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 $-1$.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 $\sim0.5 - 1.1 M_{\odot}$ and an orbital period of $0.03 - 20$ d, in which pulsars have a spin-period of $1.4 - 200$ 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 $\rm M_{\odot}$ 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 $\mathcal{P}$ and time reversal $\mathcal{T}$ symmetry; the individual $\mathcal{P}$ and $\mathcal{T}$ symmetries must be broken. The band exhibits nontrivial topology that each nodal loop carries a $\pi$ 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 $C^{\infty}({\bf R}^n,G)$(where $G$ is an arbitrary matrix Lie group).Comment: 4 pages, REVTE