Influence of a Time-Dependent Applied Electric Field on the Photocurrent in Photorefractive Materials

From the differential material rate equations, we found an additional term to the usual expression of the photocurrent, which appears only when a time-dependent external electric field is applied. This term influences the photocurrent in the material. We applied our equations to a Bi₁₂SiO₂₀ sample, under an applied sinusoidal electric field. This sample is illuminated with an oscillating interference pattern formed by two plane light waves, one of which is phase-modulated with frequency ω. We found good agreement with experimental results. Besides, for this case, our prediction for the photocurrent is better than the usual prediction of the photocurrent for low values of ω.     

Hamiltonian treatment of the spherically symmetric einstein-Yang-Mills system

A canonical formalism of the dynamics of interacting spherically symmetric Yang-Mills and gravitational fields is presented. The work is based on Dirac's technique for constrained hamiltonian systems. The gauge freedom of the Yang-Mills field is treated in the same footing with the coordinate transformation freedom of the gravitational field. In particular, the fixation of coordinates and the fixation of the internal gauge are achieved by totally similar techniques. Two classes of spherically symmetric motions are considered: (i) the class for which the Yang-Mills potentials themselves are spherically symmetric (“manifest spherical symmetry”). In this case the results are valid for an arbitrary gauge group; and (ii) the class for which, in the SO(3) gauge group, a rotation in physical space is compensated by a rotation of equal magnitude but opposite direction in isospin space (“spherical symmetry up to a gauge transformation”). For manifest spherical symmetry the problem amounts to effectively dealing with an abelian gauge group and the most general solution of the field equations turns out to be the Reissner-Nordström metric with a Coulomb field. For spherical symmetry up to a gauge transformation the problem is more interesting. the formalism contains then, besides the gravitational variables, three pairs of functions of the radial coordinate that describe the degrees of freedom of the Yang-Mills field. Two pairs of these functions can be combined into a complex field ψ and its conjugate. The hamiltonian is then invariant under r-dependent rotations in the complex ψ-plane. The third degree of freedom plays the role of a compensating field associated with this invariance under localized U(l) rotations. The compensating field can always be brought to zero by a gauge transformation. After this is done the gauge is completely fixed but the problem remains invariant under position independent rotations in the ψ plane. Static solutions of the field equations in this gauge are of the form ψ(r) = (r) exp (iΘ) with Θ independent of position. The particular case Θ = 0 corresponds to the Wu-Yang ansatz. A nontrivial static solution can be found in closed form. The Yang-Mills field is of the generalized Wu-Yang type with an extra electric term, and the metric is the Reissner-Nordström one. It is pointed out that a Higgs field can be easily introduced in the formalism. The addition of the Higgs field does not destroy the invariance of the Hamiltonian under r-dependent rotations in the ψ-plane. The conserved quantity associated with the invariance under ψ → exp (i(const))ψ coincides with the electric charge as defined by 't Hooft in a more general context.     

Stark effect on an exciton or impurity in a finite width quantum wire with an electric field confined in a finite region

We study the electronic properties of a quantum system formed by two charged particles moving in a quantum wire (QW) with finite width σ and interacting through a Coulomb potential under an uniform electric field E applied over a spatially confined region of thickness 2a (-a<z<a). The number of electronic states of this finite width system is twice the number of the less realistic system with σ=0.     

Non-linear response and electron-electron interactions in mesoscopic metal rings

A time-dependent electric field gives rise to a stationary non-equilibrium current I ⁽²⁾ around a mesoscopic metal ring threaded by a magnetic flux. We show that this current, which is proportional to the intensity of the field, is closely related to the exchange part of the interaction contribution to the equilibrium persistent current, and that the corresponding non-linear conductivity directly measures the weak localization correction to the polarization. We explicitly calculate the disorder average of I ⁽²⁾ in the diffusive regime as function of the frequency of the electric field and the static flux piercing the ring, and suggest an experiment to test our theory. Received: 5 September 1997 / Accepted: 4 November 1997     

Dependence of electromagnetic form factors of hadrons on light-cone frames

A constituent quark model is developed for an arbitrary light-cone direction so that the light-front time is x_LF⁺ = ω · x with a constant lightlike four-vector ω. Form factors are obtained from free one-body electromagnetic current matrix elements. They are found to be ω-independent for spin-0 mesons, nucleons and the Λ-hyperon, while there is an ω-dependence for spin-1 systems like the deuteron.     

Carrier dynamics and dispersive terahertz Bloch gain in semiconductor superlattices

We have directly determined the spectral shape of the complex conductivities of Bloch oscillating electrons by using time-domain terahertz (THz)-electrooptic sampling technique and presented an experimental evidence for a dispersive Bloch gain in superlattices. This unique dispersive gain without population inversion arises from a non-classical nature of Bloch oscillations; i.e., the phase of the Bloch oscillation is shifted by π/2 from that of the semi-classical charged harmonic oscillation when driven by the same AC field. By increasing the bias electric field, the gain bandwidth reached in our particular sample.     

Casimir Force on a Dielectric Cylinder

The Casimir surface force density on a compact material cylinder of radius a is calculated, at zero temperature. A Green function approach is followed. The general theory is formulated so as to hold for arbitrary permittivities ε(ω) and permeabilities μ(ω), whereas when it comes to explicit calculations the condition ε(ω) μ(ω) = 1 is assumed to hold. A simple dispersion relation is chosen, implying a high frequency cutoff ω₀. The theory yet diverges, at high angular momenta. Divergences of this sort usually appear whenever there are curved boundaries present. On physical grounds an angular momentum cutoff m₀ can be introduced, being of order ω₀a. A semi-quantitative calculation of the force thereby becomes possible. The calculated force is attractive.     

Li里得堡原子在振荡场中的回归谱研究

文中应用扩展的半经典闭合轨道理论计算了Li原子在平行电磁场加一个振荡电场中的回归谱.与相同情况下平行电磁场中的回归谱比较,当附加含时振荡场时,静外场中的吸收光谱的强度被很大程度地减弱.另外,分别讨论了回归谱强度随振荡场振幅以及频率的变化规律.     

Flow overshooting in crossing flow of lattice gas

We study the lattice gas flow of two components of biased-random walkers at a crossing under a periodic boundary. The lattice gas mixture consists of two components of particles (walkers) in which one component of particles moves north and the other component of particles moves east. The current (flow) increases with ρ_x (density of the east-bound particles) at low density and displays overshooting at an intermediate density. The flow overshooting occurs only for a certain range of ρ_y (density of the north-bound particles). Then clogging occurs and the current saturates. Furthermore, when the density is high, the current decreases with increasing density. The overshooting shown in the current-density (fundamental) diagram is due to the formation of an unstable oscillating jam just before clogging occurs. It is shown that flow overshooting does not occur in unidirectional flow through a porous medium but occurs in unidirectional flow through a group of Brownian particles.     

Spectrum Invariance for a Particular Class of Optical Fields Propagated in Far Zone

This paper describes invariance of the normalized optical spectrum for a particular class of optical fields propagated in the far zone from a secondary, spatially, partially coherent source. The optical field across the secondary planar source is inhomogeneous to give the complex degree of spectral coherence such that μ = h(αω(ρ₁–ρ₂))exp(iɛω(ρ₁²–ρ₂²)), where α, ɛ are constants, ω is optical frequency, and ρ₁, ρ₂ denote two points in the secondary source. This expression for μ is the same as obtained in the Fresnel zone from a primary, spatially incoherent source. The invariance law does not hold for the spectrum of the light propagated from the primary source.     

Near extremal Schwarzschild-de Sitter black hole area spectrum from quasi-normal modes

Using the quasi-normal modes frequency of near extremal Schwar-zschild-de Sitter black holes, we obtain area and entropy spectrum for the black hole horizon. By using Bohr-Sommerfeld quantization for an adiabatic invariant I = ∫dEω(E), where E is the energy of the system and ω(E) is the vibrational frequency, we arrive at an equally spaced mass spectrum. In the other terms, we extend directly the Kunstatter’s approach kun [6] to determine mass and entropy spectrum of near extremal Schwarzschild-de Sitter black holes which are asymptotically de Sitter rather than asymptotically flat. We show the mass and area spectrum is equally spaced only for a fixed l. For different l there are multiplets with different values of spacing.     

Studies of the third-order nonlinear optical susceptibility for InxGa1−xN/GaN cylinder quantum dots

The resonant third-order susceptibilities at various directions (both parallel and vertical to Z-axis) in self-assembled quantum dots (QDs) have been investigated. The nonlinear susceptibilities associated with the intraband transition in the conduction band are theoretically calculated for wurtzite In_xGa_1−xN/GaN-strained cylinder QDs. The confined wave functions and energies of electrons in the dots have been calculated in the effective-mass approximation by solving the 3D Schrödinger equation, in which a strong built-in electric field effect due to the piezoelectricity and spontaneous polarization has been taken into account. Furthermore, it is shown that the magnitude and the resonant position of the nonlinear susceptibility χ⁽³⁾(3ω) strongly depend on the dots’ size as well as size distribution.     

AC-hopping conductance of self-organized Ge/Si quantum dot arrays

Dense (n=4×10¹¹ cm^-2) arrays of Ge quantum dots in a Si host were studied using attenuation of surface acoustic waves (SAWs) propagating along the surface of a piezoelectric crystal located near the sample. The SAW magneto-attenuation coefficient, ΔΓ=Γ(ω,H)-Γ(ω,0), and change of velocity of SAW, ΔV/V=(V(H)-V(0))/V(0), were measured in the temperature interval T=1.5–4.2 K as a function of magnetic field H up to 6 T for the waves in the frequency range f=30–300 MHz. Based on the dependences of ΔΓ on H, T and ω, as well as on its sign, we believe that the AC conduction mechanism is a combination of diffusion at the mobility edge with hopping between localized states at the Fermi level. The measured magnetic field dependence of the SAW attenuation is discussed based on existing theoretical concepts.     

Comment on “Helmholtz Theorem and the V-Gauge in the Problem of Superluminal and Instantaneous Signals in Classical Electrodynamics,” by A. Chubykalo et al.

Fundamental errors in a paper by Chubykalo et al. [2] are highlighted. Contrary to their claim that “… the irrotational component of the electric field has a physical meaning and can propagate exclusively instantaneously,” it is shown that this instantaneous component is physically irrelevant because it is always canceled by a term contained in the solenoidal component. This result follows directly from the solution of the wave equation that satisfies the solenoidal component. Therefore the subsequent inference of these authors that there are two mechanisms of transmission of energy and momentum in classical electrodynamics, one retarded and the other one instantaneous, has no basis. The example given by these authors in which the full electric field of an oscillating charge equals its instantaneous irrotational component on the axis of oscillations is proven to be false.     

Quantum Hall effect in restricted geometries

The confinement of electrons in narrow quasi-two-dimensional conducting channels, modelled with a parabolic well, leads to asymmetric Hall plateaus about complete Landau-level fillings and to saw-toothed oscillations of the dc resistivity _xx as a function of the magnetic field B. The peaks in _xx are displaced to lower B and drastically reduced from their wide-channel values. The peak values of _xx increase with increasing channel width. The corrections to σ_yx for finite channel widths and the response to oscillating electric fields are evaluated.     

Electromagnetic dipole moments and radiative decays of particles from exchange of fermionic unparticles

We construct the propagator for a free fermionic unparticle field from basic considerations of scale and Lorentz invariance. The propagator is fixed up to a normalization factor which is required to recover the result of a free massless fermion field in the canonical limit of the scaling dimension. Two new features appear compared to the bosonic case. The propagator contains both γ and non-γ terms, and there is a relative phase of π/2 between the two in the time-like regime for arbitrary scaling dimension. This should result in additional interference effects on top of the one known in the bosonic case. The non-γ term can mediate chirality flipped transitions that are not suppressed by a light fermion mass but are enhanced by a large bosonic mass in loops, compared to the pure particle case. We employ this last feature to set stringent bounds on the Yukawa couplings between a fermionic unparticle and an ordinary fermion through electromagnetic dipole moments and radiative decays of light fermions.     

Photoluminescence and electric conductivity in germanium at low temperatures

We have studied the exciton and electron-hole droplet (EHD) luminescence in optically irradiated germanium at temperatures between 1.8 and 4.2 K in the presence of an electric field. Simultaneously the electric conductivity was measured. The sample material was high-purity Ge (N _A –N _D =7·10¹⁰ cm^–3) andp-doped Ge withN _A =3·10¹⁴ cm^–3. In the high-purity Ge samples the exciton and EHD-luminescence intensity decreased nearly linearly as a function of the applied electric current, whereas the dependence upon the electric field was more complicated. Our results could be explained by a model in which carrier annihilation at the contacts following a rapid drifting process plays a dominant role (drift model). In thep-doped Ge samples the current-dependence of the luminescence intensity was qualitatively similar. However, here the drift model is not strictly valid any more because of the reduced carrier mobility and the generation of additional carriers by impurity impact ionization. During variation of the electric field, the luminescence intensity and the electric current show hysteresis. Here the capture of the moving carriers by the EHD appears to play an important role, in addition to the EHD-nucleation process.     

Two-electron absorption in a biased quantum well

Linear light absorption of 2D electrons confined within a biased quantum well is studied theoretically. We demonstrate that for light polarization perpendicular to the 2D plane, in addition to conventional absorption peak at frequency ω≈Δ, where Δ is the intersubband energy distance, there exists a peak around a double frequency ω≈2Δ. This additional peak is entirely due to electron–electron interactions, and corresponds to excitation of two electrons by one photon. The magnitude of two-electron absorption is proportional to U², where U is the applied bias.     

On conformal Killing 2-form of the electromagnetic field

Let D:C^∞Λ^pM→C^∞(T^*MΛ^pM) be the first order linear differential operator on an n-dimensional (1≤p≤n−1) pseudo-Riemannian manifold (M,g). We have by the representation theory of orthogonal group, that the tangent bundle of this operation decomposes into the orthogonal and irreducible sum of forms of degree p+1 (which gives the exterior differential d), the forms of degree p−1 (defining the codifferential d^*) and the trace-free part of the partial symmetrization (the corresponding first order operator is denoted by D). The general forms in the kernel of D are closely related to conformal Killing vector fields, called conformal Killing p-forms, while those in kernel of d are called closed conformal Killing p-forms or, according to another terminology, planar p-forms. In particular an arbitrary planar 1-form ω is dual (by g) to the special concircular vector field ξ. We consider some local properties for the closed conformal Killing p-forms. As an application we present examples of decomposition into irreducible components for the electromagnetic field 2-form ω and its covariant derivative in four-dimensional space–time. In particular, we prove that the energy–momentum tensor T of the electromagnetic field is a symmetric conformal Killing tensor if the electromagnetic field 2-form ω is a conformal Killing form.     

Al and Cu NMR study in UCu5Al

We have studied the microscopic properties of the tetragonal UCu₅Al Kondo compound by ²⁷Al and ^63,65Cu NMR in the paramagnetic state. NMR and susceptibility measurements performed on the powdered sample, but oriented along the applied field, showed χ>χ. Plots of K(T) against χ(T) at temperatures T≥100 K yield the transferred hyperfine fields of +5.9 kOe/μ_B for ²⁷Al nuclei, and +5.3 and −7.0 kOe/μ_B for ⁶⁵Cu nuclei in crystallographically inequivalent Cu(2) and Cu(1) sites, respectively. The Knight shift vs. susceptibility plots for T<100 K exhibit a deviation from the linear behaviour (absolute values of shifts become smaller than expected). We attribute this finding to the crystalline electric field effect in similar way as it was reported for several Ce-based compounds. The random distribution of the Al and Cu(2) atoms in the crystal lattice we consider as a reason of an unusual broadening of the NMR spectra, particularly at low temperatures.