Effect of Cherenkov dissipation on vortex velocity in a waveguide-coupled Josephson junction

A motion of slow and fast vortices in a waveguide-coupled Josephson junction induced by a transport current flowing through the entire structure is studied; the coupling is not assumed to be weak. For a fast vortex, conditions are established under which current oscillations due to energy dissipation via Cherenkov radiation of Swihart waves become comparable to the current compensating for ohmic losses in the Josephson junction, waveguide, and adjacent superconductors. For a slow vortex, it is proved that intermediate and strong couplings of the Josephson junction to the waveguide shift current oscillations to the velocity range below the Swihart velocity of the Josephson junction.     

Inhomogeneous current distribution in wide high-temperature superconducting small-angle grain boundaries

Using space resolved magneto-optical microscopy we have studied the development of the critical state in high temperature superconducting small-angle grain boundaries in magnetisation experiments. It has been found that with respect to the position in the grain boundary plane this critical state develops in a highly inhomogeneous manner. Towards the centre of the grain boundary there exists a distinct suppression of the inter-granular currents flowing across the grain boundary. A strong correlation between the static (critical current) and the dynamical properties (e.g. electric field or flux-line velocity) of the vortex system inside the grain boundary appears to be the principal mechanism for the observed inhomogeneous current distribution in wide bi-crystalline current bridges.Received: 6 August 2004, Published online: 14 December 2004PACS: 74.25.Sv Critical currents - 74.72.Bk Y-based cuprates - 74.78.Bz High-T _c films - 74.81.Bd Granular, melt-textured, amorphous and composite superconductors     

Two-Dimensional Coulomb Glass as a Model for Vortex Pinning in Superconducting Films

A glass model of vortex pinning in highly disordered thin superconducting films in magnetic fields B ≪ H_c2 at low temperatures is proposed. Strong collective pinning of a vortex system realized in disordered superconductors that are close to the quantum phase transition to the insulating phase, such as InO_x, NbN, TiN, MoGe, and nanogranular aluminum, is considered theoretically for the first time. Utilizing the replica trick developed for the spin glass theory, we demonstrate that such vortex system is in non-ergodic state of glass type with a large kinetic inductance per square L_K. The distribution function of local pinning energies is calculated, and it is shown that it possesses a wide gap; i.e., the probability to find a weakly pinned vortex is extremely low.

     

Fermionic entropy of the vortex state in d-wave superconductors

In d-wave superconductors the electronic entropy associated with an isolated vortex diverges logarithmically with the size of the system even at low temperatures. In the vortex array the entropy per vortex per layer, S _V , is much larger than k _B and depends on the distribution of the velocity field v _s around the vortex. If there is a first-order transition upon a change of the velocity distribution, then there will be a big entropy jump ΔS _V ∼k _B at the transition. This entropy jump comes from the electronic degrees of freedom on the vortex background, which is modified by the vortex transition. This can explain the big jump in the entropy observed in the so-called vortex-melting transition [A. Junod, M. Roulin, J-Y. Genoud et al., Physica C, to be published], in which the vortex array and thus the velocity field are redistributed. The possibility of the Berezinskii-Kosterlitz-Thouless transition in the 3-dimensional d-wave superconductor due to the fermionic bound states in the vortex background is discussed. Pis’ma Zh. éksp. Teor. Fiz. 65, No. 6, 465–469 (25 March 1997) Published in English in the original Russian journal. Edited by Steve Torstveit.     

Vortex rings moving into turbulence

We report on an experimental study of turbulent vortex rings injected with velocity U _v0 into a grid-generated turbulent flow (with RMS streamwise velocity u _*) and followed relative to the mean flow. The initial Reynolds number of the vortices varies from 4500 to 11,500. The turbulence was characterised by an intensity I_t =u _*/U _v0, which varied over the range 0<I_t <0.03. A mathematical model based on a stochastic model of the vortex core is developed to explain and interpret the results. The vortex radius grows diffusively in time with the rate of increase of the square of the vortex radius increasing linearly with I_t . As the vortices grow, they slow down sufficiently rapidly in a manner that they penetrate a finite distance into the turbulence. The vortex velocity, averaged over many experiments, showed an initial t ^?1 decay, consistent with Maxworthy’s experiments. The analysis and experiments show that such vortices ultimately only move a finite distance from their point of generation and this distance varies inversely with I_t .     

Dynamical phase transition in two-dimensional fully frustrated Josephson junction arrays with resistively shunted junction dynamics

The dynamical phase transitions in two-dimensional fully frustrated Josephson junction arrays at zero temperature are investigated numerically with the resistively shunted junction model through the fluctuating twist boundary condition. The model is subjected to a driving current with nonzero orthogonal components i _x , i _y parallel to both axes of the square lattice. We find a roughly lattice size independent phase diagram with three dynamical phases: a pinned vortex lattice phase, a moving vortex lattice phase and a moving plastic phase. The phase diagram shows a direct transition from the pinned vortex to the moving vortex phase and the separation of the pinned vortex and the moving plastic phases. The time-dependent voltages v _x and v _y are periodic in the moving vortex lattice phase. But they are aperiodic in the moving plastic phase, resulting in non-monotonic characteristics and hysteresis in the current-voltage curves. It is found that the characteristic frequency is twice the time-averaged voltage in the moving vortex phase and around the time-averaged voltage in the plastic flow regime.Received: 29 May 2003, Published online: 2 October 2003PACS: 64.60.Ht Dynamic critical phenomena - 74.25.Sv Critical currents - 74.25.Fy Transport properties     

Cherenkov radiation from an Abrikosov-Josephson vortex

The Cherenkov radiation of generalized Swihart waves is investigated in connection with the slow motion of an Abrikosov-Josephson vortex, which corresponds to a 2 π kink in the phase difference of Cooper pairs on opposite sides of a tunnel junction. The radiative friction force acting on such a vortex is determined. An evaluation is made of the steady-state vortex velocity when the accelerating influence of an electric current through the Josephson junction is compensated by radiative slowing of the vortex due to Cherenkov radiation from the Abrikosov-Josephson vortex. Fiz. Tverd. Tela (St. Petersburg) 39, 444–448 (March 1997)     

High-Frequency Backward Waves in Longitudinally Magnetized Gyrotropic Waveguides

Backward waves in waveguides completely filled with magnetoactive plasma (gaseous or semiconductor plasma) have been investigated numerically. It is shown that two types of backward waves exist in such waveguides: cyclotron backward waves and waveguide HE-modes. While the cyclotron modes are backward waves at arbitrary system parameters (plasma density, magnetic field and waveguide radius), the waveguide backward waves appear only at certain values of there parameters. In addition the cyclotron backward waves can propagate at arbitrary wave-number k_z and at arbitrary phase velocity. The backward waveguide modes exist only at limited values of k_z and of phase velocities.     

Abrikosov vortex escape from a columnar defect as a topological electronic transition in a vortex core

The microscopic scenario of vortex escape from a columnar defect under the influence of a transport current has been studied. For defect radii smaller than the superconducting coherence length the depinning process is shown to be a consequence of two subsequent topological electronic transitions in a trapped vortex core. The first transition at a critical current j _L is associated with the opening of Fermi surface segments corresponding to the creation of a vortex-antivortex pair bound to the defect. The second transition at a certain current j _d > j _L is caused by merging of different Fermi surface segments, which accompanies the formation of a freely moving vortex.     

Measurement of rotational temperature in CO2 waveguide laser medium

A small-signal gain in CO₂ waveguide laser medium has been measured on rotational-vibrational transitions in the P-branch of the (0, 0, 1)-(0, 2⁰, 0) band. It has been found that the rotational temperature is well defined in the waveguide laser system where high excitation power is injected and a large amount of energy is flowing through vibrational, rotational, and translational degrees of freedom. The rotational temperature is slightly higher than the translational temperature.     

Numerical simulation on the operational characteristics of the pulsed inductive acceleration

To analyse the ionization and acceleration properties of an inductive plasma excited by a pulsed current flowing through the planar coil, the extended GLM formulations of the MHD (EGLM‐MHD) model, combined with the high‐temperature thermodynamic and transport model, is employed to simulate the characteristics of the flow. The two‐dimensional axisymmetric calculation captures the generation, growth, and acceleration of the current sheet, and the process is completed in the first half period. The sheet is mainly comprised of lower ionization level ions in the front and higher level ions at the back, and the density is one order higher than that of the residual plasma on the coil surface. As the abscissa value of the sheet is larger than the decoupling distance, a reversed flow emerges, generating a backward impulse, and the negative velocity can be more than 15 km/s at peak intensity B₀ = 0.5 T. In the second 1/4 period, the magnetic field and current density distribute non‐linearly on the surface and regularly in the sheet, caused by the reversing of the changing rate of the magnetic field and the particles' radial diffusion. The results at different intensities show that, for the same coil size, the time at which the maximum velocity V_max appears is advanced as the intensity increases, and V_max can be greater than 20 km/s above 0.5 T.     

AC-current-induced magnetization switching in amorphous microwires

We studied the influence of AC current flowing through microwires, on magnetization dynamics. We used a previously developed Sixtus-Tonks modified setup to evaluate the domain wall (DW) velocity within the microwire. However, instead of a magnetizing solenoid, we used a current flowing through the microwire. We observed that the AC current flowing through the annealed Co-rich microwire leads to remagnetization by fast domain wall propagation. The estimated DW velocity was approximately 4.5 km/s, which is similar to and even higher than that reported for the magnetic-field-driven domain wall propagation in Fe- and Co-rich microwires. We measured the DW velocity under tensile stress, and found that the DW velocity decreases under applied stress. An observed DW propagation induced by the current flowing through the microwire is explained considering the influence of an Oersted magnetic field on the outer domain shell. This field has a circular easy magnetization direction and magnetostatic interaction between the outer circumferentially magnetized shell and the inner axially magnetized core.     

Visualisation and analysis of large-scale vortex structures in three-dimensional turbulent lid-driven cavity flow

In this paper, large eddy simulation (LES) of a three-dimensional turbulent lid-driven cavity (LDC) flow at Re = 10,000 has been performed using the multiple relaxation time lattice Boltzmann method. A Smagorinsky eddy viscosity model was used to represent the sub-grid scale stresses with appropriate wall damping. The prediction for the flow field was first validated by comparing the velocity profiles with previous experimental and LES studies, and then subsequently used to investigate the large-scale three-dimensional vortical structures in the LDC flow. The instantaneous three-dimensional coherent structures inside the cavity were visualised using the second invariant (Q), Δ criterion, λ₂ criterion, swirling strength (λ_ci) and streamwise vorticity. The vortex structures obtained using the different criteria in general agree well with each other. However, a cleaner visualisation of the large vortex structures was achieved with the λ_ci criterion and also when the visualisation is based on the vortex identification criteria expressed in terms of the swirling strength parameters. A major objective of the study was to perform a three-dimensional proper orthogonal decomposition (POD) on the fluctuating velocity fields. The higher energy POD modes efficiently extracted the large-scale vortical structures within the flow which were then visualised with the swirling strength criterion. Reconstruction of the instantaneous fluctuating velocity field using a finite number of POD modes indicated that the large-scale vortex structures did effectively approximate the large-scale motion. However, such a reduced order reconstruction of the flow based on the large-scale vortical structures was clearly not as effective in predicting the small-scale details of the fluctuating velocity field which relate to the turbulent transport.     

Propagation of the <Emphasis Type="Italic">H</Emphasis><Subscript>10</Subscript> mode in a rectangular waveguide with a dielectric discontinuity

The transmission of the H ₁₀ wave through a waveguide-capillary cavity in the form of a section of a rectangular waveguide with a square liquid-filled capillary inside is studied theoretically. The capillary passes through the center of the waveguide perpendicularly to its wide walls. The problem is solved by constructing the mirror images of the capillary on the narrow walls of the waveguide with subsequent consideration of the diffraction of the Brillouin components of the H ₁₀ waveguide mode by a periodic “array of capillaries.” It is shown that a resonance minimum in the frequency dependence of the energy coefficient of transmission of the H ₁₀ wave through the waveguide-capillary cavity can be explained by the destructive interference of diffraction harmonics propagating in antiphase. The experimentally found existence of the capillary optimum dimension at which the resonance minimum is the most pronounced is corroborated analytically.     

Flowing liquid crystal simulating the Schwarzschild metric

We show how to simulate the equatorial section of the Schwarzschild metric through a flowing liquid crystal in its nematic phase. Inside a liquid crystal in the nematic phase, a traveling light ray feels an effective metric, whose properties are linked to perpendicular and parallel refractive indexes, n _o and n _e respectively, of the rod-like molecule of the liquid crystal. As these indexes depend on the scalar order parameter of the liquid crystal, the Beris-Edwards hydrodynamic theory is used to connect the order parameter with the velocity of a liquid crystal flow at each point. This way we calculate a radial velocity profile that simulates the equatorial section of the Schwarzschild metric, in the region outside of Schwarzschild’s radius, in the nematic phase of the liquid crystal. In our model, the higher flow velocity can be on the order of some meters per second.     

Quasi-Stable Configurations of Torus Vortex Knots and Links

The dynamics of torus vortex configurations V_{n, p, q} in a superfluid liquid at zero temperature (n is the number of quantum vortices, p is the number of turns of each filament around the symmetry axis of the torus, and q is the number of turns of the filament around its central circle; radii R₀ and r₀ of the torus at the initial instant are much larger than vortex core width ξ) has been simulated numerically based on the regularized Biot–Savart law. The lifetime of vortex systems till the instant of their substantial deformation has been calculated with a small step in parameter B₀ = r₀/R₀ for various values of parameter Λ = ln(R₀/ξ). It turns out that for certain values of n, p, and q, there exist quasi-stability regions in the plane of parameters (B₀, Λ), in which the vortices remain almost invariable during dozens and even hundreds of characteristic lengths.

     

Vortex deformation and breaking in superconductors: a microscopic description

Vortex breaking has traditionally been studied for non-uniform critical current densities, although it may also appear due to non-uniform pinning force distributions. In this article we study the case of a high-pinning/low-pinning/high-pinning layered structure. We have developed an elastic model for describing the deformation of a vortex in these systems in the presence of a uniform transport current density J for any arbitrary orientation of the transport current and the magnetic field. If J is above a certain critical value, J_c , the vortex breaks and a finite effective resistance appears. Our model can be applied to some experimental configurations where vortex breaking naturally exists. This is the case for YBa₂Cu₃O_7?δ (YBCO) low-angle grain boundaries and films on vicinal substrates, where the breaking is experienced by Abrikosov–Josephson vortices (AJV) and Josephson string vortices (SV), respectively. With our model, we have experimentally extracted some intrinsic parameters of the AJV and SV, such as the line tension ? _l and compared it to existing predictions based on the vortex structure.     

Unusual vortex dynamics in the quantum-liquid phase of a-MoxSi1−x films

We find the unusual vortex dynamics in the low-temperature liquid phase of amorphous Mo_xSi_1−x films by measuring the fluctuating component of the flux-flow voltage δV(t) about the average voltage. For the thick film, in which the quantum vortex liquid (QVL) phase has been well-determined in the field-temperature plane, δV(t) originating from the vortex motion is clearly visible in the QVL phase, where the distribution of δV(t) is anomalously asymmetric, implying large velocity and/or number fluctuations of driven vortices. For the thin film, in which the QVL phase has not been determined from the static transport measurements, similar unusual vortex motion is observed in nearly the same reduced-temperature regime. We suggest that vortex dynamics in the low-temperature liquid phase of thick and thin films is dominated by common physical mechanisms related to quantum-fluctuation effects.     

无序二维约瑟夫森结阵列中磁场引起的涡旋玻璃态相变

采用电阻阻错结的无序二维约瑟夫森结阵列模型,数值研究超导薄膜中垂直磁场引起的涡旋运动.通过分析磁场激发产生的涡旋度Ne及低频电压噪声S₀的变化特性,得到如下结论：在无序超导体中固定温度不变,随着磁场的减弱涡旋液态经过准有序的布拉格相,涡旋玻璃相重新进入到低磁场下的钉扎稀磁液相. 由于在涡旋玻璃相中,电流驱动下的噪声值表现出一个峰,表明系统处于无序与有序相互竞争的亚稳态,并且临界电流应有峰值效应. 计算得到噪声值的变化与Okuma等得到的无序超导Mo_xSi_1-x膜实验现象一致,并能解释磁场降低引起的重新进入钉扎的稀磁液相行为. 关键词： 约瑟夫森结阵列 磁通玻璃 重新进入 峰值效应