Holographic End-Point of Spatially Modulated Phase Transition

In the previous paper [arXiv:0911.0679], we showed that the Reissner-Nordstrom black hole in the 5-dimensional anti-de Sitter space coupled to the Maxwell theory with the Chern-Simons term is unstable when the Chern-Simons coupling is sufficiently large. In the dual conformal field theory, the instability suggests a spatially modulated phase transition. In this paper, we construct and analyze non-linear solutions which describe the end-point of this phase transition. In the limit where the Chern-Simons coupling is large, we find that the phase transition is of the second order with the mean field critical exponent. However, the dispersion relation with the Van Hove singularity enhances quantum corrections in the bulk, and we argue that this changes the order of the phase transition from the second to the first. We compute linear response functions in the non-linear solution and find an infinite off-diagonal DC conductivity in the new phase.Comment: 21 pages, 22 figures. v2: a note and a reference adde

Negative Differential Resistance of Oligo(Phenylene Ethynylene) Self-Assembled Monolayer Systems: The Electric-Field-Induced Conformational Change Mechanism

We investigate here a possible mechanism for the room temperature negative differential resistance (NDR) in the Au/AN-OPE/RS/Hg self-assembled monolayer (SAM) system, where AN-OPE = 2′-amino,5′-nitro-oligo(phenylene ethynylene) and RS is a C_(14) alkyl thiolate. Kiehl and co-workers showed that this molecular system leads to NDR with hysteresis and sweep-rate-dependent position and amplitude in the NDR peak. To investigate a molecular basis for this interesting behavior, we combine first-principles quantum mechanics (QM) and mesoscale lattice Monte Carlo methods to simulate the switching as a function of voltage and voltage rate, leading to results consistent with experimental observations. This simulation shows how the structural changes at the microscopic level lead to the NDR and sweep-rate-dependent macroscopic I−V curve observed experimentally, suggesting a microscopic model that might aid in designing improved NDR systems

Spin polarization amplification within nonmagnetic semiconductors at room temperature

We demonstrate theoretically that the spin polarization of current can be electrically amplified within nonmagnetic semiconductors by exploiting the fact the spin current, compared to the charge current, is weakly perturbed by electric driving forces. As a specific example, we consider a T-shaped current branching geometry made entirely of a nonmagnetic semiconductor, where the current is injected into one of the branches (input branch) and splits into the other two branches (output branches). We show that when the input current has a moderate spin polarization, the spin polarization in one of the output branches can be higher than the spin polarization in the input branch and may reach 100% when the relative magnitudes of current-driving electric fields in the two output branches are properly tuned. The proposed amplification scheme does not use ferromagnets or magnetic fields, and does not require low temperature operation, providing an efficient way to generate a highly spin polarized current in nonmagnetic semiconductors at room temperature.Comment: 11 pages, 2 figures, to appear in Phys. Rev.

Effect of cyclic chain architecture on properties of dilute solutions of polyethylene from molecular dynamics simulations

We have used molecular dynamics methods to investigate the effects of cyclic chain architecture on the properties of dilute solutions. In order to include solvent effects in estimating these properties, we use a van der Waals scaling factor determined for each solvent by matching to the theta condition. We predict that the theta temperature (theta) of cyclic PE (c-PE) is ~10% lower than for the linear case (l-PE). This can be compared to the experimental results for polystyrene (PS), where theta for cyclic PS is 2% lower. For conditions corresponding to n-pentane solvent, we predict that g2>cyclic/g2>linear is 0.59 for all temperatures above 350 K. The deviation from the ratio of 0.50–0.53 expected from analytic theory is due to the competition between chain stiffness and excluded volume effects. To calculate the intrinsic viscosity of c-PE and l-PE we extended the Bloomfield–Zimm type theory to include chain stiffness corrections. We find that for the theta temperature, the ratio of viscosities for c-PE and l-PE is 0.71, which is 7% higher than the value of 0.66 from the freely jointed chain model. This difference is caused by the larger value of g2>cyclic/g2>linear from the simulations

BRST extension of the Faddeev model

The Faddeev model is a second class constrained system. Here we construct its nilpotent BRST operator and derive the ensuing manifestly BRST invariant Lagrangian. Our construction employs the structure of Stuckelberg fields in a nontrivial fashion.Comment: 4 pages, new references adde

Conformations and charge transport characteristics of biphenyldithiol self-assembled-monolayer molecular electronic devices: A multiscale computational study

We report a computational study of conformations and charge transport characteristics of biphenyldithiol (BPDT) monolayers in the (sqrt(3)×sqrt(3))R30° packing ratio sandwiched between Au(111) electrodes. From force-field molecular-dynamics and annealing simulations of BPDT self-assembled monolayers (SAMs) with up to 100 molecules on a Au(111) substrate, we identify an energetically favorable herringbone-type SAM packing configuration and a less-stable parallel packing configuration. Both SAMs are described by the (2sqrt(3)×sqrt(3))R30° unit cell including two molecules. With subsequent density-functional theory calculations of one unit cell of the (i) herringbone SAM with the molecular tilt angle theta[approximate]15°, (ii) herringbone SAM with theta[approximate]30°, and (iii) parallel SAM with theta[approximate]30°, we confirm that the herringbone packing configuration is more stable than the parallel one but find that the energy variation with respect to the molecule tilting within the herringbone packing is very small. Next, by capping these SAMs with the top Au(111) electrode, we prepare three molecular electronic device models and calculate their coherent charge transport properties within the matrix Green's function approach. Current–voltage (I–V) curves are then obtained via the Landauer–Büttiker formula. We find that at low-bias voltages (|V|~0.5 V), the I–V characteristics of the three models show noticeable differences due to different phenyl band structures. We thus conclude that the BPDT SAM I–V characteristics in the low-bias voltage region are mainly determined by the Si–Au interaction within the individual molecule-electrode contact, while both intramolecular conformation and intermolecular interaction can affect the BPDT SAM I–V characteristics in the high-bias voltage region

Possible performance improvement in [2]catenane molecular electronic switches

Mechanically interlocked bistable supramolecular complexes are promising candidates of molecular electronics. Applying a multiscale computational approach, here we study the coherent charge transport properties of catenane monolayers sandwiched between Cu(111) electrodes. We demonstrate the robust nature of electrical switching behavior with respect to the variations in the monolayer packing density and the type of electrodes, as well as the thermal fluctuations of the molecules. We propose that the asymmetry of molecule-electrode barriers can be utilized to improve the switching ratio

BRST invariance and de Rham-type cohomology of 't Hooft-Polyakov monopole

We exploit the 't Hooft-Polyakov monopole to define closed algebra of the quantum field operators and the BRST charge $Q_{BRST}$. In the first-class configuration of the Dirac quantization, by including the $Q_{BRST}$-exact gauge fixing term and the Faddeev-Popov ghost term, we find the BRST invariant Hamiltonian to investigate the de Rham-type cohomology group structure for the monopole system. The Bogomol'nyi bound is also discussed in terms of the first-class topological charge defined on the extended internal 2-sphere.Comment: 8 page

On the Design of Secure Full-Duplex Multiuser Systems under User Grouping Method

Consider a full-duplex (FD) multiuser system where an FD base station (BS) is designed to simultaneously serve both downlink users and uplink users in the presence of half-duplex eavesdroppers (Eves). Our problem is to maximize the minimum secrecy rate (SR) among all legitimate users by proposing a novel user grouping method, where information signals at the FD-BS are accompanied with artificial noise to degrade the Eves' channel. The SR problem has a highly nonconcave and nonsmooth objective, subject to nonconvex constraints due to coupling between the optimization variables. Nevertheless, we develop a path-following low-complexity algorithm, which invokes only a simple convex program of moderate dimensions at each iteration. We show that our path-following algorithm guarantees convergence at least to a local optima. The numerical results demonstrate the merit of our proposed approach compared to existing well-known ones, i.e., conventional FD and nonorthogonal multiple access.Comment: 6 pages, 3 figure