It is an established fact that the Standard Model has to be extended to explain the so-called Beyond the Standard Model (BSM) phenomena: dark matter, matter-antimatter asymmetry of the Universe and... Show moreIt is an established fact that the Standard Model has to be extended to explain the so-called Beyond the Standard Model (BSM) phenomena: dark matter, matter-antimatter asymmetry of the Universe and neutrino flavour oscillations. The difficulty of direct detection of new particles lies in the huge parameter space of the possible candidates. Hence, data coming from the cosmological and astrophysical observations provide invaluable directions for laboratory experiments.In this thesis we explore two methods of constraining new-physics candidates: through their influence on the primordial nucleosynthesis and through observable differences in the matter distribution caused by free-streaming of the dark-matter particles. We concentrate on the well-motivated extension of the SM that aims at explaining all 3 BSM problems at the same time: the Neutrino Minimal Standard Model. In this extension, there are 3 additional heavy neutral leptons, one of which plays the role of dark matter, while the other two are necessary for induction of matter-antimatter asymmetry and neutrino oscillations. The dark-matter candidate is an example of a Warm Dark Matter particle, the free-streaming of which might be detected in the Lyman-α forest spectra of distant quasars. The other two particles have lifetimes that make them relevant to the primordial nucleosynthesis. Show less
This thesis described the development of novel scanning tunneling microscopy techniques to investigate strongly correlated electronic states in quantum matter.
In the research presented in this thesis, the effects of humidity on a series of Ru-complex molecular layers is investigated with the Conductive Atomic Force Microscopy technique. One specific... Show moreIn the research presented in this thesis, the effects of humidity on a series of Ru-complex molecular layers is investigated with the Conductive Atomic Force Microscopy technique. One specific molecule out of this series called '2-Ru-N' shows remarkable humidity sensitive diode behavior: in low humidity it behaves as a conventional resistive tunneling junction, wheras in high humidity, it behaves as a diode. The rectification ratios found for 2-Ru-N are among the highest reported for molecular junctions in the literature. By comparing the behavior of 2-Ru-N with its other counterparts, a model is presented that is based on two localized molecular orbitals that are misaligned during the addition of water. This is due to the water pulling on the counter-ions that are found in the molecular layer and hence, effectively gate one of the two localized molecular orbitals, resulting in a misalignment and thus, a diode-like behavior. Show less
This thesis is a collection of theoretical works aiming at adjusting quantum algorithms to the hardware of quantum computers. The overarching topic of these efforts is to enable digital quantum... Show moreThis thesis is a collection of theoretical works aiming at adjusting quantum algorithms to the hardware of quantum computers. The overarching topic of these efforts is to enable digital quantum simulation, the process of approximating the ground state of an arbitrary physical system with elementary operations of a quantum computer. For fermionic systems, a class including molecules and materials, the impact of quantum computing would be undoubtedly high, and algorithms exist for their simulation. However, there is a certain gap between the requirements of those algorithms and what actual quantum devices can provide: it seems that our expectations of a fully-fledged quantum computer still exceed our capabilities to build it. To make quantum simulation feasible, we seek to adapt quantum algorithms to three different types of device limitations within this thesis. Show less
Like a mixture of oil and water, lipid membranes separate into two liquid phases. While it is known that on a flat surface coexisting liquid phases result in the formation of circular domains,... Show moreLike a mixture of oil and water, lipid membranes separate into two liquid phases. While it is known that on a flat surface coexisting liquid phases result in the formation of circular domains, little is known about liquid-liquid phase separation on curved surfaces. In this thesis, a collection of experimental tools are offered to unravel this problem. Novel model systems in vitro consisting of supported lipid bilayers, or bilayers stabilised on solid substrates, are developed by using colloidal particles and micro-printed structures. In this way, both the composition and the geometry of the membrane can be controlled allowing for a direct comparison with simulation and theory. Show less
Why do black holes emit thermal radiation? And how does a closed quantum system thermalize? These apparently unrelated questions might be both connected to an essential feature of quantum... Show moreWhy do black holes emit thermal radiation? And how does a closed quantum system thermalize? These apparently unrelated questions might be both connected to an essential feature of quantum techanics: the dynamics of quantum information and its chaotic properties. Indeed, regardless of the unitary time evolution, quantum information seems to be dissipated. The solution to these contradictions may heavily affect the near future technologies, in light of the recent progresses towards building a quantum computer.In this thesis we investigate the fascinating idea that such chaotic properties leave traces on the late time hydrodynamic excitations. We do this from two opposite directions, both from weakly coupled field theories, using a combination of field theory techniques, and from strongly-coupled field theories, using the AdS/CFT correspondence. Moreover, we studied a fermionic and bosonic quantum critical point, which are 'exotic' states of matter where quantum information plays an important role. The main results of this thesis consist of the formulation of a Boltzmann-like equation for many-body chaos, the discovery of a new property of thermal correlation functions (pole-skipping), and the analysis of which is the correct and meaningful observable to measure experimentally in order to probe quantum chaos. Show less
In this thesis we study the landscape of gravitational models which modify GR by introducing an additional scalar degree of freedom (d.o.f.) to source Cosmic Acceleration. In particular we... Show moreIn this thesis we study the landscape of gravitational models which modify GR by introducing an additional scalar degree of freedom (d.o.f.) to source Cosmic Acceleration. In particular we answer the question "What is the complete set of theoretical conditions a gravitational model must satisfy, in order to give a theoretically viable cosmology?". In order to study the theoretical stability of extended models of gravity we emplout the E ective Field Theory of Dark Energy and Modied Gravity (EFToDE/MG). This is a unifying framework which allows us to study the landscape of gravitational models in a broad and model independent way. This will allow us to answer the research question in a model independent way. Show less
The focus of the dissertation "Aspects of cosmic acceleration" is the study of possible mechanisms responsible for the late-time accelerated expansion of the universe. It has 5 main chapters. In... Show moreThe focus of the dissertation "Aspects of cosmic acceleration" is the study of possible mechanisms responsible for the late-time accelerated expansion of the universe. It has 5 main chapters. In the first chapter I have given an overview of modern cosmology. Particularly, an introduction to cosmological perturbation theory, and a general overview of the cosmological standard model, as well as an overview to beyond-standard-model scenarios is presented. Chapter two discusses cosmological models based on the so-called alpha-attractor framework. Chapter three is dedicated to the study of constraints imposed by gravitational wave observations on the so-called doubly-coupled massive bimetric gravity theory. Chapter four studies the cosmological stability of the so-called massive mimetic gravity theory. Finally, Chapter five studies the effect of the Symmetron gravity on the so-called splashback radius of dark matter halos. Show less
This thesis explores interfacial conductance and electric field-effects in LaAlO3/SrTiO3 heterostructures. High quality epitaxial LaAlO3 films were grown on SrTiO3 substrates by 90° off-axis sputter... Show moreThis thesis explores interfacial conductance and electric field-effects in LaAlO3/SrTiO3 heterostructures. High quality epitaxial LaAlO3 films were grown on SrTiO3 substrates by 90° off-axis sputter deposition. The conductance properties of the interfaces were modulated by applying external electric fields in different geometries, namely back-gating (applying an electric field to the back side of the substrate) and ionic liquid gating (which applies an electric field on the side of the LaAlO3). Show less
In human cells, a meter-long DNA is condensed inside a micrometer-sized cell nucleus. Simultaneously, the genetic code must remain accessible for its replication and transcription to functional... Show moreIn human cells, a meter-long DNA is condensed inside a micrometer-sized cell nucleus. Simultaneously, the genetic code must remain accessible for its replication and transcription to functional proteins. Such plasticity of the genome is maintained by dynamic folding and unfolding of DNA-protein spools called nucleosomes. It is unclear, however, how this process is controlled when multiple nucleosomes stack on top of each other and form compact chromatin fibers. This is particularly important since nucleosomes are rarely present in isolation inside a densely packed cell nucleus. Therefore, the aim of this thesis was to increase the understanding of the chromatin fiber structure and its dynamics. Knowing these details would provide many new insights into the mechanisms of gene expression (epigenetic regulation) which, upon malfunction, may cause severe diseases. The presented work consists of an experimental approach involving the application of single-molecule force spectroscopy, and makes use of theoretical modelling based on statistical mechanics. By using magnetic tweezers, we stretched and twisted individual chromatin fibers reconstituted in vitro in order to unfold its nucleosomes. These studies show that folding of nucleosomes into chromatin fibers opens up a plethora of regulatory pathways for controlling the level of DNA organization in cells. Show less
In this thesis, we describe the latest advances in SQUID-detected Magnetic Resonance Force Microscopy (MRFM). We have developed a new MRFM setup, which we describe in great detail, in particular... Show moreIn this thesis, we describe the latest advances in SQUID-detected Magnetic Resonance Force Microscopy (MRFM). We have developed a new MRFM setup, which we describe in great detail, in particular our efforts to remove vibrational noise from our dry dilution refrigerator, whilst maintaining the lowest possible operating temperatures, and our solution to reduce the crosstalk between the B1 field and SQUID (Superconducting QUantum Interference Device) detection. We have used the setup to further investigate the MRFM signals of copper nuclei, with a specific focus on the usage of higher modes of the cantilever as source for the B1 field, resulting in an MRFM frequency shift signal from the Boltzmann polarization of spins in a voxel as small as (40 nm)3. Furthermore, we have investigated the spin system in diamond, where we found evidence of the suppression of spin-diffusion in the layer of surface spins due to our high magnetic field gradients. Show less
The Copenhagen interpretation of quantum mechanics states that a measurement collapses a wavefunction onto an eigenstate of the corresponding measurement operator. This causes a quantum... Show moreThe Copenhagen interpretation of quantum mechanics states that a measurement collapses a wavefunction onto an eigenstate of the corresponding measurement operator. This causes a quantum mechanical wavefunction to break its unitary evolution described by the Schrödinger equation and is the source of the quantum measurement problem. In this thesis we take the first steps to an experiment that might shed light on this century-old problem. We envision a single-microwave-photon interferometer that has a travelling-wave parametric amplifier (TWPA) added to each of its arms. We wonder if the process of amplification of the quantum state causes the wavefunction to collapse, as it might turn into a detector while smoothly increasing the amplifier's gain. To this end we introduce necessary concepts from the field of microwave engineering. Then, we develop a quantum theory to describe TWPAs. This is followed by preliminary calculations on the expected interference visibility of the envisioned interferometer as a function of amplifier gain and losses. Furthermore, we describe how wavefunction collapse might reveal itself in the experimental results. Finally, the thesis describes our efforts to develop a low-loss TWPA based on Josephson junctions. Show less
Materials with strongly correlated electrons show some of the most mysterious and exotic phases of quantum matter, such as unconventional superconductivity, quantum criticality and strange... Show more Materials with strongly correlated electrons show some of the most mysterious and exotic phases of quantum matter, such as unconventional superconductivity, quantum criticality and strange metal phase. In this thesis, we study strongly-correlated electron materials using spectroscopic-imaging scanning tunneling microscopy. We first describe the design and construction of a novel, ultra-stiff, scanning tunneling microscope that is optimized to have the high signal-to-noise ratio required to study these materials. We then present the discovery of the melting of the Mott insulating phase in the iridate Sr2IrO4 upon electron doping, that results in the formation of puddles of pseudogap and charge order. This is striking similar to the cuprate unconventional superconductors and for the first time we show the universality of these phenomena using scanning tunneling microscopy. We moreover discuss the effect of electric field penetration in a poorly conducting sample, and how this affects STM measurements on lightly doped Mott insulators in general. Finally, we show quasiparticle interference measurements on the correlated metal Sr2RhO4, and we discuss its comparison with photoemission results. Show less
On a structural level, the properties featured by a majority of mechanical metamaterials can be ascribed to the finite number of soft internal degrees-of freedom allowing for low-energy... Show moreOn a structural level, the properties featured by a majority of mechanical metamaterials can be ascribed to the finite number of soft internal degrees-of freedom allowing for low-energy deformations. Ideally, these low-energy deformation modes can be represented through mechanisms consisting of movable rigid geometrical units. Conversely, these mechanisms also serve as an intuitive starting point to initiate and adapt the design of mechanical metamaterials to requirements. Traditional design methods mainly comprising trial and testing can only well handle simple design tasks, not to mention that the final designs can be periodic and non-generic. In order to solve complex design problems, computer algorithm based inverse strategies provide state-of-the-art solutions. One way in which they can be utilized is by framing the material design problem as an optimization problem, where we optimize the values of control parameters (design variables) - in order to meet the desired target response. In this thesis, we present novel inverse strategies to design 2D mechanical metamaterials, whose zero-energy deformations can be modeled by one degree-of-freedom mechanisms consisting of pin-jointed polygons. We demonstrate that by optimizing the characteristic trajectory of these mechanisms, one can design generic metamaterials that exhibit complex programmable mechanics, atypical zero-energy deformations and shape-transformable behavior. Show less
