49 research outputs found
Topological protection of bound states against the hybridization
Topological invariants are conventionally known to be responsible for
protection of extended states against disorder. A prominent example is the
presence of topologically protected extended-states in two-dimensional (2D)
quantum Hall systems as well as on the surface of three-dimensional (3D)
topological insulators. Distinct from such cases, here we introduce a new
concept, that is, the topological protection of bound states against
hybridization. This situation is shown to be realizable in a 2D quantum Hall
insulator put on a 3D trivial insulator. In such a configuration, there exist
topologically protected bound states, localized along the normal direction of
2D plane, in spite of hybridization with the continuum of extended states. The
one-dimensional edge states are also localized along the same direction as long
as their energies are within the band gap. This finding demonstrates the dual
role of topological invariants, as they can also protect bound states against
hybridization in a continuum.Comment: 21 pages, 7 figure
Rich structural phase diagram and thermoelectric properties of layered tellurides Mo1-xNbxTe2
MoTe2 is a rare transition-metal ditelluride having two kinds of layered
polytypes, hexagonal structure with trigonal prismatic Mo coordination and
monoclinic structure with octahedral Mo coordination. The monoclinic distortion
in the latter is caused by anisotropic metal-metal bonding. In this work, we
have examined the Nb doping effect on both polytypes of MoTe2 and clarified a
structural phase diagram for Mo1-xNbxTe2 containing four kinds of polytypes. A
rhombohedral polytype crystallizing in polar space group has been newly
identified as a high-temperature metastable phase at slightly Nb-rich
composition. Considering the results of thermoelectric measurements and the
first principles calculations, the Nb ion seemingly acts as a hole dopant in
the rigid band scheme. On the other hand, the significant interlayer
contraction upon the Nb doping, associated with the Te p-p hybridization, is
confirmed especially for the monoclinic phase, which implies a shift of the
p-band energy level. The origin of the metal-metal bonding in the monoclinic
structure is discussed in terms of the d electron counting and the Te p-p
hybridization.Comment: 16 pages, 6 figures, 1 table, to be published in APL Material
Giant thermoelectric effect in graphene-based topological insulators with nanopores
Designing thermoelectric materials with high figure of merit requires fulfilling three often irreconcilable conditions, i.e., the
high electrical conductance , small thermal conductance and high
Seebeck coefficient . Nanostructuring is one of the promising ways to
achieve this goal as it can substantially suppress lattice contribution to
. However, it may also unfavorably influence the electronic transport
in an uncontrollable way. Here we theoretically demonstrate that this issue can
be ideally solved by fabricating graphene nanoribbons with heavy adatoms and
nanopores. These systems, acting as a two-dimensional topological insulator
with robust helical edge states carrying electrical current, yield a highly
optimized power factor per helical conducting channel. Concurrently,
their array of nanopores impedes the lattice thermal conduction through the
bulk. Using quantum transport simulations coupled with first-principles
electronic and phononic band structure calculations, the thermoelectric figure
of merit is found to reach its maximum at K. This
paves a way to design high- materials by exploiting the nontrivial topology
of electronic states through nanostructuring.Comment: 7 pages, 4 figures; PDFLaTe
Theory of topological quantum phase transitions in 3D noncentrosymmetric systems
We have constructed a general theory describing the topological quantum phase
transitions in 3D systems with broken inversion symmetry. While the
consideration of the system's codimension generally predicts the appearance of
a stable metallic phase between the normal and topological insulators, it is
shown that a direct topological phase transition between two insulators is also
possible when an accidental band crossing (ABC) occurs along directions with
high crystalline symmetry. At the quantum critical point (QCP), the energy
dispersion becomes quadratic along one direction while the dispersions along
the other two orthogonal directions are linear, which manifests the zero
chirality of the band touching point (BTP). Due to the anisotropic dispersion
at QCP, various thermodynamic and transport properties show unusual temperature
dependence and anisotropic behaviors.Comment: 5+6 pages, 3+5 figures, 1 tabl
Hidden spin-orbital texture at the -located valence band maximum of a transition metal dichalcogenide semiconductor
Finding stimuli capable of driving an imbalance of spin-polarised electrons
within a solid is the central challenge in the development of spintronic
devices. However, without the aid of magnetism, routes towards this goal are
highly constrained with only a few suitable pairings of compounds and driving
mechanisms found to date. Here, through spin- and angle-resolved photoemission
along with density functional theory, we establish how the -derived bulk
valence bands of semiconducting 1T-HfSe possess a local, ground-state spin
texture spatially confined within each Se-sublayer due to strong
sublayer-localised electric dipoles orientated along the -axis. This hidden
spin-polarisation manifests in a `coupled spin-orbital texture' with
in-equivalent contributions from the constituent -orbitals. While the
overall spin-orbital texture for each Se sublayer is in strict adherence to
time-reversal symmetry (TRS), spin-orbital mixing terms with net polarisations
at time-reversal invariant momenta are locally maintained. These apparent
TRS-breaking contributions dominate, and can be selectively tuned between with
a choice of linear light polarisation, facilitating the observation of
pronounced spin-polarisations at the Brillouin zone centre for all . We
discuss the implications for the generation of spin-polarised populations from
1T-structured transition metal dichalcogenides using a fixed energy, linearly
polarised light source.Comment: 11 pages, 6 figure