7 research outputs found
Spectral Flow and Global Topology of the Hofstadter Butterfly
We study the relation between the global topology of the Hofstadter butterfly
of a multiband insulator and the topological invariants of the underlying
Hamiltonian. The global topology of the butterfly, i.e., the displacement of
the energy gaps as the magnetic field is varied by one flux quantum, is
determined by the spectral flow of energy eigenstates crossing gaps as the
field is tuned. We find that for each gap this spectral flow is equal to the
topological invariant of the gap, i.e., the net number of edge modes traversing
the gap. For periodically driven systems, our results apply to the spectrum of
quasienergies. In this case, the spectral flow of the sum of all the
quasienergies gives directly the Rudner invariant.Comment: 5 pages, 3 figure
Localization, delocalization, and topological transitions in disordered two-dimensional quantum walks
A Short Course on Topological Insulators: Band-structure topology and edge states in one and two dimensions
This course-based primer provides newcomers to the field with a concise introduction to some of the core topics in the emerging field of topological band insulators in one and two dimensions. The aim is to provide a basic understanding of edge states, bulk topological invariants, and of the bulk--boundary correspondence with as simple mathematical tools as possible. We use noninteracting lattice models of topological insulators, building gradually on these to arrive from the simplest one-dimensional case (the Su-Schrieffer-Heeger model for polyacetylene) to two-dimensional time-reversal invariant topological insulators (the Bernevig-Hughes-Zhang model for HgTe). In each case the model is introduced first and then its properties are discussed and subsequently generalized. The only prerequisite for the reader is a working knowledge in quantum mechanics, the relevant solid state physics background is provided as part of this self-contained text, which is complemented by end-of-chapter problems
Detecting topological invariants in chiral symmetric insulators via losses
We show that the bulk winding number characterizing one-dimensional
topological insulators with chiral symmetry can be detected from the
displacement of a single particle, observed via losses. Losses represent the
effect of repeated weak measurements on one sublattice only, which interrupt
the dynamics periodically. When these do not detect the particle, they realize
negative measurements. Our repeated measurement scheme covers both
time-independent and periodically driven (Floquet) topological insulators, with
or without spatial disorder. In the limit of rapidly repeated, vanishingly weak
measurements, our scheme describes non-Hermitian Hamiltonians, as the lossy
Su-Schrieffer-Heeger model of Phys. Rev. Lett. 102, 065703 (2009). We find,
contrary to intuition, that the time needed to detect the winding number can be
made shorter by decreasing the efficiency of the measurement. We illustrate our
results on a discrete-time quantum walk, and propose ways of testing them
experimentally.Comment: 4.5 pages, 3 figures + 4 pages of Supplemental Materia
Medencegyűrű-sĂ©rĂĽlĂ©sek műtĂ©ti rögzĂtĂ©sĂ©nek vĂ©geselemes modellezĂ©se
A vertikálisan Ă©s rotáciĂłban is instabil C tĂpusĂş medencegyĂ»rĂ»-sĂ©rĂĽlĂ©sek rögzĂtĂ©sĂ©re használt kĂĽlönbözõ lemezes rögzĂtĂ©si technikák stabilitási vizsgálata az általunk kidolgozott vĂ©geselemes medencemodellen. KĂ©t lábon állás mellett C tĂpusĂş medencegyĂ»rĂ»-sĂ©rĂĽlĂ©st hoztunk lĂ©tre Ăşgy, hogy Denis I., illetve Denis II. keresztcsont (sacrum) törĂ©st Ă©s a szemĂ©remcsonti ĂzĂĽlet szakadását (symphyseolysist) modelleztĂĽnk. A symphyseolysist 4 lyukas rekonstrukciĂłslemezzel, a Denis I. sacrumtörĂ©st 2 db 2 lyukas rekonstukciĂłs lemezzel stabilizáltuk a kismedence felõl (ventralisan), majd az általunk használt transsacralisan, hátulrĂłl (dorsalisan) felhelyezett keskeny, illetve szĂ©les DC-lemezzel. A Denis II. sacrumtörĂ©st ventral felõl ugyancsak2 db 2 lyukas rekonsrukciĂłs lemezzel fixáltuk, majd dorsal felõl KFI-H-lemezes rögzĂtĂ©st modelleztĂĽnk. A vĂ©geselemes modellezĂ©s ALGOR programmal törtĂ©nt. A medencĂ©t alkotĂł csontok mellett az ĂzĂĽleteket Ă©s a mechanikai szempontbĂłl jelentõs szalagokat is modelleztĂĽk. A modell validálása megtörtĂ©nt, párhuzamosan vĂ©gzett hullai csont-szalagos preparátumokon vĂ©gzett mĂ©rĂ©si eredmĂ©nyekkel. A törĂ©si rĂ©s kĂ©t oldala közötti elmozdulást, valamint a rögzĂtõfĂ©mekben Ă©s a környezõ csontokban fellĂ©põ feszĂĽltsĂ©geket mĂ©rtĂĽk. A transsacralis lemezes synthesis mellett nagyobb elmozdulások mĂ©rhetõk, mint direkt lemezes rögzĂtĂ©s esetĂ©n. A KFIH-lemezekkel vĂ©gzett mĂ»tĂ©t stabilitása a direkt lemezes Ă©s a transsacralis lemezes synthesis stabilitása között van. Az eredmĂ©nyek korrelálnak a párhuzamosan elvĂ©gzett csont-szalagos hullai medencepreparátumokon vĂ©gzett mĂ©rĂ©sek eredmĂ©nyeivel. A vĂ©geselemes modell alkalmas a fentleĂrt sĂ©rĂĽlĂ©seket rögzĂtõ synthesisformák összehasonlĂtására. Mivel a cadaver-preparátumokon vĂ©gzett vizsgálatok számos nehĂ©zsĂ©gbe ĂĽtköznek, a modell használatának lĂ©tjogosultságavitathatatlan.A vertikálisan Ă©s rotáciĂłban is instabil C tĂpusĂş medencegyĂ»rĂ»-sĂ©rĂĽlĂ©sek rögzĂtĂ©sĂ©re használt kĂĽlönbözõ lemezes rögzĂtĂ©si technikák stabilitási vizsgálata az általunk kidolgozott vĂ©geselemes medencemodellen. KĂ©t lábon állás mellett C tĂpusĂş medencegyĂ»rĂ»-sĂ©rĂĽlĂ©st hoztunk lĂ©tre Ăşgy, hogy Denis I., illetve Denis II. keresztcsont (sacrum) törĂ©st Ă©s a szemĂ©remcsonti ĂzĂĽlet szakadását (symphyseolysist) modelleztĂĽnk. A symphyseolysist 4 lyukas rekonstrukciĂłs lemezzel, a Denis I. sacrumtörĂ©st 2 db 2 lyukas rekonstukciĂłs lemezzel stabilizáltuk a kismedence felõl (ventralisan), majd az általunk használt transsacralisan, hátulrĂłl (dorsalisan) felhelyezett keskeny, illetve szĂ©les DC-lemezzel. A Denis II. sacrumtörĂ©st ventral felõl ugyancsak 2 db 2 lyukas rekonsrukciĂłs lemezzel fixáltuk, majd dorsal felõl KFI-H-lemezes rögzĂtĂ©st modelleztĂĽnk. A vĂ©geselemes modellezĂ©s ALGOR programmal törtĂ©nt. A medencĂ©t alkotĂł csontok mellett az ĂzĂĽleteket Ă©s a mechanikai szempontbĂłl jelentõs szalagokat is modelleztĂĽk. A modell validálása megtörtĂ©nt, párhuzamosan vĂ©gzett hullai csont-szalagos preparátumokon vĂ©gzett mĂ©rĂ©si eredmĂ©nyekkel. A törĂ©si rĂ©s kĂ©t oldala közötti elmozdulást, valamint a rögzĂtõ fĂ©mekben Ă©s a környezõ csontokban fellĂ©põ feszĂĽltsĂ©geket mĂ©rtĂĽk. A transsacralis lemezes synthesis mellett nagyobb elmozdulások mĂ©rhetõk, mint direkt lemezes rögzĂtĂ©s esetĂ©n. A KFIH-lemezekkel vĂ©gzett mĂ»tĂ©t stabilitása a direkt lemezes Ă©s a transsacralis lemezes synthesis stabilitása között van. Az eredmĂ©nyek korrelálnak a párhuzamosan elvĂ©gzett csont-szalagos hullai medencepreparátumokon vĂ©gzett mĂ©rĂ©sek eredmĂ©nyeivel. A vĂ©geselemes modell alkalmas a fent leĂrt sĂ©rĂĽlĂ©seket rögzĂtõ synthesisformák összehasonlĂtására. Mivel a cadaver-preparátumokon vĂ©gzett vizsgálatok számos nehĂ©zsĂ©gbe ĂĽtköznek, a modell használatának lĂ©tjogosultsága vitathatatlan. DOI: 10.17489/biohun/2008/1/3
Scattering theory of topological phases in discrete-time quantum walks
One-dimensional discrete-time quantum walks show a rich spectrum of topological phases that have so far been exclusively analysed based on the Floquet operator in momentum space. In this work we introduce an alternative approach to topology which is based on the scattering matrix of a quantum walk, adapting concepts from time-independent systems. For quantum walks with gaps in the quasienergy spectrum at 0 and π, we find three different types of topological invariants, which apply dependent on the symmetries of the system. These determine the number of protected boundary states at an interface between two quantum walk regions. Unbalanced quantum walks on the other hand are characterised by the number of perfectly transmitting unidirectional modes they support, which is equal to their non-trivial quasienergy winding. Our classification provides a unified framework that includes all known types of topology in one dimensional discrete-time quantum walks and is very well suited for the analysis of finite size and disorder effects. We provide a simple scheme to directly measure the topological invariants in an optical quantum walk experiment