B. G. Shen

Transition of stoichiometric Sr2VO3FeAs to a superconducting state at 37.2 K

The superconductor Sr4V2O6Fe2As2 with transition temperature at 37.2 K was successfully fabricated. It has a layered structure with the space group of p4/nmm and with the lattice constants a=3.9296 A degrees and c=15.6732 A degrees. The observed large diamagnetization signal and zero resistance demonstrated the bulk superconductivity. The broadening of resistive transition was measured under different magnetic fields revealing a rather high upper critical field. The results also suggest a large vortex liquid region that reflects high anisotropy of the system. The Hall-effect measurements revealed dominantly electronlike charge carriers in this material. The superconductivity in the present system may be induced by oxygen deficiency or the multiple valence states of vanadium.

A high-mobility two-dimensional electron gas at the spinel/perovskite interface of γ-Al2O3/SrTiO3.

The discovery of two-dimensional electron gases at the heterointerface between two insulating perovskite-type oxides, such as LaAlO(3) and SrTiO(3), provides opportunities for a new generation of all-oxide electronic devices. Key challenges remain for achieving interfacial electron mobilities much beyond the current value of approximately 1,000 cm(2) V(-1) s(-1) (at low temperatures). Here we create a new type of two-dimensional electron gas at the heterointerface between SrTiO(3) and a spinel γ-Al(2)O(3) epitaxial film with compatible oxygen ions sublattices. Electron mobilities more than one order of magnitude higher than those of hitherto-investigated perovskite-type interfaces are obtained. The spinel/perovskite two-dimensional electron gas, where the two-dimensional conduction character is revealed by quantum magnetoresistance oscillations, is found to result from interface-stabilized oxygen vacancies confined within a layer of 0.9 nm in proximity to the interface. Our findings pave the way for studies of mesoscopic physics with complex oxides and design of high-mobility all-oxide electronic devices.

Surface spin-glass behavior in La2/3Sr1/3MnO3 nanoparticles

The low-temperature magnetic and transport properties of La2/3Sr1/3MnO3 nanoparticles have been investigated. It is found that a surface spin-glass behavior exists in La2/3Sr1/3MnO3 nanoparticles, which undergo a magnetic transition to a frozen state below 45 K. The low-temperature surface spin-glass behavior exists even at the highest field used (H=50 kOe). Moreover, the spin-glass-like transition disappears for particles above 50 nm. In addition, the suppressed low-field magnetoconductivity (LFMC) observed at low temperature for nanosized La2/3Sr1/3MnO3 is obviously lower than the expected upper limit of LFMC, 1/3, for polycrystalline manganites, which is proposed to arise from the higher-order tunneling through the insulating spin-glass-like surface layers.

Contrasting spin dynamics between underdoped and overdoped Ba(Fe1-xCox)2As2.

We report the first NMR investigation of spin dynamics in the overdoped nonsuperconducting regime of Ba(Fe1-xCox)2As2 up to x=0.26. We demonstrate that the absence of interband transitions with large momentum transfer Q{AF} approximately (pi/a,0) between the hole and electron Fermi surfaces results in complete suppression of antiferromagnetic spin fluctuations for x greater than or approximately 0.15. Our experimental results provide direct evidence for a correlation between T{c} and the strength of Q{AF} antiferromagnetic spin fluctuations.

Strain-dependent vacuum annealing effects in La0.67Ca0.33MnO3−δ films

The effects of vacuum annealing on the structural and transport properties of the La0.67Ca0.33MnO3−δ films grown on SrTiO3 (LCMO/STO) and NdGaO3 (LCMO/NGO) substrates have been studied. A lattice expansion due to oxygen release during the annealing is observed. Under the same condition, the change of the out-of-plane lattice parameter in LCMO/STO is two to three times larger than that in LCMO/NGO, indicating a strong tendency for the oxygen in the former to escape. Correspondingly, the metal-to-semiconductor transition shifts to lower temperatures, linearly with lattice constant until a critical value, Δd=0.03 A for LCMO/STO and Δd=0.05 A for LCMO/NGO, after which a sudden drop of the transition temperature to zero occurs. The different lattice strains in both films are presumably responsible for the different critical oxygen contents for the occurrence of the resistive transition.

Roles of multiband effects and electron-hole asymmetry in the superconductivity and normal-state properties of Ba(Fe(1-x)Cox)(2)As-2

We report a systematic investigation, together with a theoretical analysis, of the resistivity and Hall effect in single crystals of Ba(Fe1-xCox)(2)As-2 over a wide doping range. We find a surprisingly great disparity between the relaxation rates of the holes and the electrons in excess of one order of magnitude in the low-doping, low-temperature regime. The ratio of the electron to hole mobilities diminishes with temperature and doping (away from the magnetically ordered state) and becomes more conventional. We also find a straightforward explanation of the large asymmetry (compared to cuprates) of the superconducting dome: in the underdoped regime the decisive factor is the competition between antiferromagnetism and superconductivity, while in the overdoped regime the main role is played by degradation of the nesting that weakens the pairing interaction. Our results indicate that spin fluctuations due to interband electron-hole scattering play a crucial role not only in the superconducting pairing but also in the normal transport.

Influence of nanocrystallization on the evolution of domain patterns and the magnetoimpedance effect in amorphous Fe73.5Cu1Nb3Si13.5B9 ribbons

The evolution of the magnetic domain patterns and the giant magnetoimpedance (MI) effect in the Fe73.5Cu1Nb3Si13.5B9 amorphous ribbons are studied as a function of the annealing time ta at 550 °C. It is shown that by annealing at 550 °C for ta from 1 to 150 h, very fine nanocrystalline bcc-FeSi grains are homogeneously formed in the amorphous matrix. Although with increasing ta the grain size remains very fine (⩽11 nm), the coercivity Hc increases rapidly from 0.0014 Oe for ta=3 h to 1.67 Oe for ta=150 h. For the nanocrystalline ribbons with ta=3 h, the domain structure is characterized by a few broad longitudinal together with some broad transverse domain patterns, connected to the minimum coercitive field. With increasing annealing time ta⩽10 h, only transverse domain patterns are observed and the transverse domain width gradually becomes narrow. Considering the different magnetic softness, domain structure, and magnetization process, various types of the MI effect are observed in these nanocryastalline...

Improvement of reproducible resistance switching in polycrystalline tungsten oxide films by in situ oxygen annealing

The electric-field-induced resistance switching in polycrystalline tungsten oxide films was investigated. Compared with the as-deposited film, the film annealed in an oxygen atmosphere shows a more stable switching behavior, a higher low-to-high resistance ratio, and a better endurance and retention. Based on the x-ray photoemission spectroscopy analysis, the resistance switching was attributed to the change in the interfacial barrier potential, due to the electron trapping/detrapping in the surface states, and the switching improvement was attributed to the decrease in the surface density of states. The present work suggests a possible approach controlling the resistance switching property by surface modification.

High-Tc superconductivity induced by doping rare-earth elements into CaFeAsF

We have successfully synthesized the fluoride-arsenide compounds Ca1?xRExFeAsF (RE=Nd,?Pr; x=0,0.6). The X-ray powder diffraction confirmed that the main phases of our samples are Ca1?xRExFeAsF with the ZrCuSiAs structure. By measuring resistivity, superconductivity was observed at 56?K in Nd-doped and 52?K in Pr-doped samples with x=0.6. Bulk superconductivity was also proved by the DC magnetization measurements in both samples. Hall effect measurements revealed hole-like charge carriers in the parent compound CaFeAsF with a clear resistivity anomaly below 118?K, while the Hall coefficient RH in the normal state is negative for the superconducting samples Ca0.4Nd0.6FeAsF and Ca0.4Pr0.6FeAsF. This indicates that the rare-earth element doping introduces electrons into CaFeAsF, which induces high-temperature superconductivity.

Metal–semiconductor–metal GaN ultraviolet photodetectors on Si(111)

GaN metal–semiconductor–metal photoconductive detectors have been fabricated on Si(111) substrates. The GaN epitaxial layers were grown on Si substrates by means of metalorganic chemical-vapor deposition. These detectors exhibited a sharp cutoff at the wavelength of 363 nm and a high responsivity at a wavelength from 360 to 250 nm. A maximum responsivity of 6.9 A/W was achieved at 357 nm with a 5 V bias. The relationship between the responsivity and the bias voltage was measured. The responsivity saturated when the bias voltage reached 5 V. The response time of 4.8 ms was determined by the measurements of photocurrent versus modulation frequency.