54 research outputs found
Quasi 2D electronic states with high spin-polarization in centrosymmetric MoS bulk crystals
Time reversal dictates that nonmagnetic, centrosymmetric crystals cannot be
spin-polarized as a whole. However, it has been recently shown that the
electronic structure in these crystals can in fact show regions of high
spin-polarization, as long as it is probed locally in real and in reciprocal
space. In this article we present the first observation of this type of
compensated polarization in MoS bulk crystals. Using spin- and
angle-resolved photoemission spectroscopy (ARPES) we directly observed a
spin-polarization of more than 65% for distinct valleys in the electronic band
structure. By additionally evaluating the probing depth of our method we find
that these valence band states at the point in the
Brillouin zone are close to fully polarized for the individual atomic trilayers
of MoS, which is confirmed by our density functional theory calculations.
Furthermore, we show that this spin-layer locking leads to the observation of
highly spin-polarized bands in ARPES since these states are almost completely
confined within two dimensions. Our findings prove that these highly desired
properties of MoS can be accessed without thinning it down to the monolayer
limit
Kink far below the Fermi level reveals new electron-magnon scattering channel in Fe
Many properties of real materials can be modeled using ab initio methods
within a single-particle picture. However, for an accurate theoretical
treatment of excited states, it is necessary to describe electron-electron
correlations including interactions with bosons: phonons, plasmons, or magnons.
In this work, by comparing spin- and momentum-resolved photoemission
spectroscopy measurements to many-body calculations carried out with a newly
developed first-principles method, we show that a kink in the electronic band
dispersion of a ferromagnetic material can occur at much deeper binding
energies than expected (E_b=1.5 eV). We demonstrate that the observed spectral
signature reflects the formation of a many-body state that includes a photohole
bound to a coherent superposition of renormalized spin-flip excitations. The
existence of such a many-body state sheds new light on the physics of the
electron-magnon interaction which is essential in fields such as spintronics
and Fe-based superconductivity.Comment: 6 pages, 2 figure
Hard X-ray standing-wave photoemission insights into the structure of an epitaxial Fe/MgO multilayer magnetic tunnel junction
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, nondestructive, 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 kilo-electron-volt energy range to probe the structure of an epitaxially grown MgO/Fe superlattice. The superlattice consists of 9 repeats of MgO grown on Fe by magnetron sputtering on an MgO(001) substrate, with a protective Al2O3 capping layer. We determine through SW-HXPS that 8 of the 9 repeats are similar and ordered, with a period of 33 ± 4 Å, with the minor presence of FeO at the interfaces and a significantly distorted top bilayer with ca. 3 times the oxidation of the lower layers at the top MgO/Fe interface. There is evidence of asymmetrical oxidation on the top and bottom of the Fe layers. We find agreement with dark-field scanning transmission electron microscope (STEM) and X-ray reflectivity measurements. Through the STEM measurements, we confirm an overall epitaxial stack with dislocations and warping at the interfaces of ca. 5 Å. We also note a distinct difference in the top bilayer, especially MgO, with possible Fe inclusions. We thus demonstrate that SW-HXPS can be used to probe deep buried interfaces of novel magnetic devices with few-angstrom precision
Direct observation of the band gap transition in atomically thin ReS
ReS is considered as a promising candidate for novel electronic and
sensor applications. The low crystal symmetry of the van der Waals compound
ReS leads to a highly anisotropic optical, vibrational, and transport
behavior. However, the details of the electronic band structure of this
fascinating material are still largely unexplored. We present a
momentum-resolved study of the electronic structure of monolayer, bilayer, and
bulk ReS using k-space photoemission microscopy in combination with
first-principles calculations. We demonstrate that the valence electrons in
bulk ReS are - contrary to assumptions in recent literature - significantly
delocalized across the van der Waals gap. Furthermore, we directly observe the
evolution of the valence band dispersion as a function of the number of layers,
revealing a significantly increased effective electron mass in single-layer
crystals. We also find that only bilayer ReS has a direct band gap. Our
results establish bilayer ReS as a advantageous building block for
two-dimensional devices and van der Waals heterostructures
Bulk Electronic Structure of Lanthanum Hexaboride (LaB6) by Hard X-ray Angle-Resolved Photoelectron Spectroscopy
In the last decade rare-earth hexaborides have been investigated for their
fundamental importance in condensed matter physics, and for their applications
in advanced technological fields. Among these compounds, LaB has a special
place, being a traditional d-band metal without additional f- bands. In this
paper we investigate the bulk electronic structure of LaB using hard x-ray
photoemission spectroscopy, measuring both core-level and angle-resolved
valence-band spectra. By comparing La 3d core level spectra to cluster model
calculations, we identify well-screened peak residing at a lower binding energy
compared to the main poorly-screened peak; the relative intensity between these
peaks depends on how strong the hybridization is between La and B atoms. We
show that the recoil effect, negligible in the soft x-ray regime, becomes
prominent at higher kinetic energies for lighter elements, such as boron, but
is still negligible for heavy elements, such as lanthanum. In addition, we
report the bulk-like band structure of LaB determined by hard x-ray
angle-resolved photoemission spectroscopy (HARPES). We interpret HARPES
experimental results by the free-electron final-state calculations and by the
more precise one-step photoemission theory including matrix element and phonon
excitation effects. In addition, we consider the nature and the magnitude of
phonon excitations in HARPES experimental data measured at different
temperatures and excitation energies. We demonstrate that one step theory of
photoemission and HARPES experiments provide, at present, the only approach
capable of probing true bulk-like electronic band structure of rare-earth
hexaborides and strongly correlated materials.Comment: Total 26 pages, Total 11 figure
Does Exchange Splitting persist above ? A spin-resolved photoemission study of EuO
The electronic structure of the ferromagnetic semiconductor EuO is
investigated by means of spin- and angle-resolved photoemission spectroscopy
and density functional theory (GGA+). Our spin-resolved data reveals that,
while the macroscopic magnetization of the sample vanishes at the Curie
temperature, the exchange splitting of the O 2 band persists up to .
Thus, we provide evidence for short-range magnetic order being present at the
Curie temperature
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Interface properties and built-in potential profile of a LaCr O3/SrTi O3 superlattice determined by standing-wave excited photoemission spectroscopy
LaCrO3(LCO)/SrTiO3(STO) heterojunctions are intriguing due to a polar discontinuity along [001], exhibiting two distinct and controllable charged interface structures [(LaO)+/(TiO2)0 and (SrO)0/(CrO2)-] with induced polarization, and a resulting depth-dependent potential. In this study, we have used soft- and hard-x-ray standing-wave excited photoemission spectroscopy (SW-XPS) to quantitatively determine the elemental depth profile, interface properties, and depth distribution of the polarization-induced built-in potentials. We observe an alternating charged interface configuration: a positively charged (LaO)+/(TiO2)0 intermediate layer at the LCOtop/STObottom interface and a negatively charged (SrO)0/(CrO2)- intermediate layer at the STOtop/LCObottom interface. Using core-level SW data, we have determined the depth distribution of species, including through the interfaces, and these results are in excellent agreement with scanning transmission electron microscopy and electron energy-loss spectroscopy mapping of local structure and composition. SW-XPS also enabled deconvolution of the LCO and STO contributions to the valence-band (VB) spectra. Using a two-step analytical approach involving first SW-induced core-level binding-energy shifts and then VB modeling, the variation in potential across the complete superlattice is determined in detail. This potential is in excellent agreement with density functional theory models, confirming this method as a generally useful tool for interface studies
Characterization of free standing InAs quantum membranes by standing wave hard x-ray photoemission spectroscopy
Free-standing nanoribbons of InAs quantum membranes (QMs) transferred onto a
(Si/Mo) multilayer mirror substrate are characterized by hard x-ray
photoemission spectroscopy (HXPS), and by standing-wave HXPS (SW-HXPS).
Information on the chemical composition and on the chemical states of the
elements within the nanoribbons was obtained by HXPS and on the quantitative
depth profiles by SW-HXPS. By comparing the experimental SW-HXPS rocking curves
to x-ray optical calculations, the chemical depth profile of the InAs(QM) and
its interfaces were quantitatively derived with angstrom precision. We
determined that: i) the exposure to air induced the formation of an InAsO
layer on top of the stoichiometric InAs(QM); ii) the top interface between the
air-side InAsO and the InAs(QM) is not sharp, indicating that
interdiffusion occurs between these two layers; iii) the bottom interface
between the InAs(QM) and the native oxide SiO on top of the (Si/Mo)
substrate is abrupt. In addition, the valence band offset (VBO) between the
InAs(QM) and the SiO/(Si/Mo) substrate was determined by HXPS. The value of
eV is in good agreement with literature results obtained
by electrical characterization, giving a clear indication of the formation of a
well-defined and abrupt InAs/SiO heterojunction. We have demonstrated that
HXPS and SW-HXPS are non-destructive, powerful methods for characterizing
interfaces and for providing chemical depth profiles of nanostructures, quantum
membranes, and 2D layered materials.Comment: three figure
Depth-resolved resonant inelastic x-ray scattering at a superconductor/half-metallic-ferromagnet interface through standing wave excitation
We demonstrate that combining standing wave (SW) excitation with resonant inelastic x-ray scattering (RIXS) can lead to depth resolution and interface sensitivity for studying orbital and magnetic excitations in correlated oxide heterostructures. SW-RIXS has been applied to multilayer heterostructures consisting of a superconductor La1.85Sr0.15CuO4 (LSCO) and a half-metallic ferromagnet La0.67Sr0.33MnO3 (LSMO). Easily observable SW effects on the RIXS excitations were found in these LSCO/LSMO multilayers. In addition, we observe different depth distribution of the RIXS excitations. The magnetic excitations are found to arise from the LSCO/LSMO interfaces, and there is also a suggestion that one of the dd excitations comes from the interfaces. SW-RIXS measurements of correlated-oxide and other multilayer heterostructures should provide unique layer-resolved insights concerning their orbital and magnetic excitations, as well as a challenge for RIXS theory to specifically deal with interface effects
Depth-resolved resonant inelastic x-ray scattering at a superconductor/half-metallic-ferromagnet interface through standing wave excitation
We demonstrate that combining standing wave (SW) excitation with resonant inelastic x-ray scattering (RIXS) can lead to depth resolution and interface sensitivity for studying orbital and magnetic excitations in correlated oxide heterostructures. SW-RIXS has been applied to multilayer heterostructures consisting of a superconductor La1.85Sr0.15CuO4 (LSCO) and a half-metallic ferromagnet La0.67Sr0.33MnO3 (LSMO). Easily observable SW effects on the RIXS excitations were found in these LSCO/LSMO multilayers. In addition, we observe different depth distribution of the RIXS excitations. The magnetic excitations are found to arise from the LSCO/LSMO interfaces, and there is also a suggestion that one of the dd excitations comes from the interfaces. SW-RIXS measurements of correlated-oxide and other multilayer heterostructures should provide unique layer-resolved insights concerning their orbital and magnetic excitations, as well as a challenge for RIXS theory to specifically deal with interface effects
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