Linear Dichroic Signals Research Articles

Critical analysis of proximity-induced magnetism in MnTe/Bi2Te3 heterostructures

An elegant approach to overcome the intrinsic limitations of magnetically doped topological insulators is to bring a topological insulator in direct contact with a magnetic material. The aspiration is to realize the quantum anomalous Hall effect at high temperatures where the symmetry-breaking magnetic field is provided by a proximity-induced magnetization at the interface. Hence, a detailed understanding of the interfacial magnetism in such heterostructures is crucial, yet its distinction from structural and magnetic background effects is a rather nontrivial task. Here, we combine several magnetic characterization techniques to investigate the magnetic ordering in $\mathrm{MnTe}/{\mathrm{Bi}}_{2}{\mathrm{Te}}_{3}$ heterostructures. A magnetization profile of the layer stack is obtained using depth-sensitive polarized neutron reflectometry. The magnetic constituents are characterized in more detail using element-sensitive magnetic x-ray spectroscopy. Magnetotransport measurements provide additional information about the magnetic transitions. We find that the supposedly antiferromagnetic MnTe layer does not exhibit an x-ray magnetic linear dichroic signal, raising doubt that it is in its antiferromagnetic state. Instead, Mn seems to penetrate into the surface region of the ${\mathrm{Bi}}_{2}{\mathrm{Te}}_{3}$ layer. Furthermore, the interface between MnTe and ${\mathrm{Bi}}_{2}{\mathrm{Te}}_{3}$ is not abrupt, but extending over $\ensuremath{\sim}2.2$ nm. These conditions are the likely reason that we do not observe proximity-induced magnetization at the interface. Our findings illustrate the importance of not solely relying on one single technique as proof for proximity-induced magnetism at interfaces. We demonstrate that a holistic, multitechnique approach is essential to gain a more complete picture of the magnetic structure in which the interface is embedded.

Probing band parity inversion in the topological insulator GeBi2Te4 by linear dichroism in ARPES

Here we report on experiments performed by linear-dichroism angle resolved photoelectron spectroscopy (LD-ARPES) on septuple layers (SL) topological insulator GeBi2Te4. The linear dichroic signals, both for the valence band states and in the topologically protected surface state are interpreted as a probe of the symmetry of the states. The observation of a change in the LD-ARPES spectrum at the top of VB is attributed to the band parity inversion which is predicted by theory for all materials entering the topologically protected phase. Finally, the opening of a gap in the surface state dispersion along the non-high symmetry direction is reported. This is proposed to originated from the interaction between the surface state and the VB, leading to the formation of surface resonances.

Magnetic Field Induced Orbital Polarization in CubicYbInNi4: Determining the Quartet Ground State Using X-Ray Linear Dichroism

We have been able to induce a linear dichroic signal in the Yb M(5) x-ray absorption white line of cubic YbInNi(4) by the application of a magnetic field. The nonzero integrated intensity of the magnetic field induced dichroic spectrum indicates a net noncubic 4f orbital polarization. A quantitative analysis of the temperature and field strength dependence establishes that the crystal-field ground state is a Γ(8) quartet. The results demonstrate the potential of magnetic field induced linear dichroism as a new powerful approach for the investigation of the degeneracy and orbital degrees of freedom of cubic heavy-fermion and Kondo systems.

Crystal-field ground state of the noncentrosymmetric superconductorCePt3Si: A combined polarized soft x-ray absorption and polarized neutron study

We determined the crystal-field split Hund's rule ground state of the non-centrosymmetric heavy fermion superconductor CePt3Si with polarization dependent soft X-ray absorption spectroscopy (XAS) and polarized neutron scattering. We are also able to give the sequence of the crystal-field states from the temperature evolution of the linear dichroic signal in the XAS. The quantitative analysis of the XAS temperature dependence together with the neutron transition energies complete the identification of the crystal-field level scheme.

Structural and magnetic properties of epitaxialFe∕CoObilayers on Ag(001)

We have investigated the magnetic coupling between a metallic ferromagnet and an oxidic antiferromagnet in epitaxial single-crystalline $\mathrm{Fe}∕\mathrm{Co}\mathrm{O}$ bilayers on Ag(001) using x-ray absorption spectroscopy. Absorption spectra taken from bilayers with different amounts of deposited Fe show only a weak indication for the formation of Fe oxide at the $\mathrm{Fe}∕\mathrm{Co}\mathrm{O}$ interface. From the spectral shape, it is concluded that an FeO type of oxide is formed. X-ray magnetic circular dichroism (XMCD) measurements exhibit a sizable induced ferromagnetic signal at the $\mathrm{Co}\phantom{\rule{0.2em}{0ex}}{L}_{2,3}$ absorption edges, corresponding to an interface layer of 1.1 ML of CoO in which the Co magnetic moments couple with the Fe moments. The angular dependence of the Fe XMCD and Co x-ray magnetic linear dichroic signals at the ${L}_{2,3}$ edges shows that the orientation of the Co and Fe spins is parallel along the crystallographic ⟨110⟩ directions.

Quantitative Mapping of the Orientation of Fibroin β-Sheets in B. mori Cocoon Fibers by Scanning Transmission X-ray Microscopy

The spatial distribution of the linear dichroic signal associated with aligned beta-sheets in a microtomed section of a Bombyx mori cocoon silk fiber was derived from scanning transmission X-ray microscopy (STXM). The intense C 1s --> pi(amide) peak at 288.25 eV was found to have negligible dichroic signal in transverse sections but a large dichroic signal in longitudinal sections. This is consistent with other measurements of the orientation of the aligned beta-sheets in silk fibers, in particular with those obtained by polarized Raman microspectroscopy to which our results are compared. When the dichroic signal strength is mapped at better than 100 nm spatial resolution, microscopic variations are found. Although the magnitude of the dichroic signal changes over a fine spatial scale, the direction of the maximum signal at any position does not change. We interpret the spatial variation of the intensity of the dichroic signal as a map of the quality of local orientation of the beta-sheets in the fiber. At sufficiently high magnification and resolution, this technique should image individual beta-sheet crystallites, although the present implementation does not achieve that. A map of the orientation parameter P(2) is derived. The average value of P(2) (-0.20 +/- 0.04) from STXM is smaller than that derived from the analysis of the amide I band in polarized Raman spectra (-0.41 +/- 0.03). This deviation is attributed to the fact that the STXM results also include the signal from unaligned regions of the protein.

Structural relaxations around Ti, Cr and Fe impurities inα-Al2O3 probed by x-ray absorption near-edge structure combined with first-principlescalculations

We determine the structural relaxations around paramagnetic impurities (Ti, Cr, Fe) in corundum(α-Al2O3), by combining x-ray absorption near-edge structure (XANES) experiments and ab initiocalculations. The structural relaxations are found to be very local. We then show thatXANES is sensitive to small variations in interatomic distances within the coordinationshell of the absorbing atom. The experiments were carried out on single crystals of rubyand sapphires. Linear dichroic signals are essential to characterize the geometry of theimpurity site. The calculations were performed within a self-consistent ‘non-muffin-tin’framework, that uses pseudopotentials, a plane-wave basis set, and the continued fractionfor the absorption cross section.

Magnetic versus crystal-field linear dichroism in NiO thin films

We have detected strong dichroism in the Ni $L_{2,3}$ x-ray absorption spectra of monolayer NiO films. The dichroic signal appears to be very similar to the magnetic linear dichroism observed for thicker antiferromagnetic NiO films. A detailed experimental and theoretical analysis reveals, however, that the dichroism is caused by crystal field effects in the monolayer films, which is a non trivial effect because the high spin Ni $3d^{8}$ ground state is not split by low symmetry crystal fields. We present a practical experimental method for identifying the independent magnetic and crystal field contributions to the linear dichroic signal in spectra of NiO films with arbitrary thicknesses and lattice strains. Our findings are also directly relevant for high spin $3d^{5}$ and $3d^{3}$ systems such as LaFeO$_{3}$, Fe$_{2}$O$_{3}$, VO, LaCrO$_{3}$, Cr$_{2}$O$_{3}$, and Mn$^{4+}$ manganate thin films.

Calculated x-ray dichroic signals and resonant Bragg diffraction structure factors forDyB2C2

Low-temperature properties of dysprosium borocarbide display evidence of two phase transitions; it has been proposed that the first involves ordering of Dy quadrupole moments and the second, at a lower temperature, involves ordering of Dy magnetic moments. The latter has been established by magnetic neutron diffraction. Insight into the first phase transition, at ${T}_{Q},$ has been sought by resonant x-ray Bragg diffraction, which reveals charge-forbidden (Templeton-Templeton) intensities $(00l+\frac{1}{2})$ that increase with decreasing temperature. We show that this scattering is absent if the space group is $P4/mbm,$ which has been proposed for the high-temperature crystal structure. Tanaka et al. report evidence that at ${T}_{Q}$ the lattice distorts and the new space group is ${P4}_{2}/mnm.$ The lower point-group symmetry of Dy ions in this space group is shown by us to allow diffraction at $(00l+\frac{1}{2}).$ In the magnetically ordered phase two neighboring Dy moments along the c axis are mutually perpendicular and two neighboring moments in the plane normal to the c axis are almost oppositely aligned. This magnetic configuration and point-group symmetry $2/m$ for Dy sites lead us to predict magnetic and charge scattering at reflections $(00l+\frac{1}{2})$ in the magnetically ordered phase. At reflections $(h0l)$ and h odd calculated structure factors are purely magnetic. Our finding is consistent with data collected by Hirota et al. at the reflection (102). We report expressions for circular and linear dichroic signals and structure factors for Bragg scattering for the interpretation of future experiments. The structure factors are appropriate for azimuthal-angle scans in which the crystal is rotated about the $(00l+\frac{1}{2})$ or the $(h0l)$ Bragg scattering vectors.

Quantitative analysis of the effect of probe convergence on electron energy loss spectra of anisotropic materials

We present a quantitative analysis of core level electron energy loss (EEL) spectra for uniaxial anisotropic materials, taking into consideration the presence of a convergent electron probe as is the case in a scanning transmission electron microscope (STEM) or transmission electron microscope in convergent probe mode. The analysis assumes the dipole allowed, non-relativistic scattering angular distribution for inelastic electrons which have suffered core losses and the anisotropic effect is included by taking proper account of the projection of the electric field along different crystal axes. The physical response of a uniaxial material is represented by the dielectric function tensor, which has two components ε ‖ and ε ⊥. The detected core loss intensity is shown to be a linear superposition of the imaginary parts of these two components, with the `weighting' coefficients dependent on the experimental conditions. Their calculated values compare very well with the experimental measurement acquired from graphite, which is a well-known uniaxial material. We have presented the numerical result in a parameterized form for easy applications, such as to minimize or maximize the linear dichroic signal from uniaxial materials. We also discuss the condition in detail for minimizing the orientation dependence of the EELS signal in anisotropic materials.

Electric-dipole absorption and the scattering of x-rays: analytic results for a ion in an octahedral crystal field

A formulation describing electric-dipole absorption and resonance-enhanced diffraction of x-rays is applied to a ion in an octahedral crystal field and subject to a magnetic field. Analytical results are given for both the idealized and the complete forms of the scattering length. The contributing atomic quantities are discussed in detail, e.g. the spin - orbit interaction and the quadrupole moment of the valence holes. The model calculation was chosen to illustrate the use of a completely general formulation of the scattering length, from which one obtains the attenuation coefficient and cross-sections for elastic and inelastic scattering. Relevant tensor operators are referred both to unit tensors, and to appropriately coupled operators of spin and orbital angular momentum. The quantities that appear in the linear dichroic signal differ from corresponding results in the literature, and reasons for this finding are discussed.

A theory of the absorption and the resonance-enhanced diffraction of soft x-rays by anisotropic ferrous magnetic moments in

Some of the interesting magnetic properties of ferrous niobate have recently been attributed to the orbital angular momentum of the iron atom. In the present work, the spin and orbital components of the magnetic moment are shown to make distinctive appearances in the resonant contribution to the scattering length for the electrostatic scattering of x-rays, from which the dichroic signals in the attenuation coefficient and the Bragg diffraction cross-section are derived. Quantitative predictions are made for the circular and linear dichroic signals of ferrous atoms in , and these are compared to the corresponding signals predicted for free ferrous atoms. In all cases, the analytic expressions for the dichroic signal possess a relatively simple structure.