Superconductivity and properties of FeTeOx films

Films of the parent compound FeTe can be made superconducting via the addition of interstitial oxygen. The process is reversible. We have characterized the new superconductors with a variety of experiments. X-ray diffraction shows that the superconductor has the same overall structure but a small lattice constant change compared to pure FeTe. X-ray absorption shows that superconducting FeTeO_x has a nominal valence of 3+. DFT calculations show the most likely position for interstitial oxygen and confirm that such oxygen incorporation does not produce a large change in structure.     

Observation of Two-Level Critical State in the Superconducting FeTe Thin Films

FeTe, a non-superconducting parent compound in the iron-chalcogenide family, becomes superconducting after annealing in oxygen. Under the presence of magnetism, spin-orbit coupling, inhomogeneity and lattice distortion,the nature of its superconductivity is not well understood. Here we combine the mutual inductance technique with magneto transport to study the magnetization and superconductivity of FeTe thin films. It is found that the films with the highest T_C show non-saturating superfluid density and a strong magnetic hysteresis distinct from that in a homogeneous superconductor. Such a hysteresis can be well explained by a two-level critical state model and suggests the importance of granularity to superconductivity in this compound.     

Metallization and demetallization of clean and oxygen-covered ultrathin alkali metal films on GaAs(1 0 0)

The structure and the electronic valence state occupation of ultrathin K, Rb, and Cs films grown on a GaAs(1 0 0)-(4×2) surface have been studied by means of metastable He atom scattering (MHAS), He atom scattering (HAS), and low-energy electron diffraction (LEED) at temperatures ranging from 150 to 400 K. From the survival probability of the scattered He^* atoms, detailed information on the coverage-dependent filling of the alkali metal valence states and their emptying upon subsequent exposure to oxygen were derived. These data reveal for K and Rb a nearly linear band filling with increasing coverage starting at about 0.5 ML whereas a more rapid filling is observed for Cs which is almost completed at about 0.7 ML. Subsequent oxygen adsorption causes a demetallization of the metallic alkali metal monolayers. In case of Cs, a distinct minimum of the He^* signal appears at an oxygen exposure of about 0.8 L, presumably indicating the onset of subsurface oxidation.     

FeTe合金结构分析及其薄膜制备

利用固态反应法制备了名义成分为FeTe的合金, 采用X射线粉末衍射技术和Rietveld全谱拟合分析方法测定了其相组成和晶体结构. 研究表明,主相为Fe_1.08Te,空间群为P4/nmm,点阵参数 a = 3.8214(3) Å, c = 6.2875(3) Å, Z = 2, Fe原子占据2a和2c晶位, Te原子占据2c晶位. 利用脉冲激光沉积技术制备的FeTe薄膜超导转变起始温度为13.2 K,零电阻温度为9.8 K. 关键词： FeTe Rietveld结构精修 超导薄膜     

Effects of the oxygen partial pressure and annealing atmospheres on the microstructures and optical properties of Cu-doped ZnO films

Cu-doped zinc oxide (ZnO:Cu) films were deposited on p-Si (1 0 0) substrates at 200 °C under various oxygen partial pressures by using radio frequency reactive magnetron sputtering. The properties of the films were characterized by the X-ray diffraction spectroscopy (XRD), energy dispersive spectrometer, X-ray photoelectron spectroscopy (XPS) and fluorescence spectrophotometer with the emphasis on the evolution of microstructures, element composition, valence state of Cu, optical properties. The results indicated that the properties of ZnO:Cu films were significantly affected by oxygen partial pressures. XRD measurements revealed that the sample prepared at the ratio of O₂:Ar of 15:10 sccm had the best crystal quality among all ZnO:Cu films. XPS analysis results suggested that the valence of Cu in the ZnO films was a mixed state of +1 and +2, and the integrated intensity ratio of Cu²⁺ to Cu⁺ increased with the increment of oxygen partial pressure. The photoluminescence measurements at room temperature revealed a violet, two blue and a green emission. We considered that the origin of green emission came from various oxygen defects when the ZnO:Cu films grew in oxygen poor and enriched environment. Furthermore, the influence of annealing atmosphere on the microstructures and optical properties of ZnO:Cu films were discussed.     

Charge stripe structure in FeTe

We report the STM study on a single-crystalline sample of FeTe at 7.8 K. FeTe is one of the iron-based superconductor. We measured the resistivity and the magnetization of FeTe. FeTe shows SDW transition at 58 K on these measurements. We study the electronic state of FeTe by using STM/STS for observing FeTe from a microscopic viewpoint. We observed the iron layer and the tellurium layer with atomic resolution. Moreover, we discover the charge stripe structure on STM/STS measurement. We find the charge stripe structure is caused by iron atoms from the analysis. The gap structure of 9 meV was observed in tunneling spectra. This gap size is consistent with the SDW gap which is expected from mean field theory with T_N=58 K.     

Oxygen adsorption on ultrathin Co/Ir(1 1 1) films: Compositional anomaly

Adsorption of oxygen on ultrathin Co/Ir(1 1 1) films thinner than 4 monolayers in an ultrahigh vacuum environment was studied. For oxygen adsorption on cobalt films, the complex adsorption kinetics emerges partly due to the incorporation of oxygen. The amount of oxygen adsorbed at the surfaces is higher than that incorporated into the film as revealed from sputter profiling measurements. At room temperature the CoO layer exhibits paramagnetism and could not contribute to the remanent Kerr intensity. As oxygen exposure increases, the reduction of the Kerr intensity is due to the reduction of the effective layer for the magnetic measurements. Compared with oxygen saturated cobalt films, the concentration of adsorbed oxygen per Co atom shows an oscillatory behavior. A compositional anomaly of a great amount of adsorbed oxygen in submonolayer Co coverage occurs because of the maximized number of adsorption and incorporation sites for oxygen on the surface. A larger charge transfer between Co and oxygen was observed for thinner Co overlayers as revealed from the larger chemical shifts of Auger lines.     

Oxygen depth profiling of high pressure DC-sputtered amorphous silicon

We have measured the in-depth oxygen concentration of high pressure DC-sputtered a-Si films. The measurements were performed by means of AES and IR spectroscopy, immediately after sample preparation and after air exposure during two weeks. We found that post incorporation of oxygen is not located at the thin film surface, supporting the idea that a-Si films prepared by high pressure sputtering present an open pore network, as it has been previously proposed.     

Observation of lateral long range order in superconducting FeTe thin films

<正>We find that the superconductivity in the thin films of the formerly believed non-superconducting parent compound FeTe is accompanied by an emergence of second order with a correlation length of 742 nm and 258 nm at 10 K and 300 K,respectively.The structural phase transition found in iron pnictide superconductors,in non-superconducting FeTe bulk samples,and in FeSe superconducting thin films is not observed in the superconducting FeTe thin films.The interplay between superconductivity and long range order may suggest the crucial role of competition between electronic localization and itinerancy which leads to strong quantum fluctuations in the FeTe system.     

Effect of oxygen exposure on the magnetic properties of ultrathin Co/Ge(1 1 1) films

Influences of oxygen exposure on the magnetic properties of Co/Ge(1 1 1) ultrathin films have been investigated by surface magneto-optic Kerr effect technique. As the oxygen exposure increases on Co/Ge(1 1 1) films, their magnetic properties could be modified. As an example for 15 ML Co/Ge(1 1 1) films, the coercivity increases from 730 to 920 Oe and the remanence Kerr intensity is reduced for 500 Langmuir (L) of oxygen exposure. Corresponding compositions analyzed by Auger electron spectroscopy measurement shows that the amount of oxygen on the surface layers increases with increasing the oxygen exposure time. Oxygen distributes on the topmost layers of the film. The adsorbed oxygen influences the electronic density of states of Co and results in the changes of the magnetic properties. Besides, the appearance of O/Co/Ge interface could modify the stress anisotropy, and as a result the coercivity of ultrathin Co/Ge(1 1 1) film is enhanced.     

Effect of oxygen exposure on the magnetic properties of ultrathin Co/Si(1 1 1)-7×7 films

Effect of oxygen exposure on the magnetic properties of ultrathin Co/Si(1 1 1)-7×7 films have been studied. In ultrahigh vacuum environment, Auger electron spectroscopy (AES) analysis shows that no oxygen adsorption occurs on Si(1 1 1)-7×7 surface and Co-Si compound interfaces. As the thickness of Co films increases above 5 monolayers (ML), pure cobalt islands form on the surface and the amount of oxygen on the surface layers increases with increasing the oxygen exposure time. From the results of slight chemical shift and depth profiling measurements, the oxygen is weakly adsorbed on the topmost layer of 15 ML Co/Si(1 1 1) films. The adsorbed oxygen influences the electronic density of states of Co and leads to the changes of the magnetic properties. The appearance of the O/Co interface could modify the stress anisotropy, as a result, the coercivity of ultrathin Co/Si(1 1 1) films are enhanced. As an example for 15 ML Co/Si(1 1 1), the coercivity increases from 140 to 360 Oe with 5000 Langmuir of oxygen exposure.     

Formation of oxygen-induced Si(1 1 3)-3 × 2 facets on the Si(5 5 12) surface

We report the formation of Si(1 1 3)-3 × 2 facets upon exposing oxygens on the Si(5 5 12) surface at an elevated temperature. These facets are found to form only for a limited range of oxygen exposure and exhibit a well-defined 3 × 2 LEED pattern. We also find the surface electronic state unique only to the facets in the valence band. The spectral feature of these electronic states and the behavior of a (1/3 1/2) LEED spot upon oxygen contents in the facets indicate that the formation is a heterogeneous mixture of the clean Si(1 1 3) facets free of oxygens and other facets containing oxygen atoms.     

The production and oxidation of uranium nanoparticles produced via pulsed laser ablation

Depleted uranium samples were ablated using five nanosecond pulses from a Nd:YAG laser and produced films of ∼1600 Å thickness that were deposited with an angular distribution typical of a completely thermal ablation (cos¹ θ). The films remained contiguous for many months in vacuum but blistered due to tensile stress induced in the films several days after being brought into air. While under vacuum (2 × 10⁻¹⁰ Torr base pressure) the films were allowed to oxidize from the residual gases, of which water vapor was found to be the primary oxidizer. During the oxidation, the samples were monitored with both X-ray and ultraviolet photoemission spectroscopy (XPS and UPS) and were found to oxidize following Langmuir kinetics. That a 2D-surface growth model described the oxidation indicates that, even at these low pressures, oxygen accumulation on the surface is a much faster process than diffusion into the bulk. While bulk diffusion did occur, the oxygen present at the surface saturated the measurements taken using photoemission and diffusion was difficult to observe. A method for determining oxide concentration via photoemission from the valence level, as opposed to the more conventional core levels, is also presented.     

Hall effect of FeTe and Fe(Se1–xTex) thin films

The Hall effect is investigated in thin-film samples of iron–chalcogenide superconductors in detail. The Hall coefficient (R_H) of FeTe and Fe(Se_1–xTe_x) exhibits a similar positive value around 300 K, indicating that the high-temperature normal state is dominated by hole-channel transport. FeTe exhibits a sign reversal from positive to negative across the transition to the low-temperature antiferromagnetic state, indicating the occurrence of drastic reconstruction in the band structure. The mobility analysis using the carrier density theoretically calculated reveals that the mobility of holes is strongly suppressed to zero, and hence the electric transport looks to be dominated by electrons. The Se substitution to Te suppresses the antiferromagnetic long-range order and induces superconductivity instead. The similar mobility analysis for Fe(Se_0.4Te_0.6) and Fe(Se_0.5Te_0.5) thin films shows that the mobility of electrons increases with decreasing temperature even in the paramagnetic state, and keeps sufficiently high values down to the superconducting transition temperature. From the comparison between FeTe and Fe(Se_1–xTe_x), it is suggested that the coexistence of ‘itinerant’ carriers both in electron and hole channels is indispensable for the occurrence of superconductivity.     

Electronic structure of CdTe probed by Cd and Te M4,5 X-ray emission spectra

We have investigated the bulk electronic structure of CdTe focusing on the Cd 5p and Te 5p valence states by X-ray emission spectroscopy (XES). Despite the very low fluorescence yields the Cd and Te M_4,5 (5p → 3d_3/2,5/2) spectra have been recorded successfully. A good correspondence has been found between the valence band XES and X-ray photoelectron spectra (XPS) by comparison on a common binding energy scale. We also performed a density functional theory calculation of the CdTe valence band, obtaining the Cd 4d, 5s, 5p and Te 5s, 5p local partial densities of states. The experimental Cd 5p and Te 5p derived from the X-ray emission spectra are in good agreement with the calculation. The intensity ratio of the Cd M_4,5 to the Te M_4,5 spectrum is obtained to be 0.25, in agreement with the ratio of the calculated Cd 5p to the Te 5p density of states in the CdTe upper valence band (0.22).     

Photoconductivity in amorphous chalcogenide thin films: Effect of incorporation of Bi,Sb and Te IN Ge22Se78 binary alloy

Temperature and intensity dependence of steady state photoconductivity is studied in amorphous thin films of various chalcogenide glasses with a view to see the effect of incorporation of Bi, Sb and Te in Ge-Se system. Our results show that the photosensitivity decreases as 10 at % of Bi, Sb and Te are incorporated in Ge₂₂Se₇₈.Transient photoconductivity measurements have also been made on the same samples. These results show that the decay of photoconductivity becomes slower when Bi, Sb and Te are incorporated. The results have been explained in terms of the defect states which are introduced after incorporation of these elements in the binary system.     

Effect of nitrogen incorporation and oxygen vacancy on electronic structure and the absence of a gap state in HfSiO films

The effect of nitrogen (N) incorporation into HfSiO on the electronic structure and band alignment of HfSiO films was investigated. N depth profile data obtained by medium energy ion scattering (MEIS) showed that the concentration of N or the bonding or electronic state of N in the film was stable when the film was annealed at 950 °C, while the oxygen (O) in HfSiON films was present in dissociated form, as evidenced by the unoccupied electronic state of O. The valence band offsets of the HfSiO films were strongly affected by N incorporation due to the presence of N in a 2p state. Moreover, a reduction in the conduction band offset of a HfSiO film was confirmed after the film was annealed in an atmosphere of N₂. The unoccupied state of the O vacancy is responsible for the change in the conduction band offset. The results of ab-initio calculations for the density of states (DOS) of HfSiO and HfSiON supercells were in agreement with the experimental results. The incorporation on N into HfSiO prevents the formation of a gap-state inside the band gap despite the fact that an O vacancy is generated in the film.     

纳米WO_3块体材料的电致变色效应

WO3薄膜能够通过电、光、热变色 .实验发现纳米WO3块材具有明显的常规块材不具有的电致变色特性 ,即当样品中有电流通过时 ,样品从负极到正极颜色由黄色变为深蓝色 .变色前后样品的显微形貌和相结构都没有明显变化 .x射线光电子能谱说明变色后的样品中出现了低价的钨离子 (W5 ) .根据实验结果 ,变色过程被认为是一种电子注入效应 .纳米样品中的高价钨离子 (W6 )因为纳米材料的表面效应而具有足够大的活性 ,能与电子结合生成低价的离子 ,从而使样品变色     

第一性原理研究Te-N共掺p型ZnO

采用基于密度泛函理论的第一性原理赝势法对Te-N共掺杂ZnO体系的晶格结构、 杂质态密度和电子结构进行了理论分析.研究表明, N掺杂引起晶格收缩,而Te的掺入引起晶格膨胀, 从而减小晶格应力促进N的掺杂,并且Te由于电负性小于O而带正电, Te在ZnO中作为等电子施主而存在.研究发现, N掺杂体系中在费米能级附件形成窄的深受主能级, 而Te-N共掺体系中, N杂质带变宽,空穴更加离域,同时,空穴有效质量变小,受主能级变浅, 更有利于实现p型特性.因此, Te-N共掺有望成为一种更为有效的p型掺杂手段.     

金属Sn薄膜的高温氧化与表面特征

侧重研究了高温氧化（300－550℃）引起金属Sn薄膜的表面显微形貌和表面氧化状态的变化．利用原子力显微镜（AFM）的测量，观察到金属Sn薄膜表面的金属晶粒呈现近似方形的显微形貌，但是金属Sn氧化薄膜表面的金属氧化物颗粒却具有近似圆形的显微形貌，因此，金属晶粒的高温氧化是一个各向异性的过程．在X射线光电子能谱（XPS）测量的基础上，不仅发现在金属Sn薄膜和金属Sn氧化薄膜的表面都存在大量的吸附氧粒子，而且发现吸附氧粒子的吸附形式与表面的氧化程度有关 关键词：