Quantum spin circulator in Y junctions of Heisenberg chains

We show that a quantum spin circulator, a nonreciprocal device that routes spin currents without any charge transport, can be achieved in Y junctions of identical spin-$1/2$ Heisenberg chains coupled by a chiral three-spin interaction. Using bosonization, boundary conformal field theory, and density-matrix renormalization group simulations, we find that a chiral fixed point with maximally asymmetric spin conductance arises at a critical point separating a regime of disconnected chains from a spin-only version of the three-channel Kondo effect. We argue that networks of spin-chain Y junctions provide a controllable approach to construct long-sought chiral spin liquid phases.Comment: 9 pages, 3 figure

Nonlinear photocurrents in two-dimensional systems based on graphene and boron nitride

DC photoelectrical currents can be generated purely as a non-linear effect in uniform media lacking inversion symmetry without the need for a material junction or bias voltages to drive it, in what is termed photogalvanic effect. These currents are strongly dependent on the polarization state of the radiation, as well as on topological properties of the underlying Fermi surface such as its Berry curvature. In order to study the intrinsic photogalvanic response of gapped graphene (GG), biased bilayer graphene (BBG), and hexagonal boron nitride (hBN), we compute the non-linear current using a perturbative expansion of the density matrix. This allows a microscopic description of the quadratic response to an electromagnetic field in these materials, which we analyze as a function of temperature and electron density. We find that the intrinsic response is robust across these systems and allows for currents in the range of pA cm/W to nA cm/W. At the independent-particle level, the response of hBN-based structures is significant only in the ultra-violet due to their sizeable band-gap. However, when Coulomb interactions are accounted for by explicit solution of the Bethe-Salpeter equation, we find that the photoconductivity is strongly modified by transitions involving exciton levels in the gap region, whose spectral weight dominates in the overall frequency range. Biased bilayers and gapped monolayers of graphene have a strong photoconductivity in the visible and infrared window, allowing for photocurrent densities of several nA cm/W. We further show that the richer electronic dispersion of BBG at low energies and the ability to change its band-gap on demand allows a higher tunability of the photocurrent, including not only its magnitude but also, and significantly, its polarity.Comment: Updating with published version and respective references; 14 pages, 11 figure

Navegação e Desvio de Obstáculos Usando um Robô Móvel Dotado de Sensor de Varredura Laser

Este trabalho trata da navegação de robôs móveis em ambientes semi-estruturados. Para isso, foram desenvolvidos controladores baseados na abordagem reativa. Os controladores implementados permitem que o robô navegue de forma segura entre dois pontos e por corredores, evitando os obstáculos que surgirem no seu caminho. São apresentados dois métodos de desvio de obstáculos, um tangencial e outro não tangencial. As informações sobre o ambiente de trabalho do robô são proporcionadas por um sensor de varredura laser instalado a bordo do robô móvel utilizado, o PIONEER 2-DX, da ActivMedia. Os controladores desenvolvidos são validados através de simulações e experimentos reais, sempre utilizando o robô móvel equipado com o sensor laser de varredura. Boa parte dos controladores aqui apresentados já foram apresentados em outros trabalhos, porém utilizando outros sensores para propiciar ao sistema de controle as informações sobre o ambiente. Neste sentido, a contribuição deste trabalho é explorar as vantagens de sua utilização com o sensor laser de varredura

Coupled electronic and morphologic changes in graphene oxide upon electrochemical reduction

Peer reviewedPostprin