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Register a new userIntrinsic nonlinearity probed by intermodulation distortion microwave measurements on high quality MgB2 thin films
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The two tone intermodulation distortion arising in MgB2 thin films synthesized by hybrid physical-chemical vapour deposition is studied in order to probe the influence of the two bands on the nonlinear response of this superconductor. The measurements are carried out by using a dielectrically loaded copper cavity operating at 7 GHz. Microwave data on samples having critical temperatures above 41 K, very low resistivity values, and residual resistivity ratio larger than 10, are shown. The dependence of the nonlinear surface losses and of the third order intermodulation products on the power feeding the cavity and on the temperature is analyzed. At low power, the signal arising from distortion versus temperature shows the intrinsic s-wave behavior expected for this compound. Data are compared with measurements performed on Nb and YBCO thin films using the same technique.
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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. The 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.
The carrier concentration of Tl2Ba2CaCu2O8 films was modified by annealing in N2 gas. X-ray analysis of the structure and the oxygen content revealed a correspondence between carrier concentration and oxygen depletion. The TC and nonlinear surface impedance was measured using a dielectric resonator and the nonlinearity slope parameter r=dXS/dRS was found to converge to unity at the critical temperature, indicating a dominance of Josephson fluxon hysteresis on the nonlinearity. Highly inductive nonlinearity was observed in a small range of doping levels between 0.180<p<0.195 holes/Cu, which does not include the optimal doping level of 0.16 holes/Cu.
We investigated the effect of alloying on the upper critical field $H_{c2}$ in 12 $MgB_2$ films, in which disorder was introduced by growth, carbon doping or He-ion irradiation, finding a significant $H_{c2}$ enhancement in C-alloyed films, and an anomalous upward curvature of $H_{c2}(T)$. Record high values of $H_{c2}^{perp}(4.2) simeq 35T$ and $H_{c2}|(4.2) simeq 51T$ were observed perpendicular and parallel to the ab plane, respectively. The temperature dependence of $H_{c2}(T)$ is described well by a theory of dirty two-gap superconductivity. Extrapolation of the experimental data to T=0 suggests that $H_{c2}|(0)$ approaches the paramagnetic limit of $sim 70T$.
Investigations of MgB2 and Fe-based superconductors in recent years have revealed many unusual effects of multiband superconductivity but manifestations of anisotropic multiband effects in the critical current density Jc have not been addressed experimentally, mostly because of the difficulties to measure Jc along the c-axis. To investigate the effect of very different intrinsic anisotropies of sigma and pi electron bands in MgB2 on current transport, we grew epitaxial films with tilted c-axis (THETA ~ 19.5{deg}), which enabled us to measure the components of Jc both along the ab-plane and the c-axis using magneto-optical and transport techniques. These measurements were combined with scanning and transmission electron microscopy, which revealed terraced steps on the surface of the c-axis tilted films. The measured field and temperature dependencies of the anisotropic Jc(H) show that Jc,L parallel to the terraced steps is higher than Jc,T perpendicular to the terraced steps, and Jc of thinner films (50 nm) obtained from transport experiments at 0.1 T reaches ~10% of the depairing current density Jd in the ab plane, while magneto-optical imaging revealed much higher Jc at lower fields. To analyze the experimental data we developed a model of anisotropic vortex pinning which accounts for the observed behavior of Jc in the c-axis tilted films and suggests that the apparent anisotropy of Jc is affected by current pairbreaking effects in the weaker {pi} band. Our results indicate that the out-of-plane current transport mediated by the {pi} band could set the ultimate limit of Jc in MgB2 polycrystals.
Superconducting epitaxial FeSe0.5Te0.5 thin films were prepared on SrTiO3 (001) substrates by pulsed laser deposition. The high purity of the phase, the quality of the growth and the epitaxy were studied with different experimental techniques: X-rays diffraction, reflection high energy electron diffraction, scanning tunnelling microscopy and atomic force microscopy. The substrate temperature during the deposition was found to be the main parameter governing sample morphology and superconducting critical temperature. Films obtained in the optimal conditions show an epitaxial growth with c axis perpendicular to the film surface and the a and b axis parallel to the substrates one, without the evidence of any other orientation. Moreover, such films show a metallic behavior over the whole measured temperature range and critical temperature above 17K, which is higher than the target one.
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