Search

We report on a comparative analysis of three different laboratory terahertz (THz) sources emitting into open space based on a backward wave oscillator with a frequency multiplier, a microwave frequency multiplier to large numbers of harmonics based on semiconductor superlattices, and a recently developed long Josephson junction-based oscillator. The weight‒size parameters, frequency responses, and radiation powers of the sources have been qualitatively and quantitatively compared, along with the complexity of their tuning and operation under laboratory conditions.

A scheme for the controlled generation of single traveling sine- Gordon breathers in a long Josephson junction is described. In the presence of dissipation, a direct current source, and thermal fluctuations, the theoretical analysis shows that, through the nonlinear supratransmission effect, tailored magnetic pulses at the junction’s edge can effectively yield breather modes only.

An electromagnetic source of terahertz radiation on the basis of a tunnel long Josephson junction was proposed and experimentally studied. Output radiation was transmitted to open space by means of a slot antenna, which was located on the same microcircuit with the junction and matched with the collecting lens. Several Oscillator structures designed for operation within the frequency ranges of 250–410, 330–530, and 390–700 GHz with the possibility of continuous frequency tuning were manufactured and tested. The amplitude-frequency characteristics of output radiation with a spectral resolution of nearly 0.1 MHz were studied with the use of a terahertz-spectrometer based on a superconducting receiver. The study of transmitting antenna characteristics throughout the entire bandwidth was performed with the use of a cooled highly sensitive silicon based bolometer. The experimental results correspond to numerical calculations.

Interaction of the fluxon with the finite size dipole impurity in the long Josephson junction is investigated. The impurity has polarity and will be referred to as a {\it dipole} impurity because it also has a direction and, consequently, changes its sign under the space inversion transform $x \to -x$. Such a model is used to describe the inductively coupled to the Josephson transmission line qubit and the misaligned Abrikosov vortex that penetrates into the long Josephson junction. We derive the approximate equations of motion for the fluxon center of mass and its velocity. With the help of these equations we demonstrate that pinning and scattering of the fluxon on the impurity differs significantly from the case of the point impurity which is modelled by the derivative of the Dirac's $\delta$-function, Comment: 7 figures; Phys. Lett. A (in press)

We have developed a theoretical model describing the dynamics of the Josephson vortices in the long planar Josephson junction under the influence of the alternating heterogeneous magnetic field created by the tip of magnetic force microscope (MFM), and external uniform, constant magnetic field. The fitting of the model parameters ensured the coincidence of the calculated dependencies of the junction critical current on the external magnetic field with the experimental data, with a standard deviation of less than 0.1 percent. We considered the range of values of the field corresponding to the penetration of more than 10 Josephson vortices at different positions of the MFM tip. From comparison with the experiment, we obtained the estimates of Josephson length, effective area of the junction, critical current distribution along the junction boundary, inductance of the lead wires of the experimental sample and the distribution of the magnetic flux inside the junction created by the MFM tip.

In the present work, the current-voltage (I-V) characteristics in a coupled long Josephson junction based on magnesium diboride are studied by establishing a system of equations of phase differences of various inter- and intra-band channels starting from the microscopic Hamiltonian of the junction system and simplifying it through the phenomenological procedures such as action, partition function, Hubbard-Stratonovich transformation (bosonization), Grassmann integral, saddle-point approximation, Goldstone mode, phase dependent effective Lagrangian and, finally, Euler-Lagrange equation of motion. The system of equations are solved using finite difference approximation for which the solution of unperturbed sine-Gordon equation is taken as the initial condition. Neumann boundary condition is maintained at both the ends so that the fluxon is capable of reflecting from the end of the system. The phase dependent current is calculated for different tunnel voltage and averaged out over space and time. The current-voltage characteristics are almost linear at low voltage and non-linear at higher voltage which indicates that the more complicated physical phenomena at this situation may occur. At some region of the characteristics, there exist a negative resistance which means that the junction system can be used in specific electronic devices such as oscillators, switches, memories etc. The non-linearity is also sensitive to the layer as well as to the junction thicknesses. Non-linearity occurs for lower voltage and for higher junction and layer thicknesses., Comment: 17 pages, 7 figures

A study of the output terahertz (THz) emission to open space from Nb/AlO x /Nb-based long Josephson junction (LJJ) was carried out. Both bolometric technique with a frequency resolution of about1 GHz and heterodyne technique with spectral resolution of about 0.1 MHz were used for detection of the THz emission. A comparative analysis with other types of THz sources was made, such as backward wave oscillator (BWO) and frequency multiplier based on a semiconductor superlattice GaAs/AlAs with large harmonic numbers. A qualitative and quantitative comparison with respect to weight and size characteristics, the spectral properties, the emission power, as well as a general usability in laboratory conditions, was made. The highest detected power was achieved with an LJJ-based oscillator, and the most convenience was demonstrated using a superlattice-based oscillator with large harmonic numbers.

The response of the Cold-Electron Bolometers (CEBs), integrated into a 2-D array of dipole antennas, has been measured by irradiation from YBa2Cu3O7&minus, &delta, (YBCO) 50 &mu, m long Josephson junction into the THz region at frequencies from 0.1 to 0.8 THz. The possibility of controlling the amplitude-frequency characteristic is demonstrated by the external magnetic field in the traveling wave regime of a long Josephson junction. The YBCO junction has been formed on the bicrystal Zr1&minus, xYxO2 (YSZ) substrate by magnetron sputtering and etching of the film. CEBs have been fabricated using an Al multilayer structure by a self-aligned shadow evaporation technique on Si substrate. Both receiver and oscillator have been located inside the same cryostat at 0.3 K and 2.7 K plates, respectively.

We analyze the Josephson,$I_{J}$, and dissipative,$I_{V}$, currents in a magnetic Andreev interferometer in the presence of the long-range spin triplet component (LRSTC). Andreev interferometer has a cross-like geometry and consists of a SF$_{l}$ - F - F$_{r}$S circuit and perpendicular to it a N - F - N circuit, where S, F$_{l,r}$ are superconductors and weak ferromagnets with non-collinear magnetisations $\mathbf{M}_{l,r}$, F is a strong ferromagnet with a high exchange energy. The ferromagnetic wire F can be replaced with a non-magnetic wire n. In the limit of a weak proximity effect (PE), we obtain simple analytical expressions for the currents $I_{J}$ and $I_{V}$. In particular, the critical Josephson current in a long Josephson junction (JJ) is $I_{c}(\alpha ,\beta )=I_{0c}\chi (\alpha ,\beta )$, where the function $% \chi (\alpha ,\beta )$ is a function of angles $(\alpha ,\beta )_{l,r}$ that characterize the orientations of $\mathbf{M}_{l,r}$. The oscillating part of the dissipative current $I_{osc}(V)=\chi (\alpha ,\beta )\cos \varphi I_{0}(V)$ in the N - F(n)- N circuit depends on the angles $(\alpha ,\beta )_{l,r}$ in the same way as the critical Josephson current $I_{c}(\alpha ,\beta )$, but can be much greater than the $I_{c}(\alpha ,\beta )$. At some angles the current $I_{c}(\alpha ,\beta )$ changes sign. We briefly discuss a relation between the negative current $I_{c}$ and paramagnetic response. We argue that the measurements of the conductance in N - F(n) - N circuit can be used as another complementary method to identify the LRSTC in S/F heterostructures., Comment: 15 pages, 6 figures. Final version, some formulas are corrected

A long Josephson junction comprising regions with phase discontinuities driven by an external ac-drive is studied. An inhomogeneous sine-Gordon equation is used, that depicts the dynamics of long Josephson junctions with phase discontinuities. Perturbation technique along with asymptotic analysis and the method of averaging are applied to obtain an average dynamics in the form of double sine-Gordon equations for both small and large driving amplitudes. From the obtained average dynamics, it is determined that, the external ac-drive may affect the presence of the ground state of the junction. Specifically, the cases for 0 − κ and 0 − π − 0 junctions are discussed. In the presence of an external ac-drive, the critical facet length bc is analyzed for 0 − π − 0 junction above which the ground state is non-uniform. The critical bias current γc for 0 − κ junction is investigated, at which the junction switches to a resistive state. Further, the interaction of localized defect modes and the fast oscillating drive is studied for both 0 − κ and 0 − π − 0 junctions, which is explained by using Lagrangian approach. Numerical simulations are performed to support our analytical calculations.

We demonstrate the double-peaked switching current distributions of a long Josephson junction from the zero to the finite voltage state in the case of asymmetrical bias feed profile. The considered asymmetry stands to be a natural feature of YBCO junctions fabricated on bicrystal substrates. We argue that such a picture is associated with the formation of solitons at the opposite junction edges for slightly different bias currents, arisen due to asymmetry. System parameters such as junction length, noise intensity and character of bias feed inhomogeneity leading to atypical distribution of switching currents are discussed.

We investigate analytically and numerically a long Josephson junction on an infnite domain, having arbitrary periodic phase shift of k, that is, the so-called 0-k long Josephson junction. The system is described by a one-dimensional sine-Gordon equation and has relatively recently been proposed as artificial atom lattices. We discuss the existence of periodic solutions of the system and investigate their stability both in the absence and presence of an applied bias current. We find critical values of the phase-discontinuity and the applied bias current beyond which a solution switches to its complementary counterpart. Due to the periodic discontinuity in the phase, the system admits regions of allowed and forbidden bands. We perturbatively investigate the Arnold tongues that separate the region of allowed and forbidden bands, and discuss the effect of an applied bias current on the band-gap structure. We present numerical simulations to support our analytical results.

The standard theory of dynamical Coulomb blockade [$P(E)$ theory] in ultra-small tunnel junctions has been formulated on the basis of phase-phase correlations by several authors. It was recently extended by several experimental and theoretical works to account for novel features such as electromagnetic environment-based renormalization effects. Despite this progress, aspects of the theory remain elusive especially in the case of linear arrays. Here, we apply path integral formalism to re-derive the Cooper-pair current and the BCS quasi-particle current in single small Josephson junctions and extend it to include long Josephson junction arrays as effective single junctions. We consider renormalization effects of applied oscillating voltages due to the impedance environment of a single junction as well as its implication to the array. As is the case in the single junction, we find that the amplitude of applied oscillating electromagnetic fields is renormalized by the same complex-valued weight $\Xi(\omega) = |\Xi(\omega)|\exp i\eta(\omega)$ that rescales the environmental impedance in the $P(E)$ function. This weight acts as a linear response function for applied oscillating electromagnetic fields driving the quantum circuit, leading to a mass gap in the thermal spectrum of the electromagnetic field. The mass gap can be modeled as a pair of exotic `particle' excitation with quantum statistics determined by the argument $\eta(\omega)$. In the case of the array, this pair corresponds to a bosonic charge soliton/anti-soliton pair injected into the array by the electromagnetic field. Possible application of these results is in dynamical Coulomb blockade experiments where long arrays are used as electromagnetic power detectors., Comment: 24 pages, 7 figures, 2 tables

We investigate the low-energy dynamics of two coupled anisotropic Bose-Einstein condensates forming a long Josephson junction. The theoretical study is performed in the framework of the two-dimensional Gross-Pitaevskii equation and the Bogoliubov--de Gennes formalism. We analyze the excitation spectrum of the coupled Bose condensates and show how low-energy excitations of the condensates lead to multiple-frequency oscillations of the atomic populations in the two wells. This analysis generalizes the standard bosonic Josephson equation approach. We also develop a one-dimensional hydrodynamic model of the coupled condensates, that is capable of reproducing the excitation spectrum and population dynamics of the system.

Compared to irreversible gate operations, reversible digital logic gates can provide a fundamental advantage in energy efficiency. Here we discuss previously discovered 1-bit ballistic reversible logic gates and new experimental plans to measure it. The gates consist of long Josephson junctions (LJJs) connected by a circuit interface. The gate dynamics evanescently extends into the LJJs to realize the physically resonant gate operation. It differs from known adiabatic superconducting gates because it does not work in the adiabatic limit. Depending on the gate type, that is the Identity or NOT gate, the polarity of the outgoing fluxon should be preserved or inverted, respectively. The dynamics of the scattering process, originally discovered in full numerical simulation, is understood using collective coordinate analysis. We plan to experimentally study how well a fluxon can travel ballistically towards the interface, and scatter with the designed gate operation. Here we present gate and SQUID-sensing layout designs for a NOT gate. The LJJ circuit layout uses niobium trilayer short junctions with connecting wiring for inductors. Part of the design includes the shunting capacitors at the gate interface which is key for the resonant dynamics in the gates.

A new implementation for a terahertz flux-flow oscillator (FFO) based on a long Josephson junction is presented. The oscillator is integrated with the lens antenna on a single chip providing the THz emission in the open space, and with a harmonic mixer (HM) based on a SIS junction for a frequency and phase locking. We used the double slot antenna to output the emission from the planar transmission line to the open space, this antenna is coupled to the elliptical silicon lens forming narrow output beam of THz radiation. Two designs of antenna and matching structures coupled to the oscillator are developed and fabricated, the operating ranges 0.33 - 0.57 THz and 0.42 - 0.73 THz were obtained experimentally for these designs. The emission to open space was measured by an external superconducting receiver located in the separate cryogenic system.

This article presents the phase dynamics of an inline long Josephson junction in voltage state under the influence of constant external magnetic field. Fractional calculus approach is used to model the evolution of the phase difference between the macroscopic wave functions of the two superconductors across the junction. The governing non-linear partial differential equation is then solved using finite difference-finite element schemes. Other quantities of interest like Josephson current density and voltage across the junction are also computed. The effects of various parameters in the model on phase difference,Josephson current density and voltage are analyzed graphically with the help of numerical simulations.

One of the crucial factors in achieving high performance of superconducting integrated circuits, such as analog-to-digital converters (ADCs), is sampling using a high-frequency clock source with a low cycle-to-cycle jitter. As the superconductor ADC technology matures toward more complex designs for higher dynamic range performance, the need for synchronous clocking of multiple comparators continues to grow. Since high-frequency external clock sources are expensive and make a significant contribution to the heat load of the system, a high-frequency and low-jitter on-chip clock source using long Josephson junction (LJJ) is considered the preferred long-term solution. Toward that end, we are working on improving an on-chip 110-GHz clock source based on an unshunted LJJ in annular geometry. Minimizing the additional jitter added by each fan-out of the clock signal is the effort's goal. For synchronous clocking of up to three comparators, we compare clock distribution using superconducting passive transmission lines and a new approach using novel active transmission lines. We also introduce a method of synchronizing LJJ clock source with an external stable oscillator by injection locking.

By applying a voltage to a Josephson junction, the charge in superconducting layers (S-layers) will oscillate. Wavelength of the charge oscillations in S-layers is related to external current in junction, by increasing the external current, the wavelength will decrease which cause in some currents the wavelength be incommensurate with width of junction, so the CVC shows Fiske like steps. External current throwing along junction has some components, resistive, capacitive and superconducting current, beside these currents there is a current in lateral direction of junction, (x direction). On the other hand, the emitted electromagnetic wave power in THz region is related to AC component of electric field in junction, which itself is related to charge density in S-layers, which is related to currents in the system. So we expect that features of variation of current components reflect the features of emitted THz power form junction. Here we study in detail the superconductive current in a long Josephson junction (JJ), the current voltage characteristics (CVC) of junction and emitted THz power from the system. Then we compare the results. Comparing the results we see that there is a good qualitative coincidence in features of emitted THz power and supercurrent in junction.

Quantum fluids of light are an emerging platform for energy-efficient signal processing, ultrasensitive interferometry and quantum simulators at elevated temperatures. Here we demonstrate all-optical control of the topological excitations in a large polariton condensate realizing the bosonic analogue of a long Josephson junction and inducing the nucleation of Josephson vortices. When a phase difference is imposed at the boundaries of the condensate, two extended regions become separated by a sharp phase jump of π radians and a solitonic depletion of the density, forming an insulating barrier with a suppressed order parameter. The superfluid behaviour—characterized by a smooth phase gradient across the system instead of the sharp phase jump—is recovered at higher polariton densities and is mediated by the nucleation of Josephson vortices within the barrier. Our results contribute to the understanding of dissipation and stability of elementary excitations in macroscale quantum systems. All-optical control of the topological excitations of a superfluid of light is demonstrated in a high-quality-factor semiconductor microcavity. Recovery of superfluid behaviour at high polariton densities and bosonic Josephson vortices are observed.

In the current work long Josephson junctions are being studied. Magnetic flux distribution is the physical measure for phase difference of the wave functions in the superconducting layers of the junction. The current phase relation, in most cases, can be considered as an odd strict 2π-periodic function, and hence, it can be presented in Fourier series of sinuses. It is well-known from the physical experiment that with a sufficient degree of precision, a number of physical systems are reliably described with the contribution of only first two harmonics. The adequate mathematical model for the distribution of the magnetic flux is then the double sine-Gordon equation with Neumann boundary conditions at the ends of the junction. Even in the stationary case, the boundary problem is highly nonlinear and the only tool for its comprehensive study is numerical methods. The aim of the present work is to show that in the case of zero external current, the stationary equation turns out to be a fully integrable model, derived from a variation principle with a cosine potential. In our work analytical solutions for the magnetic flux distributions described in the terms of Jacobi elliptic sinuses are derived. Analytical studies in this case serve to further numerically study of this multiparametric nonlinear boundary problem, which is so important in the applied nanophysics.In the current work long Josephson junctions are being studied. Magnetic flux distribution is the physical measure for phase difference of the wave functions in the superconducting layers of the junction. The current phase relation, in most cases, can be considered as an odd strict 2π-periodic function, and hence, it can be presented in Fourier series of sinuses. It is well-known from the physical experiment that with a sufficient degree of precision, a number of physical systems are reliably described with the contribution of only first two harmonics. The adequate mathematical model for the distribution of the magnetic flux is then the double sine-Gordon equation with Neumann boundary conditions at the ends of the junction. Even in the stationary case, the boundary problem is highly nonlinear and the only tool for its comprehensive study is numerical methods. The aim of the present work is to show that in the case of zero external current, the stationary equation turns out to be a fully integrable model,...

This report discusses the following topics on superconductivity: nonlinearities in hard superconductors such as surface impedance of a type II superconductimg half space and harmonic generation and intermodulation due to alternating transport currents; and nonlinearities in superconducting weak links such as harmonic generation by a long Josephson Junction in a superconducting slab.

Spin-torque-biased magnetic dynamics in an easy-plane ferromagnet (EPF) is theoretically studied in the presence of a weak in-plane anisotropy. While this anisotropy spoils U(1) symmetry, thereby quenching the conventional spin superfluidity, we show that the system instead realizes a close analog of a long Josephson junction (LJJ) model. The traditional magnetic-field and electric-current controls of the latter map, respectively, onto the symmetric and antisymmetric combinations of the out-of-plane spin torques applied at the ends of the magnetic strip. This suggests an alternative route towards realizations of superfluidlike transport phenomena in insulating magnetic systems. We study a spin-torque-biased phase diagram, providing an analytical solution for static multidomain phases in the EPF. We adapt an existing self-consistency method for the LJJ to develop an approximate solution for the EPF dynamics. The LJJ-EPF mapping has the potential for producing applications with superconductor-based circuit functionality at elevated temperatures. The results apply equally to antiferromagnets with suitable effective free energy in terms of the N\'eel order instead of in-plane magnetization.

Typical treatments of superconducting or superfluid Josephson junctions rely on mean-field or two-mode models; we explore many-body dynamics of an isolated, ultracold, Bose-gas long Josephson junction using time-evolving block decimation simulations. We demonstrate that with increasing repulsive interaction strength, localized dynamics emerge that influence macroscopic condensate behavior and can lead to formation of solitons that directly oppose the symmetry of the junction. Initial state population and phase yield insight into dynamic tunneling regimes of a quasi one-dimensional double well potential, from Josephson oscillations to macroscopic self-trapping. Population imbalance simulations reveal substantial deviation of many-body dynamics from mean-field Gross-Pitaevskii predictions, particularly as the barrier height and interaction strength increase. In addition, the sudden approximation supports localized particle-hole formation after a diabatic quench, and correlation measures unveil a new dynamic regime: the Fock flashlight.

on the PDFJournal of Institute of Science and Technology, 2015, 20(2): 115-121

We investigate both analytically and numerically an infinitely long Josephson junction with two π -discontinuities in the phase characterized by a jump of π in the phase difference of the junction, i.e. a 0– π –0 long Josephson junction on an infinite domain. The dynamics of the system is described by a modified one-dimensional perturbed sine-Gordon equation. We investigate an instability region of the trivial zero solution in which semifluxons are spontaneously generated. A perturbation technique is used to show that the existence of static semifluxons depends on the length of the junction, the facet length, and the applied bias current. Numerical simulations are presented accompanying our analytical results.

This paper describes current transport through а long Josephson junction with an alternating critical–current density. This alternating density can be achieved in experiments bу incorporating a magnetic layer to the weak link in а special manner. The prospects for the practical use of such structures are related to the possibility of obtaining bistable Josephson elements on their basis. Joint analysis of both current–phase relations and dynamic characteristics made it possible to optimize the operation mode for a fast superconducting memory cell based on bistable contact and to assess the energy dissipation for the read and write operations.

We investigate the superconducting lifetime of a long overdamped current-biased Josephson junction, in the presence of telegraph noise sources. The analysis is performed by randomly choosing the initial condition for the noise source. However, in order to investigate how the initial value of the dichotomous noise affects the phase dynamics, we extend our analysis using two different fixed initial values for the source of random fluctuations. In our study, the phase dynamics of the Josephson junction is analyzed as a function of the noise signal intensity, for different values of the parameters of the system and external driving currents. We find that the mean lifetime of the superconductive metastable state as a function of the noise intensity is characterized by nonmonotonic behavior, strongly related to the soliton dynamics during the switching towards the resistive state. The role of the correlation time of the noise source is also taken into account. Noise-enhanced stability is observed in the investigated system.