525 research outputs found
Tunable quantum spin Hall effect in double quantum wells
The field of topological insulators (TIs) is rapidly growing. Concerning
possible applications, the search for materials with an easily controllable TI
phase is a key issue. The quantum spin Hall effect, characterized by a single
pair of helical edge modes protected by time-reversal symmetry, has been
demonstrated in HgTe-based quantum wells (QWs) with an inverted bandgap. We
analyze the topological properties of a generically coupled HgTe-based double
QW (DQW) and show how in such a system a TI phase can be driven by an
inter-layer bias voltage, even when the individual layers are non-inverted. We
argue, that this system allows for similar (layer-)pseudospin based physics as
in bilayer graphene but with the crucial absence of a valley degeneracy.Comment: 9 pages, 8 figures, extended version (accepted Phys. Rev. B
Particle dynamics and ergodicity-breaking in twisted-bilayer optical lattices
Recent experiments have realized a twisted bilayer-like optical potential for
ultra-cold atoms, which in contrast to solid-state set ups may allow for an
arbitrary ratio between the inter- and intra-layer couplings. For commensurate
Moir\'e twistings a large-enough inter-layer coupling results in particle
transport dominated by channel formation. For incommensurate twistings, the
interlayer coupling acts as an effective disorder strength. Whereas for weak
couplings the whole spectrum remains ergodic, at a critical value part of the
eigenspectrum transitions into multifractal states. A similar transition may be
observed as well as a function of an energy bias between the two layers. Our
study reveals atoms in optical twisted-bilayer lattices as an interesting new
platform for the study of ergodicity breaking and multifractality.Comment: 7 pages, 5 figure
Enhanced quasiparticle dynamics of quantum well states: the giant Rashba system BiTeI and topological insulators
In the giant Rashba semiconductor BiTeI electronic surface scattering with
Lorentzian linewidth is observed that shows a strong dependence on surface
termination and surface potential shifts. A comparison with the topological
insulator Bi2Se3 evidences that surface confined quantum well states are the
origin of these processes. We notice an enhanced quasiparticle dynamics of
these states with scattering rates that are comparable to polaronic systems in
the collision dominated regime. The Eg symmetry of the Lorentzian scattering
contribution is different from the chiral (RL) symmetry of the corresponding
signal in the topological insulator although both systems have spin-split
surface states.Comment: 6 pages, 5 figure
The Josephson light-emitting diode
We consider an optical quantum dot where an electron level and a hole level
are coupled to respective superconducting leads. We find that electrons and
holes recombine producing photons at discrete energies as well as a continuous
tail. Further, the spectral lines directly probe the induced superconducting
correlations on the dot. At energies close to the applied bias voltage eV, a
parameter range exists, where radiation proceeds in pairwise emission of
polarization correlated photons. At energies close to 2eV, emitted photons are
associated with Cooper pair transfer and are reminiscent of Josephson
radiation. We discuss how to probe the coherence of these photons in a SQUID
geometry via single photon interference.Comment: Main text: 4 pages, 4 figures, Supplementary material: 8 pages, 4
figure
On the Eigenvalue Density of Real and Complex Wishart Correlation Matrices
Wishart correlation matrices are the standard model for the statistical
analysis of time series. The ensemble averaged eigenvalue density is of
considerable practical and theoretical interest. For complex time series and
correlation matrices, the eigenvalue density is known exactly. In the real
case, however, a fundamental mathematical obstacle made it forbidingly
complicated to obtain exact results. We use the supersymmetry method to fully
circumvent this problem. We present an exact formula for the eigenvalue density
in the real case in terms of twofold integrals and finite sums.Comment: 4 pages, 2 figure
Helical edge states in multiple topological mass domains
The two-dimensional topological insulating phase has been experimentally
discovered in HgTe quantum wells (QWs). The low-energy physics of
two-dimensional topological insulators (TIs) is described by the
Bernevig-Hughes-Zhang (BHZ) model, where the realization of a topological or a
normal insulating phase depends on the Dirac mass being negative or positive,
respectively. We solve the BHZ model for a mass domain configuration, analyzing
the effects on the edge modes of a finite Dirac mass in the normal insulating
region (soft-wall boundary condition). We show that at a boundary between a TI
and a normal insulator (NI), the Dirac point of the edge states appearing at
the interface strongly depends on the ratio between the Dirac masses in the two
regions. We also consider the case of multiple boundaries such as NI/TI/NI,
TI/NI/TI and NI/TI/NI/TI.Comment: 11 pages, 15 figure
Correspondence between Andreev reflection and Klein tunneling in bipolar graphene
Andreev reflection at a superconductor and Klein tunneling through an n-p
junction in graphene are two processes that couple electrons to holes -- the
former through the superconducting pair potential Delta and the latter through
the electrostatic potential U. We derive that the energy spectra in the two
systems are identical, at low energies E<<Delta and for an antisymmetric
potential profile U(-x,y)=-U(x,y). This correspondence implies that bipolar
junctions in graphene may have zero density of states at the Fermi level and
carry a current in equilibrium, analogously to superconducting Josephson
junctions. It also implies that nonelectronic systems with the same band
structure as graphene, such as honeycomb-lattice photonic crystals, can exhibit
pseudo-superconducting behavior.Comment: 7 pages, 7 figures; much expanded version, with a revised title, test
of the analytics by computer simulation, temperature dependence of the
persistent current, and an appendix with details of the calculatio
Quantum control on entangled bipartite qubits
Ising interaction between qubits could produce distortion in entangled pairs
generated for engineering purposes (as in quantum computation) in presence of
parasite magnetic fields, destroying or altering the expected behavior of
process in which is projected to be used. Quantum control could be used to
correct that situation in several ways. Sometimes the user should be make some
measurement upon the system to decide which is the best control scheme; other
posibility is try to reconstruct the system using similar procedures without
perturbate it. In the complete pictures both schemes are present. We will work
first with pure systems studying advantages of different procedures. After, we
will extend these operations when time of distortion is uncertain, generating a
mixed state, which needs to be corrected by suposing the most probably time of
distortion.Comment: 10 pages, 5 figure
- …