QDev Seminar: Leonid Levitov

Department of Physics, MIT

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. 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-controlled
artificial compound nuclei.