Events - Daily
November 04, 2013 >>
|Monday, November 04|
Biophysics Seminar Series - Stefano Fusi (Columbia University)
The importance of mixed selectivity in complex cognitive tasks
Jadwin 111 · 12:00 p.m.– 1:00 p.m.
Condensed Matter Seminar, Leonid Levitov , "Atomic collapse in graphene"
Since the discovery that electrons in graphene behave as massless Dirac fermions, the single-atom-thick material has become a fertile playground for testing exotic predictions of quantum electrodynamics, such as Klein tunneling and the fractional quantum Hall effect. Now add to that list atomic collapse, the spontaneous formation of electrons and positrons in the electrostatic field of a superheavy atomic nucleus. The atomic collapse was predicted to manifest itself in quasistationary states which have complex-valued energies and which decay rapidly. However, the atoms created artificially in laboratory have nuclear charge only up to Z = 118, which falls short of the predicted threshold for collapse, Interest in this problem has been revived with the advent of graphene, where because of a large fine structure constant the collapse is expected for Z of order unity. I this talk I will discuss the symmetry aspects of atomic collapse, in particular the anomalous breaking of scale invariance. I will also describe recent experiments that use scanning tunneling microscopy (STM) to probe atomic collapse near STM-controlled artificial compound nuclei.
PCTS Seminar Room · 1:15 p.m.– 2:30 p.m.
High Energy Theory Seminar - Xi Dong, Stanford - "Holographic Entanglement Entropy for General Higher Derivative Gravity"
We propose a general formula for calculating the entanglement entropy in theories dual to higher derivative gravity where the Lagrangian is a contraction of Riemann tensors. Our formula consists of Wald's formula for the black hole entropy, as well as corrections involving the extrinsic curvature. We derive these corrections by noting that they arise from naively higher order contributions to the action which are enhanced due to would-be logarithmic divergences. Our formula reproduces the Jacobson-Myers entropy in the context of Lovelock gravity, and agrees with existing results for general four-derivative gravity.
We emphasize that the formula should be evaluated on a particular bulk surface whose location can in principle be determined by solving the equations of motion with conical boundary conditions. This may be difficult in practice, and an alternative method is desirable. A natural prescription is simply minimizing our formula, analogous to the Ryu-Takayanagi prescription for Einstein gravity. We show that this is correct in several examples including Lovelock and general four-derivative gravity.
PCTS Seminar Room · 2:30 p.m.– 3:30 p.m.