Person: Fang, Shiang
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Fang
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Shiang
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Fang, Shiang
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Publication Dirac Fermions and Flat Bands in the Ideal Kagome Metal FeSn(Springer Science and Business Media LLC, 2019-12-09) Fang, Shiang; Han, Minyong; Graff, David; Kaxiras, Efthimios; Kang, Mingu; Ye, Linda; You, Jhih-Shih; Levitan, Abe; Facio, Jorge; Jozwiak, Chris; Bostwick, Aaron; Rotenberg, Eli; Chan, Mun; McDonald, Ross; Kaznatcheev, Konstantine; Vescovo, Elio; Bell, David; van den Brink, Jeroen; Richter, Manuel; Prasad Ghimire, Madhav; Checkelsky, Joseph; Comin, RiccardoA kagome lattice of 3d transition metal ions is a versatile platform for correlated topological phases hosting symmetry-protected electronic excitations and magnetic ground states. However, the paradigmatic states of the idealized two-dimensional kagome lattice – Dirac fermions and flat bands – have not been simultaneously observed. Here, we utilize angle-resolved photoemission spectroscopy and de Haas-van Alphen quantum oscillations to reveal coexisting surface and bulk Dirac fermions as well as flat bands in the antiferromagnetic kagome metal FeSn, that has spatially-decoupled kagome planes. Our band structure calculations and matrix element simulations demonstrate that the bulk Dirac bands arise from in-plane localized Fe-3d orbitals, and evidence that coexisting Dirac surface state realizes a rare example of fully spin-polarized two-dimensional Dirac fermions due to spin-layer locking in FeSn. The prospect to harness these prototypical excitations in kagome lattice is a frontier of great promise at the confluence of topology, magnetism, and strongly-correlated physics.Publication Theory of Graphene Raman Scattering(American Chemical Society (ACS), 2016) Heller, Eric; Yang, Yuan; Kocia, Lucas; Chen, Wei; Fang, Shiang; Borunda, Mario; Kaxiras, EfthimiosRaman scattering plays a key role in unraveling the quantum dynamics of graphene, perhaps the most promising material of recent times. It is crucial to correctly interpret the meaning of the spectra. It is therefore very surprising that the widely accepted understanding of Raman scattering, i.e., Kramers–Heisenberg–Dirac theory, has never been applied to graphene. Doing so here, a remarkable mechanism we term“transition sliding” is uncovered, explaining the uncommon brightness of overtones in graphene. Graphene’s dispersive and fixed Raman bands, missing bands, defect density and laser frequency dependence of band intensities, widths of overtone bands, Stokes, anti-Stokes anomalies, and other known properties emerge simply and directly.Publication Ab initiotight-binding Hamiltonian for transition metal dichalcogenides(American Physical Society (APS), 2015) Fang, Shiang; Kuate Defo, Rodrick; Shirodkar, Sharmila N.; Lieu, Simon; Tritsaris, Georgios; Kaxiras, EfthimiosWe present an accurate ab initio tight-binding Hamiltonian for the transition metal dichalcogenides, MoS2, MoSe2, WS2, WSe2, with a minimal basis (the d orbitals for the metal atoms and p orbitals for the chalcogen atoms) based on a transformation of the Kohn-Sham density functional theory Hamiltonian to a basis of maximally localized Wannier functions. The truncated tight-binding Hamiltonian, with only on-site, first, and partial second neighbor interactions, including spin-orbit coupling, provides a simple physical picture and the symmetry of the main band-structure features. Interlayer interactions between adjacent layers are modeled by transferable hopping terms between the chalcogen p orbitals. The full-range tight-binding Hamiltonian can be reduced to hybrid-orbital k .p effective Hamiltonians near the band extrema that capture important low-energy excitations. These ab initio Hamiltonians can serve as the starting point for applications to interacting many-body physics including optical transitions and Berry curvature of bands, of which we give some examples.Publication Bounds on nanoscale nematicity in single-layerFeSe/SrTiO3(American Physical Society (APS), 2016) Huang, Dennis; Webb, Tatiana; Fang, Shiang; Song, Can-Li; Chang, Cui-Zu; Moodera, Jagadeesh S.; Kaxiras, Efthimios; Hoffman, JennyWe use scanning tunneling microscopy (STM) and quasiparticle interference (QPI) imaging to investigate the low-energy orbital texture of single-layer FeSe/SrTiO3. We develop a T -matrix model of multiorbital QPI to disentangle scattering intensities from Fe 3dxz and 3dyz bands, enabling the use of STM as a nanoscale detection tool of nematicity. By sampling multiple spatial regions of a single-layer FeSe/SrTiO3 film, we quantitatively exclude static xz/yz orbital ordering with domain size larger than δr2 = 20 nm × 20 nm, xz/yz Fermi wave vector difference larger than δk = 0.014 π, and energy splitting larger than δE = 3.5 meV. The lack of detectable ordering pinned around defects places qualitative constraints on models of fluctuating nematicityPublication Revealing the Empty-State Electronic Structure of Single-Unit-Cell FeSe/SrTiO3(American Physical Society (APS), 2015) Huang, Dennis; Song, Can-Li; Webb, Tatiana; Fang, Shiang; Chang, Cui-Zu; Moodera, Jagadeesh S.; Kaxiras, Efthimios; Hoffman, JennyWe use scanning tunneling spectroscopy to investigate the filled and empty electronic states of superconducting single-unit-cell FeSe deposited on SrTiO3(001). We map the momentum-space band structure by combining quasiparticle interference imaging with decay length spectroscopy. In addition to quantifying the filled-state bands, we discover a Γ-centered electron pocket 75 meV above the Fermi energy. Our density functional theory calculations show the orbital nature of empty states at Γ and explain how the Se height is a key tuning parameter of their energies, with broad implications for electronic properties.Publication Magnetic resonance spectroscopy of an atomically thin material using a single-spin qubit(American Association for the Advancement of Science (AAAS), 2017) Lovchinsky, Igor; Lukin, Mikhail; Sanchez-Yamagishi, Javier; Urbach, Elana; Choi, Soonwon; Fang, Shiang; Andersen, Trond; Watanabe, Kenji; Taniguchi, Takashi; Bylinskii, Alexei; Kaxiras, Efthimios; Kim, Philip; Park, HongkunTwo-dimensional (2D) materials offer a promising platform for exploring condensed matter phenomena and developing technological applications. However, the reduction of material dimensions to the atomic scale poses a challenge for traditional measurement and interfacing techniques that typically couple to macroscopic observables. We demonstrate a method for probing the properties of 2D materials via nanometer-scale nuclear quadrupole resonance (NQR) spectroscopy using individual atom-like impurities in diamond. Coherent manipulation of shallow nitrogen-vacancy (NV) color centers enables the probing of nanoscale ensembles down to ∼30 nuclear spins in atomically thin hexagonal boron nitride (h-BN). The characterization of low-dimensional nanoscale materials could enable the development of new quantum hybrid systems, combining atom-like systems coherently coupled with individual atoms in 2D materials.Publication Microscopic structure of three-dimensional charge order in kagome superconductor AV3Sb5 and its tunability(Research Square Platform LLC, 2022-02-24) Kang, Mingu; Fang, Shiang; Yoo, Jonggyu; Ortiz, Brenden R.; Oey, Yuzki M.; Choi, Jonghyeok; Ryu, Sae Hee; Kim, Jimin; Jozwiak, Chris; Bostwick, Aaron; Rotenberg, Eli; Kaxiras, Efthimios; Checkelsky, Joseph G.; Wilson, Stephen D.; Park, Jae-Hoon; Comin, RiccardoCorrelated electronic systems are naturally susceptible to develop collective, symmetry-breaking electronic phases as observed in Cu- and Fe-based high-temperature superconductors, and twisted Moiré superlattices. The family of kagome metals AV3Sb5 (A = K, Rb, Cs) is a recently discovered, rich platform to study many of these phenomena and their interplay. In these systems, three-dimensional charge order (3D-CO) is the primary instability that sets the stage in which other ordered phases emerge, including unidirectional stripe order, orbital flux order, and superconductivity. Therefore, determining the exact nature of the 3D-CO is key to capture the broader phenomenology in AV3Sb5. Here, we use high-resolution angle-resolved photoemission spectroscopy to resolve the microscopic structure and symmetry of 3D-CO in AV3Sb5. Our approach is based on identifying an unusual splitting of kagome bands induced by 3D-CO, which provides a sensitive way to refine the spatial charge patterns in neighboring kagome planes. Notably, we found a marked dependence of the 3D-CO structure on alkali metal and doping: the 3D-CO in CsV3Sb5 is composed of kagome layers with alternating Star-of-David and Tri-Hexagonal distortions, while KV3Sb5, RbV3Sb5, and Sn-doped CsV3Sb5 realize a staggered charge pattern breaking C6 rotational symmetry. These results establish the microscopic structure of 3D-CO and its evolution with chemical composition for the first time, providing fresh insights on the origin of the cascade of exotic electronic phases in AV3Sb5.Publication Observation of the nonlinear Hall effect under time-reversal-symmetric conditions(Springer Nature, 2018-12-17) Ma, Qiong; Xu, Su-Yang; Shen, Huitao; MacNeill, David; Fatemi, Valla; Chang, Tay-Rong; Mier Valdivia, Andrés M.; Wu, Sanfeng; Du, Zongzheng; Hsu, Chuang-Han; Fang, Shiang; Gibson, Quinn D.; Watanabe, Kenji; Taniguchi, Takashi; Cava, Robert J.; Kaxiras, Efthimios; Lu, Hai-Zhou; Lin, Hsin; Fu, Liang; Gedik, Nuh; Jarillo-Herrero, PabloPublication Observation of Interband Collective Excitations in Twisted Bilayer Graphene(Springer Science and Business Media LLC, 2021-09-27) Hesp, Niels C. H.; Torre, Iacopo; Rodan Legrain, Daniel; Novelli, Pietro; Cao, Yuan; Carr, Stephen; Fang, Shiang; Stepanov, Petr; Barcons-Ruiz, David; Herzig Sheinfux, Hanan; Watanabe, Kenji; Taniguchi, Takashi; Efetov, Dmitri K.; Kaxiras, Efthimios; Jarillo-Herrero, Pablo; Polini, Marco; Koppens, Frank H. L.The single-particle and many-body properties of twisted bilayer graphene (TBG) can be dramatically 1 different from those of a single graphene layer, in particular when the two layers are rotated relative 2 to each other by a small angle ��≈��∘1–6. Here, we probe for the first time collective excitations of TBG 3 graphene with 20 nanometer spatial resolution, by applying mid-infrared (MIR) near-field optical 4 microscopy. We unveil a propagating plasmon mode in charge-neutral TBG with ��=��.��−��.��∘, which 5 is dramatically different from the ordinary single-layer graphene intraband plasmon7,8. We interpret it 6 as an interband plasmon associated with the optical transitions between minibands originating from 7 the moiré superlattice9,10. The details of the plasmon dispersion are directly related to the motion of 8 electrons in the moiré superlattice and offer invaluable insight into a plethora of physical properties, 9 such as the band nesting between flat band and remote band10, local interlayer coupling, losses etc. We 10 find a strongly reduced interlayer coupling in the regions with AA-stacking, pointing at screening due 11 to electron-electron (e-e) interactions. Optical nano-imaging studies of TBG pave the way to spatially 12 probe interactions effects at the nanoscale11, it could potentially elucidate the contribution of collective excitations to many-body ground states12, and it unveils itself as a new platform for strong light-matter 14 interactions and quantum plasmonic studies and devices13.Publication Twofold Van Hove Singularity and Origin of Charge Order in Topological Kagome Superconductor CsV3Sb5(Springer Science and Business Media LLC, 2022-01-13) Kang, Mingu; Fang, Shiang; Kim, Jeong-Kyu; Ortiz, Brenden R.; Ryu, Sae Hee; Kim, Jimin; Yoo, Jonggyu; Sangiovanni, Giorgio; Di Sante, Domenico; Park, Byeong-Gyu; Jozwiak, Chris; Bostwick, Aaron; Rotenberg, Eli; Kaxiras, Efthimios; Wilson, Stephen D.; Park, Jae-Hoon; Comin, Riccardo