• Title/Summary/Keyword: 작은 기울기 근사법

Search Result 2, Processing Time 0.021 seconds

Derivation of Coherent Reflection Coefficient at Mid and Low Frequency for a Rough Surface (불규칙 경계면에 대한 중저주파수 간섭 반사 계수 유도)

  • Chu, Young-Min;Seong, Woo-Jae;Byun, Sung-Hoon;Kim, Sea-Moon
    • The Journal of the Acoustical Society of Korea
    • /
    • v.28 no.3
    • /
    • pp.174-186
    • /
    • 2009
  • When we apply a propagation model to the ocean with boundaries, we can calculate reflected wave using reflection coefficient suggested by Rayleigh assuming the boundaries are flat. But boundaries in ocean such as sea surface and sea bottom have an irregular rough surface. To calculate the reflection loss for an irregular boundary, it is needed to compute the coherent reflection coefficient based on an experimental formula or scattering theory. In this article, we derive the coherent reflection coefficients for a fluid-fluid interface using perturbation theory, Kirchhoff approximation and small-slope approximation respectively. Based on each formula, we can calculate coherent reflection coefficients for a rough sea surface or sea bottom, and then compare them to the Rayleigh reflection coefficient to analyze the reflection loss for a random rough surface. In general, the coherent reflection coefficient based on small-slope approximation has a wide valid region. Comparing it with the coherent reflection coefficients derived from the Kirchhoff approximation and perturbation theory, we discuss a valid region of them.

First-principles Study on the Magnetic Properties of Gd doped Bithmuth-Telluride (Gd 도핑된 비스무스 텔루라이드의 자기적 성질에 대한 제일원리 계산 연구)

  • Van Quang, Tran;Kim, Miyoung
    • Journal of the Korean Magnetics Society
    • /
    • v.26 no.2
    • /
    • pp.39-44
    • /
    • 2016
  • Determination of the structural, electronic, and magnetic properties of the magnetically doped bismuth-telluride alloys are drawing lots of interest in the fields of the thermoelectric application as well as the research on magnetic interaction and topological insulator. In this study, we performed the first-principles electronic structure calculations within the density functional theory for the Gd doped bismuth-tellurides in order to study its magnetic properties and magnetic phase stability. All-electron FLAPW (full-potential linearized augmented plane-wave) method is employed and the exchange correlation potentials of electrons are treated within the generalized gradient approximation. In order to describe the localized f-electrons of Gd properly, the Hubbard +U term and the spin-orbit coupling of the valence electrons are included in the second variational way. The results show that while the Gd bulk prefers a ferromagnetic phase, the total energy differences between the ferromagnetic and the antiferromagnetic phases of the Gd doped bismuth-telluride alloys are about ~1meV/Gd, indicating that the stable magnetic phase may be changed sensitively depending on the structural change such as defects or strains.