Bosen Wang

Intrinsic room temperature ferromagnetism in boron-doped ZnO

We report room temperature ferromagnetism in boron-doped ZnO both experimentally and theoretically. The single phase Zn1−xBxO films deposited under high oxygen pressure by pulsed laser deposition show ferromagnetic behavior at room temperature. The saturation magnetization increases monotonically from 0 to 1.5u2002emu/cm3 with the increasing boron component x from 0% to 6.8%. The first-principles calculations demonstrate that the ferromagnetism in the B-doped ZnO originates from the induced magnetic moment of oxygen atoms in the nearest neighbor sites to B–Zn vacancy pairs. The calculated total magnetic moment shows an increasing trend with the boron component which is consistent with experiment.

Different origins of the yellow luminescence in as-grown high-resistance GaN and unintentional-doped GaN films

The yellow luminescence (YL) in as-grown high-resistance (HR) and unintentional-doped (UID) GaN films grown by metal organic chemical vapor deposition has been investigated by means of photoluminescence and monoenergetic positron annihilation spectroscopy. It is found there is stronger YL in UID-GaN with higher concentration of gallium vacancy (VGa), suggesting that VGa-involved defects are the origin responsible for the YL in UID-GaN. Contrastly, there is much stronger YL in HR-GaN that is nearly free from VGa, suggesting that there is another origin for the YL in HR-GaN, which is thought as the carbon-involved defects. Furthermore, it is found that the HR-GaN film with shorter positron diffusion length Ld exhibits stronger YL. It is suggested that the increased wave function overlap of electrons and holes induced by the extremely strong space localization effect of holes deduced from the short Ld is the vital factor to enhance the YL efficiency in HR-GaN.

Enhancement of critical current density of (Pb,Sn)-doped Bi-2212 superconductors at high temperature

Abstract The magnetic properties of ceramic samples of Bi 2−2 x (PbSn) x Sr 2 CaCu 2 O y ( x =0,0.1,0.2,0.3 and 0.4) superconductors have been investigated using a vibrating sample magnetometer from 20 to 50 K and up to 8 T. Phase analysis shows that most of the (Pb,Sn) dopant enters the Bi-2212 lattice and occupies Bi sites. The current-carrying scale-length analysis has been determined from the reverse leg of the magnetization hysteresis loops and compared to the actual dimensions of the sample. Magnetization hysteresis loops of the (PbSn)-doped sample exhibit strong pinning behaviour in the higher temperature range. The effect of (Pb,Sn) doping on intragranular critical current J c was found to be enhanced by one order of magnitude at 50 K. We discuss the experimental results within a model of (PbSn) cluster pinning in blocking layers. Doping by (PbSn) clusters into the Bi–O layers provides effective pinning centres, as well as a large reduction of the resistivity anisotropy.

Zur Elektronenstruktur metallorganischer komplexe der f- Elemente Teil 37. die elektronenstruktur von Pr3+bei ψ- trigonal-bipyramidaler organischer koordination

Zusammenfassung Durch Dotierung einer Cp 3 La(NCCH 3 ) 2 -Matrix mit unterschiedlichen Mengen von Pr 3+ konnten relativ stabile Cp 3 La 1 − x Pr x (NCCH 3 ) 2 ( 1 ) ( x = 0,6; 0,3; 0,1)-Einkristalle gezuchtet werden. Auf der Grundlage absorptions- und lumineszenzspektroskopischer Untersuchungen sowie teilweise von Messungen des magnetischen Zirkulardichroismus wurden die Kristallfeld (KF)-Aufspaltungsmuster von 1 und (CH 3 OCH 2 CH 2 C 5 H 4 ) 3 Pr ( 2 ) abgeleitet. Die Parameter eines empirischen Hamilton-Operators wurden den KF-Aufspaltungsmustern von 1 und 2 angepaβt, wobei r.m.s.-Abweichungen von 27 cm −1 bzw. 24 cm −1 bei 25 bzw. 24 Zuordnungen erzielt wurden. Bei Verwendung der berechneten Wellenfunktionen und Eigenwerte konnte die Temperaturabhangigkeit der mittleren paramagnetischen Suszeptibilitat von Cp 3 Pr(NCCH 3 ) 2 sowie von 2 in ausgezeichneter Weise reproduziert werden.

Critical behavior of double perovskite La2NiMnO6

The critical behavior of the double perovskite La(2)NiMnO(6) was investigated by measurement of the magnetization around the Curie temperature T(C). The magnetic data were analyzed in the critical region using the Kouvel-Fisher method to yield the critical exponents of β = 0.408 ± 0.011 with T(C) = 270.50 (from the temperature dependence of the spontaneous magnetization below T(C)) and γ = 1.295 ± 0.015 with T(C) = 271.10 (from the temperature dependence of the inverse initial susceptibility above T(C)). The critical magnetization isotherm M(T(C),H) gives δ = 4.139 ± 0.090. The critical exponents obtained by this method obey the Widom scaling relation δ = 1+γ/β, implying the critical exponents are reliable. The values of critical exponents are close to those predicted by the three-dimensional (3D) Heisenberg model with short-range interactions.

Unusual ferromagnetic critical behavior owing to short-range antiferromagnetic correlations in antiperovskite Cu1-xNMn3-x (0.1 ≤x ≤0.4)

For ferromagnets, varying from simple metals to strongly correlated oxides,the critical behaviors near the Curie temperature (TC) can be grouped into several universal classes. In this paper, we report an unusual critical behavior in manganese nitrides Cu1-xNMn3+x (0.1 ≤ x ≤ 0.4). Although the critical behavior below TC can be well described by mean field (MF) theory, robust critical fluctuations beyond the expectations of any universal classes are observed above TC in x = 0.1. The critical fluctuations become weaker when x increases, and the MF-like critical behavior is finally restored at x = 0.4. In addition, the paramagnetic susceptibility of all the samples deviates from the Curie-Weiss (CW) law just above TC. This deviation is gradually smeared as x increases. The short-range antiferromagnetic ordering above TC revealed by our electron spin resonance measurement explains both the unusual critical behavior and the breakdown of the CW law.

Anisotropic intermediate coupling superconductivity in Cu0.03TaS2

The anisotropic superconducting state properties in Cu(0.03)TaS(2) have been investigated by magnetization, magnetoresistance and specific heat measurements. They clearly show that Cu(0.03)TaS(2) undergoes a superconducting transition at T(C) = 4.03xa0K. The obtained superconducting parameters demonstrate that Cu(0.03)TaS(2) is an anisotropic type-II superconductor. Combining specific heat jump ΔC/γ(n)T(C) = 1.6(4), gap ratio 2Δ/k(B)T(C) = 4.0(9) and the estimated electron-phonon coupling constant λ∼0.68, the superconductivity in Cu(0.03)TaS(2) is explained within the intermediate coupling BCS scenario. First-principles electronic structure calculations suggest that copper intercalation of 2H-TaS(2) causes a considerable increase of the Fermi surface volume and the carrier density, which suppresses the CDW fluctuation and favors the raise of T(C).

Thermal history dependent photoconductivity in Pr0.5Sr0.5MnO3 thin film

Thermal history dependent photoconductivity has been observed in the thermal hysteresis region of Pr0.5Sr0.5MnO3 thin film. In the cooling process, the film shows an upward resistance relaxation in darkness. Under light illumination, the transient photoconductivity effect is observed. However, it exhibits a downward resistance relaxation without illumination and persistent photoconductivity (PPC) behavior with light illumination in the warming process. The PPC ratio is 18.1% at 110 K with a light density of 1.25u2002mWu2009mm−2. It is found that the external magnetic field plays a similar role as the light illumination. The persistent magnetoresistance ratio reaches 44.4% at 110 K with a low magnetic field of 0.25 T. The underlying mechanism is discussed based on phase competition and phase stability switch between ferromagnetic metal and charge- and orbital-ordered insulator states. The results may be important for practical applications in photo-/magnetic field sensitive and memory devices.

Positron annihilation in (Ga, Mn)N: A study of vacancy-type defects

The vacancy-type defects in (Ga,Mn)N films grown by metal organic chemical vapor deposition were studied by positron annihilation technique. Doppler broadening spectra were measured for the films. Compared to the undoped GaN film, the positron trapping defects in the (Ga,Mn)N films have been changed to a new type defects and its concentration increases with the increasing Mn concentration. By analyzing the S-W correlation plots and our previous results, we identify this type defects in the (Ga,Mn)N as VN-MnGa complex. This type of defects should be considered when understand the magnetic properties in a real (Ga,Mn)N system.

Strain-tunable Josephson current in graphene-superconductor junction

Strain effects on Josephson current in a graphene-superconductor junction are explored theoretically. It is demonstrated that the supercurrent is an oscillatory function of zigzag direction strain with a strain-dependent oscillating frequency. Interestingly, it is found that the Josephson current under armchair direction strain can be turned on/off with a cutoff strain. In view of the on/off properties of the Josephson current, we propose the strained graphene Josephson junction to be utilized as a supercurrent switch.