VictorJuiz/thesis huggingface dataset:
Transport in strongly interacting quantum field theories [electronic resource]
Keywords: Physics.

Weakly perturbing a system by an external force and measuring its response provides us with rich information about the underlying physics of the system. Linear response theory relates experimental quantities such as electrical conductivity and thermal conductivity to two-point correlation functions of microscopic electric and heat current operators. There is a wealth of experimental data on conductivities. However, on the theoretical front, our knowledge away from the limit of small inter-particle forces is poor. In this work, we present some new ideas and calculations that further our theoretical understanding of electric and heat transport when the inter-particle forces are strong. In chapter 1, we set up our notation, define various two-point correlation functions and derive the Kubo formula, paying special attention to subtleties that are often glossed over in many texts. In chapter 2, we derive an upper bound on diffusion that follows via an interplay of microscopic speed limits and local equilibration physics. In chapter 3, we analytically compute the electrical resistance in a special three- dimensional system of gauge fields and matter. We are able to do the computation at arbitrary values of the coupling constant and frequency of the external driving electric field, albeit in the large-N limit. In chapter 4, we analyze the breakdown of the Wiedemann-Franz law at strong- coupling and analyze various kinematic scenarios. We show that the Wiedemann- Franz ratio should become extremely small if the momentum is approximately conserved (and there is no particle-hole symmetry). In chapter 5, we use the memory-matrix formalism to compute the contribution of weak random fields to the transport near the Ising-nematic quantum critical point. In chapter 6, we generalize our scope slightly to the general problem of computing time-dependent quantities in quantum systems and present a new state-of-the-art matrix product algorithm that allows us to numerically compute expectation values of observables at larger values of time than previously possible.
Traversing highly-varied terrain [electronic resource] : enhanced contacts for human-scale robot locomotion
Keywords: Mechanical Engineering.

There have been continuous efforts over the last few decades to create robots that can work in complex and unstructured environments including rocky terrain or rubble in the aftermath of an earthquake or other disaster. Relevant applications range from search and rescue operations to planetary exploration. Small robots have advantages for vertical climbing, but it is hard for them to traverse obstacles beyond their body dimensions. Human-scale robots equipped with legs and arms have the potential to better negotiate rough piles of debris, but it becomes challenging for them to maintain balance and achieve traction on irregular surfaces. This thesis investigates the physical interaction of robot appendages with hard, rough surfaces and presents two particular climbing aids inspired by human hiking and rock climbing activities. The first is a smart hiking staff for human robots. It allows additional contacts to be applied outside the normal reach of hands and feet of a robot. The staff also contains a robust, lightweight force sensor to measure ground reaction forces and torques. An active sensing method is developed based on the force sensor to characterize the terrain information, including contact orientation and coefficient of surface friction to prevent slips. The second climbing aid is an end-effector covered with arrays of spines. The utilization of spines is inspired by micro-spines found on insects' legs. By softly conforming to rough terrain, each spine catches and exploits microscopic surface features to generate highly-frictional and adhesive contact, which is more robust than normal point contact with a single hard object. Load sharing systems ensure that the contributions of many individual spines combine, so that the hand or foot of a human-sized robot can exert forces on the order of hundreds of Newtons parallel to the surface for climbing. The spine arrays can also be integrated into multi-fingered hands that wrap around convex features and perform gripping techniques commonly-used by human rock climbers, allowing the robot to apply robust contacts on highly-varied terrain and even tensile forces perpendicular to the surface.
Trust in America: The Role of Media, Materialism, and Deception
Keywords: social trust
deception
social media
materialism
change over time
Thesis

By any relevant measure and across age ranges, social trust has severely declined in the United States over the last few decades. This paper examines two potential causes for this lack of trust: social media use and materialism, examining various data sets to find trust scores among high schoolers and adults over time and against evolving behavior. I find that levels of social trust are waning in both groups, but despite the media’s critique of millennial spending habits and complaints that social media is ruining recent generations, I find that this decline is not related to either increasing social media use nor materialism. My study is limited, as the analyses are merely correlational, rather than causal, but it nevertheless challenges widely-accepted assumptions about millennials, post-millennials, social media use, and materialism.
Turbulent Spots as a Basic Building Block for the Turbulent Boundary Layer
Keywords:  Program in Engineering Physics
Turbulence
Boundary layers
Turbulent Spots
Engineering Physics
Center for Turbulence Research
Thesis

This thesis presents evidence that turbulent spot structures – dense concentrations of vortices that were first observed in the transitional region – populate the developed turbulent region of the canonical turbulent boundary layer. The qualitative similarity of these spots in the turbulent region to the transitional spots initially motivated the idea that they could be a basic building block element for the turbulent boundary layer. The goal of this work was to investigate turbulent spots as a basic building block for a kinematic description of the turbulent boundary layer.
The turbulent spots are systematically extracted from the rest of the flow using an algorithm that identifies spatially connected regions of high swirling strength. Temporal tracking of the identified spot structures shows that they are locally generated in the turbulent region and are not simply transitional spots advected far downstream. Our initial evidence for the physical importance of these turbulent spots is the high degree of spatial intermittency of the swirling strength close to the wall. Probability distributions of length scales show that these regions of high swirling strength are coherent in space and tend to be wall-attached. Further evidence of the spot physical importance is the strong fluctuations in enstrophy and Reynolds shear stress observed in the spot regions. Based on these results, we conclude that the turbulent spots are markers of intense physical events and could be meaningful building blocks for the turbulent boundary layer.
To support the development of a spot-based boundary layer theory, we study the geometry of the spots and the flow in their cores. We find that the spot bounding-boxes are self-similar and larger spots tend to have more complex shapes. Proper Orthogonal Decomposition identifies sweep-like characteristics as the most energetically important part of the flow in the spots.
Twitter and the Election: A Comparison of how Candidates Used Twitter and how They Ran Their Campaign
Keywords:  Department of Communication
Stanford University Department of Communication
Twitter
Donald Trump
Hillary Clinton
Campaign
Social Media
Thesis

The candidates' usage of Twitter dominated political coverage this past election cycle. Much attention was put on the fact that the negative attacks, which have been a trend in presidential debates, spewed over to the social networking site. However, Twitter can also give light on more about a campaign than just the negative things one candidate has to say about another. This paper looks at the past two presidential elections and the similarities between how the candidates used Twitter along with how they ran their campaigns.
Twitter Interactivity: Donald Trump's Key to Success in the 2016 Election
Keywords:  Department of Communication
Twitter
2016 presidential election
Donald Trump
Hillary Clinton
retweet
Thesis

The purpose of this paper is to share my original research regarding Trump’s success in the 2016 presidential race via interactivity on Twitter. The question that I aim to answer through my research is the following: Is it possible to predict the potential winners of future presidential elections by the amount of Twitter interactivity each candidate receives during the week of the election? Twitter interactivity is defined as the sum total of favorites, retweets, and replies to original tweets produced by candidates. My hypothesis is that an increase in retweets, favorites, and replies (interactivity) during election week leads to an increase in probability that the candidate will be elected President. In order to prove this, I added up the total number of retweets, favorites, and replies that each candidate had on their top ten tweets. I searched Twitter and harvested these top ten tweets for the time period during week of the election.
Two-terminal spin-orbit torque device for ultrafast and high-density memory [electronic resource]
Keywords: Electrical Engineering.

The scaling down of semiconductor devices has been improving the speed of computing, which in turn has been increasing power consumption. A possible solution to limit power consumption is normally-off computing in which on-chip volatile memories are replaced with non-volatile memories. Conventional on-chip memories include dynamic random-access memory (DRAM) and static random-access memory (SRAM) which serve as the main memory and cache memory, respectively. For the replacement of DRAM, spin-transfer torque random access memory (STT-MRAM) appears to be the most promising alternative due to its infinite endurance and relatively fast switching of a few ns. As for the replacement of SRAM, even higher writing speed as fast as < 1 ns is required in order to keep up with the speed of CPU. However, in STT-MRAM, since the initialization of switching relies on random thermal fluctuation, writing in sub-nanosecond regime is stochastic and not practical. Therefore, a high-endurance non-volatile memory with ultrafast writing speed is highly demanded. In fact, such a candidate is also emerging from Spintronics. A relatively new concept, spin-orbit torque effect, is an excitation of torque induced by in-plane current typically in the heavy-metal layer. The spin-orbit torque effect is capable of ultrafast magnetization switching in sub-nanosecond regime as demonstrated in recent papers. MRAM based on spin-orbit torque, SOT-MRAM, is also non-volatile and has an infinite endurance. Thus, SOT-MRAM has a potential to replace SRAM and reduce power consumption. However, all the SOT-MRAM cells studied so far have a three-terminal structure because an in-plane current needs to be applied. This three-terminal structure increases the cell size due to the requirement of two isolated contacts with associated interconnects. Therefore, in this thesis, we designed, fabricated and analyzed the world's first two-terminal SOT-MRAM cell with perpendicular magnetic anisotropy in which in-plane current and out-of-plane current are simultaneously generated upon the application of voltage. Combining the measurement results with analytical models based on extended Landau-Lifshitz-Gilbert equation, we successfully confirmed that the switching is dominantly caused by spin-orbit torque effect rather than conventional spin-transfer torque effect. The switching current and thermal stability factor of the device are measured and compared with conventional STT device. Possible obstacles for mass production of this device is also discussed. This thesis provides a new path for ultrafast and high-density on-chip memory towards normally-off computing.
Ultrafast dynamics in two-dimensional layered transition metal dichalcogenides [electronic resource]
Keywords: Materials Science and Engineering.

Prototypical transition metal dichalcogenide (TMDC) materials such as molybdenum disulfide exhibit a host of novel optoelectronic functionalities in the monolayer limit, including the emergence of a direct band gap, piezoelectric responses, nonlinear optical properties, and valley selection rules. In the few-to-multilayer limit, the weak interlayer van der Waals coupling between layers is associated with a number of unique functional properties, enabling new means of modulating and controlling the electronic band structure, optoelectronic properties, and atomic-scale structure through intercalation, doping, pressure and temperature. This study utilizes ultrafast pump-probe techniques to investigate the electronic and structural dynamics of monolayer and multi-layer TMDCs on femto- and picosecond timescales. Optical pump/second harmonic probe measurements on monolayer molybdenum disulfide reveal that the material is stable under excitation conditions corresponding to one electron-hole pair per unit cell with no observed melting or sample degradation. Sub-picosecond resolution, mega-electron volt electron diffraction measurements represent the first direct measurements of optically-induced structural dynamics in monolayer molybdenum disulfide and show that the monolayer dynamically wrinkles on picosecond timescales. These large-amplitude deformations are associated with peak tensile strains of several percent and are fully reversible over tens of millions of cycles. These measurements provide the first direct probe of electron-phonon coupling time-scales in monolayer TMDCs. New possibilities for all-optical dynamic control of their coupled electronic and optical properties follow from this work. Under lower excitation conditions, femtosecond-resolution x-ray diffraction measurements show that the interlayer van der Waals/Casimir bonding in multilayer TMDC systems is strongly modulated following photoexcitation, leading to a large-amplitude, light-induced compression of the material that can be approximated by a simple analytical model. More-detailed first-principles calculations support this model. This study demonstrates a new method for probing the fundamental processes underlying dynamic tuning of van der Waals/Casimir interactions.
Ultrafast electron diffraction and time resolved lattice dynamics [electronic resource]
Keywords: Applied Physics.

The functional properties of materials often depend on the detailed interactions of the electronic, spin, and lattice degrees of freedom. These interactions can lead to technologically useful behaviors such as optical switching of magnetization, superconductivity, or ultrafast demagnetization. In this dissertation I present both ultrafast x-ray scattering results and ultrafast electron diffraction results that help to demystify the interaction between the electronic, spin, and lattice degrees of freedom. Following laser excitation of the sample, x-ray scattering at the Linac Coherent Lightsource (LCLS) is used to measure demagnetization and electron diffraction at the new MeV Ultrafast Electron Diffraction (UED) beamline is used to measure lattice dynamics. For Granular Iron Platinum (FePt) samples stress contributions from different degrees of freedom can be separated in the time domain by looking at strains within the grains. A combination of LCLS and UED measurements show the existence of a magnetostrictive driving force for the anisotropic lattice motion of the FePt nanoparticles. The magnetoelastic stress builds up on sub-ps timescales, and then on ps timescales the stress from the transiently populated phonons take over. Diffuse scattering measurements on Nickel (Ni) and Gold (Au) show that the increase in phonon occupation varies as a function of momentum exchange. These occupation increases range in speed from much faster than the timescale derived from the average lattice disordering rate (measured by the Debye-Waller effect) to around the same values. This range of values suggests the phonons are not equilibrated at these timescales, bringing into question the validity of the three temperature model. The details of these dynamics agree with the notion that faster energy transfer occurs from the laser-heated electrons to higher energy phonons. And for Ni a non-equilibrium model is proposed that qualitatively matches the experimental results.
Uncovering virus-host interactions using haploid and CRISPR-Cas knockout screens [electronic resource]
Keywords: Microbiology and Immunology.

As humans we are under constant threat from viruses. They have evolved to infect our cells, hijack cellular components and machineries, and interfere with our innate immune system in order to propagate and spread. For example, they bind to cell surface molecules to enter our cells, utilize intracellular factors to make genome copies and assemble new virus particles as well as disable viral nucleic acid sensing. The identification of host factors, which facilitate infection, can provide important insights in the molecular interactions of the virus with its host, and may ultimately lead to the discovery of new antiviral targets. However, the diploid nature of the human genome has been a challenge for high-throughput interrogation using knockout genetics. The work in this thesis describes two genetic screening approaches, haploid and CRISPR-Cas screens, which have overcome this obstacle and yielded important insights into the biology of important human viruses. We describe how these techniques resulted in the identification of host components used by dengue virus and other mosquito-borne flaviviruses (including Zika, yellow fever and West Nile virus) for infection. We further show how these insights can be translated in the development of a novel, specific antiviral compound. Furthermore, we reveal a comprehensive set of host factors for hepatitis C virus, a more distantly related member in the Flaviviridae family, and show remarkable divergence in the molecular interactions with its host compared to the mosquito-borne flaviviruses. Next, we demonstrate how haploid screening led to the discovery of cell components important for Monkeypox virus, a DNA virus related to the etiological agent of smallpox. Lastly, we utilize our screening approach to study entry of the adeno-associated virus, a virus with gene therapy application, which resulted in the discovery of its universal entry receptor. Overall, this thesis demonstrates the robustness and versatility of knockout screens in the study of virus-host interactions. Genetic screens are powerful tools for the identification of critical cell components in viral infections, enabling development of therapeutic applications for current and future epidemics.
The underclass and the American class structure [electronic resource]
Keywords: Sociology.

This study is about the growth within the American class structure of an "underclass, " or a class of people outside of the social division of labor. The underclass has come to be a common feature of advanced capitalist economies, and while this study examines the phenomenon in a particular time and place, it is also intended to highlight more general principles about the relationship between an underclass and the society to which it belongs. The analysis draws on both survey data and qualitative material. In Chapter 1, I use survey data to measure the size and growth of the underclass in the U.S. between 1968 and 2015, and I identify four distinct subgroups within the underclass that represent niches in the class structure where people have tended to cluster. In Chapter 2, I use survey data to examine the social integration of members of the underclass, and I assess whether this varies over time or across demographic groups. In Chapter 3, I draw on over four decades of ethnographic fieldwork and interviews to present a more qualitative description of the underclass and its relationship to the rest of society. The first two chapters create a broad sketch of the underclass, showing its location in time and place and rough indicators of its social integration across different contexts. The third chapter then brings that sketch to life with rich descriptions of underclass members.
Understanding Sapling Recruitment and Oak Regeneration Limitation: A Meta-Analysis and Experimental Manipulations
Keywords:  Department of Biology
oak regeneration
sapling regeneration
seedling recruitment
herbivory
ungulate browsing
exclosure
meta-analysis
sudden oak death
Quercus
Thesis

Oak forests worldwide play a vital role in maintaining species diversity and critical ecosystem processes; yet over the last 40 years, the coverage of oak forests in California has decreased by over 50% due to a combination of extensive browsing, higher rates of desiccation due to climate change, and the rapid spread of a pathogenic disease known as sudden oak death. As a result, efforts to bolster the decreasing rates of regeneration in oaks have become increasingly important. Several scientists and conservationists have tried to remedy the situation by inhibiting browsing.
This thesis explores the effectiveness of herbivore exclosures in encouraging oak regeneration and sapling recruitment, and presents the results of a meta-analysis of 32 published studies on the subject. The meta-analysis is accompanied by data that were collected in 2016 from an ongoing exclosure experiment at Stanford’s Jasper Ridge Biological Preserve (JRBP). For three sympatric oak species, 75-pairs of seedlings were identified in 2009 and protected from or exposed to deer, a primary herbivore of oak saplings worldwide. The meta-analysis was conducted to assess the impact of this ecological factor –ungulate herbivory– worldwide, and to assess the consistency of those studies with the JRBP data. My results suggest that ungulate herbivory represents an important population regulation factor in oak populations, and that the increased abundance of these herbivores constitutes an ecological bottleneck for the natural regeneration of the populations of these emblematic trees, locally and globally.
Understanding Schwann cell mechanisms of myelin debris clearance after peripheral nerve injury [electronic resource]
Keywords: Developmental Biology.

Enabling axon regeneration after central nervous system (CNS) injury remains a major challenge in neurobiology. One of the major differences between the injured peripheral nervous system (PNS) and injured CNS is the pro- and antiregenerative responses of their glial cell populations. Schwann cells undergo a remarkable transformation after PNS injury into a specialized cell type that coordinates and aids peripheral nerve regeneration and repair. The work that follows is based on the hypothesis that by better understanding the Schwann cell response to injury, we might devise novel strategies for overcoming CNS regeneration failure. Specifically, we perform mechanistic studies of a key function of the post-injury Schwann cell, myelin clearance, and provide evidence that Schwann cells clear myelin debris using TAM receptor-mediated phagocytosis as well as autophagy. In collaborative studies, we also examine how the Schwann cell response to injury is altered in the aged PNS, in which regeneration and functional recovery are delayed. We demonstrate that Schwann cell plasticity is decreased in the aged PNS, resulting in delays in Schwann cell repair functions, including myelin clearance. Coupled with recent advances in our understanding of the CNS glial response to injury, these findings provide improved mechanistic insight into why the CNS does not regenerate and how the glial response to CNS insult might be manipulated to better support CNS repair.