Transport of a Brownian particle moving along the axis of a three-dimensional asymmetric periodic tube is investigated in the presence of asymmetric unbiased forces. The reduction of the coordinates may involve not only the appearance of entropic barrier but also the effective diffusion coefficient. It is found that in the presence of entropic barrier, the asymmetry of the tube shape and the asymmetry of the unbiased forces are the two ways of inducing a net current. The current is a peaked function of temperature which indicates that the thermal noise may facilitate the transport even in the presence of entropic barrier. An optimized radius exists at the bottleneck at which the current takes its maximum value. Competition between the two opposite driving factors may induce current reversal.

The logistic differential equation is used to analyze cancer cell population, in the presence of a correlated Gaussian white noise. We study the steady state properties of tumor cell growth and discuss the effects of the correlated noise. It is found that the degree of correlation of the noise can cause tumor cell extinction.

The thermal conductivity of graphene nanoribbons (layer from 1 to 8 atomic planes) is investigated by using the nonequilibrium molecular dynamics method. We present that the room-temperature thermal conductivity decays monotonically with the number of the layers in few-layer graphene. The superiority of zigzag graphene in thermal conductivity is only available in high temperature region and disappears in multi-layer case. It is explained that the phonon spectral shrink in high frequency induces the change of thermal conductivity. It is also reported that single-layer graphene has better ballistic transport property than the multi-layer graphene.In the past decade, more and more attentions have been given to the question of what happens with thermal conductivity when goes to low-dimensional materials [1]. A two-dimensional materials-graphene [2], in addition to its exceptional electric [3] and optical properties [4] , [5], reveals unique high thermal conductivity. Thermal conductivity of single-layer graphene as well as of carbon nanotubes is dependent on the chirality [6]. Recent theoretical studies suggest that the thermal conductivity of single-layer zigzag graphene is 20-50% larger than that of the singlelayer armchair graphene [7]. However, whether the superior thermal conductivity of zigzag graphene remains available for multi-layer graphene has not got enough attention and concern.Additionally, experimental demonstrations have shown that the thermal conductivity gets a decrease at the twoto three-dimensional (2D to 3D) crossover of few-layer graphene [8]. The fact that the thermal conductivity of large enough graphene sheets should be higher than that of basal planes of bulk graphite was predicted theoretically by Klemens [9]. Generally, thermal transport in conventional thin films still retains 'bulk' features because the crosssections of these structures are measured in many atomic layers. Heat conduction in such nanostructures is dominated by extrinsic effects, for example, phonon-boundary or phonon-defect scattering [10]. A recent experimental observation of high-quality few-layer graphene materials shows that the room-temperature thermal conductivity changes from˜2,800 to˜1,300 Wm −1 K −1 when the number of atomic planes in few-layer graphene increases from 2 to 4. It is explained that the observed evolution from two dimensions to bulk attributed to the cross-plane coupling of the low-energy phonons and changes in the phonon Umklapp scattering [8].Recently, the method of molecular dynamics simulation has been successful in discovering thermal conductivity and thermal rectification of the nanostructures [7] , [11]. This method, which builds the system from the bottom up, is useful to understand the intrinsic behavior, i.e., the phonon spectral behavior behind the significant change of a material's ability to conduct heat [12]. In this paper, we will study the thermal conductivity of graphene ribbons (layer from 1 to 8 atomic planes) by using the nonequilibrium molecular dynamics method. By inve...

Thermal rectification in thickness asymmetric graphene nanoribbons connecting single-layer with multi-layer graphene is investigated by using classical nonequilibrium molecular dynamics. It is reported that the graphene nanoribbons with thickness-asymmetry have a good thermal rectification. The thermal rectification factor depends on temperature as well as the thickness-ratio of the two-segment. Our results provide a direct evidence that the thermal rectifier can be achieved in a nanostructure crossing two-and three-dimension.

No abstract