Critical Supercurrents Research Articles

Field-Free Superconducting Diode Effect and Magnetochiral Anisotropy in FeTe0.7Se0.3 Junctions with the Inherent Asymmetric Barrier.

Nonreciprocal electrical transport, characterized by an asymmetric relationship between the current and voltage, plays a crucial role in modern electronic industries. Recent studies have extended this phenomenon to superconductors, introducing the concept of the superconducting diode effect (SDE). The SDE is characterized by unequal critical supercurrents along opposite directions. Due to the requirement on broken inversion symmetry, the SDE is commonly accompanied by electrical magnetochiral anisotropy (eMCA) in the resistive state. Achieving a magnetic-field-free SDE with field tunability is pivotal for advancements in superconductor devices. Conventionally, field-free SDE has been achieved in Josephson junctions by intentionally intercalating an asymmetric barrier layer. Alternatively, internal magnetism was employed. Both approaches pose challenges in the selection of superconductors and fabrication processes, thereby impeding the development of SDE. Here, we present a field-free SDE in FeTe0.7Se0.3 (FTS) junction with eMCA, a phenomenon absent in FTS single nanosheets. The field-free property is associated with the presence of a gradient oxide layer on the upper surface of each FTS nanosheet, while eMCA is linked to spin splitting arising from the absence of inversion symmetry. Both SDE and eMCA respond to magnetic fields with distinct temperature dependencies. This work presents a versatile and straightforward strategy for advancing superconducting electronics.

Asymmetric edge supercurrents in MoTe2 Josephson junctions.

To investigate the higher order topology in MoTe2, the supercurrent interference phenomena in Nb/MoTe2/Nb planar Josephson junctions have been systematically studied. By analyzing the obtained interference pattern of the critical supercurrents and performing a comparative study of the edge-touched and untouched junctions, it's found that the supercurrent is dominated by the edges, rather than the bulk or surfaces of MoTe2. An asymmetric Josephson effect with a field-tunable sign is also observed, indicating the nontrivial origin of the edge states. These results not only provide initial evidence for the hinge states in the higher order topological insulator MoTe2, but also demonstrate the potential applications of MoTe2-based Josephson junctions in rectifying the supercurrent.

Edelstein Effect Induced Superconducting Diode Effect in Inversion Symmetry Breaking MoTe2 Josephson Junctions

AbstractSuperconducting diode effect (SDE) with nonreciprocal supercurrent transport has attracted intense attention recently, not only for its intriguing physics, but also for its great application potential in superconducting circuits. It is revealed in this work that planar Josephson junctions (JJs) based on type‐II Weyl semimetal (WSM) MoTe2 can exhibit a prominent SDE due to the emergence of asymmetric Josephson effect (AJE) in perpendicular magnetic fields. The AJE manifests itself in a very large asymmetry in the critical supercurrents with respect to the current direction. The sign of this asymmetry can also be effectively modulated by the external magnetic field. Considering the special noncentrosymmetric crystal symmetry of MoTe2, this AJE is understood in terms of the Edelstein effect, which induces a nontrivial phase shift in the current phase relation of the junctions. Besides these, it is further demonstrated that the rectification of supercurrent in such MoTe2 JJs with the rectification efficiency up to 50.4%, unveiling the great application potential of WSMs in superconducting electronics.

Ubiquitous Superconducting Diode Effect in Superconductor Thin Films.

The macroscopic coherence in superconductors supports dissipationless supercurrents that could play a central role in emerging quantum technologies. Accomplishing unequal supercurrents in the forward and backward directions would enable unprecedented functionalities. This nonreciprocity of critical supercurrents is called the superconducting (SC) diode effect. We demonstrate the strong SC diode effect in conventional SC thin films, such as niobium and vanadium, employing external magnetic fields as small as 1Oe. Interfacing the SC layer with a ferromagnetic semiconductor EuS, we further accomplish the nonvolatile SC diode effect reaching a giant efficiency of 65%. By careful control experiments and theoretical modeling, we demonstrate that the critical supercurrent nonreciprocity in SC thin films could be easily accomplished with asymmetrical vortex edge and surface barriers and the universal Meissner screening current governing the critical currents. Our engineering of the SC diode effect in simple systems opens the door for novel technologies while revealing the ubiquity of the Meissner screening effect induced SC diode effect in superconducting films, and it should be eliminated with great care in the search for exotic superconducting states harboring finite-momentum Cooper pairing.

The supercurrent diode effect and nonreciprocal paraconductivity due to the chiral structure of nanotubes

The phenomenon that critical supercurrents along opposite directions become unequal is called the supercurrent diode effect (SDE). It has been observed in various systems and can often be understood by combining spin-orbit coupling and Zeeman field, which break the spatial-inversion and time-reversal symmetries, respectively. Here, we theoretically investigate another mechanism of breaking these symmetries and predict the existence of the SDE in chiral nanotubes without spin-orbit coupling. The symmetries are broken by the chiral structure and a magnetic flux through the tube. With a generalized Ginzburg-Landau theory, we obtain the main features of the SDE in its dependence on system parameters. We further show that the same Ginzburg-Landau free energy leads to another important manifestation of the nonreciprocity in superconducting systems, i.e., the nonreciprocal paraconductivity (NPC) slightly above the transition temperature. Our study suggests a new class of realistic platforms to investigate nonreciprocal properties of superconducting materials. It also provides a theoretical link between the SDE and the NPC, which were often studied separately.

Spin-Wave Doppler Shift by Magnon Drag in Magnetic Insulators.

The Doppler shift of the quasiparticle dispersion by charge currents is responsible for the critical supercurrents in superconductors and instabilities of the magnetic ground state of metallic ferromagnets. Here we predict an analogous effect in thin films of magnetic insulators in which microwaves emitted by a proximity stripline generate coherent chiral spin currents that cause a Doppler shift in the magnon dispersion. The spin-wave instability is suppressed by magnon-magnon interactions that limit spin currents to values close to but below the threshold for the instability. The spin current limitations by the backaction of magnon currents on the magnetic order should be considered as design parameters in magnonic devices.

Critical supercurrent and φ0 state for probing a persistent spin helix

We theoretically study the profile of a supercurrent in two-dimensional Josephson junctions with Rashba-Dresselhaus spin-orbit interaction (RDSOI) in the presence of a Zeeman field. Through investigating self-biased supercurrent (so called $\varphi_0$-Josephson state), we obtain explicit expressions for the functionality of the $\varphi_0$ state with respect to RDSOI parameters ($\alpha,\beta$) and in-plane Zeeman field components ($h_x,h_y$). Our findings reveal that, when the chemical potential ($\mu$) is high enough compared to the energy gap ($\Delta$) in superconducting electrodes, i.e., $\mu \gg \Delta$, RSOI and DSOI with equal strengths ($|\alpha|=|\beta|$) cause vanishing $\varphi_0$ state independent of magnetization and the type of RDSOI. A Zeeman field with unequal components, i.e., $|h_x|\neq |h_y|$, however, can counteract and nullify the destructive impact of equal-strength RDSOIs (for one type only), where $\mu\sim\Delta$, although $|h_x|= |h_y|$ can still eliminate the $\varphi_0$ state. Remarkably, in the $\mu\sim\Delta$ limit, the $\varphi_0$ state is proportional to the multiplication of both components of an in-plane Zeeman field, i.e., $h_xh_y$, which is absent in the $\mu \gg \Delta$ limit. Furthermore, our results of critical supercurrents demonstrate that the persistent spin helices can be revealed in a high enough chemical potential regime $\mu\gg \Delta$, while an opposite regime, i.e., $\mu\sim\Delta$, introduces an adverse effect. In the ballistic regime, the "maximum" of the critical supercurrent occurs at $|\alpha|=|\beta|$ and the Zeeman field can boost this feature. The presence of disorder and nonmagnetic impurities change this picture drastically so the "minimum" of the critical supercurrent occurs at and around the symmetry lines $|\alpha|=|\beta|$.

Spin and mass superfluidity in a ferromagnetic spin-1 Bose-Einstein condensate

The paper investigates the coexistence and interplay of spin and mass superfluidity in a ferromagnetic spin-1 Bose-Einstein condensate. Superfluidity is possible only in the presence of uniaxial anisotropy (linear and quadratic Zeeman effect). This follows from the topology of the order-parameter space (vacuum manifold). According to the Landau criterion, the critical phase gradients, both for mass and spin supercurrents, vanish at the phase transition from the easy-plane to the easy-axis anisotropy. However, mass superfluidity is still possible at the phase transition. This is because the Landau criterion signals instability only with respect to nonsingular vortices with special ratio between circulations of mass and spin currents. Phase slips produced by these vortices are not sufficient for complete decay of supercurrents. Full decay of supercurrents requires phase slips with vortices of another topological class and larger energy. These phase slips are suppressed by energetic barriers up to the upper critical velocity (gradient) exceeding the Landau critical velocity. The upper critical velocity does not vanish nor has any anomaly in the critical point at the phase transition from the easy-plane to the easy-axis anisotropy.

Josephson Supercurrent through the Topological Surface States of Strained Bulk HgTe

Strained bulk HgTe is a three-dimensional topological insulator, whose surface electrons have a high mobility (30,000 cm^2/Vs), while its bulk is effectively free of mobile charge carriers. These properties enable a study of transport through its unconventional surface states without being hindered by a parallel bulk conductance. Here, we show transport experiments on HgTe-based Josephson junctions to investigate the appearance of the predicted Majorana states at the interface between a topological insulator and a superconductor. Interestingly, we observe a dissipationless supercurrent flow through the topological surface states of HgTe. The current-voltage characteristics are hysteretic at temperatures below 1 K with critical supercurrents of several microamperes. Moreover, we observe a magnetic field induced Fraunhofer pattern of the critical supercurrent, indicating a dominant 2\pi-periodic Josephson effect in the unconventional surface states. Our results show that strained bulk HgTe is a promising material system to get a better understanding of the Josephson effect in topological surface states, and to search for the manifestation of zero-energy Majorana states in transport experiments.

Critical Supercurrents and Self-Organization in Quantum Hall Bilayers

We present a theory of interlayer tunneling in a disordered quantum Hall bilayer at total filling factor one, allowing for the effect of static vortices. In agreement with recent experiments [Phys. Rev. B 80, 165120 (2009); Phys. Rev. B 78, 075302 (2008)], we find that the critical current is proportional to the sample area and is comparable in magnitude to observed values. This reflects the formation of a Bean critical state as a result of current injection at the boundary. We predict a crossover to a critical current proportional to the square-root of the area in smaller samples. We also predict a peak in the critical current as the electron density varies at fixed layer separation.

Biaxially Textured IBAD-MgO Templates for YBCO-Coated Conductors

AbstractThis article reports on the development of second-generation Y1Ba2Cu3O7–(YBCO)-coated conductors deposited on biaxially textured MgO templates fabricated using ion-beam-assisted deposition (IBAD).The materials system architecture and processing techniques used to achieve high critical supercurrents on flexible superalloy substrates is described.The texturing of YBCO films on metal substrates approaches that of films deposited on single-crystal oxide substrates.Critical currents and critical current densities of YBCO films on metal substrates are also equivalent to YBCO films deposited on single-crystal oxides.

Proximity effect in superconductor–carbon nanotube–superconductor tunnel junctions

We have studied the proximity effect in a short superconductor-carbon nanotube-superconductor (S/CN/Sl junction made of a metallic carbon nanotube embedded between two conventional superconducting electrodes. It is found that some junctions possess constant supercurrents, while others have oscillating supercurrents with increasing nanotube length. Moreover, unlike superconductor-normal metal-superconductor (S/N/S) junctions, the S/CN/S junctions have fixed open channels and therefore. fixed critical supercurrents with increasing nanotube diameter. Especially, the chiral symmetry effects of the carbon nanotubes are well reflected in the dependence of the critical supercurrent on the Fermi level, from which an experimental method to explicitly measure the chiral angle of a nanotube is suggested.

Consistency of superconducting correlations with one-dimensional electron interactions in carbon nanotubes.

We show that a model of interacting electrons in one dimension is able to explain the order of magnitude as well as the temperature dependence of the critical supercurrents recently measured in nanotube samples placed between superconducting contacts. We use bosonization methods to deal with the long-range Coulomb interaction, ending up with a picture in which the critical current does not follow the temperature dependence of the gap in the contacts. Our results also reveal the presence of a short-range attractive interaction in the nanotubes, which accounts for a significant enhancement of the critical supercurrents.

Reentrant superconducting behavior of the Josephson SFS junction. Evidence for the π-phase state

Critical supercurrents, I c in Nb Cu 1− x Ni x Nb Josephson SFS junctions with F-layers prepared from ferromagnetic Cu 1− x Ni x alloys have been studied. For value x=0.52 and particular F-layer thickness we have observed I c ( T) oscillations with I c vanishing for some values of T. We associate this reentrant superconducting behavior with a crossover of the SFS junction from ‘0’- to ‘π’-state that is related to temperature dependence of spatial oscillation period of induced superconducting order parameter in the weak ferromagnet. We argue this is the first experimental evidence of the π-behavior of a Josephson junction, that is the special feature of superconducting pair flow through a ferromagnet predicted for SFS junctions by Bulaevskii, Buzdin et al [1].

Supercurrents through the superconductor–ferromagnet–superconductor (SFS) junctions

We have observed and studied critical supercurrents, I c, in cross-type Nb–Cu/Ni–Nb junctions with Josephson layers prepared from diluted ferromagnetic alloys. For suitable values of the exchange energy in the ferromagnet and the F-layer thickness we have observed I c( T) oscillation with vanishing I c for definite values of T and d F.

NbN/AlN/NbN tunnel junctions with high current density up to 54 kA/cm2

We report on progress in the development of high current density NbN/AlN/NbN tunnel junctions for applications as submillimeter wave superconductor-insulator-superconductor mixers. A very high current density up to 54 kA/cm2, roughly an order of magnitude larger than any reported results for all-NbN tunnel junctions, was achieved in the junctions with about 1 nm thick AlN barriers. The magnetic field and temperature dependence of critical supercurrents were measured to investigate the Josephson tunneling behavior of critical supercurrents in the high-Jc junctions. The junctions showed high-quality junction characteristics with a large gap voltage of 5 mV and sharp quasiparticle current rise (ΔVg=0.1 mV). The Rsg/RN ratio was about 5 with a Vm value of 14 mV measured at 4.2 K.

The critical current of the superconductive tunnel junctions containing superconductive grains embedded in the oxide barrier

The superconductive tunnel junctions containing small superconductive grains embedded in the oxide barrier are investigated by the model of one dimensional atomic chains. The distribution of the Fermi levels of the grains relative to the Fermi level of the bulk superconductor is also taken into account. The results obtained indicate that the supercurrent appears only in the case that the charge energy of the grains is less than some certain value. The critical supercurrent of the junctions at zero temperature decreases as the charge energy of the grains increases. Temperature dependences of the critical supercurrents of junctions with different charge energies are also presented.

Critical supercurrents and the pinning of vortices in commercial Ng-60 at% Ti

Heavily cold worked Nb-60 at% Ti wires have been studied. The wires had previously been heat treated and characterized by electron microscopy (Neal et al 1971). The critical supercurrents have been measured as a function of applied magnetic field at temperatures between 4.2 K and the transition temperature, and interpreted in terms of the pinning force per unit volume on the vortex lattice. Empirically the pinning force is given by Fp=f(b)Bc2n (2<or approximately=n<or approximately=2.3) where b is the reduced field B/Bc2 and f(b) depends on the particular heat treatment but not on the temperature at which the critical current is measured. Present pinning theories are reviewed but, as none account satisfactorily for the results, a simple model based on pinning by the cell walls of the defect structure has been developed. It depends upon the Ginsburg-Landau parameter being enhanced in the cell walls and fits the data for the material given the optimum heat treatment very well. It is suggested that the other heat treatments produced cell walls with a different internal structure such that the basic pinning interaction and hence f(b) and n are different.

Microwave-Enhanced Critical Supercurrents in Constricted Tin Films

Microwave-Enhanced Critical Supercurrents in Constricted Tin Films

Magnetization and Critical Supercurrents

The magnetization in high-fteld superconductors has been investigated using tubular samples. When the sample assumes a critical state, wherein every region of the sample carries critical current density J(B) determined only by the local magnetic field B, the magnetization can be predicted quantitatively from the critical current density J(B). Using observed magnetization data, a critical current density relation alpha /J = B/sub 0/ + B is deduced for Nb/sub 3/Sn and 3Nb-Zr. Alpha is a direct measure of the current carrying capacity of a sample, and B/sub 0/ coincides approximately with the thermodynamic critical field of the material. Since this relation implies JB = alpha = const for B >> B/sub 0/, the Lorertz force plays an important role in determining the critical current density. (auth)