1
|
Abstract
Lifted Kramers spin degeneracy (LKSD) has been among the central topics of condensed-matter physics since the dawn of the band theory of solids1,2. It underpins established practical applications as well as current frontier research, ranging from magnetic-memory technology3-7 to topological quantum matter8-14. Traditionally, LKSD has been considered to originate from two possible internal symmetry-breaking mechanisms. The first refers to time-reversal symmetry breaking by magnetization of ferromagnets and tends to be strong because of the non-relativistic exchange origin15. The second applies to crystals with broken inversion symmetry and tends to be comparatively weaker, as it originates from the relativistic spin-orbit coupling (SOC)16-19. A recent theory work based on spin-symmetry classification has identified an unconventional magnetic phase, dubbed altermagnetic20,21, that allows for LKSD without net magnetization and inversion-symmetry breaking. Here we provide the confirmation using photoemission spectroscopy and ab initio calculations. We identify two distinct unconventional mechanisms of LKSD generated by the altermagnetic phase of centrosymmetric MnTe with vanishing net magnetization20-23. Our observation of the altermagnetic LKSD can have broad consequences in magnetism. It motivates exploration and exploitation of the unconventional nature of this magnetic phase in an extended family of materials, ranging from insulators and semiconductors to metals and superconductors20,21, that have been either identified recently or perceived for many decades as conventional antiferromagnets21,24,25.
Collapse
|
2
|
Persistence of Structural Distortion and Bulk Band Rashba Splitting in SnTe above Its Ferroelectric Critical Temperature. NANO LETTERS 2024; 24:82-88. [PMID: 38109843 PMCID: PMC10786156 DOI: 10.1021/acs.nanolett.3c03280] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Received: 08/30/2023] [Revised: 12/06/2023] [Accepted: 12/07/2023] [Indexed: 12/20/2023]
Abstract
The ferroelectric semiconductor α-SnTe has been regarded as a topological crystalline insulator, and the dispersion of its surface states has been intensively measured with angle-resolved photoemission spectroscopy (ARPES) over the past decade. However, much less attention has been given to the impact of the ferroelectric transition on its electronic structure, and in particular on its bulk states. Here, we investigate the low-energy electronic structure of α-SnTe with ARPES and follow the evolution of the bulk-state Rashba splitting as a function of temperature, across its ferroelectric critical temperature of about Tc ≈ 110 K. Unexpectedly, we observe a persistent band splitting up to room temperature, which is consistent with an order-disorder contribution of local dipoles to the phase transition that requires the presence of fluctuating dipoles above Tc. We conclude that no topological surface state can occur under these conditions at the (111) surface of SnTe, at odds with recent literature.
Collapse
|
3
|
Soft X-ray Fermi surface tomography of palladium and rhodium via momentum microscopy. Ultramicroscopy 2023; 253:113820. [PMID: 37586245 DOI: 10.1016/j.ultramic.2023.113820] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/14/2023] [Revised: 06/10/2023] [Accepted: 07/25/2023] [Indexed: 08/18/2023]
Abstract
Fermi surfaces of transition metals, which describe all thermodynamical and transport quantities of solids, often fail to be modeled by one-electron mean-field theory due to strong correlations among the valence electrons. In addition, relativistic spin-orbit coupling pronounced in heavier elements lifts the degeneracy of the energy bands and further modifies the Fermi surface. Palladium and rhodium, two 4d metals attributed to show significant spin-orbit coupling and electron correlations, are ideal for a systematic and fundamental study of the two fundamental physical phenomena and their interplay in the electronic structure. In this study, we explored the Fermi surface of the 4d noble metals palladium and rhodium obtained via high-resolution constant initial state momentum microscopy. The complete 3D-Fermi surfaces of palladium and rhodium were tomographically mapped using soft X-ray photon energies from 34 eV up to 660 eV. To fully capture the orbital angular momentum of states across the Fermi surface, the Fermi surface tomography was performed using p- and s- polarized light. Applicability and limitations of the nearly-free electron final state model in photoemission are discussed using a complex band structure model supported by experimental evidence. The significance of spin-orbit coupling and electron correlations across the Fermi surfaces will be discussed within the context of the photoemission results. State-of-the-art fully relativistic Korringa-Kohn-Rostoker (KKR) calculations within the one-step model of photoemission are used to support the experimental results.
Collapse
|
4
|
Ultrafast Hidden Spin Polarization Dynamics of Bright and Dark Excitons in 2H-WSe_{2}. PHYSICAL REVIEW LETTERS 2023; 131:066402. [PMID: 37625042 DOI: 10.1103/physrevlett.131.066402] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 05/17/2023] [Accepted: 07/06/2023] [Indexed: 08/27/2023]
Abstract
We performed spin-, time- and angle-resolved extreme ultraviolet photoemission spectroscopy of excitons prepared by photoexcitation of inversion-symmetric 2H-WSe_{2} with circularly polarized light. The very short probing depth of XUV photoemission permits selective measurement of photoelectrons originating from the top-most WSe_{2} layer, allowing for direct measurement of hidden spin polarization of bright and momentum-forbidden dark excitons. Our results reveal efficient chiroptical control of bright excitons' hidden spin polarization. Following optical photoexcitation, intervalley scattering between nonequivalent K-K^{'} valleys leads to a decay of bright excitons' hidden spin polarization. Conversely, the ultrafast formation of momentum-forbidden dark excitons acts as a local spin polarization reservoir, which could be used for spin injection in van der Waals heterostructures involving multilayer transition metal dichalcogenides.
Collapse
|
5
|
High-energy photoemission final states beyond the free-electron approximation. Nat Commun 2023; 14:4827. [PMID: 37563126 PMCID: PMC10415355 DOI: 10.1038/s41467-023-40432-5] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/30/2022] [Accepted: 07/26/2023] [Indexed: 08/12/2023] Open
Abstract
Three-dimensional (3D) electronic band structure is fundamental for understanding a vast diversity of physical phenomena in solid-state systems, including topological phases, interlayer interactions in van der Waals materials, dimensionality-driven phase transitions, etc. Interpretation of ARPES data in terms of 3D electron dispersions is commonly based on the free-electron approximation for the photoemission final states. Our soft-X-ray ARPES data on Ag metal reveals, however, that even at high excitation energies the final states can be a way more complex, incorporating several Bloch waves with different out-of-plane momenta. Such multiband final states manifest themselves as a complex structure and added broadening of the spectral peaks from 3D electron states. We analyse the origins of this phenomenon, and trace it to other materials such as Si and GaN. Our findings are essential for accurate determination of the 3D band structure over a wide range of materials and excitation energies in the ARPES experiment.
Collapse
|
6
|
Circular dichroism in hard X-ray photoelectron diffraction observed by time-of-flight momentum microscopy. Ultramicroscopy 2023; 250:113750. [PMID: 37178606 DOI: 10.1016/j.ultramic.2023.113750] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/15/2023] [Revised: 04/01/2023] [Accepted: 05/04/2023] [Indexed: 05/15/2023]
Abstract
X-ray photoelectron diffraction (XPD) is a powerful technique that yields detailed structural information of solids and thin films that complements electronic structure measurements. Among the strongholds of XPD we can identify dopant sites, track structural phase transitions, and perform holographic reconstruction. High-resolution imaging of kll-distributions (momentum microscopy) presents a new approach to core-level photoemission. It yields full-field kx-ky XPD patterns with unprecedented acquisition speed and richness in details. Here, we show that beyond the pure diffraction information, XPD patterns exhibit pronounced circular dichroism in the angular distribution (CDAD) with asymmetries up to 80%, alongside with rapid variations on a small kll-scale (0.1 Å-1). Measurements with circularly-polarized hard X-rays (hν = 6 keV) for a number of core levels, including Si, Ge, Mo and W, prove that core-level CDAD is a general phenomenon that is independent of atomic number. The fine structure in CDAD is more pronounced compared to the corresponding intensity patterns. Additionally, they obey the same symmetry rules as found for atomic and molecular species, and valence bands. The CD is antisymmetric with respect to the mirror planes of the crystal, whose signatures are sharp zero lines. Calculations using both the Bloch-wave approach and one-step photoemission reveal the origin of the fine structure that represents the signature of Kikuchi diffraction. To disentangle the roles of photoexcitation and diffraction, XPD has been implemented into the Munich SPRKKR package to unify the one-step model of photoemission and multiple scattering theory.
Collapse
|
7
|
Influence of Disorder on the Bad Metal Behavior in Polar Amalgams. Inorg Chem 2023; 62:3965-3975. [PMID: 36821862 DOI: 10.1021/acs.inorgchem.2c04430] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/25/2023]
Abstract
The two new ternary amalgams K1-xRbxHg11 [x = 0.472(7)] and Cs3-xCaxHg20 [x = 0.20(3)] represent two different examples of how to create ternary compounds from binaries by statistical atom substitution. K1-xRbxHg11 is a Vegard-type mixed crystal of the isostructural binaries KHg11 and RbHg11 [cubic, BaHg11 structure type, space group Pm3̅m, a = 9.69143(3) Å, Rietveld refinement], whereas Cs3-xCaxHg20 is a substitution variant of the Rb3Hg20 structure type [cubic, space group Pm3̅n, a = 10.89553(14) Å, Rietveld refinement] for which a fully substituted isostructural binary Ca phase is unknown. In K1-xRbxHg11, the valence electron concentration (VEC) is not changed by the substitution, whereas in Cs3-xCaxHg20, the VEC increases with the Ca content. Amalgams of electropositive metals form polar metal bonds and show "bad metal" properties. By thermal analysis, magnetic susceptibility and resistivity measurements, and density functional theory calculations of the electronic structures, we investigate the effect of the structural disorder introduced by creating mixed-atom occupation on the physical properties of the two new polar amalgam systems.
Collapse
|
8
|
Understanding the Role of Electronic Effects in CO on the Pt–Sn Alloy Surface via Band Structure Measurements. ACS Catal 2021. [DOI: 10.1021/acscatal.1c04566] [Citation(s) in RCA: 4] [Impact Index Per Article: 1.3] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/28/2022]
|
9
|
Néel Vector Induced Manipulation of Valence States in the Collinear Antiferromagnet Mn 2Au. ACS NANO 2020; 14:17554-17564. [PMID: 33236903 DOI: 10.1021/acsnano.0c08215] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 06/11/2023]
Abstract
The coupling of real and momentum space is utilized to tailor electronic properties of the collinear metallic antiferromagnet Mn2Au by aligning the real space Néel vector indicating the direction of the staggered magnetization. Pulsed magnetic fields of 60 T were used to orient the sublattice magnetizations of capped epitaxial Mn2Au(001) thin films perpendicular to the applied field direction by a spin-flop transition. The electronic structure and its corresponding changes were investigated by angular-resolved photoemission spectroscopy with photon energies in the vacuum-ultraviolet, soft, and hard X-ray range. The results reveal an energetic rearrangement of conduction electrons propagating perpendicular to the Néel vector. They confirm previous predictions on the origin of the Néel spin-orbit torque and anisotropic magnetoresistance in Mn2Au and reflect the combined antiferromagnetic and spin-orbit interaction in this compound leading to inversion symmetry breaking.
Collapse
|
10
|
Revealing Hidden Orbital Pseudospin Texture with Time-Reversal Dichroism in Photoelectron Angular Distributions. PHYSICAL REVIEW LETTERS 2020; 125:216404. [PMID: 33274965 DOI: 10.1103/physrevlett.125.216404] [Citation(s) in RCA: 20] [Impact Index Per Article: 5.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 06/02/2020] [Accepted: 10/13/2020] [Indexed: 06/12/2023]
Abstract
We performed angle-resolved photoemission spectroscopy (ARPES) of bulk 2H-WSe_{2} for different crystal orientations linked to each other by time-reversal symmetry. We introduce a new observable called time-reversal dichroism in photoelectron angular distributions (TRDAD), which quantifies the modulation of the photoemission intensity upon effective time-reversal operation. We demonstrate that the hidden orbital pseudospin texture leaves its imprint on TRDAD, due to multiple orbital interference effects in photoemission. Our experimental results are in quantitative agreement with both the tight-binding model and state-of-the-art fully relativistic calculations performed using the one-step model of photoemission. While spin-resolved ARPES probes the spin component of entangled spin-orbital texture in multiorbital systems, we unambiguously demonstrate that TRDAD reveals its orbital pseudospin texture counterpart.
Collapse
|
11
|
Large magnetic gap at the Dirac point in Bi2Te3/MnBi2Te4 heterostructures. Nature 2019; 576:423-428. [DOI: 10.1038/s41586-019-1826-7] [Citation(s) in RCA: 125] [Impact Index Per Article: 25.0] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/10/2018] [Accepted: 10/18/2019] [Indexed: 11/09/2022]
|
12
|
Solid Solutions of Grimm-Sommerfeld Analogous Nitride Semiconductors II-IV-N 2 (II=Mg, Mn, Zn; IV=Si, Ge): Ammonothermal Synthesis and DFT Calculations. Chemistry 2019; 25:15887-15895. [PMID: 31529651 PMCID: PMC6916306 DOI: 10.1002/chem.201903897] [Citation(s) in RCA: 15] [Impact Index Per Article: 3.0] [Reference Citation Analysis] [Abstract] [Key Words] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/26/2019] [Indexed: 11/15/2022]
Abstract
Grimm–Sommerfeld analogous II‐IV‐N2 nitrides such as ZnSiN2, ZnGeN2, and MgGeN2 are promising semiconductor materials for substitution of commonly used (Al,Ga,In)N. Herein, the ammonothermal synthesis of solid solutions of II‐IV‐N2 compounds (II=Mg, Mn, Zn; IV=Si, Ge) having the general formula (IIa1−xIIbx)‐IV‐N2 with x≈0.5 and ab initio DFT calculations of their electronic and optical properties are presented. The ammonothermal reactions were conducted in custom‐built, high‐temperature, high‐pressure autoclaves by using the corresponding elements as starting materials. NaNH2 and KNH2 act as ammonobasic mineralizers that increase the solubility of the reactants in supercritical ammonia. Temperatures between 870 and 1070 K and pressures up to 200 MPa were chosen as reaction conditions. All solid solutions crystallize in wurtzite‐type superstructures with space group Pna21 (no. 33), confirmed by powder XRD. The chemical compositions were analyzed by energy‐dispersive X‐ray spectroscopy. Diffuse reflectance spectroscopy was used for estimation of optical bandgaps of all compounds, which ranged from 2.6 to 3.5 eV (Ge compounds) and from 3.6 to 4.4 eV (Si compounds), and thus demonstrated bandgap tunability between the respective boundary phases. Experimental findings were corroborated by DFT calculations of the electronic structure of pseudorelaxed mixed‐occupancy structures by using the KKR+CPA approach.
Collapse
|
13
|
Topological electronic structure and Rashba effect in Bi thin layers: theoretical predictions and experiments. JOURNAL OF PHYSICS. CONDENSED MATTER : AN INSTITUTE OF PHYSICS JOURNAL 2019; 31:283001. [PMID: 30933942 DOI: 10.1088/1361-648x/ab1529] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 06/09/2023]
Abstract
The goal of the present review is to cross-compare theoretical predictions with selected experimental results on bismuth thin films exhibiting topological properties and a strong Rashba effect. The theoretical prediction that a single free-standing Bi(1 1 1) bilayer is a topological insulator has triggered a large series of studies of ultrathin Bi(1 1 1) films grown on various substrates. Using selected examples we review theoretical predictions of atomic and electronic structure of Bi thin films exhibiting topological properties due to interaction with a substrate. We also survey experimental signatures of topological surface states and Rashba effect, as obtained mostly by angle- and spin-resolved photoelectron spectroscopy.
Collapse
|
14
|
Ab initio exploration and prediction of AE-containing nitrido(litho/magneso)tetrelates (AE = Ca, Sr; Tt = Si, Ge) with [Si 2N 6] 10- or [Ge 2N 6] 10- units. Dalton Trans 2019; 48:8671-8677. [PMID: 31140530 DOI: 10.1039/c9dt01158g] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/21/2022]
Abstract
Recently, a number of different structurally related nitrides characterized by pairs of edge-sharing Si-N tetrahedra forming [Si2N6]10- units have emerged via different synthesis methods. Concurrently, upon doping with rare earth elements (e.g. Eu2+ and Ce3+), numerous applications in the field of luminescent materials were revealed, ranging from the visible spectrum to the near IR. This compound class in turn emphasizes the extraordinary large tuning range with respect to relative composition by formal cation exchange. In this contribution, we study the dynamical stabilities of the existing Si-based nitridotetrelates and hypothetical Ge analogues promising for future synthesis efforts of luminescent materials by means of extensive phonon calculations. Further calculations of electronic and mechanical properties corroborate the fundamental suitability of the predicted compounds for the applications of potential luminescent materials with regard to band gap (Eg) and Debye temperature (ΘD). Calculated enthalpies of the reaction provide further beneficial insights for future experimental attempts. Our study hence highlights a potential range of novel stable nitridogermanates with isotypic structures and suitable electronic properties for optoelectronic applications.
Collapse
|
15
|
Ba 6
( M
N 4
)N 2-x
( M
= Mo VI
/Ta V
), a Subvalent Nitridometalate with Perovskite-like Crystal Structure. Z Anorg Allg Chem 2019. [DOI: 10.1002/zaac.201800400] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.2] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/09/2022]
|
16
|
Ca and S K-edge XANES of CaS calculated by different methods: influence of full potential, core hole and Eu doping. JOURNAL OF SYNCHROTRON RADIATION 2019; 26:152-158. [PMID: 30655480 DOI: 10.1107/s1600577518016144] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.2] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Subscribe] [Scholar Register] [Received: 09/21/2018] [Accepted: 11/14/2018] [Indexed: 06/09/2023]
Abstract
Ca and S K-edge spectra of CaS are calculated by the full-potential Green's function multiple-scattering method, by the FLAPW method and by the finite-difference method. All three techniques lead to similar spectra. Some differences remain close to the edge, both when comparing different calculations with each other and when comparing the calculations with earlier experimental data. Here it is found that using the full potential does not lead to significant improvement over the atomic spheres approximation and that the effect of the core hole can be limited to the photoabsorbing atom alone. Doping CaS with Eu will not affect the Ca and S K-edge XANES of CaS significantly but may give rise to a pre-edge structure not present for clean CaS.
Collapse
|
17
|
Distinctive Picosecond Spin Polarization Dynamics in Bulk Half Metals. PHYSICAL REVIEW LETTERS 2018; 121:077205. [PMID: 30169049 DOI: 10.1103/physrevlett.121.077205] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.2] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 03/01/2018] [Indexed: 06/08/2023]
Abstract
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.
Collapse
|
18
|
Element- and momentum-resolved electronic structure of the dilute magnetic semiconductor manganese doped gallium arsenide. Nat Commun 2018; 9:3306. [PMID: 30120237 PMCID: PMC6098022 DOI: 10.1038/s41467-018-05823-z] [Citation(s) in RCA: 14] [Impact Index Per Article: 2.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/14/2018] [Accepted: 07/23/2018] [Indexed: 11/09/2022] Open
Abstract
The dilute magnetic semiconductors have promise in spin-based electronics applications due to their potential for ferromagnetic order at room temperature, and various unique switching and spin-dependent conductivity properties. However, the precise mechanism by which the transition-metal doping produces ferromagnetism has been controversial. Here we have studied a dilute magnetic semiconductor (5% manganese-doped gallium arsenide) with Bragg-reflection standing-wave hard X-ray angle-resolved photoemission spectroscopy, and resolved its electronic structure into element- and momentum- resolved components. The measured valence band intensities have been projected into element-resolved components using analogous energy scans of Ga 3d, Mn 2p, and As 3d core levels, with results in excellent agreement with element-projected Bloch spectral functions and clarification of the electronic structure of this prototypical material. This technique should be broadly applicable to other multi-element materials.
Collapse
|
19
|
Finite lifetime broadening of calculated X-ray absorption spectra: possible artefacts close to the edge. JOURNAL OF SYNCHROTRON RADIATION 2018; 25:523-528. [PMID: 29488932 DOI: 10.1107/s1600577518000048] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Subscribe] [Scholar Register] [Received: 09/22/2017] [Accepted: 01/02/2018] [Indexed: 06/08/2023]
Abstract
X-ray absorption spectra calculated within an effective one-electron approach have to be broadened to account for the finite lifetime of the core hole. For methods based on Green's function this can be achieved either by adding a small imaginary part to the energy or by convoluting the spectra on the real axis with a Lorentzian. By analyzing the Fe K- and L2,3-edge spectra it is demonstrated that these procedures lead to identical results only for energies higher than a few core-level widths above the absorption edge. For energies close to the edge, spurious spectral features may appear if too much weight is put on broadening via the imaginary energy component. Special care should be taken for dichroic spectra at edges which comprise several exchange-split core levels, such as the L3-edge of 3d transition metals.
Collapse
|
20
|
First-principles and experimental characterization of the electronic properties of CaGaSiN 3 and CaAlSiN 3: the impact of chemical disorder. Phys Chem Chem Phys 2018; 19:9292-9299. [PMID: 28322381 DOI: 10.1039/c6cp08764g] [Citation(s) in RCA: 9] [Impact Index Per Article: 1.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/21/2022]
Abstract
We report a detailed investigation of the electronic, mechanical and optical properties of the recently discovered nitridogallosilicate CaGaSiN3 which has potential as a LED-phosphor host material. We focus on chemical disorder effects, originating from the Ga/Si site, and compared them to those of isostructural CaAlSiN3. We calculate the elastic moduli and the Debye temperature in terms of quasi harmonical approximation. Spectral properties like the joint density of states (JDOS) are evaluated and the absorption, reflectance and energy loss function are obtained from the dielectric function. The optical band gap of CaGaSiN3 from experiment is compared to the electronic band gap in terms of electronic DOS and band structure calculations. All properties are evaluated for different ordering models of Ga/Si while the experimentally observed substitutional disorder is accounted for by utilizing the Coherent Potential Approximation (CPA). We conclude a shrinking of the band gap for both CaGaSiN3 and CaAlSiN3 due to atomic disorder, which is unfavorable for potential phosphor applications. This study contributes to materials design considerations, and provides a close look on the electronic impact of substitutional disorder. Moreover, we open the scope for future investigations on solid solutions and phosphor host materials with low doping concentrations.
Collapse
|
21
|
Ammonothermal Synthesis and Optical Properties of Ternary Nitride Semiconductors Mg-IV-N 2 , Mn-IV-N 2 and Li-IV 2 -N 3 (IV=Si, Ge). Chemistry 2017; 24:1686-1693. [PMID: 29205562 DOI: 10.1002/chem.201704973] [Citation(s) in RCA: 29] [Impact Index Per Article: 4.1] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/19/2017] [Indexed: 11/09/2022]
Abstract
Grimm-Sommerfeld analogous nitrides MgSiN2 , MgGeN2 , MnSiN2 , MnGeN2 , LiSi2 N3 and LiGe2 N3 (generally classified as II-IV-N2 and I-IV2 -N3 ) are promising semiconductor materials with great potential for application in (opto)electronics or photovoltaics. A new synthetic approach for these nitride materials was developed using supercritical ammonia as both solvent and nitride-forming agent. Syntheses were conducted in custom-built high-pressure autoclaves with alkali metal amides LiNH2 , NaNH2 or KNH2 as ammonobasic mineralizers, which accomplish an adequate solubility of the starting materials and promote the formation of reactive intermediate species. The reactions were performed at temperatures between 870 and 1070 K and pressures up to 230 MPa. All studied compounds crystallize in wurtzite-derived superstructures with orthorhombic space groups Pna21 (II-IV-N2 ) and Cmc21 (I-IV2 -N3 ), respectively, which was confirmed by powder X-ray diffraction. Optical bandgaps were estimated from diffuse reflectance spectra using the Kubelka-Munk function (MgSiN2 : 4.8 eV, MgGeN2 : 3.2 eV, MnSiN2 : 3.5 eV, MnGeN2 : 2.5 eV, LiSi2 N3 : 4.4 eV, LiGe2 N3 : 3.9 eV). Complementary DFT calculations were carried out to gain insight into the electronic band structures of these materials and to corroborate the optical measurements.
Collapse
|
22
|
Spin-filtered time-of-flight k-space microscopy of Ir - Towards the "complete" photoemission experiment. Ultramicroscopy 2017; 183:19-29. [PMID: 28705441 DOI: 10.1016/j.ultramic.2017.06.025] [Citation(s) in RCA: 22] [Impact Index Per Article: 3.1] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/07/2017] [Revised: 06/16/2017] [Accepted: 06/28/2017] [Indexed: 11/28/2022]
Abstract
The combination of momentum microscopy (high resolution imaging of the Fourier plane) with an imaging spin filter has recently set a benchmark in k-resolution and spin-detection efficiency. Here we show that the degree of parallelization can be further increased by time-of-flight energy recording. On the quest towards maximum information (in earlier work termed "complete" photoemission experiment) we have studied the prototypical high-Z fcc metal iridium. Large partial bandgaps and strong spin-orbit interaction lead to a sequence of spin-polarized surface resonances. Soft X-rays give access to the 4D spectral density function ρ (EB,kx,ky,kz) weighted by the photoemission cross section. The Fermi surface and all other energy isosurfaces, Fermi velocity distribution vF(kF), electron or hole conductivity, effective mass and inner potential can be obtained from the multi-dimensional array ρ by simple algorithms. Polarized light reveals the linear and circular dichroism texture in a simple manner and an imaging spin filter exposes the spin texture. One-step photoemission calculations are in fair agreement with experiment. Comparison of the Bloch spectral function with photoemission calculations uncovers that the observed high spin polarization of photoelectrons from bulk bands originates from the photoemission step and is not present in the initial state.
Collapse
|
23
|
Determination of surface and interface magnetic properties for the multiferroic heterostructure Co/BaTiO3 using spleed and arpes. JOURNAL OF PHYSICS. CONDENSED MATTER : AN INSTITUTE OF PHYSICS JOURNAL 2016; 28:436004. [PMID: 27603180 DOI: 10.1088/0953-8984/28/43/436004] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 06/06/2023]
Abstract
Co/BaTiO3(0 0 1) is one of the most interesting multiferroic heterostructures as it combines different ferroic phases, setting this way the fundamentals for innovative technical applications. Various theoretical approaches have been applied to investigate the electronic and magnetic properties of Co/BaTiO3(0 0 1). Here we determine the magnetic properties of 3 ML Co/BaTiO3 by calculating spin-polarized electron diffraction as well as angle-resolved photoemission spectra, with both methods being well established as surface sensitive techniques. Furthermore, we discuss the impact of altering the BaTiO3 polarization on the spectra and ascribe the observed changes to characteristic details of the electronic structure.
Collapse
|
24
|
Entanglement and manipulation of the magnetic and spin-orbit order in multiferroic Rashba semiconductors. Nat Commun 2016; 7:13071. [PMID: 27767052 PMCID: PMC5078730 DOI: 10.1038/ncomms13071] [Citation(s) in RCA: 25] [Impact Index Per Article: 3.1] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/18/2016] [Accepted: 08/31/2016] [Indexed: 11/17/2022] Open
Abstract
Entanglement of the spin–orbit and magnetic order in multiferroic materials bears a strong potential for engineering novel electronic and spintronic devices. Here, we explore the electron and spin structure of ferroelectric α-GeTe thin films doped with ferromagnetic Mn impurities to achieve its multiferroic functionality. We use bulk-sensitive soft-X-ray angle-resolved photoemission spectroscopy (SX-ARPES) to follow hybridization of the GeTe valence band with the Mn dopants. We observe a gradual opening of the Zeeman gap in the bulk Rashba bands around the Dirac point with increase of the Mn concentration, indicative of the ferromagnetic order, at persistent Rashba splitting. Furthermore, subtle details regarding the spin–orbit and magnetic order entanglement are deduced from spin-resolved ARPES measurements. We identify antiparallel orientation of the ferroelectric and ferromagnetic polarization, and altering of the Rashba-type spin helicity by magnetic switching. Our experimental results are supported by first-principles calculations of the electron and spin structure. In α-GeTe, ferroelectric polarization acts to break inversion symmetry of the lattice and induce a strong Rashba-type spin splitting of the electronic band structure. Here, the authors study how this effect competes with Zeeman splitting due to ferromagnetic exchange coupling in Mn-doped GeTe.
Collapse
|
25
|
Nonmagnetic band gap at the Dirac point of the magnetic topological insulator (Bi(1-x)Mn(x))2Se3. Nat Commun 2016; 7:10559. [PMID: 26892831 PMCID: PMC4762886 DOI: 10.1038/ncomms10559] [Citation(s) in RCA: 31] [Impact Index Per Article: 3.9] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/13/2015] [Accepted: 12/26/2015] [Indexed: 12/04/2022] Open
Abstract
Magnetic doping is expected to open a band gap at the Dirac point of topological insulators by breaking time-reversal symmetry and to enable novel topological phases. Epitaxial (Bi1−xMnx)2Se3 is a prototypical magnetic topological insulator with a pronounced surface band gap of ∼100 meV. We show that this gap is neither due to ferromagnetic order in the bulk or at the surface nor to the local magnetic moment of the Mn, making the system unsuitable for realizing the novel phases. We further show that Mn doping does not affect the inverted bulk band gap and the system remains topologically nontrivial. We suggest that strong resonant scattering processes cause the gap at the Dirac point and support this by the observation of in-gap states using resonant photoemission. Our findings establish a mechanism for gap opening in topological surface states which challenges the currently known conditions for topological protection. Doping a topological insulator with magnetic impurities is expected to induce ferromagnetism and open a band gap in its surface states. Here, the authors study Mn-doped Bi2Se3, finding a mechanism for band gap opening in topologically-protected surface states which is not of magnetic origin.
Collapse
|
26
|
Anomalous d-like surface resonances on Mo(110) analyzed by time-of-flight momentum microscopy. Ultramicroscopy 2015; 159 Pt 3:453-63. [PMID: 26363904 DOI: 10.1016/j.ultramic.2015.07.008] [Citation(s) in RCA: 38] [Impact Index Per Article: 4.2] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/14/2014] [Revised: 05/26/2015] [Accepted: 07/16/2015] [Indexed: 11/28/2022]
Abstract
The electronic surface states on Mo(110) have been investigated using time-of-flight momentum microscopy with synchrotron radiation (hν=35 eV). This novel angle-resolved photoemission approach yields a simultaneous acquisition of the E-vs-k spectral function in the full surface Brillouin zone and several eV energy interval. (kx,ky,EB)-maps with 3.4 Å(-1) diameter reveal a rich structure of d-like surface resonances in the spin-orbit induced partial band gap. Calculations using the one-step model in its density matrix formulation predict an anomalous state with Dirac-like signature and Rashba spin texture crossing the bandgap at Γ¯ and EB=1.2 eV. The experiment shows that the linear dispersion persists away from the Γ¯-point in an extended energy- and k∥-range. Analogously to a similar state previously found on W(110) the dispersion is linear along H¯-Γ¯-H¯ and almost zero along N¯-Γ¯-N¯. The similarity is surprising since the spin-orbit interaction is 5 times smaller in Mo. A second point with unusual topology is found midway between Γ¯ and N¯. Band symmetries are probed by linear dichroism.
Collapse
|
27
|
Momentum-resolved spin dynamics of bulk and surface excited States in the topological insulator Bi(2)Se(3). PHYSICAL REVIEW LETTERS 2015; 114:097401. [PMID: 25793848 DOI: 10.1103/physrevlett.114.097401] [Citation(s) in RCA: 19] [Impact Index Per Article: 2.1] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 09/15/2014] [Indexed: 06/04/2023]
Abstract
The prospect of optically inducing and controlling a spin-polarized current in spintronic devices has generated wide interest in the out-of-equilibrium electronic and spin structure of topological insulators. In this Letter we show that only measuring the spin intensity signal over several orders of magnitude by spin-, time-, and angle-resolved photoemission spectroscopy can provide a comprehensive description of the optically excited electronic states in Bi_{2}Se_{3}. Our experiments reveal the existence of a surface resonance state in the second bulk band gap that is benchmarked by fully relativistic ab initio spin-resolved photoemission calculations. We propose that the newly reported state plays a major role in the ultrafast dynamics of the system, acting as a bottleneck for the interaction between the topologically protected surface state and the bulk conduction band. In fact, the spin-polarization dynamics in momentum space show that these states display macroscopically different temperatures and, more importantly, different cooling rates over several picoseconds.
Collapse
|
28
|
Trends in magnetism of free Rh clusters via relativistic ab-initio calculations. JOURNAL OF PHYSICS. CONDENSED MATTER : AN INSTITUTE OF PHYSICS JOURNAL 2015; 27:056004. [PMID: 25604525 DOI: 10.1088/0953-8984/27/5/056004] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.1] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 06/04/2023]
Abstract
A fully relativistic ab-initio study on free Rh clusters of 13-135 atoms is performed to identify general trends concerning their magnetism and to check whether concepts which proved to be useful in interpreting magnetism of 3d metals are applicable to magnetism of 4d systems. We found that there is no systematic relation between local magnetic moments and coordination numbers. On the other hand, the Stoner model appears well-suited both as a criterion for the onset of magnetism and as a guide for the dependence of local magnetic moments on the site-resolved density of states at the Fermi level. Large orbital magnetic moments antiparallel to spin magnetic moments were found for some sites. The intra-atomic magnetic dipole Tz term can be quite large at certain sites but as a whole it is unlikely to affect the interpretation of x-ray magnetic circular dichroism experiments based on the sum rules.
Collapse
|
29
|
Mixed dimensionality of confined conducting electrons in the surface region of SrTiO3. PHYSICAL REVIEW LETTERS 2014; 113:086801. [PMID: 25192117 DOI: 10.1103/physrevlett.113.086801] [Citation(s) in RCA: 11] [Impact Index Per Article: 1.1] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 02/02/2014] [Indexed: 06/03/2023]
Abstract
Using angle-resolved photoemission spectroscopy, we show that the recently discovered surface state on SrTiO(3) consists of nondegenerate t(2g) states with different dimensional characters. While the d(xy) bands have quasi-2D dispersions with weak k(z) dependence, the lifted d(xz)/d(yz) bands show 3D dispersions that differ significantly from bulk expectations and signal that electrons associated with those orbitals permeate the near-surface region. Like their more 2D counterparts, the size and character of the d(xz)/d(yz) Fermi surface components are essentially the same for different sample preparations. Irradiating SrTiO(3) in ultrahigh vacuum is one method observed so far to induce the "universal" surface metallic state. We reveal that during this process, changes in the oxygen valence band spectral weight that coincide with the emergence of surface conductivity are disproportionate to any change in the total intensity of the O 1s core level spectrum. This signifies that the formation of the metallic surface goes beyond a straightforward chemical doping scenario and occurs in conjunction with profound changes in the initial states and/or spatial distribution of near-E(F) electrons in the surface region.
Collapse
|
30
|
Correlation effects in fcc-Fe(x)Ni(1-x) alloys investigated by means of the KKR-CPA. JOURNAL OF PHYSICS. CONDENSED MATTER : AN INSTITUTE OF PHYSICS JOURNAL 2014; 26:274206. [PMID: 24935908 DOI: 10.1088/0953-8984/26/27/274206] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 06/03/2023]
Abstract
The electronic structure and magnetic properties of the disordered alloy system fcc-FexNi1-x (fcc: face centered cubic) have been investigated by means of the KKR-CPA (Korringa-Kohn-Rostoker coherent potential approximation) band structure method. To investigate the impact of correlation effects, the calculations have been performed on the basis of the LSDA (local spin density approximation), the LSDA + U as well as the LSDA + DMFT (dynamical mean field theory). It turned out that the inclusion of correlation effects hardly changed the spin magnetic moments and the related hyperfine fields. The spin-orbit induced orbital magnetic moments and hyperfine fields, on the other hand, show a pronounced and element-specific enhancement. These findings are in full accordance with the results of a recent experimental study.
Collapse
|
31
|
Magnetocrystalline anisotropy energy for adatoms and monolayers on non-magnetic substrates: where does it come from? JOURNAL OF PHYSICS. CONDENSED MATTER : AN INSTITUTE OF PHYSICS JOURNAL 2014; 26:196002. [PMID: 24762802 DOI: 10.1088/0953-8984/26/19/196002] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 06/03/2023]
Abstract
The substrate contribution to the magnetic anisotropy energy (MAE) of supported nanostructures can be assessed by a site-selective manipulation of the spin-orbit coupling (SOC) and of the effective exchange field Bex. A systematic study of Co adatoms and Co monolayers on the (1 1 1) surfaces of Cu, Ag, Au, Pd and Pt is performed to study common trends in this class of materials. It is found that for adatoms, the influence of the substrate SOC and Bex is relatively small (10-30% of the MAE) while for monolayers, this influence can be substantial. The influence of the substrate SOC is much more important than the influence of the substrate Bex, except for highly polarizable substrates with a strong SOC (such as Pt). The substrate always promotes the tendency to an out-of-plane orientation of the easy magnetic axis for all the investigated systems.
Collapse
|
32
|
Identifying the electronic character and role of the Mn states in the valence band of (Ga,Mn)As. PHYSICAL REVIEW LETTERS 2013; 111:097201. [PMID: 24033065 DOI: 10.1103/physrevlett.111.097201] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.2] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 06/04/2013] [Indexed: 06/02/2023]
Abstract
We report high-resolution hard x-ray photoemission spectroscopy results on (Ga,Mn)As films as a function of Mn doping. Supported by theoretical calculations we identify, for both low (1%) and high (13%) Mn doping values, the electronic character of the states near the top of the valence band. Magnetization and temperature-dependent core-level photoemission spectra reveal how the delocalized character of the Mn states enables the bulk ferromagnetic properties of (Ga,Mn)As.
Collapse
|
33
|
Atomic-scale engineering of magnetic anisotropy of nanostructures through interfaces and interlines. Nat Commun 2013; 3:1313. [PMID: 23271648 PMCID: PMC3535417 DOI: 10.1038/ncomms2316] [Citation(s) in RCA: 48] [Impact Index Per Article: 4.4] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/10/2012] [Accepted: 11/22/2012] [Indexed: 11/11/2022] Open
Abstract
The central goals of nanoscale magnetic materials science are the self-assembly of the smallest structure exhibiting ferromagnetic hysteresis at room temperature, and the assembly of these structures into the highest density patterns. The focus has been on chemically ordered alloys combining magnetic 3d elements with polarizable 5d elements having high spin–orbit coupling and thus yielding the desired large magneto-crystalline anisotropy. The chemical synthesis of nanoparticles of these alloys yields disordered phases requiring annealing to transform them to the high-anisotropy L10 structure. Despite considerable efforts, so far only part of the nanoparticles can be transformed without coalescence. Here we present an alternative approach to homogeneous alloys, namely the creation of nanostructures with atomically sharp bimetallic interfaces and interlines. They exhibit unexpectedly high magnetization reversal energy with values and directions of the easy magnetization axes strongly depending on chemistry and texture. We find significant deviations from the expected behaviour for commonly used element combinations. Ab-initio calculations reproduce these results and unravel their origin. The design and assembly of nanostructures exhibiting ferromagnetic hysteresis at room temperature are recognized goals for high-density data storage. Here, the authors engineer nanostructures with atomically sharp bimetallic interfaces and interlines, which exhibit large magnetic anisotropy and high temperature hysteresis.
Collapse
|
34
|
Reversal of the circular dichroism in angle-resolved photoemission from Bi2Te3. PHYSICAL REVIEW LETTERS 2013; 110:216801. [PMID: 23745908 DOI: 10.1103/physrevlett.110.216801] [Citation(s) in RCA: 8] [Impact Index Per Article: 0.7] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 07/04/2012] [Revised: 03/09/2013] [Indexed: 06/02/2023]
Abstract
The helical Dirac fermions at the surface of topological insulators show a strong circular dichroism which has been explained as being due to either the initial-state spin angular momentum, the initial-state orbital angular momentum, or the handedness of the experimental setup. All of these interpretations conflict with our data from Bi(2)Te(3) which depend on the photon energy and show several sign changes. Our one-step photoemission calculations coupled to ab initio theory confirm the sign change and assign the dichroism to a final-state effect. Instead, the spin polarization of the photoelectrons excited with linearly polarized light remains a reliable probe for the spin in the initial state.
Collapse
|
35
|
Bulk electronic structure of the dilute magnetic semiconductor Ga(1-x)Mn(x)As through hard X-ray angle-resolved photoemission. NATURE MATERIALS 2012; 11:957-962. [PMID: 23064495 DOI: 10.1038/nmat3450] [Citation(s) in RCA: 13] [Impact Index Per Article: 1.1] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 10/31/2011] [Accepted: 09/10/2012] [Indexed: 06/01/2023]
Abstract
A detailed understanding of the origin of the magnetism in dilute magnetic semiconductors is crucial to their development for applications. Using hard X-ray angle-resolved photoemission (HARPES) at 3.2 keV, we investigate the bulk electronic structure of the prototypical dilute magnetic semiconductor Ga(0.97)Mn(0.03)As, and the reference undoped GaAs. The data are compared to theory based on the coherent potential approximation and fully relativistic one-step-model photoemission calculations including matrix-element effects. Distinct differences are found between angle-resolved, as well as angle-integrated, valence spectra of Ga(0.97)Mn(0.03)As and GaAs, and these are in good agreement with theory. Direct observation of Mn-induced states between the GaAs valence-band maximum and the Fermi level, centred about 400 meV below this level, as well as changes throughout the full valence-level energy range, indicates that ferromagnetism in Ga(1-x)Mn(x)As must be considered to arise from both p-d exchange and double exchange, thus providing a more unifying picture of this controversial material.
Collapse
|
36
|
Fe spin reorientation across the metamagnetic transition in strained FeRh thin films. PHYSICAL REVIEW LETTERS 2012; 109:117201. [PMID: 23005667 DOI: 10.1103/physrevlett.109.117201] [Citation(s) in RCA: 11] [Impact Index Per Article: 0.9] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 05/31/2012] [Indexed: 06/01/2023]
Abstract
A spin reorientation accompanying the temperature-induced antiferromagnetic (AFM) to ferromagnetic (FM) phase transition is reported in strained epitaxial FeRh thin films. (57)Fe conversion electron Mössbauer spectrometry showed that the Fe moments have different orientations in FeRh grown on thick single-crystalline MgO and in FeRh grown on ion-beam-assist-deposited (IBAD) MgO. It was also observed, in both samples, that the Fe moments switch orientations at the AFM to FM phase transition. Perpendicular anisotropy was evidenced in the AFM phase of the film grown on IBAD MgO and in the FM phase of that grown on regular MgO. Density-functional theory calculations enabled this spin-reorientation transition to be accurately reproduced for both FeRh films across the AFM-FM phase transition and show that these results are due to differences in strain.
Collapse
|
37
|
Magnetic properties of CrSb compounds with zinc-blende and wurtzite structures. JOURNAL OF PHYSICS. CONDENSED MATTER : AN INSTITUTE OF PHYSICS JOURNAL 2012; 24:306005. [PMID: 22771910 DOI: 10.1088/0953-8984/24/30/306005] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 06/01/2023]
Abstract
The electronic structure and magnetic properties of Cr-Sb compounds with zinc-blende and wurtzite structure have been studied by means of the Korringa-Kohn-Rostoker (KKR) band structure method. The occurrence of a half-metallic behavior has been investigated for the bulk systems as a function of lattice parameter, as well as for thin films deposited on different substrates. In the latter case the influence of the surface and interface on the electronic structure is discussed in addition. To study magnetic order in the bulk and within the films, exchange coupling parameters have been calculated from first principles. They have been used for subsequent Monte Carlo simulations, based on a classical Heisenberg Hamiltonian, to obtain the Curie temperature.
Collapse
|
38
|
Probing bulk electronic structure with hard X-ray angle-resolved photoemission. NATURE MATERIALS 2011; 10:759-764. [PMID: 21841798 DOI: 10.1038/nmat3089] [Citation(s) in RCA: 21] [Impact Index Per Article: 1.6] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 11/25/2010] [Accepted: 07/05/2011] [Indexed: 05/31/2023]
Abstract
Traditional ultraviolet/soft X-ray angle-resolved photoemission spectroscopy (ARPES) may in some cases be too strongly influenced by surface effects to be a useful probe of bulk electronic structure. Going to hard X-ray photon energies and thus larger electron inelastic mean-free paths should provide a more accurate picture of bulk electronic structure. We present experimental data for hard X-ray ARPES (HARPES) at energies of 3.2 and 6.0 keV. The systems discussed are W, as a model transition-metal system to illustrate basic principles, and GaAs, as a technologically-relevant material to illustrate the potential broad applicability of this new technique. We have investigated the effects of photon wave vector on wave vector conservation, and assessed methods for the removal of phonon-associated smearing of features and photoelectron diffraction effects. The experimental results are compared to free-electron final-state model calculations and to more precise one-step photoemission theory including matrix element effects.
Collapse
|
39
|
Recurrent landslides predisposed by fault-induced weathering of flysch in the Western Carpathians. ACTA ACUST UNITED AC 2010. [DOI: 10.1144/egsp23.11] [Citation(s) in RCA: 5] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/04/2022]
Abstract
AbstractThe interrelationship between slope deformation and fault-induced weathering as a predisposing factor for the development of sliding is analysed through several case studies from the Western Carpathians in the Czech Republic. The study area comprises flysch nappes with alternating sandstone and shale of different permeability. These lithological structures are affected by systems of faults. Recurring slope instability is found associated with zones of deep weathering in tectonically weakened areas. Climatic variability of landslide activity can be identified during the Holocene by means of radiocarbon dating and pollen analysis. Areas affected by recurring landsliding suggest gradual and cyclic landslide frequency.
Collapse
|
40
|
Strength of correlation effects in the electronic structure of iron. PHYSICAL REVIEW LETTERS 2009; 103:267203. [PMID: 20366340 DOI: 10.1103/physrevlett.103.267203] [Citation(s) in RCA: 10] [Impact Index Per Article: 0.7] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 10/16/2009] [Indexed: 05/29/2023]
Abstract
The strength of electronic correlation effects in the spin-dependent electronic structure of ferromagnetic bcc Fe(110) has been investigated by means of spin and angle-resolved photoemission spectroscopy. The experimental results are compared to theoretical calculations within the three-body scattering approximation and within the dynamical mean-field theory, together with one-step model calculations of the photoemission process. This comparison indicates that the present state of the art many-body calculations, although improving the description of correlation effects in Fe, give too small mass renormalizations and scattering rates thus demanding more refined many-body theories including nonlocal fluctuations.
Collapse
|
41
|
Presence of the mosquito Anopheles hyrcanus in South Moravia, Czech Republic. MEDICAL AND VETERINARY ENTOMOLOGY 2009; 23:284-286. [PMID: 19712159 DOI: 10.1111/j.1365-2915.2009.00810.x] [Citation(s) in RCA: 5] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 05/28/2023]
Abstract
During a survey of mosquitoes in the South Moravian lowland area, the mosquito Anopheles hyrcanus (Pallas) (Diptera: Culicidae) was found breeding in an ancient fishpond (Nesyt). It is not clear whether this southern Palaearctic species, a known vector of malaria in Asia which has not been recorded in the Czech Republic until this year, has gone undetected in the past or whether it has recently moved into the region as a result of climate change.
Collapse
|
42
|
Inhomogeneous alloying in FePt nanoparticles as a reason for reduced magnetic moments. JOURNAL OF PHYSICS. CONDENSED MATTER : AN INSTITUTE OF PHYSICS JOURNAL 2009; 21:336002. [PMID: 21828614 DOI: 10.1088/0953-8984/21/33/336002] [Citation(s) in RCA: 6] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 05/15/2023]
Abstract
The reduced magnetic moments of oxide-free FePt nanoparticles are discussed in terms of lattice expansion and local deviation from the averaged composition. By analyses of the extended x-ray absorption fine structure of FePt nanoparticles and bulk material measured both at the Fe K and Pt L(3) absorption edge, the composition within the single nanoparticles is found to be inhomogeneous, i.e. Pt is in a Pt-rich environment and, consequently, Fe is in an Fe-rich environment. The standard Fourier transformation-based analysis is complemented by a wavelet transformation method clearly visualizing the difference in the local composition. The dependence of the magnetic properties, i.e. the element-specific magnetic moments on the composition in chemically disordered Fe(x)Pt(1-x) alloys, is studied by fully relativistic SPR-KKR band structure calculations supported by experimental results determined from the x-ray magnetic circular dichroism of 50 nm thick films and bulk material.
Collapse
|
43
|
Abstract
The complex correlation of structure and magnetism in highly coercive monoatomic FePt surface alloys is studied using scanning tunneling microscopy, x-ray magnetic circular dichroism, and ab initio theory. Depending on the specific lateral atomic coordination of Fe either hard magnetic properties comparable to that of bulk FePt or complex noncollinear magnetism due to Dzyaloshinski-Moriya interactions are observed. Our calculations confirm the subtle dependence of the magnetic anisotropy and spin alignment on the local coordination and suggest that 3D stacking of Fe and Pt layers in bulk L1_{0} magnets is not essential to achieve high-anisotropy values.
Collapse
|
44
|
Evidence for a magnetic proximity effect up to room temperature at Fe/(Ga, Mn)As interfaces. PHYSICAL REVIEW LETTERS 2008; 101:267201. [PMID: 19113784 DOI: 10.1103/physrevlett.101.267201] [Citation(s) in RCA: 7] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 01/15/2008] [Revised: 09/30/2008] [Indexed: 05/27/2023]
Abstract
We report x-ray magnetic circular dichroism and superconducting quantum interference device magnetometry experiments to study magnetic order and coupling in thin Fe/(Ga, Mn)As(100) films. We observe induced magnetic order in the (Ga, Mn)As layer that extends over more than 2 nm, even at room temperature. We find spectroscopic evidences of a hybridized d configuration of Mn atoms in Fe/(Ga, Mn)As, with negligible Mn diffusion and/or MnFe intermixing. We show by experiment as well as by theory that the magnetic moment of the Mn ions couples antiparallel to the moment of the Fe overlayer.
Collapse
|
45
|
Spin-orbit hybridization points in the face-centered-cubic cobalt band structure. PHYSICAL REVIEW LETTERS 2008; 101:066402. [PMID: 18764479 DOI: 10.1103/physrevlett.101.066402] [Citation(s) in RCA: 6] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 01/08/2008] [Indexed: 05/26/2023]
Abstract
Linear magnetic dichroism is observed in spin-, time-, and energy-resolved two-photon photoemission from valence bands of epitaxial fcc cobalt on Cu(001). With image-potential states as spectator states we identify initial bulk and surface states with minority spin character as the source for dichroic intensities and apparent dichroic lifetimes. Excellent agreement with ab initio fully relativistic calculations of the cobalt fcc band structure allows us to precisely determine spin-orbit hybridization points close to the Fermi level. These spin hot spots enhance spin-flip scattering by several orders of magnitude and are therefore assumed to be crucial in ultrafast demagnetization.
Collapse
|
46
|
Spectral function of ferromagnetic 3d metals: a self-consistent LSDA+DMFT approach combined with the one-step model of photoemission. PHYSICAL REVIEW LETTERS 2006; 97:227601. [PMID: 17155842 DOI: 10.1103/physrevlett.97.227601] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.1] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 08/02/2006] [Indexed: 05/12/2023]
Abstract
The electronic structure of ferromagnetic 3d-transition metals in the vicinity of the Fermi level is dominated by the spin-polarized d bands. Experimentally, this energy region can be probed in detail by means of angle-resolved ultraviolet photoemission and inverse photoemission. In several earlier studies the measured spectra were described either within a single-particle approach based on the local spin-density approximation including matrix-element effects within the so-called one-step model or by sophisticated many-body approaches neglecting these effects. In our analysis we combine for the first time correlation with matrix-element effects to achieve an improved interpretation of photoemission data from ferromagnetic nickel.
Collapse
|
47
|
Experimental observation and theoretical description of the pure Fano effect in the valence-band photoemission of ferromagnets. PHYSICAL REVIEW LETTERS 2005; 95:166401. [PMID: 16241823 DOI: 10.1103/physrevlett.95.166401] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 05/23/2005] [Indexed: 05/05/2023]
Abstract
The pure Fano effect in angle-integrated valence-band photoemission of ferromagnets has been observed for the first time. A contribution of the intrinsic spin polarization to the spin polarization of the photoelectrons has been avoided by an appropriate choice of the experimental parameters. The theoretical description of the resulting spectra reveals a complete analogy to the Fano effect observed before for paramagnetic transition metals. While the theoretical photocurrent and spin-difference spectra are found in good quantitative agreement with experiment in the case of Fe and Co, only a qualitative agreement could be achieved in the case of Ni by calculations on the basis of plain local spin-density approximation. Agreement with experimental data could be improved in this case in a very substantial way by a treatment of correlation effects on the basis of dynamical mean field theory.
Collapse
|
48
|
Theoretical calculation of x-ray magnetic circular dichroism spectra for Gd/Cu multilayers at the Cu K edge. JOURNAL OF PHYSICS. CONDENSED MATTER : AN INSTITUTE OF PHYSICS JOURNAL 2005; 17:5785-5794. [PMID: 32397048 DOI: 10.1088/0953-8984/17/37/014] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 06/11/2023]
Abstract
To explain the remarkable oscillations observed in the x-ray magnetic circular dichroic absorption spectra from Gd/Cu multilayers at the Cu K edge, ab initio calculations have been made using the fully relativistic Korringa-Kohn-Rostoker formalism including the spin-orbit coupling. The result reproduces well the oscillatory profiles in the near-edge region, but the peaks and valleys do not correspond to those in the difference density of states [Formula: see text] for the unoccupied Cu 4p band above the Fermi level. We find small spin and orbital moments on the interfacial Cu sites, which decay towards the core of the Cu layer. Surprisingly, neither the spin nor the orbital moments die out on the Cu sites four atomic layers away from the Co interface. This extended polarization is ascribed to the hybridization of the Cu 4p and the Gd 5d states. The accuracy of the calculation is supported by the near-bulk spin and orbital moments found on the Gd sites away from the interface.
Collapse
|
49
|
[Myiasis in a skin tumor]. EPIDEMIOLOGIE, MIKROBIOLOGIE, IMUNOLOGIE : CASOPIS SPOLECNOSTI PRO EPIDEMIOLOGII A MIKROBIOLOGII CESKE LEKARSKE SPOLECNOSTI J.E. PURKYNE 1998; 47:32-4. [PMID: 9511285] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Subscribe] [Scholar Register] [Indexed: 02/06/2023]
Abstract
A case of myiasis of a cutaneous tumour, a melanoblastoma in the right axilla of an 87-year old woman is described. After two years of illness in the last months of life an attack of the tumour by larvae of the fly Lucilia sericata (Meigen, 1826) was observed. This fly is the most frequent causal agent of tissue, cavity, ocular and urogenital human myiasis in Central Europe. Usually, injured or seriously ill patients are attacked by the larvae in urban as well as rural conditions. This is the first described case of myiasis located in a tumour.
Collapse
|
50
|
The role of the National Institute of Public Health in the field of infections with natural focality. Cent Eur J Public Health 1996; 4:123-6. [PMID: 8996726] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/03/2023]
Abstract
In the post-war period the National Institute of Public Health, later Institutes of Epidemiology and Microbiology, headed by K. Raska, ranked among famous laboratories in the world due to its priority findings and original results. Research results of the Institute stimulated further research not only in Czechoslovakia but also abroad, in laboratories of Europe and America. The authors emphasize the significance of certain results in the epidemiology and ecology of infections characterized by natural focality. In the first place they discuss the isolation of TBE in 1948 and 1949 by Gallia et al., and the study of the role of birds and bats as hosts of TBE. Significant for the recognition of zoonotic influenza viruses are papers by Tůmová, and as regards rabies in rodents the studies of Sodja et al. The institute paid attention to the introduction of Coxiella burnetii into the north-west of Bohemia. The institute's activities in the study of tularaemia, leptospirosis, Lyme borreliosis, and toxoplasmosis are also described. Raska's concept of epidemiological surveillance in the prevention of zoonoses with natural focality was fully enforced by workers of the institute. Many results of the Institute have been adopted by the WHO; it was demonstrated that it is possible by appropriate methods not only to detect human diseases in places where they are known but also to discover them in nature extensively altered by man.
Collapse
|