2,323 research outputs found
D-branes and Deformation Quantization
In this note we explain how world-volume geometries of D-branes can be
reconstructed within the microscopic framework where D-branes are described
through boundary conformal field theory. We extract the (non-commutative)
world-volume algebras from the operator product expansions of open string
vertex operators. For branes in a flat background with constant non-vanishing
B-field, the operator products are computed perturbatively to all orders in the
field strength. The resulting series coincides with Kontsevich's presentation
of the Moyal product. After extending these considerations to fermionic fields
we conclude with some remarks on the generalization of our approach to curved
backgrounds.Comment: 12 pages, Late
Generalization of the Knizhnik-Zamolodchikov-Equations
In this letter we introduce a generalization of the Knizhnik- Zamolodchikov
equations from affine Lie algebras to a wide class of conformal field theories
(not necessarily rational). The new equations describe correlations functions
of primary fields and of a finite number of their descendents. Our proposal is
based on Nahm's concept of small spaces which provide adequate substitutes for
the lowest energy subspaces in modules of affine Lie algebras. We explain how
to construct the first order differential equations and investigate properties
of the associated connections, thereby preparing the grounds for an analysis of
quantum symmetries. The general considerations are illustrated in examples of
Virasoro minimal models.Comment: 13 pages, uses amssym
Boundary Correlators in Supergroup WZNW Models
We investigate correlation functions for maximally symmetric boundary
conditions in the WZNW model on GL(1|1). Special attention is payed to volume
filling branes. Generalizing earlier ideas for the bulk sector, we set up a
Kac-Wakimoto-like formalism for the boundary model. This first order formalism
is then used to calculate bulk-boundary 2-point functions and the boundary
3-point functions of the model. The note ends with a few comments on
correlation functions of atypical fields, point-like branes and generalizations
to other supergroups.Comment: 33 page
Nonhermitian transport effects in coupled-resonator optical waveguides
Coupled-resonator optical waveguides (CROWs) are known to have interesting
and useful dispersion properties. Here, we study the transport in these
waveguides in the general case where each resonator is open and asymmetric,
i.e., is leaky and possesses no mirror-reflection symmetry. Each individual
resonator then exhibits asymmetric backscattering between clockwise and
counterclockwise propagating waves, which in combination with the losses
induces non-orthogonal eigenmodes. In a chain of such resonators, the coupling
between the resonators induces an additional source of non-hermiticity, and a
complex band structure arises. We show that in this situation the group
velocity of wave packets differs from the velocity associated with the
probability density flux, with the difference arising from a non-hermitian
correction to the Hellmann-Feynman theorem. Exploring these features
numerically in a realistic scenario, we find that the complex band structure
comprises almost-real branches and complex branches, which are joined by
exceptional points, i.e., nonhermitian degeneracies at which not only the
frequencies and decay rates coalesce but also the eigenmodes themselves. The
non-hermitian corrections to the group velocity are largest in the regions
around the exceptional points.Comment: 11 pages, 9 figure
Towards a Worldsheet Description of N=8 Supergravity
In this note we address the worldsheet description of 4-dimensional N=8
supergravity using ambitwistors. After gauging an appropriate current algebra,
we argue that the only physical vertex operators correspond to the N=8
supermultiplet. It has previously been shown that worldsheet correlators give
rise to supergravity tree level scattering amplitudes. We extend this work by
proposing a definition for genus-one amplitudes that passes several consistency
checks such as exhibiting modular invariance and reproducing the expected
infrared behavior of 1-loop supergravity amplitudes.Comment: 6 page
On boundary RG-flows in coset conformal field theories
We propose a new rule for boundary renormalization group flows in fixed-point
free coset models. Our proposal generalizes the 'absorption of boundary
spin'-principle formulated by Affleck and Ludwig to a large class of
perturbations in boundary conformal field theories. We illustrate the rule in
the case of unitary minimal models.Comment: 3 pages, uses RevTeX
Helical scattering and valleytronics in bilayer graphene
We describe an angularly asymmetric interface-scattering mechanism which allows to spatially separate the electrons in the two low-energy valleys of bilayer graphene. The effect occurs at electrostatically defined interfaces separating regions of different pseudospin polarization, and is associated with the helical winding of the pseudospin vector across the interface, which breaks the reflection symmetry in each valley. Electrons are transmitted with a preferred direction of up to 60° over a large energetic range in one of the valleys, and down to −60° in the other. In a Y-junction geometry, this can be used to create and detect valley polarization
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