5,516 research outputs found
Microscopic theory of quantum anomalous Hall effect in graphene
We present a microscopic theory to give a physical picture of the formation
of quantum anomalous Hall (QAH) effect in graphene due to a joint effect of
Rashba spin-orbit coupling and exchange field . Based on a
continuum model at valley or , we show that there exist two distinct
physical origins of QAH effect at two different limits. For ,
the quantization of Hall conductance in the absence of Landau-level
quantization can be regarded as a summation of the topological charges carried
by Skyrmions from real spin textures and Merons from \emph{AB} sublattice
pseudo-spin textures; while for , the four-band low-energy
model Hamiltonian is reduced to a two-band extended Haldane's model, giving
rise to a nonzero Chern number at either or . In the
presence of staggered \emph{AB} sublattice potential , a topological phase
transition occurs at from a QAH phase to a quantum valley-Hall phase. We
further find that the band gap responses at and are different when
, , and are simultaneously considered. We also show that the
QAH phase is robust against weak intrinsic spin-orbit coupling ,
and it transitions a trivial phase when
. Moreover, we use a tight-binding
model to reproduce the ab-initio method obtained band structures through doping
magnetic atoms on and supercells of graphene, and explain
the physical mechanisms of opening a nontrivial bulk gap to realize the QAH
effect in different supercells of graphene.Comment: 10pages, ten figure
Bootstrap Confidence Intervals for Medical Costs With Censored Observations
Medical costs data with administratively censored observations often arise in cost-effectiveness studies of treatments for life threatening diseases. Mean of medical costs incurred from the start of a treatment till death or certain timepoint after the implementation of treatment is frequently of interest. In many situations, due to the skewed nature of the cost distribution and non-uniform rate of cost accumulation over time, the currently available normal approximation confidence interval has poor coverage accuracy. In this paper, we proposed a bootstrap confidence interval for the mean of medical costs with censored observations. In simulation studies, we showed that the proposed bootstrap confidence interval had much better coverage accuracy than the normal approximation one when medical costs had a skewed distribution. When there is light censoring on medical costs (less than or equal to 25%), we found that the bootstrap confidence interval based on the simple weighted estimator is preferred due to its simplicity and good coverage accuracy. For heavily censored cost data (censoring rate greater than or equal to 30%) with larger sample sizes (n greater than or equal to 200), the bootstrap confidence intervals based on the partitioned estimator has superior performance in terms of both efficiency and coverage accuracy. We also illustrated the use of our methods in a real example
The edge engineering of topological Bi(111) bilayer
A topological insulator is a novel quantum state, characterized by
symmetry-protected non-trivial edge/surface states. Our first-principle
simulations show the significant effects of the chemical decoration on edge
states of topological Bi(111) bilayer nanoribbon, which remove the trivial edge
state and recover the Dirac linear dispersion of topological edge state. By
comparing the edge states with and without chemical decoration, the Bi(111)
bilayer nanoribbon offers a simple system for assessing conductance fluctuation
of edge states. The chemical decoration can also modify the penetration depth
and the spin texture of edge states. A low-energy effective model is proposed
to explain the distinctive spin texture of Bi(111) bilayer nanoribbon, which
breaks the spin-momentum orthogonality along the armchair edge.Comment: 5 pages, 5 figure
Topological phases in gated bilayer graphene: Effects of Rashba spin-orbit coupling and exchange field
We present a systematic study on the influence of Rashba spin-orbit coupling,
interlayer potential difference and exchange field on the topological
properties of bilayer graphene. In the presence of only Rashba spin-orbit
coupling and interlayer potential difference, the band gap opening due to
broken out-of-plane inversion symmetry offers new possibilities of realizing
tunable topological phase transitions by varying an external gate voltage. We
find a two-dimensional topological insulator phase and a quantum valley
Hall phase in -stacked bilayer graphene and obtain their effective
low-energy Hamiltonians near the Dirac points. For stacking, we do not
find any topological insulator phase in the presence of large Rashba spin-orbit
coupling. When the exchange field is also turned on, the bilayer system
exhibits a rich variety of topological phases including a quantum anomalous
Hall phase, and we obtain the phase diagram as a function of the Rashba
spin-orbit coupling, interlayer potential difference, and exchange field.Comment: 15 pages, 17figures, and 1 tabl
- …