We investigated magneto-optical response of undoped Bi2Te3 films in the terahertz frequency range (0.3–5.1 THz, 10–170 cm−1) in magnetic fields up to 10 T. The optical transmission, measured in the Faraday geometry, is dominated by a broad Lorentzian-shaped mode, whose central frequency linearly increases with applied field. In zero field, the Lorentzian is centered at zero frequency, representing hence the free-carrier Drude response. We interpret the mode as a cyclotron resonance (CR) of free carriers in Bi2Te3. Because the mode’s frequency position follows a linear magnetic-field dependence and because undoped Bi2Te3 is known to possess appreciable number of bulk carriers, we associate the mode with a bulk CR. In addition, the cyclotron mass obtained from our measurements fits well the literature data on the bulk effective mass in Bi2Te3. Interestingly, the width of the CR mode demonstrates a behavior non-monotonous in field. We propose that the CR width is defined by two competing factors: impurity scattering, which rate decreases in increasing field, and electron-phonon scattering, which exhibits the opposite behavior.

A promising route to the realization of Majorana fermions is in non-centrosymmetric superconductors, in which spin-orbit-coupling lifts the spin degeneracy of both bulk and surface bands. A detailed assessment of the electronic structure is critical to evaluate their suitability for this through establishing the topological properties of the electronic structure. This requires correct identification of the time-reversal-invariant momenta. One such material is BiPd, a recently rediscovered non-centrosymmetric superconductor which can be grown in large, high-quality single crystals and has been studied by several groups using angular resolved photoemission to establish its surface electronic structure. Many of the published electronic structure studies on this material are based on a reciprocal unit cell which is not the actual Brillouin zone of the material. We show here the consequences of this for the electronic structures and show how the inferred topological nature of the material is affected.

Scalable and routine integration of chemically exfoliated, graphene-based materials such as graphene oxide (GO) and reduced graphene oxide (rGO) into standard microelectronic fabrication is a tremendous technological challenge, blocking their advancement toward real applications. A unique approach for wafer-scale fabrication of rGO devices by a synergistic combination of chemically exfoliated GO with photolithography processing is realized. Using graphite powder as source material, a GO solution is produced in a newly optimized, low-temperature exfoliation and desalination protocol, resulting in high-quality GO and confirmed by various characterization techniques. As substrates, 4 in. Si/SiO2 or glass wafers were first silanized in a well-controlled, gas-phase procedure. Large-area GO thin films are then realized by standard spin-coating resulting in highly homogeneous, covalently bound layers of controllable thicknesses of 3-7 nm depending on the amount of spin-coatings. The robust thin films undergo routine photolithography for device fabrication, including reduction via thermal annealing into conductive rGO. The top-down fabricated rGO devices display high uniformity with electrical resistances varying within only one order of magnitude over wafer-scale and device yields as high as approximate to 93% on a wafer. The novel front-end-of-line GO integration protocol offers robust electrical performances for future implementation toward various sensor applications.

Materials with strong spin-orbit coupling (SOC) have in recent years become a subject of intense research due to their potential applications in spintronics and quantum information technology. In particular, in systems which break inversion symmetry, SOC facilitates the Rashba-Dresselhaus effect, leading to a lifting of spin degeneracy in the bulk and intricate spin textures of the Bloch wave functions. Here, by combining angular resolved photoemission spectroscopy and low temperature scanning tunneling microscopy measurements with relativistic first-principles band structure calculations, we examine the role of SOC in single crystals of noncentrosymmetric BiPd. We report the detection of several Dirac surface states, one of which exhibits an extremely large spin splitting. Unlike the surface states in inversion-symmetric systems, the Dirac surface states of BiPd have completely different properties at opposite faces of the crystal and are not trivially linked by symmetry. The spin splitting of the surface states exhibits a strong anisotropy by itself, which can be linked to the low in-plane symmetry of the surface termination.

Electronic devices based on graphene technology are catching on rapidly and the ability to engineer graphene properties at the nanoscale is becoming, more than ever, indispensable. Here, we present a procedure of graphene functionalization on SiC(0001) that paves the way towards the fabrication of complex graphene electronic chips. The procedure resides on the well-known ion-implantation technique. The efficiency of the working principle is demonstrated by the intercalation of the epitaxial graphene layer on SiC(0001) with Bi atoms, which was not possible following standard procedures. The investigation of the obtained graphene system reveals no clear spin-orbit coupling enhancement expected by theory in addition to the presence of residual structural defects. Our graphene/SiC(0001) intercalation procedure puts forward the ion-beam lithography to nanostructure and functionalize desired graphene chips.

Bi2Se3 has recently attracted a lot of attention because it has been reported to be a platform for the realization of three-dimensional topological insulators. Due to this exotic characteristic, it supports excitations of a two-dimensional electron gas at the surface and, hence, formation of Dirac-plasmons. In addition, at higher energies above its bandgap, Bi2Se3 is characterized by a naturally hyperbolic electromagnetic response, with an interesting interplay between type-I and type-II hyperbolic behaviors. However, still not all the optical modes of Bi2Se3 have been explored. Here, using mainly electron energy-loss spectroscopy and corresponding theoretical modeling we investigate the full photonic density of states that Bi2Se3 sustains, in the energy range of 0.8 eV-5 eV. We show that at energies below 1 eV, this material can also support wedge Dyakonov waves. Furthermore, at higher energies a huge photonic density of states is excited in structures such as waveguides and resonators made of Bi2Se3 due to the hyperbolic dispersion.

Theoretical and experimental studies agree that Bi1-xSbx (0.07 <= x <= 0.21) is a three-dimensional topological insulator. However, there is still a debate on the corresponding Bi1-xSbx (111) surface band structure. While three spin polarized bands have been claimed experimentally, theoretically, only two surface bands appear, with the third band being attributed to surface imperfections. Here, we address this controversy using angle-resolved photoemission spectroscopy (ARPES) on Bi1-xSbx films. To minimize surface imperfections, we have optimized the sample growth recipe. We have measured the evolution of the surface band structure of Bi1-xSbx with x increasing gradually from x = 0 to x = 0.6. Our ARPES data show better agreement with the theoretical calculations, where the system is topologically nontrivial with two surface bands.

Journal Article Unconventional Surface Plasmon Excitations in Bi2Se3 Get access Cigdem Ozsoy-Keskinbora, Cigdem Ozsoy-Keskinbora Stuttgart Center for Electron Microscopy (StEM), MPI for Intelligent Systems, Stuttgart, Germany Search for other works by this author on: Oxford Academic Google Scholar Nahid Talebi, Nahid Talebi Stuttgart Center for Electron Microscopy (StEM), MPI for Intelligent Systems, Stuttgart, Germany Search for other works by this author on: Oxford Academic Google Scholar Hadj M Benia, Hadj M Benia Nanoscale Science, MPI for Solid States Research, Stuttgart, Germany Search for other works by this author on: Oxford Academic Google Scholar Christoph T Koch, Christoph T Koch Institute for Experimental Physics, Ulm University, Ulm, Germany Search for other works by this author on: Oxford Academic Google Scholar Peter A van Aken Peter A van Aken Stuttgart Center for Electron Microscopy (StEM), MPI for Intelligent Systems, Stuttgart, Germany Search for other works by this author on: Oxford Academic Google Scholar Microscopy and Microanalysis, Volume 21, Issue S3, 1 August 2015, Pages 2057–2058, https://doi.org/10.1017/S143192761501106X Published: 23 September 2015

Three-dimensional topological insulators comprise topologically protected surface states displaying massless, Dirac-like linear dispersion with spin-momentum locking. Electrical conduction through such surface states has been documented to manifest itself in a two-dimensional character of the angle-dependent magnetotransport behavior. Here, we explore the size-dependent electronic properties of nanostructures made of the lead-containing mineral Aleksite, a naturally occurring topological insulator. Compared to its sister compound Kawazulite, a member of the well-studied Tetradymite crystal class, the crystal structure of Aleksite is distinguished by its lack of any counterpart within the group of synthetic topological insulators. Low temperature Hall measurements on thin Aleksite nanosheets reveal a significant carrier mobility on the order of 1000 cm 2 /(Vs) and a high carrier density of n = 3.9 × 10 25 m −3 . Importantly, for Aleksite nanoribbons with a width below 150 nm, a 1D weak antilocalization effect along with 1D universal conductance fluctuations emerges, which transforms into 2D behavior for larger ribbon widths

The earth's crust and outer space are rich sources of technologically relevant materials which have found application in a wide range of fields. Well-established examples are diamond, one of the hardest known materials, or graphite as a suitable precursor of graphene. The ongoing drive to discover novel materials useful for (opto)electronic applications has recently drawn strong attention to topological insulators. Here, we report that Kawazulite, a mineral with the approximate composition Bi2(Te,Se)2(Se,S), represents a naturally occurring topological insulator whose electronic properties compete well with those of its synthetic counterparts. Kawazulite flakes with a thickness of a few tens of nanometers were prepared by mechanical exfoliation. They exhibit a low intrinsic bulk doping level and correspondingly a sizable mobility of surface state carriers of more than 1000 cm(2)/(V s) at low temperature. Based on these findings, further minerals which due to their minimized defect densities display even better electronic characteristics may be identified in the future.

We study the band structure of the Bi2Se3 topological insulator (1 1 1) surface using angle-resolved photoemission spectroscopy. We examine the situation where two sets of quantized subbands exhibiting different Rashba spin splitting are created via bending of the conduction (CB) and the valence (VB) bands at the surface. While the CB subbands are strongly Rashba spin split, the VB subbands do not exhibit clear spin splitting. We find that CB and VB experience similar band bending magnitudes, which means, a spin-splitting discrepancy due to different surface potential gradients can be excluded. On the other hand, by comparing the experimental band structure to first-principles LMTO band structure calculations, we find that the strongly spin-orbit coupled Bi 6p orbitals dominate the orbital character of CB, whereas their admixture to VB is rather small. The spin-splitting discrepancy is, therefore, traced back to the difference in spin-orbit coupling between CB and VB in the respective subbands' regions.

Using angular resolved photoemission spectroscopy we studied the evolution of the surface electronic structure of the topological insulator Bi(2)Se(3) as a function of water vapor exposure. We find that a surface reaction with water induces a band bending, which shifts the Dirac point deep into the occupied states and creates quantum well states with a strong Rashba-type splitting. The surface is thus not chemically inert, but the topological state remains protected. The band bending is traced back to Se abstraction, leaving positively charged vacancies at the surface. Because of the presence of water vapor, a similar effect takes place when Bi(2)Se(3) crystals are left in vacuum or cleaved in air, which likely explains the aging effect observed in the Bi(2)Se(3) band structure.

The nucleation and growth of ultrathin MgO films on Mo(001) have been investigated with scanning tunneling microscopy and spectroscopy. In the initial growth stage, the MgO forms rather uniform islands with rectangular shapes and defined orientation. This behavior reflects a preferential binding of the oxide O ions to the top positions in the Mo support, which can be realized only in confined areas due to the MgO/Mo lattice mismatch. At monolayer coverage, a characteristic square pattern becomes visible in the STM, indicating the formation of an MgO/Mo coincidence lattice. In the coincidence cell, the interface registry alternates between O and Mg ions being in Mo top positions. The resulting imaging contrast in the STM is dominated by a work-function modulation and not by a topographic effect, as demonstrated with STM-conductance and light-emission spectroscopy. The modulated work function in the coincidence cell is assigned to a small buckling of the oxide film with either O or Mg ions being closer to the Mo surface.

The CO binding behavior to gold particles supported on MgO thin films has been analyzed with scanning tunneling microscopy (STM) and infrared spectroscopy (IRAS). The ad-particles accommodate excess electrons that originate either from a charge transfer through the thin oxide film or from a local interaction with electron-rich oxide defects that act as Au nucleation centers. The enhanced electron density in the Au aggregates affects both the spatial distribution and the vibrational properties of adsorbed CO species. Whereas preferential CO attachment to the chemically unsaturated and electron-rich boundary sites of the Au islands is deduced from the STM data, a continuous downshift of the CO stretching frequency with decreasing particle size is observed in IRAS. Both results are interpreted in the light of CO adsorption to negatively charged metal aggregates and used to draw general conclusions on the interplay between charge and adsorption properties of confined metal systems.

Misfit dislocations in a thin MgO/Mo(001) film have been investigated by conductance and light-emission spectroscopy using scanning tunneling microscopy and electron-paramagnetic resonance (EPR) spectroscopy. The line defects exhibit a higher work function than the pristine MgO, being explained by their ability to trap electrons. The electron traps are associated with a nonstoichiometric defect composition in thin oxide films and attractive pockets in the Madelung potential in thicker ones. The latter traps can be reproducibly filled by the adsorption of atomic hydrogen, which gives rise to a free-electronlike signal in EPR spectroscopy.

Using a combination of reciprocal and real-space techniques, the structural evolution and its effect on the surface morphology is investigated for MgO films of 1--30 ML thickness epitaxially grown on Mo(001). The strain induced by the mismatch with the substrate is relieved between 1 and 7 ML MgO due to the formation of an ordered network of interfacial misfit dislocations aligned along the MgO $⟨110⟩$ directions, particularly evident after annealing the film at 1070 K. A dislocation periodicity of about $60\text{ }\text{\AA{}}$ has been determined by means of grazing incidence x-ray diffraction. The dislocations induce a tilting of the surface that appears in electron diffraction along the $⟨100⟩$ MgO directions for thin films and changes to $⟨110⟩$ directions when the oxide thickness increases. Scanning tunneling microscopy (STM) shows the presence of a regular pattern on the surface below 7 ML thickness associated to the dislocation network. With increasing thickness, screw dislocations connected by nonpolar steps appear on the oxide surface. Thanks to the combination of different diffraction techniques and STM measurements, a comprehensive picture of the relaxation mechanisms in MgO films on Mo(001) can be drawn.