arXiv subjects

Daniel Steil

Publications and source records attributed to Daniel Steil.

25 records · Page 2Linked to original sources

arXiv2021

Formation of moiré interlayer excitons in space and time

Moiré superlattices in atomically thin van-der-Waals heterostructures hold great promise for an extended control of electronic and valleytronic lifetimes, the confinement of excitons in artificial moiré lattices, and the formation of novel exotic quantum phases. Such moiré-induced emergent phenomena are particularly strong for interlayer excitons, where the hole and the electron are localized in different layers of the heterostructure. In order to exploit the full potential of correlated moiré and exciton physics, a thorough understanding of the ultrafast interlayer exciton formation process and the real-space wavefunction confinement in the moiré potential is indispensable. However, direct experimental access to these parameters is limited since most excitonic quasiparticles are optically dark. Here we show that femtosecond photoemission momentum microscopy provides quantitative access to these key properties of the moiré interlayer excitons. We find that interlayer excitons are dominantly formed on the sub-50~fs timescale via interlayer tunneling at the K valleys of the Brillouin zones. In addition, we directly measure energy-momentum fingerprints of the moiré interlayer excitons by mapping their spectral signatures within the mini Brillouin zone that is built up by the twisted heterostructure. From these momentum-fingerprints, we gain quantitative access to the modulation of the exciton wavefunction within the moiré potential in real-space. Our work provides the first direct access to the interlayer moiré exciton formation dynamics in space and time and reveals new opportunities to study correlated moiré and exciton physics for the future realization of exotic quantum phases of matter.

cond-mat.mes-hall↗

arXiv2021

Ultrafast element-resolved magneto-optics using a fiber-laser-driven extreme ultraviolet light source

We present a novel setup to measure the transverse magneto-optical Kerr effect in the extreme ultraviolet spectral range at exceptionally high repetition rates based on a fiber laser amplifier system. This affords a very high and stable flux of extreme ultraviolet light, which we use to measure element-resolved demagnetization dynamics with unprecedented depth of information. Furthermore, the setup is equipped with a strong electromagnet and a cryostat, allowing measurements between 10 and 420 K using magnetic fields up to 0.86 T. The performance of our setup is demonstrated by a set of temperature- and time-dependent magnetization measurements showing distinct element-dependent behavior.

physics.optics↗

arXiv2020

Direct Access to Auger recombination in Graphene

Auger scattering channels are of fundamental importance to describe and understand the non-equilibrium charge carrier dynamics in graphene. While impact excitation increases the number of carriers in the conduction band and has been observed experimentally, direct access to its inverse process, Auger recombination, has so far been elusive. Here, we tackle this problem by applying our novel setup for ultrafast time-resolved photoelectron momentum microscopy. Our approach gives simultaneous access to charge carrier dynamics at all energies and in-plane momenta within the linearly dispersive Dirac cones. We thus provide direct evidence for Auger recombination on a sub-10~fs timescale by identifying transient energy- and momentum-dependent populations far above the excitation energy. We compare our results with model calculations of scattering processes in the Dirac cone to support our experimental findings.

cond-mat.other↗

arXiv2020

Electromagnetic dressing of the electron energy spectrum of Au(111) at high momenta

Light-engineering of quantum materials via electromagnetic dressing is considered an on-demand approach for tailoring electronic band dispersions and even inducing topological phase transitions. For probing such dressed bands, photoemission spectroscopy is an ideal tool, and we employ here a novel experiment based on ultrafast photoemission momentum microscopy. Using this setup, we measure the in-plane momentum-dependent intensity fingerprints of the electromagnetically-dressed sidebands from a Au(111) surface for s- and p-polarized infrared driving. We find that at metal surfaces, due to screening of the driving laser, the contribution from Floquet-Bloch bands is negligible, and the dressed bands are dominated by the laser-assisted photoelectric effect. Also, we find that in contrast to general expectations, s-polarized light can dress free-electron states at large photoelectron momenta. Our results show that the dielectric response of the material must carefully be taken into account when using photoemission for the identification of light-engineered electronic band structures.

cond-mat.mes-hall↗

arXiv2017

Nanoscale Magnetic Imaging using Circularly Polarized High-Harmonic Radiation

This work demonstrates nanoscale magnetic imaging using bright circularly polarized high-harmonic radiation. We utilize the magneto-optical contrast of worm-like magnetic domains in a Co/Pd multilayer structure, obtaining quantitative amplitude and phase maps by lensless imaging. A diffraction-limited spatial resolution of 49 nm is achieved with iterative phase reconstruction enhanced by a holographic mask. Harnessing the unique coherence of high harmonics, this approach will facilitate quantitative, element-specific and spatially-resolved studies of ultrafast magnetization dynamics, advancing both fundamental and applied aspects of nanoscale magnetism.

physics.optics↗

arXiv2012

Light-induced magnetization reversal of high-anisotropy TbCo alloy films

Magnetization reversal using circularly polarized light provides a new way to control magnetization without any external magnetic field and has the potential to revolutionize magnetic data storage. However, in order to reach ultra-high density data storage, high anisotropy media providing thermal stability are needed. Here, we evidence all-optical magnetization switching for different TbxCo1-x ferrimagnetic alloy composition and demonstrate all-optical switching for films with anisotropy fields reaching 6 T corresponding to anisotropy constants of 3x106 ergs/cm3. Optical magnetization switching is observed only for alloys which compensation temperature can be reached through sample heating.

cond-mat.mtrl-sci↗

arXiv2009

Ultrafast demagnetization of ferromagnetic transition metals: The role of the Coulomb interaction

The Elliott-Yafet mechanism is arguably the most promising candidate to explain the ultrafast demagnetization dynamics in ferromagnetic transition metals on timescales on the order of 100 femtoseconds. So far, only electron-phonon scattering has been analyzed as the scattering process needed to account for the demagnetization due to the Elliott-Yafet mechanism. We show for the first time that the electron-electron scattering contribution to the Elliott-Yafet mechanism has the potential to explain time-resolved magneto-optical Kerr effect measurements on thin magnetic cobalt and nickel films, without reference to a phononic "spin bath."

cond-mat.mtrl-sci↗