Response of high-Tc superconductor totime-increasing magnetic field

Using the extended critical state model, we investigate theoretically the magnetic field penetration into a semi-infinite slab of a high-Tc superconductor in the flux flow region when an external magnetic field increases monotonically with time according to power law: H=At^α. The dependencies of the flux flow resistivity and critical current density are taken to depend on magnetic field according to the generalized power law. The behavior of the solutions vs the exponent α and “amplitude” A is analyzed. It is shown that there is the critical value of the exponent α_c separating the regions with different characters of the magnetic field penetration. The value of α_c is determined by the exponents in the dependencies of the superconductor characteristics on magnetic field. For α<α_c, the electric field in a superconductor decreases with time and the magnetic field distribution tends to the one given by Bean’s critical state model. For α>α_c the electric field increases with time and the pattern of magnetic field differs increasingly from Bean’s model; the behavior of a superconductor at large times is described by the model of a normal metal with the resistivity dependent on magnetic field. Thus, the response of a superconductor is described by different approximations on different stages of the process. This result is distinct from that presented in the literature. In the boundary case of α=α_c, the character of the field penetration is time-independent.     

Terahertz Oscillating Devices Based Upon the Intrinsic Josephson Junctions in a High Temperature Superconductor

Recent developments of coherent terahertz (THz) oscillators based on the intrinsic Josephson junctions (IJJs) in mesas of the high temperature superconductor Bi₂Sr₂CaCu₂O_8+δ are reviewed. Experimental and theoretical studies of the emission from equilateral, right-angled isosceles, and acute isosceles triangular mesas are compared with those obtained from rectangular, square, and disk mesas, in order to determine the role of the mesa geometry. The superconducting properties and emission frequency f spectra are presented for a variety of triangular mesa geometries. Analytic and finite difference time domain numerical calculations of the emissions from the internal electromagnetic (EM) cavity modes of triangular mesas are compared with experiment. The experimental f always satisfies the ac Josephson relation, and its narrow linewidth arises from the synchronized emissions from many IJJs. For some mesa geometries, f also strongly locks onto an EM cavity mode frequency, enhancing the emission’s stability and output power. For other geometries, such cavity mode locking is weak, and f is highly tunable.     

Application of chemical effects in X-ray spectra for characterization of the high-Tc superconductors

Chemical effects in X-ray spectra (energy shifts, emission probability changes, satellite formation and peak profile changes) may be used for characterization of the chemical circumstances and chemical bonding of the monitored element in the sample. For that purpose, an investigation of the chemical effects in ion induced X-ray spectra of the high-T_c superconductor materials Sr₂Ln_yCe_0.5TaCu₂O_10+δ (Ln=Eu,Sm) was carried out using a high resolution crystal X-ray spectrometer. The applicability of the chemical effects for the mapping of the crystal phases in the superconductor sample using an ion microbeam is discussed.     

Crossed Andreev reflection in superconducting junctions

In this work crossed Andreev reflection (CAR) and elastic cotunneling (EC) are studied for junctions (N₁ISIN₂), where N₁ and N₂ are normal metals, S is a high T_c superconductor and I is an insulator. This study is carried out based on the analytical solutions of Bogoliubov de Gennes equations for anisotropic superconductors. The influence of different pair potential symmetries on the CAR and crossed conductance is analyzed. We show that CAR and EC are higher in d_x²-y² symmetry than in s symmetry. In the case of normal electrodes without magnetization, EC is the predominantly process for d_x²-y² symmetry, while in s symmetry, both processes decay with the same amplitude.     

Bi(2223) thick films (TC(0)=109K) on Ba2GdNbO6: a new perovskite ceramic substrate for BSCCO superconductor

Ba₂GdNbO₆ has been developed as a new substrate suitable for the BSCCO superconductor. Ba₂GdNbO₆ has a complex cubic perovskite (A₂BB′O₆) structure with a lattice constant a=8.587 Å. The DTA studies revealed that there is no phase transition occurring in Ba₂GdNbO₆ in the temperature range of 30–1300°C. The thermal expansion coefficient of Ba₂GdNbO₆ is found to be 7.913×10⁻⁶°C⁻¹. The dielectric constant and loss factor of Ba₂GdNbO₆ are in a range suitable for its use as a substrate for microwave applications. The Bi(2223) superconductor does not show any detectable chemical reaction with Ba₂GdNbO₆ even under extreme processing conditions. The thick films of Bi(2223) dip-coated on polycrystalline Ba₂GdNbO₆ substrate gave a T_C(0) of 109 K and a current density of approximately 4×10³ A/cm² at 77 K and zero magnetic field.     

Far-infrared Fabry Perot filter of high transmission with high-Tc superconductor reflectors

We report on a far-infrared Fabry Perot filter with high-T _c superconductor reflectors that has a high peak-transmission. The filter consists of two plane parallel MgO plates, with the adjacent surfaces coated with YBa₂Cu₃O₇-gd thin films. The resonance frequencies are mainly determined by the distance between the films. We present data for a filter that has for the first order resonance at a frequency of 80 cm^?1a quality factor of 55 and a peak-transmissivity of 0.16. The shape of the transmission curve is determined by standing waves between the films and within the plates.     

Evaluation of Cavity Modes in Superconducting Intrinsic-Josephson-Junction Oscillators for Terahertz-wave Generation

To realize antenna-coupled terahertz (THz)-wave oscillators using intrinsic Josephson junctions, rectangular mesas, 98-158 μm long, 78 μm wide and 1.04 μm high were fabricated on a Bi₂Sr₂CaCu₂O_8+δ high-temperature superconductor, and the cavity resonant modes in the mesas were evaluated based on their radiative frequencies. For appropriate bias voltages in the return curve of an I-V characteristic of a 138-μm-long mesa, two peaks in the radiative power were observed. At the voltages where the peaks first appeared at high and low biases, frequencies of 0.646 and 0.499 THz were measured using a lamellar interferometer. The cavity modes corresponding to these two frequencies are TE₁₂₁ and TE₁₁₁ modes, respectively. The temperature and size dependencies of the calculated resonant frequencies based on a 3D cavity model agree with measurements, validating the mode assignments.     

Induced current in a Bi2Sr2CaCu2Ox superconductor tube

Experiments were conducted to measure the induced current in a high-temperature Bi₂Sr₂CaCu₂O_x superconductor tube that was excited by an external coil driven by an AC sinusoidal voltage source. Experimental data were obtained for tests without and with an iron core inside the superconductor tube. All of the tests were conducted at 77 K and an excitation frequency of 60 Hz. The results showed that immediately after field penetration, the induced current (RMS value) decreased from the critical current, then began to recover, and eventually approached the critical current again at a high excitation current. Before field penetration, the induced current was mainly shielding current, which is 180 ° out of phase with the excitation current. After field penetration, the induced current consisted of two parts, a shielding current that led the excitation current by 180 ° and an inductive current (Faraday's law) that led the excitation current by 90 °. The presence of the iron core amplified the drop in induced current immediately after field penetration and delayed the growth of the inductive current after field penetration.     

Source/drain extension region engineering in nanoscale double gate SOI MOSFETs: Novel design methodology for low-voltage analog applications

The present paper proposes for the first time, a novel design methodology based on the optimization of source/drain extension (SDE) regions to significantly improve the trade-off between intrinsic voltage gain (A_VO) and cut-off frequency (f_T) in nanoscale double gate (DG) devices. Our results show that an optimally designed 25 nm gate length SDE region engineered DG MOSFET operating at drain current of 10 μA/μm, exhibits up to 65% improvement in intrinsic voltage gain and 85% in cut-off frequency over devices designed with abrupt SDE regions. The influence of spacer width, lateral source/drain doping gradient and symmetric as well as asymmetrically designed SDE regions on key analog figures of merit (FOM) such as transconductance (g_m), transconductance-to-current ratio (g_m/I_ds), Early voltage (V_EA), output conductance (g_ds) and gate capacitances are examined in detail. The present work provides new opportunities for realizing future low-voltage/low-power analog circuits with nanoscale SDE engineered DG MOSFETs.     

Source/drain optimization of underlapped lightly doped nanoscale double-gate MOSFETs

The impact of the spacer length at the source (L_s) and drain (L_d) on the performance of symmetrical lightly-doped double-gate (DG) MOSFET with gate length L = 20 nm is analyzed, with the type and doping concentration of the spacers kept the same as in the channel material. Using the transport parameters extracted from experimental data of a double-gate FinFET, simulations were performed for optimization of the underlapped gate-source/drain structure. The simulation results show that the subthreshold leakage current is significantly suppressed without sacrificing the on-state current for devices designed with asymmetrical source/drain extension regions, satisfying the relations L_s = L/2 and L_d = L. In independent drive configuration, the top-gate response can be altered by application of a control voltage on the bottom-gate. In devices with asymmetrical source/drain extension regions, simulations demonstrate that the threshold voltage controllability is improved when the drain extension region length is increased.     

Fabrication of nanosized superconductor structures with a critical temperature exceeding 100 K

The technology of preparation of layers (up to 100 nm thick) of a superconductor of composition (Bi,Pb)₂Sr₂Ca₂Cu₃O₁₀ with a superconducting transition temperature above 100 K is considered. It is shown that, upon preparation of thin layers, the critical condition reduces to the maintenance of a stoichiometric composition on the surface of the substrate in the process of magnetron-sputtering deposition and the formation of the crystal lattice of the superconductor at a high temperature.     

Preparation and lifetest of niobium Josephson junction tunnel diodes and arrays

Photoresist technology can be used to prepare superconductive tunnel junctions and arrays. Niobium thin films, which produce durable junctions, are chemically etched to form the base superconductor. NbNbO_xPb junctions continue to function after more than 800 days of testing. The failure mechanism for these junctions is the decrease of conductance. The niobium photoresist technology is being used to investigate neuristor-type devices.     

Single crystal growth and solidification processing of RE123 superconductive oxides

Single crystal growth and bulk solidification processing in air are reviewed for the RE_1+xBa_2−xCu₃O_6+d solid solutions (RE123; RE=Sm, Nd). By changing the solvent and/or the initial composition from the usual composition of RE123—Sm₂Ba₁Cu₁O₅ (Sm211)/Nd₄Ba₂Cu₂O₁₀ (Nd422) to the Ba enriched side, RE123 crystals with a small amount of substitution were produced, which resulted in high T_c values of about 94–96 K for both single crystals and bulk materials. Another unique and interesting finding for the bulk processing includes the reduction of the RE211 particle size with increasing the BaO/CuO ratio in the initial composition. Uniform dispersion of the fine RE211 particles was attained in the RE123 matrix, which led to improve J_c values in relatively low magnetic fields of about 1 T. Changing the initial compositions toward the Ba enriched side is a simple and fundamental process for obtaining the bulk RE123 superconductor with high T_c and J_c values even in air, which in turn is an important and useful approach from the industrial point of view.     

Properties of YBa2Cu3O7−δ-BaBiO3 composite superconductors

The effect of BaBiO₃ addition (5–30 mol.%) on the structural, thermal, and superconducting properties of YBa₂Cu₃O_7−δ(123) superconductor have been studied. BaBiO₃ addition leads to the formation of YBa₂BiO₆ and yttrium- and barium-deficient 123. Samples with up to 20 mol.% BaBiO₃ exhibit superconductivity with onset at a≈90 K. The lattice parameters of the superconductor phase decrease as more BaBiO₂ is added. Upon heating, the composites seem to have more stable oxygen content than does pure 123.     

Low temperature magnetic penetration depth in d-wave superconductors

We briefly review the experimental results of the low temperature magnetic penetration depth λ(T) of d-wave superconductors. In very clean high-T_c superconductors (HTSC) the low temperature in-plane penetration depth λ_‖(T) varies linearly with T. A weaker power law temperature dependence is found for the out-of-plane penetration depth λ_⊥(T) in most of HTSC. In the organic superconductors κ-(BEDT-TTF)₂X [X=Cu(NCS)₂, Cu[N(CN)₂]Br] and slightly dirty HTSC λ_‖∝bT²/(T*+T), where is deduced from an impurity scattering model. In clean CeCoIn₅ a crossover from linear to quadratic temperature behavior at low T was interpreted as the first evidence for nonlocal effects in a d-wave superconductor.     

Analysis of the structure of a terahertz-band 2D array with sensors on the edge of a superconductor transition

Film detectors on the edge of a Ti or Mo/Cu (T _c ≈ 0.4 K) superconductor transition are included in an N × M 2D array of planar polarization-separated antennas. The detectors are simultaneously sensors and absorbers of the total electric power from an antenna and bias circuits. The detectors are heat-insulated because of weak electron-phonon interaction with the substrate and because of the effect of the Andreev reflection of electrons in Nb electrodes. Readout based on the projection method necessitates only N + M channels in the case when the signal is continuously accumulated for all N × M detectors. Simulation of a 3 × 3 2D array at a frequency of 600 GHz shows that the matching band is ～30%.     

On the metal and oxygen content determination in Bi-2212 and Bi-2223 HTSCs

The oxygen content in Bi₂Sr₂Ca₁Cu₂O_8+δ and Bi₂Sr₂Cu₃O_10+δ superconductor samples is determined using a iodometric titration technique and using a thermogravimetric reduction technique. The reliability of the iodometric oxygen determination was exemplified by the reproducibility of at least 0.01 in δ and the excellent agreement between results obtained separately from iodometry and thermal analysis on a series of samples. Compared to similar determinations in YBa₂Cu₃O_7-δ superconductors, the determination of the metal content is an extra analytical requirement in the case of Bi and Pb containing ceramic materials. For the determination of the metal content, a reproducible and rapid method, based on differential pulse polarography, was optimized. Using EDTA-HCl as supporting electrolyte, we were able to determine the Pb, Bi and Cu content in a single experiment with a relative standard deviation (RSD) < 1 %. Using the techniques mentioned above, we were able to investigate the relationship between T_c and δ in a series of Bi-2212 and Bi-2223 HTSC's. There appears to be a maximum in the T_cvs δ curves for both phases.     

A new systematic design approach for low-power analog integrated circuits

In this paper, a new design approach for systematic design and optimization of low-power analog integrated circuits is presented based on the proper combination of a simulation-equation based optimization algorithm using geometric programming as an optimization approach and HSPICE as a simulation and verification tool by a knowledge-based transistor sizing tool which uses physical-based gm/I_D characteristic in all regions of transistor operation to increase the accuracy in a reasonable simulation time. The proposed design methodology is successfully used for automated design and optimization of an operational amplifier with hybrid-cascode compensation using 0.18 μm CMOS technology parameters with the main purpose of minimizing the power consumption of the circuit.     

Resonance characteristics of Y1 Ba2 Cu3 O7-8 superconductor ceramic coil

This research is concerned with Y₁ Ba₂ Cu₃ O_7-8 a superconductor ceramics. This ceramic pressed into a cylindrical form was cut into a wire coil by manual work and the Q-value as against frequency was measured in parallel resonance of a 5 turn coil. When the coil was cooled below the critical temperature, the electrical resistance of the superconductor vanished and the Q-value increased by about 9 times from 13 to 112 for a parallel capacity of 100 pF. The inductance of the coil was about 0-201μ H for a frequency of 25-2 MHz     

An experimental investigation of the off-diagonal Seebeck effect on textured YBCO high-Tc superconductor

The off-diagonal Seebeck effect was investigated on a melt-textured YBa₂Cu₃O_7−δ (YBCO) high-T_c superconductor. It was found that the transverse voltage V_x was proportional to the longitudinal temperature difference Δ T_z, measured directly with a pair of differential thermocouples, for textured samples with their c-axes tilted with respect to the surface’s normal. This supported the idea that the off-diagonal thermoelectric effect accounts for the anomalously high laser-induced transverse voltage on the oriented YBCO superconducting thin films. The variation of the V_x against the sample’s thickness d, at a given Δ T_z, deviated from the inversely proportional relationship when the sample was too thin. The deviation was discussed qualitatively in terms of deteriorated surface layers. The data of V_x(d) was fitted to obtain a reasonably reliable ∣ S_c−S_ab∣ value of 12 μ V K⁻¹.