A two-orbital quantum gas with tunable interactions

Im letzten Jahrzehnt haben sich Quantengasexperimente als gut kontrollierbare Modellsysteme zur Untersuchung komplexer Fragestellungen aus diversen Bereichen der Physik etabliert. Ultrakalte Quantengase zeichnen sich insbesondere dadurch aus, dass sie einen direkten und experimentell einfach realisierbaren Zugang zu ihrerWechselwirkung bieten. Das gezielte Einstellen der Wechselwirkungsstärke und die Erforschung der daraus resultierenden Aggregatzustände erlaubt es ein tiefes Verständnis der kondensierten Materie zu gewinnen. Insbesondere erdalkaliähnliche Atome wie Ytterbium bieten die Möglichkeit Phänomene der Festkörperphysik zu untersuchen, die durch die Wechselwirkung von Elektronen in verschiedenen Orbitalen oder durch eine größere Rotationssymmetrie des Spins als in gewöhnlichen Spin-1/2 Systemen hervorgerufen werden. Diese Doktorarbeit präsentiert die experimentelle Charakterisierung der Wechselwirkung ultrakalter, fermionischer Ytterbium-Atome (173Yb) in verschiedenen elektronischen Orbitalen. Dabei wird nachgewiesen, dass sich die Wechselwirkungsstärke mit Hilfe eines externen Magnetfeldes, analog zu einer Feshbach-Resonanz bei Alkali-Atomen, einstellen lässt. Bei Ytterbium wird diese Resonanz durch eine starke Spinaustauschwechselwirkung zwischen den verschiedenen Orbitalen hervorgerufen. Der Nachweis der einstellbaren Wechselwirkung erfolgt über Thermalisierungsexperimente in einer harmonischen Falle und mit Hilfe von hochauflösender Spektroskopie in einem dreidimensionalen Gitter. Des Weiteren wird mit Hilfe der neu entdeckten Resonanz zum ersten Mal experimentell ein stark wechselwirkendes Fermigas in verschiedenen Orbitalen erzeugt und spektroskopisch untersucht. Die Möglichkeit, die interorbitale Wechselwirkung direkt zu manipulieren und somit stark wechselwirkende Quantengase zu erzeugen, ebnet den Weg für die Realisierung und Untersuchung neuartiger Aggregatzustände der kondensierten Materie.In the last decade, quantum gas experiments have been established as well-controllable model systems for the investigation of complex problems originating from different fields of physics. Ultracold quantum gases are of particular interest, as they offer a direct and experimentally feasible access to their interaction. The precise control of the interaction and the exploration of resulting phases of matter grants a profound understanding of condensed matter. In particular, alkaline-earth-like atoms, such as ytterbium, allow for an implementation of condensed matter phenomena, arising from the interaction of electrons in different orbitals or exhibiting an enhanced spin rotation symmetry beyond the conventional spin-1/2 case. This thesis is dedicated to the experimental characterisation of the interaction of ultracold fermionic ytterbium atoms (\YbA) in different electronic orbitals. In the course of this thesis, we reveal the tunability of the interaction by means of an external magnetic field, similar to the case of Feshbach resonances in alkali atoms. For ytterbium, the scattering resonance is induced by the strong spin-exchange interaction between the different orbitals. Experimentally, the tunability of the interaction is demonstrated by a thermalisation experiment in a harmonic trap, as well as high-resolution spectroscopy in a three-dimensional lattice. For the first time, by means of an orbital interaction-induced Feshbach resonance, a strongly interacting two-orbital quantum gas is created and spectroscopically characterised. Controlling the interorbital interaction strength and creating strongly interacting two-orbital quantum gases paves the way towards the implementation of new states of condensed matter

Localized magnetic moments with tunable spin exchange in a gas of ultracold fermions

We report on the experimental realization of a state-dependent lattice for a two-orbital fermionic quantum gas with strong interorbital spin exchange. In our state-dependent lattice, the ground and metastable excited electronic states of $^{173}$Yb take the roles of itinerant and localized magnetic moments, respectively. Repulsive on-site interactions in conjunction with the tunnel mobility lead to spin exchange between mobile and localized particles, modeling the coupling term in the well-known Kondo Hamiltonian. In addition, we find that this exchange process can be tuned resonantly by varying the on-site confinement. We attribute this to a resonant coupling to center-of-mass excited bound states of one interorbital scattering channel

Direct probing of the Mott crossover in the SU(NN) Fermi-Hubbard model

The Fermi-Hubbard model (FHM) is a cornerstone of modern condensed matter theory. Developed for interacting electrons in solids, which typically exhibit SU($2$) symmetry, it describes a wide range of phenomena, such as metal to insulator transitions and magnetic order. Its generalized SU($N$)-symmetric form, originally applied to multi-orbital materials such as transition-metal oxides, has recently attracted much interest owing to the availability of ultracold SU($N$)-symmetric atomic gases. Here we report on a detailed experimental investigation of the SU($N$)-symmetric FHM using local probing of an atomic gas of ytterbium in an optical lattice to determine the equation of state through different interaction regimes. We prepare a low-temperature SU($N$)-symmetric Mott insulator and characterize the Mott crossover, representing important steps towards probing predicted novel SU($N$)-magnetic phases

Observation of coherent multiorbital polarons in a two-dimensional Fermi gas

We report on the experimental observation of multiorbital polarons in a two-dimensional Fermi gas of $^{173}\mathrm{Yb}$ atoms formed by mobile impurities in the metastable $^3\mathrm{P}_0$ orbital and a Fermi sea in the ground-state $^1\mathrm{S}_0$ orbital. We spectroscopically probe the energies of attractive and repulsive polarons close to an orbital Feshbach resonance and characterize their coherence by measuring the quasiparticle residue. For all probed interaction parameters, the repulsive polaron is a long-lived quasiparticle with a decay rate more than 2 orders of magnitude below its energy. We formulate a many-body theory, which accurately treats the interorbital interactions in two dimensions and agrees well with the experimental results. Our work paves the way for the investigation of many-body physics in multiorbital ultracold Fermi gases.Comment: 6 pages, 4 figures; Supplementary Materia

MYCN mediates cysteine addiction and sensitizes neuroblastoma to ferroptosis

Aberrant expression of MYC transcription factor family members predicts poor clinical outcome in many human cancers. Oncogenic MYC profoundly alters metabolism and mediates an antioxidant response to maintain redox balance. Here we show that MYCN induces massive lipid peroxidation on depletion of cysteine, the rate-limiting amino acid for glutathione (GSH) biosynthesis, and sensitizes cells to ferroptosis, an oxidative, non-apoptotic and iron-dependent type of cell death. The high cysteine demand of MYCN-amplified childhood neuroblastoma is met by uptake and transsulfuration. When uptake is limited, cysteine usage for protein synthesis is maintained at the expense of GSH triggering ferroptosis and potentially contributing to spontaneous tumor regression in low-risk neuroblastomas. Pharmacological inhibition of both cystine uptake and transsulfuration combined with GPX4 inactivation resulted in tumor remission in an orthotopic MYCN-amplified neuroblastoma model. These findings provide a proof of concept of combining multiple ferroptosis targets as a promising therapeutic strategy for aggressive MYCN-amplified tumors

A two-orbital quantum gas with tunable interactions

Im letzten Jahrzehnt haben sich Quantengasexperimente als gut kontrollierbare Modellsysteme zur Untersuchung komplexer Fragestellungen aus diversen Bereichen der Physik etabliert. Ultrakalte Quantengase zeichnen sich insbesondere dadurch aus, dass sie einen direkten und experimentell einfach realisierbaren Zugang zu ihrerWechselwirkung bieten. Das gezielte Einstellen der Wechselwirkungsstärke und die Erforschung der daraus resultierenden Aggregatzustände erlaubt es ein tiefes Verständnis der kondensierten Materie zu gewinnen. Insbesondere erdalkaliähnliche Atome wie Ytterbium bieten die Möglichkeit Phänomene der Festkörperphysik zu untersuchen, die durch die Wechselwirkung von Elektronen in verschiedenen Orbitalen oder durch eine größere Rotationssymmetrie des Spins als in gewöhnlichen Spin-1/2 Systemen hervorgerufen werden. Diese Doktorarbeit präsentiert die experimentelle Charakterisierung der Wechselwirkung ultrakalter, fermionischer Ytterbium-Atome (173Yb) in verschiedenen elektronischen Orbitalen. Dabei wird nachgewiesen, dass sich die Wechselwirkungsstärke mit Hilfe eines externen Magnetfeldes, analog zu einer Feshbach-Resonanz bei Alkali-Atomen, einstellen lässt. Bei Ytterbium wird diese Resonanz durch eine starke Spinaustauschwechselwirkung zwischen den verschiedenen Orbitalen hervorgerufen. Der Nachweis der einstellbaren Wechselwirkung erfolgt über Thermalisierungsexperimente in einer harmonischen Falle und mit Hilfe von hochauflösender Spektroskopie in einem dreidimensionalen Gitter. Des Weiteren wird mit Hilfe der neu entdeckten Resonanz zum ersten Mal experimentell ein stark wechselwirkendes Fermigas in verschiedenen Orbitalen erzeugt und spektroskopisch untersucht. Die Möglichkeit, die interorbitale Wechselwirkung direkt zu manipulieren und somit stark wechselwirkende Quantengase zu erzeugen, ebnet den Weg für die Realisierung und Untersuchung neuartiger Aggregatzustände der kondensierten Materie.In the last decade, quantum gas experiments have been established as well-controllable model systems for the investigation of complex problems originating from different fields of physics. Ultracold quantum gases are of particular interest, as they offer a direct and experimentally feasible access to their interaction. The precise control of the interaction and the exploration of resulting phases of matter grants a profound understanding of condensed matter. In particular, alkaline-earth-like atoms, such as ytterbium, allow for an implementation of condensed matter phenomena, arising from the interaction of electrons in different orbitals or exhibiting an enhanced spin rotation symmetry beyond the conventional spin-1/2 case. This thesis is dedicated to the experimental characterisation of the interaction of ultracold fermionic ytterbium atoms (\YbA) in different electronic orbitals. In the course of this thesis, we reveal the tunability of the interaction by means of an external magnetic field, similar to the case of Feshbach resonances in alkali atoms. For ytterbium, the scattering resonance is induced by the strong spin-exchange interaction between the different orbitals. Experimentally, the tunability of the interaction is demonstrated by a thermalisation experiment in a harmonic trap, as well as high-resolution spectroscopy in a three-dimensional lattice. For the first time, by means of an orbital interaction-induced Feshbach resonance, a strongly interacting two-orbital quantum gas is created and spectroscopically characterised. Controlling the interorbital interaction strength and creating strongly interacting two-orbital quantum gases paves the way towards the implementation of new states of condensed matter

Transition towards a renewable energy infrastructure: spatial interdependencies and stakeholder preferences

This dissertation analyzes the past and the expected future impacts of the energy transition in Germany on the electricity infrastructure. The first part of the dissertation examines the costs for reducing the output of renewables in order to maintain the stability of the electricity infrastructure - the so-called renewable energy curtailment costs. This part employs a spatial econometric model to estimate the regionally varying curtailment costs of different renewable energy technologies. The second part evaluates four possible future energy transition pathways by taking multiple stakeholder preferences into account. It establishes a participatory decision-making framework by using Value-Focused Thinking and Multi-Attribute Utility Theory. The key finding of the first part is that wind energy systems constitute the main driver for the rising curtailment costs. The political recommendation derived from this is to set different regional price signals for renewables. One insight of the second part of the dissertation is that an energy transition as envisaged by the German transmission system operators is considered as not sufficient for the stakeholders involved. Instead, the stakeholders prefer an energy transition with more ambitious climate protection targets that is internationally well-coordinated and that fosters citizen participation