82 research outputs found
Interplay between Rashba interaction and electromagnetic field in the edge states of a 2D topological insulator
The effects of Rashba interaction and electromagnetic field on the edge
states of a two-dimensional topological insulator are investigated in a
non-perturbative way. We show that the electron dynamics is equivalent to a
problem of massless Dirac fermions propagating with an inhomogeneous velocity,
enhanced by the Rashba profile with respect to the bare Fermi value .
Despite the inelastic and time-reversal breaking processes induced by the
electromagnetic field, no backscattering occurs without interaction. The
photoexcited electron densities are explicitly obtained in terms of the
electric field and the Rashba interaction, and are shown to fulfil generalised
chiral anomaly equations. The case of a Gaussian electromagnetic pulse is
analysed in detail. When the photoexcitation occurs far from the Rashba region,
the latter effectively acts as a "superluminal gate" boosting the photoexcited
wavepacket outside the light-cone determined by . In contrast, for an
electric pulse overlapping the Rashba region the emerging wavepackets are
squeezed in a manner that depends on the overlap area. The electron-electron
interaction effects are also discussed, for both intra-spin and inter-spin
density-density coupling. The results suggest that Rashba interaction, often
considered as an unwanted disorder effect, may be exploited to tailor the shape
and the propagation time of photoexcited spin-polarised wave packets.Comment: 15 pages, 6 figure
Noise and current correlations in tunnel junctions of Quantum Spin Hall edge states
The edge channels of two-dimensional topological systems are protected from
elastic reflection and are noiseless at low temperature. Yet, noise and
cross-correlations can be induced when electron waves partly transmit to the
opposite edge via tunneling through a constriction. In particular, in a quantum
spin Hall (QSH) system tunnelling occurs via both spin-preserving () and
spin-flipping () processes, each fulfilling time-reversal symmetry. We
investigate the current correlations of a four-terminal QSH setup in the
presence of a tunneling region, both at equilibrium and out-of-equilibrium. We
find that, although and processes do not commute and the generic
current correlation depends on both, under appropriate conditions a direct
detection of two types of partition noise is possible. In particular, while the
spin-preserving partitioning can be probed for any arbitrary tunnel junction
with a specific configuration of terminal biases, the spin-flipping
partitioning can be directly detected only under suitably designed setups and
conditions. We describe two setups where these conditions can be fulfilled, and
both types of partitioning can be detected and controlled.Comment: 11 pages, 4 figure
Tuning Excess Noise by Aharonov-Bohm Interferometry
A voltage bias applied to a conductor induces a change of the current noise
with respect to the equilibrium noise known as excess noise. We analyze the
excess noise of the electronic current flowing through a mesoscopic
Aharonov-Bohm ring threaded by a magnetic flux, coupled to a side gate, and
contacted by two metallic electrodes. It is shown that the excess noise can be
controlled both magnetically and electrostatically, demonstrating the full
tunability of the system. At zero frequency, the ratio of the noise strength to
the current (Fano factor) can thereby be minimized. Remarkably, at finite
frequency, regions of negative excess noise emerge.Comment: 6 pages, 5 figures, final version, corrected typos and updated
reference
Effects of disorder on electron tunneling through helical edge states
A tunnel junction between helical edge states, realized via a constriction in
a Quantum Spin Hall system, can be exploited to steer both charge and spin
current into various terminals. We investigate the effects of disorder on the
transmission coefficient of the junction, by modelling disorder with a
randomly varying (complex) tunneling amplitude . We show that, while for a clean junction is only determined by
the absolute value and is independent of the phase , the
situation can be quite different in the presence of disorder: phase
fluctuations may dramatically affect the energy dependence of of any
single sample. Furthermore, analysing three different models for phase disorder
(including correlated ones), we show that not only the amount but also the way
the phase fluctuates determines the localisation length
and the sample-averaged transmission. Finally, we discuss the physical
conditions in which these three models suitably apply to realistic cases.Comment: 14 pages, 7 figure
Photoexcitation in two-dimensional topological insulators: Generating and controlling electron wavepackets in Quantum Spin Hall systems
One of the most fascinating challenges in Physics is the realization of an
electron-based counterpart of quantum optics, which requires the capability to
generate and control single electron wave packets. The edge states of quantum
spin Hall (QSH) systems, i.e. two-dimensional (2D) topological insulators
realized in HgTe/CdTe and InAs/GaSb quantum wells, may turn the tide in the
field, as they do not require the magnetic field that limits the
implementations based on quantum Hall effect. Here we show that an electric
pulse, localized in space and/or time and applied at a QSH edge, can
photoexcite electron wavepackets by intra-branch electrical transitions,
without invoking the bulk states or the Zeeman coupling. Such wavepackets are
spin-polarised and propagate in opposite directions, with a density profile
that is independent of the initial equilibrium temperature and that does not
exhibit dispersion, as a result of the linearity of the spectrum and of the
chiral anomaly characterising massless Dirac electrons. We also investigate the
photoexcited energy distribution and show how, under appropriate circumstances,
minimal excitations (Levitons) are generated. Furthermore, we show that the
presence of a Rashba spin-orbit coupling can be exploited to tailor the shape
of photoexcited wavepackets. Possible experimental realizations are also
discussed.Comment: 17 pages, 3 Figure
Tunnel junction of helical edge states: Determining and controlling spin-preserving and spin-flipping processes through transconductance
When a constriction is realized in a 2D quantum spin Hall system, electron
tunneling between helical edge states occurs via two types of channels allowed
by time-reversal symmetry, namely spin-preserving ({p}) and spin-flipping ({f})
tunneling processes. Determining and controlling the effects of these two
channels is crucial to the application of helical edge states in spintronics.
We show that, despite the Hamiltonian terms describing these two processes do
not commute, the scattering matrix entries of the related 4-terminal setup
always factorize into products of p-terms and f-terms contributions. Such
factorization provides an operative way to determine the transmission
coefficient and related to each of the two processes, via
transconductance measurements. Furthermore, these transmission coefficients are
also found to be controlled independently by a suitable combination of two gate
voltages applied across the junction. This result holds for an arbitrary
profile of the tunneling amplitudes, including disorder in the tunnel region,
enabling us to discuss the effect of the finite length of the tunnel junction,
and the space modulation of both magnitude and phase of the tunneling
amplitudes.Comment: 16 pages, 8 figures. Extended Discussion Section, updated reference
lis
Full electrical control of Charge and Spin conductance through Interferometry of Edge States in Topological Insulators
We investigate electron interferometry of edge states in Topological
Insulators. We show that, when inter-boundary coupling is induced at two
quantum point contacts of a four terminal setup, both Fabry-P\'erot-like and
Aharonov-Bohm-like loop processes arise. These underlying interference effects
lead to a full electrically controllable system, where the magnitude of charge
and spin linear conductances can be tuned by gate voltages, without applying
magnetic fields. In particular we find that, under appropriate conditions,
inter-boundary coupling can lead to negative values of the conductance.
Furthermore, the setup also allows to selectively generate pure charge or pure
spin currents, by choosing the voltage bias configuration.Comment: 12 pages, 5 figures (expanded discussion section, corrected typos
Dispersionless propagation of electron wavepackets in single-walled carbon nanotubes
We investigate the propagation of electron wavepackets in single-walled
carbon nanotubes via a Lindblad-based density-matrix approach that enables us
to account for both dissipation and decoherence effects induced by various
phonon modes. We show that, while in semiconducting nanotubes the wavepacket
experiences the typical dispersion of conventional materials, in metallic
nanotubes its shape remains essentially unaltered, even in the presence of the
electron-phonon coupling, up to micron distances at room temperature.Comment: 4 pages, 2 figures, accepted by Appl. Phys. Let
Evidence for Majorana bound states in transport properties of hybrid structures based on helical liquids
Majorana bound states can emerge as zero-energy modes at the edge of a
two-dimensional topological insulator in proximity to an ordinary s-wave
superconductor. The presence of an additional ferromagnetic domain close to the
superconductor can lead to their localization. We consider both N-S and S-N-S
junctions based on helical liquids and study their spectral properties for
arbitrary ferromagnetic scatterers in the normal region. Thereby, we explicitly
compute Andreev wave-functions at zero energy. We show under which conditions
these states form localized Majorana bound states in N-S and S-N-S junctions.
Interestingly, we can identify Majorana-specific signatures in the transport
properties of N-S junctions and the Andreev bound levels of S-N-S junctions
that are robust against external perturbations. We illustrate these findings
with the example of a ferromagnetic double barrier (i.e. a quantum dot) close
to the N-S boundaries.Comment: In v2, section IV about Josephson junctions has been partly rewritten
in order to discuss the ABS spectrum in the presence of a double
ferromagnetic barrier. The list of references has been update
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