共查询到20条相似文献,搜索用时 26 毫秒
1.
A. I. Belousova Yu. E. Lozovikb 《The European Physical Journal D - Atomic, Molecular, Optical and Plasma Physics》2000,8(2):251-264
A two dimensional (2D) classical system of dipole particles confined by a quadratic potential is studied. This system can
be used as a model for rare electrons in semiconductor structures near a metal electrode, indirect excitons in coupled quantum
dots etc. For clusters of N ≤ 80 particles ground state configurations and appropriate eigenfrequencies and eigenvectors for the normal modes are found.
Monte-Carlo and molecular dynamic methods are used to study the order-disorder transition (the “melting” of clusters). In
mesoscopic clusters (N < 37) there is a hierarchy of transitions: at lower temperatures an intershell orientational disordering of pairs of shells
takes place; at higher temperatures the intershell diffusion sets in and the shell structure disappears. In “macroscopic”
clusters (N > 37) an orientational “melting” of only the outer shell is possible. The most stable clusters (having both maximal lowest
nonzero eigenfrequencies and maximal temperatures of total melting) are those of completed crystal shells which are concentric
groups of nodes of 2D hexagonal lattice with a number of nodes placed in the center of them. The picture of disordering in
clusters is compared with that in an infinite 2D dipole system. The study of the radial diffusion constant, the structure
factor, the local minima distribution and other quantities shows that the melting temperature is a nonmonotonic function of
the number of particles in the system. The dynamical equilibrium between “solid-like” and “orientationally disordered” forms
of clusters is considered. 相似文献
2.
Fujikawa Y Akiyama K Nagao T Sakurai T Lagally MG Hashimoto T Morikawa Y Terakura K 《Physical review letters》2002,88(17):176101
The structure of Ge(105)-(1 x 2) grown on Si(105) is examined by scanning tunneling microscopy (STM) and first-principles calculations. The morphology evolution with an increasing amount of Ge deposited documents the existence of a tensile surface strain in Si(105) and its relaxation with increasing coverage of Ge. A detailed analysis of high-resolution STM images and first-principles calculations produce a new stable model for the Ge(105)-(1 x 2) structure formed on the Si(105) surface that includes the existence of surface strain. It corrects the model developed from early observations of the facets of "hut" clusters grown on Si(001). 相似文献
3.
B. C. Curley G. Rossi R. Ferrando R. L. Johnston 《The European Physical Journal D - Atomic, Molecular, Optical and Plasma Physics》2007,43(1-3):53-56
Ag–Au bimetallic “nanoalloy” clusters with 38 atoms have been
studied using a Gupta many-body potential combined with a genetic
algorithm search technique. Clear changes in structure are observed as
a function of Ag/Au composition and there is a clear tendency for
surface segregation of the Ag atoms. Cluster stability is found to
increase with increasing number of Au-Au and Ag-Au bonds and the
segregation has been rationalised in terms of bonds strengths and
elemental surface energies. 相似文献
4.
We investigate, using numerical simulations and analytical arguments, a simple one-dimensional model for the swelling or the
collapse of a closed polymer chain of size N, representing the dynamical evolution of a polymer in a Θ-solvent that is rapidly changed into a good solvent (swelling)
or a bad solvent (collapse). In the case of swelling, the density profile for intermediate times is parabolic and expands
in space as t
1/3, as predicted by a Flory-like continuum theory. The dynamics slows down after a time ∝N
2 when the chain becomes stretched, and the polymer gets stuck in metastable “zig-zag” configurations, from which it escapes
through thermal activation. The size of the polymer in the final stages is found to grow as . In the case of collapse, the chain very quickly (after a time of order unity) breaks up into clusters of monomers (“pearls”).
The evolution of the chain then proceeds through a slow growth of the size of these metastable clusters, again evolving as
the logarithm of time. We enumerate the total number of metastable states as a function of the extension of the chain, and
deduce from this computation that the radius of the chain should decrease as 1/ln(ln t). We compute the total number of metastable states with a given value of the energy, and find that the complexity is non-zero
for arbitrary low energies. We also obtain the distribution of cluster sizes, that we compare to simple “cut-in-two” coalescence
models. Finally, we determine the aging properties of the dynamical structure. The subaging behaviour that we find is attributed
to the tail of the distribution at small cluster sizes, corresponding to anomalously “fast” clusters (as compared to the average).
We argue that this mechanism for subaging might hold in other slowly coarsening systems.
Received 23 October 2000 相似文献
5.
R. Thissen P. Lablanquie R.I. Hall M. Ukai K. Ito 《The European Physical Journal D - Atomic, Molecular, Optical and Plasma Physics》1998,4(3):335-342
Photoionization of rare gas clusters in the innervalence shell region has been investigated using threshold photoelectron
and photoion spectrometers and synchrotron radiation. Two classes of states are found to play an important role: (A) valence
states, correlated to dissociation limits involving an ion with a hole in its innervalence ns shell, (B) Rydberg states correlated to dissociation limits involving an ion with a hole in its outervalence np shell plus an excited neutral atom. In dimers, class A states are “bright”, that is, accessible by photoionization, and serve
as an entrance step to form the class B “dark” states; this character fades as the size of the cluster increases. In the dimer,
the “Mulliken” valence state is found to present a shallow potential well housing a few vibrational levels; it is predissociated by the
class B Rydberg states. During the predissociation a remarkable energy transfer process is observed from the excited ion that
loses its innershell electron to its neutral partner.
Received: 10 February 1998 / Revised: 17 July 1998 / Accepted: 31 July 1998 相似文献
6.
It is shown that, irrespective of the application of carbon or alumina as a support, the local structure of the “Co-sulfide”
phase formed during sulfidation of Co-and CoMo-catalysts is the same. A relation is found between the quadrupole splitting
(Q.S. value) of the “Co-sulfide” phase and its dispersion. The higher the dispersion, the larger the Q.S. value. The so-called
“Co-Mo-S” doublet is observed in all cases and it turns out to be related to a highly dispersed “Co-sulfide” phase instead
of a Co, Mo and S containing phase. 相似文献
7.
This paper discusses the effect of magnetic translational symmetry on the vortex structure in superconducting crystals with
a large basis in artificial Josephson media (regular lattices of superconducting clusters) prepared with opal as the base
material. For external magnetic fields lower than the upper critical field, the lattice Ginzburg-Landau model reduces to the
two-dimensional Frenkel’-Kontorova model which in some cases is exactly solvable, in which the crystal lattice plays the role
of an “hard sublattice” while the deformable vortex lattice plays the role of a “soft sublattice.” It is shown that static
shear waves in the vortex lattice are solutions to the two-dimensional sine-Gordon equation with an additional condition of
incompressibility implied by flux quantization. The pinning energy is found as a function of the magnetic field, nearness
to the transition line, and the crystal lattice constant.
Fiz. Tverd. Tela (St. Petersburg) 39, 1158–1162 (July 1997) 相似文献
8.
Astrophysical observations are pointing out huge amounts of “dark matter” and “dark energy” needed to explain the observed
large scale structure and cosmic dynamics. The emerging picture is a spatially flat, homogeneous Universe undergoing the today
observed accelerated phase. Despite of the good quality of astrophysical surveys, commonly addressed as Precision Cosmology, the nature and the nurture of dark energy and dark matter, which should constitute the bulk of cosmological matter-energy,
are still unknown. Furthermore, up to now, no experimental evidence has been found, at fundamental level, to explain such
mysterious components. The problem could be completely reversed considering dark matter and dark energy as “shortcomings”
of General Relativity in its simplest formulation (a linear theory in the Ricci scalar R, minimally coupled to the standard perfect fluid matter) and claiming for the “correct” theory of gravity as that derived
by matching the largest number of observational data, without imposing any theory a priori. As a working hypothesis, accelerating
behavior of cosmic fluid, large scale structure, potential of galaxy clusters, rotation curves of spiral galaxies could be
reproduced by means of extending the standard theory of General Relativity. In other words, gravity could acts in different ways at different scales and the
above “shortcomings” could be due to incorrect extrapolations of the Einstein gravity, actually tested at short scales and
low energy regimes. After a survey of what is intended for Extended Theories of Gravity in the so called “metric” and “Palatini” approaches, we discuss some cosmological and astrophysical applications where the
issues related to the dark components are addressed by enlarging the Einstein theory to more general f (R) Lagrangians, where f (R) is a generic function of Ricci scalar R, not assumed simply linear. Obviously, this is not the final answer to the problem of “dark-components” but it can be considered
as an operative scheme whose aim is to avoid the addition of unknown exotic ingredients to the cosmic pie. 相似文献
9.
Yu. Kh. Vekilov I. A. Gordeev É. I. Isaev 《Journal of Experimental and Theoretical Physics》1999,89(5):995-999
The electronic spectrum and wave functions of a new quasicrystal structure—a two-dimensional Fibonacci lattice—are investigated
in the tight-binding approximation using the method of the level statistics. This is a self-similar structure consisting of
three elementary structural units. The “central” and “nodal” decoration of this structure are examined. It is shown that the
electronic energy spectrum of a two-dimensional Fibonacci lattice contains a singular part, but in contrast to a one-dimensional
Fibonacci lattice the spectrum does not contain a hierarchical gap structure. The measure of allowed states (Lebesgue measure)
of the spectrum is different from zero, and for “central” decoration it is close to 1. The character of the localization of
the wave functions is investigated, and it is found that the wave functions are “critical.”
Zh. éksp. Teor. Fiz. 116, 1834–1842 (November 1999) 相似文献
10.
T. Tamir 《Applied Physics A: Materials Science & Processing》1981,25(3):201-210
Guided-wave methods used in the past to treat electromagnetic problems and applications in the microwave area have recently
been extended to cover work in fiber and integrated optics. The basic principles of these methods are reviewed briefly and,
in particular, the “open” properties of optical configurations are contrasted to the “closed” characteristics that describe
most microwave applications. These aspects are illustrated in the context of beam couplers of uniform and periodic varieties,
which are shown to lend themselves to rigorous treatment by microwave guided-wave methods that include both theoretical and
experimental facets. 相似文献
11.
We apply the time-dependent local density approximation to the small carbon clusters. The carbon clusters of chain and ring
shapes are found to show strong π-electron transitions. An interpretation is given for them as the one-dimensional plasmon
excitation.
Presented by K. Yabana at the International Conference on “Atomic Nuclei and Metallic Clusters”, Prague, September 1–5, 1997. 相似文献
12.
The question of whether the linear absorption spectra of metal clusters can be interpreted as density oscillations (collective
“plasmons”) or can only be understood as transitions between distinct molecular states is still a matter of debate for clusters
with only a few electrons. We calculate the photo-absorption spectra of Na2 and Na5
+ comparing two different methods: quantum fluid dynamics and time-dependent density functional theory. The changes in the
electronic structure associated with particular excitations are visualized in “snapshots” via transition densities. Our analysis
shows that even for the smallest clusters, the observed excitations can be interpreted as intuitively understandable density
oscillations. For Na5
+, the importance of self-interaction corrections to the adiabatic local density approximation is demonstrated.
Received: 1 July 2001 / Published online: 10 October 2001 相似文献
13.
14.
C. G. Jesudason A. P. Lyubartsev A. Laaksonen 《The European physical journal. E, Soft matter》2009,30(4):341-350
The behaviour of a flexible anionic chain of 150 univalent and negatively charged beads connected by a harmonic-like potential
with each other in the presence of an equal number of positive and free counterions, is studied in molecular dynamics simulations
with Langevin thermostat in a wide range of temperatures. Simulations were carried out for several values of the bending parameter,
corresponding to fully flexible polyion, moderately and strongly stiff polyion as well as for the case when bend conformation
is preferable to the straight one. We have found that in all cases three regimes can be distinguished, which can be characterized
as “random coil”, observed at high temperatures; “extended conformation” observed at moderate temperatures (of the order of
1 in reduced units), and compact “globular conformation” attained at low temperatures. While the transition between high-temperature
random and extended conformations is gradual, the transition from the extended coil to the globular state, taking place at
a temperature of about 0.2 in reduced units, is of abrupt character resembling a phase transition. 相似文献
15.
G. Chen J. G. Chen Y. J. Yang J. Q. Lin 《The European Physical Journal D - Atomic, Molecular, Optical and Plasma Physics》2010,57(1):145-149
We numerically calculate the high-order harmonic generation (HHG)
power spectra from a one-dimensional model atom irradiated by linearly
polarised 12 fs two-colour laser pulses composed of a fundamental pulse from
Ti:sapphire laser and its second harmonic. It is found that a distinct
double plateau structure appears when the relative phase of the two pulses
is set as π/8, 2π/8 or 3π/8, and the double plateau structure
disappears when the relative phase is set as 4π/8, 5π/8, 6π/8
or 7π/8. The relative-phase-dependent plateau structure is explained by
the temporal profile of the synthesised electric fields as well as the
semi-classical “three-step” model. Moreover, our numerical result shows
that cut-off frequencies of the two-colour pulse HHG spectra can be exactly
predicted by use of the semi-classical “three-step” model. 相似文献
16.
A. Faccinetto K. Thomson M. Ziskind C. Focsa 《Applied Physics A: Materials Science & Processing》2008,92(4):969-974
The response of polycyclic aromatic hydrocarbons (PAHs) to different desorption and ionization fluences has been investigated
in a laser desorption/multiphoton ionization/time-of-flight mass spectrometry scheme. The results evidence an intricate relationship
between the desorption and ionization steps, tentatively attributed to the amount of internal energy acquired by the desorbed
molecules. Different behaviors have been found for the various PAHs considered, leading to a parametric “signature” for each
species. Moreover, some insights on the fragmentation mechanism of the desorbed PAHs have been obtained, with possible interpretation
in the frame of a “ladder-switching” model. 相似文献
17.
Ronhovde P Chakrabarty S Hu D Sahu M Sahu KK Kelton KF Mauro NA Nussinov Z 《The European physical journal. E, Soft matter》2011,34(9):105
We elaborate on a general method that we recently introduced for characterizing the “natural” structures in complex physical
systems via multi-scale network analysis. The method is based on “community detection” wherein interacting particles are partitioned into an “ideal
gas” of optimally decoupled groups of particles. Specifically, we construct a set of network representations (“replicas”)
of the physical system based on interatomic potentials and apply a multiscale clustering (“multiresolution community detection”)
analysis using information-based correlations among the replicas. Replicas may i) be different representations of an identical
static system, ii) embody dynamics by considering replicas to be time separated snapshots of the system (with a tunable time
separation), or iii) encode general correlations when different replicas correspond to different representations of the entire
history of the system as it evolves in space-time. Inputs for our method are the inter-particle potentials or experimentally
measured two (or higher order) particle correlations. We apply our method to computer simulations of a binary Kob-Andersen
Lennard-Jones system in a mixture ratio of A80B20 , a ternary model system with components “A”, “B”, and “C” in ratios of A88B7C5 (as in Al88Y7Fe5 , and to atomic coordinates in a Zr80Pt20 system as gleaned by reverse Monte Carlo analysis of experimentally determined structure factors. We identify the dominant
structures (disjoint or overlapping) and general length scales by analyzing extrema of the information theory measures. We
speculate on possible links between i) physical transitions or crossovers and ii) changes in structures found by this method
as well as phase transitions associated with the computational complexity of the community detection problem. We also briefly
consider continuum approaches and discuss rigidity and the shear penetration depth in amorphous systems; this latter length
scale increases as the system becomes progressively rigid. 相似文献
18.
G. E. Astrakharchik A. I. Belousov Yu. E. Lozovik 《Journal of Experimental and Theoretical Physics》1999,89(4):696-703
A two-dimensional mesoscopic cluster of “dusty plasma” particles, which can be interpreted as a system of microparticles in
an rf gas discharge, is investigated. The ground-state configurations and corresponding eigenfrequencies and eigenvectors
are found for clusters of N=22–40 particles in a harmonic confining potential. It is shown that a change in the Debye screening length R of the particle charge in the plasma can cause structural transformations of the ground state of the system, manifested as
first-order or second-order phase transitions with respect to the parameter R. The disorder (“melting”) of the clusters is analyzed in detail by Monte Carlo simulation and molecular dynamics. By varying
the characteristic range of particle interaction in a cluster, it is possible to modulate its thermodynamic properties and
the character of the phase transitions, thereby causing a controlled transition of the system into the fully ordered, orientationally
disordered, or fully disordered state. The possibility of dusty plasma clusters coexisting in different states is discussed.
Zh. éksp. Teor. Fiz. 116, 1300–1312 (October 1999) 相似文献
19.
Giant magnetoresistance in Hg
1−x−y
Mn
x
Fe
y
Te crystals is caused by clusters with “antiferromagnetic” (Mn-Te-Mn-Te, Mn-Te-Fe-Te) and “ferromagnetic” (Fe-Fe-Fe) ordering. The effect is due to the fact that the charge carriers taking part in electric current interact with the “ferromagnetic”
cluster subsystem (Fe-Fe-Fe) magnetized to saturation and become spin-polarized. These spin-polarized charge carriers are strongly scattered by the “antiferromagnetic”
Mn-Te-Mn-Te and Mn-Te-Fe-Te clusters, because the magnetic moments inside the clusters and resultant moments of clusters have chaotic orientations. Investigations
of kinetic coefficients of Hg
1−x−y
Mn
x
Fe
y
Te crystals before and after thermal treatment show that there is no marked correlation between the giant magnetoresistance
and charge-carrier concentration, mobility, and band parameters of crystals.
__________
Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 10, pp. 28–33, October, 2007. 相似文献
20.
A dynamical non-Abelian 2-form gauge theory (with B∧F term) is endowed with the “scalar” and “vector” gauge symmetry transformations. In our present endeavor, we exploit the latter
gauge symmetry transformations and perform the Becchi–Rouet–Stora–Tyutin (BRST) analysis of the four (3+1)-dimensional (4D)
topologically massive non-Abelian 2-form gauge theory. We demonstrate the existence of some novel features that have, hitherto,
not been observed in the context of BRST approach to 4D (non-)Abelian 1-form as well as Abelian 2-form and 3-form gauge theories.
We comment on the differences between the novel features that emerge in the BRST analysis of the “scalar” and “vector” gauge
symmetries. 相似文献