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تسجيل مستخدم جديدEffect of damage by 2-MeV He ions and annealing on Hc2 in MgB2 thin films
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The effect of damage induced by 2-MeV alpha particles, followed by annealing, on the critical temperature (Tc), resistivity, and upper critical field (Hc2), of three MgB2 films made by different deposition processes has been studied. Damage creates a linear decrease in Tc with residual resistivity, and produces maxima in both Hc2(0)-perpendicular and Hc2(0)-parallel. Below Tcs of about 25 K, Hc2(0) depends roughly linearly on Tc, while the anisotropy of Hc2(0) decreases as Tc decreases. Annealing the films reproduces the Tc vs. residual resistivity dependence but not the Hc2(0) values induced by damage.
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We have studied the effect of damage induced by 2-MeV alpha particles on the critical temperature, Tc, and resistivity of MgB2 thin films. This technique allows defects to be controllably introduced into MgB2 in small successive steps. Tc decreases l
inearly as the intragrain resistivity at 40 K increases, while the intergrain connectivity is not changed. Tc is ultimately reduced to less than 7 K and we see no evidence for a saturation of Tc at about 20 K, contrary to the predictions of the Tc of MgB2 in the dirty limit of interband scattering.
The amount of oxygen incorporated into MgB2 thin films upon exposure to atmospheric gasses is found to depend strongly on the materials stoichiometry. Rutherford backscattering spectroscopy was used to monitor changes in oxygen incorporation resultin
g from exposure to: (a) ambient atmosphere, (b) humid atmospheres, (c) anneals in air and (d) anneals in oxygen. The study investigated thin-film samples with compositions that were systematically varied from Mg0.9B2 to Mg1.1B2. A significant surface oxygen contamination was observed in all of these films. The oxygen content in the bulk of the film, on the other hand, increased significantly only in Mg rich films and in films exposed to humid atmospheres.
MgB2 thin films were cold-grown on sapphire substrates by pulsed laser deposition (PLD), followed by post-annealing in mixed, reducing gas, Mg-rich, Zr gettered, environments. The films had Tcs in the range 29 K to 34 K, Jcs (20K, H=0) in the range 3
0 kA/cm2 to 300 kA/cm2, and irreversibility fields at 20 K of 4 T to 6.2 T. An inverse correlation was found between Tc and irreversibility field. The films had grain sizes of 0.1-1 micron and a strong biaxial alignment was observed in the 950C annealed film.
Our Rutherford backscattering spectrometry (RBS) study has found that concentrations up to 7 atomic percent of Rb and Cs can be introduced to a depth of ~700 A in MgB2 thin films by annealing in quartz ampoules containing elemental alkali metals at <
350 degree centigrade. No significant change in transition temperature (Tc) was observed, in contrast to an earlier report of very high Tc (>50 K) for similar experiments on MgB2 powders. The lack of a significant change in Tc and intra-granular carrier scattering suggests that Rb and Cs diffuse into the film, but do not enter the grains. Instead, the observed changes in the electrical properties, including a significant drop in Jc and an increase in delta rho (rho300-rho40), arise from a decrease in inter-granular connectivity due to segregation of the heavy alkaline metals and other impurities (i.e. C and O) introduced into the grain boundary regions during the anneals.
A series of MgB2 thin films were fabricated by pulsed laser deposition (PLD), doped with various amounts of Si up to a level of 18wt%. Si was introduced into the PLD MgB2 films by sequential ablation of a stoichiometric MgB2 target and a Si target. T
he doped films were deposited at 250 C and annealed in situ at 685 C for 1min. Up to a Si doping level of ~11wt%, the superconducting transition temperature (Tc) of the film does not change significantly, as compared to the control, undoped film. The magnetic critical current density (Jc) of the film at 5K was increased by 50% for a Si doping level of ~3.5wt%, as compared to the control film. Also, the irreversibility field of Si-doped MgB2 films (Hirr) at low temperature is higher than for the undoped film.
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