Stability of Matter in Magnetic Fields

The proof of the stability of matter is three decades old, but the question of stability when arbitrarily large magnetic fields are taken into account was settled only recently. Even more recent is the solution to the question of the stability of relativistic matter when the electron motion is governed by the Dirac operator (together with Dirac's prescription of filling the ``negative energy sea"). When magnetic fields are included the question arises whether it is better to fill the negative energy sea of the free Dirac operator or of the Dirac operator with magnetic field. The answer is found to be that the former prescription is unstable while the latter is stable. This paper is a brief, nontechnical summary of recent work with M. Loss, J.P. Solovej and H. Siedentop.Comment: Review article, 8 pages, Tex, Zeits. f. Physik (in press

Singlets and reflection symmetric spin systems

We rigorously establish some exact properties of reflection symmetric spin systems with antiferromagnetic crossing bonds: At least one ground state has total spin zero and a positive semidefinite coefficient matrix. The crossing bonds obey an ice rule. This augments some previous results which were limited to bipartite spin systems and is of particular interest for frustrated spin systems.Comment: 11 pages, LaTeX 2