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In situ investigation of conducting interface formation in LaAlO3/SrTiO3 heterostructure
유향근,Moreschini Luca,Bostwick Aaron,Walter Andrew L.,노태원,Rotenberg Eli,장영준 한국물리학회 2021 Current Applied Physics Vol.30 No.-
The high-mobility conducting interface (CI) between LaAlO3 (LAO) and SrTiO3 (STO) has revealed many fascinating phenomena, including exotic magnetism and superconductivity. But, the formation mechanism of the CI has not been conclusively explained. Here, using in situ angle-resolved photoemission spectroscopy, we elucidated the mechanisms for the CI formation. In as-grown samples, we observed a built-in potential (Vbi) proportional to the polar LAO thickness starting from the first unit cell (UC) with CI formation appearing above 3 UCs. However, we found that the Vbi is removed by synchrotron ultraviolet (UV)-irradiation; The built-in potential is recovered by oxygen gas (O2(g))-exposure. Furthermore, after UV-irradiation, the CI appears even below 3UC of LAO. Our results demonstrate not only the Vbi-driven CI formation in as-grown LAO/STO, but also a new route to control of the interface state by UV lithographic patterning or other surface modification.
Effective screening and the plasmaron bands in graphene
Walter, Andrew L.,Bostwick, Aaron,Jeon, Ki-Joon,Speck, Florian,Ostler, Markus,Seyller, Thomas,Moreschini, Luca,Chang, Young Jun,Polini, Marco,Asgari, Reza,MacDonald, Allan H.,Horn, Karsten,Rotenberg, American Physical Society 2011 Physical review. B, Condensed matter and materials Vol.84 No.8
Enhanced tunability of two-dimensional electron gas on SrTiO3 through heterostructuring
유향근,Luca Moreschini,Aaron Bostwick,Andrew L. Walter,노태원,Eli Rotenberg,장영준 한국물리학회 2020 Current Applied Physics Vol.20 No.11
Two-dimensional electron gases (2DEGs) on the SrTiO3 (STO) surface or in STO-based heterostructures have exhibited many intriguing phenomena, which are strongly dependent on the 2DEG-carrier density. We report that the tunability of the 2DEG-carrier density is significantly enhanced by adding a monolayer LaTiO3 (LTO) onto the STO. Ultraviolet (UV) irradiation induced maximum carrier density of the 2DEG in LTO/STO is increased by a factor of ~4 times, compared to that of the bare STO. By oxygen gas exposure, it becomes 10 times smaller than that of the bare STO. This enhanced tunability is attributed to the drastic surface property change of a polar LTO layer by UV irradiation and O2 exposure. This indicates that the 2DEG controllability in LTO/STO is more reliable than that on the bare STO driven by defects, such an oxygen vacancy.
Evidence for indirect band gap in BaSnO3 using angle-resolved photoemission spectroscopy
주범수,장영준,Luca Moreschini,Aaron Bostwick,Eli Rotenberg,한문섭 한국물리학회 2017 Current Applied Physics Vol.17 No.5
Transparent BaSnO3 thin films have been proposed as an alternative transparent conducting oxide (TCO). Although bulk synthesis and high-quality fabrication of epitaxial films are well established, there are still unsolved aspects about their electronic structure, such as the direct or indirect nature and the size of the band gap. We investigated the electronic structure of epitaxial BaSnO3 thin films using in situ angleresolved photoemission spectroscopy. We directly measured an indirect band gap of 3.7 eV, a value compatible with those of previous reports, but we also identified additional in-gap states at 1.6 eV below the conduction band minimum that we attribute to intrinsic defects, mainly oxygen vacancies.
Electronic structure and charge-density wave transition in monolayer VS2
김혁진,최병기,이인학,김민재,천승현,Jozwiak Chris,Bostwick Aaron,Rotenberg Eli,Chang Young Jun 한국물리학회 2021 Current Applied Physics Vol.30 No.-
Vanadium disulfide (VS2) attracts elevated interests for its charge-density wave (CDW) phase transition, ferromagnetism, and catalytic reactivity, but the electronic structure of monolayer has not been well understood yet. Here we report synthesis of epitaxial 1T VS2 monolayer on bilayer graphene grown by molecular-beam epitaxy (MBE). Angle-resolved photoemission spectroscopy (ARPES) measurements reveal that Fermi surface with six elliptical pockets centered at the M points shows gap opening at low temperature. Temperature-dependence of the gap size suggests existence of CDW phase transition above room temperature. Our observations provide important evidence to understand the strongly correlated electron physics and the related surface catalytic properties in two-dimensional transition-metal dichalcogenides (TMDCs).
Kim, Yong Su,Jeon, Sang Ho,Bostwick, Aaron,Rotenberg, Eli,Ross, Philip N.,Walter, Andrew L.,Chang, Young Jun,Stamenkovic, Vojislav R.,Markovic, Nenad M.,Noh, Tae Won,Han, Seungwu,Mun, Bongjin Simon The Royal Society of Chemistry 2013 Physical chemistry chemical physics Vol.15 No.43
<P>We report on the chemical adsorption mechanism of atomic oxygen on the Pt(111) surface using angle-resolved-photoemission spectroscopy (ARPES) and density functional calculations. The detailed band structure of Pt(111) from ARPES reveals that most of the bands near the Fermi level are surface-states. By comparing band maps of Pt and O/Pt, we identify that d<SUB><I>xz</I></SUB> (d<SUB><I>yz</I></SUB>) and d<SUB><I>z</I><SUP>2</SUP></SUB> orbitals are strongly correlated in the surface-states around the symmetry point M and K, respectively. Additionally, we demonstrate that the s- or p-orbital of oxygen atoms hybridizes preferentially with the d<SUB><I>xz</I></SUB> (d<SUB><I>yz</I></SUB>) orbital near the M symmetry point. This weak hybridization occurs with minimal charge transfer.</P> <P>Graphic Abstract</P><P>Using ARPES and DFT, the chemical bonding structure of atomic oxygen on the Pt(111) is explored. <IMG SRC='http://pubs.rsc.org/services/images/RSCpubs.ePlatform.Service.FreeContent.ImageService.svc/ImageService/image/GA?id=c3cp53376j'> </P>
Walter, Andrew L.,Sahin, Hasan,Kang, Jun,Jeon, Ki-Joon,Bostwick, Aaron,Horzum, Seyda,Moreschini, Luca,Chang, Young Jun,Peeters, Francois M.,Horn, Karsten,Rotenberg, Eli American Physical Society 2016 Physical Review B Vol.93 No.7
<P>The application of graphene to electronic and optoelectronic devices is limited by the absence of reliable semiconducting variants of this material. A promising candidate in this respect is graphene oxide, with a band gap on the order of similar to 5 eV, however, this has a finite density of states at the Fermi level. Here, we examine the electronic structure of three variants of half-fluorinated carbon on Sic(0001), i.e., the (6 root 3 x 6 root 3) R30 degrees C/SiC 'buffer layer,' graphene on this (6 root 3 x 6 root 3) R30 degrees C/SiC buffer layer, and graphene decoupled from the SiC substrate by hydrogen intercalation. Using angle-resolved photoemission, core level photoemission, and x-ray absorption, we show that the electronic, chemical, and physical structure of all three variants is remarkably similar, exhibiting a large band gap and a vanishing density of states at the Fermi level. These results are explained in terms of first-principles calculations. This material thus appears very suitable for applications, even more so since it is prepared on a processing-friendly substrate. We also investigate two separate UV photon-induced modifications of the electronic structure that transform the insulating samples (6.2-eV band gap) into semiconducting (similar to 2.5-eV band gap) and metallic regions, respectively.</P>