Normal-metal Junctions Research Articles

Self-consistent evaluation of proximity and inverse proximity effects with pair-breaking in diffusive superconducting–normal metal junctions

Self-consistent evaluation of proximity and inverse proximity effects with pair-breaking in diffusive superconducting–normal metal junctions

Application of the pseudo-Voigt function to the analysis of single-particle tunneling spectra of [formula omitted]-wave superconductors

In this study, we focused on the peak–dip–hump structure, which is a hallmark of cuprate superconductivity and is often used to explore the relationship between strong electron–phonon coupling and superconductivity. Specifically, we examined the peak–dip–hump structure and other features of cuprate tunneling spectra in d-wave superconductor–insulator–normal metal (dSC–I–N) junctions. To demonstrate the peak–dip–hump structure and other features of cuprate tunneling spectra, we applied the pseudo-Voigt function as an energy-dependent gap. We also analyzed two important experimental features of the tunneling spectra, namely the temperature and doping evolution of the differential conductance. Finally, we compared our calculation results with experimental data and found a good quantitative agreement between theory and experimental data in the considered cases.

Generation of polarized spin-triplet Cooper pairings by magnetic barriers in superconducting junctions

We investigate the proximity effect in an s-wave superconductor/ferromagnetic metal with a Rashba spin-orbit coupling/diffusive normal metal junction and an s-wave superconductor/noncollinear magnetic metal/diffusive normal metal junction. We show the generation of equal spin-triplet pairings in the diffusive normal metal due to spin-flip scattering in the intermediate magnetic regions. The emergence of the spin-triplet odd-frequency Cooper pairings can generate a zero-energy peak in the quasiparticle density of states in the diffusive normal metal.

Microscopic theory of photoassisted electronic transport in normal-metal/BCS-superconductor junctions

We investigate photoassisted electronic transport in a normal-metal/BCS-superconductor junction with a microscopic Hamiltonian approach, for several types of periodic voltage drives applied on the normal-metal side. The time-dependent current and the photoassisted noise are computed to all orders of the tunneling Hamiltonian using a Keldysh-Nambu-Floquet approach. An excess noise analysis allows one to determine to what extent pure electronic excitations with a small number of electrons per period can be generated by the different drives. When the superconducting gap is small compared to the drive frequency, the junction behaves like a normal-metal junction and minimal excess noise is reached for Lorentzian voltage drives carrying an integer charge (levitons). In the opposite regime of a large gap, the excess noise vanishes for half-quantized levitons, giving rise to the perfect transmission of a Cooper pair on the superconducting side. This microscopic approach also allows us to address the intermediate regime, when the drive frequency is comparable to the gap, allowing us to study the nontrivial interplay between Andreev reflection and quasiparticle transfer processes. Our analysis also shows the appearance of Tien-Gordon--type relations connecting the current and noise in the AC-driven junction to their DC counterpart, which we investigate in details. Finally, the possibility to build a reliable on-demand source of Cooper pairs with this setup is examined using realistic experimental parameters.

Electron transmission across normal metal-strained graphene–normal metal junctions

The transmission of the electron across the single normal metal–graphene (NG) and normal-metal–graphene–normal-metal (NGN) junctions has been investigated. For the single NG junction, the profile of the maximum transmission bears similarity to that of the conductance of the system and the minor effect of the incidence energy on transmission is found in conductance of the single NG junction. Concerning with NGN junction, the increase of strain strength always induces more resonance peaks at different angles in transmission and can therefore enhance the conductance. The increase of length of the middle graphene segment can accommodate more quasi-resonance states, leading to the more resonance peaks and richer structures in transmission. In both single NG and NGN junctions, the increase of the wavefunction period on metal side(s) can be observed due to the enhancement of strain strength, which can serve as the sensor for the detection of the strain strength in graphene.

Evidence of Majorana fermions in the noise characteristic of normal metal–topological superconductor junctions

A finite topological superconductor nanowire bears a Majorana fermion at its ends, leading to unique transport properties when connected to normal metal leads. We consider in this review two theoretical proposals based on noise measurements in normal metal–topological superconductor junctions. The first one considers a Hanbury Brown and Twiss setup where a topological superconductor is connected to two normal metal leads. The second proposal deals with the finite frequency noise of a single normal metal–topological superconductor junction. Both are computed using a unified framework of non equilibrium Keldysh Green’s functions using a Hamiltonian approach. Calculations are performed non-perturbatively in the tunnel hopping parameter and address both subgap and above gap regimes. Concerning the Hanbury Brown and Twiss setup, we find in the subgap case that when the two normal metal leads are biased with equal voltage, the noise crossed correlations are negative, as in the case of a three terminal normal metal junction. On the other hand when subgap voltages are opposite, the noise crossed correlations are positive. Predictions when the two Majoranas at the end of the topological superconductors hybridize, and when the chemical potential of the topological superconductor drives the system out of the topological phase are discussed. In the second proposal, the finite frequency emission and absorption noises are computed for a single junction. We observe noticeable structures in these quantities, related to simple transport processes involving the Majorana bound state. Both results offer an original tool for the further characterization of the presence of Majorana bound states in condensed matter systems.

Study on Green's function on topological insulator surface.

In the theory of superconducting junctions, Green's function has an important role for obtaining Andreev bound states, local density of states and Josephson current in a systematic way. In this article, we show how to construct Green's function on the surface of a topological insulator following McMillan's formalism where the energy spectrum of electrons obeys a linear dispersion. For a model of a superconductor (S)/ferromagnet (F)/normal metal (N) junction, we show that the generation of a Majorana fermion gives rise to the enhanced local density of states and pair amplitude of odd-frequency pairing. We also derive an extended Furusaki-Tsukada's formula of DC Josephson current in S/F/S junctions. The obtained Josephson current depends on the direction and magnitude of the magnetization.This article is part of the theme issue 'Andreev bound states'.

High Quality Superconductor–Normal Metal Junction Made on the Surface of MoS2 Flakes

A superconductor–normal metal (SN) junction is fabricated on the surface of a few‐layer MoS2 flake. Superconductivity is induced by ionic liquid gating, and an h‐BN flake is used to locally separate ionic liquid from the surface of MoS2. The h‐BN covered channel remains semiconducting, therefore an SN junction forms at the interface between the exposed and h‐BN covered MoS2 channel. Transport properties at the SN junction are studied at low temperatures and under magnetic fields. The probability of observing Andreev reflection is discussed based on the BTK model.

Proximity-induced superconductivity in all-silicon superconductor /normal-metal junctions

We have realised laser-doped all-silicon superconducting (S)/ normal metal (N) bilayers of tunable thickness and dopant concentration. We observed a strong reduction of the bilayers critical temperature when increasing the normal metal thickness, a signature of the highly transparent S/N interface associated to the epitaxial sharp laser doping profile. We extracted the interface resistance by fitting with the linearised Usadel equations, demonstrating a reduction of one order of magnitude from previous superconductor/doped Si interfaces. In this well controlled crystalline system we exploited the low resistance S/N interfaces to elaborate all-silicon lateral SNS Josephson junctions with long range proximity effect. Their dc transport properties, such as the critical and retrapping currents, could be well understood in the diffusive regime. Furthermore, this work lead to the estimation of important parameters in ultra-doped superconducting Si, such as the Fermi velocity, the coherence length, or the electron-phonon coupling constant, fundamental to conceive an all-silicon superconducting electronics.

Transport and noise properties of a normal metal–superconductor–normal metal junction with mixed singlet and chiral triplet pairings

We study transport and zero frequency shot noise properties of a normal metal–superconductor–normal metal (NSN) junction, with the superconductor having mixed singlet and chiral triplet pairings. We show that in the subgapped regime when the chiral triplet pairing amplitude dominates over that of the singlet, a resonance phenomena emerges out at zero energy where all the quantum mechanical scattering probabilities acquire a value of 0.25. At the resonance, crossed Andreev reflection mediating through such junction, acquires a zero energy peak. This reflects as a zero energy peak in the conductance as well depending on the doping concentration. We also investigate shot noise for this system and show that shot noise cross-correlation is negative in the subgapped regime when the triplet pairing dominates over the singlet one. The latter is in sharp contrast to the positive shot noise obtained when the singlet pairing is the dominating one.

Andreev reflection through a junction of a 3D topological insulator and an s-wave superconductor

We examine the retro and specular Andreev reflections (ARs) through a junction consisting of a three-dimensional (3D) Bi2Se3 topological insulator (TI) and an s-wave superconductor (SC) using spatially-resolved nonequilibrium Green function formalism in Nambu space. A 3DTI Bogoliubov–de Gennes Hamiltonian is constructed. The retro (specular) AR is identified by the spatial distribution of charge (spin-y) current. When approaching the interface, unlike the specular AR, the retro AR exhibits a large amount of net charge flow near the top surface of the 3DTI. In the specular AR, a large parallel component of the spin-y flow is developed in the interface, which is not found in the retro AR. Diminished charge currents are found near the onset of the specular AR. The AR mechanism enhances the current-induced spin-y polarization flowing through the junction interface. In the TI phase, larger normal-transmission conductance in the normal-metal junction yields larger AR conductance in the corresponding SC junction. The Zeeman splitting perpendicular to the quintuple layers partially (completely) suppresses the in the finite (infinite thick) TI.

Time-dependent Landauer—Büttiker formalism for superconducting junctions at arbitrary temperatures

We discuss an extension of our earlier work on the time-dependent Landauer- Buttiker formalism for noninteracting electronic transport. The formalism can without complication be extended to superconducting central regions since the Green's functions in the Nambu representation satisfy the same equations of motion which, in turn, leads to the same closed expression for the equal-time lesser Green's function, i.e., for the time-dependent reduced one-particle density matrix. We further write the finite-temperature frequency integrals in terms of known special functions thereby considerably speeding up the computation. Simulations in simple normal metal - superconductor - normal metal junctions are also presented.

Spin Hall voltages from a.c. and d.c. spin currents.

In spin electronics, the spin degree of freedom is used to transmit and store information. To this end the ability to create pure spin currents—that is, without net charge transfer—is essential. When the magnetization vector in a ferromagnet–normal metal junction is excited, the spin pumping effect leads to the injection of pure spin currents into the normal metal. The polarization of this spin current is time-dependent and contains a very small d.c. component. Here we show that the large a.c. component of the spin currents can be detected efficiently using the inverse spin Hall effect. The observed a.c.-inverse spin Hall voltages are one order of magnitude larger than the conventional d.c.-inverse spin Hall voltages measured on the same device. Our results demonstrate that ferromagnet–normal metal junctions are efficient sources of pure spin currents in the gigahertz frequency range.

Nonequilibrium and proximity effects in superconductor–normal metal junctions

We study the consequences of non-equilibrium heating and inverse proximity effect in normal metal - insulator - superconductor - insulator - normal metal (NISIN) junctions with a simple quasi-one-dimensional model. We especially focus on observables and parameter regions that are of interest in the design of SINIS coolers with quasiparticle traps. We present numerical results calculated by solving the Usadel equation and also present analytical approximations in two limiting cases: a short junction with a non-negligible resistance in both ends and a long junction with a transparent contact at one end.

Adiabatic quantum pumping through surface states in 3D topological insulators

We investigate adiabatic quantum pumping of ballistic Dirac fermions on the surface of a strong three-dimensional topological insulator. Two different geometries are studied in detail, a normal metal–ferromagnetic–normal metal (NFN) junction and a ferromagnetic–normal metal–ferromagnetic (FNF) junction. Using a scattering matrix approach, we show that each time a new resonant mode appears in the transport window the pumped current exhibits a maximum and provide a detailed analysis of the position of these maxima. We also predict a characteristic difference between the pumped current in NFN- and FNF-junctions: whereas the former vanishes for carriers at normal incidence, the latter is finite due to the different nature of wavefunction interference in the junctions. Finally, we predict an experimentally distinguishable difference between the pumped current and the conductance.

Effects of disorder on the transmission of nodal fermions through ad-wave superconductor

The bulk microwave conductivity of a dirty d-wave superconductor is known to depend sensitively on the range of the disorder potential: long-range scattering enhances the conductivity, while short- range scattering has no effect. Here we show that the three-terminal electrical conductance of a normal-metal-d-wave superconductor-normal-metal junction has a dual behavior: short-range scattering suppresses the conductance, while long-range scattering has no effect.

Hot-electron effects in InAs nanowire Josephson junctions

At a superconductor (S)-normal metal (N) junction pairing correlations can "leak-out" into the N region. This proximity effect [1, 2] modifies the system transport properties and can lead to supercurrent flow in SNS junctions [3]. Recent experimental works showed the potential of semiconductor nanowires (NWs) as building blocks for nanometre-scale devices [4-7], also in combination with superconducting elements [8-12]. Here, we demonstrate an InAs NW Josephson transistor where supercurrent is controlled by hot-quasiparticle injection from normal-metal electrodes. Operational principle is based on the modification of NW electron-energy distribution [13-20] that can yield reduced dissipation and high-switching speed. We shall argue that exploitation of this principle with heterostructured semiconductor NWs opens the way to a host of out-of-equilibrium hybrid-nanodevice concepts [7, 21].

Theoretical Study on Transport Properties of Normal Metal–Zigzag Graphene Nanoribbon–Normal Metal Junctions

We investigate transport properties of the junctions in which the graphene nanoribbon with the zigzag shaped edges consisting of the $N$ legs is sandwiched by the two normal metals by means of recursive Green's function method. The conductance and the transmission probabilities are found to have the remarkable properties depending on the parity of $N$. The singular behaviors close to E=0 with $E$ being the Fermi energy are demonstrated. The channel filtering is shown to occur in the case with $N=$ even.

Transport properties of normal metal–graphene nanoribbon–normal metal junctions

Transport properties are investigated theoretically for the junctions consisting of two normal metals and a graphene nanoribbon which has zigzag shaped edges with the electron hopping weaker than that in the other region. We find that the conductance of the present model has the same asymptotic behavior for N L ⪢ 1 as that with the spatially uniform hopping in the N ≥ 3 case, where N L and N are integers corresponding to length and width, respectively. The result seems to indicate that the zigzag edges do not contribute to the transport properties in the case with N L ⪢ 1 .

Andreev multiple reflection and current in a junction with a quantum dot embedded in a superconducting ring

We investigate the properties of Andreev multiple reflection and supercurrent in a normal-metal junction with a quantum dot embedded in a mesoscopic superconducting ring. The quantum dot is in the Coulomb blockade regime and contains several energy levels. The supercurrent through the junction is created by an external magnetic flux threaded through the ring. The supercurrent is related to the multiple Andreev reflection at interfaces between the superconductor and the normal metal, which is influenced by the Coulomb blockade on the quantum dot. Using a tight-binding model with Bogoliubov-de Gennes pairing potential, the supercurrent versus the external flux is calculated. By switching on the gate voltage applied on the dot, the supercurrent shows a series of peaks and valleys for various values of the flux through the ring. The results suggest the strong effect of the quantum dot on the supercurrent in such a structure, and the possible methods of controlling the supercurrent.