We have performed bulk-sensitive spin-resolved photoemission spectroscopy in order to clarify the intrinsic spin-resolved electronic states of half-metallic ferromagnet CrO2. We used CrO2 epitaxial films on TiO2(100), which shows a peak at 1 eV with a clear Fermi edge, consistent with the bulk-sensitive PES spectrum for CrO2. In spin-resolved spectra at 40 K, while the Fermi edge was observed in the spin up (majority spin) state, no states at the Fermi level (EF) with an energy gap of 0.5 eV below EF was observed in the spin down (minority spin) state. At 300 K, the gap in the spin down state closes. These results are consistent with resistivity measurements and magnetic hysteresis curves of the fabricated CrO2 film, constituting spectroscopic evidence for the half-metallicity of CrO2 at low temperature and reducing the spin polarization at room temperature. We also discuss the electron correlation effects of Cr 3d.
We report on the epitaxial growth of single layer NbS$_2$ on Au(111) and determine both its crystalline and electronic structure by a combination of low-energy electron diffraction, scanning tunnelling microscopy and angle-resolved photoemission spectroscopy. The layer is found to grow in the 1H structural phase with a lattice constant of (3.29$\pm$0.03)~Å, a value comparable to the bulk 2H NbS$_2$ lattice constant. The photoemission data reveals a metallic band structure down to a temperature of 30~K. The observed bands are rather broad and consistent with either a strong NbS$_2$-substrate in…
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。
Hole-doping-induced changes in the electronic structure of La$_{1-x}$Sr$_x$FeO$_3$ : soft x-ray photoemission and absorption study of epitaxial thin films
The Fe/MgO magnetic tunnel junction is a classic spintronic system, with current importance technologically, and interest for future innovation. The key magnetic properties are linked directly to the structure of hard-to-access buried interfaces, and the Fe and MgO components near the surface are unstable when exposed to air, making a deeper probing, non-destructive, in-situ measurement ideal for this system. We have thus applied hard x-ray photoemission spectroscopy (HXPS) and standing-wave (SW) HXPS in the few keV energy range to probe the structure of an epitaxially-grown MgO/Fe superlattic…
We have investigated the electronic and optical properties of epitaxial La1-xSrxFeO3 for x from 0 to 1 prepared by molecular beam epitaxy. Core-level and valence-band x-ray photoemission features monotonically shift to lower binding energy with increasing x, indicating downward movement of the Fermi level toward to the valence band maximum. Both Fe 2p and O 1s spectra broaden to higher binding energy with increasing x, consistent with delocalization of Sr-induced holes in the Fe 3d/O 2p hybridized valence band. Combining X-ray valence band photoemission and O K-edge x-ray absorption data, we m…
Combining scanning tunneling microscopy and angle-resolved photoemission spectroscopy, we demonstrate how to tune the doping of epitaxial graphene from p to n by exploiting the structural changes that occur spontaneously on the Ge surface upon thermal annealing. Furthermore, using first principle calculations we build a model that successfully reproduces the experimental observations. Since the ability to modify graphene electronic properties is of fundamental importance when it comes to applications, our results provide an important contribution towards the integration of graphene with conven…
In intercalated transition metal dichalcogenides (I-TMDC), transition metal intercalation introduces magnetic phases which in some cases induce topological band crossing. However, evidence of the topological properties remains elusive in such materials. Here we employ angle-resolved photoemission spectroscopy to reveal the band structure of epitaxially grown ferromagnetic Cr1/3NbSe2. Experimental evidence of the Weyl crossing shows Cr1/3NbSe2 to be a topological ferromagnet. This work highlights I-TMDC as platform towards the interplay of magnetic and topological physics in low-dimensional sys…
Electron-boson interaction is fundamental to a thorough understanding of various exotic properties emerging in many-body physics. In photoemission spectroscopy, photoelectron emission due to photon absorption would trigger diverse collective excitations in solids, including the emergence of phonons, magnons, electron-hole pairs, and plasmons, which naturally provides a reliable pathway to study electron-boson couplings. While fingerprints of electron-phonon/-magnon interactions in this state-of-the-art technique have been well investigated, much less is known about electron-plasmon coupling, a…
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…
The strain dependent functional properties of epitaxial transition metal oxide films can be significantly modified via substrate selection. However, large lattice mismatches preclude dislocation-free epitaxial growth on ferroelectric substrates, whose strain states are modified by applied electric fields. Here we overcome this mismatch problem by depositing an epitaxial film of ferromagnetic La0.7Sr0.3MnO3 on a single crystal substrate of well lattice matched SrTiO3 via a film of SrRuO3 that we subsequently dissolved, permitting the transfer of unstrained La0.7Sr0.3MnO3 to a ferroelectric subs…
Topological materials are derived from the interplay between symmetry and topology. Advances in topological band theories have led to the prediction that the antiperovskite oxide Sr$_3$SnO is a topological crystalline insulator, a new electronic phase of matter where the conductivity in its (001) crystallographic planes is protected by crystallographic point group symmetries. Realization of this material, however, is challenging. Guided by thermodynamic calculations we design and implement a deposition approach to achieve the adsorption-controlled growth of epitaxial Sr$_3$SnO single-crystal f…
The properties of photoemission electron sources determine the ultimate performance of a wide class of electron accelerators and photon detectors. To date, all high-efficiency visible-light photocathode materials are either polycrystalline or exhibit intrinsic surface disorder, both of which limit emitted electron beam brightness. In this letter we demonstrate the synthesis of epitaxial thin films of Cs$_3$Sb on 3C-SiC (001) using molecular-beam epitaxy. Films as thin as 4 nm have quantum efficiencies exceeding 2\% at 532 nm. We also find that epitaxial films have an order of magnitude larger …
We report the molecular beam epitaxial growth, structure, and electronic measurements of single-crystalline LaAuSb films on Al$_2$O$_3$ (0001) substrates. LaAuSb belongs to a broad family of hexagonal $ABC$ intermetallics in which the magnitude and sign of layer buckling have strong effects on properties, e.g., predicted hyperferroelecticity, polar metallicity, and Weyl and Dirac states. Scanning transmission electron microscopy reveals highly buckled planes of Au-Sb atoms, with strong interlayer Au-Au interactions and a doubling of the unit cell. This buckling is four times larger than the bu…
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…
Metal-induced gap states in epitaxial organic-insulator/metal interfaces
我们已经表明，在实验和理论上，金属诱导的间隙状态（MIGS）可以存在于外延生长的有机绝缘体/金属界面。实验是针对具有元素选择性近边X射线吸收精细结构（NEXAFS）的烷烃/Cu（001）进行的，其表现出指示MIGS的前峰。{ \it ab initio}电子结构计算支持MIGS的存在。当用Ni代替Cu衬底时，预测界面磁性（在界面处的自旋极化有机晶体）可能具有载流子掺杂。
Room temperature spin filtering in epitaxial cobalt-ferrite tunnel barriers
我们报告的直接实验证据，室温自旋过滤的磁性隧道结（MTJ）含有CoFe 2 O 4隧道势垒通过隧道磁阻（TMR）测量。Pt（111）/CoFe_2 O_4（111）/γ-Al_2 O_3（111）/Co（0001）全外延MTJ的生长是为了获得能够在室温下工作的高质量系统。自旋极化输运测量显示，在2K和290 K下，TMR值分别为-18%和-3%。此外，TMR比遵循一个独特的偏置电压依赖性，这在理论上已经被预测为在MTJ中包含磁势垒的自旋过滤的签名。因此，CoFe 2 O 4隧道势垒提供了一个模型系统，以调查在很宽的温度范围内的自旋过滤。
Highly ordered epitaxial interfaces between organic semiconductors are considered as a promising avenue for enhancing the performance of organic electronic devices including solar cells, light emitting diodes, and transistors, thanks to their well-controlled, uniform electronic properties and high carrier mobilities. Although the phenomenon of organic epitaxy has been known for decades, computational methods for structure prediction of epitaxial organic interfaces have lagged far behind the existing methods for their inorganic counterparts. We present a method for structure prediction of epita…
Spin-conserving and reversing photoemission from the surface states of Bi$_2$Se$_3$ and Au (111)
我们提出了一个基于第一性原理计算的理论，解释了（i）为什么Bi $_2$ Se $_3$（111）的光电子自旋极化的可调谐性取决于表面态的能带指数和布洛赫波矢，以及（ii）为什么在等对称Au（111）的情况下不存在这种可调谐性。这些结果不仅解释了最近令人困惑的实验观察结果，而且还为从拓扑绝缘体制造高度可调谐的自旋极化电子源提供了指导。
Graphene is a 2D material that displays excellent electronic transport properties with prospective applications in many fields. Inducing and controlling magnetism in the graphene layer, for instance by proximity of magnetic materials, may enable its utilization in spintronic devices. This paper presents fabrication and detailed characterization of single-layer graphene formed on the surface of epitaxial FeRh thin films. The magnetic state of the FeRh surface can be controlled by temperature, magnetic field or strain due to interconnected order parameters. Characterization of graphene layers by…
We report on domain pattern transfer from a ferroelectric BaTiO$_{\mathrm{3}}$ substrate with a $(111)$-orientation of the surface to an epitaxial Co film grown on a Pd buffer layer. Spatially modulated interfacial strain transfer from ferroelectric/ferroelastic domains and inverse magnetostriction in the ferromagnetic film induce stripe regions with a modulation of the in-plane uniaxial magnetic anisotropy direction. Using spin-polarized low energy electron microscopy, we observe the formation of two distinct anisotropy configurations between stripe regions. Moreover, through application of a…
Polar metals are an intriguing class of materials that simultaneously host free carriers and polar structural distortions. Despite the name "polar metal," however, most well-studied polar metals are poor electrical conductors. Here, we demonstrate the molecular beam epitaxial (MBE) growth of LaPtSb and LaAuGe, two polar metal compounds whose electrical resistivity is an order of magnitude lower than the well studied oxide polar metals. These materials belong to a broad family of $ABC$ intermetallics adopting the stuffed wurtzite structure, also known as hexagonal Heusler compounds. Scanning tr…
Ge atoms segregating on zirconium diboride thin films grown on Ge(111) were found to crystallize into a two-dimensional bitriangular structure which was recently predicted to be a flat band material. Angle-resolved photoemission experiments together with theoretical calculations verified the existence of a nearly flat band in spite of non-negligible in-plane long-range hopping and interactions with the substrate. This provides the first experimental evidence that a flat band can emerge from the electronic coupling between atoms and not from the geometry of the atomic structure.
Epitaxial graphene grown on metallic substrates presents, in several cases, a long-range periodic structure due to a lattice mismatch between the graphene and the substrate. For instance, graphene grown on Ir(111), displays a corrugated supercell with distinct adsorption sites due to a variation of its local electronic structure. This type of surface reconstruction represents a challenging problem for a detailed atomic surface structure determination for experimental and theoretical techniques. In this work, we revisited the surface structure determination of graphene on Ir(111) by using the u…
