Superconducting critical current of sputtered Nb-Ge films

The temperature dependence of the superconducting critical current density J_c(T) in zero applied field from 4.2 K to T_c has been measured for 15 films of Nb-Ge with varying composition, deposition conditions, and radiation damage. The results show (i) the enhanced superconductivity observed in stoichiometric Nb₃Ge as well as nonstoichiometric films is “bulk like” rather than filamentary, (ii) a simple correlation of the form which is reasonably independent of how the T_c has been achieved, and (iii) some additional evidence that the enhanced T_c of the films is not due to their chemical composition alone.     

Anomalous angular dependence of critical current in niobium films

An anomalous angular dependence of the critical current is observed in niobium films. This phenomenon manifests itself in the fact that, under small intensities of the external magnetic field, the critical current attains its maximum in a slightly tilted magnetic field. It is found that the position of the maximum depends on the external magnetic field, as well as on the initial conditions under which the samples were kept. A theoretical model is proposed to explain the results obtained. This model takes into account the effect of diamagnetic properties, pinning of vortices, and the initial conditions on the vortex system in Nb films.     

Radiative and nonradiative recombination times in semiconducting films

The radiation emitted spontaneously by a semiconductor which has been excited for a very short time decays exponentially with a time constant that depends on the recombination rate of electrons and holes. This recombination rate is the combination of radiative and nonradiative transition rates between conduction and valence bands of the semiconductor. The radiative recombination rate depends on the density of states of the electromagnetic field, which can be made to be dependent on the geometry. In this paper, we report on the dependence of the fluorescence lifetime upon the thickness of active thin films. For systems in which the radiative recombination rate is dominant over the nonradiative ones, the total recombination time can be changed by suitable modifications of the thickness of the film. In this situation, the nonradiative rate can be evaluated. We present experimental results for the case of cadmium sulphide (CdS) thin films.     

Ferromagnetism in transition-metal-doped semiconducting oxide thin films

A rather complete work on transition-metal (TM)-doped TiO₂ thin films has been done and room ferromagnetism (FM) is found in the whole series of Sc/V/Cr/Mn/Fe/Co/Ni-doped TiO₂ films. Not only is it remarkable that for the first time, FM at high temperature was achieved in TM-doped TiO₂, but also a very big magnetic moment of 4.2μ_B/atom could be obtained, and direct evidences of real ferromagnets with big domains were shown as well. A similar chemical trend was achieved in TM-doped In₂O₃ films, however, the observed magnetic moment is rather modest, with the maximal value is of only 0.7μ_B/atom for Ni-doped In₂O₃ films. As regards TM-doped SnO₂ films, observed magnetic moments could be very large, with the maximum saturation of 6μ_B per impurity atom for Cr-doped SnO₂ thin films, but it could be influenced very much depending on substrate types. On the other hand, results on TM-doped ZnO films interestingly have revealed that in these systems, the magnetism more likely resulted from defects and/or oxygen vacancies.     

Temperature dependence of the critical current density in Y-Ba-Cu-O thin films

We present a study of the temperature dependence of the critical currentJ _c of several dc magnetron sputtered thin Y-Ba-Cu-O films on single crystalline SrTiO₃, ZrO₂ and Al₂O₃ substrates. Near the critical temperature T_c it is found thatJ _c(1–T/T_c)ⁿ withn=3 for the SrTiO₃ and ZrO₂ substrates, whilen=1·3 for the Al₂O₃ substrate. The temperature dependence in our samples approximately agrees with standard theories for weak links or with the Ambegaokar-Baratoff equation.     

Laser-induced synthesis of compound semiconducting films

Compound semiconducting films are synthesized by laser irradiation of sandwich films of the constituants, under various conditions (time of irradiation, laser power, temperature, etc.). Experimental data give evidence of the roles of various parameters. After a presentation of the experimental results, the fundamental physico-chemical mechanisms of laser material interactions are analyzed. Particular emphasis is given to effects that need to be treated by using very-far-from-equilibrium concepts.     

Depletion layers and domain walls in semiconducting ferroelectric thin films

Commonly used ferroelectric perovskites are also wide-band-gap semiconductors. In such materials, the polarization and the space-charge distribution are intimately coupled, and this Letter studies them simultaneously with no a priori ansatz on either. In particular, we study the structure of domain walls and the depletion layers that form at the metal-ferroelectric interfaces. We find the coupling between polarization and space charges leads to the formation of charge double layers at the 90 degrees domain walls, which, like the depletion layers, are also decorated by defects like oxygen vacancies. In contrast, the 180 degrees domain walls do not interact with the defects or space charges. Implications of these results to domain switching and fatigue in ferroelectric devices are discussed.     

高温超导薄膜临界电流密度的无损测量研究

临界电流密度Jc是超导薄膜的一个重要参量,它可以衡量超导薄膜的功率承载能力。大面积高温超导薄膜制成后,其Jc需要被无损精确测量。文中提出了一种新的交流磁场下的高温超导薄膜临界态模型:(1)基于此模型对薄膜的临界电流密度进行了精确无损测量;(2)并将实验测量的三次谐波电压曲线进行拟合研究。首先,根据麦克斯韦方程和伦敦方程,计算外加直流磁场超导薄膜Meissner态下电流和磁场在薄膜内的分布;然后分析薄膜进入临界态后内部电流的变化,在考虑顶扎力作用的情况下,提出了临界态电流和磁场非均匀分布模型;最后根据其模型,推导出三次谐波电压的表达式。为了验证该理论,分别对四片超导薄膜在不同频率下进行了三次谐波和临界电流密度测量。实验结果表明:三次谐波电压的理论与实验曲线一致;与四点传输法的测量结果相比较,该方法测量超导薄膜临界电流密度的误差在5%左右,具有高精度、无损伤、方便快捷等优点。     

Dynamical properties of thin ferromagnetic semiconducting films

A Green's function technique is used to investigate the temperature dependence of the spin-wave energies of ferromagnetic semiconducting thin films including the damping effects. It is shown that the frequencies of thin ferromagnetic films are smaller, whereas the damping effects are larger compared to the bulk.     

Photoconductivity studies of Te-substituted Sn-Sb-Se semiconducting films

Amorphous thin films of chalcogenide Sn₁₀Sb₂₀Se_70?X Te _X (0≤X≤8) composition were deposited using the thermal evaporation technique. The dark conductivity measurement showed a thermally activated conduction process with single activation energy in a studied temperature regime. Photoconductivity showed no maxima in the measured temperature regime revealing that the material belongs to the type II photoconductor. The observed small difference between activation energy for photoconduction ΔE _ph and dark conduction ΔE accounts for low photosensitivity of the material. The intensity variation of the photocurrent obeys the power law with the exponent γ～0.56–0.64 revealing the dominant bimolecular recombination mechanism in the studied compositions. Transient photoconductivity revealed that initial rise of the photocurrent becomes slow with tellurium content in the sample. The change in the shape of the transient photocurrent with composition is qualitatively explained based upon change in defect statistics introduced by the tellurium content in the sample. The decay process after the initial decay was found to be nonexponential and is described with a differential life time of charge carrier that showed a decreasing trend with the tellurium content in the sample.     

Dramatic role of critical current anisotropy on flux avalanches in MgB2 films

Anisotropic penetration of magnetic flux in MgB(2) films grown on vicinal sapphire substrates is investigated using magneto-optical imaging. Regular penetration above 10 K proceeds more easily along the substrate surface steps, the anisotropy of the critical current being 6%. At lower temperatures the penetration occurs via abrupt dendritic avalanches that preferentially propagate perpendicular to the surface steps. This inverse anisotropy in the penetration pattern becomes dramatic very close to 10 K where all flux avalanches propagate in the strongest pinning direction. The observed behavior is fully explained using a thermomagnetic model of the dendritic instability.     

Effects of surface damage on critical current density in MgB2 thin films

We investigated the influence of surface damage on the critical current density (J_c) of MgB₂ thin films via 140-keV Co-ion irradiation. The J_c(H) of the surface-damaged MgB₂ films was remarkably improved in comparison with that of pristine films. The strong enhancement of J_c(H) caused by a surface damage in MgB₂ films can be ascribed to additional point defects along with an atomic lattice displacement introduced through low-energy Co-ion irradiation, which is consistent with the change in the pinning mechanism, from weak collective pinning to strong plastic pinning. The irreversible magnetic field (H_irr) at 5 K for surface-damaged MgB₂ films with a thickness of 850 and 1300 nm was increased by a factor of approximately 2 compared with that of a pristine film. These results show that the surface damage produced by low energy ion irradiation can serve as an effective pinning source to improve J_c(H) in a MgB₂ superconductor.     

Electrical and optical properties of semiconducting camphoric carbon films

In situ variation in resistance of camphoric carbon versus time of pyrolysis, temperature of pyrolysis and effect of sintering are studied to perceive the time required for the completion of pyrolysis and the activation energy from the electrical conductance plot. Variation in the electrical conductance versus temperature and activation energies derived from these measurements, are reproducible when film is thermally treated below 750 °C. Thermal treatment above 750 °C changes the anatomy of the film causing a change in the conductance profile as well as decreases its band gap to 0.1 eV. Camphor pyrolyzed at 650 °C gives semiconducting carbon with optical band gaps 1 eV (direct) and 0.8 eV (indirect). Increase in pyrolysis temperature also shifts G-band of Raman spectrum from 1605 to 1586 cm⁻¹ i.e., towards value corresponding to graphitic carbon. SEM micrograph of camphoric film shows absence of any carbon nanobeads or fibers as normally observed with camphoric carbon pyrolysed in this temperature range.     

Photonic crystal narrow filters with two neighboring waveguides and a semiconducting point defect

A tunable filter is demonstrated based on a squared lattice two-dimensional photonic crystal. The filter is formed by a single semiconducting point defect and two neighboring waveguides. Modal properties of the defect modes and the transmittance of the proposed system are analyzed using supercell method and finite difference time domain method, respectively. We show that there is a narrow pass band for each temperature between 218 and 240 Kelvin. The peak of the pass band transmittance and the frequency can be highly tunable with the temperature of an intrinsic semiconducting point defect. Also, we have showed that the frequency and temperature of the desired high transmitted filter do not sensitive on the cavity size.     

Crystal orientation dependence of the critical current density in a-axis- and c-axis-oriented MgB2 films

We have investigated the critical current density for MgB₂ films having various crystal orientations prepared by using a hybrid physical-chemical vapor deposition system. An enhancement of the critical current density is clearly presented in MgB₂ films with an a-axis or a b-axis orientation rather than a c-axis orientation. X-ray diffraction patterns reveal a suppression of c-axis orientations while a (100) orientation becomes dominant, and the surface morphology of the a-axis-oriented film shows that the orientation of the c-axis-oriented MgB₂ grains parallel to the plane of the substrate. As the a-axis orientation becomes more dominant in the MgB₂ films, the field performance of the critical current density clearly becomes better. These results suggest that the synthesis of MgB₂ with high ab-plane orientations is one of the keys to enhancing the critical current density in MgB₂.     

Thermopower of doped semiconducting hydrogenated amorphous carbon films

The thermopower S(T) and electrical conductivity have been measured from 25 to 250C for semiconducting a-C:H films doped with boron or phosphorus. S has the expected sign (positive for B-doping and negative for P-doping), is low for all films (10–50 μ V/K), and increases nearly linearly with T. This behavior, along with that observed for the electrical conductivity, is consistent with conduction via hopping at or near the Fermi level which has been shifted via doping from near mid-gap into broad bands of tail states at the appropriate band edges.     

Studies on electrosynthesized semiconducting zinc selenide thin films

The electrosynthesis of ZnSe thin films from aqueous acidic bath onto transparent conducting oxide coated glass substrates is described. The deposition potential range suitable for the deposition has been optimized using cyclic voltammetry. The influence of various deposition parameters on the structural and optical properties of the films is described. The optical constants ‘n’ and ‘k’ and the complex dielectric constants of the electrosynthesized ZnSe thin films are estimated for various wave length region. The surface morphological studies and the composition analysis are carried out and the results are discussed. Paper presented at the 2nd International Conference on Ionic Devices, Anna University, Chennai, India, Nov. 28–30, 2003.