244 research outputs found
Gapless interface states at the junction between two topological insulators
We consider a junction between two topological insulators, and calculate the
properties of the interface states with an effective low energy Hamiltonian for
topological insulators with a single cone on the surface. This system bears a
close resemblance to bilayer graphene, as both result from the hybridization of
Dirac cones. We find gapless interface states not only when the helicity
direction of the topological surface states are oppositely oriented, but they
can also exist if they are equally oriented. Furthermore, we find that the
existence of the interface states can be understood from the closing of the
bulk gap when the helicity changes orientation. Recently, superluminal
tachyonic excitations were also claimed to exist at the interface between
topological insulators. However, here we show that these interface states do
not exist
Extended homologous series of Sn-O layered systems: a first-principles study
Apart from the most studied tin-oxide compounds, SnO and SnO2, intermediate
states have been claimed to exist for more than a hundred years. In addition to
the known homologous series (Seko et al., Phys. Rev. Lett. 100, 045702 (2008)),
we here predict the existence of several new compounds with an O concentration
between 50 % (SnO) and 67 % (SnO2). All these intermediate compounds are
constructed from removing one or more (101) oxygen layers of SnO2. Since the
van der Waals (vdW) interaction is known to be important for the Sn-Sn
interlayer distances, we use a vdW-corrected functional, and compare these
results with results obtained with PBE and hybrid functionals. We present the
electronic properties of the intermediate structures and we observe a decrease
of the band gap when (i) the O concentration increases and (ii) more SnO-like
units are present for a given concentration. The contribution of the different
atoms to the valence and conduction band is also investigated.Comment: 8 page
Crystalline topological states at a topological insulator junction
We consider an interface between two strong time-reversal invariant
topological insulators having surface states with opposite spin chirality, or
equivalently, opposite mirror Chern number. We show that such an interface
supports gapless modes that are protected by mirror symmetry. The interface
states are investigated with a continuum model for the Bi2Se3 class of
topological insulators that takes into account terms up to third order in the
crystal momentum, which ensures that the model has the correct symmetry. The
model parameters are obtained from ab initio calculations. Finally, we consider
the effect of rotational mismatch at the interface, which breaks the mirror
symmetry and opens a gap in the interface spectrum.Comment: 10 pages, 5 figure
Rutherford scattering of electron vortices
By considering a cylindrically symmetric generalization of a plane wave, the
first Born approximation of screened Coulomb scattering unfolds two new
dimensions in the scattering problem: transverse momentum and orbital angular
momentum of the incoming beam. In this paper, the elastic Coulomb scattering
amplitude is calculated analytically for incoming Bessel beams. This reveals
novel features occurring for wide angle scattering when the incoming beam is
correctly prepared. The result successfully generalizes the well known
Rutherford formula, incorporating transverse and orbital angular momentum into
the formalism.Comment: 9 pages, 5 figure
Inelastic electron-vortex-beam scattering
Recent theoretical and experimental developments in the field of electron
vortex beam physics have raised questions on what exactly this novelty in the
field of electron microscopy (and other fields, such as particle physics)
really provides. An important part in the answer to those questions lies in
scattering theory. The present investigation explores various aspects of
inelastic quantum scattering theory for cylindrically symmetric beams with
orbital angular momentum. The model system of Coulomb scattering on a hydrogen
atom provides the setting to address various open questions: How is momentum
transferred? Do vortex beams selectively excite atoms, and how can one employ
vortex beams to detect magnetic transitions? The analytical approach presented
here provides answers to these questions. OAM transfer is possible, but not
through selective excitation; rather, by pre- and post-selection one can filter
out the relevant contributions to a specific signal
Exceeding the Shockley-Queisser limit within the detailed balance framework
The Shockley-Queisser limit is one of the most fundamental results in the
field of photovoltaics. Based on the principle of detailed balance, it defines
an upper limit for a single junction solar cell that uses an absorber material
with a specific band gap. Although methods exist that allow a solar cell to
exceed the Shockley-Queisser limit, here we show that it is possible to exceed
the Shockley-Queisser limit without considering any of these additions. Merely
by introducing an absorptivity that does not assume that every photon with an
energy above the band gap is absorbed, efficiencies above the Shockley-Queisser
limit are obtained. This is related to the fact that assuming optimal
absorption properties also maximizes the recombination current within the
detailed balance approach. We conclude that considering a finite thickness for
the absorber layer allows the efficiency to exceed the Shockley-Queisser limit,
and that this is more likely to occur for materials with small band gaps.Comment: 6 pages, 3 figure
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