9,960 research outputs found
Gutzwiller description of non-magnetic Mott insulators: a dimer lattice model
We introduce a novel extension of the Gutzwiller variational wavefunction
able to deal with insulators that escape any mean-field like description, as
for instance non-magnetic insulators. As an application, we study the Mott
transition from a paramagnetic metal into a non-magnetic Peierls, or
valence-bond, Mott insulator. We analyze this model by means of our Gutzwiller
wavefunction analytically in the limit of large coordination lattices, where we
find that: (1) the Mott transition is first order; (2) the Peierls gap is large
in the Mott insulator, although it is mainly contributed by the electron
repulsion; (3) singlet-superconductivity arises around the transition.Comment: 15 pages, 9 figure
The long memory of the efficient market
For the London Stock Exchange we demonstrate that the signs of orders obey a
long-memory process. The autocorrelation function decays roughly as
with , corresponding to a Hurst exponent
. This implies that the signs of future orders are quite
predictable from the signs of past orders; all else being equal, this would
suggest a very strong market inefficiency. We demonstrate, however, that
fluctuations in order signs are compensated for by anti-correlated fluctuations
in transaction size and liquidity, which are also long-memory processes. This
tends to make the returns whiter. We show that some institutions display
long-range memory and others don't.Comment: 19 pages, 12 figure
Phase diagram of doped spin-Peierls systems
The phase diagram of a model describing doped CuGeO is derived. The model
emphasizes the role of local moments released by the impurities and randomly
distributed inside the gaped singlet background. The phase diagram is
investigated by two methods: (i) in a mean field treatment of the interchain
coupling and (ii) in a real space decimation procedure in a two dimensional
model of randomly distributed moments. Both methods lead to similar results, in
a qualitative agreement with experiments. In particular, a transition to an
inhomogeneous N\'eel phase is obtained for arbitrary small doping. From the
decimation procedure, we interpret this phase at very low doping as a {\sl
Griffith antiferromagnet}. Namely, it does not have a true long range order
down to zero temperature. Nonetheless, large magnetically ordered clusters
appear already at relatively high temperatures. This demonstrates the role of
disorder in the theoretical description of doping in CuGeO. A detailed
comparison with other approaches is also given.Comment: 31 pages, 9 figure
There's more to volatility than volume
It is widely believed that fluctuations in transaction volume, as reflected
in the number of transactions and to a lesser extent their size, are the main
cause of clustered volatility. Under this view bursts of rapid or slow price
diffusion reflect bursts of frequent or less frequent trading, which cause both
clustered volatility and heavy tails in price returns. We investigate this
hypothesis using tick by tick data from the New York and London Stock Exchanges
and show that only a small fraction of volatility fluctuations are explained in
this manner. Clustered volatility is still very strong even if price changes
are recorded on intervals in which the total transaction volume or number of
transactions is held constant. In addition the distribution of price returns
conditioned on volume or transaction frequency being held constant is similar
to that in real time, making it clear that neither of these are the principal
cause of heavy tails in price returns. We analyze recent results of Ane and
Geman (2000) and Gabaix et al. (2003), and discuss the reasons why their
conclusions differ from ours. Based on a cross-sectional analysis we show that
the long-memory of volatility is dominated by factors other than transaction
frequency or total trading volume.Comment: 25 pages, 9 figure
Competitive Equilibria With Limited Enforcement
This study demonstrates how constrained efficient allocations can arise endogenously as equilibria in an economy with a limited ability to enforce contracts and with private agents behaving competitively, taking a set of taxes as given. The taxes in this economy limit risk-sharing and arise in an equilibrium of a dynamic game between governments of sovereign nations. The equilibrium allocations depend on governments choosing to tax both the repayment of international debt and the income from capital investment in their countries.
Segmentation algorithm for non-stationary compound Poisson processes
We introduce an algorithm for the segmentation of a class of regime switching
processes. The segmentation algorithm is a non parametric statistical method
able to identify the regimes (patches) of the time series. The process is
composed of consecutive patches of variable length, each patch being described
by a stationary compound Poisson process, i.e. a Poisson process where each
count is associated to a fluctuating signal. The parameters of the process are
different in each patch and therefore the time series is non stationary. Our
method is a generalization of the algorithm introduced by Bernaola-Galvan, et
al., Phys. Rev. Lett., 87, 168105 (2001). We show that the new algorithm
outperforms the original one for regime switching compound Poisson processes.
As an application we use the algorithm to segment the time series of the
inventory of market members of the London Stock Exchange and we observe that
our method finds almost three times more patches than the original one.Comment: 11 pages, 11 figure
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