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ARPES/Spin-PES Systems

Angle-Resolved and Spin-Resolved Photoelectron Spectroscopy is very sensitive regarding the demands for ultimate energy and angle resolution. Thus, in ARPES and Spin-PES special system requirements are necessary: optimized geometries, lowest magnetic fields, special manipulators for ultralow sample temperatures, special UV or laser sources. Usually, a preparation chamber is needed for sample preparation and surface quality control by LEED. Finally, best pressures are needed to keep the sample surfaces clean.



  1. (2022) Polarization-Modulated Angle-Resolved Photoemission Spectroscopy: Toward Circular Dichroism without Circular Photons and Bloch Wave-function Reconstruction

    Angle-resolved photoemission spectroscopy (ARPES) is the most powerful technique to investigate the electronic band structure of crystalline solids. To completely characterize the electronic structure of topological materials, one needs to go beyond band structure mapping and access information about the momentum-resolved Bloch wave function, namely, orbitals, Berry curvature, and topological invariants. However, because phase information is lost in the process of measuring photoemission intensities, retrieving the complex-valued Bloch wave function from photoemission data has yet remained elusive. We introduce a novel measurement methodology and associated observable in extreme ultraviolet angle-resolved photoemission spectroscopy, based on continuous modulation of the ionizing radiation polarization axis. Tracking the energy- and momentum-resolved amplitude and phase of the photoemission intensity modulation upon polarization axis rotation allows us to retrieve the circular dichroism in photoelectron angular distributions (CDAD) without using circular photons, providing direct insights into the phase of photoemission matrix elements. In the case of two relevant bands, it is possible to reconstruct the orbital pseudospin (and thus the Bloch wave function) with moderate theory input, as demonstrated for the prototypical, layered, semiconducting, transition metal dichalcogenide 2HWSe2. This novel measurement methodology in ARPES, which is articulated around the manipulation of the photoionization transition dipole matrix element, in combination with a simple tight-binding theory, is general and adds a new dimension to obtaining insights into the orbital pseudospin, Berry curvature, and Bloch wave functions of many relevant crystalline solids.

    Michael Schüler, Tommaso Pincelli, Shuo Dong, Thomas P. Devereaux, Martin Wolf, Laurenz Rettig, Ralph Ernstorfer, and Samuel Beaulieu
    Phys. Rev. X 12, 011019
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  2. (2020) Directly visualizing the momentum-forbidden dark excitons and their dynamics in atomically thin semiconductors

    Resolving momentum degrees of freedom of excitons, which are electron-hole pairs bound by the Coulomb attraction in a photoexcited semiconductor, has remained an elusive goal for decades. In atomically thin semiconductors, such a capability could probe the momentum-forbidden dark excitons, which critically affect proposed opto-electronic technologies but are not directly accessible using optical techniques. Here, we probed the momentum state of excitons in a tungsten diselenide monolayer by photoemitting their constituent electrons and resolving them in time, momentum, and energy. We obtained a direct visual of the momentum-forbidden dark excitons and studied their properties, including their near degeneracy with bright excitons and their formation pathways in the energy-momentum landscape. These dark excitons dominated the excited-state distribution, a surprising finding that highlights their importance in atomically thin semiconductors.

    Julien Madéo, Michael K. L. Man, Chakradhar Sahoo, Marshall Campbell, Vivek Pareek, E. Laine Wong, Abdullah Al-Mahboob, Nicholas S. Chan, Arka Karmakar, Bala Murali Krishna Mariserla, Xiaoqin Li, Tony F. Heinz, Ting Cao, Keshav M. Dani
    Science 370, 1199–1204 (2020)
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  3. (2022) Structure of the moiré exciton captured by imaging its electron and hole

    Interlayer excitons (ILXs) — electron–hole pairs bound across two atomically thin layered semiconductors — have emerged as attractive platforms to study exciton condensation, single-photon emission and other quantum information applications. Yet, despite extensive optical spectroscopic investigations, critical information about their size, valley configuration and the influence of the moiré potential remains unknown. Here, in a WSe2/MoS2 heterostructure, we captured images of the time-resolved and momentum-resolved distribution of both of the particles that bind to form the ILX: the electron and the hole. We thereby obtain a direct measurement of both the ILX diameter of around 5.2 nm, comparable with the moiré-unit-cell length of 6.1 nm, and the localization of its centre of mass. Surprisingly, this large ILX is found pinned to a region of only 1.8 nm diameter within the moiré cell, smaller than the size of the exciton itself. This high degree of localization of the ILX is backed by Bethe–Salpeter equation calculations and demonstrates that the ILX can be localized within small moiré unit cells. Unlike large moiré cells, these are uniform over large regions, allowing the formation of extended arrays of localized excitations for quantum technology.

    Ouri Karni, Elyse Barré, Vivek Pareek, Johnathan D. Georgaras, Michael K. L. Man, Chakradhar Sahoo, David R. Bacon, Xing Zhu, Henrique B. Ribeiro, Aidan L. O’Beirne, Jenny Hu, Abdullah Al-Mahboob, Mohamed M. M. Abdelrasoul, Nicholas S. Chan, Arka Karmakar
    Nature 603, pages 247–252 (2022)
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  4. (2019) Excited-state band mapping and momentum-resolved ultrafast population dynamics in In/Si(111) nanowires investigated with XUV-based time- and angle-resolved photoemission spectroscopy

    We investigate the excited state electronic structure of the model phase transition system In/Si(111) using femtosecond time- and angle-resolved photoemission spectroscopy (trARPES). An extreme ultraviolet 500 kHz laser source at 21.7 eV is utilized both to map the energy of excited states above the Fermi level and follow the momentum-resolved population dynamics on a femtosecond timescale. Excited-state band mapping is used to characterize the normally unoccupied electronic structure above the Fermi level in both structural phases of In/Si(111): the metallic (4 x 1) and the gapped (8 x 2) phases. The extracted band positions are compared withband-structure calculations utilizing density functional theory within both the local density approximation and GW approximations (single-particle Green's function (G) + screened Coulomb interaction (W)). While good overall agreement is found between the GW-calculated band structure and experiment, deviations in specific momentum regions may indicate the importance of excitonic effects not accounted for at this level of approximation. To probe the dynamics of these excited states, their momentum-resolved transient population dynamics are extracted with trARPES. The transient intensities are compared to a simulated spectral function modeled by a state population employing a transient elevated electronic temperature as determined experimentally. This allows the momentum-resolved population dynamics to be quantitatively reproduced, revealing important insights into the transfer of energy from the electronic system to the lattice. In particular, a comparison between the magnitude and relaxation time of the transient electronic temperature observed by trARPES with those of the lattice as probed in previous ultrafast electron diffraction studies implies a highly nonthermal phonon distribution at the surface following photo-excitation. This suggests that the energy from the initially excited electronic system is initially transferred to high-energy optical phonon modes followed by cooling and thermalization of the photo-excited system by much slower phonon-phonon coupling

    C. W. Nicholson, M. Puppin, A. Lücke, U. Gerstmann, M. Krenz, W. G. Schmidt, L. Rettig, R. Ernstorfer, and M. Wolf
    Phys. Rev. B 99, 155107
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  5. (2011) Large Tunable Rashba Spin Splitting of a Two-Dimensional Electron Gas in Bi2Se3

    We report a Rashba spin splitting of a two-dimensional electron gas in the topological insulator Bi2Se3 from angle-resolved photoemission spectroscopy. We further demonstrate its electrostatic control, and show that spin splittings can be achieved which are at least an order-of-magnitude larger than in other semiconductors. Together these results show promise for the miniaturization of spintronic devices to the nanoscale and their operation at room temperature.

    P. D. C. King, R. C. Hatch, M. Bianchi, R. Ovsyannikov, C. Lupulescu, G. Landolt, B. Slomski, J. H. Dil, D. Guan, J. L. Mi, E. D. L. Rienks, J. Fink, A. Lindblad, S. Svensson, S. Bao, G. Balakrishnan, B. B. Iversen, J. Osterwalder, W. Eberhardt, F. Baumbe
    Phys. Rev. Lett. 107, 096802
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  6. (2013) Spin-orbit splitting of the Shockley surface state on Cu(111)

    We present angle-resolved photoemission data from Cu(111). Using a focused 6 eV continuous-wave laser for photoexcitation, we achieve a high effective momentum resolution, enabling detection of the Rashba spin splitting in the Shockley surface state on Cu(111). The magnitude of the spin splitting of Δk0.006 Å1 is surprisingly large and exceeds values predicted for the analogous surface state on Ag(111), but is reproduced by first-principles calculations. We further resolve a kink in the dispersion, which we attribute to electron-phonon coupling.

    A. Tamai, W. Meevasana, P. D. C. King, C. W. Nicholson, A. de la Torre, E. Rozbicki, and F. Baumberger
    Phys. Rev. B 87, 075113
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  7. (2018) Beyond the molecular movie: Dynamics of bands and bonds during a photoinduced phase transition

    Ultrafast nonequilibrium dynamics offer a route to study the microscopic interactions that govern macroscopic behavior. In particular, photoinduced phase transitions (PIPTs) in solids provide a test case for how forces, and the resulting atomic motion along a reaction coordinate, originate from a nonequilibrium population of excited electronic states. Using femtosecond photoemission, we obtain access to the transient electronic structure during an ultrafast PIPT in a model system: indium nanowires on a silicon(111) surface. We uncover a detailed reaction pathway, allowing a direct comparison with the dynamics predicted by ab initio simulations. This further reveals the crucial role played by localized photoholes in shaping the potential energy landscape and enables a combined momentum- and real-space description of PIPTs, including the ultrafast formation of chemical bonds.

    C. W. Nicholson, A. Lücke, W. G. Schmidt, M. Puppin, L. Rettig, R. Ernstorfer, M. Wolf
    Science 362, 821–825 (2018)
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  8. (2020) Observation and control of maximal Chern numbers in a chiral topological semimetal

    Topological semimetals feature protected nodal band degeneracies characterized by a topological invariant known as the Chern number (C). Nodal band crossings with linear dispersion are expected to have at most C = 4, which sets an upper limit to the magnitude of many topological phenomena in these materials. Here, we show that the chiral crystal palladium gallium (PdGa) displays multifold band crossings, which are connected by exactly four surface Fermi arcs, thus proving that they carry the maximal Chern number magnitude of 4. By comparing two enantiomers, we observe a reversal of their Fermi-arc velocities, which demonstrates that the handedness of chiral crystals can be used to control the sign of their Chern numbers.

    Niels B. M. Schröter, Samuel Stolz, Kaustuv Manna, Fernando de Juan, Maia G. Vergniory,
    Jonas A. Krieger, Ding Pei, Thorsten Schmitt, Pavel Dudin, Timur K. Kim, Cephise Cacho,
    Barry Bradlyn, Horst Borrmann, Marcus Schmidt, Roland Widmer,
    Vladimir N. St
    Science 369, 179–183 (2020)
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  9. (2019) Chiral topological semimetal with multifold band crossings and long Fermi arcs

    Topological semimetals in crystals with a chiral structure (which possess a handedness due to a lack of mirror and inversion symmetries) are expected to display numerous exotic physical phenomena, including fermionic excitations with large topological charge, long Fermi arc surface states, unusual magnetotransport and lattice dynamics, as well as a quantized response to circularly polarized light. So far, all experimentally confirmed topological semimetals exist in crystals that contain mirror operations, meaning that these properties do not appear. Here, we show that AlPt is a structurally chiral topological semimetal that hosts new four-fold and six-fold fermions, which can be viewed as a higher spin generalization of Weyl fermions without equivalence in elementary particle physics. These multifold fermions are located at high symmetry points and have Chern numbers larger than those in Weyl semimetals, thus resulting in multiple Fermi arcs that span the full diagonal of the surface Brillouin zone. By imaging these long Fermi arcs, we experimentally determine the magnitude and sign of their Chern number, allowing us to relate their dispersion to the handedness of their host crystal.

    Niels B. M. Schröter, Ding Pei, Maia G. Vergniory, Yan Sun, Kaustuv Manna,
    Fernando de Juan, Jonas. A. Krieger, Vicky Süss, Marcus Schmidt, Pavel Dudin,
    Barry Bradlyn, Timur K. Kim, Thorsten Schmitt, Cephise Cacho, Claudia Felser,
    Vladimir N. Strocov a
    Nat. Phys. 15, 759–765 (2019)
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  10. (2014) Soft-X-ray ARPES facility at the ADRESS beamline of the SLS: concepts, technical realisation and scientific applications

    Soft-X-ray angle-resolved photoelectron spectroscopy (ARPES) with photon energies around 1 keV combines the momentum space resolution with increasing probing depth. The concepts and technical realisation of the new soft-X-ray ARPES endstation at the ADRESS beamline of SLS are described. The experimental geometry of the endstation is characterized by grazing X-ray incidence on the sample to increase the photoyield and vertical orientation of the measurement plane. The vacuum chambers adopt a radial layout allowing most efficient sample transfer. High accuracy of the angular resolution is ensured by alignment strategies focused on precise matching of the X-ray beam and optical axis of the analyzer. The high photon flux of up to 1013 photons s-1 (0.01% bandwidth)-1 delivered by the beamline combined with the optimized experimental geometry break through the dramatic loss of the valence band photoexcitation cross section at soft-X-ray energies. ARPES images with energy resolution up to a few tens of meV are typically acquired on the time scale of minutes. A few application examples illustrate the power of our advanced soft-X-ray ARPES instrumentation to explore the electronic structure of bulk crystals with resolution in three-dimensional momentum, access buried heterostructures and study elemental composition of the valence states using resonant excitation.

    V. N. Strocov, X. Wang, M. Shi, M. Kobayashi, J. Krempasky, C. Hess,
    T. Schmitt and L. Patthey
    J. Synchrotron Rad. (2014). 21, 32–44
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  11. (2022) Electronic Structure of InAs and InSb Surfaces: Density Functional Theory and Angle-Resolved Photoemission Spectroscopy

    The electronic structure of surfaces plays a key role in the properties of quantum devices. However, surfaces are also the most challenging to simulate and engineer. Here, the electronic structure of InAs(001), InAs(111), and InSb(110) surfaces is studied using a combination of density functional theory (DFT) and angle-resolved photoemission spectroscopy (ARPES). Large-scale first principles simulations are enabled by using DFT calculations with a machine-learned Hubbard U correction [npj Comput. Mater. 6, 180 (2020)]. To facilitate direct comparison with ARPES results, a “bulk unfolding” scheme is implemented by projecting the calculated band structure of a supercell surface slab model onto the bulk primitive cell. For all three surfaces, a good agreement is found between DFT calculations and ARPES. For InAs(001), the simulations clarify the effect of the surface reconstruction. Different reconstructions are found to produce distinctive surface states, which may be detected by ARPES with low photon energies. For InAs(111) and InSb(110), the simulations help elucidate the effect of oxidation. Owing to larger charge transfer from As to O than from Sb to O, oxidation of InAs(111) leads to significant band bending and produces an electron pocket, whereas oxidation of InSb(110) does not. The combined theoretical and experimental results may inform the design of quantum devices based on InAs and InSb semiconductors, for example, topological qubits utilizing the Majorana zero modes.

    Shuyang Yang, Niels B. M. Schröter, Vladimir N. Strocov, Sergej Schuwalow, Mohana Rajpalk, Keita Ohtani, Peter Krogstrup, Georg W. Winkler, Jan Gukelberger, Dominik Gresch, Gabriel Aeppli, Roman M. Lutchyn, Noa Marom
    Adv. Quantum Technol. 2022, 5, 2100033
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  12. (2021) Momentum-resolved electronic structure and band offsets in an epitaxial NbN/GaN superconductor/semiconductor heterojunction

    The electronic structure of heterointerfaces is a pivotal factor for their device functionality. We use soft x-ray angle-resolved photoelectron spectroscopy to directly measure the momentum-resolved electronic band structures on both sides of the Schottky heterointerface formed by epitaxial films of the superconducting NbN on semiconducting GaN, and determine their momentum-dependent interfacial band offset as well as the band-bending profile. We find, in particular, that the Fermi states in NbN are well separated in energy and momentum from the states in GaN, excluding any notable electronic cross-talk of the superconducting states in NbN to GaN. We support the experimental findings with first-principles calculations for bulk NbN and GaN. The Schottky barrier height obtained from photoemission is corroborated by electronic transport and optical measurements. The momentum-resolved understanding of electronic properties of interfaces elucidated in our work opens up new frontiers for the quantum materials where interfacial states play a defining role.

    Tianlun Yu, John Wright, Guru Khalsa, Betül Pamuk, Celesta S. Chang, Yury Matveyev,
    Xiaoqiang Wang, Thorsten Schmitt, Donglai Feng, David A. Muller, Huili Grace Xing, Debdeep Jena, Vladimir N. Strocov
    Sci. Adv. 7, eabi5833 (2021)
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  13. (2022) Field-induced ultrafast modulation of Rashba coupling at room temperature in ferroelectric α-GeTe(111)

    Rashba materials have appeared as an ideal playground for spin-to-charge conversion in prototype spintronics devices. Among them, α-GeTe(111) is a non-centrosymmetric ferroelectric semiconductor for which a strong spin-orbit interaction gives rise to giant Rashba coupling. Its room temperature ferroelectricity was recently demonstrated as a route towards a new type of highly energy-efficient non-volatile memory device based on switchable polarization. Currently based on the application of an electric field, the writing and reading processes could be outperformed by the use of femtosecond light pulses requiring exploration of the possible control of ferroelectricity on this timescale. Here, we probe the room temperature transient dynamics of the electronic band structure of α-GeTe(111) using time and angle-resolved photoemission spectroscopy. Our experiments reveal an ultrafast modulation of the Rashba coupling mediated on the fs timescale by a surface photovoltage, namely an increase corresponding to a 13% enhancement of the lattice distortion. This opens the route for the control of the ferroelectric polarization in α-GeTe(111) and ferroelectric semiconducting materials in quantum heterostructures.

    Geoffroy Kremer, Julian Maklar, Laurent Nicolaï, Christopher W. Nicholson, Changming Yue, Caio Silva, PhilippWerner, J. Hugo Dil, Juraj Krempaský, Gunther Springholz, Ralph Ernstorfer,
    Jan Minár, Laurenz Rettig, Claude Monney
    Nat Commun 13, 6396 (2022)
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  14. (2021) Ultrafast dynamical Lifshitz transition

    Fermi surface is at the heart of our understanding of metals and strongly correlated many-body systems. An abrupt change in the Fermi surface topology, also called Lifshitz transition, can lead to the emergence of fascinating phenomena like colossal magnetoresistance and superconductivity. While Lifshitz transitions have been demonstrated for a broad range of materials by equilibrium tuning of macroscopic parameters such as strain, doping, pressure, and temperature, a nonequilibrium dynamical route toward ultrafast modification of the Fermi surface topology has not been experimentally demonstrated. Combining time-resolved multidimensional photoemission spectroscopy with state-of-the-art TDDFT+U simulations, we introduce a scheme for driving an ultrafast Lifshitz transition in the correlated type-II Weyl semimetal Td-MoTe2. We demonstrate that this nonequilibrium topological electronic transition finds its microscopic origin in the dynamical modification of the effective electronic correlations. These results shed light on a previously unexplored ultrafast scheme for controlling the Fermi surface topology in correlated quantum materials.

    Samuel Beaulieu, Shuo Dong, Nicolas Tancogne-Dejean, Maciej Dendzik, Tommaso Pincelli, Julian Maklar, R. Patrick Xian, Michael A. Sentef, Martin Wolf, Angel Rubio, Laurenz Rettig, Ralph Ernstorfer
    Sci. Adv. 2021; 7 : eabd9275
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  15. (2008) A possible source of spin polarized electrons: The inert graphene/Ni(111) system

    We report on an investigation of spin-polarized secondary electron emission from the chemically inert system: graphene/Ni(111). An ordered passivation graphene layer (monolayer of graphite) was formed on Ni(111) surface via cracking of propylene gas. The spin polarization of secondary electrons obtained from this system upon photoemission is only slightly lower than the one from the clean Ni surface but does not change upon large oxygen exposure. These results suggest to use such passivated Ni(111) surface as a source of spin-polarized electrons stable against adsorption of reactive gases.

    Yu. S. Dedkov, M. Fonin, C. Laubschat
    Appl. Phys. Lett. 92, 052506
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  16. (2022) Excited-state band structure mapping

    Angle-resolved photoelectron spectroscopy is an extremely powerful probe of materials to access the occupied electronic structure with energy and momentum resolution. However, it remains blind to those dynamic states above the Fermi level that determine technologically relevant transport properties. In this work we extend band structure mapping into the unoccupied states and across the entire Brillouin zone by using a state-of-the-art high repetition rate, extreme ultraviolet femtosecond light source to probe optically excited samples. The wide-ranging applicability and power of this approach are demonstrated by measurements on the two-dimensional semiconductor WSe2, where the energy-momentum dispersion of valence and conduction bands are observed in a single experiment. This provides a direct momentum-resolved view, not only on the complete out-of-equilibrium band gap but also on its renormalization induced by electronic screening. Our work establishes a benchmark for measuring the band structure of materials, with direct access to the energy-momentum dispersion of the excited-state spectral function.

    M. Puppin, C. W. Nicholson, C. Monney, Y. Deng, R. P. Xian, J. Feldl, S. Dong, A. Dominguez, H. Hübener, A. Rubio, M. Wolf, L. Rettig, and R. Ernstorfer
    Phys. Rev. B 105, 075417
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  17. (2021) Direct measurement of key exciton properties: Energy, dynamics, and spatial distribution of the wave function

    Excitons, Coulomb-bound electron–hole pairs, are the fundamental excitations governing the optoelectronic properties of semiconductors. Although optical signatures of excitons have been studied extensively, experimental access to the excitonic wave function itself has been elusive. Using multidimensional photoemission spectroscopy, we present a momentum-, energy-, and time-resolved perspective on excitons in the layered semiconductor WSe2. By tuning the excitation wavelength, we determine the energy–momentum signature of bright exciton formation and its difference from conventional single-particle excited states. The multidimensional data allow to retrieve fundamental exciton properties like the binding energy and the exciton–lattice coupling and to reconstruct the real-space excitonic distribution function via Fourier transform. All quantities are in excellent agreement with microscopic calculations. Our approach provides a full characterization of the exciton properties and is applicable to bright and dark excitons in semiconducting materials, heterostructures, and devices.

    Shuo Dong, Michele Puppin, Tommaso Pincelli, Samuel Beaulieu, Dominik Christiansen, Hannes Hübener, Christopher W. Nicholson, Rui Patrick Xian, Maciej Dendzik, YunpeiDeng, Yoav William Windsor, Malte Selig, Ermin Malic, Angel Rubio, Andreas Knorr, Martin
    NatSci. 2021;1:e10010
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  18. (2022) Observation of Multi-Directional Energy Transfer in a Hybrid Plasmonic–Excitonic Nanostructure

    Hybrid plasmonic devices involve a nanostructured metal supporting localized surface plasmons to amplify light–matter interaction, and a non-plasmonic material to functionalize charge excitations. Application-relevant epitaxial heterostructures, however, give rise to ballistic ultrafast dynamics that challenge the conventional semiclassical understanding of unidirectional nanometal-to-substrate energy transfer. Epitaxial Au nanoislands are studied on WSe2 with time- and angle-resolved photoemission spectroscopy and femtosecond electron diffraction: this combination of techniques resolves material, energy, and momentum of charge-carriers and phonons excited in the heterostructure. A strong non-linear plasmon–exciton interaction that transfers the energy of sub-bandgap photons very efficiently to the semiconductor is observed, leaving the metal cold until non-radiative exciton recombination heats the nanoparticles on hundreds of femtoseconds timescales. The results resolve a multi-directional energy exchange on timescales shorter than the electronic thermalization of the nanometal. Electron–phonon coupling and diffusive charge-transfer determine the subsequent energy flow. This complex dynamics opens perspectives for optoelectronic and photocatalytic applications, while providing a constraining experimental testbed for state-of-the-art modelling.

    Tommaso Pincelli, Thomas Vasileiadis, Shuo Dong, Samuel Beaulieu, Maciej Dendzik,
    Daniela Zahn, Sang-Eun Lee, Hélène Seiler, Yingpeng Qi, R. Patrick Xian, Julian Maklar,
    Emerson Coy, Niclas S. Mueller, Yu Okamura, Stephanie Reich, Martin Wolf,
    Advanced Materials 2023, 2209100
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  19. (2022) A machine learning route between band mapping and band structure

    The electronic band structure and crystal structure are the two complementary identifiers of solid-state materials. Although convenient instruments and reconstruction algorithms have made large, empirical, crystal structure databases possible, extracting the quasiparticle dispersion (closely related to band structure) from photoemission band mapping data is currently limited by the available computational methods. To cope with the growing size and scale of photoemission data, here we develop a pipeline including probabilistic machine learning and the associated data processing, optimization and evaluation methods for band-structure reconstruction, leveraging theoretical calculations. The pipeline reconstructs all 14 valence bands of a semiconductor and shows excellent performance on benchmarks and other materials datasets. The reconstruction uncovers previously inaccessible momentum-space structural information on both global and local scales, while realizing a path towards integration with materials science databases. Our approach illustrates the potential of combining machine learning and domain knowledge for scalable feature extraction in multidimensional data.

    R. Patrick Xian, Vincent Stimper, Marios Zacharias, Maciej Dendzik, Shuo Dong, Samuel Beaulieu, Bernhard Schölkopf, Martin Wolf, Laurenz Rettig, Christian Carbogno, Stefan Bauer, Ralph Ernstorfer
    Nat Comput Sci (2022) https://doi.org/10.1038/s43588-022-00382-2
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  20. (2014) Transient excitons at metal surfaces

    Excitons, electron–hole pairs bound by the Coulomb potential, are the fundamental quasiparticles of coherent light–matter interaction relevant for processes such as photosynthesis and optoelectronics. The existence of excitons in semiconductors is well established. For metals, however, although implied by the quantum theory of the optical response, experimental manifestations of excitons are tenuous owing to screening of the Coulomb interaction taking place on timescales of a few femtoseconds. Here we present direct evidence for the dominant transient excitonic response at a Ag(111) surface, which precedes the full onset of screening of the Coulomb interaction, in the course of a three-photon photoemission process with 15 fs laser pulses. During this transient regime, electron–hole pair Coulomb interactions introduce coherent quasiparticle correlations beyond the single-particle description of the optics of metals that dominate the multi-photon photoemission process on the timescale of screening at a Ag(111) surface.

    X. Cui, C. Wang, A. Argondizzo, S. Garrett-Roe, B. Gumhalter, H. Petek
    Nature Physics 10, pp. 505–509
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  21. (2018) The graphene/n-Ge(110) interface: structure, doping, and electronic properties

    The implementation of graphene in semiconducting technology requires precise knowledge about the graphene–semiconductor interface. In our work the structure and electronic properties of the graphene/n-Ge(110) interface are investigated on the local (nm) and macro (from μm to mm) scales via a combination of different microscopic and spectroscopic surface science techniques accompanied by density functional theory calculations. The electronic structure of freestanding graphene remains almost completely intact in this system, with only a moderate n-doping indicating weak interaction between graphene and the Ge substrate. With regard to the optimisation of graphene growth it is found that the substrate temperature is a crucial factor, which determines the graphene layer alignment on the Ge(110) substrate during its growth from the atomic carbon source. Moreover, our results demonstrate that the preparation route for graphene on the doped semiconducting material (n-Ge) leads to the effective segregation of dopants at the interface between graphene and Ge(110). Furthermore, it is shown that these dopant atoms might form regular structures at the graphene/Ge interface and induce the doping of graphene. Our findings help to understand the interface properties of the graphene–semiconductor interfaces and the effect of dopants on the electronic structure of graphene in such systems.

    J. Tesch, F. Paschke, M. Fonin, M. Wietstruk, S. Böttcher, R. J. Koch, A. Bostwick, C. Jozwiak, E. Rotenberg, A. Makarova, B. Paulus, E. Voloshina, Y. Dedkov
    Nanoscale, 10, pp. 6088-6098
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  22. (2008) Rashba effect in the graphene/Ni(111) system

    We report on angle-resolved photoemission studies of the electronic π states of high-quality epitaxial graphene layers on a Ni(111) surface. In this system the electron binding energy of the π states shows a strong dependence on the magnetization reversal of the Ni film. The observed extraordinarily large energy shift up to 225 meV of the graphene-derived π band peak position for opposite magnetization directions is attributed to a manifestation of the Rashba interaction between spin-polarized electrons in the π band and the large effective electric field at the graphene/Ni interface. Our findings show that an electron spin in the graphene layer can be manipulated in a controlled way and have important implications for graphene-based spintronic devices.

    Yu. S. Dedkov, M. Fonin, U. Rüdiger, C. Laubschat
    Phys. Rev. Lett. 100, 107602
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  23. (2006) Ferromagnetic coupling in Eu/Gd (0001) observed by spin-resolved photoelectron spectroscopy

    We report on a magnetic analysis by means of spin-resolved photoelectron spectroscopy of an atomically flat heteromagnetic rare-earth interface of 1 ML Eu∕Gd(0001). The measurements reveal a high net Eu magnetization at low temperatures reflected by a spin polarization ∼15% of the Eu 4f state. This magnetic Eu configuration is due to a strong ferromagnetic interlayer exchange coupling across the Eu∕Gd interface which overcomes a weak negative intralayer coupling between Eu spins in the hexagonal two-dimensional lattice.

    Yu. S. Dedkov, Th. Kleissner, E. N. Voloshina, S. Danzenbächer, S. L. Molodtsov, C. Laubschat
    Physical Review B 73, 012402
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  24. (2007) Evidence for the short-period oscillations in spin-resolved photoemission of thin Cr(110) films

    The spin-resolved electronic structure of thin Cr overlayers on top of the Fe(110) surface was investigated by means of spin- and angle-resolved photoelectron spectroscopy. The initial fast drop of photoelectron spin-polarization at the Fermi level, followed by weak oscillatory behavior with the period of about 2 ML, can give an evidence for the first time spectroscopic observation of the short period oscillations in (110)-oriented thin Cr films.

    Yu. S. Dedkov
    Eur. Phys. J. B 57, pp. 15-19
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  25. (2006) Electron localization in metallic quantum wells: Pb versus In on Si(111)

    Two-dimensional quantum well states in ultrathin metal films generally exhibit a dispersion relation of s-p-derived states that can be described through an effective mass of the corresponding bulk band. By contrast, the effective masses in Pb quantum well states on Si(111), measured through angle-resolved photoemission, are up to an order of magnitude larger than those from the bulk states or predicted by slab calculations, while similar anomalies are not observed in the related In∕Si(111) system. We interpret these data in terms of an enhanced electron localization, and use them to interpret recent scanning tunneling microscopy results.

    J. H. Dil, J. W. Kim, Th. Kampen, K. Horn, A. R. H. F. Ettema
    Physical Review B 73, 161308 (R)
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  26. (2014) Direct observation of spin-resolved full and empty electron states in ferromagnetic surfaces

    We present a versatile apparatus for the study of ferromagnetic surfaces, which combines spin-polarized photoemission and inverse photoemission spectroscopies. Samples can be grown by molecular beam epitaxy and analyzed in situ. Spin-resolved photoemission spectroscopy analysis is done with a hemispherical electron analyzer coupled to a 25 kV-Mott detector. Inverse photoemission spectroscopy experiments are performed with GaAs crystals as spin-polarized electron sources and a UV bandpass photon detector. As an example, measurements on the oxygen passivated Fe(100)-p(1×1)O surface are presented.

    G. Berti, A. Calloni, A. Brambilla, G. Bussetti, L. Duò, F. Ciccacci
    Review of Scientific Instruments 85, 073901
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  27. (2017) Direct 3D mapping of the Fermi surface and Fermi velocity

    We performed a full mapping of the bulk electronic structure including the Fermi surface and Fermi-velocity distribution vF(kF) of tungsten. The 4D spectral function ρ(EB; k) in the entire bulk Brillouin zone and 6 eV binding-energy (EB) interval was acquired in ∼3 h thanks to a new multidimensional photoemission data-recording technique (combining full-field k-microscopy with time-of-flight parallel energy recording) and the high brilliance of the soft X-rays used. A direct comparison of bulk and surface spectral functions (taken at low photon energies) reveals a time-reversal-invariant surface state in a local bandgap in the (110)-projected bulk band structure. The surface state connects hole and electron pockets that would otherwise be separated by an indirect local bandgap. We confirmed its Dirac-like spin texture by spin-filtered momentum imaging. The measured 4D data array enables extraction of the 3D dispersion of all bands, all energy isosurfaces, electron velocities, hole or electron conductivity, effective mass and inner potential by simple algorithms without approximations. The high-Z bcc metals with large spin–orbit-induced bandgaps are discussed as candidates for topologically non-trivial surface states.

    K. Medjanik, O. Fedchenko, S. Chernov, D. Kutnyakhov, M. Ellguth, A. Oelsner, B. Schönhense, T. R. F. Peixoto, P. Lutz, C.-H. Min, F. Reinert, S. Däster, Y. Acremann, J. Viefhaus, W. Wurth, H. J. Elmers, G. Schönhense
    Nature Materials 16, pp. 615–621
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  28. (2007) Angle-resolved photoemission spectroscopy with a femtosecond high harmonic light source using a two-dimensional imaging electron analyser

    An experimental setup for time- and angle-resolved photoemission spectroscopy using a femtosecond 1 kHz high harmonic light source and a two-dimensional electron analyzer for parallel energy and momentum detection is presented. A selection of the 27th harmonic (41.85 eV) from the harmonic spectrum of the light source is achieved with a multilayer Mo∕Si double mirror monochromator. The extinction efficiency of the monochromator in selecting this harmonic is shown to be better than 7:1, while the transmitted bandwidth of the selected harmonic is capable of supporting temporal pulse widths as short as 3fs. The recorded E(k) photoelectron spectrum from a Cu(111) surface demonstrates an angular resolution of better than 0.6° (=0.03 Å−1 at Ekin,e=36 eV). Used in a pump-probe configuration, the described experimental setup represents a powerful experimental tool for studying the femtosecond dynamics of ultrafast surface processes in real time.

    S. Mathias, L. Miaja-Avila, M. M. Murnane, H. Kapteyn, M. Aeschlimann, M. Bauer
    Review of Scientific Instruments 78, 083105
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