Electric Switching of the Charge-Density-Wave and Normal Metallic Phases in Tantalum Disulfide Thin-Film Devices

We report on switching among three charge-density-wave phases - commensurate, nearly commensurate, incommensurate - and the high-temperature normal metallic phase in thin-film 1T-TaS2 devices induced by application of an in-plane electric field. The electric switching among all phases has been achieved over a wide temperature range, from 77 K to 400 K. The low-frequency electronic noise spectroscopy has been used as an effective tool for monitoring the transitions, particularly the switching from the incommensurate charge-density-wave phase to the normal metal phase. The noise spectral density exhibits sharp increases at the phase transition points, which correspond to the step-like changes in resistivity. Assignment of the phases is consistent with low-field resistivity measurements over the temperature range from 77 K to 600 K. Analysis of the experimental data and calculations of heat dissipation suggest that Joule heating plays a dominant role in the electric-field induced transitions in the tested 1T-TaS2 devices on Si/SiO2 substrates. The possibility of electrical switching among four different phases of 1T-TaS2 is a promising step toward nanoscale device applications. The results also demonstrate the potential of noise spectroscopy for investigating and identifying phase transitions in materials.Comment: 32 pages, 7 figure

Structural and Magnetic Characterization of Large Area, Free-Standing Thin Films of Magnetic Ion Intercalated Dichalcogenides Mn0.25TaS2 and Fe0.25TaS2

Free-standing thin films of magnetic ion intercalated transition metal dichalcogenides are produced using ultramicrotoming techniques. Films of thicknesses ranging from 30nm to 250nm were achieved and characterized using transmission electron diffraction and X-ray magnetic circular dichroism. Diffraction measurements visualize the long range crystallographic ordering of the intercalated ions, while the dichroism measurements directly assess the orbital contributions to the total magnetic moment. We thus verify the unquenched orbital moment in Fe0.25TaS2 and measure the fully quenched orbital contribution in Mn0.25TaS2. Such films can be used in a wide variety of ultrafast X-ray and electron techniques that benefit from transmission geometries, and allow measurements of ultrafast structural, electronic, and magnetization dynamics in space and time

Concurrent probing of electron-lattice dephasing induced by photoexcitation in 1T-TaSeTe using ultrafast electron diffraction

It has been technically challenging to concurrently probe the electrons and the lattices in materials during non-equilibrium processes, allowing their correlations to be determined. Here, in a single set of ultrafast electron diffraction patterns taken on the charge-density-wave (CDW) material 1T-TaSeTe, we discover a temporal shift in the diffraction intensity measurements as a function of scattering angle. With the help of dynamic models and theoretical calculations, we show that the ultrafast electrons probe both the valence-electron and lattice dynamic processes, resulting in the temporal shift measurements. Our results demonstrate unambiguously that the CDW is not merely a result of the periodic lattice deformation ever-present in 1T-TaSeTe but has significant electronic origin. This method demonstrates a novel approach for studying many quantum effects that arise from electron-lattice dephasing in molecules and crystals for next-generation devices.Comment: 13 pages and 4 figures in main tex

Ultrafast transmission electron microscopy of a structural phase transition

Große Hoffnungen für zukünftige Anwendungen im Gebiet der Energieumwandlung werden auf Materialien mit abstimmbaren Eigenschaften und Anregungen gesetzt. Die Funktionalität miniaturisierter Systeme ergibt sich jedoch nicht nur aus den Eigenschaften der einzelnen Materialien, sondern auch aus deren Zusammenspiel und nanoskaliger Strukturierung. Während eine Reihe etablierter experimenteller Techniken in der Lage ist, elektronische Anregungen auf Femtosekunden-Zeit- und Nanometer-Längenskalen zu verfolgen, wurde bisher über keine zeitaufgelöste Nano-Abbildung eines strukturellen Ordnungsparameters berichtet. Die vorliegende kumulative Dissertation behandelt die Entwicklung zeitaufgelöster Dunkelfeld-Bildgebung am Ultraschnellen Transmissions-Elektronenmikroskop (UTEM) in Göttingen. Dieser Ansatz kombiniert Femtosekunden-Zeitauflösung und eine räumliche Auflösung von 5 nm mit einer Empfindlichkeit für die strukturelle Komponente eines Ladungsdichtewellen-Phasenübergangs im 1T-Polytyp des Materials Tantaldisulfid. Ultrakurze Laserpulse induzieren lokal den Phasenübergang, während die raumzeitliche Relaxationsdynamik des strukturellen Ordnungsparameters mit ultrakurzen Elektronenpulsen verfolgt wird. Die Empfindlichkeit für den Ordnungsparameter wird mithilfe einer komplexen Dunkelfeld-Apertur erreicht. In einer ersten Veröffentlichung wird die Technik zur Präparation der dünnen Schichten aus Tantaldisulfid vorgestellt. Die durch Ultramikrotomie gewonnenen Proben sind ideal für Elektronen- und Röntgenexperimente in einer Transmissionsgeometrie, wie die exemplarische Untersuchung von mit Mangan und Eisen interkaliertem Tantaldisulfid zeigt. Statische optische Mikroskopie, Elektronenbeugung und Messungen des zirkularen magnetischen Röntgendichroismus dienen dazu, diese ferromagnetischen Dünnschichten zu charakterisieren und zu bestätigen, dass ihre Eigenschaften denen der ursprünglichen Kristalle entsprechen. Ein zweiter Artikel beschreibt die Umsetzung der zeitaufgelösten Nano-Abbildung. Ein zentraler Aspekt des Experiments ist die Herstellung einer Probe, die das optische Anregungsprofil räumlich strukturiert und gleichzeitig eine stroboskopische Untersuchung des Phasenübergangs in Tantaldisulfid bei Wiederholraten von hunderten Kilohertz ermöglicht. Basierend auf Parametern, die in einem stationären Heizexperiment gewonnen wurden, kann das Verhalten von nanoskaligen Ladungsdichtewellen-Domänen in der freistehenden Dünnschicht in zeitabhängigen Ginzburg-Landau-Simulationen reproduziert werden. Abschließend werden Perspektiven für zukünftige Experimente auf Basis des vorgestellten Ansatzes diskutiert. Ultraschnelle Dunkelfeld-Bildgebung ermöglicht eine Empfindlichkeit auch für weitere strukturelle Freiheitsgrade in komplexen Materialien und wird so zu einem besseren Verständnis aktiv kontrollierter Prozesse auf dem Gebiet der Energieumwandlung beitragen.High hopes are placed on materials with tunable properties and excitations for future applications in energy conversion devices. Functionality of devices, however, not only arises from the properties of individual materials but also from their interplay and nanoscale structuring. While a number of established experimental techniques are capable of tracking electronic excitations on femtosecond time and nanometer length scales, no time-resolved nanoimaging of a structural order parameter had previously been reported. Addressing this challenge, the present cumulative thesis reports on the development and application of a time-resolved dark-field electron microscopy scheme implemented at the Göttingen Ultrafast Transmission Electron Microscope (UTEM). This nanoimaging approach combines femtosecond temporal and 5 nm spatial resolution with sensitivity to the structural component of a charge-density wave phase transition in 1T-polytype tantalum disulfide. Ultrashort laser pulses locally induce the phase transition, while the subsequent spatiotemporal relaxation dynamics of the structural order parameter is tracked using ultrashort electron pulses. Order parameter sensitivity is obtained by means of a dark-field aperture array, tailored to filter the periodicities of the charge-density wave in the diffraction plane of the microscope. In the first publication contributing to this thesis, the preparation technique for the thin films of tantalum disulfide is introduced. Specimens obtained by ultramicrotomy are ideal for electron and x-ray experiments in a transmission geometry, as exemplified by the investigation of manganese- and iron-intercalated tantalum disulfide. Static optical microscopy, electron diffraction and x-ray magnetic circular dichroism measurements serve to characterize these ferromagnetic thin films and to verify that the properties reflect those of the bulk crystals. The second article describes the implementation of the ultrafast nanoimaging approach. A central aspect of the experiment is the design of a specimen that spatially structures the optical excitation pattern and allows for stroboscopic probing of the phase transition in tantalum disulfide at hundreds of kilohertz repetition rates. Based on parameters extracted from a steady-state heating experiment, the optically induced evolution of nanoscale charge-density wave domains in the free-standing thin film is reproduced in time-dependent Ginzburg-Landau simulations. Finally, perspectives for future nanoimaging experiments are discussed. Allowing for sensitivity to further structural degrees of freedom in complex materials, ultrafast dark-field imaging will contribute to a better understanding of actively controlled processes in energy conversion devices.2021-08-1

ULTRAFAST ELECTRON MICROSCOPY INNOVATIONS AND DUAL ULTRAFAST PROBE STUDIES OF VANADIUM DIOXIDE

Thesis (Ph.D.)--Michigan State University. Physics - Doctor of Philosophy, 2024This dissertation investigates the nonequilibrium physics of VO2 phase transition using a dual-probe approach combining ultrafast electron diffraction (UED) and ultrafast optical differential transmittance measurements, enabled by advancements in RF compression techniques for ultrafast electron microscopy and diffraction. By simultaneously tracking the structural and electronic responses with enhanced momentum resolution, this work provides new insights into the cooperativity and competing mechanisms underlying the photoinduced phase transition (PIPT) in VO2.The development of a cascade RF control system, featuring a two-level PID feedback loop, significantly reduces noise and instabilities in the RF system. The experimental validation of this upgraded RF system demonstrates a temporal resolution of 48 50 fs (FWHM) and a spatial resolution of 10 femtometers, pushing the limits of ultrafast electron probe technology. Leveraging these advancements, the dual-probe measurements reveal a multi-threshold nonequilibrium phenomenology in VO2 that deviates from the behavior of thermally-induced phase transitions. A critical fluence threshold Fc 48 4.5 mJ/cm2 is identified for ultrafast insulator-to-metal transition (IMT) mediated by polaron formation localized to the V-V sublattice, establishing a transient polaronic metallic (pM) state. This ultrafast IMT pathway is distinct from the thermally driven process and unaffected by lattice strain, indicating a different organizing principle at the initial stage. However, at longer timescales, the system converges back to the thermal phases governed by strong electron-phonon coupling, where IMT and structural phase transition (SPT) remain tightly coupled. The dual-probe measurements, supported by an effective medium theory, disentangle the competing effects of photoexcitation, polaron formation, and metallic domain growth, providing a unified picture connecting the nonequilibrium and equilibrium regimes. These findings advance the understanding of PIPT in VO2 and highlight the power of combining UED and ultrafast optical probes for unraveling complex phase transition dynamics in strongly correlated materials.Description based on online resource. Title from PDF t.p. (Michigan State University Fedora Repository, viewed ).Includes bibliographical references

Excitation and Relaxation Dynamics of the Photo-Perturbed Correlated Electron System 1T-TaS2

We investigate the perturbation and subsequent recovery of the correlated electronic ground state of the Mott insulator 1T-TaS2 by means of femtosecond time-resolved photoemission spectroscopy in normal emission geometry. Upon an increase of near-infrared excitation strength, a considerable collapse of the occupied Hubbard band is observed, which reflects a quench of short-range correlations. It is furthermore found that these excitations are directly linked to the lifting of the periodic lattice distortion which provides the localization centers for the formation of the insulating Mott state. We discuss the observed dynamics in a localized real-space picture

Probing transitions and phase-ordering of charge-density waves

Due to their reduced dimensionality, surfaces and quasi two-dimensional materials exhibit numerous intriguing physical phenomena that drastically differ from the bulk. To resolve these effects and the associated dynamics at their intrinsic timescales requires experimental methodologies combining a high surface sensitivity with the essential temporal resolution. However, to date, there are still very few methods that facilitate investigation of the structural degrees of freedom of surfaces on the atomic scale along with a temporal resolution of femtoseconds or picoseconds. Addressing these challenges, this thesis covers the development and application of ultrafast low-energy electron diffraction in a backscattering geometry to study structural dynamics at surfaces. In this context, a central aspect is the development of a miniaturized and laser-driven electron source based on a nanometric needle photocathode. Using such a sharp metal tip, the photoemitted electron bunches offer a particularly high coherence and remarkably short pulse durations, which were also successfully implemented recently in ultrafast transmission electron microscopy, as well as in time-resolved transmission low-energy electron diffraction. Employing the capabilities of this novel technique, so-called transition metal dichalcogenides constitute an ideal prototype system. Specifically, in the present work, the transient structural disorder of charge-density waves at the surface of 1T-TaS2 has been examined. Following the optically induced transition between two temperature-dependent charge-density wave phases, this method enables the observation of a highly disordered transient state and the subsequent phase-ordering kinetics. More precisely, the temporal evolution of the growing charge-density correlation length is traced over several hundreds of picoseconds and found to obey a power-law scaling behavior. Due to the particular properties of the charge-density wave system at hand, the observed transient disorder can be explained by the ultrafast formation of topological defects and their subsequent annihilation. These results are complemented by a numerical modeling using a timedependent Ginzburg-Landau approach. Finally, two different excitation schemes demonstrating the possibility to study the relaxation of the investigated sample on the nanosecond and microsecond timescale are presented, as well as future prospects of ultrafast low-energy electron diffraction, including other promising surface sample systems