arXiv subjects

Lei Zhou

Publications and source records attributed to Lei Zhou.

170 records · Page 10Linked to original sources

arXiv2009

Fractal plasmonic metamaterials for subwavelength imaging

We show that a metallic plate with fractal-shaped slits can be homogenitized as a plasmonic metamaterial with plasmon frequency dictated by the fractal geometry. Owing to the all-dimensional subwavelength nature of the fractal pattern, our system supports both transverse-electric and transverse-magnetic surface plasmons. As a result, this structure can be employed to focus light sources with all-dimensional subwavelength resolutions and enhanced field strengths. Microwave experiments reveal that the best achievable resolution is only, and simulations demonstrate that similar effects can be realized at infrared frequencies with appropriate designs.

physics.optics↗

arXiv2005

Vortex-like surface wave and its role in the transient phenomena of meta-material focusing

We show that a slab of meta-material (with $ε=μ=-1+iΔ$) possesses a vortex-like surface wave with no ability to transport energy, whose nature is completely different from a localized mode or a standing wave. Through computations based on a rigorous time-dependent Green's function approach, we demonstrate that such a mode inevitably generates characteristic image oscillations in two dimensional focusing with even a monochromatic source, which were observed in many numerical simulations, but such oscillations are weak in three dimensional focusing.

physics.optics↗

arXiv2004

Orbital Polarization, Surface Enhancement and Quantum Confinement in Nano-cluster Magnetism

Within a rather general tight-binding framework, we studied the magnetic properties of Ni$_{n}$ clusters with $n=9$ through 60. In addition to usual hopping, exchange, and spin-orbit coupling terms, our Hamiltonian also included orbital correlation and valence orbital shift of surface atoms. We show that orbital moment not only contributes appreciably to the total moment in this range of cluster size, but also dominates the oscillation of total moment with respect to the cluster size. Surface enhancement is found to occur not only for spin but, even stronger, also for orbital moment. The magnitude of this enhancement depends mainly on the coordination deficit of surface atoms, well described by a simple interpolation. For very small clusters ($n \leq 20$), quantum confinement of 4s electrons has drastic effects on 3$d$ electron occupation, and thus greatly influences both spin and orbital magnetic moments. With physically reasonable parameters to account for orbital correlation and surface valence orbital shift, our results are in good quantitative agreement with available experiments, evidencing the construction of a unified theoretical framework for nano-cluster magnetism.

cond-mat.mtrl-sci↗

arXiv2003

Anisotropy and oblique total transmission at a planar negative-index interface

We show that a class of negative index (n) materials has interesting anisotropic optical properties, manifest in the effective refraction index that can be positive, negative, or purely imaginary under different incidence conditions. With dispersion taken into account, reflection at a planar negative-index interface exhibits frequency selective total oblique transmission that is distinct from the Brewster effect. Finite-difference-time-domain simulation of realistic negative-n structures confirms the analytic results based on effective indices.

cond-mat.mtrl-sci↗

arXiv1996

An extension of the characteristic angle method to the easy-plane spin-3/2 ferromagnet

The Characteristic Angle (CA) method [Lei Zhou and Ruibao Tao, J. Phys. A, {\bf 27} 5599] developed previously for the easy-plane spin-1 magnetic systems has been successfully extended to the spin-3/2 case. A compact form of the CA spin-3/2 operator transformation is given, then the ground state energy, the magnon dispersion relation and the spontaneous magnetization are discussed for an easy-plane spin-3/2 ferromagnet by using the CA method. Comparisons with the old theoretical methods are made in the end.

cond-mat↗

arXiv1996

A quantum model for the magnetic multi-valued recording

We have proposed a quantum model for the magnetic multi-valued recording in this paper. The hysteresis loops of the two-dimensional systems with randomly distributed magnetic atoms have been studied by the quantum theory developed previously. The method has been proved to be exact in this case. We find that the single-ion anisotropies and the densities of the magnetic atoms are mainly responsible for the hysterisis loops. Only if the magnetic atoms contained by the systems are of different (not uniform) anistropies and their density is low, there may be more sharp steps in the hysteresis loops. Such materials can be used as the recording media for the so-called magnetic multi-valued recording. Our result explained the experimental results qualitativly.

cond-mat↗

arXiv1996

Quantum model for magnetic multivalued recording in coupled multilayers

In this paper, we discuss the possibilities of realizing the magnetic multi-valued (MMV) recording in a magnetic coupled multilayer. The hysteresis loop of a double-layer system is studied analytically, and the conditions for achieving the MMV recording are given. The conditions are studied from different respects, and the phase diagrams for the anisotropic parameters are given in the end.

cond-mat↗

arXiv1996

Spin-one ferromagnets with single-ion anisotropy in a perpendicular external field

In this paper, the conventional Holstein-Primakoff method is generalized with the help of the characteristic angle transformation [Lei Zhou and Ruibao Tao, J. Phys. A {\bf 27} 5599 (1994)] for the spin-one magnetic systems with single-ion anisotropies. We find that the weakness of the conventional method for such systems can be overcome by the new approach. Two models will be discussed to illuminate the main idea, which are the ``easy-plane" and the ``easy-axis" spin-one ferromagnet, respectively. Comparisons show that the current approach can give reasonable ground state properties for the magnetic system with ``easy-plane" anisotropy though the conventional method never can, and can give a better representation than the conventional one for the magnetic system with ``easy-axis" anisotropy though the latter is usually believed to be a good approximation in such case. Study of the easy-plane model shows that there is a phase transition induced by the external field, and the low-temperature specific heat may have a peak as the field reaches the critical value.

cond-mat↗