| 1. |
- Kramer, K. P., et al.
(författare)
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Band structure of overdoped cuprate superconductors: Density functional theory matching experiments
- 2019
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Ingår i: Physical Review B. - 2469-9969 .- 2469-9950. ; 99:22
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Tidskriftsartikel (refereegranskat)abstract
- A comprehensive angle-resolved photoemission spectroscopy study of the band structure in singlelayer cuprates is presented with the aim of uncovering universal trends across different materials. Five different hole-and electron-overdoped cuprate superconductors (La1.59Eu0.2Sr0.21CuO4, La1.77Sr0.23CuO4, Bi1.74Pb0.38Sr1.88CuO6+delta, Tl2Ba2CuO6+delta, and Pr1.15La0.7Ce0.15CuO4) have been studied with special focus on the bands with a predominately d-orbital character. Using a light polarization analysis, the e(g) and t(2g) bands are identified across these materials. A clear correlation between the d(3z2-r2) band energy and the apical oxygen distance d(A) is demonstrated. Moreover, the compound dependence of the d(x2-y2) band bottom and the t(2g) band top is revealed. A direct comparison to density functional theory (DFT) calculations employing hybrid exchange-correlation functionals demonstrates excellent agreement. We thus conclude that the DFT methodology can be used to describe the global band structure of overdoped single-layer cuprates on both the hole-and electron-doped side.
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| 2. |
- Matt, C. E., et al.
(författare)
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Direct observation of orbital hybridisation in a cuprate superconductor
- 2018
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Ingår i: Nature Communications. - : Nature Publishing Group. - 2041-1723. ; 9
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Tidskriftsartikel (refereegranskat)abstract
- The minimal ingredients to explain the essential physics of layered copper-oxide (cuprates) materials remains heavily debated. Effective low-energy single-band models of the copper-oxygen orbitals are widely used because there exists no strong experimental evidence supporting multi-band structures. Here, we report angle-resolved photoelectron spectroscopy experiments on La-based cuprates that provide direct observation of a two-band structure. This electronic structure, qualitatively consistent with density functional theory, is parametrised by a two-orbital (d(x2-y2) and d(z2)) tight-binding model. We quantify the orbital hybridisation which provides an explanation for the Fermi surface topology and the proximity of the van-Hove singularity to the Fermi level. Our analysis leads to a unification of electronic hopping parameters for single-layer cuprates and we conclude that hybridisation, restraining d-wave pairing, is an important optimisation element for superconductivity.
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| 3. |
- Rubensson, Jan-Erik, et al.
(författare)
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Rydberg-Resolved Resonant Inelastic Soft X-Ray Scattering: Dynamics at Core Ionization Thresholds
- 2015
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Ingår i: Physical Review Letters. - 1079-7114 .- 0031-9007. ; 114:13
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Tidskriftsartikel (refereegranskat)abstract
- Resonant inelastic x-ray scattering spectra excited in the immediate vicinity of the core-level ionization thresholds of N-2 have been recorded. Final states of well-resolved symmetry-selected Rydberg series converging to valence-level ionization thresholds with vibrational excitations are observed. The results are well described by a quasi-two-step model which assumes that the excited electron is unaffected by the radiative decay. This threshold dynamics simplifies the interpretation of resonant inelastic x-ray scattering spectra considerably and facilitates characterization of low-energy excited final states in molecular systems.
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| 4. |
- Sassa, Y., et al.
(författare)
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Probing two- and three-dimensional electrons in MgB2 with soft x-ray angle-resolved photoemission
- 2015
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Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 91:4, s. 045114-
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Tidskriftsartikel (refereegranskat)abstract
- electronic band structure of MgB2 involves a unique combination of two-and three-dimensional (3D) electrons derived from the boron sigma and pi states, respectively. We have mapped out the sigma and pi bands over the complete Brillouin zone, including the full disconnected Fermi surface, using high-resolution soft x-ray angle-resolved photoelectron spectroscopy. The measured band structure, which is closely related to that of graphene, is in overall good agreement with the density functional theory-general gradient approximation (DFT-GGA), though differences in Fermi surface volume are seen. Surprisingly, the measured bands are wider than calculated, by similar to 8% for the s and similar to 10-15% for the p bands. This solves the long-standing challenge of establishing the full 3D electronic structure of the model compound MgB2, and it demonstrates the tendency of DFT-GGA to overestimate the band narrowing due to exchange correlations effects.
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| 5. |
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| 6. |
- Bouravleuv, A. D., et al.
(författare)
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Electronic structure of (In, Mn) As quantum dots buried in GaAs investigated by soft-x-ray ARPES
- 2016
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Ingår i: Nanotechnology. - : IOP Publishing. - 0957-4484 .- 1361-6528. ; 27:42
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Tidskriftsartikel (refereegranskat)abstract
- Electronic structure of a molecular beam epitaxy-grown system of (In, Mn) As quantum dots (QDs) buried in GaAs is explored with soft-x-ray angle-resolved photoelectron spectroscopy (ARPES) using photon energies around 1 keV. This technique, ideally suited for buried systems, extends the momentum-resolving capabilities of conventional ARPES with enhanced probing depth as well as elemental and chemical state specificity achieved with resonant photoexcitation. The experimental results resolve the dispersive energy bands of the GaAs substrate buried in similar to 2 nm below the surface, and the impurity states (ISs) derived from the substitutional Mn atoms in the (In, Mn) As QDs and oxidized Mn atoms distributed near the surface. An energy shift of the Mn ISs in the QDs compared to (In, Mn) As DMS is attributed to the band offset and proximity effect at the interface with the surrounding GaAs. The absence of any ISs in the vicinity of the VBM relates the electron transport in (In, Mn) As QDs to the prototype (In, Mn) As diluted magnetic semiconductor. The SX-ARPES results are supported by measurements of the shallow core levels under variation of probing depth through photon energy. X-ray absorption measurements identify significant diffusion of interstitial Mn atoms out of the QDs towards the surface, and the role of magnetic circular dichroism is to block the ferromagnetic response of the (In, Mn) As QDs. Possible routes are drawn to tune the growth procedure aiming at practical applications of the (In, Mn) As based systems.
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| 7. |
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| 8. |
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| 9. |
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| 10. |
- Brauer, H.E., et al.
(författare)
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Na intercalation of VSe2 studied by photoemission and scanning tunneling microscopy
- 1997
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Ingår i: Physical Review B Condensed Matter. - 0163-1829 .- 1095-3795. ; 55:15, s. 10022-10026
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Tidskriftsartikel (refereegranskat)abstract
- In situ Na intercalation of the layered compound VSe2 has been studied with photoemission and scanning tunneling microscopy. Core-level spectroscopy proves that Na deposited in UHV onto the VSe2 surface rapidly intercalates, leaving only small amounts at the surface. The scanning tunneling microscopy measurements show that the intercalated Na is not uniformly distributed between the VSe2 layers, but preferentially in two-dimensional islands. Thus the surface region is divided into intercalated and nonintercalated areas. Hole-like features in the intercalated areas are interpreted as locally missing Na.
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| 11. |
- Chikina, Alla, et al.
(författare)
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Band-Order Anomaly at the gamma-Al2O3/SrTiO3 Interface Drives the Electron-Mobility Boost
- 2021
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Ingår i: ACS Nano. - : American Chemical Society (ACS). - 1936-0851 .- 1936-086X. ; 15:3, s. 4347-4356
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Tidskriftsartikel (refereegranskat)abstract
- The rich functionalities of transition-metal oxides and their interfaces bear an enormous technological potential. Its realization in practical devices requires, however, a significant improvement of yet relatively low electron mobility in oxide materials. Recently, a mobility boost of about 2 orders of magnitude has been demonstrated at the spinel-perovskite gamma-Al2O3/SrTiO3 interface compared to the paradigm perovskite-perovskite LaAlO3/SrTiO3 interface. We explore the fundamental physics behind this phenomenon from direct measurements of the momentum-resolved electronic structure of this interface using resonant soft-X-ray angle-resolved photoemission. We find an anomaly in orbital ordering of the mobile electrons in gamma-Al2O3/SrTiO3 which depopulates electron states in the top SrTiO3 layer. This rearrangement of the mobile electron system pushes the electron density away from the interface, which reduces its overlap with the interfacial defects and weakens the electron-phonon interaction, both effects contributing to the mobility boost. A crystal-field analysis shows that the band order alters owing to the symmetry breaking between the spinel gamma-Al2O3 and perovskite SrTiO3. Band-order engineering, exploiting the fundamental symmetry properties, emerges as another route to boost the performance of oxide devices.
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| 12. |
- Glawion, S., et al.
(författare)
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Two-Spinon and Orbital Excitations of the Spin-Peierls System TiOCl
- 2011
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Ingår i: Physical Review Letters. - 0031-9007 .- 1079-7114. ; 107:10, s. 107402-
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Tidskriftsartikel (refereegranskat)abstract
- We combine high-resolution resonant inelastic x-ray scattering with cluster calculations utilizing a recently derived effective magnetic scattering operator to analyze the polarization, excitation energy, and momentum-dependent excitation spectrum of the low-dimensional quantum magnet TiOCl in the range expected for orbital and magnetic excitations (0-2.5 eV). Ti 3d orbital excitations yield complete information on the temperature-dependent crystal-field splitting. In the spin-Peierls phase we observe a dispersive two-spinon excitation and estimate the inter- and intradimer magnetic exchange coupling from a comparison to cluster calculations.
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| 13. |
- Horio, M., et al.
(författare)
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Three-Dimensional Fermi Surface of Overdoped La-Based Cuprates
- 2018
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Ingår i: Physical Review Letters. - : American Physical Society. - 0031-9007 .- 1079-7114. ; 121:7
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Tidskriftsartikel (refereegranskat)abstract
- We present a soft x-ray angle-resolved photoemission spectroscopy study of overdoped high-temperature superconductors. In-plane and out-of-plane components of the Fermi surface are mapped by varying the photoemission angle and the incident photon energy. No k(z) dispersion is observed along the nodal direction, whereas a significant antinodal k(z) dispersion is identified for La-based cuprates. Based on a tight-binding parametrization, we discuss the implications for the density of states near the van Hove singularity. Our results suggest that the large electronic specific heat found in overdoped La2-xSrxCuO4 cannot be assigned to the van Hove singularity alone. We therefore propose quantum criticality induced by a collapsing pseudogap phase as a plausible explanation for observed enhancement of electronic specific heat.
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| 14. |
- Pietzsch, A, et al.
(författare)
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Snapshots of the Fluctuating Hydrogen Bond Network in Liquid Water on the Sub-Femtosecond Timescale with Vibrational Resonant Inelastic x-ray Scattering.
- 2015
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Ingår i: Physical Review Letters. - 1079-7114. ; 114:8
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Tidskriftsartikel (refereegranskat)abstract
- Liquid water molecules interact strongly with each other, forming a fluctuating hydrogen bond network and thereby giving rise to the anomalous phase diagram of liquid water. Consequently, symmetric and asymmetric water molecules have been found in the picosecond time average with IR and optical Raman spectroscopy. With subnatural linewidth resonant inelastic x-ray scattering (RIXS) at vibrational resolution, we take sub-femtosecond snapshots of the electronic and structural properties of water molecules in the hydrogen bond network. We derive a strong dominance of nonsymmetric molecules in liquid water in contrast to the gas phase on the sub-femtosecond timescale of RIXS and determine the fraction of highly asymmetrically distorted molecules.
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| 15. |
- Pietzsch, Annette, et al.
(författare)
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Spatial Quantum Beats in Vibrational Resonant Inelastic Soft X-Ray Scattering at Dissociating States in Oxygen
- 2011
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Ingår i: Physical Review Letters. - 0031-9007 .- 1079-7114. ; 106, s. 153004-153008
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Tidskriftsartikel (refereegranskat)abstract
- Resonant inelastic soft x-ray scattering (RIXS) spectra excited at the 1σg→3σu resonance in gas-phase O2 show excitations due to the nuclear degrees of freedom with up to 35 well-resolved discrete vibronic states and a continuum due to the kinetic energy distribution of the separated atoms. The RIXS profile demonstrates spatial quantum beats caused by two interfering wave packets with different momenta as the atoms separate. Thomson scattering strongly affects both the spectral profile and the scattering anisotropy.
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| 16. |
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| 17. |
- Strocov, V.N., et al.
(författare)
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Band structure effects in nitrogen K-edge resonant inelastic X-ray scattering from GaN
- 2004
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Ingår i: Physica status solidi. B, Basic research. - : Wiley. - 0370-1972 .- 1521-3951. ; 241:7
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Tidskriftsartikel (refereegranskat)abstract
- Systematic experimental data on resonant inelastic X-ray scattering (RIXS) in GaN near the N K-edge are presented for the first time. Excitation energy dependence of the spectral structures manifests the band structure effects originating from momentum selectivity of the RIXS process. This finding allows obtaining k-resolved band structure information for GaN crystals and nanostructures. © 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
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| 18. |
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| 19. |
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| 20. |
- Strocov, V. N., et al.
(författare)
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GaSb/GaAs quantum dot systems: In situ synchrotron radiation x-ray photoelectron spectroscopy study
- 2005
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Ingår i: Nanotechnology. - : IOP Publishing. - 0957-4484 .- 1361-6528. ; 16:8, s. 1326-1334
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Tidskriftsartikel (refereegranskat)abstract
- GaSb/GaAs quantum dot systems are fabricated using MBE under various growth modes. The as-grown samples are studied with in situ synchrotron radiation XPS covering the As 3d, Sb 4d and Ga 3d core levels and the valence band region. The XPS spectra show dramatic changes with the growth modes, reflecting changes in the local electronic structure and chemical environments of the surface and interface atoms in both quantum dots and wetting layer. A quantum dot specific contribution near the valence band maximum is identified and related to the hole accumulation process. Local valence band offsets measured in the GaSb/GaAs systems evolve over the interface region and depend on the growth modes, which adds another degree of freedom to band engineering on the nanoscale.
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| 21. |
- Strocov, V.N., et al.
(författare)
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Momentum selectivity and anisotropy effects in the nitrogen K -edge resonant inelastic x-ray scattering from GaN
- 2005
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Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 72:8
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Tidskriftsartikel (refereegranskat)abstract
- High-resolution soft x-ray emission and absorption spectra near the N K -edge of wurtzite GaN are presented. The experimental data are interpreted in terms of full-potential electronic structure calculations. The absorption spectra, compared with calculations including core hole screening, indicate partial core hole screening in the absorption process. The resonantly excited x-ray emission spectra show pronounced dispersion of spectral structures, which is attributed to effects of momentum conservation in the resonant inelastic x-ray scattering (RIXS) process. In development of GaN based optoelectronics, momentum selectivity in RIXS can be utilized to control development of band structure in GaN nanostructures. © 2005 The American Physical Society.
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| 22. |
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| 23. |
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| 24. |
- Sun, Yu-Ping, et al.
(författare)
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Internal symmetry and selection rules in resonant inelastic soft x-ray scattering
- 2011
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Ingår i: Journal of Physics B. - : IOP Publishing. - 0953-4075 .- 1361-6455. ; 44:16, s. 161002-
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Tidskriftsartikel (refereegranskat)abstract
- Resonant inelastic soft x-ray scattering spectra excited at the dissociative 1 sigma(g) -> 3 sigma(u) resonance in gas-phase O(2) are presented and discussed in terms of state-of-the-art molecular theory. A new selection rule due to internal spin coupling is established, facilitating a deep analysis of the valence excited final states. Furthermore, it is found that a commonly accepted symmetry selection rule due to orbital parity breaks down, as the core hole and excited electron swap parity, thereby opening the symmetry forbidden 3 sigma(g) decay channel.
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