Showing papers by "Hsin Lin published in 2013"

Gated silicene as a tunable source of nearly 100% spin-polarized electrons

Abstract: Silicene is a one-atom-thick two-dimensional crystal of silicon with a hexagonal lattice structure that is related to that of graphene but with atomic bonds that are buckled rather than flat. This buckling confers advantages on silicene over graphene, because it should, in principle, generate both a band gap and polarized spin-states that can be controlled with a perpendicular electric field. Here we use first-principles calculations to show that field-gated silicene possesses two gapped Dirac cones exhibiting nearly 100% spin-polarization, situated at the corners of the Brillouin zone. Using this fact, we propose a design for a silicene-based spin-filter that should enable the spin-polarization of an output current to be switched electrically, without switching external magnetic fields. Our quantum transport calculations indicate that the proposed designs will be highly efficient (nearly 100% spin-polarization) and robust against weak disorder and edge imperfections. We also propose a Y-shaped spin/valley separator that produces spin-polarized current at two output terminals with opposite spins.

Surface electronic structure of the topological Kondo-insulator candidate correlated electron system SmB6.

Abstract: The Kondo insulator SmB6 has long been known to exhibit low-temperature transport anomalies whose origin is of great interest. Here we uniquely access the surface electronic structure of the anomalous transport regime by combining state-of-the-art laser and synchrotron-based angle-resolved photoemission techniques. We observe clear in-gap states (up to ~4 meV), whose temperature dependence is contingent on the Kondo gap formation. In addition, our observed in-gap Fermi surface oddness tied with the Kramers' point topology, their coexistence with the two-dimensional transport anomaly in the Kondo hybridization regime, as well as their robustness against thermal recycling, taken together, collectively provide strong evidence for protected surface metallicity with a Fermi surface whose topology is consistent with the theoretically predicted topological Fermi surface. Our observations of systematic surface electronic structure provide the fundamental electronic parameters for the anomalous Kondo ground state of correlated electron material SmB6.

Observation of Dirac node formation and mass acquisition in a topological crystalline insulator.

Abstract: In topological crystalline insulators (TCIs), topology and crystal symmetry intertwine to create surface states with unique characteristics. The breaking of crystal symmetry in TCIs is predicted to impart mass to the massless Dirac fermions. Here, we report high-resolution scanning tunneling microscopy studies of a TCI, Pb1-xSnxSe, which reveal the coexistence of zero mass Dirac fermions protected by crystal symmetry with massive Dirac fermions consistent with crystal symmetry breaking. In addition, we show two distinct regimes of the Fermi surface topology separated by a Van-Hove singularity at the Lifshitz transition point. Our work paves the way for engineering the Dirac band gap and realizing interaction-driven topological quantum phenomena in TCIs.

Tunable topological electronic structures in Sb(111) bilayers: A first-principles study

Abstract: Electronic structures and band topology of a single Sb(111) bilayer in the buckled honeycomb configuration are investigated using first-principles calculations. A nontrivial topological insulating phase can be induced by tensile strain, indicating the possibility of realizing the quantum spin Hall state for Sb thin films on suitable substrates. The presence of buckling provides an advantage in controlling the band gap through an out-of-plane external electric field, making a topological phase transition with six spin-polarized Dirac cones at the critical point. With a tunable gap and reversible spin polarization, Sb thin films are promising candidates for spintronic applications.

Tunable topological electronic structures in Sb(111) bilayers: A first-principles study

Abstract: Electronic structures and band topology of a single Sb(111) bilayer in the buckled honeycomb configuration are investigated using first-principles calculations. A nontrivial topological insulating phase can be induced by tensile strain, indicating the possibility of realizing the quantum spin Hall state for Sb thin films on suitable substrates. The presence of buckling provides an advantage in controlling the band gap through an out-of-plane external electric field, making a topological phase transition with six spin-polarized Dirac cones at the critical point. With a tunable gap and reversible spin polarization, Sb thin films are promising candidates for spintronic applications.

Nontrivial topological electronic structures in a single Bi(111) bilayer on different substrates: A first-principles study

Abstract: Electronic structures, minimum energy configurations, and band topology of strained Bi(111) single bilayers placed on a variety of semiconducting and insulating substrates are investigated using first-principles calculations. A topological phase diagram of a free-standing Bi bilayer is presented to help guide the selection of suitable substrates. The insulating hexagonal-BN is identified as the best candidate substrate material for supporting nontrivial topological insulating phase of Bi bilayer thin films. A planar hexagonal Bi layer is predicted under tensile strain, which we show could be realized on a SiC substrate. The Bi bilayer becomes metallic under the compressive strain induced by Si and Ge substrates.

Nontrivial spin texture of the coaxial Dirac cones on the surface of topological crystalline insulator SnTe

Abstract: We present first-principles calculations of the nontrivial surface states and their spin textures in the topological crystalline insulator SnTe. The surface state dispersion on the [001] surface exhibits four Dirac cones centered along the intersection of the mirror plane and the surface plane. We propose a simple model of two interacting coaxial Dirac cones to describe both the surface state dispersion and the associated spin texture. The out-of-plane spin polarization is found to be zero due to the crystalline and time-reversal symmetries. The in-plane spin texture shows helicity with some distortion due to the interaction of the two coaxial Dirac cones, indicating a nontrivial mirror Chern number of $\ensuremath{-}2$, distinct from the value of $\ensuremath{-}1$ in a ${Z}_{2}$ topological insulator such as Bi/Sb alloys or Bi${}_{2}$Se${}_{3}$. The surface state dispersion and its spin texture would provide an experimentally accessible signature for determining the nontrivial mirror Chern number.

Imaging the evolution of metallic states in a correlated iridate

Abstract: Iridate materials are at present the focus of interest because the combination of strong spin–orbit effects and many-body electronic correlations makes their physics non-trivial. Now, the density of states of Sr3Ir2O7 is mapped out spatially using scanning tunnelling microscopy and spectroscopy, yielding insights into the influence of nanoscale heterogeneities on the electronic structure.

Reversal of the Circular Dichroism in Angle-Resolved Photoemission from Bi 2 Te 3

Abstract: The helical Dirac fermions at the surface of topological insulators show a strong circular dichroism which has been explained as being due to either the initial-state spin angular momentum, the initial-state orbital angular momentum, or the handedness of the experimental setup. All of these interpretations conflict with our data from ${\mathrm{Bi}}_{2}{\mathrm{Te}}_{3}$ which depend on the photon energy and show several sign changes. Our one-step photoemission calculations coupled to ab initio theory confirm the sign change and assign the dichroism to a final-state effect. Instead, the spin polarization of the photoelectrons excited with linearly polarized light remains a reliable probe for the spin in the initial state.

Imaging the evolution of metallic states in a spin-orbit interaction driven correlated iridate

Abstract: The Ruddlesden-Popper (RP) series of iridates (Srn+1IrnO3n+1) have been the subject of much recent attention due to the anticipation of emergent physics arising from the cooperative action of spin-orbit (SO) driven band splitting and Coulomb interactions[1-3]. However an ongoing debate over the role of correlations in the formation of the charge gap and a lack of understanding of the effects of doping on the low energy electronic structure have hindered experimental progress in realizing many of the predicted states[4-8] including possible high-Tc superconductivity[7,9]. Using scanning tunneling spectroscopy we map out the spatially resolved density of states in the n=2 RP member, Sr3Ir2O7 (Ir327). We show that the Ir327 parent compound, argued to exist only as a weakly correlated band insulator in fact possesses a substantial ~130meV charge excitation gap driven by an interplay between structure, SO coupling and correlations. A critical component in distinguishing the intrinsic electronic character within the inhomogeneous textured electronic structure is our identification of the signature of missing apical oxygen defects, which play a critical role in many of the layered oxides. Our measurements combined with insights from calculations reveal how apical oxygen vacancies transfer spectral weight from higher energies to the gap energies thereby revealing a path toward obtaining metallic electronic states from the parent-insulating states in the iridates.

Observation of a bulk 3D Dirac multiplet, Lifshitz transition, and nestled spin states in Na3Bi

Abstract: Symmetry or topology protected Dirac fermion states in two and three dimensions constitute novel quantum systems that exhibit exotic physical phenomena. However, none of the studied spin-orbit materials are suitable for realizing bulk multiplet Dirac states for the exploration of interacting Dirac physics. Here we present experimental evidence, for the first time, that the compound Na3Bi hosts a bulk spin-orbit Dirac multiplet and their interaction or overlap leads to a Lifshitz transition in momentum space - a condition for realizing interactions involving Dirac states. By carefully preparing the samples at a non-natural-cleavage (100) crystalline surface, we uncover many novel electronic and spin properties in Na3Bi by utilizing high resolution angle- and spin-resolved photoemission spectroscopy measurements. We observe two bulk 3D Dirac nodes that locate on the opposite sides of the bulk zone center point $\Gamma$, which exhibit a Fermi surface Lifshitz transition and a saddle point singularity. Furthermore, our data shows evidence for the possible existence of theoretically predicted weak 2D nontrivial spin-orbit surface state with helical spin polarization that are nestled between the two bulk Dirac cones, consistent with the theoretically calculated (100) surface-arc-modes. Our main experimental observation of a rich multiplet of Dirac structure and the Lifshitz transition opens the door for inducing electronic instabilities and correlated physical phenomena in Na3Bi, and paves the way for the engineering of novel topological states using Na3Bi predicted in recent theory.

Oscillatory surface dichroism of the insulating topological insulator Bi 2 Te 2 Se

Abstract: Using circular dichroism-angle resolved photoemission spectroscopy, we report a study of the effect of angular momentum transfer between polarized photons and topological surface states on the surface of the insulating topological insulator Bi${}_{2}$Te${}_{2}$Se. The photoelectron dichroism is found to be strongly modulated by the frequency of the helical photons including a dramatic sign flip. Our results suggest that the observed dichroism and its sign flip are consequences of strong coupling between the photon field and the spin-orbit nature of the Dirac modes on the surface. Our studies reveal the intrinsic dichroic behavior of topological surface states and point toward the potential utility of bulk insulating topological insulators in opto-spintronics device applications.

Topological dangling bonds with large spin splitting and enhanced spin polarization on the surfaces of Bi2Se3.

Abstract: We investigate the topological surface state properties at various surface cleaves in the topological insulator Bi2Se3, via first principles calculations and scanning tunneling microscopy/spectrosc...

Adiabatic transformation as a search tool for new topological insulators: Distorted ternary Li2AgSb-class semiconductors and related compounds

Abstract: We demonstrate that the first-principles based adiabatic continuation approach is a very powerful and efficient tool for constructing topological phase diagrams and locating nontrivial topological insulator materials. Using this technique, we predict that the ternary intermetallic series Li${}_{2}{M}^{\ensuremath{'}}X$, where ${M}^{\ensuremath{'}}=\text{Cu}$, Ag, Au, or Cd and $X=\text{Sb}$, Bi, or Sn, hosts a number of topological insulators with remarkable functional variants and tunability. We also predict that several III-V semimetallic compounds are topologically nontrivial. We construct a topological phase diagram in the parameter space of the atomic numbers of atoms in Li${}_{2}{M}^{\ensuremath{'}}X$ compounds, which places a large number of topological materials presented in this work as well as in earlier studies within a single unified topological framework. Our results demonstrate the efficacy of adiabatic continuation as a useful tool for exploring topologically nontrivial alloying systems and for identifying new topological insulators even when the underlying lattice does not possess inversion symmetry, and the approaches based on parity analysis are not viable.

Topological phase transition and two-dimensional topological insulators in Ge-based thin films

Abstract: We discuss possible topological phase transitions in Ge-based thin films of Ge(Bi${}_{x}$Sb${}_{1\ensuremath{-}x}$)${}_{2}$Te${}_{4}$ as a function of layer thickness and Bi concentration $x$ using the first-principles density functional theory framework. The bulk material is a topological insulator at $x=1.0$ with a single Dirac cone surface state at the surface Brillouin zone center, whereas it is a trivial insulator at $x=0$. Through a systematic examination of the band topologies, we predict that thin films of Ge(Bi${}_{x}$Sb${}_{1\ensuremath{-}x}$)${}_{2}$Te${}_{4}$ with $x=0.6$, 0.8, and 1.0 are candidates for two-dimensional (2D) topological insulators, which would undergo a 2D topological phase transition as a function of $x$. A topological phase diagram for Ge(Bi${}_{x}$Sb${}_{1\ensuremath{-}x}$)${}_{2}$Te${}_{4}$ thin films is presented to help guide their experimental exploration.

Minority-spin t(2g) states and the degree of spin polarization in ferromagnetic metallic La(2-2x)Sr(1+2x)Mn(2)O(7) (x = 0.38).

Abstract: A half-metal is a material with conductive electrons of one spin orientation. This type of substance has been extensively searched for due to the fascinating physics as well as the potential applications for spintronics. Ferromagnetic manganites are considered to be good candidates, though there is no conclusive evidence for this notion. Here we show that the ferromagnet La2−2xSr1+2xMn2O7 (x = 0.38) possesses minority-spin states, challenging whether any of the manganites may be true half-metals. However, when electron transport properties are taken into account on the basis of the electronic band structure, we found that the La2−2xSr1+2xMn2O7 (x = 0.38) can essentially behave like a complete half metal.

Surface electronic structure of a topological Kondo insulator candidate SmB6: insights from high-resolution ARPES

Abstract: Author(s): Neupane, M; Alidoust, N; Xu, S-Y; Kondo, T; Kim, D-J; Liu, Chang; Belopolski, I; Chang, T-R; Jeng, H-T; Durakiewicz, T; Balicas, L; Lin, H; Bansil, A; Shin, S; Fisk, Z; Hasan, MZ | Abstract: The Kondo insulator SmB6 has long been known to exhibit low temperature (T l 10K) transport anomaly and has recently attracted attention as a new topological insulator candidate. By combining low-temperature and high energy-momentum resolution of the laser-based ARPES technique, for the first time, we probe the surface electronic structure of the anomalous conductivity regime. We observe that the bulk bands exhibit a Kondo gap of 14 meV and identify in-gap low-lying states within a 4 meV window of the Fermi level on the (001)-surface of this material. The low-lying states are found to form electron-like Fermi surface pockets that enclose the X and the Gamma points of the surface Brillouin zone. These states disappear as temperature is raised above 15K in correspondence with the complete disappearance of the 2D conductivity channels in SmB6. While the topological nature of the in-gap metallic states cannot be ascertained without spin (spin-texture) measurements our bulk and surface measurements carried out in the transport-anomaly-temperature regime (T l 10K) are consistent with the first-principle predicted Fermi surface behavior of a topological Kondo insulator phase in this material.

Symmetry-broken electronic structure and uniaxial Fermi surface nesting of untwinned CaFe2As2

Abstract: We used angle-resolved photoemission spectroscopy to make direct measurements of the electronic structure of the untwinned uniaxial state of CaFe2As2, the parent compound of an iron-based superconductor. The very small photon beam size, combined with the relatively large single-domain area on the crystal surfaces, allowed us to obtain the intrinsic symmetry-broken dispersions and Fermi surface (FS) geometries along the orthogonal Fe-Fe bond directions without any mechanical or magnetic detwinning processes. Comparing the optimized local density approximation calculations, an orbital-dependent band shifting is introduced to obtain better agreement, which is consistent with the development of orbital ordering. More interestingly, unidirectional straight and flat FS segments are observed near the zone center, which indicates the existence of a unidirectional charge density wave order. Our results indicate strong electronic anisotropy in CaFe2As2 and put strong constraints on theories for the iron-pnictide system.

Imaging the Nanoscale Band Structure of Topological Sb

Abstract: Many promising building blocks of future electronic technology - including non-stoichiometric compounds, strongly correlated oxides, and strained or patterned films - are inhomogeneous on the nanometer length scale. Exploiting the inhomogeneity of such materials to design next-generation nanodevices requires a band structure probe with nanoscale spatial resolution. To address this demand, we report the first simultaneous observation and quantitative reconciliation of two candidate probes - Landau level spectroscopy and quasiparticle interference imaging - which we employ here to reconstruct the multi-component surface state band structure of the topological semimetal antimony(Sb). We thus establish the technique of band structure tunneling microscopy (BSTM), whose unique advantages include nanoscale access to non-rigid band structure deformation, empty state dispersion, and magnetic field dependent states. We use BSTM to elucidate the relationship between bulk conductivity and surface state robustness in topological materials, and to quantify essential metrics for spintronics applications.

Corrigendum: Topological crystalline insulators in the SnTe material class

Abstract: Nature Communications 3: Article number: 982 (2012); Published: 31 July 2012; Updated: 21 May 2013 In the Discussion section of this Article, we incorrectly claimed that an in-plane magnetic field will generate Dirac mass terms for the surface states Instead, the in-plane magnetic field merely shifts the location of the Dirac points

Minority-spin $t_{2g}$ states and the degree of spin polarization in ferromagnetic metallic La$_{2-2x}$Sr$_{1+2x}$Mn$_2$O$_7$ ($x=0.38$)

Abstract: Using angle-resolved photoemission spectroscopy (ARPES), we investigate the electronic band structure and Fermi surface of ferromagnetic La$_{2-2x}$Sr$_{1+2x}$Mn$_2$O$_7$ ($x=0.38$). Besides the expected two hole pockets and one electron pocket of majority-spin $e_g$ electrons, we show an extra electron pocket around the $\Gamma$ point. A comparison with first-principles spin-polarized band-structure calculations shows that the extra electron pocket arises from $t_{2g}$ electrons of minority-spin character, indicating this compound is not a complete half-metallic ferromagnet, with similar expectations for lightly-doped cubic manganites. However, our data suggest that a complete half-metallic state is likely to be reached as long as the bandwidth is mildly reduced. Moreover, the band-resolved capability of ARPES enables us to investigate the band structure effects on spin polarization for different experimental conditions.

Pressure-induced spin-state and insulator-metal transitions in Sr3Fe2O5 from first principles

Abstract: Using hybrid functional calculations, we find that under the experimental pressure of 36.7 GPa, layered Sr3Fe2O5 undergoes a spin-state transition (SST) from high spin to intermediate spin. This transition is triggered by an electron depletion of the highest crystal-field level x 2 �y 2 . On the other hand, a reduced inter-layer distance lifts up the antibonding 3z 2 �r 2 state, and thus its electron occupation is partially shifted to the otherwise unoccupied x 2 �y 2 conduction band. This gives rise to an insulator-metal transition (IMT). We note that the IMT is closely related to the SST, but they somewhat have a different origin. Copyright c EPLA, 2013

An experimental algorithm for identifying the topological nature of Kondo and mixed valence insulators

Abstract: Possible topological nature of Kondo and mixed valence insulators has been a recent topic of interest in condensed matter physics Attention has focused on SmB6, which has long been known to exhibit low temperature transport anomaly, whose origin is of independent interest We argue that it is possible to resolve the topological nature of surface states by uniquely accessing the surface electronic structure of the low temperature anomalous transport regime through combining state-of-the-art laser- and synchrotron-based angle-resolved photoemission spectroscopy (ARPES) with or without spin resolution A combination of low temperature and ultra-high resolution (laser) which is lacking in previous ARPES studies of this compound is the key to resolve the possible existence of topological surface state in SmB6 Here we outline an experimental algorithm to systematically explore the topological versus trivial or mixed (topological and trivial surface state admixture as in the first 3D TI Bi$_{1-x}$Sb$_x$) nature of the surface states in Kondo and mixed valence insulators We conclude based on this methodology that the observed topology of the surface Fermi surface in our low temperature data considered within the level of current resolution is consistent with the theoretically predicted topological picture, suggesting a topological origin of the dominant in-gap ARPES signal in SmB6}

Commensurate magnetic excitations induced by band splitting and Fermi surface topology in n-type cuprates

Abstract: The antiferromagnetic correlation plays an important role in high-Tc superconductors. Considering this effect, the magnetic excitations in n-type cuprates near the optimal doping are studied within the spin-density-wave description. The magnetic excitations are commensurate in the low-energy regime and further develop into spin-wave-like dispersion at higher energy, consistent with the inelastic neutron scattering measurements. We clearly demonstrate that the commensurability originates from the band splitting and Fermi surface topology. The commensurability is a normal state property and has nothing to do with d-wave superconductivity. Our results strongly suggest the essential role of antiferromagnetic correlations in the cuprates.