arXiv subjects

Hsin Lin

Publications and source records attributed to Hsin Lin.

At least 199 records · Page 11Linked to original sources

arXiv2015

Non-Kondo-like Electronic Structure in the Correlated Rare-Earth Hexaboride YbB$_6$

We present angle-resolved photoemission studies on the rare-earth hexaboride YbB$_6$, which has recently been predicted to be a topological Kondo insulator. Our data do not agree with the prediction and instead show that YbB$_6$ exhibits a novel topological insulator state in the absence of a Kondo mechanism. We find that the Fermi level electronic structure of YbB$_6$ has three 2D Dirac cone like surface states enclosing the Kramers' points, while the f-orbital which would be relevant for the Kondo mechanism is $\sim1$ eV below the Fermi level. Our first-principles calculation shows that the topological state which we observe in YbB$_6$ is due to an inversion between Yb $d$ and B $p$ bands. These experimental and theoretical results provide a new approach for realizing novel correlated topological insulator states in rare-earth materials.

cond-mat.str-el↗

arXiv2015

Surface Versus Bulk Dirac States Tuning in a Three-Dimensional Topological Dirac Semimetal

Recently, crystalline-symmetry-protected three-dimensional (3D) bulk Dirac semimetal phase has been experimentally identified in a stoichiometric high-mobility compound, Cd3As2. The Dirac state observed in Cd3As2 has been attributed to originate mostly from the bulk state while calculations show that the bulk and surface states overlap over the entire Dirac dispersion energy range. In this study, we unambiguously reveal doping induced evolution of the ground state of surface and bulk electron dynamics in a 3D Dirac semimetal. We develop a systematic technique to isolate the surface and bulk states in Cd3As2, by simultaneously utilizing angle-resolved photoemission spectroscopy (ARPES) and in-situ surface deposition. Our experimental results provide a method for tuning the chemical potential as well as to observe surface states degenerate with bulk states, which will be useful for future applications of 3D Dirac semimetal.

cond-mat.mes-hall↗

arXiv2014

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

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.

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arXiv2014

A Minimal tight-binding model for ferromagnetic canted bilayer manganites

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.

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arXiv2014

Observation of metallic surface states in the strongly correlated Kitaev-Heisenberg candidate Na2IrO3

We report high-resolution angle-resolved photoemission spectroscopy measurements on the honeycomb iridate Na2IrO3. Our measurements reveal the existence of a metallic surface band feature crossing the Fermi level with nearly linear dispersion and an estimated surface carrier density of 3.2 $\times$ 10$^{13}$ cm$^{-2}$, which has not been theoretically predicted or experimentally observed, and provides the first evidence for metallic behavior on the boundary of this material, whereas the bulk bands exhibit a robust insulating gap. We further show the lack of theoretically predicted Dirac cones at the $\overline{M}$ points of the surface Brillouin zone, which confirms the absence of a stacked quantum spin Hall phase in this material. Our data indicates that the surface ground state of this material is exotic and metallic, unlike as predicted in theory, and establishes Na2IrO3 as a rare example of a strongly correlated spin-orbit insulator with surface metallicity.

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arXiv2014

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

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.

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arXiv2014

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

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 $Γ$-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$_2$.

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arXiv2014

Observation of Quantum-Tunneling Modulated Spin Texture in Ultrathin Topological Insulator Bi2Se3 Films

Understanding the spin-texture behavior of boundary modes in ultrathin topological insulator films is critically essential for the design and fabrication of functional nano-devices. Here by using spin-resolved photoemission spectroscopy with p-polarized light in topological insulator Bi2Se3 thin films, we report tunneling-dependent evolution of spin configuration in topological insulator thin films across the metal-to-insulator transition. We observe strongly binding energy- and wavevector-dependent spin polarization for the topological surface electrons in the ultra-thin gapped-Dirac-cone limit. The polarization decreases significantly with enhanced tunneling 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 which captures this delicate relationship between quantum tunneling 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 nano-device.

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arXiv2014

Spin correlated electronic state on the surface of a spin-orbit Mott system (Layered Iridates)

Novel phases of two dimensional electron systems resulting from new surface or interface modified electronic structures have generated significant interest in material science. We utilize photoemission spectroscopy to show that the near-surface electronic structure of a bulk insulating iridate Sr$_3$Ir$_2$O$_7$ lying near metal-Mott insulator transition exhibit weak metallicity signified by finite electronic spectral weight at the Fermi level. The surface electrons exhibit a unique spin structure resulting from an interplay of spin-orbit, Coulomb interaction and surface quantum magnetism, distinct from a topological insulator state. Our results suggest the experimental realization of a novel quasi two dimensional interacting electron surface ground state, opening the door for exotic quantum entanglement and transport phenomena in iridate-based oxide devices.

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arXiv2014

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

Quantum systems in confined geometries are host to novel physical phenomena. Examples include quantum Hall systems in semiconductors and Dirac electrons in graphene. Interest in such systems has also been intensified by the recent discovery of a large enhancement in photoluminescence quantum efficiency and a potential route to valleytronics in atomically thin layers of transition metal dichalcogenides, MX2 (M = Mo, W; X = S, Se, Te), which are closely related to the indirect to direct bandgap transition in monolayers. Here, we report the first direct observation of the transition from indirect to direct bandgap in monolayer samples by using angle resolved photoemission spectroscopy on high-quality thin films of MoSe2 with variable thickness, grown by molecular beam epitaxy. The band structure measured experimentally indicates a stronger tendency of monolayer MoSe2 towards a direct bandgap, as well as a larger gap size, than theoretically predicted. Moreover, our finding of a significant spin-splitting of 180 meV at the valence band maximum of a monolayer MoSe2 film could expand its possible application to spintronic devices.

cond-mat.mtrl-sci↗

arXiv2013

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

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 upon the Kondo gap formation. In addition, our observed in-gap Fermi surface oddness tied with the Kramers' points 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 by-far the strongest evidence for protected surface metallicity with a Fermi surface whose topology is consistent with the theoretically predicted topological surface Fermi surface (TSS). Our observations of systematic surface electronic structure provide the fundamental electronic parameters for the anomalous Kondo ground state of the correlated electron material SmB6.

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arXiv2013

Mapping the unconventional orbital texture in topological crystalline insulators

The newly discovered topological crystalline insulators (TCIs) harbor a complex band structure involving multiple Dirac cones. These materials are potentially highly tunable by external electric field, temperature or strain and could find future applications in field-effect transistors, photodetectors, and nano-mechanical systems. Theoretically, it has been predicted that different Dirac cones, offset in energy and momentum-space, might harbor vastly different orbital character, a unique property which if experimentally realized, would present an ideal platform for accomplishing new spintronic devices. However, the orbital texture of the Dirac cones, which is of immense importance in determining a variety of materials properties, still remains elusive in TCIs. Here, we unveil the orbital texture in a prototypical TCI Pb$_{1-x}$Sn$_x$Se. By using Fourier-transform (FT) scanning tunneling spectroscopy (STS) we measure the interference patterns produced by the scattering of surface state electrons. We discover that the intensity and energy dependences of FTs show distinct characteristics, which can directly be attributed to orbital effects. Our experiments reveal the complex band topology involving two Lifshitz transitions and establish the orbital nature of the Dirac bands in this new class of topological materials, which could provide a different pathway towards future quantum applications.

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arXiv2013

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

We discuss possible topological phase transitions in Ge-based thin films of Ge(Bi$_x$Sb$_{1-x}$)$_2$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$_{1-x}$)$_2$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$_{1-x}$)$_2$Te$_4$ thin films is presented to help guide their experimental exploration.

cond-mat.mes-hall↗

arXiv2013

Imaging the Nanoscale Band Structure of Topological Sb

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.

cond-mat.mes-hall↗

arXiv2013

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

In the recently discovered topological crystalline insulators (TCIs), topology and crystal symmetry intertwine to create surface states with a unique set of characteristics. Among the theoretical predictions for TCIs is the possibility of imparting mass to the massless Dirac fermions by breaking crystal symmetry, as well as a Lifshitz transition with a change of Fermi surface topology. Here we report high resolution scanning tunneling microscopy studies of a TCI, Pb1-xSnxSe. We demonstrate the formation of zero mass Dirac fermions protected by crystal symmetry and the mechanism of mass generation via symmetry breaking, which constitute the defining characteristics of TCIs. In addition, we show two distinct regimes of fermiology 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.

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arXiv2013

Non-trivial spin-texture of the coaxial Dirac cones on the surface of topological crystalline insulator SnTe

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. While the out-of-the-plane spin polarization is 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 -2, distinct from the value of -1 in $Z_{2}$ 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 way to determine the nontrivial mirror Chern number.

cond-mat.mtrl-sci↗

arXiv2013

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

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.

cond-mat.mtrl-sci↗

arXiv2013

Reversal of the circular dichroism in the angle-resolved photoemission from Bi2Te3

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 Bi2Te3 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. The spin polarization of the photoelectrons, instead, remains a reliable probe for the spin in the initial state.

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