Kyung Seon Baek

Anisotropic optical properties of free and bound excitons in highly strained A-plane ZnO investigated with polarized photoreflectance and photoluminescence spectroscopy

We have investigated the polarization dependence of the near-band-edge photoluminescence and photoreflectance spectra in nonpolar (A-plane) ZnO films under strong biaxial compressive strain. We show that anisotropic strain and the orientation of the nonpolar plane play an important role in determining the polarization selectivity and properties of excitonic transitions. We identified four distinct band-edge transitions at 3.449, 3.420, 3.386, and 3.326eV. They were identified as E2 and E1 free excitons, E1 excitons bound to a donor, and free-electron-to-bound-hole transition, respectively. Unlike previously reported results on relatively thick nonpolar films, the E1 exciton (lowest energy) was mainly polarized to E⊥c and weakly polarized to E∥c under strong biaxial compressive strain in the 100nm thick film. The E2 exciton (next higher energy) was exclusively polarized to E∥c. The localization energy of DX is 34meV, which is much larger than that in polar ZnO, and the DX was not thermally delocalized even...

Mössbauer Studies of Superexchange Interaction in Tetragonal CuFe2O4

CuFe 2 O 4 has been studied by Mossbauer spectroscopy and X-ray diffraction. The crystal is found to have a tetragonal spinel structure with the lattice constants a 0 =(5.805±0.005) A and C 0 = (8.669 ± 0.005) A. The iron ions are in ferric Fe 3+ states. The temperature dependence of the magnetic hyperfine fields at 57 Fe nuclei at the tetrahedral (A) and octahedral (B) sites is analyzed by the Neel theory of ferrimagnetism. The intersublattice superexchange interaction is found to be antiferromagnetic with its strength of J A-B = -19.5k B while intrasublattice superexchange interactions are ferromagnetic with their strengths of J A-A = 3.1k B and J B-B = 4.4k B .

Mössbauer study of

has been studied by means of M?ssbauer spectroscopy and x-ray diffraction. The crystal is found to have a cubic spinel structure with the lattice constant . The iron ions are in ferric states. The temperature dependences of the magnetic hyperfine fields at the nuclei at the tetrahedral (A) and octahedral (B) sites are analysed using the N?el theory of ferrimagnetism. The inter-sublattice superexchange interaction is found to be antiferromagnetic with a strength of while the intra-sublattice superexchange interactions are ferromagnetic with strengths of and . The Debye temperatures of the tetrahedral and octahedral sites are determined to be and , respectively.

Temperature-dependent photoluminescence of ZnSe/ZnS quantum dots fabricated under the Stranski–Krastanov mode

Self-assembled ZnSe/ZnS quantum dots (QDs) have been grown in the Stranski–Krastanov (S–K) mode using a metalorganic chemical vapor deposition technique under the atomic-layer epitaxy mode. Atomic-force-microscopy measurements on the uncapped ZnSe/ZnS QDs reveal that lens-shaped ZnSe QDs are formed after 1–2 monolayer ZnSe is deposited. The ZnSe QDs are estimated 1–2 nm in height and 25–35 nm in radius. The temperature-dependent behavior of confined carriers in the ZnSe QDs has been investigated through photoluminescence (PL) measurements. PL spectra show a substantial PL linewidth narrowing accompanied by a large redshift of the emission peak energy with increasing temperature. This unusual temperature-dependent behavior is interpreted as the dot-to-dot carrier transfer through the wetting layer, which is common to QDs grown in the S–K mode.

Mössbauer study of “3c” superstructure of the ferrimagnetic Fe7Se8

Abstract Fe7Se8 has been studied by Mossbauer spectroscopy and X-ray diffraction. The crystal structure is found to be a “3c” hexagonal superstructure of the NiAs structure. It is shown that the Mossbauer results obtained at temperatures below 60 K are also consistent with the Okazakis “3c” superstructure model. Abrupt change of quadrupole shifts near 130 K suggests that the spin-rotation transition proceeds abruptly. The iron ions at all three sites are found to be in a highly covalent ferrous state. The Neel temperature is measured to be 447 ± 2 K in fair agreement with the previously reported value. The Debye temperature is also found to be 248 ± 5 K.

A Study of Co Substituted Mn-Ferrite, Mn

The Mn1- xCoxFe2O4 (x=0.0, 0.5, 1.0) materials were prepared by the high-temperature thermal decomposition method and studied for its unique magnetic phenomena of magnetic nanoparticles. The crystal structure is found to be an inverse cubic spinel with a space group of Fd-3m and the lattice constants (a 0) of 8.432, 8.486, and 8.466 Aring , respectively. The samples show magnetization (Ms) of 54.2, 29.4, and 46.9 emu/g, respectively. Also, the coercivities (Hc) of all samples are 32.4, 86.9, and 90.7 Oe, respectively. Mossbauer spectra of all samples were obtained at various temperatures. Mossbauer spectra show a ferrimagnetic state of six-line having the hyperfine field (Hhf) values of 456, 472, and 475 kOe for the tetrahedral sites and 400, 422, and 430 kOe for the octahedral sites, respectively, which increases with doping Co concentration.

_{1 - x}

Abstract Cu5FeS4 has been investigated over a temperature range from 4 to 300 K using Mossbauer technique and X-ray diffraction. At room temperature the structure is orthorhombic with lattice constants a 0 = 10.967 A , b 0 =21.920 A , and c 0 = 10.967 A . The iron ions are found to be in ferric states. As the temperature increases toward the Neel temperature, line broadening and a pronounced central peak appear, suggesting superparamagnetic relaxation. Temperature dependence of anisotropy energy is calculated from the relaxation rate.

Co

Strain effects on sizes and emission energies of ZnSe∕ZnS quantum dots (QDs) have been investigated. The initial strain in the ZnSe QD layer was altered by adjusting the thickness of the ZnS buffer. Consistent blueshifts of the ground-state emission from 4 monolayer ZnSe QDs were observed with increasing the thickness of the ZnS buffer from 20 to 40nm. Atomic-force microscopy revealed that the blueshifts are due to a continuous QD size reduction. We estimated the emission energy as a function of the initial strain in the ZnSe QD layer, which shows that the band-gap engineering is possible through the strain modification.

_{x}

Abstract Zn 0.4 Cu 0.6 Fe 1.2 Cr 0.8 O 4 has been studied by Mossbauer spectroscopy, SQUID magnetometry, and X-ray diffraction. The crystal is found to have a cubic spinel structure with the lattice constant a 0 =8.3667 A . The iron ions are in ferric states and occupy both the tetrahedral (A) and octahedral (B) sites; the fractions of the iron ions at the A-sites and B-sites are 0.52 and 0.34, respectively. While spin orderings are collinear at higher temperatures, spin canting begins to appear around 25 K and increases with decreasing temperature; the canting angle at 4.7 K reaches up to 27°. Debye temperatures of the tetrahedral and octahedral sites are determined to be 339 and 335 K, respectively.

Fe

Abstract The mixed spinel systems Co x Fe 1− x Cr 2 S 4 has been studied using the Mossbauer effect. Spectra have been collected over the temperature range 83–300 K. The isomer shifts indicate that the charge states of Fe ions are ferrous in character throughout the series. Absence of quadrupole splitting above the magnetic ordering temperature T c suggests that iron ions occupy only tetrahedral sites. It is notable that as the temperature decreases below T c , both quadrupole shift and asymmetrical line-broadening appear and increase with decreasing temperature, suggesting the presence of electric field gradient and accompanying relaxation effects.