8,402 research outputs found
Mirrorless Negative-index Parametric Micro-oscillator
The feasibility and extraordinary properties of mirrorless parametric
oscillations in strongly absorbing negative-index metamaterials are shown. They
stem from the backwardness of electromagnetic waves inherent to this type of
metamaterials.Comment: 4 pages, 2 figure
Dislocations in stacking and commensurate-incommensurate phase transition in bilayer graphene and hexagonal boron nitride
Dislocations corresponding to a change of stacking in two-dimensional
hexagonal bilayers, graphene and boron nitride, and associated with boundaries
between commensurate domains are investigated using the two-chain
Frenkel-Kontorova model on top of ab initio calculations. Structural
transformations of bilayers in which the bottom layer is stretched and the
upper one is left to relax freely are considered for gradually increased
elongation of the bottom layer. Formation energies of dislocations, dislocation
width and orientation of the boundary between commensurate domains are analyzed
depending on the magnitude and direction of elongation. The second-order phase
transition from the commensurate phase to the incommensurate one with multiple
dislocations is predicted to take place at some critical elongation. The order
parameter for this transition corresponds to the density of dislocations, which
grows continuously upon increasing the elongation of the bottom layer above the
critical value. In graphene and metastable boron nitride with the layers
aligned in the same direction, where elementary dislocations are partial, this
transition, however, is preceded by formation of the first dislocation at the
elongation smaller than the critical one. The phase diagrams including this
intermediate state are plotted in coordinates of the magnitude and direction of
elongation of the bottom layer.Comment: 15 pages, 9 figure
Comparison of performance of van der Waals-corrected exchange-correlation functionals for interlayer interaction in graphene and hexagonal boron nitride
Exchange-correlation functionals with corrections for van der Waals
interactions (PBE-D2, PBE-D3, PBE-D3(BJ), PBE-TS, optPBE-vdW and vdW-DF2) are
tested for graphene and hexagonal boron nitride, both in the form of bulk and
bilayer. The characteristics of the potential energy surface, such as the
barrier to relative sliding of the layers and magnitude of corrugation, and
physically measurable properties associated with relative in-plane and
out-of-plane motion of the layers including the shear modulus and modulus for
axial compression, shear mode frequency and frequency of out-of-plane
vibrations are considered. The PBE-D3(BJ) functional gives the best results for
the stackings of hexagonal boron nitride and graphite that are known to be
ground-state from the experimental studies. However, it fails to describe the
order of metastable states of boron nitride in energy. The PBE-D3 and vdW-DF2
functionals, which reproduce this order correctly, are identified as the
optimal choice for general studies. The vdW-DF2 functional is preferred for
evaluation of the modulus for axial compression and frequency of out-of-plane
vibrations, while the PBE-D3 functional is somewhat more accurate in
calculations of the shear modulus and shear mode frequency. The best
description of the latter properties, however, is achieved also using the
vdW-DF2 functional combined with consideration of the experimental interlayer
distance. In the specific case of graphene, the PBE-D2 functional works very
well and can be further improved by adjustment of the parameters.Comment: 22 pages, 4 figue
Transformation of amorphous carbon clusters to fullerenes
Transformation of amorphous carbon clusters into fullerenes under high
temperature is studied using molecular dynamics simulations at microsecond
times. Based on the analysis of both structure and energy of the system, it is
found that fullerene formation occurs in two stages. Firstly, fast
transformation of the initial amorphous structure into a hollow sp shell
with a few chains attached occurs with a considerable decrease of the potential
energy and the number of atoms belonging to chains and to the amorphous domain.
Then, insertion of remaining carbon chains into the sp network takes place
at the same time with the fullerene shell formation. Two types of defects
remaining after the formation of the fullerene shell are revealed: 7-membered
rings and single one-coordinated atoms. One of the fullerene structures
obtained contains no defects at all, which demonstrates that defect-free carbon
cages can be occasionally formed from amorphous precursors directly without
defect healing. No structural changes are observed after the fullerene
formation, suggesting that defect healing is a slow process in comparison with
the fullerene shell formation. The schemes of the revealed reactions of chain
atoms insertion into the fullerene shell just before its completion are
presented. The results of the performed simulations are summarized within the
paradigm of fullerene formation due to selforganization of the carbon system.Comment: 35 pages, 9 figure
Four-wave mixing, quantum control and compensating losses in doped negative-index photonic metamaterials
The possibility of compensating absorption in negative-index metatamterials
(NIMs) doped by resonant nonlinear-optical centers is shown. The role of
quantum interference and extraordinary properties of four-wave parametric
amplification of counter-propagating electromagnetic waves in NIMs are
discussed.Comment: 3 pages, 3 figures, LaTeX, corrected typos in eqs. (1) and (2
Merging Nonlinear Optics and Negative-Index Metamaterials
The extraordinary properties of nonlinear optical propagation processes in
double-domain positive/negative index metamaterials are reviewed. These
processes include second harmonic generation, three- and four-wave frequency
mixing, and optical parametric amplification. Striking contrasts with the
properties of the counterparts in ordinary materials are shown. We also discuss
the possibilities for compensating strong losses inherent to plasmonic
metamaterials, which present a major obstacle in numerous exciting
applications, and the possibilities for creation of unique ultracompact
photonic devices such as data processing chips and nonlinear-optical sensors.
Finally, we propose similar extraordinary three-wave mixing processes in
crystals based on optical phonons with negative dispersion.Comment: A review paper, 27 pages, 16 figures, 81 reference
Explicit Non-Abelian Monopoles and Instantons in SU(N) Pure Yang-Mills Theory
It is well known that there are no static non-Abelian monopole solutions in
pure Yang-Mills theory on Minkowski space R^{3,1}. We show that such solutions
exist in SU(N) gauge theory on the spaces R^2\times S^2 and R^1\times S^1\times
S^2 with Minkowski signature (-+++). In the temporal gauge they are solutions
of pure Yang-Mills theory on T^1\times S^2, where T^1 is R^1 or S^1. Namely,
imposing SO(3)-invariance and some reality conditions, we consistently reduce
the Yang-Mills model on the above spaces to a non-Abelian analog of the \phi^4
kink model whose static solutions give SU(N) monopole (-antimonopole)
configurations on the space R^{1,1}\times S^2 via the above-mentioned
correspondence. These solutions can also be considered as instanton
configurations of Yang-Mills theory in 2+1 dimensions. The kink model on
R^1\times S^1 admits also periodic sphaleron-type solutions describing chains
of n kink-antikink pairs spaced around the circle S^1 with arbitrary n>0. They
correspond to chains of n static monopole-antimonopole pairs on the space
R^1\times S^1\times S^2 which can also be interpreted as instanton
configurations in 2+1 dimensional pure Yang-Mills theory at finite temperature
(thermal time circle). We also describe similar solutions in Euclidean SU(N)
gauge theory on S^1\times S^3 interpreted as chains of n
instanton-antiinstanton pairs.Comment: 16 pages; v2: subsection on topological charges added, title
expanded, some coefficients corrected, version to appear in PR
Enhanced four-wave mixing via elimination of inhomogeneous broadening by coherent driving of quantum transition with control fields
We show that atoms from wide velocity interval can be concurrently involved
in Doppler-free two-photon resonant far from frequency degenerate four-wave
mixing with the aid of auxiliary electromagnetic field. This gives rise to
substantial enhancement of the output radiation generated in optically thick
medium. Numerical illustrations addressed to typical experimental conditions
are given.Comment: LaTeX2e, hyperref, 7 pages, 5 figures, to appear in PRA 1 august 200
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