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
During my PhD research, I studied the photon statistics of light emitted by a microcavity that contains a single quantum dot (QD) on resonance. The work encompasses an experimental part,... Show moreDuring my PhD research, I studied the photon statistics of light emitted by a microcavity that contains a single quantum dot (QD) on resonance. The work encompasses an experimental part, simulations and a theoretical element. In the experimental part, we developed a fiber-coupled single-photon source, which can easily be integrated with existing quantum technologies. The developed source is state of the art in terms of single photon rate and purity. Further, I performed extensive simulations on the quantum master model. This is a theoretical model describing the interaction between light and the quantum dot in a microcavity. These simulations lead to a better theoretical understanding of the physics behind single photon light and other non- classical states of light. Through a theoretical study, we reported of an alternative way to produce single photons called unconventional photon blockade. In my PhD research, I learned to develop new quantum technologies such as single photon sources, thoroughly analyze them using numerical simulations for improvement, as well as being able to perform a theoretical analysis for physical understanding. Show less
Graphene nanoribbons (GNRs) are used as a current carrying substrate in investigation of current-induced forces in a low-temperature STM (chapter 2). We demonstrate induced migration of Co adatoms... Show moreGraphene nanoribbons (GNRs) are used as a current carrying substrate in investigation of current-induced forces in a low-temperature STM (chapter 2). We demonstrate induced migration of Co adatoms on GNRs and on Au(111) using voltage pulses from the STM tip and we argue that motion is due to thermal excitations rather than the wind force. In chapter 3 we show that voltage signal is induced in a graphene strip when a droplet of ionic liquid is moved across its surface. Here we show that even deionized water can induce voltage over charged graphene surface due to the polarizability of water molecules. In chapter 4 we present a method for fabrication of graphene nanoelectrodes which we further test electrically in a modified STM. For the first time we demonstrate that the gap between two graphene nanoelectrodes can be tuned with subnanometric precision Show less
A difficulty of studies on chemical kinetics are the reaction time scales and detection of their intermediates. Rapid Freeze-Quench (RFQ) is one of the most common techniques to investigate... Show moreA difficulty of studies on chemical kinetics are the reaction time scales and detection of their intermediates. Rapid Freeze-Quench (RFQ) is one of the most common techniques to investigate chemical kinetics. Since the intermediates of many reactions are paramagnetic, coupling RFQ to Electron Paramagnetic Resonance (EPR) is a desirable goal, especially at high-frequency (HF-EPR). HF-EPR offers high resolution and better spectral definition. However, collection of RFQ samples for HF-EPR is troublesome. In Chapter 2, the successful coupling of RFQ to HF-EPR at 275 GHz is described. Chapter 3 describes the development of Temperature-Cycle EPR (T-Cycle EPR), a novel high-frequency EPR technique that couples laser-induced T-jumps of the sample to a high-frequency 275 GHz EPR spectrometer, to detect short-lived paramagnetic intermediates and kinetics of chemical reactions in aqueous solutions. Chapter 4 discusses the application of T-Cycle EPR on a model reaction unfolding over hundreds of milliseconds, proving the technique is suitable to study many (bio)chemical systems. Chapter 5 shows an attempt to apply T-Cycle EPR to an enzymatic system on the sub-second time. T-Cycle EPR experiments at 275 GHz are performed on the reoxidation of a mutant of Small Laccase in the sub-second time regime, without making use of RFQ. Show less
Triplet superconductivity refers to a condensate of equal-spin Cooper pairs (pairs of electrons with equal spin). While exceptionally rare in nature, triplet pairing of electrons can occur if... Show moreTriplet superconductivity refers to a condensate of equal-spin Cooper pairs (pairs of electrons with equal spin). While exceptionally rare in nature, triplet pairing of electrons can occur if either the temporal or spatial component of the superconducting wavefunction can be represented by an odd function. These are often referred to as odd-frequency and odd-parity triplets, respectively. We use hybrid magnetic devices to study the former, while the latter is investigated in mesoscopic structures of strontium ruthenate (Sr\tss{2}RuO\tss{4}). Show less
This thesis is about cosmological inflation and its relation to observations. In part I we study the observational consequences of an additional scalar field besides the inflaton field. In... Show moreThis thesis is about cosmological inflation and its relation to observations. In part I we study the observational consequences of an additional scalar field besides the inflaton field. In particular, we focus on several different regimes where we vary both the coupling between the fields and the mass of the second field. In part II we perform a statistical analysis to understand whether we can extract some of the traces of new physics present during inflation from the three-dimensional map of galaxies in our universe. Show less
This thesis discusses the discovery potential of Intensity Frontier experiments. As a particular examples of such experiment the SHiP and MATHUSLA experiments where taken. The reach of these... Show moreThis thesis discusses the discovery potential of Intensity Frontier experiments. As a particular examples of such experiment the SHiP and MATHUSLA experiments where taken. The reach of these experiments was studied for the number of specific model of the Beyond Standard Model physics, namely neutrino Minimal Standard Model, Higgs-like scalar, axion-like particles and light dark matter. Show less
Electrons in a crystal lattice have properties that may differ from those of a free electron in vacuum. The effective mass can be different from the bare electron mass, and it may even vanish, ... Show moreElectrons in a crystal lattice have properties that may differ from those of a free electron in vacuum. The effective mass can be different from the bare electron mass, and it may even vanish, so that the electron behaves in some respects as a relativistic massless particle such as a photon. The magnetic moment of the intrinsic angular momentum, the electron spin, may be also different from that of an elementary particle. Spin-like degrees of freedom, referred to as "pseudospin" or "valley isospin", can also arise in the effective low-energy description of electrons in the lattice fields. These various degrees of freedom are of interest as ways to store and transport information: one speaks of "spintronics" and "valleytronics" as alternatives to "electronics". For these purposes it is of interest to study the interplay between the orbital motion of electrons and their spin (spin-like) degrees of freedom, the so-called "spin-orbit coupling". In some systems where this interaction is strong, it causes the electron spin to be tied to the direction of motion. This thesis contains results about the effects of this "spin-momentum locking" on two classes of materials, oxide interfaces and Weyl semimetals, with a focus on their electrical transport properties. Show less
