THz Cherenkov radiation of Josephson vortex

It is shown that Josephson vortices travelling in sandwich embedded in dielectric media radiate electromagnetic waves with THz frequencies. This phenomenon is caused by the Cherenkov effect and takes place if vortex velocity exceeds the speed of light in dielectric.     

Relaxation oscillators made of bridge-type Josephson contacts

Nb, NbN, and Nb₃Ge nanobridges on sapphire substrates with hysteretic I–V-characteristics are used as active elements in relaxation oscillators. Amplitudes of up to 350 mV and frequencies of up to 500 MHz could be generated, which make these oscillators useful for many applications. Linewidth and amplitude of the relaxation oscillations in dependence of nanobridge geometry are studied in some detail.Dedicated to Prof. Dr. Dres. h.c. A. Scharmann on the occasion of his 60^th birthday     

Noise properties of SQUIDs with Superconductor-Semiconductor-Superconductor proximity effect Josephson junctions

SQUIDs using Superconductor-Semiconductor-Superconductor (SSmS) proximity effect Josephson junctions were prepared and noise measurements were carried out. Since SSmS junctions are basic elements of Josephson field effect transistors (JoFETs), information about dynamic properties of JoFETs can be gained in this way. A planar geometry was used for the SSmS junctions, with a single crystalline silicon wafer acting as both, substrate and proximity layer. Rf- and dc-SQUIDs could be realized. When the SQUIDs were operated in a flux locked loop, flux noise values comparable to conventional tunnel junction SQUIDs were measured.     

Vertical Josephson interferometers for quantum computation

We characterize a niobium-based vertical Josephson interferometer which we propose to include in a superconducting loop for applications to quantum computation using flux qubits. The most interesting feature of this device is that the Josephson current is precisely modulated by a small transversal magnetic field parallel to superconducting loop plane from a maximum to zero, with fine control and precision. This device can be used to independently control the off-diagonal Hamiltonian terms of flux qubits and/or to control the flux transfer function of a superconducting transformer for inter-qubits coupling.     

Modified Josephson Equation for Mesoscopic Parallel LC Circuit Including a Josephson Junction

By introducing the entangled state representation, parallel LC circuit including a 3osephson junction equation associated with the modification of the motion equation. the Cooper-pair number-phase quantization of the mesoscopic is realized. In the Heisenberg picture, the modified Josephson Faraday equation about the inductance is deduced from the     

Rectifying effect in a Josephson junction ratchet array

We have studied numerically a rectifying effect in an underdamped Josephson junction ratchet array driven by dc and ac current. The array consists of both alternating potential barriers and alternating inter-capacitances along the direction of vortex flow. The guide banks of high critical currents are assigned for all the longitudinal junctions to prevent the percolative pattern of vortex motion. In some junction parameters, we see a rectifying effect which indicates a finite value of the time-averaged voltage at zero dc bias. The directional dependence of the vortex motion becomes fairly large when the junction parameters lie in an optimal range which gives rise to a Shapiro step at zero dc bias. Such a rectifying effect survives for small thermal fluctuation, but eventually disappear beyond a certain critical temperature.     

Cooper-Pair Number--Phase Quantization for Inductance Coupling Circuit Including Josephson Junctions

Based on the entangled state representation and a bosonic phase operator formalism, we tackle with Cooperpair number-phase quantization for the inductance coupling circuit including Josephson junctions, and then investigate how Josephson current equations change due to the presence of the coupling inductance and obtain bosonic operator Faraday formula, as well as the corresponding number-phase uncertainty relation.     

Determination of Relaxation Time of a Josephson Tunnel Junction

We propose a non-stationary method to measure the energy relaxation time of Josephson tunnel junctions from microwave enhanced escape phenomena. Compared with the previous methods, our method possesses simple and accurate features. Moreover, having determined the energy relaxation time, we can further obtain the coupling strength between the microwave source and the junction by changing the microwave power.     

Properties of Josephson junctions in the inhomogeneous magnetic field of a system of ferromagnetic particles

The effect of an array of ferromagnetic nanoparticles on the field-dependent critical current of the short overlap Josephson junction is experimentally studied. Large reversible variations of the maximum critical current are observed depending on the magnetic state of the particles. The pronounced commensurability effects are detected which are proved by the additional peaks of magnetic field induced diffraction pattern.     

Quantum electrodynamic fluctuations of the macroscopic Josephson phase

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 〈φ²〉 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 only indirectly to the particle number. The simultaneous inclusion of both the radiation field fluctuations and quasiparticle tunneling leads to a novel type of particle-bath Hamiltonian in which the quantum particle couples through its position and momentum to two independent bosonic heat baths. We study the interplay between the two mechanisms in the present context and find interference contributions to the quantum fluctuations of the phase. We explore the observability of the QED effects discussed here.     

Phase Locking and Chaos in a Josephson Junction Array Shunted by a Common Resistance

The dynamics of a Josephson junction array shunted by a common resistance are investigated by using numerical methods. Coexistence of phase locking and chaos is observed in the system when the resistively and capacitively shunted junction model is adopted. The corresponding parameter ranges for phase locking and chaos are presented. When there are three resistively shunted junctions in the array, chaos is found for the first time and the parameter range for chaos is also presented. According to the theory of Chernikov and Schmidt, when there are four or more junctions in the array, the system exhibits chaotic behavior. Our results indicate that the theory of Chernikov and Schmidt is not exactly appropriate.     

Novel magnetoinductance effects in Josephson junction arrays: A single-plaquette approximation

Using a single-plaquette approximation, novel magnetoinductance effects in Josephson junction arrays (JJAs) are predicted, including the appearance of steps in the temperature behavior of magnetic susceptibility. The number of steps (as well as their size) is controlled by the kinetic inductance of the plaquette whose field dependence is governed by the Abrikosov vortices penetrating superconducting regions of the array. The experimental conditions under which the predicted effects should manifest themselves in artificially prepared JJAs are discussed.     

Continuous-Variable Entanglement Transfer from Cavity Field to Two Mesoscopic Josephson Junctions

We consider a composite system of two remote mesoscopic dosephson junctions interacting locally with a two-mode non-classical cavity field and investigate entanglement transfer from a bipartite continuous-variable （CV） system to a pair of localized mesoscopic dosephson junctions. We obtain analytically the time-dependent characteristic functions in the Wigner representation for the two CV subsystems, where two cases are considered for the zero and finite temperatures. Furthermore, we analyse the influences of the temperature on the period recovery of the entanglement.     

Sensitivity of Josephson junctions to high-pass filtered noise of black-body radiation

The influence of the nonwhite noise on the Josephson junction is considered by using the averaging principle. It is shown that in contrast to the case of white noise there is a non-zero critical current, but it has high sensitivity to the noise power. The dependence of noise power on the critical current can be used for the detection of cold black-body radiation.     

Wide‐band phase locking of Josephson junctions in a resonator

We found that the chain of junctions acts both as the source of radiation and as a part of the superconducting resonator when the effective capacitance of the resonator is larger than the total capacitance of all junctions. At this condition junctions are synchronized in‐phase not only at the resonance steps but also in the whole hysteretic region of I –V characteristics below the resonant frequency. The maximal allowable spread of critical currents for this effect is about 5–10%. We analyzed the origin of the effect both numerically and by the method of slowly varying amplitudes.

     

Cooper-Pair Number-Phase Quantization for a Mesoscopic LC Circuit Including a Josephson Junction

By introducing the entangled state representation and Feynman assumption that ＇electron pairs are bosons, ..., a bound pair acts as a Bose particle ＇, we construct an operator Hamiltonian for a mesoscopic inductance-capacitance （LC） circuit including a Josephson junction, then we use the Heisenberg equation of motion to derive the current equation and the voltage equation across the inductance as well as across the Josephson junction. The result manifestly shows how the junction voltage is affected by the capacitance coupling. In this way the Cooper-pair number-phase quantization for this system is completed.     

dc Josephson effect in metallic single-walled carbon nanotubes

The dc Josephson effect is investigated in a single-walled metallic carbon nanotube connected to two superconducting leads. In particular, by using the Luttinger liquid theory, we analyze the effects of the electron-electron interaction on the supercurrent. We find that in the long junction limit the strong electronic correlations of the nanotube, together with its peculiar band structure, induce oscillations in the critical current as a function of the junction length and/or the nanotube electron filling. These oscillations represent a signature of the Luttinger liquid physics of the nanotube, for they are absent if the interaction is vanishing. We show that this effect can be exploited to reverse the sign of the supercurrent, realizing a tunable π-junction.     

Warp of the Invariant Circle and Onset of Chaos in Josephson Junction Equation

The dynamics of the dc and ac driving Josephson junction equation is studied in terms of the two-dimensional Poincaré map. The smooth invariant circle on the phase cylinder in over-damped case a ） 2 loses smoothness as a decreases and becomes a strange attractor eventually. This triggers two kinds of chaos, one occurs in the regions between two Arnold tongues and the other occurs within the tongues.     

Pertubation analysis of the dynamical behavior of two-junction interferometers

The dynamical properties of symmetric quantum interferometers with equal junctions of negligible capacitance have been studied by means of perturbation analysis in the limit of small values of the parameter β. In this limit, two characteristic time constants arise. These quantities may be linked to two different dynamical processes in the system: the first is related to the time evolution of the average superconducting phase difference across the two junctions; the second defines the time scale for flux motion. The response of the system to constant and time-dependent externally applied magnetic fields is considered and a general perturbed solution for the average superconducting phase difference and the fluxon number variable is derived to first order in β.     

Josephson current in ferromagnetic d-wave superconductor/ferromagnetic d-wave superconductor junction

We solve a self-consistent equation for the d-wave superconducting gap and the effective exchange field in the mean-field approximation, study the Zeeman effects on the d-wave superconducting gap and thermodynamic potential. The Josephson currents in the d-wave superconductor (S)/insulating layer (I)/d-wave S junction are calculated as a function of the temperature, exchange field, and insulating barrier strength under a Zeeman magnetic field on the two d-wave Ss. It is found that the Josephson critical currents in d-wave S/d-wave S junction depend to a great extent on the relative orientation of the effective exchange field of the two S electrodes, and the crystal orientation of the d-wave S. The exchange field can under certain conditions enhance the Josephson critical current in a d-wave S/I/d-wave S junction.