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Li-Gang Wang

Publications and source records attributed to Li-Gang Wang.

At least 55 records · Page 3Linked to original sources
arXiv2013

Electronic band gaps and transport properties in aperiodic bilayer graphene superlattices of Thue-Morse sequence

We investigate electronic band structure and transport properties in bilayer graphene superlattices of Thue-Morse sequence. It is interesting to find that the zero-$\bar{k}$ gap center is sensitive to interlayer coupling $t'$, and the centers of all gaps shift versus $t'$ at a linear way. Extra Dirac points may emerge at $k_{y}\ne$0, and when the extra Dirac points are generated in pairs, the electronic conductance obeys a diffusive law, and the Fano factor tends to be 1/3 as the order of Thue-Morse sequence increases. Our results provide a flexible and effective way to control the transport properties in graphene.

cond-mat.str-el↗
arXiv2013

Goos-Hänchen Shifts of Partially Coherent Light Fields

We investigate the Goos-Hänchen (GH) shifts of partially coherent fields (PCFs) by using the theory of coherence. We derive a formal expression for the GH shifts of PCFs in terms of Mercer's expansion, and then clearly demonstrate the dependence of the GH shift of each mode of PCFs on spatial coherence and beam width. We discuss the effect of spatial coherence on the resultant GH shifts, especially for the cases near the critical angles, such as totally reflection angle.

physics.optics↗
arXiv2013

Comment on "Spatial Coherence and Optical Beam Shifts"

This comment is to show that our simulation data, based on our theory and method in Ref. [J. Phys. B 41, 055401 (2008)], are also in agreement with the experimental data presented for $D_{p}-D_{s}$ in Ref. [Phys. Rev. Lett. \textbf{109}, 213901 (2012)]. We also demonstrate how to show the effect of spatial coherence on the GH shifts in this comment, therefore we disagree with the claims in Ref. [Phys. Rev. Lett. \textbf{109}, 213901 (2012)].

physics.optics↗
arXiv2012

Electronic band gaps and transport in aperiodic graphene superlattices of Thue-Morse sequence

We have studied the electronic properties in aperiodic graphene superlattices of Thue-Morse sequence. Although the structure is aperiodic, an unusual Dirac point (DP) does exist and its location is exactly at the position of the zero-averaged wave number (zero-$\bar{k})$. Furthermore, the zero-$\bar{k}$ gap associated with the DP is robust against the lattice constants and the incident angles, and multi-DPs can appear under the suitable conditions. A resultant controllability of electron transport in Thue-Morse sequence is predicted, which may facilitate the development of many graphene-based electronics.

cond-mat.mes-hall↗
arXiv2011

Electronic beam shifts in monolayer graphene superlattice

Electronic analogue of generalized Goos-Hänchen shifts is investigated in the monolayer graphene superlattice with one-dimensional periodic potentials of square barriers. It is found that the lateral shifts for the electron beam transmitted through the monolayer graphene superlattice can be negative as well as positive near the band edges of zero-$\bar{k}$ gap, which are different from those near the band edges of Bragg gap. These negative and positive beam shifts have close relation to the Dirac point. When the condition $q_A d_A= -q_B d_B= m π$ ($m=1,2,3...$) is satisfied, the beam shifts can be controlled from negative to positive when the incident energy is above the Dirac point, and vice versa. In addition, the beam shifts can be greatly enhanced by the defect mode inside the zero-$\bar{k}$ gap. These intriguing phenomena can be verified in a relatively simple optical setup, and have potential applications in the graphene-based electron wave devices.

cond-mat.mes-hall↗
arXiv2011

Scaling of quantum Zeno dynamics in thermodynamic systems

We study the quantum Zeno effect (QZE) in two many-body systems, namely the one-dimensional transverse-field Ising model and the Lipkin-Meshkov-Glick (LMG) model, coupled to a central qubit. Our result shows that in order to observe QZE in the Ising model, the frequency of the projective measurement should be of comparable order to that of the system sizes. The same criterion also holds in the symmetry broken phase of the LMG model while in the model's polarized phase, the QZE can be easily observed.

cond-mat.str-el↗
arXiv2011

Tunneling states in graphene heterostructures consisting of two different graphene superlattices

We have theoretically investigated the properties of electronic transport in graphene heterostructures, which are consisted of two different graphene superlattices with one-dimensional periodic potentials. It is found that such heterostructures possess an unusual tunneling state occurring inside the original forbidden gaps, and the electronic conductance is greatly enhanced and Fano factor is strongly suppressed near the energy of the tunneling state. Finally we present the matching condition of the impedance of the pseudospin wave for occuring the tunneling state by using the Bloch-wave expansion method.

cond-mat.mes-hall↗
arXiv2010

Robust zero-averaged wave-number gap inside gapped graphene superlattices

In this paper, the electronic band structures and its transport properties in the gapped graphene superlattices, with one-dimensional (1D) periodic potentials of square barriers, are systematically investigated. It is found that a zero averaged wave-number (zero-$\overline{k}$ ) gap is formed inside the gapped graphene-based superlattices, and the condition for obtaining such a zero-$\overline{k}$ gap is analytically presented. The properties of this zero-$\overline{k}$ gap including its transmission, conductance and Fano factor are studied in detail. Finally it is revealed that the properties of the electronic transmission, conductance and Fano factor near the zero-$\overline{k}$ gap are very insensitive to the structural disorder for the finite graphene-based periodic-barrier systems.

cond-mat.mes-hall↗
arXiv2010

Electronic Band gaps and transport properties inside graphene superlattices with one-dimensional periodic squared potentials

The electronic transport properties and band structures for the graphene-based one-dimensional (1D) superlattices with periodic squared potentials are investigated. It is found that a new Dirac point is formed, which is exactly located at the energy which corresponds to the zero (volume) averaged wavenumber inside the 1D periodic potentials. The location of such a new Dirac point is robust against variations in the lattice constants, and it is only dependent on the ratio of potential widths. The zero-averaged wavenumber gap associated with the new Dirac point is insensitive to both the lattice constant and the structural disorder, and the defect mode in the zero-averaged wavenumber gap is weakly dependent on the insident angles of carriers.

cond-mat.mes-hall↗
arXiv2009

Repeater-assisted Zeno effect in classical stochastic processes

As a classical state, for instance a digitized image, is transferred through a classical channel, it decays inevitably with the distance due to the surroundings' interferences. However, if there are enough number of repeaters, which can both check and recover the state's information continuously, the state's decay rate will be significantly suppressed, then a classical Zeno effect might occur. Such a physical process is purely classical and without any interferences of living beings, therefore, it manifests that the Zeno effect is no longer a patent of quantum mechanics, but does exist in classical stochastic processes.

physics.data-an↗
arXiv2009

Transmission gap, Bragg-like reflection, and Goos-Hänchen shifts near the Dirac point inside a negative-zero-positive index metamaterial slab

Motivated by the realization of the Dirac point (DP) with a double-cone structure for optical field in the negative-zero-positive index metamaterial (NZPIM), the reflection, transmission, and Goos-Hänchen (GH) shifts inside the NZPIM slab are investigated. Due to the linear Dirac dispersion, the transmission as the function of the frequency has a gap, thus the correspond reflection has a frequency or wavelength window for the perfect reflection, which is similar to the Bragg reflection in the one-dimensional photonic crystals. Near the DP, the associated GH shifts in the transmission and reflection can be changed from positive to negative with increasing the wavelength. These negative and positive shifts can also be enhanced by transmission resonances, when the frequency is far from that at the DP. All these phenomena will lead to some potential applications in the integrated optics and optical devices.

physics.optics↗
arXiv2009

The reversibility of the Goos-Hänchen shift near the band-crossing structure of one-dimemsional photonic crystals containing left-handed metamaterials

We perform a theoretical investigation on the Goos-Hächen (GH) shift in one-dimensional photonic crystals (1DPCs) containing left-handed metamaterials (LHMs). We find an unusal effect of the GH shift near the photonic band-crossing structure, which is located at the condition, $% -k_{z}^{(A)}d_{A}=k_{z}^{(B)}d_{B}=mπ$ $(m=1,2,3...)$, under the inclined incident angle, here A denotes the LHM layer and B denotes the dielectric layer. Above the frequency of the band-crossing point (BCP), the GH shift changes from negative to positive as the incident angle increases, while the GH shift changes reversely below the BCP frequency. This effect is explained in terms of the phase property of the band-crossing structure.

physics.optics↗
arXiv2009

Fidelity and Fidelity Susceptibility of Pulses in Dispersive Media

Motivated by the growing importance of the fidelity and fidelity susceptibility (FS) in quantum critical phenomena, we use these concepts to describe the pulse propagation inside the dispersive media. It is found that there is a dramatic change in the fidelity and the FS of the pulse at a critical propagation distance inside a dispersive medium, and whether such a dramatic change for a light pulse occurs or not strongly depends on both the dispersive strength of the media and the pulse property. We study in detail about the changes of the fidelity and the FS for both a smooth and a truncated Gaussian pulse through the abnormal and normal dispersive media, where the group velocities are well defined. Our results show that both the fidelity and the FS could be very useful to determine whether the pulse is completely distorted or not at the critical distance, therefore it would be very helpful to find the maximal effective propagation region of the pulse's group velocity, in terms of the changes of the pulse's fidelity and FS.

physics.optics↗
arXiv2008

Realization of Dirac Point with Double Cones in Optics

The Dirac point with a double-cone structure for optical fields, an optical analogy Dirac fermions in graphene, can be realized in optically homogenous metamaterials. The condition for the realization of Dirac point in optical systems is the varying of refractive index from negative to zero and then to positive. Our analytical and numerical analysis have verified that, similar to electrons in graphene, the light field near the Dirac point possesses of the pseudodiffusive property, obeying the 1/L scaling law, where L is the propagating distance of light inside the media.

physics.optics↗
arXiv2008

Is it possible for a superluminal signal velocity without violating causality?

We present a model for a vacuum-like effective medium composed of the absorbing and gain media under the special designed parameters. Within the linear response theory, we prove that any pulse signal (with or without a discontinuity) through such a kind of vacuum-like effective media is always equal to the light speed in vacuum () without any distortion. As well known that the group velocity in anomalous or normal dispersive media may be smaller or larger than, or even become negative, but the discontinuous point always propagates at the velocity . Therefore we present some discussions on different definitions of the signals, based on the light pulses with a well-defined shape or with a sudden change, for trying to understand two possibilities for the signal velocity without violating the causality.

physics.optics↗
arXiv2008

Hanbury Brown-Twiss effect and thermal light ghost imaging

We show that the essential physics of the Hanbury Brown-Twiss (HBT) and the thermal light ghost imaging experiments is the same, i.e., due to the intensity fluctuations of the thermal light. However, in the ghost imaging experiments, a large number of bits information needs to be treated together, whereas in the HBT there is only one bit information required to be obtained. In the HBT experiment far field is used for the purpose of easy detection, while in the ghost image experiment near (or not-far) field is used for good quality image.

physics.optics↗