587 research outputs found
Mesoscopic and microscopic dipole clusters: Structure and phase transitions
Two dimensional (2D) classical system of dipole particles confined by a
quadratic potential is studied. For clusters of N < 81 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 in
detail 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 that 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 study of different quantities shows that the melting temperature is a
nonmonotonic function of the number of particles in the system. The dynamical
equilibrium between "solidlike" and "orientationally disordered" forms of
clusters is considered.Comment: 12 pages, 16 Postscript figures. Submitted to Phys. Rev.
Time evolution of a pair of distinguishable interacting spins subjected to controllable and noisy magnetic fields
The quantum dynamics of a
-conserving
Hamiltonian model describing two coupled spins and
under controllable and fluctuating time-dependent magnetic
fields is investigated. Each eigenspace of is dynamically
invariant and the Hamiltonian of the total system restricted to any one of such
eigenspaces, possesses the SU(2) structure of the
Hamiltonian of a single fictitious spin acted upon by the total magnetic field.
We show that such a reducibility holds regardless of the time dependence of the
externally applied field as well as of the statistical properties of the noise,
here represented as a classical fluctuating magnetic field. The time evolution
of the joint transition probabilities of the two spins and
between two prefixed factorized states is examined,
bringing to light peculiar dynamical properties of the system under scrutiny.
When the noise-induced non-unitary dynamics of the two coupled spins is
properly taken into account, analytical expressions for the joint Landau-Zener
transition probabilities are reported. The possibility of extending the
applicability of our results to other time-dependent spin models is pointed
out.Comment: 11 pages, 5 figure
Phase diagram of 2D array of mesoscopic granules
A lattice boson model is used to study ordering phenomena in regular 2D array
of superconductive mesoscopic granules, Josephson junctions or pores filled
with a superfluid helium. Phase diagram of the system, when quantum
fluctuations of both the phase and local superfluid density are essential, is
analyzed both analytically and by quantum Monte Carlo technique. For the system
of strongly interacting bosons it is found that as the boson density is
increased the boundary of ordered superconducting state shifts to {\it lower
temperatures} and at approaches its limiting position corresponding
to negligible relative fluctuations of moduli of the order parameter (as in an
array of "macroscopic" granules). In the region of weak quantum fluctuations of
phases mesoscopic phenomena manifest themselves up to . The mean
field theory and functional integral - expansion results are shown to
agree with that of quantum Monte Carlo calculations of the boson Hubbard model
and its quasiclassical limit, the quantum XY model.Comment: 7 pages, 5 Postscript figure
Quantum orientational melting of mesoscopic clusters
By path integral Monte Carlo simulations we study the phase diagram of two -
dimensional mesoscopic clusters formed by electrons in a semiconductor quantum
dot or by indirect magnetoexcitons in double quantum dots. At zero (or
sufficiently small) temperature, as quantum fluctuations of particles increase,
two types of quantum disordering phenomena take place: first, at small values
of quantum de Boer parameter q < 0.01 one can observe a transition from a
completely ordered state to that in which different shells of the cluster,
being internally ordered, are orientationally disordered relative to each
other. At much greater strengths of quantum fluctuations, at q=0.1, the
transition to a disordered (superfluid for the boson system) state takes place.Comment: 4 pages, 6 Postscript figure
New model for system of mesoscopic Josephson contacts
Quantum fluctuations of the phases of the order parameter in 2D arrays of
mesoscopic Josephson junctions and their effect on the destruction of
superconductivity in the system are investigated by means of a quantum-cosine
model that is free of the incorrect application of the phase operator. The
proposed model employs trigonometric phase operators and makes it possible to
study arrays of small superconducting granules, pores filled with superfluid
helium, or Josephson junctions in which the average number of particles
(effective bosons, He atoms, and so on) is small, and the standard approach
employing the phase operator and the particle number operator as conjugate ones
is inapplicable. There is a large difference in the phase diagrams between
arrays of macroscopic and mesoscopic objects for and ( is
the characteristic interaction energy of the particle per granule and is
the Josephson coupling constant). Reentrant superconductivity phenomena are
discussed.Comment: 4 pages, 3 Postscript figure
Josephson array of mesoscopic objects. Modulation of system properties through the chemical potential
The phase diagram of a two-dimensional Josephson array of mesoscopic objects
is examined. Quantum fluctuations in both the modulus and phase of the
superconducting order parameter are taken into account within a lattice boson
Hubbard model. Modulating the average occupation number of the sites in
the system leads to changes in the state of the array, and the character of
these changes depends significantly on the region of the phase diagram being
examined. In the region where there are large quantum fluctuations in the phase
of the superconducting order parameter, variation of the chemical potential
causes oscillations with alternating superconducting (superfluid) and normal
states of the array. On the other hand, in the region where the bosons interact
weakly, the properties of the system depend monotonically on . Lowering
the temperature and increasing the particle interaction force lead to a
reduction in the width of the region of variation in within which the
system properties depend weakly on the average occupation number. The phase
diagram of the array is obtained by mapping this quantum system onto a
classical two-dimensional XY model with a renormalized Josephson coupling
constant and is consistent with our quantum Path-Integral Monte Carlo
calculations.Comment: 12 pages, 8 Postscript figure
Chemical analysis of bioactive substances in seven siberian Saussurea species
Main groups of biologically active substances of seven siberian Saussurea species (S. controversa DC., S. latifolia Ledeb., S. parviflora (Poir.) DC., S. frolowii Ledeb, S. amara (L.) DC., S. salicifolia (L.) DC. and S. daurica Adams) have been studied using paper, thin-layer, performance liquid chromatography, IR spectroscopy, spectrophotometry and mass spectrometry with inductively coupled plasma. Siberian Saussurea species have a rich elemental composition and contain a variety of phenolic compounds, amino acids, polysaccharides. The majority of polysaccharides are accumulated by S. controversa, S. salicifolia and S. frolowii. These plants contain a significant amount of calcium that may be a species characteristic. All plants contain quercetin and its glycosides, in some species luteolin, kaempferol, glycosides of apigenin and myricetin were revealed. Phenolic acids with predominant content of caffeic, chlorogenic and cinnamic acids were found in all the species. The maximum amount of phenolic acids and flavonoids was determined in the grass of S. latifolia, S. controversa and S. daurica. Characteristic absorption bands of lactone carbonyl of sesquiterpenoids in IR spectrum found in S. latifolia, S. controversa, S. daurica, S. amara and S. salicifolia. HPLC / UV analysis showed that peaks with absorption maxima of 242-246 nm due to the presence of α,β-unsaturated ketone group in the structure of ecdysteroids were found in S. salicifolia, S. controversa, S. daurica and S. latifolia
A New High Energy Photon Tagger for the H1 - Detector at HERA
The H1 detector at HERA has been upgraded by the addition of a new
electromagnetic calorimeter. This is installed in the HERA tunnel close to the
electron beam line at a position 8m from the interaction point in the electron
beam direction. The new calorimeter extends the acceptance for tagged
photoproduction events to the high y range, 0.85 < y < 0.95, and thus
significantly improves the capability of H1 to study high energy gamma-p
processes. The calorimeter design, performance and first results obtained
during the 1996-1999 HERA running are described.Comment: 17 pages, 16 figure
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