15,698 research outputs found
Symmetry-protected topological phases with charge and spin symmetries: response theory and dynamical gauge theory in 2D, 3D and the surface of 3D
A large class of symmetry-protected topological phases (SPT) in boson / spin
systems have been recently predicted by the group cohomology theory. In this
work, we consider SPT states at least with charge symmetry (U(1) or Z) or
spin rotation symmetry (U(1) or Z) in 2D, 3D, and the surface of 3D.
If both are U(1), we apply external electromagnetic field / `spin gauge field'
to study the charge / spin response. For the SPT examples we consider (i.e.
U(1)Z, U(1)Z,
U(1)[U(1)Z]; subscripts and are short for
charge and spin; Z and Z are time-reversal symmetry and
-rotation about , respectively), many variants of Witten effect in
the 3D SPT bulk and various versions of anomalous surface quantum Hall effect
are defined and systematically investigated. If charge or spin symmetry reduces
to Z by considering charge- or spin- condensate, instead of the
linear response approach, we gauge the charge/spin symmetry, leading to a
dynamical gauge theory with some remaining global symmetry. The 3D dynamical
gauge theory describes a symmetry-enriched topological phase (SET), i.e. a
topologically ordered state with global symmetry which admits nontrivial ground
state degeneracy depending on spatial manifold topology. For the SPT examples
we consider, the corresponding SET states are described by dynamical
topological gauge theory with topological BF term and axionic -term in
3D bulk. And the surface of SET is described by the chiral boson theory with
quantum anomaly.Comment: 23 pages, 1 figure, REVTeX; Table II and Table III for summary of
part of key result
Mechanism of unidirectional movement of kinesin motors
Kinesin motors have been studied extensively both experimentally and
theoretically. However, the microscopic mechanism of the processive movement of
kinesin is still an open question. In this paper, we propose a hand-over-hand
model for the processivity of kinesin, which is based on chemical, mechanical,
and electrical couplings. In the model the processive movement does not need to
rely on the two heads' coordination in their ATP hydrolysis and mechanical
cycles. Rather, the ATP hydrolyses at the two heads are independent. The much
higher ATPase rate at the trailing head than the leading head makes the motor
walk processively in a natural way, with one ATP being hydrolyzed per step. The
model is consistent with the structural study of kinesin and the measured
pathway of the kinesin ATPase. Using the model the estimated driving force of ~
5.8 pN is in agreements with the experimental results (5~7.5 pN). The
prediction of the moving time in one step (~10 microseconds) is also consistent
with the measured values of 0~50 microseconds. The previous observation of
substeps within the 8-nm step is explained. The shapes of velocity-load (both
positive and negative) curves show resemblance to previous experimental
results.Comment: 22 pages, 6 figure
Control of spiral waves and turbulent states in a cardiac model by travelling-wave perturbations
We propose a travelling-wave perturbation method to control the
spatiotemporal dynamics in a cardiac model. It is numerically demonstrated that
the method can successfully suppress the wave instability (alternans in action
potential duration) in the one-dimensional case and convert spiral waves and
turbulent states to the normal travelling wave states in the two-dimensional
case. An experimental scheme is suggested which may provide a new design for a
cardiac defibrillator.Comment: 9 pages, 5 figure
Model for processive movement of myosin V and myosin VI
Myosin V and myosin VI are two classes of two-headed molecular motors of the
myosin superfamily that move processively along helical actin filaments in
opposite directions. Here we present a hand-over-hand model for their
processive movements. In the model, the moving direction of a dimeric molecular
motor is automatically determined by the relative orientation between its two
heads at free state and its head's binding orientation on track filament. This
determines that myosin V moves toward the barbed end and myosin VI moves toward
the pointed end of actin. During the moving period in one step, one head
remains bound to actin for myosin V whereas two heads are detached for myosin
VI: The moving manner is determined by the length of neck domain. This
naturally explains the similar dynamic behaviors but opposite moving directions
of myosin VI and mutant myosin V (the neck of which is truncated to only
one-sixth of the native length). Because of different moving manners, myosin VI
and mutant myosin V exhibit significantly broader step-size distribution than
native myosin V. However, all three motors give the same mean step size of 36
nm (the pseudo-repeat of actin helix). Using the model we study the dynamics of
myosin V quantitatively, with theoretical results in agreement with previous
experimental ones.Comment: 18 pages, 7 figure
Quasiparticle interference of C2-symmetric surface states in LaOFeAs parent compound
We present scanning tunneling microscopy studies of the LaOFeAs parent
compound of iron pnictide superconductors. Topographic imaging reveals two
types of atomically flat surfaces, corresponding to the exposed LaO layer and
FeAs layer respectively. On one type of surface, we observe strong standing
wave patterns induced by quasiparticle interference of two-dimensional surface
states. The distribution of scattering wavevectors exhibits pronounced two-fold
symmetry, consistent with the nematic electronic structure found in the
Ca(Fe1-xCox)2As2 parent state.Comment: 13 pages, 4 figure
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