Search results for "PHONON"

Conductivity of the two-dimensional electron gas atLaAlO3/SrTiO3interface

2017

We propose an analytical theory of metallic conductivity in the two-dimensional (2D) ${\mathrm{LaAlO}}_{3}/{\mathrm{SrTiO}}_{3}$ (LAO/STO) interface. For that we consider the electron-phonon interaction at the interface. The electronic part is taken from our previous work [Phys. Chem. Chem. Phys. 18, 2104 (2016)], considering the conditions for the interfacial charge carrier (electron or hole) to become itinerant. The second ingredient deals with the atomic oscillations localized near the interface and decaying rapidly at its both sides, which can be regarded as 2D acoustic phonons. The dispersion of such phonons depends on the characteristics of phonon spectra of LAO and STO. Calculating t…

Growth zoning and strain patterns inside diamond crystals as revealed by Raman maps

2005

The Raman mapping technique provides a non-destructive means of studying internal growth textures and other micro-structural heterogeneity inside diamond single-crystals. Raman maps showing distribution patterns of the bandwidth (FWHM) of the main first-order lattice vibration of diamond ( LO=TO phonon at ~1332 cm −1 ) along two-dimensional planes inside diamond crystals may reveal the internal growth zoning of these crystals. The observed zoning is affected, and in some cases even obscured in micro-areas adjacent to inclusions, by patterns of heterogeneous strain in the diamond. We present Raman maps obtained from diamond crystals containing large, single-crystal graphite inclusions, from …

Far-infrared and THz spectroscopy of 0.4PMN–0.3PSN–0.3PZN relaxor ferroelectric ceramics

2007

Abstract Temperature dependence of the optic phonons in 0.4PbMg 1/3 Nb 2/3 O 3 –0.3PbSc 1/2 Nb 1/2 O 3 –0.3PbZn 1/3 Nb 2/3 O 3 (0.4PMN–0.3PSN–0.3PZN) ceramics were studied by means of FTIR reflection and THz transmission spectroscopy in the temperature range of −253.15 to 226.85 °C. On heating from low temperatures, the A 1 component of the strongly split TO 1 mode softens towards the Burns temperature, but the softening ceases near 126.85 °C which could be a signature of polar cluster percolation temperature. Surprisingly, the TO 2 mode also softens on heating and follows the Cochran law with extrapolated critical temperature close to the melting point.

Soft mode in PMN–PSN ceramics

2008

Various ordered and disordered (1–x)Pb(Mg1/3Nb2/3)O3 –x Pb(Sc1/2Nb1/2)O3 (PMN–PSN) ceramics were studied by THz transmission spectroscopy in the temperature range of 10–300 K. It is found that the dielectric relaxation dominates in the spectra in higher temperatures, T > 170 K. In contrast, below 150 K, the strength of relaxation becomes so weak that the phonon contribution can be separated. The phonon contributribution in the investigated frequency range (200 GHz–2 THz) is presumably caused by the E component of the Last mode. The component exhibits hardening on cooling in all investigated ceramics and its critical temperature in ferroelectric PMN–PSN ceramics (x = 1, 0.95) is lower than t…

Surface optical phonon modes in GaN nanowire arrays: Dependence on nanowire density and diameter

2012

We present a systematic study, by means of Raman scattering, of the surface optical modes of GaN nanowires (NW) as a function of the fill factor, defined as the relative concentration of GaN NWs and the surrounding air. We show that changes in the fill factor from 0.8 to 0.3 result in a shift of the surface optical mode of more than 60 cm${}^{\ensuremath{-}1}$, which is explained by theoretical calculations of surface vibrations for a cylindrical nanowire based on the dielectric continuum model. Two cases are considered: the effective dielectric function approximation (Maxwell-Garnett approximation) and a model for isolated NWs with various diameters. We conclude, in agreement with the Maxw…

Localized Motion in Supercooled Glycerol as Measured by 2 H-NMR Spin-Lattice Relaxation and Incoherent Neutron Scattering

1991

Selectively deuterated glycerol has been subjected to 2H-NMR spin-lattice relaxation and quasi-elastic neutron scattering experiments. The measurements yield relaxation rates and a non-Gaussian Q-dependence of the Debye-Waller factor which are different for the two hydrogen sites. The data analysis shows that below the onset of the glass transition α-process the hydrogens perform a local motion (≈ 10-12 s) in addition to what is expected from harmonic phonons. The resulting mean-square displacements are highly temperature dependent but are significantly smaller than those found in van der Waals glasses. Amplitudes and activation energies of the carbon-bonded and oxygen-bonded hydrogens are …

Behavior of the phonon gas in restricted geometries at low temperatures

1999

Review on Raman scattering in semiconductor nanowires: I. theory

2013

Raman scattering is a nondestructive technique that is able to supply information on the crystal and electronic structures, strain, temperature, phonon-phonon, and electron-phonon interaction. In the particular case of semiconductor nanowires, Raman scattering provides addi- tional information related to surfaces. Although correct, a theoretical approach to analyze the surface optical modes loses critical information when retardation is neglected. A comparison of the retarded and unretarded approaches clarifies the role of the electric and magnetic polarization in the Raman selection rules. Since most III-V compounds growing in the zincblende phase change their crystal structure to wurtzite…

Reduction of phonon thermal conductivity in nanowires and nanoribbons with dynamically rough surfaces and edges

2009

We present an analytical model and molecular-dynamics simulations of the phonon heat transport in nanowires and nanoribbons with anharmonic lattices and dynamically rough surfaces and edges. In agreement with recent experiments on heat transport in single-crystalline silicon nanowires with rough surfaces, our model and simulations predict finite and length-independent phonon thermal conductivity in such quasi–one-dimensional systems, in contrast to anomalous phonon thermal conductivity of corresponding momentum-conserving systems with atomically smooth surfaces, divergent with the system length. Within our model, the main cause of thermal conductivity reduction is the momentum-nonconserving…

Strain effects and phonon-plasmon coupled modes in Si-doped AlN

2009

The E 2h and A 1 (LO) phonon modes of AlN films grown on sapphire are analyzed by Raman scattering as a function of silicon doping for concentrations covering from 5.5 x 10 19 cm ―3 to 5.2 x 10 21 cm ―3 . For high doping levels the appearance of a mode around 520 cm ―1 indicates the precipitation of crystalline silicon in the samples and its inhomogeneous incorporation to the AlN layer. The frequency of this mode shifts to lower energies with doping, indicating that the silicon crystals are embedded in the AlN lattice and under tensile strain. On the other hand, the AlN phonon modes are blue-shifted due to the compressive strain as a result of the silicon incorporation. This strain is parti…