Spin current generation and control in carbon nanotubes by combining rotation and magnetic field.

We study the quantum dynamics of ballistic electrons in rotating carbon nanotubes in the presence of a uniform magnetic field. When the field is parallel to the nanotube axis, the rotation-induced electric field brings about the spin-orbit interaction which, together with the kinetic, inertial, and Zeeman terms, compose the Schrödinger-Pauli Hamiltonian of the system. Full diagonalization of this Hamiltonian yields the eigenstates and eigenenergies leading to the calculation of the charge and spin currents. Our main result is the demonstration that, by suitably combining the applied magnetic field intensity and rotation speed, one can tune one of the currents to zero while keeping the other one finite, giving rise to a spin current generator.

Thermal and shape topological robustness of heat switchers using nematic liquid crystals.

One interesting way to control heat is to use devices designed by transformation thermics, where artificial media are used. However, once manufactured (either repelling or concentrating heat, for example), besides being mono-purpose, such devices are designed according to a specific geometric boundary conditions. Another problem is the temperature dependence of the materials employed, since their properties are sometimes considered temperature-invariant. In this paper, we show that a previously proposed bi-objective heat switcher (Phys. Rev. E 89, 020501(R) (2014)) is in fact robust against temperature and geometric deformations, due to the topological properties of the molecular nematic orientation. Using a geometrical approach for heat propagation, by performing finite element simulations, we show that a device made by concentric cylinders with thermotropic nematic liquid crystal between them, sustains its functionality even with their molecular thermal conductivities depending on the temperature, achieving a 60% increase and a 44% decrease in the heat flux for each mode. Utilizing topological arguments we show that deformations on the surface of the outer cylinder do not break the operating mode (repeller or concentrator). We present a comparison between our geometrical approach and the transformation thermodynamics to give an additional explanation for the obtained results. We hope the presented device is useful for heat control under mechanical and thermal influence of the external environment.

Retrieving the saddle-splay elastic constant K24 of nematic liquid crystals from an algebraic approach.

The physics of light interference experiments is well established for nematic liquid crystals. Using well-known techniques, it is possible to obtain important quantities, such as the differential scattering cross section and the saddl-splay elastic constant K24. However, the usual methods to retrieve the latter involve adjusting of computational parameters through visual comparisons between the experimental light interference pattern or a (2) H-NMR spectral pattern produced by an escaped-radial disclination, and their computational simulation counterparts. To avoid such comparisons, we develop an algebraic method for obtaining of saddle-splay elastic constant K24. Considering an escaped-radial disclination inside a capillary tube with radius R0 of tens of micrometers, we use a metric approach to study the propagation of the light (in the scalar wave approximation), near the surface of the tube and to determine the light interference pattern due to the defect. The latter is responsible for the existence of a well-defined interference peak associated to a unique angle [Formula: see text] . Since this angle depends on factors such as refractive indexes, curvature elastic constants, anchoring regime, surface anchoring strength and radius R0, the measurement of [Formula: see text] from the interference experiments involving two different radii allows us to algebraically retrieve K24. Our method allowed us to give the first reported estimation of K24 for the lyotropic chromonic liquid crystal Sunset Yellow FCF: K 24 = 2.1 pN.

Geometric effects in the electronic transport of deformed nanotubes.

Quasi-two-dimensional systems may exibit curvature, which adds three-dimensional influence to their internal properties. As shown by da Costa (1981 Phys. Rev. A 23 1982-7), charged particles moving on a curved surface experience a curvature-dependent potential which greatly influence their dynamics. In this paper, we study the electronic ballistic transport in deformed nanotubes. The one-electron Schrödinger equation with open boundary conditions is solved numerically with a flexible MAPLE code made available as supplementary data. We find that the curvature of the deformations indeed has strong effects on the electron dynamics, suggesting its use in the design of nanotube-based electronic devices.

Applying a new computational method for biological tissue optics based on the time-dependent two-dimensional radiative transfer equation.

Optical tomography is a medical imaging technique based on light propagation in the near infrared (NIR) part of the spectrum. We present a new way of predicting the short-pulsed NIR light propagation using a time-dependent two-dimensional-global radiative transfer equation in an absorbing and strongly anisotropically scattering medium. A cell-vertex finite-volume method is proposed for the discretization of the spatial domain. The closure relation based on the exponential scheme and linear interpolations was applied for the first time in the context of time-dependent radiative heat transfer problems. Details are given about the application of the original method on unstructured triangular meshes. The angular space (4πSr) is uniformly subdivided into discrete directions and a finite-differences discretization of the time domain is used. Numerical simulations for media with physical properties analogous to healthy and metastatic human liver subjected to a collimated short-pulsed NIR light are presented and discussed. As expected, discrepancies between the two kinds of tissues were found. In particular, the level of light flux was found to be weaker (inside the medium and at boundaries) in the healthy medium than in the metastatic one.