Metal Insulator Superconductor Research Articles

Metal–insulator–superconductor transition in nickelate-based heterostructures driven by topotactic reduction

The discovery of superconductivity in doped infinite-layer nickelates has attracted great interest recently. Here, a metal–insulator–superconductor transition is demonstrated by engineering the process of topotactic reduction. By employing topotactic reduction, a superconducting Nd0.8Sr0.2NiO2 layer is obtained from high-quality Nd0.8Sr0.2NiO3, which is characterized by layer-by-layer growth and low room-temperature resistivity, rather than the low-quality Nd0.8Sr0.2NiO3 with mixed phases. Moreover, an insulating intermediate state is uncovered within the transition from metallic Nd0.8Sr0.2NiO3 to superconducting Nd0.8Sr0.2NiO2, corresponding to a non-monotonic modulation of resistivity driven by topotactic reduction. In the incompletely reduced Nd0.8Sr0.2NiO2+δ, residual oxygen atoms in the Nd/Sr plane disrupt the long-range order of the infinite-layer structure, resulting in suppressed superconductivity with a low transition temperature and non-zero residual resistivity. On the other hand, the superconductivity is optimized in the fully reduced sample, where a sign change in the Hall coefficient is always observed at low temperatures. Our study highlights the diverse electronic states achievable by controlling topotactic reduction, providing valuable insight into the understanding and manipulating of superconductivity in infinite-layer nickelates.

The inverse proximity effect in tunnel heterostructure ferromagnetic–insulator–superconductor Co2CrAl–I–Pb

Ferromagnetic–insulator–superconductor Co2CrAl–I–Pb tunnel junctions were created and their current-voltage characteristics (CVCs) were studied. It has been established that they differ from the usual CVCs of tunnel junctions of a normal metal–insulator–superconductor. It is proposed to assume that the observed effect of changing the type of CVC is associated with the existence of the destruction of Cooper pairs due to the inverse effect of proximity between the Co2CrAl and Pb electrodes. The CVCs were calculated based on the approach of P. Fulde.

Electrical and thermal transport through NIS junction

We investigate the electrical and thermal transport properties of the based normal metal–insulator–superconductor (NIS) junction using Blonder–Tinkham–Klapwijk theory. We show that the tunneling conductance of the NIS junction is an oscillatory function of the effective barrier potential (χ) of the insulating region up to a thin barrier limit. The periodicity and the amplitudes of the oscillations largely depend on the values of α and the gate voltage of the superconducting region, namely, U 0. Further, the periodicity of the oscillation changes from π to as we increase U 0. To assess the thermoelectric performance of such a junction, we have computed the Seebeck coefficient, the thermoelectric figure of merit, maximum power output, efficiency at the maximum output power of the system, and the thermoelectric cooling of the NIS junction as a self-cooling device. Our results on the thermoelectric cooling indicate practical realizability and usefulness for using our system as efficient cooling detectors, sensors, etc and hence could be crucial to the experimental success of the thermoelectric applications of such junction devices. Furthermore, for an lattice, whose limiting cases denote a graphene or a dice lattice, it is interesting to ascertain which one is more suitable as a thermoelectric device and the answer seems to depend on the U 0. We observe that for an lattice corresponding to , graphene (α = 0) is more feasible for constructing a thermoelectric device, whereas for , the dice lattice (α = 1) has a larger utility.

Microwave SINIS Detectors

This review presents the main characteristics and mechanisms of operation of superconductor–insulator–normal metal–insulator–superconductor (SINIS) microwave detectors. An analysis of the detectors’ performance against a quantum detector and a photon counter is given. Methods for cooling a superconductor using normal metal traps and the role of electron cooling in optimizing the current response to terahertz radiation are discussed. Fabrication methods using shadow evaporation as well as magnetron sputtering are described.

Fast Variable-Temperature Cryogenic Blackbody Sources for Calibration of THz Superconducting Receivers

An electrically heated blackbody radiation source comprising thin metal film on a dielectric substrate and an integrating cavity was designed, fabricated, and experimentally studied at frequencies from 75 to 500 GHz. Analytical and numerical modeling were performed to optimize the emissivity, spectral uniformity, and modulation frequency of the radiation source with the spherical integrating cavity and thin film absorber. The blackbody emissivity (absorptivity) increased from 0.3 to 0.5 for the bare thin film on dielectric substrate, and up to 0.95 when it was placed inside the integrating cavity. The fabricated source mounted at the 0.5 K stage was used to measure the response time of a few microseconds and for sensitivity measurement down to 10−18 W/Hz1/2 of the superconductor–insulator–normal metal–insulator–superconductor (SINIS) detector at 100 mK.

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.

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.

Superconducting tunnel junction fabrication on three-dimensional topography based on direct laser writing

Superconducting junctions are widely used in a multitude of applications ranging from quantum information science and sensing to solid-state cooling. Traditionally, such devices must be fabricated on flat substrates using standard lithographic techniques. In this study, we demonstrate a highly versatile method that allows for superconducting junctions to be fabricated on a more complex topography. It is based on maskless direct laser writing and two-photon lithography, which allows writing in 3D space. We show that high-quality normal metal–insulator–superconductor tunnel junctions can be fabricated on top of a 20-μm-tall three-dimensional topography. Combined with conformal resist coating methods, this technique could allow sub-micron device fabrication on almost any type of topography in the future.

Superconducting Receivers for Space, Balloon, and Ground-Based Sub-Terahertz Radio Telescopes

We give a review of both our own original scientific results of the development of superconducting receivers for sub-terahertz astronomy and the main leading concepts of the global instrumentation. The analysis of current astronomical problems, the results of microwave astroclimate research, and the development of equipment for sub-terahertz radio astronomy studies justify the need and feasibility of a major infrastructure project in Russia to create a sub-terahertz telescope, as well as to enhance the implementation of the ongoing Millimetron and Suffa projects. The following results are discussed: i) superconducting coherent receivers and broadband subterahertz detectors for space, balloon, and ground-based radio telescopes have been developed and tested; ii) ultrasensitive receiving systems based on tunnel structures such as superconductor—insulator—superconductor (SIS) and superconductor—insulator—normal metal—insulator—superconductor (SINIS) have been created, fabricated, and examined; iii) a receiving array based on SINIS detectors and microwave readout system for such structures has been implemented; iv) methods for manufacturing high-quality tunnel structures Nb/AlOx/Nb and Nb/AlN/NbN based on niobium films with a current density of up to 30 kA/cm2 have been developed. Receivers operated at 200 to 950 GHz and having a noise temperature only a factor of 2 to 5 higher than the quantum limit have been created and tested.

SINIS Bolometer with Microwave Readout

A bolometer, which is based on a superconductor–insulator–normal metal–insulator–superconductor (SINIS) structure, integrated into a twin-slot antenna with a central frequency of 90 GHz, and connected with a superconducting microwave readout resonator, has been designed, fabricated, and experimentally studied. Such an elementary cell is designed for the multi-element array of a high-sensitive radioastronomic receiver, in which the readout from a great number of channels is performed by a single coaxial cable instead of separate wires and amplifier in each channel.

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.

Tunable refrigeration properties of nano-scale Rashba coupled junction devices

We propose a tunable nano-scale thermoelectric cooling device based on a metal - insulator - superconductor (MIS) junction at ultra-low temperatures with Rashba spin orbit coupling (RSOC) being present at the interfaces of the junctions. The operational range of temperature of this junction device is set by the transition temperature (Tc) of the superconducting lead. We have investigated the working principles and performance of this nano-scale junction as a cooling device using a modified version of well-known Blonder-Tinkham-Klapwijk (BTK) theory. It is observed that the refrigeration property of this device is restricted to a very narrow range of driving voltage that biases the junction and the value of which lies in the vicinity of the superconducting order parameter. So a careful choice of the superconducting lead may provide a possibility of controlling the operating bias voltage. Since the electronic charge current and also the thermal current being strongly sensitive to the strength of RSOC, a further tunability of this junction device with regard to its thermoelectric cooling properties can be achieved by experimentally achievable techniques that can manipulate the value of the Rashba coupling strength. We have numerically computed the thermal current and coefficient of performance through this junction and investigated the role of the RSOC parameter and the effective barrier potential pertaining to the insulating regime, where we have obtained a very interesting interplay between the effective barrier potential and the strength of the RSOC on the behaviour of the thermoelectric cooling and the performance coefficient of the junction. Thus this device may be suitable for tunable nano-refrigeration properties, where the thermoelectric cooling and the coefficient of performance can be manipulated by tuning the RSOC parameter and the effective barrier potential of the insulating region. Our results indicate the practical usefulness of tunability of RSOC and effective barrier potential for efficiently cooling detectors, sensors, and quantum devices and this could be crucial to experimental success of the refrigeration applications of such junction devices.

Investigation of the Speed of a SINIS Bolometer at a Frequency of 350 GHz

We have measured the time of the optical response of a 350-GHz radiation detector based on a superconductor–insulator–normal metal–insulator–superconductor tunnel structure with a suspended normal-metal bridge integrated into a planar log-periodic antenna. The transient characteristics of the detector were recorded when irradiated by a fast cryogenic blackbody source with a rise time of the order of microseconds. For this purpose, a short intense heating pulse was fed to an emitting NiCr film radiation source with a low heat capacity. The measured response time was 1.8 ± 0.5 μs at a bolometer electron temperature of 0.17 K, with a detection sensitivity of 10–17–10–18 W Hz–1/2 being potentially achievable.

Tunneling conductance oscillations in spin–orbit coupled metal–insulator–superconductor junctions

The tunneling conductance for a device consisting of a metal–insulator–superconductor (MIS) junction is studied in presence of Rashba spin–orbit coupling (RSOC) via an extended Blonder–Tinkham–Klapwijk formalism. We find that the tunneling conductance as a function of an effective barrier potential that defines the insulating layer and lies intermediate to the metallic and superconducting electrodes, displays an oscillatory behavior. The tunneling conductance shows high sensitivity to the RSOC for certain ranges of this potential, while it is insensitive to the RSOC for others. Additionally, when the period of oscillations is an odd multiple of a certain value of the effective potential, the conductance spectrum as a function of the biasing energy demonstrates a contrasting trend with RSOC, compared to when it is not an odd multiple. The explanations for the observation can be found in terms of a competition between the normal and Andreev reflections. Similar oscillatory behavior of the conductance spectrum is also seen for other superconducting pairing symmetries, thereby emphasizing that the insulating layer plays a decisive role in the conductance oscillations of a MIS junction. For a tunable Rashba coupling, the current flowing through the junction can be controlled with precision.

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

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

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.

A Robust Cooling Platform for NIS Junction Refrigeration and sub-Kelvin Cryogenic Systems

Recent advances in Normal metal–insulator-superconductor (NIS) tunnel junctions (Clark et al. Appl Phys Lett 86: 173508, 2005, Appl Phys Lett 84: 4, 2004) have proven these devices to be a viable technology for sub-Kelvin refrigeration. NIS junction coolers, coupled to a separate cold stage, provide a flexible platform for cooling a wide range of user-supplied payloads. Recently, a stage was cooled from 290 to 256 mK (Lowell et al. Appl Phys Lett 102: 082601 2013), but further mechanical and electrical improvements are necessary for the stage to reach its full potential. We have designed and built a new Kevlar suspended cooling platform for NIS junction refrigeration that is both lightweight and well thermally isolated; the calculated parasitic loading is $$<\!\!300$$ pW from 300 to 100 mK. The platform is structurally rigid with a measured deflection of 25 $$\upmu $$ m under a 2.5 kg load and has an integrated mechanical heat switch driven by a superconducting stepper motor with thermal conductivity G $$ = 4.5 \times 10^{-7}$$ W/K at 300 mK. An integrated radiation shield limits thermal loading and a modular platform accommodates enough junctions to provide nanowatts of continuous cooling power. The compact stage size of 7.6 cm $$\times $$ 8.6 cm $$\times $$ 4.8 cm and overall radiation shield size of 8.9 cm $$\times $$ 10.0 cm $$\times $$ 7.0 cm along with minimal electrical power requirements allow easy integration into a range of cryostats. We present the design, construction, and performance of this cooling platform as well as projections for coupling to arrays of NIS junctions and other future applications.

Overdamped Josephson junctions for digital applications

An interesting feature of Superconductor–Normal metal–Superconductor Josephson junctions for digital applications is due to their non-hysteretic current–voltage characteristics in a broad temperature range below Tc. This allows to design Single-Flux-Quantum (SFQ) cells without the need of external shunts. Two advantages can be drawn from this property: first the SFQ cells can be more compact which leads to a more integrated solution towards nano-devices and more complex circuits; second the absence of electrical parasitic elements associated with the wiring of resistors external to the Josephson junctions increases the performance of SFQ circuits, in particular regarding the ultimate speed of operation. For this purpose Superconductor–Normal metal–Insulator–Superconductor Nb/Al–AlOx/Nb Josephson junctions have been recently developed at INRiM with aluminum layer thicknesses between 30 and 100nm. They exhibit non-hysteretic current–voltage characteristics with IcRn values higher than 0.5mV in a broad temperature range and optimal Stewart McCumber parameters at 4.2K for RSFQ applications. The main features of obtained SNIS junctions regarding digital applications are presented.

Refrigeration of separate, user-supplied payloads with Normal–Insulator–Superconductor tunnel junctions

Normal metal–Insulator–Superconductor (NIS) tunnel junctions can be used to selectively remove the hottest electrons in the normal metal, thereby causing it to cool. NIS tunnel junctions have already been used to cool lithographically integrated payloads [1], but this requires integration of two disparate fabrication processes. To increase the flexibility of NIS refrigerators, we have designed a stage cooler based on NIS tunnel junctions that will be able to cool arbitrary, user-supplied payloads from 300mK to 100mK. This stage cooler can be backed by a helium-3 refrigerator to provide a lightweight and simple means of reaching 100mK in space applications. In this paper, we describe the design of our stage cooler and present calculations of the cooling power and time required to reach 100mK.

Tunneling conductance in gapped graphene-based normal metal–insulator–superconductor junctions: Case of massive Dirac electrons

We study the quantum transport property in a gapped graphene-based normal metal–insulator–superconductor junctions ( NG/ IG/ SG), in the limit of a thin barrier. The charged fermions in NG/ IG/ SG structure are treated as massive relativistic particles. Based on Andreev and normal reflections in normal-superconductor graphene-based junction and BTK formalism, the tunneling conductance’s in terms of some different electrostatic superconductor, U 0 and barrier, V 0 potential are obtained. Using the experimental based values of the Fermi energy in the NG and SG ( E FN and E FN + U 0, respectively), energy gap in graphene 2 mv F 2 and superconducting order parameter, Δ, it is shown that the conductance spectra of such system represent a new behavior, i.e. if we take | E FN - mv F 2 | → 0 , it becomes as a step function of V 0. This behavior of charge transportation can be considered as a nano switch.