15,850 research outputs found
Weyl semimetals in optical lattices: moving and merging of Weyl points, and hidden symmetry at Weyl points
We propose to realize Weyl semimetals in a cubic optical lattice. We find
that there exist three distinct Weyl semimetal phases in the cubic optical
lattice for different parameter ranges. One of them has two pairs of Weyl
points and the other two have one pair of Weyl points in the Brillouin zone.
For a slab geometry with (010) surfaces, the Fermi arcs connecting the
projections of Weyl points with opposite topological charges on the surface
Brillouin zone is presented. By adjusting the parameters, the Weyl points can
move in the Brillouin zone. Interestingly, for two pairs of Weyl points, as one
pair of them meet and annihilate, the originial two Fermi arcs coneect into
one. As the remaining Weyl points annihilate further, the Fermi arc vanishes
and a gap is opened. Furthermore, we find that there always exists a hidden
symmetry at Weyl points, regardless of anywhere they located in the Brillouin
zone. The hidden symmetry has an antiunitary operator with its square being
.Comment: 10 pages, 5 figure
Hidden symmetry and protection of Dirac points on the honeycomb lattice
The honeycomb lattice possesses a novel energy band structure, which is
characterized by two distinct Dirac points in the Brillouin zone, dominating
most of the physical properties of the honeycomb structure materials. However,
up till now, the origin of the Dirac points is unclear yet. Here, we discover a
hidden symmetry on the honeycomb lattice and prove that the existence of Dirac
points is exactly protected by such hidden symmetry. Furthermore, the moving
and merging of the Dirac points and a quantum phase transition, which have been
theoretically predicted and experimentally observed on the honeycomb lattice,
can also be perfectly explained by the parameter dependent evolution of the
hidden symmetry.Comment: 5 pages, 2 figures, +6 pages of supplementary information. Welcome
any comments
Evolution of neutron star + He star binaries: an alternative evolutionary channel to intermediate-mass binary pulsars
It is difficult for intermediate-mass X-ray binaries to form compact
intermediate-mass binary pulsars (IMBPs) with a short orbital-period (\la 3
\rm d), which have a heavy (\ga 0.4 M_{\odot}) CO or ONeMg white dwarf
companions. Since neutron star + He star binaries may experience
common-envelope evolution, they have some advantage to account for the
formation of short orbital-period IMBPs. In this work, we explore the
probability of IMBPs formed by this evolutionary channel. Using Eggleton's
stellar evolution code, considering that the dead pulsars were spun up by the
accreting material and angular momentum from the He star companions, we have
calculated the evolution of a large number of neutron star + He star binaries.
Our simulated results indicate that, the NS + He star evolutionary channel can
produce IMBPs with a WD of and an orbital period of
d, in which pulsars have a spin-period of ms. Comparing
the calculated results with the observational parameters (spin period and
orbital period) of 9 compact IMBPs, the NS + He star evolutionary channel can
account for the formation of 4 sources. Therefore, NS + He star binaries offer
an alternative evolutionary channel to compact IMBPs.Comment: 5 pages, 4 figures, accepted for publication in MNRAS letter
Magnetic braking of Ap/Bp stars: an alternative formation mechanism of compact intermediate-mass binary pulsars
It is difficult for the intermediate-mass X-ray binaries (IMXBs) evolutionary
channel to form intermediate-mass binary pulsars (IMBPs) with a short orbital
period (less than 3 d) via stable mass transfer. The main reason is that the
magnetic braking mechanisms are generally thought not to work for donor stars
with a mass of greater than 1.5 in the canonical model.
However, some intermediate-mass stars have anomalously strong magnetic fields
(about 100 -- 10000 G), i. e. so-called Ap or Bp stars. With the coupling
between the magnetic field and the irradiation-driven wind from the surface of
Ap/Bp stars, a plausible magnetic braking mechanism should be expected. In this
work, we attempt to investigate if IMXBs with Ap/Bp stars can produce IMBPs
with a short orbital period (less than 3 d) by such an anomalous magnetic
braking mechanism. Using a stellar evolution code, we have simulated the
evolution of a large number of IMXBs consisting of a NS and an Ap/Bp star. For
the spin evolution of the NS, we consider the accretion torque, the propeller
torque, and the spin-down torque caused by the interaction between the magnetic
field and the accretion disc. The calculated results show that, employing
anomalous magnetic braking of Ap/Bp stars, IMXBs can evolve into compact IMBPs
with short orbital periods of less than 3 d. However, there exists significant
discrepancy between the spin periods of IMBPs in our simulated results and
those observed.Comment: 8 pages, 6 figures. Accepted for publication in MNRA
Steady state current fluctuations and dynamical control in a nonequilibrium single-site Bose-Hubbard system
We investigate nonequilibrium energy transfer in a single-site Bose-Hubbard
model coupled to two thermal baths. By including a quantum kinetic equation
combined with full counting statistics, we investigate the steady state energy
flux and noise power. The influence of the nonlinear Bose-Hubbard interaction
on the transfer behaviors is analyzed, and the nonmonotonic features are
clearly exhibited. Particularly, in the strong on-site repulsion limit, the
results become identical with the nonequilibrium spin-boson model. We also
extend the quantum kinetic equation to study the geometric-phase-induced energy
pump. An interesting reversal behavior is unraveled by enhancing the
Bose-Hubbard repulsion strength.Comment: 12 pages,6 figure
Topological semimetals with double-helix nodal link
Topological nodal line semimetals are characterized by the crossing of the
conduction and valence bands along one or more closed loops in the Brillouin
zone. Usually, these loops are either isolated or touch each other at some
highly symmetric points. Here, we introduce a new kind of nodal line semimetal,
that contains a pair of linked nodal loops. A concrete two-band model was
constructed, which supports a pair of nodal lines with a double-helix
structure, which can be further twisted into a Hopf link because of the
periodicity of the Brillouin zone. The nodal lines are stabilized by the
combined spatial inversion and time reversal
symmetry; the individual and symmetries must be
broken. The band exhibits nontrivial topology that each nodal loop carries a
Berry flux. Surface flat bands emerge at the open boundary and are
exactly encircled by the projection of the nodal lines on the surface Brillouin
zone. The experimental implementation of our model using cold atoms in optical
lattices is discussed.Comment: 10 pages, 7 figures. The title is changed, the main text and
Supplemental Material are update
Negative differential thermal conductance and heat amplification in a nonequilibrium triangle-coupled spin-boson system at strong coupling
We investigate the nonequilibrium quantum heat transfer in a triangle-coupled
spin-boson system within a three-terminal setup. By including the
nonequilibrium noninteracting blip approximation approach combined with the
full counting statistics, we analytically obtain the steady state populations
and heat currents. The negative differential thermal conductance and giant heat
amplification factor are clearly observed at strong qubit-bath coupling. %and
the heat amplification is dramatically suppressed in the moderate coupling
regime. Moreover, the strong interaction between the gating qubit and gating
thermal bath is unraveled to be compulsory to exhibit these far-from
equilibrium features.Comment: 9 pages, 6 figure
Some Problems in Defining Functional Integration over the Gauge Group
We find that sometimes the usual definition of functional integration over
the gauge group through limiting process may have internal difficulties.Comment: 2 pages revtex, no figur
A Note on Functional Integral over the Local Gauge Group
We evaluated some particular type of functional integral over the local gauge
group C^{\infty}({\bf R}^n, U(1)) by going to a discretized lattice. The
results explicitly violates the property of the Haar measure. We also analysed
the Faddeev-Popov method through a toy example. The results also violates the
property of the Haar measure.Comment: 7 pages, Revte
A Note on Invariant Measure on the Local Gauge Group
In this paper we investigated the problem of the existence of invariant
meaures on the local gauge group. We prove that it is impossible to define a
{\it finite} translationally invariant measure on the local gauge group
(where is an arbitrary matrix Lie group).Comment: 4 pages, REVTE
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