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Zhong Wang

Publications and source records attributed to Zhong Wang.

At least 127 records · Page 7Linked to original sources
arXiv2017

Nodal-knot semimetals

Topological nodal-line semimetals are characterized by one-dimensional lines of band crossing in the Brillouin zone. In contrast to nodal points, nodal lines can be in topologically nontrivial configurations. In this paper, we study the simplest topologically nontrivial forms of nodal line, namely, a single nodal line taking the shape of a knot in the Brillouin zone. We introduce a generic construction for various "nodal-knot semimetals", which yields the simplest trefoil nodal knot and other more complicated nodal knots in the Brillouin zone. The knotted-unknotted transitions by nodal-line reconnections are also studied. Our work brings the knot theory to the subject of topological semimetals.

cond-mat.str-el↗
arXiv2017

Majorana Zero Modes Protected by Hopf Invariant in Topologically Trivial Superconductors

Majorana zero modes are usually attributed to topological superconductors. We study a class of two-dimensional topologically trivial superconductors without chiral edge modes, which nevertheless host robust Majorana zero modes in topological defects. The construction of this minimal single-band model is facilitated by the Hopf map and the Hopf invariant. This work will stimulate investigations of Majorana zero modes in superconductors in the topologically trivial regime.

cond-mat.supr-con↗
arXiv2017

Signature of chiral fermion instability in the Weyl semimetal TaAs above the quantum limit

We report the electrical transport properties for Weyl semimetal TaAs in an intense magnetic field. Series of anomalies occur in the longitudinal magnetoresistance and Hall signals at ultra-low temperatures when the Weyl electrons are confined into the lowest Landau level. These strongly temperature-dependent anomalies are ascribed to the electron-hole pairing instability. Our measurements show that the Weyl semimetal TaAs in the ultraquantum regime provides a good platform for studying electron-electron interaction in topological nontrivial semimetals.

cond-mat.mes-hall↗
arXiv2016

Topological magnetoplasmon

Classical wave fields are real-valued, ensuring the wave states at opposite frequencies and momenta to be inherently identical. Such a particle-hole symmetry can open up new possibilities for topological phenomena in classical systems. Here we show that the historically studied two-dimensional (2D) magnetoplasmon, which bears gapped bulk states and gapless one-way edge states near zero frequency, is topologically analogous to the 2D topological $p+\Ii p$ superconductor with chiral Majorana edge states and zero modes. We further predict a new type of one-way edge magnetoplasmon at the interface of opposite magnetic domains, and demonstrate the existence of zero-frequency modes bounded at the peripheries of a hollow disk. These findings can be readily verified in experiment, and can greatly enrich the topological phases in bosonic and classical systems.

cond-mat.mes-hall↗
arXiv2016

Topological one-way fiber of second Chern number

Optical fiber is a ubiquitous and indispensable component in communications, sensing, biomedicine and many other lightwave technologies and applications. Here we propose topological one-way fibers to remove two fundamental mechanisms that limit fiber performance: scattering and reflection. We design three-dimensional~(3D) photonic crystal fibers, inside which photons propagate only in one direction, that are completely immune to Rayleigh and Mie scatterings and significantly suppress the nonlinear Brillouin and Raman scatterings. A one-way fiber is also free from Fresnel reflection, naturally eliminating the needs for fiber isolators. Our finding is enabled by the recently discovered Weyl points in a double-gyroid~(DG) photonic crystal. By annihilating two Weyl points by supercell modulation in a magnetic DG, we obtain the photonic analogue of the 3D quantum Hall phase with a non-zero first Chern number~($C_1$). When the modulation becomes helixes, one-way fiber modes develop along the winding axis, with the number of modes determined by the spatial frequency of the helix. These single-polarization single-mode and multi-mode one-way fibers, having nearly identical group and phase velocities, are topologically-protected by the second Chern number~($C_2$) in the 4D parameter space of the 3D wavevectors plus the winding angle of the helixes. This work suggests a unique way to utilize higher-dimensional topological physics without resorting to artificial dimensions.

cond-mat.mes-hall↗
arXiv2016

Tunable Weyl Points in Periodically Driven Nodal Line Semimetals

Weyl semimetals and nodal line semimetals are characterized by linear band touching at zero-dimensional points and one-dimensional lines, respectively. We predict that a circularly polarized light drives nodal line semimetals into Weyl semimetals. The Floquet Weyl points thus obtained are tunable by the incident light, which enables investigations of them in a highly controllable manner. The transition from nodal line semimetals to Weyl semimetals is accompanied by the emergence of a large and tunable anomalous Hall conductivity. Our predictions are experimentally testable by transport measurement in film samples or by pump-probe angle-resolved photoemission spectroscopy.

cond-mat.str-el↗
arXiv2016

Collective modes in nodal line semimetals

Recently, the nodal line semimetals have attracted considerable interests in condensed matter physics. We show that their distinct band structure can be detected by measuring the collective modes. In particular, we find that the dependence of the plasmon frequency $ω_p$ on the electron density $n$ follows a $ω_p \sim n^{1/4}$ law in the long wavelength limit. Our results will be useful in the ongoing search for new candidates of nodal line semimetals.

cond-mat.str-el↗
arXiv2015

Unidirectional Transport in Electronic and Photonic Weyl Materials by Dirac Mass Engineering

Unidirectional transports have been observed in two-dimensional systems, however, so far they have not been experimentally observed in three-dimensional bulk materials. In this theoretical work we show that the recently discovered Weyl materials provide a platform for unidirectional transports inside bulk materials. With high experimental feasibility, a complex Dirac mass can be generated and manipulated in the photonic Weyl crystals, creating unidirectionally propagating modes observable in transmission experiments. Possible realization in (electronic) Weyl semimetals is also studied. We show in a lattice model that, with a short-range interaction, the desired form of the Dirac mass can be spontaneously generated in a first-order transition.

cond-mat.str-el↗
arXiv2015

Topological Superfluid and Majorana Zero Modes in Synthetic Dimension

Recently it has been shown that multicomponent spin-orbit-coupled fermions in one-dimensional optical lattices can be viewed as spinless fermions moving in two-dimensional synthetic lattices with synthetic magnetic flux. The quantum Hall edge states in these systems have been observed in recent experiments. In this paper we study the effect of an attractive Hubbard interaction. Since the Hubbard interaction is long-range in the synthetic dimension, it is able to efficiently induce Cooper pairing between the counterpropagating chiral edge states. The topological class of the resultant one-dimensional superfluid is determined by the parity (even/odd) of the Chern number in the two-dimensional synthetic lattice. We also show the presence of a chiral symmetry in our model, which implies ${\rm Z}$ classification and the robustness of multiple zero modes when this symmetry is unbroken.

cond-mat.quant-gas↗
arXiv2015

Bayesian group Lasso for nonparametric varying-coefficient models with application to functional genome-wide association studies

Although genome-wide association studies (GWAS) have proven powerful for comprehending the genetic architecture of complex traits, they are challenged by a high dimension of single-nucleotide polymorphisms (SNPs) as predictors, the presence of complex environmental factors, and longitudinal or functional natures of many complex traits or diseases. To address these challenges, we propose a high-dimensional varying-coefficient model for incorporating functional aspects of phenotypic traits into GWAS to formulate a so-called functional GWAS or fGWAS. The Bayesian group lasso and the associated MCMC algorithms are developed to identify significant SNPs and estimate how they affect longitudinal traits through time-varying genetic actions. The model is generalized to analyze the genetic control of complex traits using subject-specific sparse longitudinal data. The statistical properties of the new model are investigated through simulation studies. We use the new model to analyze a real GWAS data set from the Framingham Heart Study, leading to the identification of several significant SNPs associated with age-specific changes of body mass index. The fGWAS model, equipped with the Bayesian group lasso, will provide a useful tool for genetic and developmental analysis of complex traits or diseases.

stat.AP↗
arXiv2015

Helical Spin Order from Topological Dirac and Weyl Semimetals

We study dynamical mass generation and the resultant helical spin orders in topological Dirac and Weyl semimetals, including the edge states of quantum spin Hall insulators, the surface states of weak topological insulators, and the bulk materials of Weyl semimetals. In particular, the helical spin textures of Weyl semimetals manifest the spin-momentum locking of Weyl fermions in a visible manner. The spin-wave fluctuations of the helical order carry electric charge density; therefore, the spin textures can be electrically controlled in a simple and predictable manner.

cond-mat.str-el↗
arXiv2015

Magnetic Order Driven Topological Transition in the Haldane-Hubbard Model

In this letter we study the Haldane model with on-site repulsive interactions at half-filling. We show that the mean-field Hamiltonian with magnetic order effectively modifies parameters in the Haldane Hamiltonian, such as sublattice energy difference and phase in next nearest hopping. As interaction increases, increasing of magnetic order corresponds to varying these parameters and consequently, drives topological transitions. At the mean-field level, one scenario is that the magnetic order continuously increases, and inevitably, the fermion gap closes at the topological transition point with nonzero magnetic order. Beyond the mean-field, interaction between fermions mediated by spin-wave fluctuations can further open up the gap, rendering a first-order transition. Another scenario is a first-order transition at mean-field level across which a canted magnetic order develops discontinuously, avoiding the fermion gap closing. We find that both scenarios exist in the phase diagram of the Haldane-Hubbard model. Our predication is relevant to recent experimental realization of the Haldane model in cold atom system.

cond-mat.quant-gas↗
arXiv2014

SMT CO (2-1) Observations of Nearby Star-Forming Galaxies

We present CO $J$=2-1 observations towards 32 nearby gas-rich star-forming galaxies selected from the ALFALFA and WISE catalogs, using the Sub-millimeter Telescope. Our sample is selected to be dominated by intermediate-$M_{\rm *}$ galaxies. The scaling-relations between molecular gas, atomic gas and galactic properties (stellar mass, NUV$- r$ and WISE color W3$-$W2) are examined and discussed. Our results show that (1). In the galaxies with stellar mass $M_{\rm *}$ $\leqslant 10^{10}$ $M_{\odot}$, HI fraction ($f_{\rm HI}$ $\equiv$ $M_{\rm HI}$/$M_{\rm *}$) is significantly higher than that of more massive galaxies, while H$_2$ gas fraction ($f_{\rm H_2}$ $\equiv$ $M_{\rm H_2}$/$M_{\rm *}$) remain nearly unchanged. (2). Comparing with $f_{\rm H_2}$, $f_{\rm HI}$ correlates better with both $M_{\rm *}$ and NUV$- r$. (3). A new parameter, WISE color W3$-$W2 (12\,$μ$m$-$4.6\,$μ$m) is introduced, which is similar to NUV$- r$ in tracing star formation activity, and we find that W3$-$W2 has a tighter anti-correlation with log $f_{\rm H_2}$ than the anti-correlation of (NUV$- r$) - $f_{\rm HI}$, (NUV$- r$) - $f_{\rm H_2}$ and (W3$-$W2) - $f_{\rm HI}$. This indicates that W3$-$W2 can trace the H$_2$ fraction in galaxies. For gas ratio $M_{\rm H_2}$/$M_{\rm HI}$, only in the intermediate-$M_{\rm *}$ galaxies it appears to depend on $M_{\rm *}$ and NUV$- r$. We find a tight correlation between the molecular gas mass $M_{\rm H_2}$ and 12\,$μ$m (W3) luminosities ($L_{\rm 12\,μm}$), and the slope is close to unity (1.03 $\pm$ 0.06) for the SMT sample. This correlation may reflect that the cold gas and dust are well mixed on global galactic scale. Using the all-sky 12\,$μ$m (W3) data available in WISE, this correlation can be used to estimate CO flux for molecular gas observations and can even predict H$_2$ mass for star-forming galaxies.

astro-ph.GA↗
arXiv2014

Topological number and Fermion Green's function of Strongly Interacting Topological Superconductors

It has been understood that short range interactions can reduce the classification of topological superconductors in all dimensions. In this paper we demonstrate by explicit calculations that when the topological phase transition between two distinct phases in the noninteracting limit is gapped out by interaction, the bulk fermion Green's function $G(iω)$ at the "transition" approaches zero as $G(iω) \sim ω$ at certain momentum $\vec{k}$ in the Brillouin zone.

cond-mat.str-el↗
arXiv2014

Fractionalized Metals and Superconductors in Three Dimensions

We study three-dimensional metals with nontrivial correlation functions and fractionalized excitations. We formulate for such states a gauge theory, which also naturally describes the fractional quantization of chiral anomaly. We also study fractional superconductors in this description. This formulation leads to the "three-dimensional chiral Luttinger liquids" and fractionalized Weyl semimetals, which can arise in both fermion and boson models. We also propose experiments to detect these fractionalized phases.

cond-mat.str-el↗
arXiv2014

Topological Invariants and Ground-State Wave Functions of Topological Insulators on a Torus

We define topological invariants in terms of the ground states wave functions on a torus. This approach leads to precisely defined formulas for the Hall conductance in four dimensions and the topological magneto-electric $θ$ term in three dimensions, and their generalizations in higher dimensions. They are valid in the presence of arbitrary many-body interaction and disorder. These topological invariants systematically generalize the two-dimensional Niu-Thouless-Wu formula, and will be useful in numerical calculations of disordered topological insulators and strongly correlated topological insulators, especially fractional topological insulators.

cond-mat.str-el↗
arXiv2013

Chiral anomaly, Charge Density Waves, and Axion Strings from Weyl Semimetals

We study dynamical instability and chiral symmetry breaking in three dimensional Weyl semimetals, which turns Weyl semimetals into "axion insulators". Charge density waves (CDW) is found to be the natural consequence of the chiral symmetry breaking. The phase mode of this charge density wave state is identified as the axion, which couples to electromagnetic field in the topological $θ{\bf E}\cdot{\bf B}$ term. One of our main results is that the "axion strings" can be realized as the (screw or edge) dislocations in the charge density wave, which provides a simple physical picture for the elusive axion strings. These axion strings carry gapless chiral modes, therefore they have important implications for dissipationless transport properties of Weyl semimetals with broken symmetry.

cond-mat.str-el↗