Higgs Spectroscopy of Superconductors in Nonequilibrium

Superconductors exhibit a collective amplitude mode of the order parameter, which is called Higgs mode. With the upcoming of new THz laser technology in recent years, it was possible to excite Higgs oscillations in nonequilibrium experiments. This thesis studies Higgs oscillations in unconventional superconductors with nontrivial pairing symmetry. In comparison to conventional superconductors, unconventional superconductors show a much richer spectrum of Higgs modes. The occurring Higgs modes depend on the pairing symmetry and the excitation scheme. These modes are interesting to study as their oscillation frequency encodes multiple ground state properties of the energy gap like number of nodes in the pairing symmetry, sign of the symmetry function, number of gaps or even coupling to other modes. In this thesis, different kinds of excitation schemes are explored, including interaction quenches, state quenches, coupling to ultrashort THz light pulses or periodic driving with light. Excited by these methods, several observables are investigated, including the induced gap oscillation itself, time-resolved optical conductivity, time-resolved ARPES spectra and third-harmonic generation responses. In all of these quantities, Higgs oscillations can be observed, which allows for an experimental observation. This thesis is embedded in a new upcoming field, termed Higgs spectroscopy, which aims for the systematic investigation and identification of Higgs modes in superconductors. As a future perspective, the tool of Higgs oscillations might be used as a novel spectroscopic tool for studies on unconventional superconductors.In Supraleitern existiert eine kollektive Amplitudenmode des Ordnungsparameters, welche Higgs- Mode genannt wird. Mit Hilfe der in den letzten Jahren aufkommenden THz Laser Technologie war es möglich, Higgs-Oszillationen im Nichgleichgewicht anzuregen. Diese Arbeit untersucht Higgs-Oszillationen in unkonventionellen Supraleitern mit nichttrivialer Paarungssymmetrie. Im Vergleich zu konventionellen Supraleitern weisen unkonventionelle Supraleiter ein viel komplexeres Spektrum auf. Die auftretenden Higgs-Moden hängen dabei von der Paarungssymmetrie und der Anregungs- art ab. Diese Moden sind interessant zu untersuchen, da ihre Oszillationsfrequenz eine Vielzahl von Grundzustandseigenschaften enthält, wie Anzahl von Knoten der Paarungssymmetrie, das Vorzeichen der Symmetriefunktion, Anzahl an Energielücken oder sogar die Kopplung an andere Moden. In dieser Arbeit werden unterschiedliche Anregungsarten untersucht, wie Wechselwirkung- Quenches, Zustand-Quenches, Kopplung an ultrakurze THz-Lichtpulse oder periodisches Treiben mit Licht. Nach der Anregung durch diese Methoden werden unterschiedliche Beobachtungsgrö- ßen ausgewertet, darunter die induzierten Energielückenoszillationen, die zeitaufgelöste optische Leitfähigkeit, zeitaufgelöste ARPES-Spektren und die erzeugte dritte-harmonische Antwort. In all diesen Größen können Higgs-Oszillationen beobachtet werden, wodurch eine experimentelle Mes- sung möglich wird. Diese Arbeit ist in einem neuen aufstrebenden Feld, der Higgs-Spektroskopie, eingebettet, welches darauf abzielt, die Higgs-Moden in Supraleitern systematisch zu untersuchen und zu identifizieren. Perspektivisch könnten Higgs-Oszillationen als ein neues spektroskopisches Werkzeug zur Untersuchung von unkonventionellen Supraleitern verwendet werden

Fano resonances in scattering: an alternative perspective

In a previous paper it has been shown that the interference of the first and second order pole of the Green's function at an exceptional point, as well as the interference of the first order poles in the vicinity of the exceptional point, gives rise to asymmetric scattering cross section profiles. In the present paper we demonstrate that these line profiles are indeed well described by the Beutler-Fano formula, and thus are genuine Fano resonances. Also further away from the exceptional points excellent agreement can be found by introducing energy dependent Fano parameters.Comment: 4 pages, 3 figures, 3 tables, additional reference

D=5 M-theory radion supermultiplet dynamics

We show how the bosonic sector of the radion supermultiplet plus d=4, N=1 supergravity emerge from a consistent braneworld Kaluza-Klein reduction of D=5 M--theory. The radion and its associated pseudoscalar form an SL(2,R)/U(1) nonlinear sigma model. This braneworld system admits its own brane solution in the form of a 2-supercharge supersymmetric string. Requiring this to be free of singularities leads to an SL(2,Z) identification of the sigma model target space. The resulting radion mode has a minimum length; we suggest that this could be used to avoid the occurrence of singularities in brane-brane collisions. We discuss possible supersymmetric potentials for the radion supermultiplet and their relation to cosmological models such as the cyclic universe or hybrid inflation.Comment: 18 pages, 4 figures, plain Late

Controlled generation of a pn-junction in a waveguide integrated graphene photodetector

With its electrically tunable light absorption and ultrafast photoresponse, graphene is a promising candidate for high-speed chip-integrated photonics. The generation mechanisms of photosignals in graphene photodetectors have been studied extensively in the past years. However, the knowledge about efficient light conversion at graphene pn-junctions has not yet been translated into high-performance devices. Here, we present a graphene photodetector integrated on a silicon slot-waveguide, acting as a dual-gate to create a pn-junction in the optical absorption region of the device. While at zero bias the photo-thermoelectric effect is the dominant conversion process, an additional photoconductive contribution is identified in a biased configuration. Extrinsic responsivities of 35 mA/W, or 3.5 V/W, at zero bias and 76 mA/W at 300 mV bias voltage are achieved. The device exhibits a 3 dB-bandwidth of 65 GHz, which is the highest value reported for a graphene-based photodetector.Comment: 19 pages, 16 figure