Session C71: Poster Session I (2:00pm - 5:00pm)

Using GALFIT to Determine Galaxy Morphologies of Quasars

Little is known about the morphologies of quasars. Quasars are the brightest of the active galaxies, in which a supermassive black hole is accreting material, creating an accretion disk that outshines the host galaxy. We model images obtained from the Hubble Space Telescope of 22 nearby quasars and the galaxies in their immediate neighborhood using the GALFIT software to determine how well they are fit by typical galaxy profiles. GALFIT is an image analysis algorithm which fits parametric functions to create light profiles from two-dimensional images. We fit the quasars using two main components - a Point Spread Function (PSF), which represents a point source object (the light of the accretion disk), and a Sérsic profile, which models galaxy structures, such as a disk or a bulge. We will present details on the models, including the types of components used in each fit as well as the residual maps after subtracting the model from the data.   

SuperCDMS: Energy Calibration of a Cryogenic Ge HV Particle Detector

The goal of the SuperCDMS collaboration is to directly detect dark matter. Potential candidates for dark matter are Weakly Interacting Massive Particles (WIMPs). To detect WIMPs, it is important to be able to predict how a Ge/Si particle detector will respond to a dark matter signal. In particular, it is necessary to calibrate the recoil energy measured by these detectors. This paper presents the energy calibration spectrum of a SuperCDMS-HV Ge particle detector using Am-241 and a PuBe neutron source. Due to high event rate, criteria were developed to remove low-quality data arising from particle interactions that occur too soon after a previous interaction. Peaks in histograms of pulse amplitudes were identified as energy peaks from the various radioactive sources, and fits of these peaks formed the basis for generating an energy calibration function. The calibration function was used to generate the calibrated energy spectrum.   

Effect of polyethylene oxide on camphor sulfonic acid doped polyaniline thin film field effect transistor with ionic liquid gating

Field effect transistors (FET) using camphor sulfonic acid (CSA) doped polyaniline (PANi) blended with several polyethylene oxide (PEO) concentrations were investigated via ionic liquid gating. The pure PANi-CSA FET could not be turned “off” and had an on/off ratio of 2. Blending with 22wt%-PEO retained a high “on” state throughput current and improved the mobility, while the on/off ratio increased by 10³. Superior film quality and PEO assisted electrostatic long range interactions with the PANi chains led to device parameter enhancement. For higher PEO concentrations the field effect was suppressed due to disorder. Analysis of the UV/Vis spectra polaron band peak area near 810 nm show an increase in the mobility with decrease in the peak area, consistent with the observed results. Enhanced device parameters, high throughput current and low operating voltages (±2V), make PANi-CSA/PEO blends attractive materials for use in low power consumption electronics.   

Fabrication of femtosecond laser-induced crystals in lithium nioboSilicate 30: the effects of polarization angle on orientation and growth rate.

Optical and Electron microscope inspections reveal a dependence of light polarization on the growth of crystals in glass using a femtosecond laser. Critical for these experiments is an algorithm based on a half-wave plate that insured constant laser intensity for varying polarization directions. Laser-induced crystallization is an effective way of fabricating crystals in glass and other transparent materials owing to the fact that the process is clean, precise and contactless. The crystals obtained are of interest because of their potential use in optical data transmission. The issue tackled in this paper was that of maintaining consistent power when the polarization angle was changed. For this work, LiNbO3 crystals were fabricated in Lithium NioboSilicate glass with 30 mol percentage of silicon dioxide in the glass. Cao et al observed that at moderate pulse energies, (0.5-0.9μJ/ pulse, 300kHz) textured nanocrystals are obtained with their polar axis perpendicular to the writing laser polarization direction[1].
CIS codes: 130.3730, 130.5296

References:
[1] J. Cao, M. Lancry, F. Brisset, L. Mazerolles, R. Saint-Martin, and B. Poumellec, Crystal Growth & Design 2019 19 (4), 2189-2205   

Ultrasonic Acoustic Probing Based on Gaussian Beam Analysis

By analyzing differences in phase and amplitude of a signal, information about differing acoustic contrast between materials can be quantified. Prior methods used a scanning acoustic microscope which allowed for phase shifts to be identified by reflections but could not quantify phase shifts besides 0° or 180°. The new method uses a continuous signal to identify more precisely the phase and amplitude to analyze the transmitted signal. We have utilized the knowledge that the acoustic signal used behaves as a Gaussian beam travelling through a material. As the beam comes into contact with a defect, it is theorized to split into multiple source waves on either side which interfere as they travel across the plate to the receiver. By keeping the transmitter stationary and probing the receiver, we examine how the phase and amplitude change as the distance between transducers varies (transverse profiles). This approach potentially has a biological application because the acoustic contrast between healthy cells and cancerous cells is difficult to identify using current ultrasonic methods. This testing method could potentially gather more precise data with respect to the slightly differing contrasts between the cells and help better identify the presence and location of unhealthy cells.   

Quantum Key Distribution Using Optimum Expectation Values of Maximally Entangled GHZ States

We propose a new quantum key distribution scheme that is based on the optimum expectation values of maximally entangled Greenberger-Horne-Zeilinger states. Our protocol makes use of the degrees of freedom in continuously variable angles, thereby increasing the security of the key distribution. Outlined are two protocols that distribute a key from Alice to Bob using the above idea, followed by an extension that allows for the same key to be shared with Charlie. We show how this scheme provides for certain detection of any eavesdropper through absolute violation rather than the probabilistic violation used in many protocols.   

Temperature dependent charge transport in graphene with ferroelectric gating

CVD graphene was electrically characterized in a field effect transistor configuration with ferroelectric (FE) gating in the temperature range 300K < T < 350K. Saturated hysteresis loops of the FE co-polymer poly(vinylidene fluoride-trifluoroethylene)-PVDF-TrFE(75/25) showed that the memory window width decreased as temperature increased. Device trans-conductance (I-Vg) curves exhibit hysteresis behavior with two charge neutrality points (CNP) corresponding to the up/down polarization of the ferroelectric gate. Increasing the temperature decreased the change of the gate voltage and increased the change in the channel current measured between the two CNP’s. The electron mobility showed a steeper decrease compared to the hole mobility as temperature was increased. Desorption of O₂ and H₂O was used to explain these observations. Finally, non-volatile switching was realized using the charge storage property of the gate insulator.
  

Developing a Method for Measuring the Optical Scattering of Silica Nanosprings

Nanosprings have been shown to have a variety of applications in medicine, industry and material science due to their unique nano-scale characteristics. Recent developments have vastly improved the ability to produce large quantities of high-quality silica nanosprings, making possible new avenues of research and applications. Current research includes applications in prosthetic-bone interfaces, detection of small quantities of gases, and the development of new composite materials. As such, there is a growing interest in better understanding their properties. The motivation for this work is to develop a method for studying the optical properties of these silica nanosprings. An apparatus for measuring the optical scattering of light off of the nanosprings has been constructed and measurements are ongoing. The first objective is to confirm visual observation of diffraction effects in nanosprings. The apparatus uses a visual microscope, a spectrograph, and illumination by a variety of lamps via fiber optics. A single nanospring is held by a micromanipulator; light (typically white) is then scattered off the spring and collected to produce spectra. This data is then analyzed for diffraction effects.
  

Phase-Modulated Local Oscillator Effects on RF-DNA Fingerprints in IEEE 802.11a Wi-Fi Signals

With the increasing dependence on the internet in more and more consumer products, there is an urgent need to enhance existing digital security systems. RF-DNA fingerprints are one such approach to utilize discriminating waveform characteristics to augment the detection (rejection) of approved (unapproved) Wi-Fi devices. This work investigates a time-dependent approach to manipulate the RF-DNA fingerprints of a transmitting device through local oscillator phase modulation of the clocking system in Wi-Fi transmitters. Experimental results are used to investigate the ability for an unaffected receiver to detect a corresponding clock-modulated transmitter, as well as the changes of 802.11a Wi-Fi preambles of clock phase modulated transmissions. Changes in the waveforms are further analyzed using the Discrete Gabor Transform in the time-frequency domain. Analysis shows a predictable pattern of change over time, proportional in frequency to the phase-modulation frequency, and the ability for preamble structures to remain intact up to 30 kHz of phase modulation.   

Investigation of the Damage to Hydrophobic Self-Assembled Monolayers by Aqueous Salt Solutions

When water is forced into contact with a hydrophobic surface, a depletion layer, or a low-density region of water, is formed. To study the depletion layer in more natural applications, we look at how aqueous salt solutions interact with hydrophobic surfaces made from self-assembled monolayers (SAMs) of octadecanethiol. In order to investigate this interaction we need smooth, homogeneous SAMs. Aqueous solutions can corrode the SAM, so we experiment salt molarity and cation size to eliminate this problem. The experimental methods include contact angle measurements, scanning electron microscopy, surface plasmon resonance.   

Optimization of Microgel Imaging Using SEM

Microgels are polymer-based nanoparticles suspended in water that exhibit. The standard, noninvasive method for characterizing microgels is dynamic light scattering (DLS), which measures collective diffusion of microgels. While DLS provides reliable estimates for particle structure/dynamics, more direct methods of imaging are useful for studying polydisperse samples. Traditionally, scanning electron microscopy (SEM) uses an electron beam under high vacuum to characterize individual dried particles of dried. The dry microgel imaging suffers from two drawbacks: dehydrated particles collapse under vacuum and their dynamics is not observable. This project explored wet particle imaging in an ionic liquid stable under high vacuum. Particles were suspended in an ionic liquid film on a copper grid. Still images/movies were recorded to analyze microgel size distribution and dynamics. The average SEM size generally agreed with DLS both in ionic liquid and in water at room temperature. Variation was observed in individual particle sizes, but the average SEM size for both samples were close to the DLS size. Initial attempts at diffusion analysis using SEM particle tracking yielded mixed results as it requires tracing of many particles and further optimization.   

Effect of cerium substitution on the superconducting state of Ba0.6K0.4BiO3

The superconducting compound Ba_0.6K_0.4BiO₃ has one of the highest critical temperatures (T_c = 30 K) of any non-cuprate oxide. In this study, we probed its superconducting properties by studying the impact of chemical substitution with Ce on the system Ba_0.6-xCe_xK_0.4BiO₃. The objective of this study was to measure the effect of introducing magnetic moments by comparing the x-dependence of T_c in Ba_0.6-xCe_xK_0.4BiO₃ with the previously established behavior of Ba_0.6-xLa_xK_0.4BiO₃. Our polycrystalline samples, synthesized using the molten salt technique, exhibited suppressed T_c values compared with that of Ba_0.6K_0.4BiO₃; however, we were unable to observe a clear correlation between measured lattice parameter values and x, suggesting that the molten salt synthesis method we used is unable to finely control the Ce content of the samples. Additional measurements will be required to determine the exact stoichiometry of the samples and x-ray photoemission spectroscopy will be required to determine the oxidation state of Ce ions. In this poster, x-ray diffraction and magnetic susceptibility measurements will be presented and compared with results from Ba_0.6-xLa_xK_0.4BiO₃.   

Fragmentation and Desorption of Surface-Immobilized DNA on PMMA and PAA Substrates for Sequencing Applications

Producing ordered fragments of DNA would greatly simplify sequencing’s assembly problem. We studied the enzymatic cutting and desorption of surface-immobilized λ-DNA molecules on PMMA and PAA coated silicon wafers. Surfaces were dipped into 0.5-5.0 μg/mL DNA solutions and retracted at 1-10 mm/s speeds to stretch and immobilize (“molecularly comb”) DNAs on the surface. For PMMA, desorption was done in heated buffer solutions (DNase buffer, NEBuffer 3.1, pH-altered NEBuffer 3.1 (pH≈10) at 50-70°C). For PAA, which switches water-solubility by immersion in CaCl₂ or NaCl solutions, complete desorption into saline solution by PAA dissolution was observed.
Relative to previous soft lithography methods, we employed a microfluidic method to improve DNA fragmentation. Holes in 1mm thick PDMS membranes were produced by piercing the PDMS with microneedles (d =250-400 μm) or by curing PDMS around the microneedles. PDMS layers with holes were pressed onto adsorbed DNAs and a DNAse I cutting enzyme solution was deposited on top. By applying a vacuum (125 Torr) above the solution, the low-volume chambers fill with solution and efficiently cut the DNA, as confirmed by fluorescence microscopy.   

Observation on WSe2/WS2 hetero-bilayer spectroscopic and transport features

Semiconducting transition metal dichalcogenide (TMD) monolayers are one of the most promising materials for future optoelectronics. When two different TMD monolayers are brought together, the interlayer electronic coupling effects generate a new band structure inherent to the bilayer. We aim to fabricate hetero-bilayers comprising WSe₂ monolayer and WS₂ monolayer and analyze their optical spectroscopic and electronic transport features.
We synthesized WSe₂ and WS₂ monolayers on Si/SiO₂ substrates by chemical vapor deposition. After growth, both monolayers were sequentially transferred onto a pristine Si/SiO₂ substrate to have high quality overlapped areas of heterobilayers. The samples were then annealed to enhance the coupling between layers. Lastly, we patterned Au/Ti contacts on the bilayers by e-beam lithography for electrical property characterization.
The Raman and photoluminescence spectra of the bilayers were compared before and after annealing, and we found a significant decrease in the photoluminescence intensity for both WSe₂ and WS₂. Future work will include the analysis of the electronic transport data to obtain the carrier mobility as a function of stacking angle and possible tunability in the exciton energy emission.   

Investigation of Track Formation in CR-39 for Various Hydrated Enviornments.

CR-39, a thermoset resin, is a well characterized integrative detector that, when etched, shows tracks created by energetic charged particles produced in nuclear reactions. It has been questioned whether this detection method can be used in Pd/D electrolytic cell environments. Of concern is whether the pyrophoric nature of hydrogen’s interaction with palladium and its recombination with oxygen within the cell can create similar tracks. The validity of this detection method in an electrolytic cell environment is investigated. Additionally, track comparisons from detectors used in a Pd/D co-deposition experiments utilizing K-40 or Li-6 electrolytes were done to determine if Li-6 contributes to the observed tracks.   

Topological Effects in Knotted Arrays of One-Dimensional Quantum Rings

Quantum ring arrays (QRAs) offer insight into the impact of topology in quantum systems, displaying phenomena which often have implications in quantum technology. We study the energy spectra and wavefunction behavior of small (two and three ring) QRAs by building up a model of tunnel-coupled one-dimensional quantum rings from existing models of crossed one-dimensional quantum wires. An ambiguity arises in how we connect the ends of these crossed wires, allowing us to create QRAs with the same tunnel coupling, but topologically distinct boundary conditions. We solve these systems numerically for various strengths of the tunnel coupling and find that topological differences in hole count manifest in observable differences in the single electron QRA energy spectrum in the absence of external fields. We also consider these QRAs in the presence of a uniform external magnetic field that induces an Aharonov-Bohm phase in the electron as it tunnels. By varying the induced phase, we explore magnetic phase commensuration effects in the QRA energy spectra and find that these QRAs have additional topological qualities that manifest in further differences in the energy spectra. We propose knot theory as the tool for distinguishing these systems and explaining phase commensuration effects.   

Effect of transverse magnetic fields on an isotopically engineered diamond NV ensemble

The nitrogen vacancy (NV) in diamond is a leading candidate for quantum sensing applications such as magnetometry. Here, we study the effect of a transverse external magnetic field on the spin coherence of an ensemble of nitrogen vacancy (NV) centers in diamond with applications to high-sensitivity magnetometry. A transverse magnetic field has been shown to suppress electronic spin decoherence [1]. By utilizing 15N ions for implantation, the nuclear hyperfine interaction between the 13C spin bath and the defects is minimized. We utilize optically detected magnetic resonance and pulsed methods to explore this effect in isotopically engineered diamond films.

[1] PRB 88, 161412R (2013) Shin, et al   

Link Testing between the Data Formatter and Second Stage Boards for the Fast Tracker Trigger in the ATLAS Experiment at CERN

At CERN, link testing was performed between the Data Formatter (DF) board and the Second Stage Board (SSB). This was to advance the installation of the Fast Tracker Trigger (FTK) for the ATLAS collaboration by the end of the 2^nd Long Shutdown (LS2). Link testing was performed in the Underground Service ATLAS hall (USA 15) where the FTK is located. Different combinations of the Data Formatter board and Second Stage Board were tested using Xilinx’s Integrated Bit Error Ratio Tester (IBERT) to vary the transceiver and receiver settings between the boards. To analyze the bit error ratio between the boards, the IBERT core would generate eye scans and bathtub plots to get a visualization of the digital signal integrity.   

Highly non-stoichiometric amorphous oxide semiconductors: the structure and electronic properties of defects in a-InOx

Oxygen non-stoichiometry in crystalline In2O3 leads to formation of oxygen vacancies that play key role in carrier generation and transport in this transparent conducting oxide and have been well-studied both theoretically and experimentally. In contrast to the crystalline oxide, the structure and electronic properties of oxygen defects in amorphous indium oxide (a-InOx) are not understood and render to be fundamentally different due to the lack of lattice sites and periodicity as well as an increased number of degrees of freedom in disordered materials. Strikingly, the observed carrier concentration in a-InOx is two orders of magnitude higher than that in the crystalline oxide. Moreover, given the ionic nature of indium-oxygen bonding, it is possible to grow highly non-stoichiometric amorphous oxides that helps tune the properties over an extremely wide range.
Here, we employ ab-initio Molecular Dynamics simulations and density-functional calculations to study the structural and electronic properties of a-InOx. Based on a thorough analysis of the interatomic distances, coordination, distortions, charge localization, and defect state energies, we derive a microscopic model of defects in disordered oxides and discuss their effect on the electrical and optical properties.   

In situ ellipsometry of epitaxially grown bismuth antimony telluride on sapphire

Ellipsometry uses elliptically polarized light to characterize thin film and bulk materials. The light undergoes a change in polarization as it interacts with the sample structure. The measurement is typically expressed as two values: Psi (Ψ) and Delta (Δ). Bismuth telluride and its alloys are widely used as materials for thermoelectric and spintronic devices. This project uses in situ rotating compensator spectroscopic ellipsometer to measure properties of Bismuth Antimony Telluride ((BiSb)₂Te₃). If properties of bismuth antimony telluride such as thickness and composition can be determined by ellipsometry, it can be used as a tool to improve growth parameters in real-time, improving throughput and precision when growing these materials.   

Study of Optical Properties of composite layers of MEH-PPV nanopillars and PEDOT:PSS films

Fabrication of MEH-PPV nanopillars was completed using porous Anodic Aluminium Oxide (AAO) templates. The diameter and height of the nanopillars was controlled by adjusting the dimensions of the template. The morphology of the nanopillars was studied using scanning electron microscopy. The absorption of light of MEH-PPV nanopillars plus thin film was greater than that of the thin film only, both being the same thickness.The dimensions of nanopillars are essential for carrier processes such as exciton generation, exciton diffusion and carrier dissociation and transport. As PEDOT: PSS can enhance hole collection and exciton diffusion, addition of PEDOT: PSS improves the performance of solar cells. The variation in optical properties of composite material consisting of MEH-PPV nanopillars and PEDOT:PSS films will be investigated using UV-Vis spectroscopy and fluorescence spectroscopy with change in height and diameters of the MEH-PPV nanopillars.   

Modeling directed self-assembly of nanoparticles under perpendicular electric fields

We design and model an experiment to study the effect of electric bias on particle-coverage densities produced during ionic nanoparticle self-assembly. The experiment involves the application of a uniform external electric field parallel to a glass substrate during the self-assembly of silica nanoparticles. We refer to this procedure as directed self-assembly of monolayers (DSAM). In our theoretical analysis, we modify existing cooperative sequential adsorption models to account for diffusion under an applied electric field. We use the mean field approximation to solve for particle-coverage densities. To ascertain the validity of this method, we compare our solutions to Monte Carlo simulations of the system. We also discuss particular experimental implementations of an ionic self-assembled monolayer under the influence of perpendicular external electric fields.
  

Nonlinear Magnon Scattering Observations Via The Magneto-Optical Kerr Effect

Nonlinear Magnon Scattering is observed in Yttrium Iron Garnet (YIG) thin films. Nonlinearity is excited in the YIG thin films by high power microwave pumping. The magnetization of the films due to the microwave pumping is then observed through the Magneto Optical Kerr Effect (MOKE). The MOKE is of interest as a measurement technique because of its potential to observe magnon scattering in real time. Well-defined thresholds of the linear and nonlinear regimes are examined by ferromagnetic resonance measurements using this setup.
  

Thermal metamorphism study of carbonaceous chondritic meteorites

Carbonaceous chondritic meteorites are some of the most primitive materials in our solar system. They did not experience melting or other processes on their parent bodies (e.g. asteroids) during their initial formation, and thus, they preserve information of physical and chemical mechanisms in the solar nebula, which can unveil evidence about the origin of the planets and their components. However, most carbonaceous chondrites are exposed to secondary processes on their parent bodies, such as thermal metamorphism and aqueous alteration, modifying the primary properties of the carbonaceous chondritic constituents. Hence, in order to understand how these relics formed, it is important to analyze the modifications they have experienced induced by these secondary processes. In this work, we study the thermal metamorphism of these chondrites examining their carbon composition by Raman spectroscopy. We analyze the Raman spectra of carbon allotropes to obtain specific parameters that we use for thermal metamorphism mathematical models. In addition, we correlate the Raman results with those acquired using SEM/EDS (Scanning Electron Microscopy/Energy Dispersive X-ray Spectroscopy), and we compare these findings with the results obtained from previously studied meteoritic samples.
  

Temporal super-resolution differential dynamics microscopy for detecting fast dynamics

We introduce dual-color differential dynamic microscopy (DDM) for detecting fast dynamics. DDM has been used extensively to measure the diffusive or ballistic motion of small particles, macromolecules and bacteria. Rather than localizing and tracking individual particles, DDM works by measuring the intensity fluctuations in images across a range of detectable spatial frequencies and provides data similar to that provided by dynamic light scattering. However, DDM is limited by the camera frame rate. Fast dynamics can be measured with high-speed cameras but those are typically expensive. We have developed a dual-color imaging setup which allows us to detect dynamics faster than the camera’s frame rate. We trigger blue and red light at well-defined times within a single image exposure. By analyzing each color channel separately and in combination we detect dynamics that are several times faster than the camera frame rate.   

Gradient-Based Algorithms for Characterizing the Structure of Fibrin Clots

The human body’s ability to close wounds through clotting is vital to everyday function—but irregular clotting can cause diseases like deep vein thrombosis, Von Willebrand disease, or hemophilia, which lead to hundreds of thousands of deaths each year. Understanding how various clotting mechanisms affect the mechanical and structural properties of a blood clot’s fibrin fiber network is integral in working to prevent and treat these clot-related diseases. Two structural characteristics of the network, the average fiber diameter and branch point density, lend themselves to discovery by applying various computational image analysis techniques to images acquired using scanning electron microscopy or fluorescence microscopy. Algorithms using gradient-based thresholding were implemented in Python to minimize data loss from classic image analysis techniques and to quantify the network’s structural characteristics.   

Progress on the Development of a Magnetic Field Sensor

This project aims to develop a magnetic field sensor by using the special properties of quantum entangled photons to intensify the sensitivity of a Faraday effect based sensor. The Faraday effect occurs when the polarization of light rotates as it passes through select materials in the presence of a magnetic field. For our set up, we chose crystals with high Verdet constants: Cd0.57Mn0.43Te andCd0.86Mn0.14Te. The introduction of a super magnet creates a measurable polarization rotation in light passing through the selected crystals. Using a magnet in the shape of an annulus, we experimented with a variety of geometrical configurations in an attempt to maximize the rotation produced by the magnet and crystal. In the first set up, we mounted the magnet in close proximity to the crystal. In later designs, we placed the crystal inside of the super magnet by 3-D printing a structure to hold the crystal. Generating position-momentum entangled photons through parametric down-conversion, our goal is to use the quantum nature of the photons to create a highly sensitive magnetic field sensor.   

Adsorption of CO2 and N2 on Graphite and Cone Grid

Adsorption is the phenomenon that occurs when a gas is in the presence of any material and is uniquely fit for different elements. Using molecular dynamics simulations, the adsorption of a CO₂ and N₂ mixture on graphite is studied at differing temperatures. Attempts to increase selectivity are made by introducing cones, inverted and upright, on top of the graphite. Results show that at molecules are more easily adsorbed at 300K than 400K and that the presence of the cones decreases selectivity, though upright cones perform worse than inverted cones. Further study at different ranges of temperature and strengths of interactions with the inverted cones should provide useful in determining the most effective way to use adsorption to separate CO₂ and N₂.   

The Effect of Si Nanoparticles and SiGe Nanoparticles on the Photovoltaic Conversion Efficiency CdS/CdTe Thin Film Solar Cells

The addition of embedded Si nanoparticles and SiGe nanoparticles in CdS/CdTe thin films deposited on ITO coated glass has been investigated in this study. We’re testing the effects of Si nanoparticles and SiGe nanoparticles on the efficiency of CdS/ CdTe thin film solar cells and comparing the effects of different deposition times, particles sizes, and how they compare with each other. Si was originally tested in order to compare to the results of Au and Ag nanoparticles. SiGe nanoparticles are also being tested because Ge is likely to form more stable nanoparticles. SiGe alloys will have higher mobility as charge carriers than Si. In the process of creating these solar cells, CdTe and CdS are deposited onto ITO coated glass substrates using the method of Pulsed Laser Deposition. Si or SiGe nanoparticles were deposited between the CdS layer and the CdTe layer using the PLD method with various deposition parameters and durations to obtain variations in nanoparticles coverage at the interface. In order to characterize the Si nanoparticle embedded thin films, XRD, AFM, and SEM/EDX were used.   

Engine of Life: Biophysics and Tyrosine

Carbon dioxide emissions have increased sharply within the last few decades, which has resulted in climate change and pollution. This has led to the search for alternative energy sources, using photosynthesis as an inspiration. Within photosynthesis, photosystem II (PSII) uses light energy to drive the energetically demanding four-electron oxidation of water to dioxygen. There are two symmetric redox-active tyrosine residues, Y_Z and Y_D, in the D1 and D2 protein subunits of PSII. While these tyrosine residues are chemically identical, they are functionally distinct. It is proposed that Y_Z is directly involved in the primary electron transfer pathway of PSII. In contrast, the Y_D residue is proposed to be involved in the assembly of the catalytic Mn₄Ca-oxo cluster. My research is focused on understanding the structure and function of the Y_Zand Y_Dresidues of PSII. In my presentation, I will describe the methodology of cyanobacterial cell cultures, isolation and purification of PSII and the application of pulsed electron paramagnetic resonance spectroscopy to study the Y_Z and Y_D radicals of PSII.   

Ideal Diode Behavior at the Graphene - WSe2 Schottky Junction

The metal-semiconductor interface is a core component of many nanoelectronic devices. The Schottky junction that can form at these interfaces is key to our understanding of device behavior. We have demonstrated that in graphene-silicon junctions, the current-voltage behavior based on bulk analysis no longer applies, and the diode is best characterized by the Landauer quantum transport formalism. We extend this analysis from silicon to transition metal dichalcogenides, a class of 2D semiconductors. We construct Van der Waals heterostructures of graphene and tungsten diselenide encapsulated in hexagonal Boron Nitride, and vary the geometry to analyze the physics of this new Schottky junction.   

Ferromagnetic Positron Beam Guidance for Single Shot Measurement in 2D-ACAR Spectrometers

Angular correlation of electron-positron annihilation radiation (ACAR) is a technique to determine the electronic structure of solids. It is based on detecting the annihilation gammas' deviation from collinearity in order to measure the electron momentum before annihilation.
This study deals with the experimental feasibility of single-shot ACAR measurements as a way of efficiently mapping the Fermi surface of crystals with a low electron momentum directions, corresponding to a quasi 2D electronic system.
Single-shot measurements require the sample normal to be aligned with the detector axis, with the positron source being outside the field of view at the same time.
A novel approach of ferromagnetically guiding positrons under high external magnetic field (1.0T) conditions is explored both in simulation and experimentally. Furthermore, new radiographic methods to quickly assay positron beams are evaluated.   

Symmetries of Two Atoms with Contact Interactions in a Ring Trap

We consider an idealized model of ultracold atoms in a ring-shaped optical trap with s-wave and p-wave contact interactions. The relative dynamics for two interacting particles are mapped onto a single particle traveling on a ring disrupted by zero-range delta-function barriers and delta-prime barriers. We categorize the symmetry groups of these potentials and find the exact solutions for stationary states. By classifying the energy spectrum in terms of the irreducible representations of the symmetry groups, we reveal that there are additional symmetries “hidden” in contact interactions.   

Measurements of the Size Variation of Optically Trapped Aerosol Water Droplets Using Cavity Enhanced Raman Spectroscopy (CERS)

Optical traps have numerous applications including the study of aerosol droplets. It has been observed that in a single beam optical trap, micro-meter sized aerosol droplets exhibit hysteric axial motion in the direction of beam propagation. Two meta-stable positions have been observed and it is thought that the cause of this motion between positions may be due to morphology dependent resonances (MDRs). For this to occur, thermal expansion of the droplet from a varying laser beam power must occur. Using Cavity Enhanced Raman Spectroscopy (CERS) we can investigate the size of the droplet as the position of the droplet and the laser power varied.   

Time-integrated Four-wave Mixing Measurements on Transition Metal Dichalcogenides in High Magnetic Fields

Monolayer transition metal dichalcogenides are 2D materials with unique optical properties that make them potential candidates for use in optoelectronic devices and even quantum logic gates. We can observe changes in exciton dephasing rate in TMDs under high magnetic fields and use four wave mixing spectroscopy to better understand and manipulate exciton dynamics. Using the MONSTR apparatus, we took FWM measurements of a monolayer MoSe₂ sample, scanning from negative to positive delay signal to analyze the dephasing time of excitons. We applied magnetic fields up to 25T and altered the polarization scheme of the excitation pulses from cross-circular to co-circular to observe the dependence of exciton dephasing rate on these factors. The preliminary data suggests non-Markovian behavior, which can be seen in the broadening of the FWM signal. This may be a result of increased biexciton formation, which we learned may be tuned using a magnetic field. In the future, the application of a magnetic field onto heterostructure TMDs may also provide interesting insights, particularly into the dynamics of interlayer excitons.   

The magnetic structure of a strongly correlated rare-earth based intermetallic system Au2PrIn..

The electronic structure of the rare-earth based intermetallic system, Au₂PrIn, is studied using computational methods. The material, a member of a large family of Heusler compounds, was recently discovered, and only crystallographic data was reported. The never reported Lanthanum version (Au₂LaIn) was also studied for the comparability of non-magnetic analogue. Both Au₂LaIn and Au₂PrIn show three-dimensional metallic nature with large band dispersion at Fermi level in electronic structure. Au d-electrons are dominated in the electronic band structure of La system, whereas rare-earth f-electrons are dominated in the electronic structure of Pr system. The study suggests that the magnetism develops as the f-orbital fills up from non-magnetic La-compound to magnetic Pr-compound. The electronic band structure of Pr compound shows local magnetism with f-electron bands near the Fermi level. Calculated magnetic moment is comparable with the expected magnetic moment of Pr. Calculated Curie temperatures are comparable with the stable magnetic structure predicted by spin polarized calculations. Detailed results of electronic and magnetic properties of Pr-system with the predicted magnetic structure will be presented.   

MCG modelling and eccentricity calculations for Pb+Pb and Au+Au collisions at various √sNN

Glauber models provide insight into the initial state of nuclear collisions by treating them in terms of the interactions of the constituent nucleons, in accordance with theories about the scattering of composite particles. These phenomenological techniques are commonly used to determine various geometric quantities associated with such femtoscopic many-body systems. The Glauber Monte Carlo approach uses a random impact parameter and measured nuclear densities to investigate quantifiable properties such as the particle multiplicity and the average geometric eccentricity for heavy ion collisions. The former involves the incorporation of a particle production model to calculate the total transverse energy, a measure of centrality. The latter delves into the eccentricity of different event classes, which can be used to characterize various collision shapes for measurements of elliptic flow of heavy mesons. The results of both applications are then compared with analyses of data from the CMS and STAR experiments.   

Utilizing Electronics to Extend the Life of Musical Instruments

Musical instruments are often discarded when the cost of fixing an issue becomes an inconvenience. Millions of dollars worth of instruments are thrown away each year while communities around the country struggle with funding art and music programs. This causes music to become a privilege and for certain instruments to be only associated with certain socioeconomic classes. A new system needs to be designed where students and teachers can be exposed to musical instruments without having to worry about financial constraint. Instead of disposing of an expensive instrument and adding to landfills, mass produced electronic components can be used to extend its functionality as an educational tool. Affordable light sensors are used in place of the core sound producing material on the instrument. Software is combined with the light sensors in a manner to reproduce the sound it would make with original materials along with the ability to manually change the tone. Due to an electronic interface, performance can be easily recorded in notation software to keep track of progress and enhance the musicians understanding of music.   

Study on the Spontaneous Elemental Lysis of an Electrolyte due to the presence of a Static Magnetic Field from contact with a so-called Permanent Magnet

This is a project to verify the results of a controversial experiment performed by Prof. Felix Ehrenhaft (1944). The absence of any constructive follow up research work for 74 years has motivated this work.
This experiment is focused on studying the effects of the static magnetic field due to standard laboratory and industrial grade AlNiCo bar magnets on a 0.1 M solution of CuCl₂. The deposition of copper granules begins near the south pole of the bar magnet and gradually progresses to cover the entire surface of the magnet. Gas bubbles form near the north pole of the magnet. This hints at the lysis of the CuCl₂ electrolyte. However, this happens without any external induced potential difference acting between the terminals, and only when a magnet is present, not otherwise. The study of the lysis of electrolytes in the presence of such a static magnetic field and the implications that this phenomenon presents are the aims of this experiment.   

Synthesis and physical properties study of polar magnet HoFeWO6

Polar oxides are important system to study due to their technologically relevant properties such as ferroelectricity, multiferroicity, piezoelectricity, magnetoelectricity. Recently, RFeWO₆ (R=Dy, Y, Tb, Eu) type polar oxides were synthesized and type-II multiferroic property was reported [1]. We have extended this series and successfully synthesized HoFeWO₆ in polar structure. We have also characterized this material using dielectric and magnetization measurements. We will present the synthesis method and the results from the dielectric, structural and magnetic measurements.
[1] Ghara, Somnath, et al. "Ordered aeschynite-type polar magnets R FeW O 6 (R= Dy, Eu, Tb, and Y): A new family of type-II multiferroics." Physical Review B 95.22 (2017): 224416.   

The Development of Four-Wave Mixing Spectroscopy to Measure Vibrational Spectra in the Low-Frequency Terahertz Range

We report the progress of developing four-wave mixing spectroscopy to measure vibrational spectra in the low-frequency terahertz range. The optical tabletop system, we have improved in the Laser Spectroscopy Lab at the University of West Florida is a multipurpose system [1], capable of executing a variety of spectroscopy methods such as e.g. Raman, Coherent Raman, and Laser-Induced Breakdown spectroscopy. For this project, our focus is on Coherent Raman Spectroscopy, through three-color two-beam broadband nonlinear frequency mixing. The three-color nature of the four-wave mixing signal allows for an effective non-resonant signal suppression relative to the polarized and depolarized Raman bands. This type of nonlinear four-wave mixing has not been fully utilized for ultrafast coherent Raman microscopy. To demonstrate the precision and merit of our modified system, we present measured and processed spectra of liquid and crystalline samples and provide a characterization of the critical components that constitute our system.

[1] Laszlo Ujj, “Contribution to the development of low-frequency terahertz coherent Raman micro-spectroscopy and microscopy “, Research article, Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy, 2018, Volume 199, 15, Pages 448-454.   

Thermal Analysis of Pr1-xNdxOs4Sb12 in Normal State

At low temperatures, the filled-skutterudite compounds PrOs₄Sb₁₂ and NdOs₄Sb₁₂ exhibit states of unconventional heavy-fermion superconductivity (SC) and ferromagnetism (FM), respectively. Thus, we have interest in studying the doping system of Pr_1-xNd_xOs₄Sb₁₂ in order to identify the competing mechanisms between the two states. Because the multiband superconductivity in this system may still be influenced by the electron-phonon mechanism, the normal state properties may give us insight into the properties of conduction electrons and phonons. The molar specific heat of Pr_1-xNd_xOs₄Sb₁₂ is measured in the normal state 10K-300K, and the thermodynamic parameters are extracted by incorporating Debye Temperature, Einstein Temperature, and the electronic specific heat coefficient, which provide information about the lattice stiffness, rattling effect, and electron correlation, respectively. Our findings convey that the Debye and Einstein temperatures have a V shaped x dependence that decreases towards central values of x, following the trend in transition temperature of low temperature ordered state.   

Ferromagnetic Resonance of All Oxide Core / Shell Nanoparticle Variants

Core-shell magnetic nanoparticles (MNPs) are being considered for various applications in spintronics as well as in the bio-medical field. Ferromagnetic Resonance (FMR) is a widely-used technique for determining magnetic properties such as bulk magnetization and anisotropy, gyromagnetic ratio / g-factor, and damping. Broadband FMR using a co-planar waveguide (CPW) is well-suited for examining properties of thin film samples, but MNPs present some challenges. MNPs can have a wide distribution of sizes and crystalline orientation. Also, at room temperature MNPs are generally superparamagnetic, where the magnetic orientation is unstable against thermal fluctuations. This research sought to adapt the standard CPW-FMR methods for thin films to characterize the resonant responses of MNPs, starting with commercially available magnetite (Fe₃O₄) MNPs and progressing to more complex Fe₃O₄/CoFe₂O₄ core-shell nanoparticle variants. Protocols were developed for mounting the different MNPs onto substrates suitable for CPW-FMR. The temperature- and frequency-dependent resonant response of the MNPs was explored. The determination of specific sample properties extracted from the CPW-FMR methods will be discussed.   

Generation Optical Beating between Hyperfine levels in the Decay of Rb atoms Excited by Different Pulse Shapes.

We are investigating quantum interference in the atomic decay signal in a dilute thermal atomic gas with an intense pulsed laser beam. A short pulse of laser light (~ 3 ns) is used to drive atoms from the ground state to an excited state hyperfine manifold of levels. We are exploring how quantum beating and other excitation properties depend on the shape of the driving frequency (e.g., cw, chirped, white-noise, etc).   

Analysis of Time Evolution Algorithms to be Utilized in Modeling Classical and Quantum Mechanical Systems

Time evolution in classical and quantum mechanical systems has been the focus of research because the behavior of physical systems with time are determined by their initial conditions. Calculation methods for the system values as it changes throughout time have to be very accurate in order to have predictions that we can rely on. This is the challenge of time evolution and its modeling.

In this project several time evolution algorithms were implemented and evaluated according to criteria of accuracy, stability, and consistency. Several scalar algorithms were examined as well as a technique for matrix exponentiation. Results show that the matrix exponentiation technique has significant advantages and is suitable for use in extending the time evolution models for various systems.   

Verifying 2D device potentials and conduction with Kelvin Probe Force Microscopy

In a 2D field effect transistor (FET), the electrical properties of the channel are modulated using a gate voltage. The electrostatic doping of the channel and the contact resistance of the interacting layers both contribute to the overall device conductivity, which plateaus above a certain gate voltage. Other factors, such as surface cleanliness and microscopic details of the films also influence the conductivity as can be deduced from variations between multiple devices. In this study, we explore the ability of Kelvin probe force microscopy (KPFM) to separate out the different factors influencing overall device conductivity. By applying a potential bias to a simple device, we build confidence in the linear response and reproducibility of the KPFM technique. We then directly visualize the uniformity of the surfaces, the potential barriers between layers, and the characteristics of the WSe₂ film as a function of the applied voltage. These data deepen our understanding of device potentials and conduction in 2D FETs.


  

Detecting topological superconductivity by using a dc-SQUID

We theoretically investigate the current and phase response of a dc-SQUID composed of two Josephson junctions (JJs) in parallel. The JJs are exposed to an in-plane magnetic field and their chemical potential and Rashba spin-orbit coupling strength are tuned by top gates acting separately on each of the junctions. By tuning the system parameters, each JJ can individually be driven from the trivial to the topological phase and vice versa. We investigate the 3 possible dc-SQUID configurations: the two junctions are topological, one junction is trivial and the other topological, and the two junctions are trivial. We perform theoretical simulations of the phase difference and critical current of the dc-SQUID for the 3 different configurations, and by comparing them we identify the signatures of the topological superconducting phase and its dependence on junction transparency, magnetic field, and spin-orbit coupling.   

Water and Carbon Dioxide in Hydrated Hyperbranched Polyethylenimine Membrane Using Molecular Dynamics Simulation and Density Functional Theory

Since excessive use of fossil fuels is releasing large amounts of CO₂ and exacerbating global warming, developing efficient materials to capture CO₂ is crucial. In this study, we investigate the hydrated hyperbranched polyethylenimine (HB-PEI) membrane in the presence/absence of water and carbon dioxide using molecular dynamics (MD) simulation to characterize their distribution and reaction mechanism in the HB-PEI membrane. For this, we prepare a model HB-PEI molecule and construct a condensed HB-PEI phase in the amorphous phase with various concentration of water and carbon dioxide molecules. Large-scale Atomic/Molecular Massively Parallel Simulator (LAMMPS) package is used in our MD simulations to establish the equilibrium state of HB-PEI membranes. Through our MD simulations, we obtain samples of the local structure of water/carbon dioxide nearby amine groups in HB-PEI membranes and scrutinize a possible molecular mechanism and corresponding energy barrier for carbon dioxide capturing, via density functional theory (DFT) approach.   

An evaluation of methods for measuring thermomagnetic transport properties of bulk and thin film materials

In the “method of four coefficients,” experimental data for electrical conductivity, Seebeck, Hall, and Nernst coefficients are fit to models based on Boltzmann transport theory to estimate quantities such as the carrier effective mass, concentration, mobility, and scattering exponent. This powerful method provides experimental information about the electronic structure and charge carrier scattering mechanisms in solids, and has been used to understand enhanced performance in some thermoelectric materials. Nevertheless, there is relatively little literature available regarding descriptions of experimental apparatuses and methods of measurement of all four coefficients on a single sample in a single measurement cycle. We recently constructed a custom system for measurement of thermomagnetic properties from 8 K to 400 K. Here we report on a study of different approaches for experimentally measuring these four coefficients on both bulk and thin films samples. For example, we have investigated the effect of mounting the sample in a flat vs. vertical geometry, which can be important for adiabatic vs. isothermal Nernst measurements. Our results provide useful guidance for constructing such apparatuses to measure the thermomagnetic properties used in the method of four coefficients.   

Analyzing Challenging Scientific Thoughts in a Christian Academic Setting

Throughout a significant portion of history and within modern culture, the fields of science and religion appear to be competing for the same holds in a person’s belief system. Universities are where academics and the sciences are the prevailing held truth, while in churches, the Bible reigns as supreme authority. However, in a Christian academic setting, the predominate school of thought in belief systems might turn into a little more of a melting pot. By analyzing gathered personal data (via surveys and interviews), one can begin to piece together the predominate thoughts on the apparent conflict between religion and science at such a setting. Institutions such as this, while always diverse in thought, can present a cohesion between the two seemingly opposed parties in a way where neither side loses ground culturally. Data gathered on personal religious experience, beliefs and experiences gathered around evolution, creationism, the Big Bang, and other issues that fall from that have been collected to present an image of how science and religion might be in understanding at a Christian house of higher education.   

Electronic and Structural Properties of Yb-based Materials

We have investigated the electronic, structural, and optical spectroscopy of YbT₂Zn₂₀-based (T = Co, Rh, and Ir) 1-2-20 compounds using first-principles calculations that account for the strong on-site-f-electron Coulomb interactions. We observed the strong hybridization and screening of the f-levels by the itinerant conduction electrons that led to massive electron behavior with an unenhanced electronic Sommerfeld coefficient of over ~600 mJ mol^-1 K^-2. The optical spectroscopy exhibits renormalized Drude response, a partial enhancement of the dynamical conductivity below ~1.0 eV, the emergence of the midinfrared peak structures at ~0.46 eV, and low photon-energy dynamical effective mass and scattering rate with nontrivial energy-dependent scattering resonances from free carriers. Surprisingly, both the electronic and optical spectroscopy of YbT₂Zn₂₀ are distinctly different from the rest of the studied materials, with characteristic features that support its closeness to quantum criticality.   

395 GHz Sample Holder Waveguide for Dynamic Nuclear Polarization

A major issue in nuclear magnetic resonance (NMR) is the low sensitivity which can be achieved as a result of the small magnetic moment in nuclei. At the MagLab, we exploit dynamic nuclear polarization (DNP), which increases NMR sensitivity through a process in which electron spins transfer their larger polarization to the nuclei of interest, to yield larger DNP signals at 14 T. For DNP experiments, our spectrometer uses a sample holder which doubles as a waveguide for the microwave used to saturate the electrons in the sample under study. The lack of axial symmetry in this sample holder limits our range of experiments.

To overcome this limitation, we are manufacturing axially symmetric sample holder-waveguides by coating sample tubes with a conductive silver layer. The layer must have thickness and conductivity such that it is transparent to a 600 MHz radio frequency, yet reflective to a 395 GHz microwave. Our process consists of creating silver layer prototypes using various deposition methods, characterizing the prototypes, replicating an optimized prototype on our sample tubes, and implementing the final product in DNP experiments.   

Effect of Post-deposition Thermal Treatment on the Structural and Electrical Properties of Oxygen Deficient Perovskite Metal Oxide Thin Films

Thin films of perovskite metal oxides such as SrTiO3 (STO), La_0.67Ca _0.33MnO₃ (LCMO) and CaMnO3 (CMO) exhibit many interesting electronic properties that are useful for technological applications. These properties are highly sensitive to the oxygen stoichiometry in the thin films, which can be varied by subjecting the thin films to heat treatment (annealing) after the film is deposited. We will discuss the results of our experiments to study changes in the lattice constants and electrical resistivity induced by post-deposition annealing in various gas ambients and in vacuum. We will also present the effects of annealing in the presence of a fluorine-containing polymer with the goal of incorporating fluorine in the film crystal structure. Thin films in this study are grown using pulsed laser deposition, lattice constants are determined using 4-circle x-ray diffraction and resistivity is measured using a 4-probe VanDerpauw technique.   

Unsupervised Machine Learning for Rare Signal Detection in CMS Detector Data

We present here a method for identifying high-p_t beyond the Standard Model (BSM) signals, with no dependence on the expected physics signatures of these models. We do this by constraining our detector data to the expected signal region of a given model and training a deep neural autoencoder to 'recognize' jets belonging to the Standard Model (SM) background data. In this way, our models learn the background signature of SM jets in our signal region. We then evaluate this model on a mixture of SM and BSM jets, flagging jets with a high autoencoder reconstruction error as 'anomalous' or signal jets. This method of searching independently of any physics model is especially useful when the BSM model in question involves particles of unknown mass, branching fraction, etc. This is especially relevant in Dark Matter searches, such as the search for Semi-Visible jets in missing-E_T events at the LHC. In these cases, our models are capable of flagging a range of BSM models with different parameters within our signal region, as opposed to a supervised model (such as a Boosted Decision Tree) which is only able to flag the exact signal it was trained on. We show that this technique is a promising method of identifying arbitrary BSM signals to high precision in LHC data.   

Monte Carlo simulation on ferromagnetic monolayer of honeycomb CrI3

Since the discoveries of robust two-dimensional ferromagnetism in CrI₃ [1] and Cr₂Ge₂Te₆ [2], many research interests have focused on thermal stabilities of magnetism on van der Waals monolayers. In this work, we investigated the thermal evolutions of classical spins on 2D ferromagnetic honeycomb monolayers during simulated cooling and/or annealing by using the Monte Carlo method. Using the Heisenberg Hamiltonian with parameters proposed for CrI₃ [3], we closely reproduce the experimentally observed temperature- and in-plane magnetic field-dependences of the magnetization including the size of the Curie temperature [1]. We also find that in zero-field cooling CrI₃ monolayers exhibit multiple domains with high densities of domain walls. We will present how the domains behave as the temperature or magnetic field is changed.

[1] B. Huang, et al. Layer-dependent ferromagnetism in a van der Waals crystal down to the monolayer limit, Nature 546.7657 (2017)
[2] C. Gong, et al. Discovery of intrinsic ferromagnetism in two-dimensional van der Waals crystals, Nature 546. 265 (2017)
[3] L. Chen, J.-H. Chung, B. Gao, T. Chen, M.B. Stone, A.I. Kolesnikov, Q. Huang, and P. Dai, 1 (2018).
  

Models and Simulations of Electric Field-Biased Nanoparticle Self-Assembly

We design, model, and simulate an experiment to study the effect of electric bias on particle-coverage densities produced during ionic nanoparticle self-assembly. The experiment involves the application of a uniform external electric field parallel to a glass substrate during the
self-assembly of silica nanoparticles. We refer to this procedure as directed self-assembly of monolayers (DSAM). In our theoretical analysis, we modify existing cooperative sequential adsorption models to account for diffusion under an applied electric field. We use the mean field
approximation to solve for particle-coverage densities. To ascertain the validity of this method, we compare our solutions to Monte Carlo simulations of the system.   

Correlating Multiphoton-Absorption-Induced Luminescence (MAIL) with Morphology in Noble-Metal Nanostructures

The optical properties of nanostructured noble metals differ drastically from those of their bulk counterparts due to surface plasmon (SP) resonance. When incident light couples with the SPs of a noble-metal nanoparticle, it gives rise to strong, localized electromagnetic field enhancement. In particular, surfaces with high curvature experience especially strong field enhancement. Silver and gold SP absorption cross sections lie in the visible region, which make nanostructures of these metals ideal for studying optical phenomena. Multiphoton-absorption-induced luminescence (MAIL) is a nonlinear optical phenomenon that involves ultrafast pulses of light impingent on a nanostructured surface. The SP field enhancement allows for more efficient multiphoton absorption, and highly-efficient, broadband luminescence over the visible spectrum is produced as a result. We examine how the morphologies of noble-metal nanostructures correlate with their MAIL signals. Specifically, we focus on silver nanodendrites, synthesized from galvanic displacement reactions, and gold nanorings and nanotriangles, grown through colloidal synthesis. The absorption order, intensities, and spatial distribution of MAIL signals are examined for different morphologies.   

Characterizing the Mechanical Properties of 3D Printed Structures for Growth Plate Tissue Engineering

Stem cell differentiation is highly sensitive to the biochemical and mechanical environment. A major concern in tissue engineering is the need of a robust technique for producing structures with the precise amount of rigidity to guide stem cell differentiation. Damaged cartilage within the physis does not regenerate easily and can lead asymmetric growth arrest, making it an ideal model application. The physis has three distinct zones where cells evolve differently depending on the environmental conditions. Previous in vivo rabbit studies indicate that the mechanical properties of the 3D printed structures implanted in this area have an effect on stem cell differentiation in the area. In this work we use a solvent solution consisting of a multifunctional acrylate-based resin and ethanol to control the overall modulus of the structure. 3D printed pillar structures of varying solvent concentrations and backfills (air and a poly(ethylene glycol) diacrylate based hydrogel ) were put through compression testing and we found that there was an apparent reduction in the modulus of higher concentrations of the solvent solution of over 30 percent. This work allows us to control the modulus of 3D printed structures, which can be applied to future stem cell differentiation studies.
  

The Effect of Short Range Attractions on Sequence Defined Polyelectrolyte Coacervation

Complex coacervation is the liquid-liquid phase separation of polyelectrolytes in aqueous salt solution into a polymer-dense phase, the coacervate, and a polymer-dilute phase, the supernatant. Previous work using Monte Carlo simulations demonstrated that changing the sequence of charged and neutral monomers on polyelectrolytes while keeping the charge fraction constant alters the extent of phase separation. However, previous data does not account for the hydrophobicity of different neutral monomers. To understand the coacervation of polymers with various chemical structures, van der Waals interactions at various strengths are included in Monte Carlo simulations to show hydrophobic effects in the system. Comparisons are made to existing Monte Carlo simulations and experimental data. Understanding these patterning effects will enhance the knowledge of biomacromolecule phase separation, as well as expand the understanding of sequence-dependent polymer physics.   

All-Dielectric Nanophotonics Research Involving Undergraduates at Illinois State University

Resonant excitation and manipulation of high-index dielectric nanostructures (such as Silicon, Germanium) provide great opportunities for engineering novel optical phenomena and applications. Here, we report selective excitation and enhancement of multipolar resonances, and non-radiating anapoles in silicon nanospheres using cylindrical vector beams (CVBs). Our approach can be used as a spectroscopy tool to enhance and identify multipolar resonances as well as a straightforward alternate route to excite electrodynamic anapoles at the optical frequencies.
  

Silica Polypeptide Composite Janus Particles

Janus particles are interesting materials due to their inherent dual face properties. The ability to be able to have different functionalization and properties on a single particle allows these materials to be used for drug delivery and interfacial chemistry. Silica polypeptide composite Janus particles are unique because they are stimuli-responsive particles containing an organic polypeptide shell with an inorganic colloidal silica core, half of which is coated in chromium. The responsive nature comes from the polypeptide shell consisting of the homopolypeptide, poly (ε-carbobenzyloxy-L-lysine) (PCBL), which is known to exhibit a reversible coil-helix transition when dissolved in m-cresol. The silica core particle can be removed from the particle via etching to retain a hollow polypeptide vesicle, potentially with the chromium still bound to part of the vesicle. These can be used to study the conformational transition of the polypeptides while being structured but without being tethered to a solid surface. This will help to better understand the polypeptides role as a drug delivery vesicle.   

Arbitrary Super-Resolution Patterns Formed in Quantum Dots

In this research, interference patterns were etched onto quantum nanoparticle thin-film samples using the second harmonic of a nanosecond Nd: YAG laser. By forming one interference pattern on a sample and then phase shifting the beam, we are able to produce patterns with twice the resolution due to the nonlinear nature of the quantum nanoparticles. By using the interferometric method with phase and amplitude masks arbitrary 1-D (with two beams) and 2-D (with four beams) patterns can be formed using this lithographic technique.   

Using Depolarized Dynamic Light Scattering to Characterize Microgel Dependence on Synthesis Temperature

Microgels formed by crosslinking polysaccharide polymer chains exhibit a thermally reversible volume phase transition due to the amphiphilic properties of the parent polymer. Specifically, the microgels deswell above a volume phase transition temperature, T_v. Microgel dynamics above and below T_v has been studied extensively by dynamic light scattering (DLS) before. Here, the structure and dynamics of microgels synthesized at various temperatures are investigated through the use of depolarized dynamic light scattering (DDLS). The technique has previously been used in our lab to examine solely geometric anisotropies in non-spherical particles. It has also been used in the literature to study shape fluctuations in microgels that have a hard, polystyrene core and a soft, polymer shell. This research project looks into a possibility that the observed DDLS signal above T_v, for microgels synthesized at various temperatures, arises from microgel shape fluctuations.   

Neutron Spectrum Unfolding with Deuterated Liquid Scintillator Detectors

The detection of neutrons from nuclear reactions is becoming increasingly important in nuclear physics experiments. Neutron measurements are needed for understanding reactions that drive stellar explosions and provide insight into the behavior of exotic nuclei. Traditionally, neutron energies are obtained by the time-of-flight (ToF) method which consists in measuring the time that neutrons take to travel from the target to the detector. The quality of the measurements is limited by the distance traveled, the solid angle spanned by the detector array, and a background of gamma-rays and scattered neutrons. Pulse-shape-discrimination (PSD) allows one to distinguish, in certain detectors, neutron and gamma-ray interactions due to their decay times. A promising method in neutron research involves the use of deuterated liquid scintillators as neutron detectors where a unique correlation in the pulse-height (PH) information can also be extracted. In this work a method of unfolding neutron spectra will be presented. A response matrix is created from deuterated liquid scintillator detectors by combining PSD and PH data to obtain neutron energies alongside ToF. This method will enhance neutron studies relevant for nuclear structure and nuclear astrophysics research.   

Gold-Aluminum Thin Films as an Alternative to Pure Metals for Plasmon Resonance Sensors

Noble metal alloys have been widely investigated as an alternative to pure metals for improving the optical response of optoelectronic devices in the visible range of the electromagnetic spectrum. However, their use is hardly extended to the ultra-violet (UV) range. As an alternative, aluminum (Al)-based alloys could expand the functionality of photonic devices into the UV range. In this work, we fabricate and measure the dielectric constant of binary mixtures of gold (Au) and Al thin films. The films were deposited on a glass substrate via the co-sputtering method. The dielectric functions were measured using spectroscopic ellipsometry. We investigated how the optical response of the samples changed under a wide range of temperatures, from 25°C to 200°C. Also, we performed AFM, SEM, XRD, and EDS measurements. We demonstrate that, in all our cases, a bimetallic material outperforms their pure metal counterparts in the near-IR range after the temperature treatment, e.g., Al0.51Au0.49 shows an increased quality factor of its surface plasmon polariton (QSPP) than pure Au and Al. To verify our calculations, we measured the SPP of pristine and temperature treated samples.   

Water Wheel Generator

The world continues to move toward eco-friendly renewable energy by creating new, more advanced technologies. Solar panels, wind turbines, and dams are some of our biggest contributors. This project takes a second look at the potential of bringing the vertical water wheel back into the limelight. Historically water mills have been crucial in the progress and growth of civilization by being a major source of strength and power. Combining this power with a continuously recycled water flow has the potential to create a constant source of renewable energy. Using the principles of water wheel designs that have been perfected over centuries and securing a small generator creates a viable configuration for a tabletop water powered generator. The goal is to create an enclosed system where a water wheel powers its own pump for recycling water flow and still outputs enough wattage to be used to charge batteries. This prototype has gone through rapid prototyping using 3D printed wheel designs for the optimization of electrical output. On a small scale this design can be a tabletop charger however, on a larger scale, this design could power homes and office buildings without the hazards of daming.   

Reaction Kinetics and Mechanical Properties of Reversible Epoxies by Diels-Alder reaction

<p>Epoxies are an important class of thermosetting polymers for many long-term applications such as adhesives, structural materials, and coatings. Epoxies have durable and robust mechanical properties; but, they are difficult to remove, recycle and rework. Epoxies capable of reversible polymerization could solve these problems. In this experiment, reversible epoxies were synthesized by introducing the Diels-Alder reaction groups to epoxy monomers. 1,1'-(Methylenedi-4,1-phenylene) bismaleimide and furfuryl glycidyl ether were reacted to form a Diels-Alder cycloadduct. This cycloadduct was confirmed using Fourier Transform-Infrared (FTIR) Spectrometry. The forward and reverse Diels-Alder reaction was monitored by FTIR measurements at 90 ^oC and 110 ^oC as a function of exposure time. IR absorption peaks relevant to the reverse Diels-Alder reaction grew due to longer exposure time at 90 ^oC and 110 ^oC. The equilibrium shift was observed toward the reverse reaction dominant side at higher temperature by comparison of FTIR spectra at 90 ^oC and 110 ^oC. Mechanical properties of the reversible epoxies was examined to confirm two states of reversible epoxies: a durable network at lower temperature and soft segments at higher temperature.</p>   

Hyperentangled Bell-states analysis of two-photon 2n-degree-of-freedom system

It is known that it is impossible to unambiguously distinguish all the Bell states in one system using only linear optics even if you also use hyperentanglement. However, we can find out how many Bell states can be unambiguously distinguished with a given n in two-photon 2n-degree-of-freedom hyperentangled system. This work, based on the criterion of Peter van Loock and Norbert Lutkenhaus, uses the symmetric matrix to express Bell states and the number of distinguished Bell states can be obtained by the rank of a matrix made with all the Bell states. The results show that with the increase of n, the number of distinguished Bell states increases exponentially by 2ⁿ⁺¹-1, while the efficiency of discrimination decreases exponentially by 1/2^n-1. This work lays a foundation for Bell-state analysis of three-photon system.   

Electronic Stucture of Negative Trions in Semiconducting Quantum Dots

A trion system is one that includes an exciton system pairing with either an electron (called a negative trion) or a hole (called a positive trion). Trions in bulks typically has infinitesimal binding energies, but recent research has shown that trions in quantum well have their binding energy increased by an order of magnitude, i.e. the confinement configuration supports the formation of trions. The measurements of the photoluminescence spectra of the trions in a single, self-assembled quantum dot also show the dependence of the photoluminescence lines on the size and geometry of the quantum dot.
In this project, we will theoretically investigate the electronic structure of negative trions in a spherical quantum dot of a direct band gap semiconductor. The effect of an electric field on the trions will be considered to obtain the splitting of trion energy levels under the electric field. The respective wave functions of the resulting states have been obtained as well. The dependence of the electric field effect on the size of the dots will be investigated.   

Floquet Engineering in 1D Non-Hermitian Model

Topological properties of non-hermitian and periodically driven (floquet) systems have attacted great attention in recent years. Here, I find we can introduce robust topological edge state in 1D non-hermitian time-periodic system while its static counterpart is trivial. Also when we adjust the frequency of the outer driving within the regime around the bandwidth Δ, topological phase transition of the system has a similar behavior with the Hermitian counterpart[1]. This will help us understand non-hermitian and floquet systems better.

[1]Dal Lago, V. , M. Atala , and L. E. F. Foa Torres . "Floquet topological transitions in a driven one-dimensional topological insulator." Phys. Rev. A 92(2),023624(2015).   

Low-Pressure Plasma Device for Activation Bonding Polydimethylsiloxane(PDMS) to Glass

Surface activation of PDMS through plasma treatment is a common technique used in the fabrication of microfluidic chips for a strong bond between PDMS and glass. Plasma treatment can be done in an open lab or low-pressure environment, in which commonly produces more activated stronger bonds. The activation levels of plasma treatment of PDMS and glass can be measured using atomic force microscopy. A low-pressure plasma bonder is being built and tested in order to produce stronger microfluidic devices. The configuration and experimentation of the self-built low-pressure plasma generator will be presented.   

Electrocolonography: Non-Invasive detecton of colonic cyclic motor activity from multielectrode body surface recordings

Approximately 20% of Americans suffer from colonic motility disorders, including slow transit constipation (STC) and irritable bowel syndrome (IBS), with significant physical and social morbidity. Accurate clinical diagnosis is often challenging due to the non-specificity of symptoms. In addition, the only direct assessment tool available in current clinical practice is colonic manometry, which involves placing a small, flexible tube, or catheter, through the rectum and into the colon. There is a substantial need in both colon research and clinical practice for an accurate, non-invasive method to analyze colon motor activity. This work validates a novel non-invasive method to identify periods of cyclic motor activity in the colon using multichannel skin-surface electrical recordings on the lower abdominal region, termed electrocolonography (EColG). We also explore several spatial filtering techniques to identify wave propogation of the colon.   

Exploring the Memory Capacity of Embedded Synfire Chains

Memory is a long-studied property of neural systems that is not well understood yet essential for the success of any animal. The synfire chain is a popular sequence generating model which has been hypothesized to represent a building block for neural computation and memory. Our research was conducted to explore what affects the storage capacity of memory networks composed of embedded synfire chains. Using computational modeling, we compared the memory capacity of two neuron models – Leaky Integrate-and-Fire (LIF) and Izhikevich – and systematically varied factors that showed to affect each network. In both cases, we used a simplified model of global inhibition to control run-away excitation. Our studies showed that the memory capacity for networks consisting of LIF neurons depends strongly on the inhibition, excitation, and width of the chains. The observed memory capacity was low and insufficient for practical applications. The memory capacity for networks consisting of Izhikevich neurons was considerably larger. More research needs to be done on this. Future directions would also explore more targeted models for inhibition in hope for achieving larger capacity for memory networks.   

Enhancement of Nonlinear Optical and Piezoelectric Properties in Ferroelectrics from Symmetry-Breaking Strains

In the pursuit of lead-free piezoelectrics, recent studies have used temperature, pressure, applied voltage, and other methods to enhance piezoelectric properties in ferroelectrics. Here, we investigate changes in symmetry, polarization, and domain structure near phase boundaries in BaTiO3 crystals and BaTiO3-xBaSnO3 thin films in order to understand how property enhancement depends on external stimuli and replicate the property enhancements in versatile thin films. We use second harmonic generation (SHG) polarimetry to detect changes in point group symmetry and polarization direction, supplemented by scanning SHG microscopy, which reveals the domain structure and allows polarimetry of specific domains. We observe lower-symmetry monoclinic phases with SHG and piezoelectric property enhancements of up to 4 times near temperature phase boundaries and with applied voltage in BaTiO3 crystals. In addition, we observe similar symmetry lowering near a compositional phase boundary of BaTiO3-xBaSnO3 thin films, indicating the potential for similar property enhancements in the thin film system which is more easily integrable into a variety of devices.   

Compaction and Creep in a Photoelastic Granular System

Creep is the subsurface, slow movement and deformation of constituents in a granular packing, such as sand or sediments, due to the applied stress and disordered nature of its grain-scale interactions. The phenomenon of creep in dense granular systems is relatively understudied, leaving many open questions. We explored creep through experiments in which we observed the influence of a controlled disturbance on rearrangements in a granular packing. Our granular system consists of disks that are made from a birefringent material; this allows us to use image acquisition to observe both the movement of the grains and the changing stress distribution within the system. In the experiment, we deliver disturbances via taps of a pendulum to one side of the chamber that contains the granular packing. The tapping strength is measured using an accelerometer. We tilt the chamber to varying slopes to observe changes in system response as we approach the critical slope for more rapid granular flow. Using image analysis, we examine grain rearrangements, changes in packing density, and stress redistribution as the grains creep due to these disturbances.   

Influence of Spin-coating Speed and Mixing Ratio on Polymer-Fullerene Films

Due to increasing environmental concerns and depletion of nonrenewable resources, different forms of renewable energy must be studied. Organic solar cells are a very popular and promising form of renewable energy; however, they are still relatively new and not as well studied in comparison to traditional silicon solar cells. Thus, it is important to gain more knowledge on how those organic solar cells could be enhanced in different areas. Among the organic solar cells (OSC), OSC based on polymer-fullerene bulk heterojunctions have attracted much attention due to their low cost and easy processing conditions. The most common polymer-fullerene bulk heterojunction is poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl-C61 butyric acid methyl ester (PCBM).
In this work, we show our studies on how the characteristics of P3HT:PCBM films depend on spin coating speed and mixing ratios. By varying the ratio of PCBM:P3HT as well as using different spin coating speeds, the impact on the absorbance spectrum of the films were studied. The results help us choose the optimum processing conditions for P3HT:PCBM films to be used in bulk heterojunction solar cells.   

The Durability of a Chiral Auxetic Structure

Metamaterials are bulk objects with special mechanism properties defined by their repetitive inner structures, rather than the materials they are made of. One of the special mechanisms is ``auxetic behavior'': when the materials are stretched in one direction, unlike conventional materials, they will also expand in the lateral direction. In this project we studied the dependence of Poisson's Ratio on the geometric parameters of a chiral auxetic structure. This chiral auxetic structure can be potentially applied in deployable antennas. We used 3-D printer to efficiently design and produce the chiral auxetic structures. With an innovative experimental setup and image analysis techniques, we were able to study the durability of the chiral auxetic structures with various geometric parameters.   

Thermodynamic and Magnetic Properties of the System Ca3Co2-xZnxO6

The crystal structure of Ca₃Co₂O₆ is built from chains of Co³⁺ ions that are separated by Ca ions. Magnetic moments associated with low-spin Co ions are located on trigonal prismatic sites within the spin chains. Ferromagnetic intrachain interactions are an order of magnitude stronger than antiferromagnetic interchain interactions, leading to a quasi-one-dimensional magnetic structure. In an effort to study its magnetic state, we substituted Zn²⁺ for Co³⁺ ions. Zinc ions occupy the trigonal prismatic (magnetic) sites and charge balance causes low spin Co³⁺ ions occupying octrahedral sites to become low-spin Co⁴⁺. Thus, Zn substitution dilutes moments on trigonal prismatic sites and induces them on octahedral sites. Samples of Ca₃Co_2-xZn_xO₆ were synthesized via solid state reaction and phase purity was evaluated via x-ray diffraction. X-ray diffraction elucidated a solubility limit near x = 0.1. Heat capacity and magnetization were measured as a function of temperature and were used to construct a T vs. x phase diagram for Ca₃Co_2-xZn_xO₆. In this poster, this phase diagram will be compared to the previously reported phase diagrams for Ca₃Co_2-xR_xO₆ with R = Cu, Cr, Fe, Mn, Sc, and Ho.
  

Image Analysis and Model of Cytoskeletal Filament Density of C17.2 Cells

Single-Walled Carbon Nanotubes (SWCNTs) have great potential in the biomedical field as a way to deliver materials into the body. However, the long-term effects on the body are unknown, especially in regards to the cytoskeletal filaments. Many cellular functions such as differentiation, cell shape, and motility rely on the aid of filaments, such as actin and nestin. Recent studies have demonstrated that internal and external stimuli can affect these filaments thus affecting cellular behavior. In this project, neural stem cells, specifically C17.2 cells were used and the distribution of the cytoskeletal filaments were modeled. A data set was created through Filament Sensor, a program created by Benjamin Eltzner, and ImageJ, to quantitatively process and enhance images. This poster will look into the distribution of actin and nestin throughout the cell. Future work will look at how SWCNTs affect the distribution of these filaments.   

Photo-reduction was used to reduce the oxygen content in Graphene Oxide and correlated to Young’s modulus measurements

Finding a cost-effective way to make graphene would be beneficial to our world. Mechanical properties of graphene oxide were studied using and AFM system. Specifically, Young's modulus, stiffness, and hardness of graphene oxide samples were measured as a function of sample thickness. Results obtained for Young’s modulus ranged from 24 GPa to 277 GPa for samples of thicknesses 1.4 µm to 93 nm. Once the Young’s modulus values were obtained, photo-reduction was used to reduce oxygen content in graphene oxide samples using a 405 nm ultraviolet laser for 36 hours. Young’s modulus was reevaluated after photo reduction and its value increased from 24GPa to 44GPa for the 1.4 µm thick sample.
  

RF Antenna for Generation of Spin-Dependent Force on Cold Lithium Atoms

This study aims to design a small radio-frequency (RF) antenna that can generate magnetic fields with magnitude of 1 Gauss. This RF antenna is useful because the generated fields can magnify the hyperfine state of lithium atoms which are used to study the quantum atomic gas at low temperature. However, the function of the RF antenna is restricted when the diameter of the antenna is constrained in less than 1 inch. This restriction is the result of a high energy loss caused by signal reflection in the circuits if the antenna is directly connected with the power supply. In order to reduce the loss, an impedance matching configuration should be introduced between the antenna and the power supply. At the end of the experiment, a small RF antenna with impedance matching configuration that can generate a magnetic field with desired frequency and amplitude is designed.   

Investigation of a LCoS-SLM and Progressively More Difficult Applications of the SLM Including Optical Trapping

At the heart of all phase modulation applications is a Spatial Light Modulator (SLM). A Liquid Crystal on Silicon Spatial Light Modulator (LCoS-SLM) uses a voltage to phase retardation relationship that allows for easily programmable phase shifts. The programmable phase shifts are often programmed using phase masks. The LCoS-SLM is investigated and various applications of the SLM, including using the SLM as a “Projector”, to generate Bessel Beams, to generate a Vortex Beam with Orbital Angular Momentum, and flat-top beams of various shapes are performed. All applications are performed using a similar optical set-up with various phase masks. Finally, a vortex beam is used to optically trap micron sized nanoparticles.   

Achieving Flat Gold Surfaces for the Organization of Organic Molecules

Organic electronics are interesting for use in electronic devices but suffer in competition with inorganics due to their lower conductivity. Crystallizing organic semiconductors can increase their conductivities and a possible crystallization method is self-assembly driven by the topography and chemistry of an atomic surface reconstruction. This work aims to develop an atomically smooth Au(111) surface, which has a herring bone reconstruction. Many methods are known for samples in UHV, however these methods do not work in atmosphere or a glove box, such as is used in our lab. This work expands upon that of Maver et al. using torch annealing, which allows the atoms to rearrange into a lower energy state. We anneal the sample at 710±10°C for two minutes, and then three minutes at 410±10°C. This yields larger and flatter terraces compared to the unannealed material. The unannealed gold had mounds with no flat areas and a max depth of 7.6 nm. The annealed Au(111) had flat terraces, 13-30 nm in size, with step heights in the order of .235 nm, which matches the interplanar spacing for Au(111). In the future, we will optimize this process to achieve large terraces by adjusting the temperatures and times used.   

Immunofluorescent biomarkers for distinguishing cell phenotypes in zebrafish somitogenesis and autonomous cellular oscillators.

During zebrafish embryogenesis, coordinated genetic oscillations occur in a population of cells in the posterior-most tissues of the body axis, the tailbud and presomitic mesoderm (PSM), which will subdivide the embryonic body into morphological segments, called somites. It has been proved previously that single cells dispersed from tailbud will oscillate automatically. However, it remains unclear that which phenotype of the cells will present as autonomous oscillators. T-domain transcription factors Ntla and Tbx16 will both express in the period of somitogenesis but in different regions. Immunofluorescence experiments for both genes demonstrated the distribution of cells in different phenotypes in zebrafish embryo during somitogenesis. Comparison of immunofluorescence results for 5-somite stage embryos and high-somite stage embryos showed the change of PSM region. Combined with results for single-cell oscillation and statistical analysis, immunofluorescence for cell dispersals was able to tell the phenotypes of the oscillating cells.   

Stabilization of Francium Materials Using Cluster Compounds

We present a Quantum Mechanical study of cluster compounds Fr_l X_mY_n and Ra_l X_mY_n (X, Y = other; 0 =< l, m, n =< 10). Half-life λ of the most stable isotope of francium (²²³Fr) is 22 minutes, and only 20–30 g of the element exists naturally at any given time. The melting point, the boiling point, and density of Fr are uncertain. Isotopes of radium are radioactive, but the most stable isotope ²²⁶Ra has a half-life of 1600 yr. The stabilization of radium is known to have been experimentally achieved, using a solution in which an effluent and a metal chloride are mixed, then the previously obtained mixture reacted with a sulfate ion, to obtain effluent containing stabilized radium. The chloride can be a barium, strontium or lead chloride. We are looking to see if there are similar trends, in properties, for francium. So far, we have achieved a factor of 2.7 stabilization for some of the francium cluster compounds, compared to bare Fr in our calculations. There is the possibility of application of a stabilized Fr material as a catalyst promoter, or a remover of oxygen from vacuum tubes and light bulbs, or in atomic clocks, or in petroleum exploration, analogous to the practical uses of Cs.   

Arduinos and the X-Ray Diffractometer

For Ithaca College's Senior Project, it is required that a student develop an idea for a project, as well as the budget, goals, proposal, and design of the project itself. This project discussed in this presentation is developing a connection between an Arduino and the X-Ray Diffractometer owned by Ithaca College. The goal is to have the diffractometer and Arduino communicate easily and allow for data to be taken during the experiment easily. We wanted to make sure that students would be able to take, as well as save, data in the simplest way possible.   

Assessing iOLab-based Laboratories in Online Instruction

We implement an online introductory mechanical physics laboratory course that features the Interactive Online Lab (iOLab) device and a class structure that encourages student engagement and collaboration. The students in the online class achieved a learning gain of g = 0.55 on the FCI. The end of class survey showed that the students overwhelmingly (93%) valued the lab portion of the course. However, we can only infer their mastery of the lab learning objectives from lab related questions on exams and lab reports. We present an ongoing study that seeks to understand student attitudes towards the labs, and the processes students use to complete them. We observe student help-seeking strategies, the amount of time they spend performing the lab, and student motivation. Student attitudes towards online physics labs will be accessed through the use of in-person interviews and a pre and post Colorado Learning Attitudes About Science Survey for Experimental Physics (E-CLASS). The overall goal is to form a more detailed picture of how students complete online physics labs to improve the quality of online physics instruction.   

The Effects of Heat Exposure Over Various Time Intervals on the Performance Characteristics of Monocrystalline Silicon Photo-voltaic Cells

With the rise of awareness for climate change and environmental protection, research into alternative energy sources has becoming increasingly important. One of the most popular forms of renewable energy is solar power. Most commonly built solar panels utilize a collection silicon photovoltaic cells to generate electricity. Our research specifically looks into the efficiency characteristics of monocrystalline silicon photovoltaic cells after they are exposed to intense heat for a predetermined amount of time. The cells were heated at temperatures of 190^oC, 200^oC, and 210^oC, for times ranging from 10 minutes to 110 minutes. An analysis of our data shows an average increase in fill fraction with time for all three temperatures. These results indicate a permanent overall increase in efficiency of the cells compared to baseline values. This outcome proves that we do not need to significantly change the process of production of the cells in order to increase the efficiency.   

Optimization of CoFeB Electrodes for Magnetic Tunnel Junctions

A magnetic tunnel junction (MTJ) is the current standard for converting magnetic information into electrical information. It is formed by sandwiching an insulating barrier between two ferromagnetic electrodes. An external magnetic field is then used to switch the electrodes between parallel and antiparallel magnetic alignment. The focus of this research is to optimize the switching between magnetic states through the optimization of electrode thicknesses, especially when non-conventional barrier materials are used. The goal is to decouple the two FM layer switching fields, which allows read-out of two resistance states of the device. The method used is three-fold: First, MTJ stacks are grown using sputter deposition. Next, vibrating sample magnetometry is used to test for switching of magnetic alignment. Finally, atomic force microscopy is used to test for interfacial roughness, which could cause the layers to couple and prohibit antiparallel alignment. I will discuss our work developing a method for minimizing the coupling between the FM electrodes by focusing on understanding the interfacial roughness.   

Predicting and Synthesizing Photocatalytic Semiconductor Materials

The photocatalytic generation of hydrogen fuel is a promising way to store and utilize solar energy and mitigate carbon dioxide emissions. Water-spitting photocatalysts use solar energy to convert water into molecular hydrogen and oxygen. Yet, many of the photoactive materials required for water splitting are costly or inefficient. To accelerate the discovery of photocatalytic materials, a high-throughput computational screening protocol was developed, coupled with experimental synthesis and characterization, to select photoactive materials that can efficiently split water. Several candidates from the list were synthesized and their bandgaps and ability to produce hydrogen were characterized. Many of the compounds exhibited desirable photocatalytic water-splitting properties. This screening provides a framework from which viable photocatalytic materials can be discovered.   

Effects of Nanoparticle Size and Density on the Critical Current and Creep in (Y,Gd)BCO Films: Comparisons to Strong Pinning Theory

Incorporating nanoparticle inclusions into superconducting films is a well-established route for boosting current carrying capacity. (Y,Gd)Ba₂Cu₃O₇ films containing various nanoparticles have been shown to demonstrate inexplicably slow rates of thermally activated vortex motion (creep, S). Understanding the microscopic source of these slow rates is key to determining how to reduce S in superconductors. In this project, we report on the effects of different sizes and densities of nanoparticle inclusions on the critical current and creep in (Y,Gd)Ba₂Cu₃O₇ films. Samples in this study all contain a low density of R₂Cu₂O₅ (R = Y,Ba) inclusions (naturally occurring during growth), and each contain either no other nanoparticles, BaHfO₃, BaSnO₃, or BaZrO₃ nanoparticles. The data presented was collected from low temperature magnetization measurements in fields of 0.3 T up to 35 T using a VSM at the NHMFL and a local commercial SQUID.   

Design Parameters of Liquid Crystal Elastomer Multi-Laminates

Liquid crystal polymer networks or elastomers (LCEs) can be surface aligned into complex orientations that produce cooperative deformation and out-of-plane shape transformation.^[1] When subject to thermal stimuli, thermotropic LCE films actuate and display significant weight lifting capabilities that scale with thickness.^[2] Here, we investigate parameters that affect the mechanical output of LCE actuators including thickness, composition, disclination type, and geometrical packing. Specifically, we develop an LCE composition amenable to photoalignment processing that actuates below 70°C. The mechanical response of this material (stroke and force displacement) are contrasted to canonical LCE material systems. The actuation output of the LCE is increased by lamination.^[3] A multi-laminate LCE material actuator over 1mm in thickness is demonstrated, capable of lifting large objects (>1 lb).

[1] T. H. Ware, M. E. McConney, J. J. Wie, V. P. Tondiglia, T. J. White, Science 2015, 347, 982.
[2] C. D. Modes, K. Bhattacharya, M. Warner, Phys. Rev. E - Stat. Nonlinear, Soft Matter Phys. 2010, 81.
[3] T. Guin, M. J. Settle, B. A. Kowalski, A. D. Auguste, R. V Beblo, G. W. Reich, T. J. White, Nat. Commun. 2018, 9, 2531.   

Solid State NMR Physics Laboratory

We report on the progress of the newly developed physics laboratory at the University of the Incarnate Word (UIW). This laboratory is the first physics research laboratory at this institution. It includes a new solid state NMR experimental setup. The experimental setup is comprised of a 400 MHz variable temperature, cryogen free, superconducting magnet manufactured by Cryogenic. It also includes a Tecmag Redstone NMR Spectrometer. This spectrometer is very flexible and therefore many different samples may be analyzed. Field calibration was conducted by using Nuclear Magnetic Resonance (NMR) with liquid D2O. This allowed for successfully tracking the Te NMR signal of an iron-chalcogenide FeTeSe sample. The superconducting transition of different purity iron-chalcogenide FeTeSe samples will be analyzed in detail.   

Simulation of Efficient Perovskite Solar Cells

While Perovskite popularity has sky-rocketed since the efficiency reached 22% in 2018, there is still ample research to be done as to what characteristics and parameters affect the solar cell. In order to obtain a greater understanding of the mechanics of how a perovskite cell becomes more efficient, this project focuses on the simulation program wvAMPS and how parameters alter main characteristics such as Voc (open circuit voltage) and Jsc (short circuit current). The perovskite solar cells were obtained from National Renewable Energy Laboratory. The structure of the solar cell is Ag(120nm)orAu(100nm)/spiro-OMeTAD(180-200nm)/perovskite(550nm)/mesoporous-TiO_2(200nm)/compact-TiO₂(40-60nm)/FTO/glass. The measured solar cell parameters are: Voc = 1.06V, Jsc = 16.53 mA/cm^2 , Fill Factor = 63.5% and Efficiency = 11.2%.
The simulation attempted to parallel the measured Perovskite solar cell. With given parameters, we were able to simulate a solar cell with a Voc = 0.93V, Jsc = 21.27 mA/cm^2 , Fill Factor = 85.5% and Efficiency = 16.9%, which are very close to the measured parameters. We also varied the layer thickness to check how the device performance can be improved. These results will help determine how to optimize device structures for Perovskite solar cells.   

An Algorithm to Optimize the Search for Electromagnetic Counterparts to Gravitational Wave Events

Our understanding of gravitational wave (GW) events is greatly enhanced by identifying and studying their electromagnetic (EM) counterparts. For nearby GW events with a small localization uncertainty, an effective strategy is to search for new transient sources in previously catalogued galaxies, whose properties are consistent with the GW data. Even with a limited field of view, it is plausible to discover the EM counterparts using an efficient observational strategy. But because many galaxies must be observed and the EM counterparts are faint and fade rapidly, a reliable automatic procedure is crucial to schedule observations efficiently. To meet these challenges, we designed an algorithm in Python that uses a catalogue of nearby galaxies and the three-dimensional GW localization map to create a prioritized list of galaxies based on GW error-map probability, observability, and absolute magnitude. We tested our algorithm with past GW events and, within a few minutes, obtained consistent results with previous observations. We conclude highlighting how this algorithm can more generally assist in formulating effective followup plans with various types of small field telescopes at a variety of wavelengths, including radio.   

Machine learning for classifying the chiral phase transition in AdS/QCD

AdS/QCD is a phenomenological application of the gauge/gravity duality to strongly-interacting nuclear matter, including the quark-gluon plasma (QGP). This work uses machine learning to classify the order of the chiral phase transition between ordinary nuclear matter and the QGP. Our machine learning method is a supervised-learning synthesis of four standard classification algorithms: classification and regression trees (CART), k-Nearest Neighbors (kNN), Support Vector Machines (SVM) with a linear kernel, and Random Forest (RF). It is trained on a subset of data with known behavior, and tested on the remaining data, with a 100% success rate. We also discuss the application of this machine learning method to the development of an AdS/QCD model featuring a critical point in the QCD phase diagram, which aligns with the current experimental program at Brookhaven National Laboratory.   

Development of a zinc manganese dioxide flow battery

With larger penetrations of renewable energy on our electrical grid, there is a need for large-scale energy storage devices. A redox flow battery is an electrochemical device with great promise as a large-scale energy storage technology given the scalability of the technology and that power and energy output are decoupled. This study aims to incorporate the strengths of flow batteries with the abundance and relatively high safety of manganese dioxide batteries. However, manganese dioxide batteries historically suffer from poor reversibility. To improve reversibility, the effects of different electrolyte additives, as well as the synthesis of manganese nano-rods, were investigated. These solutions were then incorporated into a prototype flow battery to address the need for a safe and reliable energy storage device.   

Achieving an efficient control of antiferromagnetic order in artificial layered iridates

Antiferromagnetic (AFM) materials started to gain traction owing the advantages of reliability, ultrafast dynamics, etc. in spintronic applications. In our recent work, we investigated AFM order in layered iridates, which is a newly established Mott system similar to cuprates but features a strong spin-orbit coupling. By building the spin-orbit Mott insulators as SrIrO₃/SrTiO₃ superlattices, we gained controllability in the strength and sign of interlayer exchange interaction. This enables one to reach the 2D limit of a magnet, where the ordering temperature is only governed by magnetic anisotropy. The 2D antiferromagnet preserves a hidden SU(2) symmetry, which was first proposed in cuprates but never experimentally realized. Specifically, we unveiled that Dzyaloshinskii-Moriya interaction in the square-lattice magnet does not contribute to the spin anisotropy. The extremely strong 2D critical fluctuations enable us to achieve giant AFM responses to sub-tesla magnetic fields. The observed field-induced logarithmic increase of the AFM ordering demonstrates a new pathway for designing efficient AFM spintronics. These results were recently published on Phys. Rev. Lett. [119,027204, (2017)] and Nat. phys. [14, 806--810 (2018)].   

Multimodal x-ray and electron microscopy of an Allende meteorite sample

Correlative multimodal microscopy that combines complementary nanoscale imaging techniques is essential for extracting comprehensive chemical, structural, and functional information of heterogeneous complex samples. Advanced electron microscopy provides atomic-scale spatial resolution with quantitative elemental composition, while x-ray microscopy can achieve high-resolution imaging of bulk materials with chemical, magnetic, electronic, and bond orientation contrast. In our recent work (Science Advances 5, eaax3009, 2019), we combine x-ray ptychography and scanning transmission x-ray spectromicroscopy with 3D energy-dispersive spectroscopy and electron tomography to perform structural and chemical mapping of an Allende meteorite sample as a model system. We use textural and quantitative elemental information to infer its mineral composition and discuss potential processes that occurred. This multimodal x-ray and electron microscopy of the same sample overcomes the limitations of individual imaging modalities and opens up a route to future multiscale microscopies of complex functional materials and biological systems.   

Rationale Design of Polymeric Materials for Biological and Energy Applications using Multisclae Simulation Methods

Two broad areas are investigated in this poster: (i) Self-assembly of Methylcellulose in Solution: We employ coarse-grained molecular dynamics simulations to show that the current theories of stacked toroid model for fibril formation is valid only at certain polymer concentrations. Rather, we showed the existence of a nucleation mechanism and the importance of conformational fluctuations for systems containing randomly coiled chains. We also show how the conformational and structural characteristics change with grafting the backbone with flexible polymers; (ii) Polyelectrolyte Complexation: We utilize implicit solvent coarse-grained molecular dynamics to probe the influence of charge sequence along the polymer backbone, on their adsorption efficacy onto grafted polyelectrolytes. Our work show that adsorption is strongest when both the grafted and the free polyelectrolyte in the solution possess a block charge sequence architecture and is weakest when both the grafted and the free polyelectrolyte possess an alternating architecture. We then showed that the sequence dependence on adsorption efficacy is enthalpic in nature and not entropic. We also show the effect of influence of charge sequence on polydisperse polyelectrolytes.   

Proximity effects in two-dimensional transition-metal dichalcogenide materials for quantum information science

Significant strides have been made towards the storage and processing of quantum information. However, scalable robust material platforms are scarce. The two-dimensional (2D) transition-metal dichalcogenides provide a possible path forward due to their valley degrees of freedom, which may be probed by circularly polarized light. Unfortunately, these levels are commonly degenerate in energy. Therefore, to create a viable platform for valley-based qubits, it is crucial to break time reversal symmetry in a controllable manner, allowing for direct manipulation. Using state-of-the-art ab initio techniques, we demonstrate that controllable valley splitting can be achieved through a magnetic exchange proximity effect generated by a ferromagnetic 2D material substrate. Furthermore, by introducing vacancies into the transition-metal dichalcogenide layer, long-lived two-level impurity states may be stabilized. This approach reveals a new path towards the rational design of new complex multilayer systems for direct application in quantum information technologies and spin-optoelectronic devices.   

Simultaneous Study of Structure and Correlation-Driven Transitions via X-ray Standing Waves

The possibility of decoupling electronic phenomena from those of the lattice has been a hot topic when discussing correlated metal-insulator transition materials such as VO₂, NbO₂, or V₂O₃.[1] A mainly electronic transition could enable ultra-fast switching, thin film electronics, with little risk of the inevitable physical degradation associated with bulk structural transitions. However, the roles of structural (Peierls) and electron correlation (Mott) effects in driving these transitions continue to be debated in the literature.[2] Using x-ray standing waves (XSW) and high quality epitaxial thin films, we have now concurrently investigated both the structural and electronic transition within some of these correlated materials using a single technique, directly measuring their simultaneity or lack thereof for the first time. We discuss these results and their wider implications.

[1] Kalcheim, Y. et al. Robust Coupling between Structural and Electronic Transitions in a Mott Material. Phys. Rev. Lett. 122, 57601 (2019).
[2] Lee, W. C. et al. Cooperative effects of strain and electron correlation in epitaxial VO₂ and NbO₂. J. Appl. Phys. 125, 082539 (2019).   

Electronic Correlation Effects on the Fermi Surface Topology of d-Plutonium

Due to its position at the boundary between the light and heavy actinides, Pu has exotic physical properties that are complex and challenging to model. The difficulties in obtaining a full theoretical understanding for this elemental solid stem from the transitional characteristics of Pu-5f orbital electrons and understanding the role of the fluctuating magnetism in the electronic structure. Focusing on the δ-phase of elemental Pu, we perform a careful comparison of Fermi surface topology calculations using DFT and DFT+U methods. The de Haas-van Alphen (dHvA) frequencies at the Fermi surface and band masses are calculated in both magnetic and nonmagnetic states. We also analyze the effective mass enhancement due to 5f-electron correlation effects with DMFT as compared to the Gutzwiller approximation (GA). The comparison study will be helpful for future experiments to validate theoretical modeling.   

van der Waals photothermoelectric effect in atomic layer heterojunctions

Two-dimensional (2D) van der Waals (vdW) heterostructures provide exceptional opportunities for new physics and devices due to their unprecedented ability to tune the electronic, optical, magnetic and spintronic properties by atomic layer stacking and electrostatic gating. Harnessing this versatility requires a fundamental understanding of light-matter interactions and establishing new functionalities for photon-charge and photon-spin conversions. Here, we report the first observation of a highly-tunable vdW photothermoelectric effect in dual-gated MoS /graphene junctions with a striking multiple-polarity switching of photocurrent as a function of junction bias and carrier density. In stark contrast to photovoltaic effects arising from excitonic absorption in MoS₂, the vdW photothermoelectric effect originates from photoexcitation of hot electrons in graphene and thermoelectric transport across the vdW junction. Systematic studies of photoconductance as a function of photon energy and intensity reveal vdW photothermoelectric effect as the dominant mechanism for photocurrent generation at room temperature, as opposed to excitonic absorption. These findings provide an important step for understanding and control of vdW-interface devices.   

New Approaches and Observations in Scaled Contacts for 2D FETs

Atomically thin 2D crystals are promising channel materials for extremely scaled field-effect transistors (FETs). For devices at the scaled regime, both channel and contact length must be scaled, with channel length being the distance from source to drain contacts and contact length being the length of the source/drain covering the 2D semiconductor channel. Contacting 2D materials at these scaled contact lengths (< 30 nm) has rarely been pursued or studied in depth. Moreover, the device community has not yet determined how contacts can be scaled without causing significant degradation in device performance; i.e., how long is the transfer length, below which current crowding effects appear? Here, we demonstrate new measurement approaches and results for determining the transfer length of MoS₂ FETs by physically scaling the contact length. We found that, contrary to previous reports, top contacts can be scaled to ~20 nm without obvious degradation in transistor performance. Our data from measurements of over 100 devices with different contact lengths statistically imply that contact resistance variation increases in the scaled contact regime. Our work illustrates the impact of current crowding in scaled contacts and the ultimate scalability of metal-2D contact interfaces.   

Investigation into form factors for mechanical-resonance-based methods of information storage

Non-traditional information storage has become increasingly ubiquitous as a means of providing interactive, environment-specific information. With this in mind, we have investigated potential form-factors for a PZT-based information storage method that has visibly indistinguishable features for improved security. By manipulating the poling of PZT transducers, we developed a frequency-dependent, embedded array of transducers which, when excited at the relevant frequencies, reveal an engineered velocity profile. This velocity profile is then measured using a scanning laser Doppler vibrometer and decoded. Since this storage method is capable of storing frequency-dependent information, it can also store multiple layers of information that can be easily separated by applying a fast-Fourier transform. These multiple layers can be used for storing additional information or further hiding the encoded data. Two potential form factors are simulated and discussed in this proceedings, one has a flat profile while the other has a cylindrical profile.   

Spontaneous thermal Hall conductance in superconductors with broken time-reversal symmetry

The thermal Hall conductivities(THCs), $\kappa_{ij}$s have extensively been studied in recent condensed matter experiments. THC can spontaneously become non-zero for a time-reversal symmetry (TRS) broken system, and have a contribution from topologically protected edge states. In this talk, we focus on an additional bulk effect, the impurity pair breaking mechanism(IPM) in superconductors (SCs). Previously, the THCs were calculated for the chiral p-wave[1-2] SCs for point impurities. Motivated by d-wave TRS broken SCs; URu$_2$Si$_2$, SrPtAs including Sr$_2$RuO$_4$ which is recently suggested to be also possibbly, we calculate THCs at finite temperatures and for finite size impurities using the non-equilibrium quasiclassical Green's functions.

We find that the IPM is dominant in $\kappa_{yx}$ at finite temperatures when compared to the topological contribution except at very low temperatures. There are two experimental signatures of IPM: 1. a non-monotonic temperature dependence, 2. sign change as a function of temperature depending on the scattering process.

[1] S. Yip, Sup. Sci. Technol. 29, 085006 (2016)

[2] Arfi et al., Phy. Rev. B 39, 8959 (1989)   

Do Levinthals arguments lead to a paradox for Si20H20?

Levinthal argued that the folding of a protein should require a time longer than the age
of the universe. However, since
it is experimentally well established that proteins do fold on a quite short time scale, these
arguments are known as the Levinthal paradox. The paradox is resolved by folding funnel
hyphothesis. Here we will present a system whose global
minimum is not embedded in a large funnel. The global minimum is virtually
not accessible on a reasonable time scale.
The potential energy surface of Si₂₀H₂₀ is considered for this study. The dodecahedron
cage of Si₂₀H₂₀ is the theoretically established ground state. However it has never been
observed experimentally. Based on an extensive exploration of possible reaction pathways
of Si₂₀H₂₀ we show that there exists a huge number of intermediate structures that consist
mainly of collapsed cages. Among all these reaction pathways that lead into the ground
state, the system has to go through energetically flat regions before it reaches the Si₂₀H₂₀
ground state. This implies that there is no clear driving force towards this ground state
that would give rise to a short directional reaction pathway that would lead rapidly into the
ground state.   

Understanding the acoustic emission from gas bubble dynamics: a signature of CO2 leakage

The identification and characterization of CO₂ leakage signatures in water and other fluids are of interest in the geophysical study of geysers and aquifers. As several DOE programs are investigating the feasibility of operationally injecting CO₂ into the subsurface, detecting and characterizing the signatures associated with the interaction of CO₂ with water shows important implications for monitoring large scale Carbon storage. This project proposes the development of a laboratory-scale test bench to carry out experimental studies of the acoustic emission emanating from gas bubble dynamics in a bi-phasic (water and gas mixture) fluid system. Different air bubble sizes, varying from 1 cm to 10 cm, are injected from the bottom of a water bath. As the bubbles migrate to the top, the vibrational response of those bubbles is captured by an acoustic pressure sensor placed within the fluid. The interaction is expected to be dependent on the size of the bubble, which can be characterized using the recorded acoustic signal. The result provides a non-invasive technique for characterizing the air bubble-size in the water/gas system and enables us to develop a framework for determining signatures pertaining to the presence of CO₂ in water.   

Experimental Demonstration of a Superconducting 0-π Qubit

Encoding a qubit in logical quantum states with wavefunctions characterized by disjoint support and robust energies can offer simultaneous protection against relaxation and pure dephasing. Using a two-dimensional circuit-quantum-electrodynamics architecture, we experimentally realize a superconducting 0-π qubit, which hosts protected states suitable for quantum-information processing. Our multi-tone spectroscopy measurements reveal the energy level structure of the system, which can be precisely described by a simple two-mode Hamiltonian. The parity symmetry of the qubit results in charge-insensitive levels connecting the protected states, allowing for logical operations. The measured relaxation (1.6 ms) and dephasing times (25 μs) demonstrate that our implementation of the 0-π circuit not only broadens the family of superconducting qubits but also represents a promising candidate for the building block of a fault-tolerant quantum computer.   

Long-lived Floquet phases in interacting three-dimensional topological semimetals via bicircular laser fields

The use of carefully tailored light fields to manipulate quantum states of matter is an important technique in condensed matter physics. By coupling to the electronic degrees of freedom, they can induce electronic phases that were otherwise absent, for example, by selectively breaking a certain symmetry. One important example is the breaking of time-reversal symmetry by circulary polarized light, which can lead to a photo-induced Hall effect or the splitting of a three-dimensional Dirac node into Weyl nodes. Interestingly, it was recently showed that bicircular laser fields can also break spatial symmetries such as inversion or rotation symmetries, thereby inducing a charge (or spin) density wave order. Importantly, the density wave order can persist even after the light field is turned off, leading to a long-lived light-induced phase. This idea was recently explored in graphene, where it was shown to lead to a long-lived Floquet charge-density wave phase due to a dynamic synchronization transition. Here, we report our findings on coupling bicircular laser light to three-dimensional materials in order to induce spin and/or charge density wave phases. We specifically discuss the effects on topological phases of matter in Dirac and Weyl semimetals.   

Band engineering for quantum simulation with superconducting circuits

Quantum simulation has been implemented on a variety of experimental platforms such as neutral atoms, ions, quantum dots, and superconducting circuits, each offering unique features. Superconducting circuits can and have been used to realize artificial photonic materials in a wide range of lattice geometries and graph connectivities, due to the flexibility of on-chip fabrication. In addition, photon-photon interactions are possible using nonlinearities such as superconducting qubits, leveraging the vast toolkit developed for quantum computation. Here I report on recent progress towards engineering flat bands for studies of strongly correlated many-body physics.   

Using Machine Learning to Classify Phase Behavior of Oil/Water/Surfactant Systems

According to BP Statistical Review of World Energy in 2019, total oil production from the US in 2018 was about 15 million barrels per day (MBPD). Thanks to the booming shale oil production, United States has become the world’s top oil producer. In fact, estimates show that up to two thirds of conventional crude oil in mature fields remains unproduced due to the physics of fluid flow. The techniques of chemical enhanced oil recovery could overcome the physical force holding hydrocarbons, and turn these accumulations into oil reserve, which would enhance the US energy security and maintain economic growth. For the oil/water/surfactant system, the goal is to form a microemulsion phase achieving the lowest interfacial tension, which increases the capillary number and dramatically recovers the remaining oil fraction within the pore. Therefore, it is critically important to understand the phase behavior for the oil/water/surfactant systems. In collaboration with Pioneer Oil Company, our current effort is to optimize the selection of surfactants and the constituents of the surfactant blend, which turns to be a high dimensional problem. In addition to the conventional analysis, we employ machine learning techniques to solve the system as a multinomial classification problem.   

Resonant Raman Spectroscopy of the Chiral Antiferromagnet CoNb3S6

We report here the first full Raman characterization of the chiral antiferromagnet CoNb3S6 ; i.e. cobalt-doped NbS2. CoNb3S6 exhibits a large c-axis anomalous Hall effect (AHE) not entirely attributable to the small intrinsic ferromagnetic component (Co) along the c-axis [1]. This interesting behavior suggests that the enhancement in AHE may be because of a combination of magnetic field in the presence of the near-Fermi energy Weyl nodes as predicted by theory [2]. Neutron scattering experiments of CoNb3S6 [3] showed incommensurate peaks; however, it was not clear whether the peaks are due to a spin spiral or a spin density wave. Magneto-Raman spectroscopy represents an optimal method to differentiate these structures. Wavelength- and polarized-dependent Raman spectra collected at room temperature from CoNb3S6 flakes are analyzed. Experiments of temperature- and magnetic field-dependent Raman spectroscopy to seek magnon (spin wave) signatures detected no change in the spectral weight at the Neel temperature implying absence of spin density waves suggesting that the magnetic structure is helical.

References
(1) Nirmal J. Ghimire et al., Nature Commun. Vol. 9, 3280 (2018)
(2) Zachary M. Raines et al., Phys. Rev. B Vol. 96, p. 161115(R) (2017)
(3) Private communication N. Ghimire   

Physics-based models and simulations of cancer drug response in solid tumors

Over the past few decades, cancer related deaths have fallen significantly as noted by the National Cancer Institute. However, assessing cancer treatments is still predominantly a trial and error process. This approach may result in delays to administer the correct treatment, the use of more invasive procedures than necessary, or an increase in toxicity due to superfluous treatments. Although these procedures may end up saving the patient, the treatment may also have an adverse effect on their quality of life. Relaible mechanistic models of drug response can potentially be used to aid oncologists and doctors in deciding on an optimal treatment strategy for the patient. We develop a modeling framework for tumor ablation, and present coupled transport - population models of varying complexity. First, we present a radially symmetric drug diffusion and binary cell death model, which produces a theoretical dose for optimal effiacy to toxicity ratios. Further, we investigate inhomogeneous - anisotropic drug diffusion, and develop an algorithm to locate the optimla injection points. Finally, we derive stochastic tumor population models that can be coupled to transport models in our framework. Importantly, the mechanistic models outperform data-driven models in statistical tests.   

A Patterning Approach to Untangling Critical Interface Phenomena with In-Situ Resonant Scattering

Resonant soft x-ray scattering (RSoXS) is a powerful spatiochemical mesoscale characterization tool that is often overlooked across the field of interfacial science. Herein, we present a simple, yet highly sensitive, patterning approach for interface characterization that takes advantage of the physical processes intrinsic to small angle resonant X-ray scattering in order to selectively probe and enhance signals from interfacial regions of a vast array of material systems. Using several case studies, we show how patterns with simple nanoscale features can be used to 1) decouple the bulk from the interface scattering signal, 2) extract interfacial morphology with sub-nm precision, and 3) collect site-specific x-ray absorption spectra (XAS).
First, as an operando demonstration, we leverage a custom in-situ cell compatible with many synchrotron X-ray beamlines and use it conduct an operando study of Ni/Ni(OH)2 core/shell electrode undergoing electrochemical cycling under aqueous conditions. Next, we apply the technique to study the femtosecond dynamics of a 1.5 nm SiO2 surface with a broadband XUV table-top source. Finally, we introduce the potential of this technique to study material interfaces under conditions critical to the future of the next generation of electronics.   

Tuning block copolymer rheology and structure via low strength magnetic fields

The control of block polymer (BCP) structural ordering is of significant scientific interest because of their wide applications including nanotemplates, drug delivery and biomineralization. BCP properties have been controlled using various external fields including electric, magnetic, and shear fields, and via plasticizing additives, interfacial effects, and thermal methods. To control ordering in BCPs using magnetic fields, previous studies had used field strengths (≥5T), liquid crystalline mesogens, high magnetic susceptibility anisotropy groups or combinations therein. Here, we show anomalous behavior upon application of low strength magnetic fields (≥0.1 T) in coil-coil BCPs. Magneto-shear rheology shows up to a six order increase in modulus upon the field application. This magnetic response is a function of temperature, concentration, field strength, ionic content and shear strain. A minimum molecular weight and block length are required for this liquid to gel transition via low strength magnetic field. In situ small angle scattering (SAXS and SANS) and imaging techniques showed field-induced orientation in the BCPs and this orientation direction can be tuned by changing the direction of field lines.
  

Can Fermi energy be estimated experimentally?

Fermions, particles with half integral spin, follow Fermi-Dirac distribution. For free electron gas at absolute zero, Fermi energy is the energy of highest occuped level by electron. For n, which is density of free electrons, Fermi energy can be estimated by E_F= h² (3π²n)^2/3/(8mπ²) [1]. Theoretical calculation of Fermi energy of a metal involves estimation of density of electrons utilizing number of valence electrons. Experimentally, Fermi energy can be estimated by measurement of density of electrons in Hall effect measurement at room temperature. The measurement at room temperature serves as a good approximation because of very less difference between electron distribution at 0K and 300K.
[1] Modern Physics, K. Krane Wiley
Unscientific determination of Fermi energy by heating and/or cooling a copper wire, C. Kotabage, A. C. Abhyankar, Resonance- Journal of Science Education, 24 (12), 1439-1443.   

Two Content Pathways in Presenting Electromagnetism in Introductory Algebra-Based Physics Textbooks

Vector cross products play a fundamental role in determining the directions of electromagnetism in introductory physics courses. After examining thirteen introductory algebra-based physics textbooks, we found authors adopt the following two content pathways in presenting the electromagnetic phenomena: Over 90% of the textbooks follow pathway one which presents only algebraic formulas and mnemonic techniques for right- and left-hand rules. Scarcely few follow pathway two, which presents vector cross products as a mathematical model and reinforces this model by presenting the specific cross product formula for each electromagnetic phenomenon. Physics instructors teaching college physics courses similarly present electromagnetism using these two content pathways. In light of Bloom’s taxonomy of educational objectives and constructivist learning theory, we recommend the second pathway.   

Using Group Exams to Address Persistent Intuitively Appealing but Incorrect Student Reasoning

Many students tend to provide intuitively appealing (but incorrect) responses to some physics questions despite demonstrating (on similar questions) the formal knowledge necessary to reason correctly. While these inconsistencies are typically persistent even in active learning environments, we believe that adding a group component to the exam may engage students sufficiently to resolve these instances of inconsistent reasoning. In our study, students were given opportunities to revisit their answers to questions known to elicit strong intuitively appealing (but incorrect) responses in a collaborative group component of an exam immediately following a traditional individual component. Students discussed their responses with group members but were required to submit their own answers and reasoning. On this poster, we examine the effectiveness of a collaborative group exam approach in addressing and resolving inconsistencies in student reasoning and will compare the effectiveness of this approach to a more traditional peer instruction technique.   

Incorporating realistic aspects of experimentation into senior physics labs

Here we present design changes to the senior physics lab course at Penn State, which were implemented in order to increase student engagement with modeling,experiment design, and the writing process in our lab curriculum. The goal of these changes was to restructure the course to emphasize planning, prediction, and assessment of experimental data during an experimental process, in mimicry of the scientific research process, while still providing students with exposure to a breadth of experiments and experimental techniques which are historically a required part of the course. Both structural and content changes made in the course will be presented along with initial assessment of changes in student perceptions and critical thinking skills.   

Machine Learning Towards Optical Spectrum Estimation Using Nanomaterial Thin Films

Some of the major challenges in optical spectrum estimation include the necessity to create an array of thousands of identical photodetectors, or intricate mechanical systems that make the estimation system bulky and expensive. Using the spectral transmittance of an array of 11 solution-processed nanomaterial thin film filters fabricated from two layered semiconducting materials, Molybdenum-Disulfide and Tungsten-Disulfide, we have estimated the wavelength of any incoming light in a wide spectrum range. By applying machine learning techniques we have used the variations in spectral transmittance of nanomaterials as the alternative method for optical spectrum estimation. We have studied the efficacy of various machine learning algorithms including k-nearest neighbors, artificial neural networks, support vector machines, and Bayesian statistics in spectrum estimation problem and identified the key advantages and limitations of each algorithm for real-time applications such as accuracy and speed. Furthermore, we have modeled the temporal drift of filters' spectral transmittance over a period of one year and showed that it is possible to overcome the drift-induced inaccuracies over time using a modeled drift function.   

Improve the synaptic performance of resistive switching devices through interface engineering

Transition metal based resistive switching devices (like HfO₂, TiO₂₎ has been shown to be a good candidate for neuromorphic computing for its bio-inspired synaptic properties, however, the non-linear conductance change synaptic behaviour prohibits further improvement due to poor accuracy of neural network training. Here, we provide a way to eliminate the intrinsic non-linearity through electrode-oxide interface engineering, including oxygen profile control and oxide heterostructure stacking, which can improve the neural network training accuracy and shorten the training time.   

High Thermoelectric Performance in Hexagonal 2D PdTe2 Monolayer at Room Temperature

Motivated by the recent fabrication of hexagonal PdTe₂ monolayer, we investigated the thermoelectric properties of the hexagonal and pentagonal PdTe₂ structures using two approaches. The pentagonal monolayer has not been synthesized yet. The hexagonal layer had an indirect band gap of 0.17 eV while the pentagonal structure had an indirect bandgap of 1.18 eV. By applying the semi-empirical Wiedemann–Franz law to calculate the electronic thermal conductivity, we found that both hexagonal and pentagonal structures had very high ZT more than 3. However, the Wiedemann–Franz law underestimated the electronic thermal conductivity and this resulted in high ZT. Thus, we employed the Boltzmann transport equation for the electronic thermal conductivity. At high temperature ( > 500 K), the pentagonal PdTe₂ structure showed a better thermoelectric performance than the hexagonal structure. However, both structures displayed the same ZT of 0.8 at 300 K. We propose that the hexagonal PdTe₂ can be a potential high performance thermoelectric material at room temperature.
  

Textile Thermoelectric Generator Based on Carbon Nanotubes

Smart textile based on seamless device integration into clothing will dominate the wearable technology field in the near future. Carbon nanotubes (CNTs) are a favorable candidate for use in these flexible and wearable electronic devices due to tunability of their electrical and thermal properties, mechanical strength, and lightweight. The wearable devices demand wearable micro-sources of energy. Here, we propose a textile thermoelectric generator based on CNT that can utilize waste heat to generate power. The generator is fiber-structured to make it easily woven into clothing. We simulated a ~5 cm long fiber structured generator with p-n junctions at every 3 mm interval based on CNTs. To model CNTs, experimental data were used in our simulation. This generator generates a thermoelectric voltage of ~55 μV for just a 1 K temperature difference. The thermoelectric figure of merit, ZT for this CNT based device was smaller than previously reported materials. However, it is possible to fabricate CNT based devices with large ZT by proper doping and utilizing the Van Hove singularities in the density of states of CNTs.   

Structural Phase Transition of Multilayer VSe2

Vanadium diselenide (VSe2), a member of the transition metal dichalcogenides (TMDs) family, is emerging as a promising two-dimensional (2D) candidate for the electronic and spintronic device with exotic properties including charge density wave and ferromagnetism. The bulk crystal VSe2 exists in a crystallographic form of 1T phase with metallic behavior. In this paper, we report a structural phase transition of multilayer VSe2 from 1T to 2H, which occurs at about 650 K, accompanying a metal-insulator transition. The phase transition is verified by Raman spectra, as different polymorphs have different Raman signals related to different characteristic vibration modes. The electrical characteristics are also studied on VSe2 nanoflakes, as the phase transition occurs along with the metal-semiconductor transition, which is consistent with the electronic band structure calculation. The results of in-situ selected area electron diffraction (SAED) is are accord with our simulated diffraction patterns, which is strong evidence of the structural phase transition. Moreover, we observe that the 2H phase is more thermodynamically stable than the 1T phase at the multilayer level.   

Investigating the metal to insulator transition in crystalline NbO2 for neuromorphic computing applications

The metal-insulator transition (MIT) of NbO2 is promising for technological applications where a self-regulated resistivity is needed like in neuromorphic computing. Though the Mott nature (electron correlation) of the MIT of NbO2 arises purely by comparison to VO₂, the actual transition mechanism in NbOx-based memristors remains unclear mainly due to the degree of participation of multi-phases of niobium oxides, likely induced after electroforming. By investigating phase-pure, crystalline NbO2, we avoid the uncertainty created in electro-formed NbOx-based devices. Using surface sensitive techniques like LEED, HAXPES, and LEEM on NbO2(440)/Al2O3(006) thin films, we show that the phase transition does not extend to the film surface. XPS of the crystalline NbO2 reveals a second-order Peierls transition in the bulk, in agreement with DFT results, indicating electron correlation effects do not play a significant role. In addition, the observed temperature dependence of the Nb-Nb dimer distance which controls the NbO2 resistivity suggests that the switching of future phase-pure NbO2 memristors could be controlled by resistive heating in an analog rather than digital fashion.   

Magnetically driven carbon nanotubes for a highly efficient mechanical single cell damage.

Magnetized carbon nanotubes have previously been demonstrated as excellent candidates for the nanospearing transfection technique. In the presence of the moderate, spacially varying magnetic field, these magnetic nanorods exert motion, capable of piercing cell membranes, and allowing for highly efficient drug delivery. Here we study the possibility of enhancing this nanospearing technique, by varying magnetic strength and its time-space patterns, in order to inflict a highly efficient and selective, single cell mechanical damage.   

Quasiparticle and Optical Properties of Bulk and Monolayer Zirconium Disulfide

Zirconium disulfide (ZrS2) is a member of layered transition metal dichalcogenide (TMD) family that has rich and versatile chemical and physical properties. Despite much recent renewed interest in ZrS2, especially in few-layer ZrS2 systems, an accurate and systematic understanding of the quasiparticle and optical of ZrS2 from monolayer to bulk phase is still lacking. In this work, we present a fully converged GW+BSE study of the quasiparticle and optical properties of monolayer and bulk ZrS2, aiming at illustrating the subtle interplay between structural and electronic properties and the importance of the convergence issues in 2D many-body perturbation calculations.   

Ab-Initio Computations of Electronic and Related Properties of cubic Magnesium Silicide (Mg2Si)

We have performed ab-initio, self-consistent calculations of electronic, transport, and bulk properties of cubic magnesium silicide (Mg₂Si). Our computations employed the local density approximation (LDA) potential of Ceperley and Alder and the linear combination of atomic orbital (LCAO) formalism. We followed the BZW-EF method to reach the ground state of the maerials, verifiable, without using over-complete basis sets. For a room temperature lattice constant, our calculated, indirect band gap, from Γ to X, is 0.86 eV. We discuss the total and partial densities of states, electron and hole effective masses, and the bulk modulus.   

Ab-initio Calculations of Electronic Properties of Tin Selenide (SnSe)

We present results from ab-initio, self-consistent density functional theory (DFT) calculations of electronic properties of tin selenide (SnSe) in the orthorhombic B16 crystal structure. We utilized a local density approximation (LDA) potential and the linear combination of atomic orbital (LCAO) formalism. Our calculations performed a generalized minimization of the energy to reach the ground state, as required by the second DFT theorem. This process ensures the full, physical content of our findings that include electronic energy bands, total and partial densities of states, and electron and hole effective masses.   

Calculations of third order nonlinear susceptibilities focused in two photon absorption on semiconductors.

We presented the calculation of nonlinear third order susceptibility focused on Two-photon absorption phenomena (TPA) on bulk crystal semiconductors. Our calculation considers the longitudinal gauge approximation to the interaction field, the non-local part of the crystal potential and the scissor operator as correction to LDA energies calculations.   

Observation of B 1s trion and A 2s trion in 2d limit

The distinct excitonic effect in two-dimensional (2d) layered material attracts lot of attentions in recent years. The largely reduced screening effect in 2d materials allows tightly bonding exciton. With gate tunable photoluminescence spectrum, we observe B 1s trion and A 2s trion in h-BN capsuled MoSe₂. The two states show much larger valley polarization compared with their low energy counterparts A 1s exciton and trion.
  

Excited carrier relaxation in GaAs by mid-IR pump-probe spectroscopy

Ultrafast laser spectroscopy is a versatile technique that is commonly used to both understand and manipulate carrier scattering mechanisms in semiconductors. We report the observation of excited carrier relaxation times in GaAs using optical pump mid IR probe spectroscopy. We find a sharp change in relaxation times occurs at the intervalley transitions of GaAs. Our results also show the relaxation times becomes faster with increasing photoexcitation fluence. These results provide direct evidence that excited carrier decay at greatly different rates based on their energy relative to the conduction band minimum. These findings provide additional insight into the energy-dependent nature and rate of phonon emission during electronic relaxation.   

Temperature dependent vibrational modes of TNT and CL-20 cocrystal in terahertz regime

We employed terahertz time domain spectroscopy (THz-TDS), a non-invasive technique, in the range of 0.30 THz to 2.50 THz to measure the effective dielectric properties of a 1:1 molar ratio cocrystal of TNT and CL-20 in the temperature range 150K to 400K. We observed two distinguished absorption peaks at 0.80 THz and 1.3 THz at or below room temperature. The absorption peaks disappear at higher temperatures. We extracted the dielectric constants of the cocrystal using effective medium theories.   

Point-of-care ultrahigh sensitivity magnetic lateral flow assay

Today, many highly quantitative biomarker detection tools for medical diagnostics are readily available in state-of-the-art centralized clinical laboratories. However, there remains a critical need for inexpensive, versatile, and high-sensitivity diagnostic platforms which can bring the performance to the point of care (POC) or doctor’s office. Our team has developed an ultrasensitive point of care biosensor based on the miniaturized inductive detector of magnetic reporter nanoparticles (10¹⁰ emu detection limit) in a test line of a lateral flow assay (like a pregnancy test). This technology represents a new general biosensor platform that can be broadly useful in various areas of molecular diagnostics, therapy monitoring, biomarker detection, and biomedical research. The biosensor prototype performance was evaluated using a standard hCG model system with well established LFA immunochemistry based on readily available, well-characterized, and inexpensive antibodies. The first target after proof of concept is to make a biosensing platform for detecting the recurrence of prostate cancer.   

Obstacles to Undergraduate Research in Thin Film Semi-Conductor Characterization: The Creation of Inexpensive Hall Measurement Apparatus

The goal of my project was to grow and characterize thin semiconducting ZnO films grown on PET plastic via pulsed laser deposition (PLD) with a KrF laser. To characterize the electrical properties of these films, I had to create an automated Hall Measurement apparatus, as I did not have access to industrial equipment due to its unfeasible cost. The apparatus required a few components: I started by creating a Labview program to interface with an ArduinoMega, which operates a 16-switch relay. This relay was then connected to a nano-voltmeter, current source, and my sample film (which would be in a uniform magnetic field for some measurements). Labview would then compile the data into a .csv file, which my Python code would interpret and record measurements for the sheet resistance, bulk resistivity, semiconductor type, sheet carrier density, bulk carrier density, and hall mobility. The apparatus functions correctly, but improvements need to be made to get consistent data.   

Thermoelectric effect in suspended Bi2Se3 grown by molecular beam epitaxy on GaAs(111)A

Bi₂Se₃ is well known as an efficient thermoelectric (TE) material. We report growth optimization of Bi₂Se₃ thin films and thermoelectric study of suspended Bi₂Se₃ beam structures. The Bi₂Se₃ thin films were grown on GaAs(111)A substrates by molecular beam epitaxy (MBE) and confirmed by (00n) series peaks of x-ray diffraction (XRD) that the material was grown along the c-axis of GaAs(111)A. Optimizing the desorption process of GaAs(111)A surface and growth conditions of GaAs buffer layers leads to low RMS roughness and less-defective structural aspect. Using the optimizied Bi₂Se₃ films, we realize the suspended beam structures with minimal heat dissipation to GaAs substrate. The beam structures are fabricated by selective wet etching, and on the two membranes, platinum heaters and voltage channels are patterned to measure the thermoelectric effect, the conversion of temperature difference to electric voltage. Nanomachining technique enables to calculate the dimensionless figure of merit (ZT) of Bi₂Se₃ with minimal environmental factors.   

High quality Tantalum pentoxide thin film growth and its application for low loss nonlinear waveguide

Tantalum pentoxide (Ta₂O₅) of a large bandgap material has shown its potential for Si photonics due to its low absorption loss coefficient in visible to infrared regions, high Kerr coefficients, and nonlinear absorption(TPA/FCA) free. Development of large scale high optical quality thin film for integrated photonics is therefore desired. In this work, by using e-gun evaporation, large scale Ta₂O₅ thin film growth was performed. Beside the measurement of x-ray and Ramen pattern for identifying the structure, the AFM and SEM results all show the grown thin film with high quality. In addition, a low-loss and high-Q Ta₂O₅ based micro-ring resonator was fabricated and characterized. The propagating loss of 0.3 dB/cm was obtained and unloaded quality as high as 300000 was accordingly estimated. This is, in our knowledge, the largest unloaded quality for Ta₂O₅ waveguide. This all show that electron beam thermal evaporation (E-gun) grown Ta₂O₅ film is with great potential for photonic integrated circuit.   

Dry surface cleaning of twisted bilayer graphene and angle dependent oxydation by UV irradiation.

Since production of single layer graphene (SLG) by mechenical exfoliation, it has been focal to many researches thanks to its extraordinary electrical conductivity and surface to volume ratio. these characteristics of graphene have benefit for apply gas sensor devices. Recently, twisted bilayer graphene (tBLG) attracted much attention of its extra ordinary physical properties, especially for its twist angle opto-electircal dependant characteristic. specifically, the magic angle (~1.1°) which shows super conducting characteristics. Moreover, tBLG shows higher mobility and conductivity than SLG. However, stacking two layers of sigle crystaline SLG has been reported to show side effect of introducing interlayer impurities during transfer process. Furthermore, Impurities of graphene surface, such as PMMA residues, degrade electrical conductivity and mobility. Here we report a CVD synthesis of clean interlayer tBLG to containing various twist angles. Top layer of tBLG was cleaned and oxidized by angle dependency under UV irradiation at ambient condition and characterized by Raman and electrical properties. We suggest that UV cleaning is the most effective method for surface cleaning of graphene. Also, we expect a correlation between oxidation rate and twist angle.   

High transmittance Er doped ZnO thin films as electrodes for organic light-emitting diodes

Transparent conducting oxides (TCO) have attracted great attention since the first demonstration in 1907 by Baedeker. TCO thin films with high transparency and electrical conductivity have been widely used in optoelectronic devices, such as thin-film transistors, perovskite solar cells and light emitting diodes (LEDs). Er doped ZnO films exhibit higher optical transparency (~95 %) than other reported metal elements doped ZnO TCO thin films. The effect of Er doping concentration on photoelectric properties of ErZO thin films was investigated in the range of 0-2.0 wt.%. The Er impurity substitutes the Zn site and Er_Zn serves as charge donor in the ZnO crystal structure, thus resulting in the improvement of n-type conductivity as compared with intrinsic ZnO thin films. The optimized ErZO thin films present the low resistivity of 3.4×10⁻⁴ Ω/cm, high carrier concentration of 5.9×10²⁰ /cm³ and high visible optical transmittance (~93%) when the Er content is 1.0 wt.%. The ErZO thin films were used as transparent anodes to fabricate organic light-emitting diodes (OLEDs). Impressively, with the ErZO as anode, the current efficiency of the OLEDs device can reach as high as 86.5 cd/A, which was increased by 14% when compared with the reference OLEDs device (76.0 cd/A) using ITO as anode.   

Thermally induced metal-to-insulator transition in NbO2 thin films

Modification of the carrier dynamics in correlated oxide systems via epitaxial strain is a promising pathway for the practical realization of energy-efficient electronic devices. Here we present on the thermally induced metal-to-insulator transition (MIT) of epitaxial NbO₂ films grown on Al₂O₃ substrates and the modulation of the MIT temperature via epitaxial strain from the substrate. The metal-insulator transition temperature increased from 910 K to 1066 K with increasing strain. An ultrathin 3.9 nm film consisting of a single strained layer with minimal structural defects yielded a bulk-like sharp transition. The substrate-induced strain offers a new degree of freedom to improve device functionality of MIT materials.   

Synthesis and characterization of new multinary chalcogenides

Multinary chalcogenides have drawn a lot of attention due to their interesting properties. We report on newly synthesized quaternary and quintenary chalcogenide family materials. The thermal, electrical, and magnetic properties of the new materials will be discussed in detail as well as their potential technological applications. Structural results will be presented and structure-property relationships will be discussed.   

Structural Design and Manufacturing of Three-Dimensional Porous Superstructures with Additive and Subtractive Electrochemistry for Flexible Self-Powered Electronics and Electromechanical Devices

The recent search for advanced materials with desired properties for the next-generation flexible energy and electronics has been focused on the unique class of three-dimensional (3D) porous superstructures made of 2D materials, such as graphene, graphite, and molybdenum disulfide.
In this work, we report an innovative and rational approach to create 3D metallic foams with tunable multi-level porosity by using a process consisting of an additive electrodeposition and subtractive electroetching. The resulted metallic foams can readily serve as catalytic templates for the growth of 3D hierarchically porous thin graphite. The 3D graphite is freestanding after selective etching of the metallic template. They provide enhanced mechanical properties and electrochemical performances when applied as supercapacitor electrode supports. They can be readily integrated with our wearable GF/polymer strain sensors developed recently and nanomotor manipulation systems into self-powered sensing and electromechanical devices, respectively.   

In-situ Raman investigation of Laser-Induced Graphene using Machine Learning

High-quality graphene was laser-induced from precursors graphene oxide and commercial polyimide. Laser annealing is a promising method to create graphene-based devices including sensors, biomedical equipment and thin-film transistors. A Bayesian optimization-based machine-learning strategy was used to predict optimal process parameters. We custom-built our system to allow simultaneous laser patterning processing and in-situ Raman spectroscopy characterizations. The Raman G/D ratio is a good indication of the quality of the laser-induced graphene. Rapid and significant improvements in the G/D ratios were seen with the machine-learning predicted process parameters. This experimental setup has enormous potential in Autonomous Research Systems for new materials discovery and can be scaled up for advanced-manufacturing patterned graphene-based electronics.
  

Optical phenomena of irradiation induced molybdenum disulfide

Irradiation introduces damage cascades, destroys the periodicity of the lattice, and pushes the material away from equilibrium. In this contribution, we utilize heavy ion and gamma irradiations to alter the optical properties of two-dimensional (2D) materials such as MoS₂. Irradiation offers a pathway to introduce desired defect densities to any given material, and, in case of low-dimensional materials, can be used to control physical properties. Defects introduced using irradiation were characterized using transmission electron microscopy (TEM) and photoluminescence (PL) measurements. We primarily focus our attention on the behavior of valley excitons, as they dominate the optical response of the material even when the system is in equilibrium and reveal the emergence of dark excitonic states in the K-valley. With this work, we offer insight into how increased defect density can potentially engineer magnetism in 2D MoS₂.

1. A. Molina-Sánchez et al., Phys. Rev. B 93, 155435 (2016)

2. A. V. Stier et al., Nat Commun., 7:10643 (2016)

3. M. Zhang et al., J. Phys.: Condens. Matter 30 265502 (2018)   

Non-Destructive Thickness Mapping of Wafer-Scale Hexagonal Boron Nitride Down to a Monolayer

Characterization of the thickness and continuity of wide band gap 2D materials with monolayer sensitivity over large areas has proven to be very challenging. A prime example is 2D hexagonal boron nitride (hBN). Optical contrast methods suffer from the lack of visible absorption in the material; Raman spectral signatures are weak and often not conclusive; and electrical measurements are not possible due to a high electrical resistivity. In this contribution, we will demonstrate an experimental method based on the ellipsometry technique, which makes it possible to map the thickness and continuity of large-area hBN monolayers and bilayers transferred to Si/SiO₂ substrates. The method has sub-monolayer thickness sensitivity, is relatively fast, non-destructive, and can be easily automated. The hBN thicknesses measured in this study have been confirmed by Raman spectroscopy, x-ray photoemission spectroscopy, and by a series of ellipsometry control experiments. We will present a workflow of our experimental procedure, so that other researchers can extend this characterization method to other 2D materials and hopefully accelerate their development.   

Raman Enhancement effect using hexagonal Boron Nitride with a substrate

The Raman enhancement effect is a phenomenon for fundamental studies of both light-matter and matter-matter interactions and applications. Among the several Raman enhancement techniques, the surface-enhanced Raman scattering (SERS) has been the most studied. There are two different mechanisms for SERS effect, the first one is the electromagnetic mechanism when is the local electromagnetic fields around the metallic structures can be amplified. The second is the chemical mechanism which is lower understood, and its magnitude is smaller than the electromagnetic effect. The substrate surface is very important to control the necessary enhancement to make the technique as valuable as it has become. In this work, we study hexagonal boron nitride (hBN) a two-dimensional layered material with a substrate for a type of SERS, an insulator whose gap is approximately 5eV.
  

Dual comb spectroscopy for measuring the group velocity dispersion of optical fiber realized by fast locking of the beat note with a piezoelectric module

Dual-comb spectroscopy (DCS) emerged a decade ago as a powerful tool for gas spectroscopy and metrology. DCS is based on two optical frequency combs (OFCs) with slightly different repetition rates. Using DCS, one can simultaneously obtain amplitude and phase of the light, resulting in the direct determination of complex refractive index. Thus, DCS promises for investigating the solid state materials.
Here, we demonstrate an application of measuring the group velocity dispersion (GVD) of single-mode fiber (SMF) by using DCS. We constructed two Er-fiber-based OFCs and achieved the fast locking of the beat note by using PZT module based on Ref. [1]. The beat note was phase locked with the servo bandwidth of over 100 kHz and the integrated phase noise as 0.21 rad and we realized the DCS system. By using this system, we obtained a wavelength-dependent GVD of SMF and the GVD as -22.0 ps^2/km at 1550 nm with the standard deviation of 0.02 ps^2/km. It agrees well with the nominal value of the GVD. Thus, we conclude that our DCS works as a brilliant tool for solid state physics.
Reference
[1] L. C. Sinclair et al., Rev. Sci. Instrum. 86, 081301 (2015).   

Heavy Mediator at Quantum Dot/Graphene Heterojunction for Efficient Charge Carrier Transfer to Enhance the Performance of the Optoelectronic Devices

The two-dimensional mediator with heavy effective mass possesses a large density of states that can be inserted at the two-dimensional heterostructure interface. In general, the 2D heterostructure interfaces suffer from enhanced depletion region which deteriorates the charge carrier transfer efficiency and hence the device performance. The insertion of the mediator reduces the depletion region and form type–II band alignment which speeds up the carrier dissociation efficiency and hence eventually enhances the carrier transfer phenomena.
2D ReS₂ as a mediator has been employed at the heterojunction of graphene and perovskite quantum dots which results in enhancing photosensitivity (> 10⁷ A/W). Moreover, the device can withstand 100% longitudinal strain making it ideal for the future wearable optoelectronic device industry¹.
Reference:
¹Ghosh, R., et al. (2019). "Heavy Mediator at Quantum Dot/Graphene Heterojunction for Efficient Charge Carrier Transfer: Alternative Approach for High-Performance Optoelectronic Devices." ACS Applied Materials & Interfaces 11(29): 26518-26527.

  

Nonlinear Refractive Index Measurement of E-beam Evaporation Ta2O5 film

Tantalum pentoxide(Ta₂O₅) has been realized as a promising material for waveguide due to its great linear optical properties, such as high refractive index, large bandgap and so on. Recently, the nature of athermal property and high nonlinear refractive coefficient exhibit its potential for Si photonics application. In this work, using e-gun deposition growth Ta₂O₅ thin film, high quality micro-ring resonator was fabricated. The propagation loss as low as 0.3/cm was characterized from the transmission spectrum. Additionally, the nonlinear refractive coefficient (n₂) was investigated by using all optical modulation technique. The n₂ of 1.42 x 10^-14 cm²/W was estimated. Compared to the conventional materials for Si photonics, such as Si₃N₄, SiO₂ and so on, the larger n₂ value reveal the value for application.   

Investigation of the Resistive Switching Mechanism in LixNbO2 Memristors

A complete understanding of the origin of resistive switching in memristors is necessary for implementation into neuromorphic computing applications. However, the resistive switching of memristors is typically attributed to a complex combination of processes (e.g., redox reactions, ionic transport, phase changes, etc.) post an electroforming step, which limits tunability and scalabilitiy of the device. Li_xNbO₂ analog memristors circumvent the electroforming step and demonstrate a promising new mechanism wherein the diffusion of Li⁺ ions enables precise control of the resistive states [1]. Here we utilize synchotron-based x-ray spectroscopy techniques to examine the electronic strucure of Li_xNbO₂ memristors. We employ x-ray absorption spectroscopy (XAS) across the active to observe variations in the Li content. Additionally, we investigate the origin of non-volatility by probing the buried interface at the metal contacts via hard x-ray photoelectron spectroscopy (HAXPES). This work opens a new avenue of methodology for illuminating resistive switching mechanisms in memristors from a fundamental perspective.

[1] S.A. Howard et. al., APL Mater. 7, 071103 (2019)   

Tunable Dirac points and zero-energy modes in periodic curved graphene superlattices

We combined periodic ripples and electrostatic potentials to form curved graphene superlattices and studied the effects of space-dependent Fermi velocity induced from curvature on their electronic properties. With equal periods and symmetric potentials, the Dirac points do not move, but their locations shift under asymmetric potentials. This shift can be tuned by curvature and potentials. Tunable extra gaps in band structures can appear with unequal periods. The existence of new Dirac points is proposed, such that these new Dirac points can appear under smaller potentials with curvature, and their locations can be changed even under a fixed potential by adjusting the curvature. Our results suggest that curvature provides a new possible dimension to tune the electronic properties in graphene superlattices and a platform to more easily study physics near new Dirac points.   

On degradation and reliability testing GaN High-electron-mobility transistor (HEMT)

Gallium nitride (GaN) based high electron mobility transistors (HEMTs) have shown a lot of promise in high voltage, high power, and high radiation applications. However, the full realization of the III-nitride potential and large-scale adoption of this technology has been hindered by the existence of electrically active defects that manifest as deep levels in the energy bandgap. These deep levels can potentially act as charge trapping centers limiting device performance and long-term reliability. It is therefore imperative to monitor these traps in operational GaN HEMTs as close as possible to their real-world operational conditions. With that goal in mind, in this work, a suite of advanced thermal and optical-based trap spectroscopy methods and models are reported and expanded upon to directly probe and track traps in three-terminal operational of GaN HEMTs.
  

Stress/Strain Effects on Phonon Modes and Phonon Deformation Potentials in Monoclinic β-Ga2O3

Strain-induced shifts of phonon energies provide a powerful tool for modeling strain patterns in heterostructures and thin films. In the case β-Ga₂O₃, an emerging wide-bandgap semiconducting oxide, the low symmetry of its monoclinic structure is responsible for the high anisotropy and unusual ordering of phonon modes, while the effects of stress and strain on the phonon properties in general are not yet well understood. We present a rigorous, symmetry-based analysis on how the frequencies of optical phonon modes depend on the components of stress and strain tensors in monoclinic crystals, and we confront it with the results of density functional theory (DFT) calculations involving several distinct deformation scenarios, and the resulting shifts in phonon mode frequencies for β-Ga₂O₃. We derive sets of phonon deformation potential parameters for all phonon modes, including infrared-active (A_u and B_u) and Raman-active (A_g and B_g) modes. Additionally, we discuss how stress affects the order of phonon modes with B_u symmetry.   

Thermal and Magnetic Properties of Landau–Quantized Group VI Dichalcogenide Carriers in the Approach to the Degenerate Limit

This work is concerned with determination of the thermal and magnetic behavior of the Group VI Dichalcogenides in a quantizing magnetic field. Our analysis treats the principal statistical thermodynamic functions (grand partition function and the ordinary partition function, as well as the grand potential, Helmholtz free energy and the entropy), providing the basis for calculation of the specific heat: Their dependencies on temperature and magnetic field strength are carefully examined, particularly in the degenerate regime and the approach to zero temperature. The joint dependence on magnetic field and temperature is also determined for the Dichalcogenide magnetic moment in the degenerate statistical regime, replete with de Haas–van Alphen oscillatory phenomenology and also above the zero–temperature limit.   

Effect of electric potential fluctuations in Coulomb drag in double layer graphene systems

We study theoretically the drag transresistivity in a graphene double layer system exhibiting electric potential fluctuations. The fluctuations are modelled as sinusoidal oscillations in the first layer, which induces a sinusoidal variation in the electron density in that layer, which in turn induces an electron density oscillation in the second layer. We calculate the drag resistivity as a function of average charge densities in each layer for various amplitudes of the electric potential fluctuations. Recent experiments have found that the drag transresistivity in graphene double layers systems exhibit a sign change when one layer is held at the charge neutrality point, and the average density of the electrons in the other layer is varied through the charge neutrality point. Our simple model is able to qualitatively reproduce these experimental results. We discuss extensions of the model which characterize more realistically the electric potential fluctuations that are caused by the presence of charge density impurities near the graphene layers.   

Thermal expansion coefficients of high thermal conducting BAs and BP materials

Recently reported very high thermal conductivities in cubic boron arsenide (BAs) and boron phosphide (BP) crystals could potentially provide a revolutionary solution in the thermal management of high power density devices. To fully facilitate such an application, the compatible coefficient of thermal expansion (CTE) between the heat spreader and the device substrate, in order to minimize the thermal stress, needs to be considered. Here, we report our experimental CTE studies of BAs and BP in the temperature range from 100 K to 1150 K, through a combination of X-ray single crystal diffraction and neutron powder diffraction. We demonstrated that the room temperature CTEs,3.6 ± 0.15*10^-6 /K for BAs and 3.2 ± 0.2*10^-6 /K for BP, are more compatible with most of the semiconductors including Si and GaAs, in comparison with diamond, and thus could be better candidates for the future heat spreader materials in power electronic devices.   

Dying Fundamental Physics and Rising Digital Era of Engineering

From a study based on personal teaching experiences and one-to-one and in-group interaction with a large number of students (9^th to 12^th Grade and UG levels), teachers, research scholars and lab-experts across India, we found that most of the students in the science stream no longer “view” Physics as a branch of science to study the nature, rather Physics for them is a giant toolbox to design more sophisticated devices and medicines for “social use”. To most of them, knowing physics means becoming smarter at handling electricity, electronic gadgets, computers, smart-phones and apps. Most of the science students were found not driven to learn physics as a way to understanding and relishing the hidden deeper beauty of nature; for them the new wonderland is the “on-line” world.
Given the trend in students’ attitude towards physics in current time, theoretical physics might severely suffer in near future. In this talk, I discuss the issues we found and the possible course of action to better cement the foundation of tomorrow’s fundamental physics.   

Research Integrity, the Responsible Conduct of Research, and Plagiarism Analysis

Among its duties, the National Science Foundation (NSF) Office of Inspector General (OIG) is responsible for helping ensure the integrity of research programs at NSF. We investigate allegations of research misconduct (plagiarism, falsification, and fabrication) in NSF proposals and awards. We also handle allegations conflict of interests and violations of the confidentiality of NSF’s merit review to ensure the integrity of that process. We completed a review of how grantees implemented NSF’s requirement to provide responsible conduct of research training to undergraduate students, graduate students, and postdoctoral researchers. We have analyzed our plagiarism cases of the past decade to accumulate data and potentially identify institutional strategies for preventing and reducing plagiarism. We are also reviewing grants for characteristics of serial spending. I will briefly discuss these topics and present our results.   

Perovskite Electronic Ratchet for Energy Harvesting

Hybrid organic-inorganic perovskite semiconductors (HOIS) have demonstrated great potential as absorbers in thin-film solar cells, but recently there is emerging research demonstrating their application as the electronic materials in transistors, diode, and optoelectronic devices due to their unique mixed ionic-electronic properties. Here we demonstrate a novel energy harvesting application based on both ionic and electronic transport in HOIS — Perovskite electronic ratchets. The electronic ratchet is a new type of energy harvesting device that can convert (rectify) a non-directional electrical signal into stable direct current through an asymmetric potential distribution across the device (i.e., the device acts as a charge pump). Here we demonstrate the first lead-halide perovskite electronic ratchet by manipulating the ion distribution within a transistor channel to realize an asymmetric potential distribution in the perovskite device. This asymmetric potential distribution allows the perovskite ratchet device to convert both electronic noise and unbiased, periodic alternating potentials into stable direct current. Such devices have the potential for providing low-voltage power in remote and portable applications.   

Ferrofluid-Based Generator Harvesting Waste Heat and Ambient Vibration

Ferrofluid is a stable colloidal suspension of permanently magnetized nanoparticles, in which Brownian motion keeps the nanoparticles from settling under gravity, and a surfactant is placed around each particle to provide short range steric repulsion between particles to prevent particle agglomeration. Since magnetic field device has no limitations analogous to electrical breakdown in its electric field counterpart, ferrofluids have become an excellent choice for micro/nanoelectromechanical systems technology. On the other hand, ferrofluids have applications in the cooling of electronic devices, which has led to several designs of power generators using waste heat. In this work, we study a new version of ferrofluid generators harvesting both heat and vibration energy. A theoretical model based on fundamental physics principles is developed to evaluate the system's energy conversion efficiency (the ratio of generated power and the heat input). Further, we discuss the dependence of the output power on various design parameters (including the scale of the system) and methods to enhance the performance of the ferrofluid generator for potential aerospace applications.   

Thermoelectric properties of As-based 1111-Zintl compounds La1-xSrx(Zn,Cd)AsO

1111-system with the ZrCuSiAs-type structure has attracted great attention not only as the high-Tc superconductors but the high-performance thermoelectric compounds. The highest ZT in 1111-system was found in BiCuSeO with the value of ZT to be 1.4 at 923 K [1], demonstrating the high potential of the 1111-system as thermoelectric compounds. Previously, we found that LaFeAsO_1-y exhibits large power factor with values of PF = 4.1 mW/mK² at T = 75 K [2], demonstrating that As-based 1111-system can also be a candidate.
In this study, we focused on LaZnAsO and LaCdAsO (space group: P4 / nmm) compounds. Hole carriers were induced by Sr doping. Polycrystalline samples were synthesized by the solid reaction method. High dense samples were obtained by hot-press. As results, peak of the power factor of LaZnAsO was shift to high temperature comparing with LaFeAsO_1-y, achieving the value of 0.118 mW/mK2 at T=747 K. Details of sample preparation as well as thermoelectric properties will be discussed in the conference.

[1] Li-Dong Zhao, et al, Energy Environ. Sci. 7, 2900 (2014).
[2] K. Kihou, C. H. Lee, M. Miyazawa, P. M. Shirage, A. Iyo and H. Eisaki, J. Appl. Phys. 108, 033703 (2010).   

Understanding pressure-driven thermoelectric properties of PdP2 and PdAs2 nanowires via band engineering

In the race of searching for alternative clean and sustainable energy sources, thermoelectric materials have shown potential hope by converting low-grade waste heat into electricity. The efficiency of thermoelectric devices is characterized by power factor or ZT. Therefore, enhancing ZT in nanowires for the thermoelectric application is highly desired. In recent work, we propose semi-metallic nanowire with asymmetry density of states near the Fermi energy as an alternative class of thermoelectric materials. We have considered the pentagonal structure of PdP₂ and PdAs₂ nanowires (NWs) and confirmed its dynamical stability by the phonon dispersion study. The PdX₂ structure shows the transition from semiconducting to semi-metallic behaviour at a compressive strain of 8% within sustainable pressure of 0.2~0.3GPa. The semi-metallic behaviour with the asymmetric density of states near the Fermi energy boosts Seebeck co-efficient value and therefore, ZT_e value is enhanced for both the nanowires. Our study stimulates both nanowire synthetization feasibility and thermoelectric applications for the conversion of waste heat into electricity.   

Optimized Efficiency of a stochastically driven quantum dot Heat Engine

We take a stochastically driven single level quantum dot embedded between two metallic leads at
different temperatures which works as a heat engine. We analytically study the optimized efficiency
that lies between the maximum efficiency and minimum efficiencies. The minimum efficiency
either takes the efficiency value at maximum power or the lowest possible value which is zero.
We study the optimized efficiency of a quantum dot heat engine according to the
optimization criteria, to find their corresponding optimized quantities in an external magnetic field
(stochastic driving force). Accordingly, we found 1) efficiency-wise optimized efficiency is better
than efficiency at maximum power; 2) power-wise, the optimized power is smaller than its value at
maximum power by 35% and 3) period-wise, is performs the task in a cycle twice that of the period
at maximum power. We study the overall performance of the heat engine by introducing a figure of
merit that takes into account the contribution of each of the above quantities as a function of Carnot
efficiency. Based on the proposed figure of merit, the model shows that the 1 st optimization criteria
is 3 times better than the 2nd optimization criteria as a function of ηC   

Thermoelectric Properties of Solvothermal Grown Bismuth Telluride-Carbon Nanomaterials Composites

Current state of the art portable power source faces problems such as low-life, high production cost and weight penalty. As such, thermoelectric (TE) power generation and cooling have been considered as an alternative as low-cost, more efficient, environmentally responsible approach. One of more promising TE material of interest are alloys based on bismuth telluride (Bi₂Te₃) and thus much efforts have been made to improve TE properties by (1) reducing the materials’ dimension to nanoscale, and (2) incorporating other nanostructures such as graphene, to enhance electrical properties and thermal conductivity. This geometry offers a natural architecture for thermoelectric devices due to reduced thermal transport and enhanced electronic tunneling at the nanomaterial interfaces. Building upon this mechanism, we present thermoelectric properties of Bi₂Te₃ composited with various carbon nanostructures. Our TE materials are synthesized via solvothermal method resulting in uniformly dispersed nanoplates interfaced with 1D and 2D carbon nanostructures. Our results not only show change in thermoelectric properties of Bi₂Te₃ upon incorporation of carbon nanostructures, but provide characterization of Bi2Te3-nanomaterial interface for development of next-gen, low power, portable power source.
  

Defects and band positions in the p-type transparent conductor CuI

While high-performance n-type transparent conductive materials (TCMs) have existed for decades, heavy p-type doping of wide band-gap materials has proven much more challenging. The simple cubic compound CuI was recently rediscovered for this application and is currently the p-type TCM with the highest figure of merit. However, the native defects responsible for p-type conductivity in CuI, as well as compensating defects limiting the maximum achievable doping levels, have not yet been identified experimentally. Furthermore, there is disagreement in the literature on the work function and absolute band positions of CuI relative to vacuum. In this experimental study, we employ temperature- and intensity-dependent photoluminescence to draw new conclusions on the defect landscape of CuI. We then employ a combined photoemission spectroscopy-Kelvin probe system to show that the measured band positions depend critically on surface phenomena. Finally, we demonstrate that terahertz spectroscopy is an ideal tool for characterizing the electrical properties of CuI reliably and non-destructively.   

Computational Screening and Designing High-performance Solid Sorbents for CO2 Capture Technology

CO₂ is one of the major combustion products which once released into the air can contribute to global climate change. Solid sorbents have been reported to be promising candidates for CO₂ sorbents due to their high CO₂ absorption capacities. With ab initio thermodynamic calculations, we proposed a theoretical screening methodology to identify the most promising CO₂ sorbent candidates from vast array of possible solid materials. The advantage of this method is that it identifies the thermodynamic properties of the CO₂ capture reaction as a function of temperature and pressure without any experimental input beyond crystallographic structural information of the solid phases involved. According to the requirements imposed by the pre- and post- combustion technologies and based on our calculated thermodynamic properties for the CO₂ capture reactions by the solids of interest, we were able to identify only those solid materials for which lower capture energy costs are expected at the desired pressure and temperature conditions. In addition, through investigating several mixed sorbent materials, we demonstrate that by mixing/doping different types of solids it’s possible for a sorbent to shift its turnover temperature to the range of practical operating conditions.   

Improved optoelectronic properties in CdSexTe1-x through controlled composition and short-range order

We employ first principles methods based on density functional theory and beyond to study CdSe_xTe_1-x alloys in the zincblende and wurtzite structures. From the cluster expansion formalism, we provide phase diagram showing consolute temperature of 325 K where zincblende-to-wurtzite phase boundary is found for Se concentrations of x = 0.5-0.6 owing to increasing ionic character of the Cd-anion bonds. Disordered CdSe_xTe_1-x configurations are modeled using special quasirandom structures, for which optoelectronic properties are computed with the hybrid HSE06 functional. Downward bowing in the band gap and effective hole mass of the zincblende structure is highlighted for its potential benefits in photovoltaics through increased net photocurrent. Absorption coefficient and reflectivity are also reported, showing promising results in zincblende CdSe_xTe_1-x as indicated by substantial optical absorption throughout all Se concentrations. Lastly, we identify the presence of short-range order in CdSe_xTe_1-x characterized by clustering among like atoms in order to minimize strain. The degree of clustering, which may be tuned by temperature, also controls the magnitude of the band gap.   

A Multidimensional Approach to Structural Transformation Through Functionalization of Tellurene

Two dimensional (2D) Tellurene (Te) structures have recently been synthesized, and have been shown to possess high mobility and stability. Using density functional theory (DFT) and molecular dynamics (MD), we investigated the stability and electronic structure of 2D, and phase sheets, and their hydrogen, oxygen, and fluorine functionalized counterparts. Our calculations show that bare - and -Te sheets are stable and have band gaps of 0.44 eV and 1.02 eV respectively. We see that H, O and F destabilize -Te; F and H cause -Te layers to separate into functionalized atomic chains; and O causes -Te to totally transform into a Te₃O₂-like structure. Also, we studied the coverage effects of different concentrations of H and O on and β-Te and found that the full coverage case results in the highest binding energy and stability for both adatoms. Finally, we examined the stability and binding nature of functionalized β-Te on a GaSe substrate. We noted that having O and H impurities not only enhances the stability of Te layer, but also results in a strong binding energy on GaSe substrate. Our results indicate that Tellurene monolayers and functionalized counterparts are suitable for future optoelectronic devices and as metallic contacts in nanoscale junctions.   

Investigation of Au(111)/Li3PO4 Interface Structures using Neural Network Potential

Recently, the construction of interatomic potentials using first-principles calculation data and machine-learning technique has been widely tried because of higher reliability and low computational costs. In the present study, we have tried to construct the four-element neural network potential (NNP) [1,2] to investigate the Au(111)/Li₃PO₄ interface system, where the understanding of the interface structures and Li-ion distribution near the interface is of significance for the development of all-solid state Li-ion batteries and novel memory devices [3]. Using the constructed NNP, we then performed structure optimization with a large interface model of Au(111)/Li₃PO₄. In the meeting, we will discuss the calculated interface structures and the Li defect formation energies.

[1] J. Behler et al., Phys. Rev. Lett. 98, 146401 (2007).
[2] W. Li et al., J. Chem. Phys. 147, 214106 (2017).
[3] I. Sugiyama et al., APL Mater. 5, 046105 (2017).   

Strong interplay between Na- and O-related defects in Cu-based chalcogenides

Recent advances in thin-film photovoltaics became possible by controlling the incorporation of impurities. The most notable among them are alkalis in-diffusing from soda-lime glass and oxygen incorporating from various sources during the baseline processing. Herein, we investigate the interaction between Na and O in co-sputtered Cu₂ZnSnS₄(CZTS) by combining theoretical and experimental techniques. First, using secondary ion mass spectrometry (SIMS), we demonstrate that Na and O distributions in the absorber are correlated. Then, employing first-principles methods, we show that the correlation is driven by a strong ionic Na-O bonding that triggers the formation of NaO and Na₂O complexes. The remarkably high binding energies for these complexes are proven to cause Na-O clustering at all temperatures of the baseline processing. Hence, the overall character of the impurity profiles is explained through O immobilizing Na by forming the defect complexes and leading to the Na accumulation near the CZTS surface. This defect interplay paves the way for a more accurate impurity control needed for fabricating high-performance devices.
Reference: S. Grini, K. V. Sopiha, et al., Adv. Energy Mater. 9, 1900740 (2019).   

Energetic and vibrational properties of carbon monoxide adsorption on platinum nanoparticles under applied voltage

Carbon monoxide poisoning is a significant limitation to the performance of platinum-based transition metals. Manipulating the size of the catalytic particles could help inhibit carbon monoxide adsorption and increase the life cycle of the catalyst. This study models the energetic and vibrational properties of carbon monoxide on platinum nanoparticles under applied voltage using the self-consistent continuum solvation (SCCS) model. We determine adsorption patterns as the function of nanoparticle size and site coordination. It is found that the local surface charge strongly affects carbon monoxide adsorption particularly along the (111) and (001) facets of the nanoparticle. These results could prove useful in optimizing electrocatalytic systems such as proton exchange membrane (PEM) fuel cells where trace amounts of carbon monoxide in the hydrogen fuel can poison the nanostructured platinum electrodes and decrease the durability of the fuel cell.


  

Evanescent field polarization for giant chiroptical modulation from achiral gold half-rings: Theoretical insight from simulations

Metal nanoantennas have been under intense investigation due to their strong light−matter interactions and significant polarization sensitivities determined by their nano-structure. For applications seeking to realize on-chip polarization-discriminating nanoantennas, efficient energy conversion from surface waves to far-field radiation is desirable. However, the response of individual nanoantennas to the particular polarization states achievable in surface waves, such as evanescent fields, has not yet been thoroughly studied.
Here, we report the giant modulation of the visible light scattering predicted from gold half-ring, pinwheel, and other nanoantennas excited through total internal reflection of left- and right-handed circularly polarized light, by exploiting the distinct polarization properties of surface evanescent waves [ACS Nano, 12, 11657 (2018)]. This result provides a fundamentally different mechanism for chiroptical responses requiring a phase delay between transverse and longitudinal electric field oscillations, not found in free-space light. Specifically, we focus on the insight provided by theory, and in particular by the electromagnetic simulations, performed with COMSOL Multiphysics software, of the systems of interest and aspects of their chiroptical response.   

Free Energy Landscape of Sodium Solvation into Graphite

Sodium Ion Batteries (SIBs) are the most cost-effective alternative to current generation lithium ion batteries (LIBs), ^[1,2] but Na is known to deliver very low energy capacity for sodium intercalation compared to Lithium. We report here first principles moleculars dynamics aided by metadynamics ^[3] to obtain the free energy landscape including changes in the electronic coupling as a sodium ion in Dimethyl sulfoxide solvent intercalates into graphite, the first step in understanding how the local environment affects the free energy of solvation. We analyze the free energy landscape for all the possible sodium solvation scenarios, while quantifying their free energy barriers. Our simulations indicate that solvent plays an important role in stabilizing the sodium intercalation into graphite through shielding of the sodium while modulating the interaction of the solvent with the graphite sheets ^[4].

References

1. N. Yabuuchi, K. Kubota, M. Dahbi, S. Komaba. Chem. Rev. 2014, 114, 11636-11682.
2. H. Kim, J. Hong, G. Yoon, H. Kim, K. Y. Park, M. S. Park, W. S. Yoon, K. Kang. Energy Environ. Sci. 2015, 8, 2963−2969.
3. CP2K version 2.6.2 (Development Version), CP2K is freely available from http://www.cp2k.org
4. A. Kachmar, W. A. Goddard. J. Phys. Chem. C. 2018, 122(35), 20064-20072.   

Catalytic growth of N-doped Multi-Layer Graphene/Carbon Composites for Supercapacitor Applications

Multi-layer graphene (MLG) composites are excellent candidates for supercapacitor applications, but current approaches to device fabrication and the cost to produce graphene poses challenges for commercialization. Here, we use iron nitrate nonahydrate as a catalyst for the growth of graphene, melamine formaldehyde resin as a solid carbon source, and Pluronic F127 surfactant as a sacrificial porogen to enhance porosity and surface area. The MLG composites are produced via thermal carbonization. The resulting composites exhibit high surface area (2200 m²/g) and significant nitrogen doping (4-5%), yielding specific capacitance values as high as 300 F/g at 0.5 A/g. The role of temperature and coordination between functional group of the carbon precursor & metal source has been investigated to understand MLG formation.   

Determination of the Enthalpy of Adsorption of Hydrogen in Activated Carbon at Room Temperature

A very important quantity for adsorption performance is the enthalpy of adsorption ΔH. The determination of ΔH for weakly adsorbing gases (e.g., H₂) in carbonaceous porous materials is difficult, normally requiring measuring adsorption isotherms at two cryogenic temperatures and calculation of ΔH using Clausius-Clapeyron’s (CC) equation. Here we demonstrate a calculation of ΔH based on ca. room temperature isotherms at 273K, 296K. CC requires absolute isotherms; however, excess adsorption is measured; conversion between these requires the adsorbed film volume. We show that the film volume can be estimated by fitting the excess adsorption with an Ono-Kondo model and the auxiliary use of a fixed point corresponding to the saturation film density (estimated 100±20 g/L) which seems to be remarkably sample- and temperature-independent, i.e., an adsorbate property. We find that for high-quality porous carbons the film volumes are ~40%, ~12% of the total pore volume at 77K, 296K, respectively. Using these, we calculate ΔH = 8.3±0.4 kJ/mol at room temperature, an excellent agreement with the low-coverage cryogenic determination. The methodology proposed facilitates reliable calculations of the ΔH at room temperatures for weakly-adsorbing gases.   

Thermal conductivity of hydrogen sorbent materials for onboard storage applications

Measuring the thermal conductivity (TC) of H₂-physisorption storage materials under system conditions are critical in designing the most practical onboard H₂ storage vessels. Furthermore, knowing the TC helps understanding how the heat associated with the sorption/desorption of H₂ is distributed throughout the storage material.
In this work, we will show that the thermal properties of the powder materials can differ significantly from those of the bulk materials as heat transfer within a powder mainly occurs via: a) conduction through the solid particles and the gas within the interstices; and b) convection through the gas. To this end, we measured the TC on pellets of MOF-5 and powder Cu-MFU-4l with our state-of-the-art TC apparatus. We studied the behavior under He and H₂ pressures ranging from 0.03 - 80 bar, at temperatures ranging from 40 K - 380 K, and for different degrees of powder compaction. We will discuss that we measure an effective thermal conductivity that depends on the TC of the powder particles, on the powder compaction, the TC of the surrounding gas, the number of binding sites occupied, and the possible expansion or contraction of sorbents during the gas sorption/desorption which in turn all depend on temperature and pressure.   

Thermal and Spectroscopic (IR, NMR) Studies of Ammonia Borane-Polyethylene Oxide Hydrogen Storage Composites: Effect of Catalyst

Ammonia Borane (NH₃BH₃, AB) has been a potential candidate for chemical hydrogen storage due to its high hydrogen content (19.6 wt%). A downfall to the applicability of AB is its slow dehydrogenation kinetics and production of unwanted byproducts/gases. Studies show that introduction of a polymer, such as polyvinylpyrrolidone and polyacrylamide, can improve the performance of AB and decrease the release of harmful byproducts. Furthermore, catalytic additions such as, magnesium chloride (MgCl₂) and calcium chloride (CaCl₂), have proven to lower the activation energy (E_a), improve kinetics, and lower the hydrogen release temperature. This study explores thermal and vibrational analysis of pristine AB blended with polyethylene oxide (PEO) and individual additions of MgCl₂ and CaCl₂ as catalysts. Dehydrogenation kinetics were studied using a differential scanning calorimeter and the data was then compared with AB, bulk composites and analyzed. The results with CaCl₂ catalyst and high PEO content exhibited the most improved properties (i.e. lower E_a). In addition, evidence of the interactions between AB:PEO:Catalyst were given by Fourier-transform infrared and nuclear magnetic resonance spectroscopy.   

The Impacts of Synthesis Routes and Phase-Purity on Water-Splitting Behavior in the Barium-Cerium-Manganese System

Solar thermochemical hydrogen production (STCH) is a promising technology for solar fuels with high theoretical solar-thermal water-splitting efficiency. However, many candidate STCH oxides are complex ternary oxides, and they have the potential for secondary phase formation during synthesis and subsequent redox cycling. During synthesis, pervasive refractory impurity phases can occur, which impede the ability to develop a basic understanding of the STCH potential and complicate structural refinement of chemical expansion during isothermal reduction/oxidation. This work focuses on high phase-purity synthetic routes for known water-splitting oxides; in particular, BaCe_0.25Mn_0.75O₃ (BCM). These are approached by synthesizing both bulk ceramic and thin films, which are analyzed via in-operando synchrotron-based X-ray diffraction (XRD) to provide the highest quality structural refinements on BCM and other complex STCH oxides to date. The relationship between chemical shift and oxygen vacancy concentration is determined using a combination of flow-cell measurements and in-operando XRD during isothermal reduction and oxidation. In addition, key data for training computational simulations of vacancy formation and the effect of vacancies on structures is presented.   

Frustrated Lewis Pairs with Applications in Hydrogen Storage

Due to the abundance and large gravimetric energy density, hydrogen has been considered a potential source of energy.¹ Major challenges with utilizing hydrogen for energy include efficient and safe storage and transportation of it. Frustrated Lewis pairs (FLPs) have recently proven their importance in hydrogen storage.² In contrast to commonly known Lewis acid-base pairs, functionalizing the acid and base with bulky groups prevents them from binding to their counterpart causing them to be “frustrated”.³ The acid and base are held together by weak intermolecular forces without neutralization, which allows small molecules such as hydrogen to weakly bond to the Lewis acid and base in a heterolytic mechanism. FLPs as catalysts have unique characteristics including a wide range of compatible donors and acceptors.² These attributes may assist with lowering the sorption temperature and pressure, reducing the activation energy barrier and potentially allowing for hydrogenation and dehydrogenation to occur. This work focuses on the novel synthesis and characterization of an FLP system.
References: [1] C. Milanese et. al., Int. J. Hydrogen Energy. 2019, 44, 7860-7874. [2] A. Jupp, D. Stephan, Trends in Chem. 2019, 1, 35-48. [3] D. Camaioni et. al., J. Phys. Chem. A. 2012, 116, 7228-7237.   

Improving Cyclability of Metal Borohydrides for Hydrogen Storage via Non-Metallic Borohydride Additives.

The theoretical ~15 wt% hydrogen content of magnesium borohydride is inaccessible for cyclical desorption/hydrogenation cycles at reasonable temperatures (<300°C) and pressures (<350 bar). However, a variety of additive compounds lower the Mg(BH₄)₂ hydrogen evolution temperature, changing the end state when hydrogen is released, and lowering the melting point of the system. These additives have included other metal borohydrides, metal hydrides, and organics (ether/glyme). Despite many of these additives lowering the liquidus temperature, desirable due to better mixing and higher theoretical kinetics in the melt, these systems often resulted in undesirable matrices upon re-cooling and cease to thermally cycle as expected. This work investigated an alternative family of additives, non-metallic borohydrides; a broad class of low melting point borohydrides. The temperature and hydrogen-evolution dependent phase diagram of these magnesium borohydride and non-metallic borohydride systems was investigated via calorimetry and in-situ diffraction analysis. Results of heat flow and temperature programmed desorption measurements demonstrate the improved thermal cyclability of these systems. The effect of non-metallic cation size on the behavior of these systems will be discussed in detail.   

Tailoring cations in a perovskite cathode for proton-conducting solid oxide fuel cells with high performance

A rational design of a high-performance cathode for proton-conducting solid oxide fuel cells (SOFCs) is proposed in this study with the aim of improving the hydration properties of conventional perovskite cathode materials, thus leading to the development of new materials with enhanced proton migration. Herein, potassium is used to dope traditional Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3-δ (BSCF), which is demonstrated to be a beneficial way for improving hydration, both experimentally and theoretically. The theoretical study was needed to overcome practical limits that hindered direct hydrogen mobility measurements. The novel material Ba_0.4K_0.1Sr_0.5Co_0.8Fe_0.2O₃ d (BKSCF) shows a lower overall proton migration energy compared with that of the sample without K, suggesting that K-doping enhances proton conduction, which shows an improved performance by extending the catalytic sites to the whole cathode area. As a result, a fuel cell built with the novel BKSCF cathode shows an encouraging fuel cell performance of 441 and 1275 mW cm^-2 at 600 and 700 C^o, respectively, which is significantly higher than that of the cell using the pristine BSCF cathode. This study provides a new and rational way to design a perovskite cathode for proton-conducting SOFCs with high performance.   

Solid-state NMR Studies of Non-Fullerene Acceptor-Based Organic Photovoltaic Active Layers

The efficiency of organic photovoltaics has undergone remarkable increases in power conversion efficiencies (PCEs) recently achieving values in excess of 17% due, primarily, to the advent of non-fullerene acceptors (NFAs). To further increase PCEs, understanding and manipulating the morphology over broad range of length scales is necessary. The similarity in the chemical composition of the donors and NFAs limit current methods based on electron densities or refractive indices in discerning details of the morphology. To fill this gap we have used solid-state NMR (ssNMR) where donor and acceptor nuclei have distinct relaxation times that can be used to characterize the crystallinity, crystal size and state of mixing. Here, we present results on several donors and NFA combination where characteristics of the morphology were obtained. Donor and NFA blend film and their pristine films were studied by ssNMR. For PTB7:ITIC, the crystalline forms of ITIC show sensitive dependence on processing conditions. Furthermore, T₁ and T_1ρ relaxation times were used to elucidate the phase separation behavior. For PM6:Y6, spin diffusion experiments based on the contrast between the T_1ρ relaxation times of PM6 and Y6 are able to detect phase separation at a length scale of <15 nm.   

Single-junction and Tandem Cu2BaSnS4 Solar Cells with a TaS2 hole contact

Solar cells based on the wide band-gap Cu₂BaSnS₄ (CBTS) absorber have achieved open circuit voltages up to 1.1 V over a short development period, making CBTS an attractive material for tandem photovoltaic and photoelectrochemical cells. In this work, we explore an alternative CBTS growth route based on sulfurization of reactively sputtered oxide precursors, and we propose TaS₂ as an alternative back contact material. Compared to direct deposition of CBTS films from ceramic targets, reactive sputtering of oxide precursors can ultimately achieve a higher throughput and a lower cost, with the additional advantage that sulfur contamination of vacuum systems is avoided. The TaS₂ compound is selected as a prospective hole-selective contact due to its high work function and its metallic conductivity, which could prevent the losses associated with carrier transport across the semiconducting MoS₂ layer. By comparing CBTS solar cells with Mo and TaS₂ back contacts, the latter shows a significantly lower series resistance, resulting in a 10% relative efficiency improvement. Finally, we fabricate a proof-of-concept monolithic CBTS/Si tandem cell using a thin Ti(O,N) interlayer intended both as a diffusion barrier and as a recombination layer between the two subcells.   

Development of a Surface Forces Apparatus Featuring Ultrafast Non-resonant Imaging for Measuring Contact Electrification

The charging of surfaces via physical contact, known as contact electrification, is an off-equilibrium phenomenon that spans many lengthscales and can exhibit extreme energy focusing by conversion of diffuse mechanical energy into high energy x-rays. Due to the challenges of performing in-situ measurements of charge separation from buried interfaces, the fundamental mechanisms of contact electrification are still unknown. To make experimental advances in this field, we have developed a new multimodal platform that combines the angstrom precision of a surface forces apparatus with the interfacial access provided by a nonlinear optical measurement, i.e., second harmonic generation. Details of the instrument’s design and operation will be presented, which will include preliminary experimental results that tests how contact electrification is affected by monolayer-levels of surface absorbents.   

Revealing signitures of topologically protected surface states with STM

Owing to their bulk band topology, 3D topological insulators possess a massless Dirac dispersion with spin–momentum locking at the surface. The onset of a spontaneous magnetization or a broken time-reversal symmetry leads to the formation of an exchange gap in the Dirac band dispersion. In this work, we will present two salient examples to show that STM spectroscopy can be used to detect these signatures of topological surface states. The first is to measure spin–momentum-locked conduction on topological insulator Bi₂Te₂Se. A multi-probe STM with spin-polarized tips allows us to perform in situ transport measurement to differentiate surface conductance from the bulk and spin-up chemical potential from the spin-down. As a result, a spin-momentum-locked current is revealed which shows ultra-high mobility and polarization. The second is to detect topologically nontrivial magnetic states in MnBi₂Te₄. Quasi-particle interference patterns are used to probe local dispersions of both surface and bulk electronic structures. The theoretically predicted gaped surface states are evaluated with high spatial resolution. It is expected that tuning of the Fermi level in the exchange gap will result in the emergence of a quantum anomalous Hall effect.   

Neutron Scattering at Missouri: Current Status and Future Prospects

The University of Missouri has been operating a nuclear research reactor for more than 50 years and thus has a long history in neutron scattering research. Currently, we have four neutron scattering instruments in service: a triple-axis spectrometer (TRIAX), an unpolarized neutron reflectometer (GANS), and two double-axis diffractometers (2XC and PSD). We will give an overview of the performance of the instruments and of the ongoing research projects. The PSD powder diffractometer has recently been upgraded with new electronics and software and with an expansion from 5 to 15 linear position sensitive He-3 detector tubes. We will illustrate the vast improvement of signal-to background ratio and highlight ongoing research projects. Future plans to expand our suite of neutron instruments include a thermal neutron beam imaging station. We present a conceptual design and Monte Carlo calculations. The science case concerning Plant Imaging and Tomography will be discussed.   

A robust software interface for a green astro-comb: remote control and automation

Using the Doppler effect to search for exoplanets is a powerful technique. State-of-the-art spectrographs such as HARPS-N at the Italian National Telescope are the workhorse tools in such searches. Though their stability is excellent, it isn’t sufficient to detect Earth-like planets around Sun-like stars due to the extremely small Doppler shifts associated with such systems (~100 kHz shifts of optical transitions). A visible wavelength, broadband 16 GHz laser frequency comb (astro-comb) can precisely calibrate spectra to resolve these Doppler shifts, but operation of this instrument requires maintenance and laser expertise. Here, we report on the successful development and implementation of a LabVIEW program that remotely operates an astro-comb and has automation features, relieving the need for on-site scientists at the Italian National Telescope on the Canary Islands and making this tool accessible to a larger community of researchers.   

The Structure of Degassed Water-Enabled Oil-in-Water Microemulsions

Most anti-cancer agents are hydrophobic and their use on patients often requires an oil & drug delivery vehicle. The drug delivery vehicle tends to be the primary cause of side effects in patients. Growing evidence suggests that it may be possible to mix oil in water at higher concentrations if dissolved gases are removed from water. Understanding the structure of oil/water microemulsions could shed light on mechanisms of mixing. This project uses small-angle x-ray scattering, dynamic light scattering, and turbidity measurements to assess the structure of hydrophobic molecules mixed with degassed water. Results of nanostructure as a function of alkane molecule chain length and concentration will be presented. These results will be compared to the same measurements of biocompatible fatty acids. Determining the properties that enable their miscibility with an aqueous environment will be helpful for future drug delivery.   

Increasing the Nanoparticle Size Detection Sensitivity of Dynamic Light Scattering using Wavelength Dependent Excitation

We report on the development of homebuilt Dynamic Light Scattering (DLS) instrumentation to measure the size of monodisperse (MD), spherical nanoparticles (NPs) of gold. HeNe and Ar-ion lasers constitute the excitation sources for the scattering experiment, while an avalanche photodiode detects the scattered light, and an autocorrelation card analyzes the resulting signal to provide a measurement of the translational diffusion coefficient, which allows for the determination of NP diameter. We characterized our instrumentation using commercially-produced gold NPs with diameters ranging from 10nm to 200nm in aqueous solution. The strong wavelength λ dependence of the scattered light intensity (1/λ⁴) provides increased sensitivity for smaller excitation wavelengths. We present DLS measurements on gold NPs using excitation from both a HeNe laser (λ = 632.8nm) and a tunable Argon laser (457nm < λ < 515nm). The increased scattering from the shorter wavelengths should increase our sensitivity to smaller particles.
  

Application of edge detection techniques to ARPES data

Edge detection and similar image analysis techniques are commonly used in computer vision but have not been fully realized for the purpose of ARPES data analysis. Without applying any Image analysis, the interpretation of ARPES data is left to the eyes of researchers and can be tricky and unreliable due to many sources of noise and distortion from the experimental processes, as a result, some of the finer, defining, details required to classify a material can be missed. By applying edge detection techniques, we are able to highlight key features such as distinct, clustered bands and other fine details that may otherwise have been obscured by noise and other experimental artefacts. Here we show the implementations of various image processing techniques applied to ARPES data and how they not only aid the interpretation of results, but can also be looked upon as stepping stones for better data processing techniques and potential automation of the classification of quantum materials through ARPES.   

Constructing New Patterns for Structured Illumination Microscopy

Ernst Abbe derived his famous formula of diffraction limit in 1873. The implication of this formula is that resolution of light microscopy is limited, which is in the magnitude of half of the wavelength of the light used in microscope. With the advancement in information theory and fluorescence techniques, going beyond theoretical limit stated by Abbe became possible. These methods are known as ‘super-resolution microscopy’ and one of them is structured illumination microscopy(SIM).
The basic idea of SIM is similar to that of Moiré pattern. By overlapping two finely spaced patterns, a slowly varying pattern of large fringes can be observed, in analogous to sound beats. By constructing a known finely spaced pattern, and illuminate it onto an unknown finely spaced pattern, fine details of unknown pattern can be reconstructed from the overlapped pattern observed with computer algorithms. Our group would like to explore and design new interference light patterns by manipulating optics and investigate whether a better lateral and axial resolution could be achieved.   

Focused Ion Beam Pt for Cryogenic Resistive Thermometry

Local temperature measurements at cryogenic temperatures are important for studying thermal effects, both classical and quantum, in nanoscale devices. Resistive thermometry is a relatively simple way to obtain these measurements. However, most metals’ electron transport saturates at low temperatures, leading to constant resistances and making them ineffective for cryogenic thermometry. An exploration of Pt thermometers, deposited using a focused ion beam (FIB) system, reveals that they are very effective for low temperature thermometry. This is believed to be because of the carbon contamination of the Pt. A detailed analysis of the dependence of composition of the contaminants on deposition parameters and its effect on thermometer performance is presented here. Through this research, it is clear that FIB Pt offers a tunable, sensitive, template-free thermometer for nanoscale, cryogenic thermometry.   

Aggregation Dynamics of Blood Platelets

Platelet aggregation at sites of vascular injury is necessary for hemostasis and arterial thrombosis and occurs via platelet–platelet adhesion, tethering and rolling on the injured endothelium, a critical initial step in blood clot formation. In straight vessels, the presence of erythrocytes, red blood cells (RBCs) is known to push platelets toward walls, which may affect platelet aggregation and thrombus formation.
Blood Cells for required optimizing coagulation. Model predictions on platelet to platelet collisions and of platelets to the capillary wall showed that platelet size, platelet Young’s modulus and shear rate are most significant variables controlling capillary aggregation and thrombus. Results are compared with available experimental results.   

A Novel Energy-Resolved X-Ray Semiconductor Detector

The hyperspectral X-ray imaging has long been sought in various fields from material analysis to medical diagnosis. Here we propose a new semiconductor detector structure to realize energy-resolved imaging at potentially low cost. The device is designed based on the strong energy-dependent absorption of X-ray in solids. Namely, depending on the energy, X-ray photons experience dramatically different attenuation. An array or matrix of semiconductor cells is used to map the X-ray intensity along its trajectory. With known X-ray attenuation coefficient, the X-ray spectrum could be extracted from a Laplace like transform or a supervised machine learning. We conceptually demonstrated an energy-resolved X-ray detection with a regular silicon camera.
  

Improved Sensitivity of a Single-Sided Magnetic Particle Imaging Scanner

Magnetic Particle Imagining (MPI) is the new frontier of medical imaging, capable of imaging the distribution of superparamagnetic nanoparticles (SPIOs) in an expeditious and sensitive manner. For instance, the accumulation of SPIOs in tumor tissue, serving as tumor markers, presents the MPI device as a practical means of imaging for in vivo cancer imaging. The single-sided design is beneficial for this application allowing imaging of larger subjects. In our design of a scanner we utilize a field-free line (FFL) geometry of the magnetic field zero, which has a potential advantage of an increased sensitivity over the traditional approach with field-free point (FFP). Our prototype device uses a single primary coil to generate a magnetic excitation field. The SPIOs response to the excitation is detected by a planar receive coil on the surface of the device. Due to a single-sided geometry, differentiating a small signal on a strong excitation background becomes a challenging task and further impinges the potential sensitivity gain, thus a new planar gradiometer coil configuration was developed. Results from the numerical simulations and experimental data imply the improved sensitivity of the device over the single coil design.   

Retrospective quantitative harmonization in PET using deconvolution and optimal filtering

The reliability of longitudinal quantitative PET image analysis suffers if scans are acquired on different PET scanners. Here, we describe a post-reconstruction harmonization method that can be implemented to enable quantitative PET analysis across scanners.
First, images are unfiltered by Wiener deconvolution. Then, the differences in contrast recovery coefficients (CRC) from NEMA phantom scans are minimized by finding isotropic 3D Gaussian filters for each scanner/reconstruction setting combination with a downhill-simplex optimizer. After optimal filters are established, PET images to be harmonized are unfiltered using Wiener deconvolution, and then re-filtered with the determined optimal settings.
We demonstrate that the method minimizes differences in CRC values for a set of 7 heterogeneous reconstruction protocols acquired on 3 different PET/CT scanners. We apply our method retrospectively to 18F-FDG PET/CT scans of a cohort of metastatic melanoma patients to show that performing harmonization has a significant impact in assigning lesion response as measured by change in PET SUV metrics.
In conclusion, the described method for harmonization enables accurate quantitative PET image analysis for retrospective datasets, and has clinical impact in response assessment settings.   

Magnetic Resonance Imaging Thermography with Uniform Gd Microstructures

Magnetic resonance imaging is an important technique in imaging living tissue and composite structures. Many medical procedures now use MRI as a critical component including MRI guided thermal ablation therapy used to treat cancer. Such procedures require real-time, spatially and thermally accurate temperature maps. We demonstrate an MRI temperature contrast agent consisting of uniform gadolinium microstructures dispersed within a media. We report on the performance of 6 micron wide, disk-shaped Gd microstructures passivated by a layer of chromium. A SQUID magnetometer was used to determine the mass magnetization of these disks. The temperature dependence of the mass magnetization was then correlated to the nuclear magnetic resonance linewidth broadening of water protons in the presence of Gd disks. We used this correlation to demonstrate the MRI image brightness of the Gd microstructures suspended in a tissue-mimicking phantom can be related to the temperature of the sample indicating these Gd disks are a good candidate for use as an MRI temperature contrast agent.
  

Simple physical modeling for comparing respirator breathing resistance standards

Air purifying respirators are designed to reduce the number of hazardous particles respired by wearers. One barrier to respirator use is their inherent increase in breathing difficulty. The National Institute for Occupational Safety and Health (NIOSH) evaluates respirators using Standard Test Procedures (STP) for various factors, including measuring the breathing resistance as the pressure drop across the filter at a certain relatively high flow rate. The International Standards Organization (ISO) has a competing standard based on a ‘Work of Breathing’ (WOB) concept, which considers Pressure-Volume (PV) work for a series of sinusoidal simulated breaths with frequencies and magnitudes reflecting a variety of occupational scenarios, from resting to intense physical exertion. We compare the WOB and NIOSH breathing resistance limits using a simple linear model, which reflects the most common type of particulate filtering respirators. We find that the NIOSH result is more stringent than the ISO standard for most of the ISO work rates, Resting, W1, W2, and W4, but is less stringent for the moderately high W3 work rate. The effect of the non-linear flow characteristics associated with exhalation valves will be discussed along with the results of a less rigorous treatment.   

Deflector Cavity Design for Rapid 2-D Proton Beam Scanning

We investigate the design of a 2.856 GHz deflecting cavity to provide rapid 2-D beam scanning for hadron therapy. We consider geometries for both conventional TM₁₁ modes and TE₁₁–like modes. Designs are optimized for the case of sub-relativistic protons with 150 MeV kinetic energy using simulations in HFSS to characterize the full 3-D field profile. We discuss the challenge of maximizing the transverse shunt impedance while mitigating variations in the transverse voltage as a function of distance from the nominal beam axis. These changes in the effective transverse kick can lead to significant beam profile aberrations for non-relativistic particles.   

Comparative Pharmacokinetics, Biodistribution and Dosimetry of 212Pb labeled Antibody vs Peptide vs Small Molecule

We study the impact of carrier molecules in biodistribution and absorbed dose in normal tissue and tumors using ²¹²Pb radionuclide. ²¹²Pb (T_1/2 =10.64 hrs) decays to ²¹²Bi (T_1/2 = 60.6 mins) via β emission then finally stable ²⁰⁸Pb via α and γ decays. The effective T_1/2 could vary a lot depending on the carrier molecule. In this study, rats and mice data of ¹³¹I, ¹⁶⁶Ho, ¹⁵³Sm and ¹⁷⁷Lu labeled DOTATATE, ¹¹¹In and ²¹²Pb labelled trastuzumab and ¹⁸⁸Re-HEDP were utilized and fitted via SAAM II software. To calculate absorbed doses, the AUC of each organ was multiplied by the S values provided in MIRD scheme. RBE value of 5 was used to take into account the biological effect of the doses.
The highest level of radioactivity indicating the amount of absorbed dose was detected in bladder, pancreas, intestines, bone and kidneys for DOTATATE, while it was cleared out of the blood within a few hours post injection. In contrast to the peptide carrier, the antibody carrier, remained in the blood significantly longer with increased uptake in the liver after 78 hrs while decreasing everywhere else steadily. For the small molecule carrier HEDP, the main concentration was found in bone followed by kidney, thyroid, stomach, liver and blood. Quantitative analysis of effective doses is in progress.   

Rotation- and reflection-encompassing multispectral nonlocal means denoising filter for magnetic resonance imaging

Image denoising is used extensively in medical image processing. Compared to other advanced filters, nonlocal means (NLM) shows excellent performance while being straightforward to implement. NLM takes advantage of structural redundancy in images by comparing local neighborhoods (local patches) around voxels throughout the whole image. The index voxel is then restored based on a weighted average of all voxels, with the weighting dependent on the degree of similarity. We have extended this by exploiting multispectral (MS) similarity in related images, obtained with a sequence of imaging parameters (1). In addition, we have developed an adaptive user-independent method to define similar voxels (2). The filter exhibits excellent edge preservation and noise reduction in magnetic resonance (MR) images and has been applied to improve parameter determination (3, 4). Here, we introduce a rotation- and reflection-encompassing implementation of our filter, motivated by the fact that patches may be similar only upon rotation. Our results demonstrate the improved performance as compared to our original implementation. This filter has wide applicability in imaging.
1. IEEE TMI 2017;36:181-93
2. MRI 2019;55:133-9
3. JNM 2018;309:121-131
4. JON 2018;28:640-649   

Deterministic single photon subtraction for engineering exotic non-classical states of light

We present an analytical and numerical simulation study of deterministic single photon subtraction using Single Photon Raman Interaction (SPRINT) [1] in a three level Λ-type quantum emitter coupled with a chiral waveguide or cavity. This process of deterministic single photon subtraction is fundamentally different from the usual probabilistic single photon subtraction, which is inherently just the application of the annihilation operator on the incoming light field. Unlike probabilistic single photon subtraction, the photon subtraction probability relying on SPRINT is independent of the number of photons in the input field. We investigate the effects of the repeated application of the deterministic subtraction operator on photonic fields and the resulting changes in phase space statistics that lead to exotic non-classical states of light, such as states with negative Wigner functions and squeezed states. We also discuss the prospects of experimental implementation of this approach.

[1] S. Rosenblum et al., Extraction of a single photon from an optical pulse, Nature Photonics 10, 19(2015).   

Using Trapped Ion Chains for Realization of Quantum Error Correction

We are constructing a quantum processor based on a register of up to 32 trapped 171Yb+ ions, with high connectivity provided by high-fidelity entangling gates. Our goals are to implement an error-corrected logical qubit and to perform a range of quantum algorithms with a system size and gate depth that move beyond what has previously been demonstrated.
Here, we report on the current status of our first-generation integrated system, including gate fidelity, hardware performance, and automatic control. We also present the progress toward implementing fault-tolerant logical qubits.   

Exploration of the Circularly Polarized Attosecond Pulse Generation Mechanisms by Polarization Gating

We perform a fully ab initio investigation of the generation of circularly polarized attosecond pulse of atomic H driven by polarization gating. The time-dependent Schrödinger equation is solved accurately and efficiently by means of the time-dependent generalized pseudospectral method. We investigate the physical mechanism of this process by solving the time-dependent Schrödinger equation in the integral form. In this way, the contribution of the wave function from different time segment to the induced dipole can be analyzed. The dynamic behavior during this process can be unveiled by the Bohmian trajectories of the dominant part of the wave function.   

High resolution AC magnetometry NMR spectrometer development using diamond nitrogen-vacancy centers

Nitrogen-vacancy (NV) quantum defects in diamond are sensitive detectors of magnetic fields. Due to their atomic size and optical readout capability, they have been used for magnetic resonance spectroscopy of exceptionally low volume samples on diamond surfaces. Here we present our approaches to develop and construct a simple, low-cost NV-dectected NMR spectrometer. The underlying theory and preliminary experimental data will be presented.   

Microwave to Optical Transducer with Single Emitters

In this project, we aim to develop a quantum interface between microwave and optical photons. This is an essential component in realizing a hybrid quantum network where information needs to be interfaced between superconducting microwave circuits, which provide quantum information processing, and optical photons, which are suitable for long-distance communication. Our work investigates the potential of micro-fabricated devices with integrated optical and microwave cavities that use individual three-level solid-state emitters, such as NV centres, for the efficient conversion between the microwave and optical regimes. We present analytical and numerical simulation results that explore the required characteristics of the microwave and optical cavities to achieve high conversion efficiency between the microwave and optical regimes.   

Growth and characterization of Si/SiGe heterostructures towards scalable qubit architecture

Strained Si/SiGe Quantum Well (QW) structures are a promising material system for the realization of spin qubits based on spatially confined electrons. A major advantage for Si-based structures is their compatibility with mature Si-CMOS technology, providing high scalability. In this study we show a first step towards determining the relationship between the material properties (e.g. interface roughness and defect density) and qubit performance of these structures. In order to discuss the influence of the material properties of the QW, tensile strained Si QW embedded in Si_0.7 Ge_0.3 layers with different types of SiGe buffer layers are fabricated on 200 mm Si substrates. The material properties are characterized by various methods (X-Ray Diffraction, Secondary Ion Mass Spectrometry, Scanning Transmission Electron Microscopy).
  

Electric manipulation of a valley qubit in a silicon quantum dot

In a silicon quantum dot, the valley states can be encoded as a qubit for quantum information processing. Here, we study theoretically the electric manipulation of valley qubit in a silicon quantum dot. The valley qubit frequency and electric manipulation speed are studied as a function of the location of interface steps and the applied electric field. We find that the applied electric field or the interface state can enhance the electric manipulation of the valley qubit. We further study the combined effect of QD confinement and interface steps on the electric manipulation of valley qubit. We will discuss the consequence of the results on the valley qubit based quantum information processing.   

A Platform for Light-Hole Qubits in Group IV Semiconductors

Si-compatible quantum devices have been exploiting either tensile strained Si or compressively strained Ge QWs, which are the only group IV systems that can currently be routinely obtained using SiGe as growth template and barrier layers. For quantum information, the former has been used as the building block for electron spin qubits, whereas the latter has been explored in new schemes for hole spin qubits. Herein, we present a third low-dimension system consisting of highly tensile strained Ge QW integrated on an optically active platform and discuss its basic properties experimentally and theoretically. The growth of tensile strained Ge QW is achieved using direct bandgap GeSn as barrier layers grown on silicon wafers. This heterostructure yields high tensile strain in Ge QW and band structure corresponding to a sizable LH-HH splitting exceeding 100 meV. Unlike compressively strained Ge, the top of the valence band is occupied by LH in tensile strained Ge. We also found a high LH g-factor anisotropy in Ge/GeSn QW, with g = 21.8 for in-plane B-field and g = 0.69 for perpendicular-to-plane field. These properties lay the groundwork to implement LH spin qubits with potentially easier manipulation due to the combined effects of the large Rashba-type SOI, and the spin ½ of LH.   

Observation and stabilization of photonic Fock states in a hot radio-frequency resonator

Detecting and manipulating single-photons at MHz frequencies presents a challenge as, even at cryogenic temperatures, thermal fluctuations are significant. In our work [1], we use a GHz superconducting qubit to directly observe the quantization of a MHz radio-frequency electromagnetic field. Using the qubit, we achieve quantum control over thermal photons, cooling to the ground-state and stabilizing photonic Fock states. Releasing the resonator from our control, we directly observe its re-thermalization dynamics with the bath with nanosecond resolution. Extending circuit QED to a new regime, we enable the exploration of thermodynamics at the quantum scale and allow interfacing quantum circuits with MHz systems such as spins or mechanical oscillators. The tool used to design such a circuit, QuCAT [2], the "quantum circuit analyzer tool in Python" will also be featured in this poster.
[1] Science 363.6431 (2019): 1072-1075.
[2] arXiv:1908.10342 (2019).   

Single-Photon Dispersive Readout of a Qubit with a Photodetector: Theory Beyond the Rotating-Wave Approximation

We propose to use a single-photon pulse and a photodetector for the dispersive readout of a qubit. The scheme avoids the shot noise errors. However, another source of errors makes it challenging to perform a single-photon readout. To boost its performance, we propose to detune the qubit and the resonator further than usual, while coupling them stronger. The Bloch-Siegert shift then should be taken into account. It is shown how it can improve the readout. We provide simple analytical estimates for the readout contrast. Results of more complicated calculations that take the qubit relaxation into account are also presented. A contrast more than 75% can be achieved in 1μs for ideal detector and photon source.   

Study on the cavity photon induced dephasing of a transmon qubit in circuit QED due to the measurement microwave leakage

Qubit coherence is one of key features in quantum computing. In circuit QED architecture, it is known that the cavity photon induces dephasing of the qubit even in the dispersive regime [1, 2]. In this study, we studied in detail the dephasing of a 3D transmon qubit generated by the measurement microwave tone. First, we observed that the intense measurement microwave tone decreases the dephasing time. In order to avoid the dephasing, we controlled the mixer leakage by applying dc-offset voltage. We observed clear dependence of the measurement microwave leakage and dephasing, so that we can optimize our measurement setup to achieve maximum T₂^*. This calibration method is one of the standard features in our measurement system to preserve the qubit coherence.

[1] D. I. Schuster et al., Phys. Rev. Lett. 94, 123602 (2005).
[2] J. Gambetta et al., Phys. Rev. A 74, 042318 (2006).   

Enhancement in the cross-resonance gate performance

We theoretically model an experiment on a superconducting circuit made of a capacitively shunted flux qubit (CSFQ) and a transmon both capacitively coupled to a bus resonator in dispersive regime. We apply external driving microwave pulses over all energy levels and consider the transitions they impose effectively within the computational subspace. More specifically we apply entirely microwave two-qubit gate, the so called cross-resonance, on CSFQ at sweet spot and away from it. Interestingly the two-qubit fidelity is largely enhanced at certain external flux away from the sweet spot. This enhancement takes place as the result of suppressed leakage out of computational subspace.   

Spectrum and Coherence of the Current-Mirror Circuit

The current-mirror circuit [1] exhibits a robust ground-state degeneracy and wave functions with disjoint support for appropriate circuit parameters. In this protected regime, Cooper-pair excitons form the relevant low-energy excitations. Based on a full circuit analysis of the current-mirror device, we introduce an effective model that systematically captures the relevant low-energy degrees of freedom, and is amenable to diagonalization using Density Matrix Renormalization Group (DMRG) methods. We find excellent agreement between DMRG and exact diagonalization, and can push DMRG simulations to much larger circuit sizes than feasible for exact diagonalization. We discuss the spectral properties of the current-mirror circuit, and predict coherence times exceeding 1 ms in parameter regimes believed to be within reach of experiments.

[1] A. Kitaev, arXiv:cond-mat/0609441 (2006)   

Qubit fast reset with QubiC

Fast reset is a basic qubit control feedback loop where the hardware latency is critical. We develop the fast reset logic on the QubiC system by implementing the qubit status classification and the feedback loop in the FPGA firmware. The experiment setup and initial testing results are presented here.   

Exploring 3D architectures and superconducting interconnects using highly conformal ALD and CVD: a BEOL-compatible process for superconducting MgB2

The ability to fabricate 3D architectures and superconducting interconnects can be an enabling technology towards the scale up of quantum computing architectures. One of the key challenges is that physical vapor deposition methods, such as evaporation or sputtering, struggle to coat or infiltrate high aspect ratio structures, being traditionally limited to aspect ratios of 10:1 or lower. In this work we explore how to take advantage of the high conformality of ALD and some CVD processes to overcome this limitation. In particular, we have developed a BEOL-compatible, diborane-free process that is capable of growing superconducting magnesium diboride at low temperatures. This process has allowed us to demonstrate critical temperatures exceeding 20K in films that are just a few nm thick, and the formation of stable interfaces with oxide materials. Together with existing ALD processes for superconducting nitrides, we believe that this process can help us explore new device configuration, including the design of architectures with vertical junctions, or the development of superconducting interposers for advanced packaging applications.   

Impact of noise reduction in measurement of a super conducting flux qubit at 4.2 K

A flux state of a super conducting flux qubit is evaluated at 4.2 K prior to measurement of quantum state at mK. At the 4.2 K measurement, a customized rod, where a sample is attached, is inserted in the Dewar vessel filled in the liquid helium. An elimination of artifact from an experimental system is crucial. In order to prevent signal quality deterioration by environmental noise, countermeasures have been taken by strengthening the electrostatic shield and introducing the twisted pair wiring. Low noise voltage source (LP6016) for bias current control of SQUID is also effective in suppressing signal fluctuation from SQUID. As a result, noise density at 50 Hz and 150 Hz in the measurement became 1/300 and 1/500 compared to the previous experiment, respectively. We evaluated the flux state in the super conducting flux qubit through an observation of signal in a readout-SQUID. This sample was fabricated by AIST Nb standard process 2, where J_c was 25 μA/μm². On a flux dependence of threshold current (I_th) in the readout-SQUID, standard deviations (σ) of I_th was 0.16. This indicates that we can control an operating point of the readout-SQUID in an accuracy of the measurement with an order less than 1 μA. Further, σ << 1 was evaluated when we observed flux conditions in the qubit.   

Search by Lackadaisical Quantum Walk with Nonhomogeneous Weights

The lackadaisical quantum walk, a quantum walk with weighted self-loops, speeds up dispersion on a line and improves spatial search on the complete graph and periodic square lattice. In these investigations, each self-loop had the same weight, due to each graph's vertex-transitivity. In this paper, we propose lackadaisical quantum walks with self-loops of different weight. We investigate spatial search on the complete bipartite graph, which can be irregular with partitions of size N₁ and N₂, which naturally leads to self-loops having different weights l₁ and l₂, respectively. We prove that if the k marked vertices are confined to one partite set, then with the typical initial uniform state over the vertices, the success probability is improved from its non-lackadaisical value when l₁=kN₂/2N₁ and N₂>(3−2√2)N₁, regardless of l₂. When the initial state is stationary under the quantum walk, however, the success probability is improved when l₁=kN₂/2N₁ without a constraint on the ratio of N₁ and N₂. Next, when marked vertices lie in both partite sets, then for either initial state, there are many configurations for which the self-loops yield no improvement in quantum search, no matter what weights they take.   

Trimming Grover's Quantum Search Algorithm

Grover’s quantum algorithm is a complicated search algorithm made to run on a quantum computer. The algorithm allows users to search a database for a word in a significantly shorter time than a conventional computer. Past experiments have attempted to run iterations of the algorithm on a quantum computer and failed because of the complexity. My research modifies and simplifies the theory of Grover’s algorithm by utilizing results from previous work that studied the diffusion of wave functions across a hypercube. I have directly applied this diffusion process to an edited and simplified version of Grover’s algorithm based on the premise of simplifying the connections and paths between q-bits represented on a hypercube. The algorithm I have created has been run and tested on the open-source quantum computer simulations provided by IBM on their IBM Q website. My results display a trimmed and practical version of Grover’s quantum search algorithm.   

Quantum simulations and force sensing experiments with 2D arrays of 100’s of ions

We perform quantum simulations and quantum sensing with two-dimensional arrays of >100 trapped ions in a Penning trap [1, 2]. In our quantum sensing experiments, we measure small displacements of the ion crystal. Electric fields excite center-of-mass (COM) motion of the crystal. By measuring the motion-induced spin precession we can determine the amplitude of the COM motion and the strength of the exciting field [2]. Improvements in the phase stability of our coupling lasers enabled phase coherent detection protocols, resulting in a single measurement sensitivity of 50 pm for the COM motion that was far off-resonant with the trap axial frequency. 50 pm is about a factor 40 below the size of the ground state wavefunction.Further, we show results of electromagnetically induced transparency (EIT) cooling for the drumhead motion of the single-plane arrays [3, 4]. We present results that show simultaneous cooling of all the drumhead modes to close to the ground state of motion.

[1] A. Safavi-Naini et al., PRL 121, 040503 (2018)
[2] K.A. Gilmore et al., PRL 118, 263602 (2017)
[3] E. Jordan et al., PRL 122, 053603 (2019)
[4] A. Shankar et al., PRA 99, 023409 (2019)   

QAOA Algorithms for Traveling Salesman Problem Revisited

The traveling salesman problem is a traditionally NP-hard problem. Earlier quantum algorithm encodes the qubits based on vertices, while we encode the qubits based on edges, with a resources requirement halved. With edge encoding scheme, QAOA algorithm is demoed with small cases, on a classical enumlator, with the optimal solutions successfully found. The algorithm has polynomial complexity in terms of the number of quantum operations.   

Implementation of excited state energy and its analytical derivatives for photochemical reaction simulations on NISQ devices

A primitive but still powerful form of quantum computers called Noisy Intermediate-Scale Quantum (NISQ) devices is about to be utilized for the real-world problems. NISQ devices have a few hundreds to thousands of qubits under highly precise control although they are not fault-tolerant.

A quantum chemistry calculation is one of the most promising applications of NISQ devices in the near future, and Variational Quantum Eigensolver (VQE) is the most featured algorithm to take advantage of NISQ devices in quantum chemistry.
Recently, numerous algorithms based on the VQE are proposed for solving problems that are hard for classical computers, such as calculations of electronic excited states of large molecular systems.

However, VQE-type algorithms are in general based on the variational principle, which makes it difficult to predict their performances when applied to actual molecular systems due to their heuristic nature.

In this study, using the high-speed simulator Qulacs, we implement VQE-type algorithms to calculate several physical properties (analytical energy derivatives and oscillator strengths) required for photochemical reaction simulations and create their unified benchmark of various proposed methods for small molecules compared with results from the classical computation.   

The Quantum Alternating Operator Ansatz on Max-k Vertex Cover

We study the performance of the Quantum Alternating Operator Ansatz (a generalization of the QAOA for problems with hard constraints) on the problem of Max-k Vertex Cover due to its modest complexity, while still being more complex than the well studied problems of Max-Cut and Max-E3LIN2. Our approach includes (i) a performance comparison between easy-to-prepare classical states and Dicke states, (ii) a performance comparison between two XY-Hamiltonian mixing operators: the ring mixer and the complete graph mixer, (iii) an analysis of the distribution of solutions via Monte Carlo sampling, and (iv) the exploration of efficient angle selection strategies. Our results are: (i) Dicke states improve performance compared to easy-to-prepare classical states, (ii) an upper bound on the simulation of the complete graph mixer, (iii) the complete graph mixer improves performance relative to the ring mixer, (iv) the standard deviation on the distribution of solutions decreases exponentially in p (the number of rounds in the algorithm), requiring an exponential number of random samples find a better solution in the next round, and (iv) a correlation of angle parameters which exhibit high quality solutions that behave similarly to a discretized version of the Quantum Adiabatic Algorithm.   

Optimizer-aware Circuit Design for Error Unfolding in Variational Quantum Eigensolver

Quantum hardware suffers from errors and any measured output is thus a convolution of the intended result and some error distribution. The Variational Quantum Eigensolver has a classical optimizer step that takes the output from the quantum chip to determine the input parameters for the next iteration. Errors can prevent the optimizer from making progress; and if they are too large, make it impossible to calculate, let alone find, the global minimum. If the error distribution were known, it could be unfolded, but this is generally not the case, especially if the errors do not commute with the whole circuit. Exploiting ideas from randomized compilation, we introduce twirl gates into the circuit, generating logically equivalent circuits with the same number of gates and same output, but with different error behavior. The physical processes do not change, thus error rates remain the same as well. Using a Markov chain as error model, we apply a maximum likelihood fit to find those rates that produce migration matrices for unfolding consistent with all observed distributions. This results in improved output estimates of the quantum computer, and better defined uncertainties as input to the classical optimizer leading to faster convergence and a more robustly defined global minimum.   

Benchmarking Noise Extrapolation with OpenPulse

In order to augment digital qubit metrics, such as gate fidelity, we discuss analog error mitigability, i.e. the ability to accurately distill precise observable estimates, as a hybrid quantum-classical computing benchmarking task. Specifically, we characterize single-qubit error rates on IBM’s Poughkeepsie superconducting quantum hardware, incorporate control mediated noise dependence into a generalized rescaling protocol, and analyze how noise characteristics influence Richardson extrapolation-based error mitigation. Our results identify regions in the space of Hamiltonian control fields and circuit-depth which are most amenable to reliable noise extrapolation, as well as shedding light on how low-level hardware characterization can be used as a predictive tool for uncertainty quantification in error mitigated NISQ computations.   

Modeling Quantum Devices and the Reconstruction of Physics in Practical Systems

Modeling quantum devices is to find a model according to quantum theory that can explain the result of experiments in a quantum device. We find that usually we cannot correctly identify the model describing the actual physics of the device regardless of the experimental effort given a limited set of operations. According to sufficient conditions that we find, correctly reconstructing the model requires either a particular set of pure states and projective measurements or a set of evolution operators that can generate all unitary operators.   

Continuous-variable QKD network in Qingdao

Continuous-variable quantum key distribution (QKD) is a very attractive method for the physical layer protection of information transmission by secure distribution of private keys, as it uses standard telecom components that operate at room temperature and it has higher secret key rates (bits per channel use) over metropolitan areas. The longest distance of continuous-variable QKD field tests has reached 50 km, which is enough to support the construction of metropolitan networks. Here, we report the long-term performance of three nodes continuous-variable QKD network in Qingdao, which is the first continuous-variable QKD application demonstration with clear application scenarios over a long period of time through existing commercial optical fiber links. The average secret key rate achieves higher than 12.00 kbps over 71.03 km optical fiber line. The system has been running for 28 days stably with a reliable performance, which paves the way to deploy continuous-variable QKD in metropolitan settings.   

Continuous-variable source-device-independent quantum key distribution

The continuous-variable quantum key distribution with entanglement in the middle, a semi-device-independent protocol, places the source in the untrusted third party between Alice and Bob, and thus has the advantage of high levels of security with the purpose of eliminating the assumptions about the source device. However, previous works considered the collective-attack analysis, which inevitably assumes that the states of the source has an identical and independently distributed (i.i.d) structure, and limits the application of the protocol. To solve this problem, we modify the original protocol by exploiting an energy test to monitor the potential high energy attacks an adversary may use. Our analysis removes the assumptions of the light source and the modified protocol can therefore be called source-device-independent protocol. Moreover, we analyze the security of the continuous-variable source-device-independent quantum key distribution protocol with a homodyne-homodyne structure against general coherent attacks by adapting a state-independent entropic uncertainty relation. The simulation results indicate that, in the universal composable security framework, the protocol can still achieve high key rates against coherent attacks under the condition of achievable block lengths.   

Resource-Adaptable Quantum Algorithms for Scalable Simulation of the Schwinger Model

The Schwinger model (quantum electrodynamics in 1+1 dimensions) is a testbed for the study of field theories underpinning the Standard Model. Quantum computers are anticipated to enable unprecedented simulations of field theories. In this work, we give scalable and explicit digital algorithms to simulate the Schwinger model using fault-tolerant quantum computation. We upper bound a relevant metric of computational complexity, the number of T-gates, in terms of the simulation parameters, confirming that the time evolution can be simulated efficiently. Additionally, we give circuits that could be used in a nearer-term (NISQ) implementation of our simulation algorithms.   

Space-Time Mixing of Quantum Computing in an Entangled Atomic Chain and Time Crystals

Experimentally observed space-time crystals are modeled here as a 1-D 4-State Cellular Automata (CA) under a specific set of “cyclic” state-transition rules, three of which are analyzed here. The generations of these specific CA rule sets have been previously shown to model the properties of the recently described space-time crystals due to the underlying mathematical model present in these rules. The rules are shown here to display three space-time symmetry rotations in the time crystals generated. Hence, the rotational symmetry property of the computational states of these CA generations is of physical significance to the rules governing space and time in a space-time crystal. The time-crystal generations are mapped onto the surface of a space-time sphere, where the non-Euclidean geometry allows for the interchangeability between space and time in producing space-time crystals.   

Employing Variational Principles to Define the Effective Hamiltonian of a Periodically Driven Qubit

The trajectory of a linearly driven qubit in the rotating frame can be determined using the effective Hamiltonian introduced in [1], which improves on the rotating wave approximation. By its definition, this Hamiltonian (i) is analytic in time and, (ii), yields an effective driven-qubit trajectory that coincides with the exact trajectory at equally-spaced points in time. In general, these effective trajectories are significantly smoother than the exact ones. The effective Hamiltonian is determined by an infinite sum whose convergence, however, is not always guaranteed [1]. As the requirements (i) and (ii) are fulfilled by a continuum of Hamiltonians, here we hypothesize that the effective Hamiltonian additionally satisfies, (iii), a variational principle that is motivated by the smoothed trajectories mentioned above. Our variational principles state that the integral of a certain function of the Hamiltonian, e.g., its positive eigenvalue, over the full pulse duration is minimized by the effective Hamiltonian of [1]. To find evidence for or against our hypotheses, we carry out variational calculations aimed at numerically minimizing the corresponding integrals.

[1] D. Zeuch et al., arXiv preprint: 1807.02858 (2018).   

Entangled Excitons via Spontaneous Downconversion

A class of centrosymmetric molecules support excitons with a well-defined quasi-angular momentum. Cofacial arrangements of these molecules can be engineered so that quantum cutting produces a pair of excitons with angular momenta that are maximally entangled. The Bell state constituents can subsequently travel in opposite directions down molecular chains as ballistic wave packets. This is a direct excitonic analog to the entangled polarization states produced by the spontaneous parametric downconversion of light. As in optical settings, the ability to produce Bell states should enable foundational experiments and technologies based on non-local excitonic quantum correlation. The idea is elucidated with a combination of quantum electrodynamics theory and numerical simulation.   

Practical route to entanglement-enhanced communication over noisy bosonic channels

Entanglement can offer substantial advantages in quantum information processing, but loss and noise hinder its applications in practical scenarios. Although it has been well known for decades that the classical communication capacity over lossy and noisy bosonic channels can be significantly enhanced by entanglement, no practical encoding and decoding schemes are available to realize any entanglement-enabled advantage. Here, we report structured encoding and decoding schemes for such an entanglement-assisted communication scenario. Specifically, we show that phase encoding on the entangled two-mode squeezed vacuum state saturates the entanglement-assisted classical communication capacity and overcomes the fundamental limit of covert communication without entanglement assistance. We then construct receivers for optimum hypothesis testing protocols under discrete phase modulation and for optimum noisy phase estimation protocols under continuous phase modulation. Our results pave the way for entanglement-assisted communication and sensing in the radiofrequency and microwave spectral ranges.   

Quantum-Computing(QC)? Not New and Not News! Merely Trendy Fashionable "Buzzwordism, Bandwagonism and Sloganeering for Fun, Profit, Survival, Ego" aka Showbiz!

QC has been alive and well since WWII in artificial neural-network(ANN) artificial-intelligence(AI), vs computing-quanta since quantum-theory, if not Newcomb(1881)-Poincare(1911)-Weyl(1916)-...-Benford(1938)[benfordonline.net] digits log-law inverse[Antonoff/Siegel(2002)]: digits=bosons. What does QC mean? In ANN AI, not counting Turing(1936 machine plagarism of Lenz-Ising model(1911) of localized magnetism, starting with Rosen(SRI). In the 1980s Charles Rosen(Machine-Intelligence), Jacob Goldman(Xerox PARC), Irwin Wunderman(HP), Vesco Marinov and Adolph Smith(Exxon Enterprises/AI) and Edward Siegel realized that ubiquitous standard sigmoid on-site switching-function N/[1+exp(-E/T)]=N/[+1+exp(-E/T)]=N/[exp(-E/T)+1] Fermi-Dirac quantim-statistics trapping the ANN in non-optimal local-minima necessitating time and memory costly Boltzmann-machine followed by simulated-annealing to eliminate the +1 converting Fermi-Dirac quantum-statistics into Maxwell-Boltzmann classical-statistics. They realized this time and memory costly crutch could be eliminated by reversing sigmoid-function to N/[exp(-E/T)-1] Bose-Einstein quantum-statistics["Eureka!"] and then taking the limit as N approaches infinity. Bose-Einstein condensation (BEC) ["Shazam!"] blindingly quick ANN AI QC (N)-BEC-machine.   

Quantum feedback error correction of a monitored system evolving adiabatically

We devise a quantum feedback error correction method to reverse the effect of thermal excitations in quantum annealing. Conditioned on the output signal I(t) from continuous measurement records, feedback is applied to an adiabatically evolving system in the hopes of increasing the ground state population at the end of the anneal. We propose an experimental setting for such continuous measurement and feedback in the case of superconducting flux qubits. We simulate the error correction performance of a system weakly coupled to a thermal bath based on methods like quantum trajectories and quantum bayesian updates. We also derive a feedback master equation for markovian feedback (feedback delay $\tau\rightarrow 0$) and further give the timescale condition for feedback Markovianity. Realistic feedbacks are also subjected to non-negligible feedback delay, detector efficiency, and restrictions on the form of the feedback Hamiltonian due to experimental challenges. We therefore study the effectiveness of feedback correction under such limitations and explore how the optimized feedback delay time depends on the annealing schedule and limitations in other experimental parameters.   

Constructing parsimonious control functions using B-splines with carrier waves

We consider the optimal control problem for realizing logical gates in closed quantum systems, where the evolution of the state vector is governed by the time-dependent Schroedinger equation. The number of parameters in the control functions is made independent of the number of time steps by expanding them in terms of B-spline basis functions, with and without carrier waves. We use an interior point gradient-based technique from the IPOPT package to minimize the gate infidelity subject to amplitude constraints on the control functions. The symplectic Stromer-Verlet scheme is used to integrate a real-valued formulation of Schroedinger's equation in time and the gradient of the gate infidelity is obtained by solving the corresponding adjoint equation. This allows all components of the gradient to be calculated at the cost of solving 3 Schroedinger systems, independently of the number of parameters in the control functions. The method is applied to Hamiltonians that model the dynamics of several coupled super-conducting qubits. We find that including judiciously chosen frequencies in the carrier waves of the basis functions can significantly reduce the number of parameters and lead to smoother control functions.   

Learning Quantum Error Models

In this abstract we propose a methodology for learning quantum error models from experimental data. This information is useful for characterizing the effectiveness of hardware, predicting how well a circuit should run in practice, and synthesizing corrected circuits that attempt to perform better by taking the error model into account.

We learn the error model by taking each gate in the original circuit and replacing it with a parameterized probability distribution over potential gates. For example, we could replace a Pauli X gate with a distribution having probability p of performing a random unitary and probability 1-p of performing the Pauli X. We then perform bayesian inference to deduce the most likely error model that gave us the desired error.

We test our methodology on experimental data, and evaluate the learned error models in its predictive power.   

An information entropy interpretation of photon absorption by dielectric media

We measured photon absorption in dielectric media and proposed the photon-version Beer–Lambert’s law to quantify the absorption. We used a Hong–Ou–Mandel interferometer and 810 nm twin-photons. We found that the depth ratio of the null point in the interference patterns of the interferometer agreed with the classical transmittance of the samples. We established a statistical model of the photon absorption process and proposed an information entropy interpretation to understand the meaning of the Beer–Lambert law. Comparisons of the results of the photon absorption experiments with classical experiments demonstrate the validity of our model and interpretation.   

Simulating broken PT-symmetric Hamiltonian systems by weak measurement

By embedding a PT-symmetric (pseudo-Hermitian) system into a large Hermitian one, we disclose the relations between PT-symmetric quantum theory and weak measurement theory. We show that the weak measurement can give rise to the inner product structure of PT-symmetric systems, with the pre-selected state and its post-selected state resident in the dilated conventional system. Typically in quantum information theory, by projecting out the irrelevant degrees and projecting onto the subspace, even local broken PT-symmetric Hamiltonian systems can be effectively simulated by this weak measurement paradigm.   

Knowledge is the Foundation of the Functioning of the Physical World in QM:Everything Goes Through the Wave Function,IncludingNullMeasurements,Expansion of On the Nature of the Change in the Wave Function in A Measurement in Quantum Mechanics(arxiv,1995)

Nothing in quantum mechanics (qm) occurs without the involvement of the wave function (wf); the wf is non-physical. When a measurement is made of 1 quantity, a new wf accompanies the measurement wherein another quantity of the particle is uncertain due to the wf, eg, position requiring many superposed waves and momentum requiring only 1 wave. Many situations showing the indispensability of the wf are discussed. What prevents knowledge based on the wf from being acknowledged as the foundation of the physical world is the assumed unavoidable physical interaction (pi) between the measuring instrument and the physical entity measured in the change in the wf in a measurement. This is explored in part through Feynman’s jolt of a photon on an electron in his 2 slit experiment with a light source between the 2 slits. The origin of this assumed unavoidable pi appears to be Bohr’s description of complementarity. The source for Bohr appears the psychologist W. James who did not include this pi in his complementarity. Without this pi, the central role of the wave function and knowledge derived from it is clearly seen in quantum mechanics. With null measurements (no pi), the problem that appears to originate with Bohr cannot be avoided. It should not have been avoided in 1995.   

The Quantum Theory of Entanglement and Alzheimer's

"Feynman argued that if gravity is indeed a quantum phenomenon, a superposition of a particle in two places at once would create two separate gravitational fields in case of a small mass in a quantum superpostion (entanglement), two different spacetimes would coexist side by side, almost like two separate universes, a state of affairs that should not exist in Einstein's theory [1]." We show gravity as a quantum phenomenon in [2] leading to [3], [4] and [5]. Here, we illustrate such coexistances. As a light bulb filled with white light, one universe so to say, is equivalent to one filled with seven different colors of light. [1] Folger T (2019) Quantum gravity in the lab. Scientific American, [2] Goradia SG Newtonian gravity in natural units. (2012) Journal of Physical Science and Application 2: 265-268, [3] Goradia SG Dark matter from our probabilistic gravity. (2015) J of Physical Science and Application 5: 373-376, [4] Goradia SG The Quantum Theory of Entanglement and Brain Physics, (2018) Journal of Clinical Review & Case Reports, [5] Goradia SG The Quantum Theory of Entanglement and Alzeimer's. (2019) J Alzheimers Neurodegener Dis 5: 23.   

How Knowledge Leads to the Demise of Schrodinger’s Cat Through a Null Measurement (A Quantum Mechanical Measurement in Which There is No Physical Interaction Between the Measuring Instrument and the Entity Measured)

The Schrodinger cat experiment (SCE) is presented. An alteration follows where the LACK of radioactive decay (rd) leads to the demise of the cat instead of the ACT of rd. The lack of rd is a negative (null) measurement (nm) (where there is NO physical interaction between the radioactive material and the Geiger counter). The nm is non-trivial because all knowledge about the radioactive material (rm) is derived from its associated wave function (wf) which itself has no physical existence. The wf is how we make probabilistic predictions regarding systems in quantum mechanics. Before the box in the SCE is opened, the wf for the rm is: psi_rm = 1/√2 [psi_rm does not decay + psi_rm does decay] which leads to the possibility of interference before the cat is observed. As Schrodinger wrote: “The psi_function of the entire system [including rm and cat] would express this by having in it the living and the dead cat (pardon the expression) mixed or smeared out in equal parts.” The wf is the foundation for knowledge since the probabilities are derived from the wf and the wf contains all the information concerning a system. This alteration of the SCE emphasizes that the lack of rd in the original SCE is also a nm that leads to the continued life of the cat.   

A quantum mechanical interpretation of gravitational redshift of photon

It was observed that electro-magnetic waves can undergo a frequency shift in a gravitational field. This effect is important for satellite communication and astrophysical measurements. Previously, this redshift phenomenon was interpreted exclusively as a relativistic effect. Recently we found this effect can also be explained based on a quantum mechanical consideration. We propose that, due to the quantum nature of the photon, its effective mass is not zero. In a gravitational field, the total energy of the photon includes both its quantum energy and its gravitational energy. Then, the condition of energy conservation will require a frequency shift when the photon travels between two points with different gravitational potentials. This result suggests that the gravitational redshift effect of a photon is essentially a quantum phenomenon. This new understanding has a strong implication; it suggests that some of the previous experimental tests of general relativity need to be re-interpreted.
  

From Hopf fiberation to duality-entanglement relation

We show that the entanglement-duality relation [Optica 5, 942 (2018)] can indeed be obtained, in its most generic form, from Hopf fiberation and stereographic geometry. We demonstrate that this relation is a natural consequence of the stereographic geometry, providing the first geometric approach to the notion of complementarity problem. We show that this geometry is duality-entanglement sensitive. This means that, it is sensitive to the wave nature, particle nature and entanglement nature of a single qubit.   

Programming multi-level quantum gates in disordered computing reservoirs via machine learning and TensorFlow

Novel machine learning computational tools open new perspectives for quantum information systems. Here we adopt the open-source programming library TensorFlow to design multi-level quantum gates including a computing reservoir represented by a random unitary matrix. In optics, the reservoir is a disordered medium or a multi-modal fiber. We show that trainable operators at the input and the readout enable one to realize multi-level gates. We study various qudit gates, including the scaling properties of the algorithms with the size of the reservoir. Despite an initial low slope learning stage, TensorFlow turns out to be an extremely versatile resource for designing gates with complex media, including different models that use spatial light modulators with quantized modulation levels.[1]
[1] arXiv:1905.05264   

Quantum Boosting

Boosting is one of the most famous classical machine learning techniques (in theory and practice) that can construct a strong machine learning algorithm given access to a weak learning algorithm. It is natural to consider boosting in the quantum setting for the following reason: suppose we implement a quantum machine learning (QML) algorithm on a NISQ device, then the guarantees of the QML algorithm could be much worse than it was designed for, due to the noise in the quantum system. This motivates the question whether we can ``boost" the performance of a weak QML algorithm (implemented on a NISQ machine) to a *strong* QML algorithm?

Classically the time complexity for boosting the performance of a weak learner for a concept class C is characterized by the VC dimension of C. Here, we ask if quantum techniques can provide an improvement over the well-known AdaBoost algorithm introduced by Freund and Schapire '93 (for which they were awarded the Godel prize in 2003). In this work, we introduce the first quantum boosting algorithm which is provably faster than classical Adaboost with time complexity that scales quadratically in the VC dimension of C.   

Layerwise learning for quantum neural networks

We introduce a layerwise learning strategy for parameterized quantum circuits. The circuit depth is incrementally grown during optimization, starting with a shallow circuit and adding depth until the required loss is attained. By training only varying subsets of the circuit’s parameters, we keep a fixed number of training parameters while increasing circuit depth until it is sufficient to represent the data. We then show that this approach avoids the problem of saddle points, or barren plateaus, of the error surface to a large extent due to the low depth of circuits, low number of parameters, and larger magnitude of gradients compared to training the full circuit. These properties make our algorithm ideal for execution on noisy intermediate-scale quantum (NISQ) devices. We demonstrate our approach on an image-classification task on handwritten digits, and show that the number of trained parameters can be decreased substantially while keeping gradient magnitudes larger than those of quantum circuits of the same size trained on a fixed architecture.   

Magnetic and electrical field detection using NV-based AFM

We develop a homemade NV-based scanning microscopy to detect both magnetic and electrical field induced by tips we used. As for the magnetic field, the Ni tip is required to provide magnetic field and field gradient on the surface of diamond membrane. The ODMR shift is observed during the Ni tip moving around a NV center, and we observe isomagnetic circle by ODMR mapping which shrinks to about 20nm indicating the actual location of the NV center.
Besides, the sensor we made allow us to apply bias voltage between metal tip and diamond. And the electrical field can be also measured by ODMR and Pulsed ODMR shift. Our results show the electrical field up to 100V/um which maybe controls the charge state of the NV center.   

Time resolved diamond magnetic microscopy of single transition metal magnetic nanoparticles

Understanding the magnetic properties of small transition metal magnetic nanoparticles (TMMNPs) can help in exploiting their useful properties such as high surface-to-volume ratio and tailorable surface chemistry for applications in catalysis, biosensing, and ultra-high-density magnetic recording. At size below 10 nm, surface effects play a major role in determining their magnetic properties. Symmetry breaking of the crystal structure at the particle surface, dangling bonds, and surface strain can alter their properties. Accurate measurements of magnetic properties of single TMMNPs would therefore shed light on their critical properties. However, measuring these properties of such small particles at ambient conditions presents a great challenge due to the lack of sensitivity of current nonperturbative magnetic imaging techniques. Here we show recent measurements using magnetic microscopy based on nitrogen vacancy centers in diamond to measure the static and dynamic magnetic properties of individual 2-10 nm TMMNP (CoPt and FePt). Histograms of critical parameters such as magnetic anisotropy, saturation magnetization, and magnetic relaxation are extracted and correlated with morphology data taken by AFM, SEM, and TEM.   

Phase estimation in a multimode nonlienar interferometer.

Simultaneous estimation of multiple phase shifts is very important in quantum metrology as it has implications for the single-shot quantum imaging. Multiparameter estimation has been already demonstrated in linear optics even though its performance is greatly limited by computational difficulty associated with matrix permanent. Here, we propose a scheme that uses nonlinear optical parametric amplification (OPA) to estimate multiple simultaneous phase shifts. The performance of our scheme is compared with the SNL and the Heisenberg limit (HL). We choose homodyne as the measurement strategy in our scheme. As photon loss is the biggest experimental limitation, we also investigate the effect of photon loss in the phase sensitivity.   

Rare Earth Quantum Information Science Decoherence TImes and Hosts

We present a brief review of the current state of Quantum Information Science (QIS), with a focus on Quantum Information Processing (QIP) and the challenges posed by short decoherence times. Rare earth based systems have the potential to resolve many of the issues faced in QIS. Due to their 4f orbitals, rare earth atoms, when implanted into an appropriate host, demonstrate relatively high coherence times. We cover benefits and challenges regarding QIP, specifically quantum processing and optical quantum memory. We emphasize important results in recent experiments regarding rare earth-based quantum computers, and compare these results to the current state of the art in qubit design. Specifically, we perform electronic structure calculations for a rare earth ion-doped wide band gap crystal and control quantum states offered by the doped ion. G.N. is grateful for an assistantship in the US DOE Office of Science, Science Undergraduate Laboratory Internship program.   

Experimental demonstration of the validity of the quantum heat-exchange fluctuation relation in an NMR setup.

We experimentally explore the validity of the Jarzynski and Wöjcik quantum heat-exchange fluctuation relation by implementing an interferometric technique in liquid-state nuclear magnetic resonance setup and study the heat-exchange statistics between two coupled spin-1/2 quantum systems. We experimentally emulate two models—(i) the XY-coupling model, containing an energy conserving interaction between the qubits, and (ii) the XX-coupling model—and analyze the regimes of validity and violation of the fluctuation symmetry when the composite system is prepared in an uncorrelated initial state with individual spins prepared in local Gibbs thermal states at different temperatures.We further extend our analysis for heat exchange by incorporating correlation in the initial state. We support our experimental findings by providing exact analytical results. Our experimental approach is general and can be systematically extended to study heat statistics for more complex out-of-equilibrium many-body quantum systems.   

Collective enhancements in charging quantum batteries via a quantum heat engine

As a model of so-called quantum battery, a quantum system as an energy storage, we study the charging of many two-level systems via a quantum heat engine. Especially, we focus on the collective effects in the charging. We show that the collective charging outperforms in the amount of the stored energy, its fluctuation, and the charging speed in comparison to the individual one. In the collective charging, symmetry of the interaction between the engine and the battery causes emergent bosonic quantum statistics, which result in a collective high probability excitation.   

Controlling quantum dot level spacing: moving beyond the constant interaction model

Proposed large scale implementation of gate defined semiconductor quantum dots requires effective means to tune each dot to desired tunnel coupling, capacitive coupling to gates, and coupling to sources of decoherence such as the phonon bath. These parameters are currently understood within the context of the constant interaction model that predicts a periodic level spacing based on a constant capacitance of each dot to the relevant gates. Yet, the actual capacitance of the dot is not a constant and depends on parameters including the electron number, voltage of the associated gates, and size of the dot, leading to a nonlinear level spacing. We show how taking these parameters into consideration enables greater control of each quantum dot. We also present a means to change the dot level spacing, thereby changing the coupling to the phonon bath, while maintaining desired tunnel coupling in a framework similar to virtual gating for cross capacitance. The results are relevant to minimizing decoherence in quantum dot qubits.   

Auto-tuned quantum dots in silicon as a candidate platform for scalable quantum computing and quantum neuromorphic devices

Spin qubits in gate defined quantum dots in silicon present a robust architecture for quantum information processing due to long coherence times, tune-ability, and scalability. However, the fabrication of such qubits leads to strong device variability, making it challenging to scale up the number of qubits for quantum computing applications. To address this challenge, we present our new device design and an automation protocol development to tune individual qubits. This automation protocol works to counteract device variability, resulting in greater scalability and versatility for the device. From this versatility we will explore the potential of using these auto-tuned quantum dots as quantum neuromorphic devices by engineering a non-Markovian bath for one or more qubits.   

A tunable coupler for suppressing ZZ crosstalk and realizing multiform two qubit-gates

Controllable interaction between superconducting qubits is desirable for scalability architectures and quantum simulation applications. We experimentally demonstrate a simple-design tunable coupler, achieving a continuous tunability and almost zero coupling strength. Based on the tunable interaction between two qubits via the coupler, we first construct two-qubit iSWAP and control-Z(CZ) gate by applying standard rectangular pulses. We then propose a new ‘Dynamic-Total-Closing’(DTC) technology to implement the CZ gate, which adopts a graded flux pulse for the qubit to approach the avoided crossing slowly, meanwhile dynamically closing the qubit-qubit coupling with a special pulse shape during the approaching process(fidelity up to 98.4%). Furthermore, we also try iSWAP gate by parametrically oscillating coupler at the qubit-qubit detuning and cross-resonance gate by tuning coupler frequency to explore the influence of ZZ interaction.   

THz Sommerfeld Wave Propagation on Superconducting Niobium Wire for Millimeter-Wave Interconnects

Although there has been a stupendous development of superconducting microwave circuits, scalability and transmission of quantum information remain important issues. One approach involves the transduction of quantum information from microwave to millimeter-wave frequencies prior to transmission. For efficient transmission of THz waves, the development of millimeter-wave interconnects is a vital challenge. In this pursuit, we propose to transmit Sommerfeld waves in the THz regime on the surface of a single superconducting Niobium wire connected between a pair of horn antennas and W-band (75 – 110GHz) waveguides. The Sommerfeld mode on single conductors in the THz frequency range have demonstrated very low losses with room temperature conductors. Compared to the room temperature, our calculations show an increase in conductivity of superconducting Niobium by two orders of magnitude, which indicates significantly lower losses are possible for links at a few Kelvin.   

Disorder-free localization in the Kitaev Honeycomb Model

We study operator propagation and scrambling in the two-dimensional Kitaev honeycomb model. An exact solution for this model allows us to calculate the infinite-temperature out-of-time-ordered correlator for Pauli observables, where we find that the infinite-temperature average over gauge sectors manifests as a disorder average for this quantity. This induces a localization effect for observables in the bond algebra of the Hamiltonian, while observables outside of the algebra completely scramble. We interpret our result in terms of the diffusion of Pauli strings, whose endpoints are localized yet whose interiors propagate freely. We further find that, in the ground state gauge sector of the model, operators do not localize. We further extend our analysis to the finite temperature regime using Monte Carlo Metropolis-Hastings methods.   

Combinatorial fabrication and spectroscopic analysis of Fe-Co-Cr ternary alloy thin film

The development of a high-throughput experiment is an important issue in materials informatics, especially for materials discovery. Recently, combinatorial deposition and high-throughput analysis are rapidly progressed [1]. However, the combination of material fabrication and property analysis still leaves much room for development. Particularly, a question is raised about whether comprehensive phase diagram mapping is the optimum solution for materials discovery.
Here, we focus on exploring the phase transition for the materials discovery and combined the combinatorial wedged thin film deposition, the synchrotron-based micro-spectroscopic characterization, and the statistical analysis of experimental bigdata. X-ray absorption spectrum and magnetic circular dichroism (MCD) information were recorded by photoelectron emission microscopy. We obtained chemical components and magnetic contrast in each pixel of the image and analysed the histogram of MCD contrast as a variation of the chemical composition of Fe-Co-Cr. We fitted MCD histogram by Landau theory to discuss the ferromagnetic phase transition.

[1]M.L.Green.et.all, Applications of high throughput methodologies to electronic, magnetic, optical, and energy related materials. J. Appl. Phys. 113, 231101 (2013).   

Normal mode analysis of a relaxation process with Bayesian inference

Relaxation processes provide many insights into atomic/molecular structures and the kinetics and mechanisms of chemical reactions.
For the analysis, the extraction of modes that are specific to the phenomenon of interest is unavoidable.
In this study, to construct a data-driven technique for relaxation processes, we propose a framework to systematically extract normal modes from the viewpoint of model selection with Bayesian inference.
Our approach consists of a well-known method called sparsity-promoting dynamic mode decomposition, which decomposes a mixture of damped oscillations, and the Bayesian model selection framework.
We numerically verify the performance of our proposed method by using coherent phonon signals of a bismuth crystal and virtual data as typical examples of relaxation processes.
Our method succeeds in extracting the normal modes even from experimental data with significant trends.
Moreover, the selected set of modes is robust to observation noise, and our method can estimate the level of observation noise.
From these observations, our method is applicable to normal mode analysis, especially for data with significant trends, which broadens the applicability of the data-driven approach in analyzing relaxation phenomena in material science.   

Predicting Ionic Liquids Properties with Machine-Learning

Since the last decade, there has been a dramatic increase in the number of ionic liquids (ILs) synthesized, tested and utilized in various applications (e.g. energy storage, CO₂ capture, catalysis, lubricant additives, etc.). The intense interest has been due to the various novel properties that can be found in ionic liquids, such as tunable ionic conductivity, negligible vapor pressure, liquid phase within a wide temperature range, non-flammability at ambient condition, etc. Among these properties, the melting point (T_m) of ionic liquids (IL) are very important in various applications. However, the T_m can change considerably depending on the molecular structures of the anions and cations. In this study, we will explore the use of various machine learning methods to predict the melting points of various ILs that consists of several different cation and anion classes. In addition to melting point prediction, some of the related structures-properties studies of ILs will be discussed.   

Interpretable modeling by linearly independent descriptor generation method

For interpretable modeling, linear regression combining descriptor generation and selection operator, such as approach of Ghiringhelli[1] and Ouyan[2], is very effective. However, multicollinearity (MC) (and near multicollinearity) between descriptors that are problematic in linear regression analysis can reduce the quality of the descriptor selection. Therefore, we proposed the linearly independent descriptor generation (LIDG) method[3] that generates descriptors while removing MC. This approach can improve their approach.
In this presentation we will present some application examples.


References:
[1] L. M. Ghiringhelli, J. Vybiral, S. V. Levchenko, C. Draxl, and M. Scheffler, Phys. Rev. Letter {\bf 114}, 105503 (2015).
[2] R. Ouyang, S. Curtarolo, E. Ahmetcik, M. Scheffler, and L. M. Ghiringhelli, arXiv:1710.03319v2 [cond-mat.mtrl-sci] (2017).
[3] https://github.com/Hitoshi-FUJII/LIDG.
  

Artificial Intelligence to detect gravitational waves from a network of detectors

The application of artificial intelligence in astronomical data analysis is becoming more popular among the scientific community. In our recent work [1,2], we have shown the effectiveness of Particle Swarm Optimization (PSO), a well-known algorithm in the field of swarm intelligence, in addressing signal detection and parameter estimation problem related to gravitational waves from compact binary coalescences (CBCs). The fully coherent network analysis of data from multiple gravitational wave (GW) detectors is computationally expensive since it is associated with a high dimensional numerical optimization problem. In our previous work, we showed using a non-spinning 2.0 post-Newtonian order waveform and four gravitational wave detectors (two LIGO detectors, Virgo and Kagra) that PSO can achieve the same performance as a grid search with less than 200,000 templates for a component mass range of 1.0 to 10.0 solar masses at a network signal to noise ratio of 9. Currently, we are increasing the dimensionality of the optimization problem by adding spin parameters to the waveform and exploring the effectiveness of the algorithm.

[1] TS Weerathunga, SD Mohanty, Phys. Rev. D 95 (12), 2017
[2] ME Normandin, SD Mohanty, TS Weerathunga, Phys. Rev. D 98 (04), 2018   

Machine learning to predict microbial community traits driving carbon fixation

Microbial communities are ubiquitous influencers of macroscopic environments, yet overwhelming complexity makes it difficult to decipher functional relationships between specific microbes and ecosystem properties. Integrating advances in DNA sequencing technology with computational approaches like machine learning (ML) could address this problem. In [Thompson et al, PLoS One, 2019], we applied neural networks, random forest models, and indicator species analyses to correlate microbiome data (16S rRNA gene profiles) with dissolved organic carbon (DOC) content after 44 days of plant litter decomposition. We analyzed 300+ soil microcosms, including 1709 total bacterial operational taxonomic units (OTUs), and performed multiple-model feature reduction. We are now leveraging Bayesian network structure learning to infer mechanistic interactions between microbial species abundance and DOC. After training a Bayesian network model using pine litter data, we predict DOC results of independent oak litter experiments and demonstrate that relationships between microbial species abundance and DOC are conserved across litter types.   

ParaMonte: A high-performance parallel library for MonteCarlo optimization, sampling, and integration

At the foundation of predictive science lies the scientific methodology, which involves multiple steps of observational data collection, developing testable hypotheses, and making predictions. Once a scientific theory is developed, it can be cast into a mathematical model whose parameters have to be fit via observational data. This leads to the formulation of a mathematical objective function for the problem at hand, which has to be then optimized to find the best-fit parameters of the model or sampled to quantify the uncertainties associated with the parameters, or integrated to assess the performance of the model. Toward this goal, a highly customizable, user-friendly high-performance parallel Monte Carlo optimizer, sampler, and integrator library is presented here which, can be used on a variety of platforms with single to many-core processors, with interfaces to popular programming languages including Python, and Fortran/C/C++. In particular, we discuss the parallel implementation of a variant of Markov Chain Monte Carlo known as Delayed Rejection Adaptive Metrolpolis (DRAM) and its scalability   

Building a DLS Device to Measure In Situ Glycine Cluster Formation

Laser trapping nucleation uses optical tweezers focused at the air-solution interface to grow crystals. Research has shown that focusing a continuous wave (CW) laser at the glass-solution interface of a solution of glycine in deuterium oxide, does not lead to nucleation, but to laser-induced phase separation (LIPS) droplet formation. The LIPS droplet is about double the concentration of the initial solution, past the metastable zone. This droplet is believed to be made up of large, liquid-like clusters of glycine that are organized in a size gradient with larger clusters in the center of the solution.
Minutes after the LIPS droplet relaxes over a few minutes, clusters in the LIPS droplet spontaneously organize into a critical nucleus. Microscopic demixing of the solution is believed to leads to the observed polymorph-selectivity for LIPS droplet-grown crystals. To better understand the LIPS droplet, a dynamic light scattering (DLS) device will be constructed to observe the LIPS droplet in situ. By measuring the hydrodynamic radius of the glycine particles in situ, one can better understand this crystal growth mechanism.   

First-principles calculations study of electronic structure, optoelectronic, vibrational analysis, linear and nonlinear optical properties of 4,5-dibromo-2,7 dinitrofluorescien

Theoretetical inverstigation on 4,5-dibromo-2,7 dinitrofluorescien at the RHF level and different DFT with the cc-pVDZ basis set with the help of Gaussian 09 suit of program software. We have firstly modeled and optimized the geometry of the structure and further, we have computed the frequencies' analysis to understand the thermodynamic, optoelectronic, linear and nonlinear optical properties. As far as this analysis is concerned, we have also been able to come out successful with a well calculated chemical reactivity and stability of the molecule through frontier molecular orbitals defined by the higher occupied and lower unoccupied molecular orbitals (EHOMO and ELUMO). Our results insinuate that this molecule has a potential application in linear and nonlinear optical materials, and optoelectronic devices due to his large hyperpolarizability and can be a promising compound for optical limiting applications. Presently, no experimental and theoretical values in literature were determined for the above properties are available, we are hopeful that our final results will provide important information for further studies of this compound in NLO materials and optoelectronic devices.   

Computational Synthesis of 2D Materials: A High-Throughput Approach to Materials Design

The emergence of two dimensional materials opened up many potential avenues for novel device applications such as nanoelectronics, topological insulators, field effect transistors, microwave and terahertz photonics and many more. To date there are over 1,000 theoretically predicted 2D materials. Only 55 2D materials have been experimentally synthesized. Computational methods such as density functional theory can be used to determine the suitable substrates to synthesize 2D materials. Using various 2D materials databases and van der Waals corrected density functional theory we investigate the suitability of 12 substrates to stabilize 2D growth. For materials which meet the criteria for suitable substrate-assisted synthesis methods such as chemical vapor deposition or mechanical exfoliation, the density of states is computed to characterize the electronic properties of these materials for device applications.   

Predicting the properties of ultrathin magnetic HxCrS2 from first principles and machine learning

An air stable H_xCrS₂ layered material has been synthesized by soft chemical methods, which can be exfoliated down to ultrathin layers, providing a promising path for the synthesis of two-dimensional (2D) magnets¹. Although a reliable synthesis method has been developed, the atomic structure is still unknown. Variables such as Cr vacancies and H impurities must be determined in order to understand the properties of this material for device applications. We used a combination of density functional theory (DFT), molecular dynamics (MD) and cluster expansion formalism to study the energetics as a function of Cr vacancies and H impurities. From here, we studied the stability, electronic and magnetic properties of these H_xCrS₂ structures and examined the effect of layering on these properties. In order to extend our calculations to a wider range of structures, we trained a machine learning algorithm with our DFT and MD calculated data to predict the properties of other H_xCrS₂ based materials outside our training set.

¹X. Song et. al, J. Am. Chem. Soc. 141, 15634 (2019)
  

Novel Solid-State Electrolytes for Li Ion Batteries by Computational Design and High-throughput Ab Initio Calculation

Solid-state electrolytes (SSEs) can alleviate many of the issues of Li-ion batteries arising from the utilization of the organic liquid electrolytes. Up to date, several SSEs have been proposed from previous experiments or computational screening from publicly available materials databases, however, none of those SSEs are fully satifactory. In this work, we take one step further; instead of simply exploring a pre-existing materials database, we try to design new materials with aliovalent substitution of cations. This aliovalent substituion is known to facilitate kinetics of Li diffusion. We first screen potential host materials for generating new SSEs by aliovalent substituion, considering fundamental properties such as the presence of transition metals, thermodynamic stability, and band gap. Afterward, we crudely examine the potential energy surface (PES) around the Li ion at interstitial sites. The materials with the most smooth PES are then chosen for the possible candidate for the host material. Finally, we choose some materials from the candidate list and demonstrate that indeed they become a Li ion conductor after aliovalent doping. Starting from 42,337 structures, we find a number of SSEs with predicted Li ionic conductivity comparable to the state-of-the-art Li₁₀GeP₂S₁₂.   

COMPUTATIONAL STUDY OF THE INTERACTION OF A WATER MOLECULE WITH 2D-HBN(TWO-DIMENSIONAL HEXAGONAL BORON NITRIDE) AND WITH TI-2D-HBN.

After graphene synthesis in 2004 many two dimensional systems has been studied, since the possible applications of these are many. In this work I made a computational study of the interaction of a water molecule with two dimensional hexagonal boron nitride(2D-HBN). First the stability of the 2D-HBN is obtained using Density Functional Theory, Atomic Pseudopotentials,Born-Oppenheimer approximation and molecular dynamics. Next I cause this system to interact with a water molecule. In a second moment, I made a computational study the stability of the system 2D-HBN, but adding a Titanium atom, the system TI-2D-HBN. Again I provoke the interaction between a water molecule and the TI-2D-HBN system. I show the results of my calculations, and these are compared with other computational and experimental results.   

Real-space Moiré potential, lattice corrugation, and band gap variation in a MoTe2/MoS2 heterobilayer

To have a first-principles description of the Moiré pattern in a transition metal dichalcogenide heterobilayer, we have carried out DFT calculations, taking full accounts of both atomic registry in and the lattice corrugation out of the atomic layers, on a MoTe₂/MoS₂ system which has a moderate size of superlattice larger than an exciton yet not large enough to justify a continuum model treatment. We find that the local potential in the midplane of the bilayer can serve as an excellent illustration of the Moiré pattern in the van der Waals heterostructure. In the Mo atomic planes, the array of local potential planar maximum (LPPM), rather than the local potential itself, makes the Moiré pattern more obvious. Significant lattice corrugation is found in both MoTe2 (0.30Å) and MoS2 (0.77Å) layers. The interlayer Moiré potential, defined as the LPPM difference between the Mo atomic planes, has a depth of 0.20 eV and changes in direct correlation to the band gap variation in the Moiré cell, which has an amplitude of 0.04 eV. Wrinkling of the MoTe₂/MoS₂ bilayer enhances the spatial variation of the local band gap by 5 meV; by contrast, its influence on the global band gap is within 1 meV.   

Simultaneous Prediction of the Magnetic and Crystal Structure of Materials

The coupling between magnetic properties and crystal structure in many magnetic materials means that it is essential to take a holistic approach to predicting their structure. We present for the first time a genetic algorithm that performs a simultaneous global optimisation of both magnetic structure and crystal structure, and illustrate its utility in a range of systems including a complex interface structure between a half-metallic Heusler alloy and a semiconductor substrate.   

Minimization studies of elemental carbon using LCBOP semi-empirical potential.

Elemental carbon is interesting to study thanks to its material properties and structural diversity, which ranges from nanotubes through graphite to diamond. In our study of slow-cooled carbon droplets condensed in cool-star atmospheres, computational study with semi-empirical potentials is complementary to experiment and ab initio work. We’ve used the long-range carbon bond-order potential to relax, via conjugate gradient, 1.8 g/cc liquid-like carbon tiled-cube and isolated-cluster systems with 13, 20 and 100 atoms, as well as tetrahedral nanodiamond clusters of 17, 22 and 29 atoms. Tiled-cube simulation nearest neighbor histograms show a bond defining gap between 1.7-2.0 Å. Coordination statistics then show a high percentage of sp and sp2 coordination. Ring sizes of 5 - 7 atoms form more prominently than others, with 5 and 6 atom rings especially abundant. Isolated cluster relaxations show a high amount of sp chains forming, and less ring formation than the tiled-cube simulations. We also see a volume increase for the isolated clusters, unlike comparable density functional theory (DFT) simulations. Our isolated diamond-cluster relaxations saw less surface reconstruction than with DFT.   

Line integral approach for fiber-fiber interactions implemented in LAMMPS

Fiber networks are currently modelled using discrete bead-spring type molecular models or cylindrical discrete-element (DEM) models. Bead-spring models often suffer from issues arising from artificial friction originating at the bead-bead contacts, and DEM-type models suffer from high computational demands. Thus, there is a need for better models. Here, the cohesive interaction energy between straight fibers is expressed as a double integral. This double integral can be converted into a single integral that has two terms in its integrand: g(r), which depends on the relative geometrical positioning of the fibers, and V(r), which reflects the interaction energy. Although V(r) is readily available in analytical form, (can be of the lennard-jones type), g(r), which is essentially a distance distribution function of point-pairs located across 2 fibers, requires computation. The method to compute g(r) involves elliptic integrals and is computationally demonstrated in this work. An implementation of the resulting fiber-fiber interaction is demonstrated in LAMMPS, a popular molecular dynamics package, and its computational and scientific performance is evaluated.   

First Principles Evaluations of the Spin-Orbit Couplings in Crystals

The spin-orbit coupling (SOC) plays a crucial role in many spin-related phenomena such as anomalous Hall effect and spin Hall effect. However, the strength of the SOC in solids and its relation to the isolated limit have not been well understood. Here, we quantify the atomic SOC in solids by constructing the tight-binding Hamiltonian from the Wannier functions obtained by the first-principles calculation. By calculating the atomic SOC in monatomic crystals and binary compounds systematically, we find that the strength of the atomic SOC is not the atom-specific property but depends much on the crystal structure and chemical composition. We show that its crystal dependence is well explained by the spread of the Wannier function.   

Time-evolution of a Schrödinger electron driven by a localized oscillating potential

We have obtained a semi-analytical solution of the Schrödinger equation for a massive particle in the presence of a harmonic time-dependent ∼ cos (ω t) single-point ∼ δ(x) potential. We present a detailed analysis for determining the dependence of the wave function on the amplitude and frequency of the oscillatory potential, and the chosen initial conditions for the particle wave function. We have considered separately when either the potential is negative and leads to a bound state or ispositive thus yielding scattering. Our derived results could be interpreted as being related to the dynamics of an electronic Floquet state subjected to a localized laser-induced dressing field.
  

Theory of phonon glass behaviour in host-guest thermoelectrics with avoided crossing

An analytical model is developed to describe the phonon dispersion relations of thermoelectrics with the presence of heavy guest atoms (rattlers). The model also accounts for anharmonic effects in phonon damping. The spectrum of low energy states contains acoustic-like and (soft) optical-like modes, which display the typical avoided crossing, and which can be derived analytically by considering the dynamical coupling between host lattice and guest rattlers. Inclusion of anharmonic damping in the model allows us, for the first time, to compute the vibrational density of states (VDOS) and the specific heat, unveiling the anomalous boson peak (BP) relating to the glassy behaviour of phonons in the otherwise crystalline material. We discuss the dynamics of the BP as a function of the strength of the interaction between the soft modes and the anharmonic lattice, and of the energy gap between the two avoided-crossing branches. Moreover, we find a robust linear correlation between the BP frequency and the energy of the soft optical-like modes.   

Giant effect of spin-lattice coupling on the thermal transport in two-dimensional ferromagnetic CrI3

Recently, two-dimensional monolayer chromium triiodide (CrI3) with intrinsic magnetism was discovered, which shows promising applications in many technologies from sensing to data storage where thermal transport play a key role. So far the effect of spin-lattice coupling on the thermal transport properties has not been explored yet. In this talk, I would like to present the giant effect of spin-lattice coupling on the thermal conductivity of monolayer CrI3. The thermal conductivity is more than two orders of magnitude enhanced by the spin-lattice coupling. The effect is found to be especially stronger for the acoustic phonon modes, which dominates thermal transport with spin-lattice coupling. Deep analysis shows that the reason lies in the weakened phonon anharmonicity by spin-lattice coupling. The bond angle and atomic position are changed due to the spin-lattice coupling, making the structure more stiff and more symmetric, which lead to the weaker phonon anharmonicity, and thus the enhanced thermal conductivity. This study uncovers the giant effect of spin-lattice coupling on the thermal transport, which would deepen our understanding on thermal transport and shed light on future research of thermal transport in magnetic materials.   

Resonantly enhanced polariton wave mixing and Floquet parametric instability

Recent experiments by Cartella et al. [1] demonstrated terahertz optical amplification in SiC insulator following a strong mid-IR pump that resonantly excited the SiC stretching mode. Motivated by these experiments we study the problem of light reflection from a slab of insulating material with a strongly excited polariton mode. We introduce a new theoretical approach for analyzing this system using the Floquet formalism, in which polariton oscillations provide periodic time modulation. We present an analytic solution of the Fresnel light reflection problem. We demonstrate discontinuous dependence of the reflection coefficient on the incoming photon frequency, which we attribute to the existence of unstable polariton modes. We interpret these instabilities as resonantly enhanced polariton parametric wave mixing. Our results provide a simple physical interpretation of light amplification observed in recent experiments by Cartella et. al. [1]. Moreover, we argue that Floquet parametric instability could already be present in these experiments. Our approach utilizing the Floquet formalism is applicable to a broad class of systems.
[1] A. Cartella et. al, PNAS 115, 12148 (2018),   

Response at finite temperature

We present recent developments using the temperature dependent effective potential technique (TDEP) to model strongly non-harmonic materials. The method employs model Hamiltonians that explicitly depend on temperature. I will present applications pertaining to thermal conductivity, inelastic neutron spectra and phase stabilities. In addition, we investigate the cross-terms between anharmonicity and non-adiabatic electron-phonon coupling and its influence on transport properties.   

Deep Spectral Coarse Graining: Learning Simple, Dynamically Consistent Protein Models

Coarse grain models of proteins offer promising gains in both computational efficiency for molecular simulations and the development of simple physical interpretations. Recent efforts have focused on formulating the development of coarse grained force fields as a supervised learning problem, taking advantage of deep learning techniques for handling highly non-linear multibody effects produced by imposing coarse grained representations. In this work, we present a deep learning method that utilizes spectral information from simulation data to preserve essential dynamics of the original system. Following a Koopman-motivated approach, we optimize the dynamical consistency between fine grain and coarse grain systems by forming a cost from the dynamical generator eigenequation. Through this method, we can recover coarse grain empirical free energy landscapes that preserve essential dynamical information from the fine grain system.   

Crystalin Protine Structure Prediction Algorithm

The acurate prediction of protine structures is a continusly ongoing process, with the traditinal methood beign a two step process of creating restraints and constructing a basic structre from that. Recent advancements in Machien Learnign and Nural Networkds have prvided new avenus to increase acuracy for these predicted protien structures. By utilizing a Nural Network to estimat moleculare distances, and runing basic torsion angle predictions, enables us to more acuaratle fold protines and create acurate protien structure predictions for use. Hopefuly this nural network will allow further reaserch an dutilization of protines that we are curently are unable crystalize and study by traditinal means.   

Constructing the Neural Network Potential With the Energies of the Atom and Its Derivatives

Neural Network Potentials (NNPs) are highly anticipated as a possible breakthrough to overcome the trade-off between accuracy and speed in atomistic simulations. NNP learns the potential energy surface from the reference first principle calculations. The most widely used first principle calculation is based on the density functional theory (DFT) with the plane-wave basis, which does not provide atomic energy. Thus it is not possible to directly train the NNP to learn target atomic energy from plane-wave DFT. To overcome this limitation Behler and Parrinello designed high-dimensional NNP such that it represents the atomic energy as the sum of total energy.
However, the mapping of atomic energy from the total energy is not unique, and if not carefully considered, there could be a loss of information leading to errors. We call this ad-hoc mapping problem. In contrast, because a pseudo-atomic localized basis can define atomic energy by its decomposition formalism, it enables NNP to avoid the above problem. In this presentation, we demonstrate the advantages of training NNPs directly from atomic energy and its derivatives, free from the high dimensional structure.

[1] Phys. Rev. Lett. 98, 14641 (2007)
[2] Phys. Rev. Mater. 3, 093802 (2019)   

Grain boundary structures of elemental metals using machine learning potential

Global optimization algorithms, such as multi-start local optimizations and Bayesian optimization, have been useful to determine the microscopic structure and its grain boundary energy for a given macroscopic grain boundary model. Together with the global optimization algorithms, the density functional theory calculation and interatomic potentials have been employed to estimate the grain boundary energy. However, the former is computationally demanding, and the latter often lacks the predictive power for a variety of grain boundary structures. Recently, several machine-learning potentials have been proposed, which are expected to enable computing the grain boundary energy accurately with less computational costs. In this study, we investigate symmetric tilt grain boundary structures and their grain boundary energy surfaces in elemental metals using a combination of global optimization algorithms and linearized machine learning potentials [1]. We compare the grain boundary structures and grain boundary energy surfaces obtained from machine learning potentials and embedded atom method potentials to examine the accuracy and stability of our procedure.

[1] A. Seko, A. Togo, and I. Tanaka, Phys. Rev. B 99, 214108 (2019).   

Machine Learning-Aided Development of Empirical Force-Fields for Glasses

The development of reliable, yet computationally efficient interatomic forcefields is key to facilitate the modeling of glasses. However, the parametrization of novel forcefields is challenging as the high number of parameters renders traditional optimization methods inefficient or subject to bias. Here, we present a new parameterization method based on machine learning, which combines ab initio molecular dynamics simulations, Gaussian Process Regression, and Bayesian optimization. By taking the examples of silicate and chalcogenide glasses, we show that our method yields new interatomic forcefields that offer an unprecedented agreement with ab initio simulations. This method offers a new route to efficiently parametrize new interatomic forcefields for disordered solids in a non-biased fashion.   

A theoretical study on crystallization of chalcogenides via neural network potential

Phase-change materials (PCM) have attracted wide interests in fields such as data storage and neuromorphic computing. Ge-Sb-Te alloys are a representative PCM, which show large contrast of optical and electrical properties between crystalline and amorphous phases. Atomic scale modeling of PCM has relied on ab-initio molecular dynamics (AIMD), but its large computational costs have limited simulation size and time. Neural network potential (NNP) can deal with more than thousands of atoms upto microseconds, while accurate potential energy surface can be obtained by learning the data of density functional theory (DFT).
In this work, we suggest the accurate and effective training scheme to develop reliable NNP for chalcogenides (e.g. GeTe and Ge₂Sb₂Te₅). It seems that NNP trained in the conventional way moderately predicts the basic properties but rapidly crystallizes an amorphous phase without any incubation time. We then propose a simple descriptor to diagnose it and an active learning scheme to manage it. Dependence of crystallization kinetics upon temperature and density is investigated using about 4000-atom cells. It shows that incubation and crystallization time is dependent on temperature and pressure, qualitatively consistent with experiments.   

Direct prediction of quantum accurate forces for multicomponent systems

Phase-change materials (PCM) are routinely exploited in optical data storage, flexible devices and neuromorphic computing due to the extreme electro-optical contrast between crystalline and amorphous states. However, high-quality force-field models for molecular-dynamics (MD) simulations of PCM are not available, while quantum molecular dynamics simulation is limited by the small size of system. We have developed neural-network (NN) force fields for GeTe and Ge₂Sb₂Te₅, which are trained using atomic forces computed by density functional theory. Radial and angular feature vectors are designed and trained, which feature permutational and translational invariance and rotational covariance of forces. The accuracy of the NN force fields is validated by performing MD simulation involving up to 100,000 atoms and computing multiple structural and dynamical properties.   

IFF-R Models to Accurately Simulate Stress-Strain and Failure Porperties of Carbon Allotropes and Polymer Composites

Stress induced mechanical failure of polymeric materials is a result of the breakage of covalent bonds. The occurrence of bond breakage and formation is only observed during reactions. This study focuses on the novel bond-breaking capabilities of the Interface Forcefield (IFF-R). Traditional IFF models simulated systems using harmonic potentials. The IFF-R model incorporates Morse potentials; thereby, eliminating the restoring force experienced by bonded atoms stretched at large distances. This enables accurate predictions of mechanical responses in a variety of periodic systems. This study shows moduli and strength predictions of a single-walled carbon nanotube, poly(acrylonitrile) crystal, cellulose β crystal, and steel FCC lattice to be comparable to experimental values. Mechanical property predictions using IFF-R models are realized magnitudes faster than reactive forcefield (ReaxFF).
  

Enhanced Optical Properties of Single-Walled Carbon Nanotubes (via SP3-Hybridization Defects) from Many-Body Perturbation Theory Based on Density Functional Theory Calculations

The optical consequences of functionalized carbon nanotubes (CNTs) (via a pair of SP³-hybridized functional groups attached to a carbon ring) have been explored in great depth due to their promise of superior electronic properties for tunable emission in infrared energies. These studies have relied on time-dependent density functional theory (TD-DFT) calculations to model the excited states of these particles, but very little work has been completed on multiple exciton generation (MEG) processes within these systems. Here we employ a novel method based in non-equilibrium, finite-temperature, many-body perturbation theory (MBPT) calculations that utilize output from density functional theory (DFT) to accurately model excited states of these systems. We solve the Boltzmann transport equation (BE), including phonon absorption/emission and biexciton formation/recombination terms [1,2]. With this approach we compute an array of CNTs of varying chirality and functionalization scheme. We see that SP³-defect functionalization of pristine CNTs that had high-energy biexciton MEG thresholds (E ≈ 2.4E_g) can be reduced to 2E_g, which drastically increases their value as efficient multiple exciton sources.

[1] J. Chem. Phys. 147, 154106 (2017)
[2] J. Phys. Chem. Lett., 2018, 9, 19, 5759-5764   

Quasi-Diabatic Scheme for Non-adiabatic On-the-fly Simulations

We use the quasi-diabatic (QD) propagation scheme to perform on-the-fly non-adiabatic simulations of the photodynamics of ethylene. The QD scheme enables a seamless interface between accurate diabatic-based quantum dynamics approaches and adiabatic electronic structure calculations, explicitly avoiding any efforts to construct global diabatic states or reformulate the diabatic dynamics approach to the adiabatic representation. Using diabatic dynamics methods, the QD propagation scheme enables direct non-adiabatic simulation with complete active space self-consistent field on-the-fly electronic structure calculations. The population dynamics obtained from both approaches are in a close agreement with the quantum wavepacket based method and outperform the widely used trajectory surface hopping approach. Further analysis of the ethylene photo-deactivation pathways demonstrates the correct predictions of competing processes of non-radiative relaxation mechanism through various conical intersections. This work provides the foundation of using accurate diabatic dynamics approaches and on-the-fly adiabatic electronic structure information to perform ab-initio non-adiabatic simulation.   

Dielectric Screening by 2D Substrates

While electronic screening within 2D materials has been studied extensively, the question of how 2D substrates screen charge perturbations or electronic excitations adjacent to them still remains.

In this work, we use first principles calculations to study the fully non-local dielectric screening properties of 2D material substrates. Our calculations give results in good qualitative agreement with those from electrostatic force microscopy (EFM) and Kelvin probe force miscroscopy (KPFM) experiments, indicating that the experimentally observed thickness-dependent screening effects and reduced charge potential fluctuations are intrinsic to 2D materials.

2D material substrates can also dramatically change the HOMO-LUMO gaps of small molecule adsorbates. We suggest an easy and reliable method to predict the HOMO- LUMO gaps of small physisorbed molecules on 2D and 3D substrates, using only the band gap of the substrate and the gas phase gap of the molecule.   

Universal metric for plasmonicity of excitations at the nanoscale

A promising trend in plasmonics involves shrinking the size of plasmon-supporting structures down to a few nanometers, thus enabling control over light−matter interaction at extreme-subwavelength scales. In this limit, quantum mechanical effects, such as nonlocal screening and size quantization, strongly affect the plasmonic response, rendering it substantially different from classical predictions. For very small clusters and molecules, collective plasmonic modes are hard to distinguish from other excitations, such as single-electrons ones. Using rigorous quantum mechanical computational techniques for a wide variety of physical systems, we describe how the plasmonic character of a nanostructure’s optical resonance can be quantified. We define a universal metric, the generalized plasmonicity index (GPI), which can be straightforwardly implemented in any computational electronic-structure or classical electromagnetic approach to discriminate plasmons from single-particle excitations and photonic modes [ACS Nano, 11, 7321 (2017); PNAS, 115, 9134 (2018)]. The GPI metric deepens our fundamental understanding of what is a plasmon down to the molecular limit of plasmon-supporting nanostructures and provides a rigorous foundation for further development in molecular plasmonics.
  

Particle-in-cell simulation of plasmons

Plasmons are collective oscillations of the free electron gas density in conducting media such as metals. In many cases of interest, plasmons are typically characterized by solving the Maxwell equations, where the electromagnetic response can be described by the bulk permittivity. Motivated by the physical similarity of plasmas and oscillating conduction electrons, we present a particle-in-cell-based method of simulating plasmons. By solving for the instantaneous particle position and momentum, which are connected to the electromagnetic fields through current, we demonstrate the capability of this novel approach in elucidating information on the formation of the plasmons. Specifically, we demonstrate plasmon formation in gold nanorods through laser irradiation and single-electron excitation. Lastly we investigate the non-local effects of ultra-small nanoparticles approaching quantum limit.   

Heat transport between two critical one-dimensional systems

Heat transport can reveal information about interacting many-body systems beyond other transport probes. In particular, in one dimension it has been shown that the energy current is directly proportional to the central charge, thus revealing information about the degrees of freedom of critical systems. In this work, we explicitly verify this result in two cases for translationally invariant systems based on explicit microscopic calculations. More importantly, we generalise the result to non-translation invariant setups and use this to study a composite system of two subsystems possessing different central charges. We find a bottleneck effect meaning the smaller central charge limits the energy transport.   

Thermal conductivity prediction from basic properties using a developed artificial neural network

High-throughput screening and material informatics have shown a great power in material discovery including Li-battery materials, alloys, photocatalysts, and nanowires. In this talk, we will present the accurate thermal conductivity prediction from machine learning technique using a developed artificial neural network (ANN). With 231 datasets of the basic properties describing materials calculated from first-principles and the corresponding thermal conductivity from experimental measurements as training data, the constructed ANN is well trained by iterating to reduce the loss function. The trained ANN model for thermal transport successfully captures the general correlation between basic properties and thermal conductivity for different types of materials, which is predictive spanning 4 orders of magnitude of the thermal conductivity. The developed ANN model in our work for fast and accurately predicting thermal conductivity provides a powerful tool for the large-scale thermal material screening with targeted thermal transport property.   

Exploring structure and magnetism of collapsed lanthanides

By using DFT within VASP [1] and DFT+DMFT within CASTEP codes [2], we study the collapsed phases of Tb, Gd, Dy, Sm, Nd and Y at extreme pressure up to 240GPa.
In Tb, Gd, Dy we show that the collapsed phases have a 16-atom orthorhombic structure (oF16) not previously seen in the elements, whereas in Nd we show that it has an eight-atom orthorhombic structure (oF8) previously reported in several actinide elements. oF16 and oF8 are members of a new family of layered elemental structures, the discovery of which reveals that the high-pressure structural systematics of the lanthanides, actinides, and group-III elements (Sc and Y) are much more related than previously imagined. Our electronic structure calculations of Tb, combined with quantum many-body corrections, confirm previous and recent experimental observations by using synchrotron x-ray diffraction (see the Ref.[3] and the references therein), and predict that the collapsed orthorhombic phase is a ferromagnet, nearly degenerate with an antiferromagnetic state between 60 and 80 GPa. We also discuss in detail the magnetic properties of the lanthanides studied.

[1] G. Kresse and et al., Phys. Rev. B 59, 1758 (1999).
[2] M. I. McMahon, et al. Phys. Rev. B 100, 024107 (2019).
[3] E. Plekhanov et al. Phys. Rev. B 98, 075129 (2018).   

DFT studies of graphene in carbon droplets condensed in stellar atmospheres

Elemental carbon at low (ambient) pressure sublimates to vapor near 4000K, but liquid carbon is reported after laser ablation. Some meteoritic carbon particles, formed in red giant atmospheres, show a “graphene-core”/graphite-rim structure likely from super-cooled carbon droplets that nucleated graphene sheets on randomly-oriented 5-member rings. Similar core-rim particles form by slow cooling of carbon vapor in the lab (HAL-02238804). Our computations target growth of carbon rings & graphene sheets at the experimental 1.8 g/cc density estimate, by relaxing random liquid-like configurations of 13/20-atom clusters in a supercell. Inter-atom distances characteristic of covalent vs. metallic interactions (with a gap in 1.7-2 Å range) allow us to identify covalent “bonds” with small separation. Local energy minima at T = 0K show sp² & sp coordination numbers, as in the literature. Ring sizes vary from triangle to heptagon, but pentagons are more abundant than hexagons, also consistent with previous reports. Work remains to see if pent-loops can nucleate the growth of faceted pentacones, as suggested by HRTEM imaging. Unlayered graphene sheets in a frozen matrix may be an effective diffusion barrier.   

Dark Matter and Baryons (Surplus Quarks) Generated by Nonequilibrium Confinement of Quarks

The emergence of baryons (surplus quarks ) at Big Bang, required a nonequilibrium binding and superconductor-like condensation of quark-antiquark pairs before the electroweak (EW) symmetry breakdown. (Similar for leptons). As will be further shown, the formerly unknown dimensionless coupling to the Ginsburg-Landau like potential and the scale parameter in the EW theory then become microscopic functions of the massive quark and antiquark fields, thus defining the matter-antimatter asymmetry and the dark matter content in the Universe at correct orders of magnitude. The number of free parameters in the Standard Model has thereby been reduceed.
Key words: Baryogenesis; Quark Confinement; Matter-Antimatter Asymmetry, Dark Matter; Black Holes.

Earlier publications on this matter:
1. Leif Matsson, Higgs-Like Mechanism by Confinement of Quarks in a Chemical Non-Equilibrium Model.
World Journal of Mechanics, 2016,6, 441-445.
2. Leif Matsson, On Dark Matter Identification. World Journal of Mechanics, 2017,7, 133-141.   

Effect of Surface Defects on Field-Induced Hot-Carrier Chemistry in Dielectric Polymers

Performance of dielectric polymers under high electric field is limited by the electrical breakdown, which is commonly understood as an avalanche of processes such as carrier multiplication and defect generation. We model the hot-carriers transport in dielectric polymer, polyethylene, with excited-state quantum molecular dynamics simulations in presence of electric field, which reveal multiple microscopic processes induced by hot electrons and holes under an electric field. The key chemical damage occurs due to localization of holes at the surface of slab which weaken carbon-carbon bonds on the surface. Introducing surface defects alter the valenca-band maximum (VBM) state in polyethylene leading to bond breaking at lower field. Further, we have isolated C-C bond lengths and VBM localization as a proxy for dielectric breakdown. Such proxies allow us to perform one simulation to understand the effect of defect rather than scanning the complete electric field range. Such quantitative and qualitative information can be incorporated into first principles-informed, predictive modeling of dielectric breakdown.   

Simulation of the effect of electric field on the performance of ion separation and water desalination using a graphene-carbon nanotube membrane

Using molecular dynamics (MD) simulations, a graphene-carbon nanotube membrane is exposed to external electric field with intensities E = 0.1, 1, 10, and 100 mV/Å in order to separate Na⁺ and Cl^- ions from a salt water solution to produce fresh water. The results show that by increasing the strength of the applied electric field, the ion separation will be increased to 100% for E = 100 mV/Å. It is found that the ion separation efficiency of this filter is already greater than 90% for E = 10 mV/Å. Based on the results of this study, it is suggested that the graphene-carbon nanotube membrane can be used as a device of water desalination under the application of electric field.   

Effects of surface transition and adsorption on ionic liquid capacitors

Room temperature Ionic liquids (RTILs) are a type of synthesized electrolytes possessing superb electrochemical stability and low vapor pressure compared with conventional aqueous-based electrolytes, which offers significantly enlarged electrochemical window and ease of maintenance for capacitors. Experiments measuring capacitance in concentrated RTILs often found hysteresis indicating that an underlying phase transition might exist. Current theories explaining the hysteresis in terms of phase transition either assume RTIL mixtures with neutral solvents or omit ion-ion correlations in RTILs. In this study, a variant of an existing RTIL model [1] is established for solvent-free RTIL capacitors incorporating both ion-ion correlations and nonelectrostatic interactions. We first use the model to explore the spontaneous charge separation in the capacitors and find that this transition is a common feature for realistic choices of the model parameters for most RTILs. Next, we investigate the effects of preferential ion adsorption on this charge separation transition. The results show that preferential ion adsorption can be a useful design parameter for optimizing the energy storage of the capacitors.

Ref. [1] M. Z. Bazant, B. D. Storey, and A. A. Kornyshev, Phys. Rev. Lett. 106 (2011).   

Disintegration of Surfactant Micelles at Metal-Water Interfaces Promotes their Strong Adsorption

We have studied adsorption behavior of surfactant micelles at metal-water interfaces via fully atomistic simulations. We show that micelles experience a free energy barrier to adsorption. Near the metal surface, surfactant molecules in the micelles slowly rearrange leading to complete disintegration of the micelles. Disintegration of the micelles results in much stronger adsorption. After the disintegration, surfactant molecules adsorb by lying flat on the metal surface. By simulating adsorption of micelles treated as rigid bodies, we show that when the micelles remain intact, they have a weak tendency to adsorb.   

Targeted Synthesis Conditions of Layered Bismuth Oxychalcogenides Via Electrochemical Phase Diagrams

We investigated how density functional theory calculations of electrochemical stability can guide the experimental hydrothermal synthesis of bismuth oxychalcogenides. The target phases, which are layered thermoelectrics and exhibit electronic anisotropy, represent exciting materials for catalysis, photoluminescence, and energy applications. We coupled experimental synthesis with DFT-calculated multielement Pourbaix diagrams to identify target pH and potential synthesis regions that ultimately proved successful. We further investigated numerous degrees of freedom present in the reaction, such as mass ratio of reactants, temperature, and concentration of ions, to qualitatively understand how changes in the chemistry of the oxychalcogenides alter phase stability. Last we discuss how these materials can be driven towards target phase production.   

Resolution of the identity approximation applied to PNOF family of correlation calculations

PNOFi (i = 1-7) family of functionals deal with electronic structure correlation calculations by means of the reduced electron density matrix approach (1-RDM). The major advantage of a 1-RDM formulation is that the unknown functional only needs to incorporate electron correlation. PNOFi family of functionals provide an efficient way of including dynamic and static correlation as compared to wave function methods. However, a transformation from atomic orbital integrals to molecular orbital integrals has to be performed in order to computate the Coulomb and Exchange matrices in molecular orbital representation J^OM and K^OM respectively, which involves an overall fifth power time scaling factor.
Resolution of the identity (RI) approximation reduces the scaling factor of both, J^OM and K^OM matrices, and lowers the global scaling to fourth power. As a consequence, less memory is required to perform PNOFi calculations with an overall speed up in the calculation. RI is applied directly in the evaluation of J^OM and K^OM matrices, thus, improvement would be valid for all PNOFi family, however, the PNOF5 functional has been used in this work. Impact of RI on the convergence, and the obtained error in the correlation energy is analyzed for selected large size systems.   

Coherent states, Gram matrix, and Hofstadter butterfly with flat band

A method to construct flat band model is exhibited. If a Gram matrix built upon a set of vectors is regarded as a Hamiltonian of certain physical system, then we can control the ground state degeneracy by modifying the vectors. If the vectors are chosen to be certain subsets of coherent states, the resulting Hamiltonians describe single-particle lattice models with a gauge field. The massive degeneracy of the lowest band in these models is a universal property independent from the shape of lattice, which is controled by the completeness of the subsets of coherent states. The excitation spectrums show Hofstadter-butterfly-like patterns which vary when the lattice changes. The models also feature ground state wave functions in universal form, which is like Landau lowest levels, but the dynamics differs. Experimental realization of the models is promising.   

Computational simulation of patterns in a reaction-diffusion model

Recent studies in reaction-diffusion models has been oriented to produce patterns for studying polycrystalline materials as graphene oxide and among others, here in show optimized algorithm for obtaining patterns of a reaction-diffusion equation. The optimization of algorithm were carried out by using fortran code and the respective visualization was employing the Visit software. The results suggest that is possible to obtain theoretical patterns of reaction-diffusion equation as expected, which could be comparable with the experimental patterns of high resolution transmission electron microscopy (HR-TEM) in graphite oxide samples.   

Finite-Temperature Correlation Functions Using the Quantum Minimally Entangled Typical Thermal States Algorithm

Finite-temperature correlation functions provide fundamental information about the excitations and response properties of quantum many-body systems. Recently, the quantum minimally entangled typical thermal states (QMETTS) algorithm was introduced for calculating thermal averages of certain observables on near-term quantum devices. However, due to the computational cost of the quantum imaginary time evolution (QITE) subroutine underlying the QMETTS algorithm, the calculation of general thermal quantities with QMETTS remains challenging. Here, we report the calculation of finite-temperature correlation functions of quantum spin models with QMETTS. We describe how to reduce the cost of calculations by exploiting Hamiltonian symmetries and other constraints to eliminate qubits and reduce measurements. Our work advances efforts to study finite-temperature properties of quantum many-body systems on quantum computers.   

Electrosatic screening, dynamics and structure of [BMIM+][BF4-] and [BMIM+][PF6-] with confinement

Ionic liquids are spotlighted as electrolytes for batteries and supercapacitors. Such applications have nanoconfined systems, where deviation of electrostatic screening, dynamics, and structure of ionic liquids from bulk properties is caused. Although electrostatic screening is a fundamental interaction for any charged particles, electrostatic screening condition for ionic liquids is not discovered yet in confined systems. To fully utilize ionic liquids in electronic devices, it should be fully revealed how confinement alters physical properties of ionic liquids. Here, we conducted molecular dynamics simulations of [BMIM+][BF4-] and [BMIM+][PF6-] between two graphene sheets with gap of 1.5 to 4.5 nm. Confined ionic liquids show slower dynamics by 2 orders of magnitude compared to bulk dynamics and have nonmonotonic behavior in dynamics. In addition to dynamics, we observed that capacitance is independent of confinement length. To explain this confinement effect, we computed electrostatic screening condition derived from Stillinger-Lovett sum rules. Furthermore, we investigated polarization effect on electrolytes and electrodes in confined system.   

Finite-temperature auxiliary-field Quantum Monte Carlo study of dynamical correlation functions in correlated fermion systems

We compute dynamical (imaginary-time) correlation functions in correlated fermion systems using the finite-temperature auxiliary-field Quantum Monte Carlo method. The sign problem is eliminated by introducing constraints in auxiliary-field space. We carry out a systematic benchmark study of dynamical correlation functions in the two-dimensional repulsive Hubbard model, for various interaction strengths, density, and temperatures. At high temperatures, essentially exact results are obtained independent of the form of the constraint, similar to calculations of static quantities [1]. With decreasing temperature, we discuss how the constraint can be optimized to improve the accuracy of dynamical correlation functions. In the context of studying the pseudogap behavior, we apply the method to compute the self-energy and spectral functions in the Hubbard model.
References:
[1] Yuan-Yao He, Mingpu Qin, Hao Shi, Zhong-Yi Lu, Shiwei Zhang, Phys. Rev. B 99, 045108 (2016).   

Low-energy physics in the critical phase of the bilinear-biquadratic spin-1 chain

We use an efficient density matrix renormalization group (DMRG) algorithm to compute precise dynamic structure factors for the bilinear-biquadratic spin-1 chain with Hamiltonian H = Σ_i [cosθ (S_i * S_i+1) + sinθ (S_i * S_i+1)²]. Here, we focus on explaining the physics in the extended critical phase (π/4 ≤ θ < π/2) of the model. The phase transition from the Haldane phase to the critical phase is marked by the SU(3)-symmetric ULS point (θ = π/4), where the elementary excitations are spinons that can be obtained from the Bethe ansatz solution. As we move deeper into the critical phase, the spinon continua contract, and new striking features appear at higher energies. In the vicinity of the transition point from the critical to the ferromagnetic phase, a dispersion with a surprisingly simple functional form emerges, suggesting integrability of the model in the limit θ → π/2^-.   

Entanglement decomposition for the simulation of quantum many-body dynamics

Nonequilibrium dynamics in quantum matter are at the frontier of current research. Efficient and precise simulation techniques are needed to improve our understanding of equilibration and thermalization, dynamical phase transitions, decoherence effects, quantum transport etc. A major obstacle is the growth of entanglement with time which generally implies an increased complexity of the quantum state. For instance, the computation costs of simulations based on tensor network states generally grow rapidly in time, limiting the maximum reachable times. I will show how this problem can be addressed through entanglement decomposition. We can follow the dynamics, starting from an initial state, until the entanglement has grown to a point where our simulation resources are exhausted. We then decompose the current state into lower entangled components and continue by simulating the evolution of these components, decomposing them again when needed. I will demonstrate a specific entanglement decomposition scheme for matrix product state simulations and discuss its efficiency for the study of dynamics in quantum magnets.   

Counter-diabatic Spin Squeezing

Spin squeezing states (SSS) are atomic entangled states that can be used to enhance the atomic precision measurements beyond the standard quantum limit. Existing implementations relying on unitary evolution are primarily based on one-axis twisting induced through coherent cavity feedback. By adding an auxiliary driving Rabi field, our work can generalize this one-axis twisting to an equivalent two-axis twisting, which features robustness against technical imperfections, as well as the potential to reach higher squeezing and therefore greater metrological gain. Moreover, by optimizing the time-varying auxiliary field, we obtain a fast preparation of SSS with high metrological gain, which we call as counter-diabatic spin squeezing.   

Quantum Many-Body Effects in Optical Kerr Media

Nonlinear quantum optics is emerging as one of the most important research directions in photonics. These include quantum solitons (QSs), and highly nonclassical supercontinuum generation (SCG). We need new strategies to design nonlinear optical devices operating at low photon numbers and theory to describe how they work.

Following Drummond et al. [1], we adopt a phase-space analysis to map the second quantized field theory of optical propagation in nonlinear dispersive media into a system of two coupled stochastic nonlinear Schrödinger equations (SNLSEs). This mapping allows numerical simulations of QSs, quantum dispersive shock waves (QDSWs) and quantum rogue waves (QRWs) generation by the nonlinear box problem [2] at low photon number.

As recently proved [3], quantum control furnishes new opportunities. By combining SNLSEs and CRAB optimization, we look for the best control function to limit effects as quantum spreading of QSs, to maximize the spectral generation of QDSWs, and to enhance the peak intensity of QRWs.

We believe that this approach unveils essential insights into the design of new quantum sources.

[1] P. D. Drummond et al., Nature 365 (1993).
[2] G. Marcucci et al., arXiv:1908.05212, accepted on Nat. Commun.
[3] P. Doria et al., PRL 106 (2011).   

Bosonic entanglement crossover from groundstate scaling to volume laws

The crossover behavior of eigenstate entanglement entropies from an area law or log-area law for low energies and small subsystem sizes to volume laws for high energies and large subsystems can be described by scaling functions. We demonstrate this for two bosonic systems. The harmonic lattice model describes a system of coupled harmonic oscillators and is a lattice regularization for free scalar field theories. For dimensions d ≥ 2, the ground state of this model displays an entanglement area law, even at criticality, because excitation energies vanish only at a single point in momentum space. In contrast, Bose metals feature a finite Bose surface with zero excitation energy. One hence finds log-area laws for the groundstate entanglement. For both models, we sample excited states. The distributions of their entanglement entropies are sharply peaked around subsystem entropies of corresponding thermodynamic ensembles in accordance with the eigenstate thermalization hypothesis. In this way, we determine the scaling functions numerically. Eigenstates for quasi-free bosonic systems are not Gaussian. We resolve this problem by considering appropriate squeezed states instead, for which entanglement entropies can be evaluated efficiently.   

A New Graphical Method for Designing Exactly Solvable Models

Exactly solvable lattice spin models have played important roles in physics. Since Onsager’s work, some lattice spin models were exactly solved by treated as free fermion models (FFMs). For example, the one-dimensional XY model, the one-dimensional transverse field Ising model, and the Kitaev’s two-dimensional honeycomb lattice model (KHLM) can be solved in this way ([1],[2]).
Based on these facts, in this talk we will present a new method to obtain a series of solvable models that include the Jordan-Wigner transformation and the Kitaev’s method of the KHLM as special cases [3]. For any given well-behaved graphs, we can construct FFMs corresponding to them. We can also design a set of operators from them. Therefore, we can create a lot of exactly solvable Hamiltonians. By this method we will introduce new solvable models, containing three-dimensional lattice models and spin systems corresponding to a fractal-like graph.

[1] A. Kitaev and C. Laumann, arXiv:0904.2771.
[2] K. Minami, Nucl. Phys. B 939 (2019) 465.
[3] M. Ogura et al., in preparation.   

Application of the Recursive Projection Method to Electronic Structure Calculation

In this work, we focus on the mixing scheme used during the Self-Consistent (SC) process of solving the Kohn-Sham (KS) equations. The two most common mixing schemes used in practice are the “Simple Mixing” and the “Modified Broyden Mixing”, the former is a fixed point method while the latter is a Quasi-Newton method. A characterization of the former method is that it takes less computation time per iteration but required a lot of iterations to converge; the latter gives quadratic convergence hence fewer iteration but takes more computational time per iteration. It can be shown that Simple Mixing converges if the eigen-values of the Jacobian lie within some ellipse. Thus even if only a few eigen-values lie outside the ellipse, Simple Mixing will not converge. We proposed the “Recursive Projection Method” (RPM) modified for electronic structure calculation, where we estimate the subspace spanned by the eigen-vectors whose eigen-values lie within the ellipse, call this the “stable-subspace”. We perform Simple Mixing on the stable-subspace and Modified Broyden on the complementary subspace. We expect that this method will improve convergence for systems of large size.   

The Energy Landscape Governs Brittleness and Ductility in Glasses

Based on their structure, non-crystalline phases can fail in a brittle or ductile fashion. However, the nature of the linkages between structure and propensity for ductility in disordered materials has remained elusive. Here, based on molecular dynamics simulations, we investigate the fracture of several disordered phases (metallic glass, glassy silica, colloidal gel, etc.) with varying degrees of disorder. We find that that, in general, structural disorder results in an increase in ductility. By applying the activation-relaxation technique (an accelerated sampling method to identify transition states), we show that the degree of plasticity is controlled by the topography of the energy landscape.   

Interface of Hydrated Perfluorosulfonic Acid Electrolyte with a Platinum Catalyst: Structural Analyses with Dissipative Particle Dynamics Simulations

Dissipative particle dynamics (DPD) simulations were performed to study structures of hydrated perfluorosufuonic acids (PFSA) in contact with platinum surfaces. Two types of interfacial systems were simulated, where PFSA molecules interact with a cube and a slab of platinum, respectively representing a small catalyst particle used in a regular fuel cell and a platinum substrate popularly used in a model system experiment. The calculated results suggest that the two systems form a shell structure and a layer structure, respectively, of PFSA components near the platinum surfaces. The water component covers the platinum surfaces popularly in both systems, of which the coverage depends on the hydration levels of PFSA. Detailed analyses revealed that the water component networks are quite different in the two systems, which are three-dimensionally connected in the systems with a platinum cube, while disrupted by hydrophobic components in the systems with a platinum slab.   

Universal properties of creep flow in amorphous solids

Amorphous solids, such as atomic glasses, colloidal suspensions, granular matter or foams, begin to deform plastically when exposed to external stress Σ. Steady state deformation rate ∂_tε of these materials in absence of thermal fluctuations is usually described as ∂_tε ∼ (Σ − Σ_c)^β for stresses above critical stress Σ_c and vanishes below it, while in presence of thermal fluctuations flow persists below Σ_c, but is exponentially suppressed. The transient plastic deformation rate, called creep flow, is much less understood despite its importance in practical applications. Creep flow often displays a power-law decay in time ∂_tε ∼ t^−μ after which it can either arrest or eventually yield at fluidisation time τ_f. In recent years various numerical values and/or laws have been suggested for the exponent μ and time τ_f in particular experimental or numerical studies. We propose that mechanism underlying creep flow is the same as that of the steady state flow, which allows us to predicts parameters μ and τ_f of creep flow in terms of the steady state flow parameters, both in athermal and thermally activated systems. We successfully tested all our predictions using different mesoscopic elasto-plastic models of amorphous solids and found them to be consistent with published experimental results.   

Effect of structural collapse on electron density distribution and magnetic properties of ThCr2Si2-type pnictides

The structural phase transitions in the ThCr₂Si₂-type materials involve a gradual or abrupt (first-order) collapse along the tetragonal c axis. Despite many examples of such transitions in the AT₂X₂ structures, the direct experimental assessment of changes in the electron density redistribution between the A and [T₂X₂] layers upon the formation and breaking of the X–X bonding interactions is largely lacking. Earlier studies have revealed fascinating pressure-induced transitions in EuCo₂Pn₂ (Pn = P, As) which are accompanied by the change in the Eu oxidation state and the transition from the localized (4f) magnetism to itinerant (3d) magnetism. In the present contribution, we demonstrate that the changes in the electron concentration in the [Co₂Pn₂] layer defy the formal electron-counting rules that are often used for Zintl-like phases. X-ray absorption measurements offer the direct insight into the changes in the Eu oxidation state and magnetism and the associated redistribution of the electron density in the [Co₂Pn₂] layer. Calculations of the band structure and analysis of chemical bonding have provided a satisfactory interpretation for the observed changes in the X-ray absorption spectra.   

Combining theoretical and experimental methods to study some zircon-type orthovanadates at high pressure.

Many studies of zircon-type orthovanadates under high pressure have been performed. Some of these experiments using methanol-ethanol-water as pressure transmission medium report a zircon-to-monazite phase transition. In this work, we will focus on the study of the zircon-to-scheelite phase transition of some orthovanadates, combining high pressure experimental hydrostatic studies and ab initio simulations. Our approach provides information on the structural, electronic, dynamical, and elastic properties of these compounds. From our simulations, we found that around the transition pressure the zircon phase becomes dynamically unstable due to the softening of one B_1u silent mode. Our study of the elastic constants and the analysis of the generalized mechanical stability criteria show a mechanical instability also appears near the transition pressure. We will also report on the new high pressure phase that appear after the zircon-to-scheelite transition.   

New developments in large volume static compression with in situ synchrotron x-ray diffraction at High Pressure Collaborative Access Team (HPCAT) at the Advanced Photon Source

The integration of x-ray diffraction measurements with physical properties characterizations in a large volume cell provides a unique opportunity to investigate in-situ correlation between the atomic structure and the macroscopic properties of matter at high pressure (P) and high temperature (T) conditions. The beamline 16-BM-B capable of near comprehensive large-volume sample characterization at high P and high T conditions in a Paris-Edinburgh (PE) cell by using a multitude of in-situ x-ray-based techniques. An overview of the currently supported techniques that are available to users, with emphasis on new developments, is presented. Available techniques include: double-stage PE for amorphous structural measurements above 100 GPa, ultrasonic echo, fluid viscosity, high P-T synthesis, x-ray absorption density measurement, powder diffraction, phase contrast radiography, liquid (im)miscibility, and thermoelectric properties measurements.   

Pressure induced phase transitions in topological crystalline insulator SnTe and its comparison with semiconducting SnSe: Raman and First-principles studies

We report Raman study of a topological crystalline insulator (TCI) SnTe and a normal semiconductor SnSe, as a function of pressure at room temperature along with first-principles density functional theory calculations. Under pressure, iso-structural transition is observed in SnTe as revealed by the anomalous softening of the strongest Raman mode up to 1.5 GPa. Further, SnTe undergoes structural transitions at ~ 5.8, ~ 12 and ~ 18.3 GPa. The 5.8 GPa transition is associated with a structural transition from ambient cubic (Fm-3m) to orthorhombic (Pnma) phase which is no longer a topological insulator. Above the transition pressure of 12 GPa another orthorhombic Pnma[GeS] phase is stabilized coexisting with Pnma phase. Above 18.3 GPa enthalpy calculations show a transition from orthorhombic Pnma to a more symmetric cubic (Pm -3m) structure. Our high-pressure study of SnSe on the other hand reveals that it undergoes two phase transitions: one from the orthorhombic (Pnma) structure to orthorhombic (Cmcm) structure at ~ 6.2 GPa and the other at ~ 12.9 GPa in which the Cmcm phase undergoes a semi-metal to metal transition. Density functional theory calculations capture the contrast in pressure dependent behaviour of topological crystalline insulator SnTe and a normal semiconductor SnSe.   

Study of thermal properties of HMX in molten TNT.

Octol composed of 70 wt% of HMX and 30 wt% of TNT is one of the melt cast explosives that is widely used in many military applications, namely for shaped charges. Due to the presence of low melt TNT component octol exhibits complex thermal behavior. At the temperature above TNT melt, octol forms liquid-solid system that with further heating undergoes phase change and decomposition of its constituencies with release of thermal energy. In this presentation the thermal decomposition properties of octol were investigated via DSC/TG method and X-ray diffraction at various heating conditions. The thermal reactivity of heated octol was compared to other TNT- based melt cast compositions, such as Composition B and cyclotol.   

Electrical and optical properties of tetragonal polymeric C60 under high pressure

C60 having unique icosahedral truncated structure is known to form polymer or dimer between fullerene cages under high pressure and temperature. In this work the electrical and optical properties of 2-dimensional tetragonal polymeric C60 were studied under high uniaxial pressure, p, to investigate the phase transformation.
The electrical resistance value, R, decreased from 10¹⁰ to 5×10⁵ Ω as a result of pressure cycling (with ramp pressure increase in each cycle) to a maximum pressure of 32 GPa by using a diamond anvil cell (DAC). A substantial drop in R after decompression (~10¹⁰ Ω) was reached in a cycle with a ramp pressure of 22 GPa that we consider as a cross-over/phase transition p. A subsequent p-cycling to 32 GPa resulted in further (20 times!) drop in the R of the recovered sample and exhibited anomalies in R vs p behavior.
The results of Raman measured after decompression showed that the ratio of the 2-dimensional T-phase to 1-dimensional O-phase decreased as increasing the cycling p. Raman-mapping of the recovered material showed Raman spectra inhomogeneities across the sample stemming from a non-hydrostatic pressure distribution in the DAC and the relationship between shear and normal stress.   

Structural behavior of silicate liquids and glasses under extreme conditions by using synchrotron X-ray diffraction and Raman spectroscopy

The behavior of silicate melts under high-pressure and high-temperature conditions is of primary interest in the field of geophysical, chemical, material science, and technological glass process industry, both for their fundamental properties and for their significant roles in thermal transport and chemical differentiation within Earth and other terrestrial planetary interiors. Recently, considerable progress has been achieved in understanding the structural differentiation of liquid and glass silicates by both theoretical predictions and various spectroscopic experiments, yet still many issues are puzzling and several challenges must be overcome to expand our understanding of silicate liquids and glasses. Here, we report spectroscopic properties of enstatite (MgSiO3), wollastonite (CaSiO3), diopside (CaMgSi2O6) and silica (SiO2). The local atomic structure of various silicates has been studied using synchrotron angle-dispersive X-ray diffraction combined with a multi-channel collimator. Atomic pair distribution functions (PDFs) were obtained from the X-ray data. Also, we have obtained the local vibrational modes by using Raman spectroscopy, providing additional structural information that cannot be obtained by X-ray diffraction.   

Automated iterative forward analysis for pressure determination in dynamic compression experiments

High powered laser compression experiments can provide insights into material behavior at solar and planetary cores - some of the most extreme conditions in the Universe. To fully understand material response at such extreme conditions, the pressure within the sample must be accurately determined. However, pressure cannot be measured directly. Experiments must measure the velocity of a sample and use knowledge of the equation of state to extract a pressure history. With new driver developments laser shot rate is increasing, going from once a day at NIF, to every few minutes at the Stanford Materials in Extreme Conditions (MEC) endstation and the near-term sub-Hz operation at the European X-ray Field Electron Laser (XFEL) facility in Hamburg. This increase creates a need for an automated process to convert experimentally measured velocities into pressure histories. The work reported here implements an automated iterative forward analysis using the HYADES hydrocode that can scale with the growth of modern laser facilities to accurately retrieve pressure histories from a broad range of experimental conditions. We discuss this work in the context of ramp compression of various materials on the OMEGA laser facility.   

Liner Material Dependence on Penetration Ability of Metal Jet generated from Mini-Shaped Charge Devices against Iron Target

New experimental data for the mass and density gradients of Cu and Zn metal jet using the original mass-estimation method are reported. The metal jets were generated by using shaped charges, which consist of conical Cu / Zn liners and PBX explosive charge. To compare the penetration ability of these two metal jets, Fe target-blocks were used as the target. At 3 CD (3 times charge diameter; ca. 100 mm from the shaped charge device), the breakup of Cu jet was observed, and the Cu metal segment at the jet tip was generated. On the other hand, the breakup of the Zn jet was not observed at 3 CD. The tip of the Zn jet exhibited a comet-like tail, which is unique compared to the morphology of the Cu leading jet. This result shows that the Zn jet tip reaches a state of liquid, and/or becomes a fine particle at the initial formation process. Against expectations, the penetration depth of the Zn jet against the Fe target placed at 5CD from the shaped charge device was higher than that of the Cu jet having a relatively large initial density. This result suggests that the penetration depth is more affected by the total mass of the metal jet and that the liner geometry to generate the jet mass effectively is differ depending on the material properties such as density and melting point.   

Size-dependent toughness and strength in defective brittle nanowires

Vacancy defects are ubiquitous in growing materials at the nanoscale. Yet their role in low-dimensional materials such as nanowires remains less understood. Here we report the observation of two mechanisms: elastic softening and stress localization that govern effective mechanical behavior of diamond and silicon carbide nanowires. They control different mechanical aspects of the nanowire at finite deformation. Elastic softening controls the effective mechanical properties such as stiffness in the linear regime of mechanical deformation, whereas stress-localization affects the effective toughness and strength of the nanowire. As a result, the condition for crack nucleation and the direction of crack growth are directly controlled by the stress-localization mediated by the higher-order elastic behavior of the lattice. With the increasing size of the defective regime, the nanowire shows softer effective elastic behavior that arises from the low-coordinated atoms forming the basis for the softening state of the defective regime. Results show that defect-size dependent effective stiffness is controlled by softening at the defective and surface regimes, whereas defect-size dependent toughness and strength are controlled by stiffening of the material by second-order elastic modulus.   

Different Types of Shear Stress Release in Metastable Olivine

A key element to understand the mechanism of shallow-intermediate and very deep earthquakesis here provided by an accurate model description of the stress-strain curves of the subducting material, i.e. the metastable Olivine. Specifically, atomistic modeling was carried out throughab initiotechniques for the Mg₂SiO₄forsterite end-member at different pressure ranges. The achieved stress-strain relationships were finally compared to the Moment Rate Source-Time Functions (MRSTFs) and Fracture Energies values. We found that deep events have a common rupture pattern that differs substantially from that of shallow-intermediate earthquakes. This finding is in line with the different behavior observed in the stress-strain curves as a function of depth.
  

X-Ray Absorption Spectroscopy Fingerprints for bcc and hcp Iron at Earth’s Core Conditions

The shapes of Fe K-edge of X-ray absorption spectra (XAS) were theoretically computed at Core conditions by using a number of molecular dynamics snapshots from a previously equilibrated iron system at 360 GPa and 7000 K. Reference fingerprints for the different iron polymorphs, namely Fe-bcc and Fe-hcp, were thus obtained within the multi-scattering method and therefore proposed as specific spectroscopic fingerprints. It is shown that is possible to unfold the long-standing controversy about the structural complexity of iron at the Earth’s inner core conditions. Specifically, high pressure studies from static laser heated diamond anvil cell and dynamic compression can take advantage of the proposed spectroscopic fingerprints.   

Phase diagram of solid hydrogen

We present new results for the phase diagram of low-temperature high-pressure solid hydrogen, which are obtained using independent-particle and many-body wave function-based approaches. To discover the nature of phase III, density functional theory calculations within the meta-generalized gradient approximation by means of the strongly constrained and appropriately normed (SCAN) semilocal density functional are employed to investigate eleven molecular structures within wide pressure ranges of 100-500 GPa. The SCAN-DFT predicts two structures of C2/c and P6₁22 as the best candidates for phase III. We employ the diffusion Monte Carlo (DMC) method to verify the stability of competitive phases which requires an accurate description of exchange interaction. Our DMC results indicate that the optimised percentage of exact-exchange in many-body wave function equals to 40%. We name the corresponding exchange and correlation functional as PBE₁. The PBE₁-predicted phase diagram shows that the phase III of high-pressure solid hydrogen is polymorphic [1].

References:
[1] Sam Azadi, and T. D. Kuehne, Phys. Rev. B 100 (15), 155103 (2019)   

Coarse-grain simulation study of the shear-band deformation mechanism in molecular crystals

Computationally inexpensive particle-based coarse-grained (CG) models are crucial for simulations of mesoscopically slow cooperative phenomena such as plastic deformations in solids. Molecular crystals possessing complex symmetry present an enormous practical challenge for coarse-graining at molecularly resolved scales, where the molecule is mapped into a CG particle and beyond. In this paper, we present the successful bottom-up coarse-graining of a molecular energetic crystal, cyclotrimethylene trinitramine in the alpha phase (α-RDX), using the force-matching based multiscale coarse-graining (a.k.a. MSCG/FM) approach.¹ The new MSCG/FM model offers a potentially powerful free-energy tool to analyze the lattice instabilities that lead to plastic response. A specific application of this model involves a study of the molecular-level mechanisms of shear microband formation, which is observed in the atomistic simulations of α-RDX under both static and shock-wave uniaxial compressions. These were hypothesized to contribute to plasticity, and potential shock initiation in RDX-based explosives.
¹ J. D. Moore, B. C. Barnes, S. Izvekov, M. Lísal, M. S. Sellers, D. E. Taylor, and J. K. Brennan, J. Chem. Phys. 144 (10), 104501 (2016).   

Atomistic Predictions for Reaction Mechanisms, Kinetics, and Detonation Properties of the Insensitive Explosive LLM-105

Understanding the mechanical and chemical characteristics of insensitive high explosives (IHEs) is key for the design of new insensitive materials with improved response. We explore high temperature reaction kinetics and identify reaction mechanisms for the IHE LLM-105 through all-atom molecular dynamics performed at two levels of quantum chemical theory and with classical reactive potentials. Short timescale DFT-MD simulations are used to cross-validate density functional tight-binding (DFTB) predictions, which in turn are compared against multiple ReaxFF parametrizations. High-throughput DFTB simulations are coupled with the Hugoniostat technique to simulate shock loading and to characterize the Hugoniot curves for both unreacted LLM-105 and its products. DFTB predictions for the CJ state and detonation properties are in good agreement with ReaxFF-LG. Effects of pressure on reaction products and pathways are identified and isothermal-isobaric simulations are used to study reactions at CJ conditions. Prepared by LLNL under Contract DE-AC52-07NA27344. Approved for unlimited release, LLNL-ABS-793765.   

Development of low-adiabat drives for Rayleigh-Taylor strength experiments

We have used the expansion of a shocked reservoir assembly across a gap to induce ramp loading, and hence infer strength from the growth of ripples at an interface. For multi-megabar loading, the reservoir comprises a sandwich of several materials, and the resulting load history has a large amount of structure, including shocks. This structure leads to a degree of shock heating, and some uncertainty in the heating that actually occurs. We report on progress in studies to improve the reservoir drive by reducing the shock heating, including the performance of the models used for the components of the experiment in hydrodynamic simulations.   

Electron cyclotron emission with a helical wavefront in millimeter-wave regime from an electron accelerated by a circularly polarized wave

In this study, we calculated an Electron Cyclotron Emission (ECE) with a helical wavefront in the millimeter-wave regime from an electron accelerated by a Circularly Polarized (CP) wave. Recently, it has been shown that radiation from a charged particle with spiral motion has a helical wavefront. Since an electron cyclotron motion is also a type of spiral motion, the ECE should also have a helical wavefront. In particular, we attempt to generate the high power coherent ECE with a helical wavefront from a multi-electron system. Because the radiation with helical wavefront from a single electron is low power, the superposition of the electric field is necessary to detect the radiation. For this reason, the CP wave plays an important role to control the electron’ rotation motion. We have confirmed in the calculation that the rotation phase of the electrons is controlled by an externally applied CP electric field. It was also confirmed by calculation where the radiation from such the electron has a helical wavefront. We will show the details of the calculation results at the meeting.   

Evolution from Periodic Oscillation to Chaos in Q-switched Vortex Lasers of a Unit Topological Charge

Recently direct vortex lasing from azimuthal symmetry breaking resonator were demonstrated both in continuous wave [1] and in Q-switched [2] operations. Regarding to the investigation of laser dynamics we rebuild the Q-switched vortex laser system following the same configuration reported in REF [2]. Produced Q-switched laser emits annular intensity pattern and its positive unit topological charage is quantified by spiral phase pattern sensed using a interferometer. Clear transition from periodic oscillation of every 2 pulses, 4 pulses and 6 pulses to completely disorders in pulse sequences can be identified in delay maps using the pulse-to-pulse duration as an observable. This spatial-temporal dynamic can be explained by Tang-Statz-DeMars model that describes the coherent coupling and nonlinear interactions of the off-axis multiple pass resonance modes. Moreover pumping a KTP crystal by this Q-switched vortex laser at the wavelength of 1064 nm, second harmonic wave at 532 nm exhibits unconventional intensity distribution other than a vortex with doubling topological charges because of the interactions among the resonating off-axis modes in fundamental wave.
[1] Yuan-Yao Lin, et al., J. Opt 20 075203 (2018).
[2] Yuan-Yao Lin, et al., J. Opt. 21, 085201 (2019).   

Discrete evolution on temporal separation and phase difference for bound solitons in an Yb mode-locked fiber laser

Multiple pulse regime is a common feature in passively mode-locked fiber lasers (MLFL). They could either be equally spaced, randomly distributed or arranged in a state of close interacting pulses. The later is often called bound solitons and presents strong modulation in the optical spectrum, which contains information about average relative phase, Δφ, and temporal separation between the pulses, τ. In this work, we study how increments in the total intracavity energy act on τ and Δφ for an Yb MLFL. For this, we continuously monitored the optical spectrum and the autocorrelation as the pump power was increased. We observed a discrete nature of τ, which is in agreement with previous works that observed its quantization for Er-doped fiber lasers [1] and predicted the quantization of binding energy between bound solitons [2]. Our work provides a systematic experimental study capable of capturing the jumps in τ, besides reporting a discrete behavior that is also present in Δφ.

[1] J. M. Soto-Crespo, N. Akhmediev, Ph. Greluand F. Belhache, Opt. Lett. 28, 1757-1759 (2003).
[2] A. Komarov, K. Komarov and F. Sanchez, Phys. Rev. A 79, 033807 (2009).   

Using Light to Simulate the Quantum Mechanics of the Simple Pendulum

We present experiments with a type of non-diffracting optical beams that satisfy a form the 2-dimensional Helmholtz equation that is identical to the Schrodinger equation for the simple pendulum. This form is known as the Mathieu equation. As a result the optical beams are in spatial modes that are related to the quantum solutions of the pendulum. The light intensity in the far field is propostional to the probability of the pendulum bob as a function of the angular position. Depending on the parameters of the problem we can select either states that describe the pendulum swinging or rotating. This system also allows us to investigate an array of interesting quantum phenomena: stationary states with non-classical analogs, wavepacket revivals and even cat states.   

Effects of Spin-orbit-coupling-induced Resonance in Ultracold Bosonic Systems

We calculate renormalized two-body interactions using T-matrix method, which is the sum of all orders of ladder diagrams in the bare basis. Apart from corrections to three types of diagonal interactions, there emerge many off-diagonal ones. We interpret the divergence of T-matrix as SOC-induced resonance, with the aid of which, the contrast of density modulation in stripe phase in Rb87 can be enhanced by ten times of its original value and wavelength enlarged by two times so that the stripe phase may be observed directly with conventional absorption image technique. Besides SOC-induced resonance, we also recalculate the critical value for Stripe-Plane wave transiton which is shifted by a small value. Finally, we find the correction to the density profile when Raman coupling Ω>4E_r.   

Phonon Stability and Sound Velocity of Bose Mixture Droplet

When the mean-field energy is as small as the quantum fluctuation, the self-energies under Bogoliubov approximation do not necessarily include all the contributions of the same order. For quantum droplets of a Bose mixture, the lower Bogoliubov mode is unstable at small momentum. Considering all the same order self-energies, the excitation spectrum is qualitatively different from the Bogoliubov mode. We develop Beliaev's method to calculate the elementary excitation and phonon velocity accurate to the order of gas parameter (na³)^½ in the homogeneous droplet. We discuss quasi-particles' Beliaev damping and find that density phonons are almost undamped . The sound velocity is found to be positive, which determines the nonzero superfluid critical velocity of droplet. In experiments, our results can be tested by measuring sound speed, superfluid critical velocity or collective mode.   

Quench-produced solitons in a box-trapped Bose-Einstein condensate

We describe a protocol to prepare solitons in a quasi-1d box-trapped Bose-Einstein condensate using only a quench of the isotropic s-wave scattering length. A quench to exactly four times the initial scattering length creates one soliton at each boundary of the box, which then propagate in a uniform background density and collide with one another. No nonsolotonic excitations are created during the quench. We investigate the robustness of this procedure to the scattering length ramp rate and a mismatch of the final scattering length.   

Manipulation of cold chiral molecules using electronic and rotational spectroscopy

The two non-superimposable mirror images of a chiral molecule are referred to as enantiomers; that is, structures that cannot be transformed into each other by pure translation or rotation. Many molecules of biological interest have a stereogenic center that determines their functionality. However, most physical properties of enantiomers are identical, thus, chiral analysis remains a challenge, and there is a need for fast and reliable methods that can differentiate and/or separate enantiomers. Recently, the enantiomer-specific state transfer method¹ was developed. This method provides the means to populate or depopulate a rotational state of an enantiomer. We have designed, built, and characterized a compact spectrometer capable of performing chirped-pulse Fourier transform microwave and electronic spectroscopy. By combining optical methods with microwave spectroscopy, we seek to maximize the state-specific enantiomeric enrichment. We also implement more sensitive detection schemes such that small population changes can be detected. Recent experimental results and details on the new spectrometer will be discussed.

¹Eibenberger, S., Doyle, J. & Patterson, D. Enantiomer-Specific State Transfer of Chiral Molecules. Phys Rev Lett 118, 123002, (2017).   

Photoassociation of Fermionic 87Sr via the 1S0 - 1P1 Transition

The fermionic isotope of strontium, ⁸⁷Sr, is of interest for the development of optical frequency standards and the study of quantum many-body phenomena. In many of these experiments, ⁸⁷Sr is confined in an optical lattice. Detecting the presence of doubly occupied lattice sites is a valuable tool for studies of atomic gases in optical lattices, and this is typically done with photoassociation, in which two gound-state atoms in a scattering state are photo-excited to a molecular state. No resonance frequencies have been reported for transitions to molecular states of any excited electronic potential for ⁸⁷Sr. Here we present results for photoassociation of ⁸⁷Sr atoms via the ¹S₀ - ¹P₁ transition at 461nm (Γ=(2π*30.5)s−1), and measurements of optical lengths for select photoassociation spectra.   

Cold polar molecules in superfluid helium nanodroplets: Electrostatic deflection of imidazole, its complexes and fragments

Helium nanodroplet isolation is a unique method for investigating molecules and molecular structures captured in an inert, superfluid matrix. In particular, it is a highly productive tool for the study of very cold polar molecules and their assemblies. We recently demonstrated that beams of nanodroplets doped with polar molecules can be interrogated by electrostatic deflection, leading to remarkably large deflections on the order of millimeters. Here we apply this method to the imidazole molecule, which is a five-membered polar ring which plays an important role in biological processes. We are able to differentiate polar complexes of imidazole by their dipole moments, and identify their fragmentation pathways.   

Structures with local minima at low energies in above-threshold ionization electron spectra

We perform a theoretical and computational study of multiphoton above-threshold ionization (ATI) of several one-electron atomic (H and He⁺) and diatomic (H₂⁺ and HeH²⁺) quantum systems. The driving laser fields are in the near- and mid-infrared frequency range and have moderate peak intensities corresponding to the Keldysh parameter slightly larger than unity. Our calculations reveal structures with local minima in the low-energy part of the ATI electron energy spectra if the targets are initially in the excited electronic states: 2s states for H and He⁺, 1σ_u state for H₂⁺, and 2σ state for HeH²⁺. The effect is non-perturbative and depends on the frequency and intensity of the driving field. The structures in the ATI electron energy spectra have the same nature for all the systems under consideration. Although the ionization process is generally nonresonant, our analysis shows an important role of intermediate electronic energy levels between the initial state and the onset of the continuum. For the hydrogen atom initially in the 2s state, we identify the intermediate states with the principal quantum number n=3 as those responsible for shaping the structure with the local minimum in the low-energy part of the ATI electron energy spectrum.   

A relativistic single-particle potential for photoexcited electron orbitals of open-shell atoms

A single-particle potential for the calculation of relativistic photoexcited electron orbitals is introduced. Based upon the earlier work of Qian et al. (Phys. Rev. A 33, 1751 (1986)) and Boyle (Phys. Rev. A 48, 2860 (1993)) for non-relativistic electron orbitals, this relativistic potential is defined to include exactly all of the first-order electron correlations that appear in the diagrammatic perturbation series of the dipole polarizability and that contain potential corrections in the intermediate state. Analytic relationships associated with initial state jj-couplings are also considered. For example, among other things, the ability to rewrite the defined relativistic potential for a final state as an average term plus a correction to the defined average is investigated and presented.   

The Link between Artificial Neural Networks and Propagation in Random Media

Random media (RM) with tailored optical properties are attractive for their many applications. Transmission channels (TCs) in RM can be effectively controlled, and their rich behavior is due to the multitude of interacting optical modes. We demonstrate that TCs in RM act as an untrained artificial neural network (ANN), as deep as the amount of perturbations. This lets us obtain a random optical machine (ROM), able to do computation by reservoir computing (RC).

TCs in RM can be modulated by tuning the transmission matrix (TM) through iterative algorithms that modify the input until a designed output is obtained. This approach treats RM as black boxes, i.e., it treats the TM as an ANN hidden layer of a reservoir computing (RC) strategy, a machine learning technique that left untrained the ANN internal part and optimizes weights only at input and readout.

By electromagnetic perturbation theory, we prove that weakly tampering the medium generates a new TM, given by the product between the previous TM and the perturbative one. We then design the ANN depth of our ROM by optimizing the amount of perturbations, moving from an extreme learning machine (unperturbed system) to untrained deep learning (many perturbations).   

Infinite temperature adiabatic flows and quantum many-body scar states in a chaotic system

Under any operation in many-body chaotic systems, quantum states usually experience dissipation. However, this dissipation can be drastically suppressed through the application of additional appropriate control fields, as is done in counterdiabatic driving. Here, we restrict control to few-body operations and draw flows of the optimal path for quantum control in a non-integrable Ising chain. We find that these flows are highly anisotropic; we can realize almost dissipationless driving in one direction while dissipation is hard to reduce in the perpendicular direction. This indicates that the choice of the path is crucial for quantum operations in many-body chaotic systems. Furthermore, states which are eigenstates of the counterdiabatic term are shown to exhibit small dissipation even without counterdiabatic fields, and are closely related to quantum many-body scar states and many-body dark states.   

Atom-Light Interactions in Integrated Photonics: en Route to an Atomic Lab on a Photonic Chip

The integration of photonic structures with thermal atomic vapors on a chip provides efficient atom-light coupling on a miniaturized scale well beyond the diffraction limit hence, opening a new regime in the field of cavity quantum electrodynamics. In this talk, we present the results of our study on interactions of thermal Rb atoms with integrated SiN and Si Nano-devices. In the former case, the atoms are probed with a laser at the D₂ transition, whereas in latter the atoms are further excited to the 4D states with an additional excitation at telecom wavelength. Our studies on Si structures benefit from stronger mode confinement due to the large reflective index as well as a larger dipole moment. Moreover, we demonstrate novel measurements on the effects of Si surface potentials on Rb 4D states. Promising results on ring resonators pave the way towards further investigations of high-Q photonic crystal cavities in order to reach the strong coupling regime.   

An optical fiber simulator of three interacting atoms in one dimensional parabolic trap

We introduce an optical fiber that simulates a system of three trapped ultracold and strongly interacting atoms in one-dimension. To simulate the contact interactions among the atoms, we consider a sharp and narrow jump in the refractive index. To this end, we consider here three thin metallic slabs. To simulate the parabolic trap we assume that the fiber has a graded refractive index profile. While the wave-nature of single quantum particles leads well-known analogies with classical optics, for interacting many-particle systems such analogs are not straightforward. We discuss: i) how by spatially modulating the incident field, one can select the atomic statistics, i.e., emulate a system of three bosons, fermions or two bosons or fermions plus a distinguishable particle; ii) how this system can produce classical non-separability resembling that found in the analogous atomic system.

[1] M.A. Garcia-March et al. Arxiv: 1902.01748.   

Surface plasmon-polariton waves propagation and excitation along the direction of periodicity of the one-dimensional photonic crystal

The surface plasmon-polariton (SPP) waves guided by one-dimensional photonic crystals (1DPCs) are usually excited at the interface perpendicular to the direction of periodicity. In this work, we will present the formulation and the numerical results delineating the excitation of the SPP waves on the interface perpendicular to the direction of periodicity. Both the eigenvalue problem of finding and solving the dispersion equation and the problem of the excitation of the SPP waves will be discussed. The eigenvalue problem is solved using the rigorous coupled-wave approach. The results indicate the presence of the plasmonic bandgap that can help in designing the plasmonic optical filters.

References:
1- M. Rasheed and M. Faryad, “Rigorous formulation of surface plasmon-polariton-waves propagation along the direction of periodicity of one-dimensional photonic crystal,” Journal of Optical Society of America B, 35, 2957-2962 (2018).
2- M. Kamran and M. Faryad, “Excitation of surface plasmon-polariton waves along the direction of periodicity of a one-dimensional photonic crystal,” Physical Review A, 99, 053811 (2019).   

Anomalous magnetic moment of electron for an adiabatically changed Finslerian manifold.

In present work, we resolve the unified equation of motion for a quantum system on the adiabatically changed Finsler manifold, suggested by Lipovka (2017), for the particular case of the hydrogen atom. The radial part for the equation of motion is obtained to model a hydrogen atom, where the electron and the nucleus rotate around a center of mass. By using this expression, the total energy of the system under consideration (charged particles and EM field) is obtained.
By expanding in series in the small parameter α = v/c , the value of the anomalous magnetic moment of the electron is obtained. The value calculated by J. Swinger = α /2π≈0.0011614 is obtained as a particular case.   

Optical spectroscopy of TaH: Six low-lying electronic states and their agreement with computational studies

We have recorded electronic spectra of tantalum hydride (TaH) over the region 605–665 nm. The spectra were recorded by laser excitation spectroscopy at Doppler-limited resolution using a continuous-wave ring dye laser. We have assigned and fitted a total of 12 different bands, which originate from one of three low-lying electronic states. Calculations by Mark Gordon’s group and previous experimental work in our group confirm that the Ω = 2 ground state is largely derived from a σ²δπ, ³Φ₂ state. An Ω = 0⁺ state, largely a mixture of ³Σ^– and ³Π, lies only 76 cm^–1 above the Ω" = 2 ground state. By recording dispersed fluorescence, we found and characterized a total of six low-lying states of the molecule. The electronic states have been fitted by least-squares using a Hund’s case (c) Hamiltonian. We report term energies, vibrational frequencies, rotational and centrifugal distortion constants, and bond lengths for these states, which were found to be in good agreement with the computational results.   

The Physics of Critical Heat Flux Studies for Innovative ATF Cladding Surfaces

Overall Objective
The goal of this project is to investigate the engineering physics phenomenon of critical heat flux (CHF) for surfaces of accident tolerant fuel (ATF) cladding materials that are being considered for implementation jointly by industry, utilities, and the Department of Energy (DOE). This effort involves testing at small scale in a pool boiling environment and testing at intermediate scale in flow boiling under prototypical conditions, along with associated analysis.   

Floquet approach to the study of the dynamical Lamb effect

The dynamics of N qubits coupled to a harmonic oscillator with time-periodic coupling is investigated in the framework of Floquet theory. This system can be used to model nonadiabatic phenomena that require a periodic modulation of the parameters of the system. An example is the dynamical Lamb effect, the simultaneous excitation of the qubits and the harmonic oscillator out of the ground state due to the nonadiabatic change in boundary condition of the system. The time-dependent Schroedinger equation describing the system’s dynamics is solved within the Floquet formalism and two other equivalent methods that rely on a perturbative approach in the time- and Laplace-domain. Because of the periodicity of the problem analyzed, the Floquet formalism provides a framework where analytical and numerical results are closest to each other. Nonetheless, the time- or Laplace-domain perturbative approaches can be used in the presence of simple or complicated, respectively, aperiodic time-dependent terms in the Hamiltonian.   

Symmetric Rotating Wave Approximation for the Generalized Two-Mode Two-Photon System Coupled with a Bosonic Field

We study the analytically approximated eigenenergies of the two-mode two-photon quantum Rabi model coupled with a Bosonic field. By equipping the previously established generalized symmetric rotating wave approximation (G-SRWA) method used on single-mode spin-boson systems with a unitary squeezing operator and applying it onto the two-mode two-photon system, we obtain a more generalized and simplified closed-form expression for eigenenergies of the system compared to other expressions derived using the functional Bethe ansatz method. We further investigate the effectiveness of the squeezing operator by examining the eigenenergies of the generalized resonant and nonresonant case for the two-level two-photon system. The accuracy of the updated G-SRWA method in the ultra and deep-strong coupling regimes reveals interesting symmetries within the mathematical structure of the two-level two-photon systems.   

Topological Control of Extreme Nonlinear Waves

Controlling nonlinear optical processes is a significant challenge in photonics. Shock waves, rogue waves and solitons are widespread, from optics to hydrodynamics. Intense research is dedicated to advanced techniques for tailoring extreme waves and finding the conditions to induce transitions between different waves.

We develop a new strategy to supervise, modify and tune a laser beam in third-order nonlinear materials, when light propagation is ruled by the nonlinear Schrödinger equation (NLSE). We denote our approach topological control (TC) [1]. TC is based on the one-to-one correspondence between the number of wave packet oscillating phases and the genus of toroidal surfaces associated with the NLSE solutions by the Riemann theta function.

We prove that our method is experimentally realizable in a photorefractive crystal [1]. Specifically, we use the parametric time-dependence of photorefractive nonlinearity to shape the asymptotic wave profile. We tailor propagation coefficients, as nonlinearity and dispersion, to observe all the phases in the nonlinear wave evolution of a rectangular-shaped beam, and to control transitions from shock to rogue waves.

[1] G. Marcucci et al., arXiv:1908.05212, accepted on Nat. Commun.   

Theory of High-Energy Electron Thermionic Emission from Graphene

Graphene thermionic electron emission across high barrier involves energetic electrons residing far away from the Dirac point. In this work, we construct a full-band model beyond the simple Dirac cone approximation for the thermionic injection of high-energy electrons in graphene [1]. We show that the thermionic emission model based on the Dirac cone approximation is valid only in the graphene/semiconductor Schottky interface operating near room temperature but breaks down in the cases involving high-energy electrons. We further reveal a critical potential barrier height beyond which the Dirac cone approximation crosses over from underestimation to overestimation. In the high-temperature thermionic emission regime, the Dirac cone approximation severely overestimates the electrical and heat current densities by more than 50% compared to the more accurate full-band model. Our findings reveal the fallacy of Dirac cone approximation in the thermionic injection of high-energy electrons in graphene. The full-band model developed here can be readily generalized to other 2D materials and shall provide an improved theoretical avenue for the accurate analysis, modeling and design of graphene-based thermionic energy devices.
[1] Y. S. Ang et al, Phys. Rev. Appl. 12, 014057 (2019)   

Generation of optical vortices from a spatial light modulator, vortex phase plate, and mode converter

First introduced in 1992 by Prof. Allen et al., light with orbital angular momentum (OAM), also called the Laguerre–Gaussian mode, possesses an OAM of per photon. Its twisted phase wavefront is a manifestation of the azimuthal phase term in its wavefunction, while the phase singularities along the beam axis are defined as the optical vortices. The twisted light which possesses OAM can be generated by laser cavities, spiral phase plate, metalens, and spatial light modulator (SLM). In this work, we utilize a vortex phase plate, mode converter, and spatial light modulator to generate high-order vector vortex beams. The mode converter is used to transform the vortex beam with circular symmetry into rectangular symmetry and form the vortex array. This is a convenient and powerful method to produce and control the optical vortex array of the vector superposed optical field, which is composed of different orders of crossed Hermite-Gaussian bases with opposite helicity of circular polarization. The SLM provides an extra degree of freedom to increase and control the order of the bases in the vector superposed optical field, which can induce optical vortices of different sizes and quantities.   

Multipole Excitation of C60 Molecules in a Semi-Classical Approach

This work investigated the collective dynamic response of the 240 delocalized valence electrons of the C₆₀ molecules to external stimulations such as electromagnetic radiation or scattering of the charged particle, using local current approximation (LCA), which is a semiclassical approach based on general variational principle. Here the bulk of the valence electrons is treated as a semi-classical fluid that exhibits both the translational and compressional vibrations. The coupling of these two modes has been studied and the results are in good agreement with the experimental data from photoionization experiments on C₆₀.The same semi-classical approach has been applied to study the energies of higher angular momentum resonance.   

Ab Initio Potentials for Low-Energy Positronium Scattering

This poster presents ab initio potentials for positronium-atom and positronium-molecule interactions. Two methods are used to calculate the potential energy: self-consistent solution of the Hartree-Fock equations and solution of the Kohn-Sham equations at the level of the local density approximation. The total energy is computed for a wide range of positronium-molecule separations, molecular orientations, and basis sets. Comparing the two methods illustrates the importance of both electron-electron and electron-positron correlations in obtaining realistic potentials.

Low-energy positronium scattering may soon become experimentally feasible in facilities such as the positronium beam line at the University College in London. This goal of this study is to provide a starting point for the theoretical analysis of positronium scattering at low energies from first principles, to complement existing studies based on semi-empirical potentials.   

New Atomic Model from the spectra of Hydrogen, Helium, Beryllium, Boron, Carbon, and Deuterium and their ions

A cohesive unifying theory of the atom does not currently exist in Quantum Physics. In this research, the atomic spectra are allowed to determine the model for the atom based upon the finding of patterns of the Balmer-Rydberg formula in the first 20 ions and neutral atoms of the periodic table. From this data, the model postulates a standing wave of varying energy antinodes originating from the particles in the nucleus of each atom which is able to predict the ionization energies of these atoms. The transitions of the electrons in atoms are defined by the energies of each antinode represented by the difference in energy between each spectral line. The spectral patterns for H, He-I, He-II, Li-I, Li-II, Li-III, Be-I, Be-II, Be-III, Be-IV, B-I, B-II, B-III, B-IV, B-V, C-I, C-II, C-III, C-IV, and Deuterium are charted and the ionization energies are calculated from the data including general inferences this model predicts about the unification of atomic forces, electron transitions, heat, and electromagnetism. This model predicts that the nucleus of every atom is held together by energy in the form of a standing wave originating from the nucleus and surrounding it. This is the Sollism Theory of the atom.   

Casimir Juggling

Casimir forces can dominate system behavior at the nanoscale;
increasingly, efforts are being made to control and exploit them [1-3].
Here we propose a simple method for contact-free, dynamic levitation,
handling, and physical diagnosis of micron and sub-micron particles in
vacuum using these forces. This `Casimir juggling' should sidestep the
negative effects of stiction and contact contamination. Analytic
calculations and numerical modeling show particles can be
levitated, transported and deposited with nanometer precision using
kHz-MHz active-feedback Casimir probes. Standard laboratory techniques
appear adequate to experimentally test this proposal.
[1] V.A. Parsegian, Van der Waals Forces: A Handbook for Biologists,
Chemists, Engineers, and Physicists (Cambridge University Press, 2006).
[2] D.P. Sheehan and S.H. Nogami, Micro and Nanosystems 3, 348 (2011).
[3] D.P. Sheehan, D.P., J. Chem. Phys. 131, 104706 (2009).   

Survey of Molecular Rotational Transitions for Millimeter-Wave Frequency Metrology.

User-defined, spectrally pure and tunable millimeter-wave oscillators, using emerging photonic technologies, enables the probing of pure rotational molecular transitions (from 30 GHz to 1 THz). This work promises to open new applications in frequency metrology, precision molecular spectroscopy, and trace gas sensing. Here we report a theoretical survey of a wide range of molecular candidates to compare their suitability for use in a rotational based clock. We will report calculations that lead to theoretical estimations of clock factors of interest such as line positions and strengths, and sensitivity to environmental perturbations. This work will provide an understanding of the utility of ultra-precise measurements of the rotational spectrum in blooming applications such as radio-astronomy and remote sensing, non-invasive sensing, wireless communications, and for THz frequency metrology.
  

Design of a viable radiation-balanced fiber laser or amplifier

Recent advances in power scaling of fiber lasers are hindered by the transverse mode instability (TMI), which deteriorates the output beam quality. TMI is blamed on the overheating of the fibers. Anti-Stokes fluorescence (ASF) cooling of the rare-earth-doped gain material has been suggested as a potentially viable heat removal scheme. In this scheme, the fiber is pumped at a pump wavelength, which is longer than the mean fluorescence wavelength of the active ions. Therefore, the fluorescence removes some of the excess heat. It may be possible even to balance the ASF cooling against all other sources of heating and make a non-heating radiation-balanced laser (RBL). I will focus on the general scaling laws that govern the design of RBLs. I will show that the magnitude of the parasitic absorption plays an essential role in the feasibility of such designs, especially in a double-cladding setting, and the measures that need to be taken to achieve net heat-balancing in a viable RBL.

S. R. Bowman, IEEE J. Quantum Electron. 35, 115-122 (1999).
E. Mobini et al. J. Opt. Soc. Am. B. 35, 2484-2493 (2018).
M. Peysokhan et al., arXiv: 1909.13439 (2019).
M. Peysokhan et al. Appl. Opt. 58(7), 1841-1846 (2019).
J. Knall et al. Opt. Lett. 44, 2338-2341 (2019).   

Uncovering the Role of Excited States of Dication in Controlling the Dissociative Double Ionization of Ethane

With the aid of newly developed coincidence detection imaging system, we demonstrate that the branching ratios of dissociative double ionization channels of ethane can be controlled by varying the ellipticity of the intense ultrashort laser pulses. The CH₃⁺ formation channel and H⁺ formation channel show a significant yield changes, producing the highest and lowest at ellipticity of 0.59 respectively. With the help of theoretical calculations, we attribute such a control to both angle dependent ionization and intensity dependent ionization to excited dication states.
  

High bandwidth, bidirectional microwave to telecom conversion using an electro-optic transducer operating at millikelvin temperatures

Long distance connectivity of superconducting qubits poses an interesting challenge because their fragile microwave quantum states need to be protected from thermal noise. A hybrid quantum network, which combines the advantages of microwave quantum information processing and the robustness of optical telecommunication, appears as the natural solution. In this context, coherent conversion between optical and microwave photons is the key ingredient.
We present a triply resonant electro-optic modulator [1] as a bidirectional and low noise microwave to optics converter. Our system is based on a LiNbO3 whispering gallery mode resonator coupled to a superconducting 3D microwave cavity. Unlike electro-opto-mechanical systems, a low frequency mechanical mediator is not needed, allowing for a 10 MHz conversion bandwidth and low added conversion noise of less than 1 quanta for continuous wave optical pump powers up to 25 µW.

[1] A. Rueda, et al., Optica 3, (2016).   

Towards cavity quantum circuit electromechanics with millimiter-sized silicon nitride membranes

Due to their extremely high mechanical quality factors, silicon nitride membranes are commonplace in optomechanical experiments conducted with light fields. However, to strongly couple optical radiation to the vibratory motion of such membranes down in the quantum regime is still a sought-after target in these experiments.
We use here an original assembly to couple instead microwave radiation to a vibrating membrane of submillimiter dimensions, obtaining interaction strengths approaching state of the art values achieved in the optical realm.
The assembly is based on a mechanically adjustable pressing frame through spring screws, that holds a chip with an integrated lumped element resonator antenna facing another chip hosting the membrane. The membrane is coated with a layer of metal so that the two chips couple capacitively to form a resonant circuit, the capacitance of which, and thus its resonance frequency, is modulated by the membrane vibrations.
Besides its potential to challenge the foundations of quantum mechanics at unconventional mass and length scales, our electromechanical device may be well suited to study nonlinear phenomena in the framework of classical mechanics such as the emergence of chaotic dynamics in the weak damping limit.   

Acoustic modes of superfluid helium in a cross geometry

Superfluid helium is a low-loss optomechanical element, and an acoustic quality factor value up to 10⁸ has been realized experimentally in a macroscopic quantum system using a cylindrical microwave cavity [1]. It has been predicted that three orders higher quality factor may be attained with improvements to the experimental system. With these parameters, superfluid helium is a potential candidate for detecting continuous gravitational waves [2]. Here, we study the acoustic modes of superfluid helium inside a cross geometry using a re-entrant microwave cavity that provides improved detection of the acoustic modes. The cross shaped geometry is also predicted to be more sensitive than a cylinder for detection of gravitational waves.

1. L. A. De Lorenzo and K. C. Schwab, New Journal of Physics, 16, 113020 (2014).
2. S. Singh, L. A. De Lorenzo, I. Pikovski, K. C. Schwab, New Journal of Physics, 19, 073023 (2017).   

Coherent coupling completes an unambiguous optomechanical classification framework

Cavity optomechanics studies the dynamics of systems in which optical and mechanical degrees of freedom are coupled, e.g., optical resonators with movable/deformable boundaries and/or shapes. A system's behavior depends crucially on the nature of optomechanical coupling. In addition to the previously well-known dispersive (when real parts of optical eigenfrequencies depend on mechanical displacement) and dissipative coupling (when imaginary parts of optical eigenfrequencies depend on mechanical displacement), we identify coherent coupling as a new type of coupling (when neither depend on mechanical displacement) and show that the three together fit into a general, unambiguous classification framework. We discuss in detail a ring cavity with a movable mirror inside, a system that has pure coherent coupling without any dispersive or dissipative coupling.   

Two-tone optomechanical instability and its fundamental implications for backaction-evading measurements

We report a new type of optomechanical instability that arises in two-tone backaction-evading (BAE) measurements of mechanical motion, a protocol designed to overcome the standard quantum limit. We demonstrate the effect both in the optical and microwave domains using different optomechanical systems, and find excellent agreement with theory. In contrast to the well-known parametric instability that occurs in single-tone, blue-detuned pumping, and results from a two-mode squeezing interaction between the optical and mechanical modes, the two-tone instability results from single-mode squeezing of the mechanical mode due to small detuning errors in the two pump frequencies. The instability can occur even with balanced intracavity fields and for both signs of detuning errors. The required tuning accuracy increases with pump power, putting an intrinsic limit on the sensitivity of BAE measurements and on other two-tone schemes.   

Ultraviolet resonator integrated in a hollow-core fiber for Xenon plasma lasing

We integrate a cavity in a large-diameter hollow-core optical fiber based on inhibited coupling as a step towards realization of a fiber-integrated gas laser in the ultraviolet (UV) region. This is accomplished by attaching highly reflective photonic crystal (PC) membranes onto the ends of a fiber segment to form a Fabry-Perot cavity. The PC membranes are fabricated using e-beam lithography and reactive ion etching, which are then mounted on the fiber face using a micromanipulator stage. The presence of the PC holes allow for injection loading of atomic species into the fiber-cavity. Specifically, Xenon gas can be introduced through the perforated membrane into the hollow-core of the fiber to act as a gain medium for UV lasing when exposed to RF discharge.   

Numerical Modeling of Optomechanical Sensors for Dark Matter Detection

Ultralight scalar dark matter can be represented as an atomic strain that can drive the acoustic breathing modes of an elastic body. We propose various laboratory-scale mechanical resonators for measuring the acoustic excitations at frequencies ranging from kilohertz (kHz) to Gigahertz (GHz)[1]. These devices include bulk acoustic wave resonators, phononic crystals, superfluid helium detectors and suspended sapphire micropillars. We use numerical modeling techniques to characterize each device’s performance as a dark matter detector. These techniques can be applied to an arbitrary mechanical resonator to measure its viability as a sensor for these signals.   

Observation of dynamical quantum phase transitions in spinor condensates with no correspondence in ground-state phase diagram

Dynamical quantum phase transitions are closely related to equilibrium quantum phase transitions for ground states. Here, we report an experimental observation of a dynamical quantum phase transition in a spinor condensate beyond this conventional wisdom. We observe that the quench dynamics exhibits a non-analytical change with respect to a control parameter in the absence of a corresponding phase transition for the ground state there. We make a connection between this singular point and a phase transition point for the highest energy level in a subspace with zero spin magnetization of a Hamiltonian. We further show the existence of dynamical phase transitions for finite magnetization corresponding to the phase transition of the highest energy level in the subspace with the same magnetization. Our results open a door for studying dynamical phase transitions beyond the conventional ground-state phase diagram and using them as a tool to probe the phase transitions at higher energy eigenlevels of many-body Hamiltonians.   

Hydrodynamics of shock waves in ultracold non-degenerate dipolar gases.

The hydrodynamics of an ultracold, non-degenerate gas of harmonically trapped dipolar atoms is studied. We numerically simulate this system using direct simulation Monte Carlo methods in the regime where interactions are dominated by two-body collisions, as for instance can be achieved in ultracold gases of Dy or Er. We subject the gas to a short-pulse perturbation which is common in hydrodynamic studies of shock waves, emphasizing the anisotropies in hydrodynamic phenomena resulting from the anisotropic collision cross sections of the diploles. This work extends observations of anisotropic rethermalization of a cold, dipolar gas previously seen in [1,2]. Here, shock wave hydrodynamics is studied in the less explored low-temperature regime, leading to a emergence of rich phenomena. This work was supported by the NSF.

[1] K. Aikawa et al, PRL 113, 263201 (2014).
[2] Y. Tang et al, PRA 92, 022703 (2015).   

Self-organization and stroboscopic dynamics of a driven Rydberg gas

Motivated by recent experiments observing signatures of self-organized criticality (SOC) in driven Rydberg ensembles, we develop a Langevin description to explore how SOC can emerge from the microscopic interactions in experimental Rydberg setups. We demonstrate that drift and diffusion of atoms arising from an inhomogeneous trapping potential capture the stroboscopic evolution observed in experiments. The trap dynamics induces a continuous reorganization of the atoms, which pins the central density to a critical point and gives rise to scale invariant excitation avalanches. We further discuss how additional external driving can be used to extend and manipulate the SOC avalanche dynamics. This offers a detailed perspective on how avalanche dynamics can be controlled in driven Rydberg gases.   

Photon blockade in the Tavis Cummings model

We use the scattering matrix formalism to study single- and two-photon transport through the Tavis Cummings model as a function of the number of emitters coupling to the cavity mode. It is shown that for a resonant Tavis Cummings system, photon blockade at the output of the cavity mode worsens with the number of emitters while for a detuned Tavis Cummings system, photon blockade at the output of the cavity mode improves with the number of emitters. By explicitly calculating the two-photon scattering properties of this system in the thermodynamic limit of an infinite number of emitters, we show that the light emitted from Tavis Cumming system under excitation by a coherent drive transitions from bunching to anti-bunching as the emitters are detuned from the cavity mode. Finally, we analyze the impact of inhomogeneous broadening in the emitter frequencies on both resonant and detuned photon blockade through the Tavis Cumming system.   

Point-coupling Hamiltonian for frequency-independent linear optical devices

We present the point-coupling Hamiltonian as a model for frequency-independent linear optical devices acting on propagating optical modes described as a continua of harmonic oscillators. We formally integrate the Heisenberg equations of motion for this Hamiltonian, calculate its quantum scattering matrix, and show that an application of the quantum scattering matrix on an input state is equivalent to applying the inverse of classical scattering matrix on the annihilation operators de- scribing the optical modes. We show how to construct the point-coupling Hamiltonian corresponding to a general linear optical device described by a classical scattering matrix, and provide examples of Hamiltonians for some commonly used linear optical devices. Finally, in order to demonstrate the practical utility of the point-coupling Hamiltonian, we use it to rigorously formulate a matrix- product-state based simulation for time-delayed feedback systems wherein the feedback is provided by a linear optical device described by a scattering matrix as opposed to a hard boundary condition (e.g. a mirror with less than unity reflectivity).   

Efficient numerical integration of the adiabatic master equation

The capability to simulate open quantum systems under the adiabatic condition is critical for verifying the behavior of quantum annealers. While adiabaticity is a useful condition for theoretical analysis, straightforward numerical implementations of the adiabatic master equation (AME) suffer from the need to integrate an adiabatic but highly oscillatory quantum state over a very long period of time. We show that the AME in the adiabatic frame can be split in a way that prevents large superoperator evaluations and is amenable to geometric integration methods for split time-depedent problems.   

A perturbative view from the master equation: Electromagnetically induced transparency revisited

We show that by treating the weak probe field as a perturbation to the strong coupling
fields in the atomic system and using the perturbative method in a master equation, the features of
linear response of phenomena of electromagnetically induced transparency (EIT) can be uniformly
demonstrated, regardless of the details of atomic energy level configuration. We compare our
estimation with both typical and atypical EIT-observed configurations and find that our model
indeed provides a description of the sharp transmission window in central area of typical EIT curve.
It can also be inferred hereby that for various systems other than atomic gas, as long as the description
of the system’s dynamics comes down to the simplified form of master equation, the corresponding
EIT analogs and EIT-like phenomena can also be explained in this way.   

Vacancies effect on the BEC critical temperature of an ideal Bose gas within an imperfect crystal

After we have shown [1] that an ideal Bose gas in one, two or three-dimensional crystal with one structural vacancy presents Bose-Einstein condensation (BEC) at higher finite critical temperature than that when the crystal is perfect; here we report the BEC critical temperature as a function of the number of random vancancies. The particle energy spectrum is obtained using finite difference numerical method, which we use to calculate the critical temperature as the temperature at which the isobaric specific heat has its maximum [2]. When our finite system is taken to the thermodynamic limit, the critical temperature starts at the one vacancy value, it increases reaching a maximum when we have about 17 % of vacancies and then decays almost linearly to zero when the crystal has been removed.
[1] J. García, et al., Raising the Bose-Einstein critical temperature with vacancies, in progress; V.E. Barragán, et al., Int. J. of Mod. Phys. B Vol. 31, 1750100 (2017). [2] H.R. Pajkowski and R.K. Pathria, J. Phys. A: Math. Gen., Vol. 10, No. 4, 1977.   

Quantum system chaos in a PT-symmetry breaking potential

We studied the dynamics of N bosons confined in a triple well potential in phase space. The system is tackled by using the truncated Wigner approximation (TWA) and exact diagonalization of Bose-Hubbard model including a perturbative PT-symmetry breaking potential. We find a region in which the symmetry breaking develops smoothly with the control parameters enabling to break many symmetries of the Hamiltonian and reduce energy degeneracies. The model shows signatures of stationarity, chaos and mixing as a consequence of the inter-particle interactions. Generation of entangled states and loss of purity is also observed and monitored by the Meyer’s measure of the impurity and the von Neumann entropy S. Finally, the system chaotic behavior is conveniently tracked within the phase space by the growth rate of the out-of-time ordered correlator.   

Continuous protection of a quantum state from inhomogeneous dephasing

Room-temperature atomic vapors are known for their simplicity and their potential scaling-up in applications. In spite of these benefits, laser-cooled atoms have evolved to be the prevalent systems for studying strong and coherent light-matter interactions, as the latter are unhindered by Doppler broadening.
Here we present several methods to overcome the effective decrease of both atom-photon cross-section [1] and coherence time in vapors, and in fact in any inhomogeneously broadened atom-like system.
The mechanism we study can be understood as the counteraction of the inhomogeneous dephasing of two coupled states, where one state has enhanced sensitivity to the source of dephasing. A far-detuned dressing field admixes a fraction Ω²/△² of this "sensor" state into the "protected" state, yielding a velocity-insensitive state and effective line-narrowing in two-color transitions [2]. Finally, we apply this method to extend the lifetime of collective excitations stored in a thermal atomic vapor. This method is continuous, in contrast to pulsed echo-based techniques.
[1] Lahad O., Finkelstein R., Davidson O., Michel O., Poem E. & Firstenberg O. (2019). Phys. Rev. Lett. 123 173203
[2] Finkelstein R., Lahad O., Michel O., Davidson O., Poem E. & Firstenberg O. (2019). New J. Phys. 21 103024
  

Combined spontaneous symmetry-breaking and symmetry-protected topological order from cluster charge interaction

The study of symmetry-protected topological states in presence of electron correlations has recently aroused great interest as rich and exotic phenomena can emerge. Here, we report a concrete example by employing large-scale unbiased quantum Monte Carlo study of the Kane-Mele model with cluster charge interactions. The ground-state phase diagram for the model at half filling is established. Our simulation identifies the coexistence of a symmetry-protected topological order with a symmetry-breaking Kekule valence bond order and shows that the spontaneous symmetry-breaking is accompanied by an interaction-driven topological phase transition (TPT). This TPT features appearance of zeros of single-particle Green's function and gap closing in spin channel rather than single-particle excitation spectrum, and thus has no mean-field correspondence.   

Quantum Monte Carlo Simulations of the Attractive SU(3) Hubbard Model on a Honeycomb Lattice

We perform the projector quantum Monte Carlo simulation of the half-filled attractive SU(3) Hubbard model on a honeycomb
lattice, exploring the effects of SU(3) symmetry on the correlated attractive Dirac fermions. Our simulation indicates the absence of pairing order in the system and shows a quantum phase transition from the semimetal to charge density wave (CDW) at the critical point U_c = −1.52(2). We demonstrate that this quantum phase transition belongs to the chiral Ising universality class according to the numerically determined critical exponents ν = 0.82(3) and η = 0.58(4). With the increase of coupling strength, the trion formation is investigated, and the change in probability of the on-site trion occupancy infers the coexistence of on-site trionic and off-site trionic CDW states at half-filling.   

Signatures of Majorana-like Quasiparticles in Few-body Lattice Models

There is a strong interest in number conserving systems that exhibit Majorana fermions as quasiparticles. We utilize a number-conserving analog of the Kitaev wire model due to Iemini et al. in the small-lattice limit to examine signatures of topological properties in small systems. This interacting model exhibits several signatures of topological regime, such as identical entanglement spectra for the ground states in distinct parity sectors. Additionally, the Hamiltonian is akin to the BCS Hamiltonian, suggesting Bogoliubov rotations and approximations, allowing us to further examine this model in this regard. In this work, we demonstrate these signatures of different topological order in small lattice sizes to find evidence of isolated Majorana quasiparticles.

Refs:
[1] A. Yu Kitaev 2001 Phys.-Usp. 44 131
[2] F. Iemini, et al. 2017 PRL 118 200404   

Effect of Core Size on Vortex Interactions in Light

Optical vortices are characterized by a helical wavefront with integer winding, also referred to as
the topological charge, around the center of the vortex. These vortices exhibit dynamic behavior
when implanted in different locations of a Gaussian beam, and it is possible to observe vortex-
vortex interactions as the beam propagates. In the case of an oppositely charged vortex pair
separated symmetrically about the beam axis, the vortices begin with a downward motion in the
transverse plane and then accelerate toward each other. When the beam propagates, these
vortices will at some point annihilate each other, without reemergence on the other side of the
collision. The exact dynamics depend on the shape and size of the core. For smaller cores, the
diffraction of the core has an effect on the dynamics and the annihilation distance is farther along
the propagation axis. I will present our theoretical and experimental results including the impact
of charge and core size on the vortex trajectories. I will also discuss the role of the underlying
phase and amplitude gradients at the location of a given vortex on its motion as the beam
propagates.   

Finite Orbital Angular Momentum Pairing in Atomic Fermi Gases with Spin-Orbital-Angular-Momentum Coupling

Fulde-Ferrell-Larkin-Ovchinnikov states represent superconducting states with finite momentum Cooper pairs and have become one of most sought-after exotic states of matter. The recent realization of a new type of spin-orbit coupling in ultracold atomic gases, namely the spin-orbital-angular-momentum coupling, provides a novel perspective to study Fulde-Ferrell-Larkin -Ovchinnikov states in orbital angular momentum space. In this letter, we demonstrate that the combined effects of spin-orbital-angular-momentum coupling and level detuning can induce an exotic Fulde-Ferrell-Larkin-Ovchinnikov phase, where Cooper pairs have finite center-of-mass angular momenta. This implies that vortex with high-vorticity can be generated without rotation or effective vector potential field. The orientation of the phase winding can be altered by adjusting the sign of level detuning.   

Quantum Defects in Diamond: Identifying Nitrogen Isotopes of Nitrogen-Vacancy Centers

Nitrogen-vacancy (NV) centers are point defects in diamond formed by one substitutional nitrogen atom and an adjacent vacancy. Low spectral diffusion is a necessary property for NV centers to be qubit candidates. To characterize differences between naturally formed and ion implanted NV centers, diamond samples were studied that contained both types. The ion implantation used ¹⁵N to be able to differentiate from the ¹⁴N naturally formed NV centers. This project focused on identifying the isotope of a single NV center, which is the first step toward understanding differences in their emissive properties. Code was developed to execute, and then automate, the three experiments necessary to identify the isotope of a single NV center: Continuous Wave Optically Detected Magnetic Resonance (CW ODMR), Rabi oscillations, and Pulsed ODMR. These experiments resolve the hyperfine interaction of the nuclear spin states. The code was implemented on a test sample, where it successfully identified the isotope of several NV centers. The next step in this project is to link the isotopes of NV centers to their emissive properties, with a goal of producing reliable qubits for quantum information processing circuits.   

Dynamics of Inhalation

The process of inhalation relies on the complex interplay between muscular contraction in the thorax, elasto-capillary interactions in the individual airway branches, connectivity between different branches, and overall air flow into the lungs. Sophisticated pulmonary fluid dynamics models have been developed to elaborate the competition between capillarity, which tends to keep flexible branches closed, and elasticity, which favors opening, for single airway branches. However, a quantitative model combining the physiological opening process of flexible airway branches with the biomechanics and interconnected geometry of the lungs is still missing. To address this issue, we develop a statistical model of the lungs as a symmetrically-branched network of liquid-lined flexible cylinders coupled to a viscoelastic thoracic cavity. Each branch opens at a rate and a pressure that is determined by input biomechanical parameters, enabling us to test the influence of changes in the mechanical properties of lung tissues and secretions on inhalation dynamics. By summing the dynamics of all the individual branches, we quantify the evolution of overall lung pressure and volume during inhalation, and find good agreement with typical breathing curves obtained in the literature.   

Charge Carrier Transport in Cuprous Oxide: A Puzzle

Deciphering the carrier transport mechanism in Cu₂O has been elusive, as none of the classical scattering mechanisms seems to be operative. Towards this, we study electrical properties of Cu₂O, wherein samples are prepared via thermal oxidation (TO), pulsed laser deposition (PLD), and electrodeposition. These methods provide a large range of grain sizes (100nm to 5mm), type of grain boundaries (high & low angle), and intrinsic defect concentration (10¹³ to 10¹⁸ cm⁻³). T dependent Hall measurement is used to study carrier concentration and hole mobility. We observe the presence of two acceptor levels; a normal and a split Cu vacancy for the first time experimentally. The mobility vs. T data exhibits a maximum at 200 K for polycrystalline samples, while, monotonically decreases for single crystal and textured PLD thin film in the entire T range of study (80 – 300 K). Grain boundary (GB) scattering mechanism explains the dependence of transport at T < 200 K for samples with high angle GB. We find that trap mediated scattering is dominant for single crystal and poly-crystal at T > 200 K. This suggests that instead of increasing grain size or doping, the neutralization of trap centres is the only possible way to increase the mobility and thereby, performance of Cu₂O based devices.