DSpace Community:http://hdl.handle.net/10174/252020-05-26T10:51:42Z2020-05-26T10:51:42ZChanging the Topology of Electronic Systems Through Interactions or DisorderAraújo, MiguelCastro, EduardoSacramento, Pedrohttp://hdl.handle.net/10174/272392020-02-26T11:35:11Z2019-03-12T00:00:00ZTitle: Changing the Topology of Electronic Systems Through Interactions or Disorder
Authors: Araújo, Miguel; Castro, Eduardo; Sacramento, Pedro
Editors: Huixia, Luo
Abstract: Recent work on the influence of interactions or disorder on the topological properties of insulators is reviewed. We show that a purely local interaction can cause topological transitions by renormalizing kinetic energy terms alone, without phase transitions associated with order parameters. Disorder is also a mean of changing the topology of Chern insulators, as it localizes every state except for those carrying the topological invariant. With increasing disorder, states with opposite topological invariant meet and annihilate. But considering the sub-lattice degree of freedom, Chern insulators may evade localization: an anomalous Hall metal may be stabilized with strong disorder in one sublattice, while the disorder in the other sublattice remains below some critical value. Also, a ferromagnetically ordered chain of magnetic impurities on conventional singlet superconductors with spin-orbit interaction induces topological Majorana modes at the ends of the chain. A ferromagnetic chain at the surface of a superconductor with spin–orbit coupling may eliminate the helical edge states of a finite system, while localized zero modes at the edges of the chain appear. Increasing the impurity density from a magnetic chain to an island produces a finite Chern number. At half-filling small concentrations may already induce chiral modes.2019-03-12T00:00:00ZMechanisms of Acoustic Surface Wave Generation by Rigid Cyllinder ArraysBerry, Davidhttp://hdl.handle.net/10174/272282020-02-26T11:15:54Z2019-04-30T23:00:00ZTitle: Mechanisms of Acoustic Surface Wave Generation by Rigid Cyllinder Arrays
Authors: Berry, David
Abstract: The propagation of an acoustic impulse over a surface constructed from regularly spaced rigid cylinders on a rigid plane has been simulated using multiple scattering theory. The simulations show that the surface causes a complex signal comprising a main pulse and a tail. The tail has a rich frequency spectrum with peaks that can, in general, be interpreted using Bragg diffraction theory. However, this interpretation is not valid for the lowest frequency peak. The spacing between the rigid pipes generates an “organ-pipe” type resonance. Above a height corresponding to three times the cylinder diameter these resonances combine to form a signal, demonstrating the characteristics of a surface wave. Results of experimental measurements confirm this interpretation.2019-04-30T23:00:00ZOn Generation and Propagation of Acoustic Surface Waves over Rough, Periodic SurfacesBerry, Davidhttp://hdl.handle.net/10174/272112020-02-21T15:26:08Z2019-09-10T23:00:00ZTitle: On Generation and Propagation of Acoustic Surface Waves over Rough, Periodic Surfaces
Authors: Berry, David
Abstract: An acoustic surface wave is an evanescent wave that spreads cylindrically along the surface but decays exponentially with height above the surface. This wave travels at a little less than the speed of sound in air and is primarily related to the pore or surface. Surface waves are generated also when sound propagates over rough hard surface. This phenomenon is related to coherent multiple scattering from roughness elements. This paper investigates conditions under which surface waves are generated and propagated and the relationship between this surface wave frequency and the geometry are explored by measurements of the sound field due to a point source above periodically-spaced rigid strips placed on a rigid surface and by a Boundary Element Method in time-domain.2019-09-10T23:00:00ZAcoustic Surface Wave Generation over Rigid Cylinder Arrays on a Rigid PlaneBerry, Davidhttp://hdl.handle.net/10174/271702020-02-20T13:27:16Z2019-09-30T23:00:00ZTitle: Acoustic Surface Wave Generation over Rigid Cylinder Arrays on a Rigid Plane
Authors: Berry, David
Abstract: Propagation of an airborne acoustic pulse from a point source above an array of regularly spaced rigid cylinders on a rigid plane has been investigated using a two-dimensional multiple scattering theory. Time domain simulations show a main arrival and a separate delayed “tail.” Fourier analysis of the tail shows that, for a sufficiently sparse array of cylinders, it is composed of a series of spectral peaks resulting from constructive interference consistent with Bragg diffraction theory and amplitudes depending on the spacing and size of the cylinders. For increasingly compact distributions of cylinders, the lowest frequency peak is dominated by a quarter wavelength “organ pipe” or “gap” resonance in the space between the cylinders. Simulated pressure maps show that there is a transition region in the acoustic field with an extent that depends on the spacing and size of the cylinders.
Beyond this region, individual gap resonances combine to create a field that declines exponentially with height, consistent with the behaviour of a surface wave. Data from measurements of acoustic pulses above copper cylinders on rigid fibreboard under anechoic conditions demonstrate
some of the predicted characteristics.2019-09-30T23:00:00Z