arXiv subjects

Zhigang Wang

Publications and source records attributed to Zhigang Wang.

At least 145 records · Page 8Linked to original sources

arXiv2011

The fast light of CsI(Na) crystals

The responds of different common alkali halide crystals to alpha-rays and gamma-rays are tested in our research. It is found that only CsI(Na) crystals have significantly different waveforms between alpha and gamma scintillations, while others have not this phenomena. It is suggested that the fast light of CsI(Na) crystals arises from the recombination of free electrons with self-trapped holes of the host crystal CsI. Self-absorption limits the emission of fast light of CsI(Tl) and NaI(Tl) crystals.

physics.ins-det↗

arXiv2011

Neutron-gamma discrimination of CsI(Na) crystals for dark matter searches

The luminescent properties of CsI(Na) crystals are studied in this report. By using a TDS3054C oscilloscope with a sampling frequency of 5 GS/s, we find out that nuclear recoil signals are dominated by very fast light pulse with a decay time of ~20 ns, while γ-ray signals have a decay time of ~600 ns. The wavelength of nuclear recoil and γ-ray signals are also different. The study of n/γ separation shows that the rejection factor can reach an order of 10-7 with signal efficiency more than 80% at an equivalent electron recoil energy of 20 keV or more. Such a property makes CsI(Na) an ideal candidate for dark matter searches.

physics.ins-det↗

arXiv2010

Chiral topological excitonic insulator in semiconductor quantum wells

We present a scheme to realize the chiral topological excitonic insulator in semiconductor heterostructures which can be experimentally fabricated with a coupled quantum well adjacent to twoferromagnetic insulating films. The different mean-field chiral topological orders, which are due to the change in the directions of the magnetization of the ferromagnetic films, can be characterized by the TKNN numbers in the bulk system as well as by the winding numbers of the gapless states in the edged system. Furthermore, we propose an experimental scheme to detect the emergence of the chiral gapless edge state and distinguish different chiral topological orders by measuring the thermal conductance.

cond-mat.mes-hall↗

arXiv2010

On Edwards-Child's inequality

In this paper, by making use of one of Chen's theorems and the method of mathematical analysis, we refine Edwards-Child's inequality and solve a conjecture posed by Liu.

math.MG↗

arXiv2010

Determining global mean-first-passage time of random walks on Vicsek fractals using eigenvalues of Laplacian matrices

The family of Vicsek fractals is one of the most important and frequently-studied regular fractal classes, and it is of considerable interest to understand the dynamical processes on this treelike fractal family. In this paper, we investigate discrete random walks on the Vicsek fractals, with the aim to obtain the exact solutions to the global mean first-passage time (GMFPT), defined as the average of first-passage time (FPT) between two nodes over the whole family of fractals. Based on the known connections between FPTs, effective resistance, and the eigenvalues of graph Laplacian, we determine implicitly the GMFPT of the Vicsek fractals, which is corroborated by numerical results. The obtained closed-form solution shows that the GMFPT approximately grows as a power-law function with system size (number of all nodes), with the exponent lies between 1 and 2. We then provide both the upper bound and lower bound for GMFPT of general trees, and show that leading behavior of the upper bound is the square of system size and the dominating scaling of the lower bound varies linearly with system size. We also show that the upper bound can be achieved in linear chains and the lower bound can be reached in star graphs. This study provides a comprehensive understanding of random walks on the Vicsek fractals and general treelike networks.

cond-mat.stat-mech↗

arXiv2010

Orbital magnetization of the electron gas on a two-dimensional kagome lattice under a perpendicular magnetic field

The orbital magnetization of the electron gas on a two-dimensional kagome lattice under a perpendicular magnetic field is theoretically investigated. The interplay between the lattice geometry and magnetic field induce nontrivial $k$-space Chern invariant in the magnetic Brillouin zone, which turns to result in profound effects on the magnetization properties. We show that the Berry-phase term in the magnetization gives a paramagnetic contribution, while the conventional term brought about by the magnetic response of the magnetic Bloch bands produces a diamagnetic contribution. As a result, the superposition of these two components gives rise to a delicate oscillatory structure in the magnetization curve when varying the electron filling factor. The relationship between this oscillatory behavior and the Hofstadter energy spectrum is revealed by selectively discussing the magnetization and its two components at the commensurate fluxes of $f$=1/4, 1/3, and 1/6, respectively. In particular, we reveal as a typical example the fractal structure in the magnetic oscillations by tuning the commensurate flux around $f$=1/4. The finite-temperature effect on the magnetization is also discussed.

cond-mat.mes-hall↗

arXiv2009

An alternative approach to determining average distance in a class of scale-free modular networks

Various real-life networks of current interest are simultaneously scale-free and modular. Here we study analytically the average distance in a class of deterministically growing scale-free modular networks. By virtue of the recursive relations derived from the self-similar structure of the networks, we compute rigorously this important quantity, obtaining an explicit closed-form solution, which recovers the previous result and is corroborated by extensive numerical calculations. The obtained exact expression shows that the average distance scales logarithmically with the number of nodes in the networks, indicating an existence of small-world behavior. We present that this small-world phenomenon comes from the peculiar architecture of the network family.

physics.soc-ph↗

arXiv2009

Fractional topological excitations and quantum phase transition in a bilayer 2DEG adjacent to a superconductor film

We study a bilayer two-dimension-electron-gas (2DEG) adjacent to a type-II superconductor thin film with a pinned vortex lattice. We find that with increasing interlayer tunneling, the system of half filling presents three phases: gapped phase-I (topological insulator), gapless critical phase-II (metal), and gapped phase-III (band insulator). The Hall conductance for phase-I/III is 2/0 $e^{2}/h$, and has non-quantized values in phase-II. The excitation (response to topological defect, a local vortex defect) in these three phases shows different behaviors due to the topological property of the system, including fractional charge $e/2$ for each layer in phase-I. While in the case of quarter filling, the system undergoes a quantum phase transition from metallic phase to topological insulator phase (with excitation of fractional charge $e/4$).

cond-mat.mes-hall↗

arXiv2009

Topological winding properties of spin edge states in Kane-Mele graphene model

We study the spin edge states in the quantum spin-Hall (QSH) effect on a single-atomic layer graphene ribbon system with both intrinsic and Rashba spin-orbit couplings. The Harper equation for solving the energies of the spin edge states is derived. The results show that in the QSH phase, there are always two pairs of gapless spin-filtered edge states in the bulk energy gap, corresponding to two pairs of zero points of the Bloch function on the complex-energy Riemann surface (RS). The topological aspect of the QSH phase can be distinguished by the difference of the winding numbers of the spin edge states with different polarized directions cross the holes of the RS, which is equivalent to the Z2 topological invariance proposed by Kane and Mele [Phys. Rev. Lett. 95, 146802 (2005)].

cond-mat.mes-hall↗

arXiv2009

Magnetization of Two Dimensional Heavy Holes with Boundaries in a Perpendicular Magnetic Field

The magnetization of heavy holes in III-V semiconductor quantum wells with Rashba spin-orbit coupling (SOC) in an external perpendicular magnetic field is theoretically studied. We concentrate on the effects on the magnetization induced by the system boundary, the Rashba SOC and the temperature. It is found that the sawtooth-like de Haas--van Alphen (dHvA) oscillations of the magnetization will change dramatically in the presence of such three factors. Especially, the effects of the edge states and Rashba SOC on the magnetization are more evident when the magnetic field is more small. The oscillation center will shift when the boundary effect is considered and the Rashba SOC will bring beating patterns to the dHvA oscillations. These effects on the dHvA oscillations are preferred to be observed at low temperature. With increasing the temperature, the dHvA oscillations turn to be blurred and eventually disappear.

cond-mat.mes-hall↗

arXiv2009

Quantum blockade and loop current induced by a single lattice defect in graphene nanoribbons

We investigate theoretically the electronic transport properties in narrow graphene ribbons with an adatom-induced defect. It is found that the lowest conductance step of a metallic graphene nanoribbon may develop a dip even down to zero at certain values of the Fermi energy due to the defect. Accompanying the occurrence of the conductance dip, a loop current develops around the defect. We show how the properties of the conductance dip depend on the parameters of the defect, such as the relative position and severity of the defect as well as the width and edges of the graphene ribbons. In particular, for metallic armchair-edges graphene nanoribbons, whether the conductance dip appears or not, they can be controlled by choosing the position of the single defect.

cond-mat.mes-hall↗

arXiv2009

Magnetization in two-dimensional electron gas in a perpendicular magnetic field: the roles of edge states and spin-orbit coupling

We study the de Haas--van Alphen (dHvA) oscillations in the magnetization of a two-dimensional electron gas (2DEG) under the influence of the edge states and/or the Rashba spin-orbit interaction (SOI). The boundaries of the systems lift partially the degeneracies of Landau levels (LL's) and the resulting edge states lead to the changes of both the center and the amplitude of the sawtoothlike magnetization oscillation. The SOI mixes the spin-up and spin-down states of neighboring LL's into two unequally spaced energy branches. The inclusion of SOI changes the well-defined sawtooth pattern of the dHvA oscillations in the magnetization. The weaker the magnetic field is, the larger is the change of the dHvA oscillations due to the edge effect and/or the spin-orbit coupling. Some theoretical results are compared with the experimental data.

cond-mat.mes-hall↗

arXiv2008

Topological edge states and quantum Hall effect in the Haldane model

We study the topological edge states of the Haldane's graphene model with zigzag/armchair lattice edges. The Harper equation for solving the energies of the edge states is derived. The results show that there are two edge states in the bulk energy gap, corresponding to the two zero points of the Bloch function on the complex-energy Riemann surface. The edge-state energy loops move around the hole of the Riemann surface in appropriate system parameter regimes. The quantized Hall conductance can be expressed by the winding numbers of the edge states, which reflects the topological feature of the Haldane model.

cond-mat.mes-hall↗

arXiv2008

Spin Hall effect in the kagome lattice with Rashba spin-orbit interaction

We study the spin Hall effect in the kagomé lattice with Rashba spin-orbit coupling. The conserved spin Hall conductance $σ_{xy}^{s}$ (see text) and its two components, i.e., the conventional term $σ_{xy}^{s0}$ and the spin-torque-dipole term $σ_{xy}^{sτ}$, are numerically calculated, which show a series of plateaus as a function of the electron Fermi energy $ε_{F}$. A consistent two-band analysis, as well as a Berry-phase interpretation, is also given. We show that these plateaus are a consequence of the various Fermi-surface topologies when tuning $ε_{F}$. In particular, we predict that compared to the case with the Fermi surface encircling the $\mathbfΓ$ point in the Brillouin zone, the amplitude of the spin Hall conductance with the Fermi surface encircling the $\mathbf{K}$ points is twice enhanced, which makes it highly meaningful in the future to systematically carry out studies of the $\mathbf{K}$-valley spintronics.

cond-mat.mes-hall↗

arXiv2008

Edge states and the integer quantum Hall effect of spin-chiral ferromagnetic kagome lattice with a general spin coupling

The chiral edge states and the quantized Hall conductance (QHC) in the two-dimensional kagomé lattice with spin anisotropies included in a general Hund's coupling region are studied. This kagomé lattice system is periodic in the $x$ direction but has two edges in the $y$ direction. Numerical results show that the strength of the Hund's coupling, as well as the spin chirality, affects the edge states and the corresponding QHC. Within the topological edge theory, we give the expression of the QHC with the winding number of the chiral edge states on the Riemman surface. This expression is also compaired with that within the topological bulk theory and they are found to keep consistent with each other.

cond-mat.mes-hall↗

arXiv2008

Orbital magnetization and its effects in spin-chiral ferromagnetic kagome lattice in the general spin-coupling region

The orbital magnetization and its effects on the two-dimensional kagomé lattice with spin anisotropies included in the general Hund's coupling region have been theoretically studied. The results show that the strength of the Hund's coupling, as well as the spin chirality, contributes to the orbital magnetization $\mathcal{M}$. Upon varying both these parameters, it is found that the two parts of $\mathcal{M}$, i.e., the conventional part $\mathbf{M}_{c}$ and the Berry-phase correction part $\mathbf{M}_Ω$, oppose each other. The anomalous Nernst conductivity is also calculated and a peak-valley structure as a function of the electron Fermi energy is obtained.

cond-mat.mes-hall↗

arXiv2007

Edge states and the integer quantum Hall conductance in spin-chiral ferromagnetic kagome lattice

We investigate the chiral edge states in the two-dimensional ferromagntic kagomé lattice with spin anisotropies included. The system is periodic in the $x$ direction but has two edges in the $y$ direction. The Harper equation for solving the energies of edge states is derived. We find that there are two edge states in each bulk energy gap, corresponding to two zero points of the Bloch function on the complex-energy Riemann surface (RS). The edge-state energy loops parametrized by the momentum $k_{x}$ cross the holes of the RS. When the Fermi energy lies in the bulk energy gap, the quantized Hall conductance is given by the winding number of the edge states across the holes, which reads as $σ_{xy}^{\text{edge}}$=$-\frac{e^{2}}{h}% $sgn$(\sinϕ) $, where $ϕ$ is the spin chiral parameter (see text). This result keeps consistent with that based on the topological bulk theory.

cond-mat.mes-hall↗

arXiv2007

Atomic oxygen adsorption and incipient oxidation of the Pb(111) surface: A density-functional theory study

We study the atomic oxygen adsorption on Pb(111) surface by using density-functional theory within the generalized gradient approximation and a supercell approach. The atomic and energetic properties of purely on-surface and subsurface oxygen structures at the Pb(111) surface are systematically investigated for a wide range of coverages and adsorption sites. The fcc and tetra-II sites (see the text for definition) are found to be energetically preferred for the on-surface and subsurface adsorption, respectively, in the whole range of coverage considered. The on-surface and subsurface oxygen binding energies monotonically increase with the coverage, and the latter is always higher than the former, thus indicating the tendency to the formation of oxygen islands (clusters) and the higher stability of subsurface adsorption. The on-surface and subsurface diffusion-path energetics of atomic oxygen, and the activation barriers for the O penetration from the on-surface to the subsurface sites are presented at low and high coverages. In particular, it is shown that the penetration barrier from the on-surface hcp to the subsurface tetra-I site is as small as 65 meV at low coverage ($Θ$=0.25). The other properties of the O/Pb(111) system, including the charge distribution, the lattice relaxation, the work function, and the electronic density of states, are also studied and discussed in detail, which consistently show the gradually stabilizing ionic O-Pb bond with increase of the oxygen coverage.

cond-mat.mtrl-sci↗