71 research outputs found
New Approach to Nonlinear Dynamics of Fullerenes and Fullerites
New type of nonlinear (anharmonic) excitations -- bushes of vibrational modes
-- in physical systems with point or space symmetry are discussed. All infrared
active and Raman active bushes for C60 fulerene are found by means of special
group-theoretical methods.Comment: LaTeX, 8 pages, to be published in Fizika Tverdogo Tela, 200
Stability of Nonlinear Normal Modes in the FPU- Chain in the Thermodynamic Limit
All possible symmetry-determined nonlinear normal modes (also called by
simple periodic orbits, one-mode solutions etc.) in both hard and soft
Fermi-Pasta-Ulam- chains are discussed. A general method for studying
their stability in the thermodynamic limit, as well as its application for each
of the above nonlinear normal modes are presented
Properties of discrete breathers in graphane from ab initio simulations
A density functional theory (DFT) study of the discrete breathers (DBs) in
graphane (fully hydrogenated graphene) was performed. To the best of our
knowledge, this is the first demonstration of the existence of DBs in a
crystalline body from the first-principle simulations. It is found that the DB
is a robust, highly localized vibrational mode with one hydrogen atom
oscillating with a large amplitude along the direction normal to the graphane
plane with all neighboring atoms having much smaller vibration amplitudes. DB
frequency decreases with increase in its amplitude, and it can take any value
within the phonon gap and can even enter the low-frequency phonon band. The
concept of DB is then used to propose an explanation to the recent experimental
results on the nontrivial kinetics of graphane dehydrogenation at elevated
temperatures.Comment: 20.07.14 Submitted to PhysRev
Stability analysis of dynamical regimes in nonlinear systems with discrete symmetries
We present a theorem that allows to simplify linear stability analysis of
periodic and quasiperiodic nonlinear regimes in N-particle mechanical systems
(both conservative and dissipative) with different kinds of discrete symmetry.
This theorem suggests a decomposition of the linearized system arising in the
standard stability analysis into a number of subsystems whose dimensions can be
considerably less than that of the full system. As an example of such
simplification, we discuss the stability of bushes of modes (invariant
manifolds) for the Fermi-Pasta-Ulam chains and prove another theorem about the
maximal dimension of the above mentioned subsystems
Turbulent structure of the Arctic boundary layer in early summer driven by stability, wind shear and cloud-top radiative cooling: ACLOUD airborne observations
Clouds are assumed to play an important role in the Arctic amplification process. This motivated a detailed investigation of cloud processes, including radiative and turbulent fluxes. Data from the aircraft campaign ACLOUD were analyzed with a focus on the mean and turbulent structure of the cloudy boundary layer over the Fram Strait marginal sea ice zone in late spring and early summer 2017. Vertical profiles of turbulence moments are presented from contrasting atmospheric boundary layers (ABLs) from 4 d. They differ by the magnitude of wind speed, boundary-layer height, stability, the strength of the cloud-top radiative cooling and the number of cloud layers. Turbulence statistics up to third-order moments are presented, which were obtained from horizontal-level flights and from slanted profiles. It is shown that both of these flight patterns complement each other and form a data set that resolves the vertical structure of the ABL turbulence well. The comparison of the 4 d shows that especially during weak wind, even in shallow Arctic ABLs with mixing ratios below 3 g kg−1, cloud-top cooling can serve as a main source of turbulent kinetic energy (TKE). Well-mixed ABLs are generated where TKE is increased and vertical velocity variance shows pronounced maxima in the cloud layer. Negative vertical velocity skewness points then to upside-down convection. Turbulent heat fluxes are directed upward in the cloud layer as a result of cold downdrafts. In two cases with single-layer stratocumulus, turbulent transport of heat flux and of temperature variance are both negative in the cloud layer, suggesting an important role of large eddies. In contrast, in a case with weak cloud-top cooling, these quantities are positive in the ABL due to the heating from the surface.
Based on observations and results of a mixed-layer model it is shown that the maxima of turbulent fluxes are, however, smaller than the jump of the net terrestrial radiation flux across the upper part of a cloud due to the (i) shallowness of the mixed layer and (ii) the presence of a downward entrainment heat flux. The mixed-layer model also shows that the buoyancy production of TKE is substantially smaller in stratocumulus over the Arctic sea ice compared to subtropics due to a smaller surface moisture flux and smaller decrease in specific humidity (or even humidity inversions) right above the cloud top.
In a case of strong wind, wind shear shapes the ABL turbulent structure, especially over rough sea ice, despite the presence of a strong cloud-top cooling. In the presence of mid-level clouds, cloud-top radiative cooling and thus also TKE in the lowermost cloud layer are strongly reduced, and the ABL turbulent structure becomes governed by stability, i.e., by the surface–air temperature difference and wind speed. A comparison of slightly unstable and weakly stable cases shows a strong reduction of TKE due to increased stability even though the absolute value of wind speed was similar. In summary, the presented study documents vertical profiles of the ABL turbulence with a high resolution in a wide range of conditions. It can serve as a basis for turbulence closure evaluation and process studies in Arctic clouds.</p
Bushes of Nonlinear Normal Modes in Single-Layer Graphene
In-plane vibrations in uniformly stretched single-layer graphene (space group P6mm), which are described by
the Rosenberg nonlinear normal modes (NNMs) and their bushes, are studied with the aid of group-theoretical
methods developed by authors in some earlier papers. It was found that only 4 symmetry-determined NNMs
(one-dimensional bushes), as well as 14 two-dimensional bushes are possible in graphene. They are exact
solutions to the dynamical equations of this two-dimensional crystal. The verification of group-theoretical
results with the aid of ab initio simulations based on density functional theory is discussed
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