Yuri Boikov

Fabrication and investigation of YBa2Cu3O7−δ/Ba0.05Sr0.95TiO3 thin film structures for voltage tunable devices

High-temperature superconducting/ferroelectric thin film structures were deposited by laser ablation. Three types of voltage tunable devices were fabricated and investigated-trilayer capacitor, planar interdigital capacitor and coplanar waveguide. As ferroelectric the solid solution BaxSr1-xTiO3 was chosen with barium content x = 0.05 because its Curie temperature is close to the liquid nitrogen boiling point (77 K). Temperature and voltage dependences of the Ba0.05Sr0.95TiO3 dielectric constant epsilon(r,BSTO) were studied. High epsilon(r,BSTO) values were determined-up to 3000 at 75 K, 20 GHz and zero de bias. Efficient voltage tunability was demonstrated (up to 40%) at loss level tan delta=0.01-0.1. Compact YBa2Cu3O7-delta/Ba0.05Sr0.95TiO3 coplanar waveguide with as narrow gap as 18 mu m was tested as electrically tunable phase-shifter and field-induced phase shifts of more than 180 degrees were obtained by 35 V de bias at 20 GHz. This improvement was attained by proper choice of the ferroelectric material, accomplished epitaxial growth of the films and decrease of the specific dimensions

Optimized transport properties of LaAlO3/SrTiO3 heterointerfaces by variation of pulsed laser fluence

We show the influence of pulsed laser deposition fluence on the transport properties of the LaAlO(3)/SrTiO(3) (LAO/STO) heterointerface. Structural characterization by x-ray diffraction and medium energy ion spectrometry enables us to deduce that the electronic behaviour is extremely sensitive to the stoichiometry of the LAO layer as well as the structural quality of the STO surface. An optimum balance of these two quantities is demonstrated for an intermediate laser fluence.

Response of the electrical resistivity and magnetoresistance of La0.67Ca0.33MnO3 films to biaxial tensile strains

The structure, electrical resistivity, and magnetoresistance of (50-nm)La0.67Ca0.33MnO3 epitaxial films grown on a [(80 nm)Ba0.25Sr0.75TiO3/La0.3Sr0.7Al0.65Ta0.35O3] substrate with a substantial positive lattice misfit have been studied. The tensile biaxial strains are shown to account for the increase in the cell volume and in the relative concentration of Mn+3 ions in the manganite films as compared to those for the original material (33%). The peak in the temperature dependence of the resistivity ρ of La0.67Ca0.33MnO3 films was shifted by 30–35 K toward lower temperatures relative to its position in the ρ(T) graph for a manganite film grown on (001)La0.3Sr0.7Al0.65Ta0.35O3. For T < 150 K, the temperature dependences of ρ of La0.67Ca0.33MnO3/Ba0.25Sr0.75TiO3/La0.3Sr0.7Al0.65Ta0.35O3 films could be well fitted by the relation ρ = ρ0 + ρ1T4.5, where ρ0 = 0.35 mΩ cm and the coefficient ρ1 decreases linearly with increasing magnetic field. In the temperature interval 4.2–300 K, the magnetoresistance of manganite films was within the interval 15–95% (μ0H = 5 T).

Magnetoresistance anisotropy in La0.67Ba0.33MnO3 films laterally compressed by a neodymium gallate substrate

The mismatch in the crystal lattice parameters induces biaxial lateral compression of 35-nm La0.67Ba0.33MnO3 films coherently grown on neodymium gallate substrates. Mechanical stresses emerging during the nucleation and growth of the manganite layer facilitate the depletion of this layer in the alkali-earth element. This results in an increase in the unit cell volume in the grown films and a decrease in temperature TM at which the resistivity attains the maximal value. The extremal values of the negative magnetoresistance (MR ≈ 17% for μ0H = 1 T) of the grown films are observed at temperatures close to room temperature. At T < TM, the response of the resistivity of the films to the magnetic field depends on the direction of this field relative to the normal to the substrate plane and to the direction of the measuring current. At T = 95 K, scattering of holes from 90°-domain walls leads to an increase in the resistivity of the manganite films by approximately 1.1%, while the negative anisotropic magnetoresistance reaches 1.5%.

Degradation of the SrRuO3/SrTiO3 Interface Capacitance Induced by Mechanical Stresses

Trilayer epitaxial heterostructures in which a 700-nm-thick SrTiO3 interlayer is integrated with two SrRuO3 electrodes have been grown by laser ablation. In the top electrode, twenty contact pads (S ≈ 0.1 mm2) have been formed using photolithography and ion etching. The bottom SrRuO3 electrode grown on a MgO(001) substrate is common for all film capacitors on the chip. As the temperature decreases in the range of 300-50 K, the capacitance C of the capacitors increases by a factor more than two due to an increase in the permittivity ɛ of the interlayer. At T = 4.2 K, as the bias voltage of ±2.5 V is applied to the oxide electrodes, the capacitance C decreases by ∼40%. In the temperature range of 100–300 K, the ratio ɛ0/ɛ increases almost linearly with increasing temperature (ɛ0 is the permittivity in vacuum). At T > 250 K, the dielectric loss tangent of the SrTiO3 interlayer increases exponentially with increasing temperature and substantially depends on the bias voltage applied to the oxide electrodes.

Nanometer Structured Epitaxial Films and Foliated Layers Based on Bismuth and Antimony Chalcogenides with Topological Surface States

The thermoelectric and galvanomagnetic properties of nanometer structured epitaxial films and foliated layers based on bismuth and antimony chalcogenides were investi‐ gated, and an increase in the figure of merit Z up to 3.85 × 10‐3 K‐1 was observed in the Bi0.5Sb1.5Te3 films over the temperature range of 180–200 K. It is shown that an increase in the Seebeck coefficient and the change in the slope on temperature, associated with changes in the effective scattering parameter of charge carriers and strong anisotropy of scattering in the films, lead to enhance power factor due to the growth of the effective mass of the density of states. These features are consistent with the results of research of oscillation effects in strong magnetic fields at low temperatures and research of Raman scattering at normal and high pressures in the foliated layers of solid solutions (Bi, Sb)2(Te, Se)3, in which the topological Dirac surface states were observed. The unique properties of topological surface states in the investigated films and layers make topological insulators promising material for innovation nanostructured thermoelec‐ trics.

Dielectric response of Ba0.05Sr0.95TiO3(110) films to variations in temperature and electric field

Three-layer epitaxial heterostructures, in which a 1000-nm-thick intermediate layer of Ba0.05Sr0.95TiO3 is integrated with strontium ruthenate conducting electrodes, have been grown by laser evaporation. Using photolithography and ion etching, film parallel-plate capacitors SrRuO3/Ba0.05Sr0.95TiO3/SrRuO3 are formed based on the grown heterostructures. A sharp maximum in the temperature dependence of the capacitor capacitance is observed at T ≈ 75 K. At T < 100 K, the capacitance decreases by 50–60% upon applying a bias voltage Vb = ±2.5 V to the oxide electrodes. The estimate of the specific capacitance (~2.1 μF/cm2) of the Ba0.05Sr0.95TiO3(110)/SrRuO3(110) interface is obtained. For T > 250 K and the measuring signal frequency of 1 kHz, the dielectric loss tangent of the film capacitors increases exponentially with increasing temperature.

Strain enhanced anisotropy of in-plane resistivity of YBa2Cu3O7-delta films

Highly bi-axially stressed, thin epitaxial films of YBa2Cu3O7-delta were deposited on mismatched, orthorhombic (001) YAlO3 substrates. Strain may be used to enhance anisotropy and affect spin and charge ordering in cuprate superconductors. The resistivity was anisotropic along the a and b axes and the ratio rho(a)/rho(b) approximate to 2 agreed well with that for untwinned single crystals. rho(a)(T), but not rho(b)(T), peaked sharply at a temperature closely above the superconducting transition temperature, while the latter estimated from rho(b)(T) exceeded the one determined from rho(a)(T). The height of the peak Delta rho(a) in rho(a)(T) decreased with increased bias current and applied magnetic field. The behavior may be explained by partly relaxed regions, preferably aligned along the b axis, but may also depend upon spin/charge ordering.

Inhomogeneous Microstructure and Electrical Transport Properties at the LaAlO3/SrTiO3 Interface

Medium-energy ion spectroscopy (MEIS), scanning transmission electron microscopy (STEM) and X-ray photoemission spectroscopy (XPS) were used to investigate the composition and microstructure of LaAlO3/SrTiO3 (LAO/STO) interfaces grown by pulsed laser deposition of LAO on TiO2-terminated STO substrates under different oxidizing conditions. MEIS and XPS indicated Sr/La and Al/Ti intermixing within several atomic layers at all studied interfaces. XPS and STEM revealed that La diffuses deeper than Al. Analysis of the MEIS data suggests inhomogeneous lateral distribution of the diffused elements. This is further supported by the observation of a large positive magneto-resistance at low temperatures. We discuss the role of lateral inhomogeneities on the formation of the electron gas at the LAO/STO interface.

Features of growth and galvanomagnetic properties of the Bi2Te3-based epitaxial films

Galvanomagnetic properties of the thermoelectric Bi2-xSbxTe3 (x=1.5) films grown by hat wall technique were studied in the wide (5-14 T) range of the magnetic field through the temperature interval 77-300 κ The results were analyzed in the terms of the many-valley model of energy spectrum with isotropic carrier scattering. An anisotropy of the constant-energy surface becomes weaker along the binary and bisector axes (m1/m2) in a view of turn on the tilt angle (Θ) between the principal axes of the constant-energy ellipsoids and the crystallographic axes for the film in comparison with bulk Bi2-xSbxTe3. Features of temperature dependence of the degeneracy parameter (βd) for the films can be explained by additional charge carrier scattering at block boundaries. In the model with anisotropic scattering mechanism, carrier scattering along bisector directions are primary in the films. Obtained data on anisotropy of the constant-energy surface and scattering mechanism looks important for formation of the thermoelectric Bi2-xSbxTe3 film heterostructures with enhanced figure of merit.Galvanomagnetic properties of the thermoelectric Bi2-xSbxTe3 (x=1.5) films grown by hat wall technique were studied in the wide (5-14 T) range of the magnetic field through the temperature interval 77-300 κ The results were analyzed in the terms of the many-valley model of energy spectrum with isotropic carrier scattering. An anisotropy of the constant-energy surface becomes weaker along the binary and bisector axes (m1/m2) in a view of turn on the tilt angle (Θ) between the principal axes of the constant-energy ellipsoids and the crystallographic axes for the film in comparison with bulk Bi2-xSbxTe3. Features of temperature dependence of the degeneracy parameter (βd) for the films can be explained by additional charge carrier scattering at block boundaries. In the model with anisotropic scattering mechanism, carrier scattering along bisector directions are primary in the films. Obtained data on anisotropy of the constant-energy surface and scattering mechanism looks important for formation of the thermoe...