1,720,991 research outputs found
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
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
Characterization of gate oxides and microwave resonators for silicon spin qubit devices
Get PDFSilicon quantum dots present themselves as a promising implementation for quantum
information processing due to the fact that they possess a small chip foot-print, yield high
coherence times and are able to leverage the semiconductor industry. These nanoscopic
devices rely on forming an electrostatic confinement for individual electrons. To accomplish
this, an overlapping metallic gate geometry is implemented. The overlapping metallic gates
are typically electrically isolated from one another by an insulating asher oxide layer. Due
to unreliable processes during quantum dot fabrication, we substitute the asher oxide for
a more robust and reliable oxide. For this, gate-oxide test structures are fabricated to
simulate the overlapping gate geometry while various oxides are grown and deposited. It is
found that oxides, grown by either ashing or hotplate in tandem with atomic layer deposited
Al2O3, yield substantially higher breakdown voltages than conventional methods.
One issue single electron spin qubits face is noise generated by charge traps. Charge
traps appear at the Si/Oxide interface and seriously impedes many aspects of a quantum
processor such as qubit coherence times, electrostatic screening effects and two-qubit gate
fidelities. Here, we characterize the density of interface traps on a multitude of oxides
found in the Quantum Nano-Fabrication and Characterization Facility. These oxides in clude plasma enhanced chemical vapour deposition (PECVD) SiO2, Tystar dry oxidation,
commercial thermal SiO2, atomic layer deposition (ALD) Al2O3 and ALD HfO2. We find
that each of these oxides is plagued by a high density of fixed charge and interface traps.
We also implement forming gas anneals at high temperature to help passivate the charge
traps. It is found that a forming gas anneal at 400C for 10 minutes reduces the number
of interface traps by several orders of magnitude and that PECVD SiO2 and ALD Al2O3
host the smallest interface trap density of approximately 1010 eV−1
cm−2
.
A major issue facing this implementation of quantum computing is the ability to scale up. Current methods of single qubit rotation are electron spin resonance and electron
dipole spin resonance. In either method, a high frequency (HF) transmission line is placed
nearby the quantum dots. For a large scale quantum computer, hundreds to thousands of
transmission lines and therefore HF interconnects will be necessary, dramatically increasing
the complexity of the device and reducing the qubit packing density. In this thesis, we
present an elegant solution, dramatically reducing the need for many interconnects. A
superconducting microresonator sits directly above the quantum processor providing an
oscillatory magnetic field to perform single qubit rotations over an area of 1 mm2
. With
a modest quantum dot pitch of 100 nm, approximately 40 million qubits can fit within
this region. The resonator possess a unique shape to minimize the electric field component
while maximizing the magnetic field ‘felt’ by the qubits. Electrons are tuned on and off of
iv
resonance by electrostatic tuning of the electron g-factor. We fabricate and characterize a
prototype resonator’s transmission coefficient to determine its resonant frequency at room
temperature and at 1.4K. Both measurements agree with simulations with a resonance
frequency of approximately 16 GHz
Development and Electronic Characterization of Graphene-Based Hall Effect Devices
No full textGraphene is a two-dimensional carbon material with a unique honeycomb lattice structure and exceptional electronic properties. Its band structure confines carriers to a single plane, allowing them to act like relativistic massless particles at low carrier densities. This has made graphene a focal point in condensed matter physics, particularly following the groundbreaking discovery of the first topological state using a graphene lattice. Research into graphene's potential as a platform for quantum topological computing has surged. In addition to its distinct band interactions, graphene is also being studied as a potential standard for electrical resistance. However, progress in its isolation since its initial synthesis in 2004 has been limited.
This thesis focuses on the synthesis of single-layer graphene (SLG) through low-pressure chemical vapor deposition (LPCVD) on copper films at temperatures above 1000 °C. The graphene films are transferred using a wet transfer technique and characterized with atomic force microscopy (AFM) and Raman spectroscopy. Hall devices for electrical transport studies are patterned using maskless alignment photolithography, with palladium as ohmic contacts. Electronic transport measurements are conducted at cryogenic temperatures up to a magnetic field of 5T using 4-terminal measurement techniques.
Moreover, this work explores electronic transport in twisted bilayer graphene (TBG) - tungsten diselenide WSe2 Hall devices. This structure facilitates the study of strongly correlated electronic states enhanced by spin-orbit coupling induced by WSe2. Preliminary experiments to detect unconventional Hall states in similar devices are carried out at millikelvin temperatures and in magnetic fields up to 18 Teslas
Carbon nanotube electromechanical systems: Non-linear dynamics and self-oscillation
Get PDFThis thesis is motivated by the many sensing applications of carbon nanotube (CNT) nano-electromechanical systems (NEMS), both previous state-of-the-art demonstrations and proposed new uses. This research is particularly focused on the long term goal of realizing the magnetic force sensing of molecular nanomagnets, proposed in reference [1].
The fabrication of micron long, small diameter, high quality suspended carbon nanotubes is a challenging task. Integrating ferromagnetic structures which are incompatible with the CNT growth procedures increases this challenge. In this thesis, devices suitable for magnetic force sensing experiments are realized by separating the chemical vapour deposition growth of CNTs from the device contacts and gates, while maintaining CNT quality.
Using conventional readout techniques, the low-temperature measurement of the CNT NEMS mechanical state is usually limited by the CNT contact resistance and capacitance of the measurement cabling/circuit. I describe the use of a heterojunction bipolar transistor (HBT) amplifying circuit operating at cryogenic temperatures near the device to measure the mechanical amplitude at microsecond timescales. A Coulomb rectification scheme, in which the probe signal is at much lower frequency than the mechanical drive signal, allows investigation of the transient response with strongly non-linear driving. The transient dynamics in both the linear and non-linear regimes are measured and modeled by including Duffing and non-linear damping terms in a harmonic oscillator equation. The non-linear regime can result in faster sensing response times, on the order of 10 μs for the device and circuit presented.
Self-driven oscillations in suspended carbon nanotubes can create apparent instabilities in the electrical conductance of the CNT. In literature, such instabilities have been observed in kondo regime or high bias transport. In this thesis, I observed self-driven oscillations which created significant conduction within the nominally Coulomb-blockaded low-bias transport. Using a master equation system model, these oscillations are shown to be the result of strongly energy dependent electron tunneling to the contacts of high quality CNT NEMS operated at sub-Kelvin temperatures.
Finally, in a separate research project, I consider the noise characterization of spin qubits interacting with the environment. In particular, I address the problem of probing the spectral density S(ω) of semi-classical phase noise using a spin interacting with a continuous-wave (CW) resonant excitation field. Previous CW noise spectroscopy protocols have been based on the generalized Bloch equations (GBE) or the filter function formalism, and assumed weak coupling to a Markovian bath.
However, those protocols can substantially underestimate S(ω) at low frequencies when the CW pulse amplitude becomes comparable to S(ω). I derive the coherence decay more generally by extending to higher orders in the noise strength and discarding the Markov approximation. Numerical simulations show that this provides a more accurate description of the spin dynamics compared to a simple exponential decay, especially on short timescales. Exploiting these results, a new protocol is developed that uses an experiment at a single CW pulse amplitude to extend the spectral range over which S(ω) can be reliably determined, down to ω=0
Proximity Superconductivity in Indium Arsenide Two-Dimensional Electron Gas Devices
No full textOf the many theoretical proposals for quantum computers, topological quantum computing is unique in its resistance to decoherence and the reliability of its gate operations. One proposed method for achieving these topological qubits is to harness the unusual non-Abelian exchange statistics of quasiparticle excitations known as Majorana bound states. Historically, research devoted to realizing these states has primarily been in nanowires, but purely one-dimensional devices are limited in their applications. Two-dimensional electron gas devices are an alternative with the benefit of future scalability and increased options for device geometries. To this end, we developed InAs/AlGaSb surface quantum well devices compatible with the proximity-induced superconductivity required to realize a Majorana device.
Magnetotransport measurements investigating mobility-density relationships, I-V characteristics, the Shubnikov de Haas effect, and the quantum Hall effect confirm the very high quality of our dielectric deposition method and growths. Even with quantum wells so near the surface of the device, we achieve high mobilities and stable, reproducible gating characteristics. These devices have high spin-orbit coupling coefficients, confirming that we can simultaneously benefit from the inherent bulk properties of InAs and properties imparted by the rest of the growth and lithography steps. Analytical comparisons of devices with different quantum well widths, interface characters, and dielectric deposition methods reinforce the need for the rigorous optimization of numerous factors. From this analysis, we conclude that devices with smooth surface morphologies, SiO2 dielectric
deposited by atomic layer deposition, and InSb-like interfaces provide the ingredients necessary to achieve near-record mobilities and consistent gating properties.
On these same excellent wafers, we fabricated superconductor-normal-superconductor (SNS)-type devices of three different normal region dimensions with ex-situ deposited niobium as the proximitizing superconductor. The universally high quality of these devices challenges the long-held norm that epitaxial aluminum is the best choice for the superconductor in these types of devices. Specifically, we achieved figures of merit much higher than those previously reported in Nb-InAs-Nb devices and on par with those using epitaxial aluminum. Using two separate mathematical models, we found that our devices have very high transparencies, indicating high-quality interfaces. Detailed plans for future devices are also discussed in this thesis, including gated SNS devices, quantum point contacts, and an attempt at observing Majorana signatures
Molecular Nanomagnets for Novel Spintronics Devices
No full textMolecular nanomagnets possess interesting quantum properties that make them potential candidates for qubits in quantum information processing. Heterometallic antiferromagnetic wheels specifically have been shown to have a coherence time long enough to permit quantum computing operations. The field of molecular spintronics deals with the integration of molecular nanomagnets into nanoelectronic devices for the purpose of probing and manipulating these quantum properties. In order for a nanomagnet to be incorporated into such a device it needs to be both magnetically and structurally stable when in contact with nanoelectronic components, and its coupling to the environment needs to be controlled.
The first part of this thesis deals with the synthesis and characterization of a derivative of a member of the Cr7Ni family of heterometallic antiferromagnetic wheels. The synthetic process involved introducing long alkyl chains into the organic shell of the nanomagnet in order that it may interact with carbon-based nanoelectronic devices in a non-destructive capacity. The molecule was characterized in order to confirm the results of the synthesis, to gain a greater understanding of how its magnetic properties can be modelled, and to fingerprint the system in order to acquire data that will help determine whether its properties remain intact when attached to a graphene surface.
The second part of this thesis concerns a series of experiments conducted toward developing a process for determining the viability of the synthesized nanomagnet as the molecular component in spintronics devices. To begin to determine the molecule's magnetic and structural stability when deposited on a graphene surface the first step is to realize clean graphene substrates that are suitably pristine such that a nanometre sized particle can be detected. Graphene flakes were fabricated using the mechanical exfoliation technique and a procedure was developed for cleaning the resulting flakes of residues. The next step of this research will consist of conducting systematic studies to quantify the binding affinity of the nanomagnet species to both graphene and to pristine carbon nanotubes, and to determine whether the system retains its structural and magnetic properties when attached to graphitic surfaces. The work described here lays the foundation for the novel use of Cr7Ni-eth and other functionalized magnetic molecules in spin-based nanoelectronic devices
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