170 research outputs found

    Critical Fields and Critical Currents in MgB2

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    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.Comment: BOROMAG Conference Invited paper. To appear in Supercond. Sci. Tec

    Superconducting gap structure and pinning in disordered MgB2 films

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    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.Comment: Paper presented at EUCAS'0

    Comparative study of in situ and ex situ MgB2 films deposited by pulsed laser deposition

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    Two types of MgB2 films were prepared by pulsed laser deposition (PLD) with in situ and ex situ annealing processes respectively. Significant differences in properties between the two types of films were found. The ex situ MgB2 film has a Tc of 38.1K, while the in situ film has a depressed Tc of 34.5K. The resistivity at 40K for the in situ film is larger than that of the ex situ film by a factor of 6. The residual resistivity ratios (RRR) are 1.1 and 2.1 for the in situ and ex situ films respectively. The Jc-H curves of the in situ film show a much weaker field dependence than those of the ex situ film, attributable to stronger flux pinning in the in situ film. The small-grain feature and high oxygen level may be critical for the significant improvement of Jc in the in situ annealed MgB2 film.Comment: 6 pages, 6 figure

    Angular dependence of the bulk nucleation field Hc2 of aligned MgB2 crystallites

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    Studies on the new MgB2 superconductor, with a critical temperature Tc ~ 39 K, have evidenced its potential for applications although intense magnetic relaxation effects limit the critical current density, Jc, at high magnetic fields. This means that effective pinning centers must be added into the material microstructure, in order to halt dissipative flux movements. Concerning the basic microscopic mechanism to explain the superconductivity in MgB2, several experimental and theoretical works have pointed to the relevance of a phonon-mediated interaction, in the framework of the BCS theory. Questions have been raised about the relevant phonon modes, and the gap and Fermi surface anisotropies, in an effort to interpret spectroscopic and thermal data that give values between 2.4 and 4.5 for the gap energy ratio. Preliminary results on the anisotropy of Hc2 have shown a ratio, between the in-plane and perpendicular directions, around 1.7 for aligned MgB2 crystallites and 1.8 for epitaxial thin films. Here we show a study on the angular dependence of Hc2 pointing to a Fermi velocity anisotropy around 2.5. This anisotropy certainly implies the use of texturization techniques to optimize Jc in MgB2 wires and other polycrystalline components.Comment: 10 pages + 4 Figs.; Revised version accepted in Phys. Rev.

    Suppression of Superconducting Critical Current Density by Small Flux Jumps in MgB2MgB_2 Thin Films

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    By doing magnetization measurements during magnetic field sweeps on thin films of the new superconductor MgB2MgB_2, it is found that in a low temperature and low field region small flux jumps are taking place. This effect strongly suppresses the central magnetization peak leading to reduced nominal superconducting critical current density at low temperatures. A borderline for this effect to occur is determined on the field-temperature (H-T) phase diagram. It is suggested that the small size of the flux jumps in films is due to the higher density of small defects and the relatively easy thermal diffusion in thin films in comparison with bulk samples.Comment: 7 figures Phys. Rev. B accepted scheduled issue: 01 Feb 200

    Enhancement of the high-field critical current density of superconducting MgB2 by proton irradiation

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    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.Comment: 5 pages, 4 figures, to be published in Nature (in press

    Excess current in point contacts on two-band superconductor MgB2_2 in magnetic field

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    Series of I(V)I(V) characteristic and bias-dependent differential resistance dV/dI(V)dV/dI(V) for point-contacts on the single crystals of two-band superconductor MgB2_2 were measured in magnetic field up to 9 T. We have obtained magnetic field dependences of the excess current on the I(V)I(V) curves and analyzed them using Koshelev and Golubov (Phys. Rev. Lett. {\bf 90}, 177002 (2003)) theoretical results for the mixed state of a dirty two-band superconductor. Introducing a simple model for the excess current in the point contact in the mixed state our data can be qualitatively described utilizing theoretical magnetic filed behavior of the σ\sigma and π\pi-band superconducting order parameters and the corresponding averaged electronic density of states in MgB2_2.Comment: 4 pages, 3 figs, RevTex4; V2: plain text improved, BTK fit is added to insets of Fig.
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