Phase Change Observed in Ultrathin Ba0.5Sr0.5TiO3 Films by in-situ Resonant Photoemission Spectroscopy
Y. -H. Lin, K. Terai, H. Wadati, M. Kobayashi, M. Takizawa, J. I. Hwang, A. Fujimori, C. -W. Nan, J. -F. Li, S. -I. Fujimori, T. Okane, Y. Saitoh, K. Kobayashi
Epitaxial Ba0.5Sr0.5TiO3 thin films were prepared on Nb-doped SrTiO3 (100)substrates by the pulsed laser deposition technique, and were studied by measuring the Ti 2p - 3d resonant photoemission spectra in the valence-band region as a function of film thickness, both at room temperature and low temperature. Our results demonstrated an abrupt variation in the spectral structures between 2.8 nm (~7 monolayers) and 2.0 nm (~5 monolayers) Ba0.5Sr0.5TiO3 films, suggesting that there exists a critical thickness for phase change in the range of 2.0 nm to 2.8 nm. This may be ascribed mainly to the intrinsic size effects.
We demonstrate a novel approach for non-destructive in-situ characterization of phase transitions of ultrathin nickel silicide films using 3D medium-energy ion scattering. The technique provides simultaneously composition and real-space crystallography of silicide films during the annealing process using a single sample. We show, for 10 nm Ni films on Si, that their composition follows the normal transition sequence, such as Ni-Ni2Si-NiSi. For samples with initial Ni thickness of 3 nm, depth-resolved crystallography using a position-sensitive detector, shows that the Ni film transform from an …
We have studied the chemical and electronic properties of LaAlO3/SrVO3 ultrathin films by combining hard x-ray photoemission spectroscopy and transport measurements. We compare single SrVO3 (SVO) ultrathin films and SrVO3 buried below a polar LaAlO3 (LAO) thin layer, both epitaxially grown on SrTiO3. While ultrathin films (4 unit cells) of SVO do show insulating behavior over the entire temperature range, the LAO/SVO interface has a resistivity minimum at 250 K. When increasing the SVO layer thickness, the minimum is observed to shift to higher temperatures, but the resistivity stays always sm…
We characterize composition and structure of ultrathin nickel silicide during formation from 3 nm Ni films on Si(100) using in-situ high resolution ion scattering and high resolution transmission electron microscopy. We show the transition to occur in discrete steps, in which an intermediate phase is observed within a narrow range of temperature from 230 oC to 290 oC. The film composition of this intermediate phase is found to be 50% Ni:50% Si, without evidence for long-range structure, indicating the film to be a homogeneous monosilicide NiSi phase. The final phase is resemblant of the cubic …
Sputter-Induced Cross-Contaminations in Analytical AES and XPS Instrumentation: Utilization of the effect for the In-situ Deposition of Ultrathin Functional Layers
We investigated the Dirac-cone state and its modulation when an ultrathin film of topological insulator Bi2Se3 was epitaxially grown on a van-der-Waals ferromagnet Cr2Si2Te6 (CST) by angle-resolved photoemission spectroscopy. We observed a gapless Dirac-cone surface state in 6 quintuple-layer (QL) Bi2Se3 on CST, whereas the Dirac cone exhibits a gap of 0.37 eV in 2QL counterpart. Intriguingly, this gap is much larger than those for Bi2Se3 films on Si(111). We also revealed no discernible change in the gap magnitude across the ferromagnetic transition of CST, suggesting the very small character…
Magnetic anisotropy is a key parameter of magnetic materials as it decides the response in the presence of an external magnetic field. The artificial tailoring of magnetic anisotropy by manipulating surface and interface morphology is attracting widespread interest for its application in spintronic and magnetic memory devices. In this perspective, the primary focus of this thesis has been inducing and tailoring magnetic anisotropy in thin film-based systems via the engineering of structure and surface/interfacial morphology. For this purpose, instead of multistep, expensive lithographic techni…
Cobalt ferrite ultrathin films with inverse spinel structure are among the best candidates for spin-filtering at room temperature. We have fabricated high-quality epitaxial ultrathin CoFe2O4 layers on Ag(001) following a three-step method: an ultrathin metallic CoFe2 alloy was first grown in coherent epitaxy on the substrate, and then treated twice with O2, first at RT and then during annealing. The epitaxial orientation, the surface, interface and film structure were resolved combining LEED, STM, Auger and in situ GIXRD. A slight tetragonal distortion was observed, that should drive the easy …
Spin-charge interconversion (SCI) phenomena have attracted a growing interest in the field of spintronics as means to detect spin currents or manipulate the magnetization of ferromagnets. The key ingredients to exploit these assets are a large conversion efficiency, the scalability down to the nanometer scale and the integrability with opto-electronic and spintronic devices. Here we show that, when an ultrathin Bi film is epitaxially grown on top of a Ge(111) substrate, quantum size effects arising in nanometric Bi islands drastically boost the SCI efficiency, even at room temperature. Using x…
In this paper we report the effect of hydrogen on the structural properties of AISI-H13 steel nitrogen-implanted samples in low oxygen partial pressure atmosphere. The samples were implanted in a high vacuum chamber by using a broad ion beam source. The H2+/N2+ ion composition of the beam was varied and the surface composition studied in situ by photoemission electron spectroscopy (XPS). The samples were also ex situ analyzed by X-ray diffraction and scanning electron microscopy (SEM), including energy-dispersive spectroscopy measurements. It was found that hydrogen has the effect of modifying…
Lithium (Li) is the simplest metal and the lightest solid element. Here we report the first demonstration of controlled growth of two-dimensional (2D) ultrathin Li nanosheets with large lateral dimensions up to several hundreds of nanometres and thickness limited to just a few nanometres by in-situ transmission electron microscopy (TEM). The nanoscale dynamics of nanosheets growth were unravelled by real-time TEM imaging, which, in combination with density function theory (DFT) calculations indicates that the growth of bcc structured Li into 2D nanosheets is a consequence of kinetic control as…
Angle-resolved photoemission spectroscopy (ARPES) is the key momentum-resolved technique for direct probing of the electronic structure of a material. However, since it is very surface-sensitive, it has been applied to a relatively small set of complex oxides that can be easily cleaved in ultra-high vacuum. Here we describe a new multi-module system at Brookhaven National Laboratory (BNL) in which an oxide molecular beam epitaxy (OMBE) is interconnected with an ARPES and a spectroscopic-imaging scanning tunneling microscopy (SI-STM) module. This new capability largely expands the range of comp…
Alkali-metal adsorption on the surface of materials is widely used for in situ surface electron doping, particularly for observing unoccupied band structures by angle-resolved photoemission spectroscopy (ARPES). However, the effects of alkali-metal atoms on the resulting band structures have yet to be fully investigated, owing to difficulties in both experiments and calculations. Here, we combine ARPES measurements on cesium-adsorbed ultrathin bismuth films with first-principles calculations of the electronic charge densities and demonstrate a simple method to evaluate alkali-metal induced ban…
In this study, the microscopic origins of the voltage-controlled magnetic anisotropy (VCMA) in 3d-ferromagnetic metals are revealed. Using in-situ X-ray fluorescence spectroscopy that provides a high quantum efficiency, electric-field-induced changes in orbital magnetic moment and magnetic dipole Tz terms in ultrathin Co films are demonstrated. An orbital magnetic moment difference of 0.013μB. was generated in the presence of electric fields of +(-)0.2 V/nm. The VCMA of Co was properly estimated by the induced change in orbital magnetic moment, according to the perturbation theory model. The i…
Two-Dimensional Altermagnetism in Epitaxial CrSb Ultrathin Films
School of Physics Beihang University（北京航空航天大学物理学院）；Institute of High Energy Physics Chinese Academy of Sciences（中国科学院高能物理研究所）；Singapore University of Technology and Design（新加坡科技设计大学）
The presence of inherently strong spin-orbit coupling in bismuth, its unique layer-dependent band topology and high carrier mobility make it an interesting system for both fundamental studies and applications. Theoretically, it has been suggested that strong quantum size effects should be present in the Bi(110) films, with the possibility of Dirac fermion states in the odd-bilayer (BL) films, originating from dangling $p_z$ orbitals and quantum-spin hall (QSH) states in the even-bilayer films. However, the experimental verification of these claims has been lacking. Here, we study the electroni…
Through ${in~situ}$ photoemission spectroscopy, we investigated the change in the electronic and crystal structures of dimensionality-controlled VO$_2$ films coherently grown on TiO$_2$(001) substrates. In the nanostructured films, the balance between the instabilities of a bandlike Peierls transition and a Mott transition is controlled as a function of thickness. The characteristic spectral change associated with temperature-driven metal-insulator transition in VO$_2$ thick films holds down to 1.5 nm (roughly corresponding to five V atoms along the [001] direction), whereas VO$_2$ films of le…
Fundamental understanding of interfacial magnetic properties in ferromagnetic heterostructures is essential to utilize ferromagnetic materials for spintronic device applications. In this paper, we investigate the interfacial magnetic and electronic structures of epitaxial single-crystalline LaAlO$_3$ (LAO)/La$_{0.6}$Sr$_{0.4}$MnO$_3$ (LSMO)/Nb:SrTiO$_3$ (Nb:STO) heterostructures with varying LSMO-layer thickness, in which the magnetic anisotropy strongly changes depending on the LSMO thickness due to the delicate balance between the strains originating from both the Nb:STO and LAO layers, usin…
Growth of Co film on SiO2 substrates with the surface roughness of 0.5 nm and 1.6 nm has been studied in situ using the magneto-optical Kerr effect (MOKE) and four probe resistivity measurements. In-situ measurements jointly suggest the Volmer-Weber growth process and proceed via a nonmagnetic, superparamagnetic and ferromagnetic phase formation on both substrates. Islands are found to coalesce at film thicknesses of 0.6 nm and at 1.5 nm with continuous film formation around film thicknesses of 1.5 nm and 3.0 nm for smooth and rough substrates, respectively. Ferromagnetic long-range ordering i…
Thickness scaling of ferroelectricity in AlScN is a determining factor for its potential application in neuromorphic computing and memory devices. In this letter, we report on ultrathin (10 nm) Al0.72Sc0.28N films that are ferroelectrically switchable at room temperature. All-epitaxial Al0.72Sc0.28N/Pt heterostructures are grown by magnetron sputtering onto GaN/sapphire substrates followed by an in situ Pt capping approach to avoid oxidation of the Al0.72Sc0.28N film surface. Structural characterization by X-ray diffraction and transmission electron microscopy reveals the established epitaxy. …
Layered quasi-two-dimensional transition metal dichalcogenides (TMDCs), which can be readily made in ultrathin films, offer excellent opportunities for studying how dimensionality affects electronic structure and physical properties. Among all TMDCs, NbSe2 is of special interest; bulk NbSe2 hosts a charge-density-wave phase at low temperatures and has the highest known superconducting transition temperature, and these properties can be substantially modified in the ultrathin film limit. Motivated by these effects, we report herein a study of few-layer NbSe2 films, with a well-defined single-do…
Thermodynamic approach to the dewetting instability in ultrathin films
薄膜中经典去湿不稳定性的流体动力学是一个非线性过程。然而，在特征长度和时间尺度方面的不稳定性的物理表现可以通过薄膜动力学的初始条件的线性化形式来描述。或者，基于将自由能降低速率等同于粘性耗散的热力学方法[de Gennes，C. R. Acad. [298]可以提供类似的信息。在这里，我们已经评估了去湿的存在下所产生的薄膜厚度（h）依赖于温度的热毛细作用力。这种情况可以在脉冲激光熔化金属薄膜的过程中发现，其中纳米级效应导致局部h依赖性温度。热力学方法提供了一个分析描述这种热毛细去湿。只要使自由能的减小率等于粘性耗散的最小值，这种方法的结果与线性理论和实验观察的结果是一致的。产生这种最小粘性耗散的流动边界条件是当膜-基底切向应力为零时。这一发现的物理含义是，自发脱湿不稳定遵循的路径的能量损失率最小。
Ultrathin films of Na3Bi on insulating substrates are desired for opening a bulk band gap and generating the quantum spin Hall effect from a topological Dirac semimetal, though continuous films in the few nanometer regime have been difficult to realize. Here, we utilize alternating layer molecular beam epitaxy (MBE) to achieve uniform and continuous single crystal films of Na3Bi(0001) on insulating Al2O3(0001) substrates and demonstrate electrical transport on films with 3.8 nm thickness (4 unit cells). The high material quality is confirmed through in situ reflection high-energy electron diff…
Metallic oxide SrVO3 represents a prototype system for the study of the mechanism behind thickness-induced metal-to-insulator transition (MIT) or crossover in thin films due to its simple cubic symmetry with one electron in the 3d state in the bulk. Here we report a deviation of chemical composition and distortion of lattice structure existing in the initial 3 unit cells of SrVO3 films grown on SrTiO3 (001) from its bulk form, which shows a direct correlation to the thickness-dependent MIT. In-situ photoemission and scanning tunneling spectroscopy indicate a MIT at the critical thickness of ~3…
We report on the electronic structure of $α$-Sn films in the very low thickness regime grown on InSb(111)A. High-resolution low photon energies angle-resolved photoemission (ARPES) allows for the direct observation of the linearly dispersing 2D topological surface states (TSSs) that exist between the second valence band and the conduction band. The Dirac point of this TSS was found to be 200meV below the Fermi level in 10-nm-thick $α$-Sn films, which enables the observation of the hybridization gap opening at the Dirac point of the TSS for thinner films. The crossover to a quasi-2D electronic …
Perovskite ferroelectric oxides are usually considered to be brittle materials, however, recent work [Dong et al., Science 366, 475 (2019)] demonstrated the super-elasticity in the freestanding BaTiO3 thin films. This property may originate from the ferroelectric domain evolution during the bending, which is difficult to observe in experiments. Therefore, understanding the relation among the bending deformation, thickness of the films, and the domain dynamics is critical for their potential applications in flexible ferroelectric devices. Here, we reported the dynamics of ferroelectric polariza…
CoFe2O4 thin films (5 nm and 20 nm thick) were grown by oxygen assisted molecular beam epitaxy on Pt(111) at 523~K and subsequently annealed at 773 K in vacuum or oxygen. They were characterized in-situ using Auger Electron Spectroscopy, Low-Energy Electron Diffraction, Scanning Tunneling Microscopy and Conversion Electron Mössbauer Spectroscopy. The as-grown films were composed of small, nanometric grains. Annealing of the films produced an increase in the grain size and gave rise to magnetic order at room temperature, although with a fraction of the films remaining in the paramagnetic state.…
We studied the structural and magnetic properties of \textit{in-situ} grown iron mononitride (FeN) thin films. Initial stages of film growth were trapped utilizing synchrotron based soft x-ray absorption near edge spectroscopy (XANES) at the N $K$-edge and nuclear resonant scattering (NRS). Films were grown using dc-magnetron sputtering, separately at the experimental stations of SXAS beamline (BL01, Indus 2) and NRS beamline (P01, Petra III). It was found that the initial stages of film growth differs from the bulk of it. Ultrathin FeN films, exhibited larger energy separation between the t$_…
We report the electronic structure of CuTe with a high charge density wave (CDW) transition temperature Tc = 335 K by angle-resolved photoemission spectroscopy (ARPES). An anisotropic charge density wave gap with a maximum value of 190 meV is observed in the quasi-one-dimensional band formed by Te px orbitals. The CDW gap can be filled by increasing temperature or electron doping through in situ potassium deposition. Combining the experimental results with calculated electron scattering susceptibility and phonon dispersion, we suggest that both Fermi surface nesting and electron-phonon couplin…
The superconductor PdTe$_2$ is known to host bulk Dirac bands and topological surface states. The coexistence of superconductivity and topological surface states makes PdTe$_2$ a promising platform for exploring topological superconductivity and Majorana bound states. In this work, we report the layer-by-layer molecular beam epitaxy growth and spectroscopic characterization of high quality PdTe$_2$ films with thickness down to 3 monolayers (ML). In the 3 ML PdTe$_2$ film, we observed spin-polarized surface resonance states, which are isolated from the bulk bands due to the quantum size effects…
In-situ Polarized Neutron Reflectometry: Epitaxial Thin Film Growth of Fe on Cu(001) by DC Magnetron Sputtering
本文报道了用原位极化中子反射法研究Fe在Cu（001）/Si（001）上外延生长的实验结果。溅射沉积系统集成到瑞士中子溅射源SINQ的中子反射计AMOR中，这使得能够分析Fe层的所有沉积步骤期间的微观结构和磁矩。我们报告的渐进演变的可访问的参数描述的微观结构和磁性的Fe膜，再现已知的功能和扩展我们的知识的行为的Fe 3 O 4铁膜。
Observation of mirror-odd and mirror-even spin texture in ultrathin epitaxially strained RuO2 films
University of Minnesota-Twin Cities（明尼苏达大学双城分校）；Kyung Hee University（庆熙大学）；Rice University（莱斯大学）；Paul Scherrer Institute（保罗谢勒研究所）；Gwangju Institute of Science and Technology (GIST)（光州科学技术院）
This study investigated the electronic structure of SrTi$_{1-x}$V$_x$O$_3$ (STVO) thin films, which are solid solutions of strongly correlated transparent conductive oxide (TCO) SrVO$_3$ and oxide semiconductor SrTiO$_3$, using ${in situ}$ photoemission spectroscopy. STVO is one of the most promising candidates for correlated-metal TCO because it has the capability of optimizing the performance of transparent electrodes by varying ${x}$. Systematic and significant spectral changes were found near the Fermi level (${E_{\rm F}}$) as a function of ${x}$, while the overall electronic structure of …
Heusler compounds are promising materials for spintronics with adjustable electronic properties including 100% spin polarization at the Fermi energy. We investigate the electronic states of AlOx capped epitaxial thin films of the ferromagnetic half metal Co2MnSi ex-situ by soft X-ray angular resolved photoemission spectroscopy (SX-ARPES). Good agreement between the experimental SX- ARPES results and photoemission calculations including surface effects was obtained. In particular, we observed in line with our calculations a large photoemission intensity at the center of the Brillouin zone, whic…
Magnetism of ordered Sm/Co(0001) surface structures
研究了Sm/Co（0001）外延系统，在Sm/Co/W（110）外延膜上的Sm覆盖率达到1个单层（ML）。发现1/3和1 ML Sm分别有两个有序相。利用光电子能谱确定了Sm的价态，利用磁光克尔效应测量了Sm的磁性。我们发现，1 ML Sm引起的相对于下面的10 ML Co膜的矫顽力的强烈增加。元素特定的磁滞回线，通过使用共振软X射线反射率测量，显示出相同的磁性行为的两个元素。
In this manuscript, we have demonstrated that the antenna-enhanced infrared vibrational spectroscopy can be a powerful platform for high-sensitivity detection of ultrathin layer of polymer molecules and in situ monitoring of their crosslinking reaction as well as their kinetics of network formation. Our precisely designed dual-band perfect absorber (DPA) made of singles-sized resonator array shows strongly-enhanced nearfield intensity and polarization independence, providing a powerful antenna platform for the bond-selective vibrational sensing.
