Bulletin of the American Physical Society
APS March Meeting 2015
Volume 60, Number 1
Monday–Friday, March 2–6, 2015; San Antonio, Texas
Session H1: Poster Session I (2:00 pm- 5:00 pm) |
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Sponsoring Units: APS Room: Exhibit Hall C |
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H1.00001: UNDERGRADUATE RESEARCH |
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H1.00002: Electromagnetic cavities as an analog to chaos regularization of quantum tunneling rates Rachel Owen, John Rodgers For double-well potentials separated by a tunneling barrier, it has been shown theoretically that quantum mechanical tunneling rates vary greatly with well geometry. Chaotic wells exhibit statistically smaller fluctuations in energy level splitting than those characterized by nonchaotic dynamics. This phenomenon (chaos regularization) can be analyzed by examining the statistical spreads in symmetric and anti-symmetric wave states produced by tunneling. Exploiting the similarity of transverse electromagnetic waves in large cavities and quantum mechanical wave functions in symmetric double-wells, chaos regularization in electromagnetic structures was studied experimentally and numerically. The resonant frequencies in rectangular (integrable) and bowtie (chaotic) cavities coupled via short sections of cutoff waveguides were simulated using finite element code and measured using a vector network analyzer. The ensemble difference in the measured anti-symmetric and symmetric resonant frequencies squared (analogous to splitting in energy levels) showed remarkably good agreement with theory. In the rectangular cavity we observed quantized resonances spaced across a wide range of frequencies whereas in the bowtie cavity the resonances were grouped randomly close to the theoretical curve. [Preview Abstract] |
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H1.00003: Quantifying the Relationship between Surface Hydrophobicity and Depletion Layer Thickness Jared Nutter, Shannon Petersen, Ryan Sayko, Adele Poynor When water comes into contact with an extended hydrophobic surface a uniform region of low density forms, called the depletion layer. This phenomenon has only been experimentally verified on surfaces with contact angles \textgreater 100$^{\circ}$, but understanding how the thickness of the depletion layer changes with the hydrophobicity at intermediate contact angles is one of the underlying mechanisms behind several biological systems including colloidal self-assembly, protein folding, and fluid flow across membranes. We aim to quantify this relationship by using self-assembled monolayers of 1-octadecanthiol and 11-mercaptoundecanoic acid on gold to produce surfaces with contact angles between 55$^{\circ}$ and 107$^{\circ}$. We then use surface plasmon resonance spectroscopy to determine the thickness of the depletion layer formed for each surface. [Preview Abstract] |
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H1.00004: Dependence of the Contact Angle on Self-Assembled Monolayer Production Method Brooke Ollander, Ryan Sayko, Jared Nutter, Shannon Petersen, Adele Poynor When water is forced in contact with a hydrophobic surface, it attempts to reduce its contact by forming a depletion layer. A depletion layer is defined as a nanometer scale low density region of water molecules at the surface. To alter the hydrophobicity of the slide, self-assembled monolayers (SAMs) are formed by utilizing the following organothiol solutions: 11-mercaptoundecanoic acid (hydrophilic) and 1-octadecanethiol (hydrophobic). The contact angle of slides with different organothiol solution exposure times is measured using a homemade goniometer and ImageJ software. [Preview Abstract] |
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H1.00005: Flow Rate In Microfluidic Pumps As A Function Of Tension and Pump Motor Head Speed Anthony Irwin, Krista McBride As the use of microfluidic devices has become more common in recent years the need for standardization within the pump systems has grown. The pumps are ball bearing rotor microfluidic pumps and work off the idea of peristalsis. The rapid contraction and relaxation propagating down a tube or a microfluidic channel. The ball bearings compress the tube (occlusion) and move along part of the tube length forcing fluid to move inside of the tube in the same direction of the ball bearings. When the ball bearing rolls off the area occupied by the microfluidic channel, its walls and ceiling undergo restitution and a pocket of low pressure is briefly formed pulling more of the liquid into the pump system. Before looking to standardize the pump systems it must be known how the tension placed by the pumps bearing heads onto the PDMS inserts channels affect the pumps performance (mainly the flow rate produced). The relationship of the speed at which the bearings on the motor head spin and the flow rate must also be established. This research produced calibration curves for flow rate vs. tension and rpm. These calibration curves allow the devices to be set to optimal user settings by simply varying either the motor head tension or the motor head speed. [Preview Abstract] |
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H1.00006: Photoemission spectroscopy and X-ray diffraction analysis of 3D topological and Kondo insulators Pavel Shibayev The advantage of studying 3D topological insulators (TIs), compounds that have attracted the attention of many in the condensed matter field, is the ability for their existence at room temperature and no magnetic fields, allowing both for resolving their band structure via angle-resolved photoemission spectroscopy (ARPES) and understanding electrical transport and other properties via X-ray diffraction (XRD) and point-contact spectroscopy (PCS). A comprehensive quantitative analysis of Bi$_{\mathrm{2}}$Se$_{\mathrm{3}}$, a 3D TI, was carried out using these methods. The Bi$_{\mathrm{2}}$Se$_{\mathrm{3\thinspace }}$crystals were synthesized in-house at Princeton University. A first-principles calculation based on density functional theory, DFT, was performed using the Abinit software. The band structure of the crystal was then resolved via ARPES at the Advanced Light Source in LBNL, resulting in a surprisingly stark and clear single Dirac cone. A large band gap was confirmed, suggesting an increased potential for applications. In contrast, Kondo insulators are found in rare-earth based materials with f-electron degrees of freedom. Photon energy dependent dispersion relationships and temperature dependence studies were performed on a Kondo candidate CeB$_{\mathrm{6}}$ via ARPES, showing an even number of Dirac cones and a non-TI behavior. Analysis of I-V characteristics through PCS will follow, in addition to characterization via Bruker XRD for both compounds. [Preview Abstract] |
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H1.00007: Structure Property Relationships in Imidazole-based Deep Eutectic Mixtures Logan Terheggen, Tyler Cosby, Joshua Sangoro Deep eutectic mixtures of levulinic acid with a systematic series of imidazoles are measured by broadband dielectric spectroscopy, differential scanning calorimetry, and Fourier transform infrared spectroscopy to investigate the impact of steric interactions on charge transport and structural dynamics. An enhancement of dc conductivity is found in each of the imidazoles upon the addition of levulinic acid. However, the extent of increase is dependent upon the alkyl substitution on the imidazole ring. These results highlight the importance of molecular structure on hydrogen bonding and charge transport in deep eutectic mixtures. [Preview Abstract] |
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H1.00008: Laser Induced Breakdown Spectroscopy of Metals Andria Palmer, Carlos Lawhead, Laszlo Ujj Laser Induced Breakdown Spectroscopy (LIBS) is a very practical spectroscopy to determine the chemical composition of materials. Recent technical developments resulted in equipment used on the MARS Rover by NASA. It is capable of measuring the emission spectra of laser induced plasma created by energetic laser pulses focused on the sample (rocks, metals, etc.). We have develop a Laser Induced Breakdown Spectroscopy setup and investigated the necessary experimental and methodological challenges needed to make such material identification measurements. 355 and 532 nm laser pulses with 5 ns temporal duration was used to generate micro-plasma from which compositions can be determined based on known elemental and molecular emission intensities and wavelengths. The performance of LIBS depends on several parameters including laser wavelength, pulse energy, pulse duration, time interval of observation, geometrical configuration of collecting optics, and the properties of ambient medium. Spectra recorded from alloys (e.g. US penny coin) and pure metals will be presented. [Preview Abstract] |
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H1.00009: Differential Conductance Measurements of MgB$_{2}$/I/Pb Heterojunctions and all-MgB$_{2}$ Junctions David Cusick, Matthew Eckhardt, Wenqing Dai, Qi Li, Ke Chen, Daniel Cunnane, C.G. Zhuang, X.X. Xi, Michio Naito, Roberto Ramos We present our work characterizing several types of Magnesium Diboride Josephson junctions, including MgB$_{2}$/I/Pb heterojunctions and all-MgB$_{2}$ junctions. We will report on the I-V and dI/dV-V data collected at various temperatures using both a cryocooler-based experimental platform between 2 and 20 Kelvin and using a $^{3}$He probe platform between 0.3 and 1.0 Kelvin. These were both developed by undergraduates in a liberal arts university. Using high-sampling rates with a 24-bit data acquisition card and access to a broad of range of temperatures, we track and report energy gap distributions and temperature-dependent features of dI/dV peaks of MgB$_{2}$, comparing these with theoretical predictions. [Preview Abstract] |
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H1.00010: Volume phase transition and corresponding change in composition of polymeric microgels. Janna Mino, Christian Gunder, Kiril Streletzky Temperature sensitive polysaccharide microgels and parent amphiphilic polymer solution were studied in parallel with Dynamic (DLS) and Static Light Scattering (SLS) spectroscopies. The microgels showed a reversible volume phase transition which was accompanied by a significant change in microgel volume and composition. Coupling DLS and SLS techniques on microgels and polymer solutions allowed to deduce microgel size, structure, molecular weight and a relative change in microgel water content during the volume phase transition. It also allowed comparing controlled dewetting transition in microgels with corresponding phase transition in parent polymer solution. Light scattering findings were tested with SEM imaging. [Preview Abstract] |
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H1.00011: Synthesis and Characterization of Tetrathiafulvalene Derivatives Mark Bartolo, Stephen Tsui, Eric Reinheimer We synthesize tetrathiafulvalene (TTF) derivative materials in an effort to identify conducting and magnetic properties. The doping of these TTF derivatives include tetracyanobenze (TCNB), 7,7,8,8-tetracyanoquinodimethane (TCNQ), and para-dinitrobenzene (pDNB). We examine these TTF containing donor-acceptor complexes through their structural and electronic transport characteristics. [Preview Abstract] |
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H1.00012: Determination of stimulation focality in heterogeneous head models during transcranial magnetic stimulation (TMS) Erik Lee, Ravi Hadimani, David Jiles Transcranial Magnetic Stimulation (TMS) is an increasingly popular tool used by both the scientific and medical community to understand and treat the brain. TMS has the potential to help people with a wide range of diseases such as Parkinson's, Alzheimer's, and PTSD, while currently being used to treat people with chronic, drug-resistant depression. Through computer simulations, we are able to see the electric field that TMS induces in anatomical human models, but there is no measure to quantify this electric field in a way that relates to a specific patient undergoing TMS therapy. We propose a way to quantify the focality of the induced electric field in a heterogeneous head model during TMS by relating the surface area of the brain being stimulated to the total volume of the brain being stimulated. This figure would be obtained by conducting finite element analysis (FEA) simulations of TMS therapy on a patient specific head model. Using this figure to assist in TMS therapy will allow clinicians and researchers to more accurately stimulate the desired region of a patient's brain and be more equipped to do comparative studies on the effects of TMS across different patients. [Preview Abstract] |
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H1.00013: Electronic Resonance Enhancement in Raman and CARS Spectroscopy: Surface Enhanced Scattering of Highly Fluorescent Molecules Carlos Lawhead, Laszlo Ujj Surface enhanced Raman spectroscopy (SERS) is an extremely useful tool in increasing sensitivity of Raman spectroscopy; this technique significantly increases the signal from vibrational resonances which can overcome background fluoresces. Silver nanoparticles coated substrates and the silver nanoparticles in solution were used on a variety of fluorescent molecules in order to overcome sample complexities and measure the vibrational spectra. The possible enhancement of SERS using a coherent Raman (CARS) method was investigated, but enhancement factors due to Surface Enhanced CARS have yet to be verified. The instrument used was developed in the University of West Florida Physics Department utilized the second harmonic of a Nd:YAG laser to provide the excitation wavelength at 532 nm and is capable of both transmission and reflection Raman measurements. [Preview Abstract] |
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H1.00014: Fluorescence and Bonding of Quantum Dots on DNA Origami Constructs Matthew Kessinger, Timothy Corrigan, David Neff, Michael Norton Semiconductor quantum dots (QDots) have historically been of interest to the scientific community since their creation for various applications ranging from solar energy to optical labeling. In this study, bioconjugated CdSe/ZnS core/shell QDots were synthesized and functionalized with 3-mercaptopropionic acid using both traditional ligand exchange as well as newly developed \textit{in situ} functionalization techniques used to increase the quantum yield of the QDots. Their fluorescence and bonding to both gold as well as DNA origami were investigated for use in self assembled DNA constructs. It is believed that controlling the attachment and spacing of these nanoparticles on DNA origami could be used in a variety of optical labeling and sensing applications. Commercially available biotin and streptavidin functionalized quantum dots were also examined, and subject to the same experiments with gold nanoparticles as the MPA functionalized QDots. [Preview Abstract] |
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H1.00015: Analysis of layer-by-layer thin-film oxide growth using RHEED and Atomic Force Microscopy Eli Adler, M.C. Sullivan, Araceli Gutierrez-Llorente, H. Joress, A. Woll, J.D. Brock Reflection high energy electron diffraction (RHEED) is commonly used as an \textit{in situ} analysis tool for layer-by-layer thin-film growth. Atomic force microscopy is an equally common \textit{ex situ} tool for analysis of the film surface, providing visual evidence of the surface morphology. During growth, the RHEED intensity oscillates as the film surface changes in roughness. It is often assumed that the maxima of the RHEED oscillations signify a complete layer, however, the oscillations in oxide systems can be misleading. Thus, using only the RHEED maxima is insufficient. X-ray reflectivity can also be used to analyze growth, as the intensity oscillates in phase with the smoothness of the surface. Using x-ray reflectivity to determine the thin film layer deposition, we grew three films where the x-ray and RHEED oscillations were nearly exactly out of phase and halted deposition at different points in the growth. Pre-growth and post-growth AFM images emphasize the fact that the maxima in RHEED are not a justification for determining layer completion. [Preview Abstract] |
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H1.00016: Developing a protocol for creating microfluidic devices with a 3D printer, PDMS, and glass Robyn Collette, Eric Novak, Kathryn Shirk Microfluidics research requires the design and fabrication of devices that have the ability to manipulate small volumes of fluid, typically ranging from microliters to picoliters. These devices are used for a wide range of applications including the assembly of materials and testing of biological samples. Many methods have been previously developed to create microfluidic devices, including traditional nanolithography techniques. However, these traditional techniques are cost-prohibitive for many small-scale laboratories. This research explores a relatively low-cost technique using a 3D printed master, which is used as a template for the fabrication of polydimethylsiloxane (PDMS) microfluidic devices. The masters are designed using computer aided design (CAD) software and can be printed and modified relatively quickly. We have developed a protocol for creating simple microfluidic devices using a 3D printer and PDMS adhered to glass. This relatively simple and lower-cost technique can now be scaled to more complicated device designs and applications. [Preview Abstract] |
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H1.00017: Highly Porous Regenerated Cellulose Fiber Mats via the Co-Forcespinning of Cellulose Acetate for Separator Applications Alejandro Castillo, Yuanbing Mao Improvements in battery technology are necessary for the transition away from a fossil fuel based economy. An important bottle-neck in battery efficiency is the quality of the separator, which separates the cathode and anode to prevent a short-circuit while still allowing the ions in solution to flow as close to unabated as possible. In this work solutions of cellulose acetate, polyvinyldiflourine (pvdf), and polyvinylpyrrolidone (pvp) dissolved in a 2:1 v/v acetone/dimethylacetamide ~~solvent mixture were Forcespun to create nonwoven fiber mats of nanoscale diameter. These mats were then soaked in a NaOH solution so as to both strip the pvp from the fiber as well as regenerate cellulose from its acetate derivative for the purpose of creating ~high surface area, nanoporous, hydrophilic, and ioniclly conductive cellulose/pvdf nonwoven mats for the purposes of testing their suitability as battery separators [Preview Abstract] |
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H1.00018: IR spectroscopy of an OH hindered rotor in ZnO Ellen Farmer, Philip Weiser, Michael Stavola, W. Beall Fowler An infrared absorption band at 3326 cm$^{\mathrm{-1}}$ has been assigned to H$^{\mathrm{+}}$ in an antibonding configuration in the vicinity of another defect, perhaps Ca [1,2]. The 3326 cm$^{\mathrm{-1}}$ band has a distinctive dependence on temperature, consisting of several overlapping components whose intensities show thermally activated behavior [3] over the temperature range 4K to 50K. We have measured a series of high-resolution spectra for OH and OD to determine the energy level structure of the 3326 cm$^{\mathrm{-1}}$ center and its dependence on hydrogen isotope. The results of our analysis suggest an OH hindered rotor that is reminiscent of previous results for an off-axis OD-Li center in MgO [4]. * Permanent address: Truman State University [1] S.J. Jokela and M.D. McCluskey, Phys. Rev. B \textbf{72}, 113201 (2005). [2] M.D. McCluskey and S.J. Jokela, Physica B \textbf{401-402}, 355 (2007). [3] F. Herklotz \textit{et al.}, Phys. Rev. B \textbf{82}, 115206 (2010). [4] K. Martin \textit{et al.}, Phys. Rev. B \textbf{75}, 245211 (2007). [Preview Abstract] |
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H1.00019: Temperature dependence measurements for Cadmium Telluride (CdTe) solar cells Fernanda Duarte, Weining Wang Traditional silicon (Si)-based solar cells have been studied broadly and have already reached their maximum efficiency. However, their cost is relatively high, preventing them from being widely used. Unlike Si-based solar cells, Cadmium Telluride (CdTe) solar cells are considerably cheap, yet the record efficiency is still lower than that of traditional silicon-based solar cells. More studies are needed to understand and improve the efficiency of CdTe solar cells. In this work, we report our studies of the temperature dependence of CdTe solar cell parameters using the temperature-varying apparatus designed and built by us in-house. This temperature-varying apparatus will be incorporated with a solar cell testing station in order to measure the solar cell parameters while varying the temperature. The solar cell parameters will be measured at different temperatures (with a 100 K temperature range), and the effects of temperature on the open-circuit voltage, short-circuit current and efficiency of the solar cells will be reported. These results allow us to further understand the physics of CdTe solar cells and shine light on how to improve the efficiency of CdTe solar cells. [Preview Abstract] |
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H1.00020: Systematic Studies of Phase Transitions in Thermo-Responsive Polymers Used in Targeted Drug Delivery Janae Bradley, Daniel Denmark, Sarath Witanachchi Thermo-responsive polymers such as poly-N-isopropylacrylamide (PNIPAM) can undergo reversible phase transitions in aqueous solutions under varying temperatures. They are ideal candidates for the polymer shell of a targeted drug delivery capsule. Concentration and pH can affect the lower critical solution temperature (LCST) of the PNIPAM polymer and its physical properties. In this work, a systematic study of the factors that influence the LCST of the PNIPAM polymer mixed with Fe3O4 nanoparticles (MNPs) during thermal bath heating is presented. A series of PNIPAM solutions with varying concentrations of PNIPAM with MNPs were prepared and characterized using scanning electron microscopy. In-situ transmission measurements were used to determine the LCST of PNIPAM concentrations. A systematic decrease in the LCST was observed as the concentration of PNIPAM was increased. In addition, the impact of pH on the LCST of PNIPAM was examined by increasing the basicity of the PNIPAM solutions by adding adjusted KOH pellets. An increase in the thermal stability of the LCST was observed when the basicity of the PNIPAM solution was increased. The results from this study provide valuable information towards using these thermo-responsive polymers in targeted drug delivery. [Preview Abstract] |
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H1.00021: Superconducting Resonators: Protecting Schrodinger's Cat Jose Chavez, Philip Mauskopf Over the past decade, superconducting resonators have played a fundamental role in various novel astronomical detectors and quantum information processors. One example is the microwave kinetic inductance detector that is able to resolve photon energies by measuring shifts in its resonant frequency. Similar resonators have been integrated with superconducting qubits, specifically the transmon, to substantially improve quantum coherence times. The purpose of this investigation is to survey various resonant structures within the requirements of circuit quantum electrodynamics giving special attention to quality factors, TLS noise, and quasi-particle generation. Specifically, planar and three dimensional cavities with varying geometries and materials are characterized - primarily focusing on NbTiN and Nb. [Preview Abstract] |
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H1.00022: Capacitance measurements of defects in solar cells: checking the model assumptions Justin Davis, Thaddeus Cox, Jennifer Heath Capacitance measurements of solar cells are able to detect minute changes in charge in the material. For that reason, capacitance is used in many methods to electrically characterize defects in the solar cell. Standard interpretations of capacitance rely on many assumptions, which, if wrong can skew the results. We explore possible alternate explanations for capacitance transitions, which may not be linked directly to defects, such as a non-ideal back contact, and series resistance. [Preview Abstract] |
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H1.00023: Does a simple lattice protein exhibit self-organized criticality? Dana Gibbon, Alissa Runyon, Arun Bajracharya, Joelle Murray There are many unanswered questions when it comes to protein folding. These questions are interesting because the tertiary structure of proteins determines its functionality in living organisms. How do proteins consistently reach their final tertiary structure from the primary sequence of amino acids? What explains the complexity of tertiary structures? Our research uses a simple hydrophobic-polar lattice-bound computational model to investigate self-organized criticality as a possible mechanism for generating complexity in protein folding and protein tertiary structures. [Preview Abstract] |
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H1.00024: Development of TiO$_{2}$-Xwt{\%}InVO$_{4}$ Photocatalytic Nano-composites for Ambient Light Assisted Water Detoxification Sesha Srinivasan, Jeremiah Wilson, Eric Vickers, Ryan Integlia We have developed nano-composites of TiO$_{2}$-Xwt{\%}InVO$_{4}$ for environmental and biomedical research applications. TiO$_{2}$ is commonly used as catalyst that utilizes the UV portion of the sun light spectrum to induce photo-oxidation and photo-reduction processes. We hypothesized that the combination of InVO$_{4}$ and TiO$_{2}$ will result in a material that will catalyze organic contaminants through photo-oxidation under visible light. We combined TiO$_{2}$ with 2,4,6,8,10wt{\%} of InVO$_{4}$ via wet ball milling process. We have compared the various concentrations of InVO$_{4}$ on TiO$_{2}$ matrix by SEM, BET surface area analyzer, FTIR, XRD, and photodegradation of the organic contaminant Methyl Orange. After characterization we found that 4wt{\%} InVO$_{4}+$TiO$_{2}$ mixture displayed the most promising characteristics for photo-oxidation under visible light; From the BET surface area analysis it showed the largest surface area out of the prior mentioned TiO$_{2}=$ Xwt{\%} InVO$_{4}$ mixtures and a degradation amount equivalent to 50{\%} of Methyl Orange contaminant over 7 hours under visible light. In conclusion, TiO$_{2}$-Xwt{\%}InVO$_{4}$ displayed evidence of photo-oxidation under visible light conditions. [Preview Abstract] |
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H1.00025: Toward a Rb MOT for Undergrad Research and Advanced labs at Bridgewater State University Edward Deveney The seminal paper for the undergraduate MOT appeared in AJP (\textbf{63} (4), 1995) by C. Wieman, G. Flowers and S. Gilbert; `Inexpensive laser cooling and trapping experiment for undergraduate laboratories'. They wrote: ``Because of this visual appeal and the current research excitement in this area, we felt that it was highly desirable to develop an atom trapping apparatus that could be incorporated into the undergraduate laboratory classes.'' From our observations, it seems that while there are extraordinary examples of MOTs thriving in a few undergraduate labs, MOT experiments have yet to be widely incorporated into the undergraduate curriculum - likely because they are, in fact, not trivial to make. With the benefit of 20 years evolution since this 1st undergraduate MOT paper, we report the progress at BSU of constructing a $^{\mathrm{85}}$Rb MOT that incorporates significant simplifications and straightforward techniques that include: using a single ECDL laser for both trapping and re-pumping (using an EOM to add FM sidebands) and combining a purchased stabilized HeNe with the ECDL in a Fabry-Perot Interferometer to correct and sufficiently stabilize the ECDL for trapping. When completed we will revisit the question of do-ability for the undergraduate research/advanced lab. The BSU MOT was planned with and is currently being built with the help and guidance of David DeMille and his research group at Yale University [including J. Barry Thesis, Yale]. [Preview Abstract] |
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H1.00026: Optical V-Band Observations of Active Galactic Nuclei Taylor Hutchison, Raina Musso, Francis MacInnis, Mark Bottorff Students at Southwestern University participated in an international observing campaign to study twelve active galactic nuclei (AGN). As part of this project, the students measured optical V-band light variations of four targets within the range of the Southwestern University Fountainwood Observatory research telescope. Target images and a sample light curve of one target (NGC 5548) are presented. [Preview Abstract] |
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H1.00027: ABSTRACT WITHDRAWN |
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H1.00028: ABSTRACT WITHDRAWN |
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H1.00029: ABSTRACT WITHDRAWN |
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H1.00030: Origins of Nonlinearity in Superconductive Passive Circuits Sean Hamilton, Stephen Remillard The distinct origins of even and odd order nonlinear behavior in type II cuprate superconductors have yet to be fully elucidated. Microwave intermodulation distortion (IMD) was examined in a YBCO superconducting thin-film hairpin resonator at 840 MHz. Measurements of the temperature dependence of IMD near T$_{\mathrm{C\thinspace }}$support the view that the nonlinear Meissner effect is responsible for the occurrence of both 2$^{\mathrm{nd}}$ and 3$^{\mathrm{rd}}$ order IMD tones near T$_{\mathrm{C\thinspace }}$as well as their suppression in an applied magnetic field. However, at lower reduced temperatures (T/ T$_{\mathrm{C}}$ less than 0.95), where the influence of the nonlinear Meissner effect is less pronounced, 3$^{\mathrm{rd}}$ order IMD is unaffected by a static magnetic field, while 2$^{\mathrm{nd}}$ order IMD decays exponentially after a static magnetic field is removed with a temperature dependent time constant. It is apparent that the magnetically induced remnant vortex state contributes to the 2$^{\mathrm{nd}}$ order nonlinearity, but not to the 3$^{\mathrm{rd}}$ order nonlinearity, and that this effect is diminished close to T$_{\mathrm{C\thinspace }}$due to degradation of the remnant vortex state. [Preview Abstract] |
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H1.00031: Measuring and Modeling the Plasma Temperature and Density in WIRX Michael Morken, Darren Craig We develop a theoretical model and experimental techniques to provide a better picture of how the adjustable parameters such as the current, and electrode geometry affect the temperature and density of WIRX plasmas. Our model predicts the plasma temperature and density by balancing the Ohmic heating with convective losses. The Ohmic heating is measured directly as the product of the voltage drop between the electrodes and the plasma current. Temperature and density are measured independently using spectroscopic methods. Stark broadening of the H-Beta line is used to measure density. To measure the electron temperature of the plasma the line ratios of various hydrogen transitions were compared with the predictions of the Boltzmann thermal equilibrium model and the coronal equilibrium model. Preliminary experimental results are consistent with the plasma parameters predicted by our model. This work will be used to inform future modifications to the experiment with the intent of producing higher temperatures in WIRX plasma, making magnetic reconnection more probable. Work supported by US DOE. [Preview Abstract] |
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H1.00032: Two Dimensional Intermodulation Distortion Scanning of Superconducting Filter Resonators Michael Bischak, Stephen Remillard Nonlinear superconducting conductivity produces distortion that has usually been measured globally across the entire sample. In order to fully understand the origin of non linearity, local methods must be used to examine specific points in the sample. The nonlinear Ohm's law, V$=$IZ(I) includes the current dependence in the impedance. The method in this work raster scans a magnetic loop probe across a sample. In order to address limited resolution, we reduced the size of the magnetic loop probe. Using the electromagnetic field solver, sonnet, two dimensional current simulations of superconducting microwave filters composed of Tl2Ba2CaCu2O8 or of YBa2Cu3O7 reveal microwave current which is bunched up at the corners and sides of the sample. Two dimensional images of third order intermodulation distortion made with the magnetic probe at the same corners and edges reveal elevated distortion in the same places. Using the magnetic probe, third order intermodulation was seen to come from the same corners and edges where the current is bunched. This research was funded by the National Science Foundation under grant number DMR-1206149. [Preview Abstract] |
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H1.00033: Raman spectroscopy of single-walled carbon nanotubes of different lengths exposed to microwave radiation P. Bhatnagar, S. Ferguson, G. Sestric, I. Wright, S. Williams Carbon nanotubes have been observed to emit ultraviolet, visible, and infrared radiation when exposed to microwave fields. Although there is considerable controversy concerning the mechanism responsible for the emissions, the results of recently-performed experiments suggest that the emissions may be the result of field emission-induced luminescence. We have performed experiments in which both short (0.5 $\mu $m - 2 $\mu $m) and long (5 $\mu $m - 30 $\mu $m) single and double-walled carbon nanotubes were exposed to 2.46 GHz microwaves at a pressure of approximately 10$^{-6}$ torr. A comparison of the spectra of the radiation emitted from the nanotubes suggests that the longer nanotubes emitted radiation of greater intensity than the shorter nanotubes, which is consistent with field emission-induced luminescence. Moreover, structural modification of the carbon nanotubes due to microwave irradiation has been studied using the Raman spectroscopy G-band and D-band intensities, which suggests that microwave irradiation at relatively low pressures results in a decrease in nanotube defects, especially in the case of the long nanotube samples. [Preview Abstract] |
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H1.00034: Study of the angular distributions of X-rays emitted following L$_{3}$ ionization of gold atoms by electron impact I. Wright, G. Sestric, S. Ferguson, S. Williams Theoretical work suggests that when an atomic inner-shell vacancy with total angular momentum j greater than 1/2 is created by interaction with a photon or charged particle the vacancy will be aligned due to the magnetic sublevels of the ion having nonstatistical populations. The experiments we performed, testing this theory, involved measurements of the angular distributions of gold L$_{\alpha}$, L$_{\beta}$, and L$_{l}$ X-rays at forward angles in the range 0 degrees to 25 degrees emitted after being bombarded with 15-keV electrons. After corrections for absorption of the characteristic X-rays within the gold target, our results suggest that the angular distributions of the L$_{\alpha}$ and L$_{\beta}$ X-rays are essentially isotropic, as no angular dependence was observed in our data outside of experimental uncertainties. However, the results of our experiments suggest that the angular distribution of the gold L$_{l}$ X-rays may be weakly anisotropic. [Preview Abstract] |
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H1.00035: A Study of Two Dimensional Electron Gas Using 2D Fourier Transform Spectroscopy Carl McIntyre, Jagannath Paul, Denis Karaiskaj The dephasing of FES was measured in a symmetrically modulation doped 12 nm single quantum well GaAs/AlGaAs two dimensional electron gas system using time integrated four wave mixing (TIFWM) and a two dimensional Fourier transform spectroscopy (2DFTS). At high in-well carrier densities of $\sim $4 x 10$^{11}$ cm$^{-2}$, many body effects that are prevalent and measurable with non-linear optical spectroscopy. Effects of exciton-exciton and exciton-phonon scattering events, exciton populations, and biexciton formation are detectable at these carrier concentrations. Homogeneous linewidths obtained from 2DFT and TIFWM yield a zero Kelvin linewidth of 1.42 meV and an acoustic phonon scattering coefficient of 158 $\mu$ eV/K. These observations indicate a rapid increase in homogeneous linewidth with increased temperature. [Preview Abstract] |
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H1.00036: Analytically Evaluating Sums in Quantum and Statistical Physics Using Integral Transforms John Vastola Evaluating sums analytically is a problem that is easy to pose and to give approximate solutions to, but that is difficult to exactly solve in general. Many results that are known are byproducts of Fourier analysis, which requires guessing that a series corresponds to a function. A more systematic method of evaluating sums using integral transforms is proposed which can reproduce many results obtained using other techniques. In particular, representing polynomials as Laplace transforms gives some nontrivial exact results. Some applications of the method are demonstrated, and extensions of the method using integral representations of frequently appearing functions are suggested. One useful representation of the gamma function is supplied, and used to provide both well-known and more obscure results. Interestingly, the application of this integral representation to evaluating sums suggests the introduction of a novel integral transform, which itself can be used to evaluate sums. Some of the transform's properties are given, and its usefulness in other areas (like solving differential equations) is touched upon. Some physical problems involving the partition functions of statistical mechanics, and some infinite sums appearing in quantum mechanics, are considered. [Preview Abstract] |
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H1.00037: In-air Rutherford Backscattering and Particle Induced X-ray Emission for Biophysics and Material Science Research James Becker Rutherford Backscattering (RBS) and Particle Induced X-ray Emission (PIXE) are methods of nondestructive analysis of elemental composition. Rebounding particles or emitted x-rays can be ``collected'' and then analyzed to reveal the number ratio of the elements in a sample. Due to the nondestructive feature of these processes, RBS and PIXE are useful in many diverse fields of study such as archaeology, art, and biology; however, these experiments usually require large, expensive particle accelerators and detectors. Instead, I am attempting to use a radioactive source, photodiodes, and computer software to perform the same methods at a fraction of the cost. I am exploring cost, time, and resolution benefits and losses of my approach versus the traditional accelerator-based approach. [Preview Abstract] |
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H1.00038: Relativistic Quantum Mechanical Calculations on Alkali Atoms and Dimers from Cesium to Ununennium Chukwunonso Arinze, Walter Ermler Ab initio calculations using relativistic effective core potentials, and intermediate angular momentum coupling of electrons are carried out on the alkali metal atoms, and dimers from cesium through ununennium. A spin-orbit configuration interaction (SOCI) method is employed that includes a spin-orbit coupling operator and a relativistic effective core potential in the Schrodinger Hamiltonian operator. The energy levels from these calculation are found to reproduce the positions of the experimental spectral lines and predict lines not heretofore observed for both of these atoms. [Preview Abstract] |
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H1.00039: Re-Examining Gravitational Tunneling Radiation when taking into account Quantum Gravity Effects John Valentine, Trevor Prescott, Gardo Blado Although shown to theoretically exist, Hawking Radiation has yet to be detected. The paper entitled ``Gravitational Tunneling Radiation'' [1] by Mario Rabinowitz proposed a possible explanation by considering the gravitational tunneling effects in the presence of other bodies in the vicinity of the black hole. Rabinowitz showed that the power radiated (through gravitational radiation) by a black hole,$P_{R} $, is related to the power generated by Hawking Radiation, $P_{SH} $ by $\frac{P_{R} }{T}\sim P_{SH} $ where $T$ is the gravitational tunneling probability. The presence of other bodies lowers the gravitational barrier which in turn increases the gravitational tunneling probability thereby decreasing the Hawking radiation, $P_{SH} $. In this paper, we examine the modification of $T$ in the presence of quantum gravity effects by incorporating the Generalized Uncertainty Principle. \\[4pt] [1] M Rabinowitz, ``Gravitational Tunneling Radiation'' Phys Essays, 12 (1999), 346-357 arXiv:astro-ph/0212249 [Preview Abstract] |
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H1.00040: A Novel Method of Line Detection using Image Integration Method Daniel Lin, Bo Sun We developed a novel line detection algorithm based on image integration method. Hough Transformation uses spatial image gradient method to detect lines on an image. This is problematic because if the image has a region of high noise intensity, the gradient would point towards the noisy region . Denoising the noisy image requires an application of sophisticated noise reduction algorithm which increases computation complexity. Our algorithm can remedy this problem by averaging the pixels around the image region of interest. We were able to detect collagen fiber lines on an image produced by confocal microscope. [Preview Abstract] |
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H1.00041: Exploring the Power Output of Small Wind Turbines in Urban San Antonio, Texas Jose Casillas, Stephanie Sperduti, Rosa Cardenas The means of transporting power from a centralized power plant by transmission lines has several disadvantages. Electricity transmission and distribution networks are costly, require long planning processes and are unsightly to residents. These networks are also susceptible to natural disasters creating massive disruptions to consumers. For these reasons distributed power sources such as solar panels and small wind turbines are becoming a more desirable and viable means of energy production. We report on the status of a study to determine the maximum output power of small wind turbines in urban San Antonio, Texas. Wind speed data along with power measurements from small wind turbines in urban San Antonio will be reported. [Preview Abstract] |
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H1.00042: Analyzing Hurricane Sandy Angelyn Convertino, Stephan Meyer, Becca Edwards Post-tropical Storm Sandy underwent extratropical transition shortly before making landfall in southern New Jersey October 29 2012. Data from this system was compared with data from Hurricane Ike (2008) which represents a classic hurricane with a clear eye wall and symmetry after landfall. Storm Sandy collided with a low pressure system coming in from the north as the hurricane made landfall on the US East coast. This contributed to Storm Sandy acting as a non-typical hurricane when it made landfall. Time histories of wind speed and wind direction were generated from data provided by Texas Tech's StickNet probes for both storms. The NOAA Weather and Climate program were used to generate radar loops of reflectivity during the landfall for both storms; these loops were compared with time histories for both Ike and Sandy to identify a relationship between time series data and storm-scale features identified on radar. [Preview Abstract] |
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H1.00043: Using budget-friendly methods to analyze sport specific movements Lindsay Jackson, Sarah Williams, Davon Ferrara When breaking down the physics behind sport specific movements, athletes, usually professional, are often assessed in multimillion-dollar laboratories and facilities. Budget-friendly methods, such as video analysis using low-cost cameras, iPhone sensors, or inexpensive force sensors can make this process more accessible to amateur athletes, which in-turn can give insight into injury mechanisms. Here we present a comparison of two methods of determining the forces experienced by a cheerleader during co-education stunting and soccer goalies while side-diving. For the cheerleader, accelerometer measurements were taken by an iPhone 5 and compared to video analysis. The measurements done on the soccer players were taken using FlexiForce force sensors and again compared to video analysis. While these budget-friendly methods could use some refining, they show promise for producing usable measurements for possibly increasing our understanding of injury in amateur players. Furthermore, low-cost physics experiments with sports can foster an active learning environment for students with minimum physics and mathematical background. [Preview Abstract] |
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H1.00044: Porphyrin Induced Laser Deactivation of Trypsinogen-Trypsin Conversion Joanna Perido, Lorenzo Brancaleon Pancreatitis is caused by the inflammation of the pancreas, where the digestive enzyme trypsin is activated from the precursor enzyme trypsinogen while still in the pancreas. The presence of trypsin in the pancreas causes auto-activation of trypsinogen, resulting in greater inflammation and auto-digestion of the pancreas. In severe cases, this cascade effect can lead to organ failure, diabetes, and pancreatic cancer. Our hypothesis is that if trypsinogen is prevented from auto-activating into trypsin, then this cascade can be stopped. We propose to do this by inducing conformational changes in trypsinogen when bound to a photoactive porphyrin dye. Porphyrins are comprised of four linked heterocyclic groups forming a flat ring, and bind well with proteins such as trypsinogen. In this study we used spectroscopic techniques to probe the binding of meso-tetrakis (4-sulfonatephenyl) porphyrin (TSPP) to trypsinogen in vitro, as a preliminary step to then prompt and characterize conformational changes of trypsinogen through irradiation. If conformational changes are detected the trypsinogen will be tested for trypsin inactivation. This investigation may provide promising initial results to the possible use of porphyrins as an inhibitor of the self-activation of trypsinogen into trypsin, and a potential inhibitor of pancreatitis. [Preview Abstract] |
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H1.00045: Synthesizing new, high-temperature superconductors Claire Weaver, Meigan Aronson Currently, there is no accepted theory behind type-II, high-temperature superconductors, but there is a distinct relationship between anti-ferromagnetism and superconductivity. Our research focuses on synthesizing new superconducting materials by observing the link between atomic structure and magnetic moments of anti-ferromagnetic compounds and attempting to reproduce the molecular physics of these known materials in new compounds. Consider the square-planar arrangement of the transition metal Fe in the Fe-pnictide superconductors of the ZrCuSiAs ``11 11'' and the ThCr$_{\mathrm{2}}$Si$_{\mathrm{2}}$ ``122'' structure types. We believe that the physics behind this superconductor, where Fe has d$^{\mathrm{6}}$ valence electrons, contributes to the superconducting state, not the presence of Fe itself. For this reason, we are synthesizing materials containing neighboring transition metals, like Mn and Co, combined with other elements in similar crystal lattice arrangements, having ionization properties that hopefully impose d$^{\mathrm{6}}$ valence electrons on the transition metals. [Preview Abstract] |
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H1.00046: Newton-Cartan Gravity in Noninertial Reference Frames Leo Rodriguez, James St. Germaine-Fuller, Sujeev Wickramasekara We study Newton-Cartan gravity under transformations into all noninertial, nonrelativistic reference frames. These transformations form an infinite dimensional Lie group, called the Galilean line group, which contains as a subgroup the Galilei group. The fictitious forces of noninertial reference frames are encoded in the Cartan connection transformed under the Galilean line group. These fictitious forces, which are coordinate effects, do not contribute to the Ricci tensor. Only the $00$-component of the Ricci tensor is non-zero and equals ($4\pi$ times) the matter density in all reference frames. While the Ricci field equation and Gauss' law are fulfilled by the physical matter density in inertial and linearly accelerating reference frames, in rotating reference frames Gauss' law holds for an effective mass density that differs from the physical matter density. This effective density has its origin in the simulated magnetic field of rotating frames, highlighting a striking difference between linearly and rotationally accelerating frames. The equations governing the simulated fields have the same form as Maxwell's equations, a surprising result given that these equations obey special relativity (and $U(1)$-gauge symmetry), rather than Galilean symmetry. [Preview Abstract] |
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H1.00047: CHEMICAL PHYSICS |
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H1.00048: Adsorption of CO Molecules on Si(001) at Room Temperature Eonmi Seo, Daejin Eom, Hanchul Kim, Ja-Yong Koo Initial adsorption of CO molecules on Si(001) is investigated by using room-temperature (RT) scanning tunneling microscopy (STM) and density functional theory calculations. Theoretical calculations show that only one adsorption configuration of terminal-bond CO (T-CO) is stable and that the bridge-bond CO is unstable. All the abundantly observed STM features due to CO adsorption can be identified as differently configured T-COs. The initial sticking probability of CO molecules on Si(001) at RT is estimated to be as small as $\sim$ 1 x 10$^{-4}$ monolayer/Langmuir, which is significantly increased at high-temperature adsorption experiments implying a finite activation barrier for adsorption. Thermal annealing at 900 K for 5 min results in the dissociation of the adsorbed CO molecules with the probability of 60-70{\%} instead of desorption, indicating both a strong chemisorption state and an activated dissociation process. The unique adsorption state with a large binding energy, a tiny sticking probability, and a finite adsorption barrier is in stark contrast with the previous low-temperature (below 100 K) observations of a weak binding, a high sticking probability, and a barrierless adsorption. We speculate that the low-temperature results might be a signature of a physisorption state in the condensed phase. [Preview Abstract] |
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H1.00049: Multidimensional stimulated emission with a single phase-shaped pulse Arkaprabha Konar, Rachel Glenn, Vadim. V. Lozovoy, Marcos Dantus Two-dimensional optical signals from a single-pulse excitation are experimentally and theoretically investigated as a function of a sharp spectral phase-step. The phase-step introduces asymmetric diagonal and off-diagonal features into the stimulated emission spectra that are sensitive to the single-photon transitions of IR-125. The sharp phase-step causes enhanced absorption of the high frequency components and a sharp narrow-band emission of low frequency. We calculate the frequency-dispersed nonlinear spectrum versus the phase step to fourth-order in the field. We show that the third-order polarization of the sample is sensitive to phase changes in the excitation spectrum and it is responsible for the observed narrowband feature in the experiments. [Preview Abstract] |
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H1.00050: Spectroscopic Studies of Imidizolium and Pyridinium Based Ionic Liquids Ryan Booth, Jaime Stearns Ionic liquids (ILs) have been shown to be extremely useful in areas ranging from chemical synthesis to energetic materials. Furthermore, ILs are thought to be a potential replacement for hydrazine as satellite propellants because a subset are hypergolic with nitric acid. While ILs are useful, however, there is a lack of understanding of the microscopic origins for their macroscopic properties (e.g. viscosity). An example of this is that [emim$^{\mathrm{+}}$][tf2N$^{\mathrm{-}}$] is three times less viscous than its methylated counterpart [emmim$^{\mathrm{+}}$][tf2N$^{\mathrm{-}}$] and there is some discord regarding the reason. We have investigated the molecular properties of such IL pairs using UV and IR spectroscopy in the gas phase on both imidozolium and pyridinium-based ([pyr$^{\mathrm{+}}$]) ILs. UV data show that the photophysics of [emmim$^{\mathrm{+}}$][tf2N$^{\mathrm{-}}$] is different than [emim$^{\mathrm{+}}$][tf2N$^{\mathrm{-}}$] in that there is a lack of evidence for the existence of a charge transfer (CT) state (as was seen in [emim$^{\mathrm{+}}$][tf2N$^{\mathrm{-}}$]). Preliminary UV spectra for the [pyr$^{\mathrm{+}}$] ILs show at least two distinct peaks in the region from 208-270 nm, which are tentatively established as CT states between the anion and cation. IR spectra deliver structural information for both sets of ILs and should provide insight into the correlation between microscopic and macroscopic properties. [Preview Abstract] |
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H1.00051: Strong field ionization tomography with two-color circularly polarized femtosecond laser fields Maithreyi Gopalakrishnan, Chris Mancuso, Daniel Hickstein, Patrik Grychtol, Ronny Knut, Franklin Dollar, Dmitriy Zusin, Christian Gentry, Emrah Turgut, Jennifer Ellis, Henry Kapteyn, Margaret Murnane, Ofer Kfir, Oren Cohen, Avner Fleischer, Xiao-Min Tong, Ming-Chang Chen Recent experiments using two-color circularly polarized laser fields have demonstrated that high-harmonic generation (HHG), a versatile tabletop source of extreme ultraviolet (EUV) light, can be now extended to from linear to circular polarization. Here we present the first experiments using these uniquely polarized light fields to study strong field ionization (SFI), which is the complementary process to HHG. Using a velocity map imaging photoelectron spectroscopy and tomographic reconstruction techniques, we identify low-energy structures in the 3D photoelectron angular distributions that correspond to the rescattering of electrons with the ion. The observation of rescattering structures confirms the proposed explanation for HHG under two-color fields and paves the way for next-generation spectroscopies to investigate molecular structure. [Preview Abstract] |
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H1.00052: DC Sliced Ion Imaging Study of Photodissociation Dynamics: Comparison between Conditions of Simulations and Experiments Colin J. Wallace, Wei Wei, Simon W. North Vector correlations between parent molecule transition dipole moments, photofragment velocity angular momentum vectors, can provide valuable information about excited states symmetry, non-adiabatic dynamics and the forces and torques operating during fragmentation. Accurate molecular descriptions of photo-induced chemical reactions require detailed experimental results which measure vector properties. Sliced velocity mapped ion imaging is a powerful method to measure the vector correlations in the products of photo-initiated reactions. Simulations of different DC sliced imaging conditions using Simion 8.1 program have performed to assist in the interpretation of experimental data. We have also recently re-assembled and modified a ion imaging apparatus and collected ion images of several photodissociation systems. Comparisons between these simulations and our experimental images will be presented.~We are optimistic that our newly developed mathematical methods of extracting vector correlation information from sliced imaging anisotropy, will permit detailed study of a variety of benchmark dynamical systems. [Preview Abstract] |
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H1.00053: ABSTRACT WITHDRAWN |
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H1.00054: Applications of laser streaking at X-ray free electron lasers Gilles Doumy, Chris Roedig, Lou DiMauro, Adrian Cavalieri, Ivanka Grguras, Michael Meyer, John Costello, Wolfram Helml, Andreas Maier, Reinhard Kienberger, Markus illchen, Nick Hartmann, Ryan Coffee, Christoph Bostedt X-ray radiation has been long used to address selectively atoms and to yield structural information with atomic precision. The advent of X-ray Free Electron Lasers (XFEL) is revolutionizing the field of time resolved x-ray techniques. The availability of tunable pulses ranging from the soft to the hard x-ray region, and lasting only few tens of femtoseconds, or perhaps less, is enabling access to unprecedented temporal resolution. However, knowledge of the temporal properties of the x-ray pulses is poor, and synchronization to external sources introduces a timing jitter that dominates the fast dynamics and needs to be corrected for every shot. Using laser streaking techniques developed by the attosecond community, one can measure the pulse duration, and possibly improve the temporal resolution of pump probe experiments where electrons are collected to follow the processes by use of a self-referencing measurement. Illustration is presented following Auger decay in the time domain. [Preview Abstract] |
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H1.00055: Ion Transport and Local Structural Dynamics in Analogous Quaternary Ammonium and Phosphonium-Based Room Temperature Ionic Liquids Adam Holt, Philip Griffin, Katsuhiko Tsunashima, Joshua Sangoro, Alexei Sokolov The ion transport and structural dynamics in a homologous pair of quaternary ammonium and phosphonium based room temperature ionic liquids (IL), [N2228][NTF2] and [P2228][NTF2], are investigated by depolarized dynamic light scattering and dielectric spectroscopy. The atomic identity of the cation center has a pronounced effect on both long-range ion conduction as well as structural relaxation in these quaternary ILs. The dc conductivity is significantly higher in the phosphonium based IL. While the increase in dc conductivity can be attributed to a lower glass transition temperature, i.e. faster structural dynamics, of the phosphonium IL, we also have found the atomic identity of the cation center strongly influences the local secondary relaxations. The secondary relaxations in the ammonium IL exhibit an unexpected non-Arrhenius temperature dependence --in stark contrast to its phosphonium counterpart. In addition to structural dynamics, changes in the secondary relaxations suggest the differences in dc conductivity may also be attributed to a change in counter-ion coordination and could lead to a difference in the mesoscale aggregation of alkyl moieties which is known to exist in these ILs. Therefore, subtle changes of inter-ionic interactions have a direct consequence on local, structural, and long-range dynamics in these analogous ILs. [Preview Abstract] |
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H1.00056: Determination of Volatility of Ionic Liquids at the Nanoscale by means of Ultra-Fast Scanning Calorimetry -- the Method Mathias Ahrenberg, Martin Beck, Christin Schmidt, Sergey P. Verevkin, Olaf Kessler, Udo Kragl, Christoph Schick We present a new method for the determination of the vapour pressure of low volatile compounds using differential fast scanning calorimetry. We have developed and proven this method using the ionic liquids [EMIm][NTf$_{2}$] and [EMIm][NO$_{3}$] at temperatures up to 750~K and in different atmospheres to distinguish between decomposition and evaporation$^{1}$. It was demonstrated that evaporation is still the dominating process of mass loss even at temperatures 100 K above the onset of decomposition as measured with common techniques, e.g TGA. Since the method allows very high heating rates (up to 10$^{6}$~K/s)$^{2}$, much higher temperatures can be reached in the measurement of the vapour pressure as compared to common devices without significant decomposition of the ionic liquid. Furthermore, this method represents an improvement of the boiling point estimation of ILs due to the large accessible temperature range of mass loss rate determination. 1. M. Ahrenberg et al., Physical Chemistry Chemical Physics, 2014, 16, 2971-2980. 2. E. Zhuravlev and C. Schick, Thermochim. Acta, 2010, 505, 1-13. [Preview Abstract] |
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H1.00057: Morphological and Chemical Analysis of Degraded Single Junction Amorphous Silicon Module Gilbert Osayemwenre Photovoltaic solar modules have different defects and degradation characteristic modes. These defects/degradation modes normally heats up some regions in the PV module, depending on the degree and size of the localised heat or hot spot, the localized heat can rise above the temperature limit of the module thereby cause damage to the structural orientation. The presence of severe defect and degradation correlates with high temperature gradients that usually results in morphological damage especially under outdoor conditions. The present study investigates the effect of defect/degradation on the surface morphology of the single junction amorphous silicon modules (a-Si:H) during outdoor deployment. The observed structural damage was analysed using scanning electron microscope (SEM) and energy dispersion X-ray (EDX) to ascertain the elemental composition. Results show huge discrepancies in the chemical composition constitute alone different regions. The presence of high concentration of carbon and oxygen was found in the affected region. [Preview Abstract] |
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H1.00058: Cross-relaxation quenching of x-ray excited luminescence in Eu-activated phosphors Joseph Pacold, Devon Mortensen, William Reichlin, Zou Finfrock, Anthony Diaz, Gerald Seidler Compounds, molecules, and nanoparticles containing lanthanides as primary constituents or as dopants are widely used in applications including luminescent dyes and lighting phosphors. Recent work has shown that x-ray spectroscopy methods can be used to monitor the sequence of excited states that leads to luminescence in lanthanide materials. Here, we use x-ray excited optical luminescence (XEOL) to identify a nonradiative process that quenches the emissive excited state of Eu${}^{3+}$ in the phosphors YVO${}_4$:Eu${}^{3+}$ and YVO${}_4$:Bi${}^{3+}$,Eu${}^{3+}$. Taking advantage of the high flux (up to $2 \times 10^{12}$ photons/second) and focusing capability (beam FWHM 5 $\mu$m) of a modern synchrotron beamline, we observe saturation of the XEOL yield at high x-ray flux densities. The saturation effect is interpreted with a kinetic model in which pairs of excited Eu ions undergo an Auger-like cross-relaxation. This effect is well documented in the literature on cathode-ray phosphors, and allows us to estimate the excited fraction of Eu${}^{3+}$ ions. We discuss applications of this method to the broader problem of studying energy transfer in luminescent materials, as well as technical implications for future x-ray spectroscopy studies that require high flux. [Preview Abstract] |
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H1.00059: Inner-shell photoionization and core-hole decay of Xe and XeF$_2$ Stephen Southworth, Antonio Pic\'on, C. Stefan Lehmann, Ralf Wehlitz, Lan Cheng, John F. Stanton Molecular effects on inner-shell photoionization and core-hole decay are explored by comparing cross sections and partial ion yields of Xe and XeF$_2$ from Xe 3d and F 1s subshells in the 660-740 eV range. The Xe 3d-$\epsilon$f continuum shape resonances dominate the total cross sections, but prominent resonances appear in the XeF$_2$ cross section due to excitation of Xe 3d and F 1s electrons to the lowest unoccupied molecular orbital (LUMO), a delocalized anti-bonding MO. Relativistic coupled-cluster calculations were performed to identify the subshell ionization thresholds, the LUMO resonances and their oscillator strengths. Comparison of the Xe charge state distributions of the atom and molecule show a general shift to lower charge states in XeF$_2$. The measurements support a model of core-hole decay in which charge is redistributed from Xe to the F ligands and energetic F ions are produced by Coulombic fragmentation. [Preview Abstract] |
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H1.00060: Intermolecular coupling and dynamics through infrared nano-spectroscopic imaging Benjamin Pollard, Eric A. Muller, Omar Khatib, Markus B. Raschke Intermolecular interactions and coupling on the nanoscale lead to a variety of structural phases and degrees of crystallinity in soft-matter and biological systems, producing unique functional properties. We combine multi-spectral vibrational scattering-scanning near-field optical microscopy (s-SNOM) with multimodal scanning probe imaging to investigate structure-function relationships in soft matter. Using vibrational resonances as sensitive reporters of local structure, coupling, and dynamics, we resolve spectral shifts and line broadening on the nanoscale. These spectral shifts allow us to map intermolecular electric fields across nanoscale domains through solvatochromism or transition dipole coupling. Similarly, linewidths relate directly to the spatially-varying coupling dynamics of vibrational oscillators. Comparing spectral maps of peak position and linewidth to maps of adhesion and Young's modulus, for example, provides insight into the structure-function relationship dictating nanoscale self assembly in, e.g., block copolymer thin films. [Preview Abstract] |
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H1.00061: Sucralose Destabilization of Protein Structure Inha Cho, Lee Chen, Nimesh Shukla, Christina Othon Sucralose is a commonly employed artificial sweetener. Sucralose behaves very differently than its natural disaccharide counterpart, sucrose, in terms of its interaction with biomolecules. The presence of sucralose in solution is found to destabilize the native structure of the globular protein Bovine Serum Albumin (BSA). The melting temperature decreases as a linear function of sucralose concentration. We correlate this destabilization with the increased polarity of the sucralose molecule as compared to sucrose. The strongly polar nature is observed as a large dielectric friction exerted on the excited state rotational diffusion of tryptophan using time-resolved fluorescence anisotropy. Tryptophan exhibits rotational diffusion proportional to the measured bulk viscosity for sucrose solutions over a wide range of concentrations, consistent with a Stokes-Einstein diffusional model. For sucralose solutions however, the diffusion is linearly dependent with the concentration, strongly diverging from the viscosity predictions. The polar nature of sucralose causes a dramatically different interaction with biomolecules than natural disaccharide molecules.~ [Preview Abstract] |
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H1.00062: ABSTRACT WITHDRAWN |
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H1.00063: Time-resolved phase-sensitive second harmonic generation spectroscopy of electron injection at the water/air interface Jan Verlet A new methodology is developed to probe the real-time dynamics of species at interfaces based on phase-sensitive second harmonic generation. The key attributes of the method are that the measured signal is linear with concentration and can independently measure the real and imaginary part of the second-order non-linear susceptibility. We apply this methodology to probing the dynamics of the hydrated electron at the water/air interface, produced by charge-transfer-to-solvent from a precursor ion (iodide in this case). Preliminary results indicate a fast decay on a sub-picosecond timescale of the real (non-resonant) part of the second-order non-linear susceptibility, which we associate with the charge-transfer dynamics at the water/air interface. [Preview Abstract] |
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H1.00064: Electric Field: Maker \textit{and} Breaker of Molecular co-operativity: the paradigm of (CH$_{3}$OH)---(H$_{2}$O)$_{n}$ [$n=$1-4] hetero-clusters Rajeev Pathak Hydrogen-bonded hetero-clusters of methanol and water: (CH$_{3}$OH)---(H$_{2}$O)$_{n}$ [$n=$1-4], when subjected to an externally applied dipolar electrostatic field, exhibit remarkable stability up to their characteristic maximal threshold field strengths. Moderate fields below the threshold endow a given cluster with enhanced stability against dissociation, whereas beyond the threshold its HOMO-LUMO gap abruptly closes down, resulting into structural breakdown. The electric field thus plays a dual role of a maker \textit{and} breaker of molecular co-operativity in a hydrogen-bonding scenario. \textit{Ab initio} investigations on these clusters employing M06-2X hybrid functional with a 6-311$++$G(2d, 2p) basis set reveal the structural evolution of the conformers with increasing electric field strengths: structural deformations, enhancement in their dipole moments with concomitant decrease in their HOMO-LUMO gaps interspersed with abrupt changes in these attributes when the clusters ``open out.'' Energetics quantify the stability of the clusters in field, while the IR vibrational spectra bring out shifts in the normal modes with marked emergence of exotic low-frequency O-H stretches as precursors of breakdown at the threshold field strengths. [Preview Abstract] |
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H1.00065: Two-step nucleation: Monte Carlo simulation of heterogeneous nucleation of liquid droplets on soluble nanoscopic aerosols Oleksandr Zavalov, Sophie McGibbon-Gardner, Peter Poole, Richard Bowles, Ivan Saika-Voivod We use a 2D Ising lattice gas model to conduct Monte Carlo simulations of the heterogeneous nucleation of a liquid droplet from the supersaturated vapor phase, as triggered by a soluble nanoscopic aerosol particle. The liquid droplet that forms is a cluster of both solvent and solute particles. As a function of the size n of the droplet, we observe a solubility transition: At smaller n, the droplet consists of a compact solute cluster wetted by solvent, while at larger n the solute dissolves and is more uniformly distributed within the droplet. We evaluate the free energy of formation of a droplet as a function of n, and identify conditions at which nucleation is a two-step process. That is, two free energy barriers are encountered as the droplet grows, one associated with the solubility transition, and the other with the nucleation of the bulk solvent phase. We also evaluate the nucleation rate from the mean first passage time for the droplet to reach the critical size for the formation of the bulk phase, and quantify the influence on the rate due to solute solubility, and due to the relative heights of the barriers for the solubility and bulk transitions. [Preview Abstract] |
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H1.00066: Landscape, kinetics, paths and statistics of curl flux, coherence, entanglement and energy transfer in non-equilibrium quantum systems Zhedong Zhang, Jin Wang We developed a population and flux landscape theory for general non-equilibrium quantum systems. We illustrated our theory by modelling the quantum transport of donor-acceptor energy transfer. We found two driving forces for the non-equilibrium quantum dynamics. The symmetric part of the driving force corresponds to the population landscape contribution which mainly governs the equilibrium part of dynamics while the anti-symmetric part of the driving force generates the non-equilibrium curl quantum flux which leads to the detailed-balance-breaking and time-irreversibility. The multi-loop structure of the flux emerges which forms the flux-landscape. Improving the voltage and electronic coupling in general facilitates the quantum transport by reducing the population landscape barriers between major states and increasing the mean value of the flux. A limit-cycle mode emerges when the underlying flux-landscape becomes funnelled with a significant gap between the largest flux loop and the rest of them. On the kinetic level, we found that multiple kinetic paths between quantum states emerge and illustrate the interference effects [Preview Abstract] |
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H1.00067: Sensitivity of chiral 3-methylcyclopentanone structure, electric moment and thermodynamic parameters to medium polarity Watheq Al-Basheer, Said Al Azar $R$-($+)$-3-methylcyclopentanone ($R$3MCP) is a chiral ketone which can exist in as many as five conformers with two dominant conformers at room temperature; equatorial-methyl and axial-methyl. Density Functional Theory (DFT) calculations of the optimized geometries of $R$-($+)$-3-methylcyclopentanone ($R$3MCP) individual dominant conformers were performed in 10 common solvents of wide polarity range, under the framework of polarizable continuum model (PCM). DFT correlation function type B3LYP using a powerful basis set (aug-cc-pVDZ) yielded different linear correlation between solvent polarity and $R$3MCP equatorial and axial conformers Gibbs free and zero-point energies, entropies, vibrational modes frequencies, in addition to heat capacity resulting from translational, electronic, rotational and vibrational motion. Furthermore, DFT calculations of the $R$3MCP equatorial and axial conformers electric dipole and quadrupole moments components in 3D were also carried out and found to have a linear correlation with solvent polarity and cavitation energy. An observed trend for the standard Gibbs energies for the rotational equilibrium of $R$3MCP to be strongly-solvent dependent will be presented. [Preview Abstract] |
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H1.00068: 1 D Simulation of Capacitively Coupled Water Vapor Plasma Ziane Kechidi The results of a 1D simulation of a capacitively coupled water vapor discharge is reported. The simulated plasma consists of two electrodes separated by gap distance of 1 mm operating at 13.56 MHz with 26 species and 62 dominant reaction channels. The input parameters under which the plasma can be created is explored and space and time profiles of the electron densities are presented. The model finds that plasma ignition cannot be obtained in the present configuration and at pressures of greater than 0.1 atmosphere. The model has also be used to demonstrate the impact of rotational and vibrational excitation of water molecules in suppressing electrical breakdown. [Preview Abstract] |
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H1.00069: Charge Conservation in BDG Formalism and its Effect in Calculating Berry Phase of Transporting a Localized BDG Quasiparticle around a Vortex in Superfluids/Superconductors Yiruo Lin, Tony Leggett We examine charge conservation in BdG formalism and discuss the consequence of violating the charge conservation in Berry phase calculation of transporting a localized BdG quasiparticle around a vortex in superfluids/superconductors. We calculate explicitly the Berry phase in a model system in which the vortex is replaced by a 1D annulus ring geometry with quantized superfluid center-of-mass winding number and a localized Zeeman field is imposed to trap a quasiparticle with definite spin orientation. [Preview Abstract] |
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H1.00070: Parahydrogen Induced Polarization by Pairwise Replacement on Pt and Ir Nanoparticles Clifford Bowers, Ronghui Zhou, Evan Zhao, Wei Cheng, Luke Neal, Helena Weaver Parahydrogen Induced Polarization (PHIP) is a robust and scalable method for production of bulk quantities of hyperpolarized fluids. The symmetrization order inherent in parahydrogen is transformed via symmetry breaking hydrogenation reaction into NMR-observable hyperpolarization. Spin polarization of order unity can be obtained. A key requirement of PHIP is pairwise hydrogenation by addition of H atoms originating from the same H$_{\mathrm{2}}$ molecule. PHIP using supported metal catalysts is a promising recent development because it exploits the advantages over homogeneous and supported metal complexes. The present work demonstrates a new PHIP mechanism involving the pairwise replacement of parahydrogen into propene (the substrate) over TiO$_{\mathrm{2}}$ supported Ir and Pt nanoparticle catalysts. Analysis of the stereoselectivity of the pairwise replacement process is facilitated by density matrix spectral simulations. The cis and trans dispositions of the symmetrization order give strikingly different PHIP spectra. The observed stereoselectivity of the pairwise replacement step, together with control experiments, rule out an alternative mechanism involving dehydrogenation of free propane over the catalyst. [Preview Abstract] |
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H1.00071: Promoting alkali and alkaline-earth metals on MgO for enhancing CO2 capture by first-principles calculations Hyoseok Kim, Won Bo Lee, Kiwoong Kim The CO2 capture properties of Alkali(Li-, Na-, K, Rb-, and Cs-) and Alkaline-Earth metal(Be-, Ca-, Sr-, and Ba-) promoted MgO sorbents are investigated by first principles density functional theory on the basis of PW91/GGA augmented with DFT$+$D2. Calculated Adsorption energy on the metal-promoted MgO sorbents is higher than Pure MgO sorbents, except for the Na-promoter. These results indicate that the CO2 capture capacity is improved by metal promotion. Li, Ca, and Sr were identified as adequate promoters among 9 metals, considering bind stability and regenerability [Preview Abstract] |
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H1.00072: Diffusion of a particle on a static rugged energy landscape with spatial correlations Biman Bagchi, Saikat Banerjee Despite the broad applicability of the problem, we have limited knowledge about the effect of ruggedness on diffusion at a quantitative level. Every study seems to use the expression of Zwanzig [Proc. Natl. Acad. U.S.A, 85, 2029 (1988)] who derived the effective diffusion coefficient, $D_{eff} =D_{0} \exp \left( {-\beta^{2}\varepsilon^{2}} \right)$. We introduce and study two models of Gaussian random energy surface; a discrete lattice and a continuous field. Our simulations show that Zwanzig's expression overestimates diffusion in the uncorrelated Gaussian random lattice. The disparity originates from the presence of ``three-site traps'' (TST) on the energy landscape -- which are formed by the presence of deep minima flanked by high barriers on either side. Using mean first passage time (MFPT) formalism, we derive a general expression for the effective diffusion coefficient, $D_{eff} =D_{0} \exp \left( {-\beta^{2}\varepsilon^{2}} \right)\left[ {1+\mbox{erf}\left( {{\beta \varepsilon } \mathord{\left/ {\vphantom {{\beta \varepsilon } 2}} \right. \kern-\nulldelimiterspace} 2} \right)} \right]^{-1}$ in the presence of TST. In presence of spatial correlation we derive a more general form of the expression, which reduces to Zwanzig's form in certain limits. We characterize the same using non-Gaussian order parameter, and show that this ``breakdown'' scales with ruggedness following an asymptotic power law. The breakdown of Zwanzig's elegant expression was perhaps anticipated but was not clearly demonstrated earlier. [Preview Abstract] |
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H1.00073: New insides in the characterization of HDS industrial catalysts by HAADF-STEM Paz del Angel, Arturo Ponce, Josefina Arellano, Miguel J. Yacaman, Martha Hernandez-Pichardo, J. Ascencion Montoya, Jose Escobar Hydrodesulfurization (HDS) catalysts are of great importance in the petroleum industry. Transition metal sulphides catalysts of Ni(Co)Mo(W)/Al$_{\mathrm{2}}$O$_{\mathrm{3}}$ are widely used for hydrotreating reactions, like hydrodenitrogenation and HDS. One of the main issue in these catalysts is to understand the mechanism of the reaction, where MoS$_{\mathrm{2}}$ plays the most important role in the catalytic activity. We studied an industrial NiMo/Alumina sulfide catalyst highly active by using aberration-corrected HAADF-STEM techniques. The used catalysts was a state-of- the art commercial nickel-molybdenum alumina-supported formulation, including organic agent modifier. This type of material belongs to a novel family of catalysts specially designed for ultra-low sulfur production from straight-run gas oil (SRGO), cycle oil, coker gas oil, or their combinations at operating conditions of commercial interest in hydrotreating units at industrial scale. Aberration corrected HAADF-STEM allowed to observe the nanostructure and location of MoS$_{\mathrm{2}}$ and his interaction with the alumina. The results indicate that the MoS$_{\mathrm{2}}$ is highly dispersed on the alumina, however the location of Ni is one of the task of this kind of catalyst. [Preview Abstract] |
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H1.00074: Effective Reaction Coordinates in Competitive Nucleation of Gold Nanoclusters Cletus Asuquo, Richard Bowles Many materials exhibit crystal polymorphism such that they can freeze to form a variety of different structures under the same conditions. Which structure is formed, and how, is determined by the nucleation kinetics that involves the creation of a critical embryo for the new phase. In classical nucleation theory, the embryo size is usually used as the sole order parameter to describe the reaction coordinate, but this does not always contain sufficient information to describe the formation of the different phases observed in a competitive nucleation process. We present an extension of the transition path sampling algorithm to the sampling of transition paths in a competitive process, as well as the development of a multiple paths maximum likelihood analysis used to obtain accurate reaction coordinates for the different transitions. The new techniques are used to study competitive nucleation in gold nanoclusters where non-crystalline structures such as icosahedra, decahedra and face-centered cubic crystals can form. The reaction coordinates, and analyses of the nucleation pathways, give new insights to how correlated local structures arrange to form more complex structures on longer length scales. In particular, we show that the formation of the tetrahedral subunits are important [Preview Abstract] |
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H1.00075: Structural Properties of Finite MoS$_2$ Nanowires Shaylyn Clark, Andres Salgado, Lucas Fernandez-Seivane, Xochitl Lopez-Lozano Molybdenum disulfide (MoS$_2$) has been one of the most important catalysts used in refineries worldwide for hydrodesulfurization over the past century. In the last decade, and with the advent of nanotechnology, there has been a special interest in MoS$_2$ nanostructures due to their high potential as novel nanocatalysts. The study of the properties of these systems is of fundamental interest for the experimental design of their catalytic activity and efficiency. In this work, we have performed ab initio density-functional calculations (DFT) to investigate the structural properties of finite MoS$_2$ nanostrutures. All the models here presented were based on newly experimentally observed morphologies in MoS$_2$ industrial catalysts using high-angle annular dark field scanning transmission electron microscopy (HAADF-STEM) images. We simulated STEM images of the theoretical models to compare it with the experimental ones. In contrast with infinite models, the finite models prefer a rippled/twisted structure morphology over the planar or helical ones. The rippled/twisted models appear to be structurally more stable. [Preview Abstract] |
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H1.00076: Theoretical DFT Study of Homonuclear and Binary Transition-Metal Dimers Alvaro Posada-Amarillas, Alvaro Posada-Borbon A DFT study of homonuclear, and heteronuclear Pd-M, Pt-M (M$=$Cu, Ag, Au, Ni) and Pt-Pd neutral dimers is presented using different XC functionals and basis sets. Bond length and vibrational frequencies were determined for ground state configurations. Doublet and triplet states were obtained for heteronuclear dimers while dissociation energy exhibits unambiguous dependency on the HF exchange term. Electronic configurations were determined for Pd-Ag ($^{2}\Sigma )$ and Pt-Ni ($^{3}\Sigma )$ dimers. Hybrid functionals provide results in close agreement with experimental data for Pt-Ni, Pt-Pd, and Pd-Ni dimers. The hybrid mPW1PW91 functional predicts a dissociation energy value for Pt-Cu dimer of about 3.3 eV, consistent with experimental information. Overall PBE and B3PW91 are reliable functionals to predict bond lengths and harmonic frequencies of heteronuclear dimers. [Preview Abstract] |
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H1.00077: An integral method to compute Helmholtz free energies of crystalline solids Zhiyue Lu, Christopher Jarzynski We describe a method to compute the Helmholtz free energy of a crystalline solid by direct evaluation of the partition function. In the many-dimensional conformation space of all possible arrangements of $N$ particles inside a periodic box, the energy landscape consists of localized islands corresponding to different solid phases. Calculating the partition function for a specific phase involves integrating over the corresponding island. Introducing a natural order parameter that quantifies the net displacement of particles from lattices sites, we write the partition function in terms of a one-dimensional integral along the order parameter, and evaluate this integral using umbrella sampling. We validate the method by computing free energies of both face-centered cubic (FCC) and hexagonal close-packed (HCP) hard sphere crystals with a precision of $10^{-5}k_BT$ per particle. [Preview Abstract] |
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H1.00078: The effect of excitonic interactions on singlet fission dynamics in crystalline tetracene Chunfeng Zhang, Bo Zhang, Rui Wang, Min Xiao Singlet fission in organic semiconductors is interesting for its potential application in boosting the efficiency of solar conversion. Singlet-singlet annihilation induced by excitonic interaction has been regarded as a process that competes against singlet fission in high-density regime. In this work, we conduct a systematic transient optical study to investigate the density-dependent singlet fission dynamics in crystalline tetracene. Surprisingly, the transient absorption data indicate the rate of singlet fission is actually increased with increasing the excitation density [1], which is further supported by probing the quantum beating between the manifold states of triplet pairs following Burdett's approach. Our result suggests it is necessary to re-examine the role of excitonic interactions to uncover the physical mechanism underlying singlet fission in crystalline tetracene. [1] Zhang et al., J. Phys. Chem. Lett. 5, 3462 (2014). [2] Burdett {\&} Bardeen, J. Am. Chem. Soc. 134, 8597(2012). [Preview Abstract] |
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H1.00079: A Single Diblock Molecular Diode Tobechukwu Joshua Obodo, Altynbek Murat, Udo Schwingenschl\"{o}gl We investigate the rectification behavior of the diblock dipyrimidinyldiphenyl molecule and its derivates with increasing donor groups using self-interaction corrected density functional theory combined with the non-equilibrium Green's function method. In particular, we study a tandem setup for the representative optimized rectifier, finding that it significantly improves the rectification behavior of the molecular diode. Moreover, we find that the molecule consisting of donor and acceptor mimics a pn-junction, whereas the tandem setup does not behave as a pn-pn junction, rather like a p-np-n junction. Our results help explain the mechanism behind the experimentally observed rectification behavior of the molecule. [Preview Abstract] |
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H1.00080: AuCl$_4$ Functionalized Carbon Nanotubes: Origin of the p-Type Doping Altynbek Murat, Ivan Rungger, Stefano Sanvito, Udo Schwingenschl\"{o}gl The microscopic origin of the p-type doping of AuCl$_3$ functionalized carbon nanotubes (CNTs) is investigated using first-principles self-interaction corrected density functional theory. Although the system has been studied as potential candidate for highly sensitive and selective gas sensors, a clear identification of the source of the p-type doping is not achieved. Recent experimental and theoretical works suggest that it is due to the adsorbed Cl atoms. We test this hypothesis and show that adsorbed Cl atoms only lead to a p-type character for very specific concentrations and arrangements, which furthermore are not the lowest energy configurations. We therefore propose and investigate alternative mechanisms while considering all possible configurations and concentrations. In particular, we study the possible formation of different conformations of AuCl$_3$ as well as the effect of the adsorbate concentration. As a result, we find that especially AuCl$_4$ molecules bind strongly to the CNT and that they lead to an electron transfer to the molecules and thus a shift of the Fermi energy below the valence band maximum. We conclude that the origin of the p-type doping in AuCl$_3$ functionalized CNT is due to the adsorption of AuCl$_4$ molecules. [Preview Abstract] |
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H1.00081: First-Principles Investigations of Lead-Free Formamidinium Based Hybrid Perovskites Altynbek Murat, Udo Schwingenschl\"{o}gl Hybrid organic-inorganic perovskite solar cells have recently emerged as the next-generation photovoltaic technology. Most of the research work has been focused on the prototype MAPbI$_3$ perovskite (MA = Methylammonium = CH$_3$NH$_3$$^+$) and its analogues that have lead to power conversion efficiencies in excess of 15\%. Despite the huge success, these materials are still non-optimal in terms of optical absorption where the bandgaps are greater than~1.6 eV as well as the toxicology issue of lead. Thus, investigation and development of lead-free perovskites with bandgaps closer to optimal, allowing greater spectral absorption, is of great interest. In this work, we perform first principles calculations to study the structural, optical, and electronic properties of new derivatives of MAPbI$_3$ in which the organic MA cation is replaced by other organic amines of similar size such as Formamidinium (FA) and/or the Pb cation replaced by similar elements such as Sn. In particular, we investigate the role and effect of FA and Pb cations on the electronic and optical properties and analyze to which extend the bandgaps can be tuned. [Preview Abstract] |
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H1.00082: Dye attached fullerene and P3HT complexes Amanda Garnica, Luis Basurto, Rajendra Zope, Tunna Baruah We study the electronic structure of C60 fullerenes functionalized with thiophene-diketo-pyrrolopyrole-thiophene based chromophores using density functional theory combined with large polarized basis sets. These chromophores have electron donor character and thus the functionalization of the fullerene produces donor-acceptor (DA) systems. We examine in detail the effect of the linker and the addition site on the electronic structure of the fullerenes. We further study the charge transfer excited states of these DA complexes and also that of the complexes of these functionalized fullerenes with the poly(3-hexylthiophene-2,5-diyl) (P3HT) are studied using the perturbative $\Delta$-SCF method. The exciton binding energies in the functionalized fullerene-P3MT complexes are found to be smaller compared to similarly prepared phenyl-C61-butyric acid methyl ester (PCBM)-P3MT complex. [Preview Abstract] |
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H1.00083: Electronic structure and charge transfer excitation energies of three endohedral fullerene- ZnTPP/ZnPc dyads Fatemeh Amerikheirabadi, Luis Basurto, Rajendra Zope, Tunna Baruah Organic donor-acceptor (D-A) moieties make up the main component of organic photovoltaics (OPVs). It has been proved that the open circuit voltage of these devices which is a parameter in efficiency determination, is directly related to the charge transfer excited states of the D-A pairs. Fullerenes having lots of interesting acceptor properties and porphyrins as well as phthalocyanines possessing intriguing donor characteristics, are shown to be promising nominees. In this work, we computationally analyze three donor-acceptor dyads of Zn-tetraphenyl porphyrin and Zn-phthalocyanine with novel endohedral fullerenes: Sc$_{\mathrm{3}}$N@C$_{\mathrm{80}}$\textunderscore ZnTPP, Y$_{\mathrm{3}}$N@C$_{\mathrm{80}}$\textunderscore ZnTPP and Sc$_{\mathrm{3}}$N@C$_{\mathrm{80}}$\textunderscore ZnPc$_{\mathrm{.}}$ The Sc$_{\mathrm{3}}$N@C$_{\mathrm{80}}$\textunderscore and Y$_{\mathrm{3}}$N@C$_{\mathrm{80}}$ belong to a particular class of fullerenes called trimetallic nitride endohedral fullerenes where the trimellatic nitrides form the endohedral units. Density functional theory, as implemented in NRLMOL code, is used to study the electronic structure and the related properties of these D-A complexes. The charge transfer excitation energies are calculated using the perturbative delta self-consistent field method recently developed in our group. We find that the CT excitation energies are larger for endohedral fullerene based dyads compared to similar C$_{\mathrm{60}}$ based dyads. [Preview Abstract] |
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H1.00084: Current flow in biased bilayer graphene: the role of sublattices Carlos Paez, Dario Bahamon, Ana pereira We investigate here how the current flows over a bilayer graphene in the presence of an external electric field perpendicularly applied (biased bilayer). Charge density polarization between layers in these systems is known to create a layer pseudospin, which can be manipulated by the electric field. Our results show that current does not necessarily flow over regions of the system with higher charge density. Charge can be predominantly concentrated over one layer, while current flows over the other layer. We find that this phenomenon occurs when the charge density becomes highly concentrated over only one of the sublattices, as the electric field breaks layer and sublattice symmetries for a Bernal-stacked bilayer. For bilayer nanoribbons, the situation is even more complex, with a competition between edge and bulk effects for the definition of the current flow. We show that, in spite of not flowing trough the layer where charge is polarized to, the current in these systems also defines a controllable layer pseudospin. [Preview Abstract] |
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H1.00085: PHYSICS EDUCATION |
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H1.00086: Visualizations of Illinois Educational Data Cacey Stevens, Michael Marder, Sidney Nagel We examine data from scores on standardized exams taken by students in the state of Illinois. In order to analyze the factors affecting school performance in mathematics, we represent the data through visualizations, an approach commonly used to identify patterns in studies of physical systems. Exam scores for different schools are shown to depend on program type, location, and poverty concentration. For most schools in Illinois, test scores decline linearly with increased poverty concentration. However schools in Chicago show deviations from the linear trend. For any given poverty level, schools in Chicago perform better than those in other communities of Illinois. We also compare different school types, such as neighborhood, magnet, and charter programs, at each grade level. The city's selective enrollment programs show notably superior achievement at the high school level. This is less pronounced at earlier grades. [Preview Abstract] |
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H1.00087: Lab-in-a-box @ school: Exiting hands-on experiments in soft matter physics Karin Jacobs, Martin Brinkmann, Frank M\"uller Soft materials like liquids and polymers are part of everyday life, yet at school, this topic is rarely touched. Within the priority program SPP 1064 'Nano- and Microfluidics' of the German Science Foundation, we designed an outreach project that allows pupils (age 14 to 18) to perform hands-on experiments (www.labinabox.de). The experiments allow them e.g. to feel viscosity and viscoelasticity, experience surface tension or see structure formation. We call the modus operandi 'subjective experiments' to contrast them with the scientifically objective experiments, which pupils often describe as being boring. Over a dozen different experiments under the topic 'physics of fluids' are collected in a big box that travels to the school. Three other topics of boxes are available, 'physics of light, 'physics of liquid crystals', and 'physics of adhesion and friction'. Each experiment can be performed by 1-3 pupils within 10 - 20 min. That way, each scholar can perform 6 to 8 different small experiments within one topic. 'Subjective experiments' especially catch the attention of girls without disadvantaging boys. Both are fascinated by the hands-on physics experience and are therefore eager to perform also 'boring' objective experiments. Morover, before/after polls reveal that their interest in physics has greatly advanced. The project can easily be taken over and/or adapted to other topics in the natural sciences. [Preview Abstract] |
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H1.00088: QUANTUM INFORMATION, CONCEPTS AND COMPUTATION |
(Author Not Attending)
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H1.00089: Approach to solving spin-boson dynamics via non-Markovian quantum trajectories Zeng-Zhao Li, Cho-Tung Yip, Hai-Yao Deng, Mi Chen, Ting Yu, J. Q. You, Chi-Hang Lam We develop a systematic and efficient approach for numerically solving the non-Markovian quantum state diffusion equation for an open quantum system that can be strongly coupled to an environment. As an important application, we consider a real-time simulation of a spin-boson model in a strong coupling regime that is difficult to deal with using conventional methods. We show that the non-Markovian stochastic Schr\"{o}dinger equation can be efficiently implemented as a real--time simulation for this model, so as to give an accurate description of spin-boson dynamics beyond the rotating-wave approximation. [Preview Abstract] |
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H1.00090: Relation between Full Counting Statistics and the flow of Shannon entropy Mohammad Ansari, Yuli Nazarov We show that microscopic theory of counting statistics that is applicable for many quantum noise analysis, has an internal and fundamental correspondence to the Renyi entropy flow in the quantum system. This analogy reveals an interesting step towards understanding quantum thermodynamics. We also further extend the formulation for the flows of entropy using full counting statistics and determine novel quantum terms in the statistical moment measurements. [Preview Abstract] |
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H1.00091: Coherent control of multipartite excitonic entanglement in quantum dot arrays Juan E. Rolon, Joaquin E. Drut We propose a coherent control scheme for multipartite entanglement of exciton states in optically driven quantum dot arrays (QDAs) coupled by charge tunneling and resonant energy transfer (RET) processes. An adiabatic manipulation of the entanglement dynamics is devised by pulse shaping and time-dependent electric field sweeps. By varying the inter-dot distance and number of quantum dots (QDs) comprising the QDA, the excitonic qubit manifolds are obtained by a Feshbach projection over the resulting multilevel exciton configurations. We identify regimes in which the dynamics is confined to decoherence-free excitonic qubit manifolds taking into account spontaneous recombination and non-Markovian effects introduced by a phonon bath. We present results for entanglement monotones and measures such as the entanglement of formation and entanglement entropy for different QDA geometries and carrier injection conditions. Our results indicate that in spite of the effects of phonon-assisted relaxation, entanglement can be optimized and transferred between QDs by the controlled interplay of system geometry, pulse shaping, RET and carrier tunneling. [Preview Abstract] |
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H1.00092: Anisotropic exchange coupling in a nanowire double quantum dot with strong spin-orbit coupling Rui Li, J.Q. You A spin-orbit qubit is a hybrid qubit that contains both orbital and spin degrees of freedom of an electron in a quantum dot. Here we study the exchange coupling between two spin-orbit qubits in a nanowire double quantum dot (DQD) with strong spin-orbit coupling (SOC). We find that while the total tunneling in the DQD is irrelevant to the SOC, both the spin-conserved and spin-flipped tunnelings are SOC dependent and can compete with each other in the strong SOC regime. Moreover, the Coulomb repulsion between electrons can combine with the SOC-dependent tunnelings to yield an anisotropic exchange coupling between the two spin-orbit qubits. Also, we give an explicit physical mechanism for this anisotropic exchange coupling. [Preview Abstract] |
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H1.00093: Spin decoherence of mobile impurity in a one dimensional spin bath Trithep Devakul, Adrian Feiguin We study the spin decoherence of a mobile impurity interacting locally with a one dimensional spin bath. In contrast to the central spin model, where a single central spin interacts with the bath via long ranged interactions, our model considers only local exchange interactions, while allowing the impurity to move to neighboring sites via hopping $t$. We consider a spin-$1/2$ impurity, and study the decoherence, tracing over the position degree of freedom. In the large $t$ limit, the delocalized impurity behaves identically to a localized spin interacting with the bath, same as a central spin. This model allows one to treat a central spin problem - which inherently builds up long-range entanglement within the bath - instead as a Hamiltonian with only local interactions. Numerical calculations are done at various regimes of parameters, and comparison with the central spin model is discussed. [Preview Abstract] |
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H1.00094: Finite-temperature reservoir engineering and entanglement dynamics Sergue\"i Fedortchenko, Arne Keller, Thomas Coudreau, Perola Milman We propose experimental methods to engineer reservoirs at arbitrary temperature which are feasible with current technology. Our results generalize to mixed states the possibility of quantum state engineering through controlled decoherence. Finite-temperature engineered reservoirs can lead to the experimental observation of thermal entanglement--the appearance and increase of entanglement with temperature--to the study of the dependence of finite-time disentanglement and revival with temperature, quantum thermodynamical effects, and others, enlarging the comprehension of temperature-dependent entanglement properties. Our proposal is discussed in detail in two model systems, consisting of different modes of a single photon and a trapped-ion system. [Preview Abstract] |
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H1.00095: Reproducing the D-Wave Entanglement Results in Candidate Models Tameem Albash, Itay Hen, Federico Spedalieri, Daniel Lidar The demonstration of entanglement on the D-Wave devices [1] relies on the assumption that the populations derived from measurement correspond to energy eigenstate populations of the quantum Hamiltonian. We therefore ask whether leading model candidates for the D-Wave devices are able to reproduce this entanglement signature. We focus our work on a quantum adiabatic Markovian master equation (ME) [2] and a Monte Carlo rotor model (SSSV) [3]. We show that the ME reproduces both the energy spectrum and the thermal state populations of the quantum Hamiltonian extremely well, hence agreeing with the experimental results. SSSV on the other hand fails to reproduce either, a consequence of the absence of discrete energy states in this model.\\[4pt] [1] T. Lanting, A. J. Przybysz, A. Y. Smirnov, F. M. Spedalieri, M. H. Amin, A. J. Berkley, R. Harris, F. Altomare, S. Boixo, P. Bunyk, N. Dickson, C. Enderud, J. P. Hilton, E. Hoskin- son, M. W. Johnson, E. Ladizinsky, N. Ladizinsky, R. Neufeld, T. Oh, I. Perminov, C. Rich, M. C. Thom, E. Tolkacheva, S. Uchaikin, A. B. Wilson, and G. Rose, Physical Review X 4, 021041 (2014).\\[0pt] [2] T. Albash, S. Boixo, D. A. Lidar, and P. Zanardi, New J. of Phys. 14, 123016 (2012).\\[0pt] [3] S. W. Shin, G. Smith, J. A. Smolin, and U. Vazirani, arXiv:1401. [Preview Abstract] |
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H1.00096: Using the SLUG as a First Stage, Low Noise Microwave Amplifier for Superconducting Qubit Readout Edward Leonard Jr., Ted Thorbeck, Shaojiang Zhu, Robert McDermott The SLUG (Superconducting Low-inductance Undulatory Galvonometer) microwave amplifier is a large bandwidth, high saturation power, high gain, and low noise microwave element designed as a first stage cryogenic amplifier for dispersive readout of superconducting qubits. High forward gain is paired with simultaneous high reverse isolation such that bulky, expensive cryogenic circulators and isolators might be eliminated from the microwave readout chain. Here we present recent experimental data on SLUG gain, noise, and reverse isolation. We achieve gain over 10 dB at 7 GHz across a band of several hundred MHz, with system added noise of order one photon. For appropriate flux bias of the device, reverse isolation is better than -20 dB. These qualities make the SLUG a very desirable first stage amplifier for a scalable superconducting qubit readout. [Preview Abstract] |
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H1.00097: Dynamic nuclear polarization of nitrogen-vacancy centers in diamond Wen-Hui Hu Single nitrogen-vacancy (NV) centers in diamond triggered the research for wild applications in quantum information processing and quantum metrology. One of the most important advantages of the NV centers is the long coherence time of the center electron spins. Dynamic nuclear polarization (DNP) has been introduced as an efficient method to protect the spin coherence. The coherence time $T_2^{\ast}$ should have been prolonged of two orders theoretically, nevertheless less than one order in experiments. In this work, we theoretically study the DNP process in a high-purity diamond, where the dipole-dipole hyperfine interaction between the center electron spins and the bath $^{\mathrm{13}}$C nuclear spins is dominant. The simulations show that the saturated polarization of the nuclear bath depends on the spin-lock period and the efficiency of the initialization laser, accompanied with the magnitude of the external magnetic field. The polarization saturation comes from the capability of the polarization transfer and the equilibrium of probability distribution between the polarized and unpolarized states. [Preview Abstract] |
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H1.00098: Phase noise of a cavity electromechanical oscillator at millikelvin temperatures Junho Suh, Sungwan Cho, Sang Goon Kim, Seung-Bo Shim The frequency stability of a mechanical resonator is an important factor in its application to quantum information technology. We investigate the phase noise in a self-oscillation of a micromechanical resonator, parametrically driven by a superconducting microwave resonator at millikelvin temperatures. Possible physical origins of the noise are also discussed. [Preview Abstract] |
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H1.00099: Investigation of the thermal motion and mode coupling in a micromechanical resonator Seung-Bo Shim, Sungwan Cho, Sang Goon Kim, Sung Un Cho, Yun Park, Junho Suh We have investigated the thermal motion and mode coupling in a micromechanical resonator. The mechanical resonator was designed for dielectric gradient force actuation scheme. The laser reflection measurement method enabled multi-mode detection of the thermal motion up to 5$^{th}$ mode at room temperature. With these multi-modes, we could investigate the energy transfer between first and second mode by applying mechanical sideband signals. We have utilized the second mode as a phonon cavity and observed the coupling and interaction between two modes. Here, we will discuss about the room temperature mechanical mode detection method and mode coupling effect in the micromechanical resonator. [Preview Abstract] |
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H1.00100: Construction of a Confocal Fluorescence Microscope to Image Nitrogen Vacancy Centers Jordan Stroman, James Griffin, Gary Harris Long term atomic memory can be achieved using nitrogen vacancy centers (NV). Howard University is optimizing the process of creating nitrogen vacancy centers using hot filament chemical vapor deposition (HFCVD). In order to provide reliable feedback concerning the presence, concentration, and orientation of these color centers, an optical system capable of performing confocal laser scanning fluorescence microscopy has been constructed. This system consists of a 200mw laser that emits light with a wavelength of 532nm. This light is focused on a sample using a Nikon Oil Objective Lens with a numerical aperture of 1.3. The sample rest on a piezoelectric stage with a resolution of 20nm in the x, y and z direction. This optical system can confirm and locate NV centers with a resolution of 200nm. [Preview Abstract] |
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H1.00101: Digital quantum simulation of Heisenberg spin systems in circuit QED Markus Oppliger, Yves Salathe, Mintu Mondal, Johannes Heinsoo, Philipp Kurpiers, Anton Potocnik, Stefan Filipp, Andreas Wallraff, Antonio Mezzacapo, Urtzi Las Heras, Lucas Lamata, Enrique Solano A Quantum simulator realized by a well-controlled quantum system allows to simulate a wide range of complex quantum systems that are very difficult to study with classical computing. We use a promising quantum simulator based on circuit quantum electrodynamics (QED) to digitally simulate the isotropic Heisenberg XYZ interaction between two spin 1/2 particles. Since the XYZ interaction does not occur directly in the Jaynes-Cummings Hamiltonian, the interaction is decomposed into a set of single- and two-qubit gates. The resulting evolution of the quantum state is analyzed by state tomography for different interaction times after each step. As our approach can be generalized further, this experiment is a first step towards simulating large spin systems in a circuit QED architecture. [Preview Abstract] |
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H1.00102: High-fidelity quantum memory utilizing inhomogeneous nuclear polarization in a quantum dot Wenkui Ding, Anqi Shi, Jianqiang You, Wenxian Zhang We numerically investigate the encoding and retrieval processes for a quantum memory realized in a semiconductor quantum dot, by focusing on the effect of inhomogeneously polarized nuclear spins whose polarization depends on the local hyperfine coupling strength. We find that the performance of the quantum memory is significantly improved by the inhomogeneous nuclear polarization, as compared to the homogeneous one. Moreover, the narrower the nuclear polarization distribution is, the better the performance of the quantum memory is. We ascribe the performance improvement to the full harnessing of the highly polarized and strongly coupled nuclear spins, by carefully studying the entropy change of individual nuclear spins during encoding process. Our results shed new light on the implementation of a quantum memory in a quantum dot.\\ 1. Wenkui Ding, Anqi Shi, J. Q. You and Wenxian Zhang. arXiv:1407.7242 [cond-mat.mes-hall] [Preview Abstract] |
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H1.00103: A photon-photon quantum gate using a multilevel atomic system Yuuki Tokunaga We propose a method for a quantum gate between photons assisted by a multilevel atomic system. The atomic system is supposed to be in a cavity or a one-dimensional waveguide. The system can transfer a quantum state between a photon and the atom, and also works as a quantum gate for consecutively input photons. This system could be used for a building block for a universal quantum computation. We also discuss the characteristics of such quantum gates with several different multilevel system. [Preview Abstract] |
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H1.00104: Riemannium nucleus and quantum solution to the Riemann hypothesis (RH) Carlos Figueroa-Navarro, Julio Campos-Garcia, Mart\'In Molinar-Tabares, Lamberto Castro-Arce Is there an energy spectrum according to the primes numbers? This raises the question whether physical systems whose spectrum is the Riemann zeta function; ie, is there a spectrum of resonances of a nucleus that are the zeros of the zeta function? The renowned HR is basically preceded by the Euclid principle, the Euler product and the Gauss theorem. The best mathematical interpretation is the one that threw Riemannian revolution. HR holds that the nontrivial zeros of the zeta function keep the harmony of primes, but this is not fully tested. There are two possibilities: The HR can be true, then the primes have harmony, but if false, they nest in the chaos. The idea of this is important for studying models of quantum chaos is getting stronger in the last decade. These investigations lead us to believe that a highly mathematical problem becomes a physical problem; because according to Hugh Montgomery and Michael Berry the variations of the zeros, with drums and quantum billiards, can provide a physical model for explain the primes. In this paper, we generate the Mount Riemann by using MATHEMATICA and we get the profile that relates the prime numbers with the zeros of the zeta function; also we explain how quantum helps to explain the famous old problem known as the conjecture of Riemann. [Preview Abstract] |
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H1.00105: Distribution of quantum Fisher information in asymmetric cloning machines Xing Xiao, Yao Yao, Lei-Ming Zhou, Xiaoguang Wang An unknown quantum state cannot be copied and broadcast freely due to the no-cloning theorem. Approximate cloning schemes have been proposed to achieve the optimal cloning characterized by the maximal fidelity between the original and its copies. Here, from the perspective of quantum Fisher information (QFI), we investigate the distribution of QFI in asymmetric cloning machines which produce two nonidentical copies. As one might expect, improving the QFI of one copy results in decreasing the QFI of the other copy, roughly the same as that of fidelity. It is perhaps also unsurprising that asymmetric phase-covariant cloning outperforms universal cloning in distributing QFI since a priori information of the input state has been utilized. However, interesting results appear when we compare the distributabilities of fidelity (which quantifies the full information of quantum states), and QFI (which only captures the information of relevant parameters) in asymmetric cloning machines. Unlike the results of fidelity, where the distributability of symmetric cloning is always optimal for any $d$-dimensional cloning, we find that any asymmetric cloning outperforms symmetric cloning on the distribution of QFI for $d\leq18$, whereas some but not all asymmetric cloning strategies could be worse [Preview Abstract] |
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H1.00106: Quantum Information in Biological Systems Shantilal Goradia We can derive strong coupling and also recover Newtonian gravity with a quantum mechanical approach implicitly impling the variation of universal contant of gravity on a cosmic scale. Since constancy of G does not lead to cosmological constnat, we derive 137 (described as the hand of God by Feynman) as a natural logarithm of the age of the universe in Planck times (10E60). Since we use Boltzann equation on his tomb to do that, we describe that equation as the heart of God! Since otherwise, no theory including the string theory can so far come up with 137 mathematically. Maximum potential ON and OFF signals of interactions of 10E60 ever since the big bang could provide the fundamental basis of information system in the universe including in biology, implying the sperm of a man carries the genetic information of the male which when combined with the genetic information of a female egg decides the characteristics of the offspring. We may never know the exact language of nature. We speculate the string theory may be able to step in to show some deeper light in that direction. [Preview Abstract] |
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H1.00107: Two-Slit Particle Experiment and the CMB Alfred Phillips Jr. Both Einstein and Feynman discussed the difficulty of understanding the two slit experiment for particles. We show a connection between this experiment and the Cosmic Microwave Background. We have not yet determined whether this new connection necessitates a modification of metrics such as that of Robertson-Walker or how much light this new connection sheds on the dark energy (cosmological constant) problem.. [Preview Abstract] |
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H1.00108: MATTER AT EXTREME CONDITIONS |
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H1.00109: RKKY and Dzyaloshinsky-Moriya Interaction in the Electron Gas Mohammad Mahdi Valizadeh, Sashi Satpathy We illustrate the origin of the Dzyaloshinsky-Moriya interaction $ \vec S_1 \times \vec S_2$ between two localized spins embedded in a solid by considering the example of the electron gas with spin-split bands, which serves as a simple pedagogical model for the understanding of this interaction. In this case, where symmetry is broken, the magnetic interaction acquires the Dzyaloshinsky-Moriya term in addition to the well-known RKKY interaction term $\vec S_1 \cdot \vec S_2 $, so that the net interaction has the form $H = J \vec S_1 \cdot \vec S_2 + \vec D \cdot \vec S_1 \times \vec S_2$. For the standard electron gas with spin degenerate states, the DM term vanishes yielding the well known RKKY interaction results. Explicit expressions for the magnitudes of the interactions are obtained for the electron gas in two and three dimensions. This simple model serves as a pedagogical example for the origin of the Dzialoshinski-Moriya interaction in a system with broken time-reversal symmetry. [Preview Abstract] |
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H1.00110: Chromium based Spinels under Compressions Yuejian Wang, Ilias Efthimiopoulos, Thomas Ahearn, Vladimir Tsurkan, Joachim Deisenhofer, Alois Loidl The Chromium (Cr) based spinels, ACr2X4, represent a prototype system for the study of magnetism in solid [1]. More recently, multiferroicity has been found in members of this series [2]. However, the origin of the ferroic properties is not well understood; Given the strong interplay between structural and ferroic properties in this system, the structural evolution induced by pressure may shed light on the multiferroicity [3]. High-pressure X-ray diffraction and Raman spectroscopic studies have been conducted on ZnCr2Se4 and ZnCr2S4. The study elucidated the phase transformation of these spinels under high pressures by the X-ray data and the complementary information from Raman spectra. In the meantime, the unit cell volumes as well as the lattice parameters versus pressures of each individual phase were illustrated, and the corresponding elastic moduli were obtained by the fitting of the second order Birch Murnaghan equation of state. Furthermore, the vibrational properties as well as the possible changes of electric and magnetic properties induced by the structural transitions were discussed. \\[4pt] [1] T. Rudolf \textit{et al}., N. J. Phys. 9, 27 (2007) and refs. therein\\[0pt] [2] S. Weber \textit{et al}., PRL 96, 157202 (2006)\\[0pt] [3] V. Gnezdilov \textit{et al}., PRB 84, 045106 (2011) [Preview Abstract] |
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H1.00111: SURFACES, INTERFACES AND THIN FILMS |
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H1.00112: Directed Chemical Transport and Separation by Hydrogel Films containing Static and Dynamic Chemical Potential Gradients Tsung-Han Tsai, Chunjie Zhang, Hyung-Jun Koo, Paul V. Braun Materials that can manipulate the anisotropic molecular transport through built-in chemical potential gradients offer new opportunities to process chemical agents. Different from electrophoresis and microfluidics, here the chemical potential gradients, which provide the driving forces for molecular transport, are incorporated in the diffusion media. The autonomous systems can independently control the anisotropic flux of molecules and thus do not need external inputs such as electric fields and flowing carrier phases. As model systems, we used hydrogels containing static and dynamic built-in chemical potential gradients to direct molecular transport, concentrate dilute analyte and separate mixtures. The static gradients are based on electrostatic and supramolecular interactions which are specifically designed for the target molecules. The dynamic active chemical potential gradients triggered by reversible ion exchange process can alter the hydrophilicity gradually to transport or separate target molecules. [Preview Abstract] |
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H1.00113: A density functional theory investigation of select transition metal dichalcogenides Rodrick Kuate Defo, Georgios Tritsaris, Shiang Fang, Efthimios Kaxiras As a result of the extensive work done on graphene, leading to advances in sample preparation, optical detection, and transfer and manipulation of 2D materials, there has been a resurgence of interest in layered materials from which single sheets can be extracted, such as molybdenum disulfide. Particularly intriguing is the fact that layered MoS$_2$ transitions from an indirect band gap in the bulk to a direct band gap in the monolayer opening up the possibility of optoelectronic applications. These results have been verified using density functional theory and, further, dependence of the band gap on lattice strain has been investigated. This dependence is crucial in understanding emergent properties of compounds consisting of MoS$_2$ layered with other materials where there is a lattice mismatch. MoSe$_2$, MoSSe, WS$_2$, WSe$_2$ and WSSe have also been studied. Finally, dielectric functions have also been obtained for these compounds to explore the effect particularly of the asymmetric atom configurations on polarization of the material. [Preview Abstract] |
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H1.00114: Monomer Adsorption on 6-Atoms Wide Zigzag (111) Terracess Alain Phares, David Grumbine, Jr We study monomer adsorption on six-atoms wide, zigzag ($111$) terraces, with first- ($V$), second- ($W$), and third-neighbor ($U$) interactions, specializing to repulsive first-neighbors. All possible crystallization patterns, or phases, that may exist are expected to occur at relatively low temperatures. Under these conditions, the energy phase diagram is three-dimensional and depends on the dimensionless variables, $v = \mu/|V|$, $u = U/|V|$, and $w = W/|V|$. The chemical potential energy of the monomers, $\mu$, in the medium to which the terrace is exposed depends on the pressure, if the medium is a gas, or the concentration if the medium is solution. There are $95$ phases, or crystallization patterns, of the adsorbates with coverages ranging from $1/5$ to $8/9$. In particular, we find that there are $10$ distinct $1/2$, $2/3$, and $4/9$ coverage phases, and $9$ distinct $1/3$ coverage phases. [Preview Abstract] |
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H1.00115: Interplay between Self-Assembled Structures and Energy Level Alignment of Benzenediamine on Au(111) Surfaces Guo Li, Jeffrey Neaton Using van der Waals-corrected density functional theory (DFT) calculations, we study the adsorption of benzene-diamine (BDA) molecules on Au(111) surfaces. We find that at low surface coverage, the adsorbed molecules prefer to stay isolated from each other in a monomer phase, due to the inter-molecular dipole-dipole repulsions. However, when the coverage rises above a critical value of 0.9nm-2, the adsorbed molecules aggregate into linear structures via hydrogen bonding between amine groups, consistent with recent experiments [Haxton, Zhou, Tamblyn, et al, Phys. Rev. Lett. 111, 265701 (2013)]. Moreover, we find that these linear structures at high density considerably reduces the Au work function (relative to a monomer phase). Due to reduced surface polarization effects, we estimate that the resonance energy of the highest occupied molecular orbital of the adsorbed BDA molecule relative to the Au Fermi level is significantly lower than the monomer phase by more than 0.5 eV, consistent with the experimental measurements [DellAngela, Kladnik, and Cossaro, et al., Nano Lett. 10, 2470 (2010)]. [Preview Abstract] |
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H1.00116: Benzene Derivatives Adsorbed to the Ag(111) Surface: A Binding Site Study Daniel Miller, Scott Simpson, Nina Tyminska, Eva Zurek Dispersion corrected Density Functional Theory (DFT) calculations were employed to study the interaction of benzenes mono and disubstituted with functional groups encompassing a region of the activated/deacivated spectrum. Benzenes substituted with weak activating or deactivating groups like methyl and fluoro, respectively, do not have a strong site preference for adsorption to the Ag(111) surface. Strong activating (N(CH3)2) and deactivating (NO2) groups, on the other hand, have a distinct site preference. The nitrogen in the former prefers to lie above a silver atom (top site), but in the latter an Hhcp site of the Ag(111) surface is favored. Benzenes derivatized with classic activating groups donate electron density from the highest occupied molecular orbital (HOMO) of the molecule to the surface, and those functionalized with deactivating groups withdraw electron density from the surface into orbitals that are unoccupied in the gas phase. In the case of disubstituted benzenes, the strong activating/deactivating groups control the site preference and other groups assume sites that are, to a large degree, dictated by their positions on the benzene ring. Surface adsorption alters the relative stabilities of the ortho, meta and para positional isomers of disubstituted benzenes. [Preview Abstract] |
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H1.00117: Quasi-1D States Confined in a Self-Assembled Organic Super-Lattice of TTF-TCNQ on Ag(111) Seokmin Jeon, Panchapakesan Ganesh, Bobby Sumpter, Jorge Iribas Cerd\'a, Petro Maksymovych Organic charge transfer complexes (CTC) have drawn much attention due to their potential applications to conducting or semiconducting organic thin films and contacts in devices. TTF-TCNQ is a historic organic CTC with one of the highest conductivity values among numerous organic conductors. As a two-component molecular material, TTF-TCNQ in a low-dimension form on a surface naturally creates monolayer super-lattices with corrugated electrostatic potential and adsorbate-induced strain. Generally this will lead to strong confinement of the surface states, although the detailed response of the surface electronic structure remains to be understood. We investigated TTF-TCNQ monolayer films grown on Ag(111), Au(111) and Ag(100) surfaces using STM/STS at 4.3 K. Confinement of sp-derived surface states was indeed ubiquitous, including spontaneous formation of quantum dots and quasi-1D bands. The small periodicity of the lattice caused a complete depopulation of the surface states, with up to 1 eV upshift of the band minimum - much stronger effect than normally observed in assemblies. This also allows us to infer the height of the confining potential using 1D Kronig-Penney model and critically assess the long-standing problem of molecule-surface charge transfer. [Preview Abstract] |
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H1.00118: Adsorption of pentacene on (100) vicinal surfaces: role of coordination, surface chemistry and vdWs effects Jeronimo Matos, Abdelkader Kara In contrast to low miller index surfaces, vicinal surfaces are characterized by steps and step edges that not only present an interesting atomic landscape for the adsorption organic molecules, but also a unique electronic structure resulting in part from the low coordinated atoms at the step edges. The adsorption of pentacene on the stepped (511), (711), (911) surfaces (respectively 3, 4 and 5-atom wide terraces) of Cu and Ag (coinage transition metals); Pt (reactive transition metal); and Ni (reactive, magnetic transition metal) are studied using density functional theory, in order to investigate the support effects arising from differing surface chemistry. We compare the adsorption energy, adsorption geometry and electronic structure predicted by the PBE functional with those obtained from one of the optimized vdW-DF methods: optB88-vdW. [Preview Abstract] |
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H1.00119: Electron configuration and correlation effects in organometallic molecules from constraint density functional theory Kenji Nawa, Kohji Nakamura, Toru Akiyama, Tomonori Ito, Michael Weinert Interest in single organometallic molecule and that adsorbed on solid surfaces has rapidly increased because of possible novel applications. For molecules with transition metals (TMs), the $d$-electron configuration is an essential aspect of their electronic and magnetic properties, and correlation effects can not be excluded. Here, we investigate systematically the electron configuration and correlation effects for prototypical organometallic molecules of tridimensional metallocene (TMCp$_{\mathrm{2}})$ and planer phthalocyanine (TMPc). Calculations were carried out based on the constraint density functional theory (DFT) by using the full-potential linearized augmented plane wave method that incorporates an on-site Coulomb interaction correction $+U$. We find that these correlation effects play a key role in determining the ground state of the electron configuration for the organometallic molecules. The calculated ground states of TMCp$_{\mathrm{2}}$, where TM$=$Cr, Mn, Fe, Co, and Ni, obtained by constraint DFT with $+$U reproduce the experimentally determined structures of $^{\mathrm{3}}E_{\mathrm{2g}}$, $^{\mathrm{6}}A_{\mathrm{1g}}$, $^{\mathrm{1}}A_{\mathrm{1g}}$, $^{\mathrm{2}}E_{\mathrm{1g}}$, and $^{\mathrm{3}}A_{\mathrm{2g}}$, respectively. Results for the TMPc will be also presented. [Preview Abstract] |
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H1.00120: MAGNETISM |
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H1.00121: Structural and Magnetic Properties of Mn$_{1.5}$X$_{0.5}$Sn (X $=$ Cr, Mn, Fe, Co) Melt-spun Ribbons R. Fuglsby, P. Kharel, W. Zhang, S. Valloppilly, Y. Huh, D.J. Sellmyer Mn$_{1.5}$X$_{0.5}$Sn (X $=$ Cr, Mn, Fe, Co) nanomaterials in a hexagonal Ni$_{2}$In-type crystal structure have been prepared using arc-melting and melt spinning. All the samples show moderate saturation magnetization at 100 K with a highest value of 458 emu/cm$^{3}$ for Mn$_{1.5}$Fe$_{0.5}$Sn, but their Curie temperatures (Tc) are less than 300 K. The highest Tc is 206 K for the Fe containing sample. All samples except the Cr containing one show irreversibility between the zero-field-cooled and field-cooled measurements at the low temperature, showing a spin reorientation or spin-glass-like behavior. The magnetic anisotropy constants calculated at 100 K are on the order of 1 Merg/cm$^{3}$. The magnetic properties of these materials have substantially improved due to vacuum annealing, where the Tc for Mn$_{2}$Sn annealed at 450 $^{\circ}$C has increased by about 75 K from 190 K to 265 K. [Preview Abstract] |
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H1.00122: Noncollinear ferromagnetic easy axes in spin valves induced by oblique deposition Roberto Rodriguez, Thiago Bueno, Daniel Parreiras, Sebastian Michea, Mario Araujo, Waldemar Macedo, Klaus Krambrock, Roberto Pianago In this work we investigate the magnetic properties of Py/Ru/FeCo/IrMn spin valves grown by oblique magnetron sputtering. The in-plane angular dependence of the ferromagnetic resonance (FMR) was used to obtain the relevant magnetic anisotropies. As we show, the deposition geometry employed in the sample preparation setup can be used to induce noncollinear easy axes of the ferromagnetic (FM) layers of the spin valve. We could directly observe the non-collinearity on the symmetry shift of the angular dependence of the FMR resonances fields of the two FM layers. The observations of the present study suggest that, by combining oblique deposition and appropriate angles of incidence of the deposition flux, the uniaxial (and unidirectional) axes of individual FM layers can be precisely engineered in spin valve fabrication. [Preview Abstract] |
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H1.00123: Temperature and size dependent magnetic hyperthermia studies of Dextran coated Fe$_{3}$O$_{4}$ and Co$_{x}$Fe$_{\mathrm{3-x}}$O$_{4}$ ferrofluids H. Nemala, M. Palihawadana Arachchige, G. Lawes, V.M. Naik, R. Naik Magnetic hyperthermia (MHT) using magnetic nanoparticles (MNPs) is a promising technique for cancer therapy. The dominant mechanism of heat generation in MHT using superparamagnetic MNPs is the N\'{e}el relaxation in response to an applied ac magnetic field. The efficiency of heating depends on the particle size, particle size distribution and the intrinsic magnetic properties of the MNPs. In this study, we have prepared Fe$_{3}$O$_{4}$(8-14 nm) and Co$_{0.1}$Fe$_{2.9}$O$_{4}$ (10 nm) MNPs by the co-precipitation method and characterized using XRD, TEM, Zeta potential and DC magnetometry measurements. The MNPs are found to be polydispersed and form stable colloidal suspensions in weakly basic solutions (zeta potential $\sim$ -20 mV) with their hydrodynamic radii ranging from 80 to 120 nm. The specific power loss (SPL) was determined as a function of temperature using MHT measurements (140-235 Oe and 188-375 kHz) by incorporating heat losses due to nonadiabatic sample conditions. The SPL values at 298 K measured with 235 Oe and 375 kHz range from 20-95 W/g for the MNPs, and SPL monotonically decrease with increasing in temperature. The results are in agreement with the linear response theory. Details of the measurement and analyses will be presented. [Preview Abstract] |
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H1.00124: First Successful Fabrication of Nanoparticles of magnetocaloric Gd$_{5}$Si$_{4}$ Ravi L. Hadimani, Shalabh Gupta, Shane M. Harstad, Vitalij K. Pecharsky, David C. Jiles The Gd$_{5}$(Si$_{\mathrm{x}}$Ge$_{\mathrm{1-x}})_{4}$ system has been widely studied in bulk form due to its interesting properties at the phase transition. There are a few reports on the fabrication of thin films of this material but, there are no reports in the literature on synthesis and characterization of nanoparticles of this material. Unlike films, which are expected to have low refrigeration capacity due to low volume, nanoparticles have the potential to overcome the problem if a scalable and cost-effective method of nanoparticle fabrication can be developed. In this work, we have synthesized sub-micron particles of Gd$_{5}$Si$_{4}$ by high-energy ball-milling varying milling times and milling intensity. We have investigated their microstructure, crystal structure, composition and magnetic properties. We have determined the milling time beyond which the particles become non-crystalline and lose the long range ordering. We also show that the coercivity of the particles increases with increasing the milling time. Particles agglomerate at long milling times and the particles that are milled longer than 20 min no longer undergo magnetic phase transition close to 340 K, which is present in a bulk material. [Preview Abstract] |
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H1.00125: Experimental Realization of Artificial Skyrmion Lattices Dustin Gilbert, Brian Maranville, Andrew L. Balk, Brian J. Kirby, Peter Fischer, Daniel T. Pierce, John Unguris, Julie A. Borchers, Kai Liu Magnetic skyrmions exhibit topologically protected states, offering new mechanisms for high density/low dissipation information storage, and also exhibiting a host of unique topological phenomena. In bulk crystals, chiral spin textures are only found in certain systems and in limited regions in the temperature-magnetic field parameter space. We present experimental evidence of room-temperature artificially structured skyrmion lattices fabricated by carefully controlling the three dimensional anisotropy of a Co on Co/Pd hybrid structure. The hybrid structures were fabricated by patterning chirality controlled vortex-state Co nanodot arrays on top of a Co/Pd multilayer with perpendicular anisotropy; chirality control was confirmed by microscopy and magnetometry. The vortex polarity is set by an external magnetic field to manifest the skyrmion state, and confirmed by magnetometry measurements. The chiral structure of the nanodots is imprinted into the Co/Pd underlayer, as revealed by polarized neutron reflectometry and spin-transport studies. These artificial skyrmion lattices offer a convenient platform to explore skyrmion physics. This work has been supported by the NSF (DMR-1008791 and ECCS-1232275). [Preview Abstract] |
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H1.00126: Anatomy of Dzyaloshinskii-Moriya Interaction at Co/Pt Interfaces Hongxin Yang, Stanislas Rohart, Andre Thiaville, Albert Fert, Mairbek Chshiev Dzyaloshinskii-Moriya Interaction (DMI)[1] was recognized to play a crucial role at ferromagnetic (FM)/heavy metal (NM) interfaces to create magnetic skyrmions[2]. DMI also plays an essential role for fast domain wall dynamics driven by spin-orbit (SO) torques[3]. Here, we clarify the main features and microscopic mechanisms of DMI in Co/Pt bilayers by ab initio. We find that large anticlockwise DMI of the bilayers has a predominant contribution from DMI pair couplings between spins of interfacial Co layer. This DMI between interface Co spins is directly related to the change of SO energy in the adjacent Pt when Co spin chirality is reversed. DMI does not extend significantly into other Co layers and is very weak between the proximity-induced spins in Pt. It was suggested[4] that DMI at FM/NM interfaces is directly related to the proximity induced moment in NM. However, we find the opposite result, i.e. Pt moment reduction slightly increases the DMI[5]. [1] I. E. Dzialoshinskii, Sov. Phys. JETP 5, 1259 (1957); T. Moriya, Phys. Rev. 120, 91 (1960). [2] A. Fert et al. Nat. Nanotech. 8, 152 (2013). [3] A. Thiaville, et al, Europhys. Lett. 100, 57002 (2012). [4] K. Ryu et al, Nat. Nanotech. 8, 527 (2013). [5] H. Yang et al, submitted [Preview Abstract] |
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H1.00127: Theory of damping for the standing spin waves Irina Bariakhtar, Victor Bariakhtar It is well known, that the thin magnetic films exhibit dependency of the magnetic dispersion on the wave vector. This is due to the fact that in films with their thickness comparable to the exchange length, the wave vector of the spin waves becomes of the order of the exchange length because of the boundary conditions for magnetization. These kinds of thin films were studied at first by Kittel [1]. The standing spin waves are characterized by the fact, that under certain conditions they do not correlate to an alternating magnetic field within or outside the film [2]. The damping theory for the standing spin waves was not well studied yet. This problem appears interesting, since the distance between the neighboring standing spin waves increases with increasing number of frequency as $n$, and the attenuation increases with a mode number increase as $n^{4}$. In other words, high-frequency modes of the standing spin waves are not created if the exchange relaxation mechanism is valid. The standing spin waves properties are being well studied experimentally lately [3]. The authors would like to compare their theoretical results to the experimental data.\\[4pt] [1] Phys. Rev. 110, 1295 (1958).\\[0pt] [2] Spin Waves. Interscience (Wiley), New York (1968).\\[0pt] [3] Phys. Rev. Lett. 107, 037202 (2011). [Preview Abstract] |
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H1.00128: Controlling strain anisotropy in iron-palladium thin films using perovskite-oxide substrates Renee Harton, Vladimir Stoica, Roy Clarke In this study, iron-palladium (FePd) thin films were deposited on (100) barium-titanate ($BaTiO_3$) and (100) strontium-titanate ($SrTiO_3$) substrates. Both $BaTiO_3$ and $SrTiO_3$ have a perovskite crystal structure and exhibit similar structural phases, such as tetragonal and cubic, at various temperatures. In contrast to $SrTiO_3$, $BaTiO_3$ exhibits ferroelectric and piezoelectric behavior in all of its structural phases except the cubic phase. In the tetragonal phase, the strain anisotropy of $BaTiO_3$ is two-fold about the in-plane c-axis, while in the cubic phase the epitaxial strain in the substrate plane is four-fold. In this investigation, the effect of strain on the magnetism and structure of FePd/$SrTiO_3$ and FePd/$BaTiO_3$ heterostructures was studied using the Magneto-Optic Kerr Effect (MOKE), Atomic Force Microscopy (AFM) and X-Ray diffraction (XRD) analysis to investigate the correlation between the magnetic anisotropy, morphology and structure of the FePd films. [Preview Abstract] |
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H1.00129: Synthesis of magnetic GdC$_{2}$ nanoparticles using cavitation plasma Rakesh Chaudhary, Ali R. Koymen Gadolinium dicarbide (GdC$_{2})$ nanoparticles were synthesized using Gd electrodes in toluene. Gd nanoparticles are formed in plasma caused due to collapse of cavitation bubbles using ultrasonication in electric field between Gd wire electrodes. The presence of a single phase of GdC$_{2}$ nanocrystals have been determined by X-Ray Diffraction (XRD) and High Resolution Transmission Electron Microscopy (HRTEM). The GdC$_{2}$ nanoparticles have tetragonal crystal structure. Transmission Electron Microscopy (TEM) shows that the nanoparticles range in size of 4-45 nm in diameter. Magnetization measurements performed using a Superconducting Quantum Interference Device (SQUID) magnetometer shows GdC$_{2\, }$nanoparticles are paramagnetic in nature. To the best of our knowledge, this is the first synthesis of GdC$_{2}$ in single phase form, allowing further characterization of physical properties. [Preview Abstract] |
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H1.00130: Magnetic and Structural characterization of Co nanowires using advanced electron microscopy techniques Jesus Cantu-Valle, Francisco Ruiz-Zepeda, John Eder Sanchez, Fernando Mendoza-Santoyo, Arturo Ponnce We report the magnetic imaging and crystalline structure of high aspect ratio cobalt nanowires. Experimental results of magnetization reversal in cobalt nanowires are presented to illustrate the functionality of the \textit{in situ} magnetization process through the manipulation of the objective lens. By making use of this applicability, we measure the magnetization and show experimental evidence of the magnetic flux distribution in polycrystalline cobalt nanowires using off-axis electron holography. The retrieved phase map can distinguishes the magnetic contribution from the crystalline contribution with high accuracy. To determine the size and orientation of the grains within the Co nanowires, PED-assisted orientation mapping was performed. Finally, the magnetic analysis performed at individual nanowires was correlated with the crystalline orientation map, obtained by PED-assisted crystal phase orientation mapping. The large shape anisotropy determines the mayor magnetization direction rather than the magneto-crystalline anisotropy in the studied nanowires. The combination of the two techniques allowed us to directly visualize the effects of the crystallographic texture on the magnetization of the nanowire. [Preview Abstract] |
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H1.00131: Magnetic properties of Ru-Ti doped Strontium hexaferrite nanocrystalline particles Abdel Alsmadi, S. Mahmood, I. Bsoul We carried out a systematic study on the effect of the substitution of Ti$^{\mathrm{2+}}$ and Ru$^{\mathrm{4+}}$ ions for Fe$^{\mathrm{3+}}$ ions on the magnetic properties of the strontium ferrite SrFe$_{\mathrm{12-2x}}$Ru$_{\mathrm{x}}$Ti$_{\mathrm{x}}$O$_{\mathrm{19}}$ nanoparticles with ($0\le x\le 1)$, using vibrating sample magnetometry, electrical resistivity, and M\"{o}ssbauer spectroscopy. A clear irreversibility between the zero-field-cooled and field-cooled curves was observed below room temperature and the zero-field-cooled magnetization curves displayed a broad peak at a temperature T$_{\mathrm{M}}$. These results were discussed within the framework of random particle assembly model and associated with the magnetic domain wall motion. The resistivity data show some kind of a transition from insulator to perfect insulator around $T_{M} $. With Ru-Ti substitution at 5 K, the saturation magnetization showed small variations were it slightly increased up to x $=$ 0.2 and then starts to decrease for x between 0.2 and 0.5, while the coercivity decreased monotonically, recording a reduction of about 93{\%} at x $=$ 0.4. These results were discussed in light of the single ion anisotropy model and the cationic distributions based on the results of the M\"{o}ssbauer spectroscopy data. [Preview Abstract] |
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H1.00132: Doping controlled spin reorientation in dysprosium-samarium orthoferrite single crystals Shixun Cao, Weiyao Zhao, Baojuan Kang, Jincang Zhang, Wei Ren As one of the most important phase transitions, spin reorientation (SR) in rare earth transition metal oxides draws much attention of emerging materials technologies. The origin of SR is the competition between different spin configurations which possess different free energy. We report the control of spin reorientation (SR) transition in perovskite rare earth orthoferrite Dy$_{1-x}$Sm$_{x}$FeO$_{3}$, a whole family of single crystals grown by optical floating zone method from x$=$0 to 1. Temperature dependence of the magnetizations under zero-field-cooling (ZFC) and field-cooling (FC) processes are studied. We have found a remarkable linear change of SR transition temperature in Sm-rich samples for x\textgreater 0.2, which covers an extremely wide temperature range including room temperature. The $a$-axis magnetization curves under FCC process bifurcate from and then jump down to that of warming process (ZFC and FCW curves) in single crystals when x$=$0.5-0.9, suggesting complicated 4f-3d electron interactions among Dy$^{3+}$-Sm$^{3+}$, Dy$^{3+}$-Fe$^{3+}$, and Sm$^{3+}$-Fe$^{3+}$ sublattices of diverse magnetic configurations for materials physics and design. The magnetic properties and the doping effect on SR transition temperature in these single crystals might be useful in the spintronics device application. [Preview Abstract] |
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H1.00133: Crystal structure and magnetic properties of 5d double perovskite oxide Sr2EuOsO6 Jianfeng He, Hai L. Feng, Yahua Yuan, Yoshihiro Tsujimoto, Kazunari Yamaura Polycrystalline Sr2EuOsO6 has been synthesized with high-pressures and temperatures. It crystallizes in a monoclinic double perovskite structure and shows an antiferromagnetic-like transition at 51 K in a magnetic susceptibility measurement. The transition has been further characterized by specific measurements and electrical resistivity measurements. The results are compared with the magnetic properties of Ba2EuOsO6 [1] and other double perovskite oxides containing Os(V) atom. We will discuss role of spin-orbit coupling and spin polarization on the gapped electronic structure of Sr2EuOsO6 and other related compounds.\\[4pt] [1] Y. Hinatsu et al. J. Solid State Chem. 206 (2013) 300. [Preview Abstract] |
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H1.00134: Thermal fluctuations in novel artificial spin ice Henry Stopfel, Erik Ostman, Aaron Stein, Unnar Arnalds, Vassilios Kapaklis, Bjorgvin Hjorvarsson Artificial spin ice(ASI) is used as a model material to investigate frustrated systems. The square and kagome ASI has been extensively investigated since there discovery. Novel ASI structures like the Shakti lattice, have been proposed and already realized. In this structure what is not an adaption of natural magnetic materials the lattice topology leads to a high degree of degeneracy. We present here the results of Photoemission electron microscopy (using XMCD) to image the magnetization of nano-islands in a Shakti ASI. By using a three layer of Pd-Fe-Pd we can tune the Curie temperature of our magnetic material by varying the thickness of the Fe-layer. Beside a statistical analysis of the frozen-in ground state, we present also a temperature series, in which we could visualize the two energy levels of the small and large islands and due to this the different blocking temperatures for these islands. The comparison of these measurements with previous measurements on squared ASI give us a better understanding of the magnetic ordering and the thermal fluctuations in the novel Shakti ASI. [Preview Abstract] |
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H1.00135: Negative magnetization and exchange bias in Y1-xPrxCrO3 with (0\textgreater x\textgreater 0.3) E. Verdin, A. Duran, F. Morales, E. Escudero Rare earth orthochromites compounds with perovskite structure have attracted great interest because its potential applications as data storage and spintronic. We report studies of the crystalline structure, thermal, and magnetic properties performed in the compound Y$_{1-x}$Pr$_{x}$CrO$_{3\, }$with 0\textless x\textless 0.3. We found changes in the specific heat and in the magnetization when the Pr atoms are substituted in the compound. The antiferromagnetic transition, T$_{N}$, increases when the Pr atoms are added into the compound which is clearly observed by specific heat and magnetization measurements. We also found an exchange bias and magnetization reversal when the magnetization-temperature (M-T) curves were measured in field cooled mode (FC). All those changes are attributed to the influence of the Dzialoshinskii-Moriya indirect interaction that we related to the octahedral distortion, because the Pr substitution affecting the Cr-O bond lengths. [Preview Abstract] |
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H1.00136: Neutron diffraction study on the phase diagram in muliferroic DyFeO$_{3}$ Jinchen Wang, Juanjuan Liu, Jieming Sheng, Zhiying Zhao, Xia Zhao, Xuefeng Sun, Sergey Danilkin, Wei Bao The discovery of the multiferroic effect in perovskite DyFeO$_{3}$ has generated many interests due to a witnessed strong magnetoelectric coupling. In this low temperature and high magnetic-field single-crystal neutron diffraction study, we determined the magnetic phase-diagram of DyFeO$_{3}$. Although the weak ferromagnetic phase of Fe spins has been suggested to be instrumental to the strong multiferroic effect in current experimental and theoretical works, the multiferroic effect is observed only in the phase area where the applied magnetic field breaks the long-range ordered (LRO) AF order of the Dy ions into a short-range order (SRO). Our results suggest the mechanism of the remarkably strong multiferroic effect in the prototype rare-earth orthoferrite DyFeO$_{3}$ ought to be investigated through the interplay between the weak ferromagnetism of Fe and the antiferromagnetic SRO of Dy spins. [Preview Abstract] |
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H1.00137: Engineering magnetic properties and microstructure of La$_{2}$CoMnO$_{6}$ thin films by tailoring the oxygen stoichiometry Benjamin Martinez, Regina Galceran, Carlos Frontera, Lluis Balcells, Jose Cisneros-Fernandez, Jaume Roqueta, Jose Santiso, Alberto Pomar, Felip Sandiumenge We report on the magnetic and structural properties of ferromagnetic-insulating La$_{2}$CoMnO$_{6}$ thin films grown on top of (001) STO substrates by means of RF sputtering technique. Insulating ferromagnets are of interest because of the exchange splitting of the bands allowing obtaining tunnel barriers with different height for spin-up and spin-down carriers. Belonging to the perovskite family, this material can be easily integrated in spintronic devices, such as magnetic tunneling junctions and spin filters, with upgraded and distinctive functionalities. An exhaustive structural analysis, by using synchrotron X-ray diffraction, allows identifying a close correlation between the film composition and their magnetic properties. Both Curie temperature and the features of the hysteresis loops turn out to be dependent on the oxygen stoichiometry. In situ annealing conditions allow tailoring the oxygen content of the films, therefore controlling their microstructure and magnetic properties. On the other hand, transport measurements confirm the insulating character of the films. [Preview Abstract] |
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H1.00138: Magnetic and thermoelectric properties of Fe$_{\mathrm{3-x}}$Co$_{\mathrm{x}}$O$_{4}$ thin films and CoFe$_{2}$O$_{4}$/Fe$_{3}$O$_{4}$ superlattices Quang Nguyen Van, Meny Christian, Anh Tuan Duong, Yooleemi Shin, Rhim S. H, Minh Hai Nguyen Thi, Sunglae Cho Microcrystalline ferrites are used as a medium for the magnetic recording and storage of information. Magnetite, Fe$_{3}$O$_{4}$, is a ferrimagnet with a cubic inverse spinel structure and exhibits a metal-insulator, Verwey, transition at about 120 K. It is predicted to possess as half-metallic nature, $\sim $ 100{\%} spin polarization, and high T$_{\mathrm{C}}$ (850 K). Cobalt ferrite, Co$_{3}$O$_{4}$, is one of the most important members of the ferrite family, which is characterized by its high H$_{\mathrm{C}}$, moderate magnetization and very high magnetocrystalline anisotropy. Here we report on the magnetic and thermoelectric properties of Fe$_{\mathrm{3-x}}$Co$_{\mathrm{x}}$O$_{4}$ (x $=$ 0 to 1) thin films and CoFe$_{2}$O$_{4}$/Fe$_{3}$O$_{4}$ superlattices grown on MgO (100) by MBE. XRD and RHEED patterns confirmed the inverse spinel structure of the Fe$_{3}$O$_{4}$ films. Magnetic properties of the Fe$_{\mathrm{3-x}}$Co$_{\mathrm{x}}$O$_{4}$ films are markedly sensitive to the Co content. The Verwey transition was disappeared in Co-doped films. A negative MR curve with butterfly shape was observed with low Co content but disappeared for the samples with x $=$ 0.8 and 1. Seebeck coefficients increased with Co concentration; -70 $\mu $V/K for x$=$0 and -220 $\mu $V/K for x$=$1. We will also discuss on the relationship between magnetic and thermoelectric characteristics in CoFe$_{2}$O$_{4}$/Fe$_{3}$O$_{4}$ superlattices with the modulations of 5, 10, and 20 nm. [Preview Abstract] |
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H1.00139: Magnetic and electrical properties on possible room temperature hybrid multiferroic BaTiO$_{3}$/La$_{2/3}$Sr$_{1/3}$MnO$_{3}$ John Edward Ordo\~nez, Mar\'Ia Elena G\'omez, Wilson Lopera Mu\~noz, Pedro Antonio Prieto We addressed to deposit the ferromagnetic phase of the La$_{\mathrm{1-x}}$Sr$_{\mathrm{x}}$MnO$_{3}$ and the ferroelectric BaTiO$_{3}$ for possible hybrid multiferroic heterostructure. We have optimized the growth parameters for depositing BaTiO$_{3}$(BTO) / La$_{2/3}$Ca$_{1/3}$MnO$_{3}$(LCMO) / (001) SrTiO$_{3}$ by sputtering RF and DC, respectively, in pure oxygen atmosphere and a substrate temperature of 830$^{\circ}$C. Keeping fixed the magnetic layer thickness (t$_{\mathrm{LSMO}}=$40 nm) and varying the thickness of the ferroelectric layer (t$_{\mathrm{BTO}}=$20, 40, 80, 100 nm). We want to point out the influence of the thicknesses ratio (t$_{\mathrm{BTO}}$/t$_{\mathrm{LSMO}})$ on electrical and magnetic properties. From x-ray diffraction (XRD) analysis, we found the bragg peaks for LSMO maintain its position but BTO peak shift to lower Bragg angle indicating a strained BTO film. Magnetization and polarization measurements indicate a possible multiferroic behavior in the bilayers. Hysteresis loop measurements of bilayers show ferromagnetic behavior. [Preview Abstract] |
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H1.00140: Magnetization fluctuation in FeB nanomagnets under asymmetric magnetization-potential Shinji Miwa, Hitoshi Kubota, Kay Yakushiji, Shota Ishibashi, Takeshi Saruya, Akio Fukushima, Shinji Yuasa, Yoshishige Suzuki Thermal fluctuation of magnetizations gives understandings of physics in magnetic materials and noise in magnetic devices. It is theoretically calculated using the Fokker-Planck equation and the fluctuation-dissipation theorem, [1] and is experimentally characterized using magnetoresistive devices [2]. In the present study, the magnetization-fluctuation under asymmetric magnetization potential has been investigated. Magnetic tunnel junctions (MTJs) [CoFeB(3 nm)/ MgO(1 nm)/ FeB (2 nm)] were employed to conduct the study. The FeB layer (120 nm in a diameter) is a magnetic free layer whose magnetic anisotropies are 8 mT (in-plane) and 97 mT (perpendicular). The asymmetric magnetization-potential was prepared using magnetic field application (110 mT) tilted from the film normal (10 deg.). [3] Unlike the first-order response to the thermal fluctuation, [2] the second-order response is identified as a Lorentzian power spectrum whose peak appears at 0 Hz. To derive the analytic formula, fourth-order moments are calculated using the quasi-normality hypothesis ($\left\langle {ABCD} \right\rangle =\left\langle {AB} \right\rangle \left\langle {CD} \right\rangle +\left\langle {AC} \right\rangle \left\langle {BD} \right\rangle +\left\langle {AD} \right\rangle \left\langle {BC} \right\rangle )$. As a results, the obtained formula quantitatively reproduces the experiment. \\[4pt] [1] W. F. Brown, Phys. Rev. 130, 1677 (1963).\\[0pt] [2] S. Petit et al., PRL 98, 077203 (2007).\\[0pt] [3] S. Miwa et al., Nat. Mater. 13, 50 (2014). [Preview Abstract] |
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H1.00141: Exchange interaction reduction as a precursor to laser-induced demagnetization in ferromagnets Guoping Zhang, Yihua Bai, Thomas F. George Laser-induced femtosecond demagnetization in a ferromagnet presents an opportunity to develop all-optical ultrafast magnetic storage devices, but its underlying mechanism is under intense debate. The controversy has been on the first several hundred femtoseconds, where the spin moment is reduced sharply up to 50\% or higher, but the optically accessible electrons are very few. This apparent contradiction is puzzling. Here we show that a small number of excited electrons is enough to trigger a strong band structure relaxation. In all of the three 3d ferromagnets investigated here, this band relaxation sharply reduces the exchange splitting and spin moment. For fcc Ni, for every electron excited, the spin moment can be reduced by 0.23 $\rm \mu_B$ or more. Our first-principles calculation, free of fitting parameters of any kind, finally explains the experimental findings and presents a paradigm for future experiments. [Preview Abstract] |
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H1.00142: A mechanical analogy for spin currents and torques Yaroslaw Bazaliy We map the problem of spin-transfer torques in nanostructures with diffusive spin currents on a mechanical problem involving elastic strings connecting points of attachement that slide along the fixed straight rails. The obtained mechanical analogy provides a qualitative understanding of the effects of spin relaxation on spin torques, and of the phenomenon of sign-changing angular dependence [1-3] of the efficiency factor $g(\theta)$. \\[4pt] [1] A. A. Kovalev, A. Brataas, and G. E. W. Bauer, Phys. Rev. B {\bf 66}, 224424 (2002).\\[0pt] [2] Jan Manschot, A. Brataas, and G. E. W Bauer, Phys. Rev. B {\bf 69}, 092407 (2004).\\[0pt] [3] J. Barnas, A. Fert, M. Gmitra, I. Weymann, and V. K. Dugaev, Phys. Rev. B {\bf 72}, 024426 (2005). [Preview Abstract] |
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H1.00143: Ultra Low Energy Switching of Ferromagnet with Perpendicular Anisotropy on Topological Insulator by Voltage Controlled Magnetic Anisotropy Bahniman Ghosh, Tanmoy Pramanik, Rik Dey, Urmimala Roy, Leonard Register, Sanjay Banerjee We propose and demonstrate, through simulation, an ultra low energy memory device on a topological insulator thin film. The device consists of a thin layer of Fe deposited on the surface of a topological insulator, Bi$_2$Se$_3$. The top surface of Fe is covered with MgO so that the ferromagnetic layer has perpendicular anisotropy. Current is passed on the surface of the topological insulator which switches the magnetization of the Fe ferromagnet through strong exchange interaction, between electrons contributing to the surface current on the Bi$_2$Se$_3$ and the d electrons in the ferromagnet, and spin transfer torque due to shunting of current through the ferromagnet. Voltage controlled magnetic anisotropy enables ultra low energy switching. Our micromagnetic simulations, predict switching time of the order of 2.4 ns and switching energy of the order of 0.16 fJ for a ferromagnetic bit with thermal stability of 90 k$_B$T. The proposed structure combines the advantages of both large spin torque from topological insulators and those of perpendicular anisotropy materials. [Preview Abstract] |
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H1.00144: Capturing the Magnetic and Structural Phase Transition of Ferh using Extreme Ultraviolet Light Dmitriy Zusin, Patrik Grychtol, Christian Gentry, Margaret Murnane, Henry Kapteyn, Sophie Canton, Ronny Knut, Justin Shaw, Hans Nembach, Thomas Silva, Alejandro Ceballos, Catherine Bordel, Peter Fischer, Frances Hellman The temperature dependent transition from the anti-ferromagnetic to the ferromagnetic phase in FeRh is accompanied by a modification of its crystal lattice. The interplay between the magnetic and the structural transition is a matter of strong debate. It is important to better understand the mechanism(s) of the transition since it can be induced by femtosecond laser pulses and, unlike slower (nanosecond) magnetic phase transitions, does not seem to be limited by heat transfer. In this work, we use extreme ultraviolet light generated by a tabletop high harmonics source to perform element-selective investigations of the temperature-dependent magneto-optical response of a thin film FeRh sample. We study the optically induced phase transition using two ultrafast pump-probe spectroscopy approaches: by monitoring the time-resolved transversal magneto-optical Kerr effect (T-MOKE) and the transient change in reflectivity. [Preview Abstract] |
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H1.00145: The effects of shape anisotropy and exchange coupling on spin precession frequencies in exchange coupled Co/Cu/Py trilayers Sam Keramati, Uday Singh, Seth Kurfman, Ch. Binek, S. Adenwalla Ultrafast high-power laser systems have successfully opened up the field of magnetization dynamics, studying subpicosecond laser-induced spin precession dynamics, demagnetization processes and magnetization reorientation. Here we investigate laser-induced magnetization dynamics in a series of photolithographically patterned microstructures of exchange coupled trilayers of Co/Cu/Py grown on Si substrates. The microstructures have different shape anisotropies as well as different exchange coupling parameters. The latter determines the magnetization state, varying from ferromagnetically to anti-ferromagnetically coupled. We explore how the different spin precession frequencies of the constituent exchange coupled magnetic layers with unequal relaxation times can trade-off with the differing shape anisotropies. The key physical point is that the precession frequency of ferromagnetic materials and their damping parameter vary with the effective field which depends on both the shape anisotropy, and exchange coupling, while their corresponding effects can be modulated through the action of the intense pump beam. Precession frequency maps of the behavior of the exchange coupling parameter of the samples with respect to their shape anisotropy and their laser-induced modulated precession frequencies will be generated through a pump-probe experiment to address the above-mentioned objective of our work. [Preview Abstract] |
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H1.00146: Electron Irradiation Induced Modification of Ferromagnetism in (Ga,Mn)As Jia Luo, Gang Xiang The ferromagnetism properties of diluted magnetic semiconductor (Ga,Mn)As firstly improved by energetic electron irradiation, through a sequence of irradiation doses. We did a systematic study of magnetization as a function of temperature and additional magnetic field. SQUID measurements demonstrate the T$_{\mathrm{c}}$ of all (Ga,Mn)As film increased from 40K to 60K after irradiation. At the same time, electron irradiation improved the crystal quality and electric properties. The irradiation process decreases the resistance by a factor of 1/2 in the range of 10K to 50K, and transforms (Ga,Mn)As samples from insulator behavior to metallic behavior. SIMS and transport measurements confirm that the rearrangement of Mn interstitials plays a key role in the improvement of ferromagnetism properties. We infer that electron irradiation paves a new path to room-temperature ferromagnetism of (Ga,Mn)As. [Preview Abstract] |
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H1.00147: ABSTRACT WITHDRAWN |
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H1.00148: Dynamics of bright and dark localized excitonic magnetic polarons in CdMnTe spin glass compound Yuriy Gnatenko, Petro Bukivskij, Yuriy Piryatinski The measurements of the magnetic properties of spin glass (SG) system indicate that the magnetic relaxation is characterized by a broad range of times below $T_{f}$. Here, for the first time, we have investigated time-resolved photoluminescence spectra of Cd$_{\mathrm{0.70}}$Mn$_{\mathrm{0.30}}$Te SG compound at the temperature below the freezing temperature $T_{f}$. This enables us to study the dynamics of different localized excitonic magnetic polarons (LEMPs) at $T=$\textit{0.7T}$_{f} $excited in the crystal regions where various microscopic magnetic spin states (MMSSs) are formed. It was found that there is a broad distribution of the lifetimes of the LEMPs which have different lifetimes but same energies. It was shown that the presence of the long-lived LEMPs is caused by the admixture of the optically active bright exciton states to the dark exciton states as a result of the local magnetic fields formation. The lifetimes of these dark LEMPs correspond to hundreds of nanoseconds. It was also found that the decay process of the PL exciton band intensity is described by the Kohlrausch--Williams--Watts stretched exponential function which describes the recombination processes which correspond to the emission of the LEMPs formed in the crystal region of the finite clusters as well as the infinite cluster. These complex dynamical phenomena, observed for Cd$_{\mathrm{1-X}}$Mn$_{\mathrm{X}}$Te at low temperatures, reflect the spatially heterogeneous dynamics in the SG system which is due to the presence of different MMSSs below $T_{f}$. [Preview Abstract] |
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H1.00149: Ferruquadrupolar phase of the Heisenberg model with bilinear and biquadratic interactions Antonio Pires The Heisenberg antiferromagnet with bilinear and biquadratic exchange interactions has been studied using several techniques. In contrast to bilinear interactions models, quantum spin models with biquadratic interactions present a phase diagram qualitatively different from their classical counterparts, as for instance nonmagnetic phases such as the quadrupolar phase. In this work I will study the ferruquadrupolar phase of the S = 1 Heisenberg model with bilinear and biquadratic exchange interactions on the square lattice using a SU(3) Schwinger boson formalism in a mean field approximation. This nonmagnetic phase is characterized by a finite quadrupole moment. I will calculate the quadrupole moment and the static spin structure factor for several values of the parameters involved in the model. The results obtained will also be compared with the ones obtained from other theories. [Preview Abstract] |
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H1.00150: Effect of Zn substitution on the magnetic properties of Skyrmion Cu$_{2}$OSeO$_{3}$ Tien-Yu Wei, Hung-Cheng Wu, Kakarla Devi Chandrasekhar, Hung-Duen Yang There is a considerable interest in the new magnetic state, the ``Skyrmion state (A-phase),'' whose magnetic properties have a remarkable characteristic as a vortex-like spin orientation. The polycrystalline (Cu$_{\mathrm{1-x}}$Zn$_{\mathrm{x}})_{2}$OSeO$_{3}$ (x$=$0 to 0.20) samples were synthesized using solid state reaction method and studied by X-ray diffraction, X-ray absorption spectra and magnetic measurements. Variation of lattice constant with Zn doping follows the Vegard's law which signifies the successful substitution of Zn in place of Cu up to x less than 0.20. The Cu L$_{2,3}$ spectra show 2$+$ valence state for all samples. The Curie temperature decrease with Zn doping indicate the ferrimagnetic ordering is gradually suppressed. Moreover, we have notice another magnetic phase for the doping level x between 0.05 and 0.2, whose magnetic transition also shifted to low temperature for higher Zn doping. The H-T magnetic phase diagrams of the samples from ac susceptibility have been established with increasing Zn doping. The explanations for the observations of doping effects on the A-phase of Skyrmion Cu$_{2}$OSeO$_{3}$ will be discussed. [Preview Abstract] |
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H1.00151: Effect of Ni substitution on the magnetic properties of Skyrmion Cu$_{2}$OSeO$_{3}$ Chung-Lun Huang, Hung-Cheng Wu, Kakarla Devi chandrasekhar, Hung-Duen Yang Chiral magnetic lattice shows many exotic physical properties such as spin ice/spin liquid order, topological insulators and magneto-electric coupling. The chiral magnetic lattice of Cu$_{2}$OSeO$_{3}$ exhibits such kind of unique magnetic ordering where spins form the vortex like ordering called as Skyrmion. In this poster, the effects of isovalent ion doping on the Skyrmion phase of Cu$_{2}$OSeO$_{3}$ were presented. Polycrystalline (Cu$_{\mathrm{1-X}}$Ni$_{\mathrm{X}}$)$_{2}$OSeO$_{3}$ (x$=$0.0 to 0.1) samples were prepared by standard solid-state methods. Temperature and magnetic field dependent AC and DC magnetic measurements were performed. The Curie temperature decreases obviously with increasing Ni concentration by using ac susceptibility ($\chi_{\mathrm{ac}}$-T). Systematic H-T phase diagrams indicating the effects of Ni doping are established and will be discussed. [Preview Abstract] |
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H1.00152: High pressure and doping effect on the magnetic properties of CaMn$_{2}$O$_{4}$ Kun-Ju Hsieh, Hong-Cheng Wu, Kakarla D. Chandrasekhar, Hung-Duen Yang Polycrystalline Ca1-xSrxMn$_{2}$O$_{4}$ (x$=$0.05, 0.10, 0.15, and 0.20) compounds were synthesized using solid state reaction method. Samples were characterized by X-ray diffraction and magnetization measurements. The lattice constants (a, b, and c) determined by Rietveld refinement increases with Sr substitution. CaMn$_{2}$O$_{4}$ shows antiferromagnetic transition T$_{\mathrm{N}}$ near 220 K due to the Mn(III)-O-Mn(III) superexchange interaction. High-pressure effect on the T$_{\mathrm{N}}$ of CaMn2O4 has been investigated using piston-cylinder-type high-pressure apparatus designed for MPMS-XL7 magnetometer. It is found that the T$_{\mathrm{N}}$ increases with applying pressure up to 17.7 kbar at a rate of 0.487(22) K/kbar and decreases with higher Sr content. [Preview Abstract] |
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H1.00153: Exact Diagonalization of a Quantum Ising Model with Long-Range Interactions Shanna Muehe, Thomas Gunn, C.C.-Joseph Wang, Christopher Varney Due to the rapid advance of quantum spin simulators in ultra-cold ions, the varying interaction for spin models in two-dimensional lattices have become feasible for experimental exploration for exotic states of collective states of multiple spins. It is particularly interesting for the case of a triangular lattice with antiferromagnetic interaction between spins. When the Ising spin-spin interaction is uniform and restricted between nearest neighbors, the spins are geometrically frustrated. When the system interaction becomes long ranged, the geometric frustration is lost but the spins are frustrated by the long-range interaction. In the latter case, the underlying orders present in the ground state are unclear and understanding these states in finite spin systems is crucial for the benchmarking of experimental observations. Here, we investigate the quantum dipolar Ising model with exact diagonalization to analyze the ground state, order parameters, and excitations and provide a baseline for comparison with experiments. [Preview Abstract] |
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H1.00154: Exchange interaction between $J$-multiplets Naoya Iwahara, Liviu Chibotaru The crystal-field levels in lanthanides and other metal complexes with unquenched orbital momentum originate from the ground atomic $J$-multiplet. It was long believed that the exchange interaction between $J$-multiplets is basically described by Heisenberg form, $\mathbf{J}_1 \cdot \mathbf{J}_2$. In this work, Anderson's superexchange model is applied for analytical derivation of exchange interaction between arbitrary $\mathbf{J}_1$ and $\mathbf{J}_2$ multiplets [1]. The structure and the energy spectrum of the obtained exchange Hamiltonian are significantly different from those of a Heisenberg Hamiltonian. Besides, it is also found that the $1/U$ approximation [2] is not applicable for the description of exchange spectrum, since it gives qualitatively different predictions compared to the present treatment. Similar results are obtained for the exchange interaction between $J$-multiplet ($\mathbf{J}_1$) and isotropic magnetic center ($\mathbf{S}_2$). \\[4pt] [1] N. Iwahara and L. F. Chibotaru, submitted to Phys. Rev. Lett.\\[0pt] [2] P. Santini, S. Carretta, G. Amoretti, R. Caciuffo, N. Magnani, and G. Lander, Rev. Mod. Phys. {\bf 81}, 807, (2009). [Preview Abstract] |
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H1.00155: The origin of magnetic ordering in quasi-two-dimensional quantum magnets Cu(\textit{tn})Cl$_{2}$ and Cu(\textit{en})(H$_{2}$O)$_{2}$SO$_{4}$ Alzbeta Orendacova, Lucia Baranova, Robert Tarasenko, Martin Orendac, Alexander Feher, Rudolf Sykora, Dominik Legut A comparative analysis of magnetic properties of Cu(\textit{en})(H$_{2}$O)$_{2}$SO$_{4}$ (\textit{en} $=$ C$_{2}$H$_{8}$N$_{2})$ (1) and Cu(\textit{tn})Cl$_{2}$ (\textit{tn} $=$ C$_{2}$H$_{10}$N$_{2})$ (2) has been performed to search for the origin of magnetic ordering observed in (1) at $T_{c} =$ 0.9 K while hidden in (2). Previously, both materials were approximated by a quasi-two-dimensional (2d) spin 1/2 Heisenberg model on the square lattice with effective intralayer and interlayer coupling $J$/$k_{B} =$ 3 K and $J$'$=$ 10$^{-3}J, $ respectively. The first principles calculations revealed in (1) a spatial anisotropy of exchange coupling within a layer, $J_{1}$/$J_{2} =$ 0.15, in accordance with a proximity of data to 2d behavior. Considering only effect of interlayer coupling, $T_{c} =$ 0.8 K was evaluated, while $T_{c} =$ 0.85 K, when a weak ising-like spin anisotropy, $\Delta =$ 0.015 was introduced into Heisenberg layers. The effects of spin and spatial anisotropy on the ordering of (1) and the absence of a phase transition in (2) are discussed. [Preview Abstract] |
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H1.00156: Thermodynamic properties of a layered S $=$ 7/2 Heisenberg magnet Gd(OH)CO$_{3}$ Martin Orendac, Martin Ulicny, Erik Cizmar, Alzbeta Orendacova, Yan-Cong Chen, Zhao-Sha Meng, Ming-Liang Tong Thermodynamic quantities and ESR spectra of Gd(OH)CO$_{3}$ (I) are reported. The material may be considered to consist of weakly coupled layers with potentially triangular arrangement of exchange paths within each layer. Different bridging groups and distances among Gd$^{3+}$ ions may be responsible for spatial anisotropy of magnetic coupling. Preliminary analysis of magnetic susceptibility using Curie-Weiss law yielded $\theta =$-1.05 K indicating weak antiferromagnetic coupling and consequently, spin frustration in (I). More detailed simultaneous analysis of specific heat, susceptibility and magnetization studied down to nominally 0.45 K revealed non-negligible role of single-ion anisotropy. Using the model of weakly interacting S$=$7/2 trimers, the gross features of measured data may be explained while assuming single-ion anisotropy $D/k_{B}\approx $0.6 K and effective \textit{intra}trimer magnetic coupling \textbar $J/k_{B}$\textbar $\approx $0.3 K. The obtained $D$ value reasonably reproduces the position and shape of ESR line. The performed analysis suggests that magnetism in (I) is governed predominantly by crystal field effects and frustration plays a minor role. [Preview Abstract] |
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H1.00157: The structural, electronic and magnetic properties of Ga8-xMnxAs8 clusters Gangxu Gu, Gang Xiang We systematically investigate the ground-state magnetic properties of Ga8-xMnxAs8 clusters (x $=$ 0, 2, 4, 6, and 8) within the framework of density functional theory (DFT) using a strategy that successively adopts the particle swarm optimization (CALYPSO) code and fixed spin-moment (FSM) method. The results show that for Ga8-xMnxAs8 in the ground states or low-lying isomers, Mn atoms tend to assemble at the core of the clusters and the ferrimagnetic Mn-Mn couplings are identified for Ga8-xMnxAs8 (x$=$4, 6, and 8), while Ga8As8 and Ga6Mn2As8 are nonmagnetic. The possibility of multiple ground states of Ga8-xMnxAs8 (x $=$ 4, 6, and 8) is also demonstrated. The binding energy and LUMO-HOMO gap analysis show that Ga8-xMnxAs8 clusters with large x are more likely synthesized and exhibit stronger chemical reactivity. [Preview Abstract] |
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H1.00158: Qunatum oscillations of the mechanical forces in rotating molecular magnets Gwang-Hee Kim We study a rotating nanomagnet that exhibits beat structure of the quantum forces. We show that such forces are originated from tunneling between two entangled states of spin and mechanical angular momentum. They can be observed in the presence of a static magnetic field gradient with ac magnetic field and disappear on increasing total angular momentum and parameter which depends on the moment of inertia and the tunnel splitting. [Preview Abstract] |
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H1.00159: Uniaxial-pressure dependence of the magnetization dynamics in the high-symmetry single-molecule magnet Mn12-MeOH James H. Atkinson, Lakshmi Bhaskaran, Stephen Hill, Yuri Myasoedov, Eli Zeldov, Enrique del Barco, Jonathan Friedman, Adeline Fournet, George Christou The single-molecule magnet [Mn12O12(O2CCH3)16(CH3OH)4]CH3OH (``Mn12-MeOH") is a high-symmetry sibling of the Mn12-Acetate SMM that offers a prime opportunity to explore the consequences of molecular symmetry. A previous study [1] has shown that applied pressure induced changes in the Mn12-Acetate's anisotropy parameters. Here we present the results of a study in which uniaxial pressure was applied to a crystalline sample of Mn12-MeOH in order to examine how the pressure affects the quantum tunneling of magnetization at low temperature. We find that the pressure induces an increase in the resonant tunneling rate manifested as a change in the height of the tunneling steps in the magnetic hysteresis. These results suggest that pressure is altering symmetry-breaking terms in the molecule's spin Hamiltonian, giving rise to increased tunneling. \\[4pt] [1] J. H. Atkinson, K. Park, C. C. Beedle, D. N. Hendrickson, Y. Myasoedov, E. Zeldov and J. R. Friedman, EPL 102, 47008 (2013) [Preview Abstract] |
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H1.00160: Phase transitions of a quantum chain with four-spin interactions in longitudinal and transverse magnetic fields B. Boechat, J. Florencio, A. Saguia, O. F. de Alcantara Bonfim We study the ground-state properties of a spin-1/2 model on a chain containing four-spin Ising-like interactions in the presence of both transverse and longitudinal magnetic fields. We use entanglement entropy and finite-size scaling methods to obtain the phase diagrams of the model. Our numerical calculations reveal a rich variety of phases and the existence of multi-critical points in the system. We identify phases with both ferromagnetic and anti-ferromagnetic orderings. We also find periodically modulated orderings formed by a cluster of like-spins followed by another cluster of opposite like-spins. The quantum phases in the model are found to be separated by either first or second order transition lines. [Preview Abstract] |
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H1.00161: Quantum criticality in a magnetic chain with two- and four-spin interactions in a transverse field O. F. de Alcantara Bonfim, A. Saguia, B. Boechat, J. Florencio We use entanglement entropy and finite-size scaling methods to investigate the ground-state properties of a spin$-1/2$ Ising chain with two-spin ($J_2$) and four-spin ($J_4$) interactions in a transverse magnetic field ($B$). We concentrate our study on the unexplored critical region $B=1$ and obtain the phase diagram of the model in the ($J_4$-$J_2$) plane. The phases found include ferromagnetic (F), antiferromagnetic (AF), as well as more complex phases involving spin configurations with multiple periodicity. The system presents both first and second order transitions separated by tricritical points. We find an unusual phase boundary on the semi-infinite segment ($J_{4}$$<$$-1$, $ J_{2}$$=$$0$) separating the F and AF phases. [Preview Abstract] |
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H1.00162: Unusual Phase Transitions in Single Crystals of Gd$_{5}$Si$_{1.3}$Ge$_{2.7}$ and Gd$_{5}$Si$_{1.4}$Ge$_{2.6}$ R.L. Hadimani, Y. Melikhov, D.L. Schlagel, T.A. Lograsso, K.W. Dennis, R.W. McCallum, D.C. Jiles Gd$_{5}$(Si$_{\mathrm{x}}$Ge$_{\mathrm{1-x}})_{4}$ has been widely studied over the composition range 0.41\textless x\textless 0.51 where the coupled magnetic and structural first order phase transitions occur close to room temperature. It has a mixed phase region in the phase diagram with both orthorhombic I and orthorhombic II phases for compositions 0.32\textless x\textless 0.41. Previously we have used modified Arrott plots to determine the second order phase transition temperature when it is suppressed by the first order phase transition in samples with compositions x\textless 0.51. we also used these modified Arrott plots on the mixed phase composition of Gd$_{5}$Si$_{1.5}$Ge$_{2.5}$ (x $=$ 0.375) to determine the second order phase temperatures of both the monoclinic and the orthorhombic II phases. We have now investigated two more single crystals of Gd$_{5}$Si$_{1.3}$Ge$_{2.7}$ and Gd$_{5}$Si$_{1.4}$Ge$_{2.6}$ whose compositions fall in the mixed phase regions of orthorhombic I and orthorhombic II in the phase diagram. The second order phase transition temperatures of the samples were estimated to be 383~K for Gd$_{5}$Si$_{1.3}$Ge$_{2.7}$ and 365~K for Gd$_{5}$Si$_{1.4}$Ge$_{2.6}$. These temperatures are much higher than the expected second order phase transition temperature of orthorhombic II phase (280~K). This may be due to the presence of the orthorhombic I phase in larger volume fraction. [Preview Abstract] |
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H1.00163: Evidence for a magnetic metallic R phase in Vanadium dioxide VO2 Hui Xing, Payam Taheri, Peihong Zhang, Hao Zeng Vanadium dioxide VO$_{\mathrm{2}}$ has garnered extensive research interests for over decades due to its metal-insulator transition (MIT) around 340 K (Ref. 1). Much is known for the physics behind the MIT (including a correlated structural transition and the involvement of several intermediate states). On the other hand, the magnetic property across the MIT is much less known. Although there are no fundamental arguments against the possibility of forming local magnetic moments in VO$_{\mathrm{2}}$. So far, only the M2 phase has been confirmed to possess local magnetic moments. However, our temperature-dependent magnetic susceptibility measurements of VO$_{\mathrm{2}}$ show a sudden jump at the MIT that cannot be attributed to a simple Pauli susceptibility from conducting electrons. In a recent paper$^{\mathrm{2}}$, we pointed out local magnetic moments may form in the metallic R phase. The formation of local moment would naturally explain the extremely high magnetic susceptibility of VO$_{\mathrm{2}}$ above the phase transition temperature. We further discuss the magnetoresistance (MR) measured across the MIT, which shows different magnitude and field dependence in M1 and R phase, including the MR in the metallic phase suppressed to lower temperature in a VO$_{\mathrm{2}}$ electric double layer transistor device using ionic liquid as gate dielectrics. 1. F. J. Morin, Phys. Rev. Lett. 3, 34 (1959). 2. Xun Yuan et al., Phys. Rev. B 86, 235103 (2012). [Preview Abstract] |
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H1.00164: Control of proliferation rate of N27 dopaminergic neurons using Transcranial Magnetic Stimulation orientation Yiwen Meng, Ravi Hadimani, Vellareddy Anantharam, Anumantha Kanthasamy, David Jiles Transcranial magnetic stimulation (TMS) has been used to investigate possible treatments for a variety of neurological disorders. However, the effect that magnetic fields have on neurons has not been well documented in the literature. We have investigated the effect of different orientation of magnetic field generated by TMS coils with a monophasic stimulator on the proliferation rate of N27 neuronal cells cultured in flasks and multi-well plates. The proliferation rate of neurons would increase by exposed horizontally adherent N27 cells to a magnetic field pointing upward through the neuronal proliferation layer compared with the control group. On the other hand, proliferation rate would decrease in cells exposed to a magnetic field pointing downward through the neuronal growth layer compared with the control group. We confirmed results obtained from the Trypan-blue and automatic cell counting methods with those from the CyQuant and MTS cell viability assays. Our findings could have important implications for the preclinical development of TMS treatments of neurological disorders and represents a new method to control the proliferation rate of neuronal cells. [Preview Abstract] |
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H1.00165: Accurate projected augmented wave (PAW) datasets for rare-earth elements (RE=La-Lu) Mehmet Topsakal, Renata Wentzcovitch We provide accurate projected augmented wave (PAW) datasets for rare-earth (RE) elements with some suggested Hubbard U values allowing efficient plane-wave calculations. Solid state tests of generated datasets were performed on rare-earth nitrides. Through density of state (DOS) and equation of state (EoS) comparisons, generated datasets were shown to yield excellent results comparable to highly accurate all-electron full-potential linearized augmented plane-wave plus local orbital (FLAPW+LO) calculations. Hubbard U values for trivalent RE ions are determined according to hybrid functional calculations. We believe that these new and open-source PAW datasets will allow further studies on rare-earth materials. [Preview Abstract] |
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H1.00166: Half-metallic magnetism and the search for better spin valves Karin Everschor-Sitte, Matthias Sitte, Allan MacDonald We propose a simple formula for the temperature dependence of tunneling magnetoresistance to shed light on ongoing efforts to optimize spin valves. It captures a mechanism in which spin valve performance at finite temperatures is limited by uncorrelated thermal fluctuations of magnetization orientations on opposite sides of a tunnel junction. Furthermore, it directly reveals the advantages for spin-valve optimization by using materials with a high spin polarization of Fermi-level tunneling electrons, and by using materials with high ferromagnetic transition temperatures. We show that our theory is in good agreement with recent experimental studies of the temperature-dependent magnetoresistance of high-quality tunnel junctions with MgO barriers. We conclude that half-metallic ferromagnets can yield better spin-value performance than current elemental transition metal ferromagnet/MgO systems only if their ferromagnetic transition temperatures exceed $\sim 950~\mathrm{K}$.\\[0.2em] \noindent [1] K.\ Everschor-Sitte, M.\ Sitte and A.\ H.\ MacDonald, J.\ Appl.\ Phys.\ \textbf{116}, 083906 (2014). [Preview Abstract] |
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H1.00167: Nonlinear damping effects in spin torque dynamics of magnetic tunnel junctions Igor Barsukov, Yu-Jin Chen, Han Kyu Lee, Alexandre Goncalves, Jordan Katine, Rodrigo Arias, Boris Ivanov, Ilya Krivorotov Performance of nanoscale spin torque devices such as memory (STT-MRAM) and auto-oscillators critically depends on magnetic relaxation. It is commonly assumed that magnetization dynamics in the presence of spin torque can be understood as simple competition between antidamping arising from spin torque and Gilbert damping of the free layer. However our experiments reveal that the situation is more complex and that nonlinear damping processes in the free layer of magnetic tunnel junction (MTJ) nanopillars can strongly alter spin torque driven dynamics. We study elliptical MTJ nanopillars with in-plane magnetizations of the free layer and SAF layers by spin torque ferromagnetic resonance. We find an excitation spectrum associated with standing spin waves of the free layer. By varying the external field, the energy of a higher-order spin wave mode becomes twice the energy of the main mode. This opens up a nonlinear, resonant relaxation channel, giving rise to a damping increase of approximately 20 percent. With increasing spin torque provided by a DC bias current, we find that this relaxation channel competes with antidamping in a nonlinear manner, increasingly contributing to and even dominating the relaxation at subcritical currents. [Preview Abstract] |
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H1.00168: Generation of Spin and Orbital Current in Carbon Nanotubes by Spin-rotation Coupling Masato Hamada, Shuichi Murakami Spin-rotation coupling represents a coupling between the electron spins and mechanical rotations, and may be used for generation of spin currents by mechanical rotation. In our presentation we consider carbon nanotubes, and use one of the phonon modes called a twist mode. This mode gives rise to a rotation around the tube axis and eventually an effective Zeeman field parallel to the axis is generated by spin-rotation coupling. We calculate a generated spin current by solving the spin diffusion equation. In addition to the effective Zeeman field along the axis, the rotation also generates an effective orbital magnetic field in the radial direction. We calculate diamagnetic susceptibility for the radial magnetic field, and discuss the generated orbital current. [Preview Abstract] |
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H1.00169: Spin Troque Driven Anti-vortex Dynamics in Patterned Nanomagnets Mustafa Mete, Ahmet Coskuner, Ali Taha Habiboglu, Vedat Karakas, Yemliha Bilal Kalyoncu, Aisha Gokce, Ozhan Ozatay, Anna Giordano, Mario Carpentieri, Giovanni Finocchio, Federica Celegato, Paola Tiberto Recent studies have shown that unconventional spin configurations in patterned nanomagnets like vortices are potentially applicable to ultrafast memory, rf oscillators and detectors utilizing the static and dynamic response of these structures under external magnetic field and current bias. Due to the difficulties of stabilizing an isolated anti-vortex, there is still much to be explored about the static and dynamic properties of such spin structures. In this study, we report on our investigation of stable anti-vortex formation conditions and the subsequent magnetic field/dc current driven excitations. Permalloy based asteroid geometry devices exhibit anti-vortex nucleation at the center with the application of an in-plane AC demagnetizing field and an out of plane magnetic field. Changes in the stable localization of the spins immediately motivates the characterization of the dynamic response to the application of spin torque from a spin-polarized current as sensed using the anisotropic magnetoresistance effect (AMR). We will present the field and current dependence of the anti-vortex gyration frequency, the bandwidth and power in the asteroid devices. This work allows the evaluation of anti-vortex structures to be utilized in practical on-chip microwave oscillators. [Preview Abstract] |
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H1.00170: Magnetic properties of layered III-VI Diluted Magnetic Semiconductors (DMS) Thomas Pekarek, I. Miotkowski, A.K. Ramdas The new class of quasi-two-dimensional III-VI Diluted Magnetic Semiconductors (DMS) exhibits a rich collection of magnetic behavior. The Ga$_{1-x}$Mn$_{x}$S system exhibits a 3-D spin-glass transition, which was unexpected given its four atom thick two dimensional structure. The best scaling fit was found for critical exponents ($\gamma $ $=$ 4.0, $\beta =$ 0.8, and $\delta =$ 5.5) similar to the three dimensional Zn$_{1-x}$Mn$_{x}$Te system. Ga$_{1-x}$Fe$_{x}$Se exhibits a prominent magnetic anisotropy over the temperature range from 10 to 400 K. Theoretical models for In$_{1-x}$Mn$_{x}$Se, In$_{1-x}$Mn$_{x}$S, and Ga$_{1-x}$Mn$_{x}$S provide good agreement with experimental results over a wide range of temperatures and fields. The mechanism behind an unusually large thermal hysteresis ($\Delta T$~$\approx $~200~K) in In$_{1-x}$Mn$_{x}$Se, which extends up to room temperature, is not completely understood at this time. Typically, thermal hysteresis in most materials has a $\Delta T$~$\approx $~20~K occurring well below room temperature. The host III-VI semiconductors themselves are among the best non-linear optical materials. [Preview Abstract] |
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H1.00171: Computational Aspects of Anisotropy Calculations M. Daene, D. Aberg, L.X. Benedict In order to predict magnetic properties from first principles, an accurate and reliable determination of the magneto crystalline anisotropy is needed. We present results using multiple techniques and codes on the Fe$_2$B-Co$_2$B system. Furthermore, we investigate the influence of disorder in this system on the anisotropy. [Preview Abstract] |
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H1.00172: Terbium-Aluminum (TbAl$_{2}$) Binary Alloy as High Magnetostrictive Material Mary Boghosian, Carlos Sanchez, Oscar Bernal, Armen Kocharian Magnetic phase diagram for the cubic intermetallic terbium-aluminum (Tb-Al) binary alloy is being investigated for the purpose of developing material with high magnetostrain properties that can be used for energy harvesting. Low temperature magnetizations, specific heat, combined with structural examinations are few of the techniques that are being used for this purpose. Preliminary DC magnetization results on as-cast material show magnetic ordering of around 109 K in zero applied fields that varies in magnitude and direction with the increase of applied magnetic field. The preliminary results will be discussed. [Preview Abstract] |
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H1.00173: Cold plasma cleaning of SmCo5 nano-flakes prepared by surfactant-assisted Ball-milling Guangbing Han, Ke Wang, Kevin E. Elkins, Zhaoguo Qiu, Richard B. Timmons, Charles R. Savage, J. Ping Liu Surfactant assisted high energy ball milling has recently been successfully utilized in producing nanostructured hard magnetic powders. However, it is challenging to remove the surfactant on the surface which deteriorates magnet's properties during heat treatments. Cold plasma is suitable for removing organic surfactant on surface because of the high energetic ions etching at relatively low (room) temperature, while the surface of the powders will not be chemically altered by the physical method. In this work, SmCo5 nano-flakes were prepared by oleic acid (OA) assisted ball milling technique, and then Argon cold plasma was used to clean the OA from the SmCo5 flakes' surface on a homemade facility. The results show that the remaining carbon atoms can be removed effectively by Ar plasma. Good magnetic properties can be retained in the SmCo5 flakes when the plasma power, Ar pressure, and cleaning time were properly chosen. [Preview Abstract] |
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H1.00174: Magnetic and magneto-transport studies of MBE grown Cr$_2$Te$_3$ thin films with perpendicular magnetic anisotropy Anupam Roy, Samaresh Guchhait, Rik Dey, Tanmoy Pramanik, Cheng-Chih Hsieh, Amritesh Rai, Sanjay Banerjee Cr$_2$Te$_3$ is one of the very intriguing compounds in chromium chalcogenides family because of its unusual magnetic and magneto-transport properties. Here we have presented studies of molecular beam epitaxy (MBE) grown (001)-oriented Cr$_2$Te$_3$ thin films on Al$_2$O$_3$(0001) and Si(111)-(7$\times$7) surfaces. Reflection high energy electron diffraction (RHEED), scanning tunneling microscopy (STM), vibrating sample magnetometry (VSM) and other physical property measurements are used to investigate the structure, morphology, magnetic and magneto-transport properties of as-grown films. Sharp streaks in RHEED patterns imply smooth film growth on both the substrates. STM studies show hexagonal arrangements of surface atoms and measured lattice parameters agree well with the bulk crystal structures. Magnetic studies confirm the film to be ferromagnetic having a Curie temperature of about 180 K and a spin glass-like behavior is observed below 35 K. The grown films are metallic and show perpendicular magnetic anisotropy (PMA). Magneto-transport measurements reveal that the film possesses a magnetic easy axis perpendicular to the surface and this may be very useful for spintronics applications. [Preview Abstract] |
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H1.00175: Influence of the magnetic properties and repetitions on the energy product in layered thin film hard soft magnetic nanocomposites David Zagardo, Jamileh Beik Mohammadi, Andrew Tuggle, Claudia Mewes, Tim Mewes, Takao Suzuki Exchange spring composites (hard-soft magnetic composites) are interesting for many applications such as rare-earth free permanent magnets [1] and information storage [2]. One key aspect is the figure of merit, the energy product, also called (BH)max. The system of study is a magnetic nano composite where each bilayer consists of a soft and hard magnetic material of total height of 22 nm. Using micromagnetic simulations we have investigated the influence of different ratios of the volume of the hard and soft layers on the energy product and the number of bilayer repetitions. Our findings indicate that the maximum energy product depends strongly on the volume ratio as well as on the number of repetitions. In addition we have studied the influence of different anisotropy contributions of the hard and soft magnetic layer on the energy product. Finally we have studied the influence of the interlayer exchange coupling on the energy product, which show that strong interlayer exchange coupling is necessary to reach a high energy product. \\[4pt] [1] T. Hozumi et al., J. Appl. Phys. 115, 17A737 (2014); R. Houkawa et al., J. Appl. Phys. 115, 17A707 (2014).\\[0pt] [2] J. Lee et al., Nanotechnology 25, 045604 (2014); T. Tanaka et al., IEEE Transactions on Magnetics 150, 3000503 (2014). [Preview Abstract] |
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H1.00176: Hard magnetic phase evolution in nanocrystalline mechanically milled amorphous Pr$_{2}$Co$_{14}$B powder Cajetan Nlebedim, Huseyin Ucar, Parans Paranthaman, R.W. McCallum In this work, the evolution of the structural and magnetic properties of Pr$_{2}$Co$_{14}$B with mechanical milling and heat-treatment is presented. Understanding the phase evolution of magnetic properties in hard magnetic materials is crucial for developing high performance permanent magnets. Mechanical alloying/milling offers a traditional and easily deployable approach to synthesizing nanostructured materials. Nevertheless, such can result in amorphization due to high defect density leading to disorder in atomic arrangement. The crystalline phase can be thermally recovered but requires the understanding of how the properties evolve with temperature, in order to achieve useful hard magnetic properties desired for developing permanent magnets. This work shows how properties such as energy product, coercivity, remanent magnetization, saturation magnetization and Curie temperature evolve when PrCoB alloy transitions from amorphous to crystalline phase. The presentation will also include how different levels of amorphization affect the magnetic properties. [Preview Abstract] |
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H1.00177: Effect of 3d-metal doping on magnetic properties of Fe3Se4 Renat Sabirianov, Nabil Al-Aqtash, Debasis Sengupta Fe$_{\mathrm{3}}$Se$_{\mathrm{4}}$ exhibits large magnetocrystalline anisotropy (MAE) and coercivity up 40kOe. The large anisotropy of Fe$_{\mathrm{3}}$Se$_{\mathrm{4}}$ should be accompanied by large magnetization for permanent magnet applications. The magnetization of Fe$_{\mathrm{3}}$Se$_{\mathrm{4}}$ suffers from antiferromagnetic (AFM) superexchange coupling of Fe across the Se planes. We present density functional theory study of the magnetic properties of Fe$_{\mathrm{3}}$Se$_{\mathrm{4}}$ doped with TM (Co, Cr, Ni and Mn), TM ions doped in Fe sites, Fe$_{\mathrm{3-x}}$(TM)$_{\mathrm{x}}$Se$_{\mathrm{4}}$ (x $=$ 0.5), to examine a potential increase of the magnetization and Curie temperature of Fe$_{\mathrm{3}}$Se$_{\mathrm{4}}$. We performed screening of the exchange interactions and magnetization modifications upon the substitution of Fe by 3d-transition metals at various Fe sites in the Fe$_{\mathrm{3}}$Se$_{\mathrm{4}}$ We find that the doping of Fe$_{\mathrm{3}}$Se$_{\mathrm{4}}$ with 3d-elements does not remove AFM coupling across layers. The increase in the strength of exchange interactions on doping with Cr should increase the Curie temperature of the system. We compare the results of doped alloy with the ones for Cr$_{\mathrm{3}}$Te$_{\mathrm{4}}$. This compound is analogous to Fe$_{\mathrm{3}}$Se$_{\mathrm{4}}$ because Te has same electron configuration as Se, but Cr has only four d-electrons. We find that Cr$_{\mathrm{3}}$Te$_{\mathrm{4}}$ has ferromagnetic coupling and magnetization larger than one possible in Fe$_{\mathrm{3}}$Se$_{\mathrm{4}}$. Magnetization per unit cell is 18.24$\mu _{\mathrm{B}}$. MAE of this material is large (MAE $=$1.67 MJ/m$^{\mathrm{3}})$ [Preview Abstract] |
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H1.00178: Effects of TiN buffer layer on microstructure and magnetic anisotropy of FePt thin films Zhaoguo Qiu, Guangbing Han, Dechang Zeng, J. Ping Liu FePt films were deposited at room temperature on TiN buffer layer followed by annealing. The effects of thickness of the TiN layer on the microstructure and magnetic properties of FePt films were investigated. It was found that TiN layer has significant effects on the magnetic anisotropy of the FePt films. The L1$_{\mathrm{0}}$ phase transformation of the FePt films with TiN layer was more completely than that without a TiN layer. The FePt film with TiN (111) layer of 30nm thickness exhibited out-of-plane anisotropy and enhanced ordering parameter. When the thickness of TiN (111) layer further increased, the coercivity tended to decrease. The anisotropy gradually switched from out-of-plane to in-plane when the annealing temperature was increased to 700 degree. The in-plane coercivity was increased to 0.96 kOe when the thickness of randomly oriented TiN layer was 80nm.The high in-plane coercivity may come from the smooth surface morphology of FePt films induced by the small relaxation of internal stress of the thick TiN layer. [Preview Abstract] |
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H1.00179: Transient Reflectivity of a Low-Dimensional Quantum Magnet Michael Bishop, Haidong Zhou, Stephen McGill Frustrated, low-dimensional spin systems have drawn wide interest due to their ability to exhibit novel quantum phenomena such as superconductivity, spin-liquid phases, and gapped spin excitations (e.g. Haldane, Spin-Peierls, etc.). SrCu$_{\mathrm{2}}$(BO$_{\mathrm{3}})_{\mathrm{2}}$, or SCBO, a close experimental realization of the Shastry-Sutherland model, is one such quantum system in which the singlet ground state is separated from the excited triplet state by an energy gap (35 K) that can be closed by high magnetic fields (\textgreater 20 T). Furthermore, high magnetic field magnetization measurements reveal an unusual series of plateaus which occur when the magnetic field-tuned density of triplets becomes commensurate with the lattice periodicity. We have investigated the coupling of singlet and triplet pairing in SCBO to changes in its electronic structure using transient near-infrared reflectivity measurements. We investigated the temperature and magnetic field dependences of the transient reflectivity, and we will discuss these behaviors and their correlation with dimer spin excitations. [Preview Abstract] |
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H1.00180: Strong effect of low-dimensional Fe-doped cobalt niobate on a strongly ferrimagnetic system Cajetan Nlebedim, David Jiles In this work, the first investigation of the effect of Fe-doped cobalt niobate (CoNb$_{2}$O$_{6})$ imbedded in the matrix of a strongly ferrimagnetic cobalt-iron oxide, is presented. The temperature dependence of the magnetic properties and how they change with variations in the concentration of CoNb$_{2}$O$_{6}$ is also presented. CoNb$_{2}$O$_{6}$ is a prototypical low-dimensional material belonging to the pyrochlore-type AB$_{2}$O$_{6}$ systems. Its low-dimensional magnetic characteristics can help in understanding the magnetic properties of higher order systems. It has been investigated for applications in resonators and capacitors. This work shows that the magnetization of the ferrimagnetic phase is strongly affected by the concentration of Co ions in the low-dimensional phase, below 15 K but changes in coercivity with temperature were predominantly due to the ferrimagnetic phase. The systematic variation in the concentration of both phases and the cation ratio in each phase, enable us to understand the variation of the magnetic properties with temperature. This work provides useful insights into tuning the magnetism in strongly magnetic materials with transition metal AB$_{2}$O$_{6}$ systems imbedded in their matrices. [Preview Abstract] |
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H1.00181: Crystallization behavior and recoilless fraction determination of amorphous and nanocrystalline Fe56Co24Nb4B13Si2Cu1 system Monica Sorescu, Julia Limongelli, Christopher Stroh, Kevin Byerly Amorphous ferromagnetic alloy with the composition Fe56Co24Nb4B13Si2Cu1 was obtained by rapid quenching from the melt. Samples cut from the ribbons were annealed at 450, 550, 650 and 750 C in a vacuum furnace. 57Fe Mossbauer spectroscopy was used to identify the phases formed based on the refined values of the hyperfine parameters. The as-quenched specimen was analyzed with a hyperfine magnetic field distribution and corresponded to an in-plane orientation of the magnetic moment directions. The sample annealed at 450 C was found to be in a nanocrystalline state due to observation of the (FeCo)-Si alloy with the DO3 structure. The balance of the composition was represented by a metalloid-enriched amorphous grain boundary phase. In contradistinction to this, the samples annealed at 550-750 C were totally crystallized, but the new phases formed were alpha-(FeCo), (FeCo)2(BSi) and (FeCo)3(BSi). The f factor value dropped from 0.6 to 0.37 for the sample annealed at 450 C, consistent with the onset of nanocrystallization in the system. For the completely crystallized specimens, the f factor maintained values close to 0.5. This indicates that the presence of quenched-in stresses may play a role in the ability of samples to undergo recoilless emission and absorption of gamma rays. [Preview Abstract] |
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H1.00182: Scaling and memory effects in the reentrant spin glass phase of nanostructured Mn$_{x}$TaS$_{2}$ Paul Shand, John Danker, Xun Xiao, Tim Kidd, Laura Strauss We have investigated the nature of the reentrant spin glass phase of nanostructured Mn-intercalated TaS$_{2}$. The sample consisted of bundles of nanoscale fibers with an average atomic concentration of intercalated Mn of 22{\%}. The sample exhibits a ferromagnetic transition at 74 K and a transition to a cluster glass state at 40 K. The ac susceptibility measured in small dc bias fields near the cluster glass transition exhibited scaling behavior, indicating a magnetic-field dependent crossover to glassy dynamics. At temperatures below the cluster-glass transition, the nature of the dynamics was probed by ac susceptibility and zero-field cooled (ZFC) magnetization measurements. Aging and memory effects were observed, consistent with the non-equilibrium dynamics exhibited by glassy magnetic systems. In particular, we probed the ZFC magnetization memory effect as a function of cooling rate, aging time and magnetic field. The behavior is explained in terms of domain growth within the framework of droplet theory. [Preview Abstract] |
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H1.00183: Enhanced Tunneling Magnetoresistance in Voltage-controlled CoFeB/MgO Junctions Hamid Almasi, Meng Xu, Christian Gentry, Di Yu, Ty Newhouse-Illige, Y.H. Liu, J.W. Freeland, S.G.E. te Velthuis, Weigang Wang Perpendicular magnetic anisotropy (PMA) at the CoFeB/MgO interface originates from the hybridization of d orbitals of Fe and Co and the Pz orbital of Oxygen. Due to different electronic band structures, the hybridization of the $d$ orbitals of Fe and Co is likely different, therefore contributing unequally to the total PMA. This difference has been probed by an X-ray magnetic circular dichroism (XMCD). The orbital moment of Fe was found to be much larger than that of Co by XMCD.These results demonstrated that Fe contributes most to the PMA at the interface. MTJs with Fe-rich electrodes were fabricated and a substantially larger PMA was achieved. With further optimization in post-growth thermal annealing, we have achieved over 150{\%} TMR in these voltage-controllable CoFeB/MgO MTJs. [Preview Abstract] |
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H1.00184: Fermi level tuning of highly spin-polarized complex Heusler alloys via materials genome Sudip Pandey, Abdiel Quetz, Anil Aryal, Michael Fralaide, Tapas Samanta, Kamran Munira, William Butler, Igor Dubenko, Dipanjan Mazumdar, Shane Stadler, Naushad Ali Heusler alloys are the largest family of half-metals (100{\%} spin polarized at the Fermi level) and most promising for spintronic device applications. Many half-metallic full-Huesler alloys are predicted from \textit{ab-initio} calculations, but may or may not be experimentally realizable. Here, we present a novel strategy to utilize these predicted materials to tune the Fermi level of well-known, highly spin-polarized Heusler alloys. We start with the test sample of [Co$_{2}$MnSi]$_{\mathrm{1-x}}$[Co$_{2}$CrGe]$_{\mathrm{x}}$, and, by controlling the ratio of these materials, we were able to shift the Fermi level of Co$_{2}$MnSi. Experimentally, we study the structural and magnetic properties of such Heusler alloys by room temperature X-ray diffraction (XRD) and taking magnetization measurements; It was found that these complex combinations of materials are single phase even though some components (Co$_{2}$CrGe for example) might not be stable in bulk form alone. [Preview Abstract] |
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H1.00185: Phase Transitions and Magnetocaloric Effects in GdNi$_{2}$Mn$_{X}$ Anil Aryal, Abdiel Quetz, Sudip Pandey, Tapas Samanta, Igor Dubenko, Shane Stadler, Naushad Ali The structural and magnetic properties of the GdNi$_{2}$Mn$_{x}$ system (for x $=$ 0.5, 0.6, 0.8, 1.0, 1.2, 1.4, 1.5) have been studied by x-ray diffraction and magnetization measurements. A rhombohedral PuNi$_{3}$-type structure was observed in the XRD data. A second order magnetic phase transition from ferromagnetic (FM) to paramagnetic (PM) was found, characterized by a long-range exchange interaction as predicted by mean field theory. A magnetic entropy change of $| \Delta $S$_{M}| = $ 3.1 J/kg K and 2.9 J/kg K for $\Delta $H $=$ 5 T was observed in the vicinity of the Curie temperature (T$_{C})$ for GdNi$_{2}$Mn$_{0.8}$ and GdNi$_{2}$Mn$_{1.4}$ respectively. In spite of the low values of $\Delta $S$_{M}$, the relative cooling power (RCP) was found to be 176 J/Kg for the GdNi$_{2}$Mn$_{0.8}$ compound. . [Preview Abstract] |
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H1.00186: Influence of Ga content on structure and anomalous Hall effect of Fe$_{\mathrm{1-x}}$Ga$_{\mathrm{x}}$ thin films on GaSb(100) Thi Minh Hai Nguyen, Anh Tuan Duong, Yooleemi Shin, Van Quang Nguyen, Sunglae Cho The Fe-Ga alloys have recently attracted great interests because they exhibited ferromagnetic properties with high Curie temperature, high saturation magnetization and unique magnetostriction properties which are promising to real applications such as actuators, acoustic sensors, torque sensors, and positioning devices in particular for micro and nano-electromechanical systems and the integrated magnetostrictive devices. Recently, electrical spin injection from Fe$_{\mathrm{0.5}}$Ga$_{\mathrm{0.5}}$ produces an electron spin polarization above 70{\%} on GaAs(001). However, the out-of-plane saturation field and magnetization decrease rapidly with Ga content. The Fe$_{\mathrm{1-x}}$Ga$_{\mathrm{x}}$ thin films (x$=$0.4,0.5) have been grown on GaSb(100) substrate using MBE. An epitaxial film with bcc $\alpha $-Fe crystal structure (A2) was observed in Fe$_{\mathrm{60}}$Ga$_{\mathrm{40}}$ film, while an impure Fe$_{\mathrm{3}}$Ga phase with DO$_{\mathrm{3}}$ structure appeared in Fe$_{\mathrm{0.5}}$Ga$_{\mathrm{0.5}}$ film. The saturated magnetizations are 570emu/cm$^{\mathrm{3}}$ and 180emu/cm$^{\mathrm{3}}$ and the coercivities are 170 and 364Oe at room temperature for Fe$_{\mathrm{0.6}}$Ga$_{\mathrm{0.4}}$ and Fe$_{\mathrm{0.5}}$Ga$_{\mathrm{0.5}}$, respectively. A hysteresis trend in Hall resistance vs. magnetic field was observed for Fe$_{\mathrm{0.5}}$Ga$_{\mathrm{0.5\thinspace }}$film. However, there is a weak hysteresis in Fe$_{\mathrm{0.4}}$Ga$_{\mathrm{0.6}}$ film. [Preview Abstract] |
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H1.00187: Magnetic Properties of MnFe$_{2}$Ga Heusler Alloys Ahmed A. ElGendy, Mohammad Salehi-Fashami, David Sellmyer, George Hadjipanayis Recently, MnFe2Ga Heusler alloys have attracted significant attention due to their interesting physical properties such as large magnetic-field-induced strain, giant magnetocaloric effects,large magnetoresistance, and exchange bias behavior [1-2]. These properties make them promising candidates for various practical applications in the field of smart materials, magnetic refrigeration and spintronics. In this work, we prepared MnFe2Ga alloys by melt-spinning and sputtering and studied the structural and magnetic properties. The melt-spun ribbons were prepared with a wheel speed of 30 m/s. The ribbons were annealed at different temperatures for 1 hour and grinded to make fine powders. The grinded powders were also used to make the target that is used in the cluster gun for the fabrication of MnFe2Ga nanoparticles. The structure of the as made, annealed ribbons, and powders displayed a face-centered-cubic structure. The microstructure of the as-made ribbons showed equiaxed grains with an average size of 3-5 $\mu$m while the annealed ribbons showed bigger grains with small particles covering homogeneously their surface. The magnetic properties show an enhancement of magnetization while coercivity remains the same with values M(3T) and HC of 85 emu/g and 150 Oe, respectively Transmission electron microscopy with elemental mapping is currently underway to determine the structure and composition of the surface nanoparticles. The work was supported by DOE-BES-DMSE (Grant No. DE-FG02-04ER4612). \\[4pt] [1] C. W. Shih et.al. J. App Phys. 2014, 115, 17D709.\\[0pt] [2] X. D. Tang et.al. J. App. Phys. Lett. 2010, 97, 242513. [Preview Abstract] |
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H1.00188: Growth and Structural Study of Epitaxial NaMnF$_{3}$ Thin Films on SrTiO$_{3}$ Amit KC, Trent Johnson, Pavel Borisov, David Lederman Perovskite fluorides (ABF$_{3})$ exhibit many interesting phenomena, e.g. dipolar and magnetic long-range order superconductivity, as well as magnetoelectric coupling. Recently, G. C. Garcia-Castro et al. predicted that orthorhombically distorted \textit{Pnma} NaMnF$_{3}$ perovskite should have a particularly soft ferroelectric mode, and is expected to demonstrate ferroelectric order regardless of elastic strain, despite the competing antiferrodistortive instability. Thus, in combination with weak ferromagnetic order, this material is expected to be multiferroic$^{1}$. Here, we report the growth of epitaxial NaMnF$_{3}$ thin films on SrTiO$_{3} $(100) single crystal substrates via Molecular Beam Epitaxy (MBE). Structural qualities of the films were studied as a function of the substrate temperature and film thickness by the techniques of X-ray diffraction (XRD), in-situ reflection high-energy electron diffraction (RHEED), and atomic force microscopy (AFM). The best films were smooth single phase NaMnF$_{3}$, grown with four in-plane and two out-of-plane twin domains. [Preview Abstract] |
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H1.00189: Developing Nuclear Magnetic Resonance Force Microscopy (NMRFM) as an Electronic Probe of Nanoscale Condensed Matter Systems Jeremy W. Paster, Daniel M. Tennant, Shirin Mozaffari, John T. Markert The investigation of NMR via magnetic force coupling in a large field gradient has led to vast improvements in spatial resolution over the conventional inductive method. It has been demonstrated that nanoscale force sensors could be scaled to distinguish a single nuclear spin, assuming experimental noise can be minimized and other specious force signatures stifled. Accordingly, there are many efforts aimed at repurposing NMR for 3D imaging on the atomic scale [1]. In addition to proof-of-concept experiments aimed at separately resolving some of the eventual experimental barriers to atomic resolution, some of us have directed our attention to using NMR to probe the electronic environment in larger condensed matter systems which are not well suited for other scanning probe microscopy techniques and which are prohibitively small for inductive NMR detection. Previously, we proposed using NMRFM to probe superconducting transitions in microcrystals. In parallel, we revamped our investigation of thin films [2] to explore two-dimensional conducting interfaces between insulating oxides. Presented here is a survey of the technical impediments as well as current strategies for unlocking this exciting potential for NMRFM, as a tool to investigate sub-surface electronic transport in microscale and nanoscale condensed matter systems.\\[4pt] [1] Sidles JA, \textit{Appl. Phys. Lett.} 58: 2854, 1991.\\[0pt] [2] Choi JH \textit{et al}., \textit{Proc. SPIE} 5389: 399, 2004. [Preview Abstract] |
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H1.00190: Competition between itinerant ferromagnetism and spin-density wave antiferromagnetism in FeGa Yan Wu, Huibo Cao, Gregory McCandles, Julia Chan, Amar Karki, Rongying Jin, John DiTusa The metallic magnetFeGadisplays a rich magnetic behavior that includes transitions between a FM ground state to a AFM intermediate state at 68 K and back to a FM state at 360 K. The phase transition at 360 K is accompanied by a discontinuous hysteretic change in the electrical resistivity. In addition, the application of moderate magnetic fields cause a sharp transformation from the AFM to FM state with a critical $H$that grows dramatically with $T$.To explore the cause of this unusual competition of magnetic states, we investigated the magnetic structure of FeGavia extensive single crystal neutron diffraction measurements. These measurements revealed a FM ordering with magnetic moments lying along the crystallographic $c$-axis both below 68 K and above 360 K as well as incommensurate spin density wave order between these temperatures. Our refinement of the diffraction data has uncovered the existence of a small non-coplanar moment which may be the origin of our previously discovered topological Hall Effect. [Preview Abstract] |
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H1.00191: COMPLEX STRUCTURED MATERIALS INCLUDING GRAPHENE |
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H1.00192: Thickness dependence of mechanical properties of free-standing graphene oxide papers Tao Gong, Do Van Lam, Sejeong Won, Renlong Liu, Hwangbo Yun, Sanghyuk Kwon, Jinseon Kim, Ke Sun, Seungmo Lee, Changgu Lee We have characterized thickness dependence of mechanical properties, such as Young's modulus, fracture strength, fracture strain and toughness, of graphene oxide papers using tensile and bulge test methods. The GO papers were made from Hummer's method and the fabricated GO paper's thickness varied from 0.1 $\sim$ 100 $\mu$m. The measured Young's modulus and fracture strength decreased with increasing thickness ranging from 44.6 $\sim$ 8.5GPa and 170.2 $\sim$ 40MPa respectively. Through TEM, SEM and AFM characterization, the inner structure and surface morphology such as crack formation and roughness change are the keys to the variation of mechanical properties in the GO papers by the thickness. The thicker GO papers are weaker because it has more manufacturing voids in it that cause it to fail easily and less stiff. Surface wrinkle and residual stress are the mechanism of terraced fracture strain. [Preview Abstract] |
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H1.00193: ABSTRACT WITHDRAWN |
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H1.00194: Transition metals-graphene interaction: the role of the screened van der Waals energy Alberto Ambrosetti, Pier Luigi Silvestrelli The interaction of graphene with transition metals is of particular interest for practical applications, which include for instance the efficient production of high-quality graphene. The accurate theoretical description of transition metals-graphene interfaces, however, is a particularly challenging problem due to the complex interplay between van der Waals (vdW) and hybridization effects. Here we apply the DFT/vdW-WF2s method [1], which allows to augment semi-local Density Functional Theory through the introduction of screened vdW interactions. Notably, we find that a reliable modeling of the van der Waals interaction should account for complex metal screening effects, that are due to the combined contributions of the p- and s-like \textit{quasi-free} electrons, and the more \textit{localized} d-states. The resulting geometry and energetic properties are in good agreement with experimental data and sophisticates theoretical calculations. Moreover, the Maximally Localized Wannier Functions underlying the DFT/vdW-WF2s method allow for an intuitive understanding of the complex binding mechanism.\\[4pt] [1] P. L. Silvestrelli and A. Ambrosetti, Phys. Rev. B 87, 075401 (2013). [Preview Abstract] |
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H1.00195: Synthesis of carbon nano-structures using organic-molecule intercalated taeniolite layered silicates Takaaki Maezumi, Noboru Wada By calcinating organic-molecule intercalated taeniolite layered silicates, carbon nano-structures were made between the 2:1 layered silicate sheets. Raman scattering, XRD, TGA and SEM were used to characterize the samples. Large taeniolite crystals (NaLiMg$_{2}$Si$_{4}$O$_{10}$F) were first prepared by melting appropriate chemicals at high temperatures using a platinum crucible. Then, the taeniolite crystals made were cation-exchanged with Li$^{+}$, K$^{+}$, NH$_{4}^{+}$, Ca$_{2}^{+}$ and Mg$_{2}^{+}$ in salt solution. Finally, various organic molecules such as ethylene glycol, pyridine and so on were intercalated into the taeniolite crystals, and calcinated under a N$_{2}$ atmosphere at about 1000K. The resulting crystals are usually gray or black. X-ray (00l) diffraction patterns suggested that the carbon structures may be monolayer thick (i.e., graphene-like). Raman scattering spectra which exhibited a sharp G-band peak with a high G-band/D-band ratio indicated that the carbon structures were relatively well crystallized. Cation and organic-molecule dependence on the carbon structures will be discussed. In addition, evidence for stage-2 taeniolite will be presented. [Preview Abstract] |
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H1.00196: A first principles study on CVD graphene growth on copper surfaces: C-C bonding reactions at graphene edges Nobuo Tajima, Tomoaki Kaneko, Jun Nara, Ohno Takahisa Graphene has attracted considerable research interest owing to its potential application to future electronic devices. Large area and high quality graphene is needed for device applications. Chemical vapor deposition (CVD) using a copper surface with a hydrocarbon source is one of the practical methods to produce graphene. This method is appropriate for creating large area graphene with low cost, and the graphene growth control to obtain a high quality product is a remaining challenge. The carbon atom nucleation and cluster growth processes in the CVD reactions have been studied extensively as key steps that affect the graphene growth behavior. We have been studying the carbon atom reactions in these processes by theoretical approaches In the present study, we have focused on the later stage of CVD reaction, that is, carbon atom reactions at graphene edges by which carbon clusters grow in the Cu-CVD We have found that these reactions have energy barriers of $\sim$1 eV. First principles simulation code PHASE http://www.ciss.iis.u-tokyo.ac.jp/riss/english/project/device/) was used in the theoretical calculations. [Preview Abstract] |
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H1.00197: ABSTRACT WITHDRAWN |
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H1.00198: Ab-initio investigation of optical absorption spectra properties of doped Graphene Girija Dubey, Pooja Rani, Vijay Jindal Ab-initio calculations based on density functional theory(DFT) have been performed to study the changes in the absorption spectrum of graphene substituted with B, N and BN. The dielectric function and hence the absorption spectrum of single layer graphene sheet have been calculated. The present study can be concluded as, the individual B and N doping does not significantly affect the imaginary dielectric function and hence the absorption spectrum.However, red shift in the absorption towards visible range of the radiation at high doping is found to occur in case of B/N co-doping at high doping concentration. It can be inferred the B/N co-doping of graphene can alter the optical properties of graphene to make it reflect in the visible region. [Preview Abstract] |
(Author Not Attending)
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H1.00199: Molecular dynamics simulation of graphene friction: the interplay of tip, graphene and substrate Minwoong Joe, Changgu Lee Graphene is a promising future platform material that could be harnessed in wealth of new applications by virtue of its superior electronic and mechanical properties. Also, atomically thin graphene provides an ideal testbed for investigating fundamental aspects of nanoscale friction. Here, molecular dynamics simulations are performed to study frictional behaviors of atomic force microscope tip on graphene. The effects of tip crystallinity, scan direction (or graphene orientation), and graphene thickness are examined. Puckering hypothesis is revisited by comparison with rigid/supported/suspended graphene cases. Our studies provide broader perspectives into the friction mechanism on graphene. [Preview Abstract] |
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H1.00200: Effect of Ar Overpressure Ratio on the Growth of Graphene on Cu(111) Heike Geisler, Seamus Murray, Eng Wen Ong, Tyler Mowll, Parul Tyagi, Carl A. Ventrice, Jr. A graphene growth study was performed on Cu(111) in a UHV chamber by CVD using ethylene. The sample holder consisted of an oxygen series button heater with Ta heat shields to allow annealing the crystal to 900 $^{\circ}$C at pressures as high as 100 mTorr. The crystal structure of the surface was determined using LEED. Growth attempts on the clean Cu(111) surface at ethylene pressures as high as 5 mTorr only resulted in trace amounts of graphene being grown on the surface. This is attributed to the low catalytic activity of the Cu(111) surface and the high vapor pressure of Cu at the growth temperature. To suppress the sublimation of Cu, an Ar overpressure was used. Ethylene partial pressures of 2, 5, 10, and 50 mTorr were used, keeping the total pressure at 50 mTorr. The films for 2 and 5 mTorr showed predominately single domain epitaxy. At 10 mTorr ethylene partial pressure, additional diffraction spots 30$^{\circ}$ out of phase with the Cu(111) substrate were observed. At 50 mTorr of ethylene and no Ar overpressure, broad diffraction arcs were observed in LEED that were $\pm15^{\circ}$ out of phase with the substrate. Therefore, the carbon deposition rate, which depends on the ethylene partial pressure, has a large effect on the quality of the graphene film. [Preview Abstract] |
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H1.00201: Origin of the 2450 cm$^{-1}$ peak (G* band) in the Raman spectrum of graphene Ramakrishna Podila, Rahul Rao, Mehmet Karakaya, Jingyi Zhu, Apparao Rao Here, we report the Raman studies of mechanically exfoliated and chemical vapor deposited (CVD) pristine, ion{\-}irradiated, and N{\-}doped graphene (SLG, BLG, and FLG), which identify the origin of the so-called $G$*-band in graphene $\sim$ 2450 cm$^{-1}$. Our results show that the asymmetry of the $G$*-band clearly increases with interlayer stacking, with the high frequency peak exhibiting more sensitivity to intralayer defects compared to the lower component. The sub-peaks (i.e., low and high frequency components) in the $G$*{\-}band were observed to merge with increasing excitation energy and could be understood in terms of the energy dependent scattering rates of photo-excited carriers. [Preview Abstract] |
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H1.00202: Chemical Vapour Deposition of Graphene with Re-useable Pt and Cu substrates for Flexible Electronics Shumaila Karamat, Selda Sonusen, Umit Celik, Yigit Uysalli, Ahmet Oral Graphene has gained the attention of scientific world due to its outstanding physical properties. The future demand of flexible electronics such as solar cells, light emitting diodes, photo-detectors and touch screen technology requires more exploration of graphene properties on flexible substrates. The most interesting application of graphene is in organic light emitting diodes (OLED) where efforts are in progress to replace brittle indium tin oxide (ITO) electrode with a flexible graphene electrode because ITO raw materials are becoming increasingly expensive, and its brittle nature makes it unsuitable for flexible devices. In this work, we grow graphene on Pt and Cu substrates using chemical vapour deposition (CVD) and transferred it to a polymer material (PVA) using lamination technique. We used hydrogen bubbling method for separating graphene from Pt and Cu catalyst to reuse the substrates many times. After successful transfer of graphene on polymer samples, we checked the resistivity values of the graphene sheet which varies with growth conditions. Furthermore, Raman, atomic force microscopy (AFM), I-V and Force-displacement measurements will be presented for these samples. [Preview Abstract] |
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H1.00203: Non-covalent interactions in the colloidal graphene dispersions Dorsa Parviz, Ziniu Yu, Sriya Das, Fahmida Irin, Ronald Hedden, Micah Green We have studied stabilization mechanisms in colloidal dispersions of pristine graphene. Electrostatic and steric stabilization in presence of pyrene derivative as dispersants depends on the dispersant concentration, functional groups and the solution pH. The graphene/dispersant yield obtained by pyrene derivatives was considerably higher compared to conventional dispersants. Pyrene-graphene $\pi $-$\pi $ interactions were combined with a designer functional group (polydimethylsiloxane (PDMS)) to synthesize a polymer with dual functionality as dispersant and polymer matrix. The same strategy was applied to produce graphene/ PMMA and graphene/ PS films. Controllable crumpling of graphene nanosheets was induced through rapid evaporation of dispersion droplets within a spray dryer. Dimensional transition of 2D nanosheets to 3D crumpled particles was directly observed. Multi-faced dimpled morphology of pristine graphene was different than highly wrinkled morphology of crumpled graphene oxide. Changing the compressive forces during drying allowed for controllable folding of the nanosheets, while the unfolding of the redispersed crumpled particles was controlled by the solvent choice. [Preview Abstract] |
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H1.00204: Surface plasmons in doped graphene excited by the Attenuated Total Reflection technique in the THz regime F. Ramos-Mendieta, J.A. Hernandez-Lopez, M. Palomino-Ovando Surface plasmons of transverse electric (TE) and transverse magnetic (TM) polarization in doped free-standing graphene are numerically investigated at THz frequencies. For detecting these modes sufficient sensitivity of the prism-based Otto configuration is demonstrated. Complete agreement with the TM dispersion relation is found in doped graphene of Fermi level $\mu =$ 0.8 eV; perfect absorption due to wave interference is also observed. On the other hand, TE surface plasmons are special surface vibrations without induced surface charge; they are self-sustained current oscillations (unique of graphene) that arise in frequency ranges where the imaginary part of the graphene dynamical conductivity is negative. We found that TE plasmons are excited for angles of incidence very close to the critical angle between prism and air, as predicted from their dispersion relation. Reflection profiles and field intensities of these waves are presented for $\mu =$ 0.2, 0.3 eV. [Preview Abstract] |
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H1.00205: Hetero epitaxial graphene on various substrates Gary Harris, Gurpreet Kaut, Crawford Taylor Large-scale production of graphene is pivotal for the development of graphene-based electronics. These results focus on the synthesis and characterization of graphene layers. Two methods were used to grow graphene films. First, graphene films were epitaxially grown on silicon carbide substrates by thermal decomposition of SiC at high temperature and low pressure. In-house built reactor consisting of induction furnace was used to form epitaxial films for electronic applications. Second, chemical vapor deposition method was used for direct graphene synthesis on 3C-SiC with the use of copper as a catalyst. In thermal CVD process, hydrogen and methane gases were used as precursors. Methane acts as a carbon source and annealing and cooling were done hydrogen environment. Different polytypes of silicon carbide (6H-SiC and 3C-SiC) and their crystal orientations were exploited as substrates to form epitaxial graphene. Hetero epitaxial 3C-SiC epilayer was first deposited on Si substrate using chemical vapor deposition technique in cold wall, low pressure, and horizontal CVD reactor. The reactor temperature, argon pressure, flow rates and concentration of different gases (propane, silane, hydrogen and argon) was investigated to control the growth of 3C-SiC and silicon sublimation rate. The resulting graphene films were confirmed using Raman spectroscopy. Further, graphene films have been characterized with the tools of atomic force microscopy (AFM) and scanning electron microscopy (SEM). Mobility, electrical resistivity and carrier density measurements were taken using hall measurements. [Preview Abstract] |
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H1.00206: Graphene oxide membrane for liquid phase organic molecular separation Renlong Liu, Girish Arabale, Jinseon Kim, Ke Sun, Yongwoon Lee, Changkook Ryu, Changgu Lee The selective permeation of organic solvents and water through graphene oxide (GO) membranes has been demonstrated. Water was found to permeate through GO membranes faster than various alcohols. The permeation rates of propanol are about 80 times lower than that of water. Taking advantage of the differences in the permeation rates, we separated water from the alcohols and obtained alcohols with high purity. For ethanol and 1-propanol, binary solutions of the alcohol and water were filtered efficiently to produce alcohols with concentration of about 97{\%}. However, the selectivity of the filtration of methanol is significantly lower than those of the other alcohols. To understand the mechanism we followed the structural changes in the GO membranes by X-Ray diffraction analysis. From the X-ray diffraction results we speculate that the selectivity of the permeation of water and alcohols is closely related to the molecular sizes of the solvents and their polarity. In order to demonstrate the potential applications of this process for the selective removal of water from aqueous organic mixtures, we performed the separation of water from a bio-oil containing 73{\%} of water. The majority of the water was filtered out resulting in a higher purity bio-oil. [Preview Abstract] |
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H1.00207: Magneto-electronic and optical properties of transition metal dicalcogenide monolayers Yen-Hung Ho, Chih-Wei Chiu, Ming-Fa Lin, Wu-Pei Su A generalized tight-binding model is utilized to study the Landau level spectra of various transition metal dicalcogenide monolayers. The intrinsic spin-orbit coupling effectively gives rise to multiple splitting of Landau levels. With a close inspection of wavefunction characteristics, these levels can be classified into specific groups in terms of their orbital, spin and valley signatures. In the calculation of magneto-absorption spectra, the physical origins of optical selection rules are clearly resolved. Compounds are different from one another in terms of transition energies and appearance of twin peaks. Our numerical results clearly demonstrate the magnetic control of spin and valley charge carriers and provide a basis for future experiments. [Preview Abstract] |
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H1.00208: Understanding the optical and electronic properties of Ga-doped graphene N.C. Creange, C. Constantin, J.-X. Zhu, A.V. Balatsky, J.T. Haraldsen We simulate the optical and electrical responses in gallium-doped graphene, using density functional theory with a local density approximation. We show the effects of impurity doping (0-3.91\%) in the graphene sheet and for each doping percentage the change in electron density, refractive index, and optical conductivity are reported. Here, gallium atoms are placed randomly (using a 5-point average) throughout a 128-atom sheet of graphene. These calculations demonstrate the effects of hole doping due to direct atomic substitution, where we find a disruption in the electron density for small doping levels, which is due to impurity scattering of the electrons. However, there seems to be a doping percentage, above which we have calculated, at which the system transitions to produce metallic or semi-metallic behavior. These calculations are compared to a purely theoretical 100\% Ga sheet for comparison of conductivity. Furthermore, we examine the change in the electronic band structure and density of states, where the introduction of gallium electronic bands produces a shift in the electron bands and dissolves the characteristic Dirac cone within graphene. [Preview Abstract] |
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H1.00209: Van der Waals Epitaxy of Two-Dimensional $\alpha $-MoO$_{3}$ Nanosheets using Mica as Grown Templates Wang Di, Zhou Yu, Wang Mu, Peng Ruwen, Xiong Xiang The orthorhombic Molybdenum trioxide, $\alpha $-MoO$_{3}$, is one kind of graphene-like layered materials. Since the great promise for future electronic and optoelectronic application, this molybdenum-based two-dimensional (2D) layer material has recently attracted much attention. In this work, we report a Van der Waals epitaxy of $\alpha $-MoO$_{3}$ nanosheets on mica substrate under ambient pressure. This simple physical vapor-phase deposition process is proposed to medicate through the weak Van der Waals interaction between layered $\alpha $-MoO$_{3}$ and mica substrate. As a result, the grown $\alpha $-MoO$_{3}$ nanosheets, whose lateral dimension is up to 0.1mm and thickness less than 2.8 nm (about bilayer MoO$_{3}$ octahedral structure), exhibit defined lattice orientation. From bulk to bilayer, Raman spectra of $\alpha $-MoO$_{3}$ nanosheets show independed with layered number, which unlike to MoS$_{2}$. The measurement of electric resistances at room temperature shows the conductance of original $\alpha $-MoO$_{3}$ nanosheets is already high, moreover, which can be greatly improved by hydrogen doping. Our works indicate that VDWE with mica templates is a simple and feasible strategy to grow high-quality ultrathin $\alpha $-MoO$_{3}$ nanosheets, which have superiorities for investigating its novel physical properties and potential application in future. [Preview Abstract] |
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H1.00210: Photoluminescence Quenching in Single-Layer MoS$_{2}$ via Oxygen Plasma Treatment Narae Kang, Hari P. Paudel, Michael N. Leuenberger, Laurene Tetard, Saiful I. Khondaker Ultrathin two-dimensional (2D) layered transition dichalcogenides (TMDs) families have emerged as a new class of semiconducting candidates due to its intrinsic bandgap. The ability to control the properties of 2D TMDs will become a key in the development of future electronic and optoelectronic applications; however, altering the properties via creating and manipulating defects through external control is not fully investigated yet. In this work, we studied tunable optical properties of single-layer (SL) MoS$_{2}$ by applying time-dependent oxygen plasma exposure. As the exposure time increased, the strong photoluminescence (PL) of SL MoS$_{2}$ changed to complete quenching accompanied by clear changes in Raman spectra with gradual reduction of MoS$_{2}$ peaks as well as an appearance of oxidization-induced peak of Mo-O bonds formation. Using band structure calculations, we found that the creation of MoO$_{3}$ disordered-domains led to plasma-induced direct-to-indirect bandgap transition in defected SL MoS$_{2}$, resulting in PL quenching with lattice distortion. Our results suggest new opportunities of tailoring and understanding the properties of 2D TMDs. [Preview Abstract] |
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H1.00211: Scanning Photocurrent Microscopy on Single-Layer CVD MoS2, MoSe2, and Alloys Edwin Preciado, Velveth Klee, David Barroso, Kristopher Erickson, Mark Triplett, Ariana Nguyen, Chris Lee, I-Hsi Lu, Sarah Bobek, John Mann, Alec Talin, Francois Leonard, Ludwig Bartels We report scanning photocurrent measurements on CVD-grown single-layer films of MoS2, MoSe2, and MoS2(1-x)Se2x alloys. Measurements at different irradiation intensity reveal a superlinear photoresponse irrespective of the material composition. We find photocurrents that decrease and IV-behavior that becomes more linear with increasing selenium contents. Measurements in dependence of the channel width indicate carrier diffusion length on the order of 0.9 micron for MoS2 and 0.5 micron for MoSe2. Concomitant thermal imaging rule out thermal defects. Time-dependent measurements suggest long-lived traps for MoS2 based materials that are absent for MoSe2-based materials. [Preview Abstract] |
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H1.00212: Lateral hydrogenated graphene/h-BN Tunneling Magnetoresistance Devices Shayan Hemmatiyan, Cristian Cernov, Artem Abanov, Marco Polini, Allan H. MacDonald, Jairo Sinova Based upon first principle calculations, we propose a practical heterostructure of hydrogenated graphene on the top of hexagonal-boron nitride, which exhibits ferromagnetic properties and relatively large spin orbit coupling. We propose to use this functional substrate for the lateral spin valve systems. [Preview Abstract] |
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H1.00213: The Optical Properties of Germanane Patrick Odenthal, Walid Amamou, Dante O'Hara, Luyi Yang, W. D. Rice, Scott Crooker, Roland Kawakami Hydrogenated 2D germanium, known as germanane (GeH), is predicted to be a direct bandgap semiconductor with high electron mobility ($\sim$ 10$^{\mathrm{4}}$ cm$^{\mathrm{2}}$/Vs) and circularly polarized optical selection rules. However, very little experimental work on the optical properties of germanane has been reported to date. Here, we present temperature-dependent photoluminescence (PL) and photoluminescence excitation spectroscopy (PLE) data on several-layer germanane grown by Molecular Beam Epitaxy (MBE). We observe a broad PL band that is Stokes-shifted by hundreds of meV below the predicted direct bandgap of 1.53 eV. The PL intensity increases by several orders of magnitude upon decreasing the temperature from room temperature to 4K. [Preview Abstract] |
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H1.00214: Adsorption of Fluorine on single layer MoS$_2$: a first principles study Conrad Troha, Duy Le, Talat S. Rahman One of the effective methods for tuning properties of single layer MoS$_2$ is to impose interactions with adsorbates. Using density functional theory, with the optB88-vdW exchange correlation functional, we have studied the adsorption of fluorine atoms on a single layer MoS$_2$. We find that fluorine atoms prefer to adsorb on top of sulfur atoms in $(2\times 2)$, $(3\times 3)$, and $(4\times 4)$ overlayer structures and that at 1/9 ML or smaller coverage, the interactions between fluorine atoms are small and can be ignored. The band structures of the considered overlayer structures suggest that the adsorption of fluorine atoms introduces defect state inside the band-gap of MoS$_2$. This state disperses near the Fermi level in the $(2\times 2)$ overlayer structure and is dispersionless in the $(3\times 3)$ or larger overlayer structures. [Preview Abstract] |
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H1.00215: Femtosecond Time Resolved Spectroscopic Measurements of Mono and Few-layers WS$_{2}$ Shrouq Aleithan, Sudiksha Khadka, Max Livshits, Jeffery Rack, Eric Stinaff Methods for producing samples of transition metal dichalcogenide monolayers, a new two-dimensional direct-band gap semiconductor potentially useful for applications in~electronics and photonics, have dramatically improved from mechanical and chemical exfoliation, to current chemical vapor deposition (CVD) techniques. We present an experimental investigation carried out to study the exciton dynamics in mono-to-few layer sheets of WS$_{2}$. The experiment was performed on commercially CVD grown WS$_{2}$ on a sapphire substrate employing femto-second transient absorption spectroscopy and microscopy. The sample was excited using a pump pulse of 405nm and then probed with differential absorption spectra over the spectral range of 350 nm - 800 nm, using white continuum. These results should help further the understanding of exciton dynamics in two dimensional sheets of WS$_{2}$. [Preview Abstract] |
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H1.00216: Raman Characterization of Graphene and 2D TMD Heterostructures Benjamin Derby, Angela Hight Walker We report efforts to produce and characterize graphene and two-dimensional transition-metal dichalcogenides (TMD) heterostructures. Using PDMS stamps, exfoliation of graphene, MoS$_{2}$, h-BN, and TaS$_{2}$ precedes the stacking of these mono- and few layers into heterostructures. The goal is to engineer mis-orientation to enhanced Raman signatures of various layers within the heterostructures. Previous studies have reported a Raman signal strength that is angle dependent between bi-layers [1]. Using resonant Raman spectroscopy, we probe the quality of these constructed heterostructures. Ultimately, we plan to combine our optical measurements with an applied magnetic field to probe the complex magneto-Raman interaction. Previous studies [2] show a magneto-phonon resonance at specific field strengths and laser excitations. Our results to date will be summarized.\\[4pt] [1] K. Kwanpyo \textit{et al. }Phys. Rev. Lett. \textbf{108}, 246103 (2012)\\[0pt] [2] C. Qiu \textit{et. al. }Phys. Rev. Lett. \textbf{88, }165407 (2013) [Preview Abstract] |
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H1.00217: Transport Properties of CVD Grown TMDs on Flat and Patterned Substrates Joseph Martinez, Ariana Nguyen, Thomas Scott, Edwin Preciado, Velveth Klee, Dezheng Sun, Pankaj Sharma, I-Hsi Lu, David Barroso, SukHyun Kim, V. Ya. Shur, Alexei Gruverman, Peter A. Dowben, Ludwig Bartels Transition Metal Dichalcogenides (TMDs), MX2 (M$=$Mo, W, etc., X$=$S, Se, Te), have shown great promise for applications as electronic, spintronic and photonic materials. We show growth of MX2 materials under UHV (ultrahigh vacuum) and via CVD (chemical vapor deposition) on both flat and patterned substrates. Deposition on periodically-poled ferroelectric substrates reveals the impact of poling domains and the ability to reversibly invert the transport characteristics from n- to p-doped. 3D geometric patterning of substrates permits the growth across trenches and at angles to the substrate plane leading to modifications of the commonly-addressed in-plane transport properties. [Preview Abstract] |
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H1.00218: Temporal evolution of Trion-Exciton coupling in transition metal dichalcogenide Kha Tran, Akshay Singh, Galan Moody, Sanfeng Wu, Jason Ross, Xiaodong Xu, Xiaoqin Li Transition Metal Dichalcogenides (TMD's), especially in the two-dimensional limit, show remarkable physical phenomena including large light absorption by single layers (up to 10{\%}) and coherent many-body effects. Increased interactions in these two-dimensional materials have been attributed to reduced screening and these are reasoned to cause large binding energies of quasiparticles including excitons (coulomb bound electron-hole pairs) and trions (charged excitons). The coupling amongst these quasiparticles is an interesting fundamental problem relating to strength of electronic interactions as well as having applications in photo-detectors working beyond diffusive transport. We use two-color pump-probe spectroscopy to measure these quasiparticle interactions as a function of time. We concentrate on the special condition of pumping at the trion transition, which is lower in energy than the exciton, and probing the exciton transition. We observe an optical response with distinct temporal response compared to other excitation conditions. The observations are well explained by a phenomenological model incorporating Optical Bloch Equations which suggests many-body effects, including excitation induced shifts, are involved. [Preview Abstract] |
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H1.00219: Intrinsic transport of h-BN encapsulated few-layer black phosphorus Ghidewon Arefe, Young Duck Kim, Daniel Chenet, Xu Cui, Damien Chang, James Hone Few-layer black phosphorus (BP) is an exciting two-dimensional material with ambipolar behavior, large on/off ratio, and high mobility with a direct bandgap. The anisotropic atomic nature of black phosphorus exhibits unique angle dependent electronic and optical features. One of the primary difficulties in fabricating few-layer BP devices to study transport is the reactive nature of the material in ambient conditions as it degrades in the presence of air and moisture. In order to characterize the intrinsic physical properties of BP, we fabricated few-layer BP flakes that are fully encapsulated by hexagonal boron nitride (h-BN) with a clean stacking technique. We also characterized the electrical transport of h-BN encapsulated BP devices that show greatly improved environmental stability and high mobility at low temperature due to the suppression of extrinsic scattering effects such as charge impurities, surface polar optical phonons, and absorbents from air. H-BN encapsulated BP devices will be an essential platform for the observation of new physics from BP and realization of BP based advanced opto-electronic application devices body. [Preview Abstract] |
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H1.00220: ABSTRACT WITHDRAWN |
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H1.00221: Ultrafast Dynamics in Bulk and Monolayer MoS$_{2}$ Measured with Femtosecond Pump-Probe Technique Xianghai Meng, Wenzhi Wu, Avinash Nayak, Jung-Fu Lin, Deji Akinwande, Yaguo Wang MoS$_{2}$ is a typical material of transition-metal dichalcogenide family. It exhibits unique properties when thickness reduces to monolayer. Recent studies have shown strong photoluminescence (PL) and high carrier mobility on monolayer MoS$_{2}$, which makes it a promising candidate for future photonic and field-effect transistor (FET) applications. Our ultrafast measurement utilizes optical 400nm-pump 800nm-probe spectroscopy to reveal the relaxation dynamics of photo-excited carriers in both bulk and monolayer MoS$_{2}$. Measurement is carried out at ambient pressure for different pump fluences. Both a fast and a slow carrier lifetime are acquired in monolayer MoS$_{2}$ due to different carrier scattering mechanism. Carrier lifetimes are measured at different pump fluences, from which we propose possible carrier relaxation mechanisms. In suspended bulk MoS$_{2}$, coherent acoustic phonons with a peak frequency around 38GHz are observed. Phonon lifetime and amplitude at different pump fluences have also been investigated. [Preview Abstract] |
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H1.00222: ABSTRACT WITHDRAWN |
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H1.00223: Hydrogen Molecule Adsorption on a Borophene-Titanium System Gregorio Ruiz-Chavarria From the synthesis of graphene have developed a wide range of researchs on their use, both theoretical and experimental. So there have been research on graphene-based electronics, but also on issues of energy, particularly hydrogen adsorption on graphene-based systems. Given the potential represented by these structures is very natural to wonder about similar structures, but based in elements near carbon. One of the lines developed very recently consider the boron as the element to build graphene-like structures. Different studies, both theoretical and experimental have been made where the studied structures are graphene type or fullerene, where boron is used in place of carbon. We will use as a starting point the proposed structures by Xiaobao\footnote{Xiaobao Y, et al, Phys Rev B, {\bf 77}, 041402 (2008)} and Tang.\footnote{Tang H. and Ismail-Beigi S., PRL, {\bf 99}, 115501 (2007)} This structure is known as the borophene, which in first place will be decorated with titanium and then, this system interact with hydrogen molecule. In our calculation we use functional density theory, atomic pseudopotentials, Born approximation and molecular dynamic. [Preview Abstract] |
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H1.00224: Transport Properties Across Misoriented Bilayer MoS2 using Ab-initio Calculations Kuan Zhou, Supeng Ge, Darshana Wickramaratne, Roger Lake Fabrication of electrical and opto-electronic devices with vertically stacked transition metal dichalcogenides (TMDCs), leads to interfaces that are misoriented. Prior experimental and theoretical studies of misorientation in graphene bilayers demonstrated that a few degrees of misorientation is sufficient to decouple the low energy states of the individual layers. Experimental and ab-initio calculations have shown the bandgap of misoriented bilayer MoSremains indirect. The transport properties across the misoriented interface of the bilayer TMDCs is currently unknown. The coherent interlayer transmission across two stacks of MoSis calculated for unrotated and rotated MoSbilayers using ab-initio calculations. The energy dependence of the interlayer transmission is analyzed.. [Preview Abstract] |
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H1.00225: Fluorinated graphene as an efficient diffusion barrier in Ge semiconductor devices Wei Ren, Heng Gao We evaluate the efficient diffusion barrier effects of the fluorinated graphene from the first principles. By taking its advantage of impermeability, we discover such monoatomic layer can suppress the formation of the unstable interfacial oxide in Ge-based semiconductor devices. To elucidate the physical mechanism governing this shielding functionality, nudged elastic band method is adopted to calculate the barrier height of one oxygen or one germanium atom penetrating the pristine graphene and fluorinated graphene. The energy of the adsorbed O or Ge atom on different sites of the graphene is calculated, namely three positions on the honeycomb lattice, bridge, hollow, and top. Our results reveal that both the O and Ge atoms adsorbed on the graphene are most stable on the bridge site, followed by the top and hollow sites with higher energies. Different penetration paths of O and Ge atoms are considered, and the calculated values of the energy barriers for both graphene and fluorinated graphene exhibit superior impermeability and hence to hinder diffusion of O and Ge atoms across the graphene and fluorinated graphene. This latter insulating structure is expected to expedite the implementation of germanium as channel materials in next-generation nanoelectronic devices. [Preview Abstract] |
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H1.00226: Fabrication of graphene field-effect transistor on top of ferroelectric single-crystal substrate Nahee Park, Haeyong Kang, Yourack Lee, Jeong-gyun Kim, Joong-gyu Kim, Yoojoo Yun, Jeongmin Park, Taesoo Kim, Jung Ho Kim, Youngjo Jin, Yong Seon Shin, Young Hee Lee, Dongseok Suh In the analysis of Graphene field-effect transistor, the substrate material which has the direct contact with Graphene layer plays an important in the device performance. In this presentation, we have tested PMN-PT(i.e.(1-x)Pb(Mg$_{1/3}$Nb$_{2/3})$O$_{3}$-xPbTiO$_{3})$ substrate as a gate dielectric of Graphene field-effect transistor. Unlike the case of previously used substrates such as silicon oxide or hexagonal Boron-Nitride(h-BN), the PMN-PT substrate can induce giant amount of surface charge that is directly injected to the attached Graphene layer due to its ferroelectric property. And the hysteresis of polarization versus electric field of PMN-PT can cause the device to show the ferroelectric nonvolatile memory operation. We had successfully fabricated Graphene field-effect transistor using the mechanically exfoliated Graphene layer transferred on the PMN-PT(001) substrate. Unlike the case of mechanical exfoliation on the surface of silicon-oxide or the Poly(methyl methacrylate) (PMMA), the weak adhesion properties between graphene and PMNPT required the pretreatment on PMMA before the exfoliation process. The device performance is analyzed in terms of the effect of ferro- and piezo-electric effect of PMNPT substrate. [Preview Abstract] |
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H1.00227: Study of quantum capacitance in N doped few layer graphene Mehmet Karakaya, Jingyi Zhu, Ramakrishna Podila, Anurag Srivastava, Apparao Rao The intrinsically small density of states at the Fermi level in graphene results in a small serial quantum capacitance $C_{Q}$, which diminishes the total device capacitance value ($C_{tot})$ in supercapacitors. In this work, we studied $C_{Q}$ of N doped graphene in pyrrolic(N1), graphitic (N2) and pyridinic (N3) configurations. The observed $C_{Q}$ value for sample N1 was significantly different from samples N2 and N3, as predicted by DFT calculations, thus implying that precisely engineered dopant configurations, rather than concentration, can enhance $C_{Q}$. Such approaches are pivotal for alleviating the existing bottlenecks in both graphene-based device scaling and supercapacitor electrode limitations. [Preview Abstract] |
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H1.00228: Effect of adsorbed gases on the G and D' peaks of the Raman spectrum of graphene Gina Greenidge, Joshua Halpern The ability of graphene to function as a gas sensor for polar molecules has been widely investigated by monitoring the conductivity of graphene near the Dirac point. Here we demonstrate that Raman spectroscopy can also monitor the interaction of these molecules with graphene. Using a Raman microscope we observe measureable changes in the width and position of the G peak, and the intensity of the D' prime peak upon exposure to water vapor (H$_{\mathrm{2}}$O and D$_{\mathrm{2}}$O). The changes are reversible. Baking the material at 350 $^{\mathrm{o}}$C restores the graphene spectrum to its original state. We are investigating the effects of additional gases as well as the relationship of these shifts to the conductivity and the gas-graphene interaction. [Preview Abstract] |
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H1.00229: Effect of Al$_2$O$_3$ deposition on electronic Transport in Graphene: DFT-NEGF study Tomoaki Kaneko, Takahisa Ohno In order to screen the charged impurities and to prevent the adsorption of contaminant on graphene, the deposition of high-k materials such as Al$_2$O$_3$ on graphene surface is important issue for graphene device application. Since the interfacial structure of graphene and high-k materials are not identified, theoretical study on the interfacial structure dependence on electronic transport is highly demanded. In this paper, we performed the electronic transport simulation in graphene under Al$_2$O$_3$ based on the density functional theories (DFT) and nonequilibrium Green's function method (NEGF). We investigated the effect of Al$_2$O$_3$ surface termination on the electronic transport properties. According to the calculation of stability of interfaces and electronic structures, the graphene's linear band structure is preserved in O$_2$ deficient condition. In O$_2$ rich condition, on the other hand, the graphene's unique electronic structure is disturbed. These properties are important for the electronic transport in graphene under Al$_2$O$_3$. Graphene shows relatively good transport properties in O$_2$ deficient condition, but transport is considerably suppressed in O$_2$ rich condition. Our results suggest O$_2$ deficient condition is desirable for the device application. [Preview Abstract] |
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H1.00230: Asymmetric capacitance and ambipolar metal insulator transition in black phosphorus Yu Saito, Yoshihiro Iwasa Black phosphorus is a van der Waals type semiconducting layered material with a puckered honeycomb structure where each phosphorus atom is covalently bonded with three adjacent phosphorus atoms and has a direct band gap of 0.3 (bulk) - 2 (monolayer) depending on the number of layers [1], which can be promising material for optoelectronics devices such as photodetector. In this presentation, by using ionic liquid gating method, we report the ambipolar transistor operation and the field effect controlled ambipolar metal-insulator transition in black phosphorus thin flake. We observed a large modulation of the sheet resistance by more than 4 orders of magnitude in both electron channel and hole channel. These results suggest black phosphorus will be a key material for not only understanding physics of the conduction channel produced by ionic gating, but also potential functions including formation of p-n junction and therefore lateral tunnel diode utilizing intrinsic narrow band gap. \\[4pt] [1] Li et al, Nature Nanotechnology, 9, 372 (2014) [Preview Abstract] |
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H1.00231: Graphene/GaN diodes for ultraviolet and visible photodetectors Fang Lin, Shaowen Chen, Jie Meng, Geoffrey Tse, Xuewen Fu, Fujun Xu, Bo Shen, Zhimin Liao, Dapeng Yu The Schottky diodes based on graphene/GaN interface are fabricated and demonstrated for the dual-wavelength photodetection of ultraviolet (UV) and green lights. The physical mechanisms of the photoelectric response of the diodes with different light wavelengths are different. For UV illumination, the photo-generated carriers lower the Schottky barrier and increase the photocurrent. For green light illumination, as the photon energy is smaller than the bandgap of GaN, the hot electrons excited in graphene via internal photoemission are responsible for the photoelectric response. Using graphene as a transparent electrode, the diodes show a $\sim$ mS photoresponse, providing an alternative route toward multi-wavelength photodetectors. [Preview Abstract] |
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H1.00232: Exciton Radiative Lifetimes in Layered Transition Metal Dichalcogenides Maurizia Palummo, Marco Benardi, Jeffrey C. Grossman Light emission in two-dimensional (2D) transition metal dichalcogenides (TMDs) changes significantly with number of layers and stacking sequence. While the electronic structure and optical absorption are well understood in 2D-TMDs, much less is known about exciton dynamics and radiative recombination. In this talk, we show first-principles calculations of intrinsic exciton radiative lifetimes at low temperature (4 K) and room temperature (300 K) in TMD monolayers with chemical formula MX2 (M=Mo,W and X=S,Se), in bilayer and bulk MoS2, and in two MX2 hetero-bilayers. Our results elucidate the time scale and microscopic origin of light emission in TMDs, which have been the subjects of recent intense investigation. We find radiative lifetimes of a few ps at low temperature and a few ns at room temperature in the monolayers, and slower radiative recombination in bulk and bilayer than in monolayer MoS2. The MoS2/WS2 and MoSe2/WSe2 hetero-bilayers exhibit long-lived ($\sim$30 ns at room temperature) inter-layer excitons constituted by electrons localized on the Mo-based and holes on the W-based monolayer; this finding agrees with recent ultrafast spectroscopy experiments. We discuss how the radiative lifetime tunability can be employed to manipulate excitons in 2D-TMDs. [Preview Abstract] |
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H1.00233: ABSTRACT WITHDRAWN |
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H1.00234: Photoconductivity spectroscopy of epitaxial germanane devices Elizabeth Bushong, Yunqiu Kelly Luo, Jeremiah van Baren, Walid Amamou, Patrick Odenthal, Dante O'Hara, Igor Pinchuk, Jyoti Katoch, Adam Ahmed, Roland Kawakami Germanane, a 2D sheet of hydrogen terminated germanium atoms, has recently generated a great deal of interest. Unlike graphene, which has been the focus of 2D materials for almost a decade, germanane has a direct band gap ($\sim$ 1.5 eV) and strong spin-orbit coupling. Additionally, it is predicted that changing the surface functionalization of germanane will allow tuning of the band gap, which makes it promising for electronic and opto-electronic applications. Here, we investigate wavelength dependent absorption and transport properties of optically excited carriers via photoconductivity on germanane. Samples with thicknesses ranging from several atomic layers up to 250 nm have been studied. We observe an absorption edge as a function of wavelength, as expected for a semiconducting material. There appears to be a small shift to shorter wavelengths for thinner samples. The dependence on intensity and temperature are also investigated, as well as the bias dependence. These results have implications for the use of germanane in future applications. [Preview Abstract] |
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H1.00235: Planar Nanoscale Capacitors from Laterally Stacked Graphene - Boron Nitride Layers V. Ongun Ozcelik, Engin Durgun, Salim Ciraci We propose a nanoscale planar capacitor model consisting of laterally stacked two-dimensional insulating BN layers placed between two commensurate and metallic graphene layers. First-principles calculations of structure optimized total energy and self-consistent field potential performed on these nanoscale capacitors for different levels of charging and different number of BN layers mark the values of capacitance per unit mass, which are larger than those measured values for the supercapacitors made from other carbon based materials. [Preview Abstract] |
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H1.00236: High Photoresponsivity and Extrinsic Quantum Efficiency in Tri-Layer Tungsten Diselenide Phototransistors Zhengguang Lu, Nihar Pradhan, Komalavalli Thirunavukkuarasu, Jonathan Ludwig, Daniel Rhodes, Dmitry Smirnov, Luis Balicas We report on the photoresponse properties of three-layer tungsten diselenide field-effect transistors (FETs) fabricated by mechanical exfoliation of bulk crystals on SiO2/Si substrates connected with Au/Ti contacts. These devices exhibit two-terminal field-effect hole mobilities of about 350 cm2/Vs at 300K increasing up to 500 cm2/Vs at T\textless 50 K. The photoresponse properties were carried out at room temperature using various excitation wavelengths (405nm, 532nm, 670nm). In addition, we mapped photocurrent of these device with a laser spot size considerably smaller than the area of the conducting channel which indicate the photo-electrical response is not from the area surrounding the electrical contacts. It was found that the three-layer WSe2 FETs display a strong photocurrent response leading up to 0.5 A/W photoresponsivities and high external quantum efficiencies of up to $\sim$ 90{\%}. Also, these transistors display fast photoresponsive transient times of several ms. [Preview Abstract] |
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H1.00237: Study of Charge Density Wave Modulations in the Extended Hubbard Model Saumya Biswas, Roger Lake Charge density wave (CDW) modulations in a two-dimensional lattice are modeled in the mean field approximation using the extended Hubbard Hamiltonian. The electron phonon coupling is included with an on-site interaction term. The effect of coupling strength and Fermi level on the CDW wavelength and amplitude is examined. Periodic and closed boundary conditions are considered. The effect of potential modulation by electrostatic gating on the CDW phase and wavelength is calculated. [Preview Abstract] |
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H1.00238: Magneto-photoconductivitity of atomically thin transition metal dichalcogenides M. Eginligil, C. Zou, N. Peimyou, B. Cao, X. Shen, J. Shang, C. Cong, T. Yu Photoinduced effects of two-dimensional (2D) transition metal dichalcogenides (TMDs) are of great interest since the bandgap of these materials corresponds to visible range of spectrum. For instance, in molybdenum disulphide (MoS$_{2})$ -- a 2D semiconductor TMD and a non-centrosymmertic crystal, inherent broken inversion symmetry in monolayers leads to a large spin-orbit interaction which splits the valence band (VB) by 160 meV and gives rise to strong excitonic transitions due to the direct band gap at low energy \textbf{\textit{K}} and \textbf{\textit{--K}} valleys. The same broken inversion symmetry together with time reversal symmetry is responsible for spin-valley coupling in monolayer MoS$_{2}$ and similar TMDs (such as tungsten disulphide, WS$_{2})$. Spin-valley coupled band edges in TMDs result in different localization behaviors for different scattering mechanisms. In this work, we present our magneto-photoconductivity studies of mono- and bilayer field-effect transistor devices of MoS$_{2}$ and WS$_{2}$, and discuss our results in terms of localization effects. [Preview Abstract] |
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H1.00239: Lateral Mo$S_2$ p-n junctions formed by chemical doping method Won Jong Yoo, Min Sup Choi, Deshun Qu, Daeyeong Lee, Xiaochi Liu, Youngdae Jang, Changsik Kim, Jungjin Ryu Interests on transition metal dichalcogenides, especially Mo$S_2$, are growing immensely due to its semiconducting nature with visible light range bandgap and strong light absorption property, which can pave the way to replace Si-based electronics and realize flexible and transparent electronics. For more versatile applications and industrialization, however, a proper doping process is required because various devices such as photonics and tunneling devices are composed of p-n junctions. Here, we demonstrated the formation of lateral Mo$S_2$ p-n junction by using partially stacked of hBN and p-doping with Au$Cl_3$ solution. The fabricated devices showed an ideal rectifying behavior with ideality factor about 1. Under the exposure of monochromatic light, it revealed the properties of conventional p-n diode and also highly efficient photonic properties, showing feasibility to be applied for photovoltaic cells and photodetectors. Furthermore, we fabricated novel tunneling devices with similar device structure where local gates are located under Mo$S_2$. Its Fermi level can be effectively controlled by local gate modulation, so that the tunneling current can flow by band-to-band tunneling. This study provides an effective way to realize the practical devices such as photonics and tun [Preview Abstract] |
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H1.00240: ABSTRACT WITHDRAWN |
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H1.00241: Magneto-resistance of multiwall carbon nanotube Fermat yarn and coil yarn Kieu Truong, Haeyong Kang, Yourack Lee, Joong-gyu Kim, Young Hee Lee, Dongseok Suh Multiwall carbon nanotube (MWCNT) based yarn has attracted a great attention for the development of multifunctional super-fiber due to its light weight, high flexibility, high conductivity, and strong mechanical properties (Lima et al. 2011, Science). Recently the importance of coiled yarn structure was demonstrated for practical applications (Haines et al. 2014, Science). In this study, we measured the electrical resistance of neat yarns and coiled yarns at different temperatures and magnetic fields. The coiled yarn was formed by twist-insertion into the neat yarn, and the transverse and longitudinal magnetoresistance (MR) measurements were carried out. The logarithmic temperature dependence of normalized resistance and the negative MR can be explained by the combined contribution of weak-localization effect and the tunneling transport at different temperature ranges. The magnitude difference of MR between two configurations and the survival of such difference even at room temperature indicate that one-dimensional transport features are quite significant in this system. Developing Route for sub-micrometer-scale coil is discussed. [Preview Abstract] |
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H1.00242: ABSTRACT WITHDRAWN |
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H1.00243: Carbon nanotube networks grown on varios carbon nanostructures: SWCNT, MWCNT and Graphene Youngwoo Kwon, Anvar Zakhidov Secondary growth of carbon nanotubes (CNT) on the various nanoscale substrates has been performed by using chemical vapor deposition (CVD). Spinnable CNT yarns, single wall CNT sheets and graphene flakes, in NMP have been used as scaffolds for such secondary networks, The CNT yarn drawn from spinnable CNT forest is one of the promising applications of the CNT. However, orientation of the yarn and comparatively high sheet resistance make them harder for applications. Processing secondary CVD grows CNTs on the CNT yarn without any orientation of the secondary grown CNTs. Thus, this decreases the effect of the orientation of the CNT yarn and also decreases sheet resistance since the yarn have more contact each other. This after-treating will make more application possible. Furthermore, since CNT yarn does not make perfect surface and have gap between each bundle, arranging yarns to certain directions allows to growth CNT forest with specific pattern such as check pattern. Also it is possible not to make vertical CNT forest to the substrate by stack multi-layer of CNT yarn so that make felt-like-sheet of CNTs. The secondary growth of CNTs on CNTs is the useful method of fabricating of CNT yarn. [Preview Abstract] |
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H1.00244: Structural transition and mechanical properties of one dimensional boron ribbons and chains from first principles Mingjie Liu, Vasilii I. Artyukhov, Boris I. Yakobson The past decade has brought great progress in fabrication and characterization of single-atom chains of carbon (carbyne). Very recently novel atomic chain compositions such as BN and CsI were reported. The extreme and unusual properties of such 1D materials\footnote{M. Liu \emph{et al.}, \emph{ACS Nano} 7, 10075 (2013)}\footnote{V. I. Artyukhov, M. Liu, and B. I. Yakobson, \emph{Nano Lett.} 14, 4224 (2014)} motivate the search for other possible compositions with interesting behaviors. We use first-principles calculations to uncover the rich structural and mechanical properties of 1D boron. While the ground state structure of linear boron is a two-atoms-wide ribbon, tension can unravel it into a single-atom string structure. We analyze the mechanical and electronic properties of these two ``phases'' and study the thermodynamics and kinetics of transition between them using static first-principles calculations and semiempirical (DFTB) molecular dynamics. The interesting properties of 1D boron nanostructures make them an attractive system for experimental investigations. [Preview Abstract] |
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H1.00245: Non-covalent functionalization of single wall carbon nanotubes and graphene by a conjugated polymer Jilili Jiwuer, Ayjamal Abdurahman, O\u{g}uz G\"ulseren, Udo Schwingenschl\"ogl We report first-principles calculations on the binding of poly[(9,9-bis-(6-bromohexylfluorene-2,7-diyl)-co-(benzene-1,4-diyl)] to a (8,0) single wall carbon nanotube (SWCNT) and to graphene. Considering different relative orientations of the subsystems, we find for the generalized gradient approximation (GGA) a non-binding state, whereas the local density approximation (LDA) predicts reasonable binding energies. The results coincide after inclusion of van der Waals (vdW) corrections, which demonstrates a weak interaction between the polymer and SWCNT/graphene, mostly of van der Waals type. Accordingly, the density of states shows essentially no hybridization. The physisorption mechanism explains recent experimental observations and suggests that the conjugated polymer can be used for non-covalent functionalization. (Reference: Appl. Phys. Lett. 105, 013103, 2014) [Preview Abstract] |
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H1.00246: Design of Inorganic Electrides Yunwei Zhang, Hui Wang, Yanchao Wang, Yanming Ma Electrides, in which all of or part of the valence electrons occupy interstitial regions in the crystal and behave as anions, have been synthesized at ambient or high-pressure conditions [1]. Their loosely bound anionic electrons make electrides good candidates for electro-active materials. Here, we report a developed methodology to systematically design electrides for given chemical systems. The new approach is based on the swarm-intelligence CALYPSO algorithm on structure prediction [2] and requires only the chemical compositions to predict the electride phases. In contrast to the traditional ground state structure prediction method where the total energy was solely used as the fitness function, we adopted a new fitness function in combination with the first-principles calculation to select the optimal solutions for a description of given chemical systems. The experimentally know electrides have been successfully reproduced. The results suggested that our approach is reliable and can be widely applied into design of new electrides. \\[4pt] [1] Ma, Y. et al. Transparent dense sodium. Nature 485, 182-185 (2009).\\[0pt] [2] Wang, Y., Lv, J., Zhu, L. {\&} Ma, Y. Crystal structure prediction via particle-swarm optimization. Phys. Rev. B 82, 094116 (2010). [Preview Abstract] |
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H1.00247: Monte Carlo(MC) simulation study on ammonia anchored TON zeolite for carbon dioxide capture Hansol Wee, WonBo Lee If zeolites are modified by ammonia, the electronic effect in ammonia resulted in different surface basicity of the zeolite materials. So, ammonia anchored materials show better adsorption rate of CO2 than pure materials at low pressure. MC simulations for CO2 adsorption were performed at 298K. The results show that, at pressure 1000 kpa CO2 loading is 1.404 mol/kg at ammonia anchored TON, and 0.529 mol/kg at pure TON. However, at high pressure, the ammonia effect becomes marginal. Ammonia anchored TON structures may be used to adsorb CO2 more effective than normal TON structure.. [Preview Abstract] |
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H1.00248: Anisotropic mechanical properties of hexagonal SiC sheet: a molecular dynamics study Ming Yu, Emily Liu, Congyan Zhang The anisotropic mechanical properties of hexagonal SiC sheet have been studied using an efficient quantum mechanics molecular dynamics scheme based on a robust semi-empirical Hamiltonian (refereed as SCED-LCAO) [PRB 74, 15540; PHYSE 42, 1]. It was found that the SiC sheet could sustain the heavy load up to about 20 {\%}. In particular, it was found that the SiC sheet also shows large difference in the strain direction. It will quickly crack after 20 {\%} of strain in armchair the direction, but it will be slowly destroyed after 30{\%} in the zigzag direction, indicating the anisotropic nature of the mechanical properties of the SiC sheet. The nominal and 2D membrane stresses will be analyzed, from where we will obtain the 2D Young's modulus at infinitesimal strain and the third-order (effective nonlinear) elastic modulus for the SiC sheet. The detail results and discussions will be reported in the presentation. [Preview Abstract] |
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H1.00249: Towards first-principles based prediction of highly accurate electrochemical Pourbiax diagrams Zhenhua Zeng, Maria Chan, Jeff Greeley Electrochemical Pourbaix diagrams lie at the heart of aqueous electrochemical processes and are central to the identification of stable phases of metals for processes ranging from electrocatalysis to corrosion. Even though standard DFT calculations are potentially powerful tools for the prediction of such Pourbaix diagrams, inherent errors in the description of strongly-correlated transition metal (hydr)oxides, together with neglect of weak van der Waals (vdW) interactions, has limited the reliability of the predictions for even the simplest bulk systems; corresponding predictions for more complex alloy or surface structures are even more challenging . Through introduction of a Hubbard U correction, employment of a state-of-the-art van der Waals functional, and use of pure water as a reference state for the calculations, these errors are systematically corrected. The strong performance is illustrated on a series of bulk transition metal (Mn, Fe, Co and Ni) hydroxide, oxyhydroxide, binary and ternary oxides where the corresponding thermodynamics of oxidation and reduction can be accurately described with standard errors of less than 0.04 eV in comparison with experiment. [Preview Abstract] |
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H1.00250: Preferential adsorption positions for an adsorbed Li atom on the layered black phosphorus structures Congyan Zhang, Ming Yu The preferential adsorption positions for an adsorbed Li atom on the layered black phosphorus were determined by mapping out the total energy as a function of its positions on the layered black phosphorus using the density functional theory based method (referred as VASP [Phys. Rev. B 48, 13115 (1993)]). Various possible adsorption positions including the top of the bridge, the valley, and the interstitial positions of the puckered layers have been studied. It is found that the adsorption energy strongly depends on these positions with different environment. In particular, the most preferential adsorption positions for an adsorbed Li atom are found at the valleys and the interstitials of the puckered layers. The analysis of structural and electronic properties of the black phosphorus layer with the adsorbed Li atom will be discussed. [Preview Abstract] |
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H1.00251: Ab initio NMR Confirmed Evolutionary Structure Prediction for Organic Molecular Crystals Cong-Huy Pham, Emine Kucukbenli, Stefano de Gironcoli Ab initio crystal structure prediction of even small organic compounds is extremely challenging due to polymorphism, molecular flexibility and difficulties in addressing the dispersion interaction from first principles [1]. We recently implemented vdW-aware density functionals and demonstrated their success in energy ordering of aminoacid crystals [2]. In this work we combine this development with the evolutionary structure prediction method [1] to study cholesterol polymorphs. Cholesterol crystals have paramount importance in various diseases, from cancer to atherosclerosis. The structure of some polymorphs (e.g. ChM, ChAl, ChAh) have already been resolved while some others, which display distinct NMR spectra and are involved in disease formation [3], are yet to be determined. Here we thoroughly assess the applicability of evolutionary structure prediction to address such real world problems. We validate the newly predicted structures with ab initio NMR chemical shift data using secondary referencing for an improved comparison with experiments [4]. [1]Zhu et al. Acta Cryst B68, 215 (2012) [2]Quantum ESPRESSO, Sabatini et al. J Phys Cond Matt 24, 424209 (2012) [3]Jayalakshmi et al. SS Nucl Magn Res 36, 60 (2009) [4]Kucukbenli et al. JCP A 116, 3765 (2012) [Preview Abstract] |
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H1.00252: Exceptional Optoelectronic Properties of Si-related compounds Bing Huang, Houlong Zhuang, Mina Yoon, Su-Huai Wei, Bobby Sumpter The search of new silicon-related functional compounds are of great interests but still very changeling. In the last few decades, researchers have heavily studied the structural and electronic properties of silicon in order to improve its optical absorption in the visible light range using analyses of metastable silicon phases, silicon-based alloys, and silicon-based superlattices. In this talk, I will present our recent theoretical efforts on searching and designing new silicon phases, from bulk to two-dimensional (2D) silicon, with exceptional optoelectronic properties.~ Especially, we find that chemically functionalized 2D silicon and silicon alloys could be the best candidates to create efficient thin-film solar absorbers and silicon-based, white-light-emitting diodes, paving the way for new ``green'' energy applications. [Preview Abstract] |
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H1.00253: Maximizing the bandgap of BCN nanoribbons Raisi Baldez, Paulo Piquini, Alex Schmidt, Marcelo Kuroda Carbon and boron-nitride based compounds share many electronic and structural features. This fact permits mixed carbon-boron-nitride compounds to be easily synthesized without significant structural changes. Further, the partially ionic character of the boron-nitrogen bonds allow to modify the electronics of carbon based materials, introducing a variability that can be used to tune the electronic properties according to the boron and nitrogen contents. Graphene and graphene nanoribbons have emerged as promising materials for electronic applications, due to the high mobility of its charge carriers. In this work we use the genetic algorithm approach to search for configurations with variable B, C and N contents that maximize the band gap of B$_{\mathrm{x}}$C$_{\mathrm{y}}$N$_{\mathrm{z}}$ nanoribbons. Different stoichiometries are analyzed and the structural patterns that lead to the maximal band gaps are presented. [Preview Abstract] |
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H1.00254: Pd/W(110) as a highly CO tolerant electrocatalyst for hydrogen oxidation: insight from first principles Nagendra Dhakal, Sergey Stolbov Platinum perfectly catalyzes hydrogen oxidation reaction on the hydrogen fuel cell anodes. However, it has at least two drawbacks: a) it is too expensive; b) it has a low tolerance to CO poisoning. Pt-Ru bi-functional catalysts are more tolerant to CO, but they are still very expensive. In this work, we performed first-principle studies of stability reactivity of M/W(110) structures, where M$=$Pd,Ru,Au monolayers. All three systems are found to be stable: formation energy of MLs is significantly higher than cohesive energy of the M-elements. The calculated binding energies of H, H$_{2}$, OH, CO, and H$_{2}$O were used to obtain the reaction free energies. Analysis of the free energies suggests that Au-W bonding does not activate sufficiently Au monolayer, whereas Ru/W(110) is still too reactive for the CO removal. Meanwhile, Pd/W(110) is found to catalyze hydrogen oxidation and at the same time to be highly tolerant to the CO poisoning. The latter finding is explained by the fact that CO binds much weaker to Pd on W(110) than to Pt, while the OH binding energy is strong enough to ensure CO oxidation. The obtained results are traced to the electronic structure of the systems. [Preview Abstract] |
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H1.00255: Electronic and vibrational properties of monolayer and bilayer TaSe$_2$ Mack Adrian Dela Cruz, Jia-An Yan Distinct from MoS$_2$, two-dimensional atomic crystal of tantalum diselenide (TaSe$_2$) is metallic and exhibits charge-density wave (CDW) transitions. Using density-functional theory, we present a first-principles study of the electronic and vibrational properties of monolayer and bilayer TaSe$_2$ without including the CDW-induced structural distortions. For monolayer 1T-TaSe$_2$, the frequencies of the Raman active modes are 159 cm$^{-1}$ ($E_g$) and 226 cm$^{-1}$ ($A_{1g}$), while the Raman-active modes for monolayer 2H-TaSe$_2$ are at 138 cm$^{-1}$ ($E''$), 214 cm$^{-1}$ ($E'$), and 241 cm$^{-1}$ ($A_1'$). For bilayer TaSe$_2$, different stackings of monolayer 2H-TaSe$_2$ and 1T-TaSe$_2$ phases have been calculated. Electronic band structures and vibrational properties of four energetically favorable configurations will be presented. Finally, the spin-orbit coupling on the structural and electronic properties will also be discussed. [Preview Abstract] |
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H1.00256: Density Functional Theory Studies of Li-ion interaction with defected group 14 heteronuclear nanotubes and nanosheets Tichakunda Pasipanodya, Prabath Wanaguru, Raymond Atta-Fynn Nanomaterials show significant promise in enhancing Lithium ion (Li-ion) battery properties. Using density functional theory, we study the binding and diffusion of Li on defected nanotubes and nanosheets of silicon carbide (SiC) and silicon germanium (SiGe). Point and extended defects are considered to fully evaluate the influence of defects on the adsorption and diffusion properties. The trends in the adsorption-induced changes in the geometric and electronic properties will be presented. Furthermore, room temperature ab initio molecular dynamics simulations will be carried out to investigate finite temperature effects on the binding mechanisms and electronic structure. [Preview Abstract] |
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H1.00257: Ferromagnetism in SrTiO$_{3}$ Single Crystals Induced by Laser irradiation Srinivasa Rao Singamaneni, Y.F. Lee, J.T. Prater, A.I. Smirnov, J. Narayan SrTiO$_{3}$ (STO) is diamagnetic in pristine state, important in emerging field of complex oxide electronics. No attention has been paid to explore the magnetic properties of STO crystal upon laser irradiation/annealing. In this presentation, we demonstrate [1-2] that STO single crystals show ferromagnetic order up to 400 K upon KrF (248 nm) laser irradiation. The high resolution x-ray photo emission spectroscopy (XPS) measurements reveal a strong shift of Sr-, Ti- and O-related peaks. X-ray diffraction (XRD) of laser annealed STO does not reveal a signature of either secondary magnetic or amorphous phases. 300 K X-band ($\sim$ 9.543 GHz) angle-dependent electron paramagnetic resonance (EPR) measurements showed no evidence of additional magnetic peaks up on laser irradiation. XPS and EPR data did not provide a strong evidence of Ti$^{3+}$ formation upon laser annealing. No differences in the visible 300 K Raman spectra of pristine and laser annealed STO are noticed. Interestingly, the magnetic moment is decreased by almost 10-fold upon oxygen annealing of laser annealed STO, inferring that oxygen vacancies play an important role in establishing the observed ferromagnetism.\\[4pt] [1] S.S. Rao et al Appl. Phys. Lett. \textbf{105}, 042403 (2014); J. Appl. Phys., \textbf{116}, 094103 (2014). [Preview Abstract] |
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H1.00258: Cross-over from antiferromagnetic to ferromagnetic interface exchange coupling in epitaxial ferromagnetic oxides Srinivasa Rao Singamaneni, John T. Prater, Jay Narayan Interface magnetism in La$_{0.7}$Sr$_{0.3}$MnO$_{3}$/SrRuO$_{3}$ (LSMO/SRO) bilayer (BL) has been the subject of great interest in the recent past owing to interesting physics and potential applications. Through a novel approach [1-3], LSMO (131nm)/SRO (45nm) and LSMO (33nm)/SRO (45nm) bilayers have been epitaxially integrated with Si (100). Notably, in the former sample, positive exchange bias is observed --indication of antiferromagnetic exchange coupling and is found to be absent in the later. Interestingly, in the former sample, the cross-over from antiferromagnetic to ferromagnetic interface exchange coupling is noticed by varying the cooling field. We have verified that the coupling is of magnetic origin, not due to electrostatic interaction by inserting a thin ($\sim$ 10nm) SrTiO$_{3}$ layer between LSMO and SRO. We believe that the formation of interface domain walls and strong interplay among Zeeman, anisotropy and exchange energies could play a dominant role. Our results would have important implications for the physics of magnetic exchange coupled systems. \\[4pt] [1] S. S. Rao \textit{et al.,} J. Appl. Phys. (in print, 2014);\\[0pt] [2] Nano Lett., (under review, 2014);\\[0pt] [3] Narayan \textit{et al.,} J. Appl. Phys.\textbf{93}, 278 (2003). [Preview Abstract] |
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H1.00259: Exchange bias study of epitaxial LSMO/Cr2O3 thin film heterostructures integrated on Si(100) Sandhyarani Punugupati, Frank Hunte, Jagdish Narayan FM/AFM exchange bias continues to be an interesting phenomenon from both a fundamental physics and an applications point of view. Recent studies of multiferroic materials have also seen a revival of interest in the magnetoelectric (ME) and antiferromagnetic (AFM) material Cr2O3. The study of exchange bias in heterostructures consisting of ferromagnet (FM) and ME thin films provides an additional mechanism of switching the magnetization of the FM by the application of an electric field. La0.7Sr0.3MnO3 (LSMO) is a FM material with TC above room temperature and shows colossal magnetoresistance. We have studied exchange bias in epitaxial thin film heterostructures of LSMO/Cr2O3 grown on C-YSZ/Si(100) by the PLD technique. We present a detailed structural characterization of the films by XRD (2O and $\Phi )$ and TEM which confirm that the films were grown epitaxially. The heterostructures exhibited exchange bias as measured by SQUID magnetometry. The effects of LSMO deposition conditions, crystal orientation, temperature, and cooling field on the exchange bias will be discussed. [Preview Abstract] |
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H1.00260: Probing defect ordering in the Curie-Weiss metallic phase of NaxCoO2 Ben-Li Young, P.-Y. Chu, J.Y. Juang, G.J. Shu, F.C. Chou Single crystals of Na$_{2/3}$CoO$_{2}$, Na$_{2/3}$CoO$_{1.98}$, and Na$_{0.71}$CoO$_{2}$, which are metallic Curie-Weiss paramagnets, have been investigated by nuclear magnetic resonance (NMR) techniques, in order to clarify the Na atomic ordering among these samples. By analyzing the $^{23}$Na and $^{59}$Co NMR spectra, we confirm that the Na vacancies arrange orderly in Na$_{2/3}$CoO$_{1.98}$ and Na$_{0.71}$CoO$_{2}$, so that a superlattice structure is formed due to such Na ordering. In addition, the oxygen vacancies in Na$_{2/3}$CoO$_{1.98}$ can be located by the NMR spectra. As for the Na$_{2/3}$CoO$_{2}$ single crystal, a long-range Na order is not observed. [Preview Abstract] |
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H1.00261: Extended X-ray Absorption Fine Structure (EXAFS) Analysis of Vitreous Rare Earth Sodium Phosphates Changhyeon Yoo, Kanishka Marasinghe, Carlo Segre, Tomohiro Shibata The local structure around rare-earth ions (RE$^{3+})$ in rare-earth ultraphosphate (REUP) glasses has been studied using RE L$_{III}$ edge (RE $=$ Nd, Er, Dy, and Eu) and K edge (RE $=$ Nd, Pr, Dy, and Eu) extended X-ray absorption fine structure (EXAFS) spectroscopy. (RE$_{2}$O$_{3})_{x}$(Na$_{2}$O)$_{y}$(P$_{2}$O$_{5})_{1-x-y}$ glasses in the compositional range 0 $\le $ x $\le $ 0.14 and x $+$ y $=$ 0.3 and 0.4 were studied. RE-oxygen (RE-O) coordination number decreases from $\sim$ 10 to $\sim$ 7.5 with increasing RE-content for Nd, Pr, Eu, and Dy. For Er, RE-O coordination number increases from $\sim$ 8.7 to $\sim$ 10 with increasing RE-content. For the first oxygen shell, the RE-O distance ranges between 2.41-2.43 {\AA}, 2.44-2.46 {\AA}, 2.24-2.26 {\AA}, 2.28-2.32 {\AA}, and 2.32-2.36 {\AA} for Nd, Pr, Er, Dy, and EU glasses, respectively. Second shell around RE ions consists of phosphorus atoms, with RE-P distance about 3.0-3.5 {\AA} and coordination number ranging from 1 to 3. The third shell primarily contains oxygen and is at a distance about 4.0-4.1 {\AA} from RE ions. [Preview Abstract] |
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H1.00262: Extended X-ray Absorption Fine Structure (EXAFS) Analysis of Zirconium-Doped Lithium Silicate / Borate Glass-Ceramics Changhyeon Yoo, Kanishka Marasinghe, Carlo Segre, Richard K. Brow Results of Zr K-edge Extended X-ray Absorption Fine Structure (EXAFS) spectroscopy analysis of a series of Zr-doped ($\sim$ 3-10 mol{\%} Zr and atomic ratio Li/Si $\sim$ 0.8) lithium silicate glass ceramics (ZLS) and their parent glasses and a series of Zr-doped ($\sim$ 2-6 mol{\%} Zr and atomic ratio Li/B $\sim$ 0.25-0.18) lithium borate (ZLB) glasses are presented. Immediate coordination environment of all ZLS samples, i.e. the parent glasses and glass ceramics prepared via two different techniques, are remarkably similar. This observation suggests that zirconium ions may remain in the glass phase during nucleation and crystallization process. In contrast, immediate coordination environment of ZLB glasses appear to change markedly with the Zr concentration. These results also suggest that the structural role of Zr ions in ZLS and ZLB glasses may be significantly different. Details of analysis and results will be presented. [Preview Abstract] |
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H1.00263: Direct Calculation of Modal Contributions to Thermal Conductivity via Green-Kubo Modal Analysis Wei Lv, Asegun Henry In studying the physics of thermal conductivity, tremendous progress has been made over the last 20 years toward understanding lattice thermal conductivity in crystalline solids. However, most of the existing methods are based on ``phonon gas model'', which is the dominant paradigm. It essentially treats vibrations as gas particles, which scatter with each other. This analogy works well for crystals, but it hinges on the assumption that particle velocity being well defined. Because amorphous materials and molecules lack periodicity, it is difficult to define the phonon velocity. We used molecular dynamics simulations and a new formalism for calculating the modal contributions to thermal conductivity to study the amorphous materials, a-Si and a-SiO2. It is the first method that is able to obtain the modal details of phonon transport in amorphous materials including full anharmonicity. This method offers a different perspective on phonon-phonon interactions and allows for direct calculation of phonon contributions to thermal conductivity, which will advance our understanding of the phonon transport mechanism and facilitate heat transfer applications in disordered solids and polymers. [Preview Abstract] |
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H1.00264: Effect of electron-electron interaction on the conductance plateaus of a quantum wire Yonatan Abranyos, Godfrey Gumbs, Michael Pepper, Danhong Huang We present a model which is employed to explain recent experimental results for the conductance in a channel within GaAs/AlGaAs heterostructures. These measurements show that the hierarchy of conductance plateaus interestingly depend on the quantum wire widths. We propose that the data may be interpreted by the role played by entangled and non-entangled electron-pair ballistic transport. For non-entangled electrons, two electrons with different spins move through the 1D conduction channel independently without impurity or phonon scattering. For entangled states, the orbital-singlet state corresponds to the anti-symmetric two-electron state in which one spin up (down) electron is in the $n=0$ level of a parabolic confining potential while another spin down (up) electron simultaneously stays in the $n=1$ level. We demonstrate the conditions under which agreement with experiments for the conductance may be achieved. [Preview Abstract] |
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H1.00265: Study of optical properties of titania nanotube arrays by FDTD Method Oomman Varghese, Pawanjit Kaur Finite Difference Time Domain (FDTD) method is a powerful tool for understanding the propagation of electromagnetic waves through materials. The idea behind FDTD technique is to discretize both in time and space, the Maxwell equations with central difference approximations. The study of interaction between the nanostructured semiconductor materials and light is of high relevance in recent years primarily due to the applications of these materials in solar energy conversion process such as solar photovoltaics and solar photocatalysis. Titania nanotube arrays fabricated by anodic oxidation have already attracted considerable attention due to their unique properties and applications. This material has already demonstrated high light scattering and antireflection properties. To obtain a better understanding of nanotube-light interaction we used opti-FDTD software to study the influence of nanotube growth conditions on the optical properties. Simulations were carried out by defining the material properties and using the Lorenz drude model in 380-700nm range. In this presentation we will detail our findings on the correlation between the nanotube array fabrication conditions and its optical properties. [Preview Abstract] |
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H1.00266: The magneto-optical properties of non-uniform graphene nanoribbons Hsien-Ching Chung, Ming-Fa Lin When synthesizing few-layer graphene nanoribbons (GNRs), non-uniform GNRs would be made simultaneously. Recently, the non-uniform GNRs, which is a stack of two GNRs with unequal widths, have been fabricated by mechanically exfoliated from bulk graphite. Some theoretical predictions have been reported, such as gap opening and transport properties. Under the influence of magnetic fields, magnetic quantization takes place and drastically changes the electronic properties. By tuning the geometric configuration, four categories of magneto-electronic spectra are exhibited. (1) The spectrum is mostly contributed by quasi-Landau levels (QLLs) of monolayer GNRs. (2) The spectrum displays two groups of QLLs, and the non-uniform GNR behaves like a bilayer one. (3) An intermediate category, the spectrum is composite disordered. (4) The spectrum presents the coexistence of monolayer and bilayer spectra. In this work, the magneto-electronic and optical properties for different geometric configurations are given, such as energy dispersions, density of states, wave functions, and magneto-absorption spectra are presented. Furthermore, the transformation between monolayer and bilayer spectra as well as the coexistence of monolayer and bilayer spectra are discussed in detail. [Preview Abstract] |
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H1.00267: Resonance Raman Spectroscopy of Single-Wall Carbon Nanotubes Separated via Aqueous Two-Phase Extraction J. R. Simpson, J. A. Fagan, A. R. Hight Walker We report resonance Raman Spectroscopy measurements of single-wall carbon nanotube (SWCNT) samples dispersed in aqueous solutions via surfactant wrapping and separated using aqueous two-phase extraction (ATPE) into chirality-enriched semiconducting and metallic SWCNT species. ATPE provides a rapid, robust, and remarkably tunable separation technique that allows isolation of high-purity, individual SWCNT chiralities via modification of the surfactant environment.\footnote{J. A. Fagan, \textit{et al.}, Adv. Mat. \textbf{26}, 2800 (2014).} We report RRS measurements of individual SWCNT species of various chiral index including, semiconductors, armchair and zigzag metals. Raman provides a powerful technique to quantify the metallic SWCNTs in ATPE fractions separated for metallicity. We measure Raman spectra over a wide range of excitation wavelengths from (457 to 850)\,nm using a series of discrete and continuously tunable laser sources coupled to a triple-grating spectrometer. The spectra reveal Raman-active vibrational modes, including the low-frequency radial breathing mode (RBM) and higher-order modes. SWCNT chiral vectors are determined from Raman spectra, specifically the RBM frequencies and corresponding energy excitation profiles, together with input from theoretical models. [Preview Abstract] |
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H1.00268: Roles of Multi-Walled Structures in Thermal Transport Properties of Nanotubes Tomoyuki Hata, Hiroki Kawai, Ryota Jono, Koichi Yamashita The molecular structures of carbon nanotubes are thought to be deeply related with various physical properties. Understanding the relationship is one of the challenges in designing potential materials. In this research, we theoretically investigated the thermal transport properties of carbon nanotubes, focusing the multi-walled structures. We investigated the thermal conductance of the double-walled carbon nanotubes (DWCNTs) by using the nonequilibrium Green's function method. It is found that the inter-layer interaction causes the suppression of thermal conductance at low temperature. The analysis of the transmission coefficients revealed that this suppression was attributed to the energy shifts of the normal modes from the synchronized vibrations. The mechanism of such energy shifts is examined by the coupled vibration model with the parameters extracted from our simulations, and we grasp the multi-wall effects on the thermal transport properties of the nanotube structures. [Preview Abstract] |
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H1.00269: Coulomb excitations for Gapped Graphene in a perpendicular magnetic field Andrii Iurov, Godfrey Gumbs, Danhong Huang We investigate numerically the Coulomb excitations for gapped graphene and other buckled honeycomb lattices (such as silicene and germanene) in the present of a perpendicular magnetic field. The plasmons are calculated within the random phase approximation. The collective excitations in the presence of a perpendicular magnetic field for such gapped systems are shown to be different from those for both intrinsic gapless graphene as well as a standard two-dimensional electron gas. We present a theoretical description of Bernstein modes that appear due to the coupling between inter-Landau-level excitations and plasmons. [Preview Abstract] |
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H1.00270: Calculating Observable Quantities for the Hofstadter-Type Spectrum of Graphene Liubov Zhemchuzhna, Danhong Huang, Godfrey Gumbs, Andrii Iurov, Sanjay Krishna We numerically obtain density of states and the conductivity of the periodically modulated graphene in the presence of magnetic field. These quantities play most important role since they could be measured directly in experiment, so we compare our results with those from the existing experimental papers. The density of states has been calculated and shows a remarkable self-similarity like the energy bands. We estimate that for modulation period of $10 \, nm$ the region where the Hofstadter butterfly is revealed at $B \leq 2 \,T$. Both single layer and bilayer graphene have been considered.\\[4pt] [1] Godfrey Gumbs, Andrii Iurov, Danhong Huang, and Liubov Zhemchuzhna: {\em Revealing Hofstadter Spectrum for Graphene in a Periodic Potential\/}, Physical Review B {\bf 89}, 241407(R) ({\bf Rapid Communications}) (2014). [Preview Abstract] |
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H1.00271: Dielectric function for doped graphene layer with barium titanate Manuel Martinez Ramos, Eric Garces Garcia, Fernado Magana, Gerardo Jorge Vazquez Fonseca The aim of our study is to calculate the dielectric function for a system formed with a graphene layer doped with barium titanate. Density functional theory, within the local density approximation, plane-waves and pseudopotentials scheme as implemented in Quantum Espresso suite of programs was used. We considered 128 carbon atoms with a barium titanate cluster of 11 molecules as unit cell with periodic conditions. The geometry optimization is achieved. Optimization of structural configuration is performed by relaxation of all atomic positions to minimize their total energies. Band structure, density of states and linear optical response (the imaginary part of dielectric tensor) were calculated. [Preview Abstract] |
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H1.00272: Direct exfoliation of graphene in ionic liquids with aromatic groups Rozana Bari, George Tamas, Fahmida Irin, Adelia Aquino, Edward Quitevis, Micah Green The imidazolium cation of the designed and synthesized novel ionic liquids (ILs) having aromatic groups interact non-covalently with graphene. The Graphene stabilized by the IL is neither covalently functionalized nor requires the presence of additive stabilizer and such process results in dispersion of pristine graphene. This graphene dispersion is stable against centrifugation and the concentration of the resulting graphene is high as well. It was observed that the ILs are less effective in dispersing graphene if the cation does not have these aromatic groups. The interaction between the cation and the graphene surface plays an important role in the final yield of graphene. The graphene dispersion was characterized by Raman spectroscopy, X-ray Diffraction, and X-ray photoelectron spectroscopy. The experimental observations were compared with the density functional theory (DFT-D3) calculations and the comparison indicated that the experimental observations and the theoretical calculations were in good agreement. These validated theoretical calculations can further be used in future to design and synthesize the ILs in order to optimize the graphene yield without the need for additional experimentation. [Preview Abstract] |
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H1.00273: Physical Adsorption of noble gases on a monolayer graphene sheet using Grand Canonical Monte Carlo Simulation Sidi Maiga, Silvina Gatica Adsorption is defined as the attachment of atoms, or molecules of a gas, liquid or dissolved solid onto a surface, creating a film or monolayer of material onto the adsorbing surface. Using the Method of Grand Canonical Monte Carlo we computed the adsorption of Ar, Kr, and Xe on a monolayer graphene sheet, at various temperatures for each gas. For each temperature, we compute the adsorption isotherm, Energy gas-surface and Energy gas-gas, radial distribution function and structure function. We constructed the phase diagrams for 2D Ar and Kr on graphene. [Preview Abstract] |
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H1.00274: Probing weak localization in chemical vapor deposition graphene with wide constriction by scanning gate microscopy Chiashain Chuang, M. Matsunaga, Fan-Hung Liu, Tak-Pong Woo, Nobuyiki Aoki, Li-Hung Lin, Y. Ochiai, Chi-Te Liang We observe weak localization effect in different wide channels on this disordered CVD graphene device. We also perform the low temperature-scanning gate microscopy experiments under weak localization regime on CVD graphene with wide constriction. The movable local gate can sensitively perturb the total conductance in the wide constriction CVD graphene under magnetic field, suggestive the advantages in the local electric perturbation for the interference behaviors of transport carriers rather than that in fully covered and high consumption magnetic field, a great step for applications in graphene-based spintronics. [Preview Abstract] |
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H1.00275: Micron scale ballistic Josephson junctions in edge-contacted graphene Srijit Goswami, Victor Calado, Gaurav Nanda, Mathias Diez, Anton Akhmerov, Kenji Watanabe, Takashi Taniguchi, Lieven Vandersypen Despite recent improvements in the electronic quality of graphene, it has remained challenging to make superconducting contacts to it while preserving its high quality. Here, we integrate monolayer graphene encapsulated in hexagonal Boron Nitride with a type-II superconductor (Molybdenum Rhenium - MoRe) via one-dimensional contacts along the edge of the graphene. We observe gate-tunable supercurrents over distances as long as 1.5~$\mu m$. Ballistic, phase coherent transport in these devices causes the switching current to oscillate periodically with the Fermi wave number, thus providing clear evidence of a ballistic Josephson junction. Furthermore, the large critical field of MoRe allows us to resolve several broken symmetry states in the quantum Hall regime, while the MoRe remains superconducting. [Preview Abstract] |
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H1.00276: Tunable dichroism and optical absorption of graphene by strain engineering Maurice Oliva-Leyva, Gerardo G. Naumis Recently, the concept of strain engineering has been experimentally extended to the optical domain. However, strain-induced modifications of optical absorption in graphene have been only quantified in the case of a uniaxial strain. In this sense, our work opens a more rich scenario to explore strained-graphene transparency because it is applicable to an arbitrary uniform strain (e.g., uniaxial, biaxial, and so forth). From the corresponding Dirac-like equation [1], we compute the optical conductivity of graphene under a uniform strain [2,3]. This result allowed us to study the transmittance of linearly polarized light between two media separated by a strained graphene sheet [4]. We analytically characterize the degree of dichroism and the transparency of graphene as a function of an arbitrary uniform strain and the incident polarization. Also, we discuss how measurements of dichroism and transparency for two different polarization directions can be used to determine the magnitude and direction of strain. Ours findings result in very useful tools to tune the graphene absorption by mechanical strain [4]. [1] Phys. Rev. B 88, 085430 (2013). [2] J. Phys.: Condens. Matter 26, 125302 (2014). [3] J. Phys.: Condens. Matter 26, 279501 (2014). [4] arXiv: 1411.1376. [Preview Abstract] |
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H1.00277: Enhanced hot-carrier luminescence in multilayer reduced graphene oxide nanospheres Qi Chen, Chunfeng Zhang, Min Xiao We report a method to promote photoluminescence emission in graphene materials by enhancing carrier scattering instead of directly modifying band structure in multilayer reduced graphene oxide (rGO) nanospheres. We intentionally curl graphene layers to form nanospheres by reducing graphene oxide with spherical polymer templates to manipulate the carrier scattering. These nanospheres produce hot-carrier luminescence with more than ten-fold improvement of emission efficiency as compared to planar nanosheets. With increasing excitation power, hot-carrier luminescence from nanospheres exhibits abnormal spectral redshift with dynamic feature associated to the strengthened electron-phonon coupling. These experimental results can be well understood by considering the screened Coulomb interactions. With increasing carrier density, the reduced screening effect promotes carrier scattering which enhances hot-carrier emission from such multilayer rGO nanospheres. This carrier-scattering scenario is further confirmed by pump-probe measurements. [Preview Abstract] |
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H1.00278: Momentum resolved optical pump-probe spectroscopy in monolayer graphene: An analytical model and measurements Maxim Trushin, Alexander Grupp, Giancarlo Soavi, Arne Budweg, Domenico De Fazio, Antonio Lombardo, Ugo Sassi, Andrea C. Ferrari, Wolfgang Belzig, Alfred Leitenstorfer, Daniele Brida Further development in graphene based photonics and optoelectronics requires fundamental information on the evolution of the strongly non-equilibrium charge carrier distribution created by the light-carrier interaction. Here, we report polarization and fluence dependent ultrafast optical pump-probe spectroscopy of high quality CVD-grown monolayer graphene. An analytical model has been developed and employed to describe the experiments. Graphene offers a unique opportunity to probe the photocarrier occupation, not only at different energies using a two-color setup, but also in different directions in momentum space applying linearly polarized beams. The latter approach is possible due to the pseudospin-momentum coupling which results in an optical pseudospin-selection rule. Our method allows us to quantify and control the relative contributions of both the strongly non-equilibrium anisotropic occupation and the hot Fermi-Dirac photocarrier distribution to the total differential transmission measured. We provide a conclusive and quantitative evidence for an anisotropic photocarrier occupation with a life-time of about $100\,\mathrm{fs}$ and claim that its relaxation towards the isotropic distribution occurs mostly due to optical phonon emission. [Preview Abstract] |
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H1.00279: Probing the Doping level in Graphene Using Surface Plasmon Resonance. Kamrul Alam, Yang Li, Jiming Bao The present work describes an investigation of the electrochemically doped large area CVD grown graphene by using surface plasmon resonance (SPR). As graphene was doped electrochemically its conductance changes based on electron and hole concentration, that have an effect on its permittivity which has influence on the refractive index. We have used SPR angle interrogation scheme, generally known as Kretschmann configuration, to detect this change in refractive index of graphene as a shift in the angle of the SPR curve. To verify our results we have use Raman spectroscopy of the graphene-Au hybrid sample that was used for SPR measurement. Shift in the G peak signifies that graphene is doped electrochemically which is also in agreement with the shift in the angle of the SPR curve. [Preview Abstract] |
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H1.00280: Strain-Engineering of Graphene Based Topological Quantum Devices Ginetom S. Diniz, Marcos R. Guassi, Fanyao Qu We have investigated the spin-charge transport in quantum devices based on graphene nanoribbons (GNR). Our calculation is based on the surface Green's function technique, considering the presence of an uniform uniaxial strain, spin-orbit interactions (SOIs), exchange field and a smooth staggered potential. We propose the use of uniaxial strain as an efficient mechanism to tune the conductance profiles of GNR with different edge terminations. Our results show that distinct behaviors can be achieved: for armchair GNR there is a complete suppression of the conductance close to the Fermi level with the formation of a band gap that depends on the direction and strength of the strain deformation, while for zigzag GNR there is only a small conductance suppression. We also discuss the effects of SOIs and the appearance of spin-resolved conductance oscillations, and the local density of states of these GNR devices in the quantum anomalous Hall regime. Furthermore, we demonstrate that the local density of states show that depending on the smoothness of the staggered potential, the edge states of AGNR can either emerge or be suppressed. These emerging states can be probed by scanning tunneling microscope. Our findings can be potentially used in novel GNR based topological quantum devices. [Preview Abstract] |
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H1.00281: Physisorption of Fullerenes in Graphene layers and carbon Nanoribbons Monica Pacheco, Pedro Orellana, Julian Correa The study of nanostructures based on carbon allotropes has captured the interest of the scientific community in the last two decades, due to its great versatility. In such structures a simple change of geometry leads to important changes in their physicochemical properties. In this context it has been studied different carbon allotropes complexes in particular for the development of photovoltaic systems. In this paper we show a study of opto-electronic properties of fullerenes physisorbed on graphene nanoribbons. Our calculations are carried out within the framework of density functional theory (DFT) using the SIESTA package. Our results show that effectively the fullerenes bind both to the layer of graphene as well as to the nanoribbons, with binding energies of about 0.5EV. We find that when the complex is formed, the physical properties of fullerenes, graphene and nanoribbons are preserved and when graphene is functionalized with various fullerenes the electronic spectrum is composed of bands of energy which increases the intensity of the optical absorption spectrum of the complex. [Preview Abstract] |
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H1.00282: Thermoelectric properties of a trilayer graphene nanoribbon Pedro Orellana, Natalia Cortes, Luis Rosales, Monica Pacheco, Leonor Chico In this work the electronic and thermoelectric properties of a three-layer graphene with AAA stacking type are studied. By using a tight-binding model analytical expressions for the transmission and density of states are obtained. Thermoelectric properties are analyzed by numerical integration and results for thermopower and figure of merit, electronic conductance and thermal conductance are obtained. The results show that the interference effects present in this system, like Fano effect, directly affect the behavior of these thermoelectric properties [1] and as well as the Wiedemann-Franz law [2]. There is an enhancement of the thermopower of the system and a violation of the Wiedemann-Franz law in the region of energies close the Fano antiresonances and this has as a consequence an enhancement of the figure of merit of the system. \\[4pt] [1] G. G\'{o}mez-Silva, O. \'{A}valos-Ovando, M. L. Ladr\'{o}n de Guevara and P. A. Orellana, J. Appl. Phys. \textbf{111}, 053704 (2012). \\[0pt] [2] D. Boese and R. Fazio, Europhys. Lett. \textbf{56}, 576 (2001). [Preview Abstract] |
(Author Not Attending)
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H1.00283: Polarized spin and valley transport across ferromagnetic silicene junctions V. Vargiamidis, P. Vasilopoulos, V. Fessatidis We study ballistic transport of Dirac fermions through silicene barriers, of width $d$, with an exchange field $M$ and metallic gates above them that provide tunable potentials of height $U$. Away from the Dirac point (DP) the spin- and valley-resolved conductances, as functions of $U$, exhibit resonances and close to it a pronounced dip that becomes a transport gap when an appropriate electric field $E_z$ is applied. The charge conductance $g_c$ of such a junction changes from oscillatory to a monotonically decreasing function of $d$ beyond a critical $E_z$. This tuning of $g_c$ can be used to realize electric-field-controlled switching. Further, the field $M$ splits each resonance of $g_c$ in two spin-resolved peaks. The spin polarization $p_s$ of the current near the DP increases with $E_z$ or $M$ and becomes nearly perfect above certain of their values. We also show that $p_s$ can be inverted either by varying $U$ or by reversing the direction of $M$. For two barriers there is no splitting in $g_c$ when the fields $M$ are in opposite directions. Most of these phenomena have no analogs in graphene. [Preview Abstract] |
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H1.00284: Graphene optical-to-thermal converter Alejandro Manjavacas, Sukosin Thongrattanasiri, Jean-Jacques Greffet, Javier Garcia de Abajo Infrared plasmons in doped graphene nanostructures produce large optical absorption that can be used for narrow-band thermal light emission at tunable frequencies that strongly depend on the doping charge. By virtue of Kirchhoff's law, thermal light emission is proportional to the absorption, thus resulting in narrow emission lines associated with the electrically controlled plasmons of heated graphene. Here [1] we show that realistic designs of graphene plasmonic structures can release over 90{\%} of the emission through individual infrared lines with 1{\%} bandwidth. We examine anisotropic graphene structures in which efficient heating can be produced upon optical pumping tuned to a plasmonic absorption resonance situated in the blue region relative to the thermal emission. An incoherent thermal light converter is thus achieved. Our results open a new approach for designing tunable nanoscale infrared light sources. \\[4pt] [1] A. Manjavacas, S. Thongrattanasiri, J. J. Greffet, and F. J. Garcia de Abajo, Appl. Phys. Lett. (2014). [Preview Abstract] |
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H1.00285: Evanescent field coupled graphene plasmon waveguide Won Jong Yoo, Jaehwan Kweon, Euyheon Hwang Surface plasmon polaritons (SPPs) is propagating electron-light coupled oscillation. There had been various methods to excite graphene plasmon such as fabricating graphene nano-ribbon and NSOM micro-tip assisted methods. These methods are used for reducing wavelength of incident light and finally matching wave vector to propagate graphene surface plasmon. However, when graphene sheet is located between two different media, evanescent field coupling and excited graphene plasmon properites are still unclear. There are several advantages in using this coupling method. We can control the direction of propagating plasmon and effectively find plasmon modes. Interestingly, propagating wavelength is scaled to 50 -- 100 times depending on its modes and light is confined into extremely small mode size. Also its optical properties can be easily controlled by biasing voltage on its surface. To analyze graphene plasmon, we use random phase analysis (RPA) so as to figure out optical permittivity of graphene, and then we use the FDTD (finite difference time domain) and FDFD (finite difference frequency domain) computational methods so as to theoretically figure out the propagation of graphene plasmon waveguide. Also we will report various plasmon properties of graphene plasmon propagation such [Preview Abstract] |
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H1.00286: All-Metallic Vertical Transistors Based on Stacked Dirac Materials Yangyang Wang, Zeyuan Ni, Qihang Liu, Ruge Quhe, Jiaxin Zheng, Meng Ye, Dapeng Yu, Junjie Shi, Jinbo Yang, Ju Li, Jing Lu All metallic transistor can be fabricated from pristine semimetallic Dirac materials (such as graphene, silicene, and germanene), but the on/off current ratio is very low. In a vertical heterostructure composed by two Dirac materials, the Dirac cones of the two materials survive the weak interlayer van der Waals interaction based on density functional theory method, and electron transport from the Dirac cone of one material to the one of the other material is therefore forbidden without assistance of phonon because of momentum mismatch. First-principles quantum transport simulations of the all-metallic vertical Dirac material heterostructure devices confirm the existence of a transport gap of over 0.4 eV, accompanied by a switching ratio of over 10$^{4}$. Such a striking behavior is robust against the relative rotation between the two Dirac materials and can be extended to twisted bilayer graphene. Therefore, all-metallic junction can be a semiconductor and novel avenue is opened up for Dirac material vertical structures in high-performance devices without opening their band gaps. [Preview Abstract] |
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H1.00287: Identification of the existence of quantum Hall edge-state in graphene field-effect transistor at high temperatures Joong Gyu Kim, Haeyong Kang, Jeongmin Park, Yoojoo Yun, Thuy Kieu Truong, Jeong-gyun Kim, Nahee Park, Yourack Lee, Hoyeol Yun, Sang Wook Lee, Young Hee Lee, Dongseok Suh Quantum Hall effect (QHE) is one of the unique properties of two-dimensional electronic systems providing the universal standard of electrical resistance. Due to edge-state transport features in quantum Hall regime, the two-terminal graphene field-effect transistor (FET) is frequently examined for the study of the integer as well as the fractional QHEs of graphene. In this work, we present a simple method to identify the existence of quantum Hall state in the graphene FET especially at high temperatures. Using the monolayer graphene FET sample with fully broken degeneracy, we modified the equipotential line inside graphene FET by the addition of extra electrode for the clear identification of the quantum Hall state formation at given temperature and magnetic field. We suggest a simple model to explain the difference and similarity between two-terminal and multi-terminal configurations, including the discussion about the QHE devices connected in series. [Preview Abstract] |
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H1.00288: SUPERLATTICES, NANOSTRUCTURES AND OTHER ARTIFICIALLY STRUCTURED MATERIALS |
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H1.00289: Three-Dimensional Lattice Matching for Epitaxially Embedded Nanoparticles Brelon May, Peter Anderson, Roberto Myers Since Mathews-Blakeslee developed a theory of atomic lattice matched thin films, epitaxy has been modeled using only 2D lattice matching conditions between arbitrary films. For a given degree of in-plane lattice mismatch, the theory predicts a critical film thickness above which interface defects form to relax the film strain. Here we present a three-dimensional model to predict the conditions for epitaxially encased nanoparticles, which includes not only the in-plane lattice matching, but also the out-of-plane mismatch. We find that the consideration of the out-of-plane strain, due to the Poisson effect and particle shape change, can greatly alter the critical volume compared to what the Mathews Blakeslee model predicts. Our results provide new insight to nanoepitaxy of low dimensional structures especially quantum dots and nanoprecipitates. [Preview Abstract] |
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H1.00290: A DFT analysis of structure and energetics of Mg/Nb multilayers Anil Kumar, Irene Beyerlein, Jian Wang Magnesium and its alloys, the lightest structural materials, have attracted the attention of the automotive industry for reducing the vehicle's weight to increase its fuel efficiency. The magnesium phase characterized within Mg/Nb multilayers can adopt either body-centered cubic (bcc-Mg) or hexagonal close packed (hcp-Mg) structure depending on the Mg layer thickness. The bcc-Mg has a similar weight density as the hcp-Mg, but low Young's modulus, high shear modulus and conventional slip systems of bcc structure. In this work, using first-principles density functional theory, we studied both structural and mechanical properties of bcc-Mg and hcp-Mg in Mg/Nb multilayers as a function of Mg layer thickness and developed a simple theoretical model to predict the structural stability of the bcc-Mg/Nb and hcp-Mg/Nb multilayers. We show that the bcc-Mg/Nb multilayer is energetically favorable when the bcc-Mg layer is less than 4.2 nm. We also studied the mechanism such as inter-mixing of Mg and Nb atoms, creation of vacancies and doping of solute atoms at Mg/Nb interface to minimize the Mg/Nb interface energy. We found that solute atoms such as Zr, Cd and Zn, whose metallic radius are smaller than Mg, can easily segregate at Mg/Nb interface and lower the interface energy. [Preview Abstract] |
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H1.00291: Layer by Layer Fabrication of 3D Photonic Crystals by Substrate Conformal Imprint Lithography (SCIL) from Titania Nanoparticle Solutions Irene Howell, Marc Verschuuren, Rohit Kothari, James Watkins We demonstrate a method for fabricating well-aligned, large-area, log-pile 3D photonic crystals. Using 15 nm titania (anatase) nanoparticles dispersed in a mixture of propanediol and methanol, we show that these nanoparticles can be patterned to produce robust, reproducible, 1 um pitch, 500 nm line-width gratings. By planarizing the grating with an organic UV-curable resin (Norland Optical Adhesive 60), subsequent titania layers can be patterned in the proper orientation, ending with a calcination step to remove the organic resin and create a 3D photonic crystal. This method allows for a photonic stop band in the infrared region, but the limitations in feature size and alignment capability prevent fabrications of a photonic crystal with a band gap at shorter wavelengths. By applying Substrate Conformal Imprint Lithography (SCIL) technology to this method, we can produce 3D photonic crystals with features suitable for a band gap in the visible region. SCIL utilizes a higher modulus PDMS (X-PDMS) to easily achieve sub-500 nm features. Additionally, the alignment method, involving box-in-box and Moire patterns, enables reproducible, precise alignment of sequential layers within 5 nm. [Preview Abstract] |
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H1.00292: Phonon Scattering at Nanoparticles in LuAs:InGaAs Nanocomposites Feng He, Rodolfo Salas, Seth Robert Bank, Yaguo Wang Nanocomposites of III-V semiconductors embedded with semi-metallic rare earth nanoparticles grown with molecular beam epitaxy (MBE) have been widely using in optoelectrics and thermoelectrics. Manipulating the growth parameters, e.g, growth rate, particle concentration and surfactant, will allow precise control of electronic and thermal transport properties for specific applications. Fundamental understanding of phonon scattering at nanoparticles in these nanocomposites under various growth conditions is not only scientifically important, but also will facilitate the material growth and device design. We have investigated the ultrafast dynamics of coherent acoustic phonons (CAP) in LuAs:InGaAs nanocomposites with standard two-color femtosecond pump-probe technique. Phonon frequency and life time have been measured in samples grown under different conditions. Our results show that CAP lifetime increases with increasing LuAs deposition, as well as pump fluences. We have also studied the influences of growth rate and surfactant on phonon lifetime. [Preview Abstract] |
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H1.00293: Study of plasmon-polariton in 1-D photonic crystals of dielectric and magneto optical layers under the effects of external magnetic field in the polar configuration Jaime Andres Giron Sedas, Edwin Moncada Villa, Nelson Porras Montenegro Photonic crystals (PCs) are artificial microstructures with a periodic spatial distribution of the dielectric constant, which enables us to manipulate and control the photons. In particular, the existence of photonic bands in the energy spectrum as well as photonic band gaps, forbidden frequency regions for light propagation have permitted quite a number of analogies with physical properties of semiconductor, this provides a perfect base for the construction of great varieties of photonic devices. In this case the superlattice is composed of alternating slabs of two materials, one is a dielectric, and the other is magneto-optical material. The interaction between plasma excitations and the electromagnetic radiation in the frequency region around null electric and magnetic responses in these arrangements, it gives a requisite for the existence of longitudinal waves, results in the excitation of modes that couple plasmons and optical fields. Those are known as Plasmon- Polaritons. We analyze the magneto-optic response of this PCs and the Plasmon-Polaritons behavior when an external magnetic field is placed in the growth direction of the structure using the scattering matrix approach for anisotropic layer stacks. [Preview Abstract] |
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H1.00294: Electronic structure of palladium and gold-palladium nano clusters, both free and supported on MgO(100) Carlos Quintanar, Reyna Caballero, Raul Espejel, Elizabeth Chavira, Magali Ugalde, Francisco Espinoza, Samuel Trickey Ideal (or model) metal nano-clusters, both free and supported on MgO(100), have been the subject of numerous experimental studies. This work probes the characteristics of non-ideal systems. For that, palladium nano clusters first were synthesized using a sol-gel-microwave method. A mono-phase of metallic Pd was obtained as corroborated by thermo gravimetric analysis, x-ray powder diffraction, scanning electron microscopy, and high resolution transmission electron microscopy studies (HRTEM). Among the HRTEM micro-graphs we found an almost planar nano-surface (facet) with only forty six atoms (Pd$_{46}$). From that micro-graph we obtained the coordinates of the atoms in the Pd$_{46}$ nano-surface. With those coordinates, we did a DFT study of the Pd$_{46}$ nano-surface electronic structure. From the Pd$_{46}$nano surface, nine and eighteen Pd atoms were chosen to build A$_8$Pd$_9$ and A$_8$Pd$_{18}$ clusters respectively. Starting from a near-planar Au8 cluster, the Au$_8$ geometry was optimized over the fixed Pd$_9$ and Pd$_{18}$ nano-surfaces, charge transfer was determined with charge density difference analysis (CDDA) and Fukui analysis was done and. The Au$_8$Pd$_9$ cluster was positioned on an MgO(100) surface with an O vacancy and charge transfer was determined with CDDA [Preview Abstract] |
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H1.00295: Formation of finite layer MoS2 using ultrasonic agitation Tim Kidd, Rui He, Eric Clausen We have developed a process in which finite layer MoS2 can be produced using ultrasonic agitation. The material shows optical properties consistent with an average layer thickness of less than five layers. The process uses ultrasonic agitation of MoS2 in an suspension using isopropanol. Interestingly, side products involving carbon nanoparticles are also produced. These side products are quite small, and become the dominant material when using a centrifuge to separate out the smallest particles. These carbon nanoparticle side products appear to include nanometer scale particles as well as materials with sizes consistent with fullerenes and graphene with nanoscale lateral dimensions. This process appears to represent a novel method for producing finite layer MoS2 and some forms of carbon nanoparticles using a relatively simple method. [Preview Abstract] |
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H1.00296: Gyroid photonic crystal with Weyl points Siying Peng, Hongjie Chen, Harry Atwater Weyl points are degenerate energy states resulting from band crossing of linear dispersions in three dimensional momentum space. Unlike Dirac points in the two dimensional systems, Weyl points have been shown to be stable and the associated surface states are predicted to be topological surface states. These topologically protected surface states may potentially lead to various interesting phenomena such as backscattering immune transport. We fabricate and characterize photonic crystals in the infrared regime with Weyl points present in their band structures. Full wave FDTD simulations were utilized to optimize the unit cell size and material index of the gyroid structures. Three dimensional two-photon lithography method was used to fabricate optimized geometry from simulations in to polymers. We used sputtering process to coat the polymer structure with high index materials such as amorphous silicon at low temperature conformally. Optical properties of these gyroid geometries with high effective refractive index are characterized with angled resolved FTIR in order to map out the bulk and surface band structures in the momentum space. Initial FTIR measurement at normal incidence has shown strong absorption related to both structured polymer and a-Si structures. [Preview Abstract] |
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H1.00297: Bimodal Distribution of Cadmium Selenide Quantum Dots Prepared by UV-Photolithography Ajith DeSilva, M. Kaveh, Raghuveer R. Gadipalli, Sarah G. Martino, H.P. Wagner We employed wet chemical and UV photolithography methods to synthesize CdSe quantum dots (QDs). The dynamics of excitons in the QDs were studied using temperaturedependent photoluminescence (PL) ranging from 17 to 300 K The inhomogeneous shape and size of the QDs led to an asymmetric PL spectrum at 17 K, which was approximately decomposed into two Gaussian emission bands, with peak energies at 2.182 and 2.299 eV and widths of 40 and 30 meV, respectively. These bands are attributed to the existence of two CdSe QDs ensembles with differently sized QDs. With increasing temperature the PL intensities of both bands weakly change, the PL yield of the larger QDs being higher at low temperatures while the smaller QDs show the stronger emission at higher temperatures. The stronger PL quenching of the larger QDs with increasing temperature is tentatively assigned to a higher density of defects at the grain boundary compared with small QDs. TEM images of the sample revealed a distribution of nano-particles with average sizes around 10 and 15 nm supporting the existence of a bimodal QD distribution. [Preview Abstract] |
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H1.00298: SANS and SAXS Investigations of Selective Distribution of Single-Walled Carbon Nanotubes in a Polymeric System Jae-Min Ha, Hyung-Sik Jang, Sung-Hwan Lim, Sung-Min Choi Single-walled carbon nanotubes (SWNTs) have remarkable electrical, thermal, and mechanical properties which provide new possibilities for various applications, such as transparent conductive film, bio-sensor, composite and energy storage. For those applications, the fabrication of the self-assembly or guided assembly of SWNTs into highly ordered superstructures with well-defined morphology, density, and direction is demanded to enhance their physical properties and is the key to the realization of various potential applications of SWNTs. Block copolymers exhibit rich phase behaviors and have been extensively used as excellent templates for various nanostructured materials. Many efforts have been made to incorporate various nanoparticles into self-assembling block copolymers as an efficient and scalable way of fabricating well-controlled and highly ordered nanoparticle superstructures with various architectures. Recently, this approach has been successfully applied for SWNTs, but it is in its early stage. In this presentation, we will report the investigations of selective distribution of functionalized SWNTs in block copolymer systems using small-angle neutron and x-ray scattering. [Preview Abstract] |
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H1.00299: AlN nanowire growth using InN crystalline powders by physical vapor deposition Naoto Kenmochi, Hiraku Ota, Mikka Nishitani-Gamo, Noboru Wada AlN nanowires were grown by heating an evacuated quartz ampule which contained InN crystalline powders and an Al substrate at 1300 $\sim$ 1500K. The nanowire samples made were examined by SEM, TEM, EDX, XRD and Raman spectroscopy. Both EDX and Raman spectroscopy yielded that the nanowires should be crystalline InN. Almost all the nanowires exhibited a spherical head at the end, implying that the growth mechanism might be the vapor-liquid-solid (VLS) growth. The diameter typically varied from 30 nm to 500 nm, while the length could be several micron meters long. The nanowire growth was quite significant on the Al substrate close to the InN powder source. When the samples were kept at high temperatures for a longer time, both the spherical heads and nanowires were found to be thicker. The detailed mechanism for the growth and the growth conditions will be discussed. [Preview Abstract] |
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H1.00300: Metallic Nanoparticles Confined in Silica Matrices Shin-Hyun Kang, Min-Jae Lee, Jeeun Lee, Jun-Ki Lee, Sung-Min Choi Metallic nanoparticles are widely studied due to their noble properties based on the high surface area. In order to increase the practical applications, the nanoparticles should be protected from thermal damage which can cause agglomeration. A facile way of protecting metallic nanoparticles with a silica matrix is presented. Metal nanoparticles are synthesized and functionalized in aqueous solution, and are collectively confined in a silica matrix which is thermally stable enough to protect the embedded nanoparticles. The structure and morphology are investigated by small angle x-ray scattering measurements, transmission electron microscopy and scanning electron microscopy measurements. Physical and chemical properties of the heterogeneous system would be presented, too. [Preview Abstract] |
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H1.00301: Synthesis, Structural and Electrical Properties of $Mg_{x}Pb_{(1-x)}TiO_{3}$ produced by mechanosynthesis Jaldair Nobrega, Ariano De Giovanni Rodrigues, Michel Venet Zambrano, Paulo Sergio Silva Junior, Julio Cesar Camilo Albornoz Diaz, Paulo Sergio Pizani Over last decades scientific studies about ceramic materials based in metallic oxides have pointed to an increasing wide range of applications. Among them, $PbTiO_{3}$ has been significantly applied to electronic components and optoelectronics devices. Another example can be found in the microwave dielectric $MgTiO_{3}$, which has been used on the production of devices. The development of a compound that combines the physical properties of these well-known materials aims the achievement of a new type of ceramics presenting distinctive applications. We report the production of $Mg_{x}Pb_{(1-x)}TiO_{3}$, by means of mechanosynthesis techniques. Electric measurements were carried out in order to verify the dielectric behavior of the system. By analyzing the signatures of characteristic phases of $PbTiO_{3}$ and $MgTiO_{3}$ in X-Ray diffractograms, we could confirm that the stoichiometry of our solid solutions could be controlled by properly adjusting the amounts of the oxides used as precursors. The Raman spectroscopy allowed us to study the behavior of the soft mode, typical of ferroelectric, which is present in all composition. By determining the dependence of its energy with temperature variation, we could estimate the temperatures of phase transition for each composition. [Preview Abstract] |
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H1.00302: Synthesis and Characterization of Magnetic Nanoparticle Assemblies Min-Jae Lee, Ho-Hyun Kim, Shin-Hyun Kang, Sung-Min Choi The superstructure of magnetic nanoparticles, which may provide new properties, are of great interests for various potential applications as well as its own scientific merits. Despite recent advances in the fabrication and characterization of magnetic nanoparticle structures, it remains challenges to exploit for controllable organization of magnetic nanoparticles at the nanoscale. Here,we will report the synthesis and functionalization of monodisperse magnetic nanoparticles and their self-assembled structure. [Preview Abstract] |
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H1.00303: Microwave metal-dielectric metamaterials with magnetic inclusions Brittany Bates, Brandon Allison, Nicole Greene, Natalia Noginova Use of natural magnetic materials as a constituent part of metamaterials is attractive as they provide a possibility to tune material parameters at microwave range and THz frequency with external magnetic fields. Metal-dielectric multilayers and wire arrays structures were fabricated using both ferromagnetic and nonmagnetic metals. These structures were studied in free space microwave propagation experiment. We show that a cube of such a metamaterial operates as a focusing lens and a polarizer with a possibility of tuning with external magnetic field. [Preview Abstract] |
(Author Not Attending)
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H1.00304: Magnetic, structural and adsorption properties for methylene blue of PAA/MnFe2O4 nanocomposite Wei Wang, Zui Ding, J. Ping Liu PAA/MnFe$_{\mathrm{2}}$O$_{\mathrm{4}}$ nanocomposite was fabricated by a hydrothermal procedure and ultrasonic wave-assisted method. The morphology of the synthesized MnFe$_{\mathrm{2}}$O$_{\mathrm{4}}$ ferrite nanocrystals is the exclusive octahedral structure. The saturation magnetization Ms of as-synthesized nanoparticles reached 74.6emu/g. FTIR spectrum confirms the coating of PAA on the surface of MnFe$_{\mathrm{2}}$O$_{\mathrm{4}}$ ferrite nanoparticles. Here, the PAA coating does not lead to a deterioration in magnetic performance. Moreover, the PAA/MnFe$_{\mathrm{2}}$O$_{\mathrm{4}}$ nanocomposites were applied to remove Methylene Blue (MB) from wastewater. Compared with the nanoparticles without coating, the PAA coating significantly enhanced the adsorption capacity of MB onto MnFe$_{\mathrm{2}}$O$_{\mathrm{4}}$ magnetic nanoparticles, where a rapid and efficient removal of MB was observed. The research suggests that as-synthesized PAA/ MnFe$_{\mathrm{2}}$O$_{\mathrm{4}}$ nanocomposites have promising and potential applications in water treatment for removal of dyes. [Preview Abstract] |
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H1.00305: Synthesis and Characterization of Au@Pd@Au core-shell nanostructures Alejandra Londono-Calderon, J. Jesus Velazquez-Salazar, Miguel Jose Yacaman In this work we present a systematic study on the synthesis of (Au@Pd)@Au nanostructures by a seed mediated method in aqueous solution. In the first step, single crystal Au octahedra nanoparticles are used as seeds to produce bimetallic Au@Pd core-shell nanocubes of 40 nm in size. The growth mechanism of successive Au layers over the Au@Pd nanocubes and the crystallinity on the Au/Pd and Pd/Au interfaces are studied by the use of Scanning Electron Microscopy (SEM), High Resolution Transmittion Electron Microscopy (HRTEM) and Scanning Transmittion Electron Microscopy (STEM). A transformation from cubes to truncated polyhedrons is observed by Electron Tomography in the reconstruction of the surface. [Preview Abstract] |
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H1.00306: Real-space and nanoscopic observation of phase transition behaviors of VO$_{2}$ thin films using Kelvin probe force microscopy Dong-Wook Kim, Ahrum Sohn, Teruo Kanki, Koutaro Sakai, Hidekazu Tanaka VO$_{2}$ undergoes a metal-insulator transition (MIT) and a tetragonal-monoclinic structural phase transition (SPT) near room temperatureWe carry out transport and KPFM (Kelvin probe force microscopy) measurements on epitaxial VO$_{2}$/TiO$_{2}$ thin films simultaneously in the temperature range of 285-330 K. The samples have large grains due to very small lattice mismatch, which allows us to study inherent nature of the phase transition in quasi-2D VO$_{2}$ system. The sample's work function decreases from 5.1 eV to 4.9 eV, while spanning the transition temperature. The work function maps can clearly reveal coexistence of the two distinct states at the intermediate temperate range, well explained by the 2D percolation theory. [Preview Abstract] |
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H1.00307: Plasmon Excitations for a Coulomb-coupled Graphene Layer and a Thick Conductor Godfrey Gumbs, Andrii Iurov, Norman Horing Self-consistent field theory is used to obtain the plasmon dispersion relation of monolayer graphene which is Coulomb coupled to a thick conductor. We calculate numerically the undamped plasmon excitation spectrum for arbitrary wave number. For gapped graphene, both the low-frequency (acoustic) and high frequency (surface) plasmons may lie within the opening within the particle-hole region. Additionally, we obtain plasmon excitations in a region of the frequency-wave vector space which does not exist for free standing gapped graphene. [Preview Abstract] |
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H1.00308: One and two dimensional shock waves of light Ricardo Ferro, Hasanuzzaman Rahman, German Kolmakov, Manas Kulkarni By using numerical simulations for the dynamics of an exciton polariton condensate in an optical microcavity, we demonstrate that strongly nonlinear, spatially localized waves can be formed during the propagation of the condensate perturbations. We show that at the terminal stage of their evolution, the condensate density waves acquire the universal shape of a shock wave, which is similar to that known from a classical rarefied interacting gas dynamics. Since the exciton-polaritons in the condensate include photons as their integral part, such nonlinear waves can be treated as shock waves of light propagating in a microcavity. By numerically solving the chiral nonlinear wave equation for the condensate perturbation dynamics, we studied the shock front structure and then, investigate the propagation of light shock waves in a two-dimensional geometry in an unrestricted microcavity as well as in quasi-one dimensional polariton channels. We also discuss the effects of the scattering of the polariton shock waves on the structural defects in the cavity, and the effects of the phase coherence during mutual scattering of two and more shock waves. [Preview Abstract] |
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H1.00309: A quantum interferometer for studies of the exciton and polariton drag effects Andy He, Roman Ya. Kezerashvili, German V. Kolmakov Recently, the drag effects of excitons and cavity polariton condensates by an electric current running in a quantum well embedded in a cavity were theoretically predicted. These effects provide one with a useful tool to control the exciton and photons propagation in optical integrated circuits by external electric signals. Applications of such the drag effects in the design of semiconductor- and graphene-based devices for optical computing have recently been discussed in the literature. In our report, we propose a setup suitable for the studies of the exciton and polariton condensate drag effects based on self-interference of a split condensate in the presence of the driving current. By numerically simulating an output signal of a ring-shaped interferometer, we determine the range of parameters, at which the exciton and polariton drag effects in a microcavity can be observed and utilized in optical nanodevices. [Preview Abstract] |
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H1.00310: Dynamics of self-trapped excitons in layered Pb$_{\mathrm{1-X}}$Cd$_{\mathrm{X}}$I$_{2}$ semiconductors Yuriy Gnatenko, Anatolii Bukivskii, Yuriy Piryatinski The dynamics of self-trapped excitons, localized on stretched Pb-I chemical bonds which are formed on PbI$_{2}$ nanocluster surface was investigated. It should be noted that these nanoclusters are naturally formed in Pb$_{\mathrm{1-X}}$Cd$_{\mathrm{X}}$I$_{2}$ (X~$=$~0.5 and X~$=$~0.7) layered semiconductor solid solutions. They have different sizes (from several nm to several tens of nm). The measurements of photoluminescence (PL) spectra and kinetics of PL intensity decay for those materials were performed at T$=$300 K. The kinetic dependencies were obtained for the maximum of PL band (600 nm) and for its short-wave shoulder (550 nm). It was shown that PL decay kinetics is approximated by Kohlrausch-Williams-Watts (KWW) function, i.e. by stretched exponential function I(t)~$=$~I$_{0}$exp(-(t/$\tau _{\mathrm{ef}})^{\beta})$. Obtained values of $\tau _{\mathrm{ef}}$ and $\beta $ for X~$=$~0.5 are equal about 800~ns and 0.76 at 600~nm. At 550 nm these values are 700~ns and 0.74, respectively. Similarly for X~$=$~0.7 these values correspond about 800~ns and 0.80 at 600~nm. At 550 nm they are 800~ns and 0.82. Application of the Inverse Laplace Transformation (ILT) to our experimental data gave us opportunity to estimate the probability density function of self-trapped exciton state lifetimes for Pb$_{\mathrm{1-X}}$Cd$_{\mathrm{X}}$I$_{\mathrm{2}}$ (X $=$ 0.3, 0.5 and 0.7). The position of the maximum of F($\tau )$ gives us the average decay time \textless t\textgreater which is about 1250~ns which significantly differs from $\tau_{\mathrm{ef}}$ which is about 800~ns (for $\lambda $~$=$~550~nm). This complex dynamics of excitons is associated with strong heterogeneity of the investigated system. [Preview Abstract] |
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H1.00311: Half-metallicity in a BiFeO$_3$/La$_\frac{2}{3}$Sr$_\frac{1}{3}$MnO$_3$ heterostructure: A first-principles study Jilili Jiwuer, Ulrich Eckern, Udo Schwingenschl\"ogl We present first principles results for the electronic, magnetic, and optical properties of the BiFeO$_3$/La$_\frac{2}{3}$Sr$_\frac{1}{3}$MnO$_3$ heterostructure as obtained by spin polarized calculations using density functional theory. The electronic states of the heterostructure are compared to those of the bulk compounds. Structural relaxation turns out to have only a minor impact on the chemical bonding, even though the oxygen octahedra in La$_\frac{2}{3}$Sr$_\frac{1}{3}$MnO$_3$ develop some distortions due to the interface strain. While a small charge transfer affects the heterointerfaces, our results demonstrate that the half-metallic character of La$_\frac{2}{3}$Sr$_\frac{1}{3}$MnO$_3$ is fully maintained. (Reference: EPL, 102, 67009, 2013) [Preview Abstract] |
(Author Not Attending)
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H1.00312: Magnetoresistance Anomalies in LaAlO$_{3}$/ SrTiO$_{3}$(110): Fingerprints of Flat $d_{yz}$/$d_{xz}$ Band along [1-10] zigzag chain Haijiao Ma, Qinfang Zhang, Yang Liu, Anil Annadi, Wenxiong Zhou, Shengwei Zeng, Thirumalai Venky Venkatesan, Ariando Ariando We report novel features in the in-plane magnetoresistance (MR) of anisotropic two-dimensional electron gas (2DEG) at LaAlO$_{3}$/SrTiO$_{3}$(110) heterostructures, which unveil the existence of a flat band in this system. One of the striking features is an anisotropy of the MR with a ``$\mbox{d}_{z^{2}} $-wave''-like symmetry upon rotating the magnetic field H within (110) plane at low temperature, which is caused by the existence of flat $d_{yz}$/$d_{zx}$ band. Combining with theoretical and experimental results, we confirm that a flat $d_{yz}$/$d_{xz}$ band formed along [1-10] Ti-O-Ti zigzag chain direction which might be very interesting for searching for topological state such as in superconducting regime. [Preview Abstract] |
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H1.00313: Double perovskites nanoparticles: synthesis and magnetic properties (La$_{2}$NiMnO$_{6}$ vs La$_{2}$CoMnO$_{6}$) Yuanbing Mao Nanoparticles of double perovskite La$_{2}$BMnO$_{6}$ (B $=$ Ni and Co) with average particle size of about 50 nm were successfully prepared using a facile, environmentally friendly, scalable molten-salt synthesis method at 700 $^{\circ}$C in air. Their structural and morphological properties were characterized by X-ray diffraction and transmission electron microscopy. Their magnetic properties were evaluated using dc magnetic M--T and M--H, and ac magnetic susceptibility versus frequency, temperature, and field for the first time. The magnetic properties of these double perovskite nanoparticles indicate that they possess very different magnetic behaviors. The following results will be discussed: (i) field-cooled and zero-field-cooled magnetization curves become divergent at their Curie temperature, i.e. 275 K and 210 K for La$_{2}$NiMnO$_{6}$ (LNMO) and La$_{2}$CoMnO$_{6}$ (LCMO) nanoparticles, respectively, which are almost unchanged from their bulk and thin film counterparts. (ii) ac susceptibility indicates that the LNMO particles are much more complex structurally and may have anti-site defects or a second-phase with a different transition temperature. For a better understanding of the nature of the magnetic state and dynamic characteristics observed here for these LNMO and LCMO nanoparticles, further detailed studies are needed. [Preview Abstract] |
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H1.00314: Possible Mechanisms in Atomic Force Microscope-Induced Nano-Oxidation Lithography (negative AFM tip case) in La$_{0.67}$Ba$_{0.33}$MnO$_{3-\delta }$ Thin Films on SrTiO$_{3}$(001) Grace Yong, William Vanderlinde, E. Kevin Tanyi, David Schaefer, Christopher Stumpf, Rajeswari M. Kolagani In this paper, we present possible microscopic mechanisms for La$_{0.67}$Ba$_{0.33}$MnO$_{3-\delta }$ films that have been nano-oxidized by an AFM tip that is negatively biased with respect the sample. Further analysis of comparative EDS elemental profile for an unmodified film versus AFM (negative tip) modified films yield fresh insights. We can qualitatively explain many of the observations with electrochemical half reactions, electrochemical migration and electromigration. [Preview Abstract] |
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H1.00315: Heat flow and $\delta $-layers in Si nanowires Mehmet B. Bebek, T. Michael Gibbons, Stefan K. Estreicher Modern semiconductor growth techniques allow the use of heterostructures in semiconductor devices such as $\delta $-layers or superlattices, and their behavior regarding heat flow is generating considerable interest. However, there is no fully `first-principles' theoretical description of the interactions between heat flow and the interface between two dissimilar materials. In this contribution, we present the result of ongoing ab-initio, microcanonical, non-equilibrium MD simulations on Si/Ge or Si/C interfaces in a Si nanowire. We show that the `spatially-localized vibrational modes' (SLMs) associated with the interface trap incoming bulk phonons for lengths of time ranging from dozens to hundreds of periods of oscillation. Then, the trapped phonons decay into lower frequency bulk phonons. This decay depends on the availability of receiving modes on either side of the interface rather than on the origin of the incoming flow of heat. [Preview Abstract] |
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H1.00316: Local electrical imaging of tetragonal domains and field-induced ferroelectric domains in conducting SrTiO$_{3}$ Haijiao Ma, S. Scharinger, S.W. Zeng, M. Lange, A. Stöhr, Z. Huang, T. Venkatesan, R. Kleiner, M. Coey, D. Koelle, A. Ariando We report intrinsic electric mapping of local conductivity due to tetragonal domains and twin boundaries in conducting STO. Multidomains and stripe monodomains were observed in different samples at low temperatures. The distribution of these domains changes on thermal cycling above the STO cubic-to-tetragonal structural transition temperature and on electric field gating. The domains split into narrower domains when we applied side gating and we attributed this to field-induced ferroelectric domain. Twin boundaries with different orientations were observed. Angles of these domain boundaries in (110) plane are 0, 55, 125 and 145 degrees. These angles were calculated from the intersection of twin planes and substrate cutting orientation. The domains split into narrower domain segments when T decreases below 30 K. [Preview Abstract] |
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H1.00317: Probing the mechanical properties and microstructure of WSi$_{2}$/Si$_{x}$Ge$_{1-x}$ multiphase thermoelectric material by nanoindentation, electron and focused ion beam microscopy methods Francisco Sola, Frederick Dynys Silicon germanium (SiGe) thermoelectric (TE) alloys have been traditionally used in radioisotope thermoelectric generators (RTG) NASA applications. While RTG applications is the main driver of our current research, we are exploring other applications in the energy harvesting arena. There is still a need to improve the TE figure of merit (ZT) of SiGe based TE alloys and we have been working on ways to improve it by incorporating tungsten di-silicide (WSi$_{2}$) phases in to the matrix by directional solidification process. Considerable efforts have been focused until now in microstructural engineering methods that can lead to ZT improvement by microstructure optimization. Although critical for the previous mentioned applications, work pertinent to the mechanical integrity of WSi$_{2}$/SiGe based TE materials is lacking. In this presentation, we report local mechanical properties (hardness, modulus and fracture toughness) and microstructure of WSi$_{2}$/SiGe multiphase thermoelectric material by nanoindentation, scanning electron microscopy, focused ion beam and transmission electron microscopy methods. [Preview Abstract] |
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H1.00318: Nanophononic metamaterial: Thermal conductivity reduction by dispersion-resonance hybridization Mahmoud I. Hussein, Hossein Honarvar, Lina Yang Engineered manipulation of phonons can yield beneficial thermal properties in semiconducting materials. One pivotal application relates to thermoelectric materials, or the concept of converting energy in the form of heat into electricity and vice-versa. The ability to use nanostructuring to reduce the thermal conductivity without negatively impacting the power factor provides a promising avenue for achieving high values of the thermoelectric energy conversion figure-of-merit, ZT. In this work, we propose a novel nanostructured material configuration that seeks to achieve this goal. Termed ``nanophononic metamaterial,'' the configuration is based on a silicon thin-film with a periodic array of pillars erected on one or two of the free surfaces. The pillars qualitatively alter the base thin-film phonon spectrum due to a hybridization mechanism between their local resonances and the underlying atomic lattice dispersion. Using lattice dynamics calculations and molecular dynamics simulations, we predict a drop in the thermal conductivity to as low as 50{\%} of the corresponding uniform thin-film value despite the fact that the pillars add more phonon modes to the spectrum. [Preview Abstract] |
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H1.00319: Magneto-thermoelectric effects in the two-dimensional electron gas of a HgTe quantum well due to THz laser heating by cyclotron resonance absorption Mehdi Pakmehr, Christoph Bruene, Hartmut Buhmann, Laurens Molenkamp, Bruce McCombe HgTe quantum wells (QWs) have shown a number of interesting phenomena over the past 20 years, most recently the first two-dimensional topological insulating state. We have studied thermoelectric photovoltages of 2D electrons in a 6.1 nm wide HgTe quantum well induced by cyclotron resonance absorption (B $=$ 2 - 5 T) of a focused THz laser beam. We have estimated thermo-power coefficients by detailed analysis of the beam profile at the sample surface and the photovoltage signals developed across various contacts of a large Hall bar structure at a bath temperature of 1.6 K. We obtain reasonable values of the magneto-thermopower coefficients. [Preview Abstract] |
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H1.00320: The concept of position-dependent mass and its consideration in the study of a particle subjected to different types of potential Martin Molinar-Tabares, Lamberto Castro-Arce, Carlos Figueroa-Navarro, Julio Campos-Garc\'Ia We present a study where is used the concept of position-dependent mass for a particle subjected to three kind of potential: infinite quantum potential well, harmonic oscillator potential and step potential. We solve the time-independent Schr\"{o}dinger equation for each potential, considering different forms for the functional dependence of the mass respect the position. We obtain the ground state energy, the energies of some excited states and the corresponding probability densities. We make a comparison of the results with those that we would obtain if we consider an average mass for the particle. [Preview Abstract] |
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H1.00321: Investigation of insulator-sandwich MCBJ device for single molecule detection Akihide Arima, Makusu Tsutsui, Masateru Taniguchi Mechanically controllable break junction (MCBJ) is one of the most excellent methods for accurate measurements of electron transport through single molecules because of its stability and repeatability of nanometer-scale gap distance. This method has been recently used to investigate electric conductivity of individual nucleotides in an aqueous solution. However, traditional bare electrodes of MCBJ substrate generates unexpected ionic current, which deteriorates S/N ratio and disturbs accurate control of the gap distance. To solve this problem, we report the novel MCBJ device architecture. Briefly, we covered whole junctions with insulating material. This insulator-sandwich architecture enables us to suppress such ionic current and flesh electrode surface can be used in measurement because the junction is broken in the measurement circumstance for the first time. In this time, we will present basic evaluation of this device. We conducted measurements in vacuum, water, and buffers. We were able to repeat junction breaking and forming hundreds of time. We also observed that the ionic current was suppressed by 1/10 via the insulator coating compared to the traditional one. This device would contribute to investigation of physical property about single molecule. [Preview Abstract] |
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H1.00322: Sheared graphene: Electronic properties shaped by a mechanical instability Andres Concha, Shengfeng Cheng, Lucian Covaci, L. Mahadevan We explore the effects of shearing graphene ribbons on its geometry, and electronic properties. Inspired by macroscopic experiments, we show that spontaneous patterns appear when a wide ribbon is subject to shear. We compared this pattern and different regimes obtained via MD simulations with macroscopic experiments, and find good agreement between them. Beyond the low shear regime a second generation of wrinkles emerge when the system relaxes trying to keep the bond lengths as close to the relaxed length as possible. Remarkably, for all shear ratios the induced superlattice generates a momentum kick when electronic excitations enter the deformed region, an effective pseudo-magnetic superlattice, and a strong Fermi velocity renormalization. These effects modify electronic properties and suggest a simple route to engineer electronic waveguides and switches at the nanoscale. Our proposal is a concrete realization of a quantum device that takes full advantage of an elastic instability that spans from the nano to macro$-$scales. [Preview Abstract] |
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H1.00323: Ultrafast optical measurements of surface waves on a patterned layered nanostructure Brian Daly, Matteo Bjornsson, Aine Connolly, Sushant Mahat, Bryan Rachmilowitz, George Antonelli, Alan Myers, Hui-Jae Yoo, Kanwal Singh, Sean King We report ultrafast optical pump-probe measurements of 12 -- 54 GHz surface acoustic waves (SAWs) on patterned layered nanostructures. These very high frequency SAWs were generated and detected on the following patterned film stack: 25 nm physically vapor deposited TiN / 180 nm porous PECVD-grown a-SiOC:H dielectric / 12 nm non-porous PECVD-grown a-SiOC:H etch-stop / 100 nm CVD-grown a-SiO$_{\mathrm{2}}$ / Si (100) substrate. The TiN layer was dry plasma etched to form lines of rectangular cross section with pitches of 420 nm, 250 nm, 180 nm, and 168 nm and the lines were oriented parallel to the [110] direction on the wafer surface. The absorption of ultrafast pulses from a Ti:sapphire oscillator operating at 800 nm generated SAWs that were detected by time-delayed probe pulses from the same oscillator via a reflectivity change ($\Delta R)$. In each of the four cases the SAW frequency increased with decreasing pitch, but not in a linear way as had been seen in previous experiments of this sort. By comparing the results with mechanical simulations, we present evidence for the detection of different types of SAWs in each case, including Rayleigh-like waves, Sezawa waves, and leaky or radiative waves. [Preview Abstract] |
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H1.00324: Molecular Dynamics Simulations of Surface Acoustic Waves on Patterned Layered Nanostructures Matteo Bjornsson, Aine Connolly, Sushant Mahat, Bryan Rachmilowitz, Brian Daly, George Antonelli, Alan Myers, Kanwal Singh, Hiu-Jae Yoo, Sean King We report coarse-grained molecular dynamics (MD) simulations of surface acoustic waves on patterned layered nanostructures. The simulations were designed for comparison with samples consisting of the following patterned film stack: 25 nm physically vapor deposited TiN / 180 nm porous PECVD-grown a-SiOC:H / 12 nm non-porous PECVD-grown a-SiOC:H etch-stop / 100 nm CVD-grown a-SiO$_{\mathrm{2}}$ / Si (100) substrate. The TiN film was etched with lines of rectangular cross-section with pitch $=$ 168 to 420 nm. Ultrafast optical experiments on these samples have detected high frequency surface waves in the range of 10's of GHz. The MD simulation demonstrates the presence of strongly excited modes at frequencies that closely match those found in the experiments. Moreover, the simulation predicts that the type of surface wave mode detected should change depending on the pitch. For larger pitch, Rayleigh-like waves are predicted, but for smaller pitch, Sezawa waves (surface waves with properties similar to free plate modes of the thin films) are predicted. The MD simulation also demonstrates the cutoff wavelength for the Sezawa modes, as is reflected in the experimental results and as is also predicted by isotropic elastic calculations of the surface modes of a thin film on an infinite substrate. [Preview Abstract] |
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H1.00325: Evolution of interface and surface structures of ZnO$\backslash $Al2O3 multilayers upon rapid thermal annealing H.H. Liu, Q.Y. Chen, C.F. Chang, W.C. Hsieh, P.V. Wadekar, H.C. Huang, H.H. Liao, H.W. Seo, W.K. Chu ZnO$\backslash $Al2O3 multilayers were deposited on sapphires by atomic layer deposition at 85$^{\circ}$C. This low substrate temperature ensures good interface smoothness useful for study of interfacial reaction or interdiffusion. Our study aimed at the effects of rapid thermal annealing at different annealing temperatures, times and P$_{\mathrm{Ar}}$:P$_{\mathrm{O2}}$. XRR and XRD techniques were used to investigate the kinetics from which various terms of the activation energies could be determined. HR-TEM and electron diffraction were carried out to correlate the microstructures and interfacial alignments as a result of the reactions. AFM were used to assist SEM profiling of the surface morphological evolution in association with the TEM observations. [Preview Abstract] |
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H1.00326: Pulsed Laser Deposition and Reflection High-Energy Electron Diffraction studies of epitaxial long range order, nano- and microstructured Ag thin films grown on MgO, Al2O3, STO and Si Daniel Velazquez, Rachel Seibert, Hamdi Man, Linda Spentzouris, Jeff Terry Pulsed Laser Deposition is a state-of-the-art technique that allows for the fine tunability of the deposition rate, highly uniform and epitaxial sample growth, the ability to introduce partial pressures of gases into the experimental chamber for growth of complex materials without interfering with the energy source (laser). An auxiliary in situ technique for growth monitoring, Reflection High-Energy Electron Diffraction, is a powerful characterization tool for predictability of the surface physical structure both, qualitatively and quantitatively. RHEED patterns during and post deposition of Ag thin films on MgO, Al2O3, Si and STO substrtates are presented and their interpretations are compared with surface imaging techniques (SEM, STM) to evidence the usefulness of the technique. [Preview Abstract] |
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H1.00327: Indeterminate form 0/0 and tunneling in double quantum wells Igor Filikhin, Branislav Vlahovic We study single electron tunneling between localized and delocalized states in double InAs/GaAs quantum wells (DQWs). Spectral distribution of localized (or delocalized) states demonstrates high sensitivity on inter-dot distance. The tunneling goes consecutively from the higher energy levels to the ground state when the inter-dot distance decreases. The spectrum is presented by set of quasi-doublets and may be described by three parts: localized states, delocalized states, and states with different probability for localization in each QW of DQW. For the last states, the ratio W/ $\Delta $E of the wave functions overlapping integral W and the electron energy difference $\Delta $E of isolated left and right QWs is a weight coefficient in the expansion of wave function on the basis of the wave functions of isolated QWs. In case of weakly coupled QWs in DQW the indeterminate form 0/0 takes a place for the electron wave function. It is found that a small violation of the DQW shape symmetry drastically affects tunneling. This effect also appears as a numerical instability calculations for small variations of input parameters of numerical procedure. [Preview Abstract] |
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H1.00328: Emission energy control of semiconductor quantum dots using phase change material Shohei Kanazawa, Yu Sato, Ariyoshi Yamamura, Toshiharu Saiki Semiconductor quantum dots have paid much attention as it is a promising candidate for quantum, optical devices, such as quantum computer and quantum dot laser. We propose a local emission energy control method of semiconductor quantum dots using applying strain by volume expansion of phase change material. Phase change material can change its phase crystalline to amorphous, and the volume expand by its phase change. This method can control energy shift direction and amount by amorphous religion and depth. Using this method, we matched emission energy of two InAs/InP quantum dots. This achievement can connect to observing superradiance phenomenon and quantum dot coupling effect. [Preview Abstract] |
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H1.00329: Magnetic field effects and nodal ground states in InP nanowire Tiago de Campos, Paulo Eduardo de Faria Junior, Igor Zutic, Guilherme Sipahi Semiconductor nanowires (NWs) have attracted great interest in the last decade because of their unique optical, electronic, and spin-dependent properties. They are among the leading candidates to observe exotic states, such as Majorana Fermions [1]. In a seemingly trivial situation of a single particle confined in a quantum dot, it was predicted that the valence band ground state with a node is possible and was attributed to the formation of orbital textures [2]. This peculiar behavior, may also be present in wurtzite InP NWs with a diameter less then 10 nm [3]. The presence of the nodal state modifies the its basic optical properties, such as the degree of linear polarization. Here we study the change in these states when an external magnetic field is applied along nanowire axis. We studied wurtzite [0001] and zincblende [111] InP nanowires calculated within a k.p method formulation that describes both crystal phases in a single-particle Hamiltonian [4] and accounts for the applied magnetic field. \\[4pt] [1] J. Alicea, Rep. Prog. Phys. 75, 076501 (2012).\\[0pt] [2] J. Lee, K. V\'yborn\'y, J. E. Han and I. \v{Z}utic, Phys. Rev. B 89, 045\\[0pt] [3] A. Molina-S\'anchez and A. Garc\'ia-Crist\'obal, J. Phys. Cond. Matter 29, 295301 (2012).\\[0pt] [4] P. E. Faria Junior and G. M. Sipahi, J. Appl. Phys. 112, 103716 (2012). [Preview Abstract] |
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H1.00330: Polarization dependent Optical Reflectance and Electroreflectance measurements of GaAs/AlGaAs multiple quantum well Bragg structure Mim Nakarmi, Naresh Shakya, Vladimir Chaldyshev Electroreflectance spectroscopy measurement provides sharp and derivative-like spectral features in the energy region of excitonic transitions, while suppressing uninteresting background effects due to electro-modulation. We employed both electroreflectance and optical reflectance spectroscopies to probe excitonic transitions in a GaAs/AlGaAs multiple quantum well (MQW) Bragg structure. The sample used in this experiment consists of 60 periods of quantum well structures with GaAs well layer (13 nm) and AlGaAs barrier layer (94 nm), grown by molecular beam expitaxy on a semi-insulating GaAs substrate. We observed a significant enhancement of excitonic features at the x(e2-hh2) exciton transitions due to double resonance along with sharp features of heavy-hole and light-hole ground state x(e1-hh1) and x(e1-lh1) exciton transitions around incident angle of 23 degree. We will present results on polarization dependent optical reflectance and electroreflectance measurements of this structure and discuss the effect of polarization in the first and second energy states. [Preview Abstract] |
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H1.00331: Effective masses of Quasi-2D electrons in InGaAs/GaAsSb modulation-doped heterostructures Imtiaz Tanveer, Bruce McCombe, Hermann Detz, Gottfried Strasser The electronic properties of In$_{0.53}$Ga$_{0.47}$As/GaAs$_{0.51}$Sb$_{0.49}$ 2D electron gas (2DEG) systems, in spite of their use in high power electronics, have not been extensively investigated. Recently, they have been suggested as potential materials for IR quantum devices such as quantum cascade lasers (QCL), and they also show a strong Rashba effect$^{1,2}$. Here accurate values of the effective masses are important. Two remotely donor (Si)-doped samples grown by MBE with a 2DEG at the single heterostructure interface were studied by FIR magneto-transmission spectroscopy with a BOMEM FTIR spectrometer. The maximum mobilities (near 70 K) are 43,000 cm$^{2}$/Vs and 36,000 cm$^{2}$/Vs with corresponding carrier densities of 1.07 x 10$^{12}$ cm$^{-2}$ and 2.13 x 10$^{12}$ cm$^{-2}$, respectively. Cyclotron resonance measurements between 4T and 9T yielded m* = 0.0495m$_{0}$ for the more heavily doped sample. Individual transmission profiles in this case showed broadening toward high-energy, which may be due to contributions to the overall absorption profile from higher occupied subbands. The lower density sample shows an energy vs B dependence that does not extrapolate to zero at B = 0. The origin of this behavior will be discussed. [Preview Abstract] |
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H1.00332: Carrier thermalization dynamics in single Zincblende and Wurtzite InP nanowires Yuda Wang, Howard Jackson, Leigh Smith, Tim Burgess, Suriati Paiman, Philippe Caroff, Hoe Tan, Chennupati Jagadish Using transient Rayleigh scattering (TRS) measurements, we obtain photoexcited carrier thermalization dynamics for both zincblende (ZB) and wurtzite (WZ) InP single nanowires (NW) with picosecond resolution. A phenomenological fitting model based on direct band to band transition theory is developed to extract the electron-hole-plasma density and temperature as a function of time from TRS measurements of single nanowires which have complex valence band structures. We find that the thermalization dynamics of hot carriers depends strongly on material (GaAs NW vs. InP NW) and less strongly on crystal structure (ZB vs. WZ). The thermalization dynamics of ZB and WZ InP NWs are similar. But a comparison of the thermalization dynamics in ZB and WZ InP NWs with ZB GaAs NW reveals more than an order of magnitude slower relaxation for the InP NWs. We interpret these results as reflecting their distinctive phonon band structures which lead to different hot phonon effects. Knowledge of hot carrier thermalization dynamics is an essential component for effective incorporation of nanowire materials into electronic devices. We acknowledge the NSF through DMR-1105362, 1105121 and ECCS-1100489, and the Australian Research Council. [Preview Abstract] |
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H1.00333: Fluorescent DNA-templated silver nanoclusters Dmytro Nykypanchuk, Ruoqian Lin, Yolanda Small Silver nanoclusters synthesized in the presence of templating DNA molecules show tunable optical properties that depend on cluster size and structure as well as on templating DNA sequence. In this paper we study the effect of DNA sequence and temperature on the cluster photoluminescence and discuss the results in the context of thermodynamics of DNA bases binding to the silver atoms in the clusters. [Preview Abstract] |
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H1.00334: Plasmon Enhanced Ultrathin Film Broad-Band Nanoporous Absorber Jin-You Lu, Dong Liu, Kyle Wilke, Sumaya Noorulla, Nicholas Fang, TieJun Zhang Ultrathin absorbing films have attracted much attention due to their strong interference persisted inside the lossy dielectric film, which has much smaller thickness compared with conventional resonators. The absorber was realized by coating a lossy dielectric film with tens of nanometers in thickness on a metallic substrate. The ultrathin absorber was further developed to obtain perfect absorption at a given wavelength by inserting a spacer TiO2 between the dielectric and metallic substrate. However, this interference mode just contributes to the narrow band absorption. Here, we propose to combine the strong interference inside the ultrathin film absorber with localized surface plasmons (LSPs) to achieve broad-band absorption. This concept is realized by coating ultrathin absorbing Ge/Au films on nanoporous substrate, where the LSP mode is supported by pore-shape cavities. The near-field optical properties of ultrathin film on nanoporous substrate are analyzed by using the finite difference time domain method to study the spectroscopy and energy flow patterns. Simulation shows the absorption increases with the pore radius until the pore is too large to sustain LSP. Light is trapped in nanopores and penetrated into the lossy dielectric film around the pore entrance. [Preview Abstract] |
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H1.00335: Optical Properties of CdSe Nanocrystalline Photoanodes Amanda Lesar, Rohana Garuthara Cadmium selenide (CdSe) nanocrystalline photoanodes were prepared by chemical solution deposition, with deposition time varied from 24 hours to 120 hours. Photoluminescence (PL) spectroscopy, reflectance and transmittance spectra, and photoelectric current were measured to optically characterize each sample. Photoelectric current was measured in a liquid junction configuration, with sodium sulfide as the electrolyte and platinum foil as the electrode. The PL, reflectance, and transmittance spectra were measured for each sample from 79 K to room temperature. Chemical solution deposition should lead to quantum size effects, as longer deposition times form larger size nanocrystals. Quantum size effects were observed, as longer depositions times led to a shift towards lower energy in the peak of the PL spectra. The temperature dependence of the PL peak energy position was also analyzed; as the temperature increased, the peak shifted towards higher energy. Using the reflectance and transmittance spectra, the absorption coefficient $\alpha$ was calculated, and the Tauc`s plot of $(\alpha h\nu)^2$ versus $(h\nu)$ was graphed. A correlation between the observed absorption edge and the PL spectra was seen, as the absorption edge energy was approximately equal to the PL energy peak. [Preview Abstract] |
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H1.00336: Plasmon excitations of multi-layer graphene interacting with a conducting substrate Paula Fekete, Godfrey Gumbs We generalize the procedure for calculating the plasmon excitations of a 2D graphene layer that is Coulomb-coupled to a thick conducting substrate to the case when there is an arbitrary number of layers. In this work, we will present results for the plasmon excitations for up to five layers with arbitrary separation, energy gap between the valence and conduction bands for graphene and doping concentrations. Our procedure involves determining the inverse dielectric function for the composite hybrid system in the random-phase approximation (RPA). Effects due to nonlocality will be investigated. [Preview Abstract] |
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H1.00337: Invisibility's Flicker: Detecting Thermal Cloaks via Transient Effects Sophia Sklan, Xue Bai, Baowen Li, Xiang Zhang Recent research on the development of a thermal cloak has concentrated on engineering an inhomogeneous thermal conductivity and homogeneous volumetric heat capacity. While the perfect cloak of inhomogeneous $\kappa$ and $\rho c_p$ is known to be exact (no signals scattering or penetrating to the cloak's interior), no such analysis has been considered for this case. Using analytic, computational, and experimental techniques, we demonstrate that these approximate cloaks are detectable. Although they work as perfect cloaks in the steady-state, their transient (time-dependent) response is imperfect and a detectable amount of heat is scattered. This is sufficient to determine the presence of a cloak and any heat source it contains, but the material composition hidden within the cloak is not detectable in practice. [Preview Abstract] |
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H1.00338: Generating Steep Phase Anisotropy With Zero-Backscattering By Arrays of Coupled High Permittivity Dielectric Nanoresonators Feng Wang, Qi-Huo Wei, Han Htoon Simultaneous excitation of electric and magnetic dipolar modes in high-permittivity dielectric nano-resonators can lead to zero-backscattering, i.e. full transmission. Here, we numerically demonstrate that stable or unstable zero-backscattering by 2-dimensional (2D) arrays of Si nano-resonators can be realized. We also show that this Si nano-resonator array with anisotropic periodicity can generate approximate 2$\pi $ optical phase anisotropy for the transmitted light at the wavelength of zero-backscattering. By introducing strong Fano-type coupling into unit cells of the array, ultra-steep phase anisotropy can be achieved. These special optical properties promise applications in various transmissive photonic devices, and we show their potential applications in transmissive polarization conversion and sensing. [Preview Abstract] |
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H1.00339: Calculus of bands and profiles of study the system mirror - resonance of the Fibonacci Pt/Zn Lamberto Castro-Arce, Carlos Figueroa-Navarro, Julio Campos-Garcia, Martin Molinar-Tabares, Felipe Ramos-Mendieta, Betzabe Manzanares-Martinez In order to analyze the behavior of a mirror -- located resonance of 2pi, in a given system and with a given filling factor equal to 0.4 a study has been realized in an arrangement fibonacci, also in periodic slabs jobs. It is observed how in a study of profile that some waves are annulled giving birth to the mirror placed in 2pi. With regard to the resonance in a profile study the maxima are in certain structure Pt Pt Zn Pt Pt. Even if we increase the number of repetitions these are preserved, that means that they are related to effects of segments isolated inside the multilayer. [Preview Abstract] |
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H1.00340: Atomic structure prediction of metal clusters using the evolutionary algorithm Nabil Al-Aqtash, Khaldoun Tarawneh, Renat Sabirianov The evolutionary algorithm coupled with density functional (DFT) method is used to identify the global energy minimum atomic structure of metal clusters. Using evolutionary crystal structure optimization algorithm, as implemented in USPEX, we studied the atomic structure, binding energies, and magnetic properties of 13-atom Cu, Co and Cr clusters. A set of metastable and global minimum atomic structures are identified. Several new lower energy configurations were identified for 13- atom Cu, Co and Cr clusters and previous known atomic structures were confirmed by our calculations. We found that the Cu$_{13}$ cluster has a distorted hexagonal bilayer (HBL) --like structure, which is composed by two layers as in the ideal HBL structure. The distorted HBL Cu$_{13}$ is 1.17 eV lower in total energy compared to close-packed icosahedral (ICO) configuration, which reported as the lowest-energy structure for Cu$_{13}$ in previous studies. Our calculations show that Co$_{13}$ has an ideal HBL structure and Cr$_{13}$ cluster has distorted ICO structure, which are consistent with the previous studies. Moreover, our calculations show that Cr$_{13}$ has another lower energy atomic configuration with 0.003 eV difference form ICO. Cr$_{13}$ has ferrimagnetic (FIM) interaction which plays an important role in finding the lowest energy structure. We discuss the predictive capabilities of evolutionary algorithms for nanoclusters. [Preview Abstract] |
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H1.00341: Electron with position-dependent mass confined in a two-dimensional infinite square well Martin Molinar-Tabares, Carlos Ruvalcaba-Cornejo In order to have a case of study for introduce the concept of position-dependent mass, we propose to analyze the following case. Creating a rectangular crystal structure from the two-dimensional deposit of GaAa and Al$_{\mathrm{0.35}}$Ga$_{\mathrm{0.65}}$As on a substrate, we study the confinement of an electron with position-dependent effective mass. Knowing how the electron mass of the electron and its potential energy varies with the concentration of the semiconductor, we solve the time-independent Schr\"{o}dinger equation using a linear combination of wave functions of a particle enclosed inside a two-dimensional square well with infinite potential walls. The ground state energy and the energies of some excited states with the probability density of these states are found. Making a two-dimensional growth of the structure we analyze if appears sub-bands energy and if the Bloch theorem manifests. We compare our results with those that we would obtain if we consider and constant effective mass inside the crystal. [Preview Abstract] |
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H1.00342: Conformational electroresistance and hysteresis in nanoclusters Xiang-Guo Li, Xiao-Guang Zhang, Hai-Ping Cheng Abstract: Existence of multiple thermodynamically stable isomer states is one of the most fundamental properties of small clusters. We show that the conformational dependence of the Coulomb charging energy of a nanocluster leads to a giant electroresistance, where charging induced conformational distortion changes the blockade voltage. The intricate interplay between charging and conformation change is demonstrated in a nanocluster Zn$_{3}$O$_{4}$ by combining a first-principles calculation with a temperature-dependent transport model. The predicted hysteretic Coulomb blockade staircase in the current-voltage curve adds another dimension to the rich phenomena of tunneling electroresistance. The new mechanism provides a better controlled and repeatable platform to study conformational electroresistance. [Preview Abstract] |
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H1.00343: Finite-size scaling study of the one-dimensional Bose-Hubbard model via matrix product state representations Sung-Been Park, Min-Chul Cha The Bose-Hubbard model is a prototypical simple model showing quantum phase transition with a continuous symmetry. In one dimension, the quantum critical properties of the model has been studied via various methods, but still some basic properties remain unknown, such as the exact location of the critical point. It is a computational challenge to study this model with more elaborated numerical methods. The matrix product state (MPS) representations are new variational solutions to one-dimensional quantum systems. By unsing this method to find the ground state, we study the critical properties of the one-dimensional Bose-Hubbard model with a periodic boundary condition. Finite-size scaling analysis provides the phase diagram and the critical exponents. [Preview Abstract] |
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H1.00344: Solution of the homogeneous electromagnetic wave equation with velocity time depending on the discrete space-time Julio Campos-Garcia, Martin Molinar-Tabares, Lamberto Castro-Arce, Carlos Figueroa-Navarro, Rodrigo Rosas-Burgos We present in this meeting the solution of the homogeneous electromagnetic wave equation with a speed of light time depending on the discrete space-time. The solutions are compared with those that are obtained for a standard case, where the speed of light is constant. In addition, the limiting cases of the discrete and continuum space-time are analyzed. [Preview Abstract] |
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H1.00345: POSTDEADLINE ABSTRACTS |
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H1.00346: Fabrication and In-situ TEM Characterization of Freestanding Graphene Nanoribbons Devices Qing Wang, Ryo Kitaura, Shoji Suzuki, Hisanori Shinohara Edge-dependent electronic properties of graphene nanoribbons (GNRs) have attracted intensive interests. To fully understand the electronic properties of GNRs, the combination of precise structural characterization and electronic property measurement is essential. For this purpose, a new experimental technique using freestanding GNR devices has been developed, leading to the simultaneous characterization of electronic properties and edge structure of GNRs. To prepare freestanding GNR devices, graphene was first transferred on a Si substrate with an open slit covered by a silicon dioxide layer, and then silicon dioxide membrane underneath the graphene was etched away by buffered hydrogen fluoride acid. The so-prepared freestanding graphene device was assembled to a home-made TEM holder for in-situ characterization. The freestanding graphene was sculpted by a focused electron beam in TEM, purified and narrowed by Joule heating down to several nanometers width. Structure-dependent electronic properties were performed in TEM. We have observed significant increase in resistance and semiconductive behavior became more dominant with decreasing width of GNR. [Preview Abstract] |
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H1.00347: Surface Premelting Coupled with Bulk Phase Transitions in Colloidal Crystals Bo Li, Feng Wang, Di Zhou, Xin Cao, Yi Peng, Ran Ni, Maijia Liao, Yilong Han Colloids have been used as outstanding model systems for the studies of various phase transitions in bulk, but not at interface yet. Here we obtained equilibrium crystal-vapor interfaces using tunable attractive colloidal spheres and studied the surface premelting at the single-particle level by video microscopy. We found that monolayer crystals exhibit a bulk isostructural solid-solid transition which triggers the surface premelting. The premelting is incomplete due to the interruption of a mechanical-instability-induced bulk melting. By contrast, two- or multilayer crystals do not have the solid-solid transition and the mechanical instability, hence they exhibit complete premelting with divergent surface-liquid thickness. These novel interplays between bulk and surface phase transitions cast new lights for both types of transitions. [Preview Abstract] |
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H1.00348: Entrance Pressure Fluctuation of LLDPE in Capillary Flow Haiqing Hu, Shuai Li, Luyao Yan, Tongjie Sun, Linlin Liu, He Cheng Oscillating flow, which usually refers to the whole capillary pressure oscillation under constant piston speed, has been widely studied as an important instability phenomenon in capillary flow. The coil-stretch transition of entangled polymer molecules can be considered as a critical factor resulting in oscillating flow, which is only observed under controlled piston speed. It has been theorized by Weill since 1980 that the appearance of surface distortions may originate from a high-frequency oscillatory flow created at the die entry, but no experimental evidence has been found to prove it over 30 years. Wall slippage plays an important role in capillary extrusion flow instability for LLDPE melt. Local stick-slip transition leads to perturbations on the exit stress and sharkskin distortion, while global stick-slip transition results in oscillatory flow and the second glossy region or quasi periodic variation of extrudates. This article has revealed the correspondence relationship between entrance pressure fluctuation and exit stress perturbation experimentally and illuminated it by Uhland model. We have further confirmed the idea that local wall stick-slip transition can induce the entrance pressure fluctuation. In brief, the molecular disentanglement in die exit determines the critical shear stress of entrance pressure fluctuation. [Preview Abstract] |
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H1.00349: Superconductivity and Magnetism from First Principles Andreas Linscheid, Antonio Sanna, Frank Essenberger, E.K.U. Gross Magnetism has intriguing effects in superconductors. On the one hand static magnetic fields are known to suppress the uperconducting state while dynamic spin-fluctuations are the probable candidate to explain the pairing in the Fe-based Superconductors. Achieving an ab-initio description is important. First, because this allows to compute the critical field and whether a local coexistence of magnetic and superconducting phases exist. Second, the critical temperature of a material is among the predicted properties which allows to search yet unknown superconductors on a computer. The Density Functional Theory for Superconductors (SCDFT) has been very successful in predicting T c of phonon mediated superconductors. We include the magnetic density into SCDFT so that the electronic Kohn-Sham system now reproduces the electronic density $n(\mathbf{r})$, the order parameter of superconductivity $\chi(\mathbf{r}, \mathbf{r}^{\prime} )$ and the magnetic density $\bf{m}(\mathbf{r})$. We derive the xc-potential and discuss some first results. Furthermore, we discuss an effective electron interaction mediated by spin-flip processes based on the exact spin-susceptibility. We drive a xc-functional for SCDFT that includes this effective interaction and present some results. [Preview Abstract] |
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H1.00350: Iterative backflow renormalization procedure for many-body ground state wave functions of strongly interacting normal Fermi liquids Michele Ruggeri, Michele Taddei, Saverio Moroni, Markus Holzmann We propose a new trial wavefunction for the ground state of a normal Fermi liquid. We apply iterative backflow transformations to obtain a sequence of renormalized coordinates. At each iteration two and three body correlations between quasiparticles are taken into account. We use these wavefunctions to compute the ground state energy of liquid $^3$He at freezing density in two dimensions with Variational and Diffusion - Fixed Node Monte Carlo simulations. Comparing with exact transient estimate results for systems with small number of particles, we find that variance extrapolations provide accurate results for the true ground state together with stringent lower bounds. For larger systems these bounds can in turn be used to quantify the systematic bias of fixed-node calculations. These wave functions are size consistent and the scaling of their computational complexity with the number of particles is the same as for standard backflow wave functions. [Preview Abstract] |
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H1.00351: Comparative Study Between GGA and LDA Approximation Using First- Principles Calculations of Structural, Electronic, Optical and Vibrational Properties of CaTiO$_{ 3}$ Crystal Subenia Medeiros, Maeva Araujo The structural, electronic, vibrational, and optical properties of perovskite CaTiO$_{3}$ in the cubic, orthorhombic, and tetragonal phase are calculated in the framework of density functional theory (DFT) with different exchange-correlation potentials by CASTEP package. The calculated band structure shows an indirect band gap of 1.88 eV at the $\Gamma $\textbf{-R} points in the Brillouin zone to the cubic structure, a direct band gap of 2.41 eV at the $\Gamma $\textbf{ -- }$\Gamma $ points to the orthorhombic structure, and an indirect band gap of 2.31 eV at the\textbf{ M -- }$\Gamma $ points to the tetragonal phase. It is still known that the CaTiO$_{3}$ has a static dielectric constant that extrapolates to a value greater than 300 at zero temperature, and the dielectric response is dominated by low frequency ($\nu \approx $ 90cm$^{-1})$ polar optical modes in which cation motion opposes oxygen motion. Our calculated lattice parameters, elastic constants, optical properties, and vibrational frequencies are found to be in good agreement with the available theoretical and experimental values. The results for the effective mass in the electron and hole carriers are also presented in this work. [Preview Abstract] |
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H1.00352: Electron relaxation of DNP free radicals BDPA and DPPH at W-band Armin Khamoshi, Pavanjeet Kaur, Likai Song, Lloyd Lumata The stable, spin-1/2 organic free radicals BDPA and DPPH are efficient polarizing agents for dissolution dynamic nuclear polarization (DNP). Despite the hydrophobic nature of these two free radicals, BDPA and DPPH can be dissolved in specialized solvents such as sulfolane or dimethyl sulfoxide. In this work, we have investigated the temperature dependence of the spin-lattice relaxation rate 1/T$_{1}$ of these two DNP free radicals at W-band from 250 K down to 4 K. We have found that at high temperature above 40 K the relaxation rates of these free radicals (at optimum DNP concentration) behave closely according to the Raman process prediction. At lower temperature below 40 K, the relaxation rate slows down according to the direct process behavior. The results obtained here may elucidate the correlation between the relaxation of electrons and the efficiency of these free radicals in DNP. [Preview Abstract] |
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H1.00353: Power laws and extreme values in antibody repertoires Sebastien Boyer, Dipanwita Biswas, Natale Scaramozzino, Ananda Soshee Kumar, Cl\'ement Nizak, Olivier Rivoire Evolution by natural selection involves the succession of three steps: mutations, selection and proliferation. We are interested in describing and characterizing the result of selection over a population of many variants. After selection, this population will be dominated by the few best variants, with highest propensity to be selected, or highest ``selectivity.'' We ask the following question: how is the selectivity of the best variants distributed in the population? Extreme value theory, which characterizes the extreme tail of probability distributions in terms of a few universality class, has been proposed to describe it. To test this proposition and identify the relevant universality class, we performed quantitative {\it in vitro} experimental selections of libraries of $> 10^5$ antibodies using the technique of phage display. Data obtained by high-throughput sequencing allows us to fit the selectivity distribution over more than two decades. In most experiments, the results show a striking power law for the selectivity distribution of the top antibodies, consistent with extreme value theory. [Preview Abstract] |
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H1.00354: Single Crystal Diffuse X-ray Scattering Using Continuous Rotation Matthew Krogstad, Omar Chmaissem, Keith Taddei, Jared Allred, Raymond Osborn, Stephan Rosenkranz, Justin Wozniak Single crystal diffuse scattering provides a measure of the 3D pair distribution function and is thus useful for investigating short-range order in materials. Using very bright synchrotron x-ray sources and fast area detectors, large volumes of reciprocal space can be mapped quickly with a dynamic range large enough to measure both Bragg peaks and the much weaker diffuse scattering. With the appropriate tools for processing and analyzing large data sets (10 to 30GB), this technique can be used to track changes in the defect structures of a material as a function of different parameters, providing a sensitive and efficient method for investigating phenomena associated with disorder. We have been developing methods of measuring diffuse scattering using continuous sample rotations (shutterless mode) at the Advanced Photon Source, and will show data from several systems, including iron pnictides, for a range of temperatures and doping levels. [Preview Abstract] |
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H1.00355: Chemical Bonding Forces and Metallization of Hydrogen Ivan Naumov, Russell Hemley Recent theoretical and experimental studies reveal that compressed molecular hydrogen at 200-350 GPa transforms to layered structures consisting of distorted graphene sheets. The new phases of dense solid hydrogen contrast with the long-held view that symmetric close-packed, ambient alkali-metal-like structures form at these high pressures --this raises the question about the nature and fate of molecular bonding in hydrogen on compression. The realization of such unexpected structures can be explained by consideration of simple low-dimensional model systems- H6 rings and graphene-like monolayers. Both molecular quantum chemistry and well-tested solid state approaches show that these model systems like aromatic hydrocarbons exhibit a special stability, associated with the completely filled set of bonding orbitals or valence bands. This close-shell effect persists in progressing to the real layered structures where it prevents the dielectric energy gap from closing, thus delaying the pressure-induced metallization. The latter nevertheless can occur upon further compression via destroying the closed shell electronic structure which is mainly determined by the 1s electrons. The most likely scenario is the lowering of the bonding bands (their bottoms) stemming from the unoccupied atomic 2s and 2p orbitals [1]. This research was supported by EFree, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Basic Energy Sciences under Award DESC0001057. [1] I. I. Naumov and R. J. Hemley, Acc. Chem. Res. 47, 3551-3559 (2014). [Preview Abstract] |
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H1.00356: Generation of internal gravity waves by tidal flow over random oceanic topography Jiajun Zhao, Likun Zhang, Harry Swinney Internal waves (IWs) are gravity waves that propagate within density-stratified fluids such as the ocean, atmosphere, and protoplanetary disks. IWs generated by tidal flow over oceanic topography provide much of the energy needed to sustain vertical mixing, which plays a critical role in ocean circulation and global climate. Therefore, it is important to determine the amount of energy that is extracted from tidal flow over topography and radiated into IWs. We conduct 2D numerical simulations to determine the IW power generated by tidal flow over random topographies that have the seafloor spectrum. The power is found to saturate with increasing topographic roughness, and to scale linearly with the characteristic height of the topography. The linear dependence on the topographic height is, surprisingly, nearly independent of the value of the exponent characterizing the topographic spectrum. Our results should lead to improved predictions of the IW power generated by tidal flow over global ocean topography. [Preview Abstract] |
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H1.00357: Photoresponse of Single Mn doped ZnO nanowires in UV application Mon-Shu Ho This paper reports the fabrication of Mn doped ZnO nanowires(NWs) using a low temperature hydrothermal method. The resulting nanowires were characterized using field emission scanning electron microscopy, energy dispersive spectroscopy, X-ray diffraction analysis, transmission electron microscopy and photoluminescence spectroscopy. A single Mn doped Zno NW UV sensor with high performance sensing capabilities was then assembled using a focused ion beam technique. The photoresponse of the ZnO NW sensors was investigated under irradiation from 365 nm and 400 nm ultra-violet lamps. The proposed sensor exhibited rapid photoresponse speeds and short recovery times with a photocurrent ratio ($\Delta $I $=$ I$_{\mathrm{light\thinspace }}$/ I$_{\mathrm{dark}}$ ) superior to that of pure ZnO NW sensor. A possible mechanism to account for adsorption-desorption of oxygen and water molecules on Mn/ZnO NW surfaces was finally proposed to give the expression. [Preview Abstract] |
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H1.00358: Power-law-like correlation between condensation energy and superconducting transition temperatures in iron pnictide/chalcogenide superconductors: Beyond the BCS understanding Jie Xing, Sheng Li, Bin Zeng, Gang Mu, Bing Shen, J. Schneeloch, R.D. Zhong, T.S. Liu, G.D. Gu, Hai-Hu Wen Superconducting condensation energy $U_0^{int}$ has been determined by integrating the electronic entropy in various iron pnictide/chalcogenide superconducting systems. It is found that $U_0^{int}\propto T_c^n$ with $n$ = 3 to 4, which is in sharp contrast to the simple BCS prediction $U_0^{BCS}=1/2N_F\Delta_s^2$, with $N_F$ the quasiparticle density of states at the Fermi energy and $\Delta_s$ the superconducting gap. A similar correlation holds if we compute the condensation energy through $U_0^{cal}=3\gamma_n^{eff}\Delta_s^2/4\pi^2k_B^2$, with $\gamma_n^{eff}$ the effective normal state electronic specific heat coefficient. This indicates a general relationship $\gamma_n^{eff} \propto T_c^m$ with $m$ = 1 to 2, which is not predicted by the BCS scheme. A picture based on quantum criticality is proposed to explain this phenomenon. [Preview Abstract] |
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H1.00359: ABSTRACT WITHDRAWN |
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