Licht-induzierte Ladungsträgerdynamik und Licht-induziertes Schmelzen von Ladungsdichtewellen in Indiumdrähten

The work presented in this thesis is focused on the analysis of the transient changes in the electronic structure of quasi-one-dimensional indium wires during a light-induced insulator-to-metal transition. Due to large parallel sections on their Fermi surface, quasi-one-dimensional metals are unstable at low temperatures with respect to a lattice distortion, transforming the metallic system into an insulator. The In/Si(111)-(4 × 1) surface, made of zig-zag chains of indium atoms, is a well-known system of in-situ grown atomic wires serving as a prime example to explore the physics of one dimensional (1D) systems experimentally. Below a critical temperature of Tc ∼ 125 K the system undergoes a structural transition into an (8×2) reconstruction, accompanied by a gap opening of Egap = 300 meV at the Fermi level. Light pulses impinging on the low-symmetry phase provide the required energy to overcome the condensation energy and transiently turn the system into the metallic state. Photoexcitation changes the occupation of different electronic orbitals, which breaks the bonds responsible for maintaining the low symmetry structure and creates new ones that favor the high symmetry structure. The light-induced phase transition was achieved in two different regimes; single-photon absorption (hω > Egap ) and multiphoton absorption (hω 100 ps of the transient metallic phase was found, explained by the existence of a metastable (4 × 1) phase at temperatures below the transition temperature Tc . For multiphoton absorption the melting time was found to be slightly longer (τ ∼ 1 ps) with a significantly shorter lifetime for the transient metallic state of ∼ 6 ps, indicating an incomplete structural transition. These timescales suggest similar microscopic mechanisms for charge density wave (CDW) melting on indium-wires in single-photon and multiphoton absorption regimes.Die in dieser Dissertation vorgestellte Arbeit konzentriert sich auf die Analyse der transienten Änderungen in der elektronischen Struktur von quasi-1D Indium-Drähten während eines lichtinduzierten Übergangs zwischen Isolator und Metall. Auf grund großer paralleler Bereiche auf ihrer FermiFläche sind quasi-1D-Systeme bei niedrigen Temperaturen instabil hinsichtlich einer Gitterverzerrung, die das metallische System in einen Isolator umwandelt. Die In/Si(111)-(4 × 1)-Oberfläche aus Zick-Zack-Ketten von Indiumatomen ist ein bekanntes Beispiel für in-situ gewachsene Atomdrähte, um die 1D-Physik experimentell zu untersuchen. Unterhalb von einer kritischen Temperatur T c ∼ 125 K durchläuft das System einen strukturellen Übergang in eine (8 × 2) Struktur, die einer Bandlücke von 300 meV aufweist. Lichtpulse, die auf das System in der Phase niedriger Symmetrie treffen, können von ihm absorbiert werden, wodurch die notwendige Energie zur Überwindung der Kondensationsenergie bereitgestellt und das System vorübergehend in den metallischen Zustand versetzt wird. Dies verändert die Besetzung der verschiedenen elektronischen Orbitale, welche die für die Aufrechterhaltung der niedrigen Symmetriestruktur verantwortlichen Bindungen durchbrechen und solche erzeugen, die eine hohe Symmetrie begünstigen. Der lichtinduzierte Phasenübergang wurde in zwei verschiedenen Bereichen untersucht: Einzelphotonenabsorption (hω > E gap ) und Multiphotonenabsorption (hω 100 ps des transienten metallischen Zustandes gefunden, die durch das Vorhandensein einer metastabilen Phase (4 × 1) bei Temperaturen unter der Übergangstemperatur T c erklärt werden kann. Für die Multiphotonenabsorption wurde festgestellt, dass die Schmelzzeit etwas länger ist (τ ∼ 1 ps), und es wurde eine deutlich kürzere Lebensdauer für den Metastabilezustand von τ ∼ 6 ps gefunden, was auf einen unvollständigen Phasenübergang hinweist. Die gefundenen Zeitskalen deuten auf ähnliche mikroskopische Mechanismen für das Schmelzen von der Ladungsdichtewelle auf Indium-Drähten im Einzel- und Multiphotonenabsorptionsbereich hin

Direct evidence for efficient ultrafast charge separation in epitaxial WS₂/graphene heterostructures

We use time- and angle-resolved photoemission spectroscopy (tr-ARPES) to investigate ultrafast charge transfer in an epitaxial heterostructure made of monolayer WS2 and graphene. This heterostructure combines the benefits of a direct-gap semiconductor with strong spin-orbit coupling and strong light-matter interaction with those of a semimetal hosting massless carriers with extremely high mobility and long spin lifetimes. We find that, after photoexcitation at resonance to the A-exciton in WS2, the photoexcited holes rapidly transfer into the graphene layer while the photoexcited electrons remain in the WS2 layer. The resulting charge-separated transient state is found to have a lifetime of ∼1 ps. We attribute our findings to differences in scattering phase space caused by the relative alignment of WS2 and graphene bands as revealed by high-resolution ARPES. In combination with spin-selective optical excitation, the investigated WS2/graphene heterostructure might provide a platform for efficient optical spin injection into graphene

Ultrafast Momentum Imaging of Pseudospin-Flip Excitations in Graphene

The pseudospin of Dirac electrons in graphene manifests itself in a peculiar momentum anisotropy for photo-excited electron-hole pairs. These interband excitations are in fact forbidden along the direction of the light polarization, and are maximum perpendicular to it. Here, we use time- and angle-resolved photoemission spectroscopy to investigate the resulting unconventional hot carrier dynamics, sampling carrier distributions as a function of energy and in-plane momentum. We first show that the rapidly-established quasi-thermal electron distribution initially exhibits an azimuth-dependent temperature, consistent with relaxation through collinear electron-electron scattering. Azimuthal thermalization is found to occur only at longer time delays, at a rate that depends on the substrate and the static doping level. Further, we observe pronounced differences in the electron and hole dynamics in n-doped samples. By simulating the Coulomb- and phonon-mediated carrier dynamics we are able to disentangle the influence of excitation fluence, screening, and doping, and develop a microscopic picture of the carrier dynamics in photo-excited graphene. Our results clarify new aspects of hot carrier dynamics that are unique to Dirac materials, with relevance for photo-control experiments and optoelectronic device applications.Comment: 23 pages, 12 figure

Tracking primary thermalization events in graphene with photoemission at extreme timescales

Direct and inverse Auger scattering are amongst the primary processes that mediate the thermalization of hot carriers in semiconductors. These two processes involve the annihilation or generation of an electron-hole pair by exchanging energy with a third carrier, which is either accelerated or decelerated. Inverse Auger scattering is generally suppressed, as the decelerated carriers must have excess energies higher than the band gap itself. In graphene, which is gapless, inverse Auger scattering is instead predicted to be dominant at the earliest time delays. Here, $<8$ femtosecond extreme-ultraviolet pulses are used to detect this imbalance, tracking both the number of excited electrons and their kinetic energy with time- and angle-resolved photoemission spectroscopy. Over a time window of approximately 25 fs after absorption of the pump pulse, we observe an increase in conduction band carrier density and a simultaneous decrease of the average carrier kinetic energy, revealing that relaxation is in fact dominated by inverse Auger scattering. Measurements of carrier scattering at extreme timescales by photoemission will serve as a guide to ultrafast control of electronic properties in solids for PetaHertz electronics.Comment: 16 pages, 8 figure

Ferromagnetic Resonance Measurement Using a Novel Short Circuited Coaxial Probe Technique

A versatile technique to characterize the ferromagnetic resonance (FMR) of ferrite samples using a short circuited coaxial probe is presented. The technique has sensitivity comparable to that of well-established methods besides its non-contact nature, broadband and local. Detailed theoretical approach and simulation studies (Proof of Concept) using HFSS are presented. Microwave measurements on different single crystal and polycrystalline samples (Yttrium Iron Garnet (Y3Fe5O12) YIG and Nickel Ferrite (NiFe2O4) NFO have been performed. We measured the FMR response of these samples as a function of frequency and the data showed the expected variation for both in plane and out of plane magnetic fields

Microscopic understanding of ultrafast charge transfer in van-der-Waals heterostructures

Van-der-Waals heterostructures show many intriguing phenomena including ultrafast charge separation following strong excitonic absorption in the visible spectral range. However, despite the enormous potential for future applications in the field of optoelectronics, the underlying microscopic mechanism remains controversial. Here we use time- and angle-resolved photoemission spectroscopy combined with microscopic many-particle theory to reveal the relevant microscopic charge transfer channels in epitaxial WS$_2$/graphene heterostructures. We find that the timescale for efficient ultrafast charge separation in the material is determined by direct tunneling at those points in the Brillouin zone where WS$_2$ and graphene bands cross, while the lifetime of the charge separated transient state is set by defect-assisted tunneling through localized sulphur vacanices. The subtle interplay of intrinsic and defect-related charge transfer channels revealed in the present work can be exploited for the design of highly efficient light harvesting and detecting devices.Comment: 37 pages, 16 figure

First results from a multiplexed and massive instrument with sub-electron noise Skipper-CCDs

We present a new instrument composed of a large number of sub-electron noise Skipper-CCDs operated with a two stage analog multiplexed readout scheme suitable for scaling to thousands of channels. New, thick, $1.35$ Mpix sensors, from a new foundry, are glued into a Multi-Chip Module (MCM) printed circuit board on a ceramic substrate which has 16 sensors each. The instrument, that can hold up-to 16 MCMs, a total of 256 Skipper-CCD sensors (called a Super-Module with $\approx 130$ grams of active mass and $346$ Mpix), is part of the R$\&$D effort of the OSCURA experiment which will have $\approx 94$ super-modules. Experimental results with $10$ MCMs and $160$ Skipper-CCDs sensors are presented in this paper. This is already the largest ever build instrument with single electron sensitivity CCDs using nondestructive readout, both, in terms of active mass and number of channels.Comment: Corrected minor typo

Ultrafast Charge Separation in Bilayer WS2/Graphene Heterostructure Revealed by Time- and Angle-Resolved Photoemission Spectroscopy

Efficient light harvesting devices need to combine strong absorption in the visible spectral range with efficient ultrafast charge separation. These features commonly occur in novel ultimately thin van der Waals heterostructures with type II band alignment. Recently, ultrafast charge separation was also observed in monolayer WS2/graphene heterostructures with type I band alignment. Here we use time- and angle-resolved photoemission spectroscopy to show that ultrafast charge separation also occurs at the interface between bilayer WS2 and graphene indicating that the indirect band gap of bilayer WS2 does not affect the charge transfer to the graphene layer. The microscopic insights gained in the present study will turn out to be useful for the design of novel optoelectronic devices

Angle-resolved photoemission spectroscopy with 9-eV photon-energy pulses generated in a gas-filled hollow-core photonic crystal fiber

A recently developed source of ultraviolet radiation, based on optical soliton propagation in a gas-filled hollow-core photonic crystal fiber, is applied here to angle-resolved photoemission spectroscopy (ARPES). Near-infrared femtosecond pulses of only few {\mu}J energy generate vacuum ultraviolet (VUV) radiation between 5.5 and 9 eV inside the gas-filled fiber. These pulses are used to measure the band structure of the topological insulator Bi2Se3 with a signal to noise ratio comparable to that obtained with high order harmonics from a gas jet. The two-order-of-magnitude gain in efficiency promises time-resolved ARPES measurements at repetition rates of hundreds of kHz or even MHz, with photon energies that cover the first Brillouin zone of most materials.Comment: 8 pages, 3 figure

Results of the engineering run of the coherent neutrino nucleus interaction experiment (CONNIE)

The CONNIE detector prototype is operating at a distance of 30 m from the core of a 3.8 GWth nuclear reactor with the goal of establishing Charge-Coupled Devices (CCD) as a new technology for the detection of coherent elastic neutrino-nucleus scattering. We report on the results of the engineering run with an active mass of 4 g of silicon. The CCD array is described, and the performance observed during the first year is discussed. A compact passive shield was deployed around the detector, producing an order of magnitude reduction in the background rate. The remaining background observed during the run was stable, and dominated by internal contamination in the detector packaging materials. The in-situ calibration of the detector using X-ray lines from fluorescence demonstrates good stability of the readout system. The event rates with the reactor ON and OFF are compared, and no excess is observed coming from nuclear fission at the power plant. The upper limit for the neutrino event rate is set two orders of magnitude above the expectations for the standard model. The results demonstrate the cryogenic CCD-based detector can be remotely operated at the reactor site with stable noise below2 e RMS and stable background rates. The success of the engineering test provides a clear path for the upgraded 100 g detector to be deployed during 2016.Fil: Aguilar Arevalo, A.. Universidad Nacional Autónoma de México; MéxicoFil: Bertou, Xavier Pierre Louis. Comisión Nacional de Energía Atómica; Argentina. Comisión Nacional de Energía Atómica. Fundación José A. Balseiro; ArgentinaFil: Bonifazi, C.. Universidade Federal do Rio de Janeiro; BrasilFil: Butner, M.. Fermi National Accelerator Laboratory; Estados UnidosFil: Cancelo, G.. Fermi National Accelerator Laboratory; Estados UnidosFil: Castañeda Vazquez, A.. Universidad Nacional Autónoma de México; MéxicoFil: Cervantes Vergara, B.. Universidad Nacional Autónoma de México; MéxicoFil: Chavez, C. R.. Universidad Nacional de Asunción; ParaguayFil: Da Motta, H.. Centro Brasileiro de Pesquisas Físicas; BrasilFil: D'Olivo, J. C.. Universidad Nacional Autónoma de México; MéxicoFil: Dos Anjos, J.. Centro Brasileiro de Pesquisas Físicas; BrasilFil: Estrada, J.. Fermi National Accelerator Laboratory; Estados UnidosFil: Fernández Moroni, Guillermo. Universidad Nacional del Sur. Departamento de Ingeniería Eléctrica y de Computadoras. Instituto ; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas; ArgentinaFil: Ford, R.. Fermi National Accelerator Laboratory; Estados UnidosFil: Foguel, A.. Centro Brasileiro de Pesquisas Físicas; Brasil. Universidade Federal do Rio de Janeiro; BrasilFil: Hernandez Torres, K. P.. Universidad Nacional Autónoma de México; MéxicoFil: Izraelevitch, F.. Fermi National Accelerator Laboratory; Estados UnidosFil: Kavner, A.. University of Michigan; Estados UnidosFil: Kilminster, B.. Universitat Zurich; SuizaFil: Kuk, K.. Fermi National Accelerator Laboratory; Estados UnidosFil: Lima Jr, H. P.. Centro Brasileiro de Pesquisas Físicas; BrasilFil: Makler, M.. Centro Brasileiro de Pesquisas Físicas; BrasilFil: Molina, J.. Universidad Nacional de Asunción; ParaguayFil: Moreno Granados, G.. Universidad Nacional Autónoma de México; MéxicoFil: Moro, Juan Manuel. Universidad Nacional del Sur. Departamento de Ingeniería; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas; ArgentinaFil: Paolini, Eduardo Emilio. Universidad Nacional del Sur. Departamento de Ingeniería Eléctrica y de Computadoras. Instituto ; ArgentinaFil: Sofo Haro, Miguel Francisco. Comision Nacional de Energia Atomica. Gerencia D/area de Energia Nuclear; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas; ArgentinaFil: Tiffenberg, Javier Sebastian. Fermi National Accelerator Laboratory; Estados Unidos. Consejo Nacional de Investigaciones Científicas y Técnicas; ArgentinaFil: Trillaud, F.. Universidad Nacional Autónoma de México; MéxicoFil: Wagner, S.. Centro Brasileiro de Pesquisas Físicas; Brasil. Pontificia Universidade Católica do Rio Grande do Sul; Brasi