Stefan Mathias
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View article: Hybrid Frenkel–Wannier excitons facilitate ultrafast energy transfer at a 2D–organic interface
Hybrid Frenkel–Wannier excitons facilitate ultrafast energy transfer at a 2D–organic interface Open
Two-dimensional transition metal dichalcogenides and organic semiconductors have emerged as promising material platforms for optoelectronic devices. Combining the two is predicted to yield emergent properties while retaining the advantages…
View article: Pathways to bubble and skyrmion lattice formation in Fe/Gd multilayers
Pathways to bubble and skyrmion lattice formation in Fe/Gd multilayers Open
The creation and control of magnetic spin textures is of great interest in fundamental research and future device-oriented applications. Fe/Gd multilayers host a rich variety of magnetic textures including topologically trivial type-2 bubb…
View article: Observation of Floquet states in graphene
Observation of Floquet states in graphene Open
Floquet engineering—the coherent dressing of matter via time-periodic perturbations—is a mechanism to realize and control emergent phases in materials out of equilibrium. However, its applicability to metallic quantum materials and semimet…
View article: Algorithmic approaches to avoiding bad local minima in nonconvex inconsistent feasibility
Algorithmic approaches to avoiding bad local minima in nonconvex inconsistent feasibility Open
The main challenge of nonconvex optimization is to find a global optimum, or at least to avoid ``bad'' local minima and meaningless stationary points. We study here the extent to which algorithms, as opposed to optimization models and regu…
View article: Table-top three-dimensional photoemission orbital tomography with a femtosecond extreme ultraviolet light source
Table-top three-dimensional photoemission orbital tomography with a femtosecond extreme ultraviolet light source Open
Following electronic processes in molecules and materials at the level of the quantum mechanical electron wavefunction with angstrom-level spatial resolution and with full access to its femtosecond temporal dynamics is at the heart of ultr…
View article: Interlayer charge transfer in graphene–2D polyimide heterostructures
Interlayer charge transfer in graphene–2D polyimide heterostructures Open
The vertical integration of multiple two-dimensional (2D) materials in heterostructures, held together by van der Waals forces, has opened unprecedented possibilities for modifying the (opto-)electronic properties of nanodevices. This not …
View article: Ultrafast nano-imaging of dark excitons
Ultrafast nano-imaging of dark excitons Open
Saudi Arabian Ministry of Higher Education
View article: Hybrid Frenkel-Wannier excitons facilitate ultrafast energy transfer at a 2D-organic interface
Hybrid Frenkel-Wannier excitons facilitate ultrafast energy transfer at a 2D-organic interface Open
Two-dimensional transition metal dichalcogenides (TMDs) and organic semiconductors (OSCs) have emerged as promising material platforms for next-generation optoelectronic devices. The combination of both is predicted to yield emergent prope…
View article: Dynamics of electronic phase separation at the laser-induced insulator-metal transition in <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:msub><mml:mrow><mml:mo>(</mml:mo><mml:msub><mml:mi>La</mml:mi><mml:mrow><mml:mn>0.6</mml:mn></mml:mrow></mml:msub><mml:msub><mml:mi>Pr</mml:mi><mml:mrow><mml:mn>0.4</mml:mn></mml:mrow></mml:msub><mml:mo>)</mml:mo></mml:mrow><mml:mrow><mml:mn>0.7</mml:mn></mml:mrow></mml:msub><mml:msub><mml:mi>Ca</mml:mi><mml:mrow><mml:mn>0.3</mml:mn></mml:mrow></mml:msub><mml:msub><mml:mi>MnO</mml:mi><mml:mn>3</mml:mn></mml:msub></mml:mrow></mml:math>
Dynamics of electronic phase separation at the laser-induced insulator-metal transition in Open
Ultrafast optical excitations allow creating new metastable and hidden states in quantum materials. However, the fundamental material properties required to support new emergent order are largely unknown. Here, we show for two colossal mag…
View article: Time-resolved photoelectron spectroscopy at surfaces
Time-resolved photoelectron spectroscopy at surfaces Open
Light is a preeminent spectroscopic tool for investigating the electronic structure of surfaces. Time-resolved photoelectron spectroscopy has mainly been developed in the last 30 years. It is therefore not surprising that the topic was har…
View article: All-Optical Control of Bubble and Skyrmion Breathing
All-Optical Control of Bubble and Skyrmion Breathing Open
Controlling the dynamics of topologically protected spin objects by all-optical means promises enormous potential for future spintronic applications. Excitation of bubbles and skyrmions in ferrimagnetic [Fe(0.35 nm)/Gd(0.40 nm)]160 multi…
View article: Probing excitons with time-resolved momentum microscopy
Probing excitons with time-resolved momentum microscopy Open
Excitons – two-particle correlated electron-hole pairs – are the dominant low-energy optical excitation in the broad class of semiconductor materials, which range from classical silicon to perovskites, and from two-dimensional to organic m…
View article: Interlayer charge transfer in graphene 2D polyimide heterostructures
Interlayer charge transfer in graphene 2D polyimide heterostructures Open
The vertical integration of multiple two-dimensional (2D) materials in heterostructures, held together by van der Waals forces, has opened unprecedented possibilities for modifying the (opto-)electronic properties of nanodevices. Graphene,…
View article: Fast spectroscopic imaging using extreme ultraviolet interferometry
Fast spectroscopic imaging using extreme ultraviolet interferometry Open
Extreme ultraviolet pulses as generated by high harmonic generation (HHG) are a powerful tool for both time-resolved spectroscopy and coherent diffractive imaging. However, the integration of spectroscopy and microscopy to harness the uniq…
View article: All optical control of bubble and skyrmion breathing
All optical control of bubble and skyrmion breathing Open
Controlling the dynamics of topologically protected spin objects by all optical means promises enormous potential for future spintronic applications. Excitation of bubbles and skyrmions in ferrimagnetic [Fe(0.35 nm)/Gd(0.40 nm)]$_{160}$ mu…
View article: Observation of Floquet states in graphene
Observation of Floquet states in graphene Open
Recent advances in the field of condensed-matter physics have unlocked the potential to realize and control emergent material phases that do not exist in thermal equilibrium. One of the most promising concepts in this regard is Floquet eng…
View article: Author Correction: Disentangling the multiorbital contributions of excitons by photoemission exciton tomography
Author Correction: Disentangling the multiorbital contributions of excitons by photoemission exciton tomography Open
View article: A minimalist approach to 3D photoemission orbital tomography: algorithms and data requirements
A minimalist approach to 3D photoemission orbital tomography: algorithms and data requirements Open
Photoemission orbital tomography provides direct access from laboratory measurements to the real-space molecular orbitals of well-ordered organic semiconductor layers. Specifically, the application of phase retrieval algorithms to photon-e…
View article: Laser‐Induced Real‐Space Topology Control of Spin Wave Resonances
Laser‐Induced Real‐Space Topology Control of Spin Wave Resonances Open
Femtosecond laser excitation of materials exhibiting magnetic spin textures promises advanced magnetic control via the generation of non‐equilibrium spin dynamics. Ferrimagnetic [Fe(0.35 nm)/Gd(0.40 nm)] 160 multilayers are used to explore…
View article: Fast spectroscopic imaging using extreme ultraviolet interferometry
Fast spectroscopic imaging using extreme ultraviolet interferometry Open
Extreme ultraviolet pulses as generated by high harmonic generation (HHG) are a powerful tool for both time-resolved spectroscopy and coherent diffractive imaging. However, the integration of spectroscopy and microscopy to harness the uniq…
View article: Verification of ultrafast spin transfer effects in iron-nickel alloys
Verification of ultrafast spin transfer effects in iron-nickel alloys Open
The optical intersite spin transfer (OISTR) effect was recently verified in Fe 50 Ni 50 using extreme ultraviolet magneto-optical Kerr measurements. However, one of the main experimental signatures analyzed in this work, namely a magnetic …
View article: Disentangling the multiorbital contributions of excitons by photoemission exciton tomography
Disentangling the multiorbital contributions of excitons by photoemission exciton tomography Open
View article: Probing excitons with time-resolved momentum microscopy
Probing excitons with time-resolved momentum microscopy Open
Excitons -- two-particle correlated electron-hole pairs -- are the dominant low-energy optical excitation in the broad class of semiconductor materials, which range from classical silicon to perovskites, and from two-dimensional to organic…
View article: Probing electron-hole Coulomb correlations in the exciton landscape of a twisted semiconductor heterostructure
Probing electron-hole Coulomb correlations in the exciton landscape of a twisted semiconductor heterostructure Open
In two-dimensional semiconductors, cooperative and correlated interactions determine the material’s excitonic properties and can even lead to the creation of correlated states of matter. Here, we study the fundamental two-particle correlat…
View article: Unraveling femtosecond spin and charge dynamics with extreme ultraviolet transverse MOKE spectroscopy
Unraveling femtosecond spin and charge dynamics with extreme ultraviolet transverse MOKE spectroscopy Open
The magneto-optical Kerr effect (MOKE) in the extreme ultraviolet (EUV) regime has helped to elucidate some of the key processes that lead to the manipulation of magnetism on ultrafast timescales. However, as we show in this paper, the rec…
View article: Dynamics of electronic phase separation at the laser-induced insulator-metal transition in (La$_{0.6}$Pr$_{0.4}$)$_{0.7}$Ca$_{0.3}$MnO$_3$
Dynamics of electronic phase separation at the laser-induced insulator-metal transition in (La$_{0.6}$Pr$_{0.4}$)$_{0.7}$Ca$_{0.3}$MnO$_3$ Open
Ultrafast optical excitations allow creating new metastable and hidden states in quantum materials. However, the fundamental material properties required to support new emergent order are largely unknown. Here we show for two colossal magn…
View article: Laser-induced real-space topology control of spin wave resonances
Laser-induced real-space topology control of spin wave resonances Open
Femtosecond laser excitation of materials that exhibit magnetic spin textures promises advanced magnetic control via the generation of ultrafast and non-equilibrium spin dynamics. We explore such possibilities in ferrimagnetic [Fe(0.35 nm)…
View article: Hybrid Exciton Signatures in ARPES Spectra of van der Waals Materials
Hybrid Exciton Signatures in ARPES Spectra of van der Waals Materials Open
Van der Waals heterostructures show fascinating physics including trapped moire exciton states, anomalous moire exciton transport, generalized Wigner crystals, etc. Bilayers of transition metal dichalcogenides (TMDs) are characterized by l…
View article: Ultrafast dynamics of bright and dark excitons in monolayer WSe<sub>2</sub> and heterobilayer WSe<sub>2</sub>/MoS<sub>2</sub>
Ultrafast dynamics of bright and dark excitons in monolayer WSe<sub>2</sub> and heterobilayer WSe<sub>2</sub>/MoS<sub>2</sub> Open
The energy landscape of optical excitations in mono- and few-layer transition metal dichalcogenides (TMDs) is dominated by optically bright and dark excitons. These excitons can be fully localized within a single TMD layer, or the electron…
View article: Fast Twist Angle Mapping of Bilayer Graphene Using Spectroscopic Ellipsometric Contrast Microscopy
Fast Twist Angle Mapping of Bilayer Graphene Using Spectroscopic Ellipsometric Contrast Microscopy Open
Twisted bilayer graphene provides an ideal solid-state model to explore correlated material properties and opportunities for a variety of optoelectronic applications, but reliable, fast characterization of the twist angle remains a challen…