462 research outputs found
A First-Principles Approach to Insulators in Finite Electric Fields
We describe a method for computing the response of an insulator to a static,
homogeneous electric field. It consists of iteratively minimizing an electric
enthalpy functional expressed in terms of occupied Bloch-like states on a
uniform grid of k points. The functional has equivalent local minima below a
critical field E_c that depends inversely on the density of k points; the
disappearance of the minima at E_c signals the onset of Zener breakdown. We
illustrate the procedure by computing the piezoelectric and nonlinear
dielectric susceptibility tensors of III-V semiconductors.Comment: 4 pages, with 1 postscript figure embedded. Uses REVTEX and epsf
macros. Also available at
http://www.physics.rutgers.edu/~dhv/preprints/is_ef/index.htm
Symmetry breaking and gap opening in two-dimensional hexagonal lattices
9 páginas, 3 figuras.-- et al.The inhibition in wave propagation at band gap energies plays a central role in many areas of technology such as electronics (electron gaps), nanophotonics (light gaps) and phononics (acoustic gaps), among others. Here we demonstrate that metal surfaces featuring free-electron-like bands may become semiconducting by periodic nanostructuration. We combine scanning tunneling spectroscopy and angle-resolved photoemisssion to accurately determine the energy-dependent local density of states and band structure of the Ag/Cu(111) noble metal interface patterned with an array of triangular dislocations, demonstrating the existence of a 25 meV band gap that extends over the entire surface Brillouin zone. We prove that this gap is a general consequence of symmetry reduction in close-packed metallic overlayers; in particular, we show that the gap opening is due to the symmetry lowering of the wave vector group at the K point from C3v to C3.This work was supported in part by the Spanish MICINN (MAT2007-66050, MAT2007-63083
and Consolider NanoLight.es), the EU (NMP4-SL-2008-213669-ENSEMBLE), the Basque
Government (IT-257-07) and the Centre National de la Recherche Scientifique (CNRS).Peer reviewe
Enhancement of piezoelectricity in a mixed ferroelectric
We use first-principles density-functional total energy and polarization
calculations to calculate the piezoelectric tensor at zero temperature for both
cubic and simple tetragonal ordered supercells of Pb_3GeTe_4. The largest
piezoelectric coefficient for the tetragonal configuration is enhanced by a
factor of about three with respect to that of the cubic configuration. This can
be attributed to both the larger strain-induced motion of cations relative to
anions and higher Born effective charges in the tetragonal case. A normal mode
decomposition shows that both cation ordering and local relaxation weaken the
ferroelectric instability, enhancing piezoelectricity.Comment: 5 pages, revtex, 2 eps figure
Self-Organization and the Physics of Glassy Networks
Network glasses are the physical prototype for many self-organized systems,
ranging from proteins to computer science. Conventional theories of gases,
liquids, and crystals do not account for the strongly material-selective
character of the glass-forming tendency, the phase diagrams of glasses, or
their optimizable properties. A new topological theory, only 25 years old, has
succeeded where conventional theories have failed. It shows that (probably all
slowly quenched) glasses, including network glasses, are the result of the
combined effects of a few simple mechanisms. These glass-forming mechanisms are
topological in nature, and have already been identified for several important
glasses, including chalcogenide alloys, silicates (window glass, computer
chips), and proteins.Comment: One PDF file contains 10 figures and tex
Формирование городской идентичности как способ развития туристского потенциала г.Томска
Следствием глобализации становится унификация современных городов. Происходит нивелирование их уникальных черт: их культурной, социальной и визуальной идентичности. Процесс постепенной утраты уникальности и традиций города неизбежно влечет за собой снижение его привлекательности для туристов. Город, без ярко выраженного "лица": узнаваемого визуального образа и устойчивых смысловых ассоциаций, неизбежно, сдает свои позиции на современном высоко конкурентном рынке туристских услуг.The consequence of globalization is the unification of modern cities. There is a leveling of their unique features: their cultural, social and visual identity. The process of gradual loss of uniqueness and traditions. The city, without a pronounced "face": a recognizable visual image and stable semantic associations, inevitably, surrenders its positions in the modern highly competitive market of tourist services
Vibrational signature of broken chemical order in a GeS2 glass: a molecular dynamics simulation
Using density functional molecular dynamics simulations, we analyze the
broken chemical order in a GeS glass and its impact on the dynamical
properties of the glass through the in-depth study of the vibrational
eigenvectors. We find homopolar bonds and the frequencies of the corresponding
modes are in agreement with experimental data. Localized S-S modes and 3-fold
coordinated sulfur atoms are found to be at the origin of specific Raman peaks
whose origin was not previously clear. Through the ring size statistics we
find, during the glass formation, a conversion of 3-membered rings into larger
units but also into 2-membered rings whose vibrational signature is in
agreement with experiments.Comment: 11 pages, 8 figures; to appear in Phys. Rev.
Ab initio Hartree-Fock Born effective charges of LiH, LiF, LiCl, NaF, and NaCl
We use the Berry-phase-based theory of macroscopic polarization of dielectric
crystals formulated in terms of Wannier functions, and state-of-the-art
Gaussian basis functions, to obtain benchmark ab initio Hartree-Fock values of
the Born effective charges of ionic compounds LiH, LiF, LiCl, NaF, and NaCl. We
find excellent agreement with the experimental values for all the compounds
except LiCl and NaCl, for which the disagreement with the experiments is close
to 10% and 16%, respectively. This may imply the importance of many-body
effects in those systems.Comment: 11 pages, Revtex, 2 figures (included), to appear in Phys. Rev. B
April 15, 200
Raman scattering study of the a-GeTe structure and possible mechanism for the amorphous-to-crystal transition
We report on an inelastic (Raman) light scattering study of the local
structure of amorphous GeTe films. A detailed analysis of the
temperature-reduced Raman spectra has shown that appreciable structural changes
occur as a function of temperature. These changes involve modifications of
atomic arrangements such as to facilitate the rapid amorphous-to-crystal
transformation, which is the major advantage of phase-change materials used in
optical data storage media. A particular structural model, supported by
polarization analysis, is proposed being compatible with the experimental data
as regards both the structure of a-GeTe and the crystallization transition. The
remarkable difference between the Raman spectrum of the crystal and the glass
can thus naturally be accounted for.Comment: Published in: J. Phys. Condens. Matter. 18, 965-979 (2006
The physics of dynamical atomic charges: the case of ABO3 compounds
Based on recent first-principles computations in perovskite compounds,
especially BaTiO3, we examine the significance of the Born effective charge
concept and contrast it with other atomic charge definitions, either static
(Mulliken, Bader...) or dynamical (Callen, Szigeti...). It is shown that static
and dynamical charges are not driven by the same underlying parameters. A
unified treatment of dynamical charges in periodic solids and large clusters is
proposed. The origin of the difference between static and dynamical charges is
discussed in terms of local polarizability and delocalized transfers of charge:
local models succeed in reproducing anomalous effective charges thanks to large
atomic polarizabilities but, in ABO3 compounds, ab initio calculations favor
the physical picture based upon transfer of charges. Various results concerning
barium and strontium titanates are presented. The origin of anomalous Born
effective charges is discussed thanks to a band-by-band decomposition which
allows to identify the displacement of the Wannier center of separated bands
induced by an atomic displacement. The sensitivity of the Born effective
charges to microscopic and macroscopic strains is examined. Finally, we
estimate the spontaneous polarization in the four phases of barium titanate.Comment: 25 pages, 6 Figures, 10 Tables, LaTe
Thermally Induced Nano-Structural and Optical Changes of nc-Si:H Deposited by Hot-Wire CVD
We report on the thermally induced changes of the nano-structural and optical properties of hydrogenated nanocrystalline silicon in the temperature range 200–700 °C. The as-deposited sample has a high crystalline volume fraction of 53% with an average crystallite size of ~3.9 nm, where 66% of the total hydrogen is bonded as ≡Si–H monohydrides on the nano-crystallite surface. A growth in the native crystallite size and crystalline volume fraction occurs at annealing temperatures ≥400 °C, where hydrogen is initially removed from the crystallite grain boundaries followed by its removal from the amorphous network. The nucleation of smaller nano-crystallites at higher temperatures accounts for the enhanced porous structure and the increase in the optical band gap and average gap
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