The continuous development of superconducting electronics is encouraging several studies on hybrid Josephson junctions (JJs) based on superconductor/ferromagnet/superconductor (SFS) heterostructures, as either spintronic devices or switchable elements in quantum and classical circuits. Recent experimental evidence of macroscopic quantum tunneling and of an incomplete 0-pi transition in tunnel-ferromagnetic spin-filter JJs could enhance the capabilities of SFS JJs also as active elements. Here, we provide a self-consistent electrodynamic characterization of NbN/GdN/NbN spin-filter JJs as a function of the barrier thickness, disentangling the high-frequency dissipation effects due to the environment from the intrinsic low-frequency dissipation processes. The fitting of the IV characteristics at 4.2K and at 300mK by using the Tunnel Junction Microscopic model allows us to determine the subgap resistance Rsg, the quality factor Q and the junction capacitance C. These results provide the scaling behavior of the electrodynamic parameters as a function of the barrier thickness, which represents a fundamental step for the feasibility of tunnel ferromagnetic JJs as active elements in classical and quantum circuits, and are of general interest for tunnel junctions other than conventional SIS JJs.
Eigen oscillations in a superconducting parallel plate resonator with the Josephson-coupled plates are investigated. While the insulator thickness S changes from tens of microns down to the decay lengthscale of the superconducting wavefunction into a dielectric, ξ~1 nm, both the resonant frequency and Q-factor vary non-monotonically by up to three orders in magnitude. A crossover between the Swihart waves and Josephson plasmons causes a global minimum in the resonant frequency and a local maximum in the Q-factor at S~10ξ.
Triplet supercurrents in multilayer ferromagnetic Josephson junctions with misaligned magnetization can penetrate thicker ferromagnetic barriers compared to the singlet component. Although the static properties of these junctions have been extensively studied, the dynamic characteristics remain largely unexplored. Here we report a comprehensive electrodynamic characterization of multilayer ferromagnetic Josephson junctions composed of Co and Ho. By measuring the temperature-dependent current-voltage characteristics and the switching current distributions down to 0.3 K, we show that phase dynam…
We study temperature dependence of the critical current modulation Ic(H) for two types of planar Josephson junctions: a low-Tc Nb/CuNi/Nb and a high-Tc YBa2Cu3O7 bicrystal grain-boundary junction. At low T both junctions exhibit a conventional behavior, described by the local sine-Gordon equation. However, at elevated T the behavior becomes qualitatively different: the Ic(H) modulation field deltaH becomes almost T-independent and neither deltaH nor the critical field for penetration of Josephson vortices vanish at Tc. Such an unusual behavior is in good agreement with theoretical predictions …
We experimentally and theoretically investigate electromagnetic cavity modes in ultra-narrow Al-AlO$_x$-Al and Nb-AlO$_x$-Nb long Josephson junctions. Experiments show that the voltage spacing between the Fiske steps on the current-voltage characteristics of sub-$μ$m wide and several hundred $μ$m long Al-AlO$_x$-Al and Nb-AlO$_x$-Nb Josephson junctions increases when decreasing the width of a junction. This effect is explained by stray magnetic fields, which become important for narrow junctions. Theoretical estimates of the Fiske step voltage based on a nonlocal wave propagation equation are …
We demonstrate Josephson tunneling in vacuum tunnel junctions formed between a superconducting scanning tunneling microscope tip and a Pb film, for junction resistances in the range 50-300 k$Ω$. We show that the superconducting phase dynamics is dominated by thermal fluctuations, and that the Josephson current appears as a peak centered at small finite voltages. In the presence of microwave fields (f=15.0 GHz) the peak decreases in magnitude and shifts to higher voltages with increasing rf power, in agreement with theory.
We have developed a scheme for a high resolution measurement of the switching current distribution of a current biased Josephson tunnel junction using a timing technique. The measurement setup is implemented such that the digital control and read-out electronics are optically decoupled from the analog bias electronics attached to the sample. We have successfully used this technique to measure the thermal activation and the macroscopic quantum tunneling of the phase in a small Josephson tunnel junction with a high experimental resolution. This technique may be employed to characterize current-b…
The expression for additional subgap current in the presence of electron-phonon interaction is derived. We show that the phonon assisted tunneling leads to appearance of peaks on current-voltage characteristics at the Josephson frequencies corresponding to the Raman-active phonons. The relation of the obtained results to experimental observations are discussed.
Tunnel junction between two superconductors is considered in the vicinity of the critical temperature. Superconductive fluctuations above Tc give rise to the noise of the ac Josephson current although the current itself is zero in average. As a result of fluctuations, current noise spectrum is peaked at the Josephson frequency, which may be considered as precursor of superconductivity in the normal state. Temperature dependence and shape of the Josephson current noise resonance line is calculated for various junction configurations.
Collective excitations reveal fundamental properties and potential applications of superconducting states. We theoretically study macroscopic quantum tunneling (MQT) in a Josephson junction composed of multi-band superconductors, focusing on a phase mode induced by inter-band fluctuations: the Josephson-Leggett (JL) collective excitation mode. Using the imaginary-time path-integral method, we derive a formula for the MQT escape rate for low-temperature switching events. We clarify that the JL mode has two major effects on the MQT: (i) the zero-point fluctuations enhance the escape rate, and (i…
The physics of Josephson tunnel junctions drastically depends on their geometrical configurations. The shape of the junction determines the specific form of the magnetic-field dependence of the its Josephson current. Here we address the magnetic diffraction patterns of specially shaped planar Josephson tunnel junctions in the presence of an in-plane magnetic field of arbitrary orientations. We focus on a wide ensemble of junctions whose shape is invariant under point reflection. We analyze the implications of this type of isometry and derive the threshold curves of junctions whose shape is the…
We theoretically study Josephson vortex structures in Josephson junctions which have multiple tunneling channels caused by multiple superconducting gaps. Deriving "coupled sine-Gordon equations" from the free-energy taking account of the multiple tunneling channels, we examine two typical situations, a heterotic junction composed of multigap-superconductor, insulator, and single-gap superconductor, and a grain-boundary junction formed by two identical multigap superconductors. Then, we reveal in both situations that the magnetic field distribution of the Josephson vortex for $\pm s$-wave super…
The most important practical characteristic of a Josephson junction is its critical current. The shape of the junction determines the specific form of the magnetic-field dependence of the its Josephson current. Here we address the magnetic diffraction patterns of specially shaped planar Josephson tunnel junctions. We focus on a wide ensemble of generalized ellipses, called superellipses, which retain the second order symmetry. We analyze the implications of this type of isometry and derive the explicit expressions for the threshold curves of superelliptic Josephson junctions. A detailed study …
The magnetic field distribution in the barrier of small planar Josephson tunnel junctions is numerically simulated in the case when an external magnetic field is applied perpendicular to the barrier plane. The simulations allow for heuristic analytical solutions for the Josephson static phase profile from which the dependence of the maximum Josephson current on the applied field amplitude is derived. The most common geometrical configurations are considered and, when possible, the theoretical findings are compared with the experimental data.
Traditionally, since the discovery of the Josephson effect in 1962, the magnetic diffraction pattern of planar Josephson tunnel junctions has been recorded with the field applied in the plane of the junction. Here we discuss the static junction properties in a transverse magnetic field where demagnetization effects imposed by the junction geometry and configuration of the electrodes are important. Measurements of the critical current versus magnetic field in planar Nb-based high-quality junctions with different geometry, size and critical current density show that it is advantageous to use a t…
We have developed a method for the reproducible fabrication of superconducting scanning tunneling microscope (STM) tips. We use these tips to form superconductor/insulator/superconductor tunnel junctions with the STM tip as one of the electrodes. We show that such junctions exhibit fluctuation dominated Josephson effects, and describe how the Josephson product IcRn can be inferred from the junctions' tunneling characteristics in this regime. This is first demonstrated for tunneling into Pb films, and then applied in studies of single crystals of NbSe2. We find that in NbSe2, IcRn is lower than…
Within the tunneling Hamiltonian formulation for the eight-component spinors,the Josephson critical super-current has been calculated in a planar superconductor-normal graphene-superconductor junction. Coupling between superconductor regions and graphene is taken into account by a tunneling Hamiltonian which contains two types of tunneling, intra-valley and inter-valley tunneling. Within the present tunneling approach, we find that the contributions of two kinds of tunneling to the critical super-current, are completely separable. Therefore, it is possible to consider the effect of the inter-v…
We present a theoretical description of the phase dynamics and its corresponding electrodynamics in a stack of inductively coupled intrinsic Josephson junctions of layered high-$T_c$ superconductors in the absence of an external magnetic field. Depending on the spatial structure of the gauge invariant phase difference, the dynamic state is classified into: state with kink, state without kink, and state with solitons. It is revealed that in the state with phase kink, the plasma is coupled to the cavity and the plasma oscillation is enhanced. In contrast, in the state without kink, the plasma os…
It has been argued by Zurek and Kibble that the likelihood of producing defects in a continuous phase transition depends in a characteristic way on the quench rate. In this paper we discuss our experiment for measuring the Zurek-Kibble scaling exponent sigma for the production of fluxons in annular symmetric Josephson Tunnel Junctions. The predicted exponent is sigma = 0.25, and we find sigma = 0.27 +/- 0.05. Further, there is agreement with the ZK prediction for the overall normalisation.
The implications of the two-pocket Fermi surface for macroscopic quantum phenomena are considered. We demonstrate that in the case of the two-pocket Fermi surface the g-wave pairing is closely related to the d-wave one. As a result two macroscopic condensates arise. The Josephson tunneling for such two-component system has very special properties. We prove that the presence of the g-wave does not contradict the existing experimental data on tunneling. We also discuss the possible ways to experimentally reveal the g-wave component.
We investigate superconductor/insulator/ferromagnet/superconductor (SIFS) tunnel Josephson junctions in the dirty limit, using the quasiclassical theory. We formulate a quantitative model describing the oscillations of critical current as a function of thickness of the ferromagnetic layer and use this model to fit recent experimental data. We also calculate quantitatively the density of states (DOS) in this type of junctions and compare DOS oscillations with those of the critical current.
We study the equilibrium dynamics of the relative phase in a superconducting Josephson link taking into account the quantum fluctuations of the electromagnetic vacuum. The photons act as a superohmic heat bath on the relative Cooper pair number and thus, indirectly, on the macroscopic phase difference ϕ. This leads to an enhancement of the mean square <ϕ^2> that adds to the spread due to the Coulomb interaction carried by the longitudinal electromagnetic field. We also include the interaction with the electronic degrees of freedom due to quasiparticle tunneling, which couple to the phase and o…
We report on far infrared measurements of interplane conductivity for underdoped single-crystal YBa2Cu3Oy in magnetic field and situate these new data within earlier work on two other high-Tc cuprate superconductors, La(2-x)SrxCuO4 and Bi2Sr2CaCu2O(8+d). The three systems have displayed apparently disparate electrodynamic responses in the Josephson vortex state formed when magnetic field H is applied parallel to the CuO2 planes. Specifically, there is discrepancy in the number and field dependence of longitudinal modes observed. We compare and contrast these findings with several models of the…
The temperature and angular dependence of the c-axis Josephson current and the superfluid density in layered d-wave superconductors are studied within the framework of an extended Ambegaokar-Baratoff formalism. In particular, the effects of angle-dependent tunneling matrix elements and Andreev scattering at grain boundaries are taken into account. These lead to strong corrections of the low-temperature behavior of the plasma frequency and the Josephson current. Recent c-axis measurements on the cuprate high-temperature superconductors HgBa_2CaCu_{1+δ} and Bi_2Sr_2CaCu_2O_{8+δ} can therefore be…
It will be shown that a pronounced interfacial pair breaking can be identified in Josephson tunnel junctions provided the first j_{1c} and the second j_{2c} harmonics of the supercurrent, as well as the depairing current in the bulk j_{dp}, are known. Namely, within the Ginzburg-Landau theory a strong interfacial pair breaking results in the relation j_{2c}j_{dp}>>j_{1c}^2, while in standard junctions, with negligibly small pair breaking, the relation of opposite character takes place.
We have investigated macroscopic quantum tunneling in Bi$_2$Sr$_2$CaCu$_2$O$_{8+δ}$ intrinsic Josephson junctions at millikelvin temperatures using microwave irradiation. Measurements show that the escape rate for uniformly switching stacks of N junctions is about $N^2$ times higher than that of a single junction having the same plasma frequency. We argue that this gigantic enhancement of macroscopic quantum tunneling rate in stacks is boosted by current fluctuations which occur in the series array of junctions loaded by the impedance of the environment.
The physics of Josephson tunnel junctions drastically depends on their geometrical configurations and here we show that also tiny geometrical details play a determinant role. More specifically, we develop the theory of short and long confocal annular Josephson tunnel junctions in the presence of an in-plane magnetic field of arbitrary orientations. The behavior of a circular annular Josephson tunnel junction is then seen to be simply a special case of the above result. For junctions having a normalized perimeter less than one the threshold curves are derived and computed even in the case with …
We study the phenomenon of macroscopic quantum tunneling (MQT) in small Josephson junctions (JJ) with an externally applied magnetic field. The latter results in the appearance of the Fraunhofer type modulation of the current density along the barrier. The problem of MQT for a point-like JJ is reduced to the motion of the quantum particle in the washboard potential. In the case of a finite size JJ under consideration, this problem corresponds to a MQT in potential which itself, besides the phase, depends on space variables. Finally, the general expression for the crossover temperature T_0 betw…
We tested oxidized titanium layers as barriers for hybrid Josephson junctions with high $I_cR_n$-products and for the preparation of junctions for tunneling spectroscopy. For that we firstly prepared junctions with conventional superconductor electrodes lead and niobium, respectively. By tuning the barrier thickness we were able to change the junctions' behavior from a Josephson junction to tunnel-like behavior applicable for quasi-particle spectroscopy. Subsequently, we transferred the technology to junctions using Co-doped BaFe$_2$As$_2$ thin films prepared by pulsed laser deposition as base…
Phase transitions executed in a finite time show a domain structure with defects, that has been argued by Zurek and Kibble to depend in a characteristic way on the quench rate. In this letter we present an experiment to measure the Zurek-Kibble scaling exponent sigma. Using symmetric and long Josephson Tunnel Junctions, for which the predicted index is sigma = 0.25, we find sigma = 0.27 +/- 0.05. Further, there is agreement with the ZK prediction for the overall normalisation.
We study nonequilibrium effects in current transport through voltage biased tunnel junction with long diffusive superconducting leads at low applied voltage, $eV \ll 2Δ$, and finite temperatures. Due to a small value of the Josephson frequency, the quasiparticle spectrum adiabatically follows the time evolution of the superconducting phase difference, which results in the formation of oscillating bound states in the vicinity of the tunnel junction (Andreev band). The quasiparticles trapped by the Andreev band generate higher even harmonics of the Josephson ac current, and also, in the presence…
We calculate Ic(T) and Rn for both coherent and incoherent electron tunneling across a c-axis break junction between nu=s,d_x^2-y^2-wave superconducting half spaces, each with c-axis bandwidth 2J. Coherent quasiparticle tunneling only occurs for voltages V<2J/e, leading to difficulties in measuring Rn for underdoped samples. The coherent part of Ic(0) is independent of Delta_nu(0) for J/Delta_nu(0)<<1, and can be large. Our results are discussed with regard to recent experiments.
We elaborate a theoretical description of large Josephson junctions which is based on the Werthamer's microscopic tunneling theory. The model naturally incorporates coupling of electromagnetic radiation to the tunnel currents and, therefore, is particularly suitable for description of the self-coupling effect in Josephson junction. In our numerical calculations we treat the arising integro-differential equation, which describes temporal evolution of the superconducting phase difference coupled to the electromagnetic field, by the Odintsov-Semenov-Zorin algorithm. This allows us to avoid evalua…
We have observed photon-assisted Cooper-pair tunneling in an atomic-scale Josephson junction formed between a superconducting Nb tip and a superconducting Nb sample in a scanning tunneling microscope (STM) at 30 mK. High-resolution tunneling spectroscopy data show a zero-bias conduction peak and other sharp sub-gap peaks from coupling of the STM junction to resonances in the electromagnetic environment. The sub-gap peaks respond to incident microwave radiation by splitting into multiple peaks with the position and height depending on the frequency and amplitude of the microwaves. The inter-pea…
It has been argued by Zurek and Kibble that the likelihood of producing defects in a continuous phase transition depends in a characteristic way on the quench rate. In this paper we discuss an improved experiment for measuring the Zurek-Kibble scaling exponent $σ$ for the production of fluxons in annular symmetric Josephson Tunnel Junctions. We find $σ\simeq 0.5$. Further, we report accurate measurements of the junction gap voltage temperature dependence which allow for precise monitoring of the fast temperature variations during the quench.
We analyze the consequences resulting from the asymmetric boundary conditions imposed by a non-uniform external magnetic field at the extremities of a planar Josephson tunnel junction and predict a number of testable signatures. When the junction length $L$ is smaller than its Josephson penetration depth $λ_j$, static analytical calculations lead to a Fresnel-like magnetic diffraction pattern, rather than a Fraunhofer-like one typical of a uniform field. Numerical simulations allow to investigate intermediate length ($L\approx λ_j$) and long ($L>λ_j$) junctions. We consider both uniform and $δ…
We report on the spontaneous production of fluxons in the presence of a symmetry-breaking magnetic field for annular Josephson tunnel junctions during a thermal quench. The dependence on field intensity $B$ of the probability $\bar{f_1}$ to trap a single defect during the N-S phase transition drastically depends on the sample circumferences. We show that the data can be understood in the framework of the Kibble-Zurek picture of spontaneous defect formation controlled by causal bounds.
We measured the magnetic field dependence of the critical current of high quality Nb-based planar Josephson tunnel junctions in the presence of a controllable non-uniform field distribution. We found skewed and slowly changing magnetic diffraction patterns quite dissimilar from the Fraunhofer-like ones typical of a homogeneous field. Our findings can be well interpreted in terms of recent theoretical predictions [R. Monaco, J. Appl. Phys. vol.108, 033906 (2010)] for a uniform magnetic field gradient leading to Fresnel-like magnetic diffraction patterns. We also show that Fiske resonances can b…
A novel dynamical state has been observed in the dynamics of a perdurbed sine-Gordon system. This resonant state, has been experimentally observed as a singularity in the dc current voltage characteristic of an annular Josephson tunnel junction, excited in the presence of a magnetic field. With this respect, it can be assimilated to self-resonances known as Fiske steps. Differently from these, however, we demonstrate, on the basis of numerical simulations, that its detailed dynamics involves rotating fluxon pairs, a mechanism associated, so far, to self-resonances known as zero-field steps.
The magnetic field effect in the Josephson tunneling between two d-wave superconductors are investigated. When the crystal orientation of one (or each) superconductor relative to the interface normal is such that midgap states exist at the interface, there is a component of the tunneling current due to the midgap states. For a junction with a flat {001}|{110} or {100}|{110} interface, this component is the predominant contribution to the current. The predicted current-field dependence differs entirely from the conventional Fraunhofer pattern, in agreement with a published measurement. This is …
The escape of a small current-biased Josephson tunnel junction from the zero voltage state in the presence of weak microwave radiation is investigated experimentally at low temperatures. The measurements of the junction switching current distribution indicate the macroscopic quantum tunneling of the phase below a cross-over temperature of $T^{\star} \approx 280 \rm{mK}$. At temperatures below $T^{\star}$ we observe both single-photon and \emph{multi-photon} transitions between the junction energy levels by applying microwave radiation in the frequency range between $10 \rm{GHz}$ and $38 \rm{GH…
Specific features of the equilibrium current-carrying state of a Josephson tunnel junction between diffusive superconductors are studied theoretically in the 1D geometry. It is found that the Josephson current induces localized states of electron excitations in the vicinity of the tunnel barrier, which are a continuous analog of Andreev levels in a ballistic junction. The depth of the corresponding ``potential well'' is much greater than the separation between an Andreev level and the continuous energy spectrum boundary for the same transmissivity of the barrier. In contrast to a ballistic jun…
We demonstrate that shunting of Superconductor-Insulator-Superconductor Josephson junctions by Superconductor-Insulator-Normal metal (S-I-N) structures having pronounced non-linear I-V characteristics can remarkably modify the Josephson dynamics. In the regime of Josephson generation the phase behaves as an overdamped coordinate, while in the superconducting state the damping and current noise are strikingly small, that is vitally important for application of such junctions for readout and control of Josephson qubits. Superconducting Nb/AlO${_x}$/Nb junction shunted by Nb/AlO${_x}$/AuPd juncti…
Josephson tunnel junctions with the strong ferromagnetic alloy $\Fe_{0.75}\Co_{0.25}$ as the barrier material were studied. The junctions were prepared with high quality down to a thickness range of a few monolayers of Fe-Co. An oscillation length of $ξ_{F2}\approx 0.79\:{\rm {nm}}$ between 0 and $π$-Josephson phase coupling and a very short decay length $ξ_{F1}\approx 0.22\:{\rm {nm}}$ for the amplitude of the superconducting pair wave function in the Fe-Co layer were determined. The rapid damping of the pair wave function inside the Fe-Co layer is caused by the strong ferromagnetic exchange …
The behavior of a long Josephson tunnel junction drastically depends on the distribution of the dc bias current. We investigate the case in which the bias current is fed in the central point of a one-dimensional junction. Such junction configuration has been recently used to detect the persistent currents circulating in a superconducting loop. Analytical and numerical results indicate that the presence of fractional vortices leads to remarkable differences from the conventional case of uniformly distributed dc bias current. The theoretical findings are supported by detailed measurements on a n…
We have made in-situ [103] and [110]YBCO/Pb junctions, which display an hysteretic behaviour with both a well-defined Quasi-Particule branch and a Josephson current. A zero energy conductance peak is observed in the tunneling conductance which is a signature of Andreev Bound States (ABS) at a surface of a d-wave superconductor. The observed splitting of this peak at low temperature may confirm the existence of a surface state which breaks time-reversal symmetry. However, up to now, we have not seen any evidence of a coupling between Josephson current and the zero-energy ABS, predicted by Y. Ta…
The ordering trend of magnetic impurities at low temperature results in the frustration of the pair-breaking effect and induces a ``recovery'' of superconducting properties. We show that this effect manifests itself in the deviation of the Josephson current amplitude from the values obtained within the Ambegaokar-Baratoff and the Abrikosov-Gor'kov models. We consider both weak and strong-coupling cases. The theory is applied to describe the experimental data obtained for the low-$T_c$ superconductor SmRh$_4$B$_4$. We further predict a ``recovery'' effect of the Josephson current in high-temper…
We fabricated high quality Nb/Al_2O_3/Ni_{0.6}Cu_{0.4}/Nb superconductor-insulator-ferromagnet-superconductor Josephson tunnel junctions. Using a ferromagnetic layer with a step-like thickness, we obtain a 0-pi junction, with equal lengths and critical currents of 0 and pi parts. The ground state of our 330 microns (1.3 lambda_J) long junction corresponds to a spontaneous vortex of supercurrent pinned at the 0-pi step and carrying ~6.7% of the magnetic flux quantum Phi_0. The dependence of the critical current on the applied magnetic field shows a clear minimum in the vicinity of zero field.
We investigate the charge transport property of superconductor (S) /normal metal (N) / ferromagnet insulator (FI) /(normal metal) N' and S/N/FI/N'/S Josephson junctions on a three-dimensional topological insulator surface. We find asymmetric local density of states (LDOSs) in a S/N/FI/N' junction and show that the finite length of the N interlayer gives rise to subgap resonant spikes in the differential conductance and LDOSs. In a S/N/FI/N'/S junction, the Josephson current shows a non-sinusoidal current-phase relation and the N (or N') interlayer decreases the magnitude of the critical curren…
It has been argued by Zurek and Kibble that the likelihood of producing defects in a continuous phase transition depends in a characteristic way on the quench rate. In this paper we discuss our experiment for measuring the Zurek-Kibble scaling exponent $σ$ for the production of fluxons in annular symmetric Josephson Tunnel Junctions. The predicted exponent is $% σ=0.25$, and we find $σ=0.27\pm 0.05$. Further, there is agreement with the ZK prediction for the overall normalisation.
The interfacial coupling of two materials with different ordered phases, such as a superconductor (S) and a ferromagnet (F) is driving new fundamental physics and innovative applications. For example, the creation of spin-filter Josephson junctions and the demonstration of triplet supercurrents have suggested the potential of a dissipationless version of spintronics based on unconventional superconductivity. Here we demonstrate evidence for active quantum applications of S-F-S junctions, through the observation of macroscopic quantum tunneling in Josephson junctions with GdN ferromagnetic insu…
We study a current-biased 0-π-0 Josephson junction made by high-T_c superconductors, theoretically. When a length of the πjunction is large enough, this junction contains a vortex-antivortex pair at both ends of the πjunction. Magnetic flux carried by the vortices is calculated using the sine-Gordon equation. The result shows that the magnetic flux of the vortices is suppressed to zero as the distance between the vortices is reduced. By applying an external current, the orientation of the vortices is reversed, and a voltage pulse is generated. The current needed for this transition and generat…
The dc Josephson critical current of a (S/M)IS tunnel structure in a parallel magnetic field has been investigated (here S is a superconductor, S/M is the proximity coupled S and paramagnet M bilayer and I is an insulating barrier). We consider the case when, due to the Hund's rule, in the M metal the effective molecular interaction aligns spins of the conducting electrons antiparallel to localized spins of magnetic ions. It is predicted that for tunnel structures under consideration there are the conditions when the destructive action of the internal and the applied magnetic fields on Cooper …
We report a unique experimental approach to create topological superconductors by inducing superconductivity into epitaxial metallic thin film with strong spin-orbit coupling. Utilizing molecular beam epitaxy technique under ultra-high vacuum condition, we are able to achieve (111) oriented single phase of gold (Au) thin film grown on a well-oriented vanadium (V) s-wave superconductor film with clean interface. We obtained atomically smooth Au thin films with thicknesses even down to below a nanometer showing near-ideal surface quality. The as-grown V/Au bilayer heterostructure exhibits superc…
We present experimental studies of high quality underdamped 0, pi, and 0-pi ferromagnetic Josephson tunnel junctions of intermediate length L (lambda_J < L < 5 lambda_J, where lambda_J is the Josephson penetration depth). The junctions are fabricated as Nb/Al_2O_3/Cu_40Ni_60/Nb Superconductor-Insulator-Ferromagnet-Superconductor heterostructures. Using microwave spectroscopy, we have investigated the eigenfrequencies of 0, pi, and 0-pi Josephson junctions in the temperature range 1.9K...320mK. Harmonic, subharmonic and superharmonic pumping is observed in experiment, and the experimental data …
Pulse activated transitions from the metastable to the running state and viceversa have been observed in a stacked double tunnel Nb-based Josephson system. Experimental results are compared with numerical simulations based on the Sine-Gordon model of the stacked junctions by injecting pulses with variable amplitude in one of the junctions of the stack, and observing the voltage response of the other junction. Both experimental and numerical results show the possibility to induce both direct and back-switching transitions from the metastable to the running state simply by changing the amplitude…
We propose Josephson junction with a high-spin magnetic impurity sandwiched between two superconductors. This system shows a $π$ junction behavior as a function of the spin magnetic moment state of the impurity, the interface transparency, exchange coupling and electron-electron interactions in the system. The system is theoretically analyzed for possible reason behind the $π$ shift. The crucial role of spin flip scattering is highlighted. Possible applications in quantum computation of our proposed tunable high spin magnetic impurity $π$ junction is underscored.
Magic-angle twisted bilayer graphene (MATBG) has recently emerged as a highly tunable two-dimensional (2D) material platform exhibiting a wide range of phases, such as metal, insulator, and superconductor states. Local electrostatic control over these phases may enable the creation of versatile quantum devices that were previously not achievable in other single material platforms. Here, we exploit the electrical tunability of MATBG to engineer Josephson junctions and tunneling transistors all within one material, defined solely by electrostatic gates. Our multi-gated device geometry offers com…
