Superconductivity In Nanowires Research Articles

Out-of-equilibrium phonons in gated superconducting switches

Recent experiments have suggested that superconductivity in metallic nanowires can be suppressed by the application of modest gate voltages. The source of this gate action has been debated and either attributed to an electric-field effect or to small leakage currents. Here we show that the suppression of superconductivity in titanium nitride nanowires on silicon substrates does not depend on the presence or absence of an electric field at the nanowire, but requires a current of high-energy electrons. The suppression is most efficient when electrons are injected into the nanowire, but similar results are obtained when electrons are passed between two remote electrodes. This is explained by the decay of high-energy electrons into phonons, which propagate through the substrate and affect superconductivity in the nanowire by generating quasiparticles. By studying the switching probability distribution of the nanowire, we also show that high-energy electron emission leads to a much broader phonon energy distribution compared with the case where superconductivity is suppressed by Joule heating near the nanowire.

Single-Photon Detector with Multilayer and Low-Filling-Factor Superconducting Nanowire

Single-Photon Detector with Multilayer and Low-Filling-Factor Superconducting Nanowire

Creation of functional nanostructures under ion irradiation

The paper describes a method developed in National Research Centre "Kurchatov Institute" for creating different functional nanostructures using ion beam irradiation. As an example, the formation of an integrated resistive elements in a low-temperature NbN superconductor nanowire is demonstrated. The possibility of producing an insulating layer of aluminum oxide with a thickness of 15 nm on the aluminum surface at room temperature under 0.2 keV oxygen ions irradiation was shown. Keywords: Ion irradiation, NbN thin superconducting films, integrated cryogenic resistors, creation of dielectric layers under irradiation.

Improved Response Model of a Superconducting Nanowire Array for High Photon Count Rate Communication

Improved Response Model of a Superconducting Nanowire Array for High Photon Count Rate Communication

Two-layer logic elements for classic cryogenic computers

In this paper, for the first time, two-layer logic elements "NOT", "AND", "OR" and "NOR" were manufactured, in which the heating element of a nanowire in one layer is located in the place where the heated element of another superconductive nanowire is located in a nearest NbN layer. The film thickness of NbN (5-7 nm) was used to form of superconducting nanowires in different layers, and the thickness of Al2O3 (25 nm) was used as an interlayer dielectric. The paper shows the design of logic elements, and also demonstrates their operation in pulse mode. The use of multilayer structures without galvanic coupling between nanowires allows the design of multi-level logic devices. Keywords: NbN thin superconducting films, contactless switching of the superconductor state, cryoelectronic devices, integrated cryogenic resistors, logic elements "NOT"/"AND"/"OR"/"NOR", cryogenic computers.

Improved pulse discrimination for a superconducting series nanowire detector by applying a digital matched filter

Photon number resolving (PNR) is an important capacity for detectors working in quantum and classical applications. Although a conventional superconducting nanowire single-photon detector (SNSPD) is not a PNR detector, by arranging nanowires in a series array and multiplexing photons over space, such series PNR-SNSPD can gain quasi-PNR capacity. However, the accuracy and maximum resolved photon number are both limited by the signal-to-noise ratio (SNR) of the output pulses. Here, we introduce a matched filter, which is an optimal filter in terms of SNR. Experimentally, the normalized spacing between pulse amplitudes from adjacent photon number detections increased by a maximum factor of 2.1 after the matched filter. Combining with a cryogenic amplifier to increase SNR further, such spacing increased by a maximum factor of 5.3. In contrast to a low pass filter, the matched filter gave better SNRs while maintaining low timing jitters. The minimum timing jitter of 55 ps was obtained experimentally. Our results suggest that the matched filter is a useful tool for improving the performance of the series PNR-SNSPD. The maximum resolved photon number can be expected to reach 65 or even larger.

Josephson and Persistent Currents in a Quantum Ring between Topological Superconductors

AbstractIn this work, the spectra in an Aharonov–Bohm quantum‐ring interferometer forming a Josephson junction between two topological superconductor (TSC) nanowires are investigated. The TSCs host Majorana bound states at their edges, and both the magnetic flux and the superconducting phase difference between the TSCs are used as control parameters. A tight‐binding approach is used to model the quantum ring coupled to both TSCs, described by the Kitaev effective Hamiltonian. The problem is solved by means of exact numerical diagonalization of the Bogoliubov‐de Gennes Hamiltonian and obtain the spectra for two sizes of the quantum ring as a function of the magnetic flux and the phase difference between the TSCs. Depending on the size of the quantum ring and the coupling, the spectra display several patterns. Those are denoted as line, point, and undulated nodes, together with flat bands, which are topologically protected. The first three patterns can be possibly detected by means of persistent and Josephson currents. Hence, the results could be useful to understand the spectra and their relation with the behavior of the current signals.

Spin Seebeck Effect in a Hybridized Quantum-Dot/Majorana-Nanowire With Spin Heat Accumulation

Properties of spin Seebeck effect (SSE) in a quantum dot (QD) connected to a topological superconductor or semiconductor nanowire with strong spin-orbit interaction are theoretically studied by the noneqilibrium Green’s function method combined with Dyson equation technique. At low temperatures, Majorana zero modes (MZMs) are prepared at the ends of topological superconductor or semiconductor nanowire, and are hybridized to the QD with spin-dependent strength. We consider that the QD is coupled to two leads in the presence of spin heat accumulation (SHA), i.e., spin-dependent temperature in the leads. We find that the thermopower is spin-polarized when the hybridization strength between the QD and one mode of the MZMs depends on electron spin direction, and its spin-polarization can be effectively adjusted by changing the magnitude of SHA. By proper variation of the spin-polarization of the QD-MZM hybridization strength, magnitude of the SHA, dot level, or the direct coupling between the MZMs, 100% spin-polarized or pure thermopower can be generated. Our results may find real usage in high efficiency spintronic devices or detection of the MZMs, which are under current extensive study. The present model is within the reach of current nano-technologies and may by used in high efficiency spin caloritronics devices.

BCS Effect on Quantum Correlation and Tripartite Quantum Entanglement in Spinless Gapless Tomonaga-Luttinger Liquid and Cuprate Superconducting Nanowire

BCS Effect on Quantum Correlation and Tripartite Quantum Entanglement in Spinless Gapless Tomonaga-Luttinger Liquid and Cuprate Superconducting Nanowire

Dual Negative Differential of Heat Generation in a Strongly Correlated Quantum Dot Side-Coupled to Majorana Bound States

We study theoretically the properties of local heat originated from energy exchange between electrons passing through a quantum dot (QD) coupled to a phonon bath. The dot is sandwiched between two normal metal leads and also side-coupled to Majorana bound states (MBSs) formed at opposite ends of a topological superconductor nanowire. We find that in addition to the negative differential of heat generation (NDHG) in the Coulomb blockade regime, another NDHG emerges near the leads’ Fermi level due to the dot-MBS coupling. This dual NDHG effect is robust against the variation of intradot Coulomb interaction strength, and disappears if the QD is coupled to regular Fermions. Direct hybridization between the MBSs reduces their impacts on the electronic transport processes, and eliminates the dual NDHG effect. Our results show that the dual NDHG effect is quite efficient for inferring the existence of MBSs, and may remedy some limitations of the detection schemes relying on tunneling spectroscopy technique.

Photon-Assisted Average Current Through a Quantum Dot Coupled to Majorana Bound States

We investigate the properties of photon-assisted current through a quantum dot (QD) coupled to two leads and Majorana bound states (MBSs) formed at two ends of a topological superconductor nanowire. Our numerical results show that the peak’s height of the average current is reduced to half by the half fermionic character of MBSs. When a microwave (MW) field is applied on one of the leads, the negative current is converted into positive by the MBSs. This abnormal phenomenon may serve as a detection method for MBSs. Moreover, the photon-assisted tunneling sideband peak appears while a MW field is applied on the QD. Turning on the coupling between the QD and MBSs, the magnitude of the average current decreases monotonously because that the MBSs now is changed to the non-zero energy mode by emitting (absorbing) photons from the MW field. Correspondingly, the energy level in the QD is split into two by the non-zero energy mode MBSs, and then the photon-assisted tunneling occurs with the help of the MBSs.

Single‐Shot Fabrication of Semiconducting–Superconducting Nanowire Devices

AbstractSemiconducting–superconducting hybrids are vital components for the realization of high‐performance nanoscale devices. In particular, semiconducting–superconducting nanowires attract widespread interest owing to the possible presence of non‐abelian Majorana zero modes, which are quasiparticles that hold promise for topological quantum computing. However, systematic search for Majoranas signatures is challenging because it requires reproducible hybrid devices and reliable fabrication methods. This work introduces a fabrication concept based on shadow walls that enables the in situ, selective, and consecutive depositions of superconductors and normal metals to form normal‐superconducting junctions. Crucially, this method allows to realize devices in a single shot, eliminating fabrication steps after the synthesis of the fragile semiconductor/superconductor interface. At the atomic level, all investigated devices reveal a sharp and defect‐free semiconducting–superconducting interface and, correspondingly, a hard induced superconducting gap resilient up to 2 T is measured electrically. While the cleanliness of the technique enables systematic studies of topological superconductivity in nanowires, it also allows for the synthesis of advanced nano‐devices based on a wide range of material combinations and geometries while maintaining an exceptionally high interface quality.

Photon-Assisted Seebeck Effect in a Quantum Dot Coupled to Majorana Zero Modes

The Seebeck effect is investigated within the framework of a non-equilibrium Green's function technique in a quantum dot (QD) sandwiched between the left and right leads held at different temperatures. We consider that the QD is shelled by a photon field and is side-coupled to a topological superconductor nanowire hosting Majorana zero modes (MZMs). It is found that the thermopower (Seebeck coefficient) can be obviously enhanced by weak QD-MZMs coupling at low temperatures, in addition to its sign reversion that may be used for detecting the existence of MZMs. In the presence of a photon field, the thermopower can be further enhanced due to decreased electrical conductance when electrons' transport probability through each channel is reduced by photon-assisted tunneling (PAT). The hybridization between the MZMs will also induce sign reversion of the thermopower in the absence of a photon field, whereas it has less impact on the thermopower when the QD is shelled by the photon field.

Thermoelectric Transport in a Double-Quantum-Dot Coupled to Majorana Zero Modes

Thermoelectric transport through a double-quantum-dot (DQD) connected to the left and right leads is theoretically investigated in the framework of non-equilibrium Green’s function technique. We consider that the dots are also coupled to Majorana zero modes (MZMs) prepared at the two ends of a topological superconductor nanowire. It is found that the sign change of thermopower, which is promising in the detection of MZMs, can be realized by tuning several system’s parameters related to the MZMs, such as the coupling strength between the dots and the MZMs, the direct coupling between the MZMs, or even the magnetic flux penetrating through the structure. The above parameters also lead to significant enhancement of the thermopower and thermoelectric figure of merit (FOM), which indicates the conversion efficiency between heat and electrical energies. We also find that in this DQD system, both the thermopower and FOM are simultaneously enhanced by the MZMs around the electron-hole symmetric point, an ideal phenomenon in applications of thermoelectric effect. In addition, the thermoelectric effect is remarkably enhanced by the direct hybridization between the MZMs, which is very different from the case in single-dot structure.

Demonstration of Atmospheric Lidar Measurement in the Infrared Wavelength Domain with a Superconducting Nanowire Single Photon Detector

Lidar measurement are widely used today for remote sensing and atmosphere monitoring. Indeed, depending on the specific wavelength adopted, it is possible to determine the presence of specific molecules and pollutants in the atmosphere. Several studies claim that the use of Lidar systems with wavelength larger than 1 µm would provide a special tool for aerosols and carbon dioxide observations. Aerosols measurements are typically performed up to the wavelength ? = 1064 nm, while for longer wavelengths they are limited by the detector noise. In this work the results of a preliminary Lidar measurement at ? = 1064 nm by using a Superconducting Nanowire Single Photon Detector (SNSPD) are presented. SNSPDs are characterized by high efficiency in the infrared wavelength domain (IR), low noise and dead time, which can in turn enhance the signal quality in Lidar atmospheric measurements at ? > 1 µm, an interesting range for environmental applications. Preliminary measurements have been performed at the University of Naples Federico II by using a NbTiN SNSPD cooled at a temperature of 4.2 K and the Lidar system MALIA. The experimental results at 1064 nm demonstrate the feasibility of the approach for aerosol measurements in the IR, opening the way to further investigations on the use of these detectors for Lidar atmospheric measurements in the IR. The backscattering coefficient, ?? has been also estimated and its value is around 10-6 m-1sr-1.

Topological superconductivity in nanowires proximate to a diffusive superconductor–magnetic-insulator bilayer

We study semiconductor nanowires coupled to a bilayer of a disordered superconductor and a magnetic insulator, motivated by recent experiments reporting possible Majorana-zero-mode signatures in related architectures. Specifically, we pursue a quasiclassical Usadel equation approach that treats superconductivity in the bilayer self-consistently in the presence of spin-orbit scattering, magnetic-impurity scattering, and Zeeman splitting induced by both the magnetic insulator and a supplemental applied field. Within this framework we explore prospects for engineering topological superconductivity in a nanowire proximate to the bilayer. We find that a magnetic-insulator-induced Zeeman splitting, mediated through the superconductor alone, cannot induce a topological phase since the destruction of superconductivity (i.e., Clogston limit) preempts the required regime in which the nanowire's Zeeman energy exceeds the induced pairing strength. However, this Zeeman splitting does reduce the critical applied field needed to access the topological phase transition, with fields antiparallel to the magnetization of the magnetic insulator having an optimal effect. Finally, we show that magnetic-impurity scattering degrades the topological phase, and spin-orbit scattering, if present in the superconductor, pushes the Clogston limit to higher fields yet simultaneously increases the critical applied field strength.

Quantum Interference Effects in Quantum Dot Molecular With Majorana Bound States

Non-invasive detection of the Majorana bound state (MBSs), a kind of quasiparticle without charge and mass, is one of the core issues in current condensed matter physics. Here we study in theory the quantum interference effect in parallel-coupled double quantum dots which are connected either by Majorana bound states (MBSs) or regular fermions. We find that the zero-energy conductance develops a sharp peak when the dots are connected by the MBSs, whereas that in the case of the dots are coupled via regular fermions shows a valley. By varying the coupling strength between the dots and the electron reservoirs, the conductance in the two structures changes in different ways. By comparing the properties of the linear conductance in the two systems, the information of the MBSs formed at the two ends of a topological superconductor nanowire then can be inferred. We also find that the MBSs in the present structure also induces the Fano effect, and is favorable in quantum information processing.

Electronic Transport Through Double Quantum Dot Coupled to Majorana Bound States and Ferromagnetic Leads

We have studied theoretically the properties of electrical current and tunnel magnetoresistance (TMR) through a serially connected double quantum dot (DQD) sandwiched between two ferromagnetic leads by using the nonequilibrium Green’s function technique. We consider that each of the DQD couples to one mode of the Majorana bound states (MBSs) formed at the ends of a topological superconductor nanowire with spin-dependent coupling strength. By adjusting the sign of the spin polarization of dot–MBS coupling strength and the arrangement of magnetic moments of the two leads, the currents’ magnitude can be effectively enhanced or suppressed. Under some conditions, a negative TMR emerges which is useful in detection of the MBSs, a research subject currently under extensive investigations. Moreover, the amplitude of the TMR can be adjusted in a large regime by variation of several system parameters, such as direct hybridization strength between the MBSs or the dots and the positions of the dots’ energy levels. Such tunable currents and TMR may also find use in high-efficiency spintronic devices or information processes.

Создание функциональных наноструктур под действием ионного облучения

The paper describes a method developed in National Research Centre “Kurchatov Institute” for creating different functional nanostructures using ion beam irradiation. As an example, the formation of an integrated resistive elements in a low-temperature NbN superconductor nanowire is demonstrated. The possibility of producing an insulating layer of aluminum oxide with a thickness of 15 nm on the aluminum surface at room temperature under 0.2 keV oxygen ions irradiation was shown.

Low Voltage Time-Resolved Emission (TRE) Measurements of VLSI Circuit

Advanced technology nodes with small feature sizes and increased design complexity make it increasingly time-consuming to determine the root cause of yield loss. Several of the defects also occur inside a circuit making physical failure analysis (PFA) and electrical failure analysis (EFA) much more challenging. EFA has been instrumental in driving product yield and reliability for consumer products such as mobile phones and computer chips. It involves the use of state-of-the-art tools and techniques. One of the main changes EFA analyses is an enhancement of dynamic EFA in circuit failed in functional test. We propose a technique for advanced Electrical Failure Analysis (EFA) tool with a Superconducting Nanowire Single Photon Detector (SnSPD) system and its application to low voltage Time-Resolved Emission (TRE) measurements (also known as Picosecond Imaging Circuit Analysis, PICA) of scaled VLSI circuits with enhanced sensitivity for discussing Time Resolved Emission (TRE). In order to understand the figures of advantage that a single-photon detector should have to enable the acquisition of time resolved emission waveforms for low voltage applications. We will provide that measurements down to a low 1 V supply voltage were made possible by a careful optimization of the detector front-end electronics. We also characterized the emission from devices with different threshold voltages in order to understand how the emission contributions depend on this parameter and how this affects the resulting waveform. we hope to be able to show soon even better results that should allow continued application of the non-invasive TRE and PICA technology towards future scaled nodes with smaller gates and lower supply voltages.