Quality improvement of GaInNAs/GaAs quantum wells grown by plasma-assisted molecular beam epitaxy

The optimum growth condition of GaInNAs/GaAs quantum wells (QWs) by plasma-assisted molecular beam epitaxy was investigated. High-resolution X-ray diffraction and photoluminescence (PL) measurements showed that ion damage drastically degraded the quality of GaNAs and GaInNAs QWs and that ion removal magnets can effectively remove the excess ion damage. Remarkable improvement of PL intensity and obvious appearance of pendellosung fringes were observed by removing the N ions produced in the plasma cell. When the growth rate increased from 0.73 to 1.2 ML/s, the optimum growth temperature was raised from 460 degreesC to 480 degreesC and PL peak intensity increased two times. Although the N composition decreased with increasing growth rate, degradation of optical properties of GaInNAs QWs was observed when the growth rate was over 0.92 ML/s. Due to low-temperature growth of GaInNAs QWs, a distinctive reflection high-energy electron diffraction pattern was observed only when the GaAs barrier was grown under lower As-4 pressure. The samples with GaAs barriers grown under lower As-4 pressure (V/III ratio about 24) exhibited seven times increase in PL peak intensity compared with those grown under higher As-4 pressure (V/III ratio about 50). (C) 2001 Elsevier Science B,V. All rights reserved.

Effect of ion-induced damage on GaNAs/GaAs quantum wells grown by plasma-assisted molecular beam epitaxy

The effect of ion-induced damage on GaNAs/GaAs quantum wells (QWs) grown by molecular beam epitaxy employing a DC plasma as the N source was investigated. Ion-induced damage results in: (i) an observed disappearance of pendellosung fringes in the X-ray diffraction pattern of the sample; (ii) a drastic decrease in intensity and a broadening in the full-width at half-maximum of photoluminescence spectra. It was shown that ion-induced damage strongly affected the bandedge potential fluctuations of the QWs. The bandedge potential fluctuations for the samples grown with and without ion removal magnets (IRMs) are 44 and 63 meV, respectively. It was found that the N-As atomic interdiffusion at the interfaces of the QWs was enhanced by the ion damage-induced defects. The estimated activation energies of the N-As atomic interdiffusion for the samples grown with and without IRMs are 3.34 and 1.78 eV, respectively. (C) 2001 Elsevier Science B.V. All rights reserved.

New modulation structures observed in Bi2.13Sr1.87CuO6+delta superconductors

A new two-dimensional structure modulation along c- and b-axes has been discovered in superconducting single crystals of Bi2.13Sr1.87CuO6+delta (Bi2201) by x-ray scattering. Such modulation structure does not exist in non-superconducting Bi2201 single crystals, but instead lattice distortions are observed in the a-b-plane. This phenomenon may indicate that both strain relaxation and charge modulation in the a-b-plane are important to the occurrence of superconductivity in the copper oxides.

Hard magnetic properties of Sm3Fe26.7VxN4 and Sm3Fe26.7VxCy

Sm3Fe26.7V2.3N4 nitrides and Sm3Fe26.7V2.3Cy carbides have been synthesized by gas-solid phase reaction. Their hard magnetic properties have been investigated by means of additional ball-milling at room temperature. The saturation magnetization of Sm3Fe26.7V2.3N4 almost decreases linearly with increasing ball-milling time t, but that of Sm3Fe26.7V2.3Cy has no obvious change when the ball-milling time increases from t = 1 to 28 h. As a preliminary result, the maximum remanence B-r of 0.94 and 0.88 T, the coercivity mu(0i)H(C) of 0.75 and 0.25 T, and the maximum energy product (BH) of 108.5 and 39.1 kJ/m(3) for their resin-bonded permanent magnets are achieved, respectively, by ball-milling at 293 K. (C) 1999 Published by Elsevier Science B.V. All rights reserved.

Crystallographic and magnetic properties of nitride R3Fe29-xCrxN4 (R = Y, Ca, Nd, Sm, Gd, Tb, and Dy)

A systematic investigation of crystallographic and magnetic properties of nitride R3Fe29-xCrxN4 (R=Y, Ce, Nd, Sm, Gd, Tb, and Dy) has been performed. The lattice constants and unit cell volume decrease with increasing rare earth atomic number from Nd to Dy, reflecting the lanthanide contraction. After nitrogenation the relative volume expansion of each nitride is around between 5% and 7%. The nitrogenation results in a good improvement in the Curie temperature, the saturation magnetization and anisotropy fields at 4.2 K, and room temperature for R3Fe29-xCrxN4. Magnetohistory effects of R3Fe29-xCrxN4 and R3Fe29-xCrx (R=Nd and Sm) are observed in a low field of 0.04 T. First order magnetization process occurs in Sm3Fe24.0Cr5.0N4 in magnetic fields of 2.8 T at 4.2 K. After nitrogenation, the easy magnetization direction of Sm3Fe24.0Cr5.0 is changed from the easy-cone structure to the uniaxial. The good intrinsic magnetic properties of Sm3Fe24.0Cr5.0N4 make this compound a hopeful candidate for new high-performance hard magnets. (C) 1998 American Institute of Physics.

Structure and magnetic properties of nitrides R3Fe29-xCrxN4

A systematic investigation of nitrides R3F29-xCrxN4 (R = Y, Ce, Nd, Sm, Gd, Tb, and Dy) has been performed. The nitrogen concentration in the nitride R3Fe29-xCrxNy was determined to be y = 4. Nitrogenation leads to a relative volume expansion of about 5.3%. The lattice constants and unit cell volume decrease with increasing rare earth atomic number from Nd to Dy, reflecting the lanthanide contraction. In average, the increase of Curie temperature upon nitrogenation is about 200 K, compared with its parent compound. The nitrogenation also results in a remarkable improvement in the saturation magnetization and anisotropy fields for R3Fe29-x CrxN4 at 4.2 K and room temperature, compared with their parent compounds. A spin reorientation of Nd3Fe24.5Cr4.5N4 occurs at around 368 K, which is 138 K higher than that of Nd3F24.5Cr4.5. Magnetohistory effects of R3Fe29-xCrxN4 (R = Nd and Sm) are observed in a low field of 0.04 T. First-order magnetization process occurs in Sm3Fe24.0Cr5.0N4 in magnetic fields of around 3.0 T at 4.2 K. After nitrogenation the easy magnetization direction of Sm3Fe24.0C5.0 is changed from the easy-cone structure to the uniaxial. The excellent intrinsic magnetic properties of Sm3Fe24.0Cr5.0N4 make this compound a hopeful candidate for new high-performance permanent magnets.

Structural and magnetic properties of GaN:Sm:Eu films fabricated by co-implantation method

Diluted-magnetic GaN:Sm:Eu films have been fabricated by co-implantation of Sm and Eu ions into c-plane (0001) GaN films and a subsequent annealing process. The structural, morphological and magnetic characteristics of the samples have been investigated by means of high-resolution X-ray diffraction (HRXRD), atomic force microscopy (AFM), and superconducting quantum interference device (SQUID). The XRD and AFM analyses show that the annealing process can effectively recover the crystalline degradation caused by implantation. Compared with GaN:Sm films, more defects have been introduced into GaN:Sm:Eu films due to the Eu implantation process. According to the SQUID analysis, GaN:Sm:Eu films exhibit clear room-temperature ferromagnetism. Moreover, GaN:Sm:Eu films show a lower saturation magnetization (Ms) than GaN:Sm films.

Superconductivity in Iron Telluride Thin Films under Tensile Stress

By realizing in thin films a tensile stress state, superconductivity of 13 K was introduced into FeTe, a nonsuperconducting parent compound of the iron pnictides and chalcogenides, with a transition temperature higher than that of its superconducting isostructural counterpart FeSe. For these tensile stressed films, superconductivity is accompanied by a softening of the first-order magnetic and structural phase transition, and also, the in-plane extension and out-of-plane contraction are universal in all FeTe films independent of the sign of the lattice mismatch, either positive or negative. Moreover, the correlations were found to exist between the transition temperatures and the tetrahedra bond angles in these thin films.

EFFECT OF A SI LAYER ON THE SUPERCONDUCTIVE PROPERTIES OF A BIPBSRCACUO THIN-FILM

The Pb-doped BiSrCaCuO superconducting films were grown by the single source mixed evaporation technique. The microbridges of dimensions 50 mum x 40 mum were fabricated by standard photolithography technologies. Si films with a thickness of 2500 angstrom were deposited on the microbridge area surfaces of BiPbSrCaCuO superconducting films by rf-magnetron sputtering. A greatly lowered zero resistance temperature of the microbridge area of the BiPbSrCaCuO film after Si sputtering was found. A non-linear effect of the current-voltage (I-V) characteristics at 78 K was shown. The high-frequency capacitance-voltage (C-V) curve of this structure at 78 K was symmetrical with the maximum capacitance at V = 0, and the capacitance decreased with increasing applied bias voltage. Afl experimental results are discussed.

Competitions of magnetism and superconductivity in FeAs-based materials

Using the numerical unrestricted Hartree-Fock approach, we study the ground state of a two-orbital model describing newly discovered FeAs-based superconductors. We observe the competition of a (0, π) mode spin-density wave and the superconductivity as the doping concentration changes. There might be a small region in the electron-doping side where the magnetism and superconductivity coexist. The superconducting pairing is found to be spin singlet,orbital even, and coexisting sxy + dx~2-y~2 wave (even parity).

Quality improvement of GaInNAs/GaAs quantum wells grown by plasma-assisted molecular beam epitaxy

The optimum growth condition of GaInNAs/GaAs quantum wells (QWs) by plasma-assisted molecular beam epitaxy was investigated. High-resolution X-ray diffraction and photoluminescence (PL) measurements showed that ion damage drastically degraded the quality of GaNAs and GaInNAs QWs and that ion removal magnets can effectively remove the excess ion damage. Remarkable improvement of PL intensity and obvious appearance of pendellosung fringes were observed by removing the N ions produced in the plasma cell. When the growth rate increased from 0.73 to 1.2 ML/s, the optimum growth temperature was raised from 460 degreesC to 480 degreesC and PL peak intensity increased two times. Although the N composition decreased with increasing growth rate, degradation of optical properties of GaInNAs QWs was observed when the growth rate was over 0.92 ML/s. Due to low-temperature growth of GaInNAs QWs, a distinctive reflection high-energy electron diffraction pattern was observed only when the GaAs barrier was grown under lower As-4 pressure. The samples with GaAs barriers grown under lower As-4 pressure (V/III ratio about 24) exhibited seven times increase in PL peak intensity compared with those grown under higher As-4 pressure (V/III ratio about 50). (C) 2001 Elsevier Science B,V. All rights reserved.

Study of the aberrations of the IMP heavy ion microbeam irradiation system

A high energy heavy ion microbeam irradiation system is constructed at the Institute of Modern Physics (IMP) of the Chinese Academy of Sciences (CAS). A quadrupole focusing system, in combination with a series of slits, has been designed here. The IMP microbeam system is described in detail. The intrinsic and parasitic aberrations associated with the magnets are simulated. The ion beam optics of this microbeam system is investigated systematically. Then the optimized initial beam parameters are given for high spatial resolution and high hitting rates.

超导ECR源SECRAL轫致辐射谱的测量

热电子在ECR(Electron Cyclotron Resonance)源中有着非常重要的作用,为了研究ECR源的工作参数(微波功率、磁场等)对热电子的影响,我们对SECRAL(Superconducting ECR ion source with Advanced design in Lanzhou)等离子体在轴向发出的轫致辐射谱进行了系统的测量。从测得的轫致辐射谱中我们得到用来衡量热电子能量的参考量——光谱温度Tspe,并且对ECR源的几个工作参数与Tspe的关系进行了讨论。

Measurements of bremsstrahlung spectra of Lanzhou ECR Ion Source No. 3 (LECR3)

In order to diagnose the electron cyclotron resonance (ECR) plasma, electron bremsstrahlung spectra were measured by a HPGe detector on Lanzhou ECR Ion Source No. 3 at IMP. The ion source was operated with argon under various working conditions, including different microwave power, mixing gas, extraction high voltage (HV), and so on. Some of the measured spectra are presented in this article. The dependence of energetic electron population on mixing gas and extraction HV is also described. Additionally, we are looking forward to further measurements on SECRAL (Superconducting ECR Ion Source with Advanced design at Lanzhou).

Development of an intermediate energy heavy-ion micro-beam irradiation system

The micro-beam irradiation system, which focuses the beam down to micron order and precisely delivers a predefined number of ions to a predefined spot of micron order, is a powerful tool for radio-biology, radio-biomedicine and micromachining. The Institute of Modern Physics of Chinese Academy of Sciences is developing a heavy-ion microbeam irradiation system up to intermediate energy. Based on the intermediate and low energy beam provided by Heavy Ion Research Facility of Lanzhou, the micro-beam system takes the form of the magnetic focusing. The heavy-ion beam is conducted to the basement by a symmetrical achromatic system consisting of two vertical bending magnets and a quadrupole in between. Then a beam spot of micron order is formed by a magnetic triplet quadrupole of very high gradient. The sample can be irradiated either in vacuum or in the air. This system will be the first opening platform capable of providing heavy ion micro-beam, ranging from low (10MeV/u) to intermediate energy (100MeV/u), for irradiation experiment with positioning and counting accuracy. Target material may be biology cell, tissue or other non-biological materials. It will be a help for unveiling the essence of heavy-ion interaction with matter and also a new means for exploring the application of heavy-ion irradiation.