2,159 research outputs found
Phonons and related properties of extended systems from density-functional perturbation theory
This article reviews the current status of lattice-dynamical calculations in
crystals, using density-functional perturbation theory, with emphasis on the
plane-wave pseudo-potential method. Several specialized topics are treated,
including the implementation for metals, the calculation of the response to
macroscopic electric fields and their relevance to long wave-length vibrations
in polar materials, the response to strain deformations, and higher-order
responses. The success of this methodology is demonstrated with a number of
applications existing in the literature.Comment: 52 pages, 14 figures, submitted to Review of Modern Physic
Coherent phenomena in semiconductors
A review of coherent phenomena in photoexcited semiconductors is presented.
In particular, two classes of phenomena are considered: On the one hand the
role played by optically-induced phase coherence in the ultrafast spectroscopy
of semiconductors; On the other hand the Coulomb-induced effects on the
coherent optical response of low-dimensional structures.
All the phenomena discussed in the paper are analyzed in terms of a
theoretical framework based on the density-matrix formalism. Due to its
generality, this quantum-kinetic approach allows a realistic description of
coherent as well as incoherent, i.e. phase-breaking, processes, thus providing
quantitative information on the coupled ---coherent vs. incoherent--- carrier
dynamics in photoexcited semiconductors.
The primary goal of the paper is to discuss the concept of quantum-mechanical
phase coherence as well as its relevance and implications on semiconductor
physics and technology. In particular, we will discuss the dominant role played
by optically induced phase coherence on the process of carrier photogeneration
and relaxation in bulk systems. We will then review typical field-induced
coherent phenomena in semiconductor superlattices such as Bloch oscillations
and Wannier-Stark localization. Finally, we will discuss the dominant role
played by Coulomb correlation on the linear and non-linear optical spectra of
realistic quantum-wire structures.Comment: Topical review in Semiconductor Science and Technology (in press)
(Some of the figures are not available in electronic form
Intrinsic dissipation in high-frequency micromechanical resonators
We report measurements of intrinsic dissipation in micron-sized suspended resonators machined from single crystals of galium arsenide and silicon. In these experiments on high-frequency micromechanical resonators, designed to understand intrinsic mechanisms of dissipation, we explore dependence of dissipation on temperature, magnetic field, frequency, and size. In contrast to most of the previous measurements of acoustic attenuation in crystalline and amorphous structures in this frequency range, ours is a resonant measurement; dissipation is measured at the natural frequencies of structural resonance, or modes of the structure associated with flexural and torsional motion. In all our samples we find a weakly temperature dependent dissipation at low temperatures. We compare and contrast our data to various probable mechanisms, including thermoelasticity, clamping, anharmonic mode-coupling, surface anisotropy and defect motion, both in bulk and on surface. The observed parametric dependencies indicate that the internal defect motion is the dominant mechanism of intrinsic dissipation in our samples
Coherent exciton transport in semiconductors
We review the topic of Bose-Einstein condensation of excitons in
semiconductors, focusing on the signatures of the macroscopic order of the
exciton condensate.Comment: Some references were updated with respect to the published version.
appears as Chapter 19 in Novel Superfluids Volume 2, edited by K. H.
Bennemann and J. B. Ketterson, International Series of Monographs on Physics
no. 157, pages 423-474 (Oxford University Press, Oxford, 2014
Size effect on thermodynamic properties of free nanocrystals
We demonstrate that the discrete character of the vibrational spectrum of a
small crystal accounts for size dependence of its thermodynamic properties and
melting temperature. Using a self-consistent statistical method [Phys. Rev.B
66, 054302 (2002)] we derive the Gibbs free energy of free nanocrystalline
plates and calculate the thermodynamic parameters as functions of plate
thickness for Cu.Comment: 24 pages, 11 figure
Equilibrium and Non-Equilibrium Ultrafast Carrier Transport and Dynamics in Chalcopyrite Semiconductors
Chalcopyrite crystals in the II-IV-V2 family have received significant interest due to their high nonlinearity, composition-tunable bandgaps, wide transparency windows, and high damage threshold. These semiconductors have been explored for electromagnetic (EM) screening, spintronic and photovoltaic applications, making them good optical and optoelectronic materials. This thesis uses terahertz spectroscopy to understand optical, electronic, and vibrational dynamical processes in CdGeP2, ZnGeP2 and CdSiP2 chalcopyrite semiconductors.
We have employed Terahertz time-domain spectroscopy to investigate temperature-dependent ground-state properties of bulk chalcopyrite crystals that can be related to electronic transport and electron-lattice interactions. The complex spectra provide refraction and absorption from which electron-phonon coupling and average phonon energies are extracted. AC conductivity spectra provide carrier densities and electron scattering times, the temperature dependence of which are associated with unintentional shallow dopants. Temperature dependence of the scattering time is converted into carrier mobility and modeled with microscopic transport mechanisms such as polar optical phonon, acoustic phonons, deformation potential, ionized impurity, and dislocation scattering. Hence, analysis links the terahertz response to properties that can only be obtained using high-frequency AC Hall-effect measurements, Raman scattering, and conventional continuous-wave spectroscopy.
Ultrafast photoexcited charge-carrier dynamics are investigated using time-resolved terahertz spectroscopy. Analysis of the differential THz transient reveals a two-component exponential relaxation, with decay times that increase with pump fluence due to state filling. Inverting the transients recaptures the canonical rate-equation for the system and from which the recombination dynamics are attributed to the entire excitation range. To support the recombination information, THz photoconductivity determines the excited-state carrier transport as a function of temperature. Both elevated temperatures and increased excitation density decreases excited-state carrier mobility and are attributed to carrier-carrier interactions and scattering from phonons.
These model semiconductors, which have application in photonics, allow for further demonstration of pulsed-THz-based spectroscopy to capture both charge transport and dynamics. Additionally, several charge transport and carrier-lattice properties for CdGeP2, ZnGeP2, and CdSiP2 have been reported that can assist with the design and optimize of devices that comprise them
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