First, I introduce the concept of heavy fermion systems and discuss the ease of tuning their properties with external parameters including pressure, chemical doping, and magnetic fields to induce new quantum states such as unconventional superconductivity. I then delve into the limited use of dimensionality as a tuning knob for quantum criticality and highlight the new possibilities available if heavy fermion behavior is discovered in the single-layer limit. Chapter 1 establishes the van der Waals material, CeSiI, as a heavy fermion system and is the first material where heavy fermion behavior exists down to the few-layer limit. The chapter further explores the bulk magnetic properties and electronic structure of CeSiI at high magnetic fields. The quasi-two-dimensional electronic character of CeSiI leads to anisotropic hybridization between local moments and conduction electrons, a phenomenon previously only realized in theoretical calculations. With the heavy fermion properties of CeSiI established, Chapter 2 investigates the effects of pressure and La-doping on CeSiI, aiming to push it from the antiferromagnetic region of the Doniach phase diagram towards a quantum critical point. Preliminary evidence suggests that CeSiI is too distant from quantum criticality. Instead, La-doping is utilized to explore single-ion Kondo physics at the dilute Ce limit in CeSiI. Additionally, CeGaI, with a crystal structure similar to CeSiI, is examined. Although no Kondo physics is observed, the magnetic and electronic properties remain coupled to each other. Chapter 3 delves into a separate project focusing on the study of polymers composed of perylene diimide and various organic linkers. It explores how the structure of the polymer influences its pseudocapacitance properties. The chapter demonstrates the significance of contortion in device performance, aiming to provide insights for future endeavors in developing environmentally friendly energy storage systems.

This dissertation focused on the potential use of highly charged nanoparticles to stabilize dispersions of weakly charged microparticles. The experimental components of the project centered on a model colloidal system containing silica microparticles at the isoelectric point where the suspensions are unstable and prone to flocculation. The stability of the silica suspensions was studied in the presence of highly charged nanoparticles. Initial experiments used polystyrene latex with either sulfate or amidine surface groups. Effective zeta potentials were measured with nanoparticle concentrations ranging from 0.001% to 0.5% vol. Adsorption levels were determined through direct SEM imaging of the silica microparticles, showing that the nanoparticles directly adsorbed to the microparticles (amidine more than sulfate), producing relatively large effective zeta potentials. However, stability experiments showed that the latex nanoparticles did not stabilize the silica but merely provided a reduction in overall flocculation rate. It was concluded that the zeta potential was an insufficient predictor of stability as there was still sufficient patchiness on the surface to allow for the silica surfaces to aggregate. Experiments using zirconia and alumina nanoparticles did achieve effective stabilization; both types stabilized the silica suspensions for longer than the observation period of approximately 15 hours. Stability was observed at concentrations of 10^-4% to 1.0% (zirconia) and 10^-2% vol. (alumina). These particles adsorbed directly to the microparticles (confirmed via SEM) and produced increasing effective zeta potentials with increasing nanoparticle concentrations. The adsorption resulted in significant electrostatic repulsion that was determined to be effectively irreversible using colloidal probe AFM. The improved stabilizing ability was attributed to the increased van der Waals attraction between the oxide nanoparticles (compared to polystyrene). Finally, an unexpected result of the CP-AFM force measurements showed that the repulsive forces between a nanoparticle-coated particle and plate lacked the normal dependence on the radius of the probe as predicted by the Derjaguin approximation. The forces observed in nanoparticle suspensions were virtually identical for 5 µm and 30 µm probes. Based on calculations of the shear rate in the gap, it was theorized that this phenomenon may have resulted from the shearing of adsorbed particles from the surfaces, which leads to similar interaction geometries for the two probe sizes. / Ph. D.

No description available.

Understanding and controlling how phonons, or lattice vibrations, affect the behavior of photo-excited energy carriers is a longstanding goal of physical and materials chemistry. Thermal phonons, the observables in steady-state measurements such as infrared (IR) and Raman spectroscopies, are random fluctuations in nuclear displacements that scatter carriers, reducing electronic transport efficiency, and resulting in Joule heating. In contrast, coherent phonons exhibit synchronized atomic motion with maintained phase relations over extended spatial scales, often resulting in long-range vibrational energy propagation at well-defined group velocities. This thesis demonstrates that coherent phonons can enhance the transport of photo-excited electronic carriers, including electrons and excitons – strongly bound electron-hole pairs – through dynamic trapping by the propagating deformation potential they create. Our primary material platform is tungsten diselenide (WSe₂), a van der Waals semiconductor prone to strong carrier–phonon interactions due to dimensional confinement and large deformation potential interactions. Using ultrafast spatiotemporal microscopy, we directly image nonequilibrium electronic carrier transport in the presence versus the absence of either coherent acoustic phonons or phonon-polaritons. First, we show that laser-generated coherent acoustic phonons (sound/strain waves) in multilayer WSe₂ can carry free charges and excitons over microns at the speed of sound, paving the way for the miniaturization of surface acoustic wave-driven devices that are gaining increasing attention in quantum information science. Second, we show for the first time that hyperbolic phonon-polaritons in boron nitride can drag excitons in monolayer WSe₂across a van der Waals gap, leading to exciton transport at light-like speeds approaching the exciton Fermi velocity over picosecond scales. These studies highlight the potential of leveraging coherent species such as phonons to achieve orders-of-magnitude enhancement of electronic carrier transport via inter-particle interactions.

In the first study, superconducting 2D NbSe₂ was examined towards its prototypical demonstration as a transition-edge sensor, where photoexcitation caused a thermodynamic phase transition in NbSe₂ from the superconducting state to the normal state. The efficacy of the optical absorption was found to depend on the wavelength of the incoming radiation used, which ranged from the ultra-violet (405 nm), visible (660 nm), to the infrared (1060 nm). In the second case involving WSe₂, the UV-ozone treatment revealed the presence of localized excitonic emission in 1L WSe₂ that was robust and long-lived. Our third material platform dealt with hybrid 0D-2D ensembles based on graphene and WSe₂, specifically graphene–endohedral, WSe₂–fullerene (C₆₀), and WSe₂–Au nanoparticles, and exhibited exceptional performance gains achieved with both types of hybrid structures. Next, we investigated WSe₂ based mixed dimensional hybrids. Temperature T-dependent and wavelength λ-dependent optoelectronic transport measurements showed a shift in the spectral response of 1L WSe₂ towards the SPR peak locations of Au-Sp and Au-BP, fostered through the plexciton interactions. Models for the plexcitonic interactions are proposed that provide a framework for explaining the photoexcited hot charge carrier injection from AuNPs to WSe₂ and its influence on the carrier dynamics in these hybrid systems. Last, we studied interactions of vdWs hybrid structures composed of WSe₂ with 0D buckminsterfullerene (C₆₀) spheres. Our results indicate that the C₆₀-WSe₂ vdWs hybrid heterostructure appears to be an attractive architecture for enabling charge transfer and high performance photodetection capabilities. T-dependent electrical transport measurements after C₆₀ deposition revealed a dominant p-type conduction behavior and a significant ×10³ increase in WSe₂ field-effect mobility, with a maximum field-effect mobility of 281 cm²V⁻¹s⁻¹ achieved at 350 K and room-T mobility of 119.9 cm²V⁻¹s⁻¹ for the C₆₀-WSe₂ hybrid.

In this work, we study the properties and device applications of MoS2, black phosphorus, MoOx, and NbSe2. We first start with the design, fabrication, and characterization of ultra-high responsivity photodetectors based on mesoscopic multilayer MoS2. The device architecture is comprised of a metal-semiconductor-metal (MSM) photodetector, where Mo was used as the contact metal to suspended MoS2 membranes. The dominant photocurrent mechanism was determined to be the photoconductive effect, while a contribution from the photogating effect was also noted from trap-states that yielded a wide spectral photoresponse from UV-to-IR with an external quantum efficiency (EQE) ~ 104. From time-resolved photocurrent measurements, a fast decay time and response time were obtained with a stream of incoming ON/OFF white light pulses. Another interesting semiconductor 2D material that has attracted special attention due to its small bandgap and ultra-high hole mobility is the black phosphorus. An analysis of the optoelectronic properties and photocurrent generation mechanisms in two-dimensional (2D) multilayer crystallites of black phosphorus (BP) was conducted from 350 K down to cryogenic temperatures using a broad-band white light source. The Mo-BP interface yielded a low Schottky barrier "φ" _"SB" ~ -28.3 meV and a high photoresponsivity R of ~ 2.43 x 105 A/W at a source-drain bias voltage of ~ 0.5 V (300 K, and incident optical power ~ 3.16 μW/cm2). Our report is the first to highlight the empirical use of Mo as a contact metal with BP. From the analysis conducted on the BP devices, the thermally driven photocurrent generation mechanism arising from the photobolometric effect (PBE) dominated the carrier dynamics for T > 181 K since the photocurrent Iph and the bolometric coefficient β undergo a transition in polarity from positive to negative. Our results show the promise of BP to potentially advance thermoelectric and optoelectronic devices stemming from this mono-elemental, direct bandgap 2D van der Waals solid. Another intriguing metallic 2D material is superconducting 2H-NbSe2. Here we present the temperature-dependent Raman spectroscopy and electronic transport on bulk NbSe2, carried out to investigate the scattering mechanisms. We report on the photoresponse of direct probed mesoscopic 2H-NbSe2 as a function of laser energy for lasers at 405 nm, 660 nm, and 1060 nm wavelengths used to irradiate the device, where the modulation from the superconducting-to-normal-state is detected through photomodulation. Additionally, the various oxidation levels of molybdenum oxide have interesting optical and electrical properties as a function of the oxygen vacancy and stoichiometry. The substoichiometric MoOx (2 < x < 3) behaves as a high work function conductor due to its metallic defect band. As a result, one of the potential applications of MoOx is for electrical contacts providing high hole injection or extraction. In this work, we have synthesized MoOx nanosheets via chemical vapor deposition and a four-terminal device was fabricated via e-beam lithography and electronic transport was measured as a function of temperature. Outstanding properties were obtained from our MoOx nanosheets, including a high conductivity of ~ 6,680.3 S cm-1, a superior temperature coefficient of resistance ~ -0.10%, and a high sensitivity based on the bolometric coefficient β of ~ 0.152 mS K-1. In summary, this work pushes the state-of-the-art in enabling 2D van der Waals materials for next-generation high-performance detectors.

This thesis considers the problem of determining Casimir-Lifshitz forces in inhomogeneous media. The ground-state energy of the electromagnetic field in a piston-geometry is discussed. When the cavity is empty, the Casimir pressure on the piston is finite and independent of the small-scale physics of the media that compose the mirrors. However, it is demonstrated that, when the cavity is filled with an inhomogeneous dielectric medium, the Casimir energy is cut-off dependent. The local behavior of the stress tensor commonly used in calculations of Casimir forces is also determined. It is shown that the usual expression for the stress tensor is not finite anywhere within such a medium, whatever the temporal dispersion or index profile, and that this divergence is unlikely to be removed by modifying the regularisation. These findings suggest that the value of the Casimir pressure may be inextricably dependent on the detailed behavior of the mirror and the medium at large wave vectors. This thesis also examines two exceptions to this rule: first, the case of an idealised metamaterial is considered which, when introduced into a cavity, reduces the magnitude of the Casimir force. It is shown that, although the medium is inhomogeneous, it does not contribute additional scattering events but simply modifies the effective length of the cavity, so the predicted force is finite and can be stated exactly. Secondly, a geometric argument is presented for determining a Casimir stress in a spherical mirror filled with the inhomogeneous medium of Maxwell's fish-eye. This solution questions the idea that the Casimir force of a spherical mirror is repulsive, but prompts additional questions concerning regularisation and the role of non-local effects in determining Casimir forces.

Les atomes neutres sont des candidats prometteurs pour la réalisation et l’étude d’états intriqués à quelques dizaines de particules. Pour générer de tels états, une approche consiste à utiliser le mécanisme de blocage dipolaire résultant des fortes interactions dipôle-dipôle entre atomes de Rydberg.Suivant cette approche, cette thèse présente la conception et la caractérisation d’un dispositif expérimental permettant de manipuler des atomes de 87Rb individuels piégés dans des pinces op- tiques microscopiques, et à les exciter vers des états de Rydberg. Un environnement électrostatique stable et des électrodes de contrôle permettent une manipulation fine de ces états. Avec deux pinces optiques séparées de quelques microns, nous démontrons le blocage de Rydberg entre deux atomes, et nous observons leur excitation collective.Enfin, en opérant en régime de blocage partiel, nous développons une méthode permettant de mesurer l’interaction de van der Waals ∆E = C6 /R6 entre deux atomes séparés par une distance R contrôlée. Les coefficients C6 obtenus pour différents états de Rydberg sont en bon accord avec des calculs théoriques ab initio, et nous observons l’augmentation spectaculaire de l’interaction en fonction du nombre quantique principal n de l’état de Rydberg. / Neutral atoms are promising candidates for the realization of entangled states involving up to a few tens of particles. To generate such states, one approach consists in using the dipole blockade mechanism, which results from the strong dipole-dipole interactions between Rydberg atoms.Following this approach, this thesis describes the design and the characterization of an experimental apparatus allowing to manipulate single 87Rb atoms trapped in microscopic optical tweezers, and to excite them towards Rydberg states. A stable electrostatic environment and controlled electrodes enable the fine manipulation of these states. Using two optical tweezers separated by a few microns, we demonstrate the Rydberg blockade between two single atoms, and we observe their collective excitation.Finally, by operating in the partial blockade regime, we develop a method allowing to measure the van der Waals interaction ∆E = C6 /R6 between two atoms separated by a controlled distance R. The C6 coefficients obtained for various Rydberg states agree well with ab initio theoretical calculations, and we observe the dramatic increase of the interaction with the principal quantum number n of the Rydberg state.

La plupart des objets quantiques individuels développés jusqu’à aujourd’hui ne permettent pas de satisfaire toutes les conditions nécessaires pour la construction d’un simulateur quantique. Une possibilité pour obtenir un système quantique robuste est de combiner plusieurs de ces approches. Dans cette thèse, nous décrivons les résultats obtenus sur deux systèmes expérimentaux développés dans ce but.La première partie de cette thèse décrit un système hybride d’atomes neutres couplés à des qubits supraconducteurs, en construction à l’Université du Maryland. La solution envisagée pour placer un ensemble d’atomes froids à proximité de la surface supraconductrice est de piéger les atomes dans le champ évanescent se propageant autour d’une fibre optique nanométrique. Nous avons développé un dispositif permettant la production de fibres optiques nanométriques de transmission optique supérieure à 99.95% dans le mode fondamental. Nous avons également optimisé la transmission de quelques modes d’ordres supérieurs, ce qui pourra s’avérer utile pour le piégeage d’atomes.La seconde partie de cette thèse décrit un système développé à l’Institut d’Optique et comprenant des atomes neutres piégés dans des matrices de pinces optiques. Dans ce cas, nous excitons les atomes dans des états de Rydberg afin de bénéficier de fortes interactions interatomiques. Nous avons caractérisé les interactions de van der Waals et les interactions résonantes entre deux atomes individuels, et démontré le caractère cohérent de l’interaction dipolaire. Nous avons enfin simulé la dynamique d’une chaine élémentaire de spins dans une matrice de trois atomes / Most platforms that are being developed to build quantum simulators do not satisfy simultaneously all the requirements necessary to implement useful quantum tasks. Robust systems can be constructed by combining the strengths of multiple approaches while hopefully compensating for their weaknesses. This thesis reports on the progress made on two different setups that are being developed toward this goal.The first part of this thesis focuses on a hybrid system of neutral atoms coupled to superconducting qubits that is under construction at the University of Maryland. Sub-wavelength diameter optical fibers allow confining an ensemble of cold atoms in the evanescent field surrounding the fiber, which makes them ideal for placing atoms near a superconducting surface. We have developed a tapered fiber fabrication apparatus, and measured an optical transmission in excess of 99.95% for the fundamental mode. We have also optimized tapered fibers that can support higher-order optical modes with high transmission, which may be useful for various optical potential geometries.The second part of this thesis focuses on a system of neutral atoms trapped in arrays of optical tweezers that has been developed at the Institut d’Optique. Placing the atoms in highly excited Rydberg states allows us to obtain strong interatomic interactions. Using two individual atoms, we have characterized the pairwise interactions in the van der Waals and resonant dipole-dipole interaction regimes, providing a direct observation of the coherent nature of the interaction. In a three-atom system, we have finally simulated the dynamics of an elementary spin chain

The Casimir-Polder interaction between a single neutral atom and a nearby surface, arising from the (quantum and thermal) fluctuations of the electromagnetic field, is a cornerstone of cavity quantum electrodynamics (cQED), and theoretically well established. Recently, Bose-Einstein condensates (BECs) of ultracold atoms have been used to test the predictions of cQED. The purpose of the present thesis is to upgrade single-atom cQED with the many-body theory needed to describe trapped atomic BECs. Tools and methods are developed in a second-quantized picture that treats atom and photon fields on the same footing. We formulate a diagrammatic expansion using correlation functions for both the electromagnetic field and the atomic system. The formalism is applied to investigate, for BECs trapped near surfaces, dispersion interactions of the van der Waals-Casimir-Polder type, and the Bosonic stimulation in spontaneous decay of excited atomic states. We also discuss a phononic Casimir effect, which arises from the quantum fluctuations in an interacting BEC. / Die durch (quantenmechanische und thermische) Fluktuationen des elektromagnetischen Feldes hervorgerufene Casimir-Polder-Wechselwirkung zwischen einem elektrisch neutralen Atom und einer benachbarten Oberfläche stellt einen theoretisch gut untersuchten Aspekt der Resonator-Quantenelektrodynamik (cavity quantum electrodynamics, cQED) dar. Seit kurzem werden atomare Bose-Einstein-Kondensate (BECs) verwendet, um die theoretischen Vorhersagen der cQED zu überprüfen. Das Ziel der vorliegenden Arbeit ist es, die bestehende cQED Theorie für einzelne Atome mit den Techniken der Vielteilchenphysik zur Beschreibung von BECs zu verbinden. Es werden Werkzeuge und Methoden entwickelt, um sowohl Photon- als auch Atom-Felder gleichwertig in zweiter Quantisierung zu beschreiben. Wir formulieren eine diagrammatische Störungstheorie, die Korrelationsfunktionen des elektromagnetischen Feldes und des Atomsystems benutzt. Der Formalismus wird anschließend verwendet, um für in Fallen nahe einer Oberfläche gehaltene BECs Atom-Oberflächen-Wechselwirkungen vom Casimir-Polder-Typ und die bosonische Stimulation des spontanen Zerfalls angeregter Atome zu untersuchen. Außerdem untersuchen wir einen phononischen Casimir-Effekt, der durch die quantenmechanischen Fluktuationen in einem wechselwirkenden BEC entsteht.