Messung nichtlinearer Dynamiken in stark getriebenen Quantenmaterialien

The possibility to enhance desirable functional properties of correlated materials through optical excitation with intense mid-infrared pulses motivated a series of studies of their nonlinear terahertz physics. These mid-infrared light pulses are used to resonantly excite infrared-active vibrations, so that the energy of the light pulses is directly transferred into the lattice vibration. This allows for the accurate tracing of the energy flow to other nonlinearly coupled excitations of correlated materials and ultimately better understanding of their competing ground states. The balance among different low energy states, with different macroscopic properties, can be tipped by minute changes of the electron-electron interactions which in turn are sensitive to changes of the atomic arrangement. This was demonstrated in a set of experiments where the strong-field resonant excitation of the crystal lattice was observed to entail the emergence of hidden states of matter. These states are inaccessible under equilibrium conditions and show fascinating phenomena such as light-induced superconductivity and ferroelectricity, insulator-metal transitions and melting of electronic and magnetic order. To reveal these hidden states new nonlinear optical tools are required. The aim of this work is therefore to extend the well-established equilibrium technique of second harmonic generation (SHG), which is sensitive to dynamics invisible to a linear optical probing, to ultra-fast pump-probe experiment. This technique will be used to investigate how nonlinear lattice dynamics can couple to macroscopic material properties and how this can be used to manipulate them. In many correlated materials their macroscopic properties are determined by frozen in lattice distortions. The direct control of the crystal lattice therefore would allow for the ultra-fast manipulation of these properties. First experiments achieved this type of control through non-linear coupling between a resonantly driven infrared active mode and the lattice distortion that is responsible for the macroscopic material properties. We used this pathway to transiently reverse the polarization state of the ferroelectric LiNbO3 and studied the ensuing dynamics with SHG, which solely is sensitive to the microscopic polarization state. The amplitude and phase sensitive detection of the lattice dynamics revealed a transient reversal of the polarization state and allowed us to reconstruct the lattice potential energy. In high-Tc cuprates, the large amplitude excitation of the apical oxygen lattice vibration has been shown to induce transient features in the reflectivity suggestive of non-equilibrium superconductivity. Yet, notwithstanding intense research efforts a microscopic mechanism for these observations is still lacking. To address this problem, we measured time- and scattering-angle-dependent second-harmonic generation in YBa2Cu3O6+δ after exciting the apical oxygen vibration that transiently induces a superconductor-like terahertz reflectivity. Made possible by the tr-SHG technique, we observed a four-order-of-magnitude amplification of a 2.5-THz electronic mode, which displays a unique symmetry, momentum, and temperature dependence. We developed a theory involving parametric three-wave amplification of Josephson plasmons, which explains all these observations and provides a mechanism for non-equilibrium superconductivity.Die Möglichkeit, durch optische Anregung mit intensiven Laserpulsen im mittleren Infrarot die funktionellen Eigenschaften korrelierter Materialien zu manipulieren, motivierte die Untersuchung ihrer nichtlinearen Terahertz-Physik. In jenen Studien wurde - unter Umgehung elektronischer Anregungen - direkt Energie durch resonante Anregung infrarot-aktiver Schwingungen auf die Gitterschwingungen übertragen. So kann der Energiefluss in andere, nichtlinear gekoppelte fundamentale Anregungen der korrelierten Materialien genauestens verfolgt werden um damit letztlich ein tieferes Verständnis der konkurrierenden Grundzustände dieser Materialklasse zu gewinnen. Schon kleinste Änderungen der Elektron-Elektron-Wechselwirkung, die sehr empfindlich von der Konfiguration des Kristallgitters abhängen, rufen übergänge zwischen den verschiedenen niederenergetischen Zuständen hervor und erzwingen makroskopische Phasenübergänge. Dies konnte in einer Reihe von Experimenten demonstriert werden, bei denen die direkte, resonante Anregung des Kristallgitters das Material in einen verborgenen Zustand beförderte, der im Gleichgewicht nicht erreichbar ist. Diese verborgenen Zustände zeigen faszinierende Eigenschaften, wie lichtinduzierte Supraleitung und Ferroelektrizität, Isolator-Metall-Übergänge und das Schmelzen elektronischer und magnetischer Ordnung. Da viele dieser verborgenen Zustände jedoch auch konventionellen linearen optischen Sonden unzugänglich sind, sind neue nichtlineare optische Werkzeuge erforderlich, um sie sichtbar zu machen. Daher ist es Ziel dieser Arbeit, die bereits etablierte Gleichgewichtstechnik der Generation der zweiten Harmonischen (SHG) eines Laserpulses auf ultraschnelle Anrege-Abfrage-Experimente zu erweitern. Mit dieser Technik soll untersucht werden, wie nichtlineare Gitterdynamik an makroskopische Materialeigenschaften koppelt und wie diese dadurch modifiziert werden können. Die makroskopischen Eigenschaften vieler korrelierter Materialien sind durch eingefrorene Gitterverzerrungen bestimmt und die direkte Kontrolle des Kristallgitters erlaubt damit die ultra-schnelle Manipulation der mit dieser Gitterverzerrung verbundenen Eigenschaften. Frühere Experimente konnten die Kontrolle des Kristallgitters durch nichtlineare Kopplung bereits demonstrieren und zeigten, dass Kopplung zwischen der resonant angetriebenen infrarotaktiven Mode und der Gitterverzerrung, die für die makroskopischen Materialeigenschaften verantwortlich ist, die kohärente Kontrolle dieser erlaubt. Dies nutzten wir, um den Polarisationszustand des Ferroelektrikums LiNbO3 transient umzukehren, und untersuchten die sich daraus ergebende Dynamik mit SHG, die alleinig empfindlich auf den mikroskopischen Polarisationszustand ist. Die amplituden- und phasenempfindliche Detektion der Gitterdynamik zeigte eine transiente Umkehrung des Polarisationszustandes und erlaubte, die potentielle Energie des Gitters zu rekonstruieren. Vergleichbare Experimente in Hochtemperatur-Kuprat-Supraleitern wie YBa2Cu3O6+δ zeigten, dass die Anregung der apikalen Sauerstoffgitterschwingung transiente Merkmale in der Reflektivität bewirken, die auf induzierte Nicht-Gleichgewichts-Supraleitung hindeuten. Trotz intensiver Forschungsbemühungen fehlt jedoch bisher ein mikroskopischer Mechanismus, der diese Beobachtungen erklärt. Wir nutzten dieselbe Anregung, die transient eine supraleiterähnliche Terahertz-Reflektivität induziert, und messen die zeit- und streuwinkelabhängige Generation der zweiten Harmonischen in YBa2Cu3O6+δ. Ermöglicht durch die tr-SHG-Methode, beobachten wir eine Verstärkung einer elektronischen 2.5-THz-Mode, die eine einzigartige Symmetrie-, Impuls-und Temperaturabhängigkeit aufweist, um vier Größenordnungen. Diese Beobachtungen motivierten eine Theorie der parametrischen Drei-Wellen-Verstärkung von Josephson-Plasmonen, die eine Erklärung für die Nicht-Gleichgewichts-Supraleitung liefert

Violation of Boltzmann Equipartition Theorem in Angular Phonon Phase Space Slows down Nanoscale Heat Transfer in Ultrathin Heterofilms

Heat transfer through heterointerfaces is intrinsically hampered by a thermal boundary resistance originating from the discontinuity of the elastic properties. Here, we show that with shrinking dimensions the heat flow from an ultrathin epitaxial film through atomically flat interfaces into a single crystalline substrate is significantly reduced due to violation of Boltzmann equipartition theorem in the angular phonon phase space. For films thinner than the phonons mean free path, we find phonons trapped in the film by total internal reflection, thus suppressing heat transfer. Repopulation of those phonon states, which can escape the film through the interface by transmission and refraction, becomes the bottleneck for cooling. The resulting nonequipartition in the angular phonon phase space slows down the cooling by more than a factor of 2 compared to films governed by phonons diffuse scattering. These allow tailoring of the thermal interface conductance via manipulation of the interface

Surface Morphology and Strain Relief in Surfactant Mediated Growth of Germanium on Silicon (111)

The growth of Ge on Si is strongly modified by adsorbates called surfactants. The relevance of the stress on surface morphology and the growth mode of Ge on Si(111) is presented in a detailed in situ study by high resolution low energy electron diffraction (LEED) during the deposition. The change from islanding to layer-by-layer growth mode is seen in the oscillatory intensity behaviour of the 00-spot. As a strain relief mechanism, the Ge-film forms a microscopic rough surface of small triangular and defect-free pyramids in the pseudomorphic growth regime up to 8 monolayers. As soon as the pyramids are completed and start to coalesce, strain relieving defects are created at their base, finally arranging to the dislocation network. Without the driving force for the micro-roughness, the stress, the surface flattens again showing a much larger terrace length. The formation process of the dislocation network results in a spot splitting in LEED, since the periodic dislocations at the interface give rise to elastic deformation of the surface. Surprisingly the Ge-film is relaxed to 70% immediately after 8 monolayers of coverage, which is attributed to the micro rough surface morphology, providing innumerous nucleation sites for dislocation

Selecting a single orientation for millimeter sized graphene sheets

We have used Low Energy Electron Microscopy (LEEM) and Photo Emission Electron Microscopy (PEEM) to study and improve the quality of graphene films grown on Ir(111) using chemical vapor deposition (CVD). CVD at elevated temperature already yields graphene sheets that are uniform and of monatomic thickness. Besides domains that are aligned with respect to the substrate, other rotational variants grow. Cyclic growth exploiting the faster growth and etch rates of the rotational variants, yields films that are 99 % composed of aligned domains. Precovering the substrate with a high density of graphene nuclei prior to CVD yields pure films of aligned domains extending over millimeters. Such films can be used to prepare cluster-graphene hybrid materials for catalysis or nanomagnetism and can potentially be combined with lift-off techniques to yield high-quality, graphene based electronic devices

Non-Equilibrium Pathways for Excitation of Bulk and Surface Phonons through Anharmonic Coupling

Upon impulsive optical excitation of solid-state materials, the non-equilibrium flow of energy from the excited electronic system to the lattice degrees of freedom typically happens in a few picoseconds. Here we identified the surface of thin Bi films grown on Si(001) as an additional subsystem which is excited much slower on a 100 ps timescale that is caused by decoupling due to mismatched phonon dispersions relations of bulk and surface. Anharmonic coupling among the phonon systems provides pathways for excitations which exhibits a 1/T-dependence causing a speed-up of surface excitation at higher temperatures. A quantitative justification is provided by phonon Umklapp processes from lattice thermal conductivity of the Bi bulk. Three-temperature model simulations reveal a pronounced non-equilibrium situation up to nanoseconds: initially, the surface is colder than the bulk, that situation is then inverted during cooling and the surface feeds energy back into the bulk phonon system

Optical Stabilization of Fluctuating High Temperature Ferromagnetism in YTiO₃

In quantum materials, degeneracies and frustrated interactions can have a profound impact on the emergence of long-range order, often driving strong fluctuations that suppress functionally relevant electronic or magnetic phases. Engineering the atomic structure in the bulk or at heterointerfaces has been an important research strategy to lift these degeneracies, but these equilibrium methods are limited by thermodynamic, elastic, and chemical constraints. Here, we show that all-optical, mode-selective manipulation of the crystal lattice can be used to enhance and stabilize high-temperature ferromagnetism in YTiO3, a material that exhibits only partial orbital polarization, an unsaturated low-temperature magnetic moment, and a suppressed Curie temperature, Tc = 27 K. The enhancement is largest when exciting a 9 THz oxygen rotation mode, for which complete magnetic saturation is achieved at low temperatures and transient ferromagnetism is realized up to Tneq> 80 K, nearly three times the thermodynamic transition temperature. First-principles and model calculations of the nonlinear phonon-orbital-spin coupling reveal that these effects originate from dynamical changes to the orbital polarization and the makeup of the lowest quasi-degenerate Ti t2g levels. Notably, light-induced high temperature ferromagnetism in YTiO3 is found to be metastable over many nanoseconds, underscoring the ability to dynamically engineer practically useful non-equilibrium functionalities

In situ observation of stress relaxation in epitaxial graphene

Upon cooling, branched line defects develop in epitaxial graphene grown at high temperature on Pt(111) and Ir(111). Using atomically resolved scanning tunneling microscopy we demonstrate that these defects are wrinkles in the graphene layer, i.e. stripes of partially delaminated graphene. With low energy electron microscopy (LEEM) we investigate the wrinkling phenomenon in situ. Upon temperature cycling we observe hysteresis in the appearance and disappearance of the wrinkles. Simultaneously with wrinkle formation a change in bright field imaging intensity of adjacent areas and a shift in the moire spot positions for micro diffraction of such areas takes place. The stress relieved by wrinkle formation results from the mismatch in thermal expansion coefficients of graphene and the substrate. A simple one-dimensional model taking into account the energies related to strain, delamination and bending of graphene is in qualitative agreement with our observations.Comment: Supplementary information: S1: Photo electron emission microscopy and LEEM measurements of rotational domains, STM data of a delaminated bulge around a dislocation. S2: Movie with increasing brightness upon wrinkle formation as in figure 4. v2: Major revision including new experimental dat

Optical Stabilization of Fluctuating High Temperature Ferromagnetism in YTiO3_3

In quantum materials, degeneracies and frustrated interactions can have a profound impact on the emergence of long-range order, often driving strong fluctuations that suppress functionally relevant electronic or magnetic phases. Engineering the atomic structure in the bulk or at heterointerfaces has been an important research strategy to lift these degeneracies, but these equilibrium methods are limited by thermodynamic, elastic, and chemical constraints. Here, we show that all-optical, mode-selective manipulation of the crystal lattice can be used to enhance and stabilize high-temperature ferromagnetism in YTiO$_3$, a material that exhibits only partial orbital polarization, an unsaturated low-temperature magnetic moment, and a suppressed Curie temperature, $T_c$ = 27 K. The enhancement is largest when exciting a 9 THz oxygen rotation mode, for which complete magnetic saturation is achieved at low temperatures and transient ferromagnetism is realized up to $T_{neq} >$ 80 K, nearly three times the thermodynamic transition temperature. First-principles and model calculations of the nonlinear phonon-orbital-spin coupling reveal that these effects originate from dynamical changes to the orbital polarization and the makeup of the lowest quasi-degenerate Ti $t_{2g}$ levels. Notably, light-induced high temperature ferromagnetism in YTiO$_3$ is found to be metastable over many nanoseconds, underscoring the ability to dynamically engineer practically useful non-equilibrium functionalities.Comment: 14 pages, 4 figure

Driving forces for Ag-induced periodic faceting of vicinal Cu(111)

Adsorption of submonolayer amounts of Ag on vicinal Cu(111) induces periodic faceting. The equilibrium structure is characterized by Ag-covered facets that alternate with clean Cu stripes. In the atomic scale, the driving force is the matching of Ag(111)-like packed rows with Cu(111) terraces underneath. This determines the preference for the facet orientation and the evolution of different phases as a function of coverage. Both Cu and Ag stripe widths can be varied smoothly in the 3-30 nm range by tuning Ag coverage, allowing to test theoretical predictions of elastic theories.Comment: 1 text, 4 figure