Noise characteristics of an ideal shunted Josephson junction

Detailed computer simulations have been made of the effect of intrinsic noise current on the I-V characteristic and voltage noise spectral density S _v (f) of an ideal Josephson element shunted by a capacitor and resistor. The minimum value of the parameter _c at which hysteresis appears in the quasistatic I-V characteristic is greater than the noise-free value of 1. Moreover, as _c is increased, a region of large differential resistance R _d appears in the I-V characteristic that is associated with a large increase in S _V(0). In this regime the noise current is seen to cause random switching between the superconducting and nonzero voltage states. For all bias conditions, S _V(f) > R _d ²S_I(f) where S _I(f) is the spectral density of the intrinsic current noise. A comparison is made between these results and previous calculations. The implications for low-noise superconducting devices are discussed.     

Modelling the Resistive State in a Transition Edge Sensor

We have developed a model for the resistive transition in a transition edge sensor (TES) based on the model of a resistively shunted junction, taking into account phase-slips of a superconducting system across the barriers of the tilted washing board potential. We obtained analytical expressions for the resistance of the TES, R(T,I), and its partial logarithmic derivatives α _I and β _I as functions of temperature and current. We have shown that all the major parameters describing the resistive state of the TES are determined by the dependence on temperature of the Josephson critical current, rather than by intrinsic properties of the S-N transition. The complex impedance of a pristine TES exhibits two-pole behaviour due to its own intrinsic reactance.     

Experimental study of organic zero-gap conductor α-(BEDT-TTF)2I3

Abstract

A zero-gap state with a Dirac cone type energy dispersion was discovered in the organic conductor α-(BEDT-TTF)₂I₃ under high hydrostatic pressures. This is the first two-dimensional (2D) zero-gap state discovered in bulk crystals with a layered structure. In contrast to the case of graphene, the Dirac cone in this system is highly anisotropic. The present system, therefore, provides a new type of massless Dirac fermion system with anisotropic Fermi velocity. This system exhibits remarkable transport phenomena characteristic to electrons on the Dirac cone type energy structure. The carrier density, written as n∝T², is a characteristic feature of the 2D zero-gap structure. On the other hand, the resistivity per layer (sheet resistance R_S) is given as R_S=h/e² and is independent of temperature. The effect of a magnetic field on samples in the zero-gap system was examined. The difference between zero-gap conductors and conventional conductors is the appearance of a Landau level called the zero mode at the contact points when a magnetic field is applied normal to the conductive layer. Zero-mode Landau carriers give rise to strong negative out-of-plane magnetoresistance.     

Effects of electric fields on the silver photodoping of As2Se3 films

The effects of different electric fields (4.2, 8.3, 12.5, 16.7 and 20.8 Vcm_–1) on the sheet resistance, R _s, and optical band gap, E _obg, of As₂Se₃ samples (1×10⁵nm) that were photodoped by Ag (5×10³nm) have been studied. The R _s and E _obg of samples subjected to an electric field of 12.5 Vcm_–1 decrease linearly to a distance of 5 mm from both electrodes, and then saturate at larger distances. This result suggests that there is a critical value of the electric field which affects photodoping. The dependence of R _s and E _obg on the distance from the electrodes shows similar profiles for these electrodes.     

An experimental investigation of YBa2Cu3O7 films at millimeter-wave frequencies

We have measured the millimeter-wave (100 GHz) surface resistance of high-quality laserdeposited YBa₂Cu₃O₇ films on SrTiO₃ and LaA10₃ substrates. Due to finite film thickness, radiation losses are important in the normal state and in the superconducting state nearT _c. These effects are calculated andR _s characteristic of the ohmics losses in the film are extracted from the data. The surface resistanceR _s drops rapidly atT _c, and a detailed comparison with calculations which include finite mean free path effects suggests a gap which exceeds the weak coupling BCS limit.     

Microwave Response of Perfect YBa<Subscript>2</Subscript>Cu<Subscript>3</Subscript>O<Subscript>7−<Emphasis Type="Italic">x</Emphasis></Subscript> Thin Films Deposited on CeO<Subscript>2</Subscript>-Buffered Saphire: A Probe for Pairing Symmetry

Microwave surface impedance, Z _s(T), of epitaxial YBCO thin films deposited on CeO₂-buffered sapphire substrates, was measured at several discrete frequencies within the range 5–134 GHz by use of coplanar resonator and end-plate cavity resonator techniques. The main features of obtained experimental results are as follows: (i) surface resistance R _s(T) at low temperatures obeys the exponential law: R _s(T) = R _res+R ₀⋅exp [−δ/T] with a small gap δ value (δ≈ 0.7 T _c); (ii) the most perfect quasi-single-crystalline films reveal a distinct two-peak structure of R _s(T) dependence, which is not observable in films with a less ordered crystal structure. These features are believed to reveal some intrinsic electron properties of such films, namely: (i) mixed (d+is) type symmetry of electron pairing, and (ii) dominant role of extended c-oriented defects (e.g., edge dislocation arrays or twin planes) in quasiparticles scattering for the most perfect films, which demonstrate the two-peak anomalous R _s(T) behavior.     

Microwave and Terahertz Surface Resistance of MgB2 Thin Films

The knowledge of the surface resistance R _s of superconducting thin film at microwave and terahertz (THz) regions is significant to design, make and assess superconducting microwave and THz electronic devices. In this paper we reported the R _s of MgB₂ films at microwave and THz measured with sapphire resonator technique and the time-domain THz spectroscopy, respectively. Some interesting results are revealed in the following: (1) A clear correlation is found between R _s and normal-state resistivity right above T _c, ρ₀, i.e., R _s decreases almost linearly with the decrease of ρ₀. (2) A low residual R _s, less than 50 μΩ at 18 GHz is achieved by different deposition techniques. In addition, between 10 and 14 K, MgB₂ has the lowest R _s compared with two other superconductors Nb₃Sn and the high-temperature superconductor YBa₂Cu₃O_7−δ(YBCO). (3) From THz measurement it is found that the R _s of MgB₂ up to around 1 THz is lower than that of copper and YBCO at the temperature below 25 K. (4) The frequency dependence of R _s follows ω ⁿ , where ω is angular frequency, and n is power index. However, n changes from 1.9 at microwave to 1.5 at THz. The above results clearly give the evidences that MgB₂ thin film, compared with other superconductors, is of advantage to make superconducting circuits working in the microwave and THz regions.     

One-Dimensional Chiral Copper Iodide Chain-Like Structure Cu4I4(R/S-3-quinuclidinol)3 with Near-Unity Photoluminescence Quantum Yield and Efficient Circularly Polarized Luminescence

Chiral organic−inorganic hybrid metal halide materials have shown great potential for circularly polarized luminescence (CPL) related applications for their tunable structures and efficient emissions. Here, this work combines the highly emissive Cu₄I₄ cubane cluster with chiral organic ligand R/S-3-quinuclidinol, to construct a new type of 1D Cu-I chains, namely Cu₄I₄(R/S-3-quinuclidinol)₃, crystallizing in noncentrosymmetric monoclinic P2₁ space group. These enantiomorphic hybrids exhibit long-term stability and show bright yellow emission with a photoluminescence quantum yield (PLQY) close to 100%. Due to the successful chirality transfer from the chiral ligands to the inorganic backbone, the enantiomers show intriguing chiroptical properties, such as circular dichroism (CD) and CPL. The CPL dissymmetry factor (g_lum) is measured to be ≈4 × 10⁻³. Time-resolved photoluminescence (PL) measurements show long averaged decay lifetime up to 10 µs. The structural details within the Cu₄I₄ reveal the chiral nature of these basic building units, which are significantly different than in the achiral case. This discovery provides new structural insights for the design of high performance CPL materials and their applications in light emitting devices.     

Microwave surface resistance and power dependence of thallium-based films deposited onto large-area silver substrates

Microwave surface resistance (R _s ) measurements on large-area (11.4 cm²) Tl-based films deposited onto Ag (Consil 995) substrates have been made at a frequency of 18 GHz as a function of temperature. Deposition onto unoriented Ag substrates yields unoriented films characterized byR _s values of 8.2 and 33.6 m at 10.6 and 77 K, respectively. In contrast, similar deposition onto oriented Ag substrates yields oriented films with values of 12.6 and 14.6 m at 11.2 and 77 K, respectively; corresponding Cu values are 9 and 21 m. Additionally, it is found that the dependence ofR _s on microwave surface magnetic fieldH _s is weaker for the oriented films. These results suggest that the effect of orienting thec-axis in Tl films is twofold: (1) the high-frequency superconducting transition is made considerably sharper, resulting in a lowerR _s value at 77 K, and (2) the rate of increase inR _s with appliedH _s is reduced. Both effects have significant ramifications for the potential application of these materials to high-frequency accelerating cavities.     

Microwave characterization of laser ablated Y1Ba2Cu3O7−x thin films

In the present study we report the measurements of microwave surface resistance (R_s) of YBCO thin films on LaAlO₃ substrate as a function of temperature, thickness and magnetic field by microstrip resonator technique. The T_c(R = 0) of the films is 90 K and J_c > 10⁶ A/cm² at 77 K. The microwave surface resistance has been measured for films of various thicknesses. The value of R_s has been found to be initially decreased with increasing film thickness due to increase in number of defects. A minimum microwave surface resistance has been obtained for film thickness of about 300 nm. The increase of R_s with film thickness above 300 nm is possibly due to degradation of the film microstructure as observed with Atomic Force Microscopy. Temperature dependence of surface resistance has been studied for best quality films. The field induced variations of surface resistance are also investigated by applying dc magnetic field perpendicular to stripline structure and surface of the film. A general linear and square field dependence of R_s at low and high value of fields has been observed with critical field value of 0.4 T which confirms the microwave dissipation induced by flux flow in these resonators at 10 GHz frequency. The hysteresis of R_s in dc field observed for field value above critical field shows the higher value of surface resistance in decreasing field than in increasing field which is in agreement with one state critical model and is a characteristic of homogeneous superconductors.     

Experimental evaluation of the nonlinear surface resistance of YBa2Cu3O7?x step-edge grain boundary Josephson junctions

The temperature, microwave current, and dc magnetic field dependencies of the effective surface resistanceR _s of YBa₂Cu₃O_7–x grain boundary Josephson junctions have been investigated at 6 GHz. The junctions were prepared on stepped LaAlO₃ substrates and incorporated into tapered linear half-wavelength microstrip resonators. The characteristic parameters of the heterogeneous resonator were analyzed theoretically considering the junction as a lumped element of complex impedance. The microwave properties of the junction could then be extracted and related to their characteristic dc transport properties. The critical currentI _c of the junction was found to limit the linear power dissipation in the superconducting strip. At microwave currents aboveI _c , highly nonlinear microwave losses occurred, which displayed a characteristic magnetic field modulation. At even higher currents,R _s saturated at a level corresponding to the normal resistanceR _n of the junction.     

Model for small superconducting variable-thickness bridge constrictions

We present a simple model to simulate small three-dimensional superconducting constrictions of variable thickness (VTBs) for which the cross-sectional dimensions are of the same order as the length, and both are less than (T). We study the behavior of the modulus of the order parameterf and the supercurrent densityJ _s as a function of the various dimensions. We find that the extent to whichf is depressed in the banks depends on the ratio of width to length of the constriction. We show that even for very abrupt geometries,J _s is not zero andf does not reach its equilibrium value immediately outside the constriction. On the basis of our results, we propose a more general expression for the effective length valid also when all dimensions are of the same order. The calculated values ofdI _c /dT are consistent with experimental data from well-characterized VTBs. We suggest that the discrepancy between the calculated and experimental values ofI _c R _n may be due to the nonuniformity of the normal current density in the constriction.     

Intrinsic surface depression of the order parameter under mixed (s+id)-wave pair symmetry and its effect on the critical current of high-Tc SIS Josephson junctions

An intrinsic gap depression at the superconductor-insulator interface due to the very short value of the coherence length in high-T _c superconductors [HTS] is considered in the framework of a mixed (s+id)-wave pair symmetry for the order parameter ranging from pures to pured-wave. This gap depression acts as the main physical agent causing the relevant reduction ofI _c (T)R _n (T) values with respect to BCS expectations in HTS SIS Josephson junctions. Good agreement with various experimental data is obtained with both pures-wave and pured-wave symmetries of the order parameter, but with amounts of gap depression depending on the pair symmetry adopted. Regardless of the apir symmetry considered, these results prove the importance of the surface order-parameter depression in the correct interpretation of theI _c (T)R _n (T) data in HTS SIS junctions. In the case of a planar YBCO-based junction the use of the de Gennes condition allowed us to tentatively obtain an upper limit for the amount ofd-wave present in the gap of YBCO.     

The Edge Effect of a Microstrip Resonator on the Field Dependence of the Surface Resistance of a High-Tc Superconducting Thin Film

A new behavior of the field dependence of microwave surface resistance (R _s), which was observed on a microstrip resonator and may be caused by the edge of the center strip, is reported in this paper for epitaxial high-T _c superconducting (HTSC) thin films. The exhibited behavior is that R _s remains almost unchanged below a certain rf magnetic field H _rf, and then increases abruptly at this field, after which it increases in proportion to H _rf. To explain the behavior, the morphology of the microstrip resonator was examined by atomic force microscopy (AFM), which showed that the edge of the resonator was damaged in some regions because of the acid etching. If the damaged edge is considered as a weakened granular superconductor, the observed R _s behavior could be explained well in terms of the high-frequency critical state model. This implies that the edge condition should be considered in studying the field dependence of R _s when the planar resonator technique is used.     

A Colorimetric Multifunctional Sensing Method for Structural‐Durability‐Health Monitoring Systems

A colorimetric multifunctional phototransmittance‐based structural durability monitoring system is developed. The system consists of an array with four indium gallium zinc oxide (IGZO)‐based phototransistors, a light source at a wavelength of 405 nm through a side‐emitting optical fiber, and pH‐ and Cl‐selective color‐variable membranes. Under illumination at the wavelength of 405 nm at corrosion status, the pH‐ and Cl‐responsive membrane, showing a change in their color, generates a change in the intensity of the transmitted light, which is received by the phototransistor array in the form of an electrical current. I_ds and R (I_ds/I_{pH 12}) are inversely proportional to the pH, which ranges from 10 to 12. When the pH drops from 12 to 10, the magnitude of I_ds and R increases to ≈10³. In the case of Cl detection, I_ds and R (I_ds/I_{Cl 0 wt%}) increase nearly 50 times with an increase in Cl concentration of 0.05 wt%, and when the Cl concentration reaches 0.30 wt%, I_ds and R increase to ≈10³ times greater. This multifunctional colorimetric durability sensing system demonstrates considerable potential as a novel smart‐diagnostic tool of structural durability with high stability, high sensitivity, and multifunction.     

Low‐cycle fatigue life behaviour of BS 460B and BS B500B steel reinforcing bars

This paper investigates the low‐cycle fatigue resistance of BS 460B and BS B500B steel reinforcing bars and proposes models for predicting their fatigue life based on plastic‐strain (?_ap) and total‐strain (?_a) amplitudes. Constant‐amplitude, strain‐controlled low‐cycle fatigue tests were carried out on these bars under cyclic load with a frequency of 0.05 Hz. The maximum applied axial strain amplitude (?_s,max) ranges from 3 to 10% with zero and non‐zero mean strains. The strain ratios (R = ?_s,min/?_s,max) used are R =?1, ?0.5 and 0. Hysteresis loops were recorded and plastic and total strain amplitudes were related to the number of reversals (2N_f) to fatigue failure and models for predicting the number of reversals to fatigue failure were proposed. It is concluded that the predicted fatigue life of these bars is very accurate when compared with the measured experimental fatigue life results for wide range of values of strain ratios. It is also observed that based on plastic‐strain amplitude, BS B500B consistently has a longer life (higher number of cycles to failure) than those of BS 460B for all R values; however, at low plastic‐strain amplitudes they tend to behave similarly, irrespective of R value. Other observations and conclusions were also drawn.     

A Real-time Monitoring Technique for Local Plasticity in Metals Based on Lamb Waves and a Directional Actuator/Sensor Set

A real-time monitoring technique for local plasticity using Lamb waves was developed. Tensile test of a thin aluminum plate with a circular hole where high stress concentration was induced was conducted to verify this technique. During the tensile test, a series of wave signals passing through the local plastic region were collected using a directional actuator/sensor set to monitor plasticity evolution. A pulse compression technique was used to process the wave signals. With the increase of tensile stress in the specimen, the amplitude changes of S₀ and A₀ modes were obtained and the difference of Lamb wave signals was further evaluated using a proposed signal index I calculated by wavelet analysis. Combined with the numerical stress analysis of the tensile specimen, the influence of the plasticity on the amplitudes of S₀ and A₀ wave modes was analyzed. As the plastic zone grows gradually, the wave amplitudes and I of S₀ and A₀ wave modes show their different change tendencies compared with those in elastic stage. The amplitude change is more sensitive to mild plasticity than that of I, while the change of I caused by severe plasticity is more obvious than the amplitude change.     

The Distribution of the Energy Gap and Josephson IcRn Product in Bi2Sr2CaCu2O8+x by Tunneling Spectroscopy

We present direct measurements of the density of states by tunneling spectroscopy on slightly overdoped Bi₂Sr₂CaCu₂O_8+x (Bi2212) single crystals at low temperature using break-junction and point-contact techniques. We find that (i) the variation of the gap magnitude, , between 20 and 36 meV is likely to be intrinsic to Bi2212, and (ii) there is a correlation between the maximum value of the Josephson I _c R _n product and the gap magnitude: I _c R _n decreases with the increase of . The maximum I _c R _n value of 26 mV is observed at = 20.5 meV. For = 36.5 meV, the maximum measured value of I _c R _n is 7.3 mV. We conclude that (i) the distribution of the Josephson I _c R _n product as a function of gap magnitude cannot be explained by the presence of a single energy gap in Bi2212, and (ii) the coherence energy scale in Bi2212 has the maximum Josephson strength.     

Effect of Ag on the electrical transport and iso-thermal transformation of amorphous As2S3

The step-wise technique is used to follow the possible changes in d.c. transport of three different glasses of the system Ag-As₂S₃ isothermally annealed atT _g<T<T _m. The isothermal time-dependence of different characteristic electrical quantities (e.g.E _σ,σ_20°C, andσ ₀) has been drawn and correlated with the microstructural changes recorded for samples annealed at the same conditions. The effect of silver content (up to 25 at %) on the interplanar spacings (d _hkl) and relative intensities (I/I ₀) of the diffraction lines of As₂S₃ illustrates that the structure of the three-component system Ag-As₂S₃ can be represented as a solid solution of an element (e.g. S), binary phases (e.g. AsS, As₂S₃), and/or ternary phases (e.g. AgAs₂S₃, Ag₆As₂S₃), depending on the concentration of silver in the composition.     

Conductance-capacitance and voltage behavior of 1% Os-doped YMnO3/p-Si contacts under varying frequencies

The admittance characterizations of the ferroelectric Perovskite Materials 1% Osmium (Os)-doped YMnO₃ and p-Si contacts were conducted by capacitance–voltage (C–V) and conductance–voltage (G–V) curves at various frequencies. In measurements performed in this study, it was seen that the device behaves like a metal/oxide layer/semiconductor (MOS) or a metal/insulator layer/semiconductor (MIS) capacitor, just because of the ferroelectric thin film at the metal/Si substrate interface. Therefore, an accumulation region (saturation region) formed in the forward bias C–V and G–V curves at each frequency. The accumulation region series resistance value R_sc was determined from the C–V and G–V curves. The R_sc value ranged from 652 Ω at 20 kHz to 56 Ω at 1000 kHz. Furthermore, the series resistance R_s versus V curves with frequency as a parameter were plotted from the C–V and G–V data. A peak appeared around 0.0 V in the R_s vs V curve at each frequency. The peak position of the R_s shifted towards positive voltages from approximately ? 0.2 V at 20 kHz to about 0.10 V at 1000 kHz. The peak position in the corrected conductance vs voltage curves shifted towards positive voltages from approximately 0.0 V at 20 kHz to about 0.20 V at 1000 kHz. It was seen that the series resistance effect caused the highest error in the capacitance occurs in accumulation and in some part of the depletion region at C–V and G–V results depending on frequency. So to conclude, it is not possible to disregard the series resistance everytime. Therefore, the series resistance must be measured and must be considered at the measured admittance.