Superconducting Hybrid Structures Research Articles

Full counting statistics of superconductor hybrid structures involving a time-dependent field

Here we discuss the charge transfer statistics of superconductor-quantum dot-superconductor contacts in nonequilibrium and involving a time-dependent field via the applied voltage. In this case, the effects of multiple Andreev reflections and the Josephson effect exhibit distinctive features. In addition to the effects observable in the conductance, we will also discuss the effects observable in the noise.

Probing the exciton condensate via shot noise spectroscopy in superconducting hybrid structures based on excitonic insulators

Probing the exciton condensate via shot noise spectroscopy in superconducting hybrid structures based on excitonic insulators

Phase diagrams of the diode effect in superconducting heterostructures

At present the superconducting diode effect (SDE) attracts a lot of attention due to new possibilities in the superconducting electronics. One of the possible realizations of the SDE is the implementation in superconducting hybrid structures. In this case the SDE is achieved by means of the proximity effect. However, the optimal conditions for the SDE quality factor in hybrid devices remain unclear. In this study we consider the Superconductor/Ferromagnet/Topological insulator (S/F/TI) hybrid device and investigate the diode quality factor at different parameters of the hybrid structure. Consequently, we reveal important parameters that have crucial impact on the magnitude of the SDE quality factor.

Dendritic flux avalanches in superconducting hybrid structures

Magneto-optical imaging was employed to study dendritic flux avalanches in metal/superconductor and superconductor/superconductor hybrid structures over an extended range of magnetic field ramping rates. Our results in Cu/NbN show that the previously reported suppression of dendritic flux avalanches in metal coated superconducting films is limited to low ramping rates; as the ramping rate increases, the metal coating becomes less and less effective. A more complex behavior is exhibited in superconductor/superconductor hybrid structures. Our measurement in NbN partially coated with Nb, reveal three distinctive types of dendritic avalanches: those propagating in only one layer, either as regular dendrites in the uncoated NbN or as surface dendrites in the Nb layer, and hybrid dendrites that propagate in both the Nb and NbN layers simultaneously. These three types of dendrites are distinguished by their morphology, temperature dependence and instability threshold field. The overall stability of the hybrid structure significantly exceeds that of its weak component.

Reentrant superconductivity in a hybrid heterostructure with a high transparency barrier

Within the framework of the self-consistent effective field approximation of the time-dependent perturbation theory, an influence of the electron tunneling on the spontaneously induced order parameters in a normal metal–superconductor hybrid structure is considered. For a normal-metal model, which does not take into account electron-electron scattering, as well as electron-phonon coupling, a critical barrier transparency, corresponding to the disappearance of superconductivity in the ground state, was obtained. The presence of incoherent excitations leads to a complex relationship between the effects of ordering, thermal fluctuations, and tunneling. Near the critical barrier transparency, this can stabilize a superconducting state in the certain temperature intervals. As a result, a reentrant superconductivity phenomenon was observed. The studied spectral properties of the hybrid structure reflect the existence of both coherent and incoherent elementary excitations.

Phase-Coherent Thermoelectricity in Superconducting Hybrids (Brief Review)

We review some recent advances in studies of phase-coherent thermoelectric effects in superconducting hybrid structures such as, e.g., Andreev interferometers. We elucidate a number of mechanisms of electron-hole symmetry breaking in such systems causing dramatic enhancement of thermoelectric effects. We demonstrate that the flux-dependent thermopower exhibits periodic dependence on the applied magnetic flux $\Phi_x$ which in some limits may reduce to either odd or even functions of $\Phi_x$ in accordance with experimental observations. We also show that dc Josephson current in Andreev interferometers can be controlled and enhanced by applying a temperature gradient which may also cause a nontrivial current-phase relation and a transition to a $\pi$-junction state.

Anomalous Josephson effect modulated by magnetic misorientation in a topological unconventional superconductor hybrid structure

We study the anomalous Josephson effect induced by magnetic misorientation of asymmetric s-wave superconductor (SC)/ferromagnet insulator (FI)/normal layer/FI/d-wave SC junctions on a surface of topological insulator (TI). For the out-of-plane case, the magnetic misorientation not only leads to anomalous Josephson effects, but also realizes state transitions. However, for the in-plane case, the state transitions do not turn up any more. In particular, the plateaus phenomenon for d xy -wave SC junction emerges all the time. In addition, in the context of the in-plane case, the supercurrent rectification can be modulated by the magnetic misorientation. These unique phenomena can be employed to identify different d-wave superconductivities of three-dimensional TI and manufacture the supercurrent devices with such functions as phase storing, rectification, and quantum computation.

Disorder-robust p -wave pairing with odd-frequency dependence in normal metal–conventional superconductor junctions

We investigate the induced superconducting pair correlations in junctions between a conventional spin-singlet $s$-wave superconductor and a disordered normal metal. Decomposing the pair amplitude based on its symmetries in the time domain, we demonstrate that the odd-time, or equivalently odd-frequency, spin-singlet $p$-wave correlations are both significant in size and entirely robust against random non-magnetic disorder. We find that these odd-frequency correlations can even be generated by disorder. Our results show that anisotropic odd-frequency pairing represent an important fraction of the proximity-induced correlations in disordered superconducting hybrid structures.

Controllable spin pairing states in silicene-based superconducting hybrid structures with noncollinear magnetizations

We have theoretically investigated the modulation between the opposite- and equal-spin pairings and the related transport properties in silicene-based ferromagnet/ferromagnet/superconductor hybrid structures with noncollinear magnetizations. Due to the exotic electronic properties of silicene, the exclusive fully spin-polarized equal-spin pairing state appears without any contamination from the opposite-spin pairing state in a perpendicular magnetic configuration. Furthermore, the switch effect between fully spin-polarized opposite- and equal-spin pairings can be realized by tuning the Fermi level. In addition, the fully spin-polarized equal-spin pairing correlation can be enhanced by modulation of the magnitude and orientation of the exchange field in the central region. It is also a significant finding that for the formation of the fully spin-polarized equal-spin pairing state, the length of the central layer can be taken to be large, but not for opposite-spin pairing. Our findings provide an ideal platform to explore fully spin-polarized opposite- and equal-spin pairing states separately.

Reentrant superconductivity in proximity to a topological insulator

Superconducting hybrid structures with topological order and induced magnetization offer a promising way to realize fault-tolerant quantum computation. However, the effect of the interplay between magnetization and the property of the topological insulator surface, otherwise known as spin-momentum locking on the superconducting proximity effect, still remains to be investigated. We relied on the quasiclassical self-consistent approach to consider the superconducting transition temperature in the two-dimensional superconductor/topological insulator (S/TI) junction with an in-plane helical magnetization on the TI surface. It has emerged that the presence of the helical magnetization leads to the nonmonotonic dependence of the critical temperature on the TI thickness for both cases when the magnetization evolves along or perpendicular to the interface. The results obtained can be helpful for designing novel superconducting nanodevices and better understanding the nature of superconductivity in S/TI systems with nonuniform magnetization.

ПОДАВЛЕНИЕ СВЕРХПРОВОДИМОСТИ В ГИБРИДНЫХ СТРУКТУРАХ

Materials that possess strong spin-orbit interaction have provoked great interest over the past few years, in particular, in the actively developing field of quantum calculations. A topological insulator is a good example of such a material. The topological insulator has high surface conductivity, whereas in the body it shows the properties of an insulator, this being a purely phenomenological definition of a substance. Superconducting hybrid structures also present a promising elemental basis for quantum calculations and spintronics. This paper considers hybrid nanostructures like superconductor/ ferromagnet/ superconductor (S/F/S) and superconductor/ topological insulator/ superconductor (S/TI/S), where a uniform magnetic field is imposed onto the surface of the topological insulator. The paper investigates the behaviour of critical temperature in the superconducting layer Тс depending on different parameters of the systems, in particular, the dependence of Тс on the thickness of the layer dn in the topological insulator TI (or the ferromagnetic layer F). To solve the problem, we use the formalism of the quasiclassical Green's functions . The model assumes a diffusion mode, which holds true when the electron free path length is much less than the characteristic scale of the system. As a rule, such a limit is easier to be performed, since the commonly made structures have admixtures. To solve the problem on self-consistency of the superconducting energetic slot D we use a unimode approximation. As the results of our calculations, we give curves of the critical temperature behaviour in the systems under consideration. It is demonstrated that while the critical temperature exhibits a nonmonotonic behaviour and can make oscillations because of phase 0 -p transitions in the S/F/S structures, in the S/TI/S structures the temperature of the superconducting transition exhibits a trivial behaviour characterized by the monotonic attenuation.

Zero-Bias Conductance Peaks Effectively Tuned by Gating-Controlled Rashba Spin-Orbit Coupling.

Zero-bias conductance peaks (ZBCPs) can manifest a number of notable physical phenomena and thus provide critical characteristics to the underlying electronic systems. Here, we report observations of pronounced ZBCPs in hybrid junctions composed of an oxide heterostructure LaAlO_{3}/SrTiO_{3} and an elemental superconductor Nb, where the two-dimensional electron system (2DES) at the LaAlO_{3}/SrTiO_{3} interface is known to accommodate gate-tunable Rashba spin-orbit coupling (SOC). Remarkably, the ZBCPs exhibit a domelike dependence on the gate voltage, which correlates strongly with the nonmonotonic gate dependence of the Rashba SOC in the 2DES. The origin of the observed ZBCPs can be attributed to the reflectionless tunneling effect of electrons that undergo phase-coherent multiple Andreev reflection, and their gate dependence can be explained by the enhanced quantum coherence time of electrons in the 2DES with increased momentum separation due to SOC. We further demonstrate theoretically that, in the presence of a substantial proximity effect, the Rashba SOC can directly enhance the overall Andreev conductance in the 2DES-barrier-superconductor junctions. These findings not only highlight nontrivial interplay between electron spin and superconductivity revealed by ZBCPs, but also set forward the study of superconducting hybrid structures by means of controllable SOC, which has significant implications in various research fronts from superconducting spintronics to topological superconductivity.

Tunable spin-valley polarized transport channel in silicene-based superconducting hybrid structures

We investigate the influence of spin-valley polarized transport channel (SVPTC) mismatch modulated by the perpendicular electric field and the exchange field in a silicene-based ferromagnet/ferromagnet/superconductor junction and the barrier strength in a ferromagnet/insulator/ferromagnet/superconductor junction. In the former junction, due to the mismatch of SVPTC caused by the different electric fields applied in the two ferromagnet (F) layers, the zero-bias Andreev reflection and zero-bias conductance peak (ZBCP) are suppressed. Moreover, by shifting the band, the exchange field can lead to the different mismatch of SVPTC between the two F layers with opposite magnetization orientations, and thus, the conversion from ZBCP to a zero-bias conductance valley can be observed. For the latter junction, due to the electrically tunable SVPTC, the phase shift of conductance oscillation with barrier strength is created by changing the electric field but not by altering the exchange field. Particularly, for the variation from the parallel to the antiparallel magnetic configuration, there is a phase shift π/2 of conductance vs the barrier strength.

Spontaneous Currents and Topologically Protected States in Superconducting Hybrid Structures with the Spin\u2013Orbit Coupling (Brief Review)

The results of recent theoretical studies of features of superconducting states in hybrid structures whose properties are significantly determined by the spin–orbit effects have been reported. The two main phenomena appearing in such systems in the presence of additional spin splitting caused either by the Zeeman effect in a magnetic field or by the exchange field: (i) the generation of spontaneous currents and (ii) the appearance of topologically nontrivial superconducting phases. It has been shown that the spin–orbit coupling can be a key mechanism that allows implementing new inhomogeneous phase structures, in particular, the so-called “phase batteries.” The effect of geometric factors on the properties of topologically nontrivial superconducting states has been analyzed. New types of topological transitions in vortex states of Majorana wires have been proposed.

Progresses on topological phenomena, time-driven phase transitions, and unconventional superconductivity (a)Contribution to the Focus Issue Progress on Statistical Physics and Complexity edited by Roberta Citro, Giorgio Kaniadakis, Claudio Guarcello, Antonio Maria Scarfone and Davide Valenti.

In this perspective we discuss three emerging fields of condensed matter physics which in recent years have attracted considerable attention. In particular, we consider the recent challenging topics on time-dependent phase transitions, topological phenomena, and unconventional superconductivity, with the aim to foster the community towards new applications and technological advancements. As for the time-dependent phase transitions, in recent years the experiments have shown light-induced phase transitions and new fields of application are emerging, including material design. Regarding topological materials, new challenges have arisen to detect the Majorana origin of quantized conductance in superconducting hybrid structures, as well as the effect of interaction on edge channels. Finally, concerning superconductivity, non-conventional pairing and correlation effects dominate the physics of a vast class of two-dimensional materials and novel devices were recently conceived. This work offers a comprehensive overview on these topics for promoting new ideas in these fertile fields of research.

Anomalous thermal phenomena in a temperature biased graphene-based N/I/p-wave SC junction

We investigate the nonequilibrium energy transport across a graphene-based unconventional superconductor (SC) hybrid structure with a barrier region sandwiched between normal and superconducting regions, which is evaluated within an extended Dirac Bogoliubov-de Gennes formalism. Three different scenarios of the hybrids having p-wave triplet SCs with (i) px-, (ii) py-, and (iii) unitary chiral px + ipy-parings, are involved. Between them, the thermal conductance, asymmetric Kapitza resistance, and negative differential thermal conductance, respectively, characteristic by much different anomalous thermal properties, are uncovered. More interestingly, thermal energies Klein tunneling also turns up.

Identifying the graphene d-wave superconducting symmetry by an anomalous splitting zero-bias conductance peak

Not until recently, was a gate-tunable, high-temperature superconducting proximity effect in graphene demonstrated experimentally. And usually in d-wave superconductor (SC) hybrid structure, ferromagnetism and spin-triplet states could result in a splitting zero-bias conductance peak (ZBCP). Herein, we theoretically present an anomalous splitting ZBCP in a graphene-based ferromagnet/Rashba spin–orbit coupling (RSOC)/insulator/d-wave SC hybrid structure. With increasing the exchange field from h/EF = 0, the ZBCP starts to turn into a splitting one with a zero-bias conductance dip (ZBCD) sandwiched in between two subpeaks, while from h/EF = 1, the two subpeaks and ZBCD begin to gradually shrink till the ZBCP reappears. The anomalous splitting ZBCP can be modulated by the RSOC strength, magnitude of Fermi wave vector mismatch as well as insulator barrier strength. These peculiar features are ascribed to the novel spin-triplet Andreev reflection in the context of the RSOC, characteristic by the anisotropic d-wave pair symmetry combined with the relativistic nodal fermions, which in turn can be experimentally used to directly identify not only the proximity-induced ferromagnetism and RSOC but d-wave pair symmetry in graphene. These results pave the way to a new class of tunable, high-temperature superconducting spintronic devices based on large-scale graphene.

Dimensional tuning of Majorana fermions and real space counting of the Chern number

We demonstrate that it is possible to use atomic manipulation techniques to adiabatically tune between one-dimensional and two-dimensional topological superconductors in magnet--superconductor hybrid (MSH) structures. This allows one to change the nature of the associated Majorana fermions between localized bound states and chiral Majorana edge modes, and provides a new approach for counting the topological invariant of the system -- the Chern number -- in real space. Moreover, we show that the topological nature of finite-size MSH structures can be characterized using the Chern number density which adiabatically connects MSH structures to macroscopic, translationally invariant topological superconductors characterized by an integer Chern number. These results open new possibilities for studying the nature of topological superconductivity in MSH structures using atomic manipulation techniques.

Valley controlled Andreev reflection and valley-switch effect in graphene-based superconducting hybrid structures

The presence of the valley degree of freedom in two-dimensional materials leads to the valleytronics, in which information is encoded by the valley quantum number of the electron. For graphene-based superconducting hybrid structures, with the incoming electron and the Andreev reflected hole belonging to two opposite valleys, the Andreev reflection (AR) can be controlled by the valley degree of freedom. Here, we investigate theoretically the AR in graphene-based normal/supercondcutor junctions. Due to the interplay of the staggered potentials and the off-resonant circularly polarized light, the conductances demonstrate rich features. In particular, when the Fermi level of the normal lead is smaller than the superconducting gap, the retro AR and specular AR are well separated in energy, so we can easily distinguish them by studying the conductance spectra. More importantly, for normal/supercondcutor/normal junctions, the local AR is forbidden due to the vanishing density of states in one valley, and one may observe a valley-switch effect between perfect elastic cotunneling and perfect crossed AR, by reversing the handedness of the circularly polarized light.

Quasiclassical theory for interfaces with spin-orbit coupling

In recent years, substantial progress has been made regarding the application of quasiclassical theory on superconducting hybrid structures. This theoretical framework is reliant on a proper set of boundary conditions in order to describe multilayered systems. With the advent of the field of superconducting spintronics, systems which combine heavy metal layers, in which there is large spin--orbit coupling, with ferromagnets have received a great deal of attention, due to their potential for generating long range triplet superconductivity. In contrast to interfaces of strongly spin polarized materials, which are well understood, a quasiclassical theory for interfaces in systems where there is significant spin--orbit coupling does not yet exist. After reviewing the quasiclassical theory for interfaces, we here solve this problem by deriving a new set of boundary conditions which take spin--orbit coupling explicitly into account. We then go on to apply these boundary conditions to a superconductor-ferromagnet (SF) bilayer and an SFS Josephson weak link, demonstrating the emergence of long range triplet superconductivity in these systems.