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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.
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.
A novel quasi-one-dimensional topological insulator in bismuth iodide β-Bi<sub>4</sub>I<sub>4</sub>
Autè,s, Gabriel,Isaeva, Anna,Moreschini, Luca,Johannsen, Jens C.,Pisoni, Andrea,Mori, Ryo,Zhang, Wentao,Filatova, Taisia G.,Kuznetsov, Alexey N.,Forró,, Lá,szló,Van den Broek, Nature Publishing Group, a division of Macmillan P 2016 NATURE MATERIALS Vol.15 No.2
Recent progress in the field of topological states of matter has largely been initiated by the discovery of bismuth and antimony chalcogenide bulk topological insulators (TIs; refs ,,,), followed by closely related ternary compounds and predictions of several weak TIs (refs ,,). However, both the conceptual richness of Z<SUB>2</SUB> classification of TIs as well as their structural and compositional diversity are far from being fully exploited. Here, a new Z<SUB>2</SUB> topological insulator is theoretically predicted and experimentally confirmed in the β-phase of quasi-one-dimensional bismuth iodide Bi<SUB>4</SUB>I<SUB>4</SUB>. The electronic structure of β-Bi<SUB>4</SUB>I<SUB>4</SUB>, characterized by Z<SUB>2</SUB> invariants (1;110), is in proximity of both the weak TI phase (0;001) and the trivial insulator phase (0;000). Our angle-resolved photoemission spectroscopy measurements performed on the (001) surface reveal a highly anisotropic band-crossing feature located at the point of the surface Brillouin zone and showing no dispersion with the photon energy, thus being fully consistent with the theoretical prediction.
Insulating-layer formation of metallic LaNiO<sub>3</sub> on Nb-doped SrTiO<sub>3</sub> substrate
Yoo, Hyang Keun,Chang, Young Jun,Moreschini, Luca,Kim, Hyeong-Do,Sohn, Chang Hee,Sinn, Soobin,Oh, Ji Seop,Kuo, Cheng-Tai,Bostwick, Aaron,Rotenberg, Eli,Noh, Tae Won American Institute of Physics 2015 Applied Physics Letters Vol.106 No.12
Thickness-dependent electronic structure in ultrathinLaNiO3films under tensile strain
Yoo, Hyang Keun,Hyun, Seung Ill,Chang, Young Jun,Moreschini, Luca,Sohn, Chang Hee,Kim, Hyeong-Do,Bostwick, Aaron,Rotenberg, Eli,Shim, Ji Hoon,Noh, Tae Won American Physical Society 2016 Physical Review B Vol.93 No.3
<P>We investigated electronic-structure changes of tensile-strained ultrathin LaNiO3 (LNO) films from ten to one unit cells (UCs) using angle-resolved photoemission spectroscopy (ARPES). We found that there is a critical thickness t(c) between four and three UCs below which Ni e(g) electrons are confined in two-dimensional space. Furthermore, the Fermi surfaces (FSs) of LNO films below t(c) consist of two orthogonal pairs of one-dimensional (1D) straight parallel lines. Such a feature is not accidental as observed in constant-energy surfaces at all binding energies, which is not explained by first-principles calculations or the dynamical mean-field theory. The ARPES spectra also show anomalous spectral behaviors, such as no quasiparticle peak at the Fermi momentum but fast band dispersion comparable to the bare-band one, which is typical in a 1D system. As its possible origin, we propose 1D FS nesting, which also accounts for FS superstructures observed in ARPES.</P>
Universal Mechanism of Band-Gap Engineering in Transition-Metal Dichalcogenides
Kang, Mingu,Kim, Beomyoung,Ryu, Sae Hee,Jung, Sung Won,Kim, Jimin,Moreschini, Luca,Jozwiak, Chris,Rotenberg, Eli,Bostwick, Aaron,Kim, Keun Su American Chemical Society 2017 NANO LETTERS Vol.17 No.3
<P>van der Waals two-dimensional (2D) semiconductors have emerged as a class of materials with promising device characteristics owing to the intrinsic band gap. For realistic applications, the ideal is to modify the band gap in a controlled manner by a mechanism that can be generally applied to this class of materials. Here, we report the observation of a universally tunable band gap in the family of bulk 2H transition metal dichalcogenides (TMDs) by in situ surface doping of Rb atoms. A series of angle-resolved photoemission spectra unexceptionally shows that the band gap of TMDs at the zone corners is modulated in the range of 0.8-2.0 eV, which covers a wide spectral range from visible to near-infrared, with a tendency from indirect to direct band gap. A key clue to understanding the mechanism of this band-gap engineering is provided by the spectroscopic signature of symmetry breaking and resultant spin-splitting, which can be explained by the formation of 2D electric dipole layers within the surface bilayer of TMDs. Our results establish the surface Stark effect as a universal mechanism of band-gap engineering on the basis of the strong 2D nature of van der Waals semiconductors.</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>