Tunable heat pump by modulating the coupling to the leads

We follow the nonequilibrium Green's function formalism to study time-dependent thermal transport in a linear chain system consisting of two semi-infinite leads connected together by a coupling that is harmonically modulated in time. The modulation is driven by an external agent that can absorb and emit energy. We determine the energy current flowing out of the leads exactly by solving numerically the Dyson equation for the contour-ordered Green's function. The amplitude of the modulated coupling is of the same order as the interparticle coupling within each lead. When the leads have the same temperature, our numerical results show that modulating the coupling between the leads may direct energy to either flow into the leads simultaneously or flow out of the leads simultaneously, depending on the values of the driving frequency and temperature. A special combination of values of the driving frequency and temperature exists wherein no net energy flows into or out of the leads, even for long times. When one of the leads is warmer than the other, net energy flows out of the warmer lead. For the cooler lead, however, the direction of the energy current flow depends on the values of the driving frequency and temperature. In addition, we find transient effects to become more pronounced for higher values of the driving frequency.Comment: 10 pages; version 2 accepted for publication in PR

Role of the on-site pinning potential in establishing quasi-steady-state conditions of heat transport in finite quantum systems

We study the transport of energy in a finite linear harmonic chain by solving the Heisenberg equation of motion, as well as by using nonequilibrium Green's functions to verify our results. The initial state of the system consists of two separate and finite linear chains that are in their respective equilibriums at different temperatures. The chains are then abruptly attached to form a composite chain. The time evolution of the current from just after switch-on to the transient regime and then to later times is determined numerically. We expect the current to approach a steady-state value at later times. Surprisingly, this is possible only if a nonzero quadratic on-site pinning potential is applied to each particle in the chain. If there is no on-site potential a recurrent phenomenon appears when the time scale is longer than the traveling time of sound to make a round trip from the midpoint to a chain edge and then back. Analytic expressions for the transient and steady-state currents are derived to further elucidate the role of the on-site potential.Comment: version accepted for publication in PR

Precise QCD predictions on top quark pair production mediated by massive color octet vector boson at hadron colliders

We present a theoretical framework for systematically calculating next-to-leading order (NLO) QCD effects to various experimental observables in models with massive COVB in a model independent way at hadron colliders. Specifically, we show the numerical results for the NLO QCD corrections to total cross sections, invariant mass distribution and AFB of top quark pairs production mediated by a massive COVB in both the fixed scale (top quark mass) scheme and the dynamical scale (top pair invariant mass) scheme. Our results show that the NLO QCD calculations in the dynamical scale scheme is more reasonable than the fixed scheme and the naive estimate of the NLO effects by simple rescaling of the LO results with the SM NLO K-factor is not appropriate.Comment: 6 pages, 5 figures, 2 tables; version published in EPJ

Conductance spectra of metallic nanotube bundles

We report a first principles analysis of electronic transport characteristics for (n,n) carbon nanotube bundles. When n is not a multiple of 3, inter-tube coupling causes universal conductance suppression near Fermi level regardless of the rotational arrangement of individual tubes. However, when n is a multiple of 3, the bundles exhibit a diversified conductance dependence on the orientation details of the constituent tubes. The total energy of the bundle is also sensitive to the orientation arrangement only when n is a multiple of 3. All the transport properties and band structures can be well understood from the symmetry consideration of whether the rotational symmetry of the individual tubes is commensurate with that of the bundle

Next-to-leading order QCD predictions for graviton and photon associated production in the Large Extra Dimensions model at the LHC

We present the calculations of the complete next-to-leading order(NLO) QCD corrections to the inclusive total cross sections for the Kaluza-Klein(KK) graviton and photon associated production process $pp \to \gamma G_{KK} + X$ in the large extra dimensions(LED) model at the LHC. We show that the NLO QCD corrections in general enhance the total cross sections and reduce the dependence of the total cross sections on the factorization and renormalization scales. When jet veto is considered, the NLO corrections reduce the total cross sections. We also calculate some important differential cross sections for this process at NLO: the missing transverse momentum distribution, the transverse momentum distribution and the pseudorapidity distribution of photon.Comment: 28 pages, 14 figures; minor changes, version published in Phys.Rev.

One-loop Helicity Amplitudes for Top Quark Pair Production in Randall-Sundrum Model

In this paper, we show how to calculate analytically the one-loop helicity amplitudes for the process $q\bar{q} rightarrow t\bar{t}$ induced by KK gluon, using the spinor-helicity formalism. A minimal set of Feynman rules which are uniquely fixed by gauge invariance and the color representation of the KK gluon are derived and used in the calculation. Our results can be applied to a variety of models containing a massive color octet vector boson.Comment: 37 pages, 10 figures, journal versio

Semiquantum key distribution using entangled states

Recently, Boyer et al. presented a novel semiquantum key distribution protocol [M. Boyer, D. Kenigsberg, and T. Mor, Phys. Rev. Lett. 99, 140501 (2007)], by using four quantum states, each of which is randomly prepared by Z basis or X basis. Here we present a semiquantum key distribution protocol by using entangled states in which quantum Alice shares a secret key with classical Bob. We also show the protocol is secure against eavesdropping.Comment: 6 page

Characteristics of phonon transmission across epitaxial interfaces: a lattice dynamic study

Phonon transmission across epitaxial interfaces is studied within the lattice dynamic approach. The transmission shows weak dependence on frequency for the lattice wave with a fixed angle of incidence. The dependence on azimuth angle is found to be related to the symmetry of the boundary interface. The transmission varies smoothly with the change of the incident angle. A critical angle of incidence exists when the phonon is incident from the side with large group velocities to the side with low ones. No significant mode conversion is observed among different acoustic wave branches at the interface, except when the incident angle is near the critical value. Our theoretical result of the Kapitza conductance $G_{K}$ across the Si-Ge (100) interface at temperature $T=200$K is 4.6\times10^{8} {\rm WK}^{-1}{\rmm}^{-2}. A scaling law $G_K \propto T^{2.87}$ at low temperature is also reported. Based on the features of transmission obtained within lattice dynamic approach, we propose a simplified formula for thermal conductanceacross the epitaxial interface. A reasonable consistency is found between the calculated values and the experimentally measured ones.Comment: 8 figure

Thermal expansion in carbon nanotubes and graphene: nonequilibrium Green's function approach

The nonequilibrium Green's function method is applied to investigate the coefficient of thermal expansion (CTE) in single-walled carbon nanotubes (SWCNT) and graphene. It is found that atoms deviate about 1% from equilibrium positions at T=0 K, resulting from the interplay between quantum zero-point motion and nonlinear interaction. The CTE in SWCNT of different sizes is studied and analyzed in terms of the competition between various vibration modes. As a result of this competition, the axial CTE is positive in the whole temperature range, while the radial CTE is negative at low temperatures. In graphene, the CTE is very sensitive to the substrate. Without substrate, CTE has large negative region at low temperature and very small value at high temperature limit, and the value of CTE at T=300 K is $-6\times 10^{-6}$ K$^{-1}$ which is very close to recent experimental result, $-7\times 10^{-6}$ K$^{-1}$ (Nat. Nanotechnol. \textbf{10}, 1038 (2009)). A very weak substrate interaction (about 0.06% of the in-plane interaction) can largely reduce the negative CTE region and greatly enhance the value of CTE. If the substrate interaction is strong enough, the CTE will be positive in whole temperature range and the saturate value at high temperature reaches $2.0\times 10^{-5}$ K$^{-1}$.Comment: final version, to appear in PR

Unequal Intra-layer Coupling in a Bilayer Driven Lattice Gas

The system under study is a twin-layered square lattice gas at half-filling, being driven to non-equilibrium steady states by a large, finite `electric' field. By making intra-layer couplings unequal we were able to extend the phase diagram obtained by Hill, Zia and Schmittmann (1996) and found that the tri-critical point, which separates the phase regions of the stripped (S) phase (stable at positive interlayer interactions J_3), the filled-empty (FE) phase (stable at negative J_3) and disorder (D), is shifted even further into the negative J_3 region as the coupling traverse to the driving field increases. Many transient phases to the S phase at the S-FE boundary were found to be long-lived. We also attempted to test whether the universality class of D-FE transitions under a drive is still Ising. Simulation results suggest a value of 1.75 for the exponent gamma but a value close to 2.0 for the ratio gamma/nu. We speculate that the D-FE second order transition is different from Ising near criticality, where observed first-order-like transitions between FE and its "local minimum" cousin occur during each simulation run.Comment: 29 pages, 19 figure