Field-induced ultrafast modulation of Rashba coupling at room temperature in ferroelectric α\alpha-GeTe(111)

Rashba materials have appeared as an ideal playground for spin-to-charge conversion in prototype spintronics devices. Among them, $\alpha$-GeTe(111) is a non-centrosymmetric ferroelectric (FE) 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 (fs) 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 $\alpha$-GeTe(111) using time and angle-resolved photoemission spectroscopy (tr-ARPES). Our experiments reveal an ultrafast modulation of the Rashba coupling mediated on the fs timescale by a surface photovoltage (SPV), namely an increase corresponding to a 13 % enhancement of the lattice distortion. This opens the route for the control of the FE polarization in $\alpha$-GeTe(111) and FE semiconducting materials in quantum heterostructures.Comment: 31 pages, 12 figure

Field-induced ultrafast modulation of Rashba coupling at room temperature in ferroelectric alpha-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.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

Ultraschnelle Elektronendynamik in quasi-zweidimensionalen Quantenmaterialien

Quantum materials are solids with tantalizing properties arising from special symmetry, dimensionality, topology, and many-body interactions between elementary degrees of freedom (charge, spin, orbital, and lattice). They are host to fascinating emergent phenomena such as unconventional superconductivity, Mott transitions, charge density waves (CDWs), and topologically-protected electronic states, and hold promise for revolutionizing electricity generation and distribution, (quantum) computing, and data storage. Gaining a microscopic understanding of quantum materials to experimentally realize and control many-body phases is one of the overarching goals of modern condensed-matter physics. A promising pathway to fulfilling this goal are ultrashort optical excitations. Tracking the response of a broken-symmetry state after perturbation by a light pulse grants access to the relevant many-body interactions governing the emergence of equilibrium quantum states. Additionally, the interaction of quantum materials with light can induce novel emergent phenomena by steering a system towards specific transient or metastable states, facilitating control over additional functionalities within the light-enriched phase diagram. This thesis explores the electronic structure and ultrafast dynamics of several single-layer and layered quasi-2D quantum materials using femtosecond time- and angle-resolved photoemission spectroscopy (trARPES). We first establish a novel time-of-flight-based photoelectron detector for trARPES, a momentum microscope, and benchmark its performance against the widely used hemispherical analyzers. Next, we utilize the complementary nature of both detectors to characterize the electronic nonequilibrium properties of a novel 2D topological insulator, bismuthene. We map the transiently occupied conduction band after photoexcitation, observe faint signatures of topological edge states within the large fundamental bulk band gap, and track the full relaxation pathway of hot photocarriers. Next, using trARPES in combination with a complementary time-resolved structural probe, we investigate the dynamics of a prototypical layered CDW compound, TbTe3, after optical excitation. Tracking the system's order parameter during the photoinduced CDW melting and recovery reveals a surprising reemergence of CDW order at elevated electronic temperatures far greater than the thermal critical temperature, which we attribute to strong nonequilibrium between coupled electronic and lattice degrees of freedom. Additionally, we show how changes of the CDW energy gap during the CDW-to-metal transition can lead to a transient modulation of the relaxation rates of excited high-energy photocarriers. Theoretical calculations based on a nonequilibrium Green's function formalism reveal the critical role of the phase space of electron-electron scattering and the interplay of elementary interactions and the electronic band structure. Lastly, we study the ultrafast nonthermal pathway to a long-lived metastable quantum state in bulk 1T-TaS2 after optical excitation. Utilizing a double-pulse excitation of a vibrational CDW coherence, we demonstrate a high degree of control over the phase transition, laying the basis for actively controlling macroscopic material properties on ultrafast timescales. The thesis concludes with an outlook on future research of quantum materials enabled by time-resolved momentum microscopy.Quantenmaterialien weisen emergente Phänomene mit faszinierenden Eigenschaften auf, wie beispielsweise unkonventionelle Supraleitung, Mott-Übergänge, Ladungsdichtewellen (CDWs) und topologisch geschützte elektronische Randzustände, welche zukunftsweisende Anwendungen in der Stromerzeugung und -verteilung, dem (Quanten-) Computing und der Datenspeicherung ermöglichen. Die außergewöhnlichen Eigenschaften von Quantenmaterialien resultieren aus einem komplexen Zusammenspiel von Symmetrie, Dimensionalität, Topologie, und Vielteilchen-Wechselwirkungen zwischen elementaren Freiheitsgraden (Ladung, Spin, Orbital und Gitter). Um emergente Phänomene zu realisieren und die makroskopischen Eigenschaften von Quantenmaterialien zu kontrollieren, ist jedoch ein mikroskopisches Verständnis dieses Zusammenspiels erforderlich. Einen vielversprechenden Ansatz hierzu bieten ultrakurze optische Anregungen. Die dynamische Antwort einer geordneten Phase auf eine optische Anregung ermöglicht Rückschlüsse auf fundamentale Wechselwirkungen. Darüber hinaus kann die Manipulation von Quantenmaterialien mit Licht neue Phänomene mit vielversprechenden Eigenschaften hervorrufen, indem Materialien gezielt in transiente oder metastabile Zustände überführt werden. Die vorliegende Arbeit untersucht die elektronische Struktur und ultraschnelle Dynamik verschiedener Quantenmaterialien mithilfe von zeit- und winkelaufgelöster Photoemissionsspektroskopie (trARPES). Zunächst etablieren wir einen neuartigen Photoelektronendetektor, das sogenannte Flugzeit-Impulsmikroskop, und vergleichen dessen Leistungsfähigkeit mit einem konventionellen Halbkugelanalysator im Rahmen üblicher trARPES Experimente. Anschließend nutzen wir die komplementäre Funktionsweise beider Detektoren um die elektronischen Eigenschaften des neuartigen 2D topologischen Isolators Bismuthen nach optischer Anregung zu charakterisieren. Wir bestimmen den Verlauf des Leitungsbands, beobachten Signaturen topologischer Randzustände innerhalb der Bandlücke und identifizieren den gesamten Relaxationspfad angeregter Elektronen. Daraufhin untersuchen wir mittels trARPES und komplementärer struktureller Messmethoden die Dynamik des quasi-2D CDW-Materials TbTe3 nach optischer Anregung. Der Verlauf des Ordnungsparameters während der photoinduzierten Unterdrückung und Rückkehr der CDW zeigt ein überraschendes Wiederauftreten der CDW-Ordnung bei elektronischen Temperaturen, die weit über der thermischen Übergangstemperatur liegen, was wir auf das Nicht-Gleichgewicht zwischen gekoppelten elektronischen und Gitter-Freiheitsgraden zurückführen. Zudem zeigen wir, wie sich das Schließen der elektronischen CDW-Bandlücke infolge des photoinduzierten Phasenübergangs auf die Relaxationsrate angeregter Elektronen auswirkt, was wir auf ein komplexes Wechselspiel zwischen Elektron-Elektron-Streuung und Bandstruktur zurückführen. Zuletzt untersuchen wir den photoinduzierten Übergang in eine langlebige metastabile Nicht-Gleichgewichtsphase in 1T-TaS2. Dabei nutzen wir eine Sequenz optischer Pulse zur Anregung einer CDW Kohärenz und erzielen ein hohes Maß an Kontrolle über die Stabilisierung der Nicht-Gleichgewichtsphase. Die Arbeit schließt ab mit einem Ausblick auf zukünftige Forschung an Quantenmaterialien mit einem Fokus auf zeitaufgelöste Impulsmikroskopie

Temperature-dependent single-event photoemission data of 1T-TaS2

<p>RAW single-event temperature-dependent angle-resolved photoemission spectroscopy data of bulk 1T-TaS2, acquired using the XUV trARPES setup at the Fritz-Haber-Institute of the Max-Planck Society, Berlin, Germany and a SPECS METIS 1000 momentum microscope. We thank P. Sutar (Jožef Stefan Institute) for providing the samples. </p><p>The dataset here contains the RAW data used to analyze the temperature-dependent transition from the H-phase to the C-phase and further to the NC-phase on a heating cycle.</p><p>Analysis scripts can be found at https://github.com/OpenCOMPES/sed/tree/main/tutorial</p&gt

Ultrafast extreme-ultraviolet ARPES studies of electronic dynamics in two-dimensional materials

The intriguing electronic properties of two-dimensional materials motivates experiments to resolve their rapid, microscopic interactions and dynamics across momentum space. Essential insight into the electronic momentum-space dynamics can be obtained directly via time- and angle-resolved photoemission spectroscopy (trARPES). We discuss the development of a high-repetition rate trARPES setup that employs a bright source of narrowband, extreme-UV harmonics around 22.3 eV, and its application to sensitive studies of materials dynamics. In the bulk transition-metal dichalcogenide MoSe2 momentum-space quasiparticle scattering is observed after resonant excitation at the K-point exciton line, resulting in the time-delayed buildup of electrons at the Σ-point conduction band minimum. We will discuss this and other aspects of the non-equilibrium electronic response accessible with the extreme-UV trARPES probe

Orbital-resolved Observation of Singlet Fission

22 pages, 4 main figures, 5 supplementary figuresSinglet fission may boost photovoltaic efficiency by transforming a singlet exciton into two triplet excitons and thereby doubling the number of excited charge carriers. The primary step of singlet fission is the ultrafast creation of the correlated triplet pair. While several mechanisms have been proposed to explain this step, none has emerged as a consensus. The challenge lies in tracking the transient excitonic states. Here we use time- and angle-resolved photoemission spectroscopy to observe the primary step of singlet fission in crystalline pentacene and show that it occurs in a charge-transfer mediated mechanism. We gained intimate knowledge about the localization and the orbital character of the exciton wavefunctions recorded in momentum maps. This allowed us to directly compare the localization of singlet and bitriplet excitons and decompose energetically overlapping states based on their orbital character. Orbital- and localization-resolved many-body dynamics promise deep insights into the mechanics governing molecular systems and topological materials

Unveiling the orbital texture of 1T-TiTe2 using intrinsic linear dichroism in multidimensional photoemission spectroscopy

The momentum-dependent orbital character in crystalline solids, referred to as orbital texture, is of capital importance in the emergence of symmetry-broken collective phases, such as charge density waves as well as superconducting and topological states of matter. By performing extreme ultraviolet multidimensional angle-resolved photoemission spectroscopy for two different crystal orientations linked to each other by mirror symmetry, we isolate and identify the role of orbital texture in photoemission from the transition metal dichalcogenide 1T-TiTe2. By comparing our experimental results with theoretical calculations based on both a quantitative one-step model of photoemission and an intuitive tight-binding model, we unambiguously demonstrate the link between the momentum-dependent orbital orientation and the emergence of strong intrinsic linear dichroism in the photoelectron angular distributions. Our results represent an important step towards going beyond band structure (eigenvalues) mapping and learning about electronic wavefunction and orbital texture of solids by exploiting matrix element effects in photoemission spectroscopy

Odhalení skryté orbitální pseudospin textury pomocí time-revesal dichroismu v úhlové distribuci fotoelektronů

Zavedli jsme novou fyzikální pozorovatelnou, která poskytuje informaci o orbitální textuře materiálů pevných látek. Více v anglické verzi.We performed angle-resolved photoemission spectroscopy (ARPES) of bulk 2H-WSe2 for different crystal orientations linked to each other by time-reversal symmetry. We introduce a new observable called time-reversal dichroism in photoelectron angular distributions (TRDAD), which quantifies the modulation of the photoemission intensity upon effective time-reversal operation. We demonstrate that the hidden orbital pseudospin texture leaves its imprint on TRDAD, due to multiple orbital interference effects in photoemission. Our experimental results are in quantitative agreement with both the tight-binding model and state-of-the-art fully relativistic calculations performed using the one-step model of photoemission. While spin-resolved ARPES probes the spin component of entangled spin-orbital texture in multiorbital systems, we unambiguously demonstrate that TRDAD reveals its orbital pseudospin texture counterpart