1,720,980 research outputs found
Scanning Tunneling Microscopy Studies of Correlated and Topological Electronic States
No full textUnderstanding the dramatically diverse behavior of electrons in different crystal environments demands the precise knowledge of their nature on the atomic length scale. The ability of scanning tunneling microscopy (STM) to map coherent quasiparticle interference (QPI) and emerging Landau levels (LL) has opened up a new avenue to visualize quantum mechanical phenomena and provided a new toolset to study various exotic materials. In this dissertation, I describe STM studies focusing on three distinct classes of materials: heavy fermions, topological materials and bismuth.
In systems hosting an array of magnetic moments, the interaction between the itinerant and localized electrons leads to the formation of heavy fermions. STM measurements performed on the (100) surface of CeCoIn demonstrates the quasi-two-dimensional, confined nature of the emerging excitations. The response of the superconducting order parameter to potential defects and magnetic field reveals a nodeless behavior of the order parameter in z direction, an elongated vortex structure and a spatially modulated pseudogap phase. By utilizing the tunability of the band structure of CeCoIn, a combined resonant x-ray scattering (RXS) and STM measurements indicate that the QPI can be detected by RXS.
Strong spin-orbit coupling in certain materials can lead to an inverted bulk band structure and consequently to topologically non-trivial phases. Exploring the QPI on topological (crystalline) insulators demonstrates the scattering of symmetry protected surface states, which can be understood based on the shape and spin texture of these surface states. Combined QPI and LL spectroscopy are used to explore the bulk topological band structure of Dirac semimetals, whereas on a Weyl semimetal the rich variety of scattering wavevectors demonstrates the momentum-dependent delocalization of the surface states into the bulk.
The surface of Bi(111) harbors a two-dimensional electron gas consisting of spin-split surface states of multiple electron and hole pockets. Spectroscopic mapping of the arising quantum Hall state shows that a combination of local strain and many-body Coulomb interactions lift the LL degeneracy to form valley-polarized quantum Hall states. The anisotropic LL wavefunctions with orientation corresponding to broken-symmetry state are imaged on the surface providing a direct spatial signature of a nematic electronic phase
Going Beyond Counting First Authors in Author Co-citation Analysis
Get PDFThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Measuring Non-Gaussian Magic in Fermions
Get PDFClassically hard-to-simulate quantum states, or "magic" states, are prerequisites to quantum advantage, highlighting an apparent separation between classically and quantumly tractable problems. Classically simulable states such as Clifford circuits on stabilizer states, free bosonic states, free fermions, and matchgate circuits are all in some sense Gaussian. While free bosons and fermions arise from quadratic Hamiltonians, recent works have demonstrated that bosonic and qudit systems converge to Gaussians and stabilizers under convolution. In this work, we similarly identify convolution for fermions and find efficient measures of non-Gaussian magic in pure fermionic states. We demonstrate that three natural notions for the Gaussification of a state—(1) the Gaussian state with the same covariance matrix, (2) the fixed point of convolution, and (3) the closest Gaussian in relative entropy—coincide by proving a central limit theorem for fermionic systems. We then utilize the violation of Wick's theorem and the matchgate identity to quantify non-Gaussian magic in addition to a SWAP test.</p
Least Action Approach To Lumped Element Circuit Mechanics
Get PDFIn the endeavor to design and implement useful quantum computers, various platforms haveseen varying levels of success, and the route to the era of fully scalable quantum computing remains shrouded in mystery. One such platform for quantum computation is superconducting circuits, which are a fruitful environment for the observation of quantum physics on mesoscopic scales. In any effort to characterize the quantum behavior of superconducting circuits, a classical theory of circuits with a straightforward route to quantization is necessary. In this thesis, we approach the problem of establishing a maximally general theory of circuits. First, we formalize a framework wherein a classical Hamiltonian may be derived for an arbitrary nondissipative circuit, assuming only that Kirchhoff’s laws admit a unique solution. We also provide an algorithm for deriving such Hamiltonians, and prove that it may always be executed successfully for nonsingular circuits. Secondly, we generalize the aforementioned framework in such a way that circuit duality is manifestly a relabeling transformation while providing novel insight into the ill–behaved properties of nonplanar circuit duals. Finally, we produce an even more general framework that captures the classical behavior of even dissipative circuits, including a formal argument that Johnson–Nyquist noise applies to all linear resistors and a derivation of Johnson–Nyquist noise for nonlinear resistors. We derive a principle of least action from which one may derive Langevin and Fokker–Planck equations describing the dynamics of circuits with linear and nonlinear dissipative elements alike.</p
Variations on the Author
Get PDF“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Get PDFWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
Mass Loaded Tensioned Mechanical Resonators
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Highly tensioned micromechanical and nanomechanical resonators have enabled novel experiments in quantum optomechanics and numerous applications in the field of precision sensing. This is in part due to engineering efforts that have allowed for resonator modes with ultrahigh quality factors. Typically, sensing applications require functionalizing resonators by adding local mass to them. However, this may dramatically lower the resonator mode quality factor, and hence its sensitivity. This thesis studies the effect of a local mass load on the shape and quality factor of a tensioned resonator mode. Through analytical models, finite element analysis, and tabletop experiments, we show that in the limit of a large mass load, the resonator mode quality factor saturates. This effect paves the way for engineering resonators with improved sensitivity for certain applications such as magnetic force detection, accelerometry, and gravitational force sensing.
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Exploration of Protected Qubit Architectures and Two-Qubit Gate Design
Get PDFThis dissertation explores three types of protected superconducting qubits: the rhombusqubit, the 0-π qubit, and the gyrator qubit. For the rhombus qubit, we analyze both symmetric and asymmetric circuit variants, quantizing the system and comparing their energy spectra. We find that the symmetric rhombus exhibits exponential protection against both depolarization and charge noise-induced dephasing. Importantly, we show that introducing asymmetry yields a firstorder flux noise sweet spot, which can significantly extend qubit coherence and improve gate depth.
For the soft 0-π qubit, we study single- and two-qubit gates using microwave drives. We demonstrate that driving nθ achieves higher-fidelity single qubit X gate than driving nφ, with simulated fidelities ∼ 99.9% and gate time ∼ 30 ns for T1 = Tφ = 30µs. We also develop two kinds of two-qubit gates, CZ and CNOT gates. These gates are realized using both direct and Raman-type transitions, with fidelities ∼ 99.9% and gate times around 160 ns for T1 = Tφ = 30µs.
We also propose a method to generate GKP states using parametrically induced gyration between coupled oscillators. We describe a circuit implementation using a modulated inductor and derive the corresponding quantum Hamiltonian. Importantly I show that having an imperfect gyration ratio degrades the code space of gyrator qubit.
Finally I develop a graph-theoretic approach to enumerate superconducting circuits and identify candidates with favorable coherence properties. Applying this framework, we compare transmon, fluxonium, and rhombus qubits in terms of relaxation time, dephasing, and expected gate count, finding that the asymmetric rhombus offers competitive, and potentially superior, performance.</p
Signal and Background Classification Model (BDT) Improvement & Systematics Study of J/ψ Radiative Tail and BDT Scale Factor
Get PDFThe study of Lepton Flavor Universality (LFU) has gained significant attention due
to its potential to reveal new physics (NP) beyond the Standard Model (BSM). Rare
B-meson decays, particularly the ratio R(K) between the branching fractions of B+ →
K+e+e− and B+ →K+µ+µ−, serve as key probes. While the Standard Model (SM)
predicts R(K) to be close to unity, experimental results show a deviation of up to 3.1σ
from SM expectations [1]. The analysis of R(K) involves several components, including
signal-background classification and systematic uncertainty studies. This thesis presents
improvements to a machine-learning-based classification model (Boosted Decision Tree,
BDT) for background discrimination through hyperparameter tuning. The performance
is validated using ROC and PR curves, AUC scores, and the Signal Significance method,
demonstrating an increase in signal significance from 4.88 at a BDT score cut of 3.4
(default model) to 5.61 at a cut of 3.61 (optimized model). Additionally, this thesis
evaluates two systematic uncertainties. The first arises from the J/ψ radiative tail due
to electron pair Bremsstrahlung, assessed by relaxing the lower dilepton mass (q2) window
from q2 = [2.9,3.2] GeV to q2 = [2.5,3.2] GeV, leading to a systematic uncertainty of
3.7 ±0.9%. The second concerns the BDT scale factor efficiency, accounting for residual
differences between data and Monte Carlo (MC) simulations. By varying the BDT
selection criteria, the efficiency ratio is determined to be 1.0047 ±0.0075 with a loose
BDT baseline cut (BDT>0), yielding a systematic uncertainty of 0.47 ±0.75%. These
studies contribute to a deeper understanding of R(K) measurements and the potential
violation of LFU.</p
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