9,190 research outputs found
Carbanions. II. Carbon-13 nuclear magnetic resonance study of Meisenheimer complexes and their charge distribution pattern
Stable Carbocations. 198. Formation of Allyl Cations via Protonation of Alkynes in Magic Acid Solution. Evidence for 1,2-Hydrogen and Alkyl Shifts in the Intermediate Vinyl Cations
K3--Fibrations and Heterotic-Type II String Duality
We analyze the map between heterotic and type II N=2 supersymmetric string
theories for certain two and three moduli examples found by Kachru and Vafa.
The appearance of elliptic j-functions can be traced back to specializations of
the Picard-Fuchs equations to systems for surfaces. For the three-moduli
example we write the mirror maps and Yukawa couplings in the weak coupling
limit in terms of j-functions; the expressions agree with those obtained in
perturbative calculations in the heterotic string in an impressive way. We also
discuss symmetries of the world-sheet instanton numbers in the type II theory,
and interpret them in terms of S--duality of the non-perturbative heterotic
string.Comment: 16p, harvmac with hyperlink
BPS States of Exceptional Non-Critical Strings
We study the BPS states of non-critical strings which arise for zero size
instantons of exceptional groups. This is accomplished by using F-theory and
M-theory duals and by employing mirror symmetry to compute the degeneracy of
membranes wrapped around 2-cycles of the Calabi-Yau threefold. We find evidence
for a number of novel physical phenomena, including having infinitely many
light states with the first lightest state including a nearly massless
gravitino.Comment: 29 pages, 1 figure, references added, to appear in the proceedings of
the conference "Advanced Quantum Field Theory'' (in memory of Claude
Itzykson
Stable carbocations. 211. 1-Phenylallyl cations and their rearrangement to indanyl cations in superacidic media
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Concept talk cannot be avoided
Distinct systems for representing concepts as prototypes, exemplars, and theories are closely integrated in the mind, and the notion of concept is required as a framework for exploring this integration. Eliminating the term “concept” from our theories will hinder rather than promote scientific progress
Cooperative dynamics in doped manganite films: phonon anomalies in the ferromagnetic state
We present optical measurements of phononic excitations in
LaCaMnO (LCMO) and LaSrMnO (LSMO)
thin films covering the full temperature range from the metallic ferromagnetic
to the insulating paramagnetic phase. All eight phonons expected for the
Rc symmetry in LSMO and 17 out of the expected 25 phonons for the Pnma
symmetry in LCMO have been determined. Close to the
ferromagnetic-to-paramagnetic transition both compounds reveal an anomalous
behavior but with different characteristics. Anomalies in the phononic spectra
are a manifestation of the coupling of lattice degrees of freedom (DOF) to
electronic DOF. Specifically, the low-frequency external group proves to be an
indicator for lattice modifications induced by electronic correlations. The
enhanced electron-phonon coupling in LCMO is responsible for Fano-like
interference effects of distinct phonon modes with electronic continuum
excitations: we observe asymmetric phonon line shapes, mode splitting and
spectral weight transfer between modes.Comment: 10 pages, 10 figure
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