OBSERVED NONLINEAR RESPONSES IN PATTERNED SUPERCONDUCTING, FERROMAGNETIC, AND INTERACTING THIN FILMS

Many advances in technology ranging from biology and medicine through engineering and computer science to fundamental physics and chemistry depend upon the capability to control the fabrication of materials and devices at the submicron scale. Quantum mechanical effects become increasingly important to atomic and molecular interactions as the distances between neighbors decrease. These effects will provide materials and device designers with additional flexibility to establish properties of the designers choice, but the cost of this additional flexibility must be paid in the complexity of nonlinearities entering the interactions and the design process. The work presented here has provided several early results on three such interactions among closely-spaced submicron material structures: 1) the properties of superconductivity have been studied, 2) the properties of ferromagnetism have been studied, and 3) the interactions between superconductivity and ferromagnetism have been studied. Since our work was published, there has been considerable interest in all three of these wide-open areas and hundreds or thousands of additional results are now in the literature. We have used standard methods from the semiconductor industry as well as innovative methods to fabricate micron and submicron devices for observation. Standard optical lithography and standard electron beam lithography have been implemented to shape micron and submicron structures, respectively. Additionally, a laser interferometric lithography method has been invented and used to shape submicron structures. The materials used were vanadium, niobium, nickel, and/or permalloy. We have utilized SQUID magnetometry and Hall effect magnetometry to observe the properties of superconductor structures and superconductorferromagnetic mixed systems. We have used SQUID magnetometry and ferromagnetic resonance to observe the physical properties of ferromagnetic structures and the interactions between adjacent structures. Using these materials and methods we have discovered an unusual paramagnetic Meissner effect in thin Nb films that exists at igh-applied magnetic fields. We have discovered fluxoid matching anomalies at low sample temperature. And we have discovered interactions between electron exchange and magnetic dipole forces. Additionally, we have found clear evidence to support several past hypotheses advanced by other authors

Latent Room-Temperature Tc_c in Cuprate Superconductors

The ancient phrase, "All roads lead to Rome" applies to Chemistry and Physics. Both are highly evolved sciences, with their own history, traditions, language, and approaches to problems. Despite all these differences, these two roads generally lead to the same place. For high temperature cuprate superconductors however, the Chemistry and Physics roads do not meet or even come close to each other. In this paper, we analyze the physics and chemistry approaches to the doped electronic structure of cuprates and find the chemistry doped hole (out-of-the-CuO$\mathrm{_2}$-planes) leads to explanations of a vast array of normal state cuprate phenomenology using simple counting arguments. The chemistry picture suggests that phonons are responsible for superconductivity in cuprates. We identify the important phonon modes, and show that the observed T$\mathrm{_c} \sim 100$ K, the T$\mathrm{_c}$-dome as a function of hole doping, the change in T$\mathrm{_c}$ as a function of the number of CuO$\mathrm{_2}$ layers per unit cell, the lack of an isotope effect at optimal T$\mathrm{_c}$ doping, and the D-wave symmetry of the superconducting Cooper pair wavefunction are all explained by the chemistry picture. Finally, we show that "crowding" the dopants in cuprates leads to a pair wavefunction with S-wave symmetry and T$\mathrm{_c}\approx280-390$ K. Hence, we believe there is enormous "latent" T$\mathrm{_c}$ remaining in the cuprate class of superconductors.Comment: 100 pages, 61 figure

Model Realization and Numerical Studies of a Three-Dimensional Bosonic Topological Insulator and Symmetry-Enriched Topological Phases

We study a topological phase of interacting bosons in (3+1) dimensions which is protected by charge conservation and time-reversal symmetry. We present an explicit lattice model which realizes this phase and which can be studied in sign-free Monte Carlo simulations. The idea behind our model is to bind bosons to topological defects called hedgehogs. We determine the phase diagram of the model and identify a phase where such bound states are proliferated. In this phase we observe a Witten effect in the bulk whereby an external monopole binds half of the elementary boson charge, which confirms that it is a bosonic topological insulator. We also study the boundary between the topological insulator and a trivial insulator. We find a surface phase diagram which includes exotic superfluids, a topologically ordered phase, and a phase with a Hall effect quantized to one-half of the value possible in a purely two-dimensional system. We also present models that realize symmetry-enriched topologically-ordered phases by binding multiple hedgehogs to each boson; these phases show charge fractionalization and intrinsic topological order as well as a fractional Witten effect.Comment: 26 pages, 16 figure

Dimensional reduction, quantum Hall effect and layer parity in graphite films

The quantum Hall effect (QHE) originates from discrete Landau levels forming in a two-dimensional (2D) electron system in a magnetic field. In three dimensions (3D), the QHE is forbidden because the third dimension spreads Landau levels into multiple overlapping bands, destroying the quantisation. Here we report the QHE in graphite crystals that are up to hundreds of atomic layers thick - thickness at which graphite was believed to behave as a 3D bulk semimetal. We attribute the observation to a dimensional reduction of electron dynamics in high magnetic fields, such that the electron spectrum remains continuous only in the direction of the magnetic field, and only the last two quasi-one-dimensional (1D) Landau bands cross the Fermi level. In sufficiently thin graphite films, the formation of standing waves breaks these 1D bands into a discrete spectrum, giving rise to a multitude of quantum Hall plateaux. Despite a large number of layers, we observe a profound difference between films with even and odd numbers of graphene layers. For odd numbers, the absence of inversion symmetry causes valley polarisation of the standing-wave states within 1D Landau bands. This reduces QHE gaps, as compared to films of similar thicknesses but with even layer numbers because the latter retain the inversion symmetry characteristic of bilayer graphene. High-quality graphite films present a novel QHE system with a parity-controlled valley polarisation and intricate interplay between orbital, spin and valley states, and clear signatures of electron-electron interactions including the fractional QHE below 0.5 K

A Monte Carlo Approach for Studying Microphases Applied to the Axial Next-Nearest-Neighbor Ising and the Ising-Coulomb Models

The equilibrium phase behavior of microphase-forming systems is notoriously difficult to obtain because of the extended metastability of their modulated phases. In this paper we present a systematic simulation methodology for studying layered microphases and apply the approach to two prototypical lattice-based systems: the three-dimensional axial next-nearest-neighbor Ising (ANNNI) and Ising-Coulomb (IC) models. The method involves thermodynamically integrating along a reversible path established between a reference system of free spins under an ordering field and the system of interest. The resulting free energy calculations unambiguously locate the phase boundaries. The simple phases are not observed to play a particularly significant role in the devil's flowers. With the help of generalized order parameters, the paramagnetic-modulated critical transition of the ANNNI model is also studied. We confirm the XY universality of the paramagnetic-modulated transition and its isotropic nature. Interfacial roughening is found to play at most a small role in the ANNNI layered regime.Comment: 15 pages, 11 figures, 2 table

Strongly Correlated Quantum Fluids: Ultracold Quantum Gases, Quantum Chromodynamic Plasmas, and Holographic Duality

Strongly correlated quantum fluids are phases of matter that are intrinsically quantum mechanical, and that do not have a simple description in terms of weakly interacting quasi-particles. Two systems that have recently attracted a great deal of interest are the quark-gluon plasma, a plasma of strongly interacting quarks and gluons produced in relativistic heavy ion collisions, and ultracold atomic Fermi gases, very dilute clouds of atomic gases confined in optical or magnetic traps. These systems differ by more than 20 orders of magnitude in temperature, but they were shown to exhibit very similar hydrodynamic flow. In particular, both fluids exhibit a robustly low shear viscosity to entropy density ratio which is characteristic of quantum fluids described by holographic duality, a mapping from strongly correlated quantum field theories to weakly curved higher dimensional classical gravity. This review explores the connection between these fields, and it also serves as an introduction to the Focus Issue of New Journal of Physics on Strongly Correlated Quantum Fluids: from Ultracold Quantum Gases to QCD Plasmas. The presentation is made accessible to the general physics reader and includes discussions of the latest research developments in all three areas.Comment: 138 pages, 25 figures, review associated with New Journal of Physics special issue "Focus on Strongly Correlated Quantum Fluids: from Ultracold Quantum Gases to QCD Plasmas" (http://iopscience.iop.org/1367-2630/focus/Focus%20on%20Strongly%20Correlated%20Quantum%20Fluids%20-%20from%20Ultracold%20Quantum%20Gases%20to%20QCD%20Plasmas

Scale-free static and dynamical correlations in melts of monodisperse and Flory-distributed homopolymers: A review of recent bond-fluctuation model studies

It has been assumed until very recently that all long-range correlations are screened in three-dimensional melts of linear homopolymers on distances beyond the correlation length $\xi$ characterizing the decay of the density fluctuations. Summarizing simulation results obtained by means of a variant of the bond-fluctuation model with finite monomer excluded volume interactions and topology violating local and global Monte Carlo moves, we show that due to an interplay of the chain connectivity and the incompressibility constraint, both static and dynamical correlations arise on distances $r \gg \xi$. These correlations are scale-free and, surprisingly, do not depend explicitly on the compressibility of the solution. Both monodisperse and (essentially) Flory-distributed equilibrium polymers are considered.Comment: 60 pages, 49 figure

Exploring twisted bilayer graphene with nano-optics

Nano-optics studies the behaviour of light on the nanoscale. In particular, it probes the interaction of light with objects, often of nanometre-size, and reveals fine details of the material's optical properties. Optoelectronics is an integral part of optics and describes the interaction between light and electronics, such as the detection of light and subsequent conversion to an electrical signal. Understanding such mechanisms at the nanoscale is of importance for improving imaging and light-harvesting applications. In this Thesis, we apply near-field microscopy to study optics on the nanoscale. It probes optical properties using light interacting with the near-field electromagnetic field near the material's surface. Twisted bilayer graphene (TBG) is formed by stacking two layers of graphene - a one-atom-thick sheet of carbon atoms - with a small twist angle. This causes an interference pattern in the atomic lattice called a moiré pattern, which affects the electronic properties dramatically. The discovery of unconventional superconductivity in TBG in 2018 made it a thriving field of research. Adding to this, TBG revealed strongly correlating states and topological features, making it a host of tunable exotic phases that may shed light on the origins of unconventional superconductivity. These phenomena motivate us to study the optical properties of TBG on a nanoscale, which have received little attention thus far. In the first part of this Thesis, I describe spatially oscillating patterns within selected regions of TBG that we detected using near-field microscopy. We interpret them as a manifestation of plasmons --- electrons moving collectively in a wave-like pattern --- driven by interband transitions. We model these areas with a reduced interlayer coupling, which enhances the strength of interband transitions and explains the observed plasmon dispersion. After this, I discuss large-scale periodic features observed in minimally twisted bilayer graphene (¿ < 0.1 deg) by photocurent nanoscopy. For these small twist angles, the atoms rearrange in triangular domains separated by a network of domain walls. We find that the domain walls convert heat injected in the domains into a measurable current via the photothermoelectric effect. Our results uncover the sharp changes in electronic properties at the domain walls, which govern the optoelectronic response. I focus in the second part of the Thesis on the development of new experimental techniques, which enable nano-optical studies on exotic states of TBG and its relatives. I show that the semiconducting material WSe2 can be used as an ambipolar transparent top gate for infrared near-field experiments. This enables full control of the carrier density and transverse displacement field without blocking near-field access. Hereafter, I describe a commercial cryogenic near-field microscope with a base temperature of 10 K, which required modifications for reliable operation. I present an active damping system to oppose the vibrations in the system and enhance the mechanical stability. We further improve the AFM stability by changing the AFM excitation position. In the final two Chapters I examine the photoresponse of TBG at low temperature. We observe semi-periodic modulations across our sample, which we believe manifests a second-order superlattice arising from TBG aligned to the hBN substrate in combination with strain. In a different sample, we reveal a spatially inhomogeneous response from which we deduce a map of the local twist angle.La nano-óptica estudia el comportamiento de la luz en la nanoescala. En particular, mide la interacción de la luz con objetos, normalmente de tamaño nanométrico, y revela los detalles de las propiedades ópticas del material. La optoelectrónica es una parte integral de la óptica y describe la interacción entre la luz y la electrónica, como por ejemplo la detección de la luz y su sucesiva conversión a una señal eléctrica. Entender estos mecanismos en la nanoescala es de vital importancia para mejorar sus aplicaciones en imagen y en captación de luz. En esta Tesis, aplicamos la técnica de microscopía de campo-cercano para estudiar óptica en la nanoescala. Medimos las propiedades ópticas usando luz que interacciona con el campo electromagnético cercano a la superficie del material. Una bicapa de grafeno rotada (TBG por sus siglas en inglés) se forma al apilar dos capas de grafeno -una lámina de carbono de un solo átomo de grosor- con un pequeño ángulo entre ellas. Esto provoca un patrón de interferencia en la red atómica que se llama patrón moiré, que afecta las propiedades electrónicas dramáticamente. El descubrimiento de superconductividad no-convencional en TBG en el 2018 lo convirtió en un campo de investigación en auge. Además, el TBG ha revelado estados fuertemente correlacionados y características topológicas, convirtiéndolo en un portador de fases exóticas ajustable que podría arrojar luz sobre los orígenes de la superconductividad no-convencional. Estos fenómenos nos motivan a estudiar las propiedades ópticas del TBG en la nanoescala, que hasta ahora has recibido poca atención. En la primera parte de esta Tesis, describo patrones que oscilan espacialmente dentro de las regiones seleccionadas de TBG que detectamos usando microscopía de campo-cercano. Los interpretamos como la manifestación de plasmones -electrones moviéndose colectivamente en un patrón ondulatorio- promovidos por transiciones inter-banda. Modelamos estas áreas con un acoplamiento inter-capa, lo cual mejora la fuerza de las transiciones inter-banda y explica la dispersión plasmónica observada. Después de esto, hablo de características periódicas de gran escala observadas en bicapas de grafeno rotadas mínimamente (θ < 0.1°) usando nanoscopía de fotocorriente. Para estos pequeños ángulos, los átomos se reagrupan en dominios triangulares separados por una red de paredes de dominio, que gobiernan la respuesta optoelectrónica. En la segunda parte de la Tesis me concentro en el desarrollo de nuevas técnicas experimentales, que permiten estudios nano-ópticos en estados exóticos de TBG y familiares. Enseñaré que el material semiconductor WSe2 puede ser usado como una puerta superior para experimentos de campo-cercano en el infrarrojo. Esto permite un control completo de la densidad de portadores y del campo de desplazamiento eléctrico sin bloquear el acceso del campo-cercano. Sucesivamente, describo un sistema comercial de microscopía de campo-cercano con una temperatura base de 10 K, que requirió modificaciones para una operación fidedigna. Presento un sistema de amortiguación activa para contrarrestar vibraciones en el sistema y mejorar la estabilidad mecánica. Continuamos mejorando la estabilidad del AFM cambiando la posición de su excitación mecánica. En los dos capítulos finales examino la fotorespuesta del TBG a temperaturas bajas. Observamos modulaciones semi-periódicas en nuestra muestra, que creemos que manifiesta una super-red que proviene del TBG estando alineado con el substrato de hBN en combinación con deformación. En una muestra diferente, revelamos una respuesta espacial inhomogénea con la que deducimos un mapa del ángulo de rotación.Postprint (published version