4,265 research outputs found
Activity Dependent Branching Ratios in Stocks, Solar X-ray Flux, and the Bak-Tang-Wiesenfeld Sandpile Model
We define an activity dependent branching ratio that allows comparison of
different time series . The branching ratio is defined as . The random variable is the value of the next signal given
that the previous one is equal to , so . If
, the process is on average supercritical when the signal is equal to
, while if , it is subcritical. For stock prices we find
within statistical uncertainty, for all , consistent with an ``efficient
market hypothesis''. For stock volumes, solar X-ray flux intensities, and the
Bak-Tang-Wiesenfeld (BTW) sandpile model, is supercritical for small
values of activity and subcritical for the largest ones, indicating a tendency
to return to a typical value. For stock volumes this tendency has an
approximate power law behavior. For solar X-ray flux and the BTW model, there
is a broad regime of activity where , which we interpret as an
indicator of critical behavior. This is true despite different underlying
probability distributions for , and for . For the BTW model the
distribution of is Gaussian, for sufficiently larger than one, and
its variance grows linearly with . Hence, the activity in the BTW model
obeys a central limit theorem when sampling over past histories. The broad
region of activity where is close to one disappears once bulk dissipation
is introduced in the BTW model -- supporting our hypothesis that it is an
indicator of criticality.Comment: 7 pages, 11 figure
Test of a Jastrow-type wavefunction for a trapped few-body system in one dimension
For a system with interacting quantum mechanical particles in a
one-dimensional harmonic oscillator, a trial wavefunction with simple structure
based on the solution of the corresponding two-particle system is suggested and
tested numerically. With the inclusion of a scaling parameter for the distance
between particles, at least for the very small systems tested here the ansatz
gives a very good estimate of the ground state energy, with the error being of
the order of ~1% of the gap to the first excited state
Signatures of Wigner Localization in Epitaxially Grown Nanowires
It was predicted by Wigner in 1934 that the electron gas will undergo a
transition to a crystallized state when its density is very low. Whereas
significant progress has been made towards the detection of electronic Wigner
states, their clear and direct experimental verification still remains a
challenge. Here we address signatures of Wigner molecule formation in the
transport properties of InSb nanowire quantum dot systems, where a few
electrons may form localized states depending on the size of the dot (i.e. the
electron density). By a configuration interaction approach combined with an
appropriate transport formalism, we are able to predict the transport
properties of these systems, in excellent agreement with experimental data. We
identify specific signatures of Wigner state formation, such as the strong
suppression of the antiferromagnetic coupling, and are able to detect the onset
of Wigner localization, both experimentally and theoretically, by studying
different dot sizes.Comment: 4 pages, 4 figure
Decomposing the stock market intraday dynamics
The correlation matrix formalism is used to study temporal aspects of the
stock market evolution. This formalism allows to decompose the financial
dynamics into noise as well as into some coherent repeatable intraday
structures. The present study is based on the high-frequency Deutsche
Aktienindex (DAX) data over the time period between November 1997 and September
1999, and makes use of both, the corresponding returns as well as volatility
variations. One principal conclusion is that a bulk of the stock market
dynamics is governed by the uncorrelated noise-like processes. There exists
however a small number of components of coherent short term repeatable
structures in fluctuations that may generate some memory effects seen in the
standard autocorrelation function analysis. Laws that govern fluctuations
associated with those various components are different, which indicates an
extremely complex character of the financial fluctuations.Comment: 15 pages, 13 PostScript figure
The Combinatorial World (of Auctions) According to GARP
Revealed preference techniques are used to test whether a data set is
compatible with rational behaviour. They are also incorporated as constraints
in mechanism design to encourage truthful behaviour in applications such as
combinatorial auctions. In the auction setting, we present an efficient
combinatorial algorithm to find a virtual valuation function with the optimal
(additive) rationality guarantee. Moreover, we show that there exists such a
valuation function that both is individually rational and is minimum (that is,
it is component-wise dominated by any other individually rational, virtual
valuation function that approximately fits the data). Similarly, given upper
bound constraints on the valuation function, we show how to fit the maximum
virtual valuation function with the optimal additive rationality guarantee. In
practice, revealed preference bidding constraints are very demanding. We
explain how approximate rationality can be used to create relaxed revealed
preference constraints in an auction. We then show how combinatorial methods
can be used to implement these relaxed constraints. Worst/best-case welfare
guarantees that result from the use of such mechanisms can be quantified via
the minimum/maximum virtual valuation function
Scanning Tunneling Microscopy and Tunneling Luminescence of the Surface of GaN Films Grown by Vapor Phase Epitaxy
We report scanning tunneling microscopy (STM) images of surfaces of GaN films
and the observation of luminescence from those films induced by highly
spatially localized injection of electrons or holes using STM. This combination
of scanning tunneling luminescence (STL) with STM for GaN surfaces and the
ability to observe both morphology and luminescence in GaN is the first step to
investigate possible correlations between surface morphology and optical
properties.Comment: 12 pages, Revtex 3.0, submitted to Appl. Phys. Lett., three figures
available from Jian Ma at [email protected]
Testing Consumer Rationality using Perfect Graphs and Oriented Discs
Given a consumer data-set, the axioms of revealed preference proffer a binary
test for rational behaviour. A natural (non-binary) measure of the degree of
rationality exhibited by the consumer is the minimum number of data points
whose removal induces a rationalisable data-set.We study the computational
complexity of the resultant consumer rationality problem in this paper. This
problem is, in the worst case, equivalent (in terms of approximation) to the
directed feedback vertex set problem. Our main result is to obtain an exact
threshold on the number of commodities that separates easy cases and hard
cases. Specifically, for two-commodity markets the consumer rationality problem
is polynomial time solvable; we prove this via a reduction to the vertex cover
problem on perfect graphs. For three-commodity markets, however, the problem is
NP-complete; we prove thisusing a reduction from planar 3-SAT that is based
upon oriented-disc drawings
Lineshape of the thermopower of quantum dots
Quantum dots are an important model system for thermoelectric phenomena, and
may be used to enhance the thermal-to-electric energy conversion efficiency in
functional materials. It is therefore important to obtain a detailed
understanding of a quantum-dot's thermopower as a function of the Fermi energy.
However, so far it has proven difficult to take effects of co-tunnelling into
account in the interpretation of experimental data. Here we show that a
single-electron tunnelling model, using knowledge of the dot's electrical
conductance which in fact includes all-order co-tunneling effects, predicts the
thermopower of quantum dots as a function of the relevant energy scales, in
very good agreement with experiment.Comment: 10 pages, 5 figure
Bethe-Ansatz density-functional theory of ultracold repulsive fermions in one-dimensional optical lattices
We present an extensive numerical study of ground-state properties of
confined repulsively interacting fermions on one-dimensional optical lattices.
Detailed predictions for the atom-density profiles are obtained from parallel
Kohn-Sham density-functional calculations and quantum Monte Carlo simulations.
The density-functional calculations employ a Bethe-Ansatz-based local-density
approximation for the correlation energy, which accounts for Luttinger-liquid
and Mott-insulator physics. Semi-analytical and fully numerical formulations of
this approximation are compared with each other and with a cruder
Thomas-Fermi-like local-density approximation for the total energy. Precise
quantum Monte Carlo simulations are used to assess the reliability of the
various local-density approximations, and in conjunction with these allow to
obtain a detailed microscopic picture of the consequences of the interplay
between particle-particle interactions and confinement in one-dimensional
systems of strongly correlated fermions.Comment: 14 pages, 11 figures, 1 table, submitte
- …