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Consequences of a non-trivial band-structure topology in solids: Investigations of topological surface and interface states
KTH, School of Information and Communication Technology (ICT), Materials- and Nano Physics.ORCID iD: 0000-0002-7833-3943
2013 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

The development and characterization of experimental setups for angle-resolved photoelectron spectroscopy (ARPES) and spin- and angle-resolved photoelectron spectroscopy (SARPES) is described. Subsequently, the two techniques are applied to studies of the electronic band structure in topologically non-trivial materials.

The laser-based ARPES setup works at a photon energy of 10.5 eV and a typical repetition rate in the range 200 kHz to 800 kHz. By using a time-of-flight electron energy analyzer electrons emitted from the sample within a solid angle of up to ±15 degrees can be collected and analyzed simultaneously. The SARPES setup is equipped with a traditional hemispherical electron energy analyzer in combination with a mini-Mott electron polarimeter. The system enables software-controlled switching between angle-resolved spin-integrated and spin-resolved measurements, thus providing the possibility to orient the sample by mapping out the electronic band structure using ARPES before performing spin-resolved measurements at selected points in the Brillouin zone.

Thin films of the topological insulators (TIs) Bi2Se3, Bi2Te3 and Sb2Te3 are grown using e-beam evaporation and their surface states are observed by means of ARPES. By using a combination of low photon energies and cryogenic sample temperatures the topological states originating from both the vacuum interface (surface) and the substrate interface are observed in Bi2Se3 films and Bi2Se3/Bi2Te3 heterostructures, with total thicknesses in the ultra-thin limit (six to eight quintuple layers), grown on Bi-terminated Si(111) substrates. Band alignment between Si and Bi2Se3 at the interface creates a band bending through the films. The band bending is found to be independent of the Fermi level (EF) position in the bulk of the substrate, suggesting that the surface pinning of EF in the Si(111) substrate remains unaltered after deposition of the TI films. Therefore, the type and level of doping of the substrate does not show any large influence on the size of the band bending.

Further, we provide experimental evidence for the realization of a topological crystalline insulator (TCI) phase in the narrow-band semiconductor Pb1−xSnxSe. The TCI phase exists for temperatures below the transition temperature Tc and is characterized by an inverted bulk band gap accompanied by the existence of non-gapped surface states crossing the band gap. Above Tc the material is in a topologically trivial phase where the surface states are gapped. Thus, when lowering the sample temperature across Tc a topological phase transition from a trivial insulator to a TCI is observed. SARPES studies indicate a helical spin structure of the surface states both in the topologically trivial and the TCI phase.

Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology, 2013. , x, 82 p.
Series
Trita-ICT/MAP AVH, ISSN 1653-7610 ; 2013:02
Keyword [en]
spin- and angle-resolved photoelectron spectroscopy, time-of-flight analyzer, laser based light source, topological insulator, topological crystalline insulator, thin films, surface state, interface state, Bi2Se3, Pb1-xSnxSe
National Category
Physical Sciences Condensed Matter Physics
Identifiers
URN: urn:nbn:se:kth:diva-121974ISBN: 978-91-7501-735-8 (print)OAI: oai:DiVA.org:kth-121974DiVA: diva2:619836
Public defence
2013-05-31, Sal D, KTH-Forum, Isafjordsgatan 39, Kista, 10:00 (English)
Opponent
Supervisors
Funder
Swedish Research CouncilKnut and Alice Wallenberg Foundation
Note

QC 20130507

Available from: 2013-05-07 Created: 2013-05-06 Last updated: 2013-05-07Bibliographically approved
List of papers
1. A spin- and angle-resolving photoelectron spectrometer
Open this publication in new window or tab >>A spin- and angle-resolving photoelectron spectrometer
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2010 (English)In: Review of Scientific Instruments, ISSN 0034-6748, E-ISSN 1089-7623, Vol. 81, no 3Article in journal (Refereed) Published
Abstract [en]

A new type of hemispherical electron energy analyzer that permits angle and spin resolved photoelectron spectroscopy has been developed. The analyzer permits standard angle resolved spectra to be recorded with a two-dimensional detector in parallel with spin detection using a mini-Mott polarimeter. General design considerations as well as technical solutions are discussed and test results from the Au(111) surface state are presented.

Keyword
photoelectron spectroscopy, spectroscopy, analyzer
National Category
Engineering and Technology
Identifiers
urn:nbn:se:kth:diva-19360 (URN)10.1063/1.3342120 (DOI)000276210200058 ()2-s2.0-77950563091 (Scopus ID)
Funder
Knut and Alice Wallenberg FoundationSwedish Research Council
Note
QC 20100525Available from: 2010-08-05 Created: 2010-08-05 Last updated: 2017-12-12Bibliographically approved
2. An experimental setup for high resolution 10.5 eV laser-based angle-resolved photoelectron spectroscopy using a time-of-flight electron analyzer
Open this publication in new window or tab >>An experimental setup for high resolution 10.5 eV laser-based angle-resolved photoelectron spectroscopy using a time-of-flight electron analyzer
2011 (English)In: Review of Scientific Instruments, ISSN 0034-6748, E-ISSN 1089-7623, Vol. 82, no 9, 095113- p.Article in journal (Refereed) Published
Abstract [en]

We present an experimental setup for laser-based angle-resolved time-of-flight photoemission. Using a picosecond pulsed laser, photons of energy 10.5 eV are generated through higher harmonic generation in xenon. The high repetition rate of the light source, variable between 0.2 and 8 MHz, enables high photoelectron count rates and short acquisition times. By using a time-of-flight analyzer with angle-resolving capabilities, electrons emitted from the sample within a circular cone of up to +/- 15 degrees can be collected. Hence, simultaneous acquisition of photoemission data for a complete area of the Brillouin zone is possible. The current photon energy enables bulk sensitive measurements, high angular resolution, and the resulting covered momentum space is large enough to enclose the entire Brillouin zone in cuprate high-T(c) superconductors. Fermi edge measurements on polycrystalline Au shows an energy resolution better than 5 meV. Data from a test measurement of the Au(111) surface state are presented along with measurements of the Fermi surface of the high-T(c) superconductor Bi(2)Sr(2)CaCu(2)O(8+delta) (Bi2212).

Keyword
Brillouin zones, Fermi surface, high-temperature superconductors, lasers, photoelectron spectroscopy, photoemission, time of flight spectroscopy
National Category
Physical Sciences
Identifiers
urn:nbn:se:kth:diva-46858 (URN)10.1063/1.3637464 (DOI)000295621100060 ()2-s2.0-80053541533 (Scopus ID)
Funder
Knut and Alice Wallenberg FoundationSwedish Research Council
Note

QC 20111107

Available from: 2011-11-07 Created: 2011-11-07 Last updated: 2017-12-08Bibliographically approved
3. Direct observation of decoupled Dirac states at the interface between topological and normal insulators
Open this publication in new window or tab >>Direct observation of decoupled Dirac states at the interface between topological and normal insulators
2013 (English)In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 88, no 19, 195132Article in journal (Refereed) Published
Abstract [en]

Several proposed applications and exotic effects in topological insulators rely on the presence of helical Dirac states at the interface between a topological insulator and a normal insulator. In the present work, we have used low-energy angle-resolved photoelectron spectroscopy to uncover and characterize the interface states of Bi2Se3 thin films and Bi2Te3/Bi2Se3 heterostructures grown on Si(111). The results establish that Dirac fermions are indeed present at the topological-normal-insulator boundary and absent at the topological-topological-insulator interface. Moreover, it is demonstrated that band bending present within the topological-insulator films leads to a substantial separation of the interface and surface states in energy. These results pave the way for further studies and the realization of interface-related phenomena in topological-insulator thin-film heterostructures.

Place, publisher, year, edition, pages
American Physical Society, 2013
Keyword
Topological insulator, bismuth selenide, interface state, surface state
National Category
Condensed Matter Physics
Identifiers
urn:nbn:se:kth:diva-121940 (URN)10.1103/PhysRevB.88.195132 (DOI)000327158600003 ()2-s2.0-84888309073 (Scopus ID)
Funder
Knut and Alice Wallenberg FoundationSwedish Research Council
Note

QC 20131217

Available from: 2013-05-06 Created: 2013-05-06 Last updated: 2017-12-06Bibliographically approved
4. Topological crystalline insulator states in Pb1-xSnxSe
Open this publication in new window or tab >>Topological crystalline insulator states in Pb1-xSnxSe
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2012 (English)In: Nature Materials, ISSN 1476-1122, E-ISSN 1476-4660, Vol. 11, no 12, 1023-1027 p.Article in journal (Refereed) Published
Abstract [en]

Topological insulators are a class of quantum materials in which time-reversal symmetry, relativistic effects and an inverted band structure result in the occurrence of electronic metallic states on the surfaces of insulating bulk crystals. These helical states exhibit a Dirac-like energy dispersion across the bulk bandgap, and they are topologically protected. Recent theoretical results have suggested the existence of topological crystalline insulators (TCIs), a class of topological insulators in which crystalline symmetry replaces the role of time-reversal symmetry in ensuring topological protection(1,2). In this study we show that the narrow-gap semiconductor Pb1-xSnxSe is a TCI for x = 0.23. Temperature-dependent angle-resolved photoelectron spectroscopy demonstrates that the material undergoes a temperature-driven topological phase transition from a trivial insulator to a TCI. These experimental findings add a new class to the family of topological insulators, and we anticipate that they will lead to a considerable body of further research as well as detailed studies of topological phase transitions.

Keyword
Inversion, Bands, Performance, Alloys, Phase, Snte
National Category
Engineering and Technology
Identifiers
urn:nbn:se:kth:diva-109176 (URN)10.1038/NMAT3449 (DOI)000311432600019 ()2-s2.0-84870037398 (Scopus ID)
Funder
Knut and Alice Wallenberg FoundationSwedish Research Council
Note

QC 20150623

Available from: 2013-01-07 Created: 2012-12-21 Last updated: 2017-12-06Bibliographically approved
5. Spin-polarized (001) surface states of the topological crystalline insulator Pb0.73Sn0.27Se
Open this publication in new window or tab >>Spin-polarized (001) surface states of the topological crystalline insulator Pb0.73Sn0.27Se
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2013 (English)In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 87, no 11, 115106- p.Article in journal (Refereed) Published
Abstract [en]

We study the nature of (001) surface states in Pb0.73Sn0.27Se in the newly discovered topological-crystalline-insulator (TCI) phase as well as the corresponding topologically trivial state above the band-gap-inversion temperature. Our calculations predict not only metallic surface states with a nontrivial chiral spin structure for the TCI case, but also nonmetallic (gapped) surface states with nonzero spin polarization when the system is a normal insulator. For both phases, angle- and spin-resolved photoelectron spectroscopy measurements provide conclusive evidence for the formation of these (001) surface states in Pb0.73Sn0.27Se, as well as for their chiral spin structure.

Keyword
Pb1-Xsnxse, Inversion, Snte, Alloys, Phase, Bands
National Category
Physical Sciences
Identifiers
urn:nbn:se:kth:diva-120162 (URN)10.1103/PhysRevB.87.115106 (DOI)000315731400002 ()2-s2.0-84874844135 (Scopus ID)
Funder
Knut and Alice Wallenberg FoundationSwedish Research CouncilEU, European Research Council
Note

QC 20150623

Available from: 2013-04-02 Created: 2013-04-02 Last updated: 2017-12-06Bibliographically approved

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