arXiv subjects

Elia Strambini

Publications and source records attributed to Elia Strambini.

At least 19 recordsLinked to original sources

arXiv2026

Graphene Josephson diodes from inherent asymmetric disorder

Josephson diodes are non-reciprocal superconducting devices characterized by different switching currents depending on the current flow direction. They recently attracted considerable theoretical and experimental attention, in view of their possible application as rectifying elements in the field of superconducting electronics, and as probes to investigate symmetry breaking mechanisms in mesoscopic systems. In this work, we show that graphene Josephson junctions provide rectification of supercurrent with an efficiency exceeding 20%. The effect appears applying a mT out-of-plane magnetic field and is enhanced close to the nodes of the Fraunhofer interference pattern. Our theoretical model identifies long-range scattering potentials in the junction as the symmetry-breaking mechanism, which yields supercurrent rectification in highly transparent junctions. While graphene stands as an ultra-clean transmission medium, our work shows that unavoidable residual disorder in a clean two-dimensional system is sufficient to promote this effect. Tailoring of the inversion (mirror) symmetry breaking could be obtained via proper design of external gates.

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arXiv2026

Magnetically Induced Switching-Current Jumps in InAs/Al Josephson Junctions

We report Barkhausen-like switching at millitesla fields in an $n$-doped InAs/Al nanowire Josephson junction, which serves as an interferometric probe of intrinsic magnetic reconfigurations, as evidenced by discrete switching-current jumps. At $T=30$~mK the device displays a Fraunhofer-like modulation with $I_{\mathrm{sw}}(0)\approx 0.24~\mu\mathrm{A}$ and an abrupt transition at $|B|\approx 3~\mathrm{mT}$ between two branches differing by $\Delta I_{\mathrm{sw}}\approx 0.13~\mu\mathrm{A}$. By tracking the characteristic field scales from $30$ to $900$~mK, we find that the jump field is essentially temperature-independent, whereas the superconducting critical field decreases with temperature, as expected for thin Al films. The sharp discontinuity, sweep-direction asymmetry, and reproducibility across repeated scans point to avalanche-like switching between metastable magnetic configurations of the local magnetic texture, which are directly coupled to the weak link. Within an effective-field framework, each reconfiguration modifies a local field offset, thereby reshaping the interference response and leading to an abrupt reorganization of the switching-current pattern.

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arXiv2025

Diode effect in the Fraunhofer pattern of disordered planar Josephson junctions

The Josephson diode effect describes the property of a Josephson junction to have different values of the critical current for different direction of applied bias current and it is the focus of intense research thanks to the possible applications. The ubiquity of the effect experimentally reported calls for a study of the impact that disorder can have in the appearance of the effect. We study the Fraunhofer pattern of planar Josephson junctions in presence of different kinds of disorder and imperfections and we find that a junction that is {\it mirror} symmetric at zero-field forbids the diode effect and that the diode effect is typically magnified at the nodal points of the Fraunhofer pattern. The work presents a comprehensive treatment of the role of pure spatial inhomogeneity in the emergence of a diode effect in planar junctions, with an extension to the multi-terminal case and to systems of Josephson junctions connected in parallel.

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arXiv2025

Supercurrent diode with high winding vortex

Nonreciprocal supercurrent refers to the phenomenon where the maximum dissipationless current in a superconductor depends on its direction of flow. This asymmetry underlies the operation of superconducting diodes and is often associated with the presence of vortices. Here, we investigate supercurrent nonreciprocal effects in a superconducting weak-link hosting distinct types of vortices. We demonstrate how the winding number of the vortex, its spatial configuration, and the shape of the superconducting lead can steer the sign and amplitude of the supercurrent rectification. We identify a general criterion for optimizing the rectification amplitude based on vortex patterns, focusing on configurations where the first harmonic of the supercurrent vanishes. We prove that supercurrent nonreciprocal effects can be used to diagnose high-winding vortex and to distinguish between different types of vorticity. Our results provide a toolkit for controlling supercurrent rectification through vortex phase textures and detecting unconventional vortex states.

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arXiv2025

Supercurrent Time Division Multiplexing with Solid-State Integrated Hybrid Superconducting Electronics

Time-division multiplexing of cryogenic signals is a promising approach to reduce space requirements, shorten cooldown times, and increase the number of quantum devices measured per cooldown. We demonstrate time-division multiplexing of non-dissipative supercurrents using voltage-controlled hybrid superconducting demultiplexers. These chips integrate superconducting Josephson Field Effect Transistors including Al superconducting electrodes, proximitized semiconducting InAs channels, and hafnium oxide gate insulators. Each transistor fully suppresses the switching current and increases the resistance 20 times under a gate voltage of -4.5 V. A demultiplexer with one input and eight outputs showed a non-dissipative input range of +-2 uA, operating up to 100 MHz in signal frequency and 100 kHz in switching frequency at 50 mK. It achieved near-zero insertion loss in the superconducting state and an ON/OFF ratio of 17.5 dB. By optimizing the signal layout, the operation was extended up to 4 GHz using a demultiplexer with two outputs.

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arXiv2025

Quasi-$Φ_0$-periodic supercurrent at quantum Hall transitions

The combination of superconductivity and quantum Hall (QH) effect is regarded as a key milestone in advancing topological quantum computation in solid-state systems. Recent quantum interference studies suggest that QH edge states can effectively mediate a supercurrent across high-quality graphene weak links. In this work we report the observation of a supercurrent associated with transitions between adjacent QH plateaus, where transport paths develop within the compressible two-dimensional bulk. We employ a back-gated graphene Josephson junction, comprising high-mobility CVD-grown graphene encapsulated in hexagonal Boron Nitride (hBN) and contacted by Nb leads. Superconducting pockets are detected persisting beyond the QH onset, up to 2.4 T, hence approaching the upper critical field of the Nb contacts. We observe an approximate $Φ_0=h/2e$ periodicity of the QH-supercurrent as a function of the magnetic field, indicating superconducting interference in a proximitized percolative phase. These results provide a promising experimental platform to investigate the transport regime of percolative supercurrents, leveraging the flexibility of van der Waals devices.

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arXiv2025

The Ferroelectric Superconducting Field Effect Transistor

The ferroelectric field-effect transistor (Fe-FET) is a three-terminal semiconducting device first introduced in the 1950s. Despite its potential, a significant boost in Fe-FET research occurred about ten years ago with the discovery of ferroelectricity in hafnium oxide. This material has been incorporated into electronic processes since the mid-2000s. Here, we observed ferroelectricity in a superconducting Josephson FET (Fe-JoFET) operating at cryogenic temperatures below 1 Kelvin. The Fe-JoFET was fabricated on the InAsOI platform, which features an InAs epilayer hosted by an electrical insulating substrate, using HfO2 as the gate insulator, making it a promising candidate due to its ferroelectric properties. The Fe-JoFET exhibits significant hysteresis in the switching current and normal-state resistance transfer characteristics, which depend on the range of gate voltages. This phenomenon opens a new research area exploring the interaction between ferroelectricity and superconductivity in hybrid superconducting-semiconducting systems, with potential applications in cryogenic data storage and computation. Supporting this, the Fe-JoFET was operated as a cryogenic superconducting single memory cell, exhibiting both dissipative and non-dissipative states. Its non-volatility was tested over a 24-hour measurement period. We also demonstrated that the Fe-JoFET can retain information at temperatures above the superconductor critical temperature, resulting in a temperature-fault-tolerant memory cell resistant to temperature oscillations or, in the worst case, cryostat faults.

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arXiv2025

Josephson Field Effect Transistors with InAs on Insulator and High Permittivity Gate Dielectrics

InAs on Insulator (InAsOI) has been recently demonstrated as a promising platform to develop hybrid semiconducting-superconducting Josephson Junctions (JJs) and Josephson Field Effect Transistors (JoFETs). The InAsOI consists of an InAs epilayer grown onto a cryogenic-electrically-insulating InAlAs metamorphic buffer, which allows the electrical decoupling of surface-exposed adjacent devices together with a high critical current density integration. The miniaturization of Si microchips has progressed significantly due to the integration of high permittivity (high-k) gate insulators, allowing an increased gate coupling with the transistor channel with consequent reduced gate operating voltages and leakages. As well as for Si-based FETs, integrating high-k gate insulators with JoFETs promises similar advantages in superconducting electronics. Here, we investigate the gate-tunable electrical properties of InAsOI-based JoFETs featuring different high-k gate insulators, namely, HfO2 and Al2O3. We found that both the ungated and gate-tunable electrical properties of the JoFETs are strongly dependent on the insulator chosen. With both dielectrics, the JoFETs can entirely suppress the switching current and increase the normal state resistance by 10-20 times using negative gate voltages. The HfO2-JoFETs exhibit improved gate-tunable electrical performance compared to those achieved with Al2O3-JoFETs, which is related to the higher permittivity of the insulator. Gate-dependent electrical properties of InAsOI-based JoFETs were evaluated in the temperature range from 50 mK to 1 K. Moreover, under the influence of an out-of-plane magnetic field, JoFETs exhibited an unconventional Fraunhofer diffraction pattern, from which an edge-peaked supercurrent density distribution was calculated.

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arXiv2025

Electrostatic field-driven supercurrent suppression in ionic-gated metallic Josephson nanotransistors

Recent experiments have shown the possibility of tuning the transport properties of metallic nanosized superconductors through a gate voltage. These results renewed the longstanding debate on the interaction between electrostatic fields and superconductivity. Indeed, different works suggested competing mechanisms as the cause of the effect: an unconventional electric field-effect or quasiparticle injection. Here, we provide conclusive evidence for the electrostatic-field-driven control of the supercurrent in metallic nanosized superconductors, by realizing ionic-gated superconducting field-effect nanotransistors (ISFETs) where electron injection is impossible. Our Nb ISFETs show giant suppression of the superconducting critical current of up to 45%. Moreover, the bipolar supercurrent suppression observed in different ISFETs, together with invariant critical temperature and normal-state resistance, also excludes conventional charge accumulation/depletion. Therefore, the microscopic explanation of this effect calls upon a novel theory able to describe the nontrivial interaction of static electric fields with conventional superconductivity.

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arXiv2025

Back-action supercurrent diodes

Back-action refers to a response that retro-acts on a system to tailor its properties with respect to an external stimulus. This effect is at the heart of many electronic devices such as amplifiers, oscillators, and sensors. Here, we demonstrate that back-action can be exploited to achieve non-reciprocal transport in superconducting circuits. In our devices, dissipationless current flows in one direction whereas dissipative transport occurs in the opposite direction. Supercurrent diodes presented so far rely on magnetic elements or vortices to mediate charge transport or external magnetic fields to break time-reversal symmetry. Back-action solely turns a conventional reciprocal superconducting weak link with no asymmetry between the current bias directions into a rectifier, where the critical current amplitude depends on the bias sign. The self-interaction of the supercurrent stems from the gate tunability of the critical current in metallic and semiconducting systems, which promotes nearly ideal magnetic field-free rectification with selectable polarity.

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arXiv2023

Sign reversal diode effect in superconducting Dayem nanobridges

Supercurrent diodes are nonreciprocal electronic elements whose switching current depends on their flow direction. Recently, a variety of composite systems combining different materials and engineered asymmetric superconducting devices have been proposed. Yet, ease of fabrication and tunable sign of supercurrent rectification joined to large efficiency have not been assessed in a single platform so far. We demonstrate that all-metallic superconducting Dayem nanobridges naturally exhibit nonreciprocal supercurrents under an external magnetic field, with a rectification efficiency up to $\sim 27\%$. Our niobium nanostructures are tailored so that the diode polarity can be tuned by varying the amplitude of an out-of-plane magnetic field or the temperature in a regime without magnetic screening. We show that sign reversal of the diode effect may arise from the high-harmonic content of the current phase relation in combination with vortex phase windings present in the bridge or an anomalous phase shift compatible with anisotropic spin-orbit interactions.

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arXiv2023

Superconductor-ferromagnet hybrids for non-reciprocal electronics and detectors

We review the use of hybrid thin films composed of superconductors and ferromagnets for creating non-reciprocal electronic components and self-biased detectors of electromagnetic radiation. We begin by introducing the theory behind these effects, as well as discussing various potential materials that can be used in the fabrication of these components. We then proceed with a detailed discussion on the fabrication and characterization of Al/EuS/Cu and EuS/Al/Co-based detectors, along with their noise analysis. Additionally, we suggest some approaches for multiplexing such self-biased detectors.

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arXiv2023

Half-integer Shapiro steps in highly transmissive InSb nanoflag Josephson junctions

We investigate a ballistic InSb nanoflag-based Josephson junction with Nb superconducting contacts. The high transparency of the superconductor-semiconductor interfaces enables the exploration of quantum transport with parallel short and long conducting channels. Under microwave irradiation, we observe half-integer Shapiro steps that are robust to temperature, suggesting their possible non-equilibrium origin. Our results demonstrate the potential of ballistic InSb nanoflags Josephson junctions as a valuable platform for understanding the physics of hybrid devices and investigating their non-equilibrium dynamics.

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arXiv2022

Josephson Diode Effect in High Mobility InSb Nanoflags

We report evidence of non-reciprocal dissipation-less transport in single ballistic InSb nanoflag Josephson junctions, owing to a strong spin-orbit coupling. Applying an in-plane magnetic field, we observe an inequality in supercurrent for the two opposite current propagation directions. This demonstrates that these devices can work as Josephson diodes, with dissipation-less current flowing in only one direction. For small fields, the supercurrent asymmetry increases linearly with the external field, then it saturates as the Zeeman energy becomes relevant, before it finally decreases to zero at higher fields. We show that the effect is maximum when the in-plane field is perpendicular to the current vector, which identifies Rashba spin-orbit coupling as the main symmetry-breaking mechanism. While a variation in carrier concentration in these high-quality InSb nanoflags does not significantly influence the diode effect, it is instead strongly suppressed by an increase in temperature. Our experimental findings are consistent with a model for ballistic short junctions and show that the diode effect is intrinsic to this material. Our results establish InSb Josephson diodes as a useful element in superconducting electronics.

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arXiv2022

Single charge transport in a fully superconducting SQUISET locally tuned by self-inductance effects

We present a single-electron device for the manipulation of charge states via quantum interference in nanostructured electrodes. Via self-inductance effects, we induce two independent magnetic fluxes in the electrodes and we demonstrate sensitivity to single charge states and magnetic field at variable temperature. Moreover, our approach allows us to demonstrate local and independent control of the single-particle conductance between nano-engineered tunnel junctions in a fully-Superconducting Quantum Interference Single-Electron Transistor (SQUISET), thereby increasing the flexibility of our single-electron transistors. Our devices show a robust modulation of the current-to-flux transfer function via control currents, while exploiting the single-electron filling of a mesoscopic superconducting island. Further applications of the device concept to single-charge manipulation and magnetic-flux sensing are also discussed.

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arXiv2022

Thermal superconducting quantum interference proximity transistor

Superconductors are known to be excellent thermal insulators at low temperature owing to the presence of the energy gap in their density of states (DOS). In this context, the superconducting \textit{proximity effect} allows to tune the local DOS of a metallic wire by controlling the phase bias ($φ$) imposed across it. As a result, the wire thermal conductance can be tuned over several orders of magnitude by phase manipulation. Despite strong implications in nanoscale heat management, experimental proofs of phase-driven control of thermal transport in superconducting proximitized nanostructures are still very limited. Here, we report the experimental demonstration of efficient heat current control by phase tuning the superconducting proximity effect. This is achieved by exploiting the magnetic flux-driven manipulation of the DOS of a quasi one-dimensional aluminum nanowire forming a weal-link embedded in a superconducting ring. Our thermal superconducting quantum interference transistor (T-SQUIPT) shows temperature modulations up to $\sim 16$ mK yielding a temperature-to-flux transfer function as large as $\sim 60$ mK/$Φ_0$. Yet, phase-slip transitions occurring in the nanowire Josephson junction induce a hysteretic dependence of its local DOS on the direction of the applied magnetic field. Thus, we also prove the operation of the T-SQUIPT as a phase-tunable \textit{thermal memory}, where the information is encoded in the temperature of the metallic mesoscopic island. Besides their relevance in quantum physics, our results are pivotal for the design of innovative coherent caloritronics devices such as heat valves and temperature amplifiers suitable for thermal logic architectures.

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arXiv2021

Evidence of Josephson coupling in a few-layer black phosphorus planar Josephson junction

Setting up strong Josephson coupling in van der Waals materials in close proximity to superconductors offers several opportunities both to inspect fundamental physics and to develop novel cryogenic quantum technologies. Here we show evidence of Josephson coupling in a planar few-layer black Phosphorus junction. The planar geometry allows us to probe the junction behavior by means of external gates, at different carrier concentrations. Clear signatures of Josephson coupling are demonstrated by measuring supercurrent flow through the junction at milli Kelvin temperatures. Manifestation of Fraunhofer pattern with a transverse magnetic field is also reported, confirming the Josephson coupling. These findings represent the first evidence of proximity Josephson coupling in a planar junction based on a van der Waals material beyond graphene and open the way to new studies, exploiting the peculiar properties of exfoliated black phosphorus thin flakes.

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arXiv2021

Thermodynamics in topological Josephson junctions

We study the thermodynamic properties of topological Josephson junctions using a quantum spin Hall (QSH) insulator-based junction as an example. In particular, we propose that phase-dependent measurements of the heat capacity offer an alternative to Josephson-current measurements to demonstrate key topological features. Even in an equilibrium situation, where the fermion parity is not conserved, the heat capacity exhibits a pronounced double peak in its phase dependence as a signature of the protected zero-energy crossing in the Andreev spectrum. This double-peak feature is robust against changes of the tunneling barrier and thus allows one to distinguish between topological and trivial junctions. At short time scales fermion parity is conserved and the heat capacity is $4π$-periodic in the superconducting phase difference. We propose a dispersive setup coupling the Josephson junction to a tank LC circuit to measure the heat capacity of the QSH-based Josephson junction sufficiently fast to detect the $4π$-periodicity. Although explicitly calculated for a short QSH-based Josephson junction, our results are also applicable to long as well as nanowire-based topological Josephson junctions.

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