The transport diffusion in one-dimensional molecular systems is investigated through non-equilibrium molecular dynamics and Monte Carlo methods. We have proposed the power law relationship of the transport diffusion coefficient with the temperature, the mass and the transport length, D* ∝ T*m*−1L*β, where β equals to 0.8 for small systems and zero for large systems. It is found that Fick’s law is valid in long transport length but invalid in short transport length. Our results can provide a new perspective for understanding the microscopic mechanism of the molecular transport phenomena in low-dimensional systems.

We study the impacts of thermal expansion, arising from the asymmetric interparticle potential, on thermal conductance in the FPU-α-β model. A nonmonotonic dependence of the temperature gradient and thermal conductance on the cubic interaction parameter α are shown, which corresponds to the variation of the coefficient of thermal expansion. Three domains with respect to α can be identified. The results are explained based on the detailed analysis of the asymmetry of the interparticle potential. The self-consistent phonon theory, which can capture the effect of thermal expansion, is developed to support our explanation in a quantitative way. Our result would be helpful to understand the issue that whether there exist normal thermal conduction in the FPU-α-β model.

Negative differential thermal resistance (NDTR) can be generated for any one-dimensional heat flow with a temperature-dependent thermal conductivity. In a system-independent scaling analysis, the general condition for the occurrence of NDTR is found to be an inequality with three scaling exponents: n(1)n(2) < -(1+n(3)), where n(1) ∈ (-∞,+∞) describes a particular way of varying the temperature difference, and n(2) and n(3) describe, respectively, the dependence of the thermal conductivity on an average temperature and on the temperature difference. For cases with a temperature-dependent thermal conductivity, i.e. n(2) ≠ 0, NDTR can always be generated with a suitable choice of n(1) such that this inequality is satisfied. The results explain the illusory absence of a NDTR regime in certain lattices and predict new ways of generating NDTR, where such predictions have been verified numerically. The analysis will provide insights for a designing of thermal devices, and for a manipulation of heat flow in experimental systems, such as nanotubes.

We have numerically studied heat conduction in a few one-dimensional momentum-conserving lattices with asymmetric interparticle interactions by the nonequilibrium heat bath method, the equilibrium Green-Kubo method, and the heat current power spectra analysis. Very strong finite-size effects are clearly observed. Such effects make the heat conduction obey a Fourier-like law in a wide range of lattice lengths. However, in yet longer lattice lengths, the heat conductivity regains its power-law divergence. Therefore, the power-law divergence of the heat conductivity in the thermodynamic limit is verified, as is expected by many existing theories.

We study the thermal transport in graphene nanoribbons by using nonequilibrium molecular dynamics simulations. It is reported that the three-terminal graphene nanoribbons can perform some functions of thermal devices such as thermal valve, thermal switch and thermal amplifier. Electronic devices have transformed almost all aspects of our lives. It has not escaped our attention that the graphene nanoribbons we have presented here may have similar surprising applications in devices that allow the flow of heat to be controlled in a short future.

Heat conduction in three two-dimensional (2D) momentum-conserving nonlinear lattices are numerically calculated via both nonequilibrium heat-bath and equilibrium Green-Kubo algorithms. It is expected by mainstream theories that heat conduction in such 2D lattices is divergent and the thermal conductivity κ increases with lattice length N logarithmically. Our simulations for the purely quartic lattice firmly confirm it. However, very robust finite-size effects are observed in the calculations for the other two lattices, which well explain some existing studies and imply the extreme difficulties in observing their true asymptotic behaviors with affordable computation resources.

Negative differential thermal resistance (NDTR) in approximate graphene nanoribbons (GNRs) is investigated from one dimension to three dimensions by using classical molecular dynamics method. For single-layer GNRs, NDTR cannot be observed for very narrow GNRs (one dimension), and NDTR appears when the width of GNRs increases (two dimensions). However, NDTR disappears gradually on further increasing the width. For multiple-layer GNRs, when the number of the layers increases, GNRs change from a 2-D system to 3-D system, and NDTR regime reduces and eventually disappears. In addition, when the length of GNRs increases, NDTR regime also reduces and vanishes in the thermodynamic limit. These effects may be useful for designing thermal devices where NDTR plays an important role.

Spin-torque nanooscillators (STNOs), which have both the common properties of nanosized oscillators (small size, tunable operating frequency) and some particular ones (wide operating range, easy on-chip integration, etc.), have received a great deal of attention due to their high potential in applications. Yet synchronization of serially connected STNOs has been considered essential for applications. In this paper, we present findings concerning the following properties of synchronized serially connected STNOs: (i) multiple synchronization attractors coexist, and the attracting basins are entangled in a complicated manner; (ii) these attractors have different synchronized frequencies and output powers; and (iii) switching among these attractors can be induced by a small noise, which causes a resonance peak in the power spectra to vanish. These characteristics can be understood using saddle-node bifurcations and have direct impact on laboratory experiments and the potential applications of STNO-based devices.

Multichannel and omnidirectional transmission can be realized in one-dimensional periodic metamaterial layers, where double-negative materials and single-negative materials are alternately stacked. The proposed structures can produce as many resonance transmission modes as desired by adjusting the number of the periods. Such transmission modes with perfect transmission are robust against incident angles and polarizations. The frequencies and frequency intervals of these modes can be tuned by changing the thicknesses of the metamaterial layers. Composite right/left-handed transmission lines with lumped elements (capacitors and inductors) were used to verify the characteristics of the considered system. Our findings provide an efficient way to select useful multiple channel signals from all directions, and it is useful in optoelectronic device applications.

We study heat conduction through one-dimensional homogeneous lattices in the presence of the nonlinear on-site potentials containing the bounded and unbounded parts, and the harmonic interaction potential. We observe the occurrence of double negative differential thermal resistance (NDTR); namely, there exist two regions of temperature difference, where the heat flux decreases as the applied temperature difference increases. The nonlinearity of the bounded part contributes to NDTR at low temperatures and NDTR at high temperatures is induced by the nonlinearity of the unbounded part. The nonlinearity of the on-site potentials is necessary to obtain NDTR for the harmonic interaction homogeneous lattices. However, for the anharmonic homogeneous lattices, NDTR even occurs in the absence of the on-site potentials, for example, the rotator model.

Heat conduction through the Frenkel-Kontorova lattices is numerically investigated in the presence of a deformable substrate potential. It is found that the deformation of the substrate potential has a strong influence on heat conduction. The thermal conductivity as a function of the shape parameter is nonmonotonic. The deformation can enhance thermal conductivity greatly, and there exists an optimal deformable value at which thermal conductivity takes its maximum. Remarkably, we also find that the deformation can facilitate the appearance of the negative differential thermal resistance.

Following our Rapid Communication [Li et al. Phys. Rev. B 82, 140407(R) (2010)], we present a more detailed analysis of the synchronization of spin-torque oscillators (STOs), as well as some other nonlinear characteristics of STO, by using the perturbed heteroclinic cycle model rather than a linear phase oscillator model. We analyze four critical points of the system and numerically get the fifth one and meanwhile systematically study the bifurcation process of the system. Three of the fixed points limit the parameter regions for sustaining an oscillation, and another one is the boundary between a global oscillation (out-of-plane mode oscillation) and a local oscillation (in-plane mode oscillation). The frequency of the physically more relevant global oscillation is also estimated depending on the system parameters. The results are not only significant for the design of useful devices but also important for further understanding the response of an STO to an external signal and interpreting the difficulty of synchronizing serially connected STOs. We also discuss in detail several other synchronization schemes.

A two-segment Fermi-Pasta-Ulam lattice has been investigated by using nonequilibrium molecular dynamics. Here we present an anomalous negative differential thermal resistance (NDTR) that has not been reported in Frenkel-Kontorova and φ(4) lattices up to the present. The NDTR disappears in the low-temperature region. The region of NDTR shifts from the large to the small temperature difference region as the system size increases. The anomalous dependence of NDTR on the temperature can be explained as the negative effect induced by the nonlinear coupling. The explanation can also cover the phenomenon of NDTR in momentum-nonconserved lattices.

We investigate the self-sustained current oscillation of a weakly coupled semiconductor superlattice in the dynamical voltage band using a microscopic sequential tunneling model. With the voltage as a control parameter, two types of branches of current oscillation period versus voltage have been observed, which correspond to various oscillation scenarios. The first branch type consists of a series of period branches in accordance with how many charge dipoles need to be created at the emitter side to trigger a dipole-tripole oscillation scenario. For the second branch type, charge dipoles are generated periodically at the emitter, but all of them fail to develop completely and die out, thereby leading to a low-period oscillation scenario without the dipole-tripole process. The bistability between different branches is also observed by voltage up-sweeping and down-sweeping.

We present a theoretical study of vortical plasmonic lattice solitons, which form in two-dimensional arrays of metallic nanowires embedded into nonlinear media with both focusing and defocusing Kerr nonlinearities. Their existence, stability, and subwavelength spatial confinement are investigated in detail.

Fractional synchronization is one of the most interesting collective behaviors in coupled or driving-response oscillators system, very important for both a deep understanding of a particular oscillator and for its applications. We numerically investigate the fractional synchronization of a spin-torque oscillator by injected ac current. Multiple p : q locking regions are found, which display some sophisticated overlaps. The system can be analyzed as a perturbed heteroclinic cycle rather than a phase oscillator. Both the modulations on the output frequency and power are mainly due to the modulation by the external signals on the distance between the dynamical orbit and the saddle point in phase space. By using this dynamical picture, we can well understand all the numerical results, including the variation of the locking region with the amplitude vertical bar J(a)vertical bar or frequency f of the injected signal, the influence by noise, and the difference among the output powers of coexisting locking attractors. These understandings are significant for both potential applications in electronic communications and a deep investigation into this novel device.

We report a numerical study on heat conduction in one-dimensional homogeneous lattices in both the linear and the nonlinear response regime, with a comparison among three prototypical nonlinear lattice models. In the nonlinear response regime, negative differential thermal resistance (NDTR) can occur in both the Frenkel-Kontorova model and the phi4 model. In the Fermi-Pasta-Ulam- beta model, however, only positive differential thermal resistance can be observed, as shown by a monotonous power-law dependence of the heat flux on the applied temperature difference. In general, it was found that NDTR can occur if there is nonlinearity in the onsite potential of the lattice model. It was also found that the regime of NDTR becomes smaller as the system size increases, and eventually vanishes in the thermodynamic limit. For the phi4 model, a phenomenological description of the size-induced crossover from the existence to the nonexistence of a NDTR regime is provided.

We investigate the combined effects of weak disorder and a two-dimensional (2D) optical lattice on the collective excitations of a harmonically trapped Bose-Einstein condensate (BEC) at zero temperature. Accordingly, we generalize the hydrodynamic equations of superfluid for a weakly interacting Bose gas in a 2D optical lattice to include the effects of weak disorder. Our analytical results for the collective frequencies beyond the mean-field approximation reveal the peculiar role of disorder, interplaying with the 2D optical lattice and interatomic interaction, on elementary excitations along the 3D to 1D crossover. In particular, consequences of disorder on the phonon propagation and surface modes are analyzed in detail. The experimental scenario is also proposed.

We investigate the ionization of model diatomic molecules exposed in intense laser fields both numerically and analytically. Our simulations show that, in the tunneling region, the interplay of the tunneling effect and molecular two-center structure has an important role in ionization. It can lead to the enhancement of ionization in molecules as compared to their reference atoms with similar ionization potential. Furthermore, it also plays a dominating role in the angle dependence of molecular ionization. This effect is different from that of two-center interference in ionization. The latter manifests itself remarkably in the multiphoton region and can cause the ionization suppression of molecules in this region. Our further comparisons suggest the significant influence of the orbital symmetry of the reference atom on molecular ionization comparison experiments.

Previously, synchronization of spin-torque oscillator (STO) has usually been analyzed by phase oscillator model. Here we show that STO is more precisely described as a perturbed heteroclinic cycle where the frequency is sensitive to the distance between the cycle and a saddle. In the presence of external signal or coupling, the frequency changes mainly due to the modification on this distance rather than on the phase directly. Multiple attractors coexist and synchronization depends sensitively on the initial conditions when a STO is driven by an external ac. We reveal that these properties underlay the mechanisms why synchronization region in two serially connected STOs is quite small and why time delay can enhance synchronization. When more STOs are added, the parameter region for a globally attracting synchronization state may disappear. Our analysis suggests that initial conditions have to be controlled or different designs have to be fabricated in order to obtain robust synchronization of a large number of STOs for the purpose of applications.