Showing papers in "Acta Physica Polonica A in 1991"

Physics of Solid-State Laser Materials

Abstract: A survey of the physical properties of solid state materials activated with d2 , d3 and d8 transition metal ions is presented in the context of tunable laser operation. An emphasis is put on common characteristics of all three systems, like a strong electron-phonon coupling and similar electronic structures. The conditions necessary to obtain a tunable operation and to avoid an overlap of the excited state absorption and emission are formulated. It is shown that the d3 configuration system has the largest range of allowed values of the crystal field parameter 10Dq. PACS numbers: 42.55Rz, 78.50.-w 78.55.—ni

Optical Properties of Thermally Deposited Bismuth Telluride in the Wavelength Range of 2.5-10 µm

Abstract: The optical constants (the refractive index n, the absorption index k and the absorption coefficient α) of Bi2 Te3 thin films were determined in the wavelenght range of 2.5 to 10 μm. The shape of the absorption edge in Bi2Te3 thin films has been determined from transmittance and reflectance measurements. The edge is of the form expected for direct transition corresponding to Eg = 0.21 eV. The optical constants were used to determine the high frequency dielectric constant eo = 58, the optical conductivity σ = + σ2 as well as the volume and surface energy loss functions. All these parameters were used to get some information about the intraband and interband transitions.

Site percolation thresholds of fcc lattice

Abstract: c is equal to 0.198, 0.136, 0.061, and 0.05 respectively. These results agree with the Roberts approximate formula for p c . Estimation of pc to even longer-ranged couplings is presented. For instance for pc = 0.01 the range of the couplings should extend at 1east to the eight shell of neighbors.

Mobility spectrum analysis of the electrical conduction

Abstract: The integral expressions describing the electrical conduction in a periodic system is usually derived from the transport theory in the effective mass approximation. The integration over the k vector space was obtained by McClure in 1956 [1]. A commonly used form of integration in the energy space E was derived by Kolodziejczak in 1961 [2]. However, the most useful approach to the analysis of the electrical conduction versus magnetic field was proposed in 1987 by Beck and Anderson [3]. There the integration runs over the mobility All above-mentioned methods express the electrical conduction as an integral of the spectum of the electrical conductivity density S(k), S(E) or s(μ). The electrical conductivity density s(μ) has been called by Beck and Anderson the mobility spectum [3]. The electrical conductivity tensor components versus magnetic field σxx(H) and σxy(H) can be transformed into the mobility spectum s(μ) [4]. This transformation is related to the inverse Laplace transform [4, 5]. The mobility spectum s(μ) is a new tool in the analysis of the experimental data, complementary to resistivity tensor or conductivity tensor components. On the other hand, the mobility spectum theoretically calculated or experimentally determined is a new form of

Pulsed laser evaporation and epitaxy

Abstract: The application of pulsed 1asers for vaporization (ablation) of solid targets appears to be the most natural way to produce high purity fluxes of atoms/ions suitable for epitaxial growth of thin films. Since the early 1960's this unique approach has been the subject of steadily growing interest in the deposition of metals, dielectrics, semiconductors and since 1987, high-T c superconductors. Laser induced target surface morphology changes, properties of laser induced vapours and pulsed deposition rate associated with the use of a pulsed 1aser for vacuum epitaxy are discussed. A pulsed 1aser evaporation and epitaxy (PLEE) system is described and the results of PLEE application for the growth of

Anisotropic Fluorescence of Polar Molecules in External Electric Field, Dipole Moments of Para-Disubstituted Transstilbenes in the First Excited Singlet State

Abstract: The electric dipole moments μe in the lowest excited singlet state of nine short-lived para-disubstituted trans-stilbenes were determined experimentally based on the effect of the external electric freld on the fluorescence anisotropy. The values of μe obtained from Liptay's theory with the employment of the Onsager effective field are in satisfactory agreement with those calculated based on the solvatochromic effect. In the case of para-substituted trans-stilbenes with the same donor substituent R = Ν(CH3)2 in position 4 and different acceptors R, in position 4,, the enhancing electron properties of the acceptor are accompanied by the increase in μg — the dipole moment in the ground state, and μ — the dipole moment in the excited state. There exists a linear relation between the values of μc and μ5 .

Peano dynamics as a model for turbulence and strange nonchaotic behaviour

Abstract: Recent investigations [1-4] revealed the existence of strange, yet nonchaotic attraction in quasi periodically driven oscillators. These are fractal attractors which look topologically similar to the more familiar strange attractors, yet possess negatve Liapunov exponents and are thus nonchaotic. Also recently O. Roessler et al. gave convincing reasons for anticipating new strange chaotic phenomena in four dimensions [5, 6]. Now the simplest quasi periodically forced oscillation have a four-dimensional phase space. Considering that Ruelle, Takens and Newhouse envisage chaos as a sequence of finite number of Hopf bifurcations leading to a totally unstable torus in four dimensions [7] then it is understandable that one may be inclined to speculate on possible cross connections between all these different lines of thought. In what follows we outline a scenario which is similar to a Smale horse shoe [7] and may serve as a prototype for strange, but nonchaotic behaviour. We show that the action of contracting, stretching and special form of twist-folding of the phase space in a way similar but not identical to the horse shoe, leads to a distinct form of dynamics. The invariant set of this dynamics are Cantor-like objects and may be shown to be oriented on a Peano curve-like discrete manifold [8]. The immediate consequence of this picture is that we may anticipate a Poincare map of a system

Stimulated Raman Scattering of Nonclassical Light by Squeezed Phonons

Abstract: Quantum statistical properties are derived for stimulated Raman scattering by squeezed phonons including squeezing of vacuum fluctuations in radiation modes and sub-Poisson photon behaviour. Initial radiation can be in squeezed thermalized and/or sub-Poisson states, phonon system can also be squeezed. In particular, effects of initial phase conditions, losses, initial nonclassical behaviour and phonon squeezing are demonstrated in terms of reduced factorial moments up to the fifth order in combined photon-photon and photon-phonon modes. Also photon number distribution, quadrature variances and principał squeeze parameters are shown.

Pseudorotational Averaging of EPR Spectrum of Cu(II)O5Complex in YBa2Cu3O7-δin Low Temperatures

Abstract: In low temperatures the condensation of oxygen was found to occur at 05 site in an elementary cell of YBa2Cu3O7-δ being the fifth ligand forming the CuO5 complex around Cul copper in chain. This change of coordinantion from CuO4 to CuO5 is the origin of a pseudorotation related to a strong vibronic coupling of two distorted configurations: a tetragonal pyramid C4v and a trigonal bipyramid D3h, which yields a pseudocubic EPR spectrum in low temperatures. The averaged spectroscopic splitting coefficient is related to a superposition of vibronically coupled orbital states |x 2 — y2) and 3z2 -r2 ) . The averaged spectrum was for the first time observed in low temperatures since the oxygen condensation in YBa2Cu3O7-δ at 05 site of the chain occur only when oxygen undergoing fast diffusion among the chains, gets localized with decreasing temperature. The activation energy of oxygen desorption from the 05 site is 36 K.

Roughness Effects in Surface Enhanced Raman Scattering - Evidence for Electromagnetic and Charge Transfer Enhancement Mechanism

Abstract: The Surface Enhanced Raman Scattering (SERS) phenomenon was discovered in 1974 by Fleischman et al [1] The authors investigated the Raman scattering of laser light by pyridine molecules adsorbed on the surface of silver electrode activated by the oxidation-reduction procedure The obtained results were rather spectacular The intensity of Raman scattered light appeared to be about 10 6 times larger than one could expect from the number of eXcited molecules Thus, one can assume the existence of surface mechanisms, which amplify the Raman scattered light intensity The study of these mechanisms was the subject of great number of both experimental and theoretical works [2-6] Numerous theoretical models have been proposed, however, no one of them could explain that effect individually and completely [7] It was experimentally confirmed that the surface roughness plays

SPATIAL CORRELATION OF IMPURITY CHARGES INDUCED BY COULOMB INTERACTIONS APPLICATION TO DX CENTERS IN GaAs

Abstract: The electron mobility enhancement observed in heavily doped GaAs under hydrostatic pressure is interpreted in terms of spatial correlation between the donor charges within partially occupied system of impurities induced by strong inter-donor Coulomb interaction. A simple analytic theory is given for both DX 0 and DXmodels of the impurity state. The mobility is shown to increase together with pressure in both models. Estimates of the energy of the DX level are strongly perturbed by the inter-donor Coulomb interactions.

Guest Influence on the Dipolar Association of the Nematic Liquid Crystal Host

Abstract: The vast majority of nematogens is composed of polar molecules. The part of permanent dipoles in determining nematic liquid crystalline properties has been the subject of considerable discussion. The first theory of the nematic phase, proposed by Born [1], was based on purely dipolar interactions. However, when the compounds were found that do not possess permanent dipole moments, but do nevertheless exhibit the nematic phase (e.g. alkylazobenzenes [2]), Boo I's theory was abandoned. In the first successful theory of the nematic phase, given by Maier and Saupe [3], only interactions between induced dipoles have been taken into account. Although the interactions between permanent dipoles do not play

Effect of Hydrostatic Pressure on InP:Yb Luminescence

Abstract: Tle effects of hydrostatic pressure on the InP:Yb luminescence were explored using a gasketed diamond anvil cell (DAC). The pressure dependence of the Yb3+ luminescence shows a small positive shift (0.96 meV/GPa) at low pressures (< 4 GPa) and a negative one (-0.04 meV/GPa) above 4 GPa. The spectra of the Yb3+ emission differ markedly in these two pressure ranges. It was concluded that intra-4f-shell transitions of the Yb 3 + on indium substitutional (Td) site dominate in the spectrum above 4 GPa, whereas at lower pressures the emission has a different nature.

Remarks on Fluorescence Concentration Quenching of Rhodamine 6G in Methanol

Abstract: It has been shown that the concentration changes in the quantum yield η and decay time τ of fluorescence of rhodamine 6G in methanol can be explained quantitatively by non-radiative excitation energy transport (NET) from monomers to quenching centres considered as perfect traps. The good agreement of the experimental data with the theoretical curves for the critical radii ROMM = 55.4Ǻ and ROMT = 51.8Ǻ determined spectroscopically has been obtained. The excitation energy migration and the concentration dependence on the orientation factor occurring in the Förster rate for the NET have been taken into account. The nature of the quenching centres and the effect of material diffusion of active molecules on η and τ have been discussed.

Rigid and Relaxed Diagonal Orbital Hardnesses of the Valence Shell of Transition Metal Ions from Xα Calculations

Abstract: Valence shell diagonal hardnesses (electron repulsion) parameters are reported for the firstand second-row transition metal ions, determined via finite differences from the Xα eigenvalues for alternative configurations and charges. Both rigid (unrelaxed orbit als) and orbitally relaxed hardnesses have been calculated. The relaxed parameters provide a convenient basis for generating a realistic hardness tensor of catalytic systems involving transition metals at the atoms-in-molecules (AIM) oxidation state and configurations, thus facilitating the charge-sensitivity.analysis of chemisorption systems both at the AIM and orbital resolutions. The observed trends in the orbital relaxation effects are briefly discussed.

Magnetic Properties of Zinc Doped Ferrites

Abstract: Α series of samples of the system Coι-xΖnxFe2O4 (x = 0.0, 0.3, 0.4, 0.5 and 0.8) have been prepared by the usual ceramic technique. X-ray analysis shows that they are cubic spinel (single phase). The lattice parameter a and the bulk density D are measured for the samples. The substitution experiments replacing Coe+ by Ζn2+ furnish new significant fact concerning the magnetic behaviour of ferrites. The magnetic strength of the magnetic ions on the Band A-sites can be varried relative to each other by the substitution of Ζn2+ The increase of density of the composition increases the magnetic interaction of the dipole moment at the B-sites.

Influence of High Power Laser Beam on Physical Properties of Epitaxial Films of Hg1-xCdxTe (x≈0.2)

Abstract: Models have been proposed explaining differences in electrophysical and photoelectrical properties of epitaxial films of Hg1-xCdxTe (x ≈0.2) before and after treating by single pulses of Nd:YAG laser. These models were obtained on the basis of the calculated distributions of temperature, temperature gradient and concentration of interstitial mercury during and after exposing to the laser pulse. These calculations were done using the net method.

Molecular beam epitaxy of ii-vi based heterostructures

Abstract: The nonequilibrium growth technique of molecular beam epitaxy (MBE) has provided for the fabrication and investigation of a multitude of novel layered heterostructures based on II—VI compound semiconductors. The ability to grow epitaxial metastable magnetic and semimagnetic semiconductors 1ayered with conventional II-VI semiconductors has resulted in structures which, for example, exhibit frustrated antiferromagnetism, and a wide wavelength tunability due to selftrapping of excitons in ZnTe-containing layered structures and due to extremely 1arge ( ≈1 eV) quantum shifts of light emission from MnTe/CdTe superlattice structures. In addition, the control in the stoichiometry of surfaces and the composition of molecular beams used in the MBE growth technique has allowed for the fabrication of very advanced heterostructures which have combined the II—VI and III-V families of compound semiconductors. The work which will be described in the following review represents a very small sampling of the many important results achieved in the field of II-VI based heterostructures. The topics have been selected to illustrate and provide an example of the utility of MBE and the potential of "engineered" II—VI heterostructures and quantum wells.

Semi-insulating transition metal-doped iii-v materials

Abstract: This review surveys the properties of transition metal (TM) doped semi- insulating (SI) IlI-V semiconductors. After a general definition of a SI ma- terial, a simple model of a SI crystal with a midgap donor and shallow impurities is discussed. A short history, main properties, and thermal stabil- ity problems of SI Cr-doped GaAs are presented. The puzzling problem of SI V-doped GaAs is explained. Several dopants (Cr, Fe, Co, and Ti) in SI InP are discussed in terms of the resistivities obtained, as well as thermal stability. Finally, GaP and GaInAs high resistivity systems are considered.

Conductance Fluctuations in Microstructures of HgCdMnTe Bicrystals

Abstract: Microscopic four-contact probes to semimagnetic HgCdMnTe grain-boundary inversion 1ayers have been photolithographically patterned. Magnetoresistance measurements performed on these samples revealed aperiodic conductance fluctuations of the magnitude of the order of e2 /h. Quantitative analysis of both fluctuation amplitude and their mean period indicate that we have approached the mesoscopic regime in our system. This opens new possibilities in studies of spin-subsystem dynamics in semimagnetic semiconductors.

Interface Effective Pinning and Dynamical Coupling of Spin Waves in Bilayer Ferromagnetic Films with Canted Magnetizations

Abstract: We present a theory for the spin-wave excitations in bilayer magnetic systems with exchange coupling across the interface between two ferromagnetic films having the same crystallographic structure. The Hamiltonian includes exchange and anisotropy terms within each magnetic film of the composite system, as well as at the interface and at the free surfaces. We examine, in particular, the effect of interface canting between the magnetization directions for each film, and we derive effective pinning parameters and dynamic coupling parameters for the interface as functions of the canting angle. We show that, in the special case when the magnetization directions are antiparal lel in the bilayer system, the two films may be considered as effectively decoupled as regards the spin-wave dynamics. We discuss the relevance of this theory to recent experimental studies of bilayer ferromagnetic systems that exhibit antiferromagnetic interface exchange.

Modulational Instability of Nonlinear Exponential Schrödinger Waves

Abstract: One-dimensional propagation of nonlinear waves has been extensively studied both experimentally and theoretically in many branches of physics. Especially, the nonlinear Schrodinger equation (NS, henceforth) has attracted much attention [1]. This equation describes wave propagation in weakly nonlinear and strongly dispersive media and is valid for small amplitude waves. A generalization of the theory for arbitrary amplitude waves leads to the exponential nonlinear Schrodinger equation (ENS, hereafter)

Modified Nonlinear Schrödinger Equation for Nonlinear Waves in Optical Fibres

Abstract: A new model equation governing the propagation of nonlinear pulses in optical fibres has been derived on the assumption of a saturated nonlinearity of the refractive index. This equation is a combination of the exponential nonlinear Schrodinger equation and the derivative one. It is valid for the long fibres. A modulational stability has been calculated to find out a cut-off in an angular frequency of a carrier wave. Moreover, it has been shown that the equation possesses family of stationary solutions. An initial value problem has been discussed on the basis of the implicit pseudo-spectral scheme.

Collapse of semiflexible polymers in two dimensions. Monte Carlo simulations

Abstract: We present Monte Carlo simulations of a two-dimensional square lattice semiflexible polymer model. Short-range repulsive potential (excluded volume) and long-range attractive (segment-segment) potential are included. Particular attention was paid to a phase transition of a model polymer chain from a random coil state to a dense globule. The transition temperature increases with the chain length and decreases with the stiffness of the model chain. The cooperativity of the transition increases with the polymer stiffness.

Quantum mechanical studies of bond and molecular polarizabilities of gas-phase metal halides

Abstract: The bond and molecular polarizabilities are studied for the gas-phase metal halides using delta-function potential model of chemical binding on the basis of quantum mechanical approach. The applicability of the model is demonstrated. The contribution of the polarity corrections for the metal halides is thoroughly investigated and it has been found that it plays a significant role for fluorides. Our polarizability calculations resolve the discrepancy about the conformation of the barium dichloride and mercury dichloride and favour the nonlinear structure. Due to lack of experimental results for most of halides of transition metals, the decision whether to incorporate polarity corrections or not remains uncertain at this stage: we must await measurements of more experimental values before we reach any final conclusion. The ionic bond orders have been used for the first time to investigate polarizabilities of monomers and dimers of alkali halides. The results for dimers reveal that polarity contribution is essential for the lithium halides dimers and not for other dimers. The computed results will be asset for the interpretation of experimental measurements.

Laser induced fluorescence of indium vapour

Abstract: Atomic states of indium are well-known up to the ionization limit [1-3] and, for doubly excited states, even in higher energy range [1]. The information available about indium diatomic molecule is much more scarce, mostly due to experimental difficulties in producing indium vapour of sufficiently high density. Up to now In2 has been studied only in a few classical spectroscopy experiments and some unresolved bands of unidentifled origin, placed in the near UV and in the violet and green parts of the visible spectrum, have been reported [4-6]. On the other hand, even the first two atomic asymptotes 5 2P1/2,3/2+ 52P1/2,3/2 and 5 2P1/2,3/2+ 62S1/2 lead to several tens of molecular states and one can expect a rich and complicated spectrum of Int in the visible region. In our letter we report the first observation of a laser induced fluorescence (LIF) of indium dimer. Some experimental difficulties present at cell measurements in indium vapour are also discussed. Additionally we study a variation of the effective lifetime of the 6 2S1 / 2 atomic state with the temperature which in our opinion gives an indirect evidence of the population of high atomic levels upon excitation of indium vapour by 410.3 nm resonance light.