Shapiro Steps in Flux-Trapped Surface Intrinsic Junctions of Bi2Sr2CaCu2O8＋δ

Microwave-field responses of the surface intrinsic Josephson junctions (IJJs) of Bi2Sr2CaCu2O8 δ superconductors are investigated. The IJJs are fabricated using an in situ low-temperature cleavage technique, which leads to the well-characterized surface CuO2 double layers and surface junctions. For the surface junctions in the largejunction limit, usually no Shapiro steps appear when a microwave field is applied. It is found that when the junctions are in a flux-trapped state, which is produced by a pulsed current and in which the critical current is significantly suppressed, clear Shapiro steps can be observed. These results are important for the study of the microwave-field properties of vortex-carrying IJJs and may find their use in device applications.     

Fiske steps in surface intrinsic Josephson junctions of Bi2Sr2CaCu2O8+δ single crystals

Current－voltage characteristics of the surface intrinsic Josephson junctions under a magnetic field parallel to CuO_2 planes have been studied experimentally. Fiske steps in such a junction have been observed. The Swihart velocity for the junction is found to be 7.2×10^5m/s, which corresponds to the velocity mode of a single junction.     

NON-EQUILIBRIUM MICROWAVE RESPONSE OF Yba2Cu3O7-δ GRANULAR THIN FILMS UNDER MAGNETIC FIELDS

Microwave responses of YBa₂Cu₃O_7-δ(YBCO) granular film have been studied at the microwave frequency of 30.5 GHz. In the absence of a magnetic field the dependence of a normal microwave response on the bias current is observed at a temperature close to T_c. When a magnetic field ranged from 5.0 mT to 33.0 mT is applied, the responses broaden and shift toward a lower temperature. In the superconducting state, the responses were found to be highly dependent on the magnetic field. For the current equal to 5.0mA and a magnetic field above 17.0mT the response increases and did not vanish even at a very low temperature, the fact is believed to be correlated to the anisotropic character of the structure.     

Photoinduced Resistance Change in an Oxygen-Deficient La 0.9 Sr 0.1 MnO 3-δ Thin Film

Photoinduced resistance change （ △ R/R） in an oxygen-deficient La0.9Sr0.1MnO3-δ thin film is studied. At room temperature, the resistance change of about 30% and response time of about 75 ns are observed under the illumination with a 532nm laser pulse of 7ns and light power of 750mW. It is also found that △ R/R changes with the light power. The phenomena are explained in terms of the photoinduced hole carriers and localized insulator-to-metal transition, which may have potential applications in optoelectronic devices.     

ADSORPTION BEHAVIOR OF AMINO ACIDS ON COPPER SURFACES

The major results of a series of our recent investigations on the adsorption of eight amino acids on Cu(001) and (111) surfaces are reviewed in the present paper. In all studied cases the molecules adsorb onto the surface in their anionic form. With the increase of the coverage three different 2D phases of the adsorbates, that is, the 2D lattice gas, intermediate, and solid phases, appear sequentially, although for few systems one or two of them do not appear. In both the 2D lattice gas and intermediate phases the molecules "stand" with their two oxygen "feet" on the surface and the intermolecular interactions are repulsive, although in the former they can diffuse frequently whereas in the latter they are discommensurate in one direction with the substrate. In the solid phase the molecules "lie" down on the surface to form commensurate superstructures. Adsorption of amino acids may often induce step faceting as well as bunching to form facets. Adsorption of L-lysine on Cu(001) may cause steps bunching to form facets with all the same chirality. Our preliminary results show that it is possible to manipulate individual molecules with the STM tip even at room temperature. These results may have applications in nano-materials, nano-technology, and very likely also in chiral separations or enantioselective heterogeneous catalysis.     

The transverse laser induced thermoelectric voltages in step flow growth （1-x）Pb(Mg1/3Nb2/3)O3-xPbTiO3 thin films

This paper reports that the transverse laser induced thermoelectric voltages (LITV) are observed for the first time in the step flow growth (1-x)Pb(Mg1/3Nb2/3)O3-xPbTiO3 (PMN-PT,x = 0.20, 0.33, 0.50) thin films deposited on vicinal-cut strontium titanate single crystal substrates. Because lead magnesium niobate-lead titanate is a solid solution of lead magnesium niobate (PMN) and lead titanate (PT), there are two types of signals. One is wide with a time response of a microsecond, and the other superimposed with the wide signal is narrow with a time response of a nanosecond. The transverse LITV signals depend on the ratio of PMN to PT drastically. Under the irradiation of 28-ns pulsed KrF excimer laser with the 248-nm wavelength,the largest induced voltage is observed in the 0.50Pb(Mg1/Nb2/3)O3-0.50PbTiO3 films. Moreover, the effects of film thickness, substrates, and tilt angles of substrates are also investigated.     

Study of Nb/Nb_xSi_(1-x)/Nb Josephson junction arrays

Owing to the adjustable characteristics and superior etching properties of co-sputtered Nbx Si1-x film, we are trying to fabricate Nb/Nbx Si1-x/Nb Josephson junction arrays for voltage standard. It is important to find the suitable Nbx Si1-x barrier for the junctions. Josephson junctions with different barrier content are fabricated. Current–voltage characteristics are measured and analyzed. It is demonstrated in this paper that critical current can be adjusted by using different barrier content and thickness. Shapiro steps of five hundred junctions in series are observed.     

Effect of transparency on Josephson junction between s+g-wave superconductors

In this paper, a dc Josephson junction between borocarbide superconductors has been studied theoretically. The s+g-wave pairing symmetry which is observed in rare earth complex of borocarbides has a huge anisotropy and is an interesting form of unconventional superconductivity. We calculate the Josephson current in a superconductor--insulator--superconductor (SIS) Josephson junction with s+g-wave superconducting pairing symmetry. In our planar junction c-axis is parallel to an interface with finite transparency but ab-planes of two tetragonal superconductors are misorientated by angle α. We obtain that the Josephson current is strongly dependent on mis-orientation between the left and the right ab-planes. An insulator sandwiched between two superconductors which acts as a potential barrier is demonstrated by a transparency coefficient. The effects of the potential barrier and the mis-orientation on the current are studied analytically and numerically. Occurrence of 0--π transition in this s+g-wave junction is investigated in this paper. A comparison between d-wave Josephson junction and s+g-wave one is also made in the present paper.     

Polarization Properties of Quantum-Dot-Based Single Photon Sources

Polarization properties of single photons emitted by optical pumping from a single quantum dot （ QD） are studied by using a four-level system model. The model is capable of explaining the polarization uncertainty observed in single photon emission experiments. It is found that the dependence of photon emission efficiency and polarization visibility on pump power are opposite in general cases. By employing QDs with small size and strong carrier confinement, the photon polarization visibility under high pump power can be improved. In addition, embedding a QD into a well designed microcavity is also found to be favourable, whereas the trade-off between high polarization visibility and multi-photon emission is noted.     

Time-and-space resolved measurements of the emission uniformity of carbon fibre cathode in high-current pulsed discharge

The remaining challenges, confronting high-power microwave (HPM) sources and pulsed power generators, stimulate the developments of robust relativistic electron beam sources. This paper presents a carbon fibre cathode which is tested in a single pulsed power generator. The distribution and the development of cathode plasma are observed by time-and-space resolved diagnostics, and the uniformity of electron beam density is checked by taking x-ray images. A quasistationary behaviour of cathode plasma expansion is observed. It is found that the uniformity of the extracted electron beam is satisfactory in spite of individual plasma jets on the cathode surface. These results show that carbon fibre cathodes can provide a positive prospect for developing a high-quality electron beam.     

Electron Heat Transport during Center Fuelling by Pellet Injection on HL-2A Tokamak

The improved energy confinement has been observed in many tokamaks with center pellet injection since 1980＇s. The pellet enhanced performance （ PEP ） was achieved with high power additional heating in JET and other large tokamaks. The mechanism of the PEP mode has been analyzed theoretically . The analysis shows that a few mechanisms are involved in the reduction of anomalous transport and the relative weight of these mechanisms depends on the experimental conditions. In this paper we report the pellet injection experiment results without auxiliary heating on HL-2A tokamak. Our works focus on the investigation of the electron heat transport in ohmic discharges with center pellet fuelling.     

Reversal transient laser-induced voltages in La2/3Ca1/3MnO3 films

This paper reports that the transient laser-induced voltages have been observed in La2/3Ca1/3MnO3 thin films on MgO （001） in the absence of an applied current. A peak voltage of - 0.15 V was detected in response to 0.015J pulse of 308 nm laser. It is demonstrated that the signal polarity is reversed when the films are irradiated through the substrate rather than at the air/film interface. Off-diagonal thermoelectricity may support the inversion of the signal when the irradiation direction is reversed.     

Effects of microwave pulse-width damage on a bipolar transistor

This paper presents a theoretical study of the pulse-width effects on the damage process of a typical bipolar transistor caused by high power microwaves (HPMs) through the injection approach. The dependences of the microwave damage power, P, and the absorbed energy, E, required to cause the device failure on the pulse width τ are obtained in the nanosecond region by utilizing the curve fitting method. A comparison of the microwave pulse damage data and the existing dc pulse damage data for the same transistor is carried out. By means of a two-dimensional simulator, ISE-TCAD, the internal damage processes of the device caused by microwave voltage signals and dc pulse voltage signals are analyzed comparatively. The simulation results suggest that the temperature-rising positions of the device induced by the microwaves in the negative and positive half periods are different, while only one hot spot exists under the injection of dc pulses. The results demonstrate that the microwave damage power threshold and the absorbed energy must exceed the dc pulse power threshold and the absorbed energy, respectively. The dc pulse damage data may be useful as a lower bound for microwave pulse damage data.     

Novel transit-time oscillator(TTO) combining advantages of radial-line and axial TTO

A novel transit-time oscillator(TTO) is proposed in this paper. An axial cathode which has been widely used in high power microwave(HPM) source and an extractor with radial feature are adopted. In this way, the inherent advantages of axial and radial TTO, both of which can be utilized as high-quality intense relativistic electron beam(IREB), can be generated and the power capacity is also increased. The working mode is π/2 mode of TM01 based on small-signal theory, and under the same energy storage, the maximum electric field in extractor decreases 16.3%. Besides, by utilizing the natural bending of the solenoid, this TTO saves over 60% of the length required by the uniform magnetic field, and consequently reduces the energy consumed by solenoid. The PIC simulation shows that by using 1.0-T decreasing magnetic field generated by the shorter solenoid, 3.37-GW microwave at 12.43 GHz is generated with 620-kV and 13.27-kA input,and the overall conversion efficiency is 41%.     

Universal Minimum Conductivity in Disordered Double-Weyl Semimetal

We report an exact numerical study on disorder effect in double-Weyl semimetals, and compare exact numerical solutions for the quasiparticle behavior with the Born approximation and renormalization group results. It is revealed that the low-energy quasiparticle properties are renormalized by multiple-impurity scattering processes,leading to apparent power-law behavior of the self-energy. Therefore, the quasiparticle residue surrounding nodal points is considerably reduced and vanishes as Z_E ∝E~r with nonuniversal exponent . We show that such unusual behavior of the quasiparticle leads to strong temperature dependence of diffusive conductivity. Remarkably, we also find a universal minimum conductivity along the direction of linear dispersion at the nodal point, which can be directly observed by experimentalist.     

Magnetic Field Dependence of the Critical Current of Bi2Sr2CaCu2O8＋δ-Based Intrinsic Josephson Junctions

Well-characterized surface intrinsic Josephson junctions (IJJs) on Bi2Sr2CaCu2O8 δ single crystals are fabricated by in situ cryogenic cleavage of the crystals and immediate evaporation of A u films on the crystal surface. Magnetic field dependences of the critical currents of the surface and inner I33s are carefully measured. We find that the     

Physics design of SSRF synchrotron radiation security

High brightness of SSRF brings about synchrotron radiation security problems, which will be solved in physics design. The main radiations are generated from bending magnets and insertion devices. Since the fact that radiation power and radiating area are different in these two kinds of synchrotron radiation, the arrangements of photon absorbers, diaphragms and other vacuum components need to be treated distinctively. In addition, SSRF interlock protection threshold is defined and the beam orbit in the straight line is limited. Hence, beam orbit in the bending magnets and IDs are also restricted by the threshold. The orbit restriction is calculated and helps us to arrange the vacuum components. In this paper, beam orbit distortion restricted by interlock protection threshold, radiation power, radiation angle and illuminating area are calculated. From the calculation results, the physics designs in manufacture and installation vacuum components are put forward. By commissioning, it is shown that physics requirements are met rigidly in the engineering process.     

Optical phase front control in a metallic grating with equally spaced alternately tapered slits

A technique capable of focusing and bending electromagnetic （EM） waves through plasmonic gratings with equally spaced alternately tapered slits has been introduced. Phase resonances are observed in the optical response of transmission gratings, and the EM wave passes through the tuning slits in the form of surface plasmon polaritons （SPPs） and obtains the required phase retardation to focus at the focal plane. The bending effect is achieved by constructing an asymmetric phase front which results from the tapered slits and gradient refractive index （GRIN） distribution of the dielectric material. Rigorous electromagnetic analysis by using the two-dimensional （2D） finite difference time domain （FDTD） method is employed to verify our proposed designs. When the EM waves are incident at an angle on the optical axis, the beam splitting effect can also be achieved. These index-modulated slits are demonstrated to have unique advantages in beam manipulation compared with the width-modulated ones. In combination with previous studies, it is expected that our results could lead to the realization of ootimum designs for plasmonic nanolenses.     

Fano Resonances Can Provide Two Criteria to Distinguish Majorana Bound States from Other Candidates in Experiments

There are still debates on whether the observed zero energy peak in the experiment by Stevan et al.[Science 346(2014) 602]reveals the existence of the long pursued Majorana bound states(MBSs).We propose that,by mounting two scanning tunneling microscopic tips on top of the topological superconducting chain and measuring the transmission spectrum between these two metallic tips,there are two kinds of characteristics on the spectrum that are caused by MBSs uniquely.One is symmetric peaks with respect to zero energy and the other is 4π period caused by a nearby Josephson junction.The former refers to the fact that MBSs are composited by Majorana fermions which distributed in the particle and hole subspaces equally.The latter is based on the well known 4π period of Josephson effect in topological superconductor.We think that such two characteristics can be used as criteria to distinguish MBSs from other candidates,such as impurities,Kondo effect and traditional Andreev bound states.     

Angle-dependent spin waves in antidot bilayers

Ferromagnetic resonance is introduced to examine the microwave frequency response of Ni Fe/Ir Mn bilayers, patterned as antidot arrays. In the experiment, field direction dependence on mode is obtained by rotating the applied magnetic field. We find that at a given resonance frequency, the dependence of the resonance field on the angle has a tendency of sinusoid/cosine variation in the experiment. From micromagnetic simulation it can be seen that spin waves are localized between dots from a given mode profile. This is caused by a demagnetization distribution with a larger value in the center of the two nearest dots than that of the next-nearest dots.