Thermoelectric transport properties of armchair graphene nanoribbon heterostructures.

In this paper, we numerically analyze the thermoelectric (TE) properties of recently synthesized graphene nanoribbon (GNR) heterostructures that are obtained as extensions of pristine armchair graphene nanoribbons (AGNRs). After simulating their band structure through a nearest-neighbor tight-binding model, we use the Landauer formalism to calculate the necessary TE coefficients, with which we obtain the electrical conductanceG, thermopowerS, thermal conductanceKe, linear-response thermocurrentIth/ΔT=GS, and figure of meritZT(using literature results for the phonon thermal conductanceKph), at room temperature. We then compare the results for the nanoribbon heterostructures with those for the pristine AGNR nanoribbons. The comparison shows that the metallic AGNRs become semiconducting (with much higherZTvalues) after the inclusion of the extensions that transform them into heterostructures and that some heterostructures have higher values ofZTwhen compared to the semiconducting pristine AGNRs from which they have originated.

Electric-field-generated topological states in a silicene nanotube.

Applying an electric field perpendicular to the axis of a silicene armchair nanotube allows us to numerically study the formation of eight topological edge states as silicene's intrinsic spin-orbit gap is closed by the sublattice-staggered electrostatic potential created by the electric field. Following their evolution with electric field, it is revealed that, at very small fields, these eight states are very broad, spin-locked, and sublattice constrained, inheriting their properties from the K and K' states in a silicene two-dimensional honeycomb lattice. Four of those states are centered at the very top of the nanotube and the other four states are centered at the very bottom. As the field increases, each state starts to become narrower and to spread its spectral weight to the other sublattice. With further increase of the field, each state starts to spatially split, while the sublattice spreading continues. Once the spectral weight of each state is distributed evenly among both sublattices, the state has also effectively split into two spatially disconnected parts, after which, further increasing of the field will spread apart the two halves, moving them to the lateral regions of the nanotube, at the same time that the state halves become narrower. This is consistent with the formation of topological edge states, which delimit four ribbon-like topologically different regions: top and bottom topologically trivial 'ribbons' (where the electric field has induced a topological phase transition) that are adjacent to two topologically nontrivial 'ribbons' located at opposing sides of the nanotube. We also briefly access the possibility of observing these edge states by calculating the electronic properties for an electric field configuration that can be more readily produced in the laboratory.

Manganese requirements of broiler breeder hens.

The present research was conducted to assess Mn requirements of broiler breeder hens. One hundred and twenty Cobb 500 hens, 22 wk of age, were individually allocated in cages. After fed a Mn-deficient diet (22.2 ppm), hens were randomly placed in treatments having 6 increments of 30-ppm Mn. All trace minerals were from laboratory grade sources being Mn from Mn sulfate (MnSO4H2O). Treatments were fed for 4 periods of 28 d. There were no interactions between dietary Mn and period for any evaluated response (P > 0.05). Requirements of Mn for hen day egg production and settable egg production were 115.8 and 56.6 ppm and 122.1 and 63.6 ppm (P < 0.05), respectively, using quadratic polynomial (QP) and broken line quadratic (BLQ) models, whereas total eggs and total settable eggs per hen had Mn requirements estimated at 115.7 and 56.6 and 121.8 and 61.7 ppm (P < 0.05), respectively. Number of cracked, defective, and contaminated eggs decreased, whereas hatchability, hatchability of fertile eggs, eggshell percentage, and eggshell palisade layer increased when hens were fed diets having 48.5 to 168.2-ppm Mn (P < 0.05). Maximum responses for egg weight and eggshell percentage were 117.7 and 63.6 ppm as well as 131.6 and 71.0 ppm (P < 0.05), respectively, using QP and BLQ models. Breaking strength and egg specific gravity had Mn requirements estimated at 140.2 and 112.7 ppm as well as 131.3 68.5 ppm (P < 0.05), whereas eggshell palisade layer and eggshell thickness were maximized with 128.8 and 68.8 ppm and 140.2 134.2 ppm, respectively, for QP and BLQ models (P < 0.05). Maximum yolk Mn content values were obtained using 118.0- and 118.4-ppm Mn by QP and BLQ models, respectively. The average Mn requirements estimated for QP and BLQ models is 128.4 and 92.3 ppm Mn (18.7 and 13.5 mg/hen/d), respectively, which is much lower than what has been currently recommended in commercial production.

Kondo effect under the influence of spin-orbit coupling in a quantum wire.

The analysis of the impact of spin-orbit coupling (SOC) on the Kondo state has generated considerable controversy, mainly regarding the dependence of the Kondo temperature T K on SOC strength. Here, we study the one-dimensional (1D) single impurity Anderson model (SIAM) subjected to Rashba (α) and Dresselhaus (ß) SOC. It is shown that, due to time-reversal symmetry, the hybridization function between impurity and quantum wire is diagonal and spin independent (as it is the case for the zero-SOC SIAM), thus the finite-SOC SIAM has a Kondo ground state similar to that for the zero-SOC SIAM. This similarity allows the use of the Haldane expression for T K, with parameters renormalized by SOC, which are calculated through a physically motivated change of basis. Analytic results for the parameters of the SOC-renormalized Haldane expression are obtained, facilitating the analysis of the SOC effect over T K. It is found that SOC acting in the quantum wire exponentially decreases T K while SOC at the impurity exponentially increases it. These analytical results are fully supported by calculations using the numerical renormalization group (NRG), applied to the wide-band regime, and the projector operator approach, applied to the infinite-U regime. Literature results, using quantum Monte Carlo, for a system with Fermi energy near the bottom of the band, are qualitatively reproduced, using NRG. In addition, it is shown that the 1D SOC SIAM for arbitrary α and ß displays a persistent spin helix SU(2) symmetry similar to the one for a 2D Fermi sea with the restriction α = ß.

Multiband superconductivity in BiS2-based layered compounds.

A mean-field treatment is presented of a square lattice two-orbital-model for [Formula: see text] taking into account intra- and inter-orbital superconductivity. A rich phase diagram involving both types of superconductivity is presented as a function of the ratio between the couplings of electrons in the same and different orbitals ([Formula: see text]) and electron doping x. With the help of a quantity we call orbital-mixing ratio, denoted as [Formula: see text], the phase diagram is analyzed using a simple and intuitive picture based on how [Formula: see text] varies as electron doping increases. The predictive power of [Formula: see text] suggests that it could be a useful tool in qualitatively (or even semi-quantitatively) analyzing multiband superconductivity in BCS-like superconductors.

Fano-Andreev effect in a T-shape double quantum dot in the Kondo regime.

In the present work, we investigate the electronic transport through a T-shape double quantum dot system coupled to two normal leads and to one superconducting lead. We explore the interplay between Kondo and Andreev states due to proximity effects. We find that Kondo resonance is modified by the Andreev bound states, which manifest through Fano antiresonances in the local density of states of the embedded quantum dot and normal transmission. This means that there is a correlation between Andreev bound states and Fano resonances that is robust under the influence of high electronic correlation. We have also found that the dominant couplings at the quantum dots are characterized by a crossover region that defines the range where the Fano-Kondo and the Andreev-Kondo effect prevail in each quantum dot. Likewise, we find that the interaction between Kondo and Andreev bound states has a notable influence on the Andreev transport.

Transport and strong-correlation phenomena in carbon nanotube quantum dots in a magnetic field.

Transport through carbon nanotube (CNT) quantum dots (QDs) in a magnetic field is discussed. The evolution of the system from the ultraviolet to the infrared is analyzed; the strongly correlated (SC) states arising in the infrared are investigated. Experimental consequences of the physics are presented-the SC states arising at various fillings are shown to be drastically different, with distinct signatures in the conductance and, in particular, the noise. Besides CNT QDs, our results are also relevant to double-QD systems.

Transport properties of strongly correlated electrons in quantum dots studied with a simple circuit model.

Numerical calculations are shown to reproduce the main results of recent experiments involving nonlocal spin control in quantum dots [Craig, Science 304, 565 (2004).]. In particular, the experimentally reported zero-bias-peak splitting is clearly observed in our studies. To understand these results, a simple "circuit model" is introduced and shown to qualitatively describe the experiments. The main idea is that the splitting originates in a Fano antiresonance, which is caused by having one quantum dot side connected in relation to the current's path. This scenario provides an explanation of the results of Craig et al. that is an alternative to the RKKY proposal, also addressed here.

Electron transport through a molecular conductor with center-of-mass motion.

The linear conductance of a molecular conductor oscillating between two metallic leads is investigated numerically both for Hubbard interacting and noninteracting electrons. The molecule-leads tunneling barriers depend on the molecule displacement from its equilibrium position. The results present an interesting interference which leads to a conductance dip at the electron-hole symmetry point that could be experimentally observable. It is shown that this dip is caused by the destructive interference between the purely electronic and phonon-assisted tunneling channels, which are found to carry opposite phases. When an internal vibrational mode is also active, the electron-hole symmetry is broken but a Fano-like interference is still observed.

Prediction of ferromagnetic correlations in coupled double-level quantum dots.

Numerical results for transport properties of two coupled double-level quantum dots (QDs) strongly suggest that under appropriate conditions the dots develop a novel ferromagnetic (FM) correlation at quarter filling (one electron per dot). In the strong coupling regime (Coulomb repulsion larger than electron hopping) and with interdot tunneling larger than tunneling to the leads, an S=1 Kondo resonance develops in the density of states, leading to a peak in the conductance. A qualitative "phase diagram," incorporating the new FM phase, is presented. In addition, the necessary conditions for the FM regime are less restrictive than naively believed, leading to its possible experimental observation in real QDs.

Superconductivity in the two-dimensional t-J model.

Using computational techniques, it is shown that pairing is a robust property of hole-doped antiferromagnetic insulators. In one dimension and for two-leg ladder systems, a BCS-like variational wave function with long-bond spin singlets and a Jastrow factor provides an accurate representation of the ground state of the t-J model, even though strong quantum fluctuations destroy the off-diagonal superconducting long-range order in this case. However, in two dimensions it is argued-and numerically confirmed using several techniques, especially quantum Monte Carlo-that quantum fluctuations are not strong enough to suppress superconductivity.

Doped stripes in models for the cuprates emerging from the one-hole properties of the insulator.

The extended and standard t-J models are computationally studied on ladders and planes, with emphasis on the small J/t region. At couplings compatible with photoemission results for undoped cuprates, half-doped stripes separating pi-shifted antiferromagnetic (AF) domains are found, as in Tranquada's interpretation of neutron experiments. Our main result is that the elementary stripe "building block" resembles the properties of one hole at small J/t, with robust AF correlations across the hole induced by the local tendency of the charge to separate from the spin. This suggests that the seed of half-doped stripes already exists in the unusual properties of the insulating parent compound.