158 research outputs found
Enhancing the stability of a fractional Chern insulator against competing phases
We construct a two-band lattice model whose bands can carry the Chern numbers
C=0,pm1,pm2. By means of numerical exact diagonalization, we show that the most
favorable situation that selects fractional Chern insulators (FCIs) is not
necessarily the one that mimics Landau levels, namely a flat band with Chern
number 1. First, we find that the gap, measured in units of the on-site
electron-electron repulsion, can increase by almost two orders of magnitude
when the bands are flat and carry a Chern number C=2 instead of C=1. Second, we
show that giving a width to the bands can help to stabilize a FCI. Finally, we
put forward a tool to characterize the real-space density profile of the ground
state that is useful to distinguish FCI from other competing phases of matter
supporting charge density waves or phase separation.Comment: 10 pages, 6 figure
Noncommutative geometry for three-dimensional topological insulators
We generalize the noncommutative relations obeyed by the guiding centers in
the two-dimensional quantum Hall effect to those obeyed by the projected
position operators in three-dimensional (3D) topological band insulators. The
noncommutativity in 3D space is tied to the integral over the 3D Brillouin zone
of a Chern-Simons invariant in momentum-space. We provide an example of a model
on the cubic lattice for which the chiral symmetry guarantees a macroscopic
number of zero-energy modes that form a perfectly flat band. This lattice model
realizes a chiral 3D noncommutative geometry. Finally, we find conditions on
the density-density structure factors that lead to a gapped 3D fractional
chiral topological insulator within Feynman's single-mode approximation.Comment: 41 pages, 3 figure
Excitation spectrum of the homogeneous spin liquid
We discuss the excitation spectrum of a disordered, isotropic and
translationally invariant spin state in the 2D Heisenberg antiferromagnet. The
starting point is the nearest-neighbor RVB state which plays the role of the
vacuum of the theory, in a similar sense as the Neel state is the vacuum for
antiferromagnetic spin wave theory. We discuss the elementary excitations of
this state and show that these are not Fermionic spin-1/2 `spinons' but spin-1
excited dimers which must be modeled by bond Bosons. We derive an effective
Hamiltonian describing the excited dimers which is formally analogous to spin
wave theory. Condensation of the bond-Bosons at zero temperature into the state
with momentum (pi,pi) is shown to be equivalent to antiferromagnetic ordering.
The latter is a key ingredient for a microscopic interpretation of Zhang's
SO(5) theory of cuprate superconductivityComment: RevTex-file, 16 PRB pages with 13 embedded eps figures. Hardcopies of
figures (or the entire manuscript) can be obtained by e-mail request to:
[email protected]
Zero-modes in the random hopping model
If the number of lattice sites is odd, a quantum particle hopping on a
bipartite lattice with random hopping between the two sublattices only is
guaranteed to have an eigenstate at zero energy. We show that the localization
length of this eigenstate depends strongly on the boundaries of the lattice,
and can take values anywhere between the mean free path and infinity. The same
dependence on boundary conditions is seen in the conductance of such a lattice
if it is connected to electron reservoirs via narrow leads. For any nonzero
energy, the dependence on boundary conditions is removed for sufficiently large
system sizes.Comment: 12 pages, 11 figure
Conductance scaling at the band center of wide wires with pure non--diagonal disorder
Kubo formula is used to get the scaling behavior of the static conductance
distribution of wide wires showing pure non-diagonal disorder. Following recent
works that point to unusual phenomena in some circumstances, scaling at the
band center of wires of odd widths has been numerically investigated. While the
conductance mean shows a decrease that is only proportional to the inverse
square root of the wire length, the median of the distribution exponentially
decreases as a function of the square root of the length. Actually, the whole
distribution decays as the inverse square root of the length except close to
G=0 where the distribution accumulates the weight lost at larger conductances.
It accurately follows the theoretical prediction once the free parameter is
correctly fitted. Moreover, when the number of channels equals the wire length
but contacts are kept finite, the conductance distribution is still described
by the previous model. It is shown that the common origin of this behavior is a
simple Gaussian statistics followed by the logarithm of the E=0 wavefunction
weight ratio of a system showing chiral symmetry. A finite value of the
two-dimensional conductance mean is obtained in the infinite size limit. Both
conductance and the wavefunction statistics distributions are given in this
limit. This results are consistent with the 'critical' character of the E=0
wavefunction predicted in the literature.Comment: 10 pages, 9 figures, RevTeX macr
Spectral Statistics in Chiral-Orthogonal Disordered Systems
We describe the singularities in the averaged density of states and the
corresponding statistics of the energy levels in two- (2D) and
three-dimensional (3D) chiral symmetric and time-reversal invariant disordered
systems, realized in bipartite lattices with real off-diagonal disorder. For
off-diagonal disorder of zero mean we obtain a singular density of states in 2D
which becomes much less pronounced in 3D, while the level-statistics can be
described by semi-Poisson distribution with mostly critical fractal states in
2D and Wigner surmise with mostly delocalized states in 3D. For logarithmic
off-diagonal disorder of large strength we find indistinguishable behavior from
ordinary disorder with strong localization in any dimension but in addition
one-dimensional Dyson-like asymptotic spectral singularities. The
off-diagonal disorder is also shown to enhance the propagation of two
interacting particles similarly to systems with diagonal disorder. Although
disordered models with chiral symmetry differ from non-chiral ones due to the
presence of spectral singularities, both share the same qualitative
localization properties except at the chiral symmetry point E=0 which is
critical.Comment: 13 pages, Revtex file, 8 postscript files. It will appear in the
special edition of J. Phys. A for Random Matrix Theor
Random Dirac Fermions and Non-Hermitian Quantum Mechanics
We study the influence of a strong imaginary vector potential on the quantum
mechanics of particles confined to a two-dimensional plane and propagating in a
random impurity potential. We show that the wavefunctions of the non-Hermitian
operator can be obtained as the solution to a two-dimensional Dirac equation in
the presence of a random gauge field. Consequences for the localization
properties and the critical nature of the states are discussed.Comment: 5 pages, Latex, 1 figure, version published in PR
The effects of weak disorders on Quantum Hall critical points
We study the consequences of random mass, random scalar potential and random
vector potential on the line of clean fixed points between integer/fractional
quantum Hall states and an insulator. This line of fixed points was first
identified in a clean Dirac fermion system with both Chern-Simon coupling and
Coulomb interaction in Phys. Rev. Lett. {\bf 80}, 5409 (1998). By performing a
Renormalization Group analysis in 1/N (N is the No. of species of Dirac
fermions) and the variances of three disorders , we find that is irrelevant along this line, both
and are marginal. With the presence of all the three
disorders, the pure fixed line is unstable. Setting Chern-Simon interaction to
zero, we find one non-trivial line of fixed points in plane
with dynamic exponent z=1 and continuously changing , it is stable against
small in a small range of the line ,
therefore it may be relevant to integer quantum Hall transition. Setting
, we find a fixed plane with z=1, the part of this plane with
is stable against small , therefore it may be relevant to
fractional quantum Hall transition.Comment: 16 pages, 19 figure
Density of states in the non-hermitian Lloyd model
We reconsider the recently proposed connection between density of states in
the so-called ``non-hermitian quantum mechanics'' and the localization length
for a particle moving in random potential. We argue that it is indeed possible
to find the localization length from the density of states of a non-hermitian
random ``Hamiltonian''. However, finding the density of states of a
non-hermitian random ``Hamiltonian'' remains an open problem, contrary to
previous findings in the literature.Comment: 6 pages, RevTex, two-column
Confinement of Slave-Particles in U(1) Gauge Theories of Strongly-Interacting Electrons
We show that slave particles are always confined in U(1) gauge theories of
interacting electron systems. Consequently, the low-lying degrees of freedom
are different from the slave particles. This is done by constructing a dual
formulation of the slave-particle representation in which the no-double
occupany constraint becomes linear and, hence, soluble. Spin-charge separation,
if it occurs, is due to the existence of solitons with fractional quantum
numbers
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