We investigate atomic scale friction between clean graphite surfaces by using molecular dynamics. The simulation reproduces atomic scale stick-slip motion and low frictional coefficient, both of which are observed in experiments using frictional force microscope. It is made clear that the microscopic origin of low frictional coefficients of graphite lies on the honeycomb structure in each layer, not only on the weak interlayer interaction as believed so far.

We propose an electroweakly interacting spin-1 dark matter (DM) model. The electroweak gauge symmetry, SU(2)L×U(1)Y , is extended into SU(2)0×SU(2)1×SU(2)2×U(1)Y . A discrete symmetry exchanging SU(2)0 and SU(2)2 is imposed. This discrete symmetry stabilizes the DM candidate. The spin-1 DM particle (V0) and its SU(2)L partners (V±) interact with the Standard Model (SM) electroweak gauge bosons without any suppression factors. Consequently, pairs of DM particles efficiently annihilate into the SM particles in the early universe, and the measured value of the DM energy density is easily realized by the thermal freeze-out mechanism. The model also predicts a heavy vector triplet (W′± and Z′) in the visible sector. They contribute to the DM annihilation processes. The mass ratio of Z′ and V0 determines values of various couplings, and constraints on W′ and Z′ restrict regions of the parameter space that are viable for DM physics. We investigate the constraints from perturbative unitarity of scalar and gauge couplings, the Higgs signal strength, W′ search at the LHC, and DM direct detection experiments. It is found that the relic abundance of V0 explains the right amount of the DM energy density for 3 TeV $$ \underset{\sim }{<}{m}_Vo\underset{\sim }{<} $$ < ∼ m V o < ∼ 19 TeV.

By molecular dynamics simulation, we have investigated classical Heisenberg spins, which are arrayed on a finite simple cubic lattice and interact with each other only by the dipoledipole interaction, and have found its peculiar from-Edge-to-interior freezing process. As the temperature is decreased, spins on each edge predominantly start to freeze in a ferromagnetic alignment parallel to the edge around the corresponding bulk transition temperature, then from each edges grow domains with short-range orders similar to the corresponding bulk orders, and the system ends up with a unique multi-domain ground state at the lowest temperature. We interpret this freezing characteristics is attributed to the anisotropic and long-range nature of the dipole-dipole interaction combined with a finite-size effect.In the last decade systems consisting of arrayed ferromagnetic nanoparticles have attracted much attention as a possible element with the high storage density. 1 Magnetic properties of such systems have been analyzed based on various theoretical model, in which the magnetic moment of each nanoparticle is represented by a classical Heisenberg spin with a proper magnitude. The spins are interacting with each other by the dipole-dipole interaction, and each spin suffers from the magnetic anisotropy energy which represents the shape and/or bulk lattice anisotropies of the original nanoparticle magnetic moments. There appeared studies on the susceptibility, 2 magnetic hysteresis, 3, 4 and energy relaxation 5 in such models of a finite size. However, there have been little work on systems only with the dipole-dipole interaction which we call here simply dipolar systems. Considering that to clarify magnetic properties of dipolar systems of a finite size is of importance as the first step in researches of arrayed magnetic nanoparticles as well as of interest from a theoretical viewpoint in statistical physics, *

We studied the current-induced magnetization dynamics of a domain wall confined in a magnetic wire with bi-axial anisotropy. We showed that above the threshold current density, breathing-mode excitation, where the thickness of the domain wall oscillates, is induced by spin-transfer torque.We found that the breathing-mode can be applied as a source of microwave oscillation because the resistance of the domain wall is a function of the domain wall thickness. In a current sweep simulation, the frequency of the breathing-mode exhibits hysteresis because of the confinement.

Plant cells are surrounded by a cell wall and do not migrate, which makes the regulation of cell division orientation crucial for development. Regulatory mechanisms controlling cell division orientation may have contributed to the evolution of body organization in land plants. The GRAS family of transcription factors was transferred horizontally from soil bacteria to an algal common ancestor of land plants. SHORTROOT ( SHR ) and SCARECROW ( SCR ) genes in this family regulate formative periclinal cell divisions in the roots of flowering plants, but their roles in nonflowering plants and their evolution have not been studied in relation to body organization. Here, we show that SHR cell autonomously inhibits formative periclinal cell divisions indispensable for leaf vein formation in the moss Physcomitrium patens , and SHR expression is positively and negatively regulated by SCR and the GRAS member LATERAL SUPPRESSOR , respectively. While precursor cells of a leaf vein lacking SHR usually follow the geometry rule of dividing along the division plane with the minimum surface area, SHR overrides this rule and forces cells to divide nonpericlinally. Together, these results imply that these bacterially derived GRAS transcription factors were involved in the establishment of the genetic regulatory networks modulating cell division orientation in the common ancestor of land plants and were later adapted to function in flowering plant and moss lineages for their specific body organizations.