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Search Results: 1 - 10 of 50545 matches for " Y. Bugoslavsky "
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Andreev Reflection Spectroscopy in Niobium Point Contacts in Magnetic Field
Y. Miyoshi,Y. Bugoslavsky,L. F. Cohen
Physics , 2004, DOI: 10.1103/PhysRevB.72.012502
Abstract: The Blonder-Tinkham-Klapwijk model of the Point-Contact Andreev Reflection spectroscopy does not provide an adequate description of experiment in the presence of magnetic field. We demonstrate this using a junction between a niobium tip and a copper film. We modify the the-ory to explicitly take into account the contribution to conductance that stems from the normal vortex cores in the superconductor. These results have important implications for interpretation of transport spin polarisation measurements using PCAR technique. We demonstrate that stray magnetic fields can be responsible for the experimentally observed dependence of the spin po-larisation on the strength of the interface barrier, and potential misassignment of the inferred spin polarisation.
Structural, magnetic and transport properties of thin films of the Heusler alloy Co2MnSi
L. J. Singh,Z. H. Barber,Y. Miyoshi,Y. Bugoslavsky,W. R. Branford,L. F. Cohen
Physics , 2003, DOI: 10.1063/1.1690868
Abstract: Thin films of Co2MnSi have been grown on a-plane sapphire substrates from three elemental targets by dc magnetron co-sputtering. These films are single phase, have a strong (110) texture and a saturation magnetization of 4.95 uB/formula unit at 10 K. Films grown at the highest substrate temperature of 715 K showed the lowest resistivity (47 uOhm cm at 4.2 K) and the lowest coercivity (18 Oe). The spin polarization of the transport current was found to be of the order of 54% as determined by point contact Andreev reflection spectroscopy. A decrease in saturation magnetization with decreasing film thickness and different transport behaviour in thinner films indicate a graded disorder in these films grown on non-lattice matched substrates.
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.
Investigation of the growth and magnetic properties of highly oriented films of the Heusler alloy Co2MnSi on GaAs(001)
L. J. Singh,Z. H. Barber,A. Kohn,A. K. Petford-Long,Y. Miyoshi,Y. Bugoslavsky,L. F. Cohen
Physics , 2004,
Abstract: Highly (001) oriented thin films of Co2MnSi have been grown on lattice matched GaAs(001) without a buffer layer. Stoichiometric films exhibited a saturation magnetization slightly reduced from the bulk value and films grown at the highest substrate temperature of 689 K showed the lowest resistivity (33 micro.ohm.cm at 4.2 K) and the lowest coercivity (14 Oe). The spin polarization of the transport current was found to be of the order of 55% as determined by point contact Andreev reflection spectroscopy. The reduced magnetization obtained was attributed to the antiferromagnetic Mn2As phase. Twofold in-plane magnetic anisotropy was observed due to the inequivalence of the <110> directions, and this was attributed to the nature of the bonding at the reconstructed GaAs surface.
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.
Superconducting gap structure and pinning in disordered MgB2 films
Y. Bugoslavsky,Y. Miyoshi,G. K. Perkins,A. D. Caplin,L. F. Cohen,H. Y. Zhai,H. M. Christen,A. V. Pogrebnyakov,X. X. Xi,O. V. Dolgov
Physics , 2003, DOI: 10.1088/0953-2048/17/5/052
Abstract: We have performed a comparative study of two thin films of magnesium diboride (MgB2) grown by different techniques. The critical current density at different temperatures and magnetic fields was evaluated from magnetisation curves, the structure of superconducting order parameter was obtained from point-contact spectroscopy, and the scattering rates were evaluated by fitting the temperature dependent normal-state resistivity to the two-band model. The films have similar critical temperatures close to 39 K, but the upper critical fields were different by a factor of 2 (5.2T and 2.5 T at 20 K). We have found that the film with higher Hc2 also had stronger scattering in the sigma band and smaller value of the superconducting gap in this band. As the scattering in sigma band is primarily due to the defects in boron plane, our results are consistent with the assumption that disordering the boron planes leads to enhanced Hc2 and better pinning properties in magnetic field.
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