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We investigate coupled excitonic and vibrational effects in carbon nanotubes using a time-dependent Hartree-Fock approach. The results reveal intricate details of excited-state dynamics. In the ground state, spontaneous uneven distribution of the pi electrons over the bonds (i.e., Peierls dimerization) is observed throughout the entire nanotube, particularly in large-radius CNTs. However, we demonstrate that vibrational relaxation following photoexcitations leads to substantial local distortion of the tube surface, overriding the Peierls dimerization. This mutually affects the electronic system, resulting in localized states (self-trapped excitons). These phenomena critically control photoinduced dynamics and charge transport in nanotube materials.

A skyrmion in a nanosized disk of a chiral magnet can be used as a bit of information. To this end, it is desirable to control the creation and removal of a skyrmion only by currents without using external magnetic fields. Here we propose to create a skyrmion by applying a current pulse to a nanodisk. The skyrmion can be removed from the disk by applying a dc current. We show that the dynamics of the created skyrmion can lead to a rectification effect, in which a dc voltage is generated by the motion of skyrmion in the presence of an ac current. PACS numbers: 75.10.Hk, 75.30.Kz, A new spin texture called skyrmion has been found in certain chiral magnets without inversion symmetry, such as MnSi and Fe 0.5 Co 0.5 Si. [1-7] A skyrmion can be described in terms of spins which wrap a sphere. When a conduction electron passes through the skyrmion, its spin is fully polarized by the spin texture of the skyrmion, yielding a quantized Berry phase 4π that corresponds to a quantized magnetic flux Φ = hc/e. This emergent electromagnetic field significantly affects the dynamics of the skyrmions and produces a Magnus force perpendicular to the skyrmion velocity. [8,9] Application of a spin polarized current exerts a spin transfer torque on the skyrmion, causing it to move at an angle to the current direction determined by the dissipation.[10] The resulting voltage is perpendicular to the velocity; thus, a Hall (transverse) voltage is induced, which has been measured experimentally. [11] Since skyrmions are stable topological excitations in chiral magnets, they have promising applications in spintronics. [12] Due to the presence of the Magnus force [8,9], the threshold current to drive the skyrmion from the pinning sites is extremely small, 4 − 5 orders of magnitude smaller than that for magnetic domain walls. [11,13,14] Thus, skyrmions have tremendous advantages for information storage applications, because the lower current implies less Joule heating. From a technological point of view, it is desirable to be able to manipulate, create, or destroy skyrmions by applying only a current without using an external magnetic field. This requires use of magnetic disk that is is typically the same size as the skyrmion, of the order of 100 nm, such that only one skyrmion can be accommodated in the disk. The goal of producing a single magnetic vortex in a nanodisk was recently achieved [15,16]. In most known chiral magnets, the skyrmion is not stable without an external magnetic field, and in the absence of a field a spiral structure is favored. Upon increasing the magnetic field, a skyrmion state becomes the ground state.[17] Two immediate questions regarding the dynamic creation of skyrmions arise: first, how to stabilize skyrmions without magnetic fields, and secondly, how to create a skyrmion using currents.The answer to the first question is to exploit the magnetic anisotropy in thin films. [18] Due to the reduced dimensionality, an out-of-plane anisotropy develops that is inversely proportional to the film thickness. Such anisotrop...

Investigation of a great number of physical systems shows that a Landau free energy of the form F(φ)=Hφ+(A/2)φ2+(B/3)φ3+(C/4)φ4 describes a first-order phase transition in an internal or external field H. To study the formation of static domain walls in these systems we include a spatial gradient (Ginzburg) term of the scalar order parameter φ. From the variational derivative of the total free energy we obtain a static equilibrium condition. By solving this equation exactly for different physical parameters and boundary conditions, we obtained different quasi-one-dimensional soliton-like solutions. These solutions correspond to three different types of domain walls between the two different phases which are created in the system. In addition, we obtain soliton lattice (domain wall array) solutions, calculate their formation energy and the asymptotic interaction between the solitons. By introducing certain transformations, we show that the solutions obtained here can be used to study domain walls in other physical systems such as described by asymmetric double Morse potentials. Finally, we apply our results to the specific cases of liquid crystals and the jam phenomena in traffic flows.