Superconductor Junctions Research Articles

Signatures of the Higgs mode in transport through a normal-metal–superconductor junction

A superconductor subject to electromagnetic irradiation in the terahertz range can show amplitude oscillations of its order parameter. However, coupling this so-called Higgs mode to the charge current is notoriously difficult. We propose to achieve such a coupling in a particle-hole-asymmetric configuration using a DC-voltage-biased normal-metal--superconductor tunnel junction. Using the quasiclassical Green's function formalism, we demonstrate three characteristic signatures of the Higgs mode: (i) The AC charge current exhibits a pronounced resonant behavior and is maximal when the radiation frequency coincides with the order parameter. (ii) The AC charge current amplitude exhibits a characteristic nonmonotonic behavior with increasing voltage bias. (iii) At resonance for large voltage bias, the AC current vanishes inversely proportional to the bias. These signatures provide an electric detection scheme for the Higgs mode.

Vanadium gate-controlled Josephson half-wave nanorectifier

Recently, the possibility to tune the critical current of conventional metallic superconductors via electrostatic gating was shown in wires, Josephson weak-links, and superconductor-normal metal–superconductor junctions. Here, we exploit such a technique to demonstrate a gate-controlled vanadium-based Dayem nano-bridge operated as a half-wave rectifier at 3 K. Our devices exploit the gate-driven modulation of the critical current of the Josephson junction and the resulting steep variation of its normal-state resistance, to convert an AC signal applied to the gate electrode into a DC one across the junction. All-metallic superconducting gated rectifiers could provide the enabling technology to realize tunable photon detectors and diodes useful for superconducting electronics circuitry.

Nonadiabatic charge pumping across two superconductors connected through a normal metal region by periodically driven potentials

Periodically driven systems exhibit resonance when the difference between an excited state energy and the ground state energy is an integer multiple of ℏ times the driving frequency. On the other hand, when a superconducting phase difference is maintained between two superconductors, subgap states appear which carry a Josephson current. A driven Josephson junction therefore opens up an interesting avenue where the excitations due to applied driving affect the current flowing from one superconductor to the other. Motivated by this, we study charge transport in a superconductor–normal metal–superconductor junction where oscillating potentials are applied to the normal metal region. We find that for small amplitudes of the oscillating potential, driving at one site reverses the direction of current at the superconducting phase differences when difference between the subgap eigenenergies of the undriven Hamiltonian is integer multiple of ℏ times the driving frequency. For larger amplitudes of oscillating potential, driving at one site exhibits richer features. We show that even when the two superconductors are maintained at same superconducting phase, a current can be driven by applying oscillating potentials to two sites in the normal metal differing by a phase. We find that when there is a nonzero Josephson current in the undriven system, the local peaks and valleys in current of the system driven with an amplitude of oscillating potential smaller than the superconducting gap indicates sharp excitations in the system. In the adiabatic limit, we find that charge transferred in one time period diverges as a powerlaw with pumping frequency when a Josephson current flows in the undriven system. Our calculations are exact and can be applied to finite systems. We discuss possible experimental setups where our predictions can be tested.

Equivalent critical behavior of a helical point contact and a two-channel Luttinger liquid–topological superconductor junction

This paper unifies the Teo-Kane theory of a point-contact in the quantum spin Hall effect and the Affleck-Giuliano theory of a junction between a topological superconductor and two quantum wires. The authors show that the two problems are related by duality

Observation of Frequency Up-Conversion Gain in SIS Junctions at W Band

We have developed a test setup to measure frequency up-conversion gain in superconductor–insulator–superconductor (SIS) junctions at W band (75–110 GHz). A conventional SIS mixer with Nb/AlOx/Nb tunnel junctions was used as a frequency up-converter. An up-converted signal is measured by a room-temperature down-converter system. We observed distinct intermediate frequency responses to signal inputs from a continuous microwave source. The measured single-sideband up-conversion gain in SIS junctions was clearly positive (> 0 dB) with input frequency less than 2 GHz, which was well-calibrated by using an input from the continuous-wave source.

Charge transport in silicene-based ferromagnetic p-wave superconductor junctions

Using Blonder-Tinkham-Klapwijk (BTK) formalism we studied the transport properties in silicene-based normal/ferromagnetic p-wave superconductor (N/FpS) junctions. Our study shows that the amplitude of the normalized conductance strongly depends on the pairing symmetry in superconductor such as px, py and px+ipy-wave symmetries, the applied electric field (EZ) and the exchange field (h) in FpS. In N/FpxS-silicene junctions the zero-bias conductance peak is obtained due to the Andreev bound states. When the electric field and magnetic field are parallel, called P configuration, there is one conductance dip (CD) for the voltage eV=lEz−λSO with λSO the spin obit coupling term and 2l the distance between two sublattices in the buckled silicene. In the anti-parallel (AP) configuration there is one CD at eV=lEz−λSO for 0<lEz−λSO<h and for lEz−λSO≥h there are two CDs at eV=h and eV=lEz−λSO, respectively. A sudden transition of conductance from zero to maximum (henceforth referred to as“switching of conductance”) at eV=lEz−λSO occurs in both N/FpyS-silicene junction and N/Fpx+ipyS-silicene junction. In N/FpyS-silicene junction the Andreev resonant peak appears at eV=h for lEz−λSO−h=0.1Δ00 in the AP case, where Δ00 is the zero temperature gap of the FpS. In N/Fpx+ipyS-silicene junctions the maximum value of the conductance appears at eV=Δ00−h in P junction while in AP junction at eV=Δ00+h.

Properties of grooved Dayem bridge based YBa2Cu3O7−δ superconducting quantum interference devices and magnetometers

The transport properties of a YBa2Cu3O7−δ superconducting quantum interference device (SQUID) based on grooved Dayem bridge weak links are studied as a function of temperature: at high temperatures (60 K&amp;lt;T&amp;lt;Tc=89 K), the weak links show properties similar to superconductor–normal conductor–superconductor junctions, while at temperatures below 60 K, the weak links behave like short Dayem bridges. Using these devices, we have fabricated SQUID magnetometers with galvanically coupled in-plane pickup loops: at T = 77 K, magnetic field white noise levels as low as 63 fT/Hz have been achieved.

Magnetic-field-induced transition in a quantum dot coupled to a superconductor

The magnetic moment of a quantum dot can be screened by its coupling to a superconducting reservoir, depending on the hierarchy of the superconducting gap and the relevant Kondo scale. This screening-unscreening transition can be driven by electrostatic gating, tunnel coupling, and, as we demonstrate here, magnetic field. We perform high-resolution spectroscopy of subgap excitations near the screening-unscreening transition of asymmetric superconductor - quantum dot - superconductor (S-QD-S) junctions formed by the electromigration technique. Our measurements reveal a re-entrant phase boundary determined by the competition between Zeeman energy and gap reduction with magnetic field. We further track the evolution of the phase transition with increasing temperature, which is also evidenced by thermal replicas of subgap states.

High dynamic resistance elements based on a Josephson junction array.

A chain of superconductor–insulator–superconductor junctions based on Al–AlOx–Al nanostructures and fabricated using conventional lift-off lithography techniques was measured at ultra-low temperatures. At zero magnetic field, the low current bias dynamic resistance can reach values of ≈1011 Ω. It was demonstrated that the system can provide a decent quality current biasing circuit, enabling the observation of Coulomb blockade and Bloch oscillations in ultra-narrow Ti nanowires associated with the quantum phase-slip effect.

Electrical response of superconductor/ferromagnet/topological-insulator/superconductor junctions to magnetic texture dynamics

We consider a hybrid structure consisting of superconducting or normal leads with a combined ferromagnet-3D topological insulator interlayer. We compare responses of a Josephson junction and a normal junction to magnetic texture dynamics. In both cases the electromotive force resulting from the magnetization dynamics generates a voltage between the junction leads. For an open circuit this voltage is the same for normal and superconducting leads and allows for electrical detection of magnetization dynamics and a structure of a given magnetic texture. However, under the applied current the electrical response of the Josephson junction is essentially different due to the strong dependence of the critical Josephson current on the magnetization direction and can be used for experimental probing of this dependence. We propose a setup, which is able to detect a defect motion and to provide detailed information about the structure of magnetic inhomogeneity. The discussed effect could be of interest for spintronics applications.

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.

Disorder and critical current variability in Josephson junctions

We investigate theoretically the origins of observed variations in the critical currents of Nb/Al-AlOx/Nb Josephson junctions in terms of various types of disorder. We consider the following disorder sources: vacancies within the Al layer, thickness variations in the AlOx layer, and “pinholes” (i.e., point contacts) within the AlOx layer. The calculations are all performed by solving the microscopic Bogoliubov–de Gennes Hamiltonian self-consistently. It is found that a small concentration of vacancies within the Al layer is sufficient to suppress the critical current, while the presence of a small number of thick regions of the oxide layer induces a similar effect as well. The pinhole scenario is found to result in anomalous behavior that resembles neither that of a pure tunnel junction nor that of a superconductor–normal–superconductor junction but a regime that interpolates between these two limits. We comment on the degree to which each of the three scenarios describes the actual situation present in these junctions.

Characterization of Sr2RuO4 Josephson junctions made of epitaxial films

We have studied fundamental properties of weak-link Sr2RuO4/Sr2RuO4 Josephson junctions fabricated by making a narrow constriction on superconducting Sr2RuO4 films through laser micro-patterning. The junctions show a typical overdamped behavior with much higher critical current density, compared with those previously reported for bulk Sr2RuO4/s-wave superconductor junctions. Observed magnetic field and temperature dependences of the Josephson critical current suggest that the chiral p-wave is unlikely for the superconducting symmetry, encouraging further theoretical calculations of the Sr2RuO4/Sr2RuO4 type junctions.

Fractional charges in conventional sequential electron tunneling

The notion of fractional charges was up until now reserved for quasiparticle excitations emerging in strongly correlated quantum systems, such as Laughlin states in the fractional quantum Hall effect, Luttinger quasiparticles, or parafermions. Here, we consider topological transitions in the full counting statistics of standard sequential electron tunneling, and find that they lead to the same type of charge fractionalization - strikingly without requiring exotic quantum correlations. This conclusion relies on the realization that fundamental integer charge quantization fixes the global properties of the transport statistics whereas fractional charges can only be well-defined locally. We then show that the reconciliation of these two contradicting notions results in a nontrivially quantized geometric phase defined in the detector space. In doing so, we show that detector degrees of freedom can be used to describe topological transitions in nonequilibrium open quantum systems. Moreover, the quantized geometric phase reveals a profound analogy between the fractional charge effect in sequential tunneling and fractional Josephson effect in topological superconducting junctions, where likewise the Majorana- or parafermions exhibit a charge which is at odds with the Cooper pair charge as the underlying unit of the supercurrent. In order to provide means for an experimental verification of our claims, we demonstrate the fractional nature of transport statistics at the example of highly feasible transport models, such as weakly tunnel-coupled quantum dots or charge islands. We then show that the geometric phase can be accessed through the detector's waiting time distribution. Finally, we find that topological transitions in the transport statistics could even lead to new applications, such as the unexpected possibility to directly measure features beyond the resolution limit of a detector.

Enhanced shot noise at bilayer graphene–superconductor junction

Transport properties of graphene-superconductor junction have been studied extensively to understand the interplay of the relativistic Dirac quasiparticles and superconductivity. Though shot noise measurements in graphene have been performed to realize many theoretical predictions, both at zero magnetic field as well as quantum Hall (QH) regime, its junction with superconductor remain unexplored. Here, we have carried out the shot noise measurements in an edge contacted bilayer graphene--niobium superconductor junction at zero magnetic field as well as QH regime. At the Dirac point we have observed a Fano factor $\ensuremath{\sim}1/3$ above the superconducting gap $(\mathrm{\ensuremath{\Delta}})$ and a transition to an enhanced Fano factor $\ensuremath{\sim}0.5$ below the superconducting gap. By changing the carrier density we have found a continuous reduction of Fano factor for both types of carriers; however, the enhancement of Fano factor within the superconducting gap by a factor of $\ensuremath{\sim}1.5$ is always preserved. The enhancement of shot noise is also observed in the QH regime, where the current is carried by the edge state, below the critical magnetic field and within the superconducting gap. These observations clearly demonstrate the enhanced charge transport at the graphene-superconductor interface.

Dirac point formation revealed by Andreev tunneling in superlattice-graphene/superconductor junctions

A graphene superlattice is formed by a one-dimensional periodic potential and is characterized by the emergence of new Dirac points in the electronic structure. The group velocity of graphene's massless Dirac fermions at the new points is drastically reduced, resulting in a measurable effect in the conductance spectroscopy. We show here that tunnel spectroscopy using a superconducting hybrid junction is more sensitive to the formation of Dirac points in the spectrum of graphene superlattices due to the additional contribution of Andreev processes. We examine the transport properties of a graphene-based superlattice--superconductor hybrid junction and demonstrate that a superlattice potential can coexist with proximity-induced superconducting correlations. Both effects contribute to change graphene's spectrum for subgap energies and, as a result, the normalized tunneling conductance features sharp changes for voltages proportional to the energy separation between the original and the newly generated Dirac points.

Double Andreev reflections at surface states of the topological insulators with hexagonal warping

We study the Andreev reflection (AR) at the interface of the topological insulator with hexagonal warping and superconductor junction. Due to the hexagonal warping effect, the double ARs are found in a certain range of the incident angle, where for one incident electron beam, two beams of holes are reflected back. Interestingly, both the beams of holes are reflected as retro-AR on the same side of the normal line of the interface but with different reflection angles, different from the previously reported double AR with one retro-AR and one specular-AR. The double reflections owing to the warping effect show the optics-like property of the Dirac fermion and can stimulate the double reflections of light in anisotropic crystals. In addition, we find that the double ARs are dependent on the hexagonal warping parameter nonmonotonically, and in an intermediate strength the double AR phenomenon is prominent, providing a possibility to explore the warping parameter of topological insulators.

Design and Performance of a Sideband Separating SIS Mixer for 800–950 GHz

We present the design and results of characterization of a new sideband separating (2SB) mixer for 800–950 GHz, based on superconductor–insulator–superconductor (SIS) junctions. This is the first waveguide 2SB SIS mixer demonstrated at such a high frequency. The design is following the classical quadrature hybrid architecture, meanwhile additional attention was put on the reduction of reflections in the RF structure in order to minimize the RF imbalance, to achieve a high image rejection ratio (IRR). The RF waveguide block was manufactured by micromilling and populated by single-ended SIS mixers developed earlier for upgrade of the CHAMP+ high-band array on the APEX telescope. These SIS mixers have double-sideband (DSB) noise temperatures from 210 to 400 K. The assembled 2SB mixer yields a SSB noise temperature from 450 to 900 K, with an IRR above 15 dB in 95% of the band. Comparing the DSB and the SSB sensitivities, we find that the waveguide losses are as low as expected and do not exceed 0.6 dB. The presented mixer is a prototype for use in a 2SB dual polarization receiver planned for deployment on the APEX telescope.

Superconducting order parameter and bosonic mode in hydrogen-substituted NdFeAsO0.6H0.36 revealed by multiple-Andreev-reflection spectroscopy

Synthesized under high pressure, hydrogen-substituted NdFeAsO${}_{0.6}$H${}_{0.36}$ (with a critical temperature of 48 K) possibly demonstrates a double-dome superconducting state in its phase diagram, thus foreshadowing extraordinary physics as compared with other iron-based oxypnictides. Ballistic Andreev transport in superconductor--normal metal--superconductor junctions provides direct information about the superconducting order parameter at any temperatures up to ${T}_{c}$, and specific boson emission. The authors discuss here the characteristics found of the multiple-band superconducting state, the boson energy, and their nature.

Negative differential thermal conductance in a borophane normal metal–superconductor junction

We study the charge and heat transport in a normal metal/superconductor (NS) junction of the tilted anisotropic Dirac cone material borophane, using the extended Blonder–Tinkham–Klapwijk formalism. In spite of the large mismatch in the Fermi wave vector of the normal metal and superconductor sides of the borophane NS junction, the electron–hole conversion happens with unit probability at normal incidences. Furthermore, in the heavily doped superconducting regime, for heavily doped normal borophane, the electron–hole conversion happens with unit probability, at almost any incident angle. Finding the dependence of the differential Andreev conductance on the Fermi energy and excitation energy gives us a handle to distinguish specular from retro Andreev reflection. We numerically find that, independent of the Fermi energy, the temperature dependence of the differential thermal conductance in borophane can be modelled as an inverse Gaussian function, reflecting the d-wave symmetry of the borophane superconductor. We propose a scheme for achieving negative differential thermal conductance, as a key building block of thermal circuits, at intermediate Fermi energies. Our findings will have potential applications in developing borophane-based thermal management and signal manipulation mesoscopic structures such as heat transistors, heat diodes, and thermal logic gates.