Robustness of topologically protected surface states in layering of Bi2Te3 thin films

Bulk Bi2Te3 is known to be a topological insulator. We investigate surface states of Bi2Te3(111) thin films of one to six quintuple layers using density-functional theory including spin-orbit coupling. We construct a method to identify topologically protected surface states of thin film topological insulators. Applying this method to Bi2Te3 thin films, we find that the topological nature of the surface states remains robust with the film thickness and that the films of three or more quintuple layers have topologically nontrivial surface states, which agrees with experiments.     

Magnetic field-induced surface electron states in thin films

Analytical expressions are derived for the energy spectrum of magnetic field-induced surface electron states in thin films. The theory is based on investigations of asymptotic solutions of the Weber equation.     

Spin-dependent resonant tunneling through quantum-well states in magnetic metallic thin films

Quantum-well (QW) states in nonmagnetic metal films between magnetic layers are known to be important in spin-dependent transport, but QW states in magnetic films remains elusive. Here we identify the conditions for resonant tunneling through QW states in magnetic films and report first principles calculations of Fe/MgO/FeO/Fe/Cr and Co/MgO/Fe/Cr. We show that, at resonance, the current increases by 1 to 2 orders of magnitude. The tunneling magnetoresistance ratio is much larger than in simple spin tunnel junctions and is positive (negative) for majority- (minority-) spin resonances, with a large asymmetry between positive and negative biases. The results can serve as a basis for novel spintronic devices.     

Superconducting surface resistance of thin amorphous Ga films on metallic substrates

The surface resistance of a metal coated with amorphous superconducting films is investigated. With an improved microwave cryostat and a two layer model considering the influence of the substrate, much thinner films can be used than previously. As an example the surface resistance of amorphous Ga is determined. A set of parameters ₀/k _BT_c=2,27; _L=13,4 nm; ₀=223 nm andl=1,0 nm describes the temperature dependence independent of thickness. As expected a model of free electrons is in agreement with the experimental results. For crystalline-Ga ₀/k _B T _c =1,91; _L=24 nm; ₀=163 nm and a much larger mean free pathl=22 nm is found.     

H2O在SrTiO3-(001)TiO2表面上吸附和解离的密度泛函理论研究

利用密度泛函理论研究了0.25单层(ML),0.5ML,0.75ML和1ML吸附率下H2O在SrTiO3-(001)TiO2表面上的吸附行为.比较了不同吸附率下分子吸附和解离吸附的稳定性,利用微动弹性带(nudged elastic band)方法计算了H2O的解离势垒.结果表明:在低吸附率(0.25ML和0.5ML)时,H2O表现为解离吸附;在0.75ML吸附率下,分子吸附和解离吸附同时存在;而在全吸附(吸附率为1ML)时,分子吸附更稳定.基于对H2O分子与表面之间以及H2O分子之间的电荷转移和相互作用的分析,讨论了吸附率对H2O吸附和解离的影响.     

A diatomic network model for electronic states of bulk,surface and thin films

This paper reports our investigations on an exactly solvable network model of solids. General properties of diatomic lattices, in particular the dispersion relations for the energy levels, the structure of the energy bands and the density of states for bulk, surface and thin films are compared with the corresponding properties of the monatomic lattices which we have previously reported. For a thin film of finite thickness, we present a simple but informative method of analyzing Van Hove singularities in the density of states. Effects due to the inclusion of second neighbor interactions are also studied. Calculations are exact and the method has apparent advantages over other methods of discussing similar problems.     

Dynamics of Skyrmion crystals in metallic thin films

We study the collective dynamics of the Skyrmion crystal in thin films of ferromagnetic metals resulting from the nontrivial Skyrmion topology. It is shown that the current-driven motion of the crystal reduces the topological Hall effect and the Skyrmion trajectories bend away from the direction of the electric current (the Skyrmion Hall effect). We find a new dissipation mechanism in noncollinear spin textures that can lead to a much faster spin relaxation than Gilbert damping, calculate the dispersion of phonons in the Skyrmion crystal, and discuss the effects of impurity pinning of Skyrmions.     

二氧化锆薄膜表面粗糙度的研究

采用电子束蒸发工艺,利用泰勒霍普森相关相干表面轮廓粗糙度仪,研究了不同基底粗糙度、不同二氧化锆薄膜厚度以及不同的离子束辅助能量下所沉积的二氧化锆薄膜的表面粗糙度。结果表明：随着基底表面粗糙度的增加,二氧化锆薄膜表面粗糙度呈现出先缓慢增加,当基底的粗糙度大于10nm后呈现快速增加的趋势;随着二氧化锆薄膜厚度的增加,其表面均方根粗糙度（RMS）先减小后增大;随着辅助沉积离子能量的增加,其表面粗糙度呈现出先减小后增加的趋势。     

Nano-scale and surface precipitation of metallic particles in laser interference patterned noble metal-based thin films

Laser interference patterning (also known as “laser interference metallurgy”) is used to locally tailor the microstructure of oxide (Pd_0.25Pt_0.75O_x) and nitride (Cu₃N) thin films to induce a chemical decomposition, which is responsible for a decrease of electrical resistivity. This technique allows hereby a laser-induced chemical decomposition of as-deposited oxide and nitride films, resulting locally in a porous microstructure due to the simultaneous emission of gaseous nitrogen and oxygen. The process locally generates at the nanometer scale metal precipitatation of Pt or Cu in the oxide or nitride matrix. Thus, isolated metallic clusters with low resistivity coexist with a high resistivity phase, establishing a preferential electrical conduction path and giving the system a lower effective resistivity. The decomposition process is investigated by four-point probe method, X-ray diffraction, spectrophotometry, white light interference, scanning and transmission electron microscopies.     

Temperature dependence of SSW linewidth in metallic thin films

Measurements of the standing spin wave linewidth H _n at 17·6 GHz from 6 °K to 400 °K are presented for two permalloy films whose structure is either polycrystalline or epitaxial. It is shown that in both films, H _n is proportional ton ² (n is the mode number). Although H _n is little modified by temperature (20% variation maximum over the entire temperature range), we experimentally state that the slopep of H _n (n ²) is a linear function of the saturation magnetizationM which agrees with a planar inhomogeneity model.The authors wish to thank H. Pascard and F. Hoffmann who have prepared and selected the films as also E. Villain and A. Gatineau for invaluable technical assistance.     

A new approach to the calculation of tight-binding surface density of states in thin films

We present a new approach to the calculation of planar electronic density of states of thin films within the tight-binding scheme. The diagonal elements of the electronic Green's function are obtained by an iteration procedure derived from the transfer-matrix approach for semi-infinite crystals. An application is made to the study of the (100) planes of a model transition metal film with a simple cubic structure.     

Synthesis of superdispersed surface phases: An example from the sulfidization of thin metallic films

The chemical composition and forms of element present in superdispersed (nanosized) surface phases were studied by X-ray photoelectron spectroscopy and X-ray spectral electron probe microanalysis. The phases were obtained during the sulfidization of thin metallic films of iron and copper on monocrystals of arsenopyrite and chalcopyrite. The dependence of the phase composition and the stoichiometry of sulfide phases on the sulfur concentration in the system was determined by physicochemical modeling (minimization of the Gibbs energy).     

Laser interference metallurgy: A new method for periodic surface microstructure design on multilayered metallic thin films

Methods for micro- and nanostructuring are essential for functionalization of materials surfaces. In particular, photon-assisted methods for synthesis of functional surfaces have been intensively investigated in the last years. In this study, a new method for surface modification and production of long-range order periodical structures called “laser interference metallurgy” is explored. A metallic thin film sample consisting of three layers composed of Fe, Cu and Al (from top to bottom) on a glass substrate was irradiated with an interference pattern using a Nd:YAG laser (wavelength of 355 nm, 10 ns of pulse duration). For the interference pattern, a configuration producing a line-type energy distribution was chosen. The laser fluence was high enough to melt the aluminium and copper layers at the interference maxima but the iron layer remained in the solid state. Thus, diffusive and convective exchange occurred between aluminium and copper at the energy maxima positions leading to periodical alloy formation with a long-range order. Because it remained in solid state, the iron layer at the top acted as a protective layer effectively preventing removal of the molten layers. The interaction of the different layers was characterized using FIB, TEM and EDX in STEM mode.     

Investigation of the influence of low-concentration hydrogen on the surface potential of thin metallic films for sensor applications

In this paper, a study of the influence of hydrogen (concentrations 6 ppm − 1%) on the work function of thin metallic films at moderately elevated temperatures is presented. The work function was measured indirectly by the observation of the surface potential of dedicated test structures using scanning surface potential microscope. Metallic layers with thicknesses of 10, 20, 30, and 50 nm were deposited on semiconductor substrates as well as on a thick gold layer. The investigations were focused on palladium thin films although a comparison to results obtained for platinum layers was also discussed.     

Andreev states and the Josephson effect in superconducting heterojunctions on thin YBa2Cu3Ox films

Conductance anomalies at low bias voltages and superconducting currents in Au/YBa₂Cu₃O_x and Nb/Au/YBa₂Cu₃O_x heterojunctions in which the c axis of the YBa₂Cu₃O_x (YBCO) epitaxial film is rotated in the (110) YBCO plane through 11° with respect to the normal to the substrate plane were studied experimentally. The films were prepared by laser deposition onto (7 2 10)-oriented NdGaO₃ substrates. The current-voltage characteristics of the heterojunctions exhibit conductance anomalies at low voltages. The behavior of these anomalies is studied at various temperatures and in various magnetic fields. The critical current and Shapiro steps observed in the current-voltage characteristics of Nb/Au/YBa₂Cu₃O_x were evidence of the Josephson effect in these heterojunctions. The experimental results are analyzed in terms of the model of the arising of bound states caused by Andreev reflection in superconductors with d-type symmetry of the superconducting order parameter.     

关于MgB2/Al2O3超导薄膜电阻转变和各向异性的研究

利用电子束蒸发方法将MgB2超导薄膜沉积到Al2O3(001)衬底上.采用标准的四引线法研究了磁场平行和垂直超导薄膜ab平面下的电阻转变.一个激活能模型 U(T,H)= U0(1-T/(Tc+δ))n (1-H/Hc2(0))m被建立用来分析超导薄膜磁通线的激活能和电阻转变,结果表明该模型能够在整个转变温度范围描述超导体磁通线的激活能和电阻转变.另外,利用多项式Hc2(t)=Hc2(0)+At+Bt2分析了MgB2/Al2O3超导薄膜的上临界磁场,获得了该超导薄膜的各向异性参数γ=Hc2ab(0)/Hc2c(0)= 2.26.