3,337 research outputs found
Stacking Order dependent Electric Field tuning of the Band Gap in Graphene Multilayers
The effect of different stacking order of graphene multilayers on the
electric field induced band gap is investigated. We considered a positively
charged top and a negatively charged back gate in order to independently tune
the band gap and the Fermi energy of three and four layer graphene systems. A
tight-binding approach within a self-consistent Hartree approximation is used
to calculate the induced charges on the different graphene layers. We found
that the gap for trilayer graphene with the ABC stacking is much larger than
the corresponding gap for the ABA trilayer. Also we predict that for four
layers of graphene the energy gap strongly depends on the choice of stacking,
and we found that the gap for the different types of stacking is much larger as
compared to the case of Bernal stacking. Trigonal warping changes the size of
the induced electronic gap by approximately 30% for intermediate and large
values of the induced electron density
Photon recoil momentum in a Bose-Einstein condensate of a dilute gas
We develop a "minimal" microscopic model to describe a
two-pulse-Ramsay-interferometer-based scheme of measurement of the photon
recoil momentum in a Bose-Einstein condensate of a dilute gas [Campbell et al.,
Phys. Rev. Lett. 94, 170403 (2005)]. We exploit the truncated coupled
Maxwell-Schroedinger equations to elaborate the problem. Our approach provides
a theoretical tool to reproduce essential features of the experimental results.
Additionally, we enable to calculate the quantum-mechanical mean value of the
recoil momentum and its statistical distribution that provides a detailed
information about the recoil event.Comment: 6 pages, 4 figure
Diagrammatic quantum field formalism for localized electrons
We introduce a diagrammatic quantum field formalism for the evaluation of
normalized expectation values of operators, and suitable for systems with
localized electrons. It is used to develop a convergent series expansion for
the energy in powers of overlap integrals of single-particle orbitals. This
method gives intuitive and practical rules for writing down the expansion to
arbitrary order of overlap, and can be applied to any spin configuration and to
any dimension. Its applicability for systems with well localized electrons has
been illustrated with examples, including the two-dimensional Wigner crystal
and spin-singlets in the low-density electron gas.Comment: 13 pages, 0 figure
D- shallow donor near a semiconductor-metal and a semiconductor-dielectric interface
The ground state energy and the extend of the wavefunction of a negatively
charged donor (D-) located near a semiconductor-metal or a
semiconductor-dielectric interface is obtained. We apply the effective mass
approximation and use a variational two-electron wavefunction that takes into
account the influence of all image charges that arise due to the presence of
the interface, as well as the correlation between the two electrons bound to
the donor. For a semiconductor-metal interface, the D- binding energy is
enhanced for donor positions d>1.5a_B (a_B is the effective Bohr radius) due to
the additional attraction of the electrons with their images. When the donor
approaches the interface (i.e. d<1.5a_B) the D- binding energy drops and
eventually it becomes unbound. For a semiconductor-dielectric (or a
semiconductor-vacuum) interface the D- binding energy is reduced for any donor
position as compared to the bulk case and the system becomes rapidly unbound
when the donor approaches the interface.Comment: Submitted to Phys. Rev. B on 19 November 200
Low-temperature kinetics of exciton-exciton annihilation of weakly localized one-dimensional Frenkel excitons
We present results of numerical simulations of the kinetics of
exciton-exciton annihilation of weakly localized one-dimensional Frenkel
excitons at low temperatures. We find that the kinetics is represented by two
well-distinguished components: a fast short-time decay and a very slow
long-time tail. The former arises from excitons that initially reside in states
belonging to the same localization segment of the chain, while the slow
component is caused by excitons created on different localization segments. We
show that the usual bi-molecular theory fails in the description of the
behavior found. We also present a qualitative analytical explanation of the
non-exponential behavior observed in both the short- and the long-time decay
components.Comment: Published in J. Chem. Phys. 114, 1 April (2001
Photon Recoil in Light Scattering by a Bose-Einstein Condensate of a Dilute Gas
Abstract: Photon recoil upon light scattering by a Bose–Einstein condensate (BEC) of a dilute atomic gas is analyzed theoretically accounting for a weak interatomic interaction. Our approach is based on the Gross–Pitaevskii equation for the condensate, which is coupled to the Maxwell equation for the field. The dispersion relations of recoil energy and momentum are calculated, and the effect of weak nonideality of the condensate on the photon recoil is ubraveled. A good agreement between the theory and experiment [7] on the measurement of the photon recoil momentum in a dispersive medium is demonstrated
The Relative Role of Land in Climate Policy
Replaced with revised version of paper 06/26/09.Climate policy, land use related emissions, carbon forest sequestration, CGE, Environmental Economics and Policy,
Molecular analysis of sensory axon branching unraveled a cGMP-dependent signaling cascade
Axonal branching is a key process in the establishment of circuit connectivity within the nervous system. Molecular-genetic studies have shown that a specific form of axonal branching - the bifurcation of sensory neurons at the transition zone between the peripheral and the central nervous System - is regulated by a cyclic guanosine monophosphate (cGMP)-dependent signaling cascade which is composed of C-type natriuretic peptide (CNP), the receptor guanylyl cyclase Npr2, and cGMP-dependent protein kinase Iα (cGKIα). In the absence of any one of these components, neurons in dorsal root ganglia (DRG) and cranial sensory ganglia no longer bifurcate, and instead turn in either an ascending or a descending direction. In contrast, collateral axonal branch formation which represents a second type of axonal branch formation is not affected by inactivation of CNP, Npr2, or cGKI. Whereas axon bifurcation was lost in mouse mutants deficient for components of CNP-induced cGMP formation; the absence of the cGMP-degrading enzyme phosphodiesterase 2A had no effect on axon bifurcation. Adult mice that lack sensory axon bifurcation due to the conditional inactivation of Npr2-mediated cGMP signaling in DRG neurons demonstrated an altered shape of sensory axon terminal fields in the spinal cord, indicating that elaborate compensatory mechanisms reorganize neuronal circuits in the absence of bifurcation. On a functional level, these mice showed impaired heat sensation and nociception induced by chemical irritants, whereas responses to cold sensation, mechanical stimulation, and motor coordination are normal. These data point to a critical role of axon bifurcation for the processing of acute pain perception
CEMENT SELECTION IN DENTAL PRACTICE
The cementation procedure is considered to be the most important stage of fixed prosthodontics, while the correct selection of cement is the guarantee of successful restoration which is conditioned by the its durability. Over the recent years, numerous cementing substances have been introduced to the dental practice that differ greatly from conventional cements with their properties and application methods and that is why even experienced dentists often have certain difficulties in the variety of cements to choose the one that is right for each clinical case. The selection of cement depends on a number of factors, such as the type of resorption substance, the shape of prepared tooth, the possibility to isolate the area, subject to cementation in the oral cavity as well as the patient’s aesthetic requirements. Thus, the objective of the article is to analyze currently used dental cements in order to help the dentists make the right selection of cement for different clinical cases.The cementation procedure is considered to be the most important stage of fixed prosthodontics, while the correct selection of cement is the guarantee of successful restoration which is conditioned by the its durability. Over the recent years, numerous cementing substances have been introduced to the dental practice that differ greatly from conventional cements with their properties and application methods and that is why even experienced dentists often have certain difficulties in the variety of cements to choose the one that is right for each clinical case. The selection of cement depends on a number of factors, such as the type of resorption substance, the shape of prepared tooth, the possibility to isolate the area, subject to cementation in the oral cavity as well as the patient’s aesthetic requirements. Thus, the objective of the article is to analyze currently used dental cements in order to help the dentists make the right selection of cement for different clinical cases
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