Thickness-dependent surface morphology of La0.9Sr0.1MnO3 ultrathin films

La_0.9Sr_0.1MnO₃ (LSMO) ultrathin films with various thickness (in the range of 5-50 unit cells) are grown on (0 0 1) substrates of the single-crystal SrTi_0.99Nb_0.01O₃ by laser molecular-beam epitaxy (laser-MBE), and the surface morphology of these films were measured by scanning tunneling microscopy (STM). STM images of LSMO ultrathin film surface reveal that surface morphology becomes more flat with increasing film thickness. This study highlights the important effect of strain caused by the lattice mismatch between substrates and ultrathin films. And the results should be useful to the investigations on growing manganite perovskite materials.     

Coulomb gap: how a metal film becomes an insulator

Electron tunneling measurements of the density of states (DOS) in ultrathin Be films reveal that a correlation gap mediates their insulating behavior. In films with sheet resistance R<5000 Omega the correlation singularity appears as the usual perturbative ln(V) zero bias anomaly (ZBA) in the DOS. As R is increased further, however, the ZBA grows and begins to dominate the DOS spectrum. This evolution continues until a nonperturbative |V| Efros-Shklovskii Coulomb gap spectrum finally emerges in the highest R films. Transport measurements of films which display this gap are well described by a universal variable range hopping law R(T) = (h/2e(2))exp(T0/T)(1/2).     

Tunneling interferometry and measurement of the thickness of ultrathin metallic Pb(111) films

Spectra of the differential tunneling conductivity for ultrathin lead films grown on Si(111) 7 × 7 single crystals with a thickness of 9 to 50 ML have been studied by low-temperature scanning tunneling microscopy and spectroscopy. The presence of local maxima of the tunneling conductivity is characteristic of such systems. The energies of maxima of the differential conductivity are determined by the spectrum of quantum-confined states of electrons in a metallic layer and, consequently, the local thickness of the layer. It has been shown that features of the microstructure of substrates, such as steps of monatomic height, structural defects, and inclusions of other materials covered with a lead layer, can be visualized by bias-modulation scanning tunneling spectroscopy.     

Scanning tunneling microscopy of adsorbates on insulating films. From the imaging of individual molecular orbitals to the manipulation of the charge state

Ultrathin insulating films on metal substrates are unique systems for using a scanning tunneling microscope to study the electronic properties of single atoms and molecules that are electronically decoupled from the metallic substrate. Individual gold atoms on an ultrathin insulating sodium chloride film supported by a copper surface exhibit two different charge states, which are stabilized by the large ionic polarizability of the film. The charge state and associated physical and chemical properties such as diffusion can be controlled by adding or removing a single electron to or from the adatom with a scanning tunneling microscope tip. The simple physical mechanism behind the charge bistability in this case suggests that this is a common phenomenon for adsorbates on polar insulating films. In the case of molecules on ultrathin NaCl films, the electronic decoupling allows the direct imaging of the unperturbed molecular orbitals, as will be shown in the case of individual pentacene molecules. PACS 68.37.Ef; 73.61.Ng; 73.20.Hb     

Crystallographic structure of ultrathin Fe films on Cu(100)

We report the observation of bcc-like crystal structures in 2-4 monolayer (ML) Fe films grown on fcc Cu(100) using scanning tunneling microscopy. The local bcc structure provides a straightforward explanation for their frequently reported outstanding magnetic properties, i.e., ferromagnetic ordering in all layers with a Curie temperature above 300 K. The nonpseudomorphic structure, which becomes pseudomorphic above 4 ML film thickness, is unexpected in terms of conventional rules of thin film growth and stresses the importance of finite thickness effects in ferromagnetic ultrathin films.     

氮流量对磁控溅射法制备氮化钛薄膜光学性能的影响

采用磁控溅射方法在载波片和Al基片上制备了氮化钛薄膜;通过改变N2流量来改变薄膜中N／Ti原子比例。采用分光光度计和扫描隧道显微镜测试手段对氮化钛薄膜光学性能随N2流量变化的规律进行了研究。薄膜反射率光谱和扫描隧道图谱分析表明,氮化钛薄膜主要遵循自由载流子光吸收机制,随着N含量的增加,溥膜中的自由电子数目不断减少,等离子体频率逐渐红移,反射率降低,薄膜颜色发生变化。从薄膜扫描隧道谱（STS）可知,TiN薄膜表现出类似金属的光学性能,并且其禁带宽度Eg=1．64eV。     

Effect of CoSi2 buffer layer on structure and magnetic properties of Co films grown on Si（001） substrate

Buffer layer provides an opportunity to enhance the quality of ultrathin magnetic films.In this paper,Co films with different thickness of Co Si2buffer layers were grown on Si(001)substrates.In order to investigate morphology,structure,and magnetic properties of films,scanning tunneling microscope(STM),low energy electron diffraction(LEED),high resolution transmission electron microscopy(HRTEM),and surface magneto-optical Kerr effect(SMOKE)were used.The results show that the crystal quality and magnetic anisotropies of the Co films are strongly affected by the thickness of Co Si2buffer layers.Few Co Si2monolayers can prevent the interdiffusion of Si substrate and Co film and enhance the Co film quality.Furthermore,the in-plane magnetic anisotropy of Co film with optimal buffer layer shows four-fold symmetry and exhibits the two-jumps of magnetization reversal process,which is the typical phenomenon in cubic(001)films.     

Two-dimensional fractal-like growth on semiconductors: The formation of continuous manganese monosilicide ultrathin films on Si(111)

Continuous crystalline MnSi ultrathin films with atomically flat surfaces, which are highly expected to find an application in Si-based spintronics, are grown on Si(111)-7×7 by solid phase epitaxy method. The interfacial reaction between Mn and Si and the formation processes of the films as well as their morphological variation with annealing temperature are investigated using scanning tunneling microscopy. The MnSi ultrathin films form in a two-dimensional (2D) fractal-like mode only at a Mn coverage above ∼2 ML and at an annealing temperature in the narrow range of ∼250-300 °C. Above ∼300 °C, the growth mode gradually transforms into Volmer-Weber mode and correspondingly, the continuous films transform into 2D compact islands. The films grow with a thickness unit of quadruple layer, which is consistent with the B20-type MnSi structure.     

Selective formation of ultrathin PbSe on Ag(111)

Two-dimensional (2D) semiconductors, such as lead selenide (PbSe), locate at the key position of next-generation devices. However, the ultrathin PbSe is still rarely reported experimentally, particularly on metal substrates. Here, we report the ultrathin PbSe synthesized via sequential molecular beam epitaxy on Ag(111). The scanning tunneling microscopy is used to resolve the atomic structure and confirms the selective formation of ultrathin PbSe through the reaction between Ag₅Se₂ and Pb, as further evidenced by the theoretical calculation. It is also found that the increased accumulation of Pb leads to the improved quality of PbSe with larger and more uniform films. The detailed analysis demonstrates the bilayer structure of synthesized PbSe, which could be deemed to achieve the 2D limit. The differential conductance spectrum reveals a metallic feature of the PbSe film, indicating a certain interaction between PbSe and Ag(111). Moreover, the moiré pattern originated from the lattice mismatch between PbSe and Ag(111) is observed, and this moiré system provides the opportunity for studying physics under periodical modulation and for device applications. Our work illustrates a pathway to selectively synthesize ultrathin PbSe on metal surfaces and suggests a 2D experimental platform to explore PbSe-based opto-electronic and thermoelectric phenomena.     

Phase transition-induced superstructures of β-Sn films with atomic-scale thickness

The ultrathin β-Sn(001) films have attracted tremendous attention owing to its topological superconductivity(TSC),which hosts Majorana bound state(MBSs) for quantum computation.Recently,β-Sn(001) thin films have been successfully fabricated via phase transition engineering.However,the understanding of structural phase transition of β-Sn(001)thin films is still elusive.Here,we report the direct growth of ultrathin β-Sn(001) films epitaxially on the highly oriented pyrolytic graphite(HOPG) substrate and the characterization of intricate structural-transition-induced superstructures.The morphology was obtained by using atomic force microscopy(AFM) and low-temperature scanning tunneling microscopy(STM),indicating a structure-related bilayer-by-bilayer growth mode.The ultrathin β-Sn film was made of multiple domains with various superstructures.Both high-symmetric and distorted superstructures were observed in the atomic-resolution STM images of these domains.The formation mechanism of these superstructures was further discussed based on the structural phase transition of β to α-Sn at the atomic-scale thickness.Our work not only brings a deep understanding of the structural phase transition of Sn film at the two-dimensional limit,but also paves a way to investigate their structure-sensitive topological properties.     

Persistent superconductivity in ultrathin Pb films: a scanning tunneling spectroscopy study

By using a low temperature scanning tunneling microscope we have probed the superconducting energy gap of epitaxially grown Pb films as a function of the layer thickness in an ultrathin regime (5-18 ML). The layer-dependent energy gap and transition temperature (Tc) show persistent quantum oscillations down to the lowest thickness without any sign of suppression. Moreover, by comparison with the quantum-well states measured above Tc and the theoretical calculations, we found that the Tc oscillation correlates directly with the density of states oscillation at E(F) . The oscillation is manifested by the phase matching of the Fermi wavelength and the layer thickness, resulting in a bilayer periodicity modulated by a longer wavelength quantum beat.     

Optical and Structural Phenomena at Multipulse Interference Femtosecond Laser Fabrication of Metasurfaces on a Thin Film of Amorphous Silicon

JETP Letters - The spatial dependence of the differential conductivity of ultrathin Pb films deposited on the Si(111)7 × 7 surface has been studied by low-temperature scanning tunneling...     

表面吸附K原子的多层FeSe/SrTiO3(001)薄膜的抗磁响应的原位测量

SrTiO₃（001）单晶表面上生长的单层FeSe薄膜显示出了超乎寻常的高温超导电性,其超导增强机制的一个重要因素是电子由衬底转移到了单层FeSe薄膜当中.基于此认识,研究者们在吸附了钾（K）原子的多层FeSe薄膜表面上观察到了类似超导能隙的隧穿能谱和光电子能谱.但这种自上而下的电子掺入方式在多层FeSe薄膜表面上可能引起的高温超导电性,还缺乏零电阻或迈斯纳效应等物性测量实验的直接证实.本研究利用自行研制的一台特殊的多功能扫描隧道显微镜,在生长于SrTiO₃（001）衬底上的多层FeSe薄膜表面上,不但观察到了超导能隙随K吸附量的变化,而且利用原位双线圈互感测量技术,成功地的观察到了该薄膜的抗磁响应,并由此确定了该薄膜样品呈现迈斯纳效应的超导转变温度为23.9 K.其穿透深度随温度的变化呈二次幂指数关系,表明该体系的超导序参量很可能具有S^±配对对称性.     

Stable nontrivial Z2 topology in ultrathin Bi (111) films: a first-principles study

Recently, there have been intense efforts in searching for new topological insulator materials. Based on first-principles calculations, we find that all the ultrathin Bi (111) films are characterized by a nontrivial Z(2) number independent of the film thickness, without the odd-even oscillation of topological triviality as commonly perceived. The stable nontrivial Z(2) topology is retained by the concurrent band gap inversions at multiple time-reversal-invariant k points with the increasing film thickness and associated with the intermediate interbilayer coupling of the Bi film. Our calculations further indicate that the presence of metallic surface states in thick Bi (111) films can be effectively removed by surface adsorption.     

氮分压对氮化铜薄膜结构及光学带隙的影响

在不同的氮分压r(r=N₂/［N₂+Ar］)和射频功率P下，使用反应射频磁控溅射法，在玻璃基片上制备了氮化铜薄膜样品.用台阶仪测得了薄膜的厚度，用原子力显微镜、X射线衍射仪、紫外-可见光谱仪对薄膜的表面形貌、结构及光学性质进行了表征分析.结果表明，薄膜的沉积速率随P和r的增加而增大.薄膜表面致密均匀，晶粒尺寸为30nm左右.随着r的增加，薄膜颗粒增大，且薄膜由(111)晶面转向(100)晶面择优生长.薄膜的光学带隙E_g在1.47—1.82eV之间，随r的增加而增大. 关键词： 氮化铜薄膜 反应射频磁控溅射 晶体结构 光学带隙     

Superconductivity of the Pb/Ag superlattice

Layered Pb/Ag thin films were produced by vacuum evaporation on cooled glass substrates. Measured x-ray diffraction spectra showed peaks characteristic of a superlattice. The superconducting energy gap and the transition temperature of the Pb/Ag superlattice were measured by tunneling experiments. BCS-reduced tunneling conductance revealed the dependence the phonon structure on the superlattice period value. It was also observed that the order of the crystal structure has an influence on the superconducting parameters.     

Molecular Beam Epitaxy Growth of Tetragonal FeS Films on SrTiO_3(001) Substrates

We report the successful growth of the tetragonal FeS film with one or two unit-cell(UC) thickness on SrTiO_3(001)substrates by molecular beam epitaxy. Large lattice constant mismatch with the substrate leads to high density of defects in single-UC FeS, while it has been significantly reduced in the double-UC thick film due to the lattice relaxation. The scanning tunneling spectra on the surface of the FeS thin film reveal the electronic doping effect of single-UC FeS from the substrate. In addition,at the Fermi level,the energy gaps of approximately 1.5 meV are observed in the films of both thicknesses at 4.6 K and below. The absence of coherence peaks of gap spectra may be related to the preformed Cooper-pairs without phase cOherence.     

Structure and electronic properties calculation of ultrathin α-Al2O3 films on (0 0 0 1) α-Cr2O3 templates

Ab initio total energy Hartree-Fock calculations of ultrathin films of α-Al₂O₃ on (0 0 0 1) α-Cr₂O₃ templates are presented. The surface relaxation, the in-plane reconstruction and the surface and strain energies of the slabs are studied as a function of alumina film thickness. The surface Al layer is found to relax inwards considerably, with the magnitude of the inwards relaxation depending on the thickness of the ultrathin alumina film in a non-linear manner. The calculations also reveal that ultrathin films of alumina lower the surface energy of (0 0 0 1) α-chromia substrates. This indicates that the (0 0 0 1) α-chromia surface provides favourable conditions for the templated growth of α-alumina. However, increasing the alumina film thickness is found to give rise to a significant increase in strain energy. Finally, the electronic properties at the surface of the (0 0 0 1) α-Al₂O₃/α-Cr₂O₃ slabs are investigated. Here it is found that the alumina coating gives rise to an increase in the covalency of the bonds at the surface of the slabs. In contrast, the influence of an alumina layer on the electrostatic potential at the surface of the chromia slab is relatively minor, which should also be beneficial for the templated growth of α-alumina on (0 0 0 1) α-chromia substrates.     

Photochemistry on thin metal films: probe of electron dynamics in metal-semiconductor heterosystems

The surface photochemistry of on ultrathin epitaxial Ag films on Si(100) substrates has been studied with the goal to employ it as a tool to unravel the electron dynamics in such films. An increase of the photodesorption cross section is observed--a factor of 5 for 266 nm light and 12 nm film thickness--when the film thickness is decreased, despite the fact that the optical absorbtivity decreases. The increased photodesorption cross section is interpreted to result from photon absorption in the Si substrate producing electrons at energies and parallel momenta which are not allowed in Ag. These electrons penetrate through the Ag film despite the gap in the surface projected band structure utilizing quantum resonances.     

X-ray characterization of as-deposited, epitaxial films of Bi(0 1 2) on Au(1 1 1)

Electrodeposition is used to produce epitaxial single-crystal films on Au(1 1 1) substrates without annealing or other post-deposition modification. X-ray techniques show that the Bi(0 1 2) plane is parallel to the underlying Au(1 1 1) surface, and the azimuthal orientation of the films is determined. Combination of the X-ray data with in situ scanning tunneling microscopy (STM) images suggests a common growth mode from the first few layers up to thick films.