Impurity effect on weak antilocalization in the topological insulator Bi2Te3

We study the weak antilocalization (WAL) effect in topological insulator Bi(2)Te(3) thin films at low temperatures. The two-dimensional WAL effect associated with surface carriers is revealed in the tilted magnetic field dependence of magnetoconductance. Our data demonstrate that the observed WAL is robust against deposition of nonmagnetic Au impurities on the surface of the thin films, but it is quenched by the deposition of magnetic Fe impurities which destroy the π Berry phase of the topological surface states. The magnetoconductance data of a 5 nm Bi(2)Te(3) film suggests that a crossover from symplectic to unitary classes is observed with the deposition of Fe impurities.     

Gate-voltage control of chemical potential and weak antilocalization in Bi₂Se₃

We report that Bi?Se? thin films can be epitaxially grown on SrTiO? substrates, which allow for very large tunablity in carrier density with a back gate. The observed low field magnetoconductivity due to weak antilocalization (WAL) has a very weak gate-voltage dependence unless the electron density is reduced to very low values. Such a transition in WAL is correlated with unusual changes in longitudinal and Hall resistivities. Our results suggest a much suppressed bulk conductivity at large negative gate voltages and a possible role of surface states in the WAL phenomena.     

Emerging weak antilocalization effect in Ta0.7Nb0.3Sb2 semimetal single crystals

Weak antilocalization (WAL) effect is commonly observed in low-dimensional systems, three-dimensional (3D) topological insulators and semimetals. Here, we report the growth of high-quality Ta_0.7Nb_0.3Sb₂ single crystals via the chemical vapor transport (CVT). Clear sign of the WAL effect is observed below 50 K, probably due to the strong spin−orbital coupling in 3D bulk. In addition, it is worth noting that a relatively large MR of 120% appears under 1 T magnetic field at T = 2 K. Hall measurements and two-band model fitting results reveal high carrier mobility (>1000 cm²·V⁻¹·s⁻¹ in 2–300 K region), and off-compensation electron/hole ratio of ~8:1. Due to the angular dependence of the WAL effect and the fermiology of the Ta_0.7Nb_0.3Sb₂ crystals, interesting magnetic-field-induced changes of the symmetry of the anisotropic magnetoresistance (MR) from two-fold (≤ 0.6 T) to four-fold (0.8–1.5 T) and finally to two-fold (≥ 2 T) are observed. This phenomenon is attributed to the mechanism shift from the low-field WAL dominated MR to WAL and fermiology co-dominated MR and finally to high-field fermiology dominated MR. All these signs indicate that Ta_0.7Nb_0.3Sb₂ may be a topological semimetal candidate, and these magnetotransport properties may attract more theoretical and experimental exploration of the (Ta,Nb)Sb₂ family.     

A metallic surface state with uniaxial spin polarization on Tl/Ge(111)-(1 × 1)

We show that a metallic surface state is formed on Tl/Ge(111)-(1 × 1). The surface state forms electron pockets around K of the surface Brillouin zone. A first-principles calculation reveals that the electron pockets are composed of a single branch of a spin-split surface-state band. The spin quantization axis is along the surface normal and inverts according to the time-reversal symmetry. Since this spin-split branch is the unique metallic band on this surface, the surface conductivity should be governed by this spin-split branch, suggesting a possible spin-polarized electric current.     

Electron-phonon interaction and localization of surface-state carriers in a metallic monolayer

Temperature-dependent electron transport in a metallic surface superstructure, Si(111)sqrt[3] x sqrt[3]-Ag, was studied by a micro-four-point probe method and photoemission spectroscopy. The surface-state conductivity exhibits a sharp transition from metallic conduction to strong localization at approximately 150 K. The metallic regime is due to electron-phonon interaction while the localization seemingly originates from coherency of electron waves. Random potential variations, caused by Friedel oscillations of surface electrons around defects, likely induce strong carrier localization.     

Effect of electron-phonon interaction on surface states in zinc-blende GaN,AlN, and InN under pressure

A variational approach is used to study the surface states of electrons in a semi-infinite polar semiconductor under hydrostatic pressure. The effective Hamiltonian and the surface-state levels are derived including the effects of electron-optical phonon interaction and pressure. The numerical computation has been performed for the surface-state energies versus pressure for zinc-blende GaN, AlN, and InN. The results show that the effect of electron-optical phonon interaction lowers the surface-state energy. It is also found that the effect of electron-surface optical phonon interaction is much bigger than the effect of electron-half space longitudinal optical phonon interaction for surface-state levels. It indicates that the surface-state energies and the influence of electron-phonon interaction increase with pressure obviously.Received: 12 June 2003, Published online: 22 September 2003PACS: 63.20.Kr Phonon electron and phonon-phonon interactions - 71.38.-k Polarons and electron-phonon interactions - 73.20.At Surface states, band structure, electron density of states     

Synthesis and magnetotransport properties of Bi_2Se_3 nanowires

Bi_2Se_3, as a three-dimensional topological insulator, has attracted worldwide attention for its unique surface states which are protected by time-reversal symmetry. Here we report the synthesis and characterization of high-quality singlecrystalline Bi_2Se_3 nanowires. Bi_2Se_3 nanowires were synthesized by chemical vapor deposition(CVD) method via goldcatalyzed vapor-liquid-solid(VLS) mechanism. The structure and morphology were characterized by scanning electron microscopy(SEM), transmission electron microscopy(TEM), x-ray photoelectron spectroscopy(XPS), and Raman spectroscopy. In magnetotransport measurements, the Aharonov–Bohm(AB) effect was observed in a nanowire-based nanodevice, suggesting the existence of surface states in Bi_2Se_3 nanowires.     

Weak localization in disordered spin-1 chiral fermions

We theoretically investigate the quantum interference theory of magnetotransport of the three-component or spin-1 chiral fermions, which possess two linear Dirac bands and a flat band. For isotropic scalar impurities, the correction of conductivity from the coherent backscatter and non-coherent backscatter contributions cancel out in the intravalley scattering, leading to a weak localization correction to the Drude conductivity from the intervalley scattering. For the anisotropic impurities, the above cancelation is removed, we find the approximative quantum interference conductivity in the weak anisotropy case. The contributions from the chiral anomaly and classical Lorentz force are also discussed. Our work reveals some intriguing and detectable transport signatures of the novel spin-1 chiral fermions.     

Electron-phonon interaction effects on the surface states in wurtzite nitride semiconductors

A variational approach is used to study the surface states of an electron in a semi-infinite wurtzite nitride semiconductor. The surface-state energy of the electron is calculated, by taking the effects of the electron-surface optical phonon interaction and structure anisotropy into account. The numerical computation has been performed for the energies of the electronic surface states as a function of the surface potential V₀ for wurtzite GaN, AlN, and InN, respectively. The results show that the electron-phonon interaction lowers the surface state energy. It is also found that the energies of the electronic surface-state in wurtzite structures are lower than that in the zinc-blende structures by hundreds of meV for the materials calculated. The influence of e-p-interactions on the surface state of electron cannot be neglected.     

The nature of the vectorial photoelectric effect in the threshold energy region

Summary An immediate correlation has been proved to exist between the vectorial photoelectric effect and electronic surface structure modified due to adsorption. Experimental studies were performed for Cs overlayers on W(110) and Si(111) faces using visible exciting photons with energies below those of both bulk and surface plasmons. Considerable (more than 20 fold) growth of the effect was observed forp-polarized light in the vicinity of threshold as the light frequency approached the position of the maximum of local density in the surface-state band. A special case of the vectorial photoelectric effect forp-polarized light has been found in the absence of bulk photoemission excited bys-polarized light. Our results prove that the anomalous vectorial photoelectric effect is caused by the normal electric vector component of thep-polarized light under appropriate conditions for the particular photoemission enhancement on the surface. The effect can be related to optical absorption by the surface-state band and to subsequent electron emission into the vacuum, the normal electric vector component of thep-polarized light being essential for both processes.     

The effect of carrier density gradients on magnetotransport data measured in Hall bar geometry

We have measured magnetotransport of the two-dimensional electron gas in a Hall bar geometry in the presence of small carrier density gradients. We find that the longitudinal resistances measured at both sides of the Hall bar interchange by reversing the polarity of the magnetic field. We offer a simple explanation for this effect and discuss implications for extracting conductivity flow diagrams of the integer quantum Hall effect.     

Dispersion and localization of electronic states at a ferrocene/Cu(111) interface

Low-temperature scanning tunneling microscopy and spectroscopy combined with first-principles simulations reveal a nondissociative physisorption of ferrocene molecules on a Cu(111) surface, giving rise to ordered molecular layers. At the interface, a 2D-like electronic band is found, which shows an identical dispersion as the Cu(111) Shockley surface-state band. Subsequent deposition of Cu atoms forms charged organometallic compounds that localize interface-state electrons.     

Surface-state parameters from very low energy electron reflection data

A parametric form of the amplitude of elastic reflection of very low energy electrons is derived. The amplitude expression conforms to the results of an earlier analysis of a simple case of electron reflection called the quasi two-beam case. The parameters in the amplitude expression refer to: (1) the surface states of the crystal; (2) the band structure of the substrate crystal; and (3) absorption (inelastic scattering) in the energy range of the experiment The amplitude expression also includes parameters relating to (4) the behavior of the amplitude at infinity and at negative energy.The amplitude expression is used to parameterize existing experimental results for nickel (001) and for the surface formed by adsorption of sodium on nickel (001) to form the centered (2 × 2) structure. The parameterization employs previously-computed values of parameters relating to the nickel band structure [category (2) above], and parameters in categories (1), (3) and (4) are adjusted to fit the electron-reflection data. In the case of the sodium-covered surface it is shown that the shape of the intensity-energy curve and the general level of intensity relative to that for clean nickel depends critically on the surface-state parameters. Two surface states are needed to fit the intensity data The values of the surface-state parameters are: location relative to vacuum level: 2.5 ± 0.1, 6.9 ± 0.2eV; width: 4.2 ± 0.4, 7.5 ± 1.0eV. The classification and significance of surface-state resonances is discussed briefly.     

Effect of self-assembled Ge nanostructures on Si surface electronic properties

Incorporating self-assembled Ge islands on Si surfaces into electronic devices has been suggested as a means of forming small features without fine-scale litho- graphy. For use in electronic devices, the electrical properties of the deposited Ge and their relation to the underlying Si substrate must be known. This report presents the results of a surface photovoltage investigation of the surface energy barrier as increasing amounts of Ge are added to a Si surface by chemical vapor deposition. The results are interpreted in terms of band discontinuities and surface states. The surface barrier increases as a wetting layer is deposited and continues to increase as defect-free islands form. It saturates as the islands grow. As the amount of Ge continues increasing, defects form, and the surface barrier decreases because of the resulting allowed states at the Ge/Si interface. Qualitatively similar behavior is found for Si(001) and Si(111). Covering the Ge with Si reduces the surface-state density and possibly modifies the wetting layer, decreasing the barrier to one more characteristic of Si. Initial hydrogen termination of the surface decreases the active surface-state density. As the H desorbs, the surface barrier increases until it stabilizes as the surface oxidizes. The behavior is briefly correlated with scanning-tunneling spectroscopy data. Received: 13 November 2000 / Accepted: 14 November 2000 / Published online: 23 May 2001     

The electrical transport properties of thin metal films containing ionized impurities

Summary Analytical expressions for the electrical conductivity and thermoelectric power are calculated for thin metal films containing ionized impurities that affect the value and the energy dependence of the electron relaxation time. The electrical conductivity decreases, whereas the thermoelectric power increases on addition of such impurities. For pure lattice scattering there is no size effect in the thermoelectric power. To speed up publication, the author of this paper has agreed to not receive the proofs for correction.     

Tunable quantum dot arrays formed from self-assembled metal-organic networks

The confinement of Ag(111) surface-state electrons by self-assembled, nanoporous metal-organic networks is studied using low-temperature scanning tunneling microscopy and spectroscopy as well as electronic structure calculations. The honeycomb networks of Co metal centers and dicarbonitrile-oligophenyl linkers induce surface resonance states confined in the cavities with a tunable energy level alignment. We find that electron scattering is repulsive on the molecules and weakly attractive on Co. The tailored networks represent periodic arrays of uniform and coupled quantum dots.     

三维拓扑绝缘体antidot阵列结构中的磁致输运研究

利用聚焦离子束刻蚀技术在拓扑绝缘体Bi_2Se_3薄膜中刻蚀了纳米尺度的反点(antidot)阵列,并对制作的三个器件进行了系统的电学输运测量研究.低温下,所有器件中都观察到明显的弱反局域化效应.通过对弱反局域化效应的分析,发现器件一(Dev-1,不含有antidot阵列)和器件二(Dev-2,含有周期较大的antidot阵列)是始终由一个导电通道主导的量子输运系统,但在器件三(Dev-3,含有周期较小的antidot阵列)中能明确观察到较低温度下存在两个独立的导电通道,而在较高温度下Dev-3表现为由一个导电通道主导的量子输运系统.     

Impurity-induced localisation of the 2D surface-state continuum on a metal surface

The local density of states of two model systems formed by an individual adsorbate (Cs or Ar) on a Cu(111) surface is investigated using a wave-packet propagation approach. In both cases a sharp peak appears at the threshold of the 2D Shockley surface-state continuum. This peak is attributed to quasi-stationary states located just below the 2D continuum. Following the existence criterion for a bound state in two dimensions by Simon, such states splitting off the surface-state bottom should appear whenever the surface-state continuum is perturbed by an attractive potential, independently of the existence of atomic-like states bound by the perturbing potential. These findings are discussed in connection with recent observations of peaks located at the surface-state threshold in scanning tunnelling microscopy experiments. PACS 68.37.Ef; 73.20.At; 73.20.Hb; 68.43.-h     

A model for dissipative magnetotransport in the quantum Hall regime

Summary A model which describes the dissipative quantum magnetotransport in a noninteracting two-dimensional electron gas under the quantum Hall conditions is presented. The resulting dissipative conductivity is proportional to a damping factor γ. The assumption of an exponential dependence of γ on the Hall electric field gives a good fit to the available experimental data.