Triangle Seminars
Tuesday, 12 Oct 2010
Blood and metal flowing down helical pipes
Jonathan Mestel
(Imperial College)
Abstract:
Helically symmetry is an exact generalisation of two-dimensionality and
axisymmetry.
The flow down a helical pipe is investigated under the assumption of
helical symmetry. The implications for blood flow in the body are
discussed. It is shown that the observed torsion of arteries may have
fluid dynamical benefits.
The blood is then replaced by a liquid metal, and it is found that the
same flow can give rise to the spontaneous generation of magnetic field,
known as a dynamo. Animations, but no experiments will be shown, in the
absence of a volunteer for this surgical procedure.
Helically symmetry is an exact generalisation of two-dimensionality and
axisymmetry.
The flow down a helical pipe is investigated under the assumption of
helical symmetry. The implications for blood flow in the body are
discussed. It is shown that the observed torsion of arteries may have
fluid dynamical benefits.
The blood is then replaced by a liquid metal, and it is found that the
same flow can give rise to the spontaneous generation of magnetic field,
known as a dynamo. Animations, but no experiments will be shown, in the
absence of a volunteer for this surgical procedure.
Posted by: KCL
Wednesday, 13 Oct 2010
Unusual singular behavior of the entanglement entropy in one dimension
Francesco Ravanini
(Bologna)
Abstract:
We study the Renyi entropy of the one-dimensional XYZ spin-1/2 chain in the entirety of its phase diagram. The model has several quantum critical lines corresponding to rotated XXZ chains in their paramagnetic phase, and four tri-critical points where these phases join. Two of these points are described by a conformal field theory and close to them the entropy scales as the logarithm of its mass gap. The other two points are not conformal and the entropy has a peculiar singular behavior in their neighbors, characteristic of an essential singularity. At these non-conformal points the model undergoes a discontinuous transition, with a level crossing in the ground state and a quadratic excitation spectrum. We propose the entropy as an efficient tool to determine the discontinuous or continuous nature of a phase transition also in more complicated models.
We study the Renyi entropy of the one-dimensional XYZ spin-1/2 chain in the entirety of its phase diagram. The model has several quantum critical lines corresponding to rotated XXZ chains in their paramagnetic phase, and four tri-critical points where these phases join. Two of these points are described by a conformal field theory and close to them the entropy scales as the logarithm of its mass gap. The other two points are not conformal and the entropy has a peculiar singular behavior in their neighbors, characteristic of an essential singularity. At these non-conformal points the model undergoes a discontinuous transition, with a level crossing in the ground state and a quadratic excitation spectrum. We propose the entropy as an efficient tool to determine the discontinuous or continuous nature of a phase transition also in more complicated models.
Posted by: KCL
W-algebras and surface operators in 4d N=2 gauge theories
📍 London
Niclas Wyllard
(Chalmers)
Abstract:
We discuss relations between two a priori unrelated classes of objects: (i) W-algebras, which are certain symmetry algebras of two-dimensional conformal field theories, and (ii) four-dimensional N=2 gauge theories in the presence of surface operators (certain two-dimensional defects). In particular, we relate the classifications of W-algebras and surface operators.
We discuss relations between two a priori unrelated classes of objects: (i) W-algebras, which are certain symmetry algebras of two-dimensional conformal field theories, and (ii) four-dimensional N=2 gauge theories in the presence of surface operators (certain two-dimensional defects). In particular, we relate the classifications of W-algebras and surface operators.
Posted by: QMW
Black Holes and Exotic Geometries
📍 London
Jan de Boer
(Amsterdam)
Thursday, 14 Oct 2010
D-Brane Wess-Zumino Terms and U-Duality
Eric Bergshoeff
(Groningen)
Abstract:
I will show how to construct gauge-invariant and U-duality covariant expressions for
Wess-Zumino terms corresponding to general Dp-branes in arbitrary D less than 10 dimensions.
A distinguishing feature of these Wess-Zumino terms is that they contain twice as
many scalars as the 10-D compactified dimensions, in line with
doubled geometry. It turns out that For D less than 10 the charges of the
higher-dimensional branes can all be expressed as products of the
0-brane charges, which include the D0-brane and the NS-NS 0-brane
charges. I will show how the general expressions for these charges
determine the non-trivial conjugacy class
to which some of the higher-dimensional D-branes belong.
Some implications and extensions of our work will be discussed.
I will show how to construct gauge-invariant and U-duality covariant expressions for
Wess-Zumino terms corresponding to general Dp-branes in arbitrary D less than 10 dimensions.
A distinguishing feature of these Wess-Zumino terms is that they contain twice as
many scalars as the 10-D compactified dimensions, in line with
doubled geometry. It turns out that For D less than 10 the charges of the
higher-dimensional branes can all be expressed as products of the
0-brane charges, which include the D0-brane and the NS-NS 0-brane
charges. I will show how the general expressions for these charges
determine the non-trivial conjugacy class
to which some of the higher-dimensional D-branes belong.
Some implications and extensions of our work will be discussed.
Posted by: QMW