Transient band structures in the ultrafast demagnetization of ferromagnetic gadolinium and terbium

We compare the laser-driven demagnetization dynamics of the rare earths gadolinium and terbium by mapping their transient valance band structures with time- and angle-resolved photoelectron spectroscopy. In both metals, the minority and majority spin valence bands evolve independently with different time constants after optical excitation. The ultrafast shift of the partially unoccupied minority spin bulk band to higher binding energy and of the majority spin surface state to lower binding energy suggests spin transport between surface and bulk. The slower response of the fully occupied majority spin band follows the lattice temperature and is attributed to Elliott-Yafet type spin-flip scattering. Terbium shows a stronger and faster decay of the exchange splitting, pointing to ultrafast magnon emission via 4f spin-to- lattice coupling

Influence of Magnetic Phase Transitions on Electronic Band Structures and Fermi Surfaces of Heavy Lanthanide Metals

Titelseite Kurzfassung und Abstract Inhaltsverzeichnis 1\. Einleitung 1 2\. Grundlagen 7 3\. Durchführung der Experimente 25 4\. Elektronische Bandstrukturen 43 5\. Schnitte der Fermiflächen mit Hochsymmetrieebenen 67 6\. Fermiflächen-Nesting und magnetische Phasenübergänge 81 7\. Zusammenfassung und Ausblick 107 Anhang 113 Literaturverzeichnis 115 Publikationen 127 Danksagung 129Aufgrund ihrer einzigartigen magnetischen Eigenschaften sind die Lanthanidmetalle seit einigen Jahrzehnten Gegenstand intensiver experimenteller und theoretischer Forschung. In den späten 1950ern wurde der Durchbruch für das Verständnis der magnetischen Kopplung der in hohem Maße lokalisierten 4f-Momente erzielt, indem die RKKY-Theorie der indirekten magnetischen Kopplung auf die Lanthanidmetalle angewendet wurde. Die elektronischen Zustände am Ferminiveau, d.h. die Fermifläche (FS), spielen dabei die entscheidende Rolle. Das Vorhandensein von parallelen Segmenten der FS (Nesting) wurde als Ursache der antiferromagnetischen Phasen identifiziert (die sog. Nestinghypothese), bei denen die magnetischen Momente eine Helix bilden. Zum größten Teil basieren unsere heutigen Kenntnisse der Bandstrukturen und FS der Lanthanide auf theoretischen Rechnungen, wobei nicht klar ist, wie genau diese Vorhersagen zutreffen. Die meisten experimentellen Untersuchungen beschränken sich auf die ΓA-Richtung der Brillouinzone (BZ) und liefern daher kein vollständiges Bild. Positronenannihilationsmessungen zeigen ein Nesting der FS bei Y und Y-Gd-Legierungen. Bis zur vorliegenden Arbeit stand jedoch der direkte experimentelle Nachweis von Nesting bei einem reinen Lanthanidmetall noch aus. In dieser Dissertation berichte ich detailliert über die Bandstrukturen und FS von Gd, Tb, Dy sowie Y; letzteres dient als unmagnetische Referenz. Die Metalle wurden mittels winkelauflösender Photoemission am Strahlrohr 7.0.1 der Advanced Light Source in Berkeley (USA) untersucht. Als Proben wurden einkristalline Filme von 10--20~nm Dicke verwendet, die in situ auf ein W(110)-Substrat aufgedampft wurden. Mittels Variation der Emissionswinkel und der Photonenenergien wurden Daten aufgenommen, die einen weiten Bereich des Impulsraums abdecken und dabei mehrere BZ umspannen. Dies liefert Zugang zum besetzten Teil der Bandstrukturen von Y, Gd, Tb und Dy. Es werden Photoemissionsdaten für alle Hochsymmetrierichtungen, die parallel zur Probenoberfläche liegen, gezeigt sowie die Veränderungen untersucht, die auf der ferromagnetischen Ordnung bei tiefen Temperaturen beruhen. Weiterhin erfolgt ein Vergleich mit theoretischen Ergebnissen. Für alle vier Metalle werden Abbildungen der FS gezeigt, die eine vollständige BZ umfassen. Sie liefern erstmals den direkten experimentellen Nachweis für Nesting bei Tb und Dy und stützen damit die Nestinghypothese. Die Aufnahme solcher Datensätze erfordert nur wenige Stunden. Dies eröffnet die Möglichkeit temperaturabhängiger Messungen und somit die Untersuchung der Umstrukturierung der FS von Gd, Tb und Dy im Zuge der magnetischen Ordnung. Die vorliegende Arbeit stellt meines Wissens die bislang umfassendste experimentelle Studie der elektronischen Eigenschaften von Lanthanidmetallen dar.Due to their fascinating and unique magnetic properties, lanthanide metals have been in the focus of intense experimental and theoretical research for several decades. A breakthrough in the understanding of the origin of the coupling of the highly localized magnetic 4f moments was achieved in the late 1950s, by applying the RKKY theory of indirect magnetic coupling to lanthanide systems. The electronic states at the Fermi energy, i.e. the Fermi surface (FS), were identified as playing the key role. The existence of parallel sections of the FS---the so-called FS nesting---was linked to the development of antiferromagnetically ordered phases, with a helical arrangement of the spins (this link is commonly referred to as the nesting hypothesis). It is fair to say that most of our knowledge on the electronic structures of the lanthanides and the shapes of their FS is based on theoretical calculations, which is not satisfactory, as it is unclear how accurately the band structures are described within the framework of the theoretical models. Most experimental studies to date have been restricted to the ΓA line of the Brillouin zone (BZ) and provide no complete picture. Positron annihilation studies reveal a FS nesting for Y and Y-Gd alloys; however, for a pure lanthanide metal, nesting has, prior to my work, not been observed in a direct experiment. In this thesis I report in detail on the electronic band structures and FS of Gd, Tb, Dy, and---as a non-magnetic reference---Y metal. These were investigated by means of angle-resolved photoemission at beamline 7.0.1 of the Advanced Light Source, LBNL, UC Berkeley, Cal., USA. Single- crystalline films of 10--20~nm thickness, grown in situ on a W(110) substrate, were used as samples. By varying emission angles and photon energies, I have obtained data that cover a wide region of momentum space, including several BZ. This provides full access to the occupied part of the electronic structures of Y, Gd, Tb, and Dy. I show photoemission data for all high- symmetry lines parallel to the sample surface. I investigate changes that come along with ferromagnetic ordering at low temperatures, and compare experimental with theoretical results. I present data for the FS of the four metals, covering a full BZ. Thus I give the first direct experimental evidence of the existence of nesting of the paramagnetic FS of Tb und Dy, and I provide strong support for the nesting hypothesis. The short time required for data acquisition---only a few hours per data set---opens the possibility to investigate the temperature-induced reconstruction of the Gd, Tb and Dy FS due to magnetic ordering. To my knowledge, this thesis represents the most comprehensive experimental study of the electronic properties of lanthanide metals to date

A femtosecond X-ray/optical cross-correlator

For a fundamental understanding of ultra fast dynamics in chemistry, biology and materials science it has been a longstanding dream to record a molecular movie, where both the atomic trajectories and the chemical state of every atom in matter is followed in real time. Free-electron lasers (FEL) provide this perspective as they deliver brilliant femtosecond X-ray pulses spanning a wide photon energy range, which is necessary to gather element-specific and chemical-state-selective information with femtosecond time resolution. The key challenge lies in synchronizing the FEL with separate optical lasers. We exploit the peak brilliance of the FEL in Hamburg (FLASH) and establish X-ray pulse induced transient changes of the optical reflectivity in GaAs as a powerful tool for X-ray/optical cross-correlation. This constitutes a breakthrough en route to a molecular movie and – equally important – opens the novel field of femtosecond X-ray induced dynamics, only accessible with high brilliance X-ray free-electron lasers

Searching for long-lived particles beyond the Standard Model at the Large Hadron Collider

International audienceParticles beyond the Standard Model (SM) can generically have lifetimes that are long compared to SM particles at the weak scale. When produced at experiments such as the Large Hadron Collider (LHC) at CERN, these long-lived particles (LLPs) can decay far from the interaction vertex of the primary proton–proton collision. Such LLP signatures are distinct from those of promptly decaying particles that are targeted by the majority of searches for new physics at the LHC, often requiring customized techniques to identify, for example, significantly displaced decay vertices, tracks with atypical properties, and short track segments. Given their non-standard nature, a comprehensive overview of LLP signatures at the LHC is beneficial to ensure that possible avenues of the discovery of new physics are not overlooked. Here we report on the joint work of a community of theorists and experimentalists with the ATLAS, CMS, and LHCb experiments—as well as those working on dedicated experiments such as MoEDAL, milliQan, MATHUSLA, CODEX-b, and FASER—to survey the current state of LLP searches at the LHC, and to chart a path for the development of LLP searches into the future, both in the upcoming Run 3 and at the high-luminosity LHC. The work is organized around the current and future potential capabilities of LHC experiments to generally discover new LLPs, and takes a signature-based approach to surveying classes of models that give rise to LLPs rather than emphasizing any particular theory motivation. We develop a set of simplified models; assess the coverage of current searches; document known, often unexpected backgrounds; explore the capabilities of proposed detector upgrades; provide recommendations for the presentation of search results; and look towards the newest frontiers, namely high-multiplicity ‘dark showers’, highlighting opportunities for expanding the LHC reach for these signals