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Cornelius Gahl

Publications and source records attributed to Cornelius Gahl.

13 recordsLinked to original sources
arXiv2025

Magnon-mediated terahertz spin transport in metallic Gd|Pt stacks

We study femtosecond spin transport in a Gd|Pt stack induced by a laser pulse. Remarkably, the dynamics of the spin current from Gd to Pt suggests that its dominant driving force is the ultrafast spin Seebeck effect. As the contribution of a transient spin voltage in the metal Gd is minor, Gd acts akin a magnetic insulator here. This view is supported by time- and spin-resolved photoemission, which indicates that a buildup of spin voltage is suppressed by exchange scattering, leading to similar amplitudes and relaxation rates of hot majority- and minority-spin electron populations.

cond-mat.mes-hall↗
arXiv2025

Strain engineering of valley-polarized hybrid excitons in a 2D semiconductor

Encoding and manipulating digital information in quantum degrees of freedom is one of the major challenges of today's science and technology. The valley indices of excitons in transition metal dichalcogenides (TMDs) are well-suited to address this challenge. Here, we demonstrate a new class of strain-tunable, valley-polarized hybrid excitons in monolayer TMDs, comprising a pair of energy-resonant intra- and intervalley excitons. These states combine the advantages of bright intravalley excitons, where the valley index directly couples to light polarization, and dark intervalley excitons, characterized by low depolarization rates. We demonstrate that the hybridized state of dark KK' intervalley and defect-localized excitons exhibits a degree of circular polarization of emitted photons that is three times higher than that of the constituent species. Moreover, a bright KK intravalley and a dark KQ exciton form a coherently coupled hybrid state under energetic resonance, with their valley depolarization dynamics slowed down a hundredfold. Overall, these valley-polarized hybrid excitons with strain-tunable valley character emerge as prime candidates for valleytronic applications in future quantum and information technology.

cond-mat.mes-hall↗
arXiv2024

Excitons under large pseudomagnetic fields

Excitons in Transition Metal Dichalcogenides (TMDs) acquire a spin-like quantum number, a pseudospin, originating from the crystal's discrete rotational symmetry. Here, we break this symmetry using a tunable uniaxial strain, effectively generating a pseudomagnetic field exceeding 40 Tesla. Under this large field, we demonstrate pseudospin analogs of spintronic phenomena such as the Zeeman effect and Larmor precession. Moreover, we determine previously inaccessible fundamental properties of TMDs, including the strength of the depolarizing field responsible for the loss of exciton coherence. Finally, we uncover the bosonic -- as opposed to fermionic -- nature of many-body excitonic species using the pseudomagnetic equivalent of the $g$-factor spectroscopy. Our work is the first step toward establishing this spectroscopy as a universal method for probing correlated many-body states and realizing pseudospin analogs of spintronic devices.

cond-mat.mes-hall↗
arXiv2024

Ultrafast Optical Control of Rashba Interactions in a TMDC Heterostructure

We investigate spin relaxation dynamics of interlayer excitons in a MoSe2/MoS2 heterostructure induced by the Rashba effect. In such a system, Rashba interactions arise from an out-of-plane electric field due to photo-generated interlayer excitons inducing a phonon-assisted intravalley spin relaxation. We develop a theoretical description based on a microscopic approach to quantify the magnitude of Rashba interactions and test these predictions via time-resolved Kerr rotation measurements. In agreement with the calculations, we find that the Rashba-induced intravalley spin mixing becomes the dominating spin relaxation channel above T = 50 K. Our work identifies a previously unexplored spin-depolarization channel in heterostructures which can be used for ultrafast spin manipulation.

cond-mat.mes-hall↗
arXiv2024

Local work-function manipulation by external optical stimulation

Strongly differing static dipole moments of the trans and cis isomers of photochromic azobenzene allow for optical switching the work function of azobenzene-functionalized self-assembled monolayers (SAMs). We apply these properties in a fundamental experiment to manipulate the area size of the switched SAM. Azobenzene molecules were excited by ultraviolet laser illumination and the transient isomerization profile of the SAM was spatially resolved recording photoemission electron microscopy (PEEM) images. Thereby we demonstrate the spatial tuning of the SAM work-function and discuss the role of the laser spot-profile in generating sharp edges or gradual changes of the work function.

cond-mat.mes-hall↗
arXiv2023

Strain fingerprinting of exciton valley character

Momentum-indirect excitons composed of electrons and holes in different valleys define optoelectronic properties of many semiconductors, but are challenging to detect due to their weak coupling to light. The identification of an excitons' valley character is further limited by complexities associated with momentum-selective probes. Here, we study the photoluminescence of indirect excitons in controllably strained prototypical 2D semiconductors (WSe$_2$, WS$_2$) at cryogenic temperatures. We find that these excitons i) exhibit valley-specific energy shifts, enabling their valley fingerprinting, and ii) hybridize with bright excitons, becoming directly accessible to optical spectroscopy methods. This approach allows us to identify multiple previously inaccessible excitons with wavefunctions residing in K, $Γ$, or Q valleys in the momentum space as well as various types of defect-related excitons. Overall, our approach is well-suited to unravel and tune intervalley excitons in various semiconductors.

cond-mat.mes-hall↗
arXiv2023

Probing the formation of dark interlayer excitons via ultrafast photocurrent

Optically dark excitons determine a wide range of properties of photoexcited semiconductors yet are hard to access via conventional spectroscopies. Here, we develop a time-resolved ultrafast photocurrent technique (trPC) to probe the formation dynamics of optically dark excitons. The nonlinear nature of the trPC makes it particularly sensitive to the formation of excitons occurring at the femtosecond timescale after the excitation. As proof of principle, we extract the interlayer exciton formation time 0.4~ps at 160 $μ$J/cm$^2$ fluence in a MoS$_2$/MoSe$_2$ heterostructure and show that this time decreases with fluence. In addition, our approach provides access to the dynamics of carriers and their interlayer transport. Overall, our work establishes trPC as a technique to study dark excitons in various systems that are hard to probe by other approaches.

cond-mat.mes-hall↗
arXiv2021

Spin/valley coupled dynamics of electrons and holes at the ${\text{MoS}_{2}-\text{MoSe}_{2}}$ interface

The coupled spin and valley degrees of freedom in transition metal dichalcogenides (TMDs) are considered a promising platform for information processing. Here, we use a TMD heterostructure ${\text{MoS}_{2}-\text{MoSe}_{2}}$ to study optical pumping of spin/valley polarized carriers across the interface and to elucidate the mechanisms governing their subsequent relaxation. By applying time-resolved Kerr and reflectivity spectroscopies, we find that the photoexcited carriers conserve their spin for both tunneling directions across the interface. Following this, we measure dramatically different spin/valley depolarization rates for electrons and holes, $\sim 30\,{\text{ns}}^{-1}$ and $< 1\,{\text{ns}}^{-1}$, respectively and show that this difference relates to the disparity in the spin-orbit splitting in conduction and valence bands of TMDs. Our work provides insights into the spin/valley dynamics of free carriers unaffected by complex excitonic processes and establishes TMD heterostructures as generators of spin currents in spin/valleytronic devices.

cond-mat.mes-hall↗
arXiv2020

Ultrafast photocurrents in MoSe$_2$ probed by terahertz spectroscopy

We use the terahertz (THz) emission spectroscopy to study femtosecond photocurrent dynamics in the prototypical 2D semiconductor, transition metal dichalcogenide MoSe$_2$. We identify several distinct mechanisms producing THz radiation in response to an ultrashort ($30\,$fs) optical excitation in a bilayer (BL) and a multilayer (ML) sample. In the ML, the THz radiation is generated at a picosecond timescale by out-of-plane currents due to the drift of photoexcited charge carriers in the surface electric field. The BL emission is generated by an in-plane shift current. Finally, we observe oscillations at about $23\,$THz in the emission from the BL sample. We attribute the oscillations to quantum beats between two excitonic states with energetic separation of $\sim100\,$meV.

cond-mat.mes-hall↗
arXiv2016

Optical properties of azobenzene-functionalized self-assembled monolayers: Intermolecular coupling and many-body interactions

In a joint theoretical and experimental work the optical properties of azobenzene-functionalized self-assembled monolayers (SAMs) are studied at different molecular packing densities. Our results, based on density-functional and many-body perturbation theory, as well as on differential reflectance (DR) spectroscopy, shed light on the microscopic mechanisms ruling photo-absorption in these systems. While the optical excitations are intrinsically excitonic in nature, regardless of the molecular concentration, in densely-packed SAMs intermolecular coupling and local-field effects are responsible for a sizable weakening of the exciton binding strength. Through a detailed analysis of the character of the electron-hole pairs, we show that distinct excitations involved in the photo-isomerization at low molecular concentrations are dramatically broadened by intermolecular interactions. Spectral shifts in the calculated DR spectra are in good agreement with the experimental results. Our findings represent an important step forward to rationalize the excited-state properties of these complex materials.

cond-mat.mtrl-sci↗
arXiv2011

Hot electron driven enhancement of spin-lattice coupling in 4f ferromagnets observed by femtosecond x-ray magnetic circular dichroism

Femtosecond x-ray magnetic circular dichroism was used to study the time-dependent magnetic moment of 4 fs electrons in the ferromagnets Gd and Tb, which are known for their different spin-lattice coupling. We observe a two-step demagnetization with an ultrafast demagnetization time of 750 fs identical for both systems and slower times which differ sizeably with 40 ps for Gd and 8 ps for Tb. We conclude that spin-lattice coupling in the electronically excited state is enhanced up to orders of magnitude compared to equilibrium.

cond-mat.mtrl-sci↗
arXiv2001

Anisotropy of Quasiparticle Lifetimes and the Role of Disorder for Ultrafast Electron Dynamics in Graphite

Femtosecond time-resolved photoemission of photoexcited electrons in highly oriented pyrolytic graphite (HOPG) provides strong evidence for anisotropies of quasiparticle (QP) lifetimes. Indicative of such anisotropies is a pronounced anomaly in the energy dependence of QP lifetimes between 1.1 eV and 1.5 eV - the vicinity of a saddle point in the graphite bandstructure. This is supported by recent ab initio calculations and a comparison with experiments on defect-enriched HOPG which reveal that disorder, e.g. defects or phonons, increases electron energy relaxation rates.

cond-mat↗