Electronic Structures of Giant Magnetocaloric $Gd_5Si_2Ge_2$ Alloy

Rhee, Joo-Yull The Korean Vacuum Society 2002 Journal of Korean Vacuum Science & Technology Vol.6 No.4

The electronic structures of Gd$_{5}$Si$_2$Ge$_2$ compound, which has a giant magnetocaloric effect, in the monoclinic and orthorhombic phases were calculated using the tight-binding linear-muffin-tin-orbital method within the atomic-sphere approximation. The calculated total energies of the monoclinic and orthorhombic structures in the paramagnetic phase confirm that the orthorhombic structure is more stable than monoclinic structure. The density of states (DOS) at the Fermi level of the orthorhombic phase is higher than that of the monoclinic phase in the paramagnetic phase, fulfilling the Stoner criterion. The calculated charge density verified the breaking of Ge(Si)-Ge(Si) bonding in the basal plane upon the orthorhombic-monoclinic phase transition. The DOS curve fairly well reproduces the photoemission spectrum.m.

Metamagnetism in $Fe_3$Al Alloy

Rhee, Joo-Yull,Lee, Young-Pak The Korean Vacuum Society 2003 Applied Science and Convergence Technology Vol.12 No.1

In this study we report the results of ab initio first-principles calculations to investigate the possibility of metamagnetic behavior in Fe$_3$Al alloy. We used the WIEN2k package of full-potential linearized-augmented- plane-wave method within the local-spin-density approximation to the density-functional theory. The exchange-correlation functional is the generalized-gradient approximation of Perdew-Burke-Ernzerhof. The theoretical lattice constant, which is about 0.5% smaller than the experimental one, is obtained by minimizing the total energy. If the volume decreases about 9 % from the equilibrium, the total magnetic moment decreases abruptly from 4.6 $\mu_{B}$/f.u. to 4.0 $\mu_{B}$/f.u. Since this change is considerably large (∼14%), it is possible to measure by a simple high-pressure experiment at about 180 kbar.

New Method of Determination of Density of States in the Pseudogap of Amorphous Semiconductor

Rhee, Joo-Yull 울산대학교 1984 연구논문집 Vol.15 No.2

장효과 실험을 이용한 비정질 반도체의 에너지갭 안에서의 상태밀도를 정하는 새롭고 간단한 방법을 구했다. finite-temperature근사를 제외하고는 아무런 가정도 사용되지 않았다. 이 방법은 세 개의 간단히 가정된 상태밀도 분포들-일정한 상태밀도, 포물형 상태밀도 그리고 지수형 상태밀도-의해 검토되었다. 이러한 검토 결과 이 이론은 타당하다는 것이 입증되었다. Simple and direct method of calculation of the density of states in the pseudogap of amorphous semiconductor from the field-effect-conductance-change measurement is derived. There is no assumptions but finite temperature approximation. this theory is checked by such simplified assumptions as constant, parabolic and exponential density of states distributions and these prove that the theory is valid.

Theoretical Interpretation of Field Effect Experiment of Amorphous Semiconductor

Rhee, Joo-Yull 울산대학교 1984 연구논문집 Vol.15 No.2

직류 글로우 방전에 의한 사일렌의 분해로 제작된 수소화된 비정질규소에 대한 장효과 실험결과를 a-SiFEF이론으로 해석해 보았다. 이론은 log ID-log(VF-VFB)의 그래프가 직선이 될 것으로 예상하였는데 실험결과와 잘 일치하였다. The field-effect-experiment data of the hydrogenated a-Si prepared by the glow discharge decomposition of silane was analyzed by the theoretical method of a-Si Field-Effect-Transister(FET) under assumption of exponential N(E). The theory predicts that log ID-log(VF-VFB)curve becomes linear and the prediction and the experimental results are qualitatively in good agreements.

Optical properties of giant magnetocaloric Gd5Si2Ge2

Joo Yull Rhee 한국물리학회 2003 THE JOURNAL OF THE KOREAN PHYSICAL SOCIETY Vol.43 No.42

The electronic structures of the Gd$_{5}$Si$_{2}$Ge$_{2}$ compound, which has a giant magnetocaloric effect, in the monoclinic phase were calculated using an all-electron full-potential linearized-augmented-plane-wave method within the local-density approximation. The obtained self-consistent charge density and potential were used to calculate the optical conductivity spectra for both the diagonal and the off-diagonal components. The off-diagonal component is negligibly small. Although the crystal structure is monoclinic, the optical anisotropy is rather insignificant in the 2 $-$ 6 eV range while there is some anisotropy in the 0 $-$ 2 eV range. A possible explanation is discussed.

Metamagnetism in Fe₃Al Alloy

Joo Yull Rhee,Young-Pak Lee 한국진공학회(ASCT) 2003 Applied Science and Convergence Technology Vol.12 No.S1

In this study we report the results of ab initio first-principles calculations to investigate the possibility of metamagnetic behavior in Fe3Al alloy. We used the WIEN2k package of full-potential linearized-augmented- plane-wave method within the local-spin-density approximation to the density-functional theory. The exchange-correlation functional is the generalized-gradient approximation of Perdew-Burke-Ernzerhof. The theoretical lattice constant, which is about 0.5% smaller than the experimental one, is obtained by minimizing the total energy. If the volume decreases about 9 % from the equilibrium, the total magnetic moment decreases abruptly from 4.6 μB/f.u. to 4.0 μB/f.u. Since this change is considerably large (~14%), it is possible to measure by a simple high-pressure experiment at about 180 kbar.

Electronic Structures and Optical Properties of Fe2VAl and Fe2VGa Alloys

Joo Yull Rhee 한국물리학회 2005 THE JOURNAL OF THE KOREAN PHYSICAL SOCIETY Vol.47 No.4

The electronic structures and the optical conductivity (OC) spectra of the Fe2VAl Heusler alloy (HA) were calculated by using the WIEN2k package utilizing an all-electron full-potential linearized- augmented-plane-wave method and compared with the experimental results. Those of the Fe3Al alloy, the parent phase of the Fe2VAl HA, and the Fe2VGa HA, which is an isoelectronic alloy of the Fe2VAl HA, were also calculated for the omparison. For the case of the Fe3Al alloy, the calculated OC spectrum was in good agreement with the experiment, while, for the case of the Fe2VAl alloy, the agreement was very poor. This is presumably due to the well-known of the local-density approximation. Nevertheless, the present calculational results can reproduce relatively well the signant reduction in the intensity of peak at low energies (1 { 2 eV) in the OC spectra for the Fe3Al alloy compared to that for the Fe2VAl alloy. The Fe2VGa alloy exhibits an OC spectrum very similar to that of the Fe2VAl alloy. The calculated pseudogap is much wider than the experimental one, which might be due to an overestimated hybridization between Fe 3d and Al sp states.

Pressure dependent metamagnetism of Fe3M (M=Al and Si)

Joo Yull Rhee 한국물리학회 2004 THE JOURNAL OF THE KOREAN PHYSICAL SOCIETY Vol.45 No.1

In this study we report the results of rst-principles calculations to investigate the possibility of pressure dependent metamagnetic behavior in Fe3M (M = Al and Si) alloys. We used the WIEN2k package of the full-potential linearized-augmented-plane-wave method. For the case of Fe3Al (Fe3Si), the theoretical lattice constant, which is about 0.7 % (1 %) larger (smaller) than the experimental value, is obtained by minimizing the total energy. If the volume decreases by about 16 % (9 %) from equilibrium, the total magnetic moment decreases abruptly from 5.0 B/cell (4.0 B/cell) to 3.2 B/cell (4.6 B/cell). Since this change is considerably large (67 % for Fe3Al and 14 % for Fe3Si), it is possible to measure by a simple high-pressure experiment at about 80 kbar for Fe3Al and 50 kbar for Fe3Si. The Fe atoms, similar to fcc-Fe (FeI), exhibit metamagnetism.

Electronic Structures of Giant Magnetocaloric Gd5Si₂Ge₂ Alloy

Joo Yull Rhee 한국진공학회(ASCT) 2002 Journal of Korean Vacuum Science & Technology Vol.6 No.4

The electronic structures of Gd_5Si₂Ge₂ compound, which has a giant magnetocaloric effect, in the monoclinic and orthorhombic phases were calculated using the tight-binding linear-muffin-tin-orbital method within the atomic-sphere approximation. The calculated total energies of the monoclinic and orthorhombic structures in the paramagnetic phase confirm that the orthorhombic structure is more stable than monoclinic structure. The density of states (DOS) at the Fermi level of the orthorhombic phase is higher than that of the monoclinic phase in the paramagnetic phase, fulfilling the Stoner criterion. The calculated charge density verified the breaking of Ge(Si)-Ge(Si) bonding in the basal plane upon the orthorhombic→monoclinic phase transition. The DOS curve fairly well reproduces the photoemission spectrum.

PUBLIC HEALTH PROBLEMS OF WATER POLLUTION

( G Yull Rhee ) 대한보건협회 1990 90년 보건종합학술대회 연제지 Vol.- No.-