68 research outputs found
Intercalation of graphene on SiC(0001) via ion-implantation
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 new 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. Our
results put forward the ion-beam lithography to nanostructure and functionalize
desired graphene chips
Origin of Rashba-splitting in the quantized subbands at Bi2Se3 surface
We study the band structure of the topological
insulator (111) 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 6 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
Ripple modulated electronic structure of a 3D topological insulator
3D topological insulators, similar to the Dirac material graphene, host
linearly dispersing states with unique properties and a strong potential for
applications. A key, missing element in realizing some of the more exotic
states in topological insulators is the ability to manipulate local electronic
properties. Analogy with graphene suggests a possible avenue via a topographic
route by the formation of superlattice structures such as a moir\'e patterns or
ripples, which can induce controlled potential variations. However, while the
charge and lattice degrees of freedom are intimately coupled in graphene, it is
not clear a priori how a physical buckling or ripples might influence the
electronic structure of topological insulators. Here we use Fourier transform
scanning tunneling spectroscopy to determine the effects of a one-dimensional
periodic buckling on the electronic properties of Bi2Te3. By tracking the
spatial variations of the scattering vector of the interference patterns as
well as features associated with bulk density of states, we show that the
buckling creates a periodic potential modulation, which in turn modulates the
surface and the bulk states. The strong correlation between the topographic
ripples and electronic structure indicates that while doping alone is
insufficient to create predetermined potential landscapes, creating ripples
provides a path to controlling the potential seen by the Dirac electrons on a
local scale. Such rippled features may be engineered by strain in thin films
and may find use in future applications of topological insulators.Comment: Nature Communications (accepted
Emergent quantum confinement at topological insulator surfaces
Bismuth-chalchogenides are model examples of three-dimensional topological
insulators. Their ideal bulk-truncated surface hosts a single spin-helical
surface state, which is the simplest possible surface electronic structure
allowed by their non-trivial topology. They are therefore widely
regarded ideal templates to realize the predicted exotic phenomena and
applications of this topological surface state. However, real surfaces of such
compounds, even if kept in ultra-high vacuum, rapidly develop a much more
complex electronic structure whose origin and properties have proved
controversial. Here, we demonstrate that a conceptually simple model,
implementing a semiconductor-like band bending in a parameter-free
tight-binding supercell calculation, can quantitatively explain the entire
measured hierarchy of electronic states. In combination with circular dichroism
in angle-resolved photoemission (ARPES) experiments, we further uncover a rich
three-dimensional spin texture of this surface electronic system, resulting
from the non-trivial topology of the bulk band structure. Moreover, our study
reveals how the full surface-bulk connectivity in topological insulators is
modified by quantum confinement.Comment: 9 pages, including supplementary information, 4+4 figures. A high
resolution version is available at
http://www.st-andrews.ac.uk/~pdk6/pub_files/TI_quant_conf_high_res.pd
Observation of Dirac surface states in the noncentrosymmetric superconductor BiPd
Funding from the MPG-UBC center and the Engineering and Physical Sciences Research Council (EP/I031014/1 and EP/L505079/1) are acknowledged. This work was supported by the DFG within projects STA315/8-1 and BE5190/1-1.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 (ARPES) and low temperature scanning tunneling microscopy (STM) 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.PostprintPeer reviewe
Growth of High-Mobility Bi2Te2Se Nanoplatelets on hBN Sheets by van der Waals Epitaxy
The electrical detection of the surface states of topological insulators is
strongly impeded by the interference of bulk conduction, which commonly arises
due to pronounced doping associated with the formation of lattice defects. As
exemplified by the topological insulator Bi2Te2Se, we show that via van der
Waals epitaxial growth on thin hBN substrates the structural quality of such
nanoplatelets can be substantially improved. The surface state carrier mobility
of nanoplatelets on hBN is increased by a factor of about 3 compared to
platelets on conventional Si/SiOx substrates, which enables the observation of
well-developed Shubnikov-de Haas oscillations. We furthermore demonstrate the
possibility to effectively tune the Fermi level position in the films with the
aid of a back gate
Photon emission spectroscopy of thin MgO films with the STM: from a tip-mediated to an intrinsic emission characteristic
Light emission spectroscopy of self-assembled arrays of silver nano-crystals with the STM
The optical properties of ordered and disordered assemblies of Ag nano-crystals on HOPG have been characterized by photon emission spectroscopy with the STM. The spectra reveal two emission maxima, which are attributed to plasmon modes oscillating parallel and perpendicular to the substrate plane. The interpretation of the emission mechanism is supported by polarization measurements and model calculations. The effect of long-range order in the particle layer on the optical properties is discussed
Structure and morphology of thin MgO films on Mo(001)
Using a combination of reciprocal and real-space techniques, the structural evolution and its effect on thesurface morphology is investigated for MgO films of 1–30 ML thickness epitaxially grown on Mo(001). Thestrain induced by the mismatch with the substrate is relieved between 1 and 7 ML MgO due to the formationof an ordered network of interfacial misfit dislocations aligned along the MgO (110) directions, particularlyevident after annealing the film at 1070 K. A dislocation periodicity of about 60 Å has been determined bymeans of grazing incidence x-ray diffraction. The dislocations induce a tilting of the surface that appears inelectron diffraction along the (100) MgO directions for thin films and changes to (110) directions when theoxide thickness increases. Scanning tunneling microscopy (STM) shows the presence of a regular pattern onthe surface below 7 ML thickness associated to the dislocation network. With increasing thickness, screwdislocations connected by nonpolar steps appear on the oxide surface. Thanks to the combination of differentdiffraction techniques and STM measurements, a comprehensive picture of the relaxation mechanisms in MgOfilms on Mo(001) can be drawn
Structural and optical properties of vanadium and hafnium nitride nanoscale films: effect of stoichiometry
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