552 research outputs found
On class visualisation for high dimensional data: Exploring scientific datasets
Parametric Embedding (PE) has recently been proposed as a general-purpose
algorithm for class visualisation. It takes class posteriors produced by a
mixture-based clustering algorithm and projects them in 2D for visualisation.
However, although this fully modularised combination of objectives (clustering
and projection) is attractive for its conceptual simplicity, in the case of
high dimensional data, we show that a more optimal combination of these
objectives can be achieved by integrating them both into a consistent
probabilistic model. In this way, the projection step will fulfil a role of
regularisation, guarding against the curse of dimensionality. As a result, the
tradeoff between clustering and visualisation turns out to enhance the
predictive abilities of the overall model. We present results on both synthetic
data and two real-world high-dimensional data sets: observed spectra of
early-type galaxies and gene expression arrays.Comment: to appear in Lecture notes in Artificial Intelligence vol. 4265, the
(refereed) proceedings of the Ninth International conference on Discovery
Science (DS-2006), October 2006, Barcelona, Spain. 12 pages, 8 figure
Developments in Correlated Fermions
This manuscript is based on the Summary and Overview talk given at the "The
International Conference of Strongly Correlated Electronic Systems" (SCES '04),
July 26-30, at Karlsruhe, Germany. After highlighting some of the principal new
experimental developments in heavy fermions presented at the conference, I turn
to two kinds of theoretical questions. (1) What is understood of the
fermi-liquid state of the heavy fermions and what is not, but may reasonably
well be understood by systematic calculations. (2) The profound issues raised
by the observed correlations near the quantum critical points in the heavy
fermions. The numbers and letters in the parenthesis in the text refer to the
listing of the talks in the "Program and Abstracts" book of the conference
Sources of intrinsic rotation in the low flow ordering
A low flow, gyrokinetic formulation to obtain the intrinsic
rotation profiles is presented. The momentum conservation equation in the low
flow ordering contains new terms, neglected in previous first principles
formulations, that may explain the intrinsic rotation observed in tokamaks in
the absence of external sources of momentum. The intrinsic rotation profile
depends on the density and temperature profiles and on the up-down asymmetry.Comment: 20 page
Spin Gap Fixed Points in the Double Chain Problem
Applying the bosonization procedure to weakly coupled Hubbard chains we
discuss the fixed points of the renormalization group flow where all spin
excitations are gapful and a singlet pairing becomes the dominant instability.Comment: 15 pages, TeX, C Version 3.
Fermi surface renormalization in Hubbard ladders
We derive the one-loop renormalization equations for the shift in the
Fermi-wavevectors for one-dimensional interacting models with four Fermi-points
(two left and two right movers) and two Fermi velocities v_1 and v_2. We find
the shift to be proportional to (v_1-v_2)U^2, where U is the Hubbard-U. Our
results apply to the Hubbard ladder and to the t_1-t_2 Hubbard model. The
Fermi-sea with fewer particles tends to empty. The stability of a saddle point
due to shifts of the Fermi-energy and the shift of the Fermi-wavevector at the
Mott-Hubbard transition are discussed.Comment: 5 pages, 4 Postscript figure
Charge Dynamics in the Planar t-J Model
The finite-temperature optical conductivity in the planar
model is analysed using recently introduced numerical method based on the
Lanczos diagonalization of small systems (up to 20 sites), as well as by
analytical approaches, including the method of frequency moments and the
retraceable-path approximation. Results for a dynamical mobility of a single
hole at elevated temperatures reveal a Gaussian-like
spectra, however with a nonanalytical behavior at low . In the single
hole response a difference between the ferromagnetic (J=0) and the
antiferromagnetic () polaron shows up at . At larger dopings
numerical results in studied systems are consistent with the thermodynamical
behavior for . spectra show a non-Drude
falloff at large frequencies. In particular for `optimum' doping
we obtain in the low- regime the relaxation rate with , being consistent with the marginal Fermi
liquid concept and experiments. Within the same regime we reproduce the nearly
linear variation of dc resistivity with . This behavior is weakly
dependent on , provided that .Comment: 21 pages of text plus 17 figures, postscrip
Pharmacokinetic profile and quantitation of protection against soman poisoning by the antinicotinic compound MB327 in the guinea-pig
Current organophosphorus nerve agent medical countermeasures do not directly address the nicotinic effects of poisoning. A series of antinicotinic bispyridinium compounds has been synthesized in our laboratory and screened in vitro. Their actions can include open-channel block at the nicotinic receptor which may contribute to their efficacy. The current lead compound from these studies, MB327 1,1âČ-(propane-1,3-diyl)bis(4-tert-butylpyridinium) as either the diiodide (I2) or dimethanesulfonate (DMS) has been examined in vivo for efficacy against nerve agent poisoning. MB327 I2 (0â113 mg kgâ1) or the oxime HI-6 DMS (0â100 mg kgâ 1), in combination with atropine and avizafone (each at 3 mg kgâ1) was administered to guinea-pigs 1 min following soman poisoning. Treatment increased the LD50 of soman in a dose-dependent manner. The increase was statistically significant (p < 0.01) at the 33.9 mg kgâ1 (MB327) or 30 mg kgâ1 (HI-6) dose with a comparable degree of protection obtained for both compounds. Following administration of 10 mg kgâ1 (i.m.), MB327 DMS reached plasma Cmax of 22 ÎŒM at 12 min with an elimination t1/2 of 22 min. In an adverse effect study, in the absence of nerve agent poisoning, a dose of 100 mg kgâ1 or higher of MB327 DMS was lethal to the guinea-pigs. A lower dose of MB327 DMS (30 mg kgâ1) caused flaccid paralysis accompanied by respiratory impairment. Respiration normalised by 30 min, although the animals remained incapacitated to 4 h. MB327 or related compounds may be of utility in treatment of nerve agent poisoning as a component of therapy with atropine, anticonvulsant and oxime, or alternatively as an infusion under medical supervision
Thirty Years of heavy Fermions: Scientific Setting for their Discovery and Partial Understanding
Heavy-Fermions provide an extreme example of the utility of the idea of
continuity and analyticity in physics. Their discovery and study in the past
thirty years has added a fascinating chapter to condensed matter physics. I
briefly review the origins of the heavy-fermion problem out of the study of
magnetic moments in metals and the study of mixed-valent rare-earth compounds.
I also review the principal ideas underlying the features understood in their
fermi-liquid phase as well as in their anisotropic superconductivity. The
unsolved issues are also briefly mentioned.Comment: This is the text of one of the talks given at the plenary symposium
entitled "Thirty years of heavy Fermions" at the beginning of the
International conference on Strongly correlated Electrons in Vienna in July
200
Competing Orders in Coupled Luttinger Liquids
We consider the problem of two coupled Luttinger liquids both at half filling
and at low doping levels, to investigate the problem of competing orders in
quasi-one-dimensional strongly correlated systems. We use bosonization and
renormalization group equations to investigate the phase diagrams, to determine
the allowed phases and to establish approximate boundaries among them. Because
of the chiral translation and reflection symmetry in the charge mode away from
half filling, orders of charge density wave (CDW) and spin-Peierls (SP)
diagonal current (DC) and -density wave (DDW) form two doublets and thus can
be at most quasi-long range ordered. At half-filling, umklapp terms break this
symmetry down to a discrete group and thus Ising-type ordered phases appear as
a result of spontaneous breaking of the residual symmetries. Quantum disordered
Haldane phases are also found, with finite amplitudes of pairing orders and
triplet counterparts of CDW, SP, DC and DDW. Relations with recent numerical
results and implications to similar problems in two dimensions are discussed.Comment: 16 pages, 5 figures, 4 tables. Revised manuscript; a misprint in Eq.
B3 has been corrected. The paper is already in print in PR
Role of Orbital Degeneracy in Double Exchange Systems
We investigate the role of orbital degeneracy in the double exchange (DE)
model. In the limit, an effective generalized ``Hubbard''
model incorporating orbital pseudospin degrees of freedom is derived. The model
possesses an exact solution in one- and in infinite dimensions. In 1D, the
metallic phase off ``half-filling'' is a Luttinger liquid with
pseudospin-charge separation. Using the solution for our effective
model, we show how many experimental observations for the well-doped () three-dimensional manganites can be qualitatively
explained by invoking the role of orbital degeneracy in the DE model.Comment: 8 pages, 2 figures, submitted to Phys. Rev.
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