Velocimetry of superconducting vortices based on stroboscopic resonances

Causes and Consequences of Ordering and Dynamic Phases of Confined Vortex Rows in Superconducting Nanostripes

Understanding the behaviour of vortices under nanoscale confinement in superconducting circuits is important for the development of superconducting electronics and quantum technologies. Using numerical simulations based on the Ginzburg-Landau theory for non-homogeneous superconductivity in the presence of magnetic fields, we detail how lateral confinement organises vortices in a long superconducting nanostripe, presenting a phase diagram of vortex configurations as a function of the stripe width and magnetic field. We discuss why the average vortex density is reduced and reveal that confinement influences vortex dynamics in the dissipative regime under sourced electrical current, mapping out transitions between asynchronous and synchronous vortex rows crossing the nanostripe as the current is varied. Synchronous crossings are of particular interest, since they cause single-mode modulations in the voltage drop along the stripe in a high (typically GHz to THz) frequency range.

Anisotropic transport induced by DC electrical current bias near the critical current

We investigated the transport characteristics of a square shape superconducting Ta thin film under DC electrical current bias along the diagonal direction. The resistance parallel (R_∥) and perpendicular (R_⊥) to the DC current, I_DC, is measured with various magnetic fields. R_∥ and R_⊥ show contrasting dependence on I_DC. First, the critical current of R_∥ is smaller than that of R_⊥. Second, R_⊥ shows an unexpected reduction at current bias where R_∥ shows a rapid increase near the transition from a flux flow state to a normal state. The intriguing anisotropic transport characteristics can be understood by the inhomogeneous current density profile over the square sample. Diagonal DC current induces an anisotropic current density profile where the current density is high near the biasing electrode and low at the center of the sample. Accordingly, the electrical transport in the perpendicular direction could remain less affected even near the critical current of R_∥, which leads to the higher critical current in R_⊥. Complicated conduction profile may also allow the anomalous reduction in the R_⊥ before finally shifting to the normal state.

Ultra-fast kinematic vortices in mesoscopic superconductors: the effect of the self-field

Within the framework of the generalized time-dependent Ginzburg–Landau equations, we studied the influence of the magnetic self-field induced by the currents inside a superconducting sample driven by an applied transport current. The numerical simulations of the resistive state of the system show that neither material inhomogeneity nor a normal contact smaller than the sample width are required to produce an inhomogeneous current distribution inside the sample, which leads to the emergence of a kinematic vortex–antivortex pair (vortex street) solution. Further, we discuss the behaviors of the kinematic vortex velocity, the annihilation rates of the supercurrent, and the superconducting order parameters alongside the vortex street solution. We prove that these two latter points explain the characteristics of the resistive state of the system. They are the fundamental basis to describe the peak of the current–resistance characteristic curve and the location where the vortex–antivortex pair is formed.

Dynamical regimes of kinematic vortices in the resistive state of a mesoscopic superconducting bridge

In this work, we studied the dynamics of the kinematic vortices (kV) in a mesoscopic superconductor under the influence of external applied magnetic fields (H) and transport currents (Jtr). AJtr(H) phase diagram is determined. We show that the vortex dynamics are profoundly affected by the presence of constrictions and byH. We find that between the Meissner and normal states, the resistive state is comprehended by three distinct states. One of these states is a kinematic vortex-antivortex pair for low values ofH. For moderate values of fields, the kV state is unpaired and only a single kinematic vortex exists. In the high field regime, the kinematic vortex becomes an Abrikosov-like state with a lower velocity. It was also shown that for tenths of picoseconds, a surface barrier effect acts over the instantaneous velocity of the kV.

Use of thermal gradients for control of vortex matter in mesoscopic superconductors

Usually, the measurements of electronic and magnetic properties of superconducting samples are carried out under a constant temperature bath. On the other hand, thermal gradients induce local variation of the superconducting order parameter, and the vortex dynamics can present interesting behaviors. In this work, we solved the time-dependent Ginzburg-Landau equations simulating samples under two different thermal gradients, and considering two values of the Ginzburg-Landau parameter, [Formula: see text]. We find that both parameters, i.e. [Formula: see text] and thermal gradients, play an important role on the vortex dynamics and on the magnetization behavior of the samples.

Josephson vortex loops in nanostructured Josephson junctions

Linked and knotted vortex loops have recently received a revival of interest. Such three-dimensional topological entities have been observed in both classical- and super-fluids, as well as in optical systems. In superconductors, they remained obscure due to their instability against collapse - unless supported by inhomogeneous magnetic field. Here we reveal a new kind of vortex matter in superconductors - the Josephson vortex loops - formed and stabilized in planar junctions or layered superconductors as a result of nontrivial cutting and recombination of Josephson vortices around the barriers for their motion. Engineering latter barriers opens broad perspectives on loop manipulation and control of other possible knotted/linked/entangled vortex topologies in nanostructured superconductors. In the context of Josephson devices proposed to date, the high-frequency excitations of the Josephson loops can be utilized in future design of powerful emitters, tunable filters and waveguides of high-frequency electromagnetic radiation, thereby pushing forward the much needed Terahertz technology.