Ze Cheng

Velocity-controlled guiding of electron in graphene: Analogy of optical waveguides

Motivated by the realization of the Dirac point with tunable Fermi velocity in low-dimensional systems, we investigate the guided modes in graphene waveguides corresponding to the electron motion (or the hole motion) in a symmetric velocity barrier. We find that the fundamental mode always exists, but the higher-order mode may disappear. These discrete guided modes imply that there is a lowest cutoff frequency for an incident electron and that the incident electrons with different angles may have different minimum cutoff frequencies. These interesting features will be helpful for the investigation on an electronic fiber.

Electron transport in a ferromagnetic/normal/ferromagnetic tunnel junction based on the surface of a topological insulator

We theoretically study the electron transport properties in a ferromagnetic/normal/ferromagnetic tunnel junction, which is deposited on the top of a topological surface. The conductance at the parallel (P) configuration can be much bigger than that at the antiparallel (AP) configuration. Compared P with AP configuration, there exists a shift of phase which can be tuned by gate voltage. We find that the exchange field weakly affects the conductance of carriers for P configuration but can dramatically suppress the conductance of carriers for AP configuration. This controllable electron transport implies anomalous magnetoresistance in this topological spin valve, which may contribute to the development of spintronics. In addition, there shows an existence of Fabry-Perot-like electron interference in our model based on the topological insulator, which does not appear in the same model based on the two dimensional electron gas.

Quantum entanglement and phase transition in a two-dimensional photon–photon pair model

Abstract We propose a two-dimensional model consisting of photons and photon pairs. In the model, the mixed gas of photons and photon pairs is formally equivalent to a two-dimensional system of massive bosons with non-vanishing chemical potential, which implies the existence of two possible condensate phases. Using the variational method, we discuss the quantum phase transition of the mixed gas and obtain the critical coupling line analytically. Moreover, we also find that the phase transition of the photon gas can be interpreted as enhanced second harmonic generation. We then discuss the entanglement between photons and photon pairs. Additionally, we also illustrate how the entanglement between photons and photon pairs can be associated with the phase transition of the system.

Additional hyper-Raman scattering mechanism due to polar modes

Abstract We establish a unified theoretical framework to describe hyper-Raman scattering of light by nonpolar modes and longitudinal or transverse polar modes. An additional mechanism is found for hyper-Raman scattering from polar modes, which stems from the electric-dipole moments of polar modes. In this framework, we derive an analytical, temperature-dependent expression of the hyper-Raman scattering intensity of light and observe two distinct peaks of hyper-Raman scattering from the split polar modes.

THERMOFIELD ANALYSIS OF MAGNON SQUEEZING STATE IN THE FERROMAGNET

In this paper, we introduce the self-consistent field approximation to treat with the nonlinear interaction among spin waves. Then temperature-dependent Bogoliubov transformation is introduced to generate a new representation which engenders the transition from the zero temperature to the finite temperature. At last, temperature-dependent quantum fluctuation properties of magnons are discussed in the thermal field. At lower temperature, we find that the fluctuation of spin-component at some given time regions can be below the zero-point fluctuation level of the vacuum state and exhibit a periodical squeezing behavior. In particular, these squeezed effects vanish with the increasing of temperature. These squeezing effects differ from the previous studies.

TWO-MODE ENTANGLEMENT VIA SUPERPOSITIONS OF TWO-MODE COHERENT STATES

Two-mode entanglement of superposition states from several two-mode coherent states is investigated according to the entanglement criterion.9 The dependence of entanglement on relative phase angle, coherent amplitude of each mode and phase angle is discussed. It is shown that under certain conditions the superposition states can exhibit entanglement properties.

Localization of surface exciton-polaritons

We investigate the localization of surface exciton-polaritons in the presence of random roughness and spatial dispersion. The localization criteria are examined. The localization effects are embodied in the large enhancement and rapid decay of the field intensity on the surface. The calculation shows that the localization occurs in a limited frequency range above the resonant frequency of transverse excitons.