arXiv subjects

Shengyuan A. Yang

Publications and source records attributed to Shengyuan A. Yang.

241 records · Page 14Linked to original sources

arXiv2011

Spin Polarized and Valley Helical Edge Modes in Graphene Nanoribbons

Inspired by recent progress in fabricating precisely zigzag-edged graphene nanoribbons and the observation of edge magnetism, we find that spin polarized edge modes with well-defined valley index can exist in a bulk energy gap opened by a staggered sublattice potential such as that provided by a hexagonal Boron-Nitride substrate. Our result is obtained by both tight-binding model and first principles calculations. These edge modes are helical with respect to the valley degree of freedom, and are robust against scattering, as long as the disorder potential is smooth over atomic scale, resulting from the protection of the large momentum separation of the valleys.

cond-mat.mtrl-sci↗

arXiv2010

Scattering universality classes of side jump in anomalous Hall effect

The anomalous Hall conductivity has an important extrinsic contribution known as side jump contribution, which is independent of both scattering strength and disorder density. Nevertheless, we discover that side jump has strong dependence on the spin structure of the scattering potential. We propose three universality classes of scattering for the side jump contribution, having the characters of being spin-independent, spin-conserving and spin-flip respectively. For each individual class, the side jump contribution takes a different unique value. When two or more classes of scattering are present, the value of side jump is no longer fixed but varies as a function of their relative disorder strength. As system control parameter such as temperature changes, due to the competition between different classes of disorder scattering, the side jump Hall conductivity could flow from one class dominated limit to another class dominated limit. Our result indicates that magnon scattering plays a role distinct from normal impurity scattering and phonon scattering in the anomalous Hall effect because they belong to different scattering classes.

cond-mat.mes-hall↗

arXiv2010

Theory of I-V Characteristics of Magnetic Josephson Junctions

We analyze the electrical characteristics of a circuit consisting of a free thin-film magnetic layer and source and drain electrodes that have opposite magnetization orientations along the free magnet's two hard directions. We find that when the circuit's current exceeds a critical value there is a sudden resistance increase which can be large in relative terms if the currents to source or drain are strongly spin polarized and the free magnet is thin. This behavior can be partly understood in terms of a close analogy between the magnetic circuit and a Josephson junction.

cond-mat.mtrl-sci↗

arXiv2010

Quantum Anomalous Hall Effect in Graphene from Rashba and Exchange Effects

We investigate the possibility of realizing quantum anomalous Hall effect in graphene. We show that a bulk energy gap can be opened in the presence of both Rashba spin-orbit coupling and an exchange field. We calculate the Berry curvature distribution and find a non-zero Chern number for the valence bands and demonstrate the existence of gapless edge states. Inspired by this finding, we also study, by first principles method, a concrete example of graphene with Fe atoms adsorbed on top, obtaining the same result.

cond-mat.mtrl-sci↗

arXiv2009

Electron-Magnon Scattering in Anomalous Hall Effect

We study the role played by electron-magnon scattering in the anomalous Hall effect. We find that it has important contributions distinct from other scattering processes like impurities scattering and phonon scattering. As a demonstration, we calculate the Hall conductivity for a two dimensional Dirac model. The result indicates that as system control parameter varies, the competition between magnon scattering and other types of scattering changes the Hall conductivity drastically. In particular, the side jump contribution could acquire a strong temperature dependence.

cond-mat.mtrl-sci↗

arXiv2008

Edge states in Graphene: from gapped flat band to gapless chiral modes

We study edge-states in graphene systems where a bulk energy gap is opened by inversion symmetry breaking. We find that the edge-bands dispersion can be controlled by potentials applied on the boundary with unit cell length scale. Under certain boundary potentials, gapless edge-states with valley-dependent velocity are found, exactly analogous to the spin-dependent gapless chiral edge-states in quantum spin Hall systems. The connection of the edge-states to bulk topological properties is revealed.

cond-mat.mes-hall↗

arXiv2008

Theory of Electromotive Force Induced by Domain Wall Motion

We formulate a theory on the dynamics of conduction electrons in the presence of moving magnetic textures in ferromagnetic materials. We show that the variation of local magnetization in both space and time gives rise to topological fields, which induce electromotive forces on the electrons. Universal results are obtained for the emf induced by both transverse and vortex domain walls traveling in a magnetic film strip, and their measurement may provide clear characterization on the motion of such walls.

cond-mat.mtrl-sci↗