Session S1: Poster Session III (1:00-4:00PM)

Charge Transport Anisotropy in n-Type Disk-Shaped Triphenylene-Tris(aroyleneimidazole)s

Two novel n-type disk-shaped molecules containing a triphenylene core and three fused naphthaleneimide imidazole or peryleneimide imidazole ``arms'' are synthesized and characterized. Their optical and electronic properties are consistent with the pi-extended structural feature of the aromatic cores. The n-type charge carrier mobilities of these molecules are evaluated by both field effect transistors and space-charge limited-current measurements, which show drastically different mobility anisotropy. A strong correlation between film morphology and the charge transport behavior is established by X-ray scattering and atomic force microscopic analyses.   

Identification of the Amorphous $AlB_4H_{11}$ Structure

In recent experimental work, $AlB_4H_{11}$ has been identified as a potential hydrogen storage material with a good desorption temperature and partial reversibility. It is an amorphous, white solid at room temperature and its molecular structure is presently unknown. We combine experimental measurements (NMR, neutron vibrational spectra and IR) and a theoretical structure prediction method to identify the (local) structure of the amorphous $AlB_4H_{11}$ phase. The theoretical structure prediction method is a combination of the Monte-Carlo based prototype electrostatic ground state search (PEGS) method and first-principles calculation (DFT). The PEGS+DFT method has successfully predicted many crystalline solid structures, but has never been applied to the prediction of amorphous solid structures. The PEGS predictions of the $AlB_4H_{11}$ structure are quite successful: we find the calculated phonon density of states (pDOS) of our PEGS+DFT predicted structures is in close agreement with the experimental vibrational measurements. More broadly, our findings indicate that first-principles theoretical design of new amorphous materials for energy storage is now possible, paving a promising way for similar studies in the future.   

Influence of $xc$ functional on thermal-elastic properties of metal oxides: A DFT-based Debye-Gr\"{u}neisen model approach

For high-temperature applications, the chemical stability, as well as the mechanical integrity of the oxide material used is of utmost importance. Solving these problems demands a thorough and fundamental understanding of their thermal-elastic properties. In this work, we report density-functional theory (DFT) calculations to investigate the influence of the $xc$ functional on specific thermal-elastic properties of some common oxides CeO$_2$, Cu$_2$O, and MgO. Namely, we consider the local-density approximation (LDA), the generalized gradient approximation due to Perdew, Burke, and Ernzerhof (GGA-PBE), as well as a recently popularized hybrid functional due to Heyd-Scuseria-Ernzehof (HSE06). In addition, we will also report DFT+$U$ results where we introduce a Hubbard $U$ term to the Cu $3d$ and the Ce $4f$ states. Upon obtaining the DFT total energies, we then couple this to a volume-dependent Debye-Gr\"{u}neisen model [1] to determine the thermodynamic quantities of these oxides at arbitrary pressures and temperatures. We find an explicit description of the strong correlation (e.g. via the DFT+$U$ approach and using HSE06) is necessary to have a good agreement with experimental values. $[1]$ A. Otero-de-la-Roza, D. Abbasi-P\'{e}rez et al. Com. Phys. Com. 182 (2011) 2232   

Towards solar cells combining silicon nanopilars and various semiconductor nanoparticles

We have developped solar cells combining silicon nanopilars and various semiconductor nanoparticles-NPs- (silicon or CdSe ones obtained by chemical synthesis or silicon ones obtained by thermal annealing of multilayer structure Si-SiO(2)/SiN(x) fabricated by reactive magnetron sputtering.). Nanopilars are obtained by standard RIE process using silica spheres as active mask for the etching. NPs are chemically grafted or deposited by Langmuir Blodgett technique on the p-type pillars and therefore covered by a thin polysilicon n type layer. The doping level and of the different interfaces are monitoring by 3D Atom Probe Tomography. Photocurrents curve without and with NPs will be presented, evidencing the NPs active role.   

An Antireflective Top Contact for Optoelectric Devices Using Niobium Doped Titanium Dioxide

We present an investigation of an integrated top contact system for optoelectric semiconductor devices (e.g. solar cells, photo-detectors, etc.). Niobium-doped titanium dioxide (TNO) is used as a transparent conductor and a graded TiO$_{2}$/SiO$_{2}$ layer with a nanostructured SiO$_{2}$ surface is added to suppress reflection and improve durability. TNO was chosen to replace indium tin oxide (ITO), the industry standard transparent conductor, in an effort to reduce dependence on increasingly scarce resources (i.e. indium). TNO offers other potential benefits over ITO, including superior durability, higher index of refraction, and superior transparency in the infrared. The graded antireflective layer takes advantage of the common crystalline structure of TiO$_{2}$ and SiO$_{2}$, making fabrication simpler and more reliable than would be possible if the materials had differing structures. The nanostructured SiO$_{2}$ surface uses a ``moth's eye'' pattern of sub-wavelength-scale cones to create a gradual optical transition from ambient air to the SiO$_{2}$. All materials are deposited using RF magnetron sputtering. Results are compared to current standards and the strengths and weaknesses of the TNO system are discussed.   

Thermal Properties of Polarization Switching in Vertical-Cavity Surface-Emitting Lasers

This investigation studied the thermal properties of the polarization switching (PS) in vertical-cavity surface-emitting lasers (VCSELs). The studies were performed by experiments. In experiments, the current modulation frequency and ambient temperature of VCSELs were varied to study their thermal effects on PS. The hysteresis loop of polarization switching broadens as the modulation frequency increased and narrows as the modulation frequency decreased. We assumed that PS is activated as the temperature in the active region reaches a certain temperature. The PS of the VCSEL controlled by continuously varied ambient temperature is also studied. The experiments results show that the thermal effect plays a major role in PS and the hysteresis of PS. These results contribute to the understanding of the mechanism of VCSEL's polarization switching.   

Temperature dependence of the null hysteresis loop in polarization switching of vertical-cavity surface-emitting lasers

This research investigated the frequency-dependent polarization switching hysteresis loop (PSHL) of vertical-cavity surface-emitting lasers (VCSELs) in constant ambient temperature. The special feature of VCSELs is the polarization switching (PS). In most case, the switching current in the rise and decline bias are not the same, and shows hysteresis. The ambient temperature was controlled in this experiment. The hysteresis loop width of polarization switching disappears as the modulation frequency is lower than a critical frequency. Experimental results show that the critical frequency dependents on the ambient temperature. Experimental data presents that the thermal effect plays a major role in the hysteresis of PS. These results greatly contribute to the understanding of the VCSEL's PSHL.   

Linear and nonlinear properties in soft glass optical fibers for device applications

Optical fiber technology is predominantly based on silica glass fibers. Non-silica soft glass fibers exhibit substantially different optical properties such as higher refractive index, larger nonlinear coefficient and structural fabrication flexibility. We aim to exploit these novel properties for device applications such as sensing and light generation. We report measurement of linear dispersion and nonlinear coefficient in the range of 1.5 $\mu $m in two custom designed soft glass microstructure optical fibers. The fibers are composed of SF57 (Schott) and Bismuth-doped silica (Asahi Glass Co.) respectively with Hexagonal Wagonwheel microstructure design. These fibers are designed to allow phase matching of nonlinear optical processes near 1.6$\mu $m. Our measurements indicate nonlinear coefficients 1000 times that of standard silica fiber. Transverse modes in these fibers are difficult to separate leading to a complicated dispersion results. Next steps include observation of parametric generation and Brillouin gain.   

Observation of New Photoluminescence Emission in the YAG Structure and Its significant Effects on the Material Optical Properties

Yttrium Aluminum Garnets, Y$_{3}$Al$_{5}$O$_{12}$ (YAG) are widely used as host materials for transition and rare earth elements in laser, optoelectronic and scintillation applications. They are the most important solid state laser host materials and the most promising scintillation materials. New photoluminescence peaks were observed at 700 and 800 nm in undoped yttrium aluminum garnet (YAG) single crystals. This luminescence has great effects on the optical properties of rare-earth doped YAG crystals and their performance in laser and scintillation applications. Photoluminescence measurements revealed a number of luminescence peaks in all YAG crystals regardless of the growth conditions due to native defects and low-level impurities. The strong 700 and 800 nm emissions were attributed to low level of iron impurities as confirmed by Glow Discharge Mass Spectrometry analysis. This study reveals that iron is a native impurity in all YAG crystals that plays a significant role on modifying the optical and scintillation properties of this important class of photonic materials.   

Development of new semi-rigid coaxial cables for low temperature experiments

Coaxial cables with seamless outer electrical conductors are promising for low-noise readout from novel cryogenic devices such as superconducting radiation or particle detectors, operating below helium temperature. Low thermal conductance as well as small signal attenuation is essential in wiring with coaxial cables for array detectors consisting of hundreds of pixels. We developed thin semi-rigid coaxial cables employing normal alloys (CuNi, SUS and beryllium copper) and superconducting alloys (NbTi and Nb). Superconducting coaxial cables with outer diameters from 0.86 mm to 2.19 mm showed good superconducting transitions at 9~10 K, and flat attenuation properties up to above 5 GHz.   

Investigation of 1/f flux noise in superconducting circuits

Low-frequency 1/f flux noise is a dominant source of dephasing in the Josephson phase and flux qubits. Recent work has revealed the presence of a high density of unpaired spins at the surfaces of superconducting thin films; it is now believed that these spins are the source of the noise, although the microscopic noise mechanism is not understood. Here we describe experiments on SQUIDs and Josephson phase qubits designed to shed light on the underlying noise mechanism, and we describe efforts to develop novel materials with reduced levels of noise.   

Biomcompatible gold nanorods conjugated with photosensitizers assisted for photostability and photodestructive ability

Light-exposure-mediated higher temperatures that markedly accelerate the degradation of indocyanine green (ICG) in aqueous solutions by thermal decomposition have been a serious medical problem. In this work, we present the example of using gold nanorods (Au NRs) simultaneously serving as photodynamic and photothermal agents to destroy malignant cells. Au NRs were successfully conjugated with hydrophilic photosensitizer, indocyanine green (ICG), to achieve photodynamic therapy (PDT) and photothermal therapy (PTT). We also demonstrated that Au NRs conjugated with ICG displayed high chemical stability and acted as a promising diagnostic probe. Due to its stability even via higher temperatures mediated by laser irradiation, the combination of PDT and PTT proved to be efficiently killing cancer cells as compared to PDT or PTT treatment alone and enhanced the effectiveness of photodestruction and was demonstrated to enhance its photostability.   

Encapsulation of Protoporphyrin IX in PGLA for Biological Applications

In biological studies, Protoporphyrin IX has been utilized in researching with photodynamic therapy. With different situations that arise, it is beneficial to have products tailored to the given investigation. Here we propose encapsulating Protoporphyrin IX into poly(lactic-co-glycolic acid) (PLGA) with different parameters in hopes of obtaining products with necessary properties, including photoluminescent intensity, size and loading levels. We hypothesize the intensity of Protoporphyrin IX can be tuned, and this study will help to find the conditions for intensity change. We use chloroform and PVA solutions in the emulsion extraction/evaporation. The extraction and evaporation phases are different such that we can get particles sooner or later, respectively, which can lead to differences in loading levels. We vary diverse parameters, including volumes of the organic solvent, the aqueous solution, the drug volume (consisting of Protoporphryin IX in a solvent). The products PLGA-Protoporphyrin IX (Pppix) were obtained, characterized and optical properties were observed.   

Hydrothermal Solution-Processed Reduced Nano-Graphene Oxide as Blue Photoluminescence Quantum Dots

Chemical derived graphene oxide, an atomically thin sheet of graphite with 2-D construction, offers interesting electronic, chemical and mechanical properties that are currently being explored for advanced electronics, membranes and composites. Herein, we synthesize and explore the blue photoluminescence (PL) nano-graphene quantum dots (QD) through hydrothermal-solution-processed reduced graphene oxide. The PL investigation indicated that graphene oxide solution showed weak fluorescence. However, when the nano-graphene oxide solution samples were heated at different temperatures, from 200-300 $^{\circ}$, the blue PL intensity of the solution improved radically as heating temperature increased. We also investigated time dependence at a certain heating temperature and the PL Intensity and peak based on graphene QDs under different pH values by adding NaOH. The FT-IR measurements showed that the functional groups of the graphene oxide had been altered due to the hydrothermal routes. In addition, we also investigated the absorption spectrum of the graphene QDs under different conditions, XRD and XPS images of the graphene oxide, TEM and SEM images based on graphene QDs under different conditions.   

Nanocomposite for radiation detection

Cerium fluoride is among the widely studied inorganic scintillators for radiation detection, because of its high light output and high stopping power. Herein, platelets shape CeF$_{3}$ nanoparticles for radiation detection was synthesis by bisolvent solvothermal method. The characterization of nanoprticles was done by photoluminescence, XRD and SEM measurement. The synthesized CeF$_{3}$ nanoparticles have broad emission peak around 330 nm. ZnO is a semiconductor scintillator, having fast decay time. ZnO nanoparticles were synthesized using solvothermal method and UV-Vis, photoluminescence and SEM measurement were done for their characterization. The absorption spectrum of the ZnO nanoparticles is dependent on the size of the nanoparticles. By changing the temperature and the concentration of zinc salt and surfactant, ZnO with absorption spectra overlapping with the emission spectra of CeF$_{3 }$were made for the purpose of creating nanocomposites, with improved scintillation properties. The energy transfer between two nanoparticles was also studied and the changes in Photoluminescence intensity of the nanocomposites were described.   

Simulating slow strain-rate processes in disordered solids

A generic computational scheme is developed in this work to investigate extremely slow strain rate processes, including the experimentally accessible strain rates and even lower ones, for disordered materials systems. We use a self-learning metabaisn escape algorithm to capture the strain-rate and temperature dependent stress relaxation events for a binary Lennard-Jones alloy. It is found that the yield stress decreases when the strain rate decreases and when the temperature increases.   

Plasmon Resonance Energy of Nickel Manganese Films

Plasmonics studies how light interacts with conductors at the nanoscale level. Plasmons result from a collective oscillation of charges. Surface plasmonics has become of interest due to the ability to confine optical energy into subwavelength volumes, however the ability to control and manipulate them has yet to be developed. In the future, plasmonics can revolutionize optical devices with increased performance speed and functionality. To alter the properties of a surface plasmon wave, an active layer is grown beneath a metal surface. Since, part of the plasmon's electromagnetic field resides in this layer, the layer couples to the surface plasmons. Thus, each sample studied was composed of an amorphous nickel manganese film grown on top of silicon with a capping copper layer. Optical absorption measurements were made with these films using a Cary-OLIS spectrophotometer from 190 -- 450 nm. Two peaks in the absorption of each sample were found: one around 3.30 eV which is the surface plasmon peak as seen in previous literature and a bound electron peak at 4.45 eV. The full width half maximum of the surface plasmon peak is observed to increase when increasing the manganese concentration in each sample. A discussion as to possible reasons why will be presented.   

Electronic States of Nickel Effected By Magnetic Doping

Spin currents have a great potential to replace charge currents. This would revolutionize how we read/write information. The generation and switching of spin currents however must be well understood. Transport measurements suggest that magnetic impurities can alter the mean free path of carriers and thereby create spin currents. Angle-resolved photoemission is used to determine the change in the electronic states of Ni induced by doping with iron, chromium, and manganese near the Fermi Energy. The samples were single crystals of nickel(110) with variable amounts of dopant diffused into it. Alloy single crystals were used over epitaxial thin films due to the sharper features at the Fermi Energy that they produced. The mean free path, magnetic splitting, and carrier density are affected by a few percent of each of the dopants. Iron suppresses the mean free path of minority spins only, while chromium and manganese suppresses both spins and decreases the magnetic splitting. The strong variation of these affects from one impurity to the other supports the concept of tailoring spin transport by magnetic doping. [1] K. N. Altmann et al., Phys. Rev. Lett.~\textbf{87}, 137201 (2001) [2] K.R. Podolak, Ph.D. Thesis, Penn. State (2008)   

Fabrication of graphene device from graphite intercalation compound

The mechanical exfoliation of graphite is possibly the simplest and practical method in laboratories to obtain graphene flakes for scientific research. However efficiency for obtaining graphene, with desired layer-number and size, depends largely on crystal specific characters, eg., dislocations. To improve the issue, we have adopted graphite intercalation compound (GIC) instead of graphite for a starting material. Generally, GIC is chemically active. We used SbCl$_5$- GIC, which is stable in the atmosphere. Stage structure of SbCl$_5$-GIC could be tuned by temperature of intercalation. We found that considerable number of undoped graphene flakes coexisted with thin SbCl$_5$-GIC flakes, on a substrate where flakes were transferred.?Statistical inspection of number of graphene layer indicated that it is significantly dependent on the stage number of GIC.   

Graphene synthesis by calcination of ethylene-glycol intercalated layered silicates: x-ray and Raman scattering studies

Excellent electrical, mechanical and chemical properties of graphene make it fascinating for various applications including high-speed and high-performance electric devices. Although there have been many synthesis methods innovated to obtain graphene in the past, a simple and controllable synthesis method of graphene mass production is still being pursued for such applications. Here, layered silicates are chosen as a host material to intercalate ethylene glycol (EG). EG-intercalated layered silicates are then calcinated in air or vacuum to synthesize graphene between the silicate layers. Layered silicates used are taeniolite, fluorinated mica and vermiculite. Our Raman scattering and x-ray diffraction experiments suggest that graphene may be made between the silicate layers after calcination. In particular, second-order Raman peaks found in vermiculite samples indicate that vermiculite may be most suited to synthesize well-crystallized graphene among the layered silicates used.   

AFM and SThM Characterization of Graphene

We report on detailed characterization of epitaxial grown graphene on SiC and chemical vapor deposition grown graphene on Cu foil using atomic force microscopy (AFM) and scanning thermal microscopy (SThM). We focus on the electronic and thermal properties of graphene grain boundaries, and thus providing valuable feedback to materials growth. Specifically, we perform thermal conductivity contrast mapping and surface potential mapping of graphene, and compare with that obtained on the Au electrodes and the substrate.   

Wafer-scale synthesis of defect-negligible monolayer graphene at reduced temperature on hydrogen-rich evaporated (111) copper films

In contrast to commercially available copper foils, evaporated copper film on wafer scale supporting substrates holds great promise in chemical vapor deposition (CVD) of graphene for direct integration into device manufacturing processes. Monolayer graphene with negligible defects ($<$5{\%}) was synthesized on evaporated copper films at temperatures $<$ 900 $^{\circ}$C using hydrogen-free methane precursor that has not been previously reported. In this work, high-quality monolayer graphene obtained on evaporated copper film was likely enabled by the distinct properties of hydrogen-rich (111) preferred crystal orientation as indicated by X-ray diffraction (XRD) and electron back scattering diffraction (EBSD). The distinct difference in the crystal orientation of copper films versus foils resulted in dissimilar interplay with the precursor gas, as confirmed by time-of-flight secondary ion mass spectroscopy (TOF-SIMS). This study demonstrates experimental evidence for differences in the growth dynamics of CVD graphene on copper film versus conventional foils.   

Anisotropic Hydrogen Etching of Chemical Vapor Deposited Graphene

In terms of the preparation of graphene, chemical vapor deposition (CVD) has raised its popularity as a scalable and cost effective approach for graphene synthesis. While the formation of graphene on copper foil has been intensively studied, the reverse reaction of graphene reacts with hydrogen has not been systematically studied. In this talk we will present a simple, clean, and highly anisotropic hydrogen etching method for CVD graphene catalyzed by the copper substrate. By exposing CVD graphene on copper foil to hydrogen flow around 800 $^{\circ}$C, we observed that the initially continuous graphene can be etched to have many hexagonal openings. In addition, we found that the etching is temperature dependent and the etching of graphene at 800 ${\circ}$C is most efficient and anisotropic. 80{\%} of the angles of graphene edges after etching are 120$^{\circ}$, indicating the etching is highly anisotropic. No increase of D band along the etched edges indicates that the crystallographic orientation of etching is zigzag direction. Furthermore, we observed that copper played an important role in catalyzing the etching reaction, as no etching was observed for graphene transferred to Si/SiO$_{2}$ under similar conditions. This highly anisotropic hydrogen etching technology may work as a simple and convenient way to determine graphene crystal orientation and grain size, and may enable the etching of graphene into nanoribbons for electronic applications.   

Density functional theory calculations on the effects of oxygen adsorption on graphene in the presence of defects

Density functional theory calculations were carried out to investigate effects of oxygen adsorption on the electronic and optical properties of single layer graphene (SLG) in the presence of defects. Stone-Wales and single and double vacancy defects, also including the 555-777 and 5555-6-7777, were considered. Analysis of changes upon oxygen adsorption will be reported in detail, regarding morphology, binding energies and electronic structure, as compared to pristine SLGs, for pure GGA, hybrid and van der Waals corrected functionals. Nudged elastic band calculations of various pathways were investigated in order to understand the diffusion and dissociation of oxygen on SLGs in the presence of defects. Calculation of the optical response carried out at different levels of theory will be discussed, using both hybrid and long-range corrected hybrid functionals, with consideration of comparison to experimental characterization. Finally, implications of the effects of oxygen adsorption on graphene in the presence of defects for aspects of catalyst-free growth or electron transport will be suggested.   

Using the G' Raman cross-section to understand the phonon dynamics in bilayer graphene systems

The integrated area (IA) of the four peaks (P$_{11}$, P$_{12}$, P$_{21}$ and P$_{22}$) of the G$^{\prime}$ Raman band of AB stacked bilayer graphene are analyzed as a function of the laser power for different laser lines. We show that the IA of each peak depends on temperature and also depends on the laser excitation energy. This special dependence is explained in terms of the electron-phonon coupling and the relaxation time of the photon-excited electron. Due to the short relaxation time of the photo-excited electron by emitting phonons, the relative intensities of the four peaks are determined by a different combination of relaxation processes that give rise to some G$^{\prime}$ peaks increasing in IA at the expense of others, thereby making the IA of the peaks different from each other and dependent on laser excitation energy and power level. Also, we report an anomalous behavior of the G$^{\prime}$ IA for the 532\,nm laser energy, that shows a resonance regime in which a saturation of what we call the P$_{12}$ process occurs. This effect is a relevant phenomenon that gives important information about the electron and phonon dynamics and needs to be taken into account for certain applications of bilayer graphene in the field of nanotechnology.   

Adhesive interaction between graphene membranes and amorphous substrates

To integrate graphene in functional devices, it is essential to understand interfacial adhesion between graphene and surrounding materials for mechanical support and encapsulation. In complement with recent efforts aiming to measure the adhesion energy experimentally, we present a theoretical model for adhesive interaction between a graphene monolayer and an amorphous substrate. The model is extended to analyze the morphological stability of a graphene membrane on an oxide substrate. It is found that the bending modulus, which increases drastically from monolayer to multilayered graphene, plays an important role in the transition from conformal to non-conformal morphology of the graphene membranes on a corrugated surface. Furthermore, the work of adhesion is predicted to drop considerably from monolayer to bilayer graphene, in good agreement with recent measurements. The theoretical results suggest that tunable adhesion of graphene can be achieved by controlling the surface roughness of the substrate.   

Structural Properties of Epitaxial Graphenes on 4H-SiC(0001): A Theoretical Study

The structural properties of the epitaxial graphene including the buffer layer on 4H-SiC(0001) are studied using the recently developed SCED-LCAO molecular dynamics scheme [PRB 74, 15540 (2006)]. Both Si-terminated and C-terminated SiC substrates are considered. In a previous work [BAPS, Vol. 56, 463 (2011)], we had shown that 10x10 monolayers of carbon on 8x8 4H-SiC(0001) substrate as well as 13x13 monolayers of carbon on 6$\surd $3x6$\surd $3 4H-SiC(0001) substrate are minimum in strain between the buffer layer and the substrate and that they are nearly commensurate. In this work, we have added an epitaxial layer of graphene to the above geometries and both C-terminated and Si-terminated SiC surfaces will be considered. We will highlight the roles played by the buffer layer, the epitaxial layer, and the substrate on the structural properties of this system. Analysis of the structure in terms of local bonding arrangements and charge distributions will also be provided.   

Intercalation of metal islands and films at the interface of epitaxially grown graphene and Ru(0001) surfaces

Epitaxial graphene on Ru(0001) provides a high quality adlayer, but has a strong interaction with the substrate. This interaction can be effectively weakened via intercalating a layer of other element between epitaxial graphene and its substrate. In this work, we intercalated seven kinds of metals including noble metals Pt, Pd and Au, magnetic metals Ni and Co, a IIIA group metal In and a rare earth metal Ce, at the interface of epitaxially grown graphene and Ru(0001), to show the universality of metal intercalations in this system. These metals form different intercalated structures that have different impacts on the corrugation of graphene. Atomic resolution images of a perfect graphene lattice on the intercalated structures can always be obtained, which confirms that the intercalation of these metals is a non-damaging process. Additionally, based on primary DFT calculations, we propose a model involving metal-atom-aided-defect and self-healing of C-C bonds at high temperature, which can well explain the intercalation process of some metals and provide the potential to better understand the intercalation mechanism.   

Superconductivity in Graphene-Lithium Systems

We present first-principles calculations on systems consisting of a few layers of graphene and lithium. In particular, we investigate the evolution of the electron-phonon coupling strength with an increasing number of layers. We find that for intercalated systems such as C$_6$-Li-C$_6$ or C$_6$-Li-C$_6$-Li-C$_6$ the electron-phonon coupling is weak. However, for systems of equal number of graphene and lithium layers, such as C$_6$-Li or C$_6$-Li-C$_6$-Li, the electron-phonon coupling is strong. We investigate the optimal configuration that yields the highest superconducting transition temperature.   

Impermeable ``single-monolayer'' Graphenic encasement of bacteria for high vacuum Transmission electron microscopy

Biological cells are hygroscopic, permeable, and electron-absorbing, and imaging them \textit{via} electron microscopes has been an important challenge due to the volumetric-shrinkage and structural degradation of cells under high vacuum and fixed electron beam. In this talk, we will show that ``single-monolayer'' graphenic encasement of individual whole ``wet'' bacterial cells can enable wet-phase TEM imaging by preserving their dimensional, topological characteristics and cellular water under high vacuum (10$^{-5}$ Torr) and beam current (150 A/cm$^{2})$. Ultrathin and impermeable ``single monolayer'' graphene microsheet was wrapped around or laid on bacteria. The combination of strongly-packed honeycomb-lattice, high Young's modulus, high electrical and thermal conductivity, and mesoscale flexibility of the single graphene monolayer reduced the permeability of cells under TEM conditions, significantly abated electron beam damage and cell-delamination from substrate.   

$\eta ^{6}$ Chemical Modification of Epitaxial Graphene: An Avenue for Non Destructive Surface Functionalization and Atomic Layer Deposition

Graphene's superior properties are expected to develop next generation electronic applications. However, a major challenge which still remains is to incorporate it into different systems via its functionalization. Moreover, such functionalization must not alter graphene's superior properties. In most cases, functionalization leads to conversion of the planar sp$^{2}$ hybridized state of carbon into the tetrahedral sp$^{3}$ states. This conversion leads to an increase in carrier-scattering and a reduction in carrier-density. In this talk, we demonstrate a novel $\eta ^{6}$ functionalization route, where the d-orbital of a transition metal binds with the pi-cloud at the center of graphene's aromatic rings resulting in a unique grafting of a monolayer of metal atoms. As a result, the sp$^{2}$ state of the carbon atoms is preserved and its superior properties (high carrier density and low carrier scattering) are maintained. We envision that this functionalization route will allow graphene to be interfaced with several systems, thus significantly broadening the scope of its applications.   

A three-dimensional architecture of vertically aligned multilayer graphene facilitates heat dissipation across joint solid surfaces

Low operation temperature and efficient heat dissipation are important for device life and speed in current electronic and photonic technologies. Being ultra-high thermally conductive, graphene is a promising material candidate for heat dissipation improvement in devices. In the application, graphene is expected to be vertically stacked between contact solid surfaces in order to facilitate efficient heat dissipation and reduced interfacial thermal resistance across contact solid surfaces. However, as an ultra-thin membrane-like material, graphene is susceptible to Van der Waals forces and usually tends to be recumbent on substrates. Thereby, direct growth of vertically aligned free-standing graphene on solid substrates in large scale is difficult and rarely available in current studies, bringing significant barriers in graphene's application as thermal conductive media between joint solid surfaces. In this work, a three-dimensional vertically aligned multi-layer graphene architecture is constructed between contacted Silicon/Silicon surfaces with pure Indium as a metallic medium. Significantly higher equivalent thermal conductivity and lower contact thermal resistance of vertically aligned multilayer graphene are obtained, compared with those of their recumbent counterpart. This finding provides knowledge of vertically aligned graphene architectures, which may not only facilitate current demanding thermal management but also promote graphene's widespread applications such as electrodes for energy storage devices, polymeric anisotropic conductive adhesives, etc.   

Ballistic Transport in Graphene pnp Junctions Formed by Embedded Local Gates

Due to its gapless energy spectrum, one enables to tune the carrier type and density in graphene and realize pnp-type potential barriers in situ by using electrostatic gating. Such potential barriers provide opportunities to investigate novel phenomena such as Klein tunneling and quantum-Hall edge-state equilibration. In this study, we obtained high-quality graphene pnp junctions by embedding pre-patterned local gates in a substrate without dielectric-layer deposition or electron-beam exposure of the graphene sheet. We achieved ballistic and phase-coherent carrier transport in a graphene pnp device with a 130-nm-wide local gate, which is almost an order magnitude wider than reported previously[1]. In a high magnetic field, device with a 1-micro meter-wide local gate exhibited the $2e^{2}/h$ quantum-Hall plateau, indicating no backscattering in the local gate region. The conductance across our pnp junctions shows a gate-voltage dependence that is very distinctive from that of top-gated junctions, indicating a strong screening of the electric field by the embedded local gate. [1] A. F. Young and P. Kim, Nature Physics $\textbf{5}$, 222 (2009)   

Carbon Nucleation from ring seeds on Icosahedral Fe$_{13}$

Using ab initio molecular dynamics calculation, we computed the initial stages of carbon nanotube growth from ten -- atom carbon rings on icosahedral Fe$_{13}$. The carbon ring interacts with Fe$_{13}$ to form a zigzag structure due to a mismatch between the number of atoms on the ring and the atoms on the pentagonal ring of Fe$_{13}$, with formation energy of 2.2 ev. A mixture of carbon dimers and carbon atoms interact with the Fe$_{13}$ + C$_{10}$ structure to yield a tubular structure with the lowest energy. Another configuration of carbon atoms distributed in the vicinity hexagonal lattice sites interacts with Fe$_{13}$ + C$_{10}$ structure to form also a tubular structure that is almost degenerate in energy to the ground state. The results indicate that the icosahedral symmetry of the Fe$_{13}$ with its magnetic moment almost intact is retained whenever the tubular structure is formed.   

Anomalous dependence of band gaps of binary nanotubes on diameters

Using cluster approximation, AlN, BN, GaN, SiGe, SiC, and GeC armchair type 1 nanotubes have been spin optimized using the hybrid functional B3LYP, a double $\zeta $ basis set and the GAUSSIAN 03 software. The electronic structures of group III nitride and group IV-IV nanotubes indicate that the band gap increases with tube diameter contrary to behavior expected from quantum size effects. A detailed study indicates that, in a class of binary nanotubes with partial ionic contributions in the bonds, for example, AlN, BN, GaN, GeC, and SiC, ionicity of the bonds decreases as diameter decreases due to increased sp$^{3}$ contribution. This causes the band gap to increase with diameter. But in nanotubes with covalent bonding, for example SiGe, the gap decreases with diameter. A general trend for a class of binary nanotubes is established.   

Ultra high Transparent and Conductive Electrodes Based on As-Grown SWNT with Metallic Conductivity

Carbon based materials have been proven to be a unique material for transparent conducting films, with potential for application on liquid crystal displays, touch screens and solar cells. We successfully grew SWNT films by Chemical Vapor Deposition method using Fe nanocatalysts on quartz substrates. The ratio of semiconductor/metallic nanotubes varied depending on the treatment conditions of the catalyst nanoparticles, according to Raman analysis. SEM analysis of the samples revealed homogeneous coverage of the quartz substrates by SWNTs, which exhibit transparencies higher than 98{\%}. Sheet resistance measurements of these SWNT films, by Van der Pauw method, demonstrated the correlation between the conductivity and the abundance of semiconductor and metallic nanotubes in the films. Increasing the content of metallic SWNTs in the film up to 90{\%} decreased the sheet resistance down to 4-5 K$\Omega $/, while maintaining a high transparency of over 98{\%}. For comparison, transparent electrodes based on high quality monolayer graphene sheets were also fabricated. The conductivity and transparency of the electrodes of as grown SWNTs were comparable to the electrodes based on monolayer graphene.   

Determination of the vibration frequencies of Carbon Nanotubes and Carbon Nanoribbons

The optical properties of organic materials are similar to the properties of the molecules that constitute it. This motivates the study of molecular complexes that would form new materials. Interesting optical properties are observed in conjugated polymers due to delocalized $\pi $ electrons, which allow transitions in the visible spectrum. In this work we present results of the vibration spectrum for pristine and functionalized carbon nanoclusters. The electronic properties are obtained by first principles calculations, based on the Becke-Perdew GGA approximation. The frequencies are computed numerically by differentiation of the energy gradients in slightly displaced geometries. The three stages of calculations will be performed using the ADF code. The geometric optimization protocols for pristine and functionalized structures are made adopting the ``quasi Newton approach.'' The vibration spectra show interesting properties when the structures are functionalized with organic molecules.   

High surface area electrode for high efficient microbial electrosynthesis

Microbial electrosynthesis, a process in which microorganisms directly accept electrons from an electrode to convert carbon dioxide and water into multi carbon organic compounds, affords a novel route for the generation of valuable products from electricity or even wastewater. The surface area of the electrode is critical for high production. A biocompatible, highly conductive, three-dimensional cathode was fabricated from a carbon nanotube textile composite to support the microorganism to produce acetate from carbon dioxide. The high surface area and macroscale porous structure of the intertwined CNT coated textile ?bers provides easy microbe access. The production of acetate using this cathode is 5 fold larger than that using a planar graphite electrode with the same volume. Nickel-nanowire-modified carbon electrodes, fabricated by microwave welding, increased the surface area greatly, were able to absorb more bacteria and showed a 1.5 fold increase in performance   

In-Air Growth of Carbon Nanoflowers

A new class of carbon nanostructures was fabricated under rapid heating in air. Solid carbon sources and metal catalysts were used to facilitate growth. Their microstructures were characterized using scanning electron microscopy and micro Raman spectroscopy. The results indicate patches of graphene and carbon tapered tubules. Two methods of fabrication are presented.   

A Molecular Dynamics Study on the Confinement of Carbon Dioxide Molecules in Carbon Nanotubes

The influence of atmospheric carbon dioxide (CO$_{2})$ concentration on global warming is considered as one of the primary environmental issues of the past two decades. The main source of CO$_{2}$ emission is human activity, such as the use of fossil fuels in transportation and industrial plants. Following the release of Kyoto Protocol in 1997, effective ways of controlling CO$_{2}$ emissions received much attention. As a result, various materials such as activated carbon, zeolites, and carbon nanotubes (CNTs) were investigated for their CO$_{2}$ adsorbing properties. CNTs were reported to have CO$_{2}$ adsorption capability twice that of activated carbon, hence they received the most attention. In the current study, single walled carbon nanotubes (SWNTs) were used as one dimensional nanoporous materials and their CO$_{2}$ adsorption capacity was analyzed with Molecular Dynamics simulations. Results indicated that SWNTs are excellent CO$_{2}$ adsorbers and their effectiveness increase at low CO$_{2}$ concentrations. In addition, we showed that by varying temperature, CO$_{2}$ can be removed from the SWNTs, providing a simple method to reuse SWNTs.   

A Molecular Dynamics Study on the Exfoliation of Single Walled Carbon Nanotubes in Supercritical Carbon Dioxide

Carbon nanotubes (CNTs) could be used in various technological applications due to their structural, electrical and mechanical properties. However, in order to get the theoretically predicted benefits, nanotubes have to be succesfully dispersed in the polymer matrix. Supercritical fluids were previously shown to result in good dispersion of nanofillers (in the case of clays and spherical nanofillers). In the current study, we investigated the potential use of supercritical carbon dioxide to unbundle single walled carbon nanotubes via molecular dynamics simulations. Various carbon nanotube systems were simulated with XenoView simulation software, and the effect of surface modification of nanotube with CO$_{2}$-philic chemicals was investigted on nanotube dispersion. Results showed that surface modification of nanotubes improves their dispersion in supercritical carbon dioxide.   

Interplay of Wetting and Elasticity in the Nucleation of Carbon Nanotubes

Controlling the structure of single-walled carbon nanotubes during synthesis is one of the outstanding challenges of nanoscale science. Some degree of control has been demonstrated through catalytic synthesis, where the interaction with the catalyst is believed to play a key role, but the exact mechanisms leading to preferential growth remain unclear. Here we report on the development of a model that focuses on the lift-off of carbon nanotube caps after nucleation. We test the model using atomistic molecular dynamics simulations and discuss the implications of the model for understanding growth processes that may control chirality. We illustrate the role of the competition between cap strain energy and adhesive forces in the lifting of these caps from the catalyst surface prior to elongation. We show that, given a particular cap structure, there is a lower bound on the catalyst size from which the cap can lift. This lower bound explains the mismatch between nanotube and catalyst diameters observed in experiment. These findings offer new insight into the nucleation of single-walled carbon nanotubes, and they many lead to the design of catalysts that can better control nanotube structure.   

Electronic properties of carbon nanodisks and nanocones

The electronic properties of graphene nanodisks and nanocones are calculated following a tight-binding approach. The total density of states of both structures are found to be quite similar when a large number of atoms is considered, including the peak at the Fermi energy associated with the contribution of the border atoms. We have also calculated, for both systems, the local density of states which highlight the differences of the electronic structures due to the particularities of the spatial symmetry of the nanodisks and nanocones. Moreover, the electronic properties are throroughly discussed via the analysis of the joint density of states and absorption spectra of the nanostructures.   

Synthesis and field emission properties of periodic arrays of vertically aligned carbon nanotubes on copper

Periodic arrays of carbon nanotubes (CNTs) with different densities were synthesized on copper substrate by employing nanosphere lithography (NSL) and plasma enhanced chemical vapor deposition. Vertically aligned CNTs were formed using Ni as catalyst at a growth pressure of 8 torr of C$_{2}$H$_{2}$/NH$_{3}$ mixture and a temperature of 520\r{ }C. Electron emissions of the CNTs with different densities were investigated to reveal the dependence of the electron emission properties of the CNTs on their densities. Experimental results showed that low-density CNTs exhibited better field emission properties as compared to the high-density CNTs. Low-density CNTs exhibited lower turn-on and threshold electric fields, and a higher field enhancement factor. The high density of CNTs resulted in the deterioration of the FE properties due to the screening of the electric field by the adjoining CNTs.   

Fabrication of Micro-Electromagnetic Devices for the Manipulation of Carbon Nanotubes

With advances in microscopy techniques, such as transmission electron and scanning transmission electron microscopy, it has become increasingly more interesting to study and manipulate the structures of solid state materials at the atomic scale. One of the issues with the study of these properties is that these advanced microscopes require that the sample be confined to a space not very conducive to traditional methods of actuation, such as atomic force microscopy and electrostatic methods. In our research, we are primarily focused on building tiny ``electro-magneto-mechanical'' devices on carbon nanotubes in order to manipulate them without breaking vacuum or removing them from the microscope. Our current project is focused on putting small, magnetizable paddles on to the carbon nanotubes, using an SEM for electron beam lithography and a metal evaporation deposition system, then running current to induce a magnetic field with which the paddles, which can have any magnetic dipole we want, will align. In order to create these devices we must first choose a type of material and a method for creating the magnetic paddles, in our work we use iron. The main problem with iron is the rate at which it oxidizes at the nanoscale, it is practically instantaneous. Because of this we have developed a method, using what basically amounts to simple geometry, for encasing our iron paddles with gold and thus preventing the oxidation. It is our hope that this research opens the doors for many new opportunities in nanoscale materials science, and at the least greatly reduce the time required for many current experiments.   

2D Fluidization of Nanomaterials by Biomimetic Membranes

The last decade has seen much progress in the synthesis and manufacturing of a large variety of nanometer sized particles of different materials, geometries and properties. If they can be assembled into larger structures, these manmade nano-objects are posed to be the ``atoms'' and ``molecules'' of new materials. In order to facilitate their dynamic rearrangements we have developed a method that uses material specific binding peptides to anchor nano-objects to lipids in supported bilayers (SLB). In this study we use single walled carbon nanotubes (CNT) with a mean length of 1 micrometer as model of a potential nano-building block. By fluorescently labeling CNTs we are able to use video-microscopy to investigate the dynamic behavior of membrane anchored CNTs. We show that the 2D fluidity of the lipid membrane can be successfully templated on the CNTs and that they stay laterally mobile while being confined to a plane. Furthermore, the dependence of CNT mobility on specific binding stoichiometries is discussed.   

SWNT Imaging Using Multispectral Image Processing

A flexible optical system was developed to image carbon single-wall nanotube (SWNT) photoluminescence using the multispectral capabilities of a typical CCD camcorder. The built in Bayer filter of the CCD camera was utilized, using OpenCV C++ libraries for image processing, to decompose the image generated in a high magnification epifluorescence microscope setup into three pseudo-color channels. By carefully calibrating the filter beforehand, it was possible to extract spectral data from these channels, and effectively isolate the SWNT signals from the background.   

Absorption of Carbon Monoxide Molecules On Carbon Nanotubes

Carbon nanotubes (CNTs) have been demonstrated to be promising nanoscale molecular sensors for detecting gas molecules such as NH3, NO2 and O2. But pristine CNTs show limitations in detection of highly toxic gases such as carbon monoxide (CO). In our work, interaction of CO molecules with both armchair and zigzag single-walled CNTs (SWCNTs) has been investigated through Density Functional Theory (DFT) simulations using the software VASP. SWCNTs with structural deformation, Stones-Wales (SW) defects or single vacancy (SV) defects are considered. It has been found that structural deformation, resulting in the significant change of local physical properties, enables strong chemical absorption of CO molecules on the surface of CNTs. In addition, a SV defect also brings about dramatic change in the local geometrical structure and higher chemical reactivity, facilitating strong binding of one and two CO molecules on CNTs. Compared to pristine and SW-defect CNTs, deformed and SV-defect CNTs are more promising as gas sensors to detect toxic carbon monoxide molecules.   

Van der Waals Interactions of Organic Molecules on Semiconductor and Metal Surfaces: a Comparative Study

We present a comparative investigation of vdW interactions of the organic molecules on semiconductor and metal surfaces using the DFT method implemented with vdW-DF. For styrene/H-Si(100), the vdW interactions reverse the effective intermolecular interaction from repulsive to attractive, ensuring preferred growth of long wires as observed experimentally. We further propose that an external $E$ field and the selective creation of Si dangling bonds can drastically improve the ordered arrangement of the molecular nanowires [1]. For BDA/Au(111), the vdW interactions not only dramatically enhances the adsorption energies, but also significantly changes the molecular configurations. In the azobenzene/Ag(111) system, vdW-DF produces superior predictions for the adsorption energy than those obtained with other vdW corrected DFT approaches, providing evidence for the applicability of the vdW-DF method [2].   

Analyzing the frequency shift of physi-adsorbed CO2 in metal organic framework materials

Combining first-principles density functional theory simulations with IR and Raman experiments, we determine the frequency shift of vibrational modes of CO$_2$ when physi-adsorbed in the iso-structural metal organic framework materials Mg-MOF74 and Zn-MOF74. Surprisingly, we find that the resulting change in shift is rather different for these two systems and we elucidate possible reasons. We explicitly consider three factors responsible for the frequency shift through physi-absorption, namely (i) the change in the molecule length, (ii) the asymmetric distortion of the CO$_2$ molecule, and (iii) the direct influence of the metal center. The influence of each factor is evaluated separately through different geometry considerations, providing a fundamental understanding of the frequency shifts observed experimentally.   

Zinc in +III oxidation state

The possibility of Group 12 elements, such as Zn, Cd, and Hg existing in an oxidation state of +III or higher has fascinated chemists for decades. Significant efforts have been made in the past to achieve higher oxidation states for the heavier congener mercury (since the 3$^{rd}$ ionization potential of the elements decrease as we go down the periodic table). It took nearly 20 years before experiment could confirm the theoretical prediction that Hg indeed can exist in an oxidation state of +IV. While this unusual property of Hg is attributed to the relativistic effects, Zn being much lighter than Hg has not been expected to have an oxidation state higher than +II. Using density functional theory we show that an oxidation state of +III for Zn can be realized by choosing specific ligands with large electron affinities i.e. superhalogens. We demonstrate this by a systematic study of the interaction of Zn with F, BO$_{2}$, and AuF$_{6}$ ligands whose electron affinities are progressively higher, namely, 3.4 eV, 4.4 eV, and 8.4 eV, respectively. Discovery of higher oxidation states of elements can help in the formulation of new reactions and hence in the development of new chemistry.   

A multi-scale model using single ion impacts predicts large-scale pattern formation in irradiated solids

Energetic particle irradiation of solids can lead to highly regular nanoscale pattern formation under the right environmental conditions. The widespread use of this technology in current industrial settings for doping and hardening suggests the promise of harnessing spontaneous pattern formation to create novel devices. However, existing theoretical models of ion irradiation fail to agree broadly with experiment even in the linear regime of small-amplitude patterns. A central challenge in modeling this system is to rigorously connect the pico-second effect of single ion impacts, spanning mere nanometers of materal, to the long-time evolution of surface structures spanning tens to hundreds of nanometers. Here, we present a multi-scale framework that achieves this end. We start with molecular dynamics simulations of single ion impacts, and upscale this data into a continuum PDE for the surface topography. Stability analysis of the PDE predicts the characteristic pattern wavelength as the incidence angle is varied. The predictions agree remarkably well with experiment, and overturn the long-held assumption that pattern formation is due to the removal of target atoms by sputter erosion.   

Multiscale atomistic and coarse-grained-particle analysis of fracture of graphene sheet

The problems related to material strength such as crack propagation, dislocation motion, and response to nano-indentation involve multiscale phenomena which atomistic or non-linear events occur at a certain confined region and elastic or linear deformation takes place at surrounding wide-spread region. In order to tackle these multiscale problems, we have developed the hybrid molecular-dynamics/coarse-grained-particle (MD-CGP) method which eliminates the artificial effects often occurred at the interface of atomistic and coarse regions by using extra atoms and particles at the interface and applying the Langevin thermostat for the extra atoms and particles. The method allows us to perform multiscale simulations of any materials such as metals, semiconductors, and insulators and of dynamic phenomena at finite-temperatures. We have performed the multiscale simulation of fracture of defect-free graphene membrane by nano-indentation and clarified that decrease of the fracture strength with respect to temperature is almost same as that by existence of a vacancy. We will discuss detail about the computational efficiency of this method and simulation results.   

Interface magnetism in LSMO-BiFeO$_{3}$ heterostructures

The interplay of the magnetic and ferroelectric order parameter is currently a topic of intense research. This magnetoelectric coupling can potentially be enhanced at the interface of a FM-FE heterostructure and, therefore, study of such prototype interfaces could provide fundamental insight to design of multiferroic materials. In this work interface magnetism investigations of multiferroic BFO (3-5nm) with a high-spin polarization ferromagnet (LSMO, 10nm) is presented. We have performed detailed resonant reflectivity studies at the Mn (for LSMO) and Fe (for BFO) L$_{3}$ edges in LSMO/BFO as a function of temperature. Analysis of the data using rigorous scattering theory employing the tensor nature of the refractive index has elucidated the charge and magnetic density depth profiles along the depth of the film. Investigations at room temperature do not show magnetism at the Fe-edge which imply that the antiferromagnetic order in BFO persists both in bulk and interface. A comprehensive interface characterization as a function of temperature will be presented.   

Designing Smart Surfaces to Segregate Cells

Our aim is to utilize smart surfaces to separate specific populations of cells from a heterogeneous sample. There are a number of diseases (e.g., malaria and various cancers) that alter the elasticity of biological cells. In this work, we use the mechanical stiffness of the cells as a key parameter since it can reveal the presence of disease. By integrating mesoscale models for hydrodynamics of surrounding fluids and for micromechanics of cells, and the Hierarchical Bell Model for specific cell-substrate interactions, we examine the fluid-driven motion of cells in a microchannel over a hard, weakly adhesive surface that contains ``sticky'' diagonal stripes. We show that as cells roll along the surface, they obtain a net displacement perpendicular to the direction of the fluid flow. This displacement is a function of the cell's stiffness, meaning that two cells with different compliances or adhesive properties exhibit different amounts of displacement, and in this manner are effectively sorted. We compare the results of the simulations with the data obtained in the experiments with HL-60 cells.   

Biocompatibility of Carbon Nanotubes in Mammalian Cells: An Imaging Based Approach

Carbon nanotubes have been widely researched for ultrasensitive biomolecule detection and drug delivery. However, its impact on cells is yet to be fully characterized, mainly due to the complex biological in vivo environment. We report here a mammalian cell-imaging paradigm to study the cellular response to single-walled carbon nanotubes (SWNTs) at the single-cell level. Chinese Hamster Ovarian cells were exposed to SWNTs resuspended in phosphate buffered saline (PBS) at various concentrations. Upon exposure, we optically imaged the cells (1) to visually quantify the SWNTs' crossing of the cell membrane in real-time; (2) to both qualitatively and quantitatively assess the morphological changes associated with cellular stress in the presence of SWNTs; and (3) to serially quantify cell survival with highly sensitive bioluminescence-based imaging. Consistent with literature reports, high concentration of SWNTs acutely compromised cell division and decreased cell survival. Low concentrations were well tolerated by the cells initially but had similar effects after prolonged exposure. We discuss the inter-relationships among the cell morphology, viability, and intracellular SWNT uptake parameters, as a function of nanotube concentration and exposure time.   

Radial Distribution Function of Silicene

Silicene is the counterpart of graphene and its potential applications as a part of the current electronics, based in silicon, make it a very important system to study. We perform molecular dynamics simulations and analyze the structure of two and three dimensional arrays of Si atoms by means of the radial distribution function at diferent temperatures and densities. As a first approach, for the 2D case, the Lennard-Jones potential is used and two sets of parameters are tested. We found that the radial distribution function does not change with the parameters and resembles the corresponding to the (111) surface of the FCC structure, which is similar to that of the honeycomb lattice although with different peak heights. The liquid phase appears at very high temperatures, suggesting a very stable system in the solid phase. In 3D, a comparison with potentials developed specifically for silicon, as suggested by Stillinger and Weber[F. H. Stillinger and T. A. Weber, Phys. Rev. B 31, 5262 (1985)] will be presented.   

Electronic and transport properties of metallic hexaboride nanorods

In this work, we performed electronic structure calculations of quasi one-dimensional metallic hexaboride XB$_{6}$ nanorods, where X are mostly rare-earth metals with 4$f$ levels such as La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu, which usually regarded as good thermoelectric materials because of their low work functions and transport properties. Our purpose is to facilitate the research and manufacture of metal boride probes, thus we study extensively the size-dependence and element-specificity of the electronic properties. In these nanorods, we uncovered few general features that elucidate their excellent thermionic and field emission property. To further investigate the transport properties, we adopted the combined Landauer-Buttiker formalism and non-equilibrium Green's function technique to compute the transmission coefficients near the Fermi level and found that hexaboride nanorods can be converted from metallic to semiconducting by applying a gate voltage larger than 10 V.   

Electronic transport in aperiodic nanosheets, nanotubes and nanowires

Based on the Kubo-Greenwood formula, we study the electronic transport in cubically-structured quasiperiodic nanosheets, nanotubes and nanowires, where the nanotubes are made by connecting one of the boundaries of a square-lattice sheet and the nanowires have a cross section of 90$\times $90 atoms. Calculations of electrical conductivity were performed by using a tight-binding Hamiltonian and by combining the convolution theorem with the real-space renormalization method [1]. The \textit{dc} conductance shows quantized spectra, where nanotubes have steps with a double height in comparison with those of a nanosheet, both in contrast to an inhomogeneous step structure derived from nanowires. The \textit{ac} conductivity shows a Drude and an oscillating behavior, when the electric field is along a periodic or quasiperiodic direction, respectively. Finally, the theoretical results are compared with experimental data. \\[4pt] [1] V. Sanchez and C. Wang, \textit{Phys. Rev. B} \textbf{70}, 144207 (2004).   

Trainable solar cell effect in TPPS4 nanowires.

We have measured photovoltaic currents for self-assembled \textit{meso-}tetra(4-sulfonato-phenyl)porphine (TPPS4) nanowires. This is surprising, given that our devices are symmetrical. However, by ``training'' the nanowires with a bias voltage, we can break this symmetry. After training, when the nanowires are illuminated with a 488 nm laser under zero bias voltage, a solar cell current is observed, with polarity opposite to the training voltage. The magnitude of solar cell current depends on both the time of training and the magnitude of the training voltage. After the training voltage is removed, the solar cell current decays very quickly initially and then transitions to a much slower decay. This suggests that there are different populations of trapped charges that are repositioned by the electric field of the training voltage, and that are responsible for the solar cell current. We present a qualitative model in which the Schottky barriers at the interfaces with electrodes are affected by these trapped charges, leading to asymmetrical charge transport and thus to the observed current.   

Structural Properties and Stability of Double Walled Armchair Silicon Nanotubes

A systematic study of armchair double-walled Si nanotubes (DWNT) (n,n)@(m,m) (3 $\le $ n $\le $ 6 ; 7 $\le $ m $\le $ 12) using the finite cluster approximation is presented. The geometries of the tubes have been spin optimized with an all electron 3-21G* basis set and the B3LYP functional. The study indicates that the stabilities of the double-walled Si nanotubes are of the same order as those of single-walled Si nanotubes suggesting the possibilities of experimental synthesis of both single-walled and double-walled Si nanotubes. The binding energy per atom or the cohesive energy of the double-walled nanotubes depends not only on the number of atoms but also on the coupling of the constituent single-walled nanotubes. Some nanotubes with small interlayer separations do not hold the coaxial cylindrical structure after optimization. The NTS (n, n)@(n+3, n+3) are found to have large formation energies and binding energies per atom. For example, (3,3)@(6,6), (4,4)@(7,7), (5,5)@(8,8), and (6,6)@(9,9) all have large binding energies per atom, around 3.7eV/atom. All double-walled Si nanotubes are found to be semiconductors. However, the band gap, in general, is observed to decrease from single walled nanotubes to double walled nanotubes.   

Hierarchically driven nanostructure electrocatalysts for direct sensing of biomolecules

Applying nanoscale device fabrications toward biomolecules, ultra sensitive, selective, robust, and reliable chemical or biological microsensors have been one of the most fascinating research directions in our life science. It is still challenging to make miniaturized sensors having the sensitive delectability to biologically relevant species due to limitations in terms of catalytic efficiency and sustainability. We introduce hierarchically driven iridium dioxide nanowires (IrO$_{2}$ NWs) directly on a platinum (Pt) microwire, which allows a simple fabrication of the amperometric sensor and shows a favorable electronic property desired for sensing of H$_{2}$O$_{2}$ and NADH without aid of enzymes. We have prepared highly crystalline IrO$_{2}$ needlelike-NWs by an atmospheric pressure chemical vapor deposition of IrO$_{2}$ powder without catalyst on a Pt wire surface, IrO$_{2}$ NWs-Pt, which was tested as an amperometric microsensor. The structures and morphologies of the IrO$_{2}$ NWs were characterized using FE-SEM, HRTEM and Raman spectroscopy. This rational engineering of a nanoscale architecture based on the direct formation of the 1-D nanostructures on an electrode can offer a useful platform for high performance electrochemical biosensors efficiently, sensitive detection of biologically important molecules.   

Hierachically grown RuO$_{2}$ nanowires on electrospun IrO$_{2}$ nanofiber

Electrospinning is a well known tool to synthesize nanofibers with the various diameters. Iridium oxide(IrO$_{2})$ and Ruthenium oxide(RuO$_{2})$ have a great potential as materials for electrodes in electrochemical devices due to their high electrical conductivity, chemical stability, and characteristics. So when they are mixed, we expect superior electrochemical properties and stability. We synthesize IrO$_{2}$ nanofibers from mixture of Iridium precursor and polymer. At certain condition, we were able to obtain uniform and continuous fibers that the average diameter of nanofibers is approximately 150 nm. After calcination, RuO2 nanowires were then hierarchically grown on IrO$_{2}$ nanofibers by APCVD at about 650 ${^\circ}$ without any catalyst. The diameters of nanowires are about 50 nm and the length is $\sim $ 1.5 $\mu $m. The structures and morphologies were examined using scanning electron microscopy (FE-SEM), high resolution electron microscopy (HRTEM), X-ray diffraction (XRD) spectrum and Raman spectroscopy.   

Synthesis of metallic ReO$_{3}$ nanowires

Rhenium trioxide (ReO$_{3})$ is well known as an unusual transition metal oxide with unexpectedly high electrical conductivity close to that of copper. We present the synthesis of highly crystalline metallic rhenium trioxide (ReO$_{3})$ nanowires. ReO$_{3}$ nanowires were grown on a 200 nm silica-covered Si (001) wafer by atmospheric-pressure chemical vapor deposition (APCVD) at about 300 ${^\circ}$ without any catalyst. The wafer was placed in the quartz boat approximately 15 cm downstream from the fine mashed ReO3 powder and heated at 320 ${^\circ}$ with flowing of high purity Ar (500 sccm) for 2 h and then kept at 450 ${^\circ}$ for an additional 3 h. The two-step heating enhanced the growth of ReO$_{3}$ nanowires. The structures and morphologies were examined using scanning electron microscopy (FE-SEM) and high-resolution electron microscopy (HRTEM). Based on HRTEM, the ReO$_{3}$ nanowires exhibit a core of perfect cubic perovskite type single crystal structure with a shell of thin amorphous and disordered structures of less than 2 nm in the near surface layers. Possibly this is due to proton intercalation induced by the surface reaction of single crystal ReO$_{3}$ with water.   

The effect of frequency noise on the spectral response of nanomechanical oscillators

We study the spectral response of an underdamped nanomechanical oscillator in the presence of frequency noise. Motion of a nanobeam is excited and measured using the magnetomotive technique. Telegraph noise is applied on nearby side gates, so that the eignenfrequency of the nanobeam randomly jumps back and forth between two values due to electrostatic coupling. This arrangement is analogous to a mechanical oscillator dispersively coupled to a classical or quantum two-level system. The spectrum displays two distinct peaks when the two eigenfrequencies are separated by a distance larger than the damping constant $\lambda $ of the oscillator. As the switching rate $W$ of the telegraph noise increases, we observe that the two peaks merge into one. The width of the single spectral peak decreases with increasing $W$. At $W>>\lambda $, the spectral width approaches its intrinsic value as if frequency noise is absent. The results are in agreement with theoretical predictions.   

Self-standing Hybrid Nanofibers of TiO2 and TiO2/Hydroxyapatite: Application in Photocatalytic and Photovoltatic Systems

A Hybrid fibers of Hydroxyapatite TiO$_{2}$, HAp/TiO$_{2}$ with modified photocatalytic properties were synthesized using a template method. Liquid phase deposition (LPD) technique was employed to grow TiO$_{2}$ layers on cellulose fibers, followed by deposition of HAp from a pseudo body solution, and finally heat removing the cellulose template. The resulting material has a fibrous structure, mimicking the cellulose fibers shape, and have a typical surface area of 114 m$^{2}$/g, compared to 74 m$^{2}$/g for pure TiO$_{2}$ fibers. Adsorption and photocatalytic degradation tests showed that addition of HAp to TiO$_{2}$ fibers increased the adsorptive from 17{\%} to 35{\%}. Nano particulated TiO$_{2}$ fibers as one-dimensional long structures were introduced into TiO$_{2}$ P25 nano particle films using co-electrophoretic deposition. This resulted in less porosity and higher roughness factor of the films that provided more favorable conditions for electron transport. The films used as the photoanode of a dye solar cell (DSC) produced 65{\%} higher photovoltaic efficiency. TiO$_{2}$ fibers can be excellent binders in single-step, organic-free electrophoretic deposition of TiO$_{2}$ for DSC photoanode.   

Family-Dependent Rectification Characteristics in Ultra-Short Graphene Nanoribbon p-n Junctions

We present the first transport property investigation of a-few-nm-long armchair graphene nanoribbon (AGNR) p-n junctions by using first-principles method. Intriguingly, family-dependent rectification is observed. To be specific, traditional rectification effect in the forward direction is observed in the AGNR p-n junctions with 3n and 3n+2 widths whereas reverse rectification effect is observed in the AGNR p-n junctions with 3n+1 width. The analysis of the spatial distribution of molecular projected self-consistent Hamiltonian eigenstates and the projected density of states give an insight of the observed results.   

Chirality Distribution Measurements of the NIST Single-Wall Carbon Nanotube Reference Material Using Resonance Raman Spectroscopy

The ability to rapidly and easily determine the chiral vector distribution within a nanotube population remains a key measurement need for carbon nanotube processing and applications. We report Resonance Raman Spectroscopy (RRS) measurements of a SWCNT reference material from NIST. The SWCNT samples were synthesized using the CoMoCat method, dispersed in aqueous solutions by wrapping in deoxycholate surfactant, and separated by length using ultracentrifugation. We measure Raman spectra over a wide range of excitation wavelengths from 457 nm to 850 nm using a series of discrete and continuously tunable laser sources coupled to a triple-grating spectrometer with a liquid-nitrogen-cooled detector. The spectra reveal Raman-active vibrational modes including the low-frequency radial breathing mode and higher-order modes. Chirality distributions are determined from the Raman spectra, specifically the RBM frequency and energy excitation profiles, together with input from theoretical models. RRS is sensitive to both major and minor chiral species in the sample. We will compare the resulting chirality distribution obtained from RRS with those obtained from other orthogonal measurement techniques.   

A study of 3-dimensionally periodic carbon nanostructures

Electronic structures with intricate periodic 3-dimensional arrangements at the submicron scale were investigated. These may be fabricated using artificial porous opal substrates as the templates in which the targeted conducting medium is introduced. In the past these materials were reported to show interesting electronic behaviors. [Michael Bleiweiss, et al ``Localization and Related Phenomena in Multiply Connected Nanostructured,'' BAPS, Z30.011, Nanostructured Materials Session, March 2001, Seattle]. Several materials were studied in particular disordered carbon which has been reported to show quantum transport including fractional hall steps. The results of these measurements, including the observation of localization phenomena, will be discussed. Comparisons will be made with literature data.   

Charge redistribution and interlayer coupling in twisted bilayer graphene under electric fields

There is an ongoing controversy on the electronic characteristics of rotated bilayer graphene. Several experiments on rotated few-layer graphene grown on SiC show an electronic behavior similar to that of single-layer graphene, with the same carriers' velocity as that of an isolated graphene monolayer; for this reason, these systems have been considered as composed of uncoupled graphene sheets. We investigate the electronic density redistribution of rotated bilayer graphene under a perpendicular electric field, showing that the layers are actually coupled even for large angles. This layer-layer coupling is evidenced by the charge transfer on these structures as a function of the external voltage. We find an inhomogeneous excess charge distribution that is related to the moir\'e patterns for small angles, but that persists for larger angles where the carriers' velocity is equal to that of single layer graphene. Our results show that rotated bilayer systems are coupled for all rotation angles.   

Spin-density-wave instability in graphene doped near the van Hove singularity

We study the instability of the metallic state towards the formation of a new ground state in graphene doped near the van Hove singularity. The system is described by the Hubbard model and a field theoretical approach is used to calculate the charge and spin susceptibility. We find that for repulsive interactions, within the random phase approximation, there is a competition between ferromagnetism and spin-density wave (SDW). It turns out that a SDW with a triangular geometry is more favorable when the Hubbard parameter is above the critical value $U_c (T)$, which depends on the temperature $T$, even if there are small variations in the doping. Our results can be verified by ARPES or neutron scattering experiments in highly doped graphene.   

Effects of static charging and exfoliation of layered crystals

Using first-principle plane wave method we investigate the effects of static charging on structural, electronic and magnetic properties of suspended, single layer graphene, graphane, fluorographene, BN and MoS(2) in honeycomb structure. The limitations of periodic boundary conditions in the treatment of charged layers are clarified. Upon positive charging the band gaps between the conduction and valence bands increase, but the single layer nanostructures become metallic owing to the Fermi level dipping below the maximum of valence band. Moreover, their bond lengths increase leading to phonon softening. As a result, the frequencies of Raman active modes are lowered. High level of positive charging leads to structural instabilities in single layer nanostructures, since their specific phonon modes attain imaginary frequencies. Similarly, excess positive charge is accumulated at the outermost layers of metallized BN and MoS(2) sheets comprising a few layers. Once the charging exceeds a threshold value the outermost layers are exfoliated. Charge relocation and repulsive force generation are in compliance with classical theories.   

Effects of Peptide Adsorption on the Electronic Properties of Graphene

In this work we aim to explain the increase in the electrical conductance in a single-layer graphene (SLG) field-effect transistor (FET) upon binding the peptide HSSYWYAFNNKT, which we previously demonstrated. The adsorption of the peptide onto the SLG has been carefully modeled by applying empirical molecular dynamics simulations with the AMOEBA force-field. The peptide adsorbed SLG structure demonstrates $\pi -\pi $ stacking with aromatic amino acids, namely His,Tyr, Thr and Ph. Based on this large-scale peptide-SLG system, calculations on the electron transport using the nonequilibrium Green's function formalism at the extended Huckel level were carried out. Transmission eigenchannels and spectra, projected density of states, effects of modeling realistic leads (gold vs. graphene) and $I-V$ characteristics will be discussed in detail. In this context, suppositions as to the mechanism of increased conductance for the peptide-SLG FET will be proposed.   

Heat transfer at interfaces with graphene

Graphene has an ultrahigh in-plane thermal conductivity (5500 W/mK), but simultaneously a much lower conductivity along the c-axis in graphite or at the interfaces with other materials. As graphene finds more and more applications in nanoelectronics and high-performance composites, these interfaces become critically important in defining their heat dissipation and conduction performance. Unlike conventional interfaces in materials such as grain boundaries, the interfaces with graphene can be tuned by chemically modifying the graphene monolayer or intercalating the interfaces. These nano-engineering proposals require fundamental understanding of the heat transfer mechanisms. In order to obtain some insights on the transfer processes of mechanical and thermal energy across these interfaces, we perform series of molecular dynamics simulations, in combination with theoretical analysis by considering the quasi-ballistic nature of phonon transport at nanoscale. The result shows that heat dissipation or transport can be divided into two stages, beginning with an interface-controlled process. The effects of interface structures and binding properties on the whole process will be covered in this talk, with several examples showing how the interfacial thermal transfer can be engineered.   

Nonlocal resistance in a bilayer graphene under a magnetic field

We have performed nonlocal resistance measurement in bilayer graphene with Hall-bar geometry under high magnetic fields. We observed large nonlocal resistance near the charge neutral point (CNP) which grows rapidly with field intensity. At 15 Tesla, the relative increase of nonlocal resistance near CNP is an order of magnitude larger than the local resistance at the same condition. This behavior is similar to the recent nonlocal measurement result in a single layer graphene. The detailed field and temperature dependences will be presented.   

Edge currents and an array of nanopores in zigzag and chiral graphene nanoribbons as a route toward high-ZT thermoelectrics

We exploit peculiar transport properties of graphene nanoribbons (GNRs) with zigzag or chiral edges where local current density of quasiparticles close to the Dirac point is largely confined around their edges. Thus, drilling an array of nanopores in the interior of such nanoribbons will not affect much the electronic transport while substantially suppressing the phonon transport in sufficiently long wires. For nanoribbons of length $\sim 1$ $\mu$m and width 4.1 nm and nanopores of diameter 2 nm, the ZT for both zigzag GNRs and chiral GNRs is $\simeq 4$ at $T=77$ K and $\simeq 1.5$ at $T=300$ K. With slightly asymmetric arragement of nanopores, the $ZT$ of zigzag GNRs can be further enhanced to reach $\simeq 13$ at T=77 K and $\simeq 5$ at T=300 K.   

Low-frequency noise in graphene FETs

We report on the low-frequency electronic noise in graphene-based FET structures. Samples were created using standard e-beam lithography and exfoliated, epitaxially-grown, and CVD-grown single-layer graphene films. The lowest overall noise was observed in epitaxially-grown films on SiC. We also investigated the gate dependence of the noise amplitude. Previous studies have suggested that the noise dependence should be either $\Lambda$-shaped in keeping with the Hooge model, or M-shaped as described by the charge-noise model. We here propose a new noise model based upon resonant scattering theory, which not only explains both types of gate dependence on noise, but also models the general noise behavior in graphene.   

Magnetic Properties of Graphene Superlattices

We studied graphene superlattices (SL) in the presence of magnetic fields. We found magnetic properties of single layer and bilayer graphene SLs in a weak magnetic field are strongly determined by emergent Dirac physics. Moreover, the spatial anisotropy of diagonal conductivities can be reversed when magnetic field is tuned from weak to intermediate strength. In a strong magnetic field, all anisotropies disappear and results from pristine graphene are restored.   

Co-adsorption of \textit{n} Monomer Species on Terraces and Nanotubes

We consider the partition function, $Z$, of the system of $n$ monomer species adsorbed on a terrace or a nanotube of arbitrary periodic lattice geometry, $L $atomic sites in length, and $M$' sites in the width of the terrace or in the normal cross-section of the nanotube. $Z$ is related to the eigenvalues of a real and non-negative matrix (\textbf{T} matrix) of rank ($n$+1)$^{M}$, where $M$ is an integer multiple of $M$'. In the infinite-$L$ limit, we also prove that $Z$ is the largest eigenvalue of the \textbf{T}-matrix, raised to the power 1/$M$. Because the rank of this matrix increases exponentially with $M$, we develop a technique for its recursive construction applicable to any lattice geometry, which is easily programmed and efficiently adaptable for supercomputing and multiparallel processing. As examples, we consider the co-adsorption on square, equilateral triangular, and honeycomb surfaces. This general formulation can now be applied to model a whole new set of experiments involving the coadsorption of two or more monomer species, on terrace or nanotube surfaces with various periodic lattice structures.   

Temperature dependence of optoelectronic properties of CdS$_{x}$Se$_{1-x}$, CdS$_{x}$Te$_{1-x}$, CdSe$_{x}$Te$_{1-x}$

Temperature dependence of the optoelectronic properties of ternary compounds, CdS$_{x}$Se$_{1-x}$, CdS$_{x}$Te$_{1-x}$ and CdSe$_{x}$Te$_{1-x}$, are presented. The analysis of the temperature dependence of the energy gap of these compounds, for various compositions, is discussed in light of the Varshni formula. Extension of this study is then made to include the contributions of the temperature dependence of the gap of the component binary semiconductors. The model takes into account the implications of the above results on the refractive indices of these ternary compound semiconductors.   

Modeling Random Dopant Fluctuation Effects in Nanoscale Tri-Gate MOSFETs

The tri-gate FET has been hailed as the biggest breakthrough in transistor technology in the last 20 years. The increase in device performance (faster switching, low power, improved short channel effects, etc.), coupled with the reduction in device size, would allow for huge gains in the electronics industry. In this work, an atomistic quantum-corrected Monte Carlo 3-D device simulator was used to not only investigate the validity of these claims, but also how quantum size quantization and random dopant fluctuation (RDF) affect the tri-gate FET performance and how to curb these issues. The main findings are as follow: 1) carrier scattering leads to ON current degradation of $\sim $30{\%} and hence cannot be ignored; 2) deviations in threshold voltage due to random channel doping are smaller in the tri-gate FET; 3) RDF due to the source/drain discreteness can be engineered by adjusting the source/drain junction depth. With randomness reduced, the overall performance should increase when used in ICs, where consistency in device characteristics is essential.   

Spin Torque and Its Efficiency in 3D Rashba Models with Localized Spins

In addition to the magnetization reversal by the spin-transfer torque, the magnetization reversal driven by a strong spin-orbit coupling has been intensely investigated experimentally and theoretically. In this presentation, we focus on 3D Rashba models coupled with localized spins, and study the spin torque theoretically. This is motivated by the recent discovery of BiTeI as 3D Rashba systems. As a result, we find that the spin torque in 3D Rashba models is largely enhanced in the high-carrier-density regime compared with 2D Rashba models. We also find that the spin-torque efficiency defined as the ratio between the spin torque and the electric current is enhanced when the Fermi energy lies on only lower band, both in 3D and 2D. As we change the Rashba spin-orbit coupling, the spin-torque efficiency becomes maximum when the Rashba spin-orbit coupling is comparable to the exchange coupling to the localized spins. The optimum spin-torque efficiency becomes large when the Rashba spin-orbit coupling is large, and it is preferable for the magnetization reversal with smaller amount of current injection.   

All-optical patterning of Nuclear Polarization in Gallium Arsenide

We employ Stray-Field NMR Imaging for $^{69}$Ga and $^{71}$Ga to monitor the spatially-dependent polarization of nuclear spins in semi-insulating bulk GaAs at 9.4 T. By exploiting two competing mechanism as hyperfine and quadrupolar interaction for optical nuclear polarization at different illumination intensities and wavelengths, we demonstrate all-optical creation of three-dimensional patterns of positive and negative nuclear polarization. We also observe the isotope dependence of the patterning process, where different isotopes ($^{69}$Ga and $^{71}$Ga) have opposite polarization at the same location in the sample. By combining the optical pumping with varied NMR pulse sequences, we demonstrate the control of nuclear polarization.   

Dynamical simulation of a two-electron spin qubit based on hyperfine interaction

The interaction with the nuclear field is one of the main sources of decoherence in solid state quantum bits. However, recently it has been shown that hyperfine interaction can also be used to manipulate the spin state of electrons in double quantum dots. In this work, we simulate numerically the dynamics of a spin-based quantum bit consisting of two electrons confined in a double quantum dot, including the interaction with a random nuclear field. The two electron wavefunction is built using an antisymmetric function expansion and the total hamiltonian is discretized spatially using finite differences. For the dynamic simulation, an exponential approximation of the time propagator is used. In order to model the hyperfine interaction, a normally-distributed random magnetic field is assigned to each simulation grid point. The dynamics of the system is calculated by averaging over an ensemble of quantum dots. It is shown how the hyperfine field can be used to drive transitions between singlet and triplet states, an effect that has already been found experimentally. Particularly, it is shown how these transitions can be controlled using an applied electric field, thus allowing for the realization of a two-electron quantum gate.   

The interaction of hydrogen donors with compensating acceptors in semiconducting oxides

Hydrogen is a common impurity that can act as a donor and an acceptor in a semiconductor, leading to a variety of behavior that may impact the desired performance. In one class of materials, wide-band-gap semiconducting oxides, the role of H has been strongly linked to n-type conductivity. In these systems H acts predominantly or exclusively as a shallow donor as an interstitial or while substituting on an O site. In addition to the resulting n-type conductivity, the incorporation of H or other donor dopants also leads to an increase in compensating acceptor defects. The interaction of H with cation vacancies, the dominant acceptor impurity involved in compensation, has yet to be fully explored. Using first-principles calculations employing hybrid functionals, we investigate the complexes formed between cation vacancies and interstitial H donors. We report on the formation energies, binding energies, and vibrational frequencies of the hydrogenated cation vacancies in the transparent semiconducting oxides SnO2, In2O3 and Ga2O3 [1]. We find that H can interact strongly with vacancies in both atomic and molecular form, and can have a significant impact on the electrical properties of devices employing these oxides. \\[4pt] [1] J.B.Varley et al., J.Phys. Cond. Matter 23, 334212 (2011).   

Electronic properties of doped cesium niobate

Antiferroelectric cesium niobate Cs$_{2}$Nb$_{4}$O$_{11}$ (CNO) is known to have novel water splitting capability. It was reported that the broad blue photoluminescence was observed at 77 K. Particularly when treated with NiO$_{2}$, the water-splitting activity under UV-light irradiation increased five times. In this work we present systematic studies of the electronic properties of doped CNO by using density functional theory calculations. We first obtain direct band gap of 3.12 eV for the pure sample that agrees well with experimental value of 3.55 $\pm$ 0.05 eV. Then we notice, at both the high and low-density regions, the band gap effectively reduced after oxygen atoms substituted by sulfur and nitrogen, and niobium by vanadium. As a final point, we present and discuss relations between impurity concentration and band gap reduction.   

Inspecting the microstructure of electrically active defects at the Ge/GeO$_{x}$ interface

High mobility substrates are important key elements in the development of advanced devices targeting a vast range of functionalities. Among them, Ge showed promising properties promoting it as valid candidate to replace Si in CMOS technology. However, the electrical quality of the Ge/oxide interface is still a problematic issue, in particular for the observed inversion of the n-type Ge surface, attributed to the presence of dangling bonds inducing a severe band bending [1]. In this scenario, the identification of electrically active defects present at the Ge/oxide interface and the capability to passivate or anneal them becomes a mandatory issue aiming at an electrically optimized interface. We report on the application of highly sensitive electrically detected magnetic resonance (EDMR) techniques in the investigation of defects at the interface between Ge and GeO$_{2}$ (or GeO$_{x})$, including Ge dangling bonds and defects in the oxide [2]. In particular we will investigate how different surface orientations, e.g. the (001) against the (111) Ge surface, impacts the microstructure of the interface defects. [1] P. Tsipas and A. Dimoulas, Appl. Phys. Lett. 94, 012114 (2009) [2] S. Baldovino, A. Molle, and M. Fanciulli, Appl. Phys. Lett. 96, 222110 (2010)   

Quantum Transport in Ultra-scaled Junctionless Transistors

As the dimensions of metal-oxide-semiconductor field-effect transistors have been scaled down to a few nano-meters, short channel effects have started to significantly degrade their performance. The junctionless transistor is an alternative device structure which is expected to reduce short channel effects. However, an extreme device scaling raises another issue, namely, source-to-drain tunneling. Junctionless transistors contain several doping atoms in the channel which can enhance tunneling processes and cause electrons to scatter with them. Through self-consistent quantum transport simulations based on the tight-binding model with elelctron-phonon scattering included, it is found that junctionless nanowire transistors with a gate length of 5 nm do not outperform conventional inversion-mode transistors with the same dimension in terms of their on-state characteristics, mainly due to impurity scattering in the channel. The advantage of the junctionless transistor in the the subthreshold region vanishes due to large tunneling currents through doping impurities.   

Band crossing in isovalent semiconductor alloys with large size mismatch

Mixing isovalent compounds AC with BC to form alloys A$_{1-x}$B$_{x}$C has been an effective way in band structure engineering to enhance the availability of material properties. In most cases, the mixed isovalent atoms A and B, such as Al and Ga in Al$_{1-x}$Ga$_{x}$As or As and Sb in GaAs$_{1-x}$Sb$_{x}$ are similar in their atomic sizes and chemical potentials; therefore, the physical properties of A$_{1-x}$B$_{x}$C change smoothly from AC to BC. However, in some cases when the chemical and size differences between the isovalent atoms A and B are large, adding a small amount of B to AC or vice versa can lead to a discontinuous change in the electronic band structure. These large size- and chemicalmismatched (LSCM) systems often show unusual and abrupt changes in the alloys' material properties, which provide great potential in material design for novel device applications. In this report, based on first-principles band-structure calculations we show that for LSCM GaAs$_{1-x}$N$_{x}$ and GaAs$_{1-x}$Bi$_{x}$ alloys at the impurity limit the N (Bi)-induced impurity level is above (below) the conduction-(valence-) band edge of GaAs. These trends reverse at high concentration, i.e., the conduction-band edge of GaAs$_{1-x}$Nx becomes an N-derived state and the valence-band edge of GaAs$_{1-x}$Bi$_{x}$ becomes a Bi-derived state, as expected from their band characters. We show that this band crossing phenomenon cannot be described by the popular BAC model but can be naturally explained by a simple band broadening picture.   

Engineering Efficiency Droop in InGaN/GaN Multiple Quantum Well LEDs

In this work, we address the technologically important issue of efficiency droop pronounced in InGaN/GaN multiple quantum well (QW) LEDs. A two-fold modeling approach is employed where: 1) the NEMO 3-D tool is used to compute the atomistic strain fields and associated polarization potentials in the active region, and 2) the outputs from NEMO 3-D are then coupled to the Synopsys TCAD tool to determine the terminal electrical and optical properties of the device. Next, a series of numerical experiments are performed that mainly aims to improve the efficiency droop without compromising the internal quantum efficiency (IQE) of the device. These include:1) varying the QW thickness, 2) employing different configurations of tri-material barriers, 3) varying the molar concentration of the barrier materials, and 4) varying the doping density in the barrier region.   

Using Polarization Effects in Deep UV Nitride Emitters

We numerically and experimentally investigate the effects of compositional grading in high Al content AlGaN, with applications to deep UV emitters. The large polarization fields create a fixed charge density in the graded regions, which by necessity, are screened by mobile electrons and holes. The net effect is a pn junction band structure in which bound polarization charge assumes the role of immobile ionized dopants, and the screening charge assumes the role of mobile carriers released from the dopants. Using this phenomenon it should be possible to overcome the difficulties in doping high Al content AlGaN, and make efficient deep UV light emitting diodes, and lasers.   

Improvement of $M$-plane GaN thin film grown on pre-annealing $\beta $-LiGaO$_{2}$ (100) substrate

\textbf{\textit{M}}-plane GaN thin films have been grown on $\beta $-LiGaO$_{2}$ (100) substrates by plasma-assisted molecular-beam epitaxy. In order to improve the quality, we tried to grow \textbf{\textit{M}}-plane GaN thin films on pre-annealed in vacuum and in air ambient LiGaO$_{2}$ (100) substrates. X-ray diffraction data indicated that the \textbf{\textit{M}}-plane GaN thin film grown on the LiGaO$_{2}$ (100) substrate pre-annealed in air ambient has better crystal quality than that grown on the LiGaO$_{2}$ (100) substrate pre-annealed in vacuum. In addition, we found that the strain between GaN and LiGaO$_{2}$ substrate can be relaxed by growing GaN thin film on pre-annealed LiGaO$_{2}$ substrate in air. It reveals that LiGaO$_{2}$ substrate annealing in air ambient can suppress the formation of lithium-rich surface effectively to grow a high quality \textbf{\textit{M}}-plane GaN thin film on the LiGaO$_{2}$ substrate.   

The Growth of InGaN on Single Crystal ZnO (000-1) Substrate by Metal Modulation Epitaxy Method

The growth of InGaN film on single crystal ZnO (000-1) substrate by plasma assisted-molecular beam epitaxy has been investigated. The metal modulation epitaxy (MME) technique was applied in this experiment. The growth mechanism of InGaN thin film on ZnO (000-1) was in the form of 2D growth model by \textit{in-situ} observation of reflection high-energy electron diffraction (RHEED). From the observation of atomic force microscope (AFM), we found that surface roughness of InGaN thin film can be improved using MME technique. The crystal quality and the indium content of InGaN thin film was determined by the X-ray diffraction method. The full width at half-maximum (FWHM) and the indium content of InGaN thin film is 298.64 arc-sec and 17 {\%}, respectively. According to the calculation of bowing parameters, the 17 {\%} indium content of InGaN thin film closed to In$_{0.18}$Ga$_{0.82}$N which is perfect lattice-matching to ZnO (000-1). The micro-structure analysis of InGaN grown on ZnO by high-resolution transmission electron microscope (HR-TEM) was performed, which shows that the interface between the substrate and the film is clearly indicating In$_{0.17}$Ga$_{0.83}$N to be lattice-matching with ZnO (000-1).   

Studies on the Growth and Characterization of hexagonal boron nitride thin films

Hexagonal Boron Nitride (h-BN) with a direct band gap of $\sim $5.9 eV has emerged as a promising deep ultraviolet photonic material. We will present the studies on the growth and characterization of h-BN thin films grown on different substrates such as sapphire, YSZ, quartz, and metal substrates. The samples were grown by electron beam evaporation technique. Atomic force microscopy, x-ray diffraction, optical spectroscopy, and Hall effect measurement were employed to characterize surface morphology, structural, optical and electronic, and electrical properties, respectively. We will also present the results of Mg-doped h-BN thin films in an effort to make p-type BN. Implementation of our findings on the development of deep ultraviolet photonic devices will also be discussed.   

Electrical characterization of homoepitaxially deposited boron-doped diamond

Homoepitaxial boron doped single crystal diamond films were deposited on 100 oriented Ib type synthetic diamond substrates via microwave plasma chemical vapor deposition. The gas phase chemistry of 6{\%} methane/ hydrogen ratio and 5000 ppm of (B/C)$_{gas}$ was used for all samples deposited at different temperature from 900 to 1200$^{\circ}$C. The deposited films were characterized by FTIR, AFM, Raman spectroscopy and XRD Rocking curve measurement to assess doping level, surface morphology and crystalline quality. The high growth rate 16 micron per hour has been obtained by optimizing growth parameters such as microwave power, chamber pressure, and gas flow rate. The four-probe electrical resistance measurements on boron-doped samples were conducted between 14 K and 350 K and show competing effects of intrinsic and hoping conductivity.   

A simple approach to the polytypism in boron nitride

Boron nitride (BN) is well known as having a polytype including cubic (c-BN) with zinc blende structure (3C) and hexagonal BN (h-BN) phases similarly to C. From the experimental viewpoints, it is found that h-BN appears at ambient temperature and pressure whereas c-BN is preferable at high temperature and high pressure. In order to clarify its polytypism, there have been some ab initio calculations mainly focusing on four-fold coordinated structures such as 3C-, 6H-, 4H-, and 2H-BN. Although the ab initio calculations predict the polytypes for bulk BN, the physical interpretation of the polytypism is still unclear because of the complexity of identifying individual contribution in the ab initio calculations. In order to make up this deficiency, we investigate the polytypism for BN in bulk form using a simple approach on the basis of empirical interatomic potentials. The calculated respective energy differences between 6H and 3C, between 4H and 3C, and between 2H and 3C are 5.0 meV, 7.5 meV, and 17.1 meV when the ionicity fi=0.143 for BN is employed. These results are consistent with previously reported ab initio calculations. Furthermore, we also discuss the relative stability between h-BN and c-BN using this simple approach.   

Morphological Studies on GaN Nanowire Growth Modes

We describe our results on growth of single crystal GaN nanowires in three different growth modes (straight, serrated and epitaxial) on catalyst-patterned substrates by means of chemical vapor deposition. The growth is carried out in a tube furnace wherein gallium oxide is used as reactor source and a mixture of ammonia and hydrogen gas is used as precursor. Growth of GaN nanowires are demonstrated on both Au and Ni-catalyst patterned substrates. We show that by controlling the deposition parameters, specifically the size of the catalyst and amount of gallium oxide, we can control the growth morphology. While straight GaN nanowires typically grow on substrates patterned with catalyst particles with dimensions of the order of ~100nm, the epitaxial nanowires grow on substrates with much smaller dimensions of catalyst particles ~50nm. The newly demonstrated GaN wire growth mode with periodic serrations, typically grow under conditions involving large catalyst size $\sim$300nm and excess gallium oxide. In this work, a detailed investigation is carried out on the structural properties of the three different growth modes by means of high resolution transmission microscopy, x-ray diffraction and Raman spectroscopy studies in order to obtain a better understanding of their growth mechanism.   

Pulse-duration dependent structural change mechanism in Ge$_{2}$Sb$_{2}$Te$_{5}$

The mechanism of ultrafast structural change in Ge$_{2}$Sb$_{2}$Te$_{5}$ (GST) induced by optical excitation was studied by using pump-probe coherent optical phonon (COP) spectroscopy. The structural change mechanism showed strong dependency on the duration of the pump pulse. While the frequency of displacively generated A$_{1}$ COP was observed to red-shift (by more than $\sim $ 5{\%}) in the 2-3 ps time-scale for a long pulse excitation ($\sim $100fs), it was instantaneous in the case of short pulse excitation. Furthermore, the shifted COP frequency in the intermediate, transient state was almost independent of the pulse duration or the fluence of the optical pump, which, along with the pulse duration dependency, cannot be explained by photoexcited high-density carriers or thermal effect. The strong dependency of the observed time-scale on the pulse duration and the fluence-independent COP frequency in the transient state indicate that coherent atomic motion due to ultrafast COP generation plays a dominant role in the transition to intermediate, transient state.   

The Calculation for the Microscopic Capacitance of Gate Electrode in High Electron Mobility Transistors

Novel nanodevices have developed below 60 nm. Accurate characterization requires detailed electronic structure. In field effect transistor, a dielectric layer isolates the channel from the gate electrode. The model is based on the nanoscale MOSFET with gate length 60 nm, channel length 25 nm and insulator thickness 10 nm. The capacitance of the gate is the series combination of the geometric capacitance. Its dielectric constant is associated with the local electron density. The carrier density decreases from the source to drain in the channel. The capacitance is assumed in parallel connection along this direction.   

Characterizing Heat Spreading and Performance Degradation in Organic Light-Emitting Diodes

In this work, we present for the first time high resolution thermal images of operating organic light-emitting diodes (OLEDs) and show that the surface temperature of these devices can be used to map current density and identify the origin of localized defects and performance degradation. Both luminance and lifetime of OLEDs decrease dramatically with increased operating temperature due to self-heating. Furthermore, localized Joule heating at defects results in local hot spots, thus degrading the brightness homogeneity, altering the electro-optical characteristics of the OLED, and leading to electrode delamination and black spots. Increasing the lifetime of OLEDs clearly relies at least in part on improved thermal management. Using thermoreflectance microscopy, we observed evidence of the correlation between structural defects and areas of low current density by examining areas of operating devices which showed visual damage and had a low relative surface temperature. We also show the validity of using thermoreflectance microscopy to perform basic characterization of operating OLEDs, such as examining diode behavior, extrapolating material qualities such as diffusivity and conductivity, and quantifying the heat flow through working devices.   

Spatially Resolved Thermal Analysis of High Power LEDs Using Thermoreflectance Microscopy

The efficiency, reliability, and lifetime of high power light emitting diodes (LEDs) depend critically on their operating temperature. The lateral temperature distribution is of particular importance with large area, high power LEDs as defects related to overheating in high power LEDs usually occur at a high rate at the surface. In this work, we present the use of lock-in thermoreflectance imaging to measure the spatially resolved surface thermal distribution of operating LEDs. This non-invasive thermography technique offers high spatial and thermal resolutions. We show that results of thermoreflectance surface temperature are quantitatively consistent with temperature measurements obtained using forward voltage bias and wavelength shift techniques. We demonstrate the power of spatially resolved thermoreflectance by imaging the highly non-uniform surface temperature distribution of an operating LED at high electrical bias power. We conclude that the non-uniform surface temperature distribution is resulted from non-uniformly distributed inject current and overheating at the contacts. We also investigate the thermal impact of encapsulating commercial LEDs with a plastic lens and silicone epoxy.   

Optical Properties of PbTe and PbSe

We report optical properties of PbTe and PbSe as obtained from first principles calculations with the Tran-Blaha modified Becke-Johnson potential. The results are discussed in relation to existing experimental data, particularly in relation to the temperature dependence of the band gap.   

Strained layer relaxation effect on current crowding and efficiency improvement of GaN based LED

Efficiency droop effect of GaN based LED at high power and high temperature is addressed by several groups based on career delocalization and photon recycling effect(radiative recombination). We extend the previous droop models to optical loss parameters. We correlate stained layer relaxation at high temperature and high current density to carrier delocalization. We propose a third order model and show that Shockley-Hall-Read and Auger recombination effect is not enough to account for the efficiency loss. Several strained layer modification scheme is proposed based on the model.   

Wigner-crystallization of Rydberg-Polaritons in the lowest Landau level

For electrons and dipolar fermions in the lowest Landau level the critical filling for Wigner-crystallization was shown to be $\nu_c \approx 1/7$ [Baranov et. al., Phys. Rev. Lett. 100 (2008)]. We investigate the fractional quantum Hall effect for Van-der-Waals interacting bosons as realized e.g. by stationary-light polaritons in a Rydberg gas and find no transition to the Wigner crystal (WC). Our numerical studies suggest a crystalline groundstate below $\nu=1/6$ which is expected to be described by a correlated WC of composite quasiparticles. Taking into account a cut-off in the Van-der-Waals interaction we find the WC to be favorable for large cut-offs. Numerical results for different geometries are presented and realistic implementations are discussed.   

Kelvin Probe Microscopy and Electrostatic Force Microscopy of reduced graphene oxide platelets

We present combined Scanning Kelvin Probe Microscopy (SKPM) and Electrostatic Force Microscopy (EFM) measurements on reduced graphene oxide (rGO). Although the as-synthesized graphene oxide is insulating, controlled reduction chemistry can render this material semiconducting or even semi-metallic. The availability of several types of oxygen functional groups allows rGO to interact with a wide range of organic and inorganic compounds. We perform sensitive SKPM and EFM measurements on patterned rGO electronic circuits and show that the electrical potential and charge distribution are significantly changed when the device is exposed to various organic dopants. We also demonstrate that these experiments allow a systematic study of the conducting channels through rGO as a function of the chemical dopant.   

Length, Radius, and Tilt Angle Control of Carbon Nanotube Probes for High Resolution Atomic Force Microscopy

The lateral spatial resolution of modern atomic force microscopy (AFM) is largely limited by the radius of curvature of the probe. Owing to their extraordinary mechanical strength, large aspect-ratio, and sub-nanometer radius, carbon nanotubes (CNTs) have emerged as the ideal AFM probe tip material, yet existing methods for CNT-AFM probe fabrication have not been optimized. In this work, we present a fabrication method that yields direct control over the CNT's length, radius, and tilt angle by using a positioning stage operated in a transmission electron microscope (TEM) to directly attach a single-walled CNT to the apex of an AFM probe tip. The CNT probes are then utilized to image gold nanoparticles and DNA with tapping-mode AFM in ambient conditions. While imaging gold nanoparticles, we report a full-width radius dilation of 5.5 \AA$\,$ and nearly 8 nm resolution enhancement compared to commercially available super sharp Si AFM probes. We also measure a DNA fullwidth of less than 5.0 nm and observe, in some cases, the fine structure associated with the DNA double-helix with a pitch of 3.32 nm, which agrees well with the theoretical value of 3.4 nm.   

Experiments in NMR Force Microscopy

We report details of the construction and use of three nuclear magnetic resonance force microscopy (NMRFM) probes, as well as the development of control systems for three-dimensional nanoscale imaging and spectroscopy. Our variable temperature probe performed position-dependent $^{1}$H NMR force measurements on a 25x15x7 $\mu $m$^{3}$ single crystal of ammonium sulfate (NH$_{4})_{2}$SO$_{4}$ at room temperature in a sample-on-oscillator geometry. Force signals were detected with a signal-to-noise ratio of 6, and 12 $\mu $m resolution, in a one-dimensional scan. Measurements of NMR relaxation times T$_{2}^{\ast }$=1.5$\pm $0.2 $\mu $s, T$_{2}$= 44$\pm $2 $\mu $s, and T$_{1}$=5.6$\pm $0.7 s were obtained. We describe the upgrade of our $^{3}$He NMRFM probe for measurements towards the base temperature of 0.3K for investigation of nanoscale structures and metal oxide interfaces using the iOSCAR technique and perpendicular-cantilever geometry. Force-detected $^{11}$B NMR signals in a 30 $\mu $m crystal of superconductor MgB$_{2}$ have also been achieved using this probe. Efforts in the development of our NMRFM probe for the study of biological samples in liquid media are reported. Magnetic field effects on micromagnet films on cantilevers are being studied for the characterization of the mechanical sensors to be used in these liquid experiments.   

A Technique for In Situ Ballistic Electron Emission Microscopy

Ballistic electron emission microscopy (BEEM) is a scanning tunneling microscopy (STM) technique that can measure transport of hot electrons through materials and interfaces with high spatial and energetic resolution. BEEM requires an additional contact to ground the metal base layer of a metal semiconductor junction. Performing BEEM \emph{in situ} with the sample fabrication requires a custom built STM or modifying a commercial one to facilitate the extra contact, which leaves the technique to highly trained experts. This poster will describe our work to develop a special silicon substrate that has the extra contact built in to enable \emph{in situ} BEEM without modifications to the STM. Electrically isolated contact traces are lithographically patterned \emph{ex situ} onto the silicon substrate and connected to the BEEM sample plate which is then inserted into the ultra-high vacuum chamber. The metal is then deposited through a shadow mask and then mounted \emph{in situ} onto the STM for BEEM measurements. BEEM measurements comparing both \emph{in situ} and \emph{ex situ} deposited films will be presented.   

Nano and Micro Structures Image Based on Lens-Crystal System For Hard X-Ray Radiation

We present results of imaging properties of the lens-crystal system for hard x-ray radiation. The system is based on a beryllium parabolic refractive lens placed in front of the sample, and an asymmetric silicon single crystal placed behind the sample. We record the magnified x-ray phase contrast image at the x-ray energy 15~keV. The peculiarities of image transformation are investigated both experimentally and theoretically when the focus of refractive lens is moved across and along the optical axis. The computer program was elaborated for a simulation of image formation in the system based on the refractive lens and the crystal with asymmetric Bragg diffraction. The algorithm is based on the FFT procedure.   

High Throughput Tomography at the Advanced Photon Source 2BM Beamline enabling the Study of the Morphological Changes in MgH2 Destabilized LiBH4 Systems

Understanding morphologies in two phase hydride systems, such as MgH2 and LiBH4 mixtures, will permit the study of mass transport (i.e. diffusion), interface reaction(i.e. H2 desorption reactions) and ultimately models for H2 desorption and uptake rates. Many hydride systems are prepared by high energy ball milling which delivers stochastic microstructures from which many images are needed in order to collect reliable particle size and interfacial area data. The high throughput tomographic imaging system at 2BM of the Advanced Photon Source permitted data collection from a series of mixed hydrides---with the goal of optimizing energy for absorption contrast from a two phase system and determining relative amounts of hydride phase as well as interfacial area between the hydrides. Two-phase mixtures at LiBH4:MgH2 ratios of 1:3, 1:1, and 2:1 were imaged. The optimal energy for measurement was determined to be 15 keV (having 18{\%} transmission for the MgH2 phase and above 90{\%} transmission for the LiBH4 phase). Results showed that the {\%} of interfacial area for the mixed composite system was always higher in the catalyzed system---increasing from 15{\%} to 34{\%} in the 1:3 system, from 27{\%} to 60{\%} in the 1:1 system, and 22{\%} to 37{\%} in the 2:1 system.   

A study of biofunctionalized silica nanospring surface for immunosensor applications

A study of biofunctionalized VANS (vertically aligned (silica) nanospring) surface for immunosensor applications is presented. VANS surface treated with 3-aminopropyltriethoxysilane (APTES) leaves a primary amine groups on the VANS surface. Glutaraldehyde (GA) reacts with APTES modified VANS surface forming imine bonds at one end of glutaraldehyde, leaving aldehyde groups at the other end to react with the antibody. X-ray photoelectron study verifies each step of VANS surface functionalization. A goat anti mouse antibody (G$\alpha $M IgG I) is immobilized as a biorecognition layer on the APTES-GA modified surface and targeted to mouse IgG. It is investigated that mouse IgG captured from the solution phase specifically binds to goat anti mouse IgG on APTES-GA- G$\alpha $M IgG I. Then layer of G$\alpha $M IgG II attached to the APTES-GA- G$\alpha $M IgG I-mouse IgG surface reacts only when there is mouse IgG instead of rabbit IgG. A modeling of a resistor-inductor-capacitor (RLC) circuit of impedance spectra measured after the addition of successive layer indicates the these biological layers behave as insulating layers. It is explored that there is a greater magnitude of change between successive bio-layers below 10 kHz. Changes in the magnitudes of the elements of the RLC equivalent circuit indicate that the addition of biological layers impedes ionic motion thereby changing the effective dielectric response by the biomolecule polarization.   

Terahertz detection of nanorectifiers

Despite of a broad range of applications in the terahertz (THz) region, there is still a lack of compact, low-cost, solid-state devices that can function at room temperature. Here we report on THz detection by a novel type of unipolar nanodiode, known as self-switching diode (SSD), which is realized by breaking the symmetry of a nanochannel. The SSD has a completely different working principle from conventional diodes since it does not rely on any doping junction or Schottky barrier. Its intrinsically low parasitic capacitance enables electrical rectification at ultrahigh speed. In this report, the SSDs are coupled with standard bow-tie antennas. The THz signals are generated by means of a free-electron laser, at several THz frequencies ranging from 1.3 THz to 2 THz. The THz detection measurements were carried out at room temperature and 20 K. We find that the highest estimated voltage sensitivity achieved at room temperature and 20 K operations are around 390 mV/mW and 46 V/mW, respectively. For detection at room temperature, this is the highest speed reported in nanorectifiers to date. Furthermore, we also observe evidence of THz detection enhancement by localised plasma oscillation before the cut-off frequency as reported in the Monte-Carlo simulations.   

Characterization of Solar Cells via LED Spectrophotometer with Consideration of Spatial and Spectral Distributions of the LED Light Sources

Fast and cheap spectral response measurement systems are required in solar cell manufacturing lines in order to produce high quality solar cells. Using LEDs as light sources is the most promising technique. However, since LEDs inherently have 10 - 15 nm spreads in wavelength, the resolution for the wavelength is about 10 times worse than conventional spectrophotometers. In this report, a method to compensate for the wavelength spread of LEDs has been developed. The output power from a solar cell can be expressed as, $P_{out} =\int {P_{in} (\lambda )\times \varepsilon (\lambda )d\lambda \cong } \int {P_{in} (\lambda )\times \sum\limits_{i=0}^n {a_i \times \lambda ^id\lambda } } $, where $P_{in} (\lambda )$ is the distribution function of LED light power, $\varepsilon (\lambda )$is the efficiency of a solar cell, $\lambda $ is the wavelength and $a_i $ is the coefficient of an $n^{th}$ order polynomial approximation of $\varepsilon (\lambda )$. Since 8 different wavelength LEDs were used, the maximum value of $n$is 7. With the same method, quantum efficiency was also approximated. Two different sizes of Si single crystal solar cells were examined with this method. For the larger solar cell, the effect of spatial distribution of LED light intensity was also calibrated. By comparing the data from larger solar cell with existing data sheet, it was confirmed that the solar cell efficiency can be accurately measured by an LED spectrophotometer without assuming monochromatic light sources.   

Simulation and Modeling of charge particles transport using SIMION for our Time of Flight Positron Annihilation Induce Auger Electron Spectroscopy systems

Time of flight Positron Annihilation Induced Auger Electron Spectroscopy system, a highly surface selective analytical technique using time of flight of auger electron resulting from the annihilation of core electrons by trapped incident positron in image potential well. We simulated and modeled the trajectories of the charge particles in TOF-PAES using SIMION for the development of new high resolution system at U T Arlington and current TOFPAES system. This poster presents the SIMION simulations results, Time of flight calculations and larmor radius calculations for current system as well as new system.   

Solar cell evaluation using electron beam induced current with the large chamber scanning electron microscope

An initial study using electron beam induced current (EBIC) to evaluate solar cells has been carried out with the large chamber scanning electron microscope (LC-SEM) at the Western Kentucky University Nondestructive Analysis Center. EBIC is a scanning electron microscope technique used for the characterization of semiconductors. To facilitate our studies, we developed a Solar Amplification System (SASY) for analyzing current distribution and defects within a solar cell module. Preliminary qualitative results will be shown for a solar cell module that demonstrates the viability of the technique using the LC-SEM. Quantitative EBIC experiments will be carried out to analyze defects and minority carrier properties. Additionally, a well-focused spot of light from an LED mounted at the side of the SEM column will scan the same area of the solar cell using the LC-SEM positioning system. SASY will then output the solar efficiency to be compared with the~minority carrier properties found using EBIC.   

Circular dichroism in double layer metallic crossed-gratings

We report on the fabrication of double layer gold crossed-gratings consisting of a top convoluted layer of gold grating over a bottom gold grating using an e-beam direct write technique together with a lift-off process. The crossed-gratings exhibit, in the visible range, strong circular dichroism which is dependent upon the incident direction due to the convoluted top gold grating and also the conducting substrate. Resonance dips in the transmittance of circularly polarized light are also observed. The experimental results are explained qualitatively by simulations using a finite-integration technique. The simulations confirm that the dips in the transmittance are electromagnetic resonances corresponding to parallel and anti-parallel current flows in the crossed-gratings. (Accepted by J.Opt )   

Enhancement of magnetic moment in powder Mn11Si19

Higher-manganese silicides (HMSs), having a formula of MnSi$_{1.75-x}$, have drawn much attention in terms of future spintronics materials, following provisional findings of ferromagnetism in them. However, their magnetic properties have still been controversial. In this paper, magnetic moment in Mn$_{11}$Si$_{19}$ was measured at temperatures 5K to 290K, both in bulky and in powdery state. The bulk specimen was carefully prepared using a temperature gradient solution growth (TGSG) method and confirmed to be of single crystal by x-ray characterisation. It was found that Mn$_{11}$Si$_{19}$ had paramagnetism. Moreover, its magnetic moment was enhanced from 0.16 Wb m/$\mu_{B}$ to 0.22 Wb m/$\mu_{B}$ by changing its structure from bulky and powdery, respectively.   

Blue Afterglow from ZnS:Ag,Co Water Soluble Nanoparticles

Blue afterglow has been obtained from water soluble ZnS:Ag,Co nanoparticles by using a simple wet chemistry synthesis. The nanoparticles have a cubic zinc blende structure with average sizes of about 4 nm as determined by high-resolution transmission electron microscopy (HRTEM) and X-ray diffraction (XRD). The blue emission peaked at 441 nm is due to the transition from surface defects to Ag$^{+}$ luminescent centers. However, the afterglow from ZnS:Ag,Co nanoparticles is centered at 475 nm. The presence of Co$^{2+}$ is necessary to obtain the afterglow. The X-ray photoelectron spectroscopy (XPS) measurement indicates that oxidation occurred on the particle surfaces is also important. The blue afterglow ZnS:Ag,Co nanoparticles may open new applications in biological imaging, detection and treatment.   

Magnetic and structural properties of ferrofluids based on Cobalt-Zinc ferrite nanoparticles

Ferrofluids are colloidal systems composed of a single domain of magnetic nanoparticles with a mean diameter around 10 nm, dispersed in a liquid carrier. Magnetic Co$_{(1-x)}$Zn$_{x}$Fe$_{2}$O$_{4}$ ferrite nanoparticles were prepared via co-precipitation method from aqueous salt solutions in an alkaline medium. The composition and structure of the samples were characterized through EDX and XRD, respectively. Transmission Electron Microscopy studies permitted determining nanoparticle size. Grain size of nanoparticle conglomerates was established via Atomic Force Microscopy. The magnetic behavior of ferrofluids was characterized by Vibrating Sample Magnetometer; and finally, a Magnetic Force Microscope was used to visualize the magnetic domains of nanoparticles. The mean size of the crystallite of nanoparticles determined by using the Scherrer approximation diminished when the Zn concentration increases. The size of the nanoparticles obtained by TEM is in good agreement with the crystallite size calculated from XRD measures. The magnetic properties investigated at room temperature presented super-paramagnetic behavior, determined by the shape of the hysteresis loop. Finally, our magnetic nanoparticles are considered a soft magnetic material.   

Magnetic Quenching of Plasmon-photonic Activities in Fe$_{3}$O$_{4}$-Elastomer Composite

We present preliminary results on the effect of particle size on the optical response of Fe$_{3}$O$_{4}$-silicone elastomer composite in the presence of external static magnetic field. The optical response of composites containing 2wt{\%} of Fe$_{3}$O$_{4}$ particles of diameter range 20-30nm, 40-60 nm and d$<$500nm in silicone elastomer were measured using a PerkinElmer lambda 950 UV/vis/NIR spectrometer. We observed a systematic redshift in the optical response of composites containing nanoparticles (20nm-30nm and 40-60nm) with increasing static magnetic field strength, which saturates near 600 Gauss. However, we observed a dramatic suppression to near quenching of the plasmonic activities in the micron size particle (d $<$ 500nm) elastomer composites. This occurred even at very low applied static magnetic fields, suggesting particle size limitations in modulation of plasmon-photonics by external magnetic field.   

Photo-Switching of Magnetization in Iron Nanoparticles

We report the theoretical studies of light induced switching in core-shell nanoparticles. The core of the nanoparticle is made of Fe coated with the shell of azobenzene. The latter is a photochromic material with the reversible \textit{trans-cis} photoisomerization upon irradiation by UV and visible light. The magnetization of nanoparticles can be reversibly switched by using specific wavelengths of light. \textit{trans-cis} photoisomerization of azobenzene induces both the change in surface local magnetic moments and alters the exchange interactions on the surfaces of the nanoparticles. These two mechanisms can lead to induced magnetization switchable by light pulse. We study the effects of photoisomerization of azobenzene on iron (Fe) nanoparticle. Ab initio calculations using SIESTA code show that the ferromagnetic (FM) and antiferromagnetic (AFM) exchange interaction in Fe dimer increase by 40{\%} due to photoisomerization of azobenzene. While an infinite flat Fe monolayer shows variation on the exchange interactions on the surfaces as result of photoisomerization. The local magnetic moments of Fe sheet increase by 6{\%} due to photoisomerization. Using an \textit{ab initio }parameterization of magnetic interactions, we propose statistical model based on competing exchange interactions for the investigation of Fe nanoparticle magnetization. We performed Monte Carlo simulations of magnetization of the core-shell nanoparticle as a function of temperature. The results show that Fe nanoparticles magnetization at room temperature can change by at least 40{\%} due to photoisomerization of azobenzene.   

On the scattering of polarized neutrons by solitons in low dimensional magnetic systems

The cross section of the scattering of polarized neutrons by solitons in low dimensional systems with the magnetic structure is calculated. The authors consider solitons corresponding to the formation of a kink in a system of adatoms on the surface of a substrate or a crowdion in a chain of atoms in crystals described by the sine-Gordon equation, a breather with zero topological charge, and also solitons in a bound electron-phonon quasi-one-dimensional molecular chain. It is shown that study of the polarized neutrons scattering provides the possibility to investigate the~ static and dynamic properties of the solitons. In addition, the information obtained from the the neutron scattering allows for experimental reconstruction of the magnetic momentum distribution in these structures. We suggest that prospective experiments on observing and studying the solitons can deepen the insight into the physics of the strongly correlated systems.   

Theory of electrically-tunable coupling between magnetic nanoparticles on graphene

We will present a study of the dependence on graphene carrier density of the coupling between magnetic nanoparticles adsorbed on a graphene sheet. Small system {\it ab initio} coupling strength results will be compared with semi-analytic results based on a phenomenological Dirac-band model for graphene's $\pi$-bands. The model calculation uses kinetic exchange coupling parameters for the interaction between a magnetic nanoparticle and the graphene $\pi$-bands that are extracted from {\it ab initio} results for graphene/ferromagnetic metal interfaces.   

Synthesis and thermoelectric properties of PbTe nanostructures

Lead Telluride (PbTe) nanostructures were synthesized via the solvothermal/hydrothermal route. X-ray diffraction (XRD), scanning electron microscopy (SEM), and transmission emission microscopy (TEM) were used to characterize the as-prepared samples and indicated that the PbTe nanoparticles were cubical in shape and had face centered cubic structure. The effects of the addition of surfactants on the shapes and sizes of the nanocubes were investigated. PbTe nanocubes synthesized with the addition of surfactants showed uniform well-defined shapes. The effect of the synthesis temperature on the structure and morphology was also investigated; it was found that the particle size increased with the synthesis temperature. Thermoelectric property of the as-synthesized PbTe nanostructures was also investigated.   

Ab-initio Calculation of the Magnetic Properties of BN Nanoribbon

Recently, the search for new spintronics materials has also included graphene-based materials due to the theoretical prediction that this type of material may show the half-metallic property. Here, we present the results of a density functional theory within a generalized gradient approximation study of narrow Boron Nitride nanoribbon (BNNR). The objective of this study is to determine whether this type of material will be ferromagnetic (FM), antiferromagnetic (AFM), or ferrimagnetic. Our results show that the narrow zigzag shaped BNNR prefers antiferromagnetic state. The energy difference among the three states (FM, AFM, ferrimagnetic), however, is very small. Increasing the width of BNNR, was predicted to cause the stabilization of FM state instead of the AFM state.   

Ab initio nonadiabatic molecular dynamics of the ultrafast excitation energy transfer in small semiconducting carbon nanotube aggregates

Outstanding physical properties of carbon nanotubes (CNTs), such as well-defined optical resonance and ultrafast nonlinear response, result in CNTs gaining popularity in academic and industrial endeavors as potential effective energy generating devices. Following recent experiments on ultrafast excitation energy transfer in small semiconducting carbon nanotube aggregates [1], we report results of ab initio nonadiabatic molecular dynamics simulation of the energy transfer taking place in two carbon nanotube systems. We investigate the energy transfer between (8,4) and (10,0) CNTs, as well as (8,4) and (13,0) CNTs. In both cases, the CNTs are orthogonal to each other. Luer et al. in [1] elucidate the second excitonic transitions followed by fast intratube relaxation and energy transfer from the (8,4) CNT toward other acceptor tubes. Our project aims to provide a better understanding of the energy transfer mechanism in the given systems, which should foster development of a theory for the electronic structure and dynamics of CNT networks, hence enhancing their tailoring and application in the future. References 1.Larry Luer, Jared Crochet, Tobias Hertel, Giulio Cerullo, Gugliermo Lanzani. ACSNano. Vol.4, No. 7, 4265-4273   

Transport Imaging in the Near-Field Regime for the Determination of Carrier Diffusion Behavior in ZnO and GaN Nanostructures

Optical imaging of the spatial dependence of recombination luminescence generated at a point source is used to determine minority carrier diffusion lengths in GaN and ZnO nanostructures. By combining the resolution of near-field optics with the localized carrier generation capability of the scanning electron microscope, both diffusion and waveguiding behavior can be directly imaged in a contact-free manner for any luminescent material. This approach has been used to investigate the effect of different shell materials on GaN core/shell nanowires on surface recombination and carrier transport [Baird et. al. Appl. Phys. Lett. 98, 132104(2011)]. In ZnO, transport imaging shows variations in transport behavior depending on growth technique and morphology. Diffusion lengths in excess of 1 $\mu$m have been observed in ZnO nanobelts, consistent with reports of low concentrations of point and extended defects in these materials.   

Effect of Magnetic-Dielectric Interface on Capacitor Functions of Fe-Doped Barium Titanate Nanocomposites at Low Frequency ( $<$ 1 MHz)

Metal-dielectric interfaces have been shown to greatly influence the capacitor functions of nanodielectric composites. However, the effect of magnetic metal impurities in nanocomposites on dielectric properties is not well understood. Nanodielectric composites of iron in off stoichiometric barium titanate were fabricated by sol-gel process with varying pH concentrations. The fabricated samples containing 1-2{\%} iron reveal unique interfacial structure as studied by X-ray and atomic force microscopy. The capacitance of the as prepared samples was measured with an Agilent LCR meter at frequencies ranging from 20 Hz to 1 MHz. All nanodielectric composites show similar capacitor characteristic and reveal continuous decrease in capacitor density with increasing frequency. For a particular sample, this continuous decrease is a direct result of the porous interface between the iron granules and the dielectric material. This behavior, however, is not measurably influenced by an applied magnetic field in all samples.   

Spin dependent thermoelectric effect in magnetic tunnel junctions

Thermoelectric effects in magnetic nanostructures and the so-called spin caloritronics are attracting much interests . Indeed it provides a new way to control and manipulate spin currents which are key elements of spin-based electronics. Here we report on the magneto thermoelectric effect in a magnetic tunnel junction. The thermovoltage in this geometry can reach 1 mV. Moreover a magneto-thermovoltage effect could be measured with ratio similar to the tunnel magnetoresistance ratio. The Seebeck coefficient can then be tuned by changing the relative magnetization orientation of the two magnetic layers in the tunnel junction. Therefore our experiments extend the range of spintronic devices application to thermoelectricity and provide a crucial piece of information for understanding the physics of thermal spin transport.   

Enhanced thermoelectric properties using modulation-doping strategy in nanocomposites

We introduce the modulation-doping strategy in bulk SiGe nanostructures to improve the thermoelectric power factor. By separating carriers from their parent atoms via embedding heavily doped nanoparticles inside a host matrix, the ionized impurity scattering rate could be largely reduced, resulting in enhanced mobility. By band engineering, the carriers can spill over from nanoparticles into the host matrix, resulting in similar carrier concentrations, Fermi levels and consequently Seebeck coefficients as those of the uniform nanocomposites. In addition, nanoparticles with low thermal conductivities can further reduce the overall thermal conductivity of the sample. Combining the enhanced electrical conductivity, the reduced thermal conductivity and the unaffected Seebeck coefficient, we were able to enhance the thermoelectric properties of Ge-dilute Si95Ge5. And therefore were able to fabricate a low-cost sample with a competitive performance as those of the state of the art Si80Ge20. The fabricated modulation-doped sample has a higher figure of merit, compare to its equivalent uniform nanocomposite sample and compare to ionized impurity-doped host matrix.   

Ternary p-type half-Heuslers: another perspective bulk thermoelectrics

By employing the nanocomposite approach, we have achieved peak ZT of 0.8 at 700 $^{\circ}$C for p-type Hf$_{0.5}$Zr$_{0.5}$CoSb$_{0.8}$Sn$_{0.2}$. A larger difference in atomic mass and size of Hf and Ti than Hf and Zr in the crystal structure has produced a peak ZT of $\sim $1.1 at 800 $^{\circ}$C for p-type Hf$_{0.8}$Ti$_{0.2}$CoSb$_{0.8}$Sn$_{0.2}$. However the ZT peak is on the high temperature side. A ternary combination of Ti, Zr, and Hf at M site (MCoSb) has given rise to higher ZT in n-type MNiSn system. Will ternary on the M site yield better ZT for p-type? In this report, we will present our recent achievement of 10-25{\%} ZT improvement for temperature range 25-700 $^{\circ}$C without sacrificing the peak ZT value of 1.1 at 800 $^{\circ}$C.   

Potassium resonant doping in Bi$_{92.5}$Sb$_{7.5}$ material

Bi$_{1-x}$Sb$_{x}$ alloy material is a narrow band semiconductor when x is in the range of 0.07 -- 0.4. The energy gap reaches a maximum value of 0.014 eV in the composition range of x = 0.15 to 0.2. Thermoelectric properties on this material have been investigated for half a century due to the promising cooling applications. However, the low efficiency originating from the medium thermoelectric Power Factor (PF) prohibited this material from real applications. Here, we demonstrated that by potassium doping, both the Seebeck Coefficient and electrical conductivity can be enhanced by 25{\%} for the ingot samples for a broad temperature range. Further more, by combining with our nano grain size approach, thermal conductivity of ingot sample was reduced by 20{\%}, this lead to the figure of merit ZT increase of 50{\%} comparing to ingot counterpart. The potassium resonant doping was accounted for the PF enhancement for ingot Bi$_{92.5}$Sb$_{7.5}$ sample. We also find that the resonant doping can be realized for nano composite samples through parameters optimization.   

Simple solvothermal synthesis of large scale single crystal PbS nanorods and their properties

PbS nanorods and quantum dots have lots of applications such as in solar cells, gas sensors, infra-red detectors etc. PbS nanorods were synthesized via a very simple and inexpensive solvothermal method. Two reagents viz. Ethylenediaminetetraacetic acid (EDTA) and PEG--PPG--PEG triblock copolymer (P123) were employed in this growth for two special purposes. EDTA helps in slowing down the reaction and P123 plays the role of 1D structure directing agent. Comparison of the X--ray diffraction patterns of the nanowires synthesized using EDTA and without using EDTA clearly shows that the crystalline quality is significantly improved with EDTA. Transmission Electron Microscopy images and Selected Area Electron Diffraction patterns demonstrate that the nanorods are single crystalline. The average diameter of the nanorods was found to be about 18 -- 20 nm. Thus, the nanorods are within the quantum regime as the exciton Bohr radius of PbS is 18 nm. Raman spectra of these samples are compared.   

Superlattice formed by quantum dot sheets: density of states and IR absorption

Low-energy electronic states in heterostructures formed by periodically placed quantum dot (QD) sheets are studied taking into account (i) ultranarrow (single or several monolayers thickness) wetting layers and (ii) scattering by randomly-placed QDs. The host material is described within the effective mass approximation, scattering by the QD sheets is governed by the periodical Dyson equation, and effect of wetting layers is taken into account within the transfer matrix approach. Using the current conservation and inversion symmetry requirements, the transfer matrix is evaluated beyond effective mass approximation. The binding energy of localized state, the reflection (transmission) coefficient for the single QD sheet case, and the energy spectrum of superlattice are determined for the case of GaAs host matrix with InAs QD sheets. Spectral dependency of mid-IR absorption in superlattice due to photoexcitation of electrons from localized states into minibands is determined by the QD's concentration, period of sheets, and wetting layer characteristics. Such a dependency modifies the characteristics of optoelectronic devices based on heterosrtuctures with QD sheets.   

Voltage-tunable IR photodetector based on asymmetrically doped coupled quantum wells

We report on the mid-IR two-color photodetector with spectral photoresponse tunable by biased voltage. The device is based on GaAs/AlGaAs asymmetrically doped coupled quantum well structures grown by molecular beam epitaxy. A single unit of the detector is composed of 6.5 nm-thick GaAs layer doped with Si up to 5$\times $10$^{11 }$cm$^{-2}$ and 6.5 nm-thick undoped GaAs separated by 3.1 nm-thick Al$_{0.2}$Ga$_{0.8}$As. Units are separated by 50 nm-thick Al$_{0.2}$Ga$_{0.8}$As barriers. Our devices consist of 25 identical units. We demonstrated that in temperature range 20 - 70 K the peak detection wavelengths can be switched from 7.5 $\mu $m to 11.1 $\mu $m by varying bias from -5 V to +5 V. Our modeling shows that spectral tunability is basically determined by three factors. First, electron energy levels are shifted due to Stark effect. Second, electric field leads to the charge redistribution in the coupled wells and electron energy levels are shifted by electron-electron interaction. Third, tunneling processes from photoexcited quasi-localized electron states in wells to conducting states are enhanced in the electric field due to Fowler-Nordheim effect. The modeling results are in very good agreement with the experimental data. The proposed detectors are promising candidates for adaptive tunable sensing in the IR range.   

Analytical minimization of total spread of generalized Wannier functions in one-dimensional crystals

Since their introduction in 1937, Wannier functions have become key theoretical and computational tools in Solid State Physics. In this work, generalized Wannier functions of several bands in a one-dimensional crystal are investigated. For the case of two bands, necessary condition for minimum of total variance leads to a non-linear set of four second-order differential equations having a simple analytical solution. This is a new alternative to current iterative numerical procedures. Results are displayed for diatomic crystals with inversion symmetry. The relation between generalized Wannier functions and orthogonalized atomic orbitals is discussed. It is shown that such functions may present increased exponential localization when compared with Wannier functions of separate bands. Our findings should be relevant in studies of layered semiconductor and photonic structures and linear atomic chains. Hopefully, it will speed progress both in understanding and applications of Wannier functions.   

Capacitively coupled double quantum dot system in the Kondo regime

We present a detailed study of the low-temperature physics of an interacting double quantum dot system in a T-shape configuration is presented. Each quantum dot is modeled by a single Anderson impurity and we include an inter-dot electron-electron interaction to account for capacitive coupling that may arise due to the proximity of the quantum dots. By employing a numerical renormalization group approach to a multi-impurity Anderson model, we study the thermodynamical and transport properties of the system in and out of the Kondo regime. We find that the two-stage-Kondo effect reported in previous works is drastically affected by the inter-dot Coulomb repulsion. In particular, we find that the Kondo temperature for the second stage of the two-stage-Kondo effect increases exponentially with the inter-dot Coulomb repulsion, providing a possible path for its experimental observation.   

Electrical Characterization of a Hybrid 1D-0D System of Quantum Dot Chains

Recent developments in the area of material science and nanotechnology open new horizons to fabricate novel artificial materials with the possibility to engineer the electron and phonon material properties. Our objective is to create structures with high electrical conductivity and low thermal conductivity. The outcome will allow efficient transfer of thermal energy into the electrical output. Early theoretical and experimental research indicates that one-dimensional (1D) quantum wires and zero-dimensional (0D) quantum dots can be used to fabricate very efficient thermoelectric devices. This project focuses on the development of artificial material systems, called quantum dot chains, where 1D states, which are important for high electrical conductivity, coexist with 0D states, causing efficient acoustic phonon scattering, reducing the thermal conductivity. The samples are InGaAs/GaAs grown using an MBE technique of strain-induced self-assembly. Current analysis of our results shows anisotropies of temperature-dependent measurements of sheet resistance and electron mobility, along with physical characterization via AFM scans. Continued studies of electron and phonon physics in this system will help determine the potential for chains of quantum dots in thermoelectric applications.   

Tunable Quantum Phase Transition in a Dissipative Resonant Level

We show that quantum phase transitions (QPT) exist in a simple dissipative resonant level system. The electromagnetic environment couples both to tunneling processes (characterized by lead resistance Re) and to voltage fluctuations of the gate (characterized by gate resistance Rg). We bosonize this model and map it to a Tomonaga-Luttinger type model. Using a ``Coulomb-gas'' RG analysis, we relate our dissipative resonant level model to the double barrier problem in a Luttinger liquid. For the symmetric case and Re + Rg $>$ 2h/e$^2$, a Kosterlitz-Thouless QPT separates strong-coupling and weak-coupling phases. Interestingly, in the symmetric case, all relevant couplings between tunneling processes and the environment disappear, leading to perfect transmission at T=0. A second order QPT is also induced by coupling asymmetry for Re + Rg $<$ 2h/e$^2$. The two phases correspond to the resonant level merging with the right lead while the left lead decouples, and vice versa.   

Effects of intrasubband coupling on the scattering phases and density of states in a quantum wire

The properties of scattering phases and density of states in a quantum wire with an attractive scatterer are analyzed. We consider two bound states, belonging to different subbands, which couple to a scattering channel and give rise to two Fano resonances. It is shown that varying the parameters of the scatterer (such as its strength and position) produces significantly different effects on the phase behavior and density of states, depending on the subband they occur. These effects stem mainly from the difference between the coupling matrix elements of the two resonant levels with the propagating channel mode. As a consequence, the phase evolution in one subband may exhibit opposite behavior from the phase evolution in another subband. These findings may prove experimentally useful in ballistic transport through narrow channels.   

Porous silicon nanowire arrays decorated by Ag nanoparticles for surface enhanced Raman scattering study

A large scale and highly ordered Ag nanoparticle-decorated porous silicon nanowire array was fabricated for a uniform and reproducible surface-enhanced Raman scattering (SERS) substrate. The overall process for the proposed structure is simple and reliable with the use of only chemical etching and metal reduction processes. The SERS sensitivity of the novel substrate as low as 10$^{-16}$ M for rhodamine 6G (R6G) and the Raman enhancement factor as high as 10$^{14}$ were obtained. The excellent SERS performances were mainly attributed to the strong local electromagnetic effect which is associated with the formation of large-quantity Ag nanoparticles on porous silicon nanowire array and the existence of semiconductor silicon nanowires. Significantly, the quadratic relation between the logarithmic concentrations and the logarithmic integrated Raman peak intensities provided quantitative detection of R6G. Our results open new possibilities for applying SERS to trace detection of low-concentration biomolecules.   

Atomistic Modeling of Degradation Mechanisms in Nanoscale HEMT Devices

In this work, through atomistic numerical simulations, we investigate how performance degradation of state-of-the-art AlGaN HEMTs is governed by an intricate coupling of the underlying thermo-electro-mechanical processes while operating at high power and/or high-temperature. The polarization induced charge density is shown to be strongly dependent on the thickness of the AlN barrier layer. This further demonstrates that the degradation in these HEMT devices is related to the reduction of the effective thickness of the AlN barrier layer, which, during operation at high device temperature, could arise from the diffusion of gate metal into the barrier material matrix. This finding has been validated using the massively parallel LAMMPS molecular dynamics tool and available experimental data. We have also demonstrated that the polarization fields alone can induce channel carriers at zero external bias and lead to a significant increase in the ON current.   

Electron transport properties in self switching nano-diodes

Ultra-fast novel nanodevices, namely the self-switching diode (SSD) were fabricated from the InAs/AlGaSb heterostructures grown by solid-source molecular beam epitaxy on a semi-insulating GaAs (100) substrate. The two lines were etched thorough the 2DEG layer and become insulating. The effective channel width was actually smaller because of the depletion layer at the etched boundaries. Depending on the sign of the applied voltage the effective channel width will increase or reduce, giving rise to the diode-like characteristics. The diode-like characteristics were clearly observed for the InAs SSDs at 300K, and turn on voltage is strongly dependent on the channel width of the SSDs. In the SSD with $W$ = 460 nm, the InAs channel of was fully pinched of under the equilibrium condition, and positive voltage of 1.46 V was needed to drive a current thorough the InAs channel. On the other hand, the positive voltage of 2.32 V was needed to drive a current in the SSD with $W$ = 230 nm. We found to need the higher the turn-on voltage in the SSD of the narrower channel width. Furthermore, multi-channel SSDs were fabricated. $I-V$ characteristics and the AFM images of the SSD array of the symmetric nanowires, which are 1.5 $\mu $m long and approximately $W$ = 170 nm. The current densities were clearly increased with increasing the number of the nano-wires. The clear diode-like nonlinear behavior and rectification reflected ballistic nature of electrons in InAs-based SSD were observed.   

Modeling NIR-to-Visible Upconversion Kinetics in Er$^{3+}$, Yb$^{3+}$:NaYF$_{4}$ Nanocrystals and Finite-difference Time-domain Simulations of Noble-metal Enhancement Substrates

Upconversion (UC) phosphors are able convert infra-red light to visible wavelengths. The (erbium) Er$^{3+}$, (ytterbium) Yb$^{3+}$: NaYF$_{4}$ system is the most efficient upconversion phosphor known, and yet the quantitative aspects of the mechanism responsible for upconversion, such as the values of key microscopic rate constants, have not been determined. In this work, the dynamics of the photo-physical processes leading to near-infrared (NIR) to visible upconversion in Er$^{3+}$, Yb$^{3+}$: NaYF$_{4}$ nanocrystals are investigated using a nonlinear rate-equation model. In tandem with this effort, we use finite difference time domain (FDTD) simulations to analyze noble-metal nanostructures designed to enhance the absorption and emission of the upconverting phosphor. Simulation results are compared with time-resolved luminescence and reflectivity of the plasmonic substrates, which show systematic improvement in fluorescence yield and good agreement with simulated kinetics.   

Optical properties of Benzylpiperazine/CuI (111) system

We are interested in understanding the mechanism responsible for the recently observed [1] strong orange fluorescence in systems that contain Benzylpiperazine (BZP) molecules adsorbed on CuI film. To study possible optical transitions we use ab-initio electronic structure calculations based on density functional theory with the generalized-gradient (PBE) approximation for the exchange-correlation functional. In particular, we examine in detail the band structure including defect states of the CuI film with the lowest energy (111) surface and the excited states of the BZP molecule. It is shown that while optically non-active in the corresponding energy window, the two parts of the system, CuI slab and BZP, produce strong visible light emission when coupled together. Our numerical analysis demonstrates that the reason for such emission is the optical transitions between the excited molecule and the iodine vapor atom states on the CuI (111) surface. We discuss possible applications of the effect and that of defect vapor atom states on other optical properties of the system, including excitonic effects in the ultrafast response. [1] R. Blair, unpublished.   

Room temperature sub-diffraction-limited plasmon laser by total internal reflection

Plasmon lasers are a new class of coherent optical amplifiers that generate and sustain light well below its diffraction limit. Their intense, coherent and confined optical fields can enhance significantly light-matter interactions and bring fundamentally new capabilities to bio-sensing, data storage, photolithography and optical communications. However, metallic plasmon laser cavities generally exhibit both high metal and radiation losses, limiting the operation of plasmon lasers to cryogenic temperatures, where sufficient gain can be attained. Here, we present room temperature semiconductor sub-diffraction limited laser by adopting total internal reflection of surface plasmons to mitigate the radiation loss, while utilizing hybrid semiconductor-insulator-metal nano-squares for strong confinement with low metal loss. High cavity quality factors, approaching 100, along with strong lambda/20 mode confinement lead to enhancements of spontaneous emission rate by up to 18 times. By controlling the structural geometry we reduce the number of cavity modes to achieve single mode lasing.   

Probing semiconductor band structures and heterojunction interface properties with ballistic carrier emission: GaAs/AlxGa1-xAs as a model system

Utilizing tunnel emission of ballistic electrons and holes in a tunnel transistor with a Mott-barrier collector, we have developed a method to self-consistently determine the energy gap of a semiconductor and band offsets at a semiconductor heterojunction without using a priori material parameters. As a model system, electronic band gaps of the AlGaAs alloys together with conduction and valence band offsets at the GaAs/AlGaAs (100) interfaces are measured with a resolution of several meV at 4.2 K. The direct-gap band offset ratio for the GaAs/AlGaAs (100) interface is found to be 59:41. In the indirect-gap regime, ballistic electrons from direct tunnel emissions probe the X valley in the conduction band, while those from Auger-like scattering processes in the metal base film probe the higher-lying L valley. Such selective electron collection may be explained by their different momentum distributions and parallel momentum conservation at the quasiepitaxial Al/GaAs (100) interface. We argue that the present method is in principle applicable to arbitrary type-I semiconductor heterostructures. [W. Yi. at al. Phys. Rev. B 81, 235325 (2010)]   

Chaotic dynamics of electron resonant tunneling in a quasi-one-dimensional superlattice

A novel approach is employed for studying the dynamics of electron resonant tunneling in a quasi-one-dimensional superlattice. The semiclassical force-balance transport equation is used to describe the time-dependence of the dynamical equation for the electron center-of-mass velocity in the presence of an applied DC electric field and two external AC fields. Evolution of phase space trajectories from initial non-equilibrium states is obtained. The attractors of this dynamical system are determined and their stability for different sets of system parameters is obtained. Coupling between the electron's velocity and the external fields gives rise to rich spatiotemporal behavior including chaos.   

Cloaking Effect of Superlens in Time Domain

A ``perfect lens'' (ideal absorption-less slab with $\mu $=?=-1) or ``superlens'' (a perfect lens with small absorption) can cloak a small object located in its vicinity such that no far field observer can detect the small particle, i.e., being invisible. While the problem is well understood in the steady state by solving the Maxwell equation in frequency domain, its time domain properties, such as how the cloaking effect started, remained unknown. In this paper, by using the time-dependent Green's function approach, we present a time domain study of the cloaking properties of the ``superlens'' As a current source is turned ``on,'' the system's response will be consisted of a transient response in the beginning and a steady state response in the long run. It turns out that it takes a long time (tens of thousands of cycle) for the ``perfect lens'' to build up the cloaking effect, and this required period depends on a number of factors, such as the separation between the lens and the particle, the absorption of the slab, and the dispersion of the slab. Moreover, along with many other interesting effects, we also find that, the dipole moment, on its way to be invisible, it oscillates with a decreasing amplitude.   

Quantum mechanical investigation of CO, NO and HCN adsorption on Ag8 nanoclusters; (structural, electronic and topological properties)

We used DFT-GW method to study pure Ag8 nanocluster and CO, NO, HCN adsorption on it. The pure Ag8 stabilizes in Td structure and the behavior of binding energy is consistent with the total charge density at bond points while bond length exhibits a behavior similar to the mean bond charge. Adsorption of CO, NO and HCN molecules on different atomic sites of Ag8 were studied and it was found that all these molecules, mainly due to electron donation, prefer to be adsorbed by LUMO orbital. This finding is in qualitative agreement with the experimental observation of the enhanced binding of CO molecule on positively charged silver atoms, compared with the neutral and negatively charged samples [J. Phys. Chem. A 2006, 110, 7167-7172]. Our calculation shows no significant charge transfer during NO adsorption on Ag8. The CO, NO and HCN adsorbed nanoclusters as well as cationic and anionic samples do not stabilize in Td structure. This charge induced geometry transition is related to the high (low) value of LUMO (HOMO) state in Td structure which indicates high energy cost of electron donation (acceptation) in this isomer. The topological analysis of electron density clarify that the adsorption energy of molecules on Ag8 is consistent with the value of the charge density at the topological bond point between molecule and nanocluster.   

Electronic transport and scanning tunneling microscopy study of bismuth quantum films

Semimetal films, such as bismuth films, have attracted great interest due to their unique electronic properties around the Fermi level. Bulk Bi has three conduction band minima at L points about 40mV lower than the single valance band maximum at T point. However, with decreasing Bi layer thickness, an energy gap is expected to open when the size confinement becomes sufficient to raise the lowest electron subband above the highest hole subband. We have utilized quantum growth method to fabricate atomically flat Bi thin films with thickness controllable down to single atomic layer. The quantum films are then studied in situ with a cryogenic four-probe STM to examine the correlation of the electronic and transport properties. An oscillatory resistivity change is observed as function of film thickness, which is discussed in comparison with the local density of states.   

First-order phase transition in In/Si(111)-4$\times $1

In/Si(111)-4$\times $1 surface is known to undergo a temperature-induced structural phase transition between a high-temperature (HT) 4$\times $1 phase and a low-temperature (LT) 8$\times $2 phase. The issues on the nature of this temperature-dependent phase transition, whether it is of order-order (displacive) or order-disorder type and first-order or second-order, are unsettled. We investigated the 4$\times $1-to-8$\times $2 phase transition by using LEED and STM. We observed a hysteresis of the LEED beam intensities as the temperature changes during cooling and heating. This hysteresis indicates that the structural phase transition is of first order. STM images revealing the coexistence of the 4$\times $1-HT and 8$\times $2-LT phase domains during the transition are consistent with the first-order transition. An influence of the defects on the phase transition of this In/Si(111) surface will be discussed.   

Glancing Angle Deposition of Ag on Si(111)7x7

Ag(111) films were vapor-deposited in ultra-high vacuum on Si(111)7x7 substrates. The angle of deposition was varied from normal incidence to 80 degrees and the films were studied by x-ray reflectivity. It is found that, even for very thin films, the film roughness increased dramatically with the angle of deposition. This poster will highlight experimental results as well as the development of a UHV chamber that enables a laboratory x-ray source to monitor low angle reflectivity during film growth. Funding is acknowledged from the Ronald E. McNair Post-baccalaureate Achievement Program and NSF DMR-0706278. Some measurements were performed on the 6IDC beam line, supported by the US-DOE (through Ames Lab, W-7405-Eng-82), at the Advanced Photon Source (US-DOE, W-31-109-Eng-38) located at Argonne National Laboratory.   

Photoluminescence characterization of Cl -- doped Cu$_{2}$O thin film photoelectrodes

Electrodepostion was used to deposit ~Cl-doped Cu$_{2}$O thin films on ITO substrates. Photocurrent and the Photouminescence (PL) measurements were done on prepared electrodes. CuCl$_{2}$ and different pH values of the solution bath were used to control the doping level in Cu$_{2}$O. Photocurrent responses in photoelectrochemical cells clearly showed improved performance over the un-doped Cu$_{2}$O thin films as seen by the difference in the light and the dark current . A deconvoluted PL spectra assuming Gaussian spectral profile showed three underlying peaks. The temperature dependence of the ~peak energy position and intensity was analyzed. Furthermore, the electrodeposition and the nature of the conductivity of the films were also analyzed. These results of the Cl-doped Cu$_{2}$O films will be compared with those for the Cu$_{2}$O un-doped films and presented.   

High transconductance zinc oxide thin-film transistors on flexible plastic substrates

We report the fabrication and characterization on high-performance ZnO based TFTs on unheated plastic substrate. ZnO films were grown by pulsed laser deposition (PLD) on polyethylene napthalate (PEN) substrates. Top-gate ZnO-TFTs were fabricated by photolithography and wet chemical etching. The source and drain contacts were formed by lift-off of e-beam deposited Ti(20 nm)/Au(200 nm). An HfO$_{2}$ with thickness 100 nm was selected as the gate insulator, and top gate electrode Ti(20 nm)/Au(200 nm) was deposited by e-beam evaporation. We prepared a set of the structure with SiO$_{2}$/TiO$_{2}$ to investigate the characteristic changes that appear in the film characteristics in response to bending. From the $I_{D}-V_{DS}$ and the transfer characteristics which are affected by bending and return for the ZnO-TFT with SiO$_{2}$/TiO$_{2}$ buffers, the TFTs were bent to a curvature radius of 8.5 mm. The transconductance, $g_{m}$ is obtained 1.7 mS/mm on flat, 1.4 mS/mm on bending and 1.3 mS/mm on returning the film, respectively. The $I_{D}-V_{DS}$ characteristics were therefore not changed by bending. All of the devices exhibited a clear pinch-off behavior and a high on/off current ratio of $\sim $10$^{6}$. The threshold voltages, $V_{th}$ were not changed drastically. Furthermore, TFT structures were changed from a conventional top-gate type to a bottom-gate type. A high transconductance of 95.8 mS/mm was achieved in the bottom-gate type TFT by using Al$_{2}$O$_{3}$ oxide buffer.   

Transition Metal Disulfide Films by Reactive Sputtering: Structure, Magnetism, and Electronic Transport

Transition metal disulfides ($i.e.,$ \textit{TM}S$_{2}$, where \textit{TM} = Cu, Co, Fe, Ni, etc.) are a unique class of materials displaying diverse functional properties such as highly spin-polarized ferromagnetism (Co$_{1-x}$Fe$_{x}$S$_{2})$, superconductivity (CuS$_{2})$, a Mott insulating ground state (NiS$_{2})$, and unusually high potential for solar absorber applications (FeS$_{2})$. Significant research has been performed on bulk crystals of these materials but little has been done on thin films, despite the obvious potential for heterostructured systems due to their diverse functionality and intrinsic epitaxial compatibility. In this work we report on the wide applicability of reactive sputtering from metallic targets in an Ar/H$_{2}$S gas environment as a reliable deposition method. Via detailed characterization of crystal structure, microstructure, and electronic, magnetic, and optical properties, we demonstrate successful deposition of a wide variety of single phase polycrystalline thin films with bulk-like properties. A brief survey of optimized growth conditions, resulting physical properties, and general trends based on metal reactivity, will be presented. We argue that this method holds great promise for the synthesis of a novel family of all-sulfide-based heterostructured devices. Work funded by NSF MRSEC and UMN IREE.   

Titanium Isopropoxide Precursor Volume Consumption as a Function of Temperature for Titanium Dioxide Thin Films Grown by Atomic Layer Deposition

Atomic layer deposition (ALD) offers tremendous opportunities for controlling material synthesis on an atomic level and for creating nanolayers with unique new functionalities. ALD is a chemical gas phase thin film deposition method based on alternating surface reactions that employs two or more precursors. ALD is often used for growth of high k dielectric constant oxide films. Titanium dioxide material have a k value of 80, and a band gap of $\sim $ 3 eV, and due to strong oxidizing properties thin films coated on construction materials and glass have fog proof, and self cleaning properties. Our ALD reactor employs liquid Titanium Isopropoxide [Ti{\{}OCH(CH$_{3})_{2}${\}}$_{4}$] as a metal precursor and distilled H$_{2}$O as an oxygen source to grow thin films of titanium dioxide [TiO$_{2}$] on silicon [Si], gallium nitride [GaN], and Aluminium foil [Al-foil] substrates. Titanium Isopropoxide exhibit a vapor pressure surge above 40$^{\circ}$ C and we report the volume precursor consumption as a function of precursor temperature and thin film thickness for ALD grown TiO$_{2 }$on Si, GaN, and Al-foil substrates. We will also present dielectric constants of the TiO$_{2}$ thin films measured with a variable angle spectroscopic ellipsometer.   

Epitaxial growth of BiFeO$_{3}$ thin films on SrTiO$_{3}$/Si substrates

We are using molecular beam epitaxy (MBE) to grow BiFeO$_{3}$ (BFO) thin films. SrTiO$_{3 }$ (STO) on Si is used as a virtual substrate to enable the growth of BFO. Commensurate growth of STO on Si using MBE has been achieved by using co-deposition with the fluxes adjusted for stoichiometric growth and the growth rate is determined using RHEED intensity oscillations. The native oxide of the Si substrates is removed in-situ by deoxidation at around 750\r{ }C using a flux of Sr. The substrate is cooled to 500\r{ }C and additional Sr is added to form template with a (2x1) surface structure. BFO is then deposited on well-characterized STO (2-20nm thick) on Si using Fe and oxygen plasma with an overpressure of Bi flux- the growth rate being controlled by the incoming Fe flux. The RHEED pattern taken during deposition of BFO shows 2-D growth front with a 6-fold surface reconstruction. The structural and magnetic properties of the BFO samples have also been measured.   

Disorder trapping during crystallization of the B2 ordered NiAl system

Using molecular dynamics (MD) simulations, disorder trapping associated with solidification is studied for the (100), (110) and (111) growth directions in the B2 NiAl ordered alloy compound. At the high interface velocities studied we observe pronounced disorder trapping, i.e., the formation of antisite defects and vacancies in the crystal at higher than equilibrium concentrations upon rapid solidification. The vacancies are located primarily on the Ni sublattice and the majority of antisite defects are Ni atoms on the Al sublattice, while the concentration of Al on the Ni sublattice is negligibly small. The defect concentration is found to increase in an approximately linear relationship with an increase of the interface velocity or undercooling. Further there is no significant anisotropy in the defect concentrations for different interface orientations.   

Strain Engineering in Self-Assembled Vertically Aligned Nanocomposite Thin Films

Self-assembled vertically aligned nanocomposite (VAN) thin films hold great promises in engineering material functionalities including ferroelectric, ferromagnetic, and multiferroics by tunable intrinsic strains. The VAN exhibits a well ordered vertically columnar structure with highly epitaxial quality in thin film. In this work, we describe the approach of strain tuning in (BiFeO$_{3})_{x}$:(Sm$_{2}$O$_{3})_{1-x}$, (La$_{0.7}$Sr$_{0.3}$MnO$_{3})_{0.7}$:(Mn$_{3}$O$_{4})_{0.3}$ and (BaTiO$_{3})_{x}$:(Sm$_{2}$O$_{3})_{1-x}$ VAN thin films grown by pulsed laser deposition (PLD). We investigate the intrinsic strain mechanism in VAN thin films as well as the effects of film growth kinetics and thermodynamic stability to the final VAN architecture. We also demonstrate the tunable ``microstructure - strain - physical properties'' relationships in VAN thin films. Experiment results reveal that: 1) Enhanced vertical strain can cause systematic reduction of dielectric loss of VAN thin films; 2) Strain tuning in ferromagnetic VAN thin films can lead to tunable T$_{c}$ and magneto resistance; and 3) RT ferroelectric can be achieved by strain tuning of BTO and results the highest T$_{c}$ in VAN thin film. Furthermore, geometrically controlled nanoporous structure can also be processed by thermal treatment on VAN thin films.   

STM Examination of Trithiapentacone on Vicinal Gold (788)

Trithiapentacone (TTPO) is a member of a group of promising novel pentacene derivatives of interest in organic electronics for their excellent resistance to photooxidation and a range of band gaps. TTPO is a robust molecule with a HOMO-LUMO gap of 1.95 eV that can be thermally evaporated onto an electrode. TTPO is a polar species of pentacene with centered oxygen and sulfur bridge substituents. The vicinal gold (788) surface is a well studied surface on which pentacene molecules and other pentacene derivatives such as 6,13-dichloropentacene self assemble in long range order. We will present a STM study of TTPO on Au (788) addressing the possibility of self-assembly for organic semiconductor applications.   

High resolution STM imaging of a unit cell of SrTiO$_3$(100)-$\sqrt{5}\times\sqrt{5}-R26.6^\circ$ surface superstructures

SrTiO$_3$(100)-$\sqrt{5}\times\sqrt{5}-R26.6^\circ$ surfaces were studied by scanning tunneling microscope (STM) in ultra-high vacuum conditions at room temperatures. In the our previous report, we showed the arrangement of titanium and oxygen atoms in the unit cells of a ($\sqrt{5}\times\sqrt{5}$) surface superstructure with scanning tunneling microscope and concluded the TiO$_2$ layer is the terminating plane of the ($\sqrt{5}\times\sqrt{5}$) surface [1]. Recently, we succeeded in imaging the surfaces in filled states with much higher spatial resolution. Oxygen atomic orbitals are individually recognized and the local structures at the center of the O fourfold hollow site with ($\sqrt{5}\times\sqrt{5}$) periodicity are more clearly seen. Comparing our experimental results with the previous works, especially a theoretical study of O-vacancy model [2] and an experimental and theoretical study of Sr adatom model [3], detailed discussion on $\sqrt{5}\times\sqrt{5}$ surfaces became possible.\\[4pt] [1] I. Shiraki and K. Miki, Surf. Sci. 605, 1304(2011)\\[0pt] [2] Z. Fang and K Terakura, Surf. Sci. 470, L75(2000)\\[0pt] [3] T. Kubo and H. Nozoye, Phys. Rev. Lett. 86, 1801(2001)   

Real-Time Grazing Incidence Small Angle X-Ray Scattering Studies of the Growth Kinetics of Sputter-Deposited Silicon Thin Films

The fundamental kinetics of thin film growth remains an active area of investigation. In this study, silicon thin films were grown at room temperature on silicon substrates via both on-axis and off-axis plasma sputter deposition, while the evolution of surface morphology was measured in real time with \textit{in-situ} grazing incidence small angle x-ray scattering (GISAXS) at the National Synchrotron Light Source. GISAXS is a surface-sensitive, non-destructive technique, and is therefore ideally suited to a study of this nature. In addition to investigating the effect of on-axis versus off-axis bombardment, the effect of sputter gas partial pressure was examined. \textit{Post-facto}, \textit{ex-situ} atomic force microscopy (AFM) was used to measure the final surface morphology of the films, which could subsequently be compared with the surface morphology determined by GISAXS. Comparisons are made between the observed surface evolution during growth and theoretical predictions. This work was supported by the Department of Energy, Office of Basic Energy Sciences.   

Structure of electrolyte decomposition products on high voltage spinel cathode materials determined by in situ neutron reflectometry

Interfacial reactions on electrical energy storage (EES) materials mediate their stability, durability, and cycleablity. Understanding these reactions in situ is difficult since they occur at the liquid-solid interface of an optically absorbing material that hinders the use of techniques such as infra-red spectroscopy. Furthermore, since the interfaces involve liquids classic vacuum-based analytical methods can only probe reaction products, which are stable under vacuum. Here, we present the results of an in situ neutron reflectometry study detailing the formation of a thick solid-electrolyte interphase (SEI) on a high voltage spinel cathode material. The cathode/electrolyte system used in this study is a LiMn1.5Ni0.5O4 thin film subjected to a 1.2 molar LiPF6 in 1:1 ethylene carbonate - dimethyl carbonate electrolyte solution.   

Dependence of optical absorption of thin nanocomposite films on inclusions distribution

It is well known, that one of the ways to widen the spectral range of the elements of photovoltaic systems is using the nanocomposite films as part of its active element. The problem of characterization of the optical properties of nanocomposite thin films, especially with inhomogeneous distribution of inclusions, arises. In this work, we propose an approach which allows us to determine the parameters of the nanocomposite film which cannot be measured directly. The approach is based on the previous works where the ultrathin films electrodynamics was studied earlier. The absorption spectrum sensibly depends on the film parameters. They are the inclusions' concentration in the film, the presence of a shell of the inclusion particles and the size of this shell, the spatial orientation of particles in a layer and the presence of different types of particles and their relative spatial distribution in the film. The strong dependence of the absorption profiles on the type of distribution of particles along the film, \textit{ceteris parebus}, is demonstrated in this report.   

Precise Control of Areal Density of `Click' Functional Groups on Nanoparticle Surfaces

Tremendous effort has been invested into devising methods that enable full control of surface functionalities by use of a mixture of two self-assembled monolayers (SAM). By assuming control of the relative amounts of SAM moieties on a surface it is possible to fabricate materials with well-known molecular compositions and, thus, well-characterized properties. This could be achieved by mixing one reactive and another inert modifying agent or by utilizing two agents with complementary properties. These surfaces have shown benefits in various applications ranging from sensing to DNA sequencing. Two methods are performed to control the exact amount of 'click' functionalities on the curved surfaces of silica and iron oxide. The density of alkyne is controlled by mixing two silanes, of which one bears the alkyne functionality, and the other does not. Azide density, however, is controlled in a novel method using the kinetic control of the substitution reaction of bromide to azide on a bromide-terminated silanized surface. We report herein a novel method to quantitatively ascertain no phase separation of the silanes occurred on the curved surface, and that true mixing was achieved. We also show that true mixing of silanes occurs regardless of surface geometry or identity for both systems   

First-principles study of adsorption and dissociation of CO$_{2}$ on $\alpha $-Pu (020) surface

The adsorption and dissociation of CO$_{2}$ on the $\alpha $-Pu (020) surface has been investigated using density functional theory (DFT) within generalized gradient approximation (GGA). The full-potential FP/LAPW+lo method has been used to calculate the adsorption energies at the scalar relativistic with no spin-orbit coupling (NSOC) and fully relativistic with spin-orbit coupling (SOC) theoretical levels. The completely dissociated configurations (C+O+O) exhibit the strongest binding with the surface, followed by the partially dissociated configurations (CO+O), with the molecular CO$_{2}$ adsorption having the weakest binding with the Pu surface. It is found that the SOC effect increases the adsorption energies for all considered adsorptions. For all initial vertically upright orientations, the geometry and orientation of the CO$_{2}$ molecule do not change after optimization. For all initial flat lying orientations, the final state of the CO$_{2}$ molecule corresponds to a bent geometry with a bond angle of $\sim $130$^{\circ}$. For CO+O co-adsorption, the stable configurations corresponded to CO dipole moment orientations of 105$^{\circ}-167^{\circ}$ with respect to the normal surface. The local density of states and difference charge densities are also discussed.   

Calculating thermodynamics properties of quantum systems by a non-Markovian Monte Carlo procedure

We present a history-dependent Monte Carlo scheme for the efficient calculation of the free energy of quantum systems inspired by Wang-Landau and metadynamics. In the two-dimensional quantum Ising model, chosen here for illustration, the accuracy of free energy, critical temperature, and specific heat is demonstrated as a function of simulation time and successfully compared with the best available approaches. The approach is based on a path integral formulation of the quantum problem and can be applied without modifications to quantum Hamiltonians of any level of complexity. The combination of high accuracy and performance with a much broader applicability is a major advance with respect to other available methods.   

Application of Group Representation Theory to Derive Hermite Interpolation Polynomials on a Triangle

Current methods used to devise sets of Hermite interpolation polynomials of minimal order that ensure $C^{(n)}$ continuity across triangular element boundaries in two dimensions are not readily extensible to higher dimensions. The extension of such methods is especially difficult when the number of degrees of freedom afforded by data at points is different from the number of degrees of freedom determined by the coefficients of a complete polynomial basis to a particular order. This work introduces a formalism based on group representation theory that can accomplish this task in general. The method is introduced through the derivation of $C^{(1)}$ continuous Hermite polynomials that interpolate data at the three vertices of an equilateral triangular element. These interpolation polynomials are reported here for the first time. The polynomials derived here are compared to the standard polynomials defined in a right triangle by using the two sets of polynomials to solve the Laplace equation over finite elements, and also by interpolating example functions over a single triangle element.   

Finite Temperature Properties of Clusters by Replica Exchange Metadynamics: The Water Nonamer

We introduce an approach for the accurate calculation of thermal properties of classical nanoclusters. Based on a recently developed enhanced sampling technique, replica exchange metadynamics, the method yields the true free energy of each relevant cluster structure, directly sampling its basin and measuring its occupancy in full equilibrium. All entropy sources, whether vibrational, rotational anharmonic and especially configurational -- the latter often forgotten in many cluster studies -- are automatically included. For the present demonstration we choose the water nonamer (H$_2$O)$_9$, an extremely simple cluster which nonetheless displays a sufficient complexity and interesting physics in its relevant structure spectrum. Within a standard TIP4P potential description of water, we find that the nonamer second relevant structure possesses a higher configurational entropy than the first, so that the two free energies surprisingly cross for increasing temperature.   

The predictive integration method for dynamics of infrequent events

With the increasing prominence and availability of multi-processor computers, recasting problems in a form amenable to parallel solution is becoming a critical step in effective scientific computation. We present a method for parallelizing molecular dynamics simulations in time scale, by using predictive integration. Our method is closely related to Voter's parallel replica method, but goes beyond that approach in that it involves speculatively initializing processors in more than one basin. Our predictive integration method requires predicting possible future configurations while it does not suffer from restrictions due to correlation time after transitions between basins.   

Coarse graining approach to First principles modeling of radiation cascade in large Fe super-cells

First principles techniques employed to understand systems at an atomistic level are not practical for large systems consisting of millions of atoms. We present an efficient coarse graining approach to bridge the first principles calculations of local electronic properties to classical Molecular Dynamics (MD) simulations of large structures. Local atomic magnetic moments in crystalline Fe are perturbed by radiation generated defects. The effects are most pronounced near the defect core and decay with distance. We develop a coarse grained technique based on the Locally Self-consistent Multiple Scattering (LSMS) method that exploits the near-sightedness of the electron Green function. The atomic positions were determined by MD with an embedded atom force field. The local moments in the neighborhood of the defect cores are calculated with first-principles based on full local structure information. Atoms in the rest of the system are modeled by representative atoms with approximated properties. This work was supported by the Center for Defect Physics, an Energy Frontier Research Center funded by the US Department of Energy, Office of Science, Office of Basic Energy Sciences.   

Modeling Nonlinear Non-Stationary Self-Organized Asymptotic States in High Energy Density Plasmas Driven by Intense Crossing Laser Beams

The multiscale dynamics of Vlasov-Maxwell and Vlasov-Poisson systems where non-stationary self-organized states can be formed is a grand challenge. From chaotic particle orbits there arise collective self-consistent fields which are highly coherent. We will explain the novel aspects of these states and contrast them with electron plasma waves which are small amplitude disturbances that are naturally resonant. We will also point out connections between these systems and the high Reynolds number limit of 2D incompressible (Euler) turbulence, galaxy formation models and applications in the nonlinear optics of laser-matter interactions. Techniques will be given of capturing the multiscale dynamics and the conformal invariance hypothesis in the fine scale structures beneath the large scale order found in such systems.   

Multiscale modeling of nanofoams under irradiation

Nanoscale porosity appears in solids under a number of conditions: radiation damage in nuclear reactors, initial stages of ductile failure, in astro-materials, etc. Using molecular dynamics (MD) simulations, we analyze the radiation damage and surface modification of materials with various nanoscale porosities, where experimental techniques can be difficult to use and interpret. We consider (a) irradiation with ions with energies in the range 1-25 keV, of interest for fusion and fission energy applications; (b) swift heavy ion irradiation, with energies up to few GeV, relevant for track formation and interstellar grain evolution. We find that irradiation effects have larger spatial extent than for full-density solids and include the production of point-defects and twins which change the mechanical properties of the samples. We use our MD results as input for a Monte Carlo (MC) code to calculate sputtering yields from nanofoams of different geometries under different irradiation conditions. We also use our MD results to build models which predict possible radiation endurance under intense irradiation.   

Application of Gaussian Approximation Potentials to Barium Titanate

The computational study of complex phenomena over long time and distance scales not accessible to first principles calculations requires empirical potentials usually derived by integrating out the electronic degree of freedoms. Recently, a parameter free approach called Gaussian Approximation Potentials (GAP) has been shown that it can duplicate first principles density functional theory (DFT) total energies and atomic forces accurately [1]. In a GAP the atomic neighborhood of an atom is projected onto the angular momentum channels of Wigner-D functions, yielding a bispectrum of the expansion coefficients of the atomic density. A non-parametric Gaussian Process regression is used to fit a database of total energies and forces to the combination of angular momentum channels constituting the bispectrum. We report initial results of using a GAP to describe the ferroelectric perovskite Barium Titanate (BaTiO$_3$). \\[4pt] [1] A.~P. Bartok, M. C. Payne, R. Kondor, and G. Csanyi, Phys. Rev. Lett. {\bf 104} 136403 (2010).   

Ab initio simulation of radiation damage in nuclear reactor pressure vessel materials

Using Kinetic Monte Carlo we developed a code to study point defect hopping in BCC metallic alloys using energetics and attempt frequencies calculated using VASP, an electronic structure software package. Our code provides a way of simulating the effects of neutron radiation on potential reactor materials. Specifically we will compare the Molybdenum-Chromium alloy system to steel alloys for use in nuclear reactor pressure vessels.   

Influence of Nutation of the Projectile on Fracture of the Anisotropic Target

In actual practice the direction of the vector of velocity, as a rule, doesn't coincide with the direction of a longitudinal axis of a moving body, and makes with it some angle named the angle of nutation. The nutation influence on process of interaction of the projectile and a target is defined not only by its size, but also geometrical and kinematic parameters of the process. It is obvious that for a case of the prolonged projectile the angle of nutation influence is more considerable, than for the compact projectile because in this case the nutation angle presence changes not only a picture of strain-stress state of interaction bodies, but also leads to loss of stability in the projectile. The three-dimensional problem of oblique high-velocity interaction of the prolonged cylindrical projectile from steel with an anisotropic target from organoplastic is considered. Lengthening of the projectile makes from 15 to 30 calibers, the range of initial velocities of the projectile from 700 to 3000 m/sec is researched. Modeling is carried out numerically by a method of finite elements. Influence of nutation angle and rotation of the projectile on fracture of target and stability of the projectile is analyzed.   

Mechanical Behavior of Ceramic Composites under Pulse Loadings

The prediction of mechanical behavior ceramic composites under pulse loadings is the complicated problem owing to insufficient knowledge about laws of structure evolution and nucleation and accumulation of damages. Computer simulation of mechanical behavior of ceramic composites at single and repeated pulse influences of submicrosecond duration are presented in the given work. The model of the structured representative volume of ultrafine-grained ceramics composites was developed using the data of microscopic researches. Deformation and damage of structured representative volumes of some ceramic composites on meso-scale level were simulated under pulse loadings having amplitudes near several GPa. The critical fracture stress on meso-scale level depends not only on relative volumes of voids and strengthened phases, but also sizes of corresponding structure elements. It was shown that the isolated micro- and meso-scale cracks can be generated in ceramic composites at pulse amplitude less than the Hugoniot elastic limit. In the studied ceramic composites the critical failure stress in spall zone is changed nonmonotonically with growth of the volume concentration of strengthened phases.   

Interaction of free boundary of fluid layer with Taylor wave created by laser pulse

In the hydrodynamic laboratory of SarFTI NRNU MEPhI experiments carry out to study the instability of the free surface of a thin (about 2 mm) layer of fluid, developing after the going to surface nonsteady decaying shock wave (Taylor wave) with amplitude of about 1 kbar. The Taylor wave was produced by evaporating a thin (about 1 mm) target located on the surface layer under the influence of the laser pulse duration of 10 ns. The results of the experiments and discussion are presented.   

Superconductivity and Magnetism in isovalent substituted Europium Iron-Asenide Superconductor EuFe$_{2}$As$_{2-x}$P$_{x}$

The EuFe2As2-xPx system is known as a coexisting system of superconductivity and magnetism with varying isovalent substitution concentration of P in place of As. Magnetism seems to originate from localized Eu$^{2+}$ moments around 19 K, whereas Superconductivity occurs at 28 K in x = 0.4. We have studied magnetic and transport properties in various single crystals grown by the self-flux method in a vertical Bridgman furnace, whose compositions are characterized by EPMA and ICP-AES. From temperature dependence of magnetic susceptibility Eu has a magnetic moment of 7.9$\mu _{B}$ indicating Eu$^{2+}$ obtained previously, and did not depend on x upto x = 0.3, then coexists with superconductivity above x = 0.4. At the coexisting region we showed a reentrant behavior of the superconductivity in resistivity and it shows a peculiar magnetic field dependence.We show the precise phase diagram and superconductivity as a function of x.   

Four Types of Single-Walled SiGe Nanotubes: Existence and Stability

Four types of SiGe armchair nanotubes from (3, 3) to (12, 12) have been studied using the cluster approximation and the dangling bonds saturated by H atoms. The tubes have been spin- and geometry optimized using the hybrid functional B3LYP, the all electron 6-31G**//3-21G* basis set and the GAUSSIAN 03/09 suite of software. Cohesive energies, band gaps, bond lengths, hybridization of Ge and Si atoms, among others will be presented in details. In general, cohesive energy of type I tubes decrease and saturate around 3.389eV and that of, type IV tubes increase and saturate around 3.411eV. The cohesive energy of both type II and type III tubes indicates oscillatory behavior eventually reaching saturation as the tube diameter increases. Band gaps, in general for all four types decrease as the tube diameter increases. The band gap of type I tubes saturate at around 1.0eV and type IV tubes indicate the lowest saturation gap of about 0.3eV. While most types I-III tubes show sp$^{2}$ or a mixture of sp$^{2}$ and sp$^{3}$ bonding, some type IV tubes show predominantly p bonding. Results of types I-III will be compared with previous results using the cluster approximation and the B3LYP/LANL2DZ level of theory.\footnote{S. Rathi and A. K. Ray, Chem. Phys. Lett. 466, 79 (2008).}   

Rational computational design of high energy density capacitor dielectrics through navigation of the polymer chemical space

Next generation capacitor dielectrics should provide an attractive combination of high dielectric constant, fast response, low dielectric loss, high breakdown field and high temperature stability. With these objectives in mind, we adopt a research strategy in which newly developed high throughput first principles computational techniques are used to rapidly, and accurately, navigate the polymer chemical space to identify promising candidates. After validating our high throughput approach against computationally expensive dispersion corrected density functional theory calculations, we systematically study a number of C, Si and Ge backbone containing polymers with various side chain functional groups. Our initial accomplishment is the identification of new class of polymer systems with a large dielectric constant, and band gap large enough to provide reasonable insulating properties. These predictions are currently being validated by parallel experimental work, and are being further refined.   

Constraint density functional calculations for multiplets in ligand-fields: Applications to Fe-phthalocyanine and Al$_2$O$_3$:Cr$^{3+}$

In transition-metal-based complexes and molecules, multiplet structures are essential in understanding the electronic structure. However, it is often difficult to evaluate a {\it true} ground state or the {\it lowest} state within a given ligand (or crystal) symmetry from first principles calculations based on density-functional theory (DFT). Here, we propose a simple DFT approach, implemented into the FLAPW method\footnote{Wimmer, Krakauer, Weinert, Freeman, PRB{\bf 24}, 864; Weinert, Wimmer, Freeman, PRB{\bf 26}, 4571}, to treat multiplets in ligand-fields, by imposing a density matrix constraint on the $d$-orbital occupation numbers. We demonstrate the utility of this approach for the case of an isolated single Fe phthalocyanine (FePc) and a Cr impurity in a corundum Al$_2$O$_3$. For the FePc, results predict that there are three stationary states of $^3E_g$, $^3B_2$, and $^3A_2$ in the Fe$^{2+}$ ion, and our total energy calculations clearly demonstrate that the ground state is $^3A_{2g}$. In the case of the Al$_2$O$_3$:Cr$^{3+}$, where an on-site Coulomb correlation correction (+$U$) is incorporated, the ground state is $^4A_2$ and the total energy difference between the ground state and the excited state $^4T_2$, 2.9~eV, roughly agrees with an experimental value of 2.23~eV.   

The effects of micro-alloying elements on a BCC iron grain boundaries

The effects of micro-alloying elements, vanadium and niobium, on a few of BCC Fe grain boundaries are studied using Density Functional Theory calculations to develop an advanced high-strength steel alloy material. The lowest energy configuration of the grain boundaries structures are obtained from the first-principles calculations. The substitutional and interstitial point defect formation energies of micro-alloying elements in the grain boundaries are compared. The segregation energies of micro-alloying elements onto the grain boundaries and its fractured surfaces are computed. The cohesive energy calculations of the grain boundaries with and without micro-alloying elements are performed to investigate the effects of micro-alloying elements on the cohesive strength of the grain boundaries. The tensile and shear tests on the grain boundaries with and without the segregants are carried out to study the effect on the grain boundaries.   

Diamond-like carbon-metal nanocomposites for solar thermal energy

Solar thermal energy systems are a possible technology for clean energy production. Carbon-based materials, including diamond-like carbon (DLC), have been suggested as promising for solar energy harvesting [1]. A strategy for tuning the properties of DLC is the incorporation of transition metal nanoparticles in the material [2]. Using density functional theory calculations, we study the structural, mechanical, and optical properties of DLC with transition metal nanoinclusions such as silver and copper. Key properties of the nanocomposites, such as the sp$^{2}$/sp$^{3}$ bonding ratio, elastic properties, and optical band gaps are reported. Such carbon-metal alloys are calculated to show enhanced performance over DLC. \\[4pt] [1] P. Patsalas, Thin Solid Films 519, 3990-3996 (2011). \\[0pt] [2] P. Oelhafen and A. Schler, Solar Energy 79, 110-121 (2005).   

The effects of 5-7 defects on the mechanical characteristics of carbon nanotubes intramolecular junctions and their influences on electronic properties

This study utilizes molecular dynamics simulations and first-principles techniques to investigate mechanical and electronic properties of the single-walled carbon nanotube (SWCNT) intramolecular junctions (IMJs). Results show that the mechanical behaviors are mainly affected by the diameter of thinner segment of two composed tubes, whereas are barely able to be influenced by the number of 5-7 defects in the interface region. With applied compression, the yielding stress and Young's modulus of the SWCNT IMJs are found to be strongly associated with the orientation of two contacted heptagon rings where their ordering values are vertical $>$ tilted $>$ parallel to the axial direction. Under applied tensile, a lowest value in the yielding stress and Young's modulus is found on the parallel status, but the other two reveal alternating relations. Moreover, the magnitude of the stress amplitude is proportional to the Young's modulus and yielding stress and the buckling location is shown to be dependence on different orientations of two contacted heptagon rings. Finally, our first-principles calculations indicated that the considered SWCNT (5,0)/(8,0) IMJs at a certain length with different distributions of 5-7 defects on the tube wall exhibit diverse electronic properties, such as the changes of the density of states and modulation of energy gap. With increasing length, the numbers of states near the Fermi level region increase significantly and the energy gap tends to shrink as well.   

Optical absorption in degenerately doped semiconductors: Mott transition or Mahan excitons?

In the exploration of material properties, parameter-free calculations are a modern, sophisticated complement to cutting-edge experimental techniques. \emph{Ab-initio} calculations are now capable of providing a deep understanding of the interesting physics underlying the electronic structure and optical absorption, e.g., of the transparent conductive oxides. Due to electron doping, these materials are conductive even though they have wide fundamental band gaps. The degenerate electron gas in the lowest conduction-band states drastically modifies the Coulomb interaction between the electrons and, hence, the optical properties close to the absorption edge. We describe these effects by developing an \emph{ab-initio} technique which captures also the Pauli blocking and the Fermi-edge singularity at the optical absorption onset, that occur in addition to quasiparticle and excitonic effects. We answer the question whether free carriers induce an excitonic Mott transition or trigger the evolution of Wannier-Mott excitons into Mahan excitons. The prototypical $n$-type zinc oxide is studied as an example.   

Fast and Accurate Modeling of Molecular Energies with Machine Learning

A machine learning model for the prediction of atomization energies of a diverse set of organic molecules, based on nuclear charges and atomic positions only, will be discussed. The problem of obtaining molecular energies across chemical compound space, aka. as solving Schroedinger's equation, is mapped onto a non-linear statistical regression problem of reduced complexity. We use eigenvalues of a ``Coulomb''-matrix to encode all Cartesian and atomic number variables of a molecule. Regression models are trained on, and compared to, atomization energies computed with hybrid density-functional theory for a sub-set of the GDB-13 database consisting of more than seven thousand molecules. Cross-validation yields a mean absolute error of less roughly 10 kcal/mol. Applicability and transferability is demonstrated for the prediction of potential energy curves of unseen molecules.   

Development of a relativistic impurity embedding code based on the KKR Green function method

We present a new implementation of the KKR Green function method for electronic structure calculations of impurity atoms embedded in a crystalline host. Our code is able to treat impurity atoms not necessarily positioned at host sites. This became possible by a two step approach: For large deviations of the impurity from the host position a virtual atom method is used where the host Green function is expanded around the new impurity position. Then, small displacements are treated by an expansion of the Green function. In addition, we include in our code a newly developed accurate method to directly solve the coupled Lippmann-Schwinger equations for non-spherical potentials via a system of algebraic equations. When spin-orbit effects are included in the presence of spin polarization, this is especially important for the irregular solutions because of the coupling of different angular momenta up to the origin. We apply the method to solve the Schr\"odinger, the scalar-relativistic as well as the Dirac equation by using appropriate source terms.   

A constraint method for variational total energy calculations of electronic excitations within the Kohn-Sham scheme

We propose a general constraint on the occupations of single-particle orbitals within (hybrid) density-functional theory that allows a variational total energy formulation for extended systems containing arbitrary electronic excitations. This method is simple to implement and is ideal for studying structural relaxations and molecular-dynamics simulations in the presence of excitations since accurate forces can be calculated via the Hellman-Feynman theorem.   

Quantum phase transitions of 2-d dimerized spin-1/2 Heisenberg models with spatial anisotropy

Motivated by the unexpected Monte Carlo results as well as the theoretical proposal of a large correction to scaling regarding the critical theory for the staggered-dimer spin-1/2 Heisenberg model on the sqare lattice, we study the phase transitions induced by dimerization of several dimerized quantum Heisenberg models with spatial anisotropy using first principles Monte Carlo method. We focus on investigating the finite-size scaling of the observables $\rho_{s1}2L$ and $\rho_{s2}2L$ since such strategy might reveal the subtlety of determining the corresponding critical theory as suggest in [1]. Here $\rho_{si}$ with $i \in \{1,2\}$ and $L$ refer to the spin stiffness in the i-direction and the spatial box size, respectively. Surprisingly, similar to the results found in [1], while we do observe a large correction to scaling for $\rho_{s1}2L$ of the staggered-dimer model on the honeycomb lattice, the observable $\rho_{s2}2L$ of the same model receives a negligible correction to its scaling. Further, our simulation data for all the models considered here including the herringbone- and ladder-dimer models are compatible with the established numerical values for $\nu, \beta/\nu$ and $\omega$ in the $O(3)$ universality class. To explain the results presented in this study, a deepened theoretical understanding for the critical theories of the models considered here is required.   

Local-TQO and Stability of Frustration-Free Hamiltonians

The attention of the condensed matter and mathematical physics communities has recently focused on Hamiltonians with low-energy sectors exhibiting some form of topological order. In our work [1], we present a generalization of the result of Bravyi et al. [2,3] on the stability of topological quantum order for Hamiltonians composed of commuting projections with a common zero-energy subspace. In particular, the commutativity condition can be removed: We prove stability of the spectral gap for gapped, frustration-free Hamiltonians under general, quasi-local perturbations. Also, we will discuss the ``Local Topological Quantum Order'' and ``Local-Gap'' conditions sufficient for proving stability. \\[4pt] [1] S. Michalakis and J. Pytel, {\em Stability of Frustration-Free Hamiltonians.} arXiv:1109.1588 (2011).\\[0pt] [2] S. Bravyi and M.B. Hastings, {\em A short proof of stability of topological order under local perturbations.} arXiv:1001.4363. \\[0pt] [3] S. Bravyi, M.B. Hastings, and S. Michalakis, {\em Topological quantum order: stability under local perturbations.} J. Math. Phys. {\bf 51}, 093512 (2010).   

New limits on violations of the equivalence principle from solar system observations

The equivalence principle is the foundation of general relativity theory, but violations of this principle are generically predicted by theories that attempt to unify gravity with the other fundamental interactions of nature. Violations of equivalence could lead to measurable deviations of solar system bodies from their expected positions near stable or semi-stable Lagrange points. We extend earlier constraints using new observational data on Trojan and artificial satellites near Jupiter, Neptune, Mars and Earth.   

Is Dark Energy Composed of Photons with Negative Mass (Dark Photons)?

Dark energy may be composed of photons with negative inertial and gravitational mass. Since the photons comprising dark energy would have negative mass, they could not be observed even though they exert a repulsive gravitational force on photons that have positive mass. Furthermore, dark photons possess little mass which is a feature of dark energy. Dark photons are small enough to comprise the needed density to support a universe that is close to flat. They also may allow for repelling entities with positive mass while they are attracted to particles with positive mass, a possible requirement of dark energy. First presented at Space Telescope Science Institute Spring Symposium: A Decade of Dark Energy, May 5-8, 2008, Baltimore, MD.   

Visualization and Analysis of Synthetic Observations of Star Forming Regions

We present multidimensional visualizations used for the exploration, analysis, and comparison of simulated synthetic observations and real astronomical observational data cubes. By comparing synthetic observations of simulated star forming regions to real observational radio data cubes utilizing 2D and 3D visualization techniques we are able to more effectively and efficiently compare these types of data, in particular their hierarchical structure and kinematic features such as outflows or expanding shells. For our synthetic data we use simulations performed with the ORION adaptive mesh refinement (AMR) three-dimensional gravito-radiation-hydrodyanics code which follow the collapse and evolution of protostars down to AU size scales. The synthetic observations are produced using MOLLIE, a molecular line radiative transfer code. Through comparisons of 2D dendrogram representations of the star forming region's hierarchical structure, 2D column maps, and 3D data visualizations we are able to gain a better understanding of the physical structures and kinematic features, and enhance the interpretation of astronomical data cubes.   

Implementation of cryptographic hash function SHA256 in C++

This abstract explains the implementation of SHA Secure hash algorithm 256 using C++. The SHA-2 is a strong hashing algorithm used in almost all kinds of security applications. The algorithm consists of 2 phases: Preprocessing and hash computation. Preprocessing involves padding a message, parsing the padded message into m-bits blocks, and setting initialization values to be used in the hash computation. It generates a message schedule from padded message and uses that schedule, along with functions, constants, and word operations to iteratively generate a series of hash values. The final hash value generated by the computation is used to determine the message digest. SHA-2 includes a significant number of changes from its predecessor,~SHA-1. SHA-2 consists of a set of four hash functions with digests that are 224, 256, 384 or 512 bits. The algorithm outputs a 256 bits message block with an internal state block of 256 bits and initial block size of 512 bits. Maximum message length in bit is generated is 2$^{64 }$-1, over all computed over a series of 64 rounds consisting or several operations such as and, or, Xor, Shr, Rot. The code will provide clear understanding of the hash algorithm and generates hash values to retrieve message digest.   

Pressure-Induced Structural, Magnetic and Transport Transitions in the Two-Legged Ladder Sr$_{3}$Fe$_{2}$O$_{5}$

The layered compound SrFeO$_{2}$ with an FeO$_{4}$ square-planar motif exhibits an unprecedented pressure-induced spin state transition ($S$ = 2 to 1), together with an insulator-to-metal (I-M) and an antiferromagnetic-to-ferromagnetic (AFM-FM) transition. In this study, we have studied the pressure effect on the structural, magnetic and transport properties of the structurally related two-legged spin ladder Sr$_{3}$Fe$_{2}$O$_{5}$. When pressure was applied, this material first exhibited a structural transition from \textit{Immm} to \textit{Ammm }at $P_{s}$ = 30 $\pm $ 2 GPa. This transition involves a phase shift of the ladder blocks from (1/2,1/2,1/2) to (0,1/2,1/2), by which a rock-salt type SrO block with a seven-fold coordination around Sr changes into a CsCl-type block with eight-fold coordination, allowing a significant reduction of volume. However, the $S$ = 2 antiferromagnetic state stays the same. Next, a spin state transition from $S$ = 2 to $S$ = 1, along with an AFM-FM transition was observed at $P_{c}$ = 34 $\pm $ 2 GPa, similar to that of SrFeO$_{2}$. A sign of an I-M transition was also observed at pressure around $P_{c}$.   

Uncover the nature of cubic-rhombohedral transition in Fe1-xO

Transition metal monoxide Fe1-xO is an archetypal Mott insulator and an important geological compound. Despite considerable study during the past few decades, the origin of the high-pressure cubic-rhombohedral transition in this fundamental material is still not fully understood. Combining high-pressure nanoscale x-ray diffraction imaging techniques, we conducted density-functional theory (DFT) based first-principles calculations to reveal the nature of the transition. Our theoretical calculations confirm the conclusions drawn from our imaging experiments that the pressure-induced rhombohedral distortion of Fe1-xO is associated with $<$111$>$ stacking defects cluster and is ferroelectric in nature.   

Structural, magnetic, electronic properties of the filled skutterudite EuFe$_4$As$_{12}$

The filled skutterudite EuFe$_4$As$_{12}$ has been synthesized and its structural, electronic, magnetic and thermodynamic properties have been carefully investigated. In this compound, the Fe and Eu moments order ferrimagnetically at T$_C$ = 151 K, the highest magnetic ordering temperature among filled skutterudite compounds. LDA+$U$ band structure calculations confirm the observed magnetic polarizations and suggest that the conduction electrons in EuFe4As12 have a large spin polarization, although slightly smaller than in the isostructural EuFe$_4$Sb$_{12}$. We present a joint experimental and theoretical study of the electronic and magnetic properties for both compounds, including the isostructural EuFe$_4$P$_{12}$, where the exchange of the pnictide can be considered as chemical pressure. To separate the influence of mere volume effects and a change of the pnictide we also studied the behaviour under hydrostatic pressure for EuFe$_4$As$_{12}$, both experimentally and theoretically.   

Bi3-xM3O11+? (M=Cr, Rh, Ir, Pt, Pd): A series of new KSbO3-type structural magnetic materials

KSbO3-type family is interesting because it can adopt three interpenetrating networks with the composition changing from ABO3 (KSbO3 and KIrO3) to ABO3.667 (Bi3Ru3O11, La3Ru3O11, and Bi3GaSb2O11). Recently Belik et. al reported a new KSbO3-type random ferrimagnet Bi3Mn3O11 with high Tc. Here we reported a series of new KSbO3-type structural materials Bi3-xM3O11+? (M=Cr, Rh, Ir, Pt, Pd) synthesized by high pressure and high temperature (HPHT). We investigated the effects of oxygen content on the structural, physical, and chemical properties of these materials, because a wide variation of ? value (changed from -0.5 to 0.6) in this system keeps the same cubic structure. In addition, we also studied the effects of Bi content on the structure, physical, and chemical properties. The value of x was changed from 0 to 0.4 in Bi3-xCr3O11+?.   

Electronic and Magnetic Structures in 2D Graphene and BN Nanoflakes

The two-dimensional nanomaterials have been attracting great attention for potential applications. Using first-principles calculations, we have studied the geometric and electronic properties of zigzag graphene nanoflake(ZGNF)and single-layer BN nanoflake (ZBNNF)before and after hydrogen and oxygen adsorption. The interplay between atomic adsorption, crumpling structures, zigzag edges and magnetic structures has been investigated systematically. Our calculated results show that the total magnetic moments of the ZGNF with the adsorbed atoms increase significantly. The adsorbed atoms induce the crumpling structures and central spin polarization, while the magnetic orders of zigzag borders are controlled. A detailed electronic structure analysis is given. The single-layer BN nanoflake has a special crumpling struture different from the ZGNF. The left-right symmetry of atomic height displacements is broken. This study demostrates a new approach to manipulate the properties of 2D nanomaterials. This work was supported by the SRFROCS,SEM and Shandong OYS Foundation.   

Non-destructive generation of nano-scale periodic pinning potentials for magnetic domain walls: a way to bias domain wall propagation

The stray magnetic field of an array of ferromagnetic nanodots is used to generate a spatially periodic pinning potential for domain walls moving through a physically separate, weakly disordered, magnetic layer lying beneath the array. This technique represents a non-destructive method to create tunable and localised pinning sites for domain walls which are consequently subject to co-existing (but independent) periodic and disordered pinning potentials. Beyond the fundamentally attractive application of creating a model experimental system to study interface motion through multiple co-existing pinning potentials, our system interestingly exhibits many characteristics that are normally associated with exchange bias. This is a direct result of the fact that pinning effects induced by the periodic pinning potential depend upon the polarity of the applied magnetic field which drives the domain wall motion, a phenomenon which manifests itself in field-polarity-dependent domain wall mobilities and profiles.   

Probing antiferromagnetism in NiMn/Ni/(Co)/Cu$_{3}$Au(001) single-crystalline epitaxial thin films

Antiferromagnetism of equi-atomic single-crystalline NiMn thin film alloys grown on Ni/Cu$_{3}$Au(001) is probed by means of magneto-optical Kerr effect (MOKE). Thickness-dependent coercivity enhancement of NiMn/Ni/Cu$_{3}$Au(001) showed that 7 atomic monolayers (ML) NiMn order antiferromagnetically at room temperature. It is found that NiMn can couple to out-of-plane (OoP) as well as in-plane (IP) magnetized Ni, the latter stabilized by Co under-layer deposition. The antiferromagnetic (AFM) ordering temperature (T$_{AFM})$ of NiMn coupled to OoP Ni is found to be much higher (up to 110 K) than in the IP case, for otherwise identical interfacial conditions. This is attributed to the `magnetic proximity effect' in which the ferromagnetic (FM) layer substantially influences the T$_{AFM}$ of the adjacent AFM layer and can be explained by either (i) a higher interfacial coupling strength or/and (ii) more thermally stable NiMn distorted spin structure when coupled to Ni magnetized in OoP direction than in IP. An exchange-bias effect could only be observed for the thickest NiMn film studied (35.7 ML); the exchange-bias field is higher in the OoP exchange-coupled system than in the IP one due to the same reason/s.   

Tunable carrier spin polarization in PbMnS throught quantum confinement

We report magneto-photoluminescence studies on manganese-doped lead sulfide (Pb$_{1-x}$Mn$_{x}$S, x = 0 to 5{\%}) quantum dot system synthesized by solution-phase chemical method. The size (3 to 15nm), temperature (7K to 100K), magnetic field (0 to 7T), and laser power dependence (1mW to 20mW) of photoluminescence were systematically investigated, with a focus on carrier spin polarization. Depending on the sizes and growth conditions, the spin polarization can be tuned from being positive to negative. Core/shell structured quantum dots with Mn$^{2+ }$ions doped in the core or shell were also studied. The sign change in the spin polarization suggests tunable exchange interactions caused by quantum confinement effect and wave function engineering through heterostructure design.   

Spin Dynamics in Recombinant H-chain Ferritin Reconstituted with 500Fe/Protein

Ferrihydrite mineral cores averaging 500 Fe/protein shell were grown within recombinant human H-chain ferritin and characterized by M$\phi $ssbauer spectroscopy. At 4.2 K, two magnetic phases ascribed to iron at surface and interior sites of the antiferromagnetic nanolattice of the mineral core were identified, registering hyperfine fields of 445 kOe and 486 kOe, respectively. With increasing temperature, from 4.2 K to 20 K, their values decrease at widely different rates; at a rate of 3.52 kOe/K for interior and 19.33 kOe/K for surface sites, before spectral lines collapse to quadrupolar doublets at a blocking temperature of $TB$ = 22 K. Below $TB$, spin relaxation at interior sites is consistent with the theory of collective magnetic excitations of a superparamagnetic particle, whereas spin dynamics at surface sites are more consistent with theories of many-spin nano-magnet systems with greater complexity in the potential energy landscape that supports low energy spin-wave excitations. The entire M$\phi $ssbauer temperature spectral profile will be presented and discussed in the light of the above observations.   

Magnetic Anisotropy of GaAs/Fe/Au Core-Shell Nanowires Grown by MBE

In order to increase the storage density of magnetic memories while maintaining thermal stability, new types of recording media and technologies need to be explored. In this context, perpendicular recording geometry is among the most attractive ideas. In this work we demonstrate a novel method of fabricating such perpendicular media, in the form of GaAs/Fe/Au core-shell nanowires (NWs) grown by molecular beam epitaxy on GaAs(111)B substrates. Scanning electron microscopy images show that the Fe shell has successfully coated the sidewalls of GaAs nanowires. Magnetic anisotropy of GaAs/Fe core-shell NWs was studied by ferromagnetic resonance and by superconducting quantum interference device (SQUID) magnetometry. Our results show that the magnetic anisotropy of this novel core-shell NW system cannot be simply described by any known theory, as revealed by our attempts to perform micromagnetic simulation using the object oriented micromagnetic framework (OOMMF). The observed features thus suggest the existence of a domain structure that is specific to this new system. We will now attempt to identify the domain structure of this complex system by magnetic force microscopy.   

Anomalous field dependence of magnetoresistance in magnetic multilayers based on the Fibonacci sequence

The discovery of giant magnetoresistance and concomitant effects such as oscillatory exchange coupling in magnetic multilayers was the starting point of the spintronics era. In the present study, a multilayer of Fe and Cr was designed, using the Fibonacci sequence for the growth of the non-magnetic spacers. Using the gradient method we simulated the magnetic field dependence of these nanostructures for in-plane magnetic fields. The simulations included magnetocrystalline anisotropy for the [100] and [110] directions, as well as bilinear and biquadratic exchange coupling. The resultant magnetization and magnetoresistance behavior shows unusual behavior, such as a Devil's staircase for the magnetization and increasing magnetoresistance with increasing applied magnetic fields. When the ratio of the biquadratic to bilinear coupling exceeds 30{\%} the magnetic field response is expected to show even fractal behavior. These numerical simulations will also be compared to experimental measurements on samples prepared with nominally the same thicknesses.   

Large exchange anisotropy in nanostructured Cu$_{30}$Mn$_{70}$ ribbons

Exchange bias (H$_{ex}$), arising from coupling between antiferromagnetic (AFM) and ferromagnetic (FM) materials, is a diagnostic metric of magnetic interactions in inhomogeneous systems. An extremely large H$_{ex}$ of 10 kOe is found in the rapidly-solidified alloy Cu$_{30}$Mn$_{70}$ at 10 K [1-2], but disappears at temperatures above the system blocking temperature of 123 K. X-ray diffraction reveals the presence of two well-crystallized FCC $\gamma$-phases with cubic a-parameters 3.744 and 3.750 {\AA}, unit cell volumes of 52.5 and 52.7 {\AA}$^{3}$, respectively, and a crystallite size of $\sim$30 nm. We hypothesize that the $\gamma$-phase with larger a-parameter is Mn-rich while the other is Mn-poor. The observed magnetic behavior is attributed to exchange interactions between Mn-rich regions where AFM coupling between nearest-neighbor Mn atoms dominates, and Mn-poor regions where FM coupling between next-nearest-neighbors dominates [1]. Compositional fluctuations result in an additional cluster-glass-like freezing behavior due to magnetically frustrated interactions between Mn atoms [2]. \\[4pt] [1] Kouvel, J.S., J. Appl. Phys. S31 (1961) 5, S142-147. \\[0pt][2] Mydosh, J. A. (1995), Spin Glasses, Taylor \& Francis.   

Spin polarized surface states on stepped magnetic surfaces: ab-initio approach

It was shown that surface states electrons become spin polarized above magnetic layers and nanoislands [1]. In the present work we perform the state of the art ab-initio studies of surface state electrons at steps of magnetic metals. We focus on steps of 3d metals on Cu(111) surface. We have revealed a spin-dependent charge transfer at step ages which is explained by Smoluchowski effect. Strongly inhomogeneous spin polarization of surface statates [1] at steps is revealed. Our results indicate that tunneling magnetoresistance at steps can exhibit very strong changes at the atomic scale. \\[4pt] [1] L. Diekhoner et. al. Phys. Rev. Lett. 90, 236801   

Core/Shell and High Aspect Ratio Magnetic Oxide Nanoparticles for Antenna Applications

Improved antenna gain, reduced antenna aperture size, and improved bandwidth are of interest to an increasingly mobile world. To obtain these improvements our efforts are directed at developing new magnetic oxide nanoparticle/polymer composites with modifiable permeability and permittivity and low electrical losses. Our approach consists of producing core/shell and shape controlled magnetic nanoparticles. Methods of synthesis utilize microwave and traditional heating to perform hydrothermal and solvothermal reactions. Decomposition of metal acetylacetonates is performed using various alcohols resulting in spherical nanoparticles with diameters of $\approx$8-16 nm and 3-7 nm for Fe$_{3}$O$_{4}$ and CoFe$_{2}$O$_{4}$, respectively. Microwave methods result in similar particles, but are produced in an hour or less as compared to 48 hrs via the traditional solvothermal method. Successive growths are used to produce larger monolithic particles as well as core/shell systems where exchange coupling between the core and shell is observed. Hexaferrite particles have been produced via hydrothermal synthesis, while high aspect ratio Fe$_{3}$O$_{4}$ nanoparticles ($\approx$10-100 nm) produced via hydrothermal synthesis result in nanoneedles with high $\mu _{r}$.   

Magnetism and structure of amorphous Co-W alloyed nanoparticles

W-capped Co nanoparticles dispersed in an alumina matrix are studied by means of high-resolution transmission electron microscopy, extended x-ray absorption fine structure, SQUID-based magnetic measurements, ac magnetic susceptibility, and x-ray magnetic circular dichroism. Results show the formation of amorphous Co-W alloy nanoparticles, the magnetic properties of which are modified by the amount of W or Co present in the samples. The average Co magnetic moment depends on the number of W atoms surrounding it. Co-W particles show superparamagnetic behavior and are described as an array of noninteracting particles with random anisotropy axes and an average moment per particle proportional to the particle volume and to the average Co moment for each alloy composition. Values of the magnetic anisotropy constant of the particles are on the order of $10^6$ erg/cm$^3$, higher than that of bulk Co. Evidence of short-range ordering within each amorphous particle is found that provides insight of the origin of their magnetic anisotropy.   

Competition between quantum criticality and Fermi liquid behavior revealed by transport measurements in ultrapure Sr$_{3}$Ru$_{2}$O$_{7}$

The layered perovskite metal Sr$_{3}$Ru$_{2}$O$_{7}$ has gained considerable interest since the discovery of its field-tuned quantum criticality [1] and the subsequent discovery of a new electronic phase with a high magnetoresistive anisotropy, consistent with the existence of an electronic nematic fluid [2]. Further studies have explored the properties of the ``normal state'' from which the anisotropic phase emerges in terms of thermodynamics [3] and quantum oscillations [4], but a comprehensive study of the transport properties of the purest samples has so far been lacking. Here, we present the first study of the normal state resistivity of ultrapure Sr$_{3}$Ru$_{2}$O$_{7}$, involving temperature control over more than two orders of magnitude and a 15T magnet system. The high signal to noise of this measurement allows for precise extraction of the temperature dependence exponent and provides detailed understanding of the transport signatures of field-tuned quantum criticality. \newline [1] S. A. Grigera {\it et al}., Science {\bf 294}, 329 (2001). \newline [2] R. A. Borzi {\it et al}., Science {\bf 315}, 214 (2007). \newline [3] A. W. Rost {\it et al}., Science {\bf 325}, 5946 (2009). \newline [4] J.-F. Mercure {\it et al}., Phys. Rev. B {\bf 81}, 235103 (2010).   

The Influence of Layer Thickness-Ratio on Magnetoresistance in La$_{2/3}$Ca$_{1/3}$MnO$_{3}$/La$_{1/3}$Ca$_{2/3}$MnO$_{3}$ Exchange Biased System

Isothermal magnetic field dependence of the resistance in La$_{2/3}$Ca$_{1/3}$MnO$_{3}$ (F-LCMO)/ La$_{1/3}$Ca$_{2/3}$MnO$_{3}$(AF-LCMO) bilayer and AF-LCMO/F-LCMO/AF-LCMO trilayer at temperatures below N\'{e}el temperature of the antiferromagnetic layer were carried out to study the thickness layers influence on magneto transport properties. We grew multilayers using a high oxygen pressure sputtering technique. We systematically varied the thickness of the F-LCMO layer, t$_{F}$, maintaining constant the thickness of the AF-LCMO layer, t$_{AF}$. We studied the influence of the thickness ratio t$_{F}$/t$_{AF}$ on the ZFC and FC magnetoresistance (MR) loops. H$_{FC}$ was varied from 100 Oe to 400 Oe. We found that MR has hysteretic behavior as observed in [La$_{2/3}$Ca$_{1/3}$MnO$_{3}$/La$_{1/3}$Ca$_{2/3}$MnO$_{3}$]$_{N}$ superlattices, where MR increases with the increasing field from H=0 to a maximum and then it decreases continuously. The position and magnitude of the maximum is not symmetric with respect to the axis H=0 for both FC and ZFC loops. We found that magnetoresistance behavior of the bilayer and trilayer is thickness-ratio dependent for both ZFC and FC loops.   

Micromagnetic simulations and experimental calibration of magnetic behavior in multiferroic BiFeO$_{3}$

BiFeO$_{3}$ (BFO) is a magnetoelectric multiferroic that exhibits ferroelectric and antiferromagnetic (AFM) ordering at room temperature. BFO films are frequently coupled to a ferromagnet (FM) layer grown on the BFO surface, but the behavior of the magnetization in BFO films and at the BFO/FM interface is not understood. We propose a micromagnetic model for the magnetization in BFO in which spin canting throughout the film, induced by the Dzyaloshiskii-Moriya (DM) effect, creates weak ferromagnetism on the BFO surface. This surface ferromagnetism is significantly enhanced by an exchange interaction with the FM at the BFO/FM interface. We perform OOMMF micromagnetic simulations of BFO/CoFe bilayers and reproduce experimentally reported one-to-one mapping between BFO and CoFe domains as well as enhanced magnetic moment on the BFO surface, on the order of 0.5 $\mu _{B}$ per unit cell. From this model, we can extract AFM/FM exchange energies and DM energies by fitting experimentally reported values for BFO surface magnetic moment and hysteresis. We also demonstrate that magnetization switching of the BFO domains can induce switching in the FM. We reproduce experimentally observed 180\r{ } total magnetization switching through 90\r{ } switching in stripe-like domains.   

Growth of ferromagnetic La$_{2/3}$Ca$_{1/3}$MnO$_{3}$ / ferroelectric BaTiO$_{3}$ heterostructures

Multiferroic materials exhibiting simultaneous ferroelectricity and ferromagnetism have potential applications in information storage and in the emerging field of spintronics. Ferromagnetic / ferroelectric multilayers could be a way to obtain a multiferroic heterostructure. We addressed to deposit the ferromagnetic phase of the La$_{1-x}$Ca$_{x}$MnO$_{3}$ and the ferroelectric BaTiO3 seeking a multiferroic properties in these hetero structures. We have optimized the growth parameters for depositing BaTiO$_{3 }$(BTO) / La$_{2/3}$Ca$_{1/3}$MnO$_{3 }$(LCMO) / (001)SrTiO$_{3}$ by pulsed laser deposition (PLD) at pure oxygen atmosphere and a substrate temperature of 820 \r{ }C. The bilayer structure and microstructure were studied by x-ray diffraction (XRD) and atomic force microscopy (AFM). For individual layers, lattice parameter is a$_{BTO}$=4.068 {\AA}, and a$_{LCMO}$=3.804 {\AA}, whereas in the bilayer, Bragg peaks for LCMO maintain its position but BTO peak shift to lower Bragg angle indicating a strained BTO film. Magnetization and polarization measurements indicate a possible multiferroic heterostructures   

Characterization of Nano-oxidation Lithography on La0.7Ba0.3MnO3 Thin Films Grown by Pulse-Laser-Deposition

Nano-oxidation using Atomic Force Microscopy has been used to produce nano-scale patterns on a variety of materials, including semiconductors, insulators, metals and superconductors. Perovskite Oxide thin films have demonstrated unique material characteristics. The ability to make localized modifications on these films would allow nanofabrication of devices utilizing these unique properties. In this study, the atomic force microscope was used to modify the surface of La$_{0.7}$Ba$_{0.3}$MnO$_{3}$ thin films grown using Pulsed Laser Deposition Techniques. Surface patterns were produced and studied as a function of humidity, applied tip voltage, temperature, and growth conditions. Reproducible patterns were produced using both positive and negative tip voltages. Two growth modes were observed. A moderate growth rate was common for positive tip voltages up to 15 V (at T=75.1$\pm $1; H=78$\pm $2{\%}) which allowed the controlled formation of features considerably larger than those produced on materials like silicon. Additionally, a significantly larger growth rate was observed for negative tip voltages exceeding -12V. This growth mode provides the potential to produce structures at significantly higher write speeds, making these manganese films candidates for data storage devices.   

Finite-Size Scaling Effects in Chromia thin films

Controlling magnetism by electrical means remains a key challenge in the area of spintronics. The use of magnetoelectrically active materials is one of the most promising approaches to this problem. Utilizing Cr$_2$O$_3$ as the magnetoelectric pinning layer in a magnetic heterostructure both temperature assisted and isothermal electrical control of exchange bias have been achieved [1,2]. Interestingly, this ME switching of exchange bias has only been achieved using bulk Cr$_2$O$_3$ crystals, isothermal switching of exchange bias using thin film chromia remains elusive. We investigate the origin of unusually pronounced finite-size scaling effects on the properties of Cr$_2$O$_3$ grown by Molecular Beam Epitaxy; in particular we focus on the different temperature dependencies of the magnetic susceptibility of bulk vs. thin film chromia, the change in N\`{e}el temperatures, and the implications for the magneto electric properties of chromia thin films. \\[4pt] [1] P. Borisov et al., Phys. Rev. Lett. 94, 117203 (2005).\\[0pt] [2] X. He et al., Nature Mater. 9, 579 (2010).   

Control of noise in magnetic multilayers by spin torque

In this work we show that the stability of magnetic nanostructures can be enhanced by time-dependent spin momentum transfer. Building reliable magnetic devices at smaller scales need to address the issue of thermal noise. Using two commonly studied magnetic systems with multiple stable states at zero temperature as examples, we show that periodic spin torques can enhance the stability of the system and hence suppress the noise due to interwell transitions. In the case of weak periodic spin torques, stochastic resonance which is usually associated with ac magnetic fields is also manifested for non-conservative torques. In more complex systems with a relatively low energy barrier, it is shown that high frequency spin torques can inhibit interwell transitions and in effect suppress the telegraph noise due to the switching between neigboring states.   

Full Field-Frequency Study of Standing Spin Wave Modes in FM / NM / FM Trilayers

Spin waves constitute an attractive avenue for spintronic devices as they can be used to carry information through magnetic media as well as to store information in form of localized modes. As a way of manipulating these entities, external microwave fields can be employed. For a given precession frequency, the amplitude of the localized spin wave mode in a single ferromagnetic layer is determined by the confining geometry and applied magnetic field strength. By introducing a non-magnetic spacer layer into the layer, the development of spin waves can be modified in a frequency range by varying the spacer layer thickness [1,2]. Here we investigate the role of the spacer layer thickness in a Permalloy/Copper/Permalloy trilayer by means of full field-frequency ferromagnetic resonance (FMR) spectroscopy. \\[4pt] [1] H. Bosse and H. G\"artner, JMMM 80 339 (1989).\\[0pt] [2] M. Belmeguenai, T. Martin, G. Woltersdorf, M. Maier, and G. Bayreuther, PRB 76 104414 (2007).   

Electronic and Optical properties of Mn $\delta$-doping InGaAs/GaAs Quantum Wells

Recent magneto-optical measurements in InGaAs quantum wells (QWs) with GaAs barriers with $\delta$-doped layers of Mn and C, show a strong oscillations of circularly polarized QW emission as a function of the magnetic field. The oscillations amplitude increases with the Mn content and they persist up to 25 K [Appl. Phys. Lett. {\bf 98}, 251901 (2011)]. In order to understand these magneto-oscillations we studied the electronic and optical properties of the two-dimensional hole gas (2DHG) formed in the QW. The holes are provided by both Mn and C doping. We used the spin-density functional theory within the ${\bf k} \cdot {\bf p}$, the envelope function and the virtual crystal approximations to describe the electronic states of the system. The {\it sp-d} interaction was described by the Zener kinetic-exchange model. Our results show that when an external magnetic field is applied a spin-polarized 2DHG is formed at the Mn $\delta$-doped layer, inducing an effective magnetic field. This field affects the charge redistribution of the system inducing oscillations in the Fermi and energy levels, and consequently in the QW emission. The strength of these oscillations is strongly dependent on the nature of the Mn doping, in particular the distance between the QW and the Mn layers.   

Structural and magnetic properties of GaAs-MnAs nanowires grown by MBE

Ferromagnetic (FM) Nanowires (NWs) have been proposed recently as a base of a new type of nano-magnetic memory structures. In this context, the investigations of NWs combining FM materials with commercially used semiconductors (SCs) like Si or GaAs are desirable. NW structures of GaAs combined with (Ga,Mn)As FM SC and MnAs FM metal were grown by MBE. Primary core GaAs NWs growth was induced by different nanocatalysts on two substrates for comparison reasons: by the autocatalytic growth mode on Si(100), Si(111) and by Au nanodroplets induced catalytic growth on GaAs(111)B. In both cases NW cores of GaAs were grown at high temperature (550 C for GaAs(111)B and 600 C for Si). The (Ga,Mn)As and MnAs shells were grown on both types of primary GaAs NW cores at low temperature (250 C). Structural properties of the core-shell GaAs-(Ga,Mn)As NWs were investigated by electron microscopy (SEM, TEM) and magnetic properties were studied by SQUID magnetometry and FM resonance. The NWs grown on GaAs(111)B are thinner (50 nm), shorter (3 microns) and denser than the NWs grown on Si (typical diameters of 100 nm, lengths up to 15 microns). The FM behavior observed in these NW structures up to room temperature is related rather to MnAs nanoclusters than to individual NW.   

Electrical spin injection and detection in Si nanowires

While electrical spin injection and detection in bulk Si has now been established, the availability of high quality single crystalline silicon nanowires has stimulated considerable interest in demonstrating spin-polarized transport in these 1-dimensional structures. Here, we report our efforts to electrically inject spin-polarized electrons from cobalt contacts into n-type silicon nanowires through Al$_{2}$O$_{3}$ tunnel barriers. The electrically doped nanowires were synthesized by the vapor-liquid-solid process with a cold wall chemical vapor deposition system using silane and phosphine precursors. Low temperature magneto-transport studies have shown spin-valve like behavior in both two-terminal and four-terminal lateral devices. The spin-valve signal as functions of temperature and bias voltage will be discussed. We will also show the influence of defects and trap states on the stability of nanowire devices.   

Bond operator theory for the frustrated anisotropic Heisenberg antiferromagnet on a square lattice

In this work we use the bond operator formalism in a mean field approximation to study quantum phase transitions in the S = 1 Heisenberg antiferromagnet with single ion anisotropy up to the next-next-nearest neighbor coupling (the J1-J2-J3 model) on a square lattice. The model features a complex T = 0 phase diagram, whose ordering vector is subject to quantum corrections with respect to the classical limit. The phase diagram shows a quantum paramagnetic phase situated among Ne\'el, spiral and collinear states.   

Behavior of the overlap distribution of various spin glasses at low temperature

Numerical results for the probability distribution, $P(q,T)$, of the spin-overlap $q$ as a function of temperature $T$, is reported for several randomly frustrated systems. These include (i) random bond Ising systems, such as the Edwards-Andreson spin-glass model, (ii) site diluted systems which are geometrically frustrated, such as FCC lattices with $40\%$ of their sites occupied with up-down spins, and (iii) random-field systems. $P(q,T)$ stands for an average of $P_{\cal J}(q,T)$ over many system samples with different realizations of quenched randomness ${\cal J}$. We also report statistical fluctuations of $P_{\cal J}(q,T)$ which are relevant to the issue of self-averaging.   

Second Frustration for Artificial Spin Ice

Since its introduction six years ago, artificial spin ice has been employed to successfully study frustration and disorder, to explore extensions of thermodynamics to granular systems, to investigate topological defects and information encoding, and has become ground for direct imaging of ``magnetic monopoles.'' The research has concentrated so far on a few basic geometries (square, ladder, honeycomb, triangular) in which the frustration of the magnetic interaction at the vertices could (or not) bring about a degeneracy. Here we propose new topologically non-trivial geometries, which we call ``of second frustration.'' In these arrays each vertex, while frustrated, has a unique low energy configuration, and is therefore non degenerate; yet a second frustration is regained globally and vertex excitations are topologically protected on loops inside the array. These topological excitations, which control the entropy, cannot be suppressed, can move, merge and exchange topological charge. As novel, more dynamical artificial spin ice is being developed by many, these new lattices could provide an interesting playground for driving and controlling topological excitations, and for taylor-design of probe-response properties.   

Low temperature spin glass transition in Gallium ferrite single crystals

Magnetoelectric gallium ferrite (GaFeO$_{3}$ or GFO) manifests close to room temperature ferrimagnetism owing to inherent cationic site disorder in an otherwise antiferromagnetic ground state structure. In GFO, Fe ions at Fe1 and Fe2 sites are antiferromagnetically coupled while Fe and Ga at Fe2 and Ga2 sites respectively are ferromagnetically coupled. Ga1 site is magnetically inactive. Here, we present a detailed study to probe phase transitions in GFO using ac and dc magnetic characterization methods to demonstrate spin glass behavior in GFO below 200 K. Our dc magnetization measurement exhibits that while GFO undergoes standard para (PM) to ferromagnetic (fM) transition at T$_{c} \quad \sim $ 290 K, splitting between field cooled and zero-field cooled plots is observed at low temperatures hinting at the spin-glass like behavior. Further, temperature dependent ac susceptibility measurements at different frequencies and at different dc fields demonstrate that the system exhibits a non-equilibrium canonical spin glass (SG) state below the spin glass transition temperature $\sim $ 210 K. The spin glass state has been further characterized by memory effect and aging measurements. The origin of such a spin-glass phase is proposed to arise from a network of geometrically frustrated spin system attributed to combination of antiferromagnetic interaction among the Fe ions in the two Fe sites and Ga2 site as well as inherent cation site disorder.   

Spin $S=1$ ``Quantum spin liquid'': quantum criticality in 6H-B-Ba$_3$NiSb$_2$O$_9$

We present a minimal model for a recently discovered material 6H-B-Ba$_3$NiSb$_2$O$_9$ which was proposed as a candidate for $S=1$ quantum spin liquid on a triangular lattice. Our spin-1 model lies on a stacked multilayer triangular lattice. In our minimal model, we point out the competition between Heisenberg exchange interactions, which favor magnetic ordering, and the easy-plane single-ion anisotropy, which favors a uniform quantum paramagnetic state with $S^z =0$ state at each site. We argue that the system is close to the quantum critical point separating these two phases and on the quantum paramagnetic phase side. Viewing the system as a three dimensional multilayer structure, we find that the frustrated interlayer and intralayer exchange interaction induces nodal lines of low energy spin excitations at the quantum critical point. Moreover, due to the quasi-2D nature of the system and proximity to the quantum critical point, we show there exists a broad intermediate temperature regime with linear temperature dependence of specific heat. Various other predictions and suggestions for experiments are discussed.   

Order by disorder in kagome-type magnets

Magnets on kagome lattice have been and continue to be the archetypical setting in which to study exotic order induced by geometrical frustration in theory and experiment. Apprxoimate realizations of kagome magnet has been observed in compounds such as SCGO and herbertsmithite. Artificial kagome spin ice has also been created using lithographically fabricated arrays of nanoscale magnets. The frustration comes from the fact that nearest-neighbor spin interactions on a network of corner-sharing triangles cannot be satisfied simultaneously, leading to a huge degeneracy in its classical ground state, for both discrete and continuous spins. Here, we present some recent progress in simulating numerically, and analysing field-theoretically, kagome magnets with differing sets of interactions.   

Exact Diagonalization of the Kondo Necklace Model on a Triangular Lattice

We consider the Kondo necklace model on a triangular lattice. The interplay of magnetic ordering and Kondo screening is investigated through the use of exact diagonalization of finite clusters with periodic boundary conditions. In the absence of Kondo screening, the system is in the magnetically ordered phase whereas the magnetic moments reduce monotonically as the screening strength is increased. It is found that the system has a unique ground state in the entire range of the parameters studied, which excludes the possibility of partially ordered degenerate ground states. We also discuss the effects of quantum fluctuations in the model, with emphasis on the lifted degeneracy of partially ordered states.   

Tensor networ simulation of phase diagram of frustrated J1-J2 Heisenberg model on a checkerboard lattice

We use the recently developed tensor network algorithm based on infinite projected entangled pair states (iPEPS) to study the phase diagram of frustrated antiferromagnetic J1-J2 Heisenberg model on a checkerboard lattice. The simulation indicates a Neel ordered phase when J2 $<$ 0.88J1, a plaquette valence bond solid state when 0.88 $<$ J2/J1 $<$ 1.11, and a stripe phase when J2 $>$ 1.11J1, with two first-order transitions across the phase boundaries. The calculation shows the cross-dimer state proposed before is unlikely to be the ground state of the model, although such a state indeed arises as a metastable state in some parameter region.   

Fabrication of organic spin-valve devices using indirect deposition

The organic spin valves (OSVs) comprised of two ferromagnetic (FM) electrodes separated by an organic spacer are the principal device structures to study the spin-dependent transport. Normally, the top FM electrodes are directly deposited on top of the organic layer. The vaporized FM atoms can penetrate or diffuse into organic layer and form the so-called ill-defined layer, which is as thick as 80-100 nm. In this work we report a reliable method for fabricating high quality, reproducible OSVs. We use indirect deposition method, which relies on the scattering of the evaporated metallic atoms with inert gas to reduce kinetic energy, to deposit the Co electrode on top of the organic layer Alq$_{3}$. This method significantly suppresses the penetration of Co atoms into Alq$_{3}$ layer during deposition process, in comparison with devices fabricated by conventional direct deposition method. The improved Alq$_{3}$/Co interface is further confirmed by comparing the magnetic moment of depositing Co onto Alq$_{3}$ and Si substrates by indirect and direct deposition methods. And an OSV effect in La$_{0.7}$Sr$_{0.3}$MnO$_{3}$/Alq$_{3}$/Co devices is demonstrated at room temperature, indicating the improvement of spin injection efficiency at sharp Alq$_{3}$/Co interface.   

Spin Transport in the XXZ Chain at Finite Temperature and Momentum

We investigate the role of momentum for the transport of magnetization in the spin-$1/2$ Heisenberg chain above the isotropic point at finite temperature and momentum [1]. Using numerical and analytical approaches, we analyze the autocorrelations of density and current and observe a finite region of the Brillouin zone with diffusive dynamics below a cut-off momentum, and a diffusion constant independent of momentum and time, which scales inversely with anisotropy. Lowering the temperature over a wide range, starting from infinity, the diffusion constant is found to increase strongly while the cut-off momentum for diffusion decreases. Above the cut-off momentum diffusion breaks down completely.\\[4pt] [1] Robin Steinigeweg and Wolfram Brenig, arXiv:1107.3103   

Chemisorption of Co-based cyanol pyridyl valence tautomers on Au(111)

Organic paramagnetic bistable molecules such as transition metal valence tautomers, where a magnetic transitions can be indiced by an external perturbation, are attracting considerable attention due to their potential utilization in molecular electronic and spintronic devices. Using calculations from first principle based on density functional theory we have investigated the chemisorbtion of Co-based cyanol pyridyl valence tautomer (VT) molecules on the Au(111) surface via a thiol head group. Among the possible adsorption sites on the Au(111) surface, we considered both the fcc hollow site and the bridge site, which are suggested to be the two lowest energy adsorption configurations in previous investigations. We have characterized the stability and the influence of the substrate bonding on the electronic and magnetic properties of the VT molecule. Moreover, by adding a top Au contact, we have studied the electron and spin transport properties of these systems.   

Analysis on the Magnetic and Thermodynamic behavior of a Linear Magnetic Chain [Co(bpdc)(H$_{2}$O)$_{2}$].H$_{2}$O

[Co(bpdc)(H$_{2}$O)$_{2}$].H$_{2}$O (bpdc=biphenyldicarboxylate) was found to be a one dimensional ferromagnetic Co (II) chain system that orders below 5.5 K due to a weak antiferromagnetic inter-chain coupling. Classical Fisher Model (CFM), Mean Field Theory (MFT), and Ising Model (IM) were applied to the high temperature susceptibility data of [Co(bpdc)(H$_{2}$O)$_{2}$].H$_{2}$O. The fit to CFM yielded a J/k$_{B}$ = + 7.40 K with R$^{2}$ = 0.993; MFT gave a J/k$_{B}$ = + 5.31 K with a low R$^{2}$ = 0.391. The ratio interchain interactions (J') to that of intra-chain interactions was estimated to be J'/J = - 0.005. The magnetic specific heat was obtained via direct subtraction of the specific heat data for Zinc analogue from that of [Co(bpdc)(H$_{2}$O)$_{2}$].H$_{2}$O . The magnetic specific heat data was fit to the Ising Model for spin $\raise.5ex\hbox{$\scriptstyle 1$}\kern-.1em/ \kern-.15em\lower.25ex\hbox{$\scriptstyle 2$} $ yielding J/k$_{B}$ = + 15.27 K. The interpretations on the spin state of the Co(II) at different temperatures in the compound are consistent with the behavior of Co(II) in other compounds with similar octahedral sites.   

Magnetic properties of 3d transition metal-phthalocyanine molecules on oxided Cu(110) surface

After being extensively interesting topics in both fundamental researches and application practices for a decade, spintronics is now on its way of diversity. Molecular spintronics has attracted much attention in recent years, due to the accelerating miniaturization of electronic devices. In this work, based on first-principles calculations, we studied the electronic and magnetic properties of both isolated metal-phthalocyanines (commonly referred to as MPc) molecules (M=Mn, Fe and Co) and a single MPc molecule adsorbed on oxided Cu(110) [O-Cu(110)] surface. We find that the easy axis of FePc molecule switches from in-plane direction to perpendicular direction when it is adsorbed on O-Cu(110) surface. However, such a switch of direction of magnetization could not be observed for MnPc and CoPc molecules which are two neighbors of FePc with one electron less and more, respectively. Furthermore, we find that the magnetization of these MPc molecules on O-Cu(110) surface are rather stable, so they could not be altered by moderate hole/electron doping. The e$_{g}$ orbitals (d$_{xz}$ and d$_{yz})$ of MPc molecules are found to be crucial for their magnetization on O-Cu(110) surface. \textbf{Acknowledgement.} This work was supported by DOE Grant DE-FG02-05ER46237.   

Dependence of magnetic field and electronic transport of Mn4 Single-molecule magnet in a Single-Electron Transistor

Dependence of magnetic field and electronic transport of Mn4 Single-molecule magnet in a Single-Electron Transistor A. Rodriguez, S. Singh, F. Haque and E. del Barco Department of Physics, University of Central Florida, 4000 Central Florida Blvd., Orlando, Florida 32816 USA T. Nguyen and G. Christou Department of Chemistry, University of Florida, Gainesville, Florida 32611 USA Abstract We have performed single-electron transport measurements on a series of Mn-based low-nuclearity single-molecule magnets (SMM) observing Coulomb blockade. SMMs with well isolated and low ground spin states, i.e. S = 9/2 (Mn4) and S = 6 (Mn3) were chosen for these studies, such that the ground spin multiplet does not mix with levels of other excited spin states for the magnetic fields (H = 0-8 T) employed in the experiments. Different functionalization groups were employed to change the mechanical, geometrical and transport characteristics of the molecules when deposited from liquid solution on the transistors. Electromigration-broken three-terminal single-electron transistors were used. Results obtained at temperatures down to 240 mK and in the presence of high magnetic fields will be shown.   

Magnetic characterization of a Mn6 Single-molecule Magnet

Single Molecule magnets (SMMs) are excellent candidates for the exploration of fundamental quantum effects at the nanoscale, as well as for potential applications in emerging technologies, such as quantum computation. Of particular interest are Mn6 SMMs, one of which has the highest effective energy barrier to magnetization reversal reported so far in the literature. We have performed Hall-effect magnetometry on single crystals of a new kind of Mn6 at cryogenic temperatures ($>$230mK). We will discuss the dependence of the hysteresis loops on the temperature and different sweep rates of the applied magnetic field. The high saturation fields observed in this SMM will be discussed.   

Stoner Instability in a Strongly Repulsive Fermi Gas

We study the existence of itinerant ferromagtism using a ultracold Fermi gas with short range repulsion in a harmonic trap. A degenerate Fermi gas is rapidly quenched into the regime of strong effective repulsion near a Feshbach resonance. The spin fluctuations are monitored using speckle imaging and, contrary to several theoretical predictions, the samples remain in the paramagnetic phase for arbitrarily large scattering length. We also observe a rapid decay into bound pairs over times on the order of $10\hbar/E_{F}$ in a wide range of interaction strengths, which is intrinsic by the nature of Feshbach resonance and preventing the study of equilibrium phases of strongly repulsive fermions. Our work suggests that a Fermi gas with strong short-range repulsive interactions does not undergo a ferromagnetic phase transition.   

Magnetism of Cobalt Dibromide Dihydrate and Monohydrate

The magnetic properties of two hardly studied bromide compounds among the large family of 3d divalent transition metal halide hydrates are examined, with Co(II) the metal ion. Of interest is comparison between magnetic behaviors in the much more common dihydrate form and in the rarely made monohydrate form, as well as comparison between known properties of the far better studied chloride materials and those containing bromide. In the title dihydrate material a magnetic susceptibility maximum appears at 9.5 K, about half the temperature of a similar feature in the chloride dihydrate system. But in the title monohydrate material an enhanced susceptibility maximum appears at 15.5 K, virtually the same as in the similarly behaving chloride monohydrate system. As for chloride systems, a major difference between di- and monohydrates appears in that magnetic irreversibilities are far stronger in the latter. The magnetic susceptibility in both title systems is non-Curie-Weiss like at moderate to high temperatures, but can be accounted for well on the basis of a ground and an excited Kramers doublet, along with exchange interactions. The susceptibility in the vicinity of the maximum in the title dihydrate material is approximately accounted for with a three-dimensional Ising model.   

Crystal structure and ferromagnetic phase transitions in CeCu$_{1-x}$Ge$_{1+x}$ (0.0$\mathbin{\lower.3ex\hbox{$\buildrel<\over {\smash{\scriptstyle=}\vphantom{_x}}$}}$$x\mathbin{\lower.3ex\hbox{$\buildrel<\over {\smash{\scriptstyle=}\vphantom{_x}}$}} $0.3)

As revealed in the powder x-ray diffraction and crystallographic data, the single phase sample in the series CeCu$_{1-x}$Ge$_{1+x}$ (0.0$\mathbin{\lower.3ex\hbox{$\buildrel<\over {\smash{\scriptstyle=}\vphantom{_x}}$}} $$x\mathbin{\lower.3ex\hbox{$\buildrel<\over {\smash{\scriptstyle=}\vphantom{_x}}$}} $0.3) crystallizes in the AlB$_{2}$-type structure with space group P6/mmm. The maximum ferromagnetic transition temperature T$_{c}$ in CeCu$_{1-x}$Ge$_{1+x}$, as determined from the electrical- resistivity and magnetic susceptibility measurements, is 10.6 K for the compound CeCu$_{0.8}$Ge$_{1.2}$. The magnetic susceptibility for each sample in CeCu$_{1-x}$Ge$_{1+x}$ (0.0$\mathbin{\lower.3ex\hbox{$\buildrel<\over {\smash{\scriptstyle=}\vphantom{_x}}$}} $$x\mathbin{\lower.3ex\hbox{$\buildrel<\over {\smash{\scriptstyle=}\vphantom{_x}}$}} $0.3) follows Curie's behavior between 100 and 300 K with an effective moment 2.6$\pm $0.1 $\mu _{B}$/Ce atom, a value close to that of Ce$^{3+}$. However, the observed saturation magnetic moment values (0.96 $\sim $ 1.15 $\mu _{B})$ at low temperatures for all these compounds are well less than the theoretically expected value 2.14 $\mu _{B}$ for the free Ce$^{3+}$ ion tangling the entire six-fold J = 5/2 multiplet. Subtracting the estimated phonon contribution from LaCuGe, the entropy associated with the magnetic structure of CeCuGe is found to meet the theoretical value of Rln2, which would be expected for a doublet ground state of Ce$^{3+}$ ion in the compound CeCuGe. The reduced saturation moment in CeCu$_{1-x}$Ge$_{1+x}$ is reasonably ascribed to partial lifting of the 4f-electron level degeneracy.   

Deuterated Mn and Ni Dibromide Dihydrate and Contrast with Normal Water Systems

First row transition metal hydrated bromide compounds are much less studied magnetically than chlorides, and for each of the common hydration states 2, 4 or 6 waters. Examination of rarer monohydrate chlorides was initiated only more recently. Quite intriguing similarities and differences in the magnetism, with respect to that of the most closely related dihydrates especially, occur. It is of interest to extend measurements to bromide systems, especially mono- and dihydrate forms. And given the role, structural and magnetic, that hydrogen bonding can play, deuteration effects are also worth exploring (and little studied in general). Magnetic measurements on the new deuterated title systems are presented. For the deuterated Mn compound the susceptibility is of similar general appearance to that of the normal water system, but with a maximum at 2.1 K, only one-third the 6.3 K of the latter; the maximum is much larger in the deuterated system. For the deuterated Ni compound a maximum appears about 10\% lower than the 6.0 K location in the normal water system, and is of moderately larger size; it is however substantially broader. Comparisons with deuterated vs normal water behaviors in related materials, where differences are often far less striking, will be made.   

Magnetic Domain Dynamics Study in Dysprosium

We have studied magnetic domain fluctuations in a Yttrium-Dysprosium-Yttrium tri-layer with X-ray photon correlation spectroscopy (XPCS) in conjunction with resonant soft x-ray magnetic scattering. Dysprosium possesses a helical antiferromagnetic ordering below its N\'{e}el temperature (T$_{N }$= 180K) and above its Curie temperature (T$_{C }\sim $ 85K). With resonant x-ray scattering, we observed the magnetic satellite peak at (0,0,q$_{m})$ due to the helical magnetic structure. We determined the transition temperature and found a shift in T$_{C}$ from the bulk value and hysteresis behavior around T$_{C}$. The transverse correlation length showed a minimum at both T$_{C}$ and T$_{N}$. With the coherent x-rays, we observed magnetic speckles from both static disorder and magnetic domains. The XPCS studies as a function of temperature showed that the magnetic domains are static up to 2000 seconds time scale when T $<$ 175K. Close to T$_{N}$, there appears to be slow dynamics which might be due to domain wall fluctuation.   

Giant Electromechanical Response in Graphene Nanoribbons

The demonstration of spin injection into graphene has proposed that graphene could play a role in spintronic devices. Specifically, it has been found that zigzag graphene nanoribbons (ZGNR) have spin states at their edges. In this study, first principles quantum mechanical calculations have been performed to investigate the effect of twist on the electronic, magnetic and transport properties of ZGNR. We investigate the electronic and magnetic structures of nanoribbon of ZGNR in the flat geometry and with 180$^{0}$ twisting. Using density functional theory coupled with nonequilibrium Green's function method implemented in SIESTA code, we examine the local magnetic moments and the quantum conductance of twisted ZGNR in its ground state (antiferromagnetic) and in case of ferromagnetic spin orientations. Our calculations show that ZGNR in its ground state is insensitive to twisted deformation, since the conductance of the twisted ZGNR is almost unchanged, as well as, no band gap change. However, we observe electromechanical switch via twisting a ferromagnetic ZGNR in hypothetical ferromagnetic nanoribbons. The transmission in a hypothetical ferromagnetic state for 4-ZGNR is 2 quantum of conductance, while the transmission becomes zero in case of oppositely polarized leads (after twisting), i.e. we observe an ideal spin valve. We relaxed both the atomic positions and the spin directions in our calculations allowing for a Bloch/Neel-like domain wall in the latter case.   

Simulations of random defects for fast domain wall motion in ferromagnetic nanowires

Domain walls in ferromagnetic nanowires move slowly when driven by large magnetic fields due to a process known as Walker breakdown. During Walker breakdown vortices are formed which slow the domain wall leading to low average speeds. Vortex formation is driven by the precessional motion of the magnetic moments in the domain wall. Techniques which disturb the coherent precession can be used to disrupt Walker breakdown and therefore recover fast domain wall speeds which could be useful for future technological devices. We used a combination of micromagnetic simulation and theoretical modeling to simulate the effects of random defects (voids) on the motion of a domain wall. Simulations find there is critical defect density associated with the destruction of the breakdown. We then use a magnetostatic charge model to calculate the strength of the perturbing field, created by the defects, necessary to disrupt the precessional motion of the moment in the domain wall. For an appropriate defect density the domain walls move quickly, without experiencing breakdown or a change in magnetic structure, for applied fields at least an order of magnitude greater than the typical breakdown field.   

Anisotropic Magnetoresistance Effects in Fe, Co, Ni, Fe$_4$N, and Half-Metallic Ferromagnet: A Systematic Analysis

We theoretically analyze the anisotropic magnetoresistance (AMR) effects of bcc Fe ($+$), fcc Co ($+$), fcc Ni ($+$), Fe$_4$N ($-$), and a half-metallic ferromagnet ($-$) [1]. The sign in each (~~) represents the sign of the AMR ratio observed experimentally. We here use the two-current model for a system consisting of a spin-polarized conduction state and localized d states with spin--orbit interaction. From the model, we first derive a general expression of the AMR ratio. The expression consists of a resistivity of the conduction state of the $\sigma$ spin ($\sigma=\uparrow$ or $\downarrow$), $\rho_{s \sigma}$, and resistivities due to s--d scattering processes from the conduction state to the localized d states. On the basis of this expression, we next find a relation between the sign of the AMR ratio and the s--d scattering process. In addition, we obtain expressions of the AMR ratios appropriate to the respective materials. Using the expressions, we evaluate their AMR ratios, where the expressions take into account the theoretical values of $\rho_{s \downarrow}/\rho_{s \uparrow}$ of the respective materials. The evaluated AMR ratios correspond well to the experimental results. \\[4pt] [1] S. Kokado {\it et al}., submitted to J. Phys. Soc. Jpn.   

Molecular Dynamics Simulation of a Two Dimensional Heisenberg Fluid

In this work we use numerical Monte Carlo and Molecular Dynamics to study a classical two-dimensional compressible magnetic fluid. The magnetic interactions are realized through a Yukawa-like potential while particles interact through Lenard-Jones forces. Our preliminary results point to a very rich phase transition picture. At high density the system seems to undergo a magnetic transition, as suggested by the magnetization and susceptibility results.   

Ultrafast spin precession dynamics in exchange-biased FeNi/FeMn/FeNi films

We investigated the spin precession dynamics in exchange-biased FeNi/FeMn/FeNi films by means of time-resolved magneto-optical Kerr effect (TR-MOKE). We observed the spin precession of all FeNi/FeMn/FeNi films in the TR-MOKE signals. The precession oscillations of the films changed rapidly as varying a thickness of antiferromagnet (FeMn). The period of the precession oscillations was not single and in the range of 10 to 20 ps. It is supposed that this is not only related to the exchange bias between ferromagnet and antiferromagnet but also double exchange biases in FeNi/FeMn/FeNi films.   

Tilted polarizer devices with a generic spin torque efficiency coefficient

Using the effective planar description [1-3] we study spin transfer devices with tilted polarizer. The dynamics of the free layer are investigated in the case of spin torque with substantially non-constant efficiency factor $g$. The switching diagram is constructed and discussed in qualitative terms.\\[4pt] [1] Ya. B. Bazaliy, Appl. Phys. Lett. {\bf 91}, 262510 (2007).\\[0pt] [2] Ya. B. Bazaliy, Phys. Rev. B {\bf 76}, 140402(R) (2007).\\[0pt] [3] Ya. B. Bazaliy, arXiv:1109.1331 (2011).   

Exchange coupled graded/soft/graded film with well-isolated grains fabricated by post-annealing

In order to solve the trilemma issue in perpendicular recording media, exchange spring and exchange coupled composite media have been proposed to reduce the coercivity of the hard magnetic layer without sacrificing the thermal stability. Thereafter, Suess proposed the graded media with a continuous variation in anisotropy to reduce the coercivity even further. Here we present an anisotropy-graded [FePt/C]5/Fe/[C/FePt]5 film, where C layer thickness gradually and symmetrically increases from two FePt ends to the soft Fe layer. In contrast with FePt/Fe bilayer, the graded/soft/graded film has a large reduction of 74{\%} in coercivity. Adding C layers with different thickness not only tailors the Ku gradient, but also refines the grain size and weakens the intergranular exchange interaction. Micromagnetic simulation reveals that the magnetization reversal is initiated by domain wall formation at the center of the soft Fe layer followed by domain wall propagation in two FePt layers simultaneously. This work was supported by the National Science Foundation for Distinguished Young Scholars (Grant No. 51025101), NSFC (Grant No. 60776008, 51101095).   

Anomalous magnetic anisotropy in Mn$_{3}$O$_{4}$ investigated by $^{55}$Mn$^{2+}$ and $^{55}$Mn$^{3+}$ Nuclear magnetic resonance

Mn3O4 has Yafet-Kittel type spin structure below Neel Temperature (41K). The magnetization along the c-axis is smaller than that along the ab-plane even in external magnetic field of 30 Tesla, implying huge magnetic anisotropy in the ab-plane. We measured $^{55}$Mn$^{2+}$ and $^{55}$Mn$^{3+}$ Nuclear Magnetic Resonance (NMR) of a Mn$_{3}$O$_{4}$ single crystal in external magnetic field. The canting angles of Mn$^{2+}$ and Mn$^{3+}$ magnetic moments were calculated from the spectral shift obtained for various magnetic field directions between the a, b and c-axes. With the canting angle data, we estimated the anisotropy energies of the Mn$^{2+}$ and Mn$^{3+}$ magnetic moments and the exchange energy between them. The result also shows that Mn$^{3+}$ spins in the Yafet-Kittel structure lie in the ab-plane contrary to the previous reports.   

Magnetic property enhancement of modified nanocrystalline ZrCo$_{5}$-based magnets

The metastable ZrCo$_{5}$ compound may be a good candidate for the development of rare-earth-free high-performance hard magnetic materials because of its high magnetocrystalline anisotropy field. Melt spinning is a good approach to synthesize metastable phase because of its high quench rate. In this work, the effect of Zr and Fe addition on structure and magnetic properties of melt-spun nanocrystalline Zr$_{1+x}$Co$_{5}$(0--0.3) alloys has been investigated. All the samples consist of orthorhombic ZrCo$_{5}$ hard magnetic and Co/Zr$_{6}$Co$_{23}$ soft magnetic phases. Proper Zr addition causes nanostructure refinement and the increase of the hard magnetic phase content, which strengthens intergrain exchange coupling. As a result, coercivity and maximum energy product of ZrCo$_{5}$-based magnet are significantly enhanced. The best magnetic properties: $_{i}$H$_{c}$ = 2.8 kOe, (BH)$_{max}$ = 4MGOe, which is the best value among Co-Zr binary alloys, are achieved in Zr$_{1.1}$Co$_{5}$. The temperature coefficient of its coercivity between 10 and 380K is -0.05{\%}/K. The saturation magnetization of nanocrystalline Zr$_{1.1}$Co$_{5}$ is greatly increased due to 16 at{\%} Fe addition.   

Competing magnetic interactions and interfacial frozen spin in Ni-NiO core-shell nano-rods

This work investigates the complex interfacial magnetism of free- standing Ni-NiO core-shell rods fabricated by electroless plating and an anodic aluminum oxide template. Vertical magnetization shift, arising from opposite field cooling conditions, suggests frozen spin (FS) at the Ni-NiO interface. The FS was related to the pinning effects of the NiO on the Ni, which mediated the interfacial antiferromagnetic (AFM)-ferromagnetic (FM) coupling, leading to the temperature-dependent properties of the rods. The FS was evident below 100 K, at which point the NiO-AFM dominated the properties with a suppressed coercive field and non-saturated magnetization. Above 100 K, however, the Ni-FM dominated and the FM phase was restored, due to the disappearance of the FS.   

Magnetic Properties of Nanostructured Mn$_{x}$TaS$_{2}$

We have fabricated tapelike nanostructures of Mn$_{x}$TaS$_{2}$ that consist of bundles of TaS$_{2}$ nanotubes intercalated with Mn. The magnetic properties of samples with Mn concentrations of 15{\%} and 23{\%} (nominal 25{\%}) were investigated. The Mn$_{0.23}$TaS$_{2}$ sample exhibited ferromagnetic characteristics with a Curie temperature near 90 K, which is close to that of bulk Mn$_{0.25}$TaS$_{2}$ crystals; however, the behavior of the linear and nonlinear ac susceptibility in the vicinity of the transition indicates departures from standard 3D ferromagnetic behavior. The Mn$_{0.15}$TaS$_{2}$ sample exhibits a spin-glass-like transition near 8 K, with a frequency-dependent ac susceptibility near the transition and hysteretic and aging effects below the transition. The imaginary part of the ac susceptibility becomes non-zero at temperatures significantly higher than the transition temperature, which is unusual for spin glasses. Overall, our measurements indicate strong similarities between our nanostructured samples and their bulk crystalline counterparts but some intriguing differences as well. It is not yet clear whether these differences can be attributed to the quasi-one-dimensional nature of the nanostructured Mn$_{x}$TaS$_{2}$ samples.   

The contactless measurement of forces which affect a magnetic particle in a fluid and its manipulation

The magnetic force has been used for the drug delivery, the cell/DNA manipulation, and other handlings of micro or nanoparticles. When magnetic particles are suspended in a fluid, they are influenced by the magnetic force caused by the magnetic field gradient, the gravity force, and the buoyance force. The magnetic torque also affects them to align their magnetic moments to the direction of the applied magnetic field. Furthermore, the viscous force or the force between the magnetized magnetic particles themselves cannot be neglected. In this study, methods of the quantitative measurement of these forces and the manipulation of a magnetic particle were developed. Four electromagnets were used to apply magnetic fields to ferrite particles (300nm-300$\mu$m) in a fluid in a vessel. The particle tracking velocimetry method was used to visualize the behavior of the particles. Based on the theory of the magnetic suspension and balance system, the vertical and horizontal forces affected a magnetic particle were estimated from the current in the coil of each electromagnet without any physical contact to it. And, the contactless manipulations of the magnetic particle suspended in the stagnated or flowing fluid by controlling the coil currents were demonstrated successfully.   

Determination of second order phase transition temperature of monoclinic phase of Gd$_{5}$(Si$_{x}$Ge$_{1-x})_{4}$

Gd$_{5}$(Si$_{x}$Ge$_{1-x})_{4}$ has a first order phase transition from high temperature paramagnetic monoclinic to low temperature ferromagnetic orthorhombic phase for 0.4$<$x$<$0.51. It is not possible to determine experimentally the second order phase transition temperature of orthorhombic or monoclinic phase. Previous studies have estimated second order phase transition temperature of the orthorhombic phase using a modified Arrott plot technique. The composition 0.3$<$x$<$0.4 Gd$_{5}$(Si$_{x}$Ge$_{1-x})_{4}$ has mixed monoclinic and orthorhombic phases without a clear transition between the two phases. We have determined the second order phase transition temperatures of both orthorhombic and monoclinic phases for Gd$_{5}$Si$_{1.5}$Ge$_{2.5 }$ using modified Arrott plots. Magnetic moment vs. magnetic field for various temperatures was measured using a SQUID magnetometer. Arrott plots were plotted from M$^{1/\beta }$ and (H/M)$^{1/\gamma }$ isotherms using critical exponents of 1/$\gamma $= 0.85 and 1/$\beta $=1.85. We obtained two sets of parallel lines for the orthorhombic phase and the monoclinic phase. The temperature of the isotherm that passes through the origin for both orthorhombic and monoclinic phase was estimated to be 299.5~K and 197~K respectively.   

Boron diffusion due to annealing in CoFeB/MgO/CoFeB interfaces: A combined HAXPES and NEXAFS study

We report the hard x-ray photo-electron spectroscopy (HAXPES) and near edge x-ray absorption fine structure (NEXAFS) of CoFeB$\vert $MgO$\vert $CoFeB tunnel junctions as a function of annealing time. Upon annealing, the oxidation state of B changes from predominantly elemental (0 valence) boron in the as deposited sample to higher oxidation in annealed samples as evident from HAXPES spectra. The NEXAFS spectroscopy results showed that upon heating, B species migrate towards the MgO and interact with it. A comparison of the tunnel junction NEXAFS signature with some standards suggests that the B forms a 3-fold coordinated boron compound in the MgO environment and 4-fold coordinated boron resembling Kotoite mineral in the CoFe/MgO interface.   

M\"{o}ssbauer study of corrosion and abrasion products in oil transporting pipes

It is known that one of the main technological problems in carbon steel oleoducts is the corrosion produced by different substances, such as water, carbon dioxide, sulfur, and microorganisms. In addition, if in such mixture there is sand, aggressive sludge can be form that abrasions material from the oleoduct. A room temperature M\"{o}ssbauer study of corroded material taken from different sites of oleoducts is presented. Most of the M\"{o}ssbauer spectra reveal the presence of nanoparticles, indicating that in these pipes the abrasion problem is severe. A preliminary identification of the oxidized samples suggests the presence of magnetite, and some Iron hydroxides. Further studies are in course in order to identify unambiguously the products present in the corroded materials.   

Performance of hybrid density functional theory for $\alpha $ \textit{versus} $\delta $-Pu

Hybrid density functional theory, which replaces a fraction of density functional theory exchange with exact Hartree-Fock exchange, has been used to study the electronic, geometric, and magnetic properties of $\alpha $--Pu and compared with our previous results for $\delta $--Pu. A non-magnetic (NM) ground state was realized for $\alpha $=0.55 ($\alpha $ indicating the fraction of the HF exchange) for $\delta $--Pu but the equilibrium atomic volume deviated from experiment by 19{\%}, the 5$f$ electron population was close to 4 and a 5$f$ DOS that shows anomalous localization and failed to match experimentally obtained PES data.\footnote{R. Atta-Fynn and A. K. Ray, Europhys. Lett. 85, 27008 (2009)} For $\alpha $-Pu, a NM ground state was obtained at 40{\%} HF exchange. Comparing the two phases at the NM ground state, the 5$f$ population is about the same, but energy differences between the different magnetic configurations for the two phases are observed. For $\delta $-Pu NM-FM and NM-AFM \textit{$\Delta $E} are 86.33 and 82.28 mRy/atom, respectively, and for $\alpha $-Pu the NM-FM and NM-AFM \textit{$\Delta $E} are 144.98 and 60.72 mRy/atom, respectively. The 5$f$ DOS for $\delta $-Pu show no DOS at the Fermi level but the presence of localized states, while for $\alpha $-Pu DOS show delocalization. Though hybrid density functional theory might not perform better compared to DFT for $\delta $-Pu, it shows promise for $\alpha $-Pu.   

Mott behavior of ultrathin epitaxial LaNiO3 films and interfaces via hard x-ray and standing-wave excited photoemission

In this study we apply several emerging x-ray photoemission techniques to study Mott behavior of ultrathin LaNiO3 films and interfaces in a depth-resolved manner. In order to understand the effects of thickness and strain on the electronic structure, we apply hard x-ray photoemission (HAXPES) at 6 keV to epitaxial LaNiO3 films of varying thickness under compressive and tensile strain. Mott metal-to-insulator transition is observed for the thinnest films. Furthermore, standing-wave-excited photoemission is used to study the electronic structure of ultrathin LaNiO3 in a SrTiO3/LaNiO3 superlattice. Standing-wave measurements of core-level and valence band spectra are used to derive layer-resolved densities of states, revealing a suppression of electronic states near the Fermi level in the multilayer as compared to bulk LaNiO3. Further analysis shows that the suppression of these states is not homogeneously distributed over the LaNiO3 layers but is more pronounced near the interfaces.   

Depth-resolved ARPES of buried layers and interfaces via soft x-ray standing-wave excited photoemission

We introduce a new depth-selective photoemission technique, achieved by combining soft x-ray ARPES with standing-wave (SW) excited photoelectron spectroscopy, wherein the intensity profile of the exciting x-ray radiation is tailored within the sample. This effect is accomplished by setting-up an x-ray standing-wave field within the sample by growing it on a synthetic periodic multilayer mirror substrate, which in first-order Bragg reflection acts as the standing-wave generator. The antinodes of the standing wave function as epicenters for photoemission and can be moved vertically through the buried layers and interfaces by scanning the x-ray incidence angle. The new SW-ARPES technique is then applied to the investigation of the electronic properties of the buried interface within a magnetic tunnel junction La0.7Sr0.3MnO3/SrTiO3. The experimental results are compared to the state-of-art one-step photoemission theory including matrix element effects.   

Dependence of Polarization and Dielectric response on Epitaxial Strain in (Ba$_{x}$Sr$_{1-x}$)TiO$_{3}$ Ultrathin Films from First-Principles

A first-principles-derived schemes is used to use a first-principles-derived technique to construct the temperature-versus-misfit strain phase diagrams for the whole BST composition rang (i.e., x=0.00,0.20,0.40,0.60,0.80,1.00). Moreover, we investigate the dependence of their dielectric and ferroelectric properties on the strain and the concentration. Our results reveal that the predicated phase diagrams show a topology similar to those calculated by Shirokov et. al. Phy. Rev. B. \textbf{79} 144118 (2009) with quantitative discrepancies that will be revealed and explained. Our results also indicate that in-plane strain increases (respectively, decreases) the in-plane (respectively, out-of-plane) dielectric constants. Furthermore, the out-of-plane component of dielectric permittivity $\varepsilon _{33}$ enhances with lowering x in (Ba$_{x}$Sr$_{1-x})$TiO$_{3}$ films. We hope that our results will be benefits to many scientists and will lead to new strategies for material design.   

A first principles thermodynamic study of Si-HfO$_{2}$ and Pt-HfO$_{2}$ interfaces

Atomic-level control of dielectric-semiconductor and dielectric-metal interfaces has become critical in determining the properties of the emerging high-$k$ oxide-based MOSFETs. Using Si-HfO$_{2}$ and Pt-HfO$_{2}$ as an example, we investigate the evolution of these interface structures and electronic properties as a function of processing conditions by combining density functional theory results and statistical thermodynamics. Firstly, using first principles thermodynamics (FPTs), we determine the phase diagrams of Si-HfO$_{2}$ and Pt-HfO$_{2}$ interfaces. The vibrational and configurational entropic contributions to the free energies of the condensed phases are explicitly included. We demonstrate that the predictions of the FPT approach are in quantitative agreement with experiments for the interfaces considered. Secondly, a parameter-free methodology to determine the work function shift (or effective work function, EWF) of metal when interfaced with oxide is developed. This strategy is combined with statistical thermodynamics to predict the most probable EWF at given processing condition. The favorable agreement between the computed and experimental EWFs under generally adopted conditions is indicative of the usefulness of such full first-principles property-processing relationship studies.   

Spiral spin order induced ferroelectricity in various type-II multiferroics

In the past decade, one of the milestones associated with multiferroicity researches has been the experimental and theoretical identification of spin orders induced ferroelectricity in various spin frustrated oxide materials. These specific orders include the noncollinear spiral spin order, collinear spin order of exchange striction, $E$-type antiferromagnetic (AFM) order of double-exchange nature, and so on. Both the cross-product type and dot-product type spin interactions may contribute to the ferroelectricity generation. In consequence, experimentally observable multiferroic phenomena can be complex and reflected in multifold dimensions. In this talk, we address the spiral spin order induced ferroelectricity in various multiferroics where the cross-product type spin interaction is believed to contribute to the ferroelectricity. A modulation of such spiral spin order by various approaches is investigated and in particular the complex spin interactions in $R$MnO$_{3}$ with magnetic and non-magnetic doping at the R-site and Mn site will be discussed. Hopefully this talk may allow additional facts to our comprehensive understanding of the multifold interactions which eventually have impact on the magnitude of ferroelectric polarization in those multiferroics.   

Ultrafast Photoinduced Magnetic Phase Transition in Manganites

The process of manipulating collective spin ordering and inducing magnetic phase transitions in highly \textit{non-equilibrium, non-thermal }states at \textit{femtosecond} time scales has received much current interest. These ultrafast processes offer opportunities for significant improvement over modern magneto-optical recording and magnetic storage/logic devices. One prominent system for such femtosecond magnetism is the strongly correlated manganites, which are truly responsive near the phase boundary, exhibiting extreme sensitivity to external stimuli such as light, electric and magnetic fields. Here, using ultrafast two-color magnetic circular dichroism spectroscopy, we observed distinct fs spin dynamics and critical behaviors in the strongly correlated manganites. We will discuss the origin of the responses as well as present a thorough study on differently doped and thin film samples.   

$\delta$-doped SrTiO3 heterostructure in high magnetic fields

High mobility 0.1\% $\delta$-doped STO magneto-transport has been measured in high magnetic fields as a function of angle. The Nb:SrTiO3 doping layer is 25 nm thick and sandwiched between insulating SrTiO3 buffer and cap layers on an SrTiO3 substrate, putting it well within the 2-dimensional limit. The system exhibits Shubnikov-de Haas oscillations over the entire angle range measured. The resulting interplay between multiple sub-bands as the system approaches the quantum limit will be discussed.   

Magnetic order and structural phase transition in strained ultrathin SrRuO$_{3}$/SrTiO$_{3}$ superlattice

Strain is one of the key parameters to control the properties of functional materials in fabrication. With first-principles simulations, we find for the first time that SrRuO$_{3}$/SrTiO$_{3}$ superlattice undergoes robust phase transitions with the in-plane lattice strain ranging from -4.5\% to 6\%. In the high tensile strain region, the magnetic ordering among neighboring Ru ions changes from a ferromagnetic to an antiferromagnetic phase, together with a metal-to-Mott-insulator transition, a unique character in our superlattice which is absent in the bulk. On the other hand, in the low strain region, the suppression of the octahedra tilting was also observed. The driving force of the suppression of tilting was investigated and found to be the charge redistribution in the Sr-O plane. All of these are important to the strategy of controlling the structural, electronic and magnetic properties in Ru-based perovskite systems.   

Unexpected high conductivity at emerging twin boundaries in LSMO thin films

Transport properties of high quality La$_{2/3}$Sr$_{2/3}$MnO$_{3}$ (LSMO) thin films are studied by using Conducting Scanning Force Microscopy (C-SFM) measurements. Current images were acquired in a non-invasive manner by using the contact operation mode at the lowest possible applied load needed. LSMO thin film surface consists of one unit cell steps separating atomically flat terraces, with a low surface roughness on the terraces reproducing the STO substrate morphology. The existence of twin boundaries within the film is evidenced by conducting scanning force microscopy data. I(V) characteristics curves obtained at specific surface points indicate an important enhancement of the electronic response at the twin boundaries locations as compared to that measured on the twin surface. The absolute values of the measured current for a given voltage may differ by up to nearly one order of magnitude depending on whether the tip contacts the location where the twin boundary emerges at the surface or it is placed on a region on top of one of the twin crystallites. The origin of this large difference is not clear yet, but preliminary analysis seems to indicate that an increase in the density of states (DOS) at the boundaries might have an important contribution.   

The role of correlations on oxygen orbitals in late transition-metal oxides

We investigate the effect on transition-metal oxide physics of including interactions on the oxygen sites as well as on the transition-metal site using a generalization of the single-site Dynamical Mean Field method. On-site repulsive and Hund's interactions in the full Slater-Kanamori form are treated using a numerically exact continuous-time quantum Monte-Carlo solver. We determine the metal-insulator and magnetic phase diagrams as a function of charge-transfer tendency and interaction strengths. The results are compared directly to models with no oxygen correlations, yielding insights about the role of oxygen-specific correlations.   

Dynamical mean-field embedding of the dual fermion dynamical cluster approach for strongly correlated systems

We extend the recently developed dual fermion dynamical cluster approach with a further embedding of the dual fermion lattice into a larger, third length scale. The resulting approach is a complete multi-scale many-body technique for strongly correlated electron systems. It treats the short length scales explicitly by the dynamical cluster approach, intermediate length scales diagrammatically with the dual fermion technique, and the largest length scales approximately at a dynamical mean-field level. This technique iterated to self-consistency on all the three length scales. To illustrate the implementation and applicability of this method, we test it with the one and two dimensional Falicov-Kimball model. We will specifically address the convergence and critical scaling behavior of the charge-density-wave transition temperature.   

Single domain VO$_{2}$ single crystals exhibiting metal-insulator transition and insulator-insulator transition

Electrical transport, optical microscopy, and synchrotron-based x-ray micro-diffraction measurements have been carried out on single domain VO$_{2}$ single crystals, exhibiting an abrupt first-order metal--insulator transition (MIT) with a high resistance ratio $\sim $ 10$^{5}$ near $\sim $67 $^{\circ}$C and a gradual insulator--insulator transition (IIT) at $\sim $48 $^{\circ}$C. Low temperature (T$<$ 45 $^{\circ}$C), intermediate ($\sim $50 $^{\circ}$C $<$T$<\sim $67 $^{\circ}$C), above MIT (T$>$67 $^{\circ}$C) phases were insulating monoclinic M2 with half vanadium ion dimerization, monoclinic M1 with full vanadium ion dimerization, and rutile (R) phases, respectively. In this poster, we will present unusual structural phases of M1, M2, and R of single-domain VO$_{2}$ single crystals and discuss the relationships between structures and the characteristics of the MIT of VO$_{2}$.   

Mechanism of orbital-selective phase transition by different magnetic states

Using a dynamical cluster approximation we analyzed the behavior a degenerated two-orbital anisotropic Hubbard model at half-filling where both orbitals have equal bandwidths and one orbital is in paramagnetic (PM) state while the other one is in antiferromagnetic (AF) solution. We found an orbital-selective metal-insulator transition in which the PM orbital undergoes a transition from a Fermi liquid (FL) to a Mott insulator through a non-FL, while the AF orbital displays a transition from a FL to an AF insulator through an AF metallic phase. Finally, we would discuss that our intermediate phases are possibly related to the puzzling AF metallic state iron-pnictide superconductors.   

Lattice-Symmetry-Driven Phase Competition in Vanadium Dioxide

We performed group-theoretical analysis of the symmetry relationships between lattice structures of R, M1, M2, and T phases of vanadium dioxide in the frameworks of the general Ginzburg-Landau phase transition theory. The analysis leads to a conclusion that the competition between the lower-symmetry phases M1, M2, and T in the metal-insulator transition is pure symmetry driven, since all the three phases correspond to different directions of the same multi-component structural order parameter. Therefore, the lower-symmetry phases can be stabilized in respect to each other by small perturbations such as doping or stress.   

Valence fluctuation driven quantum phase transition

In recent years quantum critical phenomenon have acquired a great interest in the condensed matter community. Many rare earth intermetallic compounds, which are also heavy fermions can be tuned easily to quantum critical point by application of external perturbations like magnetic field and pressure. YbRh$_2$Si$_2$ and CeCu$_2$Si$_2$ are a few examples. The periodic Anderson model (PAM) is a paradigm for studying these kind of systems. We investigate the extended periodic anderson model (EPAM), which includes Coulomb interaction of conduction and localised electrons using local moment approach (LMA) within dynamical mean field theory (DMFT) with the objective of developing an understanding of quantum phase transitions due to valence fluctuations. We show that tuning c$-$f interaction and on-site energy of localised electron (which can be achieved by varying external perturbation like pressure) leads to some exotic phenomena like vanishing of Fermi liquid scale. We study transport properties near quantum critical point and highlight the anomalies due to the proximity of QCP.   

Band Insulators and Hubbard interactions

We investigate the theoretical models which are band insulators in the abscence of interaction and evolve into Mott insulators under strong mutual repulsive interaction. We use the cellular dynamical mean-field theory to study the nature of the interaction-driven transitions in the systems. In various systems we compute the renormalized band gap which is defined by the self-energy corrected band gap and show that it is a convenient measure to characterize band insulating phases in such systems; the spectral gap is fully explained by the renormalized band gap when the systems remain in the band insulating phase. Finally we discuss interesting transition behavior from a band insulator to a Mott insulator.   

Electronic structure of Lu$_{1-x}$La$_{x}$VO$_{3}$ single crystals using soft x-ray spectroscopy

The rare-earth vanadates, $R$VO$_{3}$, offer a rich phase diagram of both orbital and spin ordering phenomena, stemming from their two-fold occupation of the three-fold degenerate V $t_{2g}$ orbitals. It has been discussed that, in $R$VO$_{3}$, which shows the $t_{2g}$ orbital ordering, the Jahn-Teller coupling suppression is much weaker than that in the $e_{g}$ electron systems. In order to address the orbital ordering effects, we report soft x-ray measurements of Lu$_{1-x}$La$_{x}$VO$_{3}$ single crystals, which approach both the smallest and largest rare-earth ionic sizes. X-ray absorption spectroscopy and x-ray emission spectroscopy, which reveal both the unoccupied and occupied partial density of states, are employed to observe the changes in the V 3$d$ and O 2$p$ states, across the orbital ordering transitions and $R$-site ionic radii. Also, resonant inelastic x-ray scattering is applied to probe the O 2$p$-V 3$d$* charge transfer excitations and V 3$d$-3$d$* transitions. Together, these complementary techniques provide a picture of the electronic structure of Lu$_{1-x}$La$_{x}$VO$_{3}$ to test the role of the orbital ordering during phase transitions with varying rare-earth ionic sizes.   

Singularity in self-energy and composite fermion excitations of interacting electrons

We propose that a composite fermion operator $f_{i\sigma}(2n_{i{\bar \sigma}}-1)$ could have coherent excitations, where $f_{i\sigma}$ is the fermion operator for interacting electrons and $n_{i{\bar \sigma}}$ is the number operator of the opposite spin. In the two-impurity Anderson model, it is found that the excitation of this composite fermion has a pseudogap in the Kondo regime, and has a finite spectral weight in the regime where the excitation of the regular fermion $f_{i\sigma}$ has a pseudogap. In the latter regime, the self-energy of $f_{i\sigma}$ is found to be singular near Fermi energy. We argue that this composite fermion could develop a Fermi surface with Fermi liquid behaviors but ``hidden'' from charge excitations in lattice generalizations. We further illustrate that this type of excitations is essential in addressing the pseudogap state and unconventional superconductivity.   

Finite pulse relaxation calorimetry and specific heat of NdOs$_4$Sb$_{12}$

The compound NdOs$_4$Sb$_{12}$ is a mean-field ferromagnet at about 1 K. As inferred from specific heat measurements below 10 K, the electronic specific heat coefficient of this compound is very large ($\sim$ 520 mJ/mol-K$^2$). Intriguingly, a recent ultrasonic measurement shows that this compound has double ultrasonic dispersions at $\sim$15 K and 40 K. We have used our newly developed relaxation calorimeter to measure the specific heat of NdOs$_4$Sb$_{12}$ in the temperature range 11 K to 300 K. In this presentation, we will describe the experimental setup used for the finite pulse relaxation calorimetry in a crycooler and the results of our measurements on the NdOs$_4$Sb$_{12}$ sample.   

Magnetization measurements in site disordered UCu$_4$Ni

UCu$_4$Ni is a non-Fermi liquid, quantum critical system, obtained from dilution of Cu for Ni in antiferromagnetic (AFM) UCu$_5$. Introduction of Ni in the lattice structure of the parent compound suppresses the AFM transition temperature from 16.5 Kelvin down to zero at $x=1$. Because Ni ions are slightly smaller than Cu ones (Pd and Cu ions are similar in size), Ni can be expected to distribute itself randomly between the two available crystal sites. UCu$_4$Ni is therefore a site-disordered material. We present a magnetization study of a random powder of UCu$_4$Ni as a function of applied magnetic field (-9.0--9.0 T) and temperature (2--300 K). We analyze the data from the point of view of magnetic disorder, as would be produced by a distribution of local magnetic susceptibilities, within the context of the Kondo Disorder Model. Comparison with similar existing studies in polycrystalline samples by others will be discussed.   

Finite Size Effects on Electrical Transport of Nanoribbons of the Charge Density Wave Conductor NbSe$_{3}$

NbSe$_{3}$ is a textbook example of a charge density wave material, and although its properties are well known in bulk, its nanoscale characteristics are relatively unexplored. In particular, owing to the chain-like atomic structure of NbSe$_{3}$, electric transport studies on quasi-one-dimensional devices remain a new frontier in scientific research in which unique finite size effects occur as a result of geometrical confinement and lattice reconstruction in the nanoscale. We used a novel, facile chemical vapor transport method for synthesizing nanoscale, single-crystalline NbSe$_{3}$ nanoribbons. This bottom-up method for preparing free-standing nanoribbon devices avoids potential sample degradation observed in top-down approaches. Typical ribbons have cross sectional area 10$^{4}$ $nm^{2}$ or less and lengths 10 - 50 $\mu$m. Single nanoribbon electrical transport measurements show expected charge density wave transitions at 59 and 141 K. We also observe significant enhancement in the depinning effect and sliding regimes mainly attributed to finite size effects.   

The magnetoelectirc effect on the novel multiferroic Co$_{3}$TeO$_{6}$

The magnetic, thermal, and dielectric measurements were performed on a single crystal sample Co$_{3}$TeO$_{6}$. Two anomalies are observed at $T_{1 }\sim $ 26 K and $T_{2} \quad \sim $ 18 K in magnetic susceptibility and specific heat measurements. Dielectric constant data show a step anomaly at 18 K, which does not display frequency-dependent behavior but a magnetoelectric effect. Furthermore, the values of the magnetoelectric coupling constant$\gamma $ were calculated, which are 0.0268 and 0.0239 at 7 K and 13 K, respectively. The temperature-dependent X-ray diffraction suggests that the lattice parameters slightly deviate form linear trend as temperature down to 26 K, and then shows an anomalous variation around 18 K, where a structural distortion probably appears. All phenomena of our results indicate that Co$_{3}$TeO$_{6}$ is one member of multiferroic materials   

Quantum phase transition of the sub-Ohmic rotor model

We investigate the behavior of an $N$-component quantum rotor coupled to a bosonic dissipative bath having a sub-Ohmic spectral density $J(\omega) \propto \omega^s$ with $s<1$. With increasing dissipation strength, this system undergoes a quantum phase transition from a delocalized phase to a localized phase. We determine the exact critical behavior of this transition in the large-$N$ limit. For $1>s>1/2$, we find nontrivial critical behavior corresponding to an interacting renormalization group fixed point while we find mean-field behavior for $s<1/2$. The results agree with those of the corresponding long-range interacting classical model. The quantum-to-classical mapping is therefore valid for the sub-Ohmic rotor model.   

Pomeranchuk instability from a black hole

Within the AdS/CFT correspodence, we introduce a probe spinor field and a neutral symmetric traceless spin-two field which are dual to a fermionic operator and a neutral tensor order parameter in the boundary field theory. By considering a certain coupling between the two probe fields, we show that a transition induced by condensation of the neutral order parameter dual to the neutral spin-two field can lead to the breaking of rotational symmetry of Fermi surface and hence a Pomeranchuk instability. The critical properties of this transition are computed.   

Superfluidity Without Boson Condensation

Superfluidity is generally attributed to condensation of boson wavefunctions. He-4 atoms have zero net spin in the nucleus and in the electrons, and so are bosons, while He-3 atoms have a nuclear spin of $\raise.5ex\hbox{$\scriptstyle 1$}\kern-.1em/ \kern-.15em\lower.25ex\hbox{$\scriptstyle 2$} $, and are fermions. This fits the conventional understanding that the superfluid transition of He-4 at 2.2 K is due to boson condensation, while the lack of such a transition in He-3 is due to the absence of bosons until atomic pairing into boson-like Cooper pairs occurs in the mK range. However, Kadin [1] has extended a novel non-boson superconducting condensation mechanism to superfluids. This is based on an interleaved two-phase structure of close-packed electron orbitals (analogous to an ionic liquid), whereby each orbital is surrounded by multiple anti-phase orbitals, with nodes between them to maintain orthogonality. This entire structure moves together as a superfluid, preserving long-range phase coherence. The lack of superfluidity in He-3 is attributable \textit{not} to these atoms being fermions, but rather to the presence of unpaired nuclear spins that destroy the phase coherence due to spin-flip scattering. This is consistent with the behavior of mixtures of He-3 and He-4. Only at mK temperatures, when the nuclear spins order and spin-flip scattering is suppressed, is superfluidity again possible. A similar picture may also be extended to ``Bose-Einstein condensates'' in dilute concentrations of alkali metal atoms, but \textit{only} if one assumes the presence of close-packed atomic clusters or droplets, rather than a dilute gas. [1] A.M. Kadin, \underline {http://arxiv.org/abs/0909.2901} (2009); \underline {http://arxiv.org/abs/1107.5794} (2011).   

3He-4He liquid mixtures investigated by neutron imaging technique at low temperatures

Helium is a unique element which exhibits a variety of different phases and unusual behaviors. It can be found in nature in two stable isotopic forms: $^{3}$He and $^{4}$He. One of the most profound quantum mechanical effects, superfluidity, occurs below 2.17 K in liquid helium $^{4}$He and 0.003 K in liquid $^3$He. There are also interesting phenomena occurring in mixtures of the two isotopes. One demonstrative example is the finite solubility of liquid $^{3}$He (a Fermi system) in superfluid $^{4}$He (a Bose system) even at T = 0 K. This is the basic principle in the operation of a $^{3}$He-$^{4}$He dilution refrigerator capable of continuously producing 2 mK. While much has been done in studies of the thermodynamical, quantum properties of liquid helium mixtures, there has not been any attempt to visualize the dynamics of $^{3}$He in liquid $^{4}$He. Presented results of neutron imaging experiments on 0.3 bar liquid $^{3}$He-$^{4}$He mixtures, at 1.5 K have shown a clear diffusion of $^{3}$He driven by the difference in chemical potential. The data were taken for over 12 hours using a high resolution CCD camera.   

Traces of Vortices in Superfluid Helium Droplets

We report on the observation of vortices in superfluid $^{4}$He droplets produced by a free fluid-jet expansion. The vortices were traced by introducing silver atoms into the droplets, which clustered along the vortex lines. The silver clusters were subsequently surface deposited and imaged via electron microscopy. The prevalence of elongated track-shaped deposits shows that vortices are ubiquitous in droplets larger than about 300 nm and that their lifetime exceeds a few milliseconds. In these experiments the droplets become superfluid within a few microseconds, which is at least 10$^{6}$ times faster than in previous bulk helium experiments. We discuss possible formation mechanisms and the stability of the vortices obtained during this rapid phase transition.   

Size effect of properties in hybrid of gold nanoparticles and calamitic mesogen

In this contribution we describe the synthesis and investigation of Au NPs functionalized with organic mesogenic groups which self-assemble into liquid crystalline phases. A nematic mesogenic liquid crystalline laterally reacts with functionalized gold via a siloxane group using Karstedt's catalyst. Compared to earlier work where NPs were in the size of 1.5-2.5 nm, we report here on larger than 5 nm sized particles. The synthesis of the particles is based on a new methodology using digestive ripening and synthesis functionalisation of the organic groups. This example may open up a protocol for liquid crystal molecules encapsulated in nanoparticles. Properties of the calamitic mesogen encapsulated gold nanoparticles have been investigated by HRTEM, NMR, UV/Vis, differential scanning calorimetriy (DSC), optical polarizing microscopy (OPM).   

Trimeric surfactant as superior boundary lubricant

Surfactants are widely used to modify surfaces and interfaces properties. A unique cationic trimeric surfactant, tri(dodecyldimethylammonioacetoxy)-diethyltriamine trichloride (DTAD), was found to form liposome-like aggregates in solution or a highly ordered bilayer patterns on mica surface [1,2]. We have investigated the normal and shear forces between two atomically smooth mica surfaces across this surfactant using the surface force balance (SFB) technique. Shear forces measured across the trimeric surfactant solution demonstrate ultra-low friction coefficient ($\mu $=5x10$^{-5})$ under pressure of tens of atmospheres. AFM scans under the trimeric surfactant solution demonstrate the presence of large spheres (ca. 100nm in diameter) on the mica. The contribution of the compressed charged spheres to the ultra-low friction measured can be explained by the efficient hydration lubrication mechanism exist between the hydrated surfactant head-groups and the mica surface. A weak dependence of the friction coefficient to the shear rate or amplitude was observed.\\[4pt] [1] Hou et al., \textit{Langmuir}, \textbf{24}, 10572 (2008).\\[0pt] [2] Wu et al., \textit{Langmuir}, \textbf{26}, 7922 (2010).   

Freely Suspended Smectic Films in Aqueous Environment

Smectic liquid crystals easily form thin films which are freely suspended on a solid frame in air. These systems have been thoroughly studied for various purposes such as structural studies of smectic phases, investigating phase transitions in two-dimensional systems, and studying various physical properties of liquid crystals. In the present study, we explore the preparation of freely suspended smectic films in water. A prerequisite is the presence of a surfactant which accumulates at the liquid-crystal/water interface and induces a homeotropic anchoring of the director, so that the smectic layers align parallel to the two film surfaces. The presence of the surfactant might also serve as a handle to tune properties such as the surface tension of the films (which is hardly possible for freely suspended films in air). We study the formation of films in water using different frames and different surfactants, and we focus especially on the thinning behaviour which occurs when the temperature is increased towards the smectic - nematic or smectic - isotropic transition.   

Stiffness Dependent Separation of Cells in a Microfluidic Device

Abnormal cell mechanical stiffness can point to the development of various diseases including cancers and infections. We report a new high-throughput technique for continuous cell separation utilizing variation in cell stiffness. We use a microfluidic channel decorated by periodic diagonal ridges to separate K562 lymphoblastic cell line modified to different mechanical stiffness values. Diagonal ridges within the microfluidic flow channel compress and deform the cells in rapid succession to translate each cell perpendicular to the channel axis in proportion to its stiffness. Atomic force microscopy (AFM) was used to directly measure the Young's modulus of modified K562 cells to verify the stiffness variation. We demonstrate that soft cells can be separated from stiff cells at physiological concentrations with a fivefold enrichment of cell populations. This microfluidic device opens the way for conducting rapid and low-cost cell analysis and purification through physical markers.   

Spin-orbital models using dipolar fermions in zig-zag optical lattices

Ultra-cold dipolar spinor fermions in zig-zag type optical lattices can mimic spin-orbital models relevant in solid-state systems, as pyroxene titanium and layered vanadium oxides, with the interesting advantage of reviving the quantum nature of orbital fluctuations. We discuss two different physical systems in which these models may be simulated, showing that the interplay between lattice geometry and spin and orbital quantum dynamics produces a wealth of novel quantum phases.   

Spin-dependent low-energy $^4$He$^+$ ion scattering on non-magnetic surfaces

We investigated electron-spin-polarized $^4$He$^+$ ion scattering on various non-magnetic surfaces at kinetic energies below 2 keV [1]. It was observed that the scattered He$^+$ ion yield depends on the He$^+$ ion spin. We interpret this spin-dependent scattering in terms of the spin-orbit coupling (SOC) that acts transiently on the He$^+ 1s$ electron spin in the He$^+$-target binary collision. This interpretation qualitatively explains the relationship between the spin-dependent scattering and the scattering geometry, incident velocity, and magnetic field arrangement. This is the first study to report SOC caused by projectile electron spin in ion scattering.\\[4pt] [1] T.Suzuki, Y.Yamauchi, and S.Hishita, Phys.Rev.Lett. 107(2011)176101.   

Iron chalcogenide photovoltaic absorbers -- problems and opportunities

Realizing new, efficient solar absorbers containing earth-abundant materials represents a critical element for expanding the reach of photovoltaic (PV) technologies, meeting growing energy needs. The use of Fe in PV was proposed more than 25 years ago in the form of FeS$_{2}$ pyrite. We report a concerted and integrated theoretical and experimental study that provides new insight into the problem of FeS$_{2}$. Computational results on FeS$_{2}$ reveal high formation energies for bulk point defects and small formation energies for S vacances near the surface. These findings are consistent with the formation of metallic S-deficient binary Fe-S phases at low temperatures that affect the electrical and optical properties of thin films. We have used this new understanding to propose and implement design rules for identifying new Fe-containing materials---Fe$_{2}$SiS$_{4}$ and Fe$_{2}$GeS$_{4}$-- that may circumvent the limitations of pyrite. These ternary materials are $p$-type with direct allowed optical band gaps near 1.5 eV.   

Normal force and interpenetration between polyelectrolyte brushes

We examine the normal force between two opposing polyelectrolyte brushes and the interpenetration of their chains that is responsible for sliding friction. We focus on the special case of semi-dilute brushes in a theta solvent, for which the classical strong-stretching theory (SST) can be solved analytically. Interestingly, SST predicts that the brushes contract as they are compressed together maintaining a polymer-free gap, which provides an explanation for the ultra-low frictional forces observed in experiment. We examine the degree to which the SST predictions are affected by chain fluctuations by employing self-consistent field theory (SCFT). While the normal force is relatively unaffected, fluctuations are found to have a strong impact on brush interpenetration. Even still, the contraction of the brushes does significantly prolong the onset of interpenetration, implying that a sizeable normal force can be achieved before the sliding friction becomes significant.   

Optical Properties of III-V Semiconductors in Wurtzite Phase

A number of recent experiments have shown that the photoluminescence intensity in free standing nanowires is polarization dependent. One contribution to this effect is the optical anisotropy arising from the tendency of the nanowires to crystallize in wurtzite form. We calculate the frequency dependent dielectric functions for nine non-Nitride wurtzite phase III-V semiconductors (AlP, AlAs, AlSb, GaP, GaAs, GaSb, InP, InAs and InSb), based on our maximally constrained empirical pseudopotential bulk band structure calculations. Their complex dielectric functions are calculated in the dipole approximation for polarization perpendicular and parallel to the c-axis of the crystal. We also predict their reflectivity spectra and the static dielectric constants. Optical selection rules are used to explain key features of the dielectric functions. In general it is seen that the III-V wurtzite phase semiconductors exhibit strong optical anisotropy which suggests that they could be potentially useful as nonlinear optical crystals.   

Studies to Enhance Superconductivity in Thin Film Carbon

With research in the area of superconductivity growing, it is no surprise that new efforts are being made to induce superconductivity or increase transition temperatures (T$_{c})$ in carbon given its many allotropic forms. Promising results have been published for boron doping in diamond films, and phosphorus doping in highly oriented pyrolytic graphite (HOPG) films show hints of superconductivity.. Following these examples in the literature, we have begun studies to explore superconductivity in thin film carbon samples doped with different elements. Carbon thin films are prepared by pulsed laser deposition (PLD) on amorphous SiO$_{2}$/Si and single-crystal substrates. Doping is achieved by depositing from (C$_{1-x}$M$_{x})$ single-targets with M = B$_{4}$C and BN, and also by ion implantation into pure-carbon films. Previous research had indicated that Boron in HOPG did not elicit superconducting properties, but we aim to explore that also in thin film carbon and see if there needs to be a higher doping in the sample if trends were able to be seen in diamond films. Higher onset temperatures, T$_{c}$ , and current densities, J$_{c}$, are hoped to be achieved with doping of the thin film carbon with different elements.   

Anomalous resistivity peak in a ferromagnetic nanowire proximity-coupled to superconducting electrodes

Motivated by the recent experiment of Wang et al. [Nature Physics 6, 389 (2010)], we study temperature-dependent transport in such a mesoscopic structure consisting of a ferromagnetic nanowire proximity-coupled to two conventional superconducting electrodes. It is assumed that the asymmetry in the tunneling barrier gives rise to the Rashba spin-orbit-coupling in the barrier that enables induced p-wave superconductivity in the ferromagnet to exist. First, we develop a microscopic theory of Andreev scattering at the spin-orbit-coupled interface, derive a set of self-consistent boundary conditions, and find an expression for the p-wave minigap in terms of the microscopic parameters of the contact. Second, we study temperature-dependence of the resistance near the superconducting transition and find that it should generally feature a fluctuation-induced peak. The upturn in resistance is related to the suppression of the single-particle density of states due to the formation of fluctuating pairs, whose tunneling is suppressed. We find a good agreement between the data and our fluctuation theory. Then, we discuss this and related setups involving ferromagnetic nanowires in the context of one-dimensional topological superconductors.   

Soft phonon mode and superconducting properties of 2H-NbS2 compared to 2H-NbSe2

I will report on several recent results on 2H-NbS$_{2}$. This compound is the only superconducting 2H-dichalcogenide which does not develop a charge density wave (CDW). I will start with the temperature dependence of the phonon spectra of 2H-NbS$_{2}$ measured by Inelastic X-ray Scattering (IXS). Along $\Gamma$M, a huge softening of two phonon modes was observed on a wide part of the Brillouin zone. This is almost the same as the CDW precursor soft phonons modes that appear above $T_{CDW}\approx33\,K$ in 2H-NbSe${_2}$[Weber11]. It clearly indicates that 2H-NbS$_{2}$ is also at proximity of a CDW instability. In the second part I will show measurement of H$_{c1}$ and magnetic penetration depth, which show signs of a small energy scale in the superconducting gap of 2H-NbS$_{2}$, very similar to 2H-NbSe${_2}$ [Fletcher07], and also in good agreement with STS measurement[Guillamon08]. In view of these two facts and as an open question, we would like to discuss the hypothesis of a quantum critical point (QCP) lying between 2H-NbS$_{2}$ and 2H-NbSe${_2}$, where the 2$^{nd}$ order phase transition would be the CDW instability.   

Evidence for a hidden-order pseudogap in URu$_2$Si$_2$

Through an analysis and modeling of data from various experimental techniques, we present evidence for the presence of a hidden order pseudogap in URu$_2$Si$_2$ in the temperature range between 25 K and 17.5 K. Considering fluctuations of the hidden order energy gap at the transition as the origin of the pseudogap, we evaluate the effects that gap fluctuations would produce on observables like tunneling conductance, neutron scattering, and nuclear resonance, and relate them to the experimental findings. We show that the transition into hidden order phase is likely second order and is preceded by the onset of non-coherent hidden order fluctuations.   

In Situ Electronic Characterization of Graphene Nanoconstrictions Fabricated in a Transmission Electron Microscope

We report electronic measurements on high quality graphene nanoconstrictions (GNCs) fabricated in a transmission electron microscope (TEM), and the first measurements on GNC conductance with an accurate measurement of constriction width down to 1 nm. To create the GNCs, freely suspended graphene ribbons were fabricated using few-layer graphene grown by chemical vapor deposition. The ribbons were loaded into the TEM, and a current-annealing procedure was used to clean the material and improve its electronic characteristics. The TEM beam was then used to sculpt GNCs to a series of desired widths in the range 1-700 nm; after each sculpting step, the sample was imaged by TEM and its electronic properties were measured in situ. GNC conductance was found to be remarkably high, comparable to that of exfoliated graphene samples of similar size. The GNC conductance varied with width approximately as G(w) = (e$^{2}$/h)w$^{0.75}$, where w is the constriction width in nanometers. GNCs support current densities greater than 120 $\mu $A/nm$^{2}$, 2 orders of magnitude higher than that which has been previously reported for graphene nanoribbons and 2000 times higher than that reported for copper.   

One-dimensional Si nanolines in hydrogenated Si(001)

We present a detailed study of the structural and electronic properties of a self-assembled silicon nanoline embedded in the H-terminated silicon (001) surface, known as the Haiku stripe. The nanoline is a perfectly straight and defect free endotaxial structure of huge aspect ratio; it can grow micrometre long at a constant width of exactly four Si dimers (1.54 nm). Another remarkable property is its capacity to be exposed to air without suffering any degradation. The nanoline grows independently of any step edges at tunable densities, from isolated nanolines to a dense array of nanolines. In addition to these unique structural characteristics, scanning tunnelling microscopy and density functional theory reveal a one-dimensional state confined along the Haiku core. This nanoline is a promising candidate for the long sought after electronic solid-state one-dimensional model system to explore the fascinating quantum properties emerging in such reduced dimensionality. Phys. Rev. B, 84, 035328 (2011)   

Formation of complex emulsion in microfluidic channel

We first report a novel emulsification method using two binary mixtures that produces complex emulsion by phase separation, triggered by external diffusion of separation agent dissolved in continuous phase. A disperse phase consists of monomer and D-solvent (good solvent for disperse phase), while mixture of separation-triggering agent (SA) and C-solvent (good solvent for continuous phase) is used as a continuous phase. Individual droplet was formed by microfluidic system, allowing how the separation dynamically occurs as a function of time. This system consists of the three major steps involving the transformation of a single droplet into the complex emulsion. In the first, when two immiscible phases meet at cross-junction, droplets are generated and dispersed in continuous phase containing the SA. Since the SA is only selectively soluble in the monomer of disperse phase, external diffusion of SA into the droplets through the interface occurs which initiates phase separation. In transient state, the external diffusion of the SA generates both partial separated region and SA/monomer complex at the near interface of partial separated region. In the last step, where the phase separation fully occurs, single droplets transform into complex emulsion such as double emulsion or multiple emulsion with high order. In addition, we demonstrate a capability for generation of complex emulsions gives a great opportunity to fabricate functional materials using template synthesis.   

Heralded entanglement between two single atoms at remote locations

Entanglement between single trapped atoms at large distances is a key resource for future applications in quantum communication, like quantum networks and the quantum repeater. We have set up two independently operating atomic traps situated in two neighboring laboratories separated by 20 meter. On each side we capture a single Rb-87 atom in an optical dipole trap and generate a spin-entangled state between the atom and a single spontaneously emitted photon [1]. These photons are collected with high-NA objectives, coupled into single-mode optical fibers, and guided to the same fiber beam splitter (BS) where they interfere. A coincident detection of two orthogonally polarized photons leaving the BS allows us to project them onto two out of four maximally entangled Bell states. This Bell-state-projection on the photons swaps the entanglement onto the atoms. Here we report the faithful generation and analysis of entanglement between two single trapped atoms at remote locations. The observed entanglement fidelity is high enough, opening up the possibility for a first loophole-free test of Bell's inequality [2].\\[4pt] [1] J. Volz, et al., Phys. Rev. Lett. 96, 030404 (2006). \\[0pt] [2] W. Rosenfeld, et al., Adv. Sci. Lett. 2, 469 (2009).   

Quantum network with trusted and untrusted relays

Quantum key distribution offers two distant users to establish a random secure key by exploiting properties of quantum mechanics, whose security has proven in theory. In practice, many lab and field demonstrations have been performed in the last 20 years. Nowadays, quantum network with quantum key distribution systems are tested around the world, such as in China, Europe, Japan and US. In this talk, I will give a brief introduction of recent development for quantum network. For the untrusted relay part, I will introduce the measurement-device-independent quantum key distribution scheme and a quantum relay with linear optics. The security of such scheme is proven without assumptions on the detection devices, where most of quantum hacking strategies are launched. This scheme can be realized with current technology. For the trusted relay part, I will introduce so-called delayed privacy amplification, with which no error correction and privacy amplification is necessarily to be performed between users and the relay. In this way, classical communications and computational power requirement on the relay site will be reduced.   

Entanglement witness operator for quantum teleportation

The ability of entangled states to act as resource for teleportation is linked to a property of the fully entangled fraction. We show that the set of states with their fully entangled fraction bounded by a threshold value required for performing teleportation is both convex and compact. This feature enables for the existence of hermitian witness operators the measurement of which could distinguish unknown states useful for performing teleportation. We present an example of such a witness operator illustrating it for different classes of states. We further discuss the experimental measurability of the witness operator. (arXiv:1108.1493v2 [quant-ph]; to appear in Phys. Rev. Lett.)   

Entanglement Entropy in 1-D integrable chains

We study analytically the Renyi entropy of a bipartite lattice in the limit of two semi-infinite chains joined at the origin, for a few integrable 1-dimensional models, by using the techniques of Corner Transfer Matrices of the corresponding 2-D classical systems, namely the 8-vertex model and the RSOS. In the scaling limit, close to a conformal point, we reproduce the leading behavior expected from CFT prediction. The sub-leading corrections, however, differ from na\"ive expectations and we show that lattice effect can give rise to additional relevant terms in any numerical approach. Moreover, in the vicinity of a non-conformal (ferromagnetic) point, we observe a violation of universality and a behavior of the entropy characteristic of an {\it essential singularity}.   

Constant Tip-Surface Distance with Atomic Force Microscopy via Quality Factor Feedback

The atomic force microscope (AFM) is a powerful and widely used instrument to image topography and measure forces at the micrometer and nanometer length scale. Because of the high degree of operating accuracy required of the instrument, small thermal and mechanical drifts of the cantilever and piezoactuator systems hamper measurements as the AFM tip drifts spatially relative to the sample surface. To compensate for the drift, we control the tip-surface distance by monitoring the cantilever quality factor (Q) in a closed loop. Brownian thermal fluctuations provide sufficient actuation to accurately determine cantilever Q by fitting the thermal noise spectrum to a Lorentzian function. We show that the cantilever damping is sufficiently affected by the tip-surface distance so that the tip position of soft cantilevers can be maintained within 70 nm of a setpoint in air and within 3 nm in water with 95 percent reliability. Utilizing this method to hover the tip above a sample surface, we have the capability to study sensitive interactions at the nanometer length scale over long periods of time.   

Fractal contours of scalar in smooth flows

A passive scalar field was studied under the action of pumping, diffusion and advection by a smooth flow with a Lagrangian chaos. We present theoretical arguments showing that scalar statistics is not conformal invariant and formulate a new effective semi-analytic algorithm to model scalar turbulence. We then carry out massive numerics of scalar turbulence focusing on nodal lines. The distribution of contours over sizes and perimeters is shown to depend neither on the flow realization nor on the resolution (diffusion) scale for scales exceeding this scale. The scalar isolines are found fractal/smooth at the scales larger/smaller than the pumping scale. We characterize the statistics of bending of a long isoline by the driving function of the Loewner map, show that it behaves like diffusion with diffusivity independent of resolution yet, most surprisingly, dependent on the velocity realization and time (beyond the time on which the statistics of the scalar is stabilized).   

Laplace pressure induced droplet generation in micromold for synthesizing monodisperse microspheres.

Microspheres are widely used in applications such MEMS, chemical release systems, optical materials and various biological applications. Here, we report the new micromolding technique for synthesizing spherical monodisperse particles through surface-tension-induced flow. The spherical droplets were prepared using Laplace pressure difference, which is highly depending on geometries of the mold shape, without any pumping system to make flow. We calculated the minimum pressure difference to make the flow moves and form the droplets. It provides a synthetic tool for generating the microspheres using different reaction schemes; UV-polymerization, sol-gel reaction and colloidal assemblies. The monodisperse spherical particles, which are made of various materials, were successfully generated without any surfactant because each droplet can be separately positioned in mold patterns during solidification process.   

Tuning the Ferroelectric Properties through a Magnetic Field

Preparation and characterization of multiferroic materials in which ferroelectricity and ferromagnetism coexist would be a milestone for functionalized materials and devices. We have demonstrated that the electric polarization of ferroelectric polymer, poly vinylidene fluoride (PVDF), can be controlled by applying an external magnetic field. Samples were created in a layered heterostructure, with the key part of a PVDF layer sandwiched by two layers of Fe thin films. We found that as the applied magnetic field is changed, the switching of electric polarization for the PVDF displayed a dependence on the external magnetic field. We also noticed that both coercivity and polarization for the PVDF polymer display hysteretic features according to the change of an applied magnetic field. Our study showed that the thickness of both the iron layer and the PVDF layer have an effect on the magnetoelectric coupling in our samples. The same magnetostriction strain applied to a thicker PVDF layer becomes tougher to flip the polarization compared to a thinner PVDF layer. As the iron film thickness increases, the magnetoelectric strain also increases, and the PVDF polymer can be easily saturated and the polarization is more easily flipped. We have shown that it is possible to control the ferroelectric properties of a PVDF film by tuning the magnetic field in heterostructures. Our study shows that this system could have show promising applications for new information technology and devices.   

Anderson localization and correlation-induced delocalization in N-leg optical lattices

Recent experiments demonstrated localization of ultracold neutral atoms in a disordered optical potential. The great advantage of these atomic systems is the control of disorder form and strength, interactions, dimensionality, etc., in principle allowing for unambiguous observation of the phenomenon under a variety of circumstances. However, the localization observed so far is qualitatively indistinguishable from classical localization of particles in a disordered trapping potential. We propose a realization of the one-dimensional random dimer model and certain N-leg generalizations using cold atoms in an optical lattice. These models exhibit multiple delocalization energies that depend strongly on the symmetry properties of the corresponding Hamiltonian. We demonstrate that the N-leg systems possess similarities with their 1D ancestors but are demonstrably distinct. The existence of critical delocalization energies leads to dips in the momentum distribution which serve as a clear signal of the localization-delocalization transition.   

Multidimensional scaling of ideological landscape on social network sites

Social network sites (SNSs) are valuable source of information on various subjects in network science. Recently, political activity of SNSs users has increasing attention and is an interesting interdisciplinary subject of physical and social science. In this work, we measure ideological positions of the legislators of U.S. and South Korea (S.K.) evaluated by Twitter users, using the information employed in the bipartite network structure of the legislators and their Twitter followers. We compare the result with ideological positions constructed from roll call record of the legislators. This shows there is a discrepancy between the ideological positions evaluated by Twitter users and actual positions estimated from roll call votes in S.K. We also asses the ideological positions of the Twitter users themselves and analyze the distribution of the positions.   

Some Recent Developments in Structure and Glassy Behavior of Proteins

We have used ARVO developed by us to find that the ratio of volume and surface area of proteins in Protein Data Bank distributed in a very narrow region [1]. Such result is useful for the determination of protein 3D structures. It has been widely known that a spin glass model can be used to understand the slow relaxation behavior of a glass at low temperatures [2]. We have used molecular dynamics and simple models of polymer chains to study relaxation and aggregation of proteins under various conditions and found that polymer chains with neighboring monomers connected by rigid bonds can relax very slowly and show glassy behavior [3]. We have also found that native collagen fibrils show glassy behavior at room temperatures [4]. The results of [3] and [4] about the glassy behavior of polymers or proteins are useful for understanding the mechanism for a biological system to maintain in a non-equilibrium state, including the ancient seed [5], which can maintain in a non-equilibrium state for a very long time. (1) M.-C. Wu, M. S. Li, W.-J. Ma, M. Kouza, and C.-K. Hu, EPL, in press (2011); (2) C. Dasgupta, S.-K. Ma, and C.-K. Hu. Phys. Rev. B 20, 3837-3849 (1979); (3) W.-J. Ma and C.-K. Hu, J. Phys. Soc. Japan 79, 024005, 024006, 054001, and 104002 (2010), C.-K. Hu and W.-J. Ma, Prog. Theor. Phys. Supp. 184, 369 (2010); S. G. Gevorkian, A. E. Allahverdyan, D. S. Gevorgyan and C.-K. Hu, EPL 95, 23001 (2011); S. Sallon, et al. Science 320, 1464 (2008).   

Effective spectral dimension of hub in scale-free networks

Exploring the World Wide Web has become one of the key issues in information science, specifically in context of its application to the PageRank-like algorithms used in search engine. The random walk approach has been employed to study such a problem. The probability of the return to the origin (RTO) of random walks is inversely related to how information can be accessed during random surfing. We find analytically that the RTO probability for a given starting node shows a crossover from a slow to a fast decay behavior with time and the crossover time increases with the degree of the starting node. Furthermore, the RTO probability is almost constant in the early-time regime as the degree exponent approaches two. This result indicates that a random surfer can be effectively trapped at the hub and supports the necessity of the random jump strategy empirically used in the Google's search engine.   

Rings and boxes in dissipative environments

We study a particle on a ring in presence of a dissipative Caldeira-Leggett environment and derive its response to a DC field [1]. We find, through a 2-loop renormalization group analysis, that a large dissipation parameter $\eta$ flows to a fixed point $\eta^R=\hbar/2\pi$. We also reexamine the mapping of this problem to that of the Coulomb box and show that the relaxation resistance, of recent interest, has a certain average that is quantized for $\eta>\eta^R$, leading to quantized noise. We propose a box experiment to detect this noise. When the particle carries a spin with spin-orbit interactions [2] we find that the spin correlations in the direction perpendicular to the ring are finite at long times, i.e. do not dephase, while the parallel components may decay as a power law at strong dissipation. \\[4pt] [1] Y. Etzioni, B. Horovitz and P. Le Doussal, Phys. Rev. Lett. {\bf 106}, 166803 (2011). \\[0pt] [2] G. Zar\'{a}nd, G. T. Zim\'{a}nyi and F. Wilhelm, Phys. Rev. B{\bf 62}, 8137 (2000).   

JV Annealing Study of P3HT:PCBM OPV

Current-voltage (JV) analysis of poly (3-hexlythiophene) (P3HT) and phenyl-C61-butaric acid methyl ester (PCBM) organic photovoltaics (OPV) yield valuable insight into the internal physics of devices. A simple lumped circuit model, previously used to analyze various thin film photovoltaics and more recently applied to OPV has been used to study annealing parameters. An annealing study of P3HT:PCBM blend OPV was carried out using the lumped circuit model. Limiting carrier lifetime-mobility product information and barrier data were extracted from the JV analysis of these devices. Collection loss characteristics were also obtained. The data was used to better characterize annealing effects.   

Hydrogen adsorption around a hydrogen atom anchored in a graphene vacancy

In this work, we study the adsorption of hydrogen molecules around a hydrogen atom anchored in a graphene vacancy. We used density functional theory and molecular dynamics. To study the adsorption of hydrogen molecules on the system we used three hydrogen molecules around each hydrogen atom It was found that raising the temperature of the system up to 900 K the hydrogen molecules remained linked to the system of graphene doped with hydrogen. The results show that doping graphene with atomic hydrogen can be useful to store hydrogen molecules in the system.   

Beyond the Hubbard model: best effective single dressed band description of interacting atoms in optical lattices

We construct the effective lowest-band Bose-Hubbard model incorporating interaction-induced on-site correlations. The model is based on ladder operators for local correlated states, which deviate from the usual Wannier creation and annihilation operators, allowing for a systematic construction of the most appropriate single-band low-energy description in form of the extended Bose-Hubbard model. A formulation of this model in terms of ladder operators not only naturally contains the previously found effective multi-body interactions, but also contains multi-body induced single particle tunneling, pair tunneling and nearest-neighbor interaction processes of higher orders. An alternative description of the same model can be formulated in terms of occupation-dependent Bose-Hubbard parameters. These multi-particle effects can be enhanced using Feshbach resonances, leading to corrections which are well within experimental reach and of significance to the phase diagram of ultracold bosonic atoms in an optical lattice. We analyze the energy reduction mechanism of interacting atoms on a local lattice site and show that this cannot be explained only by a spatial broadening of Wannier orbitals on a single particle level, which neglects correlations.   

Doping-induced evolution of superconducting gap in iron-based superconductors: a point-contact Andreev reflection study of BaNi-122 single crystals

We report a systematic investigation on c-axis point-contact Andreev reflection (PCAR) in superconducting BaFe$_{2-x}$Ni$_x$As$_2$ single crystals from underdoped to overdoped regions (0.075 $<$ x $<$ 0.15). At optimal doping (x = 0.1) the PCAR spectrum feature a dip-hump structure at the edge of the conductance gap, which corresponds to electron-boson coupling mode in energy scale. Two-superconducting-gap structure is resolved in the PCAR spectroscopy. In the s$_{\pm}$ scenario, quantitative analysis using a generalized Blonder-Tinkham-Klapwijk (BTK) formalism with two gaps: one isotropic and another angle dependent, suggest a nodeless state in strong-coupling limit with gap minima on the Fermi surfaces. Upon crossing above the optimal doping (x $>$ 0.1), the PCAR spectrum show an in-gap sharp narrow peak at low bias, in contrast to the case of underdoped samples (x $<$ 0.1), signaling the onset of deepened gap minima or nodes in the superconducting gap. This result provides evidence of the modulation of the gap amplitude with doping concentration, consistent with the calculations for the orbital dependent pair interaction mediated by the antiferromagne   

Discontinuous percolation transition in hidden in continuous percolation transition

~Diffusion limited cluster aggregation model is the well known model which describes the aggregation of diffusive clusters. In this model, time~$t$~is defined so as to describe the situation in which clusters follow Brownian motion. Thus~$t$~defined in this model can be regarded as real time. In this presentation, we introduce a new variable~$p$which increases as much as~$\delta p=1/N$~whenever two distinct clusters aggregate. We use $p$ instead of~$t$~to study a percolation problem in diffusion limited cluster aggregation model. Then, we find that a discontinuous percolation transition occurs if we observe the growth of the largest cluster as a function of the transformed variable $p$. This result implies that a continuous percolation transition can be observed as a discontinuous percolation transition when a controlled parameter is changed.   

The magnetic flux periodicity in SNS square ring.

In this work, we study the magnetic flux periodicity of d-wave superconductor-normal-superconductor square ring. The Hamiltonian of the SNS square ring is solved by using self-consistent Bogoliubov-de Gennes equations. The total current as a function of flux exhibits the hc/e period. As we increase the length of the normal metal, the hc/e period becomes hc/2e period. Furthermore, we also investigate the behavior of the supercurrent in SNS and SIS junctions and the phase variation of the Josephson current.   

High Pressure Ultrasonic and Transport Studies of Cerium Skutterudite Thermoelectrics

In the effort to find more efficient thermoelectric materials, some skutterudite materials based on CoSb$_{3}$ have shown that these materials have the potential of being effective at elevated temperatures. This work will present high pressure ultrasonic and transport measurements on the cerium filled versions of this parent compount, CeFe$_{4}$Sb${12}$ and CeFe$_{3}$CoSb${12}$. A previous study on these materials showed an anomaly in the equation of state for these materials originally attributed to the vitrification of a Methanol:Ethanol pressure medium. However, the studies performed as a part of this work demonstrate that, while this may be partly to blame, there is some contribution due to a real effect in this material.   

Topological Superfluid in P-band Optical Lattice

By studying p-band fermionic system with nearest neighbor attractive interaction we find translation symmetry protected $Z_{2}$ topological superfluid (TSF) that is characterized by a special fermion parity pattern at high symmetry points in momentum space $k=$ $(0,0)$, ($0$, $\pi $), ($\pi $, $0$), ($\pi $, $\pi $). Such $Z_{2}$ TSF supports the robust Majorana edge modes and a new type of low energy excitation - (supersymmetric) $Z_{2}$ link-excitation.