Integrated Calendar

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. In this talk we will discuss the symmetry aspects of atomic collapse, in particular the anomalous breaking of scale invariance. We will also describe recent experiments that use scanning tunneling microscopy (STM) to probe atomic collapse near STM-controled artificial compound nuclei.

One fundamental question in neuroscience remains largely unanswered: how are computations implemented by structured neural circuits? Over the last fifty years we have learned how sensory, motor and cognitive information is represented in different regions of the mammalian brain. Anatomical studies are beginning to reveal precisely structured neural circuits, including stereotyped circuit motifs across brain areas subserving different functions. However, linking physiology and detailed anatomy remains elusive in most cases. We know little about activity in specific cell types, the nodes of the circuit diagram, in behaving animals.

In our lab we study how tactile information is represented in different brain circuits in the mice vibrissal system. We train mice to move their whiskers to judge the location of an object presented in one of several locations and record extra- and intracellularly from specific neural types in this circuit. I’ll present recent results showing how tactile information is processed and transformed by specific neural types and circuits as it ascends from the sensory periphery to cortex.

The estimated magnitude of aerosol cloud mediated radiative forcing has evolved during the IPCC assessments, but its large uncertainty remained essentially unchanged and constituted a major component of the total uncertainty in climate forcing. In fact, paradoxically, the advancements in understanding impacts of cloud aerosol interactions on radiation increase the uncertainty, because we discover additional pathways by which aerosols affects clouds much faster than developing our ability to quantify these effects observationally. In other words, we discover more of what we should know that we don't know. This inherently increases the known uncertainty. The presentation will review the various known and recently discovered aerosol cloud mediated radiative effects and the methods that may be used to quantify them. The major challenges in doing so are measuring CCN from satellites and disentangling the impacts of CCN and updrafts on cloud properties. New breakthrough capabilities that give hope that it may be achievable from current satellite measurements will be presented.