Superconducting Tunnel Junctions Research Articles

Influence of transmission line parameters on the degree of matching of the generator with the SIS-mixer in the frequency range 200-700 GHz

This work is aimed at the development, research and optimization of superconducting integrated circuits designed to match the impedances of a long Josephson junction generator, the so-called flux generator (FFO) and a superconductor--insulator--superconductor (SIS) detector in the sub-terahertz frequency range. Numerical calculations of integrated circuits designed to optimize the parameters of transmission lines are presented. The main parameters of the lines and their influence on signal propagation are determined. The results of optimization of integrated matching circuits in the range of 450-700 GHz are demonstrated. Keywords: integrated matching circuits, Josephson junctions, sub-terahertz generator.

Structural and superconducting properties of low-temperature ultrathin PbBi<sub>3</sub> films

Bismuth (Bi), as a stable heaviest element in the periodic table of elements, has strong spin-orbit coupling, which has attracted a lot of attention as the parent material of various known topological insulators. Previous calculations predicted that Bi(111) with a thickness less than eight bilayers and the ultrathin black-phosphorus-like Bi(110) films are single-element two-dimensional (2D) topological insulators. However, it is generally believed that these crystalline bismuth phases are not superconducting or their transition temperature should be lower than 0.5 mK. Lead (Pb) is a good superconducting elementary material, and there is a relatively small difference in radius between the Bi atom and Pb atom. According to the Hume-Rothery rule, it is expected that Pb/Bi alloys in an arbitrary ratio should be superconducting. One may thus expect to form crystalline Bi based superconductors by Pb substitution, which might host intriguing topological superconductivity. While our previous work has demonstrated a low-temperature stable Pb<sub>1–<i>x</i></sub>Bi<sub><i>x</i></sub> (<i>x</i>~0.1) alloy phase in which Pb in the Pb(111) structure is partially replaced by Bi, the Bi crystalline structure-based phases of the superconducting alloys still lack in-depth research. Here, we report a new low-temperature phase of Pb-Bi alloy thin film, namely PbBi<sub>3</sub>, on the Si(111)-(7 × 7) substrate, by co-depositing Pb and Bi at a low temperature of about 100 K followed by an annealing treatment of 200 K for 2 h. Using low-temperature scanning tunneling microscopy and spectroscopy (STM/STS), we characterize <i>in situ</i> the surface structure and superconducting properties of the Pb-Bi alloy film with a nominal thickness of about 4.8 nm. Two spatially separated phases with quasi-tetragonal structure are observed in the surface of the Pb-Bi alloy film, which can be identified as the pure Bi(110) phase and the PbBi<sub>3</sub> phase, respectively, based on their distinct atomic structures, step heights and STS spectra. The PbBi<sub>3</sub> film has a base structure similar to Bi(110), where about 25% of the Bi atoms are replaced by Pb, and the surface shows a <inline-formula><tex-math id="M2">\begin{document}$\sqrt 2 \times \sqrt 2 R{45^ \circ }$\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="12-20220050_M2.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="12-20220050_M2.png"/></alternatives></inline-formula> reconstructed structure. The superconducting behavior of the PbBi<sub>3</sub> phase is characterized using variable-temperature STS spectra. We obtain that the superconducting transition temperature of PbBi<sub>3</sub> is about 6.13 K, and the <inline-formula><tex-math id="M3">\begin{document}$2\varDelta (0)/{k_{\text{B}}}{T_{\text{c}}}$\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="12-20220050_M3.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="12-20220050_M3.png"/></alternatives></inline-formula> ratio is about 4.62 using the fitting parameter of <inline-formula><tex-math id="M4">\begin{document}$\varDelta (0) = 1.22{\text{ meV}}$\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="12-20220050_M4.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="12-20220050_M4.png"/></alternatives></inline-formula> at 0 K. By measuring the magnetic field dependent superconducting coherence length, the critical field is estimated at larger than 0.92 T. We further investigate the superconducting proximity effect in the normal metal-superconductor (N-S) heterojunction consisting of the non-superconducting Bi(110) domain and the superconducting PbBi<sub>3</sub> domain. The N-S heterojunctions with both in-plane configuration and step-like configuration are measured, which suggest that the atomic connection and the area of the quasi-2D Josephson junctions and the external magnetic field can affect the lateral superconducting penetration length. We also observe the zero-bias conductance peaks (ZBCPs) in the superconducting gap of the PbBi<sub>3</sub> surface in some cases at zero magnetic field. By measuring d<i>I</i>/d<i>V</i> spectra at various temperatures and by adopting a superconducting Nb tip, we identify that the ZBCP originates from the superconductor-insulator-superconductor (S-I-S) junction formed between a superconducting tip and the sample. Nevertheless, the Bi(110)-based PbBi<sub>3</sub> phase may provide a possible platform to explore the intriguing topological superconducting behaviors at the vortexes under magnetic fields, or in the vicinity of the potentially topological superconducting Bi(110) islands by considering the proximity effect.

Critical Current Modulation in Josephson Junctions Contacted by Redundant Vias

We study the impact of redundant via placement on Fraunhofer interference patterns in superconductor–insulator–superconductor Josephson junctions with thin counter electrodes. By varying the number and location of contact vias to the counter electrode in otherwise equivalent junctions and studying the magnetic field dependence of the critical current, we observe a variety of unconventional features in the interference patterns, including a lifting of critical current minima and differences in node positions and side lobe amplitudes. We attribute these features to local changes in the Josephson phase gradient arising from the nonuniform magnetic thickness resulting from the vias. Our findings highlight the significant role that contact vias on thin counter electrodes play in the response of Josephson junctions to applied magnetic fields.

Proximity-effect-induced superconductivity in Bi2Te3/FeSe0.5Te0.5 superlattices

The topological-insulator/superconductor heterostructure is predicted to offer an opportunity to investigate the proximity effect in condensed matter systems. Here, we construct a well-ordered Bi2Te3/FeSe0.5Te0.5 superlattice structure on TiO2-SrTiO3 substrate by pulsed laser deposition, and investigate its superconducting property induced by the proximity effect. Based on the electrical transport measurements, Cooper pairs in superconducting FeSe0.5Te0.5 layers can be introduced into the surface states of Bi2Te3 layers, and so induce superconductivity of the entire Bi2Te3/FeSe0.5Te0.5 superlattice. The superconducting properties in the superlattices such as the superconducting critical temperature Tc, anisotropy γ, critical current density Jc, and flux pinning mechanism depend strongly on the thicknesses of FeSe0.5Te0.5 and Bi2Te3 layers. The thicker FeSe0.5Te0.5 layer and thinner Bi2Te3 layer are beneficial for the performance of superconductivity. Our experimental results provide a pathway to studying the mechanism of proximity effect at the interfaces between superconductor and topological insulator.

Fabrication of Superconducting Nb–AlN–NbN Tunnel Junctions Using Electron-Beam Lithography

Mixers based on superconductor–insulator–superconductor (SIS) tunnel junctions are the best input devices at frequencies from 0.1 to 1.2 THz. This is explained by both the extremely high nonlinearity of such elements and their extremely low intrinsic noise. Submicron tunnel junctions are necessary to realize the ultimate parameters of SIS receivers, which are used as standard devices on both ground and space radio telescopes around the world. The technology for manufacturing submicron Nb–AlN–NbN tunnel junctions using electron-beam lithography was developed and optimized. This article presents the results on the selection of the exposure dose, development time, and plasma chemical etching parameters to obtain high-quality junctions (the ratio of the resistances below and above the gap Rj/Rn). The use of a negative-resist ma-N 2400 with lower sensitivity and better contrast in comparison with a negative-resist UVN 2300-0.5 improved the reproducibility of the structure fabrication process. Submicron (area from 2.0 to 0.2 µm2) Nb–AlN–NbN tunnel junctions with high current densities and quality parameters Rj/Rn > 15 were fabricated. The spread of parameters of submicron tunnel structures across the substrate and the reproducibility of the cycle-to-cycle process of tunnel structure fabrication were measured.

Fabrication of NIS and SIS Nanojunctions with Aluminum Electrodes and Studies of Magnetic Field Influence on IV Curves

Samples of superconductor–insulator–superconductor (SIS) and normal metal–insulator–superconductor (NIS) junctions with superconducting aluminum of different thickness were fabricated and experimentally studied, starting from conventional shadow evaporation with a suspended resist bridge. We also developed alternative fabrication by magnetron sputtering with two-step direct e-beam patterning. We compared Al film grain size, surface roughness, resistivity deposited by thermal evaporation and magnetron sputtering. The best-quality NIS junctions with large superconducting electrodes approached a resistance R(0)/R(V2Δ) factor ratio of 1000 at 0.3 K and over 10,000 at 0.1 K. At 0.1 K, R(0) was determined completely by the Andreev current. The contribution of the single-electron current dominated at V > VΔ/2. The single-electron resistance extrapolated to V = 0 exceeded the resistance R(V2Δ) by 3 × 109. We measured the influence of the magnetic field on NIS junctions and described the mechanism of additional conductivity due to induced Abrikosov vortices. The modified shape of the SINIS bolometer IV curve was explained by Joule overheating via NIN (normal metal–insulator–normal metal) channels.

Estimating the Single-Element Concentration of Intercalated Insulators for the Emergence of Superconductivity.

To predict whether a compound will superconduct and to predict its transition temperature T c prior to measurement have always been desires of the materials science community. Matthias was first to report the necessary conditions for the occurrence of superconductivity in elements, compounds, and alloys in terms of density (valence electrons per atom). This current report is motivated by somewhat similar empirical observations concerning the importance of valence electrons per unit cell; more specifically, dopant valence electrons per unit cell within intercalated insulators. In this article, though not exhaustive, a representative list of 40 superconductors will be used to show that the onset of superconductivity (insulator-superconductor boundary) within intercalated insulators can easily be modeled, almost exactly, by the ideal gas law equation. Given this observation, in contrast to Matthias, interactions are semiclassically accounted for to ultimately determine the single-element onset concentration needed to bring about superconductivity within many intercalated insulators known to date. The 13 compounds which were previously intercalated and will be discussed include inorganics, TiSe2, C60, YBa2Cu3O6, IrTe2, Bi2Se3, MoS2, ZrNCl, HfNCl, BP (black phosphorus), HoTe3, and Y2Te5, and organics, C22H14 and C14H10. In essence, the overall objective of this report is to offer a slightly different viewpoint on superconductivity, led by empirical observations, which seemingly leads to predictable experimental outcomes. If newly discovered materials further validate this approach to intercalated superconductors, with minor refinements, a route to purposefully designing superconductors may be accessible through onset conditions outlined in this article.

Spontaneous symmetry breaking induced thermospin effect in superconducting tunnel junctions

We discuss the charge and the spin tunneling currents between two Bardeen-Cooper-Schrieffer (BCS) superconductors, where one density of states is spin-split. In the presence of a large temperature bias across the junction, we predict the generation of a spin-polarized thermoelectric current. This thermo-spin effect is the result of a spontaneous particle-hole symmetry breaking in the absence of a polarizing tunnel barrier. The two spin components, which move in opposite directions, generate a spin current larger than the purely polarized case when the thermo-active component dominates over the dissipative one.

THz Range Low-Noise SIS Receivers for Space and Ground-Based Radio Astronomy

We report on research in the field of low-noise receiving systems in the sub-terahertz (THz) range, carried out in recent years, aimed at developing receivers with quantum sensitivity for implementation in space and ground-based radio telescopes. Superconductor-Insulator-Superconductor (SIS) mixers based on high-quality tunnel junctions are the key elements of the most sensitive sub-THz heterodyne receivers. Motivations and physical background for technology improvement and optimization, as well as fabrication details, are described. This article presents the results of the SIS receiver developments for the 211–275 GHz and 790–950 GHz frequency ranges with a noise temperature in the double sideband (DSB) mode of approximTELY 20 K and 200 K, respectively. These designs and achievements are implemented in the development of the receiving systems for the Russian Space Agency mission “Millimetron”, and for the ground-based APEX (Atacama Pathfinder EXperiment) telescope.

Spectroscopic signatures of gate-controlled superconducting phases

We investigate the tunneling conductance of superconductor-insulator-normal metal (SIN) and superconductor-insulator-superconductor (SIS) heterostructures with one superconducting side of the junction that is electrically driven and can exhibit $\pi$-pairing through a modification of the surface inversion asymmetric couplings. In SIN tunneling we find that the variation of the electrically driven interactions generally brings an increase of quasi-particles in the gap due to orbitally polarized depaired states, irrespective of the inter-band phase rearrangement. The peak of SIN conductance at the gap edge varies with a trend that depends both on the strength of the surface interactions as well as on the character of the gate-induced superconducting state. While this shift can be also associated with thermal effects in the SIN configuration, for the SIS geometry at low temperature the electric field does not yield the characteristic matching peak at voltages related with the difference between the gaps of the superconducting electrodes. This observation sets out a distinctive mark for spectroscopically distinguishing the thermal population effects from the quantum gate-driven signatures. In SIS the electrostatic gating yields a variety of features with asymmetric peaks and broadening of the conductance spectral weight. These findings indicate general qualitative trends for both SIN and SIS tunneling spectroscopy which could serve to evaluate the impact of gate-control on superconductors and the occurrence of non-centrosymmetric orbital antiphase pairing.

Charge dynamics in quantum-circuit refrigeration: Thermalization and microwave gain

Previous studies of photon-assisted tunneling through normal-metal–insulator–superconductor junctions have exhibited potential for providing a convenient tool to control the dissipation of quantum-electric circuits in situ. However, the current literature on such a quantum-circuit refrigerator (QCR) does not present a detailed description for the charge dynamics of the tunneling processes or the phase coherence of the open quantum system. Here, we derive a master equation describing both quantum-electric and charge degrees of freedom, and discover that typical experimental parameters of low temperature and yet lower charging energy yield a separation of time scales for the charge and quantum dynamics. Consequently, the minor effect of the different charge states can be taken into account by averaging over the charge distribution. We also consider applying an ac voltage to the tunnel junction, which enables control of the decay rate of a superconducting qubit over four orders of magnitude by changing the drive amplitude; we find an order-of-magnitude drop in the qubit excitation in 40 ns and a residual reset infidelity below 10−4. Furthermore, for the normal island, we consider the case of charging energy and single-particle level spacing large compared to the superconducting gap, i.e., a quantum dot. Although the decay rates arising from such a dot QCR appear low for use in qubit reset, the device can provide effective negative damping (gain) to the coupled microwave resonator. The Fano factor of such a millikelvin microwave source may be smaller than unity, with the latter value being reached close to the maximum attainable power.

Fast unconditional initialization for superconducting qubit and resonator using quantum-circuit refrigerator

In a quantum computation scheme, such as recycling a qubit, the initialization of the qubit is always required. At present, the initialization of the qubit is one of the bottlenecks in such computation, and high fidelity and fast initialization still remain important research subjects. In this study, by coupling a SINIS (superconductor–insulator–normal metal–insulator–superconductor) junction to a coupled system consisting of a qubit and a resonator, a photon is removed from the resonator by single-photon-assisted tunneling, and the effective relaxation rate of the resonator is increased. In addition, by applying an |e,0⟩−|f,0⟩ drive pulse and an |f,0⟩−|g,1⟩ drive pulse to the qubit, the energy of the qubit is rapidly transferred to the resonator to realize high-speed initialization. The present simulation shows that the qubit can be initialized with 99% fidelity in 80 ns and 99.9% in 137 ns when the electron temperature of the normal metal of the SINIS junction is 10 mK. The simulation also shows that the relaxation time of the qubit is about 13% longer when the resonator is interposed between the SINIS junction and the qubit than when the SINIS junction is directly coupled to the qubit.

Statistical detection of Josephson, Andreev, and single quasiparticle currents in scanning tunneling microscopy

We present a method to identify distinct tunneling modes in a tunable superconducting tunnel junction composed of a superconducting tip and a sample in a scanning tunneling microscope. Combining the relative decay constant of tunneling current extracted from $I\ensuremath{-}V\ensuremath{-}z$ spectroscopy with its statistical analysis over the atomic disorders in the sample surface, we identified the crossover of dominant tunneling modes between single charge tunneling, Andreev reflection (AR), and Josephson tunneling with respect to the bias voltage at a measurement temperature nearly half of the critical temperature. The method enables one to determine the specific tunneling regime independently of the spectral shapes and to reveal intrinsic modulation of AR and Josephson current by disorder that will be crucial for superconducting quantum information processing.

Superconducting Sub-THz Receivers for Space and Ground-Based Radio Astronomy

The developments in the field of low-noise receiving systems in the sub-THz range, performed at the Kotelnikov Institute of Radio Engineering of the Russian Academy of Sciences in recent years and aimed at the creation of receivers with quantum sensitivity for space and ground-based radio telescopes are presented. Superconductor–insulator–superconductor (SIS) mixers based on high-quality tunnel junctions are key elements of the most sensitive sub-THz heterodyne receivers. The article presents the results of the development of SIS receivers in the 211–275 GHz and 800–950 GHz ranges with a noise temperature in the double-sideband (DSB) mode of about 20 K and 220 K, respectively. These achievements will be used to develop receiving systems for APEX and LLMA ground-based telescopes, as well as for the Millimetron space mission.

Tunneling and break junction spectroscopy of the ambient-pressure semiconducting and superconducting gap structures in the ladder compound (Sr, Ca)14Cu24O41

The ladder-chain compound ${(\mathrm{Sr}, \mathrm{Ca})}_{14}{\mathrm{Cu}}_{24}{\mathrm{O}}_{41}$ is a semiconductor at ambient pressure, but becomes a bulk superconductor above the pressure of about 3 GPa. Since the compound is at the verge of the metal-insulator transition, it is reasonable to make tunnel measurements to probe the electronic density of states in its subtleties. We present the results of such measurements carried out at ambient pressure. The break junction (BJ) tunneling gives evidence for the apparent typical gap $2\mathrm{\ensuremath{\Sigma}}$ of about 140 meV at temperature $T$, equal to 4 K. The gap is smeared out at ${T}^{*}\ensuremath{\approx}90--100$ K. We interpret the gap as that induced by the charge density wave (CDW) formation, although its $T$-dependence differs from the usually observed CDW-like mean-field behavior. Quite unexpectedly, BJ spectra also exhibit distinct zero-bias peak accompanied by the low-energy gaps of $2\mathrm{\ensuremath{\Delta}}\ensuremath{\approx}4--8$ meV immediately after BJ are formed at 4 K. The thermally driven disappearance of this apparently superconducting structure at ${T}_{c}\ensuremath{\approx}7--13$ K is consistent with the conventional properties of superconducting tunnel junctions. The resultant ratio $\mathrm{\ensuremath{\Delta}}/{T}_{c}$ is consistent with similar observed values for a high-${T}_{c}$ cuprate superconductor. Therefore, we attribute this feature as well as the Josephson-like zero-bias conductance peak to the superconductivity induced at the freshly created BJ surface of the ${(\mathrm{Sr}, \mathrm{Ca})}_{14}{\mathrm{Cu}}_{24}{\mathrm{O}}_{41}$, which is semiconducting in the sample bulk. Our additional scanning tunneling microscopy data testify that the cracked atomic surface of this compound is substantially modified, which might be the reason for the superconductivity appearance.

Josephson Effect in MgB2/Pd/Nb Trilayer Josephson Junctions

This paper reports fabrication techniques and results of MgB2/Pd/Nb trilayer Josephson junctions. The MgB2 bottom electrode was co-evaporated by molecular beam epitaxy (MBE) technique from both magnesium and boron sources at a low substrate temperature ~ 300 °C, while the interlayer and the top niobium electrode (Pd/Nb bilayer) were deposited ex-situ using RF sputtering. The junctions exhibited and Josephson effect as well as a modulation of the critical current in a magnetic field applied in a direction normal to the junction plane. Fractional and integer Shapiro steps were observed at voltages corresponding to the frequency of the applied microwave radiation field. The products of the junctions compare well with the previously reported values. The results suggest that it should be possible to fabricate all-MgB2 and MgB2 as one of the electrodes Superconductor/Normal/Superconductor (SNS), Superconductor/Insulator/Superconductor (SIS) or even Superconductor/Ferromagnet/Superconductor (SFS) tunnel junctions with interesting characteristics and for various applications.
 Keywords: MgB2; all-MgB2; Josephson Tunnel junctions; trilayer devices; Niobium

Nonlocal superconducting quantum interference device

Superconducting quantum interference devices (SQUIDs) that incorporate two superconductor/insulator/superconductor (SIS) Josephson junctions in a closed loop form the core of some of the most sensitive detectors of magnetic and electric fields currently available. SQUIDs in these applications are typically operated with a finite voltage which generates microwave radiation through the ac Josephson effect. This radiation may impact the system being measured. We describe here a SQUID in which the Josephson junctions are formed from strips of normal metal (N) in good electrical contact with the superconductor (S). Such SNS SQUIDs can be operated under a finite voltage bias with performance comparable or potentially better than conventional SIS SQUIDs. However, they also permit a novel mode of operation that is based on the unusual interplay of quasiparticle currents and supercurrents in the normal metal of the Josephson junction. The new method allows measurements of the flux dependence of the critical current of the SNS SQUID without applying a finite voltage bias across the SNS junction, enabling sensitive flux detection without generating microwave radiation.

Cascaded superconducting junction refrigerators: Optimization and performance limits

We demonstrate highly transparent silicon–vanadium and silicon–aluminum tunnel junctions with relatively low sub-gap leakage current and discuss how a trade-off typically encountered between transparency and leakage affects their refrigeration performance. We theoretically investigate cascaded superconducting tunnel junction refrigerators with two or more refrigeration stages. In particular, we develop an approximate method that takes into account self-heating effects but still allows us to optimize the cascade a single stage at a time. We design a cascade consisting of energy-efficient refrigeration stages, which makes cooling of, e.g., quantum devices from above 1 K to below 100 mK a realistic experimental target.

Normal Metal-Insulator-Superconductor Tunnel Junctions With Pulsed Laser Deposited Titanium Nitride as Superconductor

Here we report the fabrication of normal metal - insulator - superconductor (NIS) tunnel junctions using superconducting titanium nitride grown by pulsed laser deposition (PLD). The films for NIS junction fabrication were deposited on two different substrates: silicon nitride film and magnesium oxide. TiN films were characterized by means of electrical transport measurements, and films with superconducting transition temperatures above the liquid helium boiling point were chosen for fabrication of NIS junctions. Tunnel junction devices were successfully fabricated using electron beam lithography and shadow evaporation techniques. The insulator layer formation was performed using two different approaches: the tunnel barrier was either formed by direct oxidation of TiN, or by fabrication of an additional aluminum oxide layer. Devices fabricated by direct oxidation showed much more transparent barriers and slightly higher subgap currents, but both types of devices could be used for thermometry. Further optimization of the direct oxidation process may allow electric cooling applications in the future.

Atomic Structure of Superconducting Tunnel Junctions using STEM and APT

Atomic Structure of Superconducting Tunnel Junctions using STEM and APT