Roland V. Sarmago

Gamma-ray irradiation effects on the optical properties of bulk ZnO single crystals

Hydrothermal-grown bulk ZnO single crystals are investigated before and after gamma-ray irradiation. The irradiation does not alter the optical transparency in the visible region. The gamma rays only induce modified near-band-edge UV emission with blue-shifted peaks and shortened response times. From the initial values before irradiation, the peaks shift by 5 to 6 nm, and the response times shorten by 140 to 440 ps. We attribute these observations to the radiation-induced defects on the bulk crystals. Our results nevertheless lead to the realization of short-wavelength ZnO scintillators that can be utilized in high-energy-radiation environments.

Terahertz emission from Indium Oxide films grown on MgO substrates using sub-bandgap photon energy excitation

Indium oxide (In2O3) films grown by thermal oxidation on MgO substrates were optically excited by femtosecond laser pulses having photon energy lower than the In2O3 bandgap. Terahertz (THz) pulse emission was observed using time domain spectroscopy. Results show that THz emission saturates at an excitation fluence of ~400 nJ/cm2. Even as two-photon absorption has been excluded, the actual emission mechanism has yet to be confirmed but is currently attributed to carriers due to weak absorption from defect levels that are driven by a strain field at the interface of the substrate and the grown film.

Growth of Bi2Sr2CaCu2O8+δ thin films with enhanced superconducting properties by incorporating CaIrO3 nanoparticles

We synthesized Bi2Sr2CaCu2O8+δ (Bi-2212) thin films with incorporated CaIrO3 (Ca-iridate) nanoparticles, by using pulsed laser deposition and post-growth ex situ annealing. We found that the density of incorporated Ca-iridate strongly affected the superconducting properties and microstructure of the Bi-2212 thin films. Incorporating a low density of Ca-iridate (450 laser pulses) in the Bi-2212 improved its superconducting properties (Tc-onset = 97 K, Tc-zero = 84 K) over those of pure Bi-2212 (Tc-onset = 94 K, Tc-zero = 80 K). However, incorporating a higher density (1800 pulses) significantly reduced Tc-zero to ≈ 57 K. Incorporating a low density of Ca-iridate also decreased the c-axis lattice constant. Films with incorporated Ca-iridate exhibited greater critical current density, Jc(B), than the pure Bi-2212 film. These results indicate that incorporating low densities of Ca-iridate nanoparticles into Bi-2212 can improve its superconducting properties.

Post Deposition Heat Treatment Effects on Ceramic Superconducting Films Produced by Infrared Nd:YAG Pulsed Laser Deposition

Pulsed laser deposition (PLD) has become a potential method in fabricating highly quality superconducting thin films suitable for electronic applications such as in Josephson junctionbased electronics and in second generation coated conductors [11, 12, 14, 20, 23]. PLD of high Tc superconductors generally utilize excimer lasers in the ultraviolet (UV) range [6, 11] . However, excimer lasers use toxic gases such as Cl and F for excitation. In contrast, flash lamp pumped Nd:YAG laser can provide stable power and better beam profile [14, 20]. Nd: YAG lasers are also easy to operate and have low maintenance costs [2, 14, 20]. To date, the third harmonic (355 nm) and fourth harmonic (266 nm) of the Nd:YAG has been used to grow high quality high-Tc superconducting films [12, 14, 20].

Growth of superconducting Bi2Sr2CaCu2O8+δ single crystals from a (KCl-Bi2O3) flux

Abstract Large-sized superconducting Bi 2 Sr 2 CaCu 2 O 8+δ single crystals with typical dimensions of up to 15 × 5 mm 2 were prepared via the KCl-Bi 2 O 3 flux method. The crystals produced were of two forms—needle-like and plate-like. These crystals were found along the walls and at the bottom of the crucible. However, it was observed that when an alumina rod was placed at the center of the crucible, crystals were found to grow along the length of the rod. The temperature program used for growth run consists of temperature cycling, temperature-holding and slow cooling processes. The temperature cycling in the range of 830–860°C, combined with the temperature-holding at 830°C and slow cooling to room temperature, is effective in growing large single crystals.

Terahertz emission from indium oxide films on MgO substrates excited at a photon energy below the bandgap

Indium oxide (In2O3) films grown on MgO substrates by thermal oxidation were excited by femtosecond laser pulses having photon energy lower than the bandgap. The emission of terahertz (THz) pulse was observed using time domain spectroscopy in the reflection-geometry excitation. Results show that THz generation saturates at an excitation fluence of ∼400 nJ/cm2. Although two-photon absorption has been ruled out, the actual THz emission mechanism and still has to be verified and is temporarily attributed to carriers from defect level absorption possibly being driven by a strain field due to the lattice mismatch between the In2O3 and the MgO substrate.

Raman and luminescence spectra in the spin–Peierls system Cu1−xNixGeO3

Abstract We have studied Raman and luminescence spectra of Cu1−xNixGeO3. The appearance of luminescence spectra in lightly Ni-doped samples suggests the formation of impurity states in the charge-transfer gap. The folded phonon mode, the spin–Peierls gap mode, and the two-magnetic-excitation bound-state mode, which are evidences of the spin–Peierls transition, have been observed in lightly Ni-doped samples, as well as in Zn- and Si-doped CuGeO3.