Superconducting Tunnel Junctions Research Articles

Tunneling spectroscopy of c -axis epitaxial cuprate junctions

Atomically precise epitaxial structures are unique systems for tunneling spectroscopy that minimize extrinsic effects of disorder. We present a systematic tunneling spectroscopy study, over a broad doping, temperature, and bias range, in epitaxial c-axis La$_{2-x}$Sr$_{x}$CuO$_{4}$/La$_{2}$CuO$_{4}$/La$_{2-x}$Sr$_{x}$CuO$_{4}$ heterostructures. The behavior of these superconductor/insulator/superconductor (SIS) devices is unusual. Down to 20 mK there is complete suppression of c-axis Josephson critical current with a barrier of only 2 nm of La$_{2}$CuO$_{4}$, and the zero-bias conductance remains at 20-30% of the normal-state conductance, implying a substantial population of in-gap states. Tunneling spectra show greatly suppressed coherence peaks. As the temperature is raised, the superconducting gap fills in rather than closing at $T_{c}$. For all doping levels, the spectra show an inelastic tunneling feature at $\sim$ 80 meV, suppressed as $T$ exceeds $T_{c}$. These nominally simple epitaxial cuprate junctions deviate markedly from expectations based on the standard Bardeen-Cooper-Schrieffer (BCS) theory.

A Compact Model for Superconductor- Insulator-Superconductor (SIS) Josephson Junctions

We present a Verilog-A based compact model for the superconductor-insulator-superconductor (SIS) Josephson junction. The model can generate both hysteretic and non-hysteretic current-voltage ( I-V ) response for the SIS junctions utilizing the Stewart-McCumber parameter. We calibrate our model with different SIS samples and demonstrate accurate matching between the simulated and experimental results. We implement temperature effect on the energy gap and the critical current of the superconductor to explore the dynamic trends in device characteristics. We calculate the junction inductance and energy as functions of junction current and temperature. We simulate the read/write operations of an SIS junction based cryogenic memory cell to illustrate the usability of our model.

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.

Very large thermal rectification in ferromagnetic insulator-based superconducting tunnel junctions

We investigate electronic thermal rectification in ferromagnetic insulator-based superconducting tunnel junctions. Ferromagnetic insulators coupled to superconductors are known to induce sizable spin splitting in the superconducting density of states and also lead to efficient spin filtering if used as tunnel barriers. The combination of spin splitting and spin filtering is shown to yield a substantial amount of self-amplification of the electronic heat diode effect due to breaking of the electron-hole symmetry in the system, which is added to the thermal asymmetry of the junction. Large spin splitting and large spin polarization (≳90%) can potentially lead to thermal rectification efficiencies exceeding ∼5×104% for realistic parameters in a suitable temperature range, thereby outperforming up to a factor of ∼250, the heat diode effect achievable with conventional superconducting tunnel junctions. These results are relevant for improved control of heat currents in innovative phase-coherent caloritronic nanodevices and for enhanced thermal management of quantum circuits at the nanoscale.

Characterization of Amplification Properties of the Superconducting-Ferromagnetic Transistor

We report on the measurement results of the superconducting-ferromagnetic transistors (SFTs). The devices were made at Northwestern University and Hypres (SeeQC), Inc. (Nevirkovets et al., 2014; 2015). SFT is a multiterminal device with the SIS'FIFS (or SFIFS'IS) structure (where S, I, and F denote a superconductor, an insulator, and a ferromagnetic material, respectively) exploiting intense quasiparticle injection in order to modify the nonlinear I-V curve of a superconducting tunnel junction. SFT is capable of providing voltage, current, and power amplification while having good input/output isolation. We characterized the devices using different measurement techniques. We measured S parameters of the single- and double-acceptor devices at frequencies up to 5 MHz. Importantly, we confirmed that the isolation between the input and output of the device is quite good. However, the techniques typically employed to characterize semiconductor devices do not allow for revealing the full potential of our low-resistive SFT devices, especially those having two acceptors. In the latter case, we also tested the devices using the battery-powered current sources with floating grounds. Analyzing double-acceptor I-V curves recorded at different levels of injection currents, for an optimal load, we deduced a small-signal voltage gain of 33 and a power gain of 2.4. We suggest that further improvement of the SFT device parameters is possible in optimized devices, so that the device potentially may serve as a preamplifier for readout of output signals of cryogenic detectors and be useful as an element of other superconductor-based circuits. In addition, we used scanning transmission electron microscopy to identify some problems in the fabrication of the devices without any planarization.

A Nonlinear Transmission Line Model for Simulating Distributed SIS Frequency Multipliers

Superconductor/insulator/superconductor (SIS) jun-ctions have extremely nonlinear electrical properties, which makes them ideal for a variety of applications, including heterodyne mixing and frequency multiplication. With SIS mixers, the SIS junctions normally have circular cross sections, but they can also be fabricated in the form of microstrip transmission lines, known as distributed SIS junctions (DSJs). By using a DSJ as an open-circuit stub, it is possible to create a large SIS junction with a low effective input reactance. This is beneficial for SIS frequency multipliers because their output power is proportional to the area of the junction. It is challenging, however, to simulate the behavior of DSJs because they have to be modeled as transmission lines and the model has to take into account the quasiparticle tunneling current, which is a nonlinear function of the ac voltage. In this article, we present a new nonlinear transmission line model to accurately describe the behavior of DSJs and to simulate the performance of distributed SIS frequency multipliers (DSMs). This model is compared to experimental data from a recent DSM device and good agreement is found between the dc tunneling currents and the output powers at the second harmonic. Based on this success, an improved DSM design is proposed that has a higher output power and a higher conversion efficiency than previous designs.

Identifying the graphene p-wave superconducting symmetry by conductance and shot noise oscillations

Herein, the quantum transport in a graphene-based normal layer/insulator/p-wave superconductor tunnel junctions is studied by an extended Blonder-Tinkham-Klapwijk theory. It is shown that at the insulating region width being finite, the conductance and shot noise spectra exhibit oscillations as a function of applied bias voltage, which is a striking novel feature and absent in thin-barrier limit. Particularly, the oscillatory behaviors between the px-, py-, and px+ipy-wave pairing symmetries are significantly different, stemming from the different types of zero-energy bound states. These features could not only be experimentally used to identify the p-wave superconductivity of graphene but also pave the way to the future preparation of graphene p-wave superconducting devices.

From quantized local Andreev reflection to perfect crossed Andreev reflection in topological insulator–superconductor hybrid systems

We systemically investigate the quantized local Andreev reflection (LAR) and crossed Andreev reflection (CAR) in four-terminal topological insulator--superconductor hybrid systems based on the Kane-Mele model and Haldane model corresponding to the quantum spin Hall (QSH) and quantum anomalous Hall (QAH) systems. We find that although both are driven by edge states, the LAR and CAR behave in different ways. The essential difference between the LAR and CAR is the coherent phases of incoming electrons and outgoing holes in the transport processes. For the LAR, the total phase remains zero, and quantum interference is always constructive. However, for the CAR, the coherent phase varies with system parameters. Fortunately, we find a weak magnetic field can smoothly modify the edge states and then the interference phase; finally, a perfect quantized CAR is realized in the QAH-superconductor hybrid systems. In addition, the quantized LAR is more robust against random disorder because the coherent phases carried by electrons are completely canceled out by holes.

Fabrication of Planar-Integrated SIS Mixer Circuits with Improved Uniformity and Yield

Single-pixel prototype superconductor–insulator–superconductor (SIS) mixer integrated circuits (ICs) for multi-beam heterodyne receivers were fabricated. We introduced plasma-enhanced chemical vapor deposition (PE-CVD) for insulator layer deposition and machine-aligned via-hole etching for contact-hole definition on SIS junctions to achieve high uniformity and yield. In the PE-CVD, we applied a compressive/tensile/compressive SiO2 trilayer technique to control the film stress. The SiO2 trilayer stress was stable and negligibly low. The uniformity and junction quality yield of the single-pixel prototype SIS mixer ICs were improved in the process applying the PE-CVD and the via-hole etching.

On-Wafer Cryogenic Characterization Technique of an SIS-Based Frequency Up and Down Converter

This paper describes an on-wafer characterization system and calibration technique for frequency up and down converters based on a superconductor–insulator–superconductor (SIS) tunnel junction. The measurement system uses a 4-K probe station in combination with a vector network analyzer which allows measurement of both up- and down-conversion gains and reflection coefficients. We employed and verified a scalar mixer calibration technique for accurate characterization of the conversion properties. We present the detailed measurement technique and verification using an SIS frequency converter sample.

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.

Non-linearities in Superconducting Tunnel Junction Radiation Detectors and Their MCA Readout

The response of cryogenic high-resolution detectors to a short-pulse laser consists of a Poisson-distributed set of equidistant peaks that correspond to integer numbers of absorbed photons. Since the laser has a negligible intrinsic line width, the peaks can be used for detailed characterization of both the detector and the data acquisition system. We have characterized our superconducting tunnel junction (STJ) photon detectors in the UV and soft X-ray range with a pulsed 355-nm laser at rates up to 5000 counts/s. The observed peaks are described by a Gaussian to very high accuracy, with a width between ~ 1 and ~ 3 eV FWHM depending on the detector area and the absorbed energy. For high statistics, centroids can be determined with a precision of order 1 meV over an energy range of several 100 eV. This allows identifying and correcting for non-linearities in the digitizer that can otherwise limit the calibration accuracy.

Small capacitance self-shunted MoRe–Si(W)–MoRe junctions for SQUIDs applications

MoRe–Si(W)–MoRe planar Josephson junctions with a hybrid barrier layer made of amorphous silicon doped with tungsten at relatively high tungsten concentrations (~ 11%) are experimentally studied. Small intrinsic (natural) capacitance and shunting by tungsten nanoclusters give an advantage to MoRe–Si(W)–MoRe junctions against traditional superconductor–insulator–superconductor (SIS) planar junctions as candidates for innovative superconducting electronics. It is shown that the use of such junctions with a Si(W) barrier layer thickness of 15–30 nm can substantially enhance the sensitivity of both RF and DC SQUIDs.

Low-Loss, Thermally Insulating, and Flexible Rectangular Dielectric Waveguide for Sub-THz—Signal Coupling in Superconducting Receivers

Dielectric waveguides have potential as transmission lines in subterahertz (sub-THz) applications due to their low-loss. A new application of a rectangular dielectric waveguide (RDW) for a THz superconducting receiver system for radio astronomy is investigated and demonstrated for the first time. The RDW can be used to transmit sub-THz local oscillator (LO) signals into the cryostat for driving the superconductor insulator superconductor mixer, with very low heat transfer. A series of RDWs with different lengths are fabricated. The measured average attenuation constants of the RDW are 0.034 and 0.069 dB/mm at 3.4 and 300 K measurement environments over 240–300 GHz, respectively. The double-sideband system noise temperature of the receiver system is also measured. With this LO signal feeding system, there is only 0.69 mW thermal transfer power from the 300 to the 3.4 K cold stage. Compared to traditional pumping signal feeding schemes, such as the horn-to-horn structure and beam splitter method, this approach outperforms the traditional schemes in terms of feeding flexibility, insertion loss, and thermal isolation for the cryostat. The proposed RDW can be widely applied to superconducting receivers for radio astronomy in the future.

Chiral Majorana fermion

The chiral Majorana fermion, is a massless fermionic particle being its own antiparticle, which was predicted to live in (1+1)D (i.e. one-dimensional space plus one-dimensional time) or (9+1)D. In condensed matter physics, one-dimensional (1D) chiral Majorana fermion can be viewed as the 1/2 of the chiral Dirac fermion, which could arise as the quasiparticle edge state of a two-dimensional (2D) topological state of matter. The appearance of an odd number of 1D chiral Majorana fermions on the edge implies that there exist the non-Abelian defects in the bulk. The chiral Majorana fermion edge state can be used to realize the non-Abelian quantum gate operations on electron states. Starting with the topological states in 2D, we illustrate the general and intimate relation between chiral topological superconductor and quantum anomalous Hall insulator, which leads to the theoretical prediction of the chiral Majorana fermion from the quantum anomalous Hall plateau transition in proximity to a conventional s-wave superconductor. We show that the propagation of chiral Majorana fermions leads to the same unitary transformation as that in the braiding of Majorana zero modes, and may be used for the topological quantum computation.

Superconductivity in topological materials

In recent years, by introducing topological invariants into condensed matter systems, new phases of mater are revealed. Of these new phases, the topological insulator, topological semimetal and topological superconductor are the most important. They are called topological materials due to nontrivial topological parameters. Topological superconductors hold Majorana zero modes at the edges, satisfying non-abelian statistics, which makes them major candidate for realizing topological quantum computation. Besides exploring intrinsic topological superconductor, a promising way to realize topological superconductor is to induce superconductivity into other kinds of topological materials. Up to now, experimentalists have developed some techniques, such as gating, doping, high pressure, interface effect and hard point contact to introduce superconductivity into various topological materials, and also they have studied the topological properties of the induced superconductivity. In this review, we summarize the representative researches on intrinsic topological superconductor candidates and induced superconductivities in topological insulators and semimetals. The advantages and disadvantages of different techniques are discussed. Besides, the potential evidences of topological superconductors are analyzed. In the end, the outlook of this actively pursued research field is given.

Matching of Radiation with Array of Planar Antennas with SINIS Bolometers in an Integrating Cavity

Abstract—Arrays of planar annular antennas in the 345 GHz range with integrated superconductor–insulator–normal metal–insulator–superconductor (SINIS) bolometers have been developed, fabricated, and experimentally investigated. To increase the absorption efficiency, the source signal was matched with the array using a back-to-back horn and a counter-reflector. The efficiency of the receiving structure was studied depending on the direction of irradiation—through the substrate and from the antenna—as well as on the thickness of the substrate. The methods for normalizing a received signal using a reference channel outside and inside a cryostat is described. The best results were obtained by irradiating the array from the antenna with a substrate thickness equivalent to a quarter wavelength in the dielectric and sputtering of the rear side with a gold film (counterreflector). The effective matching band in the main range was more than 50 GHz; the volt–watt sensitivity reaches 109 V/W.

Vortex penetration field measurement system based on third-harmonic method for superconducting RF materials

We develop a vortex penetration field (Hv) measurement system using the third-harmonic response of an applied AC magnetic field from a solenoid coil positioned above a superconducting sample. The parameter Hv is measured via detection of the third-harmonic component, which drastically changes when a vortex begins to penetrate the superconductor with temperature increase. The magnetic field locally applied to one side of the sample mimics the magnetic field within superconducting radio-frequency (SRF) cavities and prevents edge effects of the superconducting sample. With this approach, our measurement system can potentially characterize surface-engineered SRF materials such as Superconductor–Insulator–Superconductor multilayer structure (S–I–S structure). As a validation test, we measure the temperature dependence of Hv of two high-RRR bulk Nb samples and obtain results consistent with the literature. We also confirm that our system can apply magnetic fields of at least 120 mT at 4–5 K without any problem of heat generation of the coil. The field applied is higher than those reported in previous works, allowing for a more accurate estimation of Hv at lower temperatures.

Low-Noise Sis Receivers for New Radio-Astronomy Projects

We have developed, manufactured, and tested a waveguide mixer in the range 211–275 GHz on the basis of the superconductor–insulator–superconductor (SIS) tunnel structures. The methods of manufacturing high-quality tunnel structures on quartz substrates have been worked out. To extend the receiver band, the Nb/AlOx/Nb and Nb/AlN/NbN tunnel junctions with a high current density of up to 20 kA/cm2 are employed. The dependence of the characteristics of the receiving elements on the signal frequency is simulated for the intermediate-frequency band 4–12 GHz. The measurements demonstrate a good agreement of the input band of the receiving structures with the calculated results. The uncorrected noise temperature of the receiver amounts to 24 K at a frequency of 265 GHz, which is only two times higher than the quantum limit. The receivers under development are intended for a number of newly-built ground-based radio telescopes (“Suffa” and LLAMA), as well as for the “Millimetron” space program.

Platform of chiral Majorana edge modes and its quantum transport phenomena

Majorana fermions, as electronic quasi-particle modes in solid states, have been under the focus of research due to their exotic physical properties. While the evidence of Majorana fermions as zero-dimensional bound states has been well established, the existence of one-dimensional Majorana modes is still under debate. The main reason is that the current theoretical proposals of platforms supporting such states are very challenging experimentally. Here, we propose a method to create two-dimensional topological superconductors with a heterostructure of ferromagnet, topological insulator thin film and superconductor. We show that such a system supports one-dimensional chiral Majorana edge modes in a wide range of parameters which is readily achievable in experiments. We further propose a new transport measurement to detect these modes.