Shunquan Liu

Temperature-controlled epitaxy of InxGa1-xN alloys and their band gap bowing

InxGa1-xN alloys (0 ≤ x ≤ 1) have been grown on GaN/sapphire templates by molecular beam epitaxy. Growth temperature controlled epitaxy was proposed to modulate the In composition so that each InxGa1-xN layer was grown at a temperature as high as possible and thus their crystalline quality was improved. The bandgap energies of the InxGa1-xN alloys have been precisely evaluated by optical transmission spectroscopy, where the effect of residual strain and electron concentration (the Burstein-Moss effect) on the bandgap energy shift has been considered. Finally, a bowing parameter of ∼1.9 ± 0.1 eV has been obtained by the well fitting In-composition dependent bandgap energy.

Hole mobility in wurtzite InN

Hole mobility in wurtzite InN at low electric fields is studied by an ensemble Monte Carlo calculation. Scatterings of holes by polar optical phonons, nonpolar optical phonons, acoustic phonons, ionized and neutral impurities, and threading dislocations are taken into account. Mobility of holes is ∼220 cm2/V s at 300 K in the InN, where holes are only scattered by the lattice. It decreases to 20–70 cm2/V s when the present quality of InN with threading dislocation density of ∼1010 cm−2 and residual donor concentration of over 1017 cm−3 is considered. The calculated mobility coincides well with the recent experimental observation.

Improvement of coercivity and thermal stability of anisotropic Nd13Fe79.4B7Nb0.3Ga0.3 powders by diffusion of Pr-Cu alloys

Highly anisotropic Nd13Fe79.4B7Nb0.3Ga0.3 powders from a self-organized rod-like disproportionation microstructure and room temperature magnetic properties of μ0Mr = 1.4 T, iHc = 13 kOe, and (BH)max = 41 MGOe are obtained by optimizing hydrogenation disproportionation desorption recombination (HDDR) process. Diffusion of Pr68Cu32 alloy into the HDDR powders further increases the coercivity of the powders to 18 kOe. This enhancement in coercivity is mainly attributed to the modification of grain boundaries including the decrease of Fe content and the increase of boundary thickness. The modified boundaries can work as effective pinning layers for domain walls. The thermal stability of magnetic properties of HDDR powders is also improved after the diffusion treatment.

Coercivity enhancement in Dy-free Nd–Fe–B sintered magnets by using Pr-Cu alloy

The grain boundary phase of Dy-free sintered Nd-Fe-B magnets is modified by using Pr68Cu32 eutectic alloy. The coercivity of the modified magnets reaches 21 kOe, which is the highest value in Dy-free Nd-Fe-B sintered magnets. Microstructural investigations show that a smooth and thick grain boundary layer is formed, and the content of the ferromagnetic elements in the grain boundary layer decreases from 65 at. % to 9 at. %. In addition, the mean grain size (4.5 μm) in the doped sintered magnets is smaller than that (6.5 μm) in the original sintered magnets. The modification in grain boundary and grain size reduces the magnetic interactions among grains and hinders nucleation of reversed magnetic domains, resulting in a coercivity enhancement.

Deep donor state in InN: Temperature-dependent electron transport in the electron accumulation layers and its influence on Hall-effect measurements

Temperature-dependent electron transport properties in electron accumulation layers of InN are quantified by using the multilayer model. Room temperature electron densities in the electron accumulation layers are 5.83 × 1013 and 3.26 × 1014 cm−2 with Hall mobilities of 429 and 149 cm2/Vs in In- and N-polarity InN, respectively. A deep donor state with an activation energy of ∼80 meV, which is previously believed to exist in the InN bulk layer, is found to actually locate in the electron accumulation layers. The InN bulk layer shows single-shallow-donor behavior and its temperature-dependent electron mobility is in good agreement with the ensemble Monte Carlo simulation results.

Electronic structure of GaInN semiconductors investigated by x-ray absorption spectroscopy

We have investigated the electronic structure of GaInN semiconductors by performing x-ray absorption near-edge fine structure (XANES) spectroscopy at Ga K-edge and self-consistent-field real-space multiple-scattering theory calculations. It was demonstrated that the nondestructive Ga K-edge XANES spectra can be used as the fingerprints of structure and composition for GaInN. The theoretical calculations gave a reasonable reproduction of the experimental spectral features. The results revealed that the combination of the experimental XANES and the theoretical calculations is a powerful tool for studying the electronic structure of GaInN semiconductors.

Anisotropic ternary Pr13Fe80B7 powders prepared by hydrogenation disproportionation desorption recombination process

High-purity ternary Pr13Fe80B7 magnetic powder with high anisotropy is obtained by a modified hydrogenation disproportionation desorption recombination (HDDR) process in this work. This powder exhibits a record-high Br of 1.05T, and has a good crystal texture along the c axis. It is found that the formation of HDDR Pr13Fe80B7 anisotropy is related to a rodlike disproportionation microstructure. We also show that by changing the HDDR conditions, an intrinsic coercivity of 800kA∕m and an energy product of 144kJ∕m3 are obtained on this powder. The details of the modified HDDR process are also presented in this paper.

Coercivity enhancement in Pr9.5Fe83Zr2B5.5 magnetic nanomaterials

The coercivity of Pr2Fe14B/α-Fe magnetic nanomaterial has been drastically improved from 6.8 kOe to 26 kOe by the diffusion of the Pr68Cu32 alloy. Pr68Cu32 alloy reacts with α-Fe to form boundary phases among Pr2Fe14B grains. The boundary phases have weak magnetic properties and have a thickness comparable to the domain wall thickness of Pr2Fe14B phase. So boundary layers are very effective in pinning the motion of domain walls, leading to an increased coercivity. The coercivity of this material at 160 °C is improved from 2.7 kOe to 10 kOe by diffusion treatment.

Tunable Valley Polarization and Valley Orbital Magnetic Moment Hall Effect in Honeycomb Systems with Broken Inversion Symmetry

In this Letter, a tunable valley polarization is investigated for honeycomb systems with broken inversion symmetry such as transition-metal dichalcogenide MX2 (M = Mo, W; X = S, Se) monolayers through elliptical pumping. Compared to circular pumping, elliptical pumping is a more universal and effective method to create coherent valley polarization. When two valleys of MX2 monolayers are doped or polarized, a novel anomalous Hall effect (called valley orbital magnetic moment Hall effect) is predicted. Valley orbital magnetic moment Hall effect can generate an orbital magnetic moment current without the accompaniment of a charge current, which opens a new avenue for exploration of valleytronics and orbitronics. Valley orbital magnetic moment Hall effect is expected to overshadow spin Hall effect and is tunable under elliptical pumping.

Magnetic frustration and magnetocaloric effect in AlFe2-xMnxB2 (x = 0-0.5) ribbons

The crystal structure and magnetic properties of AlFe2−x Mn x B2 (x = 0–0.5) compounds were investigated. With increasing Mn content, the magnetic properties of these compounds evolve gradually and the lattice parameters change continuously: Curie temperature (T c) decreases from 312 K (x = 0) to 220 K (x = 0.5) and a new phase transition from the ferromagnetic state (FM) to spin-glass state (SPG) appears upon cooling. The lattice shrinks in the ab plane while it expands in the c direction. The formation of re-entrant SPG around 50 K was studied via temperature dependent ac and dc magnetization as well as the initial magnetization curves in 5 K. The reason for this glass state is the triangular configuration of magnetic atoms and the antiferromagnetic interactions introduced by Mn substitutions. The Curie transition leads to a conventional magnetocaloric effect (MCE). The Mn doping leads to a decrease in the MCE near room temperature, but there is a plane in entropy change (ΔS) for the x = 0.1 compound under low field which makes these compounds good candidates to produce composites used for magnetic refrigeration application in a wide temperature span.