Photoinjection of hydrogen and the nature of a giant shift of the fundamental absorption edge in highly disordered V2O5 films.

Production of atomic photochemical hydrogen under the action of light and its subsequent injection into transition metal oxides has yielded numerous interesting results. Here we report on the mechanism of the photo-induced hydrogen transfer between adsorbed organic molecules and the surface of highly disordered V(2)O(5) films. We have managed to carry out the photoinjection of hydrogen into the V(2)O(5) films at very low temperatures, which is very important both for investigations of the reaction mechanism and for the optical properties of the V(2)O(5) films.

EPR line-shape anisotropy and hyperfine shift of methyl radicals in solid Ne, Ar, Kr, and p-H2 gas matrices.

Earlier studies have shown that pure quantum mechanical effects on the "light" methyl radical at low temperature minimize the anisotropy of CW EPR spectra to a high resolution character, and new experiments under different conditions display a small additional electron paramagnetic resonance (EPR) line-shape anisotropy. In this work the effects of the solid H(2) quantum matrix and three other typical solid noble-gas matrices on the spectral anisotropy and the hyperfine interaction (hfi) constant of trapped methyl radicals presented as matrix shifts (deviation from the value in free space) are studied in some detail. Experimental EPR data at liquid-He temperatures were used to explore the dependence of the additional broadening and the spectral anisotropy of the hosted methyl radicals and to correlate the experimental spectral anisotropy to the matrix-radical interaction.

Interpreting the effect of vaccination on steady state infection in animals challenged with Simian immunodeficiency virus.

A representative vaccinated macaque challenged with SIVmac251 establishes a persistent infection with a lower virus load, higher CTL frequencies, and much higher helper cell frequencies, than a representative control animal. The reasons for the difference are not fully understood. Here we interpret this effect using a mathematical model we developed recently to explain results of various experiments on virus and CTL dynamics in SIV-infected macaques and HIV-infected humans.

Model with two types of CTL regulation and experiments on CTL dynamics.

Recently, we developed a mathematical model of interaction between the HIV and the immune system to match various dynamic experiments carried out in HIV-infected humans and SIV-infected macaques. The model includes helper cell-dependent and helper cell-independent cytotoxic lymphocytes (CTLs) and predicts two stable steady states, a state with a high virus load and few helper cells, and another state with a low virus load and many helper cells. Here we upgrade the model to take into account recent reports on the link between the activation status of infected cells and their ability to produce virus, the effect of helper cells at the time of priming on CTL differentiation, and virus dynamics in unvaccinated macaques with a broad genetic background acutely infected with SIVmac251.

Coulomb crystals in the magnetic field.

The body-centered-cubic Coulomb crystal of ions in the presence of a uniform magnetic field is studied using the rigid electron background approximation. The phonon mode spectra are calculated for a wide range of magnetic-field strengths and for several orientations of the field in the crystal. The phonon spectra are used to calculate the phonon contribution to the crystal energy, entropy, specific heat, Debye-Waller factor of ions, and the rms ion displacements from the lattice nodes for a broad range of densities, temperatures, chemical compositions, and magnetic fields.

Comparative magneto-photoluminescence study of ensembles and of individual InAs quantum dots.

We report on magneto-photoluminescence studies of InAs/GaAs quantum dots (QDs) of considerably different densities, from dense ensembles down to individual dots. It is found that a magnetic field applied in Faraday geometry decreases the photoluminescence (PL) intensity of QD ensembles, which is not accompanied by the corresponding increase of PL signal of the wetting layer on which QDs are grown. The model suggested to explain these data assumes considerably different strengths of suppression of electron and hole fluxes by a magnetic field.

Hypersonic modulation of light in three-dimensional photonic and phononic band-gap materials.

The elastic coupling between the a-SiO2 spheres composing opal films brings forth three-dimensional periodic structures which besides a photonic stop band are predicted to also exhibit complete phononic band gaps. The influence of elastic crystal vibrations on the photonic band structure has been studied by injection of coherent hypersonic wave packets generated in a metal transducer by subpicosecond laser pulses. These studies show that light with energies close to the photonic band gap can be efficiently modulated by hypersonic waves.

Nuclear spin nanomagnet in an optically excited quantum dot.

Linearly polarized light tuned slightly below the optical transition of the negatively charged exciton (trion) in a single quantum dot causes the spontaneous nuclear spin polarization (self-polarization) at a level close to 100%. The effective magnetic field of spin-polarized nuclei shifts the optical transition energy close to resonance with photon energy. The resonantly enhanced Overhauser effect sustains the stability of the nuclear self-polarization even in the absence of spin polarization of the quantum dot electron.

Stabilization of the electron-nuclear spin orientation in quantum dots by the nuclear quadrupole interaction.

The nuclear quadrupole interaction eliminates the restrictions imposed by hyperfine interaction on the spin coherence of an electron and nuclei in a quantum dot. The strain-induced nuclear quadrupole interaction suppresses the nuclear spin flip and makes possible the zero-field dynamic nuclear polarization in self-organized InP/InGaP quantum dots. The direction of the effective nuclear magnetic field is fixed in space, thus quenching the magnetic depolarization of the electron spin in the quantum dot.

Magnetic-field-induced second-harmonic generation in semiconductor GaAs.

We show that application of a magnetic field induces optical second-harmonic generation (SHG) in GaAs. This phenomenon arises from field-induced symmetry breaking causing new optical nonlinearities. A series of narrow SHG lines is observed in the spectral range from 1.

Hanle effect driven by weak localization.

The influence of weak localization on the Hanle effect in a two-dimensional system with a spin-split spectrum is considered. We show that weak localization drastically changes the dependence of a stationary spin polarization S on an external magnetic field B. In particular, the nonanalytic dependence of S on B is predicted for III-V-based quantum wells grown in the [110] direction and for the [100]-grown quantum wells having equal strengths of Dresselhaus and Bychkov-Rashba spin-orbit coupling.

Suppression of Dyakonov-Perel spin relaxation in high-mobility n-GaAs.

We report a large and unexpected suppression of the free electron spin-relaxation in lightly doped n-GaAs bulk crystals. The spin-relaxation rate shows a weak mobility dependence and saturates at a level 30 times less than that predicted by the Dyakonov-Perel theory. The dynamics of the spin-orbit field differs substantially from the usual scheme: although all the experimental data can be self-consistently interpreted as a precessional spin-relaxation induced by a random spin-orbit field, the correlation time of this random field, surprisingly, is much shorter than, and is independent of, the momentum relaxation time determined from transport measurements.

Effect of the electron gas polarizability on the specific heat of phonons in Coulomb crystals.

The effect of the background polarizability on the thermodynamic properties of a Coulomb crystal of ions is studied. The response of electrons is treated using the Thomas-Fermi (TF) and random phase approximations (RPA). For the case of ions fixed at their lattice sites, the energy of bcc and fcc crystals is calculated to first order in the screening parameter (kappa(TF)a)(2) (kappa(TF) is the TF wave number and a is the ion sphere radius).

Ultrafast quenching of the antiferromagnetic order in FeBO3: direct optical probing of the phonon-magnon coupling.

The dynamics of the optically induced phase transition from the antiferromagnetic to the paramagnetic state in FeBO3 is observed using a pump-probe magneto-optical Faraday technique employing 100 fs laser pulses. At the pump energy of 1.55 eV phonon-assisted transitions dominate in the absorption of light and ultrafast heating of the lattice occurs.

Manipulation of the spin memory of electrons in n-GaAs.

We report on the optical manipulation of the electron spin relaxation time in a GaAs-based heterostructure. Experimental and theoretical study shows that the average electron spin relaxes through hyperfine interaction with the lattice nuclei, and that the rate can be controlled by electron-electron interactions. This time has been changed from 300 ns down to 5 ns by variation of the laser frequency.

Anomalous acoustoelectric effect in La0.67Ca0.33MnO3 films.

We have studied the acoustoelectric (AE) effect produced by surface acoustic waves (SAW) in a monolithic layered structure, composed of a piezodielectric LiNbO3 substrate and a La0.67Ca0.33MnO3 film.

Evidence for a quasi-two-dimensional proton glass state in Cs5H3(SO4)(4); xH(2)O Crystals.

We describe damping of hypersonic and ultrasonic longitudinal acoustic (LA) phonons in crystals of Cs 5H (3)(SO (4))(4);xH 2O (PCHS) between 100 and 360 K. The damping of LA phonons exhibits strong dispersion caused by relaxation processes in the region of transformation into the glasslike phase (T(g) approximately 260 K). Near T(g) the damping of ultrasonic phonons propagating in the basal plane reflects the cooperative freezing of acid protons.

Qualitative modification of the high energy atomic photoionization cross section.

It is demonstrated that the nonrelativistic high energy omega-->infinity behavior of the photoionization cross section of an nl atomic subshell, sigma(nl)(omega), for l>0 is independent of l and is given by sigma(nl)(omega) approximately 1/omega(9/2), rather than the previously generally accepted sigma(nl)(omega) approximately 1/omega(l+7/2). Furthermore, for l = 1, although the exponent does not change, the coefficient is significantly altered. This modification of sigma(nl)(omega) is due to the interchannel interaction between ns photoionization channels and l not equal0 channels in the atom.

Manifestation of intrinsic defects in optical properties of self-organized opal photonic crystals.

Self-organized synthetic opals possessing a face centered cubic (fcc) lattice are promising for fabrication of a three-dimensional photonic crystal with a full photonic band gap in the visible. The fundamental limiting factor of this method is the large concentration of lattice defects and, especially, planar stacking faults, which are intrinsic to self-assembling growth of colloidal crystal. We have studied the influence of various types of defects on photonic band structure of synthetic opals by means of optical transmission, reflection and diffraction along different crystallographic directions.