1,581 research outputs found
Phononic topological insulators with tunable pseudospin physics
Efficient control of phonons is crucial to energy-information technology, but
limited by the lacking of tunable degrees of freedom like charge or spin. Here
we suggest to utilize crystalline symmetry-protected pseudospins as new quantum
degrees of freedom to manipulate phonons. Remarkably, we reveal a duality
between phonon pseudospins and electron spins by presenting Kramers-like
degeneracy and pseudospin counterparts of spin-orbit coupling, which lays the
foundation for "pseudospin phononics". Furthermore, we report two types of
three-dimensional phononic topological insulators, which give topologically
protected, gapless surface states with linear and quadratic band degeneracies,
respectively. These topological surface states display unconventional phonon
transport behaviors attributed to the unique pseudospin-momentum locking, which
are useful for phononic circuits, transistors, antennas, etc. The emerging
pseudospin physics offers new opportunities to develop future phononics
Stochastic Averaging Principle for Dynamical Systems with Fractional Brownian Motion
Stochastic averaging for a class of stochastic differential equations (SDEs)
with fractional Brownian motion, of the Hurst parameter H in the interval (1/2,
1), is investigated. An averaged SDE for the original SDE is proposed, and
their solutions are quantitatively compared. It is shown that the solution of
the averaged SDE converges to that of the original SDE in the sense of mean
square and also in probability. It is further demonstrated that a similar
averaging principle holds for SDEs under stochastic integral of pathwise
backward and forward types. Two examples are presented and numerical
simulations are carried out to illustrate the averaging principle
Stable nontrivial Z2 topology in ultrathin Bi (111) films: a first-principles study
Recently, there have been intense efforts in searching for new topological
insulator (TI) materials. Based on first-principles calculations, we find that
all the ultrathin Bi (111) films are characterized by a nontrivial Z2 number
independent of the film thickness, without the odd-even oscillation of
topological triviality as commonly perceived. The stable nontrivial Z2 topology
is retained by the concurrent band gap inversions at multiple
time-reversal-invariant k-points and associated with the intermediate
inter-bilayer coupling of the multi-bilayer Bi film. Our calculations further
indicate that the presence of metallic surface states in thick Bi(111) films
can be effectively removed by surface adsorption.Comment: 5 pages, 3 figure
Converting normal insulators into topological insulators via tuning orbital levels
Tuning the spin-orbit coupling strength via foreign element doping and/or
modifying bonding strength via strain engineering are the major routes to
convert normal insulators to topological insulators. We here propose an
alternative strategy to realize topological phase transition by tuning the
orbital level. Following this strategy, our first-principles calculations
demonstrate that a topological phase transition in some cubic perovskite-type
compounds CsGeBr and CsSnBr could be facilitated by carbon
substitutional doping. Such unique topological phase transition predominantly
results from the lower orbital energy of the carbon dopant, which can pull down
the conduction bands and even induce band inversion. Beyond conventional
approaches, our finding of tuning the orbital level may greatly expand the
range of topologically nontrivial materials
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