How can one stretch an atomically thick membrane, and what happens to it in the process? In this talk, we use controllably strained 2D materials (2DMs, such as graphene or transition metal dichalcogenides) as a new platform to study the static and dynamic response of electrons, excitons, and phonons. In the process, we find that the mechanical properties of 2DMs qualitatively differ from that of their three-dimensional counterparts.

 

First, we find that the inevitable out-of-plane crumpling modifies every mechanical property of 2DMs, making their mechanical response more akin to biological membranes than solid objects. Specifically, the out-of-plane crumpling renders the thermal expansion coefficient negative and substrate-dependent, decreases Young’s modulus, increases the bending rigidity by several orders of magnitude, and changes the sign of the Poison’s ratio. Second, we examine the effect of the mechanical strain on phonons in 2DMs. We use strain engineering to control the bandstructure of 2DM phononic crystals and to realize new nanomechanical spectrometers. Finally, we use mechanical strain to control excitons, Coulomb-bound electron-hole pairs in 2D semiconductors. We identify exciton types, bring different excitons into resonance, and control exciton flows in controllably strained devices.

In this presentation, we investigate how atomically defined heterostructures, such as 2D stacks, 1D boundaries within a 2D material, and 0D vacancies and substitutes in 2D materials, localize new protected states that host quasiparticle excitations. Quasiparticles are emergent excitations in condensed matter systems that behave like particles, but are not elementary particles like electrons or protons. In 2D solids, a wide variety of quasiparticle excitations can be experimentally accessed, including localized electrons, excitons, superconducting states, and polaritons, as well as more exotic systems like Tomonaga Luttinger liquid. Each specific quasiparticle excitation is a result of a specific crystalline symmetry. By inducing atomically defined 0D, 1D, and 2D heterostructures, it is possible to localize, protect, and create novel quasiparticle excitations.

 

We demonstrate how to engineer heterostructures into 2D Transition Metal Dichalcogenides and visualize the resulting electronic structure using Scanning Tunneling Microscopy. We discuss the resulting quasiparticle excitations that we directly capture using Scanning Tunneling Microscopy, as well as Time Resolved optical Spectroscopy. We show how 0D heterostructures or defects in 2D MoSe2 and 2D WS2 carry intrinsic point defects that substantially modify electronic properties. For instance, individual S vacancies create two-level systems within the band gap with extremely high spin-orbit coupling, and can mediate single photon emission via optical or electric stimulation. We also demonstrate how C-H for S or Se substitutes in 2D WS2 and 2D WSe2 form locally charged hydrogen-like states. Upon deprotonation, the localized carbon radical hosts a deep in-gap state with a net spin and electron-phonon coupling that is similar to NV color centers in diamond, but with atomistic control. Furthermore, individual Chalcogen Vacancies can be decorated with a variety of different adatoms, creating new types of 0D heterostructures, such as Co-S substitutes that could become relevant for Quantum Information Science.

 

In 1D heterostructures, such as Mirror Twin Boundaries in MoSe2 or WS2, we discovered atomically thin metallic conductors that undergo a quantum phase transition at low temperature to form a Tomonaga Luttinger Liquid (TTL). TTLs are correlated electron systems, where electrons couple to form a bosonic system, and that can foster partial electron and spin transport.

 

As a 2D heterostructure, we explore WS2/WSE2 stacks, where we study the formation and recombination channels of interlayer excitons formed in WS2/WSE2 stacks, which may relate to the recently reported formation of excitonic Bose-Einstein condensates. Another fascinating area of research is coupling quasiparticles to form new quasiparticles. By stacking 2D WSe on linear plasmonic cavities, we were able to couple excitons to plasmons, creating plexcitons. We use techniques such as photo low-temperature Scanning Tunneling Microscopy, near-field optical microscopy, and low-temperature time-resolved optical spectroscopy to investigate the properties and behavior of these quasiparticle excitations.

Die theoretischen Begriffe wie auch die dazugehörigen experimentellen Tests der Grundlagen der Allgemeinen Relativitästheorie werden im Rahmen eines konstruktiv axiomatischen Schemas eingeführt. Dabei werden die Experimente mit eigener Beteiligung etwas ausführlicher behandelt: Das sind der Test des Äquivalenzprinzips mit der Satellitenmission MICROSCOPE, der Test der gravitativen Rotverschiebung mit den Galileo-Satelliten und der Test der Gleichheit von aktiver und passiver gravitativer Masse mittels Lunar Laser Ranging. Nach einer kurzen Übersicht über alle beobachtbaren Konsequenzen der Allgemeinen Relativitätstheorie, die ebenfalls extensiv getestet werden, wird die Bedeutung zukünftiger Experimente mit Quantenmaterie dargestellt.

Wie genau nehmen es die Star Trek Macher eigentlich mit der Physik? Erstaunlich genau! Das ist der Inhalt dieses Vortrages. Ob Spock in Windeseile ausrechnet, dass genau 1771551 Tribbles in den Laderaum der Enterprise passen, ob sich die Enterprise wirklich in der Nähe des Sterns Sigma Draconis des Spektraltypus Gamma 9 befinden kann, oder warum man nur mit dem Warp-Antrieb die gigantischen Entfernungen des Universums überbrücken kann – all diese Fakten, die bei Star Trek immer wieder vorkommen, werden erklärt und gezeigt, dass bei Star Trek nichts dem Zufall überlassen bleibt, sondern alle genannten Daten immer auf handfester Physik beruhen.