N. Kokubo

Mode locking of vortex matter driven through mesoscopic channels.

We investigated the driven dynamics of vortices confined to mesoscopic flow channels by means of a dc-rf interference technique. The observed mode-locking steps in the IV curves provide detailed information on how both the number of vortex rows and the lattice structure in each flow channel change with magnetic field. Minima in flow stress occur when an integer number of rows is moving coherently, while maxima appear when the incoherent motion of mixed n and n+/-1 row configurations is predominant. Simulations show that the enhanced pinning at mismatch originates from quasistatic fault zones with misoriented edge dislocations induced by disorder in the channel edges.

Dynamic melting of confined vortex matter.

We study dynamic melting of confined vortex matter moving in disordered, mesoscopic channels by mode-locking experiments. The dynamic melting transition, characterized by a collapse of the mode-locking effect, strongly depends on the frequency, i.e., on the average velocity of the vortices. The associated dynamic ordering velocity diverges upon approaching the equilibrium melting line T(m,e)(B) as v(c) approximately (T(m,e)-T)(-1). The data provide the first direct evidence for velocity dependent melting and show that the phenomenon also takes place in a system under disordered confinement.

Scanning SQUID microscope study of vortex polygons and shells in weak-pinning disks of an amorphous superconducting film

Direct observation of vortices by the scanning superconducting quantum interference device microscopy was made on large mesoscopic disks of an amorphous MoGe thin film. Owing to the weak pinning nature of the amorphous film, vortices are able to form geometry induced, quasi-symmetric configurations of polygons, and concentric shells in the large disks. Systematic measurements made on selected disks allow us to trace not only how the vortex pattern evolves with magnetic field, but also how the vortex polygons change in size and rotate with respect to the disk center. The results are in good agreement with theoretical considerations for mesoscopic disks with sufficiently large diameter. A series of vortex images obtained in a disk with a pinning site reveals a unique line symmetry in vortex configurations, resulting in modifications of the shell filling rule and the magic number.

Superconductivity in transparent zinc-doped In2O3 films having low carrier density

Abstract Thin polycrystalline zinc-doped indium oxide (In2O3–ZnO) films were prepared by post-annealing amorphous films with various weight concentrations x of ZnO in the range 0x 0.06. We have studied the dependences of the resistivity ρ and Hall coefficient on temperature T and magnetic field H in the range 0.5T 300 K, H6 Tfor 350 nm films annealed in air. Films with 0x0.03 show the superconducting resistive transition. The transition temperature Tc is below 3.3 K and the carrier density n is about 1025–1026 m−3. The annealed In2O3–ZnO films were examined by transmission electron microscopy and x-ray diffraction analysis revealing that the crystallinity of the films depends on the annealing time. We studied the upper critical magnetic field Hc2 (T) for the film with x = 0.01. From the slope of dHc2 /dT, we obtain the coherence length ξ (0) ≈ 10 nm at T = 0 K and a coefficient of electronic heat capacity that is small compared with those of other oxide materials.

Transport properties and microstructures of polycrystalline In2O3–ZnO thin films

We prepared polycrystalline In2O3–ZnO films by post annealing the amorphous films (1.0 wt % ZnO) at 200 °C with various annealing times ta 0≤ta≤20 h. We have measured the electric resistivity and Hall mobility and also observed film structures by not only the x-ray diffraction but also scanning transmission electron microscopy (STEM) with electron energy-loss spectroscopy (EELS). We have found the following: (1) Hall mobility takes the maximum with respect to the carrier density and the annealed films clearly show the superconductivity of which transition temperature increases with increase in ta. (2) The data on EELS spectra mapping of indium plasmon indicate that droplets of the pure indium phase exist on grain boundaries and near the interface between the film and the glass substrate. However, it seems that these droplets do not form an electrical conducting path but contribute to the scattering centers for carrier electrons, from the dispersed distribution of these droplets in STEM-EELS spectra mappin...

Direct Imaging of Vortex Polygons and Vortex Shells in Mesoscopic Squares of a Weak Pinning Superconducting Thin Film

We report on the direct visualization of vortices in mesoscopic squares of a weak pinning amorphous superconducting thin film using a scanning superconducting quantum interference device microscope. From systematic measurements made on selected dots, we trace how the vortex pattern evolves with an applied magnetic field. The observed images clearly illustrate the formation of symmetric polygons and concentric shells of vortices, from which the rule of shell filling with increasing vorticity is formulated. We analyze the size and shape of vortex polygons and obtain insight into the flow pattern of the shielding current governing vortex configurations.

Vortex states and peak effect at low temperature in thick amorphous films with weak pinning

We report on the dynamic as well as static properties of vortices in the low-temperature (T) solid phase of a thick amorphous MoxGe1-x film with weak pinning. The vortex-solid state is considered to be a weakly disordered vortex lattice, which is inferred from the mode-locking resonance over the broad field region. The peak effect of the critical current Ic with magnetic field is clearly visible down to the lowest T measured (0.2 K), which corresponds to 3.3% of the superconducting transition temperature. We find that largest voltage noise originating from current-driven vortices appears in the field region just prior to the peak field at which Ic takes a peak. This is in accord with the result reported in NbSe2 single crystals.

Commensurate and Incommensurate Vortex States Confined in Mesoscopic Triangles of Weak Pinning Superconducting Thin Films

We report on the direct observation of vortex states confined in equilateral and isosceles triangular dots of weak pinning amorphous superconducting thin films with a scanning superconducting quantum interference device microscope. The observed images illustrate not only pieces of a triangular vortex lattice as commensurate vortex states, but also incommensurate vortex states including metastable ones. We comparatively analyze vortex configurations found in different sample geometries and discuss the symmetry and stability of commensurate and incommensurate vortex configurations against deformations of the sample shape.

Depression of positive magneto-conductance due to anti-weak localization effect in annealed In2O3-ZnO thick films

We investigated the magneto-conductivity Δ in three dimensional indium zinc oxide films with different resistivity ρ prepared by postannealing in air. The weak localization theory was fitted to data of Δ H) at temperatures below 50K by the use of suitable inelastic scattering time τi(T) and spin-orbit(S-O) scattering time τi. We found the ρ dependences of both times τ and τi in a range 1.5 × 10−3Ω < ρ 300K) <4 × 10−6Ω. As ρ increases, the ratio τi/τ increases from ≍ .005 to ≍ .5 and the Δ − at low temperatures changes from positive to negative values. We suggest a picture that the annealing in air brings the change of the S-O scattering from light to heavy atoms, namely, oxygen to indium and/or zinc atoms.

Broadband Noise of Driven Vortices at the Mode‐Locking Resonance

Vortex‐flow noise across the mode‐locking (ML) resonance has been studied in an amorphous MoxGe1−x film. We have found that broadband noise (BBN) is remarkably suppressed, when the driven vortex system undergoes ML (dynamic ordering), where current‐voltage characteristics show the step‐like behavior. The result is consistent with the view of ML freezing that the mode‐locked state is a frozen solid pinned in the moving frame of reference. By changing the amplitude of the ac drive, we find the correlation between the sharpness of the ML resonance and suppression of BBN at the ML step.