Session K1: Poster Session II: 2:00 pm - 5:00 pm

Crystal Structure and Superconductivity of FeSr$_{2}$(Y,Nd)(Cu,Zn)$_{2}$O$_{6+\delta}$

We have prepared the polycrystalline samples of FeSr$_{2}$Y$_ {0.75}$Nd$_{0.25}$(Cu$_{1-x}$Zn$_{x}$)$_{2}$O$_{6+\delta}$ solid solution system (${x}$ = 0, 0.01, 0.02, 0.05) to investigate the Zn substitution effects. The DC magnetization measurement results showing the samples exhibited decreasing in T$_{c}$ while increasing the Zn content, ${x}$, and the superconductivity was disappeared around ${x}$ = 0.05. Crystal structure has been analyzed by using X-ray and neutron powder diffraction data. The relation between the superconductivity and crystal structure is discussed based on the experimental results.   

DC Magnetization and Growth of Heavy-Fermion Superconductor CeCoIn$_{5 }$and CeIn$_{3}$ Crystal

The superconducting and magnetic state in heavy-fermion intermetallic compounds provides a promising realm of materials to study quantum critical behavior. Single crystals of the heavy-fermion superconductors CeCoIn$_{5 }$and CeIn$_{3 }$were$_{ }$ synthesized from the pure element using an excess of Indium. The material crystallizes in the body tetragonal space group P4/mmm structure of HoCoGa$_{5}$ and has alternating layers of CeIn$_{3}$ and CoIn$_{2}$. The details of the flux growth technique used to grow CeCoIn$_{5 }$and$_{ }$CeIn$_{3}$, and the result of DC magnetization and transport measurements performed in the temperature range 1.9K to 100K will be reported.   

Superconducting Properties of Al-Eu Alloys

We discuss the superconducting properties of Al-Eu alloys with Eu concentrations in the range of 0-5000ppm. The electron-transport and tunneling properties are discussed in the context of Abrikosov-Gor'kov pair-breaking and Kaiser resonant-scattering theories. This complements previous work on the Al-Mn system.   

New Superconducting Material: Mg${_2}$SnX (X = C, B)

In the Mg-Sn system, there is only one intermetallic phase of Mg${_2}$Sn composition. This intermetallic phase crystallizes in a cubic symmetry with prototype CaF${_2}$ structure, named anti-fluorite. About three years ago, our group reported the existence of superconductivity in the Mg${_2}$SnB intermetallic phase with a superconducting critical temperature close to 35 K [1]. In this intermetallic phase the boron atoms occupy the interstitial sites available in the structure. This work shows that, besides boron atoms, carbon also can occupy the interstitial sites in the same structure and can also induce superconductivity in the system with high critical superconducting temperature. This conclusion is sustained by resistivity, magnetization, and X-ray diffraction measurements. This report is based upon work supported by FAPESP (2005/01257-9). [1] B. Ferreira, C. M Franco, C. A. M. dos Santos, D. Rodrigues Jr., L. Ghivelder, and A. J. S. Machado; Phys. C 408- 410, 148 (2004)   

New superconductor of the M$_{2}$AX family with Ti$_{2}$GeC composition.

In this work the Ti$_{2}$GeC phase is investigated by x-ray diffraction, magnetic and resistivity measurements. Polycrystalline samples with nominal compositions Ti$_{2}$GeC were prepared by solid state reaction. The samples were encapsulated under argon and heat-treated at 1100\r{ }C for 240 hours. X-ray powder diffractograms suggest that all peaks can be indexed with the hexagonal phase of Cr$_{2}$AlC prototype. The electrical resistance as a function of temperature for the Ti$_{2}$GeC reveals a standard metal like behavior when this material is cooled from the heat treatment under equilibrium conditions. However, magnetic measurements display diamagnetic behavior close to 9.5K. On the other hand, when the Ti$_{2}$GeC phase is submitted a rapid quenching the R(T,B) curve shows superconducting critical temperature close to 9.5K without applied magnetic field. The magnetoresistance data with applied magnetic filed suggests bulk superconductivity. In spite of the great number of compounds which belong to this family, superconductivity has been reported for five cases. So, this work sustains the idea of the existence of a new class of superconducting materials that crystallizes in the Cr$_{2}$AlC prototype.   

Search for Magnesium Diboride like Binary Superconductors

Efforts to create compounds iso-electronic and iso-structural with magnesium diboride and also superconducting have so far met with limited success. Doping the Mg-site or the B-site have also not yielded higher transition temperatures. They have either been non-superconducting or have lower transition temperatures, Tc. Searching for magnesium diboride-like compounds using the electronegativity of MgB$_{2}$ (1.7333) has yielded a rich family of potential superconductors. The search has been facilitated using the recently designed ElectroNegativity Spectrum Maps [ENSMaps] of binary systems A$_{x}$B$_{y}$. Here we display the potential families. Using the relationship between Tc and atomic mass, we estimate the transition temperatures of the most likely MgB$_{2}$-like binary superconductors. We also suggest materials that can be doped to give an electronegativity of 1.7333.   

Study of the Pseudogap in Y1-xCaxBa2Cu3O7-d Superconductor

The generic phase diagram of doping vs. temperature for High~Temperature Superconductors (HTSC) presents in the underdoped region a zone called Pseudogap (PG). The analysis of this region is very interesting since it is believed that it is~deeply related, and therefore can help us understanding better the superconductive (SC) transition. Different theories exist up to now which try to explain the origin of the PG and its influence on the SC transition. Some of them see the PG as a precursor of the superconducting gap, while others see it as a competing phenomenon which retards the SC transition. In the present work we analyze resistivity measurements in bulk samples of the high temperature superconductor Y1-xCaxBa2Cu3O7-d, for different~concentrations of calcium (x) and oxygen (d), and its influence on the PG zone. We discuss our results on the light of some of these theories.   

MaxEnt-MuSR study of GdBCO: potential precursor effects

We analyze muon-spin resonance (muSR) data of underdoped (Tc = 81 K) and optimal doped (Tc = 93 K) superconducting GdBCO showing different precursor effects. [1] Precursors refer to anomalous behavior seen just above Tc. Transverse field muSR data recorded at 1 kOe, RT, and 100 Oe, 120 and 200 K are analyzed by Maximum Entropy. MaxEnt determines the frequency (i.e. magnetic field) distribution from the muSR time series. [2] Two Lorentzians fit the frequency signals much better than two Gaussians, one Lorentzian, or one Gaussian. Thus, in GdBCO the muon probes dynamic fields, caused by muon motion and/or magnetic fluctuations. The number of Balmer (muon-stop) sites [1] has been confirmed. Zero-field muSR curve fitting studies [1] showed irregularities in asymmetry and muon-spin relaxation just above Tc. We apply MaxEnt to these ZF GdBCO data to search for magnetic precursor effects. Research is supported by DOE-LANL and WiSE@SJSU. [1] Dawson \textit{et al,} J Appl Phys 64 (1988) 5809; [2] Lee \textit{et al,} J Appl Phys 95 (2004) 6906; also at scitation.aip.org.   

The enhanced high Tc superconductivity by ordering dopant

We discuss the high pressure improvement on superconducting transition temperature (Tc) related to ordering apical oxygen layer of a high temperature superconductor (HTS). This study became available in the high pressure synthesized Sr$_{2}$CuO$_{3+\delta }$ superconductor with K$_{2}$NiF$_{4}$ structure showing so far rarely formed partially occupied ?apical oxygen? which also act as the dopant of the HTS. The well-defined links between Tc and modulated structures suggests that optimizing the ordering at apical oxygen layer outside CuO2 plane is a promising way to further enhance Tc.   

Mo$_{2}$BC: Chemical and External Pressure Effects

The intermetallic Mo$_{2}$BC is a superconductor with a T$_{C}$ = 6.6 K and a crystalline face centered orthorhombic structure. Chemical pressure generated by changing the carbon concentration decreases T$_{C}$ in a non monotonic rate. Complete elimination of carbon, changes the crystalline structure from orthorhombic to body centered tetragonal, and reducing T$_{C}$ to about 5.8 K. At ambient pressure the compound presents a minimum in the resistivity at 50 K, which could be related to a Kondo anomaly. In polycrystalline samples we applied external pressures up to 4.8 GPa with a diamond anvil cell, which induced negative changes in the superconducting transition at a rate dT$_{C}$/dP = - 0.03 K/GPa. These results will be discussed in terms of the electronic band structure.   

Unconventional Metallic Phase of the Quasi Two-Dimensional Organic Superconductors

We show, by analyzing previously published nuclear magnetic resonance (NMR) data, that there are large antiferromagnetic fluctuations above $T_{\mathrm{NMR}}$ $\sim50$ K in the metallic phase of $\kappa$-(ET)$_2$X family of organic charge transfer salts. The proximity of the metallic phase to the antiferromagnetic Mott insulating phase and the d-wave superconductivity are thought to be the origin of the large antiferromagnetic fluctuations. The antiferromagnetic correlation lengths are estimated to be several lattice constants at $T_{\mathrm{NMR}}$ which place the materials between the isotropic triangular lattice and the square lattice. For materials close to the Mott insulating phase the nuclear spin relaxation rate per unit temperature, Knight shift and Korringa ratio all decrease significantly with decreasing temperature below $T_{\mathrm{NMR}}$, inconsistent with the renormalized Fermi liquid picture previously thought to be the correct description of the low temperature metallic phase in these materials. One plaussible explanation is that a pseudogap, similar to that observed in the underdoped cuprate superconductors, opens up in the density of states below $T_{\mathrm{NMR}}$. Such a pseudogap has recently been predicted to occur in the dimerised organic charge transfer salts materials by the resonating valence bond (RVB) theory.   

Photoemission Spectroscopy on the System of Noncentrosymmetric Lithium Ternary Borides

We performed x-ray photoemission spectroscopy at BL27SU of SPring-8 on polycrystalline samples of Li$_{2}$Pd$_{1.5}$Pt$_{1.5}$B and Li$_{2}$Pt$_{3}$B prepared by the arc melting method. We also employed a polycrystalline platinum plate commercially available for comparison. We also performed the calculation of valence band structure of Li$_{2}$Pt$_{3}$B using full-potential augmented plane wave method with local density approximation. Our experimental data on the samples and the previous photoemission study on Li$_{2}$Pd$_{3}$B support that electron correlations do not play an important role in them.   

Anomalous transport measurements in overdoped high-temperature superconductors

A fundamental question~in understanding~the phenomenon of~superconductivity in the high temperature superconductors (HTSC) is~the possibility of describing the electrons in these materials in the~frame of the~Fermi Liquid theory. In the phase diagram of temperature vs. doping~ we find four different phases: Antiferromagnetic, Pseudogap, Marginal Fermi Liquid and Fermi Liquid. This~work is focused~ in the study of Fermi Liquid zone, which appears at high doping (overdoped region).~It~ has been generaly assumed, without much study, that the behaviour of this zone is~that of~a normal metal. Nevertheless, our measurements, and data from other researchers~show~different behaviour.~ We have measured resistivity and Hall effect~vs. temperature in overdoped YBaCaCuO thin films. Theanomalous transport properties presented in this region is argued~in our conclussions to be~evidence of a non Fermi Liquid behavior.   

Magneto-thermal instabilities in irradiated high density MgB$_{2}$ compound

The effects of irradiation with low dosages of $\gamma $-rays, protons and electrons on the magnetization and critical current density of MgB$_{2}$ bulk samples were studied. Magnetic susceptibility measurements present a transition temperature with diamagnetic signal at $\sim $38.5 K. Magneto-thermal instabilities as flux jumps are observed in the magnetization hysteresis loops below 23 K, for all samples. The flux jump behavior is independent of the irradiation. The number of flux jumps decreases as the temperature increases. The magneto-thermal instabilities observed is a competing process between the Lorentz and pinning forces that depend on the bath temperature as well as on the defect density that influence the current density. The field dependence of the critical current density, J$_{C}$ was evaluated using the Bean's model for different temperatures (from 2, 10, 15, and 20 K). The results show instabilities in the critical current, J$_{C}$, below 10 K as a consequence of the flux jumps events observed in the isothermal magnetization curves. In this presentation we will analyze the influence of the flux jumps on the critical currents densities.   

Observed magnetism and its field dependence in c-axis-oriented YBCO vortex states.

Muon-spin-resonance ($\mu $SR) data of c-axis-oriented YBCO [1] vortex states are analyzed to determine the field dependence of observed AF magnetism. Field distributions are obtained from $\mu $SR data using Maximum-Entropy (ME). We found [2] that well below T$_{c}$ YBCO vortex signals are best fitted by a Gaussian and a Lorentzian; the latter indicating AF in and near the vortex cores. The field dependence of the AF Lorentzian width is about linear. [2] ME-$\mu $SR analysis of c-axis-oriented YBCO data also suggests a field \textit{direction} dependence, pointing toward 3-dim magnetism. Our results show contradictions to curve fitting and FFT results. [1] An LSCO neutron study agrees with 3-dim field-induced AF. [3] An AF presence in and near vortex cores supports theories predicting a magnetic origin for cuprate superconductivity. [3, 4] Research supported by NSF-REU, DOE-LANL and WiSE@SJSU. [1] Lichti \textit{et al}, Hpf Int's 63 (1990) 73; [2] Boekema \textit{et al}, Physica C460-462 (2007) 1255 and ref's therein; [3] Lake \textit{et al}, Nature Materials 4 (2005) 658; [4] Chen, Zhang \textit{et al}, Phys Rev B 67 (2003) 22051.   

Magnetization, Creep, and Flux Pinning in YBa$_{2}$Cu$_{3}$O$_{7-x}$ Thin Films with Nanoscale Pinning

Critical current and flux pinning have been studied for YBa$_{2}$Cu$_{3}$O$_{7-x}$ (YBCO) thin films with Y$_{2}$BaCuO$_{5}$ (211) precipitates introduced as layers and as random distributions. Magnetically determined critical current density ($J_{c})$ was fit to $J_{c} \quad \propto $ $B^{-\alpha }$ and values of \textit{$\alpha $} were suppressed from the control sample values of \textit{$\alpha $} = 0.5 to lower values for pinned samples, reaching as low as \textit{$\alpha $} = 0.2 for the layer pinned 211 sample at low temperatures. $U(J)$ vs $J$ curves were extracted from M-H measurements with various ramp rates, at temperatures from 4.2 K to 77 K for pinned and control samples. Direct magnetization decay measurements were made for the 211 layer pinned sample and good agreement was seen with ramp rate derived results. Values of \textit{$\mu $} $\cong $ 0.6-0.8 were seen for all samples, while \textit{$\nu $} $\cong $ 0.4 for control samples, 0.1 for layer pinned samples, and 0.2-0.4 for the random pinned samples. The values of \textit{$\mu $} and \textit{$\nu $} extracted were inconsistent with 2-D pinning behavior in all cases, even though the layer spacing in the layer pinned sample is smaller than the calculated collective correlation length. While the layer pinned sample is clearly in the collective pinning regime, the artificial defects in the random pinned sample may be in the isolated strong pinning regime.   

Oxidation of MgB$_{2}$ Thin Films

We report on x-ray photoelectron spectroscopy studies of the surface oxides of high quality MgB$_{2}$ thin films grown both by reactive evaporation [1] and by hybrid physical chemical vapor deposition [2]. Depending upon the treatment of the MgB$_{2}$ surface after deposition, the oxide can contain both magnesium and boron, with a substantial variation in the Mg:B ratio. Brief high temperature (T $\ge $ 400 $^{o}$C) exposure of the MgB$_{2 }$surface to even near ultra-high-vacuum conditions, results in the formation of a thin mixed oxide (MgB$_{x}$O$_{y})$ and subsurface layer of elemental B due to the greater reactivity of Mg. As a result of the higher mobility of the Mg cations, prolonged exposure to the background ambient additionally results in a progressively thicker MgO surface oxide layer, and in a larger elemental B subsurface component. The surface oxide formed at $\sim $ room temperature is more Mg rich than the initial, mixed oxide layer formed at high T. The latter is a significantly better passivating layer as indicated by the resistance of the film to water etching. We discuss these differences in surface oxide chemistry with a focus on guiding the development of thin film processes applicable to tunnel barrier formation on MgB$_{2}$. [1] B. Moeckly and W. Ruby, SUST 19, L21 (2006) [2] X. Zheng, et al., Nat. Mater. 1, 35 (2002)   

Deposition temperature dependence of YBCO transport properties

In this paper, we report a strong correlation between the stacking fault (SF) density and the critical current density of YBa2Cu3O7-$\delta $(YBCO) thin films in an applied field (Jcin-field). High quality superconducting YBCO thin films (thickness $\sim $300 - 350 nm) were deposited on SrTiO3 (STO) and LaAlO3 (LAO) substrates using a pulse laser deposition (PLD) technique. We found that theJcin-field increases as the deposition temperature increases (775$^{\circ}$C - 825$^{\circ}$C) for the samples grown on both STO and LAO substrates. Detailed microstructural studies including cross-section transmission electron microscopy (TEM) and high resolution TEM were conducted for all the films deposited on STO substrates. The YBCO SF density increases from $\sim $ 4.0x105/cm to $\sim $1.2x106/cm as the deposition temperature increases from 775$^{\circ}$C to 825$^{\circ}$C. An interesting linear relation is observed between the SF density and the Jcin-field value, which suggests that the YBCO SF density plays an important role in the YBCO in-field transport performance.   

Fluctuation Nernst-Ettingshausen Effect above Ordinary/Quantum Superconducting Transition

A problem of the definition of the heat transported in thermomagnetic phenomena has been well realized in the late sixties, but not solved up to date. Ignoring this problem, numerous recent theories grossly overestimate the thermomagnetic coefficients in strongly interacting systems. Here we develop a gauge-invariant microscopic approach, which shows that the heat transfer should include the energy of the interaction between electrons and a magnetic field. We also demonstrate that the surface currents induced by the magnetic field transfer the charge in the Nernst effect, but do not transfer the heat in the Ettingshausen effect. Only with these two modifications of the theory, the physically measurable thermomagnetic coefficients satisfy the Onsager relation. We critically revised the Gaussian fluctuation theory above the ordinary/quantum superconducting transition and show that the gauge invariance uniquely relates thermomagnetic phenomena in the Fermi liquid with the particle-hole asymmetry.   

Flux-Flow noise in YBCO thin films in the normal region, transition and superconducting state.

The dynamic of vortexes inside type II superconductor thin films in the mixed state, that is, under their critical temperature and immersed in a DC magnetic field below its critical value, can be studied by means of the measurement of flux-flow noise, before the transition, during it and in the superconducting state. We measure the fluctuation in the voltage signal in the pseudogap region for an YCaBaCuO thin film, and compare it with the response in the other two regions. The response for overdoped and underdoped samples is compared with the response of optimaldoped samples.   

Phase diagram and vortex dynamics in superconducting spherical nanoshells

Curving a superconducting film into a spherical shell changes its vortex-related properties drastically due to topological constraints. The interplay between the Lorentz force due to an applied field and the vortex superflow will force vortices away from the equator (leaving an equatorial ``Meissner band'') and towards the poles, where they may coalesce to form giant or ring-like vortices. Using the time-dependent Ginzburg-Landau equations adapted for the spherical geometry, we derive the phase diagram and identify where, as a function of the applied magnetic field, the shell thickness and the shell radius, different vortex phases occur. We also examine the dynamics of the decay of giant and ring-like vortices into a vortex lattice, when the magnetic field is adapted so that a phase boundary is crossed. Moreover, we show that the vortex dynamics are insensitive to moderate imperfections in the shell: effects due to topological constraints can overcome the pinning potential due to imperfections. This robustness, together with the tunability of the phase diagram through a limited set of controllable parameters, makes superconducting nanoshells uniquely suited for the study of vortex states.   

Out of equilibrium phase dynamics in ferromagnetic Josephson junctions

With a pump-probe measurement performed below 1K, we probe the switching mechanism of strongly underdamped ferromagnetic Josephson junctions in the classical limit. At equilibrium or for slow sweeps, we observe that the switching is governed by thermal fluctuations, as expected. When the sweep frequency is comparable to the inverse phase relaxation time, we observe premature switching due to phase bifurcation. From the frequency dependence of the switching probability we directly deduce the phase relaxation time $\tau $=1/RC, where R is the quasiparticle resistance and C the junction capacitance. Moreover, we observe a peculiar scaling of the Fiske steps (resonances between the Josephson phase and the electromagnetic cavity modes) with the junction length: the resonance frequencies are not multiples of the inverse junction length, but present a finite offset. We attribute this offset to the high frequency ferromagnetic susceptibility.   

Macroscopic quantum tunneling in a damped Josephson junction coupled to a nanomechanical resonator

Motivated by work in the field of quantum computation, we studied the tunneling rate in a damped Josephson junction (JJ) coupled to a nanomechanical resonator. The Josephson phase difference was treated as the coordinate of a particle trapped in a metastable cubic potential well, and the resonator was considered to be a simple harmonic oscillator. The damping on the system by the environment was modeled as two reservoirs of simple harmonic oscillators, one of which was coupled to the JJ, and the other coupled to the resonator. We found that damping the JJ suppresses the tunneling rate, a result already predicted by theory and verified by experiment for a single JJ. We also find that increasing the coupling strength between the JJ and the resonator suppresses the tunneling rate, while adding damping to the resonator actually enhances the tunneling rate. These results are important, as a full understanding of the tunneling rate's dependence on system parameters is essential for the proper operation of quantum bits.   

Suppression of quantum fluctuations in a Josephson junction coupled to a nanomechanical resonator

The quantum mechanical properties of a Josephson junction (JJ) in parallel with a nanomechanical resonator were studied. The JJ phase difference was treated as a ``particle'' trapped in a quadratic potential well, which was used to approximate the well-known tilted washboard potential of the junction. When coupled to the resonator, the square of the uncertainty in the position of the JJ ``particle'' was suppressed, {\it i.e.} quantum fluctuations of the JJ were reduced by the resonator. The uncertainty principle was obeyed by the system, in that the square of the uncertainty in the JJ's momentum was enhanced with resonator coupling. We also included the effects of environmental damping. Damping the junction enhanced the suppression of quantum fluctuations beyond that due to resonator coupling alone. Damping the resonator, however, suppressed the effect of JJ-resonator coupling and thus resulted in less suppression of quantum fluctuations. Preliminary results for the effects on quantum fluctuations of a weak nonlinear term in the JJ's potential energy have also been obtained.   

Indication of Both d- and s-Wave like Superconducting Gaps in YBa$_{2}$Cu$_{3}$O$_{7}$

Both of the d- and s- wave interpretations of the superconducting gaps in high Tc superconductors are separately supported by experiments, leading sometimes to conflicting views. In an effort to resolve this conflict, we performed first-principle quantum calculations as follows. We utilized self-consistent, electronic wave functions and electron-phonon interaction matrix elements, and we solved four-dimensional Eliashberg gap equations. Our results showed that on three sheets of the Fermi surfaces, the calculated superconducting gap exhibits a strong anisotropy and can lend itself to a d-wave interpretation. In contrast, the calculated superconducting gap on the small sheet of the Fermi surface around the S-Point only shows a relatively small variation from about 18 meV to 25 meV and there is no node on this sheet, leading to s-wave interpretation. Our findings point to the need for measurements of the superconducting gap on this sheet of the Fermi surface around the S-point. Such measurements are expected to shed light on the gap symmetry properties of high Tc superconductors. Work funded in part by the Department of the Navy, Office of Naval Research (ONR, Grant No. N00014-4-1-0587) and by the National Science Foundation (Award No. HRD 0503362)   

Exact thermodynamics of phase separation and pairings in Hubbard nanoclusters.

The exact numerical diagonalization and thermodynamics in an ensemble of small Hubbard nanoclusters reveal intriguing insights into the phase separation, charge and spin pairings, Bose condensation and ferromagnetism in nanometer scale. The phase diagram off half filling strongly suggests the existence of electron pairing, superparamagnetism and saturated ferromagnetism in small nanoclusters driven by electron repulsion and doping. Rigorous criteria for the existence of charge and spin pairings in the ground state and corresponding crossovers at finite temperatures are formulated. The phase separation and electron pairing, monitored by a magnetic field and electron doping, surprisingly resemble phase diagrams in the family of doped high T$_c$ cuprates. Exact theory provides incoherent electron charge pairing above T$_c$ and pair coherency with spin condensation below T$_c$. These ideas may also be linked to recent atomic scale tunnelling experiments in La cuprates on nucleation of pairing pseudogaps and microscopic inhomogeneities in a real space.   

Ferromagnetic Pairing Ground States on Two-Coupled Chains

Recently, ferromagnetic superconductivity was discovered in $\mathrm{ZrZn_2}$, $\mathrm{UGe_2}$ and $\mathrm{URhGe}$. Microscopic explanation of this phenomenon is a challenge in theoretical physics, but the problem is rather subtle and difficult, since we have to treat rotational symmetry breaking of spin and electron-pair condensation simultaneously. At the present stage, it is expected that a simple concrete model exhibiting both ferromagnetism and electron-pair condensation, even if it has somewhat artificial aspects, will shed some light on understanding of mechanisms of ferromagnetic superconductivity. Here we report an extended Hubbard model on two chains which has fullypolarized pairing ground states. The Hamiltonian consists of intra-chain electron-hopping, on-site repulsion, inter-chain charge attraction and inter-chain ferromagnetic interaction terms. The following is shown in our model. In the case where the on-site repulsion term is vanishing, the model has degenerate ground states in which electrons form spin triplet pairs, and thus the ground states exhibit electron-pair condensation but do not exhibit ferromagnetism. When the on-site repulsion is added, the model has the unique (up to spin degeneracy) ground state in which ferromagnetism and electron-pair condensation coexist. We also present an extension of the model to higher dimensional cases.   

Electron charge pairing and Nagaoka spin instabilities in nanoclusters

The electron pairings and magnetism in various frustrated Hubbard clusters are studied exactly with emphasis on under doping, magnetic field and temperature. Small clusters provide insight into charge spin separation and thermal condensation of electron charge and spin degrees [1]. The spin saturated phase in so called Nagaoka state is found equivalent to ferromagnetic Mott-Hubbard like insulator with spin pairing gap, while non maximum spin ground state is of BCS-like metallic origin with equal charge pairing and spin gaps. The calculated phase diagrams resemble a number of spatially inhomogeneous coherent and incoherent paired phases seen in nanometer scale in high T$_c$ cuprates, fullerene molecules, Co and Nb nanoparticles. [1] A.~N.~Kocharian, G.~W.~Fernando, K. Palandage, Tun Wang and J.~W.~Davenport, Phys.~Rev. B{\bf 74}, 024511 (2006); Phys.~Lett. A{\bf 364}, 57(2007).   

Dynamic Response to On/Off Signals in Nano Junctions

We report the implementation of the time-dependent nonequilibrium Keldysh Green's function theorem(TD-NEGF). It provides a promising way to study the transient transport dynamics in nano devices by first principles calculation. Very importantly, we derive an efficient technique to overcome the singularity problem in the integration of spectrum function. The reliability of this method is carefully checked by a one-dimension chain model with the analytical solution. We then perform the ab-initio calculation in a realistic molecular junction (Al-Benzene-Al). The current dynamic response arises after applying an ``upward'' or ``downward'' step pulse, which predicts a characteristic timescale in transport dynamics of nano systems.   

One-dimensional fermion pairing

We study fermion pairing in a one-dimensional fermion gas at $T=0$ interacting via a generalized two-body attractive, separable interaction [1] where the effective range was varied from zero (delta potential) to infinity. The binding energy of fermion pairs with zero center-of-mass momentum increases as a function of the interaction strength and decreases as a function of the interaction range for a given strength. Fermion pairs with finite, nonzero center-of-mass momentum have an energy dispersion relation that exhibits two excitation branches: One phonon-like for low momentum which, for weak coupling, can disappear before the second, roton-like excitation appears for values of the momentum larger than $2k_ {F}$ and only above a minimum threshold interaction strength value. The interaction range has the effect of privileging the quadratic over the linear relation dispersion as it goes from short to long range. This study completes a trilogy initiated for 3D [2] and later for 2D [3]. \, [1] P. Nozi\`{e}res y Schmitt-Rink, J. Low Temp. Phys. 59, 195 (1985); [2] M. Casas et al., Physica C 295, 93 (1998); [3] S.K. Adhikari et al., Phys. Rev. B 62, 8671 (2000).   

Numerical simulation of fluxon dynamics in a Josephson junction array

We present a numerical study of the dynamics of fluxons trapped in a parallel array of Josephson junctions. Simulations of switching current measurements have been performed in order to support experimental work in our group. Switching current measurements allow determination of the transition rate of the fluxon from its pinned state to a running state. We simulate the classical RCSJ equations of motion for a 9-junction parallel array, with and without frequency-dependent damping, and calculate switching current distributions by increasing the external current in the simulation. A new retrapping mechanism for fluxons, related to the coupling of the junctions in the array, has been identified. We present results from the simulations and comparisons to experiment.   

Signatures of Kosterlitz-Thouless behavior in the superfluid density of anisotropic layered superconductors

In quasi-two-dimensional (2D) systems, as thin films of $^4$He or of superconductors, the superfluid transition is expected to be driven by phase fluctuations, according to the Kosterlitz and Thouless (KT) theory. However, signatures of KT vortex-antivortex phase fluctuations should be observable, at some energy scale $T_d$, also in strongly anisotropic layered superconductors, where quasi-2D behavior arises due to a small Josephson coupling between neighboring planes. While in the 2D case $T_d$ is uniquely identified by the KT temperature $T_{KT}$ where the universal jump of the superfluid density is observed, in the layered case such universality is lost. Here we show this effect by means of a renormalization-group analysis of a layered version of the sine-Gordon model, appropriate to describe the occurrence of KT physics in layered superconductors. We find that in the presence of a finite interlayer coupling $T_d$ is controlled by the vortex-core energy, and can be significantly larger than the 2D scale $T_{KT}$. When applied to the superfluid-density behavior in cuprate superconductors these results allows us to determine a non-trivial behavior of the vortex-core energy in these systems. L.Benfatto, C.Castellani and T.Giamarchi, Phys. Rev. Lett. 98, 117008 (2007)   

Fermi surface arcs and the infrared conductivity of underdoped YBa$_{2}$Cu$_{3}$O$_{6.5}$

Using recent finding, that the electronic states lost below the pseudogap energy ($\Delta _{pg})$ are recovered in the energy region immediately above it, we analyze the in-plane far infrared conductivity data in underdoped orthoII YBa$_{2}$Cu$_{3}$O$_{6.5}$ and are able to find evidence for the opening of a pseudogap on part of the Fermi surface with the remaining ungaped piece proportional to the temperature. These results are similar to recent angle-resolved photoemission spectroscopy data in underdoped Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8+\delta }$.   

Ultrafast Carrier Dynamics of YBCO Films with Various In-plain Orientations Investigated by Pump-probe and Terahertz Spectroscopy

In this study, we focus on the terahertz (THz) and optical responses of superconducting YBa$_{2}$Cu$_{3}$O$_{7-\delta}$ (YBCO) films with various in-plane orientations. We report results and analyses of THz time domain spectroscopy and time-resolved photoinduced reflectivity experiments on four in-plane orientated superconducting YBCO films grown on yttria-stabilized zirconia substrates. Our study of the transmissions of THz time domain spectroscopy indicates a higher value of conductivity at room temperature for the 0$^{\circ}$-orientation films than the 45$^{\circ}$-films. Using the optical pump-THz probe scheme, we observed combinations of positive and negative THz transmission transients relative to a thermal equilibrium level with different relaxation times of about 0.9 and 9.0 ps respectively for all samples containing 45$^{\circ}$-domains. Possible physical mechanisms will be discussed.   

Influence of oxygen orbitals on impurity states in superconducting cuprates

Recent STM studies have shown that the oxygen states play a hitherto unappreciated role in the inhomogeneous electronic structure of the CuO2 plane. To gain some insight into these effects, we solve the Bogoliubov-de Gennes equations numerically for a 3-band Hubbard model with d-wave pair interaction. We consider the one-impurity problem, and discuss the role of the various atomic orbitals in the in the formation of impurity bound states and magnetic moments in the presence of correlations described by the Hubbard U. Finally, we study the interference of many impurities.   

Nature of the superconductor-insulator transition in disordered thin films

Highly disordered superconducting (SC) thin films undergo a magnetic-field (B) driven superconductor-insulator transition (SIT) whose detailed nature is still not completely understood. Starting from a microscopic description, we analyze this SIT in disordered thin films, and demonstrate that disorder leads to the formation of islands where SC order is rather high. For weak disorder, increasing B eventually results in the vanishing of the SC order parameter, implying an insulating state. At higher disorder, however, increasing B suppresses correlations between phases of the SC order parameter in different islands, giving rise to a novel kind of SIT. One of the remarkable predictions of this study is that in the latter regime, there are still SC islands in the sample even on the insulating side. This outcome, which is consistent with pertinent experiments, explains the recently observed huge magneto-resistance peak in disordered thin films. It may also be relevant in attempts to explain the occurrence of pseudo-gap in under-doped high-Tc superconductors, which have recently been found to be intrinsically disordered.   

Growth and properties of WO3, NaxWO3, and KxWO3 thin films.

We report optimization of thin-film growth conditions and films properties of WO3, NaxWO3, and KxWO3. Films are grown by pulse laser deposition and used substrates are (100) LaAlO3 (a $\sim $ 3.788 A) and (111) Y-ZrO2 (3.63 A). Growth temperature and oxygen pressure are varied from 600C to 300C and from 10 mTorr to 1000 mTorr, respectively. WO3 are formed in monoclinic or tetragonal structure on LaAlO3 substrates. Films are insulators and temperature dependence of resistivity shows the variable range hopping with Coulomb interaction like behavior [resistivity is proportional to exp(1/T)\^{}(1/2)]. On Y-ZrO2 substrates, WO3 are formed in mixed structure of hexagonal and tetragonal due to an epitaxial effect [(111) Y-ZrO2 substrate has hexagonal surface]. KxWO3 are formed in hexagonal structure on both substrates. a- and c-axis oriented films are obtained on LaAlO3 and Y-ZrO2 substrates, respectively. KxWO3 films show superconductivity at Tc(onset) $\sim $ 4 K and Tc(zero) $\sim $ 2 K. This work is supported by Air Force Office of Scientific Research.   

Vortex phase diagram and electromagnetic anisotropy of cation composition controlled Bi2212 single crystals

In the present study, vortex phase diagram and pinning properties of Bi2212 single crystals under $H$ // $c$ have been systematically studied as functions of cation and oxygen compositions, while these matters were well understood for slightly cation-nonstoichiometric samples almost ten years ago. We confirmed that cation stoichiometry largely affects vortex state and pinning strength. A particular crystal with a cation composition of nearly 2:2:1:2 ``Bi2212'' exhibited strong bulk pinning behaviors even in the high temperature region, resulting in disappearance of magnetic reversible region below the first-order-transition (FOT) temperature of vortex state. In addition, entropy change of the vortex at FOT of ``Bi2212'' crystals was found to be apparently larger than that of conventional ones. On the contrary, Bi and Ca-rich and Sr-poor single crystals showed poor pinning behaviors with low irreversibility fields and critical current density. Systematic enhancement of in-plane anisotropy in resistivity, decreases in \textit{$\rho$}$_{c}$ and increases of penetration depth with approaching the cation stoichiometric composition suggested that disordered crystal lattice due to partial substitutions of Bi and Ca for Sr-site, which is common for conventional Bi2212 single crystals, degraded superconducting properties of Bi2212.   

What is the vortex ``transport entropy"?

Below the superconducting transition the large thermomagnetic effects in the type II superconductors are determined by magnetic vortices. These topological excitations are completely different from particle-hole exctitations in the Fermi liquid and, therefore, the thermomagnetic effects do not require particle-hole asymmetry. Thermomagnetic effects in the vortex state are widely described in terms of the ``transport entropy.'' Despite of intensive theoretical and experimental investigations, this mysterious quantity is still in conflict with either the Onsager principle or the third law of thermodynamics [1]. We resolve this forty years enigma taking into account the magnetization current in the presence of the temperature gradient. Then contributions of superconducting currents of vortices are canceled in the Nernst effect, and, therefore, in agreement with the Onsager relation, both the Nernst and Ettingshausen phenomena originate solely from vortex cores. Finally, the transport entropy turns out to be by a factor of $4 ln (\lambda / \xi)$ smaller than that used in literature [1] ($\lambda $ is the magnetic field penetration depth, $\xi$ is the coherence length. For high-temperature cuprates this factor is $\sim$20. \newline [1] R.P. Huebener, {\it Magnetic flux structures in superconductors}, Springer-Verlag, Berlin, (1979).   

Variational Monte-Carlo investigation of gossamer-superconductivity

Motivated by the interesting superconducting properties in layered organic materials, and the proposed gossamer superconductivity in this context, we performed variational Monte-Carlo simulations. We investigate a previously proposed model-Hamiltonian of a Hubbard-model with additional anti-ferromagnetic coupling term. We work on a square lattice with additional diagonal bonds and with a wave-function with partly projected double-occupied states. Further factors for anti-ferromagnetic states are introduced in our wave-function.   

Superconducting Mechanism in multi-walled Carbon Nanotubes

Recently Japanese group led by Haruyama [1] reported the significant enhancement of superconductivity, i.e., Tc=12K, in end-bonded Multi-walled Carbon Nanotubes. We can explain the enhancement by the electron confinement in the lateral direction, i.e., between the inner and outer cylinders, because electron density correlation enhances the phonon-mediated superconductivity. In other words, superconductivity in the (multi-walled) Carbon Nanotubes is due to the electron-phonon interaction and Tc is enhanced due to the density correlation caused by the confinement. First, we use simple concentric rings to estimate the Tc enhancement using the BCS theory. Next, we use the tight-binding model to calculate the Tc increase more accurately. In this context, this experimental result is very similar to the enhancement of Tc=15K in 4 angstrom single-walled Carbon Nanotubes by Tang et al. [2]. [1] I. Takesue et al., Phys. Rev. Lett., Vol. 96, 057001 (2006). [2] Z. K. Tang et al., Science, Vol. 292, 2462 (2001).   

Theoretical Investigation of Fermion Pairing in Three-band Extended Hubbard Model

We analyze for fermion pairing the two-dimensional extended Hubbard model (or d-p model) on a square lattice by a slave-boson method reported in a previous work$^{1}$ . The onsite coulomb repulsion between Cu d holes is assumed to be strong. The nearest-neighbor interaction in momentum space U$_{kq}$ , introduced additionally, for transition from a momentum q to k is assumed to be separable and is expanded in terms of basis functions corresponding to d$_{xy}$ and d$_{x2-y2}$ .The possibility of a mixed (s-d)-wave symmetry also exists for the spin degeneracy N$>>$1 if bose field fluctuations are taken into consideration. For the hole doping ($\delta >$0) case, the additional holes are expected to occpy oxygen sites. This implies that the renormalized charge transfer gap $\Delta _{reg}$ tends towards zero for $\delta >$ 0. We find the approximate Fermi liquid behavior for $\Delta \quad _{reg} \to $ 0 once the pure d$_{x2-y2}$ wave singlet superconducting instability sets in; otherwise non-Fermi liquid behavior is the prevalent one. The charge and the spin ordering gaps appear in the single-particle excitation spectrum when d$_{xy}$ component is taken into account. The latter is expected to shed light on the pseudo-gap phenomenon in cuprates. 1.Partha Goswami, Presented in \textbf{SCES'07} proceedings and accepted for publication in Physica B (see http://dx.doi.org/10.1016/j.physb2007.10.076).   

Electronic structure calculations for the fluctuating-bond model of high-temperature superconductivity.

High-temperature superconductors (HTS) have been intensely studied for the past 20 years due to their scientific and technological importance. The materials are characterized by square arrays of copper-oxygen-copper bonds. Although several salient features of HTS phenomena have been characterized, a working theory of the underlying physical processes in these materials has been lacking. Collaborators at IBM have recently proposed a model for HTS (Nature Physics {\bf 3}, p. 184, 2007), involving a nonlinear coupling of the planar oxygen vibrations to the $d_{x^2-y^2}$ electrons on the coppers. This interaction is modeled as a two-phonon coupling process in the resulting fluctuating-bond model (FBM). In the present work, we investigate the FBM theory by exploring the ability of the oxygens to buckle out of the straight Cu-O-Cu configuration by adjusting the charge on the copper atoms. The study is being conducted using Car-Parrinello simulations to investigate the electronic structure of the materials, and zero temperature single point DFT energy calculations to investigate the floppiness of the oxygen bonds. The aim is to justify, from first principles, the parameterization used in the FBM theory.   

Near-Zero Modes in Superconducting Graphene

Vortices in the simplest superconducting state of graphene contain very low energy excitations, whose existence is connected to an index theorem that applies strictly to an approximate form of the relevant Bogoliubov-deGennes equations. When Zeeman interactions are taken into account, the zero modes required by the index theorem are (slightly) displaced. Thus the vortices acquire internal structure; the resulting ``modicules'' obey nonabelian quantum statistics.   

Do `magic' Electronegativities exist for superconductivity?

Studies have established a strong correlation between electronegativity and superconductivity. Here we examine the electronegativity values of many known binary superconducting systems[A-15s, Y$_{2}$C$_{3}$, CaSi$_{2}$, MgB$_{2}$,{\ldots}] with high transition temperatures and use those[magic] values and their series to predict new superconducting materials. We also estimate the transition temperatures of the predicted compounds if they could be formed under pressure.   

Bogoliubov angle and visualization of particle-hole mixture in

Superconducting excitations ---Bogoliubov quasiparticles --- are the quantum mechanical mixture of negatively charged electron (-e) and positively charged hole (+e). Depending on the applied voltage bias in STM one can sample the particle and hole content of such a superconducting excitation. Recent Scanning Tunneling Microscope (STM) experiments offer a unique insight into the inner workings of the superconducting state of superconductors. We propose a new observable quantity for STM studies that is the manifestation of the particle-hole dualism of the quasiparticles. We call it a{\em Bogoliubov angle}. This angle measures the relative weight of particle and hole amplitude in the superconducting (Bogoliubov) quasiparticle. We argue that this quantity can be measured locally by comparing the ratio of tunneling currents at positive and negative biases. Bogoliubov angle allows one to visualize robustness of superconducting state locally. It may also allow one to measure the particle-hole admixture of excitations in normal state above critical temperature and thus to measure superconducting correlations in pseudogap state.   

Electronegativity Spectrum Maps: A computational combinatorial materials synthesis and search tool.

Using Pauling's electronegativity scale of the elements from 0.7 to 4.0 we build a matrix of possible binary combinations in increments of 0.1 for binary systems A$_{x}$B$_{y}$. We get a 34 x 34 spreadsheet of electronegativities. We call this an ElectroNegativity Spectrum Map [ENSMap]. Each of the 1156 cells represents a possible combination of two electronegativities that could yield a binary compound. Using the correlation between electronegativity and superconductivity, we can identify from an ENSMap the electronegativities of known superconductors of a given binary class. We can also identify other electronegativity combinations that give the same electronegativity as a known superconductor. Here we show that ENSMAPS of binary systems can become a powerful computational combinatorial material synthesis tool and also a tool for searching for novel materials. We use ENMaps to predict twenty new binary superconductors with high transition temperatures.   

Magnetic susceptibility and M\"{o}ssbauer studies of [FeX$_{3}$](ClO$_{4}$)$_{2}$$\cdot$H$_{2}$O with X = bpz, bpy, phen or tpy

Magnetic studies have been made on several tris-chelated iron complex compounds [FeX$_{3}$](ClO$_{4})_{2}\cdot $H$_{2}$O with aromatic nitrogen heterocycle ligands X = bpz (2,2'-bipyrazine), bpy (2,2'-bipyridine), phen (1,10-phenanthroline) or tpy (2,2':6,2''-terpyridine). SQUID data (2-300 K and 0.01-1 T) yielded small effective magnetic moments, which are characteristic of low-spin Fe(II), in agreement with the isomer shift and quadrupole splitting values from M\"{o}ssbauer measurements (4-300 K, 0-5 T). Meanwhile, apart from the expected diamagnetism, a positive term of temperature-independent paramagnetic susceptibility prevails in most cases.   

Experimental Determination of Thermal Entanglement in Spin Clusters using Magnetic Susceptibility Measurements

Until a few years ago, entanglement was not believed to exist beyond atomic scale, due to the large number of constituents of macroscopic objects. Surprisingly, it was theoretically demonstrated that entangled states can exist in solids at finite temperature and this kind of entanglement is referred in literature as ``thermal entanglement''. Since then, a few experimental evidences have been reported confirming the presence of entanglement in solids state systems. The present work reports an experimental observation of thermal entanglement in a spin chain formed in the compound Na$_{2}$Cu$_{5}$Si$_{4}$O$_{14}$. The presence of entanglement was investigated through two measured quantities, an Entanglement Witness and the Entanglement of Formation, both derived from the magnetic susceptibility, determined experimentally. It was found that pairwise and tripartite entanglement exist below $\sim $200 K and $\sim $240 K, respectively. A theoretical study of entanglement evolution as a function of applied field and temperature is also presented.   

Magnetic Structure and Magnetic Properties of CaMn2Sb2

The AM2X2 ternary intermetallic (A = rare or alkaline earth, M = transition metal) compounds have revealed interesting magnetic properties due to the interplay between their magnetic sublattices. Pursuing the idea that the coupling between Mn-Mn ions can significantly affect electric transport properties, we investigated (Ca,Sr)Mn2Sb2 intermetallic compounds which presents two secondary magnetic transitions at 82K and 250K. Field dependent dc-magnetization curves for CaMn2Sb2 were obtained at two different temperatures, above and below 250K, show a relatively steep increase of the magnetization upon increasing the field to $H ${\_} 5 kOe, followed by a less steep and almost linear increase with the field and no tendency for saturation. The net macroscopic moment on the Mn at 300 K and 50 kOe is only a fraction of a Bohr magneton (ca. 0.15 \textit{\'{\i}}B/ Mn), and evidently, a simple interpretation of its value in terms of localized high/low spin Mn2+ ions is unrealistic. The low moment can be viewed as a signature of the counterbalanced coupling between Mn atoms that are sitting on two inequivalent magnetic sites as predicted by theory [1]. In order to understand CaMn2Sb2 magnetic structure, we also performed neutron scattering measurements to clarify the magnetic structure and the origin of the low temperature transition. [1] S. Boved, J. Merz, A. L. Lima, V. Fritsch, J. D. Thompson, J. L. Sarrao, M. Gillessen, and R. Dronskowski. \textit{Inorg. Chem.}, 45:4047, 2006.   

Pressure Dependent Magnetism in Magnetically Ordered Interlanthanide Chalcogenides

Several new interlanthanide chalcogenide compounds, Ln$'$/Ln$''$/Q (Ln$’$=light and Ln$''$=heavy lanthanide, Q=S or Se) have been synthesized using a novel flux-growth technique, their complex structures determined, and their magnetic properties measured. The majority, with general formula Ln$'$Ln$''$Q$_3$ are paramagnetic for T$>$ 2K, with effective moments consistent with the magnetic Ln constituents. EuLn$_2$Q$_4$ (Ln=Tb - Lu), which crystallize in the CaFe$_2$O$_4$- type three-dimensional channel structure, are all antiferromagnetic with T$_N$ $\sim$ 3-5 K. The Ln constituent is geometrically frustrated and has secondary effects on the magnetic properties,which are dominated by the Eu-Eu superexchange coupling. The sharply defined Neel temperature increases with hydrostatic pressure to P$\sim$ 7 kbar for all EuLn$_2$Q$_4$. (For example, for EuLu2Se4, dT$_N$/dP = +0.03 K/kbar at low pressures.)   

Possible Exotic Magnetism in the Anti-Perovskite Nitride Cr$_{3}$PtN

Samples of the anti-Perovskite nitride Cr$_{3}$PtN were synthesized for bulk magnetic studies. X-ray diffraction confirmed the structure and revealed no secondary phases within instrumental sensitivity ($\sim $2-4 vol. {\%}). Bulk magnetic properties were studied by SQUID magnetometry at $T$ = 5-300 K in magnetic fields $H$ up to 6.5 T. Highly hysteretic ferromagnetism was found, with a Curie temperature $T_{c}\approx $ 110 K. (Prior to nitriding, the Cr$_{3}$Pt starting material was paramagnetic.) At 5 K, the coercive field $H_{c}$ is $\sim $2.3 T. The curious and possibly exotic feature is that the saturation magnetic moment is small, 0.2 G-cm$^{3}$/gram: if the signal arises from bulk Cr$_{3}$PtN, the corresponding moment is only 0.1 $\mu _{B}$ per formula unit, which is quite small for a 100 K ferromagnetic. The saturation magnetization varies as $M_{sat}\sim $ (1-$T$/$T_{c})^{\beta }$ with critical exponent $\beta $=0.40. In isostructural Pd-based Cr$_{3}$PdN (not single phase), no ferromagnetism was found above 5 K. DFT calculations of the band structure for the ideal anti-Perovskite compounds revealed a high electronic density of states $N(E_{F})$ for Cr$_{3}$PtN and a somewhat lower value for Cr$_{3}$PdN.   

Spin-reorientation transitions in Er, Tm and Yb orthoferrites: magnetic and structural properties.

Magnetic and structural characteristics of ErFeO$_3$, TmFeO$_3$ and YbFeO$_3$ single crystals were studied over a wide temperature range. Magnetic measurements found that the spin-rotation transitions in all crystals are well described by the earlier proposed theory with no fitting parameters. Additionally, they have shown the absence of the magnetic compensation point in TmFeO$_3$, and a noticeable growth of the c-axis magnetization at low temperatures in TmFeO$_3$ and ErFeO$_3$. The X-ray measurements found no symmetry-lowering lattice distortions during the reorientation. Overall, the measurements cover a wide range of material parameters and demonstrate the generality of the modified mean field theory of the $\Gamma_4 \to \Gamma_{24} \to \Gamma_2$ orientation phase transitions in orthoferrites. // L. T. Tsymbal {\em et al.}, J. Appl. Phys {\bf 101}, 123919 (2007).   

Magnetostatic Interactions in Partially Shielded Polyaniline-Ferromagnet Composite Nanowire Arrays

Ferromagnetic nanowires have remarkable magnetic properties including high coercivities and strong magnetic shape anisotropy. These unique properties have been theoretically studied and various models attribute the observed characteristics to inter- and intra- wire magnetostatic interactions, which are a function of the structure of the nanowires and their coupling with the applied magnetic field. In this study, we use porous alumina templates and electrodeposition to fabricate Fe, Ni, and Co nanowires. We also use the same techniques to grow polyaniline nanotubes and then fill them with Fe, Ni, and Co nanowires, creating magnetically shielded ferromagnet structures. We measure the magnetic properties of these structures as a function of their diameter and temperature in order to better understand the magnetic interactions that arise in ferromagnetic nanowire arrays. By partially shielding the wires with PAni, we are able to better discern the effects of these interactions. Results will be presented and compared to theoretical models.   

Dipolar effect in the standing spin waves of dot structures

Standing spin waves in dot structures are generally described by dipole-exchange mechanisms. In this work, the role of dipolar coupling has been probed via micromagnetic simulation of spin wave mode structure on a circular, permalloy, dot (200nm diameter, 40nm thickness) containing a concentric gap (ring) void of magnetic material. This gap precludes exchange coupling between the inner dot and the outer ring of the overall dot structure, while allowing dipolar coupling. The mode structure in the dot is investigated as a function of gap width and diameter with the magnetization perpendicular to the dot disk. Spin wave modes (Magnetostatic forward volume mode) up to 4$^{th}$ order were observed for both continuous dot and dot with a narrow gap (width $<$10 nm). As the diameter of this narrow gap increases, the spin wave patterns remain intact, though higher order peaks are more affected. When ring width is $>$ 10nm, the spin wave spectra are significantly disturbed, leading to a complete disappearance of high order spin wave modes at gap widths in excess of 16nm. Influence of gap geometry on specific standing wave modes will be presented and discussed.   

Absence of Hole Confinement in Transition Metal Oxides with Orbital Degeneracy

The compounds with orbital degrees of freedom exhibit many possible scenarios for hole propagation which in most cases lead to hole localization [1]. Here we investigate the spectral properties of a hole moving in a two-dimensional Hubbard model for strongly correlated $t_{2g}$ electrons. Although superexchange interactions are Ising-like, a quasi-one-dimensional coherent hole motion arises due to effective three-site terms. This mechanism is fundamentally different either from the hole motion via quantum fluctuations in the conventional spin model with SU(2) symmetry or from the $e_g$ orbital model [2]. The present orbital model describes also propagation of a hole in some $e_g$ compounds [3], and we argue that orbital degeneracy alone does not lead to hole self-localization. [1] J. Zaanen and A.M. Ole\'{s}, Phys. Rev. B {\bf 48}, 7197 (1993). [2] J. van~den Brink, P. Horsch, and A.M. Ole\'{s}, Phys. Rev. Lett. {\bf 85}, 5174 (2000). [3] M. Daghofer, A.M. Ole\'s, and W. von der Linden, Phys. Rev. B {\bf 70}, 184430 (2004).   

Magnetic phase separation in LaMn$_{1-x}$Fe$_{x}$O$_{3+y}$

We have investigated the LaMn$_{1-x}$Fe$_{x}$O$_{3+y}$ system in the whole range of 0$\le $x$\le $1, for polycrystalline samples prepared by solid state reaction in air. All samples show orthorhombic structure (space group Pnma). For x=0 the oxygen excess, estimated to be y $\sim $ 0.1, produces vacancies in the La and Mn sites and generates a fraction around 20{\%} of Mn$^{4+}$ ions (3t$_{2g})$ and 80{\%} of the usual Mn$^{3+}$ ions (3t$_{2g}$, 1e$_{g})$, with possible double exchange interaction between them. The Fe-doping in this system is known to produce only stable Fe$^{3+}$ ions (3t$_{2g}$, 2e$_{g})$. We find an evolution from a fairly strong ferromagnetic (FM) behavior, with saturation magnetization (T=2K) m$_{S}$ $\sim $ 4 $\mu _{B}$ and Curie temperature T$_{c} \quad \sim $ 160 K, for x=0, to an antiferromagnetic (AFM) behavior, with T$_{N}$=790 K, for x=1. For intermediate Fe contents a mixed phase scenario occurs, with a gradual decrease (increase) of the FM (AFM) phase, accompanied by a systematic transition broadening for 0.2$<$x$<$0.7. A calculation based on the expected exchange interaction among the various magnetic-ion types, accounts very well for the m$_{S}$ dependence on Fe doping.   

The Friedel-Anderson Impurity with Orbital Degeneracy

A recently developed compact solution for the non-degenerate Friedel-Anderson impurity is extended to impurities with orbital degeneracy. The singlet ground state is investigated for two and three orbits (corresponding to four and six d-states). The ground state energy and the multi-d-state occupations are calculated. The magnetic moment (above the Kondo temperature) is obtained in different regions of the parameter space of V$_{sd}$ (s-d-hopping matrix element), E$_{d}$ (d-state energy), U and U$_{x}$ (Coulomb and exchange energies). The average d-state occupation can be adjusted to about one, two or three d-electrons. A comparison between different orbital degeneracy but the same d-state occupation is performed. The role of the Coulomb and the exchange interaction in the magnetic and singlet states is analyzed. The challenges for the treatment of a real d-impurity with five d-orbits is discussed.   

Magnetic and charge correlations in the frustrated 2D Hubbard model

The high temperature superconductors have motivated numerous theoretical studies of strongly correlated many-body systems for over two decades. The richness of the phase diagram of these materials belies their relatively simple quasi-two-dimensional structure of stacked CuO$_2$ planes, where copper ions form a square lattice. With the experimental observation of several complex phases, including superconductivity, in quasi-two-dimensional triangular lattice materials (e.g., Na$_x$CoO$_2$ $\cdot y$H$_2$O and $\kappa$-(ET)$_2$X) we now have material systems in which geometric frustration plays a prominent role. With this as our motivation, we investigate the 2D Hubbard model on a series of lattice geometries. We report preliminary results from mean-field calculations of the charge and magnetic properties of our model on frustrated and non-frustrated lattices. We also discuss the potential application of the constrained path quantum Monte Carlo (CPQMC) method to the study of frustrated 2D Fermi systems.   

Identification of Bosonic Mode in the Electron-Doped Superconductor Pr$_{0.88}$LaCe$_{0.12}$CuO$_{4-\delta }$

It is well known that in the superconducting state, the current carriers are the cooper pairs, where two electrons (fermions) get paired up by a mediator (glue) and behave as a single boson. In conventional superconductors, lattice vibrations (phonons) act as a glue to pair up the electrons, however, in high T$_{c}$ superconductors, the mechanism that binds these fermions together is sill unclear. There are two principal contenders for the glue: phonons and the spin excitations. Using high resolution STM we have probed an electron-doped superconductor, Pr$_{0.88}$LaCe$_{0.12}$CuO$_{4-\delta }$(T$_{c}$ = 24K) and identified a bosonic excitation at 10.5 $\pm $ 2 meV which could potentially act as the superconducting glue. The energy scale of this mode rules out an explanation in terms of the oxygen optical phonons confining the possibilities to spin excitations and the acoustic phonons. This finding potentially takes us one step closer to identifying the superconducting glue in High T$_{c}$ superconductors.   

Investigating the Structure of La$_{2}$CuGa$_{12}$, Using Neutron Powder Diffraction

Single crystals of a new phase, La$_{2}$CuGa$_{12}$, have been synthesized using flux growth. Preliminary single crystal X-ray diffraction results suggest La$_{2}$CuGa$_{12}$ to be isostructural to Ce$_{2}$PdGa$_{12 }$and crystallize in the $P$4/\textit{nbm} space group with lattice parameters of $a \quad \sim $ 6.179 {\AA} and $c \quad \sim $ 15.384 {\AA}. Residual electron density, which was observed in the Fourier difference map of the single crystal X-ray diffraction data, and the observation of satellite peaks in the data are indicative of the possibility of statistical disorder in La$_{2}$CuGa$_{12}$. In addition, unusual behavior of thermal parameters for an additional Ga position is observed in the X-ray diffraction data of La$_{2}$CuGa$_{12}$. Although a preliminary model has been constructed using single crystal X-ray diffraction experiments, due to the two electron difference between $^{29}$Cu and $^{31}$Ga, neutron powder diffraction experiments may be a more suitable probe in accurately determining the structure and site occupancy of the additional Ga atom in La$_{2}$CuGa$_{12}$. We have employed neutron powder diffraction (BT-1) to investigate the structure in the phase, La$_{2}$CuGa$_{12}$.   

Synthesis and Characterization of High Quality Single Crystal CeCoIn5

The preliminary results on the structure and electronic properties of (R, Ce)(Co, M)In$_{5}$ are presented. Here R= Pr and M= Fe. We present the results of our measurements of resistivity, specific heat and magnetization. This work is supported by the Department of Energy Grant {\#} DE-F052-05NA27036.   

Spin valve electrode for organic light emitting devices

Effects of spin filter on singlet and triplet exciton fractions in organic light-emitting devices (OLEDs) is attracting considerable attention nowadays. Electroluminescence in organic semiconductors strongly depends on the relative population of singlet and triplet excitonic states. Controlling the spin statistics by injecting and transporting carriers with defined spin orientation can amplify a chosen electronic transition increasing the device efficiency or changing the emission spectral band. A spin valve was used as hole injector, substituting the traditional indium tin oxide electrode. A comparison of electroluminescence and IV curves between similar devices with and without spin valve electrode is reported. The result shows that spin valve can successfully replace conventional electrode in OLEDs.   

Nonlinear current-voltage characteristics of oxygen-deficient La$_{0.67}$Ca$_{0.33}$MnO$_{3-y}$ films

Two different types of nonlinear current-voltage characteristics are observed in oxygen-deficient La$_{0.67}$Ca$_{0.33}$MnO$_{3-y}$ (LCMO) films at temperatures below insulator-metal transition. The parabolic-like dynamic conductance $G(V)$, defined as d$I$/d$V$, curves near zero bias observed in highly oxygen-deficient LCMO films implies the contribution from the spin-dependent tunneling transport between ferromagnetic clusters with magnetic-disordered regions serving as tunneling barriers. On the other hand, for the slightly oxygen-deficient LCMO films, dips around zero bias were observed in nonlinear $G(V)$ curves and have been attributed to spin-flip scattering with oxygen vacancies serving as scattering centers.   

Anisotropies in Quaternary Intermetallic Compounds

From the high-temperature series expansion of magnetic susceptibilities and the anisotropic Weiss temperatures, the first Steven's parameter, B$_{2}^{0}$ , and the magnetic exchange interaction constant J$_{ex}^{ll}$ of each R$^{=3}$ ions magnetic sublattice in quaternary intermetallic compounds, RNi$_{2}$B$_{2}$C B(R= Tm, Er, Ho, Dy, and Tb) were obtained. The R =Dy system shows the biggest B$_{2}^{0}$ value and the R = Tb system does the smallest one. Also we have measured and analyzed the anisotropic M(H) isotherms as a function of applied magnetic fields for H perpendicular and parallel to the c-axis for each compounds to check out our crystalline electric field (CEF) results obtained from the previous mentioned method by using the anisotropic Weiss temperatures. It turned out that most of the temperature dependence of magnetization curve M (T) for H perpendicular the c-axis at low temperature comes from the temperature dependent population of the singlet ground state in group L among groups L(low-lying levels of ground states), H(high levels of ground states), and M(first excited states).   

Anisotropy in the magnetodielectric constant in orthorhombic HoMnO3 thin film

Epitaxial HoMnO$_3$ thin films in the metastable orthorhombic structure were successfully synthesized on SrTiO$_3$ substrates by pulsed laser deposition. The crystal structure, surface roughness, and surface morphology of the films were characterized by various tools such as X-ray diffraction, atomic force microscopy, scanning electron microscopy etc.. Macroscopic physical properties were measured with a Quantum Design PPMS. It is found that an significant increase of the dielectric constant accompanies the onset of magnetic order at ~45 K in the films. This then proves that there exists a magnetodielectric coupling in orthorhombic HoMnO$_3$. Anisotropy in the magnetodielectric constant was observed according to the direction for an external magnetic field. These behaviors will be compared to the cases of YMnO$_3$ and BiMnO$_3$. Measurements of the reverse effect, that is, the variation of the magnetic susceptibility with an electric field, are being attempted.   

Diffraction-induced magneto-optical enhancement in gyrotropic gratings

The spectra of diffracted magneto-optical Kerr effect (D-MOKE), for polar magnetization in one- dimensional gyrotropic gratings, were presented by Antos \textit{et al.} [Appl. Phys. Lett. \textbf{86}, 231101 (2005)]. It was noted that the magnitude of Kerr rotation in the first-order diffraction was one order higher than that of the zeroth-order diffraction in most of the energy range. In this study, a rigorous coupled-wave approach, implemented as Airy-like internal series, was applied to investigate the diffraction-induced MO enhancement. The simulated spectra of D-MOKE are consistent with their experimental ones. Moreover, it was found that the magnitude ratio of the first-order Kerr rotation to the zeroth-order one was strongly dependent on the grating depth. In other words, D-MOKE can be effectively modulated by the groove depth. This theoretical approach is of great significance in designing and applying the diffracted MO elements.   

Synthesis and magnetic properties of Zn$_{1-x}$Mn$_{x}$O/ZnO coaxial nanocabl.

Zn$_{1-x}$Mn$_{x}$O/ZnO (x=0.04 and 0.20) coaxial nanocables were prepared by using an ultrahigh-vacuum radio-frequency magnetron sputtering system. The samples were characterized by scanning electron microscopy (SEM), x-ray diffraction (XRD), Rutherford backscattering and high-resolution transmission electron microscopy (HR-TEM), and with a superconducting quantum interference device magnetometer. The SEM images show that the morphology and the alignment of ZnO nanocables are maintained after the deposition of Zn$_{1-x}$Mn$_{x}$O layer, and the thickness of Zn$_{1-x}$Mn$_{x}$O layer is about 20 nm. The XRD analysis reveals that Mn is incorporated well into the wurtzite ZnO without forming Mn oxide. The HR-TEM image shows that both ZnO core layer and Zn$_{1-x}$Mn$_{x}$O shell layer are single crystalline and an excellent epitaxial growth has been achieved. The magnetic property measurement indicates that the Zn$_{0.96}$Mn$_{0.04}$O/ZnO coaxial nanocable is in the ferromagnetic state at 300 K as well as at 10 K, while Zn$_{0.80}$Mn$_{0.20}$O/ZnO is nonferromagnetic even at 10 K and the bare ZnO nanorod is diamagnetic. The aging effect of the magnetism for Zn$_{0.96}$Mn$_{0.04}$O/ZnO coaxial nanocable was also investigated, and it was found that the aged sample showed a mixed magnetic phase of ferromagnetism and paramagnetism.   

Enhanced magneto-optical effect in one-dimensional spin photonic crystals

Spin photonic crystals (SPCs) are very interesting for information technology which requires advanced solutions for heavy communication traffic, high-density storage, and high-speed computing. By using external magnetic field, for instance, the optical properties of SPCs can be tuned. By using the interference pattern of two femtosecond-laser beams, a selective-area annealing of the as-deposited Co$_{2}$MnSi film was achieved and one-dimensional SPCs were fabricated. The atomic-force-microscopy results confirmed that regularly-spaced alternating lines with a periodicity of 2 $\mu $m were produced, and the magnetic-force-microscopy studies revealed the same periodic patterns of magnetic domains. The longitudinal Kerr rotations of the p-polarized zeroth-order and first-order diffracted beams were measured. The longitudinal Kerr rotation of the first-order diffracted beam turns out to be nearly 28 times larger with respect to the zeroth-order one.   

Size dependent magnetic properties of magnetite (Fe$_{3}$O$_{4})$ nanoparticles.

Magnetism of magnetite (Fe$_{3}$O$_{4})$ nanoparticles was studied as a function of the particle size. Fe$_{3}$O$_{4}$ nanoparticles with different size from 3 nm to 10 nm were synthesized by high temperature organic solution phase method. Hysteresis loops of all the particles showed superparamagnetic behavior at room temperature. The blocking temperature (T$_{B})$ decreases with decreasing particle size. All hysteresis loops were fitted by the Langevin's function, where the saturation magnetization (M$_{s})$ was extracted. M$_{s}$ was further deduced by using the saturated moment and accurately measured mass of the particles. The two methods agree with each other excellently. M$_{s}$ decreases as the particle size is decreased, and is in general much smaller than that of bulk. M$_{s}$ shows a sharp drop with increasing temperature at low temperatures and deviates from the T$^{3/2}$-law. This behavior is attributed to competing ferromagnetic and antiferromagnetic exchange interactions which contribute differently at the surface and interior of the particles.   

\textit{Ab initio} study of the possibility of noncollinear magnetism in small Mn clusters

We investigated the possibility of noncollinear magnetism in small Mn$_{n}$ clusters ($n$ = 2-6) using the density-functional method SIESTA with the generalized gradient approximation (GGA) to exchange and correlation. The lowest-energy states identified were ferromagnetic for Mn$_{2}$ and Mn$_{3}$, and magnetically noncollinear for Mn$_{4}$, Mn$_{5 }$ and, most decidedly, Mn$_{6}$. These SIESTA/GGA results, which are compared with those of an earlier SIESTA study that used the local spin density approximation, are qualitatively in keeping with the result obtained by VASP/GGA calculations.   

Microscopic simulation and analysis of a spin crossover transition

In spin crossover materials, an abrupt phase transition between a low spin state and a high spin state can be driven by temperature, pressure or illumination. Of a special relevance are Fe(II) based coordination polymers where, in contrast to molecular systems, the phase transition shows a pronounced hysteresis which is desirable for technical applications. A satisfactory microscopic explanation of this large cooperative phenomenon has been sought for a long time. The lack of X-ray data has been one of the reasons for the absence of microscopic studies. In this work, we present an efficient route to prepare reliable model structures and within ab initio density functional theory analysis and effective model considerations we show that in polymeric spin crossover compounds magnetic exchange between high spin Fe(II) centres is as important as elastic couplings for explaining the considerable cooperativity and thus the width of the hysteresis.   

Statistical mechanics and thermodynamics in anisotropic Heisenberg-like nanoclusters

The single site quantum and thermal entanglement, concurrences, quantum phase transitions and quantum critical points are studied in small spin s = 1/2 and 1 in ferromagnetic and antiferromagnetic Heisenberg clusters. The grand canonical ensemble of Heisenberg clusters is also used for exact calculations of thermal properties, quantum and thermal entanglements of the spin lattice models in the presence of magnetic field and anisotropic field. We study the magnetic phase transitions and crossovers in clusters of various topologies driven by exchange interaction, external field and temperature. The comparison with the exact solution for the Heisenberg model in thermodynamic limit for the limiting cases is also provided. The small Ising and Heisenberg clusters are also used for comparison with the exact Bethe-ansatz solutions. These exact results in clusters give a novel insight into the properties of single molecule magnets, the dynamics of magnetization and can be useful for interpretation of the phase diagram in molecular nanomagnets and nanometer-sized magnetic particles.   

muSR in Ba$_{2}$CoO$_{4}$

A positive muon spin rotation and relaxation ($\mu^+$SR) experiment on the single crystal Ba$_2$CoO$_4$ indicates the existence of an antiferromagnetic (AF) transition occurring at $T_{N}\sim$25~K. Weak transverse field measurements (wTF-$\mu^+$SR) show that the paramagnetic volume fraction of the sample decreases rapidly at the magnetic transition indicating a bulk effect. Zero field measurements (ZF-$\mu^+$SR) show the presence of a magnetically ordered state below $T_{N}$. The results are compared to recent magnetic susceptibility and neutron measurements. Although there are two possible AF spin structures proposed by recent neutron experiments, the $\mu$SR results clearly exclude AF order along the $c$-axis while supporting AF order in the $ab$ plane.   

Inelastic Neutron Scattering Excitations for a Spin $\frac{3}{2}$ Tetramer: Application to Magnetic Excitations in Na$_3$RuO$_4$

We examine the magnetic properties and inelastic neutron scattering excitations for spin 3/2 tetramer using an isotropic Hamiltonian. Results on magnetic excitations observed in polycrystalline sodium ruthenate (Na$_3$RuO$_4$) are compared to the theoretical predictions. Previous work has suggested that this material consists of relatively isolated tetramers of S=3/2 Ru(V) ions, where a Heisenberg antiferromagnetic Hamiltonian was proposed. We determine that tetramer model (interacting dimers) may not be a good candidate for the magnetic structure of the system. Using three separate models, we compare parameters determined from magnetic susceptibility and inelastic neutron scattering structure factors, which suggest that separate dimers may be a more plausible model. However, future studies on single crystals are suggested to help clarify the apparent discrepancies between these model and our results.   

$\mu $SR study of spin dynamics and phase transition of the two-dimensional tetramer-cuprate Na$_{5}$RbCu$_{4(}$AsO$_{4})_{4}$Cl$_{2}$

In an effort to explain the magnetic properties of such low-dimensional systems, $^{87}$Rb Nuclear Magnetic Resonance (NMR) experiments in a Na$_{5}$RbCu$_{4}$(AsO$_{4})$Cl$_{2}$ system were performed. This novel two-dimensional (2D) cuprate contains layers of coupled Cu$_{4}$O$_{4}$ tetramers. The spin exchange interactions are confined to 2D layers and the Cu are divalent, making the system a s=1/2 antiferromagnet. In zero applied magnetic field, it orders antiferromagnetically via a second-order phase transition at T$_{N}$=15(1) K. The ordered state was characterized by $^{87}$Rb NMR, and a non-collinear rather than collinear arrangement of spins was suggested. New structural phase transition(s) around 74 and 110 K were also evidenced. We present a \textit{$\mu $}SR study of this cuprate. The investigation of the spin dynamics (via the muon longitudinal relaxation rate $\lambda $(T) ) in the temperature range 2$<$T$<$300K in zero-field, with particular attention to the order parameter below T$_{N}$ and around structural phase transitions at T$\sim $74K and T$\sim $110K is shown.   

Spin dynamics of La$_{0.845}$Sr$_{0.155}$Mn$_{1-x}$M$_{x}$O$_{3}$ (M = Mn, Cu, Co) perovskites

Influence of the spin-lattice coupling on the magnetoresistance and magnetocaloric properties of La$_{0.845}$Sr$_{0.155}$Mn$_{1-x}$M$_{x}$O$_{3}$ (M = Cu, Co) perovskites has been investigated by means of electron spin resonance (ESR) spectroscopy. It was observed that asymmetrical ESR signals due to ferromagnetic correlations at temperatures T $<$ T$_{min}$ became Lorentzian at T $>$ T$_{min}$, where T$_{min}$ corresponds to the narrowest ESR linewidth. The temperature dependence of the ESR intensity, I(T), for the samples was well described by an expression of I(T) = I$_{o}$exp(E$_{a}$/k$_{B}$T). In the high temperature region, 1/I(T) obeyed the Curie-Weiss law. The minimum linewidth, $\Delta $H$_{min}$, was determined to be 674, 890 and 750 Oe for La$_{0.845}$Sr$_{0.155}$Mn$_{1}$O$_{3}$, La$_{0.845}$Sr$_{0.155}$Mn$_{0.9}$Cu$_{0.1}$O$_{3}$ and La$_{0.845}$Sr$_{0.155}$Mn$_{0.98}$Co$_{0.02}$O$_{3}$, respectively. This indicated an improvement of the spin-lattice coupling in samples with Cu or Co addition. The strongest spin-lattice coupling resulted in the largest magnetocaloric effect in La$_{0.845}$Sr$_{0.155}$Mn$_{0.9}$Cu$_{0.1}$O$_{3}$. The addition of Cu or Co in La$_{0.845}$Sr$_{0.155}$Mn$_{1}$O$_{3}$ reduced its ferromagnetism and conductivity. The mechanism of the spin-lattice coupling is discussed.   

Competition between magnetic structures in the Fe-rich fcc FeNi alloys

We report on the results of a systematic \textit{ab initio} study of the magnetic structure of Fe rich fcc FeNi binary alloys for Ni concentrations up to 50 at. {\%}. Calculations are carried out within density functional theory using two complementary techniques, one based on the Exact Muffin-Tin Orbital theory within the coherent potential approximation and another one based on the Projector Augmented-Wave method. We observe that the evolution of the magnetic structure of the alloy with increasing Ni concentration is determined by a competition between a large number of magnetic states, collinear as well as non-collinear, all close in energy. We emphasize a series of transitions between these magnetic structures, in particular we have investigated a competition between disordered local moment configurations, spin spiral states, the double layer antiferromagnetic state, and the ferromagnetic phase, as well as the ferrimagnetic phase with a single spin flipped with respect to all others. We show that the latter should be particularly important for the understanding of the magnetic structure of the Invar alloys.   

One-Dimensional Magnetic Chain Fragmentation in Ti/Ga Modified Multiferroic DyMn$_{2}$O$_{5}$

We have studied the effect of Ga$^{3+}$ substitution into Mn$^{3+}$ site in DyMn$_{2}$O$_{5}$ on its multiferroic charateristics. Crystallographic structural, thermal, dielectric and magnetic properties are measured and discussed in terms of the dilution of magnetic Mn$^{3+}$ ions. Replacement of Mn$^{3+}$ with Ga$^{3+}$ changes the compound into a disordered multiferroic system and colossal magnetodielectric effect observed in DyMn$_{2}$O$_{5}$ diappears quickly below x$\sim $0.1 in DyMn$_{2-x}$Ga$_{x}$O$_{5}$. However as the content of Ga increases above x$\sim $0.1, a new type of ferroelectric transition identified by a peak in dielectric constant is evolved from the slowly varying dielectric background. Moreover remarkable enhancement of dielectric constant at ferroelectric transition temperature is continued even up to the solubility limit of Ga (x$\sim $0.2). This observation suggests one possible example of the channel for ferroelectricity in the disordered multiferroic system.   

Maser Generation of Hypersound From Heat Through Adiabatic Demagnetization of a Paramagnetic Crystal

It has been shown in our earlier work on thermal masers [Sov. Phys. - JETP (USA), v. 57, No. 6, pg. 1263-9 (1983)] that the energy of thermal phonons of a crystal like La$_{2}$Mg$_{3}$(NO$_{3})_{12}$*24H$_{2}$O (LaMN) doped with $^{59}$Co$^{2+}$ (ions A) and Ce$^{3+}$ (ions B) can be converted directly into the energy of coherent microwave radiation. Maser action has been observed at liquid helium temperatures in the 0.18-T static magnetic field after termination of the 1-T magnetic field pulse (which is applied for the adiabatic cooling of the electron spins $\raise.5ex\hbox{$\scriptstyle 1$}\kern-.1em/ \kern-.15em\lower.25ex\hbox{$\scriptstyle 2$} $ of the ions B). A partial population inversion is achieved in the system of hyperfine magnetic spin sublevels of the ions A as a result of the fast resonant thermal mixing between the nuclear spins I = 7/2 of the ions A and the cooled spins B. Our device acts as a \textit{quantum heat engine}, without any microwave or optical pumping. Here we propose an analogous easily tunable pulsed \underline {\textit{phonon}} maser. The laser crystals of KY(WO$_{4})_{2}$ [Eur. Phys. J. \textbf{B55}, 388-395 (\textbf{2007})] doped with $^{168}$Er$^{3+}$ and $^{171}$Yb$^{3+}$ (I = $\raise.5ex\hbox{$\scriptstyle 1$}\kern-.1em/ \kern-.15em\lower.25ex\hbox{$\scriptstyle 2$} )$ or $^{173}$Yb$^{3+}$ (I = 5/2) ions are proposed as working substances (instead of the crystal hydrate of LaMN:Ce$^{3+}$: $^{59}$Co$^{2+})$.   

Magnetic and Structural Properties of Magnetite in Radular Teeth of Chiton Acanthochiton Rubrolinestus

The major radular lateral teeth of Polyplacophora Chiton comprise a magnetite biomineral cap.We have investigated the structure and magnetic properties of the biomineralized magnetite crystallites in mature teeth of Chiton \emph{Acanthochiton Rubrolinestus}. From the measurement of magnetic properties of tooth particles using SQUID magnetometry we find that the saturation magnetization and the Verwey transition temperature (T$_v$) are 78.4 emu/g and 105 K, respectively. An in situ examination of the structure of magnetite-bearing region within individual tooth using the high resolution TEM, together with electron diffraction (ED) pattern and energy-dispersive X-ray (EDX) analyses indicates magnetite microcrystal form electron-dense polycrystalline sheets with typical length 800 nm and width 150 nm or so. These polycrystalline sheets are arranged regularly along the longitude direction of the tooth cutting surface. Furthermore, the microcrystallites in polycrystalline sheet take on the generally good crystallinity.   

Heat diffusion in a classical Heisenberg chain

We study the heat transport in a one dimensional classical Heisenberg chain by means of spin dynamics simulation. The system consists of a $N=2000$ spins in the microcanonical ensemble, and the temperature is evaluated using the so-called configurational temperature formula. We thermalize the system to an equilibrium state at low temperature, and then we give a delta function energy packet in the middle of the chain. Studying both qualitatively and quantitatively the spreading of the delta, we characterize the diffusion of heat in the chain. In fact, calculating the second moment of the pulse, and using the relation \begin{equation} \left\langle \sigma (t) ^2\right\rangle =2 D t^{\alpha}, \,~ ( 0 < \alpha < 2), \end{equation} where $\sigma$ is the width of the pulse, $D$ is the diffusion constant and $\alpha$ is a parameter which consider the kind of diffusion. According our simulation, the heat in this system transports subdiffusively. This result imply that the system becomes a thermal insulator in the thermodynamic limit $N \rightarrow \infty $.   

Magnetic properties of a mechanically alloyed metastable Cu$_{1-x}$Co$_{x}$ system

We report a detailed study of the magnetic properties of Cu$_{1-x}$Co$_{x}$ with Co concentrations between 7 and 10 at{\%} produced by mechanical alloying, through a reactive milling process by using a Retsch PM 400 planetary ball mill under argon atmosphere. We have magnetically characterized our samples by using a Vibrating Sample magnetometer, VSM, from Quantum Design$^{TM}$. We conducted magnetization hysteresis loops at different temperatures from 5 to 300 K. We also measured magnetization as a function of temperature for samples with different milling times. We analyzed the dependence of the coercive field on temperature and found that when milling time increases from 80 to 100h, it reflects an increase in the coercive field from 425 to 525 Oe at 30K; that is the maximum coercive field. We can explain these results by using a dipolar interaction model according to the Co precipitate size in the copper matrix.   

Study of the magnetic behavior of Mn$_{1-x}$Zn$_{x}$ Ferrite nanoparticles at low temperature

We report on magnetic spin resonance and magnetization hysteresis loops of Mn$_{1-x}$Zn$_{x}$ Ferrite nanoparticles with sizes ranging from 20nm to 50nm obtained via microemulsions. The samples were evaluated by VSM technique ranging from 10K$<$300K at ZFC and FC. EPR measurements at T=180K were carried out in the 0 $\le $ x $\le $ 0.75 range. Experimental results for the peak-to-peak linewidth, \textit{$\Delta $H}$_{pp}$, have been discussed by the existence of monodomain ferrimagnetic particles. The results indicate an increase of \textit{$\Delta $H}$_{pp}$ by increasing the Zn concentration. The EPR signal shows a second EPR pick for x$>$0.5 at g=4.10 associated at local magnetic fields produced for Mn chains. The anisotropy constant was calculated by means of a genetic algorithm for parameter optimization of the Jiles-Atherton model.   

NMR and spin relaxation in systems with iron oxide nanoparticles

Effect of the superparamagnetic nanoparticles to $^{1}$H NMR spectra and spin dynamics of the host systems have been studied in liquid (water and toluene), solid (polymer) and gelatin suspensions of $\gamma $-Fe$_{2}$O$_{3}$ nanoparticles. Significant broadening of $^{1}$H NMR spectra and growing relaxation rates were observed with increased concentration of nanoparticles in the liquid systems while the polymer systems demonstrate inhomogeneous broadening of the spectra and practically no dependence of T$_{1}$ upon the nanoparticle concentration. In gelatin solution, both effects were observed depending on the line position. We explain the experimental results taking into account predomination of self-diffusion as a source of the relaxation, with allowance made for the formation of magnetic aggregates.   

Progress towards Growth and Characterization of Rare-Earth Nanoparticles using the Inverse Micelle Method

Nano-sized particles and clusters have promising electrical, chemical, and magnetic properties as compared to the bulk materials. Due to their reduced dimensionality, it makes their physical properties significantly different from the bulk material. The nano-sized materials have great potential for technical applications, such as, magnetic information storage, imaging, medical devices, and magnetic refrigeration. In this report, we will present the preliminary results on the growth and characterization of rare-earth metallic nanoparticles of Gd and Nd synthesized by the inverse micelle method [1]. These results will be compared to the bulk properties of Gd and Nd, as well as, to those exhibited by metallic nanoparticles, such as Co (by inverse micelle), and Gd (by laser evaporation cluster source), which have been found to show superparamagnetic behavior, enhanced magnetization, and self-organization [2-4]. [1] X.M. Lin, et al. Langmuir. 14, 7140 (1998). [2] D.C. Douglass, et al. Phys. Rev. B. 47, 19 (1993). [3] C. Petit, et al. Advanced Materials. 10, 259 (1998). [4] J.P. Chen, et al. Phys. Rev. B. 51, 11527 (1995).   

Exchange biased anisotropic magnetoresistance in Co/CoO bilayer

We measured the anisotropic magnetoresistance of a Co(11nm)/CoO bilayer in exchange biased and unbiased states. The bilayer was fabricated on a-Al$_{2}$O$_{3}$ substrate maintained at 300$^{o}$C by molecular beam epitaxy at a base pressure of 10$^{-11}$mbar. $\theta $-2$\theta $ X-ray diffraction scans reveal hcp (001) texture of the Co film. No peak associated with the naturally formed CoO top layer was identified. Small angle X-ray reflectivity scans yield the Co and CoO thicknesses as 11 and 2.4nm, respectively. Exchange bias was obtained from field cooled magnetoresistance measurements at various temperatures carried out in a closed cycle cryostat. Exchange bias varies quasi linearly with temperature and vanishes at the blocking temperature, $T_{B}$=97K. The latter is less than 2/3 of the bulk N\'{e}el temperature allowing to estimate the T=0 antiferromagnetic correlation length of CoO to be 1.84 nm in accordance with the geometrical confinement.   

Synthesis and characterization of iron nanoparticles by high-pressure sputtering.

The study of magnetic nanoparticles is interesting because of its importance and applications in the production of nano-electromechanical systems (NEMS) and micro-electromechanical systems (MEMS). We use a sputtering technique to deposit iron nanoparticles on a silicon substrate. The nanoparticles are then analyzed using atomic force microscopy (AFM), x-ray diffraction, and superconducting quantum interference devices (SQUID). AFM data shows that the size of the particles depends on different deposition conditions. Then, x-ray diffraction data shows that the nanoparticles adopt the body-centered cubic crystal structure. Finally, SQUID measurements were performed to characterize the magnetic properties of the nanoparticles. Systematic change in the magnetic properties was observed for particles with different sizes. The results show that the size and magnetic properties could be tuned by changing the deposition conditions.   

Magnetite-Alginate-AOT nanoparticles based drug delivery platform

Iron oxide having the magnetite structure is a widely used biomaterial, having applications ranging from cell separation and drug delivery to hyperthermia. In order to increase the efficacy of drug treatments, magnetite nanoparticles can be incorporated into a composite system with a surfactant-polymer nanoparticle, which can act as a platform for sustained and enhanced cellular delivery of water-soluble molecules. Here we report a composite formulation based on magnetite and Alginate-aerosol OT (AOT) nanoparticles formulated using an emulsion-cross-linking process loaded with Rhodamine 6G [1]. We prepared two set of nanoparticles by using Ca$^{2+}$ or Fe$^{2+}$ to cross-link the alginate polymer. Additionally, we added $\sim $8 nm diameter Fe$_{3}$O$_{4}$ magnetic nanoparticles prepared by a soft chemical method to these alginate-AOT nanoparticles. The resulting composites were superparamagnetic at room temperature, with a saturation magnetization of approximately 0.006 emu/g of solution. We will present detailed studies on the structural and magnetic properties of these samples. We will also discuss HPLC measurements on Rhodamine uploading in these composites. [1] M.D.Chavanpatil, Pharmaceutical Research, vol.24, (2007) 803.   

The importance of local band effects for ferromagnetism in hole doped La$_2$CuO$_4$.

Doping is of vital importance for cuprates, since only about 0.15 holes per Cu atom is sufficient to transform them from antiferromagnetic insulators to the best known superconductors. The doping is usually made through substitution of the ternary element (Sr for La etc.), and the effects are typically described by rigid band filling of the CuO band. However, results of band calculations for supercells of La$_{(2-x)}$Ba$_x$CuO$_4$ show that the rigid band model for doping is less adequate than what is commonly assumed. In particular, weak ferromagnetism (FM) can appear locally around clusters of high Ba concentration. The clustering is important at large dilution and averaged models for magnetism, such as the virtual crystal approximation, are unable to stabilize magnetic moments. These results give a support to the idea that weak FM can be the cause of the destruction of superconductivity at high hole doping.   

Tuning magnetic interaction in orthorhombic Neodymium-Yttrium Manganites Nd$_{1-x}$Y$_{x}$MnO$_{3}$

By lowering the Mn-O-Mn bond angle in LnMnO$_{3}$ with Ln=La-Ho the Neel-temperature decreases and at Ln=Tb the A-type antiferromagnet transforms to an incommensurate (IC) spin-spiral phase for Ln=Gd,Tb,Dy. The spin-spiral breaks both inversion and time reversal symmetry leading to a strong coupling between magnetism and ferroelectric polarization. We investigate the evolution of the crystal and magnetic structure from the A-type phase to the IC spin spiral phase by systematically replacing neodymium by yttrium in NdMnO$_{3}$ resulting to a decrease of the tolerance factors to values similar to that for multiferroic TbMnO$_{3}$. One advantage of this approach is that the tolerance factor can be tuned and that neodymium and yttrium are not high neutron absorbing elements in sharp contrast to other rare earths like Gd, Dy and Eu. Compositions x=0.0 to 0.6 have been prepared, neutron and x-ray powder diffraction patterns were measured as well as the magnetic properties. It can be shown that by decreasing the tolerance factor that way, similar effects can be seen as with varying the ionic size of the rare earth ions. For example we found that between 0.4$<$x$<$0.6 the incommensurate phase co-exists with the A-type antiferromagnetic phase and with x=0.6 and higher the system is only incommensurate and seemingly multiferroic.   

Determination of Magnetic Directions in Multiferroic BiFeO$_{3}$ Thin Films

BiFeO$_{3 }$(BFO), a ferroelectric and an antiferromagnet, is the only single phase room temperature multiferroic that is currently known. Multiferroics are interesting materials not only because of their exciting order parameters, but for the potential for parameter coupling. In order to understand the magnetoelectric coupling, the individual order parameters must first be understood. A combination of in-plane and out-of-plane piezoresponse force microscopy (PFM) allows 3D mapping of the ferroelectric polarization directions in micron-sized regions of the films. The magnetic order of BFO was obtained by using x-ray linear and circular dichroism images using a photoelectron emission microscope (PEEM). Angle dependent structural measurements allow decoupling of the two order parameters, ferroelectric and magnetic, contributing to the photoemission signal. Careful analysis of linear and circular dichroism images at critical angles allows determination of magnetic directions in BiFeO$_{3}$. These studies reveal a strain-driven reduction in magnetic symmetry in thin films, leading to the formation of an easy magnetic axis as opposed to the observed easy plane for bulk films. This reduction along with the previous proof of FE-AFM coupling allows electrical control of its magnetic axis.   

Magnetization damping in epitaxial CrO$_{2}$ (110)

Epitaxial CrO$_{2}$ thin films were grown on TiO$_{2}$ (110) substrates using chemical vapor deposition (CVD) using a CrO$_{3}$ precursor as described elsewhere [1]. In-plane angular dependent ferromagnetic resonance (FMR) measurements confirm a uniaxial in-plane anisotropy with the easy axis along the c-axis. Frequency dependent FMR measurements were carried out over a frequency range from 7-60 GHz along the easy axis of the film. The resonance field dependence on the microwave frequency is well described by the Kittel formula, enabling the determination of M$_{eff}$ and $\gamma $ of the films. The ferromagnetic resonance linewidth depends only weekly on the microwave frequency: the linewidth has a minimum around 20 GHz and increasing linearly for larger frequencies with a very small slope. Based on this we estimate the Gilbert damping constant (intrinsic) to be of the order 10$^{-4}$, i.e. very small. The main contribution to the magnetization relaxation is extrinsic in nature and can therefore be further optimized. References: [1]: X. W. Li, A. Gupta, and G. Xiao, Appl. Phys. Lett. \textbf{75}, 713 (1999).   

Magnetic Clustering and Possible Chemical Nonuniformity in Bi0.125Ca0.875MnO3

The manganite system Bi1-xCaxMnO3 possesses intriguing properties in the low bismuth doping region. In this electron doped region (0.6$<$x$<$1), a large ferromagnetic (FM) moment of $\sim $1.2 Bohr magnetons per Mn site is found for x$\sim $0.875. However, the origin of this FM clustering configuration is still an open question. Chemical nonuniformity (Bi ion segregation) as a candidate interpretation has been explored with TEM/EDS, which can give a quantitative assessment of geometrical parameters, chemical composition and crystal structure of second phase particles. We have identified evidences for the possible Bi nonuniformity in nano-scale, which are consistent with the results from small-angle neutron scattering. This work is supported by NSF DMR-0512196.   

Zero and low field magnetization dynamics in tapered nanopillar spin valves caused by micromagnetic structure

Exciting spin-torque driven magnetization dynamics in a uniformly magnetized nanomagnet typically requires large magnetic fields and electric currents, both of which are undesirable for practical applications. Here, we show that by combining an angled sidewall and a low saturation magnetization material, such as permalloy (Py), in a simple nanopillar spin valve structure, a micromagnetic configuration can be established which is conducive to spin-torque excitation of magnetization dynamics in the absence of an external magnetic field. As verified by micromagnetic simulations, these magnetization oscillations occur in both nanomagnet layers of the spin valve due to a substantial out of plane magnetization component that is located at and near the ends of the tapered elliptical disks. For magnetic fields close to zero, the resultant giant magnetoresistance spectra show oscillations with peak frequencies between 5.2-6.5 GHz, and minimum linewidths on the order of 50 MHz.   

Non-Adiabatic Spin Transfer Torque in Realistic Materials

The motion of simple domain walls and more complex magnetic textures in the presence of a transport current is sensitive to the size and sign of the non-adiabatic spin transfer torque coefficient $\beta$, even though this dimensionless coefficient is believed to typically have a small value. The ratio of $\beta$ to the Gilbert damping coefficient $\alpha$ is particularly important and has been variously estimated to be close to 0, close to 1, and large or small. By identifying $\beta$ as following from the influence of a transport current on $\alpha$, we derive concise analytical expressions for $\beta$ in real materials. When spin-orbit is included in the band structure, the damping has an intra-band contribution that is proportional to the square of the quasiparticle lifetime. We will discuss estimates for $\beta$ and for the $\alpha/\beta$ ratio in common magnetic materials.   

Study of Fe/Cr Magnetic Multilayers and Periodic Arrays of Submicron Magnetic Dots by Vector Network Analyzer Technique

Vector network analyzer (VNA) technique up to 8.5 GHz was applied to measure in-plane dynamic response in Fe/Cr magnetic multilayers and for the in-plane magnetized periodic arrays of Permalloy circular magnetic dots. In the antiferromagnetically coupled [Fe/Cr]$_{n}$ multilayers (n=10,20,40) we have investigated field dependence of the acoustic resonance in a wide range of temperatures between 300K down to 2K both for the low magnetic fields and close to the saturation field. FMR studies of the array of FeNi dots with diameter of 1 micron, the aspect ratio L/R=0.1 and with centre to centre distance varying between 1.2 to 2.5 micron allowed to resolve multiple FMR resonances as a function of magnetic field. We have found the main FMR linewidth to be dependent on the magnetic history. For the magnetic fields below 300 Oe, where magnetic vortex state forms, we have observed the field dependence of the radial modes (f$_{r} \quad >$ 6GHz) to show minima close to the zero magnetic field.   

Ferromagnetic Resonance in Magnetic and Conducting Thin Films

Ferromagnetic resonance provides unique information concerning both electronic spin orientation and their interactions. A model for the simulation of magnetic resonance line shape in metallic and magnetic thin films, developed within the thermodynamic approximation is presented. The proposed model includes the contribution of the Gilbert term to the precession of magnetic moments in external magnetic fields, the effect of shape anisotropy, of magnetocrystalline anisotropy, and allows for different textures. The main features of the ferromagnetic resonance line (line position, line width, line asymmetry, and line shape) are analyzed while taking into account the competition between magnetic and conducting features. The model allows a detailed analysis of the angular dependence of the ferromagnetic resonance line. It has been tested on a multilayer consisting of 60 bilayers (Fe 0.16 nm/Pt 0.18 nm).   

Optical control of topological spin and charge transport in semiconductors

In n-doped semiconductors, the strong spin-orbit coupling in valance band can be utilized through the band renormalization by optical field off-resonantly coupled to the interband transitions. The adiabatic electronic ground state is thus reactively controlled by optical pulses, exhibiting an anomalous spin-dependent Hall conductivity. Light-matter interaction is exploited here in a unique way, i.e. light does not induce real excitations in the system but act as a control knob for switching on/off novel material properties, in contrast to most previous use of light. With the control by linearly polarized light, a pure spin Hall current of electrons can be driven by an in-plane DC electric field, which results in net spin accumulations at the edges of the optical excitation area. Effectively, one has created a spin battery powered by optical pulses together with DC electric field, or equivalently, an optically gated spin transistor. As conduction band spin-orbit coupling is not needed, the resultant electron spin accumulations can have long lifetime when control light is adiabatically switched off. In addition, circularly polarized light breaks the time reversal symmetry and can result in spin polarized anomalous Hall conductance.   

Ultrafast Photo-induced Ferromagnetic Phase Enhancement in Ion Implanted GaMnAs

Ion implantation is a far less demanding doping technique than molecular beam epitaxy (MBE), with potential for large scale application. Using femtosecond UV pump/NIR probe, polar MOKE spectroscopy, we reveal enhancement of the ferromagnetic phase on the 100s of picosecond timescale, via laser excited transient carriers in ion implanted GaMnAs. Particularly, the temperature dependence of the dynamic magnetization enhancement in ion-implanted samples exhibits abrupt quenching at low temperatures, which is a strikingly different behavior than that observed in MBE-grown samples. We tentatively assign the observed effect to the quasi-2D confinement of Mn spins in the magnetic layer due to the non-uniform doping profile obtained by ion-implantation. We discuss the implications of our results on the change of spin scattering mechanism at low temperatures, which may provide new insights into the metal to insulator transition visible at low temperatures in magneto-transport measurements.   

Intrinsic spin Hall effect in platinum metal

Spin Hall effect (SHE) is studied with first-principles relativistic band calculations for platinum, which is one of the most important materials for metallic SHE and spintronics. We find that intrinsic spin Hall conductivity (SHC) is as large as $\sim 2000 (\hbar/e)(\Omega {\rm cm})^{-1}$ at low temperature, and decreases down to $\sim 200 (\hbar/e)(\Omega {\rm cm})^{-1}$ at room temperature [1]. It is due to the resonant contribution from the spin-orbit splitting of the doubly degenerated $d$-bands at high-symmetry $L$ and $X$ points near the Fermi level. By modeling these near-degeneracies by an effective Hamiltonian, we show that SHC has a peak near the Fermi energy and that the vertex correction due to impurity scattering vanishes. We therefore argue that the large SHE observed experimentally in platinum is of intrinsic nature. [1] G.Y. Guo, S. Murakami, T.-W. Chen, and N. Nagaosa, arXiv:cond-mat/07050409.   

First-principles Study on the Magnetic Interaction in ZnO-based Dilute Magnetic Semiconductors

Using first-principles calculations, we investigate the electronic structures and magnetic properties of dilute magnetic semiconductors (DMS). The electronic structures are calculated by using Korringa-Kohn-Rostoker method combined with the coherent potential approximation. Since the d electrons of the magnetic impurity in DMS are strongly localized, we apply self-interaction correction to the local density approximation for the exchange-correlation energy. From the first-principles results, we evaluate the magnetic exchange interaction $J_{ij}$ between the pairs of magnetic impurities by using the Lichtenstein's formula. We found that the magnetic interaction in ZnO-based DMS is basically antiferromagnetic without any additional carrier because of large energy gaps between the occupied and unoccupied d states. We will also discuss about the carrier dependence of the magnetic interaction in ZnO-based DMS.   

Corresponds between Spin-Hall Effect and Ordinary Hall Effect

The ``spin-Hall effect'' has recently attracted a lot of attention and a central question is whether the effect is due to the \textit{intrinsic} spin-orbit interaction or due to spin-asymmetric scattering by \textit{extrinsic} impurities. We shed light on this question using a new approach based on the non-equilibrium Green function (NEGF) formalism, which allows us to go continuously from the ballistic to the diffusive limit and we present approximate analytical expressions that describe our results fairly well. We establish a correspondence between Spin Hall effect and Ordinary Hall effect, since from our point of view these two effects are quite similar. Our model suggests that a spin accumulation proportional to the current should be observed in clean ballistic samples and we show how this spin accumulation evolves as momentum and/or spin relaxation processes are introduced in a controlled way. We further show good quantitative agreement with recent experimental observations in GaAs suggesting that these can be understood in terms of an intrinsic effect driven by the Rashba interaction, although experiments on ZnSe likely have a different origin.   

Anisotropy, coercivity and thermal activation in $L$1$_{0}$-FePt Nanoparticles.

$L$1$_{0}$ FePt materials have attracted tremendous attention for their potential applications due to their high anisotropy and excellent mechanical properties. It is well known that the magnetization behavior of small particles is strongly affected by thermal fluctuation especially when the particle size is close to the critical size of superparamagnetic. In this work, the size effect on anisotropy constant K$_{u}$, magnetic viscosity parameter S, activation volume V$_{ac}$ and coercivity H$_{c}$ of the $L$1$_{0}$ FePt nanoparticles obtained by the salt-matrix annealing have been studied systematically. It was found that K$_{u}$ increases with increasing particles size. The maximum field-dependent viscosity parameter S$_{max}$ increases monotonously with temperature for the particles with size of 3 nm to 8 nm. Moreover, S$_{max}$ of small particles is more sensitive to temperature than that of large particles. However, the temperature dependence of both S$_{max}$ and V$_{ac}$ of the 15 nm particles are different from those for 3-8 nm particles. Further analysis of relation between H$_{c}$ and V$_{ac}$ suggested that the 3-8 nm particles are ideal single-domain particles, while the 15 nm particles can not be well described as single-domain particles even the particle size is much smaller than the single-domain size.   

Composition Dependent Properties in Disordered, Lower Dimensional Co/Ni Dichloride Monohydrate

This new mixed magnet is composed of the lower dimensional insulators Co dichloride monohydrate and Ni dichloride monohydrate, both studied previously by us. The Co system is an antiferromagnetic spin glass with an antiferromagnetic transition at 15.0 K and a spin glass transition at 8.4 K. The antiferromagnetic Ni system orders at 5.6 K. Both materials are characterized by ferromagnetic intrachain interactions and antiferromagnetic interchain interactions; an element of randomness is present in the Co system but not the Ni. Mixtures of these two components have been prepared and studied over a wide composition range. The variation in magnetic behavior with composition is not anticipated on the basis of pure component properties, e.g., the location of a susceptibility maximum and of a field induced transition. Irreversibility effects are present over a broad composition range. It seems likely that admixture of the two components enhances effects of disorder and frustration already present in the Co system, so that nonequilibrium behavior occurs even for Ni rich mixtures.   

Generic and topological features of flat bands in tight binding hopping models

We study some generic features of flat bands, that appear in a number of tight binding hopping models. Such models have recently received some attention in the literature [1,2], with a number of suggested experimental realizations. In some models the flat band touches dispersing bands at a discrete set of high-symmetry points in the Brillouin zone. In other models, the flat band is gapped. A topological argument, crucially depending on the boundary conditions of the system, is given which explains why in some models the flat band is gapped, while in others it is not. The argument is based on the observation that in flat bands in addition to quasi-local eigenstates, there invariably exist states involving non-contractible loops. In those cases where the flat band touches a dispersive band, our argument also determines at which points in momentum space this occurs. [1] C. Wu, et al., Phys. Rev. Lett. 99, 070401 (2007) [2] J. Schulenburg, et al., Phys. Rev. Lett. 88, 167207 (2002)   

Magnetic Behavior in Cobalt and Nickel Dibromide Hydrates

The magnetism of 3d transition metal dibromide hydrates has been examined far less than for corresponding chlorides. Continued here is a program of examining the especially neglected monohydrate systems, by extending earlier studies of Co and Ni dichloride monohydrates to bromide counterparts. For comparison purposes the only lightly examined Co and Ni dibromide dihydrate systems are also studied. Both Ni materials, dihydrate and monohydrate, show an antiferromagnetic susceptibility maximum at, surprisingly, virtually the same 6.0 K location. The maximum is broader and of lesser magnitude in the monohydrate, perhaps indicating lower dimensional character. Marked differences in magnetization isotherms as a function of temperature also distinguish these two materials. In Co dibromide dihydrate a susceptibility maximum occurs at 9.5 K, only about half the temperature of a similar feature in the corresponding chloride. Yet in the monohydrate an enhanced susceptibility maximum appears at 15.5 K, remarkably close to such a feature in the corresponding chloride. And as in that system, nonequilibrium magnetic properties appear, suggesting that a significant degree of frustration is present.   

Raman spectroscopy of multiferroic TbMnO$_3$

Coupling between the lattice and magnetic degrees of freedom in TbMnO$_3$ has been observed to produce magnetic excitations with electric dipole activity, or electromagnons. Recent reports of electromagnons in other multiferroic (113)-orthomanganites\footnote{R. Vald\'{e}s Aguilar \textit{et al.}, Phys. Rev. B \textbf{76}, 060404 (2007).} and related (125)-manganites\footnote{A. B. Sushkov \textit{et al.}, Phys. Rev. Lett. \textbf{98}, 027202 (2007).} indicate a complementary Raman study may provide additional insight into the importance of spin-lattice coupling. We present Raman spectra of single-crystal and polycrystalline TbMnO$_3$ using a triple-grating spectrometer in a collinear backscattering configuration as a function of temperature ($4-300\,$K) and polarization along various crystallographic axes. The absence of any observable low-frequency modes (intensity $<1000$ times that of prominent Raman-active phonons) suggests a weak scattering cross-section for the electromagnon. Additionally, we discuss the temperature dependence of Raman-active phonons and compare with results from infrared measurements.   

Magnetic behaviour of the Bi$_{2-x}$Sr$_{x}$Ir$_{2}$O$_{7}$ pyrochlore

Compounds of the Bi$_{2-x}$Sr$_{x}$Ir$_{2}$O$_{7}$ solid solution have been synthesized by the solid state reaction method. Structural modifications as well the valence states of Iridium have been studied as a function of the strontium content by Rietveld refinements and electrochemical analytical methods. Electrical properties of Bi$_{2-x}$Sr$_{x}$Ir$_{2}$O$_{7}$ show single phase and metallic behaviour in the whole range of compositions. Magnetically this system behaves as a Curie-Weiss paramagnet from 2-300 K. the magnetic moment suggests the presence of Ir$^{5+}$ valence state.   

Invar effect and non-collinear magnetism in CuFe alloys

The Invar effect has been observed in many Fe rich alloys, most famously Ni Invar. Generally the Invar behavior is associate with the strong coupling between the lattice and magnetic degrees of freedom, and therefore depends on the magnetic ordering in these alloys. Recent experimental works observed an Invar effect in fcc-FeCu solid solutions. [Gorria et al., PRB 69, 214421 (2004)] We investigate the magnetic states of fcc-FeCu solid solutions in the Invar regime and compare it with the order in low Fe concentration alloys, to establish the connection between the Invar effect and the magnetic order. To study this we employ the locally selfconsistent multiple scattering (LSMS) real space method for solving the LDA Kohn-Sham equation as well as its extension to fully relativistic calculations to investigate all effects leading to anisotropies and non collinear ordering of magnetic moments. We find that the magnetic order in the iron rich alloys (44\%-65\% Fe) is non-collinear and disordered. We will present the dependence of the magnetic disorder on iron concentration and lattice constant in both the Invar and non-Invar regimes to establish the validity of the standard magnetic disorder explanation of the Invar effect for FeCu solid solutions. Research sponsored by the Division of Materials Sciences and Engineering, U.S. Department of Energy under contract DE-AC05-00OR22725 with UT-Battelle, LLC.   

X-ray scattering study of the magnetic phase transitions in GdFe3(BO3)4

The rare earth iron borates have interesting magnetic properties due to the subtle interactions between the rare earth and the iron moments. Among these materials, GdFe3(BO3)4 has the most complex phase diagram as suggested by previous studies. [1,2] These studies suggest that iron moments order antiferromagnetically below T$_{N}\sim $36 K and that there are several additional magnetic phase transitions below T$_{N}$. Yet whether and at what temperature the Gd moments order and the nature of the additional transitions, remain largely unknown. Using x-ray magnetic scattering, we have verified that the moments order antiferromagnetically with a propagation vector (0 0 3/2). Large resonant scattering enhancements at the Gd L$_{II}$ and L$_{III}$ edges show unambiguously that Gd moments order at T$_{N}$. Both resonant and nonresonant scattering data exhibit a splitting of the magnetic peak along c* above $\sim $ 10 K which indicates an incommensurate phase transition, with the incommensurability $\delta $ increasing continuously as a function of temperature ($\delta \sim $.0038 near T$_{N})$. Use of the NSLS/BNL is supported by the U. S. DOE under Contract no. DE-AC02-98CH10886. [1] F. Yen et. al, PRB 73, 54435 (2006) [2] A. I. Pankrats et. al, JETP 99, 766 (2004)   

Non-Collinear Magnetic Orderings in Mott Insulators

Non-collinear magnetic orderings of four Cu magnetic moments in Mott insulators Rd$_{2}$CuO$_{4 }$(R =Nd, Pr) of I4/mmm symmetry and associated magnetic phase transitions are of interest in studies of transformations, when correlated electron-hole carriers are introduced in R$_{2-x}$Ce$_{x}$CuO$_{4\pm \delta }$. Orderings are determined by thermodynamic potential in representation by antiferromagnetic \textbf{l}$_{1}$, \textbf{ l}$_{2 }$ and magnetic \textbf{m} vectors, with orderings of \textbf{l}$_{1 }$, \textbf{l}$_{2}$ vectors along [100] , [010] axis,$_{ }$with values \textbf{l}$_{1}^{2 }$= \textbf{l}$_{2}^{2 }$= 1/2 \textbf{l}$_{0}^{2}$, [1], which can be presented as, $\Phi \quad =$ 1/2 A( \textbf{l}$_{1}^{2 }$+ \textbf{l}$_{2}^{2})$+ 1/2 A$_{3}$\textbf{l}$_{3}^{2}$+ 1/2 B\textbf{m}$^{2}_{ }$+ 1/2 D [(\textbf{l}$_{1}$\textbf{m})$^{2}$+ (\textbf{l}$_{2}$\textbf{m})$^{2}$]+ 1/2 D$_{3}$(\textbf{l}$_{3}$\textbf{m})$^{2 }$+ 1/4 I( \textbf{l}$_{1}^{2 }$+ \textbf{l}$_{2}^{2 })^{2}$ + 1/4 I$_{3 }$\textbf{l}$_{3}^{2 }$+1/4 E ( \textbf{l}$_{1}^{2 }-$ \textbf{l}$_{2}^{2 })^{2 }$ + 1/4 a ( \textbf{l}$_{1z}^{2 }$+ \textbf{l}$_{2z}^{2 })$ + 1/4 a\textbf{l}$_{3z}^{2} \quad -$ 1/4 b$_{2 }$[ ( \textbf{l}$_{1y}^{2 }$+ \textbf{l}$_{2x}^{2 })$ - ( \textbf{l}$_{1x}^{2 }$+ \textbf{l}$_{2y}^{2 })$ ] - 1/4 b$_{4 }$[ ( \textbf{l}$_{1y}^{2 }$+ \textbf{l}$_{2x}^{2 })^{2}$ + ( \textbf{l}$_{1x}^{2 }$+ \textbf{l}$_{2y}^{2 })^{2}$ ] -\textbf{ mH} where \textbf{l}$_{3}$=0. Magnetic phase transitions, are concerned with change of \textbf{l}$_{1 }$, \textbf{l}$_{2}$ values in fields $\sim $H$_{c1}$, $\sim $H$_{c}$, where \textbf{l}$_{1}^{2}$=0, \textbf{l}$_{2}^{2}$=\textbf{l}$_{0}^{2}$, when field is oriented along [100], [110] axis respectively, and next \textbf{l}$_{2 }$rotation to orthogonal to field direction in fields $\sim $H$_{c2}$, when field is along [110] axis. Fields H$_{c1}$, H$_{c}$, H$_{c2}$ are presented as, H$_{c1}^{2}$=2BE\textbf{l}$_{0}^{4}$; H$_{c}^{2}$=H$_{c1}$H$_{c2}$, if H$_{c2}^{2}$=b$_{2}$B\textbf{l}$_{0}^{2}$; H$_{c}^{2}=\surd $2$\cdot $H$_{c1}$H$_{c2}$ if H$_{c2}^{2}$=b$_{4}$B\textbf{l}$_{0}^{4}$. Formation of charge density waves of checkerboard structure can be detected by studies of transformation of magnetic phase transitions and fields in R$_{2-x}$Ce$_{x}$CuO$_{4\pm \delta }$. [1]. A. N. Bazhan, AIP Proceedings 850 (2006) 1241   

An Explanation for the Very Low Friction of Polyelectrolyte Brushes

It is shown using a method based on the mean field theory of Miklavic Marcelja that it should be possible for osmotic pressure due to the counterions associated with the two polyelectrolyte polymer brush coated surfaces to support a reasonable load (i.e., about $10^5 Pa$)with the brushes held sufficiently far apart to prevent entanglement of polymers belonging to the two brushes, thus avoiding what is believed to be the dominant mechanisms for static and dry friction. This is shown to be true even if the brushes are highly compressed, which is consistent with the observation by Raviv, et. al., that the friction for polyelectrolyte brush coated surfaces can be exceedingly low, even if the brushes are highly compressed.   

Temperature Dependent Raman Intensities and Calculations of Chiral ketone Conformers

Vibrational Raman spectra for the C-H stretch region of $R$-(+)-3-methylcyclopentanone ($R$3MCP) neat sample as a function of temperature variation will be presented and employed as a conformational analysis method. Probing the methyl group C-H stretch region (2850 -- 3000 cm$^{-1})$ relative peak intensities of assigned conformers, is being manipulated to determine the conformer energy differences between equatorial methyl and axial methyl isomers. Raman spectroscopy performed under liquid nitrogen (RUN) will also be presented as a preliminary tool to observe Raman vibrational modes at liquid nitrogen temperatures ($\sim$ 77 $^{\circ}$K), and to improve resolution and high signal to noise ratio. An observed few wavenumbers bathochromic (red) shift in Raman lines frequencies will be attributed to sample phase change. The validity of Density Functional Theory (DFT) calculations, of different levels basis sets, for $R$3MCP conformers Raman intensities and frequencies will be investigated and compared against experimental findings.   

Optimum Atomic Ranges in Coupled Dipole Method

In recent years, the coupled dipole method (CDM) has been applied to calculate the van der Waals dispersion interaction (VDW) between dielectric nanoclusters. Similar self-consistent method has also been used to calculate the static polarizabilities, from which the fully-retarded VDW was calculated. The breakthrough in CDM is that it can calculate all n-body terms in VDW. This allows for significant improvement upon the usual 2-body description of VDW. As the size of nanocolloids increases close to the experimentally measurable size, however, solving for eigenvalues of a large matrix (with all n-body interactions) in CDM becomes non-trivial. Since each interaction term decreases as the separation distance becomes large, certain atomic ranges within which the VDW would converge is expected in large systems. Identifying this range will improve the efficiency of CDM. Here, we report the results of a systematic study on the optimized maximum atomic ranges in CDM for different shapes and sizes of clusters. REF: Langmuir 23, 1735 (2007).   

Umbrella Sampling in the Long Range Ising Model

Umbrella sampling with the largest cluster size as the local order parameter has been used to study liquid to solid nucleation. In the absence of a rigorous definition of a solid-like cluster, an ad hoc definition of the clusters must be adopted. As a result it is not clear if the vanishing of the free energy barrier found by umbrella sampling should be interpreted in terms of a spinodal and how well the cluster found at the free energy maximum corresponds to the true nucleating droplet. To better understand umbrella sampling with a local order parameter we study nucleation in the long-range Ising model for which the cluster definition is given rigorously by a percolation mapping. We determine the free energy of the long-range Ising model as a function of the largest cluster size and the quench depth, and study the properties of the nucleating clusters as the spinodal is approached.   

TOF MS Study of Photodissociation of Borazine at 193 nm

Photofragmentation of borazine molecule has been investigated in a supersonic molecular beam condition (Ar + 1{\%} borazine mixture) by using radiation of 193 nm (250 mJ/pulse). Fragments were photo ionized using another laser (193 nm, 3 mJ/pulse) and detected by a linear time-of-flight mass spectrometer. Both lasers passed through the work area of the TOF mass spectrometer at the same time. We found that the main channel of borazine photofragmentation is formation of B$_{3}$N$_{3}$H$_{5 }$radical and hydrogen atom. The possible mechanism was proposed and discussed.   

Single Pulse Time Resolved Four Wave Mixing

We present a new experimental technique for single-shot time resolved ultrafast Coherent Anti-Stokes Raman Spectroscopy (CARS), where we use the arrival time of pulses at the intersection of broad beams as controlled time delays. The three dimensional (Boxcars) phase-matching configuration allows unique mapping of two independent time delays (pump-Stokes and pump-probe) onto the geometrical axes of the interaction region. The signal emitted from each point of the beams' intersection carries information on the molecular state at the particular time delay after the excitation, and thus the spatial profile of the CARS beam provides time resolved trace of the intra-molecular vibrational dynamics. We show that our technique allows for capturing of a few picoseconds of vibrational evolution by means of a single ultrashort pulse. Moreover, the ability to resolve two time delays between pulses enables us to study vibrational dynamics on the ground and the excited electronic states, as well as the correlation between the nuclear motions within these different vibrational potentials. The ability to record vibrational dynamics while exposing the molecules to a single optical pulse allows for characterization of short living and unstable chemical species as transitional complexes of chemical reactions.   

Spectroscopic characterization of the ionization energy, $^{3}\Sigma _{u}^{+}$, (2)$^{3}\Pi _{g}$, and (3)$^{3}\Pi _{g}$ states of Be$_{2}$

Low-lying electronic states of beryllium dimer are investigated by laser induced fluorescence (LIF) and resonance enhanced multiphoton ionization (REMPI) spectroscopies. Be$_{2}$ is formed by pulsed laser ablation and free jet expansion into vacuum. Comparing 1+1 REMPI and LIF spectra for the X$^{1}\Sigma _{g}^{+}$ (v=0) -$>$ B$^{1}\Sigma _{u}^{+}$(v) bands we find significant perturbations in the REMPI spectra, which are interpreted as autoionizing resonances in the ionization continuum. Photoionization efficiency (PIE) measurements yield an accurate value for the ionization energy, namely 7.40 eV, which is considerably larger than previous theoretical predictions. New CASSCF/MRCI calculations are presented which accurately reproduce the experimental IP. Rotationally resolved spectra for the (1)$^{3}\Sigma _{u}^{+}->$ (2)$^{3}\Pi _{g}$ and (1)$^{3}\Sigma _{u}^{+}->$ (3)$^{3}\Pi _{g}$ band systems of Be$_{2}$ have also been measured for the first time providing further experimental benchmarks for recent ab initio calculations. PIE measurements are also used to accurately determine the X$^{1}\Sigma _{g}^{+}$ $<->$ (1)$^{3}\Sigma _{u}^{+}$ interval.   

Rovibrationally Inelastic Velocity-Dependent Cross Sections from Doppler Lineshapes

We are studying rovibrationally inelastic processes in the Li$_{2}$* (A)~--~noble gas system through a spectroscopic technique that employs the Doppler shift for velocity selection of the lithium molecules. Our goals are to look for experimental evidence of a novel vibrational transfer mechanism that involves impacts with the side of the molecule, and to investigate and compare different combinations of rotational and vibrational energy transfer. The experimental results will be compared with cross sections calculated from \textit{ab initio} potential surfaces.   

Saturated nucleate pool boiling of oxygen under magnetically-enhanced effective gravity

We investigate the effect of enhancing gravity on saturated nucleate pool boiling of oxygen for effective gravities ($g_{\mathrm{eff}}$) of $1g$, $6g$, and $16g$ ($g=9.8\;\mathrm{m s^{-2}}$) at a saturation pressure of $760\;\mathrm{torr}$ and for heat fluxes of $10\sim 3000\;\mathrm{W m^{-2}}$. The effective gravity on the oxygen is increased by applying a magnetic body force generated by a superconducting solenoid. We measure the heater temperature (expressed as a reduced superheat) as a function of heat flux and fit this data to a piecewise power-law/linear boiling curve. At low heat flux ($\alt 400\;\mathrm{W m^{-2}}$) the superheat is proportional to the cube root of the heat flux. At higher heat fluxes, the superheat is a linear function of the heat flux. The value of the transition heat flux separating these two regions is proportional to $g_{\mathrm{eff}}^{0.25}$, indicating a possible link to the critical heat flux.   

Thermal Hystersis in Magnetic Phases of Solid Oxygen

Measurements of the dielectric susceptibility of solid oxygen have been carried out in the temperature range 4.2$<$T$<$54 K. Relatively large hysteresis effects $(\sim 0.4\%)$ have been observed for the dielectric response in the $\alpha$ and $\beta$ phases on thermal cycling below 44 K. The temperature for the transition between the two-sublattice antiferromagnetic $\alpha$ phase and the frustrated quasi-helical $\beta$ phase is observed to be independent of the thermal cycles. The areas of the thermal hysteresis scale with the extent of the thermal excursion in the frustrated $\alpha$ phase.   

Does Hot Water Freeze Faster than Cold? Investigation of the Reproducibility and Causes of the Mpemba Effect

An investigation into the reproducibility and possible causes of the Mpemba effect has been performed.~ The Mpemba effect is the name given to the common observation by non-scientists that hot water appears to freeze faster than cold water.$^{1}$~ Previous scientific studies of this effect have found conflicting results. These discrepancies appear to be due in part to inconsistent definitions of freezing based on visual observation.~ We have investigated the Mpemba effect by continuously monitoring the temperature of a container of water to determine the amount of time needed for the water to turn completely to ice, as indicated by the temperature falling below 0 $^{o}$C.~ We have successfully observed the effect repeatedly and have found it to be dependent on the sample's temperature history rather than the sample temperature when placed into the freezer.~ Room temperature water which had been briefly heated to 100 $^{o}$C then cooled froze approximately 50 {\%} faster than room temperature water which had not been heated. The effect on the freezing time of increasing or decreasing the amount of dissolved gas in the water will also be discussed. 1. M. Jeng. Am. J. Phys. \textbf{74} 514 (2006).   

Surface roughness effects on superhydrophobicity

Superhydrophobic surfaces, with liquid contact angle greater than 150 degree, have important practical applications ranging from self-cleaning window glasses, paints, and fabrics to low friction surfaces. Many biological surfaces, such as the lotus leaf, have hierarchically structured surface roughness which is optimized for superhydrophobicity through natural selection. Here we present a molecular dynamics study of liquid droplets in contact with self-affine fractal surfaces. Our results indicate that the contact angle for nanodroplets depends strongly on the root-mean-square (rms) surface roughness amplitude but is nearly independent of the fractal dimension D of the surface[1,2]. References: [1] C. Yang, U. Tartaglino and B.N.J. Persson, Phys. Rev. Lett. 97, 116103 (2006) [2] C. Yang, U. Tartaglino and B.N.J. Persson, arXiv:0710.3264   

Production of Polymer Core-Shell Colloids with High Uniformity via Coaxial Electrospray

Although nanofibers fabricated by electrospinning have been attracting wide interest, the production of colloids by electrospraying has not much studied so far. We have developed a simple method for the production of core-shell colloids with high uniformity by means of the coaxial electrospray. Contrary to usual coaxial setup, the inner nozzle was set to touch the inside wall of the outer nozzle for reproducible production. A polymer solution for the core was introduced through the outer nozzle and another solution for the shell was provided through the inner nozzle. The structure of the colloids was dependent on the polymer concentration, relative feed ratio between the polymer solutions. Especially, core-shell structured colloids are our primary interest due to their promising uses in drug-delivery systems, cosmetics, and food industries. This talk will present the production of core-shell colloids consisting of two polymer components.   

Janus particles at the planar water-oil interface

Amphiphilic Janus particles (hydrophobic on one side, hydrophilic on the other) were placed at the planar water-oil interface at various surface coverage and found to self-assemble into two-dimensional crystals with long-range hexagonal order, which we studied by fluorescence and phase contrast microscopy. Surprising dependence is observed not only on the surface chemical makeup of the hydrophilic side but also on the Janus balance (i.e. the relative sizes of hydrophobic and hydrophilic portions), which is analogous to the HLB balance that characterizes molecular surfactants.   

Ab Initio Simulations of Silica Alpha Quartz Surfaces and their Interaction with Water

Two types of dry silica alpha quartz (0001) surfaces have been investigated through density functional calculations. One is the dense surface proposed by Rignanese et al.[1] that has 3-fold flower like six-member rings on the top and the three-member rings underneath. The other one is a newfound surface that has zigzag-shape six-member rings on the top and the three-member rings underneath. It is found that the new one is energetically more stable than the dense surface. The interactions of the surfaces with water molecules have also been concerned. Both of the surfaces have the similar hydrophilic properties. We report our results from MD simulations at a temperature of 300K. \newline \newline [1]. G.-M. Rignanese, Alessandro De Vita, J.-C. Charlier, Roberto Car, Phys. Rev. B \textbf{61}, 13250 (2000)   

Molecular motion in alkylsilane self-assembled monolayers

We have investigated intra-molecular rotation within polar-substituted alkylsilane self-assembled monolayers (SAMs) on fused silica, utilizing surface-sensitive dielectric spectroscopy. Both trichlorosilanes (which allow crosslinking within the SAM) and monochlorosilanes (attached only to the surface) are utilized to grow monolayer and submonolayer films. Dielectric loss spectra as a function of temperature have been obtained for SAMs with varying carbon chain length, surface coverage, and alkyl terminal group. As shown by ellipsometry, contact angle measurements, and AFM, monochlorosilanes form a more disordered monolayer than trichlorosilanes. This more disordered film may result in additional degrees of freedom within the monolayer, or in the language of phase transitions, a rotator phase. Issues such as uncontrolled vertical polymerization and film growth by island formation and their effect on rotational dynamics will be discussed.   

Photoemission and reaction study of mass-selected Pt clusters on TiO$_{2}$(110) surface

Metal cluster has been speculated to have strong size dependence in catalytic activity. The clusters on surfaces would give further specificity because of the interaction between the clusters and the surface. Catalytic properties of mass-selected metal clusters on well-defined oxide surfaces have been investigated using the new ultra high vacuum cluster deposition apparatus. In this study, we have examined catalytic and electronic properties of platinum clusters used as a composition of automotive exhaust catalysts, and used titanium dioxide as the support. Pt cluster ions were produced by a DC magnetron-sputter cluster ion source [1] with an ion funnel [2], mass-selected by a quadrupole mass filter, and then deposited on TiO$_{2}$(110) single crystal surfaces. The catalytic oxidation of CO on Pt$_{n}$/TiO$_{2}$ (n$<$10) was investigated using the high-pressure reaction cell with quartz linings, which was connected to the external recirculation loop with a stainless steel bellows pump. The catalytic activity was suggested to be dependent on the size (n) of Pt$_{n}$ clusters. It was expected to be due to the electronic properties of the clusters. The size specificity will be discussed with the results of photoemission spectroscopy. [1] H. Haberland et al., J. Vac. Sci. Technol. A 10, 3266 (1992). [2] S.A. Shaffer el al., Rapid Commun. Mass Spectrom. 11, 1813 (1997).   

Structural and electronic properties of aluminum nanoclusters

The electronic properties of Al clusters containing upto 60 atoms are investigated using an all electron density funstional theory using large polarized Gaussian basis sets with 39 Gaussians per atom (NRLMOL basis). We have performed an extensive search for the lowest energy isomers for clusters upto Al$_{21}$. We build a database of candidate structures for the ground state using different strategies. First, a few structures are randomly generated and fully relaxed using planewave pseudopotential method. We also performed simulated annealing runs using ab initio molecular dynamics for clusters upto Al$_{20}$. In three sets of simulated annealing runs the clusters were heated upto 900 and 1000K and were slowly cooled to 50K at different rates. After every half picosecond, the cluster was quenched. Additionally the best basin hopping geometries obtained using empirical embedded atom potential were also fully optimized. The process generates 40-50 structures for each size. All the structures are relaxed using full-potential PAW method using a large energy cutoff. For larger clusters we used best available geometries from literature obtained from basin hopping and simulated annealing tgechniques. The electronic properties of Al clusters are subsequently determined for this datebase at the all-electron level using Gaussian basis set.   

Density Functional Study of the Photodetachment Spectra of Zinc and Calcium Cluster Anions.

The structure and electronic states of zinc cluster anions Zn$_{2}$ through Zn$_{9}$ and calcium cluster anions from Ca$_{2}$ through Ca$_{19}$ were optimized within the hybrid density functional approach. Based on these results, the photoelectron detachment spectra of Zn$_{2}$ through Zn$_{6}$ and of Ca$_{2}$ through Ca$_{6}$ anions were determined. Additionally, the electron affinity of Zn$_{N}$ (N up to 9) and of Ca$_{N }$(N up to 19) was calculated within the same approximation. Both the calculated electron affinities and theoretical photoelectron binding energies are in very good agreement with experiment. Theoretical predictions for Zn$_{3}$, Ca$_{2}$ and Ca$_{4}$ anions display additional electron detachment binding energies that are not present in the published experimental data.   

Ligand-spacer controlled size selectivity of gold clusters

It has been observed in the experiment that the presence of diphosphine ligands with varying spacers (L$_{3}$, L$_{5}$, and L$_{6})$ leads to the formation of Au clusters of characteristic size [1]. In particular, in the presence of L$_{3}$, Au$_{11}^{+3}$ clusters are formed, while the presence of L$_{5}$ leads to the formation of Au$_{8}^{+2}$, Au$_{9}^{+2}$ and Au$_{10}^{+2}$ clusters. We have carried out calculations based on the density functional theory in the projector augmented wave scheme (PAW) and the pseudopotential approach, to examine the effect of the diphosphine ligand spacer size on the stability of Au clusters containing 2 to 11 atoms through evaluations of the cluster total energy and proper corrections of spurious interactions between charged supercells. For example, to investigate the stability of Au$_{11}^{+3}$, we compare the total energy of Au$_{11}$(X)$_{5}^{+3}$ and Au$_{8}$(X)$_{4}^{+2}$ + Au$_{3}$X$^{+1}$ (X= L$_{3}$ and L$_{5}$ ligands) and find that Au$_{11}^{+3}$ is indeed preferred by L$_{3}$ rather than L$_{5}$, in agreement with the experiment. The electronic structural changes brought about by the various local environments of these clusters are presented with full details. [1] Bertino et al. Phys. Chem. B Lett. 110, 21416 (2006)   

An excitonic state-resolved approach to coherent phonons in quantum dots: generation and relaxation dynamics.

The strength of exciton-phonon couplings in quantum dots has remained controversial due in part to the complex eigenstate spectrum of electrons and holes in these systems. We recently implemented a combined time/frequency domain approach, towards exciton selective spectroscopy. This approach initially yielded a unified picture of the controversial mechanism of electron and hole relaxation dynamics in quantum dots. Recently, this approach yielded the first simultaneous observation of underdamped coherent optical and acoustic modes, thereby providing a direct measure of the controversial size dependent exciton-phonon coupling strengths for both modes. Pumping into higher excited states reveals that the Auger based electron relaxation process is vibrationally incoherent whereas non-adiabatic hole dynamics retain a memory of the vibrational coherence. Finally, phase and state-selective results reveal the electronic surface upon which the vibrational coherence is generated.   

Photochemical Processing of Carbon Dioxide Ices and Simple Ice Mixtures

The investigation of the photochemical processes that can occur in carbon dioxide ices and ice mixtures have important applications in astrophysics, planetary astronomy, and atmospheric chemistry. In this investigation, carbon dioxide ices and ice mixtures are grown at various temperatures using a closed-cycle helium cryostat. The ices are irradiated with x-rays for periods of up to six hours. The chemical changes that occur during the processing are monitored using x-ray photoelectron spectroscopy and Fourier transform infrared spectroscopy. A quadrupole mass spectrometer is also used to study the gas phase species evolving from the ice surface during photoprocessing. The XPS, FTIR and mass spectrometer results are presented and correlated. It is noted that significant differences are observed, particularly for the time dependence of the evolution of the gas phase molecules, between ices grown at 77 K as compared to those grown at 20 K and at intermediate temperatures.   

Single Electron Transport of Oligothiophene molecules

We examined the fabrication of single-electorn transistors (SETs) with oligothiophene derivative or Au nano particle covered with oligothiophene as Coulomb island. The SET device was consisted of a nanogap electrode, a molecular Coulomb island, and back gate electrode. The nanogap electrode was fabricated by the electromigration method. We could obtain the SET characteristics from the electron transport properties of both devices at 11K. Spectra that owing to a molecule was obtained in the electric transport properties of these devices. Moreover, we could obtain the SET characteristics in some devices.   

Characterization of hydroxyls on Si(001)-2$\times $1:H$_{2}$O using O 1s core-level spectroscopies and core-excited state DFT calculations

The Si(001)-2$\times $1 surface exposed to water molecules at room temperature has been chosen to single out the electron structure of the hydroxyl Si-OH, a surface species playing an important role in many technologically relevant processes. We confront here original core-electron spectroscopy DFT calculations to O 1s XPS (X-ray Photoelectron Spectroscopy) and NEXAFS (Near Edge Absorption Fine Structure) data. On various Si-OH environments (unpaired and paired hydroxyls), the impact of hydrogen bond on the calculated core-ionized and/or neutral core-excited states is examined, and compared to the limit case of the water dimer. The theoretical approach enables to label the main experimental NEXAFS transitions and to interpret their polarization-dependent dichroism. As water dissociation on the surface can go beyond the formation of hydroxyls, the DFT electron structure of bridging oxygens (Si-O-Si) is calculated. It is predicted that the XPS line associated to the latter species is shifted by 0.5-1.0 eV to lower binding energy with respect to hydroxyls.   

Structure-Property Relations in beta-Hairpin Peptide Hydrogels

A de novo designed beta hairpin peptide, capable of undergoing intramolecular folding and consequent intermolecular self-assembly into a fibril-based network that forms a hydrogel, has been studied. A combination of SANS and cryo-TEM have been used to quantitatively investigate the nanofibrillar hydrogel network morphology. An increase in the peptide concentration resulted in a denser fibrillar network as revealed via a change in the high q mass fractal exponent from 2.5 to 3. This is accompanied by a decrease in the measured correlation length from 23 to 16 Angstroms, indicative of the increase in the number of crosslinks and a reduction in the interfibril distances in the proximity of individual crosslinking points. In the USANS regime, a slope of -4 is indicative of gel microporosity. These changes, both, at the network as well as the individual fibril length scales can be directly visualized in situ by cryo-TEM. Fibrillar nanostructure and the network morphology are directly related and can be used to tune the bulk hydrogel stiffness, as studied via oscillatory rheology. Knowledge of the precise nano-through microstructure can help in the formation of specific structure-property relationships in these novel peptide-based hydogels.   

Structured Hydrogels using Micelles as Templates

Molecularly imprinted polymers can be created by crosslinking polymers in the presence of molecular templates. If the pores generated after the removing of templates have almost the same size and shape as the template, the material has a potential to be used for separation, biosensor and drug delivery applications. In this work, micelles were used as the template as they can be easily removed from the hydrogel and a range of structures are accessible by combining a (linear) polyelectrolyte and an oppositely charged surfactant. Poly(2-(dimethylamino)ethyl methacrylate) (PDMAEMA) was synthesized and quaternized using methyl iodide. We have performed small angle neutron scattering (SANS) on solutions and hydrogels of PDMAEMA with sodium dodecylsulfate (SDS) under different contrast matching conditions. A structured hydrogel was then formed by chemically crosslinking the semi-dilute PDMAEMA solution which contained SDS. It was confirmed that spherical micelle-like structures were associated along the polymer chain in a bead-and-necklace structure consistent with what has been observed in the (uncharged) poly(ethylene oxide)/SDS system. Furthermore, it was shown that the interaction between PDMAEMA and micelles is strong enough to maintain the nanoscale structure formed along the PDMAEMA chain, even after crosslinking, leading to a structured hydrogel.   

Controlled Charge-Flow in A Molecular Interferometer

We describe an electron transfer molecule interferometer capable of controlling electron flow by inelastic tunneling manipulation. The molecule consists of electron donor and acceptor groups that are connected by a bridge. Upon photo-excitation of the molecule, an electron transfers from the donor to the acceptor group by tunneling through the bridge. The structure of the bridge is such that it provides interfering electron tunneling pathways. We show how to control interferences between the pathways by isotopic substitutions of pathway-specific bridge atoms. Such substitutions create pathway-localized bridge vibrations that can be selectively IR- excited during the electron transfer event. We suggest ways of experimentally controlling pathway interferences and electron transfer rates.   

Single-crystalline rutile TiO$_{2}$ nanowires by mass selected Ni catalyst: Synthesis and electrical properties

We present a novel method for growing high quality TiO$_{2}$ nanowires using mass-selected Ni clusters of nanometer sizes produced by magnetron sputtering and also show their electric field-effect functions. Single-crystalline TiO$_{2}$ nanowires(NWs) are grown by atmospheric pressure physical vapour deposition(APPVD) process, using TiO and Ti metal powders as a Ti source and Ni nanoparticles as a catalyst, respectively. For the TiO$_{2}$ NWs growth, first, the Ti metal layer with a thickness of $\sim $50nm was then deposited on the SiO$_{2}$/Si substrate by the e-beam evaporation technique and subsequently, the mass selected Ni clusters by using magnetron sputtering source combined with a quadrupole mass filter was deposited onto the Ti layer. APPVD growth was then performed in a horizontal quartz tube furnace at 800$^{\circ}$C-950$^{\circ}$C by introducing high purity Ar carrier gas (99.999{\%}) with the flow rate of 300 sccm for 2 hours. The $I-V$ curves are linear over the entire annealing temperature range at 200 $\sim$500$^{\circ}$C, showing that the electrodes form good ohmic contacts with the nanowires. The $I $vs $V_{G}$ curves for various values of $V_{SD}$ and gate dependent $I-V $curves of a TiO$_{2}$ nanowire configured as a back-gated FET are also obtained and will be discussed.   

Accurate electronic-structure description of Mn complexes: a GGA+U approach

Conventional density-functional approach often fail in offering an accurate description of the spin-resolved energetics in transition metals complexes. We will focus here on Mn complexes, where many aspects of the molecular structure and the reaction mechanisms are still unresolved - most notably in the oxygen-evolving complex (OEC) of photosystem II and the manganese catalase (MC). We apply a self-consistent GGA + U approach [1], originally designed within the DFT framework for the treatment of strongly correlated materials, to describe the geometry, the electronic and the magnetic properties of various manganese oxide complexes, finding very good agreement with higher-order ab-initio calculations. In particular, the different oxidation states of dinuclear systems containing the [Mn$_2$O$_2$]$^{n+}$ (n= 2, 3, 4) core are investigated, in order to mimic the basic face unit of the OEC complex. [1]. H. J. Kulik, M. Cococcioni, D. A. Scherlis, N. Marzari, Phys. Rev. Lett., 2006, 97, 103001   

A density functional theory study of the benzene-water complex

We calculated the intermolecular interaction of the benzene-water complex using real-space pseudopotential density functional theory with a van der Waals density functional (vdW-DF). Developed recently, vdW-DF has been applied to a number of van der Waals complexes with promising results. Our results for the intermolecular potential energy surface between benzene and a water molecule clearly show a stable configuration with the water molecule standing above the benzene with one or both of the H atoms pointing toward the benzene plane, as predicted by previous studies. However, when the water molecule is pulled outside the perimeter of the benzene ring, the configuration of the complex becomes unstable with the water molecule attaching in a saddle point configuration to the rim of the benzene with its O atom adjacent to a benzene H. The results for ground state structure are compared with available experiments and quantum chemical calculations.   

First-principles study of the oxygen-reduction reaction on the Pt (100) surface

We have performed density-functional study of oxygen-reduction reaction on the Pt(100) surface. Equilibrium structures of oxygen on the surface are found and carefully analyzed. Our calculations show that oxygen molecules reduce into atoms when they reach the Pt(100) surface and cover the surface up to a complete monolayer with a binding energy of 2.12 eV per oxygen atom. More oxygen molecules reaching the surface will continue to reduce but also drive the ones in the monolayer away from the surface, making the surface an effective catalyst. However, oxygen does not stack up into layers on the Pt(100) surface and therefore does not play a significant role in degrading the Pt (100) surface during any catalytic process there.   

Chromism of Model Organic Aerosol

The optical properties of the atmospheric aerosol play a fundamental role in the Earth's radiative balance. Since more than half of the aerosol mass consists of complex organic matter that absorbs in the ultraviolet and visible regions of the spectrum, it is important to establish the identity of the organic chromophores. Here we report studies on the chromism vs. chemical composition of photolyzed (lambda longer than 305 nm) solutions of pyruvic acid, a widespread aerosol component, under a variety of experimental conditions that include substrate concentration, temperature and the presence of relevant spectator solutes, such ammonium sulfate. We use high resolution mass- and 13C NMR-spectrometries to track chemical speciation in photolyzed solutions as they undergo thermochromic and photobleaching cycles. Since the chemical identity of the components of these mixtures does not change in these cycles, in which photobleached solutions gradually recover their yellow color in the dark with non-conventional kinetics typical of aggregation processes, we infer that visible absorptions likely involve the intermolecular coupling of carbonyl chromophores in supramolecular assemblies made possible by the polyfunctional nature of the products of pyruvic acid photolysis.   

Spatially separating individual conformers of neutral (bio)molecules

Large (bio)molecules exhibit multiple conformers (structural isomers), even under the cold conditions present in a supersonic jet. For various applications, i.~e., scattering experiments, it would be highly desirable to prepare molecular packets of individual conformers. It is well-known that polar molecules can be manipulated using strong electric fields. Many techniques have been developed for the manipulation of small molecules in low-field-seeking quantum states. However, application of these techniques to large molecules is not straightforward, because, for larger molecules, all states are high-field seeking at the relevant electric field strengths. To manipulate the motion of large molecules one has to use Alternate Gradient (dynamic) focusing. This method has been successfully demonstrated in the Alternate Gradient deceleration of diatomic molecules. Using the same Alternate Gradient focusing principle, applying switched electric fields in a quadrupole guide, we have set up a new experiment to spatially separate individual conformers of large molecules. This experiment exploits the different mass-to-dipole ($m/\mu$) ratios, similar to a quadrupole mass-to- charge ($m/q$) filter for ions. In a proof-of-principle experiment, we have demonstrated the conformer selection of cis- and trans-3-aminophenol.   

Spectroscopy of free radicals and radical containing entrance-channel complexes in superfluid helium nanodroplets

The unique properties of superfluid helium nanodroplets, namely their low temperature (0.4 K) and fast cooling rates (10$^{16}$ Ks$^{-1})$, provide novel opportunities for the formation and high-resolution study of metastable structures or molecular complexes containing free radicals. We discuss methods for the production of radicals and their applicability for embedding the radicals in helium nanodroplets. The spectroscopy of free radicals (i.e. C$_{3}$H$_{3})$ and of radical containing entrance-channel complexes, for example X--HY (X=Cl, Br, I, CH$_{3}$; Y=F, CN), embedded in helium nanodroplets is detailed. The observed complexes provide new information on the potential energy surfaces of several fundamental chemical reactions and on the intermolecular interactions present in open-shell systems. Prospects for further experiments of radicals embedded in helium droplets are discussed.   

Calculation of spin-orbit, static polarizabilities, and alignment of cold polar molecules

Cold polar molecules offer exciting perspectives for studying strongly interacting cold quantum gases. Their creation in the absolute ground state through successive absorption/emission sequences relies on the detailed knowledge of their structure. Here we present new calculations of the electronic structure of all heteronuclear alkali molecules, based on effective core and polarization potentials [1] and quasi-diabatic perturbation theory [2]. Results on static dipolar polarizabilities and spin-orbit coupling functions are obtained for all pairs, all symmetries, and all internuclear distances, for the first time in most cases. Prospects for the alignment and orientation of these molecules under the influence of a combined strong laser field and a weak electrostatic field [3] are discussed. [1] M. Aymar and O. Dulieu, J. Chem. Phys. 122, 204302 (2005) [2] R. Cimiraglia et al, J. Phys. B: At. Mol. Phys. 18, 3073 (1985) [3] B. Friedrich et D. Herschbach, J. Phys. Chem. A 103, 10280 (1999)   

Comparison of molecular energies calculation using simulated quantum algorithm and classical computer methods

Very few quantum algorithms are currently useable today. When calculating molecular energies, using a quantum algorithm takes advantage of the quantum nature of the algorithm and calculation. A few small molecules have been used to show that this method is possible. This method will be applied to larger molecules and compared to classical computer methods.   

Growth and characterizations of m-plane GaN and InN on gamma-LiAlO$_{2}$ substrate grown by plasma-assisted molecular beam epitaxy

Non-polar nitrides are investigated in this report. Substrate used is gamma-phase LiAlO$_{2}$ (LAO) (100) grown by Czochralski pulling method. The in-plane lattice mismatch between the LAO (100) plane and the GaN (1-100) plane, is small with a lattice mismatch of [0001]GaN$\vert \vert $[010]LAO $\sim $ 0.3{\%} and [11\underline {2}0]GaN$\vert \vert $[001]LAO $\sim $1.7{\%}. M-plane GaN epilayer and InN were successfully grown by ultra-high vacuum plasma-assisted molecular beam epitaxy system. Extensive characterizations have been carried out which include x-ray diffraction theta/two-theta scan, rocking curve measurement, scanning electron microscopy, cathodoluminescence, photoluminescence, and Raman spectroscopy. Details will be presented.   

AlGaN/GaN high electron mobility transistor grown on GaN template substrate by molecule beam epitaxy system

In this study, AlGaN/GaN high electron mobility transistor (HEMT) structure was grow on GaN template substrate radio frequency plasma assisted molecular beam epitaxy (MBE) equipped with an EPI UNI-Bulb nitrogen plasma source. The undoped GaN template substrate was grown on c-sapphire substrate by metal organic vapor phase epitaxy system (MOPVD). After growth of MOVPE and MBE, the samples are characterized by double crystal X-ray diffraction (XRD), transmission electron microscopy (TEM), field emission scanning electron microscopy (SEM), atomic force microscopy (AFM), and Hall effect measurements. We found that the RMS roughness of template substrate play the major role in got the high value of mobility on AlGaN/GaN HEMT. When the roughness was lower than 0.77 nm in a 25 $\mu$m x 25 $\mu$m area, the mobility of HEMT at the temperature of 77 K was over 10000 cm$^{2}$/Vs.   

The role of oxygen vacancies and defects on Lithium intercalation capacity of composite vanadium-titanium oxide thin films

Composite films of vanadium-titanium oxides have been recognized as promising cathode materials for lithium ion batteries. While there is a consensus agreement that the cycling stability of the mixed V/Ti-oxide system is improved compared to V$_{2}$O$_{5}$ thin films, there are different findings on the Li+ intercalation capacity of V$_{2}$O$_{5}$ with the addition of TiO$_{2}$. To understand the difference we have carried out a systematic semi quantitative investigations on the defect and oxygen vacancy concentration as determined using Raman and UV-visible spectroscopy and transmission electron microscopy of V$_{2}$O$_{5}$ and 5{\%}Ti doped V$_{2}$O$_{5}$ films prepared by spin coating using two precursors: a metalorganic and a organic sol gel precursor. We observe that a critical concentration of defects and oxygen vacancies is important to have high capacity. With 5{\%} Ti doping the capacity in films prepared using solgel precursor increases, but the capacity decreases in films prepared with the metalorganic precursor. We attribute this to the different concentrations of oxygen vacancy defects in the two samples.   

Fe-Doped Sno$_{2}$ Powders Obtained By Sol-Gel Method, Mechanochemical Alloying, and Thermal Treatment

The present work is aimed to investigate experimental conditions to obtain pure Sn$_{1-x}$F$e_{x}$O$_{2-\delta }$ ($x$=0, 0.05, and 0.1) powders by three methods: (1) sol-gel method, (2) mechanochemical alloying and (3) thermal treatment. In (1), different precursors were employed: mixtures of Sn$^{4+}$ and Fe$^{3+}$ or Sn$^{2+}$ and Fe$^{2+}$. In (2), SnO$_{2}$ and $\alpha $-Fe or $\alpha $-Fe$_{2}$O$_{3}$ were used as reactants. In (3), the Fe-doped SnO$_{2}$ were obtained by mechanochemical milling and thermal treatment.$_{ }$All samples were characterized by X-Ray diffraction (XRD) using Rietveld refinement, Fourier-transformed infrared (FTIR) spectroscopy and room temperature $^{57}$Fe M\"{o}ssbauer spectrometry (MS). The XRD patterns of samples prepared by (1) showed only peaks of SnO$_{2}$. The MS showed ferromagnetic and paramagnetic signals. The samples obtained by (2) showed XRD peaks due to SnO$_{2}$ (rutile). The MS revealed the presence of Fe$^{2+}$ and Fe$^{3+}$ states as well as $\alpha $-Fe or $\alpha $-Fe$_{2}$O$_{3}$ due to the reactants. In the case of (3) was observed total incorporation of Fe$^{3+}$ in the SnO$_{2}$ structure without presence of impurities.   

ZnO films synthesized by thermal annealing of ZnSe/GaAs heterostructures

ZnO received much attention due to its potential application for the fabrication of ultraviolet light emitters and photodetectors. High crystalline quality films were grown using MBE, PLD, and CVD on Al$_{2}$O$_{3}$, GaN, SiC, and other substrates. However, further progress in this area is slowed down by the difficulties associated with doping ZnO p-type. Here, we report on the synthesis and doping of ZnO films using the annealing of MBE-grown ZnSe/GaAs heterostructures in the controlled environment. Se is displaced by oxygen through the reaction: 2ZnSe + 3O$_{2} \to $ 2ZnO + 2SO$_{2}\uparrow $. In addition, As migrating from the GaAs substrate into the ZnO layer, promotes p-type doping. While ZnGa$_{2}$O$_{4}$, ZnO$_{2}$, and other second phases form as the result of high temperature annealing ($>$700$^{\circ}$C), stoichiometric ZnO films are obtained at moderate temperatures ($\sim$500$^{\circ}$C). Films processed under optimized conditions exhibit sharp band edge emission, narrow rocking curve, and are comparable with the ZnO films grown on the GaAs substrates using other techniques. I would like to acknowledge support from the Office of Naval Research under grant N00014-06-1-1018.   

CdTe films grown on Si, GaAs and Quartz substrates

CdTe films of varying thickness were grown by the laser epitaxy technique on Si(001), GaAs(001), and quartz substrates. The quality of the resultant films was studied by x-ray diffraction and photoreflectance. Splitting of the valence band plus an increase in the band-gap were observed as the CdTe film thickness was decreased. To explain the experimental results, we have examined the contributions from quantum confinement, and from strain induced by the lattice mismatch and by the difference in coefficients of thermal expansion between CdTe and the substrate. For thicker films, we found that the strain was relaxed significantly near the film surface so that its crystalline quality and band-gap approached that of the bulk crystals.   

Electrochemical capacitance and charge relaxation resistance of mesoscopic capacitors

We investigate numerically the transport properties of a mesoscopic capacitor which consists of a quantum dot connected via a single lead to an electron reservoir. The fluctuations and distributions of electrochemical capacitance $C_\mu$ and charge relaxation resistance $R_q$ have been studied. It shows that the distribution of electrochemical capacitance $C_\mu$ at one conducting channel case in our numerical calculation is different from the former theoretical prediction obtained from the scattering matrix theory. The difference is due to the existence of necklace states which has been observed in a recent optical experiment.   

Porous Silicon Structures under action microwave Radiation: Charge Carrier Heating Effects

Porous silicon (por-Si) is one of modern nanomaterials, which is intensively investigated recently. The action of microwave radiation is only slightly investigated on por-Si, however. Basically there are papers intended to application of por-Si as substrates in microwave and opto-electronic interconnects. Action microwave radiation (MW) often manifests itself through effects of charge carrier heating in semiconductors. Since the energy quanta of MW radiation are too small to challenge any quantum jumps in common semiconductors, it is likely that carrier heating can be responsible for effects arising in por-Si under MW radiation also. This question is discussed in present contribution based on experimental study of electrical conductivity and electromotive force (emf) in por-Si structures under the action of 10 GHz frequency MW radiation pulses. Two-terminal por-Si containing structures were made by usual technology of electrochemical etching of p-type, 0, 4 Ohm$\cdot $cm Si plates in the HF: Ethanol=1:2 electrolyte. It has been shown that experimentally observed decrease of the resistance of the samples and rise of emf can be explained both assuming concept of hole heating, by MW radiation in fractal-like percolation grid of por-Si structure.   

Magnetization of Dirac electrons in Bismuth in the quantum limit

In the semimetal Bi, the Fermi Surface (FS) is comprised of 3 Dirac-like electron pockets and a hole pocket. Accurate measurements of the magnetization $M$ in Bi were previously limited to fields $H<$ 2 T. Following the recent report of fractional filling in Bi by Behnia et al. (Science 2007), we have performed extensive torque magnetization measurements in fields up to 33 T, in the geometry with $\bf H$ at angle $\theta$ to $\| Z$ (trigonal axis). At small $\theta$, we observe a set of quantum oscillations reflecting interference between hole and electron pockets. The pattern is highly sensitive to $\theta$. We have mapped out the variation of the periods vs. $\theta$ and $H$ in the quantum limit ($n$ = 1 at 9 T), and beyond. We also observe oscillations above 9 T, which correspond to fractional filling $\nu = 2/3$ and $\nu = 2/5$. Other anomalous features at high fields will also be reported. Resarch supported by PCCM Institute, Princeton University.   

Variational entropy and kinetics of hot electrons

The carrier distribution function and the effective temperature of an electronic population, optically generated in a semiconductor, are obtained by means of a variational method. An expression for the rate of entropy increment during the cooling process of the plasma is derived. We apply this expression to study the transduction processes in quantum ratchets based on resonant tunneling systems. Additionally, the relation of this rate of entropy increment to a restricted formulation of a quantum H theorem is explored.   

Phase switching in a voltage-biased Aharonov-Bohm interferometer

Recent experiment [Sigrist et al., Phys. Rev. Lett. {\bf 98}, 036805 (2007)] reported switches between $0$ and $\pi$ in the phase of Aharonov-Bohm oscillations of the two-terminal differential conductance through a two-dot ring with increasing voltage bias. Using a simple model, where one of the dots contains multiple interacting levels, these findings are explained as a result of transport through the interferometer being dominated at different biases by quantum dot levels of different ``parity'' (i.e. the sign of the overlap integral between the dot state and the states in the leads). The redistribution of electron population between different levels with bias leads to the fact that the number of switching events is not necessarily equal to the number of dot levels, in agreement with experiment. For the same reason switching does not always imply that the parity of levels is strictly alternating. Lastly, it is demonstrated that the correlation between the first switching of the phase and the onset of the inelastic cotunneling, as well as the sharp (rather than gradual) change of phase when switching occurs, give reason to think that the present interpretation of the experiment is preferable to the one based on electrostatic AB effect.   

Study of the 2DEG in InGaAs/AlInAs heterostructures by persistent photoconductivity effect

The electronic properties of the two-dimensional electron gas (2DEG) in InGaAs/AlInAs heterostructures have been studied by Shubnikov-de Haas measurement at 0.3 K. After illuminating at 0.3 K, the carrier density of the sample increased from 2.3×10$^{12}$ cm$^{-2}$ to 2.5x10$^{12}$ cm$^{-2}$ and the mobility decreased slightly from 36200 cm$^{2}$/Vs to 34900 cm$^{2}$/Vs. In order to study the effect of the channel width on the 2DEG, we made the nanometer-scaled 2DEG channels were varied with different widths of 100 nm to 500 nm. The SdH measurement was performed on these wires for the magnetic field up to 12 T at 0.3 K. We observed the persistent photoconductivity effect on these wires and the electronic properties of these wires are under investigation.   

Magneto-transport Study on the nanometer-scaled wires made of Al$_{x}$Ga$_{1-x}$N/GaN heterostructures

The electronic characteristics of nano-wires made of high-mobility Al$_{x}$Ga$_{1-x}$N/GaN heterostructures have been studied. The Al$_{x}$Ga$_{1-x}$N/GaN samples were grown on GaN-template buffer layer by plasma-assisted molecular beam epitaxy. We obtained the mobility and carrier density of two-dimensional electron gas to be 9328 cm$^{2}$/Vs and 7.917x10$^{12}$ cm$^{-2}$ by conventional van der pauw Hall measurement at temperature of 77K, respectively. We prepared the samples of field-effect-transistors and reduced the width of the conducting channel from 1$\mu $m to 100 nm by Focus Ion Beam. The Shubnikov-de Haas oscillations were observed by magneto-resistance measurement at 0.3 K and the electronic properties for the samples of different channel widths were under investigation.   

Transport characterization in silicon nanowires

In spite of the great technological interest associated with nanowires, there are only few direct electrical measurements of both the doping level and of the density of surface state. Moreover, the effect of the phonon surface scattering is not well known. We have studied the transport in silicon nanowires, fabricated using e-beam lithography and RIE etching on SOI substrate (thickness of silicon~: 20 nm N and P-type intentionally doped at 10$^{20 }$cm$^{-3})$. The nanowire widths vary from 100 nm down to 10 nm. We can extract the intrinsic resistance of the nanowires, excluding the contacts resistances. We found resistances superior to theoretical resistances. We explain these higher resistances by surface defects on nanowires naturally oxidized and we are able to determine the depletion width due to surface defects, and to deduce the doping level and the density of surface states. We found a reasonable value of 1.10$^{13}$ to 2.10$^{13 }$defects/cm$^{2}$, associated with a doping value of 5.10$^{19}$ to 9.10$^{19}$ cm$^{-3}$. Studies of the transport in the range 80 to 300 K demonstrate that the resistance of the nanowires is mainly bound to phonon scattering localized on the surface (bulk contribution is negligible).   

Study of the Mott-insulator TiOCl under pressure and doping

We will discuss recent experiments of the Mott-insulator TiOCl under pressure and Na doping in the frame of Density Functional Theory (DFT) calculations where we employ Car-Parrinello molecular dynamics with Projected Augmented Wave (PAW) wavefunctions. For TiOCl under pressure a phase transition from insulator to metal is found as observed experimentally. For the doped system we have considered supercells of Na doped TiOCl and we propose possible effective models for the mechanism of doping.   

Quantum Optics with Colloidal Nanocrystals in Solution

Semiconductor nanocrystal quantum dots (NQDs) have been increasingly utilized in developing technologies such as lasers, light-emitting diodes, and bioimaging. Despite gaining popularity as labeling sources in bioimaging, no reliable method has been developed to characterize and identify single nanoparticles in solution, a requirement for efficient labeling at single-cell/single-NQD level. Here we present our recent results addressing the aggregation problem by combining FCS with photon pair correlation spectroscopy (PPCS). The combination of these two methods together with necessary theoretical treatment allows us to quantify, for the first time, the clustering degree of different nanocrystals in solution. The extent of aggregation in a sample can be straightforwardly determined by the deviation of occupation number between PPCS and FCS measurements. CdSe nanocrystals dispersed in organic solvents such as toluene or hexane have minimal clustering, usually less than 1.1 NQD/cluster and do not show considerable aggregation over time. To the contrary, commercially available water-soluble CdSe NQDs that are typically used for cell labeling show a tendency toward aggregation into small, 2-3 NQD clusters. In addition, the clustering degree of such dots increases over time rendering their use in single-dot labeling problematic.   

Energy level shift due to the co evaporated LiF in Alq$_{3}$

Recently, one of us (FS) observed strong improvement in conductivity of LiF-doped tris (8-hydroxyquinoline) aluminum (Alq$_{3}$). We have investigated the p n-doping of the organic material. The doping induces energy level shift in frontier orbital for about 0.25 eV when the doping ratio by weight is 10 \%. Small amount of metal deposition (0.5 $\AA$ of Al, Ag, Au) on LiF-doped Alq$_{3}$ causes further shift, with Al the most ($\sim$1 eV) and Au the least (0.3 eV). Further metal depositions reverse the shift for about 0.5 eV. These results suggest that the metal induced enhancement of n-doping in LiF:Alq$_{3}$ contributes to the improvement in conductivity. After 1$\sim$2 $\AA$, the properties of metals show up and the Energy levels converge.   

Discontinuity of the dielectric function at Bragg reflexes

As an example for layered materials, the loss function of graphite was studied for momentum transfers $q$ beyond the first Brillouin zone. Surprisingly, near Bragg reflexes, the spectra are highly dependent on very small changes in $q$, which reminds the non-analyticity of the loss function in the optical limit ($q\!\to\!0$). The effect is investigated by means of first principle calculations within the random phase approximation (RPA) and is confirmed by inelastic x-ray spectroscopy (IXS) measurements. We find crystal local field effects to be crucial and propose a simple $2\!\times\! 2$ model dielectric function for explanation.   

Excitonic effects in optical absorption spectrum of CdS/ZnSe core-shell Nanostructures.

We have used a first principles, TDDFT method to study CdS (Type I) nanocrystal quantum dots and CdS/ZnSe core/shell (Type II) heteronanostructures with high accuracy. We have studied the existence of excitons and multi-excitons and the possibility of optical gain in these nanoclusters. The size dependence of the HOMO-LUMO gap, electron-hole wave function overlap in the relevant states, coordination of atoms at the surface as well as the stability of such clusters will be presented in some detail.   

Stark Shifts in the Mid-Infrared Absorption Edge of Type II Quantum Wells

We have studied electric field induced (Stark) shifts in real-space indirect mid-infrared transitions that occur in type II AlSb/InAs/GaSb quantum wells. Because of the spatial separation of the electron and hole wavefunctions, the potential drop between the layers dominates the shift in the absorption edge, and can result in either a red shift or a blue shift, depending on the ordering of the quantum wells within the intrinsic region of a p-i-n diode. Of particular interest is the case in which a reverse bias on the diode yields a blue shift in the absorption edge since this field tends to increase the overlap between the electron and hole wavefunctions, increasing the absorption strength. We will give the results of low temperature photocurrent spectroscopy on a series of samples with different layer ordering, degrees of confinement, and coupling between the wells.   

Reflectance anisotropy spectroscopy of II-VI semiconductor surfaces

The spectroscopical reflectance anisotropy (RA) response of II-VI semiconductor surfaces, which exhibits different reconstructions are studied. We use, an $ab$ $initio$ pseudopotential calculation in the framework of the density functional theory and within the local density approximation (DFT-LDA) to obtain the relaxed atomic positions, and then we use a microscopic formulation based on a semi-empirical tight binding (SETB) approach which includes spin-orbit (SO) interactions[1] to obtain the RA spectra. We show RA spectrum of each surface reconstruction and compare our theoretical results with experimental. We find a good agreement between experimental and theoretical spectra. [1] R.A. V\'azquez-Nava, B.S. Mendoza and C. Castillo, Phys. Rev. B {\bf 70}, 165306 (2004).   

Ultrafast nonlinear spectroscopy characterization of CdSe quantum dots

Frequency degenerate and nondegenerate two-photon absorption spectra of direct band gap semiconductor quantum dots, such as CdSe and CdTe, have attracted great attention recently because of their potential applications in nonlinear photonic devices. In this work, we used the femtosecond time-resolved photon echo technique to characterize the third-order nonlinear optical properties of CdSe quantum dots in toluene. The quantum dots had an average size of about 3 nm and the lowest absorption peak at 559 nm. A femtosecond laser with 150 fs pulsewidth and 1 KHz repetition rate was used for the measurement. The copolarization hyperpolarizability at 775 nm was found to be about -1x10$^{-43}$ m$^{6}$/V$^{2}$ and the dephase time was shorter than the resolution limitation of our system at room temperature. This work at Hampton University was supported by Army Research Office (W911NF-07-1-0608) and National Science Foundation (HRD-0734635, HRD-0630372, ESI-0426328/002, and EEC-0532472).   

Direct bandgap of group IV semiconductors by uni-axial stress

We theoretically examine the possibility of converting typical group IV semiconductors Si, SiGe (zinc blende), and Ge into direct bandgap materials by uniaxial stress along the $<$111$>$ and $<$100$>$ directions. For silicon, the required tensile strain is too large to be practical. For SiGe and Ge along $<$111$>$, although band splitting at the L point lowers the conduction band edge at L, a direct bandgap can still be achieved through a supralinear decrease in the energy of the conduction band edge at $\Gamma $. The required longitudinal strains along $<$111$>$ are 8{\%} and 4{\%} for GeSi and Ge, respectively. For strain along $<$100$>$, the position of the conduction band edge at the $\Gamma $ point varies sub-linearly with strain; therefore strain along $<$100$>$ is less efficient: GeSi is unlikely to achieve a direct gap by extension along $<$100$>$ and Ge requires a 6{\%} longitudinal strain. The full dependence of the indirect/direct transition on arbitrary combinations of uniaxial/hydrostatic tensile strain is given for both GeSi and Ge.   

Quasi-particle spectra of various forms of cristalline Germanium Telluride

GeTe is known to undergo a very complex structural phase transition under pressure. Also it changes the phase upon heating from crystalline NaCl structure to amorphous phase. Several models were suggested for possible structural transition pathways such as Toledano, Modified Buerger and Watanabe model. We have investigated the dielectric function of GeTe at representative structural phases using the pseudopotential density functional method within the generalized gradient approximation. To calculate the quasiparticle spectra, we carried out the GW calculations. Semicore d-electrons of Te are found to be very critical for calculating correct transition pressures, and they are included explicitly as valence. These electrons affect the electronic structures through the p-d coupling, which is found non- negligible. Our results can help analyze experimental data and investigate the transition pathway of GeTe.   

Spectroscopic characterization of Europium and Praseodymium doped Gallium Nitride powders

Rare earth (RE) doped GaN continues to be of interest for applications in display technology, solid-state light sources, and optical communications. Recently, RE doped GaN powders have been prepared using different methods including flux techniques and combustion synthesis. In this work, we report on the luminescent properties of Eu$^{3+}$ and Pr$^{3+}$ doped GaN powder prepared by a Na flux method for potential applications in light source development. Under above-gap pumping, GaN:Eu and GaN:Pr powders exhibited intense red emissions at $\sim $ 621 nm and $\sim $652 nm, which corresponds to the intra-4f RE$^{3+}$ transitions $^{5}$D$_{0} \quad \to \quad ^{7}$F$_{2}$ and $^{3}$P$_{0} \quad \to \quad ^{3}$F$_{2}$ states, respectively. A temperature dependent study of the red emission showed that the integrated PL intensity is quenched at room-temperature by $\sim $30{\%} and $\sim $50{\%} for Pr:GaN and Eu:GaN, respectively. More results of temperature dependent and time-resolved emission spectroscopy of Eu and Pr doped GaN powders will be presented at the conference.   

Electronic structure and conduction-band mass of InN under pressure

An initio calculations for n-type InN of the band structure and effective masses (optical- and curvature-) in the lowest conduction band as functions of applied pressure and varying electron concentration are presented. The calculations as well as available experimental data demonstrate the strong non-parabolicity of the InN conduction band.   

Temperature dependence of the InGaPN conduction band structure

Material properties of III-N-V alloys, such as GaAsN, InGaAsN, and InGaPN, have been intensively studied, because a small amount of nitrogen (N) incorporation results in very large bandgap bowing and dramatic change in the band structure.$^{1,2}$ Recently, temperature dependence of the parameters, $i.e.$ the localized states energy $E_N $ introduced by an isolated N and the interaction potential $V$, of the band anticrossing (BAC) model in GaAsN epilayers has been reported.$^{3}$ These properties have never been studied for InGaPN. In this work, temperature-dependent photoreflectance (PR) measurements are employed to characterize the conduction band structure of In$_{0.54}$Ga$_{0.46}$P$_{1-y}$N$_{y}$ ($y$ = 0 and 0.02) grown on GaAs substrates. The band gap and the upper subband $E_+ $ are observed in InGaPN as predicted by the BAC model. To investigate the energetic positions of the features in the PR spectra, a Kramers-Kronig analysis is proposed. Based on these PR data and the BAC model, we find $E_N =2.054$ eV and $V=1.513$ eV at 293 K. With decreasing temperature, the energy of $E_N $ shifts significantly to higher energies. Simultaneously, the interaction potential $V$ between the N states and the host conduction band also rises to higher values. The thermal shifts of $E_N $ and $V$ are $dE_N /dT\approx -0.43$ meV/K and $dV/dT\approx -0.67$ meV/K, respectively. 1.APL \textbf{88}, 031907 (2006). 2.APL \textbf{89}, 192116 (2006). 3.APL \textbf{89}, 202105 (2006).   

Electron pairing in small Hubbard Clusters.

Exact thermal studies of 4-site Hubbard Nanoclusters are carried out using the analytical eigenvalues. Electron pairing is seen when the on-site Coulomb interaction is smaller than a critical value Uc(T) in the repulsive Hubbard clusters which also show spin pairing at a lower temperature. Specific heat and probability calculations provide strong support for the existence of competing (paired and unpaired) phases near optimal doping. Attractive 4-site Hubbard model can be mapped on to the repulsive model and these studies can be used to understand ferroelectricity in certain metal clusters.   

Dipole model for Non linear response of adsorbed overlayers

We present theoretical calculations of second harmonic generation (SHG) from overlayers of alkali atoms adsorbed on a crystalline metallic surface. We assume that the overlayer is formed by an ordered two-dimensional (2D) array of adatoms that respond to the local electric field like point-like harmonic oscillators. We consider overlayers with several rational coverages, assuming that the adsorbates occupy high symmetry sites which form a Bravais lattice that is commensurate with the substrate [1,2]. SHG spectra are obtained for the five 2D Bravais lattices. We found that SHG can be used to observe ordered phases when the ordered phase has a rectangular, centered-rectangular or oblique symmetry. [1] H. Arce, W. L. Mochan and G. Cocho, Surf. Sci. 294, 108 (1993). [2] H. Arce and W. L. Mochan, J. Phys.: Condens. Matter 5, A 101 (1993).   

Magneto-exciton transitions in laterally coupled quantum dots

We present a study of the electronic and optical properties of laterally coupled quantum dots. The excitonic spectra of this system under the effects of an external magnetic field applied perpendicular to the plane of the dots is obtained, with the potential of every individual dot taken as the superposition of a quantum well potential along the axial direction with a lateral parabolic confinement potential, and the coupled two- dot system then modeled by a superposition of the potentials of each dot, with their minima at different positions and truncated at the intersection plane. The wave functions and eigenvalues are obtained in the effective-mass approximation by using an extended variational approach in which the magneto- exciton states are simultaneously obtained [1]. The allowed magneto-exciton transitions are investigated by using circularly polarized radiation in the plane perpendicular to the magnetic field. We present results on the excitonic absorption coefficient as a function of the photon energy for different geometric quantum-dot confinement and magnetic-field values. Reference: [1] Z. Barticevic, M. Pacheco, C. A. Duque and L. E. Oliveira, Phys. Rev. B 68, 073312 (2003).   

Anisotropy of plasmon spectrum due to joint Rashba and Dresselhaus spin-orbit interaction

We have investigated the combined effect of Rashba and Dresselhaus spin-orbit interaction (SOI) on the many-body polarization function of a two-dimensional electron system (2DES). The dielectric function of a 2DES is calculated within the random phase approximation and the plasmon energy spectrum as a function of the momentum magnitude for its different orientations is obtained. Our calculations show the peaked behavior of dynamical structure factor as a function of the polar angle of momentum. This strong peak corresponds to the plasmon, which is damped due to SOI. Thus, we have clearly demonstrated that due to the anisotropy of the spin-orbit interaction, the plasmons with the definite values of energy and momentum can be excited only in the certain direction.   

A STM study on temperature-dependent adsorption of H$_2$O on Si(001)

We studied the temperature dependence of water molecule adsorption on the Si(001)-2$\times$1 surface by using Scanning Tunneling Microscopy. The water molecules are known to dissociate during the adsorption on Si(001)and form Si-H and Si- OH bonds. Recently, we demonstrated that they are two adsorption configurations: ID (inter-dimer) and OD (on-dimer). These two configurations show population ratio of n(ID)/n(OD) $\sim$ 5 at Room Temperature. In order to understand the adsorption kinetics more thoroughly, we have measured n(ID)/n(OD) by varying the sample temperature from 300K to 870K. It is found that n(ID)/n(OD) show strong temperature dependence, and it even becomes smaller than 1. The cross-over temperature is at around 470K.   

Phase Transition Plasticity Response in Uniaxially Compressed Silicon Nanospheres

We present a microscopic description for the response of crystalline Si nanospheres up to 10 nm in radius for various uniaxial compression levels. The behavior at low compressions closely resembles the Hertzian predictions. At higher compressions the creation of a new beta-tin phase in the particle core leads to (i) volumetric changes (ii) an increase in elastic moduli, and (iii) significant hardening. Further, (iv) a reversible character of the transformation is obtained with molecular dynamics simulations. The agreement of (i)-(iv) with recent experimental findings~$[1]$ challenges the current exclusive view of a dislocation plasticity response in somewhat larger nanoparticles. The phase transition path should dominate in ultrasmall structures, where dislocation activity is prohibited.\newline $[1]$~W. W. Gerberich et al., J. Mech. Phys. Sol. 51, 979 (2003). \newline $[2]$~P. Valentini, W. W. Gerberich, and T. Dumitrica, Phys. Rev. Lett. 99, 175701 (2007).   

Friction measurements on InAs NWs by AFM manipulation

We discuss a new approach to measure the friction force between elastically deformed nanowires and a surface. The wires are bent, using an AFM, into an equilibrium shape determined by elastic restoring forces within the wire and friction between the wire and the surface. From measurements of the radius of curvature of the bent wires, elasticity theory allows the friction force per unit length to be calculated. We have studied friction properties of InAs nanowires deposited on SiO$_{2}$, silanized SiO$_{2}$ and Si$_{3}$N$_{4}$ substrates. The wires were typically from 0.5 to a few microns long, with diameters varying between 20 and 80 nm. Manipulation is done in a `Retrace Lift' mode, where feedback is turned off for the reverse scan and the tip follows a nominal path. The effective manipulation force during the reverse scan can be changed by varying an offset in the height of the tip over the surface. We will report on interesting static- and sliding friction experiments with nanowires on the different substrates, including how the friction force per unit length varies with the diameter of the wires.   

First-Principles Theoretical Analysis of Dopant Diffusion on Surfaces of II-VI Compound Semiconductor Nanocrystals

We present a detailed analysis of diffusion of dopants (e.g., Mn, Cu) on surfaces of ZnSe nanocrystals and discuss its implications for dopant incorporation into the growing nanocrystals. We focus on nanocrystals with diameters d $\sim $ 5 nm that have polyhedral shapes with well-defined facets. Using first-principles density functional theory calculations, we have studied the dopant diffusion and adsorption mechanisms and obtained the energetics of various possible dopant diffusion pathways. ZnSe(001)-(2$\times $1) is found to be the energetically favorable surface for dopant binding, with multiple adsorption sites. Our results indicate that dopant atoms can migrate with low activation barriers along the Se dimer rows without substantial surface relaxation. Diffusion across the dimer rows is governed by a higher-barrier pathway, which can lead to dopant incorporation into the nanocrystal through strong bonding with the nanocrystal surface at the corresponding adsorption site in the trough between adjacent dimer rows.   

Structural defects in SiC nanowires

High-resolution transmission electron microscopy (HRTEM) and selected area electron diffraction (SAED) are used to investigate structural defects in zinc blende SiC nanowires produced by a vapor-solid (VS) mechanism. It is found that the defects exist as the stacking faults and twins including single twin, double twins, and quasiperiodic placement twins. The results indicate that the important role of defects in determining the morphologies and structures i.e. stacking faults result in formation of branches or junctions, while twins cause kinks, bamboo or a zigzag appearance. Based on the characterizations, the defects formation mechanism and the influence on the nanowire growth kinetics and behavior are also discussed.   

Nonlinear modification of tunneling and conductivity in two-dimensional electron gas--impurity system in strong high-frequency electric fields

We investigate two-dimensional electron gas system coupled to adjacent impurity sites. When a high-frequency uniform electric field is applied perpendicular to the electron gas layer it significantly modifies electron correlations in the impurity-gas system. At strong magnitudes of external field the system enters nonlinear dynamical control regime, similar to double quantum dot structures. In contrast to the latter, coulomb activation of the impurity sites introduces strong scattering for conduction electrons that leads to nontrivial renormalization of the tunneling. Modification of tunneling rates as a function of the field amplitude is calculated. We show that for low enough temperatures this effect is manifested in nonlinear dependence of the conductivity of two-dimensional electron gas as a function of the ac field strength. It develops a periodic peak structure.   

Influence of inactivated dopants clusters and post annealing on electrical and optical properties of indium tin oxide on plastic substrates

Indium tin oxide (ITO) films have been introduced as a transparent electrode for large area electronics such like display and photovoltaics due to their high electrical conductivity coupled with high transmission. This study describes the influence of Sn defect clusters on the electrical and optical properties of the ITO films. Absorption coefficient analysis indicates that electrically inactive Sn clusters generate defect states within the band gap and strongly affect the electrical and optical properties of the ITO films. Electrical and optical properties of ITO films are enhanced by reduction of Sn defect clusters with activation of Sn dopants, upon heat treatment. To explain the enhanced carrier transport property, we propose the reaction model based on our Rutherford backscattering spectrometry (RBS), Hall measurement, and x-ray photoelectron spectroscopy (XPS) data. The proposed model describes Sn$^{4+}$ ions forming a neutral defect complex with Sn$^{2+}$ ions which may not contribute to electrical conductivity in the amorphous phase. These Sn defect clusters in the amorphous phase dissociate and could contribute to an increase in Sn$^{4+}$ concentration in ITO films without changing oxygen contents.   

Hole mobility in Copper-doped CdTe films

Copper-doped CdTe films have been grown by the laser epitaxy technique. X-ray diffraction, Rutherford backscattering, and photoreflectance spectroscopy were utilized to characterize the CdTe:Cu films. Structural analysis suggests that the growth of CdTe:Cu on GaAs(100) is initiated along the (100) orientation, but changes to the (111) direction after the film thickness exceeds 400 nm. Hall effect measurements indicate that copper doping can achieve hole mobility over 150 cm$^{2}$/Vs at room temperature. Changes in the hole effective mass and phonon spectra have been calculated to explain the enhanced hole mobility in Cu:CdTe.   

An experimental study on $\Gamma $(2) modular symmetry in the quantum Hall system with a small spin-splitting

Magnetic-field-induced phase transitions are investigated by performing transport measurements on the two-dimensional (2D) electron system in an AlGaAs/GaAs heterostructure at low temperatures. [1] Both the semicircle law and universal two-parameter scaling are observed as the spin-splitting becomes resolved with increasing the magnetic field $B$ perpendicular to such a 2D system. The critical resistivities, however, are not of the expected universal values, and the modular symmetry is reduced from $\Gamma _{0}$(2) to $\Gamma $(2). On the other hand, $\Gamma _{0}$(2) symmetry survives at lower $B$ where the spin-splitting is unresolved. Therefore, the reduction of the modular symmetry can be due to the resolved small spin-splitting as predicted by Dolan [2]. Such a small splitting not only reduces the modular symmetry, but also breaks the scaling on the longitudinal conductivity \textit{$\sigma $}$_{xx}$ at higher temperatures. It is found that the scaling on the Hall conductivity \textit{$\sigma $}$_{xy}$ is more robust than that on \textit{$\sigma $}$_{xx}$ under a small spin gap. References [1] C. F. Huang, Y. H. Chang, H. H. Cheng, Z. P. Yang, H. D. Yeh, C. H. Hsu, C.-T. Liang, H. H. Lin, D. R. Hang, and H. H. Lin, J. Phys.:Condens. Matt. 19, 026205 (2007). [2] B. P. Dolan, Phys. Rev. B 62, 10278 (2000).   

Probing insulator-quantum Hall transitions near the onset of Landau quantization in GaAs/AlGaAs heterostructures

Magneto-transport measurements are performed on the two-dimensional GaAs electron systems (2DESs) to study the low-field insulator (I) and quantum Hall (QH) effect. With increasing the perpendicular magnetic field B, the 2DESs undergo direct I-QH transitions to enter QH liquids from the low-field insulators near the onset of Landau quantization. The mobility obtained from Shubnikov-de Haas oscillations, however, indicates that such transitions do not occur as Landau bands become well-separated. We note that Landau quantization is significant even when the Landau-level spacing is smaller than the broadening, and it is insufficient to consider the crossover from weak localization to such a quantization in direct I-QH transitions. Our study supports the importance of the two-body interaction to direct I-QH transitions.   

Single-valley (110) and double-valley (001) AlAs quantum wells

Doubly-degenerate valley quantum number in (001)AlAs quantum wells (QWs) functions as pseudo-spin degree of freedom for understanding exchange interactions, and single valley anisotropic mass, if observed in high mobility (110)AlAs, could lead to interesting phases in quantum limit. We optimized growth of AlAs QWs [1], and grew (001) oriented double valley degenerate AlAs QW with density n=2.4 x 10$^{11}$ cm$^{-2}$ and mobility $\mu $=4.3 x 10$^{5}$ cm$^{2}$/Vs(330 mK), an improvement of almost an order of magnitude over published results. We also grew (110)oriented AlAs QW with n=4.2 x 10$^{11}$ cm$^{-2}$ and $\mu $=5.4 x 10$^{4}$ cm$^{2}$/Vs(330 mK). The (110)AlAs QW is predicted to occupy single valley, and anisotropic $\mu $ along two crystallographic directions of (100) {\&} (1-10) may be expected. Experimentally, we observed $\mu $ anisotropy $\mu _{(100)}$/$\mu _{(1-10)}$=1.6 in dark(1.4 K) and strong odd-filling factor SdH gaps, evidence consistent with single valley occupancy. We studied $\mu $ on both growth facets as function of temperature. The $\mu $ of (110)AlAs QWs is seen to saturates below the Fermi temperature but (001)AlAs QW does not saturate. Possible causes will be discussed, and measurements down to 50 mK will be presented. [1] Dasgupta, \textit{et al.} Appl. Phys. Lett. (`07)   

Analysis of spin current shot noise from a quantum dot coupled to a quantized bosonic field

We examine spin current generated by quantum dots coupled to quantized bosonic field. The dots are connected to normal leads at zero bias voltage across the dot. We model the dot as a two level spin system with one of the spin states lying below the Fermi level of the leads and the other above. Spin flips followed by subsequent tunnelling out into the leads generate a pure spin current in the absence of any charge current. The dot is coupled to a quantized bosonic field that influences the generation of spin current in one of two possible scenarios. First, we consider spin flips induced via Raman transitions in optical microcavity. Secondly, electron spin resonance via classical magnetic field induces spin flips in the presence of a quantized phonon field that modulates the energy levels of the dot. In the limit of strong Coulomb blockade our model is analogous to the Jaynes-Cummings model in quantum optics. In the case of optical cavity mediated spin flips, we show that the spin current is bistable for a coherently driven cavity and this bistability is clearly visible in the spin current shot noise. A comparison of spin current and shot noise for interactions with different bosonic fields is presented.   

Magneto-Dynamics of a Double Quantum Dot System

We have examined the microscopic dynamics of a double quantum dot system modeled by a potential having two three-dimensional Dirac delta functions of generally unequal strengths separated in position by$\vec {a}$: \[ V(\mathord{\buildrel{\lower3pt\hbox{$\scriptscriptstyle\rightharpoonup$}}\over {r}} )=-\alpha _1 \delta ^{(3)}(\mathord{\buildrel{\lower3pt\hbox{$\scriptscriptstyle\rightharpoonup$}}\over {r}} -\raise0.7ex\hbox{${\vec {a}}$} \!\mathord{\left/ {\vphantom {{\vec {a}} 2}}\right.\kern-\nulldelimiterspace}\!\lower0.7ex\hbox{$2$})-\alpha _2 \delta ^{(3)}(\mathord{\buildrel{\lower3pt\hbox{$\scriptscriptstyle\rightharpoonup$}}\over {r}} +\raise0.7ex\hbox{${\mathord{\buildrel{\lower3pt\hbox{$\scriptscriptstyle\rightharpoonup$}}\over {a}} }$} \!\mathord{\left/ {\vphantom {{\mathord{\buildrel{\lower3pt\hbox{$\scriptscriptstyle\rightharpoonup$}}\over {a}} } 2}}\right.\kern-\nulldelimiterspace}\!\lower0.7ex\hbox{$2$}). \] While these delta function potentials individually support a single energy level, the introduction of a strong magnetic field gives rise to Landau quantization and a plethora of energy levels. The relative magnitude of $\raise0.7ex\hbox{${\alpha _1 }$} \!\mathord{\left/ {\vphantom {{\alpha _1 } {\alpha _2 }}}\right.\kern-\nulldelimiterspace}\!\lower0.7ex\hbox{${\alpha _2 }$}$affects the bonding/antibonding character of the states, as well as the multiplicity of levels induced by magnetic quantization.   

Magnetotransport Properties of Ferromagnetic Semiconducting Fe$_{1-x}$Co$_{x}$Si Alloy Nanowires: Building Blocks for Silicon Based Spintronics

Fe$_{1-x}$Co$_{x}$Si alloys were recently shown to be concentrated magnetic semiconductors and can be promising materials for spin injection into silicon not only because of its high spin polarization but also because of its CMOS compatibility. By developing a rational approach to synthesizing nanowires using single source precursors, we have realized a host of new transition metal silicide nanowires through chemical vapor deposition/transport. We have synthesized FeSi nanowires using Fe(SiCl$_{3})_{2}$(CO)$_{4}$ and CoSi nanowires using Co(SiCl$_{3})$(CO)$_{4}$. Building on these initial successes, we synthesized magnetic semiconducting Fe$_{1-x}$Co$_{x}$Si alloys using mixed precursors. The as synthesized nanowires were characterized using high resolution transmission electron microscopy, energy dispersive X-ray spectroscopy, and X-ray absorption spectroscopy. The bulk Fe$_{1-x}$Co$_{x}$Si alloys were recently shown to be concentrated magnetic semiconductors and can be suitable materials for spin injection into silicon because of its CMOS compatibility. The interesting magnetic semiconducting behavior is shown using magneto-transport and X-ray magnetic circular dichroism revealing the interesting chemistry behind the magnetic properties. These novel magnetic semiconducting silicide nanowires are exploited as building blocks for silicon-based spintronic nanodevices.   

Adsorption, Symmetry and Magnetic Properties in TM-GaN Nanocrystals

Transition metal(TM)-doped dilute magnetic semiconductor nanocrystals are of interest for potential applications in spintronics due to their tunable properties. The physical properties of semiconductor nanocrystals depend on their size or shape. we present first-principles calculations for the electronic structure and magnetic properties of completely passivated, H- adsorbed and isolated Mn/Cr-doped GaN nanocrystals with different point symmetry. A novel half-metallicity and magnetic metastability has been found. The multiple metastable spin states and spin-flip for metallic channel are demonstrated in these nanocrytals with the various H coverages and point groups symmetry. A detailed electronic structure analysis is given. Our calculated results imply a new type of half-metallic and spin-crossover nanomaterial.   

Ab Initio Study of Magnetic Properties of Cr-doped Chalcopyrites: (BeSn, BeGe, MgGe)N$_2$

A Density Functional Theory within Generalized Gradient Approximation study of three thermodynamically stable Cr-doped (II-IV)-N$_2$ chalcopyrites: (BeSn, BeGe, MgGe)N$_2$ was performed. Since the chalcopyrites are ternary materials, there are possibilities of having ferromagnetic or antiferromagnetic configurations, depending on which metal site was substituted by the dopant. The results show both BeSnN$_2$ and BeGeN$_2$ to be ferromagnetic independent of the substitution sites. On the other hand, MgGeN$_2$ was found to be antiferromagnetic for Cr$_ {Mg}$ (Cr substitutes Mg site) and ferromagnetic for Cr$_{Ge}$ (Cr substitutes Ge site.)   

Electron spin splitting effect in AlSb/InAs/AlSb quantm wells

We investigated the electron spin splitting effect in AlSb/InAs/AlSb quantum wells (QW) both experimentally and theoretically. Our experiment was performed on MBE-grown high quality AlSb/InAs/AlSb QW samples whose potential symmetry was controlled by intentional/unintentional dopings near the QW layer, where we didn't observe the beating pattern in the Shubnikov de Haas (SdH) oscillations experimentally. Although the presence of the beating pattern in the SdH oscillations can be considered as a side-evidence of the zero-field spin splitting in the pertinent 2DEG (two dimensional electron gas), the absence of it does \textit{not} necessarily support the \textit{absence of the zero-field spin splitting}. Our theoretical analysis including all the Rashba, Dresselhaus and Zeeman spin-splitting Hamiltonians revealed that the absence of the SdH beating is not inconsistent with the presence of the zero-field spin splitting in our specific samples. Considering the fact that SdH oscillations itself are not visible below 1T in our experiment, the magnitude of the zero-field spin splitting energy can be as large as 1 meV according to our theoretical treatment.   

Low-temperature spin dynamics of Mn-rich Mn(Ga)As nanoclusters embedded in a GaAs matrix

Recently, the composite systems of Mn-rich Mn(Ga)As nanoclusters embedded in GaAs matrices have received an increasing attention due to the large magneto-optical and magneto-resistance effects at room temperature which could be applied to spin-electronic devices. In this work, we report the low-temperature spin dynamic behaviours including memory effects and slow magnetic relaxation of such composite systems. The systems can be formed by \textit{in situ} postgrowth annealing of (Ga,Mn)As films at 650 $^{o}$C for 10 min because of spinodal decomposition. High-resolution TEM images show zincblende Mn-rich Mn(Ga)As nanoclusters with a diameter in the range of 10-20 nm embedded in a GaAs matrix. From zero-field cooled and field cooled measurements, we can observe a clear bifurcation of the two curves demonstrating the existence of the spin-glass-like phase below the blocking temperature in the systems with high Mn concentration. Memory effects and slow magnetic relaxation, the typical characteristics of spin-glass-like phases, are also detected, and the hierarchical model is confirmed to be in accordance with such low-temperature behaviours. On the other hand, for samples with low Mn content, ferromagnetic order remains up to 360K.   

Proposal of Spin Interference Experiment Using GaAs/AlGaAs Heterostructures

We propose a spin interference (SI) experiment [1] using GaAs/AlGaAs system, where the zero-field spin-splitting ($\Delta _{0})$ is caused by the Dresselhaus term predominantly. GaAs/AlGaAs has the advantage in gating stability and long carrier mean free path relative to the other III-V materials, making it a standard semiconductor in mesoscopic physics experiment. It is shown theoretically as well as experimentally that values of $\Delta _{0}$ are generally smaller in GaAs/AlGaAs than in InGaAs/InAlAs. In the present work, however, we suggest that the gate-controllability of spins in GaAs/AlGaAs, \textit{$\beta $}$\equiv \Delta _{0}$/$k$ being the relevant parameter, should also be as good as that in InGaAs/InAlAs based on our simulations, which makes the SI experiment possible even with GaAs/AlGaAs. We fabricated a series of quantum nanowires on GaAs/AlGaAs wafers with various wire widths. We discuss their transport properties and the prospective for the future SI experiment. [1] Koga \textit{et al.}, PRB \textbf{70}, 161302(R) (2004); \textit{ibid.} \textbf{74}, 041302(R) (2006).   

Electrical spin injection and detection by ballistic transport in MnAs / GaAs / GaAs : MnAs spin--valve hybrid heterostructures

Electrical spin injection and detection by ballistic transport of spin-polarized carriers in ferromagnet (FM) / semiconductor (SC) / ferromagnet (FM) hybrid structures are key issues in semiconductor-based spintronics. By using ballistic transport of spin-polarized carriers, we can improve the spin injection / detection efficiency without using a high tunnel barrier at the FM/SC interface that decreases the current driving capability when used in active devices. In this paper, we report on the spin injection and detection by ballistic transport in perpendicular spin-valve hybrid heterostructures consisting of MnAs (20 nm) / GaAs (10 -- 30 nm) / GaAs:MnAs (5 nm) grown by molecular beam epitaxy. The GaAs:MnAs layer contains ferromagnetic MnAs nanoclusters embedded in a GaAs matrix, and acts as a spin injector and a spin detector. Several {\%} of spin-valve MR ratio was clearly observed up to 300 mV at temperature lower than 90 K. Considering the fact that all the junctions showed ohmic current-voltage characteristics, the spin-valve MR would be 10$^{-6}$ for purely diffusive transport regime. Consequently, the spin-valve MR signal of several {\%} is caused by the ballistic transport of spin-polarized carriers in the GaAs layer.   

Observation of intrinsic spin-Hall effect in transport

We study spin transport in n and p-doped mesoscopic H-bar structures fabricated from HgTe/HgCdTe quantum wells. In experiment the current is driven in one leg of H-bar structure while the second one is used to detect the voltage signal (similar to proposal presented in PRB 70, 241301(2004)) In p-doped samples, our data seem to confirm the existence of intrinsic spin-Hall effect. We also present the Landauer- Buttiker calculations with realistic parameters and details of band structure to verify experimental observation.   

Crystal Growth of Quasi-One Dimensional SrNbO$_{3.41}$ and LaTiO$_{3.41}$

Single crystals of SrNbO$_{3.41}$ and LaTiO$_{3.41}$ were grown in order to investigate the physical properties of these quasi- one dimensional conductors. Single crystals growth was accomplished with an optical image furnace; characterization was performed with X-ray powder diffraction. The resistance and heat capacity of SrNbO$_{3.41}$ were measured in the temperature range 300 K $> T >$ 0.3 K. SrNbO$_{3.41}$ was annealed to examine the influence of oxygen content on the electrical resistivity. The Debye temperature and electronic heat capacity coefficient of SrNbO$_{3.41}$ were found to be 458.5 $\pm$ 0.2 K and 0.77 $\pm$ 0.07 mJ/mol K respectively.   

Investigating the Diffusive Behavior of HPC with DLS and FPR: A Comparative Analysis of Experimental Method

The study of HPC (Hydroxy-propyl-cellulose) chains in aqueous solution through the experimental techniques of FPR (Fluorescence Photo-bleaching Recovery) and DLS (Dynamic Light Scattering) has shown empirical inconsistencies in observed polymer dynamics. The approach to analyzing the inconsistencies consisted of preparing fluorescently labeled and unlabeled HPC solutions at a range of concentrations from the same stock solution. Results from DLS have indicated the reliable presence of a slow mode of diffusion in both labeled and unlabeled samples. The slow mode appeared in FPR experiments, but not reproducibly. In addition, results from DLS on labeled and unlabeled HPC have found startling differences in line shape of correlation function indicating signal detection from an unknown mechanism. Future directions for this study include an investigation into the reasons behind the before mentioned inconsistencies and an analysis of HPC solutions with different fluorescent labels to further explore the nature of the slow diffusion mode if it is determined not to be an artifact from sample preparation, or an unknown aspect particular to DLS studies.   

Investigation of the Oxidation Growth Kinetics of La0.67Ba0.33MnO3 and LaMnO3 Perovskite Films using Atomic Force Microscopy (AFM) lithography

Manganese oxides doped with certain alkaline earth elements exhibit colossal magnetoresistance (CMR), which has great prospective applications in technological advancements. Our research is focused on the growth kinetics and electrical properties of LaMnO3 and La0.67Ba0.33MnO3. LaMnO3, behaves as an anti-ferromagnetic insulator. However, if extra oxygen is incorporated in this material, an insulator-metal transition occurs and the transport characteristics of the material shift causing ferromagnetism. In this research AFM induced nano-lithography of the LaMnO3 thin films is performed on samples of varying oxygen contents and compared to results of AFM induced nanolithography on La0.67Ba0.33MnO3. The quality and reproducibility of nanostructures produced is heavily dependent on the bias voltage direction between the film and the AFM probe.   

New Generalized Phase Shift Approach to Solve the Helomholtz Wave Equation

A new method for solving the acoustic scattering wave equation in order to facilitate the exploration for production of oil and gas. The approach is based on a new way to generalize the so called ``one way wave equation". Our approach makes use some very simple,standard ideas from differential equations and the non-iterative solution of the Lippmann-Schwinger equation in quantum scattering. The importance of this new approach is that it corrects the usual approximation made in the one way wave equation so that the approach is equivalent to full solution of the two way acoustic scattering problem. However, because it is formulated so as to deal with coupled first order differential equations, it makes the problem appear to be one way. The initial conditions for the coupled first order differential equations need to satisfy the boundary conditions associated with waves that can travel two ways. The problem of evanescent waves is being treated using a projection operator technique patterned after an approach of H. Feshbach in nuclear physics. We are currently testing the method for several simple two dimensional acoustic models.   

Characterization of 60 Hz environmental electromagnetic noise with a simple antenna.

The purpose of this project is to characterize environmental electromagnetic noise at 60Hz. We have constructed an antenna consisting of an LC circuit tuned around 60 Hz to detect the noise in the air, and made analyses in both the time and frequency domains. The data has shown considerable fluctuations in the peak frequency as well as the phase over any given period of time. Also, it has been found that the spectrum broadens depending on where the antenna is placed. We suspect that the broadening results from the superposition of seismic motion acting on the antenna to the electric signal. We are currently investigating if location and time of the day has any effect on the characteristics of the electromagnetic noise.   

Dynamics of a Planar Arm Model with Servo-regulated Viscoelastic Muscles in a Microgravity Environment

We constructed a mechanical arm model consisting of a rigid upper arm and forearm which simulates vertical planar arm motion with two degrees of freedom: shoulder rotation and elbow rotation. Computer controlled servo-motors effect rotation of the elbow and shoulder joints through tensions incited in elastic materials which represent muscles. We predicted and then observed vertical planar arm motion in the laboratory under normal Earth gravity conditions, and on NASAs Weightless Wonder in near zero gravity conditions. Because the arm only has two degrees of freedom we were able to simulate near zero gravity in the laboratory and predict the subsequent motion by operating it in the horizontal plane. We will discuss results of the actual observed motion in these three environments, and compare them to the motion predicted based on the equations of motion. We will also discuss how the project was developed physically, mathematically, and electronically.   

Why Not Solar Power?

Most of the world generally depends on energy sources such as fossil fuels and nuclear power to meet our energy consumption needs. As we all know, the excessive use of these resources has large environmental impacts, including displacing habitats, pollution, global warming, and scarcity of resources. Solar power is a clean form of energy that has the potential to fulfill our energy needs while balancing the natural state of our environment. So why do we not power our houses with solar energy? I will give a general overview of the working principles of commercially available solar power, and examine the issues relating to why we should use it and why we currently do not.   

Ice Nucleation Near the Surfactant-Water Interface

Ice nucleation is a fundamental component of the atmospheric mechanisms driving the formation of clouds. Atmospheric nucleation occurs with a variety of compounds and conditions, but understanding the behavior of water is key in all cases. We have used multiscale molecular simulations to study heterogeneous nucleation in clouds, probing the influence of long-chain alcohols on the freezing of water droplets. Ice nucleation occurs at a finite distance from the heterogeneous surface, due to the disruption of the hydrogen bond network in response to the surfactant-water interface. The penetration depth of the disturbance is found to be dependent upon the chain length and surface organization, as well as the acidity of the terminal alcohol group.   

An Extended Hydrophobic Surface Submerged in Water: The Formation of a Depletion Layer.

Hydrophobic literally means water hating. When small amounts of water come in contact with a hydrophobic surface, the water will minimize its contact area by forming a drop. What will happen when bulk water comes in contact with an extended hydrophobic surface? We have employed the surface sensitive technique of surface plasmon resonance to probe for the existence of a depletion region (an ultra-thin low-density region) at this boundary. We also explore the interaction of water and a mixed hydrophobic and hydrophilic surface and its effect on the formation of a depletion layer.   

Physics Education for Blind Students: The Teachers' Perspective

We discuss the challenge high school teachers face when teaching physics to the blind. Using the oral history method, we interview physics teachers who have dealt with the inclusion of blind students in regular classrooms. Based on our study, we find that the performance of these students varies, depending on the studied subject. The narrative makes clear the teachers' lack of preparation to deal with inclusion, and their search for alternative methods to improve blind students' learning.   

Impact of Inquiry-Based Learning on Attitudes and Science Content Knowledge of Elementary School Teachers and Students

We report on a research project studying the influence of hands-on and inquiry-based learning in K-8 classrooms. We designed learning experiences for teachers that included significant hands-on and inquiry-based lessons, and designed instruments to test attitudes and content knowledge. The lessons are meant to address topics of state and national learning standards. Teachers were invited to take part in a pilot program in the summer of 2006, and we hope to continue the project over the next few years. We will report on the activities developed, the attitudes toward and achievement in science, and any changes that were seen after learning science through active learning. Resources can be found on the project website, http://tnst.randolphcollege.edu.   

Prospective Nanoscience Lessons for High School Classroom Activities

Workshops for learning and teaching in nanoscale science in the Hampton Roads area in Virginia have been provided for high school science teachers of 7-12$^{th}$ grade. Main objectives of the workshops are to enhance teachers' awareness of the connections between nanoscience and the traditional sciences, and provide a collection of suitable classroom activities in nanoscience. Prospective nanoscience lessons for high school classroom activities have been introduced in summer 2007 and 08. The selected classroom lessons are surface area and volume, nanolight, solar cells, nanocard, allotropes of carbon, biosensors, DNA origami, ferrofluids, intermolecular forces, quantum dots, scanning probe microscopy, and space elevator. This work at Hampton University was supported by ARO (W911NF-07-1-0608) and NSF (HRD-0734635, HRD-0630372, and ESI-0426328/002).   

Modular Approach of Nanophysics for Undergraduate Science and Engineering Curriculum Development

Advances in nanoscience and nanotechnology are closely related with understanding nanoscale materials and their functionalization, which are still in their infancy. Further attention in the education of human-engineered nanoscale materials is needed before the beauty of nanoscience and nanotechnology becomes the reality of our modern life. The current urgent demand or existing challenge in nanoscience and nanotechnology is to educate, train, and prepare a new generation of skilled workers in nanoscience and nanotechnology. A modular approach of nanophysics for undergraduate curriculum development is introduced for educating students in multidisciplinary areas of science and engineering. The nanophysics educational modules are electronic dynamics and optical properties of semiconductor nanocrystals and nanometals. This work was supported by Army Research Office (W911NF-07-1-0608) and National Science Foundation (HRD-0734635, HRD-0630372, ESI-0426328/002, and EEC-0532472).   

Quantum Mechanics Laboratories using Correlated Photons.

Progress in laboratory techniques with correlated photons has allowed the implementation of table-top experiments for teaching undergraduate quantum mechanics. The experiments that we have developed [1] complement an undergraduate course on quantum mechanics. They use light at the quantum level to illustrate both fundamental and operational aspects of quantum mechanics. Laboratory experiments on interference of light with heralded photons, or with two correlated photons, going through an interferometer are vivid exercises on state superposition, state projection, and base rotation. Other experiments with entangled pairs address more fundamental aspects of quantum mechanics, including nonlocal correlations and violations of Bell Inequalities. [1] E.J. Galvez et al., Am. J. Phys. 73, 127 (2005).   

Making the Nanoworld Accessible: Nanoscience Education Using Scanning Probe Methods

A partnership between researchers and educators at the University of Washington, North Seattle Community~College~and two companies, Nanosurf, AG and nanoScience Instruments has been forged to develop a nationally replicable model of a sustainable and up-to-date undergraduate teaching laboratory of scanning probe microscopy (SPM) methods applied to nanoscience and nanotechnology. Within this partnership a new paradigm of operating and maintaining a SPM laboratory has been developed that provides a truly hands-on experience in a classroom laboratory setting with a small student to instrument ratio involving a variety of SPM techniques and topics.~ To date, we have run a first successful undergraduate~laboratory workshop, where students were able to have extensive hands-on experience on five SPM modes of operation including: electrostatic force microscopy~involving photovoltaic polymeric materials, ~tunneling microscopy and the determination of the workfunction, and nanolithography using the dip-pen method. http://depts.washington.edu/ nanolab/NUE{\_}UNIQUE/NUE{\_}UNIQUE.htm   

Utilization of recycled neutron source to teach prompt gamma analysis activation-PGNA

Neutron activation analysis based on prompt gamma ray emission has significantly developed during the past twenty years. The technique is particularly suited for the identification of low atomic number elements, as nitrogen that is a main component of drugs and explosives. Identification of these substances is important in the context of humanitarian demining, and in the control of illicit traffic of drugs and explosives. As a good example of recycling of radioactive sources, a $^{241}$Am-Be neutron source emitting 10$^{7}$neutron/s, that was not longer in use for other purposes at Ingeominas, was used to build a neutron irradiator that can be used to teach prompt gamma ray analysis, and other nuclear techniques. We irradiated individual samples, each about 4 gram, of three different elements: nitrogen in urea, silicon in milled rock, and cadmium in cadmium oxide. The prompt gamma rays emitted in the nuclear reactions $^{112}$Cd (neutron,gamma) $^{113}$Cd, $^{28}$Si (neutron,gamma) $^{29}$Si and $^{14}$N (neutron,gamma) $^{15}$N were identified using a well-type NaI (Tl) detector, connected to a multi-channel analyzer.   

Teaching Physics for Agronomy Students: Contextualizing Laboratory Classes

In this paper we discuss a method of teaching physics to agronomy majors. We use laboratory classes to apply basic physics concepts to common situations in agronomy. As an example, we report a project developed by the students involving the construction of a device, frequently used on farms, to transport water without burning fuel or using electricity. We believe that contextualizing helps to improve physics classes and increase students' motivation.   

Measurement and simulation of AlSb/InAs triple barrier resonant tunneling diodes. NanoJapan program summer 2007

Recently, Koga \textit{et al.} [1] proposed to make a spin filter out of only non-magnetic semiconductors. This spin filter consists of a triple barrier resonant tunneling structure (TB-RTS), and the Rashba spin-orbit coupling effect is utilized for matching the spin-dependent resonance levels, which would result in a high spin filtering efficiency. To test this theoretical idea, we obtained TB-RTS samples with InAs layers as quantum wells and AlSb layers as barriers from Tohoku University and studied their I-V characteristics. To understand the physics of our experimental $I-V$ curves obtained at 300, 77, and 4.2K, we also performed theoretical simulations. The authors acknowledge the growth of TB-RTS by Dr. K. Ohtani of Tohoku University. [1] T. Koga, J. Nitta, H. Takayanagi and S. Datta, Phys. Rev. Lett. \textbf{88}, 126601 (2002).   

Tracing the Rich History of Physics Research at NIST/NBS Through the Pages of the \textit{Journal of Research of the National Institute of Standards and Technology.}

What do Edward B. Rosa, William W. Coblentz, Ugo Fano, Charlotte E. Moore, William D. Phillips and Eric Cornell have in common? All are premier NBS/NIST physicists who published important research in the \textit{Journal of Research of the National Institute of Standards and Technology} and the \textit{Journal}'s precursors. The \textit{Journal of Research} is the flagship publication of the National Institute of Standards and Technology, formerly the National Bureau of Standards. The \textit{Journal} has been published under various titles and in various forms since 1904. This poster examines a sampling of the rich body of physics literature published in the \textit{Journal of Research} since the early 1900s, and analyzes the impact this literature has had on the physics and scientific communities. From Edward B. Rosa's paper, ``The Absolute Measurement of Inductance'' published in 1905, to the \textit{Journal}'s 1996 Special Issue on Bose-Einstein condensation, the \textit{Journal of Research} has been the venue for papers of some of the most influential American physicists of the twentieth century.   

The Increasingly Disordered History of Entropy

The interpretation of irreversibility had played a significant part of philosophical debates, but it was not until Carnot and his son established entropy as part of the empirical science of engines that the issue reached practical importance. It also had to wait for Maxwell, Boltzman, Gibbs and the birth of statistical mechanics that the concept of entropy was given a stronger theoretical basis, although the approximation it was based on is still a source of disagreement. This talk will focus on the debate from its early ``demonic'' times, past Szilard and Einstein building a refrigerator, to the role of von Neumann and Shannon in connecting the idea to information theory, without forgetting about the quantum mechanical master equations, all the way into its current use in quantum information theory.   

The stolen brain of Einstein

Pathologist Thomas Stoltz Harvey performed an autopsy on Einstein after his death in 1955. During the autopsy Harvey removed Einstein's brain, took pictures of it and then cut it into several pieces. A lot of scientific attention has been devoted to Einstein' brain, and it still comes up once in a while. We've all heard something or other about Einstein's brain, as it has become somewhat of a folk lore. What is less known is that Harvey in actuality did not have the permission to remove the brain. Only later Harvey convinced Einstein's Hans Albert Einstein son that this was for a good purpose. The brain would only be used for scientific purpose, which will be published reputable journals. I will try to describe in some detail the long journey this brain has taken in last fifty two years.   

Calculating cold curves for Equation of State using different types of Density Functional Theory codes

With fast computers and improved radiation-hydrodynamics simulation techniques, increasingly complex high energy-density physics systems are investigated by modeling and simulation efforts, putting unprecedented strain on the underlying Equation of State (EOS) modeling. EOS models that have been adequate in the past can fail in unexpected ways. With the aim of improving the EOS, models are often fitted to calculated data in parts of the parameter space where little or no experimental data is available. One example is the compression part of the cold curve. We show that care needs to be taken in using Density Functional Theory (DFT) codes. While being perfectly adequate for calculations in many parts of the parameter space, approximations inherent to pseudo-potential codes can limit their applicability for large compressions. Sandia is a multiprogram laboratory operated by Sandia Corporation, a Lockheed Martin Company, for the United States Department of Energy's National Nuclear Security Administration under contract DE-AC04-94AL85000.   

A Hybrid Density Functional Study of Zigzag SiC Nanotubes.

Using \textit{ab initio} hybrid density functional theory based calculations, we report here the electronic and geometric structure properties of \textit{three }different types of single-walled zigzag silicon carbide nanotubes from (3,0) to (11,0). Our calculations show type 1 structures to be most stable, with the cohesive energies of the newly proposed type 3 nanotubes intermediate between type 1 and type 2. For all nanotubes, Si atoms moved outward after relaxation making two concentric cylinders of Si and C atoms. The HOMO-LUMO (``band'') gaps for type 1 and type 2 nanotubes show an oscillatory pattern as the tube diameter increases but for type 3, the gap decreases monotonically with increasing tube diameter. All the tubes studied here appear to have triplet ground states except for type 1 (3, 0). It is expected that these tubes with significant surface reconstructions, varieties of band gaps, and magnetic properties would have interesting and important applications in the field of band gap engineering and molecular electronics.   

Resonance modulations by tunable multi-level inter-dot coupling in a parallel quantum dot interferometer

We investigate the characteristics of resonance through parallel-coupled double quantum dots in an Aharonov-Bohm interferometer. In this system, we calculate the quantum transmission for different combinations of two-level inter-dot coupling by employing an exactly solvable tight-binding formalism. First, for the case that only one level in each dot participates in transport, we present contour plots of the transmission as a function of the energy level in each dot for different strengths of inter-dot coupling and magnetic flux. As the inter-dot coupling increases, an anti-crossing of resonances for a fixed magnetic flux appears due to the coalescence of two dots into one. In addition, a doubling of the periodicity in the transmission and the swing of the Fano resonance are also observed to be modulated by the magnetic flux. Second, for the case that two-levels in each dot contribute to the transport, an extra Fano resonance in the transmission appears by tuning the inter-dot coupling between the even--odd parity states. On the other hand, the inter-dot coupling of the odd-odd parity states gives rise to a collapse of the Fano dip in the transmission resonance. Finally, we discuss the resonance effect on the interplay between the two-level inter-dot coupling and magnetic flux.   

Ab-Initio Molecular Dynamics Study of the Structural Stability of fcc-Carbon on Diamond

The observation of a new carbon phase in thin films on diamond has been reported. High Resolution Electron Microscopy studies of the carbon thin films revealed a face-centered-cubic (fcc) crystal structure with a lattice constant of 3.563 {\AA}. It is interesting to note that this value is very close to that of carbon in the cubic diamond phase (3.567 {\AA}), suggesting that the substrate is contributing to the stabilization of fcc-carbon. Thus, in the present work we have studied the structural stability of fcc-C on the surface (001) of diamond. We have used the Density Functional Theory and the Molecular Dynamics approach in the slab configuration. The calculations where performed with the pseudopotentials LCAO method (SIESTA code) and the Generalized Gradient Approximation (GGA) for the exchange-correlation potential. We have analyzed the changes in the atomic structure, density of states (LDOS), and the local orbital population. We found that the carbon layers in the fcc structure on diamond are unstable.   

A coarse-grained model for simulations of graphite and carbon nanotubes

As multi-walled nanotubes in many experimental systems are quite large in diameter and in length and they consist billions of atoms, a new multi-scale modeling approach is necessary. This modeling approach can serve as a bridge between atomistic modeling and continuum modeling of multi-walled nanotubes. For graphite nanobeams and carbon nanotubes we present a new model which is based on the hexagonal symmetry of the graphite structure. This model is based on ``N cell to one Cell'' mapping of the graphite structure. N cells in the original atomic structure are equivalent to one cell in the hexagonal mesh model. The number N can be very large thus it is possible to model large multi-walled nanotubes and graphite nanobeams.   

Linking Quantum and Classical Descriptions: Hierarchical Development of Born-Oppenheimer-like Approximations

The Born-Oppenheimer (BO) approximation is ubiquitous in quantum mechanical calculations of molecular wavefunctions. Here I report an extension of traditional BO approximation to a hierarchy of BO-like approximations which enables dynamic linking among distinct scales in complex crystals, from the quantum world to the macroscopic. At first, the usual BO approximation allows us to average out electrons to give descriptions of slow motion of nuclei. Then we describe an atom in crystals with two classes of independent variables corresponding to two distinct time scales: lattice deformation and relative atomic motion. We invoke a BO-like Ansatz as constraint to construct a ``constrained distribution function'' so that phonons are averaged out from lattice deformation. To progress further, we use BO-like approximation to modify constraints so as to account for defects. The BO-like approximations is further extended from separating distinct time scales to different length scales. The ultimate hierarchy of BO-like approximations enables formulation of nonequilibrium multiscale statistical mechanics where electrons, phonons, defects and lattice deformation are dynamically coupled.   

Gas Adsorption Characterization of Rigid and Amorphous Polymers

Nanostructured materials have unusual mechanical, electrical and optical properties and are becoming increasingly important for energy storage. A variety of materials, such as zeolites, metal organic frameworks, covalent organic frameworks, activated carbons, and hypercrosslinked polymers, have recently been explored for energy storage. Polymers of Intrisic Microporosisity (PIMs) are macromolecules that form nanoporous materials (effective pore size $<$2 nm) that are rigid at a nanometer length scale due to the structure of the selected monomers, but can be flexible at a macroscopic scale and show swelling properties due to their polymeric nature. PIMs offer an interesting alternative to the materials mentioned above, as the functionality can be directly embedded in the material framework, allowing for intrinsic control in adsorptive properties by the PIM and flexibility in alternate adsorption applications such as CO2 sequestration. In this work, we present our recent efforts to study PIMs by MC simulations, and demonstrate the effects of box size and chain length on simulated measurements including pore size distribution and surface area.   

Is the energy landscape of simple fluid a fractal?

The energy landscape of fluids is know to be very important close to a phase or glass transition. Real landscapes are fractal objects so one can ask if something similar happens with the structure of the phase space topology. Thus, a simple modification of the Monte-Carlo algortihm is proposed to explore the topography and the scaling of the energy landscape. We apply this idea to a simple hard-core fluid. The results for different packing fractions show a power law scaling of the landscape boundary, with a characteristic scale that separates the values of the scaling exponents. Finally, it is shown how the topology determines the freezing point of the system due to the increasing importance and complexity of the boundary.   

\textit{Ab initio} molecular dynamics simulations of the static, dynamic and electronic properties of liquid lead

We present results for a comprehensive study of the static, dynamic and electronic properties of liquid Pb near melting by means of 216-particle \textit{ab initio} molecular dynamics simulations based on a real-space implementation of pseudopotentials constructed within density-functional theory. The predicted results and available experimental data are in very good agreement, which confirms the adequacy of this technique to achieve a reliable description of the behavior of liquid metals, including their dynamic properties. Although some of the computed properties of liquid Pb are similar to those of simple liquid metals, others differ markedly. Our results show that an appropriate description of liquid Pb requires the inclusion of relativistic effects in the determination of the pseudopotentials of Pb.   

Critical fluctuations and phase separation in pure fluid

A series of experiments were performed in microgravity in order to study fluctuations and phase separation in pure fluids. These experiments were performed using Alice 2 apparatus containing an optical cell filled with sulfur hexafluoride (SF$_{6})$ near critical point. Fluctuations of the intensity of the transmitted light through a cell containing the fluid under investigation appear as domains of different intensities. Critical fluctuation images are very sensitive to optical noise from the experimental system. Two different methods used in image processing were tested and the results in order to find the optimal filtering method. First method is based on an n-point filter to eliminate optical noise from images obtained in microgravity. The second method is based on wavelets threshold to eliminate optical noise. We also estimated the fractal dimension of fluctuation domains using a box counting method. The fractal dimension was related to the temperature of the fluid. We found a power law similar to those observed for other phase transitions. In phase separation case, we used the texture analysis method to find the size of the clusters. Based on this approach we found a quantitative relationship between the cluster's size and temperature.   

Imaging density fluctuations in liquids using Schlieren photographic technique

Schlieren imaging method was used to visualize variations in the index of refraction or density of transparent liquid media. In one set of experiments, a colloidal suspension of silica and water was placed in a sample cell of 20 mm height. Schlieren method was used to visualize non equilibrium fluctuations caused by a concentration gradient. Using image processing techniques, the characteristic length of the fluctuations and the static structure factor were estimated. The temporal evolutions of concentration fluctuations were also analyzed and the correlation time was determined. The Schlieren technique was also used to image ultrasound beam incident on a corrugated water-brass interface and showed a backward displacement of the reflected beam at an angle of 22.5$^{0}$, confirming the observations of Breazeale and Torbett [Appl. Phys. Lett. \textbf{29}, 456 (1976)]. However, a new theory hypothesizes that this beam displacement results from excitation of a new type of leaky surface wave.   

Deep supercooling and relaxation processes in confined liquid crystals.

Broadband dielectric and photon correlation spectroscopies has been applied for investigations of the dynamic behavior of liquid crystals (LCs) confined in porous matrices with random pores. We observed deep supercooling of LC in random pores. The relaxation times of the process due to the molecular reorientaions in deeply supercooled state are slower than at the temperatures corresponding to nematic phase by a 5-6 orders of magnitude. This slowing down is accompanied by anomalous broadening of the dielectric spectra. The relaxation processes due to reorientation of molecules (of liquid crystals confined in narrow random pores) around their short axis is glass-like and the temperature dependence of relaxation times obeys Vogel-Fulcher law. Dynamics of director orientational fluctuations was also strongly affected by random confinement and these fluctuations were not frozen at temperatures much bellow bulk crystallization temperature. The small pore size (surface effects) and random pore structure (geometrical disorder) stimulate partial disorder (at least at long scales) and prevent crystallization. Therefore LC supercooled in small random pores has properties typical for glass forming liquids.   

A study of electric field response by liquid crystal molecular dynamics algorith

The response to an electric field of a liquid crystal cell is studied by the newly developed liquid crystal molecular dynamics (LCMD) algorithm. In contrast to the conventional conjugate gradient (CG) approach, which can only simulate the initial and final configurations of a LC cell under an applied electric field, the LCMD scheme can simulate intermediate configuration of the LC molecules. In addition, the electric field distribution within the LC cell at each time step is calculated directly. In this work, three cases will be presented: (i) LC molecules with spatial variation of polar angle, (ii) LC molecules with spatial variation of azimuthal angle and (iii) LC cells with nano-scale patterned substrates. The optical properties of the intermediate LC cell will also be presented.   

Electro-optic properties of liquid crystal carbon nanotube composites

We study the effects of carbon nanotubes on different properties of different liquid crystals: the elastic constant, Freedericksz transition, topological defect formation, interaction with external fields (i.e. electric). During our previous studies on the altitudinal angle of carbon nanotube doped liquid crystal electro-optic cells, we have measured a large shift of the transition voltage during Freedericksz transition. We are continuing our exploration on the physical properties of the liquid crystal nanocomposites. One of the most interesting questions is how different types of carbon nanotubes, conductive and semi-conductive affect the electrical properties of the liquid crystals. Our main tool is two dimensional microscopic transmission ellipsometry which provides information about the 3D orientation of the liquid crystalline entities.   

Environmental Mode SEM Studies of Liquid Crystal Droplets

An in-depth understanding of the liquid crystal nanometer scale structure of the holographic polymer dispersed liquid crystals (HPDLCs) is essential to optimally improve its diffraction efficiency. In this work the liquid crystal (LC) droplets are imaged for the first time in HPDLCs without LC removal using environmental mode SEM (ESEM). The field controllable HPDLCs have periodic layers of LC droplets and polymer matrix. Their applications include photonic crystals, optical pressure sensors, reconfigurable mirrors, wavelength filters and displays. Hi-vac mode SEM is a well-known technique used for high-resolution structural analysis of HPDLCs where the LC is removed prior to imaging. This results in the contraction and sometimes collapsing of grating structure. ESEM imaging aids in the study the LC microscopic structure without LC removal. A comparison of morphology features such as grating thickness, droplet structure is made between the images obtained by ESEM and hi-vac SEM.   

Diffusion and microrheology of dense liposome suspensions

Phospholipid vesicles can be stabilized against fusion, up to volume fraction around 80{\%}, which is accomplished by studding the outer leaflet with charged nm-sized particles. The diffusion of such soft, flexible and hollow objects is revealed by single-particle tracking. Image analysis of time trajectories, obtained using epifluorescence imaging, was performed at sub-pixel resolution. This poster will reveal aspects of curiously heterogeneous dynamics and also quantification of microrheology in this system. Taken together, this system of charged, polydisperse, flexible objects displays rich dynamics that contrasts acutely with known behavior for hard-sphere dense particle systems.   

Emulsions stabilized by Janus amphiphilic colloidal particles

Emulsions stabilized by amphiphilic colloidal particles were investigated by both theoretical calculation and experiments. The concept of Janus balance is defined as the dimensionless ratio of work to transfer an amphiphilic colloidal particle (a `Janus particle') from the oil-water interface into the oil phase, normalized by the work needed to move it into the water phase. The calculation shows that the emulsion will be most stable when the Janus balance is unity. Experimentally, large quantities of Janus particles with different Janus balance were synthesized. The emulsion type and emulsion stability were investigated by using these particles to stabilize oil-water emulsions. It is found that Janus particles can stabilize emulsions for extended times. Finally, the emulsion type will be shown to depend on the geometry (Janus balance) of the Janus particles.   

Flow-induced Shear of Colloidal Gels

We study colloidal gels as they break apart under shear. To make our colloidal gels, we add polystyrene polymer to PMMA colloids, inducing the depletion force. We then pump these colloidal gels through a capillary tube, and the resulting parabolic flow profile in the tube causes the gel to shear and break. We visualize this using confocal microscopy. We present two-dimensional and three-dimensional data from this experiment that has been analyzed through tracking individual particles and the movement of aggregates. We study the breaking points of the gel and characterize the structure of these points as a function of flow rate, volume fraction, and polymer concentration.   

Effect of the polydispersion in the crystallization and micro-structure of the high charged colloids

In this work we investigate the effect of the polydipersion in the crystallization and micro-structure of the high charged colloids particles with tow and three different types and different concentrations of that types. This results were obtained by computer simulation, the particles interaction was modeled by a screened Coulomb potential. We used 4000 particles in our simulation cell to let them evolution from an initial random configuration, periodic boundary conditions was imposed to simulate the bulk. The temporal evolutions of the configuration show long-ranged self-ordering and a crystalline transition, the crystalline nucleation depend of the concentrations of different kinds as well as of types of particle. The common neighbor analysis (CNA) exhibit the competition of two micro-structures, icosahedral and bcc, in the equilibrium bcc crystalline order is dominant with relative abundance over the other micro-structures. 1.- U. Gasser, Eric R. Weeks \textit{et al}, Science, \textbf{292} (258), 2001. 2.- Stefan Auer, Daan Frenkel, Letter of Nature, \textbf{409} (1020), 2001. 3.- J.P. Hoogenboom, \textit{et al} , Phys. Rev. Leeters, \textbf{89} (256104), 2002. 4.- M. Ch\'{a}vez-P\'{a}ez, E. Urrutia-Ba\~{n}uelos and M. Medina --Noyola, Phys. Rev. E, \textbf{58} (681),1998 5.- Andrew S. Clarke and Hannes J\'{o}nsson, Phys. Rev. E, \textbf{47} (3975), 1993.   

Using DLS Spectroscopy and Optical Probe Diffusion to examine structure of Brij Micelles

We studied properties of Brij-35 surfactant micelles in solution using Dynamic Light Scattering (DLS) Spectroscopy and Optical Probe Diffusion method. Aqueous solutions of Brij-35 with concentrations ranging from 2 to 100g/L were prepared, both with and without polystyrene latex probes of diameters 24, 50, 186, 282 and 792nm. Solutions were studied at four temperatures of 10, 25, 40 and 70$^{o}$C with DLS to obtain micelle and probe diffusion coefficients (D$_{m}$, D$_{p})$. Using both diffusion coefficients we deduced micelle radius (a$_{m})$, micelle water content ($\delta )$, and number of surfactant molecules per micelle (N) using two different models. First, hard sphere model of micelles/probe interaction was used to analyze the data by two methods, after a$_{m}$ was obtained from intercept of D$_{m}$(c). The first method uses the slope of D$_{m}$(c) and size of probes to determine N and $\delta $. The second method uses the linear least-squares fit of D$_{p}$(c) for different probe sizes to determine N and $\delta $. Both methods reveal that with increase in solution temperature a$_{m}$ increases by 10{\%}, N increases and $\delta $ decreases by a factor of 2. The second model treats micelles as core-shell particles with corona radius (a$_{c})$. This model used two different approaches based on linear least-squares fits of D$_{m}$(c) and D$_{p}$(c). We found a$_{m }$to be 4-4.5nm and a$_{c}$-a$_{m}$ to be 1nm without relying on Stokes-Einstein equation. Results for N and $\delta {\rm g}$ were also consistent.   

von Karman Vortex Streets Generated by Different Shaped Rods

von Karman vortex street is a pattern of vortices behind a bluff body in a uniform stream. Strouhal number St, a non-dimensional frequency of vortex shedding, depends on Reynolds number Re, however the precise relationship is not known although there were several proposals. To test these proposals and to investigate the nature of vortex shedding, we have generated Karman vortex streets in two-dimensional soap film using conic bodies with different croess-sectional geometric shapes and orientations. We found that the structure-based St-Re relationship, St=1/(A+B/Re), is in good agreement with our experimental data using different shapes and orientations. Two coefficients A and B are functions of geometries and orientations of bluff bodies. Also, we found under certain conditions, two adjacent vortices merge into a big vortex downstream.   

Electrostatic gyrokinetic turbulence

A kinetic description of turbulence becomes necessary in cases where particle collisions are not strong enough to maintain a local Maxwellian velocity distribution over relevant dynamical timescales. This is often the case with plasmas. This type of turbulence can exhibit structure formation in phase space having, in general, double the dimensionality. We study a simple limit of the gyrokinetic equation where there are two spatial dimensions and one velocity dimension. Symmetries are exploited to find scaling laws using standard arguments from neutral fluid turbulence. A discussion of closure is presented with an emphasis on the relationship to Naveier-Stokes turbulence and the kinetic extension of the Navier-Stokes equation, the Boltzmann equation.   

Microfluidic cell electroporation using a mechanical valve

A microfluidic electroporation technique is demonstrated based on the operation of an elastomeric valve in a polydimethylsiloxane (PDMS) fabricated microchip and a common dc power supply. The pulse needed for permeabilization of the cell membrane is generated by temporarily interrupting the circuit using the valve. The electropermeabilization of suspended and adherent Chinese hamster ovary cells with green DNA dye SYTOX is demonstrated. The technique eliminates the cost and complexity associated with a pulse generator and microfabricated electrodes that are often involved in microscale electroporation devices. It also offers the potential of integrating electroporation as a unit operation in large-scale microfluidic systems with the increasing application of elastomeric valves in these systems.   

Ionic currents through individual carbon nanotubes

The miniaturization from microfluidic to nanofluidic channels is a growing field of research. Many new effects are predicted and observed in nanochannels owing to the increased influence of surface interactions. One of the most intriguing features is the theoretical prediction by Hummer et al (2001) of an enhanced flow of aqueous solutions through hydrophobic carbon nanotubes. Experimental work has so far been done on membranes of carbon nanotubes (Hinds et al., Holt et al.). To the best of our knowledge, the experimental investigation of fluid flow through an individual single-wall carbon nanotube has not been conducted. We have developed a fabrication scheme that allows us to do measurements on such individual tubes. It is based on the use of a sacrificial layer etching process to fabricate connections to the nanotube. First measurements reveal that the ionic current through an individual 2 micron long single-wall nanotube is slightly smaller than expected from geometrical considerations. We hope to be able to present more extensive experimental data, e.g. on the variations between individual carbon nanotubes having the same nominal parameters.   

Toward a Carbon Nanotube-Based Capillary Rheometer

Nanofluidic devices featuring multi-walled carbon nanotubes (MWNTs) as fluid channels are fabricated for the purpose of measuring the flow of individual, submicron objects (e.g. polymers, nanoparticles) in solution. (Sun, Crooks J. Am. Chem. Soc. 122, 12340, 2000) The MWNTs serve as conduits between two electrolyte reservoirs and the passage of an analyte through the structure is detected by a decrease in the ionic conductance. Initial prototypes employ MWNTs with approximate inner diameters and lengths of 40 nm and 1 micron, respectively. However, because the devices are constructed using simple and generalizable processes, these geometric parameters can be easily varied. Such a device containing a single MWNT could be used to advance fundamental understanding of complex fluid rheology at the nanometer length scale and also function as a sensitive single-object characterization tool for nanoparticles and polymers.   

Sequential random packings

We introduce sequential random procedures to pack polydisperse particles, for both cases of spherical and ellipsoidal shape. In the case of spheres, we generalize the recent study of random space-filling bearings to a more realistic situation, where the spacing offset varies randomly during the space-filling procedure, and show that it reproduces well the size-distributions observed in recent studies of real fault gouges. In particular, we show that the fractal dimensions of random polydisperse bearings sweep predominantly the low range of values in the spectrum of fractal dimensions observed along real faults, which strengthen the evidence that polydisperse bearings may explain the occurrence of seismic gaps in nature. For ellipsoids we discuss the main difficulties in packing polydisperse ellipsoids sequentially and propose a procedure to overcome them, based in variational methods.   

The stress dip under a granular semi-pile.

The origin of stress dip under the apex of a sandpile has stimulated significant debate within the scientific community. On the other hand, it is argued that a semi-pile built against a vertical wall is of more practical interest since it serves as a model of dams, dykes and embankments. Numerical results suggest that there will not be a dip in this case. Here we show clear experimental evidence that the presence of the wall enhances the dip under the pile significantly. Moreover, our investigation provides insight into the influence of the wall on the force chains which appear to a key element in the formation of the dip.   

Discrete Element Modeling of Close Box Oscillation with Granular Particles: Force Laws and Energy Dissipation

Partially filled cavity particle dampers are widely used in aerospace applications. In comparison to the conventional viscous fluid based damping, the temperature-independent performance and design simplicity of particle dampers make them more attractive when the temperature varies significantly. Recently, the discrete element method (DEM) has been widely used to simulate the particle damper consisting of a cavity box. In order to truly represent the real damping system, the use of accurate force laws in the DEM simulation is critical. In this work, we use different force models in DEM simulation to investigate the damping behavior of a close oscillating box filled with glass and steel particles. The force models used in this work include linear, Hertz, inelastic, plastic, history-dependent, and history-independent models. We have found that the damping is very sensitive to the shear force models, but insensitive to the normal force models. The underlying mechanism has been investigated. In order to investigate the optimum filled fraction of particles and help us design the dampers, various configurations of different filled fractions are simulated. The energy dissipation through collision and friction is also investigated in this damping device.   

The Transition of Two-Dimensional Hard Spheres from Liquid to Solid Regimes Under Gravity Using a Global Equation of State

In a previous paper, Hong started with the Enskog equation for hard spheres of mass $m$ and diameter $d$ under gravity., and derived an exact equation for an equilibrium density profile at a specific temperature $T$. [\textit{Physica A}, \textbf{271}, 192 (1999)] This leads to a transition between the liquid and the solid regime that is temperature dependent. The size of the solid regime can be predicted using the temperature of the system obtained from the density profile. In a previous paper, Luding derives a new global equation of state for hard spheres in two-dimensions under gravity. [\textit{Phys. Rev. E}, 163(2001)] Using this equation, we obtain a more exact equation for an equilibrium density profile at a temperature $T $in two-dimensions. We use this equation with MD simulated data to obtain relationships of the number of solid layers, the center of mass, and the fluctuations of the center of mass as a function of $T$.   

Solitary granular avalanches: stability, fingering and theoretical modeling

Avalanching processes do not only occur in the air as we know of snow avalanches, mud flows and land-slides. Such events frequently happen below the see level as they take many forms from turbidity currents to thick sediment waves. In this study we report results on laboratory scale avalanche experiments taking place both in the air and under-water. In both cases a family of stable solitary erosion/deposition waves is observed [1]. At higher inclination angles, we show the existence of a long wavelength transverse instability followed by a coarsening and the onset of a fingering pattern. While the experiments strongly differ by the spatial and time scales, the agreement between the stability diagrams, the wavelengths selection and the avalanche morphology suggest a common erosion/deposition scenario. We also use these erosion/deposition waves to investigate the dynamics of granular flow and jamming in the frame work of the Partial Fluidization Theory (PFT) proposed by Aronson et al. to describe the dynamics of granular matter near jamming [2]. [1] F. Malloggi et al. Europhysics Letters, 2006, Erosion waves: Transverse instabilities and fingering 75, 825-831 [2] I. S. Aranson et al.. Transverse instability of avalanches in granular flows down an incline.~ Physical Review E, 2006, 73, 050302; I.S.Aronson et al., Non rheological properties of granular flows: exploring the near jamming limit, preprint (2007).   

Examination of the Angle of Repose in a Vertically Vibrated Container of Granular Materials

Experiments are conducted using various granular materials subject to a vertical vibration. The angle of repose is studied while varying certain parameters of the system, such as vibration amplitude, vibration frequency, initial height, grain size, container size, and container shape. Empirical relationships are found for the angle of repose as a function of each of these variables. Precession of the angle of repose is also examined, particularly as a function of grain size.   

Relative Permeabilities: a pore-level model study of the capillary number dependence

Relative permeabilities are widely used by the petroleum industry in reservoir simulations of recovery strategies. In recent years, pore level modeling has been used to determine relative permeabilities at zero capillary number for a variety of more and more realistic model porous media. Unfortunately, these studies cannot address the issue of the observed capillary number dependence of the relative permeabilities. Several years ago, we presented a method for determining the relative permeabilities from pore-level modeling at general capillary number. We have used this method to determine the relative permeabilities at several capillary numbers and stable viscosity ratios. In addition, we have determined these relative permeabilities using one of the standard dynamic methods for determining relative permeabilities from core flood experiments. Our results from the two methods are compared with each other and with experimental results.   

Analysis of Drop Oscillations Excited by an Electrical Point Force in AC EWOD

Recently, a few researchers have reported the oscillation of a sessile drop in AC EWOD (electrowetting on dielectrics), and some of its consequences. The drop oscillation problem in AC EWOD is associated with various applications based on electrowetting such as LOC (lab-on-a-chip), liquid lens, and electronic display. However, no theoretical analysis of the problem has been attempted yet. In the present paper, we propose a theoretical model to analyze the oscillation by applying the conventional method to analyze the drop oscillation. The domain perturbation method is used to derive the shape mode equations under the assumptions of weak viscous flow and small deformation. The Maxwell stress is exerted on the three-phase contact line of the droplet like a point force. The force is regarded as a delta function, and is decomposed into the driving forces of each shape mode. The theoretical results on the shape and the frequency responses are compared with experiments, which shows a qualitative agreement.   

Apparent Slip at Hydrophilic Surface: Fluorescence Resonance Energy Transfer Study

We have used a fluorescence resonance energy transfer (FRET) technique to measure the apparent slip velocity a separations $<$ 10 nm from a hydrophilic surface. A focused laser beam was used to excite Tb3+ dye in solution and the excited energy was transferred to Rhodamine 6G that was previously immobilized on the solid surface. In addition, with diffraction-limited spatial resolution, we have measured the fluorescence intensity profile of Rhodamine 6G to monitor the flow profile near the interface. The relevance is to present a greatly improved estimate of what determines the boundary condition of fluid flow in situations where details of the near-surface velocity profile matters, as in microfluidic devices.   

Effect of surfactants on the film thickness in the drag-out coating problem

In this paper, we give a simple proof of the thickening effect of surfactant on the thin film deposited when a flat plate is withdrawn from a liquid bath. This problem without the effect of surfactant was first considered in the seminal paper of Landau and Levich (1942). Our proof here is based on an asymptotic analysis of the lubrication model for the Navier-Stokes equations. Our result is consistent with the results obtained numerically and experimentally on similar problems by other investigators. We will discuss our result against the backdrop of these known numerical and experimental results. This result has been obtained in collaboration with Gelu Pasa.   

Self-consistent fluid-plasma simulation in small spaces

There is a great interest in understanding fluids and plasmas in small spaces as the complexity of micro technology systems increases. A self-consistent model of charged and fluid particle dynamics is applied to atmospheric small space (200 $\mu $m) discharges in helium. Hydrodynamic transport equations of the self-consistent model are described with an emphasis on the different terms involved in the close coupling among the fluid species, charged species and the electric field. The discharges are studied from an initial cloud till the stages of charged particle over-amplification in small spaces where transients are particularly important. Gas heating, neutral depletion and electric field reversals are observed, highlighting the close interaction between fluid and charged species -- both effects therefore characterize and govern the evolution of the small space discharge.   

Relaxation Dynamics in Glass-Forming Hydrogen-Bonded Liquids

We will address the relaxation dynamics in hydrogen-bonded super-cooled liquids near (but above) the glass transition, measured via Broad-Band Dielectric Spectroscopy (BDS). We propose a theory based on decomposing the relaxation of the macroscopic dipole moment into contributions from hydrogen bonded clusters of molecules. We discuss the statistical mechanics of the super-cooled liquid and with a theoretical estimate of the relaxation time of each cluster we provide predictions for the real and imaginary part of the frequency dependent dielectric response. Using glycerol as a particular example we demonstrate quantitative correspondence between theory and experiments. The theory also demonstrates that the $\alpha$ peak and the ``excess wing'' stem from the same physics in this material.   

Breakdown of effective temperature agreement near jamming

The jamming concept may unify a wide class of disparate phenomena. Central to this are the behaviors of effective temperatures as a system falls out of equilibrium. We present experimental measurements of effective temperatures in a granular system as it is gradually brought close to jamming. One effective temperature is based on the Einstein relation and defined in terms of the ratio of diffusion to mobility of a heavy test particle dragged through the system. The others are measures from local single-particle observables: the granular temperature is the average kinetic energy of the grains; the thermometer temperature is the average total mechanical energy of a weighted oscillator placed in the system. In thermal equilibrium and, surprisingly, when this non-equilibrium granular system is driven far from jamming, these effective temperatures agree. As jamming is approached, the Einstein temperature, which depends on system wide relaxation, systematically deviates from the other local measurements of effective temperature. The amount of deviation depends on the system's proximity to jamming and results in qualitative scaling differences of the relaxation timescales. These results suggest that global packing constraints play an important role in the breakdown of the thermal analogy as granular materials jam.   

Crystal growth kinetics exhibit a fragility-dependent decoupling from viscosity

We establish the temperature dependence of the kinetic coefficient associated with crystal growth into the supercooled liquid for 7 organic and 8 inorganic materials. We show that the kinetic coefficient for crystal growth scales with the shear viscosity raised to an exponent that depends systematically on the fragility of the liquid; fragility quantifies the deviation away from an Arrhenius temperature dependence for the viscosity. For strong liquids, the exponent is -1. The greater the fragility, the larger the deviation from -1. We argue that this breakdown in scaling between the crystal growth kinetics and the viscosity is a manifestation of heterogeneous dynamics in supercooled liquids.   

Dynamics in depletion gels studied with X-ray photon correlation spectroscopy

The slow, non-equilibrium, dynamics in low-concentration (particle volume fraction $\Phi \approx$ 20 \%) depletion gels consisting of colloid-polymer mixtures was studied using X-ray photon correlation spectroscopy. In some recent work (A. Fluerasu et al. PRE 76, 010401(R), 2007) we have shown that towards full aging, the intermediate scattering functions are well described by compressed exponential decays, indicating a form of ``jamming'' in the system which occurs even on length scales smaller than the particle radius. Here we extend these results by probing samples with different interaction potentials (tunned by the polymer concentration) and by characterizing the dynamical heterogeneities in these soft gels. We will also show first results on the formation of the gels in shear flow that were obtained using a combination of X-ray, direct observation, and co-flow mixing techniques.   

Exceptionally stable organic glasses: a molecular view of the glass-to-liquid transformation

Exceptionally stable organic glasses have been prepared by physical vapor deposition. Substrate temperature and deposition rate have been found to determine the degree of stabilization. When optimized, these factors allow the production of films with very slow kinetics and up to 2{\%} more dense than the ordinary glass. This is as dense as the estimated density of the equilibrium supercooled liquid at Tg -- 50 K. Translational motion, density and surface mobility were measured in films vapor deposited at a range of temperatures from Tg down to Tg -- 150 K. Stable films can be superheated well above Tg, and the very slow relaxation rates allow the investigation of the glass-to-liquid ``melting'' transition. Results suggest this process occurs by nucleation and growth, with regions of low viscosity liquid developing within the glassy matrix.   

Unexpected long range order at the early stage of spinodal decomposition

During the early stage of spinodal decomposition most of the phase separating systems lead to the formation of incoherent structures with small-range orientational and translational order. In this work we found that in the region near below the spinodal line two-dimensional systems with competing interactions can form hexagonal structures with long-range order. As a consequence of the strong mode selectivity, a network of density scars with large density fluctuations is formed at the early stage of the process of phase separation. The points of ramification of this network of scars act like nucleation centers of a hexagonal phase and ultimately define the domain structure, correlation length and statistical properties of the topological defects.   

Nonequilibrium Assembly of Polymers and Quantum Dots from a Confined Geometry

Dissipative structures, such as convection patterns and fingering instabilities, are formed when a droplet containing nonvolatile solutes (e.g., polymers, nanoparticles, colloids, or DNA) is allowed to evaporate on a solid surface. However, these self-organized structures are, in general, irregular. The evaporation is, in principle, a nonequilibrium process. Herein, we report a simple, one-step technique to produce well-ordered structures consisting of polymers or quantum dots with unprecedented regularity by allowing a drop of polymer solution to evaporate in a sphere-on-flat geometry. This technique, which dispenses with the need for lithography and external fields, is fast, cost-effective and robust. As such, it represents a powerful strategy for creating highly structured, multifunctional materials and devices.   

Quantitative analysis of buried structures in polymer, nanoparticles and their nanocomposites by GISAXS

Grazing incident small angle x-ray scattering (GISAXS) is a powerful and rapidly developing technique for the characterization of nanoscopic morphology and structures at surfaces. However, due to complicated data analysis involving distorted wave Born approximation (DWBA), most of the quantitative analysis nowadays is carried out only on surface structures. When combined with x-ray resonant-enhanced geometry, GISAXS can be extended to measure the buried structures in 3D polymer, nanoparticles and their nanocomposite thin films. In the current work, a more comprehensive approach is developed in order to extract in-plane structures as well as their depth-dependence within thin films, which cannot be readily accessible using conventional techniques, such as scanned probe microscopy.   

Self-Assembled Arrays of Non-Coalescent Water Drops

Condensation figures form over polymer dissolved in volatile solvent exposed to a stream of moist air. These patterns are hexagonally symmetric and they comprise of non-coalescent and nearly monodisperse water drops. Typical condensation figures have a range of drop sizes, resulting from the nucleation, growth and coalescence of different generations of water drops. In this study, we image the pattern formation over evaporating polymer solutions. We observe that rafts of growing drops evolve into a highly organized two dimensional lattice, which eventually evaporate away as well, templating ordered arrays of holes in polymer films. We derive the analytical and modeling framework for determining the key kinetic parameters that control the typical length scales and timescales of droplet growth, non-coalescence and assembly.   

Equilibrium Switching: Nucleosomes and Transmembrane Proteins

The problem of placing rods on a line is an old problem in statistical mechanics. It has recently found applications in biology in the context of nucleosome positioning and transmembrane helix prediction. In both of these problems, the underlying lattice possesses a heterogeneous energy landscape, making the problem nontrivial. We explain the relative ease of helix prediction over nucleosome positioning as due to the existence of ``switching'' regions, i.e. a ground state density profile that deviates from that formed by successively occupying the lowest available energy minima. We illustrate the concept with a simple disordered systems model that can be solved exactly and discuss the functional implications for both transmembrane proteins and nucleosomes.   

Flow through a reconstituted marine quartz sediment by an interacting lattice gas simulation

Regions of a reconstituted cylinder of quartz sediment (5.9 cm diameter x 13 cm long) from the Northern Gulf of Mexico were sub-sampled as 6.5 mm diameter cylinders. Images of sub-samples were made from x-ray micro-focus computed tomography data at 11 micron resolution. Using a coarse-grained approximation, each sample image is represented by a cubic lattice (100$^{3}$ voxels). Fluid, a pool of particles at the lattice base supplies fluid-particles flows against gravity to the sink at the top of the lattice. In addition to the concentration gradient, an external pressure bias, similar to a hydraulic head drives the mobile particles upward against gravity. Particles are allowed to execute stochastic motion by a Metropolis algorithm. Variations of the root mean square displacement of each fluid-particle and their center of mass with the time steps, mass transfer, and flux are examined as a function of the external pressure bias and compared to constant head permeameter measurements.   

Distribution of self-organizing driven particles and their mobility around a slit in porous media

Self-organizing structures and mobility profiles of a mixture of immiscible driven particles (A, B with molecular weights M$_{A}$ and M$_{B})$ through a porous medium are examined by an interacting lattice computer simulation. The porous medium is generated by a random distribution of barriers on a three dimensional lattice with a slit across the center. Interacting particles enter the lattice from the source and execute their stochastic motion with the Metropolis algorithm, and are driven by their concentration gradient and a pressure bias against gravity. Density and mobility profiles of particles in steady-state are studied as a function of pressure bias. A large fraction of particles flows through the slit with a relatively uniform dispersion in the surrounding porous regions in absence of the bias. Increasing the bias introduces long-range correlations among the constituents resulting in larger density further away from the slit. Effects of the size of the slit's width on the density profile of particles and their mobility are also examined.   

Pair diffusion in quasi-one- and quasi-two-dimensional binary colloid suspensions.

We report the results of measurements of the center of mass and relative pair diffusion coefficients in quasi-one-dimensional (q1D) and quasi-two-dimensional (q2D) binary colloid suspensions. The new results extend the findings of similar studies of one-component q1D and q2D colloid suspensions. Our principal new finding is that the presence of the smaller diameter component can destroy the oscillatory structure of the separation dependence of the q2D relative pair diffusion coefficient of the large particles even though the oscillatory character of the large particle equilibrium pair correlation function remains prominent, and that no such effect occurs with the q1D suspension. An interpretation of these results is proposed.   

Three-particle correlation functions of quasi-two dimensional one-component and binary colloid suspensions.

We report the results of experimental determinations of the triplet correlation functions of quasi-two-dimensional one-component and binary colloid suspensions in which the colloid-colloid interaction is short ranged. The suspensions studied range in density from modestly dilute to solid. The triplet correlation function of the one-component colloid system reveals extensive ordering deep in the liquid phase. At the same density the ordering of the larger diameter component in a binary colloid system is greatly diminished by a very small amount of the smaller diameter component. The possible utilization of information contained in the triplet correlation function in the theory of melting of a quasi-two-dimensional system is briefly discussed.   

Rotational Diffusion of Colloidal Clusters

We synthesize fluorescent PMMA particles and form them into clusters of several particles. We separate the clusters and make dilute suspensions of identical clusters in an index and density matched solvent. We scan the suspensions with a high-speed confocal microscope in 3D over time. For each cluster we determine the position and orientation with modified particle tracking software, and we follow the translational and rotational motion in time. The diffusion coefficients for translational and rotational motion agree with the Stokes-Einstein and the Stokes-Einstein-Debye relations.   

DNA Linker Mediated Crystallization of Nanocolloids

Biofunctionalized nanocolloids offer a promising platform for creation of novel materials using bio-addressable interactions. Crystalline phases are of especial interest for the development of novel functional structures. We demonstrate that crystallization of nanocolloids can be achieved via hybridization of dispersed non-complementary single stranded DNA capped colloids with flexible single-stranded linker DNA. The crystalline structure belongs to body central cubic lattice and exhibits large thermal expansion. The evolution of the structure has been studied in details using in-situ small angle x-ray scattering. The formation of crystalline structures and reduced metastability are observed for systems with longer DNA linkers.   

Translation-rotation coupling in concentrated colloidal suspensions

Single-particle tracking has been used to contrast translational and rotational diffusion in monodisperse colloidal suspensions. Visualization of angular motion reveals that rotation has different properties from translation. Collective motions of particles lead to subdiffusive rotation. Deviations from Fickian motion increase as the volume fraction increases above 50{\%}. Translational and rotational coupling of a single particle is measured directly for the first time and its dependence on the volume fraction is discussed.   

Large Scale Computer Simulation of Erythrocyte Membranes with Explicit Cytoskeleton$^{\dag }$

The erythrocyte membrane is composed essentially of a self-assembled lipid bilayer and a polymerized protein meshwork, referred to as the cytoskeleton. For the erythrocyte, the polymer meshwork is composed of spectrin and anchored to the bilayer through specialized proteins. In this investigation we extended a coarse-grained model of self-assembled lipid membranes, recently developed by us, to account for the cytoskeleton. Simulation of bilayer patches, with dimensions about 0.5 $\mu $m $\times $ 0.5 $\mu $m, were performed$^{ }$to investigate the effects of the cytoskeleton on the membrane elastic properties. The bending modulus and surface tension are extracted from the spectra of the out-of-plane thermal undulations of the membrane. Using Monte Carlo, we also extracted the compression and shear moduli. Preliminary findings suggest a measurable effect in thermal undulations resulting from the introduction of the cytoskeleton.   

Transconductance of a double quantum dot system coupled to a microcavity

The transconductance of a double quantum dot system in the coulomb blockade regime for single spin $\raise.5ex\hbox{$\scriptstyle 1$}\kern-.1em/ \kern-.15em\lower.25ex\hbox{$\scriptstyle 2$} $ electrons on each dot mutually coupled by an exchange interaction has been studied$^{1}$. We investigate the effect of this system when coupled to a microcavity and determine conditions for the external potential on each of the dots. The significant peaking in the transconductance as found without cavity mode coupling, depends to a large degree on the potential of each dot. The electron spins on each of the quantum dots are responsible for the strong mutual interaction but, also these spins result in only weak coupling to the adjacent leads. The weak Kondo condition, however is responsible for the enhanced transconductance. The addition of the dominant modes of a strongly coupled microcavity and its effect on the transconductance are discussed for differing potentials and spacing between the dots, and for some conditions result in an enhanced transconductance. 1. V. Koerting, P. Wolfle. PRL 99, 036807(2007)   

The role of state preparation in quantum process tomography

We study the affects of preparation of input states in a quantum tomography experiment. We study two preparation procedures, stochastic preparation and preparation by measurements. It turns out that the stochastic preparation procedure yields linear process maps, while the results obtained from an open system that is initially prepared using von Neumann measurements is shown to be non-linear, and can only be consistently described by a bilinear process map. A new process tomography recipe is derived for preparation by measurement for qubits. The difference between preparing states for an experiment by measurement and by stochastic process is analyzed.   

Quantifying the role of interference in quantum information processing

We present a quantitative measure of interference, applicable to any quantum mechanical process in a finite dimensional Hilbert space, and use it to examine the role that interference plays in various quantum algorithms and other quantum information theoretical tasks. We present results for the amount of interference in both Grover's search and Shor's factoring algorithms, and on how interference correlates with success probability in the case of disturbances of these algorithms through static or random unitary errors, or decoherence through bit--flips and spin flips [1]. We have also studied the statistics of interference in random quantum algorithms, both using well known random matrix ensembles (CUE, the circular unitary ensemble, and HOE, the Haar orthogonal ensemble), as well as recently introduced random circuit ensembles. We show that the interference distributions in the random circuit ensembles converge rapidly (i.e.~exponentially, or even in a Gaussian fashion) towards the universal interference distributions of CUE and HOE, which predict close to maximum interference with very high probability for a randomly picked quantum algorithm [2]. \newline [1] D. Braun and B. Georgeot, Phys. Rev. A {\bf 73}, 022314 (2006); and arXiv:0711.1513. \newline [2] L. Arnaud and D. Braun, Phys. Rev. A {\bf 75}, 062314 (2007).   

Resonant activation and multi-junction switching characteristics of Bi-2212 intrinsic Josephson junctions

Bi-2212 intrinsic Josephson junctions (IJJs) are expected to be applied to a superconductive qubit. We have studied quantum behavior of two different types of Bi-2212 IJJs, one is fabricated by FIB etching [2,3] and the other is fabricated by Double Side Etching Method (DSEM)[4]. In this report, we show their multi-junction switching properties with or without microwave irradiation. The experimental results indicate that the couplings between junctions change according to the sample fabrication process. [1] K.Inomata et al., Phys. Rev. Lett. 95, 107005 (2005) [2] S.-J.Kim et al., Appl. Phys. Lett. 74,1156 (1999) [3] Yu.I.Latyshev et. Al., JETP Lett. 69, 84 (1999) [4] H.B.Wang et al., Appl. Phys. Lett. 78, 4010 (2001)   

Macroscopic Resonant Tunneling Above the Crossover Temperature in a rf SQUID

We report studies of macroscopic resonant tunneling (MRT) between fluxoid states in an rf-SQUID qubit as function of temperature. The measured tunneling rates as a function of flux bias exhibit evidence of energy level quantization up to a temperature (900 mK) well above the crossover temperature (T$_C$) between the quantum and the thermal regime. The data agree with the level structure calculated using independently measured circuit parameters. The MRT is a useful probe of decoherence-inducing noise in the rf SQUID since the measurements are much simpler and give values for flux noise equal to those obtained from T$_2$.   

Universal quantum degeneracy point for solid-state qubits

When operated at the quantum degeneracy point, the so called ``sweet spot'', solid-state qubits can be protected from the first order decoherence of low-frequency noise in the off-diagonal coupling. Here, we show that a quantum degeneracy point can also be found for low-frequency noise in the diagonal coupling in an encoded- qubit scheme. We also study the protocols for implementing one and two bit quantum logic gates and the effects of circuit imperfection on this scheme. A practical system to realize this scheme with superconducting qubits is then presented.   

Diffusion Prefactors and Vibrational Entropic Contributions for small Cu and Ag clusters on Cu(111) and Ag(111)

In examination of the diffusion energetics and dynamics of small 2D Ag and Cu clusters on Ag(111) and Cu(111) we found the results for the 7-atom cluster to be particularly interesting. In particular the diffusion pre-exponential factor for the Cu clusters on Ag(111) is found to be one order of magnitude larger than that for the homo-systems; Cu cluster on Cu(111) and Ag cluster on Ag(111). Analysis of the vibrational entropic contributions to the system free energy points to the subtle differences in the three cases and the significant contribution of the substrate atoms that lie in the vicinity of the diffusing islands. We trace the differences in the results for the 7-atom cluster from those for smaller sized ones to the nature of the cluster-substrate interactions. The vibrational dynamics and energetics of the systems are obtained using ab initio electronic structure calculations (density functional theory) and compared to those obtained using many body interaction potentials.   

Magnetical studies on thin films of iron oxides

We have grown hematite ($\alpha $-Fe$_{2}$O$_{3})$ thin films on stainless steel and silicon dioxide (SiO$_{2})$ substrates and magnetite (Fe$_{3}$O$_{4})$ thin films on silicon substrates by RF magnetron sputtering process. Conversion Electron M\"{o}ssbauer (CEM) spectra of these films exhibit hyperfine parameter values which are characteristic of these iron oxides. Magnetization measurements parallel to the plane of the film as a function of temperature, M(T), were done at a constant field of 1 kOe to $\alpha $-Fe$_{2}$O$_{3}$ films and at 200 Oe to Fe$_{3}$O$_{4}$ films. The M(T) curve of the $\alpha $-Fe$_{2}$O$_{3}$ film showed a linear increasing of magnetization from 5 K to 160 K, related with the Morin transition. While the temperature of the Fe$_{3}$O$_{4}$ film is decreased, a sharp decrease in magnetization is observed at 123 K, associated to the Verwey transition. We carried out measurements of magnetization as a function of applied magnetic field, the loops of $\alpha $-Fe$_{2}$O$_{3}$ film exhibits hysteresis which is related to their weakly ferromagnetic behavior and the loops of the Fe$_{3}$O$_{4}$ film at 300 and 110 K show a magnetic value around 5 memu at 3 KOe in the curves.   

Far-from-equilibrium Ag-Cu thin-films on Cu(100) and Ag(100) substrates

We study the dynamics of multilayer, heteroepitaxial deposition of mixed incoming fluxes of Ag and Cu atoms and the corresponding post-deposition relaxation on Ag (100) and Cu (100) substrates. To this end, we carried out extensive temperature accelerated dynamics (TAD) simulations at different substrate temperatures to characterize the morphology of the resulting films. Depending on the flux of impinging atoms, the number of layers, and the substrate temperature, the system can exhibit kinetically trapped, far-from-equilibrium configurations. Complex multi-adatom moves, not usually accounted for in regular Monte Carlo simulations, have a non-trivial relevance in the dynamics.   

Chemical and structural dynamical equilibrium in bimetallic nanoclusters

Coupling the surface and finite-system thermodynamics is a challenging key point in modeling bimetallic clusters. This is particularly crucial when the alloy surfaces undergo first-order phase transitions. In this context, we have investigated the segregation of Ag atoms in the surfaces of Cu-Ag nanoclusters, using $N$-body interatomic potentials and the Monte Carlo method. The (111) and (100) surfaces of Cu-Ag alloys are known to exhibit first-order phase transitions coupling a jump in the surface concentration and structural rearrangements. For the Wulff polyhedron of 405 atoms, these phase transitions are replaced by a dynamical equilibrium (DE) in phase space. For the (100) facets, this DE affects both the composition and the structure of the facets. Furthermore, all the (100) facets are correlated during the DE, due to atomic relaxations which affect also the core region of the nanocluster. For the (111) facets, the DE concerns only the composition of the facets, the atomic structure remaining unchanged. Consequently, the (111) facets are not correlated each other, indicating that pure Ag and pure Cu (111) facets can be observed in a given nanocluster. These simulations allow us to propose a surface phase diagram for the studied Wulff polyhedron.   

Mechano-Chemical Stability of Gold Nanoparticles Coated with Alkanethiolate SAMs

Molecular dynamics simulations are used to probe the structure and stability of alkanethiolate self-assembled monolayers (SAMs) on gold nanoparticles. We have observed that the surface of gold nanoparticles become highly corrugated by the adsorption of the SAMs. Furthermore, as the temperature is increased, the SAMs dissolve into the gold nanoparticle, creating a liquid mixture at temperatures much lower than the melting temperature of the gold nanoparticle. By analyzing the mechanical and chemical properties of gold nanoparticles at temperatures below the melting point of gold, with different SAM chain lengths and surface coverage properties, we have determined that the system is metastable. The model and computational results that provide support for this hypothesis are presented.   

Morphology and Growth Dynamics of Buckling Polymer Surfaces

Surface buckling is an expected response of a polymer surface when an applied in-plane stress exceeds a material-defined critical stress. This onset of a prescribed surface struture has been gaining increasing attention over the last decade towards applications as diverse as enhanced adhesion and flexible circuitry. However, the majority of research has been performed for systems subjected to stresses well above their critical buckling stress. Here, we highlight recent experiments that characterize the buckle morphology and dynamic buckling behavior of an oxidized PDMS surface swollen with solvent vapor. At low stresses, we observe the spontaneous formation of hexagonally packed microlens arrays not predicted by current buckling theory. These structures coalesce at higher stresses into predicted morphologies. This understanding provides new routes for the controlled formation of surface patterns with complex arrangements and geometries.   

Preparation and Characterization of Ta$_2$O$_5$--CeO$_2$ Films

Ta$_2$O$_5$ films have been widely studied due to their chemical and thermal stability, high dielectric constant and refractive index. It is known, for certain composites of Ta$_2$O$_5$--TiO$_2$, Ta$_2$O$_5$--Al$_2$O$_3$, and Ta$_2$O$_5$--ZrO$_2$ polycrystalline ceramics, that there is a significant increase in the dielectric constant compared to pure Ta$_2$O$_5$; this has stimulated research of doped thin films of Ta$_2$O$_5$. In this study [1], the sol-gel spin coating method has been used to make Ta$_2$O$_5$--CeO$_2$ thin films. These films have been prepared in various composition ratios to observe changes in their surface morphology, optical and structural properties. Reflectance and transmittance spectra were collected in the spectral range of 300-1000 nm, and were accurately fit using the Tauc-Lorentz model. Film thicknesses, refractive indices, absorption coefficients, and optical band gaps were extracted from the theoretical fit. The highest refractive index value was found at 5\% CeO$_2$-doping. The structure of the films was characterized by XRD and FTIR spectrometry, while the surface morphology was examined through AFM. [1] D. Saygin-Hinczewski, K. Koc, I. Sorar, M. Hinczewski, F.Z. Tepehan, and G.G. Tepehan, Sol. Energy Mater. Sol. Cells 91, 1726 (2007).   

On x-ray scattering by solitons in systems of adsorbed atoms

The cross section for scattering of x-rays by solitons is calculated. The authors consider solitons corresponding to the formation of a kink in a system of adatoms on the surface of a substrate, or a crowdion in a chain of atoms in crystals described by the sine-Gordon equation, and also solitons in a bound electron-phonon quasi-one-dimensional molecular chain. It is shown that study of the x-ray scattering provides for the possibility to gather information on the static and dynamic properties of the solitons.   

On electromagnetic wave propagation in systems of adsorbed monolayers

Taking into account the discrete nature of the adsorbed layer, the authors study electromagnetic waves in the system of insulating substrate with the monolayer of adsorbed molecules. Calculated are the dispersion relations of the excitations with s- and p- polarizations localized at the adsorbed layer. To obtain the linear response of such systems, Green's function approach was undertaken. The susceptibility tensor of the system is expressed in terms of Green's function, and the pole structure of the susceptibility is investigated.   

Complex electronic interference effects in Ag films on Ge(111)

We have performed a 3-D mapping by angle-resolved photoemission of the electronic structure of atomically uniform Ag thin films grown on Ge(111). Electrons in the Ag film with energies within the absolute gap of Ge are fully confined, forming quantum well states, while electrons with energies outside this gap form quantum well resonances. In addition to these features, our results show complex interference patterns that can be attributed to umklapp processes at the interface. The lattice mismatch between Ag and Ge creates an incommensurate interface, which introduces multiple periodicities, resulting in a rich electronic structure. We have performed a detailed numerical analysis of the experimental results, and the interaction parameters, related to the interfacial potential modulation, are extracted.   

Effects of Energetic Particles on Friction Behavior of Diamond-like Carbon

This research investigates the failure mechanisms of damages induced by interactions of energetic particles and a diamond-like carbon (DLC) film. Experimental approaches include using an ultra-high vacuum tribometer with attached spectroscopic analysis techniques to generate those particles and conduct in-situ tribological testing. Analytically, using multiple-peaks deconvolution to analyze carbon 1s X-ray photoelectron spectroscopy (XPS) results, the bonding information of $C$ atoms on the surface of DLC film has been extracted. Comparing with friction test results, it is found that the frictional behavior of DLC film (after the reaction with energetic particles) strongly depends on the ratio between \textit{sp3} and \textit{sp2} hybridization of $C$ atoms on the surface.   

Nonextensivity in Magnetic Systems

Nonextensive statistics has been successfully applied to different areas of physics, whenever long-range correlations, fractality, inhomogeneity or long time memory are present. Nonextensive thermodynamics is derived from the definition of nonextensive entropy: $S_q =k(1-\sum\limits_i {p_i^q )/(q-1)} $, where $p_{i}$ are probabilities and $q$ is the so-called entropic index. From this definition one obtains the $q$-density matrix, $\rho ^q$ and, from it, thermodynamically related quantities. In condensed matter, strong correlated systems are good candidates to be approached from the nonextensive formalism. This is the case of manganese oxides, or manganites. They are magnetically inhomogeneous and present fractal grain structure. In the present work we discuss various features which are observed in manganites and, from experimental data, we give a physical interpretation for the entropic index $q$ and calculate various aspects of the magnetism of different samples, such as the magnetic susceptibility and phase diagram. We also discuss magnetic elementary excitations in inhomogeneous media using the nonextensive approach.   

Low temperature metamagnetic transitions in single crystal ErNi$_{2}$B$_{2}$C: torque magnetometry study

The phase diagram of metamagnetic transitions in single-crystal rare-earth nickel borocarbide ErNi$_2$B$_2$C has been determined at 1.9 K with a Quantum Design torque magnetometer. The critical fields of the transitions depend crucially on the angle between applied field and the easy axis [100] in the $ab$-plane. Torque measurements have been made while sweeping the magnitude of the magnetic field at a constant angular direction (parallel to basal tetragonal $ab$-planes) over an angular range greater than two quadrants. Sequences of metamagnetic transitions with increasing field differ for the fields along (or close enough to) the easy [100] axis from those near the hard [110] axis. These torque measurements reveal new metamagnetic states in ErNi$_2$B$_2$C which were not apparent in previous longitudinal-magnetization and neutron studies. Their nature is considered and clarified. In the low-field range influences of superconductivity are observed and interpreted.   

Phase diagram of Na1-xCaxV2O4 compounds synthesized at high pressure

Ambient pressure CaV2O4 and high-pressure NaV2O4 crystallize in the CaFe2O4 structure type containing double chains of edge-sharing VO6 octahedra. Recent measurements on NaV2O4 reveal low-dimensional metallicity and evidence of half-metallic ferromagnetism. In contrast, CaV2O4 is an antiferromagnetic insulator. To explore the evolution of these ground-state behaviors, we have prepared a series of Ca-doped NaV2O4 compounds with the formula Na1-xCaxV2O4 (x=0-1) using high-pressure synthesis. The lattice parameters of Na1-xCaxV2O4 samples change with nominal x according to Vegard's law. The metallic state in NaV2O4 is dramatically altered by Ca doping. Samples with higher Ca concentrations (x=0.6-0.8) exhibit a metal-insulator transition around 150 K. Samples at the Na end (x=0-0.2) show a broad antiferromagnetic transition in the 120-160 K range in accordance with earlier reports. With increased Ca doping, the antiferromagnetic transition is suppressed to $\sim $70 K at the Ca-endmember. Transport measurements show an insulator-metal transition at x$\sim $0.4. Comparison to existing studies at the Ca- and Na-rich ends will be discussed along with a schematic (T-x) phase diagram for the Na1-xCaxV2O4.   

PbSe Quantum Dot Growth using MBE

We have studied the formation of PbSe quantum dots by molecular beam epitaxy (MBE). It has been shown that PbSe can generate multiple carriers from a single photon, making it attractive for solar cell research. These quantum dots were grown onto silicon substrates that were either clean or coated with buffer layers of CaF and/or BaF. The size and distribution shapes are controlled by temperature and growth rates. The purity and stoichiometry of the sample are determined using Auger spectroscopy upon creation. We used a scanning electron microscope and atomic force microscope (AFM) to verify the size and nature of the quantum dots after they are created. Preliminary investigations indicate the buffer layers were susceptible to Fluorine deficiency in vacuum and also aged quickly in air when not capped with PbSe. Our results on thicker films of PbSe, in which thin films rather than dots are created, are smooth and uniform but do include many features with sizes on the order of 10 -- 100 nm. Thinner coatings of PbSe are highly dependent on the quality and nature of the buffer layer they are grown upon.   

Nanobubbles on a Graphite surface immersed in Water: Effect of temperature

Nanobubbles form on the surface of some solids immersed in a liquid, either spontaneously or by induction. Although their existence has been debated for some time, Atomic Force Microscopy (AFM) observations have confirmed their formation. These bubbles range in size from 10 to 100 nanometers and have important implications for the properties of the interfaces and may be responsible for long-range hydrophobic attractive forces. Interestingly, the use of nanobubbles has also been proposed for the treatment of strokes using ultrasound. The formation of nanobubbles on water-solid interfaces influences the adsorption of nanoparticles and the corresponding wetting properties. One of the parameters relevant to the stability of the bubble is the contact angle, which in turn depends on the surface tensions of the substrate, liquid and vapor involved through the so-called Young's equation. We have used a quantitative model that incorporates the attraction of the substrate to calculate the contact angles, at different temperatures. We have compared our calculated results with the experimental data available in the literature.   

Electrostatic-directed deposition of nanoparticles on a field generating substrate

In this paper we develop a Brownian dynamics model applied to position metal nanoparticles from the gas phase onto electrostatic-patterns generated by biasing P-N junction substrates. Brownian motion and fluid convection of nanoparticles, as well as the interactions between the charged nanoparticles and the patterned substrate, including electrostatic force, image force and van der Waals force, are accounted for in the simulation. Using both experiment and simulation we have investigated the effects of the particle size, electric field intensity, and the convective flow on coverage selectivity. Coverage selectivity is most sensitive to electric field, which is controlled by the applied reverse bias voltage across the p-n junction. A non-dimensional analysis of the competition between the electrostatic and diffusion force is found to provide a means to collapse a wide range of process operating conditions and an effective indicator or process performance.   

Non-linear conductance of a short quantum point contact

We have measured non-linear conductance $G$ of a very short, less than 80 nm in lithographic length, quantum point contact as a function of the source-drain voltage $V_{sd}$ and gate voltage $V_g$ at the device lattice temperature T$<$20 mK. The width/length ratio of the QPC is approximately 2.5. We observe several well-resolved plateaus in $G$ at $V_{ds}$=0, but find no prominent zero-bias peak in $G(V_{ds})$ reported by several groups in longer, lower aspect ratio contacts at near-opening gate voltage [1,2]. The peak is believed to arise due to Kondo-like correlations between a quasi-local magnetic state in the constriction [1,3] and the device leads. Our data suggest that the quasi-bound spin state does not form in short QPCs and agree qualitatively with the recent predictions [3]. [1] S.M. Cronenwett~et. al. {\em Physical Review Letters}, 88(226805), 2002. [2] E.J.~Koop et. al. {\em J Supercond Nov Magn}, 20:433, 2007.[3]Tomaz Rejec and Yigal Meir. {\em Nature}, 442:900, 2006.   

First-principle calculations of molecular conductance with semiconducting electrodes

For molecular electronics, many researches have been performed about molecular conductance with gold electrodes. However, several recent experiments have demonstrated the feasibility of attaching various molecules on silicon surfaces. So, we study molecular junction formed on silicon surface. We calculate conductance of systems included pure silicon and phosphorus-doped silicon electrodes. In order to calculate the conductance, we adopt nonequilibrium Green's function theory combined with density functional theory (NEGF-DFT). We consider several molecules and atomic wires sandwiched between pure Si (100) electrodes or P-doped Si (100) electrodes. In our DFT calculation, the band gap value of the pure bulk silicon is about 0.554 eV. In our model, phosphorus-doped silicon electrodes are composed of P/Si in the ratio 1: 63 and the resulting doped electrodes show the metallic features. In the case of pure silicon electrodes transmission coefficients are about equal to zero in the energy ranges of E$_{F}$ $\pm$ 0.38 eV, while in the case of phosphorus-doped silicon electrodes transmission coefficients take finites values.   

Conductivity of La 0.75Sr0.25BO3 perovskite-type oxides

We perform a joint experimental and theoretical study of the conductivity and electronic structures in La$_{0.75}$Sr$_{0.25}A$O$_{3}(A $= Cr, Mn, Fe and Co) with perovskite structure. The samples with pure phase are prepared by the solid reaction. The electrical conductivity measurements indicate that in La$_{0.75}$Sr$_{0.25}A$O$_{3}$ the conductivity orderly increases for Cr, Mn, Fe and Co. The first-principles simulations show that conductivity depends on electronic structure. The d electron increasing in transition metal atoms results in carrier concentration increasing.   

Structural and physical properties of Fe-doped LiMn$_{2}$O$_{4}$ oxides

A joint experimental and theoretical study of the physical property and electronic structures in Fe-doped LiMn$_{2}$O$_{4}$ has been performed. The samples with pure phase are prepared by the solid reaction. The X-ray diffraction refinement and SEM analysis show that Fe enters lattice to occupy Mn site. The physical property measurements indicate that Fe doping results in modification of microstructure. The physical properties heavily depend on Fe-doping concentration. The first-principles simulations show that charge ordering and magnetic ordering occur in Fe-doped samples. The d electron increasing in the system results in carrier concentration changing.   

Small Angle X-ray Scattering of Self-assembling Systems Composed of Phosphocholine Lipids and Dendrimers.

Polypropylenimine tetraamine (DAB-Am) and 1-directional arboral dendrimers have been studied in different molar concentrations with 1,2-Dioleoyl-sn-Glycero-3-phosphocholine (DOPC). One-directional arborols are amphiphilic dendrimers with hydrophilic head group composed of 9 hydroxyl groups and a short alkyl chain for the hydrophobic tail. Film and liposome shell thickness was determined using small angle x-ray scattering beamline at the Center for Advanced Microstructures and Devices at Louisiana State University.   

Conformational Space of Hydroxyacetone Studied by Matrix-Isolation FTIR Spectroscopy and Quantum Chemical Methods

The matrix-isolation FTIR spectrum of hydroxyacetone monomers isolated in Ar matrix at 12K was studied. Interpretation of the experiment was aided by MP2 and DFT calculations at the 6-311++G(d,p) level. A 2D potential energy surface, in the space of OCCO and HOCC dihedral angles, revealed 4 non-equivalent minima, Cc, Tt, Tg and Ct. The energy barriers for Tg-$>$Tt and Ct-$>$Cc conversions (0.7 kJ/mol both) were found to be below the zero-point vibrational level associated with the isomerization coordinate of the higher energy form in each pair (Tg and Ct). Then, only Cc and Tt forms have physical meaning. In accord with the relative energy calculated for Tt ($>$11 kJ/mol), its estimated population in gas phase at 298K is only 1{\%}. Indeed, only Cc form was experimentally detected. Its characterization included the full interpretation of the vibrational spectrum and the calculation of the NMR spectra of the compound in different media.   

Synthesis and characterization of size-selected Ag nanoparticles with icosahedral shape

We report the synthesis of Ag nanoparticles, produced by the technique of Inert Gas Aggregation, which allows a very precise selection of the nanoparticle sizes and deviations. We found the optimal experimental conditions to synthesize nanoparticles of six different sizes: 1.3$\pm $0.24, 1.7$\pm $0.35, 2.5$\pm $0.44, 3.7$\pm $0.41, 4.5$\pm $0.88, and 5.5$\pm $0.24 nm. With this, we were able to investigate the dependence of the size of the nanoparticles on the synthesis parameters. Our data suggest that the aggregation of clusters (dimmers, trimer, etc) inside of the nanocluster source is the predominant physical mechanism for the formation of the nanoparticles. In order to preserve their structural and morphological properties, the impact energy of the clusters landing into the substrate was controlled; the acceleration energy of the nanoparticles was around 0.1 eV/atom, assuring a deposition in a soft landing fashion. HRTEM and HAADF-STEM images showed that the particles were icosahedral.   

Directed polymer in random media with a defect

We investigate a directed polymer in random media with an attractive defect at the center of the one dimensional substrate. Without the defect, end to end distance $\Delta x$ of the polymer follows $\Delta x \sim t^{1/z}$ with $z=3/2$ which is related to the value of the dynamic exponent in Kardar- Parisi-Zhang equation. When the defect strength $\epsilon$ is weak, its contribution to $\Delta x$ is negligible. If $\epsilon > \epsilon_c$ then $\Delta x$ becomes constant. This kind of transition is related to a queueing phenomena in the asymmetric simple exclusion process. Various critical exponents near the transiton point are also discussed.   

Thermal conductivity measurements of Bi$_{2}$Te$_{3}$ nanowire arrays in anodic aluminum oxide template by 3$\omega $ method

Bismuth telluride is a thermoelectric material which is famous for its high figure of merit ZT$\sim $1. Theoretical predictions propose that the thermoelectric properties of nanowires could be greatly enhanced compared to its bulk form. We have prepared Anodic Aluminum Oxide (AAO) templates with 60 nm pore diameter, 80 $\mu $m thick and $\sim $30{\%} porosity, and synthesized bismuth telluride nanowire arrays into the AAO template by electrodeposition. The modified version of the 3$\omega $ slope method [1] was employed to measure the anisotropic thermal conductivity of the nanowire array in AAO template. This report presents the temperature dependent cross-plane (parallel to nanochannel) and in-plane (perpendicular to nanochannel) thermal conductivity of the nanowires. And estimate the thermal conductivity of bismuth telluride nanowire specifically by using a nanowire filling factor of $\sim $30{\%} in a temperature range of 80-300 K. 1. D. G. Cahill, Rev. Sci. Instrum. 61, 802 (1990)   

Theoretical Study on F atom diffusion on Si(111) surface

We studied diffusion mechanisms of fluorine atom adsorbed on Si(111) surface in a low coverage regime by means of the first-principles density functional calculation. Recent experiments found that the diffusion frequency of the fluorine atoms on Si(111)-(7x7) surface is very low and interestingly it is enhanced after the deposition of silicon atoms on the surface. While this measurement strongly suggests that the diffusing extra silicon atoms assist the fluorine migration, the mechanism has not been understood yet. We found that it is hard for F atoms to hop between surface Si atoms directly because of the strong Si-F bonding. Instead we suggest the SiF complex diffusion model, in which SiF bond is kept during diffusion. This model is also understood as the Si atom diffusion with carrying the F atom. This model, in which the activation energy is calculated to be 1.34 eV, can explain the experiment very well. This work was partly supported by RSS21 project in IT program and a Grant-in-Aid for Scientific Research (No.17064017) of MEXT of the Japanese Government.   

A setup for simultaneously measureing the thermopower and electrical conductivity of $\mu $m-thickness specimens

We report the concept and configuration of our new setup for measurement of thermopower and electrical conductivity for $\mu $m-thickness specimens, especially for thermoelectric materials. It is very difficult and tedious to accurately measure the thermopower for specimens with thickness less than $\sim $100 $\mu $m due to the limitations of smallest size $\sim $25$\mu $m of thermocouples. Such are obvious when applied to the measurement of nanowire arrays and multilayer . In order to resolve these difficulties, we developed a new setup with integration of Pt-film thermometers and electrical electrodes on two sapphire chips used to clamp specimens with thickness $>$40 \textbf{$\mu $}m and cross section 2 x 3 mm$^{2}$. Use this setup the thermopower and electric conductivity can be measured simultaneously for temperature range 20-400 K. The advantages of the setup are (1) accuracy: the real temperatures of both sides of the sample can be obtained. (2) convenience for loading samples: just assemble the sample between the two microchips and make sure of a good thermal and electrical contacts. A Bi$_{2}$Te$_{3}$ nanowire array in AAO template was tested, the thermopower $\sim $ 50$\mu $V/K was measured for diameter $\sim $ 60 nm of nanowires.   

SNS-Magnetism Reflectometer Initial Measurements.

The SNS Magnetism Reflectometer is one of the first three operational SNS instruments. Its construction was completed in May 2006 and the first data sets were collected in July 2006, shortly after passing the instrument readiness safety review. The beamline commissioning proceeds in parallel to the power ramp up of the SNS accelerator. While the initial reflectivity measurements were taken with only 250 Watts of proton power, by mid November 2006 proton power had already increased to 60 kW, making the SNS the most intense pulsed neutron source on a per pulse basis and in August 2007 by running at 185kW became the most intense source in the world. In this presentation, initial commissioning results for the Magnetism Reflectometer, in particular beam intensity/divergence measurements, tests of the polarized neutron equipment (super mirror polarizer, RF spin flippers) as well as transmission measurements of the bandwidth limiting chopper systems and some science reflectivity measurements will be presented. Reflectivity data for magnetic MgO and Si3N4/Mn multilayer will be discussed in this presentation. The magnetic MgO:N(2.2{\%}) is a system that is doped with 2.2{\%} Nitrogen there by introducing a local magnetic moment of about 8emu/cc. The SNS Magnetism Reflectometer on beamline 4A shares a common primary beam port and shutter with the SNS Liquids Reflectometer (BL4B) from which it is separated by 4.8\r{ } in horizontal angle. Individual secondary shutters allow operating the two instruments independently.   

Magnetic resonance force microscopy with two-dimensional spatial encoding

We demonstrate a novel method of creating Magnetic Resonance Force Microscopy (MRFM) images that eliminates the need to scan the probe-sample distance. Conventionally, scanning a magnetic tip over the sample in at least two dimensions is required for imaging with MRFM. At each position the signal from a different slice of the sample is acquired, where the slice is defined by the the rf field and the ferromagnetic gradient tip geometry. An image can be reconstructed by deconvolving the shape of the slice from the data. The new method we demonstrate keeps the sample-tip distance constant and resolves the signal origin by spatial encoding with rf pulses. For spatial encoding in one dimension rf pulses are applied with a gradient field coil. These pulses produce a Fourier-encoding in the longitudinal magnetization. In the second dimension Hadamard encoding [1] is employed. 2D images of a patterned (NH$_4$)$_2$SO$_4$ crystal sample are reconstructed from the known field distributions with a resolution of 1 $\mu$m at room temperature.\newline [1] K. W. Eberhardt et al., Phys. Rev. B 76: 180405 (2007)   

Optical and Transport Study of Nanocomposite Film of Polymer and PbS Quantum Dots

Optical studies have been done with nanocomposite of conjugated polymers with colloidal lead sulfide PbS quantum dots (QDs). Partial quenching of polymer photoluminescence by PbSe QDs showed the co-existence of energy and charge transfer within this system. Further investigation by means of cw spectroscopy (including photoluminescence action spectroscopy, photo-induced absorption spectroscopy and photoluminescence quantum efficiency) have been rendered for comprehensive study regarding photogenerated charge transfer. Transient transport and time of flight measurements have been employed to conduct mobility and recombination rate study of the same film. The size dependent optoelectronic properties were observed and explained by model fittings. Life time and activation energies were drawn from the fittings. Study also shows improvement on both optical absorption and charge transfer properties with QDs being treated with post-synthesis ligands exchange.   

Multiscale modeling of early stage growth of CNTs produced by a catalytic CVD process

The catalytic chemical vapour deposition process is a widely used method for the production of single and multi-wall carbon nanotubes (CNTs), but there remain many uncertainties concerning the precise synthesis mechanisms and therefore the degree of control over the types of CNT that can be produced. Hence, we have developed a parameterized mesoscale model to simulate the early stages of growth of CNTs, and used this to establish a connection between the carbon-catalyst interaction energy, carbon deposition rate and catalyst particle shape and size and the type of CNT produced. The interaction energies for the various components of the model were determined using molecular dynamics simulations [1] using potential functions previously derived from density functional calculations [2] for cobalt, iron and nickel catalyst particles interacting with carbon. We present results from atomistic simulations for the different surface energies of the carbon mesh on various metal nanoparticles, and also influence of additives, such as sulfur or oxygen, on the graphitization ability of transition metals via semi-empirical molecular orbital calculations. [1] Y. Shibuta, J.A. Elliott, \textit{Chem. Phys. Lett.}, \textbf{427}, 365-370 (2006). [2] Y. Shibuta and S. Maruyama, \textit{Comp. Mater. Sci.}, \textbf{39}, 842-848 (2007).   

Investigation of a potential force-generation machinery driven by a cytoskeletal Walker-type ATPase in prokaryotic cells

Cytoskeletal proteins are often involved in generating mechanical force to drive various cellular processes. A subgroup of the Walker-type ATPases acts as cytoskeletal proteins that show highly dynamic behavior in bacterial cells. One of the most prominent examples is MinD that works with other cellular components to prevent cell division at unwanted polar sites through cycles of pole-to-pole oscillation in \textit{E. coli} cells. We use fluorescence microscopy techniques to study the process of MinD assembly and disassembly on a lipid bilayer membrane surface and any possible change of membrane properties caused by MinD association with the membrane. To form a supported bilayer membrane, vesicles of the polar or total extract of \textit{E. coli} membrane or synthetic lipids of defined composition are adsorbed to a treated glass coverslip. Ca$^{2+}$ is added to enable vesicle fusion to form a continuous bilayer on a glass surface. Formation of a bilayer is examined using fluorescence recovery after photobleaching. The results on the protein assembly on membranes present an important step in understanding the intermediate stages that occur during the dynamic movement of MinD in cells.   

Resistive Small-World Networks

The focus of this study is small-world network where there are at least two network paths between any two nodes and the edges have uniform Ohmic resistance. Assuming that signals can locally propagate along the edges between nearest-neighbor nodes due to only potential difference between the nodes, the question being asked is about the global propagation of signals through the network. Simulations demonstrate that the average equivalent resistance of random conductive network follows the average geodesic path but only for highly-connected networks. One physical realization of this situation are conduction paths observed during electrical breakdown.