报告摘要：

We discuss two topics where geometry and topology play a crucial role. In the first one we present a calculation of the effective geometry-induced quantum potential for the carriers in graphene shaped as a helicoidal nanoribbon1. In this geometry the twist of the nanoribbon plays the role of an effective transverse electric field in graphene and this is reminiscent of the Hall effect. However, this effective electric field has a different sign for the two iso-spin states and translates into a mechanism to separate the two chiral species on the opposing rims of the nanoribbon. Iso-spin transitions are expected with the emission or absorption of microwave radiation which could be adjusted to be in the THz region. In the second topic we consider Dirac strings and monopole-like excitations in chiral magnets under a current drive2. In these non-centrosymmetric materials skyrmion lines carry emergent magnetic field, which is experienced by conduction electrons. The inflow and outflow of this field across a closed surface is not necessarily equal, thus it allows for the existence of emergent monopoles. One example is a segment of skyrmion line inside a crystal, where a monoplole and antimonopole pair is connected by the emergent magnetic flux line. This is a realization of Dirac string-like excitations. We study the dynamics of monopoles in chiral magnets under an electric current and show that in the process of creation of skyrmion lines, segments of skyrmion lines are first created via the proliferation of monopoles and antimonopoles.  Then these line segments join and span the whole system through the annihilation of monopoles. At high currents the skyrmion lines are destroyed via the proliferation of monopoles and antimonopoles resulting in a chiral liquid phase. The existence of monopoles in chiral magnets can be inferred from transport or imaging measurements.

报告人简介：

Prof. Avadh Saxena is Group Leader of the Condensed Matter and Complex Systems group (T-4) at Los Alamos National Lab, New Mexico, USA, where he has been since 1990. He is also an affiliate of the Center for Nonlinear Studies at Los Alamos. His main research interests include phase transitions, optical, electronic, vibrational, transport and magnetic properties of functional materials, device physics, soft condensed matter, geometry, topology and nonlinear phenomena. He is an Affiliate Professor at the Royal Institute of Technology (RTH), Stockholm, Sweden and holds adjunct professor positions at the University of Barcelona, Spain, Virginia Tech and the University of Arizona, Tucson. He is Scientific Advisor to National Institute for Materials Science (NIMS), Tsukuba, Japan. He is a Fellow of Los Alamos National Lab, a Fellow of the American Physical Society (APS), and a member of the Sigma Xi Scientific Research Society and APS.

Contact him at: avadh@lanl.gov