419 research outputs found
Spurious trend switching phenomena in financial markets
The observation of power laws in the time to extrema of volatility, volume
and intertrade times, from milliseconds to years, are shown to result
straightforwardly from the selection of biased statistical subsets of
realizations in otherwise featureless processes such as random walks. The bias
stems from the selection of price peaks that imposes a condition on the
statistics of price change and of trade volumes that skew their distributions.
For the intertrade times, the extrema and power laws results from the format of
transaction data
Quantifying the behavior of stock correlations under market stress
Understanding correlations in complex systems is crucial in the face of turbulence, such as the ongoing financial crisis. However, in complex systems, such as financial systems, correlations are not constant but instead vary in time. Here we address the question of quantifying state-dependent correlations in stock markets. Reliable estimates of correlations are absolutely necessary to protect a portfolio. We analyze 72 years of daily closing prices of the 30 stocks forming the Dow Jones Industrial Average (DJIA). We find the striking result that the average correlation among these stocks scales linearly with market stress reflected by normalized DJIA index returns on various time scales. Consequently, the diversification effect which should protect a portfolio melts away in times of market losses, just when it would most urgently be needed. Our empirical analysis is consistent with the interesting possibility that one could anticipate diversification breakdowns, guiding the design of protected portfolios
Fractional Quantum Hall Effect in a Diluted Magnetic Semiconductor
We report the observation of the fractional quantum Hall effect in the lowest
Landau level of a two-dimensional electron system (2DES), residing in the
diluted magnetic semiconductor Cd(1-x)Mn(x)Te. The presence of magnetic
impurities results in a giant Zeeman splitting leading to an unusual ordering
of composite fermion Landau levels. In experiment, this results in an
unconventional opening and closing of fractional gaps around filling factor v =
3/2 as a function of an in-plane magnetic field, i.e. of the Zeeman energy. By
including the s-d exchange energy into the composite Landau level spectrum the
opening and closing of the gap at filling factor 5/3 can be modeled
quantitatively. The widely tunable spin-splitting in a diluted magnetic 2DES
provides a novel means to manipulate fractional states
Reflectivity and Microwave Absorption in Crystals with Alternating Intrinsic Josephson Junctions
We compute the frequency and magnetic field dependencies of the reflectivity
in layered superconductors with two alternating intrinsic Josephson
junctions with different critical current densities and quasiparticle
conductivities for the electric field polarized along the c-axis. The parameter
describing the electronic compressibility of the layers and the charge
coupling of neighboring junctions was extracted for the
SmLaSrCuO superconductor from two independent
optical measurements, the fit of the loss function at zero magnetic
field and the magnetic field dependence of the peak positions in .
The experiments are consistent with a free electron value for near
the Josephson plasma frequencies.Comment: 4 pages, 4 postscript figures, misprints in table correcte
Magnetic Catalysis in AdS4
We study the formation of fermion condensates in Anti de Sitter space. In
particular, we describe a novel version of magnetic catalysis that arises for
fermions in asymptotically AdS4 geometries which cap off in the infra-red with
a hard wall. We show that the presence of a magnetic field induces a fermion
condensate in the bulk that spontaneously breaks CP symmetry. From the
perspective of the dual boundary theory, this corresponds to a strongly coupled
version of magnetic catalysis in d=2+1.Comment: 22 pages, 4 figures. v2: References added, factors of 2 corrected,
extra comments added in appendix. v3: extra comments about fermion modes in a
hard wall background. v4: A final factor of
Coupling between phonons and intrinsic Josephson oscillations in cuprate superconductors
The recently reported subgap structures observed in the current-voltage
characteristic of intrinsic Josephson junctions in the high-T_c superconductors
Tl_2Ba_2Ca_2Cu_3O_{10+\delta} and Bi_2Sr_2CaCu_2O_{8+\delta} are explained by
the coupling between c-axis phonons and Josephson oscillations. A model is
developed where c-axis lattice vibrations between adjacent superconducting
multilayers are excited by the Josephson oscillations in a resistive junction.
The voltages of the lowest structures correspond well to the frequencies of
longitudinal c-axis phonons with large oscillator strength in the two
materials, providing a new measurement technique for this quantity.Comment: 4 pages, 3 figures, revtex, aps, epsf, psfig. submitted to Physical
Review Letters, second version improved in detai
Flavor-symmetry Breaking with Charged Probes
We discuss the recombination of brane/anti-brane pairs carrying brane
charge in . These configurations are dual to co-dimension one
defects in the super-Yang-Mills description. Due to their
charge, these defects are actually domain walls in the dual gauge theory,
interpolating between vacua of different gauge symmetry. A pair of unjoined
defects each carry localized dimensional fermions and possess a global
flavor symmetry while the recombined brane/anti-brane pairs
exhibit only a diagonal U(N). We study the thermodynamics of this
flavor-symmetry breaking under the influence of external magnetic field.Comment: 21 pages, 10 figure
Magnetic Catalysis: A Review
We give an overview of the magnetic catalysis phenomenon. In the framework of
quantum field theory, magnetic catalysis is broadly defined as an enhancement
of dynamical symmetry breaking by an external magnetic field. We start from a
brief discussion of spontaneous symmetry breaking and the role of a magnetic
field in its a dynamics. This is followed by a detailed presentation of the
essential features of the phenomenon. In particular, we emphasize that the
dimensional reduction plays a profound role in the pairing dynamics in a
magnetic field. Using the general nature of underlying physics and its
robustness with respect to interaction types and model content, we argue that
magnetic catalysis is a universal and model-independent phenomenon. In support
of this claim, we show how magnetic catalysis is realized in various models
with short-range and long-range interactions. We argue that the general nature
of the phenomenon implies a wide range of potential applications: from certain
types of solid state systems to models in cosmology, particle and nuclear
physics. We finish the review with general remarks about magnetic catalysis and
an outlook for future research.Comment: 37 pages, to appear in Lect. Notes Phys. "Strongly interacting matter
in magnetic fields" (Springer), edited by D. Kharzeev, K. Landsteiner, A.
Schmitt, H.-U. Yee. Version 2: references adde
Emerging interdependence between stock values during financial crashes
To identify emerging interdependencies between traded stocks we investigate the behavior of the stocks of FTSE 100 companies in the period 2000-2015, by looking at daily stock values. Exploiting the power of information theoretical measures to extract direct influences between multiple time series, we compute the information flow across stock values to identify several different regimes. While small information flows is detected in most of the period, a dramatically different situation occurs in the proximity of global financial crises, where stock values exhibit strong and substantial interdependence for a prolonged period. This behavior is consistent with what one would generally expect from a complex system near criticality in physical systems, showing the long lasting effects of crashes on stock markets
Inverse magnetic catalysis in field theory and gauge-gravity duality
We investigate the surface of the chiral phase transition in the
three-dimensional parameter space of temperature, baryon chemical potential and
magnetic field in two different approaches, the field-theoretical
Nambu-Jona-Lasinio (NJL) model and the holographic Sakai-Sugimoto model. The
latter is a top-down approach to a gravity dual of QCD with an asymptotically
large number of colors and becomes, in a certain limit, dual to an NJL-like
model. Our main observation is that, at nonzero chemical potential, a magnetic
field can restore chiral symmetry, in apparent contrast to the phenomenon of
magnetic catalysis. This "inverse magnetic catalysis" occurs in the
Sakai-Sugimoto model and, for sufficiently large coupling, in the NJL model and
is related to the physics of the lowest Landau level. While in most parts our
discussion is a pedagogical review of previously published results, we include
new analytical results for the NJL approach and a thorough comparison of
inverse magnetic catalysis in the two approaches.Comment: 37 pages, 11 figures, to appear in Lect. Notes Phys. "Strongly
interacting matter in magnetic fields" (Springer), edited by D. Kharzeev, K.
Landsteiner, A. Schmitt, H.-U. Ye
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