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Search Results: 1 - 10 of 582617 matches for " A. D. Caplin "
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Dynamic interactions between pancake vortex stacks and Josephson vortices in Bi2Sr2CaCu2O8 single crystals: relaxation and ratchets
G. K. Perkins,A. D. Caplin,L. F. Cohen
Physics , 2005, DOI: 10.1088/0953-2048/18/10/007
Abstract: We present a detailed study of the dynamic interactions between Josephson vortices and stacks of pancake vortices in a Bi2Sr2CaCu2O8 single crystal, obtained by measuring the effect of applied inplane magnetic field pulses on the c-axis magnetisation. The predominant interaction is to relax the system towards equilibrium. However, using a highly sensitive AC technique we are able to measure also the forces acting to drive the system away from equilibrium, consistent with the existence of dragging interactions between the Josephson and pancake systems. Such forces were discussed recently as the basis of possible flux ratchet devices.
Mapping the dynamic interactions between vortex species in highly anisotropic superconductors
M. Tesei,G. K. Perkins,A. D. Caplin,L. F. Cohen,T. Tamegai
Physics , 2008, DOI: 10.1088/0953-2048/21/7/075019
Abstract: Here we use highly sensitive magnetisation measurements performed using a Hall probe sensor on single crystals of highly anisotropic high temperature superconductors $Bi_{2}Sr_{2}CaCu_{2}O_{8}$ to study the dynamic interactions between the two species of vortices that exist in such superconductors. We observe a remarkable and clearly delineated high temperature regime that mirrors the underlying vortex phase diagram. Our results map out the parameter space over which these dynamic interaction processes can be used to create vortex ratchets, pumps and other fluxonic devices.
Critical currents and vortex dynamics in super-conducting MgB2
Y. Bugoslavsky,G. K. Perkins,X. Qi,L. F. Cohen,A. D. Caplin
Physics , 2001,
Abstract: The recently-discovered MgB2 super-conductor has a transition temperature Tc approaching 40K, placing it intermediate between the families of low and high temperature super-conductors (LTS and HTS). In practical applications, super-conductors are permeated by quantised magnetic flux vortices, and when a current flows there is dissipation unless the vortices are "pinned" in some way, and so inhibited from moving under the influence of the Lorentz force. This vortex motion sets the limiting critical current density Jc in the super-conductor. Vortex behaviour has proved to be more complicated in the HTS than in LTS materials. While this has stimulated extensive theoretical and experimental research, it has impeded applications. Clearly it is important to explore vortex behaviour in MgB2; here we report on Jc, and also on the creep rate S, which is a measure of how fast the "persistent" currents decay. Our results show that naturally-occurring grain boundaries are highly transparent to supercurrent, and suggest that the steep decline in Jc with increasing magnetic field H reflects a weakening of the vortex pinning energy, possibly because this compound forms naturally with a high degree of crystalline perfection.
The effect of magnetic field on the two superconducting gaps in MgB2
Y. Bugoslavsky,Y. Miyoshi,G. K. Perkins,A. D. Caplin,L. F. Cohen,A. V. Pogrebnyakov,X. X. Xi
Physics , 2003, DOI: 10.1103/PhysRevB.69.132508
Abstract: Double-gap superconductivity in an epitaxial MgB2 film has been studied by means of point-contact spectroscopy in magnetic field up to 8 Tesla. The relatively fast disappearance of the feature associated with the pi-band gap at a field around 1 T is caused by the line broadening due to strong pair breaking rather than to a collapse of the double-gap state. This pair breaking was found to increase linearly with field. Field dependences of the order parameters Delta_pi and Delta_sigma in the pi and sigma bands were measured in field applied parallel and perpendicular to the film, at T = 4.2 K. In perpendicular field, both order parameters survive to a common Hc2, which is about 6.5 T for this direction. In parallel field, the decrease of Delta_sigma is much more gradual, consistent with the Hc2 being about 4 times greater in this ori-entation. The difference in Delta_pi measured in the two field orientations is however smaller than the difference of Delta_sigma. We compare these results with the data on tunnelling spectroscopy and specific heat measurements of MgB2 single crystals and find consistency between the different experimental approaches.
Electron diffusivities in MgB2 from point contact spectroscopy
Y. Bugoslavsky,Y. Miyoshi,G. K. Perkins,A. D. Caplin,L. F. Cohen,A. V. Pogrebnyakov,X. X. Xi
Physics , 2005, DOI: 10.1103/PhysRevB.72.224506
Abstract: We demonstrate that the variation of the Andreev reflection with applied magnetic field provides a direct means of comparing the properties of MgB2 with the theory for a dirty two-band superconductor, and we find good agreement between the two. The ratio of electron diffusivities in the s and p bands can be inferred from this experiment. We find that the field dependence of the density of states at the Fermi level in the p band is independent of the field direction, and that the anisotropic upper critical field is determined by the anisotropic diffusivity in the s band.
Critical Fields and Critical Currents in MgB2
A. D. Caplin,Y. Bugoslavsky,L. F. Cohen,L. Cowey,J. Driscoll,J. Moore,G. K. Perkins
Physics , 2002, DOI: 10.1088/0953-2048/16/2/309
Abstract: We review recent measurements of upper (Hc2) and lower (Hc1) critical fields in clean single crystals of MgB2, and their anisotropies between the two principal crystallographic directions. Such crystals are far into the "clean limit" of Type II superconductivity, and indeed for fields applied in the c-direction, the Ginzburg-Landau parameter k is only about 3, just large enough for Type II behaviour. Because m0Hc2 is so low, about 3 T for fields in the c-direction, MgB2 has to be modified for it to become useful for high-current applications. It should be possible to increase Hc2 by the introduction of strong electron scattering (but because of the electronic structure and the double gap that results, the scatterers will have to be chosen carefully). In addition, pinning defects on a scale of a few nm will have to be engineered in order to enhance the critical current density at high fields.
Enhancement of the high-field critical current density of superconducting MgB2 by proton irradiation
Y. Bugoslavsky,L. F. Cohen,G. K. Perkins,M. Polichetti,T. J. Tate,R. Gwilliam,A. D. Caplin
Physics , 2001, DOI: 10.1038/35079024
Abstract: A relatively high critical temperature, Tc, approaching 40 K, places the recently-discovered superconductor magnesium diboride (MgB2) intermediate between the families of low- and copper-oxide-based high-temperature superconductors (HTS). Supercurrent flow in MgB2 is unhindered by grain boundaries, unlike the HTS materials. Thus, long polycrystalline MgB2 conductors may be easier to fabricate, and so could fill a potentially important niche of applications in the 20 to 30 K temperature range. However, one disadvantage of MgB2 is that in bulk material the critical current density, Jc, appears to drop more rapidly with increasing magnetic field than it does in the HTS phases. The magnitude and field dependence of Jc are related to the presence of structural defects that can "pin" the quantised magnetic vortices that permeate the material, and prevent them from moving under the action of the Lorentz force. Vortex studies suggest that it is the paucity of suitable defects in MgB2 that causes the rapid decay of Jc with field. Here we show that modest levels of atomic disorder, induced by proton irradiation, enhance the pinning, and so increase Jc significantly at high fields. We anticipate that chemical doping or mechanical processing should be capable of generating similar levels of disorder, and so achieve technologically-attractive performance in MgB2 by economically-viable routes.
Super-conducting critical fields and anisotropy of a MgB2 single crystal
G. K. Perkins,J. Moore,Y. Bugoslavsky,L. F. Cohen,J. Jun,S. M. Kazakov,J. Karpinski,A. D. Caplin
Physics , 2002, DOI: 10.1088/0953-2048/15/7/330
Abstract: Despite the intense activity in the year since the discovery of superconductivity in MgB2, key parameters, in particular the upper and lower critical fields Hc2 and Hc1 and their anisotropies, are not well-established, largely because of the difficulty of growing MgB2 crystals. Attempts have been made to deduce these parameters from experiments on polycrystalline material, but they have substantial uncertainties. Hc2 is particularly important for applications, as it is the field which quenches bulk super-conductivity. In terms of understanding MgB2, it is now clear that the conventional electron-phonon interaction is strong enough to account for the high transition temperature Tc, but the consequences of the double super-conducting gap for the anisotropy and its dependence on temperature, are uncertain. Here we describe detailed direct measurements of Hc1(T) and Hc2(T) for the two principal crystallographic directions in a clean single crystal of MgB2. For fields in the c-direction, $\mu_0 H^c_{c1}(0)$ = $0.28 +- 0.01T$ and $\mu_0 H^c_{c2}(0)$ is $3 +- 0.5T$; this ratio of critical fields is rather low and implies that MgB2 is only just a Type II super-conductor. The anisotropies of both critical fields are close to 2.
High Critical Current Density and Improved Irreversibility Field in Bulk MgB2 Made By A Scaleable, Nanoparticle Addition Route
J. Wang,Y. Bugoslavsky,A. Berenov,L. Cowey,A. D. Caplin,L. F. Cohen,J. L. MacManus Driscoll,L. D. Cooley,X. Song,D. C. Larbalestier
Physics , 2002, DOI: 10.1063/1.1506184
Abstract: Bulk samples of MgB2 were prepared with 5, 10, and 15% wt.% Y2O3 nanoparticles added using a simple solid-state reaction route. Transmission electron microscopy (TEM) showed a fine nanostructure consisting of ~3-5 nm YB4 nanoparticles embedded within MgB2 grains of ~400 nm size. Compared to an undoped control sample, an improvement in the in-field critical current density Jc was observed, most notably for 10% doping. At 4.2K, the lower bound Jc value was ~2x105A.cm-2 at 2T. At 20K, the corresponding value was ~8x104A.cm-2. Irreversibility fields were ~11.5 T at 4.2K and 5.5T at 20K, compared to ~4T and ~8T, respectively, for high-pressure synthesized bulk samples.
Evidence for Nodal superconductivity in Sr$_{2}$ScFePO$_{3}$
K. A. Yates,I. T. M. Usman,K. Morrison,J. D. Moore,A. M. Gilbertson,A. D. Caplin,L. F. Cohen,H. Ogino,J. Shimoyama
Physics , 2009, DOI: 10.1088/0953-2048/23/2/022001
Abstract: Point contact Andreev reflection spectra have been taken as a function of temperature and magnetic field on the polycrystalline form of the newly discovered iron-based superconductor Sr2ScFePO3. A zero bias conductance peak which disappears at the superconducting transition temperature, dominates all of the spectra. Data taken in high magnetic fields show that this feature survives until 7T at 2K and a flattening of the feature is observed in some contacts. Here we inspect whether these observations can be interpreted within a d-wave, or nodal order parameter framework which would be consistent with the recent theoretical model where the height of the P in the Fe-P-Fe plane is key to the symmetry of the superconductivity. However, in polycrystalline samples care must be taken when examining Andreev spectra to eliminate or take into account artefacts associated with the possible effects of Josephson junctions and random alignment of grains.
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