30,472 research outputs found
Breaking Symmetry with Light: Ultra-Fast Ferroelectricity and Magnetism from Three-Phonon Coupling
A theory describing how ferroic properties can emerge transiently in the
ultra-fast regime by breaking symmetry with light through three-phonon coupling
is presented. Particular emphasis is placed on the special case when two
exactly degenerate mid-infra-red or THz phonons are resonantly pumped, since
this situation can give rise to an exactly rectified ferroic response with
damping envelopes of ~ 1 ps or less. Light-induced ferroelectricity and
ferromagnetism are discussed in this context, and a number of candidate
materials that could display these phenomena are proposed. The same analysis is
also applied to the interpretation of previous femto-magnetism experiments,
performed in different frequency ranges (visible and near-infrared), but
sharing similar symmetry characteristics.Comment: 10 page
Groups of virtual and welded links
We define new notions of groups of virtual and welded knots (or links) and we
study their relations with other invariants, in particular the Kauffman group
of a virtual knot.Comment: 22 pages, 7 figure
Representations of Braid Groups and Generalisations
We define and study extensions of Artin's representation and braid monodromy
representation to the case of topological and algebraical generalisations of
braid groups. In particular we provide faithful representations of braid groups
of oriented surfaces with boundary components as (outer) automorphisms of free
groups. We give also similar representations for braid groups of non oriented
surfaces with boundary components and we show a representation of braid groups
of closed surfaces as outer automorphisms of free groups. Finally, we provide
faithful representations of Artin-Tits groups of type as
automorphisms of free groups.Comment: Few misprints corrected, simpler proof for proposition 1
The Levi Decomposition of a Graded Lie Algebra
We show that a graded Lie algebra admits a Levi decomposition that is
compatible with the grading
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