Current Search: Medina, Fernando D. (x)
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- Title
- Experimental studies of yttrium barium(2)(copper(1-x)iron(x))(3)oxygen(7+delta) and bismuth(2)strontium(2)calcium(n-1)copper(n)oxygen(2n+4) high T(c) superconductors.
- Creator
- Lu, Huizong, Florida Atlantic University, Medina, Fernando D.
- Abstract/Description
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Fe-substituted Y123 and Bi-based high Tc superconducting samples were prepared and studied. Magnetic susceptibility measurements, x-ray diffraction and Scanning Electron Microscopy were used to study the transition temperature Tc, preferential grain orientation and phase formation of the compounds. Tc was found to decrease more rapidly with Fe concentrations above 2.5%. Thermally reducing the oxygen content shows that Fe clusters form at high Fe concentrations. Magnetic susceptibility and x...
Show moreFe-substituted Y123 and Bi-based high Tc superconducting samples were prepared and studied. Magnetic susceptibility measurements, x-ray diffraction and Scanning Electron Microscopy were used to study the transition temperature Tc, preferential grain orientation and phase formation of the compounds. Tc was found to decrease more rapidly with Fe concentrations above 2.5%. Thermally reducing the oxygen content shows that Fe clusters form at high Fe concentrations. Magnetic susceptibility and x-ray diffraction measurements show that only the 80 K phase is present in Bi4Sr3Ca3Cu6Ox samples. For Bi0.7Pb0.3SrCaCu1.8Ox and Bi1.8Pb0.2Sr2Ca2Cu3Ox samples, the 110 K phase becomes dominant after sintering for 126 hours and 48 hours respectively. Preferential grain orientation was also observed in these materials.
Show less - Date Issued
- 1990
- PURL
- http://purl.flvc.org/fcla/dt/14643
- Subject Headings
- High temperature superconductivity, High temperature superconductors
- Format
- Document (PDF)
- Title
- Photoluminescence characterization of zinc(1-x) manganese(x) sulfur under different excitation sources.
- Creator
- Ye, Yubin, Florida Atlantic University, Medina, Fernando D.
- Abstract/Description
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A study of the temperature dependence of the photoluminescence spectra from Zn1-xMnxS (x = 0.01, 0.03, 0.05, 0.10, 0.20, 0.25, 0.30, 0.35 and 0.50) with different excitation energies (514.5, 488.0, 457.9, 405.5 and 365.9 nm) is given. A new band is reported in addition to the four usual emission bands. The unusual shift of the 2.1 eV (YEL) band energy is first observed in our experiments. Models for YEL, ORG (2.0 eV) and RED (1.7 eV) emission bands have been proposed. The quenching with...
Show moreA study of the temperature dependence of the photoluminescence spectra from Zn1-xMnxS (x = 0.01, 0.03, 0.05, 0.10, 0.20, 0.25, 0.30, 0.35 and 0.50) with different excitation energies (514.5, 488.0, 457.9, 405.5 and 365.9 nm) is given. A new band is reported in addition to the four usual emission bands. The unusual shift of the 2.1 eV (YEL) band energy is first observed in our experiments. Models for YEL, ORG (2.0 eV) and RED (1.7 eV) emission bands have been proposed. The quenching with concentration and temperature and the influence of excitation energy on the photoluminescence are discussed. The spin-spin exchange interaction between the nearest neighbor Mn^+2 ions (d-d electrons), and the exchange interaction between 3d electrons of Mn^+2 ions and the conduction or valence band electrons (d-s or d-p electrons) are introduced to interpret the shift of the bands. The strong electron-phonon interaction is also analyzed, and an effective phonon energy and a Huang-Rhys factor are obtained.
Show less - Date Issued
- 1993
- PURL
- http://purl.flvc.org/fcla/dt/14988
- Subject Headings
- Semiconductors, Magnetic ions, X-ray spectroscopy, Zinc sulfide crystals
- Format
- Document (PDF)
- Title
- Photoluminescence analysis of zinc(1-x) manganese(x) selenium.
- Creator
- Xue, Jun, Florida Atlantic University, Medina, Fernando D.
- Abstract/Description
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Zn1-x is a ternary alloy belonging to a class of materials known as Diluted Magnetic Semiconductors (DMS's). Single-crystal samples of this material possesses a cubic (zinc-blende) structure for x 300 K. Three emission bands are detected and are named the YEL (centered ~2.12 eV), ORA (centered 1.91 --> 1.98 eV) and IR bands. The shift of the YEL band is observed and related to the d-d interaction of the Mn^+2 ions and to thermal lattice expansion. The electron-phonon interaction is used to...
Show moreZn1-x is a ternary alloy belonging to a class of materials known as Diluted Magnetic Semiconductors (DMS's). Single-crystal samples of this material possesses a cubic (zinc-blende) structure for x<0.30 and a hexagonal (wurtzite) structure for 0.30300 K. Three emission bands are detected and are named the YEL (centered ~2.12 eV), ORA (centered 1.91 --> 1.98 eV) and IR bands. The shift of the YEL band is observed and related to the d-d interaction of the Mn^+2 ions and to thermal lattice expansion. The electron-phonon interaction is used to explain the broadening of the YEL band. A model is proposed to explain the existence of the ORA band and the shift of its energy with temperature and Mn concentration. This model for the first time provides a detailed explanation of the anomalous behavior of the energy gap of this material for low Mn concentrations.
Show less - Date Issued
- 1997
- PURL
- http://purl.flvc.org/fcla/dt/15388
- Subject Headings
- Diluted magnetic semiconductors--Analysis, Photoluminescence
- Format
- Document (PDF)
- Title
- Photoluminescence study of zinc(1-x)manganese(x)sulfur.
- Creator
- Moreno, Miguel Angel, Florida Atlantic University, Medina, Fernando D.
- Abstract/Description
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Zinc(1-x)manganese(x)sulfur is a ternary semiconductor compound which belongs to the family of diluted magnetic semiconductors (DMS). It has two crystalline phases: cubic for x
Show moreZinc(1-x)manganese(x)sulfur is a ternary semiconductor compound which belongs to the family of diluted magnetic semiconductors (DMS). It has two crystalline phases: cubic for x < 0.10, and hexagonal for 0.10 < x < 0.50. Its energy gap is temperature and x dependent. The gap, ranging from 3.7 to 3.8 eV, is the largest of the DMS. These properties allow studies of the energy levels of the Mn^2+ ions in ZnS and their internal or ion-to-band transitions, by using photoluminescence. Samples with x = 0.01-0.50 were excited with the 5145A line of an Ar+ laser. The spectra, taken at temperatures between 10 and 300K, showed three bands: a band peaking at 2.12 eV was related to the 4T1 to 6A1 internal transition; a band peaking at 1.95-2.10 eV (depending on T) was related to the 4T1 to Gamma 1 5 ion-to-band transition; the origin of the third band, peaking at 1.65 eV, was discussed. A partial fourth band in the IR region was detected. The position of the 6A1 state with respect to the valence band was established.
Show less - Date Issued
- 1989
- PURL
- http://purl.flvc.org/fcla/dt/14560
- Subject Headings
- Diluted magnetic semiconductors, Photoluminescence
- Format
- Document (PDF)
- Title
- Spectroscopic studies of zinc(1-x) manganese(x) sulfur.
- Creator
- Chen, Xiaoming., Florida Atlantic University, Medina, Fernando D.
- Abstract/Description
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The optical properties of the diluted magnetic semiconductor Zn1-x MnxS have been studied by using photoluminescence emission, lifetime and excitation spectroscopies. In photoluminescence spectroscopy three bands were found peaking at 585 nm (yellow band), 650 nm (orange band) and 750 nm (red band). For the yellow and red bands the peak position changes little with Mn concentration. For the orange band the peak energy decreases with increasing Mn concentration. The lifetimes of the yellow,...
Show moreThe optical properties of the diluted magnetic semiconductor Zn1-x MnxS have been studied by using photoluminescence emission, lifetime and excitation spectroscopies. In photoluminescence spectroscopy three bands were found peaking at 585 nm (yellow band), 650 nm (orange band) and 750 nm (red band). For the yellow and red bands the peak position changes little with Mn concentration. For the orange band the peak energy decreases with increasing Mn concentration. The lifetimes of the yellow, orange and red bands were found to be about 70, 50 and 7 mus. The results of the lifetime measurements support the assumption that there is energy transfer from the yellow to the red band through radiationless transitions. In the excitation spectroscopy of the yellow band four peaks were found peaking near 526, 487, 461 and 426 nm. These four bands have been ascribed to transitions from the four lowest excited states of Mn ++ to the ground state. (Abstract shortened with permission of author.)
Show less - Date Issued
- 1992
- PURL
- http://purl.flvc.org/fcla/dt/14790
- Subject Headings
- Diluted magnetic semiconductors, Photoluminescence, Spectrum analysis
- Format
- Document (PDF)
- Title
- Temperature dependence of polarized Raman spectra of zinc tungsten oxide single crystals.
- Creator
- Wang, Hong, Florida Atlantic University, Medina, Fernando D.
- Abstract/Description
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In this thesis, a group-theoretical analysis of the characteristic modes of lattice vibrations for ZnWO4 single crystals is given. The assignments of the vibrational modes have been made. The temperature dependence of the Raman spectra, which is reported here for the first time, has been obtained experimentally in various polarization geometries. Anharmonic contributions and interactions between phonons are discussed.
- Date Issued
- 1991
- PURL
- http://purl.flvc.org/fcla/dt/14714
- Subject Headings
- Phonons, Raman effect, Lattice dynamics
- Format
- Document (PDF)
- Title
- RAMAN SPECTRA OF ZIRCONIA CERAMICS.
- Creator
- KIM, JAE HOON, Florida Atlantic University, Medina, Fernando D., Charles E. Schmidt College of Science, Department of Physics
- Abstract/Description
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Raman spectroscopy was used to investigate how phase structure and phase stability of zirconia mixtures depend on temperature, type and concentration of added rare earth oxides, and fabrication process. All 8wt% yttria samples exhibited the tetragonal structure between 300 and 10 K. The TZP sample, prepared under pressure and fully hardened, exhibited the most completely stabilized tetragonal structure. The plasma coating process did not improve the stability of the tetragonal structure....
Show moreRaman spectroscopy was used to investigate how phase structure and phase stability of zirconia mixtures depend on temperature, type and concentration of added rare earth oxides, and fabrication process. All 8wt% yttria samples exhibited the tetragonal structure between 300 and 10 K. The TZP sample, prepared under pressure and fully hardened, exhibited the most completely stabilized tetragonal structure. The plasma coating process did not improve the stability of the tetragonal structure. Adding ceria to zirconia did not completely stabilize the tetragonal structure, while adding alumina actually destabilized it. Mode Gruneisen parameters, obtained from changes in frequency for yttria samples with different concentration, agreed with published results under pressure, except for one Eg mode. The negative value obtained for this mode was ascribed to changes in cation-anion interactions. The temperature dependence of the linewidth for this mode was quantitatively explained by cubic anharmonic interactions.
Show less - Date Issued
- 1987
- PURL
- http://purl.flvc.org/fcla/dt/14401
- Subject Headings
- Zirconium oxide, Spectrum analysis
- Format
- Document (PDF)