11,893 research outputs found
Towards the parameterisation of the Hubbard model for salts of BEDT-TTF: A density functional study of isolated molecules
We calculate the effective Coulomb repulsion between electrons/holes, U, and
site energy for an isolated BEDT-TTF [bis(ethylenedithio)tetrathiafulvalene]
molecule in vacuo. U=4.2 \pm 0.1 eV for 44 experimental geometries taken from a
broad range of conformations, polymorphs, anions, temperatures, and pressures
(the quoted `error' is one standard deviation). Hence we conclude that U is
essentially the same for all of the compounds studied. This shows that the
strong (hydrostatic and chemical) pressure dependence observed in the phase
diagrams of the BEDT-TTF salts is not due to U. Therefore, if the Hubbard model
is sufficient to describe the phase diagram of the BEDT-TTF salts there must be
significant pressure dependence on the intramolecular terms in the Hamiltonian
and/or the reduction of the Hubbard U due to the interaction of the molecule
with the polarisable crystal environment. The renormalised value of U is
significantly smaller than the bare value of the Coulomb integral:
F_0=5.2\pm0.1 eV across the same set of geometries, emphasising the importance
of using the renormalised value of U. The site energy (for holes), xi=5.0\pm0.2
eV, varies only a little more than U across the same set of geometries.
However, we argue that this plays a key role in understanding the role of
disorder in ET salts in general and in explaining the difference between the
beta_L and beta_H phases of beta-(BEDT-TTF)_2I_3 in particular.Comment: 13 pages, 6 figures, also see animations at
http://www.youtube.com/watch?v=3K2kP8hWpZI,
http://www.youtube.com/watch?v=wIz1cRsSdEs and
http://www.youtube.com/watch?v=bNzUBAS6AFM, Expanded discussion of
renormalisation effects. To appear in J. Chem. Phy
A New Optimal Stepsize For Approximate Dynamic Programming
Approximate dynamic programming (ADP) has proven itself in a wide range of
applications spanning large-scale transportation problems, health care, revenue
management, and energy systems. The design of effective ADP algorithms has many
dimensions, but one crucial factor is the stepsize rule used to update a value
function approximation. Many operations research applications are
computationally intensive, and it is important to obtain good results quickly.
Furthermore, the most popular stepsize formulas use tunable parameters and can
produce very poor results if tuned improperly. We derive a new stepsize rule
that optimizes the prediction error in order to improve the short-term
performance of an ADP algorithm. With only one, relatively insensitive tunable
parameter, the new rule adapts to the level of noise in the problem and
produces faster convergence in numerical experiments.Comment: Matlab files are included with the paper sourc
Competition between disorder and exchange splitting in superconducting ZrZn_2
We propose a simple picture for the occurrence of superconductivity and the
pressure dependence of the superconducting critical temperature, T_{SC}, in
ZrZn_2. According to our hypothesis the pairing potential is independent of
pressure, but the exchange splitting, E_{xc}, leads to a pressure dependence in
the (spin dependent) density of states at the Fermi level,
D_\sigma(\epsilon_F). Assuming p-wave pairing T_{SC} is dependent on
D_\sigma(\epsilon_F) which ensures that, in the absence of non-magnetic
impurities, T_{SC} decreases as pressure is applied until it reaches a minimum
in the paramagnetic state. Disorder reduces this minimum to zero, this gives
the illusion that the superconductivity disappears at the same pressure as
ferromagnetism does.Comment: 7 pages, 4 figures, submitted to J. Phys. Cond. Ma
Phase diagram of the spin-1/2 triangular J1-J2 Heisenberg model on a three-leg cylinder
We study the phase diagram of the frustrated Heisenberg model on the triangular lattice with nearest- and next-nearest-neighbor spin-exchange coupling, on three-leg ladders. Using the density-matrix renormalization-group method, we obtain the complete phase diagram of the model, which includes quasi-long-range 120° and columnar order, and a Majumdar-Ghosh phase with short-ranged correlations. All these phases are nonchiral and planar. We also identify the nature of phase transitions
Haldane phase in the hubbard model at 2/3-filling for the organic molecular compound Mo3 S7 (dmit)3
We report the discovery of a correlated insulator with a bulk gap at 2/3 filling in a geometrically frustrated Hubbard model that describes the low-energy physics of Mo3S7(dmit)(3). This is very different from the Mott insulator expected at half-filling. We show that the insulating phase, which persists even for very weak electron-electron interactions (U), is adiabatically connected to the Haldane phase and is consistent with experiments on Mo3S7(dmit)(3)
Synthetic Mudscapes: Human Interventions in Deltaic Land Building
In order to defend infrastructure, economy, and settlement in Southeast Louisiana, we must construct new land to
mitigate increasing risk. Links between urban environments and economic drivers have constrained the dynamic delta
landscape for generations, now threatening to undermine the ecological fitness of the entire region. Static methods of
measuring, controlling, and valuing land fail in an environment that is constantly in flux; change and indeterminacy are
denied by traditional inhabitation.
Multiple land building practices reintroduce deltaic fluctuation and strategic deposition of fertile material to form the
foundations of a multi-layered defence strategy. Manufactured marshlands reduce exposure to storm surge further
inland. Virtual monitoring and communication networks inform design decisions and land use becomes determined
by its ecological health. Mudscapes at the threshold of land and water place new value on former wastelands. The
social, economic, and ecological evolution of the region are defended by an expanded web of growing land
Critical exponents and phase transition in gold nuclei fragmentation at energies 10.6 and 4.0 GeV/nucleon
An attempt to extract critical exponents gamma, beta and tau from data on
gold nuclei fragmentation due to interactions with nuclear emulsion at energies
4.0 A GeV and 10.6 A GeV is presented. Based on analysis of Campi's 2nd charge
moments, two subsets of data at each energy are selected from the inclusive
data, corresponding to 'liquid' and 'gas' phases. The extracted values of
critical exponents from the selected data sets are in agreement with
predictions of 'liquid-gas' model of phase transition.Comment: 21 pages, 15 figure
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