Z. G. Ivanov

Improved SrTiO3 multilayers for microwave application: Growth and properties

Au/SrTiO3 (STO), Au/YBa2Cu3O7−x (YBCO)/STO, Au/STO/CeO2/STO, and Au/STO/MgO/STO multilayer structures on LaAlO3 substrates have been fabricated. Their properties are investigated at 1 MHz and at microwave frequencies (40 MHz–40 GHz) as a function of applied voltage, and in the temperature range between 20 and 300 K. The STO thin films showed comparatively high values of dielectric constant (up to 1800 at 50 K) with low measured film losses, tan δ<0.005, (limit of the measurement setup), at 20 GHz and 20 K, indicating their applicability for microwave devices. The dielectric constant is shown to be independent of frequency to at least 40 GHz, while the losses exhibit no or weak frequency dependence. Relaxation of the STO film in oxygen atmosphere before deposition of YBCO is shown to reduce interactions between the YBCO and STO films. After an “ex situ” high-temperature (900 °C) treatment in flowing oxygen a reduction of the STO dielectric constant as well as tan δ has been observed. Using cerium dioxide a...

Epitaxial YBa2Cu3O7−δ /BaxSr1−xTiO3 heterostructures on silicon‐on‐sapphire for tunable microwave components

Epitaxial trilayer heterostructures of YBa2Cu3O7−δ/BaxSr1−xTiO3/YBa2Cu3O7−δ were grown on silicon‐on‐sapphire buffered by a double layer of CeO2/Y–ZrO2. Such structures may be considered for tunable microwave filters. The top and bottom YBa2Cu3O7−δ films were well c‐axis oriented, free from microcracks and had superconducting transitions Tc’s in the range 86–90 K. A thin antidiffusion layer of SrTiO3 (d≊70 A) between YBa2Cu3O7−δ and BaxSr1−xTiO3 (x=0.25–0.9) promoted better crystallinity and higher Tc of the top superconducting film. An Ag/BaxSr1−xTiO3/YBa2Cu3O7−δ capacitor structure was used to determine the dielectric permittivity and the high frequency loss tan δ of the BaxSr1−xTiO3 layer. Maximum values of the permittivity of the BaxSr1−xTiO3 layers were observed around the Curie temperatures of corresponding bulk monocrystals. The dielectric permittivity of the BaxSr1−xTiO3 (x=0.25–0.75) layers depended strongly (≊20%) on an applied voltage (±2.5 V) at temperatures around 77 K. The tan δ was much hig...

Microstructure of an artificial grain boundary weak link in an YBa2Cu3O7−σ thin film grown on a (100)(110), [001]-tilt Y-ZrO2 bicrystal

Abstract The microstructure of an artificial grain boundary in an YBa 2 Cu 3 O 7-σ (YBCO) thin film grown on a (100)(110), [001]-tilt yttria-stabilized-zirconia (YSZ) bicrystal substrate has been studied using transmission electron microscopy (TEM). The orientation relationship between the YBCO film and the YSZ substrate was [001] YBCO ‖[001] YSZ and [110] YBCO ‖[100] YSZ for each half of the bicrystal film. However, the exact boundary geometry of the bicrystal substrate was not transferred to the film. The substrate boundary was straight while the film boundary was wavy. In several cases there was bending of the lattice confined within a distance of a few basal-plane lattice spacings from the boundary plane and microfaceting. No intergranular secondary phase was observed but about 25% of the boundary was covered by c -axis-tilted YBCO grains and a -axis-oriented grains, both of which were typically adjacent to CuO grains or surrounded by a thin Cu-rich amorphous layer.

Voltage divider based on submicron slits in a high Tc superconducting film and two bicrystal grain boundaries

Experiments on a model of rapid single flux quantum (RSFQ) flip‐flop cell, based on high‐Tc (HTS) Josephson junctions show that it can operate as a voltage divider at frequency up to 400 GHz. The junctions were formed in YBaCuO film, deposited on novel Y–ZrO2 bicrystals with two asymmetric 32° grain boundaries, about 10 μm apart, and allow a new design of RSFQ logic based on a single HTS layer. Small inductances (≂10 pH) were made as narrow, submicron size slits. The junction widths were between 4 and 10 μm and for ten junctions located close to the tested circuits, the linear critical current densities at T=4.4 K were 10.7 μA/μm±50% for one grain boundary and 8.3 μA/μm±50% for the other one. IcRn was about 1 mV±50%. A current density of half the expected value meant that the test circuit did not act as an ideal flip–flop down to the lowest frequency. As a voltage divider it gave a half value division up to 0.82 mV at T=4.4 K and to 0.4 mV at 30 K.

Giant lasing effect in magnetic nanoconductors

We propose a new principle for a compact solid-state laser in the 1–100 THz regime. This is a frequency range where attempts to fabricate small-size lasers up to now have met severe technical problems. The proposed laser is based on a new mechanism for creating spin-flip processes in ferromagnetic conductors. The mechanism is due to the interaction of light with conduction electrons; the interaction strength, being proportional to the large exchange energy, exceeds the Zeeman interaction by orders of magnitude. On the basis of this interaction, a giant lasing effect is predicted in a system where a population inversion has been created by tunneling injection of spin-polarized electrons from one ferromagnetic conductor to another—the magnetization of the two ferromagnets having different orientations. Using experimental data for ferromagnetic manganese perovskites with nearly 100% spin polarization, we show the laser frequency to be in the range 1–100 THz. The optical gain is estimated to be of order 107 cm−1, which exceeds the gain of conventional semiconductor lasers by 3 or 4 orders of magnitude. A relevant experimental study is proposed and discussed.

Epitaxial growth and properties of YBa2Cu3Ox‐Pb(Zr0.6Ti0.4)O3‐YBa2Cu3Ox trilayer structure by laser ablation

We have grown YBa2Cu3Ox‐Pb(Zr0.6Ti0.4)O3‐YBa2Cu3Ox multilayer structure on SrTiO3 and Al2O3 substrates using laser ablation. The deposition conditions for the growth of trilayers and their properties are studied in this investigation. Scanning electron microscope images and x‐ray diffraction analyses indicate that all the constituent films in the trilayer grow epitaxially on SrTiO3 and were highly oriented on Al2O3. Transport measurements on these multilayers show that top YBa2Cu3Ox films have good superconducting properties.

Effects of substrate temperature on the microstructure of YBa2Cu3O7−δ films grown on (001) Y‐ZrO2 substrates

High-quality YBa2Cu3O7-δ films grown on (001) single-crystal Y-ZrO2 substrates by pulsed laser deposition have been studied as a function of substrate temperature using transmission electron microscopy. A transition from epitaxial films to c-axis oriented polycrystalline films was observed at 740°C. An intermediate, polycrystalline, BaZrO3 layer was formed from a reaction between the film and the substrate. A dominant orientation relationship of [001] YBCO//[001]int. layer//[001]YSZ and [110] YBCO//[110]int. layer//[100]YSZ was observed. The formation of grain boundaries in the films resulted in an increased microwave surface resistance and a decreased critical-current density. The superconducting transition temperature remained fairly constant at about 90 K.

Tailoring the temperature coefficient of capacitance in ferroelectric varactors

Two BaxSr1-xO3, films with different contents of Ba (Ba0.25Sr0.75O3 and Ba0.75Sr0.5O3), separated by a MgO film, are used to fabricate planar varactors with low temperature coefficient of capacitance (TCC), high tunability and low losses. The TCC of the varactors are small in the temperature interval between the dielectric permittivity peaks of the Ba0.25Sr0.75O3 and Ba0.75Sr0.5O3 films

Superconducting (Bi,Pb)-Ca-Sr-Cu-O thin films prepared in situ by laser ablation

Superconducting Bi‐Ca‐Sr‐Cu‐O and Bi‐Pb‐Ca‐Sr‐Cu‐O films were grown in situ by excimer laser ablation from pressed powder and sintered superconducting targets, respectively. The substrate temperature during deposition was found to be important to obtain a maximum transition temperature. Both 110 and 80 K superconducting phases were obtained in films with a zero resistance at 74 K.

Fabrication and transport measurements of YBa2Cu3O7−x nanostructures

We have developed a process to fabricate nanostructures in oxide materials, especially YBa2Cu3O7−x (YBCO) thin films. The fabrication process is based on a multilayer mask with an amorphous carbon layer as a key component. Electron-beam lithography, reactive ion etching, and ion-beam etching have been the supporting techniques for the process. Nanobridges ranging in width from 50 to 170 nm have been fabricated and characterized. The initial YBCO film was 50 nm thick and had a Tc of 86 K. Tc of the fabricated bridges decreased about 3–5 K with respect to the initial film. The level of the critical current density was 4–9×105 A/cm2 at 77 K and 1.6–2×107 A/cm2 at 4.2 K. No increase of the critical current density was observed as the cross-section area became smaller. The process has also been applied to fabricate 30–50 nm wide trenches in a-axis-oriented YBCO on top of a similar-oriented PrBa2Cu3O7−x (PBCO) layer. No supercurrent was observed in such junctions. The current transport was governed by Mott variable range hopping.