Showing papers by "Hsin Lin published in 2014"

Direct observation of the transition from indirect to direct bandgap in atomically thin epitaxial MoSe2

Abstract: The transition from an indirect to direct bandgap in transition metal dichalcogenides has been observed in samples with thicknesses ranging from 8 to 1 monolayers by angle-resolved photoemission spectroscopy.

Observation of a three-dimensional topological Dirac semimetal phase in high-mobility Cd3As2.

Abstract: Topological Dirac semimetals constitute a promising platform for the study of quantum Hall phenomena and Weyl fermion transport. Using high-resolution angle-resolved photoemission spectroscopy, Neupane et al. identify the topological bulk Dirac semimetal phase in a Cd3As2 system.

Observation of quantum-tunnelling-modulated spin texture in ultrathin topological insulator Bi2Se3 films

Abstract: Understanding the spin-texture behaviour of boundary modes in ultrathin topological insulator films is critically essential for the design and fabrication of functional nanodevices. Here, by using spin-resolved photoemission spectroscopy with p-polarized light in topological insulator Bi2Se3 thin films, we report tunnelling-dependent evolution of spin configuration in topological insulator thin films across the metal-to-insulator transition. We report a systematic binding energy- and wavevector-dependent spin polarization for the topological surface electrons in the ultrathin gapped-Dirac-cone limit. The polarization decreases significantly with enhanced tunnelling realized systematically in thin insulating films, whereas magnitude of the polarization saturates to the bulk limit faster at larger wavevectors in thicker metallic films. We present a theoretical model that captures this delicate relationship between quantum tunnelling and Fermi surface spin polarization. Our high-resolution spin-based spectroscopic results suggest that the polarization current can be tuned to zero in thin insulating films forming the basis for a future spin-switch nanodevice.

Prediction of large-gap two-dimensional topological insulators consisting of bilayers of group III elements with Bi.

Abstract: We use first-principles electronic structure calculations to predict a new class of two-dimensional (2D) topological insulators (TIs) in binary compositions of group III elements (B, Al, Ga, In, and Tl) and bismuth (Bi) in a buckled honeycomb structure. We identify band inversions in pristine GaBi, InBi, and TlBi bilayers, with gaps as large as 560 meV, making these materials suitable for room-temperature applications. Furthermore, we demonstrate the possibility of strain engineering in that the topological phase transition in BBi and AlBi could be driven at ∼6.6% strain. The buckled structure allows the formation of two different topological edge states in the zigzag and armchair edges. More importantly, isolated Dirac-cone edge states are predicted for armchair edges with the Dirac point lying in the middle of the 2D bulk gap. A room-temperature bulk band gap and an isolated Dirac cone allow these states to reach the long-sought topological spin-transport regime. Our findings suggest that the buckled honeycomb structure is a versatile platform for hosting nontrivial topological states and spin-polarized Dirac fermions with the flexibility of chemical and mechanical tunability.

Mapping the unconventional orbital texture in topological crystalline insulators

Abstract: In crystalline topological insulators, the combination of an insulating bulk with conducting surface states is due to particular crystal symmetry. The associated Dirac cones—linear crossings in the electronic band structure—exhibit non-trivial orbital textures that have now been probed by means of scanning tunnelling spectroscopy. The newly discovered topological crystalline insulators feature a complex band structure involving multiple Dirac cones1,2,3,4,5,6, and are potentially highly tunable by external electric field, temperature or strain. Theoretically, it has been predicted that the various Dirac cones, which are offset in energy and momentum, might harbour vastly different orbital character7. However, their orbital texture, which is of immense importance in determining a variety of a material’s properties8,9,10 remains elusive. Here, we unveil the orbital texture of Pb1−xSnxSe, a prototypical topological crystalline insulator. By using Fourier-transform scanning tunnelling spectroscopy we measure the interference patterns produced by the scattering of surface-state electrons. We discover that the intensity and energy dependences of the Fourier transforms show distinct characteristics, which can be directly attributed to orbital effects. Our experiments reveal a complex band topology involving two Lifshitz transitions11 and establish the orbital nature of the Dirac bands, which could provide an alternative pathway towards future quantum applications.

Hydrogenated ultra-thin tin films predicted as two-dimensional topological insulators

Abstract: Using thickness-dependent first-principles electronic structure calculations, we predict that hydrogenated ultra-thin films of tin harbor a new class of two-dimensional (2D) topological insulators (TIs). A single bilayer (BL) tin film assumes a 2D-TI phase, but it transforms into a trivial insulator after hydrogenation. In contrast, tin films with 2 and 3 BLs are found to be trivial insulators, but hydrogenation of 2 to 4 BL films results in a non-trivial TI phase. For 1 to 3 BLs, H-passivation converts the films from being metallic to insulating. Moreover, we examined iodine-terminated tin films up to 3 BLs, and found these to be non-trivial, with the films becoming semi-metallic beyond 1 BL. In particular, the large band gap of 340 meV in an iodine-terminated tin BL is not sustained in the iodine-terminated 2 BL and 3 BL tin films.

Strain driven topological phase transitions in atomically thin films of group IV and V elements in the honeycomb structures

Abstract: We have investigated topological electronic properties of freestanding bilayers of group IV (C, Si, Ge, Sn, and, Pb) and V (As, Sb, and, Bi) elements of the periodic table in the buckled and planar honeycomb structures under isotropic strain using first-principles calculations. Our focus is on mapping strain driven phase diagrams and identifying topological phase transitions therein as a pathway for guiding search for suitable substrates to grow two-dimensional (2D) topological insulators (TIs) films. Bilayers of group IV elements, excepting Pb, generally transform from trivial metal topological metal TI topological metal trivial metal phase with increasing strain from negative (compressive) to positive (tensile) values. Similarly, among the group V elements, As and Sb bilayers transform from trivial metal trivial insulator TI phase, while Bi transforms from a topological metal to TI phase. The band gap of 0.5 eV in the TI phase of Bi is the largest we found among all bilayers studied, with the band gap increasing further under tensile strain. Differences in the topological characteristics of bilayers of group V elements reflect associated differences in the strength of the spin–orbit coupling (SOC). We show, in particular, that the topological band structure of Sb bilayer becomes similar to that of a Bi bilayer when the strength of the SOC in Sb is artificially enhanced by a factor of 4. This study provides the first report that As can be a 2D TI under tensile strain. Notably, we found the existence of TI phases in all elemental bilayers we studied, except Pb.

The changing colors of a quantum-confined topological insulator.

Abstract: Bismuth selenide (Bi2Se3) is a 3D topological insulator, its strong spin–orbit coupling resulting in the well-known topologically protected coexistence of gapless metallic surface states and semiconducting bulk states with a band gap, Eg ≃ 300 meV. A fundamental question of considerable importance is how the electronic properties of this material evolve under nanoscale confinement. We report on catalyst-free, high-quality single-crystalline Bi2Se3 with controlled lateral sizes and layer thicknesses that could be tailored down to a few nanometers and a few quintuple layers (QLs), respectively. Energy-resolved photoabsorption spectroscopy (1.5 eV < Ephoton < 6 eV) of these samples reveals a dramatic evolution of the photon absorption spectra as a function of size, transitioning from a featureless metal-like spectrum in the bulk (corresponding to a visually gray color), to one with a remarkably large band gap (Eg ≥ 2.5 eV) and a spectral shape that correspond to orange-red colorations in the smallest samples...

Nanoscale interplay of strain and doping in a high-temperature superconductor.

Abstract: The highest-temperature superconductors are electronically inhomogeneous at the nanoscale, suggesting the existence of a local variable that could be harnessed to enhance the superconducting pairing. Here we report the relationship between local doping and local strain in the cuprate superconductor Bi2Sr2CaCu2O8+x. We use scanning tunneling microscopy to discover that the crucial oxygen dopants are periodically distributed in correlation with local strain. Our picoscale investigation of the intraunit-cell positions of all oxygen dopants provides essential structural input for a complete microscopic theory.

Nanoscale interplay of strain and doping in a high-temperature superconductor

Abstract: The highest temperature superconductors are electronically inhomogeneous at the nanoscale, suggesting the existence of a local variable which could be harnessed to enhance the superconducting pairing. Here we report the relationship between local doping and local strain in the cuprate superconductor Bi$_2$Sr$_2$CaCu$_2$O$_{8+x}$. We use scanning tunneling microscopy to discover that the crucial oxygen dopants are periodically distributed, in correlation with local strain. Our picoscale investigation of the intra-unit-cell positions of all oxygen dopants provides essential structural input for a complete microscopic theory.

Role of acoustic phonons in Bi 2 Se 3 topological insulator slabs: A quantum transport investigation

Abstract: We present a model for quantum transport in a ${\mathrm{Bi}}_{2}{\mathrm{Se}}_{3}$ slab, which is based on using a nonequilibrium Green's function approach in which bulk and surface states are modeled realistically, and the effects of phonon scatterings are included. Resistivity is computed for different temperatures and strengths of the electron-phonon coupling at various doping levels. Temperature dependence of resistivity is found to display an insulating trend when the slab is biased at the Dirac point even in the presence of strong electron-phonon coupling. In sharp contrast, for carrier doping, the material displays a metallic behavior induced by acoustic scattering effects, even though purely ballistic transport yields an insulating trend, explaining contradictory trends reported in transport experiments on ${\mathrm{Bi}}_{2}{\mathrm{Se}}_{3}$. Our analysis, furthermore, suggests an experimental strategy for obtaining a handle on the strength of electron-phonon coupling in topological insulators via temperature-dependent transport measurements.

Orbital density wave order and electronic correlation driven insulating 1T-TaS2 monolayer

Abstract: We present the orbital resolved electronic properties of structurally distorted 1T-TaS2 monolayers. After optimizing the crystal structures, we obtain the lattice parameters and atomic positions in the star-of-David structure, and show the low-temperature band structures of distorted bulk are consistent with recent angle resolved photoemission spectroscopy (ARPES) data. We further clearly demonstrate that $5d$ electrons of Ta form ordered orbital-density-wave (ODW) state with dominant $5d_{3{z}^2-{r}^2}$ character in central Ta, driving the one-dimensional metallic state in paramagnetic bulk and half-filled insulator in monolayer. Meanwhile, the star-of-David distortion in monolayers favors charge density wave and the flat band stabilizes ferromagnetic density wave of Ta spins with the same wavevector of ODW 4/13 b_1+1/13 b2. We propose that $1$T-TaS$_{2}$ monolayer may pave a new way to study the exciton physics, exciton-polaron coupling, and potential applications for its exciton luminescence.

Y-shape spin-separator for two-dimensional group-IV nanoribbons based on quantum spin hall effect

Abstract: An efficient spin-separator that operates in quantum spin hall phase has been investigated for two-dimensional group-IV materials. A three-terminal Y-shaped device has been simulated via non-equilibrium Green Function to demonstrate the separation of unpolarized current at source terminal into spin-polarized current of opposite polarity at the two drain terminals. Device controls, i.e., tunable buckling and perpendicular magnetic field have been modeled comprehensively to evaluate the device feasibility and performance. It is shown that these controls can preferentially steer current between the two drains to create a differential charge current with complementary spin polarization, thus enabling a convenient regulation of output signal.

Electrically tunable localized tunneling channels in silicene nanoribbons

Abstract: The topological phase of a silicene nanoribbon holding edge states in the bulk energy gap can be easily broken by an external electric field. Here, we show through low-energy Green's function calculations that it is possible to localize conducting channels anywhere in a silicene nanoribbon by applying an inhomogeneous electric field. The spin degeneracy of these channels can also be broken in the same manner, allowing conduction of spin as well as charge. On this basis, we suggest design of a ternary logic device, which could be used in low-power circuits. Our study demonstrates that silicene and related group IV elements with honeycomb structure could provide a platform for efficient manipulation of spin currents via external electric fields, without the need to switch magnetic fields for spintronics applications.

Topological phase transition and quantum spin Hall state in TlBiS2

Abstract: We have investigated the bulk and surface electronic structures and band topology of TlBiS2 as a function of strain and electric field using ab-initio calculations. In its pristine form, TlBiS2 is a normal insulator, which does not support any non-trivial surface states. We show however that a compressive strain along the (111) direction induces a single band inversion with Z2 = (1;000), resulting in a Dirac cone surface state with a large in-plane spin polarization. Our analysis shows that a critical point lies between the normal and topological phases where the dispersion of the 3D bulk Dirac cone at the Γ-point becomes nearly linear. The band gap in thin films of TlBiS2 can be tuned through an out-of-the-plane electric field to realize a topological phase transition from a trivial insulator to a quantum spin Hall state. An effective k·p model Hamiltonian is presented to simulate our first-principles results on TlBiS2.

Tunable topological electronic structure of silicene on a semiconducting Bi/Si(111)- 3 × 3 substrate

Abstract: Using first-principles calculations to obtain the crystal and electronic structures, we show that the $1\ifmmode\times\else\texttimes\fi{}1$ phase of silicene is energetically more favorable than the $\sqrt{3}\ifmmode\times\else\texttimes\fi{}\sqrt{3}$ silicene superstructure on a semiconducting Bi/Si(111)-$\sqrt{3}\ifmmode\times\else\texttimes\fi{}\sqrt{3}$ substrate. The band gap of the system is found to be influenced strongly through the participation of Bi orbitals, which possess a larger spin-orbit coupling strength compared to Si. In particular, the nontrivial (topological) band gap of a few meV in freestanding $1\ifmmode\times\else\texttimes\fi{}1$ silicene enlarges to 124 meV and becomes trivial in the presence of the substrate. We further show how an out-of-the-plane external electric field can be used to tune the band gap and restore the nontrivial topological phase.

Spin-correlated electronic state on the surface of a spin-orbit Mott system

Abstract: We use angle-resolved photoemission spectroscopy to show that the near-surface electronic structure of a bulk insulating iridate Sr3Ir2O7 lying near a metal-Mott insulator transition exhibits weak metallicity signified by finite electronic spectral weight at the Fermi level. The surface electrons exhibit a spin structure resulting from an interplay of spin-orbit, Coulomb interaction and surface quantum magnetism. Our results shed light on understanding the exotic quantum entanglement and transport phenomena in iridate-based oxide devices.

Dirac mass generation from crystal symmetry breaking on the surfaces of topological crystalline insulators

Abstract: The tunability of topological surface states (SS) and controllable opening of the Dirac gap are of fundamental and practical interest in the field of topological materials. In topological crystalline insulators (TCIs), a spontaneously generated Dirac gap was recently observed, which was ascribed to broken cubic crystal symmetry. However, this structural distortion has not been directly observed so far, and the microscopic mechanism of Dirac gap opening via crystal symmetry breaking remains elusive. In this work, we present scanning tunneling microscopy (STM) measurements of a TCI Pb$_{1-x}$Sn$_x$Se for a wide range of alloy compositions spanning the topological and non-topological regimes. STM topographies directly reveal a symmetry-breaking distortion on the surface, which imparts mass to the otherwise massless Dirac electrons - a mechanism analogous to the long sought-after Higgs mechanism in particle physics. Remarkably, our measurements show that the Dirac gap scales with alloy composition, while the magnitude of the distortion remains nearly constant. Based on theoretical calculations, we find the Dirac mass is controlled by the composition-dependent SS penetration depth, which determines the weight of SS in the distorted region that is confined to the surface. Finally, we discover the existence of SS in the non-topological regime, which have the characteristics of gapped, double-branched Dirac fermions.

Topological phase transition and quantum spin Hall state in TlBiS$_2$

Abstract: We have investigated the bulk and surface electronic structures and band topology of TlBiS$_2$ as a function of strain and electric field using \textit{ab-initio} calculations. In its pristine form, TlBiS$_2$ is a normal insulator, which does not support any non-trivial surface states. We show however that a compressive strain along the (111) direction induces a single band inversion with Z$_2$ = (1;000), resulting in a Dirac cone surface state with a large in-plane spin polarization. Our analysis shows that a critical point lies between the normal and topological phases where the dispersion of the 3D bulk Dirac cone at the $\Gamma$-point becomes nearly linear. The band gap in thin films of TlBiS$_2$ can be tuned through an out-of-the-plane electric field to realize a topological phase transition from a trivial insulator to a quantum spin Hall state. An effective $\mathbf{k \cdot p}$ model Hamiltonian is presented to simulate our first-principles results on TlBiS$_2$.

First-principles study of atomic structures and electronic properties of Bi ultrathin films on Ge(111)

Abstract: Abstract The atomic and electronic structures of ultrathin bismuth films on Ge(111) surface were investigated using first-principles calculations at Bi coverages ranging from 1/3 ML to 5 ML. Morphology of the surfaces varied as the coverage of Bi was increased. The first layer of bismuth atoms followed the well-known trimer model, exhibiting large Rashba spin-splittings. At 2 ML, bismuth atoms of the second monolayer form the second stacking layer of trimers, whereas at 3 ML and 5 ML, bismuth atoms of the two topmost monolayers form a buckled honeycomb structure. While the electronic structures of the two topmost layers exhibit two-dimensional nontrivial topological insulating phase, the bismuth atoms lying under these layers play an important role in p -type doping of the system.

Anomalous spin-momentum locked two-dimensional states in the vicinity of a topological phase transition

Abstract: We perform spin-resolved and spin-integrated angle-resolved photoemission spectroscopy measurements on a series of compositions in the BiTl(S1-xSex)2 system, focusing on x-values in the vicinity of the critical point for the topological phase transition (the band inversion composition). We observe quasi two dimensional (2D) states on the outer boundary of the bulk electronic bands in the trivial side (non-inverted regime) of the transition. Systematic spin-sensitive measurements reveal that the observed 2D states are spin-momentum locked, whose spin texture resembles the helical spin texture on the surface of a topological insulator. These anomalous states are observed to be only prominent near the critical point, thus are possibly related to strong precursor states of topological phase transition near the relaxed surface.

Stable structure of high In coverage on Si ( 111 ) − α − 3 × 3 -Au

Abstract: The structure of 0.8 ML In coverage on the Si(111)-$\sqrt{3}\ifmmode\times\else\texttimes\fi{}\sqrt{3}$-Au surface is investigated by scanning tunneling microscopy/spectroscopy and low energy electron diffraction. By depositing 1.2 ML In at room temperature followed by annealing at $500{\phantom{\rule{0.16em}{0ex}}}^{\ensuremath{\circ}}\mathrm{C}$, the surface reveals the $\sqrt{7}\ifmmode\times\else\texttimes\fi{}\sqrt{3}$ structure with 0.8 ML In coverage. The detailed atomic structure is studied by both the topography and $dI/dV$ images, and the results are further supported by the density functional theory. The observed structure has higher In coverage than the previous experiments and has not been proposed in the previous calculations.

A Minimal tight-binding model for ferromagnetic canted bilayer manganites

Abstract: Half-metallicity in materials has been a subject of extensive research due to its potential for applications in spintronics. Ferromagnetic manganites have been seen as a good candidate, and aside from a small minority-spin pocket observed in La$_{2-2x}$Sr$_{1+2x}$Mn$_{2}$O$_{7}$ $(x=0.38)$, transport measurements show that ferromagnetic manganites essentially behave like half metals. Here we develop robust tight-binding models to describe the electronic band structure of the majority as well as minority spin states of ferromagnetic, spin-canted antiferromagnetic, and fully antiferromagnetic bilayer manganites. Both the bilayer coupling between the MnO$_2$ planes and the mixing of the $|x^2 - y^2>$ and $|3z^2 - r^2>$ Mn 3d orbitals play an important role in the subtle behavior of the bilayer splitting. Effects of $k_z$ dispersion are included.