S.I. Krasnosvobodtsev

Critical magnetic field of the vortex-free state in NbC thin films and prospects for its observation in MgB2

The critical magnetic fields Hc‖ and Hc2 are measured for thin films of the isotropic superconductor NbC. It is revealed that the critical fields exhibit strong anisotropy due to the vortex-free state of the film in a magnetic field aligned parallel to its surface. The Hc‖/Hc2 ratio at 2 K exceeds 6 and increases with increasing temperature. The dependence Hc‖(T) agrees quantitatively with the concepts of microscopic theory on the vortex-free state of a thin film of a clean superconductor in the temperature range below Tc. As the electron mean free path decreases under irradiation of the film with a low dose of He+ ions, the critical field Hc‖ remains unchanged near Tc but increases significantly at lower temperatures. The well-known theoretical models are used to estimate the electronic parameters and thicknesses of MgB2 films for which the specific features associated with the vortex-free state of the two-gap superconductor can manifest themselves in the temperature dependence of the critical magnetic field Hc‖(T).

Superconducting films with Tc=39 K prepared from stoichiometric MgB2 targets

Oriented films of MgB2 high-Tc superconductor are synthesized by pulsed laser sputtering of stoichiometric MgB2 targets with subsequent annealing of the amorphous Mg-B material deposited onto MgO(111) substrates. The critical temperature of the films depends on the purity of the targets sputtered. The purification of boron powder in a vacuum makes it possible to minimize the content of impurities in the targets and to prepare MgB2 films exhibiting a critical temperature above 39 K and a sharp inductive transition. A high room-temperature to residual resistivity ratio (more than 3) indicates the good quality of the films.

Effect of ion irradiation on the upper critical magnetic field in electronic and hole d-wave superconductors

The effect of ion irradiation on the upper critical magnetic field Hc2 in electronic and hole high-temperature superconductors is studied. It is shown that the variation of Hc2 may be connected with the d-wave symmetry of the order parameter.

A direct readout high-T/sub c/ dc SQUID electronics with ac bias and a liquid-nitrogen-cooled preamplifier

Direct readout dc SQUID electronics based on a liquid-nitrogen-cooled preamplifier and alternating current bias was developed. The preamplifier with an intrinsic voltage noise of 0.18 nV/Hz/sup 1/2/ was used as the first stage of this electronics. A mathematical description of the noise suppression by this electronics was done. Low-frequency noise was reduced by a factor of 6 at 1 Hz by this technique.

Deposition of smooth high-T c superconducting films with a solid-state YAG: Nd3+ laser

A method is presented for the deposition of smooth epitaxial high-Tc superconducting films using a pulsed infrared YAG: Nd3+ laser and velocity filtration. This method is based on the removal of drops and solid particles from the flow of deposited substance with a fast shutter made of a rotating disk. The deposited smooth homogeneous YBa2Cu3O7−δ films have a critical temperature Tc > 90 K. The surface concentration of drops is less than 3 × 102 cm−2, and their amount is decreased by more than six orders of magnitude.

Critical magnetic field Hc2 and electron scattering in MgB2

The correlation between the upper critical field Hc2 and the residual resistivity ρ of the MgB2 compound is studied in a wide range of ρ values. The slope −dHc2/dT of the temperature dependence of Hc2 near Tc is found to increase steadily with increasing residual resistivity to 100 μΩ cm. Over the range from 0 to 50 μΩ cm, the dependence of −dHc2/dT on ρ is fitted well by a linear function, which is typical of a single-band superconductor. The fit is performed using the electronic parameters of the two bands (π and σ) that form the Fermi surface. The field Hc2 is assumed to depend on the electronic parameters of the σ band only. Using this approximation, the following quantities are estimated: the relative contribution of σ electrons to the total conductivity along the boron planes (which turns out to be about 1/2), the ratio of the mean free paths of electrons in the σ and π bands lσ/lπ ≈ 1.5, and the ratios between some other parameters describing electron scattering.

Dependence of the upper critical field on the defect concentration in MgB2 and the electronic structure parameters

The upper critical field Hc2 (H ‖ c) of the two-band superconductor MgB2 is studied as a function of the residual resistivity ρn. It is found that the superconductor follows the standard trend: the slope-dHc2/dT of the temperature dependence of Hc2(T) increases with the number of defects. The upper critical field in the clean limit is found, and direct estimations of the parameters of carriers in the 2D σ band (including the Fermi velocity and the coherence length) are made. The contribution of the electron scattering to the magnitude of Hc2 is determined, and the mean free path of electrons in samples with various defect concentrations is estimated. The density of states of σ electrons at the Fermi level is calculated using the dependence of the slope-dHc2/dT on ρn and a band structure model. It is impossible to estimate this density of states directly, because the upper critical field is determined by the carriers of one band, whereas the resistivity depends on the carriers in both bands.

Optical diagnostics for fabrication of YBCO thin film DC SQUIDs

Optical reflectance spectra of thin YBCO films on SrTiO/sub 3/ bicrystals were measured in a photon energy range of 1-3 eV at room temperature. Raman spectra measurements in the range 200-700 cm/sup -1/ were performed. The best samples were shown to have high reflectance anisotropy and a well-defined Raman peak /spl nu//sub 1/=500+1 cm/sup -1/. YBCO thin films on a bicrystal substrate that successfully passed optical control were used for Josephson junctions and DC SQUID fabrication. The fabricated SQUIDs operated at 77.3 K in the flux locked loop mode and had a typical white noise (1-2)/spl times/10/sup -5/ /spl Phi//sub 0//Hz/sup 1/2/ at frequencies higher than 200-500 Hz. The best magnetic field resolution was 650 fT/Hz/sup 1/2/ at frequencies f>200 Hz.<<ETX>>