New features in collective dynamics of intrinsic Josephson junctions

We discuss the breakpoint phenomenon, which reflects a resonance between Josephson and plasma oscillations in the system of coupled Josephson junctions. The resonance leads to the creation of the longitudinal plasma waves (LPW) at the breakpoint and to the oscillation dependence of the breakpoint current as a function of the dissipation and coupling parameters. A group behavior of the current-voltage characteristics (IVC) of the stacks with different number of junctions is predicted. Particularly, in the materials with dissipation parameter β=0.35 and coupling parameter α=3, we expect four groups of the IVC, which demonstrate a different character of changing of the breakpoint current with the number of junctions in the stack. Using the k-αβ-method, we find the values of the wave numbers k of the LPW and explain this group behavior of the IVC.     

Nonlinear dynamics and resistive transition in intrinsic Josephson junctions

We study the dynamics of phase differences in intrinsic Josephson junctions by using numerical simulations. We especially pay attention to the dynamics of junctions with weak dissipation under an externally applied electric current, where nonlinear character of the system appears most significant. It is found that, when an electric current is applied, the spatially localized oscillations appear. These oscillations are similar to the so-called “breather modes”, which are well known in the dynamics of the sine-Gordon systems. We show that the development of these modes depends on the number of junctions, and the dynamics significantly differs between even and odd numbers. As a result, the critical current of the system also undergoes this even-odd effect.     

Slow-fast dynamics in Josephson junctions

We report on a nontrivial type of slow-fast dynamics in a Josephson junction model externally shunted by a resistor and an inductor. For large values of the shunt inductance the slow manifold is highly folded, and different types of dynamical behavior in the fast variable are possible in dependence on the other parameters of the junction. We discuss how particular features of the dynamics are manifested in the current-voltage characteristics of the shunted junction.Received: 10 March 2003, Published online: 11 August 2003PACS: 05.45.-a Nonlinear dynamics and nonlinear dynamical systems - 85.25.Cp Josephson devices - 74.81.-g Inhomogeneous superconductors and superconducting systems     

Breathing charge density waves in intrinsic Josephson junctions

We demonstrate the creation of a charge density wave (CDW) along a stack of coupled Josephson junctions (JJs) in layered superconductors. Electric charge in each superconducting layer oscillates around some average value, forming a breathing CDW. We show the transformation of a longitudinal plasma wave to CDW in the state corresponding to the outermost branch. Transition between different types of CDW’s related to the inner branches of IV characteristic is demonstrated. The effect of the external electromagnetic radiation on the states corresponding to the inner branches differs crucially from the case of the single JJ. The Shapiro steps in the IV characteristics of the junctions in the stack do not correspond directly to the frequency of radiation ω. The system of JJs behaves like a single whole system: the Shapiro steps or their harmonics in the total IV characteristics appear at voltage $\sum {V_l } = N_R \frac{m} {n}\omega$ , where V _l is the voltage in the lth junction, N _R is the number of JJs in the rotating state, and m and n are integers.     

Superconducting phase qubits with shadow-evaporated Josephson junctions

We develop a fabrication process for the superconducting phase qubits in which Josephson junctions for both the qubit and superconducting quantum interference device(SQUID) detector are prepared by shadow evaporation with a suspended bridge. Al junctions with areas as small as 0.05 μm~2 are fabricated for the qubit, in which the number of the decoherencecausing two-level systems(TLS) residing in the tunnel barrier and proportional to the junction area are greatly reduced. The measured energy spectrum shows no avoided crossing arising from coherent TLS in the experimentally reachable flux bias range of the phase qubit, which demonstrates the energy relaxation time T_1 and dephasing time T_φ on the order of 100 ns and 50 ns, respectively. We discuss several possible origins of decoherence from incoherent or weakly-coupled coherent TLS and further improvements of the qubit performance.     

Multifluxon dynamics in driven Josephson junctions

The dynamics of fluxons in a long Josephson junction driven by time-varying nonuniform bias currents are described by a generalization of the sine-Gordon equation. This equation has solitary wave solutions which correspond to current vortices or quantized packets of magnetic flux in the junction. As with the sine-Gordon equation, multifluxon solutions may be demonstrated for the long Josephson junction. Our numerical calculations show that several fluxons may be launched or annihilated at the end of a junction. We also show multiple steady state conditions which correspond to one or more flux quanta trapped in the junction.     

Influence of coupling between junctions on breakpoint current in intrinsic Josephson junctions

We study theoretically the current-voltage characteristics of intrinsic Josephson junctions in high-Tc superconductors. An oscillation of the breakpoint current on the outermost branch as a function of coupling alpha and dissipation beta parameters is found. We explain this oscillation as a result of the creation of longitudinal plasma waves at the breakpoint with different wave numbers. We demonstrate the commensurability effect and predict a group behavior of the current-voltage characteristics for the stacks with a different number of junctions. A method to determine the wave number of longitudinal plasma waves from alpha and beta dependence of the breakpoint current is suggested. We model the alpha and beta dependence of the breakpoint current and obtain good agreement with the results of the simulation.     

Reflectivity and microwave absorption in crystals with alternating intrinsic Josephson junctions

We compute the frequency and magnetic field dependencies of the reflectivity R(omega) in layered superconductors with two alternating intrinsic Josephson junctions with different critical current densities and quasiparticle conductivities for the electric field polarized along the c axis. The parameter alpha describing the electronic compressibility of the layers and the charge coupling of neighboring junctions was extracted for the SmLa1-xSr xCuO (4-delta) superconductor from two independent optical measurements, the fit of the loss function L(omega) at zero magnetic field and the magnetic field dependence of the peak positions in L(omega). The experiments are consistent with a free electron value for alpha.     

Quantum effects on capacitively coupled intrinsic Josephson junctions

We numerically study quantum effects in intrinsic Josephson junctions of layered high-T_c superconductors in order to explain recent experimental observations on the switching rate enhancement in the low temperature quantum regime. We pay attention to the capacitive coupling between neighboring junctions and perform simulations for the Schrödinger equation derived from the Hamiltonian describing the capacitive coupling. The simulation results reveal that the phase dynamics show synchronous behaviors when entering the quantum regime. This is qualitatively consistent with the experimental result.     

Dissipative dynamics of planar d-wave Josephson junctions

We study quantum dynamics of and phase transitions in a Josephson junction between two planar d-wave superconductors where the processes of both quasiparticle and Cooper pair tunneling give rise to nonlocal dissipative terms in the effective action. By combining a perturbative weak coupling analysis in the charge representation with a variational approach in the phase representation at strong coupling, we ascertain a layout of the junction's phase diagram and discuss the corresponding behaviors.     

The current phase relation in Josephson tunnel junctions

The J(?) relation in SFIFS, SNINS, and SIS tunnel junctions is studied. A method for the analytical solution of linearized Usadel equations has been developed and applied to these structures. It is shown that the Josephson current across the structure has a sum of sin? and sin2? components. Two different physical mechanisms are responsible for the sign of sin2?. The first one is the depairing by current, which contributes positively to the sin2? term, while the second one is the finite transparency of SF or SN interfaces, which provides the negative contribution. In SFIFS junctions, where the first harmonic vanishes at the 0-π transition, the calculated second harmonic fully determines the J(?) curve.     

Controlled dynamics of sine-Gordon breather in long Josephson junctions

A possible definition of the specific heat of open quantum systems is based on the reduced partition function of the system. For a free damped quantum particle, it has been found that under certain conditions, this specific heat can become negative at low temperatures. In contrast to the conventional approaches focusing on the system degree of freedom, here we concentrate on the changes induced in the environment when the system is coupled to it. Our analysis is carried out for an Ohmic environment consisting of harmonic oscillators and allows to identify the mechanism by which the specific heat becomes negative. Furthermore, the formal condition for the occurrence of a negative specific heat is given a physical interpretation in terms of the total mass of bath oscillators and the system mass.     

Thickness dependence of the maximum DC Josephson current in superconducting proximity junctions

We investigated the maximum dc Josephson current I_M in NbNb_xO_yCu/Pb junctions as a function of normal layer thickness d_Cu. Theoretical calculations based on the McMillan proximity effect model are in good agreement with the experimental data. The monotonous decrease of RI_M (R is the normal tunneling resistance) indicates the absence of any oscillatory behaviour or anomaly in the pair amplitude F_Cu(d_Cu) of the Cu/Pb sandwich, in contradiction with theoretical prediction.     

Microwave receiver system with quantum superconducting Josephson junctions based on LTSC and HTSC

The paper is concerned with the methods for frequency measurement in millimeter and submillimeter wave ranges using AC Josephson effect. Basing on the method the Microwave Superconductive Receiver System was designed. The main results obtained for niobium Josephson junctions and bridges made of HTSC, as a measurement element are presented.