In this dissertation, we investigate charge transport in magnetic nano-objects. We use the mechanically-controlled break junction (MCBJ) technique to contact single molecules and use three different device geometries to optimize the process of contacting nanoparticles. We focus in particular on magnetic signatures in the quantum transport of these structures.

The central theme of this dissertation is the experimental investigation of electrical transport through single-molecule junctions using the mechanically controlled break-junction (MCBJ) technique at room temperature. Of particular interest is the phenomenon of quantum interference, wherein the wave-like nature of the electron plays a crucial role in the electrical conductance of various molecules. Chemical design and the influence of external stimuli such as mechanical manipulation or applying strong electric fields can impact the interference, of which the latter is also used to investigate conductance switching due to conformational changes.

Chapter 1 serves as an introduction to the field of molecular electronics, discussing the origin of the field and landmarks, and provides theoretical considerations concerning charge transport in metal-molecule-metal junctions. Finally, a general introduction on quantuminterference is given.

Chapter 2 broadly discusses the MCBJ experimental method, where the mechanical and electrical equipment for two set-ups is discussed alongside the modus operandi for doing fast-breaking experiments. Subsequently, data analysis using machine learning methods will be discussed. Ultimately, reference measurements on bare gold electrodes and an oligo(phenylene-ethynylene) (OPE) molecule were performed as a benchmark for roomtemperature high-bias and current-voltage characterization on other molecules. For high-bias experiments we note that measurements can be performed reliably and shows in the case of the OPE molecule that its conductance increases as a function of applied bias voltage in agreement with the single-level model.

Chapter 3 revolves around the study of the electrical properties of molecules containing a cyclophane core. For molecules containing a paracyclophane core we observe that ortho-connections suppress the conductance more so than a para-connection as compared to a meta-connection. Additionally, we find para-connections to cyclophane units to be the common denominator in showing mechanosensitive behaviour, i.e., in which themolecule changes its conductance strongly due to mechanical deformation.

In Chapter 4 we have developed a method to reconstruct the observed destructive quantum interference dip in a molecule with a naphthalenophane core, opting for establishing a closer link between theory and experiment. Two complementary techniques at room temperature were used for this study: (i) the MCBJ technique, which allows for large statistical sampling fortifying the robustness of the dip reconstructionmethod; (ii) the alternating-current scanning tunneling microscopy break-junction technique (ACSTM- BJ) allowing for the continuous simultaneous measurements of the conductance and the corresponding thermopower, providing additional information on the destructive quantum interference dip. We find a sinusoidal response of the thermopower across the conductance dip without a sign change. Theoretical calculations on conductance and thermopower including electrode distance and energy alignment variations emphasize the crucial role of thermal fluctuations at roomtemperature.

For Chapter 5 we change pace and shift towards molecular switches. Three differently anchored norbordaniene molecules were investigated under high-bias circumstances. For all compounds, we find two conductance states. We find no full switching between two conductance states, as the two states are present across a wide-range of applied bias voltages and no clear population differences between the states are found. Alternatively to the explanation of the switching within the molecule itself, one can argue that either we observe two different configurations of the molecular junction or that interactions of the short linkers of the molecules, by interactions with the gold surface, quench the switching between the states unlike previous published results using a molecule with the same backbone but with longer linkers.

Chapter 6 investigates the effect of chemical design on the conductance of macrocyclic structures, studying them with different substituents. We observe a clear difference in conductance between para- and meta-connections in the core using thiophene and benzene substituents, consecutively. Here, the created para- connected path shows a higher conductance than its meta counterpart. Different connections, para and meta, in molecules with the same backbone show less of an effect on the conductance of the molecular junction. Additionally, preliminary results of one of the compounds using room temperature current-voltage characteristics shows a negative differential conductance and hysteretic behaviour.

Lastly, in Chapter 7 we conclude the obtained results from this dissertation and place them in a broader perspective.

In this dissertation, the charge transport mechanism in the conductive fibres of cable bacteria is investigated. In Chapter 1, the research field of bacterial electricity is introduced. Three kinds of bacterial nanowires are discussed: Shewenella nanowires, Geobacter nanowires and the conductive fibres fromcable bacteria. Even though the three types of protein wires are all conductive, the cable bacteria’s protein wires stand out because their activation energy of conductance is much lower than that of the other nanowires and because they transport electrons over centimeter instead of micrometer distances. These differences suggest they have a distinct charge transport mechanism. To put different transport mechanisms in more context, metallic conduction, semiconduction, and hopping conduction are treated side by side and emphasis is placed on the temperature dependence of conductivity....

This thesis discusses several studies on magnetic two-dimensional (2D) materials, focusing on their nanomechanical properties and the behavior of resonance frequencies in response to temperature changes. These studies employ nanomechanical resonators, specifically suspended membranes (drum resonators) of 2D magnetic materials. The frequency response of these resonators is measured using optical excitation combined with an interferometric setup, allowing identification of resonance frequencies. By altering the temperature of the resonators, the resonance frequency shifts as the strain within the 2D material changes. This strain change is partially magnetostrictive in origin due to changes in the magnetic order within the materials, offering a method to study these magnetic characteristics...

This dissertation centers around two topics: graphene nanoribbons (GNRs) and superconductors. The aim of this thesis is to work towards combine these two topics, in order to study how superconducting correlations interact with magnetic correlations within graphene nanoribbons, such as the magnetic edge states present in the zigzag edges of graphene nanoribbons. To introduce superconducting correlations into a graphene nanoribbon, it is important that there is a highly electrically transparent interface between a superconductor and the graphene nanoribbon. This is the main scope of this work.

This work presents research towards using molybdenum rhenium (MoRe) alloy as an electrical contact material for 9 atom wide armchair edge GNRs (9-AGNRs). MoRe electrodes with nanometer-size separations (30 nm and 6 nm) are made and compared with palladium electrodes. Experiments with contacting aerosol gold nanoparticles were performed to confirm that the MoRe electrodes are superconducting and capable of making a clean contact. Beside pure superconducting contacts, Palladium is considered as a contact material, which is made superconducting by the proximity effect. To study the proximity effect, electrical measurements at a base temperature of 30 mK were performed on variable thickness Nb-Au-Nb and Nb-Pd-Nb superconductor-normal metal-superconductor (SNS) junctions made by shadow mask evaporation. A constriction in the Pd layer allows for increasing the junction resistance by feedback-controlled electromigration until a tunnel contact is formed. As the final part of this research, a superconducting diode effect was identified and studied in these SNS junctions.

In this thesis, we use methods offered by state of the art nanofabrication and single-molecule measurement techniques to capture polycyclic aromatic hydrocarbon (PAH) all-organic, di-radical molecules in solid-state devices, and study their electronic transport properties in different conditions with a focus on their magnetic properties.

Electrostatically-defined nanostructures in bilayer graphene (BLG), known for its tunable bandgap, have promising applications in spintronics and valleytronics, however, its thermo- transport phenomena have not yet been investigated. This thesis aims to fabricate a BLG field-effect transistor (FET) device and characterize the thermotransport phenomena (See- beck coefficient) in electrostatically defined quantum point contacts (QPCs) and quantum dots (QDs). For this purpose, a bilayer graphene flake is encapsulated in hexagonal boron nitride (hBN) with Ti/Au heaters, top gates and 100-nm-separated split gates placed on top of the upper hBN flake. To define a QD, the 100 nm wide finger gates were separated from the top gates by a 30 nm Al2O3 dielectric. However, the electric field induced by the back gate was being screened by a layer of charges somewhere between the back gate and the bilayer graphene. The origin of this layer of charges remains unknown. As a result, the channel could not be fully depleted (unless when B = 5 T) and showed features indicating an unintended charging and discharging effect somewhere in the sample. As a consequence, the formation of a QD or a QPC at B = 0 T was not possible. Despite that, thermal voltages were measured in the two-dimensional BLG, applying currents up to 50 μA to the aforementioned heaters. The estimated Seebeck coefficient (based on resistance characterizations) was in the range of μV/K (corresponding with theoretical predictions) and enabled an estimate of an induced temperature gradient of 0.5 ± 0.2 K.

In this dissertation, we analyse the charge transport of nanoscale molecular junctions in mechanically controllable break junction (MCBJ) experiments. In particular, we focus on the characterization of molecular features going beyond the "single-peak" picture, that is, considering features in the measurements in addition to themost prominent one. To achieve this goal, we use a combination of improved experimental techniques and data analysis...

In this thesis, we explore the interaction between multiple optical light fields. Although these light fields can interfere with each other in a vacuum, actual interaction can only occur in a medium. This interaction can be amplified in materials with nanoscale dimensions such as a monolayer of WS2 and metallic nanostructures. The shapes of these materials have characteristic length scales smaller than the wavelength of light and influence the interaction between light and matter. There are three main reasons why studying light field interactions in nanoscale materials is interesting. First, by investigating the interaction between different light waves we have a tool to probe the light behavior in, e.g., a monolayerWS2 or metallic nanostructures. Secondly, by studying the interaction of multiple light waves we can probe the characteristics of the light waves themselves. Finally, the interaction of light waves can be utilized in novel applications such as frequency conversion and photonic nanocircuitry. This thesis is divided into four parts which will be discussed in the following sections…

This thesis describes atomic spin chains subject to magnetic anisotropy. Such chains, assembled through means of scanning tunneling microscopy (STM), can be home to a plethora of magnetic states and spin physics. This includes quantum tunneling of magnetization, the injection and delocalization of magnons and, for single atoms, the application of electron spin resonance (ESR). Each phenomenon happens at their own timescale which is dependent on the isolation of the spin chain from the environment, ranging from seconds to picoseconds.

Cavity optomechanics studies the interaction between mechanical resonators and optical cavities through radiation pressure forces and aims to harness this interaction for applications in the areas of high precision metrology, tests of fundamental quantum mechanics, or quantum information processing. For the most ambitious of these applications it is necessary that the mechanical resonator has a sufficiently high mechanical quality factor such that it can undergo at least a few coherent oscillations before interacting with incoherent thermal phonons. Furthermore, the optomechanical coupling must be large enough to make the interaction between optics and mechanics probable and, ideally, deterministic.
This work pursues both goals using a thin membrane in the middle (MIM) of an optical cavity. This is a common configuration in cavity optomechanics but most experiments to date have lowmechanical quality factors and optomechanical couplings.

This dissertation concerns transport measurements in single-molecule junctions using the mechanically controlled break junction (MCBJ) technique. It describes various aspects that play a role in charge transport through single molecules, in order to develop the necessary knowledge to ultimately develop electronic devices based on intrinsic molecular functionality.

In this thesis, the microwave detection of mechanically compliant objects is investigated. This starts with a system of a suspended metal drum capacitively coupled to a high impedance microstrip resonator. The mechanical non-linear dissipation of the drums is studied. Next, a suspended nanowire coupled to a CPW resonator is studied. With an electrostatic drive at twice the mechanical resonance frequency, there occurs a parametric excitation of either the mechanical signal or the coupled microwave resonance frequency of the cavity. Then the microwave loss in flux-tunable resonators is investigated for future experiments. One of the goals of this project was to couple a suspended nanowire with a SQUID loop of a flux tunable cavity. Here, the dielectric loss in flux tunable resonators is studied in order to optimize the design of future devices.

Physics at the level of an atom is dominated by laws of quantum mechanics. Often, this is entangled with a high complexity in behavior of the systems at that length scale. Unravelling the properties of a material at the atomic level is, therefore, a challenging task that easily supersedes current computational capabilities. A route to circumvent this problem is found in physical realization of simpler quantum systems that are representative of the complex quantum systems one is interested in. These simpler physical systems, unlike their more complex counterparts, can actually be measured and information about the complex system, otherwise inaccessible, gained. This thesis describes experimental work focusing mainly on the property of magnetism in spin chains. To mimic these complex systems, we employ a scanning tunneling microscope (STM) to build atomic chains on solid state surfaces and probe their magnetic properties. The intrinsic strength of STM in building and testing structures with single atom precision makes STM a great candidate for simulation of complex quantum systems. In addition to STM having a role of a quantum simulator, I present work supporting STM as a control device determining the very existence of the magnetic excitations of the atom it measures. Finally, I present experimental findings that suggest we are able to probe the magnetic excitations of the atom with subatomic resolution. In summary, this thesis work presents STM as a powerful probing and control tool for studies on quantum magnetism at the level of a single atom.

This thesis is about measurements of the electrical properties of different families of molecules. The research was focused on fundamental questions of charge transport in organic molecules. With this aim, experiments were performed to shed light on the underlying transport mechanisms. For example, an important ingredient of the work concerned the understanding of quantum interference effects in transport. On the other hand, several bio-inspired molecules were tested as electrical components revealing, for example, that with a certain chemical design a single-curcuminoid molecule act as a switch.

This thesis revolves around nanomechanical membranes made of suspended two - dimensional materials. Chapters 1-3 give an introduction to the field of 2D-based nanomechanical devices together with an overview of the underlying physics and the measurementtools used in subsequent chapters. The research topics that are discussed can bedivided into four categories: characterisation (Chapters 4 and 5), sensors (Chapter 6),actuators (Chapters 7 and 8) and novel materials (Chapter 9).

This thesis is an experimental investigation of the physical properties of different transition metal oxide ultra-thin films. A common feature of these various materials and structures is that they exhibit a solid-state phase transition from a metallic to an insulating state, which is triggered upon changing sample composition, or by varying an external stimulus such as temperature, illumination or gas pressure. The experiments performed cover a broad spectrum of condensed matter, from material growth, structural characterisation and nanodevice fabrication to low-temperature magnetotransport, synchrotron microscopy and gas sensing.

In this work, we investigate a series of physical phenomena resulting from the confinement of electrons in those localised charge and spin quantum boxes generically called molecules.