title: Scattering experiments with microwave billiards at an exceptional point under broken time-reversal invariance.
abstract: Scattering experiments with microwave cavities were performed and the effects of broken time-reversal invariance (TRI), induced by means of a magnetized ferrite placed inside the cavity, on an isolated doublet of nearly degenerate resonances were investigated. All elements of the effective Hamiltonian of this two-level system were extracted. As a function of two experimental parameters, the doublet and the associated eigenvectors could be tuned to coalesce at a so-called exceptional point (EP). The behavior of the eigenvalues and eigenvectors when encircling the EP in parameter space was studied, including the geometric amplitude that builds up in the case of broken TRI. A one-dimensional subspace of parameters was found where the differences of the eigenvalues are either real or purely imaginary. There, the Hamiltonians were found to be PT invariant under the combined operation of parity (P) and time reversal (T) in a generalized sense. The EP is the point of transition between both regions. There a spontaneous breaking of PT occurs.
authors.affiliation: Institut für Kernphysik, Technische Universität Darmstadt, D-64289 Darmstadt, Germany and Laboratoire de Photonique Quantique et Moléculaire, CNRS UMR 8537, Institut d'Alembert FR 3242, Ecole Normale Supérieure de Cachan, F-94235 Cachan, France.
authors.forename: S
authors.initials: S
authors.lastname: Bittner
authors.affiliation: Institut für Kernphysik, Technische Universität Darmstadt, D-64289 Darmstadt, Germany.
authors.forename: B
authors.initials: B
authors.lastname: Dietz
authors.affiliation: Max-Planck-Institut für Kernphysik, D-69029 Heidelberg, Germany.
authors.forename: H L
authors.initials: HL
authors.lastname: Harney
authors.affiliation: Institut für Kernphysik, Technische Universität Darmstadt, D-64289 Darmstadt, Germany.
authors.forename: M
authors.initials: M
authors.lastname: Miski-Oglu
authors.affiliation: Institut für Kernphysik, Technische Universität Darmstadt, D-64289 Darmstadt, Germany.
authors.forename: A
authors.initials: A
authors.lastname: Richter
authors.affiliation: Division of Physics and Astronomy, Kyoto University, Kitashirakawa Oiwake-cho, Sakyo-ku, Kyoto, 606-8502, Japan.
authors.forename: F
authors.initials: F
authors.lastname: Schäfer