60 research outputs found
High-energy photoemission final states beyond the free-electron approximation
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
Dispersive charge density wave excitations and temperature dependent commensuration in Bi2Sr2CaCu2O8+{\delta}
Experimental evidence on high-Tc cuprates reveals ubiquitous charge density
wave (CDW) modulations, which coexist with superconductivity. Although the CDW
had been predicted by theory, important questions remain about the extent to
which the CDW influences lattice and charge degrees of freedom and its
characteristics as functions of doping and temperature. These questions are
intimately connected to the origin of the CDW and its relation to the
mysterious cuprate pseudogap. Here, we use ultrahigh resolution resonant
inelastic x-ray scattering (RIXS) to reveal new CDW character in underdoped
Bi2Sr2CaCu2O8+{\delta} (Bi2212). At low temperature, we observe dispersive
excitations from an incommensurate CDW that induces anomalously enhanced phonon
intensity, unseen using other techniques. Near the pseudogap temperature T*,
the CDW persists, but the associated excitations significantly weaken and the
CDW wavevector shifts, becoming nearly commensurate with a periodicity of four
lattice constants. The dispersive CDW excitations, phonon anomaly, and
temperature dependent commensuration provide a comprehensive momentum space
picture of complex CDW behavior and point to a closer relationship with the
pseudogap state
ARPES: A probe of electronic correlations
Angle-resolved photoemission spectroscopy (ARPES) is one of the most direct
methods of studying the electronic structure of solids. By measuring the
kinetic energy and angular distribution of the electrons photoemitted from a
sample illuminated with sufficiently high-energy radiation, one can gain
information on both the energy and momentum of the electrons propagating inside
a material. This is of vital importance in elucidating the connection between
electronic, magnetic, and chemical structure of solids, in particular for those
complex systems which cannot be appropriately described within the
independent-particle picture. Among the various classes of complex systems, of
great interest are the transition metal oxides, which have been at the center
stage in condensed matter physics for the last four decades. Following a
general introduction to the topic, we will lay the theoretical basis needed to
understand the pivotal role of ARPES in the study of such systems. After a
brief overview on the state-of-the-art capabilities of the technique, we will
review some of the most interesting and relevant case studies of the novel
physics revealed by ARPES in 3d-, 4d- and 5d-based oxides.Comment: Chapter to appear in "Strongly Correlated Systems: Experimental
Techniques", edited by A. Avella and F. Mancini, Springer Series in
Solid-State Sciences (2013). A high-resolution version can be found at:
http://www.phas.ubc.ca/~quantmat/ARPES/PUBLICATIONS/Reviews/ARPES_Springer.pdf.
arXiv admin note: text overlap with arXiv:cond-mat/0307085,
arXiv:cond-mat/020850
Spin-Orbital Separation in the quasi 1D Mott-insulator Sr2CuO3
As an elementary particle the electron carries spin \hbar/2 and charge e.
When binding to the atomic nucleus it also acquires an angular momentum quantum
number corresponding to the quantized atomic orbital it occupies (e.g., s, p or
d). Even if electrons in solids form bands and delocalize from the nuclei, in
Mott insulators they retain their three fundamental quantum numbers: spin,
charge and orbital[1]. The hallmark of one-dimensional (1D) physics is a
breaking up of the elementary electron into its separate degrees of freedom[2].
The separation of the electron into independent quasi-particles that carry
either spin (spinons) or charge (holons) was first observed fifteen years
ago[3]. Using Resonant Inelastic X-ray Scattering on the 1D Mott-insulator
Sr2CuO3 we now observe also the orbital degree of freedom separating. We
resolve an orbiton liberating itself from spinons and propagating through the
lattice as a distinct quasi-particle with a substantial dispersion of ~0.2 eV.Comment: 35 pages, 8 figure
Oxygen redox chemistry without excess alkali-metal ions in Na[MgMn]O
The search for improved energy-storage materials has revealed Li- and Na-rich intercalation compounds as promising high-capacity cathodes. They exhibit capacities in excess of what would be expected from alkali-ion removal/reinsertion and charge compensation by transition-metal (TM) ions. The additional capacity is provided through charge compensation by oxygen redox chemistry and some oxygen loss. It has been reported previously that oxygen redox occurs in O 2 orbitals that interact with alkali ions in the TM and alkali-ion layers (that is, oxygen redox occurs in compounds containing Li–O(2)–Li interactions). Na[MgMn]O exhibits an excess capacity and here we show that this is caused by oxygen redox, even though Mg resides in the TM layers rather than alkali-metal (AM) ions, which demonstrates that excess AM ions are not required to activate oxygen redox. We also show that, unlike the alkali-rich compounds, Na[MgMn]O does not lose oxygen. The extraction of alkali ions from the alkali and TM layers in the alkali-rich compounds results in severely underbonded oxygen, which promotes oxygen loss, whereas Mg remains in Na[MgMn]O, which stabilizes oxygen
GaSb/GaAs quantum dot systems
GaSb/GaAs quantum dot systems : in situ synchrotron radiation x-ray photoelectron spectroscopy study / R. Claessen ... - In: Nanotechnology. 16. 2005. S. 1326-133
k-space imaging of anisotropic 2D electron gas in GaN/GaAlN high-electron-mobility transistor heterostructures
A case study of novel X-ray Optics for FEL sources
We suggest optical schemes for the European X-ray Free Electron Laser facility (XFEL.EU) in Hamburg: a single element X-ray spectrometer on the basis of a reflection zone plate (RZP) for single-shot diagnostics; and a two-element soft X-ray spectrometer on the basis of two RZPs to carry out Resonant Inelastic X-ray Scattering (RIXS) experiments. With this setup, a full map of the sample spectrum is obtainable in a single measurement. The main advantage of using zone plates is the possibility to enable dispersion and focusing in one step. Moreover, highest possible X-ray transmission is achieved by using the minimum number of optical elements. Taking into account the European XFEL beam parameters, our simulations, concerning the RIXS experiment, produced very promising results, reaching an energy resolution (E/ΔE) of up to 30,000 at photon energy of 1 keV. When applied as a single shot spectrometer the energy resolution for RZP is of the same order of magnitude
Nanoscale measurements of unoccupied band dispersion in few-layer graphene
The properties of any material are fundamentally determined by its electronic band structure. Each band represents a series of allowed states inside a material, relating electron energy and momentum. The occupied bands, that is, the filled electron states below the Fermi level, can be routinely measured. However, it is remarkably difficult to characterize the empty part of the band structure experimentally. Here, we present direct measurements of unoccupied bands of monolayer, bilayer and trilayer graphene. To obtain these, we introduce a technique based on low-energy electron microscopy. It relies on the dependence of the electron reflectivity on incidence angle and energy and has a spatial resolution ∼10 nm. The method can be easily applied to other nanomaterials such as van der Waals structures that are available in small crystals only
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