Superconducting Tunnel Junctions Research Articles

Calibration of cryogenic amplification chains using normal-metal–insulator–superconductor junctions

Various applications of quantum devices call for an accurate calibration of cryogenic amplification chains. To this end, we present an experimentally feasible calibration scheme and use it to accurately measure the total gain and noise temperature of an amplification chain by employing normal-metal–insulator–superconductor (NIS) junctions. Our method is based on the radiation emitted by inelastic electron tunneling across voltage-biased NIS junctions. We derive an analytical expression that relates the generated power to the applied bias voltage which is the only control parameter of the device. After the setup has been characterized using a standard voltage reflection measurement, the total gain and the noise temperature are extracted by fitting the analytical expression to the microwave power measured at the output of the amplification chain. The 1σ uncertainty of the total gain of 51.84 dB appears to be of the order of 0.10 dB.

Comment on “Observation of nuclear gamma resonance with superconducting tunnel junction detectors” [AIP Advances 6, 025315 (2016)

Comment on “Observation of nuclear gamma resonance with superconducting tunnel junction detectors” [AIP Advances 6, 025315 (2016)

Response to “Comment on ‘Observation of nuclear gamma resonance with superconducting tunnel junction detectors’” [AIP Advances 9, 059101 (2019)

Response to “Comment on ‘Observation of nuclear gamma resonance with superconducting tunnel junction detectors’” [AIP Advances 9, 059101 (2019)

Phase relations in superconductor–normal metal–superconductor tunnel junctions

The phase difference $\phi$, between the superconducting terminals in superconductor-normal metal-superconductor tunnel junctions (SINIS), incorporates the phase differences $\chi_{1,2}$ across thin interfaces of constituent $SIN$ junctions and the phase incursion $\varphi$ between the side faces of the central electrode of length $L$. It is demonstrated here that $\chi_{1,2}$ pass through over their proximity-reduced domain twice, there and back, while $\phi$ changes over the single period. Two corresponding solutions, that describe the double-valued order-parameter dependence on $\chi_{1,2}$, jointly form the single-valued dependence on $\phi$, operating in two adjoining regions of $\phi$. The phase incursion $\varphi$ plays a crucial role in creating such a behavior. The current-phase relation $j(\phi,L)$ is composed of the two solutions and, at a fixed small $L$, is characterized by the phase-dependent effective transmission coefficient.

Electric-field-controlled superconductor–ferromagnetic insulator transition

Superconductivity beyond electron-phonon mechanism is always twisted with magnetism. Based on a new field-effect transistor with solid ion conductor as the gate dielectric (SIC-FET), we successfully achieve an electric-field-controlled phase transition between superconductor and ferromagnetic insulator in (Li,Fe)OHFeSe. A dome-shaped superconducting phase with optimal Tc of 43 K is continuously tuned into a ferromagnetic insulating phase, which exhibits an electric-field-controlled quantum critical behavior. The origin of the ferromagnetism is ascribed to the order of the interstitial Fe ions expelled from the (Li,Fe)OH layers by gating-controlled Li injection. These surprising findings offer a unique platform to study the relationship between superconductivity and ferromagnetism in Fe-based superconductors. This work also demonstrates the superior performance of the SIC-FET in regulating physical properties of layered unconventional superconductors.

Experimental Setup of Ac Thermoelectric Power Measurements in a Cryocooler PPMS System and Its Implementation to Superconductors, Topological Insulator, and Thermoelectric Materials

We have designed and developed an experimental setup to measure the Seebeck coefficient of a variety of samples at cryogenic temperatures and under magnetic fields up to 7 T employing the physical property measurement system (PPMS). The measurement technique uses a low frequency ac thermal gradient generated by two thin film heaters in thermal contact with the sample. Heaters and temperature sensors are all fitted on a standard PPMS sample puck. The validity of this method is tested by measuring the thermoelectric power of several superconductors and thermoelectric samples. We have used this technique to measure the thermoelectric power of various topological insulator single crystals (Pb0.8Sn0.2Te, Bi2Te3, Bi2Se2.1Te0.9, and Sb2Te3). The developed hardware and software control is suitable for studying the thermoelectric power of small samples (length 2 mm) in a commercial cryomagnetic system (PPMS) and it allows for studying superconductor, semiconductor, thermoelectric, or topological insulator material in wide temperature (2–300 K) and magnetic field (0–7 T) ranges.

Broadband Lamb shift in an engineered quantum system

The shift of energy levels owing to broadband electromagnetic vacuum fluctuations, the Lamb shift, has been pivotal in the development of quantum electrodynamics and in understanding atomic spectra. Currently, small energy shifts in engineered quantum systems are of paramount importance owing to the extreme precision requirements in applications such as quantum computing. However, without a tunable environment it is challenging to resolve the Lamb shift in its original broadband case. Consequently, the observations in other than atomic systems are limited to environments comprised of narrow-band modes. Here, we observe a broadband Lamb shift in high-quality superconducting resonators, a scenario also accessing static shifts inaccessible in Lamb's experiment. We measure a continuous change of several megahertz in the fundamental resonator frequency by externally tuning the coupling strength of the engineered broadband environment which is based on hybrid normal-metal--superconductor tunnel junctions. Our results may lead to improved control of dissipation in high-quality engineered quantum systems and open new possibilities for studying synthetic open quantum matter using this hybrid experimental platform.

Open-Circuit Ultrafast Generation of Nanoscopic Toroidal Moments: The Swift Phase Generator

Efficient and flexible schemes for a swift, field-free control of the phase in quantum devices have far-reaching impact on energy-saving operation of quantum computing, data storage, and sensoring nanodevices. We report a novel approach for an ultrafast generation of a field-free vector potential that is tunable in duration, sign, and magnitude, allowing to impart non-invasive, spatio-temporally controlled changes to the quantum nature of nanosystems. The method relies on triggering a steady-state toroidal moment in a donut-shaped nanostructure that serves as a vector-potential generator and quantum phase modulator. Irradiated by moderately intense, few cycle THz pulses with appropriately shaped polarization states, the nano donut is brought to a steady-state where a nearby object does not experience electric nor magnetic fields but feels the photo-generated vector potential. Designing the time structure of the driving THz pulses allows for launching picosecond trains of vector potentials which is the key for a contact-free optimal control of quantum coherent states. During the toroidal moment rise up time radiation is emitted which can be tuned in a very broad frequency band. We carry out full-fledged quantum dynamic simulations, and theoretical analysis to illuminate the underlying principles and to endorse the feasibility, robustness, and versatility of the scheme. This research could trigger a new class of ultrafast quantum devices operated and switched in an energy-efficient, contact and field-free manner, enabling new techniques for use in quantum information, magnetic nanostructures and superconducting tunnel junctions as well as in toroidally ordered systems and multiferroics.

The Superconductor-Superinsulator Transition: S-duality and the QCD on the Desktop

We show that the nature of quantum phases around the superconductor-insulator transition (SIT) is controlled by charge-vortex topological interactions, and does not depend on the details of material parameters and disorder. We find three distinct phases, superconductor, superinsulator, and bosonic topological insulator. The superinsulator is a state of matter with infinite resistance in a finite temperature range, which is the S-dual of the superconductor and in which charge transport is prevented by electric strings binding charges of opposite sign. The electric strings ensuring linear confinement of charges are generated by instantons and are dual to superconducting Abrikosov vortices. Material parameters and disorder enter the London penetration depth of the superconductor, the string tension of the superinsulator and the quantum fluctuation parameter driving the transition between them. They are entirely encoded in four phenomenological parameters of a topological gauge theory of the SIT. Finally, we point out that, in the context of strong coupling gauge theories, the many-body localization phenomenon that is often referred to as an underlying mechanism for superinsulation is a mere transcription of the well-known phenomenon of confinement into solid-state physics language and is entirely driven by endogenous disorder embodied by instantons with no need of exogenous disorder.

Violation of the Ambegaokar–Baratoff Relation in Dirty S–I–S Junctions

It has been shown that the presence of narrow-gap quantum jumpers in “dirty” (low concentrations of identical nonmagnetic impurities in the insulator layer) superconductor–insulator–superconductor (S–I–S) junctions results in a significant deviation of the critical supercurrent (Josephson current) in the temperature range 0 ≤ T ≤ Tc (Tc is the superconducting transition temperature in the S shores of the junction) from the value given by the known Ambegaokar–Baratoff relation. It has been found that the character of this deviation changes at a certain nonzero temperature Tb < Tc: this deviation is negative at 0 ≤ T< Tb (deficiency of the supercurrent), whereas it is positive at Tb< T ≤ Tc (excess of the supercurrent). Estimates indicate that experimental manifestation of this effect can be detected.

Superconducting electrode capacitor based on double-sided YBCO thin film for wireless power transfer applications

A high-temperature superconducting (HTS) electrode capacitor (HTS capacitor) has been reported for the first time in the form of a superconductor–insulator–superconductor structure. Double-sided HTS thin film might be inherently suitable for the raw material choice of an HTS capacitor. In terms of electrode material, HTS thin film has a much lower surface resistance than normal conductors, and, as a dielectric medium, its substrate generally has a larger dielectric constant but a smaller loss tangent than the dielectric layers of some conventional capacitors. Thus, it is possible to fabricate a high-quality capacitor based on double-sided HTS thin film. This type of HTS capacitor could play a role in wireless power transfer (WPT) technology, since the WPT efficiency depends on the quality factors of both the coil and capacitor. Numerous studies have been performed to optimize the WPT system with HTS coil. In contrast, we develop an HTS capacitor based on double-sided YBCO film for WPT applications and investigate its effect on the WPT system. It is found that the HTS capacitor can provide a high quality factor, enhance WPT efficiency, and work normally with an HTS coil at 77 K, avoiding the dilemma between an HTS coil and a normal capacitor.

On-chip cooling by heating with superconducting tunnel junctions

Heat management and refrigeration are key concepts for nanoscale devices operating at cryogenic temperatures. The design of an on-chip mesoscopic refrigerator that works thanks to the input heat is presented, thus realizing a solid-state implementation of the concept of cooling by heating. The system consists of a circuit featuring a thermoelectric element based on a ferromagnetic insulator-superconductor tunnel junction (N-FI-S) and a series of two normal metal-superconductor tunnel junctions (SINIS). The N-FI-S element converts the incoming heat in a thermovoltage, which is applied to the SINIS, thereby yielding cooling. The cooler's performance is investigated as a function of the input heat current for different bath temperatures. We show that this system can efficiently employ the performance of SINIS refrigeration, with a substantial cooling of the normal metal island. Its scalability and simplicity in the design makes it a promising building block for low-temperature on-chip energy management applications.

Influence of Local Correlations on the “Homogeneous Insulator–Superconductor” Transition in the Domain Boundaries of the Charge-Order Phase of a 2D System of a Mixed Valence

It is demonstrated in the (pseudo)spin S=1 formalism that the structure of antiphase domain boundaries in the phase of charge ordering of a mixed-valence system of the Cu1+, 2+, 3+ "triplet" type in cuprates on a two-dimensional square lattice depends to a considerable extent on on-site correlation parameter U. The results of computer modeling on large square lattices illustrate the change in the boundary structure (from a homogeneous monovalent nonconducting structure of the Cu2+ type to a filamentary superconducting one) induced by a relatively small variation of positive U values.

Quasi-particle density of states and bosonic modes in Bi2Sr2CaCu2O[formula omitted

The quasi-particle density of states (DOS) at temperatures from 4.2 K to high above the critical temperature Tc is obtained by inversion directly from the experimentally measured superconductor-insulator-superconductor (SIS) tunneling data performed on three different Bi2Sr2CaCu2O8+δ samples. Bosonic modes accompanying the superconductivity or related to the pseudogap, as well as their temperature-dependent energies are extracted by analyzing the fine structures exhibited in the DOS curves. Consistent with the coexistence of the two bosonic modes in the superconducting state for the optimally and underdoped samples, a kink is found in the derivative curve of the DOS with respect to the energy, which provides a kind of evidence for the coexistence of the pseudogap and superconductivity in these materials. These results shed light on the interactions between the charged carriers and the bosonic modes and may give us the fingerprints of the pairing gluons in the cuprates.

Series-connected array of superconductor–insulator–superconductor junctions in the 100 GHz-band heterodyne mixer for FOREST on the Nobeyama 45 m telescope

Abstract We have designed and experimentally evaluated a series-connected array of superconductor–insulator–superconductor (SIS) junctions in the 100 GHz-band mixer for the multi-beam receiver FOREST on the Nobeyama 45 m millimeter-wave telescope. The construction of the junction chip comprised a waveguide probe antenna, impedance-matching circuit, SIS array junction, and choke filter, which were made from a superconducting niobium planar circuit on a quartz substrate. The multi-stage impedance-matching circuit between the feed point and the SIS junction was designed as a capacitively loaded transmission line, and it comprised two sections with high (∼90 Ω) and low (∼10 Ω) characteristic impedance transmission lines. The structure of this tuning line was simple and easy to fabricate, and the feed impedance matched with the SIS junction in a wide frequency range. The signal coupling efficiency was more than 92% and the expected receiver noise temperature was approximately twice the quantum limit for 75–125 GHz based on quantum theory. The array junction devices with three to six connected junctions were fabricated and we measured their performance in terms of the receiver noise temperature and gain compression in the laboratory. We successfully developed an array junction device with a receiver noise temperature of ∼15–30 K and confirmed that the improvement in the saturation power corresponded to the number of junctions. The newly developed array junction mixer was installed in the FOREST receiver and it successfully detected the 12CO(J = 1–0) molecular line toward IRC +10216 with the Nobeyama 45 m telescope.

Superconducting qubits in Russia

Results of the two-year research in the framework of the first Russian project for superconducting qubits, supported by the Foundation for Advanced Research, Federal Agency on Atomic Energy, and the Ministry of Science and Higher Education of the Russian Federation, are presented. The main result is the demonstration of single-qubit quantum gate operations using several types of superconducting qubits with a coherence time of more than 1 μs. The project develops this breakthrough scientific and technical line of research in Russia, in fact, from scratch. The fabrication technology of submicron-scale superconducting tunnel junctions and various coherent quantum structures (qubits) based on them has been worked out; preparation and control of quantum states for such qubits, quantum tomography of prepared states, and realisation and precision control of single-qubit quantum gate operations are demonstrated.

Süperiletken Durumda Cıva Bazlı Bakır Oksit Katmanlı Süperiletkenin Özgül Kapasitanslarının Tespiti

Mercury based copper oxide layered high temperature superconductor, which consists of superconductor–insulator–superconductor (SIS) layers, can be considered as a stack of nearly ideal, intrinsic Josephson junctions (IJJ). The SIS junction, where the electrical field is confined, topologically resembles a parallel-plate capacitor. As is known, the coupling between junctions in superconductors is capacitive. Hence, the determination of the specific capacitance (Cs) of the IJJ at the superconducting state has a crucial importance in order to give information about superconductivity mechanism. In this study, the Cs values of the investigated sample have been calculated by means of the critical current density, Jcand plasma frequency, that have been obtained from magnetic measurements taken at the below temperatures than the critical transition temperature, Tc. Moreover, Cs values at superconducting temperature have been compared to that of the normal temperature.

Origin of Mott Insulating Behavior and Superconductivity in Twisted Bilayer Graphene

A remarkable recent experiment has observed Mott insulator and proximate superconductor phases in twisted bilayer graphene when electrons partly fill a nearly flat mini-band that arises a `magic' twist angle. However, the nature of the Mott insulator, origin of superconductivity and an effective low energy model remain to be determined. We propose a Mott insulator with intervalley coherence that spontaneously breaks U(1) valley symmetry, and describe a mechanism that selects this order over the competing magnetically ordered states favored by the Hunds coupling. We also identify symmetry related features of the nearly flat band that are key to understanding the strong correlation physics and constrain any tight binding description. First, although the charge density is concentrated on the triangular lattice sites of the moir$\text{\'e }$ pattern, the Wannier states of the tight-binding model must be centered on different sites which form a honeycomb lattice. Next, spatially localizing electrons derived from the nearly flat band necessarily breaks valley and other symmetries within any mean-field treatment, which is suggestive of a valley-ordered Mott state, and also dictates that additional symmetry breaking is present to remove symmetry-enforced band contacts. Tight-binding models describing the nearly flat mini-band are derived, which highlight the importance of further neighbor hopping and interactions. We discuss consequences of this picture for superconducting states obtained on doping the valley ordered Mott insulator. We show how important features of the experimental phenomenology may be explained and suggest a number of further experiments for the future. We also describe a model for correlated states in trilayer graphene heterostructures and contrast it with the bilayer case.

Performance of Tantalum STJ X-ray Detectors at Elevated Count Rates

We have developed an X-ray detector system based on Ta superconducting tunnel junctions (STJs) with an energy resolution below 10 eV under 1 keV. A principal advantage of STJs over other low-temperature detectors is their high count rate capability. In this work, we have optimized the operating conditions and digital pulse processing parameters for input count rates up to 25 kcps/pixel. Resolution below 10 eV is maintained up to 2.5 kcps/pixel, and at input rates of 25 kcps per pixel the resolution is 26 eV FWHM at 525 eV.

Toward the Absolute Spin-Valve Effect in Superconducting Tunnel Junctions.

A superconductor with a spin-split excitation spectrum behaves as an ideal ferromagnetic spin-injector in a tunneling junction. It was theoretically predicted that the combination of two such spin-split superconductors with independently tunable magnetizations may be used as an ideal absolute spin-valve. Here, we report on the first switchable superconducting spin-valve based on two EuS/Al bilayers coupled through an aluminum oxide tunnel barrier. The spin-valve shows a relative resistance change between the parallel and antiparallel configuration of the EuS layers up to 900% that demonstrates a highly spin-polarized current through the junction. Our device may be pivotal for realization of thermoelectric radiation detectors, a logical element for a memory cell in cryogenics, superconductor-based computers, and superconducting spintronics in general.