2,509 research outputs found
Cyclostationary measurement of low-frequency odd moments of current fluctuations
Measurement of odd moments of current fluctuations is difficult due to strict
requirements for band-pass filtering. We propose how these requirements can be
overcome using cyclostationary driving of the measured signal and indicate how
the measurement accuracy can be tested through the phase dependence of the
moments of the fluctuations. We consider two schemes, the mixing scheme and the
statistics scheme, where the current statistics can be accessed. We also
address the limitations of the schemes, due to excess noise and due to the
effects of the environment, and, finally, discuss the required measurement
times for typical setups.Comment: 13 pages, 3 figure
Full counting statistics of Luttinger liquid conductor
Non-equilibrium bosonization technique is used to study current fluctuations
of interacting electrons in a single-channel quantum wire representing a
Luttinger liquid (LL) conductor. An exact expression for the full counting
statistics of the transmitted charge is derived. It is given by Fredholm
determinant of the counting operator with a time dependent scattering phase.
The result has a form of counting statistics of non-interacting particles with
fractional charges, induced by scattering off the boundaries between the LL
wire and the non-interacting leads.Comment: 5 pages, 2 figure
A simple mathematical model for the effect of benzoannelation on cyclic conjugation
In a series of earlier studies, it was established that benzoannelation in the angular (resp. linear) position relative to a ring R of a polycyclic conjugated π-electron system, increases (resp. decreases) the intensity of the cyclic conjugation in the ring R. Herein, it is shown how this regularity can be explained by means of a simple, Kekulé-structurebased argument, itself based on an idea of Randić from the 1970s
Theoretical Studies on Radialenes and Related Molecules
For certain classes of molecules it is possible to obtain a general solution
·of the Ruckel problem, i.e. to derive expressions for the orbital energy, orbital coefficients, total n-electron energy, etc. in a closed analytical form1-5• General solutions are important because a large amount of numerical labour can be saved. Besides, they show the dependence of HMO quantities on the molecular topology, which has been recently investigated by various authors
The maximal energy of classes of integral circulant graphs
The energy of a graph is the sum of the moduli of the eigenvalues of its
adjacency matrix. We study the energy of integral circulant graphs, also called
gcd graphs, which can be characterized by their vertex count and a set
of divisors of in such a way that they have vertex set
and edge set . For a fixed prime power and a fixed divisor set size , we analyze the maximal energy among all matching integral circulant
graphs. Let be the elements of .
It turns out that the differences between the exponents of
an energy maximal divisor set must satisfy certain balance conditions: (i)
either all equal , or at most the two differences
and may occur; %(for a certain depending on and ) (ii)
there are rules governing the sequence of consecutive
differences. For particular choices of and these conditions already
guarantee maximal energy and its value can be computed explicitly.Comment: Discrete Applied Mathematics (2012
Suppression of geometrical barrier in crystals by Josephson vortex stacks
Differential magneto-optics are used to study the effect of dc in-plane
magnetic field on hysteretic behavior due to geometrical barriers in
crystals. In absence of in-plane field a vortex
dome is visualized in the sample center surrounded by barrier-dominated
flux-free regions. With in-plane field, stacks of Josephson vortices form
vortex chains which are surprisingly found to protrude out of the dome into the
vortex-free regions. The chains are imaged to extend up to the sample edges,
thus providing easy channels for vortex entry and for drain of the dome through
geometrical barrier, suppressing the magnetic hysteresis. Reduction of the
vortex energy due to crossing with Josephson vortices is evaluated to be about
two orders of magnitude too small to account for the formation of the
protruding chains. We present a model and numerical calculations that
qualitatively describe the observed phenomena by taking into account the
demagnetization effects in which flux expulsion from the pristine regions
results in vortex focusing and in the chain protrusion. Comparative
measurements on a sample with narrow etched grooves provide further support to
the proposed model.Comment: 12 figures (low res.) Higher resolution figures are available at the
Phys Rev B version. Typos correcte
Elementary models of 3D topological insulators with chiral symmetry
We construct a set of lattice models of non-interacting topological
insulators with chiral symmetry in three dimensions. We build a model of the
topological insulators in the class AIII by coupling lower dimensional models
of classes. By coupling the two AIII models related by
time-reversal symmetry we construct other chiral symmetric topological
insulators that may also possess additional symmetries (the time-reversal
and/or particle-hole).
There are two different chiral symmetry operators for the coupled model, that
correspond to two distinct ways of defining the sublattices. The integer
topological invariant (the winding number) in case of weak coupling can be
either the sum or difference of indices of the basic building blocks, dependent
on the preserved chiral symmetry operator. The value of the topological index
in case of weak coupling is determined by the chiral symmetry only and does not
depend on the presence of other symmetries. For topological
classes AIII, DIII, and CI with chiral symmetry are topologically equivalent,
it implies that a smooth transition between the classes can be achieved if it
connects the topological sectors with the same winding number. We demonstrate
this explicitly by proving that the gapless surface states remain robust in
classes as long as the chiral symmetry is preserved, and the
coupling does not close the gap in the bulk. By studying the surface states in
topological classes, we show that class CII and AII are
distinct, and can not be adiabatically connected
Dynamics of waves in 1D electron systems: Density oscillations driven by population inversion
We explore dynamics of a density pulse induced by a local quench in a
one-dimensional electron system. The spectral curvature leads to an "overturn"
(population inversion) of the wave. We show that beyond this time the density
profile develops strong oscillations with a period much larger than the Fermi
wave length. The effect is studied first for the case of free fermions by means
of direct quantum simulations and via semiclassical analysis of the evolution
of Wigner function. We demonstrate then that the period of oscillations is
correctly reproduced by a hydrodynamic theory with an appropriate dispersive
term. Finally, we explore the effect of different types of electron-electron
interaction on the phenomenon. We show that sufficiently strong interaction
[ where is the fermionic mass and the relevant spatial
scale] determines the dominant dispersive term in the hydrodynamic equations.
Hydrodynamic theory reveals crucial dependence of the density evolution on the
relative sign of the interaction and the density perturbation.Comment: 20 pages, 13 figure
Integral circulant graphs of prime power order with maximal energy
The energy of a graph is the sum of the moduli of the eigenvalues of its
adjacency matrix. We study the energy of integral circulant graphs, also called
gcd graphs, which can be characterized by their vertex count n and a set D of
divisors of n in such a way that they have vertex set Zn and edge set {{a, b} :
a, b in Zn; gcd(a - b, n) in D}. Using tools from convex optimization, we study
the maximal energy among all integral circulant graphs of prime power order ps
and varying divisor sets D. Our main result states that this maximal energy
approximately lies between s(p - 1)p^(s-1) and twice this value. We construct
suitable divisor sets for which the energy lies in this interval. We also
characterize hyperenergetic integral circulant graphs of prime power order and
exhibit an interesting topological property of their divisor sets.Comment: 25 page
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