Recombination of an electron-hole plasma in silicon under the action of femtosecond laser pulses

Experimental data are obtained on the dynamics of conduction-electron relaxation at the stage preceding the melting of a silicon surface layer. The energy of a quantum of probe radiation is smaller than the band gap, making it possible to obtain information about the electron-phonon relaxation processes for an electron concentration of ～ 10²¹ cm^?3 in the conduction band.     

Dynamics in molecular crystals: An application of proton NMR to selectively protonated biphenyl. Do the rings flip together or independently?

The synthesis of a specific isotopomer, C₆D₄H(ortho)-H(ortho)D₄C₆ of biphenyl is reported. The intramolecular dipolar coupling of the protons leads to a well-resolved single-crystal proton nuclear magnetic resonance (NMR) spectrum and allows one to study the dynamics of the phenyl rings in a unique way. At room temperature and above, the most conspicuous dynamical mode consists of 180° ring flips. The present data together with previous measurements of the total flip rate allow us to conclude that the rings flip almost exclusively independently of each other. Between the incommensurate (IC) phase transition of biphenyl at 38 K andT=250 K, the prominent namical mode consists of oscillatory twists ϕ(t) of the two rings. The data allow us to infer the mean square, (φ²), of these twists. (φ²) is found to grow linearly withT for 50<T<200 K. From the slope of (φ²) vs.T the frequency (the wave number[(v)\tilde]\tilde v) is derived. The result is[(v)\tilde] = 20\tilde v = 20 cm⁻¹. ForT<38 K, the spectra give direct evidence of the IC phase transition and its nature (stripelike rather than quiltlike). The temperature dependence of the magnitude of the order parameter of the IC phase is obtained.     

On two functionals connected to the Laplacian in a class of doubly connected domains in space-forms

LetB ₁ be a ball of radiusr ₁ inS ⁿ (ℍⁿ), and letB ₀ be a smaller ball of radiusr ₀ such thatB ₀ ⊂B ₁ . ForS ⁿ we considerr ₁ π. Let u be a solution of the problem- δm = 1 in Ω :=B ₁ /B ₀ vanishing on the boundary. It is shown that the associated functionalJ (Ω) is minimal if and only if the balls are concentric. It is also shown that the first Dirichlet eigenvalue of the Laplacian on Ω is maximal if and only if the balls are concentric.     

Sequent calculi for propositional star-free likelihood logic

We consider classical, multisuccedent intuitionistic, and intuitionistic sequent calculi for propositional likelihood logic. We prove the admissibility of structural rules and cut rule, invertibility of rules, correctness of the calculi, and completeness of the classical calculus with respect to given semantics.__________Published in Lietuvos Matematikos Rinkinys, Vol. 45, No. 1, pp. 3–21, January–March, 2005.     

Doppler cooling a single Ca+ ion with a violet extended-cavity diode laser

We present a scheme for employing a violet extended-cavity diode laser in experiments with single, trapped ions. For this the grating-stabilised laser is spatially and spectrally filtered and referenced to a Fabry–Pérot cavity. We measure an upper limit to the line width by observing a 305-kHz FWHM beat note with the second harmonic of a titanium sapphire laser. The laser is subsequently used to optically cool a single ⁴⁰Ca⁺ ion close to the Doppler limit. PACS 03.67.Lx; 32.80.Pj; 42.55.Px     

Energy spectrum of <Emphasis Type="Italic">p</Emphasis> electrons in a plane crystal lattice and in a strong magnetic field

Harper equations are derived for a _px, _py electronic system. Analysis is carried out for extreme points of the quasi-continuous spectrum in the cases when the number of magnetic flux quanta through a unit cell is a rational number and calculations are made for square and triangular lattices as well as for a honeycomb lattice with two nonequivalent atoms. The possibility of application of the results for explaining the fractional Hall effect is considered.     

Wave function mapping of self-assembled quantum dots by capacitance spectroscopy

We use frequency-dependent capacitance–voltage spectroscopy to study the dynamic charging of self-assembled InAs quantum dots. With increasing frequency, the AC charging becomes suppressed, beginning with the low-energy states. By applying an in-plane magnetic field, we generate an additional magnetic confinement that alters the tunneling barrier and hence the charging dynamics. In traveling through the potential barrier, the electrons acquire an additional momentum k₀, proportional to the magnetic field B. As the tunneling is enhanced, when k₀ matches the maximum of the electronic wave function Ψ (in momentum representation), we are able to map out the shape of Ψ by varying B.     

Longwave Marangoni convection in an inhomogeneously heated liquid

The supercritical Marangoni convection has been studied in a plane-parallel liquid layer, bounded by a free deformable gas-liquid interface from above and by a low-heat-conductivity wall from below, occurring under conditions of inhomogeneous heating in the horizontal plane. In a longwave approximation with a small inhomogeneity of heat flux, the process is described by a system of two-dimensional nonlinear equations for the temperature perturbations, vorticity, and free surface deformation. The concept of quasiequilibrium, implying stability of long-range flows, is introduced, which allows the inhomogeneous heat flux to be modeled by a step function. The linear stability is analyzed in the cases of planar and axisymmetric heat fluxes. The boundaries of stability of the convection regimes are determined on the plane of parameters characterizing the degree of supercriticity inside a heated spot and the depth of damping outside the spot. For an axisymmetric spot, the domains of stability with respect to perturbations for various azimuthal numbers are established.     

Optical spectroscopy of single InAs/InP quantum dots around λ=1.55 μm

We discuss the preparation and spectroscopic characterisation of a single InAs/InP quantum dot suitable for long-distance quantum key distribution applications around λ=1.55 μm. The dot is prepared using a site-selective growth technique which allows a single dot to be deposited in isolation at a controlled spatial location. Micro-photoluminescence measurements as a function of exciton occupation are used to determine the electronic structure of the dot. Biexciton emission, shell filling and many-body re-normalization effects are observed for the first time in single InAs/InP quantum dots.     

Ultrafast carrier dynamics in ferromagnetic InGaMnAs

We have carried out an ultrafast time-resolved differential reflectivity study of a ferromagnetic semiconductor InGaMnAs and made a systematic comparison with low-temperature grown and high-temperature grown InGaAs reference films. Very short carrier lifetimes (2 ps) were observed in InGaMnAs and the low-temperature grown InGaAs film, but not in the high-temperature grown InGaAs film. We attribute the short lifetimes to carrier trapping by mid-gap states introduced during low-temperature MBE growth. Furthermore, at long times, we observed periodic oscillations in the differential reflectivity signal with period 20 ps, which we interpret as coherent acoustic phonons.