28,850 research outputs found
Topological phases and phase transitions on the square-octagon lattice
We theoretically investigate a tight binding model of fermions hopping on the
square-octagon lattice which consists of a square lattice with plaquette
corners themselves decorated by squares. Upon the inclusion of second neighbor
spin-orbit coupling or non-Abelian gauge fields, time-reversal symmetric
topological Z_2 band insulators are realized. Additional insulating and gapless
phases are also realized via the non-Abelian gauge fields. Some of the phase
transitions involve topological changes to the Fermi surface. The stability of
the topological phases to various symmetry breaking terms is investigated via
the entanglement spectrum. Our results enlarge the number of known exactly
solvable models of Z_2 band insulators, and are potentially relevant to the
realization and identification of topological phases in both the solid state
and cold atomic gases.Comment: 12 pages, 9 figure
Coulomb drag between helical edge states
We theoretically investigate the Coulomb drag between the edge states of two
quantum spin Hall systems. Using an interacting theory of the one-dimensional
helical edge modes, we show that the drag vanishes at second order in the
inter-edge interaction, where it is typically finite in other systems, due to
the absence of backscattering within the edges. However, in the presence of a
small external magnetic field the drag is finite and scales as the fourth power
of the magnetic field, a behavior that sharply distinguishes it from other
systems. We obtain the temperature dependence of the drag for regimes of both
linear and quadratic edge dispersion in the presence of a finite field.Comment: 4 pages, 3 figure
Landau-Zener Interference in Multilevel Superconducting Flux Qubits Driven by Large Amplitude Fields
We proposed an analytical model to analyze the Landau-Zener interference in a
multilevel superconducting flux qubit driven by large amplitude external
fields. Our analytical results agree remarkably with those of the experiment
[Nature 455, 51 (2008)]. Moreover, we studied the effect of driving-frequency
and dephasing rate on the interference. The dephasing generally destroys the
interference while increasing frequency rebuilds the interference at large
dephasing rate. At certain driving frequency and dephasing rate, the
interference shows some anomalous features as observed in recent experiments.Comment: 7 pages, 6 figure
Diffractive dissociation including pomeron loops in zero transverse dimensions
We have recently studied the QCD pomeron loop evolution equations in zero
transverse dimensions [Shoshi:2005pf]. Using the techniques developed in
[Shoshi:2005pf] together with the AGK cutting rules, we present a calculation
of single, double and central diffractive cross sections (for large diffractive
masses and large rapidity gaps) in zero transverse dimensions in which all
dominant pomeron loop graphs are consistently summed. We find that the
diffractive cross sections unitarise at asymptotic energies and that they are
suppressed by powers of alpha_s. Our calculation is expected to expose some of
the diffractive physics in hadron-hadron collisions at high energy.Comment: 14 pages, 5 figures; numerous explanations added, matches the
published versio
Evidence for the Luttigger liquid density of states in transport across the incompressible stripe at fractional filling factors
We experimentally investigate transport across the incompressible stripe at
the sample edge in the fractional quantum Hall effect regime at bulk filling
factors and . We obtain the dependence of the equilibration
length, that is a phenomenological characteristics of the transport, on the
voltage imbalance and the temperature, at high voltage imbalances. These
dependencies are found to be of the power-law form, which is a strong evidence
for the Luttigger liquid density of states.Comment: 4 pages, to appear in EP
Gapless Fermions and Quantum Order
Using 2D quantum spin-1/2 model as a concrete example, we studied the
relation between gapless fermionic excitations (spinons) and quantum orders in
some spin liquid states. Using winding number, we find the projective symmetry
group that characterizes the quantum order directly determines the pattern of
Fermi points in the Brillouin zone. Thus quantum orders provide an origin for
gapless fermionic excitations.Comment: 23 pages. LaTeX. Homepage http://dao.mit.edu/~we
Quantum state transmission in a cavity array via two-photon exchange
The dynamical behavior of a coupled cavity array is investigated when each
cavity contains a three-level atom. For the uniform and staggered intercavity
hopping, the whole system Hamiltonian can be analytically diagonalized in the
subspace of single-atom excitation. The quantum state transfer along the
cavities is analyzed in detail for distinct regimes of parameters, and some
interesting phenomena including binary transmission, selective localization of
the excitation population are revealed. We demonstrate that the uniform
coupling is more suitable for the quantum state transfer. It is shown that the
initial state of polariton located in the first cavity is crucial to the
transmission fidelity, and the local entanglement depresses the state transfer
probability. Exploiting the metastable state, the distance of the quantum state
transfer can be much longer than that of Jaynes-Cummings-Hubbard model. A
higher transmission probability and longer distance can be achieved by
employing a class of initial encodings and final decodings.Comment: 8 pages, 7 figures. to appear in Phys. Rev.
Geometric phase and quantum phase transition in an inhomogeneous periodic XY spin-1/2 model
The notion of geometric phase has been recently introduced to analyze the
quantum phase transitions of many-body systems from the geometrical
perspective. In this work, we study the geometric phase of the ground state for
an inhomogeneous period-two anisotropic XY model in a transverse field. This
model encompasses a group of familiar spin models as its special cases and
shows a richer critical behavior. The exact solution is obtained by mapping on
a fermionic system through the Jordan-Wigner transformation and constructing
the relevant canonical transformation to realize the diagonalization of the
Hamiltonian coupled in the -space. The results show that there may exist
more than one quantum phase transition point at some parameter regions and
these transition points correspond to the divergence or extremum properties of
the Berry curvature.Comment: 6 pages, 3 figures. As a backup of a previous work and some typos in
the published version are fixe
Meson distribution amplitudes in holographic models
We study the wave functions of light and heavy mesons in both hard-wall (HW)
and soft-wall (SW) holographic models which use AdS/CFT correspondence. In the
case of massless constituents, the asymptotic behaviors of the electromagnetic
form factor, the distribution amplitudes, and the decay constants for the two
models are the same, if the relation between the dilaton scale parameter and
the size of meson is an inverse proportion. On the other hand, by introducing a
quark mass dependence in the wave function, the differences of the distribution
amplitudes between the two models are obvious. In addition, for the SW model,
the dependences of the decay constants of meson on the dilaton scale parameter
differ; especially f_{Qq}\sim \kappa^3/m_Q^2 is consistent with the
prediction of the heavy quark effective theory if \kappa\sim m_Q^{1/2}. Thus
the parameters of the two models are fit by the decay constants of the distinct
mesons; the distribution amplitudes and the \xi-moments are calculated and
compared.Comment: 30 pages, 11 figures, 2 tables, minor modifications and one short
paragraph added, some references added and removed, accepted for publication
in PR
Dynamical Domain Wall Defects in 2+1 Dimensions
We study some dynamical properties of a Dirac field in 2+1 dimensions with
spacetime dependent domain wall defects. We show that the Callan and Harvey
mechanism applies even to the case of defects of arbitrary shape, and in a
general state of motion. The resulting chiral zero modes are localized on the
worldsheet of the defect, an embedded curved two dimensional manifold. The
dynamics of these zero modes is governed by the corresponding induced metric
and spin connection. Using known results about determinants and anomalies for
fermions on surfaces embedded in higher dimensional spacetimes, we show that
the chiral anomaly for this two dimensional theory is responsible for the
generation of a current along the defect. We derive the general expression for
such a current in terms of the geometry of the defect, and show that it may be
interpreted as due to an "inertial" electric field, which can be expressed
entirely in terms of the spacetime curvature of the defects. We discuss the
application of this framework to fermionic systems with defects in condensed
matter.Comment: 12 pages, Late
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