Prospects and Limitations of Electronic Structure Imaging by Angle Resolved Photoemission Spectroscopy

Workshop Report


Satoshi Kera, Stephan Kümmel, Achim Schöll

The focus workshop PLESI16 was held in spring 2016 from April 24 to 27 at the MPI PKS. The program consisted of 13 invited talks, 4 hot topic talks, one poster session and the colloquium of Prof. Michael Ramsey from the University of Graz, Austria.

The main focus of the workshop was to bring together researchers working in the field of orbital imaging by Angle Resolved Photoelectron Spectroscopy (ARPES) in experiment and theory. In recent years ARPES has seen a rapid development with a particular focus on imaging the electronic structure of ordered molecular layers. The contributions by Prof. Ueno and Prof. Ramsey gave a sci- entifically deep and historically fascinating perspective on the development of the field of photoemission spectroscopy, reminding everyone that it is not only allowed, but even fruitful to occasionally question the ‘established truth’. We learned with joy that “sometimes, one must sacrifice precision for clarity” (Bertrand Russell cited by M. Ramsey), and pondered the concept of “unnecessarily simplifying”.

The combination of ARPES experiments and elec- tronic structure calculations in the orbital imaging approach has become increasingly popular for investigat- ing the structural, chemical and physical properties of molecular materials. However, despite its increasing popularity, the approach also leads to fascinating and press- ing questions on the fundamental level. One prominent example is the plane wave final state approximation, which in the simplest interpretation of the photoemission process is often used to describe the outgoing electron. This approximation is too narrow for describing presently studied effects such as circular dichroism and raises the general question of how deviations due to the more com- plex final state manifest themselves and whether there is evidence for deviations particularly at low kinetic ener- gies or for scattering at high energies. On the theoretical side the question arises how final state effects can be taken into account for systems with a complicated, possible low-symmetry electronic structure such as seen in layers of organic molecules and whether explicit simulation of the emission process in real time can sidestep the problem. From a theoretical perspective, the interpretation of ARPES data by single particle orbitals raises the question, why and when the single-particle interpretation is appropriate and whether theory is available that goes beyond such a single orbital interpretation. Beyond this, the workshop was intended to fathom what can be learned from ARPES about systems such as e.g. graphene or other 2D or layered materials.

The talk by Prof. Wollenhaupt gave fascinating in sights into time-resolved experiments with high-harmonic generation set-ups and broadened the workshop’s perspective from solid-state systems to atomic and molecular physics. Other speakers pointed out new experimental possibilities with respect to spin-resolved experiments. The field of invited speakers was deliberately composed to cover these different aspects.

Well-known scientists from Europe, Japan, the US and Israel attended the workshop and provided excellent con- tributions. Beyond the aforementioned Prof. Ramsey (Graz, Austria), prominent researchers as Prof. Ebert (Munich, Germany), Dr. Ferretti (Modena, Italy), Prof. Höfer (Marburg, Germany), Prof. Kronik (Rehovot, Is-rael), Prof. Osterwalder (Zürich, Switzerland), Prof. Puschnig (Graz, Austria), Prof. Reinert (Würzburg, Germany), Prof. Rubio (Hamburg, Germany), Dr. Soubach (Jülich, Germany), Prof. Ueno (Chiba, Japan), and Prof. Wolf (Berlin, Germany) and others followed the invitation to Dresden and shaped this workshop with their excellent contributions.

Among the 36 participants were 14 scientific newcomers on the PhD and PostDoc level from all over the world. Amongst these, Simon Moser (Berkeley) and Matthias Dauth (Bayreuth) gave invited and hot topis talks, respectively, and all other young colleagues contributed by presentations at the poster session.

The workshop was a successful event and inspired very fruitful discussions. It summarized the recent progress in the field, helped to shape a perspective for the future, and triggered stimulating new ideas.

We would like to thank the MPI PKS for facilitating this focus workshop in their visitors program and for providing the excellent infrastructure to us and the partici- pants. We particularly acknowledge the efficient organizational support and friendly assistance by the team of the visitors program, in particular Mandy Lochar, before, during, and after the workshop.