The group explores ultrafast electron dynamics in different low-dimensional materials using time- and angle-resolved photoemission spectroscopy. Due to confinement and reduced electronic screening low-dimensional materials exhibit peculiar electronic properties that we aim to control with tailored femtosecond light fields.
The underlying idea can be explained quite simply using a children's toy. The spinning top, which will serve as an example here, has a somewhat unusual shape: its body is spherical with a cylindrical hole in the center surrounding the central rod. When the spinning top is at rest, the rod in the center points upward (Figure 1, left). However, if one sets this particular spinning top in rotation, it will, unlike a conventional spinning top, turn upside down and rotate on the top of the rod (Figure 1, right). The inverted position is stabilized by the rotation of the spinning top.
spinning top at rest (left) and dynamical stabilization of the inverted position (right)
This idea of dynamical stabilization can be exploited to generate new electronic properties in a solid. For example, the wavelength of the driving laser pulses can be chosen to be resonant to a lattice vibration of the solid. When atoms move against each other under the influence of the laser, the overlap of electron clouds between neighboring atoms is modulated, which has a direct effect on the electronic properties of the material. In many cases, periodic driving causes the atoms to move to a new equilibrium position and a completely new crystal lattice will form, stabilized by the periodic drive and completely analogous to the dynamic stabilization of the inverted spinning top in Figure 1.
There are no limits to the imagination: using periodic driving one can turn an insulator into a metal (or vice versa), a metal into a superconductor, a trivial semiconductor into a semiconductor with topologically protected edge states, and much more. There are many exciting theoretical predictions on this topic and also first experimental successes.
To find out if periodic driving of the solid with strong laser pulses has the desired effect, one needs a method to study the electronic properties of the solid. We are using time- and angle-resolved photoelectron spectroscopy (tr-ARPES) for this purpose. Here, the sample is irradiated with UV light to release electrons from the material. With the help of a hemispherical analyzer the velocity and the emission angle of these photoelectrons are measured. From these measured quantities, the binding energy E and the momentum k of the electrons in the material can be determined, such that the measurement provides direct access to the band structure E(k) and thus to the electronic properties of the material.
We are interested in various one- and two-dimensional electron systems, some of which we fabricate ourselves in a sample preparation chamber. Due to confinement and screening the interactions between individual electrons in these materials are particularly strong. As a result, often the smallest changes, e.g. in the atomic structure, have a large influence on the electronic properties. For this reason, these low-dimensional electron systems exhibit a number of exciting properties, such as metal-insulator transitions or even superconductivity at low temperatures. Our recent achievements include the observation of light-induced phase transitions in one-dimensional indium wires [1, 2], the demonstration of ultrafast charge transfer in a heterostructure made of two different two-dimensional materials [3, 4], the study of various aspects of charge carrier dynamics in two-dimensional carbon (graphene) [5-8], and the generation of photon-dressed electronic states .
 M. Chavez-Cervantes et al., Phys. Rev. Lett. 123, 036405 (2019)
 M. Chavez-Cervantes et al., Phys. Rev. B 97, 201401 (2018)
 S. Aeschlimann et al., Sci. Adv. 6, eaay0761 (2020)
 R. Krause et al., arXiv2012.09268 (2020)
 S. Aeschlimann et al., Phys. Rev. B 96, 020301 (2017)
 E. Pomarico et al., Phys. Rev. B 95, 024304 (2017)
 I. Gierz et al., Phys. Rev. Lett. 115, 086803 (2015)
 I. Gierz et al., Nat. Mater. 12, 1119 (2013)
 S. Sato et al., J. Phys. B: At. Mol. Opt. Phys. 53, 225601 (2020)