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Kurtz F, Dauwe TN, Yalunin SV, Storeck G, Horstmann JG, Böckmann H, Ropers C. Non-thermal phonon dynamics and a quenched exciton condensate probed by surface-sensitive electron diffraction. NATURE MATERIALS 2024; 23:890-897. [PMID: 38688990 PMCID: PMC11230895 DOI: 10.1038/s41563-024-01880-6] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Received: 03/16/2023] [Accepted: 03/26/2024] [Indexed: 05/02/2024]
Abstract
Interactions among and between electrons and phonons steer the energy flow in photo-excited materials and govern the emergence of correlated phases. The strength of electron-phonon interactions, decay channels of strongly coupled modes and the evolution of three-dimensional order are revealed by electron or X-ray pulses tracking non-equilibrium structural dynamics. Despite such capabilities, the growing relevance of inherently anisotropic two-dimensional materials and functional heterostructures still calls for techniques with monolayer sensitivity and, specifically, access to out-of-plane phonon polarizations. Here, we resolve non-equilibrium phonon dynamics and quantify the excitonic contribution to the structural order parameter in 1T-TiSe2. To this end, we introduce ultrafast low-energy electron diffuse scattering and trace strongly momentum- and fluence-dependent phonon populations. Mediated by phonon-phonon scattering, a few-picosecond build-up near the zone boundary precedes a far slower generation of zone-centre acoustic modes. These weakly coupled phonons are shown to substantially delay overall equilibration in layered materials. Moreover, we record the surface structural response to a quench of the material's widely investigated exciton condensate, identifying an approximate 30:70 ratio of excitonic versus Peierls contributions to the total lattice distortion in the charge density wave phase. The surface-sensitive approach complements the ultrafast structural toolbox and may further elucidate the impact of phonon scattering in numerous other phenomena within two-dimensional materials, such as the formation of interlayer excitons in twisted bilayers.
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Affiliation(s)
- Felix Kurtz
- Department of Ultrafast Dynamics, Max Planck Institute for Multidisciplinary Sciences, Göttingen, Germany
- 4th Physical Institute, Solids and Nanostructures, University of Göttingen, Göttingen, Germany
| | - Tim N Dauwe
- Department of Ultrafast Dynamics, Max Planck Institute for Multidisciplinary Sciences, Göttingen, Germany
- 4th Physical Institute, Solids and Nanostructures, University of Göttingen, Göttingen, Germany
| | - Sergey V Yalunin
- Department of Ultrafast Dynamics, Max Planck Institute for Multidisciplinary Sciences, Göttingen, Germany
- 4th Physical Institute, Solids and Nanostructures, University of Göttingen, Göttingen, Germany
| | - Gero Storeck
- Department of Ultrafast Dynamics, Max Planck Institute for Multidisciplinary Sciences, Göttingen, Germany
- 4th Physical Institute, Solids and Nanostructures, University of Göttingen, Göttingen, Germany
| | - Jan Gerrit Horstmann
- Department of Ultrafast Dynamics, Max Planck Institute for Multidisciplinary Sciences, Göttingen, Germany
- Department of Materials, ETH Zurich, Zurich, Switzerland
| | - Hannes Böckmann
- Department of Ultrafast Dynamics, Max Planck Institute for Multidisciplinary Sciences, Göttingen, Germany
- 4th Physical Institute, Solids and Nanostructures, University of Göttingen, Göttingen, Germany
| | - Claus Ropers
- Department of Ultrafast Dynamics, Max Planck Institute for Multidisciplinary Sciences, Göttingen, Germany.
- 4th Physical Institute, Solids and Nanostructures, University of Göttingen, Göttingen, Germany.
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Epp D, Schröder B, Möller M, Ropers C. Gigahertz streaking and compression of low-energy electron pulses. STRUCTURAL DYNAMICS (MELVILLE, N.Y.) 2024; 11:024306. [PMID: 38566809 PMCID: PMC10987198 DOI: 10.1063/4.0000235] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Figures] [Subscribe] [Scholar Register] [Received: 12/08/2023] [Accepted: 03/04/2024] [Indexed: 04/04/2024]
Abstract
Although radio frequency (RF) technology is routinely employed for controlling high-energy pulses of electrons, corresponding technology has not been developed at beam energies below several kiloelectronvolts. In this work, we demonstrate transverse and longitudinal phase-space manipulation of low-energy electron pulses using RF fields. A millimeter-sized photoelectron gun is combined with synchronized streaking and compression cavities driven at frequencies of 0.5 and 2.5 GHz , respectively. The phase-controlled acceleration and deceleration of photoelectron pulses is characterized in the energy range of 50 -100 eV . Deflection from a transient space-charge cloud at a metal grid is used to measure a fourfold compression of 80 - eV electron pulses, from τ = 34 to τ = 8 ps pulse duration.
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Affiliation(s)
| | | | | | - Claus Ropers
- Author to whom correspondence should be addressed:. URL:https://www.mpinat.mpg.de/de/ropers
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Horn-von Hoegen M. Structural dynamics at surfaces by ultrafast reflection high-energy electron diffraction. STRUCTURAL DYNAMICS (MELVILLE, N.Y.) 2024; 11:021301. [PMID: 38495951 PMCID: PMC10942804 DOI: 10.1063/4.0000234] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Figures] [Subscribe] [Scholar Register] [Received: 12/08/2023] [Accepted: 02/13/2024] [Indexed: 03/19/2024]
Abstract
Many fundamental processes of structural changes at surfaces occur on a pico- or femtosecond timescale. In order to study such ultrafast processes, we have combined modern surface science techniques with fs-laser pulses in a pump-probe scheme. Grazing incidence of the electrons ensures surface sensitivity in ultrafast reflection high-energy electron diffraction (URHEED). Utilizing the Debye-Waller effect, we studied the nanoscale heat transport from an ultrathin film through a hetero-interface or the damping of vibrational excitations in monolayer adsorbate systems on the lower ps-timescale. By means of spot profile analysis, the different cooling rates of epitaxial Ge nanostructures of different size and strain state were determined. The excitation and relaxation dynamics of a driven phase transition far away from thermal equilibrium is demonstrated using the In-induced (8 × 2) reconstruction on Si(111). This Peierls-distorted surface charge density wave system exhibits a discontinuous phase transition of first order at 130 K from a (8 × 2) insulating ground state to (4 × 1) metallic excited state. Upon excitation by a fs-laser pulse, this structural phase transition is non-thermally driven in only 700 fs into the excited state. A small barrier of 40 meV hinders the immediate recovery of the ground state, and the system is found in a metastable supercooled state for up to few nanoseconds.
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Affiliation(s)
- Michael Horn-von Hoegen
- Department of Physics and Center for Nanointegration CENIDE, University of Duisburg-Essen, Lotharstrasse. 1, 47057 Duisburg, Germany
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Lee C, Kassier GH, Miller RJD. High bunch charge low-energy electron streak diffraction. STRUCTURAL DYNAMICS (MELVILLE, N.Y.) 2024; 11:024309. [PMID: 38595978 PMCID: PMC11003762 DOI: 10.1063/4.0000246] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Figures] [Subscribe] [Scholar Register] [Received: 01/15/2024] [Accepted: 03/07/2024] [Indexed: 04/11/2024]
Abstract
For time-resolved diffraction studies of irreversible structural dynamics upon photoexcitation, there are constraints on the number of perturbation cycles due to thermal effects and accumulated strain, which impact the degree of crystal order and spatial resolution. This problem is exasperated for surface studies that are more prone to disordering and defect formation. Ultrafast electron diffraction studies of these systems, with the conventional stroboscopic pump-probe protocol, require repetitive measurements on well-prepared diffraction samples to acquire and average signals above background in the dynamic range of interest from few tens to hundreds of picoseconds. Here, we present ultrafast streaked low-energy electron diffraction (LEED) that demands, in principle, only a single excitation per nominal data acquisition timeframe. By exploiting the space-time correlation characteristics of the streaking method and high-charge 2 keV electron bunches in the transmission geometry, we demonstrate about one order of magnitude reduction in the accumulated number of the excitation cycles and total electron dose, and 48% decrease in the root mean square error of the model fit residual compared to the conventional time-scanning measurement. We believe that our results demonstrate a viable alternative method with higher sensitivity to that of nanotip-based ultrafast LEED studies relying on a few electrons per a single excitation, to access to all classes of structural dynamics to provide an atomic level view of surface processes.
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Affiliation(s)
- Chiwon Lee
- Departments of Chemistry and Physics, University of Toronto, 80 St. George Street, Toronto, Ontario M5S 3H6, Canada
| | - Günther H. Kassier
- Center for Free-Electron Laser Science CFEL, Deutsches Elektronen-Synchrotron DESY, Notkestrasse 85, 22607 Hamburg, Germany
| | - R. J. Dwayne Miller
- Departments of Chemistry and Physics, University of Toronto, 80 St. George Street, Toronto, Ontario M5S 3H6, Canada
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Bie YQ, Zong A, Wang X, Jarillo-Herrero P, Gedik N. A versatile sample fabrication method for ultrafast electron diffraction. Ultramicroscopy 2021; 230:113389. [PMID: 34530284 DOI: 10.1016/j.ultramic.2021.113389] [Citation(s) in RCA: 4] [Impact Index Per Article: 1.3] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/10/2021] [Revised: 08/21/2021] [Accepted: 08/30/2021] [Indexed: 11/16/2022]
Abstract
Integral to the exploration of nonequilibrium phenomena in solid-state systems is the study of lattice motion after photoexcitation by a femtosecond laser pulse. For the past two decades, ultrafast electron diffraction (UED) has played a critical role in this regard. Despite remarkable progress in instrumental development, this technique is still bottlenecked by a demanding sample preparation process, where ultrathin single crystals of large lateral size are typically required. In this work, we describe an efficient, versatile method that yields high-quality, laterally extended (≥ 100 µm), and thin (≤ 50 nm) single crystals on amorphous films of Si3N4 windows. It applies to most exfoliable materials, including those reactive in ambient conditions, and promises clean, flat surfaces. Besides the natural extension to fabricating van der Waals heterostructures, our method can also be applied to future-generation UED that enables additional control of sample parameters, such as electrostatic gating and excitation by a locally enhanced terahertz field. Our work significantly expands the type of samples for UED studies and also finds application in other time-resolved techniques such as attosecond extreme-ultraviolet absorption spectroscopy. This method hence provides further opportunities to explore photoinduced transitions and to discover novel states of matter out of equilibrium.
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Affiliation(s)
- Ya-Qing Bie
- State Key Lab of Optoelectronic Materials and Technologies, Guangdong Province Key Laboratory of Display Material and Technology, School of Electronics and Information Technology, Sun Yat-sen University, Guangzhou, 510275, China; Massachusetts Institute of Technology, Department of Physics, Cambridge, MA 02139, United States
| | - Alfred Zong
- Massachusetts Institute of Technology, Department of Physics, Cambridge, MA 02139, United States; University of California at Berkeley, Department of Chemistry, Berkeley, CA 94720, United States
| | - Xirui Wang
- Massachusetts Institute of Technology, Department of Physics, Cambridge, MA 02139, United States
| | - Pablo Jarillo-Herrero
- Massachusetts Institute of Technology, Department of Physics, Cambridge, MA 02139, United States
| | - Nuh Gedik
- Massachusetts Institute of Technology, Department of Physics, Cambridge, MA 02139, United States.
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Horstmann JG, Böckmann H, Wit B, Kurtz F, Storeck G, Ropers C. Coherent control of a surface structural phase transition. Nature 2020; 583:232-236. [PMID: 32641815 DOI: 10.1038/s41586-020-2440-4] [Citation(s) in RCA: 40] [Impact Index Per Article: 10.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/21/2019] [Accepted: 03/31/2020] [Indexed: 11/09/2022]
Abstract
Active optical control over matter is desirable in many scientific disciplines, with prominent examples in all-optical magnetic switching1,2, light-induced metastable or exotic phases of solids3-8 and the coherent control of chemical reactions9,10. Typically, these approaches dynamically steer a system towards states or reaction products far from equilibrium. In solids, metal-to-insulator transitions are an important target for optical manipulation, offering ultrafast changes of the electronic4 and lattice11-16 properties. The impact of coherences on the efficiencies and thresholds of such transitions, however, remains a largely open subject. Here, we demonstrate coherent control over a metal-insulator structural phase transition in a quasi-one-dimensional solid-state surface system. A femtosecond double-pulse excitation scheme17-20 is used to switch the system from the insulating to a metastable metallic state, and the corresponding structural changes are monitored by ultrafast low-energy electron diffraction21,22. To govern the transition, we harness vibrational coherence in key structural modes connecting both phases, and observe delay-dependent oscillations in the double-pulse switching efficiency. Mode-selective coherent control of solids and surfaces could open new routes to switching chemical and physical functionalities, enabled by metastable and non-equilibrium states.
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Affiliation(s)
- Jan Gerrit Horstmann
- 4th Physical Institute, Solids and Nanostructures, University of Göttingen, Göttingen, Germany
| | - Hannes Böckmann
- 4th Physical Institute, Solids and Nanostructures, University of Göttingen, Göttingen, Germany
| | - Bareld Wit
- 4th Physical Institute, Solids and Nanostructures, University of Göttingen, Göttingen, Germany
| | - Felix Kurtz
- 4th Physical Institute, Solids and Nanostructures, University of Göttingen, Göttingen, Germany
| | - Gero Storeck
- 4th Physical Institute, Solids and Nanostructures, University of Göttingen, Göttingen, Germany
| | - Claus Ropers
- 4th Physical Institute, Solids and Nanostructures, University of Göttingen, Göttingen, Germany. .,Max Planck Institute for Biophysical Chemistry, Göttingen, Germany.
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Storeck G, Horstmann JG, Diekmann T, Vogelgesang S, von Witte G, Yalunin SV, Rossnagel K, Ropers C. Structural dynamics of incommensurate charge-density waves tracked by ultrafast low-energy electron diffraction. STRUCTURAL DYNAMICS (MELVILLE, N.Y.) 2020; 7:034304. [PMID: 32596414 PMCID: PMC7311179 DOI: 10.1063/4.0000018] [Citation(s) in RCA: 5] [Impact Index Per Article: 1.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Figures] [Subscribe] [Scholar Register] [Received: 04/07/2020] [Accepted: 04/13/2020] [Indexed: 06/11/2023]
Abstract
We study the non-equilibrium structural dynamics of the incommensurate and nearly commensurate charge-density wave (CDW) phases in 1T-TaS 2 . Employing ultrafast low-energy electron diffraction with 1 ps temporal resolution, we investigate the ultrafast quench and recovery of the CDW-coupled periodic lattice distortion (PLD). Sequential structural relaxation processes are observed by tracking the intensities of main lattice as well as satellite diffraction peaks and the diffuse scattering background. Comparing distinct groups of diffraction peaks, we disentangle the ultrafast quench of the PLD amplitude from phonon-related reductions of the diffraction intensity. Fluence-dependent relaxation cycles reveal a long-lived partial suppression of the order parameter for up to 60 ps, far outlasting the initial amplitude recovery and electron-phonon scattering times. This delayed return to a quasi-thermal level is controlled by lattice thermalization and coincides with the population of zone-center acoustic modes, as evidenced by a structured diffuse background. The long-lived non-equilibrium order parameter suppression suggests hot populations of CDW-coupled lattice modes. Finally, a broadening of the superlattice peaks is observed at high fluences, pointing to a non-linear generation of phase fluctuations.
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Affiliation(s)
- G. Storeck
- 4th Physical Institute, Solids and Nanostructures, University of Göttingen, 37077 Göttingen, Germany
| | - J. G. Horstmann
- 4th Physical Institute, Solids and Nanostructures, University of Göttingen, 37077 Göttingen, Germany
| | - T. Diekmann
- 4th Physical Institute, Solids and Nanostructures, University of Göttingen, 37077 Göttingen, Germany
| | - S. Vogelgesang
- 4th Physical Institute, Solids and Nanostructures, University of Göttingen, 37077 Göttingen, Germany
| | - G. von Witte
- 4th Physical Institute, Solids and Nanostructures, University of Göttingen, 37077 Göttingen, Germany
| | - S. V. Yalunin
- 4th Physical Institute, Solids and Nanostructures, University of Göttingen, 37077 Göttingen, Germany
| | | | - C. Ropers
- Author to whom correspondence should be addressed:
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Abstract
A review that summarizes the most recent technological developments in the field of ultrafast structural dynamics with focus on the use of ultrashort X-ray and electron pulses follows. Atomistic views of chemical processes and phase transformations have long been the exclusive domain of computer simulators. The advent of femtosecond (fs) hard X-ray and fs-electron diffraction techniques made it possible to bring such a level of scrutiny to the experimental area. The following review article provides a summary of the main ultrafast techniques that enabled the generation of atomically resolved movies utilizing ultrashort X-ray and electron pulses. Recent advances are discussed with emphasis on synchrotron-based methods, tabletop fs-X-ray plasma sources, ultrabright fs-electron diffractometers, and timing techniques developed to further improve the temporal resolution and fully exploit the use of intense and ultrashort X-ray free electron laser (XFEL) pulses.
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Horstmann JG, Storeck G, Wit B, Diekmann T, Epp D, Rossnagel K, Schäfer S, Vogelgesang S, Ropers C. Structural phase transitions and phase ordering at surfaces probed by ultrafast LEED. EPJ WEB OF CONFERENCES 2019. [DOI: 10.1051/epjconf/201920508005] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/15/2022] Open
Abstract
We demonstrate the capability of ultrafast low-energy electron diffraction to resolve phase-ordering kinetics and structural phase transitions on their intrinsic time scales with ultimate surface sensitivity.
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