456 research outputs found
Flat bands with higher Chern number in pyrochlore slabs
A large number of recent works point to the emergence of intriguing analogs
of fractional quantum Hall states in lattice models due to effective
interactions in nearly flat bands with Chern number C=1. Here, we provide an
intuitive and efficient construction of almost dispersionless bands with higher
Chern numbers. Inspired by the physics of quantum Hall multilayers and
pyrochlore-based transition-metal oxides, we study a tight-binding model
describing spin-orbit coupled electrons in N parallel kagome layers connected
by apical sites forming N-1 intermediate triangular layers (as in the
pyrochlore lattice). For each N, we find finite regions in parameter space
giving a virtually flat band with C=N. We analytically express the states
within these topological bands in terms of single-layer states and thereby
explicitly demonstrate that the C=N wave functions have an appealing structure
in which layer index and translations in reciprocal space are intricately
coupled. This provides a promising arena for new collective states of matter.Comment: 5+3 pages. Title extended, as publishe
Topological Flat Band Models and Fractional Chern Insulators
Topological insulators and their intriguing edge states can be understood in
a single-particle picture and can as such be exhaustively classified.
Interactions significantly complicate this picture and can lead to entirely new
insulating phases, with an altogether much richer and less explored
phenomenology. Most saliently, lattice generalizations of fractional quantum
Hall states, dubbed fractional Chern insulators, have recently been predicted
to be stabilized by interactions within nearly dispersionless bands with
non-zero Chern number, . Contrary to their continuum analogues, these states
do not require an external magnetic field and may potentially persist even at
room temperature, which make these systems very attractive for possible
applications such as topological quantum computation. This review recapitulates
the basics of tight-binding models hosting nearly flat bands with non-trivial
topology, , and summarizes the present understanding of interactions
and strongly correlated phases within these bands. Emphasis is made on
microscopic models, highlighting the analogy with continuum Landau level
physics, as well as qualitatively new, lattice specific, aspects including
Berry curvature fluctuations, competing instabilities as well as novel
collective states of matter emerging in bands with . Possible
experimental realizations, including oxide interfaces and cold atom
implementations as well as generalizations to flat bands characterized by other
topological invariants are also discussed.Comment: Invited review. 46 pages, many illustrations and references. V2:
final version with minor improvements and added reference
Quantum Hall Circle
We consider spin-polarized electrons in a single Landau level on a cylinder
as the circumference of the cylinder goes to infinity. This gives a model of
interacting electrons on a circle where the momenta of the particles are
restricted and there is no kinetic energy. Quantum Hall states are exact ground
states for appropriate short range interactions, and there is a gap to
excitations. These states develop adiabatically from this one-dimensional
quantum Hall circle to the bulk quantum Hall states and further on into the
Tao-Thouless states as the circumference goes to zero. For low filling
fractions a gapless state is formed which we suggest is connected to the Wigner
crystal expected in the bulk.Comment: 12 pages, publishe
Effective spin chains for fractional quantum Hall states
Fractional quantum Hall (FQH) states are topologically ordered which
indicates that their essential properties are insensitive to smooth
deformations of the manifold on which they are studied. Their microscopic
Hamiltonian description, however, strongly depends on geometrical details.
Recent work has shown how this dependence can be exploited to generate
effective models that are both interesting in their own right and also provide
further insight into the quantum Hall system. We review and expand on recent
efforts to understand the FQH system close to the solvable thin-torus limit in
terms of effective spin chains. In particular, we clarify how the difference
between the bosonic and fermionic FQH states, which is not apparent in the
thin-torus limit, can be seen at this level. Additionally, we discuss the
relation of the Haldane-Shastry chain to the so-called QH circle limit and
comment on its significance to recent entanglement studies.Comment: 6 pages, 5 figures. Written for a Special Issue on Foundations of
Computational and Theoretical Nanoscience in Journal of Computational and
Theoretical Nanoscience (proceedings for nanoPHYS'09 in Tokyo
The Pfaffian quantum Hall state made simple--multiple vacua and domain walls on a thin torus
We analyze the Moore-Read Pfaffian state on a thin torus. The known six-fold
degeneracy is realized by two inequivalent crystalline states with a four- and
two-fold degeneracy respectively. The fundamental quasihole and quasiparticle
excitations are domain walls between these vacua, and simple counting arguments
give a Hilbert space of dimension for holes and particles
at fixed positions and assign each a charge . This generalizes the
known properties of the hole excitations in the Pfaffian state as deduced using
conformal field theory techniques. Numerical calculations using a model
hamiltonian and a small number of particles supports the presence of a stable
phase with degenerate vacua and quarter charged domain walls also away from the
thin torus limit. A spin chain hamiltonian encodes the degenerate vacua and the
various domain walls.Comment: 4 pages, 1 figure. Published, minor change
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