Spin-resolved Photoelectron Spectroscopy Research Articles

UVSOR Synchrotron Facility Update

UVSOR is a Japanese low-energy synchrotron light source that has been operational since 1983. It has 750 MeV electron beam energy and a circumference of 53 m. The machine has undergone two major upgrades and is now called UVSOR-III, with a moderately small emittance of 17 nm·rad and is operated in the top-up mode at 300 mA. It has six undulators. Three in-vacuum undulators provide high-brightness VUV-SX light to the beamlines BL3U, BL4U, and BL6U. BL3U and BL4U are actively used for chemistry and life sciences using in situ and operando measurements, such as soft X-ray absorption and photoelectron spectroscopy. Since 2020, a unique photoelectron analyzer called the Momentum Microscope has been in operation in BL6U. Two variable polarization undulators provide high-brightness VUV light to the beamlines, BL5U and BL7U, which are used for solid-state physics through angle- and spin-resolved photoelectron spectroscopy. BL1U is equipped with two APPLE-II undulators in tandem and is used for various research purposes, including laser Compton scattering gamma rays. In addition to these beamlines, eight beamlines are operational which use bending magnet radiation, ranging from THz to tender X-rays. In 38 years of operation, we have dealt with and properly resolved machine problems, such as water leakage from magnet coils and cooling water flow paths in ultrahigh vacuum. Considering the machine’s declining competitiveness as a light source due to its aging and emerging demands in the low-energy area, we have started a design study on the future plan, as well as discussing it closely with the user community.

Direct probing of temperature-independent bulk half-metallicity in Co2MnSi by spin-resolved hard x-ray photoemission

We have performed the bulk-sensitive spin-resolved hard x-ray photoelectron spectroscopy (HAXPES) to directly clarify the spin-dependent valence band electronic states and spin polarization of $L{2}_{1}$-ordered ${\mathrm{Co}}_{2}\mathrm{MnSi}$ as a predicted half-metal. The spin-resolved valence band HAXPES spectra clearly exhibited the difference in the majority and minority spin states in the bulk region of a 30-nm-thick ${\mathrm{Co}}_{2}\mathrm{MnSi}(001)$ thin film buried under a MgO(2 nm) layer at both temperatures of 21 and 300 K. As expected from the half-metallicity in ${\mathrm{Co}}_{2}\mathrm{MnSi}$, we found that the majority (minority) spin spectrum shows a metallic Fermi edge (a band gap), leading the high spin polarization of \ensuremath{\sim}90% at around the Fermi level at a temperature of 21 K. The spin-resolved HAXPES experiments revealed that the half-metallicity of ${\mathrm{Co}}_{2}\mathrm{MnSi}$ in the bulk region is independent on the temperature up to 300 K. The experiments also revealed the slight changes in the majority spin spectral shapes and the shift of the valence band maximum of the minority spin states with temperature. By comparing the experimental results with the first-principles calculations at a finite temperature obtained by the disordered local moment method, it is suggested that the slight changes in the majority and minority spin HAXPES spectra of ${\mathrm{Co}}_{2}\mathrm{MnSi}$ with temperature is mainly caused by the temperature-dependent Co $3d$ electronic states. It is also suggested that the conduction band minimum of the minority spin states is located sufficiently above the Fermi level at temperature up to 300 K.

Bulk spin polarization of magnetite from spin-resolved hard x-ray photoelectron spectroscopy

There is broad consensus that magnetite (${\mathrm{Fe}}_{3}{\mathrm{O}}_{4}$) is a promising material for spintronics applications due to its high degree of spin polarization at the Fermi level. However, previous attempts to measure the spin polarization by spin-resolved photoemission spectroscopy have been hampered by the use of low photon energies resulting in high surface sensitivity. The surfaces of magnetite, though, tend to reconstruct due to their polar nature, and thus their magnetic and electronic properties may strongly deviate from each other and from the bulk, dependent on their orientation and specific preparation. In this study, we determine the intrinsic---i.e., bulk---spin polarization of magnetite by spin-resolved photoelectron spectroscopy on (111)-oriented thin films, epitaxially grown on ZnO(0001), with hard x-rays, making it a truly bulk-sensitive probe. This becomes possible by using a novel, specially adapted momentum microscope, featuring time-of-flight energy recording and an imaging spin-filter.

Direct observation of spin-resolved valence band electronic states from a buried magnetic layer with hard X-ray photoemission

ABSTRACT We report spin-resolved hard X-ray photoelectron spectroscopy (spin-HAXPES) for a buried Fe thin film in the valence band region. For the spin-HAXPES experiments, we developed an ultracompact built-in Mott-type spin-filter in a sample carrier, which enabled us to use the merit of two-dimensional (2D) multi-channel detector in a recent photoelectron analyser without modifying an apparatus for HAXPES. The effective Sherman function and the single-channel figure of merit (FOM) of the spin-filter were assessed to be −0.07 and 2.0 × 10−4, respectively. By utilizing the 2D detector of the photoelectron analyser, the effective FOM increased by a factor of ~4 × 104 compared to the case when only 1 channel of the 2D detector was used. We have applied spin-HAXPES to MgO(2 nm)/Fe(50 nm)/MgO(001) structures. The spin-HAXPES experiments revealed the majority and minority spin electronic states and the spin polarisation of the buried Fe thin film. Due to the large photoionization cross-section of the 4s orbital of Fe in HAXPES, the spin-resolved spectra mainly reflected the Fe 3d and 4s states. The observed spin-HAXPES and spin polarisation spectral shapes agreed well with the calculated spin-resolved cross-section weighted densities of states and spin polarisation spectra. In contrast, a small discrepancy in the energy scale was recognised due to the electron correlation effects. These results suggest that the electron correlation effects are important in the electronic structure of bulk Fe, and spin-HAXPES is useful for detecting genuine spin-resolved valence band electronic structures of buried magnetic materials.

Distinctive Picosecond Spin Polarization Dynamics in Bulk Half Metals.

Femtosecond laser excitations in half-metal (HM) compounds are theoretically predicted to induce an exotic picosecond spin dynamics. In particular, conversely to what is observed in conventional metals and semiconductors, the thermalization process in HMs leads to a long living partially thermalized configuration characterized by three Fermi-Dirac distributions for the minority, majority conduction, and majority valence electrons, respectively. Remarkably, these distributions have the same temperature but different chemical potentials. This unusual thermodynamic state is causing a persistent nonequilibrium spin polarization only well above the Fermi energy. Femtosecond spin dynamics experiments performed on Fe_{3}O_{4} by time- and spin-resolved photoelectron spectroscopy support our model. Furthermore, the spin polarization response proves to be very robust and it can be adopted to selectively test the bulk HM character in a wide range of compounds.

Observation of topological superconductivity on the surface of an iron-based superconductor.

Topological superconductors are predicted to host exotic Majorana states that obey non-Abelian statistics and can be used to implement a topological quantum computer. Most of the proposed topological superconductors are realized in difficult-to-fabricate heterostructures at very low temperatures. By using high-resolution spin-resolved and angle-resolved photoelectron spectroscopy, we find that the iron-based superconductor FeTe1-x Se x (x = 0.45; superconducting transition temperature Tc = 14.5 kelvin) hosts Dirac-cone-type spin-helical surface states at the Fermi level; the surface states exhibit an s-wave superconducting gap below Tc Our study shows that the surface states of FeTe0.55Se0.45 are topologically superconducting, providing a simple and possibly high-temperature platform for realizing Majorana states.

Nonvortical Rashba Spin Structure on a Surface with C_{1h} Symmetry.

A totally anisotropic peculiar Rashba-Bychkov (RB) splitting of electronic bands was found on the Tl/Si(110)-(1×1) surface with C_{1h} symmetry by angle- and spin-resolved photoelectron spectroscopy and first-principles theoretical calculation. The constant energy contour of the upper branch of the RB split band has a warped elliptical shape centered at a k point located between Γ[over ¯] and the edge of the surface Brillouin zone, i.e., at a point without time-reversal symmetry. The spin-polarization vector of this state is in-plane and points almost the same direction along the whole elliptic contour. This novel nonvortical RB spin structure is confirmed as a general phenomenon originating from the C_{1h} symmetry of the surface.

Development of hard X-ray photoelectron SPLEED-based spectrometer applicable for probing of buried magnetic layer valence states

A novel design of high-voltage compatible polarimeter for spin-resolved hard X-ray photoelectron spectroscopy (Spin-HAXPES) went into operation at beamline BL09XU of SPring-8 in Hyogo, Japan. The detector is based on the well-established principle of electron diffraction from a W(001) single-crystal at a scattering energy of 103.5eV. It's special feature is that it can be operated at a high negative bias potential up to 10kV, necessary to access the HAXPES range. The polarimeter is operated behind a large hemispherical analyzer (Scienta R-4000). It was optimized for high transmission of the transfer optics. A delay-line detector (20mm dia.) is positioned at the exit plane of the analyzer enabling conventional multichannel intensity spectroscopy simultaneously with single-channel spin analysis. The performance of the combined setup is demonstrated by the spin-resolved data for the valence-region of a FeCo functional layer of a tunneling device, buried beneath 3nm of oxidic material. The well-structured spin polarization spectrum validates Spin-HAXPES in the valence energy range as powerful method for bulk electronic structure analysis. The spin polarization spectrum exhibits a rich structure, originating from clearly discernible transitions in the majority and minority partial spin spectra.

Specific features of the electronic, spin, and atomic structures of a topological insulator Bi2Te2.4Se0.6

The specific features of the electronic and spin structures of a triple topological insulator Bi2Te2.4Se0.6, which is characterized by high-efficiency thermoelectric properties, have been studied with the use of angular- and spin-resolved photoelectron spectroscopy and compared with theoretical calculations in the framework of the density functional theory. It has been shown that the Fermi level for Bi2Te2.4Se0.6 falls outside the band gap and traverses the topological surface state (the Dirac cone). Theoretical calculations of the electronic structure of the surface have demonstrated that the character of distribution of Se atoms on the Te–Se sublattice practically does not influence the dispersion of the surface topological electronic state. The spin structure of this state is characterized by helical spin polarization. Analysis of the Bi2Te2.4Se0.6 surface by scanning tunnel microscopy has revealed atomic smoothness of the surface of a sample cleaved in an ultrahigh vacuum, with a lattice constant of ~4.23 A. Stability of the Dirac cone of the Bi2Te2.4Se0.6 compound to deposition of a Pt monolayer on the surface is shown.

Spin-resolved photoelectron spectroscopy using femtosecond extreme ultraviolet light pulses from high-order harmonic generation.

The fundamental mechanism responsible for optically induced magnetization dynamics in ferromagnetic thin films has been under intense debate since almost two decades. Currently, numerous competing theoretical models are in strong need for a decisive experimental confirmation such as monitoring the triggered changes in the spin-dependent band structure on ultrashort time scales. Our approach explores the possibility of observing femtosecond band structure dynamics by giving access to extended parts of the Brillouin zone in a simultaneously time-, energy- and spin-resolved photoemission experiment. For this purpose, our setup uses a state-of-the-art, highly efficient spin detector and ultrashort, extreme ultraviolet light pulses created by laser-based high-order harmonic generation. In this paper, we present the setup and first spin-resolved spectra obtained with our experiment within an acquisition time short enough to allow pump-probe studies. Further, we characterize the influence of the excitation with femtosecond extreme ultraviolet pulses by comparing the results with data acquired using a continuous wave light source with similar photon energy. In addition, changes in the spectra induced by vacuum space-charge effects due to both the extreme ultraviolet probe- and near-infrared pump-pulses are studied by analyzing the resulting spectral distortions. The combination of energy resolution and electron count rate achieved in our setup confirms its suitability for spin-resolved studies of the band structure on ultrashort time scales.

Monitoring surface resonances onCo2MnSi(100)by spin-resolved photoelectron spectroscopy

The magnitude of the spin polarization at the Fermi level of ferromagnetic materials at room temperature is a key property for spintronics. Investigating the Heusler compound Co$_2$MnSi a value of 93$\%$ for the spin polarization has been observed at room temperature, where the high spin polarization is related to a stable surface resonance in the majority band extending deep into the bulk. In particular, we identified in our spectroscopical analysis that this surface resonance is embedded in the bulk continuum with a strong coupling to the majority bulk states. The resonance behaves very bulk-like, as it extends over the first six atomic layers of the corresponding (001)-surface. Our study includes experimental investigations, where the bulk electronic structure as well as surface-related features have been investigated using spin-resolved photoelectron spectroscopy (SR-UPS) and for a larger probing depth spin-integrated high energy x-ray photoemission spectroscopy (HAXPES). The results are interpreted in comparison with first-principles band structure and photoemission calculations which consider all relativistic, surface and high-energy effects properly.

Design of a multi-channel spin polarimeter**Supported by National Natural Science Foundation of China (10979021, 11027401, 11174054, 11304338, 11227902), the Ministry of Science and Technology of China (2011CB921800) and the “Strategic Priority Research Program (B)” of the Chinese Academy of Sciences (XDB04010100)

All commercial electron spin polarimeters work in single channel mode, which is the bottleneck of researches by spin-resolved photoelectron spectroscopy. By adopting the time inversion antisymmetry of the magnetic field, we developed a multichannel spin polarimeter based on normal incident very low energy electron diffraction (VLEED). The key point to achieve the multi-channel measurements is the spatial resolution of the electron optics. The test of the electron optics shows that the designed spatial resolution can be achieved and an image type spin polarimeter with 100 times 100, totally ten thousand channels is possible to be realized.

A mobile UHV-system designed for femtosecond time, energy and spin resolved photoelectron spectroscopy

We report on the design and tests of a completely mobile photoemission end station for femtosecond time, energy and spin resolved magnetodynamics experiments. The setup is designed for the use at free-electron laser (FEL) x-ray beamlines as well as at synchrotron facilities. In situ sample preparation in an adjacent UHV chamber is performed. The assembly can be altered to obtain energy resolved measurements with the help of a hemispherical electron energy analyzer or to obtain energy integrated measurements by inserting an electron transfer lens system. The magnetic information is obtained by spin analysis with a 50 kV Mott detector. The setup is tested in house with an electron gun, at the Swiss Light Source (SLS) and has been used at the FEL at FLASH in Hamburg. Results show the practicability of the system and exemplarily the kinetic energy dependent polarization of the low energy secondary electrons emitted from a 15 ML iron film on W(110) is presented.

Spin-polarized (001) surface states of the topological crystalline insulator Pb0.73Sn0.27Se

We study the nature of (001) surface states in Pb_{0.73}Sn_{0.27}Se in the newly discovered topological-crystalline-insulator (TCI) phase as well as the corresponding topologically trivial state above the band-gap-inversion temperature. Our calculations predict not only metallic surface states with a nontrivial chiral spin structure for the TCI case, but also nonmetallic (gapped) surface states with nonzero spin polarization when the system is a normal insulator. For both phases, angle- and spin-resolved photoelectron spectroscopy measurements provide conclusive evidence for the formation of these (001) surface states in Pb_{0.73}Sn_{0.27}Se, as well as for their chiral spin structure.

The adsorption, stability and properties of Mn6Cr Single-Molecule-Magnets studied by means of nc-AFM, STM, XAS and Spin-resolved Photoelectron Spectroscopy

The ionic single-molecule-magnet [MnIII6CrIII]3 with corresponding three counterions has been deposited on different surfaces and studied with respect to its structure and its electronic and magnetic properties. This is the first time that spin polarization of photoelectrons ejected by means of circularly polarized synchrotron radiation has been measured in a single-molecule-magnet.

Negative spin polarization at the Fermi level in Fe4N epitaxial films by spin-resolved photoelectron spectroscopy

We evaluated the valence band structures of 10-nm-thick ferromagnetic Fe4N epitaxial films on SrTiO3(001) substrates at room temperature using spin-resolved photoelectron spectroscopy. Negative spin polarization is confirmed at the Fermi level. The experimentally obtained photoelectron spectra are well explained by first-principles calculations based on the psuedopotential method.

An instrument for the investigation of actinides with spin resolved photoelectron spectroscopy and bremsstrahlung isochromat spectroscopy

A new system for spin resolved photoelectron spectroscopy and bremsstrahlung isochromat spectroscopy has been built and commissioned at Lawrence Livermore National Laboratory for the investigation of the electronic structure of the actinides. Actinide materials are very toxic and radioactive and therefore cannot be brought to most general user facilities for spectroscopic studies. The technical details of the new system and preliminary data obtained therein will be presented and discussed.

Spin resolved photoelectron spectroscopy of[Mn6IIICrIII]3 + single-molecule magnets and of manganese compounds as reference layers

Properties of the manganese-based single-molecule magnet are studied. It contains six MnIII ions arranged in two bowl-shaped trinuclear triplesalen building blockslinked by a hexacyanochromate and exhibits a large spin ground state ofSt = 21/2. The dominant structures in the electron emission spectra of resonantly excited at the L3-edge are the L3M2, 3M2, 3, L3M2, 3V and L3VV Auger emission groups following the decay of the primaryp3/2 core hole state. Significant differences of the Auger spectra from intact and degraded show up. First measurements of the electron spin polarization in theL3M2, 3V andL3VV Auger emission peaks from the manganese constituents in resonantly excited at the L3-edge near 640 eV by circularly polarized synchrotron radiation are reported. Inaddition spin resolved Auger electron spectra of the reference substances MnO,Mn2O3 andMnII(acetate)2·4H2O are given. The applicability of spin resolved electron spectroscopy forcharacterizing magnetic states of constituent atoms compared to magneticcircular dichroism (MCD) is verified: the spin polarization obtained fromMnII(acetate)2·4H2O at room temperature in the paramagnetic state compares tothe MCD asymmetry revealed for a star-shaped molecule with aMn4IIO6 core at 5 K in an external magnetic field of 5 T.

Substrate-induced spin-orbit splitting of quantum-well and interface states in Au, Ag, and Cu layers of different thicknesses on W(110) and Mo(110) surfaces

The substrate-induced spin-orbit splitting of interface and quantum-well states formed in Au, Ag, and Cu layers on W(110) and Mo(110) surfaces has been revealed using angle- and spin-resolved photoelectron spectroscopy. It has been shown that the magnitude of the splitting depends noticeably on the atomic number of the substrate material and is markedly larger for layers of these metals on W(110), i.e., on the surface of a metal with a larger atomic number (Z W = 74), than on the surface of Mo(110), i.e., an element with a smaller atomic number (Z Mo = 42), while depending only weakly on the atomic number of the adsorbed metal. Measurements of the dispersion of the formed quantum-well states have shown that the substrate-induced spin-orbit splitting increases with increasing parallel component of the photoelectron momentum (which correlates with the Rashba model) for all thicknesses of deposited films (up to 10 ML). The magnitude of induced spin-orbit splitting of the interface states evolving in monolayer Au, Ag, and Cu coatings on W(110) and Mo(110) decreases with increasing parallel component of the excited photoelectron momentum.

Investigations of magnetic overlayers at the Advanced Photon Source

Magnetic overlayers of Fe and Co have been investigated with x-ray magnetic circular dichroism in x-ray absorption spectroscopy and photoelectron spectroscopy, including spin-resolved photoelectron spectroscopy, at Beamline 4 at the Advanced Photon Source. Particular emphasis was placed upon the interrogation of the 2p levels of the Fe.