Josephson Junctions Research Articles

Josephson junction of minimally twisted bilayer graphene

Josephson junction of minimally twisted bilayer graphene

Fractional AC Josephson effect in a topological insulator proximitized by a self-formed superconductor

Fractional AC Josephson effect in a topological insulator proximitized by a self-formed superconductor

Microwave signatures of topological superconductivity in planar Josephson junctions

Microwave signatures of topological superconductivity in planar Josephson junctions

Reduced decay in Josephson coupling across ferromagnetic junctions with spin–orbit coupling layers

The generation of Sz = 1 triplet Cooper pairs has been predicted theoretically in superconducting–ferromagnetic hybrid heterostructures in the presence of spin–orbit coupling [F. S. Bergeret and I. V. Tokatly, Phys. Rev. B 89, 134517 (2014) and Jacobsen et al., Sci. Rep. 6, 23926 (2016)]. In this study, we experimentally investigate vertical Josephson junctions where the weak link is formed from a ferromagnetic layer with perpendicular magnetic anisotropy sandwiched by two non-magnetic layers with weak or strong spin–orbit coupling. We find that the decay of the Josephson coupling is reduced in the latter case, possibly indicating the presence of Sz = 1 spin-triplet correlations. We speculate that the canted magnetization required for these correlations is provided by the interaction of magnetization with Meissner effect in the superconducting layers.

Noise Scaling in SQUID Arrays

Abstract We numerically investigate the noise scaling in high-T_c commensurate 1D and 2D SQUID arrays. We show that the voltage noise spectral density in 1D arrays violates the scaling rule of ~1/N_p for the number N_p of Josephson junctions in parallel. In contrast, in 2D arrays with N_s 1D arrays in series, the voltage noise spectral density follows more closely the expected scaling behaviour of ~N_s/N_p. Additionally, we reveal how the flux and magnetic field rms noise spectral densities deviate from their expected ~(N_sN_p)^{-1/2} scaling and discuss their implications for designing low noise magnetometers.

Thermal transport across a Josephson junction in a dissipative environment

Thermal transport across a Josephson junction in a dissipative environment

Spin-Degeneracy Breaking and Parity Transitions in Three-Terminal Josephson Junctions

Hybrid Josephson junctions (JJs) realized in superconductor-semiconductor heterostructures host fermionic modes known as Andreev bound states (ABSs). In these structures, a promising and yet unexplored avenue for harnessing spin and parity degrees of freedom is offered by JJs with three or more superconducting terminals, where phase-induced spin polarization and transitions of the ground state to an odd parity were predicted to arise. Here we spectroscopically probe the two-dimensional band structure of ABSs in a phase-controlled InAs/Al three-terminal JJ. Andreev bands show signatures of spin-degeneracy breaking, with level splitting in excess of ∼9 GHz, and zero-energy crossings associated to ground state fermion parity transitions. Spin splitting and parity transitions are enabled and controlled by locally applied magnetic fluxes, in the absence of Zeeman effect or Coulomb blockade. Our results underscore the potential of multiterminal hybrid devices for phase engineering ABSs, with significant implications for spin- and parity-based quantum devices. Published by the American Physical Society 2024

Full counting statistics of superconductor hybrid structures involving a time-dependent field

Here we discuss the charge transfer statistics of superconductor-quantum dot-superconductor contacts in nonequilibrium and involving a time-dependent field via the applied voltage. In this case, the effects of multiple Andreev reflections and the Josephson effect exhibit distinctive features. In addition to the effects observable in the conductance, we will also discuss the effects observable in the noise.

On mathematical analysis of synchronization of bidirectionally coupled Kuramoto oscillators under inertia effect

In this article, we analyze the phase synchronization and frequency synchronization for the bidirectionally coupled Kuramoto model under the effect of inertia. Unlike the classical Kuramoto model equipped with all-to-all coupled interaction, in the setting of this model, each oscillator θi only interacts directly with θi+1 and θi−1. The bidirectional interaction is a typical setting of the concatenation in power systems. Additionally, it is necessary to impose the effect of inertia in the Kuramoto model in the applications such as power systems and Josephson junction array. In this article, we first present a theory of the global convergence for frequency synchronization for the identical case. For the non-identical case, we prove that the second-order bidirectionally coupled Kuramoto model exhibits a frequency synchronization if the coupling strength is large, inertia is small, and all oscillators are initially confined to a sector. We emphasize that the arc length of this sector possesses a positive lower bound which is independent of the number of oscillators. If, in addition, all natural frequencies are identical, we further show that the phase synchronization emerges. Moreover, we demonstrate the numerical simulations to support the main results. On the other hand, we observe that the model equipped with large inertia can exhibit the synchronization. Exploring the synchronization theory for large inertia case is left as the future work.

Twist-and-turn dynamics of spin squeezing in bosonic Josephson junctions: Enhanced shortcuts-to-adiabaticity approach

Twist-and-turn dynamics of spin squeezing in bosonic Josephson junctions: Enhanced shortcuts-to-adiabaticity approach

(Invited) Development of Single Crystal Nbn Superconductor on Wide Bandgap GaN By Sputtering and Thermal Annealing

The field of quantum computing is rapidly evolving, and Niobium Nitride (NbN) superconductors have emerged as a pivotal component in the field of quantum computing due to their unique properties. NbN possesses relatively higher superconducting transition temperature (Tc = 15-17 K) as compared to traditional superconductors, making it a promising material for various applications, including superconductor quantum computers, single photon detectors, and hot electron bolometers [1]. The investigation of NbN's superconductivity has been a subject of extensive research over the past few decades. Given that the properties of NbN are intricately linked to its crystal structure, lattice constant, and nitrogen content, precise control of the structural and electrical characteristics of NbN films is essential for the realization of NbN based quantum devices. The (111) planes of δ-NbN exhibit relatively small lattice mismatches with group-III nitride semiconductors (~0.2% for AlN and ~2.7% for GaN) [2], suggesting the potential for integrating crystalline NbN superconductors with nitride semiconductors on a single wafer. Notably, N-polar AlGaN/GaN high electron mobility transistors have been successfully fabricated on NbN films, operating under Tc with a negative differential resistance [3], indicating that the integration of nitride semiconductors and superconductors can be harnessed to create Josephson junctions. To achieve high-quality all-nitride NbN/group-III nitride/NbN Josephson junctions, a comprehensive understanding of the NbN/GaN heterointerfaces is crucial. Despite limited reports on the epitaxial growth of NbN films on nitride semiconductors, little is known about the structural and transport properties of such NbN films. Previously, MBE growth of NbN films have been demonstrated on GaN substrate, showing superconducting critical temperatures exceeding 10 K [4]. In this presentation, our efforts on the development of single crystal NbN thin films on wide bandgap GaN substrate using cost efficient sputtering technique followed by a subsequent high-temperature annealing will be presented. Commercially available single-crystalline GaN (0001)/sapphire templates as well as differently oriented (c and a-planes) sapphire substrates are used as the substrates for the sputtered deposition of NbN films at 20 °C. After the deposition, a high temperature annealing was performed separately at 950 °C for 30 mins under Ar as well as N2 atmosphere. While the as-deposited NbN film on GaN did not exhibit a discernible peak in the high-resolution XRD measurement, a high-intensity, single crystal (111)-oriented NbN film emerged prominently after annealing in both N2 and Ar environments. This suggests a notable enhancement in the crystalline quality of the superconducting material attributable to the thermal annealing process. Although the crystallinity of the NbN layer improves post-annealing, there is an associated increase in grain size and surface roughness. Preliminary findings indicate a reduction in the critical temperature of the NbN film from 12.82 K to 8.69 K after high-temperature annealing, potentially linked to the incorporation of impurities and crystallographic defects during this process. Further investigations using atomic resolution transmission electron microscopy, secondary ion mass spectrometry and X-ray photoelectron spectroscopy will be performed to investigate the influence of annealing duration, temperature, and conditions on the critical temperature, impurity incorporation and defect formation in NbN films deposited with different thickness.In continuation of our recent exploration into depositing crystalline superconducting NbN films on III-nitride semiconductors, this presentation will also focus on our recent advancement of ultrawide bandgap semiconductor materials such as the growth of high-quality quality β-Ga2O3 films and its ternary (AlxGa1-x)2O3 alloys, using metalorganic (MOCVD) and low-pressure (LPCVD) chemical vapor deposition growth techniques. Ga2O3 and (AlxGa1-x)2O3 have emerged as a promising semiconductor material platform for applications in high-power electronics and ultraviolet optoelectronics due to their excellent chemical and thermal stability, as well as high breakdown field strengths. The structural and electrical characteristics of Si-doped β-Ga2O3 and (AlxGa1-x)2O3 thin films will be presented. Additionally, we will present recent findings on achieving phase-pure β-(AlxGa1-x)2O3 films with a record-high Al composition (< 99%) [5]. Our research efforts into understanding the orientation-dependent Al incorporation in β-(AlxGa1-x)2O3 thin films and the determination of band offsets at β-(AlxGa1-x)2O3/β-Ga2O3 heterostructures will be presented. Furthermore, the presentation will touch upon the heteroepitaxial development of other phases of Ga2O3 and (AlxGa1-x)2O3, including α, γ, and κ. Acknowledgment: Dr. Bhuiyan acknowledges the support and guidance of his PhD advisor- Dr. Hongping Zhao at The Ohio State University for the MOCVD growth and characterization of β-Ga2O3 and (AlxGa1-x)2O3 films conducted in Zhao's group. Reference: Yamashita et al., Phys. Rev. Appl. 8, 054028 (2017).Kobayashi et al., Appl. Phys. Express 13, 061006 (2020).Yan et al., Nature 555, 183 (2018).G. Wright et al., APL Mater. 10, 051103 (2022).Bhuiyan et. al, Phys. Status Solidi RRL 17, 2300224 (2023). Figure 1

Large Superconducting Diode Effect in Chiral Carbon Nanotubes

Superconducting electronics are ubiquitous in devices ranging from single-photon detectors to quantum bits. One of the most common superconducting circuit elements is a Josephson junction (JJ)—a junction of two superconductors coupled through either a small normal metal or an insulator. Recently, non-reciprocal switching currents in JJs based on traditional superconductors have been reported, demonstrating a robust diode effect. This so-called Josephson diode effects (JDEs) has since attracted interest to shed light on material and device properties, but also to enable new applications. In this talk, we will present theoretical results on the JDE in chiral carbon nanotubes (CNTs). We find that these CNT-JJs can exhibit large diode efficiencies when an external magnetic field is applied along the nanotube. Additionally, our numerical simulations show that the polarity of the diode can be tuned by electrostatically gating the CNT-JJ. We will discuss the microscopic details that give rise to large diode efficiencies and gate-tunability in chiral CNT-JJs.This work was supported by the U.S. Department of Energy (DOE), Office of Science, Basic Energy Sciences (BES), under Award DE-SC0022245. The work at Sandia is supported by a LDRD project. SNL is managed and operated by NTESS under DOE NNSA contract DE-NA0003525.

Quantum Enhanced Josephson Junction Field-Effect Transistors for Logic Applications

The need of data centers for cloud and network computing has dramatically increased power consumption. In 2014, data centers in the U.S. consumed an estimated 70 billion kWh, representing about 1.8% of total U.S. electricity consumption [“United States Data Center Energy Usage Report," published in June 2016 and supported by the Federal Energy Management Program of the U.S. Department of Energy]. Worldwide, the International Energy Agency estimates that currently about 1% of all global electricity is used by data centers. It is expected that by 2030 the electricity use of information and communications technology may exceed 20% of all global electricity. The microelectronics industry has been focusing on aggressively shrinking the size of logic and memory devices to reduce power consumption. However, this scaling is going to eventually stop as we approach the fundamental materials limit. Moreover, the power dissipation due to the increased number of transistors is also expected to be a limiting factor for processor performance.Superconducting computing has been suggested as a promising approach for low-energy, power-efficient circuit applications. Moreover, using superconducting interconnects can further reduce power consumption. Josephson junction field-effect transistors (JJFET, Fig. a) have recently emerged as a promising candidate for superconducting computing. JJFETs are particularly useful for low power consumption applications as they are operated with, in the superconducting regime, zero voltage drop across its source and drain. Feasibility of using JJFETs for logic operations has been explored [1]. Moreover, high noise margin has been demonstrated for the logic inputs [2]. Compared to single flux quantum approach, JJFET circuits can use designs similar to conventional silicon MOSFETs [1].For JJFETs to perform logic operations, the gain-factor (αR) value must be larger than 1. Here αR = dIc/d(Vg–Vt) x πΔ/Ic, Δ is the superconducting gap, Vg the gate bias voltage, Vt the threshold voltage. Ic ∝ exp(-L/ξc) is the critical supercurrent, where L is the channel length and ξc the carrier coherence length [3]. In a conventional JJFET, ξc ∝ (Vg–Vt)0.5 (Fig. b), and thus dIc/d(Vg–Vt) is small (Fig. c). This translates to a requirement of superconducting transition temperature of ~ 400K for αR larger than 1, far exceeding any recorded critical temperatures. As such, it is impossible to use conventional JJFETs for logic operations.Here, we propose a novel type of JJFET based on quantum phase transition, such as the excitonic insulator (EI) transition in a type-II InAs/GaSb heterostructure [4,5], for low-energy, power-efficient logic applications. The nature of the collective phenomenon in the EI quantum phase transition can provide a sharp transition of the supercurrent states (e.g., ξc ∝ (Vg–Vt)5 and dIc/d(Vg–Vt) very large, as shown in Figs. b and c, respectively) which will enable αR larger than 1 with an easy-to-achieve superconducting transition temperature of ~ 40K. In this talk, we will present some preliminary results demonstrating that indeed the gain factor in these quantum enhanced JJFETs can be greatly improved, thus making them a promising candidate for logic applications.Sandia National Laboratories is a multimission laboratory managed and operated by NTESS LLC, a wholly owned subsidiary of Honeywell International Inc. for the U.S. DOE’s NNSA under contract DE-NA0003525. This paper describes objective technical results and analysis. Any subjective views or opinions that might be expressed in the paper do not necessarily represent the views of the U.S. DOE or the United States Government.

One-dimensional Z4 topological superconductor

We describe the mean-field model of a one-dimensional topological superconductor with two orbitals per unit cell. Time-reversal symmetry is absent, but a nonsymmorphic symmetry, involving a translation by a fraction of the unit cell, mimics the role of time-reversal symmetry. As a result, the topological superconductor has Z4 topological phases, two that support Majorana bound states and two that do not, in agreement with a prediction based on K-theory classification [K. Shiozaki, M. Sato, and K. Gomi, ]. As with the Kitaev chain, the presence of Majorana bound states gives rise to the 4π-periodic Josephson effect. A random matrix with nonsymmorphic time-reversal symmetry may be block diagonalized, and every individual block has time-reversal symmetry described by one of the Gaussian orthogonal, unitary, or symplectic ensembles. We show how this is manifested in the energy level statistics of a random system in the Z4 class as the spatial period of the nonsymmorphic symmetry is varied from much less than to of the order of the system size. Published by the American Physical Society 2024

Topological superconductivity in a magnetic-texture coupled Josephson junction

Topological superconductivity in a magnetic-texture coupled Josephson junction

Theory of universal diode effect in three-terminal Josephson junctions

We theoretically study the superconducting diode effect in a three-terminal Josephson junction. The diode effect in superconducting systems is typically related to the presence of a difference in the critical currents for currents flowing in the opposite direction. We show that in multi-terminal systems this effect occurs naturally without the need of the presence of any spin interactions and is a result of the presence of a relative shift between the Andreev bound states carrying the supercurrent. On an example of a three-terminal junction, we demonstrate that the non-reciprocal current in one of the superconducting contacts can be induced by proper phase biasing of the other contacts, provided that there are at least two Andreev bound states in the system and the symmetry of the system is broken. This result is confirmed in numerical models describing the junctions in both the short- and long-regime. By optimizing the geometry of the junction, we show that the efficiency of the realized superconducting diode exceeds 35%. We relate our predictions to recent experiments on multi-terminal junctions, in which non-reciprocal supercurrents were observed.

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

Resonant switching current detector based on underdamped Josephson junctions

Current-biased Josephson junctions can act as detectors of electromagnetic radiation. At optimal conditions, their sensitivity is limited by fluctuations causing stochastic switching from the superconducting to the resistive state. This work provides a quantitative description of a stochastic switching current detector, based on an underdamped Josephson junction. It is shown that activation of a Josephson plasma resonance can greatly enhance the detector responsivity in proportion to the quality factor of the junction. The ways of tuning the detector for achieving optimal operation are discussed. For realistic parameters of Nb/AlOx/Nb tunnel junctions, the sensitivity and noise-equivalent power (NEP) can reach values of S≃5×1012 (V/W) and NEP≃2×10−23 (WHz−1/2), respectively. These outstanding characteristics facilitate both bolometric and single-photon detection in microwave and terahertz ranges. Published by the American Physical Society 2024

A new behavioral-level model of superconducting Josephson junctions with Simulink

A new behavioral-level model of superconducting Josephson junctions with Simulink

Pressure Induced Nonmonotonic Evolution of Superconductivity in 6R-TaS_{2} with a Natural Bulk Van der Waals Heterostructure.

The natural bulk Van der Waals heterostructure compound 6R-TaS_{2} consists of alternate stacking 1T- and 1H-TaS_{2} monolayers, creating a unique system that incorporates charge-density-wave (CDW) order and superconductivity (SC) in distinct monolayers. Here, after confirming that the 2D nature of the lattice is preserved up to 8GPa in 6R-TaS_{2}, we documented an unusual evolution of CDW and SC by conducting high-pressure electronic transport measurements. Upon compression, we observe a gradual suppression of CDW within the 1T layers, while the SC exhibits a dome-shaped behavior that terminates at a critical pressure P_{c} around 2.9GPa. By taking account of the fact that the substantial suppression of SC is concomitant with the complete collapse of CDW order at P_{c}, we argue that the 6R-TaS_{2} behaves like a stack of Josephson junctions and thus the suppressed superconductivity can be attributed to the weakening of Josephson coupling associated with the presence of CDW fluctuations in the 1T layers. Furthermore, the SC reversely enhances above P_{c}, implying the development of emergent superconductivity in the 1T layers after the melting of T-layer CDW orders. These results show that the 6R-TaS_{2} not only provides a promising platform to explore emergent phenomena but also serves as a model system to study the complex interactions between competing electronic states.