Home About us Contact
|
Home About us Contact | ||
|
|
||
Phonons
Kinds of Phonons Terms modified by Phonons Selected AbstractsInfluence of substrate temperature on the properties of electron beam evaporated ZnSe filmsCRYSTAL RESEARCH AND TECHNOLOGY, Issue 4 2010M. G. Syed Basheer Ahamed Abstract ZnSe films were deposited on glass substrates keeping the substrate temperatures, at room temperature (RT), 75, 150 and 250 °C. The films have exhibited cubic structure oriented along the (111) direction. Both the crystallinity and the grain size increased with increasing deposition temperature. A very high value of absorption co-efficient (104 cm -1) is observed. The band gap values decrease from a value of 2.94 eV to 2.69 eV with increasing substrate temperature. The average refractive index value is in the range of 2.39 , 2.41 for the films deposited at different substrate temperatures. The conductivity values increases continuously with temperature. Laser Raman spectra showed peaks at 140.8 cm -1, 246.7 cm -1and 204.5 cm -1which are attributable to 2TA LO phonon and TO phonon respectively. (© 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Preparation and characterization of electrodeposited indium selenide thin filmsCRYSTAL RESEARCH AND TECHNOLOGY, Issue 6 2005S. Gopal Abstract Indium Selenide (InSe) thin films were deposited from a mixture of Indium chloride and selenium dioxide in aqueous solution by electrodeposition technique on Indium Tin oxide coated glass substrates. The effects of the parameters during deposition such as current density, deposition potential versus saturated calomel electrode, pH value and concentration of source material were studied. X-ray diffraction studies were carried out on the films to analyze the microstructure using an x-ray diffractometer and were examined by RAMAN spectroscopy. The Raman peak position did not change much with chemical concentrations. Raman scattering due to the (LO) phonon was observed at 211 cm,1. Optical absorption studies were performed with a double beam ultra violet-visible ,NIR spectrophotometer in the wavelength 300,1100 nm. The surface morphology of the layer was examined using a scanning electron micrograph. The composition of the films was studied using an Energy Dispersive Analysis by X-Rays (EDAX). [source] Enhancement of Thermoelectric Figure-of-Merit by a Bulk Nanostructuring ApproachADVANCED FUNCTIONAL MATERIALS, Issue 3 2010Yucheng Lan Abstract Recently a significant figure-of-merit (ZT) improvement in the most-studied existing thermoelectric materials has been achieved by creating nanograins and nanostructures in the grains using the combination of high-energy ball milling and a direct-current-induced hot-press process. Thermoelectric transport measurements, coupled with microstructure studies and theoretical modeling, show that the ZT improvement is the result of low lattice thermal conductivity due to the increased phonon scattering by grain boundaries and structural defects. In this article, the synthesis process and the relationship between the microstructures and the thermoelectric properties of the nanostructured thermoelectric bulk materials with an enhanced ZT value are reviewed. It is expected that the nanostructured materials described here will be useful for a variety of applications such as waste heat recovery, solar energy conversion, and environmentally friendly refrigeration. [source] Revealing the Electron,Phonon Coupling in a Conjugated Polymer by Single-Molecule Spectroscopy,ADVANCED MATERIALS, Issue 15 2007R. Hildner Electron,phonon coupling in a ,-conjugated polymer is revealed by single-molecule spectroscopy in combination with statistical pattern recognition techniques. The technique allows to reveal the phonon-side band in the spectra of methyl-substituted ladder-type poly(para-phenylene) (see figure). For this polymer a weak electron,phonon coupling strength is found at low temperatures. The distribution of the phonon frequencies provides strong evidence that the low-energy vibrational modes, which couple to the electronic transitions, stem from vibrations of the host matrix. [source] How light gets through periodically nanostructured metal films: a role of surface polaritonic crystalsJOURNAL OF MICROSCOPY, Issue 3 2003A. V. Zayats Summary The physical origin of the enhanced optical transmission of periodically structured films related to surface plasmon polaritons is discussed from first principles. The enhancement of transmission through smooth, randomly rough and periodically nanostructured films is considered. Analysis shows that any metal (or dielectric) nanostructured film can exhibit enhanced transmission in certain spectral ranges corresponding to surface plasmon (or phonon) polariton Bloch mode states on a periodic structure. Resonant tunnelling via these states is responsible for the transmission enhancement. The properties of surface polaritonic crystals are analogous to those of photonic crystals and can find numerous applications for scaling down optical devices to nanometric dimensions as well as for designing novel nanostructured materials whose optical properties are determined by surface polariton interaction in a periodic structure. [source] Coherent acoustic phonons in a thin gold film probed by femtosecond surface plasmon resonanceJOURNAL OF RAMAN SPECTROSCOPY, Issue 11 2008Shoichi Yamaguchi Abstract We report on coherent phonon detection in a thin gold film by a new femtosecond pump-probe surface plasmon resonance (SPR) technique. Acoustic coherent phonons are generated impulsively in the gold film on a prism, and they are detected in the time domain through the reflectivity modulation for probe pulses that satisfy the SPR condition. We observed not only the fundamental vibration of a longitudinal acoustic phonon but also the overtones up to the fourth. Copyright © 2008 John Wiley & Sons, Ltd. [source] Size-dependent Raman and infrared studies of PbSe nanoparticlesJOURNAL OF RAMAN SPECTROSCOPY, Issue 9 2008F. S. Manciu Abstract Micro-probe Raman and far-infrared absorption spectroscopies were used to prove the existence of optical phonon modes of PbSe nanoparticles prepared by colloidal chemistry and preliminarily characterized by transmission electron microscopy. To the best of our knowledge, this is the first time that evidence of the surface phonon (SP) mode by Raman spectroscopy has been experimentally observed. The wavenumber of the SP mode is consistent with its prediction by a dielectric continuum model. While for different PbSe nanoparticle sizes the observed SP mode does not show any obvious change in its position, there is a clear shift by approximately 4 cm,1 toward higher wavenumber in the appearance of the LO(,) in the Raman spectra from the 3 nm to the 7 nm PbSe nanoparticles. Far-infrared measurements demonstrate the presence of the transverse optical TO(,) and of the coupled phonon modes. Copyright © 2008 John Wiley & Sons, Ltd. [source] Temperature-dependent Raman scattering studies of the geometrically frustrated pyrochlores Dy2Ti2O7, Gd2Ti2O7 and Er2Ti2O7JOURNAL OF RAMAN SPECTROSCOPY, Issue 4 2008M. Ma, czka Abstract The temperature-dependent Raman studies of A2Ti2O7(A = Dy, Er, Gd) were performed on single crystals and polycrystalline samples in the 4.2,295 K temperature range. The Raman spectra showed softening of the majority of phonon modes upon cooling in the whole temperature range studied and large decrease of linewidths. These changes have been analyzed in terms of strong third-order phonon,phonon anharmonic interactions. Moreover, the 312 and 330 cm,1 modes of Er2Ti2O7(Gd2Ti2O7) showed hardening upon cooling down to about 130 K (100 K) and then anomalous softening below this temperature. The observed anomalous behavior of the Raman modes indicates that some important changes occur in these materials at low temperatures. However, the origin of this behavior is still not clear. Copyright © 2008 John Wiley & Sons, Ltd. [source] Diamond as pressure sensor in high-pressure Raman spectroscopy using sapphire and other gem anvil cellsJOURNAL OF RAMAN SPECTROSCOPY, Issue 4 2003Valentín García Baonza Abstract We propose a new Raman pressure scale based on the shift with pressure of the fundamental Raman band of micrometer-sized diamonds. First, we confirmed that the pressure slope of the triply degenerate diamond phonon behaves in a similar fashion to that of the bulk. Our measurements were calibrated Raman against the Sm:YAG fluorescence pressure scale up to 5 GPa using a gasketed sapphire anvil cell. The most relevant features regarding the design of the anvil cell are briefly outlined. Measurements were performed under hydrostatic conditions using 4 : 1 methanol,ethanol as pressure-transmitting medium. The calibration pressures according to the relationship p(GPa) = 0.356[,(cm,1) , 1332.3] are considered to be accurate within about 0.1 GPa. The convenience of using micrometer-sized diamonds as pressure sensors in Raman studies using gem anvil devices is demonstrated with several examples. Copyright © 2003 John Wiley & Sons, Ltd. [source] Thermal Conductivity of the Rare-Earth Strontium AluminatesJOURNAL OF THE AMERICAN CERAMIC SOCIETY, Issue 5 2010Chunlei Wan The thermal conductivity of a series of complex aluminates, RE2SrAl2O7, with different rare-earth (RE) ions, has been measured up to 1000°C. There is a strong dependence on the atomic number of the RE ion, ranging from an approximately 1/T dependence for the lanthanum strontium aluminate to an almost temperature-independent behavior of the dysprosium strontium aluminate. The latter conductivity is comparable with that of yttria-stabilized zirconia, the current material of choice for thermal barrier coatings. The temperature dependence of the thermal conductivities of all the aluminates studied can be fit to a standard phonon,phonon scattering model, modified to account for a minimum phonon mean free path, in which the difference in behavior is attributed to increased phonon,phonon scattering with the atomic mass of the RE ion. Although a satisfactory parametric fit is obtained, the model does not take into account either the detailed layer structure of the aluminates, consisting of alternating rock-salt and perovskite layers in a natural superlattice structure, or the site preferences of the RE ion. This suggests that further model development is warranted. [source] Influence of Stabilizer Concentration on Transport Behavior and Thermopower of CNT-Filled Latex-Based CompositesMACROMOLECULAR MATERIALS & ENGINEERING, Issue 5 2010Yeon Seok Kim Abstract The influence of the stabilizer/SWNT ratio on the transport behavior of latex-based polymer nanocomposites is examined in an effort to improve electrical conductivity while maintaining or improving the Seebeck coefficient (i.e., thermopower). Results show that phonon and electron transport are significantly affected by tube/tube junctions, and the carrier transport across the junctions can be manipulated by altering the stabilizer concentration. Electrical conductivity of composites containing 10,wt.-% SWNT nearly doubles, becoming greater than 900,S,·,m,1, by changing the SWNT:GA ratio from 1:3 to 10:1, while thermal conductivity and Seebeck coefficient remain relatively constant (near 0.25,W,·,m-K,1 and 40,µV,·,K,1, respectively). [source] Gliding dislocations in Bi2Te3 materialsPHYSICA STATUS SOLIDI (A) APPLICATIONS AND MATERIALS SCIENCE, Issue 1 2009N. Peranio Abstract In Bi2Te3, dislocations were found with an uniquely high mobility at room temperature. The gliding dislocations were analysed and their effect on the thermoelectric properties is discussed. The glide of dislocations was induced by heating with a focused electron beam at 120 keV, external stresses were not applied. The dislocations were bowed out in the glide direction and were only pinned at the surface of the samples. Stereomicroscopy combined with image simulations yielded basal plane dislocations with a density of 109 cm,2 and Burgers vectors of type ,110,. Video sequences showing the glide of single dislocations and groups of dislocations were recorded. Isolated dislocations showed a high mobility in ±,110, direction at a velocity of 10,100 nm s,1. Dislocation dipoles were pinned and did not glide. Dislocations equidistantly arranged within the same glide plane showed a collective movement. Dislocations piled up in different glide planes were fixed and acted as barriers for gliding dislocations. The motion of dislocations was attributed to residual shear stresses of about 10 MPa, and their glide directions depended on the sign of the Burgers vector. Attractive and repulsive forces of dislocations directly visualise the forces due to the elastic strain fields of other dislocations. The relevance of phonon scattering on dislocations in Bi2Te3, particularly due to their high mobility and density, was confirmed by two inspections: (i) Dislocations decrease the lattice thermal conductivity due to phonon scattering on the elastic strain field. The phonon mean free path was estimated to about 800 µm at 3 K and agreed with published data. (ii) The dislocation resonance theory of Granato and Lücke predicts an interaction between phonons and dislocations acting as oscillating strings. The attenuation of ultrasound was estimated and was compared with published data. (© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Confinement effects on optical phonons in spherical, rod-, and tetrapod-shaped nanocrystals detected by Raman spectroscopyPHYSICA STATUS SOLIDI (A) APPLICATIONS AND MATERIALS SCIENCE, Issue 2 2007Concetta Nobile Abstract Spherical, rod- and tetrapod shaped CdSe nanocrystals are investigated by Raman spectroscopy and the longitudinal-optical and surface optical phonons are observed. We find that the position of the longitudinal-optical phonon slightly red-shifts with decreasing diameter, whereas the position of the surface optical phonon depends significantly on diameter and length of the rods or the tetrapod arms. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Excitons and defects in homoepitaxial diamond films from cathodoluminescence of p,/p+ samplesPHYSICA STATUS SOLIDI (A) APPLICATIONS AND MATERIALS SCIENCE, Issue 11 2004M. Wade Abstract We study the signals from excitons and defects from the cathodoluminescence spectra (CL) of the p,/pseudo substrate sample. The CL from the p, film are determined from the variation of the CL of the sample as the voltage of the exciting electron beam increases. At the lower voltages, it probes the p, film. The CL is dominated by the free exciton associated with the TO phonon (FETO, FWHM = 6 meV), but shows also small signals (ratio = 5 × 10,2) mainly from the A band (and other 2.6, 3.6 and 4.05 eV wide bands) and tiny signals from the 5 RL defect. At the higher voltages it probes also the Ib substrate and is dominated by the signals of the H3 defects in the Ib substrate. The p, film appears almost homogeneous, and of good quality from the characteristics of its exciton. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Tunneling current through g -factor engineered series quantum dotsPHYSICA STATUS SOLIDI (B) BASIC SOLID STATE PHYSICS, Issue 4 2009Yasuhiro Tokura Abstract We evaluate resonant tunneling current via series quantum dots with different g -factors by Bloch equation method. We found that the resonant current is always singly peaked as a function of bias and its peak position depends on the gate voltage. When the rate of spontaneous phonon emission is very small, the shift of the peak positions between single spin transport and both spin transport corresponds to the half of the difference of the Zeeman energy. In contrast, when the phonon assisted tunneling predominates, the peak always appears at one of the resonant tunneling conditions for larger bias. These features are explained by spin blockade effect. (© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Temperature dependent high resolution resonant spectroscopy on a charged quantum dotPHYSICA STATUS SOLIDI (B) BASIC SOLID STATE PHYSICS, Issue 4 2009M. Kroner Abstract We present temperature dependent high resolution resonant optical spectroscopy on a single, negatively charged InGaAs quantum dot. We performed laser transmission measurements yielding the natural linewidth of the excitonic ground state transition of a quantum dot in a temperature range from 4.2 K up to 25 K. Here, we describe the linewidth evolution and the temperature induced red shift of the resonance energy with simple models based on the exciton,phonon coupling in the quantum dot. The resonant spectroscopy measurements are complemented with results from non-resonant PL measurements on the very same quantum dot. Here we observe a simple linear behavior of the linewidth according to an effect of a fluctuating environment. (© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] First and second optical transitions in single-walled carbon nanotubes: a resonant Raman studyPHYSICA STATUS SOLIDI (B) BASIC SOLID STATE PHYSICS, Issue 11 2007H. Telg Abstract Resonant Raman spectroscopy was performed to study electron,phonon coupling in single-walled carbon nanotubes separated in solution. By varying the excitation energy from 1.26 eV to 1.93 eV we obtained radial breathing mode resonance profiles of the first and second optical transitions E11 and E22 of the (9,1) and (8,3) tubes. We observe up to 16 times stronger Raman intensities for the E11 transitions which can mostly be attributed to a two times broader linewidth of the E22 transition. Comparison of the matrix element ratio ,11/,22 to theoretical predictions on the electron,phonon coupling show a deviation of a factor 1.7 which might be associated with the change of the exciton,photon matrix element. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Influence of La doping on the properties of molybdenum perovskite Sr1,xLaxMoO3 (0 , x , 0.2)PHYSICA STATUS SOLIDI (B) BASIC SOLID STATE PHYSICS, Issue 6 2006S. B. Zhang Abstract The effects of La-doping on the structural, magnetic, electrical transport and specific-heat properties in 4d perovskites Sr1,xLaxMoO3 (0 , x , 0.2) have been investigated. The substitution of Sr ions by La ions does not change the space group of the samples, but increases the lattice parameter a (Ĺ). The resistivity , and magnetic susceptibility , decrease monotonously with the increase of x , while the electronic specific heat coefficient ,e increases. The resistivity of all samples shows a T2 dependence in the low-temperature region of 2 K < T < 125 K and a T dependence in the high-temperature range of 130 K < T < 350 K, related to the electron,electron (e,e) and electron,phonon (e,ph) scattering, respectively. The specific-heat data agrees with the classical Dulong,Petit phonon specific heat, Ccl = 3kBrNA = 124.7 J/mol K at high temperatures and Cp(T )/T = ,e + ,pT2 at low temperatures. These behaviors can be explained according to the decrease of the density of states (DOS) at the Fermi energy level (EF), N (EF). (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Photoluminescence properties of PbSe/PbS core-shell quantum dotsPHYSICA STATUS SOLIDI (C) - CURRENT TOPICS IN SOLID STATE PHYSICS, Issue 12 2009Georgy I. Maikov Abstract The present study describes the investigation of the ground-state exciton emission in the PbSe/PbS core-shell colloidal quantum dots (CQDs) with different core-radius/shell-thickness ratio, by recording temperature dependence of ground-state exciton photoluminescence (PL) properties, including line-width, integrated PL intensity, and the temperature coefficient of energy band-gap, over a temperature range from 1.4 K to 300 K. The obtained data reveal a reduction of the temperature coefficient of the energy band-gap as well as slightly decrease of exciton and optical phonon coupling in the PbSe/PbS core-shell CQDs with respect to that in the corresponding PbSe core. (© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Exciton spin decay in quantum dots: single and double phonon ssisted transitionsPHYSICA STATUS SOLIDI (C) - CURRENT TOPICS IN SOLID STATE PHYSICS, Issue 2 2009K. Roszak Abstract We study phonon assisted transitions leading to exciton spin decay in quantum dots. We concentrate on couplings specific to the hole (Bir-Pikus Hamiltonian) and the exciton as a whole (short-range exchange interaction). A system strongly confined in a self-assembled quantum dot is considered. We discuss single-phonon assisted processes arising from the interplay of these interactions and show the dominating role of transverse phonons. We have also found a new two-phonon assisted transition, connected with the interplay of different parts of the Bir-Pikus Hamiltonian in the presence of the singlet-triplet splitting. (© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Raman scattering spectra of CdS nanocrystals fabricated by a reverse micelle methodPHYSICA STATUS SOLIDI (C) - CURRENT TOPICS IN SOLID STATE PHYSICS, Issue 1 2009Aishi Yamamoto Abstract Raman scattering (RS) spectra of CdS nanocrystals (NCs) fabricated by a reverse micelle method were studied and were analysed using a phonon confinement model. One broad and red-shifted RS peak due to the LO phonon confined in CdS NCs was observed. The RS spectra were calculated using a phonon confinement model with the NC sizes estimated from the optical absorption spectra. The calculated RS spectra as suming that the NCs have the wurtzite structure reproduce the experimental spectra better than those assuming zincblende structure, which is consistent with the result of transmission electron microscope (TEM) observations. Our finding indicates that the crystal structure of the nanocrystal can be determined by the analyses of Raman scattering spectral shape. (© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Ultrafast dynamics of plasmon-phonon coupling: Estimation of electron mobility in GaAsPHYSICA STATUS SOLIDI (C) - CURRENT TOPICS IN SOLID STATE PHYSICS, Issue 1 2008*Article first published online: 29 NOV 200, Muneaki Hase Abstract Utilizing a femtosecond pump-probe technique, ultrafast relaxation of plasmon-like LO phonon-plasmon coupled (LOPC) modes is studied. The time-resolved reflectivity change exhibits strong mode beating between the LO phonon and the lower branch of the LOPC modes. It is revealed that the decay of the upper branch (or plasmon-like mode) of the coupled modes is very rapid in time-frequency space, explored by using wavelet analysis. The decay time of the plasmon-like LOPC mode, which corresponds to the relaxation time ,, is obtained to be , 79 fs and this time constant enables us to extract the electron mobility , to be 2074 ± 200 cm2/Vs. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Interplay of electron,phonon and Coulomb interaction in semiconductor quantum dotsPHYSICA STATUS SOLIDI (C) - CURRENT TOPICS IN SOLID STATE PHYSICS, Issue 7 2006J. Förstner The cover picture of this issue of physica status solidi (c) has been taken from the article [1]. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Energy relaxation in CdSe/ZnSe quantum dots under the strong exciton-phonon coupling regimePHYSICA STATUS SOLIDI (C) - CURRENT TOPICS IN SOLID STATE PHYSICS, Issue 4 2006A. V. Platonov Abstract Resonant photoluminescence and photoluminescence excitation spectroscopy have been applied to CdSe/ZnSe quantumdots where separation between the quantized levels is close to the optical phonon energy. Astrong relaxation enhancement induced by the optical phonon is observed. The fine structure of the phonon replicas is resolved. It is attributed to the optical phonons with large wave vectors from the symmetrical points of the Brillouin zone. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Polytype control and properties of AlN on siliconPHYSICA STATUS SOLIDI (C) - CURRENT TOPICS IN SOLID STATE PHYSICS, Issue 7 2005V. Cimalla Abstract In this paper we report on the optical and structural characterisation of cubic polytype AlN thin films on 3C-SiC/Si(111) pseudo-substrates prepared by carbonisation. We have found that 3C-AlN phase can be stabilised on the "waved" 3C-SiC(111) surface by polytype replication. On the other hand, 2H-AlN was grown on atomically smooth surfaces. X-ray diffraction and transmission electron microscopy imply that the cubic AlN was grown along with hexagonal inclusions up to 20%. Spectroscopic ellipsometry in the infrared region show the typical phonon lines for the two polytypes. The appearance of the TO phonon at 646 cm,1 confirms the existence of 3C-AlN. In the ultraviolet region the critical points in the band structure were identified. For 3C- and 2H-AlN the first direct transition at the ,-point was determined to be at 5.74 eV and 6.16 eV, respectively. No indirect transition at lower energies was detected. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Behavior of phonons in short period GaN-AlN superlatticesPHYSICA STATUS SOLIDI (C) - CURRENT TOPICS IN SOLID STATE PHYSICS, Issue 11 2004C. Pinquier Abstract This work deals with short period GaN-AlN superlattices, studied by Raman spectroscopy using visible and ultraviolet excitations. The E2 (high) and E1(TO) phonons show a two-mode behavior. In contrast, we observed an unique spectral line in the A1(TO) spectrum: its dependence on the mean superlattice composition is in good agreement with recent theoretical results predicting its delocalized character. Two Raman lines strongly enhanced under ultraviolet excitations, were observed in the A1(LO) spectrum. Lattice dynamical simulations within the Rigid Ion Model have been performed to assign these Raman lines. The first one corresponds to an A1(LO) phonon confined in GaN wells. The second line is assigned to a dispersive B1 - like mode with an amplitude modulation similar to that of an interface mode. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Coherence length and time of excitons in ZnSe quantum wellsPHYSICA STATUS SOLIDI (C) - CURRENT TOPICS IN SOLID STATE PHYSICS, Issue 3 2004B. Dal Don Abstract We investigate the in-plane transport of excitons in quantum wells by nano-photoluminescence. The experimental method is based on a confocal microscope with an enhanced resolution given by the introduction of a solid immersion lens. In combination with pulsed laser excitation and streak-camera detection, we have access to transport phenomena on a timescale faster than the time of scattering with acoustic phonons and a length scale of the light wavelength. We use ZnSe-based quantum wells as a model system since hot excitons with well defined excess energy can be formed assisted by the emission of optical phonons. This results e.g. in a periodic quenching of the excitonic transport length as function of excitation excess energy which, in comparison, is not found in GaAs quantum wells. Monte Carlo simulations of the nonlinear expansion of the luminescence spot observed as a function of time reveal the difference between the spatial profiles of the luminescence and the exciton density. The latter shows an oscillatory behaviour in time due to the dominant backscattering, when the first acoustic phonon is emitted. From this oscillation we can determine simultaneously the coherence time and length of the excitonic transport in ZnSe quantum wells. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Elastic properties of two-dimensional quasicrystalsACTA CRYSTALLOGRAPHICA SECTION A, Issue 4 2008Hans Grimmer Quasicrystals (QC) with two-dimensional quasiperiodic and one-dimensional periodic structure are considered. Their symmetry can be described by embedding the three-dimensional physical space VE in a five-dimensional superspace V, which is the direct sum of VE and a two-dimensional internal space VI. A displacement v in V can be written as v = u + w, where u,VE and w ,VI. If the QC has a point group P in VE that is crystallographic, it is assumed that w and a vector u,,VE lying in the plane in which the crystal is quasiperiodic transform under equivalent representations of P, inequivalent ones if the point group is 5-, 8-, 10- or 12-gonal. From the Neumann principle follow restrictions on the form of the phonon, phason and phonon,phason coupling contributions to the elastic stiffness matrix that can be determined by combining the restrictions obtained for a set of elements generating the point group of interest. For the phonon part, the restrictions obtained for the generating elements do not depend on the system to which the point group belongs. This remains true for the phason and coupling parts in the case of crystallographic point groups but, in general, breaks down for the non-crystallographic ones. The form of the symmetric 12 × 12 matrix giving the phonon, phason and phonon,phason coupling contributions to the elastic stiffness is presented in graphic notation. [source] Extension of the time-dependent dynamical diffraction theory to `optical phonon'-type distortions: application to diffraction from coherent acoustic and optical phononsACTA CRYSTALLOGRAPHICA SECTION A, Issue 1 2003Peter Sondhauss An extension of the time-dependent Takagi,Taupin theory to `optical phonon'-type distortions is presented. By splitting the susceptibility into the contributions from each atom in a unit cell, modifications to the structure factor as well as lattice parameter are taken into account. The result is a compact, surprisingly simple, equation with a strong formal similarity to the classical Takagi,Taupin equation, with the latter included as a special case. Time dependence is explicitly retained and thus the analysis is applicable to situations where the crystal is modified on time scales comparable with that for the X-rays to traverse an extinction depth. A comparison is made between the influence of coherent acoustic and optical phonons on the diffraction of X-rays. Numerical and perturbative analytical solutions of the generalized Takagi,Taupin equation are presented in the presence of such phonons. [source] Contribution to the study of elastic properties of KGd(WO4)2:Er3+ single crystals by Brillouin spectroscopyCRYSTAL RESEARCH AND TECHNOLOGY, Issue 12 2009D. Kasprowicz Abstract In this paper we present the study of the acoustic phonons propagating in Er3+ -doped KGd(WO4)2 single crystals by Brillouin spectroscopy. For the investigated crystals the velocities of the longitudinal and transverse acoustic phonons [100], [010], [001], [101] and [110] have been determined. Moreover, the values of the elastic constants: C22, C44 and C66 of Er3+ -doped KGd(WO4)2 single crystals have been estimated. It was revealed that the presence of the Er3+ -ions in KGd(WO4)2 crystals, for the used doping concentration 1 at% does not influence their elastic properties. (© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] | ||||||||||||||||||||||||||||||||||