Symmetry rules shaping spin-orbital textures in surface states

Gotlieb, Kenneth,Li, Zhenglu,Lin, Chiu-Yun,Jozwiak, Chris,Ryoo, Ji Hoon,Park, Cheol-Hwan,Hussain, Zahid,Louie, Steven G.,Lanzara, Alessandra American Physical Society 2017 Physical Review B Vol.95 No.24

<P>Strong spin-orbit coupling creates exotic electronic states such as Rashba and topological surface states, which hold promise for technologies involving the manipulation of spin. Only recently has the complexity of these surface states been appreciated: they are composed of several atomic orbitals with distinct spin textures in momentum space. A complete picture of the wave function must account for this orbital dependence of spin. We discover that symmetry constrains the way orbital and spin components of a state coevolve as a function of momentum, and from this, we determine the rules governing how the two degrees of freedom are interwoven. We directly observe this complexity in spin-resolved photoemission and ab initio calculations of the topological surface states of Sb(111), where the photoelectron spin direction near (Gamma) over bar is found to have a strong and unusual dependence on photon polarization. This dependence unexpectedly breaks down at large |k|, where the surface states mix with other nearby surface states. However, along mirror planes, symmetry protects the distinct spin orientations of different orbitals. Our discovery broadens the understanding of surface states with strong spin-orbit coupling, demonstrates the conditions that allow for optical manipulation of photoelectron spin, and will be highly instructive for future spintronics applications.</P>

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).

Direct visualization and control of SrOx segregation on semiconducting Nb doped SrTiO3 (100) surface

유향근,Schwarz Daniel,Ulstrup Søren,Kim Woojin,Jozwiak Chris,Bostwick Aaron,노태원,Rotenberg Eli,Chang Young Jun 한국물리학회 2022 THE JOURNAL OF THE KOREAN PHYSICAL SOCIETY Vol.80 No.11

We investigated how SrOx segregates on a Nb doped SrTiO3 (100) surface by in air annealing. Using atomic force and photoemission electron microscopes, we can directly visualize the morphology and the electronic phase changes with SrOx segregation. SrOx islands less than 2 μm in size and 1–5 unit cells thick nucleate frst and grow in a labyrinth domain pattern. After prolonged annealing, SrOx forms a ~ 10 nm thick flm. We show that the domain pattern can be controlled by introducing a surface miscut angle of SrTiO3. Additionally, the segregated SrOx has a lower work function, compared to that of SrTiO3. These results suggest that the control and tunability of SrOx segregation is applicable to the design of a new functional electronic devices in the semiconducting SrTiO3 based heterostructure.

Evidence for absence of metallic surface states in BiO2-terminated BaBiO3 thin films

오지섭,김민우,김기덕,이한결,유향근,신수빈,장영준,한문섭,Chris Jozwiak,Aaron Bostwick,Eli Rotenberg,김형도,노태원 한국물리학회 2018 Current Applied Physics Vol.18 No.6

We investigated the atomic configuration and the electronic structure of a BaBiO3 (BBO) thin film and its (001) surface. It was theoretically predicted that two-dimensional electron gases would be formed when the film is BiO2-terminated. We deduced depth-profile information for a BBO thin film using angle-dependent X-ray photoemission spectroscopy. Analysis of the spectral weights of the Ba 3d and Bi 4f core levels confirmed that the BBO film should have a BiO2-terminated topmost layer. We used in-situ angle-resolved photoemission spectroscopy to experimentally determine the electronic structure of a BBO thin film and found no metallic surface state. We distinguished surface states from bulk states by evaporating potassium atoms in-situ on the surface. A surface state near the bottom of the topmost bulk valence band, which was predicted by the DFT calculations, was identified. However, other surface states well separated from the bulk states were not observed. Our results provided evidence that descriptions of the BBO electronic structure require more detailed and elaborate approaches.

Emergence of a Metal-Insulator Transition and High-Temperature Charge-Density Waves in VSe₂ at the Monolayer Limit

Duvjir, Ganbat,Choi, Byoung Ki,Jang, Iksu,Ulstrup, Søren,Kang, Soonmin,Thi Ly, Trinh,Kim, Sanghwa,Choi, Young Hwan,Jozwiak, Chris,Bostwick, Aaron,Rotenberg, Eli,Park, Je-Geun,Sankar, Raman,Kim, Ki-Seo American Chemical Society 2018 NANO LETTERS Vol.18 No.9

<P>Emergent phenomena driven by electronic reconstructions in oxide heterostructures have been intensively discussed. However, the role of these phenomena in shaping the electronic properties in van der Waals heterointerfaces has hitherto not been established. By reducing the material thickness and forming a heterointerface, we find two types of charge-ordering transitions in monolayer VSe<SUB>2</SUB> on graphene substrates. Angle-resolved photoemission spectroscopy (ARPES) uncovers that Fermi-surface nesting becomes perfect in ML VSe<SUB>2</SUB>. Renormalization-group analysis confirms that imperfect nesting in three dimensions universally flows into perfect nesting in two dimensions. As a result, the charge-density wave-transition temperature is dramatically enhanced to a value of 350 K compared to the 105 K in bulk VSe<SUB>2</SUB>. More interestingly, ARPES and scanning tunneling microscopy measurements confirm an unexpected metal-insulator transition at 135 K that is driven by lattice distortions. The heterointerface plays an important role in driving this novel metal-insulator transition in the family of monolayer transition-metal dichalcogenides.</P> [FIG OMISSION]</BR>

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>

Evidence for absence of metallic surface states in BiO₂-terminated BaBiO₃ thin films

Oh, Ji Seop,Kim, Minu,Kim, Gideok,Lee, Han Gyeol,Yoo, Hyang Keun,Sinn, Soobin,Chang, Young Jun,Han, Moonsup,Jozwiak, Chris,Bostwick, Aaron,Rotenberg, Eli,Kim, Hyeong-Do,Noh, Tae Won Elsevier 2018 Current Applied Physics Vol.18 No.6

<P><B>Abstract</B></P> <P>We investigated the atomic configuration and the electronic structure of a BaBiO<SUB>3</SUB> (BBO) thin film and its (001) surface. It was theoretically predicted that two-dimensional electron gases would be formed when the film is BiO<SUB>2</SUB>-terminated. We deduced depth-profile information for a BBO thin film using angle-dependent X-ray photoemission spectroscopy. Analysis of the spectral weights of the Ba 3d and Bi 4<I>f</I> core levels confirmed that the BBO film should have a BiO<SUB>2</SUB>-terminated topmost layer. We used <I>in-situ</I> angle-resolved photoemission spectroscopy to experimentally determine the electronic structure of a BBO thin film and found no metallic surface state. We distinguished surface states from bulk states by evaporating potassium atoms <I>in-situ</I> on the surface. A surface state near the bottom of the topmost bulk valence band, which was predicted by the DFT calculations, was identified. However, other surface states well separated from the bulk states were not observed. Our results provided evidence that descriptions of the BBO electronic structure require more detailed and elaborate approaches.</P> <P><B>Highlights</B></P> <P> <UL> <LI> An atomic configuration and a surface state of a BiO<SUB>2</SUB>-terminated BaBiO<SUB>3</SUB> thin film were studied. </LI> <LI> The depth-profile information of our BaBiO<SUB>3</SUB> film was investigated and it had a BiO<SUB>2</SUB> termination layer. </LI> <LI> Contrary to the theoretical proposal for a 2DEG on a BiO<SUB>2</SUB>-terminated BaBiO<SUB>3</SUB>, no surface metallic state was observed. </LI> <LI> The absence of the metallic surface state may be attributed to misprediction on the band gap size. </LI> </UL> </P>