Neutron polarization analysis on the multiferroic TbMn2O5

Polycrystalline TbMn₂O₅ was prepared by the standard solid-state reaction method and characterized by powder X-ray diffraction and magnetization to assure it is of single phase. Heat capacity measurements on the compound reveal an antiferromagnetic phase transition at 45 K. A broad peak below 6 K in the heat capacity measurements corresponds to the crossover transition of Tb³⁺ ordering. To confirm these magnetic orderings, neutron powder diffractions on TbMn₂O₅ with XYZ neutron polarization analysis were performed at the diffuse neutron scattering (DNS) spectrometer, FRJ-II, by using neutron wavelength of 4.8 Å in the temperature range of 1.8–250 K. Magnetic scattering was separated from nuclear coherent and spin incoherent scattering contributions. Long-range ordered magnetic peaks were observed below 39 K which is consistent with the heat capacity results. The drastic increasing intensities below 6 K indicate the ferromagnetic transition in Tb³⁺ orderings.     

Nucleation, growth and morphology of Co thin films on Ag(1 1 0)

Scanning tunneling microscopy (STM) experiments reveal that Co growth on Ag(1 1 0), at coverages of Co < 1 ML and low substrate temperatures (150 K), involves a concomitant insertion of Co into the top Ag layer and exchange of Ag out onto the surface. At 300 K, coverages of Co > 1 ML gives rise to a 3D nanocluster growth on the surface, with the clusters covered by Ag. Depending slightly on coverage, the clusters have a typical diameter of 3 nm and a height of 0.4 nm. Upon annealing to 500 K, major changes are observed in the morphology of the surface. STM and AES show that there is a reduction of the number of Co islands on the surface, partly due to subsurface Co cluster migration and partly due to sintering into larger clusters.     

Quasi-distributed displacement sensor for structural monitoring using a commercial OTDR

A quasi-distributed displacement sensor for structural monitoring using an optical time domain reflectometer is demonstrated. Four displacement sensing heads are placed along a standard single mode optical fibre in several locations with different intervals. Their configurations introduce power loss through the decrease of their fibre loop radius when displacement is applied. The decrease of the light intensity with displacement variation is reported. Losses of 9 dB for a 120 mm displacement with a sensitivity of 0.027 dB/mm are reported. The quasi-distributed configuration is able to address sensors with 1 m distance resolution between them.     

Structure and properties of the one-dimensional cobalt oxide CaCo2O4

We have studied crystal structure and transport properties of the quasi one-dimensional cobalt oxide CaCo₂O₄. The CaCo₂O₄ phase crystallizes in calcium-ferrite type structure, which consists of a corner- and edge-shared CoO₆ octahedron network including one-dimensional double chains. Large thermoelectric power (S  150 μV/K at 390 K) with metallic temperature dependence of S, moderate resistivity (ρ  2.9 × 10⁻¹ Ω cm at 390 K) with carrier localization at low temperature, and normal thermal conductivity (κ  6.3 W/Km at 390 K) were observed. The phonon mean-free path was calculated from the observed data, as a function of temperature. The long phonon mean-free path (l  24 Å at 300 K) implies that the thermal conductivity could be suppressed by impurity scattering of phonons with partial element substitution.     

Photonic generation of microwave signal using a dual-wavelength single-longitudinal-mode distributed Bragg reflector fiber laser

In this paper, a novel all-optical microwave generation technique based on a dual-wavelength single-longitudinal-mode (SLM) distributed Bragg reflector (DBR) fiber laser is proposed and demonstrated. By exploiting spatial hole burning (SHB) effect, this laser could provide stable dual-wavelength SLM operation with a wavelength separation of 0.088 nm corresponding to the microwave signal at 10.484 GHz with a 3 dB bandwidth of 28 kHz. By appropriately adjusting the pump power, dual-wavelength oscillation could be maintained at different temperatures. We have theoretically analyzed the mechanism for microwave generation of the proposed DBR laser, and the calculated microwave frequency is in good agreement with our experimental results. Furthermore, experimental observation shows both of the laser wavelengths and generated microwave signals have good stability at room temperature.     

Carbon nanotube–polymer composites for photonic devices

We report the fabrication of high optical quality single wall carbon nanotube polyvinyl alcohol composites and their application in nanotube based photonic devices. These show a broad absorption of semiconductor tubes centred at 1.55 μm, the spectral range of interest for optical communications. The films are used as mode-lockers in an erbium doped fibre laser, achieving 700 fs mode-locked pulses. Raman spectroscopy shows no damage after a long time continuous laser operation.     

What can we learn about the spectrum of high-energy stray neutron fields from Bonner sphere measurements?

Extended-Range Bonner sphere spectrometers (ERBSSs) are well suited for measurements in high-energy stray neutron fields, where neutron energies extend from thermal to a few hundred MeV. These fields typically have a thermal peak, a fairly flat intermediate region, a medium energy peak at 1 MeV and a high-energy peak at a few 100 MeV. The data analysis is not straightforward: it requires unfolding with response functions that have a substantial amount of overlap, and the responses of the modified spheres (which provide information about the fluence at high energies) increase dramatically above 100 MeV. In this paper, I try to determine which of the main features of the spectrum are well determined by the data given optimal methods of analysis, and which features are subject to ambiguity. To do this, I analyse ERBSS measurements using two methods of analysis that are particularly useful for this purpose, Bayesian parameter estimation and maximum entropy unfolding.     

Comparison of Yb-doped fiber laser with one-end and double-end pumping configuration

A high-power Ytterbium-doped fiber laser (YDFL) with homemade double clad fiber (DCF) is introduced in this paper. The output power characteristics of a linear cavity fiber laser have been studied theoretically by solving the rate equations and experimentally tested with single- and double-end-pumping configurations. When both ends of the fiber are pumped by two high-power laser diodes with a launched power of 300 W each, a maximum CW output of 444 W is obtained with a slope efficiency of 75%.     

Dimensional evolution of silicon nanowires synthesized by Au–Si island-catalyzed chemical vapor deposition

This study explores the nucleation and morphological evolution of silicon nanowires (Si-NWs) on Si (0 0 1) and (1 1 1) substrates synthesized using nanoscale Au–Si island-catalyzed rapid thermal chemical vapor deposition. The Au–Si islands are formed by Au thin film (1.2–3.0 nm) deposition at room temperature followed by annealing at 700 °C, which are employed as a liquid-droplet catalysis during the growth of the Si-NWs. The Si-NWs are grown by exposing the substrates with Au–Si islands to a mixture of gasses SiH₄ and H₂. The growth temperatures and the pressures are 500–600 °C and 0.1–1.0 Torr, respectively. We found a critical thickness of the Au film for Si-NWs nucleation at a given growth condition. Also, we observed that the dimensional evolution of the NWs significantly depends on the growth pressure and temperature. The resulting NWs are 30–100 nm in diameter and 0.4–12.0 μm in length. For Si (0 0 1) substrates 80% of the NWs are aligned along the 1 1 1 direction which are 30° and 60° with respect to the substrate surface while for Si (1 1 1) most of the NWs are aligned vertically along the 1 1 1 direction. In particular, we observed that there appears to be two types of NWs; one with a straight and another with a tapered shape. The morphological and dimensional evolution of the Si-NWs is significantly related to atomic diffusion kinetics and energetics in the vapor–liquid–solid processes.     

The electrodeposition of copper onto UHV-prepared GaAs(0 0 1) surfaces

Synchrotron surface X-ray diffraction has been used to investigate in situ the morphology and epitaxy of monolayer amounts of copper electrodeposited from aqueous electrolyte onto ultra-high vacuum prepared, smooth, Ga- or As-terminated GaAs(0 0 1) surfaces. The fcc lattice of the epitaxial Cu islands is rotated by 5° and tilted by about 9° with respect to the GaAs substrate lattice, leading to eight symmetry equivalent domains of Cu islands terminated by {1 1 1} facets.     

The microwave induced resistance response of a high mobility 2DEG from the quasi-classical limit to the quantum Hall regime

Microwave induced resistance oscillations (MIROs) were studied experimentally over a very wide range of frequencies ranging from 20 GHz up to 4 THz, and from the quasi-classical regime to the quantum Hall effect regime. At low frequencies regular MIROs were observed, with a periodicity determined by the ratio of the microwave to cyclotron frequencies. For frequencies below 150 GHz the magnetic field dependence of MIROs waveform is well described by a simplified version of an existing theoretical model, where the damping is controlled by the width of the Landau levels. In the THz frequency range MIROs vanish and only pronounced resistance changes are observed at the cyclotron resonance. The evolution of MIROs with frequency is presented and discussed.     

Scanning tunneling microscopy/spectroscopy on multi-layered cuprate superconductor Ba2Ca5Cu6O12 (O1−x Fx)2

Scanning tunneling microscopy/spectroscopy (STM/STS) measurements on multi-layered cuprate superconductor Ba₂Ca₅Cu₆O₁₂ (O_1−x F_x)₂ are carried out. STM topographies show randomly distributed bright spot structures with a typical spot size of 0.8 nm. These bright spots are occupied about 28% per one unit cell of c-plane, which is comparable to the regular amount of apical oxygen of 20% obtained from element analysis. Tunneling spectra simultaneously show both the small and the large gap structures. These gap sizes at 4.9 K are about Δ 15 meV and 90 meV, respectively. The small gap structure disappears at the temperature close to T_C, while the large gap persists up to 200 K. Therefore, these features correspond to the superconducting gap and pseudogap, respectively. These facts give evidence for some ordered state with large energy scale even in the superconducting state. For the superconducting gap, the ratio of 2Δ/K_BT_C = 4.9 is obtained with T_C = 70 K, which is determined from temperature dependence of the tunneling spectra.     

Dynamics of the collective excitations of the quantum Hall system

Using the non linear optical technique of 3-pulse 4-wave mixing, we study the dynamics of the collective excitations of the quantum Hall system. We excite the system with 100 fs pulses propagating in directions k₁ and k₃ and then probe its time evolution with a delayed pulse k₂. We measure the non-linear optical response from the lowest Landau level along the direction k₁+k₂−k₃. As function of the time delay of pulse k₂, this signal shows striking beats for short time delays (500 fs), followed by a rise (20 ps) and then a decay (100 ps). We identify the microscopic origin of this dynamics by extending the standard theory of ultra fast nonlinear optics to include the effects of the correlations.     

From Zn microspheres to hollow ZnO microspheres: A simple route to the growth of large scale metallic Zn microspheres and hollow ZnO microspheres

Large scale metallic Zn microspheres and hollow ZnO microspheres are synthesized by thermal evaporation and vapor transport by heating a ZnO/graphite mixture at 1000 °C. Firstly, metallic Zn microspheres are fabricated with diameters in the range of 1–10 μm. The Zn microspheres are then annealed at 600 °C in air, which form hollow semiconducting ZnO microspheres. EDX and XRD spectra reveal that the oxidized material is indeed ZnO. Room temperature photoluminescence spectra of the oxidized material show a sharp peak at 380 nm and a wider broad peak centered at 490 nm. This growth mechanism is discussed and further investigated for other metallic and metal oxide microstructures.     

Transmission grating spectrograph with on-line recording of XUV soft X-ray spectra from laser produced plasmas

The setting up and on-line operation of a transmission grating spectrograph in the XUV soft X-ray region is described. The detector was a microchannel plate–image intensifier combination, and the output of the image intensifier was coupled to a CCD camera–frame grabber system through an imaging lens. The spectrograph could be operated in the 5–20 Å range with 0.6 Å spectral resolution and in the 5–50 Å range with 1 Å resolution, respectively. The high sensitivity of the detector enables single shot operation, which is useful for several laser plasma interaction studies.     

Compact 120 TW Ti:sapphire laser system with a high gain final amplifier

A 120 TW/36 fs laser system based on Ti:sapphire chirped-pulse amplification (CPA) has been successfully established in our lab. The final four pass Ti:sapphire amplifier pumped by an energetic single-shot Nd:YAG—Nd:glass laser was designed and optimized. With 24 J/8 ns pump energy at 532 nm, 300 mJ/220 ps chirped pulse was amplified to 5.98 J in this amplifier, and a total saturated gain of 20 was achieved. The focused intensity of compressed beam could reach to 10²⁰ W/cm² with the M² of 2.0.     

Energetics of dopant atoms in subsurface positions of diamond semiconductor

We present a set of ab initio energetics for a substitutional boron (B) impurity atom in subsurface positions, from the topmost to the fifth atomic layer, of both C(001)-2×1:H and C(111)-1×1:H. We compare the calculated surface-B binding energies with those obtained for P [T. Miyazaki, H. Kato, H. Okushi, S. Yamasaki, e-J. Surf. Sci. Nanotech. 4 (2006) 124]. The surface-P binding energies become larger as the position of P is closer to the two surfaces. They are up to 4 eV for C(001)-2×1:H and 2.6 eV for C(111)-1×1:H, respectively. For B, in contrast, the binding energies are within 0.5 eV for both surfaces. An implication of our finding in the context of a mechanism for P and B doping in diamond is discussed.     

Arsenic-related recombination in MOVPE-grown ZnO/GaAs films

In this paper, ZnO films grown by metalorganic vapour phase epitaxy on various substrates (GaAs, silicon, sapphire) and using different VI /II ratios, are investigated by photoluminescence (PL) spectroscopy. The PL spectra of layers grown on GaAs show significant recombination at 3.320 eV, 3.305 eV and 3.270 eV. These energies are remarkably similar to what have been reported for hybrid beam deposited ZnO:As [Y.R. Ryu, T.S. Lee, H.W. White, Appl. Phys. Lett. 83 (2003) 87] and arsenic-implanted ZnO crystals [T.S. Jeong, M.S. Han, C.J. Youn, Y.S. Park, J. Appl. Phys. 96 (2004) 175], and the lines are ascribed to the incorporation of arsenic, which diffuses from the substrate into the films. Two acceptor levels are deduced at 120 meV and at 140–150 meV.     

Temperature dependent photoluminescence from ZnO/MgZnO multiple quantum wells grown by pulsed laser deposition

We have studied temperature dependent photoluminescence (PL) from ZnO Multiple Quantum Wells (MQWs) of different well layer thicknesses in the range 1–4 nm grown on (0001) sapphire by a novel in-house developed buffer assisted pulsed laser deposition. At 10 K the PL peak shifted toward blue with decreasing well layer thickness and at constant well layer thickness the PL peak shifted towards red with increasing temperature. To the best of our knowledge we have observed for the first time an efficient room temperature (RT) PL emanating from such MQWs. The red shift of the PL peak with increasing temperature has been found to be due to the band gap shrinkage in accordance with the Varshni’s empirical relation. The spectral linewidth was found to increase with increasing temperature due to the scattering of excitons with acoustic and optical phonons in different temperature regimes. Both at RT and at 10 K the PL peak shifted with respect to the well layer thickness in the range of 3.35–3.68 eV with decreasing thickness in agreement with the calculated values.     

Structure and reaction pathways of methyl lactate on Pd(1 1 1)

The adsorption and reaction of methyl lactate (CH₃CH(OH)COOCH₃) is studied in ultrahigh vacuum on a Pd(1 1 1) surface using temperature-programmed desorption (TPD) and reflection–absorption infrared spectroscopy (RAIRS). Methyl lactate reacts at relatively low temperatures (220 K) by O–H bond scission. This intermediate can either react with hydrogen to reform methyl lactate at 280–300 K or undergo β-hydride elimination to form flat-lying methyl pyruvate. This decomposes to form acetyl and methoxy carbonyl species as found previously following methyl pyruvate adsorption on Pd(1 1 1). These species predominantly react to form carbon monoxide, methane and hydrogen.