627 research outputs found
Calculations of canonical averages from the grand canonical ensemble
Grand canonical and canonical ensembles become equivalent in the
thermodynamic limit, but when the system size is finite the results obtained in
the two ensembles deviate from each other. In many important cases, the
canonical ensemble provides an appropriate physical description but it is often
much easier to perform the calculations in the corresponding grand canonical
ensemble. We present a method to compute averages in canonical ensemble based
on calculations of the expectation values in grand canonical ensemble. The
number of particles, which is fixed in the canonical ensemble, is not
necessarily the same as the average number of particles in the grand canonical
ensemble
Population of isomers in decay of the giant dipole resonance
The value of an isomeric ratio (IR) in N=81 isotones (Ba, Ce,
Nd and Sm) is studied by means of the ( reaction.
This quantity measures a probability to populate the isomeric state in respect
to the ground state population. In ( reactions, the giant dipole
resonance (GDR) is excited and after its decay by a neutron emission, the
nucleus has an excitation energy of a few MeV. The forthcoming decay
by direct or cascade transitions deexcites the nucleus into an isomeric or
ground state. It has been observed experimentally that the IR for Ba
and Ce equals about 0.13 while in two heavier isotones it is even less
than half the size. To explain this effect, the structure of the excited states
in the energy region up to 6.5 MeV has been calculated within the Quasiparticle
Phonon Model. Many states are found connected to the ground and isomeric states
by , and transitions. The single-particle component of the wave
function is responsible for the large values of the transitions. The calculated
value of the isomeric ratio is in very good agreement with the experimental
data for all isotones. A slightly different value of maximum energy with which
the nuclei rest after neutron decay of the GDR is responsible for the reported
effect of the A-dependence of the IR.Comment: 16 pages, 4 Fig
Spin splitting of X-related donor impurity states in an AlAs barrier
We use magnetotunneling spectroscopy to observe the spin splitting of the
ground state of an X-valley-related Si-donor impurity in an AlAs barrier. We
determine the absolute magnitude of the effective Zeeman spin splitting factors
of the impurity ground state to be g= 2.2 0.1. We also investigate
the spatial form of the electron wave function of the donor ground state, which
is anisotropic in the growth plane
An ab initio theory of double odd-even mass differences in nuclei
Two aspects of the problem of evaluating double odd-even mass differences D_2
in semi-magic nuclei are studied related to existence of two components with
different properties, a superfluid nuclear subsystem and a non-superfluid one.
For the superfluid subsystem, the difference D_2 is approximately equal to
2\Delta, the gap \Delta being the solution of the gap equation. For the
non-superfluid subsystem, D_2 is found by solving the equation for two-particle
Green function for normal systems. Both equations under consideration contain
the same effective pairing interaction. For the latter, the semi-microscopic
model is used in which the main term calculated from the first principles is
supplemented with a small phenomenological addendum containing one
phenomenological parameter supposed to be universal for all medium and heavy
atomic nuclei.Comment: 7 pages, 10 figures, Report at Nuclear Structure and Related Topics,
Dubna, Russia, July 2 - July 7, 201
Partial level density of the n-quasiparticle excitations in the nuclei of the 39< A <201 region
Level density and radiative strength functions are obtained from the analysis
of two-step cascades intensities following the thermal neutrons capture. The
data on level density are approximated by the sum of the partial level
densities corresponding to n quasiparticles excitation. The most probable
values of the collective enhancement factor of the level density are found
together with the thresholds of the next Cooper nucleons pair breaking. These
data allow one to calculate the level density of practically any nucleus in
given spin window in the framework of model concepts, taking into account all
known nuclear excitation types. The presence of an approximation results
discrepancy with theoretical statements specifies the necessity of rather
essentially developing the level density models. It also indicates the
possibilities to obtain the essentially new information on nucleon correlation
functions of the excited nucleus from the experiment.Comment: 29 pages, 8 figures, 2 table
Nonlinear electron transport in normally pinched-off quantum wire
Nonlinear electron transport in normally pinched-off quantum wires was
studied. The wires were fabricated from AlGaAs/GaAs heterostructures with
high-mobility two-dimensional electron gas by electron beam lithography and
following wet etching. At certain critical source-drain voltage the samples
exhibited a step rise of the conductance. The differential conductance of the
open wires was noticeably lower than e^2/h as far as only part of the
source-drain voltage dropped between source contact and saddle-point of the
potential relief along the wire. The latter limited the electron flow injected
to the wire. At high enough source-drain voltages the decrease of the
differential conductance due to the real space transfer of electrons from the
wire in GaAs to the doped AlGaAs layer was found. In this regime the sign of
differential magnetoconductance was changed with reversing the direction of the
current in the wire or the magnetic field, whet the magnetic field lies in the
heterostructure plane and is directed perpendicular to the current. The
dependence of the differential conductance on the magnetic field and its
direction indicated that the real space transfer events were mainly mediated by
the interface scattering.Comment: LaTeX 2e (epl.cls) 6 pages, 3 figure
Wave function mapping conditions in Open Quantum Dots structures
We discuss the minimal conditions for wave function spectroscopy, in which
resonant tunneling is the measurement tool. Two systems are addressed: resonant
tunneling diodes, as a toy model, and open quantum dots. The toy model is used
to analyze the crucial tunning between the necessary resolution in
current-voltage characteristics and the breakdown of the wave functions probing
potentials into a level splitting characteristic of double quantum wells. The
present results establish a parameter region where the wavefunction
spectroscopy by resonant tunneling could be achieved. In the case of open
quantum dots, a breakdown of the mapping condition is related to a change into
a double quantum dot structure induced by the local probing potential. The
analogy between the toy model and open quantum dots show that a precise control
over shape and extention of the potential probes is irrelevant for wave
function mapping. Moreover, the present system is a realization of a tunable
Fano system in the wave function mapping regime.Comment: 6 pages, 6 figure
Abelian symmetries in multi-Higgs-doublet models
N-Higgs doublet models (NHDM) are a popular framework to construct
electroweak symmetry breaking mechanisms beyond the Standard model. Usually,
one builds an NHDM scalar sector which is invariant under a certain symmetry
group. Although several such groups have been used, no general analysis of
symmetries possible in the NHDM scalar sector exists. Here, we make the first
step towards this goal by classifying the elementary building blocks, namely
the abelian symmetry groups, with a special emphasis on finite groups. We
describe a strategy that identifies all abelian groups which are realizable as
symmetry groups of the NHDM Higgs potential. We consider both the groups of
Higgs-family transformations only and the groups which also contain generalized
CP transformations. We illustrate this strategy with the examples of 3HDM and
4HDM and prove several statements for arbitrary N.Comment: 33 pages, 2 figures; v2: conjecture 3 is proved and becomes theorem
3, more explanations of the main strategy are added, matches the published
versio
Chaotic oscillations in a map-based model of neural activity
We propose a discrete time dynamical system (a map) as phenomenological model
of excitable and spiking-bursting neurons. The model is a discontinuous
two-dimensional map. We find condition under which this map has an invariant
region on the phase plane, containing chaotic attractor. This attractor creates
chaotic spiking-bursting oscillations of the model. We also show various
regimes of other neural activities (subthreshold oscillations, phasic spiking
etc.) derived from the proposed model
A magnetically-induced Coulomb gap in graphene due to electron-electron interactions
Insights into the fundamental properties of graphene's Dirac-Weyl fermions
have emerged from studies of electron tunnelling transistors in which an
atomically thin layer of hexagonal boron nitride (hBN) is sandwiched between
two layers of high purity graphene. Here, we show that when a single defect is
present within the hBN tunnel barrier, it can inject electrons into the
graphene layers and its sharply defined energy level acts as a high resolution
spectroscopic probe of electron-electron interactions in graphene. We report a
magnetic field dependent suppression of the tunnel current flowing through a
single defect below temperatures of 2 K. This is attributed to the
formation of a magnetically-induced Coulomb gap in the spectral density of
electrons tunnelling into graphene due to electron-electron interactions
- …