Graphene Josephson Junctions Research Articles

Self-heating effects and switching dynamics in graphene multiterminal Josephson junctions

We experimentally investigate the electronic transport properties of a three-terminal graphene Josephson junction. We find that self-heating effects strongly influence the behavior of this multiterminal Josephson junction (MTJJ) system. We show that existing simulation methods based on resistively and capacitively shunted Josephson junction networks can be significantly improved by taking into account these heating effects. We also investigate the phase dynamics in our MTJJ by measuring its switching current distribution and find correlated switching events in different junctions. We show that the switching dynamics is governed by phase diffusion at low temperatures. Furthermore, we find that self-heating introduces additional damping that results in overdamped I−V characteristics when normal and supercurrents coexist in the device. Published by the American Physical Society 2024

Controllable Andreev Bound States in Bilayer Graphene Josephson Junctions from Short to Long Junction Limits.

We demonstrate that the mode number of Andreev bound states in bilayer graphene Josephson junctions can be modulated by controlling the superconducting coherence length insitu. By exploiting the quadratic band dispersion of bilayer graphene, we control the Fermi velocity and thus the coherence length via the application of electrostatic gating. Tunneling spectroscopy of the Andreev bound states reveals a crossover from short to long Josephson junction regimes as we approach the charge neutral point of the bilayer graphene. Furthermore, analysis of different mode numbers of the Andreev energy spectrum allows us to estimate the phase-dependent Josephson current quantitatively. Our Letter provides a new way for studying multimode Andreev levels by tuning the Fermi velocity.

Phase dependence of thermal transport in graphene Josephson junctions

Phase dependence of thermal transport in graphene Josephson junctions

Tuning the supercurrent distribution in parallel ballistic graphene Josephson junctions

Tuning the supercurrent distribution in parallel ballistic graphene Josephson junctions

Junctions and Superconducting Symmetry in Twisted Bilayer Graphene.

Junctions provide a wealth of information on the symmetry of the order parameter of superconductors. We analyze junctions between a scanning tunneling microscope (STM) tip and superconducting twisted bilayer graphene (TBG) and TBG Josephson junctions (JJs). We compare superconducting phases that are even or odd under valley exchange (s- or f-wave). The critical current in mixed (s and f) JJs strongly depends on the angle between the junction and the lattice. In STM-TBG junctions, due to Andreev reflection, the f-wave leads to a prominent peak in subgap conductance, as seen in experiments.

Observation of half-integer Shapiro steps in graphene Josephson junctions

We study quantum transport and AC Josephson effect of hexagonal boron nitride encapsulated graphene (BGB) Josephson junctions (JJs). Our experiments reveal the emergence of the half-integer Shapiro steps in the n-type regime with high electron carrier densities. We attribute this observation to the gate-tunable transmission probability of the graphene junction. Our numerical simulations are consistent with the appearance of half-integer Shapiro steps at high transparency, which suggests a skewed current phase relationship in the graphene JJ.

Nonreciprocal Supercurrents in a Field-Free Graphene Josephson Triode.

Superconducting diodes are proposed nonreciprocal circuit elements that should exhibit nondissipative transport in one direction while being resistive in the opposite direction. Multiple examples of such devices have emerged in the past couple of years; however, their efficiency is typically limited, and most of them require a magnetic field to function. Here we present a device that achieves efficiencies approaching 100% while operating at zero field. Our samples consist of a network of three graphene Josephson junctions linked by a common superconducting island, to which we refer as a Josephson triode. The three-terminal nature of the device inherently breaks the inversion symmetry, and the control current applied to one of the contacts breaks the time-reversal symmetry. The triode's utility is demonstrated by rectifying a small (nA scale amplitude) applied square wave. We speculate that devices of this type could be realistically employed in the modern quantum circuits.

Non-local Andreev reflection through Andreev molecular states in graphene Josephson junctions

We propose that a device composed of two vertically stacked monolayer graphene Josephson junctions can be used for Cooper pair splitting. The hybridization of the Andreev bound states of the two Josephson junction can facilitate non-local transport in this normal-superconductor hybrid structure, which we study by calculating the non-local differential conductance. Assuming that one of the graphene layers is electron and the other is hole doped, we find that the non-local Andreev reflection can dominate the differential conductance of the system. Our setup does not require the precise control of junction length, doping, or super conducting phase difference, which could be an important advantage for experimental realization.

The study of contact properties in edge-contacted graphene–aluminum Josephson junctions

Transparent contact interfaces in superconductor–graphene hybrid systems are critical for realizing superconducting quantum applications. Here, we examine the effect of the edge contact fabrication process on the transparency of the superconducting aluminum–graphene junction. We show significant improvement in the transparency of our superconductor–graphene junctions by promoting the chemical component of the edge contact etch process. Our results compare favorably with state-of-the-art graphene Josephson junctions. The findings of our study contribute to advancing the fabrication knowledge of edge-contacted superconductor–graphene junctions.

Effect of dilute impurities on short graphene Josephson junctions

Despite the structural simplicity of graphene, its mechanical and electronic remarkable properties make this material a credible starting point for new technologies across a wide range of fields. The recent realizations of graphene-based hybrid systems, such as Josephson junctions, make graphene a promising a platform for new generations of devices for topological quantum computing and quantum sensing. To this aim, accurate control of the electronic properties of graphene Josephson junctions in the presence of disorder is essential. Here, we study the effect of a dilute homogeneous spatial distribution of non-magnetic impurities on the equilibrium supercurrent sustained by a ballistic graphene Josephson junction in the short junction limit. Within the Dirac-Bogoliubov-de Gennes approach and modeling impurities by the Anderson model we derive the supercurrent and its equilibrium power spectrum. We find a modification of the current-phase relation with a reduction of the skewness induced by disorder, and a nonmonotonic temperature dependence of the critical current. The potentialities of the supercurrent power spectrum for accurate spectroscopy of the hybridized Andreev bound states-impurities spectrum are highlighted. In the low temperature limit, the supercurrent zero frequency thermal noise directly probes the spectral function at the Fermi energy.

Quantum-noise-limited microwave amplification using a graphene Josephson junction.

Josephson junctions (JJs) and their tunable properties, including their nonlinearities, play an important role in superconducting qubits and amplifiers. JJs together with the circuit quantum electrodynamics architecture form many key components of quantum information processing1. In quantum circuits, low-noise amplification of feeble microwave signals is essential, and Josephson parametric amplifiers (JPAs)2 are the widely used devices. The existing JPAs are based on Al-AlOx-Al tunnel junctions realized in a superconducting quantum interference device geometry, where magnetic flux is the knob for tuning the frequency. Recent experimental realizations of two-dimensional (2D) van der Waals JJs3-5 provide an opportunity to implement various circuit quantum electrodynamics devices6-8 with the added advantage of tuning the junction properties and the operating point using a gate potential. While other components of a possible 2D van der Waals circuit quantum electrodynamics architecture have been demonstrated, a quantum-noise-limited amplifier, an essential component, has not been realized, to the best of our knowledge. Here we implement a quantum-noise-limited JPA using a graphene JJ, that has a linear resonance gate tunability of 3.5 GHz. We report 24 dB amplification with 10 MHz bandwidth and -130 dBm saturation power, a performance on par with the best single-junction JPAs2,9. Importantly, our gate-tunable JPA works in the quantum-limited noise regime, which makes it an attractive option for highly sensitive signal processing. Our work has implications for novel bolometers; the low heat capacity of graphene together with JJ nonlinearity can result in an extremely sensitive microwave bolometer embedded inside a quantum-noise-limited amplifier. In general, this work will open up the exploration of scalable device architectures of 2D van der Waals materials by integrating a sensor with the quantum amplifier.

A gate-tunable graphene Josephson parametric amplifier

With a large portfolio of elemental quantum components, superconducting quantum circuits have contributed to advances in microwave quantum optics1. Of these elements, quantum-limited parametric amplifiers2-4 are essential for low noise readout of quantum systems whose energy range is intrinsically low (tens of μeV)5,6. They are also used to generate non-classical states of light that can be a resource for quantum enhanced detection7. Superconducting parametric amplifiers, such as quantum bits, typically use a Josephson junction as a source of magnetically tunable and dissipation-free non-linearity. In recent years, efforts have been made to introduce semiconductor weak links as electrically tunable non-linear elements, with demonstrations of microwave resonators and quantum bits using semiconductor nanowires8,9, a two-dimensional electron gas10, carbon nanotubes11 and graphene12,13. However, given the challenge of balancing non-linearity, dissipation, participation and energy scale, parametric amplifiers have not yet been implemented with a semiconductor weak link. Here, we demonstrate a parametric amplifier leveraging a graphene Josephson junction and show that its working frequency is widely tunable with a gate voltage. We report gain exceeding 20 dB and noise performance close to the standard quantum limit. Our results expand the toolset for electrically tunable superconducting quantum circuits. They also offer opportunities for the development of quantum technologies such as quantum computing, quantum sensing and for fundamental science14.

Steady Floquet-Andreev states in graphene Josephson junctions.

Engineering quantum states through light-matter interaction has created a paradigm in condensed-matter physics. A representative example is the Floquet-Bloch state, which is generated by time-periodically driving the Bloch wavefunctions in crystals. Previous attempts to realize such states in condensed-matter systems have been limited by the transient nature of the Floquet states produced by optical pulses1-3, which masks the universal properties of non-equilibrium physics. Here we report the generation of steady Floquet-Andreev states in graphene Josephson junctions by continuous microwave application and direct measurement of their spectra by superconducting tunnelling spectroscopy. We present quantitative analysis of the spectral characteristics of the Floquet-Andreev states while varying the phase difference of the superconductors, the temperature, the microwave frequency and the power. The oscillations of the Floquet-Andreev-state spectrum with phase difference agreed with our theoretical calculations. Moreover, we confirmed the steady nature of the Floquet-Andreev states by establishing a sum rule of tunnelling conductance4, and analysed the spectral density of Floquet states depending on Floquet interaction strength. This study provides a basis for understanding and engineering non-equilibrium quantum states in nanodevices.

Phase-dependent microwave response of a graphene Josephson junction

Gate-tunable Josephson junctions embedded in a microwave environment provide a promising platform to in-situ engineer and optimize novel superconducting quantum circuits. The key quantity for the circuit design is the phase-dependent complex admittance of the junction, which can be probed by sensing an rf SQUID with a tank circuit. Here, we investigate a graphene-based Josephson junction as a prototype gate-tunable element enclosed in a SQUID loop that is inductively coupled to a superconducting resonator operating at 3 GHz. With a concise circuit model that describes the dispersive and dissipative response of the coupled system, we extract the phase-dependent junction admittance corrected for self-screening of the SQUID loop. We decompose the admittance into the current-phase relation and the phase-dependent loss and as these quantities are dictated by the spectrum and population dynamics of the supercurrent-carrying Andreev bound states, we gain insight to the underlying microscopic transport mechanisms in the junction. We theoretically reproduce the experimental results by considering a short, diffusive junction model that takes into account the interaction between the Andreev spectrum and the electromagnetic environment, from which we deduce a lifetime of ~17 ps for non-equilibrium populations.

Critical current fluctuations in graphene Josephson junctions

We have studied 1/f noise in critical current I_c in h-BN encapsulated monolayer graphene contacted by NbTiN electrodes. The sample is close to diffusive limit and the switching supercurrent with hysteresis at Dirac point amounts to simeq 5 nA. The low frequency noise in the superconducting state is measured by tracking the variation in magnitude and phase of a reflection carrier signal v_{rf} at 600–650 MHz. We find 1/f critical current fluctuations on the order of delta I_c/I_c simeq 10^{-3} per unit band at 1 Hz. The noise power spectrum of critical current fluctuations S_{I_c} measured near the Dirac point at large, sub-critical rf-carrier amplitudes obeys the law S_{I_c}/{I{_c}}^2 = a/f^{beta } where asimeq 4times 10^{-6} and beta simeq 1 at f > 0.1 Hz. Our results point towards significant fluctuations in I_c originating from variation of the proximity induced gap in the graphene junction.

Low-frequency critical current noise in graphene Josephson junctions in the open-circuit gate voltage limit

We investigate critical current noise in short ballistic graphene Josephson junctions in the open-circuit gate-voltage limit within the McWorther model. We find flicker noise in a wide frequency range and discuss the temperature dependence of the noise amplitude as a function of the doping level. At the charge neutrality point we find a singular temperature dependence T^{-3}, strikingly different from the linear dependence expected for short ballistic graphene Josephson junctions under fixed gate voltage.

Data and Codes for "Zero Crossing Steps and Anomalous Shapiro Maps in Graphene Josephson Junctions"

Data and Codes for "Zero Crossing Steps and Anomalous Shapiro Maps in Graphene Josephson Junctions"

Data and Codes for "Zero Crossing Steps and Anomalous Shapiro Maps in Graphene Josephson Junctions"

Data and Codes for "Zero Crossing Steps and Anomalous Shapiro Maps in Graphene Josephson Junctions"

Zero Crossing Steps and Anomalous Shapiro Maps in Graphene Josephson Junctions.

The AC Josephson effect manifests itself in the form of "Shapiro steps" of quantized voltage in Josephson junctions subject to radiofrequency (RF) radiation. This effect presents an early example of a driven-dissipative quantum phenomenon and is presently utilized in primary voltage standards. Shapiro steps have also become one of the standard tools to probe junctions made in a variety of novel materials. Here we study Shapiro steps in a widely tunable graphene-based Josephson junction in which the high-frequency dynamics is determined by the on-chip environment. We investigate the variety of patterns that can be obtained in this well-understood system depending on the carrier density, temperature, RF frequency, and magnetic field. Although the patterns of Shapiro steps can change drastically when just one parameter is varied, the overall trends can be understood and the behaviors straightforwardly simulated, showing some key differences from the conventional RCSJ model. The resulting understanding may help interpret similar measurements in more complex materials.

Compact SQUID Realized in a Double-Layer Graphene Heterostructure.

2D systems that host 1D helical states are advantageous from the perspective of scalable topological quantum computation when coupled to a superconductor. Graphene is particularly promising for its high electronic quality, its versatility in van der Waals heterostructures, and its electron- and hole-like degenerate 0th Landau level. Here we study a compact double-layer graphene SQUID (superconducting quantum interference device), where the superconducting loop is reduced to the superconducting contacts connecting two parallel graphene Josephson junctions. Despite the small size of the SQUID, it is fully tunable by the independent gate control of the chemical potentials in both layers. Furthermore, both Josephson junctions show a skewed current-phase relationship, indicating the presence of superconducting modes with high transparency. In the quantum Hall regime, we measure a well-defined conductance plateau of 2e2/h indicative of counter-propagating edge channels in the two layers.