Physics
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The Physics department is committed to education and research in physics, the study of the fundamental universal laws that govern the behavior of matter and energy, and the exploration of the consequences and applications of those laws. Our department is widely known for its excellent teaching and student mentoring. Our department plays an important role in the university’s Core Curriculum. We have strong academic programs with several options for undergraduate physics majors, leading to the B.S. degree, as well as graduate curricula leading to the M.S. and Ph.D. degrees. Our research groups span a variety of fields within physics. Our principal concentrations are in Astrophysics, Relativity, Gravitation and Cosmology, Condensed Matter Physics, Lasers and Optics, Physics Education, Solar Physics, and the Space Science and Engineering Lab.
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Item Nanotwin and phase transformation in tetragonal Pb(Fe1/2Nb1/2)1-xTixO3 single crystal(2008) Tu, Chi-Shun; Tseng, C.-T.; Chien, R.R.; Schmidt, V. Hugo; Hsieh, C.-M.This work is a study of phase transformation in (001)-cut Pb(Fe1/2Nb1/2)1−xTixO3Pb(Fe1/2Nb1/2)1−xTixO3 (x=48%)(x=48%) single crystals by means of dielectric permittivity, domain structure, and in situ x-ray diffraction. A first-order T(TNT)-C(TNT)T(TNT)-C(TNT) phase transition was observed at the Curie temperature TC≅518KTC≅518 K upon zero-field heating. T, TNTTNT, and C are tetragonal, tetragonal nanotwin, and cubic phases, respectively. T(TNT)T(TNT) and C(TNT)C(TNT) indicate that minor TNTTNT domains reside in the T and C matrices. Nanotwins, which can cause broad diffraction peak, remain above TC≅518KTC≅518 K and give an average microscopic cubic symmetry in the polarizing microscopy. Colossal dielectric permittivity (>104)(>104) was observed above room temperature with strong frequency dispersion. This study suggests that nanotwins can play an important role in relaxor ferroelectric crystals while phase transition takes place. The Fe ion is a potential candidate as a BB-site dopant for enhancing dielectric permittivity.Item Field-induced intermediate orthorhombic phase in (110)-cut Pb(Mg1/3Nb2/3)0.70Ti0.30O3 single crystal(2008) Tu, Chi-Shun; Schmidt, V. Hugo; Chien, R.R.; Tsai, S H; Lee, S.-C.; Luo, H.Item Piezoelectric response and origin in (001) Pb(Mg1/3Nb2/3)0.70Ti0.30O3 crystal(2008) Tu, Chi-Shun; Hsieh, C.-M.; Schmidt, V. Hugo; Chien, R.R.; Luo, H.Converse and direct piezoelectric coefficients (dC33 and dD33) of (001)-cut Pb(Mg1/3Nb2/3)0.70Ti0.30O3 (PMN-30%PT) single crystals have been investigated as a function of poling electric (E) field. E-field-dependent domain structures were observed by using a polarizing microscope. Both dC33 and dD33 exhibit a rapid increase at E=1–2 kV/cm and reach maxima at E=2.5–4 kV/cm. This study suggests that polarization rotation from rhombohedral to monoclinic MA phases plays an important role while the high piezoelectric response builds up. Overpoling phenomenon evidenced by a sudden reduction in piezoelectric coefficient with increasing field is very sensitive to Ti content.Item Rapid piezoelectric response and origin in (001) Pb (In1/2Nb1/2)0.70Ti0.30O3 crystal(2009) Tu, Chi-Shun; Chien, R.R.; Lee, S.-C.; Schmidt, V. Hugo; Luo, H.Normal piezoelectric coefficient d33, domain structure, and x-ray diffraction of a (001)-cut Pb(In1/2Nb1/2)0.70Ti0.30O3 (PIN-30%PT) single crystal have been investigated as a function of poling electric (E) field. E-field-dependent domain structures were observed by using a polarizing microscope. Piezoelectric coefficient d33 exhibits a rapid increase at E=1–3 kV/cm and reaches a maximum of about 1200 pC/N near E=6 kV/cm. Structurald spacing and domain structure results suggest that polarization rotation from rhombohedral to monoclinic MA phases causes the high piezoelectric response. Overpoling phenomenon associated with a sudden reduction of d33 with increasing field was not evidenced.Item Thermal stability of Ba(Zr0.8-xCexY0.2)O2.9 ceramics in carbon dioxide(2009) Tu, Chi-Shun; Chien, R.R.; Schmidt, V. Hugo; Lee, S.-C.; Huang, C.-C.; Tsai, Chih-LongIn situx-ray diffraction spectra (25–1000 °C) have been measured as a function of temperature for proton-conducting Ba(Zr0.8−xCexY0.2)O2.9 (x=0.0–0.4)ceramics in CO2 atmosphere. Atomic vibrations before and after exposure to CO2 were obtained by using the micro-Raman scattering (150–1600 cm−1). Ba(Zr0.8Y0.2)O2.9 and Ba(Zr0.6Ce0.2Y0.2)O2.9 reveal a promising thermal stability in CO2 without apparent decomposition up to 1000 °C. However, Ba(Zr0.5Ce0.3Y0.2)O2.9 and Ba(Zr0.4Ce0.4Y0.2)O2.9 exhibit thermally stable below 550 °C and then proceed an obvious chemical decomposition of BaCO3 and Zr0.8−xCexY0.2O2 above 550 °C, which were clearly evidenced by the Raman vibrations of 1057 and 466 cm−1, respectively. A first-order orthorhombic-hexagonal structure transition was confirmed in BaCO3 in the region of 810–850 °C upon heating. This study suggests that the Ba(Zr0.8−xCexY0.2)O2.9ceramics with x≤0.2 are promising candidates for proton-conducting applications in CO2-containing environment.Item Intermediate phases in rhombohedral Pb(Mg1/3Nb2/3)1-xTixO3 crystal(2008) Tu, Chi-Shun; Chuang, H.-T.; Lee, S.-C.; Chien, R.R.; Schmidt, V. Hugo; Luo, H.Phase transformations of (111)-cut Pb(Mg1∕3Nb2∕3)1−xTixO3 (x=0.30) single crystals before and after an electric (E)–field poling have been investigated by means of dielectricpermittivity, hysteresis loop, polarization current, and in situx-ray diffraction as a function of temperature. An R-T-C transition sequence was observed in the unpoled sample upon zero-field heating. R, T, and C are rhombohedral, tetragonal, and cubic phases, respectively. After a prior E-field poling, an extra intermediate monoclinic (possible MA) phase was induced and the crystal underwent an R-MA-T-C phase sequence with significant responses in polarization current. The dielectric dispersion was diminished after poling and reappeared in the cubic state. The dielectricpermittivities with and without a prior poling were found to follow the Curie–Weiss equation, ε′=C/(T−T0), above the Burns temperature 505K with the same constants C=1.9×105 K and T0=460K.Item Nanotwins and phases in high-strain Pb(Mg1/3Nb2/3)1-xTixO3 crystal(2008) Tu, Chi-Shun; Hsieh, C.-M.; Chien, R.R.; Schmidt, V. Hugo; Wang, F.-T.; Chang, W.S.This work is a study of the thermal stability of (001)-cut Pb(Mg1/3Nb2/3)1−xTixO3 (x=0.30) single crystals before and after an electric (E)-field poling by means of dielectricpermittivity, hysteresis loop, domain structure,polarization current, and x-ray diffraction. An R(RNT)-R(RNT)/T(TNT)-T(TNT)-C transition sequence was observed upon heating in the unpoled sample. R, RNT, T, TNT, and C are the rhombohedral, rhombohedral nanotwin, tetragonal, tetragonal nanotwin, and cubic phases, respectively. R/T indicates coexistence of the R and T phases. R(RNT) and T(TNT) indicate that the RNT and TNTstructures mimic monoclinic phases in the R and T matrices, respectively. After a prior E-field poling, an R(RNT)-T(TNT)-C phase sequence takes place upon heating. The dielectricpermittivity and current density evidenced an additional polarization at 355 K, which is associated with the vanishing of the dielectric dispersion, which reappears near 410 K and remains up to the Burns temperature TB=510 K. This study suggests that nanotwins (RNT and TNT) can play an important role in high-strain piezoelectric crystals while phase transition takes place. Under E=38 kV/cm, [001] T domains randomly appeared in the matrix, suggesting that the matrix consists of a glassy matrix and ferroelectric nanoclusters.Item Electric-field-induced dielectric anomalies and optical birefringence in Pb(Mg1/3Nb2/3)1-xTixO3 (x=0.10) single crystal(2006) Tu, Chi-Shun; Wang, F.-T.; Chien, R.R.; Schmidt, V. Hugo; Lim, L.-C.This work aims to study thermal stability, frequency response, and refractive indices in a (001)-cut Pb(Zn1∕3Nb2∕3)0.90Ti0.10O3 (PZNT10%) crystal before and after an electric (E)-field poling, which are important for piezoelectric and optical uses. Dielectric permittivities have been measured as functions of temperature, frequency, and poling strength. The dielectric absorption reveals a broad dipolar relaxation in the region of 260–310K in the unpoled sample. Piezoelectric resonances were observed in the poled sample and show significant changes while phase transitions are taking place. The unpoled crystal shows almost no birefringence, indicating that the structure averaged over the optically anisotropic ferroelectric domains is optically isotropic. Birefringence appears after an E-field poling and shows a “negative” uniaxial distortion. The Cauchy equations for ordinary no and extraordinary nerefractive indices were determined between 0.45 and 1.4μm. However, the phase-matching criterion for second harmonic generation was not found.Item Prior poling effect on thermal phase stability in (110)-cut Pb(Mg1/3Nb2/3)0.74Ti0.26O3 single crystal(2005) Chien, R.R.; Schmidt, V. Hugo; Tu, Chi-Shun; Wang, F.-T.Phases and domains in a (110)-cut Pb(Mg1∕3Nb2∕3)0.74Ti0.26O3 (PMNT26%) single crystal have been investigated as functions of temperature and dc electric (E) field by dielectric permittivity, polarizing microscopy, and electric polarization. The unpoled sample has a dominant rhombohedral (R) phase, which coexists with monoclinic (M) phase domains, i.e., R∕M at room temperature. With 45kV∕cm dc poling applied along [110] at room temperature, a single domain of R phase with polar orientation perpendicular to the poling field, i.e., Ṟ, was obtained. After the poling was removed, the poled sample has Ṟ/M microdomains, where the M distortion is close to the R phase. The zero-field-heating domain patterns in both unpoled and poled samples exhibit continuous polarization rotation via an intrinsic M phase in the regions of 355-375 and 365-380 K, respectively. Orthohombic and tetragonal phases were not observed in the temperature-dependent study. In general, a rhombohedral/monoclinic→monoclinic→cubic transition sequence takes place upon heating for unpoled and poled samples.Item Dielectric and photovoltaic phenomena in tungsten-doped Pb(Mg1/3Nb2/3)1-xTixO3 crystal(2006) Tu, Chi-Shun; Wang, F.-T.; Chien, R.R.; Schmidt, V. Hugo; Tseng, C.-T.This work investigates dielectric and photovoltaic behaviors in Pb(Mg1∕3Nb2∕3)0.64Ti0.36O3single crystaldoped with 0.5mol% WO3. Dielectric permittivities measured as functions of temperature and frequency reveal two first-order-type phase transitions upon heating and cooling. The photovoltaic response strongly depends on illumination wavelength, sample thickness, and prior electric-field poling. The relation of photovoltage and light intensity under near-ultraviolet (λ=406nm)illumination for the poled samples can be expressed by an exponential equation. Optical transmission reveals that the cutoff wavelength is near 400nm and indicates a minimum electronic energy gap of ∼3.0eV.
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