8,865 research outputs found
A new kind of McKay correspondence from non-Abelian gauge theories
The boundary chiral ring of a 2d gauged linear sigma model on a K\"ahler
manifold classifies the topological D-brane sectors and the massless open
strings between them. While it is determined at small volume by simple group
theory, its continuation to generic volume provides highly non-trivial
information about the -branes on , related to the derived category
. We use this correspondence to elaborate on an extended notion of
McKay correspondence that captures more general than orbifold singularities. As
an illustration, we work out this new notion of McKay correspondence for a
class of non-compact Calabi-Yau singularities related to Grassmannians.Comment: 29 pages, harvmac(b), 2 fig
Large and Small Polaron Excitations in La2/3(Sr/Ca)1/3MnO3 Films
We present detailed optical measurements of the mid-infrared (MIR)
excitations in thin films of La2/3Sr1/3MnO3 (LSMO) and La2/3Ca1/3MnO3 (LCMO)
across the magnetic transition. The shape of the excitation at about 0.2 eV in
both samples is analyzed in terms of polaron models. We propose to identify the
MIR resonance in LSMO as the excitation of large polarons and that in LCMO as a
small polaron excitation. A scaling behavior for the low-energy side of the
polaronic MIR resonance in LSMO is established
Superrotation planetary atmospheres: Mechanical analogy, angular momentum budget and simulation of the spin up process
Superrotation rates observed in planetary atmospheres are analyzed based on the concept of a thermally driven zonally symmetric circulation. Specifically, how this superrotation is produced and maintained against the tendency for friction to oppose differential motions between the atmosphere and the underlying planet is addressed. The time evolution of a fluid leading from corotation under uniform heating to superrotation under globally nonuniform heating is simulated using a three dimensional zonally symmetric spectral model and Laplace transformation. The increased tendency toward geostrophy combined with the increase of surface pressure toward the poles (due to meridional mass transport), induces the atmosphere to subrotate temporarily at lower altitudes. The resulting viscous shear near the surface thus permits angular momentum to flow from the planet into the atmosphere where it propagates upwards and, combined with the change in moment of inertia, produces large superrotation rates at higher viscosities
Colossal Effects in Transition Metal Oxides Caused by Intrinsic Inhomogeneities
The influence of quenched disorder on the competition between ordered states
separated by a first-order transition is investigated. A phase diagram with
features resembling quantum-critical behavior is observed, even using classical
models. The low-temperature paramagnetic regime consists of coexisting ordered
clusters, with randomly oriented order parameters. Extended to manganites, this
state is argued to have a colossal magnetoresistance effect. A scale T* for
cluster formation is discussed. This is the analog of the Griffiths
temperature, but for the case of two competing orders, producing a strong
susceptibility to external fields. Cuprates may have similar features,
compatible with the large proximity effect of the very underdoped regime.Comment: 4 pages, 4 figure
Altitude variation of ion composition in the midlatitude trough region - Evidence for upward plasma flow
Altitude effect on ion concentration in midlatitude trough and plasmaspher
Amorphous thin film growth: theory compared with experiment
Experimental results on amorphous ZrAlCu thin film growth and the dynamics of
the surface morphology as predicted from a minimal nonlinear stochastic
deposition equation are analysed and compared. Key points of this study are (i)
an estimation procedure for coefficients entering into the growth equation and
(ii) a detailed analysis and interpretation of the time evolution of the
correlation length and the surface roughness. The results corroborate the
usefulness of the deposition equation as a tool for studying amorphous growth
processes.Comment: 7 pages including 5 figure
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