Critical Current Modulation Research Articles

A ZVS Critical Current Tracing Modulation for Current-Fed Series Resonant Converters in Fuel Cell Vehicles

A ZVS Critical Current Tracing Modulation for Current-Fed Series Resonant Converters in Fuel Cell Vehicles

Flux-pinning mediated superconducting diode effect in NbSe2/CrGeTe3 heterostructure

In ferromagnet/superconductor bilayer systems, dipolar fields from the ferromagnet can create asymmetric energy barriers for the formation and dynamics of vortices through flux pinning. Conversely, the flux emanating from vortices can pin the domain walls of the ferromagnet, thereby creating asymmetric critical currents. Here, we report the observation of a superconducting diode effect (SDE) in a NbSe2/CrGeTe3 van der Waals heterostructure in which the magnetic domains of CrGeTe3 control the Abrikosov vortex dynamics in NbSe2. In addition to extrinsic vortex pinning mechanisms at the edges of NbSe2, flux-pinning-induced bulk pinning of vortices can alter the critical current. This asymmetry can thus be explained by considering the combined effect of this bulk pinning mechanism along with the vortex tilting induced by the Lorentz force from the transport current in the NbSe2/CrGeTe3 heterostructure. We also provide evidence of critical current modulation by flux pinning depending on the history of the field setting procedure. Our results suggest a method of controlling the efficiency of the SDE in magnetically coupled van der Waals superconductors, where dipolar fields generated by the magnetic layer can be used to modulate the dynamics of the superconducting vortices in the superconductors.

Nb/a-Si/Nb Josephson junctions for high-density superconducting circuits

We present electrical characterization data of sputtered Nb/a-Si/Nb Josephson junctions (JJs) for high-speed and high-density superconducting circuits. Junctions were studied with critical current densities (Jc) ranging from 0.01 to 3 mA/μm2 at 4 K. For junctions deposited at room temperature and processed to a maximum temperature of 150 °C, the dependence of Jc on barrier thickness d is exponential, Jc∝exp (−d/d0), with d0 constant over the entire range of Jc values studied. Junctions were annealed at temperatures up to 300 °C to study changes in their electrical properties and possible compatibility with high temperature fabrication processes. Current–voltage characteristics, critical current uniformity, critical current modulation with in-plane magnetic field, and sub-gap resistance behavior of these junctions were measured at 4 K and demonstrate that the junction properties do not degrade with annealing. These data indicate that Nb/a-Si/Nb JJs are a potential candidate for higher speed and higher density superconducting circuits.

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.

Critical current modulation induced by an electric field in superconducting tungsten-carbon nanowires

The critical current of a superconducting nanostructure can be suppressed by applying an electric field in its vicinity. This phenomenon is investigated throughout the fabrication and electrical characterization of superconducting tungsten-carbon (W-C) nanostructures grown by Ga^+ focused ion beam induced deposition (FIBID). In a 45 nm-wide, 2.7 upmu m-long W-C nanowire, an increasing side-gate voltage is found to progressively reduce the critical current of the device, down to a full suppression of the superconducting state below its critical temperature. This modulation is accounted for by the squeezing of the superconducting current by the electric field within a theoretical model based on the Ginzburg–Landau theory, in agreement with experimental data. Compared to electron beam lithography or sputtering, the single-step FIBID approach provides with enhanced patterning flexibility and yields nanodevices with figures of merit comparable to those retrieved in other superconducting materials, including Ti, Nb, and Al. Exhibiting a higher critical temperature than most of other superconductors, in which this phenomenon has been observed, as well as a reduced critical value of the gate voltage required to fully suppress superconductivity, W-C deposits are strong candidates for the fabrication of nanodevices based on the electric field-induced superconductivity modulation.

Dual Fraunhofer interference and charge fluctuations in long quantum phase slip wires

Charge interference (Aharonov-Casher effect) in a long superconducting quantum phase slip wire is considered, and from this the ``dual'' Fraunhofer interference effect (dual to the critical current modulation of a short Josephson junction in an external magnetic field) is derived. The device that can be used to observe this effect is proposed. Furthermore, the impact of wire inhomogeneities, charge disorder, and noise on the phase slip amplitude is investigated. Although intrinsically protected against small fluctuations, the Aharonov-Casher interference resulting from jumps of random offset charges and quasiparticles can result in significant fluctuations of the measured current-voltage characteristics of the quantum phase slip wire, similar to the effects of Joule heating when averaged out over many fluctuations. Possible ways to identify and mitigate such disorder are discussed.

Quantum interference of currents in an atomtronic SQUID

Quantum interference of currents is the most important and well known quantum phenomenon in a conventional superconducting quantum interference device (SQUID). Here, we report the observation of quantum interference of currents in an atomtronic SQUID. Analogous to a conventional SQUID, currents flowing through two junctions in an atomtronic SQUID interfere due to the phase difference from rotation. This interference results in modulation of critical currents. This modulation was observed for three different radii with clear modulation periods which were measured to be consistent with fundamental rotation rates. This observation shows the possibility of studying various interesting SQUID physics with an atomtronic SQUID and especially, macroscopic quantum phenomena with currents may be realized with an atomtronic SQUID toward the goal of quantum metrology of rotation sensing.

Engineering quantum interference

A model for describing the interference and diffraction of wave functions of one-dimensional Josephson array interferometers is presented. The derived expression for critical current modulations accounts for an arbitrary number of square junctions, the variable distance between these, and the variable size of their area. Predictions are tested on real arrays containing up to 20 equally spaced and identical junctions and on arrays shaped with peculiar geometries. A very good agreement with the modulations predicted by the model and the experimental results is obtained for all the tested configurations. It is shown that specific designs of the arrays generate significant differences in their static and dynamical (non-zero voltage) properties. The results demonstrate that the magnetic field dependence of Josephson supercurrents shows how interference and diffraction of macroscopic quantum wavefunctions can be manipulated and controlled.

Micro-SQUIDs based on MgB2 nano-bridges for NEMS readout

We show the results obtained from the fabrication and characterisation of MgB2 loops with two nano-bridges as superconducting weak links. These ring structures are made to operate as superconducting quantum interference devices and are investigated as readout system for cryogenics NEMS resonators. The nano-constrictions are fabricated by EBL and ion beam milling. The SQUIDs are characterised at different temperatures and measurements of the noise levels have been performed. The devices show high critical current densities and voltage modulations under applied magnetic field, close to the critical temperatures.

NanoSQUID operation using kinetic rather than magnetic induction.

We report on a method of nanoSQUID modulation which uses kinetic inductance rather than magnetic inductance to manip-ulate the internal fluxoid state. We produced modulation using injected current rather than an applied magnetic field. Using this injected current, we were able to observe the triangle-wave shaped modulation of the device critical current which was periodic according to the London fluxoid quantization condition. The measurement results also confirmed that the fluxoid state inside a superconducting loop can be manipulated using primarily kinetic inductance. By using primarily kinetic inductance rather than magnetic inductance, the size of the coupling inductor was reduced by a factor of 10. As a result, this approach may provide a means to reduce the size of SQUID-based superconducting electronics. Additionally, this method provides a convenient way to perform kinetic inductance characterizations of superconducting thin films.

Niobium NanoSQUIDs Based on Sandwich Nanojunctions: Performance as a Function of the Temperature

In this paper, an experimental investigation of the main characteristics of a niobium nano Superconducting QUantum Interference Device (nanoSQUID) as a function of the temperature (9-0.3 K) is presented. The nanosensor consists of a niobium superconducting loop (0.4 × 1.0 μm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> ) interrupted by two sandwich nanojunctions (Nb/Al-AlOx/Nb) having an area of about (300 × 300) nm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> . These nanodevices have been fabricated by means of a focused ion beam sculpting method, which is used as a lithographic technique to define the various elements of the SQUID. We have performed measurements of current-voltage, critical current-magnetic flux characteristics, and switching current distributions from the zero voltage state for different temperatures. The high critical current modulation depths and the low intrinsic dissipation exhibited by these devices ensure a suitable sensitivity for nanoscale applications in the whole temperature range investigated.

Working Point Adjustable DC-SQUID for the Readout of Gap Tunable Flux Qubit

In flux qubits, qubit states are commonly read out by a dc superconducting quantum interference device (SQUID), which is inductively coupled to the qubit and works as a persistent current detector. However, neither the bottom nor the top regions of the critical current modulation curve of the SQUID can be used for the readout. In this paper, we show that a flux-trap loop inserted to the readout SQUID provides the ability to adjust the working point, which helps to avoid this constraint. This improvement is useful for some complicated flux qubit experiments.

Geometric dependence of Nb-Bi2Te3-Nb topological Josephson junction transport parameters

Superconductor-topological insulator–superconductor Josephson junctions have been fabricated in order to study the width dependence of the critical current, normal state resistance and flux periodicity of the critical current modulation in an external field. Previous literature reports suggest anomalous scaling in topological junctions due to the presence of Majorana bound states. However, for most realized devices, one would expect that trivial -periodic Andreev levels dominate transport. We also observe anomalous scaling behaviour of junction parameters, but the scaling can be well explained by mere geometric effects, such as the parallel bulk conductivity shunt and flux focusing.

Ultra High Sensitive Niobium NanoSQUID by Focused Ion Beam Sculpting

The nanosuperconducting quantum interference device (nanoSQUID) is a powerful tool for nanoscience investigations. In this paper, the main features of niobium nanoSQUID based on deep submicron Josephson tunnel junctions have been investigated. The superconductive nanosensor has a rectangular loop of (1 × 0.4 μm 2) interrupted by two square Josephson junctions having a side length of 0.3 μm. The crucial steps of the fabrication process have been performed using focusing ion beam (FIB) nanosculpting technique. A full characterization of the nanosensor has been performed including the measurement of the voltage swing, the voltage responsivity, the spectral density of the magnetic flux and spin noise and the investigation of the main characteristics as a function of the temperature. Due to a very high responsivity (2.9 mV/ Φ0), the nanodevice exhibited, at T= 4.2 K, an intrinsic spectral density of the magnetic flux as low as 600 n Φ0/Hz 1/2, corresponding to a magnetic moment noise of 9 μB/Hz 1/2. For temperature less than 4.0 K, the nanodevice shows hysteretic current-voltage characteristics. However, the high critical current modulation depths ensure a suitable sensitivity for nanoscale applications.

Local and nonlocal Fraunhofer-like pattern from an edge-stepped topological surface Josephson current distribution.

We report a surface-dominant Josephson effect in superconductor-topological insulator-superconductor (S-TI-S) devices, where a Bi1.5Sb0.5Te1.7Se1.3 (BSTS) crystal flake was adopted as an intervening TI between Al superconducting electrodes. We observed a Fraunhofer-type critical current modulation in a perpendicular magnetic field in an Al-TI-Al junction for both local and nonlocal current biasing. Fraunhofer-type modulation of the differential resistance was also observed in a neighboring Au-TI-Au normal junction when it was nonlocally biased by the Al-TI-Al junction. In all cases, the Fraunhofer-like signal was highly robust to the magnetic field up to the critical field of the Al electrodes, corresponding to the edge-stepped nonuniform supercurrent density arising from the top and rough side surfaces of the BSTS flake, which strongly suggests that the Josephson coupling in a TI is established through the surface conducting channels that are topologically protected.

Resistive state triggered by vortex entry in YBa2Cu3O7−δ nanostructures

We have realized YBa2Cu3O7−δ nanowires and nano Superconducting Quantum Interference Devices (nanoSQUID). The measured temperature dependence of the wire resistances below the superconducting transition temperature has been analyzed using a thermally activated vortex entry model valid for wires wider than the superconducting coherence length. The extracted zero temperature values of the London penetration depth, λ0≃270±15nm, are in good agreement with the value obtained from critical current modulations as a function of an externally applied magnetic field in a nanoSQUID implementing two nanowires.

Induced change of critical current density profile in Nb/Al-AlOx/Nb Josephson junctions

A technique to induce spatial modulation of critical current density in niobium based Josephson devices by using a selective thermal annealing is reported. By depositing a carbon film onto selected region of the Josephson element it is possible to induce a localized heating, with a spatial resolution less than 1 μm, exploiting the much higher absorbance coefficient of carbon than the niobium one. The effectiveness of such technique is demonstrated by experimental measurement of the critical current vs. magnetic field, measured at T = 4.2 K, showing that the change of critical current density occurs only in the region corresponding to the absorber film area. Furthermore, the theoretical behaviour, by modelling a suitable step-like junction barrier shape, has been carried out to fit the experimental data in order to verify the selective modulation of critical current. This technique can be very useful in view of quantum computing experiments, Majorana fermions detection and superconducting magnetic sensors.

Performances of niobium planar nanointerferometers as a function of the temperature: a comparative study

The growing interest in the development of nano-superconducting quantum interference devices (nanoSQUIDs) and their applications in nanoscience demands a deep study of these ultra-high-sensitivity nanodevices. In this paper, the main features of niobium planar nanointerferometers as a function of the temperature have been investigated. In particular, two types of nanodevices have been realized and studied: one based on nanobridges and the other on sub-micron Josephson tunnel junctions (JTJs). The nanobridge-based nanointerferometer, fabricated by electron beam lithography (EBL), consists of a sub-micrometric loop (effective area of 0.5 μm2), interrupted by two Dayem nanobridges having a width and length of 50 and 80 nm respectively. The JTJs-based nanointerferometer has two Nb/Al-AlOx/Nb SNIS (superconductor/normal-insulator/superconductor) junctions with an area of 0.09 μm2, connected by a rectangular loop (1 μm × 0.2 μm), and realized by the three-dimensional focused ion beam (3D FIB) sculpting technique. For both nanodevices, measurements of current–voltage characteristics and critical current versus external magnetic flux have been performed at operating temperatures ranging from 4.2 to 1.2 K. The critical current modulation depths, magnetic flux to current transfer factor (current responsivity) and magnetic flux resolution as a function of the temperature have been evaluated for both nanosensors. Furthermore, measurements of the voltage–magnetic flux characteristics and the spectral density of magnetic flux noise have been performed at T = 4.2 K.

Ultra low noise YBa2Cu3O7−δ nano superconducting quantum interference devices implementing nanowires

We present results on ultra low noise YBa2Cu3O7–δ (YBCO) nano Superconducting QUantum Interference Devices (nanoSQUIDs). To realize such devices, we implemented high quality YBCO nanowires, working as weak links between two electrodes. We observe critical current modulation as a function of an externally applied magnetic field in the full temperature range below the transition temperature TC. The white flux noise below 1μΦ0/Hz at T=8 K makes our nanoSQUIDs very attractive for the detection of small spin systems.

Fluxon dynamics of a long Josephson junction with two-gap superconductors

We investigate the phase dynamics of a long Josephson junction (LJJ) with two-gap superconductors. In this junction, two channels for tunneling between the adjacent superconductor (S) layers as well as one interband channel within each S layer are available for a Cooper pair. Due to the interplay between the conventional and interband Josephson effects, the LJJ can exhibit unusual phase dynamics. Accounting for excitation of a stable 2$\ensuremath{\pi}$-phase texture arising from the interband Josephson effect, we find that the critical current between the S layers may become both spatially and temporally modulated. The spatial critical current modulation behaves as either a potential well or barrier, depending on the symmetry of the superconducting order parameter, and modifies the Josephson vortex trajectories. We find that these changes in phase dynamics result in emission of electromagnetic waves as the Josephson vortex passes through the region of the 2$\ensuremath{\pi}$-phase texture. We discuss the effects of this radiation emission on the current-voltage characteristics of the junction.