On the role of electron correlation and disorder on persistent currents in isolated one-dimensional rings

To understand the role of electron correlation and disorder on persistent currents in isolated 1D rings threaded by magnetic flux ?, we study the behavior of persistent currents in aperiodic and ordered binary alloy rings. These systems may be regarded as disordered systems with well-defined long-range order so that we do not have to perform any configuration averaging of the physical quantities. We see that in the absence of interaction, disorder suppresses persistent currents by orders of magnitude and also removes its discontinuity as a function of ?. As we introduce electron correlation, we get enhancement of the currents in certain disordered rings. Quite interestingly we observe that in some cases, electron correlation produces kink-like structures in the persistent current as a function of ?. This may be considered as anomalous Aharonov-Bohm oscillations of the persistent current and recent experimental observations support such oscillations. We find that the persistent current converges with the size of the rings.     

Effects of spin-orbit interaction on superconductor-ferromagnet heterostructures: Spontaneous electric and spin surface currents

We find proximity-induced spontaneous spin and electric surface currents at all temperatures below the super-conducting T _c in an isotropic s-wave superconductor deposited with a thin ferromagnetic metal layer with spin-orbit interaction. The currents are carried by Andreev surface states and generated as a joint effect of the spin-orbit interaction and the exchange field. The background spin current arises in the thin layer due to different local spin polarizations of electrons and holes, which have almost opposite velocities in each of the surface states. The spontaneous surface electric current in the superconductor originates in the asymmetry of Andreev states with respect to sign reversal of the momentum component parallel to the surface. The conditions for electric and spin currents to show up in the system significantly differ from each other.     

The electron-nucleon weak neutral current and 0− -0+ nuclear isomerism

Since one photon exchange is completely forbidden in processes of the type ?0^? → ?0⁺ (? is a charged lepton) or any crossed version of such a process, weak neutral current effects are considerably enhanced. It is shown that 0̄?0⁺ nuclear isomers, if they exist, will accordingly exhibit strikingly large neutral current effects. In particular, the monoenergetic conversion electrons will have helicities h of the order of 25% which should be easily measurable. In certain favourable cases, the nuclear weak current matrix elements may be measured experimentally via the analogue β decay or electron capture so that measuring h will constitute a measurement of the electron's neutral vector current coupling constant. In any event, detection of h would prove the existence of an as yet unseen class of neutral currents.     

一种测量GaAs-GaAlAs行波激光放大器增益特性的实验方法

本文报导了一种测量光耦合效率η的新实验方法。这个方法是建立于p-n结短路光电流原理上的。本文推导出适合于行波激光放大器的光耦合效率的公式。短路光电流用一检流计测量,利用公式获得光耦合效率的实验值。利用实验所测光耦合效率,测量了行波激光放大器的增益随注入电流变化的规律,其结果和实验符合。另外本文还介绍了在脉冲注入电流条件下测行波半导体激光放大器增益的实验方法。     

Single-pion production and the structure of the weak neutral current

We investigate the restrictions on the structure of the weak neutral current imposed by single pion production cross sections on single nucleons. A general vector (V), axial-vector (A) neutral current with |ΔI|?1 is assumed, where the isovector V,A neutral currents are the neutral members of the isotriplets containing the charged weak currents. From neutrino cross sections alone we derive bounds for the neutral current coupling constants. These bounds supplement the known constraints from inclusive scattering in a very useful way. More specific assumptions about the isoscalar neutral current are also considered. We discuss the resulting bounds using the existing neutrino data. Finally, it is shown that with the advent of antineutrino data for single pion production the neutral current coupling constants will be determined uniquely.     

On hole and electron scattering effects in energy band investigations by photon-enhanced field emission

The detection of change in photo-modulated field-emission currents enables one to define not only the levels from which the excited electrons are ejected, but also on which levels the holes are created. The hole recombination processes determine the necessary absorbed light intensity in order to get measurable reduction in that part of field emission current which is due to electrons with energies between E_i and E₁ + ΔE below the Fermi level. The analysis gives an estimate of the order of the parameters involved as a function of hole scattering type, band characteristics, etc. It is shown that 10³ – 10⁴ W/cm² absorbed light intensities are necessary. The problem of separating the effects of excited electron scattering from those of the energy variations of the tunneling operator is discussed.     

Field-assisted spin-polarized electron transport through a single quantum well with spinben orbit coupling

We have investigated theoretically the field-driven electron transport through a single-quantum-well semiconductor heterostructure with spin-orbit coupling. The splitting of the asymmetric Fano-type resonance peaks due to the Dresselhaus spin-orbit coupling is found to be highly sensitive to the direction of the incident electron. The splitting of the Fano-type resonance induces the spin-polarization dependent electron current. The location and the line shape of the Fano-type resonance can be controlled by adjusting the energy and the direction of the incident electron, the oscillation frequency, and the amplitude of the external field. These interesting features may be used to devise tunable spin filters and realize pure spin transmission currents.     

Field-assisted spin-polarized electron transport through a single quantum well with spin-orbit coupling

We have investigated theoretically the field-driven electron transport through a single-quantum-well semiconductor heterostructure with spin-orbit coupling.The splitting of the asymmetric Fano-type resonance peaks due to the Dresselhaus spin-orbit coupling is found to be highly sensitive to the direction of the incident electron.The splitting of the Fano-type resonance induces the spin-polarization dependent electron current.The location and the line shape of the Fano-type resonance can be controlled by adjusting the energy and the direction of the incident electron,the oscillation frequency,and the amplitude of the external field.These interesting features may be used to devise tunable spin filters and realize pure spin transmission currents.     

Current control in periodically driven quantum dots using nonadiabatic double crossing

The purpose of this Letter is to propose a method for controlling electron currents on quantum dots driven by an oscillating electric field. The effects of nonadiabatic transition on time-averaged currents are theoretically studied in dot systems where energy levels exhibit a double crossing within one period of the driving field. The current is enhanced or suppressed as a result of the constructive or destructive interference between different transition paths at a double crossing. The current also depends on the number of dots because of the presence of dot-lead coupling.     

Superconductivity at dielectric pairing of spatially separated quasiparticles

A new mechanism of the superconductivity based upon the pairing of the spatially separated electrons and holes due to their Coulomb attraction is presented. In the systems considered the phase of the order parameter is not fixed and therefore the charges superfluid flow connected with non-dissipative electrical currents is possible. The critical temperature T_c may be high (? 100°K). The critical current has been found. The diamagnetic response and the coefficient of the electromagnetic wave absorption in the systems have been calculated. The possible experiments are discussed.     

Microstructure and flux distribution in both pure and carbon-nanotube-embedded Bi2Sr2CaCu2O8+δ superconductors

In order to improve pinning properties of bulk Bi₂Sr₂CaCu₂O_8+δ (Bi-2212) materials, samples of both pure Bi-2212 and Bi-2212 with carbon nanotubes embedded (CNTE Bi-2212) have been prepared by partial-melting processing. The preparation conditions are chosen so that a significant fraction of carbon nanotubes can be successfully embedded in the material, as indicated by thermogravimetric analysis. The microstructure and composition of non-superconducting second phases are found to be different in these two types of samples. By means of magneto-optical (MO) imaging, flux distributions in both types of samples are investigated up to T=77 K. The MO investigation reveals the propagation of a flux front in both pure and CNTE Bi-2212, showing that there is a strong coupling between grains (clusters) which enables the flow of inter-granular currents. This grain coupling persists in our field range of ±180 mT. In bulk non-textured ceramic high-T_c superconductors, the flux fronts caused by currents flowing through the entire sample are observed for the first time. Intra-granular current densities are obtained from the images as a function of grain size. The MO investigations have revealed the differences in the current densities caused by the presence of carbon nanotubes, showing that the carbon nanotubes are indeed functioning like columnar defects produced by heavy-ion irradiation. The increase of the flux penetration field is also a manifestation of the increase of the transport current density in the CNTE Bi-2212. The superconducting properties in our samples are very well reproducible.     

Weinberg's gauge model for weak and electromagnetic interactions with Han-Nambu quarks

A version of the Weinberg model is constructed using Han-Nambu quarks, in order to obtain the conventional classification of hadrons. The use of Han-Nambu quarks leads to a suppression of neutrinos coupled to the ΔS = 0 hadronic current, while the coupling to neutral leptonic currents remains as in Weinberg's theory.     

Improved photonic crystal directional coupler with short length

We investigate a photonic crystal (PC) waveguide coupler which is formed by two closely spaced linear waveguides in a two-dimensional triangular lattice of air holes. Our study shows that shifting one row of the air holes between the waveguides affects the dispersion curves of the guided modes and if the triangular lattice of air holes between the waveguides is replaced by a rectangular lattice, this modification results in an ultra-short coupling structure with coupling length less than 3a, where a is the lattice constant. Also, we investigate the effect of changing the radii of air holes that are adjacent to or between the waveguides on the coupling length and show that increasing the radius of air holes between the waveguides decreases the coupling length. We analyze the output spectrum of an ultra-short channel drop filter designed based on this structure.     

L波段相对论速调管输入腔研究

 介绍L波段强流相对论速调管放大器输入腔的数值分析、新型的结构设计和实验调试及其应用等方面的情况, 对输入腔进行了特别的结构设计, 解决了大耦合孔对谐振腔参数的不良影响, 设计符合RKA应用的要求。均匀电子束经过输入腔后, 得到了约6％的最大基波调制电流深度。     

Coherent quantum 1/f noise from electron-phonon interactions

A physical electron, or any other current carrier in condensed matter with piezoelectric electron-phonon coupling, is best described as a system with a certain energy uncertainty. The latter is caused by the coherent state of the transversal phonon modes. The energy, and the number of virtual phonons included into the physical particle are uncertain. This energy uncertainty leads to current fluctuations which are calculated here and shown to have a 1/f spectrum.     

Mesoscopic effects in adiabatic spin pumping

We show that temporal shape modulations (pumping) of a quantum dot in the presence of spin-orbital coupling lead to a finite dc spin current. Depending on the strength of the spin-orbit coupling, the spin current is polarized perpendicular to the plane of the two-dimensional electron gas, or has an arbitrary direction subject to mesoscopic fluctuations. We analyze the statistics of the spin and charge currents in the adiabatic limit for the full crossover from weak to strong spin-orbit coupling.     

Josephson effect in supercondutor/ferromagnetic semiconductor/superconductor junctions

Using a general expression for dc Josephson current, we study the Josephson effect in ballistic superconductor (SC)/ferromagnetic semiconductor (FS)/SC junctions, in which the mismatches of the effective mass and Fermi velocity between the FS and SC, spin polarization P in the FS, as well as strengths of potential scattering Z at the interfaces are included. It is shown that in the coherent regime, the oscillatory dependences of the maximum Josephson current on the FS layer thickness L and Josephson current on the macroscopic phase difference φ for the heavy and light holes, resulting from the spin splitting energy gained or lost by a quasiparticle Andreev-reflected at the FS/SC interface, are much different due to the different mismatches in the effective mass and Fermi velocity between the FS and the SC, which is related to the crossovers between positive (0) and negative (π) couplings or equivalently 0 and π junctions. Also, we find that, for the same reason, Z and P are required not to surpass different critical values for the Josephson currents of the heavy and light holes. Furthermore, it is found that, for the dependence of the Josephson current on φ, regardless of how L,Z, and P change, the Josephson junctions do not transit between 0 and π junctions for the light hole.     

Current mirror effect and correlated cooper-pair transport in coupled arrays of small Josephson junctions

We have measured the transport properties of capacitively coupled one-dimensional arrays of small aluminum Josephson junctions. Under suitable biasing conditions, the samples act as current mirrors; the currents in the two arrays couple to each other and become equally large. The coupling is found both for currents flowing in the same direction and in opposite directions. We have also measured the time dependent fluctuations of the currents in the two arrays, and we find a strong correlation or anticorrelation of the fluctuations depending on the relative direction of the two currents.