Showing papers by "Hsin Lin published in 2009"

Observation of a large-gap topological-insulator class with a single Dirac cone on the surface

Abstract: Recent experiments and theories have suggested that strong spin–orbit coupling effects in certain band insulators can give rise to a new phase of quantum matter, the so-called topological insulator, which can show macroscopic quantum-entanglement effects. Such systems feature two-dimensional surface states whose electrodynamic properties are described not by the conventional Maxwell equations but rather by an attached axion field, originally proposed to describe interacting quarks. It has been proposed that a topological insulator with a single Dirac cone interfaced with a superconductor can form the most elementary unit for performing fault-tolerant quantum computation. Here we present an angle-resolved photoemission spectroscopy study that reveals the first observation of such a topological state of matter featuring a single surface Dirac cone realized in the naturally occurring Bi_2Se_3 class of materials. Our results, supported by our theoretical calculations, demonstrate that undoped Bi_2Se_3 can serve as the parent matrix compound for the long-sought topological device where in-plane carrier transport would have a purely quantum topological origin. Our study further suggests that the undoped compound reached via n-to-p doping should show topological transport phenomena even at room temperature.

A tunable topological insulator in the spin helical Dirac transport regime

Abstract: Helical Dirac fermions—charge carriers that behave as massless relativistic particles with an intrinsic angular momentum (spin) locked to its translational momentum—are proposed to be the key to realizing fundamentally new phenomena in condensed matter physics. Prominent examples include the anomalous quantization of magneto-electric coupling, half-fermion states that are their own antiparticle, and charge fractionalization in a Bose– Einstein condensate, all of which are not possible with conventional Dirac fermions of the graphene variety. Helical Dirac fermions have so far remained elusive owing to the lack of necessary spin-sensitive measurements and because such fermions are forbidden to exist in conventional materials harbouring relativistic electrons, such as graphene or bismuth. It has recently been proposed that helical Dirac fermions may exist at the edges of certain types of topologically ordered insulators—materials with a bulk insulating gap of spin–orbit origin and surface states protected against scattering by time-reversal symmetry—and that their peculiar properties may be accessed provided the insulator is tuned into the so-called topological transport regime. However, helical Dirac fermions have not been observed in existing topological insulators. Here we report the realization and characterization of a tunable topological insulator in a bismuthbased class of material by combining spin-imaging and momentum-resolved spectroscopies, bulk charge compensation, Hall transport measurements and surface quantum control. Our results reveal a spin-momentum locked Dirac cone carrying a nontrivial Berry’s phase that is nearly 100 per cent spin-polarized, which exhibits a tunable topological fermion density in the vicinity of the Kramers point and can be driven to the long-sought topological spin transport regime. The observed topological nodal state is shown to be protected even up to 300 K. Our demonstration of room-temperature topological order and non-trivial spintexture in stoichiometric Bi_2Se_3.M_x (M_x indicates surface doping or gating control) paves the way for future graphene-like studies of topological insulators, and applications of the observed spinpolarized edge channels in spintronic and computing technologies possibly at room temperature.

Observation of time-reversal-protected single-Dirac-cone topological-insulator states in Bi2Te3 and Sb2Te3

Abstract: We show that the strongly spin-orbit coupled materials Bi_2Te_3 and Sb_2Te_3 and their derivatives belong to the Z_2 topological-insulator class. Using a combination of first-principles theoretical calculations and photoemission spectroscopy, we directly show that Bi_2Te_3 is a large spin-orbit-induced indirect bulk band gap (δ∼150 meV) semiconductor whose surface is characterized by a single topological spin-Dirac cone. The electronic structure of self-doped Sb_2Te_3 exhibits similar Z_2 topological properties. We demonstrate that the dynamics of spin-Dirac fermions can be controlled through systematic Mn doping, making these materials classes potentially suitable for topological device applications.

Origin of the Electron-Hole Asymmetry in the Scanning Tunneling Spectrum of the High-Temperature Bi 2 Sr 2 CaCu 2 O 8 + δ Superconductor

Abstract: We have developed a material specific theoretical framework for modeling scanning tunneling spectroscopy (STS) of high-temperature superconducting materials in the normal as well as the superconducting state. Results for ${\mathrm{Bi}}_{2}{\mathrm{Sr}}_{2}{\mathrm{CaCu}}_{2}{\mathrm{O}}_{8+\ensuremath{\delta}}$ (Bi2212) show clearly that the tunneling process strongly modifies the STS spectrum from the local density of states of the ${d}_{{x}^{2}\ensuremath{-}{y}^{2}}$ orbital of Cu. The dominant tunneling channel to the surface Bi involves the ${d}_{{x}^{2}\ensuremath{-}{y}^{2}}$ orbitals of the four neighboring Cu atoms. In accord with experimental observations, the computed spectrum displays a remarkable asymmetry between the processes of electron injection and extraction, which arises from contributions of Cu ${d}_{{z}^{2}}$ and other orbitals to the tunneling current.

Multicolor upconversion and color tunability in Tm3+/Ho3+/Yb3+ doped opaque aluminum tellurite ceramics

Abstract: Rare earth doped opaque aluminum tellurite ceramics exhibiting multicolor upconversion and color tunability have been synthesized and characterized. The multicolor fluorescence comprises of green, red, and blue upconversion emissions from Ho3+ and Tm3+ ions. By varying the intensity ratios between the upconversion emission bands, the fluorescence color can be tuned from multicolor to white color by changing the infrared wavelength pump power. The fluorescence color tunability in Ho3+/Tm3+/Yb3+ triply doped aluminum tellurite ceramics will lead to new applications in the field of multicolor fluorescence technology.

Origin of the high-energy kink in the photoemission spectrum of the high-temperature superconductor Bi 2 Sr 2 CaCu 2 O 8

Abstract: The high-energy kink or the waterfall effect seen in the photoemission spectra of cuprates is suggestive of the coupling of quasiparticles to a high-energy bosonic mode with implications for the mechanism of superconductivity. Recent experiments, however, indicate that this effect may be an artifact produced entirely by matrix element effects, i.e., by the way the photoemitted electron couples to incident photons in the emission process. In order to address this issue directly, we have carried out realistic computations of the photointensity in ${\text{Bi}}_{2}{\text{Sr}}_{2}{\text{CaCu}}_{2}{\text{O}}_{8}$ where the effects of the matrix element are included together with those of the corrections to the self-energy resulting from electronic excitations. Our results demonstrate that while the photoemission matrix element plays an important role in shaping the spectra, the waterfall effect is a clear signature of the presence of strong coupling of quasiparticles to electronic excitations.

Role of oxygen electrons in the metal-insulator transition in the magnetoresistive oxide La2-2xSr1+2xMn2O7 probed by compton scattering.

Abstract: We have studied the [100]-[110] anisotropy of the Compton profile in the bilayer manganite. Quantitative agreement is found between theory and experiment with respect to the anisotropy in the two metallic phases (i.e., the low temperature ferromagnetic and the colossal magnetoresistant phase under a magnetic field of 7 T). Robust signatures of the metal-insulator transition are identified in the momentum density for the paramagnetic phase above the Curie temperature. We interpret our results as providing direct evidence for the transition from the metalliclike to the admixed ionic-covalent bonding accompanying the magnetic transition. The number of electrons involved in this phase transition is estimated. Our study demonstrates the sensitivity of the Compton scattering technique for identifying the number and type of electrons involved in the metal-insulator transition.

Spectral decomposition and matrix element effects in scanning tunneling spectroscopy of Bi 2 Sr 2 CaCu 2 O 8 + δ

Abstract: We present a Green's function-based framework for modeling the scanning tunneling spectrum from the normal as well as the superconducting state of complex materials where the nature of the tunneling process---i.e., the effect of the tunneling ``matrix element,'' is properly taken into account. The formalism is applied to the case of optimally doped ${\text{Bi}}_{2}{\text{Sr}}_{2}{\text{CaCu}}_{2}{\text{O}}_{8+\ensuremath{\delta}}$ (Bi2212) high-${T}_{\text{c}}$ superconductor using a large tight-binding basis set of electron and hole orbitals. The results show clearly that the spectrum is modified strongly by the effects of the tunneling matrix element and that it is not a simple replica of the local density of states of the $\text{Cu}\text{ }{d}_{{x}^{2}\ensuremath{-}{y}^{2}}$ orbitals with other orbitals playing a key role in shaping the spectra. We show how the spectrum can be decomposed usefully in terms of tunneling ``channels'' or paths through which the current flows from various orbitals in the system to the scanning tip. Such an analysis reveals symmetry-forbidden and symmetry-enhanced paths between the tip and the cuprate layers. Significant contributions arise from not only the ${\text{CuO}}_{2}$ layer closest to the tip but also from the second ${\text{CuO}}_{2}$ layer. The spectrum also contains a longer range background reflecting the nonlocal nature of the underlying Bloch states. In the superconducting state, coherence peaks are found to be dominated by the anomalous components of Green's function.

Warping the cone on a Topological Insulator

Abstract: Department of Physics, Northeastern University, Boston, MA 02115(Dated: December 27, 2009)The energy-momentum relationship of electrons on the surface of an ideal topological insulatorforms a cone - a Dirac cone, which, when warped (no longer described by the Dirac equation), canlead to unusual phenomena such as enhanced electronic interference around defects and a magnet-ically ordered broken symmetry surface. A detailed spin-texture and hexagonal warping maps onBi

Orbitally relieved magnetic frustration in NaVO2

Abstract: The magnetic properties of ${\text{NaVO}}_{2}$ are investigated using full-potential linearized augmented plane-wave method. We perform calculations for three structures. For the rhombohedral structure at 100 K, the ${t}_{2g}$ orbitals of V ions are split into upper ${a}_{1g}$ and lower ${e}_{g}^{\ensuremath{'}}$ orbitals by a trigonal distortion of compression. For the monoclinic structure at 91.5 K, the system behaves such as a frustrated spin lattice with spatially anisotropic exchange interactions. For another monoclinic structure at 20 K, the magnetic frustration is relieved by a lattice distortion which is driven by a certain orbital ordering and the long-range magnetic ordering is thus formed. Moreover, the small magnetic moment originates from the compensation of orbital moment for the spin moment.

Observation of unconventional band topology in a superconducting doped topological insulator, Cux-Bi2Se3: Topological Superconductor or non-Abelian superconductor?

Abstract: The Cu-doped topological insulator Bi$_2$Se$_3$ has recently been found to undergo a superconducting transition upon cooling, raising the possibilities that it is the first known "topological superconductor" or realizes a novel non-Abelian superconducting state. Its true nature depends critically on the bulk and surface state band topology. We present the first photoemission spectroscopy results where by examining the band topology at many different copper doping values we discover that the topologically protected spin-helical surface states remain well protected and separate from bulk Dirac bands at the Fermi level where Copper pairing occurs in the optimally doped topological insulator. The addition of copper is found to result in nonlinear electron doping and strong renormalization of the topological surface states. These highly unusual observations strongly suggest that superconductivity on the topological surface of Cu$_x$Bi$_2$Se$_3$ cannot be of any conventional type in account of the general topological theory. Characteristics of the three dimensional bulk Dirac band structure are reported for the first time with respect to the superconducting doping state and topological invariant properties which should help formulate a specific theory for this novel superconductor.

Effects of hydrogen impurities on Ge1?xMnx semiconductors

Abstract: The structural and magnetic properties of hydrogen-doped Ge1−xMnx diluted magnetic semiconductors are investigated using a first-principles pseudopotential method. Our results show that hydrogen impurities intend to bond to Mn ions in the Mn-doped Ge system and strongly influence the magnetic properties of this system. Hydrogen impurities actually reduce the Curie temperature of this system and might be one of the reasons for the existence of paramagnetic samples of Ge1−xMnx. Alternatively, hydrogenation can provide an easy and nonvolatile way to control and pattern the ferromagnetic properties of Mn-doped Ge diluted magnetic semiconductors, which has been achieved in the Mn-doped GaAs system (Goennenwein S. T. B. et al., Phys. Rev. Lett., 92 (2004) 227202).

Viewpoint: Warping the cone on a Topological Insulator

Abstract: The energy-momentum relationship of electrons on the surface of an ideal topological insulator forms a cone, which, when warped, can lead to unusual phenomena such as enhanced interference around defects and a magnetically ordered exotic surface.

Magnetic and electronic properties of α-NaMnO2

Abstract: The magnetic and electronic properties of α-NaMnO2 are investigated by performing the first-principles density functional calculations. A semiconducting ground state with high spin Mn is found. The magnetic coupling within a Mn layer is antiferromagnetic and frustrated, while the coupling between layers is very weak. Spatially anisotropic exchange is found within a Mn layer. The large Jahn–Teller distortion lowers the energy of d3r2−z2 orbital, leading to the stabilization of high spin Mn and ferromagnetic orbital ordering. Moreover, GGA+U studies show that α-NaMnO2 has the mixed characteristics of Mott–Hubbard insulators and charge transfer insulators.

Effects of hydrogen impurities on MnxSi1−x semiconductors

Abstract: In the present work, we have chosen different configurations to perform relaxations to determine the ground states of hydrogen doping in MnxSi1−x, respectively. Our results show that hydrogen impurity intends to bond strongly to Mn ion in MnxSi1−x. After introducing hydrogen, the phase transfer occurs from a half metallic one to a metallic case, and the magnetic moment of Mn ion is reduced. In MnxSi1−x, Mn ions exhibit a strong short-range antiferromagnetic and long-range ferromagnetic interaction. However, with the hydrogen doping, the exchange interactions of Mn ions oscillate between antiferromagnetic coupling and ferromagnetic coupling as a function of the distances between Mn ions. Hydrogen significantly influences the properties of MnxSi1−x.

High Resolution Compton Scattering as a Probe of the Fermi Surface in the Iron-based Superconductor LaO1−xFxFeAs

Abstract: We have carried out first-principles all-electron calculations of the (001)-projected 2D electron momentum density and the directional Compton profiles along the [100], [001] and [110] directions in the Fe-based superconductor LaOFeAs within the framework of the local density approximation. We identify Fermi surface features in the 2D electron momentum density and the directional Compton profiles, and discuss issues related to the observation of these features via Compton scattering experiments.