Flux Transformer Geometry Research Articles

The crossovers of the resistance behaviors in the top and bottom surface layers of 2H-NbSe 2

The critical currents and the resistances in the top and the bottom surface layers have been studied in 2H-NbSe 2single crystal using the flux transformer geometry. In the peak effect region, the critical currents and the resistances in the top and bottom surface layers have distinctly different values. These results are tentatively understood in terms of an order–disorder transition and have implications for the correlations of the vortex lattice throughout the sample.

Multiterminal measurements of vortex correlations in the (K,Ba)BiO 3 system

We have studied vortex correlations in single crystals of Ba 1- x K x BiO 3 by means of multiterminal transport measurements in magnetic fields up to 12 T. The pseudo flux-transformer geometry was used to probe the vortex correlations in the vortex liquid as a function of temperature and field. The vortex system is found to be correlated over the sample thickness just above the onset of resistivity for low to intermediate applied fields. The observed loss of vortex correlation at higher fields is attributed to pinning.

Shapiro steps in superconducting films with a periodic pinning potential

Shapiro voltage steps at voltages V n =nV 0 occur in the voltage-current characteristics of a superconducting film with a periodic pinning potential in the presence of rf radiation. We report on their experimental observation in a superconducting film with a square lattice of perforated microholes (antidots) and on numerical simulations showing the Shapiro steps in the voltage-current characteristics of the dc flux transformer. For the square antidot lattice, the ratio V 0 /ν of the step height to the applied rf frequency depends on the antidot lattice period d, while in the flux transformer geometry this ratio can be tuned by the magnetic field. This tuneability of the resolution makes the flux transformer a good candidate for applications (e.g. broad-band radiation detection).

Vortex physics in confined geometries

Patterned irradiation of cuprate superconductors with columnar defects allows a new generation of experiments which can probe the properties of vortex liquids by forcing them to flow in confined geometries. Such experiments can be used to distinguish experimentally between continuous disorder-driven glass transitions of vortex matter, such as the vortex glass or the Bose glass transition, and non-equilibrium polymer-like glass transitions driven by interaction and entanglement. For continuous glass transitions, an analysis of such experiments that combines an inhomogeneous scaling theory with the hydrodynamic description of viscous flow of vortex liquids can be used to infer the critical behavior. After generalizing vortex hydrodynamics to incorporate currents and field gradients both longitudinal and transverse to the applied field, the critical exponents for all six vortex liquid viscosities are obtained. In particular, the shear viscosity is predicted to diverge as | T− T BG| − νz at the Bose glass transition, with ν≃1 and z≃4.6 the dynamical critical exponent. The scaling behavior of the AC resistivity is also derived. As concrete examples of flux flow in confined geometries, flow in a channel and in the Corbino disk geometry are discussed in detail. Finally, the implications of scaling for the hydrodynamic description of transport in the DC flux transformer geometry are discussed.

Heavy ion induced columnar defects: a sensitive probe for the 2D/3D behaviour of vortex matter in high-temperature superconductors

Heavy ion irradiation is used to create columnar defects in high-temperature superconductors (HTS). The heavy ion induced defects are not only very well controlled in shape and density, but also in the direction of the tracks with respect to the crystallographic c-axis. Pinning of the flux lines as a function of magnetic field orientation then becomes dependent on vortex dimensionality. The two-dimensional (2D)/three-dimensional (3D) behaviour of flux lines was investigated in the highly anisotropic Bi-based superconducting oxide. Results obtained from transport current measurements with epitaxial films, measurements with small single crystals in flux transformer geometry and muon spin rotation (μSR) data with relatively thick crystals are compared.

An improved analysis of resistivity from flux transformer geometry

We present two algorithms for the calculation of the resistivity tensor from current/voltage measurements for the two different contact configurations: “flux transformer” and “ I‖ c”. The application of this method is illustrated on data for an Y 0.78 Pr 0.22 Ba 2 Cu 3 O 7− δ single crystal.

Vortex coupling in Bi2Sr2CaCu2O8+y, using controlled sample geometry

The flux transformer geometry has been useful for studying vortex coupling in Bi2Sr2CaCu2O8+y crystals but is limited by the difficulty of controlling sample size. We have used an argon ion mill to etch two channels into the top face of a Bi2Sr2CaCu2O8+y crystal to reproduce the effects of the flux transformer geometry with more precise control over effective sample dimensions. The temperature dependence of the transformer voltage in such a crystal shows a pronounced peak between 87 K and 89 K which is linear with current. Application of a magnetic field (H ∥ c-axis) sharply suppresses the peak, broadens it, and shifts it to lower temperatures. The peak in the transformer voltage and its current and field dependence are consistent with interlayer Josephson coupling of thermally excited vortices.

Observation of a three-dimensional vortex-line liquid in a highly c-axis-oriented (Bi,Pb)2Sr2Ca2Cu3Ox silver-sheathed tape.

We have measured the temperature dependence of resistance of a highly c-axis-oriented (Bi,Pb${)}_{2}$${\mathrm{Sr}}_{2}$${\mathrm{Ca}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{\mathit{x}}$ silver-sheathed tape in the mixed state using the dc flux transformer geometry. In the vortex fluid regime, we have observed the onset of a nonlocal conductivity which corresponds to the peak (${\mathit{T}}_{\mathrm{peak}}$) in ${\mathit{R}}_{\mathrm{bot}}$(T) curves. This can be explained qualitatively in terms of the vortex decoupling which arises from an increased thermal fluctuation. At T${\mathit{T}}_{\mathrm{peak}}$, the vortices are three dimensional (3D) in character. This behavior is quite different from the results obtained recently by Safar et al. [Phys. Rev. Lett. 69, 824 (1992)] and Busch et al. [Phys. Rev. Lett. 69, 522 (1992)] in ${\mathrm{Bi}}_{2}$${\mathrm{Sr}}_{2}$${\mathrm{CaCu}}_{2}$${\mathrm{O}}_{\mathit{y}}$ single crystals using the same measuring method. Their results show that the vortices are two dimensional in character. The difference of the vortex dimensionality between (Bi,Pb${)}_{2}$${\mathrm{Sr}}_{2}$${\mathrm{Ca}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{\mathit{x}}$ and ${\mathrm{Bi}}_{2}$${\mathrm{Sr}}_{2}$${\mathrm{CaCu}}_{2}$${\mathrm{O}}_{\mathit{y}}$ superconductors in the vortex liquid regime may be attributed to different anisotropies due to their different crystal structures and pinning disorders. In a 3D vortex line liquid, the results reveal that dissipation seems to originate from the thermally activated plastic motion of a pinned vortex line liquid. The field and temperature dependence of the activation energy is as follows: U(H,T)=${\mathit{U}}_{0}$(1-T/${\mathit{T}}_{\mathit{c}}$)${\mathit{H}}^{\mathrm{\ensuremath{-}}0.45}$. \textcopyright{} 1996 The American Physical Society.

Influence of columnar defects on vortex dynamics in Bi2Sr2CaCu2O8 from out-of-plane and flux transformer transport measurements.

The $c$-axis resistivity, ${\ensuremath{\rho}}_{c}(B, T, \ensuremath{\theta})$, where $\ensuremath{\theta}$ is the angle between the $c$ axis and the dc field, has been measured for ${\mathrm{Bi}}_{2}$${\mathrm{Sr}}_{2}$Ca${\mathrm{Cu}}_{2}$${\mathrm{O}}_{8}$ single crystals before and after the introduction of columnar defects by heavy ion irradiation. The effects of different columnar track density and angle with respect to the basal plane are also investigated. Uniaxial enhancement of the irreversibility line for fields below the matching field and parallel to the columnar defects is observed in out-of-plane transport measurements. Measurements in the flux transformer geometry confirm that the vortices are connected lines in the irradiated crystal. We have also attempted to reconcile $c$-axis data with the predictions of the Bose-glass theory for correlated disorder.

2D-3D vortex-line transition in two-layer Josephson arrays.

We calculate the dynamical response of two-layer Josephson arrays to a bias current applied into only one layer (primary layer), in a perpendicular magnetic field. The pancake vortices in the two layers sometimes form into flux lines which move as a unit under the influence of the bias current. The lines break apart into independent pancakes in sufficiently high currents, sufficiently weak interlayer coupling, or possibly sufficiently high fields. We discuss the relevance of these results to recent experiments in high-${\mathit{T}}_{\mathit{c}}$ superconductors in the flux transformer geometry.

Experimental evidence of a non-local resistivity in a vortex line liquid

We present transport measurements on YBCO single crystals in the vortex liquid regime using the DC flux transformer geometry. We show that the data are inconsistent with the material being an anisotropic, local conductor. The results provided evidence for non-local transport which indicate that there can be correlated vortex motion over macroscopic distances in this system. We have shown that this correlated vortex motion can be destroyed by either thermal fluctuations or the applications of large driving forces.

Flux cutting in YBa2Cu3O7- delta single crystals: Experiment and phenomenological model.

We measured the current induced loss of vortex correlation in YBCO crystals using the pseudo DC flux transformer geometry. We find that the current $I_{cut}$ at which the top and bottom voltage differ is a linear function of temperature independent of pinning. Although the vortex correlation length at $I_{cut}$ coincides with the sample thickness, the experimental data show that $I_{cut}$ is sample thickness independent. The observed behavior with temperature, magnetic field and thickness is described by a phenomenological model.

Observation of a nonlocal conductivity in the mixed state of YBa2Cu3O7- delta : Experimental evidence for a vortex line liquid.

We report on transport measurements in the mixed state of ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7}$ single crystals using the dc flux transformer geometry. In the vortex fluid regime we have observed the onset of a nonlocal conductivity. At lower temperatures, close to but above the phase transition temperature the vortices become effectively three dimensional in character. This is in sharp contrast to recent flux transformer experiments on ${\mathrm{Bi}}_{2}$${\mathrm{Sr}}_{2}$${\mathrm{CaCu}}_{2}$O which have failed to see a nonlocal conductivity, confirming the two-dimensional character at high temperatures of the vortices in that system. These observations suggest that in a finite temperature interval, the vortex state in ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7}$ is a line liquid.

Observation of two-dimensional vortices in Bi2Sr2CaCu2Ox.

We report transport measurements in the mixed state of single crystals of ${\mathrm{Bi}}_{2}$${\mathrm{Sr}}_{2}$${\mathrm{CaCu}}_{2}$${\mathrm{O}}_{\mathit{x}}$ using a modified version of the dc flux transformer geometry pioneered by Giaever in low-${\mathit{T}}_{\mathit{c}}$ materials. We find that over a wide range of the phase diagram the vortices are effectively two dimensional in character. Our results suggest that vortex cutting is an efficient process and that it is unlikely that the vortices are strongly entangled in this region of the phase diagram.