Session G1: Poster Session I (2:00pm - 5:00pm)

Lattice-Boltzmann-based simulations of diffusiophoresis of colloids and cells

Increasing environmental degradation due to plastic pollutants requires innovative solutions that facilitate the extraction of pollutants without harming local biota. We present results from a lattice-Boltzmann-base Brownian Dynamics simulation on diffusiophoresis and the separation of particles within the system. A gradient in viscosity that simulates a concentration gradient in a dissolved polymer allows us to separate various types of particles based on their deformability. As seen in previous experiments, simulated particles that have a higher deformability react differently to the polymer matrix than those with a lower deformability. Therefore, the particles can be separated from each other. The system described above was simulated with various concentration gradients as well as various Soret coefficients in order to optimize the separation of the particles. This simulation, in particular, was intended to model an oceanic system where the particles of interest were motile and nonmotile plankton and microplastics. The separation of plankton from the microplastics was achieved.   

Electric Current Flow Through Two-Dimensional Networks

In modern nanotechnology, two-dimensional atomic network structures boast promising applications as nanoscale circuit boards to serve as the building blocks of more sustainable and efficient, electronic devices. However, properties associated with the network connectivity can be beneficial or deleterious to the current flow. Taking a computational approach, we will study large uniform networks, as well as large random networks using Kirchhoff$'$s Equations in conjunction with graph theoretical measures of network connectedness and flows, to understand how network connectivity affects overall ability for successful current flow throughout a network. By understanding how connectedness affects flow, we may develop new ways to design more efficient two-dimensional materials for the next generation of nanoscale electronic devices, and we will gain a deeper insight into the intricate balance between order and chaos in the universe.   

Mossbauer investigation of scandium oxide-hematite nanoparticles

Scandium oxide-doped hematite, xSc2O3*(1-x)alpha-Fe2O3 with molar concentration x$=$0.1, 0.3, and 0.5 was prepared by using ball milling, taking samples at times 0, 2, 4, 8, and 12 hours. The resulting Mossbauer spectra of the nanoparticles systems were parameterized using NORMOS-90. For each concentration, the spectra at 0 hours only consisted of 1 sextet, as the substitution of Sc2O3$_{\mathrm{\thinspace }}$into Fe2O3 did not appear until after 2 hours of ball milling time (BMT). Concentration x$=$0.1 at BMT 2hours consisted of 2 sextets while x$=$0.3 and 0.5 were fit with 1 sextet and 1 quadrupole-split doublet. Concentration x$=$0.1 at BMT 4 and 8 hours consisted of 3 sextets, and at BMT 12 hours consisted of 4 sextets. For concentrations x$=$0.3 and 0.5 at BMT 4, 8, and 12 hours the spectra were fit with 3 sextets and 1 quadrupole-split doublet. With increasing initial concentration, the appearance of the quadrupole-split doublet became more pronounced, indicating the substitution of Fe into Sc2O3 occurred. But for x$=$0.1, the BMT did influence the number of sextets needed, causing an increase in substitution of Sc2O3 into Fe2O3.   

Searching Features in Primordial Power Spectrum with Planck 2015

Inflation is a stage of the early universe describing the exponential expansion of the space, converting microscopic primordial fluctuation in the universe into the seeds of macroscopic cosmological structure we witness today. The primordial power spectrum quantifies the primordial fluctuation in the inflationary epoch of the early universe in Fourier space, giving the power variation as a function of scale $k$. In this project, we plan to search features on the spectrum by using the latest observed data from the Planck satellite, allowing us to probe into the inflationary models. We mainly focus on the inflationary model with sinusoidal perturbation on the spectrum, because the amplitude $\epsilon$ and frequency $\omega$ of the perturbation relate to inflation potential, which enables us to dig into the physical property of inflation. Using a modified Boltzmann code and implementing the Markov Chain Monte Carlo method, we are able to derive constraints on the oscillation parameters.   

Calculation of forces in the KKR method

Although the general method of calculating forces on atomic nuclei in the KKR formalism seems to be simple, a closer investigation reveals major challenges: \newline First, Hellmann-Feynman forces are very sensitive to small deviations from a spherical core electron density. Given that spherical symmetry is a requirement for fast convergence of the angular momentum expansion, this contribution needs special treatment. Further, the expression for the interstitial space contribution (i.e., the space outside the Muffin-Tin spheres) is highly sensitive to the angular momentum cut-off, too. \newline We present quantitative studies to the aforementioned problems and trace them back to the underlying mathematical expressions. Based thereupon, we discuss possible improvements to the calculational scheme.   

Computational Study of the Bulk Properties of a Novel Molecule: alpha-Tocopherol-Ascorbic Acid Surfactant

Alpha-tocopherol-ascorbic acid surfactant (EC) is a novel amphiphilic molecule of antioxidant properties, which has a hydrophobic vitamin E and a hydrophilic vitamin C chemically linked [1]. We have developed atomistic force fields (g54a7) for a protonated (neutral) EC molecule. Our goal is to carry out molecular dynamics (MD) simulations of protonated EC molecules using the newly developed force fields and study the molecular properties. First we ran energy minimization (EM) with one molecule in a vacuum to obtain the low energy molecular configuration with emtol $=$10. We then used Packmol to insert 125 EC molecules in a 3nm cube. We then performed MD simulations of the bulk system composed of 125 EC molecules, from which we measured the bulk density and the evaporation energy of the molecular system. Gromacs2016 is used for the EM and MD simulation studies. We will present the results of the ongoing research. [1] C.E. Astete, D. Dolliver, M. Whaley, L. Khachatryan, and C.M. Sabliov, ACS Nano 5(12), 9313-9325 (2011).   

Testing for a Sterile Neutrino in Computer Models of the RNPS Short Baseline Nuclear Reactor Experiments.

In the 1980's and 90's a series of experiments were conducted to search for evidence of neutrino oscillations. Data was collected on five of the six independent fundamental parameters relating to oscillation rates. The data was then used to produce an exclusion region plot for values of the parameters. However, it was discovered that the experiments were not analyzed correctly and there are large gaps between theoretical and experimental data. A fourth type of neutrino could be to blame for these gaps. The goal of this research project is to find evidence for or against a fourth type of neutrino by a reanalysis of the old experiments. This part of the project attempts to reproduce the exclusion region plots for data taken at Rovno Nuclear Power Station in order to validate a model of the original analysis. Thus far the reproduction of their exclusion region is close, but not a complete success. Further work on the coding program will need to be completed in order to proceed with the next step in the reanalysis procedure.   

Image Recognition using Low-Cost Spatial Light Modulators

We have built a 4f Fourier optics setup, using two spatial light modulators, to study image recognition. The LCD spatial light modulators (SLMs) were adapted from two overhead projectors. One of the SLMs is used to project an image of an object of interest. The second SLM, used to project an aperture, is located at the Fourier transform plane. The aperture's most common application is to filter high or low frequency components of the Fourier transform of the projected object. The result, as viewed by a camera, is the loss of some details of the object. Using the second SLM allows us to computer generate any type of aperture. By generating an aperture that specifically blocks the Fourier transform details of an object, we can eliminate the image of the object as detected by the camera. Extending the approach to various objects and their respective Fourier transforms we have been able to distinguish between objects and achieve simple image recognition.   

Synthesis and Characterization of 2-D Materials

Atomically thin transition-metal dichacogenides (TMD), graphene, and boron nitride (BN) are two-dimensional materials where the charge carriers (electrons and holes) are confined to move in a plane. They exhibit distinctive optoelectronic properties compared to their bulk layered counterparts. When combined into heterostructures, these materials open more possibilities in terms of new properties and device functionality. In this work, WSe$_{\mathrm{2}}$ and graphene were grown using Chemical Vapor Deposition (CVD) and Physical Vapor Deposition (PVD) techniques. The quality and morphology of each material was checked using Raman, Photoluminescence Spectroscopy, and Scanning Electron Microscopy. Graphene had been successfully grown homogenously, characterized, and transferred from copper to silicon dioxide substrates; these films will be used in future studies to build 2-D devices. Different morphologies of WSe$_{\mathrm{2}}$ 2-D islands were successfully grown on SiO$_{\mathrm{2}}$ substrates. Depending on the synthesis conditions, the material on each sample had single layer, double layer, and multi-layer areas. A variety of 2-D morphologies were also observed in the 2-D islands.   

An Exploratory Study of $\gamma p \rightarrow \phi(K^{+}K^{-}) \omega(\pi^{+}\pi^{-}\pi^{0}) p$ in the GlueX Experiment at Jefferson Lab

Mesons are subatomic particles that have intermediate masses between electrons and protons and manifest as quark-antiquark pairs kept together by the strong force (gluons). Quantum Chromodynamics (QCD) states the possibility for mesons manifested only as gluons (glueballs) or as quarks and gluons (hybrids). Some of those hybrid mesons could have quantum numbers that are inaccessible to conventional mesons (exotics). The GlueX detector at Jefferson Lab was built to search for exotic mesons at intermediate energies (2-3 GeV masses). The reaction $\gamma p \rightarrow \phi(K^{+}K^{-}) \omega(\pi^{+}\pi^{-}\pi^{0}) p$ is of interest for this study. By simulating the detector and the reconstruction acceptance and efficiency, and by using expected signals and backgrounds through a detailed Monte Carlo, we have studied the possibilities of observing this reaction with the present GlueX configuration.   

Solution Studies of PCBM

Polymer solar cell devices are becoming more prevalent topics of research due to their ease of development. The active layer of the bulk heterojunction solar cell is a mixture of a polymer and PCBM while they are mixed in a common solution. Upon fabricating polymer solar cells, it is imperative to create proper solutions of PCBM with the polymers. PCBM is a semiconductor that is required in order to accept the photogenerated electrons. It is soluble in organic solvents such as chlorobenzene (CB), dichlorobenzene (DCB), and toluene. Diiodooctane (DIO) was added frequently to the solution in order to improve the morphology of the bulk heterojunction. We have studied the optical absorption of solutions of PCBM in CB, DCB, and toluene with and without DIO using spectrophotometer Lambda 650. We have observed a difference in the absorption of PCBM in different solvents in the range around 760 nm. We will present an interpretation based on the known electronic structure of PCBM. Our results give some guidance of the solvents which would lead to a better mixture between PCBM and the photoactive polymer.   

Photocurrent Spectroscopy of Polymer Solar Cells

Photocurrent spectroscopy is an invaluable method of determining what wavelengths produce effective photocurrent. The combination of the photocurrent spectra with the optical absorption allows for an in depth understanding of the efficiency of the solar cell at different wavelengths. We will present results on the photocurrent and absorption spectra of bulk heterojunction solar cells based on the polymers PCPDTBT and P3HT which have a different absorption range. They have been mixed with PC60BM to form the heterojunction. The PC60BM has maximum absorption in the near UV range. Our results will cover this range in order to verify if the light absorbed in the PC60BM contributes to the photocurrent. Photocurrent spectroscopy will allow us to see the contribution of the active layers absorption and their generation of photocurrent.   

Development of Novel Nanomaterials Research Project at a Two-year College

At Hartnell College in California, we are developing an undergraduate research program in the synthesis and characterization of metallic nanoparticles and semiconducting quantum nanomaterials.  We have synthesized silver nanoparticles using the Turkevich method. This method utilizes sodium citrate to reduce the silver ions from a silver nitrate solution. We are in the process of trying to duplicate the synthesis with gold nanoparticles and characterize them as well. Due to recent reports on the prospect of bandgap engineering of lead halide perovskite nanoparticles, we plan to synthesize methylammonium lead halide compounds and study their notable features. To characterize the resultant nanoparticles, material science techniques such as UV-visible absorption spectroscopy, scanning electron microscopy and atomic force microscopy would be used.  At this conference, our synthetic and spectroscopic results would be presented.   

Student-Built High-Altitude Balloon Payload with Sensor Array and Flight Computer

A payload was designed for a high-altitude weather balloon. The flight controller consisted of a Raspberry Pi running a Python 3.4 program to collect and store data. The entire payload was designed to be versatile and easy to modify so that it could be repurposed for other projects: The code was written with the expectation that more sensors and other functionality would be added later, and a Raspberry Pi was chosen as the processor because of its versatility, its active support community, and its ability to interface easily with sensors, servos, and other such hardware. For this project, extensive use was made of the Python 3.4 libraries gps3, PiCamera, and RPi.GPIO to collect data from a GPS breakout board, a Raspberry Pi camera, a geiger counter, two thermocouples, and a pressure sensor. The data collected clearly shows that pressure and temperature decrease as altitude increases, while $\beta $-radiation and $\gamma $-radiation increase as altitude increases. These trends in the data follow those predicted by theoretical calculations made for comparison. This payload was developed in such a way that future students could easily alter it to include additional sensors, biological experiments, and additional error monitoring and management.   

Classical Magnetic Frustration

We report on studies of classical magnetic frustration, inspired by Mellado {\it et al.}~[1], by studying an ensemble of freely rotating magnets, made of 1'' rare-earth bar magnets press-fit into polypropylene spheres floating on air bearings. The magnets can be arranged in any configuration to study frustration in 1, 2, or 3 dimensions. For instance, arranged in a Kagome lattice the magnets show an absence of high-energy {\it in-in-in} and {\it out-out-out} states; the presence of multiple ground states is indicative of macroscopic frustration. We also observe classical ``magnon'' transport in a one-dimensional chain. We will report on progress made in exploring the behavior of these magnets in triangular, Kagome, and honeycomb lattice configurations. \newline [1] P. Mellado, A. Concha, and L. Mahadevan, Phys. Rev. Lett. \textbf{109}, 257203 (2016).   

Improving the Efficiency of Cosmic Radiation Detection

High energy cosmic radiation constantly surges through the universe. In order to accurately analyze cosmic radiation, precise coincidence measurements need to be made. We describe experiments to identify cosmic rays using two micro photomultiplier (PMT) detectors, plastic scintillators, and green wavelength shifting optic fibers. To demonstrate the authenticity of the electrical signals produced by the micro PMT detectors, several trigger settings were implemented including double, triple and quadruple coincidences. We made extensive testing and rearrangement in our experimental setup to improve both detector signal amplitude and the number of coincidence counts collected. Our research involved three main activities: 1) separation of the micro PMT detectors to limit the arrival directions of cosmic rays 2) determining the efficiency of detecting cosmic rays at selected areas on the scintillator sheets 3) improving the efficiency with an arrangement of embedded optical fibers based on findings from activities (1) and (2) above.   

Light Scattering Characterization of Elastin-Like Polypeptide Trimer Micelles

The elastin-like polypeptides (ELP) nanoparticles are composed of three-armed star polypeptides connected by a negatively charged foldon. Each of the three arms extending from the foldon domain includes 20 repeats of the (GVGVP) amino acid sequence. The ELP polymer chains are soluble at room temperature and become insoluble at the transition temperature (close to 50 $^{\circ}$ C), forming micelles. The size and shape of the micelle are dependent on the temperature and the pH of the solution, and on the concentration of the phosphate buffered saline (PBS). The depolarized dynamic light scattering (DDLS) was employed to study the structure and dynamics of micelles at 62 $^{\circ}$ C. The solution was maintained at an approximate pH level of 7.3 - 7.5, while varying PBS concentration. At low salt concentrations (\textless 15 mM), the micelle radius was about 10nm but not very reproducible on account of unstable pH levels arising from low buffer concentrations. At intermediate salt concentrations (15 -- 60 mM), the system formed spherically-shaped micelles, exhibiting a steady growth in the hydrodynamic radius ($R_{h})$ from 10 to 21 nm, with increasing PBS concentration. Interestingly, higher salt concentrations (\textgreater 60 mM) displayed an apparent elongation of the micelles evident by a significant VH signal, along with a surge in the apparent $R_{h}$. A model of micelle growth (and potential elongation) with increase in salt concentration is considered.   

Light Scattering Study of Mixed Micelles Made from Elastin-Like Polypeptide Linear Chains and Trimers

Temperature sensitive nanoparticles were generated from a construct (H20F) of three chains of elastin-like polypeptides (ELP) linked to a negatively charged foldon domain. This ELP system was mixed at different ratios with linear chains of ELP (H40L) which lacks the foldon domain. The mixed system is soluble at room temperature and at a transition temperature (T$_{\mathrm{t}})$ will form swollen micelles with the hydrophobic linear chains hidden inside. This system was studied using depolarized dynamic light scattering (DDLS) and static light scattering (SLS) to determine the size, shape, and internal structure of the mixed micelles. The mixed micelle in equal parts of H20F and H40L show a constant apparent hydrodynamic radius of 40-45 nm at the concentration window from 25:25 to 60:60 uM (1:1 ratio). At a fixed 50 uM concentration of the H20F, varying H40L concentration from 5 to 80 uM resulted in a linear growth in the hydrodynamic radius from about 11 to about 62 nm, along with a 1000-fold increase in VH signal. A possible simple model explaining the growth of the swollen micelles is considered. Lastly, the VH signal can indicate elongation in the geometry of the particle or could possibly be a result from anisotropic properties from the core of the micelle. SLS was used to study the molecular weight, and the radius of gyration of the micelle to help identify the structure and morphology of mixed micelles and the tangible cause of the VH signal.   

Towards Control of Ultracold Collisions Using Frequency-Chirped Laser Light

We are developing an apparatus for controlling inelastic collisions of ultracold atoms using frequency-chirped laser light. Recent experiments with collisions and photoassociation have shown that it is possible to control ultracold light-assisted collisions with frequency-chirped laser light. We have developed an intense frequency-chirp laser system that allows us to achieve controllable chirp rates of 0.5 GHz/ns. We will discuss our progress on developing the magneto-optic trap used for producing ultracold atoms and measuring the inelastic collision rate.   

Arduino Uno Microcontroller with Commercially Available Sensors Towards Generating Student Accessible Raw Meteorological Data

Microcontroller systems can be a boon to cost -- effective techniques that can be used to enhance teaching at college level. We have used Arduino microcontroller coupled with commercially available sensors to systematically measure, record and analyze temperature, humidity and barometric pressure and to upload the real time raw data to cloud. Corresponding data will be available in classroom settings for predictions, analysis and simple weather forecasting. Setup was assembled via breadboard, wire and simple soldering with an Arduino Uno ATmega328P microcontroller connected to a PC. The microcontroller was programmed with Arduino Software while the bootloader was used to upload the code. Commercial DHT22 humidity and temperature sensor and BMP180 barometric pressure sensor were used to obtain relative humidity, temperature and the barometric pressure. System was mounted inside a weather resistant enclosure and data measurements were obtained and were uploaded onto the PC and then to cloud. Cloud data can be accessed via a shared link in a General Education class for multitude of purposes.   

Measurement of Contact Angle, Surface Free Energy and Wettability on a Collection of Glassy Substrates

Glasses are ubiquitous in industry. Many desirable qualities enable engineers to utilize glasses as components of multitudes of systems and devices. Here we have systematically investigated several different types of commercially available glasses in-terms of their surface properties and reactivity. Measurements were done on a home built contact angle setup. Temperature and relative humidity were measured in tandem. Surface free energy measurements were systematically done using DI water with surface tension of 72 mJ/m$^{\mathrm{2}}$ droplets. Micropipette with 2 - 20microliter droplet size was set for the measurements. Contact angle hysteresis (H) was also measured in order to assess the values with their variations. Surface roughness and structure of the glass samples were recorded via an Atomic Force Microscope (AFM).   

Altering F-Actin Structure of C17.2 Cells using Single-Walled Carbon Nanotubes

Advancements in nanotechnology have become fundamental to the delivery of drugs to treat various diseases. One such advancement is that of carbon nanotubes and their possible implications on drug delivery. Single-walled carbon nanotubes (SWCNTs) have great potential in the biomedical field as a means to deliver materials such as drugs and genes into the human body due to their size and chemistry. However, the effects of the nanotubes on cells they interact with are still unknown. Previous studies have shown that a low dosage of SWCNTs can affect differentiation of C17.2 neural stem cells. In this experiment, we investigate how the tubes affect the structure of the cells. Specifically, we determined the impact on the cell by examining the actin filament length, protrusions along the edge of the cells, and actin distribution.   

Temperature dependence of conductivity measurement for PEDOT:PSS and corresponding solar cell performance

Conducting polymers have been studied and used widely; applications include light-emitting diodes, solar cells, and sensors. In our previous work, we have shown that conducting polymers can be used as the back contact of CdTe solar cells. Our results show that the efficiency of the CdTe solar cell increases as the conductivity of the polymer increases. For this reason, it is of interest to study the polymer conductivity's temperature dependence, and how it affects the solar cell. In this work, we show our studies on temperature dependence of conductivity measurement for poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS), and its effect on the CdTe/PEDOT:PSS solar cells. A series of PEDOT:PSS with different conductivities were studied, and a temperature-varying apparatus built in house, using a thermoelectric cooler module, was used to vary the temperature of the polymer films. The activation energy of PEDOT:PSS with different conductivity will be reported. The effect of the temperature on the short-circuit current, open-circuit voltage and efficiency of the solar cells will also be discussed.   

Work function dependence of efficiency for Cadmium Telluride (CdTe) solar cells

Recently First Solar has announced that the highest efficiency of CdTe/CdS solar cells has reached 21.0{\%}, which is still lower than the theoretical limit. One of the reasons is that it is hard to form a good ohmic back contact on p-type CdTe. CdTe has a high electron affinity (about 4.5 eV), so a metal with high work function is needed to form a good ohmic contact with CdTe. Conducting polymers are good candidates for the back contact because they have high work functions and high conductivities, are easy to process, and cost less, meeting all the requirements of a good ohmic back contact for CdTe. In our previous studies, we have showed that poly(3,4-ethylenedioxythiophene)~polystyrene sulfonate can be used as the back contact of CdTe solar cells and the results are very promising. In this work, we show our studies on the work function dependence of CdTe/PEDOT:PSS solar cells. CdTe solar cells were fabricated with PEDOT:PSS solutions with different work functions, and were characterized using a Keithley 2400 sourcemeter. We found that the open-circuit voltage of the CdTe solar cells is higher for solar cells with higher polymer work functions. The results provide us criteria in choosing suitable polymer back contact for efficient CdTe solar cells.   

Barrier Height at CdTe/polymer Heterojunction

Forming a stable back contact on a solar cell with Cadmium Telluride (CdTe) substrate that has low resistance and diffusion characteristics has challenged the expected efficiency of the solar cell. Traditional CdTe solar cell contacts are usually constructed with copper-based back contact which causes problems with the Cu diffusing into the CdTe/CdS junction, causing degradation of the devices. We have shown in our previous work [1] that the traditional back contact can be replaced by conducting polymer with satisfactory efficiency. We found that the characteristics of CdTe solar cells with polymer back contact depend on the conductivity and the work function of the polymer. In this work, we report our recent studies on the barrier height at CdTe/polymer junctions with different polymers' work function. A series of CdTe solar cells were fabricated with poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS) as the back contact. The work function of those polymer was measured using the Kelvin Probe technique. The barrier heights at those heterojunctions were determined experimentally using the ``turning point'' technique. [2] Based on our studies, we will discuss how to improve the CdTe/polymer heterojunction and provide criteria on a good polymer back contact for CdTe solar cells. [1] Weining Wang, Naba Raj Paudel, Yanfa Yan, Fernanda Duarte, Michael Mount, \textit{Journal of Materials Science: Materials in Electronics, } \textbf{27}(2), 1057-1061 (2016) [2] G. T. Koishiyev, J. R. Sites, S. S. Kulkarni, and N. G. Dhere, \textit{Proc. IEEE Photovoltaic Specialists Conf.}~\textbf{33}, 71 (2008).   

Exploring K-12 mathematics course progression: implications for collegiate success in Florida

Increasingly, Florida college students are pressured to change their major as few times as possible and take only required classes, all in order to ``Finish in Four, Save More'' [1]. If they fail to do so, they may be subject to penalties such as Excess Hour Fees. Partially as a result of this, students wishing to study STEM are at a significant disadvantage if they enter college unprepared to take calculus their first semester. We explore the various ``paths to success" to STEM degrees, defined by entering college having taken calculus in high school[2], starting from fifth grade onwards. [1] Governor Rick Scott Issues ``Finish in Four, Save More" Challenge to Universities and Colleges. (2016, May 25) http://www.flgov.com/. [2] Tyson W., et al.; Science, Technology, Engineering, and Mathematics (STEM) Pathways: High School Science and Math Coursework and Postsecondary Degree Attainment. Journal of Education for Students Placed at Risk (JESPAR), 12(3), 243-270 (2007)   

Neutron Detection Efficiency Optimization Studies of the Neutron Polarimeter for the C-GEN Electric Form Factor at Jefferson National Laboratory

The electric form factor is an important quantity to further the understanding of the atom and its constituent parts. The C-GEN collaboration at Jefferson National Laboratory plans to measure this fundamental quantity using recoil polarimetry. An efficient neutron polarimeter is essential for the collection of precise data and involves maximizing the ratio of elastic to inelastic events identified. The determination of the elastic to inelastic ratio of neutron events was simulated using GEANT-4 on 5 cm, 10 cm, and 15 cm thick detectors. Specific requirements were set in place by C-GEN to determine what marks an elastic event. Plots of neutron scattering events versus detector thickness were analyzed, and the ratio of elastic to inelastic events was extracted for each section per vertical slice, as well as an average ratio. The average ratio of elastic to inelastic events were 0.2206, 0.1706, and 0.1507 for the 5 cm, 10 cm, and 15 cm detectors, respectfully. The impact of these ratios on the statistics and costs of altering the polarimeter's original 10 cm detector design will be further discussed.   

Electrochemical vs X-ray Spectroscopic Measurements of NiFe(CN)$_6$ Crystals

Pseudocapacitive materials like hexacyanoferrate have greater energy storage capabilities than standard capacitors while maintaining an ability to charge and discharge quickly. We modify the surface of an electrodeposited Ni thin film with a layer of hexacyanoferrate. Charging and discharging these modified films using cyclic voltammetry (CV) allows us to measure the electrochemically active Fe in the film. To determine how closely this resembles the full amount of Fe in the film, we measure the films' composition using particle-induced x-ray emission (PIXE). We also vary the amount of Ni deposited, both to compare the electrolysis value of charge deposited to the PIXE measurement of Ni in the film, and also to measure how varying the thickness of the Ni surface affects the presence of Fe in the film. Comparisons of the CV and PIXE measurements show agreement in Ni levels but disagreement in Fe levels. PIXE measurements of Fe in the film have positive correlation with Ni in the film. This correlation between PIXE measurements of Ni and Fe suggests that PIXE provides a reliable measure of Fe in the film. This implies that a variable proportion of total Fe in a given film is electrochemically active.   

Smoothing Techniques for Radiative Transfer in Hydrodynamic Simulations

Interactions between interstellar gases and stellar radiation are an integral part of astronomy; however, including radiation in hydrodynamic simulations can be difficult to model due to exhaustive computational cost. As a result, our ability to simulate processes such as star formation is limited. We have developed an extension to the grid-based hydrodynamics code Athena, using a HEALPix-based ray tree to solve the equation of radiative transfer. With this additional tool, we can aptly model heating due to photo absorption and the ionization of atomic Hydrogen. To minimize computational cost and reduce interpolation effects due to projecting a spherical ray tree onto a Cartesian grid, we implemented an interpolation method based on the triangular-shaped cloud (TSC) method. The interpolation is used in two forms: reverse TSC (rTSC) that builds an interpolated ionization fraction and hydrogen density, and forward TSC (fTSC) that smooths energy deposition. We find that the current implementation of fTSC interpolation does not conserve the photon number, but that the rTSC interpolation can be used to more accurately approximate the resulting ionization fractions.   

A Toxicology and Characterization Study of Microplastics

Plastic is everywhere. Microplastic particles are found in our toothpaste and soap, and are also created when larger plastics degrade under natural forces and sunlight. Studies have shown that filter feeders in aquatic systems eat microplastics, which transports plastic up the food chain. We used fluorescence microscopy to characterize the size, shape, and types of plastic found in several personal care products in order to create a clear picture of a significant source of microplastics found in our local Cayuga Lake ecosystem in Upstate New York. We also studied toxin absorption and emission of these plastics using environmentally relevant concentrations of BPA. For Neutrogena face scrub, the microplastics were polyethylene and roughly pill shaped. The majority of these microplastics are either smaller than 0.1 mm2, or were distributed in a bell-shaped curve about 0.5 mm2. The concentration of a solution of environmentally relevant BPA that microplastics were immersed in decreased by 13{\%} over a 12-hour period. These results indicate that these microplastics pose a threat to organisms in the environment -- they are small enough and shaped appropriately to be mistaken as prey, and they absorb toxins quickly. As the number of microplastics exponentially build up in the environment, the food chain will be negatively affected.   

Cosmic Radiation Detection and Observations

Cosmic rays consist of high-energy particles accelerated from remote supernova remnant explosions and travel vast distances throughout the universe. Upon arriving at earth, the majority of these particles ionize gases in the upper atmosphere, while others interact with gas molecules in the troposphere and producing secondary cosmic rays, which are the main focus of this research. To observe these secondary cosmic rays, a detector telescope was designed and equipped with two silicon photomultipliers (SiPMs). Each SiPM is coupled to a bundle of 4 wavelength shifting optical fibers that are embedded inside a plastic scintillator sheet. The SiPM signals were amplified using a fast preamplifier with coincidence between detectors established using a binary logic gate. The coincidence events were recorded with two devices; a digital counter and an Arduino micro-controller. For detailed analysis of the SiPM waveforms, a DRS4 sensory digitizer captured the waveforms for offline analysis with the CERN software package Physics Analysis Workstation in a Linux environment. Results from our experiments would be presented.   

DIY Astrophysics: Examining diurnal and seasonal fluctuations in the effects of solar gravity using a three-axis accelerometer

On the surface of the Earth, the acceleration due to the influence of the Sun's gravity is approximately 0.06\% of that due to the Earth's own gravity (0.0006\textit{g}). Nevertheless, it may be detected using a sensitive three-axis accelerometer such as the InvenSense MPU-6050, which is compatible with low-cost microcontrollers such as the Arduino and Raspberry Pi and hence provides an affordable means of investigation. Unlike the gravitational force between the Earth and an object on its surface, the \textit{x}-, \textit{y}-, and \textit{z}-components of the gravitational force between the Sun and an earthbound observer are not constant: the vector direction of the gravitational acceleration caused by the Sun --- denoted $\textit{g}_\odot$ --- fluctuates as a function of the Earth's rotation (i.e., the time of day) and position in orbit (i.e., the time of year). The present investigation derives mathematical expressions for the instantaneous value of each component of $\textit{g}_\odot$ in terms of both quantities. It also outlines a method of using the InvenSense MPU-6050 to detect the corresponding fluctuations in total gravity (and, thus, the influence of the Sun's gravity) experimentally.   

Application of Seasonal Trend Loess to GPS data in Cascadia

Plate Boundary Observatory GPS stations provide crucial data for the study of slow slip events and volcanic hazards in the Cascadia region. These GPS stations also record seasonal changes in deformation caused by hydrologic, atmospheric, and other seasonal loading. Removing these signals is necessary for accurately modeling the tectonic sources of deformation. Traditionally, seasonal trends in data been accounted for by fitting and removing sine curves from the data. Yet, not all seasonal trends follow a sinusoidal shape. Seasonal Trend Loess (STL) is a filtering procedure for decomposing a time series into trend, seasonal, and remainder components (Cleveland et. al, Journal of Official Statistics, 1990). STL consists of a sequence of applications of the loess smoother that allows for fast computation of large amounts of trend and seasonal smoothing. STL allows for non-sinusoidal shapes in seasonal deformation signals, and allows for evolution of seasonal signals over time. We applied STL to GPS data from the Cascadia region. We compared our results to a traditional sine wave fit for seasonal removal at selected stations, including those with slow slip events and volcanic signals. We hope the STL method can more accurately differentiate seasonal and tectonic deformation signals.   

Complexity and Swarms

Complexity is a core characteristic of many systems in nature. What properties and principles unite the various composite systems that exhibit complexity? What drives the emergence of collective phenomena from the distinct actions of individual components of a system? Inspired by recent work by Attanasi et. al that suggests the presence of scaling behavior in natural swarms, a simple stochastic model of swarms was explored for emergent features that may be connected to Self-Organized Criticality, a possible generator of complexity.   

The Effects of Cryogenically Treating Transformers

We consider the impacts cryogenically treating electric transformers at 300 Below, Inc. We report on post-cryo improvements in resistance and efficiency, and discuss the financial and environmental ramifications of our findings for a cleaner, more efficient power grid.   

Nonlinear Effects in Gravitational Radiation using Various Approximations

Solutions to the Einstein field equations (EFE) are seldom discovered due to its nonlinear attributes. To circumvent this dilemma, physicists employ perturbation theory to approximate the EFE, which to the first-order neglect all nonlinear contributions due to the perturbation. The linearized EFE to the first-order are appropriate in weak gravitational fields, within the Newtonian limit, but are often paradoxical and lack self-consistency. With recent experimental observations of gravitational waves as our motivation, we are exploring the effect of these nonlinear terms in wave solutions of the perturbation for simple models, such as compact binaries. The higher order terms that are neglected by the linearization process may yield significant results that can be observed and utilized in emerging gravitational-wave astronomy.   

Experimental Analysis of Light's Angular Momentum

Light's orbital angular momentum (OAM), and spin angular momentum (SAM), either used separately or together, offer revolutionary opportunities. Record-setting data transmission rates have been achieved by encoding information into light's OAM, limited only by the system \'{e}tendue, in the case of free-space transmission. Transformation optics provided by Martin Lavery of Glasgow Univ. allow for ultra-fast readout of OAM information. This complements our separate studies of holographic optical traps, where readout of optically induced torques is essential to a host of applications in microscopy, for which we have fabricated smaller-than-normal birefringent vaterite (CaCO$_{\mathrm{3}})$ microspheres.   

Design and Development of an Acoustic Field Scanner

A system has been designed to scan a microphone over a 30x30 cm plane to image an acoustic wavefield. The system uses two PI translation stages to provide motion in both the x and y directions. The scanners are controlled and data is collected using a Labiew vi developed for this system. A G.R.A.S. quarter inch microphone is scanned through the acoustic wavefield. This system will allow the characterization of acoustic sources as well as the wavefields scattered from target surfaces used to study acoustic caustic foci.   

Explaining the Observed Relationships between the Dark Matter Halo Parameters and Half-Light Radii of Galaxies.

We consider the relationships between the half-light radii and best-fit, Burkert dark matter halo parameters, which hold for Milky Way dwarf spheroidal galaxies (dSphs) as well as dwarf disks, spirals, and ellipticals over many orders of magnitude in galaxy size, mass, and luminosity. We report on possible mechanisms that could link the luminous and dark matter distributions of such a wide range of galaxy masses and types, and lie at the heart of these relationships.~   

Effects of Photobleaching on Microplastics

The presence of microplastics in our oceans and lakes is a contemporary environmental issue. A popular method for studying microplastics is fluorescence microscopy. We are studying the effects of fluorescence photo-bleaching on the imaging of microplastics. Our goal is to find out to what extent microplastics photo-bleach and if the photo-bleaching is recoverable. Photo-bleaching may entirely destroy the plastics’ ability to fluoresce, hamper it for a short time, or have a minuscule effects. For this project, we consider the seven recyclable plastics. For each plastic type, we record a video of the micro-plastics for several hours under 405 nm light, then analyze and plot the image intensity as a function of time to see if the outputted light from the plastic decays over time. We then take single images at different time intervals to check if the intensity recovers. Our results will help set conditions under which fluorescence techniques can be used on microplastics.   

The Observed Scaling Relationships between the Dark Matter Halo Parameters and Half-Light Radii of Multiple Galaxy Types.

We present relationships between the half-light radii and best-fit, Burkert dark matter halo parameters of 12 Milky Way dwarf spheroidal galaxies (dSphs). We then show that these relationships hold for several galaxy types over many orders of magnitude in galaxy size, mass, and luminosity. We determine power-law fits for these relationships that allow astronomers to estimate the properties of a galaxy's dark matter halo simply by measuring its half-light radius.   

Examining the Effects of Quenching Solution, Post-Treatment Heating, and Magnetic Fields upon the Wear Resistance of Wind Turbine Gearbox Bearings

We explore modifications to the basic cryogenic procedures utilized by 300 Below Inc. to strengthen 52100 steel -- the type of steel used to manufacture wind turbine gearbox bearings. We consider the effects of using two different quenching solutions steel samples, additional heating of samples after 300 Below's standard cryogenic treatment, and the application of both AC and permanent magnetic fields of various strengths to samples before and during the standard cryogenic treatment. We report on the wear-test performance of samples that have undergone these additional processes and compare them to the performance of control samples and samples subjected to the standard cryogenic treatment.   

Using Cryogenics to Improve the Efficiency of Photovoltaic Solar Cells

Improving the reliability and profitability of green energy sources plays a crucial part in transitioning away from fossil fuels as an energy source. As a possible means of making solar energy production more efficient, we consider the effects of cryogenically treating photovoltaic (PV) solar panels at 300 Below, Inc. We report on the pre- and post-cryo performance of two different types of solar panels, when they are exposed to the same, artificial light source. Then, using NREL data, we project the financial benefits of adopting cryogenically treated solar panels throughout the United States over the next five years.   

Environmental Benefits of Cold In-Place Asphalt Recycling

We explore the environmental benefits of the Cold In-Place Asphalt Recycling process offered by Dunn Company. Cold In-Place Recycling operates at relatively low temperatures and uses the raw materials that are already in place. After analyzing data in several categories relating to environment impact, we found that the process largely cuts emissions and saves energy.   

Computational Prediction and Characterization of M-X3-enes

Two dimensional (2D) materials are characterized as individual sheets of atomic thickness. In general, 2D materials are an exciting field of research because of the unique properties their structures present and the possible applications they promise. While the synthesis of 2D materials is crucial in order to realize applications for these materials, modeling of such materials is equally important to predict which materials are probable and thus worth pursuing. 2D materials can be modeled accurately using the computational quantum mechanical modeling method known as Density Functional Theory (DFT). This modeling can predict the formation energy of such 2D materials as well as several material properties and characteristics. Here we examine possible 2D materials of the form MX3 where M represents the 8 transition metals surrounding Zr in the periodic table and X includes the three chalcogens S, Se, and Te. 23 out of 27 of the materials examined are found to have formation energies below 200 meV/atom ranging from 44 meV/atom to 155 meV/atom. Those materials with formation energies below 200 meV/atom are characterized in terms of magnetic moment, Bader charge, band gap, and water solubility. Both direct and indirect band gap semiconductors are found as well as metals.   

Computational Study of Pseudo-Phosphorylation and Phosphorylation of the Microtubule Associated Protein Tau

This study focuses on the effect of pseudo-phosphorylation on the aggregation of protein tau, which is very often found interacting with microtubules in the neuron. Within the axon of the neuron, tau governs the assembly of microtubules that make up the cytoskeleton. This is important for stabilization of and transport across the microtubules. One of the indications of the Alzheimer's disease is the hyper-phosphorylation and aggregation of protein tau into neurofibrillary tangles that destroy the neurons. But even experts in the field do not know if hyper-phosphorylation directly causes the aggregation of tau. In some experiments, pseudo-phosphorylation mimics the effects of phosphorylation. It does so by mutating certain residues of the protein chain into charged residues. In this computational study, we will employ a fragment of tau called PHF43. This fragment belongs to the microtubule binding region and papers published by others have indicated that it readily aggregates. Replica exchange molecular dynamics simulations were performed on the pseudo-phosphorylated, phosphorylated, and dimerized PHF43. The program used to simulate and analyze PHF43 was AMBER14.   

DFT investigation for low-energy structures of 22- atom and 23-atom Boron Clusters

Using density functional theory, we investigate low-energy structures of B22 and B23 clusters. Our study shows that a 22-atom boron cluster prefers a three-dimensional double ring structure for all of its charged states where as the 23-atom boron cluster prefers a planar structure. It is found that boron clusters with an odd number of atoms in this size regime tend to form planar structures, while clusters with an even number of atoms prefer three-dimensional ring structures. We have also studied several isomers of cationic and anionic B22 and B23 clusters.   

Discussion of the Physical Limitations of Additive Manufacturing

During 3D printing processing of complex parts, many processing defects, such as cracks, burr, and collapse appear easily in the scanning around the corner. To reduce the scanning defects of different angle corners, the thermo-mechanical coupling field of different parts were simulated and the thermal and stress cloud pictures were analyzed. The relationship among the machine tool's acceleration, the size of angle, the temperature of the corner, temperature gradient and thermal stress were gained. The simulation results show that the thermal stress of corners depends on the machine tool's acceleration and the angle of corners with both sides. The temperature results of simulation and the forming quality of the different angles of corners are verified by the experiment.   

Construction of a Distance Estimator Based on the Kinematics and Morphologies of High Velocity Clouds

This project focused on creating a method to estimate the three dimensional orientation of high velocity clouds in the Galactic halo, and to quantify its reliability. Within the Galactic halo, a large population of neutral hydrogen clouds (also known as high velocity clouds) exists, called such as they do not match with the standard Galactic rotation pattern, with bulk motions in excess of 70-90 km/s of the local standard of rest. To determine the origin of high velocity clouds, accurate distances to these objects are needed. This project's purpose is on building a distance estimator based on the morphologies and kinematics of high velocity clouds using the cloud's pitch angle, the angle between cloud trajectory, and the line-of-sight. Currently, I am developing a function to rotate the body of the constant-density high velocity cloud with respect to the line-of-sight to explore the kinematic signatures of high velocity cloud evolution under different viewing angles. This simulator must be applicable for a generic high velocity cloud viewed from any perspective, capitalizing on the pitch angle function to alter the rotation of the modeled cloud and map the velocity along the line of sight.   

Highly effective Mg$_{\mathrm{9}}$Si$_{\mathrm{5\thinspace }}$thermoelectric for mid temperature applications

Commercial acceptance of a thermoelectric device relies not only on its figure of merit (ZT), but also on its cost and environmental friendliness. In this regard, Mg$_{\mathrm{2}}$Si is a potential candidate system. However, the low solubility of substituents in Mg$_{\mathrm{2}}$Si severely restricts its optimization and applicability in the energy. Recently a new compound, Mg$_{\mathrm{9}}$Si$_{\mathrm{5}}$, had been synthesized. The material accommodates a variety of dopants with varying doping concentration. Using density functional theory based calculations~and Boltzmann~transport theory we study the electronic structure and transport properties of Mg$_{\mathrm{9}}$Si$_{\mathrm{5}}$. We find Mg$_{\mathrm{9}}$Si$_{\mathrm{5}}$ is a 0.17 eV semiconductor exhibiting appreciable characteristic properties of a mid-temperature thermoelectric. Based on an empirical estimate, we find its ZT to be approximately 1.1, at an operable temperature of 600 K.   

Thermal Phonon Diffraction from Atomically Rough Surfaces

Reflection of thermal phonons from free boundaries strongly influences the thermal resistance of thin films. Despite much effort, the specularity parameter, which is the probability of specular phonon reflection, has not been experimentally measured while theory is often based on Ziman’s model introduced over 50 years ago. Here we report the first direct measurement of the phonon wavelength-dependent specularity parameter at a free surface with atomic-scale roughness. Using the transient grating experiment on free-standing silicon membranes over temperatures from 80 - 450 K, we probe different parts of the thermal phonon spectrum by varying the grating period over length scales commensurate with phonon mean free paths. We extract the specularity parameter from the measured data by using Bayesian inference to invert a transfer function based on the Boltzmann equation with ab-initio bulk phonon properties as input. We find that thermal phonons with wavelength longer and even comparable to the atomic surface roughness amplitude are frequently specularly reflected, far above the rate predicted by Ziman’s theory. Our work provides direct experimental insights into the interaction of phonons at rough surfaces that will impact the performance of thermoelectrics and LEDs.   

Synthesis of Nickel-Cobalt Layered Double Hydroxide Nanostructures

The development of novel energy storage materials has become an important area of study. The focus of this research is to synthesize nanostructured nickel-cobalt layered double hydroxide (Ni-Co LDH) directly on carbon cloth substrate with a high electrical conductivity and electrochemical stability. The morphology and structure of the different Ni-Co ratios were analyzed with X-Ray Diffraction (XRD) and Scanning Electron Microscopy (SEM). XRD results confirmed the success synthesis of the Ni-Co LDH. SEM images show that the morphology of the nanostructures on the carbon cloth vary as the Ni-Co ratios change. Given by their unique nano-architecture, this method provides an efficient route to synthesize well-controlled three dimensional Ni-Co LDH nanostructures for nanodevice application. It is important to continue investigating the electrochemical properties in the future.   

Solvothermal synthesis of Mg-doped Li$_{\mathrm{2}}$FeSiO$_{\mathrm{4}}$/C nanocomposite cathode materials for lithium-ion batteries

Lithium transition metal orthosilicates, such as Li$_{\mathrm{2}}$FeSiO$_{\mathrm{4\thinspace }}$and Li$_{\mathrm{2}}$MnSiO$_{\mathrm{4}}$, as cathode material have attracted much attention lately due to their high theoretical capacity (\textasciitilde 330 mAh/g), low cost, and environmental friendliness. However, they suffer from poor electronic conductivity and slow lithium ion diffusion in the solid phase. Several cation-doped orthosilicates have been studied to improve their electrochemical performance. We have synthesized partially Mg-substituted Li$_{\mathrm{2}}$Mg$_{x}$Fe$_{\mathrm{1-}}_{x}$SiO$_{\mathrm{4}}$-C, ( $x =$ 0.0, 0.01, 0.02, and 0.04) nano-composites by solvothermal method followed by annealing at 600$^{\mathrm{o}}$C in argon flow. The structure and morphology of the composites were characterized by XRD, SEM and TEM. The surface area and pore size distribution were measured by using N$_{\mathrm{2}}$ adsorption/desorption curves. The electrochemical performance of the Li$_{\mathrm{2}}$Mg$_{x}$Fe$_{\mathrm{1-}}_{x}$SiO$_{\mathrm{4}}$-C composites was evaluated by Galvanostatic cycling against metallic lithium anode, electrochemical impedance spectroscopy, and cyclic voltammetry. Li$_{\mathrm{2}}$Mg$_{\mathrm{0.01}}$Fe$_{\mathrm{0.99}}$SiO$_{\mathrm{4}}$-C sample shows a capacity of \textasciitilde 278 mAh/g (at C/30 rate in the 1.5-4.6 V voltage window) with an excellent rate capability and stability, compared to the other samples. We attribute this observation to its higher surface area, enhanced electronic conductivity and higher lithium ion diffusion coefficient.   

$\alpha -$MnO$_{2}$ Nanorod-composites as Electrode Material for Supercapacitors

MnO$_{2}$-based supercapacitors as electrochemical storage systems have attracted immense interest due to their low cost, natural abundance, high theoretical specific capacitance and environmental friendliness. We have synthesized $\alpha $-MnO$_{2}$ and $\alpha $-MnO$_{2}$/CNF (carbon nanofibers, 5 wt{\%}) nanocomposites using co-precipitation method. The XRD results confirm the formation of a single phase $\alpha $-MnO$_{2}$ and SEM, TEM studies reveal the formation of nanorods of $\alpha $-MnO$_{2,\, }$but with a larger size in the case of $\alpha $-MnO$_{2}$-CNF nanocomposites. Pure $\alpha $-MnO$_{2}$ shows a larger surface area (266 m$^2$ /g), and lower electrical conductivity (0.02 S/cm) compared to that of $\alpha $-MnO$_{2}$-CNF (131 m$^2$ /g, 0.67 S/cm). Cyclic voltammetry (CV) studies and galvanostatic charge/discharge studies have been performed on $\alpha $-MnO$_{2\, }$nanocomposites, coated on Ni foam, using a potential ranging from -0.02 to 0.8 V, in a 1 M Na$_{2}$SO$_{4}$ aqueous solution. The measured specific capacitance of $\alpha $-MnO$_{2}$ is 245 F/g whereas that of $\alpha $-MnO$_{2}$-CNF is 192 F/g. Although, the electrical conductivity of $\alpha $-MnO$_{2}$-CNF is higher than that of $\alpha $-MnO$_{2}$, its observed lower specific capacitance is attributed to its reduced surface area compared to $\alpha $-MnO$_{2}$. Currently, we are optimizing the amount of CNF in $\alpha $-MnO$_{2}$-CNF nanocomposites to enhance supercapacitor performance.   

Cobalt Phosphide Nanowire Arrays for Flexible Solid-State Asymmetric Supercapacitors

Supercapacitors, owing to its fast charge-discharge rate, high power density, excellent stability and long cycle life, have received tremendous interest as promising electrochemical energy storage devices for a large variety of applications. In this study, Cobalt phosphide nanowire arrays on carbon cloth were synthesized by a simple two-step hydrothermal method. Owing to the unique nanostructures, it exhibits good performance such as high capacitance and high rate capability when utilized as supercapacitor electrodes. Moreover, the solid-state flexible asymmetric supercapacitor based on cobalt phosphide electrode demonstrates excellent performance such as high energy density and power density. In addition, the solid-state supercapacitor devices show remarkable cycle stability. This work demonstrates an example of cobalt phosphide for supercapacitor electrode application as well as a promising candidate for next-generation energy storage material.   

Optical Waveguides Written in Silicon with Femtosecond Laser

Silicon is one of the most widely used materials in modern technology, ranging from electronics and Si-photonics to microfluidic and sensor applications. Despite the long history of Si-based devices, and the strong demand for opto-electronical integration, 3D Si laser processing technology is still challenging. Recently, nanosecond-pulsed laser was used to fabricate embedded holographic elements in Si [1]. However, ~until now, there was no demonstration of femtosecond-laser-written optical elements inside Si. In this paper, we present optical waveguides written deep inside Si with 1.5 um femtosecond laser. The laser beam, with 2 uJ pulse energy and 350 fs pulse duration focused inside Si sample, produces permanent modification of Si. By moving the lens along the beam direction we were able to produce optical waveguides up to 5 mm long. The diameter of the waveguide is measured to be 10 um. The waveguides were characterized with both optical shadowgraphy and far field imaging after CW light coupling. We observed nearly single mode propagation of light inside of the waveguide. The obtained difference of refractive index inside of the waveguide, is 2.5*10-4. [1]Tokel.et.al.,\underline {arxiv.org/abs/1409.2827}   

Hybrid Pulsed Nd:YAG Laser

This work concerns the novel design of an inexpensive pulsed Nd:YAG laser, consisting of a hybrid Kerr Mode Lock (KLM) and Q-switch pulse. The two pulse generation systems work independently, non simultaneously of each other, thus generating the ability for the user to easily switch between ultra-short pulse widths or large energy density pulses. Traditionally, SF57 glass has been used as the Kerr medium. In this work, novel Kerr mode-locking mediums are being investigated including: tellurite compound glass (TeO$_{2}$), carbon disulfide (CS$_{2}$), and chalcogenide glass. These materials have a nonlinear index of refraction orders of magnitude,(\textit{n$_{2}$}), larger than SF57 glass. The Q-switched pulse will utilize a Pockels cell. As the two pulse generation systems cannot be operated simultaneously, the Pockels cell and Kerr medium are attached to kinematic mounts, allowing for quick interchange between systems. Pulse widths and repetition rates will vary between the two systems. A goal of 100 picosecond pulse widths are desired for the mode-locked system. A goal of 10 nanosecond pulse widths are desired for the Q-switch system, with a desired repetition rate of 50 Hz. As designed, the laser will be useful in imaging applications.   

Fabrication of lithium niobate for three wave mixing, quantum information and communications

Lithium Niobate (LN) is a crystal that has applications in nonlinear optics. Poling LN crystals allows quasi phase matching and three wave mixing to be achieved while allowing crystals to be longer without incurring a phase-mismatch penalty. Periodically Poled Lithium Niobate (PPLN) has a high degree of effective nonlinearity due to the increased interaction length. Fabrication of PPLN crystals starts from a Lithium Niobate wafer~doped with MgO. The wafer is periodically patterned with photoresist, then placed inside a conductive electrolyte solution. A high voltage is applied through the solution, contacting the wafer where the resist is absent. A 3-5 kV pulse is applied through the electrolyte, causing a domain reversal between the photoresist, leading to periodic poling.~An alternative fabrication process of PPLN involves the wafer periodically patterned with electrodes and placed in a dielectric oil bath held at a constant temperature. The fabrication of PPLN will be explored using various voltages, temperatures and periods. The fabricated structures will be tested in frequency upconversion and downconversion experiments for quantum information and communication applications.   

Resonant RF Photodetectors for Microwave and Infrared Applications

Room-temperature semiconductor-based photodetectors consisting of resonant RF circuits coupled to microstrip buslines, fabricated on an active substrate are demonstrated. The RF resonant circuits are characterized at RF frequencies as a function of resonator geometry, as well as for their response to incident IR radiation. The detectors are modeled analytically and using finite element method based commercial simulation software. Theoretical results from both methods show with good agreement the measured experimental results. We demonstrate that detector response can be improved by choice of photoconductive material, and further for a given material, by optimizing the position of the optical signal to overlap the RF field enhancement. The RF circuits with strong field enhancement are demonstrated to validate improve detector response. Such resonant detectors can easily be multiplexed on a single readout line and thus offers opportunities for applications in RF photonics, materials metrology, or single read-out multiplexed detector arrays and signal processing.   

Static Mixer for Heat Transfer Enhancement for Mold Cooling Application

Injection molding is the process by which a material is melted in a barrel and then it is injected through a nozzle in the mold cavity. When it cools down, the material solidifies into the shape of the cavity. Typical injection mold has cooling channels to maintain constant mold temperature during injection molding process. Even and constant temperature throughout the mold are very critical for a part quality and productivity. Conformal cooling improves the quality and productivity of injection molding process through the implementation of cooling channels that ``conform'' to the shape of the molded part. Recent years, the use of conformal cooling increases with advance of 3D printing technology such as Selective Laser Melting (SLM). Although it maximizes cooling, material and dimension limitations make SLM methods highly expensive. An alternative is the addition of static mixers in the molds with integrated cooling channels. A static mixer is a motionless mixing device that enhances heat transfer by producing improved flow mixing in the pipeline. In this study, the performance of the cooling channels will be evaluated with and without static mixers, by measuring temperature, pressure drop, and flow rate. The following question is addressed: Can a static mixer effectively enhance heat transfer for mold cooling application processes? This will provide insight on the development of design methods and guidelines that can be used to increase cooling efficiency at a lower cost.   

Applied Augmented Reality for High Precision Maintenance

Augmented Reality had a major consumer breakthrough this year with Pokemon Go. The underlying technologies that made that app a success with gamers can be applied to improve the efficiency and efficacy of workers. This session will explore some of the use cases for augmented reality in an industrial environment. In doing so, the environmental impacts and human factors that must be considered will be explored. Additionally, the sensors, algorithms, and visualization techniques used to realize augmented reality will be discussed. The benefits of augmented reality solutions in industrial environments include automated data recording, improved quality assurance, reduction in training costs and improved mean-time-to-resolution. As technology continues to follow Moore's law, more applications will become feasible as performance-per-dollar increases across all system components.   

A large-range scanner for a SQUID microscope

Scanning Superconducting QUantum Interference Device (SQUID) microscopy is an ultrasensitive technique for studies of many condensed matter phenomena. Due to the long-range nature of magnetic fields, it is often beneficial to image large areas of studied samples. While, for example, a typical magnetic force microscope scan range is a few tens of micrometers, for scanning SQUIDs, hundreds of micrometers or a few millimeters scan range is desirable. We develop a millimeter-range scanner with minimized image distortions. It is based on piezo-mechanical amplifiers and could be combined with a voltage source with an active vibration suppression feedback loop for applications in a closed cycle cryostat.   

Effect of Parkinson's Disease in Transcranial Magnetic Stimulation Treatment

Transcranial Magnetic Stimulation is a non-invasive clinical therapy used to treat depression and migraine, and shows further promise as treatment for Parkinson's disease, Alzheimer's disease, and other neurological disorders. However, it is yet unclear as to how anatomical differences may affect stimulation from this treatment. We use finite element analysis to model and analyze the results of Transcranial Magnetic Stimulation in various head models. A number of heterogeneous head models have been developed using MRI data of real patients, including healthy individuals as well as patients of Parkinson's disease. Simulations of Transcranial Magnetic Stimulation performed on 22 anatomically different models highlight the differences in induced stimulation. A standard Figure of 8 coil is used with frequency 2.5 kHz, placed 5 mm above the head. We compare cortical stimulation, volume of brain tissue stimulated, specificity, and maximum E-field induced in the brain for models ranging from ages 20 to 60. Results show that stimulation varies drastically between patients of the same age and health status depending upon brain-scalp distance, which is not necessarily a linear progression with age.   

Silk-polypyrrole biocompatible actuator performance under biologically relevant conditions

Biocompatible actuators that are capable of controlled movement and can function under biologically relevant conditions are of significant interest in biomedical fields. Previously, we have demonstrated that a composite material of silk biopolymer and the conducting polymer polypyrrole (PPy) can be formed into a bilayer device that can bend under applied voltage. Further, these silk-PPy composites can generate forces comparable to human muscle (\textgreater 0.1 MPa) making them ideal candidates for interfacing with biological tissues. Here silk-PPy composite films are tested for performance under biologically relevant conditions including exposure to a complex protein serum and biologically relevant temperatures. Free-end bending actuation performance, current response, force generation and, mass degradation were investigated . Preliminary results show that when exposed to proteins and biologically relevant temperatures, these silk-PPy composites show minimal degradation and are able to generate forces and conduct currents comparable to devices tested under standard conditions.   

Development of Active DNA Control Technique for DNA Sequencer With a Solid-state Nanopore

We have developed a technique that can control the arbitrary speeds of DNA passing through a solid-state nanopore of a DNA sequencer. For this active DNA control technique, we used a DNA-immobilized Si probe, larger than the membrane with a nanopore, and used a piezoelectric actuator and stepper motor to drive the probe. This probe enables a user to adjust the relative position between the nanopore and DNA immobilized on the probe without the need for precise lateral control. In this presentation, we demonstrate how DNA (block copolymer ([(dT)25-(dC)25-(dA)50]m)), immobilized on the probe, slid through a nanopore and was pulled out using the active DNA control technique. As the DNA-immobilized probe was being pulled out, we obtained various ion-current signal levels corresponding to the number of different nucleotides in a single strand of DNA.   

Brownian dynamics simulation of a polymer chain in a solid-state nanopore attached to a molecular stop

We study a nanopore inside a silicon dioxide membrane submerged in a $KCl$ solution with a negatively charged polymer chain of varying lengths whose movement is described using Brownian dynamics. The polymer is attached to a molecule with a radius larger than that of the nanopore's which acts as a molecular stop, allowing the chain to thread the nanopore but preventing it from translocating. We found that the polymer chain's variation of movement along the nanopore decreased when increasing applied biases and chain lengths for portions of the chain closest to the molecular stop. The chain displacement within the pore is also compared to a freely translocating polymer where preliminary results show the free polymer having a greater variation in the radial direction. Overall, our preliminary results indicate that the radial direction of the polymer chain is dominated by the confinement in the narrow nanopore with restrictions imposed by the molecular stop and bias playing a lesser role. Understanding the interaction behavior of the polymer chain-stop molecule may lead to methods that decrease movement variation, facilitating an improvement on characterizing and identification of molecules.   

Computational Study of Compact Microring Resonator Biosensors for Label-Free Detection

High Q microcavities have been investigated for chemical/biological sensing due to their highly sensitive response to binding events. In this work, we design and simulate a feasible and near minimally sized microring resonator sensor with large enough sensitivity to detect a single cellular analyte. Sensor performance is evaluated by varying waveguide material and dimension, and light polarization and wavelength, to maximize the detectable resonant wavelength shift due to a single cellular analyte. 3D simulations using a finite-element based method show a 2.5 $\mu$ m radius sensor (approximately the length of one cell) produces a 125 pm wavelength shift, Q of 1150, and 6.4dB extinction ratio for a single bound cellular analyte, making the design promising for high sensitivity cellular sensing.   

Piezophototronic Effect Enhanced Self-powered UV/Visible Photodetector Based on Type-II ZnO/ZnSe Core/Shell Heterojunction

Piezophototronic effect, coupling of piezoelectric and optical properties in semiconductor materials, has attracted much interest recently because of its capabilities of improving device performance significantly. In this paper, we report a Piezophototronic effect enhanced self-powered broadband UV/visible photodetector based on ZnO/ZnSe core/shell nanowire array. The integrated photodetector based on the ZnO/ZnSe core/shell structure is capable of detecting the whole band range of the visible spectrum as well as UV light, and it is further boosted through applying compressive load under different wavelength excitation sources by three orders of magnitude in the relative responsivity. The significant improved performance is believed to stem from piezophototronic effect and its abrupt interface between ZnSe and ZnO. Furthermore, the device exhibits good self-powered photodetection performance under UV/visible light illumination. This work is expected to generate broad interest in exploring the application using type II heterostructure for broadband UV/visible photodetection under both powered and self-powered conditions.   

Luminescent Lanthanide Complexes for Plasmonics and Metamaterials Studies.

Organic complexes, X(NO$_{\mathrm{3}})_{\mathrm{3}}$\textbullet Bpy$_{\mathrm{2}}$, where X is rare earth ion, are of interest for various optical applications, including probing effects of modified optical environment in plasmonic systems and metamaterials, or mapping optical fields in metasurfaces by spectroscopic methods. Using solution growth technique, we grow single crystals with Eu, Gd, Nd, Tm, Er and Yb, and crystals with two rare earth ions, Er$_{\mathrm{0.5}}$Yb$_{\mathrm{0.5}}$(NO$_{\mathrm{3}})_{\mathrm{3}}$\textbullet Bpy$_{\mathrm{2}}$ and Eu$_{\mathrm{0.5}}$Nd$_{\mathrm{0.5}}$(NO$_{\mathrm{3}})_{\mathrm{3}}$\textbullet Bpy$_{\mathrm{2}}$. Obtained crystals can be excited at UV or the rare earth ion transitions, demonstrate relatively high efficiency of luminescence in visible or infra-red, and are suitable for thin film fabrication. The effects of plasmonic environment on the emission and excitation spectra and energy transfer processes will be discussed.   

Characteristics of Al$_2$O$_3$ film by introducing additional oxygen and oxygen vacancy using Pt catalytic

Al$_2$O$_3$ is an attractive gate insulator for gallium nitride power device. It remains a big issue of mobility degradation because of oxygen vacancy (Vo) of Al$_2$O$_3$ film. Furthermore, little is known about influence of the Vo of Al$_2$O$_3$ on transistor property. In this paper, we study characteristics of Al$_2$O$_3$ insulator by introducing additional oxygen and Vo. We prepared p-Si(100)/SiO$_2$/Al$_2$O$_3$/Pt capacitors. These capacitors were annealed at 300 - 600 °C in N$_2$, O$_2$ and 3\% H$_2$ ambient to introduce additional oxygen and Vo into Al$_2$O$_3$ using Pt catalytic effect. The fixed charge density in Al$_2$O$_3$ film was negligible small from linear relationship between Vfb and Al$_2$O$_3$ thickness. The Vfb shift of capacitors which annealed at 300 - 600 °C in N$_2$ ambient exhibited about +0.6 V compared to the ideal Vfb. This is dominantly due to the dipole at Al$_2$O$_3$/SiO$_2$ interface. In contrast, the Vfb shift increased from +0.6 to +1.9 V with increasing the annealing temperature in O$_2$ ambient. The strength of the dipole increase because additional oxygen introduced by Pt catalytic effect piled up at Al$_2$O$_3$/SiO$_2$ interface. This suggests that the oxygen concentration at Al$_2$O$_3$/SiO$_2$ interface plays an important role of Vfb shift.   

Palladium nanoparticles functionalized graphene nanosheets for Li-O$_{\mathrm{2}}$ batteries: enhanced performance by tailoring the morphology of discharge product

Lithium oxygen (Li-O$_{\mathrm{2}})$ batteries represent a promising candidate for the next generation electric vehicle.$^{\mathrm{1-3}}$ Despite the attractive prospect, some issues including large overpotentials, poor recyclability and unstable electrolyte$^{\mathrm{4-6}}$ limit the wide applications of Li-O$_{\mathrm{2}}$ batteries. Due to the insoluble and non-conductive nature of discharge product Li$_{\mathrm{2}}$O$_{\mathrm{2}}$, it has been widely accepted that the performance of oxygen evolution reaction (OER) process is not only determined by the catalyst itself but also close linked to morphology and electronic conductivity of Li$_{\mathrm{2}}$O$_{\mathrm{2}}$ formed during oxygen reduction reaction (ORR) process. Herein, we report a strategy to improve the battery performance by tailoring the morphology of discharge product. By using graphene nanosheets (GNSs ) functionalized with Pd nanoparticles (NPs) as cathode catalyst, the growth and morphology of the discharge products of Li$_{\mathrm{2}}$O$_{\mathrm{2}}$ can be effectively tailored, thereby leading to the improved Li-O$_{\mathrm{2}}$ battery performance. Surprisingly, on bare GNSs cathode, the discharge product showed widely observed large-sized toroidal morphology. While for Pd NPs functionalized GNSs, the discharge product was homogenously distributed on the cathode in the form of small nanoparticles with an average diameter of \textasciitilde 25 nm. As a result, Pd NPs functionalized GNSs exhibited a high discharge capacity of 7690 mAh g$^{\mathrm{-1}}$. Meanwhile, the battery with tailored morphology exhibits lower charge overpotential.   

Combustion of Biofuel as a Renewable Energy Source in Sandia Flame Geometry

Energy security and climate change are two important key causes of wide spread employment of biofuel notwithstanding of problems associated with its usage. In this research, combustion of biofuel as a renewable energy source was numerically investigated in the well-known and practical Sandia flame geometry. Combustion performance of the flame has been simulated by burning biodiesel (methyl decanoate, methyl 9-decenoate, and n-heptane) oxidation with 118 species reduced/skeletal mechanism. The open-source code OpenFoam was used for simulating turbulent biodiesel-air combustion in the cylindrical chamber using the standard k-epsilon model. To check the accuracy of numerical results, the system was initially validated with methane-air Sandia national laboratories flame D experimental results. Excellent agreements between numerical and experimental results were observed at different cross sections. After ignition, temperature distributions at different distances of axial and radial directions as well as species mass fraction were investigated. It is concluded that biofuel has the capability of implementation in the turbulent jet flame that is a step forward in promotion of sustainable energy technologies and applications.   

Fixed Junction Light Emitting Electrochemical Cells based on Polymerizable Ionic Liquids

Organic photovoltaic (OPV) devices are of interest due to ease of fabrication, which increases their cost-effectiveness. OPV devices based on fixed-junction light emitting electrochemical cells (LECs) in particular have shown promising results. LECs are composed of a layer of polymer semiconductor blended with a salt sandwiched between two electrodes. As a forward bias is applied, the ions within the polymer separate, migrate to the electrodes, and enable electrochemical doping, thereby creating a p-n junction analog. In a fixed junction device, the ions are immobilized after the desired distribution has been established, allowing for operation under reverse bias conditions. Fixed junctions can be established using various techniques, including chemically by mixing polymerizable salts that will bond to the polymer under a forward bias. Previously we have demonstrated the use of the polymerizable ionic liquid allyltrioctylammonium allysulfonate (ATOAAS) as an effective means of creating a chemically fixed junction in an LEC. Here we present the application of this approach to the creation of photovoltaic devices. Devices demonstrate higher open circuit voltages, faster charging, and an overall improved device performance over previous chemically-fixed junction PV devices.   

Thin Film Organic Solar Cells: Theoretical and Experimental Band Gap Energy Calculations.

We analyze here different organic thin films with potential use for solar cells. We calculate the molecular orbitals and we obtain the band gap. We notice that the added zwitterions diminish the band gap of the film, making better solar cells. The two solar cells are obtain by depositing on the substrate of choice two different polymers, polyaniline and poly(3-hexylthiophene-2,5-diyl), and the zwitterion: p-benzoquinone monoamine. The band gap is theoretically calculated by using HyperChem package and obtained experimentally via Halogen and Deuterium spectra measurements. The I-V curves show that these films have great potential as efficient solar cells, as shown by the calculated ideality factor.   

Photovoltaic Enhancement with Ferroelectric HfO$_{\mathrm{2\thinspace }}$Embedded in the Structure of Solar Cells

Enhancing total efficiency of the solar cells is focused on the improving one or all of the three main stages of the photovoltaic effect: absorption of the light, generation of the carriers and finally separation of the carriers. Ferroelectric photovoltaic designs target the last stage with large electric forces from polarized ferroelectric films that can be larger than band gap of the material and the built-in electric fields in semiconductor bipolar junctions. In this project we have fabricated very thin ferroelectric HfO$_{\mathrm{2}}$ films (\textasciitilde 10nm) doped with silicon using RF sputtering method. Doped HfO$_{\mathrm{2}}$ films were capped between two TiN layers (\textasciitilde 20nm) and annealed at temperatures of 800\textordmasculine C and 1000\textordmasculine C and Si content was varied between 6-10 mol. {\%} using different size of mounted Si chip on hafnium target. Piezoforce microscopy (PFM) method proved clear ferroelectric properties in samples with 6 mol. {\%} of Si that were annealed at 800\textordmasculine C. Ferroelectric samples were poled in opposite directions and embedded in the structure of a cell and an enhancement in photovoltaic properties were observed on the poled samples vs unpoled ones with KPFM and I-V measurements.   

Atomic-level Design of Water-resistant Hybrid Perovskites with Optimal Band Gap for Solar Cells

Organic-inorganic hybrid perovskites hold the promise to become the next generation solar-cell materials. However, it is wellknown that these materials, exemplified by CH$_{\mathrm{3}}$NH$_{\mathrm{3}}$PbI$_{\mathrm{3}}$, will readily decompose with a trace amount of water under heat, which is perhaps the biggest and the most pressing problem in the field. In this work, we unveil the strategy to make the hybrid perovskites water-proof and also demonstrate the way to control their band gaps at the atomic level. A new family of hybrid perovskites is designed, which are both water-resistant and can achieve the optimal band gap smaller than that of CH$_{\mathrm{3}}$NH$_{\mathrm{3}}$PbI$_{\mathrm{3}}$.?   

Resonant Soft X-Ray Scattering: A Versatile Technique for Spatio-Chemical Characterization of Solar Fuel Materials

The development of complex mesoscale (nm - $\mu$ m) materials used for a wide range of solar fuel applications requires comparable evolution in the analytical instruments and techniques in order to understand the physical and chemical structure-property relationships underlying their performance. In this presentation, we will show some of the first experimental results demonstrating how Resonant Soft X-Ray Scattering (RSoXS) can be a powerful tool for the solar fuels community due to its chemical sensitivity, large accessible size scale, and polarization control. Specifically, we will reveal its ability to simultaneously interrogate the bulk, surfaces, and buried interfaces of low-Z element materials (including BiVO4), such as those used as nanostructured photoelectrodes, catalysts, and ion exchange membranes. In addition, we will outline recent developments we have made, on both the instrumental and device level, to enable operando and in-situ RSoXS characterization of electrochemical materials in liquid and gaseous environments. The practical challenges of conducting such experiments will be addressed so that the solar fuels community is well-informed about the potential of this novel time-resolved reciprocal space probe.   

BC${_3}$ as electrode for Mg ion batteries

We propose layered BC${_3}$ a novel electrode material for rechargeable magnesium ion batteries. Using dispersion-corrected density functional theory calculations, we show that layered BC${_3}$ can intercalate Mg ions between its layers to form the stoichoimetry Mg${_{0.5}}$BC${_3}$, which corresponds to a theoretical capacity of 572 mAh/g. We also propose a three step staging mechanism for Mg ion intercalation in BC$_3$ and show that it presents a moderate open circuit voltage in the range of 0.82 to 0.96 V with respect to metallic Mg anode.   

Investigation of PVDF - TiO$_{\mathrm{2}}$ Nanoparticle Composite Thin Films by XPS, SEM and EDS for Use in the Capacitive Storage of Energy

In this investigation, thin films of polyvinylidene fluoride (PVDF) containing nanoparticles of the ceramic titanium dioxide (TiO$_{\mathrm{2}})$ are synthesized using physical vapor deposition techniques. This combination of materials shows promise for possible use as the dielectric in capacitors, particularly regarding energy storage. This composite approach allows for the integration of complimentary features such as high dielectric permittivity from the integrated nanoparticles and high breakdown strength from the polymer matrix, resulting in a greatly enhanced energy density. Co-deposited films with a TiO$_{\mathrm{2}}$ content up to 8 percent have been synthesized and intermittent contact AFM and elemental mapping from EDS show that the dispersion of the nanoparticles in the material is homogeneous. Analysis from XPS indicates a defluorination of the films (C/F ratio greater than 1) from the deposition process, with the final film being a mixture of PVDF and polyvinyl fluoride (PVD). In addition, other parameters such as the dielectric constant and the breakdown voltage are given.   

Hydrogen interstitials inside bulk reduced-CeO$_{\mathrm{2}}$: charge state, defect chemistry and dynamics by \textit{ab-initio} calculations

CeO$_{\mathrm{2}}$ is a well-known and widely-used solid oxide fuel cells (SOFC) electrolyte and catalyst anode support, due to its facile oxygen vacancy formation and diffusion within its symmetric and capacious fluorite lattice. In real SOFC working conditions, hydrogen fuels will dissociate on anode surface and possibly permeate inside CeO$_{\mathrm{2}}$-based anode support and electrolyte. Studying hydrogen defect inside CeO$_{\mathrm{2}}$ lattice thus has two significant impacts: To see how hydrogen alters the ``oxygen buffering'' inside CeO$_{\mathrm{2}}$ as an anode support, and to see how it affects oxygen vacancy's diffusion and clustering inside electrolyte CeO$_{\mathrm{2}}$. Hereby \textit{ab-initio} calculations in Kohn-Sham Density Functional Theory with Hubbard model of self-interaction correction is carried out to investigate the electron polaron and vacancy formation in CeO$_{\mathrm{2}}$ affected by hydrogen, the hydrogen defect state within the band gap of CeO$_{\mathrm{2}}$, the chemistry of defect interactions and its effect on oxygen vacancy mobility.   

Understanding the good kinetics of Mo$_{6}$S$_{8}$ as cathode in Mg ion batteries by key electronic states.

Up to now, Chevrel phase (Mo$_{6}$X$_{n}$X'$_{8-n}$, X$=$S, X'$=$Se) is still the unique cathode material in Mg ion batteries that has acceptable kinetic performance. However, the origin of good kinetics still needs to be clarified for further, which is critical to the rational explore of the cathode material for the battery with both good cycling performance and specific energy, though some studies have been investigated from the point view of electrochemistry and crystal structure. The study on electronic structure is eager to get a deep insight of the intrinsic property due to the 2e$^{-}$ charge transfer in Chevrel phase and the inspiration from the success studies of the key electronic states in lithium ion batteries. In this report, the unoccupied states of a typical Chevrel phase Mo$_{6}$S$_{8}$ including Mo L-edge and S K-edge were studied with various amount of Mg$^{2+}$ inserted by tender X-ray absorption spectroscopy. A key electronic state at the pre-edge of S spectra were found to be evolved regularly with Mg$^{2+}$ inserted and extracted. It is assigned to be hybridization between Mo 4d states and S 3p states. The evolution of this state opens a gate to reveal the nature of good kinetics in Chevrel phase.   

First-Principles evaluation of the Chevrel phase intercalated with Be, Mg, Ca, Sr, and Ba

Li ion batteries are extremely useful when an item requires portability and compactness, such as laptops and cell phones; due to the lightweight/compact nature of Li ion batteries. The lightweight and compact nature of Li ion batteries comes at a high cost. It is sensible to consider Li ion battery alternatives, which are more cost effective and useable when portability is not a priority. An option for a less expensive battery source is the Ca ion battery. The Ca ion battery is interesting as many researchers overlook the potential battery source due to the perplexity of finding suitable anode materials and electrolytes. In order for this technology to work, cathodes that allow for the reversible intercalation of Ca$^{2+}$ ions and also provide a preferred voltage must be identified. We investigate the Chevrel phase compounds of $\mathrm{Mo}_6X_8$ (X = S, Se, Te) which can intercalate various ions. The concentration of the ion intercalated with the Chevrel cathode is studied. We consider doped versions of the Chevrel phase, using various dopants to substitute Mo. We use density functional theory to calculate the voltage of several intercalation ions with the Chevrel material. The resulting electronic properties of the aforementioned materials will be investigated.   

Atomistic Modeling of Cation Diffusion in Transition Metal Perovskites La$_{\mathrm{\mathbf{1-x}}}$\textbf{Sr}$_{\mathrm{\mathbf{x}}}$\textbf{MnO}$_{\mathrm{\mathbf{3\pm \delta }}}$\textbf{ for Solid Oxide Fuel Cell Cathodes Applications}

Cation diffusion in La$_{\mathrm{1-x}}$Sr$_{\mathrm{x}}$MnO$_{\mathrm{3\pm \delta }}$ (LSM) and in related perovskite materials play an important role in controlling long term performance and stability of solid oxide fuel cell (SOFCs) cathodes. Due to sluggish rates of cation diffusion and complex coupling between defect chemistry and cation diffusion pathways, currently there is still lack of quantitative theoretical model predictions on cation diffusivity vs. T and P(O$_{\mathrm{2}})$ to describe experimental cation tracer diffusivities. In this work, based on \textit{ab initio} modeling of LSM defect chemistry and migration barriers of the possible cation diffusion pathways, we assess the rates of A-site and B-site cation diffusion in a wide range of T and P(O$_{\mathrm{2}})$ at x$=$0.0 and 0.2 for SOFC applications. We demonstrate the active cation diffusion pathways in LSM involve cation defect clusters as cation transport carriers, where reduction in the cation migration barriers, which are governed by the steric effect associated with the metal-oxygen cage in the perovskite lattice, is much greater than the penalty of repulsive interaction in the A-site and B-site cation vacancy clusters, leading to higher cation diffusion rates as compared to those of single cation vacancy hopping mechanisms. The predicted Mn and La/Sr cation self-diffusion coefficients of LSM at at x$=$0.0 and 0.2 along with their 1/T and P(O$_{\mathrm{2}})$ dependences, are in good agreement with the experimental tracer diffusion coefficients.   

Low temperature thermoelectric properties of hot pressed composite samples of CrSb$_{\mathrm{2}}$: evidence for possible phonon-drag effect.

We present on the thermoelectric transport properties of CrSb$_{\mathrm{2}}$ samples prepared by hot-press densification in the temperature range of 2 - 350 K. At around 10 K, the thermal conductivity of CrSb$_{\mathrm{2}}$ decreases dramatically by three orders of magnitude compared to the single crystal counterpart. Analysis shows that the reduced thermal conductivity results from increased scattering of the phonons off the grain-boundaries within the samples. A strong interrelationship between the thermal conductivity and the Seebeck coefficient is observed; indicating a significant presence of phonon-drag effect in this system. With ZT $=$ 0.018 at 310 K for the sample hot pressed at 600 $^{\mathrm{o}}$C, an increase in ZT by 80 {\%} over the previously reported values for polycrystalline samples is achieved.   

First principles calculations of Thermoelectric properties of Bi2Te3 and PbTe

We presented first-principle calculations of electron and phonon transport in Bi2 Te3 and PbTe. We focused on the several thermoelectric properties; Seebeck coefficient, electrical conductivity, electrical thermal conductivity and lattice thermal conductivity. The electronic transport is calculated using the projector augmented wave (PAW) method implemented in Vienna Ab-initio Simulation Package (VASP) and Boltzmann transport equation (BTE). From electronic transport, the Seebeck coefficient can be estimated by simple expression containing band-gap energy. From phonon transport, we calculated the interatomic force constants (IFCs) along with a fully iterative solution of phonon-BTE. This approach allows both harmonic and anharmonic interatomic forces to be contained into the result. The calculated lattice thermal conductivity was found to be in good agreement with experimental data. We discussed that the first-principle methodology can be a tool to understand the transport details in many solid-state devices.   

NMR and calorimetry of CuAgSe and CuAgSe$_{\mathrm{0.5}}$Te$_{\mathrm{0.5}}$

Copper based chalcogenides have attracted much interest due to potential electronic and thermoelectric applications, and CuAgSe in particular has been found to have very low thermal conductivity and high mobility, with the possibility of Dirac-like electronic features. We have used $^{\mathrm{63}}$Cu, $^{\mathrm{65}}$Cu, and $^{\mathrm{77}}$Se NMR along with specific heat and DFT computation to study the structure, vibrational and electronic properties. We observe a large Einstein-like mode in the CuAgSe specific heat, which we discuss in terms of Cu hopping between half-occupied sites. DFT energy minimization and computed phonon behavior also support this result. The Cu NMR T$_{\mathrm{1}}^{\mathrm{-1}}$ is dominated by phonons at low temperatures, also indicative of strongly anharmonic vibrational behavior, which may contribute to the lower thermal conductivity. At higher temperatures, Se NMR shows evidence for Cu hopping. We also examined Cu NMR shifts and find that the metallic shift makes up a very small portion, however based on the reported effective mass the results indicate that Cu s states make a significant contribution to the conduction band, contrary to computational results.   

Photothermal Deflection Spectroscopy of materials for energy applications

A new photothermal deflection spectroscopy (PDS) setup has been constructed at Transylvania University. This poster will focus on the photothermal behavior of nanomaterials such as quantum dots as well as organic photovoltaic materials. With respect to organic photovoltaic materials, this work aims to understand differences in photothermal behavior between the solution and solid-film phases, where changes in photothermal spectra give insight into changes in electronic structure. A general overview of the PDS capabilities at Transylvania will also be given.   

Cross-Plane Thermal Conductivity Measurements of Periodical Nanoporous In$_{\mathrm{0.1}}$Ga$_{\mathrm{0.9}}$N Thin Films

Nanoporous thin films are expected to reduce lattice thermal conductivity while maintain the bulk-like electrical properties, which can yield a high thermoelectric figure of merit (ZT) [1,2]. For Si thin films, a room-temperature ZT\textasciitilde 0.4 has been reported for 55-nm-pitch nanoporous patterns [3]. Along this line, a high ZT is also anticipated for other nanoporous thin films whose bulk counterparts have superior electrical properties but high lattice thermal conductivities. In this work, the cross-plane thermal conductivities of nanoporous In$_{\mathrm{0.1}}$Ga$_{\mathrm{0.9}}$N thermoelectric thin films [4] with varied porous patterns are measured with the time-domain thermoreflectance technique. In our measurements, a remarkable thermal conductivity reduction has been observed for In$_{\mathrm{0.1}}$Ga$_{\mathrm{0.9}}$N thin films with relatively large sub-micron nanoporous features. Our studies provide guidance for ZT enhancement in nanostructured nitrides and oxides. References: [1] Marconnet et al., Journal of Heat Transfer 135, 061601-1/10 (2013). [2] Cahill et al., Appl. Phys. Rev. 1, 011305 (2014). [3] Tang et al., Nano Lett. 10, 4279-4283 (2010). [4] Lu et al., Semicond. Sci. Technol. 28, 074023 (2013).   

Hydrogen and Methane Sorption in Carbon Microspheres

One possible avenue of gaseous fuel storage is the use of physisorption. The dispersive van der Waals force results in an attraction between a gas molecule and a solid surface. This causes the density of the gas to increase. The two major categories of sorbent materials being investigated are carbon materials (activated carbons) and porous crystalline solids (metal-organic frameworks). Results will be presented on hydrogen and methane sorption measurements on activated carbon microspheres. The carbon microspheres are produced hydrothermally from sucrose. In brief, sucrose is dissolved in water and heated at 200 degrees Celsius resulting in small carbon spheres. The microspheres as first produced are solid, with no inner hollow. Transmission electron microscopy shows the microspheres as approximately one micrometer in diameter. These microspheres were then activated using high temperature carbon dioxide to create pores and increase surface areas. Results before and after activation will be presented. The effect of particle shape on packing of the sorbent powder will be discussed. Transmission electron microscopy images will be shown.   

Theoretical Studies of Carrier Diffusion in Perovskite Tantalum Oxynitride Photocatalyst

Water-splitting photocatalysts have been attracting considerable attention in the scientific community since they enable to produce clean and environmentally friendly chemical energy in the form of H$_{2}$. Perovskite BaTaO$_{2}$N (BTON) is expected to be a performing photcatalyst due to its band structure suitable for visible light absorption and overall water-splitting reaction. Indeed, successful hydrogen evolution and oxygen evolution reactions are reported under sacrificial reagent. However, to achieve highly efficient overall water-splitting, electronic properties such as carrier diffusion are need to be improved. In our study, we investigate the carrier diffusion properties in BTON by focusing on cooling process of hot carriers via phonon, by means of first-principles calculations combining Density Functional Theory (DFT) and Many-body Perturbation Theory (MBPT). In particular, we calculated the lifetime of hot carrier in BTON by evaluating electron-phonon coupling constant. We found that BTON has very short carrier lifetime with an order of 1 fs. We also clarified that anion ordering, i.e. the anionic distribution of O$^{2-}$ and N$^{3-}$ in the oxynitride, affects the lifetime of hot hole.   

The Effect of Co-catalyst on Water-splitting Photocatalyst: A DFT Analysis

Water splitting photocatalyst is getting wide attention due to their ability to produce H$_{\mathrm{2}}$ by utilizing solar energy. One of the promising way to boost its efficiency is to load co-catalysts such as noble metal nanoparticles on the photocatalyst. Co-catalysts are believed to separate photogenerated electrons and holes by forming a Schottky barrier at the metal-semiconductor interface, at which O$_{\mathrm{2}}$ and H$_{\mathrm{2}}$ evolution reaction (OER, HER) are thought to occur. Nevertheless, the details of reactive sites and reaction mechanism are still not clear due to the difficulty in detecting OER and HER. In our research, we aimed to elucidate the relations between geometrical and electronic features of co-catalysts and to clarify the absorption structure of water on the photocatalyst by means of first-principles calculations. We picked up Pt/SrTiO$_{\mathrm{3}}$ as a subject, well known cases, and found the changes in charge density difference and band structure according to the place of co-catalyst. We will also investigate the dependence of water adsorption energy on the absorption site and the role of co-catalysts in HER by analyzing the changes in electronic structures by the absorption of water on the surface.   

Adsorption-Induced Structural Changes in Graphene Oxide Frameworks

In Graphene Oxide Frameworks (GOFs), graphite oxide has been intercalated with benzene diboronic acid linker molecules to create a porous material, which has shown promise for its hydrogen storage capacity. The original model of covalent crosslinked graphene sheets$^{2}$ has recently come under question, though, due to swelling of GOFs observed in polar solvents.$^{3}$ To probe the nature of bonding between linker and graphene, we have conducted hydrogen adsorption isotherm measurements at high pressures with a gas handling system designed for \textit{in situ }neutron scattering measurements. An extended hysteresis loop in the sub-critical nitrogen isotherm indicates a possible swelling of the material upon gas adsorption. To assess pressure induced structural changes in GOF materials we are using the two-axis neutron diffractometer at the University of Missouri Research Reactor to measure the position and intensity of the (001) Bragg peak of GOF as function of pressures up to 100 bar both at 80 K and room temperature. In addition, pair distribution functions of deuterium adsorbed on GOF have been measured at the Nanoscale-Ordered Materials Diffractometer at Oak Ridge National Laboratory, showing the presence of a liquid-like phase at 42 K. $^{2.}$Burress, J. W. \textit{et al.}, \textit{Angew. Chem Int. Ed.} \textbf{49,} 8902--8904 (2010). $^{3.}$Mercier, G. \textit{et al.} \textit{J. Phys. Chem. C} \textbf{119,} 27179--27191 (2015).   

Insights into molecular architecture of terpenes using small angle neutron scattering

Understanding macromolecular architectures is vital to engineering prospective terpene candidates for advanced biofuels. Eucalyptus plants store terpenes in specialized cavity-like structures in the leaves called oil glands, which comprises of volatile (VTs) and non-volatile (NVTs) terpenes. Using small-angle neutron scattering, we have investigated the structure and phase behavior of the supramolecular assembly formed by Geranyl beta-D-glucoside (GDG), a NVT and compare the results with that of beta-octyl glucoside (BOG). The formation of micellar structures was observed in the concentration range of 0.5-5 v/v{\%} in water using small angle neutron scattering (SANS) where Schultz sphere model was used in quantifying structural parameters of micelles. SANS studies determine that GDG and BOG behave like amphiphiles forming micellar structures in aqueous solution. The micelles swell upon addition of alpha-Pinene (AP) indicating partition to the core region of the micelles. The general behavior of the micellar growth after partitioning of AP to form thermodynamically stable sizes varies with the NVT concentration. Our studies reveal that the presence of steric hindrance in the GDG via the unsaturated bonds could help stabilize VTs inside the oil glands.   

Electron-Phonon Coupling Effects in Molecular Heat Conduction

We examine electronic heat conduction via molecular complexes in the presence of local electron-phonon coupling effects. Specifically, we analyze transport characteristics of molecular junction regarding the strength of molecule-reservoir and electron-phonon coupling parameters, temperature and energy of molecular vibrations. We also perform a detailed analysis of the influence of phonon-assisted processes and the structure of phonon sidebands onto heat fluxes. For that purpose, we use non-perturbative computational scheme based on inelastic version of Landauer formula, where the Green's functions technique combined with polaron transformation was used to calculate multi-channel transmission probability function, while accessibility of individual conduction channels is governed by Boltzmann statistics. Our analysis is based on the hypothesis that the dynamics created by electron-phonon interaction onto the molecule asymmetrically connected to two thermal reservoirs will lead to thermal rectification effect.   

Dry Methane Reforming by Atmospheric Pressure Glow Discharge Plasma Reactor

Dry methane reforming to produce syngas by glow discharge plasma at the atmospheric pressure has been investigated. In this study the reactor was especially designed to have the advantage of the large treatment area because of the high electrode distance (2-2.5 cm) to initiate the uniform discharge inside the plasma reactor. The effects of feed flow rate and electrode distance on methane and carbon dioxide conversion and CO and H$_{\mathrm{2}}$ selectivity were studied. The experiment was operated in the input power of 22 W, the total feed flow rate from 60 ml/min to 150 ml/min, electrode distance 2 cm and 2.5 cm and the molar ratio of CO$_{\mathrm{2}}$/CH$_{\mathrm{4}}=$3. At the constant voltage of V$=$10 kV the best results for conversion of CH$_{\mathrm{4}}$ and CO$_{\mathrm{2}}$ were 75{\%} and 37.02{\%}, respectively. The effect of voltage in the range from 10 kV to 18 kV by constant parameters such as (F$=$30 kHz, molar ratio of CO$_{\mathrm{2}}$/CH$_{\mathrm{4}}=$3, feed flow rate$=$ 60ml/min and electrode distance d$=$2 cm) shows that the considerable results for methane and carbon dioxide conversion were 87.6{\%} and 46.3{\%}; for CO and H$_{\mathrm{2}}$ selectivity, were 70{\%} and 30{\%}, respectively. The efficiency of plasma method was achieved 54{\%} under the conditions of CO$_{\mathrm{2}}$/CH$_{\mathrm{4}}=$3, feed gases flow rate 60ml/min, electrode distance 2(cm), applied voltage 10 (kV), and input power 22(w).   

Superalkalis as building blocks of noble-metal-free CO$_{\mathrm{2}}$ activation

One of the great challenges to treat greenhouse effect is to convert CO$_{\mathrm{2}}$~into fuels. The key step for this process requires activation of the CO$_{\mathrm{2}}$~molecule. Recent experiments have shown that this can be accomplished by reacting CO$_{\mathrm{2}}$~with noble metal Au. Realizing that the addition of an electron transforms the neutral CO$_{\mathrm{2}}$~from linear to a bent structure, it was argued that the key parameter that promotes electron transfer from a metal atom to CO$_{\mathrm{2}}$~depends upon its ionization potential. We note that hetero-atomic clusters known as super-alkalies can be designed such that their ionization potential is smaller than those of alkali atoms. Using first-principles theory we have designed a variety of super-alkali species using different electron counting rules and studied their thermodynamic stability using molecular dynamics simulation. Reaction of these super-alkalies with CO$_{\mathrm{2}}$~shows that they can be ideal noble-metal free particles for CO$_{\mathrm{2}}$~activation. ~   

``H$_{\mathrm{2}}$ sponge”: pressure as a means for reversible high-capacity hydrogen storage in nanoporous Ca-intercalated covalent organic frameworks

We explore the potential and advantages of Ca-intercalated covalent organic framework-1 (CaCOF-1) as a 3 dimensional (3D) layered material for reversible hydrogen storage. Density functional theory calculations show that by varying the interlayer distance of CaCOF-1, a series of metastable structures can be achieved with the interlayer distance falling in the range of 4.3--4.8 Å. When four hydrogen molecules are adsorbed on each Ca, a high hydrogen uptake of 4.54 wt{\%} can be produced, with the binding energy falling in the ideal range of 0.2--0.6 eV per H$_{\mathrm{2}}$. While H$_{\mathrm{2}}$ absorption is a spontaneous process under H$_{\mathrm{2}}$ rich conditions, tuning the interlayer distance by reasonable external pressure could compress CaCOF-1 to release all of the hydrogen molecules and restore the material to its original state for recyclable use. This provides a new method for gradual, controllable extraction of hydrogen molecules in covalent organic frameworks, satisfying the practical demand for reversible hydrogen storage at ambient temperatures.   

The generalized Onsager model and DSMC simulations of high-speed rotating flows with product and waste baffles

The generalized Onsager model for the radial boundary layer and of the generalized Carrier-Maslen model for the axial boundary layer in a high-speed rotating cylinder ((S. Pradhan {\&} V. Kumaran, J. Fluid Mech., 2011, vol. 686, pp. 109-159); (V. Kumaran {\&} S. Pradhan, J. Fluid Mech., 2014, vol. 753, pp. 307-359)), are extended to a multiply connected domain, created by the product and waste baffles. For a single component gas, the analytical solutions are obtained for the sixth-order generalized Onsager equations for the master potential, and for the fourth-order generalized Carrier-Maslen equation for the velocity potential. In both cases, the equations are linearized in the perturbation to the base flow, which is a solid-body rotation. An explicit expression for the baffle stream function is obtained using the boundary layer solutions. These solutions are compared with direct simulation Monte Carlo (DSMC) simulations and found excellent agreement between the analysis and simulations, to within 15{\%}, provided the wall-slip in both the flow velocity and temperature are incorporated in the analytical solutions.   

Effect of the Hole Injection Layer on the Performance of Polymer Solar Cells

The hole injection layer in polymer solar cells was reported to improve the performance of the devices, mainly by increase of the open circuit voltage. In this study, we will report the use of PEDOT:PSS with different concentrations, 1{\%} and 2.4{\%} in water, as the hole injection layer. The active layers we will use are a mixture of PCBM with either P3HT or PCPDTBT. The anode will be an ITO film on a glass substrate, and the cathode will be thermally evaporated aluminum on the polymer film. The goal is to observe if the hole injection layer increases the open circuit voltage, and if the concentration of PEDOT:PSS in water will have an effect on the open circuit voltage. We will present the current-voltage characteristics of the polymer solar cells in dark and under illumination, as well as the impedance spectra of the solar cells in the frequency range of 10-10$^{\mathrm{6}}$ Hz.   

Modeling of the Electric Characteristics of Solar Cells

The purpose of a solar cell is to covert solar energy, through means of photovoltaic action, into a sustainable electrical current that produces usable electricity. The electrical characteristics of solar cells can be modeled to better understand how they function. As an electrical device, solar cells can be conveniently represented as an equivalent electrical circuit with an ideal diode, ideal current source for the photovoltaic action, a shunt resistor for recombination, a resistor in series to account for contact resistance, and a resistor modeling external power consumption. The values of these elements have been modified to model dark and illumination states. Fitting the model to the experimental current voltage characteristics allows to determine the values of the equivalent circuit elements. Comparing values of open circuit voltage, short circuit current, and shunt resistor can determine factors such as the amount of recombination to diagnose problems in solar cells. The many measurable quantities of a solar cell's characteristics give guidance for the design when they are related with microscopic processes.   

Characterization of Inverted Polymer Bulk Heterojunction Solar Cells

Inverted solar cells were proven to be an improvement over polymer solar cells in terms of durability and reliability. We have fabricated the solar cells using P3HT and PCPDTBT as the active polymer with PC60BM as the electron acceptor. The materials we deposited from solution by spin coating on glass substrates with ITO film. Molybdenum oxide was thermally evaporated overtop the spin coated polymer solar cell to realize the inverted design. The devices were finalized by thermally evaporated aluminum contacts which were then mechanically reinforced with silver paste. Current voltage characteristics were performed both in dark and under illumination to characterize the inverted solar cells and to verify the inverted solar cell design. Impedance spectroscopy in dark and under illumination were used to gain more information about the photoelectric processes in the devices and to build a realistic equivalent circuit model of the inverted solar cells. The inverted solar cells were then compared against standard polymer bulk heterojunction solar cells produced with the same active materials.   

Convergent Sequences of Thermodynamic Greens Functions

The frequency Fourier transform of thermodynamic greens functions can be represented as matrix elements of the resolvent of the Liouville operator with respect to one of two inner products. These inner products involve thermal averages of commutators or anti-commutators of various operators in which the vectors within the inner products contain factors of creation and annihilation operators. Sequences of increasingly-sized truncated resolvent matrices are guaranteed to converge to exact results. Each of the approximations is non-perturbative in that the eigen-energies of each truncation arise from secular determinants. The thermodynamic equations for detailed balance provide a means of determining an iterative sequence of thermal expectation values, whose limit points determine the thermal equilibrium values. The anti-commutator inner product space includes odd numbered factors of Fermi operators while the commutator inner product space includes spins, Bosons, and even numbers of factors of Fermions. Of particular note is the fact that the commutator space contains vectors whose norm squared can be either positive or negative. Several examples of this formulation will be illustrated.   

CMB Anisotropy and the role they play in probing cosmological parameters: WMAP satellite

We study the perturbation of Einstein's field equations and radiation angular power spectrum of the Cosmic Microwave Background (CMB) anisotropy to understand the temperature fluctuations in the early universe. Using Weinberg's approach, we plot the scalar multiple coefficient C$^{S}_{\, \ell }$ / 2$\Omega $ in square microKelvin for different cosmological parameters H$_{o\, }$, $\Omega_{b}$h$^{2}$, $\Omega_{c}$h$^{2}$, etc. (using WMAP {\&} `LAMBDA' data). The effect of the changes in various cosmological parameters on the multi-pole coefficients in the radiation angular power spectrum of the Cosmic Microwave Background (CMB) anisotropy is related; consistent with the evolution of universe.   

Ground-State of the Dimerized 1\emph{D} Heisenberg Model with Next Nearest Neighbor Interaction

A well-known variant of the one-dimensional antiferromagnetic spin $1/2$ Heisenberg model includes explicit dimerization and was first studied by Cross and Fisher many years ago. The Hamiltonian is given by H=J_{1}\sum_{l=1}^{2N-1}\left( 1-\left( -1\right) ^{l}\delta\right) \vec{S}_{l}\cdot\vec{S}_{l+1}+J_{2}\sum_{l=1}^{2N-2}\vec{S}_{l}\cdot\vec {S}_{l+2}% where $J_{1}$ is the nearest neighbor interaction (here we take $J_{1}=1$), $\delta$ ($0\leq\delta\leq1$) is the dimerization and $J_{2}$ ($0\leq J_{2}\leq2$) is the next-nearest neighbor interaction. Here we shall apply both a Lanczos matrix truncation as well as a Connected Moments approach to study both the ground-state energy as well as the energy gap.   

Quasiparticle statistics from the ground state wave function

A topologically ordered phase is a gapped state that can be characterized by the topological entanglement entropy (TEE) $\gamma$ and by the properties of its excitations when moved around one another. The literatures contains two approaches to extract $\gamma$ from the computable ground-state entanglement entropy $S$, the Levin-Wen construction and the Kitaev-Preskill construction, in 2D. Both approaches can be modified so that they are usable to obtain the modular $\mathcal{S}$- and $\mathcal{U}$-matrices that encode the quasiparticle properties. We compare the two approaches and comment on the issue of corner contributions using the Kalmeyer-Laughlin state as an example.   

Comparison and Analysis of 3,4 dihydrocylmandelic acid (DHMA) and noremetanephrine (NMN) on Amyloid-Beta 40 Monomer for treatment of Alzheimer's Disease using Molecular Dynamics Simulation

Alzheimer's disease (AD) is type of degenerative dementia caused memory loss and behavior problem. Main reason of AD is Amyloid-Beta 40(A$\beta )$ mostly composed of $\alpha $ -helix form misfolds to insoluble fibrils and soluble oilgomer. This insoluble fibrils aggregate with beta sheet structure and form the plaque which is caused nurotoxicity in brain. Both 3,4 dihydrocylmandelic acid (DHMA) and noremetanephrine (NMN) are the metabolite of norepinephrine in brain . Also these are inhibit the changing formation of fibrils and maintain the $\alpha $ --helix structure. In this computational modeling study, both NMN and DHMA molecules were modified and analyzed for specific effect on the A$\beta $-monomer using molecular dynamics simulation. Using molecular dynamic simulation, NMN and DHMA act as modulator on three A$\beta $-monomer batches and could observe the conformational changing of these A$\beta $-monomer under the physiologocal condition. This computational experiment is designed to compare and analyze both of chemicals for determining which chamecal would be more effective on the conformation of A$\beta $ 40 monomer.   

Hydrogen detection by a boron sheet: A theoretical study .

A single boron sheet is now considered as a new nanomaterial with promising applications in electronics and as a sensor device. In this study we present quantum-classical molecular dynamic (QCMD) calculation of reflection, adsorption, and transmission processes of hydrogen impacting at energy in range 0.25 to 100 eV on a single boron sheet. Quantum-mechanical component of our QCMD approach is self-consistent charge tight binding density functional theory method (SCC-DFTB, [1]). We consider the corrugated boron sheet as our target, created experimentally [2], and compare our results with those reported for graphene [3], showing noticeable differences. [1] A. Mannix et al. Science 350, 1513 (2015). [2] M. Elstner et al. Phys. Rev. B 58, 7260 (1998) [3] R. C. Ehemann et al. Nanoscale research letter 7, 198 (2012)   

Accelerated Discovery of High-Refractive-Index Polymers Using First-Principles Modeling, Virtual High-Throughput Screening, and Data Mining

Organic materials with refractive index (RI) values higher than 1.7 have attracted considerable interest in recent years due to the tremendous potential for their application in optical, optometric, and optoelectronic devices, and thus for shaping technological innovation in numerous related areas. Our work is concerned with creating predictive models for the optical properties of organic polymers, which will guide our experimentalist partners and allow them to target the most promising candidates. The RI model is developed based on a synergistic combination of first-principles electronic structure theory and machine learning techniques. The RI values predicted for common polymers using this model are in very good agreement with the experimental values. We also benchmark different DFT approximations along with various basis sets for their predictive performance in this model. We demonstrate that this combination of first-principles and data modeling is both successful and highly economical in determining the RI values of a wide range of organic polymers. To accelerate the development process, we cast this modeling approach into the high-throughput screening, materials informatics, and rational design framework that is developed in the group. This framework is a powerful tool and has shown to be highly promising for rapidly identifying polymer candidates with exceptional RI values as well as discovering design rules for advanced materials.   

Predictive Modeling for Strongly Correlated f-electron Systems: A first-principles and database driven machine learning approach

Data driven computational tools are being developed for theoretical understanding of electronic properties in $f$-electron based materials, e.g., Lanthanides and Actnides compounds. Here we show our preliminary work on Ce compounds. Due to a complex interplay among the hybridization of $f$-electrons to non-interacting conduction band, spin-orbit coupling, and strong coulomb repulsion of $f$-electrons, no model or first-principles based theory can fully explain all the structural and functional phases of $f$-electron systems. Motivated by the large need in predictive modeling of actinide compounds, we adopted a data-driven approach. We found negative correlation between the hybridization and atomic volume. Mutual information between these two features were also investigated. In order to extend our search space with more features and predictability of new compounds, we are currently developing electronic structure database. Our f-electron database will be potentially aided by machine learning (ML) algorithm to extract complex electronic, magnetic and structural properties in $f$-electron system, and thus, will open up new pathways for predictive capabilities and design principles of complex materials.   

Evaluating DFT for Transition Metals and Binaries: Developing the V/DM-17 Test Set

We have developed the V-DM/17 test set to evaluate the experimental accuracy of DFT calculations of transition metals. When simulation and experiment disagree, the disconnect in length-scales and temperatures makes determining ``who is right'' difficult. However, methods to evaluate the experimental accuracy of functionals in the context of \textit{solid-state materials science}, especially for transition metals, is lacking. As DFT undergoes a shift from a descriptive to a predictive tool, these issues of verification are becoming increasingly important. With undertakings like the Materials Project leading the way in high-throughput predictions and discoveries, the development of a one-size-fits-most approach to verification is critical. Our test set evaluates 26 transition metal elements and 80 transition metal alloys across three physical observables: lattice constants, elastic coefficients, and formation energy of alloys. Whether or not the formation energy can be reproduced measures whether the relevant physics are captured in a calculation. This is especially important question in transition metals, where active d-electrons can thwart commonly used techniques. In testing the V/DM-17 test set, we offer new views into the performance of existing functionals.   

The benchmarking of density functional theory functionals with quantum Monte Carlo for an accurate description of thermoelectric materials

A quickly progressing industrial and technological environment has necessitated the ability to create materials with tailored properties. Exploring new materials computationally has the potential to avoid the high cost of experimental synthesis. Density functional theory (DFT), a mean-field approach, is a prominent method for calculating accurate properties and energetics of solids. However, the choice of functional introduces uncontrolled approximations and often neglects dispersion effects. To describe materials with an accuracy necessary for consistent and reliable prediction, all relevant interactions must be captured. Quantum Monte Carlo (QMC), a statistical approach to solving the Sch\"odinger equation exactly, can be utilized to address these shortcomings of DFT. In our work, QMC is utilized to benchmark DFT functionals, to obtain the most accurate results of transport properties in periodic solids viable for thermoelectric application. Here, we present results related to the study of tin selenide with QMC.   

Visualizing spatial correlation: structural and electronic orders in iron-based superconductors on atomic scale

Crystalline matter on the nanoscale level often exhibits strongly inhomogeneous structural and electronic orders, which have a profound effect on macroscopic properties. This may be caused by subtle interplay between chemical disorder, strain, magnetic, and structural order parameters. We present a novel approach based on combination of high resolution scanning tunneling microscopy/spectroscopy (STM/S) and deep data style analysis for automatic separation, extraction, and correlation of structural and electronic behavior which might lead us to uncovering the underlying sources of inhomogeneity in in iron-based family of superconductors (FeSe, BaFe$_{\mathrm{2}}$As$_{\mathrm{2}})$. We identify STS spectral features using physically robust Bayesian linear unmixing, and show their direct relevance to the fundamental physical properties of the system, including electronic states associated with individual defects and impurities. We collect structural data from individual unit cells on the crystalline lattice, and calculate both global and local indicators of spatial correlation with electronic features, demonstrating, for the first time, a direct quantifiable connection between observed structural order parameters extracted from the STM data and electronic order parameters identified within the STS data.   

Hexagonal Boron Nitride: A Promising Substrate for Graphene with High Heat Dissipation

Supported graphene on standard SiO$_{\mathrm{2}}$ substrate exhibits unsatisfactory heat dissipation performance that is far inferior to the ultrahigh thermal conductivity of suspended case. A suitable substrate for enhancing the thermal transport in supported graphene is highly desirable. By using molecular dynamics simulations, we have studied thermal conductivity of sing-layer graphene (SLG) supported on bulk hexagonal boron nitride ($h$-BN) substrate. Notable length dependence and high thermal conductivity are observed in $h$-BN supported SLG, suggesting thermal transport properties are close to that in suspended SLG. At room temperature, thermal conductivity of $h$-BN supported SLG is as high as 1347.3±20.5 W/mK, which is about 77{\%} of suspended case and more than twice of SiO$_{\mathrm{2}}$ supported SLG. Furthermore, the $h$-BN substrate gives rise to a regular and weak stress distribution in graphene, which results in less suppressed phonon relaxation time and phonon mean free path. We also find stacking and rotation have significant impacts on structure dynamics and thermal conductivity of $h$-BN supported graphene. Our study provides valuable insights towards the design of realistic supported graphene devices with high performance heat dissipation.   

Kinetics and atomic mechanisms of rapid semiconductor-to-metal transitions in monolayer TMDCs

Rapid and controllable transitions between semiconducting (H) and metallic (T') phases of monolayer transition-metal dichalcogenides are of interest for 2D electronics. However, theoretical studies have been limited to calculations of thermodynamic stability of H and T' phases, while experimental investigations have uncovered only slow thermally-activated transitions that occur over $10^3-10^4$ seconds. Here, we demonstrate, through a combination of DFT and non-adiabatic QMD, softening of phonon modes located at the Brillouin zone boundary, thus exposing a hitherto unknown low barrier pathway for the H-T' phase transformation. We compare the fast kinetics of this new mechanism to previous strategies for improving the H-T' phase transition by quantifying phase transition activation barriers in strained, charge- and donor-doped monolayers using NEB. We discuss implications of this pathway in enabling fast phase transitions through irradiation for use in 2D electronics and non-volatile memories.   

Disorder effects on the charge separation pathway by intermixing of donor and acceptor molecules

The organic photovoltaics (OPVs) have recently attracted much interest as alternative energy sources. Though the power convergence efficiencies (PCEs) of OPVs have reached more than 11{\%}, the PCEs are lower than those of Si solar cells. To improve the PCEs, we need to reveal a fine mechanism of charge separations between the electrons and holes at the interface of electron donating and accepting materials. The objective of this study is revealing the mechanism of the charge separation in the OPVs by investigation of a typical pair of donor and acceptor, i.e., P3HT and PCBM, respectively. To this end, we investigated the energy profile from exciton states to charge separated states in two types of atomistic interface. Namely, we prepared bilayer and intermix interfaces of P3HT and PCBM, which were made by conducting molecular dynamics simulations with 48 P3HTs and 144 PCBMs. Then, we calculated electronically excited states in those interfaces by applying semi-empirical quantum calculations and then revealed charge separation pathway reaching 4 nm of electron-hole distance. Finally, we discuss critical effects of charge delocalization on the charge separation pathway.   

Ab-initio modelling of solvent effects in pentacene-derived systems in the context of singlet fission

Singlet fission (SF) is a multiexciton generation process that could be harnessed to improve the efficiency of photovoltaic devices. Experimentally, systems derived from the pentacene molecule have been shown to exhibit ultrafast SF with high yields. It has become apparent that the excited states and thereby SF is strongly affected by the molecular environment. This means that modelling approaches that restrict attention to only a few molecules for tractability are severely limited. In order to overcome this limitation we harness excited-state linear-scaling DFT (as implemented in the ONETEP code), combined with empirical MD to sample configurations of the environment. Specifically, we use constrained DFT and time-dependent DFT to model the excited electronic states relevant for SF. We investigate how these states are affected by the explicitly modelled molecular environment and the implications for fission.   

Modeling Photochemical Dynamics in Optically Active Energetic Materials

Most high explosives (HEs) absorb in the UV range, making it difficult to develop HEs that can be excited with standard lasers. The conventional optical initiation mechanisms require high laser intensity and occur via indirect thermal or shock processes. A photochemical initiation mechanism could allow control over the chemistry contributing to decomposition leading to initiation. We combine UV femtosecond transient absorption (TA) spectroscopy and excited state femtosecond stimulated Raman spectroscopy (FSRS) with Nonadiabatic Excited State Molecular Dynamics (NA-ESMD) to model the photochemical pathways in nitromethane (NM), a low sensitivity HE known to undergo UV photolysis. We investigate the ultrafast photodecomposition of NM from the n$\pi^*$ state excited at 266 nm. The FSRS photoproduct spectrum points to methyl nitrite formation as the dominant photoproduct. A total photolysis quantum yield of 0.27 and an n$\pi^*$ state lifetime of 20 fs were predicted from simulations. Predicted time scales reveal that NO$_2$ dissociation occurs in 81$\pm$4 fs and methyl nitrite formation is much slower at 452$\pm$9 fs corresponding to the absorption feature in the TA spectrum. The relative time scales are consistent with isomerization by NO$_2$ dissociation and ONO rebinding.   

Quantum Molecular Dynamics Study on Sufidation Process of Molybdenum Oxide

Molybdenum disulfide (MoS$_{\mathrm{2}})$ monolayer, a direct bandgap semiconductor, is a promising candidate for future electronics applications due to its unique mechanical and electronic properties, for which fundamental understanding of growth processes is indispensable. \textit{In situ} transmission electron microscopy (TEM) study has shown that MoS$_{\mathrm{2}}$ nanocrystals are formed from a submonolayer molybdenum oxide dispersed on an oxide support by sulfidation in an H$_{\mathrm{2}}$S/H$_{\mathrm{2}}$ atmosphere. Time-resolved TEM images revealed that single-layer MoS$_{\mathrm{2}}$ nanocrystals form preferentially and that multi-layer nanocrystals form later in the sulfidation process. Here, we use quantum molecular dynamics simulation to investigate the sulfidation process of molybdenum oxide monolayer in H$_{\mathrm{2}}$S/H$_{\mathrm{2}}$ atmosphere. Simulation results identify key reaction pathways and intermediate products for MoS$_{\mathrm{2}}$ formation. We also quantify the interplay between H$_{\mathrm{2}}$ and those intermediate products. These atomistic mechanisms not only explain experimental results but also shed light on controlled growth of MoS$_{\mathrm{2}}$ monolayers.   

Explosives initiation by compression of gas inclusions.

This paper presents a study of explosives initiation by impact. The ignition of solid explosives with gas inclusion has been studied at the compression gas. Adiabatic compression sensitivity test machine is similar to the Bureau of Explosives compression apparatus. During the test, the gas in contact with the explosive is rapidly compressed using a known drop weight system. Small samples of the test explosive are placed in a piston-cylinder apparatus and a drop weight is used to initiate rapid compression gas. Drop heights are varied to change the ignition conditions. The instantaneous compression in the chamber is measured by a pressure transducer. Since the pressure rise in the test chamber is rapid, the compression will be nearly adiabatic and rapid temperature rise will result. A mathematical model of impact, the gas compression and ignition are also discussed in this paper.   

Electron Transfer Mechanism in Proteins at Different Temperatures

Electron transfer probability in proteins is calculated as a function of temperature. The rate of single-step electron transfer reaction is mediated by through-bridge tunneling. The energy of electron is expressed as a function of temperature of electrons. Tunneling pathways and their interferences in proteins depend on the temperature and help to determine the structure of proteins. Deep tunneling effects are also discussed. Vibrational quantum effects as well as the inelastic tunneling and hopping of electrons in protein medium is also studied.   

Ab initio study of Cu doped KCl

Experimental studies have shown that doping alkali halides with copper atoms result in an enhancement of their optical properties. In this work, we calculate the electronic properties of KCl doped with Cu through the Density Functional Theory (DFT) scheme. The Wien2k was used, which is based on the Full-Potential Augmented Plane Waves with local orbital method (APW-lo). Structural optimization of the 3x3x3 supercell was performed using the GGA PBE96 approximation to exchange-correlation. For band gap and electronic properties, calculations were performed using the modified Becke-Johnson potential (mBJ). Band structure, density of states and optical properties are examined and compared with the properties of pure KCl.   

Achieving Chemical Accuracy in Li-rich Layered Oxide Materials via Quantum Monte Carlo Method

Density Functional Theory (DFT) is the most popular and versatile method for atomic scale modeling and design of new cathode materials such as Li-rich layered transition metal (TM) oxides . However, all current DFT functionals fail to accurately model the s-to-d or p-to-d orbital charge transfer energies present in TM atoms. Although DFT can be corrected by empirical parameters, such as Hubbard-U, their transferability on different systems can be questionable. Quantum Monte Carlo (QMC) is the method that treats electrons explicitly to solve the many-body Schrodinger equation exactly. Especially for formation energies of solids, QMC is the only method that can achieve near chemical accuracy while being applicable to systems with 100s of electrons. We discuss our recent application of QMC methods to LiCoO$_2$ and LiNiO$_2$ to understand the bonding mechanisms during charging and discharging. We show that QMC is able to accurately predict the operating voltages and energies of the localized d-states of transition-metal atoms without any empirical parameters. We highlight the possible use of QMC in designing Li-rich layer oxide alloys for future generation cathode materials, and to serve as a critical benchmark for calibrating DFT methods for the accurate materials design.   

Colossal magnetoresistance effect in double exchange models with Potts and Ising variables

Two-orbital model with Jahn-Teller phonons for colossal magnetoresistance is investigated at an electronic density of $n=0.75$ where: 1) Mn spins are treated classically with a 4-state Potts model with up/down/right/left directions, and 2) Ising spins with only up/down directions. These results are compared to those of earlier studies where a new state was found that had both FM and CE-like characteristics, which made use of classical Heisenberg spins.[1] With both the 4-state Potts and Ising spins, we see large CMR peaks, similar to that observed in [1]. We conclude that while the existence of this new state may explain nanoscale phase separation tendencies in CMR manganites, it may not be directly responsible for the large resistivity peak, confirming earlier results.[2] \\ \\ 1. C. \c{S}., Shuha Liang, and Elbio Dagotto, Phys. Rev. B {\bf 85}, 174418, 2012.\\ 2. C. \c{S}., Gonzalo Alvarez, and Elbio Dagotto, Phys. Rev. Lett. {\bf 105}, 097293, 2010.\\   

Study of the Valley Hall effect in hydrogen-doped MoS2 by DFT simulation

Molybdenum disulfide (MoS2) is one of the most interesting 2D honeycomb structure transition-metal dichalcogenide (TMDC). MoS2 has intrinsic valley physics and show the valley Hall effect induced by circular-polarized light. In this work, we study hydrogen-doped MoS2 single layer to understand various changes due to hydrogen doping. By controlling the hydrogen position in MoS2 layer and change hydrogen concentration by increase the super cell size, we investigated the valley Hall conductance, band structure, spin density and PDOS.   

The adsorption of CH$_3$ and C$_6$H$_6$ on corundum-type sesquioxides: The role of van der Waals interactions

Van der Waals (vdW) interactions play an important role in the adsorption of atoms and molecules on the surface of solids. This role becomes more significant whenever the interaction between the adsorbate and surface is physisorption. Thanks to recent developments in density functional theory (DFT), we are now able to employ different vdW methods that helps us to account for the long-range vdW forces. However, the choice of the most efficient vdW functional for different materials is still an open question. In our study, we examine different vdW approaches to compute bulk and molecular adsorption properties of M$_2$O$_3$ oxides (M: Cr, Fe, and Al) as well-known examples of the corundum family. For the bulk properties, we compare our results for the heat of formation, cohesive energy, lattice parameters and bond distances as obtained using the different vdW functionals and available experimental data. Next we compute the adsorption energies of the benzene molecule (as an example of physisorption) and CH$_3$ (as an example of chemisorption) on top of the (0001) M-terminated and MO-terminated surfaces. Calculating the vdW contributions into the adsorption energies, we find that the vdW functionals play important role not just in the weak adsorptions but even in strong adsorption.   

Transport Properties Of Van Der Waals Hybrid Heterostructures.

Here we study transport properties of van der Waals heterostructures composed of carbon nanotubes adsorbed on nanoribbons of distinct 2D materials. Calculations of the electronic density of states and conductance of the hybrid systems are obtained in single band tight-binding approximation in the Green function formalism by adopting real--space renormalization schemes. We show that an analytical approach may be derived when both systems are formed by the same type of atoms. In the coupled structures the different electronic paths along the ribbons and finite nanotubes lead to quantum interference effects which are reflected as Fano antiresonances in the conductance. The electronic and transport properties of these materials are modulated by changing geometrical and structural parameters, such as the nanotube diameter and the widths and edge type of the ribbons.   

Extended Antiferromagnetic Models for Ionic-Covalent Bonding in Crystals

Successful quasiparticle theory has been developed for the correlation effects related to antiferromagnetic (AF) phase such as those in the high-temperature superconductors. In addition, it is well-established how to construct the corresponding two-electron correlated states by considering the half-filled 4-orbital model [1], under which the covalent wavefunction represents the AF state. In this poster, we constructed quasielectrons for chemical bonding by including the ionic part to generalize the AF-type quasielectrons. The Bloch quasi-electron orbitals can be obtained after imposing the periodic condition based on crystal symmetry. [1] Phys. Stat. Sol. (b) 242, No. 2, p.p. 317-321 (2005).   

Magnetic properties and exchange interactions in transition metal oxides: Benchmarking the ACBN0 functional

We study the magnetic properties and magnetic exchange interactions in the 3d transition metal (TM= Mn, Co, Fe and Ni) oxides using the recently developed ACBN0 functional which is a parameter-free extension of traditional DFT+U functional where the Hubbard U is calculated selfconsistently and depends on the electron density of the system.ACBN0 greatly improves the electronic properties of the TMOs by improving the band-gap in MnO and NiO and making CoO and FeO insulating which is otherwise described incorrectly within DFT (LDA/GGA) functionals. The magnetic properties (magnetic moments, magnetic ordering energies, exchange coupling constants (J’s)) are all better described by ACBN0 at par with the Hybrid functionals and in closer agreement with the experimental values. For MnO and NiO, we investigated the magnetic properties at equilibrium and under pressure and found a good agreement with other advanced functionals. For all the oxides studied here, we did a thorough and e xtensive study by comparing different pseudopotentials and find overall that ACBN0-LDA is better for describing magnetic properties compared to ACBN0-PBE. We also discuss the application of ACBN0 to two mixed-valent systems Mn$_3$O$_4$ and Co$_3$O$_4$, where it is possible to evaluate U for different sites.   

Coarse-Grained Force Field Development of Novel Bolaamphiphilic Molecules: VOTCA or Martini?

We have performed a series of atomistic molecular dynamics (MD) simulations of novel bolaamphiphilic molecules, VECAR, in water in recent years. At low molar density, the VECAR molecules aggregate in water to form small-size micelles which have a potential application as a drug-carrier. With the atomistic MD simulation results, many data analyses were carried out to understand the structure of the self-assemblies and the dynamic process of the aggregation in the atomistic level. However, to be able to study the interactions of the micelle with other micelles or a lipid bilayer membrane in a biophysical environment, we need to use coarse-grained (CG) molecular dynamics simulations. We have developed CG force fields of the novel molecule using both VOTCA and Martini methods. The summary of our findings and the comparisons of using the structure-based method (VOTCA) and free-energy-based method (Martini) will be presented.   

Effect of lattice relaxation on thermal conductivity prediction via molecular dynamics simulations: study on fcc-based structures

This work studies the computational details of molecular dynamics (MD) thermal conductivity prediction with Green-Kubo method. Little consensus has been made among researchers about the choice of lattice parameter in MD thermal conductivity calculation, leading to mutually disagreeing reports. Simulations on fcc-based structures with different lattice parameters were performed to calculate lattice thermal conductivities, heat current autocorrelation functions, and phonon density of states. The results were compared to experimental reports and ab initio calculations to conclude that lattice volume relaxation in isobaric-isothermal (NpT) ensemble is crucial for accurate prediction of thermal conductivity. In addition, effect of domain size and interatomic potential cutoff distance was also studied in the context of lattice relaxation, and it was verified that conventional choice of cutoff distance may result in underestimation of thermal conductivity. After analyzing the size and cutoff dependence of lattice parameter, a new criterion for cutoff distance was suggested. Simulations were performed with the newly developed simulation parameters, and showed improved agreement with experimental and ab initio results.   

Cluster-based many-body calculations of flux quantization and superfluid density

The energy and free energy of a superconductor is periodic as a function of threaded magnetic flux when the superconductor is constructed with an annular geometry. The curvature of the energy curves with respect to the minima is related to the superfluid density of the superconductor. We present calculations for the energy curves and superfluid density that are based on self-consistent many-body perturbation theory applied to both single- and multi-band Hubbard models. We demonstrate how to adapt our scheme when a cluster approximation is used to limit the computational cost of obtaining the electron self-energy.   

Casimir Effect in Highly Dense System

Casimir force is related to the dielectric constant and other properties of a medium. The effects of high density and the corresponding chemical potential on the Casimir force are studied in detail. Casimir force for extremely dense systems such as super dense stellar cores or the highly dense portions of complex structures of polymer or proteins is investigated. It is expected that the Casimir force can provide additional information in understanding the interaction between molecules of complex structures such as proteins or extremely dense stellar cores at very high temperatures. The modified electric permittivity in dense systems is used to describe the Casimir force correctly.   

Thermalization in a closed many-body quantum system

A new high-performance algorithm was recently proposed for calculating level density in interacting many-body systems. It was applied to spin- and parity-dependent shell-model nuclear level densities using methods of statistical spectroscopy. Using this algorithm we analyze the intrinsic thermalization effect in isolated systems of interacting particles. We show examples of the approach and discuss the dependence of the level density on the interaction parameters.   

Semi-Extrapolated Finite Difference Schemes for Partial Differential Equations

When solving partial differential equations, finite difference (FD) methods are a popular choice. A variety of factors come into play when choosing a FD method, such as stability and cost of computation. Explicit methods are inexpensive to use but they have small stability ranges. Implicit methods have large stability ranges, however, they are expensive to use. To reduce the cost of implicit methods, extrapolation is often applied, yet this results in an explicit scheme that usually has a greatly reduced stability range. In a response to the small stability ranges of explicit methods, we developed a discretization technique that uniquely combines implicit schemes with extrapolation. The resulting novel explicit schemes maintain accuracy and, when compared to analogous explicit schemes, exhibit an improved stability range. In our presentation, we will review the stability ranges of several popular FD schemes. We will then discuss our novel technique and how it can be used to solve the heat and advection equations. Upon applying our technique to these equations, we will analyze the resulting stability ranges and demonstrate a non-trivial improvement in stability compared to the ranges of analogous explicit methods.   

Electromagnetic field computation at fractal dimensions

According to Mandelbrot’s work on fractals, many objects are in fractional dimensions that the traditional calculus or differential equations are not sufficient. Thus fractional models solving the relevant differential equations are critical to understand the physical dynamics of such objects. In this work, we develop computational electromagnetics or Maxwell equations in fractional dimensions. For a given degree of imperfection, impurity, roughness, anisotropy or inhomogeneity, we consider the complicated object can be formulated into a fractional dimensional continuous object characterized by an effective fractional dimension D, which can be calculated from a self-developed algorithm. With this non-integer value of D, we develop the computational methods to design and analyze the EM scattering problems involving rough surfaces or irregularities in an efficient framework. The fractional electromagnetic based model can be extended to other key differential equations such as Schrodinger or Dirac equations, which will be useful for design of novel 2D materials stacked up in complicated device configuration for applications in electronics and photonics.   

Why the enrollment in Physics Programs is decreasing

There are several reasons for the decrease in enrollment in Physics programs which includes but not limited to the (1) lack of mathematical skill, (2) part time education, (3) financial burdens, (4) students liking for teachers is given more importance over the educational standards, (5) lack of team spirit and political environment of academia. All of these factors are compared with the international education standards to find out the reasons why students from certain regions and outside US are not only more hardworking but are better prepared to accept challenges of relatively more technical subjects such as Physics and they are less distracted as well.   

Modeling and computational simulation and the potential of virtual and augmented reality associated to the teaching of nanoscience and nanotechnology

With the advent of new information and communication technologies (ICTs), the communicative interaction changes the way of being and acting of people, at the same time that changes the way of work activities related to education. In this range of possibilities provided by the advancement of computational resources include virtual reality (VR) and augmented reality (AR), are highlighted as new forms of information visualization in computer applications. While the RV allows user interaction with a virtual environment totally computer generated; in RA the virtual images are inserted in real environment, but both create new opportunities to support teaching and learning in formal and informal contexts. Such technologies are able to express representations of reality or of the imagination, as systems in nanoscale and low dimensionality, being imperative to explore, in the most diverse areas of knowledge, the potential offered by ICT and emerging technologies. In this sense, this work presents computer applications of virtual and augmented reality developed with the use of modeling and simulation in computational approaches to topics related to nanoscience and nanotechnology, and articulated with innovative pedagogical practices.   

Princeton University Materials Academy for underrepresented students

Summer 2016 gave underrepresented high school students from Trenton New Jersey the opportunity to learn materials science, sustainability and the physics and chemistry of energy storage from Princeton University professors. New efforts to place this curriculum online so that teachers across the United States can teach materials science as a tool to teach ``real'' interdisciplinary science and meet the new Next Generation Science Standards (NGSS). The Princeton University Materials Academy (PUMA) is an education outreach program for underrepresented high school students. It is part of the Princeton Center for Complex Materials (PCCM), a National Science Foundation (NSF) funded Materials Research Engineering and Science Center (MRSEC). PUMA has been serving the community of Trenton New Jersey which is only eight miles from the Princeton University campus. We reached over 250 students from 2003-2016 with many students repeating for multiple years. 100{\%} of our PUMA students have graduated high school and 98{\%} have gone on for college. This is compared with overall Trenton district graduation rate of 48{\%} and a free and reduced lunch of 83{\%}. We discuss initiatives to share the curriculum online to enhance the reach of PCCM' PUMA and to help teachers use materials science to meet NGSS and give their students opportunities to learn interdisciplinary science.   

Communicating Scientific Research to Non-Specialists

Public outreach to effectively communicate current scientific advances is an essential component of the scientific process. The challenge in making this information accessible is forming a clear, accurate, and concise version of the information from a variety of different sources, so that the information is understandable and compelling to non-specialists in the general public. We are preparing a magazine article about planetary system formation. This article will include background information about star formation and different theories and observations of planet formation to provide context. We will then discuss the latest research and theories describing how planetary systems may be forming in different areas of the universe. We demonstrate here the original professional-level scientific work alongside our public-level explanations and original graphics to demonstrate our editorial process.   

Does interactive instruction in introductory physics impact long-term outcomes for students?

Early college classroom experiences contribute greatly to students leaving STEM majors. Peer instruction is a research-based pedagogy in which students, in small groups in the classroom, discuss concepts and work short problems. A single study at Harvard found that taking peer-instruction introductory physics also increases persistence in science majors. To what degree, if at all, peer instruction helps retention and performance for STEM majors at large public institutions (like University of Texas, Austin) is not known. Here I describe the results of a retrospective pilot study comparing outcomes for students who took different sections of the same calculus-based introductory mechanics course in Fall 2012 and Fall 2014. Compared with traditional lecture sections, peer-instruction sections had a 50{\%} lower drop rate, a 40{\%} / 55{\%} higher rate of enrollment in the 2$^{nd\, }$/ 3$^{rd}$ courses in the sequence, and, for the Fall 2012 cohort, a 74{\%} / 165{\%} higher rate of graduating from UT Austin / the UT Austin College of Natural Sciences by Fall 2015. I will discuss weaknesses of this retrospective pilot study and present plans for an intentionally-designed study to be implemented beginning Fall 2017.   

Computation in Physics: Resources and Support

We will describe exciting new resources and support opportunities that have been developed by ``PICUP'' to help faculty to integrate computation into their physics courses. (``PICUP'' is the ``Partnership for Integration of Computation into Undergraduate Physics''). These resources include editable curricular materials that can be downloaded from the PICUP Collection of the ComPADRE Digital Library: www.compadre.org/PICUP. Support opportunities include week-long workshops during the summer and single-day workshops at national AAPT and APS meetings.   

Analyzing Neutral Responses to the Maryland Physics Expectation Survey

We examine the post neutral responses of students on the Maryland Physics Expectation Survey (MPEX) in introductory physics courses for physics majors at Ithaca College. Analyzing responses to the MPEX are usually done by looking at the favorable and unfavorable responses to the survey. We decided to look at neutral responses because of the range of values we received from the survey scores which fell between a range of 15\% and 25\%. This range of values was observed in other studies done using the MPEX. Our analysis of the responses to the MPEX was conducted with different statistical approaches. We analyzed student responses to the MPEX based on each item, or each instructors student responses, or by analyzing what the items on the survey were addressing.   

Patterns of Incorrect Responses on the FCI and Course Success

The Force Concept Inventory (FCI) is often used to measure the effectiveness of instructional pedagogy in introductory physics courses both at the algebra- and calculus-based level. Scores on the FCI are correlated with the performance of students in a class, as measured by their final course grade. We have collected data from several semesters of first-semester introductory mechanics courses at a public 4-year university, taught in large-scale classrooms with pedagogy including elements of Just-in-Time Teaching pedagogy along with active learning course components. The data collected includes pre- and post-test FCI scores, midterm exam grades, and final course grades. We examine whether certain patterns of incorrect answers on the FCI post-test are predictive of course grades, indicating whether certain specific student preconceptions are more detrimental than others to the success of students in an introductory mechanics course.   

A Study of Physics Faculty's Instructional Practices: Implications for Experiential STEM Faculty Development Model.

When teaching physics, many factors determine the final impact the course will have on a student. Using STEP, a teacher content professional development program, we are studying the incorporation of inquiry-based teaching strategies in the professional development of university professors through an active engagement program. Through the professors' involvement in the program, they gain experience with inquiry-based instruction that can be put into effect in their own classrooms to possibly create a shift in understanding and success ratesat physics undergraduate courses. This model consists of faculty peer mentoring, facilitating instruction within a community of practice, and implementation of undergraduate inquiry-based physics teaching strategies. Here, professors are facilitating the physics lessons to in-service high school teachers while using inquiry strategies and interactive activities rather than traditional lecture. This project aided the creation of an undergraduate inquiry-based physics course at the University of Houston. It could lead to a new form of professor professional development workshop that does not only benefit the professor, but also highschoolteachers not properly trained in the field of physics.   

Beyond Lecture and Non-Lecture Classrooms: LA-student interactions in Active Learning Classrooms

Our expanded multi-site study on active learning classrooms supported by Learning Assistants (LAs) aims to understand the connections between three classroom elements: the activity, student learning, and how LAs support the learning process in the classroom. At FIU, LAs are used in a variety of active learning settings, from large auditorium settings to studio classroom with movable tables. Our study uses the COPUS observation protocol as a way to characterize LAs behaviors in these classrooms. With a focus on LA-student interactions, our analysis of how LAs interact with students during a 'learning session' generated new observational codes for specific new categories of LA roles. Preliminary results show that LAs spend more time interacting with students in some classes, regardless of the classroom setting, while in other classrooms, LA-student interactions are mostly brief. We discuss how LA-student interactions contribute to the dynamics and mechanism of the socially shared learning activity.   

Relative vs. Absolute Space and Time

Einstein says that there is no absolute space or absolute time. But we argue that we can mathematically consider an absolute space and absolute time, in order to eliminate all paradoxes and anomalies from Theory of Relativity. Relative Space and Time are referring to Subjective Theory of Relativities, while Absolute Space and Time are referring to Objective Theory of Relativity. The observers are relative, subjective indeed, but mathematically there can be considered an Absolute Observer. \textbraceleft There are things which are absolute.\textbraceright   

Andrzej Trautman, Ivor Robinson, and the foundations of gravitational radiation theory

It is especially pertinent following the momentous detection of gravitational waves by LIGO and the death of Ivor Robinson in 2016 that we investigate the central role played by the Polish physicist Andrzej Trautman and his dear collaborator Robinson in helping to establish the foundations of gravitational wave research. Trautman was a student of Leopold Infeld who had famously rejected the reality of gravitational waves. Yet Trautman's intuition, informed in part by his training as a radio engineer, led him to be the first to correctly pose asymptotic boundary conditions that described the mass loss of an isolated system through emitted gravitational radiation. His series of papers announcing these results were published in a then obscure Polish journal. Fortunately, though, Felix Pirani visited Warsaw in 1957 and he was so impressed with Trautman that he arranged for him to visit his group at King's College in London. Trautman's lectures in London won him wide admiration, and significantly affected the subsequent work on gravitational wave solutions of Einstein's equations in the group led by Hermann Bondi. This was also the occasion in which Trautman and Robinson discovered a deep and abiding mathematical affinity, resulting in the discovery of exact solutions of Einstein's equations that could be interpreted as representing gravitational radiation. This talk is based in part on an interview with Trautman conducted in Warsaw in June, 2016.   

Updates on the African Synchrotron Light Source (AfLS) Project.

Africa is the only habitable continent without a synchrotron light source. A full steering committee was elected at the African Light Source (AfLS) conference on November 16-20, 2015 at the European Synchrotron Radiation Facility (ESRF) in Grenoble, France. The conference brought together African scientists, policy makers, and stakeholders to discuss a synchrotron light source in Africa. Firm outcomes of the Conference were a set of resolutions and a roadmap. Additionally, a collaborative proposal to promote Advanced Light Sources and crystallographic sciences in targeted regions of the world was submitted by the International Union of Pure and Applied Physics (IUPAP) and the International Union of Crystallography (IUCr) to the International Council for Science (ICSU). \underline {www.africanlightsource.org}.   

Segmentation of 9Cr Steel Samples based on Composition and Mechanical Property.

Data mining approaches were used to look at composition-process-property linkage in 9Cr steel. We present results of cluster identification using 7 principal composition elements and analyze its significance with respect to mechanical tensile properties. Data set comprises 82 compositional variants of 9Cr steel whose Cr weight fraction ranges 8-13{\%}. The alloys underwent heat treatments (homogenization, normalization, and 1 to 3 tempering cycles) and were tested for tensile and creep properties at room temperature and elevated temperatures (427/800 oC median/max). In this study, alloys were partitioned into groups, and their mechanical properties were analyzed for significant differences across groups. Normalized weight fractions were used to delineate groups of alloys. Partitioning Around Medoids (PAM) clustering was used, with dissimilarities instead of distance metrics. Dataset of 21 chemical components, with Fe being the majority component, followed by Cr and C. Major contributors of composition to PAM clustering were obtained from PCA scores. Mean ultimate tensile strength of segmented groups of alloys was analyzed with ANOVA {\&} Tukey HSD tests to identify the final 3 groups based on composition and mechanical property.   

Structural Properties of High Speed Electrodeposited Ni-Co Alloy Film on Titanium.

A new and innovative high-speed process for direct electrodeposition of Ni-Co alloy on titanium surfaces without any pretreatment or displacement reaction has recently been reported [1]. Investigations of the non-columnar growth mechanism(s) that result in high-speed adhesive coating formation are presented. Our results indicate that deposition of nanocrystalline nickel throughout the entire film growth process plays a critical role. When present, local nanowire formation is interpreted in terms of super-saturated conditions. Titanium is a metal that finds use in a wide variety of applications as a structural material in aircrafts, engines, missiles, bicycles and load-bearing bone prostheses. Conventional pretreatment methods to remove a thin tenacious oxide layer and then cap the surface with a sacrificial layer are dangerous, time-consuming and environmentally unfriendly. Extensions of the new high speed method to additional thin film systems are considered. [1] Hussain, MS. Direct Ni-Co alloy plating of titanium alloy surfaces by high speed electrodeposition. Trans Inst of Metal Finishing 90 (2012) 15. doi: 10.1179/174591911X13188464808876   

Reconstructing transient structural states from laser driven atomic diffusion in a metallic multilayer

Recent laser pump, x-ray probe measurements on metallic multilayer systems have demonstrated that it is possible to drive solid state diffusion in metallic systems with as little as one optical excitation pulse. However, reconstructing the spatially and temporally dependent concentration profile is non-trivial given the complex and dynamically changing forces that drive the atomic motion in these systems. In this work we present x-ray diffraction simulations to reconstruct the experimentally measured x-ray diffraction patterns of laser excited metallic multilayers. The resulting numerical fitting procedure retrieves the transient atomic concentration profile and lattice strain within the laser driven metallic multilayer.   

A first-principles study of the avalanche pressure of alpha zirconium

We investigate the stability of a monovacancy in alpha zirconium under various strains and pressures by examining the vacancy formation energy through first-principles calculations. There is maximum formation energy of 2.35 eV under uniaxial strain corresponding to a c/a ratio of 1.75. Under volumetric strain, the formation energy increases as the strain increases. The formation energy as a function of the volumetric stress or pressure was also examined, with a minimum value of 2.00 eV at zero pressure. Using the equations of state method, we find that the formation volume of the vacancies decreases as the pressure increases, with a value of 0.6 unit-atom-volume at zero pressure. The formation enthalpy increases monotonically as the pressure increases. We predict that the avalanche pressure of alpha zirconium is -15 GPa, where vacancy formation is exothermic, causing avalanche swelling and the failure of the material.   

Orbital-free ab initio molecular dynamics study of the dynamics of the free liquid surface of In.

We report results of an orbital-free ab initio molecular dynamics (OF-AIMD) study of the free liquid surface of In at 550 K. A key ingredient in the OF-AIMD method is the local pseudopotential describing the ions-valence electrons interaction. We have used the previously developed force-matching method [1] to derive a local ionic pseudopotential suitable to account for a rapidly varying density system, such as a free liquid surface. We obtain good results for structural properties, such as the reflectivity. Moreover, we have been able to study ab initio the evolution in some dynamical properties as we move from the central region where the system behaves like the bulk liquid, to the free liquid surface and compare them to experimental results [2]. [1] B.G. del Rio and L.E. Gonzalez, J. Phys.: Condens. Matter 26, 465102 (2014) [2] B. Wehinger, M. Krisch, and H. Reichert, New Journal of Physics 13, 023021 (2011)   

Ellipsometry Study on Intrinsic Optical Properties of Epitaxial Aluminum

Aluminum has attracted attention as a promising material for plasmonic applications. The interest in aluminum plasmonics is due to the higher plasma frequency than that of noble metals such as gold or silver, allowing for surface plasmon resonance to appear in the ultraviolet (UV) region. In order to optimize the performance of Al plasmonic nanostructure, it's important to reduce ohmic and scattering losses of surface plasmons by minimizing the structural imperfections. Here, we incorporated spectroscopic ellipsometry (SE) to report the intrinsic optical properties of epitaxial Al thin film on Si developed to minimize such losses. Accurate dielectric function of single-crystalline Al has been missing because of the limitation of optical method for chemically synthesized, single- crystalline Al nanoparticles. In this work, our epitaxial Al film allowed us to measure the intrinsic dielectric constants of it by SE and such information would be significantly useful for theoretical and experimental study of single-crystal aluminum plasmonic applications.   

Theoretical prediction of the electronic transport properties of the Al-Cu alloys based on the first-principle calculation and Boltzmann transport equation

Metal alloys, especially Al-based, are commonly-used materials for various industrial applications. In this paper, the Al-Cu alloys with varying the Al-Cu ratio were investigated based on the first-principle calculation using density functional theory. And the electronic transport properties of the Al-Cu alloys were carried out using Boltzmann transport theory. From the results, the transport properties decrease with Cu-containing ratio at the temperature from moderate to high, but with non-linearity. It is inferred by various scattering effects from the calculation results with relaxation time approximation. For the Al-Cu alloy system, where it is hard to find the reliable experimental data for various alloys, it supports understanding and expectation for the thermal electrical properties from the theoretical prediction.   

Floquet spectrum and driven conductance in Dirac materials: Effects of Landau-Zener-Stuckelberg-Majorana interferometry

Using the Landau-Zener-Stückelberg-Majorana-type (LZSM) semiclassical approach, we study both graphene and a thin film of a Weyl semimetal subjected to a strong ac electromagnetic field. The spectrum of quasienergies in the Weyl semimetal turns out to be similar to that of a graphene sheet. It has been predicted qualitatively that the transport properties of strongly irradiated graphene oscillate as a function of the radiation intensity [S. V. Syzranov et al., Phys. Rev. B 88, 241112 (2013)]. Here we obtain rigorous quantitative results for a driven linear conductance of graphene and a thin film of a Weyl semimetal. The exact quantitative structure of oscillations exhibits two contributions. The first one is a manifestation of the Ramsauer-Townsend effect, while the second contribution is a consequence of the LZSM interference defining the spectrum of quasienergies.   

Development of a Quantum Optical Setup for Single Photon Experiments

Following the work of E. Galvez (Colgate University), we constructed a quantum optical setup to control and detect single photons generated via Type-I spontaneous parametric down conversion using a barium-borate crystal. The photons were detected in coincidence using a Field Programmable Gate Array. The data acquisition and user interfaces to manipulate the photon counts were programmed in LabVIEW. We aligned a beam-splitter into our optical setup to measure the degree of second-order coherence of the Ga-N laser, a quantity used to investigate the existence of the photon. We aligned a Mach-Zhender interferometer into our optical setup to measure single photon interference and to perform the quantum eraser experiment.   

Spectral hole burning and its application in microwave photonics

In microwave photonics, electron spin ensembles are candidates for use as quantum memories with potentially long storage times. Here, we demonstrate the creation of long-lived collective dark states by spectral hole burning in the microwave regime. The coherence time in our hybrid quantum system (nitrogen–vacancy centres strongly coupled to a superconducting microwave cavity) becomes longer than both the ensemble’s free-induction decay and the bare cavity dissipation rate. The hybrid quantum system thus performs better than its individual subcomponents. We demonstrate the creation of multiple pairs of dark states, which opens the way for long-lived quantum multimode memories and solid-state microwave frequency combs.   

A SiGe Quadrature Pulse Modulator for Superconducting Qubit State Manipulation

Manipulation of the quantum states of microwave superconducting qubits typically requires the generation of coherent modulated microwave pulses. While many off-the-shelf instruments are capable of generating such pulses, a more integrated approach is likely required if fault-tolerant quantum computing architectures are to be implemented. In this work, we present progress towards a pulse generator specifically designed to drive superconducing qubits. The device is implemented in a commercial silicon process and has been designed with energy-efficiency and scalability in mind. Pulse generation is carried out using a unique approach in which modulation is applied directly to the in-phase and quadrature components of a carrier signal in the 1-10 GHz frequency range through a unique digital-analog conversion process designed specifically for this application. The prototype pulse generator can be digitally programmed and supports sequencing of pulses with independent amplitude and phase waveforms. These amplitude and phase waveforms can be digitally programmed through a serial programming interface. Detailed performance of the pulse generator at room temperature and 4 K will be presented.   

Quantum simulation using superconducting qubits coupled to a 1D transmission line

We consider a system of superconducting qubits spaced along a 1D transmission line in the regime of strong qubit-photon coupling. To each qubit we route wires for both frequency bias and dynamical control. Due to the strong, 1D interaction between qubits and photons, the Hilbert space relevant to system dynamics expands exponentially in both the qubit and photon number. This system enables the exploration of quantum models, including correlated decay processes such as super- and sub-radiance, and quantum scattering.   

A Relativistic Symmetrical Interpretation of Quantum Mechanics

This poster describes a relativistic symmetrical interpretation (RSI) which postulates: quantum mechanics is intrinsically time-symmetric, with no arrow of time; the fundamental objects of quantum mechanics are transitions; a transition is fully described by a complex transition amplitude density with specified initial and final boundary conditions, and; transition amplitude densities never collapse. This RSI is compared to the Copenhagen Interpretation (CI) for the analysis of Einstein's bubble experiment using both the Dirac and Klein-Gordon equations. The RSI has no zitterbewegung in the particle’s rest frame, resolves some inconsistencies of the CI, and gives intuitive explanations of some previously mysterious quantum effects.   

Coherence is not Catalytic

Aberg has claimed in a recent Letter," Phys. Rev. Lett. 113, 150402 (2014)", that the coherence of a reservoir can be used repeatedly to perform coherent operations without ever diminishing in power to do so. The claim has particular relevance for quantum thermodynamics because, as shown in "Phys. Rev. Lett. 113, 150402 (2014)", latent energy that is locked by coherence may be extractable without incurring any additional cost. We show here ( arXiv:1603.00003 [quant-ph]) , however, that repeated use of the reservoir gives an overall coherent operation of diminished accuracy and is necessarily accompanied by an increased thermodynamic cost.   

Heat engine by exorcism of Maxwell Demon using spin angular momentum reservoir

Landauer's erasure principle is a hallmark in thermodynamics and information theory. According to this principle, erasing one bit of information incurs a minimum energy cost. Recently, Vaccaro and Barnett (VB) have explored the role of multiple conserved quantities in memory erasure. They further illustrated that for the energy degenerate spin reservoirs, the cost of erasure can be solely in terms of spin angular momentum and no energy. Motivated by the VB erasure, in this work we propose a novel optical heat engine that operates under a single thermal reservoir and a spin angular momentum reservoir. The novel heat engine exploits ultrafast processes of phonon absorption to convert thermal phonon energy to coherent light. The entropy generated in this process then corresponds to a mixture of spin up and spin down populations of energy degenerate electronic ground states which acts as demon's memory. This information is then erased using a polarised spin reservoir that acts as an entropy sink. The proposed heat engines goes beyond the traditional Carnot engine.   

Dispersive coupling between a surface-acoustic-wave resonator and a superconducting qubit.

Hybrid quantum systems involving superconducting qubits are widely investigated in quantum information science. Surface-acoustic-wave (SAW) devices can be another low-loss physical system and a candidate for the component. The strain caused by a SAW can couple to varieties of physical systems, such as microwaves, optical electromagnetic fields and NV centers through piezo and other elastic effects. Here we propose a dispersively-coupled quantum system consisting of a high-Q SAW resonator (resonant frequency: 300 MHz) and a superconducting transmon qubit (3.5 GHz) on a quartz substrate. The qubit is also coupled to a microwave coplanar waveguide resonator (5.5 GHz) for the measurement. The coupling strength $\chi $ between the SAW resonator and the qubit is expected to be around 10 kHz, which is larger than the decay rate of the SAW resonator, about 300 Hz, already achieved in our experiment at a dilution-fridge temperature.   

Probing the permittivity from two-level system defects which are manipulated in population and swept in energy using a superconducting resonator

Two-level systems (TLSs) in a dielectric have a deleterious effect on the coherent states of superconducting resonators and qubits. By application of a microwave field outside the bandwidth of a microwave cavity mode we can invert these TLSs with the simultaneous application of time-dependent electric field bias. This latter field also changes the TLS energies, towards or away from resonance with the resonator before they decay. This changes the permittivity of the dielectric. By controlling the bias rate, we can manipulate the number of excited TLSs and their energy distribution. This changes the cavity resonance frequency by few MHz and also the loss tangent of the mode resonance. Numerical modelling explains the experimental data.   

Quantum entropy source on an InP photonic integrated circuit for random number generation

Random number generators are essential to ensuring performance in information technologies, including cryptography, stochastic simulations, and massive data processing. In this talk, we will describe a quantum entropy source for random number generation on an indium phosphide photonic integrated circuit. The proposed chip integrates all the optical elements, including lasers and detectors, and is based on a novel phase-diffusion configuration that uses two-laser interference and heterodyne detection. The resulting device offers high-speed operation and reduced form factor. In addition, its compatibility with complementary metal-oxide semiconductor technology opens the path to its integration in computation and communication electronic devices.   

Multiparameter estimation with single photons

It was suggested in [Phys. Rev. Lett. 111, 070403] that optical networks with relatively simple preparation and measurement devices -- single photon Fock states and on-off detectors -- can show significant improvements over classical strategies for multiparameter estimation when the number of modes in the network is small.~ This was further developed in [arXiv:1610.07128] for the case of single parameter estimation, and shown to be sub-shotnoise only for n\textless 7. ~In this paper, we show that this simple strategy can give asymptotically post-classical sensitivity for multiparameter estimation even when the number of modes is large.~ Additionally, we consider the effects of several other measurement techniques that can increase the efficiency of this device.   

A fundamental limit in the capability of Gaussian systems in quantum metrology

For a fixed average energy, the simultaneous estimation of multiple phases provides a better total precision than estimating them individually. We show that for a multimode passive interferometer with a phase in each mode and Gaussian inputs, this improvement is no more than a factor of 2. This suggests a fundamental limitation in the performance of Gaussian states. While such limitations are well known in quantum computation and communication, ours is the first such instance in the field of quantum metrology. While our proof of this limitation assumes equal squeezing magnitudes and an orthogonal transformation, that this factor-of-two is indeed a fundamental property of Gaussian states is supported by numerics on completely general systems. Since this limitation does not exist for a single-phase estimation problem, our work shows the richness of quantum-limited multiparameter estimation. The strength of our work lies in its generality. It considers an arbitrary number of parameters, and applies to quantum-limited imaging and possibly future gravitational wave detection. It also makes no assumption of stationarity in time, a common feature in waveform estimation. It can thus be applied to emerging areas such as pulsed optomechanics.   

Quantum parameter estimation and coherent control with a time-dependent Hamiltonian

We present our theoretical results on quantum metrology of a general parameter when the Hamiltonian is time-dependent. We obtain the optimal solution to the quantum Fisher information, and show that coherent control can give an advantage in maximizing it. We derive the optimal Hamiltonian control required, and with a minimal example of a spin-1/2 particle in a rotating magnetic field, we find that the fundamental limit of $T^2$ time scaling for the quantum Fisher information of time-independent Hamiltonian can be exceeded when the Hamiltonian is time-dependent, which reaches $T^4$ time scaling in estimating the rotation frequency of the field. This may be understood intuitively as the acquired quantum phase accelerating in time. Reference: arXiv:1606.02166   

Sensitivity, quantum limits, and quantum enhancement of noise spectroscopies

We study the fundamental limits of noise spectroscopy using estimation theory, Faraday rotation probing of an atomic spin system, and squeezed light. We find a simple and general expression for the Fisher information, which quantifies the sensitivity to spectral parameters such as resonance frequency and linewidth. For optically-detected spin noise spectroscopy, we find that shot noise imposes ``local'' standard quantum limits for any given probe power and atom number, and also ``global'' standard quantum limits when probe power and atom number are taken as free parameters. We confirm these estimation theory results using non-destructive Faraday rotation probing of hot Rb vapor, observing the predicted optima and finding good quantitative agreement with a first-principles calculation of the spin noise spectra. Finally, we show sensitivity beyond the atom- and photon-number-optimized global standard quantum limit using squeezed light.   

Studying the intermediate regime between flux qubit and fluxonium

A flux qubit is a micron-size superconducting loop intersected by several Josephson junctions. When the number of junctions over the loop is small (3 - 4), the system has a rather limited coherence time but exhibits large quantum current fluctuations, a property that opens perspectives for coupling microscopic magnetic systems. When the number of junctions over the loop is increased to a large number (typ. 50), the qubit - known in this configuration as fluxonium - can become immune to both charge and quasiparticle noise, and exhibit long coherence times. However, its quantum current fluctuations are typically much smaller. There is therefore a trade-off to be found. In this work, we develop a numerical method using photonic basis representation which makes possible quantitative predictions and fast parameter space scanning for flux qubits with more than four junctions. This enables us to study the full Hamiltonian of the system including kinetic inductance and geometric capacitance terms. We show that both typically reduce the gap of the qubit by over 1GHz. Finally, we study the intermediate regime between the flux qubit and the fluxonium namely the sensitivity to charge and quasiparticle noise as the number of junctions increases.   

New Circuit QED system based on Triple-leg Stripline Resonator.

Conventional circuit QED system consists of a qubit located inside a linear stripline resonator, which has successfully demonstrated a strong coupling between a single photon and a qubit. Here we present a new circuit QED system, where the qubit is coupled to triple-leg stripline resonator (TSR). We have shown that TSR supports two-fold degenerate photon modes among others. By coupling them to a single qubit, we have obtained the dressed states of a coupled system of a single qubit and two-fold degenerate photon modes. By locating two qubits at two legs of TSR, we have studied a potential two-bit gate operation (e.g., CNOT gate) of the system. We will discuss the main advantage of utilizing two-fold degenerate photon modes   

Ferromagnetic Josephson junctions with niobium nitride

Recently, novel physics and device applications in hybrid structures of superconductor (SC) and ferromagnet (FM), e.g., spin injection into SC, long-range Josephson effect, cryogenic memory, have been studied actively. Among various interesting phenomena in SC/FM structures, a “$\pi $ state ($\pi $ junction)” emerged in ferromagnetic Josephson junctions (SC/FM/SC) is attractive as a superconducting phase shifter for superconducting devices. In the present work, we developed the ferromagnetic Josephson junction in order to realize a “quiet” superconducting flux qubit with a $\pi $ junction. Contrary to conventional flux qubits, the qubit with a $\pi $ junction can be operated without an external magnetic field which is a noise source, and thus good coherence characteristics is expected. As a superconducting material, we adopted niobium nitride (NbN) with high superconducting critical temperature of \textasciitilde 16 K, which can be grown epitaxially on a magnesium oxide substrate. Regarding the ferromagnetic material we used copper nickel (CuNi), and fabricated the NbN/CuNi/NbN junctions and then evaluated the dependences of the Josephson critical current on the temperature, thickness and so on.   

Characterization of coplanar waveguide resonators made of nitride superconductors

Superconducting coplanar waveguide (CPW) resonator is a key component of superconducting electromagnetic field detectors and superconducting qubits based on circuit quantum electrodynamics (QED), where a high quality factor is desirable for applications. We have previously reported superconducting transmon qubits based on fullyepitaxial NbN/AlN/NbN tunnel junctions grown on a MgO substrate. However, the internal quality factor of the superconducting CPW resonator made of a (100) NbN film were at most several thousands, suggesting the existence of a loss mechanism coming from the MgO substrate or the interfacial two-level-systems (TLS). To clarify the origin of the loss mechanisms in superconducting CPW resonators, we systematically investigated the dependences on substrate materials, deposition conditions of nitride superconductors, and surface treatment conditions prior to the deposition. CPW resonators made of NbN or TiN deposited on a hydrogenterminated silicon substrate without any surface treatment showed a high internal quality factor above one million at the microwave power of a single photon level. Our results support that loss in superconducting resonators is dominated by TLS at the interface between the superconductor and the substrate.   

Implementation of all-microwave entanglement schemes in 3D transmon two qubit system

We implemented all-microwave two qubit entanglement scheme via Stark shift-induced controlled phase gate, as suggested by J. Chow et al., [1]. Our system consists of two superconducting transmon qubits, one of which is a tunable-frequency qubit and the other is a fixed-frequency qubit, embedded in a three dimensional copper cavity. As we align higher quantum states outside the computational states, i.e., \textbar 12\textgreater and \textbar 03\textgreater , we could achieve controlled phase gate by applying a microwave tone which induces the Stark shift. The gate time can be controlled depending on how close we align the levels. We will present our results on the estimation of the fidelity of generated Bell states with tomographic reconstruction of the two-qubit states as a function of the gate time. [1] J. Chow et al., New J. Phys. 15, 115012 (2013).   

Input-Output Theory for Two Qubits in a 1D Waveguide: Photon Correlations

We study the input power dependence of the photon-photon correlations, $g_2(t)$, in waveguide QED. Input-output theory is used (in the Markovian approximation) in order to go beyond wavefunction methods that are typically limited to a few photons. The system consists of two qubits (2LS) strongly coupled to propagating photons in a one-dimensional waveguide. The input is a coherent state of light with no correlations. After interacting with two qubits, transmitted and reflected photons show bunching and antibunching. We quantify the power by the mean number of photons per spontaneous decay time, $\bar{n}$. As a function of $\bar{n}$, $g_2(t=0)$ starts at $1$, peaks for $\bar{n}<1$, and then returns to $1$ at large power. Oscillations in $g_2(t)$ grow as $\bar{n}$ increases, as the input drives Rabi oscillations from which input photons then scatter. Surprisingly, when the two qubits are colocated, the reflected photons are always antibunched, even under classical driving.    

Effective medium theory for the design of superconducting nanowire single-photon detectors

Superconducting nanowire detectors are an important resource in quantum information science because of their high detection efficiency, low dark count rates, and high saturated detection rates.  To obtain high efficiency, an optical cavity is placed around the nanowire to increase the absorption.  Recent designs show that high efficiencies can also be obtained by placing the nanowire on top of a ``half cavity,'' consisting of a dielectric spacer and a metallic mirror or directly on top of a dielectric stack of alternating high and low refractive index layers.  Optimizing the design of these structure often requires numerical simulations of the optical structure, which can be quite time consuming especially for TM-polarized light.  Here, we use an effective medium model which is appropriate for thin nanowires typically used in these detectors.  Such an approach can greatly facilitate optimizing the design of detector optical structures.  We compare the results of the effective medium model to other effective approaches and numerical simulations of the full problem.   

Non-Markovian Dynamics of a Qubit Due to Photon Scattering in a Waveguide

We study the dynamics of a few photons in a 1D waveguide scattered off a qubit. We present a simple and elegant approach leading to exact solutions of the space-time evolution. A mirror terminating the waveguide drastically changes the behavior of the system by creating a feedback loop. We show that in this case the two-excitation wavefunction is described by a delayed partial differential equation, thereby extending the well-known result in the one-excitation sector. We contrast the results with and without the mirror, and address the non-Markovian properties induced by the feedback loop.   

Electrical and mechanical tuning of a silicon vacancy defect in SiC for quantum information technology

We develop coherent control via Stark effect over the optical transition energies of silicon monovacancy deep center in hexagonal silicon carbide. We show that this defect’s unique asymmetry properties of its piezoelectric tensor and Kramer’s degenerate high-spin ground/excited state configurations can be used to create new possibilities in quantum information technology ranging from photonic networks to quantum key distribution. We also give examples of its qubit implementations via precise electric field control.   

Topological entanglement negativity in Chern-Simons theories

We study the topological entanglement negativity between two spatial regions in (2+1)-dimensional Chern-Simons gauge theories by using the replica trick and the surgery method. For a bipartitioned or tripartitioned spatial manifold, we show how the topological entanglement negativity depends on the presence of quasiparticles and the choice of ground states. In particular, for two adjacent non-contractible regions on a tripartitioned torus, the entanglement negativity provides a simple way to distinguish Abelian and non-Abelian theories. Our method applies to a Chern-Simons gauge theory defined on an arbitrary oriented (2+1)-dimensional spacetime manifold.   

Relative entropy of conformal interfaces, boundary states, and the applications to Chern-Simons theories

Relative entropy is a measure to distinguish two quantum states. In this work, we study the relative entropy for a 1+1 dimensional CFT with different conformal interfaces/defects and conformal boundary conditions. By using relative entropy, we can extract the universal data of conformal interfaces/defects and conformal boundary states. In addition, we apply our methods to 2+1 dimensional Chern-Simons theories. It is found that the results are sensitive to whether the theory is Abelian or non-Abelian, and can be used to detect topological data without UV divergence.   

Entanglement manipulation via quantum walks in linear chain registers

It has been theoretically argued that continuous time quantum walks are effective in performing entangling gates in systems of two and three qubits coupled via higher energy auxiliary states. We investigate how quantum walks can perform entangling quantum gates in a scalable linear chain register. We focus on systems that have two sufficiently coherent auxiliary states available for pulse control, such as self-assembled quantum dots, diamond defect states, and transmon superconducting qubits. We show that states and Rabi frequencies in linear chain registers of such architectures can be mapped onto multidimensional hypercube graphs with the degree of asymmetry dictated by the structure of physical interactions between qubits. We analytically and numerically demonstrate that quantum walks traversing these graphs can accumulate sufficient phase and return back to boolean domain, thus manipulating entanglement.   

Entanglement Elimination Before Particle Detections of the Entangled Particles-2

An experiment has been proposed that should demonstrate entanglement elimination before photon detections are made in the case where the idler photon provides which way information to the paired signal photon and the idler photon is destroyed at the fixed ultrablack micropost located at the crossroads of the two possible idler photon paths before the signal photon reaches its detection screen. The expected result is interference in the signal photon intensity distribution without any correlation between detection events for the paired signal and idler photons. The fixed micropost does not allow for any ``record'' of a momentum transfer between the idler photon and the fixed micropost when the idler photon impacts the fixed micropost. If unexpectedly, a which way pattern in the signal photon intensity distribution is obtained instead of interference, then we would have a case where a signal photon is still affected by the paired idler photon even though the idler photon has already been destroyed and the entanglement eliminated. Given the break in logic underlying the second possibility (where the destroyed idler photon still provides which-way information to the paired signal photon), the latter result (which-way intensity distribution for the signal photons) is more unlikely than an interference intensity distribution for the signal photons. This experimental scenario can be contrasted with another where the idler photon is detected along a specific path rather than destroyed. In the latter scenario, the entanglement is maintained and the idler photon supplies which-way information to the paired signal photon. The result is a which-way intensity distribution for the signal photons.   

Surpassing the shot-noise limit by homodyne-mediated feedback.

Entangled systems with large quantum Fisher information (QFI) can be used to outperform the standard quantum limit of the separable systems in quantum metrology. However, the interaction between the system and the environments inevitably leads to decoherence and decrease of the QFI, and it is not clear whether the entanglement systems can be a better resource than separable systems in the realistic physical condition. In this work, we study the steady QFI of two driven and collectively damped qubits with homodyne-mediated feedback. We show that the steady QFI can be significantly enhanced both in the cases of symmetric feedback and nonsymmetric feedback, and the shot-noise limit of separable states can be surpassed in both cases. The QFI can even achieve the Heisenberg limit for appropriate feedback parameters and initial conditions in the case of symmetric feedback. We also show that an initial-condition-independent steady QFI can be obtained by using nonsymmetric feedback.   

Solving the quantum brachistochrone equation through differential geometry

The ability of generating a particular quantum state, or model a physical quantum device by exploring quantum state transfer, is important in many applications. Due to the environmental noise, a quantum device suffers from decoherence causing information loss. Hence, completing the state-generation task in a time-optimal way can be considered as a straightforward method to reduce decoherence. For a quantum system whose Hamiltonian has a fixed type and a finite energy bandwidth, it has been found that the time-optimal quantum evolution can be characterized by the quantum brachistochrone equation(PRL, 96, 060503 (2006)). In addition, the brachistochrone curve is found to have a geometric interpretation: it is the limit of a one-parameter family of geodesics on a sub-Riemannian model(PRL 114, 170501 (2015)). Such geodesic-brachistochrone connection provides an efficient numerical method to solve the quantum brachistochrone equation. In this work, we will demonstrate this numerical method by studying the time-optimal state-generating problem on a given quantum spin system.We also find that the Pareto weighted-sum optimization turns out to be a simple but efficient method in solving the quantum time-optimal problems.   

Lindbladians with multiple steady states: theory and applications

Lindbladians, one of the simplest extensions of Hamiltonian-based quantum mechanics, are used to describe decay and decoherence of a quantum system induced by the system's environment. While traditionally viewed as detrimental to fragile quantum properties, a tunable environment offers the ability to drive the system toward steady-state subspaces that can store, protect, and process quantum information. This poster reviews recent results about Lindbladians with multiple steady states. These results include statements about symmetries, the dependence of the infinite-time state on initial state, effects of Hamiltonian perturbations, the energy scale of leakage out of the steady-state subspace, generalizations of Berry's phase, and extensions of the quantum geometric tensor. The results will be presented using several examples of continuous-variable quantum computation using cat-codes.   

A Package of Information as the Planck Unit of Information and Also as a Fundamental Physical (Universal) Constant.

Dimension of information as the fifth dimension of the universe including packages of new information, is nested with space-time. Distributed density of information is matched on its correspondence distributed mater in space-time. Fundamental particle (string) like photon and graviton needs a package of information including its exact quantum state and law for process and travel a Planck length in a Planck time. This process is done via sub-particles (substrings). Processed information is carried by particle as the universe's history. My proposed formula for Planck unit of information ($I_{P} )$ and also for Fundamental Physical (Universal) Constant is: I_{P} =\frac{l_{P} }{ct_{P} }=1 Planck length $l_{P} $, Planck time $t_{P} $, and $c,$ is light speed. Also my proposed formula for calculation of the packages is: $I=t_{P}^{-1} .\tau $, in which, $I$ is number of packages, and $\tau $ is lifetime of the particle. ``Communication of information'' as a ``fundamental symmetry'' leads phenomena. Packages should be always up to date including new information for evolution of the Universe. But, where come from or how are created new information which Hawking and his colleagues forgot it bring inside the black hole and leave it behind the horizon in form of soft hair?   

Knowledge, as the Result of the Processed Information by Human's Sub-particles (substrings)/Mind in our Brain

In my vision, there are four animated sub-particles (mater, plant, animal and human sub-particles) as the origin of the life and creator of momentum in each fundamental particle (string). They communicate with dimension of information which is nested with space-time for getting a package of information in each Planck time. They are link-point between dimension of information and space-time. Sub-particle which identifies its fundamental particle, processes the package of information for finding its next step. Processed information carry always by fundamental particles as the history of the universe and enhance its entropy. My proposed formula for calculating number of packages is $I=t_{P}^{-1} .\tau $, Planck time $t_{P} $, and $\tau $ is fundamental particle's lifetime. For example a photon needs processes $1.8\times 10^{43}$ packages of information for finding its path in a second. Duration of each process is faster than light speed. In our bodies, human's sub-particles (substrings) communicate with dimension of information and get packages of information including standard ethics for process and finding their next step. The processed information transforms to knowledge in our mind. This knowledge is always carried by us.   

Protocol for efficient qubit initialization with a tunable environment

Fast and on-demand thermalization to milliKelvin temperatures presents a major technological challenge for superconducting quantum bits. We develop an efficient initialization protocol for a superconducting qubit by coupling it to a thermal bath through two LC resonators. The inductance of the resonator, which is coupled to the bath, is dynamically adjustable, allowing control over its natural resonance frequency. The relaxation rate of the qubit can be increased this way by several orders of magnitude by sweeping the tunable resonator into resonance with the qubit. Such setup is a part of environmental quantum state engineering by dissipation, where one aims to drive the system into a desired steady state by using a carefully tailored environment. We solve the quantum dynamics corresponding our protocol with a Markovian master equation and show that the ground-state occupation of our system is well protected during fast sweeps of the environmental coupling. Consequently, we obtain a lower bound for the duration of the protocol. Our results suggest that the current experimental state of the art for the initialization speed of superconducting qubits at a given fidelity can be considerably improved.   

Erosion in extruder flow

A detailed analysis of the fluid flow in Tadmor's unwound channel model of the single screw extruder is performed by combining numerical and analytical methods. Using the analytical solution for the longitudinal velocity field (in the limit of zero Reynolds number) allows us to devote all the computational resources solely for a detailed numerical solution of the transversal velocity field. This high resolution 3D model of the fluid flow in a single-screw extruder allows us to identify the position and extent of Moffatt eddies that impede mixing. We further consider the erosion of particles (e.g. carbon-black agglomerates) advected by the polymeric flow. We assume a particle to be made of primary fragments bound together. In the erosion process a primary fragment breaks out of a given particle. Particles are advected by the laminar flow and they disperse because of the shear stresses imparted by the fluid. The time evolution of the numbers of particles of different sizes is described by the Bateman coupled differential equations used to model radioactivity. Using the particle size distribution we compute an entropic fragmentation index which varies from 0 for a monodisperse system to 1 for an extreme poly-disperse system.   

Binary gas mixture in a high speed channel

The viscous, compressible flow in a 2D wall-bounded channel, with bottom wall moving in? the positive $x-$direction, simulated using the direct simulation Monte Carlo (DSMC) method,? has been used as a test bed for examining different aspects of flow phenomenon and separation performance of a binary gas mixture at Mach number \textit{Ma }$=$\textit{ (U\textunderscore w / }$\backslash $\textit{sqrt(}$\gamma $\textit{ k\textunderscore B T\textunderscore w /m)?) }in the range\textit{0.1 \textless Ma \textless 30}, and Knudsen number \textit{Kn }$=$\textit{ 1/(}$\backslash $\textit{sqrt(2) }$\pi $\textit{ d\textasciicircum 2 n\textunderscore d H)}in the range? \textit{.1 \textless Kn \textless 10}. The generalized? analytical model is formulated which includes the fifth order differential equation for the? boundary layer at the channel wall in terms of master potential ($\chi )$, which is derived? from the equations of motion in a 2D rectangular $(x - y)$coordinate. The starting point? of the analytical model is the Navier-Stokes, mass, momentum and energy conservation? equations in the $(x - y)$coordinate, where $x$and $y$are the streamwise? and wall-normal directions, respectively. The linearization approximation is used ((Pradhan {\&} Kumaran\textit{, J. Fluid Mech -}); (Kumaran {\&} Pradhan, \textit{J. Fluid Mech -})), where the equations of motion are truncated at linear order in the velocity and pressure perturbations to the base flow, which is anisothermal compressible Couette flow. Additional assumptions in the? analytical model include high aspect ratio \textit{(L \textgreater \textgreater H)}, constant temperature in the base state (isothermal condition), and low? Reynolds number (laminar flow). The analytical solutionsare compared with direct simulation Monte Carlo (DSMC) simulations and found good agreement (with a difference of less than 10{\%}), provided the boundary conditions are accurately incorporated in the analytical solution.   

DSMC simulations of leading edge flat-plate boundary layer flows at high Mach number

The flow over a 2D leading-edge flat plate is studied at Mach number \textit{Ma }$= (U_{inf}/ \backslash $\textit{sqrt\textbraceleft k}$_{B}T_{inf}$\textit{/ m\textbraceright ) }in the range \textit{\textless Ma \textless 10}, and at Reynolds number number \textit{Re }$= (L_{T} U_{inf}$\textit{ rho}$_{inf\thinspace }$\textit{)/ mu}$_{inf\thinspace }$ equal to 10$^{\mathrm{\thinspace \thinspace }}$using two-dimensional (2D) direct simulation Monte Carlo (DSMC) simulations to understand the flow phenomena of the leading-edge flat plate boundary layer at high Mach number. Here, $L_{T}$is the characteristic dimension, $U_{inf}$and $T_{inf}$are the free stream velocity and temperature, \textit{rho}$_{inf}$ is the free stream density, $m$is the molecular mass, \textit{mu}$_{inf\thinspace }$is the molecular viscosity based on the free stream temperature $T_{inf},$and $k_{B}$is the Boltzmann constant. The variation of streamwise velocity, temperature, number-density, and mean free path along the wall normal direction away from the plate surface is studied. The qualitative nature of the streamwise velocity at high Mach number is similar to those in the incompressible limit (parabolic profile). However, there are important differences. The amplitudes of the streamwise velocity increase as the Mach number increases and turned into a more flatter profile near the wall. There is significant velocity and temperature slip ((Pradhan and Kumaran, J. Fluid Mech-2011); (Kumaran and Pradhan, J. Fluid Mech-2014)) at the surface of the plate, and the slip increases as the Mach number is increased. It is interesting to note that for the highest Mach numbers considered here, the streamwise velocity at the wall exceeds the sound speed, and the flow is supersonic throughout the flow domain.   

Microfluidics Device Simulation in MATLAB

Microfluidics fluid channels have different dominant properties of flow than do macrofluidic channels. At small channel sizes, the calculations that model the fluid flow need to include slip velocity at the walls of the channel, the mean free path of particles, and other factors that can be difficult to compute. In order to reduce the potential for error and provide meaningful graphical representations of the computations, a computer program can be implemented. We are creating a MATLAB program suite to perform the relevant calculations quickly and accurately. Additionally, by building on this program, the potential for testing new ideas for microfluidic devices can be realized. This would reduce the costs associated with prototyping microfluidic devices as devices can be modeled in software without the need for creating physical devices until the concepts are shown to be viable.   

Air Donuts: Toroidal Bubbles Stabilized by Hydrophobin Protein Surfactant

Hydrophobins are surface-active proteins made by fungi. Whereas typical surfactants such as sodium dodecyl sulfate exhibit a great deal of molecular flexibility, hydrophobin protein surfactants behave as globular solids to create strong, thin biofilms at air-water interfaces. It has been known for a long time that hydrophobin surfactants can stabilize bubbles in unusual shapes, including rods of striking aspect ratio. Under appropriate conditions, these structures can be reformed into air-filled toroids. These ``air donuts'' are stable for hours or even days and feature high surface area.   

Surface Geometry Modification for Effective Water Spreading

Liquid spreading on solid surfaces depend on the kind of materials (liquid, solid, and vapor) and the interface geometries. Many applications involving liquid deposition by spraying and condensation require deliberate management of liquid spreading over the surfaces. In this work water spreading on solid surfaces with geometrical patterns are observed by experiments in an environmental chamber, which is controlled for temperature and humidity. Water will be deposited to the surfaces through spraying and condensation on a Peltier cooling stage. An optical microscope camera is utilized to observe static and dynamic liquid spreading behavior during the tests. Various test samples of different surface wettability and geometrical modifications are created by 3d printing technique. There are numerous studies that uses chemical coatings for wettability modification providing an effective solution. But these conventional techniques have limited long term reliability when the surface is exposed to environmental contamination which degrades the wetting behavior over time. By adding geometrical features less than a millimeter in the length scale, sustained ``highways'' are created on the surfaces, which provide an effective method for liquid drainage path with long term reliability. A successful implementation will lead to increased efficiency of applications such as dehumidifiers, heating and cooling systems in buildings and power plants.   

Granular flows in constrained geometries

Confined geometries are widespread in granular processing applications. The deformation and flow fields in such a geometry, with non-trivial boundary conditions, determine the resultant mechanical properties of the material (local porosity, density, residual stresses etc.). We present experimental studies of deformation and plastic flow of a prototypical granular medium in different nontrivial geometries--- flat-punch compression, Couette-shear flow and a rigid body sliding past a granular ‘half-space’. These geometries represent simplified scaled-down versions of common industrial configurations such as compaction and dredging. The corresponding granular flows show a rich variety of flow features, representing the entire gamut of material types, from elastic solids (beam buckling) to fluids (vortex-formation, boundary layers) and even plastically deforming metals (dead material zone, pile-up). The effect of changing particle-level properties (e.g., shape, size, density) on the observed flows is also explicitly demonstrated. Non-smooth contact dynamics particle simulations are shown to reproduce some of the observed flow features quantitatively. These results showcase some central challenges facing continuum-scale constitutive theories for dynamic granular flows.   

The generalized analytical model and DSMC simulations of high-speed rotating flow in polar $(r -- \theta ) $coordinate

The generalized analytical model for the radial boundary layer in a high-speed rotating cylinder is formulated for studying the gas flow field due to insertion of mass, momentum and energy into the rotating cylinder in the polar~ $(r - \theta )$~plane. The analytical solution includes the sixth order differential equation for the radial boundary layer at the cylindrical curved surface in terms of master potential~($\chi )$, which is derived from the equations of motion in a polar~$(r - \theta )$~plane. The linearization approximation ((Pradhan {\&} Kumaran\textit{, J. Fluid Mech-}); (Kumaran {\&} Pradhan, \textit{J. Fluid Mech-})) is used, where the equations of motion are truncated at linear order in the velocity and pressure disturbances to the base flow, which is a solid-body rotation. Additional assumptions in the analytical model include constant temperature in the base state (isothermal condition), and high Reynolds number, but there is no limitation on the stratification parameter. The analytical solutions are compared with direct simulation Monte Carlo (DSMC) simulations and found good agreement (with a difference of less than 10{\%}), provided the boundary conditions are accurately incorporated in the analytical solution. The slow down of the circumferential velocity of the bulk of the rotating fluid due to the presence of stationary intake tube is studied for stratification parameter in the range 0.707$-$3.535, and found significant slow down (between 8 to 28{\%}), which induces the secondary radial flow towards the axis, and it further excites the secondary axial flow, which could be very important for the centrifugal gas separation processes.   

3D Printed Emulsion and Janus Particle Microfluidic Devices

Microfluidic devices have the ability to manipulate volumes of fluid in the range of microliters to picoliters. Microfluidic devices have high importance in the field of bioanalysis; samples can be quickly and easily tested using complex microfluidic devices. It has been shown that inexpensive microfluidic devices can be produced quickly using a 3D printer PDMS and shrinking material. The ability to fabricate a three dimensional particle focusing device has been shown, and this will be continued by the shrinking of a device to allow a colloidal particle solution to be focused. A device allowing for the creation of an emulsion will be fabricated. This will be built upon to allow for the creation of Janus particles, or particles made of two separate materials. This research will create Janus particles with one hydrophilic side and one hydrophobic side. The creation of Janus particles has a wide variety of applications due to its ability to be amphiphilic.   

3D Printed Multi-layer Microfluidic Devices

Microfluidic devices are increasingly important to the field of bioanalysis for their ability to quickly process a sample in the microliter and picoliter scale. It has been shown that single-layered microfluidic devices can be produced quickly and inexpensively using a 3D printer, PDMS, and shrinking material. This research will expand these methods to create multi-layered microfluidic devices. This research will focus on two main obstacles when creating multi-layer microfluidic devices: layer alignment, and surface roughness. The development of multilayer microfluidic devices allows for more compact microfluidic chip design.   

Optimally Stopped Optimization

We combine the fields of heuristic optimization and optimal stopping. We propose a strategy for benchmarking randomized optimization algorithms that minimizes the expected total cost for obtaining a good solution with an optimal number of calls to the solver. To do so, rather than letting the objective function alone define a cost to be minimized, we introduce a further cost-per-call of the algorithm. We show that this problem can be formulated using optimal stopping theory. The expected cost is a flexible figure of merit for benchmarking probabilistic solvers that can be computed when the optimal solution is not known, and that avoids the biases and arbitrariness that affect other measures. The optimal stopping formulation of benchmarking directly leads to a real-time, optimal-utilization strategy for probabilistic optimizers with practical impact. We apply our formulation to benchmark the performance of a D-Wave 2X quantum annealer and the HFS solver, a specialized classical heuristic algorithm designed for low tree-width graphs. On a set of frustrated-loop instances with planted solutions defined on up to $N = 1098$ variables, the D-Wave device is between one to two orders of magnitude faster than the HFS solver.   

Quantum Entaglement and the Generalized Uncertainty Principle

We study the effects of the Generalized Uncertainty Principle on quantum entanglement by studying the modified uncertainty relation of two identical entangled particles [1] and the inseparability condition [2]. Rigolin showed a decrease (from the usual) in the lower bound in the product of the uncertainties of the position and momentum of two identical entangled particles. Duan, et.al. derived an inseparability condition for a pair of EPR-type operators for continuous variables. In both cases, the GUP correction resulted in a higher lower bound from Rigolin's result and a higher upper bound for the inseparability condition in Duan's relation. In the case of Rigolin's result, the GUP correction decreases the disagreement with the HUP while in Duan's case the inseparability and entanglement condition are enhanced. [1] G. Rigolin, Found. Phys. Lett. 15, 293 (2002), arxiv quant-ph/0008100; (2001) arxiv quant-ph/0105057 [2] L. Duan, G. Giedke, J. I. Cirac, and P. Zoller, Phys Rev Lett 84, 2722 (2000)   

A First Principles Study of the Large Anomalous Hall Effect in Noncollinear Antiferromagnets Mn$_{3}$Ge and Mn$_{3}$Sn

Anomalous Hall effect (AHE) has been thought to be present only in ferromagnetic conductors, with its size being proportional to the net magnetization. Using symmetry arguments and first principles calculations, physicists recently demonstrated that large AHE may appear in noncollinear antiferromagnets [1]. Indeed, the AHE has been recently observed in the spin liquids and antiferromagnets [2,3]. Mn$_{3}$Ge and Mn$_{3}$Sn are hexagonal chiral antiferromagnets with zero net magnetization which yet exhibit the large AHE being in the same order as in ferromagnets such as Fe [2,3]. Here we calculate the electronic and magnetic structure of Mn$_{3}$Ge and Mn$_{3}$Sn based on the density functional theory with the generalized gradient approximation. The anomalous hall conductivity of Mn$_{3}$Ge and Mn$_{3}$Sn are also calculated using efficient Wannier function interpolation. A microscopic understanding of such spin-related transports as AHE in noncollinear antiferromagnets could accelerate development of spintronics.\\ \text[1] Chen H, Niu Q and MacDonald A H \textit{Phys. Rev. Lett.} 112 017205 (2014)\\ \text[2] Nakatsuji S, Kiyohara N and Higo T \textit{Nature} 527 212–5 (2015)\\ \text[3] Kiyohara N, Tomita T and Nakatsuji S \textit{Phys. Rev. Appl.} 5 064009 (2016)   

Giant field-induced adiabatic temperature changes in Ni-Mn-In-based Heusler alloys

Direct measurements of the adiabatic temperature change ($\Delta $T$_{\mathrm{AD}})$ of Ni$_{\mathrm{50}}$Mn$_{\mathrm{35}}$In$_{\mathrm{14.5}}$B$_{\mathrm{0.5\thinspace }}$have been done using an adiabatic magnetocalorimeter in a temperature range of 250-350 K, and with magnetic field changes up to $\Delta $H$=$1.8 T. The initial susceptibility in the low magnetic field region drastically increases with temperature starting at about 300 K. Magnetocaloric effects (MCE) parameters were found to be a linear function of H$^{\mathrm{2/3\thinspace }}$in the vicinity of the second order transitions (SOT), whereas the first order transitions (FOT) do not obey the H$^{\mathrm{2/3\thinspace }}$law due to the discontinuity of the transition. The relative cooling power (RCP) based on the adiabatic temperature change for a magnetic field change of 1.8 T has been estimated. Maximum values of $\Delta $T$_{\mathrm{AD}} \quad =$ -2.6 K and 1.7 K were observed at FOT and SOT for $\Delta $H$=$1.8 T, respectively. Acknowledgement This work was supported by the Office of Basic Energy Sciences, Material Science Division of the U.S. Department of Energy, DOE Grant No. DE-FG02-06ER46291 (SIU) and DE-FG02-13ER46946 (LSU).   

Quantum Effects of Magnons Confined in Multilayered CoPd Ferromagnets

Quantum entanglement is a unique quantum mechanical effect that arises from the correlation between two or more quantum systems. The fundamental aspects of magnon entanglement has been theoretical studied [1] and the interest in developing technologies that exploits quantum entanglement is growing. We discuss the results of an experimental study of magnon entanglement in multilayered CoPd ferromagnets. Our findings are interesting and will aid in developing novel magnonic devices. [1] T. Morimae, A. Sugita, and A. Shimizu, Phys. Rev. A 71, 032317 (2005).   

MegaOhm extraordinary Hall effect in oxidized CoFeB

We report on the development of controllably oxidized CoFeB ferromagnetic films demonstrating the extraordinary Hall effect (EHE) resistivity exceeding 1 $\Omega $ cm and magnetic field sensitivity up to 10$^{6}$ $\Omega $/T. Such EHE resistivity is four orders of magnitude higher than that previously observed in ferromagnetic materials, while sensitivity is two orders larger than the best of semiconductors.   

Effect of magnetic dipolar interactions on temperature dependent magnetic hyperthermia in Fe$_{\mathrm{3}}$O$_{\mathrm{4\thinspace }}$ferrofluids

We have investigated temperature dependent magnetic hyperthermia in two ferrofluids of Fe$_{\mathrm{3}}$O$_{\mathrm{4}}$ nanoparticles prepared by co-precipitation (CP) and hydrothermal (HT) synthesis methods. Both the CP and HT prepared nanoparticles show similar physical particle size distribution ($\approx $14 nm) and saturation magnetization ($\approx $ 70 emu/g), but very different specific absorption rate (SAR) $\approx $110 W/g and $\approx $40 W/g at room temperature (measured with an ac magnetic field amplitude of 240 Oe and frequency 375 kHz). This observed reduction in SAR has been explained by taking into account the dipolar interactions and the distribution of magnetic core size of MNPs in ferrofluids. The HT ferrofluid shows a higher effective dipolar interaction and a wider distribution of magnetic core size of MNPs compared to that of CP ferrofluid. We have fitted the temperature dependent SAR data using linear response theory, incorporating an effective dipolar interaction, to determine the magnetic anisotropy constant of MNPs prepared by CP (22 $+$/- 2 kJ/m$^{\mathrm{3}})$ and HT (26 $+$/- 2 kJ/m$^{\mathrm{3}})$ synthesis methods. These values are in good agreement with the magnetic anisotropy constant determined using frequency and temperature dependent magnetic susceptibility data obtained on powder samples. The details of the study will be presented.   

Spin Polarized Transport in Multilayer Structures with Complex Magnetic Configurations

The spin transport and spin polarization in a new class of multilayer structures are investigated for non-collinear and noncoplanar magnetic configurations containing repetitive magnetic layers. The magnetic configuration of the structure dictates the existence of certain degrees of freedom that determines magnetic transport and polarization properties. We consider magnetic structures in magnetic multilayers with canted spin configurations separated by non-magnetic quantum well so that the exchange interaction between the neighbor barriers can be ignored. Configurations of magnetizations in barriers include some structures consisting of two "ferromagnetic'' or "antiferromagnetic'' domains twisted relative to each other by a certain angle (angle noncollinearity). The similar system, formed from two noncollinear domains separated by canted "magnetic defect'' is also considered. The above mentioned properties of these systems depend strongly on the type of magnetic configuration and variation of certain degrees of freedom. Simple theoretical approach with the transfer matrix method is carried out to understand and predict the magnetic properties of the multilayer systems.   

Growth and Magnetic Properties of Iron Nitride Thin Films

Iron Nitride thin films have been a significant research topic for the past few decades due to its potential applications such as magnetic sensor, magnetic hard disks, and spintronics. The biggest challenge in this area is the growth of a single phase material on a substrate. In this research project, iron nitride films were grown using reactive pulsed laser deposition on a silicon substrate. The purpose is to optimize a single phase ?-Fe3N4. The optimization was done by changing the growth parameters in the pulsed laser deposition such as the gas mixture, substrate temperature, and laser energy density. The composition, structure, and surface properties of the films were characterized by X-Ray diffraction, Raman spectroscopy, X-ray photoelectron spectroscopy, and scanning electron microscopy techniques. Temperature and field dependent magnetization has been investigated by superconducting quantum interface device (SQUID) magnetometer and ferromagnetic resonance (FMR) spectroscopy. The detailed analysis of structural and magnetization data will be discussed in this presentation.   

LaSrMnO$_{3}$ and LaMnO$_{3}$ pyramid growth technology by pulsed-laser deposition.

Thin films of La$_{0.7}$Sr$_{0.3}$MnO$_{3}$ (LSMO) and LaMnO$_{3}$ (LMO) have been actively studied for more than two decades. Our interest in these materials is focused on the layer-by-layer pyramidal growth of the thin-films on peizoelectric substrates by pulsed-laser deposition technology (PLD) for magnet-based devices. The films desired were grown in the absence of thorough substrate preparation procedures. The samples grown consisted of layers of LSMO (2 layers) and LMO (1 layer), and the area of every successive layer was twice the area of the previous layer. The deposition rate of the pyramids grown was nearly 0.07 nm per laser pulse for efficient growth. The Curie temperature of the samples is approximately 290K. These pyramidal thin films provide an opportunity to control magnetization of LSMO and the locality of the measurements.   

Polarcatrosphy and electronic reconstructions in LaAlO$_{\mathrm{3}}$/SrMnO$_{\mathrm{3}}$ (111) digital heterostructures

Based on extensive first-principle density functional theory calculations, we report different electronic phases at the LaAlO$_{\mathrm{3}}$/SrMnO$_{\mathrm{3}}$ (111) heterointerfaces. In the $n$-type LaAlO$_{\mathrm{3}}$/SrMnO$_{\mathrm{3}}$ (111) supperlattices, electrons transferred from LaAlO$_{\mathrm{3}}$ component distribute unevenly in SrMnO$_{\mathrm{3}}$ component and occupy Mn's e$_{\mathrm{g}}$ orbital, inducing half-metallic ferromagnetism in the framework of Zener double exchange. With increasing SrMnO$_{\mathrm{3}}$ layers, the sum of every Mn magmon keep a constant suggesting a fixed number of charge transferred from LaAlO$_{\mathrm{3}}$ component. For $p$-type superlattices, holes reside almost uniformly at the SrO$_{\mathrm{3}}$ and LaO$_{\mathrm{3}}$ plane drived by the polar electric field in the LaAlO$_{\mathrm{3}}$ and SrMnO$_{\mathrm{3}}$ component. With absence of the e$_{\mathrm{g}}$ states at the Mn sites, bulk-like G-type AFM ordering were obvious with almost imperceptible octahedron rotation and tilting. But $p$-type superlattices are metallic because of hole transfer. Our studies demonstrate the potential applications of perovskite heterointerfaces in spintronic devices.   

Study the growth of Pb-doped BiFeO$_{\mathrm{3}}$ and strain-relaxed SrRuO$_{\mathrm{3}}$ bottom layer on SrTiO$_{\mathrm{3}}$ (111) substrate.

Magnetoelectric multiferroics possess inherent coupling between magnetic and ferroelectric order parameters, which can be used as an indirect medium to switch magnetic moment of adjacent magnetic layer by changing the polarity of the electric field and vice-versa. To date, BiFeO$_{\mathrm{3}}$ is the only magnetoelectric multiferroic material which shows ferroelectric (T$_{\mathrm{C\thinspace }}$\textasciitilde 1103K) and antiferromagnetic ordering temperature (T$_{\mathrm{N\thinspace }}$\textasciitilde 643K) above room temperature. In our research, we are trying to find out a better growth condition for Pb-doped BiFeO$_{\mathrm{3}}$ and SrRuO$_{\mathrm{3}}$ film on atomically flat SrTiO$_{\mathrm{3}}$ (111) substrates. The polarization of Pb-doped BFO can easily be switched along out of plane direction by electric field. We observed no electric leakage at the domain walls of Pb-doped BFO which is a very distinct feature in this system.   

The realization of an artificial magnetoelectric heterostructure (FeCo/AlN) micro-beam resonator for ultra-high sensitivity magnetic sensing applications

It's becoming more and more crucial to develop high sensitivity magnetic sensors that are chip-based and cryogen-free. Recent advances in artificial multiferroics and magnetostrictive/piezoelectric materials have opened the door to novel micron-scale magnetic field tunable resonator devices [1]. Here we show how magnetostrictive FeCo can be grown in-situ on a piezoelectric AlN micro-beam with coupled heterostructural strain. The resulting magnetostrictive properties of FeCo produce a considerable resonance shift when placed in a magnetic field [2]. The piezoelectric AlN underlayer captures this signal at two regions of maximum planar strain in the first harmonic mode. Our results reveal FeCo beams with a considerable strain induced resonance shift in a DC magnetic field when driven with either a piezo-shaker, or a small AC field. Furthermore, we demonstrate how the use of a beam geometry, rather than a standard resonant cantilever, fundamentally achieves an increase sensitivity to magnetic fields. [1] E. Lage, \textit{et. Al.,} \textit{Nature Materials} \textbf{11} (2012) [2] M. Staruch, \textit{et. Al.,} \textit{Appl. Phys. Lett.} \textbf{107} (2015)   

Simultaneous imaging of strain waves and induced magnetization dynamics at the nanometer scale.

The magnetoelastic effect or inverse magnetostriction---the change of magnetic properties by elastic deformation or strain---is often a key coupling mechanism in multiferroic heterostructures and nanocomposites. It has lately attracted considerable interest as a possible approach for controlling magnetization by electric fields (instead of current) in future devices with low power consumption. However, many experiments addressing the magnetoelastic effect are performed at slow speeds, often using materials and conditions which are impractical or too expensive for device integration. Here, we have studied the effect of the dynamic strain accompanying a surface acoustic wave on magnetic nanostructures. We have simultaneously imaged the temporal evolution of both strain waves and magnetization dynamics of nanostructures at the picosecond timescale. Our experimental technique, based on X-ray microscopy, is versatile and provides a pathway to the study of strain-induced effects at the nanoscale.   

Derivation and its Application of the Generalized Spin Injection Coefficients in the Ferromagnet-Organic Semiconductor-Ferromagnet non-Local Spin Transport Structure

Be aware of the possibility of spin flipping in the interface between ferromagnet and organic semiconductor(OSC) and the special charge-spin relationship of the carriers in organic semiconductors, we theoretically investigate the improvement of spin transport coefficients and spin injection efficiency within the frame of non-local spin transport structure. Considering the redistribution for two spin bands inside the junction while the current goes through it, we derive a generalized spin injection efficiency formula and find that efficiency can be increased considerably in the presence of spin flipping in the junction region. Besides, we also numerically obtain the effects of two spin bands ratio and the polaron proportion, which is unique for OSC, on the spin injection efficiency and spin transport coefficients.   

Nanosecond spin relaxation times in single layer graphene spin valves with hexagonal boron nitride tunnel barriers.

We present an experimental study of spin transport in single layer graphene using atomic sheets of hexagonal boron nitride (h-BN) as a tunnel barrier for spin injection. While h-BN is expected to be favorable for spin injection, previous experimental studies have been unable to achieve spin relaxation times in the nanosecond regime. Here, we investigate spin relaxation in graphene spin valves with h-BN barriers and observe room temperature spin lifetimes in excess of a nanosecond, which provides experimental confirmation that h-BN is indeed a good barrier material for spin injection into graphene. By carrying out measurements with different thicknesses of h-BN, we investigate the range of h-BN thickness required to observe large MR signals and higher spin relaxation times in graphene spin valves. Our measurements suggest that monolayer h-BN may not be the optimal choice for efficient spin injection, while thicker h-BN allows the realization of larger MR signals and longer spin relaxation times in graphene spin valves.   

Quantum compass Heisenberg model on the square lattice

The quantum compass model is important for the field of strongly correlated systems with orbital degeneracy and of solid state based devices proposed for quantum computing. I study the compass model with the addition of Heisenberg interactions. I consider a model on the square lattice, with x and z axis. The nearest neighbor interactions are of two types: (a) frustrated interaction Jx and Jz, and (b) Heisenberg interaction along both axis with exchange J.The compass interactions depend on the bond direction. The model is characterized by a high level of frustration. I use a non-linear spin wave theory where four term operators are treated in a self consistent mean field approximation. I calculate all the possible ordered phases at zero temperature, either with ferromagnetic or antiferromagnetic order. I also calculate the spin structure factors and obtain the magnetization as a function of temperature for the Ising-like phases.   

Spin liquid and strip solid phases in the extended XXZ model on the Kagome lattice

Employing large-scale quantum Monte Carlo simulations, we investigate the ground state phase diagram of an extended XXZ model on kagome lattice focusing on the magnetization at $m=1/6$. As the spin exchange interactions are reduced towards the Ising limit, the system undergoes a first order phase transition from ferromagnetic ordered phase to a strip solid phase that only breaks the lattice rotational symmetry. Further reducing the transverse interaction, a $Z_2$ spin liquid phase emerges. We introduce an additional fourth-neighbor interaction that allows to tune transitions between strip solid and spin liquid. The properties of the phase transitions are investigated.   

Spin wave excitations in kagome staircase Co$_3$V$_2$O$_8$

Co$_3$V$_2$O$_8$ features spin-3/2 moments decorating a buckled kagome staircase lattice. While the ground state is a collinear ferromagnet, the $H-T$ phase diagram features a complex series of transversely polarized spin density wave states with a variable propagation vector that takes on multiple distinct commensurate and incommensurate values. The spin wave dispersion within the ($0\,K\,L)$ plane in the ferromagnetic phase has been fully mapped out using the DCS time-of-flight neutron spectrometer. While previous work has treated this compound as a quasi-two dimensional structure of weakly coupled kagome lattice planes we have measured significant spin wave dispersion along the $b$-axis, indicating a three dimensional lattice with significant coupling between the buckled kagome planes.   

Pressure effects on the physical properties of Kagome Cu$_{\mathrm{\mathbf{3}}}$\textbf{Bi(SeO}$_{\mathrm{\mathbf{3}}}$\textbf{)}$_{\mathrm{\mathbf{2}}}$\textbf{O}$_{\mathrm{\mathbf{2}}}$\textbf{Cl metamagnet}

The effects of pressure on the structural and magnetic properties have been studied in Kagome Cu$_{\mathrm{3}}$Bi(Se$_{\mathrm{1-x}}$Te$_{\mathrm{x}}$O$_{\mathrm{3}})_{\mathrm{2}}$O$_{\mathrm{2}}$Cl polycrystalline samples. The initial crystal structure $P_{mmn}$ is gradually converted to $P_{cmn}$ space group when x $\ge $ 0.6, which could be determined by synchrotron X-ray diffraction, Raman spectroscopy, and magnetization measurements. The antiferromagnetic transition temperature ($T_{N})$ and the critical field ($H_{C})$ of metamagnetic spin-flip transition increase, but the value of saturation magnetization ($M_{S})$ decreases with Te doping concentration. Under external pressure, the $T_{N}$ and $M_{S}$ increase, while the $H_{C}$ reduces. These anisotropic pressure results could be explained by the modulation of competition between ferromagnetic intralayer and antiferromagnetic interlayer interactions. The route to control the metamagnetic spin-flip transition by anisotropic pressure effects might be helpful to understand the mechanism of field- induced multiferroic Cu$_{\mathrm{3}}$Bi(SeO$_{\mathrm{3}})_{\mathrm{2}}$O$_{\mathrm{2}}$Cl   

The effects of Zn doping on magnetic properties of Cu$_{\mathrm{\mathbf{3}}}$\textbf{Bi(SeO}$_{\mathrm{\mathbf{3}}}$\textbf{)}$_{\mathrm{\mathbf{2}}}$\textbf{O}$_{\mathrm{\mathbf{2}}}$\textbf{Cl}

Recently, layered spin-frustrated Cu$_{\mathrm{3}}$Bi(SeO$_{\mathrm{3}})_{\mathrm{2}}$O$_{\mathrm{2}}$Cl has received considerable research attention due to its unusual magnetic properties. Two inequivalent Cu$^{\mathrm{2+\thinspace }}$ions form a pseudo-kagome lattice that invokes spin frustration and anisotropic magnetic properties. In this study, the influence of Zn doping on the complex magnetic properties has been explored. Polycrystalline (Cu$_{\mathrm{1-x}}$Zn$_{\mathrm{x}})$Bi(SeO$_{\mathrm{3}})_{\mathrm{2}}$O$_{\mathrm{2}}$Cl (0 x 0.5) samples were synthesized using solid-state reaction and characterized by X-ray diffraction and magnetic measurements. The Zn doping strongly modulates the magnetic ground state of the system. The antiferromagnetic transition temperature $T_{N} =$ 24 K and magnetic field-induced hysteresis observed for x $=$ 0 at low field are systematically shifted to lower temperature and reduced with Zn doping. These results can illustrate the insight of the occurrence of field-induced spin-flip type multiferroics in Cu$_{\mathrm{3}}$Bi(SeO$_{\mathrm{3}})_{\mathrm{2}}$O$_{\mathrm{2}}$Cl.   

Magnetic phase diagram slightly below the saturation field in the stacked J1-J2 model in the square lattice with the JC interlayer coupling

We study the effect of adding interlayer coupling to the square lattice, $J_1$-$J_2$ Heisenberg model in high external magnetic field. In particular, we consider a cubic lattice formed from stacked $J_1$-$J_2$ layers, with interlayer exchange coupling $J_C$. For the 2-dimensional model ($J_C=0$) it has been shown that a spin-nematic phase appears close to the saturation magnetic field for the parameter range $-0.4\leq J_2/J_1$ and $J_2>0$. We determine the phase diagram for 3-dimensional model at high magnetic field by representing spin flips out of the saturated state as bosons, considering the dilute boson limit and using the Bethe-Salpeter equation to determine the first instability of the saturated paramagnet. Close to the highly frustrated point $J_2/J_1\sim 0.5$, we find that the spin-nematic state is stable even for $|J_C/J_1|\sim 1$. For larger values of $J_2/J_1$, interlayer coupling favors a broad, phase-separated region. Further increase of $|J_C|$ stabilizes a collinear antiferromagnet, which is selected via the order-by-disorder mechanism.   

Investigation of Elastic and Magnetic Properties in Sm$_{\mathrm{\mathbf{2}}}$\textbf{Ti}$_{\mathrm{\mathbf{2}}}$\textbf{O}$_{\mathrm{\mathbf{7}}}$

Magnetically frustrated pyrochlore oxides continue to attract a great deal of attention due to the possibility of observing exotic low temperature magnetic states. Among these, Sm$_{\mathrm{2}}$Ti$_{\mathrm{2}}$O$_{\mathrm{7}}$ is a singular example as the magnetic Sm$^{\mathrm{3+}}$ ions exhibit a very small moment ($\mu =0.15\mu_{B} )$ weakly coupled via exchange interaction ($\theta_{cw} =-0.26K)$. Recent magnetization observations suggest that spin correlations begin to develop below 2K, although no long range order is observed down to 0.5 K. Therefore, we have investigated the elastic and magnetic properties of a Sm$_{\mathrm{2}}$Ti$_{\mathrm{2}}$O$_{\mathrm{7}}$ single crystal using specific heat and ultrasonic measurements down to 50 mK. The specific heat results clearly reveal a cusp at approximately 350 mK, suggesting the existence of a second order phase transition. From the ultrasonic measurements, at zero magnetic field, the principal elastic constants all show an anomaly at 350 mK, consistent with a magnetic phase transition. For a magnetic field applied along the [100] direction, the critical temperature is suppressed and a critical field is at 6 T. For a field along the [111] direction, the transition is broadened and no critical field is observed up to 12 T. Thermal and field hysteresis are observed in the variation of the elastic constants indicating a slow relaxation of the spins toward thermal equilibrium.   

$\mu $SR studies of the extended kagome systems YBaCo$_{4}$O$_{7+\delta }$ ($\delta =$ 0 and 0.1)

We present a $\mu $SR study of the extended kagome systems YBaCo$_{4}$O$_{7+\delta }$ ($\delta =$ 0 and 0.1), which are made up of an alternating stacking of triangular and kagome layers. The parent material YBaCo$_{4}$O$_{7.0}$ undergoes a structural phase transition at 310 K, releasing geometrical frustration and thereby stabilizing an antiferromagnetically ordered state below T$_{N}=$ 106 K. The $\mu $SR spectra of YBaCo$_{4}$O$_{7.0}$ exhibit the loss of initial asymmetry and the development of a fast relaxation component below T$_{N}=$ 111 K. This indicates that the Co spins in the kagome planes remain in an inhomogeneous and dynamically fluctuating state down to 4 K, while the triangular spins order antiferromagnetically below T$_{N}$. The nonstoichiometric YBaCo$_{4}$O$_{7.1}$ compound with no magnetic ordering exhibits a disparate spin dynamics between the fast cooling (10 K/min) and slow cooling (1 K/min) procedures. While the fast-cooled $\mu $SR spectra show a simple exponential decay, the slow-cooled spectra are described with a sum of a simple exponential function and a stretched exponential function. These are in agreements with the occurrence of the phase separation between interstitial oxygen-rich and poor regions in the slow-cooling measurements.   

Classical ground states of magnetic chains with twisted long range interactions in MoS$_{2}$ flakes

Magnetic impurities (MI) embedded in a metal can interact indirectly through the conduction electron host, a mechanism known as Ruderman-Kittel-Kasuya-Yosida (RKKY) interaction. Doped transition metal dichalcogenides (TMDs) can provide carriers with strong spin-orbit coupling, allowing for interesting exchange effects between MIs. A sizable Dzyaloshinskii-Moriya (DM) interaction has been shown to exist [1,2], which is long ranged when the MIs lie on/near a flake edge [3]. This opens the possibility of stable phases in MI assemblies in real TMD samples, even for 1D arrays. The combination of long range interactions and DM terms, leads to helical and strongly frustrated impurity interaction in these chains. We present results for 1D MI chains built near TMD flake edges, and study the role of long-range exchange interactions in determining the ground state configurations of the system. A Monte Carlo search of minimal energy configurations reveals interesting patterns, with characteristics that depend on impurity concentration and TMD doping levels. [1] F. Parhizgar \emph{et al.}, PRB \textbf{87}, 125401 (2013). [2] D. Mastrogiuseppe \emph{et al.}, PRB \textbf{90}, 161403(R) (2014). [3] O. \'Avalos-Ovando \emph{et al.}, PRB \textbf{93}, 161404(R) (2016); arXiv:1607.08553; 1610.02142   

Formation and visualization of individual skyrmions in confined geometries

Magnetic skyrmions are topologically stable whirlpool-like spin textures that offer great promise as information carriers for future ultra-dense memory and logic devices. Here, we report the visualization of the skyrmion chains in FeGe nanostripes and skyrmion clusters in nanodisks by high resolution Lorentz TEM, and the electrical probing of individual skyrmions in MnSi nanowires when the wire diameter is comparable to that of a skyrmion. Specifically, we found that the highly stable skyrmion chain originated from the termination of the spin helix at the edges of the nanostripes under the action of applied field, and the field-driven transition of skyrmion cluster states in nanodisks. These findings demonstrate that the geometry defects can be used to control the formation of topologically nontrivial magnetic objects. Finally, we present the electrical probing of such magnetic field-driven skyrmion cluster (SC) states in ultra-narrow single-crystal MnSi nanowires (NWs) with diameters (40 -- 60 nm), where the creation or deletion of an individual skyrmion in the cluster states leads to quantized jumps in magnetoresistance (MR), which is supported by the Monte Carlo simulations.   

Dzyaloshinskii-Moria Interaction in CoNiPt tri-layer heterostructures.

Ultrathin magnetic multilayer structures with perpendicular magnetic anisotropic (PMA) and strong Dzyaloshinskii-Moria interaction (DMI) is potentially important for non-volatile memory and logic applications. CoPt bilayer system is a well-known PMA material system with strong DMI. However, thick magnetic films with multiple CoPt layer repetitions have vanishing effective DMI due to its inversion symmetry. In this work we investigate ultrathin CoNiPt heterostructures in which Ni layer breaks the inversion symmetry. Multilayer structures with the number of CoNiPt tri-layers varying from 1 to 10 were grown by DC sputtering technique. Vibrating sample magnetometer was used to measure out-of-plane hysteresis loops indicating PMA. Magneto-optic Kerr effect microscopy was used to analyze the magnetization switching dynamics. We observed bubble-type domains for films with up to 4 CoNiPt tri-layers and labyrinth-like stripe domains for 5 and more repetitions. Domain wall motion in presence of in- and out-of-plane magnetic field pulse combination was recorded. Domain wall creep velocity model is used to extract effective DMI. DMI evolution with the number of tri-layers and as a function of individual layer thicknesses was analyzed.   

Room temperature magnetism on the zigzag edges of phosphorene nanoribbons

Phosphorene, as a promising candidate of the post-graphene, has been synthesized recently. it is a monolayer black phosphorus with a puckered honeycomb lattice structure possessing a limit band gap and high carrier mobility. In our recent article, we employ the non-perturbative numeric methods of large-scale quantum Monte-Carlo (QMC) simulations, determinant quantum Monte-Carlo and the ground state constrained path quantum Monte-Carlo simulations repectively, to investigate the edge magnetism in the bulk insulating phosphorene nanoribbons. We find that relatively weak interactions can lead to remarkable edge magnetism in the phosphorene nanoribbons. Strong ferromagnetic correlations along the zigzag edges are revealed by the ground state constrained path quantum Monte-Carlo simulations, and a high Curie temperature up to room temperature is shown by the limit temperature determinant quantum MonteCarlo calculations .   

Ising exchange interaction in lanthanides and actinides

The Ising exchange interaction is a limiting case of strong exchange anisotropy and represents a key property of many magnetic materials. Here we find the necessary and sufficient conditions to achieve Ising exchange interaction for metal sites with unquenched orbital moments [1]. Contrary to current views, the rules established here narrow much the range of lanthanide and actinide ions that can exhibit Ising exchange interaction. It is shown that the Ising interaction can be of two types: (i) coaxial, with magnetic moments directed along the anisotropy axes on the metal sites and (ii) non-coaxial, with arbitrary orientation of one of the magnetic moments. These findings will contribute to purposeful design of lanthanide- and actinide-based materials. [1] L. F. Chibotaru and N. Iwahara, New. J. Phys. 17, 103028 (2015).   

Electronic structures of (Rb,Ba)Mn[Fe(CN)$_6$] Prussian blue analogue

A$_n$M[M$^{\prime}$(CN)$_6$]$_m$$\cdot$xH$_2$O-type (A: alkali-metal ion; M, M$^{\prime}$: transition-metal ion) Prussian blue analogues (PBAs) have been studied extensively since the discovery of photo-induced spin transition in PBA.\footnote{ O. Sato et al, Science {\bf 272}, 704 (1996).} In the (Rb,Ba)Mn[Fe(CN)$_6$]-type PBAs, antiferromagnetic to ferrimagnetic-like phase transitions were observed below $\sim 5~K$ under magnetic field.\footnote{A. Thakur et al, J. Appl. Phys. {\bf 111}, 063908 (2012).} However, the mechanism of such magnetic transitions is controversial. In this work, we have investigated the electronic structures of (Rb,Ba)Mn[Fe(CN)$_6$] PBAs by employing soft X-ray absorption spectroscopy (XAS), soft X-ray magnetic circular dichroism (XMCD), and charge transfer multiplet (CTM) calculation. The measured XAS and XMCD spectra reveal different bonding characters for Fe and Mn ions, respectively. In the CTM calculation, both the metal-to-ligand charge transfer and the ligand-to-metal charge transfer are found to be necessary. We will discuss on the role of their electronic structures in their phase transitions.   

Incommensurate quantum-size oscillations in Acene-based molecular wires - effects of quantum-fluctuations

Molecular wires of the acene-family can be viewed as a physical realization of a two-rung ladder Hamiltonian. For acene-ladders, closed-shell ab-initio calculations and elementary zone-folding arguments predict incommensurate gap oscillations as a function of the number of repetitive ring units, $N_\text{R}$, exhibiting a period of about ten rings. Results employing open-shell calculations and a mean-field treatment of interactions suggest anti-ferromagnetic correlations that could potentially open a large gap and wash out the gap oscillations. Within the framework of a Hubbard model with repulsive on-site interaction, $U$, we employ a Hartree-Fock analysis and the density matrix renormalization group to investigate the interplay of gap oscillations and interactions. We confirm the persistence of incommensurate oscillations in acene-type ladder systems for a significant fraction of parameter space spanned by $U$ and $N_\text{R}$.   

Origin of minute magnetic moments in spin-orbit coupled zig-zag Phosphorene monolayer

We predict signature of induced magnetism in the electronic band structure of phosphorene monolayer in the presence of external magnetic field using \emph{ab initio} density functional theory approach with inclusion of the spin-orbit coupling. We derive a tight-binding (TB) Hamiltonian [\emph {Phys. Rev. B} $\bf{89}$(201408), 2014] with minimun energy including spin-orbit coupling for two dimensional phosphorene monolayer. Applying an extrenal magentic field in antiferromagneic order in the ranges where the structure is still stable, leads to the appearance of minute magnetic moments on phosphorous atom. Moreover, we examine the effect of spin polarization on zigzag phosphorene monolayer (PML) and the development of intrinsic moments of the corresponding active edge states when the system is simulated under external antiferromagnetic ordering. In case of PML, up to a value of 0.023 $\mu$$_{b}$ per formula unit is few orders of magnitude larger than the ferromagnetic ordering of external magnetc field. \footnotetext{$^{\dag}$Email of Corresponding Author: pdeb@tezu.ernet.in}   

Pushing the Limits of Monte Carlo Simulations for the 3d Ising Model

While no analytic solution for the 3d Ising model exists, various numerical methods like series expansion, Monte Carlo and MCRG have provided precise information about the phase transition.\footnote{For an overview of earlier work, see A. Pelissetto and E. Vicari, Phys. Rep. \textbf{368}, 549 (2002)} Using histogram techniques and quadruple precision Monte Carlo simulation that employs the Wolff cluster flipping algorithm with both 32-bit and 53-bit random number generators, we have investigated the critical behavior of the 3d Ising Model, with lattice sizes ranging from $16^3$ to $1024^3$. By analyzing data with cross-correlations\footnote{M. Weigel and W. Janke, Phys. Rev. E \textbf{81}, 06672 (2010)} between various thermodynamic quantities obtained from the same data pool, e.g. logarithmic derivatives of magnetization and derivative of magnetization cumulant,\footnote{A. M. Ferrenberg and D. P. Landau, Phys. Rev. B \textbf{44}, 5081 (1991)} we have obtained the critical inverse temperature $K_c=0.221\,654\,626(5)$ and the critical exponent of the correlation length $\nu=0.629\,73(14)$ whose precisions are comparable to those from the latest theoretical predictions.   

Magnetic, Electrical and Dielectric Properties of LaMnO$_{\mathrm{3+\eta }}$ Perovskite Manganite.

The high pure polycrystalline LaMnO$_{\mathrm{3+\eta }}$ perovskite manganite has been synthesized using conventional solid state reaction method. The studied sample crystallizes into orthorhombic O', phase indexed with Pbnm space group. The magnetization measurement exhibits that the studied sample shows paramagnetic (PM) to ferromagnetic (FM) phase transition at T$_{\mathrm{C}}=$191.6K followed with a frustration due to antiferromagnetic (AFM) kind of spin ordering at low temperature, T$_{\mathrm{f}}=$85.8K. The electrical resistivity measurements carried out at 0 tesla and 8 tesla magnetic field exhibits insulating kind of behavior throughout the measured temperature range. The resistivity at 0 tesla exhibits low temperature FM insulator to high temperature PM insulator type phase transition at T$_{\mathrm{C}}=$191.6K similarly as observed from magnetization measurement. The application of the magnetic field (8 tesla) shifts T$_{\mathrm{C}}$ to higher temperature side and the charge transport follows Shklovskii Efros variable range hopping (SE VRH) mechanism. The temperature and frequency dependent dielectric permittivity studied for the sample exhibits relaxation process explained based on Debye $+$Maxwell-Wagner relaxation mechanism.   

Magnetic entropy change associated with critical behavior in the precursor region of single crystalline FeGe

Cubic helimagnet FeGe has emerged as a class of skyrmion materials near room temperature that may impact future information technology. Experimentally identifying the detailed properties of skyrmion materials enables their practical application acceleratedly. Here we study the magnetic entropy change (MEC) of single crystalline FeGe in its precursor region and clarify its close relation to the critical exponents of a second-order phase transition in this area. The maximum MEC is found to be 2.86 J/kg.K for 7.0 T magnetic field change smaller than that of common magnetocaloric materials indicating the multiplicity and complexity of the magnetic structure phases in the precursor region. This result also implies that the competition among the multimagnetic phases can partly counteract the magnetic field driven force and establishes a stable balance. Based on the obtained MEC and the critical exponents, the exact Curie temperature of single crystalline FeGe under zero magnetic field is confirmed to be 279.1 K, higher than previously reported 278.2 K. This finding pave the way for reconstruction of FeGe phase diagram in the precursor region.   

Magnetization Reversal Dynamics in CoNi Heterostructures

Ultrathin ferromagnetic films with perpendicular magnetic anisotropy are of great potential for information processing and storage applications. Magnetization switching in these materials undergoes through a complex process that includes domain nucleation and evolution. In this work we present experimental results of magnetization reversal of (CoNi)$_{\mathrm{n}}$ multilayer films with varying Ni thickness and number of CoNi bilayer repetitions. (CoNi)$_{\mathrm{n}}$ films were grown by the magnetron sputtering technique. Vibrating sample magnetometry and magneto-optic Kerr effect magnetometry measured out-of-plane hysteresis curves with curve measurement times varying from 0.01s to 1hr. Significant dependence of the hysteresis loop shape on loop measurement times indicated slow magnetization relaxation processes taking place. Magneto-optic Kerr effect microscopy was used to study relaxation processes at constant reversing magnetic fields. Domain nucleation and their evolution into a dendritic structure was observed. Direct observation of a domain structure and its analysis revealed fast and slow magnetization reversal processes dependent on the reversing field magnitude, Ni thickness and number bi-layer repetitions. We apply several magnetization switching models to analyze experimental results.   

Effect of annealing on the magnetic and magnetocaloric properties of Ni-Mn-In-B alloys as solidified ribbons

The structural, thermal, magnetic, and magnetocaloric properties of Ni$_{\mathrm{50}}$Mn$_{\mathrm{35}}$In$_{\mathrm{14.5}}$B$_{\mathrm{0.5\thinspace }}$melt-spun ribbons have been investigated using room-temperature x-ray diffraction (XRD), differential scanning calorimetry (DSC), and magnetization measurements. Magnetic and structural transitions were found to coincide in temperature leading to large magnetocaloric effects associated with the first-order magnetostructural phase transition. In comparison to the bulk and as-spun ribbon, both the martensitic transition temperature (T$_{\mathrm{M}})$ and Curie temperature (T$_{\mathrm{C}})$ shifted to lower temperatures on annealed Ni$_{\mathrm{50}}$Mn$_{\mathrm{35}}$In$_{\mathrm{14.5}}$B$_{\mathrm{0.5\thinspace }}$ribbons. Significant increase in magnetocaloric effect has been observed between the as-spun and the annealed ribbons. A comparison of magnetic properties and magnetocaloric effects in Ni$_{\mathrm{50}}$Mn$_{\mathrm{35}}$In$_{\mathrm{14.5}}$B$_{\mathrm{0.5\thinspace }}$as-spun ribbon, bulk, and annealed ribbon have been shown in detail. Acknowledgement: This work was~supported by the Office of Basic Energy Sciences, Material Science Division of the U.S. Department of Energy, DOE Grant No. DE-FG02-06ER46291 (SIU) and DE-FG02-13ER46946 (LSU).   

Low-temperature magnetoelectric effect in multiferroic h-Yb1-xHoxMnO3

In this work, we study the low-temperature ferroelectricity, magnetic property and ME effect in Yb$_{\mathrm{1-x}}$Ho$_{\mathrm{x}}$MnO$_{\mathrm{3}}$. In YbMnO$_{\mathrm{3}}$, ferroelectric polarization (P) is closely related with the structure change derived from spin-reorientation process. The initial symmetric relationship of P between the upper and lower half of magnetic sublattice will be broken, which gives rise to the detectable polarization. Additionally, the asymmetry of the $P-T$ curves revealed the pinning effect of the defects in the material. In Ho-doped samples 2D antiferromagnetic perturbation as well as the second AFM ordering have been observed. Substitution of Yb by Ho atoms shows great influences on electric properties and the lowdoping concentration tend to be more favorable for the enhancement of P. The maximum polarization has been promoted hugely$^{\mathrm{\thinspace }}$in Yb$_{\mathrm{0.8}}$Ho$_{\mathrm{0.2}}$MnO$_{\mathrm{3}}$. We suggested the variation of P is closely related with the stronger exchange interaction in Mn-O-Ho as well as the establishment of new Ho layers with the increase of Ho.   

Relevance of Jahn-Teller effect in strongly anisotropic metal sites

Recently, heavy transition metal compounds have been intensively investigated because of their unusual magnetic properties driven by strong spin-orbit coupling. In particular, the systems with highly degenerate local electronic states, as represented by the $d^1$ double perovskites containing Mo$^{5+}$, Re$^{6+}$, Os$^{7+}$, show enhanced anisotropy in exchange interactions. Despite the four-fold (quasi) degeneracy in the $d^1$ materials, the Jahn-Teller (JT) distortion has not been observed in many of them by x-ray and neutron diffraction down to a few Kelvin. This ``violation'' of JT theorem has been often reported, whereas no explanation for it has been given so far. Here, we address the nature of the JT effect in cubic double perovskites with $ab~initio$ based methodology. We calculated the local properties such as crystal field levels, vibronic coupling, and magnetic moments, by using state-of-the-art post Hartree-Fock method. The JT stabilization for these ions is obtained weak to intermediate compared to the frequency of the local active vibration. This implies either the absence or a dynamical character of JT effect, explaining the absence of static JT distortions. The influence of the dynamical JT effect on local magnetic properties will be presented.   

Soft X-ray absorption spectroscopy and magnetic circular dichroism study of valence and spin states of half-metallic CrO$_2$ nanorods

Half-metallic ferromagnetic CrO$_2$, with the Curie temperature $T_C \sim390~K$, is very interesting because most of the transition metal oxides are antiferromagnetic insulators. It has been proposed \footnote{M. A. Korontin et al., Phys. Rev. Lett. {\bf 80}, 4305 (1998).} that the metallic ferromagnetism in CrO$_2$ originates from the oxygen-mediated double exchange interaction between mixed-valent Cr ions, caused by self-doping. But this issue is controversial.\footnote{J. H. Shim et al., Phys. Rev. Lett. {\bf 99}, 057209 (2007).} We have investigated the valence and spin states in CrO$_2$ nanorods by employing soft X-ray absorption spectroscopy (XAS) and soft X-ray magnetic circular dichroism (XMCD). The valence states of Cr ions are found to be Cr$^{3+}$-Cr$^{4+}$ mixed-valent at the surfaces, but nearly Cr$^{4+}$ in the bulk. The temperature-dependent XMCD intensity is observed, in agreement with the bulk $T_C$. We will discuss the electronic structure and its half-metallic ferromagnetism in CrO$_2$.   

The Kondo Temperature of Two-dimensional Electron Gas with Rashba Spin-orbit~Coupling

We use the Hirsch-Fye quantum Monte Carlo method to study the single magnetic impurity problem in two-dimensional electron gas with Rashba spin-orbit coupling. We calculate the spin susceptibilities for different spin-orbit couplings, different Hubbard interactions, and different chemical potentials. The Kondo temperatures for different parameters are estimated by fitting the universal curves of spin susceptibilities. We find that the Kondo temperature is almost a linear function of the Rashba spin-orbit energy when the chemical potential is close to the edge of the conduction band, and when the chemical potential is far away from the band edge, the Kondo temperature is independent of the spin-orbit coupling. These results demonstrate that, for single impurity problem in this system, the most important reason to alter the Kondo temperature is the divergence of density of states near the band edge, and the divergence is induced by the spin-orbit coupling.   

$^{\mathrm{11}}$B Pulsed NMR Study of DyNi$_{\mathrm{2}}$B$_{\mathrm{2}}$C Single Crystals

DyNi$_{\mathrm{2}}$B$_{\mathrm{2}}$C is the only compound in the $R$Ni$_{\mathrm{2}}$B$_{\mathrm{2}}$C ($R =$ rare-earth) series where superconductivity at $T_{c} $\textasciitilde 6.2 K coexists with the antiferromagnetic ordering below the Nèel temperature$ T_{N}$ \textasciitilde 10.3 K. $^{\mathrm{11}}$B pulsed NMR measurements were performed at 8.0056 T to investigate the local electronic structures and \textit{4f} spin dynamics of DyNi$_{\mathrm{2}}$B$_{\mathrm{2}}$C powders and single crystals. The spectrum for the single crystal showed three narrow resonance peaks at 295 K due to the nuclear Zeeman splitting of a nuclear spin $I =$ 3/2 with quadrupolar perturbation. The $^{\mathrm{11}}$B NMR Knight shift of the single crystal was very large and highly anisotropic at $K=-$0.60{\%} and $+$0.27{\%} for the fields parallel and perpendicular, respectively, to the $c$-axis at 295 K. Considering the anisotropy of the Knight shift, we were able to simulate the $^{\mathrm{11}}$B NMR power pattern that agreed well with the measured spectrum. The linewidth was also large and anisotropic, and the linewidth value increased rapidly at low temperatures. The $^{\mathrm{11}}$B NMR shift and linewidth were found to be proportional to the magnetic susceptibility, indicating that the hyperfine field at the B site originates from the 4$f$ spins of Dy. Above $T_{N}$, the values for 1/$T_{\mathrm{1}}$ and 1/$T_{\mathrm{2}}$ were very large, showing slight increases at low temperatures. Below $T_{N}$, the values of 1/$T_{\mathrm{1}}$ and 1/$T_{\mathrm{2}}$ were suppressed significantly because of the slowing of the 4$f $spin fluctuation. This confirmed the huge change in Dy \textit{4f} spin dynamics across the antiferromagnetic transition.   

Site specific physics in RT$_{5}$ (R $=$ rare earths and T $=$ transition metals) materials

Most of RT$_{5}$ compounds form in hexagonal CaCu$_{5}$-type structure with three non-equivalent sites: R (1a), T (2c), and T (3g). R atoms sit in the middle of the T (2c) hexagonal layers. Advanced density functional theory calculations including on-site electron correlation and spin orbit coupling show crystal field split localized R 4f states, which are responsible for the large part of the magnetic anisotropy exhibited by these systems. In addition, the hexagonal T (2c) layers help enhancing the magnetic anisotropy. Partially quenched R 4f orbital moment is the origin of magnetic anisotropy which also helps enhancing magnetic moment. The interchange of T sites by other transition metals and the partial substitution of R atoms by transition metals could optimize needed magnetic moment and magnetic anisotropy by forming a complex geometry structure favoring permanent magnetic properties.   

Spin wave propagation in perpendicular magnetized 20 nm Yttrium Iron Garnet with different antenna design

Magnonics offers a new way to transport information using spin waves free of charge current and could lead to a new paradigm in the area of computing. Forward volume (FV) mode spin wave with perpendicular magnetized configuration is suitable for spin wave logic device because it is free of non-reciprocity effect. Here, we study FV mode spin wave propagation in YIG thin film with an ultra-low damping. We integrated differently designed antenna i.e., coplanar waveguide and micro stripline with different dimensions. The $k$ vectors of the spin waves defined by the design of the antenna are calculated using Fourier transform. We show FV mode spin wave propagation results by measuring S$_{\mathrm{12}}$ parameter from vector network analyzer and we extract the group velocity of the FV mode spin wave as well as its dispersion relations.   

Theoretical Investigation of Anisotropic Damping in Exchange Bias Systems

An accurate description of the magnetization dynamics of exchange bias systems is essential for further development of computer read heads and STT-MRAM. There have been several theoretical predictions of an anisotropic Gilbert damping tensor [1], influenced by the symmetry of the crystal structure, in place of the scalar Gilbert damping parameter in the Landau-Lifshitz-Gilbert equation of motion. However, experimental confirmation is difficult as the anisotropy of the damping parameter is expected to be small for single crystals. We follow up on our experimental discovery of a strong unidirectional contribution to the relaxation of exchange bias systems [2] by implementing an anisotropic damping tensor in our Matlab-based micromagnetics code M3. We present results for a damping tensor with unidirectional anisotropy with respect to the instantaneous orientation of the magnetization. References: 1. K. Gilmore et al. Phy. Rev. B, 81, 174414 (2010) and references therein. 2. T. Mewes et al. IEEE Magn. Lett. 1, 3500204 (2010).   

Oxygen control of perpendicular magnetic anisotropy and spin-orbit torques in Ta/ CoFeB/ MgO trilayer

Current-induced magnetization switching allows the integration of magnetic capabilities into electric circuits. The spin-orbit interaction in heavy-metal/ferromagnetic heterostructures are of profound interest, since they provide an efficient way to manipulate the magnetization, via strong current driven spin orbital torques (SOTs). Materials possessing perpendicular magnetic anisotropy (PMA) are the preferred choice for the fabrication of memory devices since its magnetization can be switched with a small current density. Here we present the Oxygen control of perpendicular magnetic anisotropy of Ta/ CoFeB/ MgO, which is accomplished by fabricating a thin wedge layer of Al on top of the MgO layer followed by oxidation in Oxygen plasma. Thinner end of the Al wedge will be over oxidized and the thicker end will be under oxidized, hence degree of oxidation varies from thinner to the thicker end. This in effect provides a means to control the Oxygen content at the CoFeB/ MgO interface and to control the perpendicular magnetic anisotropy. We will further discuss the dependence of SOTs, measured with adiabatic harmonic Hall technique on varying PMA.   

Ferromagnetic resonance investigation of relaxation in IrMn/CoFe bilayers

Although the exchange bias effect has been known for 60 years, it still remains a very active field of research. Studying magnetization dynamics and relaxation mechanisms in exchange bias systems is particularly important due to technological applications such as in read-heads. Here we report on our studies of the magnetic anisotropies and magnetization relaxation in IrMn(6nm)/CoFe(t) \quad systems. We have observed a very strong interface energy of the exchange bias effect in these samples. Furthermore, a strong perpendicular anisotropy and a small in-plane uniaxial anisotropy are observed in these systems. Moreover, in-plane angle dependent ferromagnetic resonance data suggests that the common analytical model cannot fully describe the anisotropies in these systems. Instead, a numerical approach needs to be used to minimize the energy and investigate the anisotropies in these systems. We observe a strong unidirectional relaxation in these samples, which for thin films is dominated by two magnon scattering. However, our data also indicates the presence of an additional unidirectional contribution to the relaxation not caused by two magnon scattering.   

Unusual Magnetic Behavior in BaMn$_{2}$Sb$_{2}$

We have performed experimental investigation on magnetic properties of BaMn$_{2}$Sb$_{2}$ single crystals grown using the flux method. X-ray diffraction measurements show that BaMn$_{2}$Sb$_{2}$ forms the ThCr$_{2}$Si$_{2}$-type tetragonal structure at room temperature. Single crystal neutron diffraction refinement indicates the G-type antiferromagnetic (AFM) ordering below T$_{N}$. However, the transition temperature T$_{N}$ is strongly sample dependent, varying from 280 K to 800 K. At a fixed temperature and magnetic field, the DC magnetization is also strongly time dependent. These results indicate that the magnetic properties of BaMn$_{2}$Sb$_{2}$ are extremely sensitive to sample and measurement history. Possible origins will be discussed.   

Micromagnetic Study of Vortex Core Motion driven by Thermal Spin Transfer Torque

We report on micromagnetic investigations of the magnetization dynamics for vortex core structures within a thin square film of permalloy, using our finite element code M3. We have studied the dependence of the vortex motion on the temperature gradient, lateral sample dimensions and thicknesses, as well as the influence of the Landau-Lifshitz-Gilbert damping parameter on the resulting vortex motion. To further analyze our numerical results of the gyrotropic motion we use an analytic solution of the Thiele equation, which has been expanded by Thiaville et al. to include a spin polarized current density. We show that the final core deflection depends on the Landau Lifshitz Gilbert damping parameter only in second and higher orders. However, the eigenfrequency of the free vortex motion is in leading order proportional to this parameter, which becomes important for vortex motions driven by a series of heat pulses. Our results indicate that materials with a low Gilbert damping parameter will lead to a larger amplitude vortex core motion in case one utilizes heat pulses to generate spin torque to resonantly excite it.   

Structural and magnetic properties of inverse Heusler alloys Mn$_{\mathrm{2}}$CoZ ( Z$=$ Ga,Ge,Sb)

Heusler compounds are probably the single biggest family of half-metals (100{\%} spin-polarized at the Fermi Level) and most promising for spintronic device applications. Many newer half-metallic full Heusler compounds in their L21 form are predicted from ab-initio calculations. The inverse Heusler alloys (Y$_{\mathrm{2}}$XZ) are interesting in this respect, and also predicted to be stable. Experimentally, we successfully prepared arc-melt samples of Mn$_{\mathrm{2}}$CoZ (Z$=$Ga,Ge,Sb). We study the structural and magnetic properties of inverse Heusler alloys using X-ray diffraction (XRD) and SQUID and VSM magnetometry. We found that these alloys are single phase after annealing at 500 C$^{\mathrm{0}}_{\mathrm{\thinspace }}$for 48 hours with single- grain microstructure. Energy dispersive spectroscopy measurements is also conducted to verify the composition of these alloys. Their ordering properties with respect to L21 structure and other possible orientations (C1b for instance) will be discussed in this presentation.   

Magnetic properties of the layered III-VI diluted magnetic semiconductor Ga1$-$xFexTe

Magnetic properties of single crystalline Ga1-xFexTe (x $=$ 0.05) have been measured. GaTe and related layered III-VI semiconductors exhibit a rich collection of important properties for THz generation and detection. The magnetization versus field for an x $=$ 0.05 sample deviates from the linear response seen previously in Ga1-xMnxSe and Ga1-xMnxS and reaches a maximum of 0.68 emu/g at 2 K in 7 T. The magnetization of Ga1-xFexTe saturates rapidly even at room temperature where the magnetization reaches 50{\%} of saturation in a field of only 0.2 T. In 0.1 T at temperatures between 50 and 400 K, the magnetization drops to a roughly constant 0.22 emu/g. In 0 T, the magnetization drops to zero with no hysteresis present. The data is consistent with Van-Vleck paramagnetism combined with a pronounced crystalline anisotropy, which is similar to that observed for Ga1$-$xFexSe. Neither the broad thermal hysteresis observed from 100-300 K in In1$-$xMnxSe nor the spin-glass behavior observed around 10.9 K in Ga1$-$xMnxS are observed in Ga1$-$xFexTe. Single crystal x-ray diffraction data yield a rhombohedral space group bearing hexagonal axes, namely R3c. The unit cell dimensions were a $=$ 5.01 A, b $=$ 5.01 A, and c $=$ 17.02 A, with alpha $=$ 90*, Beta $=$ 90*, and gamma $=$ 120* giving a unit cell volume of 369 A\textasciicircum 3.   

Voltage Controlled Perpendicular Magnetic Anisotropy.

Here we report the voltage controlled perpendicular magnetic anisotropy of a multilayer stack composed of P-type silicon substrate/ Gd2O3/ Co/ Pt grown by pulsed laser deposition (PLD) under ultra-high vacuum conditions. For examination of the voltage effect on magnetic properties of the samples, we performed magneto optical Kerr effect (MOKE) measurements. The results show a clear inverse relationship between voltage and coercivity. The exchange of oxygen ions which occurs at the interface between gadolinium oxide and cobalt may increase the cobalt oxide concentration within the optical interface layer. One potential application for this research could be the creation of a voltage controlled magnetic tunneling junction memory storage device. Proper implementation may be able to combine non-volatility with higher areal densities and low power consumption.   

Crystalline Electric Field Effect in Ho$_{1-x}$Dy$_{x}$Ni$_{2}$B$_{2}$C System.

We have measured magnetization curves of Ho$_{1-x}$Dy$_{x}$Ni$_{2}$B$_{2}$C (x $=$ 0.0, 0.1, 0.3, 0.6, 1.0) single crystals at various temperatures with the applied magnetic fields up to 20 kG where Neel and superconducting temperatures (T$_{N}$, T$_{C})$ ratio, T$_{N}$/T$_{C,\, }$varies from 0.73 to 1.66 for x$=$ 0.0 and 1.0. All measurements show some not linear behaviors in magnetization versus applied magnetic fields at low temperature regions. From the theoretical analysis of I4/mmm group symmetry in structure with the energy level pictures of CEF (crystalline electric field) effect of magnetization isotherms anisotropy at various temperatures, we have obtained ground state energy states in D$_{4h}$ singlet $\Gamma_{4}$ and first excited doublet state $\Gamma_{5}$ in addition to excited $\Gamma _{1.}$ Current CEF energy level analysis shows some qualitative agreement between theoretical calculation and experiments only at high magnetic fields regime, which means the interplay between magnetism and superconductivity may be considered.   

Spin dynamics and thermal stability in L10 FePt

Increasing the data storage density of hard drives remains one of the continuing goals in magnetic recording technology. A critical challenge for increasing data density is the thermal stability of the written information, which drops rapidly as the bit size gets smaller. To maintain good thermal stability in small bits, one should consider materials with high anisotropy energy such as L10 FePt. High anisotropy energy nevertheless implies high coercivity, making it difficult to write information onto the disk. This issue can be overcome by a new technique called heat-assisted magnetic recording, where a laser is used to locally heat the recording medium to reduce its coercivity while retaining relatively good thermal stability. Many of the microscopic magnetic properties of L10 FePt, however, have not been theoretically well understood. In this poster, I will focus on a single L10 FePt grain, typically of a few nanometers. Specifically, I will discuss its critical temperature, size effect and, in particular, spin dynamics in the writing process, a key to the success of heat-assisted magnetic recording.   

Investigation of Barium Ferrite, Searching for Soft Magnetic Materials in High Frequency Applications

Soft ferrites have been extensively and intensively applied for high frequency device applications. Among them, Ba-ferrites substituted by Mn and Ti are particularly attractive as future soft magnetic material candidates for advanced high frequency device applications. However, very little has been known as to the intrinsic magnetic properties, such as damping parameter, which is crucial to develop high frequency devices. In the present study, much effort has been focused on fabrication of single crystal Ba-ferrites and measurements of damping parameter by FMR. Ba-ferrite samples consisted of many grains with various sizes have been prepared. The saturation magnetization and the magnetic anisotropy field of the sample are in reasonable agreement with the values in literature. The resonances positions in the FMR spectra over a wide frequency range also comply with theoretical predictions. However, the complex resonance shapes observed makes it difficult to extract dynamic magnetic property. Possible reasons are the demagnetization field originating from irregular sample shape or existence of multiple grains in the samples.   

Triangular Nanomagnet Magnetization switching dynamics.

Single domain nanomagnets are essential for magnetic memory and non-volatile logic applications. Configurational anisotropy defines ``Y'' or ``buckle'' magnetization ground states in triangular nanomagnets. In this work we investigate the magnetization switching dynamics of a triangular nanomagnet numerically and experimentally. Bar magnets with coercivity lower than that of the triangle dipolarly coupled to its vertices were used to manipulate the local magnetization of that triangle's vertices. We demonstrate that magnetization ground states can be manipulated by controlling the local magnetization within these vertices. The switching dynamics is defined by the configurational anisotropy and undergoes through counter-clockwise macrospin rotation. Equilateral concave triangles measuring 50-500nm a side were coupled to bar magnets, and both were defined by electron beam lithography followed by a lift-off process. A uniform external field was used to selectively switch the triangle vertices with use of engineered dipolarly coupled bar magnets. Magnetic force microscopy was used to image magnetization before and after switching processes within each nanomagnet. The investigated triangle switching dynamics is assessed for use in non-volatile logic applications.   

Multiple relaxation times in single-molecule magnets

Multiple relaxation times detected in the ac magnetic susceptibility of several single-molecule magnets have been always assigned to extrinsic factors, such as nonequivalent magnetic centers or effects of intermolecular interactions in the crystal. By solving quantum relaxation equations, we prove that the observed multiple relaxation times can be of intramolecular origin and can show up even in single-ion metal complexes \footnote{ L. T. A. Ho and L. F. Chibotaru, \textbf{Phys. Rev. B} 94, 104422 (2016)}. For the latter a remarkably good description of the coexistent two relaxation times is demonstrated on several experimental examples. This proves the relevance of the intramolecular mechanism of multiple relaxation times in such systems, which is even easier justified in polynuclear magnetic complexes.   

Discrete Spin Vector Approach for Monte Carlo-based Magnetic Nanoparticle Simulations.

The study of magnetic nanoparticles has gained significant popularity due to the potential uses in many fields such as modern medicine, electronics, and engineering. To study the magnetic behavior of these particles in depth, it is important to be able to model and simulate their magnetic properties efficiently. Here we utilize the Metropolis-Hastings algorithm with a discrete spin vector model (in contrast to the standard continuous model) to model the magnetic hysteresis of a set of protected pure iron nanoparticles. We compare our simulations with the experimental hysteresis curves and discuss the efficiency of our algorithm.   

Magnetism and structure of a half-metallic Heusler compound Co-Mn-Cr-Si

Half metallic ferromagnetic Heusler compounds have a potential in the development of spintronic devices for its high spin polarization at the Fermi level and lattice structure compatibility. Heusler compounds based on cobalt are considered a good candidate for room temperature half-metals due to their high Curie temperature. Co$_{\mathrm{2}}$CrSi is one of such predicted half-metal, but it is meta-stable and difficult to synthesize in the desired crystal structure. We have successfully synthesized a Heusler compound Co$_{\mathrm{2}}$Mn$_{\mathrm{0.5}}$Cr$_{\mathrm{0.5}}$Si by using arc melting and rapid quenching followed by thermal treatment under high vacuum to control any parasitic contamination. Crystal X-ray diffraction pattern shows the samples crystallize in a cubic Heusler structure with some degrees of structural disorder. Curie temperatures of the prepared samples are observed well beyond room temperature near 900 K. Magnetic anomalies present in as-prepared samples are cleared, and its magnetic properties are improved by thermal treatment.   

Investigation of Anisotropic Bonded Magnets in Permanent Magnet Machine Applications

Rare earth elements (REE) provide the high energy product necessary for permanent magnets, such as sintered Nd$_{\mathrm{2}}$Fe$_{\mathrm{14}}$B, in many applications like wind energy generators. However, REEs are considered critical materials due to risk in their supply. To reduce the use of critical materials in permanent magnet machines, the performance of anisotropic bonded NdFeB magnets, aligned under varying magnetic field strength, was simulated using 3D finite element analysis in a 3MW direct-drive permanent magnet generator (DDPMG), with sintered N42 magnets used as a baseline for comparison. For direct substitution of the anisotropic bonded magnets, approximately 85{\%} of the efficiency of the baseline model was achieved, irrespective of the alignment field. The torque and power generation of the DDPMG was not found to vary significantly with increase in the alignment field. Finally, design changes were studied to allow for the achievement of rated torque and power with the use of anisotropic bonded magnets, demonstrating the potential for reduction of critical materials in permanent magnets for renewable energy applications.   

Quadruple Cone Coil with improved focality than Figure-8 coil in Transcranial Magnetic Stimulation

Transcranial Magnetic Stimulation (TMS) is a non-invasive therapy which uses a time varying magnetic field to induce an electric field in the brain and to cause neuron depolarization. Magnetic coils play an important role in the TMS therapy since their coil geometry determines the focality and penetration's depth of the induced electric field in the brain. Quadruple Cone Coil (QCC) is a novel coil with an improved focality when compared to commercial Figure-8 coil. The results of this newly designed QCC coil are compared with the Figure-8 coil at two different positions of the head - vertex and dorsolateral prefrontal cortex, over the 50 anatomically realistic MRI derived head models. Parameters such as volume of stimulation, maximum electric, area of stimulation and location of maximum electric field are determined with the help of computer modelling of both coils. There is a decrease in volume of brain stimulated by 11.6 {\%} and a modest improvement of 8 {\%} in the location of maximum electric field due to QCC in comparison to the Figure-8 coil.   

Effects of Rare-Earth Doping on the Martensitic Transition Temperatures and Magnetocaloric and Transport Properties of Ni$_{50}$Mn$_{35}$Sn$_{15}$ Alloys

The structural, magnetic, magnetocaloric, and transport properties of rare-earth (R) doped Ni$_{49}$Sm Mn$_{35}$Sn$_{15}$ and Ni$_{49}$PrMn$_{35}$Sn$_{15}$ Heusler alloys have been studied by room temperature XRD and magnetization measurements. The studied compounds show a cubic L21-type structure at room temperature. The substitution of R $=$ Sm and Pr for Ni in Ni$_{50}$Mn$_{35}$Sn$_{15}$ resulted in the shifting of the martensitic temperature (T$_{M})$ from 160 K (for Ni$_{50}$Mn$_{35}$Sn$_{15})$ to 190 K (for Sm) and 212 K (for Pr). However the Curie temperature of the austenite phase (T$_{C})$ remained unchanged (\textasciitilde 325 K). Both conventional and inverse magnetocaloric effects were observed in these compounds. The maximum value of the positive magnetic entropy change ($\Delta $S$_{M})$ near T$_{M}$ with $\Delta $H $=$ 5T was \textasciitilde 5 J/kgK and \textasciitilde 12 J/kgK for R $=$ Sm and Pr, respectively. Large values of RCP, 278 and 315 J/kg, were obtained for R $=$ Sm and Pr, respectively. The maximum values of the magnetoresistance was found to be -18{\%} (R$=$ Sm) and -30{\%} (R$=$Pr) for $\Delta $H $=$ 5T.   

Harmonic decomposition of magneto-optical signal from superparamagnetic Fe3O4 nanoparticles

Superparamagnetic nanoparticles (SPNPs) are expected to play an increasingly important role in bio-imaging and therapy. These applications rely on understanding SPNPs magnetic properties which have been successfully characterized by AC Faraday rotation (FR). AC FR is used here to build on results presented earlier by measuring solutions of surfactant-coated magnetite nanoparticles. The setup employs a He-Ne laser, polarizing components, a sinusoidal B-field, and a lock-in detection scheme to measure the SPNPs FR. Such a setup provides a novel, economical way of determining important magnetic properties of SPNPs. The main intensity signal (1f) along with higher harmonics are collected and analyzed to calculate quantities such as the Verdet constant and the magnetic moment. We hope further analysis can also reveal details of size distribution and relaxation times of SPNPs. We will present results from samples with various concentrations as well as a particular concentration subjected to a range of B-field frequencies (between 800 Hz and 14 kHz). Findings are compared to results from more traditional techniques like magnetic susceptibility measurements, magnetic force microscopy, etc. We will also address the comparative advantages of this technique and its limitations.   

Experimental Evidence for Quantized Vortices and Superfluidity of Semiconductor Excitons in a Trap

Semiconductor excitons are electron-hole pairs bound by Coulomb attraction. Composed by two fermions, excitons undergo Bose-Einstein condensation (BEC) in an original fashion. Combescot et al. showed that quantum statistics leads to a grey condensate made by a dominant fraction of dark excitons coherently coupled to a weaker population of bright excitons. Thus, the condensate emits a weak coherent photoluminescence (PL) that can be experimentally used to probe the phase coherence of this "grey" condensate. Recently, we have reported the first signatures for a grey condensate signaled by the macroscopic spatial coherence of a confined dark gas of excitons, below a critical temperature of 1K.\\The PL radiated by the grey condensate is strongly inhomogeneous spatially. Dark spots are identified in the emission profile, signaling the local damping of bright excitons by over 50\%. Using spatial interferometry, we reveal that the dark spots observed in a grey condensate are quantized vortices with a phase singularity of the coherent PL emission due to a 2$\pi$ phase winding around the core of one quantized vortex. We then discuss the role of disorder for the localization and the formation of vortices across BEC of excitons.   

Artificial gravity field, astrophysical analogues, and topological phase transitions in strained topological semimetals

Effective gravity and gauge fields are emergent properties intrinsic for low-energy quasiparticles in topological semimetals. Here, taking two Dirac semimetals as examples, we demonstrate that applied lattice strain can generate warped spacetime, with fascinating analogues in astrophysics. Particularly, we study the possibility of simulating black-hole/white-hole event horizons and gravitational lensing effect. Furthermore, we discover strain-induced topological phase transitions, both in the bulk materials and in their thin films. Especially in thin films, the transition between the quantum spin Hall and the trivial insulating phases can be achieved by a small strain, naturally leading to the proposition of a novel piezo-topological transistor device. Our result not only bridges multiple disciplines, revealing topological semimetals as a unique table-top platform for exploring interesting phenomena in astrophysics and general relativity; it also suggests realistic materials and methods to achieve controlled topological phase transitions with great potential for device applications.   

Canted coherent spin states in p-Si.

The spin waves or magnons are collective excitations of spins in magnetically ordered materials. Spin waves are considered to be the prime candidates for future energy efficient spintronics and magnonics devices. In the absence of magnetic order, the spin transport is believed to be diffusive and spin waves are not expected to exist in non-magnetic semiconductors. Here we present the first experimental proof of spin injection induced spin waves in non-magnetic p-Si. The thermally driven spin injection, also called spin-Seebeck tunneling, leads to non-equilibrium spin accumulation in p-Si. We propose that collective spin excitations or spin waves ensue in the non-magnetic materials due to non-equilibrium spin accumulation when the specimen dimension (p-Si specimen thickness of 2 $\mu$ m) is smaller than the spin-diffusion length. The spin excitations are confirmed from magneto-thermal transport measurement showing hysteretic thermal spin crossover behavior.   

Temperature-Dependent Photoconductivity Responce and Band Gap Variation of Tl$_{\mathbf{2}}${In}$_{\mathbf{2}}${S}$_{\mathbf{3}}${Se Layered Single Crystals}

Temperature variation of indirect band gap of Tl$_{2}$In$_{2}$S$_{3}$Se layered single crystals were obtained by means of absorption and photoconductivity measurements. The temperature coefficient of $-$7.1~$\times$ ~10$^{-4}$~eV/K from absorption measurements in the temperature range of 10--300~K in the wavelength range of 520--1100~nm and $-$5.0~$\times$ ~10$^{-4}$~eV/K from PC measurements in the temperature range of 132--291~K in the wavelength range of 443--620~nm upon supplying voltage~$V$~$=$~80~V were obtained. From the analysis of dark conductivity measurements in the temperature range of 150--300~K, conductivity activation energy was obtained as 0.51~eV above 242~K. The degree of the disorder, the density of localized states near Fermi level, the average hopping distance and average hopping energy of Tl$_{2}$In$_{2}$S$_{3}$Se crystals were found as, 1.9~$\times$ ~10$^{5}$~K, $N_{f}$~$=$~4~$\times$ ~10$^{20}$~cm$^{-3}$eV$^{-1}$, 29.1~{\AA} and 24.2~meV in the temperature range of 171--237~K, respectively. Activation energy of hopping conductivity at~$T$~$=$~171~K was obtained as 41.3~meV and the concentration of trapping states was found as 1.6~$\times$ ~10$^{19}$~cm$^{-3}$.   

Heitler-London Model for Acceptor-Acceptor Interactions in Doped Semiconductors

The interactions between acceptors in semiconductors are often treated in qualitatively the same manner as those between donors. Acceptor wave functions are taken to be approximately hydrogenic and the standard hydrogen molecule Heitler-London model is used to describe acceptor-acceptor interactions. But due to valence band degeneracy and spin-orbit coupling, acceptor states can be far more complex than those of hydrogen atoms, which brings into question the validity of this approximation. To address this issue, we develop an acceptor-acceptor Heitler-London model using single-acceptor wave functions of the form proposed by Baldereschi and Lipari, which more accurately capture the physics of the acceptor states. We calculate the resulting acceptor-pair energy levels and find, in contrast to the simple singlet-triplet splitting of the hydrogen molecule, a rich ten-level energy spectrum. Our results, computed as a function of inter-acceptor distance and spin-orbit coupling strength, suggest that acceptor-acceptor interactions can be qualitatively different from donor-donor interactions, and should therefore be relevant to the magnetic properties of a variety of p-doped semiconductor systems.   

Tunable optical properties of ZnCdTe$_{\mathbf{2-x}}$\textbf{Se}$_{\mathbf{x}}$\textbf{ (x }$=$\textbf{ 0.625) chalcopyrite for photovoltaics; a mBJLDA approach }

In the present study, we have performed ab-initio simulations of sp-element defect in ZnCdTe$_{2-x}$Se$_{x}$ (x $=$0.625) chalcopyrite to check the tuning of band gap as compared to the pristine case. The exchange and correlation (XC) effects are taken into account by an orbital independent modified Becke-Johnson (mBJ) potential as coupled with Local Density Approximation (LDA) for these calculations. The calculated energy band structures show a direct band gap at the à point in the brillouin zone for the pristine as well as the defected case and the band gap decreases with inclusion of sp-disorder. The imaginary dielectric function predicts the optical band gap of pristine ZnCdTe$_{2}$ very close to the experimental value and the results are in reasonable agreement without applying any scissor operator. With inclusion of sp-element defect, the optical spectra is tuned to optimal region, suitable for photovoltaics. It is apparent that mBJ functional is well suited for calculating electronic structure of pristine as well as defected ZnCdTe$_{2\, }$chalcopyrite.   

Theoretical study on the possibility of S doping in anatase TiO$_{\mathrm{2}}$

Titanium dioxide (TiO$_{\mathrm{2}})$ is well known for its numerous and diverse applications. Usually doping is often used to tune the properties of materials. In this work, isovalent Sulfur (S) doping in anatase TiO$_{\mathrm{2}}$ (TiO$_{\mathrm{2-x}}$S$_{\mathrm{x}})$ was studied using the first principles method. The total energy calculations were used to determine the defect formation energies and the chemical potential landscape with different S doping concentrations. The results showed that anatase TiO$_{\mathrm{2-x}}$S$_{\mathrm{x}}$ with concentrations x$=$0.0278 and 0.0625 cannot exist without the co-existence of other Ti binary compounds, such as TiO$_{\mathrm{2}}$, Ti$_{\mathrm{2}}$O$_{\mathrm{3}}$, TiS, TiS$_{\mathrm{2}}$, and TiS$_{\mathrm{3}}$. Moreover, other elements doped with S together to stabilize the compounds were also investigated.   

The effects of impurities on the structural and electronic properties

In this work we explore the effects of molybdenum (Mo) and sulfur (S) atoms placed in two differents positions in the interlayer region of MoS$_{2}$ bilayer. By means density functional theory (DFT) calculations we have determinated that these impurity-atoms can be adsorbed in the interlayer region. Structurally, this impurity atoms causes the interlayer repultion and therefore separate the layers, decoupling the bond and breacking the equilibrium position between them. This could be used to exfoliate MoS$_{2}$ layers. Adittionaly, we found that the adatoms produce impurity states in the band gap region of MoS$_{2}$ pristine bilayer. The configurations with S impurities are semiconductors without spin polarization, whereas, when Mo impurities are located within the interlayer region, produce total spin polarization at the Fermi level.   

Mobility Enhancement in Buried Two-Dimensional Electron and Hole Gases by Remote Carrier Screening

Buried two-dimensional electron and hole gases (2DEGs and 2DHGs) are a promising platform for solid-state quantum computation. For effective top gating, the surface quality and its impact on the buried two-dimensional carriers are critical. We report a mobility enhancement in the buried Si 2DEG and Ge 2DHG channels due to the remote carrier screening at the surface. An electron or hole screening layer is formed at the surface by carriers tunneling from the buried channel to the surface, resulting in the mobility enhancement. The peak carrier mobilities of Si 2DEG and Ge 2DHG are 4 times and 2 times higher than those without remote surface screening. Furthermore, by increasing the gate voltage, a transition from non-equilibrium to equilibrium in a two-dimensional system due to this surface tunneling is also observed for the first time.   

Nickel and Chromium doped ZnO: electronic, magnetic, and optical properties studied via first-principles calculations.

As one of the most important transition metal oxide semiconductors, ZnO has not only been widely used, in As one of the most important transition metal oxide semiconductors, ZnO has not only been widely used, in optoelectronics, gas sensors and solar cells, but it has shown potential application for next-generation resistive nonvolatile memories. In this work, we study the electronic and magnetic properties of Nickel and Chromium doped ZnO, as well as Ni and Cr codoped ZnO using first-principles band structure calculations, performed within Density Functional Theory. We use the mBJ$+U_{\mathrm{Zn}}$ approach which allowed to better describe the electronic structure of pure zinc oxide, providing a gap value in excellent agreement with experiment. The 3d related levels arising from the Ni and Cr impurities are found to lie close to the top of the valence band and in the gap region, respectively, being responsible for a reduction in the ZnO bandgap. From the band structure calculations, we obtained the real and imaginary parts of the frequency-dependent complex dielectric function for pure and doped systems. Our findings for the dielectric function, electrical conductivity and loss function are discussed in regard to device applications. Comparisons with results on doped samples grown using hydrothermal microwave assisted technique are performed.   

Electrical behaviors of high purity germanium at low temperature

The electric behaviors of high purity germanium (HPGe) crystals at low temperature play an important role in determining the purity level of such materials used to fabricate radiation detectors. In the present work, the temperature dependence of electrical properties has been measured for the temperature range from 4.2$^{o}$K to 100$^{o}$K in two types of HPGe samples, polycrystalline crystals and single crystals, containing different impurity concentrations. The conductivity versus the inverted temperature curves for all of samples was divided into three distinctive temperature ranges: (a) high temperature where the conductivity increased to a maximum with decreasing temperature, (b) intermedium temperature where the conductivity decreased proportionally with decreasing temperature, and (c) low temperature where the conductivity continued decreasing slowly with decreasing temperature. It was also found that there was a turning point on the conductivity vs temperature curves for both types of samples. However, the turning points for them were significantly different: 30K for single crystal samples while 60K for polycrystalline samples. We report our measurements in this paper.   

Synthesis of high quality monolayer WS$_{\mathrm{2}}$ using chemical vapor deposition.

Monolayer tungsten dichalcogenide WS$_{\mathrm{2}}$ have addressed interest from material scientist for new generation of optoelectronics due to thickness dependent optical properties and mechanical flexibility. Continuous monolayers WS$_{\mathrm{2}}$ were synthesized using chemical vapor deposition (CVD) on various substrates, similar to our previous publication. By controlling growth temperature, we could yield high quality monolayer WS$_{\mathrm{2}}$. Optical, atomic force microscopic images and Raman scattering indicate that the film was mostly covered by monolayer WS$_{\mathrm{2}}$ with large grain size about 50 $\mu $m. Strong, direct gap emission at 636 nm with relatively small full width at half maxima and the absence of defect-related transitions in power-dependence photoluminescence (PL) revealed the excellent quality of as-grown film in compared with CVD-grown monolayer MoS$_{\mathrm{2}}$. Moreover, PL intensity and energy mapping at $A$-exciton also shows uniformity and continuity of our films. Our results shows monolayer WS$_{\mathrm{2}}$ could be potentially applied to optoelectronic devices such as light emission diodes/   

Ge-on-Si Materials Created by Physical Vapor Deposition

Ge nanostructures are grown on Si substrates using solid Ge sources in a compact chemical vapor deposition (CVD) system. The surface termination of the Si substrate and the flow rate of the Ar carrier gas are found to strongly impact the morphology and structure of the Ge nanostructures. The growth mechanism is revealed through a systematical analysis of the x-ray diffraction (XRD) patterns for samples grown under various conditions. The morphological, structural, and optoelectronic properties of these Ge-on-Si materials will be reported.   

Study of low-temperature resistivity minimum and Hall Effect in pulsed laser deposited single crystalline titanium nitride (TiN) films

Titanium nitride (TiN) films were grown by a pulsed laser deposition technique using a variety of deposition parameters such as substrate temperature, ambient gas pressure, target-substrate distance, substrate materials, etc. The TiN thin films fabricated at temperatures in the range of 500-800 $^{\circ}$ C in vacuum ambient are found to be epitaxial with (111) orientation. Low-temperature transport properties were systemically in TiN films with different room temperature resistivities (100-500 $\mu $ohm-cm) under an applied magnetic field from 0 to 5.0 T. The temperature dependence of resistivity shows a generally minimum behavior at low temperatures (T\textless 40 K) under various applied fields. Best fittings were made by considering both the electron-electron (e-e) interactions in terms of T$^{1/2\, }$dependence and the Kondo-like spin dependent scattering in terms of ln T dependence. The Hall measurements and data analysis have shown that the charge carriers are electron in metallic TiN films. For example, the Hall coefficient and electron density at 300 K were found to be -6.4$\times$ 10$^{-5\, }$cm$^{3}$/C and 9.7 $\times$ 10$^{22}$/cm$^{3}$, respectively.   

UV-Assisted Atmospheric Pressure Spatial Atomic Layer Deposition of ZnO

ZnO has received much renewed interest as a wide band gap semiconductor for its variety of applications. For certain applications, such as thin film transistors, it is important to have highly crystalline ZnO with few defects, as a high defect concentration introduces too many charge carriers and can contribute to source-drain leakage. In this paper, we present a new roll-to-roll process, namely UV-Assisted Atmospheric Pressure Spatial Atomic Layer Deposition, for synthesizing high quality, crystalline ZnO. Using X-ray diffraction techniques, we show that the UV-activation of diethylzinc allows us to deposit c-axis oriented ZnO at temperatures as low as 50 C with significantly improved crystallinity. This temperature is significant as it is below the glass transition temperature of polyethylene terephthalate (PET), a popular substrate in the field of flexible electronics. Our new process allows us to overcome the tendency of ZnO to be amorphous when grown below 100 C. We also present the effect of growth under UV-illumination at different wavelengths on the defect states in ZnO with the use of X-ray photoemission spectroscopy, photothermal deflection spectroscopy and photoluminescence.   

Fabricating Stoichiometric Vanadium Dioxide Thin Films for Modulating Light Emission

The near-room-temperature insulator-to-metal phase transition in vanadium dioxide (VO$_{2})$ can produce drastic changes in resistivity and optical constants. In the nanophotonics field, this property of VO$_{2}$ is particularly attractive, because it can be used to manipulate the local environment of light emitters. However, since vanadium exhibits a large number of stable oxides, it can be challenging to obtain stoichiometric VO$_{2}$. Although the published literature contains many reports of optimal fabrication conditions, such parameters can vary substantially from system to system due to differences in architecture. Here, we will present an experimental procedure for optimizing stoichiometric VO$_{2}$ thin film growth by sputtering. Temperature-dependent spectroscopic ellipsometry, x-ray-diffraction, and resistivity measurements have been used to characterize sputtered VO$_{2}$ thin films. The time-domain switching behavior of VO$_{2}$ has also been also investigated by pump-probe spectroscopy. Finally, building on the work of Cueff et al., we will show how these VO$_{2}$ thin films can be electrically switched to modulate light emission by erbium ions faster than their excited state lifetime.   

Fine structure of high-power microwave-induced resistance oscillations

We report on a pronounced fine structure of microwave-induced resistance oscillations (MIRO) in an ultra-clean two-dimensional electron gas. This fine structure is manifested by additional sharp extrema residing beside the primary ones and, according to theoretical considerations, originates from multiphoton-assisted scattering off short-range impurities. Unique properties of the fine structure allow us to access all experimental parameters, including microwave power, and to separate different contributions to photoresistance. Finally, we demonstrate that the fine structure can be used to quantitatively assess the correlation properties of the disorder potential.   

Lattice Thermal Conductivity of Silicon from DFTB Molecular Dynamics

Bulk and nano-structure silicons are promising building blocks for the design of future electronic devices. Understanding of their thermal conductivity is important not only for fundamental research, but also to develop applications, e.g., thermoelectricity. However, accurate measurement of thermal conductivity is challenging due to numerous experimental uncertainties. In this work, we develop a hybrid approach combining Born-Oppenheimer molecular dynamics (BOMD) with lattice dynamics. This approach depicts numerically phonon quasiparticles, from which the phonon lifetime $\tau(q,\lambda)$, group velocity $v(q,\lambda)$ and heat capacity $C_v(q,\lambda)$ are extracted for each phonon mode ($q,\lambda$). With these quantities, it is straightforward to evaluate the lattice thermal conductivity $\kappa$ according to the Boltzmann transport equation, $\kappa= \frac{1}{3V} \sum_{q,\lambda} v^2(q,\lambda) \tau(q,\lambda) C_{v}(q,\lambda)$ To benchmark the validity of our approach, we carry out BOMD simulation and phonon calculation of bulk silicon using Density Functional based Tight Binding (DFTB) as implemented in the package of Trocadero. A series of temperatures from 300 to 1600 K are considered, and a good agreement with experimental data is achieved.   

The Anomalous de Haas-van Alphen Effect in InAs/GaSb quantum wells

The de Haas-van Alphen effect (dHvAe) describes the periodic oscillation of the magnetisation in a material as a function of inverse applied magnetic field. It forms the basis of a well established procedure for measuring Fermi surface properties and its observation is typically taken as a direct signature of a system being metallic. However, certain insulators can show similar oscillations of the magnetisation from quantisation of the energies of electron states in filled bands. Recently the theory of such an anomalous dHvAe (AdHvAe) has been worked out but so far there is no clear experimental observation. Here, we show that the inverted narrow gap regime of InAs/GaSb quantum wells is an ideal platform for the observation of the AdHvAe. From our microscopic calculations we make quantitative predictions for the relevant magnetic field and temperature regimes, and describe unambiguous experimental signatures.   

Hall field-induced resistance oscillations in MgZnO/ZnO heterostructures

We report on a nonlinear magnetotransport study of a two-dimensional electron gas hosted in a MgZnO/ZnO heterostructure, a recently emerged high-quality material system. We find that upon application of a sufficiently high direct current, the differential resistivity exhibits pronounced Hall field-induced resistance oscillations (HIRO) which are well known from low magnetic field (B $\sim$ 0.1 T) experiments on GaAs/AlGaAs and, more recently, Ge/SiGe quantum wells. Owing to much higher effective electron mass in our system ($m^\star \approx 0.3 m_0$, $m_0$ is a free electron mass), we were able to observe HIRO extending to fields above 5 T. Exclusive sensitivity of HIRO to a short-range component of the disorder potential allows us to unambiguously conclude that the mobility of our sample is limited by large-angle scattering off impurities within or near the 2D channel.   

Electronic structural, optical and phonon lattice dynamical properties of pure- and La-doped SrTiO$_{\mathrm{\mathbf{3}}}$

To better understand the thermodynamic and optical behaviors of lanthanum doped strontium titanate (LSTO) with different La-doping levels at high temperature, the \textit{ab initio} thermodynamics by combining the first-principles density functional theory with lattice phonon dynamics have been employed to investigate their electronic structures and thermodynamic evolutions versus temperatures. The results show that when doping La into STO, the band-gap is vanished as extra electron fills into the STO conduction band. With increasing the La-doping levels, the LSTO structures become unstable with phonon soft modes. From the calculated dielectric constant matrix of LSTO with different La-doping levels, one can see that in three cases (2La-STO, 3La-STO, 6La-STO) their diagonal elements are not equal, which means that these crystal structures are uniaxial and anisotropic. With increasing La-doping levels, the calculated thermodynamic properties ($\Delta $H, $\Delta $G) with reference to DFT energy of pure STO are decreased. With increasing temperature (T), the $\Delta $H(T) is increased while the $\Delta $G(T) is decreased.   

Pressure-induced ferroelastic phase transition in SnO$_2$ from density functional theory

We studied the high-pressure ferroelastic transition of rutile- to CaCl$_2$-type SnO$_2$ within density functional theory and Landau free energy theory. Softening mechanism of B_{1g}\) mode (order parameter \emph{Q}) and the coupling mechanism between the soft B_{1g}\) mode and the soft transverse acoustic (TA) mode (strain \varepsilon\)) are clarified by calculating Landau energy map around the ground state. It is found that the Sn-O-Sn bending induced soft B_{1g}\) mode effectively reduces the excess energy increase caused by bond stretching, which however always leads to SnO$_6$ octahedral distortion. The octahedral distortion is subsequently minimized by lattice distortion strain \varepsilon\), which interacts with the soft B_{1g}\) mode to further increase the stability of system.   

Nano-scale displacement sensing based on van der Waals interaction.

We propose that a nano-scale displacement sensor with high resolution for weak-force systems can be realized based on vertically stacked two-dimensional (2D) atomic corrugated layer materials bound through van der Waals (vdW) interactions. Using first-principles calculations, we found that the electronic structure of bi-layer blue phosphorus (BLBP) vary appreciably with the lateral and vertical interlayer displacements. The variation of the electronic structure due to the lateral displacement is attributed to the change in the interlayer distance $d_{z}$ induced by atomic layer corrugation, which is in a uniform picture with vertical displacement. Despite the different stacking configurations of BLBP, we find that the change of the in-direct band gap is proportional to $d_{z}^{-2}$. Further, this $d_{z}^{-2}$ dependence is found to be applicable to other graphene-like corrugated bi-layer materials such as MoS$_{\mathrm{2}}$. BLBP represents a large family of bi-layer 2D atomic corrugated materials for which the electronic structure is sensitive to the interlayer vertical and lateral displacement, and thus could be used for nano-scale displacement sensor. This can be done by monitoring the tunable electronic structure using absorption spectroscopy.   

Magnetization of a magnetic quantum structure

The energy dispersion and magnetization of a modified magnetic dot are investigated numerically. The effects of an additional electrostatic potential, magnetic fields non-uniformity, and the Zeeman spin splitting are studied. The modified magnetic quantum dot is a magnetically formed quantum structure which has different magnetic fields inside and outside the dot. The additional electrostatic potential prohibits the ground-state angular momentum transition in the energy dispersion as a function of the magnetic field inside the dot and provides the oscillation of the magnetization as a function of the chemical potential energy. The magnetic fields non-uniformity has smoothen the shape of magnetization. The Zeeman spin splitting produces additional peaks on the magnetization.   

Strain effects in the electronic structure of CrN

Chromium nitride (CrN) has a promising future for its resistance to corrosion and hardness, and fascinating magnetic and electronic properties. CrN presents a phase transition in which the crystal structure, magnetic ordering, and electronic properties change at a (Neel) temperature ~280K. Thin films from different groups exhibit varied conductance behavior at low temperature. We have performed ab initio calculations using the LSDA+U method, and estimate the interaction between the Cr-3d and N-2p orbitals, by analyzing the band structure near the optical gap (~0.2 eV). We also calculate effective masses and investigate the effect of strain fields on the electronic structure. Our results show that for compressive strain ~1.3$\%$ the band gap closes, suggesting that realistic strains could cause a significant change in the electronic structure and could contribute to explain under what experimental conditions the material has metallic behavior. The changes in the effective mass derived from our calculations show a large anisotropy, which would result in anisotropic charge carrier mobility. The mass anisotropy is found to be connected with the magnetic ordering in the lattice.   

Low temperature photo-induced carrier dynamics in the GaAs$_{\mathrm{0.985}}$N$_{\mathrm{0.015}}$ alloy

We report the exploration of photo-induced carrier dynamics in the GaAs$_{\mathrm{0.985}}$N$_{\mathrm{0.015}}$ Alloy. The time-resolved and high magnetic field-dependent photoluminescence experiments were carried out to identify the radiative transitions, and the localized and delocalized states at various excitation power and temperature. A nonmonotonic dependence of the PL energy on temperature at low laser power, and the observation of two different decay times at the temperature below 100 K indicate the free electrons undergo a delocalization to localization transition with decreasing temperature. In the low temperature region, the localization is further enhanced by an applied high magnetic field, and an unexpected high field blocking of the diamagnetic shift was observed.   

Study of optical properties of polar and non-polar ZnO using Terahertz time domain spectroscopy

In this paper, we have studied the frequency dependent optical properties of polar and non-polar ZnO. The sample was measured in different region of the same axis for ZnO which is parallel with axis a and m in C-plane. We found the signals in the same axis perform identically and have similar data. We also observed the optical properties of A-plane ZnO before and after annealing. The calculated refractive index for extinction coefficient and conductivity. Finally, the Drude model was used find the fit data and obtain the carrier concentration and mobility. In A-plane ZnO case, there is a mobility 231.98(cm$^{\mathrm{2}}$/Vs) rise to 786.32(cm$^{\mathrm{2}}$/Vs) before and after annealing.   

The Carrier Recombination Of InAs/GaAs Quantum Dots

In this study, the Time-Correlated Single Photo Counting (TCSPC) technique was used to measure the photoluminescence (PL) spectra and time-resolved PL spectra of InAs/GaAs QDs. Results showed that at temperatures below 50K, the lifetime of QDs are below 2ns in the infrared region. However, at temperatures over 50K, lifetimes exceeding 2ns can be observed. As the temperature was increased from 14K to 100K the lifetime also increases. Finally, when we fixed the temperature at 100K, we observed that the lifetime increased as the wavelength was increased.   

The photoluminescence of InAs/GaAs Quantum Dots

The photoluminescence of InAs/GaAs quantum dots (QDs) were studied with various temperatures and photoexcitation densities. The QDs were excited with laser pulses of energy 1.5 eV. The peaks of the PL at 14 K is 1000 nm and shifts to 1020 nm at 300 K. The red-shift at peaks of the photoluminescence as temperature increases was analyzed with the Varshni's equation and the band-gap energy was derived. The activation energy was also obtained from the temperature-dependent photoluminescence. The bandwidth of the PL depends on the size of the laser beam used for photoexcitation indicates the inhomogeneous distribution of different sizes of quantum dots.   

First-principle studies on the influence of anisotropic pressures on the physical properties of AlN

AlN has been widely used in electro-acoustic mechanical sensors. The performance of those AlN based sensors are usually dominated by its intrinsic physical and electronic properties. In this work, we first performed extensive first-principle studies to discuss the effect of uniaxial and biaxial mechanical pressures on the structural and physical properties of AlN piezoelectric material, including the longitudinal elastic constant (C$_{33})$, piezoelectric constant (e$_{33})$, static dielectric constant ($\varepsilon_{33})$, and mass density ($\rho )$.We give the relationship between the paramters mentioned above and the longitudinal acoustic wave velocity (V) under anisotropic pressures. Our results show that the applied uniaxial or biaxial pressure in the basal plane has a more obvious influence on physical properties of AlN than the uniaxial pressure along hexagonal axis. The pressure-induced variations of C$_{33}$, e$_{33}$ and $\rho $ significantly change the V value, whereas that of $\varepsilon _{33}$ on V is negligible. Our theoretical results provide useful information for the performance predictions of AlN based FBAR mechanical sensors$^{1}$ 1. E. Anderås, I. Katardjiev and V. Yantchev, J. Micromech. Microeng. 21, 85010-85016(85017) (2011).   

Quantum capacitance anomalies of two-dimensional non-equilibrium states under microwave irradiation

We report our direct study of the compressibility on ultrahigh mobility two-dimensional electron system ($\mu =$1x10$^{\mathrm{7}}$ cm$^{\mathrm{2}}$/Vs) in GaAs/AlGaAs quantum wells under microwave (MW) irradiation. The field penetration current results show that the quantum capacitance oscillates with microwave induced resistance oscillations (MIRO), however, the trend is opposite with respect to the compressibility for usual equilibrium states in the theoretical proposal. The anomalous phenomena at integer $j=\omega $/$\omega_{\mathrm{C}}$ regime provide a platform for study on the non-equilibrium system under microwave. Moreover, the quantum capacitance indication for multi-photon process at $j=$1/2 can be detected under intensive microwave below 30 GHz, although it does not appear in resistances.   

Chern number of many-body states on various lattices

The Chern numbers of many body states on lattices with magnetic field are numerically evaluated. Various lattice models are considered with short/long range particle-particle interaction projected into the Hofstadter type Landau band. The non-Abelian Berry connection defined by an approximate ground state multiplet is numerically constructed and used for the computation. For the $\nu=1/m$ ($m$:odd) state for the weak field are consistently described by the Laughlin state with the $m$ fold topological degeneracy independent of the lattices. Effects of randomness are included and discussed with spectral flows as well. We have further discussed systems with strong magnetic field on the Kagome, square, honeycomb and triangular lattices. Even denominator states on lattices are also investigated in relation to the possible Fermi liquid state [K. Kudo, T. Kariyado, and Y. Hatsugai, in preparation].   

Vacancy effects on the electronic and structural properties pentacene

Defects in organic crystals are likely to affect charge transport in organic electronic devices. Vacancies can create lattice distortions and modify electronic states associated with the molecules in its surrounding. Spectroscopy experiments indicate that molecular vacancies trap charge carriers. Experimental characterization of individual defects is challenging and unambiguous. Here we use density functional calculations including van der Waals interactions in a supercell approach to study the single vacancy in pentacene, a prototype organic semiconductor. We determine formation energies, local lattice relaxations, and discuss how vacancies locally distort the lattice and affect the electronic properties of the host organic semiconductor.   

High field transport of high performance black phosphorus transistors

As an emerging two dimensional layered semiconductor, few-layer black phosphorus (BP), with high mobility and high density of states, has attracted great interest recently due to its great potential in applications for digital electronics and photonics. Despite the tremendous research efforts on BP electronic devices in the past two years, high field transport and current carry capability of BP is still largely missing. Here, we perform a first comprehensive study on most important figures-of-merit such as on-state current, mobility, velocity and interface trap density of BP FETs based on high-k HfO$_{\mathrm{2}}$ dielectrics in comparison with SiO$_{\mathrm{2}}$ and push its high field transport much further beyond the current status.   

3D nano-SQUIDs with nano-constriction junctions.

Nano-SQUIDs (superconducting quantum interference devices) significantly shrink the SQUID washer by replacing the traditional tunneling junctions with the nano-constriction junctions. In such a design, the spin sensitivity of nano-SQUIDs, which is proportional to the radius of SQUID washer, are greatly improved. Additionally, the nano-SQUID with nano-constriction junctions are also excel in a high working field range, a direct coupling from spins to the nano-constrictions. However, current planar nano-SQUIDs made of Nb and NbN showed relatively a shallow flux modulation depth. Here, we developed a fabrication method for nano-SQUIDs based on Nb and NbN by replacing the planar design with a 3D structure. We studied the main parameters limited the flux modulation depth of the Nb and NbN nanoSQUID. As a result, we made Nb and NbN nano-SQUIDs with a reversible current-voltage curve and flux modulation depth above 60{\%} and 35{\%} respectively. The working field range and flux noise of the Nb nanoSQUID is 0.5 T and 0.34 $\mu \Phi $0$\surd $Hz.   

First-principles demonstration of superconductivity at 280 K in hydrogen sulfide with low phosphorus substitution

Recently BCS superconductivity at 190 K has been discovery in a highly compressed hydrogen sulfide[1,2]. We use first-principles calculations to systematically examine the effects of partially substituting the chalcogenide atoms on the superconductivity of hydrogen chalcogenides under high pressures2. We find detailed trends of how the critical temperature changes with increasing the V-, VI- or VII-substitution rate, which highlight the key roles played by low atomic mass and by strong covalent metallicity. In particular, a possible record high critical temperature of 280 K is predicted in a stable H3S0.925P0.075 with the Im3m structure under 250 GPa[3]. [1] A. P. Drozdov, M. I. Eremets, and I. A. Troyan, Conventional superconductivity at 190 K at high pressures, arXiv:1412.0460 (2014). [2] A. P. Drozdov, M. I. Eremets, I. A. Troyan, V. Ksenofontov, S. I. Shylin, Conventional superconductivity at 203 K at high pressures in the sulfur hydride system. Nature 525, 73-76 (2015) [3] Y. F. Ge, F. Zhang, and Y. G. Yao, First-principles demonstration of superconductivity at 280 K in hydrogen sulfide with low phosphorus substitution, Phys. Rev. B 93, 224513 (2016).   

Enhancement of Critical Current Density of Yttrium Barium Copper Oxide (YBCO) Thin Films by Introducing Nano dimensional Cerium Oxide Defects

In the application of high temperature superconductors (HTSC), the critical current density, Jc, is often the most important parameter in the design and engineering of practical devices. In this work we report the enhance the critical density of YBa2Cu3O7-x (YBCO) HTSC thin films by restraining the magnetic flux using self-assembled nano-structural defects. High density extended crystalline defects were introduced into c-axis oriented YBa2Cu3O7-x (YBCO) thin films, manufactured using pulsed laser deposition (PLD). These defects consist CeO2 of various densities determined by the number of laser pulses. The structural characterizations of YBCO/ CeO2 were carried out using x-ray diffraction (XRD) and scanning electron microscopy (SEM). Superconducting proprieties were measured using a vibrating sample magnetometer (VSM). The critical current density (Jc) of pure YBCO and CeO2 embedded YBCO films were calculated from magnetization (M) versus Field (H) loops using Bean's model. The critical current density shows significant enhancement and it is robust against applied field.   

Multiple nodeless superconducting gaps in noncentrosymmetric superconductor PbTaSe$_{2}$ with topological bulk nodal lines

Low-temperature thermal conductivity measurements were performed on single crystal of PbTaSe$_2$, a noncentrosymmetric superconductor with topological bulk nodal lines in its electronic band structure. It is found that the residual linear term $\kappa_0/T$ is negligible in zero magnetic field. Furthermore, the field dependence of $\kappa_0/T$ exhibits a clear ``$S$"-shape curve. These results suggest that PbTaSe$_2$ has multiple nodeless superconducting gaps. Therefore, the spin-triplet state with gap nodes does not play a significant role in this noncentrosymmetric superconductor with strong spin-orbital coupling. The fully gapped superconducting state also meets the requirement of a topological superconductor, if PbTaSe$_2$ is indeed the case. \bf Reference: M. X. Wang {\it et al.}, Phys. Rev. B 93, 020503(R) (2016)   

Electron-hole cuprates for a possible Bose condensation?

Bose condensation, by promoting quantum behavior from the microscopic to the macroscopic world, can produce some spectacular effects: like include superconductivity, superfluidity, and coherent matter waves. This work proposes to achieve the Bose condensation of bound electron-hole pairs (excitons) in a solid at a temperature that is high compared to other Bose condensates. The route we will explore to achieve a BEC of bound excitons utilizes a modulation-doped “infinite-layer” CuO2-based compound. Using a molecular-beam epitaxy we will grow bilayer (or trilayer) of SrCuO2/La2CuO4 thin films. Structural and electrical data will be discussed.   

Spin reorientation and 'quasi-nematic' order in the re-entrant tetragonal phase of Ba$_{0.76}$K$_{0.24}$Fe$_2$As$_2$

Clarifying the phase diagram and the nature of the competing or coexisting orders between magnetism, nematicity and superconductivity is of primary importance for the understanding of the mechanism of iron-based superconductors. Here, we investigate the re-entrant tetragonal phase in Ba$_{0.76}$K$_{0.24}$Fe$_{2}$As$_{2}$ in detail by DC magnetisation, resistivity, thermal expansion, thermal conductivity and thermo-electrical measurements. The thermal expansion indicates that the transition into the re-entrant phase is incomplete, but becomes more complete in high magnetic fields. The magnetization provides strong evidence that the spin alignment in the re-entrant C$_{4}$ phase is out-of-plane. The Nernst coefficient shows a large negative value in the stripe-type spin density wave (SDW) state owing to the Fermi surface reconstruction associated with nematic order. At the transition into the re-entrant C$_{4}$ tetragonal phase it hardly changes, suggesting that a similar ‘quasi-nematic’ electronic order could be present in this phase, in a novel form that preserves the tetragonal crystal symmetry. We propose a chequerboard type of charge or orbital ordering that replaces the nematic order of the stripe-type SDW phase below the C$_{4}$ re-entrant transition.   

Evolution of High-Temperature Superconductivity from a Low-Tc Phase Tuned by Carrier Concentration in FeSe Thin Flakes

We report the evolution of superconductivity in an FeSe thin flake with systematically regulated carrier concentrations by the liquid-gating technique. With electron doping tuned by the gate voltage, high-temperature superconductivity with an onset at 48 K can be achieved in an FeSe thin flake with Tc less than 10 K. This is the first time such high temperature superconductivity in FeSe is achieved without either an epitaxial interface or external pressure, and it definitely proves that the simple electron-doping process is able to induce high-temperature superconductivity with Tc as high as 48 K. Intriguingly, our data also indicate that the superconductivity is suddenly changed from a low-Tc phase to a high-Tc phase with a Lifshitz transition at a certain carrier concentration. These results help to build a unified picture to understand the high-temperature superconductivity among all FeSe-derived superconductors and shed light on the further pursuit of a higher Tc in these materials.   

Link between superconductivity and types of carriers in FeSe thin films

In iron-based superconductors, $\beta $-FeSe possesses the simplest tetragonal structure but attracts much attention due to its unusual properties. It exhibits a great boost of the superconducting transition temperature ($T_{\mathrm{c}})$ in the monolayer form, under high pressure, via ion/cluster intercalations and electric field gating. There is a common consensus that the enhancement of $T_{\mathrm{c}}$ is accompanied with the evolution of electronic structure of the Fermi surface, that is, associated with the types of charge carriers. Although qualitative ARPES results have shown that the electron-like carriers play a key role in promoting the $T_{\mathrm{c}}$, hitherto, a quantitative link between the carrier nature and the superconductivity has not been clarified. In this work, with our successful synthesis of a series of high quality $\beta $-FeSe thin films of tunable $T_{\mathrm{c}}^{\mathrm{0}}$\textbf{`}s from 2 K to 14 K, we find by systematic transport measurements that the holes and electrons coexist in all the samples. While the concentration of electron-type carriers increases monotonically by about 6 times, the hole carrier density roughly holds a constant value. It implies an intimate relation between the electron carriers and the superconductivity. Moreover, our analysis on the monolayer FeSe samples of $T_{\mathrm{c}}$ \textasciitilde 40 K points to a vanishing hole pocket. Our results thereby unveil that the $T_{\mathrm{c}}$ enhancement in FeSe is related to the increase in the electron density, which becomes more pronounced upon a sudden decrease in the hole density.   

Superconducting fluctuation effect in CaFe$_{0.88}$Co$_{0.12}$AsF

Out-of-plane angular dependent torque measurements were performed on CaFe$_{0.88}$Co$_{0.12}$AsF single crystals. Superconducting fluctuations, featured by magnetic field enhanced and exponential temperature dependent diamagnetism, are observed above the superconducting transition temperature $T_c$, which is similar to that of cuprate superconductors, but less pronounced. In addition, the ratio of $T_c$ versus superfluid density follows well the Uemura line of high-$T_c$ cuprates, which suggests the exotic nature of the superconductivity in CaFe$_{0.88}$Co$_{0.12}$AsF   

Hund's induced Fermi-liquid instabilities and enhanced quasiparticle interactions

Hund's coupling is shown to generally favor, in a doped half-filled Mott insulator, an increase in the compressibility culminating in a Fermi-liquid instability towards phase separation. The largest effect is found about the frontier between an ordinary and an orbitally-decoupled ("Hund's") metal. The increased compressibility implies an enhancement of quasiparticle scattering, thus favoring other possible symmetry breakings. This physics is shown to happen in simulations of the Fe-based superconductors (BaFe$_2$As$_2$, FeSe, FeSe monolayer), possibly implying the relevance of this mechanism in the enhancement of the the critical temperature for superconductivity. L. de' Medici, ArXiv:1609.01303, P. Villar Arribi and L. de' Medici, unpublished.   

A Mott insulator continuously connected to iron pnictide superconductors

Whether an actual Mott insulator can be realized in the phase diagram of the iron pnictides remains an open question. Here we use transport, transmission electron microscopy, X-ray absorption spectroscopy, and neutron scattering to demonstrate that NaFe$_{1-x}$Cu$_x$As near $x\approx 0.5$ exhibits real space Fe and Cu ordering, and are antiferromagnetic insulators with the insulating behavior persisting above the N\'eel temperature, indicative of a Mott insulator. Upon decreasing $x$ from $0.5$, the antiferromagnetic ordered moment continuously decreases, yielding to superconductivity around $x=0.05$. Our discovery of a Mott insulating state in NaFe$_{1-x}$Cu$_x$As thus makes it the only known Fe-based material in which superconductivity can be smoothly connected to the Mott insulating state, highlighting the important role of electron correlations in the high-$T_{\rm c}$ superconductivity.   

Interplay of Superconductivity and Spin Density Wave - Magnetism in PrFeAsO and Hole-doped (Pr $_{\mathrm{1-x\thinspace }}$Sr $_{\mathrm{x}})$ FeAsO: Synthesis, Structure, Thermodynamic, Magnetic, Transport, Phonon properties and Pressure effect

Hole doping in iron-pnictide (1111) PrFeAsO by substituting Pr$^{\mathrm{3+}}$ by Sr$^{\mathrm{2+}}$ creates superconducting Pr$_{\mathrm{1-}}_{x}$Sr$_{x}$FeAsO tetragonal $P_{\mathrm{4/nmm}}$ phases at room temperature. Sr doping facilitate hole transfer through Pr/Sr plane {\&} FeAs layers. Hall-effect measurements at different magnetic field {\&} temperature ($+$Ve R$_{\mathrm{H}})$ confirms hole like charge carriers. Lattice constants ($a ${\&} $c)$ increase monotonously with Sr/hole concentration. Tc (SC) varies from 12.5 K to 15.5K with a maximum of 15.5K at $x =$ 0.22 with (optimal doping) largest SC volume fraction. Temperature (1.7 K \textasciitilde 300K) {\&} Magnetic field (1T\textasciitilde 7T)-dependent resistivity, magnetic susceptibility {\&} specific heat been measured {\&} calculated Cp/T (J/mole-K2) {\&} entropy (J/mole-K). Thermoelectric power $S$ (T) of PrFeAsO have W-like shape {\&} smaller amplitude with much larger spread. $p$-drop (\textasciitilde 150 K) has been identified with SDW/ lattice instability. Coexistence of SC {\&} SDW behavior were observed {\&} pressure effects on both being investigated by resistivity measurements under hydrostatic pressure up to 1.8GPa using piston-cylinder clamp cell device {\&} compared with electron doped Sm(O$_{\mathrm{1\thinspace -\thinspace x}}$ F$_{\mathrm{x}})$ FeAs. T$_{\mathrm{c}}$ increases ($+$ dT$_{\mathrm{c}}$/dP) with pressure for under-doped (Pr $_{\mathrm{1-x}}$ Sr $_{\mathrm{x}})$FeAsO similar to high-T$_{\mathrm{c}}$ cuprates {\&} -Ve pressure effect on SDW temperature. The results suggest a symmetry between electron {\&} hole-doping Fe-pnictide superconductors.   

Observation of a new possible superconducting state and anomalous insulating state in surface K-dosed (Li$_{\mathrm{\mathbf{1-x}}}$\textbf{Fe}$_{\mathrm{\mathbf{x}}}$\textbf{OH)FeSe}

By using scanning tunneling microscopy/spectroscopy, we studied the evolution of electronic structure and superconductivity of (Li$_{\mathrm{\mathbf{1-x}}}$\textbf{Fe}$_{\mathrm{\mathbf{x}}}$\textbf{OH)FeSe via surface potassium (K) dosing. We found that the }$\Gamma $\textbf{-centered electron band, which was 70meV above Fermi level (}\textbf{\textit{E}}$_{\mathbf{F}}$\textbf{), can be tuned to cross }\textbf{\textit{E}}$_{\mathbf{F}}$\textbf{ by K dosing, and contributes a new electron pocket at }$\Gamma $\textbf{ When such Lifshitz transition happens, the superconducting gap on M-centered electron pocket is slightly suppressed. With further K dosing, a new superconducting-like gap gradually opens on the }$\Gamma $\textbf{ electron pocket, and forms a dome like doping behavior. After that, the system eventually evolves into an insulating phase with gradually depleted density of states near E}$_{\mathrm{\mathbf{F}}}$\textbf{. Our results provide more detailed phase diagram of FeSe in the deep electron doping region. The novel Fermi surface with electron pockets at both }$\Gamma $\textbf{ and M points will provide more clues to understand superconductivity of Fe-based superconductors.}   

Anomalous correlation effects and unique phase diagram of electron-doped FeSe revealed by photoemission spectroscopy

FeSe layer-based superconductors exhibit exotic and distinctive properties. The undoped FeSe shows nematicity and superconductivity, while the heavily electron-doped K$_{\mathrm{x}}$Fe$_{\mathrm{2-y}}$Se$_{\mathrm{2}}$ and single-layer FeSe/SrTiO$_{\mathrm{3}}$ possess high superconducting transition temperatures. However, a comprehensive study on the doping dependence of an FeSe layer-based superconductor is still lacking. Through angle-resolved photoemission spectroscopy studies on K-dosed thick FeSe ?lms and FeSe$_{\mathrm{0.93}}$S$_{\mathrm{0.07}}$ bulk crystals, here we reveal the internal connections between these two types of FeSe-based superconductors, and obtain superconductivity below \textasciitilde 46 K in an FeSe layer under electron doping without interfacial effects. Moreover, we discover an exotic phase diagram of FeSe with electron doping, including a nematic phase, a superconducting dome, a correlation-driven insulating phase and a metallic phase. Such an anomalous phase diagram unveils the remarkable complexity, and highlights the importance of correlations in FeSe layer-based superconductors.   

Electrical resistivity of single crystals of CaFeAsO under applied pressure

Flouroarsenide CaFeAsF is a 1111-type of iron-based superconductors parent, similar to the LaFeAsO parent, but being oxygen-free. Various studies to date on the pure and doped CaFeAsF compounds have been conducted on polycrystalline samples. We have carried out high pressure electrical resistivity measurements on single crystals of CaFeAsF parent for the first time. It is observed that the insulating state above the structure transition temperatures is transformed into a metallic under pressures up to \textasciitilde 5 GPa. Furthermore, we found a pressure-induced superconductivity with zero resistivity in CaFeAsF under pressures above 15 GPa. Surprisingly, this pressure-temperature phase diagram of the CaFeAsF single crystals is in contrast with that of LaFeAsO single crystals reported previously where no superconductivity emerges under pressures up to 37 GPa..   

Imaging of doped iron pnictides across a structural phase transition

The emergent anisotropy through a structural-phase transition in an iron pnictide single crystal of Ba(Fe$_{0.987}$Au$_{0.012})_{2}$As$_{2}$ was studied using polarized laser light microscopy. The undoped parent of BaFe$_{2}$As$_{2}$'s crystal structure distorts from tetragonal to orthorhombic at the structural phase transition temperature $T_{S}$, which coincides with an antiferromagnetic transition, causing the formation of structural domains that can be observed as stripes across the sample. For Ba(Fe$_{0.987}$Au$_{0.012})_{2}$As$_{2}$, however, $T_{S}=$108 K and the Neel temperature $T_{N}=$100 K. We studied the disappearance of these domains as the sample was heated across these transitions. Images of the sample were taken using a defocused laser beam through fully crossed polarizers. The images were analyzed by aligning and averaging groups of images to reduce noise, by taking the difference of the images above and below $T_{S}$ to isolate the stripes from the background, and by using Fourier transformations and comparing them to those of simulated striped patterns.   

Energy Gap Measurements of K-Doped Iron Pnictide Superconductor Ba(1-x)KxFe2As2 using Point Contact Spectroscopy

We present results of low-temperature conductance measurements of the energy gap of K-doped iron pnictide Ba(1-x)KxFe2As2 superconductors, where x $=$ 0.6, 0.33. Multi-gap superconductors such as iron pnictides can exhibit multiple energy gaps depending on the crystal growth conditions. These energy gaps are often anisotropic relative to the crystal lattice, with some gaps primarily conducting parallel or perpendicular to the c-axis of the lattice. We discuss how undergraduate students developed the laboratory infrastructure consisting of a 2 Kelvin cryocooler and a Helium-3 cryostat, prepared the samples from single crystals, and use point contact spectroscopy (PCS) to measure the energy gaps. We present details of how our measurements are influenced by ``fritting'' which is a technique of tuning the point contact region through current impulses. We present our results that show indications of multiple gaps and compare these to those of other research groups.   

Fully gapped superconducting state in Au2Pb: a natural candidate for topological superconductor

We measured the ultra-low-temperature specific heat and thermal conductivity of Au$_2$Pb single crystal, a possible three-dimensional Dirac semimetal with a superconducting transition temperature $T_c \approx$ 1.05 K. The electronic specific heat can be fitted by a two-band $s$-wave model, which gives the gap amplitudes $\Delta_1$(0)/$k_BT_c$ = 1.41 and $\Delta_2$(0)/$k_BT_c$ = 5.25. From the thermal conductivity measurements, a negligible residual linear term $\kappa_0/T$ in zero field and a slow field dependence of $\kappa_0/T$ at low field are obtained. These results suggest that Au$_2$Pb has a fully gapped superconducting state in the bulk, which is a necessary condition for topological superconductors if Au$_2$Pb is indeed one.   

Effects of Domain Walls in Quantum Anomalous Hall Insulator/Superconductor Heterostructures

Half quantized conductance plateaus (HQCPs) of $\frac{e^2}{2h}$ have been observed recently in quantum anomalous Hall insulator(QAHI) /superconductor(SC) heterostructures. A theoretical work predicted that such half quantized plateaus could appear due to the Majorana chiral mode in the SC region when the normal QAHI regions support chiral electron modes and thus has quantized Hall conductance, $\sigma_{xy} = \frac{e^2}{h}$. However, experimentally the HQCP happens when the Hall conductance has a non-quantized value, $\sigma_{xy} \approx 0.8\frac{e^2}{h}$. In this presentation, we attribute this non-quantized $\sigma_{xy}$ to additional channels on domain walls and support the claim that the HQCPs are due to Majorana chiral mode.   

Enhancing Superconductivity in Hyperbolic Metamaterials

Recent experiments have demonstrated the viability of the metamaterial approach allowing for the enhancement of the critical temperature of a superconductor [1, 2]. Dielectric response engineering was used to enhance electron-electron interaction in core-shell epsilon near zero (ENZ) metamaterials. We have also demonstrated that an aluminum/aluminum oxide hyperbolic metamaterial geometry (aluminum/aluminum oxide multilayers) is capable of Tc enhancement [1]. This approach has superior transport and magnetic properties compared to core-shell metamaterial superconductors [2]. Here we report the Tc enhancement in tin/dielectric multilayers. Reflectivity measurements were made to confirm the hyperbolic character of fabricated metamaterial. Effect of metamaterial cell size on Tc will be discussed. [1]. Vera N. Smolyaninova et.al, Scientific Reports 6, 34140 (2016) [2]. Vera N. Smolyaninova et.al, Scientific Reports 5, 15777 (2015).   

Tuning the phase mixture state of superconducting BaPb1-xBixO3 by using heteroepitaxial strain

The crystal structure of high-$T_{\mathrm{c}}$ superconductor BaPb1-xBixO3(BPBO,$T_{\mathrm{c}}=$12 K) is associated with a doping rate $x$ and temperature $T$. At low $T$, the structure of BPBO tends to orthorhombic and an orthorhombic to tetragonal transition occurs as $T$ is increased. However, these phases coexist in a wide range of $x$ due to the first-order nature of this phase transition. Therefore, the phase mixture state of BPBO does not simply depend on the $x$ and $T$. Since superconductivity of BPBO only occurs in tetragonal phase, engineering the phase mixture state of BPBO would be critical for enhancing $T_{\mathrm{c\thinspace }}$. Here, we demonstrate that strain engineering of a BPBO thin film could be useful for controlling the volume ratio of tetragonal and orthorhombic phases. We fabricated and optimized the growth conditions of BPBO films by pulsed laser deposition technique. Also we characterized the resultant crystal structure and surface morphology using X-ray diffraction and atomic force microscopy. By utilizing buffer layer technique, we could impose proper biaxial strain to BPBO films, as revealed by X-ray diffraction. In the presentation, we will discuss the relationship between superconductivity and crystal structure in more detail.   

Superconductivity in NbSe$_{2\, }$intercalated with indium atoms.

NbSe$_{2}$ compound crystallizes in MoS$_{2}$ prototype structure and it is a very well knows superconductor, which exhibits coexistence between superconductivity and CDW instability. High quality samples of this material display maximum superconducting critical temperature close to 7.2 K and CDW transition close to 33.0 K. In this work, we present results that shown NbS$_{2}$ prototype stabilization structure when NbSe$_{2}$ is intercalated with indium atoms. These intercalations obey the In$_{x}$NbSe$_{2}$ stoichiometry with indium intercalation into 0.025 $\le $ x $\le $ 0.08 range. In this range of the composition the critical temperature reach the maximum close to 7.5 K on the In$_{0.035}$NbSe$_{2}$ stoichiometry, revealed by resistivity, magnetization and heat capacity measurements. On the resistivity measurements, we did not find any evidence of CDW instability in all range of temperature.   

ynthesis and superconducting proprieties on the Nb-Co-B system

Since the discovery of the superconductivity in MgB$_{\mathrm{2}}$ a renew interest in borides systems have increased in the last years. The interesting in the boride systems is how rare is the superconductivity occurrence. Many MRB$_{\mathrm{2}}$ (MR - Refractory metal) crystallize in the same prototype structure than MgB$_{\mathrm{2}}$ type AlB$_{\mathrm{2}}$. However just NbB$_{\mathrm{2}}$ is a known superconductor material with superconducting critical temperature close to 3.9 K. So in this work we shall show results which suggest that small substitution of Nb for Co on the Nb$_{\mathrm{1-x}}$Co$_{\mathrm{x}}$B$_{\mathrm{2}}$ stoichiometry nominal increase superconducting critical temperature from 3.9 K without Co to 5 K in the Nb$_{\mathrm{0.95}}$Co$_{\mathrm{0.05}}$B$_{\mathrm{2}}$ nominal composition. These results are sustained by magnetization, resistivity and heat capacity measurements.   

Superconductivity in a new ternary compound of the Ta-Zr-B system with FeB prototype structure

Recently was published the discovery of superconductivity in Ta$_{\mathrm{1-x}}$Hf$_{\mathrm{x}}$B which presents maximum T$_{\mathrm{c}}$ close to 6.7 K on the Ta$_{\mathrm{0.7}}$Hf$_{\mathrm{0.3}}$B nominal composition. This material display strongly signature of a new multiband compound. Within this scenario in this work we shall show a systematic study in the Ta$_{\mathrm{1-x}}$Zr$_{\mathrm{x}}$B series of the compounds. The results sustained by X-ray diffraction, resistivity, magnetization and heat capacity measurements suggest that all series crystallize in FeB prototype structure with maximum superconducting critical temperature close to 6.0 K for Ta$_{\mathrm{0.8}}$Zr$_{\mathrm{0.2}}$B nominal composition.   

Induced Superconductivity by Vanadium Substitution in the Diboride HfB$_{\mathrm{2}}$

The diboride HfB$_{\mathrm{2}}$ is a non-superconducting diamagnetic compound that crystallizes in the AlB$_{\mathrm{2}}$ prototype crystal structure. The most well known compound in the AlB$_{\mathrm{2}}$ structure is the multiband superconductor MgB$_{\mathrm{2}}$ with a critical temperature close to 39K. In this work we present the systematic substitution of hafnium for vanadium in Hf$_{\mathrm{(1-x)}}$V$_{\mathrm{x}}$B$_{\mathrm{2}}$ and report induced superconductivity with a critical temperature up to 8.4K. The superconductivity characterization by magnetization, resistivity, and heat capacity measurements are presented.   

Intergrowth of two layered buliding blocks to search for possible superconductivity

Layered compounds are promising platforms to realize novel superconductivity, exemplified by spin-triplet one in Sr$_2$RuO$_4$ and high-temperature one in cuprates and iron pnictides. Additionally, reduced dimensionality is conducive to the formation of spin- or charge-density wave order. Considering the similar phase diagrams observed in several different superconductors, it is rational to search for superconductivity by tuning an ordered phase in a layered system. LaTe$_3$ has a quasi-two dimensional structure comprising a square lattice of Te and exhibiting charge-density wave order well above room temperature. Superconductivity can be realized in the Te-square lattice by tuning its electronic structure. We try intergrowing LaTe$_3$ with other superconducting or magnetic layers to search for novel superconductivity. However, most of the cases are not successful due to lattice mismatch or chemical incompatibility between the two layers. We will present the structure and basic physical properties of the successful one with a quaternary composition K-La-Mn-Te.   

Estimating the intrinsic magnetic susceptibility in the normal state of LaO$_{0.5}$F$_{0.5}$BiS$_2$

We present magnetization and magnetic susceptbility data in LaO$_{0.5}$F$_{0.5}$BiS$_2$from 2.7 to 250 K tempratures and -4 to +4~T fields. The signal in very low-fields (10~Oe or less) is small and hard to measure accurately above the superconducting transition temperature ($T_c\sim$3~K). At higher fields, we observe that the high-temperature susceptibility is dominated by a paramagnetic component. Independent chemical analysis suggests that this component might come from Ce impurities contained in the starting materials. By analyzing the behavior of the magnetization as a function of field and temperature under the assumption of impurity contributions, we make an estimate of the intrinsic value of the susceptibility in the normal state of the material.   

Anomalous In-plane Magnetoresistance of Electron-doped Cuprate La$_{\mathrm{2-}}_{x}$Ce$_{x}$CuO$_{\mathrm{4-}}_{\delta }$

We report the systematic in-plane magnetoresistance (MR) measurements on electron-doped cuprate La$_{\mathrm{2-}}_{x}$Ce$_{x}$CuO$_{\mathrm{4-}}_{\delta }$ thin films as a function of doping and oxygen contents in the magnetic field up to 14 T. A crossover from negative magnetoresistance ($n$-MR) to positive magnetoresistance ($p$-MR) occurs between $x=$ 0.07 and 0.08, corresponding to the boundary of long-range antiferromagnetic order determined by $\mu $SR [Saadaoui, H.\textit{ et al.} \textit{Nature Commun.} \textbf{6}, 6041 (2015)]. With increasing the doping level, the $p$-MR effect becomes weaker and is hardly discernable at $x=$ 0.15, in accordance with the boundary of two-dimensional antiferromagnetic order [K. Jin, \textit{et al}. \textit{Phys. Rev. B.} 80, 012501 (2009), \textit{Nature }476,73(2011)]. At $x=$ 0.15, the as-grown samples show $n$-MR, whereas the optimally annealed ones display $p$-MR, similar to that in NCCO [J. Wu \textit{et al}. \textit{Phys. Rev. Lett.} 73, 1291(1994)] and PCCO [J. Higgins \textit{et al}. \textit{Phys. Rev. B.} 73,104510(2006)]. We also find linear MR at $x=$ 0.06 and $x=$ 0.1, and the plausible origins will be discussed.   

Quasiclassical theory for the flux-flow Hall effect in a type-II superconductor

After the sign change of the Hall conductivity have been observed in some high-$T_{\rm c}$ superconductors, intensive studies have been performed on the flux-flow Hall effect in type-II superconductors theoretically and experimentally. Despite these efforts, a microscopic understanding of the anomalous flux-flow Hall effect is still missing. This may be because the Lorentz force is missing from the standard Eilenberger equations. Recently, the Lorentz force has been incorporated successfully in a gauge-invariant manner within the real-time Keldysh formalism. The augmented quasiclassical equations in the Keldysh formalism have been used to calculate flux-flow Hall conductivity numerically for the s-wave pairing on an isotropic Fermi surface. However, the temperature dependence of the Hall conductivity have not been calculated. We calculate the temperature dependence of the ohmic and Hall resistivities induced by a motion of an isolated vortex by transforming the energy variable of the augmented quasiclassical equations in Keldysh formalism into the Matsubara energy on the imaginary axis. It is shown that linear responses can be calculated much more easily compared to the aproach based on the augmented quasiclassical equations in the Keldysh formalism.   

Characterization of Phase-Slip Centers created in superconducting \textbf{Nb}$_{\mathrm{\mathbf{x}}}$\textbf{Ti}$_{\mathrm{\mathbf{1-x}}}$\textbf{N thin films close to T}$_{\mathrm{\mathbf{c}}}$

The dissipative states induced by an over-critical (pair-breaking) current in superconducting Nb$_{\mathrm{x}}$Ti$_{\mathrm{1-x}}$N strips were investigated and characterized in the vicinity of the critical temperature T$_{\mathrm{c}}$ (\textasciitilde 8.7 K). The suppression of superconductivity then occurs locally and leads to the creation of a phase-slip center (PSC). In the case where the over-critical current is applied as a step pulse, the PSC voltage rise is preceded by a nucleation time t$_{\mathrm{d}}$ which can be analyzed through a Time-Dependent Ginzburg-Landau theory due to Tinkham. In conformity with previous work, we interpret the effective gap relaxation time of the theory as the film cooling time. By consideration of the respective weights of the electron and phonon specific heats, the phonon escape time can be derived from experiments. It is here found to be 1.8 ns for a 20 nm NbTiN film sputtered on polished crystalline Al$_{\mathrm{2}}$O$_{\mathrm{3\thinspace }}$   

Electronic structure of FeS superconductor.

A new iron-based superconductor FeS has been discovered recently. Here we report its electronic structure by performing high-resolution angle-resolved photoemission spectroscopy measurement on FeS single crystal. It contains two hole-like and two electron-like bands around the Brillouin zone center and corner, respectively, near Fermi energy. However, the other hole-like band around the zone center observed in other iron-based compounds is missing. Moreover, these four bands exhibit moderate kz dispersion with quasi-two-dimensional property. Compared between the band structures calculated by theory and obtained from our experiment, the renormalization factor is about 4, indicating strong electronic coupling. By studying the phase diagram of FeSe compounds with Te and S isovalent dopants, we revealed that the superconducting transition temperature should have intimate relation with the intensity of electronic correlation and Fermi-surface topology for the high temperature superconductors. Our results would put strong constrain on the theoretical calculations.   

Robust Upward Dispersion of the Neutron Spin Resonance in the Heavy Fermion Superconductor Ce$_{1-x}$Yb$_{x}$CoIn$_5$

The neutron spin resonance is a collective magnetic excitation that appears in copper oxide, iron pnictide, and heavy fermion unconventional superconductors. Although the resonance is commonly associated with a spin-exciton due to the $d$($s^{\pm}$)-wave symmetry of the superconducting order parameter, it has also been proposed to be a magnon-like excitation appearing in the superconducting state. Here we use inelastic neutron scattering to demonstrate that the resonance in the heavy fermion superconductor Ce$_{1-x}$Yb$_{x}$CoIn$_5$ with $x=0,0.05,0.3$ has a ring-like upward dispersion that is robust against Yb-doping. By comparing our experimental data with random phase approximation calculation using the electronic structure and the momentum dependence of the $d_{x^2-y^2}$-wave superconducting gap determined from scanning tunneling microscopy for CeCoIn$_5$, we conclude the robust upward dispersing resonance mode in Ce$_{1-x}$Yb$_{x}$CoIn$_5$ is inconsistent with the downward dispersion predicted within the spin-exciton scenario.   

Anisotropic electron-phonon coupling in the spinel oxide superconductor

Anisotropic electron-phonon coupling has been widely observed in various quasi-two dimensional superconductors like perovskite copper oxides, 2H-NbSe2 and MgB2, which exhibit unexpected critical superconducting transition temperature (Tc). Hitherto, LiTi2O4 with a remarkable Tc up to 13 K is the only known oxide superconductor in spinels, the origin of which remains obscured mainly due to the lack of high-quality single crystals. By probing tunneling spectra of single crystalline thin films in different orientations, an intrinsic anisotropic electron-phonon coupling is firstly observed in such a cubic system as well, thereby giving insight to the issue of key ingredients in understanding the novel superconducting systems.   

Ballistic graphene Josephson junctions from the short to the long regime

We explore the critical current ($I_C$) temperature scaling of ballistic Josephson junctions. Using encapsulated graphene/boron-nitride heterostructure devices, we vary device length from the short to the long junction regime. We extract the carrier-density-independent energy $\delta$E by calculating the ballistic cavity level spacing through the Fabry-Perot oscillations of the normal resistance. In the long and intermediate junction regimes, we find $I_C$ scales as exp(-$k_B$T/ $\delta$E) at higher temperatures. For short junctions, we find strong agreement with theoretically predicted $I_C$ behavior. In the zero temperature limit, $I_C$ of a long (short) junction saturates at a magnitude determined only by the product of $\delta$E ($\Delta$) and the number of transversal modes in the junction.   

A New Parametrized Description of Magnetic Josephson Junctions

We present a study of diffusive Josephson junctions made of two superconductors connected by a ferromagnetic film. When the link is sufficiently thin Cooper pairs can tunnel from one superconductor to the other, generating a Josephson current. Junctions with inhomogeneous magnetization are of particular interest because they display long range triplet pair correlations. The generation of these correlations in the hybrid structure is studied by solving numerically the Usadel equations. In previous work [1] we have shown for example that the rotation of the magnetization can be used to tune the relative weight of the singlet and triplet pair correlations, thereby tuning the current. Here, we present a novel approach to the numerical treatment of the Usadel equations, using a parametrization that takes automatically into account a required constraint of the model. We show results for the weak magnetization case. We also discuss new challenges posed in the high magnetization regime, where the parameterization is accompanied by the appearance of ‘moving singularities’. Various methods are proposed in order to address this technical issue. [1 ] T.E. Baker, A. Richie-Halford, A. Bill, New J. Phys. 16, 093048 (2014); Europhys. Lett., 107, 17001 (2014); Phys. Rev. B 94, 104518 (2016).   

Coexistence of type II Dirac transport and anisotropic superconductivity in layered material PdTe$_{\mathrm{2}}$ and improved possible topological superconductor

We demonstrate the coexistence of the type-II featured tilted Dirac cones and anisotropic superconductivity in a layered crystal PdTe$_{\mathrm{2}}$ by the combined studies using the low-temperature transport, de Haas-van Alphen oscillations and the first-principles calculations. The superconductivity appears at 1.9K and the upper critical field when Ixc is 3 times larger than that when I-c. Six conductive pockets are identified in the dHvA measurements, where one mode with frequency of 8.0T exhibits the nontrivial Berry phase. Detailed band structure of PdTe$_{\mathrm{2}}$ was analyzed by theory calculation which is consistent with the dHvA measurements and feature of the type-II Dirac fermions was confirmed. We consider type II Dirac semimetals are the optimized materials with topological superconductivity transport and PdTe$_{\mathrm{2}}$ is an improved platform to produce the possible topological superconductor.   

Cooper pair transport in 1D Josephson chains in the regime $E_C \ll E_J \approx T$.

We investigated the current-voltage characteristics (IVC) of one-dimensional arrays of SQUIDs in an unusual regime of very small Josephson energies, $E_J \approx 100-500mK$, and even smaller charging energies, $E_C \approx 10mK$. The $E_C$ values were realized by shunting the $Al-Al_2O_3-Al$ Josephson junctions with a large capacitance to the ground. The zero-bias resistance of the 1D chains is dominated by the quasiparticle transport at $T>0.2K$; below this temperature, only the Cooper pair transport is observed. We have measured the current $I_S$ corresponding to the switching between the low-voltage ``phase-diffusion'' branch of the IVC and the high-voltage branch ($eV \approx n \cdot 2 \Delta $, where $n$ is the number of SQUIDs in the chain, and $\Delta $ is the superconducting gap). At $E_J=100mK$ and $T=50mK$, the extraordinary small values of $I_S \approx 10^{-13} A$ are 1000 times smaller than the Ambegaokar-Baratoff critical current $I_C$. The IVC remains hysteretic in this regime, which indicates that dissipation in the chain is weak. The mechanisms of dissipation will be discussed.   

Studying the superconductor-ferromagnet proximity effect with polarised neutron reflectometry

At the interface between a superconductor (S) and ferromagnet (F), an inhomogeneity can convert singlet Cooper pairs into the (spin aligned) long ranged triplet component (LRTC). The manipulation of the LRTC forms the basis of the emerging field of super-spintronics. Several theoretical works predict modification to the local magnetic state inside the S layer with the inclusion of triplet Cooper pairs, however there are now several experimental observations which disagree on both the magnitude and direction of this induced moment (see for example \footnote{M. G. Flokstra, \textit{et al.}, \textbf{Nat. Phys.} doi:10.1038/nphys3486 (2015)} and \footnote{Di Bernardo, \textit{et al.}, \textbf{Physical Review X}, 5(4), 1-7. doi:10.1103/PhysRevX.5.041021 (2015)}). Here we report on measurements of the proximity effect using polarised neutron reflectometry, a technique sensitive to changes in the total magnetisation of a S-F heterostructure. Our results suggest that a `smoking gun' direct signature of the LRTC is below the sensitivity of our technique, we are able to study the inverse effect namely a modification to the ferromagnetism by proximity to singlet superconductivity. These observations are supported by XMCD measurements showing changes to the Fe and Co below the S layer T$_c$.   

Andreev reflection at a graphene-superconductor interface in the quantum Hall regime.

At metal-superconductor interfaces Andreev processes occur where an electron tunneling into the superconductor carries with it a second electron, effectively reflecting a hole with opposite momentum back into the metal. This is due to the superconducting gap, which, at low energies, only allows the formation of cooper pairs inside the superconductor, representing an accessible way to measure Cooper-pair tunneling phenomena. An important requirement for strong Andreev processes is a clean interface with a high transmission probability. Graphene is a promising candidate for achieving an extremely clean interface to superconductors, however recent results show achieving a transparent interface is non-trivial. Graphene also has a remarkably large mean free path, which allows accurate measurement of reflected and transmitted currents. In the quantum hall regime, chiral edge states open new possibilities to measure novel Andreev processes. In this work, we use controlled assembly in an inert atmosphere to create high-quality grapheme-superconductor interface. Due to the high critical field of these superconductors, we are able to reach the quantum hall regime in graphene while preserving superconductivity, we will describe the resultant Andreev processes observed at such interfaces.   

Andreev reflection at a graphene-high-temperature superconductor interface in the quantum Hall regime.

At metal-superconductor interfaces Andreev processes occur where an electron tunneling into the superconductor carries with it a second electron, effectively reflecting a hole with opposite momentum back into the metal. This is due to the superconducting gap, which, at low energies, only allows the formation of cooper pairs inside the superconductor, representing an accessible way to measure Cooper-pair tunneling phenomena. An important requirement for strong Andreev processes is a clean interface with a high transmission probability. Graphene is a promising candidate for achieving an extremely clean interface to superconductors, however recent results show achieving a transparent interface is non-trivial. In the quantum hall regime, chiral edge states open new possibilities to measure novel Andreev processes. In this work, we use controlled assembly in a well-controlled inert atmosphere to create high-quality interfaces between monolayer and bilayer graphene and high-temperature superconductors. Due to the high critical field of these superconductors, we are able to reach the quantum hall state in the graphene layer while preserving superconductivity, and we describe the resultant Andreev processes observed at such interface.   

Gyroidal Mesoporous Niobium Nitride Superconductors from Block Copolymer Self-Assembly

Superconductors with mesoscale ordering and porosity are expected to have very different properties from their bulk counterparts. The exploration of these properties has been limited, however, by the lack of tunable, versatile, and robust wet-chemical synthesis methodologies to mesostructured superconductors. We report the synthesis of gyroidal NbN superconductors from gyroidal block copolymer self-assembly-derived niobium oxide. The resulting materials have a Tc of about 7.8 K, a critical current density of 440 A cm$^{-2}$ at 100 Oe and 2.5 K, and a mesoscale lattice with the I4$_1$32 (alternating gyroid) structure with d$_{100}$ spacings between 27 and 36 nm. We will discuss recent efforts to improve the superconducting properties of these materials and to expand block copolymer-inorganic hybrid co-assembly to be a scalable, tunable platform for exploration of the impacts of mesoscale order and porosity on superconducting properties. \\ Reference \\ S. W. Robbins, P. A. Beaucage, H. Sai, K. W. Tan, J. P. Sethna, F. J. DiSalvo, S. M. Gruner, R. B. van Dover, U. Wiesner, \textit{Block copolymer self-assembly directed synthesis of mesoporous gyroidal superconductors}, Sci. Adv. \textbf{2} (2016), e1501119.   

Correlation between superconductivity and bond angle of CrAs chain in non-centrosymmetric compounds A2Cr3As3 (A$=$K, Rb)

Non-centrosymmetric superconductors have recently received special attentions due to the expectation of unconventional pairings and exotic physics associated with such pairings. The superconductors A2Cr3As3 (A$=$K, Rb) belongs to such kind of superconductor. In this study, we are the first to report the finding that the superconductivity of A2Cr3As3 (A$=$K, Rb) has a positive correlation with the extent of non-centrosymmetry. Our in-situ high pressure ac susceptibility and synchrotron x-ray diffraction measurements reveal that the larger bond angle of As-Cr-As in the CrAs chains can be taken as a key factor controlling superconductivity. While the smaller bond angle and the distance between the CrAs chains also affect the superconductivity due to their structural connections with the larger angle. We find that the value difference between large and small angle is in favor of superconductivity. These results are expected to shed a new light on the underlying mechanism of the superconductivity in these Q1D superconductors and also to provide new perspective in understanding other non-centrosymmetric superconductors.   

West Coast Swing Dancing as a Driven Harmonic Oscillator Model

The study of physics in sports not only provides valuable insight for improved athletic performance and injury prevention, but offers undergraduate students an opportunity to engage in both short- and long-term research efforts. In this project, conducted by two non-physics majors, we hypothesized that a driven harmonic oscillator model can be used to better understand the interaction between two west coast swing dancers since the “stiffness” of the physical connection between dance partners is a known factor in the dynamics of the dance. The hypothesis was tested by video analysis of two dancers performing a west coast swing basic, the sugar push, while changing the stiffness of the physical connection. The difference in stiffness of the connection from the ideal was estimated by the leader; the position with time data from the video was used to measure changes in the amplitude and phase difference between the leader and follower. While several aspects of our results agree with the proposed model, some key characteristics do not, possibly due to the follower relying on visual leads.   

Reversible spin transfer torque switching of uncompensated non-collinear antiferromagnet

New spin-transfer torque approach using uncompensated noncollinear antiferromagnets are investigated by micromagnetic simulations using the generalized Landau-Lifshitz-Gilbert equation. The adventages of antiferromagnetic spintronics include low stray field, insensitive to disturbing external magnetic field, intrinsic high frequency and fast dynamics.[1] Spin transfer torque can efficiently switch or rotate the magnetizations of antiferromagnetic free layer[2, 3] however the switching is not always reversible for collinear antiferromagnet when there is no suitable magnetic anisotropic potential. Slightly uncompensated noncollinear antiferroamgnet has almost cancel magnetization however the residue magnetization and non-collinear nature could make the switching reversible and controllable as efficiently as the ferromagnetic case.   

Surface and interface trapping of low energy positrons at a graphene / Cu interface measured using depth resolved Doppler Broadening Spectroscopy (DBS)

We report Doppler broadening spectroscopy (DBS) measurements on 6-8 layers graphene on polycrystalline Cu substrate and on polycrystalline Cu using a variable energy (\textasciitilde 2 eV to 20 keV) positron beam system. The 511 keV gamma annihilation line shape is parameterized using S and W-parameters, defined as ratios of the peak and wing regions to the total area of the 511 keV peak, respectively. Chemical information from the site of positron annihilation was derived by taking the ratio of the 511 keV annihilation gamma spectrum collected at various positron energies with respect to the annihilation spectrum collected from polycrystalline Cu. For higher positron energies, the S and W parameters as well as the ratio plots indicate that positrons are trapping at the interface between the multilayer graphene overlayer and Cu. The variation of S (W) parameters as a function of incident positron energy showed a peak (dip) at \textasciitilde 5eV which correlates with a peak in the positronium formation from multilayer graphene. Present investigations are pointing towards a resonant positron neutralization process involving a surface plasmon excitation, though more theoretical and experimental studies are required to confirm this.   

Many-body localization in an array of superconducting transmon devices

Superconducting circuits hold promise for excellent platform of quantum simulations. Up to date, fabrication disorder of superconducting circuits has been an obstacle for realizing large scale quantum simulations of realistic condensed matter models. To overcome this problem, we study theoretically the prospect of quantum simulation of strongly disordered and interacting quantum matter in an array of superconducting transmon devices. Specifically we are interested in bosonic many-body localization. Transmons interact via capacitive dipole-dipole interaction and the many-excitation interaction, that is crucial for many-body localization, is provided by the anharmonicity of the transmon energy spectrum. The disorder strength of the on-site energy is in-situ tunable over an order of magnitude through combining over-all flux-tuning to disorder in transmon loop area, providing a possibility for studying phase transitions. High controllability of superconducting circuits can be used for detailed quantum measurements and coherent driving of transmons. Using the recently developed DMRG like method for finding highly-excited eigenstates, we will explore features of many-body localization in an array of superconducting transmon devices.   

Chiral Liquid Crystals of Different Viscosities and the Detection of Volatile Organic Compounds

Cholesteric and nematic liquid crystals (LCs) confined in different geometries such as rectangular, triangular and spherical grooves, as well as prepared as thin films and droplets were studied as promising gas sensors for volatile organic compounds (VOCs), namely ethanol, toluene, cyclohexane, and acetic acid. A variety of illuminating conditions was used to find an optimal configuration that provided the best sensitivity and selectivity of LCs to the VOCs. Differences in responses were studied for planar and homeotropic orientation of LCs on the substrates. It was found that waveguide geometry has a number of advantages for detecting small concentrations of VOCs well before LCs undergo isotropization transition. The light propagation in the waveguides was analyzed. The sequence of transitions previously discovered in [1] ( change of order parameter on the surface of LC, mass transfer between areas of LC with different order parameter and isotropization) was confirmed for LCs with relatively low viscosities ( c.a. 102 ). The prototype of the VOCs “smelling nose” was built in order to selectively detect the presence of VOCs in the air. Its characteristics, functioning and optimization were analyzed.   

Optics of Confined Liquid Crystals for Gas Detection

Cholesteric liquid crystals (CLCs) of a wide range of viscosities were studied experimentally in relation to their use as gas sensors and sensors of volatile organic compounds (VOCs), specifically ethanol, cyclohexane, toluene, acetic acid, and pyridine. CLCs were obtained by mixing low molar mass liquid crystals (MBBA and cholesterol derivatives with siloxane based oligomers). The droplets of CLCs were placed in containers with controlled atmospheres. The shift of the selective reflection band, predominantly from shorter to longer wavelengths, and the color changes were observed in the CLC illuminated by light coming from the various directions. Visible optical changes were observed in droplets with viscosities of CLCs ranging from c.a. 4 Pa*s to 10$^5$ Pa*s. The most responsive droplets in which the shift of the selective reflection band occurs at lower concentrations of VOCs were prepared from CLC mixtures with the lowest viscosities. Higher viscosities of CLCs lead to a slower response to VOCs, but the rate of response is different for each pair of VOC and CLC with a certain viscosity. This finding opens a possibility for selective detection of VOCs by CLCs with different viscosities. The mechanism of VOCs diffusion, interaction with CLC matrix and optical changes is discusse   

Spatially-aligned graphene nanoribbon field-effect transistors using Au(788) templates and a Van der Waals-mediated pick-up technique

Graphene nanoribbons (GNRs) have recently garnered scientific interest due to their exciting electrical properties, acting as a semiconducting alternative to zero-gap graphene. Techniques have been developed to synthesize spatially-aligned, atomically-precise, high aspect ratio GNRs using the terraced structure of Au(788) crystals as a template. Here we report electronic transport measurements on spatially-aligned GNR field-effect transistors (FETs), grown on the stepped surface of atomically clean Au(788) crystals. The FETs were fabricated via a polymer-free pickup method mediated by the Van der Waals interactions between hexagonal boron nitride and the GNRs. We use scanning tunneling microscopy, Raman spectroscopy, and electronic transport measurements to characterized the topographic and electronic properties of the device.   

Multi-scale simulations of Na2S $+$ SiS2 glassy electrolyte

Developing solid electrolytes with high ionic conductivity is a significant challenge. Here we explore sulfide glasses as potential electrolytes. A classical molecular dynamics approach was applied to evaluate the structures and ionic conductivity of a wide range of xNa2S -- (1-x) SiS2 glassy electrolytes. Due to their amorphous nature, various starting configurations obtained using a typical melt-quench technique were explored to gather statistically reasonable structures. In order to validate the model, the results from the pair distribution functions for [0.5Na2S -- 0.5SiS2] were compared with structure factor data from experiments. Finally, ionic conductivity was calculated for varying compositions to identify the most promising electrolytes. To scale up the calculations, allowing for the determination of interface properties and large scale calculations, a kinetic Monte Carlo simulation is developed to work in conjunction with the molecular dynamics calculations. Using this approach, it is possible to model the conductivity in these glasses from the atomic level to the macroscale.   

Soft particles as emulsion stabilizers

Efficiency of soft particles to stabilize emulsions is examined by measuring their desorption energy i.e. the energy required to detach the particle form a fluid interface. Microgels as well as generic deformable particles are considered. Soft particles and the interface are modeled using molecular dynamics simulations and the free-energy is calculated using the thermodynamic integration method. It is shown that the softness affects the particle-interface binding in two opposing directions. On the one hand, a soft particle spreads at the interface, removes a larger liquid-liquid contact area, and thus is energetically more favorable than a rigid particle. On the other hand, softness gives the particle an extra degree of freedom to get reshaped instead of deforming the interface, producing a smaller excess interfacial area during the detachment, and hence faces a smaller energy barrier to detach. Eventually, the first effect prevails, and a soft spherical particle attaches stronger than a rigid particle to the fluid interface, although its binding is much weaker than the rigid particle with the same lenticular shape at the interface, suggesting that rigid oblate particles are even more effective stabilizers than soft particles.   

Duality in three-dimensional topological dynamics

Topological dynamics is a well developed approach for analyzing two-dimensional systems, such as the chaotic mixing of 2D fluids. However, extending such topological techniques into higher dimensions has been met with considerable difficulty. Recently, we have developed a technique to extract symbolic dynamics from the complex topology of intersecting stable and unstable manifolds for systems described by 3D volume-preserving maps. Such maps are physically relevant to particle transport by incompressible fluid flows or by magnetic field lines. Quite unexpectedly, the symbolic dynamics extracted from a variety of examples exhibits a remarkable duality: the symbolic description of the forward evolution of 2D surfaces is equivalent to the symbolic description of the backward evolution of 1D curves. One specific consequence of this is that the exponential growth rate in the area of a surface evolving forward is the same as the exponential growth rate in the length of a curve evolving backward in time. We illustrate this phenomenon with chaotic vortex dynamics in a 3D fluid flow.   

Synchrotron Light Sources in Developing Countries

The more than 50 light sources in operation include facilities in Brazil, Korea, and Taiwan which started in the 1980's when they were developing countries. They came on line in the 1990's and have since trained hundreds of graduate students. They have attracted mid-career diaspora scientists to return. Growing user communities have demanded more advanced facilities, leading to higher performance new light sources that are now coming into operation. Light sources in the developing world now include the following: $\backslash $textbf\textbraceleft SESAME\textbraceright in the Middle East which is scheduled to start research in 2017 ($\backslash $underline \textbraceleft www.sesame.org\textbraceright ); $\backslash $textbf\textbraceleft The African Light Source\textbraceright , in the planning stage ($\backslash $underline \textbraceleft www.safricanlightsource.org\textbraceright ); and $\backslash $textbf\textbraceleft The Mexican Light Source\textbraceright , in the planning stage ($\backslash $underline \textbraceleft http://www.aps.org/units/fip/newsletters/201509/mexico.cfm\textbraceright ). See: http://wpj.sagepub.com/content/32/4/92.full.pdf{\$}$+${\$}html; http://www.lightsources.org/press-release/2015/11/20/grenoble-resolutions-mark-historical-step-towards-african-light-source..   

Molecular Dynamical Simulation of Thermal Conductivity in Amorphous Structures

While current descriptions of thermal transport exists for well-ordered materials such as crystal latices, new methods are needed to describe thermal transport in disordered materials, including amorphous solids. Because such structures lack periodic, long-range order, a group velocity cannot be defined for thermal modes of vibration; thus, the phonon gas model cannot be applied to these structures. Instead, a new framework must be applied to analyze such materials. Using a combination of density functional theory and molecular dynamics, we have analyzed thermal transport in amorphous structures, chiefly amorphous germanium. The analysis allows us to categorize vibrational modes as propagons, diffusons, or locons, and to determine how they contribute to thermal conductivity within amorphous structures. This method is also being extended to other disordered structures such as amorphous polymers.   

Josephson current signatures of the Majorana flat bands on the surface of time-reversal-invariant Weyl and Dirac semimetals

A linear Josephson junction mediated by the surface states of a time-reversal-invariant Weyl or Dirac semimetal localizes Majorana flat bands protected by the time-reversal symmetry. We show that as a result, the Josephson current exhibits a discontinuous jump at $\pi$ phase difference which can serve as an experimental signature of the Majorana bands. The magnitude of the jump scales proportionally to the junction length and the momentum space distance between the Weyl nodes projected onto the junction. It also exhibits a characteristic dependence on the junction orientation. We demonstrate that the jump is robust against the effects of non-zero temperature and weak non-magnetic disorder.   

Ferromagnetism controlled by electric filed in tilted phosphorene nanoribbon.

We investigated the magnetic property of tilted black phosphorene nanoribbons (TPNRs) affected by an external electric field. We also studied the edge passivation effect on the magnetism and thermal stability of the nanoribbons. The pure TPNR displayed an edge magnetic state, but it disappeared in the edge reconstructed TPNR due to the self-passivation. In addition, we found that the bare TPNR was mechanically unstable because an imaginary vibration mode was obtained. However, the imaginary vibration mode disappeared in the edge passivated TPNRs. No edge magnetism was observed in hydrogen and fluorine passivated TPRNs. In contrast, the oxygen passivated TPNR was more stable than the pure TPNR and the edgeto-edge antiferromagntic (AFM) ground state was obtained. We found that the magnetic ground state could be tuned by the electric field from antiferromagnetic (AFM) to ferromagnetic (FM) ground state. Interestingly, the oxygen passivated TPNR displayed a half-metallic state at a proper electric field in both FM and AFM states.   

Hard X-ray RIXS studies of rare earth hexaborides

We present a rare-earth L$_3$ resonant inelastic X-ray scattering f divalent hexaborides RB$_6$ with R=Sm, Yb and compare existing data on X=Eu. In YbB$_6$, we find enhanced RIXS intensity at resonances not near the divalent absorption peak and interpret the nature of the generated excitations with the aid of electronic structure calculations. In addition, we find RIXS signal generated at a well-defined resonance several eV higher that it not observed in absorption and suggest a physical origin of this enhanced resonance-free RIXS signal.   

Nanoscopy of Black Phosphorus Degradation

We report on the experimental quantification of geometric properties and theoretical modeling of the chemical degradation process of black phosphorus (BP) and investigate the effectiveness of passivation coatings using infrared near-field nanoscopy. We chemically identify oxidized phosphorus species locally at the onset of degradation by nanoscale spectroscopic imaging at mid-infrared frequencies. We found that these species can form underneath 5 nm thick Al2O3 coating deposited by atomic layer deposition (ALD) indicating that thin coating is insufficient to protect BP against degradation caused by ambient medium. By performing simultaneous topographic and optical time series imaging over several months, we show that a nanolayer BP exposed to ambient environment degrades at a steadily increasing rate until saturation begins, so that the degraded area and volume of degraded regions as functions of time follow the well-known S-shaped growth curve (sigmoid growth curve). Phenomenological modeling of experimental results suggests a strong influence of degraded areas on adjacent BP. Our model is advantageous since it is based on elementary probabilities that can be related to the O2 and H2O content in the ambient medium, as well as to the chemical reaction processes that result in oxidized phosphorus species.   

Compression selective solid-state chemistry

Compression selective solid-state chemistry refers to mechanically induced selective reactions of solids under thermomechanical extreme conditions. Advanced quantum solid-state chemistry simulations, based on density functional theory with localized basis functions, were performed to provide a remarkable insight into bonding pathways of high-pressure chemical reactions in all agreement with experiments. These pathways clearly demonstrate reaction mechanisms in unprecedented structural details, showing not only the chemical identity of reactive intermediates but also how atoms move along the reaction coordinate associated with a specific vibrational mode, directed by induced chemical stress occurred during bond breaking and forming. It indicates that chemical bonds in solids can break and form precisely under compression as we wish. This can be realized through strongly coupling of mechanical work to an initiation vibrational mode when all other modes can be suppressed under compression, resulting in ultrafast reactions to take place isothermally in a few femtoseconds. Thermodynamically, such reactions correspond to an entropy minimum process on an isotherm where the compression can force thermal expansion coefficient equal to zero. Combining a significantly brief reaction process with specific mode selectivity, both statistical laws and quantum uncertainty principle can be bypassed to precisely break chemical bonds, establishing fundamental principles of compression selective solid-state chemistry. Naturally this leads to understand the "alchemy" to purify, grow, and perfect certain materials such as emerging novel disruptive energetics.   

Laser impact: from two-temperature warm dense matter to crystallized surface structures

We consider laser ablation dynamics of thin films mounted on substrate or freestanding. Optical or X-ray lasers are used. Focusing systems are based on a high aperture lens or on a phase plate. Thus or diffraction limited focal spot with maximum in the center and approximately Gaussian fluence distribution around, or ring type distribution with zero of fluence in a center are formed. Topologically different cupola like or torus like structures made from a deformed film are created under these two focusing conditions. We develop a wide set of techniques to describe thermodynamic, transport, and kinetic properties of isothermal $(T_e=T_i)$ and non-isothermal $(T_e\ll T_i)$ condensed matter in high energy density states: $T_e\sim (1\div 10)$ eV, $T_i\sim 1-10$ kK. The set includes DFT, QMD, and kinetic equations techniques that gives us equation of state, thermal conductivity, electron-ion coupling parameter. Hydrodynamics and molecular dynamics simulations give us detailed view on thermal and dynamical processes leading to melting, motion, deformation, fraction, and final solidification of material in form of cupola like or torus like structures.   

Provable classically intractable sampling with measurement-based computation in constant time

We present a constant-time measurement-based quantum computation (MQC) protocol to perform a classically intractable sampling problem. We sample from the output probability distribution of a subclass of the instantaneous quantum polynomial time circuits introduced by Bremner, Montanaro and Shepherd. In contrast with the usual circuit model, our MQC implementation includes additional randomness due to byproduct operators associated with the computation. Despite this additional randomness we show that our sampling task cannot be efficiently simulated by a classical computer. We extend previous results to verify the quantum supremacy of our sampling protocol efficiently using only single-qubit Pauli measurements.   

Characterization of a Compact Cryogenic Package Approach to Ion Trap Quantum Comuting

One challenge for the expansion of trapped ion systems to a large scale is the lack of repeatable integration technology to realize compact and stable operating environment. In this work, we present a novel ion trapping environment where conventional ultra-high vacuum (UHV) chambers are replaced with a sealed ceramic package operating in a cryogenic environment. A microfabricated surface ion trap mounted on a 100-pin ceramic pin grid array (CPGA) package is placed in a UHV environment. A titanium lid with windows for optical access is then attached to the CPGA via an indium seal which maintains the UHV conditions for the ion trap. The trap package assembly is operated at cryogenic temperatures (5K) in order to freeze out most of the residual background gas. Activated charcoal is used to pump remaining helium and hydrogen molecules. Metallic Yb ablated using a Q-switched Nd:YAG laser at 1,064 nm is used as the atomic source. A compact radio frequency resonant circuit is used to create the RF potential for trapping. A low output impedance amplifier drives a superconducting inductor of value 2 uH in series with the trap capacitance in order to produce 200V at 26 MHz with low heating at 5K. We present the experimental progress towards trapping ions in this compact cryogenic setup.   

Improving optomechanical sensors with injected squeezing

Optomechanical systems have recently entered the quantum domain and in this regime they may provide highly sensitive detection of masses, forces and displacements. We show how the performance of optomechanical systems can be significantly improved when the optomechanical cavity is driven not only by a laser but also by squeezed vacuum light with an appropriate phase. We show in particular the improvement in force sensing in the regime of coherent quantum noise cancellation [1], backaction cooling due to the possibility to suppress scattering at the Stokes sideband [2], and also the ability to entangle two mechanical resonators, even out of the resolved sideband regime [3]. [1] Ali Motazedifard, F. Bemani, M. H. Naderi, R. Roknizadeh, D. Vitali, New J. Phys. 18 (2016) 073040 [2] Muhammad Asjad, Stefano Zippilli, David Vitali, arXiv:1606.09007. [3] Muhammad Asjad, Stefano Zippilli, David Vitali Phys. Rev. A 93, 062307 (2016)   

Axioms and Measurements

The traditional postulates of quantum mechanics are formulated to mainly suit the Copenhagen interpretation. They are not suited for the Many-Worlds interpretation. Often, it has been assumed that MWI needs no postulates, but the mathematics alone can never constitute a physical theory. There have to be some rules for how the mathematical entities are related to our observations. Two axioms have been newly proposed. In short, postulate 1) The state are square normalizable. Where the particle is, is given by a distribution, which is given by the absolute square of the state; 2) The state time evolution is continuous and unitary and all interactions are spatially local. From the postulates, it can be derived that the state is a vector in Hilbert space. It can also be seen that experiments that are properly set up can measure the eigenvalues of Hermitian operators. The probability recipe as was given by Born need not be a part of the axioms in the Many-Worlds interpretation but can be derived from the given axioms. Many previous attempts to derive the probability rule has been forced to use subtle and less convincing argumentation as they were based on intuition rather than well-defined postulates.   

Oxidation and Unimolecular Decomposition of Aluminum Metalloid Nanoclusters

We have been studying molecular scale aluminum clusters (known as metalloid clusters) that are passivated against immediate oxidation via a layer of organic ligands as energetic materials that may retain the high energy density of bulk metals but offer substantially faster reaction kinetics. Several experimental efforts have also begun on lab-scale synthesis of ligated metalloid clusters that could be tailored for energetics applications and allowing low-valence metals to oxidize within the reaction zone of a detonation. But, considerable synthesis challenges remain. The air stability provided by these ligands is currently one of the key limiting factors in moving to larger scale testing of these materials. Recently, we showed that nucleation and growth of well-dispersed aluminum nanoclusters supported by functionalized-graphene layers is possible and may provide new insights toward more stable nanostructures.   

Robust operating point for capacitively coupled singlet-triplet qubits

Singlet-triplet qubits confined by electrically gated double quantum dots exhibit fast single-qubit gates via exchange interaction. In addition, two-qubit entangling gates are achieved via non-local capacitive coupling. Both of these interactions are controlled by tilting the double dots, which is sensitive to background charge fluctuations. By considering a mode of tilting where the interqubit electrostatic interaction balances the exchange interaction, we report a theoretical sweet spot such that the effective exchange is insensitive to charge fluctuations. We simulate the fidelity of the entangling gates in this regime when the qubits are perturbed by 1/f charge noise.   

Cornell-BNL Electron Energy Recovery Linac FFAG Test Accelerator (CBETA)

A novel energy recovery linac (ERL) with Non-Scaling Fixed Field Alternating Gradient (NS-FFAG) racetrack is being constructed as a result of collaboration of the Cornell University with Brookhaven National Laboratory. The existing injector and superconducting linac at Cornell University are being installed together with a single NS-FFAG arcs and straight section at the opposite side of the linac to form an ERL system. The 6 MeV electron beam from injector is transferred into the 36 MeV superconducting linac and accelerated by four successive passes: from 42 to 150 MeV using the same NS-FFAG structure made of permanent magnets. After the maximum energy of 150 MeV is reached, the electron beam is brought back to the linac with opposite Radio Frequency (RF) phase and with 4 passes electron energy is recovered and brought back to the initial energy of 6 MeV. This is going to be the first 4 pass superconducting ERL and the first NS-FFAG permanent magnet structure to bring the electron beam back to the linac.   

Interactive Biophysics with Microswimmers: Education, Cloud Experimentation, Programmed Swarms, and Biotic Games

Modern biotechnology gets increasingly powerful to manipulate and measure microscopic biophysical processes. Nevertheless, no platform exists to truly interact with these processes, certainly not with the convenience that we are accustomed to from our electronic smart devices. In my talk I will provide the rational for such Interactive Biotechnology and conceptualize its core component, the BPU (biotic processing unit), which is then connected to an according user interface. The biophysical phenomena currently featured on these platforms utilize the phototactic response of motile microorganisms, e.g., Euglena gracilis, resulting in spatio-temporal dynamics from the single cell to the self-organized multi-cellular scale. I will demonstrate multiple platforms, such as scalable biology cloud experimentation labs, tangible museum exhibits, biotic video games, low-cost interactive DIY kits using smartphones, and programming languages for swarm robotics. I will discuss applications for education as well as for professional and citizen science. Hence, we turn traditionally observational microscopy into an interactive experience.   

Novel physical constraints on implementation of computational processes

Non-equilibrium statistical physics permits us to analyze computational processes, i.e., ways to drive a physical system such that its coarse-grained dynamics implements some desired map. It is now known how to implement any such desired computation without dissipating work, and what the minimal (dissipationless) work is that such a computation will require (the so-called “generalized Landauer bound”). We consider how these analyses change if we impose realistic constraints on the computational process. First, we analyze how many degrees of freedom of the system must be controlled, in addition to the ones specifying the information-bearing degrees of freedom, in order to avoid dissipating work during a given computation, when local detailed balance holds. We analyze this issue for deterministic computations, deriving a state-space vs. speed trade-off, and use our results to motivate a measure of the complexity of a computation. Second, we consider computations that are implemented with logic circuits, in which only a small numbers of degrees of freedom are coupled at a time. We show that the way a computation is implemented using circuits affects its minimal work requirements, and relate these minimal work requirements to information-theoretic measures of complexity.   

Entanglement of condensed magnons via k-space fragmentation

Motivated by recent interest in quantum control of magnons in magnetic insulators, we propose a model for engineering momentum space entanglement of fragmented magnon condensates. We show that an applied sinusoidal magnetic field can drive a quantum phase transition into a  ground state exhibiting macroscopic quantum entanglement.  We discuss experimental signatures and prospects for realizing this model in chiral magnets and in yttrium-iron garnet (YIG).   

Random search for a dark resonance

A pair of resonant laser fields can drive a three-level system into a dark state where it seizes to absorb and emit radiation due to destructive interference. We propose a scheme to search for this resonance by randomly changing the frequency of one of the fields each time a fluorescence photon is detected. The longer the system is probed, the more likely the frequency is close to resonance and the system populates the dark state. Due to the correspondingly long waiting times between detection events, the evolution is non-ergodic and the precision of the frequency estimate does not follow from the conventional Cram\'{e}r-Rao bound of parameter estimation. Instead, a L\'{e}vy statistical analysis yields the scaling of the estimation error with time for precision probing of this kind.   

Magnon Kerr Effect in a Cavity Quantum Electrodynamics System

We experimentally demonstrate magnon Kerr effect in a cavity quantum electrodynamics (QED) system, where magnons in a small yttrium iron garnet (YIG) sphere are strongly but dispersively coupled to the photons in a three-dimensional cavity. The Kerr term comes from the magnetocrystalline anisotropy of the YIG sphere. When the YIG sphere is pumped to generate considerable magnons, the Kerr effect yields a perceptible shift of the cavity central frequency and more appreciable shifts of the magnon modes. We derive an analytical relation between the magnon frequency shift and the drive power for the uniformly magnetized YIG sphere and find that it agrees very well with the experimental results of the Kittel mode. Our study paves the way to explore nonlinear effects in the cavity QED system with magnons. The nonlinear properties may be utilized in the hybrid quantum systems.   

Anisotropic transport in disordered double Weyl semimetal

The stability of double Weyl semimetal in the presence of on-site disorder potential is examined thoroughly by numerical calculation of localization length, conductance and density of states. We use a tight binding model for a layered Chern insulator. An out-of-layer transport calculation and density of state calculation shows that the double Weyl semimetal is unstable and enters into diffusive metal even in the presence of weak disorder. Our in-layer transport calculation results found an unexpected oscillation behavior in localization length and conductance as a function of disorder strength. The oscillation behavior can be understood as the motion of doubly Weyl nodes with very long lifetime.   

Connecting the dots: Time-reversal symmetric Weyl semimetals with tunable Fermi arcs

Weyl semimetals exhibit Fermi arc surface states: a line of zero energy surface modes connecting surface projections of Weyl nodes of opposite chiralities. Generically, a set of Weyl nodes can be connected by the Fermi arcs in multiple ways, and a natural question is whether these different connectivities can be deformed into each other by only varying the bulk Hamiltonian. In this talk, we present explicit lattice models for noninteracting, time reversal symmetric ($\mathrm{T}^2 = + 1$) Weyl semimetals, in which the connectivities of the Fermi arcs can indeed be interpolated continuously by tuning a parameter in the Hamiltonian, without affecting the location and chiralities of the Weyl nodes. The bulk polarization and magnetization in the ground state are shown to vary with the tuning parameter, which can potentially be a measurable effect. This talk is based on arXiv:1608.01313.   

Effect of high electromagnetic fields on cellular growth

It is already known that high-intensity electromagnetic field affect the human lung growth and forces the T-cells to decrease by 20-30 percent. The electromagnetic field had a severe impact on human T-cells in contrast to lung cells. Due to the high-intensity electromagnetic field, the growth of T-cells becomes low and release of Ca+2 increases up to 3.5 times more than the lung cells. The high-intensity electromagnetic radiations do not directly produce cancer cells but had a severe impact on the growth of T-cells. It can also be said that electromagnetic field acts a role in the cancer initiation. It creates disordered in the structure of membranes and gesture transduction. The higher exposure to electromagnetic field increases PKC-alpha and this larger release from membranes cannot be controlled. It was concluded that greater exposure to the electromagnetic field is dangerous and had a severe impact on T-cells growth and lung cells growth and due to this greater possibility of leukemia occurrence. We show a similar effect of electromagnetic fields single celled bacteria to compare the bacterial cellular growth with the human cells using the bacteria strains which are commonly found in human body.