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Through the integration of semiconductors or complex oxides into metal resonators, tunable metamaterials have been achieved by a change of environment using an external stimulus. Metals provide high conductivity to realize a strong resonant response in metamaterials; however, they contribute very little to the tunability. The complex conductivity in high-temperature superconducting films is highly sensitive to external perturbations, which provides new opportunities in achieving tunable metamaterials resulting directly from the resonant elements. Additionally, superconducting metamaterials are expected to enable strong nonlinear response and quantum effects, particularly when Josephson junctions are integrated into the metamaterial resonant elements. Here we demonstrate ultrafast dynamical tuning of resonance in the terahertz (THz) frequency range in YBa2Cu3O7-δ(YBCO) split-ring resonator (SRR) arrays excited by near infrared femtosecond laser pulses. The photoexcitation breaks the superconducting Cooper pairs to create quasiparticles. This dramatically modifies the imaginary part of the complex conductivity and consequently the metamaterial resonance on an ultrafast timescale, although the real conductivity does not change significantly. We observed resonance switching accompanied by substantial frequency tuning as a function of photoexcitation fluence, which also strongly depends on the nanoscale thickness of the superconducting films. All of our experimental results agree with calculations using an analytical model, which takes into account the contributions of the complex conductivity of the YBCO films to SRR resistance and kinetic inductance. The theoretical calculations reveal that the increasing SRR resistance upon increasing photoexcitation fluence is responsible for the reduction of resonance strength, and changes in both the resistance and kinetic inductance cause the resonance frequency shifts.

We demonstrate thermal and ultrafast optical tuning in planar terahertz (THz) superconducting metamaterials. The fundamental resonance of an array of split-ring resonators (SRRs) fabricated from a 50-nm-thick high-temperature superconducting (HTS) YBa2Cu3O7-(YBCO) film is characterized as a function of temperature and near-infrared photoexcitation fluence. The HTS metamaterial exhibits a very strong resonant response at temperatures much lower than the transition temperature Tc. Increasing the temperature reduces the density of Cooper pairs, which results in a dramatically decreasing imaginary part of the complex conductivity, and thereby tunes the metamaterial resonance. We observe switched resonance strength and large red shift of resonance frequency when the temperature increases from 20 K to Tc. Similar resonance switching and frequency tuning is also demonstrated in an ultrafast time scale through near-infrared femtosecond laser excitation. We further compare the thermal tuning behaviour of the 50-nm-thick HTS metamaterial with a metamaterial sample comprised of gold SRRs with identical geometry and dimensions, which has negligible tunability.

In Bewegung gesetzt: Ein einfaches Verfahren zur chemischen Abscheidung aus Lösung wurde erstmals für das epitaktische Aufwachsen von Ge auf ein Si‐Substrat genutzt. Die Hall‐Beweglichkeit in den Ge‐Filmen bei Raumtemperatur und einer Ladungsträgerkonzentration von 3.45×1019cm−3ist sehr hoch (1700 cm2V−1s−1).

Epitaxial GaN thin films have been deposited on (0001) sapphire substrates by a chemical solution approach of polymer-assisted deposition. The films are smooth with no detectable cracks or pores, as observed by scanning electron microscopy and atomic force microscopy. Microstructural studies by X-ray diffraction and transmission electron microscopy show that the GaN films have a hexagonal structure with an epitaxial relationship between the film and the substrate of (0001)GaN||(0001)Al2O3and [112̅0]GaN||[101̅0]Al2O3. The films with a room temperature resistivity of around 0.13 Ω·cm exhibit photoluminescence characteristic of wurtzite hexagonal GaN.

Pinning in YBa2Cu3O7?x(YBCO) films is compared with mixed rare-earth (RE) films. The field dependence of critical current density and pinning force plots at 75.5 K are analysed. The work provides a guideline as to how to optimize pinning by inclusion of bothrandom and correlated defects. In summary, Y mixed with larger RE ions appears to be optimum for effective pinning across a wide field regime as well as permitting growth at temperatures low enough for technological use.

Metal oxides are emerging as important materials for their versatile properties such as high-temperature superconductivity, ferroelectricity, ferromagnetism, piezoelectricity and semiconductivity. Metal-oxide films are conventionally grown by physical and chemical vapour deposition. However, the high cost of necessary equipment and restriction of coatings on a relatively small area have limited their potential applications. Chemical-solution depositions such as sol–gel are more cost-effective, but many metal oxides cannot be deposited and the control of stoichiometry is not always possible owing to differences in chemical reactivity among the metals. Here we report a novel process to grow metal-oxide films in large areas at low cost using polymer-assisted deposition (PAD), where the polymer controls the viscosity and binds metal ions, resulting in a homogeneous distribution of metal precursors in the solution and the formation of uniform metal–organic films. The latter feature makes it possible to grow simple and complex crack-free epitaxial metal-oxides.

There are numerous potential applications for superconducting tapes based on YBa2Cu3O7–x(YBCO) films coated onto metallic substrates. A long-established goal of more than 15 years has been to understand the magnetic-flux pinning mechanisms that allow films to maintain high current densities out to high magnetic fields. In fact, films carry one to two orders of magnitude higher current densities than any other form of the material. For this reason, the idea of further improving pinning has received little attention. Now that commercialization of YBCO-tape conductors is much closer, an important goal for both better performance and lower fabrication costs is to achieve enhanced pinning in a practical way. In this work, we demonstrate a simple and industrially scaleable route that yields a 1.5–5-fold improvement in the in-magnetic-field current densities of conductors that are already of high quality.

Using a layer-by-layer self-assembly technique, we have deposited multilayers of nickel phthalocyanine tetrasulfonate (NiPc) and poly(diallyldimethylammonium chloride) (PDDA) onto indium tin oxide (ITO) and ITO/TiO2 substrates, respectively. To probe charge-transfer properties of these self-assembled multilayers on conductive and semiconductive surfaces, Kelvin probe and surface photovoltage measurements have been carried out. The results showed an oscillating behavior of the surface potential or work function (ΔΦ) as the surface layer was alternated between PDDA and NiPc on both substrates with or without light. The average width of this surface potential oscillation was around 350 mV. These changes in surface potential, induced by self-assembled PDDA or NiPc, were due to the modulation of electron affinity rather than band bending at ITO surfaces. The photoinduced change in band bending (δΔV), however, has little correlation to the number of layers (PDDA or NiPc).

Monolithic integration of high-temperature superconducting YBCO and nonlinear dielectric SrTiO 3 films on polycrystalline yttrium-iron-garnet (YIG) substrates represents an advance of great technical significance for dual-tuning (both electrically and magnetically) microwave components. We have solved the problems of crystallographic incompatibility among these materials and the lack of a template for epitaxial growth of SrTiO 3 and YBCO films on polycrystalline substrate. Using a template of biaxially oriented MgO, deposited by an ion-beam-assisted-deposition technique, we have produced device quality SrTiO 3 and YBCO films on polycrystalline YIG substrates. Both SrTiO 3 and YBCO films were c-axis oriented with an in-plane mosaic spread of 7° full width at half-maximum (FWHM) for SrTiO 3 and less than 7° FWHM for YBCO. The YBCO films had transition temperatures above 87 K.

We deposited epitaxial Ba0.6Sr0.4TiO3(BST) films having thickness of 400 nm on MgO(001) substrates, where a 10 nm thick Ba1-xSrxTiO3 (x = 0.1 - 0.7) interlayer was inserted between BST and MgO to manipulate the stress of the BST films. Since the main difference of those epitaxial BST films was the lattice constant of the interlayers, we were very successful in controlling the stress of the BST films. BST films under small tensile stress showed larger dielectric constant than that without stress as well as those under compressive stress. Stress relaxation was investigated using epitaxial BST films with various thicknesses grown on different interlayers. For BST films grown on Ba0.7Sr0.3TiO3 interlayers, the critical thickness was about 600 nm. On the other hand, the critical thickness of single-layer BST film was less than 100 nm.

Metal-organic chemical vapor deposition (MOCVD) has been used to deposit conductive oxide RuO2 thin films at different temperatures on (100) MgO and (100) LaAlO3 substrates. The microstructural properties of the RuO2 films have been studied using X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and high-resolution electron microscopy (HREM). Different growth and microstructural properties were observed for the films deposited on the two substrates. For example, the films on MgO are epitaxial at deposition temperatures as low as 350°C, and the films on LaAlO3 are epitaxial only at deposition temperatures of 600°C and above. The epitaxial films on MgO consist of two variants with an orientation relationship given by (110) RuO2//(100) MgO and [001] RuO2//[011]MgO. The epitaxial films on LaAlO3, on the other hand, contain four variants with an orientation relationship given by (200)RuO2//(100)LaAlO3 and [011] RuO2//[011] LaAlO3. The difference in growth and microstructural properties were explained based on geometrical considerations for the film and substrates. The RuO2/substrate interfaces and the RuO2 domain boundaries were also investigated by TEM and HREM.

For the first time, we have demonstrated that epitaxial Ba0.6Sr0.4TiO3 thin films can be grown on the epitaxial RuO2 electrodes by pulsed laser ablation. The Ba0.6Sr0.4TiO3 films, deposited on epitaxial (200) oriented RuO2 grown on (100) yttria-stabilized zirconia, exhibit single out-of-plane orientation as well as in-plane alignments. The detailed epitaxial relationships between the Ba0.6Sr0.4TiO3 films and the RuO2 electrodes have been analyzed by X-ray diffraction and transmission electron microscopy.

We present real-time studies of nucleation and growth of 180° ferroelectric domains in congruent LiTaO3 under an external electric field using optical microscopy. The measured wall velocities are an order of magnitude higher than those reported before using ex-situ techniques. Merger of growing domains form serrated fronts which show a futher order of magnitude increase in wall velocity. A time dependent velocity is required in the Avrami theory to explain the change in domain area versus time at a constant electric field. We believe that these features of domain switching must be considered when studying ferroelectrics in general.

Capacitors of Au/ferroelectric BaTiO3/p-Si with different constructions of ferroelectric BaTiO3, such as single layer amorphous, bilayer of amorphous on polycrystalline, and nanolayer amorphous on a number of stacked layers of polycrystalline on microcrystalline, were fabricated using RF magnetron sputtering. The flat-band voltage, which is a direct indication of the charges in the BaTiO3 and at the interface between BaTiO3 and Si, was a function of the construction of the BaTiO3 layers. Positive charges in the range of 1012/cm2 were found for single layer amorphous or bilayer amorphous on polycrystalline BaTiO3. However, negative charges in the range of 1010/cm2 were detected for the BaTiO3 films with nanolayer structures. The quasistatic capacitance-voltage measurements on different capacitors showed lower interface state density between BaTiO3 and Si by using nanolayer structures.

Detailed studies were conducted on YBa2Cu3O7-xthin films with multilayer Al/Cr/Yb metals as ohmic contact electrodes. Ytterbium not only provided an excellent ohmic contact to YBa2Cu3O7-xthin films, but also improved the zero resistance temperature after thermal treatment of the contacts. Superconducting thin films with zero resistance temperature of around 90 K with extremely sharp resistivity transitions, contact current density greater than 1.3 x 103A/cm2, and specific contact resistivity as low as 10-9ohm · cm2at 77 K were achieved on yttrium stabilized ZrO2substrates. This occurred after contact deposition and thermal treatment at 450° C in pure oxygen for 20 min.

Low leakage and high dielectric constant ferroelectric BaTiO3 thin film capacitors, taking advantage of the best properties of both amorphous and polycrystalline materials, were fabricated using radio frequency magnetron sputtering. The leakage current density was reduced to 10-10 A/cm2 at a field intensity of (2 ± 0.5) × 105 V/cm by using a nanolayer structure with an amorphous layer, on a number of alternating stacked layers of polycrystalline on microcrystalline material. This leakage current is believed to be the lowest reported so far for BaTiO3 with a dielectric thickness in the range of 250 nm. The dielectric constant and other electrical properties can be purposely controlled by choosing the number of layers and deposition conditions.

An improved analytical approach is introduced to calculate the solar cell parameters based on the lumped single exponential model. The main feature of this approach is that the diode junction ideality factor has been considered as an output characteristic dependent parameter under illuminated conditions. By using the experimental data of Charles, Abdelkrim, Muoy and Mialhe and Chan and Phang for five solar cells, the parameters are calculated and compared with the results of the analytical methods suggested either by Phang, Chan and Phillips or Kishore. The meaningless negative series resistance, caused by the assumption of a constant diode ideality factor along the entire current-voltage curve, can be avoided by the present analytical method.

Pulsed laser deposition was used to deposit high-quality YBa2Cu3O7-δ (YBCO) thin films directly on y-cut LiNbO3 substrates. The as-deposited YBCO films had a high degree of in-plane orientation and showed superconducting transition temperature (Tco) at 91 K with a transition width of less than 1 K. Transport critical current densities were found to be ∼ 106 A cm-2 at 77 K and zero field. An ion beam minimum channelling yield of 16% was obtained for YBCO films, indicating high crystallinity. High-resolution transmission electron microscopy studies showed that interface between the film and the substrate was quite smooth and free from interfacial interdiffusion. The defects in thin films are also identified. The work showed that high-quality high Tc thin films can be deposited directly on LiNbO3. Novel devices based on the properties of both YBCO and LiNbO3 could be realized based on these results.

We have designed, fabricated and characterized electrically tunable and adaptive microwave devices incorporating paraelectric thin films of Sr0.5Ba0.5TiO3 (SBTO) and SrTiO3 (STO) on 1 cm × 1 cm × 0.5 mm LaAlO3 substrates. The paraelectric layer thickness for these devices varied between 0.5 μm and 2 μm. We used normal metal Au and superconducting YBa2Cu3O7-x top electrodes for SBTO and STO devices, and operated them at room- and cryogenic-temperatures, respectively. The microwave devices included voltage-tunable resonators, voltage-tunable phase shifters, voltage-tunable mixers, and voltage-tunable filters. Under dc voltage bias, these compact planar devices exhibited up to 30% resonant frequency modulation, about 1°/mm-GHz phase shift, more than 40 dB change in mixed microwave power, and fine-tunable symmetric filter profile with less than 2% bandwidth and more than 15% adaptive range.

High crystallinity Ba0.5Sr0.5TiO3 thin films were deposited on LaAlO3 substrates by pulsed laser deposition. A conductive metallic oxide, SrRuO3, provided not only a good bottom electrode for Ba0.5Sr0.5TiO3 but also an excellent seed layer for epitaxial growth of Ba0.5Sr0.5TiO3 on it. The epitaxial nature of the Ba0.5Sr0.5TiO3 thin films on the LaAlO3 substrate was confirmed by x-ray diffraction, Rutherford backscattering spectroscopy, and cross-sectional transmission electron microscopy. The quite good dielectric and electrical properties of crystalline Ba0.5Sr0.5TiO3 thin films suggest that Ba0.5Sr0.5TiO3/SrRuO3 is a good combination in terms of structural, electrical, and dielectric properties of Ba0.5Sr0.5TiO3 thin films.

A processing technique is developed to fabricate single-crystal yttria-stabilized zirconia (YSZ) on amorphous SiO2 on (100) single-crystal Si. The YSZ layer shows high crystallinity with an ion beam channelling minimum of 11 % from Rutherford backscattering. Even though there is a thick intervening amorphous SiO2 layer between the Si and the YSZ, the single-crystal YSZ layer is aligned with the crystal Si. The tilt and rotation variations are 0·64° and 1·11° respectively, based on X-ray diffraction Ω-rocking curves on [100] and π scans on [202] of YSZ films. The epitaxial nature of the YSZ on the thick SiO2 layer (from 10 to 100 nm) is further confirmed by high-resolution cross-sectional transmission electron microscopy analysis. This processing and the resulting multilayer structure might provide a new option for fabricating innovative microelectronic devices.

We show that twin boundaries, a ubiquitous c-axis-correlated (i.e., extending along the c axis throughout the thickness of the sample) crystallographic defect in some high-temperature superconductors, naturally induce a Bose glass vortex phase transition in the magnetic phase diagram of heavily twinned YBa2Cu3O7-δ thick films.

The choice of the bottom electrode or barrier layer plays an important role in determining the electrical and structural properties of metal/ferroelectric/metal thin film capacitors. A substantial improvement of the electrical and structural properties of the capacitors was found by using RuO2as a bottom electrode. Electrical measurement on a capacitor with a structure of BaTiO3(246 nm)/RuO2(200 nm)/SiO2/Si, where the BaTiO3thin film was deposited at room temperature, showed a dielectric constant of around 15, leakage current density of 1.6 × 10−7A/cm2at 4 V, a dc conductivity of 9.8 × 1014S/cm, and a capacitance per unit area of 5.6 × 104pF/cm2. A similar structure but with polycrystalline BaTiO3(273 nm) as the dielectric deposited at 680°C showed a dielectric constant of 290, leakage current density of 1.7 × 10−3A/cm2at 4 V, a dc conductivity of 1.2 × 10−8S/cm, and a capacitance per unit area of 9.4 × 105pF/cm2. Scanning electron microscopy analysis on the films showed differences in the microstructure due to the use of different bottom electrode materials, such as RuO2or Pd.

Pulsed laser deposition was used to deposit high-quality YBa2Cu3O7-δ(YBCO) thin films directly on y-cut LiNbO3substrates. The as-deposited YBCO films had a high degree of in-plane orientation and showed superconducting transition temperature (Tco) at 91K with a transition width of less than IK. Transport critical current densities were found to be ∼106A/cm2at 77K and zero field. An ion beam minimum channeling yield of 16% was obtained for YBCO films, indicating high crystallinity. High-resolution transmission electron microscopy studies showed that the interface between the film and the substrate was quite smooth and free from interfacial interdiffusion. The defects in thin films are also identified. The work showed that high-quality high Tcthin films can be deposited directly on LiNbO3. Novel devices based on the properties of both YBCO and LiNbO3could be realized based on these results.

Highly crystalline SrRuO3(SRO) and La0.5Sr0.5CoO3(LSCO) thin films were deposited on (100) Pt/ MgO by pulsed laser deposition. The films were mainly (001) textured normal to the substrate surface with a high degree of in-plane orientation with respect to the substrate’s major axes. These films were characterized using x-ray diffraction, Rutherford backscattering, four-point probe resistivity measurement, and transmission electron microscopy. The room temperature resistivity for LSCO and SRO films on Pt/MgO was found to be ∼35 and ∼40 μΩ-cm, respectively. An ion beam minimum channeling yield of ∼43% and ∼33% was obtained for LSCO and SRO films, respectively. The interface between Pt and oxide was found to be smooth and free from any interfacial diffusion. This result showed that high-quality low resistivity oxide thin films can be deposited on Pt.

Epitaxial conductive RuO2 thin films were used as bottom electrodes of Ba0.5Sr0.5TiO3 (BSTO) thin-film capacitors. The BSTO showed pure (111) orientation normal to the substrate surface when grown on (200) oriented epi-RuO2 on LaAlO3. The BSTO film consisted of a main body of stoichiometric bulk layer and a slightly oxygen deficiency region on the very film surface as revealed by Rutherford backscattering spectrometry. The dielectric constant of BSTO was above 400 at 10 kHz with zero dc electric field bias. A bias dc electric field modulation of the dielectric constant was observed to be more than 80% at 100 kHz at a field intensity of 6 × 105 V/cm.

Metal/ultrathin Tantalum Nitride (TaN)/p-Si (MTS) diodes were fabricated using dc magnetron reactive sputtering to form the ultrathin (˜5 nm) resistive TaN layer. The diode quality factor and barrier height were directly related to the substrate arrangement during sputtering of the ultrathin TaN layer. An effective barrier height of 0.75±0.01 eV and near-unity diode quality factor were observed while the plane of the substrate was perpendicular to that of the target. However, an effective barrier height of 0.78±0.01 eV and a diode quality factor around 1.15±0.02 were seen while the substrate was parallel to the target. Thermal stability of the as-deposited diodes was studied using elevated-temperature treatment with different time intervals at 100°C and 150°C, respectively. Thermal stability of the MTS diodes was also related to the substrate arrangement during sputtering.

We report an experimental demonstration of thermal tuning of resonance frequency in a planar terahertz metamaterial consisting of a gold split-ring resonator array fabricated on a bulk single-crystal strontium titanate (SrTiO_3) substrate. Cooling the metamaterial starting from 409?K down to 150?K causes about a 43% shift in resonance frequency, and there is very little variation in resonance strength. The resonance shift is due to the temperature-dependent dielectric constant of the strontium titanate. The experiment opens up avenues for designing tunable terahertz devices by exploiting the temperature-sensitive characteristic of high dielectric constant substrates and complex metal oxide materials.

We have successfully coated a nanofeatured material with ZrO2and TiO2using a polymer assisted deposition technique.

In this study, two kinds of spinning solutions were prepared by dissolving bamboo cellulose pulp into 1-ethyl-3-methyl imidazole chloride salt ([EMIM] Cl) and 1-butyl-3-methyl imidazole diethyl phosphate salt ([BMIM]DEP) ionic liquids, respectively. Furthermore, the rotational rheometer was used to test the steady-state rheological properties of above as-prepared spinning solutions. The research results show that both of these two ionic liquids exhibit better solubility to the bamboo cellulose pulp. The apparent viscosities(ea) decrease with the increased temperature(T) and shear rate(g) and increase with the increased concentration. The non-Newtonian index(n) declined with the increase of both shear rate and concentration, as well as increased with the build-up temperature. The structural viscosity index(De) increased with the increased concentration and tended to decrease with temperature rise. Meanwhile, viscous flow activation energy(Ee) decreases with the increased share rate as well as the concentration. According to the results, it can be seen that the bamboo cellulose pulp/[EMIM]Cl with the concentration of 6% at 70degC exhibits better spinnability.