Supercurrent In Superconductor Research Articles

Spin phase regulated spin Josephson supercurrent in topological superconductor

Without applied bias voltage, a superconducting phase difference can drive a charge Josephson supercurrent in a superconductor junction. In analogy, we here theoretically propose a spin phase that intrinsically generates spin Josephson supercurrent, and this spin Josephson effect is studied in a junction of superconducting nanowire (SNW). We show that spin-orbit coupling and magnetic field give rise to spin-triplet superconductivity in the SNW, thus allowing the superfluid of both charge and spin. Next we introduce the concept of a spin phase that can be generated by controls of spin current, magnetic field, or electric field. By the analysis of pairing correlations and Ginzburg-Landau-type theory, it is shown that the spin phase makes spin-up and spin-down $S=1$ Cooper pairs get opposite phases and move oppositely in a Josephson junction, so a dissipationless pure spin current is induced. We also derive the formula that the spin current is equal to the derivative of an Andreev bound state to the spin phase, which is analogous to that of charge Josephson effect. At last, our calculation verifies the existence of spin supercurrent inside the superconducting gap in the topologically nontrivial phase. Our paper provides a view on spin Josephson effect and indicates the potential combination of topological superconductivity and spintronics.

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.

Theory of supercurrent in superconductors

According to the standard theory of superconductivity, the origin of superconductivity is electron pairing. The induced current by a magnetic field is calculated by the linear response to the vector potential, and the supercurrent is identified as the dissipationless flow of the paired electrons, while single electrons flow with dissipation. This supercurrent description suffers from the following serious problems: (1) it contradicts the reversible superconducting-normal phase transition in a magnetic field observed in type I superconductors; (2) the gauge invariance of the supercurrent induced by a magnetic field requires the breakdown of the global [Formula: see text] gauge invariance, or the nonconservation of the particle number; and (3) the explanation of the ac Josephson effect is based on the boundary condition that is different from the real experimental one. We will show that above problems are resolved if the supercurrent is attributed to the collective mode arising from the Berry connection for many-body wavefunctions. Problem (1) is resolved by attributing the appearance and disappearance of the supercurrent to the abrupt appearance and disappearance of topologically protected loop currents produced by the Berry connection; problem (2) is resolved by assigning the non-conserved number to that for the particle number participating in the collective mode produced by the Berry connection; and problem (3) is resolved by identifying the relevant phase in the Josephson effect is that arising from the Berry connection, and using the modified Bogoliubov transformation that conserves the particle number. We argue that the required Berry connection arises from spin-twisting itinerant motion of electrons. For this motion to happen, the Rashba spin–orbit interaction has to be added to the Hamiltonian for superconducting systems. The collective mode from the Berry connections is stabilized by the pairing interaction that changes the number of particles participating in it; thus, the superconducting transition temperatures for some superconductors is given by the pairing energy gap formation temperature as explained in the BCS theory. The topologically protected loop currents in this case are generated as cyclotron motion of electrons that is quantized by the Berry connection even without an external magnetic field. We also explain a way to obtain the Berry connection from spin-twisting itinerant motion of electrons for a two-dimensional model where the on-site Coulomb repulsion is large and doped holes form small polarons. In this model, the electron pairing is not required for the stabilization of the collective mode, and the supercurrent is given as topologically protected spin-vortex-induced loop currents (SVILCs).

Spin supercurrent in two-dimensional superconductors with Rashba spin-orbit interaction

Spin current is a central theme in spintronics, and its generation is a keen issue. The spin-polarized current injection from the ferromagnet, spin battery, and spin Hall effect have been used to generate spin current, but Ohmic currents in the normal state are involved in all of these methods. On the other hand, the spin and spin current manipulation by the supercurrent in superconductors is a promising route for dissipationless spintronics. Here we show theoretically that, in two-dimensional superconductors with Rashba spin-orbit interaction, the generation of dissipationless bulk spin current by charge supercurrent becomes highly efficient, exceeding that in normal states in the dilute limit, i.e. when the chemical potential is close to the band edge, although the spin density becomes small there. This result manifests the possibility of creating new spintronic devices with long-range coherence.

Conservation of spin supercurrents in superconductors

We demonstrate that spin supercurrents are conserved upon transmission through a conventional superconductor, even in the presence of spin-dependent scattering by impurities with magnetic moments or spin-orbit coupling. This is fundamentally different from conventional spin currents, which decay in the presence of such scattering, and this has important implications for the usage of superconducting materials in spintronic hybrid structures.

(Invited) Novel Electronic Transport in Topological Semimetals

Since the discovery of graphene, Dirac systems have attracted considerable attention due to their extraordinary properties. Recently, Dirac semimetals (DSs)-a new quantum state of matter-have been proposed. DSs can be viewed as an analogue of graphene but with linear energy dispersion in all three momentum directions. Due to the distinct electronic properties, DSs have been a fertile playground in discovering novel phenomena. In addition, it is possible to realize topological superconductivity in DSs, which plays a key role in next generation quantum computation. Shortly after theoretical predictions, several materials have been experimentally confirmed to be DSs, such as Cd3As2 and ZrTe5. Here, we report magneto-transport studies on Cd3As2 and ZrTe5. Shubnikov–de Haas oscillations were observed in both materials. A non-trivial Berry’s phase of π was obtained indicating the existence of Dirac fermions. However, angular dependence measurement shows different results in these two materials. Dirac fermions in ZrTe5 show a two-dimensional nature while in Cd3As2 they behave like three-dimensional particles. In addition, we observed supercurrent in superconductor (Al)–DS (Cd3As2)–superconductor (Al) junctions. Realizing topological superconductivity is an essential step towards finding Majorana fermions which are believed to be building blocks for quantum computers. Sandia National Laboratories is a multi-mission laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. Department of Energy’s National Nuclear Security Administration under contract DE-AC04-94AL85000.

Origin of the U(1) field mass in superconductors

Recently, a new theory for superconductivity has been put forward, in which the persistent current generation is attributed to the emergent singularities of the electronic wave function that are created by the spin-twisting itinerant circular motion of electrons. The persistent current generated by this mechanism behaves in every respect like supercurrent in superconductors, yielding the flux quantum h/2e and the Josephson frequency 2eV/h, where h is Planck’s constant, −e is the electron charge, and V is the voltage across the Josephson junction. The mass generation of the U(1) gauge field (or the Meissner effect) in the new theory is due to the emergence of topological objects, ‘instantons’ generated by the single-valued requirement of the wave function in the presence of the emergent singularities.The current standard theory of superconductivity is based on the BCS theory, and explains the emergence of superconductivity as due to the global U(1) gauge symmetry breaking realized by the Cooper pair formation. The U(1) field mass generation is believed to be due to this global U(1) gauge symmetry breaking. However, the feasibility of this mechanism has been questioned since no known interaction can prepare the global U(1) symmetry broken state from the normal state. We argue here that the U(1) mass generation in the BCS superconductor can be attributed to the one by the instanton mentioned above if the Rashba spin-orbit interaction is added. Then, the occurrence of persistent current generation becomes due to the instanton formation, and the role of the Cooper pair formation is to stabilize the instanton by providing an energy gap for perturbative excitations. Upon forming the Cooper pair, the instanton is stabilized and persistent current generation becomes possible. Thus, the superconducting transition temperature coincides with the Cooper pair formation temperature.

Dependence of proximity-induced supercurrent on junction length in multilayer-graphene Josephson junctions

We report experimental observation of the proximity-induced supercurrent in superconductor–multilayer graphene–superconductor junctions. We find that the supercurrent is a linearly decreasing function of the junction length (separation of the superconducting electrodes), which is quite different from the usual behavior of exponential dependence. We suggest that this behavior originates from the intrinsic large contact resistance between the multilayer and the superconducting electrodes.

Optical induced vortices and persistent currents in polariton condensates

Observation of quantized vortices in non-equilibrium polariton condensates, suggesting parallelisms with conventional superfluids, have been reported either by spontaneous formation and pinning in the presence of disorder [1,2] or by imprinting it onto the signal or idler of an optical parametric oscillator (OPO) [2]. Here we report the first observation of a polariton condensate receiving a quantized angular momentum by means of a short optical pulse and maintaining its rotation for a time much longer than the pulse duration, the polariton and coherence lifetimes. This observation shows a peculiar character of polariton condensates and reveals analogies with supercurrents in superconductors or persistent flow in condensates [3].

Blockade and Counterflow Supercurrent in Exciton-Condensate Josephson Junctions

We demonstrate that perfect conversion between charged supercurrents in superconductors and neutral supercurrents in electron-hole pair condensates is possible via a new Andreev-like scattering mechanism. As a result, when two superconducting circuits are coupled through a bilayer exciton condensate, the superflow in both layers is drastically modified. Depending on the phase biases the supercurrents can be completely blocked or exhibit perfect drag.

Models for transport properties of polymer thin films

AbstractWe have observed that thin films of different undoped non‐conjugated polymers exhibit high conductivity in metal–polymer–metal (M‐P‐M) structures. Below the critical temperature, Tc of the superconductivity of the contacts, a supercurrent in superconductor–polymer–superconductor (S‐P‐S) structures was obtained. Investigations of the current‐voltage characteristics as a function of contact diameter at different locations on the polymer showed that only in some small channels does a high conductivity appear, whereas at other places a reversible on/off switching was obtained. We postulate that the conductivity effect is connected with the phenomenon of electrification. The electrons or holes are supplied by the electrodes which are in contact with the polymer. These injected charge carriers accumulate in localised states on different locations in the polymer matrix. As a result, a large inner electrical field appears in the polymer giving rise to local changes between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO). In the high field region the forbidden gap can vanish, resulting in small channels with a high “metallic‐like” conductivity of the polymer. At present, there exists no theory explaining all the peculiarities of the electrical transport observed. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

High conductivity and supercurrent in superconductor–polymer–superconductor systems

We have observed that semicrystalline thin film polyimide exhibits high conductivity effect in metal–polymer–metal structures. The polymer was undoped and partially conjugated. We found that the conductivity is controlled by the effect of electrification and depends on the amount of charge accepted from the metallic electrodes. For example, in superconductor–polymer–superconductor structures, the conductivity can decrease, be constant or there can be a supercurrent flow through the polymer film due to the proximity effect.

Thermopower induced by a supercurrent in superconductor-normal-metal structures.

We examine the thermopower Q of a mesoscopic normal-metal (N) wire in contact with superconducting (S) segments and show that even with electron-hole symmetry Q may become finite due to the presence of supercurrents. Moreover, we show how the dominant part of Q can be directly related to the equilibrium supercurrents in the structure. In general, a finite thermopower appears both between the N reservoirs and the superconductors and between the N reservoirs themselves. The latter, however, strongly depends on the geometrical symmetry of the structure.