Wen-Hsien Li

Stress relaxation in GaN by transfer bonding on Si substrates

The stress state of GaN epilayers transferred onto Si substrates through a Au–Si bonding process was studied by micro-Raman scattering and photoluminescence techniques. By increasing the Au bonding thickness from 1to40μm, the high compressive stress state in GaN layer was relieved. A 10μm Au bonding layer thickness is shown to possess the maximum compressive stress relief and also the deformation potential of the quantum well was found to be ∼85meV. A nonlinear parabolic relation between luminescent bandgap and the biaxial stress of the transferred GaN epilayer in the compressive region was observed.

Superparamagnetism and magnetic defects in Fe2VAl and Fe2VGa

We have investigated the magnetic properties of Heusler-type Fe2VAl and Fe2VGa by means of susceptibility, magnetization, and nuclear magnetic resonance (NMR) measurements. As evidenced by the static and dynamic magnetic susceptibilities, both compounds exhibit the characteristic features of a superparamagnetic glass. The freezing temperatures of our samples are 23 K for Fe2VAl and 9 K for Fe2VGa. A detailed analysis indicates the coexistence of individual magnetic defects with large-moment superparamagnetic clusters. The temperature-dependent magnetization and NMR linewidth are dominated by isolated magnetic defects, identified as Fe antisite defects. The concentration of these defects, determined from NMR linewidths, is 4.5 × 10 −3 in Fe2VAl, in good agreement with the value deduced from previous field-dependent specific heat measurements.

Superconductivity in zero-dimensional indium nanoparticles

An In nanoparticle powder was fabricated by employing the thermal evaporation method. Size analysis using x-ray diffraction profiles analysis and atomic force microscopy images indicates an average particle diameter of 36(2)nm and a Gaussian size distribution with a half-width of 3nm. The superconducting critical temperature of the In nanopowder increases slightly to 3.57(3)K, which was found to be extremely insensitive to the applied magnetic field. The critical magnetic field reaches ∼5T, which is a factor of 170 times higher than that of bulk In.

Specific heat and Lindemann-like parameter of metallic clusters: Mono- and polyvalent metals

The Brownian-type molecular dynamics simulation is revisited and applied to study the thermal and geometric properties of four mono- and two polyvalent metallic clusters. For the thermal property, we report the specific heat at constant volume CV and study the solid-liquid-like transition by scrutinizing its characteristic. For the geometric property, we calculate the root mean square relative bond-length fluctuation delta as a function of increasing temperature. The thermal change in delta reflects the movement of atoms and hence is a relevant parameter in understanding the phase transition in clusters. The simulated results for the CV of alkali and aluminum clusters whose ground state structures exhibit icosahedral symmetry generally show one phase transition. In contrast, the tetravalent lead is quite often seen to exhibit two phase transitions, a premelting process followed by a progressive melting. In connection with the premelting scenario, it is found here that those (magic number) clusters identified to be of lesser stability (among other stable ones) according to the second energy difference are clusters showing a greater possibility of undergoing premelting process. This energy criterion applies to aluminum clusters nAl=28 and 38. To delve further into the thermal behavior of clusters, we have analyzed also the thermal variation of deltaT and attempted to correlate it with CV(T). It turns out that the premelting (if exist) and melting temperatures of the smaller size clusters (n less, similar 50) extracted from CV do not always agree quantitatively with that deduced from delta.

Interplay between the magnetic and electric degrees of freedom in multiferroic Co3TeO6

Neutron diffraction, magnetic susceptibility, specific heat, and dielectric constant measurements of single-crystal Co3TeO6 have been measured to study the interplay between the ferroelectricity and magnetic order. Long-range incommensurate magnetic order develops below T-M1 = 26 K, which is followed by three additional zero-field phase transitions at T-M2 = 19.5 K, T-M3 = 18 K, and T-M4 = 16 K where the incommensurate order changes and commensurate order develops. In magnetic fields up to 14 T we find that the magnetic intensities and incommensurate wave vector are dramatically altered as ferroelectricity develops, with a fifth abrupt transition around 10 T. The overall behavior characterizes Co3TeO6 as a type-II multiferroic.

Spin, charge and lattice couplings in Cu-deficient oxysulphide BiOCu0.94S

The electrical and magnetic properties of slightly Cu-deficient BiOCu(0.94)S are investigated using neutron diffraction, ac magnetic susceptibility, magnetization and electrical resistivity measurements. The Cu spins order in a ferromagnetic arrangement below T(C) = 250 K. An antiferromagnetic component develops below 180 K when the crystalline unit cell experiences a sharp thermal contraction upon cooling, resulting in a canted ferromagnetic spin arrangement at low temperatures. In the magnetically ordered state the electrical transport can be described using three-dimensional variable range hopping conduction. An applied magnetic field can effectively reduce the hopping barrier. Spin-charge couplings are clearly revealed when the resistivity departs from the hopping conduction and begins to increase with increasing temperatures above 250 K where the Cu spins become disordered.

Spin Polarization and Quantum Spins in Au Nanoparticles

The present study focuses on investigating the magnetic properties and the critical particle size for developing sizable spontaneous magnetic moment of bare Au nanoparticles. Seven sets of bare Au nanoparticle assemblies, with diameters from 3.5 to 17.5 nm, were fabricated with the gas condensation method. Line profiles of the X-ray diffraction peaks were used to determine the mean particle diameters and size distributions of the nanoparticle assemblies. The magnetization curves M(Ha) reveal Langevin field profiles. Magnetic hysteresis was clearly revealed in the low field regime even at 300 K. Contributions to the magnetization from different size particles in the nanoparticle assemblies were considered when analyzing the M(Ha) curves. The results show that the maximum particle moment will appear in 2.4 nm Au particles. A similar result of the maximum saturation magnetization appearing in 2.3 nm Au particles is also concluded through analysis of the dependency of the saturation magnetization MP on particle size. The MP(d) curve departs significantly from the 1/d dependence, but can be described by a log-normal function. Magnetization can be barely detected for Au particles larger than 27 nm. Magnetic field induced Zeeman magnetization from the quantum confined Kubo gap opening appears in Au nanoparticles smaller than 9.5 nm in diameter.

Thermal effects on the activity and structural conformation of catechol 2,3-dioxygenase from Pseudomonas putida SH1.

A bacterium, Pseudomonas putida SH1, which can catabolize phenol, naphthalene, or cresol as the sole carbon and energy source, was isolated from a petroleum-contaminated site in Taiwan. The catechol 2,3-dioxygenase (C23O) was purified from this bacterial strain when grown on naphthalene as the sole carbon and energy source. The enzyme is composed of four identical subunits with a native molecular weight of 128 +/- 5 kD. Small-angle neutron scattering (SANS) techniques were employed to study the thermal effects on the structural conformation of this enzyme in solution. The SANS measurements revealed distinct changes in the size of the enzyme between 50 and 80 degrees C, and the size was not restored during the subsequent cooling. The enzyme started to denature at 55 degrees C, and the structure was destroyed by the time the temperature reached 80 degrees C, at which the enzyme had become more than twice the original size. The optimal catalytic temperature of the enzyme was at 50 degrees C. The half-life of the activity at this temperature was 45 min. The enzyme activity increases starting from 25 degrees C and reaches its maximum at 50 degrees C, below which no obvious change in the size of the enzyme is found. Noticeable enlargement of the enzyme is revealed when the enzymatic activity starts to fall. By combination of SANS measurement and biochemical properties of the enzyme, this study demonstrates the correlation of enzyme size in solution and catalytic activity upon a heat treatment. In addition, for a protein composed of multiple subunits, the shape of the enzyme and the dissociation of the enzyme subunits in a thermal cycle were also demonstrated by SANS methodology.

Low-temperature phase separation in GaN nanowires: An in situ x-ray investigation

In this study, we in situ employed low-temperature x-ray diffraction to investigate phase separation in GaN nanowires. Our observations showed that a distinct phase separation developed below 260K, the zinc-blende phase, which is related to short range ordering. The correlation lengths of the zinc-blende phase reached their maximum at 140K but correlation length was still revealed at around 23nm. Our results may be understood using the finite size model and support the conclusion that the phase separation was reversible and occurred through the interaction of the characteristic size of the ordered domain of the GaN nanowires.

Development of Ferromagnetic Superspins in Bare Cu Nanoparticles by Electronic Charge Redistribution

We report on the results of investigating the ferromagnetic properties of bare Cu nanoparticles. Three sets of bare Cu nanoparticle assemblies with mean particle diameters of 6.6, 8.1, and 11.1 nm were fabricated, employing the gas condensation method. Curie-Weiss paramagnetic responses to a weak driving magnetic field were detected, showing the appearance of particle superspins that overcomes the diamagnetic responses from the inner core. The isothermal magnetization displays a Langevin field profile together with magnetic hysteresis appearing even at 300 K, demonstrating the existence of ferromagnetic superspins in the Cu nanoparticles. Shifting of a noticeable amount of electronic charge from being distributed near the lattice sites in bulk form toward their neighboring ions in nanoparticles was found. The extended 3d and 4s band mixture are the main sources for the development of localized 3d holes for the development of ferromagnetic particle superspins in Cu nanoparticles.