arXiv subjects

U. Höfer

Publications and source records attributed to U. Höfer.

14 recordsLinked to original sources

arXiv2023

Buildup and dephasing of Floquet-Bloch bands on subcycle time scales

Strong light fields have created spectacular opportunities to tailor novel functionalities of solids. Floquet-Bloch states can form under periodic driving of electrons and enable exotic quantum phases. On subcycle time scales, lightwaves can simultaneously drive intraband currents and interband transitions, which enable high-harmonic generation (HHG) and pave the way towards ultrafast electronics. Yet, the interplay of intra- and interband excitations as well as their relation with Floquet physics have been key open questions as dynamical aspects of Floquet states have remained elusive. Here we provide this pivotal link by pioneering the ultrafast buildup of Floquet-Bloch bands with time- and angle-resolved photoemission spectroscopy. We drive surface states on a topological insulator with mid-infrared fields - strong enough for HHG - and directly monitor the transient band structure with subcycle time resolution. Starting with strong intraband currents, we observe how Floquet sidebands emerge within a single optical cycle; intraband acceleration simultaneously proceeds in multiple sidebands until high-energy electrons scatter into bulk states and dissipation destroys the Floquet bands. Quantum nonequilibrium calculations explain the simultaneous occurrence of Floquet states with intra- and interband dynamics. Our joint experiment-theory study opens up a direct time-domain view of Floquet physics and explores the fundamental frontiers of ultrafast band-structure engineering.

cond-mat.mes-hall↗

arXiv2022

Ultrafast electron dynamics in a topological surface state observed in two-dimensional momentum space

We study ultrafast population dynamics in the topological surface state of Sb$_2$Te$_3$ in two-dimensional momentum space with time- and angle-resolved two-photon photoemission. Linear polarized mid-infrared pump pulses are used to permit a direct optical excitation across the Dirac point. We show that this resonant excitation is strongly enhanced within the Dirac cone along three of the six $\barΓ$-$\bar{M}$ directions and results in a macroscopic photocurrent when the plane of incidence is aligned along a $\barΓ$-$\bar{K}$ direction. Our experimental approach makes it possible to disentangle the decay of transiently excited population and photocurent by elastic and inelastic electron scattering within the full Dirac cone in unprecedented detail. This is utilized to show that doping of Sb$_2$Te$_3$ by vanadium atoms strongly enhances inelastic electron scattering to lower energies, but only scarcely affects elastic scattering around the Dirac cone.

cond-mat.mes-hall↗

arXiv2021

Ultrafast charge-transfer dynamics in twisted MoS$_2$/WSe$_2$ heterostructures

Two-dimensional transition metal dichalcogenides (TMD) offer a unique platform for creating van-der-Waals heterojunctions with fascinating physical properties and promising applications in optoelectronics and valleytronics. Because of their typical type-II band alignment, photoexcited electrons and holes can separate via interfacial charge transfer. To understand the nature and the dynamics of this charge transfer is of utmost importance for the design and efficiency of potential devices. However, systematic studies concerning the influence of the stacking angle on the charge transfer remain sparse. Here, we apply time- and polarization resolved second-harmonic imaging microscopy to investigate the charge-transfer dynamics for three MoS$_2$/WSe$_2$ heterostructures with different stacking angles at a previously unattainable time-resolution of $\approx$ 6 fs. For 1.70 eV excitation energy, electron transfer from WSe$_2$ to MoS$_2$ is found to depend considerably on the stacking angle with the fastest transfer time observed to be as short as 12 fs. At 1.85 eV excitation energy, ultrafast hole transfer from MoS$_2$ to hybridized states at the $Γ$-point or to the K-points of WSe$_2$ has to be considered. Surprisingly, the corresponding decay dynamics show only a minor stacking-angle dependence indicating that radiative recombination of indirect $Γ$-K excitons becomes the dominant decay route for all samples.

cond-mat.mtrl-sci↗

arXiv2021

Tip-Induced $β$-Hydrogen Dissociation in an Alkyl Group Bound on Si(001)

Atomic-scale chemical modification of surface-adsorbed ethyl groups on Si(001) was induced and studied by means of scanning tunneling microscopy. Tunneling at sample bias > +1.5V leads to tip-induced C-H cleavage of a $β$-hydrogen of the covalently bound ethyl configuration. The reaction is characterized by the formation of an additional Si-H and a Si-C bond. The reaction probability shows a linear dependence on the tunneling current at 300 K; the reaction is largely suppressed at 50 K. The observed tip-induced surface reaction at room temperature is thus attributed to a one-electron excitation in combination with thermal activation.

cond-mat.mtrl-sci↗

arXiv2020

Second-harmonic imaging microscopy for time-resolved investigations of transition metal dichalcogenides

Two-dimensional transition metal dichalcogenides (TMDC) have shown promise for various applications in optoelectronics and so-called valleytronics. Their operation and performance strongly depend on the stacking of individual layers. Here, optical second-harmonic generation (SHG) in imaging mode is shown to be a versatile tool for systematic time-resolved investigations of TMDC monolayers and heterostructures in consideration of the material's structure. Large sample areas can be probed without the need of any mapping or scanning. By means of polarization dependent measurements, the crystalline orientation of monolayers or the stacking angles of heterostructures can be evaluated for the whole field of view. Pump-probe experiments then allow to correlate observed transient changes of the second-harmonic response with the underlying structure. The corresponding time-resolution is virtually limited by the pulse duration of the used laser. As an example, polarization dependent and time-resolved measurements on mono- and multilayer MoS$_2$ flakes grown on a SiO$_2$/Si(001) substrate are presented.

cond-mat.mtrl-sci↗

arXiv2018

Second-harmonic generation as probe for structural and electronic properties of buried GaP/Si(001) interfaces

Optical second-harmonic generation is demonstrated to be a sensitive probe of the buried interface between the lattice matched semiconductors gallium phosphide and silicon with (001) orientation. Rotational anisotropy measurements of SHG from GaP/Si show a strong isotropic component of the response not present for pure Si(001) or GaP(001). The strength of the overlaying anisotropic response directly correlates with the quality of the interface as determined by atomically resolved scanning transmission electron microscopy.Optical second-harmonic generation is demonstrated to be a sensitive probe of the buried interface between the lattice matched semiconductors gallium phosphide and silicon with (001) orientation. Rotational anisotropy measurements of SHG from GaP/Si show a strong isotropic component of the response not present for pure Si(001) or GaP(001). The strength of the overlaying anisotropic response directly correlates with the quality of the interface as determined by atomically resolved scanning transmission electron microscopy. Systematic comparison of samples fabricated with different growth modes in metal organic vapor phase epitaxy reveals that the anisotropy for different polarization combinations can be used as a selective fingerprint for the occurrence of anti-phase domains and twins. This all-optical technique can be applied as an {\it in-situ} and non-invasive monitor even during growth. Systematic comparison of samples fabricated with different growth modes in metal organic vapor phase epitaxy reveals that the anisotropy for different polarization combinations can be used as a selective fingerprint for the occurrence of anti-phase domains and twins. This all-optical technique can be applied as an {\it in-situ} and non-invasive monitor even during growth.

cond-mat.mes-hall↗

arXiv2016

Perturbation of Exciton Aggregate Coupling by Optical Excitation in Crystalline Perfluoropentacene Films

Carrier multiplication by singlet exciton fission enhances photovoltaic conversion efficiencies in organic solids. This decay of one singlet exciton into two triplet states promises to overcome the Shockley-Queisser limit as up to two electrons may be harvested per absorbed photon. Intermolecular coupling is deemed mandatory for both, singlet exciton fission and a band-like transport. Such a coupling is manifested, $e.g.$, by the Davydov-splitting of the lowest-energy exciton transition in crystalline organic solids. For the model system perfluoropentacene, the corresponding transitions in the experimental, polarisation-resolved absorption spectra are identified by theoretical calculations based on the concept of H- and J- aggregation. Optical injection into the first vibronic progression of the fundamental exciton transitions significantly perturbs the higher-energy transitions that are associated to H-type aggregates of the $S_0 \rightarrow S_3$ transition during and following efficient singlet exciton fission. These findings underline the necessity for efficient carrier extraction as triplet accumulation may be detrimental to both, singlet exciton fission and any potentially band like transport. More generally, our observations indicate that electronic excitations can perturb the electronic band structure in organic crystals and highlight their correlated nature by potentially distorting the lattice.

cond-mat.mtrl-sci↗

arXiv2016

Ultrafast Energy- and Momentum-resolved Surface Dirac Photocurrents in the Topological Insulator Sb$_2$Te$_3$

We present energy-momentum mapping of surface Dirac photocurrent in the topological insulator Sb$_2$Te$_3$ by means of time- and angle-resolved two-photon photoemission spectroscopy combined with polarization-variable mid-infrared pulse laser. It is demonstrated that the direct optical transition from the occupied to the unoccupied part of the surface Dirac-cone permits the linear and circular photogalvanic effect which thereby enables us to coherently control the surface electric-current by laser polarization. Moreover, the surface current mapping directly visualizes ultrafast current dynamics in the Dirac cone in the time domain. We unravel the ultrafast intraband relaxation dynamics of the inelastic scattering and momentum scattering separately. Our observations pave the pathway for coherent optical control over surface Dirac electrons in topological insulators.

cond-mat.mtrl-sci↗

arXiv2016

Model potential for the description of metal/organic interface states

We present an analytical one-dimensional model potential for the description of electronic interface states that form at the interface between a metal surface and flat-lying adlayers of $π$-conjugated organic molecules. The model utilizes graphene as a universal representation of these organic adlayers. It predicts the energy position of the interface state as well as the overlap of its wave function with the bulk metal without free fitting parameters. We show that the interface state's energy depends systematically on the bond distance between the carbon backbone of the adayers and the metal. The general applicability and robustness of the model is demonstrated by a comparison of the calculated energies with numerous experimental results for a number of flat-lying organic molecules on different closed-packed metal surfaces that cover a large range of bond distances.

cond-mat.mes-hall↗

arXiv2016

Adsorption geometry and the interface states: The relaxed and compressed phases of NTCDA/Ag(111)

The theoretical modelling of metal-organic interfaces represents a formidable challenge, especially in consideration of the delicate balance of various interaction mechanisms and the large size of involved molecular species. In the present study, the energies of interface states, which are known to display a high sensitivity to the adsorption geometry and electronic structure of the deposited molecular species, have been used to test the suitability and reliability of current theoretical approaches. Two well-ordered overlayer structures (relaxed and compressed monolayer) of NTCDA on Ag(111) have been investigated using two-photon-photoemission to derive precise interface state energies for these closely related systems. The experimental values are reproduced by our DFT calculations using different treatments of dispersion interactions (optB88, PBE-D3) and basis set approaches (localized numerical atomic orbitals, plane waves) with remarkable accuracy. This underlines the trustworthiness regarding the description of geometric and electronic properties.

cond-mat.mtrl-sci↗

arXiv2016

Adsorption geometry and electronic properties of flat-lying monolayers of tetracene on the Ag(111) surface

The geometrical and electronic properties of the monolayer (ML) of tetracene (Tc) molecules on Ag(111) are systematically investigated by means of DFT calculations with the use of localized basis set. The bridge and hollow adsorption positions of the molecule in the commensurate $γ$-Tc/Ag(111) are revealed to be the most stable and equally favorable irrespective to the approximation chosen for the exchange-correlation functional. The binding energy is entirely determined by the long-range dispersive interaction. The former lowest unoccupied orbital remains being unoccupied in the case of $γ$-Tc/Ag(111) as well as in the $α$-phase with increased coverage. The unit cell of the $α$-phase with point-on-line registry was adapted for calculations based on the available experimental data and the computed structures of the $γ$-phase. The calculated position of the Tc/Ag(111) interface state is found to be noticeably dependent on the lattice constant of the substrate, however its energy shift with respect to the Shockley surface state of the unperturbed clean side of the slab is sensitive only to the adsorption distance and in good agreement with the experimentally measured energy shift.

cond-mat.mes-hall↗

arXiv2016

Generation of Transient Photocurrents in the Topological Surface State of Sb$_{2}$Te$_{3}$ by Direct Optical Excitation with Mid-Infrared Pulses

We combine tunable mid-infrared (MIR) pump pulses with time- and angle-resolved two-photon photoemission to study ultrafast photoexcitation of the topological surface state (TSS) of Sb$_{2}$Te$_{3}$. It is revealed that MIR pulses permit a direct excitation of the unoccupied TSS owing to an optical coupling across the Dirac point. The novel optical coupling provokes asymmetric transient populations of the TSS at ${\pm}k_{||}$, which mirrors a macroscopic photoexcited electric surface current. By observing the decay of the asymmetric population, we directly demonstrate the dynamics of the long-lived photocurrent and its protection against backscattering. Our discovery promises important advantages of photoexcitation by MIR pulses for spintronic applications.

cond-mat.mtrl-sci↗

arXiv2015

Formation of image-potential states at the graphene/metal interface

The formation of image-potential states at the interface between a graphene layer and a metal surface is studied by means of model calculations. An analytical one-dimensional model-potential for the combined system is constructed and used to calculate energies and wave functions of the image-potential states at the Gamma-point as a function of the graphene-metal distance. It is demonstrated how the double series of image-potential states of free-standing graphene evolves into interfacial states that interact with both surfaces at intermediate distances and finally into a single series of states resembling those of a clean metal surface covered by a monoatomic spacer layer. The model quantitatively reproduces experimental data available for graphene/Ir(111) and graphene/Ru(0001), systems which strongly differ in interaction strength and therefore adsorption distance. Moreover, it provides a clear physical explanation for the different binding energy and lifetime of the first (n=1) image-potential state in the valley and hill areas of the strongly corrugated moire superlattice of graphene/Ru(0001).

cond-mat.mes-hall↗

arXiv2011

Energy Shift and Wavefunction Overlap of Metal-Organic Interface-States

The properties of Shockley-type interface states between $π$-conjugated organic molecular layers and metal surfaces are investigated by time-resolved two-photon photoemission experiments and density functional theory. For perylene- and naphthalene-tetracarboxylic acid dianhydride (PTCDA and NTCDA) adsorbed on Ag(111), a common mechanism of formation of the interface state from the partly occupied surface state of the bare Ag(111) is revealed. The energy position is found to be strongly dependent on the distance of the molecular carbon rings from the metal and their surface density. Bending of the carboxyl groups enhances the molecular overlap of the interface state.

cond-mat.other↗