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Uhrberg, Roger
##### Publications (10 of 77) Show all publications
Wang, W., Olovsson, W. & Uhrberg, R. (2016). Band structure of hydrogenated silicene on Ag(111): Evidence for half-silicane. PHYSICAL REVIEW B, 93(8), 081406
Open this publication in new window or tab >>Band structure of hydrogenated silicene on Ag(111): Evidence for half-silicane
2016 (English)In: PHYSICAL REVIEW B, ISSN 2469-9950, Vol. 93, no 8, p. 081406-Article in journal (Refereed) Published
##### Abstract [en]

In the case of graphene, hydrogenation removes the conductivity due to the bands forming the Dirac cone by opening up a band gap. This type of chemical functionalization is of the utmost importance for electronic applications. As predicted by theoretical studies, a similar change in the band structure is expected for silicene, the closest analog to graphene. We here report a study of the atomic and electronic structures of hydrogenated silicene with hydrogen on one side, the so-called half-silicane. The ("2 root 3 x 2 root 3") phase of silicene on Ag(111) was used in this Rapid Communication since it can be formed homogeneously across the entire surface of the Ag substrate. Low-energy electron diffraction and scanning tunneling microscopy data clearly show that hydrogenation changes the structure of silicene on Ag(111) resulting in a (1 x 1) periodicity with respect to the silicene lattice. The hydrogenated silicene also exhibits a quasiregular ("2 root 3 x 2 root 3")-like arrangement of vacancies. Angle-resolved photoelectron spectroscopy revealed two dispersive bands which can be unambiguously assigned to half-silicane. The common top of these bands is located at similar to 0.9 eV below the Fermi level. We find that the experimental bands are closely reproduced by the theoretical band structure of free-standing silicene with H adsorbed on the upper hexagonal sublattice.

##### Place, publisher, year, edition, pages
AMER PHYSICAL SOC, 2016
##### National Category
Physical Sciences
##### Identifiers
urn:nbn:se:liu:diva-125672 (URN)10.1103/PhysRevB.93.081406 (DOI)000369729000001 ()
##### Note

Funding Agencies|Swedish Research Council (VR) [621-2010-3746, 621-2014-4764, 621-2011-4426]; Linkoping Linnaeus Initiative for Novel Functional Materials - Swedish Research Council (VR) [2008-6582]; Swedish National Infrastructure for Computing (SNIC)

Available from: 2016-03-02 Created: 2016-02-29 Last updated: 2016-03-29
Sohail, H. M. & Uhrberg, R. I. G. (2016). Electronic and atomic structures of a Sn induced 3√3x3√3 superstructure on the Ag/Ge(111) √3x√3 surface. Surface Science, 644, 29-33
Open this publication in new window or tab >>Electronic and atomic structures of a Sn induced 3√3x3√3 superstructure on the Ag/Ge(111) √3x√3 surface
2016 (English)In: Surface Science, ISSN 0039-6028, E-ISSN 1879-2758, Vol. 644, p. 29-33Article in journal (Refereed) Published
##### Abstract [en]

We have investigated sub-monolayer coverages of Sn on the Ag/Ge(111)  surface. It was found that ≈0.45 monolayer (ML) resulted in a new, well-defined, reconstruction with a $3\sqrt{3}x3\sqrt{3}$ periodicity. The periodic structure of the surface atoms was verified by low energy electron diffraction and scanning tunneling microscopy. The electronic structure was studied in detail using angle resolved photoelectron spectroscopy and core level spectroscopy at a temperature of 100 K. Several surface bands were identified and their dispersions are presented along the $\overline{M}$ and $\overline{K}$$\overline{M}$ high symmetry lines of the $3\sqrt{3}x3\sqrt{3}$ surface Brillouin zone (SBZ). The $3\sqrt{3}x3\sqrt{3}$ surface has a metallic character since there is a strong surface band crossing the Fermi level near -points coinciding with $\overline{K}$-points of the 1×1 SBZ. The Fermi contour of the metallic band showed a hexagonal shape in contrast to the circular shaped Fermi contour of the initial  surface. Both empty and filled state STM images showed a hexagonal arrangement of protrusions which show a local  periodicity and a superimposed modulation of the apparent heights with a $3\sqrt{3}x3\sqrt{3}$ periodicity.

Elsevier, 2016
Natural Sciences
##### Identifiers
urn:nbn:se:liu:diva-105221 (URN)10.1016/j.susc.2015.08.037 (DOI)000367489000005 ()
##### Funder
Swedish Research CouncilKnut and Alice Wallenberg Foundation
##### Note

Vid tiden för disputation förelåg publikationen endast som manuskript

Available from: 2014-03-13 Created: 2014-03-13 Last updated: 2018-03-21Bibliographically approved
Sohail, H. M. & Uhrberg, R. I. G. (2016). Experimental studies of an In/Pb binary surface alloy on Ge(111). Surface Science, 649, 146-151
Open this publication in new window or tab >>Experimental studies of an In/Pb binary surface alloy on Ge(111)
2016 (English)In: Surface Science, ISSN 0039-6028, E-ISSN 1879-2758, Vol. 649, p. 146-151Article in journal (Refereed) Published
##### Abstract [en]

In this study, we present a binary In/Pb surface alloy on Ge(111) formed by evaporating0.85 monolayer (ML) of In on the Pb/Ge(111)  surface with 1.33 ML of Pb. A welldefined3×3 periodicity is formed after annealing at a temperature of ≈200 °C, as verified by bothlow energy electron diffraction (LEED) and scanning tunneling microscopy (STM). OverviewSTM images, obtained at 50 K, show a clear 3×3 periodicity. Detailed STM images reveal thatthe protrusions consist of atomic sized features with a local hexagonal arrangement. Each 3×3unit cell contains nine such features indicating a structure with 9 atoms per 3×3 cell. Based onangle resolved photoelectron spectroscopy (ARPES) data, we have identified five surface bandswithin the bulk band gap. Four of them cross the Fermi level leading to a metallic character of thesurface. The dispersions of these bands have been mapped in detail along the high symmetrydirections of the 3×3 surface Brillouin zone. Fermi contours, mapped in 2D k-space, showinteresting features. In particular, the occurrence of two differently rotated hexagon like contoursis discussed.

Elsevier, 2016
Natural Sciences
##### Identifiers
urn:nbn:se:liu:diva-105222 (URN)10.1016/j.susc.2016.02.016 (DOI)000379097000021 ()
##### Funder
Knut and Alice Wallenberg FoundationSwedish Research Council
##### Note

Funding agencies:Technical support from Dr. Johan Adell and Dr. T. Balasubramanian at MAX-lab is gratefully acknowledged.

Vid tiden för disputation förelåg publikationen som manuskript

Available from: 2014-03-13 Created: 2014-03-13 Last updated: 2018-03-21Bibliographically approved
Wang, W., Olovsson, W. & Uhrberg, R. (2015). Experimental and theoretical determination of sigma bands on ("2 root 3 x 2 root 3") silicene grown on Ag(111). Physical Review B. Condensed Matter and Materials Physics, 92(20), 205427
Open this publication in new window or tab >>Experimental and theoretical determination of sigma bands on ("2 root 3 x 2 root 3") silicene grown on Ag(111)
2015 (English)In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 92, no 20, p. 205427-Article in journal (Refereed) Published
##### Abstract [en]

Silicene, the two-dimensional (2D) allotrope of silicon, has very recently attracted a lot of attention. It has a structure that is similar to graphene and it is theoretically predicted to show the same kind of electronic properties which have put graphene into the focus of large research and development projects worldwide. In particular, a 2D structure made from Si is of high interest because of the application potential in Si-based electronic devices. However, so far there is not much known about the silicene band structure from experimental studies. A comprehensive study is here presented of the atomic and electronic structure of the silicene phase on Ag(111) denoted as (2 root 3 x 2 root 3)R30 degrees in the literature. Low energy electron diffraction (LEED) shows an unconventional rotated ("2 root 3 x 2 root 3") pattern with a complicated set of split diffraction spots. Scanning tunneling microscopy (STM) results reveal a Ag(111) surface that is homogeneously covered by the ("2 root 3 x 2 root 3") silicene which exhibits an additional quasiperiodic long-range ordered superstructure. The complex structure, revealed by STM, has been investigated in detail and we present a consistent picture of the silicene structure based on both STM and LEED. The homogeneous coverage by the ("2 root 3 x 2 root 3") silicene facilitated an angle-resolved photoelectron spectroscopy study which reveals a silicene band structure of unprecedented detail. Here we report four silicene bands which are compared to calculated dispersions based on a relaxed (2 root 3 x 2 root 3) model. We find good qualitative agreement between the experimentally observed bands and calculated silicene bands of sigma character.

##### Place, publisher, year, edition, pages
AMER PHYSICAL SOC, 2015
##### National Category
Physical Sciences
##### Identifiers
urn:nbn:se:liu:diva-123801 (URN)10.1103/PhysRevB.92.205427 (DOI)000365509100011 ()
##### Note

Funding Agencies|Swedish Research Council [621-2010-3746, 621-2014-4764, 621-2011-4426, 2008-6582]; Linkoping Linnaeus Initiative for Novel Functional Materials; Swedish National Infrastructure for Computing (SNIC)

Available from: 2016-01-11 Created: 2016-01-11 Last updated: 2017-11-30
Sakamoto, K., Oda, T., Kimura, A., Takeichi, Y., Fujii, J., Uhrberg, R., . . . Woong Yeom, H. (2015). Symmetry induced peculiar Rashba effect on thallium adsorbed Si(111) surfaces. Journal of Electron Spectroscopy and Related Phenomena, 201, 88-91
Open this publication in new window or tab >>Symmetry induced peculiar Rashba effect on thallium adsorbed Si(111) surfaces
2015 (English)In: Journal of Electron Spectroscopy and Related Phenomena, ISSN 0368-2048, E-ISSN 1873-2526, Vol. 201, p. 88-91Article in journal (Refereed) Published
##### Abstract [en]

The geometric symmetry of the surface plays an important role for the spin-orbit-induced spin texture of two-dimensional electronic states. This article reviews the peculiar Rashba spins induced by a C-3 symmetry, including the completely spin polarized surface states with the polarization vector oriented perpendicular to the surface, i.e. a direction that is not expected in a typical Rashba system. This review also describes that this peculiar Rashba situation has possibility to suppress backscattering and therefore to greatly improve the efficiency of spin transport, which is an essential issue in the development of high-performance semiconductor spintronic devices. (C) 2014 Elsevier B.V. All rights reserved.

Elsevier, 2015
##### Keywords
Rashba effect; Symmetry; Silicon; Thallium
##### National Category
Physical Sciences
##### Identifiers
urn:nbn:se:liu:diva-120470 (URN)10.1016/j.elspec.2014.09.008 (DOI)000357909500016 ()
##### Note

Funding Agencies|JSPS [20244045, 25287070]

Available from: 2015-08-12 Created: 2015-08-11 Last updated: 2017-12-04
Muhammad Sohail, H. & Uhrberg, R. (2015). Umklapp induced surface band structure of Ag/Ge(111)6 x 6. Surface Science, 635, 55-60
Open this publication in new window or tab >>Umklapp induced surface band structure of Ag/Ge(111)6 x 6
2015 (English)In: Surface Science, ISSN 0039-6028, E-ISSN 1879-2758, Vol. 635, p. 55-60Article in journal (Refereed) Published
##### Abstract [en]

This study focuses on the electronic structure of a 6 x 6 surface which is formed by 0.2 monolayer of Ag on top of the Ag/Ge(111) root 3 x root 3 surface. The 6 x 6 periodicity was verified by low energy electron diffraction. Angle resolved photoelectron spectroscopy was employed to study the electronic structure along the (Gamma) over bar-(M) over bar-(Gamma) over bar and (Gamma) over bar-(K) over bar-(M) over bar high symmetry lines of the 6 x 6 surface Brillouin zone. There are six surface bands in total. Out of these, three were found to be related to the 6 x 6 phase. The surface band structure of the 6 x 6 phase is significantly more complex than that of the,root 3 x root 3 surface. This is particularly the case for the uppermost surface band structure which is a combination of a surface band originating from the underlying root 3 x root 3 surface and umklapp scattered branches of this band. Branches centered at neighboring 6 x 6 SBZs cross each other at an energy slightly below the Fermi level. An energy gap opens up at this point which contains the Fermi level. The complex pattern of constant energy contours has been used to identify the origins of various branches of the surface state dispersions. (C) 2014 Elsevier B.V. All rights reserved.

Elsevier, 2015
##### Keywords
Ge(111); Silver (Ag); Electronic band structure; Umklapp bands; LEED; ARPES
##### National Category
Physical Sciences
##### Identifiers
urn:nbn:se:liu:diva-116940 (URN)10.1016/j.susc.2014.12.008 (DOI)000350929400009 ()
##### Funder
Swedish Research Council Available from: 2015-04-13 Created: 2015-04-10 Last updated: 2018-03-21
Wang, W., Sohail, H. M., Osiecki, J. & Uhrberg, R. (2014). Broken symmetry induced band splitting in the Ag2Ge surface alloy on Ag(111). Physical Review B. Condensed Matter and Materials Physics, 89(12), 125410-1-125410-6
Open this publication in new window or tab >>Broken symmetry induced band splitting in the Ag2Ge surface alloy on Ag(111)
2014 (English)In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 89, no 12, p. 125410-1-125410-6Article in journal (Refereed) Published
##### Abstract [en]

We report a study of the atomic and electronic structures of the ordered Ag2Ge surface alloy containing ⅓ monolayer of Ge. Low-energy electron diffraction (LEED), scanning tunneling microscopy (STM), and angle-resolved photoelectron spectroscopy (ARPES) data reveal a symmetry breaking of the expected √3 × √3 periodicity, which is established for other Ag2M alloys (M = Bi, Sb, Pb, and Sn). The deviation from a simple √3 × √3 structure manifests itself as a splitting of diffraction spots in LEED, as a striped structure with a 6× periodicity including a distortion of the local hexagonal structure in STM, and as a complex surface band structure in ARPES that is quite different from those of the other Ag2M alloys. These results are interesting in view of the differences in the atomic and electronic structures exhibited by different group IV elements interacting with Ag(111). Pb and Sn form √3 × √3 surface alloys on Ag(111), of which Ag2Pb shows a surface band structure with a clear spin-orbit split. Si and C form silicene and graphene structures, respectively, with linear band dispersions and the formation of Dirac cones as reported for graphene. The finding that Ag2Ge deviates from the ideal (√3 × √3) Ag2Sn and Ag2Pb surface alloys makes Ge an interesting “link” between the heavy group IV elements (Sn, Pb) and the light group IV elements (Si, C).

##### Place, publisher, year, edition, pages
American Physical Society, 2014
Natural Sciences
##### Identifiers
urn:nbn:se:liu:diva-105259 (URN)10.1103/PhysRevB.89.125410 (DOI)000332460600003 ()
Available from: 2014-03-14 Created: 2014-03-14 Last updated: 2018-03-16Bibliographically approved
Sohail, H. M. & Uhrberg, R. I. G. (2014). Electronic and atomic structures of the Ag induced √3x√3 superstructure on Ge(111). Surface Science, 625, 23-29
Open this publication in new window or tab >>Electronic and atomic structures of the Ag induced √3x√3 superstructure on Ge(111)
2014 (English)In: Surface Science, ISSN 0039-6028, E-ISSN 1879-2758, Vol. 625, p. 23-29Article in journal (Refereed) Published
##### Abstract [en]

The Ag/Ge(111) surface together with Ag/Si(111) constitutes a set of surfaces that is ideally suited for fundamental studies related to low dimensional physics. We here focus on the atomic and electronic structures of the two-dimensional superstructure induced by Ag on Ge(111), a surface that is significantly less studied than the Si counterpart. Extensive information on the surface band structure obtained by angle resolved photoelectron spectroscopy (ARPES) is presented, complemented by atomic information from scanning tunneling microscopy (STM). The results reveal new findings that are important for the understanding of the Ag induced structure, acting as a prototype for semiconductor/metal interfaces. i) We have identified a new occupied surface band near the -point of the surface Brillouin zone. ii) The Ag/Ge(111) surface exhibits a partially occupied surface band, S1, with a parabolic-like shape at Γ¯. At low temperature (≈ 100 K) this band splits into two bands, S1U and S1D. The identification of two bands is significantly different from the case of Ag/Si(111) for which just one band has been reported. Besides these specific results, our extensive ARPES study reveals four surface bands at room temperature (RT), while five surface bands were identified at ≈ 100 K (LT). Room temperature empty state STM images show, depending on the tunneling bias, both honeycomb and hexagonal periodicities which are consistent with the honeycomb chained trimer and the in-equivalent trimer models, respectively.

Elsevier, 2014
##### Keywords
Ge(111); Ag; Monolayer; Atomic structure; Electronic band structure; Free electron like band; STM; LEED; ARPES
Natural Sciences
##### Identifiers
urn:nbn:se:liu:diva-105216 (URN)10.1016/j.susc.2014.02.013 (DOI)000336355200005 ()
Available from: 2014-03-13 Created: 2014-03-13 Last updated: 2018-03-21Bibliographically approved
Sohail, H. M., Eriksson, J., Osiecki, J. & Uhrberg, R. I. G. (2014). First principles study of electronic and atomic structures of a √3x√3 superstructures induced by Ag on Si(111) and Ge(111).
Open this publication in new window or tab >>First principles study of electronic and atomic structures of a √3x√3 superstructures induced by Ag on Si(111) and Ge(111)
##### Abstract [en]

We have employed first principles density functional theory (DFT) based calculations (WIEN2k) to study the electronic and atomic structures of the  reconstruction induced by Ag on Si(111) and Ge(111). The Ag/Si(111)  surface, in particular, has acted as a model system when it comes to the interaction between adsorbed metals and semiconductor surfaces. Two models have been studied, i.e., the honeycomb-chained-triangle (HCT) and the  in-equivalenttriangle (IET) model. The band structures of these models were calculated using density functional theory within the generalized gradient approximation (GGA) and the local density approximation (LDA). The band structures calculated from the fully relaxed versions of the two models were found to be quite similar except for the occupancy of the free electron like band at the - point. The IET model gives a slightly lower energy minimum compared to the HCT model for both Si and Ge. Further, we find that the energy minima are deeper for Ge when comparing the results with Si for the HCT and IET models, respectively. The theoretical surface band structure is qualitatively in good general agreement with the experimental dispersions of the main surface states, while the theoretical band widths are approximately half of the experimental ones. The calculated band structures show a gap between the two uppermost, fully occupied, bands at the $\overline{K}$- point only when the IET model is used to account for the electronic structure of Ag/Si(111) . Neither the IET nor the HCT model resulted in a gap when applied to Ag/Ge(111) .

Natural Sciences
##### Identifiers
urn:nbn:se:liu:diva-105218 (URN)
Available from: 2014-03-13 Created: 2014-03-13 Last updated: 2018-03-16Bibliographically approved
Sohail, H. M. & Uhrberg, R. I. G. (2014). Origin of the metal to semiconductor transition associated with the √3x√3 and 6×6surfaces of Ag/Ge(111).
Open this publication in new window or tab >>Origin of the metal to semiconductor transition associated with the √3x√3 and 6×6surfaces of Ag/Ge(111)
##### Abstract [en]

This study focuses on the electronic structure of a 6×6 surface which is formed by 0.2 monolayer of Ag on top of the Ag/Ge(111)  surface. The 6×6 periodicity was verified by low energy electron diffraction. Angle resolved photoelectron spectroscopy was employed to study the electronic structure along the $\overline{M}$ and $\overline{K}$$\overline{M}$ high symmetry lines of the 6×6 surface Brillouin zone. There are six surface bands in total. Out of these, three were found to be related to the 6×6 phase. The surface band structure of the 6×6 phase is significantly more complex than that of the  surface. This is particularly the case for the uppermost surface band structure which is a combination of a surface band originating from the underlying  surface and umklapp scattered branches of this band. Branches centered at neighboring 6×6 SBZs cross each other at an energy slightly below the Fermi level. An energy gap opens up at this point which contains the Fermi level and thus making the 6×6 semiconducting. The complex pattern of constant energy contours have been used to identify the origins of various branches of the surface state dispersions.

Natural Sciences
##### Identifiers
urn:nbn:se:liu:diva-105220 (URN)
Available from: 2014-03-13 Created: 2014-03-13 Last updated: 2018-03-16Bibliographically approved

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