715 research outputs found
Band structure engineering in (Bi1-xSbx)2Te3 ternary topological insulators
Three-dimensional (3D) topological insulators (TI) are novel quantum
materials with insulating bulk and topologically protected metallic surfaces
with Dirac-like band structure. The spin-helical Dirac surface states are
expected to host exotic topological quantum effects and find applications in
spintronics and quantum computation. The experimental realization of these
ideas requires fabrication of versatile devices based on bulk-insulating TIs
with tunable surface states. The main challenge facing the current TI materials
exemplified by Bi2Se3 and Bi2Te3 is the significant bulk conduction, which
remains unsolved despite extensive efforts involving nanostructuring, chemical
doping and electrical gating. Here we report a novel approach for engineering
the band structure of TIs by molecular beam epitaxy (MBE) growth of
(Bi1-xSbx)2Te3 ternary compounds. Angle-resolved photoemission spectroscopy
(ARPES) and transport measurements show that the topological surface states
exist over the entire composition range of (Bi1-xSbx)2Te3 (x = 0 to 1),
indicating the robustness of bulk Z2 topology. Most remarkably, the systematic
band engineering leads to ideal TIs with truly insulating bulk and tunable
surface state across the Dirac point that behave like one quarter of graphene.
This work demonstrates a new route to achieving intrinsic quantum transport of
the topological surface states and designing conceptually new TI devices with
well-established semiconductor technology.Comment: Minor changes in title, text and figures. Supplementary information
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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
Ultra-low carrier concentration and surface dominant transport in Sb-doped Bi2Se3 topological insulator nanoribbons
A topological insulator is a new state of matter, possessing gapless
spin-locking surface states across the bulk band gap which has created new
opportunities from novel electronics to energy conversion. However, the large
concentration of bulk residual carriers has been a major challenge for
revealing the property of the topological surface state via electron transport
measurement. Here we report surface state dominated transport in Sb-doped
Bi2Se3 nanoribbons with very low bulk electron concentrations. In the
nanoribbons with sub-10nm thickness protected by a ZnO layer, we demonstrate
complete control of their top and bottom surfaces near the Dirac point,
achieving the lowest carrier concentration of 2x10^11/cm2 reported in
three-dimensional (3D) topological insulators. The Sb-doped Bi2Se3
nanostructures provide an attractive materials platform to study fundamental
physics in topological insulators, as well as future applications.Comment: 5 pages, 4 figures, 1 tabl
A topological insulator surface under strong Coulomb, magnetic and disorder perturbations
Three dimensional topological insulators embody a newly discovered state of
matter characterized by conducting spin-momentum locked surface states that
span the bulk band gap as demonstrated via spin-resolved ARPES measurements .
This highly unusual surface environment provides a rich ground for the
discovery of novel physical phenomena. Here we present the first controlled
study of the topological insulator surfaces under strong Coulomb, magnetic and
disorder perturbations. We have used interaction of iron, with a large Coulomb
state and significant magnetic moment as a probe to \textit{systematically test
the robustness} of the topological surface states of the model topological
insulator BiSe. We observe that strong perturbation leads to the
creation of odd multiples of Dirac fermions and that magnetic interactions
break time reversal symmetry in the presence of band hybridization. We also
present a theoretical model to account for the altered surface of BiSe.
Taken collectively, these results are a critical guide in manipulating
topological surfaces for probing fundamental physics or developing device
applications.Comment: 14 pages, 4 Figures. arXiv admin note: substantial text overlap with
arXiv:1009.621
In-Plane Orbital Texture Switch at the Dirac Point in the Topological Insulator Bi2Se3
Topological insulators are novel macroscopic quantum-mechanical phase of
matter, which hold promise for realizing some of the most exotic particles in
physics as well as application towards spintronics and quantum computation. In
all the known topological insulators, strong spin-orbit coupling is critical
for the generation of the protected massless surface states. Consequently, a
complete description of the Dirac state should include both the spin and
orbital (spatial) parts of the wavefunction. For the family of materials with a
single Dirac cone, theories and experiments agree qualitatively, showing the
topological state has a chiral spin texture that changes handedness across the
Dirac point (DP), but they differ quantitatively on how the spin is polarized.
Limited existing theoretical ideas predict chiral local orbital angular
momentum on the two sides of the DP. However, there have been neither direct
measurements nor calculations identifying the global symmetry of the spatial
wavefunction. Here we present the first results from angle-resolved
photoemission experiment and first-principles calculation that both show,
counter to current predictions, the in-plane orbital wavefunctions for the
surface states of Bi2Se3 are asymmetric relative to the DP, switching from
being tangential to the k-space constant energy surfaces above DP, to being
radial to them below the DP. Because the orbital texture switch occurs exactly
at the DP this effect should be intrinsic to the topological physics,
constituting an essential yet missing aspect in the description of the
topological Dirac state. Our results also indicate that the spin texture may be
more complex than previously reported, helping to reconcile earlier conflicting
spin resolved measurements
Topological semimetal in a fermionic optical lattice
Optical lattices play a versatile role in advancing our understanding of
correlated quantum matter. The recent implementation of orbital degrees of
freedom in chequerboard and hexagonal optical lattices opens up a new thrust
towards discovering novel quantum states of matter, which have no prior analogs
in solid state electronic materials. Here, we demonstrate that an exotic
topological semimetal emerges as a parity-protected gapless state in the
orbital bands of a two-dimensional fermionic optical lattice. The new quantum
state is characterized by a parabolic band-degeneracy point with Berry flux
, in sharp contrast to the flux of Dirac points as in graphene. We
prove that the appearance of this topological liquid is universal for all
lattices with D point group symmetry as long as orbitals with opposite
parities hybridize strongly with each other and the band degeneracy is
protected by odd parity. Turning on inter-particle repulsive interactions, the
system undergoes a phase transition to a topological insulator whose
experimental signature includes chiral gapless domain-wall modes, reminiscent
of quantum Hall edge states.Comment: 6 pages, 3 figures and Supplementary Informatio
The space group classification of topological band insulators
Topological band insulators (TBIs) are bulk insulating materials which
feature topologically protected metallic states on their boundary. The existing
classification departs from time-reversal symmetry, but the role of the crystal
lattice symmetries in the physics of these topological states remained elusive.
Here we provide the classification of TBIs protected not only by time-reversal,
but also by crystalline symmetries. We find three broad classes of topological
states: (a) Gamma-states robust against general time-reversal invariant
perturbations; (b) Translationally-active states protected from elastic
scattering, but susceptible to topological crystalline disorder; (c) Valley
topological insulators sensitive to the effects of non-topological and
crystalline disorder. These three classes give rise to 18 different
two-dimensional, and, at least 70 three-dimensional TBIs, opening up a route
for the systematic search for new types of TBIs.Comment: Accepted in Nature Physic
Spin-Rotation Symmetry Breaking in the Superconducting State of CuxBi2Se3
Spontaneous symmetry breaking is an important concept for understanding
physics ranging from the elementary particles to states of matter. For example,
the superconducting state breaks global gauge symmetry, and unconventional
superconductors can break additional symmetries. In particular, spin rotational
symmetry is expected to be broken in spin-triplet superconductors. However,
experimental evidence for such symmetry breaking has not been conclusively
obtained so far in any candidate compounds. Here, by 77Se nuclear magnetic
resonance measurements, we show that spin rotation symmetry is spontaneously
broken in the hexagonal plane of the electron-doped topological insulator
Cu0.3Bi2Se3 below the superconducting transition temperature Tc=3.4 K. Our
results not only establish spin-triplet superconductivity in this compound, but
may also serve to lay a foundation for the research of topological
superconductivity
Fully gapped topological surface states in BiSe films induced by a d-wave high-temperature superconductor
Topological insulators are a new class of materials, that exhibit robust
gapless surface states protected by time-reversal symmetry. The interplay
between such symmetry-protected topological surface states and symmetry-broken
states (e.g. superconductivity) provides a platform for exploring novel quantum
phenomena and new functionalities, such as 1D chiral or helical gapless
Majorana fermions, and Majorana zero modes which may find application in
fault-tolerant quantum computation. Inducing superconductivity on topological
surface states is a prerequisite for their experimental realization. Here by
growing high quality topological insulator BiSe films on a d-wave
superconductor BiSrCaCuO using molecular beam epitaxy,
we are able to induce high temperature superconductivity on the surface states
of BiSe films with a large pairing gap up to 15 meV. Interestingly,
distinct from the d-wave pairing of BiSrCaCuO, the
proximity-induced gap on the surface states is nearly isotropic and consistent
with predominant s-wave pairing as revealed by angle-resolved photoemission
spectroscopy. Our work could provide a critical step toward the realization of
the long sought-after Majorana zero modes.Comment: Nature Physics, DOI:10.1038/nphys274
A road to reality with topological superconductors
Topological states of matter are a source of low-energy quasiparticles, bound
to a defect or propagating along the surface. In a superconductor these are
Majorana fermions, described by a real rather than a complex wave function. The
absence of complex phase factors promises protection against decoherence in
quantum computations based on topological superconductivity. This is a tutorial
style introduction written for a Nature Physics focus issue on topological
matter.Comment: pre-copy-editing, author-produced version of the published paper: 4
pages, 2 figure
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