Y. X. Zhang

Structural, optical, and magnetic properties of Cu-implanted GaN films

The structural, optical, and magnetic properties of Cu-implanted GaN films have been investigated. No secondary phase was found within the resolution limit of the instrument but the lattice defects such as vacancies were present in the film. Room temperature ferromagnetism was observed with saturation magnetization of 0.3μB/Cu atom. The field-cooled magnetization curves can be well fitted by a Curie-Weiss model and a standard three-dimensional spin-wave model in the low and high temperature ranges, respectively. Our findings indicate that the vacancylike defects should be considered in understanding the observed magnetic properties of the Cu-implanted GaN films.

TWO LOCAL VIBRATIONAL MODES RELATED TO HYDROGEN IN GAN

We have observed two absorption bands located at around 1730 and 2960 cm−1 in the infrared (IR) absorption spectra from undoped GaN samples which are grown using low pressure metalorganic vapor phase epitaxy and irradiated by gamma ray and then exposed to a radio frequency hydrogen plasma. Proton implantation followed by gamma-ray irradiation of the GaN samples can also activate the IR band at around 1730 cm−1. Based on the experimental results, we tentatively ascribe the 1730 cm−1 band to the local vibrational modes of Ga–H complexes in the vicinity of N vacancies and the 2960 cm−1 band to those of either N–H complexes in the vicinity of Ga vacancies or C–H complexes.

EXPERIMENTAL-STUDY ON THE ER/P-INP SCHOTTKY-BARRIER

Rare‐earth element Er was deposited onto (100) oriented Zn‐doped p‐type InP to form Schottky barriers. The Er/p‐InP Schottky barrier have been studied by current‐voltage (I‐V), temperature dependence of current‐voltage (I‐V‐T), and capacitance‐voltage (C‐V) methods and Schottky barrier heights (SBHs) measured by I‐V and I‐V‐T methods are in the range 0.83–0.87 eV, while SBHs measured by the C‐V method are in the range 0.98–1.06 eV. Ideality factor n and series resistances R are in the range 1.08–1.11 and 30–50 Ω, respectively. Combining the experimental results of SBHs reported in the literature for Schottky barriers with various metals on p‐InP (100), we conclude the Fermi level pinning for InP with (100) orientation is much stronger than that for Si or GaAs.

Magnetic and transport properties of cobalt doped La0.7Sr0.3MnO3

The magnetic and transport properties of La0.7Sr0.3Mn1−xCoxO3 (0 ≤ x ≤ 1) samples were systematically studied. A magnetic phase diagram was constructed according to the observed results. At the two ends (x ≤ 0.3 and x > 0.9), the samples exhibit the conventional ferromagnetism with the metallic conducting behavior, which can be interpreted in terms of the double exchange coupling between Mn (Co) ions. As for the intermediate Co-doping region (0.3 < x ≤ 0.9), the samples enter a mixed magnetic phase state, which displays the characteristics of the glassy states. According to the frequency-dependent results from the ac susceptibility, a representative frequency sensitivity factor K was calculated to be ∼0.1 for the typical La0.7Sr0.3Mn0.6Co0.4O3 sample, corresponding to the coexistence of superparamagnetic-like free spins and the reentrant spin glass. The results of the Curie-Weiss fitting of the dc magnetization suggest that the Co3+ ions are in the intermediate spin state with the increase of Co-doping le...

Quantum molecular dynamic simulations of warm dense carbon monoxide

Using quantum molecular dynamic simulations, we have studied the thermophysical properties of warm dense carbon monoxide under extreme conditions. The principal Hugoniot pressure up to 286 GPa, which is derived from the equation of state, is calculated and compared with available experimental and theoretical data. The chemical decomposition of carbon monoxide has been predicted at 8 GPa by means of pair correlation function and the charge density distribution. Based on Kubo-Greenwood formula, the dc electrical conductivity and the optical reflectivity are determined, and the nonmetal-metal transition for shock compressed carbon monoxide is observed around 40 GPa.

Fabrication of FeOx thin films and the modulation of transport and magnetic properties by resistance switching in Au/α-Fe2O3/Pt heterostructure

It was found that by using the same α-Fe2O3 target in fabrication process, an oxygen-deficient and an oxygen-sufficient atmosphere facilitated the formation of Fe3O4 and α-Fe2O3, respectively. The Au/α-Fe2O3/Pt heterostructure showed a memristive bipolar resistance switching. A clear difference was observed in the transport and magnetic properties between the two resistance states. The enhanced conductivity and magnetization in the Au/α-Fe2O3/Pt heterostructure are believed to result from an enhanced electron hopping between Fe3+-Fe2+ pairs and Fe3+-O-Fe2+ double exchange coupling effect. The enhanced Fe2+ comes from a redox transition between two phases (one behaves like Fe3O4 and the other like α-Fe2O3), due to the electrochemical migration of oxygen vacancies.

Electrical and optical properties of fluid iron from compressed to expanded regime

Using quantum molecular dynamics simulations, we show that the electrical and optical properties of fluid iron change dramatically from compressed to expanded regime. Along isochores, the main trend of the electrical resistivity, which is in good agreement with experimental data, is found to be well reproduced by our calculations. Near low densities, where the constant volume derivative of the electrical resistivity on internal energy becomes negative, the transition of iron from the metallic to the non-metallic state takes place. The study of the optical conductivity, absorption coefficient, and Rosseland mean opacity shows that, quantum molecular dynamics combined with the Kubo-Greenwood formula provide powerful tools to explore the electrical and optical properties of fluid iron.

Quantum molecular dynamics simulations for the nonmetal-metal transition in fluid nitrogen oxide

First-principle molecular-dynamics simulations based on density-functional theory have been used to study the thermophysical properties of fluid nitrogen oxide under extreme conditions. We have presented wide range equation of states, from which the principal Hugoniot were derived up to 200 GPa, and the results are well accordant with the experimental and theoretical data. The optical conductivity is calculated via the Kubo-Greenwood formula, from which the dc conductivity is obtained. The nonmetal-metal transition is observed at about 40 GPa, and is attributed to the dissociation of nitrogen oxide molecules. Additionally, the density of states and the distribution of the electronic charge are also investigated to study the complex behavior of fluid nitrogen oxide.

Quantum molecular dynamics simulations of thermophysical properties of fluid ethane

We have performed first-principles molecular-dynamics simulations based on density-functional theory to study the thermophysical properties of ethane under extreme conditions. We present results for the equation of state of fluid ethane in the warm dense region. The optical conductivity is calculated via the Kubo-Greenwood formula from which the dc conductivity and optical reflectivity are derived. The close correlation between the nonmetal-metal transition of ethane and its decomposition, that ethane dissociates significantly into molecular and/or atomic hydrogen and some long alkane chains, has been systematically studied by analyzing the optical conductivity spectra, pair correlation functions, electronic density of states, and charge density distribution of fluid ethane.

Ultraintense laser absorption and γ-ray synchrotron radiation in near critical density plasmas

Ultraintense laser absorption and γ-ray synchrotron radiation in near-critical-density (NCD) plasmas are investigated. Besides the known skin-depth emission and reinjected electron synchrotron emission in NCD plasmas, we find a new γ-ray emission mechanism, where γ-rays are dominantly produced by the Transversely Oscillating Electron synchrotron Emission (TOEE). In this new TOEE mechanism, electrons mainly oscillate in the transverse direction under the balance between the longitudinal laser ponderomotive force and the restoring electrostatic force. A great amount of γ photons are emitted in the transverse direction, where the peak radiation power is enhanced by twice and the photon divergence angle is relatively decreased. The features of γ-rays produced from this new TOEE mechanism have been identified and compared with the other two mechanisms by two-dimensional particle-in-cell simulations.