Enhancement of Tunneling Density of States at a Junction of Three Luttinger Liquid Wires

We study the tunneling density of states (TDOS) for a junction of three Tomonaga-Luttinger liquid wires. We show that there are fixed points which allow for the enhancement of the TDOS, which is unusual for Luttinger liquids. The distance from the junction over which this enhancement occurs is of the order of x=v/(2 omega), where v is the plasmon velocity and omega is the bias frequency. Beyond this distance, the TDOS crosses over to the standard bulk value independent of the fixed point describing the junction. This finite range of distances opens up the possibility of experimentally probing the enhancement in each wire individually.

Optimal simultaneous maximum a posterioriestimation of states, noise statistics and parameters I. Algorithm

The simultaneous state and parameter estimation problem for a linear discrete-time system with unknown noise statistics is treated as a large-scale optimization problem. The a posterioriprobability density function is maximized directly with respect to the states and parameters subject to the constraint of the system dynamics. The resulting optimization problem is too large for any of the standard non-linear programming techniques and hence an hierarchical optimization approach is proposed. It turns out that the states can be computed at the first levelfor given noise and system parameters. These, in turn, are to be modified at the second level.The states are to be computed from a large system of linear equations and two solution methods are considered for solving these equations, limiting the horizon to a suitable length. The resulting algorithm is a filter-smoother, suitable for off-line as well as on-line state estimation for given noise and system parameters. The second level problem is split up into two, one for modifying the noise statistics and the other for modifying the system parameters. An adaptive relaxation technique is proposed for modifying the noise statistics and a modified Gauss-Newton technique is used to adjust the system parameters.

Optimal simultaneous maximum a posterioriestimation of states, noise statistics and parameters II. Numerical performance

A very general and numerically quite robust algorithm has been proposed by Sastry and Gauvrit (1980) for system identification. The present paper takes it up and examines its performance on a real test example. The example considered is the lateral dynamics of an aircraft. This is used as a vehicle for demonstrating the performance of various aspects of the algorithm in several possible modes.

Critical test for Altshuler-Aronov theory: Evolution of the density of states singularity in double perovskite Sr2FeMoO6 with controlled disorder

With high-resolution photoemission spectroscopy measurements, the density of states (DOS) near the Fermi level (E-F) of double perovskite Sr2FeMoO6 having different degrees of Fe/Mo antisite disorder has been investigated with varying temperature. The DOS near E-F showed a systematic depletion with increasing degree of disorder, and recovered with increasing temperature. Altshuler-Aronov (AA) theory of disordered metals well explains the dependences of the experimental results. Scaling analysis of the spectra provides experimental indication for the functional form of the AA DOS singularity.

Edge states, spin transport, and impurity-induced local density of states in spin-orbit coupled graphene

We study graphene, which has both spin-orbit coupling (SOC), taken to be of the Kane-Mele form, and a Zeeman field induced due to proximity to a ferromagnetic material. We show that a zigzag interface of graphene having SOC with its pristine counterpart hosts robust chiral edge modes in spite of the gapless nature of the pristine graphene; such modes do not occur for armchair interfaces. Next we study the change in the local density of states (LDOS) due to the presence of an impurity in graphene with SOC and Zeeman field, and demonstrate that the Fourier transform of the LDOS close to the Dirac points can act as a measure of the strength of the spin-orbit coupling; in addition, for a specific distribution of impurity atoms, the LDOS is controlled by a destructive interference effect of graphene electrons which is a direct consequence of their Dirac nature. Finally, we study transport across junctions, which separates spin-orbit coupled graphene with Kane-Mele and Rashba terms from pristine graphene both in the presence and absence of a Zeeman field. We demonstrate that such junctions are generally spin active, namely, they can rotate the spin so that an incident electron that is spin polarized along some direction has a finite probability of being transmitted with the opposite spin. This leads to a finite, electrically controllable, spin current in such graphene junctions. We discuss possible experiments that can probe our theoretical predictions.

Tailoring local density of optical states to control emission intensity and anisotropy of quantum dots in hybrid photonic-plasmonic templates

We report results of controlled tuning of the local density of states (LDOS) in versatile, flexible, and hierarchical self assembled plasmonic templates. Using 5 nm diameter gold (Au) spherical nanoantenna within a polymer template randomly dispersed with quantum dots, we show how the photoluminescence intensity and lifetime anisotropy of these dots can be significantly enhanced through LDOS tuning. Finite difference time domain simulations corroborate the experimental observations and extend the regime of enhancement to a wider range of geometric and spectral parameters bringing out the versatility of these functional plasmonic templates. It is also demonstrated how the templates act as plasmonic resonators for effectively engineer giant enhancement of the scattering efficiency of these nano antenna embedded in the templates. Our work provides an alternative method to achieve spontaneous emission intensity and anisotropy enhancement with true nanoscale plasmon resonators. (C) 2015 AIP Publishing LLC.

Electronic Structure of Vacancy Ordered Spinels, $GaMo_{4}S_{8}$ and $GaV_{4}S_{8}$, from ab Initio Calculations

We report ab initio calculations for the band dispersions and total as well as partial densities of states for vacancy ordered, clustered spinels, GaMo4S8 and GaV4S8. Results are presented for the high temperature cubic phase for both compounds. Additionally, we discuss results of similar calculations for GaMo4S8 in an idealized cubic structure, as well as the nonmagnetic and the ferromagnetic states of the low temperature rhombohedral structure. Comparison of these results allows us to discuss the unusual aspects of the electronic structure of this interesting class of compounds, and provide estimates of the crystal-field and exchange splitting strengths.

Virtues of marginally metallic oxides

Transition-metal oxides at the metal-insulator boundary, especially those belonging to the perovskite family, exhibit fascinating phenomena such as insulator-metal transitions controlled by composition, high-temperature superconductivity and giant magnetoresistance (GMR), Interestingly, many of these marginally metallic oxides obey the established criteria for metallicity and have a finite density of states at the Fermi;level. The perovskite manganates exhibiting GMR, on the other hand, are unusual in that they possess very high resistivities in the 'metallic' state and show no significant density of states at the Fermi level, Marginal metallicity in oxide systems is a problem of great complexity and contemporary interest and its understanding is of crucial significance to the diverse phenomena exhibited by these materials.

Theoretical description of time-resolved pump/probe photoemission in TaS2: a single-band DFT+DMFT(NRG) study within the quasiequilibrium approximation

In this work, we theoretically examine recent pump/probe photoemission experiments on the strongly correlated charge-density-wave insulator TaS2.We describe the general nonequilibrium many-body formulation of time-resolved photoemission in the sudden approximation, and then solve the problem using dynamical mean-field theory with the numerical renormalization group and a bare density of states calculated from density functional theory including the charge-density-wave distortion of the ion cores and spin-orbit coupling. We find a number of interesting results: (i) the bare band structure actually has more dispersion in the perpendicular direction than in the two-dimensional planes; (ii) the DMFT approach can produce upper and lower Hubbard bands that resemble those in the experiment, but the upper bands will overlap in energy with other higher energy bands; (iii) the effect of the finite width of the probe pulse is minimal on the shape of the photoemission spectra; and (iv) the quasiequilibrium approximation does not fully describe the behavior in this system.

Tripartite Entanglement versus Tripartite Nonlocality in Three-Qubit Greenberger-Horne-Zeilinger-Class States

We analyze the relationship between tripartite entanglement and genuine tripartite nonlocality for three-qubit pure states in the Greenberger-Horne-Zeilinger class. We consider a family of states known as the generalized Greenberger-Horne-Zeilinger states and derive an analytical expression relating the three-tangle, which quantifies tripartite entanglement, to the Svetlichny inequality, which is a Bell-type inequality that is violated only when all three qubits are nonlocally correlated. We show that states with three-tangle less than 1/2 do not violate the Svetlichny inequality. On the other hand, a set of states known as the maximal slice states does violate the Svetlichny inequality, and exactly analogous to the two-qubit case, the amount of violation is directly related to the degree of tripartite entanglement. We discuss further interesting properties of the generalized Greenberger-Horne-Zeilinger and maximal slice states.

Low temperature electrical transport properties of carbon matrix containing iron nanoparticles

We present a comparative study of the low temperature electrical transport properties of the carbon matrix containing iron nanoparticles and the films. The conductivity of the nanoparticles located just below the metal-insulator transition exhibits metallic behavior with a logarithmic temperature dependence over a large temperature interval. The zero-field conductivity and the negative magnetoresistance, showing a characteristic upturn at liquid helium temperature, are consistently explained by incorporating the Kondo relation and the two dimensional electron-electron interaction. The films, in contrast, exhibit a crossover of the conductivity from power-law dependence at high temperatures to an activated hopping law dependence in the low temperature region. The transition is attributed to changes in the energy dependence of the density of states near the Fermi level. The observed magnetoresistance is discussed in terms of quantum interference effect on a three-dimensional variable range hopping mechanism.

Efficient implementations of a pseudodynamical stochastic filtering strategy for static elastography

A computationally efficient pseudodynamical filtering setup is established for elasticity imaging (i.e., reconstruction of shear modulus distribution) in soft-tissue organs given statically recorded and partially measured displacement data. Unlike a regularized quasi-Newton method (QNM) that needs inversion of ill-conditioned matrices, the authors explore pseudodynamic extended and ensemble Kalman filters (PD-EKF and PD-EnKF) that use a parsimonious representation of states and bypass explicit regularization by recursion over pseudotime. Numerical experiments with QNM and the two filters suggest that the PD-EnKF is the most robust performer as it exhibits no sensitivity to process noise covariance and yields good reconstruction even with small ensemble sizes.

Self Assembly and Electronic Structure of ZnO Nanocrystals

In this paper, we report the synthesis and self assembly of various sizes of ZnO nanocrystals. While the crystal structure and the quantum confinement of nanocrystals were mainly characterized using XRD and UV absorption spectra, the self assembly and long range ordering were studied using scanning tunneling microscopy after spin casting the nanocrystal film on the highly oriented pyrolytic graphite surface. We observe self assembly of these nanocrystals over large areas making them ideal candidates for various potential applications. Further, the electronic structure of the individual dots is obtained from the current-voltage characteristics of the dots using scanning tunneling spectroscopy and compared with the density of states obtained from the tight binding calculations. We observe an excellent agreement with the experimentally obtained local density of states and the theoretically calculated density of states.

Infrared studies on the conformation of synthetic alamethicin fragments and model peptides containing .alpha.-aminoisobutyric acid

ABSTRACT: Infrared studies of synthetic alamethicin fragments and model peptides containing a-aminoisobutyric acid (Aib) have been carried out in solution. Tripeptides and larger fragments exhibit a strong tendency to form /3 turns, stabilized by 4 - 1 10-atom hydrogen bonds. Dipeptides show less well-defined structures, though C5 and C7 conformations are detectable. Conformational restrictions imposed by Aib residues result in these peptides populating a limited range of states. Integrated intensities of the hydrogen-bonded N-H stretching band can be used to quantitate the number of intramolecular hydrogen bonds. Predictions made from infrared data are in excellent agreement with nuclear magnetic resonance and X-ray diffraction studies. Assignments of the urethane and tertiary amide carbonyl groups in the free state have been made in model peptides. Shifts to lower frequency on hydrogen bonding are observed for the carbonyl groups. The 1-6 segment of alamethicin is shown to adopt a 310 helical structure stabilized by four intramolecular hydrogen bonds. The fragments Boc-Leu-Aib-Pro-Val-Aib-OMe (1 2-1 6) and Boc-Gly-Leu-Aib-Pro-Val-Aib-OMe (1 1-1 6) possess structures involving 4 - 1 and 5 - 1 hydrogen bonds. Supporting evidence for these structures is obtained from proton nuclear magnetic resonance studies.

Metal-insulator transition of naxwo3 studied by angle-resolved photoemission spectroscopy

The electronic structure of sodium tungsten bronzes NaxWO3 is investigated by high-resolution angle-resolved photoemission spectroscopy (ARPES). The ARPES spectra measured in both insulating and metallic phases of NaxWO3 reveals the origin of metal-insulator transition (MIT) in sodium tungsten bronze system. It is found that in insulating NaxWO3 the states near the Fermi level (E-F) are localized due to the strong disorder caused by the random distribution of Na+ ions in WO3 lattice. Due to the presence of disorder and long-range Coulomb interaction of conduction electrons, a soft Coulomb gap arises, where the density of states vanishes exactly at E-F. In the metallic regime the states near E-F are populated and the Fermi level shifts upward rigidly with increasing electron doping (x). Volume of electron-like Fermi surface (FS) at the Gamma(X) point of the Brillouin zone gradually increases with increasing Na concentration due to W 5d t(2g) band filling. A rigid shift of the Fermi energy is found to give a qualitatively good description of the Fermi surface evolution. As we move from bulk-sensitive to more surface sensitive photon energy, we found the emergence of Fermi surfaces at X(M) and M(R) point similar to the one at the Gamma(X) point in the metallic regime, suggesting that the reconstruction of surface was due to rotation/deformation of WO6 octahedra.