Inhibition of the photoinduced structural phase transition in the excitonic insulator Ta2_2NiSe5_5

Femtosecond time-resolved mid-infrared reflectivity is used to investigate the electron and phonon dynamics occurring at the direct band gap of the excitonic insulator Ta$_2$NiSe$_5$ below the critical temperature of its structural phase transition. We find that the phonon dynamics show a strong coupling to the excitation of free carriers at the \Gamma\ point of the Brillouin zone. The optical response saturates at a critical excitation fluence $F_C = 0.30~\pm~0.08$~mJ/cm$^2$ due to optical absorption saturation. This limits the optical excitation density in Ta$_2$NiSe$_5$ so that the system cannot be pumped sufficiently strongly to undergo the structural change to the high-temperature phase. We thereby demonstrate that Ta$_2$NiSe$_5$ exhibits a blocking mechanism when pumped in the near-infrared regime, preventing a nonthermal structural phase transition

Ultrafast Electronic Band Gap Control in an Excitonic Insulator

We report on the nonequilibrium dynamics of the electronic structure of the layered semiconductor Ta$_2$NiSe$_5$ investigated by time- and angle-resolved photoelectron spectroscopy. We show that below the critical excitation density of $F_{C} = 0.2$ mJ cm$^{-2}$, the band gap $narrows$ transiently, while it is $enhanced$ above $F_{C}$. Hartree-Fock calculations reveal that this effect can be explained by the presence of the low-temperature excitonic insulator phase of Ta$_2$NiSe$_5$, whose order parameter is connected to the gap size. This work demonstrates the ability to manipulate the band gap of Ta$_2$NiSe$_5$ with light on the femtosecond time scale

Inhibition of the photoinduced structural phase transition in the excitonic insulator Ta2NiSe5{\mathrm{Ta}}_{2}{\mathrm{NiSe}}_{5}

Femtosecond time-resolved midinfrared reflectivity is used to investigate the electron and phonon dynamics occurring at the direct band gap of the excitonic insulator Ta2NiSe5 below the critical temperature of its structural phase transition. We find that the phonon dynamics show a strong coupling to the excitation of free carriers at the Γ point of the Brillouin zone. The optical response saturates at a critical excitation fluence FC=0.30±0.08 mJ/cm2 due to optical absorption saturation. This limits the optical excitation density in Ta2NiSe5 so that the system cannot be pumped sufficiently strongly to undergo the structural change to the high-temperature phase. We thereby demonstrate that Ta2NiSe5 exhibits a blocking mechanism when pumped in the near-infrared regime, preventing a nonthermal structural phase transition

Detection of a coherent excitonic state in the layered semiconductor BiI3_{3}

The measurement and manipulation of the coherent dynamics of excitonic states constitute a forefront research challenge in semiconductor optics and in quantum coherence-based protocols for optoelectronic technologies. Layered semiconductors have emerged as an ideal platform for the study of exciton dynamics with accessible and technologically relevant energy and time scales. Here, we investigate the sub-picosecond exciton dynamics in a van-der-Waals semiconductor upon quasi-resonant excitation, and achieve to single out an incipient coherent excitonic state. Combining broadband transient reflectance spectroscopy and simulations based on many-body perturbation theory, we reveal a transient enhancement of the excitonic line intensity that originates from the photoinduced coherent polarization that is phase-locked with the interacting electromagnetic field. This finding allows us to define the spectral signature of a coherent excitonic state and to experimentally track the dynamical crossover from coherent to incoherent exciton, unlocking the prospective optical control of an exciton population on the intrinsic quantum-coherence timescale

Halide Perovskite Artificial Solids as a New Platform to Simulate Collective Phenomena in Doped Mott Insulators

The development of quantum simulators, artificial platforms where the predictions of many-body theories of correlated quantum materials can be tested in a controllable and tunable way, is one of the main challenges of condensed matter physics. Here we introduce artificial lattices made of lead halide perovskite nanocubes as a new platform to simulate and investigate the physics of correlated quantum materials. We demonstrate that optical injection of quantum confined excitons in this system realizes the two main features that ubiquitously pervade the phase diagram of many quantum materials: collective phenomena, in which long-range orders emerge from incoherent fluctuations, and the excitonic Mott transition, which has one-to-one correspondence with the insulator-to-metal transition described by the repulsive Hubbard model in a magnetic field. Our results demonstrate that time-resolved experiments provide a quantum simulator that is able to span a parameter range relevant for a broad class of phenomena, such as superconductivity and charge-density waves

Fundamental interactions governing the non equilibrium electronic structure in low dimensions

This thesis explores the electronic structure and ultrafast dynamics of two lowdimensional materials with focus on the role of intrinsic interactions and couplings to the environment. Charge carriers in the matter are never completely independent: they interact among each other and couple to lattice vibrations (phonons) and other excitations. Their behavior is also influenced by the environment in which the material is embedded. Moreover, confining charges to low dimensions promotes interactions and enhances the impact of the environment. All these factors lead to a variety of static and dynamic properties, and potentially to the emergence of new phases of matter. Investigating a system out of its equilibrium helps the assignment of each fundamental interaction to the related physical property. Remarkably, addressing ultrafast dynamics can also uncover novel properties which otherwise would not be accessible at equilibrium. The first study of this thesis explores the modification of the unoccupied electronic structure of ultrathin films of SiO2 by electron quantum confinement and investigates the electronic coupling at the interface with the Ru(0001) substrate. By means of timeresolved two-photon photoelectron spectroscopy, the formation of quantized states is resolved, whose energies are altered by the image potential of the metal. The second and major study deals with the quasi-one-dimensional material Ta2NiSe5 which shows a combined electronic and structural phase transition upon heating and likely exhibits an excitonic insulator ground state. Here, the ultrafast charge carrier, exciton and lattice dynamics are disclosed by complementary time-resolved photoelectron and optical spectroscopies. The electron relaxation rate follows an anomalous dependence on the excess energy and is reduced by the transient increase of screening of the Coulomb interaction. The coherent phonon dynamics are generated by the photoinduced displacement of the charges. Optical absorption saturation restrains the number of photoexcited charges thereby hindering a photoinduced structural change. Also, the electronic band gap is transiently modulated by means of light. Nontrivially, it widens upon photoinduced strengthening of the excitonic insulator order parameter in remarkable agreement with Hartree-Fock calculations. These findings show that intrinsic interactions highly impact on the properties of Ta2NiSe5. Moreover, they demonstrate that it is possible to optically control the out-of-equilibrium electronic structure of a strongly interacting system on an ultrafast timescale. These studies show that unraveling the role of fundamental interactions in low dimensions provides profound understanding and potential control of the equilibrium electronic structure and the photoinduced ultrafast dynamics of very diverse materials

Fundamentale Wechselwirkungen für die elektronische Struktur in niedrigen Dimensionen im (Nicht-)Gleichgewicht

This thesis explores the electronic structure and ultrafast dynamics of two lowdimensional materials with focus on the role of intrinsic interactions and couplings to the environment. Charge carriers in the matter are never completely independent: they interact among each other and couple to lattice vibrations (phonons) and other excitations. Their behavior is also influenced by the environment in which the material is embedded. Moreover, confining charges to low dimensions promotes interactions and enhances the impact of the environment. All these factors lead to a variety of static and dynamic properties, and potentially to the emergence of new phases of matter. Investigating a system out of its equilibrium helps the assignment of each fundamental interaction to the related physical property. Remarkably, addressing ultrafast dynamics can also uncover novel properties which otherwise would not be accessible at equilibrium. The first study of this thesis explores the modification of the unoccupied electronic structure of ultrathin films of SiO2 by electron quantum confinement and investigates the electronic coupling at the interface with the Ru(0001) substrate. By means of timeresolved two-photon photoelectron spectroscopy, the formation of quantized states is resolved, whose energies are altered by the image potential of the metal. The second and major study deals with the quasi-one-dimensional material Ta2NiSe5 which shows a combined electronic and structural phase transition upon heating and likely exhibits an excitonic insulator ground state. Here, the ultrafast charge carrier, exciton and lattice dynamics are disclosed by complementary time-resolved photoelectron and optical spectroscopies. The electron relaxation rate follows an anomalous dependence on the excess energy and is reduced by the transient increase of screening of the Coulomb interaction. The coherent phonon dynamics are generated by the photoinduced displacement of the charges. Optical absorption saturation restrains the number of photoexcited charges thereby hindering a photoinduced structural change. Also, the electronic band gap is transiently modulated by means of light. Nontrivially, it widens upon photoinduced strengthening of the excitonic insulator order parameter in remarkable agreement with Hartree-Fock calculations. These findings show that intrinsic interactions highly impact on the properties of Ta2NiSe5. Moreover, they demonstrate that it is possible to optically control the out-of-equilibrium electronic structure of a strongly interacting system on an ultrafast timescale. These studies show that unraveling the role of fundamental interactions in low dimensions provides profound understanding and potential control of the equilibrium electronic structure and the photoinduced ultrafast dynamics of very diverse materials.Diese Arbeit behandelt die elektronische Struktur und die ultraschnelle Dynamik zweier niederdimensionaler Materialien, wobei das Augenmerk auf intrinsischenWechselwirkungen und Kopplungen mit der Umgebung liegt. Ladungsträger in Materialien sind niemals vollkommen unabhängig: Sie interagieren miteinander und koppeln an Gitterschwingungen (Phononen) und andere Anregungszustände. Ihr Verhalten wird auch durch die Umgebung beeinflusst, in die das Material eingebettet ist. Darüber hinaus fördert die Verringerung der Dimensionalität dieWechselwirkungen der Ladungen untereinander und erhöht den Einfluss der Umgebung auf sie. Alle diese Faktoren führen zu einer Vielzahl statischer und dynamischer Eigenschaften und immer wieder auch zu Phasenübergängen. Die Untersuchung angeregter Zustände eines Systems hilft, eine Verbindung zwischen fundamentalen Wechselwirkungen und physikalischen Eigenschaften herzustellen. Durch die Beobachtung ultraschneller Dynamik in einem Material können zudem neuartige Eigenschaften entdeckt werden, welche im Grundzustand nicht sichtbar sind. Im ersten Teil der Arbeit wird die unbesetzte elektronische Struktur ultradünner SiO2-Filme untersucht, welche durch Quanteneinschluss der Elektronen und ihre elektronische Kopplung an das Ru(0001)-Substrat erzeugt wird. Unter Verwendung von zeitaufgelöster Zweiphotonen-Photoelektronenspektroskopie werden quantisierte Zustände beobachtet, deren Energien durch das metallische Bildpotential modifiziert werden. Der zweite Teil und Schwerpunkt dieser Arbeit beschäftigt sich mit dem quasieindimensionalem Material Ta2NiSe5, das temperaturabhängig einen simultanen elektronischen und strukturellen Phasenübergang aufweist und wahrscheinlich im Grundzustand ein exzitonischer Isolator ist. Hier wird die ultraschnelle Ladungsträger-, Exzitonen- und Gitterdynamik komplementär mit zeitaufgelöster Photoelektronen- und optischer Spektroskopie untersucht. Die Elektronenrelaxationsrate hat eine ungewöhnliche Abhängigkeit von der Elektronenenergie und wird durch die transiente Abschirmungszunahme der Coulomb-Wechselwirkung verringert. Die Phononendynamik wird von photoangeregter Ladungsverschiebung getrieben. Eine optische Absorptionssättigung begrenzt die Zahl der angeregten Ladungsträger, wodurch ein photoinduzierter struktureller Phasenübergang verhindert wird. Durch Licht kann auch die elektronische Bandlücke temporär verändert werden. Bemerkenswerterweise vergrößert sie sich durch eine photoinduzierte Erhöhung des Ordnungsparameters des exzitonischen Isolators, was durch Hatree-Fock Rechnungen bestätigt wird. Diese Ergebnisse unterstreichen den großen Einfluss intrinsischer Wechselwirkungen auf die Eigenschaften von Ta2NiSe5. Darüber hinaus liefern sie den Beweis, dass die angeregte elektronische Struktur eines stark wechselwirkenden Systems auf einer ultraschnellen Zeitskala optisch kontrollierbar ist. Diese Untersuchungen belegen die essentielle Bedeutung fundamentaler Wechselwirkungen für das Verständnis von und die Kontrolle über die elektronische Struktur und photoinduzierte Dynamik in sehr unterschiedlichen Systemen

Ultrafast charge carrier and exciton dynamics in an excitonic insulator probed by time-resolved photoemission spectroscopy

An excitonic insulator phase is expected to arise from the spontaneous formation of electron-hole pairs (excitons) in semiconductors where the exciton binding energy exceeds the size of the electronic band gap. At low temperature, these ground state excitons stabilize a new phase by condensing at lower energy than the electrons at the valence band top, thereby widening the electronic band gap. The envisioned opportunity to explore many-boson phenomena in an exci-tonic insulator system is triggering a very active debate on how ground state excitons can be experimentally evidenced. Here, we employ a nonequilibrium approach to spectrally disentangle the photoinduced dynamics of an exciton condensate from the entwined signature of the valence band electrons. By means of time-and angle-resolved photoemission spectroscopy of the occupied and unoccupied electronic states, we follow the complementary dynamics of conduction and valence band electrons in the photoexcited low-temperature phase of Ta2NiSe5, the hitherto most promising single-crystal candidate to undergo a semiconductor-to-excitonic-insulator phase transition. The photoexcited conduction electrons are found to relax within less than 1 ps. Their relaxation time is inversely proportional to their excess energy, a dependence that we attribute to the reduced screening of Coulomb interaction and the low dimensionality of Ta2NiSe5. Long after (> 10 ps) the conduction band has emptied, the photoemission intensity below the Fermi energy has not fully recovered the equilibrium value. Notably, this seeming carrier imbalance cannot be rationalized simply by the relaxation of photoexcited electrons and holes across the semi-conductor band gap. Rather, a rate equation model involving different photoemission cross -sections of the valence electrons and the condensed excitons is able to reproduce the delayed recovery of the photoemission intensity below the Fermi energy. The model shows that electron quantum tunnelling between the exciton condensate and the valence band top is enabled by an extremely small activation energy of 4 x 10-6 eV and explains the retarded recovery of the exciton condensate. Our findings not only determine the energy gain of ground state exciton formation with exceptional energy resolution, but also demonstrate the use of time-resolved photoemission to unveil the re-formation dynamics of an exciton condensate with femtosecond time resolution