S. N. Song

Pulsed organometallic beam epitaxy of complex oxide films

We report the results of a pulsed organometallic beam epitaxy (POMBE) process for growing complex oxide films at low background gas pressure (10−4–10−2 Torr) and low substrate temperature (600–680 °C) using organometallic precursors in an oxygen plasma environment. Our results show that POMBE can extend the capability of organometallic chemical vapor deposition to growing complex oxide films with high precision both in composition and structure without the need for post‐deposition oxidation and heat treatments. The growth of phase‐pure, highly oriented Y‐Ba‐Cu‐O superconducting oxide films {[Tc (R=0)=90.5 K] and Jc (77 K, 50 K gauss)=1.1×105 A/cm2} is given as an example. Similar to the pulsed laser deposition process, the POMBE method has the potential for in situ processing of multilayer structures (e.g., junctions).

Anomalous Hall effect in (110)Fe/(110)Cr multilayers

We have studied the Hall effect and the magnetoresistance in [110] textured Fe/Cr multilayers grown by electron beam evaporation. We have observed a strong magnetic field dependence of the Hall coefficient as well as a large magnetoresistance. In all cases the Hall voltage is positive. The ordinary Hall coefficient is positive at room temperature and changes sign at low temperatures; this is similar to the behavior of an antiferromagnet but differs from that observed in Fe‐Cr alloys. The extraordinary Hall coefficient Rs is positive and varies with the resistivity ρ as Rs∝ρ2.6, suggesting the importance of interface scattering.

Oriented thin films of YBaCu(F)O with high Tc and Jc prepared by electron beam multilayer evaporation

Thin films of YBaCu(F)O were deposited on SrTiO3(100) substrates by multilayer deposition from three electron guns containing Y, BaF2, and Cu under a pressure of 5×10−5 Torr of O2. The films were later annealed in a separate chamber under a flowing O2‐H2O atmosphere. X‐ray diffraction studies reveal that the resulting structure is highly oriented with the a axis perpendicular to the substrate. Scanning electron micrographs show a morphology consisting of an array of orthogonal, interconnecting bars with well‐developed junctions. High‐resolution electron microscopy and electron diffraction patterns show that these junctions are atomically abrupt and that the associated c axes are mutually perpendicular. These epitaxial films show a sharp resistive transition with Tc(R=0) as high as 90 K. The zero field critical current density, determined from magnetization measurements, is 2.9×106 A/cm2 at 4.2 K and 5.0×104 A/cm2 at 77 K.

Preparation of Y‐Ba‐Cu‐O thin films on MgO by dc magnetron sputtering from a stoichiometric Y1Ba2Cu3O7−δ target

Y‐Ba‐Cu‐O thin films have been deposited on MgO substrates by dc magnetron sputtering from a stoichiometric YBa2Cu3O7−δ target, after which they are subjected to a short heat treatment. Zero resistance is routinely achieved near 60 K, and one film shows zero resistance at 68.0 K.

Transport properties in n-type InSb films grown by metalorganic chemical vapor deposition

We have measured the temperature and magnetic field dependence of the Hall mobility and transverse magnetoresistance in n‐type InSb films epitaxially grown on GaAs substrates by metalorganic chemical vapor deposition. The films show a giant magnetoresistance: e.g., at 240 K the resistivity increases over 20 times at a magnetic field of 5 T; the low field coefficient of resistivity at 77 K is as high as 47.5 μΩ cm/G. The Hall mobility decreases with magnetic field and saturates at higher fields. By taking the interface carrier transport into account, the observed field dependence of the Hall mobility and magnetoresistance may be understood based on a two‐layer model.

Magnetization of sinter‐forged YBa2Cu3O7−δ

The magnetization and critical magnetization current densities of sinter‐forged YBa2Cu3O7−δ samples of nearly theoretical density have been studied. The magnetization curves can be well fitted by Kim’s critical state equation Jc(B)=α/(B0+B) with a flat plate geometry. The results revealed an anisotropy in the lower critical field and the critical current density with respect to the forging axis. The deduced critical current density in the favorable direction is larger than the value of isotropic polycrystalline samples.

Generalized de Gennes-Takahashi-Tachiki proximity effect theory

We generalize the de Gennes-Takahashi-Tachiki theory of coupled dirty superconductors to include the effects of orbital diamagnetism, spin (Pauli) paramagnetism, spin-orbit scattering, and magnetic impurity scattering. Some new expressions for thin film sandwiches and superlattices are obtained.

Superlattices of YBa2Cu3O7−δ/PrBa2Cu3O7−δ grown by the pulsed organometallic beam epitaxy method

We report on the growth of YBa2Cu3O7−δ/PrBa2Cu3O7−δ superlattices by pulsed organometallic beam epitaxy technique. X‐ray diffraction and analytical high resolution transmission electron microscopy measurements show high‐quality superlattice crystalline structures are being grown. Electrical measurements of these layered oxides have also shown very promising results. This work demonstrates the capabilities of pulsed organometallic chemical vapor deposition as an alternative method for growing high temperature superconducting oxides in a controlled manner.

Dimensional crossover and its effects on localization/interaction and superconductivity in Si/Nb multilayers

Abstract The effect of the dimensional crossover on localization/interaction and superconductivity in ultrathin Si/Nb multilayers has been studied by systematically varying the Nb and Si layer thicknesses to separately control the degree of disorder and the anisotropy. A small coupling is sufficient to drive the strongly localized states into the weak localization regime. As a result, superconductivity may persist up to an R □ much larger than h (2e) 2 . The results are in qualitative agreement with a recently proposed anisotropic tight-binding model for localization.

Superconducting YBa2Cu3O7−δ thin films with three different orientations

Epitaxial thin films of YBaCuO have been prepared with (1) the a axis perpendicular to (100) SrTiO3 ; (2) the c axis perpendicular to (100) SrTiO3 ; and (3) the [110] axis perpendicular to (110) SrTiO3. Films were fabricated using a multilayer deposition technique involving three electron guns containing Y, BaF2, and Cu under a pressure of 5×10−5 Torr of O2. As deposited films, which contained polycrystalline and amorphous regions, were later annealed in a furnace under a flowing O2‐H2 O atmosphere. X‐ray diffraction patterns as well as scanning electron microscopy and high‐resolution electron microscopy images confirm that the films are highly oriented, essentially epitaxial. The a‐axis oriented film exhibits zero resistance at 90 K and a critical current density of 2.9×106 A/cm2 at 4.2 K while the c‐axis oriented film exhibits a Tc of 88 K and a Jc of 0.9×107 A/cm2 at 4.2K; the Jc values were determined magnetically. The [110]‐orientation film shows the sharpest transition with a transition width of 1 K...