Bulletin of the American Physical Society
2017 Annual Meeting of the APS Mid-Atlantic Section
Volume 62, Number 19
Friday–Sunday, November 3–5, 2017; Newark, New Jersey
Session N2: CMP-QM: Topological Insulators and Semimetals II |
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Chair: Yi Li, Johns Hopkins University Room: 240, Campus Center, NJIT |
Sunday, November 5, 2017 12:15PM - 12:27PM |
N2.00001: Topological Hofstadter problem in four dimensions with quantized Hall response Canon Sun, Yi Li We generalize the Hofstadter problem of two-dimensional quantum Hall systems to a time-reversal invariant Hofstadter problem in four dimensions (4D) based on a 4D quantum Hall model, where spin-1/2 particles are coupled to a Landau-type SU(2) gauge field via spin-orbit coupling. The non-trivial topology is manifested through the spatial separation of (3$+$1)d surface Weyl modes with opposite chiralities. We further investigate the bulk-edge correspondence in this 4D Hofstadter problem and show the presence of a quantized Hall response under parallel E and B fields as a consequence of the (3$+$1)d chiral anomaly. A possible realization of the 4D Hofstadter system in ultra-cold atomic systems via synthetic dimension is also proposed. [Preview Abstract] |
Sunday, November 5, 2017 12:27PM - 12:39PM |
N2.00002: Symmetry induced non local currents, an application in anyon statistics Georgios Gounaris, Fotios Diakonos The use of symmetries is in the core of physics, playing a fundamental role in the theoretical treatment of any system. In this particular study we will examine the role of symmetry in the description of anyon statistics. We introduce the concept of non-local currents, which can be defined for potentials which are invariant under symmetry transformations and correlate the values of the wavefunction in symmetry related spatial domains. These currents occur pairwise and one member of the pair acts as an order parameter for symmetric systems being zero when the system is in an eigenstate of the corresponding symmetry transform and non-zero otherwise. The other member defines an operator carrying the information of the eigenvalue of the operator describing the associated symmetry transform when its partner vanishes. We take advantage of that property by applying this method in a 2-D highly symmetric system of two interacting anyons. We will prove by working in the anyon gauge that the eigenvalue of the mapping operator is the non trivial statistical phase of the anyons. To obtain this result no further assumptions are needed. This points out the importance of the symmetry as a general tool to treat the statistics of composite particles. [Preview Abstract] |
Sunday, November 5, 2017 12:39PM - 1:15PM |
N2.00003: Monopole Harmonic Superconductivity in Doped Weyl Semimetals Invited Speaker: Yi Li In this talk, I will present a dramatic effect of Fermi surface monopole structure on the superconducting pairing symmetry: When Cooper pairs acquire non-trivial two-particle Berry phases, their pairing phases cannot be globally well-defined in the momentum space. Therefore, the conventional description of superconducting pairing symmetries in terms of spherical harmonics (e.g. s-, p-, d-waves) ceases to apply. Instead, they are represented by monopole harmonic functions. These novel superconducting states can be realized in doped Weyl semi-metals. They exhibit topologically protected nodal gap functions and rich patterns of Majorana surface arcs regardless of concrete pairing mechanism. [Preview Abstract] |
Sunday, November 5, 2017 1:15PM - 1:27PM |
N2.00004: Josephson effect and Meissner response of monopole superconductivity in Weyl semimetals Shu-Ping Lee, Yi Li In this talk, we would like to propose experimental signatures of monopole superconductivity in time-reversal symmetry broken Weyl semimetals, where nontrivial Berry phases of Cooper pairs lead to novel nodal superconducting symmetry protected by topology. By calculating the energy-phase relation of a Josephson junction between superconducting Weyl semimetals, we show the zero-energy Majorana modes emerging at the junction interface that leads to a single electron tunneling with $4\pi$ fractional Josephson effect. The topologically protected Majorana boundary modes with divergent density of state can lead to paramagnetic Meissner response in the presence of a magnetic field that is in a sharp contrast to a typical s-wave superconductor with conventional diamagnetic Meissner effect. [Preview Abstract] |
Sunday, November 5, 2017 1:27PM - 1:39PM |
N2.00005: Quasiparticle Interference of Surface States in Type-II Weyl Semimetal WTe$_{\mathrm{2}}$ Wenhan Zhang, QuanSheng Wu, Lunyong Zhang, Sang-Wook Cheong, Alexey Soluyanov, Weida Wu Topological Weyl semimetal (TWS) is a metal, where low energy excitations behave like Weyl fermions of high-energy physics. It was recently shown that due to the lower symmetry of condensed matter systems, they can realize two distinct types of Weyl fermions. The type-I Weyl fermion in a metal is formed by a linear crossing of two bands at a point in the crystalline momentum space - Brillouin zone (BZ). The second type TWSs host type-II Weyl points appearing at the touching points of electron and hole pockets, which is a result of tilted linear dispersion. The type-II TWS was predicted to exist in several compounds, including WTe$_{\mathrm{2}}$. Several ARPES studies of WTe$_{\mathrm{2}}$ were reported so far, having contradictory conclusions on the topological nature of observed Fermi arcs. In this work, we report the results of spectroscopic imaging with a scanning tunneling microscope and first principle calculations, establishing clear quasiparticle interference features of the surface states of WTe$_{\mathrm{2}}$. Our work provides a strong evidence for surface state scattering. Although the surface Fermi arcs are clearly observed, it is still difficult to prove the existence of predicted Type-II Weyl points in the bulk. [Preview Abstract] |
Sunday, November 5, 2017 1:39PM - 2:15PM |
N2.00006: Direct optical detection of Weyl fermion chirality in a topological semimetal Invited Speaker: Nuh Gedik A Weyl semimetal (WSM) is a novel topological phase of matter, in which Weyl fermions (WFs) arise as pseudo-magnetic monopoles in its momentum space. The chirality of the WFs, given by the sign of the monopole charge, is central to the Weyl physics, since it directly serves as the sign of the topological number and gives rise to exotic properties such as Fermi arcs and the chiral anomaly. Despite being the defining property of a WSM, the chirality of the WFs has never been experimentally measured. Here, we directly detect the chirality of the WFs by measuring the photocurrent in response to circularly polarized mid-infrared light. The resulting photocurrent is determined by both the chirality of WFs and that of the photons. Our results pave the way for realizing a wide range of theoretical proposals for studying and controlling the WFs and their associated quantum anomalies by optical and electrical means. More broadly, the two chiralities, analogous to the two valleys in 2D materials, lead to a new degree of freedom in a 3D crystal with potential novel pathways to store and carry information. [Preview Abstract] |
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