Session Q10: Superconductivity: Vortex II

Vortex ratchets from asymmetric weak-pinning channels

The dynamics of vortex flow in confined geometries can be explored with structured weak-pinning channels of superconducting a-NbGe surrounded by strong-pinning NbN channel edges. The lack of pinning allows the vortices to move through the channels with the dominant interaction determined by the shape of the channel walls. We have fabricated such weak-pinning channels with asymmetric sawtooth edges for controlling the motion of vortices. We present measurements of vortex dynamics in the channels and compare these with similar measurements on a set of channels with uniform width. While the uniform-width channels exhibit a symmetric response for both directions through the channel, the vortex motion through the asymmetric channels is considerably different, with substantial asymmetries in both the static depinning and dynamic flux flow. We report on the rich dependence on magnetic field and driving force amplitude for this vortex ratchet effect.   

Vortex dynamics in a single weak-pinning superconducting channel with a Corbino geometry.

We report transport measurements of vortex flow dynamics in nanofabricated weak-pinning channels of a-NbGe with strong-pinning NbN channel edges. The channels are arranged in circular patterns on a Corbino disk geometry with a radial bias current, thus eliminating the influence of edge barriers to vortex entry on the dynamics. We have developed a SQUID voltmeter arrangement for resolving the flux flow voltage from a small number of vortices moving through such a single mesoscopic channel. We discuss the configuration of this measurement scheme, along with potential applications of this system for measuring the flow of vortices around single weak-pinning channels free from edge barriers.   

Spontaneous Transverse Voltage and Amplified Switching in Superconductors with Honeycomb Pinning Arrays

Using numerical simulations, we show that a novel spontaneous transverse voltage can appear when a longitudinal drive is applied to type-II superconductors with honeycomb pinning arrays in a magnetic field near certain filling fractions. We find a coherent strongly amplified transverse switching effect when an additional transverse ac current is applied, even for very small ac current amplitudes. The transverse ac drive can also be used to control switching in the longitudinal voltage response. We discuss how these effects could be used to create new types of devices such as current effect transistors.   

Anisotropic properties of superconducting niobium wire-networks

By utilizing focused ion beam (FIB) patterning technique we were able to fabricate hole-arrays with interhole spacing down to 150 nm into superconducting niobium (Nb) films. This enabled us to have a large temperature range to explore the properties of Nb wire networks in which the superconducting strips between neighboring holes are comparable to the superconducting coherence length. We studied the anisotropy of these superconducting networks by measuring the critical temperatures and magnetoresistances at various magnetic field directions respect to the film surface. The effect of film thickness, hole diameter, interhole-spacing and the symmetry of the hole lattice on the anisotropy will be reported.   

Vortex Matter in Magnet --Superconductor Hybrids

Arrays of magnetic nanowires in alumina template can create highly inhomogeneous and tesla strong magnetic fields on the template surface. We discuss properties of vortex matter created by such magnetic nanostructures and an external field in thin superconducting film on the template surface. This includes possible phase diagrams, pinning properties and critical current. We predict possible types of hysteresis curves and I-V curves in such magnet-superconductor hybrid in a broad temperature range   

Comparing computer simulations to measurements of slow moving vortices in NbSe$_2$

We observed slow moving vortices in NbSe$_2$ using scanning tunneling microscopy (STM) at a temperature of 4.2 K driven by the slow decay of the magnetic field of our superconducting magnet. The velocity of the vortices depends on the rate of field decay (\~ 0.4 mT/day) and the distance of the STM tip from the center of the sample. In our case the velocities of the vortices are in the range of \~ pm/s allowing for high spatially and (relatively speaking) temporal data series. In order to understand the details of the measurements we wrote a simple 2D simulation for moving vortices in a static potential landscape. The simulation allows for a variety of scenarios such as periodic/fixed boundaries, constant/variable driving force, insertion/extraction of vortices, point/line defects in order to match the behavior in the simulation to the measurement. Although some phenomenas such as similar track patterns and local lattice distortions around point defects have been reproduced, the behavior of the velocity with time, showing aperiodic 'spikes', so far defied explanation. The results of the simulation as well as possible reasons for the velocity vs. time data will be discussed in detail.   

Towards ordered flux flow in A15 superconductor V$_{3}$Si at high fields

The motion of flux quanta is observed in a high-quality superconducting single crystal of V$_{3}$Si with weak pinning and significantly reduced thermal fluctuations due to a critical temperature of less than 17 K. This opens up the possibility of approaching ordered, Bardeen-Stephen flux flow (BSFF). The flux flow resistivity \textit{$\rho $}$_{ff}$ associated with dissipative flux motion is observed in V-I curves as a high-current transition to an ohmic curve whose dissipation level is \textit{below} the normal-state level. Details of overcoming technical difficulties of using high currents are described. BSFF is expected to be manifested by a linear dependence of \textit{$\rho$}$_{ff}$ on applied field $H$. Measuring from fields of 6 T up to 20 T, an approach to ohmic curves characteristic of BSFF are clearly distinguishable, along with other interesting features such as the ``peak'' effect in critical current $J_{c}(H)$ seen only when the pinning energy density is comparable to the elasticity of the flux medium. This and further data and their interesting ramifications are discussed.   

Time-resolved infrared spectroscopy of superconducting NbTiN film near H$_{c2}$

We use subnanoseconds time-resolved, pump-probe infrared spectroscopy to study vortex dynamics of a conventional superconductor, Nb$_{0.5}$Ti$_{0.5}$N near H$_{c2}$. The measurements were performed at the National Synchrotron Light Source, Brookhaven National Laboratory; Picosecond pulses from a near-infrared Ti:sapphire laser were used as a pump and, subnanosecond pulses of infrared synchrotron radiation as a probe. We report detailed magnetic field dependences of the amplitude of photoinduced quasiparticles and the effective lifetime of the quasiparticles and also discuss vortex dynamics in the system near H$_{c2}$. Near H$_{c2}$, we observe an interesting deviation from the field-independent behavior of the effective lifetime at lower fields.   

Imaging the dynamics of single vortices on grain boundaries in YBa$_2$Cu$_3$O$_{6+x}$ thin films

We use a scanning Hall probe microscope with single vortex resolution to study the dynamics of single vortices on grain boundaries in YBa$_2$Cu$_3$O$_{6+x}$ thin films with ~10 kHz bandwidth. In the presence of an applied current, we observe individual vortices hopping between pinning sites. Detecting the motion of individual vortices allows us to probe the very- low-voltage regime of the current -voltage ($I-V$) characteristic, at voltage levels of $2\cdot10^{-15}$ V. By scanning the grain boundary we show local $I-V$ curves and investigate the statistical processes at the onset of vortex motion.   

Imaging the Vortex Density of States in Bi$_2$Sr$_2$CaCu$_2$O$_{8+x}$

We use a low temperature scanning tunneling microscope (STM) to image vortices in the high temperature superconductor Bi$_2$Sr$_2$CaCu$_2$O$_{8+x}$, in magnetic fields up to 9T. We locate the vortices via their increased local density of states near $\pm$7 meV. We investigate the dependence of the vortex halo on several parameters, including applied magnetic field and proximity to surface impurities.   

Single Vortex Imaging and Shaking in a YBa$_{2}$Cu$_{3}$O$_{6.991}$ Single Crystal

We image and manipulate individual vortices in a detwinned YBa$_{2}$Cu$_{3}$O$_{6.991}$ single crystal, using magnetic force microscopy (MFM). We observe a strong dynamic asymmetry between the fast and slow directions of the raster imaging pattern, when the MFM tip-vortex force exceeds the depinning force. The vortex can be dislodged hundreds of nanometers along the raster direction, while perpendicular to it, along the slow direction, it can be stretched up to several microns by the same applied force. We attribute this effect to single vortex shaking, in analogy with vortex matter shaking induced by applying an oscillating magnetic field or current. Here, we raster the tip, shaking the top of one vortex to help it explore different configurations, which are determined by the interplay between pinning and elasticity. In this process, elastic energy associated with stretching along the slow direction can be released, thus allowing the vortex to be dragged further. We are currently studying shaking in an underdoped YBa$_{2}$Cu$_{3}$O$_{7-\delta}$ single crystal, where the Josephson coupling along crystal c-axis is much reduced, rendering elasticity weaker.   

Tunneling of an extended object in a dissipative environment: suppression of tunneling of vortices caused by a remote gate.

A strong decrease of the resistance of a superconducting film measured in magnetic field has been observed when a metallic gate is placed above the film.~\footnote{N. Mason, and A. Kapitulnik, Phys. Rev. B. 65, 220505(R) (2002).} We study how the magnetic coupling between vortices in a superconducting film and electrons in a remote unbiased gate affects the tunneling rate of the vortices. We examine two approaches to analyze tunneling in the presence of low-energy degrees of freedom: (i) the response of the electrons inside the metallic gate to a change in the vortex position is described as ``tunneling with dissipation'' (ii) the scattering of the electrons by the magnetic flux of the vortex leads to the Orthogonality Catastrophe that opposes the vortex tunneling. We show that the gate can cause a dramatic suppression of the vortex tunneling restoring the superconducting property in accord with the experiment.   

Dynamics of vortices driven by magnetic field changes observed by LT-STM

When changing the magnetic field for a type two superconductor the vortex density has to change accordingly. Vortices have to enter/leave the superconductor to facilitate that change. Since vortex-defect interactions impede the vortex motion such a change does not happen instantaneously. Observing the vortex lattice at a given distance from the center by STM allows to study that behavior. The velocity usually decays close to exponentially. Although the first 'fast' phase (\~ 30 min) is unaccessible to STM due to its limited scan speed the tail section (\~ 5 h) can be studied in detail. We will present analysis of the data and compare it to previous results on slow moving vortices driven by a slowly decaying magnetic field.   

h/e-Flux Periodicity in Superconducting Loops

We apply the BCS theory to superconducting rings with unconventional order parameter symmetries. An external magnetic flux changes the character of the states in the condensate; as a consequence the energy of the superconducting ground state varies with a flux period of h/e. This h/e periodicity is caused by the flux-induced reconstruction of the supercurrent carrying condensate.