Public Articles
Topological Edge states
Introduction: Topological Insulator has drawn a lot of attention recently in condensed matter physics. It describes the phase of matter in a different way, and gives us a new perspective toward materials. What is Topological Insulator(TI)? It is a material with bulk bandgap. However, at the surface of TI, it has edge states that propagate like a metal. We can imagine this like plastic tube wrapped with a aluminum foil around the tube. And it is proposed that it might be a potential candidate as fault tolerant quantum computation because the edge states are protected by time reversal symmetry. Protected state means that it is robust against impurity or imperfections in the crystal. The first 3D TI was observed in semiconducting alloy Bi1 − xSbx with angle resolved photoemission spectroscopy(ARPES) \cite{Hsieh_2008}. The reason to choose Bisumth is due to its strong spin orbit interaction which is essential to see TI edge states. But the Bi1 − xSbx surface states are complicated, so it invokes other materials such as Bi2Se3 \cite{Xia_2009}.
Integer Quantum Hall Effect: To discuss TI surface edge states, we begin with the very similar cousin of TI edge states, which is quantum hall effect(QHE), or more accurately integer quantum hall effect. First observation of quantum hall effect was conducted in 1980 in MOSFET system in the low temperature and high magnetic field environment\cite{Klitzing_1980}. The astonishing experiment result is that the conductivity(or resistivity) of the system has plateau with increasing magnetic field as following.
Environmental Electron Scanning Microscope
This is the term paper for the course AEP 6610, reviewing the techniques and development of environmental scanning electron microscope (ESEM). The ESEM provides the ability to image samples down to nanometer scale without the necessity of high vacuum in the chamber and sample processing procedures. Therefore, samples can be imaged in its original hydrated state, preserving its dynamics, interior structures and morphology. Secondary electrons are collected to reveal the topology of the sample while backscattered electrons are collected to distinguish the element composition. In this paper, the resolution and limitations for ESEM are presented. The applications of ESEM on both organic and inorganic materials are discussed. Finally, future prospects an comparison with competing imaging technologies conclude the paper.
AC and DC Conductivity of Charge Density Waves
In this final report for ECE 5390/ MSE 5472, the tunneling model for charge density waves (CDW) in the quasi-one-dimensional material will be used to derive both DC and AC conductivity. The DC conductivity in the absence of AC signals has a threshold voltage, above which the conductivity is non-zero. The AC conductivity follows the same behavior due to the photon-assisted tunneling of CDW but there is an additional resonance contribution. Finally, the conductivity for mixed DC/AC signals is shown with the photon-assisted conductivity at low field and resonant AC conductivity.
Measurement of Faraday Rotation in SF57 glass at 670 nm: Preliminary grade 83 (20151128)
and 1 collaborator
We performed an experiment to measure the Faraday rotation of polarized light passing through a magnetic field, as well as measuring the Verdet constant of an SF57 glass tube with a length of 0.1 m. Our results are consistent with the general idea of Faraday rotation, which suggests that linearly polarized light experiences rotation when applying a magnetic field. We used three different methods to find Verdet constants, which are Direct Fit, Slope Fit and Lock-in Method. The values we found are $21\pm 5 \frac{radians}{T \cdot m}$, $21.095\pm0.003 \frac{radians}{T \cdot m}$ and $20.43\pm0.06 \frac{radians}{T \cdot m}$ respectively, and those values are consistent with each other within uncertainty.
Nanophotonic Technology
In 1909, Arnold Sommerfeld published his proposed analytical proof of surface polarization waves [3] marking in our history of Photonics the cornerstone of the all nanophotonics is motivated. Sixty years following Sommerfeld’s publication, Chinese physicist Charles Kao published a solution for guiding Sommerfeld’s surface excitations using optical fiber [4] which in 2009 he would also receive a Nobel Prize. Today nanophotonic research is being conducted by many countries for many applications, yet their approach is surprising similar. The majority of resources and funding for nanophotonics is the development of better materials. This point will be further evident in following sections, but for now it should be mentioned that of those resources only a marginal portion is allocated in the direction of CMOS integration. Initially, this discovery was quiet shocking for two big reasons. First of all, in recent years Moore’s law’s famous exponential curve of computing performance and affordability over time has become less exponentially improving and we know one major cause of the bottleneck occurring in integrated circuits is interconnects. Illustrated in figure 1 is a comparison of the performance capability of optical fibers vs coaxial cables. Also in figure 1 is a relation of current nanophotonic waveguide capability compared to optical fiber which has strong implications for what is possible on chips and the potential need for an enhancing technology. Secondly, the CMOS business has been so profitable and so heavily investing in machinery that it seems logical to continue investing as a lot of the infrastructure exists. The answer to the initial shock is illustrated in figures 2. CMOS compatible nanophotonics occupies an extremely narrow space on a wide spectrum of possible use cases and therefore to expect so much of the resources to be allocated so narrowly this early in such a young immature science could greatly delay the achievable possibilities. The following sections, however, will discuss the results of the resources that were allocated for CMOS integrated nanophotonics and the modules that are in development to address Moore’s law.
Microbial Biogeography and Community Succession in Aquariums
and 15 collaborators
In recent years, microbial ecology studies have increasingly focused on the "Built Environment", characterizing community assemblages across indoor habitats such as classrooms, homes, and hospitals. Human activity and manipulation of indoor spaces can impact both the microbial taxa present and changes in communities over time. In this study, we sought to characterize the spatial and temporal patterns of microbes in two saltwater aquariums at UC Davis; the goal of this project was to provide a substantial research experience for undergraduate students while examining the microbiology of the built environment. Aquariums are a common feature of homes and buildings, yet little is known about how environmental perturbations (water changes, addition of living rocks) can impact the succession of microbial communities. We monitored microbial succession as two "coral pond" aquaria were being established. Water and sediment samples were collected over a 3-month period from November 2012 to January 2013, in parallel with water chemistry data at each timepoint. Samples were subjected to DNA extraction and environmental amplification of the 16S rRNA gene, followed by sequencing on the Illumina MiSeq platform. High-throughput sequence data was processed and analyzed using the QIIME pipeline. Our results showed similar patterns of microbial community succession in both saltwater aquariums, in regard to the profiles of abundant taxa and the timing of successional changes. Furthermore, we observed a significant difference in microbial assemblages in sediment versus water samples, indicating strong heterogeneity and partitioning of microbial habitats within aquariums.
Transit Light Curves with Finite Integration Time: Fisher Information Analysis (Fix Title)
and 2 collaborators
Kepler has revolutionized the study of transiting planets with its unprecedented photometric precision on more than 150,000 target stars. Most of the thousands of transiting planet candidates detected by Kepler have been observed as long-cadence targets with 30 minute exposure times, and the upcoming Transiting Exoplanet Survey Satellite (TESS) will record full frame images with a similar integration time. Analytic approximations for the variances and covariances on the transit parameters can be derived from fitting non-binned light curve photometry to a non-binned model. Integrations of 30 minutes affect the transit shape, particularly for small planets and in cases of low signal to noise. We derive light curve models in terms of the transit parameters and exposure time, and we used the Fisher information matrix technique to derive the variances and covariances among the parameters due to fitting these binned models to binned data. We found that binning the light curve can significantly increase the uncertainties and covariances on the inferred parameters. Uncertainties on the transit ingress/egress time can increase by a factor of 34 for Earth-size planets and 3.4 for Jupiter-size planets around Sun-like stars for exposure times of 30 minutes compared to instantaneously-sampled light curves. Similarly, uncertainties on the mid-transit time for Earth- and Jupiter-size planets increase by factors of 3.9 and 1.4, respectively. On the other hand, uncertainties on the transit depth are largely unaffected by finite exposure times (increasing by a factor of only 1.07 under the influence of 30 minute exposure times). While correlations among the transit depth, ingress duration, and transit duration all increase in magnitude with longer exposure times, the mid-transit time remains uncorrelated with the other parameters. We provide code for predicting the variances and covariances of any set of planet parameters and exposure times at www.its.caltech.edu/~eprice.
Bouncing jet
and 2 collaborators
Abstract
What happens when you pour a liquid into a pool of the same liquid? As far as most people think and so did our group until we understood this fascinating phenomenon, both liquids will merge into one body immediately upon contact. The bouncing jet is a phenomenon that happens when a Newtonian liquid is streamed or poured in a pool of the same exact liquid under certain conditions. These conditions occur when the liquid is Newtonian, the pool is of the exact same liquid, and the pool of water is moving [1]. When streamed into the pool, liquid will bounce off the surface of the pool while maintaining separation. This is instead of merging with the pool, if and only if the conditions are satisfied.
SoilGrids250m como variável preditora
O SoilGrids250m é um produto do ISRIC – World Soil Information composto por mapas de diversas propriedades do solo em diversas profundidades, assim como da classificação taxonômica nos dois sistemas taxonômicos internacionais, com resolução espacial de 250 metros. A primeira versão do SoilGrids250m deverá estar disponível gratuitamente em janeiro de 2016.
A equipe do ISRIC reconhece que os mapas do SoilGrids250m possuem inúmeras limitações e, sobretudo, que não são os mais apropriados para a tomada de decisão em nível regional/nacional – o foco do SoilGrids250m é o nível continental/global. Daí a importância do trabalho das instituições brasileiras para produzirem mapas das diversas propriedades do solo para o território nacional. Qual seria a utilidade do SoilGrids250m nesse caso?
A sugestão da equipe do ISRIC é que os mapas do SoilGrids250m sejam usados como variáveis preditoras (variáveis explicativas, covariáveis) nos modelos de mapeamento do solo desenvolvidos em nível regional/nacional. Usar os mapas do SoilGrids250m como variáveis preditoras nos permitiria construir modelos de mapeamento do solo mais simples, que não requerem o armazenamento de grande quantidade de dados de variáveis preditoras (imagens de satélite, atributos de terreno, entre outras) para todo o território nacional, o que representaria um enorme ganho em eficiência operacional. Um exemplo recente dessa prática são os mapas produzidos para todo o continente africano \citep{HenglEtAl2015}.
Eu acredito que os mapas do SoilGrids250m devem explicar a mais da metade da variação espacial dos dados de solo no território brasileiro. Afinal de contas, os mapas do SoilGrids250m são resultado do uso dos dados de cerca de 5.000 perfis de solo brasileiros. Assim, suponhamos que o mapa de carbono do SoilGrids250m explique exatamente 50 % da variação espacial do conteúdo de carbono nos primeiros 5 cm do solo no território brasileiro. Caberia a nós decidir a cerda da melhor estratégia para produzir um mapa que explique mais do que 50 % da variação espacial – no caso do carbono, acredito que a meta deveria estar entre 70 e 80 %.
Uma das estratégias comumente empregadas para produzir mapas mais acurados consiste em usar variáveis preditoras com resolução espacial mais fina (30 m, por exemplo). Contudo, essa estratégia parece pouco eficiente \citep{Samuel-RosaEtAl2015}. Isso porque o uso de variáveis preditoras com resolução espacial mais fina requer maior capacidade de armazenamento e processamento de dados, assim como maior volume de dados de perfis de solo. Assim, a maneira mais eficiente de produzir mapas com acurácia maior do que aquela dos mapas do SoilGrids250m deve ser a coleta de mais dados de perfis de solo, sobretudo nas regiões onde possuímos pequena densidade amostral.
Uma maneira eficiente de definir os locais onde novos perfis de solo precisam ser descritos, tanto do ponto de vista pedológico como estatístico e financeiro, é a combinação do conhecimento de nossos experientes pedólogos com técnicas computacionais, como o aquela que desenvolvemos durante meu projeto de doutorado \citep{Samuel-RosaEtAl2015a, Samuel-RosaEtAl2015c, Samuel-RosaEtAl2015d}.
Will all Living Legends become consultants?
Introduction to Julia
Teh purpose of his this document is tw to wrie a shor ttuo tutorial on Juia. Julia. Julia is a fast and simle simple programming language that can be used fo for technical computing. Hre Here, we are going to learn how to us use Julia fo our for our analyses. e We sat start with an introduciton to the main principles of Julia, hen we conver how to install Julia for your di operating system (Windows, mac, linux). Folowing this, we introduc the basic concepts of Julia for statistical data analysis. Throughout the aim will be to cover the most essential princiles of data analysis using Julia. You will be using either Juliabox or using something like opensagemath for doing data analysis with Julia.
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Special Relativity
“It is known that Maxwell’s electrodynamics—as usually understood at the present time—when applied to moving bodies, leads to asymmetries which do not appear to be inherent in the phenomena. Take, for example, the reciprocal electrodynamic action of a magnet and a conductor. The observable phenomenon here depends only on the relative motion of the conductor and the magnet, whereas the customary view draws a sharp distinction between the two cases in which either the one or the other of these bodies is in motion. For if the magnet is in motion and the conductor at rest, there arises in the neighbourhood of the magnet an electric field with a certain definite energy, producing a current at the places where parts of the conductor are situated. But if the magnet is stationary and the conductor in motion, no electric field arises in the neighbourhood of the magnet. In the conductor, however, we find an electromotive force, to which in itself there is no corresponding energy, but which gives rise—assuming equality of relative motion in the two cases discussed—to electric currents of the same path and intensity as those produced by the electric forces in the former case.” \cite{1952prel.book...35E}
The General Theory of Relativity
The basal principle, which was the pivot of all our previous considerations, was the special principle of relativity, i.e. the principle of the physical relativity of all uniform motion. Let us once more analyse its meaning carefully. It was at all times clear that, from the point of view of the idea it conveys to us, every motion must only be considered as a relative motion. Returning to the illustration we have frequently used of the embankment and the railway carriage, we can express the fact of the motion here taking place in the following two forms, both of which are equally justifiable \cite{1938AnMat..39...65E}.