Superconducting Tunnel Junctions Research Articles

Topological phases of the kicked Harper–Kitaev model with ultracold atoms

We propose using ultracold atoms trapped in a one-dimensional periodically driven optical lattice to realize the Harper–Kitaev model, where the on-site energies are periodically kicked. Such a system provides a natural platform to study both Chern insulators and Majorana fermions. Based on calculating the quasienergy spectra, we find that both Floquet Majorana modes and Hall chiral edge modes could appear at the sample boundary in the gaps between the quasienergy bands. We also study the competition of topological superconductor and Chern insulator states in the model. We calculate the index and Floquet Chern number to characterize the above two different topological states, including the topological phase transitions in the kicked Harper–Kitaev model with the increase in the strength of the kick.

Investigation of the regimes of mixing of superconducting tunneling structures

The regimes of operation of a superconductor–insulator–superconductor tunnel junction based on three-layer structures Nb/AlOx/Nb and Nb/AlN/NbN as a harmonic mixer (for frequencies of the order of 600 and 20 GHz, respectively) and a frequency up-converter (in the frequency range from 0.1 to 5.0 GHz) have been investigated experimentally. The quasiparticle and Josephson mixing regimes have been compared. It has been shown that, in some practical applications, such as the use of the superconductor–insulator–superconductor junction as a cryogenic harmonic phase detector, the Josephson mixing regime is more preferable, because it can provide a higher signal and a greater signal-to-noise ratio as compared to the quasiparticle mixing regime. It has also been demonstrated that the Josephson mixing regime is promising for the use in signal multiplexing systems for superconducting detectors.

Charge Neutral Fermionic States and Current Oscillation in a Graphene–Superconductor Hybrid Structure

The proximity properties of edge currents in the vicinity of the interface between the graphene and superconductor in the presence of magnetic field are investigated. It is shown that the edge states introduced by Andreev reflection at the graphene–superconductor (G/S) interface give rise to the charge neutral states in all Landau levels. We note that in a topological insulator–superconductor (TI/S) hybrid structure, only N = 0 Landau level can support this type of charge neutral states. The different interface states of a G/S hybrid and a TI/S hybrid is due to that graphene consists of two distinct sublattices. The armchair edge consists of two inequivalent atoms. This gives rise to unique electronic properties of edge states when connected to a superconductor. A direct consequence of zero charge states in all Landau levels is that the current density approaches zero at interface. The proximity effect leads to quantum magnetic oscillation of the current density in the superconductor region. The interface...

Блокировка туннельного тока в низкоомных структурах нормальный металл—изолятор—сверхпроводник

Блокировка туннельного тока в низкоомных структурах нормальный металл—изолятор—сверхпроводник

Low input reflection cryogenic low noise amplifier for Radio Astronomy multipixel receivers

The advancement of Radio Astronomy instruments pushes innovation in several fronts. Sensitivity aside, one way in which cryogenic receivers can be upgraded is by increasing the number of beams in single dish antennas, building what is commonly known as a Focal Plane Array (FPA). In this paper we present a novel reduced input reflection 4–12 GHz cryogenic Low Noise Amplifier (LNA) for the Intermediate Frequency (IF) of millimeter wave superheterodyne multipixel receivers with Superconductor-Insulator-Superconductor (SIS) mixers. The aim of this development is to reduce the input reflection of the amplifier to a level at which the bulky cryogenic isolators traditionally used in this type of receivers are no longer necessary and can be avoided. Ultimately this simplification would allow complying with the tight mass and volume restrictions imposed over FPAs. However, the improvement of the input reflection has a cost in terms of noise and gain performance. This effect is critically evaluated by comparing it with other alternative options built with devices of the same technology. The results show that this approach may have advantages in terms of sensitivity of the complete receiver.

THz Frequency Up-Conversion using Superconducting Tunnel Junction

In this letter we present the characterization of the first frequency up-converter using distributed superconducting tunnel junctions. The power of the frequency up-converted signal was large enough to pump an SIS mixer. The efficiency of the distributed superconducting tunnel junction is 11-22 % for a fractional bandwidth of 7% with excellent spectral line purity. The −3 dB line width of the up-converter signal is better than 1 Hz, which was the lowest resolution bandwidth of the spectrum analyzer.

Observation of unusual topological surface states in half-Heusler compounds LnPtBi (Ln=Lu, Y).

Topological quantum materials represent a new class of matter with both exotic physical phenomena and novel application potentials. Many Heusler compounds, which exhibit rich emergent properties such as unusual magnetism, superconductivity and heavy fermion behaviour, have been predicted to host non-trivial topological electronic structures. The coexistence of topological order and other unusual properties makes Heusler materials ideal platform to search for new topological quantum phases (such as quantum anomalous Hall insulator and topological superconductor). By carrying out angle-resolved photoemission spectroscopy and ab initio calculations on rare-earth half-Heusler compounds LnPtBi (Ln=Lu, Y), we directly observe the unusual topological surface states on these materials, establishing them as first members with non-trivial topological electronic structure in this class of materials. Moreover, as LnPtBi compounds are non-centrosymmetric superconductors, our discovery further highlights them as promising candidates of topological superconductors.

Gate voltage control of the AlOx/SrTiO3 interface electrical properties

Electron-beam deposition of an insulating granular aluminium or off-stoichiometric amorphous alumina layer on a SrTiO3 surface is a simple way to get a metallic interface from insulating materials. No heating nor specific preparation of the SrTiO3 surface is needed. In this paper, we investigate how the electrical properties of this interface can be tuned by the use of a back gate voltage (electrical field through the SrTiO3 substrate). We demonstrate that the slow field-effect observed at room temperature can be used to tune in a controlled, reversible way the low temperature electrical properties of the interface. In particular, important parameters of a transistor such as the amplitude of the resistance response to gate voltage changes or the existence of an ‘on’ or an ‘off’ state at zero gate voltage and at low temperature can be adjusted in a single sample. This method should be applicable to any SrTiO3-based interface in which oxygen vacancies are involved and might provide a powerful way to study the metal or superconductor insulator transition observed in such systems.

Tunnel superconducting junctions for a cryogenic multiplexing system

the possibility of using superconductor–insulator–superconductor (SIS) tunnel junctions in a cryogenic multiplexing system for setting an ac current through arrays of transition edge sensors and reading out the received signal, as well as for setting a dc current through arrays of superconducting quantum interferometers is studied. Operation of the SIS frequency converter at relatively low (below 10 GHz) working frequencies is experimentally investigated. Different operation regimes are implemented; the conversion loss in the quasiparticle mode is found to be 8 dB and the output saturation power is–58 dBm. The possibility of up-conversion of several input signal frequencies simultaneously is demonstrated. It is shown that the SIS junction can be used as a current source controlled by an external RF signal.

High-performance electronic cooling with superconducting tunnel junctions

When biased at a voltage just below a superconductor's energy gap, a tunnel junction between this superconductor and a normal metal cools the latter. While the study of such devices has long been focussed to structures of submicron size and consequently cooling power in the picoWatt range, we have led a thorough study of devices with a large cooling power up to the nanoWatt range. Here we describe how their performance can be optimized by using a quasi-particle drain and tuning the cooling junctions' tunnel barrier.

Fabrication of high sensitivity 3D nanoSQUIDs based on a focused ion beam sculpting technique

In this paper a nanofabrication process, based on a focused ion beam (FIB) nanosculpting technique, for high sensitivity three-dimensional nanoscale superconducting quantum interference devices (nanoSQUIDs) is reported. The crucial steps of the fabrication process are described, as are some peculiar features of the superconductor–normal metal–insulator–superconductor (SNIS) Josephson junctions, which may useful for applications in cryocooler systems. This fabrication procedure is employed to fabricate sandwich nanojunctions and high sensitivity nanoSQUIDs. Specifically, the superconductive nanosensors have a rectangular loop of 1 × 0.2–0.4 μm2 interrupted by two square Nb/Al–AlOx/Nb SNIS Josephson junctions with side lengths of 0.3 μm. The characterization of a typical nanoSQUID has been carried out and a spectral density of magnetic flux noise as low as 0.8 μΦ0 Hz–1/2 has been measured.

Quantum critical points in tunneling junction of topological superconductor and topological insulator

The tunneling junction between one-dimensional topological superconductor and integer (fractional) topological insulator (TI), realized via point contact, is investigated theoretically with bosonization technology and renormalization group methods. For the integer TI case, in a finite range of edge interaction parameter, there is a non-trivial stable fixed point which corresponds to the physical picture that the edge of TI breaks up into two sections at the junction, with one side coupling strongly to the Majorana fermion and exhibiting perfect Andreev reflection, while the other side decouples, exhibiting perfect normal reflection at low energies. This fixed point can be used as a signature of the Majorana fermion and tested by nowadays experiment techniques. For the fractional TI case, the universal low-energy transport properties are described by perfect normal reflection, perfect Andreev reflection, or perfect insulating fixed points dependent on the filling fraction and edge interaction parameter of fractional TI.

Fabrication of TiN/AlN/TiN tunnel junctions

We have fabricated TiN/AlN/TiN tunnel junctions with an epitaxial layer. The critical temperature of TiN can be changed in the range from 0.5 to 5.0K. Therefore, it is easy to set 5.0K as the target critical temperature. When a Superconducting Tunnel Junction (STJ) is operated as a photon detector, it is necessary to cool it to within 0.1K of the critical temperature in consideration of the noise of the thermally stimulated currents. Because 0.3K was desirable, as for the manufacture of general purpose photon detectors, the critical temperature 5.0K. TiN and AlN films were deposited by dc and rf magnetron sputtering in a load-lock sputtering system at ambient substrate temperatures. The junctions have a gap voltage of Vg=1.1mV, and critical current density of Jc=0.24A/cm2, and Rsg/Rn of 7.2, and low subgap leakage current (Isub@ 500µV=180nA). We report our experiment system, the manufacture method and the junction properties in this paper.

Harmonic and reactive behavior of the quasiparticle tunnel current in SIS junctions

In this paper, we show theoretically and experimentally that the reactive quasiparticle tunnel current of the superconductor tunnel junction could be directly measured at specific bias voltages for the higher harmonics of the quasiparticle tunnel current. We used the theory of quasiparticle tunneling to study the higher harmonics of the quasiparticle tunnel current in superconducting tunnel junction in the presence of rf irradiation. The impact of the reactive current on the harmonic behavior of the quasiparticle tunnel current was carefully studied by implementing a practical model with four parameters to model the dc I-V characteristics of the superconducting tunnel junction. The measured reactive current at the specific bias voltage is in good agreement with our theoretically calculated reactive current through the Kramers-Kronig transform. This study also shows that there is an excellent correspondence between the behavior of the predicted higher harmonics using the previously established theory of quasiparticle tunnel current in superconducting tunnel junctions by J.R. Tucker and M.J. Feldman and the measurements presented in this paper.

Optimized Nonuniform Nb SIS Parallel Josephson Junction Array Oscillators

We developed submillimeter superconducting oscillators using Josephson effects in the Nb-based superconductor–insulator–superconductor (SIS) parallel array. The radiation emitted from the array is detected using integrated SIS twin junctions. Despite the small junction number $(N=21)$ , the $(I{-}V)$ characteristics of the detector clearly exhibit photon-assisted quasiparticle steps when the array is biased upon its Josephson resonances, ranging from 370 to 500 GHz. The array has a moderate current density $(J_{c}\approx=6 \text{kA}/\text{cm}2)$ but provides an output power around 0.28 $\mu\text{W}$ , which would be sufficient in achieving sensitive heterodyne reception. The impedance array can be matched easier to microwave circuits than the usual long Josephson junctions. Furthermore, the parallel array approach can be also extended up to about 1.4 THz using Nb-based SIS junctions.

Superconducting terahertz receivers for space-borne and balloon-borne radio telescopes

A superconducting heterodyne receiver based on superconductor–insulator–superconductor (SIS) junctions and a bolometric receiver based on superconductor–insulator–normal metal (SIN) tunnel junctions developed at the Kotelnikov Institute is described. The noise temperature and noise equivalent power of such a receiver fit the requirements for the TELIS, BOOMERANG, OLIMPO, and LSPE aerostatborne radio telescopes, and the space-based SPICA and MILLIMETRON projects, among others. Applications of such receivers for medical diagnostics and remote control are described.

Multifrequency Seashell Slot Antenna With Cold-Electron Bolometers for Cosmology Space Missions

A novel type of the “seashell” multichroic polarized antenna is proposed for cosmology space missions. The polarized slot antennas are arranged in the compact form of a seashell with individual slots for each frequency and each polarization. Such an arrangement gives a unique opportunity for independent adjusting individual parameters of slots with microstrip lines (MSLs) and bolometers. For each frequency band, the seashell antenna contains two pairs of orthogonal slots for each polarization connected by MSLs with a bolometer in the middle for in-phase operation. To fit slots in the $\lambda/2$ area for the best beam shape, lumped capacitances in the form of the H-slot were introduced. Ellipticity of a beam was improved to the level of better than 1%. The seashell antenna gives a unique opportunity to select the needed bandwidth using two options. The first option is the frequency selection by resonance properties of slots with MSLs and resistive cold-electron bolometer (CEB). The second option is the frequency selection by resonant CEB with an internal nanofilter organized by a kinetic inductance of the NbN superconducting nanostrip and a capacitance of the nanoscale superconductor–insulator–normal tunnel junction.

Fabrication of superconducting tunnel junctions with embedded coil for applying magnetic field

We have proposed and demonstrated a superconducting tunnel junction (STJ) with an embedded coil for applying a magnetic field. The STJ was fabricated on the coil, which was embedded in a Si substrate. The coil in the Si substrate consists of superconducting microstrip lines and applies a magnetic field to the STJ to suppress the dc Josephson current. The embedded coil was designed with a line and space of 3µm and a thickness of 120nm. To planarize the coil, we employed chemical mechanical polishing (CMP) in our fabrication process. In this STJ, the maximum current of the embedded coil was 28mA, which corresponded to the maximum magnetic field of 11.76mT.

Superconducting tunnel junctions on MgB2 using MgO and CaF2 as a barrier

We report the fabrication of superconducting tunnel junctions, both of superconductor–insulator-normal metal (SIN) and superconductor–insulator-superconductor (SIS), on MgB2 using MgO and CaF2 as a barrier. The SIN junctions fabricated using an MgO barrier showed excellent quasi-particle characteristics, including a large superconducting gap (Δ) of 2.5–3meV and a low zero-bias conductance. We have also fabricated SIS junctions with an MgO barrier, but the quasi-particle characteristics of the SIS junctions are not as good as those of the SIN junctions, namely a reduced superconducting gap and a high zero-bias conductance. It appears that top MgB2 electrodes do not grow well on an MgO barrier, which is also suggested from in-situ RHEED observation. The SIN junctions fabricated using a CaF2 barrier showed less sharp quasi-particle characteristics than using an MgO barrier. However, the SIS junctions using a CaF2 barrier showed a fairly large IcRN value at 4.2K over 1mV and also exhibited finite Josephson current up to almost the film's Tc (∼30K). The RHEED observation revealed that top MgB2 electrodes grow well on a CaF2 barrier.

Electronically Tuned Local Oscillators for the NOEMA Interferometer

We present an overview of the electronically tuned local oscillator (LO) system developed at the Institut de RadioAstronomie millimetrique (IRAM) for the superconductor–insulator–superconductor (SIS) receivers of the NOrthern Extended Millimeter Array interferometer (NOEMA). We modified the frequency bands and extended the bandwidths of the LO designs developed by the National Radio Astronomy Observatory (NRAO) for the Atacama Large Millimeter Array (ALMA) project to cover the four NOEMA LO frequency ranges 82–108.3 GHz (Band 1), 138.6–171.3 GHz (Band 2), 207.7–264.4 GHz (Band 3), and 283–365 GHz (Band 4). The NOEMA LO system employs commercially available MMICs and GaAs millimeter MMICs from NRAO which are micro-assembled into active multiplied chain (AMC) and power amplifier (PA) modules. We discuss the problem of the LO spurious harmonics and of the LO signal directly multiplied by the SIS mixers that add extra noise and lead to detections of unwanted spectral lines from higher order sidebands. A waveguide filter in the LO path is used to reduce the higher order harmonics level of the LO at the output of the final frequency multiplier, thus mitigating the undesired effects and improving the system noise temperature.