxed, as a function of different waiting times $t$ and temperatures $T,$ allows to spectroscopyse the pinning potential of the defects.

Charging in the vortex lattice of type-II superconductors

We study the magnetic-field dependence of the vortex-core charge in the Abrikosov lattice of an $s$-wave superconductor based on the augmented quasiclassical equations, where we incorporate the pair-potential gradient (PPG) terms characteristic of charging in superconductors besides the well-known Lorentz force. Our numerical results at $T=0.2{T}_{\mathrm{c}}$ and $0.5{T}_{\mathrm{c}}$ reveal that periodic charge redistribution is superimposed on the magnetic flux-line lattice with different spatial patterns at different fields. The PPG terms are dominant at weak fields over the Lorentz force for accumulating charge in the vortex cores, whereas the Lorentz force prevails at higher fields to give rise to a peak structure in the core charge around $H\ensuremath{\sim}\frac{1}{2}{H}_{\mathrm{c}2}$. We estimate the peak value of the core charge at $T=0.2{T}_{\mathrm{c}}$ using parameters appropriate for cuprates to obtain a large value of $Q\ensuremath{\sim}{10}^{\ensuremath{-}2}|e|$ in the core region of radius $0.2{\ensuremath{\xi}}_{0}$ in the $ab$ plane and length $1\phantom{\rule{0.16em}{0ex}}\mathrm{nm}$ along the $c$ axis.

Disorder-induced transition from type-I to type-II superconductivity in the Dirac semimetal PdTe2

We report a doping study directed to intentionally induce disorder in ${\mathrm{PdTe}}_{2}$ by the isoelectronic substitution of Pt. Two single-crystalline batches ${\mathrm{Pd}}_{1\ensuremath{-}x}{\mathrm{Pt}}_{x}{\mathrm{Te}}_{2}$ have been prepared with nominal doping concentrations $x=0.05$ and $x=0.10$. Sample characterization by energy dispersive x-ray spectroscopy revealed Pt did not dissolve homogeneously in the crystals. For the nominal value $x=0.10$ small single crystals cut from the batch appeared to have $x=0.09$, as well as the nonstoichiometric composition ${\mathrm{Pd}}_{0.97}{\mathrm{Pt}}_{<0.004}{\mathrm{Te}}_{2.03}$. Magnetic and heat capacity measurements demonstrate a transition from type-I to type-II superconducting behavior upon increasing disorder. From transport measurements we calculate a residual resistivity ${\ensuremath{\rho}}_{0}=1.4\phantom{\rule{4pt}{0ex}}\ensuremath{\mu}\mathrm{\ensuremath{\Omega}}\phantom{\rule{0.16em}{0ex}}\mathrm{cm}$ suffices to turn ${\mathrm{PdTe}}_{2}$ into a superconductor of the second kind.

Quantum muon diffusion and the preservation of time-reversal symmetry in the superconducting state of type-I rhenium

Elemental rhenium exhibiting type-II superconductivity has been previously reported to break time-reversal symmetry in the superconducting state. We have investigated an arc-melted sample of rhenium exhibiting type-I superconductivity. Low-temperature zero-field muon-spin relaxation measurements indicate that time-reversal symmetry is preserved in the superconducting state. Muon diffusion is observed, which is due to quantum mechanical tunneling between interstitial sites. The normal state behavior is characterized by the conduction electrons screening the muons and thermal broadening, and is typical for a metal. Energy asymmetries between muon trapping sites and the superconducting energy gap also characterize the superconducting state behavior.

Generalized phenomenological model for the magnetic field penetration and magnetization hysteresis loops of a type-II superconductor

A generalized phenomenological model for the critical state of type-II superconductors with magnetic field parallel to the superconducting plate is proposed. This model considers the global magnetization including both the equilibrium magnetization from surface screening current and the non-equilibrium magnetization from bulk pinning in a self-consistent way. Our model can be used to simulate the magnetization-hysteresis-loops (MHLs) and flux penetrating process of different type-II superconductors, from low- to high-kappa values. Here we take an optimally doped Ba0.6K0.4Fe2As2 single crystal as a testing example. The model can fit the data quite well and several important parameters can be extracted from the fitting. Thus, the model can be extended to a general case for studying the magnetization and flux penetration in other type-II superconductors.

Motion of Magnetic Vortices in Type-Ii Superconductor with Randomly Distributed Pinning Centers

Motion of Magnetic Vortices in Type-Ii Superconductor with Randomly Distributed Pinning Centers

Role of surface functional groups to superconductivity in Nb2C-MXene: Experiments and density functional theory calculations

The recently discovered surface-group-dependent superconductivity in Nb2C-MXene fabricated by the molten salts method is attracting wide attention. However, regarding the superconductivity of Nb2C-MXene with functional F groups (Nb2CFx), there were some conflicting results in experimental and theoretical studies. Herein, we systematically carried out experimental and theoretical investigations on the superconductivity in Nb2C-MXene with the Cl functional group (Nb2CClx) and Nb2CFx. The experimental results of the Meissner effect and zero resistivity have proved that Nb2CClx is superconducting with the transition temperature (Tc) ∼ 5.2 K. We extract its superconducting parameters from the temperature dependence of resistivity and the field dependence of the magnetization. The Ginzburg-Landau parameter κGL is estimated to be 2.41, indicating that Nb2CClx is a typical type-Ⅱ superconductor. Conversely, both magnetic and electrical transport measurements demonstrate that Nb2CFx is not superconducting. The first-principles density functional theory (DFT) calculations show that the Tc of Nb2CClx is ∼ 5.2 K, while Nb2CFx is dynamically unstable with imaginary frequency in phonon spectrum, which is in good agreement with the experimental results. Our studies not only are useful for clarifying the present inconsistency but also offer referential significance for future investigations on the superconductivity of MXenes.

Superconductivity in doped Weyl semimetal Mo0.9Ir0.1Te2 with broken inversion symmetry

This work presents the emergence of superconductivity in Ir—doped Weyl semimetal T d —MoTe2 with broken inversion symmetry. Chiral anomaly induced planar Hall effect and anisotropic magneto-resistance confirm the topological semimetallic nature of MoIr x Te2. Observation of weak anisotropic, moderately coupled type-II superconductivity in T d -MoIr x Te2 makes it a promising candidate for topological superconductor.

Dynamical Mechanisms of Vortex Pinning in Superfluid Thin Films.

We characterize the mechanisms of vortex pinning in a superfluid thin film described by the two-dimensional Gross-Pitaevskii equation. We consider a vortex "scattering experiment" whereby a single vortex in a superfluid flow interacts with a circular, uniform pinning potential. By an analogy with linear dielectrics, we develop an analytical hydrodynamic approximation that predicts vortex trajectories, the vortex fixed point and the unpinning velocity. We then solve the Gross-Pitaevskii equation to validate this model, and build a phase portrait of vortex pinning. We identify two different dynamical pinning mechanisms marked by distinctive phonon emission signatures: one enabled by acoustic radiation and another mediated by vortex dipoles nucleated within the pin. Relative to obstacle size, we find that pinning potentials on the order of the healing length are more effective for vortex capture. Our results could be useful in mitigating the negative effects of drag due to vortices in superfluid channels, in analogy to maximizing supercurrents in type-II superconductors.

Casimir Effect between Superconducting Plates in the Mixed State

The Casimir effect between type-II superconducting plates in the coexisting phase of a superconducting phase and a normal phase is investigated. The dependence of the optical conductivity of the superconducting plates on the external magnetic field is described in terms of the penetration depth of the incident electromagnetic field, and the permittivity along the imaginary axis is represented by a linear combination of the permittivities for the plasma model and Drude models. The characteristic frequency in each model is determined using the force parameters for the motion of the magnetic field vortices. The Casimir force between parallel YBCO plates in the mixed state is calculated, and the dependence on the applied magnetic field and temperature is considered.