진공증착된 비정질 규소의 상태밀도 결정

이주열 울산대학교 1981 연구논문집 Vol.12 No.2

진공증착된 비정질 규소는 Plasma deposition에 의해 제작된 시료보다 gap內에 局在化된 defect states가 많기 때문에 상온의 전기 전도도(σRT)가 크다. gap內의 상태밀도, N(ε),가 field effect실험에 의해 0.4eV<(ε??-ε)<0.9eV의 에너지 영역에서 결정되었다. 진공증착된 비정질 규소의 N(ε??)-ε??는 Fermi준위-가 10??㎝??eV??로 plasma deposition에 의해 제작된 시료보다 크게 나타났다. hydrogenation된 시료는 N(ε)가 낮게 나타났는데 T??=350℃에서 가장 작았다. N(ε)는 저온에서 variable range hopping의 여러가지 이론적인 모형에 대한 결과의 分析에 사용되었다. The room temperature conductivity (σRT) of the evaporated a-Si is higher than that of the plasma deposited one, because of the many localized defect states in the gap. The density of states in the gap, N(ε), was determined in the energy range 0.4eV<(ε??-ε)<0.9eV using the field effect experiment. N(ε) of the evaporated a-Si, where ε?? is Fermi level, was about 10??㎝?? eV?? and was larger than that of plasma deposited sample. N(ε) was lowered by the hydrogenation and was minimized at T??=350℃. The knowledge of N(ε) was used in the analysis of the results of the variable range hopping at low temperature using various theoretical models.

Optical and magneto-optical properties of GdFe$_{2}$

Joo Yull Rhee 한국물리학회 2003 THE JOURNAL OF THE KOREAN PHYSICAL SOCIETY Vol.43 No.51

The electronic structures of the ferromagnetic Laves-phase GdFe2 compound were calculated to obtain the diagonal (xx) and o-diagonal (xy) components of the optical conductivity tensor by using an all-electron full-potential linear-augmented-plane-wave method. The so-called LDA+U method is applied to properly treat the Gd 4f electrons in the electronic structure calculations. It improves the agreement between experiments and calculations for the lattice constant and magnetic moments. The calculated results for the xx and xy spectra exhibit a reasonable agreement with the experimental ones; however, application of the LDA+U method has rather insignicant eects on the optical and magneto-optical properties of GdFe2 in the 1.5 .. 5.5-eV range because of the very small number of Gd 5d minority-spin electrons, resulting in almost no contributions of 5d ! 4f interband transitions to the optical and magneto-optical spectra. For xy, application of the LDA+U method results in even worse agreement between experiment and calculation.

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.

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.

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 $Gd_5Si_2Ge_2$ 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.

Electronic structures, and optical and magneto-optical properties of RFe2 (R=Y, Ce, Gd, Yb, and Lu)

Joo Yull Rhee 한국물리학회 2004 THE JOURNAL OF THE KOREAN PHYSICAL SOCIETY Vol.44 No.32

The electronic structures of ferromagnetic Laves-phase RFe2 (R = Y, Ce, Gd, Yb and Lu) compounds were calculated to obtain the diagonal (xx) and o-diagonal (xy) components of the optical-conductivity tensor by using an all-electron full-potential linear-augmented-plane-wave method. The so-called LDA+U method is applied to properly treat the R 4f electrons in the electronic structure calculations, except for YFe2 and LuFe2 whose 4f orbitals are empty and completely full, respectively. Unlike the case of GdFe2, the LDA+U method is crucial to the case of the YbFe2 compound. The application of the LDA+U method improved the agreement between experiments and calculations for the xx and xy spectra, especially for YbFe2. It was shown that the rare-earths 4f electrons do not play a signicant role in determining the optical and magnetooptical spectra of RFe2 (R = Y, Ce, Gd, Yb and Lu) compounds in the 1.5 - 5.5-eV range.

Evidences of the structural phase transition of equiatomic CoPt film

Joo Yull Rhee,G. S. Chang,C. N. Whang,K. W. Kim,Y. P. Lee 한국진공학회 1998 한국진공학회 학술발표회초록집 Vol.- No.-

Optical properties and electronic structures of FeAl alloy

Joo Yull Rhee 한국물리학회 2006 Current Applied Physics Vol.6 No.5

The electronic structures and optical conductivity (OC) spectrum of equiatomic FeAl alloy were calculated using a full-potentiallinearized-augmented-plane-wave method within a generalized-gradient approximation. The calculated OC spectrum does not agreewell with the experimental one. Since the FeAl alloy is known to have a correlation-induced paramagnetic ground state, the so-calledLDA + U method is applied to improve the agreement between the experimental and calculated spectra. If the LDA + U method isapplied, the calculated equilibrium lattice constant is closer to the experimental one than the calculation without LDA + U. Theinclusion of corrections for both the real and imaginary parts of the self-energy, as well as the LDA + U method, markedlyimproved the agreement between the experimental and calculated OC spectra.

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.