Macroscopic quantum tunnelling in spin filter ferromagnetic Josephson junctions

Design of filtering cable with defected conductor layer

Electrical cables, often referred to as 'blood vessels' and 'nerves' of the industry, play a vital role in the connection of electrical devices. However, traditional cables that lack distributed filtering functions are usually the primary coupling path for electromagnetic compatibility (EMC) problems. An innovative design for a filtering cable, which incorporates insulated electrical wires coated with a specific defected conductor layer (DCL), enables it to achieve distributed filtering advantages along its axis. Microwave network analysis is employed to build the two-port network model of filtering cable, which efficiently analyzes the cascading characteristics of periodic or aperiodic filtering cables. To validate, the flexible printed circuit board (FPCB) with sawtooth dumbbell-shaped DCL and mounted by capacitors is wrapped around the stripped section of the coaxial cable to manufacture a multi-stopband filtering cable. Simulated and measured results demonstrate that the proposed filtering cable can be effectively suppressed in the stopband, which can be adjusted by changing the values of capacitors.

Symmetry-broken Josephson junctions and superconducting diodes in magic-angle twisted bilayer graphene

The coexistence of gate-tunable superconducting, magnetic and topological orders in magic-angle twisted bilayer graphene provides opportunities for the creation of hybrid Josephson junctions. Here we report the fabrication of gate-defined symmetry-broken Josephson junctions in magic-angle twisted bilayer graphene, where the weak link is gate-tuned close to the correlated insulator state with a moiré filling factor of υ = -2. We observe a phase-shifted and asymmetric Fraunhofer pattern with a pronounced magnetic hysteresis. Our theoretical calculations of the junction weak link-with valley polarization and orbital magnetization-explain most of these unconventional features. The effects persist up to the critical temperature of 3.5 K, with magnetic hysteresis observed below 800 mK. We show how the combination of magnetization and its current-induced magnetization switching allows us to realise a programmable zero-field superconducting diode. Our results represent a major advance towards the creation of future superconducting quantum electronic devices.

Van der Waals ferromagnetic Josephson junctions

Superconductor-ferromagnet interfaces in two-dimensional heterostructures present a unique opportunity to study the interplay between superconductivity and ferromagnetism. The realization of such nanoscale heterostructures in van der Waals (vdW) crystals remains largely unexplored due to the challenge of making atomically-sharp interfaces from their layered structures. Here, we build a vdW ferromagnetic Josephson junction (JJ) by inserting a few-layer ferromagnetic insulator Cr2Ge2Te6 into two layers of superconductor NbSe2. The critical current and corresponding junction resistance exhibit a hysteretic and oscillatory behavior against in-plane magnetic fields, manifesting itself as a strong Josephson coupling state. Also, we observe a central minimum of critical current in some JJ devices as well as a nontrivial phase shift in SQUID structures, evidencing the coexistence of 0 and π phase in the junction region. Our study paves the way to exploring sensitive probes of weak magnetism and multifunctional building-blocks for phase-related superconducting circuits using vdW heterostructures.

Unconventional supercurrent phase in Ising superconductor Josephson junction with atomically thin magnetic insulator

In two-dimensional (2D) NbSe₂ crystal, which lacks inversion symmetry, strong spin-orbit coupling aligns the spins of Cooper pairs to the orbital valleys, forming Ising Cooper pairs (ICPs). The unusual spin texture of ICPs can be further modulated by introducing magnetic exchange. Here, we report unconventional supercurrent phase in van der Waals heterostructure Josephson junctions (JJs) that couples NbSe₂ ICPs across an atomically thin magnetic insulator (MI) Cr₂Ge₂Te₆. By constructing a superconducting quantum interference device (SQUID), we measure the phase of the transferred Cooper pairs in the MI JJ. We demonstrate a doubly degenerate nontrivial JJ phase (ϕ), formed by momentum-conserving tunneling of ICPs across magnetic domains in the barrier. The doubly degenerate ground states in MI JJs provide a two-level quantum system that can be utilized as a new dissipationless component for superconducting quantum devices. Our work boosts the study of various superconducting states with spin-orbit coupling, opening up an avenue to designing new superconducting phase-controlled quantum electronic devices.

Van der Waals structures provide a new platform to explore novel physics of superconductor/ferromagnet interfaces. Here, NbSe₂ Josephson junction with Cr₂Ge₂Te₆ enables non-trivial Josephson phase by spin-dependent interaction, boosting the study of superconducting states with spin-orbit coupling and phase-controlled quantum electronic device.

Low temperature characterization of high efficiency spin-filter Josephson junctions

The interplay between superconducting and ferromagnetic order pa¬rameters in S/F interfaces gives rise to a wide range of peculiar properties with applications in high-efficiency computation and in the emerging field of super¬conducting spintronics. In NbN/GdN/NbN Josephson junctions, GdN barriers give unique properties due to the double insulting and ferromagnetic nature of the material, as demonstrated in previous works. Here we focus on tunneling spectroscopy of these junctions down to 0.3 K when changing the barrier thick¬ness, which contributes to complete a consistent picture on the physics of these junctions and supports the previous indications of equal-spin Cooper pairs con¬tributing to the total supercurrent of the devices.

Superconductor to resistive state switching by multiple fluctuation events in NbTiN nanostrips

We report on measurements of the switching current distributions on two-dimensional superconducting NbTiN strips that are 5 nm thick and 80 nm wide. We observe that the width of the switching current distributions has a non-monotonous temperature dependence, where it is constant at the lowest temperatures up to about 1.5 K, after which it increases with temperature until 2.2 K. Above 2.5 K any increase in temperature decreases the distribution width which at 4.0 K is smaller than half the width observed at 0.3 K. By using a careful analysis of the higher order moments of the switching distribution, we show that this temperature dependence is caused by switching due to multiple fluctuations. We also find that the onset of switching by multiple events causes the current dependence of the switching rate to develop a characteristic deviation from a pure exponential increase, that becomes more pronounced at higher temperatures, due to the inclusion of higher order terms.

Tuning of Magnetic Activity in Spin-Filter Josephson Junctions Towards Spin-Triplet Transport

The study of superconductor-ferromagnet interfaces has generated great interest in the last decades, leading to the observation of spin-aligned triplet supercurrents and 0-π transitions in Josephson junctions where two superconductors are separated by an itinerant ferromagnet. Recently, spin-filter Josephson junctions with ferromagnetic barriers have shown unique transport properties, when compared to standard metallic ferromagnetic junctions, due to the intrinsically nondissipative nature of the tunneling process. Here we present the first extensive characterization of spin polarized Josephson junctions down to 0.3 K, and the first evidence of an incomplete 0-π transition in highly spin polarized tunnel ferromagnetic junctions. Experimental data are consistent with a progressive enhancement of the magnetic activity with the increase of the barrier thickness, as neatly captured by the simplest theoretical approach including a nonuniform exchange field. For very long junctions, unconventional magnetic activity of the barrier points to the presence of spin-triplet correlations.

Current phase relation from graphs and diagrams and application to thick ferromagnetic Josephson junctions

In this work we present a method of representing terms in the current-phase-relation of a ballistic Josephson junction by combinations of diagrams, used in previous work to represent an equivalent of the matching condition determinant of the junction. This is accomplished by the expansion of the logarithm of this determinant in Taylor series and keeping track of surviving terms, i.e. terms that do not annihilate each other. The types of the surviving terms are represented by connected graphs, whose points represent diagrammatic terms of the determinant expansion. Then the theory is applied to obtain approximations of the current-phase relation of relatively thick ballistic ferromagnetic Josephson junctions with non-collinear magnetizations. This demonstrates the versatility of the method in developing approximations schemes and providing physical insight into the nature of contributions to the supercurrent from the available particle excitations in the junction. We also discuss the strong second harmonic contribution to the supercurrent in junctions with three mutually orthogonal magnetization vectors and a weak intermediate ferromagnet.

Statistics of localized phase slips in tunable width planar point contacts

The main dissipation mechanism in superconducting nanowires arises from phase slips. Thus far, most of the studies focus on long nanowires where coexisting events appear randomly along the nanowire. In the present work we investigate highly confined phase slips at the contact point of two superconducting leads. Profiting from the high current crowding at this spot, we are able to shrink in-situ the nanoconstriction. This procedure allows us to investigate, in the very same sample, thermally activated phase slips and the probability density function of the switching current I_sw needed to trigger an avalanche of events. Furthermore, for an applied current larger than I_sw, we unveil the existence of two distinct thermal regimes. One corresponding to efficient heat removal where the constriction and bath temperatures remain close to each other, and another one in which the constriction temperature can be substantially larger than the bath temperature leading to the formation of a hot spot. Considering that the switching current distribution depends on the exact thermal properties of the sample, the identification of different thermal regimes is of utmost importance for properly interpreting the dissipation mechanisms in narrow point contacts.

Apparent Power Law Scaling of Variable Range Hopping Conduction in Carbonized Polymer Nanofibers

We induce dramatic changes in the structure of conducting polymer nanofibers by carbonization at 800 °C and compare charge transport properties between carbonized and pristine nanofibers. Despite the profound structural differences, both types of systems display power law dependence of current with voltage and temperature, and all measurements can be scaled into a single universal curve. We analyze our experimental data in the framework of variable range hopping and argue that this mechanism can explain transport properties of pristine polymer nanofibers as well.