Superconducting transition and phase diagram of single-crystal MgB_ {2}

Department of Physics, Western Michigan University, Kalamazoo, Michigan, United States
Physical Review B (Impact Factor: 3.66). 01/2003; 67(1). DOI: 10.1103/PhysRevB.67.012505
Source: arXiv

ABSTRACT The superconducting phase diagram of MgB2 was determined from magnetization, magnetotransport, and single-crystal specific-heat measurements. A zero-temperature in-plane coherence length of 9.4 nm is determined. The superconducting anisotropy γ increases from a value around 2 near Tc to above 4.5 at 22 K. For H∥c a pronounced peak effect in the critical current occurs at the upper critical field. Evidence for a surface superconducting state is presented for H∥c that might account for the widespread in reported values of γ.

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    ABSTRACT: The anisotropy of the upper critical field in MgB2 has been investigated on single crystalline, dense polycrystalline and powder samples by both transport and magnetic measurements. On the single-crystal sample, Hc2c and Hc2ab have been measured directly with applied field parallel to the c axis and ab plane. The angular dependence of Hc2 shows deviation from the anisotropic Ginzburg-Landau model at lower temperatures. On polycrystalline and powder samples, Hc2c and Hc2ab have been determined by the method proposed by Bud’ko et al. The value of anisotropy parameter γ=Hc2ab/Hc2c is temperature and sample dependent, and is about 3 and 4.5 for single and polycrystals, respectively, near Tc. However, Hc2c(T) are almost the same for all samples. These features could be an indication of anisotropic s-wave superconductivity with pancakelike energy gap or resulted from the different impurity levels in these samples. The anisotropy of Hc2 in the ab plane has also been measured on single crystal and we set an upper bound of 1% for the in-plane anisotropy.
    Physical Review B 09/2003; 68(10). DOI:10.1103/PhysRevB.68.104513 · 3.66 Impact Factor
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    ABSTRACT: Magnetic hysteresis loops (MHLs) have been measured on single-crystalline and dense polycrystalline MgB2 prepared under high pressure by micro-Hall probe and superconducting quantum interference device magnetometer. Heating effect due to the viscous force on vortices has been found by changing the field-sweep rate. Magnetic critical current densities obtained from MHLs by Bean model show different field dependence for single and polycrystalline samples. The scaling behaviors of flux-pinning force are also different. There is only one pinning force peak for polycrystalline sample, whereas there are three pinning force peaks for single-crystalline sample. The irreversibility field has been determined by different methods. The anisotropy of flux pinning and the pinning mechanism in MgB2 are discussed.
    Physical Review B 09/2003; 68(10). DOI:10.1103/PhysRevB.68.104514 · 3.66 Impact Factor
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    ABSTRACT: We report the influence of two-band superconductivity on the flux creep and the critical current densities of a MgB2 thin film. The small magnetic penetration depth of λ=50±10 nm at T=4 K is related to a clean π-band. We find a high self-field critical current density Jc, which is strongly reduced with applied magnetic field, and attribute this to suppression of the superconductivity in the π-band. The temperature dependence of the creep rate S (T) at low magnetic field can be explained by a simple Anderson–Kim mechanism. The system shows high pinning energies at low field that are strongly suppressed by high field.
    Solid State Communications 02/2015; 204. DOI:10.1016/j.ssc.2014.11.015 · 1.70 Impact Factor

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