Session V1: Poster Session III (2:00 pm - 5:00 pm)

Thought effect can not be explained by the photoelectric conversion of the solar cells

In our experiments, thought waves have been shown to be capable of changing (affecting) the output voltage of photovoltaic cells located from as far away as 1-3 meters. There are no wires between brain and photoelectric cell and so it is presumed only the thought waves act on the photoelectric cell. There were some experimental phenomena can not be explained by the photoelectric conversion of the solar cells. So the paper supposes that the thought effect is unknown thought-electric conversion. $http://meetings.aps.org/link/BAPS.2012.MAR.P33.14$ The sun and dark hole are a balance system-SDS which triggered periodic mass extinctions and created new life on our earth. The quantum orbits both of planets and dark comets of dark hole decided the period. $http://meetings.aps.org/link/BAPS.2008.DNP.LG.6$ ``Since 1950, In the oceans, 90\% of all large fish have disappeared'' (Jnho Baez, UCR, $http://math.ucr.edu/home/baez/extinction/$) Consciousness changed output voltages and a balance between Electrons and electron holes; by the nuclear energy of spacetime, it should change impaction orbit of the balance of SDS for avoiding next impaction; it would change balance system of gene-TaiJi model to evolve life during the extinction. $http://meetings.aps.org/link/BAPS.2013.MAR.H1.267$   

Single crystal growth and magnetic excitations of transistion metal oxide CoV2O6

Low-dimensional magnetic materials are an area of interest due to their unusual properties such as metamagnetism and magnetization plateaus [1]. Solid state synthesis has produced polycrystalline CoV$_2$O$_6$ which exists in two polymorphs: one with a monoclinic structure, and the other with a triclinic structure [2]. Single crystals have been grown from polycrystalline CoV$_2$O$_6$ using the flux method under vacuum and are large enough to aid in single crystal neutron diffraction [3]. Magnetic excitations have been measured using powder neutron diffraction in the low temperatures regime with variable energy. The magnetic excitations have been compared between the two phases. The energy of the system has been modelled in terms of the spin-orbit coupling, structural distortions, and the crystal field and compared to neutron data.\\[4pt] [1] M. Markkula, A. M. Arevalo-Lopez, and J. P. Attfield, J. Solid State Chem. \textbf{192}, 390 (2012).\\[0pt] [2] M. Lenertz, J. Alaria, D. Stoeffler, S. Colis, and A. Dinia, J. Phys. Chem. C \textbf{115}, 17190 (2011).\\[0pt] [3] Z. He, J.-I. Yamaura, Y. Ueda, and W. Cheng, JACS \textbf{131}, 7554 (2009).   

Atomic Processes and Diagnostics of Low Pressure Krypton Plasma

Optical emission spectroscopy along with suitable collisional-radiative (CR) model is used in plasma diagnostics. Importance of reliable cross-sections for various atomic processes is shown for low pressure argon plasma [1-2]. In the present work, radially-averaged Kr emission lines from the 2$p_{i} \to $1$s_{j}$ were recorded as a function of pressure from 1 to 50mTorr. We have developed a CR model using our fine-structure relativistic-distorted wave cross sections [3]. The various processes considered are electron-impact excitation, ionization and their reverse processes. The required rate coefficients have been calculated from these cross-sections assuming Maxwellian energy distribution. Electron temperature obtained from the CR model is found to be in good agreement with the probe measurements.\\[4pt] [1] R. K. Gangwar, L. Sharma, R. Srivastava and A. D. Stauffer, \textit{J. of Appl. Phys. }111, 053307 (2012)\\[0pt] [2] Dipti, R. K. Gangwar, R. Srivastava and A. D. Stauffer, Eur. Phys. J. D, 2013, \textbf{67}, 40244.\\[0pt] [3] R. K. Gangwar, L. Sharma, R. Srivastava, and A. D. Stauffer, Phys. Rev. A, 2010, \textbf{82}, 032710.   

Effect of nonlinear nonlinear coupling to a pure dephasing model

We investigate the influence of the nonlinear coupling to the coherence of a pure dephasing model. The total system consists of a qubit and a Bosonic bath, which are coupled by an interaction $H_I=g_1\sigma_z\otimes x+g_2\sigma_z\otimes x^2$ with $x=\frac{1}{\sqrt{2}}(a+a^\dagger)$. It's shown that no matter how small $g_2$ is, the long time behavior of the coherence is significantly changed by the nonlinear coupling for free induction decay (FID), while the effect of $g_1$ can be neglected as long as $g_1$ is much smaller than the enegy splitting of the qubit. In the case that many-pulse dynamical decoupling control is exerted on the qubit, $g_2$ also modulates the oscillation of the coherence. Our results indicate that the nonlinear coupling must be taken into account for long time dynamics.   

Hartree-Fock Theory of a Harmonically Trapped Dirty Bose-Einstein Condensate via the Replica Method

A recent non-perturbative approach towards the dirty boson problem relies on the Hartree-Fock theory which is worked out on the basis of the replica method [1]. Here we extend this approach for a weakly interacting Bose-gas at finite temperature in a quenched delta-correlated disorder potential from the homogeneous case to an anisotropic harmonic confinement within the Thomas-Fermi approximation. In this way we obtain and solve coupled self-consistency equations, which relies on a decomposition of the particle density into the condensate density, the thermal density as well as the density of fragmented local Bose-Einstein condensates within the respective minima of the random potential landscape. Although we reproduce for weak disorder and at zero temperature the seminal results of Huang and Meng from a Bogoliubov theory [2,3] only qualitatively, we yield for strong enough disorder a quantum phase transition to a Bose-glass phase [4]. [1] R. Graham and A. Pelster, I. J. Bif. Chaos 19, 2745 (2009) [2] K. Huang, H.-F. Meng, Phys. Rev. Lett. 69, 644 (1992) [3] G.M. Falco, A. Pelster, and R. Graham, Phys. Rev. A 75, 063619 (2007) [4] P. Navez, A. Pelster, and R. Graham, App. Phys. B 86, 395 (2007)   

Formation and dynamics of anti-ferromagnetic correlations of ultracold fermions in tunable lattices

The observation of anti-ferromagnetic spin correlations of ultracold fermions in optical lattices is an important milestone towards an experimental study of the Hubbard model. In this model many questions on the low-temperature phase diagram still remain open, both for simple cubic and square configurations, as well as for more complex lattice geometries. Additionally, for creating an equilibrated low-temperature state and a successful implementation of advanced cooling schemes based on entropy redistribution, an understanding of the formation time for spin correlations is of paramount importance. In our experiment we load a two-component, repulsively interacting fermionic quantum gas into an optical lattice of tunable geometry. For very low temperatures we observe anti-ferromagnetic correlations on neighbouring sites in both isotropic 3D cubic and 2D square lattices. We also study the strength of the spin correlations in more complex lattice geometries, such as honeycomb, 1D-dimerized and spin-ladder lattice configurations. Furthermore, we investigate the characteristic formation time of spin correlations in optical lattices by changing the lattice geometry on variable timescales.   

Direct probe of topological order for cold atoms

Cold-atom experiments in optical lattices offer a versatile platform to realize various topological quantum phases. A key challenge in those experiments is to unambiguously probe the topological order. We propose a method to directly measure the characteristic topological invariants (order) based on the time-of-flight imaging of cold atoms. The method is generally applicable to detection of topological band insulators in one, two, or three dimensions characterized by integer topological invariants. Using detection of the Chern number for the two-dimensional anomalous quantum Hall states and the Chern-Simons term for the three-dimensional chiral topological insulators as examples, we show that the proposed detection method is practical, and robust to typical experimental imperfections such as limited imaging resolution, inhomogeneous trapping potential, and disorder in the system.   

Collisional Microscopy and Generation of Entanglement with Ultracold Quantum Gases

We describe an apparatus for cold collisional microscopy of quantum gases, in which pairwise entanglement is produced between a many-body gas and an optical-lattice-bound array of secondary atoms used as quantum-non-destructive probes. We discuss detailed implementation of collisional entanglement schemes based on Ramsey-style interferometer sequences, as well as tunnel-assist and -inhibit schemes, in which probe atoms are conditionally shuffled according to the presence or state of a sample atom. The technical implementation for precise manipulation of multichromatic optical lattices is discussed, including achromatic holographic projection microscopy at high numerical aperture and methods for achieving sufficient quantum state control within a lattice site.~Applications of collisional microscopy will also be discussed, ranging from imaging of dynamical and non-equilibrium quantum many-body systems, to characterization of strongly entangled gases through extraction of entanglement entropy, to algorithmic cooling of quantum critical gases.   

Cutoff frequency of sound velocities for a multi-slab Bose-Einstein condensate

An inhomogeneous multi-slab 3D Bose gas is produced by applying to the gas a Kronig-Penney potential in one direction, while the bosons are free to move in the other two directions. The variable density produces a dispersive effect over the sound waves, making the phase and group sound velocities frequency dependent. Below the critical temperature the dispersion relation between wavenumber and frequency $\omega(k)$ is determined by a constant factor called \emph{the curvature} of the density, within the Klein-Gordon equation which describes the sound wave propagation in the condensate. Since the curvature of the density profiles between and inside the barriers are completely different, the sound velocities are distinct too. More importantly, in the region occupied by the slabs waves propagate only if their frequencies are greater than a \emph{cutoff frequency}, otherwise evanescent waves arise. We show the density profile, the phase and group sound velocities and we give the curvature dependent cutoff frequency as obtained from the group velocity equation for the region occupied by the barriers. For high frequencies both phase and group velocities approach to that of a homogeneous gas at the same temperature.   

Bose gas in disordered, finite-layered systems

Disorder effects in the thermodynamic properties of a Bose gas are analyzed. The gas is confined within a layered box of size $L$ in the z-direction and infinite in the other two directions. The layers are first modeled by a periodic array of $M$ Dirac delta-functions of equal intensity. We investigate the effects on the specific heat, energy and entropy when a random set of vacancies is introduced in the layered array. A dramatic increase in the maximum of the specific heat is observed when the system has a $0.1$ to $0.2$ fraction of random vacancies compared to the original, periodic array and this maximum, which is reminiscent of a Bose-Einstein condensation for an infinite array, occurs at a higher temperature.   

Specific heat for boson matter in multifilament cables

We report the isocoric specific heat for an ideal Bose gas confined in a periodic array of filaments with finite rectangular cross section and infinite length, set together to form a cable. The filament structure is created by applying to the gas two, mutually perpendicular, Kronig-Penney delta-potentials, while the bosons are free to move along the third direction. The energy spectrum accessible to the particles is obtained and introduced into the grand potential to calculate the specific heat of the system as a function of temperature for different values of the periodic structure parameters such as: the number, the area of the cross section and wall permeability, of the filaments. The specific heat shows a dimensional crossover from 3D to 1D, in temperature regions where the de Broglie wavelength of the bosons is comparable to twice the separation between the walls of the filaments. Furthermore, we provide and discuss a criterion for identifying the critical temperature based on the behavior of the chemical potential and its first derivatives, as well as the population of the ground state.   

Intrinsic Versus Extrinsic Which-Way Information and the Use of Extrinsic WW Information in a Delayed Choice Experiment to Send Information Immediately Between 2 Paired Particles

Intrinsic states characterize a quantity inherent in the particle itself, such as spin. Extrinsic states characterize a quantity that is not in the particle itself but that describes the particle. An example is the specific path a particle is taking through an interferometer. Extrinsic states such as the specific path of a particle can be eliminated before a measurement is made. An experiment is described to show the impact that this ability to eliminate extrinsic states can make. This experiment relies on a delayed choice for an idler photon that immediately affects the signal photon with which it is entangled. The delayed choice concerns whether to maintain or eliminate the entanglement before any measurements are made. The idler photon can essentially be lost before any measurements are made because the states of the idler photon that enters the optical microcavity related to its entanglement are eliminated when the idler photon enters the optical microcavity. One of the reasons is that the states of the idler photon are extrinsic to the particle itself (they characterize the particular path of the particle, a characteristic not inherent to the particle itself) where the information underlying the extrinsic state is eliminated when the particle enters the cavity situated at the confluence of the two possible particle paths. Over a number of runs with this choice, the resulting distribution of the paired signal photons shows interference. If the entanglement is instead maintained, the resulting distribution of the paired signal photons shows ww information.   

The Immediate Affect of Information in a Delayed Choice on a Distant Distribution as Seen in Different Inertial Reference Frames: The ``Effect'' May Occur Before the ``Cause''

An experiment is described in the laboratory reference frame that relies on delayed choices for idler photons that immediately affects the distribution of signal photons with which the idler photons are initially entangled. The delayed choices on the idler photons concern whether to maintain or instead eliminate the entanglement between the paired idler and signal photons before any measurements are made. Eliminating the entanglement is done through eliminating the which-way information carried by the idler photon. If the entanglement is maintained, the result is which-way information in the distribution of the signal photons. If the entanglement is instead eliminated, the result is the elimination of which-way information and the presence of interference in the distribution of the signal photons. In other inertial reference frames, the change in state in the signal photon may occur before the delayed choice on the paired idler photon is made. A Minkowski diagram depicts the situation for the laboratory reference frame and another inertial reference frame where the change in state in the signal photon occurs before the delayed choice on the paired idler photon.   

FDTD study of the formation of optical vortices associated with core-shell nanoparticle cluster

Light absorbing plasmonic metal-dielectric nanoparticles suspended in water, or nanofluids have recently been experimentally demonstrated to produce steam at high efficiencies upon solar illumination. This approach localizes high temperatures to the interior of the liquid through efficient trapping of incoming light via scattering and absorption mechanisms. In suspensions, nanoparticles may form clusters due to surface wetting properties, and little work has focused on understanding the optical properties of clusters. In this work, we use the FDTD method to accurately visualize the optical power flow through various plasmonic metal-silica core-shell nanoparticle pairs at different inter-particle separations (10-100 nm). At these separations phase singularities of the power flow can occur, such as vortices of light inside the dielectric core which can enhance the absorption cross-section of the cluster. We study the conditions required to form these vortices. We also consider titanium nitride as shell, other than the widely studied noble metals to visualize the extinction cross-section of a cluster which depends on the separation, and the permittivity of the dielectric core.   

Nanometer Scale Microscopy via Graphene Plasmons

Using graphene plasmons (GPs), we can realize a nanometer scale microscopy. Our scheme takes advantage of the extremely large wave number of GPs and the low loss of graphene. Comparing with conventional nonlinear structured illumination microscopy basing on high order nonlinearity associated with high intensity light, our proposal only requires linear response. Consequently we need very weak field, which means less damage to the sample and may play a significantly important role in imaging of the biological systems.   

Efficient Non-Resonant Absorption in Thin Cylindrical Targets: Experimental Evidence

A theoretical possibility of non-resonant, fast, and efficient (up to 40 percent) heating of very thin conducting cylindrical targets by broad electromagnetic beams was predicted in [Akhmeteli, arXiv:physics/0405091 and 0611169] based on rigorous solution of the diffraction problem. The diameter of the cylinder can be orders of magnitude smaller than the wavelength (for the transverse geometry) or the beam waist (for the longitudinal geometry) of the electromagnetic radiation. This can be used for numerous applications, such as pumping of active media of short-wavelength lasers, e.g., through efficient heating of nanotubes with laser radiation. Experimental confirmation of the above results is presented [Akhmeteli, Kokodiy, Safronov, Balkashin, Priz, Tarasevitch, arXiv:1109.1626 and 1208.0066]. Significant (up to 6\%) absorption of microwave power focused on a thin fiber (the diameter is three orders of magnitude less than the wavelength) by an ellipsoidal reflector is demonstrated experimentally. For the longitudinal geometry, experiments provide a confirmation of significant absorption (up to 35\%) of the power of a wide CO2 laser beam propagating along a thin wire (the diameter of the wire can be orders of magnitude less than the beam waist width).   

Scattering of near-zero-energy positronium by H$_2$

The scattering length and pick-off annihilation parameter for the $S$-wave scattering of zero-energy positroniums (Ps) by H$_2$ are calculated by the stabilization method using explicitly correlated Gaussians. The confined variational method is used to optimize the Gaussians in order to describe the short-range interaction of the incident Ps with H$_2$ in the fixed nucleus approximation. By applying a confining potential to the center-of-mass of Ps, the problem of continuum states can be converted to a problem of discrete energy levels. For scattering at very low energies, the convergence of the scattering parameters can be improved by including exterior basis functions to describe the asymptotic region, which are given by products of Guassians with H$_2$ wave function and Ps wave function. In addition, the effect of van der Waals interaction between the Ps and H$_2$ on scattering parameters will be taken into account.   

Experimental and theoretical studies of excited-state angular-momentum alignment and orientation signals in atomic rubidium in the presence of an external magnetic field

We present level-crossing signals for the hyperfine transitions of the $D_2$ line of rubidium and show that these signals can be described very precisely by a theoretical model that is based on optical Bloch equations. The crossings occur when the levels are shifted by the nonlinear Zeeman effect in an external magnetic field $B$, whose direction defines the quantization axis $z$. A coherent state is said to be aligned if the population of atoms varies as a function of $|m_F|$, the projection of the total atomic angular momentum $F$ on the quantization axis $z$, but is equal for $+m_F$ and $-m_F$. When the energies of two magnetic sublevels for which $\Delta{m_F}=2$ cross, an aligned state can be created by excitation with coherent radiation. An oriented state can be created for crossings with $\Delta{m_F}=1$. Because the theoretical model has been extended to include the hyperfine structure of the atomic levels, strong magnetic sublevel mixing in an external magnetic field, and the Doppler effect, precise agreement between theory and experiment is possible even at excitation power densities where optical pumping plays a role. We present experimental results and theoretical calculations, showing their dependence on laser power density and frequency.   

Stimulated and coherent Raman spectroscopy with 0? pulses

We developed a new variant of stimulated and coherent Raman spectroscopy with shaped short pulses, applicable to multi-scattering media. The technique is based on the spectral modulation of the laser pulse due to the Raman scattering. Using discrete dipole approximeation we modeled the scattering from nanoparticles and calculated response from molecules in vicinity of nanoparticles to demonstrate the effects of 0?-short laser pulses. The obtained results may have a broad range of applications from spectroscopy and pathogen detection to microscopy.   

Second quantization of squeezed light through non-linear medium

We investigate the interaction of Rb atoms and squeezed light which is treated quantum mechanically. We establish the model of Gaussian beam propagating through the non-linear medium. The spatial modes of the output beam include high order Laguerre-Gaussian (LG) modes. We find the differential equation describing the second-quantized input-output relations. Since the spatial LG modes of the output beam are entangled with the squeezed states, we apply various schemes of spatial modes selection in order to produce the squeezed states which can lead to various applications. We also explain the result of the experiment where a Gaussian pump field is put through a Rb cell and a sub-shot noise is found.   

Possibility of vibrationally resolved cross section measurements for low energy charge transfer in H $+$ H$_{2}^{+}$

Charge transfer (CT) in H $+$ H$_{2}^{+}\to $ H$^{+} \quad +$ H$_{2}$~has fundamental implications because it involves the smallest atomic ion, atom, molecular ion, and molecule possible. The current merged-beam apparatus at Oak Ridge National Laboratory (ORNL) in Oak Ridge, Tennessee, can reliably create and access low collision energies; the existing ion-atom merged beams apparatus there is the only apparatus currently able to benchmark the CT of these fundamental systems at energies below 0.1eV/u (Phys. Rev. A \textbf{84}, 062716, 2011). However, the data analysis suffers from the lack of information on the initial states of H$_{2}^{+}$~which makes comparison to state-to-state calculations (PRA \textbf{67} 022708 (2003) impossible without educated guesses. We are exploring the possibility of inserting a three-dimensional imaging technique at the end station~of the ORNL apparatus in order to measure the vibrational state distribution of H$_{2}^{+}$ that are produced by the electron cyclotron resonance ion source. Our initial design for the insertion of this technique in the aforementioned system will be presented here.   

The Utilization of Chloroform Post-Treatment to Improve the Adhesion of Au Thin Films onto PMMA

The metallization of Au onto plastics is an important processing step in the fabrication of microfluidic devices. While its corrosion resistance and excellent electrical and thermal conductivity make Au a good choice, its inertness results in poor adhesion to polymer surfaces. Previous studies have indicated that exposing commercially available Poly(methyl methacrylate) (PMMA) sheets to chloroform vapor following Au deposition significantly improves adhesion. In this study, we deposited 6 nm of Au onto 1.50 mm thick PMMA and exposed the samples to vapor released from chloroform heated on a hot plate set at 70 $^{\circ}$C. The force required to remove the Au thin films was determined by placing samples on a polisher spinning at 150 rpm and utilizing UV-VIS spectroscopy to measure the transmittance of 700 nm light through the films to quantify their removal as a function of applied polishing force. The Au thin films were also characterized using AFM. AFM images demonstrated a progressive roughening of the surface corresponding to an increase in applied force. Additionally, these images support a model in which the chloroform treatment softens the PMMA surface, producing a softened layer that the polisher removes simultaneously with the Au thin film.   

Impurity Green's function of the one-dimensional Fermi gas

We investigate the model of an impurity interacting with free Fermi gas in one spatial dimension through a delta function potential both at zero and finite temperature. Using Bethe Ansatz technique we represent time dependent correlation function and the average momentum of an impurity as a Fredholm determinant. Our results are applicable both for finite repulsive and attractive interactions as well as in a Tonks-Girardeau limit.   

Spatial Modes of a Squeezed Vacuum Field

We prepared a quantum noise suppressed squeezed vacuum field by propagating a beam with a wavelength of 795nm through a hot Rb cell. Observation of the quadrature noise showed that we achieved a noise suppression of -2.0 dB below the quantum noise limit. When a spatial mask was applied to the beam after its interaction with atoms, we observed that the detected quantum noise suppression strongly depended on the shape of the mask. An exploration of the spatial distribution of noise in the squeezed field illustrated that the squeezed field was in a different spatial mode from the pump field used as a local oscillator. Our research showed that the squeezed field consisted of several spatial modes with various squeezing parameters. If a pure squeezed mode could be extracted, it would enhance the signal to noise ratio, which would impact precision metrology and quantum memory applications.   

Experimental apparatus for quantum simulation with two-dimensional 9Be$+$ Coulomb crystals

We report on the development of a new experimental setup designed for Quantum Simulation studies at a computationally relevant scale using laser-cooled 9Be$+$ ion-crystals in a Penning trap. The trap geometry is optimized using numerical calculations for trapping large ion crystals with enhanced optical access and reduced anharmonic perturbations. Separate loading and spectroscopy zones prevent long term drifts of the trapping parameters due to contamination of the trap electrodes with Be deposits. Our customized superconducting magnet provides a homogenous (dB/B \textless 10-6) magnetic field at 3T required for ion trapping. Laser frequencies required for cooling/detection and spin-motion entanglement are generated from telecom wavelength fiber laser systems in the IR via nonlinear conversion. Our new approach employs high-efficiency telecom modulators and mode-selecting cavities to generate multiple beamlines from a single Sum-frequency-Generation step. Ultimately, this newly developed setup will allow for studies of many-body spin systems with tuneable interaction strength from infinite-range to nearest-neighbour type interaction.   

Complete and Partial Transfer of Energy in Bremsstrahlung Should Include Rotational and Vibrational Kinetic Energies

In complete braking achievement the rotational and vibrational as well as the linear kinetic energies of the charged particle results in a photon: $h\nu = 1/2mv^2 + 1/2I\omega^2 + 1/2kx^2.$ In partial transfer of kinetic energies of the deccelerating particle the resulting photon is $h\nu = [(1/2mv^2)_1 + (1/2I\omega^2)_1 + (1/2kx^2)_1] - [(1/2mv^2)_2 + (1/2I\omega^2)_2 + (1/2kx^2)_2]$. The linear kinetic energy of the charged particle is$1/2mv^2$, the rotational kinetic energy is $1/2I\omega^2$ and the vibrational kinetic energy is given by $1/2kx^2$.   

Evolution of quantum wave packets in the presence of time-dependent absorption

The dynamics of a quantum particle submitted to a barrier is strongly influenced by the wave nature of matter, and may for instance lead to the classically forbidden phenomenon of tunnelling. While the case of a real potential barrier is a standard problem in quantum mechanics, different approaches are possible to model an {\it absorbing} barrier. Here we present a quantitative comparison between two such approaches. The first one describes an absorbing time-dependent point-like barrier by means of certain time-dependent boundary conditions of Kottler's type, while the second approach treats the wave function of the moving particle as a component of a spinor evolving under the action of a matrix Hamilton operator.   

Magneto-optical resonances and relaxation mechanisms in an extremely thin cell: experiment and theory for the cesium D$_{1}$ line

Magneto-optical resonances are a sensitive effect that allows to make stringent tests for theoretical models, which in turn, can help to improve devices that measure magnetic field. The experiments were carried out with an extremely thin cell (ETC) that provides high spatial resolution and allows sub-Doppler spectroscopy. At the same time the theoretical description of the signal requires delicate treatment of effects peculiar to thin cells. The cell, manufactured in Armenia, consists of two YAG glass windows separated by a distance of less than one micrometer. The experimental measurements of magneto-optical resonances were done using LIF signals of a cesium atomic vapor layer with a thickness varying from about 350 nm to of about 900 nm. In this study we obtained an accurate theoretical description of magneto-optical resonances using a theoretical model based on the optical Bloch equations that is an expanded version of earlier models and now includes a more detailed treatment of relaxation processes and the saturation of the atom-laser interaction in the high-intensity areas of the beam.   

Efficiency of multi-wave mixing in a sphere

We consider nonlinear muti-wave mixing in a sphere. We compare the efficiency of wave mixing in a sphere with the efficiency in a bulk or in the slab where, as well-known, the phase-matching plays an important role. We have found the optimal conditions for nonlinear generation in a sphere. The obtained results can be applied to coherent Raman microscopy and allow us to maximize the signal for arbitrary shpae of nanoparticles.   

Modelling Spatial Modes of Squeezed Vacuum -- When it Comes to Squeezing, Plane Waves Are Just Too Plain

Recent research relying on the \textit{polarization self rotation} (PSR) effect in Rb$^{\mathrm{87\thinspace }}$has revealed a squeezed vacuum field consisting of several spatial modes with various squeezing parameters [Mi Zhang, Spatial Modes of a Squeezed Vacuum Field, 2015 APS March Meeting]. In order to explain these results, we re-derive the beam propagation model describing the creation of squeezed vacuum via PSR and incorporate more realistic multimode input-output relations in the paraxial approximation. We solve the propagation equation and use it to predict the spatial distribution of squeezed vacuum via the proper Laguerre-Gauss modal structure. This modal structure is instrumental in the development of a complete second quantized beam propagation formalism also being reported at this meeting [Zhihao Xiao, Second quantization of squeezed light through non-linear medium, 2015 APS March Meeting].   

Photoionization of Fe$^{7+}$ from the ground and metastable states

The $B$-spline Breit-Pauli $R$-matrix method is used to investigate the photoionization of Fe$^{7+}$ from the ground and metastable states in the energy region from ionization thresholds to 172 eV. The present calculations were designed to resolve the large discrepancies between the recent measurements and available theoretical results. The multiconfiguration Hartree-Fock method in connection with $B$-spline expansions is employed for an accurate representation of the initial and final states wavefunctions. The close-coupling expansion includes 99 fine-structure levels of Fe$^{8+}$ in energy region up to $3s^23p^54s$ states. It includes levels of the $3s^23p^6$, $3s^23p^53d$, $3s^23p^54s$, and $3s3p^63d$ configurations and some levels of the $3s^23p^43d^2$ configuration which lie in the energy region under investigation. The present photoionization cross sections agree well with the Breit-Pauli $R$-matrix calculation of Sossah et al. and the TOPbase data in the magnitude of the background cross sections, but show somewhat richer resonance structure which qualitatively agree with the measurements. The calculated cross sections, however, are several times lower than the measured cross sections depending upon photon energy. The cross sections for photoionization of metastable states   

Two-color x-ray pump x-ray probe study of the core-hole decay dynamics in XeF2

To resolve the femtosecond inner-shell dynamics and the subsequent induced electron transfer in a molecule, the core-hole decay dynamics in XeF2 have been directly studied using femtosecond time-resolved x-ray pump x-ray probe coincidence imaging. The study of XeF2 molecule allows us to compare the molecular core-hole decay with the atomic case, Xe atom. To study these processes, the recently developed capability at LCLS was used to generate two-color x-ray pulses with variable delay. A time and position sensitive detector has been used to record the ion fragments in coincidence. The correlated ion kinetic energies make it possible to select and assign different excitation pathways, being able to track the atomic and the molecular core-hole decay dynamics.   

Electron Impact Inner-shell Ionization of Ions

Electron impact ionization cross-sections (EIICS) for H to Sc isoelectronic series over incident energies ranging from threshold to 10 keV have been reported. Recently we have proposed various [1-3] easy-to-use models for total EIICS of ions. The selection of the range of atomic number $Z $for different isoelectronic series was guided by the availability of the experimental and the quantum calculated EIICS. These models agree well with the experimental results.\\[4pt] [1] A. K. F. Haque, M. A. Uddin, M. Shahjahan, M. R Talukder, A. K. Basak and B. C. Saha,`` Electron impact inner-shell ionization of atoms,'' in \textit{Advances in Quantum Chemistry, }\textbf{61, }309-373 (2011).\\[0pt] [2] A.K.F. Haque, M.Ismail Hossain, T.I.Talukder, Mahmudul Hasan, M.Alfaz Uddin, A.K.Basak, B.C.Saha, F.B.Malik. `` Electron impact ionization of $K$-shell and $H$- to \textit{Be}- isoelectronic series: an empirical model,'' Radiation Physics and Chemistry, 91, 50-59 (2013).\\[0pt] [3] A. K. F. Haque, M. Shahjahan, M. A. Uddin, M. A. R. Patoary, A. K. Basak, B. C. Saha, and F. B. Malik ``Generalized Kolbenstvedt model for electron impact ionization of the K-, L- and M-shell ions,'' Physica Scripta, \textbf{81, }045301 (2010).   

Absorption spectra of monolayer MoS2 in high magnetic field

We have measured the absorption spectra of monolayer MoS2 film at several temperatures in pulsed high magnetic fields up to 52 T. At room temperature, the observed spectrum dominated by two main peaks, which are located at 660 nm and 606 nm. These peaks are ascribed to excition and trion absorption peaks respectively [1]. At low temperature (4.2 K), two peaks show the blue shift to 633 nm and 588 nm, respectively. Irrespective of the temperature, applying magnetic field does not show pronounced influence on the peaks even in 52 T.   

Measuring the conductivity dependence of the Casimir force

The strength and distance dependence of the Casimir force can be controlled through the conductivity of the material bodies, with lower conductivity in general leading to lower Casimir forces. However low conductivity, large bandgap materials which are insulating, have drawbacks as any surface electrostatic charges cannot be easily compensated. This restricts experiments to metallic or highly doped semiconductor materials. We will report on measurements of the Casimir force gradient using the frequency shift technique. Improvements in the measurement technique will be discussed. Measurements of the Casimir force gradient using low and high conductivity silicon surfaces will be reported.   

Nonlinear dynamical phase diagram of ultra-cold atoms in periodic lattices

We study the dynamics of ultra-cold bosonic gases in periodic deep optical potential by incorporating the nonlinear inter-site coupling into the discrete nonlinear Schr\"{O}dinger equation (DNLSE). We numerically solve the DNLSE to analyze and compare the evolution of Gaussian wave packets reported in [Phys. Rev. Lett., 86, 11(2001)]. Our result corroborates the validity of the phase diagram calculated by the variational method within a certain range of parameters of the initial wave packets. We calculate the long-term evolution of solutions of DNLSE to generate a fine phase diagram to elaborate the transitions between different dynamic modes. We propose an experimental scheme to verify the phase diagram based on the Bose-Einstein condensates in optical lattices.   

Superfluidity and Chaos

The hallmark of superfluidity is the appearance of a quantized metastable circulating current. The Landau criterion links the metastability of a vortex state to its spectral stability, i.e. to the inaccessibility of elementary excitations connecting it to other states with the same energy. In low dimensional systems, superfluid vortex states can exist due to their dynamical stability even if they are spectrally unstable. This traditional paradigm associate superfluid vortex states with stationary stable fixed points in phase space. Hence, Bogoliubov de Gennes (BdG) stability analysis is normally used to determine the feasibility of such states. In this work we challenge this traditional criterion and highlight the role of chaos in the analysis, thus explaining the existence of current carrying eigenstates which are neither spectrally-stable nor dynamically- stable.   

Interaction and Disorder Effects across BCS-BEC Crossover in Two-Dimensional Fermi Gases

We investigate the effect of static impurities in two-dimensional ultracold atomic Fermi gases. We incorporate disorder from impurities through fluctuations and study its effects on the BCS-BEC crossover. We analyze the effect of quenched disorder for various physical quantities such as chemical potential, pairing gap, density of states, spectral function, and ground-state energy. We extend our study further towards the experimentally viable quantities such as condensate fraction, sound velocity and Landau critical velocity. The results are presented as a function of binding energy and scattering length. We observe negligible effect of disorder in 2D for BCS Cooper pairs and considerable amount of depletion in the BEC regime but intriguingly the results also reveal that disorder effect is masked at the crossover region.   

Direct laser cooling of the BH molecule

Ultracold polar molecules are of interest for a variety of applications, including tests of fundamental physics, ultracold chemistry, and simulation of many-body quantum systems. The laser cooling techniques that have been so successful in producing ultracold atoms are difficult to apply to molecules. Recently however, laser cooling has been applied successfully to a few molecular species, and a magneto-optical trap of SrF molecules has now been demonstrated. We have investigated the BH molecule as a candidate for laser cooling. We have produced a molecular beam of BH and have measured the branching ratios for the excited electronic state, ${\rm A}^{1}\Pi (v'{=}0)$, to decay to the various vibrational states of the ground electronic state, ${\rm X}^{1}\Sigma$. We verify that the branching ratio for the spin-forbidden transition to an intermediate triplet state is inconsequentially small. We measure the frequency of the lowest rotational transition of the X state, and the hyperfine structure in the relevant levels of both the X and A states, and determine the nuclear electric quadrupole and magnetic dipole coupling constants. Our results show that a relatively simple laser cooling scheme can be used to cool, slow and trap BH molecules.   

Trapping effect on the sound velocity of a multilayer Fermi gas

We present the trapping effect on the behavior of the isothermal compressibility and sound velocity for an interactionless Fermi gas immersed in a periodic interconnected multilayer structure created by an external Dirac comb potential which can vary both in spacing and in the intensity that controls the impenetrability of the layer edge (the wall) [1]. At $T=0$, for a given layer width and respect to the free ideal Fermi gas values, the isothermal compressibility as a function of the impenetrability starts in one and then monotonically increases to reach a larger constant value which is width dependent. The sound velocity as a function of impenetrability starts in one and for a range of impenetrabilities shows a bump which suggests that the presence of the structure increases the speed. For a finite temperature, given a separation between the walls and several values of their impenetrabilities, both properties start their evolution in temperature from the ideal Fermi gas value, unfold at temperatures near and under $T_F$, and then recover the behavior of a classical gas at higher temperatures. \\ \noindent [1] P. Salas and M.A. Sol\'is, ``Trapping effect of periodic structures on the thermodynamic properties of a Fermi gas", J. Low Temp, Phys. {\bf 175}, 427 (2014).   

Superfluidity in 1D and 3D Spin-Imbalanced Fermi Gases

The phase separation between superfluid and normal phases (both polarized and unpolarized) in trapped Fermi gases in the BEC-BCS crossover reveals the interplay between superfluid pairing, interactions, and dimensionality. We measure density profiles of both spins of a two-component, spin-polarized gas of $^6$Li atomic fermions cooled to $\sim 100$~nK. In a 3D gas, an unpolarized superfluid core is surrounded by a polarized shell. We observe gradual suppression of this core as interactions are weakened from unitarity. For a 1D gas in an optical lattice, the phase separation matches exactly-solved 1D models, where the central phase is partially polarized, and is predicted to exhibit FFLO correlations\footnote{Y.A. Liao et al., Nature {\bf 467}, 567 (2010).}. By increasing the inter-tube tunneling rate, we investigate the dimensional crossover between 1D and 3D Fermi gases. In this regime, the FFLO order parameter is predicted to be correlated between tubes\footnote{K. Sun, and C. J. Bolech, PRA {\bf 87}, 053622 (2013)}, and its modulation length constant over larger regions of the trap\footnote{M. M. Parish, et al., PRL {\bf 99}, 250403 (2007)}. These features are predicted to enhance the observable signatures of FFLO correlations; we report progress towards such measurements.   

Feature Selection via Modified Gravitational Optimization Algorithm

Feature selection is the process of selecting a subset of relevant and most informative features, which efficiently represents the input data. We proposed a feature selection algorithm based on n-dimensional gravitational optimization algorithm (NGOA), which is based on the principle of gravitational fields. The objective function of optimization algorithm is a non-linear function of variables, which are called masses and defined based on extracted features. The forces between the masses as well as their new locations are calculated using the value of the objective function and the values of masses. We extracted variety of features applying different wavelet transforms and statistical methods on FLAIR and T1-weighted MR brain images. There are two classes of normal and abnormal tissues. Extracted features are divided into groups of five features. The best feature is selected in each group using N-dimensional gravitational optimization algorithm and support vector machine classifier. Then the selected features from each group make several groups of five features again and so on till desired number of features is selected. The advantage of NGOA algorithm is that the possibility of being drawn into a local optimal solution is very low. The experimental results show that our method outperforms some standard feature selection algorithms on both real-data and simulated brain tumor data.   

Optically Pumped NMR Studies of Mechanically Induced Strain in GaAs Films

We present a new methodology for measuring strain in semiconductor films based on optically pumped NMR (OPNMR). Single crystals of GaAs were epoxy bonded to Si wafers at 100 $^{\circ}$C. The GaAs is then variably thinned by selective chemical etching. Upon cooling, biaxial tensile strains are induced in the GaAs films since the coefficient of thermal expansion in GaAs is different than in the Si support. OPNMR experiments were carried out at 6-10 K. The OPNMR spectra are selective to nuclei within a photon penetration depth from the surface. When mounted on a 0.635 mm thick Si support, the strain, which is proportional to the observed quadrupole splitting, is found to decrease with increasing thickness of the GaAs films and appears to approach a residual value. When the same GaAs film is mounted on a thicker 5mm Si block, the strain increased. To explain the observations, we consider effects of dislocation relaxation of strain and bending of the composite. The interface strain extracted from the measurements is 5.5 $\times$ 10$^{-4}$, in good agreement with the value estimated using the differential thermal contraction of Si and GaAs. The strain resolution of the technique is about 10$^{-5}$ in GaAs.   

Theoretical study of the structural, vibrational and dielectric properties of PbSnTe alloys

Thermoelectric devices have promise in dealing with the challenges of the growing demand for alternative clean energy and Te-based materials well-known candidates for them. Recently [1], we have shown that the high values for the dielectric constant, together with anharmonic LA-TO coupling, reduces the lattice thermal conductivity and enhances the electronic conductivity in PbTe. Also, it was shown that by alloying this material with Se, the electronic conductivity of the alloys is also enhanced [2]. But, it is not clear if the same occurs when alloying with Sn. We show, in this work, our \textit{ab initio} results for the structural, vibrational and dielectric properties of Pb$_{\mathrm{1-x}}$Sn$_{\mathrm{x}}$Te alloys. The calculations were carried out by using the Density Functional Theory, and the alloys were described by the Virtual Crystal Approximation. Our results show that their structural properties do not obey the Vegard rule. However, we have detected that the anharmonic LA-TO coupling still exists and the obtained values for the dielectric constant show higher values than that obtained for PbTe. \\[4pt] [1] H. W. Leite Alves, \textit{et al.}, Phys. Rev. B\underline {87}, 115204 (2013).\\[0pt] [2] Y. Pei, \textit{et al}., Nature \underline {473}, 66 (2011).   

Experimental observation of amplification death via asymmetric gain

The amplification action of two coupled RLC circuits is experimentally controlled via a spatially inhomogeneous gain. Specifically we have demonstrated that increasing the overall gain of an unstable RLC circuit can result in its stabilization. This counterintuitive phenomenon has its roots in managing impedance matching and thus can be applicable to a variety of wave systems.   

Laser Induced Breakdown Spectroscopy of Glass and Crystal Samples

Different types of quartz crystals and rare earth ions doped glasses have been identified using the laser induced breakdown spectroscopy (LIBS) technique. LIBS is a real time technique, can be used to identify samples in solid, liquid and gas phases. The advantage of LIBS technique is that no sample preparation is required and laser causes extremely minimal damage to the sample surface. The LIBS spectrum of silicate glasses, prepared by sol-gel method and doped with different concentration of rare earth ions, has been recorded. The limit of detection of rare earth ions in glass samples has been calculated. Total 10 spectrums of each sample were recorded and then averaged to get a final spectrum. The ocean optics LIBS2500 plus spectrometer along with a Q- switched Nd: YAG laser (Quantel, Big Sky) were used to record the LIBS spectrum. This spectrometer can analyze the sample in the spectral range of 200 nm to 980 nm. The spectrum was processed by OOILIBS-plus (v1.0) software. This study has application in the industry where different crystals can be easily identified before they go for shaping and polishing. Also, concentration of rare earth ions in glass can be monitored in real time for quality control.   

Optical spectrum measurement of a cell-adhered microcavity for the cell-cycle analysis applications

We build a setup and demonstrate successful measurement of the transmittance spectrum of a whispering gallery mode silica optical microcavity in which NIH 3T3 cells adhered on the top surface to achieve real-time and label-free measurement of the cell cycle. Label-free measurement is expected to prevent the cells to exhibit secondary effect. We build a system that enables the control of the gap distance between the microcavity and the tapered fiber, both of which are placed in the cell culture medium. The optimization of the tapered fiber diameter is the key to measure the spectrum of a microcavity in liquid. A swept wavelength laser light at a wavelength of 766 to 780 nm is used for the measurement. The cavity exhibit a $Q$ of $1.0\times 10^{6}$in air, where the value is $1.0\times 10^{5}$ in the medium and drops to $3.1\times 10^{4}$ after the cell-adhesion. Still the $Q$ of the microcavity is sufficiently high to detect the change at the cavity surface. Indeed we observe slight spectrum shift toward a longer wavelength, which we believe is due to the adherence of NIH 3T3 cells on the silica microcavity.\textbf{ }The successful measurement of the transmittance spectrum of a microcavity in cell culture medium is the first step to realize the analysis of the cell-cycle based on microcavity system.   

Laser performance at 1064 nm in Nd$^{3+}$ doped oxi-tellurite glasses

The search for Nd$^{3+}$ doped new solid-state laser hosts having specific thermo-mechanical and optical properties is very active. Among tellurites, the TeO$_{2}$-ZnO glass combines good mechanical stability, chemical durability, high linear and nonlinear refractive indices, low phonon energies ($\sim$750 cm$^{-1}$) and a wide transmission window (0.4-6 $\mu $m). Their high nonlinear optical properties can be used for the development of Kerr-lens mode-locked subpicosecond lasers. The present work concentrates on the luminescence properties and the laser performance of a TeO$_{2}$-ZnO tellurite glasses doped with Nd$^{3+}$. True continuous-wave laser action is achieved by pumping the sample with a CW Ti:Sapphire laser inside a standard two-mirror laser cavity. A low laser threshold of 8 mW and a laser slope efficiency of 21{\%} could be obtained for an output coupler transmission of 2.7{\%}, which is an encouraging improvement compared to what was reported in the past with other Nd-doped tellurite bulk glasses.   

Mie plasmon-polariton modes in two-dimensional metallic photonic crystals

We have studied the Mie plasmon-polariton resonances in cylindrical metallic hollow rods by calculating the scattering ($Q_{sca}$), extinction ($Q_{ext}$) and absorbtion ($Q_{abs}$) coefficients, which were compared with results for the photonic band structure (PBS) of the corresponding periodic 2D system, showing that Bloch plasmon-polariton modes in the periodic system stem from plasmon resonances at each individual rod. On the other hand, by calculating the field distribution corresponding to plasmon resonances, in both cases, we show that the symmetry properties remain similar, indicating a robustness of these localized plasmon modes.   

Optical ``Bernoulli'' forces

By Bernoulli's law, an increase in the relative speed of a fluid around a body is accompanies by a decrease in the pressure. Therefore, a rotating body in a fluid stream experiences a force perpendicular to the motion of the fluid because of the unequal relative speed of the fluid across its surface. It is well known that light has a constant speed irrespective of the relative motion. Does a rotating body immersed in a stream of photons experience a Bernoulli-like force? We show that, indeed, a rotating dielectric cylinder experiences such a lateral force from an electromagnetic wave. In fact, the sign of the lateral force is the same as that of the fluid-mechanical analogue as long as the electric susceptibility is positive ($\epsilon>\epsilon_{0}$), but for negative-susceptibility materials (e.g. metals) we show that the lateral force is in the opposite direction. Because these results are derived from a classical electromagnetic scattering problem, Mie-resonance enhancements that occur in other scattering phenomena also enhance the lateral force. [This talk is based on \textit{Phys. Rev. A} \textbf{88}, 023829 (2013).]   

Extraordinary transverse magneto-optical Kerr effect in a superlens

It has been shown that a slab of a negative index material can behave as a superlens enhancing the imaging resolution beyond the wavelength limit. We show here that if such a slab possesses in addition some magneto-optical activity, it could act as an ideal optical filter and exhibit an extraordinary transverse magneto-optical Kerr effect. Moreover, we show that losses, which spoil the imaging resolution of these lenses, are a necessary ingredient to observe this effect.   

Characterization/Selection of a Continuous Wave Laser for RIMS Analysis in Nuclear Forensics

The effort to implement the technology of resonance ionization mass spectroscopy (RIMS) to problems of nuclear forensics involves the use of multiple lasers to selectively ionize the elements of concern. While current systems incorporate pulsed lasers, we present the results of a feasibility study to determine alternative (Continuous Wave) laser technologies to be employed for analysis of the actinides and fission products of debris from a nuclear detonation. RIMS has the potential to provide rapid isotope ratio quantification of the actinides and important fission products for post detonation nuclear forensics. The current approach to ionize uranium and plutonium uses three Ti-Sapphire pulsed lasers capable of a fundamental wavelength range of 700-1000 nm. In this work, we describe the use of a COTS CW laser to replace one of the pulsed lasers used for the second resonance excitation step of plutonium near 847.282 nm. We characterize the critical laser parameters necessary to achieve high precision isotope ratio measurements including the stability over time of the mean wavelength, bandwidth and spectral mode purity. This far narrower bandwidth laser provides a simpler setup, more robust hardware (greater mobility), and more efficient use of laser irradiance.   

Multi-level Capacitive Memory Effect in Metal/Oxide/Floating-Schottky Junction

A memory computing (memcomputing) system can store and process information at the same physical location simultaneously. The essential components of memcomputing are passive devices with memory functionality, such as memristor, memcapacitor, and meminductor. We report the realization of a Schottky contact memcapacitor compatible with the current Si CMOS technology. Our memcapacitor is formed by depositing a stack of metal and oxide thin films on top of a Schottky contact. Here, the metal electrode of the Schottky contact is floating. The working principle of our memcapacitor is based on the fact that the depletion width of the Schottky contact varies according to the amount of charge stored in the floating metal electrode. The voltage pulse applied across the Metal/Oxide/Floating-Schottky junction controls charge flow in the Schottky contact and determines the amount of charge stored eventually. It is demonstrated experimentally that our memcapacitor exhibits hysteresis behaviors in capacitance-voltage curves and possesses multiple capacitance values that are switchable by the applied voltage pulse.   

High mobility field effect transistors of SnO$_{\mathrm{x}}$ on glass substrates made by reactive sputtering of Sn metal

We report on the electrical properties of SnO$_{\mathrm{x}}$ thin films and the performance of their field effect transistors on glass substrates made by reactive sputtering of a Sn metal target. We investigated the electrical properties of SnO$_{\mathrm{x}}$ films as a function of the oxygen pressure. The mobility of the SnO$_{\mathrm{x}}$ films on glass substrates after post-deposition annealing at 400 C was as high as 15.3 cm$^{2}$/Vs while its carrier density was 4.42 $\times$ 10$^{18}$ cm$^{-3}$. By x-ray diffraction, we have found that the films are mixture of SnO and SnO$_{2}$ phases, suggesting possibility of further enhancement of the electrical properties if the phase can be controlled. Nevertheless, we will report on the performance of thin film transistors using polycrystalline SnO$_{\mathrm{x}}$ as the channel layer and the atomic-layer-deposited AlO$_{\mathrm{x}}$ and HfO$_{\mathrm{x}}$ as the gate oxide.   

Integrated silicon nanophotonics: structure and electro-optic properties of BaTiO$_{3}$ on Si(001)

High-quality epitaxial BaTiO$_{3}$ (BTO) on Si has emerged as a promising material for future electro-optic (EO) devices based on BTO's large effective Pockels coefficient. In order to achieve strong EO coupling, a film must have (1) correct crystallographic orientation with respect to the applied electric field, and (2) low leakage current in the film to sustain a strong electric field. We report on the EO response of BTO films deposited on Si by molecular beam epitaxy. O$_{2}$ rapid thermal anneal at 600C for 30 min ensures full oxidation of BTO for minimal leakage current with minor change in crystalline structure. EO characterization was performed by analyzing changes of the polarization of a laser beam transmitted through pairs of lithographically defined electrodes. The EO response shows signatures of ferroelectric domains with in-plane polarization. Comparison with normalized responses of c-axis and a-axis films illustrate that a strong EO response is observed even for a mixed film. These results quantify the relationship between BTO structure and EO properties, an important step towards future silicon photonic devices based on ferroelectric oxides.   

Optimizing Energy Conversion: Magnetic Nanomaterials

We present herein the work started at SUNY Oswego as a part of a SUNY 4E grant. The SUNY 4E Network of Excellence has awarded SUNY Oswego and collaborators a grant to carry out extensive studies on magnetic nanoparticles. The focus of the study is to develop cost effective rare-earth-free magnetic materials that will enhance energy transmission performance of various electrical devices (solar cells, electric cars, hard drives, etc.). The SUNY Oswego team has started the preliminary work for the project and graduate students from the rest of the SUNY 4E team (UB, Alfred College, Albany) will continue the project. The preliminary work concentrates on analyzing the properties of magnetic nanoparticle candidates, calculating molecular orbitals and band gap, and the fabrication of thin films.   

Prevention of Initial Defect of Low-Noise Solid-State-Nanopore Device

To achieve DNA sequencing using solid-state-nanopore, it is necessary to reduce an electric noise current. In this study, the noise was decreased by reducing the capacitance($C)$ of the nanopore device. We coated an insulating material near a nanopore on a membrane of the device, and confirmed that the capacitance of the device needs to be decreased down to 100 pF in order to reduce the noise. However, electric-charge difference($\Delta Q)$ between electrolyte in the one and the other chamber occurred high voltage ($\Delta V \quad = \quad \Delta Q$/$C)$ to the membrane because the capacitance($C)$ was reduced. The electric-charge difference defected the membrane when pouring the electrolyte onto the both sides of the membrane. In order to prevent the initial defects, we established new procedures to reduce the electric-charge difference using electric bypass between the one and the other chamber. Then, we confirmed that there were no defects on the membrane with this procedure.   

Development of Multifunctional Luminomagnetic Nanoparticles as Bioimaging Contrast Agents

Trivalent rare earth doped nanocrystalline materials with multiple functionalities have drawn special attention in biomedical industry. Current research is focused on the use of various materials with dual functionality for potential multifunctional applications. In this project, we are developing near infrared(NIR) based nanocrystals (NCs) as contrast agents with multimodal features comprising of strong NIR fluorescence, X-ray fluorescence and magnetic properties by utilizing the superparamagnetic features of Gd$^{3+}$, the high X-ray excitation cross section of Lu$^{3+}$, and the NIR fluorescence of Nd$^{3+}$. Halides, such as MGdLuF$_{4}$ (M=K,Na), were doped with NIR active rare earth ions, Nd$^{3+}$, where synthesis conditions have been optimized to obtain the brightest phosphor with a size of sub-50 nm. Characterization of the NCs were performed to explore the excitation and emission properties, crystal structure, morphology, magnetization properties, and X-ray fluorescence properties. The potential use of these NCs can be utilized as contrast agents for medical imaging application such as optical imaging, magnetic resonance (MRI) and X-ray imaging.   

Intracellular Imaging Applications of Rare Earth doped Multifunctional Metal Oxysulphide Nanomaterials

Nanomaterials with multiple imaging features have a lot of attention in the medical industry where there is always a high demand for contrast agents that can give more information about the intracellular level mechanisms. Nanomaterials with specific size, shapes, surface functionalities, and properties are needed for intracellular level optical imaging. Rare-earth doped inorganic nanophosphors are the best choice for these applications due to their several advantages such as excellent optical properties, size and composition control, etc. Though there are several efficient rare earth based halide nanophosphors, an efficient halide free nanophosphor is still lacking. In this work we are presenting a series of rare earth doped metal oxysulphide host, M2O2S:Re (M=Gd,Y,La, Re=Yb,Er,Tb,Eu,Nd), as an alternate host with fluorescence efficiency equal or even higher than that of halides. Following a detailed study on the optical and magnetic properties we evaluated the potentiality of this material as nanoscale multifunctional contrast agents by \textit{in vitro} and \textit{in vivo} animal experiments. Our experimental results show that by adjusting the dopant concentrations and host structures the material property can be tuned over a wide range for multimodal imaging applications and optimized compositions can be achieved for high contrast intracellular imaging.   

MPO$_{4}$:Nd$^{3+}$ (M=Ca, Gd), Luminomagnetic Nanophosphors with Optical and Magnetic Features for Multimodal Imaging Applications

Nanomaterials with multiple functionalities play a very important role in several high technology applications. A major area of such applications is the biomedical industry, where contrast agents with multiple imaging modalities can provide better results than conventional materials. Many of the contrast agents available now have drawbacks such as toxicity, photobleaching, low contrast, size restrictions, and overall cost of the imaging system. Rare-earth doped inorganic nanophosphors are alternatives to circumvent several of these issues, together with the added advantage of super high resolution imaging due to the excellent near infrared sensitivity of the phosphors. In addition to optical imaging features, by adding a magnetic ion such as Gd$^{3+}$ at suitable lattice positions, the phosphor can be made magnetic, yielding dual imaging functionalities. In this research, we are presenting the optical and magnetic imaging features of sub-nanometer size MPO$_{4}$:Nd$^{3+}$ (M=Ca, Gd) phosphors for the potential application of these nanophosphors as multimodal contrast agents. Cytotoxicity, in vitro and in vivo imaging, penetration depth etc. are studied for various phosphor compositions, and optimized compositions are explored.   

Measurement of Quantum Yield and Upconversion Brightness in Red, Blue and Green on NIR Excited M$_{2}$O$_{2}$S:Yb/Er/Ho/Tm Phosphors

A series of broadly color tunable upconversion phosphors were synthesized from M$_{2}$O$_{2}$S (M=Y,Gd,La) using a flux fusion method. We investigate their upconversion properties as a function of the dopant concentrations and excitation power density. The phosphor compositions were determined for their upconversion characteristics under 800, 980 and 1550 nm excitations. By measuring the quantum yield and luminous brightness, we investigate their potential applications in biomedical imaging as well as NIR display applications. Results are compared with the well-known upconversion phosphor NaYF$_{4}$:Yb/Er/Ho/Tm and found that the M$_{2}$O$_{2}$S phosphor systems are more efficient compared to NaYF$_{4}$. By adopting various synthesis protocols, we were able to examine M$_{2}$O$_{2}$S in the size range of 10 nm to 10 $\mu$m.   

Synthesis and Characterization of Down-converting Ca$_{8}$(PO$_{4}$)$_{6}$:Nd$^{3+}$ Nanocrystals for Biomedical Imaging Applications

Currently, fluorescent probes (FPs), such as organic dyes and fluorescent proteins, are widely used for biomedical imaging, but exhibit undesirable characteristics such as small stokes shifts, large spectral overlaps, short fluorescence lifetimes, and photobleaching. In recent years rare earth doped nanoparticles (NPs) have shown promising results for use as FPs with properties that overcome the limitations of traditional fluorophores. Our current work utilizes the rare-earth ion, Nd3+, which exhibits NIR-NIR excitation and emission wavelengths that are within the low absorption and scattering region for biological tissues. Calcium phosphate was chosen as the host crystal because of its biocompatibility. The nanocrystals were then characterized by X-ray diffraction and TEM imaging. Spectroscopic studies were done to determine the emission and absorption intensity, quantum yield, and fluorescence lifetime. Analysis of the data was performed and the NPs were shown to possess superior properties when compared to those of traditional fluorophores.   

Novel conduction behavior in nanopores coated with hydrophobic molecules

Nanopores that are hydrophilic either by the nature of their pore surface or after suitable treatment are well studied in the context of solid-state nanopores. The ionic conduction in these nanopores typically exhibit a near no-concentration-dependence region low concentrations of salt (\textless 1 mM salt concentrations), followed by the high concentration region where it is proportional to the concentration. A simple cylindrical model for pore conduction can explain these behaviors based on surface and bulk conduction of the ions in the buffer solution [Smeets et al 2006 Nano Letters \textbf{6}, 89]. However, in nanopores coated with hydrophobic pores we find that the pore conductance is $\sim$ c$^{0.5}$ (c: concentration). This behavior is in shark contrast with the behavior of hydrophilic pores. We will present these results for different hydrophobic molecular coatings that exhibit this behavior.   

Mechanical Properties of Pentaerythritoltetranitrate (PETN) Single Crystals from nano- indentation: Depth Dependent Response at the Nano Meter Scale

This paper presents the investigation of the mechanical behaviors of the energetic material pentaerythritol tetranitrate (PETN) single crystals using a nanoindentation technique. The indentation tests have been performed on the (110) crystal face, using both spherical and wedge-shaped tips. The load displacement curves along with analysis has been used to extract the mechanical properties and to identify the anisotropic indentation elastic constants for the PETN. The calculated indentation moduli of the PETN single crystal were found to decrease as indentation depth increases and become displacement independent region when the indentation depth is higher 200nm. The indentation modulus obtained from spherical tip indentation is compared with results calculated by using literature values of the anisotropic elastic constants. The wedge indenter tip measurements at various tip orientations are different due to the anisotropy of the PETN. The yield behaviors of the PETN single crystal were also explored using both spherical and wedge tip indentation and differences are discussed. Key Words: PETN, nano-indentation, anisotropic elastic constants, single crystal, plastic yield   

All-optical near-field self-alignment in sub-nanoscale induced by Fano-resonance

Fano-resonance of photonic crystal (PC) slabs generally possesses extremely high quality-factor (Q-factor) which indicates large optical force produced via radiation pressure with low input power. Unlike atoms, nano- and micro-particles, periodicity of PC slab creates identical force field in every unit cell, which enables the manipulation of much wider area in millimeter scale. Here we developed a novel mechanism to construct a conservative optical force field to automatically align PC slabs with sub-nanometer resolution, a technique has a potential application in 3D photonic crystal fabrication. The phase response of our system can be predicted after the features of PC slab are precisely depicted using temporal coupled-mode theory. The conservation of optical force is then theoretically demonstrated based on Response Theory of Optical Force (RTOF) which has a perfect agreement with numerical simulation results of Maxwell Stress Tensor (MST) and Kelvin Force. In the end, we show no non-conservative component exists in the force field from Finite-element Method (FEM) simulation after applying Helmholtz-Hodge decomposition to it.   

Effects of Metal Nanoparticles on Optical Behavior Tunability of Multi-layer MoS$_{2}$

We investigate the effects of metal nanoparticles (NP) on tunability of the optical response from relatively thick MoS$_{2}$ flakes. The plasmonic interactions and charge transfer at the metal-semiconductor interface is studied through metal NP deposition on mechanically exfoliated MoS$_{2}$. The optical quenching effect of metal NP capping is observed. Our photoluminescence results show that the surface charge transfer at the metal-semiconductor interface and creation of possible defect points at the semiconductor surface can lead to quenching of the photoluminescence response. Our optical observation of thicker MoS$_{2}$ flakes shows that, by using a metal NP capping layer, one can selectively exclude photoluminescence response from the peak due to indirect transition. This approach provides a controlled method for tuning the optical response of relatively thick MoS$_{2}$ flakes. In this study the effects of Au, Cu and Ag NPs are investigated. Micro-PL spectroscopy of the MoS$_{2}$ flakes is performed; where, PL spectra are collected in the 1.2 eV to 2.3 eV energy range and a 532 nm CW laser is used for excitation. X-ray photoelectron spectroscopy (XPS) is performed to investigate the nature of the metal-semiconductor interface.   

Metrology for Nanoscale Manufacturing

The extension of optical techniques to the nanoscale is increasingly powerful because manufacturing requires fast, in-line, non-destructive metrology. As part of the NASCENT NSF Engineering Research Center led by the University of Texas at Austin, the focus of the effort is on the tools necessary for establishing manufacturing infrastructure required for process control of nanoscale printing. The initial exploration of scatterometry involved the use of a 244 nm laser to evaluate the zero-order reflectivity as a function of angle of incidence for two polarizations. Measurements of the wire-grid polarizer were then repeated with 405 nm to investigate the extensibility of scatterometry. In conjunction with the scatterometry data, rigorous coupled wave analysis simulations were used to determine the behavior of the reflectivity as a function of five critical dimensions. Varying the parameters led to the ability to fit the simulation curves to the experimental data, thus revealing the dimensions of the wire-grid polarizer. Grating profiles are established continuously allowing for the implementation of roll-to-roll manufacturing as envisioned by NASCENT. Scatterometry is a workhorse of Si lithography because of its fast, non-contact measurements at extreme sub-wavelength scales.   

Spin-valley coupling study in transition metal dichalcogenides

A single layer (monolayer) of transition metal dichalcogenide (TMD), a two dimensional (2D) crystal with a Honeycomb lattice structure, has attracted tremendous interests thanks to its unique optical and electronic properties. Unlike graphene, TMD monolayer is a direct-gap semiconductor with interband transition in the visible energy range. The direct gap is located at the corners of the 2D hexagonal Brillouin zone which are technically called valleys. The two degenerate but inequivalent valleys (denoted by $\pm$K) constitute a binary degree of freedom, which is potentially treated as a new kind of information carrier just like the traditional charges and spin. In TMD monolayer, the inversion symmetry is originally broken. This leads to valley-selective circular dichroism meaning that the $\pm$ K valleys can only be exclusively excited by right-($\sigma +)$ or left-circularly ($\sigma $-) polarized light, which enables optical manipulation of the valley pseudospin. Besides, the strong spin-orbital coupling (SOC) mainly stemming from the transition metal atoms gives a remarkable valence band spin splitting ranging from 0.1 eV to 0.5 eV for different TMDs. Together with the above mentioned inversion symmetry breaking, this leads to strong coupled spin and valley which makes the valley and spin robust against scattering by smooth deformations and long wavelength phonons. As a result, the valley-dependent optical selection rule is often accompanied by a spin-dependent optical selection rule. Here, we will present the experimental demonstration of the spin-valley locking in TMD monolayer. The spin polarization is realized by the circular polarized optical fields and is electrically detected.   

Coherent Manipulation of a Single Magnetic Atom Using Polarized Single Electron Transport in a Double Quantum Dot

We consider theoretically a magnetic impurity spin driven by polarized electron tunneling in a double quantum dot transport. Spin blockade effect and spin conservation in the system make the magnetic impurity to sufficiently interact with each transported electron. This effect yields the nanomagnet coherently driven by a single electron which carries information about the magnetic impurity spin. The present scheme may develop all electrical manipulation of manomagnets by means of single electrons, which is significant for the implementation of scalable logical systems in information processing.   

Spin textures for surface states with non-Rashba-type spin-orbit interaction

Surface states in Tl/Si and Bi/Si surfaces show non-Rashba-type spin splitting due to spin-orbit interaction (SOI). We construct effective tight-binding models on the triangular lattice for the surface states of Bi/Si and Tl/Si surfaces with spin-orbit interaction, respecting the crystal symmetries. Consequently, band structures and spin textures calculated from these models qualitatively agree with the experimental results. We find a new term in the Tl/Si model, which does not exist in Rashba systems. In addition, we numerically find bound states at the junction between two surface regions which have different signs of the SOI parameters in the Bi/Si system and in the Tl/Si system. Interestingly, the spin direction of the bound states is perpendicular to the crystal surface, whereas the spins of the bulk states are in-plane in the Bi/Si junction model. We compare the results with calculations using continuum models in two junction models. We also discuss physical realizations of such junctions.   

A comparative analysis of deposition methods for nickel contacts onto CZT

Our studies have demonstrated quality of material surface, interface and interface contacts must be considered in fabrication of an optimally functioning radiation detector. Dangling bonds, poor surface processing, contaminants on the surface and the quality of the contacts negatively contribute to detector energy resolution, and increased leakage currents. A Cadmium Zinc Telluride (CZT) crystal had nickel contacts deposited and characterized for these three cases: a) PVD, b) sputter cleaned plus PVD, and c) Ion Beam Assisted Deposition. In each of these cases the materials were characterized through use of current voltage (IV) measurements, Gamma Ray response and Scanning Electron Microscopy. The IV curves, resistivities, gamma responses, and surface features of the sample have been analyzed and compared. These results elucidate the influence of surface processing on quality of contacts and interfaces in optimizing the fabrication of a functioning radiation detector.   

Characterization of deposited nickel contacts for CZT

Cadmium Zinc Telluride (CZT), a material used in room temperature radiation detectors, has many surface features which contribute to suboptimal functioning as a radiation detector. Dangling bonds, quality of polished surface, and the contacts are contributing factors to reduced detector energy resolution, and increased leakage currents. Nickel contacts were deposited, characterized and compared to the gold chloride electroless contact, and deposited gold. We took current voltage (IV) characteristics of CZT-based detectors and obtained their Gamma response spectra. These measurements enabled us to determine an optimal metal contact deposition process and material with improved detector performance as measured by leakage current and charge collection efficiency. The work functions of the contacts were chosen to work well with CZT. Quality surfaces, and contacts may serve as a means to improve the functioning of CZT in detector applications.   

Effects of annealing conditions on the structural and electrical properties of a-oriented ZnO thin films

We studied the structural and optical properties of non-polar a-plane ZnO films grown on r-plane Al$_{\mathrm{2}}$O$_{\mathrm{3}}$ substrates by RF-sputtering with different annealing conditions and oxygen partial pressure. Epitaxial relationships between the ZnO films and Al$_{\mathrm{2}}$O$_{\mathrm{3}}$ substrates were determined by $\varphi $-scan to be ZnO[0001]//Al$_{\mathrm{2}}$O$_{\mathrm{3}}$[-1011], ZnO[1-100]//Al$_{\mathrm{2}}$O$_{\mathrm{3}}$[1-210]. Photoluminescence spectra showed that band gap depended on annealing times, which is sensitive to intrinsic emission and surface defects in the a-plane ZnO. Hall Effect were measured on all samples by a PPMS system to find the relations amount annealing times, carrier concentrations and mobilities.   

Enhancement of UV upconversion emission under near infrared excitation from NaYF4 nanocrystals doped with different lanthanide dopants

Lanthanide doped nanocrystals have been attracted intensive attentions recently due to their interesting photoluminescence properties. Each lanthanide exhibits its own unique optical property, allowing for emission of light at different wavelengths. Consequently, different dopants can be used at varying concentrations to fine-tune desired emission wavelengths. The hexagonal crystal structure with low phonon energy and high photon efficiency has made NaYF$_{4}$ a very efficient host for lanthanide dopants. Excitation of this material occurs at low energy in the near infrared (NIR) region and through energy transfer between dopants, emission at higher energy in visible regions is observed. A lot of work has been reported on upconversion to visible light; however, there is limited work on UV upconversion and enhancement of emission. Our current work focused on synthesizing a pure crystalline phase of NaYF$_{4}$ and doping these nanocrystals with different lanthanides, including Tm, Nd and Gd using various concentrations. These nanocrystals have been systematically characterized by X-ray diffraction (XRD), scanning electron microscope (SEM), and photoluminescence (PL).   

Transition Dynamics for Muonium in Silicon Germanium Alloys

We use longitudinal field muon spin relaxation technique to observe charge-state and site-change transitions of muonium in Si$_{\mathrm{1-x}}$Ge$_{\mathrm{x}}$ samples (x $=$ 0.45, 0.77, 0.81, 0.84, and 0.94). We primarily focus on modeling the temperature and field dependence of the relaxation data to investigate the donor and acceptor ionization energies, paramagnetic hyperfine frequencies and charge-state/ site-change cycles. We compare donor/acceptor energies from relaxation data to those from asymmetry fits and verify assignments of specific dynamics to each observed relaxation feature and access energy values not previously determined. Previous studies have shown that the T-site muonium acceptor level enters the valence band near x $=$ 0.92. We find separate muonium acceptor states with muonium trapped at a Si within the tetrahedral Si$_{\mathrm{n}}$Ge$_{\mathrm{4-n}}$ cage region and propose a new charge cycle that involves valence band resonant states.   

Modeling of Transition Metal Color Centers in Diamond

Diamond stands out among single-photon sources due to an intrinsically large band gap, efficient electrical excitation, the ability to host bright optical centers, photo-stable emission, room-temperature operation, short excited state lifetimes, and the ability to host hundreds of different color centers. Currently, most of these centers are active in the optical spectrum, but a single-photon source in the IR would represent a significant advancement. In pursuit of this end, the effects of a number of different transition metal atoms upon the diamond lattice have been investigated via cluster calculations using the General Atomic Molecular and Electronic Structure System (GAMESS) code. The importance of cluster size and electron correlation effects is considered, and time-dependent DFT and multi-configurational SCF approaches are compared.   

Vacancies and Temperature Effects on the Mechanical Properties of Silicene Nanoribbons

The study of two-dimensional materials has been increasing. In the past few years a large variety of these kinds of materials has been studied, such as boron nitride, molybdenum disulfide, and silicene. The main potential application of silicene is found in electronic devices, nevertheless, other possible applications can be found. It is necessary to understand the mechanical properties of the material, because it can help us to determine the stability and useful lifetime thereof. We study vacancies and temperature effects on the Young's Modulus (YM) of silicene nanoribbons (SNRs). To determine their mechanical properties, we perform molecular dynamics simulations at room temperature and at temperatures ranging from 400 to 600 K, and analyze the YM of three square shaped SNRs, whose lengths varies from 3 to 8 nm. Results at room temperature showed that the YM for the ribbons without defects increases with the length in both directions of chirality. On the other hand, for SNRs in the presence of defects (mono-and bivacancies) the YM depends on the defect position. It increases as the vacancy approaches the SNRs boundary. We observed that the higher the temperature, the lower the YM with and without vacancies. However, dependence on the length remains the same.   

Optical properties of TiO$_{\mathrm{2-x}}$ based Re-RAM switching devices under the effect of oxygen vacancies

The tuning of the optical properties of TiO$_{\mathrm{2-x}}$ based materials can be achieved by varying the mole fraction. The accurate calculations of linear optical function (refractive index, reflectance, coefficient of absorption, and both imaginary and real dielectric function) were carried out. The dependence of these properties under the effect of the oxygen mole fraction were analyzed. Using controllable mole fraction, various intermediate resistance states are induced. Furthermore, the presence of oxygen vacancies which is linked to the on-state conduction and resistance switching mechanism and its effect on the optical properties is studied.   

High Efficiency Alternative Current Driven Organic Light Emitting Device

In this work, we introduce the use of field-activated organic light emitter, coupled with a semiconducting gate electrode to create a novel, highly efficient lighting device. A layer of ZnO nanoparticles between the interface of an ITO contact and a PEDOT:PSS injection layer facilitates impressive control over the capacitive characteristics of the device. The advanced capacitive behavior of our devices gives rise to a barrier for carrier injection at low frequencies. Conversely, it promotes the generation of a field-induced polarization current in the active layer at high frequencies. The alternative current driven organic light emitting device obtains the power efficiency over 300 lm/W A at \textgreater 500 cd/m2, which is the highest power efficiency to date among high-luminance organic light emitting devices, to the best of the authors' knowledge. We interpret these findings as the negative magnetoresistance induces the `secondary' carriers that would contribute to light.   

Technological Concepts for Enhancing Semiconductor Ion Implantation Sources

Two novel ideas for improving ion sources for the two energy extremes of ion implantation for semiconductor manufacturing are described. Since the invention of the transistor, semiconductor devices have been miniaturized. As semiconductors become smaller shallow implantation is desired and ion energy needed for implantation decreases, resulting space charge (intra-ion repulsion) effects, which reduced beam currents and production rates. To increase production rates, molecular ions are used. Carborane, which is the most stable molecular boron ion leaves unacceptable carbon residue on extraction grids. Special O$_{2}$ elliptical cross section dissociator that injects O unto the grid can in-situ prevent carbon deposition without loading up power supplies. Pure gaseous processes are desired for enable rapid switch among ion species. For deep implantation and for avoiding the use of over molecular phosphorous and arsenic can be generated by introducing phosphine in dissociators via 4PH$_{3}$ $=$ P$_{4} +$ 6H$_{2}$ in a pure gaseous process (same for arsenic AsH$_{3}$). In the ion source molecular or high charge state phosphorous and arsenic can be generated. Concepts and devices will be presented.   

Effect of Boron doping on the structural, optical and electrical properties of ZNO nanoparticles produced by the Hydrothermal method

Effect of boron doping with 0-11 at. {\%} concentration on structural, optical and electrical properties of Zinc oxide nanopowder synthesized by Hydrothermal method has been reported. XRD results reveal that all B doped ZnO nanopowders have single phase hexagonal structure without any impurity. Positions of main diffracted peaks of ZnO shifted slightly towards small 2$\theta $ angle and grain size decreases from 60.39 nm to 34.34 nm with an increase of B doping. SEM analyses indicate that the doping concentration of B affected the surface morphology of ZnO nanostructures. Optical properties were examined by UV--Vis absorption/diffuse reflectance spectroscopy. The optical band gap of Zn$_{\mathrm{1-x}}$B$_{\mathrm{x}}$O nanostructures increased from 3.27 to 3.30 eV with increasing doping from x$=$0 to x$=$0.11. The role of B doping on the transport properties was searched by temperature dependent Hall measurements in range of 180--350 K. The carrier concentration of the samples increased from 0.11x10$^{14}$ to 4.08x10$^{14}$ cm$^{\mathrm{-3}}$, the Hall mobility decreased from 5.61 to 1.22cm$^{2}$V$^{-1}$s$^{-1}$ and electrical resistivity decreased from 10.89 $\times$ 10$^{4}$ to 1.25 $\times$ 10$^{4}$ ohm-cm with the increase of B doping at room temperature. The electrical resistivity was observed to decrease with both the increase in dopant concentration and the temperature.   

Growth and characterization of CH$_{3}$NH$_{3}$PbX$_{3}$(X$=$I, Br) single crystals by solution method

Solar energy conversion using solar cells requires materials that absorb in a broad spectral range, from visible to near infrared, to harvest most of the solar photons, as well as with the capability to convert effectively the incident light into free charges that produce electrical current and voltage. Organic-inorganic perovskite-structured hybrids CH$_{3}$NH$_{3}$PbX$_{3}$ (X $=$ Cl, Br, I or a combination) exhibit good application potentials in the next generation solid-state solar cells. In order to improve the properties of CH$_{3}$NH$_{3}$PbX$_{3}$ based solar cell, the studies on the basic materials are of great necessities. In this work, we present the results of the successful growth of large single crystals of CH$_{3}$NH$_{3}$PbI$_{3}$ and CH$_{3}$NH$_{3}$PbBr$_{3}$ with size up to Cm's using hydrohalic acid solution method. The solubilities of CH$_{3}$NH$_{3}$PbI$_{3}$ and CH$_{3}$NH$_{3}$PbBr$_{3}$ in hydrohalic acid were determined by weight method at the temperature range between 300-360K. X-ray diffractometry, scanning electron microscopy were used to study the structure and morphology, and the lattice parameters were estimated using Rietveld refinement method. The study of crystal nucleation, morphology and dimensions indicates that these are strongly dependent on the supercooling state occurred to the liquid during crystal growth, in which the \textbraceleft 100\textbraceright always exhibits the largest faces on the as-grown crystals. Optical properties of these single crystals were characterized by FT-IR, Raman, photoluminescence and cathode fluorescent spectroscopy.   

Large-scale Synthesis of monolayer MoSe$_{2}$ via Chemical Vapor Deposition

Molybdenum diselenide (MoSe$_{2})$ has a direct band gap of 1.55eV for a monolayer utilized photodetctor and optoelectronics. Recently, its synthesis methods have been briskly researched as a material for electronic devices from reason why it has similar properties with molybdenum disulfide (MoS$_{2})$. We present synthesis method for large-scale monolayer MoSe$_{2}$ through the chemical vapor deposition using Se and MoO$_{3}$ powder as a precursor. Raman and X-ray photoelectron microscopy confirmed the quality of synthesized MoSe$_{2}$. Moreover, electrical property was investigated with field effect transistor.   

Synthesis of MoS$_{2}$ Flakes by Electrical Arc Discharge of Molybdenum Wire and sulfidization of MoO$_{3}$

Molybdenum Disulfide (MoS$_{2})$ is an important inorganic compound in industry due to its robustness, ability to withstand high temperatures, and physical properties. It acts a solid lubricant in oils as well as functioning as an excellent co-catalyst in desulfurization for uses in petroleum refining. In this project, we used electrical arc discharge to first synthesize Molybdenum Trioxide (MoO$_{3})$, then sulfidized the MoO$_{3}$ at 800$^{\circ}$C to form our MoS$_{2}$ flakes. We then used a scanning transmission electron microscope (Hitachi 5500) to analyze the structure of our MoS$_{2}$ flakes and perform a chemical analysis. Electrical arc discharge is an innovative and favorable synthesis method due to the simplicity of the setup, and the large volume of particles produced. We varied parameters such as the voltage and amperage in our electrical arc discharge process to maximize the amount of MoO$_{3}$ produced; thereby increasing the amount of MoS$_{2}$ we could produce. Characterizations were done to ensure that the Molybdenum Disulfide was of correct structure (flakes) and to ensure that these flakes were of correct chemical composition. Future applications in industry of these particles directly depend on the ability for us to produce these particles in the correct shape (flakes) and the efficiency by which we can produce them.   

Pin diodes based on n-ZnO/i-Al$_{2}$O$_{3}$/p-Si

We focus on the pin-diodes fabricated with n-type ZnO thin films deposited on p-type silicon by RF-sputtering, using very-thin Al$_{2}$O$_{3}$ as an insulation layer. By annealing at various temperatures, the domain sizes of the ZnO films would vary accordingly as viewed under scanning electron microscope (SEM). With larger grains, the leakage current was lowered, suggesting that grain boundaries are responsible for the leakage. However, the ZnO films largely remain textured along the c-axis as judged by their clear ZnO (0002) peaks of the XRD theta-2theta scans that are related to the phi-scans of off-axis planes. Furthermore, the XRD data also showed the structural changes of the insulation layer after annealing above certain temperature. For the samples annealed at 750$^{\circ}$C and 850$^{\circ}$C, the I-V curves showed characteristic pin diodes behaviors. The photoconductivity was measured as a function of intensity of a pulsed laser beam of wavelengths 1064nm, 532nm, 266nm. The photoelectronic IV responses will be discussed in regards to the electronic tunneling structures of the pin junctions and their dependence on the fabrication processes.   

Calculation of effective magnetic field of phonon dynamics in the Born Oppenheimer approximation

Phonon dynamics with an effective magnetic field can lead to phonon hall effect. We study the effect magnetic field on phonon dynamics due to the coupling to electrons in the Born Oppenheimer approximation. Symmetry properties of the effective magnetic field are discussed in crystals. Two methods to calculate the effective magnetic field are brought up, with model calculations on the honey comb lattice. The application of the methods can be directly applied in first principle calculation.   

Rigid Unit Modes; their energy and temperature dependence with aluminium phosphate

One of the problems within crystallography is the concept of the average structure where due to dynamical translations or librations the structure is not truly static. This type of behaviour is common within polyhedral based compounds such as AlPO$_4$ where, in this case, the rigid tetrahedra can move or tilt with respect to each other as a consequence of the instability of the 180$^\circ$ Al-O-P bond. We explore the energy and temperature dependence of these modes using inelastic neutron scattering and heat capacity measurements as well as using computational modelling to assign the observed behaviour to these rigid unit modes.   

Phase transition of MoS$_{2}$ using laser irradiation

The multi-layer 2H- MoS$_{2}$ flakes are transferred toSiO$_{2}$/Si substrate by mechanical exfoliation method and transformed into 1T-MoS$_{2}$ by Li intercalation. The phase change by Li doping leads semiconducting 2H-MoS$_{2}$ to metallic 1T-MoS$_{2}$ that is confirmed by Raman and PL spectroscopy and I-V measurements. Then, 1T-MoS$_{2}$ flakes are locally heated to recover to 2H-MoS$_{2}$ using 532nm-laser beam that can increase the irradiated power up to 10 mW. The characteristics of thermally patterned 2H-MoS$_{2}$ are investigated by confocal PL and photo-current and I-V measurements. Also, the junction characteristics of 2H- and 1T-MoS$_{2}$ flakes will be discussed further in this presentation.   

The Calculation of the Polarizaion for Quantum voids in Applied Electrical Field

Under certain conditions voids or approximately spherical cavities may be formed when a electronic device has been long term used and they result in a swelling and deterioration of the device infrastructure. The formation and properties of such voids have been extensively studied because of the practical implications for damaged transistor devices. We calculate the electronic profile at the interface of voids with the density functional theory. We introduce the electronic profile to the dielectric constant of multilayer of the boundary area of the voids, and using the Laplace equations and the standard boundary conditions at the various layers of the voids. We obtain the electrical field around the voids. The polarization therefore can be found.   

Spin polarization driven by a charge-density wave in monolayer 1T-TaS$_2$

Using first-principles calculations, we investigate the electronic and vibrational properties of monolayer T-phase TaS$_2$. Our results demonstrate that the formation of a CDW is energetically favorable at low temperature, similar to bulk 1T-TaS$_2$. Electron-phonon coupling is found to be essential for the lattice reconstruction. In the CDW phase the electronic states near the Fermi level are strongly localized, which explains the formation of significant magnetic moments. The spin polarization transforms the material into a magnetic semiconductor. The combination of inherent spin polarization with a semiconducting nature distinguishes the monolayer fundamentally from the bulk compound as well as from other two-dimensional transition metal dichalcogenides. Monolayer T-phase TaS$_2$ therefore has the potential to enable two-dimensional spintronics.   

Giant valley drifts in uniaxially strained monolayer MoS$_2$

Using first-principles calculations, we study the electronic structure of monolayer MoS$_2$ under uniaxial strain. We show that the energy valleys drift far off the corners of the Brillouin zone (K points), about 12 times the amount observed in graphene. Therefore, it is essential to take this effect into consideration for a correct identification of the band gap. The system remains a direct band gap semiconductor up to 4\% uniaxial strain, while the size of the band gap decreases from 1.73 to 1.54 eV. We also demonstrate that the splitting of the valence bands due to inversion symmetry breaking and spin-orbit coupling is not sensitive to strain.   

Electronic structure and bonding in fluorapatites crystals: A$_{10}$(PO$_4$)$_6$F$_2$ (A=Ba, Ca, Pb and Sr)

In this work we report a computational study of electronic properties of fluorapatites (A$_{10}$(PO$_4$)$_6$F$_2$ with A=Ba, Ca, Pb and Sr) using ab initio calculation. We employed Density Functional Theory using a Plane Wave basis set and pseudopotentials to obtain the band structure, total and partial density of states, electronic charge density and electron localization function. We obtain that Ba-, Ca- and Sr-fluorapatite have a wide band gap in the range of $\sim$5.4 eV, while the Pb-fluorapatites have a band gap of $\sim$3.8 eV and different band structure compared with the rest of fluorapatites. The electron charge density and the electron localization function reveal covalent character of the bond between the oxygen and phosphorus in the tetrahedral substructure for all fluorapatites. We analyze the results and possible causes behind the differences in the electronic structure of these fluorapatites.   

Excitonic valley polarization and coherence in atomically thin MoS$_2$

We study the excitonic valley polarization and coherence in few-layer MoS$_{2}$ by circular- and linear-polarization-resolved photoluminescence. The valley polarization is largest in monolayer MoS$_{2}$ and decreases with the increase in the number of layers or temperature. Contrary to the valley polarization, the linear polarization is negligibly small in monolayer MoS$_{2}$ and increases with the increase in the number of layers or temperature. The valley decoherence in monolayer MoS$_{2}$ is at least an order of magnitude faster than the valley depolarization or exciton decay at low temperature, implying it has a pure dephasing origin. The valley coherence is steady against the increase in temperature or photoexcitation intensity, excluding phonon or carrier-carrier scattering from the dominant decoherence process. The temperature dependence of the valley polarization can be explained by the center of mass momentum dependent long range electron-hole exchange interaction, whereas that of the linear polarization may be due to relatively temperature-insensitive valley decoherence.   

Photoluminescence Saturation and Exciton Decay Dynamics in Transition Metal Dichalcogenide Monolayers

We study the photoluminescence saturation and exciton decay dynamics in monolayer transition transition-metal dichalcogenides (TMDs). Monolayer MoSe$_{\mathrm{2}}$ shows a PL saturation at a very low excitation intensity, more than two orders of magnitude lower than monolayer MoS$_{\mathrm{2}}$. Transient reflection spectroscopy shows that nonlinear exciton-exciton annihilation is the dominant exciton decay mechanism in monolayer MoSe$_{\mathrm{2}}$, in contrast to the exciton decay in monolayer MoS$_{\mathrm{2}}$ Furthermore, we measure exciton lifetime \textgreater 125 ps for monolayer MoSe$_{\mathrm{2}}$ much longer than the several-ps exciton lifetime in MoS$_{\mathrm{2}}$. We find that that the difference in their exciton lifetime can explain both the dramatically different exciton decay mechanism and PL saturation behavior of MoSe$_{\mathrm{2\thinspace }}$and MoS$_{\mathrm{2}}$ monolayers.   

Carrier recombination in m-plane InN thin film

Nonpolar m-plane InN thin films grown on LiAlO$_{\mathrm{2\thinspace }}$substrates by plasma-assisted molecular beam epitaxy have been studied using time-resolved photoluminescence (TRPL) upconversion technique. The carrier densities of 1.97 x 10$^{\mathrm{19}}$ cm$^{\mathrm{-3\thinspace }}$and mobility of 420 cm$^{\mathrm{2}}$/Vs were measured by van der Pauw--Hall geometry. The carrier temperature curves at different temperatures, derived from the time-resolved photoluminescence (TRPL) spectra at different time delay, indicate that hot carriers lost most of their excess energy by releasing LO-phonons. The effective LO phonon emission times increase with the lattice temperature, from 53 to 197 fs for 35 and 250 K, respectively. We found that the effective LO emission time of m-plane InN is smaller than that of c-plane InN. The recombination rates were derived from the TRPL measured at the energy closed to the bandgap energy. Similar to c-plane InN, the Shockley-Read-Hall recombination coefficient of m-plane InN shows a 3-fold increase for temperature increasing from 35 to 250 K. The Auger recombination was less effective as compared to that in c-plane InN.   

Optical properties of non-polar ZnO using Terahertz time domain spectroscopy

The m-plane ZnO thin films grown on m-plane sapphire substrates by atomic layer deposition were investigated with the terahertz time domain spectroscopy. The terahertz emission was generated by exciting a LT-GaAs antenna with laser pulses from Ti:sapphire at the wavelength of 800 nm. One of the ZnO samples was thermally annealed with a rapid thermal annealing system with O$_{\mathrm{2}}$ at a temperature of 700 $^{\mathrm{o}}$C. The other ZnO sample was studied without annealing. The refractive indices and extinction coefficients of m-plane ZnO along c- and a-axis were derived and found significant different. For both samples, the extinction coefficient and refractive index decreases monotonically with frequency. While the mobility along a-axis was found about the same after annealing, the mobility along c-axis has been improved significantly due to annealing. The annealing treatment has shown its impact on reducing the carrier concentration of 4.5 x 10$^{\mathrm{19}}$ to 3.5 x 10$^{\mathrm{18}}$ cm$^{\mathrm{-3}}$ for unannealed and annealed ZnO, respectively.   

Carrier localization in green emitting InGaN/GaN multiquantum well structure

Green emitting InGaN/GaN multiquantum well sample is investigated using photoluminescence (PL) and time-resolved photoluminescence (TRPL) spectroscopy. Carrier localization of energy $\sim$ 12 meV due to inhomogeneous distribution of In in the InGaN quantum well (QW) layer is observed. The temperature dependence of PL peak energy exhibits S-shape phenomenon and is comparatively discussed with the Varshni's empirical formula. The full width at half maximum (FWHM) of the PL emission band shows increasing-decreasing-increasing behavior with increase in temperature. The temperature dependence of radiative life time ($\tau_{\mathrm{r}})$ show $\tau_{\mathrm{r}}$ $\sim$ $T^{3/2}$ dependence with temperature above 200 K which confirms the insignificant effect of carrier localization at room temperature. Transmission electron microscopy (TEM) study reveals the absence of In-rich regions known for strong carrier localization in the InGaN QW layer which is consistent with the results of PL and TRPL.   

Hot carrier cooling in Si-doped InN

Temperature and excitation power dependent time-integrated photoluminescence of Si doped InN thin films are investigated. Photoluminescence (PL) spectra at low temperatures are described by single emission peak ensued due to ?free-to-bound? recombination; whereas PL spectra at higher temperatures above 150K are characterized by both ?band-to-band? and ?free-to-bound? transition. Carrier dynamics of Si doped InN thin films is studied using pump-probe reflection spectroscopy at room temperature. The hot electron cooling process is well described by electron-electron scattering. The dependence of the hot electron cooling rate on total electron density shows sublinear to linear behavior with increase of background electron density. The variation of the carrier recombination lifetime with total electron density implicates the dominance of the defect-related nonradiative recombination channel over other recombination processes.   

Comparison between stamping method and one-by-one dry-transfer method for the fabrication of h-BN sandwiched graphene FET in the quantum Hall regime

We have fabricated a dual-gate graphene field-effect-transistor (FET) for the study of integer Quantum Hall Effect in terms of its edge-state transport. The graphene was encapsulated by hexagonal boron-nitride (h-BN) flakes without any interlayer residues using the `stamping-transfer' method, which is critical for the observation of graphene's intrinsic transport properties. Using the poly propylene carbonate (PPC) and Polydimethylsiloxane (PDMS), initially top h-BN flake is picked up and then graphene flake is picked up by van der Waals' force between graphene and top h-BN. These two layers are dropped down on the bottom h-BN flake to complete the encapsulated formation. To make the source/drain (S/D) and top-gate electrodes, whole area of graphene is not covered by top h-BN flake. The open areas of graphene, located at both ends, are covered by S/D metal electrodes, which made whole graphene channel region sandwiched by top and bottom hBN. We compared this method with the old one which put the two-dimensional flakes one by one using `dry-transfer'' method, and found out a significant difference in the device quality especially at low temperatures and high magnetic fields in the quantum Hall regime.   

Exact solutions of fractional Chern insulators: interacting particles in the Hofstadter model at finite size

We show that all the bands of the Hofstadter model on the torus have an exactly flat dispersion and Berry curvature when a special system size is chosen. This result holds for any hopping and Chern number. Our analysis therefore provides a simple rule for choosing a particularly advantageous system size when designing a Hofstadter system whose size is controllable, like a qubit lattice or an optical cavity array. The density operators projected onto the flat bands obey exactly the Girvin-MacDonald-Platzman algebra, like for Landau levels in the continuum in the case of $C=1$, or obey its straightforward generalization in the case of $C>1$. This allows a mapping between density-density interaction Hamiltonians for particles in the Hofstatder model and in a continuum Landau level. By using the well-known pseudopotential construction in the latter case, we obtain fractional Chern insulator phases, the lattice counterpart of fractional quantum Hall phases, that are exact zero-energy ground states of the Hofstadter model with certain interactions. Finally, the addition of a harmonic trapping potential is shown to lead to an appealingly symmetric description in which a new Hofstadter model appears in momentum space.   

Nature of Quasielectrons and the Continuum of Neutral Bulk Excitations in Laughlin Quantum Hall Fluids

We construct model wavefunctions for a family of single-quasielectron states supported by the $\nu=1/3$ fractional quantum Hall (FQH) fluid. The charge $e^*$ = $e/3$ quasielectron state is identified as a composite of a charge-$2e^*$ quasiparticle and a $-e^*$ quasihole, orbiting around their common center of charge with relative angular momentum $n\hbar > 0$, and corresponds precisely to the ``composite fermion'' construction based on a filled $n=0$ Landau level plus an extra particle in level $n > 0$. An effective three-body model (one $2e^*$ quasiparticle and two $-e^*$ quasiholes) is introduced to capture the essential physics of the neutral bulk excitations. We also explore different ways of representing many-body wavefunctions in fractional quantum hall fluids, including the holomorphic wavefunctions, Jack polynomial formalisms and the diagrammatic representations. (Bo Yang and F.D.M. Haldane, PRL 112, 206804).   

Coexistence and competition of on-site and intersite Coulomb interactions in Mott-molecular-dimers

Recent findings of Mott-Hubbard physics in ultracold atoms trapped in periodic potentials have reinvigorated the search for quantum simulators of fermionic and bosonic Hubbard-like models. With this in mind, we performed a systematic study of a two-site realization of the Hubbard model, i.e, in a regime where this model can exactly treated. Particularly, we reveal the interplay between on-site (U) and intersite (V) Coulomb interactions in the extended two-site Hubbard model. Due to its atomic-like form quantum correlations intrinsic to Mott-molecular-dimers are exactly computed. Our results for physical quantities such as double occupancy and specific heat are consistent with those obtained for the one-band Hubbard model, suggesting that a two-site dimer model is able to capture the essential thermodynamic properties of strongly interacting electron systems. It is shown that intersite Coulomb interactions promotes the formation of doublons, which compete with the spin-singlet state induced by the on-site Coulomb repulsion. Our results are expected to be relevant for understanding electronic and thermodynamical properties of interacting electrons in strongly coupled magnetic atoms.   

Carrier dynamics in EuTiO$_{3}$ films probed by femtosecond pump-probe spectroscopy

Recently, perovskite oxide EuTiO$_{3}$ has attracted considerable attention due to its intriguing multiferroic properties. To gain a deeper understanding of its fascinating properties, it is essential to characterize the competing interactions between charge, lattice, spin, and orbital parameters. Here we present optical studies of the ultrafast carrier dynamics in EuTiO$_{3}$ films grown on SrTiO$_{3}$ substrates by probing photo-induced transient absorption (TA) in the weak excitation limit. All the signals were measured at room temperature. The transient curve of EuTiO$_{3}$ exhibits a fast rise after photo excitation (approximately 2 ps) and a long decay component with time constant of several nanoseconds, which are attributed to carrier-phonon coupling and carrier recombination respectively. The absorption change of EuTiO$_{3}$ near zero temporal delay is found to be quite different from the SrTiO$_{3}$ substrates, implying carrier-phonon interactions differ distinctively between these two materials. Our results could be helpful to understand the microscopic interactions in perovskite oxide.   

Investigation of 2-dimentional electron liquid at the interface of La(1-x)Al(1$+$x)O3/SrTiO3

The emergence of two dimensional electron liquid (2DEL) at the interface between two insulating oxides of lanthanum aluminate (LaAlO3) and strontium titanate (SrTiO3) shows unusual superconductivity and magnetism compared to conventional semiconductor-based 2DEG systems. One important issue resides on the influence of the stoichiometry to the 2DEL. Here, we report the structrual analysis and magneto-transport results on a series of La(1-x)Al(1$+$x)O3/SrTiO3with different x grown by oxide molecular beam epitaxy (OMBE) with in-situ growth monitoring using reflection high electron energy diffraction (RHEED). Detailed low temperature magneto-transport data and its correlation to the stoichiometry and film strain will be presented and discussed   

Double Charge Ordering States and Spin Ordering State Observed in a RFe$_{2}$O$_{4}$ System

Charge, spin, and lattice degrees of orderings are of great interest in the layered quantum material RFe$_{2}$O$_{4}$ (R $=$ Y, Er, Yb, Tm, and Lu) system. Recently many unique properties have been found using various experimental methods. However so far the nature of the two-dimensional (2D) charge ordering (CO) state is not clear and no observation of its fine structure in energy has been reported. Here we report unambiguous observation of double 2D CO states at relatively high temperature in a polycrystalline Er$_{0.1}$Yb$_{0.9}$Fe$_{2}$O$_{4}$ using Raman scattering. The energy gaps between the 3D and the double 2D states are 170 meV (41.2 THz) and 193 meV (46.6 THz), respectively. We also observed a spin ordering (SO) state at below 210 K with characteristic energy of 45 meV (10.7 THz). Our investigation experimentally identified new fine structures of quantum orders in the system, which also extends the capability of optical methods in investigating other layered quantum materials.   

Ultrafast dynamics of titanate octahedra tilting modes in a THz laser pulse

Using first-principles calculation, the electronic band structure and electric dipole moment for orthorhombic titanate are examined. To explore the energy dissipation after pulse excitation, the light-lattice coupling is treated with the mode oscillator model. By solving the equation of motion, we investigate the dynamics of the tilting modes. The evolution of the optical transition matrix elements as functions of different octahedra tilting modes and mode amplitudes are also evaluated, which provides theoretical base for the future experiments.   

Correlating valence state, site preference and co-substitution to the magnetoelastic properties of cobalt ferrite

Understanding how to influence the physics of magnetism, especially the relationship between magnetic susceptibility and stress, can be very useful in designing non-contact stress and torque sensors using magnetoelastic materials. This is particularly important considering that materials rarely occur in states desirable for direct applications. In this work we show that the magnetoelastic properties of cobalt ferrite are strongly dependent on the valence states and site preferences of substituted cations. It was found that co-substitution of magnetic and non-magnetic cations, is key to achieving simultaneous improvement in magnetostriction amplitude and strain sensitivity to applied magnetic field. Nevertheless, Curie temperature decreased, irrespective of the valence state, site preference or co-substitution. This presentation will show why tetravalent Ge resulted in superior magnetostrictive properties compared to other tetravalent, trivalent and divalent cations substituted into the crystal lattice of cobalt ferrite.   

Quantum Monte Carlo calculations of structural and electronic properties in the correlated oxide NiO

Transition metal oxides pose difficulties for condensed matter theories due to the presence of strong electronic correlations. The complex interplay among correlation and exchange in d subshells, crystal field effects, p-d hybridization and charge transfer gives rise to a rich variety of structural and electronic phases. NiO is one such challenging d system, where conventional band theory fails. Compared to the experimental value, the cohesive energy of bulk NiO computed within DFT-LDA differs by almost a factor of 18 {\%}. Band gap computed within standard local or semi-local functionals are off by a factor of 80 {\%}. A quasi-particle correction like the G$^{\mathrm{0}}$W$^{\mathrm{0}}$ approach cannot correct the band gap and is still by far too low. In this work we adopt the Diffusion Quantum Monte (DMC) approach to study the structural and electronic properties of NiO. Trial wave-functions were self consistently generated in a Slater-Jastrow form. To test pseudopotentials used, DMC calculations were done on atomic Ni and O and their computed ionization potentials showed excellent agreement with experiments (within 0.04{\%}). The equilibrium bond length and binding energy of the NiO dimer were also computed that were 0.001{\%} and 0.03{\%}, respectively, from experimental values. DMC calculations of equation of state and band gap of bulk NiO will be presented.   

Investigation of the Band Gap in Co$_{3}$O$_{4}$

Co$_{3}$O$_{4}$ is a strongly correlated oxide with a spinel structure and G-type antiferromagnetic order at temperatures below 40 K. It is a widely studied material owing to its applications in gas sensing, spintronics, batteries, and catalysis. The strong correlation and magnetism make it a difficult material to model from first principles. Density functional theory calculations require the use of a Hubbard U to correctly model its magnetic behavior. The band gap is sensitive to the choice of U allowing one to tailor the gap to a wide range of values. This often provides a phenomenological approach to determining U, but in the case of Co$_{3}$O$_{4}$ there is no experimental consensus on the actual value of the band gap. We utilize an alternate approach by matching the theoretical valence band structure to the actual valence band data obtained via x-ray photoemission spectroscopy. This generated set of U values is used to compute an absorption spectrum, which is in good agreement with ellipsometry results.   

Electronic Structures of the Charge Density Wave System RTe$_{2}$ (R$=$Ce, Pr) investigated by ARPES

The rare-earth (R)-based RTe$_{2}$ compounds are known as the charge-density-wave (CDW) systems. In this work, we have investigated the electronic structures of RTe$_{2}$ (R$=$Ce, Pr) by employing angle-resolved photoemission spectroscopy (ARPES) experiment and the first-principles band structure calculations. The overall shapes and sizes of the measured Fermi surfaces (FSs) of RTe$_{2}$ are found to be similar to those of the calculated FSs for the undistorted structures. The metallic states crossing the Fermi level (E$_{\mathrm{F}})$ are observed in ARPES even in the CDW state, indicating that the metallic states remain under the CDW transition with the remnant ungapped FSs. R 4$f$ PES spectra exhibit that the 4$f$ hybridization peak (4$f^{\mathrm{n}}c^{\mathrm{m-1}})$ in R$=$Pr is located deeper than in R$=$Ce, resulting in the much weaker 4$f$ spectral intensity near E$_{\mathrm{F}}$ in R$=$Pr. The shadow bands and the corresponding very weak FSs are found to arise from the band folding due to the interaction of Te(1) layers with R-Te(2) layers and the CDW-induced FS reconstruction. The E$_{\mathrm{F}}$-crossing states are stronger with the linear vertical polarization than with the linear horizontal polarization. The photon-energy maps in ARPES demonstrate the two-dimensional character of the near- E$_{\mathrm{F}}$ states.   

Circular dichroism of chiral photonic crystal liquid layers with enclosed defect inside

The photonic crystals of artificial and self-organizing structures with spatial periodic changes in dielectric and magnetic properties have attracted considerable interest recently due to unusual physical properties and wide practical applications. The chiral periodic structure in the scale of optical wavelength gives rise to strong and characteristic circular dichroism responses at visible wavelengths. Here we investigate photonic density, circular dichroism and peculiarities of absorption and emission spectra at various eigen polarizations in multilayered one-dimensional chiral soft matter with two layers of CLCs and an isotropic defect layer inside. The circular dichroism is defined by differences in light energy absorption $\it A$=1-($R+T$) by the system ($\it R$ and $\it T$ are the reflection and transmission coefficients, respectively) and $\it A^{l,r}$ are the light absorptions, if the incident light has left and right circular polarizations, respectively. This problem can be solved by the modified Ambartsumian's layer addition method. The influence of absorption and gain on the circular dichroism, absorption and emission spectra is established in cholesteric liquid crystal (CLC) cell with an isotropic defect layer inside.   

Fermion-sign-free Majarana-quantum-Monte-Carlo studies of quantum critical phenomena of Dirac fermions in two dimensions

Quantum critical phenomena may be qualitatively different when massless Dirac fermions are present at criticality. Using our recently-discovered fermion-sign-free Majorana quantum Monte Carlo (MQMC) method, we investigate the quantum critical phenomena of {\it spinless} Dirac fermions on the honeycomb lattice having $N_s=2L^2$ sites with largest $L=24$, at their charge-density-wave (CDW) phase transitions. By finite-size scaling, we accurately obtain critical exponents of this so-called Gross-Neveu chiral-Ising universality class of two (two-component) Dirac fermions in 2+1D: $\eta=0.45(2)$, $\nu=0.77(3)$, and $\beta=0.60(3)$, which are qualitatively different from the mean-field results but are reasonably close to the ones obtained from renormalization group calculations.   

Beyond structural bottleneck: the nature of nonthermal optically induced ultrafast phase switching in VO$_2$

Ultrafast manipulation of the electronic states of strongly correlated electronic crystals near room temperature, such as VO$_2$, encompasses enormous opportunities in high-speed electronics and photonics. However, its strong coupling to the first-order structural phase transition presents a bottleneck effect, which leads to cracking and various instabilities. Here, we show that repetitive ps or even sub-ps phase switching can be initiated by using mid-infrared photons where the rapid transformation is driven by instantaneous shift of chemical potential, rather than lattice or electronic heating. Using fs electron crystallography, we establish the cooperative doping-induced multi-step atomic pathway, which leads to a metal-insulator transition at a fractional energy dose.   

First-principles study of Sr$_2$Ir$_{1-x}$Rh$_x$O$_4$: charge transfer, spin-orbit coupling change, and the metal-insulator transition

Using first-principles density functional theory (DFT) calculations, we investigated the electronic structure of Rh-doped iridate, Sr$_2$Ir$_{1-x}$Rh$_x$O$_4$ for which the doping ($x$) dependent metal-insulator transition (MIT) has been reported experimentally and the controversial discussion developed regarding the origin of this transition. Our DFT+U calculation shows that the value of $\langle L.S \rangle$ remains largely intact over the entire doping range considered here ($x=0.0, 0.125, 0.25, 0.50, 0.75,$ and $1.0$) in good agreement with the branching ratio measured by x-ray absorption spectroscopy. Also contrary to a previous picture to explain MIT based on the charge transfer between the transition-metal sites, our calculation clearly shows that those sites remain basically isoelectronic while the impurity bands of predominantly rhodium character are introduced near the Fermi level. As the doping increases, this impurity band overlaps with lower Hubbard band of iridium, leading to metal-insulator transition. The results will be discussed with comparison to the case of Ru doping.   

Evidence of a quantum critical point in Ce$_{1-x}$Yb$_{x}$CoIn$_{5}$ alloys at high Yb doping

We performed this study on single crystals of Ce$_{\mathrm{1-x}}$Yb$_{x}$CoIn$_{5}$ alloys with the motivation to further explore some of the previously reported unusual behaviors such as robust coherence and superconductivity, non-Fermi liquid (NFL) behavior, and the possibility of quantum criticality in higher Yb doping. Our specific heat and electronic magneto-transport measurements on the alloy with x $=$ 0.75 nominal doping down to temperatures (T) as low as 0.5 K and magnetic fields (H) as high as 14 T. Our analysis of both specific heat and resistivity data unveils the presence of a crossover from NFL behavior at high temperatures to Fermi-liquid (FL) behavior at lower temperatures. Our analysis also indicates that the origin of the NFL behavior is a result of quantum fluctuations of unknown origin. The H-T phase diagram extracted from resistivity and specific heat shows that the crossover from NFL to FL behavior at zero temperature occurs at H $=$ 0. This implies that the alloy with x $=$ 0.75 Yb concentration is quantum critical, i.e., x$_{\mathrm{c}} = $ 0.75. This result of zero field quantum critical point at x $=$ 0.75 is also confirmed from our analysis of magneto-resistance data.   

Two-channel Kondo physics from arsenic bond oscillations in zirconium arsenide selenide

The two-channel Kondo effect is a fascinating but extremely fragile many-body state that has been theoretically discussed extensively. we address metallic compounds of a specific (PbFCl) structure for which a $-AT^{1/2}$ term to $\rho(T)$ is frequently observed, in line with the two-channel Kondo effect. The origin of this anomalous behavior has remained enigmatic since here, solely the interaction between electrons may account for this behavior, and the two-channel Kondo state is not expected to occur. By combining chemical and structural investigations with various physical property measurements we show that the magnetic field-independent $-AT^{1/2}$ term to the low-T resistivity observed over two decades in ZrAs$_x$Se$_y$ with 1.90 $\leq x+y \leq$ 1.99 originates from vacancies in the layer exclusively built up by As. Furthermore, we can trace back the two-channel Kondo effect in this material to a dynamic Jahn-Teller effect operating at these vacancies. All physical properties of the investigated compounds support this conclusion. Our findings will be relevant also for other metallic systems with pnictogen-pnictogen bondings, e.g., cage-forming compounds like the skutterudites.   

When do interactions with Goldstone bosons lead to non-Fermi liquids

There are few general physical principles that protect the low-energy excitations of a quantum phase. Of these, Goldstone's theorem and Landau-Fermi liquid theory are the most relevant to solids. In this talk, I will present a general analysis of when non-Fermi liquid behavior can arise in electronic systems due to coupling to Goldstone modes. We unify previously known cases using a single criterion and predict a new candidate involving phonons under a magnetic field.   

Evaluation of the magnetic field induced glassy behavior and oscillating magnetocalaroic effect in Zn$_{0.05}$Ni$_{0.95}$Cr$_2$O$_4$ spinel oxide

Polycrystalline Zn (5{\%}) doped NiCr$_{2}$O$_{4}$ sample was prepared by standard solid state reaction method. Room temperature structural refinement through X-ray diffraction confirm the cubic crystal structure with Fd-3m space group. Temperature dependent dc and AC magnetic measurements revealed multiple magnetic transitions. Doping small amount of non Jahn-Teller (J-T) ion (Zn$^{2+})$ in place of J-T site (Ni$^{2+})$ shows pronounced influence on the magnetostructural transitions. We have found a frequency independent magnetic transitions in the AC susceptibility measurement indicate the absence of glassy magnetic behavior under zero applied dc bias field. However, an unusual new glassy magnetic transition was discerned under the small dc magnetic field (3 kOe) in the AC susceptibility measurement. We have demonstrated oscillating magnetocaloric effect through isothermal magnetization measurements at low temperature. The observed unusual magnetic properties can be ascribed to the strong coupling and competing interaction between the spin-lattice-orbital interactions.   

Crystal structure and physical properties of Cr oxide with K$_{2}$NiF$_{4}$-type structure

Ruddlesden-Popper (RP) type structure materials have been attracted much attention due to their interesting physical properties including high-$T_{\mathrm{c}}$ superconductivity, charge stripe, itinerant ferromagnetism and so on. In this work we are presenting physical properties of some of the K$_{2}$NiF$_{4}$ type structure compounds. YSrCrO$_{4}$ is first synthesized and found to be a hetto-type K$_{2}$NiF$_{4}$ structure. The space group of YSrCrO$_{4}$ is determined to be orthorhombic Pccn by the electron diffraction and the powder X-ray diffraction. YSrCrO$_{4}$ shows two-dimensional (2D) spin correlations and a canted antiferromagnetic (AF) ordering. Evidence of AF ordering of the Cr oxides is obtained microscopically from ESR. The dielectric measurements suggest existence of in-gap states, while no magneto-dielectric coupling was observed in the above compounds.   

Creep-free ac hysteretic dynamics in epitaxial ferroelectric BiFeO$_{3}$ films

Dynamics of domain wall (DW) in ferroelectric (FE) films principally governs their switching properties under applied electric field ($E)$. At finite temperature ($T)$, the DW motion and their FE switching characteristics can be understood only by introducing the creep motion. Despite this importance, there have been little studies on creep motion of FE films under \textit{ac}-driven force. In this work, we investigate \textit{ac}-driven hysteretic dynamics of FE domains in epitaxial BiFeO$_{3}$ (BFO) films through polarization-electric field hysteresis loops with varying frequency and other switching characters. All BFO films were grown at the optimized growth condition, by employing different bottom electrodes of La$_{0.67}$Sr$_{0.33}$MnO$_{3}$ (LSMO) and SrRuO$_{3}$ (SRO); only BFO/SRO shows nearly creep-free hysteretic dynamics. We argue that inhomogeneous internal $E$ plays a significant role in such distinctive FE dynamics of BFO domains, which are affected by surface morphologies of bottom electrodes. Our results highlight that growth-mode-induced interfacial structure between an FE film and a bottom electrode result in engineering domain dynamics of FE switching characteristics.   

Application of the Wang-Landau Algorithm applied to Ferroelectrics

The conventional description of phase transitions in ferroelectrics is based on canonical thermodynamic functions and always assumes the thermodynamic limit of an infinite system. However, ferroelectrics at nanoscale recently became of high interest due to their potential applications in minuaturized devices. It is this timely and more appropriate to use the microcanonical ensemble when mimicking ferroelectric systems. Here, a first-principles-derived scheme, combined with an efficient Monte Carlo microcanonial technique, is used to gain new insight into the paraelectric to ferroelectric phase transition and the effect of the electric field on properties of BaTiO$_{3}$ systems. In this presentation, we will show the temperature variation of the specific heat for different lattice sizes in BaTiO$_{3}$ systems. The nature of the phase transition and the behaviour of the specific heat versus the lattice size will be documented. In addition, the effect of the electric field on the character of the phase transitions in BaTiO3 systems will be investigated by showing the free energy (F) versus the internal energy (U) curves. Electrocaloric effects can also be easily computed and will be discussed.   

Evolution of multiple dielectric responses and relaxor-like behaviors in pure and nitrogen-ion-implanted (Ba, Sr)TiO$_{3}$ thin films

Multiple dielectric responses are comparatively investigated in the pure and nitrogen-ion-implanted (Ba, Sr)TiO$_{3}$ (BST) films. Larger diffusive degree of phase transition and more relaxor-like features than those of pure BST films are observed in implanted ones, where the long-range-dipolar-correlated-orders were further segregated into local polar orders after the implantation. Moreover, the implanted films possess a transition from local reorientations of groups of dipoles induced nearly-constant-loss (NCL) type to oxygen vacancies (V$_{o})$ hopping type conduction at high temperature. Whereas, pure films behave as NCL type conduction along with a dielectric relaxation, which arises from the motions of defect complexes V$_{o}^{2+}$-Ti$^{3+}$.   

Effect of ferroelectric layer on the magnetic properties of ferromagnetic layer

In this presentation, we show the integration of classical two-phase multiferroic heterostructures composed of room-temperature ferroelectric BaTiO$_{3}$ (BTO) and ferromagnetic La$_{0.7}$Sr$_{0.3}$MnO$_{3}$ (LSMO) epitaxial thin films grown on technologically important substrate Si (100) [1-3]. Bilayers of BTO/LSMO thin films display ferromagnetic Curie transition temperatures of $\sim$350 K, close to the bulk value, that are independent of BTO films thickness in the range of 25-100 nm. Discontinuous magnetization jumps associated with BTO structural transitions were suppressed in M(T) curves, probably due to substrate clamping effect. Interestingly, at cryogenic temperatures, the BTO/LSMO structure with BTO layer thickness of 100nm shows almost 2-times higher magnetic coercive field, 3-times reduction in saturation magnetization and improved squareness compared to the sample without BTO. We attribute that to the strong in-plane spin pinning of the ferromagnetic layer induced by BTO layer at BTO/LSMO interface. This work demonstrates that it is possible to manipulate the magnetic properties of ferromagnetic layer by varying the thickness of ferroelectric layer, without applying external electric field.\\[4pt] [1] S.S.Rao \textit{et al}., J. Appl. Phys., \textbf{116}, 094103 (2014); \\[0pt] [2] J. Appl. Phys., (in print, 2014);\\[0pt] [3] Nano Lett., \textbf{13}, 5814 (2013).   

On Structural States of Multiferroic InMnO$_{3}$

InMnO3 (with small R site ion) was recently found to be ferroelectric and to crystallize with space group P63cm under certain preparation conditions (Appl. Phys. Lett. 102, 172901 (2013). We have conducted detailed structural studies to explore the phase diagram and to identify the structural forms of InMnO3 under varying preparation conditions. Detailed diffraction measurement results will be presented. This work is supported by DOE Grant DE-FG02-07ER46402.   

Anderson Chern insulators

Previously, it was shown that quantum spin Hall insulators (QSHI) with helical edge states can be turned into a trivial insulator with Chern number $N=0$ by applying magnetic fields. Further increase of the magnetization can result in a quantum anomalous Hall insulator (QAHI) which supports chiral edge state with $N=1$. In this work, we show that for intermediate magnetization strengths, before the QSHI is turned into a QAHI, a topologically non-trivial phase which supports a single branch of chiral edge states can be obtained by increasing non-magnetic disorder. We call this phase the Anderson Chern insulator phase. In contrast to QAHI in which the chiral edge states are protected by the bulk gap, the gapless chiral edge states in Anderson Chern insulators survive even though the bulk gap is closed by disorder. Moreover, an Anderson Chern insulator exhibits quantized conductance of $e^2/h$ instead of $2e^2/h$ as for topological Anderson insulators. Therefore, we propose that this Anderson Chern insulator phase is a new phase of topological matter.   

Topological properties of linear circuit lattices

Motivated by the topologically insulating (TI) circuit of capacitors and inductors proposed and tested in [1], we present a related circuit with less elements per site [2]. The normal mode frequency matrix of our circuit is unitarily equivalent to the tight-binding matrix of a quantum spin Hall insulator. Spinful fermionic time-reversal symmetry manifests itself in the TI circuit context as a result of a discrete symmetry of the circuit; elastic backscattering between edge normal modes does not occur whenever a circuit perturbation is invariant under such a symmetry. We also generalize the idea and provide a platform to simulate tunable and locally accessible lattices with arbitrary spin-orbit hopping. A simulation of a non-Abelian Aharonov-Bohm effect using such linear circuit designs is discussed. \newline\newline [1] N. Jia, A. Sommer, D. Schuster, and J. Simon, e-print arXiv:1309.0878. \newline [2] V. V. Albert, L. I. Glazman, and L. Jiang, e-print arXiv:1410.1243.   

Growth and Characterization of Large Scale (Sb$_{1-x}$Bi$_{x})_{2}$Te$_{3}$ Thin Films on Mica

Topological insulators (TIs) attract attentions for both fundamental science and potential applications because of their bulk band inversion arising from the strong spin orbital coupling. However, it is necessary to tune the Fermi level and Dirac cone in 3D TI (Sb$_{1-x}$Bi$_{x})_{2}$Te$_{3}$ to make an ideal system for TI applications. In this work, we report high quality (Sb$_{1-x}$Bi$_{x})_{2}$Te$_{3}$ thin films grown on mica substrate by molecular beam epitaxy. The surface roughness of the thin film can reach as low as 0.7 nm in a large area by van der Waals epitaxy. (Sb$_{1-x}$Bi$_{x})_{2}$Te$_{3}$ thin film with x $=$ 0.04 shows a local maxima in the room temperature sheet resistance, which indicates a minimization of the carrier density due to band structure engineering. Moreover, for higher Bi concentration, due to an increase of the surface roughness and corresponding reduction of electron mobility, the sheet resistance increases. Our results on the feasibility of depositing (Sb$_{1-x}$Bi$_{x})_{2}$Te$_{3}$ in wide Bi range on mica substrate will helpful for the application of TI at room temperature and flexible electronics.   

Evidence of a Positron bound state on the surface of Bi$_{2}$Te$_{2}$Se

We describe experiments aimed at probing the sticking of positrons to the surfaces of topological insulators performed at University of Texas at Arlington using the Positron Annihilation induced Auger electron Spectrometer. A magnetically guided beam was used to deposit positrons at the surface of Bi$_{2}$Te$_{2}$Se sample at energy of $\sim$ 2 eV. Peaks observed in the energy spectra and intensities of electrons emitted as a result of positron annihilation showed peaks at energies corresponding to Auger peaks in Bi and Te providing clear evidence of Auger emission associated with the annihilation of positrons in a surface bound state. Theoretical estimates of the binding energy of this state are compared with estimates obtained by measuring the incident beam energy threshold for secondary electron emission and the temperature dependence positronium emission. The experiments provide strong evidence for the existence of a positron bound state at the surface of Bi$_{2}$Te$_{2}$Se and indicate the practicality of using positron annihilation to selectively probe the critically important top most layer of topological insulator system.   

Collective modes at a surface of a topological insulator

We investigate hybrid plasmon-phonon modes of a polar topological insulator that originate from interaction among the quasiparticles of surface and bulk states, and also optical phonons. As an example, we study electron-doped Bi$_2$Se$_3$. We analyze the dispersion of the collective modes of this compound for (i) a bulk sample with a depletion layer created by acceptor adsorbates on the surface and (ii) a thin film. In the first case, we show that a depletion layer gives rise to two energy-momentum regions, where the surface states dominate the collective modes over the bulk carriers. In a thin film, the phonons are more prominent than the bulk carriers. The anisotropy of the phonon response makes the thin film behaves as a waveguide. We discuss how these various collective modes can be detected by scanning near-field optical microscopy.   

Magnetic properties of graphite nanostructures in carbon microspheres

Carbon microspheres with interesting magnetic properties are prepared by solid-phase pyrolysis using as a precursor the metal-free phthalocyanine H$_2$(C$_{32}$N$_8$H$_{16}$). By changing conditions of pyrolysis an average diameter $\it d$=2-3.8$\mu$m carbon microspheres, consisting of graphitized nanocrystallites with a thickness of 5-15 graphene layers of 5-20nm width with sufficiently narrow size distribution are formed. In particular, at $\it T_{pyr}$=700$^{0}$C and $\it t_{pyr}$=60min carbon microspheres have average $\it d$=3.4$\pm$0.15$\mu$m. Magnetic characteristics of samples are investigated by vibrational magnetometer in temperature range $T$=5-300$\mu$m and magnetic fields up to $H$=80kOe, as well as with the ESR method. Analysis of magnetization $\it M$ vs $\it H$ and $\it T$ show presence of paramagnetic centers with concentration $\it n$=3${\times}$10$^{19}$spin/g and temperature-independent diamagnetism with susceptibility, $\kappa_{dia}$=1.5${\times}$10$^{-6}$ emu/g Oe. Parameters of ESR spectrum are: $\it g$-factor 2.0031, intensity ${\sim}$5${\times}$10$^{19}$spin/g and narrow linewidth of 0.8Oe due to strong exchange. Paramagnetism in carbon microspheres is apparently driven by edge uncompensated spins of nanographitic crystallites and impurity nitrogen atoms.   

Reduction of Vanadium Oxide (VOx) under High Vacuum Conditions as Investigated by X-Ray Photoelectron Spectroscopy

Vanadium oxide thin films were formed by depositing thin films of vanadium on quartz substrates and oxidizing them in an atmosphere of oxygen. The deposition was done by the e-beam technique. The oxide films were annealed at different temperatures for different times under high vacuum conditions. The technique of x-ray photoelectron spectroscopy has been employed to study the changes in the oxidation states of vanadium and oxygen in such films. The spectral features in the vanadium 2p, oxygen 1s, and the x-ray excited Auger regions were investigated. The Auger parameter has been utilized to study the changes. The complete oxidation of elemental vanadium to V2O5 was observed to occur at 700$^{\circ}$C. At any other temperature, a mixture of oxides consisting of V2O5 and VO2 was observed in the films. Annealing of the films resulted in the gradual loss of oxygen followed by reduction in the oxidation state from $+$5 to 0. The reduction was observed to depend upon the annealing temperature and the annealing time.   

Investigation of InBi intermetallic compounds as possible topological insulators

InBi intermetallic compounds were found to be potentially 2-dimensional topological insulators from first-principle calculations. In our experiment, InBi, In$_{\mathrm{2}}$Bi$_{\mathrm{3}}$ and In$_{\mathrm{3}}$Bi$_{\mathrm{5}}$ were fabricated either in bulk or thin film, by depositing In and Bi into 2 layers by e-beam evaporation of proper thickness ratios for each stoichiometry. The bi-layers were rapid thermal annealed to accomplish the reaction or intermixing well above their temperatures of phase transition. EDS and secondary electron imaging were used to determine the obtained stoichiometry. CVD using a 3-zone furnace was as tested with various combinations of zone temperatures, types and flow rates of carrying gases to investigate the feasibility of epitaxial growth. Samples were characterized by XRD to obtain the crystalline phase, magneto-transport measurements to determine the carrier concentration and mobility using a PPMS to correlate the measured results. We will ponder on the implication of our findings in regards to the possibilities of being a candidate for topological insulator as predicted by the calculations.   

Optical and Electronic Properties of Diisopropylammonium Bromide molecular ferroelectric crystal (DIPAB)

Diisopropylammonium Bromide molecular ferroelectric crystal (DIPAB) could be considered as a potential alternative for perovskite ferroelectric materials. We report the results of \textit{ab-initio} calculations of electronic band structure and density of states to underline and explain the optical properties of $P2_1$ ferroelectric phase of DIPAB. In particular, we present the results on complex dielectric function, absorption, reflectivity, energy-loss spectra, and complex refractive index as functions of the frequency of the incident electromagnetic wave. We found that the optical band gap of the polar ferroelectric phase of DIPAB is $\approx $ 5 eV consistent with the steepest rise in the absorption spectra. Furthermore, we found that the ferroelectric phase of DIPAB exhibits two fundamental oscillator bands at 5.91 and 6.4 eV, which correspond to the optical transitions from the valence band of bromine to the conduction band of nitrogen and carbon. Analysis of optical spectra in the 0--4.8 eV photon energy range reveals that this phase is characterized by high transparency, no absorption and a small reflectivity in this range. We found that the sharp maxima in the energy-loss occur at 14.35 and 15.82 eV in polar phase. The peak value of volume loss, 15.82 eV in polar phase, coincides with the zero values of the real part of the corresponding dielectric functions.   

Electromagnetic Wave Transmission Through a Nano-Hole

An integral equation formalism is presented to describe electromagnetic wave transmission through a subwavelength nano-hole in a thin plasmonic sheet. The dyadic Green's function forthe associated Helmholtz problem is employed. Taking the subwavelength radius of the nano-hole to be the smallest length of the system, we have obtained an exact solution of the integral equation forthe dyadic Green's function analytically. This dyadic Green's function is then used in the numerical calculations of EM wave transmission through the nano-hole for normal incidence of the incoming wave train. The EM transmission involves two distinct contributions, one emanating from the nano-hole and the other is directly transmitted through the thin plasmonic layer itself. The transmitted radiation exhibits interference fringes in the vicinity of the nano-hole, and they tend to flatten as a function of increasing lateral separation from the hole.   

PT-symmetric Floquet Lattices

We investigate spectral and dynamical properties of periodically driven PT-symmetric dimer systems and show that in the Floquet space they are described by Parity-Time symmetric lattices. The topology of the Floquet lattice depends on the complexity of the driving. For the simplest driving scheme associated with a sinusoidal coupling, we show that as the gain/loss parameter increases, the Floquet spectrum and the corresponding eigenvectors undergo a transition from an exact to a broken PT-symmetric phase via an Exceptional Point singularity. The phase transition is also reflected in the associated Floquet dynamics. This paves the way to experimentally investigate extended lattice dynamics in PT-systems. Arealization in the RF domain is reported and compared with the theoretical analysis.   

Effects of stoichiometry and crystalline morphology on transport in topological insulator nanowires

In nanostructured topological insulators with increased surface-to-volume ratio the contribution of charge transport through topological surfaces relative to the bulk will be enhanced. Here we report on electrochemical synthesis and characterization of Bi$_2$Te$_3$ and Sb$_2$Te$_3$ nanowires in which the effect of materials' stoichiometry as well as nanowire size was investigated. Nanowires were grown in porous anodic aluminum oxide membranes with pore diameters varying from 18 to 150 nm. Stoichiometry and the wire morphology were tuned by electrochemical cell voltages in the 30 - 150 mV range. Topological signatures and surface conductance are affected by both crystallinity and chemical composition. We found that in the narrow range of the electrochemical potential (130 mV-150 mV) when stoichiometry changes were very small the nanowire conductance could still be hugely different. The results of high-field magnetotransport and I-V characteristic measurements on nanowires with crystallinity and morphology controlled by a suitable annealing protocol and imaged using transmission (TEM) and scanning (SEM) electron microscopies will be presented.   

Laser induced oxidation and optical properties of bismuth telluride nanoplates

Bi-Te nanoplates (NPs) grown by low pressure vapor transport method were studied by Raman spectroscopy, atomic force microscopy (AFM), energy-dispersive X-ray spectroscopy (EDS), and Auger electron spectroscopy (AES). We find that the surface of relatively thick (more than tens of nanometers) Bi2Te3 NPs is oxidized in the air and forms a bump under heating with moderate laser power, as revealed by the emergence of Raman lines characteristic of Bi2O3 and TeO2 and characterization by AFM and EDS. Further increase of laser power burns holes on the surface of the NPs. Thin (thicknesses less than 20 nm) NPs with stoichiometry different from Bi2Te3 were also studied. Raman lines from non-stoichiometric NPs are different from those of stoichiometric ones. Thin NPs with the same thickness but different stoichiometries show different color contrast compared to the substrate in the optical image. This indicates that the optical absorption coefficient in thin Bi-Te NPs strongly depends on their stoichiometry. Controlling the stoichiometry in the Bi-Te NP growth is thus very important for their thermoelectric, electronic, and optical device applications.   

Investigation of Interactions between the Doped Rare Earth Ions and Encaged Radicals in C12A7:RE3$+$ Using Optical and EPR Spectroscopy

Doped calcium aluminates (C12A7) (C12A7:Eu3$+$ and C12A7:Mn2$+)$ have been prepared using solid state reaction methods. The Eu3$+$ and Mn2$+$ dopants can both occupy the Ca2$+$ positions in C12A7.The unique cage-like structure of C12A7 allows different anions, such as oxygen, hydrogen to be trapped in cage by modifying the sample preparation or treatment conditions. The effects of these encaged anions/radicals on the local symmetries of Ca2$+$ have been studied using photoluminescence from C12A7 doped with Eu3$+$, which is a sensitive environmental probe. The effects can also be independently observed from the hyperfine structure of electron paramagnetic resonance spectra in C12A7 doped with Mn2$+$. Our results showed: 1) the presence of 5D0 to 7F0 transition implies that Eu3$+$ is at a non-centrosymmetric site in all caged radical centers; 2) this singlet transition is doubled when superoxides are encaged in C12A7, indicating that the sites of calcium (or Eu2$+)$ ions are not identical due to the distortion of the encaged anions; 3) a blue shift of the transition occurred due to nephelauxetic effects in asymmetry sites. We have also observed double sets of EPR signals of sextet hyperfine splitting for Mn2$+$ in C12A7-O but single sets in C12A7-H. We conclude that the local symmetry around Ca2$+$ positions are distorted more when superoxide is encaged in C12A7. Finally, RE3$+$ doped C12A7 samples have been systematically prepared and the interactions between the encaged ions and RE3$+$ emission centers studied using EPR spectroscopy.   

Enhanced spin orbit interaction of graphene by Ir cluster decoration

Enhancing the strength of the intrinsic spin orbit (SO) coupling in graphene is a critical issue in achieving the quantum spin Hall effect predicted by Haldane et al. Here we report the measurements of the weak localizations in graphene, which has been decorated by Ir clusters. The SO scattering rate ($\tau _{\mathrm{EM}})$ is extracted by fitting the curves using the formula of E. MacCan. It is found that $\tau_{\mathrm{EM}}$ is monotonically dependent on the electronic relaxation time. Further analysis points that it obeys an Elliot-Yafet relaxation, which can be attributed to the dominance of Kane-Mele $\tau_{\mathrm{EM}}$ interaction. The SO interaction strength can be extracted by fitting the $\tau_{\mathrm{EM}}$ data dependent on the gate voltage. After considering the temperature effect, an SO strength value of 5 $\sim$ 7meV is achieved, which has been greatly enhanced as compared to that of pristine graphene.   

Zero modes in superconducting nanowires with Desselhaus spin-orbit coupling

Using chiral decomposition, we are able to find analytically the zero modes in the Kitaev ladder model and superconducting nanowires with Desselhaus spin-orbit coupling. Analytic conditions for the existence of zero modes are obtained. As a result, we are able to predict the number of zero modes in these systems. Moreover, we find that when suitable resonance condition is satisfied exact zero modes exist even in finite systems contrary to the common belief.   

Chiral Majorana edge modes in d-wave superconductor/antiferromagnet heterostructures

In this work, we study the heterostructure of a d-wave superconductor coupled to an antiferromagnet. We show that Majorana fermion edge states can be created in the system even in the absence of spin-orbit coupling, given that a supercurrent is induced in the superconductor. The Majorana edge states exist even the bulk is gapless and they propagates perpendicular to the direction of the supercurrent. The Majorana modes can be detected through tunneling and heat transport measurements.   

Chiral magnetic effect in two-band lattice model of Weyl semimetal

Employing a two-band model of Weyl semimetal, a definite result on the existence of the chiral magnetic effect (CME) is established within the linear-response theory. The crucial role played by the limiting procedure in deriving correct transport properties is clarified. Besides, in contrast to the prediction based on linearized effective models, the value of the CME coefficient in the uniform limit shows nontrivial dependence on various model parameters. Even when these parameters are away from the region of the linearized models, such that the concept of chirality may not be appropriate, this effect still exists. This implies that the Berry curvature, rather than the chiral anomaly, provides a better understanding of this effect.   

Magnetic ordering at the topological insulator-magnetic insulator interface

The spin momentum locking property of topological insulators is the key to their practical utility. However, due to large bulk contribution and defects, it has been difficult to harness it. One of the possible solutions is to use heterostructures of topological insulators and magnetic insulators which can make the spin momentum locking robust at the interface. Here we report on the angular dependence of magnetoresistance on heterostructures of the topological insulator BiSbTeSe$_{\mathrm{2}}$ and the magnetic insulator YIG. We find that a four-fold symmetry arises in the in-plane angular dependence spectrum above a critical external field indicating the presence of magnetic ordering at the interface. We demonstrate that the interfacial magnetic ordering arises from a spin polarized interface state which is distinct from the topological surface state.   

Superlattice valley engineering for designer topological insulators

A topological insulator is a novel state of quantum matter, characterized by symmetry-protected Dirac interfacial states within its bulk gap. Tremendous effort has been invested into the search for topological insulators. To date, the discovery of topological insulators has been largely limited to natural crystalline solids. Therefore, it is highly desirable to tailor-make various topological states of matter by design, starting with but a few accessible materials or elements. Here, we establish that valley-dependent dimerization of Dirac surface states can be exploited to induce topological quantum phase transitions, in a binary superlattice bearing symmetry-unrelated interfacial Dirac states. This mechanism leads to a rich phase diagram and allows for rational design of strong topological insulators, weak topological insulators, and topological crystalline insulators. Our ab initio simulations further demonstrate this mechanism in $[111]$ and $[110]$ superlattices of calcium and tin tellurides. While our results reveal a remarkable phase diagram for the binary superlattice, the mechanism is a general route to design various topological states.   

Orbital and spin magnetization induced by electric current in crystals with helical structure

Crystals with helical lattice structure lack inversion and mirror symmetries. In such systems with low symmetry, we expect various physical phenomena which never occur in systems having higher symmetry. In particular, because a helix is similar to a solenoid, we expect that an electric current will induce orbital and spin magnetization. To confirm this scenario, we introduce a simple tight-binding model with helical lattice structure. Using this model, we calculate the orbital and spin magnetization induced by electric field along the helical axis. The resulting orbital magnetization in response to the electric field is along the helical axis. The direction of the induced orbital magnetization is opposite for the right-handed helix and the left-handed one. Furthermore, when the spin-orbit coupling is included, the spin magnetization is also induced along the helical axis as well. This spin magnetization comes from a radial spin texture on the Fermi surface, which is totally different from the Rashba system having tangential spin texture. We also show that by changing the model parameters the model shows characteristic phase transitions into a Weyl semimetal and a weak topological insulator.   

Linked Temperature Evolution of the bulk gap and helical topological surface states in SmB$_{6}$

The paradigm mixedvalent insulator SmB$_{6}$ with a temperature dependent bulk gap has recently become the first paradigm example of a strongly correlated topological insulator with f-d band inversion and with experimental evidences for in-gap surface states and surface transport. In this work temperature- and polarization-dependent angle-resolved photoemission on cleaved \textless 100\textgreater surfaces of SmB$_{6}$ quantifies the T-evolution of (i) the Sm 4f state coherence, (ii) the X-point f-conduction band energy and many-body gap destabilization, and (iii) the intimately connected fate of topologically protected in-gap states. DFT and DFT$+$DMFT calculations confirm early theory [1] that hybridization between boron 2p and Sm 4f states provides crucial assistance in the full opening of the many-body f-d gap. Also a dimensional crossover above 100K from 3D bulk d-band states crossing E$_{\mathrm{F}}$ at high T to low T 2D in-gap surface states is shown to coincide with the development of a circular dichroism signature of in-gap state helicity. \\[4pt] [1] R.M. Martin and J. W. Allen, J. Appl. Phys. 50, 7561 (1979).   

Low temperature magnetoresistance studies in MBE grown topological insulator thin films

We studied low temperature magnetoresistance in molecular beam epitaxy grown topological insulator Bi$_2$Se$_3$ and Bi$_2$Te$_3$ thin films. The surface and structural characterization of the grown films showed smooth epitaxial growth on Si(111). The magnetoresistance has been measured at low temperatures (2 - 20 K) with magnetic fields upto 9 T. The full range perpendicular field magnetoresistance has been explained with the original Hikami-Larkin-Nagaoka theory. Altshuler-Aronov theory of localization has been used to understand the full range parallel field magnetoresistance. Various scattering times have been estimated by fitting the magnetoresistance data with the theory. It is shown that the Zeeman effect is not needed to explain the magnetoresistance and has not been considered in the theory either. The angle dependent anisotropic magnetoresistance has also been observed and explained using the above theories.   

Dirac Semimetal Films as Spin Conductors on Topological Substrates

Spin-momentum locked states, notably those found on the surfaces of topological insulators, are promising for low-power electronic devices based on spin transport. Here we report a much more versatile case involving a Dirac semimetal film on a topological insulator substrate. Such a film can carry highly spin-polarized conduction channels by electronic coupling to the substrate. The spin channel width, defined by the film thickness, is at the designer's disposal, thus permitting optimization of the system parameters. The concept and underlying physics of such quasi-bulk spin channels are confirmed by calculations of a model system involving Bi$_{2}$Se$_{3}$ as the substrate and its low-Z substitute as the overlayer film. The results demonstrate Dirac semimetals as an important class of material for spintronic applications.   

Detection of entanglement by helical Luttinger liquids

A Cooper-pair or electron-hole splitter is a device capable of spatially separating entangled fermionic quasiparticles into mesoscopic solid-state systems such as quantum dots or quantum wires. We theoretically study such a splitter based on a pair of helical Luttinger liquids, which arise naturally at the edges of a quantum spin Hall insulator. Equipping each helical liquid with a beam splitter, current-current cross correlations can be used to construct a Bell inequality whose violation would indicate nonlocal orbital entanglement of the injected electrons and/or holes. Due to the Luttinger-liquid correlations, however, the entanglement is suppressed depending on ambient temperature and voltage bias.   

Hole doping study in antiferromagnetic BaFe$_{2}$Se$_{3}$ and BaMn$_{2}$As$_{2}$

Motivated by the close relationship between antiferromagnetism and superconductivity, we studied hole doping in two antiferromagnetic compounds BaFe$_{2}$Se$_{3}$ and BaMn$_{2}$As$_{2}$. BaFe$_{2}$Se$_{3}$ has a block antiferromagnetic transition around 250 K with a magnetic moment 2.8 $\mu_{\mathrm{B}}$/Fe and BaMn$_{2}$As$_{2}$ exhibits a G-type antiferromagnetism with a large N\'eel temperature $T_{\mathrm{N}}=$ 625 K and a large order moment 3.9 $\mu _{\mathrm{B}}$/Mn. We did the explicit investigations on the Ba$_{0.6}$K$_{0.4}$Fe$_{2}$Se$_{3}$ compound which had anisotropic Heisenberg-like spin glass and variable range hopping conductivity [J. K. Bao et al., J. Phys.: Condens. Matter 26, 026002 (2014)]. As for the semiconducting BaMn$_{2}$As$_{2}$, potassium doping introduces holes into this system and makes it a metal. Moreover, weak ferromagnetic transition appears for the heavily potassium doping [J. K. Bao et al., Phys. Rev. B 85, 144523 (2012)]. However, the origin of weak ferromagnetism in the heavily doped Ba$_{\mathrm{1-x}}$K$_{\mathrm{x}}$Mn$_{2}$As$_{2}$ is still an open question.   

Doping effects of transition metals on superconducting properties of (Ca,\textit{RE})FeAs$_{2}$

At the previous March Meeting, we reported new iron based superconductors (Ca,\textit{RE})FeAs$_{2}$ (Ca112) (\textit{RE} $=$ La-Nd, Sm-Gd)$^{[1,2]}$. Superconducting transition was observed in all samples except for Ce-doped sample, and $T_{c}$ of La-doped sample exceeded 30 K. In this study, we have synthesized transition metals (\textit{TM }=Mn, Co, Ni) co-doped Ca112 samples. Mn co-doping suppressed superconductivity. On the contrary, enhancement of $T_{c}$ with sharp superconducting transitions was observed in most of the Co or Ni co-doped samples. $T_{c}$ of Co co-doped samples decreased with a decrease in ionic radii of \textit{RE}$^{3+}$ from 38 K for \textit{RE} $=$ La to 29 K for \textit{RE} $=$ Gd, though Eu doped sample showed exceptionally low $T_{c} \quad =$ 21 K. $J_{c}$ value of La and Co co-doped sample estimated from magnetization measurement is approximately 2.0 x 10$^{4}$ Acm$^{-2\, }$at 2 K suggesting bulk superconductivity. [1] H. Yakita \textit{et al}., $J$. \textit{Am}. \textit{Chem}. \textit{Soc}. \textbf{136} (2014) 846 [2] H. Yakita \textit{et al}. APS March Meeting 2014 C1 00090   

Magnetic, Transport Properties, Lower Critical Field, Penetration Depth, Anisotropy and Gap Evidences of Ca$_{10}$ (Pt$_{n}$as$_{8})$ (Fe$_{2-X}$Pt$_{x}$As$_{2})_{5}$(n $=$ 3 And 4) Superconductors

Platinum iron arsenides Ca$_{10}$(Fe$_{1-x}$Pt$_{x}$As)$_{10}$(Pt$_{n}$As$_{8})$ ($n~=$ 3 {\&} 4) are first Fe based superconductors with metallic spacer layers. Crystal structure have stacks of Ca (Pt$_{n}$As$_{8})$ Ca (Fe$_{2}$As$_{2})$ consists of superconducting Fe$_{2}$As$_{2}$ layers alternating with Pt$_{n}$As$_{8}$ layers, forming a triclinic \textit{P1}, 1038phase with $n = $ 3 and tetragonal \textit{P4/n}, 1048phase with $n = $ 4. Two different negatively charged layers [(FeAs)$_{10}$]$^{n-}$ and (Pt$_{3+y}$As$_{8})^{m-}$ compete for electrons provided by Ca$^{2+}$-ions. In parent compound Ca$_{10}$(FeAs)$_{10}$(Pt$_{3}$As$_{8})$, no excess charge dopes FeAs-layer, and superconductivity is induced by Pt-substitution. Additional Pt in Pt$_{4}$As$_{8}$ layer shifts charge balance between layers and T$_{c}$ raises to 38 K, but decreases again if additionally Pt is substituted for Fe. Charge doping is supported by T$_{c}$ $\approx $ 30 K in electron-doped La-1038, x $=$ 0:2 (Ca$_{1-x}$La$_{x})_{10}$(Pt$_{3}$As$_{8})$(Fe$_{2}$As$_{2})_{5}$ without significant Pt-substitution. Magnetic properties were explored. Magnetization measurements reveal fish-tail hysteresis loop and relatively high critical current density at low $T$. Lower critical field,$ H_{c1}$~deduced from vortex penetration into single crystals. Ginsburg-Lauder parameters extracted from reversible magnetizations data. Upper critical field determined by resistive transition shows large anisotropy. With La doping, the structural/magnetic phase transitions are suppressed. $T$ dependency of the $H_{c1}$ is compared with BCS-gap models and anisotropy of $H_{c1}$ will be discussed.   

Nematic phase in strain free detwined BaFe2-xNixAs2

Here I present the transport and neutron scattering results in BaFe2-xNixAs2. The crystal is detwined using pressure and then the pressure is released at base temperature before the experiment. In the detwined sample, the anisotropy persist at a temperature higher than the both structural and magnetic transition, but the temperature range is much lower than the anisotropy induced by pressure.   

Disorder induced correlation gap suppresses superconductivity in the 5$d$ metallic perovskite Ba$_{1-x}$La$_{x}$PbO$_{3}$

We report the synthesis and characterization of the electronic structure of thin films of the perovskite Ba$_{1-x}$La$_{x}$PbO$_{3}$ grown by oxide molecular-beam epitaxy. Using angle-resolved photoemission spectroscopy our measurements reveal a Fermi surface consistent with density functional calculations at low doping, but indicate the formation of an energy gap at higher doping values ($x$ $\sim$ 0.2), consistent with electrical transport measurements. By comparison with temperature-dependent point contact tunneling spectroscopy measurements, we show this behavior is consistent with a disorder-driven correlation gap. Moreover the photoemission data reveal a density of states that is not linear at high binding energies, suggesting discrepancy with previous tunneling density of states measurements of superconducting oxides.   

Superconductivity and Eu Valence Instability in Undoped Eu3Bi2S4F4

We recently synthesized a novel bismuth sulfofluoride, EuBiSF$^{2}$,[1] a CDW-like transition occurs at 280 K, below which SC emerges at 0.3 K. The Eu ions show an anomalously mixed valence about $+$2.2. With structural design, we successfully synthesized a new europium bismuth sulfofluoride, Eu$^{3}$Bi$^{2}$S$^{4}$F$^{4}$.[2] The compound crystallizes in a tetragonal lattice (space group I4/mmm, a $=$ 4.0771(1) {\AA}, c $=$ 32.4330(6) {\AA}, and Z $=$ 2), in which CaF$^{2}$-type Eu$^{3}$F$^{4\, }$layers and NaCl-like BiS$^{2}$ bilayers stack alternately along the crystallographic caxis. There are two crystallographically distinct Eu sites, Eu(1) and Eu(2) at the Wyckoff positions 4e and 2a, respectively. Our bond valence sum calculation, based on the refined structural data, indicates that Eu(1) is essentially divalent, while Eu(2) has an average valence of $+$2.64(5). This anomalous Eu valence state is further confirmed and supported, respectively, by M\"{o}ssbauer and magnetization measurements. The Eu3$+$ components donate electrons into the conduction bands that are mainly composed of Bi 6px and 6py states. Consequently, the material itself shows metallic conduction and superconducts at 1.5 K without extrinsic chemical doping. [1] Hui-Fei Zhaiet al.,Phys. Rev. B90, 064518 (2014). [2] Hui-Fei Zhaiet al.. J. Am. Chem. Soc. 2014, 136, 15386 $-$15393.   

Polycrystalline Silicon Thin Films at Low Temperature using SiF$_{4}$ / SiH$_{4}$ mixture

Polycrystalline silicon films with a strong (110) texture were prepared at 400$^{\circ}$C by a plasma-enhanced chemical vapor deposition using different SiF$_{4}$~flow rates ([SiF$_{4}$] $=$ 0--0.5 sccm) under a fixed SiH$_{4}$~flow rate ( [SiH$_{4}$] $=$ 1 or 0.15 sccm). The effects of the addition of SiF$_{4}$ to SiH$_{4}$ on the structural properties of the films were studied by Raman scattering, X-ray diffraction (XRD), Atomic force microscopy and stress measurements. For [SiH$_{4}$] $=$ 1 sccm, the crystallinity and the (110) XRD grain size monotonically increased with increasing [SiF$_{4}$] and their respective maxima reach 90{\%} and 900 {\AA}. However, for [SiH$_{4}$] $=$ 0.15 sccm, both the crystallinity and the grain size decreased with [SiF$_{4}$]. Mechanisms causing the change in crystallinity are discussed, and it was suggested that an improvement in the crystallinity, due to the addition of SiF$_{4}$, is likely to be caused by the effect of a change in the surface morphology of the substrates along with the effect of \textit{in situ} chemical cleaning.   

Electronic conduction in Sr$_{2}$RuO$_{4}$ and Sr$_{2}$RhO$_{4}$ thin films

Transition metal oxides belonging to the Ruddelsden-Popper series, e.g., T${^\prime}$-La$_{2}$CuO$_{4}$, Sr$_{2}$RuO$_{4}$, and Sr$_{2}$RhO$_{4}$, share several geometrical- and associated electronic features. In all cases, squares of transition metal oxide layers are separated by insulating layers, hence, the observed electronic conduction is anisotropic. So far, much attention has been attributed to the metallic conduction in cuprates and ruthenates and metallic conduction in rhodates has been sparsely acknowledged. This is partly due to the absence of superconductivity in the RhO$_{2}$ planes. We show that the metallic conduction in RhO$_{2}$ planes is subject to $d_{[Rh-O]}$ distance which can be tuned by epitaxial strain.   

Investigation of the changes in the density of states in the Copper-Titanium system

The density of states and structure parameters in the copper-titanium system have been investigated by DFT. Various compounds of the copper and titanium (such as CuTi, CuTi2, CuTi3, Cu3Ti) have been studied. The DFT calculations were performed using the GGA exchange-correlation potential. For each compound the atoms were relaxed by minimizing the forces and allowing changes in the unit cell. Geometrical structure and variations in the density of states in the vicinity of the Fermi level have correlated with the near neighbors of copper/titanium.   

A Model Approach to Flux-Pinning Properties of YBCO Vortex States via Non-Superconducting Impurities

Thin film YBCO samples with added non-superconducting nanodot defects of CeOand BaSnOare the focus of recent high-temperature superconductor studies. Examining the structure shows that quantized magnetic flux vortices from within the sample arrange themselves in a self-assembled lattice. The nanodots, with non-superconducting properties, serve to present structural properties to restrict motion of these vorticies under a \textit{pinning-force}and to enhance the critical current density. A formulation of a new model for the system by a variation in the electron pair velocity via the virtual work from the nanodot defects in accordance to the well-known Superconductivity theories is tested. A solution to the expression for the magnetic flux, zero net force and pair velocity will generate a setting for the optimal deposition parameters of number density, growth geometry and mass density of these nanodot structures. With a calculation of pair velocities from a similar work, a comparison is made between experimental and theoretical velocity calculations using growth geometry and chemical potential. This will yield insight into how the current density for a doped high-temperature superconductor will be modified and tuned based on the density of the nanodots themselves.   

Superconductivity enhanced by Se-doping in Eu$_{3}$Bi$_{2}$S$_{4}$F$_{4}$

We investigated the negative chemical pressure effect of Eu$_{3}$Bi$_{2}$S$_{4}$F$_{4}$ by partially substituting S with Se. The ``parent'' compound Eu$_{3}$Bi$_{2}$S$_{4}$F$_{4}$ is a new member of the BiS$_{2}$-based superconductors. [H.F. Zhai et al., J. Am. Chem. Soc. 136, 15386$-$15393, (2014)] It shows anomalous Eu valence and superconductivity of $T_{c}=$1.5 K without chemical doping. With S/Se-doping, we found that a CDW-like anomaly is gradually suppressed to lower temperatures, and meanwhile the superconductivity (SC) is enhanced. For Eu$_{3}$Bi$_{2}$S$_{2}$Se$_{2}$F$_{4}$, $T_{c}$ reaches 3.4 K. Magnetization measurements reveal an average Eu valence of $\sim$ 2.06, which means that Se doping does not introduce extra electrons but instead, lowers down to a low electron doping level of $x $ $\sim$ 0.1. Therefore, the present system manifested itself as a rare example of existence of SC at very low doping levels.   

Characterization of Titanium Nitride thin films Sputtered at Room Temperature

Thin film titanium nitride (TiN) has become widely used in photon detection with microwave kinetic inductance detectors and recently as resonant structures in superconducting quantum information circuits. The attractive properties of the material are its widely tunable critical temperature, large surface inductance, and low losses at microwave frequencies when incorporated into resonant circuits. We report on thin films of TiN sputter-deposited on intrinsic silicon substrates at room temperature for various nitrogen flow rates and deposition pressures. Characterization of the inductance and microwave losses in lumped-element resonators fabricated from these films will also be discussed.   

Anisotropy of superconducting properties of flexible magnesium-diboride-coated carbon nanotube yarns

Flexible ultralight magnesium-diboride-coated carbon nanotube (MgB$_2$-CNT) yarns have critical temperatures up to 37~K, high critical currents and fields comparable with conventional superconducting wires. Superconducting yarns containing MgB$_2$-CNT nanofibers have been prepared by conformal coating of CNT sheets with boron in photothermal chemical vapor deposition, and annealing in magnesium vapors. Electrical transport measurements in a magnetic field, whose direction is varied relative to the sample orientation, showed anisotropy in superconducting properties. The critical field anisotropy ratio $H_{c2}^\parallel{}/H_{c2}^\perp{}$ reaches 1.2-1.4 over a wide temperature range below $T_c$, comparable to but slightly lower than the factor 1.4-2 of epitaxial MgB$_2$ thin films. An X-ray diffraction study confirmed the crystalline anisotropy of composite wires and showed, that the MgB$_2$ grains prefer to grow with the \textit{ab}-plane parallel to the carbon nanotube walls and the yarn axis.   

Plasma and thermal assisted selenization for the preparation of chalcopyrite light-absorbing layers

Chalcopyrite compound has attracted much attention most recently because of their application in high efficient photovoltaic devices. In order to obtain a decent chalcopyrite photovoltaic device, it is very critical to optimize the metallic precursor layers and choose a suitable selenization technique. We demonstrate plasma and thermal assisted selenium cracking methods for preparing Cu(In,Ga)Se$_{2}$ (CIGS) semiconductor films using elemental selenium vapor. The selenization process includes the modification of the ionization state of Se species by radio frequency plasma and/or thermal heating and homogenous control of interactions with CuInGa metallic precursors. We obtained CIGS absorber layers with improved homogeneity and crystallization. The result is explained by the enhancement of reaction kinetics between the reduced Se phase and metallic precursor layers.   

A15 compounds, HTSC and strong coupling superconductors

We suggest that it is possible to understand the origin of high T$_{C}$ in the framework of strong coupling theory of superconductors. An exploratory investigation of materials with promising structural or electronic motifs is presented. The electron-phonon coupling constant $\lambda $ has been calculated for A15 compounds assuming the width of the $\Gamma_{12}$ band as 80 meV. $\lambda $ has been found to be inversely proportional to the molecular weight. The analysis is based on the numerically derived equation for T$_{C}$ from strong coupling theory for superconductivity by McMillan. The Coulomb pseudopotential $\mu^{\ast }$ has been found to be negative for YBa$_{2}$Cu$_{3}$O$_{7}$. The T$_{C}$ of YBa$_{2}$(Cu$_{1-x}$Zn$_{x})_{3}$O$_{7}$ decreases linearly from 89 K to the nonsuperconducting state at a rate of about 15 K/at{\%} of Zn substitution. Similar analysis YBa$_{2}$(Cu$_{1-x}$Zn$_{x})_{3}$O$_{7}$ shows that $\mu^{\ast }$ changes sign from negative to positive as T$_{C}$ reduces. The isotope effect exponent $\alpha $ is found to go through a maximum as T$_{C}$ decreases and is found to be equal to 1/2 for x$=$0.026 which can be verified experimentally.   

Topological Superconductivity in Ferromagnetic Metal Chains: Part II

One most important feature of the Majorana end states observed in the STM experiments is their sharply localized spatial profile. This cannot be explained by a conventional model for 1D topological superconductors, which predicts that Majorana end states wouldn't be observable in a chain much shorter than the superconducting coherence length. In this second talk we resolve this issue by showing the fundamental difference between the na\"{\i}ve 1D model and a proper one, where the 1D-2D/3D hybrid nature of the real experimental structure is taken into account. The strong hybridization between the chain and the higher-dimensional host superconductor introduces long-range (power-law) coupling into the 1D system and significantly modifies the spatial profile of possible Majorana states. As a consequence the superconducting coherence length becomes irrelevant to the decay of the Majorana wavefunctions at a small length scale whereas the Fermi wavelength prevails. We will show concrete examples of eigenstates in a finite-size hybrid system where the Majorana end states are indeed localized within a length scale determined by the Fermi wavelength. This is in good agreement with experimental observations. We will also discuss the implication of this new regime, where the superconducting coherence length is irrelevant by realistic measure, in terms of the coupling energy between Majorana states and the operation time when braiding them.   

Magnetic Anisotropy in DyNi2B2C system

To figure out the magnetic and transport anisotropy in DyNi$_{2}$B$_{2}$C which have superconducting critical temperature T$_{c}$ lower than the antiferromagnetic Neel temperature T$_{N}$ among RNi$_{2}$B$_{2}$C (R$=$ rare earth elements) compounds, we have measured the static magnetization curves M(H,T) with the applied magnetic fields parallel and perpendicular to the crystallographic c-axis at various temperatures and applied magnetic fields. We have observed several magnetic transitions only for the applied magnetic field perpendicular to the c-axis and such magnetic transitions have shift sensitively to the higher temperature regions. We compared our results with the Dy$^{+3}$ magnetic sublattice structure previously reported from neutron scattering experiments   

Anisotropic bias dependent transport property in defective phosphorene Layer

We present the electronic band structure, defect formation energy and bias dependent transport property. Both single and divacancy defects have been considered. We found that the defect formation energy is much less than that in graphene. The defect configuration strongly affects the electronic structure. The band gap vanishes in single vacancy layer, but the band gap reappears in divacancy layers. Interestingly, a single vacancy defect behaves like a p-type impurity for transport property. Unlike the common beliefs, we observe that the vacancy defect can contribute to greatly increasing the current. Along the zigzag direction, both single and divacancy defects contribute to enhancing the current while the I-V characteristics along the armchair direction are dependent on the defect configurations. Despite this defect configuration dependency, we have found that the current along the armchair direction is always greatly larger than that found along the zigzag direction and the anisotropic current ratio of armchair to zigzag direction is an order of 10$^3$. This was supported by NRF (No. 2013R1A1A2006071) and by the Supercomputing Center/Korea Institute of Science and Technology Information with supercomputing resources including technical support (KSC-2014-C3-052)   

Enhanced superconducting properties of Bi2Sr2CaCu2O8$+\delta $ thin films by incorporating Iridates nanoparticles

We incorporated CaIrO$_{3}$ (Ca-iridate) nanoparticles at the interface of Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8+\delta }$ (Bi-2212) thin films and substrates by pulsed laser deposition and post-growth \textit{ex situ} annealing. The density of incorporated Ca-iridate strongly affected the superconducting properties and microstructure of the Bi-2212 thin films. For the incorporation of low density Ca-iridate (450 laser pulses) in the Bi-2212, its superconducting properties enhanced ($T_{c-onset} = $ 97 K, $T_{c-zero} = $ 84 K) over those of pure Bi-2212 ($T_{c-onset} = $ 94 K, $T_{c-zero} = $ 80 K). However, incorporating a higher density (1,800 pulses) significantly reduced $T_{c-zero}$ to $\approx $57 K. Incorporating a low density of Ca-iridate also decreased the $c$-axis lattice constant. Films with incorporated Ca-iridate exhibited greater critical current density, $J_{c}$(B), than the pure Bi-2212 film. These results indicate that incorporating low densities of Ca-iridate nanoparticles into Bi-2212 can improve its superconducting properties.   

Dipolar couplings in Li$_{0.9}$Mo$_{6}$O$_{17}$ purple bronze

We report the study of the internal magnetic field at the atomic scale at the Li site in the quasi-one-dimensional (Q1D) metal Li$_{0.9}$Mo$_{6}$O$_{17}$ (purple bronze), with theoretical calculations based on the structure of its crystal lattice and the result of our $^{7}$Li-NMR measurements on a single crystal sample at an externally applied magnetic field $B_{0}$ = 6 - 12 T. We find that the anisotropic dipolar couplings to the paramagnetic Mo electron spins are the dominant source of the local magnetic field at the lithium site. Other local magnetic field sources such as the dipolar couplings between the $^{7}$Li-$^{7}$Li nuclei, isotropic contact hyperfine couplings to the Mo electron spins, and demagnetization and Lorentz fields are also estimated. Significant changes of the distribution of the dipolar couplings are observed at the ``metal-insulator'' crossover temperature and lower temperatures along the direction of $B_{0}$ $\parallel$ $c$, indicating a significant local magnetic field inhomogeneity due to the spin effect from the Mo electrons. No evidence of charge effect at the ``metal-insulator'' crossover temperature or lower temperatures is observed.   

$^{11}$B NMR Study of HoB$_{4}$

$^{11}$B NMR measurements were performed on a single crystal of HoB$_{4}$ to investigate disorder induced effects on the \textit{4f} spin structures and dynamics. The $^{11}$B NMR spectrum, shift, linewidth, \textit{1/T}$_{1}$, and \textit{1/T}$_{2}$, were measured down to 3.5 K at 8 T perpendicular to the $c$-axis. Above $T_{N} \quad =$ 5.7 K, the $^{11}$B NMR linewidth is very large and the shift is also large and negative. In addition, both depend on temperature strongly and increase at lower temperature, which is similar to the susceptibility. This fact confirms that the hyperfine field at the boron site originates from the \textit{4f} spins of Ho. Below $T_{N}$, the $^{11}$B NMR spectrum shows a single broad shape with an extremely large linewidth. This behavior is an unexpected result compared with usual NMR spectra in an ordered state for a single crystal specimen, where the single broad peak splits into several narrow peaks below $T_{N}$, because of the different local magnetic fields at the each boron sites in the AF state. Considering frustration and disorder effects on the NMR data, we conclude that this behavior originates from the magnetic frustration and quadrupole moment disorder effects on the NMR \textit{static} data. Above $T_{N}$, the both rates are very large and then increase toward $T_{N}$. Below $T_{N}$, the both rates decrease tremendously.   

Casimir Levitation: Stabilization of a neutral atom above a dielectric ring

Levitation, in popular culture, is the phenomenon of an object rising against gravity by supernatural means. However, levitation is not the work of anything supernatural, nor do you need to have attended Hogwarts School of Witchcraft and Wizardry in order to learn how to achieve it. The goal of my project is to propose a method to levitate a neutral, anisotropically polarizable, atom above a dielectric ring using the Casimir effect. In particular, we have already shown that an anisotropically polarizable atom experiences a repulsive force when it approaches the dielectric ring along the symmetry axis of the ring. But, the atom is not stable on this axis. We are working on the proposal that a spinning anisotropically polarizable atom above a dielectric ring will achieve stability, and thus get trapped. Our goal is to prove this very appealing hypothesis analytically.   

RF-sputtered NbN superconducting thin film for the usage as an electrode of graphene FET and a flexible superconducting wire

Recent report on the usage of NbN as an electrode for two-dimensional electronic system such as graphene encourages the study of noble physical phenomena which makes the injection of superconducting charge carriers into the channel of graphene FET inducing the combination of superconductivity with quantum Hall effect. In this study, we examined NbN thin film deposited by rf-sputtering method in various conditions. We checked the effects of deposition temperature, working pressure, and relative flow-rate ratio between argon and nitrogen gases during sputtering. Structural analysis by XRD and SIMS showed that the NbN film was successfully deposited on the silicon-oxide substrate and the highest Tc obtained was 10.5K with high Hc2 over 14T at 5.5K for the film deposited at 600 oC. On the basis of optimal conditions, the change of superconducting properties depending on the deposition temperature was carefully examined for the development of low temperature deposition process that can be applied to the graphene FET fabrication. Additionally we tested the usage of flexible substrate for the deposition of this superconducting material on the purpose of highly flexible superconducting wire with greatly enhanced mechanical properties as reported recently for the MgB2 superconductor.   

Scanning tunneling microscopy study on new layered superconductor Ta4Pd3Te16 single crystal

Ta4Pd3Te16 is a newly discovered layered superconductor with quasi one dimensional crystal structure. Recent measurements show that it may host unconventional superconductivity. Here we report low-temperature scanning tunneling microscopy/spectroscopy study on this single crystal. Chains like atomic structure on cleaved (-103) surface is observed. There exists CDW like modulations with commensurate periods. The tunneling conductance shows an s-wave like superconducting gap. Vortex lattice is observed in magnetic field, but single vortex core is much larger than expected from bulk Hc2. The superconductivity gap is rapidly suppressed by applied field. Our observations can be well understood by multi-band superconductivity.   

Characterisation of the CDW in the novel superconducting family Na2Ti2X2O

There have been a variety of predictions for the origin of the density wave (DW) order that exists in the layered titanium oxy-pnictide superconductors Na2Ti2X2O (X $=$ As, Sb), primarily focusing on a scenario where Fermi surface nesting results in an incommensurate spin-density wave. Here, we show using a combination of muon spin-rotation and synchrotron x-ray diffraction, that the density wave order in these materials is in fact a commensurate charge density wave. We will elaborate on the details of the accompanying structural distortion, and discuss some of the implications of this result in the context of the cuprate and iron pnictide superconductors.   

Possible importance of charge fluctuation in BiS$_2$ superconductors

Studying the pairing mechanism of various layered superconductors may help extracting the essence of the high $T_c$ layered superconductors. Recently, new series of superconductors possessing BiS$_2$ layer has been discovered. Their maximum $T_c$ reaches $10.6$K in LaO$_{1-x}$F$_x$BiS$_2$, and the origin of the pairing glue is under debate. In the STM/STS experiment, ``checkerboard stripe'' charge order has been observed on the surface of NdO$_{0.7}$F$_{0.3}$BiS$_2$, and also the existence of some kind of fluctuation has been suggested in NQR. These observations suggest the importance of charge fluctuations in this system. In this present study, we study the possibility of charge/orbital fluctuation mediated superconductivity in these materials. The bands around the Fermi level consists of Bi 6p and S 3p orbitals. They are more spread than d orbitals and have strong anisotropy, so we consider orbital dependent inter-site interactions between Bi and S besides the intra-site ones. We find that the orbital dependent inter-site interaction enhances $d_{x^2-y^2}$ quadrupole susceptibility and tendency toward nematic orbital order. We discuss the possibility of superconductivity enhanced by this kind of charge/orbital fluctuations.   

Variable frequency characterization of interaction at nanoscale in linear dynamic AFM

Using electrostatic coupling between an AFM tip and a metallic surface as a test interaction, we shall present the measurement of the force between the tip and the surface, together with the measurement of the interaction stiffness and the associated dissipation. These three quantities constitute a full characterization of the interaction at nanoscale. They are measured independently, simultaneously and quantitatively at the same place. This is made possible thanks to a force feedback method that ensures the DC immobility of the tip and to the simultaneous application of a sub-nanometer oscillation to the tip. In this established linear regime, stiffness and damping are directly obtained from amplitude and phase change measurements. We shall demonstrate that this method is not restricted to the lever resonance frequency. To the contrary, this interaction characterization whose can be used at any frequencies with essentially the same performances. We believe that simultaneous and independent measurements of force, stiffness and damping, out of lever resonance, at nanoscale, and within the context of linear response define a new AFM paradigm that we call Force Feedback Microscopy (FFM).   

An ultra-low temperature scanning Hall probe microscope (SHPM) for magnetic imaging below 40 mK

We describe the design of a low temperature scanning Hall probe microscope (SHPM) for a dilution refrigerator system. A detachable SHPM head with 25.4 mm OD and 200 mm length is integrated at the end of the mixing chamber base plate of the dilution refrigerator insert (Oxford Instruments, Kelvinox MX$-$400) by means of a dedicated docking station. It is also possible to use this detachable SHPM head with a variable temperature insert (VTI) for 2 K--300 K operations. A microfabricated 1$\mu$m size Hall sensor (GaAs/AlGaAs) with integrated scanning tunneling microscopy (STM) tip was used for magnetic imaging. The field sensitivity of the Hall sensor was better than 1 mG/$\surd $Hz at 1 kHz bandwidth at 4 K. Both the domain structure and topography of LiHoF4, which is a transverse-field Ising model ferromagnet which orders below TC$=$1.53 K, was imaged simultaneously below 40 mK.   

1fm/$\surd $Hz Noise Level Low Temperature Atomic Force {\&} Magnetic Force Microscope (LT-AFM/MFM) in 20mK-300K Temperature Range

We describe the design of a new low temperature Fabry-Perot interferometer for LT-AFM/MFM operating in 20mK-300K Temperature range. We used a multilayer dielectric mirror coated optical fiber to achieve 1fm/$\surd $Hz Noise Level, while the shot noise limit was 0.16fm/$\surd $Hz. The fibre can be brought very close to the cantilever using a dedicated 2mm stroke piezo nanopositioner integrated in the piezo tube scanner. The same nanopositioner is used to park the fibre to a safe parking location during cantilever exchange. The LT-AFM/MFM can be used between 6 $\mu$W-3mW laser power. We have demonstrated performance of the LT-AFM/MFM by imaging a hard disk sample between 1.5-300K and Abrikosov vortex lattice in BSCCO single crystal at 4K.   

Development of low temperature scanning tunneling microscope and test results at university of ulsan (Korea)

Thanks to the development of software and electronics in the last few decades along with advanced UHV technology, the scanning tunneling microscope (STM) has made a tremendous impact on various fields of surface science. In order to build up an STM system and make it perform as expected, every component of STM needs to serve the others well. Recent progress on the development of low temperature STM at University of Ulsan (Korea) will be discussed. Advantages of our STM system are following: (1) compact design with \textit{in-situ} sample preparation and tip/sample exchange capability, (2) simple and effective vibration isolation damper, (3) copper-stainless steel welding technique for cryostat, (4) simple liquid helium bath pumping setup for tunneling spectroscopy. Results of recent performance test will be discussed as well. *kimjd@ulsan.ac.kr   

The SQCRAMscope: Scanning Quantum CRyogenic Atom Microscope

Microscopy techniques co-opted from nonlinear optics and high energy physics have complemented solid-state probes in elucidating exotic order manifest in condensed matter materials. Up until now, however, no attempts have been made to use modern techniques of ultracold atomic physics to directly explore properties of strongly correlated or topologically protected materials. This poster will present the SQCRAMscope, a novel Scanning Quantum CRyogenic Atom Microscope technique for imaging magnetic and electric fields near cryogenically cooled materials. With our SQCRAMscope, we aim to image inhomogeneous transport and domain percolation in technologically relevant materials whose order has evaded elucidation.   

X-ray Laue Diffraction Microscopy in 3D at the Advanced Photon Source

X-ray Laue Diffraction 3D Microscopy developed at 34-ID beamline in the Advanced Photon Source has been a unique and powerful tool for spatially-resolved structural studies at sub-micron level for materials science. It is applicable to a wide range of microstructure and evolution problems of materials at mesoscale in many diverse fields, including materials engineering, condensed matter physics, and high-pressure geophysics. With advanced focusing mirror optics and depth-resolving technique, focused polychromatic or monochromatic x-ray beams can be used to determine the local phases of crystalline materials, the local crystal orientation and therefore the grain and phase boundary structure, and the local defect distribution including elastic and plastic strains. A description of the technique will be presented with illustrations of highlighted recent applications, and the ongoing upgrade plan of pushing microdiffraction techniques towards nanodiffraction with significant improvements in smaller beam sizes and higher focusing flux, based on the new near diffraction-limited storage rings x-ray source. [1] W. Liu and G. Ice, ``X-ray Laue Diffraction Microscopy in 3D at the Advanced Photon Source," in Strain and Dislocation Gradients from Diffraction, Imperial College Press, 2014, pp. 53-81. [2] F. Hofmann, et al, ``X-ray micro-beam characterization of lattice rotations and distortions due to an individual dislocation,'' Nature Communications, 4, 2774-1-2774-7 (2013). [3] L. Zhang, et al, ``Disproportionation of (Mg,Fe)SiO3 perovskite in Earth's deep lower mantle,'' Science 344, 877-882 (2014).   

Biological Cell Manipulation by Magnetic Nanoparticles

We report experimental data on biological cells (erythrocytes) manipulation by magnetite (Fe3O4) nanoparticles. The experiments were accomplished on the top of the device consisting of two conducting contours. An electric current flowing through the contours generates a non-uniform magnetic field making magnetic nanoparticles to move towards the magnetic energy minima. In turn, magnetic nanoparticles drag biological cells in the same direction. We present experimental data showing cell manipulation by controlling the electric current. This technique allows us to capture and move cells located in the vicinity (5-10 microns) of the current-carrying wires. One of the most interesting results shows a periodic motion of erythrocytes, which frequency is controlled by the electric circuit. The obtained results demonstrate the feasibility of non-destructive cell manipulation by magnetic nanoparticles with micrometer-scale precision.   

The Extended Core Coax: A Novel Nanoarchitecture for Electrochemical Sensing of Infectious Disease Biomarkers

We report the development and fabrication of a novel nanoarchitecture for electrochemical sensing, the extended core coax (ECC). Each ECC is a vertically oriented nanocoax, comprised of an extended inner metal core and an outer metal shield, separated by a dielectric annulus. The inner (gold) and outer (chrome) metals serve as the working and counter electrodes, respectively, with $\sim$200 nm separation gap / annulus. Arrays with a base area of 0.1 mm$^{2}$ were fabricated, each containing $\sim$10$^{5}$ individual ECCs connected in parallel. Previous iterations of the nanocoax have demonstrated $\sim$100x greater electrochemical response over a planar control due to the nanoscale proximity of the working and counter electrodes [1]. We anticipate the ECC will function similarly, and offer the additional benefit of overcoming diffusion limitations due to the extended core working electrode, which protrudes $\sim$200 nm above the shield of the ECC. Additionally, the extended gold core provides a substrate for biofunctionalization, making the ECC an attractive candidate for further development towards electrochemical detection of infectious disease biomarkers such as cholera toxin. \\[4pt] [1] B. Rizal, et al., Anal. Chem. 85, 10040 (2013).   

Fabrication and characterization of MoS$_{2}$ chemiresistor for pH sensing

Chemical and biological sensing is critical in medical diagnosis, environmental monitoring, etc. As a semiconductor with a bandgap energy and naturally 2D layered atomically thin structure, molybdenum disulfide (MoS$_{2})$ has potential to provide ultrahigh sensitivity and its planar nature is preferred for monolithic integration. In this paper, we report the fabrication and characterization of a simple gate-free MoS$_{2}$ sensor device configured as a planar chemiresistor for pH sensing. MoS$_{2}$ crystals from monolayer to multiple layers were prepared by widely used mechanical exfoliation technique, and transferred onto a Si chip with a 300 nm SiO$_{2}$ layer. A mask-free fabrication process was applied to manufacture the chemiresistors with contact electrodes. With a constant bias voltage, the real-time currents following through the chemiresistors were recorded when drops of pH buffer solution in the pH range from 3 to 10 were placed on the crystal surface. The currents increase with a response time less than 2 seconds. The resistances decrease linearly with the increase of pH values, with a high sensitivity ($\Delta \Omega $/pH) derived. The repeatability, hysteresis and long-term stability of the chemiresistors were also investigated. The simple mask-free fabrication, fast response time, high sensitivity and other properties altogether prove that MoS$_{2}$ is a very promising sensor material.   

Adaptable Quantum Dot Nanomaterials for IR Sensing

IR nanomaterials with the effective control of photoelectron processes will strongly enhance sensing technologies for security, driving, navigation and other applications. Development and implementation of sensors with adaptable parameters would provide optimal use of sensing resources. Voltage-tunable, three-dimensional nanoscale profile created by charged quantum dots provides an effective tool to manage nanoscale processes. We experimentally investigate the effects of selective bipolar doping of quantum dot media on the dark current, noise current, photoresponse, and photoelectron lifetime. We also study the redistribution of the built-in dot charge under the voltage bias and tunability of the above characteristics. The preliminary results show a strong effect of nanoscale barriers on noise and photoresponse characteristics of IR nanomaterials.   

High performance multilayer surface plasmon sensors

Though high performance SPR sensors are readily available, it remains desirable to fabricate sensors with enhanced sensitivity, resolution and evanescent fields for numerous applications. Since SPR characteristics of bimetallic (Ag/Au) and bimetallic waveguide coupled (Bi-WC) sensors are known to be better than those of single metal sensors,\footnote{K. S. Lee, T. S. Lee, I. Kim, and W. M. Kim, J Phys D Appl Phys 46 (12) (2013).} we have undertaken investigations of the performance of multilayer structures.\footnote{K. Tiwari, S. C. Sharma and N. Hozhabri, submitted (2014)} We employ the transfer matrix method (TMM) for calculating SPR characteristics of such structures as functions of Ag/Au and Si$_{3}$N$_{4}$ waveguide thickness. Several quartz/Ag/Si$_{3}$N$_{4}$/Au structures were deposited in a \textit{class-100} clean room facility. The thicknesses of Ag and Au were fixed at 35 and 28 nm respectively. However, the thickness of the intermediate Si$_{3}$N$_{4}$ waveguide layer was varied from 50 - 150 nm. The SPR curves were measured for all these structures by using the Kretschmann configuration system. We observe excellent agreement between the experimental SPR data and computational results. For an optimized 150 nm thickness of Si$_{3}$N$_{4}$ waveguide, we observe high sensitivity to changes in the refractive index (S$_{\mathrm{n}}\approx $ 52$^{0}$/RIU), extremely narrow SPR curves (FWHM$\le $0.28, yielding high figure-of-merit of 60-180) and increased decay length of evanescent fields ($\delta \approx $ 258 nm).   

Study of parameters for designing Barkhausen noise sensing elements using finite element analysis

Barkhausen noise emissions occur in ferromagnetic materials on application of externally varying magnetic field. These emissions primarily occur due to the presence of pinning sites or discontinuities within the material which act as inhibitors to domain wall motion. The emissions can be sensed using an induction coil placed above the sample. This coil senses the variations in magnetic flux which translates to the induced emf. In this study, we optimize the design of the sensing coil via finite element simulations. The selection of optimum number of turns, choice of sensor core material and arrangement will be discussed. The approach to optimization of the sense-coil design will be presented.   

Design details of a new positron beam system for materials studies

We report here the current status of the development of a state of the art high flux variable energy spin-polarized positron beam facility. Monoenergetic positrons are obtained using high efficiency rare gas moderator (RGM-1). This will allow us to collect data 100 times faster than our current beam system. This beam line will include a 2 meters Time-Of-Flight (TOF) tube, which will result in a higher energy resolution for the TOF spectrometer. The design of the beam line also allows for ambient pressure two gamma coincidence measurements. The ultra-high vacuum system for the beam line has been constructed and was tested for a vacuum of $\sim$ 10$^{-8}$ mbar. The magnetic field for the positron transport has been achieved using a combination of Helmholtz and a series of short solenoid coils, and the magnetic field is $\sim$ 40-100 gauss along the beam line. The ExB positron energy filter and the transport magnetic field were successfully tested using an electron beam. We will discuss the installation of the RGM-1 and the 2 meters TOF spectrometer to the beam line and the final beam tuning in conjunction with the SIMION simulation.   

Simulation of the positron and electron trajectories for a new Time of flight (TOF) Spectrometer for positron annihilation induced Auger electron spectroscopy (PAES) with high flux positron beam

A new high flux positron beam line is under construction for TOF-PAES as well as for spin polarized coincidence Doppler broadening spectroscopy for surface characterization in the University of Texas at Arlington (UTA). This beam line has a high efficiency rare gas moderator system and employs a combination of axial and transverse magnetic fields for the selection of positron beam energy. The moderator system feeds the mono-energetic positron beam into the TOF-PAES system where transverse electromagnetic fields (trochoidal analyzer) allow the simultaneous passage of the positron beam and the electrons emitted from the sample. Here we describe the characteristics of the positron beam trajectories from the source to the target through this beam line using SIMION. These simulations have been used to optimize the axial and transverse magnetic field values at the energy selector as well the electrostatic potentials at the trochoidal energy analyzer. The trajectories of the secondary electrons ejected from the sample as a result of the positron beam interaction are also described for various ejection angles and energies. These later simulations have been utilized to optimize the height of the channel plate used for the detection of electrons.   

Anomalous scattering and redirection of sound in narrow liquid channels

Propagation of sonic waves through a finite-length channel clad between two identical liquid-immersed metal plates with accounting for excitation of coupled surface Rayleigh waves propagating near metal-liquid interfaces is studied. The transmission coefficient is calculated for the wide range of frequencies of the incident sound wave, $f =$ 0.2 $\div$ 1.4 MHz. At discrete frequencies the transmission and reflection is anomalously suppressed that is shown to be accompanied by unusual redirection of sound from the liquid into metal at the edges of the channel. Proposed theory is in excellent agreement with experimental data obtained for water channels formed by Al and Cu plates.   

Development and Characterization of Dynamic Light Scattering Instrumentation to Determine Nanoparticle Size

Dynamic Light Scattering (DLS) provides a high-throughput and accurate measurement of particle sizes for monodisperse (MD) spherical nanoparticles (NPs). We report on the development and characterization of homebuilt DLS instrumentation to measure the size of MD NPs of gold, polystyrene, and ZnO. HeNe and Ar-ion lasers comprise the excitation sources for the scattering experiment. An avalanche photodiode detects scattered light and an autocorrelation card analyzes the signal to provide a measurement of the translational diffusion coefficient, which for MD and spherical particles allows for the determination of NP radius. We have tested our apparatus using commercially produced gold NPs in the range of 10 nm to 200 nm and synthesized ZnO NPs. DLS measurements were compared to those obtained by Atomic Force Microscopy (AFM). After size characterization, the ZnO NPs will be employed in ongoing toxicity studies.   

Super-resolution imaging using fluorescent soft micro-lens

Spatial resolution of conventional optical microscope is limited by the diffraction of roughly half the wavelength of the incident light. Among strategies of obtaining resolution beyond the diffraction limit, near-field scanning optical microscopy (NSOM) is widely used. In previous work, we performed NSOM using a simple design constituted by attaching a glass micro-lens (MLs) or a liquid MLs on a cantilever. However, NSOM achieves super-high resolution sacrificing its mobility and imaging speed comparing with far-field imaging, especially when the specimen has uneven surfaces. In this work, we showed that a polydimethylsiloxane (PDMS) micron-sized sphere can be used as MLs as well. Images having enhanced contrast resolutions were achieved when the PDMS MLs was mechanically deformed along z-axis. On the other hand, the focal length of PDMS MLs can be tuned when being deformed by the pressure along x-axis. The scanning mobility of the whole device was further improved when attaching PDMS MLs onto a flexible cantilever. We also introduced different fluorophores into PDMS spheres, which resulting fluorescent MLs (FMLs). The advantages of FMLs involve the feasibility of locating MLs during a fluorescent imaging while having tunable focal length.   

Compact scanning probe microscope with 3-D positioning capabilities

We developed a new design for scanning probe microscopes (SPM) intended for low temperature operation. The main design philosophy is to make the SPM body compact and rigid, with an outer diameter of one inch. A secondary goal is to make this instrument easier to build, use and repair compared to our previous designs. While all the positioners are based on the stick-slip principle, the motion along the three axes is implemented very differently: motion along Z, or tip-sample approach, is accomplished by two vertical rods running along the length of the body. Motion along X is done by sliding on a single rod, and along Y by sliding the sample stage on top of the tube scanner used for generating images. Initial test results will be presented.   

Photon Counting with an Embedded Micro Mojo V3 FPGA

We used an inexpensive Field Programmable Gate Array (FPGA) to generate time stamps from a photon counting experiment. The FPGA was configured to receive signals from an avalanche photodiode, latch onto a 32 bit, 400 MHz clock/counter, and transmit the time stamps to a host computer. These time stamps were then analyzed on the host machine in real time to calculate the intensity auto-correlation function of the signal source, in our case a fluorescence correlation spectroscopy experiment. The basic state machines for the system are a clock/counter/trigger, FIFO data buffer, and serial I/O to an onboard processor that handles communication with the host. The trigger state machine is similar to a clock except the cycle is based on the signal positive edge. The trigger cycle stores the counter and initiates data transfer to the FIFO buffer.   

Construction for Cryogen free 3He/4He dilution refrigerator integrated with conduction cooled 15T magnet

We constructed a cryogen-free 3He/4He dilution refrigerator (DR) integrated with a conduction cooled 15T superconducting magnet. The integrated magnet and 3He/4He dilution system is precooled by a commercial two stage pulse tube refrigerator (PTR). 3He/4He mixture gas compressed at 4 Bar is first introduced into the heat exchangers mounted on the first (40K) and the second (2.5K) stage of PTR. The mixture is condensed at the second stage without Joule-Thomson stage due to its high pressure. Once the liquid 3He/4He mixture is obtained, a conventional DR design including a still, counterflow heat exchangers, and a mixing chamber is adopted for the continuous operation. The 15T superconducting magnet is directly connected to the second stage and cooled by conduction cooling down to about 3K after being pre-cooled with liquid N2 flow. The current leads for superconducting magnet up to 120A require careful considerations of low thermal-conductance with high electrical-conductance and robust electrical isolation.   

The power of three-dimensional imaging for an unambiguous identification of the ro-vibrational state of H$_{2}^{+}$, D$_{2}^{+}$, and HD$^{+}$

We are presenting a three-dimensional imaging technique that could efficiently measure the ro-vibrational states of small diatomic molecular ions such as H$_{2}^{+}$ in two steps. First, the molecular ion is sent toward a jet of alkali atoms to undergo a resonant dissociative charge exchange. Then, the positions of the fragments and their flight time difference are measured with two position sensitive detectors. From these measurements, we obtained the value of the kinetic energy release, which is directly related to the original vibrational excitation of H$_{2}^{+}$. This technique scheme was first developed by D. P. de Bruijn and J. Los (Rev. Sci. Intstrum. 53, 1020, 1982). Details and examples will be presented.   

The speed of light is the inflection point matter and dark matte

The gravitational field of the object is due to radiation and rotation. The reason of the objects can radiation is that linear velocity of objects rotation is less than the speed of light. When the linear velocity of object rotation is equal to the speed of light, the object won't radiate any more, radiate particle(electromagnetic wave particle) rotate by the radius of itself radius, now the state is the inflection point of the matter and dark matter. at this time, the object's gravitational field radius equal to the radius of the object, and then continue to change that is dark matter, the velocity of dark matter rotation is faster than the light. Dark matter radiation doesn't radiate. The gravitational field radius is equal to the radius of the dark matter, that is why dark matter is difficult to be observed also detected. The curvature of the radiation--the the linear velocity of the object divide the speed of the light. The rotation speed of the object--the speed of light multiply the linear velocity and then divide the radius of the object.   

Examination of Humidity Effects on Measured Thickness and Interfacial Phenomena of Exfoliated Graphene on SiO$_{2}$ via AC-AFM

Tapping (AC) mode Atomic Force Microscopy (AFM) is commonly used to determine the thickness of graphene samples. However, AFM measurements have been shown to be sensitive to environmental conditions such as adsorbed water, in turn dependent on relative humidity (RH). In the present study, AC-AFM is used to measure the thickness and loss tangent of exfoliated graphene on silicon dioxide (SiO$_{2})$ as RH is increased from 10{\%} to 80{\%}. We show that the measured thickness of graphene is dependent on RH. Loss tangent is an AFM imaging technique that interprets the phase information as a relationship between the stored and dissipated energy in the tip-sample interaction. This study demonstrates the loss tangent of the graphene and oxide regions are both affected by humidity, with generally higher loss tangent for graphene than SiO$_{2}$. As RH increases, we observe the loss tangent of both materials approaches the same value. We hypothesize that there is a layer of water trapped between the graphene and SiO$_{2}$ substrate to explain this observation. Using this interpretation, the loss tangent images also indicate movement and change in this trapped water layer as RH increases, which impacts the measured thickness of graphene using AC-AFM.   

Long-range interaction of anisotropic systems

The first-order electrostatic interaction energy between two far-apart anisotropic atoms depends not only on the distance between them but also on their relative orientation, according to Rayleigh Schr\"{o}dinger perturbation theory. Using the first-order interaction energy and the continuum model, we study the long-range interaction between a pair of parallel graphene sheets. The asymptotic form of the obtained potential density, $ \varepsilon(D) \propto -D^{-3}-O(D^{-4})$, is consistent with the random phase approximation and Lifshitz theory. Accordingly, neglectance of the anisotropy, especially the nonzero first-order interaction energy, is the reason that the widely used Lennard-Jones potential approach and dispersion-corrections in density functional theory give a wrong asymptotic form, $ \varepsilon(D) \propto -D^{-4}$.   

Moments and Lanczos Study of the Anisotropic One Dimensional $XY$ Model in a Skewed Magnetic Field

Here we wish to study the ground-state and energy gap of the one dimensional spin $\frac{1}{2}$ anisotropic antiferromagnetic $XY$ Heisenberg model given by \begin{equation} H=\sum_{l=1}^{N}\left[ \left( 1+\gamma\right) S_{l}^{x}S_{l+1}^{x}+\left( 1-\gamma\right) S^{y}_{l}S_{l+1}^{y}-h\left( -1\right) ^{l}S_{l}^{z}\right] \end{equation} where $\gamma$ is the anisotropy parameter and $h$ is an external magnetic field. We shall investigate the ground-state energy as well as the energy gap as a function of both the anisotropy parameter as well as the number of holes as a function of the external magnetic field.   

Moments and Lancszos Study of the Anisotropic One Dimensional $t-J$ Model with Holes

In this work we wish to study the ground-state energy as well as the energy gap of the one-dimensional $t-J$ model \[ H=\sum_{l}^{N}\left[ S_{l+1}^{z}S_{i}+\frac{1}{2}\alpha\left( S_{l+1}% ^{+}S_{l}^{-}+S_{l+1}^{-}S_{l}^{+}\right) \right] \] where $\alpha$ is the anisotropy parameter. We will investigate both the ground-state energy as well as the energy gap as the number of holes is increased for lattices of length 16 sites.   

A Variational Moments Approach to the One Dimensional Hubbard Model

In this work we shall study the one dimensional Hubbard model% \[ H=t\sum_{,\sigma}\left( c_{i\sigma}^{\dag}c_{j\sigma}+c_{j\sigma}^{\dag }c_{i\sigma}\right) +U\sum_{i}n_{i\uparrow}n_{i\downarrow}% \] using both a connected moments approach as well as a Lanczos tridiagonal scheme. Following the work of Eichenberger and Baeriswyl (PRB 76, 180504(R), 2007) we use a modified variational wavefunction which includes the hopping term of the Hamiltonian. Our results show a marked improvement in our estimation of the ground-state energy in the region of intermediate coupling $t/U\approx0.1$.   

Quantum Monte Carlo Applied to Binary Transition Metal-Oxides

Materials based on transition metal-oxides (TMO) play a central role in many applications and in the fundamental research of advanced materials. However, this class of materials is one of the most challenging for computation. The standard computational methods to study them are based on Density Functional Theory (DFT), which often fails to provide the required level of accuracy. A natural solution to overcome the intrinsic limitations of DFT approximations is to directly solve the many-body problem in TMO. For large systems, this can be made practical by applying quantum Monte Carlo (QMC) methods. These methods are very expensive computationally, but recent developments in algorithms and computational infrastructures have enabled their application to real materials. We will show that QMC methods, such as diffusion Monte Carlo (DMC), are now practical to study multiple properties of TMO. The application of DMC to study the structural, electronic and ionic defect properties of various binary TMO, including FeO, CoO, NiO, and ZnO will be discussed. We will also outline current limitations in hardware and algorithms.   

A GPU enhanced approach to identify atomic vacancies in solids materials

Identification of vacancies in atomic structures plays a crucial role in the characterization of a material, from structural to dynamical properties. In this work we introduce a computationally improved vacancy recognition technique, based in a previous developed algorithm. The procedure is based in the use of Graphics Processing Unit (GPU) instead of Central Processing Unit (CPU), taking advantage of random number generation as well the use of a large amount of simultaneous threads as available in GPU architecture, improving the spatial mapping in the sample and the speed during the identification process of atomic vacancies. The results show that with this technique, efficiency is improved. Along with the above a reduction of required parameters in comparison with the original algorithm is presented. We show that only the lattice constant and a tunable overlap are enough as input parameters in the process, and are also highly related. A study of those parameters is presented, suggesting how the parameter choice must be addressed. Benchmarks were made using one standard CPU and GPU between the original code and the present work, revealing an improvement in the execution time.   

A Time Parallel Implementation of the Time Decomposition Strategy for the Dirac equation

For certain formulations of partial differential equations, proper time-parallel pre conditioners can be successfully applied in space-time finite element simulations. Such an approach may enable the extraction of more parallelism to better utilize high performance computing resources. In this work, we examine the behavior of the gauge free, low-mass regime Dirac equation using space-time finite elements. The purpose of this research is to present a stable parallel implementation algorithm of the physical system. We discretize space and time together for the entire domain using a finite element space which does not separate time and space basis functions. We also explore the effectiveness of the time decomposition preconditioner, additive Schwarz preconditioner with KSP (Krylov Subspace Methods) solvers for this problem. We show that proper time parallel implementation allows for physically intuitive boundary conditions, improvement of numerical efficiency, and reduces the overall error of the computed solution   

A Multiscale computational approach to predict the elastic deformation fields in Moir\'{e} Patterns of 2D van der Waals interfaces and heterostructures

Recent technological advancements in isolation and transfer of different 2-dimensional (2D) materials have led to renewed interest in Van der Waals (vdW) heterostructures. We report a multiscale computational method to predict the deformation of vdW heterostructures using density functional theory (DFT) informed continuum simulations. We validate our method by comparing its predictions with all atom atomistic simulations for the graphene-hBN bilayer system and computing the in-plane strains, local curvature for different misorientation angles between two lattices. We also present closed form solutions for the elastic field as a function of lattice mismatch, relative rotations and predict the deformation fields for MoS2-WS2, MoSe2-WSe2 systems that have been recently synthesized experimentally.   

Atomistic Computational Model of Radiation Damage of Nano-sized Systems in Intense X-ray Pulses

We present a combined Monte-Carlo/molecular- dynamics (MC/MD) computational model that is suitable for monitoring the physics of intense, femtosecond XFEL pulses interacting with complex systems of various sizes, from nanometers to micrometers, and matters of various compositions. In this model, the occurrences of x-ray absorption, ionization, relaxation and electron-impact processes are treated by a MC method, and the subsequent dynamics of the all the electrons, ions and atoms are tracked using an MD method. Our model extends previous MC/MD model and provides new capabilities to probe the impacts of transient states on radiation damage dynamics. Recently, we have added LAMMPS as the driver of MD dynamics. This is a critical addition as now our code can run on Mira, a new petascale supercomputer with 786K core processors at the Argonne Leadership Computing Facility. Also, it can treat micron-sized systems with trillions of particles and both homogeneous and heterogeneous composition. Using our model, we examine the ionization dynamics of Argon clusters in an XFEL pulse as a function of particle sizes and pulse parameters, and we compare our results with the experimental data [S. Schorb \textit{et al}. PRL \textbf{108}, 233401 (2012)].   

Easy GROMACS: A Graphical User Interface for GROMACS Molecular Dynamics Simulation Package

GROMACS is a widely used molecular dynamics simulation package. Since it is a command driven program, it is difficult to use this program for molecular biologists, biochemists, new graduate students and undergraduate researchers who are interested in molecular dynamics simulations. To alleviate the problem for those researchers, we wrote a graphical user interface that simplifies protein preparation for a classical molecular dynamics simulation. Our program can work with various GROMACS versions and it can perform essential analyses of GROMACS trajectories as well as protein preparation. We named our open source program `\textit{Easy GROMACS}'. \textit{Easy GROMACS} can give researchers more time for scientific research instead of dealing with technical intricacies.   

Calculating Relativistic Transition Matrix Elements for Hydrogenic Atoms Using Monte Carlo Methods

The nonrelativistic transition matrix elements for hydrogen atoms can be computed exactly and these expressions are given in a number of classic textbooks. The relativistic counterparts of these equations can also be computed exactly but these expressions have been described in only a few places in the literature. In part, this is because the relativistic equations lack the elegant simplicity of the nonrelativistic equations. In this poster I will describe how variational Monte Carlo methods can be used to calculate the energy and properties of relativistic hydrogen atoms and how the wavefunctions for these systems can be used to calculate transition matrix elements.   

Automated discovery of novel solid-state ionic conductors enabled by large-scale molecular dynamics computations

Fast solid-state inorganic Li-ion conductors offer a path toward safer batteries with high energy density, but apart from a few material classes, the inorganic solid-state space remains mostly unexplored. Computational approaches using density functional theory (DFT) have been proven to be successful for the design of electrode materials, but have had few applications for the discovery of electrolytes. This is because the physiochemical factors that regulate ionic conductivity are poorly understood, and conductivity can be very sensitive to small structural and compositional variations. In this work, we present relationships between the ionic conductivity and several potential structural descriptors, like the size and dimensionality of ion-conducting pathways, void fraction, Li-concentration, sensitivity to volume change, etc. We identify these relationships from massive ab-intio molecular dynamics simulations on a comprehensive dataset of approximately 1500 crystalline materials. Our investigation is enabled by computational resources at the Oak Ridge Leadership Computing Facility and the high-throughput automation platform AiiDA.   

First Principles Study of Phosphor Host Materials

Solid state lighting uses light-emitting diodes (LEDs) instead of electrical filaments. Blue LEDs are combined with an inorganic phosphor consisting of a host lattice doped with Ce$^{3+}$. Electronic transitions between the 4f and 5d levels convert blue light from the diode to light ranging from green ($\sim$510 nm) to red ($\sim$700 nm) and the combination of yellow with blue creates white light. The most efficient phosphors have been found to have rigid crystal structures, along with band gaps large enough that the 4f to 5d transition of the dopant ion can occur without interference. Using Debye temperature as a proxy for structural rigidity, we use density functional theory (DFT) to calculate and plot $\Theta_D$ versus band gap to give us a prediction of phosphor suitability for forty different materials. PBE functionals and the quasi-harmonic Debye model were used to estimate $\Theta_D$, and band gaps were calculated with a hybrid functional. DFT was also used to probe six potential hosts more closely: AlO$_3$, LaBr$_3$, Ba$_2$SiO$_4$, Sr$_2$SiO$_4$, Sr$_2$Si$_5$N$_8$, and Y$_3$Al$_5$O$_{12}$. The electronic states of these compounds were aligned with vacuum using a supercell slab model and will be compared with the Ce$^{3+}$ levels to determine suitability as host materials.   

Transport properties of carbon dioxide and ammonia in water - ethylene glycol mixtures from molecular dynamics simulations

The endothermic decomposition of ammonium carbamate has been proposed as a novel heat sink mechanism for aircraft thermal management (Johnson {\it et al}. SAE Technical Paper 2012-01-2190, 2012, doi:10.4271/2012-01-2190]). The products of this decomposition are carbon dioxide and ammonia which need to be efficiently removed in order to better control the decomposition reaction. Molecular dynamics simulations can provide insight into the transport properties of carbon dioxide and ammonia in the carrier fluid. In this work, an extensive set of molecular dynamics simulations was performed to better quantify the concentration dependence of solubility and diffusivity of carbon dioxide and ammonia in water, ethylene glycol, and their mixtures at standard temperature and pressure and at elevated temperature. The simulation results confirm the experimental observations that ammonia is more soluble than carbon dioxide in either water or ethylene glycol and that both carbon dioxide and ammonia are more soluble in ethylene glycol than in water. The simulations of water - ethylene glycol mixtures show that increasing the molar fraction of ethylene glycol leads to increased solubility of carbon dioxide and ammonia in the mixture.   

A Simple Pythagorean Interpretation of E$^{2} = $ p$^{2}$c$^{2} +$ (mc$^{2})^{2}$

We are considering the relationship between the relativistic energy, the momentum, and the rest energy, $E^{2}=p^{2}c^{2} + $\textit{ (mc}$^{2})^{2},$ and using geometrical means to analyze each individual portion in a spatial setting. The aforementioned equation suggests that \textit{pc} and \textit{mc}$^{2}$ could be thought of as the two axis of a plane. According to de Broglie's hypothesis $\lambda =h/p$ therefore suggesting that the \textit{pc}-axis is connected to the wave properties of a moving object, and subsequently, the \textit{mc}$^{2}$-axis is connected to the particle properties. Consequently, these two axis could represent the particle and wave properties of the moving object. An overview of possible models and meaningful interpretations will be presented.   

A Testable Unified Theory That Works

The harmonic neutron hypothesis is a unified theory of a dimensionally consistent harmonic point set space defining physical phenomena. It is based on equality-pair transformations (EPTs), of the n$^{\mathrm{0}}$; e; $\alpha _{\mathrm{0}}$; and the Rydberg constant, R, and 3 finite integer sets: (V$_{\mathrm{f}})$, defined below; the first 12 natural numbers to derive the first generation of particles and bosons; and a finite set of primes for higher generations. All of the derivations/ predictions are made using the natural units and the 3 number sets. The purpose is to demonstrate that it is possible to derive, sets of integers, which inter-relate and predict many of the physical constants from Planck time to the Higgs boson starting with just these 4 sets within a harmonic system. All the physical constants are evaluated as frequency equivalent ratios. The fundamental EPT is based on the transformation of electromagnetic energy into matter via the set (V$_{\mathrm{f}})$, scaled from neutron pair production. Elements v$_{\mathrm{f}}$ in (V$_{\mathrm{f}})$ are based on the ratio of the annihilation frequency equivalent of the neutron and 1 Hz, 2.271859078 x 10$^{\mathrm{23}}$ Hz.   

Several notes on physics

Several notes on physics are presented, which include our views of quantum mechanics, general relativity, special relativity, electric charge, nonlinear electrodynamic, general transformation in hyperspace, solutions to equations of operators, superstring, shapes of particles and statistical mechanics.   

A direct measurement of built-in potential across LaAlO3/SrTiO3(001) heterojunctions

The quasi-two-dimensional electron gas (q2DEG) at the oxide interface LaAlO3/SrTiO3 has attracted a lot of attention in recent years due to its rich phenomena. The ?polar catastrophe? model is one of the conducting mechanisms, which relies on the polar potential built-in LaAlO3 layer. Although several experiments have been tried to measure the electrostatic potential in LaAlO3, the magnitude of the polar potential is still under debate.[1][2][3] We present a systematically study on the electronic transport properties of the LaAlO3(5 unit cells)/SrTiO3 interfaces capping with several different metals, whose work functions vary from 4.28 to 5.6 eV. The barrier height between the capping layer and the q2DEG estimated by the tunneling resistance shows strong correlation with the metal work function. The carrier density of q2DEG increases as decreasing the capping metal work function. These results strongly suggest the existence of the built-in potential in LaAlO3 and the residual polar potential in five-unit-cell LaAlO3 is estimated to be 1.3 eV. [1]E. Slooten, et al, Phys. Rev. B 87, 085128 (2013). [2]H. Liang, et al, Sci. Rep. 3, 1975 (2013). [3]G. Berner, et al, Phys. Rev. B 88, 115111 (2013).   

Surface structures and defect control during epitaxy of crystal

In this talk, I will show a few examples in which the surface structures play important roles on the defect incorporation into the crystal lattice. The surface orientations and the atomic structures have strong influence on the solubility, the type, and even the direction of the incorporated defects.   

First-principles studies of S-doping and adsorption with hematite $\alpha $-Fe$_{2}$O$_{3}$ (0001) film

Based on spin-polarized density functional theory, we have investigated the atomic, electronic, and magnetic structures of hematite $\alpha $-Fe$_{2}$O$_{3}$ (0001) film. An S atom adsorption on the surface of the film has then been studied. The preferred site on the surface has been identified. The changes of the electronic structure of the film have been analyzed when an O atom is substituted by an S atom at different locations inside the film. The change with the concentration of S-doping will also be discussed.   

Revisitation of the frictional properties of SiO2 as the LFM (lateral force microscopy) reference

Recently, experimental studies concerning frictional properties at the nanoscale using AFM(atomic force microscopy), specifically with LFM, are made on various kinds of materials including noble 2D graphene sheets and 1D nanotubes. The LFM technique requires calibration assuredly and therefore choosing a stable substrate as a reference is of importance. SiO2 is often used as the standard to calibrate LFM data obtained from a material of interest. However, according to our observation, the friction of cleansed SiO2 substrate can change gradually by long-time continuous LFM scanning. The friction increases up to about 1.5 times (50{\%}) in comparison to the initial state while minute topographical difference, at the {\AA} level, is detected. The friction depends on the number of scanning events, and the change follows an inverse exponential function, F(t) $=$ A(1-exp[-Bt])$+$F(0), where F is friction, t means time when continuous measurements are made, and A, B, F(0) are constants. Here, friction shift accompanied by z-scanner movement has been observed concurrently and corrected for the long time AFM measurements. In this regard, proper correction for the LFM shift induced by the z-scanner drift will also be introduced.   

Determination of Superlatice Effect on Hafnium nitride/Vanadium nitride Nano-structures

Binary nitrides multilayers systems were grown on silicon (100) substrates with the aim to study the coherent assembly in HfN/VN material. The multilayers films were grown via reactive r.f. magnetron sputtering technique by systematically varying the bilayer period ($\Lambda )$ and the bilayer number (n) while maintaining constant the total coating thickness ($\sim$ 2.4~$\mu $m). The multilayers were characterized by High angle X-ray diffraction (HA-XRD), low angle X-ray diffraction (LA-XRD), HfN and VN layers were analyzed by X-ray Photoelectron Spectroscopy (XPS) and electron and transmission microscopy (TEM). HA-XRD results showed preferential growth in the face-centered cubic (111) crystal structure for HfN/VN multilayers system with the epitaxial relation (111)[100]$_{\mathrm{HfN}}$//(200)[100]$_{\mathrm{VN}}$. The maximum coherent assembly was observed with presence of satellite peaks. With this idea, ternary and binary nitrides films have been designed and deposited on Si (100) substrates with bilayer periods ($\Lambda )$ in a broad range, from nanometers to hundreds of nanometers to study the structural evolution, coherent assembly progress and optical properties like The critical angle, dispersion coefficient, index of refraction for HfN/VN multilayers with decreasing bilayer thickness.   

The thickness dependence of surface energy and contact angle of water on ultrathin MoS$_{2}$ film

The properties of ultrathin 2D materials generally show a strong thickness dependence. Using first-principles methods, we have systematically calculated surface energy and surface stress of MoS$_{2}$ films as a function of thickness from one to 12 layers, using two van der Waals functional based approaches (vdW-DF and DFT-D2). Based on the calculated surface energies, which increases with the increasing thickness, we further analyze the surface contact angle of water droplet on MoS$_{2}$ film surface using Young's equation as a function of thickness, in comparison with experiments, from which the water-MoS$_{2}$ interfacial energy and its MoS$_{2}$ thickness dependence is derived. These results will be useful for future studies of physical and chemical properties of ultrathin MoS$_{2}$ films.   

Integration of a DC magnetron sputtering system into an ultra-high vacuum chamber for fabrication of Schottky diodes

A DC magnetron sputtering system was installed into a UHV chamber for sputtering of metal thin films with little contamination. Control of the DC power, chamber pressure and deposition time is crucial to deposit metal films with reproducible thicknesses and topographies. A graphical user interface was created to efficiently control all potential process variations. Thin films of tungsten were deposited on both n-Si and p-Si using Argon as the ionizing gas. Scanning tunneling microscopy was used \textit{in situ} to analyze the surface roughness. Ballistic electron emission microscopy was utilized to provide nanometer scale insight into the homogeneity of the tungsten-silicon Schottky barrier.   

Depth-Resolved X-Ray Reciprocal Space Mapping for Surface Microstructure Measurements

A depth-resolved X-ray reciprocal space mapping technology has been developed to investigate crystal microstructures from top of the surface to few micrometers under the surface. The depth-dependent microstructures were successfully used to reveal structure evolution occurred in both crystal-growth process and post growth treatments in few different thin films and/or crystals. Our results show that depth-resolved reciprocal space mapping is a powerful tool to monitor thin film and/or crystal microstructures and provide important information for optimization of the crystal-growth process and post-growth treatments.   

Depth of origin of sputtered atoms and isotopic angular distribution of atoms sputtered from metal alloys

Angular distribution of atoms sputtered from the surface and the near surface region under ion bombardment provides critical information about the sputtering mechanism. In the present study Monte-Carlo based SRIM simulation is used to explore the depth dependent energy and angular distribution of the sputtered atoms from liquid metal alloys, Ga:In and Ga:Bi; using normally incident keV Ar ions. These alloys are known to exhibit Gibbsian segregation where lightly bound species tends to segregate on the top of the alloy. The isotopic distributions of sputtered atoms from the alloy are also presented.   

Proton permeation through atomically thin crystals

It is well known that electrons can easily traverse through thin 2D crystals, including graphene, hexagonal boron nitride (hBN), molybdenum disulphide (MoS$_{2}$) etc.; at the same time pristine graphene and other 2D materials are impermeable to molecules and atoms including helium. Protons represent somewhat intermediate case, which, together with the fact that hydrogen technologies are extremely important nowadays, motivated us to study proton permeation through thin 2D crystals. Employing both liquid and solid proton conducting electrolytes we demonstrate that monolayers of graphene and hBN are permeable to protons at ambient conditions, while MoS$_{2}$, bilayer graphene and multilayer hBN show no proton conduction. Temperature dependence confirms the thermionic nature of the proton permeation with the activation energies of 0.3, 0.6 and 0.8 eV for monolayer hBN, monolayer graphene and bilayer hBN, respectively. Our findings suggest that atomically thin crystals can be promising for various hydrogen technologies, for instance, as proton exchange membranes for fuel cells.   

Visualization of Exciton Transport in Molecular and Quantum Dot Solids

Transport of nanoscale energy in the form excitons is at the core of the operation of a wide range of nanostructured optoelectronic devices such as solar cells, light emitting diodes and excitonic transistors. Particularly important is the relationship between exciton transport and nanoscale disorder, the defining characteristic of molecular and nanostructured materials. Here we report a spatial, temporal, and spectral visualization of exciton transport in molecular crystals and quantum dot solids. Using tetracene as an archetype molecular crystal, the imaging reveals that exciton transport occurs by random walk diffusion, with a transition to subdiffusion as excitons become trapped. By controlling the morphology of tetracene, we show that the transition to subdiffusive transport occurs at earlier times as disorder is increased. In colloidal quantum dot films, we show that diffusion does not occur by a random-walk process; instead, energetic disorder causes the exciton diffusivity to decrease over time. Our findings demonstrate that the mechanism of exciton transport depends strongly on the nanoscale morphology and disorder.   

Self-consistent theory of helium atom scattering by a thermally excited monolayer solid

The inelastic scattering of a helium atom beam by an incommensurate monolayer solid of Xe/Pt(111) for incident energies in the range 4 --16 meV and monolayer temperatures of 25 -- 75 K is evaluated self-consistently (SC) in the one-phonon approximation. The target is very corrugated and the final scattering state comprises strong diffraction and inelastic terms. At 50 and 75 K, the atom energy gain (phonon annihilation) processes have strength comparable to the energy loss (phonon creation) processes; there are pervasive and large departures from expectations based on weak-coupling detailed balance ratios. The SC results are compared to experimental data and to results from a simpler non-self-consistent approximation (NSC) that relies on harmonic approximations to the Debye-Waller attenuations of elastic and inelastic strengths. There are major differences in the trends seen in the SC and NSC results.   

Transient Heat Conduction in Strongly Correlated Systems

We analyze heat transport carried by electrons via quantum dots, modeled as strongly-correlated systems with discrete spectrum of available energy levels, which couple to two heat reservoirs of different temperatures. Our computational method for the electronic heat flux is based on the density matrix formalism, while the transition rates between particular quantum states are determined within the Fermi's golden rule. By taking into consideration the non-steady-state solutions for probabilities, we examine the influence of initial conductions and contact-induced time delays onto the rapid thermal switching response of the quantum system under investigation. Specifically, we use several different models for quantum dot, where the Zeeman splitting, Coulomb blockade, and the concept of dark-state are explicitly included. A special attention is devoted to thermal memory effects and the relationship between all the quantum transport expressions and the hyperbolic Cattaneo-Vernotte equation.   

Nonlinearities and Noise Properties of Electronic Heat Transfer in Molecular Junctions

We examine the electronic heat transport phenomena in nanoscale junctions composed of organic molecules weakly coupled to two heat reservoirs of different temperatures. The electronic heat flux and its dynamical noise properties are calculated within the scattering (Landauer) formalism with the transmission probability determined by using non-equilibrium Green's functions technique. The perturbative computational scheme is used to determine nonlinear corrections to the electronic heat flux and its noise power spectral density with up to the second order terms in relation to the temperature difference. Our results show the limited applicability of ballistic Fourier's law and the fluctuation-dissipation relations to nanoscale heat flow carried by electrons. Further, we discuss the influence of quantum interference and dimensionality of heat reservoirs onto the transport characteristics and shot noise spectra related to molecular systems under consideration. Importantly, the nonlinear transport theory developed by us may be extended to higher order terms to address a huge variety of problems associated with nonlinear thermal effects, which may occur at nanoscale.   

Theoretical Study of Layered Oxychalcogenides as Thermoelectric Materials

In order to restrain global warming and realize a sustainable global energy system, the researches of various energy resources are done. In these various energy resources, attracted technology is thermoelectric technology. Recently Layered oxychalcogenides has interesting properties useful for new type thermoelectric materials. Firstly, from DFT and DFPT calculations, we performed the electronic structure calculation and the thermal structure calculation about layered oxychalcogenides. In addition, we calculated thermoelectric properties Blotch-Boltzmann equation, semi-classically. We indicate the mechanism behind the high power factor from calculation about the transport properties. The key to understanding the power factor is that different effective masses contribute to different transport phenomena in the crystal. The discrepancy between the effective mass for the conductivity and the thermoelectric power showed that the conductivity and thermoelectric power are conveyed by electrons with different effective masses in the Brillouin zone. In point of view the thermal conductivity $\kappa$, we discuss the electronic part $\kappa_{el}$ from Bloch-Boltzmann equations, and $\kappa_{ph}$ from DFTP calculations.   

Separating Lattice and Electronic Thermal Conductivity Contributions in Bi$_{2}$Se$_{3}$ and Bi$_{2}$Te$_{3}$ Single Crystals

Nanostructured materials are an effective approach in reducing lattice thermal conductivity and improving overall thermoelectric efficiency. A challenge for experimental measurements of thermal conductivity is separating the contributions from both carriers and phonons. Building on the work of K. Lukas et al., Phys. Rev. B 85, 205410 (2012),$^{\, }$we report measurements of thermal and electrical conductivity of single crystal thermoelectrics: Bi$_{2}$Se$_{3}$ and Bi$_{2}$Te$_{3}$ in a transverse magnetic field up to 9 Tesla. Our experiments provide a separation of the lattice/electronic components and make possible a better theoretical model of the lattice portion of the thermal conductivity in materials.   

Computational screening of organic materials towards improved photovoltaic properties

The world today faces an energy crisis that is an obstruction to the development of the human civilization. One of the most promising solutions is solar energy harvested by economical solar cells. Being the third generation of solar cell materials, organic photovoltaic (OPV) materials is now under active development from both theoretical and experimental points of view. In this study, we constructed a parameter to select the desired molecules based on their optical spectra performance. We applied it to investigate a large collection of potential OPV materials, which were from the CEPDB database set up by the Harvard Clean Energy Project. Time dependent density functional theory (TD-DFT) modeling was used to calculate the absorption spectra of the molecules. Then based on the parameter, we screened out the top performing molecules for their potential OPV usage and suggested experimental efforts toward their synthesis. In addition, from those molecules, we summarized the functional groups that provided molecules certain spectrum capability. It is hoped that useful information could be mined out to provide hints to molecular design of OPV materials.   

First-principles study of amorphous carbon: a promising candidate for Na-ion batteries

The perspective of a widespread use of clean but intermittent sources of electricity (wind and solar) as well as that of hybrid electric vehicles calls for alternatives to Li-ion batteries as Li resources are limited. Na being abundant, cheap, and a relatively light and small atom, Na-ion batteries have attracted a lot of interest the past few years. However, while most of the Na-ion batteries studies focus on the positive electrode, the negative electrode remains little investigated and an efficient anode providing all a good capacity, a high cycle life, and a descent rate of charge/discharge, is still not available. The efficient electrode materials for Li, in particular diamond Si and graphite C, have been shown to not allow the intercalation of Na [1, 2]. Computational studies report positive intercalation energies [3, 4] and therefore suggest that the insertion of Na into the crystalline framework (C and Si) is thermodynamically not favored: Na atoms prefer to gather into Na clusters rather than to intercalate into the crystalline phase. Amorphization of Si was found to be a valid strategy to improve the interaction between Si and Na [3]. We investigate here the effects of amorphization of C on its storage properties vis-\`a-vis Na (as well as Li for reference).   

First-Principles Density Functional Theory Modeling Study on the Redox Chemistry of Graphene Oxides Affected by Placement Geometry of Oxygen Functional Groups

To date, lithium-ion batteries have been extensively gained attention due to their promising potential in the industry. Despite their promising properties, improving their poor power density is still needed for practical applications. In addition, sustaining the high redox potential in the lithium-ion batteries is prerequisite for exhibiting the high energy and power densities. Recently, layered carbon materials including graphenes and carbon nanotubes have been paid special attention as promising electrode candidates with high power densities due to their exceptionally high surface area and active oxygen functional groups on their surfaces. However, the lack of reliable information on the redox chemistry of the candidates is the obstacle to be uncovered for practical applications. In this study, we investigated the redox chemistry of graphene oxides cluster models with well-controlled hydroxyl functional groups at the edge. First-principles density functional theory approach was employed to understand the geometric effect of the incorporated hydroxyl functional groups on the redox chemistry. Our study will provide an insight on the strategy for improving the redox potentials of graphene-based electrode candidates.   

Temperature dependent study of impurities in LiFePO$_{4}$/C nanoparticles and their impact on electrochemical performance

We have synthesized LiFePO$_{4}$/C nanoparticles using a simple sol-gel method followed by calcination at various temperatures from 600 $^{\circ}$C to 900 $^{\circ}$C. X-ray diffraction shows that samples annealed at 600$^{\circ}$ C are phase pure while those treated at higher temperatures contain Fe$_{2}$P and Li$_{3}$PO$_{4}$ impurity phases, which increase with increasing annealing temperature. Mossbauer spectroscopy and magnetic measurements were used to quantify the amount of Fe$_{2}$P impurity phase. Scanning electron microscopy measurement reveals a noticeable increase in particle size as the annealing temperature increases from 700 $^{\circ}$C to 900 $^{\circ}$C. Optimal results are obtained in LiFePO$_{4}$/C samples annealed at 700 $^{\circ}$C, which show the lowest charge transfer resistance, highest Li-ion diffusion coefficient, the highest specific capacity of 166 mAh/g at a rate of 1C and the best rate capability and cycling stability among all samples.   

Porous MnO$_{2}$ prepared by sol-gel method for electrochemical supercapacitor

MnO$_{2}$ has attracted great attention as material for electrochemical pseudocapacitor due to its high theoretical specific faradic capacitance ($\sim$ 1370 F$\cdot $g$^{-1})$, environmental friendliness and wide potential window in both aqueous and nonaqueous electrolytes. However, the MnO$_{2}$ has a low surface area which depresses its electrochemical performance. The amorphous $\alpha $-MnO$_{2}$ composite was synthesized by sol gel method in the presence of the tri-block copolymer P123. Our aim is to investigate the role of P123 on the electrochemical performance of MnO$_{2}$. The samples with and without P123 were prepared and characterized by x-ray diffraction (XRD), SEM, TEM and Brunauer--Emmett--Teller (BET) method. The electrochemical performances of the amorphous MnO$_{2}$ composites as the electrode materials for supercapacitors were evaluated by cyclic voltammetry~and AC impedance measurements in a 1M Na$_{2}$SO$_{4}$ solution. The results show that the sample prepared without P123 exhibited a relatively low specific capacitance of 28F$\cdot $g$^{-1}$, whereas the porous MnO$_{2}$ prepared with P123 exhibited 117 F$\cdot $g$^{-1\, }$at 5 mV/s. The results of crystalline MnO$_{2}$ composites will also be presented.   

Synthesis and electrochemical characterization of mesoporous Li$_{\mathbf{2}}$FeSiO$_{\mathbf{4}}$/C composite cathode material for Li-ion batteries

Lithium iron silicate (Li$_{2}$FeSiO$_{4})$ has the potential as cathode for Li ion batteries due to its high theoretical capacity ($\sim$ 330 mAh/g) and improved safety. The application of Li$_{2}$FeSiO$_{4}$ as cathode material has been challenged by its poor electronic conductivity and slow lithium ion diffusion in the solid phase. In order to solve these problems, we have synthesized mesoporous Li$_{2}$FeSiO$_{4}$/C composites by sol-gel method using the tri-block copolymer (P123) as carbon source. The phase purity and morphology of the composite materials were characterized by x-ray diffraction, SEM and TEM. The XRD pattern confirmed the formation of $\sim$ 12 nm size Li$_{2}$FeSiO$_{4}$ crystallites in composites annealed at 600 $^{\circ}$C for 6 h under argon atmosphere. The electrochemical properties are measured using the composite material as positive electrode in a standard coin cell configuration with lithium as the active anode and the cells were tested using AC impedance spectroscopy, cyclic voltammetry, and galvanostatic charge/discharge cycling. The Li$_{2}$FeSiO$_{4}$/C composites showed a discharge capacity of $\sim$ 240 mAh/g at a rate of C/30 at room temperature. The effect of different annealing temperature and synthesis time on the electrochemical performance of Li$_{2}$FeSiO$_{4}$/C will be presented.   

Synthesis of Nanostructured Graphene/Metal-oxides Hybrid for High-performance Supercapacitors

Graphene has been considered as promising material for supercapacitor electrodes due to their large surface area, good chemical stability and excellent electrical conductivity. However, until now, conventional graphene-based supercapacitors cannot provide enough energy storage ability due to irreversible restacking behavior of graphene sheets. Various methods have been explored to solve this problem, but most of methods require complex and multi-step process, which will prevent scalable synthesis. Here, we present an easy and scalable synthesis method for nanostructured graphene/metal-oxides hybrid starting from graphene-oxide. The hybrid material prepared in this method provides high specific capacitance with high electrochemical stability. The sample characterization using XRD, XPS, FE-SEM, FE-TEM and Cyclic-Voltametry will be presented.   

Superstructured Carbon Nanotube/Porous Silicon Hybrid Materials for Lithium-Ion Battery Anodes

High energy Li-ion batteries (LIBs) are in great demand for electronics, electric-vehicles, and grid-scale energy storage. To further increase the energy and power densities of LIBs, Si anodes have been intensively explored due to their high capacity, and high abundance compared with traditional carbon anodes. However, the poor cycle-life caused by large volume expansion during charge/discharge process has been an impediment to its applications. Recently, superstructured Si materials were received attentions to solve above mentioned problem in excellent mechanical properties, large surface area, and fast Li and electron transportation aspects, but applying superstructures to anode is in early stage yet. Here, we synthesized superstructured carbon nanotubes (CNTs)/porous Si hybrid materials and its particular electrochemical properties will be presented.   

Charge Transport and Structural Dynamics in Deep Eutectic Mixtures

Charge transport and structural dynamics in a series of imidazole and carboxylic acid-based deep eutectic mixtures are investigated by broadband dielectric spectroscopy, dynamic light scattering, $^{1}$H nuclear magnetic resonance spectroscopy, calorimetry, and Fourier transform infrared spectroscopy. It is found that the extended hydrogen-bonded networks characteristic of imidazoles are broken down upon addition of carboxylic acids, resulting in an increase in dc conductivity of the mixtures. These results are discussed within the framework of recent theories of hydrogen bonding and proton transport.   

Understanding Iron-based catalysts with efficient Oxygen reduction activity from first-principles calculations

Catalysts based on Fe/N/C clusters can support the oxygen-reduction reaction (ORR) without the use of expensive metals such as platinum. These systems can also prevent some poisonous species to block the active sites from the reactant. We have performed spin-polarized calculations on various Fe/N/C fragments using the Vienna Ab initio Simulation Package (VASP) code. Some results are compared to similar calculations obtained with the Gaussian code. We investigate the partial density of states (PDOS) of the 3d orbitals near the Fermi level and calculate the binding energies of several ligands. Correlations of the binding energies with the 3d electronic PDOS's are used to propose electronic descriptors of the ORR associated with the 3d states of Fe. We also suggest a structural model for the most active site with a ferrous ion (Fe$^{2+}$) in the high spin state or the so-called Doublet 3 (D3).   

Plasmon Enhanced Hetero-Junction Solar Cell

Here we report a systematic study of plasmon-enhanced hetero-junction solar cells made of colloidal quantum dots (PbS) and nanowires (ZnO), with/without metal nanoparticles (Au). The structure of solar cell devices was characterized by AFM, SEM and profilometer, etc. The power conversion efficiencies of solar cell devices were characterized by solar simulator (OAI TriSOL, AM1.5G Class AAA). The enhancement in the photocurrent due to introduction of metal nanoparticles was obvious. We believe this is due to the plasmonic effect from the metal nanoparticles. The correlation between surface roughness, film uniformity and device performance was also studied.   

Local Structure and Electrical Performance of Pulsed Laser Deposited CdTe/CdS Thin-Film Solar Cells

The photovoltaic thin films of CdS/CdTe were prepared by pulsed laser deposition (PLD) on indium tin oxide (ITO) coated glass. The local structural variations in the thin films around Cd atom upon variations in the thin film growth parameters were investigated by X-ray absorption near-edge spectroscopy (XANES) and extended X-ray absorption fine-structure spectroscopy (EXAFS) and x-ray diffraction. X-ray absorption spectroscopy measurements were performed at the National Synchrotron Light Source of Brookhaven National Laboratory. The effect of the thicknesses of the CdS and CdTe layers, laser energy and the substrate temperature on the local crystal structure and coordination around the Cd atoms were investigated through quantitative multiple scattering analysis and modeling of the x-ray absorption spectroscopy data. The induced local structural modifications upon varying synthesis conditions are correlated with the electrical performance of these photovoltaic thin-films. The quantitative multiple scattering analyses and modeling of X-ray absorption spectroscopy data revealed the local environment around the Cd atoms are highly sensitive to thin film deposition parameters and the variations of the Cd local structure influences interface quality consequently, affect the electrical performance of these photovoltaic thin films.   

Electrowetting-Controlled Dual Liquid Prism for Adaptive Beam Steering

The use of concentrating photovoltaic (CPV) technology has been the most promising method of harvesting solar radiation. These CPV systems often require motor-driven tracking devices to steer the sun's beams onto solar cells. The cost of maintaining these tracking systems is the primary inhibitor for widespread application. We aim to overcome the need for mechanical trackers through the use of an electrowetting-driven solar tracking (EWST) system. The electrowetting-driven solar tracking system consists of an array of novel electrowetting-controlled dual liquid prisms, which are filled with immiscible fluids that have large differences in refractive indices. The naturally formed meniscus between the fluids can function as a dynamic optical prism. Via the full-range modulation of the liquid prisms, incident sunlight can be adaptively tracked, steered, and focused onto CPV cells through a fixed optical condenser. Furthermore, unlike the conventional and cumbersome motor-driven tracking systems used today, the liquid prism system would be suitable for rooftop applications. The results of this project reveal that the EWST system has the potential to generate $\sim$ 70{\%} more green energy at 50{\%} of the conventional capital cost.   

Role of chlorine in the performance of mixed halide perovskite solar cells

Recently introduced mixed halide perovskite solar cells have attracted considerable attention due to the demonstrated high photoconversion efficiency and the anticipated low cost fabrication. The presence of chlorine in CH3NH3PbX3 (x $=$ halogen) apparently increased the carrier diffusion length to about 1 micron. Increase in carrier diffusion length enabled use of inorganic thin films instead of high surface area nanostructures as electron transport medium. However, due to the ferroelectric behavior of organic halides, the devices often give unreliable efficiency values and other solar cell parameters. We observed that chlorine play a significant role in the reliability of cell characteristics, We will present the result of our photo-impedance spectroscopy studies on chlorine-iodine based metalorganic perovskite cells.   

A possibility as a new type of thermoelectric application on organic-inorganic hybrid perovsike ABI$_{3}$ system: A density functional theory study

The electronic structures of organic-inorganic hybrid systems ABI$_{3}$ (A $=$ CH$_{3}$NH$_{3}$, NH$_{2}$CHNH$_{2}$; B $=$ Sn, Pb; X $=$ I) in the distorted phase from their patent cubic phase are systematically studied using the first-principles calculations. Here, we examine thermoelectric properties for ABI$_{3}$ compounds based on the DFT electronic structures of their optimized crystal structures. The ABI$_{3}$ compounds should be considered for good thermoelectric application. We reveal that good thermoelectric performance of ABI$_{3}$ systems originate from large seebeck coefficients and low thermal conductivities. As a consequence, we predict that ABI$_{3}$ system is a promising material for new thermoelectric application compared to thermoelectric properties of well-known thermoelectric material Bi$_{2}$Te$_{3}$.   

Graphene oxide as a candidate material for natural gas storage: A first principles study

Alternative sources of clean energy will be much in demand in the coming days. To store methane (CH$_4$) in sorbent materials at ambient conditions for on-board vehicular usage, minimum adsorption energy of 18.8 KJ/mol is desirable. In this work, we have investigated methane adsorption on graphene oxide using first principles calculations. To accurately capture the weak interactions between CH$_4$ and the substrate we have included van der Waals interactions in our calculations. We show that the adsorption energy falls within the target range. Careful analysis of the various contributions to the binding shows that the enhancement in adsorption energy on going from graphene to graphene oxide arises from a subtle synergy between various effects.   

Solar photocatalytic conversion of CO2 to fuels by nanostructured oxides

Converting the carbon dioxide and water vapor to hdyrocarbon fuels through phocatalytic processes using sunlight is a promising route for limiting the CO2 accumulation in the atmosphere. This CO2 recycling process facilitates the unabated use of hydrocarbon fuels as well. Nevertheless, the photocatalytic CO2 conversion process has not yet demonstrated a reasonable light-to-fuel energy conversion efficiency for it to be considered as a viable technology. Nanostructured oxide semiconductors have recently shown potentials for efficiency enhancement. Appropriate band gap and band alignment, sufficient surface area for light absorption and low loss transfer of photocarriers to the surface are important criteria for the selection of photocatalysts. We will present the results of study on solar photocataltyic conversion of CO2 and water vapor using three oxide nanostructured materials, TiO2, ZnO and Cu2O, with different band gaps and flat band positions in converting CO2 to fuels. We will compare the quantum efficiencies and discuss possible reaction routes studied by using isotopic form of water for the reaction.   

Vacuum force

To study on vacuum force, we must clear what is vacuum, vacuum is a space do not have any air and also ray. There is not exist an absolute the vacuum of space. The vacuum of space is relative, so that the vacuum force is relative. There is a certain that vacuum vacuum space exists. In fact, the vacuum space is relative, if the two spaces compared to the existence of relative vacuum, there must exist a vacuum force, and the direction of the vacuum force point to the vacuum region. Any object rotates and radiates. Rotate bend radiate-- centripetal, gravity produced, relative gravity; non gravity is the vacuum force. Gravity is centripetal, is a trend that the objects who attracted wants to Centripetal, or have been do Centripetal movement. Any object moves, so gravity makes the object curve movement, that is to say, the radiation range curve movement must be in the gravitational objects, gravity must be existed in non vacuum region, and make the object who is in the region of do curve movement (for example: The earth moves around the sun), or final attracted in the form gravitational objects, and keep relatively static with attract object. (for example: objects on the earth moves but can't reach the first cosmic speed).   

Using Simple Circuits as Thermal Models for your Home

One of the most cost effect ways to improve the energy efficiency of your home is to increase your home's insulation. But would it be better to insulate your walls or by new windows? Not all options are equally effective, nor do they have equal costs. So how can you determine which option improves your homes insulation the most? I present an analogy to simple resistor circuits that can be used by introductory students to answer these questions.   

Anisotropic O vacancy formation and diffusion in LaMnO3

Anisotropy effects in solid oxide fuel cells are typically not considered because of the high operating temperatures. Focusing on the prototypical perovskite LaMnO$_3$, we apply first-principles calculations to demonstrate that this approximation is no longer valid when the operating temperature is reduced and discuss the consequences for the material properties. In addition, we show that strain and Sr doping can be used to further increase the anisotropy. Tensile strain promotes both the O formation and diffusion in pristine and Sr doped LaMnO$_3$, while Sr doping enhances the O vacancy formation but not the diffusion barrier. Both in LaMnO$_3$ and La$_{0.75}$Sr$_{0.25}$MnO$_3$ the O diffusion is found to be favorable in the [011] and [0$\bar{1}$1] directions.   

Electronic structure and vacancy formation in La(1-x)B(x)CoO3 (B=Mg,Ca,Ba and x=0.125)

The LaCoO$_3$ class of materials is of interest for cathodes of solid oxide fuel cells. Spin-polarized density functional theory is applied to cubic La$_{0.75}$(Mg/Ca/Ba)$_{0.125}$CoO$_3$. The effect of this cation doping on the electronic and magnetic properties as well as oxygen vacancy formation energy is studied. Oxygen vacancies with proximity to the dopant are energetically favourable in most cases. We discuss the effect of distortions of the CoO$_6$ octahedron on the electronic structure and the formation energy of oxygen vacancies. The order of formation oxygen is found to be Mg $>$ Ca $>$ Ba. Cation doping incorporates holes to the Co-O network which enhances the oxygen vacancy formation.   

Thin Film Evaporation of Receding Meniscus within Micro Pillar Arrays

Evaporation is a key process in power generation, water desalination, and thermal management applications. It has been proved that hydrophilic micro structured surfaces can enhance the convection heat transfer by promoting high-performance thin film evaporation and enlarging the total heat transfer surface area. When depositing a water droplet on hydrophilic structured surfaces, two distinct regions can be observed, a) central region with water level higher than the micro pillar height (droplet region), b) thin film region as a result of liquid meniscus receding among micro structures. In this study, we are able to probe the physics of thin film evaporation of receding liquid meniscus among micro pillar arrays with different pillar heights, spacings and diameters. Heat transfer is systematically studied in the droplet and thin film region for each sample. Also, Young-Laplace equation and kinetic theory of mass transport are used to model the thin film evaporation around micro pillars. With the proposed model, the shape of meniscus around micro pillars and the diameter of droplet/extended thin film region can be predicted and compared with the experimental measurement. The model can also be extended to model thin film evaporation of meniscus within nano structured surfaces.   

Positron Annihilation Spectroscopy of Common Mineral Constituents of Shale

Recent investigation of positron lifetime and Doppler broadening in Barnett Shale samples have shown a small intensity of positronium (Ps) formation. The samples studied have XRF information on 35 elements, XRD information on mineral constituents, and chemical information on total organic carbon (TOC). It is not known where Ps is formed in the shale. Previous research has shown that Ps is not formed in quartz-rich sandstone, calcite-rich limestone or dolomite-rich rocks, which contain minerals that also constitute a significant part of most shale samples. No information about Ps formation in clay minerals, which are often dominant in shale, has been available. The purpose of the present study is to determine which clay minerals form Ps. Twenty-five different common rock-forming minerals have been studied. Hydration of some of the minerals has also been varied. As a result of this work, a better idea of where Ps is formed in the shale samples has been obtained, but there still remains some uncertainty regarding the hydration in the shale and the possibility of direct Ps formation in the organic carbon itself. *Participant in the summer 2014 TCU REU program in Physics and Astronomy funded by the National Science Foundation under grant PHY-1358770.   

Pressure effect on stabilities of self-Interstitials in HCP-Zirconium

The self-interstitial atoms (SIAs) mediate the evolution of micro-structures which is crucial in understanding the instabilities of hexagonal close packed (HCP) structures. Taking zirconium as a prototype, we investigate the pressure effect on the stabilities of SIAs using first-principles calculations based on density-functional theory. We found that the pressure greatly affects the stability of the SIAs. The SIAs in basal planes are more stable under pressure. The SIA configuration of the lowest formation energy changes from basal octahedral (BO) to octahedral (O) at a pressure of 21 GPa. The lowest formation enthalpy configuration switches from BO to S (split-dumbbell) at the pressure of 30 GPa. The formation volumes of SIAs decrease monotonically in response to an increase in pressure. Our results reveal that it is important to take pressure effects into account when predicting the micro-structural evolution of HCP structures. (Scientific Reports, 4, 5735)   

Electrical Properties of the Ln$_{\mathrm{3-x}}$Y$_{\mathrm{x}}$TaO$_7$ (Ln$=$Dy, Gd) solid solution

Systems with the formula Ln$_{\mathrm{3-x}}$Y$_{\mathrm{x}}$TaO$_7$ (Ln$=$Dy, Gd) crystallizing in the weberite relatedstructure (SG C222, No. 20) were synthesizedby the solid-state reaction method. Structural characterization through Rietveld refinements indicates that a solid solution in the whole range 0$\le $x$\le $3 exist in both Gd and Dy systems. Structural cell parameters as a function of the rare-earth content areshown in this work, as well as the AC electrical properties in the 30-700$^o$C range. We have performed impedance (400-600$^{\circ}$C)and electric polarizability measurements (at room temperature); the results indicate that the polarizability reaches its maximum values at x$=$0.67, 1.67 and 2.33 for the Gd samples, meanwhile these values were x$=$0.67 and 2.0 for the Dy samples. In intermediate composition values, the Ln$_{\mathrm{3-x}}$Y$_{\mathrm{x}}$TaO$_7$ samples exhibit null polarization.   

The 4f multipole ordering effect on core-level spectroscopies of Ce intermetallics

The 3d transition metal compounds and 4f rare earth compounds show attractive phenomena, such as superconductivity and Kondo effect, due to strong electron correlations among localized 3d and 4f electrons. Especially, multipole ordering of orbital and/or spin in 4f and 5f compounds is attracting much attention these years. For example, CeB$_{\mathrm{6}}$ is known to show antiferro-quadrupolar (AFQ) ordering below 3.2K. X-ray core-level spectroscopy is an efficient technique to investigate the electronic states of strongly correlated systems. Recent years, experimental techniques have been rapidly developing and, especially, the progress in experimental resolution has enabled us to observe fine spectral features, which were not formerly observed. These advantages will enable us to observe spectral fine features related with the multipole ordering. In this study, we discuss multipole ordering effects on X-ray spectra for CeB$_{\mathrm{6}}$, especially paying attention on the polarization dependence. In order to simulate the electronic state of CeB$_{\mathrm{6}}$ with the multipole ordering, we use an impurity Anderson model including a simplified RKKY interaction.   

Low Temperature heat capacity of Uranium-Plutonium MOX single crystals

The establishment of the basic properties of actinides based materials is crucial for the understanding of conventional and advanced nuclear fuels. Accessing ground state properties at very low temperature for these systems gives a direct overview of their fundamental features. Moreover, when these materials can be produced as single crystals, side effects due to the presence of grains and impurities phases are drastically reduced, giving a very powerful add-in for theoretical and industrial oriented studies. This clearly ensures the reliability of the parameters determined while existing models of these strategic materials can be probed especially in the purpose of applications/developments and safety concerns. Here we report on heat capacity measurements performed on U-Pu MOX in single crystal form. Tiny crystals with mass of 2 to 15 mg have been produced by solid-solid chemical vapour transport technique with several different compositions ranging from pure UO$_{2}$ to PuO$_{2}$. Compositions close to UO$_{2}$ (U rich) present a persistent signature similarly to the magnetic transition reported for the pure phase T$_{N}$ $\sim$ $31$ K while plutonium rich concentrations do not show any hint of the magnetic transition down to the minimum temperature achieved.   

Thermodynamics of plastic flow of BCC metals from atomistic studies of isolated screw dislocations

The thermodynamic description of dislocation glide in BCC metals depends crucially on the shape of the Peierls barrier that $1/2\langle 111\rangle$ screw dislocations have to overcome when moving in the lattice. While the height of this barrier can be obtained unequivocally using saddle-point search algorithms such as the Nudged Elastic Band (NEB) method, its exact shape depends on the chosen approximation of the transition pathway of the system. We formulate a procedure that allows to identify the position of the dislocation directly from the displacements of atoms in its core. We investigate the performance of this model by calculating curved paths of a $1/2\langle 111\rangle$ screw dislocation in tungsten from a series of images obtained recently using the NEB method at zero applied stress and for positive/negative shear stresses perpendicular to the slip direction. The Peierls barriers plotted along these curved paths are shown to be quite different from those obtained previously by assuming a straight dislocation path. We demonstrate how these results can be utilized to develop a new thermodynamic model of plasticity of BCC metals that is systematically linked to the atomic-level properties of isolated $1/2\langle 111 \rangle$ screw dislocations.   

Temperature Dependence of the Dielectric Function of Ni near the Curie Temperature

Ornstein {\em et al.} (Physica V, 1938) observed a discontinuity in the normal-incidence reflectance of Ni at the Curie temperature. A similar phenomological observation of a temperature related slope change in resistivity versus temperature in Ni was made by Litschel {\em et al.} (J.\ Phys.\ Chem.\ Solids, 1985). To follow up on these observations we carried out measurement of the dielectric function $\epsilon$ as a function of temperature for a magnetized thin film Ni sample (1000 \AA\ Ni on SiO$_2$) and bulk polycrystalline Ni via ellipsometric measurement at a fixed photon energy (1.9 eV). For a magnetized sample, we observe a discontinuity in the dielectric function versus temperature just below the Curie temperature for both thin film and bulk polycrystalline Ni. In an unmagnetized Ni sample (obtained by heating above the Curie temperature), this continuity is not present. We believe that this observation is due to a change in the on-diagonal dielectric tensor elements for Ni in the magnetized samples, not due to the off-diagonal elements related to the Kerr effect.   

A theoretical investigation of the origins of atoms and sub-atomic particles

It seems the universe was at some very early stage shortwave energy. When the universe cooled some of this became matter, perhaps best thought of as waves confined into quanta. The change into longer wavelengths, if we think of the wavelengths of the particles that make up atoms when emitted as radioactivity, with different properties was presumably brought about by this cooling. In atoms these quantized waves are further confined by electrostatic forces and perhaps other forces. It seems that the electrostatic forces caused the coalescence of neutrons and electrons into atoms, with neutrons being there to keep like charges in the nucleus from like charges i.e., protons, and maybe providing some kind of mass force in the atom. It may be the further cooling of the universe allowed larger atoms than hydrogen to form later as well as having allowed the electrostatic forces to cause the formation of the first atoms. Note the sheer explosive forces present at early stages may have also prevented atom formation and later relative stability allowed this process to take place.   

An analysis of nuclear-electronic forces

It is known that electrons can act in waves and so can protons and neutrons, and that they have a negative, positive and neutral charge, respectively. It seems the motion of the positive charge and motion of the negative charge of the atom keep them from merging and yet the attractive forces stop them from parting, apart from when radiation activity is observed. The neutral charge may add a motion which loosens the attraction of the positive protonic attractive force for the negative electronic force. It seems thus the atom is relatively immobilized negative, positive and neutral waves held in confined motion. These waves can be loosed into travelling as waves by forces which break the balance of intra-atomic attractive forces and thus cause the emission of sub-atomic ``particles'' as waves. Thus the attractive forces in normal circumstances balance the ``wave motion'' forces, keeping stable atomic structure intact.   

Democracy is a historical urgency

Survival of humanity, on this planet, may depend, heavily, on coping with advancing technology of nuclear missiles. Let us consider critical alternatives of powerful governments: democracy, as an alternative to dictatorship. Democracy is based on free elections, as a government of the people, by the people and for the people. Democracy is a historical urgency, in the age of inter-continental nuclear missiles, computerized on a push-button, conceivably controllable by a very powerful, miscalculating and/or insane, dictator, capable of producing global nuclear holocaust, on our entire planet. Diplomacy, together with supporting activities, should be utilized, to help, in important steps, at this time, for achieving democracy in critical areas.   

Geometric Implications of Maxwell's Equations

Maxwell's synthesis of the varied results of the accumulated knowledge of electricity and magnetism, based largely on the searching insights of Faraday, still provide new issues to explore. A case in point is a well recognized anomaly in the Maxwell equations: The laws of electricity and magnetism require two 3-vector and two scalar equations, but only six dependent variables are available to be their solutions, the 3-vectors $\bf E$ and $\bf B$. This leaves an apparent redundancy of two degrees of freedom (J. Rosen, AJP $\bf 48$, 1071 (1980); Jiang, Wu, Povinelli, J. Comp. Phys. $\bf 125 $, 104 (1996)). The observed self-consistency of the eight equations suggests that they contain additional information. This can be sought as a previously unnoticed constraint connecting the space and time variables,$ \bf r$ and $t$. This constraint can be identified. It distorts the otherwise Euclidean 3-space of $\bf r$ with the extremely slight, time dependent curvature $k(t)=R_{\rm curv}^{-2}(t)$ of the 3-space of a hypersphere whose radius has the time dependence $dR_{\rm curv}/dt=\pm c$ nonrelativistically, or $dR_{\rm curv}^{\rm Lor}/dt=\pm ic$ relativistically. The time dependence is exactly that of the Hubble expansion. Implications of this identification will be explored.   

Prof. Jakob Narkiewicz-Jodko's Discoveries and his Laboratory

Prof. Jakob Narkiewicz-Jodko (1947--1905) major discoveries are: Electrography -- the method of the visualization of electric discharge from the bodies due to the application of high strength and high frequency electric fields [3,4], and the first observation of the propagation of the electromagnetic waives for information transfer over the distances [5,6]. They were made in his laboratory located at his manor home Nadniemen. We describe these experiments and the Lab equipment used for the discoveries. Unfortunately the Nadniemen manor designed and built in Neogothic style was destroyed at the WWII. Our goal is to restore the Lab of Prof. Jakob Narkiewicz-Jodko as a museum. We also introduce our hypothesis regarding architectural design of the manor home Nadniemen. [1] Decrespe M. La vie et les oeuvres de M. de Narkiewicz-Iodko, member et collaborateur de l'Institut imperial de medecineexperimentale de Saint-Petersbourg, member of correspondent de la Societe de Medecine de Paris, etc./ Marius Decrespe.- Paris, Chamuel, 1896, 51p. [2] Annalen der Physik.- Leipzig, 1896. -- Bd 293, 132 [3] Electrography// The Photographic news for amateur photographers.- 1896.- vol. 40, p.450 [4] Maack F. Elektrographie. Mit besonderer Berucksich-tigung der Versuche Narkiewicz-Jodko/ Ferdinand Maack// Wissenseschaltliche Zeitschrift\textellipsis -- 1898.- Bd 1, 1, 8-22; -1898.- Bd 1, 2/3, 89-99. [5] S\'{e}ances de la societe francaise de physique/ Societe francaise de physique. -- Paris, 1898, p. 77-79. [6] Present condition of wireless telegraphy// Consular reports: Commerce, manufacturers, etc. of their consular districts. Bureau of Foreign Commerce of United States.- Washington 1901, v.66. p. 44.   

Relativistic Navier-Stokes Equation, (Navier-Stokes Equation with Estakhr's Correction)

At relativistic speeds Navier-Stokes equation is incorrect unless Estakhr's correction is included. this equation relates energy flux as vector at relativistic speeds. $(-1/c^2)\frac {\partial\mathbf{q}}{\partial{t}}$, at low speeds Estakhr's relativistic correction vanishes. $\rho\gamma^{3}(\frac{D\mathbf{v}}{Dt})= \rho\gamma^{3}(\frac{\partial{\mathbf{v}}} {\partial{t}}+\mathbf{v}\cdot {\nabla{\mathbf{v}}})=-\nabla{p}+ \nabla\cdot{\mathbf{T}}-\frac{1} {c^2}\frac{\partial{\mathbf{q}}} {\partial{t}}+\mathbf{f}$, where $\mathbf{v}$ is the flow velocity, $\rho$ is the fluid density, $p$ is the pressure, $\mathbf{T}$ is the (deviatoric) component of the total stress tensor, which has order two, $\mathbf{f}$ represents body forces (per unit volume) acting on the fluid,$\nabla$ is the del operator, $\gamma$ is the lorentz factor.   

The Crisis in Scholarly Communication, Open Access, and Open Data Policies: The Libraries' Perspective

For years the cost of STEM databases have exceeded the rate of inflation. Libraries have reallocated funds for years to continue to provide support to their scientific communities, but they are reaching a point at many institutions where they are no longer able to provide access to many databases considered standard to support research. A possible or partial alleviation to this problem is the federal open access mandate. However, this shift challenges the current model of publishing and data management in the sciences. This talk will discuss these topics from the perspective of research libraries supporting physics and the STEM disciplines.   

Climate Change \& Social Justice: Why We Should Care

In the past several years the global impacts brought about by climate change have become increasingly apparent through the advent of numerous natural disasters. In these events the social costs of climate change have materialized demonstrating high costs in lives, livelihoods, and equity. Due to geographic bad-luck many of the countries most affected by climate change are those that contributed least, a challenge that's exacerbated by a lack of robust infrastructure in these countries. Wealthy nations remain at risk themselves and incidents such as Hurricanes Sandy \& Katrina have demonstrated that in times of crisis even institutions like the Red Cross will abandon the poor to their deaths. As necessary action on climate change would cost the fossil fuel industry \$20 trillion, money in politics has stymied action. Recently, however, a groundswell grassroots movement (e.g. People's Climate March in NYC) and great strides in energy technology and policy have begun to create necessary change. Reports quantifying the impacts of climate change will be discussed, as well as an update on the current state of the global climate justice movement. The important contributions from scientists to this movement will be highlighted.   

The Evolution of Carbon Burning Flames Inside Super-Asymptotic Giant Branch Stars

We explore how carbon burning impacts the bifurcation region separating stars whose final fate is a massive white dwarf from stars whose final fate is a massive star supernova. A dense grid of models with initial mass ($M_{ini}$) from 6.0$M_{\odot}$ to 11.0$M_{\odot}$ are evolved from pre main-sequence to the end of nuclear burning using the open-source toolkit, Modules for Experiments in Stellar Astrophysics (MESA). For stars between 7.0$M_{\odot}$ $\le$ $M_{ini}$ $\leq$ 9.0$M_{\odot}$, energy losses at the center of the core due to neutrino cooling causes a temperature inversion resulting in off-center ignition. First ignition occurs where the minimum temperature of 7$\cdot$10$^{8}$ K, and a density ($\rho_{crit}$) of 2$\cdot$10$^{6}$ g/cm$^3$ is met. We conclude that for stars within this range, the location of first ignition decreases as a function of initial mass. Moreover, we show that there exist a unique ignition density of 2$\cdot$10$^{6}$ g/cm$^3$.   

A Type of Dark Matter May be found by Neutron Emissions

This author has previously suggested that Neutrons in Neutron Stars are arranged in a Quasi-Crystal Structure and when they are ejected at Relativistic Velocities maintain some of that structure in forming Very large Nuclei of many Neutrons. When the Neutrons are ejected a Nuclear Electron and a Neutrino are emitted, making Neutron Stars Neutrino Sources, both from the Surface and from the Ejected matter. Likewise large collections of the Ejected matter form Dark Matter in Outer space as they are Super Heavy Hydrogen and sometimes just large Collections of Neutrons. As time passes the Large Collections of Neutrons break apart and form many Super Heavy Hydrogen Nuclei, but of smaller mass. Each breaking produces Neutrino emissions. The Super Heavy Hydrogen combines with Oxygen to produce Super Heavy water, which collects in Comets, on Planets like Earth and on moons such as Europa. Europa should be emitting Neutrinos, as there should be some emissions from the Earth itself and from the Earth's Atmosphere. The Neutrinos emitted from around Black Holes and Neutron stars should be particularly easy to detect, as there should be a lot of them.   

A Synchrotron Radiation Research Facility for Africa

Africa is the only habitable continent without a synchrotron light source. Dozens of African scientists use facilities abroad. Although South Africa has become a member of ESRF, the number of users is limited by distance and travel cost. A light source in Africa would give thousands of African scientists access to this tool. Momentum is now building for an African light source, as a collaboration involving several sub-Saharan African countries. An interim Steering Committee has been formed. SESAME, now nearing completion in Jordan as a collaboration of 9 countries in the Middle East (www.sesame.org.jo) may be the example followed. UNESCO became the umbrella organization for SESAME at its Executive Board 164th session, May 2002, as it did in the case of CERN in the 1950s. UNESCO's Executive Board described SESAME as ``a quintessential UNESCO project combining capacity building with vital peace-building through science'' and ``a model project for other regions''. It is likely that UNESCO, if asked, would play a similar role as a facilitator for an African light source.   

Analytical Model for the Diffusion Process in a In-Situ Combustion Tube

The in-situ combustion process (ISC) is basically an air or oxygen enriched gas injection oil recovery process, inside an extraction well. In contrast to a conventional gas injection process, an ISC process consists in using heat to create a combustion front that raises the fuel temperature, decreasing its viscosity, making extraction easier. The oil is taken toward the productor by means of a vigorous gas thrust as well as a water thrust. To improve and enhance this technique in the field wells, it has been widely perform experimental laboratory tests, in which an in-situ combustion tube is designed to simulate the extraction process. In the present work we propose to solve analytically the problem, with a parabolic partial differential equation associated to the convection-diffusion phenomenon, equation which describes the in-situ combustion process. The whole mathematical problem is established by completing this equation with the correspong boundary and initial conditions, the thickness of the combustion zone, flow velocity, and more parameters. The theoretically obtained results are compared with those reported in literature. We further, fit the parameter of our model to the mentioned data taken from the literature.   

Flexo-Opto-Electric Studies of fullerene (C$_{60}$) nano-colloids in namatic liquid crystal

We have explored the effects of fullerene (C$_{60})$ nano colloids on the elastic, dielectric and optical properties of thermotropic liquid crystal in nematic phase as a function of C$_{60}$ concentration and temperature. Data suggest softening of elastic behavior and divergence of dielectric properties as the temperature approaches to the isotropic phase. We will also report critical concentration and the critical exponent as extracted by fitting data to a model equation. These studies are important because of the potential applications in liquid crystal devices, drug delivery vehicles, and solar energy systems.   

Metallicities of Low Mass Inefficient Star Forming Dwarfs in S4G: Testing the Closed Box Paradigm

Low mass dwarf galaxies are the most numerous extragalactic population in the Local Universe. Many gas-rich dwarfs appear to be forming stars less efficiently than normal, massive disk galaxies and are therefore important laboratories for the study of star formation. Here we present new observations using the Palomar Double Spectrograph for 19 dwarf galaxies from the S4G Survey with the lowest stellar to HI mass ratios. Preliminary analysis of the data indicate a wide range of metallicities which vary by as much as 0.5 dex in a single galaxy in different star forming regions. Such a dispersion in metallicities favors an open box model and the results suggest a varied star formation history, possibly induced via minor mergers and accretion.   

A phenomenological model for tunneling rate of nonequilibrium quasiparticles in superconducting qubits

In superconducting qubits the lifetime of quantum states cannot be prolonged arbitrarily by decreasing temperature. At low temperature quasiparticles tunneling between electromagnetic environment and superconducting islands takes the condensate state out of equilibrium due to charge imbalance. We obtain tunneling rate from a phenomenological model for non-equilibrium, where a fixed non-equilibrium quasiparticle density leads to a temperature-dependent chemical potential shift. This deduces a non-monotonic behavior of the relaxation rate as function of temperature. As a result electromagnetic environment can dramatically influence qubit transitions. This leads to crucial fabrication hints for improvement in quantum control of superconducting qubits.   

Simulation of a Solid-Solid Transition in Confined Colloidal Hard Spheres

Recent experiments on a system of colloidal particles confined between two flat plates showed a two-stage nucleation process involving the transition of a solid, consisting of n$+$1 crystalline layers with a square symmetry (n$+$1 s-phase), to another solid consisting of n triangular layers (n t-phase), via an intermediate metastable liquid droplet [1]. Using event-driven molecular dynamics and Monte Carlo simulations, we study the 5s $\to $ 4t solid-solid transition in colloidal hard spheres confined between two planar hard walls. The 5s solid initially melts, forming a liquid droplet, within which the 4t solid nucleates. Calculations of the free-energy landscape confirm that the optimal kinetic pathway is a two-stage nucleation process with a critical nucleus consisting of liquid-like and t-solid-like particles. In addition, we find that the t-solid-like cluster nucleates near the planar hard walls, and contains both face-centered-cubic and hexagonal-close-packed ordered particles. \\[4pt] [1] Y. Peng, F. Wang, Z. Wang, A. M. Alsayed, Z. Zhang, A. G. Yodh and Y. Han, Nature Materials, In press (2014).   

A Pseudopotential Approach to Compute Thermodynamic Properties of Liquid Semiconductors

This paper deals with the theoretical approach for calculating the thermodynamical properties viz. Enthalpy(E),Entropy(S) and Helmholtz free energy(F) of some liquid semiconductors (Si, Ga, Ge, In, Sn, Tl, Bi, As, Se, Te and Sb). The Gibbs-Bogoliubov(GB) variational method is applied to compute the thermodynamical properties. Our well established model potential is used to define the electron-ion interaction. Charged Hard Sphere (CHS) reference system is used to describe the structural contribution to the Helmholtz free energy in the liquid phase. Local field correction function proposed by Farid \textit{et al} is adopted to see the screening effect. Lastly, our newly constructed model potential is an effective one to produce the data of thermodynamical properties of some liquid semiconductor.   

Electronic and Optical Properties of Core/Shell Pb$_{16}$X$_{16}$/Cd$_{52}$X$_{52}$ (X$=$S, Se, Te) Quantum Dots

The electronic and optoelectronic properties of semiconductor quantum dots (QDs) are mediated by surface defects due to the presence of dangling bonds producing trap states within the HOMO-LUMO energy gap, and contributing to fluorescence quenching. Surface capping ligands are generally used to alleviate this problem and increase the quantum yields of QDs. An alternative way is to synthesize core-shell QD structures; i.e., a QD core with a shell of another semiconductor material. We have investigated the effects of Cd$_{52}$X$_{52}$ shells on the photoexcited dynamics of Pb$_{16}$X$_{16}$ (X$=$S, Se, Te) QDs. The thin ($\approx $ 0.50 nm) shells were found to result largely in type I core/shell structures and a blue shift of the absorption spectra. Our studies revealed fairly strong core-shell hybridization in the electronic states close to the conduction band (CB) edge for Pb$_{16}$S$_{16}$ and Pb$_{16}$Se$_{16}$ cores, whereas for the Pb$_{16}$Te$_{16}$ core, such CB states were largely shell-like in nature. Nonadiabatic DFT-based dynamics, coupled with the surface hopping method, was used to study the effects of the core and shell compositions on energy relaxation rates in these systems.   

Role of Radio Frequency Power in the Plasma Enhanced Chemical Vapor Deposition

PECVD, Plasma Enhanced Chemical Vapor Deposition, is used to deposit thin films from a gas state to a solid state on a substrate. Experimental study from the X-ray diffraction spectra of Silicon-Oxide films deposited as a function of radio frequency (rf) power apparently indicates that RF power might be playing a stabilizing role and produces better deposition. The results show that the rf power results in smoother morphology, improved crystallinity, and lower sheet resistance value in the PECVD process. The PECVD processing allows deposition at lower temperatures, which is often critical in the manufacture of semiconductors. In this invited talk we will address two aspects of the problem, first to develop a model to study the mechanism of how the PECVD is effected by the RF power, and second to actually simulate the effect of rf power on PECVD. As the PECVD is a very important component of the plasma processing technology with many applications in the semiconductor technology and surface physics, the research proposed here has the prospect to revolutionize the plasma processing technology through the stabilizing role of the rf power. Recent results obtained after the abstract submission will also be included.   

Magnetoelectric coupling in supercoducting Sr2VO3FeAs revealed by scanning tunneling microscopy

Sr2VO3FeAs is known to exhibit high Tc ($\sim$ 37K) superconductivity with no magnetic ordering in the FeAs layer but weak magnetic moment in the V sublattice. An angle resolved photo emission spectroscopy also shows the non-trivial Fermi surface due to the V 3d orbitals. We have studied on Sr2VO3FeAs single crystal using spectroscopic imaging scanning tunneling microscopy (SI-STM) with variable temperature from 4.6K to 100K, and magnetic field up to 7T. Our results show that Sr2VO3FeAs has charge density wave (CDW) modulation in the V sublattice with the same wave vector observed in the neutron scattering experiment. The modulation strength is reduced with applying magnetic field. An electronic Fermi surface with largest V 3d character shows suppressed superconductivity possibly due to strong V-site correlation. However the multi-orbital nature of FeAs allows overall unsuppressed superconductivity at high Tc.   

Carbon Nanotube Charge Collectors in Doped Hybrid Perovskite Solar Cells

Hybrid organo-metallic solar cells based on perovskite crystals have had steadily improved power conversion efficiencies over the past two years, and within this period have achieved efficiencies over 19{\%}. We show that additions of Metal-Halide dopants, such as Cobalt (II) Iodide or Indium and Bismuth materials, can cause substitutional doping at the Lead atom. This may result in structural distortions (as in isovalent Co-doping) within the lattice causing change in the spatial distribution of charge carriers. We show that Co-doping results in an increased open circuit voltage upon light soaking due to possible higher charge accumulation. We also have investigated effects of p-doping the hole transport layer. We also incorporate composite sheets of MW carbon nanotubes and silver nanowires as charge collectors. These sheets provide a transparent and flexible electrode with lower sheet resistance due to integration of Ag nanowires. This has an effect on the work function of the sheet, making it more versatile as an electrode for use in a variety of device structures. This allows us a semi-transparent perovskite device, where incident light can be absorbed from either side of the device. This is beneficial towards achieving multi-junction perovskite solar cells.   

Pressure-induced magneto-structural transition in iron via a modified solid-state nudged elastic band method

Materials under pressure may exhibit critical electronic and structural transitions that affect equation of states, as known for superconductors and the magneto-structural transformations of iron with both geophysical and planetary implications. While experiments often use constant-pressure (diamond-anvil cell, DAC) measurements, many theoretical results address a constant-volume transitions, which avoid issues with magnetic collapse but cannot be directly compared to experiment. We establish a modified solid-state nudge elastic band (MSS-NEB) method to handle magnetic systems that may exhibit moment (and volume) collapse during transformation. We apply it to the pressure-induced transformation in iron between the low-pressure body-centered cubic (bcc) and the high-pressure hexagonal close-packed (hcp) phases, find the bcc-hcp equilibrium coexistence pressure and a transitional pathway, and compare to shock and DAC experiments.   

Theoretical Investigation Optical Properties of Si$_{12}$C$_{12}$ Clusters and Oligomers having Potential as Excitonic Materials

SiC clusters may have potential in 2-D exciton circuits. We determined the most stable Si$_{\mathrm{n}}$C$_{\mathrm{n}}$ isomer structures (n$\le $12) out of hundreds to thousands isomers using a method combining Stochastic Potential Surface Search and Pseududopotential Plane-Wave Density Functional Theory Car-Parinello Molecular Dynamics simulated annealing (PSPW-CPMD-SA). Four low-energy Si$_{\mathrm{12}}$C$_{\mathrm{12}}$ isomer structures are discussed to illustrate the varying optical properties of clusters with structures: i) cage type with C- and Si- segregations, ii) symmetric type formed having $\pi $-stacked C aromatic rings and exterior Si regions, iii) nearly planar bowl with C fullerene fragment surrounded by Si atoms, and iv) symmetrical SiC cluster having alternate SiC bonding in the structure. We employed B3LYP and PBE0 functionals and both cc-pVTZ and aug-cc-pVTZ basis sets to perform TDDFT calculations of excitation energies and photo-absorption spectra to show how structure and bonding patterns affect photo excitations in different types of SiC clusters. The electron and the hole charge distribution patterns in excitation were calculated for major photoabsorption transitions, reported for the most stable isomer, closo Si$_{\mathrm{12}}$C$_{\mathrm{12}}$. To understand electric field effects we also calculated dynamical polarizabilities for all the four structures using Coupled Perturbed Hartree-Fock (CPHF) at B3LYP/aug-cc-pVTZ and PBE0/aug-cc-pVTZ level of theory.   

Use of atomic hydrogen source in collision: technological challenges

Atomic hydrogen was extensively studied in the past due to its obvious fundamental aspect. Also, quite few investigations were dedicated to atomic hydrogen sources because the results of experimental investigations on systems involving H would provide very rigorous tests for theoretical models. But even if atomic hydrogen sources are currently widespread in experimental physics, their uses in experiments on collisions are still very challenging mainly due to threefold problem. First, there is the difficulty to create H in the laboratory in sufficiently large number densities. Second, there is the strain to adjust the velocities of the produced atomic hydrogens. And third, there is the toil to control the internal energies of these atomic hydrogens. We will present an outline of different techniques using atomic hydrogen sources in collisions, which could be found in the literatures, such as merged-beam technique, gas cell technique, and trap, and propose an experiment scheme using a turn-key atomic hydrogen source that experiments such as charge transfer could benefit from.   

Spatially and temporally resolved exciton dynamics and transport in single nanostructures and assemblies

The frontier in solar energy conversion now lies in learning how to integrate functional entities across multiple length scales to create optimal devices. To address this new frontier, I will discuss our recent efforts on elucidating multi-scale energy transfer, migration, and dissipation processes with simultaneous femtosecond temporal resolution and nanometer spatial resolution. We have developed ultrafast microscopy that combines ultrafast spectroscopy with optical microscopy to map exciton dynamics and transport with simultaneous ultrafast time resolution and diffraction-limited spatial resolution. We have employed pump-probe transient absorption microscopy to elucidate morphology and structure dependent exciton dynamics and transport in single nanostructures and molecular assemblies. More specifically, (1) We have applied transient absorption microscopy (TAM) to probe environmental and structure dependent exciton relaxation pathways in sing-walled carbon nanotubes (SWNTs) by mapping dynamics in individual pristine SWNTs with known structures. (2) We have systematically measured and modeled the optical properties of the Frenkel excitons in self-assembled porphyrin tubular aggregates that represent an analog to natural photosynthetic antennae. Using a combination of ultrafast optical microscopy and stochastic exciton modeling, we address exciton transport and relaxation pathways, especially those related to disorder.   

Pressure-induced magnetic quantum critical point in the itinerant helimagnet MnP

Manganese monophosphide, MnP, is an interesting magnetic material that has been investigated since 1960s in the context of rich magnetic phase diagram, Lifshitz multicritical point, and magnetocaloric effect [1-4]. It adopts in the orthorhombic \textit{B31-}type structure derived from the hexagonal NiAs-type structure. At ambient pressure, MnP is ferromagnetic between $T_{\mathrm{c}}=$291 K and $T_{\mathrm{s}} \approx $ 50 K, below which the magnetic structure changes into a screw-type order with Mn spins rotating in the a-b plan and propagating along the c axis [1,2]. We have performed comprehensive high-pressure studies on MnP single crystals up to 10 GPa with a suite of experimental probes, including resistivity, ac magnetic susceptibility, neutron diffraction. We found that the application of pressure reduces $T_{\mathrm{c}}$ and alters the ferromagnetic transition to an antiferromagnetic-like state above $\sim$ 3 GPa, and finally suppresses completely the magnetic transition around $P_{\mathrm{c}}$ $\sim$ 7-8 GPa. Exotic properties including the non-Fermi-liquid behavior and dramatic enhancement of effective mass are clearly evidenced near P$_{\mathrm{c}}$, signaling the occurrence of magnetic quantum critical point. Ref. [1] Phys. Rev. 135, A1033 (1964). [2] JAP 37, 1056 (1966). [3] PRL 44, 1692 (1980). [4] PRB 77, 104439 (2008).   

On The Nature Of Spacetime

While Einstein made spacetime relative for observers and an \textit{active} player in physical phenomena he tacitly assumed that all observers experience spacetimes that are \textit{always} \textit{synchronizable }We propose extension of concept of spacetime by considering possibility of an observer experiencing spacetimes that cannot synchronize with that of a system due to impossibility of transfer of any information between them. This coupled with fundamental premise of quantized action leads to increasing desynchronization between spacetime experienced by observer and that of system leading to only probability distribution functions connecting spacetime coordinates of two. This desynchronization of spacetimes is postulated as the root cause of fundamental probabilistic nature of Quantum Physics. It is shown that Schrodinger's equation models space desynchronization but not that of time inclusion of which leads to Quantum Field Theory. Desynchronization explains fundamental difference in quantum statistics and classical statistics and also existence of dynamic symmetry in addition to geometric symmetry. \textit{Nested desynchronized spacetime} model of our Universe is proposed. It is shown how desynchronization can allow modeling of elementary particles as extended systems and not \textit{point-like} explaining why these may be modeled as representations of Lie groups.   

Coexistence of three-wave, four-wave and five-wave mixing processes and Autler-Townes splittings in a superconducting artificial atomic system

We present a theoretical study of multi-wave mixing in a driven superconducting quantum qubit (artificial atom) with a $\triangle$-type three-level structure. We first show that three-wave mixing (TWM), four-wave mixing (FWM) and five-wave mixing (FIWM) processes can coexist in the microwave regime in such an artificial system due to the absence of selection rules. Because of electromagnetically induced transparency suppression of linear absorption in a standard ladder-type configuration, the generated FWM is enhanced greatly and its efficiency can be as high as 0.1\% for only a single artificial atom, which is comparable to or even larger than that of many previous schemes in atomic systems. Moreover, it is possible to obtain a more higher conversion efficiency by using an array of such artificial atoms. We also show that quantum interference between TWM and FIWM signals has a significant impact on the total signal intensity being a coherent superposition of these two signals. Our scheme for the generation of microwave signals may have potential applications in solid-state quantum information processing.   

Highly Ordered and Highly Aligned Two-Dimensional Binary Superlattice of a SWNT/Cylindrical-Micellar System

The synthesis of binary nanoparticle superlattice, which may provide new properties through synergetic coupling between different types of nanoparticles, are of great interest for various potential applications as well as its own scientific merit. While exciting progress has been made in the fabrication of binary spherical-nanoparticle superlattices with various symmetries by using an interplay of entropic and enthalpic interactions, systematic experimental studies on the formation of binary 1D nanoparticle superlattices have been very rare. Here, we report a highly ordered intercalated hexagonal binary superlattice of hydrophilically functionalized single-walled carbon nanotubes (p-SWNTs) and surfactant (C12E5) cylindrical micelles. When p-SWNTs (with a diameter slightly larger than that of the C12E5 cylinders) were added to the hexagonally packed C12E5 cylindrical-micellar system, p-SWNTs positioned themselves in such a way that the free-volume entropies for both p-SWNTs and C12E5 cylinders were maximized, thus resulting in the intercalated hexagonal binary superlattice. The binary superlattice can be highly aligned in one direction by an oscillatory shear field and remains aligned after the shear is removed.   

Optical studies of photoactive states in mixed organic-inorganic hybrid perovskites stabilized in polymers

Mixed organic-inorganic hybrid perovskites MAX-PbY$_{2}$(X,Y$=$I, Br,Cl) have been demonstrated as very attractive materials for absorbers of solar cells and active layers of light emitting diodes and optically driven lasers. The bandgap of the perovskites can be tuned by mixing halogen atoms in different ratios. In this presentation we study mixed MAX-PbY$_{2} $(X,Y$=$I, Br, Cl) particles synthesized directly in protective polymer matrices as light emitters. Both, time integrated and time resolved photoluminescence have been used to study the materials. So synthesized MAX-PbX$_{2}$ are very stable when measured at room temperature and in air with radiative recombination of photogenerated carriers as the main decay path. In contrast, MAX-PbY$_{2}$ with mixed halogen atoms display luminescence from sub-bandgap states which saturate at higher excitation levels. The density of these states depends on the used polymer matrix and increases upon illumination. We further compare the MAX-PbY$_{2}$ synthesized in polymers and as films and show that these states are inherent to the material rather than its microstructure.   

Phase Transition Study of Ammonia Borane/Polymer Composites: Potential Hydrogen Storage Systems

Ammonia Borane (NH$_{3}$BH$_{3})$, a potential hydrogen storage material exhibits a phase transition at T$_{\mathrm{p}}$ $\sim$ 223 K but the underlying mechanism is unclear. Ammonia borane (AB) blended with polymers (e.g. polyacrylamide) significantly improves the dehydrogenation kinetics and suppression of byproducts/impurities. We carried out heat capacity measurements over a temperature range of 180--300 K, and detected an anomaly at around 223 K, indicating a first-order transition. The change in enthalpy and entropy was calculated for AB as well as the composites and compared. The transition enthalphy and entropy revealed a decrease by increasing the polymer content and the transition temerature showed a small shift to lower temperatures for AB/Polymer composites both of which could be due to the interaction between polyacrylamide and AB after blending. This phenomena was also supported by infrared measurements.   

Pure spin current transport in Alq3 by spin pumping

The use of organic semiconductors (OSCs) in spintronics has aroused considerable interests, owing to their much longer spin-relaxation times of OSCs than those of inorganic counterparts. The most studied example is the organic spin valve (OSV), in which magnetoresistance (MR) effect is frequently reported. However, studies on pure spin current injection and transport in OSCs are scarce. Recently, the pioneering work by Watanabe et al. demonstrated that pure spin current can be pumped into and propagates in semiconducting polymers [1]. In the present work we extend the study to small molecule OSCs, and demonstrate that pure spin current can be injected into Alq$_{3}$ from the adjacent magnetic insulator Y$_{3}$Fe$_{5}$O$_{12}$ (YIG) by spin pumping. The pure spin current is detected by inverse spin Hall effect (ISHE) in Pd after propagation through Alq$_{3}$. From the ISHE voltage $V_{ISHE}$ as a function of the Alq$_{3}$ thickness, the spin diffusion length is determined to be $\sim$ 50 nm and does not depend on temperature. This result indicates the MR decrease as increasing temperature in OSVs is not due to the reduced spin diffusion length. \\[4pt] [1] S. Watanabe, et.al, Nat. Phys. \textbf{10}, 308 (2014).   

Second-harmonic rotational anisotropy and circular dichroism of a Sb$_{2}$Te$_{3}$ topological insulator nanoplate

Topological insulators are an exotic class of materials that are electrically insulating in the bulk but conductive at the surface due to the presence of topological surface states. Spin-momentum locking of the surface states makes topological insulators potential candidates for spintronic applications. Experimental detection of the surface states and their spin polarization due to spin-momentum locking remains difficult because of the lack of surface-specific analytical techniques. Optical second-harmonic generation (SHG) is a surface-specific probe and thus it should probe the topological surface states preferentially over the bulk states. Here we present an experimental study of SHG from a Sb$_{2}$Te$_{3}$ topological insulator nanoplate, which is a 30-$\mu$m wide, hexagonal-shaped, and (0001)-faced single crystal grown via chemical vapor deposition on an oxidized Si(001) substrate. We show that SHG rotational anisotropy can identify the crystalline symmetry of the nanoplate and also probe spin-polarized currents excited by circularly polarized light. Furthermore, by measuring SHG circular dichroism (the differential SHG of left- and right-handed circularly polarized excitation), we show that a non-zero SHG circular dichroism signal when the incident plane lies in the mirror symmetry plane of the nanoplate corresponds to time reversal symmetry breaking due to photo-induced spin polarization.   

Trapping and manipulating single nano-objects with dynamic temperature fields

One of the challenges of single molecule experiments in solution is the ability to trap and manipulate one or even multiple molecules against the erratic Brownian motion. The Brownian fluctuations are fueled by thermal energy and increase in strength with increasing temperature. Therefore, it is at first glance counterintuitive to confine Brownian fluctuations with the help of elevated temperatures. In thermal nonequilibrium, however, temperature gradients induce thermo-phoretic and thermo-osmotic drifts, which provide the means for single particle manipulation in solution. Here we describe experiments which use optically heated metal nanostructures to create dynamical temperature profiles in solution. These temperature profiles induce thermo-phoretic drift fields that act as effective potentials for objects suspended in liquid. Combined with optical feedback mechanisms, such effective potentials can be shaped to store and manipulate single or even a well-defined number of multiple objects in a small observation volume. The developed thermophoretic trapping system therefore paves the way for extended single molecule studies in solution or even well controlled bi- or multi molecular interaction studies.   

Large Chern number topological superfluids in bilayer system

We investigate the topological phase transition with large Chern number in a coupled layer system. The topological transitions between different topological superfluids can be realized by controlling the binding energy, interlay tunneling and layer asymmetry {\it etc}. These topological phase transitions can be characterized by energy gap closing and reopening at the critical points at zero momentum, where the Pfaffian and Chern number undergo a discontinuous change. The bulk-edge correspondence ensures that the number of edge modes exactly equals the Chern number. However all these edge modes localized at the same edge have the same chirality and propagate along the same direction. These topological phases can be detected by spin texture at or near zero momentum, which changes discontinuously across the phase transition point due to band inversion. This model can be easily generalized to multilayer system in which the Chern number equals any positive integer --- similar to that in integer quantum Hall effect --- can be realized. This work paves a new way in the realization of topological superfluids with large Chern number.   

Two distinct superconducting fluctuation diamagnetisms in Bi$_2$Sr$_{2-x}$La$_x$CuO$_{6+\delta}$

Superconducting fluctuations was studied through the angular dependent torque measurements on a series of Bi$_2$Sr$_{2-x}$La$_x$CuO$_{6+\delta}$ (BSLCO) single crystals. Two distinct superconducting fluctuation diamagnetism were observed: one is the superconducting thermal fluctuation, with a boundary close to the superconducting phase boundary; while another one, up to the temperature as high as about 180 K, showing maximum signal in the sample with hole carrier density $p=0.125$, could be due to preformed pairs. In addtion, we observed linearly temperature dependent paramagnetic torque signals in BSLCO samples, possibly a result of quantum criticality from a quantum critical point at the optimal doping.   

Straining graphene with low-temperature compatible electrostatic comb-drive actuators

Graphene holds great promises as an active element in future nano electromechanical systems. Therefore, thorough study of electromechanical properties of this 2D material is a crucial step towards its applications in flexible electronics. We present the fabrication and characterization of silicon-based electrostatic comb-drive actuators made for integrating individual graphene sheets. The micromachined comb-drive actuators are designed such that they can induce significant mechanical forces for straining graphene allowing to systematically investigate mechanical and electromechanical properties of high-quality graphene. By using highly doped silicon the comb-drive actuators become compatible with low temperatures, i.e. cryogenic temperatures allowing for quantum electromechanical experiments. Further device functionality is introduced by a local gate that enables the tunability of the chemical potential of the graphene. This approach makes possible a detailed study of the graphene under controlled high strain allowing simultaneous and independent tuning of other external parameters, i.e temperature, charge density, magnetic field. With Raman spectroscopy we measure and characterize mono and bilayer graphene samples at room temperature under applied strains up to 1{\%}. A detailed analysis of data allows clear separation of strain and doping. It is demonstrated that with this technique graphene sheet reproducibly experiences only strain while operating the comb-drive actuator.   

Fragmentation, vorticesand phantom vortices in rotating Bose-Eistein Condensates

Superfluids are distinguished from ordinary fluids by the quantized manner the rotation is manifested in them. Precisely, quantized vortices are known to appear in the bulk of a superfluid subject to external rotation. In this work we study a trapped ultracold Bose gas of N$=$100 atoms in two spatial dimensions that is eitherstirred by a rotating beam or by a rotating anisotropy. We use the time-dependent multiconfigurational Hartree method for bosons, that extends the mainstream mean-field theory, to calculate the dynamics of the gas in real time. As the gas is rotated the wavefunction of the system changes symmetry and topology. Fragmentation accompanies the resonances and change of symmetry of the wavefunction of the gas. We see a series of resonances as the rotating frequencyis increased and a variety of different excitations. Most importantly, we identify a novel type of topological defect as a phantom vortex. A phantom vortex cannot be seen in the density of the gas but it leaves its signature in the correlation function. We conclude that fragmentation of the gas appears hand-in-hand with resonant absorption of energy and angular momentum from the external agent of rotation.   

Influence of $^{13}$C isotopic labeling location of $^{13}$C DNP of acetate using TEMPO free radical

Dynamic nuclear polarization (DNP) via the dissolution method enhances the liquid-state magnetic resonance (NMR or MRI) signals of insensitive nuclear spins by at least 10,000-fold. The basis for all these signal enhancements at room temperature is the polarization transfer from the electrons to nuclear spins at cryogenic temperature and high magnetic field. In this work, we have studied the influence of the location of $^{13}$C isotopic labeling on the DNP of sodium acetate at 3.35 T and 1.4 K using a wide ESR linewidth free radical 4-oxo-TEMPO. The carbonyl [1-$^{13}$C]acetate spins produced a polarization level that is almost twice that of the methyl [2-$^{13}$C]acetate spins. On the other hand, the polarization of the methyl $^{13}$C spins doubled to reach the level of [1-$^{13}$C]acetate when the methyl group was deuterated. Meanwhile, the solid-state nuclear relaxation of these samples are the same and do not correlate with the polarization levels. These behavior implies that the nuclear relaxation for these samples is dominated by the contribution from the free radicals and the polarization levels can be explained by a thermodynamic picture of DNP.   

Dynamic nuclear polarization of carbonyl and methyl $^{13}$C spins in acetate using trityl OX063

Hyperpolarization via dissolution dynamic nuclear polarization (DNP) is a physics technique that amplifies the magnetic resonance signals by several thousand-fold for biomedical NMR spectroscopy and imaging (MRI). Herein we have investigated the effect of carbon-13 isotopic location on the DNP of acetate (one of the biomolecules commonly used for hyperpolarization) at 3.35 T and 1.4 K using a narrow ESR linewidth free radical trityl OX063. We have found that the carbonyl $^{13}$C spins yielded about twice the polarization produced in methyl $^{13}$C spins. Deuteration of the methyl group, beneficial in the liquid-state, did not produce an improvement in the polarization level at cryogenic conditions. Concurrently, the solid-state nuclear relaxation of these samples correlate with the polarization levels achieved. These results suggest that the location of the $^{\mathrm{13}}$C isotopic labeling in acetate has a direct impact on the solid-state polarization achieved and is mainly governed by the nuclear relaxation leakage factor.   

Key to formation of two-dimensional electron gas and two-dimensional superconductivity at LaAlO$_{3}$/SrTiO$_{3}$

In this report, we systematically studied the band alignment and interfacial atomic structure of (110) LaAlO$_{3}$/SrTiO$_{3}$ (LAO/STO) interfaces. We show that for both (110) and (001) LAO/STO heterojunctions, the intrinsic or extrinsic coexistence of La and Ti in ABO$_{3}$ perovskite unit cells at the interface reduces the valence of Ti, generating a local field and further leading to band bending of the STO. The free electrons would be trapped in the bended conduction band forming 2DEG. This opens new insight of band engineering for controlling the behavior of complex oxide heterojunctions. Besides$, $the two dimensional superconductiviy of (110) LAO/STO samples is demonstrated based on the systematical transport measurements. The two dimensional characteristics of the superconductiviy is confirmed by analying the Berezinskii-Kosterlitz-Thouless transition. The estimated superconductive thickness is about 18 nm. This discovery may inspire a new round of upsurge on study of LAO/STO interfaces.   

Nontrivial anomalous Hall effect in ultrathin Pt/permalloy bilayers

Anomalous Hall effect of Pt (2.5 nm)/permalloy bilayers with the thickness t$_{\mathrm{Py}}=$ 0.6$\sim $10 nm; Pt/permalloy (2.2 nm) bilayers with the thickness t$_{\mathrm{Pt}}=$ 1.5$\sim $10 nm and Pt (2.5 nm)/permalloy (2.2 nm) bilayers with the post-annealing temperature 100$\sim $500${^\circ}$ grown on MgO (001) substrates are investigated. The Pt/permalloy bilayer shows distinguished performance from the single permalloy layer due to the interfacial influence. Effective magnetic anisotropy of the bilayer with t$_{\mathrm{Py}}$ \textless 2.2 nm turns to be perpendicular to the film plane and it increases with decreasing measured temperature. More interestingly, the anomalous Hall effect is also greatly enhanced in these Pt/permalloy bilayers, comparing with that in bulk permalloy. The parameters presenting skew scattering, side jump and intrinsic contribution become extremely large, indicating a strong influence of spin orbit coupling coming from Pt/permalloy interface on the anomalous Hall effect.   

Evolution of Anomalous Hall Behavior in Pt/Co/Pt Trilayers

A series of Pt (t nm)/Co (0.5 nm)/Pt (5-t nm) trilayers with various thickness were prepared and post annealed for changing the proportions among the skew scattering (a), side jump and intrinsic contributions (b), which dominate the anomalous Hall effect in these films from a general point of view. The shape of Hall angle curve ($\rho_{AH}/\rho_{xx} $versus $\rho_{xx})$, as expected, turned from bending to linear and then bending again with raising the annealing temperature. The conventional scaling expression $\rho_{AH}/\rho_{xx} = \quad a + \quad b\rho_{xx} $is not adequate for the analysis of anomalous Hall effect, especially in those very thin films since side jump is suppressed by surface roughness and the skew scattering is enhanced by the interfacial scattering. The evolution of anomalous Hall behavior with temperature demonstrates the reliability of identifying the skew scattering as a cooperation of a residual resistance of impurities and a phonon scattering. Likewise, the thermal treatment can also tune the side jump contribution, so as to the ratio between the side jump scattering and the skew scattering, leading to a change of the shape of Hall angle curve. The distinct physical image was exhibited by 3D maps of correlation coefficient for $\rho_{AH}/\rho_{xx} $and $\rho_{xx}$, which may help to build a proper synergy between theory and experiment on the research of anomalous Hall effect.   

Proton transfer along water bridges in biological systems with density-functional tight-binding

When examining the dynamics of charge transfer in high dimensional enzymatic systems, the cost of quantum mechanical treatment of electrons increases exponentially with the size of the system. As a semi-empirical method, density-functional tight-binding aids in shortening these calculation times, but can be inaccurate in the regime where bonds are being formed and broken. To address these inaccuracies with respect to proton transfer in an enzymatic system, DFTB is being used to calculate small model systems containing only a single amino acid residue donor, represented by an imidazole molecule, and a water acceptor. When DFTB calculations are compared to B3LYP geometry calculations of the donor molecule, we observe a bond angle error on the order of 1.2 degrees and a bond length error on the order of 0.011 {\AA}. As we move forward with small donor-acceptor systems, comparisons between DFTB and B3LYP energy profiles will provide a better clue as to what extent improvements need to be made. To improve the accuracy of the DFTB calculations, the internuclear repulsion term may be altered. This would result in energy profiles that closely resemble those produced by higher-level theory.   

The Effect of Isotopic Substitution on Quantum Proton Transfer Across Short Water Bridges in Biological Systems

Many biological systems utilize water chains to transfer charge over long distances by means of an excess proton. This study examines how quantum effects impact these reactions in a small model system. The model consists of a water molecule situated between an imidazole donor and acceptor group, which simulate a fixed amino acid backbone. A one dimensional energy profile is evaluated using density functional theory at the 6-31G*/B3LYP level, which generates a barrier with a width of 0.6 {\AA} and a height of 20.7 kcal/mol. Quantum transmission probability is evaluated by solving the time dependent Schr\"{o}dinger equation on a grid. Isotopic effects are examined by performing calculations with both hydrogen and deuterium. The ratio of hydrogen over the deuterium shows a 130-fold increase in transmission probability at low temperatures. This indicates a substantial quantum tunneling effect. The study of higher dimensional systems as well as increasing the number of water molecules in the chain will be necessary to fully describe the proton transfer process.   

Selective Label-free Electrokinetic Cell Tracker (SELECT): a novel liquid platform for cell characterization

Characterization and analysis of rare cells provide critical cues for early diagnosis of diseases. Electrokinetic cell separation has been previously established to have greater efficiency when compared to traditional flow cytometry methods. It has been shown by many researchers that buffer solutions in which cells are suspended in, have enormous effects on producing required dielectrophoretic (DEP) forces to characterize cells. Most commonly used suspension buffers used are deionized water and cell media. However, these solutions exhibit high level of intrinsic noise, which greatly masks the electrokinetic signals from cells under study. Ionic liquids (ILs) show promise towards the creation of conductive fluids with required electrical properties. The goal of this project is to design and test ILs for enhancing DEP forces on cells while creating an environment for preserving their integrity. We analyzed two methylimidazolium based ILs as suspension medium for cell separation. These dicationic ILs possess slight electrical and structural differences with high thermal stability. The two ILs were tested for cytotoxicity using HeLa and bone cells. The effects of electrical neutrality, free charge screening due to ILs towards enhanced electrokinetic signals from cells were studied with improved system resolution and no harmful effects.   

Designing the bandgap of ZnO via Alloying of Magnesium and Sulfur

ZnO is emerging as one of the materials of choice for UV applications. It has a benign chemical nature, a deep excitonic energy level, and a direct bandgap of 3.4 eV. The latter two properties make ZnO a highly efficient light-emitter at and above room temperature. Alloying ZnO with magnesium and sulfur creates the Mg$_{\mathrm{x}}$Zn$_{\mathrm{1-x}}$O and ZnS$_{\mathrm{x}}$O$_{\mathrm{1-x}}$ alloy systems which can tune the bandgap by design and add new optical and electronic functionalities to ZnO. In Mg$_{\mathrm{x}}$Zn$_{\mathrm{1-x}}$O, annealing studies were performed to overcome the phase segregation tendency, reduce intrinsic defects, and enhance the UV luminescence. It was found that annealing under an argon environment significantly improved the material and optical properties of the films due to the removal of intrinsic defects and completion of alloying. In ZnS$_{\mathrm{x}}$O$_{\mathrm{1-x}}$, phase segregation is expected to occur during growth due to the various crystal structures of end members and large difference in anion radii of S and O. However, this alloy system may form intermediate compounds such as zinc sulfate (ZnSO$_{\mathrm{4}})$ which significantly impact material and optical properties. The removal of undesirable compounds will be discussed in terms of the growth conditions.   

Correlation between local crystal structure and physical properties in BiS2-based superconductors

Recently, layered superconductors with BiS2 conduction layer have been discovered. Since the layered structure and low-dimensional electronic states are similar to cuprates and Fe-based superconductors, studies on exploration of new superconductors and discussion of superconductivity mechanisms of the BiS2-based superconductors have got attention of researchers in the field of condensed matter physics. To understand the mechanisms of inducement of superconductivity in the BiS layer, we have investigated the physical properties and established superconductivity phase diagrams of various series of REO1-xFxBiS2. To understand those phase diagrams, powder x-ray diffraction and x-ray absorption fine structure were carried out. It was found that optimization local crystal structure and reduction of in-plane disorder should be correlating with the physical properties of BiS2-based compounds. We will discuss how superconductivity is induced and the Tc is enhanced, on the basis of local crystal structure such as atomic coordinates, atomic distances and in-plane disorder.   

Critical anomalous Hall behavior in Pt/Co/Pt trilayers grown on paper with perpendicular magnetic anisotropy.

Perpendicular magnetic anisotropy was observed in Pt/Co/Pt trilayers prepared on three kinds of paper substrates with conspicuous difference of roughness by sputtering. Anomalous Hall effect exhibits well magnetic transport properties for partial samples. The trends of Hall resistivity over longitudinal resistivity ($\rho_{\mathrm{AH}}$/$\rho $xx) versus $\rho $xx are bending instead of a traditional linear relationship for thick single-layer Co films. Further, study reveals that this behavior strongly depends on ratios among contributions from the skew scattering induced by residual resistance and phonons, the side jump and the intrinsic parts in anomalous Hall effect. A 3D map of correlation coefficients (R) of $\rho _{\mathrm{AH}}$/$\rho $xx and $\rho $xx shows the ratios locate at a critical and ultra-narrow area for our trilayers. This study may throw new light on the understanding of anomalous Hall effect as well as lead to an economical and practical method to fabricate Hall devices on flexible substrates.   

The origin of the UV Luminescence and its Enhancement in nanocrystalline ZnO film

ZnO is an excellent luminescent material in the UV range with a potentially wide range of applications. However, many as-grown films are observed to contain some intrinsic defects which can diminish UV-emission efficiency, limiting their practical usefulness. This study presents a route to enhance UV luminescence from ZnO sputtered films. The photoluminescence (PL) spectra of the as-grown film exhibits prominent visible PL attributed to zinc interstitial (Zn$_{\mathrm{i}})$ related defects, and a weak UV PL peak. To understand the route toward enhanced UV PL, one set of as-grown films were annealed in O$_{2}$ atmosphere and another set in Ar atmosphere. PL spectra of O$_{2}$-annealed samples revealed enhanced UV PL and elimination of the Zn$_{\mathrm{i}}$-related defect emission, however, an O$_{2}$-related defect emission was evolved. In contrast, Ar annealed films showed significantly enhanced UV emission with nearly completely quenched visible emissions. The origin of UV PL was studied by low temperature measurements which indicate that an emission related to structural defects is dominant in the UV region.   

Leveraging zinc interstitials and oxygen vacancies for sensitive biomolecule detection through selective surface functionalization

In this study, functionally engineered EIS technique was implemented to investigate the influence of surface functionalization on sensitivity of biomolecule detection using nanostructured ZnO platform. Organic molecules with thiol and carboxylic functional groups were chosen to control biomolecule immobilization on zinc and oxygen-terminated 2D planar and 1D nanostructured ZnO surfaces. The amount of functionalization and its influence on charge perturbations at the ZnO-electrolyte interface were studied using fluorescence and EIS measurements. We observed the dependence of charge transfer on both the polarity of platform and concentration of cross-linker molecules. Such selectively modified surfaces were used for detection of cortisol, a major stress indicator. Results demonstrated preferential binding of thiol groups to Zn terminations and thus leveraging ZnO interstitials increases the sensitivity of detection over larger dynamic range with detection limit at 10fg/mL.   

Spin transport of the frustrated quasi-two-dimensional Heisenberg antiferromagnet

We use the Self Consistent Harmonic Approximation together with the Kubo formalism of the Linear Response Theory to study the spin transport in the quasi-two-dimensional frustrated Heisenberg antiferromagnet in a square lattice with easy-plane ion single anisotropy at zero temperature. The regular part of the spin conductivity $\sigma^{reg}(\omega)$ is determined for several values of the critical ion single parameter $D_{c}$, that separates the low $D$ region from the large $D$ quantum paramagnetic phase. We have obtained an abrupt change in the spin conductivity in the point of phase transition indicating a strong influence of frustration on the spin transport properties   

The Vibrational Dynamics of 3D HOCl Above Dissociation

We have analyzed the vibrational dynamics of HOCl above dissociation using a 3D energy surface which governs the vibrational dynamics of HOCl above dissociation. The dynamics is dominated by an invariant manifold which is transversally unstable for small spacing between Cl and HO complex, and stable for large spacing. Above dissociation, the InM separates two mirror image periodic orbits, embedded in a large chaotic sea, that can hold a large number of quantum states. These periodic orbits have the capability of forming significant quasibound states of the molecule above dissociation.   

Development of Micro-Four-Point Probe Compatible with an Scanning Tunneling Microscope

As miniaturization of electronic devices goes on, while more attention has to be paid to the influence of the existence of surfaces and interfaces on the electric and/or magnetic properties of the electronic materials, some researchers have proposed to use the intrinsic surface properties for the development of future devices. On the other side, the existence of crystal surface and interface may reduce the spatial degrees of freedom of the carriers, and thus results in various novel quantum phenomena related to the reduction of dimensionality. It is highly desirable to obtain the electronic structure, morphology information and transport property of a material in situ, since they are strongly related. Based on a commercial apparatus equipped with STM and MBE systems, we developed an electronic transport measurement system that is compatible with the STM. A micro-four-point probe (MFPP) is utilized to increase the surface sensitivity in the transport measurement, which can be conducted at low temperature (Tmin$=$1 K) and high magnetic field (Bmax$=$11 T). With this system, we succeeded in detecting superconductivity above 100 K in a single unit-cell layer of FeSe film grown on an Nb-doped strontium titanate.   

Coherent single-spin source based on topological insulators

We report on the injection of quantized pure spin current into quantum conductors. In particular, we propose an on-demand single-spin source generated by periodically varying the gate voltages of two quantum dots that are connected to a two-dimensional topological insulator via tunneling barriers. Due to the nature of the helical states of the topological insulator, one or several spin pairs can be pumped out per cycle giving rise to a pure quantized alternating spin current. Depending on the phase difference between two gate voltages, this device can serve as an on-demand single-spin emitter or single-charge emitter. Again, due to the helicity of the topological insulator, the single-spin emitter or charge emitter is dissipationless and immune to disorder. The proposed single-spin emitter can be an important building block of future spintronic devices.   

Thermoelectric properties of bulk nanowire-nanoparticle composites

Towards realizing highly efficient bulk thermoelectrics based on nanowire-nanoparticle composites, the effect of microstructure and composition on thermoelectric properties of an illustrative composite system composed of copper nanoparticles and zinc phosphide (Zn$_{3}$P$_{2})$ nanowires is studied. Here, the intent is to extend high efficiencies achieved in individual nanowire devices to bulk nanowire assemblies. To study these effects of microstructure, thermoelectric performances of compositionally non-uniform copper nanoparticle- unfucntionalized Zn$_{3}$P$_{2}$ nanowire pellets (composite-I) were compared against those of compositionally uniform copper nanoparticle-benzenedithiol functionalized Zn$_{3}$P$_{2}$ nanowire pellets (composite-II). These results indicated that compositional non-uniformity, coupled with copper doping of Zn$_{3}$P$_{2}$ nanowires, offers more room for optimizing the thermoelectric performances of the composites. Overall, a high thermoelectric figure of merit of 0.23 at 770K was achieved in composite-I. This is two orders of magnitude higher than any achieved to date in Zn$_{3}$P$_{2}$ system. This study indicates that tuning the microstructures and composition of materials is a route for enhancing their thermoelectric efficiencies.   

Quantum phase transition of light in the resonator array

We give a concrete experimental scheme for engineering the insulator-superfluid transition of light in a one-dimensional (1-D) array of coupled superconducting stripline resonators. In our proposed architecture, the on-site interaction and the photon hopping rate can be tuned independently by adjusting the transition frequencies of the charge qubits inside the resonators and at the resonator junctions, respectively, which permits us to systematically study the quantum phase transition of light in a complete parameter space. By combining the techniques of photon-number-dependent qubit transition and fast read-out of the qubit state using a separate low-Q resonator mode, the statistical property of the excitations in each resonator can be obtained with a high efficiency. An analysis of the various decoherence sources and disorders shows that our scheme can serve as a guide to coming experiments involving a small number of coupled resonators.   

Frustrated spin-spin interactions between trapped ions using longitudinal and transverse phonon modes

We present a scheme of quantum simulation of many-body interactions with trapped ions via the exchange of virtual phonons, where the motion from both the longitudinal and the transverse directions is considered. By tuning the detunings of Raman lasers, the long-range and locally tunable interaction is easily obtained between different spins. We show that the competing spin-spin couplings mediated by all motion modes can give rise to higher levels of frustration and richer phase transitions than the conventional approaches based on the longitudinal or the transverse phonon modes alone.   

Single laser beam photothermal microscopy

Fluorescence microscopy provides a tool to study dynamics in softmatter materials on a molecular level. However, the observation time for fluorescent objects is limited due to bleaching. One way to overcome this limitation is the use of gold nanoparticles as labels. They are chemically inert under typical situations. These particles are selectively imaged using a modulated heating laser and a non-absorbed detection laser even in the presence of background scatterers. The absorbed power results in a localised temperature profile and to a refractive index change which only occurs for absorption. For finite thermal diffusivities the temperature profile does not instantly follow temperature changes present on the nanoparticle's surface. This results in an out-of-phase modulation of the detection laser. By exploiting the limited thermal diffusivity we show that a single laser beam being intensity modulated is enough to selectively image and quantify absorption. The use of a single laser makes photothermal microscopy easier to implement into existing microscopy setups.   

Quantum Computational Resource Quality of a Symmetry-Protected Topologically Ordered Phase

Symmetry-protected topologically ordered (SPTO) states are many-body quantum states invariant under an on-site symmetry group, which can be grouped into distinct SPTO phases based on their non-local entanglement structure. While originally arising in the context of condensed matter physics, SPT states have also attracted interest in quantum information for their ability to be used as resource states for quantum computation. We investigate entanglement naturally present in the 1D SPTO phase associated with on-site octahedral symmetry and show that, as long as certain characteristic lengths are finite, all its ground states can be used to efficiently implement any one-qubit gate operation with arbitrary accuracy. This feature is an intrinsic property of the entire phase, and we show that it can also be probed by means of a particular string-order parameter. Our approach may pave the way toward a novel program to classify quantum many-body systems based on their operational use for quantum information processing.   

Thermal and laser induced sintering in Pt nanoparticles studied by conventional and synchrotron x-ray diffraction

The thermal and laser induced sintering behavior of 5 -- 6 nm Pt nanoparticles self- assembled into 50 nm diameter spherical aggregates has been studied by conventional and synchrotron-based x-ray diffraction (XRD) measurements. In the first instance, the aggregated Pt nanoparticles were solution annealed at temperatures between 120 and 215 $^{\circ}$C over time periods from 10's to 100's of minutes. In each case the linewidth of the conventionally measured diffraction pattern consisted of a single component which systematically narrowed suggesting an increase in the size of the as-prepared nanoparticles. In a second set of experiments, the aggregated Pt nanoparticles were exposed to about 10,000 laser pulses, each with a duration of about 1 ps and an energy density of 250 mJ/cm$^{2}$. XRD spectra were acquired after each 100 lasers pulses corresponding to 100 ps of sample irradiation. A narrow line component was observed in the diffraction pattern after the first 100 laser pulses and dominated the lineshape after a few thousand laser pulses. These measurements reflect the effects of long term, low temperature atomic transport in comparison with high energy, short time transport.   

Single file diffusion in microtubules

We investigate the single file diffusion (SFD) of large particles entering a confined tubular geometry, such as luminal diffusion of proteins inside microtubules or flagella. While single-file effects have no effect on particle density, we report significant single-file effects for individually-tracked tracer particle motion. Both exact and approximate ordering statistics of particles entering semi-infinite tubes agree well with our stochastic simulations. Considering initially empty semi-infinite tubes, with particles entering at one end starting from an initial time $t=0$, tracked particles display super-diffusive effective exponents just after they enter the system and trends towards diffusive exponents at later times. Equivalently, if diffusive exponents are assumed the effective diffusivity is reduced at early times and enhanced at later times through a logarithmic factor $\log{N}$, where $N$ is the number of particles in the tube. When we number each particle from the first ($n=1$) to the most recent ($n=N$), we find good scaling collapse of the effective diffusivity for all $n$. Techniques that track individual particles, or local groups of particles, such as photo-activation or photobleaching, will exhibit single-file effects.   

Temperature dependent thermal conductivity of single- and bi- layer MoS$_{2}$ and MoSe$_{2}$

Thin layer transition metal dichalcogenide (TMDC) materials have received extensive interests in recent years. In this work, for the first time we systematically investigated and compared the thermal transport properties of two TMDC materials, MoS$_{2}$ and MoSe$_{2}$, and in single-layer (1L) and bi-layer (2L) forms. The optothermal Raman technique is used in the measurement process. With an improved and more robust experimental data analysis process, we discovered the thermal contact resistance and the interfacial thermal conductance of the four materials for the first time. These factors provide boundary conditions and are crucial in generating the final thermal conductivity for the suspended materials. For 1L MoS$_{2}$ and MoSe$_{2}$, the room-temperature thermal conductivities are (80 $\pm$ 17) W/mK and (72 $\pm$ 19) W/mK, respectively. For 2L MoS$_{2}$ and MoSe$_{2}$, we obtain values of (73 $\pm$ 25) and (39 $\pm$ 13)W/mK. The thermal conductivity of suspended 1L MoS$_{2}$ decreases to (66 $\pm$ 16) W/mK upon heating to 500K.   

Substrate-Phonon-Mediated Plasmon Hybridization in Coplanar Graphene Nanoribbons

Mode hybridization between adjacent graphene nanoribbons determines the integration density of graphene-based plasmonic devices. Here we demonstrate this plasmon hybridization by characterizing the coupling strength of plasmons in graphene nanoribbon arrays in terms of graphene Fermi level and inter-ribbon spacing. Both experimental and computational results showed that the plasmon coupling is strongly mediated by the substrate phonons. For polar substrate, the plasmon coupling strength was limited by the plasmon-phonon interaction. In contrast, nonpolar substrate affects neither the energy distribution of original plasmon modes in graphene nanostructures nor their plasmon interactions, which increase exponentially as the inter-ribbon spacing decreases. To further explore the potential of graphene broadband plasmonics on nonpolar substrate, we propose a scheme that uses a metal-dielectric heterostructure to prevent the overlap of plasmons between neighboring graphene nanoribbons. The device structures retain the plasmon resonance frequency of the graphene ribbons and maximally isolate the plasmonic components from the surrounding electromagnetic environment, allowing modular design in integrated plasmonic circuits.   

New Method for Storing Information based on the Magnetic Permeability

We present a new approach for storing information based on bits with different values for their magnetic permeability. Unlike present magnetic recording, information stored in this way should be ideal for archiving since it is unaffected by exposure to a magnetic field or moderate changes of temperature. Using heating with laser pulses as short as 100 $\mu $ sec, we have decreased the permeability of micron sized bits of an amorphous ferromagnet, Metglas, by crystallizing them. The permeability of micron sized bits of Cu/permalloy bilayers was decreased by using ohmic heating to cause the Cu to diffuse into the permalloy (80{\%} Ni 20{\%} Fe). This occurs because Ni is no longer ferromagnetic if the Ni atoms have too many Cu neighbors. The changes in the permeability are read by using either a magnetic tunnel junction or a spin transfer oscillator to measure whether the flux lines of a probe field are affected by the bits. The permeability of Cu/permalloy bilayers is not affected by 10 mega rads of gamma radiation from Co 60. Using heat assisted recording (HAMR) will permit writing permeability bits on a nm scale.   

Thermal stability of organic-inorganic hybrid perovskite structures from first principles

Organic-inorganic hybrid perovskites are currently the focus of intense research owing to their impressive efficiency as photovoltaic devices. Specifically, methylammonium lead tri-halides are of interest in this regard, however, some of their basic properties are not completely known yet. We investigate structural stability of organic-inorganic hybrid perovskites of methylammonium lead tri-halide type (MAPbI$_{3-n}$Cl$_n$: $n$ = 0-3) at different temperatures, using ab initio structure optimization, energy calculations, and molecular dynamics. Different crystal structures and free energies are compared, and effects of temperature change are discussed. The results provide insight toward understanding stable methylammonium lead tri-halides for photovoltaic applications.   

Multiple sharp lattice plasmon modes in 2D Au nanoparticle superlattices

Periodic metal-nanoparticle (NP) 1D chains and 2D arrays can produce sharp lattice plasmon modes due to the coupling between the diffraction mode of the periodic lattice and the localized surface plasmon resonance of the metal NPs. 2D Au NP superlattices, new structures that combine multiple length scales, have the potential for new optical properties such as mode coupling. They differ from periodic 2D arrays in that the spacing between NP patches introduces an additional microscale patch periodicity while the sub-microscale NP periodicity is maintained within one patch. In the reciprocal space, the small periodicity corresponds to a large wavevector while the large periodicity corresponds to a small one. For 2D NP superlattices, the two wavevectors sum together and show additional, satellite diffraction modes at higher and lower energies than the modes for periodic 2D arrays. We found that multiple sharp lattice plasmon modes exist in the 2D Au NP superlattices as the satellite diffraction modes couple with the localized surface plasmon mode. Multiple peaks with narrow linewidths in the transmission spectrum were observed in both numerical calculations and experiments. 2D Au NP superlattices provide flexibility in tuning the lattice plasmon mode through changing the microscale periodicity of the patches. The multiple sharp lattice plasmon modes can also serve as potential cavity modes for surface-emitting lasing.   

Modularity Enhances the Rate of Evolution in a Rugged Fitness Landscape

Biological systems are modular, and this modularity affects the evolution of biological systems over time and in different environments. We here develop a theory for the dynamics of evolution in a rugged, modular fitness landscape. We show analytically how horizontal gene transfer couples to the modularity in the system and leads to more rapid rates of evolution at short times. The model, in general, analytically demonstrates a selective pressure for the prevalence of modularity in biology. We use this model to show how the evolution of the influenza virus is affected by the modularity of the proteins that are recognized by the human immune system. A modular model of the fitness landscape of the virus better fits the observed virus evolution data.   

Polarized neutron scattering as a probe of spin nematicity in the iron-based superconductor BaFe$_{\mathrm{2-x}}$Ni$_{\mathrm{x}}$As$_{2}$

We use polarized neutron scattering to demonstrate that in-plane spin excitations in electron doped superconductor BaFe1.904Ni0.096As2 [1] change from isotropic to anisotropic in the tetragonal phase well above the antiferromagnetic ordering and tetragonal-to-orthorhombic lattice distortion temperatures without an uniaxial pressure [2]. The anisotropic low-energy spin excitations at the same momentum transfer share similar features with the spin nematic phase probed in the detwinned samples with uniaxial pressure, and consistent with in-plane resistivity anisotropy [3]. These results indicate that the polarized neutron scattering is a good probe of the spin nematicity in the tetragonal phase of iron pnictides [4]. References [1] Yanchao Chen, et.al., Supercond. Sci. Technol. 24, 065004 (2011) [2] Huiqian Luo, et.al., Phys. Rev. Lett. 111, 107006 (2013). [3] Xingye Lu, et.al., Science 345, 657 (2014) [4] Huiqian Luo, et.al., unpublished manuscript (2015).   

Magnetic properties of MnF3

MnF3 which is A-type antiferromagnetic material has been reported to show the negative thermal expansion (NTE) below Neel temperature. In this work, the temperature and magnetic field dependence of the magnetization of MnF3 was measured to find the spin order. The M(T) curve measured by NMR fits well with the theory for antiferromagnet with anisotropy, T$^2$ e$^{(-\beta _G)}$, and the measured energy gab(E$_G$) is about 30 K. The M(H) curve shows that a ferromagnetic phase is mixed with the antiferromagnetic phase below the transition temperature. From the comparison of the M(H) curve at 30 K with theory, the relation between K$_a$ and J$_1$ was obtained which is given by K$_a$ $\sim$ 1.9 J$_1 + 10.3$ in absolute temperature unit.   

Excitonic Josephson effect in $\nu_{\mathrm{t}} =1$ quantum Hall bilayer systems

The many similarities between exciton condensates and cooper pairs have driven intense interests to search for exciton-superfluidity effect. The Josephson-like tunneling conductance peak in a single quantum Hall (QH) has been viewed as a strong signature of exciton superfluidity. However, in fact, the Josephson-like effect is not an exact analogy of Josephson effect in superconducting junctions. Therefore, we study three kinds of excitonic Josephson junctions (JJs) composed of QH bilayers: SS, SNS, and SS'S junctions in a pseudospin picture. By solving the Landau-Lifshitz-Gilbert equations, we find, in contrast with superconducting JJs, interlayer single-particle tunneling raises spatial inhomogeneity in supercurrent and system-size dependent current-phase-relation. In addition, under the effect of the tunneling, the supercurrent could not flow through the normal metal via Andreev reflection. Interestingly, anomalous supercurrents occur in SNS junctions even in absence of Josephson interference: the tunneling would set the phase to zero at NS interfaces, which will lead to a phase bias across a Josephson-like junction (i.e. single condensate system) with a nonzero ground state phase, and hence induces the anomalous supercurrent.   

Screening effect on electronic and field-emission properties of graphene nanoribbons

The electronic and field-emission properties of zigzag graphene nanoribbons (ZGNRs) influenced by manipulated nanostructure width (Lw) and nanostructure-to-nanostructure separation (Dx) are investigated using first-principles calculations. The corresponding characteristics, including band gap, magnetic moment, field enhancement factor and work function are explored and presented. It is found that the behavior of those properties under saturation versus Dx is observed, and the corresponding values approach their limits as Dx increases to a certain value. In addition, the electric-field-induced changes in band gaps of the ZGNRs with expanded separation are much more significant than those of ZGNRs with little separation. These phenomena can be attributed to physical origins such that the greater the separation between ZGNRs, the less significant the screening effect becomes. Finally, the altered magnetic moment of the ZGNRs due to the presence of an external electric field is analyzed and discussed.   

Controlling magnetic order and quantum disorder in molecule-based magnets

Metal-organic coordination polymers are materials in which transition metal ions are linked via organic molecules into chain or plane-like structures. Strong hydrogen bonds enable these units to form three-dimensional lattices, while the underlying anisotropy causes low-dimensional magnetism to evolve. Here the magnetic properties of a number of these compounds are investigated through high-field magnetization, heat capacity, and magnetic susceptibility measurements. It is shown that [Cu(pyz)H$_2$O(gly)$_2$]ClO$_4$ is a highly one-dimensional antiferromagnet, whilst the compounds [Cu(pyz)(gly)]ClO$_4$ and Cu(H$_2$O)VCF$_4$ are dimerized with a non-magnetic singlet ground state and behave as zero-dimensional disordered magnets at zero field. Furthermore, these two materials are shown to undergo a field-induced transition through a quantum critical point into an XY ordered phase, which in the case of [Cu(pyz)(gly)]ClO$_4$, is reminiscent of Bose-Einstein condensation of triplons.   

The Structural Bases for Polymer Glass-Transition Temperatures

The glass-transition temperatures (Tgs) observed for chemically distinct polymers range over several hundred K, and the molecular bases for this wide variability are largely unknown, though the following three factors are often mentioned as being pivotal: 1. Their inherent conformational flexibilities; 2. The sizes or steric bulk of their side-chains; and 3. Their inter-chain interactions. These three factors are generally interdependent, making it difficult to predict or even rationalize the Tgs of polymers. Structurally analogous aliphatic copolyesters, copolyamides, and copoly(ester/amide)s can be synthesized to produce amorphous samples with Tgs that are unaffected either by crystallinity or polymer chain lengths. Their conformations are virtually identical, and each can be synthesized with or without side-chains, so we can begin to evaluate the relative importance of the above three factors. The Tgs of un-branched analogous samples should differ solely due to factor 3., while analogous samples with singly-branched repeat units should provide a measure of the relative importance of factors 1. and 2.   

Optically Excited Graphene - Non - Equilibrium Many Body Theory

A generalized non-equilibrium dynamical mean field theory (DMFT) for graphene is presented. The NE-DMFT describes graphene in the presence of an external field coupling to the electrons and thus changing in a severe but controllable way the elecronic properties of graphene. The non-equilibrium DMFT derives properties such as electronic density of states (LDOS) and occupation numbers of the optically driven system. It fully characterizes the system in its time dependent state. It is demonstrated, how such a setup may be employed in order to realize all-optical switching processes. Results for relevant time scales in setups as well as wave-mixing influences are presented.