Publications

@article{Stettner2026,

title = {Tracing the film structure of an organic semiconductor with photoemission orbital tomography},

author = {M. Stettner and S. Kaidisch and Andrey V. Matetskiy and E. Fackelman and S. Soubatch and C. Kumpf and F. C. Bocquet and M. G. Ramsey and P. Puschnig and F. S. Tautz},

url = {https://arxiv.org/abs/2603.06204},

doi = {10.48550/arXiv.2603.06204},

year  = {2026},

date = {2026-03-06},

journal = {arXiv:2603.06204 [cond-mat.mtrl-sci]},

keywords = {},

pubstate = {forthcoming},

tppubtype = {article}

}

@article{Eggers2026,

title = {Subcycle videography of lightwave-driven Landau-Zener-Majorana transitions in graphene},

author = {V. Eggers and G. Inzani and M. Meierhofer and L. Münster and J. Helml and R. Wallauer and S. Zajusch and S. Ito and L. Machtl and H. Yin and C. Kumpf and F. C. Bocquet and C. Bao and J. Güdde and F. S. Tautz and R. Huber and U. Höfer},

url = {https://arxiv.org/abs/2602.12844},

doi = {10.48550/arXiv.2602.12844},

year  = {2026},

date = {2026-02-13},

journal = {arXiv:2602.12844 [cond-mat.mes-hall]},

abstract = {Strong light fields have unlocked previously unthinkable possibilities to tailor coherent electron trajectories, engineer band structures and shape emergent phases of matter all-optically. Unravelling the underlying quantum mechanisms requires a visualisation of the lightwave-driven electron motion directly in the band structure. While photoelectron momentum microscopy has imaged optically excited electrons averaged over many cycles of light, actual subcycle band-structure videography has been limited to small electron momenta. Yet lightwave-driven elementary processes in quantum materials often occur throughout momentum space. Here, we introduce attosecond-precision, subcycle band-structure videography covering the entire first Brillouin zone (BZ) and visualize one of the most fundamental but notoriously elusive strong-field processes: non-adiabatic Landau-Zener-Majorana (LZM) tunnelling. The interplay of field-driven acceleration within the Dirac-like band structure of graphene and periodic LZM interband tunnelling manifest in a coherent displacement and distortion of the momentum distribution at the BZ edge. The extremely non-thermal electron distributions also allow us to disentangle competing scattering processes and assess their impact on coherent electronic control through electron redistribution and thermalization. Our panoramic view of strong-field-driven electron motion in quantum materials lays the foundation for a microscopic understanding of some of the most discussed light-driven phenomena in condensed matter physics. },

keywords = {},

pubstate = {forthcoming},

tppubtype = {article}

}

@article{Bao2026,

title = {Observation of an isolated flat band in the van der Waals crystal NbOCl2},

author = {C. Bao and V. Eggers and M. Meierhofer and J. Helml and L. Münster and S. Ito and L. Machtl and S. Zajusch and G. Inzani and L. Wittmann and M. Liebich and R. Wallauer and U. Höfer and R. Huber},

url = {https://www.nature.com/articles/s43246-025-01070-0},

doi = {10.1038/s43246-025-01070-0},

year  = {2026},

date = {2026-01-10},

journal = {Communications Materials},

volume = {7},

number = {60},

abstract = {Dispersionless electronic bands lead to an extremely high density of states and suppressed kinetic energy, thereby increasing electronic correlations and instabilities that can shape emergent ordered states, such as excitonic, ferromagnetic, and superconducting phases. A flat band that extends over the entire momentum space and is well isolated from other dispersive bands is, therefore, particularly interesting. Here, the band structure of the van der Waals crystal NbOCl2 is revealed by utilizing photoelectron momentum microscopy. We directly map out an electronic band that is flat throughout the entire Brillouin zone and features a width of only  ~ 100 meV. This band is well isolated from both the conduction and remote valence bands. Moreover, the quasiparticle band gap shows a high tunability upon the deposition of cesium atoms on the surface. By combining the single-particle band structure with the optical transmission spectrum, the optical gap is identified. The fully isolated flat band in a van der Waals crystal provides a qualitatively new testbed for exploring flat-band physics.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{Theilen2025,

title = {Observing the spatial and temporal evolution of exciton wave functions},

author = {M. Theilen and S. Kaidisch and M. Stettner and S. Zajusch and E. Fackelman and A. Adamkiewicz and R. Wallauer and A. Windischbacher and C. S. Kern and M. G. Ramsey and F. C. Bocquet and S. Soubatch and F. S. Tautz and U. Höfer and P. Puschnig},

url = {https://arxiv.org/abs/2511.23001},

doi = {10.48550/arXiv.2511.23001},

year  = {2025},

date = {2025-11-28},

urldate = {2025-11-28},

journal = { arXiv:2511.23001  [cond-mat.mtrl-sci]},

abstract = {Excitons, the correlated electron-hole pairs governing optical and transport properties in organic semiconductors, have long resisted direct experimental access to their full quantum-mechanical wave functions. Here, we use femtosecond time-resolved photoemission orbital tomography (trPOT), combining high-harmonic probe pulses with time- and momentum-resolved photoelectron spectroscopy, to directly image the momentum-space distribution and ultrafast dynamics of excitons in  -sexithiophene thin films. We introduce a quantitative model that enables reconstruction of the exciton wave function in real space, including both its spatial extent and its internal phase structure. The reconstructed wave function reveals coherent delocalization across approximately three molecular units and exhibits a characteristic phase modulation, consistent with ab initio calculations within the framework of many-body perturbation theory. Time-resolved measurements further show a  % contraction of the exciton radius within 400 fs, providing direct evidence of self-trapping driven by exciton-phonon coupling. These results establish trPOT as a general and experimentally accessible approach for resolving exciton wave functions -- with spatial, phase, and temporal sensitivity -- in a broad class of molecular and low-dimensional materials. },

keywords = {},

pubstate = {forthcoming},

tppubtype = {article}

}

@article{Kaidisch2025_arxiv,

title = {Photoemission tomography of excitons in 2D systems: momentum-space signatures of correlated electron-hole wave functions},

author = {S. Kaidisch and A. Kleiner and S. Refaely-Abramson and P. Puschnig and C. S. Kern},

url = {https://arxiv.org/abs/2511.14956},

doi = {10.48550/arXiv.2511.14956},

year  = {2025},

date = {2025-11-18},

urldate = {2025-11-18},

journal = { arXiv:2511.14956 [cond-mat.mtrl-sci]},

abstract = {The momentum-space signatures of excitons can be experimentally accessed through time-resolved (pump-probe) photoelectron spectroscopy. In this work, we develop a computational framework for exciton photoemission orbital tomography (exPOT) in periodic systems, enabling the simulation and interpretation of experimental observables within many-body perturbation theory. By connecting the  +Bethe-Salpeter Equation (BSE) approach to photoemission tomography, our formalism captures exciton photoemission in periodic systems, explicitly incorporating photoemission matrix element effects induced by the probe pulse. The correlated nature of electrons and holes introduces distinct consequences for excitonic photoemission, including a dependence on pump pulse polarization. Using the prototypical two-dimensional material hexagonal boron nitride, we demonstrate these effects and show how our framework extends to excitons with finite center-of-mass momentum, making it well-suited to studying momentum-dark excitons. This provides valuable insights into the microscopic nature of excitonic phenomena in quantum materials. },

keywords = {},

pubstate = {forthcoming},

tppubtype = {article}

}

@article{Brandhoff2025,

title = {When Aromaticity Falls Short in Molecule–Surface Interactions},

author = {J. Brandhoff and R. K. Berger and F. Otto and M. Schaal and L. Brill and O. T. Hofmann and P. Puschnig and T. Fritz and R. Forker},

url = {https://pubs.acs.org/doi/full/10.1021/acs.jpcc.5c05441},

doi = {10.1021/acs.jpcc.5c05441},

year  = {2025},

date = {2025-09-26},

urldate = {2025-09-26},

journal = {The Journal of Physical Chemistry C},

volume = {129},

issue = {46},

abstract = {Aromaticity is one of the most important concepts in organic chemistry. There are cases in which a molecule undergoes changes to increase its aromaticity. This higher aromaticity comes with an energetic gain and is commonly referred to as aromatic stabilization. Previously, it has been reported that some molecules undergo such a stabilization when adsorbing on a surface, which has been identified as the reason for charge transfer into the molecular π-system. Utilizing photoemission orbital tomography and density functional theory, we investigate changes in the molecular π-system upon adsorption and elucidate the influence on the aromaticity. We demonstrate how the energetic gain from an aromatic stabilization on surfaces can be outweighed by hybridization. Uncovering a mechanism in which the molecular π-system forms dative bonds with the surface, our study reveals that the concept of aromatic stabilization on surfaces has been incomplete so far.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{Kern2025,

title = {Circular dichroism in the photoelectron angular distribution of achiral molecules},

author = {C. S. Kern and X. Yang and G. Zamborlini and S. Mearini and M. Jugovac and V. Feyer and U. De Giovannini and A. Rubio and S. Soubatch and M. G. Ramsey and F. S. Tautz and P. Puschnig},

url = {https://arxiv.org/abs/2507.12113},

doi = {10.48550/arXiv.2507.12113},

year  = {2025},

date = {2025-07-16},

urldate = {2025-07-16},

journal = {arXiv:2507.12113 [cond-mat.mtrl-sci]},

keywords = {},

pubstate = {forthcoming},

tppubtype = {article}

}

@article{Zirwick2025,

title = {Highly Ordered Single Domain Peri-Tetracene Monolayers on Ag(110)},

author = {M. Zirwick and N. Kainbacher and J. B. Bauer and M. S. Wagner and P. Puschnig and T. Chassé and H. F. Bettinger and H. Peisert},

url = {https://pubs.acs.org/doi/10.1021/acs.jpcc.5c01482},

doi = {10.1021/acs.jpcc.5c01482},

year  = {2025},

date = {2025-04-15},

urldate = {2025-04-15},

journal = {The Journal of Physical Chemistry C},

volume = {129},

issue = {17},

pages = {8447–8454},

abstract = {The on-surface reaction of 1,1’-bitetracene (Bi4A) to peri-tetracene (tetrabenzo[bc,ef,kl,no]coronene) (4-PA) on Cu(110) and Ag(110) is studied by photoemission, scanning tunneling microscopy (STM) and low energy electron diffraction (LEED). Density functional theory (DFT) computations suggest that the Ag(110) substrate is well suited for the formation of large-area 4-PA monolayers with a preferential adsorption alignment of 4-PA molecules along the [11̅0] direction. The experiments confirm the formation of 4-PA and presence of large highly ordered 4-PA domains. Two distinct phases emerge, growing seamlessly over large areas and even spanning step edges. Evidence for charge transfer from the substrate to the molecule was found, resulting in a filling of the lowest unoccupied molecular orbital (LUMO) of 4-PA.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{Haags2025b,

title = {Tomographic identification of all molecular orbitals in a wide binding-energy range},

author = {A. Haags and D. Brandstetter and X. Yang and L. Egger and H. Kirschner and A. Gottwald and M. Richter and G. Koller and F. C. Bocquet and C. Wagner and M. G. Ramsey and S. Soubatch and P. Puschnig and F. S. Tautz},

doi = {10.1103/physrevb.111.165402},

issn = {2469-9969},

year  = {2025},

date = {2025-04-00},

urldate = {2025-04-00},

journal = {Phys. Rev. B},

volume = {111},

number = {16},

publisher = {American Physical Society (APS)},

abstract = {<jats:p>In the past decade, photoemission orbital tomography (POT) has evolved into a powerful tool to investigate the electronic structure of organic molecules adsorbed on surfaces. Here we show that POT allows for the comprehensive experimental identification of all molecular orbitals in a substantial binding energy range of more than 10 eV. Making use of the angular distribution of photoelectrons as a function of binding-energy, we exemplify this by extracting an orbital-resolved projected density of states for 15 <a:math xmlns:a="http://www.w3.org/1998/Math/MathML"><a:mi>π</a:mi></a:math> and 23 <b:math xmlns:b="http://www.w3.org/1998/Math/MathML"><b:mi>σ</b:mi></b:math> orbitals from the experimental data of the prototypical organic molecule bisanthene (<c:math xmlns:c="http://www.w3.org/1998/Math/MathML"><c:mrow><c:msub><c:mi mathvariant="normal">C</c:mi><c:mn>28</c:mn></c:msub><c:msub><c:mi mathvariant="normal">H</c:mi><c:mn>14</c:mn></c:msub></c:mrow></c:math>) on a Cu(110) surface. These experimental results for an essentially complete set of orbitals within the given binding-energy range serve as stringent benchmarks for electronic structure methods, which we illustrate by performing density functional calculations employing four frequently used exchange-correlation functionals. By computing the respective molecular-orbital-projected densities of states, a one-to-one comparison with experimental data for an unprecedented number of 38 orbital energies became possible. The quantitative analysis of our data reveals that the range-separated hybrid functional HSE performs best for the investigated organic/metal interface. At a more fundamental level, the remarkable agreement between the experimental and the Kohn-Sham orbital energies over a binding-energy range larger than 10 eV suggests that—perhaps unexpectedly—Kohn-Sham orbitals approximate Dyson orbitals, which would rigorously account for the electron extraction process in photoemission spectroscopy but are notoriously difficult to compute, in a much better way than previously thought.</jats:p>

          <jats:sec>

            <jats:title/>

            <jats:supplementary-material>

              <jats:permissions>

                <jats:copyright-statement>Published by the American Physical Society</jats:copyright-statement>

                <jats:copyright-year>2025</jats:copyright-year>

              </jats:permissions>

            </jats:supplementary-material>

          </jats:sec>},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{Mearini2025,

title = {Substrate Stabilized Charge Transfer Scheme In Coverage Controlled 2D Metal Organic Frameworks},

author = {S. Mearini and D. Brandstetter and Y. Y. Grisan Qiu and D. Baranowski and I. Cojocariu and M. Jugovac and P. Gargiani and M. Valvidares and L. Schio and L. Floreano and A. Windischbacher and P. Puschnig and V. Feyer and C. M. Schneider},

url = {https://onlinelibrary.wiley.com/doi/10.1002/smll.202500507?af=R},

doi = {10.1002/smll.202500507},

year  = {2025},

date = {2025-02-17},

urldate = {2025-02-17},

journal = {Small},

volume = {2500507},

abstract = {Recently, 2D metal-organic frameworks (2D MOFs), characterized by complexcharge transfer mechanisms, have emerged as a promising class of networksin the development of advanced materials with tailored electronic andmagnetic properties. Following the successful synthesis of a 2D MOF formedby nickel (Ni) linkers and 7,7,8,8-tetracyanoquinodimethane (TCNQ) ligands,this work investigates how the Ni-to-ligand ratio influences the electroniccharge redistribution in an Ag(100)-supported 2D MOF. The interplaybetween linker-ligand and substrate-MOF charge transfer processes leads to astable equilibrium, resulting in a robust electronic structure that remainsindependent of stoichiometric ratios. This stability is primarily based on theelectron transfer from the metal substrate, which compensates for chargeimbalances introduced by the metal-organic coordination across differentMOF configurations. Despite minor changes observed in the magneticresponse of the Ni centers, these findings emphasize the robustness of theelectronic structure, which remains largely unaffected by structural variations,highlighting the potential of these 2D MOFs for advanced applications inelectronics and spintronics.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{Nakayama2025b,

title = {Anisotropic dispersion of excitonic bands of the single-crystal pentacene (001) surface as measured by low-energy angle-resolved high-resolution electron energy-loss spectroscopy},

author = {Y. Nakayama and F. C. Bocquet and R. Tsuruta and S. Soubatch and F. S. Tautz},

url = {https://www.sciencedirect.com/science/article/pii/S0368204825000015},

doi = {10.1016/j.elspec.2025.147514},

year  = {2025},

date = {2025-01-31},

journal = {Journal of Electron Spectroscopy and Related Phenomena},

volume = {279},

issue = {147514},

abstract = {Low-energy high-resolution electron energy-loss spectroscopy (HREELS) is a useful technique for the charac-

terization of various excitation processes at solid surfaces. However, no successful work has been reported on

molecular single-crystal samples yet. In the present study, low-energy angle-resolved HREELS measurements

were conducted on single-crystal pentacene, an organic semiconductor. The results confirmed the excitonic

bands exhibiting energy–momentum dispersion and anisotropy of these depending on the surface crystallo-

graphic directions, corroborating the occurrence of exciton delocalization, contrary to the ordinary notion of the

Frenkel exciton for weakly interacting van der Waals molecular solids. The present results demonstrate that low-

energy angle-resolved HREELS is applicable to the precise examination of the excitonic characteristics of solid-

state surfaces, even for molecular semiconductor single crystals.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{Qiu2025,

title = {Conformation-Driven Nickel Redox States and Magnetism in 2D Metal–organic Frameworks},

author = {Y. Y. Grisan Qiu and D. Brandstetter and S. Mearini and D. Baranowski and I. Cojocariu and M. Jugovac and G. Zamborlini and P. Gargiani and M. Valvidares and A. Windischbacher and P. Puschnig and V. Feyer and C. M. Schneider},

url = {https://advanced.onlinelibrary.wiley.com/doi/10.1002/adfm.202418186?af=R},

doi = {10.1002/adfm.202418186},

year  = {2025},

date = {2025-01-29},

urldate = {2025-01-29},

journal = {Adv. Funct. Mater.},

volume = {2418186},

abstract = {2D metal–organic frameworks (2D MOFs) attract considerable attention because of their versatile properties and as potential candidates for single-atom catalysis, high-density information storage media or molecular electronics and spintronics devices. Their unique characteristics arise from an intricate interplay between the metal center, the surrounding ligands and the underlying substrate. Here, the intrinsic magnetic and electronic properties of a single-layer MOF on graphene is investigated with a combination of spectroscopic techniques and theoretical modeling. Taking advantage of the weak interaction between the MOF and graphene substrate, it is specifically focused on the influence of the coordination environment on these properties. Notably, two distinct coordination configurations are observed for the transition metal centers within the 2D MOF, and clarify how axial distortions in the ligand field affect the hybridization between the Ni 3d states and the π-symmetric molecular orbitals of 7,7,8,8-tetracyanoquinodimethane ligands, leading to the coexistence of two Ni redox states with different spin configurations. Furthermore, the transition from a nearly free-standing MOF is examined to metal-supported frameworks, elucidating the impact of substrate interactions on the electronic and magnetic properties. The findings advance the understanding of MOFs and offer insights into developing functional materials with tailored magnetic and electronic properties.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{Haags2025,

title = {Tomographic identification of all molecular orbitals in a wide binding energy range},

author = {A. Haags and D. Brandstetter and X. Yang and L. Egger and H. Kirschner and A. Gottwald and M. Richter and G. Koller and F. C. Bocquet and C. Wagner and M. G. Ramsey and S. Soubatch and P. Puschnig and F. S. Tautz},

url = {https://arxiv.org/abs/2501.05287},

year  = {2025},

date = {2025-01-09},

urldate = {2025-01-09},

journal = {ArXiv},

abstract = {In the past decade, photoemission orbital tomography (POT) has evolved into a powerful tool to investigate the electronic structure of organic molecules adsorbed on surfaces. Here we show that POT allows for the comprehensive experimental identification of all molecular orbitals in a substantial binding energy range, in the present case more than 10 eV. Making use of the angular distribution of photoelectrons as a function of binding energy, we exemplify this by extracting orbital-resolved partial densities of states (pDOS) for 15 π and 23 σ orbitals from the experimental photoemission intensities of the prototypical organic molecule bisanthene (C28H14) on a Cu(110) surface. In their entirety, these experimentally measured orbital-resolved pDOS for an essentially complete set of orbitals serve as a stringent benchmark for electronic structure methods, which we illustrate by performing density functional theory (DFT) calculations employing four frequently-used exchange-correlation functionals. By computing the respective molecular-orbital-projected densities of states of the bisanthene/Cu(110) interface, a one-to-one comparison with experimental data for an unprecedented number of 38 orbital energies becomes possible. The quantitative analysis of our data reveals that the range-separated hybrid functional HSE performs best for the investigated organic/metal interface. At a more fundamental level, the remarkable agreement between the experimental and the Kohn-Sham orbital energies over a binding energy range larger than 10,eV suggests that -- perhaps unexpectedly -- Kohn-Sham orbitals approximate Dyson orbitals, which would rigorously account for the electron extraction process in photoemission spectroscopy but are notoriously difficult to compute, in a much better way than previously thought. },

keywords = {},

pubstate = {forthcoming},

tppubtype = {article}

}

@article{Hurdax2025,

title = {Integer Charge Transfer Model–PTCDA on MgO(001)/Ag(001) Probing the Transition from Single to Double Integer Charge Transfer},

author = {P. Hurdax and M. Hollerer and C. S. Kern and P. Puschnig and M. Sterrer and M. G. Ramsey},

url = {https://pubs.acs.org/doi/10.1021/acs.jpcc.4c08104},

doi = {10.1021/acs.jpcc.4c08104},

issn = {1932-7455},

year  = {2025},

date = {2025-01-08},

urldate = {2025-01-08},

journal = {J. Phys. Chem. C},

volume = {129},

issue = {2},

pages = {1553--1561},

publisher = {American Chemical Society (ACS)},

abstract = {For weakly interacting adsorbate/substrate systems, the integer charge transfer (ICT) model describes how charge transfer across interfaces depends on the substrate work function. In particular, work function regimes where no charge transfer occurs (vacuum level alignment) can be distinguished from regions where integer charge transfer by electron tunneling from substrate to adsorbate or vice versa takes place (Fermi level pinning). While the formation of singly integer charged molecular anions and cations of organic semiconductors on various substrates has been well described by this model, the double integer charging regime has so far remained unexplored and experimentally elusive. Here, we extend the integer charge transfer model to the transition from single to double integer charging. This was made possible by combining a molecular adsorbate with high electron affinity (Perylenetetracarboxylic-dianhydride (PTCDA)) with a substrate with tunable work function (ultrathin MgO(001) films on Ag(001)). Our results, obtained with scanning tunneling microscopy (STM), photoemission spectroscopy (PES), work function measurements and density function theory (DFT) calculations, show that after completing the single negative charging of all molecules in a PTCDA monolayer in the first Fermi level pinning regime, the system transitions to a vacuum level alignment regime for singly charged molecules when the substrate work function is reduced, and finally enters the second Fermi level pinning regime at very low substrate work function, in which the molecules become doubly negatively charged.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{Mearini2024,

title = {Band Structure Engineering in 2D Metal–Organic Frameworks},

author = {S. Mearini and D. Baranowski and D. Brandstetter and A. Windischbacher and I. Cojocariu and P. Gargiani and M. Valvidares and L. Schio and L. Floreano and P. Puschnig and V. Feyer and C. M. Schneider},

url = {https://onlinelibrary.wiley.com/doi/10.1002/advs.202404667},

doi = {10.1002/advs.202404667},

year  = {2024},

date = {2024-08-09},

urldate = {2024-08-09},

journal = {Advanced Science},

volume = {11},

number = {2404667},

issue = {38},

abstract = {The design of 2D metal–organic frameworks (2D MOFs) takes advantage ofthe combination of the diverse electronic properties of simple organic ligandswith different transition metal (TM) centers. The strong directional nature ofthe coordinative bonds is the basis for the structural stability and the periodicarrangement of the TM cores in these architectures. Here, direct and clearevidence that 2D MOFs exhibit intriguing energy-dispersive electronic bandswith a hybrid character and distinct magnetic properties in the metal cores,resulting from the interactions between the TM electronic levels and theorganic ligand 𝝅-molecular orbitals, is reported. Importantly, a method toeffectively tune both the electronic structure of 2D MOFs and the magneticproperties of the metal cores by exploiting the electronic structure of distinctTMs is presented. Consequently, the ionization potential characteristic ofselected TMs, particularly the relative energy position and symmetry of the 3dstates, can be used to strategically engineer bands within specificmetal–organic frameworks. These findings not only provide a rationale forband structure engineering in 2D MOFs but also offer promisingopportunities for advanced material design.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{nokey,

title = {Emergence of Band Structure in a Two- Dimensional Metal−Organic Framework upon Hierarchical Self-Assembly},

author = {D. Baranowski and M. Thaler and D. Brandstetter and A. Windischbacher and I. Cojocariu and S. Mearini and V. Chesnyak and L. Schio and L. Floreano and C. Gutiérrez Bolaños and P. Puschnig and L. L. Patera and V. Feyer and C. M. Schneider},

url = {https://pubs.acs.org/doi/10.1021/acsnano.4c04191#},

doi = {10.1021/acsnano.4c04191},

year  = {2024},

date = {2024-07-17},

urldate = {2024-07-17},

journal = {ACS Nano},

volume = {18},

pages = {19618−19627},

abstract = {Two-dimensional metal−organic frameworks (2D-MOFs) represent a category of atomically thin materials that combine the structural tunability of molecular systems with the crystalline structure characteristic of solids. The strong bonding between the organic linkers and transition metal centers is expected to result in delocalized electronic states. However, it remains largely unknown how the band structure in 2D-MOFs emerges through the coupling of electronic states in the building blocks. Here, we demonstrate the on-surface synthesis of a 2D-MOF exhibiting prominent π-conjugation. Through a combined experimental and theoretical approach, we provide direct evidence of band structure formation upon hierarchical self-assembly, going from metal−organic complexes to a conjugated two-dimensional framework. Additionally, we identify the robustly dispersive nature of the emerging hybrid states, irrespective of the metallic support type, highlighting the tunability of the band structure through charge transfer from the substrate. Our findings encourage the exploration of band-structure engineering in 2D-MOFs for potential applications in electronics and photonics.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{Bennecke2024,

title = {Disentangling the multiorbital contributions of excitons by photoemission exciton tomography},

author = {W. Bennecke and A. Windischbacher and D. Schmitt and J. P. Bange and R. Hemm and C. S. Kern and G. D’Avino and X. Blase and D. Steil and S. Steil and M. Aeschlimann and B. Stadtmüller and M. Reutzel and P. Puschnig and G. S. M. Jansen and S. Mathias},

url = {https://www.nature.com/articles/s41467-024-45973-x},

doi = {10.1038/s41467-024-45973-x},

year  = {2024},

date = {2024-02-28},

urldate = {2024-02-28},

journal = {Nature Communications},

volume = {15},

number = {1804},

pages = {10},

abstract = {Excitons are realizations of a correlated many-particle wave function, specifi-cally consisting of electrons and holes in an entangled state. Excitons occurwidely in semiconductors and are dominant excitations in semiconductingorganic and low-dimensional quantum materials. To efficiently harness thestrong optical response and high tuneability of excitons in optoelectronics andin energy-transformation processes,access to the full wavefunction of theentangled state is critical, but has so far not been feasible. Here, we show howtime-resolved photoemission momentum microscopy can be used to gainaccess to the entangled wavefunction and to unravel the exciton’s multiorbitalelectron and hole contributions. For the prototypical organic semiconductorbuckminsterfullerene (C60), we exemplify the capabilities of exciton tomo-graphy and achieve unprecedented access to key properties of the entangledexciton state including localization, charge-transfer character, and ultrafastexciton formation and relaxation dynamics.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{Klein2024,

title = {Peri-Tetracene from 1,1′-Bitetracene: Zipping up Structurally Defined Graphene Nanoribbons},

author = {M. Klein and J. B. Bauer and N. Kainbacher and M. S. Wagner and K. Greulich and P. Haizmann and E. Giangrisostomi and R. Ovsyannikov and P. Puschnig and T. Chassé and H. F. Bettinger and H. Peisert},

doi = {10.1021/acs.jpcc.3c08182},

year  = {2024},

date = {2024-02-22},

journal = {J. Phys. Chem. C},

volume = {128},

pages = {4048-4059},

abstract = {Polycyclicaromatichydrocarbons(PAHs) are promising molecules for a manifold of applications in organic electronics, spintronics, or energy storage devices. Among PAHs, particular attention has been focused on the synthesis and study of acenes and fused acenes, peri-acenes, allowing tuning of the highest occupied molecular orbital−lowest unoccupied molecular orbital (HOMO−LUMO) gap with the size of the conjugated system. As a starting point for the surface synthesis of larger PAHs, we synthesized a 1,1′-bitetracene for the first time. This precursor molecule consists of two tetracene units connected via the 1,1′-position with a torsion angle of 70°.  The interface properties of the molecule before and after annealing on a Cu(111) surface are investigated. Using X-ray photoemission spectroscopy(XPS), angle-resolved photoelectron spectroscopy (ARPES), low-energy electron diffraction (LEED), and scanning-tunneling microscopy (STM), it is experimentally demonstrated that the tetracene units zip up with the help of heat forming peri-tetracene. These results and the exact adsorption geometry are in excellent agreement with calculations using density functionaltheory (DFT). Moreover, the calculations enable the identification of newly formed valence band states at the interface to Cu(111).},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{Adamkiewicz2023,

title = {Coherent and Incoherent Excitation Pathways in Time-Resolved Photoemission Orbital Tomography of CuPc/Cu(001)-2O},

author = {A. Adamkiewicz and M. Raths and M. Stettner and M. Theilen and L. Münster and S. Wenzel and M. Hutter and S. Soubatch and C. Kumpf and F. C. Bocquet and R. Wallauer and F. S. Tautz and U. Höfer},

doi = {10.1021/acs.jpcc.3c04859},

year  = {2023},

date = {2023-10-09},

urldate = {2023-10-09},

journal = {J.Phys. Chem. C},

volume = {127},

pages = {20411},

abstract = {Time-resolved photoemission orbital tomography (tr-POT) offers unique possibilities for tracing molecular electron dynamics. The recorded pump-induced changes of the angle-resolved photoemission intensities allow one to characterize unoccupied molecular states in momentum space and to deduce the incoherent temporal evolution of their population. Here, we show for the example of CuPc/Cu(001)-2O that the method also gives access to the coherent regime and that different excitation pathways can be disentangled by a careful analysis of the time-dependent change of the photoemission momentum pattern. In particular, we demonstrate by varying photon energy and polarization of the pump light how the incoherent temporal evolution of the LUMO distribution can be distinguished from coherent contributions of the projected HOMO. Moreover, we report the selective excitation of molecules with a specific orientation at normal incidence by aligning the electric field of the pump light along the molecular axis.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{Kern2023,

title = {Photoemission orbital tomography for excitons in organic molecules},

author = {C. S. Kern and A. Windischbacher and P. Puschnig},

doi = {https://doi.org/10.1103/PhysRevB.108.085132},

year  = {2023},

date = {2023-08-22},

urldate = {2023-08-22},

journal = {Phys. Rev. B},

volume = {108},

pages = {085132},

abstract = {Driven by recent developments in time-resolved photoemission spectroscopy, we extend the successful method of photoemission orbital tomography (POT) to excitons. Our theory retains the intuitive orbital picture of POT, while respecting both the entangled character of the exciton wave function and the energy conservation in the photoemission process. Analyzing results from three organic molecules, we classify generic exciton structures and give a simple interpretation in terms of natural transition orbitals. We validate our findings by directly simulating pump-probe experiments with time-dependent density functional theory.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{Kern2023b,

title = {Simple extension of the plane-wave final state in photoemission: Bringing understanding to the photon-energy dependence of two-dimensional materials},

author = {C. S. Kern and A. Haags and L.Egger and X. Yang and H. Kirschner and S. Wolff and T. Seyller and A. Gottwald and M. Richter and U. De Giovannini and A. Rubio and M. G. Ramsey and F. C. Bocquet and Soubatch and F. S. Tautz and P. Puschnig and S. Moser},

url = {https://journals.aps.org/prresearch/abstract/10.1103/PhysRevResearch.5.033075},

doi = {10.1103/PhysRevResearch.5.033075},

year  = {2023},

date = {2023-08-03},

journal = {Phys. Rev. Research},

volume = {5},

issue = {033075},

abstract = {Angle-resolved photoemission spectroscopy (ARPES) is a method that measures orbital and band structure contrast through the momentum distribution of photoelectrons. Its simplest interpretation is obtained in the plane-wave approximation, according to which photoelectrons propagate freely to the detector. The photoelectron momentum distribution is then essentially given by the Fourier transform of the real-space orbital. While the plane-wave approximation is remarkably successful in describing the momentum distributions of aromatic compounds, it generally fails to capture kinetic-energy-dependent final-state interference and dichroism effects. Focusing our present study on quasi-freestanding monolayer graphene as the archetypical two-dimensional (2D) material, we observe an exemplary 𝐸kin

-dependent modulation of, and a redistribution of spectral weight within, its characteristic horseshoe signature around the

‾‾‾

K

and

‾‾‾

K

′

points: both effects indeed cannot be rationalized by the plane-wave final state. Our data are, however, in remarkable agreement with ab initio time-dependent density functional simulations of a freestanding graphene layer and can be explained by a simple extension of the plane-wave final state, permitting the two dipole-allowed partial waves emitted from the C 2⁢𝑝𝑧

orbitals to scatter in the potential of their immediate surroundings. Exploiting the absolute photon flux calibration of the Metrology Light Source, this scattered-wave approximation allows us to extract 𝐸kin

-dependent amplitudes and phases of both partial waves directly from photoemission data. The scattered-wave approximation thus represents a powerful yet intuitive refinement of the plane-wave final state in photoemission of 2D materials and beyond.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{Ito2023,

title = {Build-up and dephasing of Floquet-Bloch bands on subcycle timescales},

author = {S. Ito and M. Schüler and M. Meierhofer and S. Schlauderer and J. Freudenstein and R. Reimann and D. Afanasiev and K. A. Kokh and O. E. Tereshchenko and J. Güdde and M. A. Sentef and U. Höfer and R. Huber

},

doi = {10.1038/s41586-023-05850-x},

year  = {2023},

date = {2023-04-12},

urldate = {2023-04-12},

journal = {Nature},

volume = {616},

pages = {pages 696–701},

abstract = {Strong light fields have created opportunities to tailor novel functionalities of solids. Floquet–Bloch states can form under periodic driving of electrons and enable exotic quantum phases. On subcycle timescales, lightwaves can simultaneously drive intraband currents and interband transition, which enable high-harmonic generation and pave the way towards ultrafast electronics. Yet, the interplay of intraband and interband excitations and their relation to Floquet physics have been key open questions as dynamical aspects of Floquet states have remained elusive. Here we provide this link by visualizing the ultrafast build-up of Floquet–Bloch bands with time-resolved and angle-resolved photoemission spectroscopy. We drive surface states on a topological insulator with mid-infrared fields—strong enough for high-harmonic generation—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 non-equilibrium calculations explain the simultaneous occurrence of Floquet states with intraband and interband dynamics. Our joint experiment and theory study provides a direct time-domain view of Floquet physics and explores the fundamental frontiers of ultrafast band-structure engineering.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{Freudenstein2022,

title = {Attosecond clocking of correlations between Bloch electrons},

author = {J. Freudenstein and M. Borsch and M. Meierhofer and D. Afanasiev and C. P. Schmid and F. Sandner and M. Liebich and A. Girnghuber and M. Knorr and M. Kira and R. Huber},

doi = {10.1038/s41586-022-05190-2},

year  = {2022},

date = {2022-10-12},

urldate = {2022-10-12},

journal = {Nature},

volume = {610},

pages = {290--295},

abstract = {Delocalized Bloch electrons and the low-energy correlations between them determine key optical, electronic and entanglement functionalities of solids, all the way through to phase transitions. To directly capture how many-body correlations affect the actual motion of Bloch electrons, subfemtosecond (1 fs = 10−15 s) temporal precision is desirable. Yet, probing with attosecond (1 as = 10−18 s) high-energy photons has not been energy-selective enough to resolve the relevant millielectronvolt-scale interactions of electrons near the Fermi energy. Here, we use multi-terahertz light fields to force electron–hole pairs in crystalline semiconductors onto closed trajectories, and clock the delay between separation and recollision with 300 as precision, corresponding to 0.7% of the driving field’s oscillation period. We detect that strong Coulomb correlations emergent in atomically thin WSe2 shift the optimal timing of recollisions by up to 1.2 ± 0.3 fs compared to the bulk material. A quantitative analysis with quantum-dynamic many-body computations in a Wigner-function representation yields a direct and intuitive view on how the Coulomb interaction, non-classical aspects, the strength of the driving field and the valley polarization influence the dynamics. The resulting attosecond chronoscopy of delocalized electrons could revolutionize the understanding of unexpected phase transitions and emergent quantum-dynamic phenomena for future electronic, optoelectronic and quantum-information technologies.},

keywords = {Selected Preliminary Work},

pubstate = {published},

tppubtype = {article}

}

@article{Yang2022,

title = {Momentum-selective orbital hybridisation},

author = {X. Yang and M. Jugovac and G. Zamborlini and V. Feyer and G. Koller and P. Puschnig and S. Soubatch and M. G. Ramsey and F. S. Tautz},

doi = {10.1038/s41467-022-32643-z},

year  = {2022},

date = {2022-01-01},

urldate = {2022-01-01},

journal = {Nat. Commun.},

volume = {13},

pages = {5148},

abstract = {When a molecule interacts chemically with a metal surface, the orbitals of the molecule hybridise with metal states to form the new eigenstates of the coupled system. Spatial overlap and energy matching are determining parameters of the hybridisation. However, since every molecular orbital does not only have a characteristic spatial shape, but also a specific momentum distribution, one may additionally expect a momentum matching condition; after all, each hybridising wave function of the metal has a defined wave vector, too. Here, we report photoemission orbital tomography measurements of hybrid orbitals that emerge from molecular orbitals at a molecule-on-metal interface. We find that in the hybrid orbitals only those partial waves of the original orbital survive which match the metal band structure. Moreover, we find that the conversion of the metal’s surface state into a hybrid interface state is also governed by momentum matching constraints. Our experiments demonstrate the possibility to measure hybridisation momentum-selectively, thereby enabling deep insights into the complicated interplay of bulk states, surface states, and molecular orbitals in the formation of the electronic interface structure at molecule-on-metal hybrid interfaces.},

keywords = {Selected Preliminary Work},

pubstate = {published},

tppubtype = {article}

}

@article{Haags2021,

title = {Momentum-space imaging of σ-orbitals for chemical analysis},

author = {A. Haags and X. Yang and L. Egger and D. Brandstetter and H. Kirschner and F. C. Bocquet and G. Koller and A. Gottwald and M. Richter and J. M. Gottfried and M. G. Ramsey and P. Puschnig and S. Soubatch and F. S. Tautz},

doi = {10.1126/sciadv.abn0819},

year  = {2022},

date = {2022-01-01},

urldate = {2022-01-01},

journal = {Sci. Adv.},

volume = {8},

pages = {eabn0819},

abstract = {Tracing the modifications of molecules in surface chemical reactions benefits from the possibility to image their orbitals. While delocalized frontier orbitals with π character are imaged routinely with photoemission orbital tomography, they are not always sensitive to local chemical modifications, particularly the making and breaking of bonds at the molecular periphery. For such bonds, σ orbitals would be far more revealing. Here, we show that these orbitals can indeed be imaged in a remarkably broad energy range and that the plane wave approximation, an important ingredient of photoemission orbital tomography, is also well fulfilled for these orbitals. This makes photoemission orbital tomography a unique tool for the detailed analysis of surface chemical reactions. We demonstrate this by identifying the reaction product of a dehalogenation and cyclodehydrogenation reaction.},

keywords = {Selected Preliminary Work},

pubstate = {published},

tppubtype = {article}

}

@article{Stallberg2022,

title = {Ultrafast Exciton Dynamics and Charge Transfer at PTCDA/Metal Interfaces},

author = {K. Stallberg and A. Namgalies and S. Chatterjee and U. Höfer},

doi = {10.1021/acs.jpcc.2c04696},

year  = {2022},

date = {2022-01-01},

journal = {J. Phys. Chem. C},

volume = {126},

pages = {12728--12734},

abstract = {The functionality of organic electronic devices is governed by the dynamics of charge carriers and excited states in organic semiconductors. In particular, the relaxation of excitons and the transfer of charge carriers at metal electrodes crucially determine the performance of organic optoelectronic devices. In a combined experimental study we apply time-resolved photoluminescence and two-photon photoemission to reveal the ultrafast exciton dynamics and charge transfer at prototype organic/metal contacts comprising thin molecular films on single-crystalline noble-metal surfaces. On the basis of experiments with systematically varied film thicknesses, we relate the strong quenching of Frenkel excitons and charge-transfer excitons to the wave function overlap with the metal, indicating charge transfer as the dominant relaxation pathway. Moreover, the presence of an electronic interface state is found to facilitate the transfer of excited carriers across the organic/metal interface.},

keywords = {Selected Preliminary Work},

pubstate = {published},

tppubtype = {article}

}

@article{Hurdax2022,

title = {Large Distortion of Fused Aromatics on Dielectric Interlayers Quantified by Photoemission Orbital Tomography},

author = {P. Hurdax and C. S. Kern and T. G. Boné and A. Haags and M. Hollerer and L. Egger and X. Yang and H. Kirschner and A. Gottwald and M. Richter and F. C. Bocquet and S. Soubatch and G. Koller and F. S. Tautz and M. Sterrer and P. Puschnig and M. G. Ramsey},

doi = {10.1021/acsnano.2c08631},

year  = {2022},

date = {2022-01-01},

journal = {ACS Nano},

volume = {16},

pages = {17435-17443},

abstract = {Polycyclic aromatic compounds with fused benzene rings offer an extraordinary versatility as next-generation organic semiconducting materials for nanoelectronics and optoelectronics due to their tunable characteristics, including charge-carrier mobility and optical absorption. Nonplanarity can be an additional parameter to customize their electronic and optical properties without changing the aromatic core. In this work, we report a combined experimental and theoretical study in which we directly observe large, geometry-induced modifications in the frontier orbitals of a prototypical dye molecule when adsorbed on an atomically thin dielectric interlayer on a metallic substrate. Experimentally, we employ angle-resolved photoemission experiments, interpreted in the framework of the photoemission orbital tomography technique. We demonstrate its sensitivity to detect geometrical bends in adsorbed molecules and highlight the role of the photon energy used in experiment for detecting such geometrical distortions. Theoretically, we conduct density functional calculations to determine the geometric and electronic structure of the adsorbed molecule and simulate the photoemission angular distribution patterns. While we found an overall good agreement between experimental and theoretical data, our results also unveil limitations in current van der Waals corrected density functional approaches for such organic/dielectric interfaces. Hence, photoemission orbital tomography provides a vital experimental benchmark for such systems. By comparison with the state of the same molecule on a metallic substrate, we also offer an explanation why the adsorption on the dielectric induces such large bends in the molecule.},

keywords = {Selected Preliminary Work},

pubstate = {published},

tppubtype = {article}

}

@article{Wallauer2020,

title = {Tracing orbital images on ultrafast time scales},

author = {R. Wallauer and M. Raths and K. Stallberg and L. Münster and D. Brandstetter and X. Yang and J. Güdde and P. Puschnig and S. Soubatch and C. Kumpf and F. C. Bocquet and F. S. Tautz and U. Höfer},

doi = {10.1126/science.abf3286},

year  = {2021},

date = {2021-01-01},

urldate = {2021-01-01},

journal = {Science},

volume = {371},

pages = {1056-1059},

abstract = {Frontier orbitals determine fundamental molecular properties such as chemical reactivities. Although electron distributions of occupied orbitals can be imaged in momentum space by photoemission tomography, it has so far been impossible to follow the momentum-space dynamics of a molecular orbital in time, for example, through an excitation or a chemical reaction. Here, we combined time-resolved photoemission using high laser harmonics and a momentum microscope to establish a tomographic, femtosecond pump-probe experiment of unoccupied molecular orbitals. We measured the full momentum-space distribution of transiently excited electrons, connecting their excited-state dynamics to real-space excitation pathways. Because in molecules this distribution is closely linked to orbital shapes, our experiment may, in the future, offer the possibility of observing ultrafast electron motion in time and space.},

keywords = {Selected Preliminary Work},

pubstate = {published},

tppubtype = {article}

}

@article{Wallauer2021,

title = {Momentum-Resolved Observation of Exciton Formation Dynamics in Monolayer WS2},

author = {R. Wallauer and R. Perea-Causin and L. Münster and S. Zajusch and S. Brem and J. Güdde and K. Tanimura and K. -Q. Lin and R. Huber and E. Malic and U. Höfer},

doi = {10.1021/acs.nanolett.1c01839},

year  = {2021},

date = {2021-01-01},

journal = {Nano Lett.},

volume = {21},

pages = {5867--5873},

abstract = {The dynamics of momentum-dark exciton formation in transition metal dichalcogenides is difficult to measure experimentally, as many momentum-indirect exciton states are not accessible to optical interband spectroscopy. Here, we combine a tunable pump, high-harmonic probe laser source with a 3D momentum imaging technique to map photoemitted electrons from monolayer WS2. This provides momentum-, energy- and time-resolved access to excited states on an ultrafast time scale. The high temporal resolution of the setup allows us to trace the early-stage exciton dynamics on its intrinsic time scale and observe the formation of a momentum-forbidden dark KΣ exciton a few tens of femtoseconds after optical excitation. By tuning the excitation energy, we manipulate the temporal evolution of the coherent excitonic polarization and observe its influence on the dark exciton formation. The experimental results are in excellent agreement with a fully microscopic theory, resolving the temporal and spectral dynamics of bright and dark excitons in WS2.},

keywords = {Selected Preliminary Work},

pubstate = {published},

tppubtype = {article}

}

@article{Reimann2018,

title = {Subcycle observation of lightwave-driven Dirac currents in a topological surface band},

author = {J. Reimann and S. Schlauderer and C. P. Schmid and F. Langer and S. Baierl and K. A. Kokh and O. E. Tereshchenko and A. Kimura and C. Lange and J. Güdde and U. Höfer and R. Huber},

doi = {10.1038/s41586-018-0544-x},

year  = {2018},

date = {2018-01-01},

journal = {Nature},

volume = {562},

pages = {396--400},

abstract = {Harnessing the carrier wave of light as an alternating-current bias may enable electronics at optical clock rates1. Lightwave-driven currents have been assumed to be essential for high-harmonic generation in solids2–6, charge transport in nanostructures7,8, attosecond-streaking experiments9–16 and atomic-resolution ultrafast microscopy17,18. However, in conventional semiconductors and dielectrics, the finite effective mass and ultrafast scattering of electrons limit their ballistic excursion and velocity. The Dirac-like, quasi-relativistic band structure of topological insulators19–29 may allow these constraints to be lifted and may thus open a new era of lightwave electronics. To understand the associated, complex motion of electrons, comprehensive experimental access to carrier-wave-driven currents is crucial. Here we report angle-resolved photoemission spectroscopy with subcycle time resolution that enables us to observe directly how the carrier wave of a terahertz light pulse accelerates Dirac fermions in the band structure of the topological surface state of Bi2Te3. While terahertz streaking of photoemitted electrons traces the electromagnetic field at the surface, the acceleration of Dirac states leads to a strong redistribution of electrons in momentum space. The inertia-free surface currents are protected by spin–momentum locking and reach peak densities as large as two amps per centimetre, with ballistic mean free paths of several hundreds of nanometres, opening up a realistic parameter space for all-coherent lightwave-driven electronic devices. Furthermore, our subcycle-resolution analysis of the band structure may greatly improve our understanding of electron dynamics and strong-field interaction in solids.},

keywords = {Selected Preliminary Work},

pubstate = {published},

tppubtype = {article}

}