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Revealing the charge density in solids
KTH, School of Information and Communication Technology (ICT), Microelectronics and Applied Physics, MAP.ORCID iD: 0000-0002-3086-9642
KTH, School of Information and Communication Technology (ICT).
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(English)Manuscript (Other academic)
National Category
Other Engineering and Technologies not elsewhere specified
Identifiers
URN: urn:nbn:se:kth:diva-8062OAI: oai:DiVA.org:kth-8062DiVA: diva2:13283
Note
QC 20100702Available from: 2007-12-12 Created: 2007-12-12 Last updated: 2010-07-02Bibliographically approved
In thesis
1. Adatoms, Quasiparticles & Photons: The Multifaceted World of Photoelectron Spectroscopy
Open this publication in new window or tab >>Adatoms, Quasiparticles & Photons: The Multifaceted World of Photoelectron Spectroscopy
2007 (English)Doctoral thesis, comprehensive summary (Other scientific)
Abstract [en]

The experimental work presented in this thesis is based on a wide assortment of very advanced and highly sophisticated photoelectron spectroscopy (PES) techniques. The objective of the present study has been to reveal and understand the electronic structure and electron dynamics in a broad spectrum of materials, ranging from wide band gap oxides, via semiconductors along with metals, and finally high-temperature superconductors.

The first part of the thesis concerns laser-based pump-and-probe PES. This unique experimental technique has permitted a study of the excited electronic structure and the electron dynamics of several semiconductor surfaces. An insight into details of the adatom to restatom charge-transfer of the Ge(111)c(2x8) surface is presented, as well as an estimate for the timescale in which the dynamic adatoms of the Ge(111):Sn(sqrt3xsqrt3)R30deg surface operate. Further results comprise a novel unoccupied surface state at the GaSb(001) surface as well as a time-resolved study of the charge accumulation layer at the InAs(111)A/B surfaces.

In the second part, high-resolution synchrotron based angle-resolved PES (ARPES) data from the cuprate high-temperature superconductor La(2-x)Sr(x)CuO(4) (LSCO) is presented. This extensive study, reveals detailed information about how the Fermi surface and electronic excitations evolve with doping in the superconducting state. The results comprise support for a connection between high- and low-energy electronic responses, the characteristics of the superconducting gap, and indication of a quantum phase transition between two different superconducting phases.

In the third group of experiments we move away from the two-dimensional systems and concentrate on fully three-dimensional compounds. By the use of soft x-ray ARPES it is possible to extract the three-dimensional electronic structure in a straightforward manner with increased k(perpendicular)-resolution. As a result the first high-quality ARPES data from Cu2O is presented, as well as a novel method for extracting the (real space) electron density by ARPES. These experiments clearly display the advantages of using soft x-ray ARPES. If the material and type of experiment is chosen wisely, the benefit of the increased k||-window and the free electron final state, surpass the drawbacks of decreased count-rate and inferior energy resolution. Finally we return to the high-temperature superconductors (NCCO & Nd-LSCO) and make use of the increased bulk-sensitivity. From an evident change in the shape of the Fermi surface when moving from low to high photon energies, the durface to bulk difference in electronic structure is highlighted.

Place, publisher, year, edition, pages
Stockholm: KTH, 2007. xvi, 54 p.
Series
Trita-ICT/MAP, 2007:12
Keyword
angle-resolved photoelectron spectroscopy, electronic structure, electron dynamics, semiconductor surfaces, oxides, high-temperature superconductors
National Category
Other Engineering and Technologies not elsewhere specified
Identifiers
urn:nbn:se:kth:diva-4659 (URN)978-91-7178-822-1 (ISBN)
Public defence
2007-12-13, N1, Isafjordsgatan 28 A/D, Kista, 10:15
Opponent
Supervisors
Note
QC 20100810Available from: 2007-12-12 Created: 2007-12-12 Last updated: 2010-08-10Bibliographically approved
2. Investigations of Transition Metal Systems: Angle-Resolved Photoelectron Spectroscopy at Low and High Energy
Open this publication in new window or tab >>Investigations of Transition Metal Systems: Angle-Resolved Photoelectron Spectroscopy at Low and High Energy
2007 (English)Doctoral thesis, comprehensive summary (Other scientific)
Abstract [en]

The work presented in this thesis aims at achieving an increased understanding of the electronic structure of materials whose properties are to a large extent governed by transition metals. All studies are based on data from angle-resolved photoelectron spectroscopy (ARPES) employed on the valence band of the studied systems. In some cases spectroscopic data have also been combined with numerical electronic structure studies performed using a density functional theory computer code.

Five of the studies make extensive use of the benefits provided by performing angle-resolved photoelectron spectroscopy at high photon energies. When probed with soft x-ray photons, the near Fermi edge distribution of spectral weight in Nd2-xCexCuO4 and La2-x-yNdySrxCuO4 display significant differences as compared to similar studies performed in the 20-100 eV range of photon energies. This effect can be attributed to the approximate two-fold increase in probing depth as compared to the low photon energy case.

However, high photon energies not only give increased probing depth, it also enables a straightforward interpretation of ARPES data from fully three-dimensional compounds in terms of valence band energy dispersion. This principle has been used to study the dispersions in the transition metal oxides CoO and Cu2O as well as in a proof-of-principle study, where copper was used as a test case.

CoO exhibits a magnetic phase transition at the Néel temperature, from a paramagnetic state into an antiferromagnetically ordered state. The presented data show a significant shift of spectral weight between different band structure regions as the sample temperature is elevated across the Néel temperature.

The Cu2O ARPES data have confirmed the existence of a hybridized Cu 3d - Cu 4s state. Through comparision with theoretically calculated results, a refined value of the Hubbard potential U has been determined.

An extensive ARPES dataset from copper, covering the entire Brillouin zone, has in a proof-of-principle study been used to derive the binding-energy resolved valence band charge density.

Two ARPES studies of the La2-x-yNdySrxCuO4 system have been performed in the cnventional low photon energy range. The high energy resolution has enabled a detailed study of dispersive features in the near Fermil level region of the valence band.

Place, publisher, year, edition, pages
Stockholm: KTH, 2007. x, 38 p.
Series
Trita-ICT/MAP, 2007:9
Keyword
angle-resolved photoelectron spectroscopy, electronic structure, oxides, high-temperature superconductors
National Category
Other Engineering and Technologies not elsewhere specified
Identifiers
urn:nbn:se:kth:diva-4568 (URN)978-91-7178-783-5 (ISBN)
Public defence
2007-12-11, Sal N2, KTH-Electrum 3, Isafjordsgatan 28 A/D, Kista, 10:00
Opponent
Supervisors
Available from: 2007-12-07 Created: 2007-12-07 Last updated: 2010-07-02Bibliographically approved

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Tjernberg, Oscar

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