We have used molecular dynamics to calculate the thermal conductivity of symmetric and asymmetric graphene nanoribbons (GNRs) of several nanometers in size (up to approximately 4 nm wide and approximately 10 nm long). For symmetric nanoribbons, the calculated thermal conductivity (e.g., approximately 2000 W/m-K at 400 K for a 1.5 nm x 5.7 nm zigzag GNR) is on the similar order of magnitude of the experimentally measured value for graphene. We have investigated the effects of edge chirality and found that nanoribbons with zigzag edges have appreciably larger thermal conductivity than nanoribbons with armchair edges. For asymmetric nanoribbons, we have found significant thermal rectification. Among various triangularly shaped GNRs we investigated, the GNR with armchair bottom edge and a vertex angle of 30 degrees gives the maximal thermal rectification. We also studied the effect of defects and found that vacancies and edge roughness in the nanoribbons can significantly decrease the thermal conductivity. However, substantial thermal rectification is observed even in the presence of edge roughness.

Abstract:A three-dimensional (3D) topological insulator (TI) is a quantum state of matter with a gapped insulating bulk yet a conducting surface hosting topologically-protected gapless surface states. One of the most distinct electronic transport signatures predicted for such topological surface states (TSS) is a well-defined half-integer quantum Hall effect (QHE) in a magnetic field, where the surface Hall conductivities become quantized in units of (1/2)e 2 /h (e being the electron charge, h the Planck constant) concomitant with vanishing resistance. Here, we observe well-developed QHE arising from TSS in an intrinsic TI of BiSbTeSe 2 . Our samples exhibit surface dominated conduction even close to room temperature, while the bulk conduction is negligible. At low temperatures and high magnetic fields perpendicular to the top and bottom surfaces, we observe well-developed integer quantized Hall plateaus, where the two parallel surfaces each contributing a half integer e 2 /h quantized Hall (QH) conductance, accompanied by vanishing longitudinal resistance. When the bottom surface is gated to match the top surface in carrier density, only odd integer QH plateaus are observed, representing a half-integer QHE of two degenerate Dirac gases. This system provides an excellent platform to pursue a plethora of exotic physics and novel device applications predicted for TIs, ranging from magnetic monopoles and Majorana particles to dissipationless electronics and fault-tolerant quantum computers.2

Bi2Se3 is an important semiconductor thermoelectric material and a prototype topological insulator. Here we report observation of Shubnikov-de Hass oscillations accompanied by quantized Hall resistances (R(xy)) in highly doped n-type Bi2Se3 with bulk carrier concentrations of few 10(19) cm(-3). Measurements under tilted magnetic fields show that the magnetotransport is 2D-like, where only the c-axis component of the magnetic field controls the Landau level formation. The quantized step size in 1/R(xy) is found to scale with the sample thickness, and average ~e(2)/h per quintuple layer. We show that the observed magnetotransport features do not come from the sample surface, but arise from the bulk of the sample acting as many parallel 2D electron systems to give a multilayered quantum Hall effect. In addition to revealing a new electronic property of Bi2Se3, our finding also has important implications for electronic transport studies of topological insulator materials.

We show that thermal rectification (TR) in asymmetric graphene nanoribbons (GNRs) is originated from phonon confinement in the lateral dimension, which is a fundamentally new mechanism different from that in macroscopic heterojunctions. Our molecular dynamics simulations reveal that, though TR is significant in nanosized asymmetric GNRs, it diminishes at larger width. By solving the heat diffusion equation, we prove that TR is indeed absent in both the total heat transfer rate and local heat flux for bulk-size asymmetric single materials, regardless of the device geometry or the anisotropy of the thermal conductivity. For a deeper understanding of why lateral confinement is needed, we have performed phonon spectra analysis and shown that phonon lateral confinement can enable three possible mechanisms for TR: phonon spectra overlap, inseparable dependence of thermal conductivity on temperature and space, and phonon edge localization, which are essentially related to each other in a complicated manner. Under such guidance, we demonstrate that other asymmetric nanostructures, such as asymmetric nanowires, thin films, and quantum dots, of a single material are potentially high-performance thermal rectifiers. KEYWORDS: Thermal rectification, phonon lateral confinement, phonon localization, edge/surface effect, molecular dynamics, phonon spectra I nspired by the impact of electric diodes on the electronics industry, extensive attention has been given to the search of rectification of various other transport processes.1−3 Thermal rectification (TR) is a diode-like behavior where the heat current changes in magnitude when the applied temperature (T) bias is reversed in direction. A perfect thermal rectifier would be one that is highly thermal conductive in one direction while insulating in the other, and it is expected to work as a promising thermal management component of electronics as chip size continues decreasing or as a stand-alone thermally driven computing system replacing the electronic ones in certain conditions.Numerous studies have predicted or demonstrated the existence of TR in bulk or nanosized systems, most of which are heterojunctions (HJ) or graded systems.3−11 For twosegment systems, TR was usually attributed to the different Tdependence of the thermal conductivity (κ), 5,7 and for interfaces TR has been interpreted as the different phonon spectra mismatch before and after reversing the applied T bias. Phonon localization was suggested to play a role as well. 12,13 Recently, TR was also predicted to occur in asymmetric pristine carbon nanostructures, 13−17 which are composed of a single material and are attractive for their simple structure and high thermal conductance. 18 However, the origin of TR in such homogeneous nanostructures remains unclear. In this work, we have observed, using molecular dynamics and analytical derivations, that phonon confinement in the lateral dimension is required for TR to occur in asymmetric homogeneous structures made of a single material. We further show that...

The cross-linking of the B cell Ag receptor (BCR) is coupled to the stimulation of multiple intracellular signal transduction cascades via receptor-associated, protein tyrosine kinases of both the Src and Syk families. To monitor changes in the subcellular distribution of Syk in B cells responding to BCR cross-linking, we expressed in Syk-deficient DT40 B cells a fusion protein consisting of Syk coupled to green fluorescent protein. Treatment of these cells with anti-IgM Abs leads to the recruitment of the kinase from cytoplasmic and nuclear compartments to the site of the cross-linked receptor at the plasma membrane. The Syk-receptor complexes aggregate into membrane patches that redistribute to form a cap at one pole of the cell. Syk is not demonstrably associated with the internalized receptor. Catalytically active Syk promotes and stabilizes the formation of tightly capped BCR complexes at the plasma membrane. Lyn is not required for the recruitment of Syk to the cross-linked receptor, but is required for the internalization of the clustered BCR complexes. In the absence of Lyn, receptor-Syk complexes at the plasma membrane are long lived, and the receptor-mediated activation of the NF-AT transcription factor is enhanced. Thus, Lyn appears to function to negatively regulate aspects of BCR-dependent signaling by stimulating receptor internalization and down-regulation.