1,372 research outputs found
Controlling conductance statistics of quantum wires by driving ac fields
We calculate the entire distribution of the conductance P(G) of a
one-dimensional disordered system --quantum wire-- subject to a time-dependent
field. Our calculations are based on Floquet theory and a scaling approach to
localization. Effects of the applied ac field on the conductance statistics can
be strong and in some cases dramatic, as in the high-frequency regime where the
conductance distribution shows a sharp cut-off. In this frequency regime, the
conductance is written as a product of a frequency-dependent term and a field
independent term, the latter containing the information on the disorder in the
wire. We thus use the solution of the Mel'nikov equation for time-independent
transport to calculate P(G) at any degree of disorder. At lower frequencies, it
is found that the conductance distribution and the correlations of the
transmission Floquet modes are described by a solution of the
Dorokhov-Mello-Pereyra-Kumar equation with an effective number of channels. In
the regime of strong localization, induced by the disorder or the ac field,
P(G) is a log-normal distribution. Our theoretical results are verified
numerically using a single-band Anderson Hamiltonian.Comment: 6 pages, 4 figures. V2: a new reference added. Minor correction
Chiral bound states in the continuum
We present a distinct mechanism for the formation of bound states in the
continuum (BICs). In chiral quantum systems there appear zero-energy states in
which the wave function has finite amplitude only in one of the subsystems
defined by the chiral symmetry. When the system is coupled to leads with a
continuum energy band, part of these states remain bound. We derive some
algebraic rules for the number of these states depending on the dimensionality
and rank of the total Hamiltonian. We examine the transport properties of such
systems including the appearance of Fano resonances in some limiting cases.
Finally, we discuss experimental setups based on microwave dielectric
resonators and atoms in optical lattices where these predictions can be tested.Comment: 9 pages, 8 figures. v2: includes results specific to honeycomb
lattice; matches published versio
Commensurability effects for fermionic atoms trapped in 1D optical lattices
Fermionic atoms in two different hyperfine states confined in optical
lattices show strong commensurability effects due to the interplay between the
atomic density wave (ADW) ordering and the lattice potential. We show that
spatially separated regions of commensurable and incommensurable phases can
coexist. The commensurability between the harmonic trap and the lattice sites
can be used to control the amplitude of the atomic density waves in the central
region of the trap.Comment: Accepted for publication in Physical Review Letter
Superradiance at the localization-delocalization crossover in chlorosomes
Conferencia invitada ; Nonequilibrium condensed matter and biological systems, Madrid, March 9-11, 2016 ; http://scala.uc3m.es/SFE16/Peer Reviewe
Interacting electron systems between Fermi leads: effective one-body transmissions and correlation clouds
In order to extend the Landauer formulation of quantum transport to
correlated fermions, we consider a spinless system in which charge carriers
interact, connected to two reservoirs by non-interacting one-dimensional leads.
We show that the mapping of the embedded many-body scatterer onto an effective
one-body scatterer with interaction-dependent parameters requires to include
parts of the attached leads where the interacting region induces power law
correlations. Physically, this gives a dependence of the conductance of a
mesoscopic scatterer upon the nature of the used leads which is due to electron
interactions inside the scatterer. To show this, we consider two identical
correlated systems connected by a non-interacting lead of length
. We demonstrate that the effective one-body transmission of the
ensemble deviates by an amount from the behavior obtained
assuming an effective one-body description for each element and the combination
law of scatterers in series. is maximum for the interaction strength
around which the Luttinger liquid becomes a Mott insulator in the used model,
and vanishes when and . Analogies with the Kondo
problem are pointed out.Comment: 5 pages, 6 figure
Generic helical edge states due to Rashba spin-orbit coupling in a topological insulator
We study the helical edge states of a two-dimensional topological insulator
without axial spin symmetry due to the Rashba spin-orbit interaction. Lack of
axial spin symmetry can lead to so-called generic helical edge states, which
have energy-dependent spin orientation. This opens the possibility of inelastic
backscattering and thereby nonquantized transport. Here we find analytically
the new dispersion relations and the energy dependent spin orientation of the
generic helical edge states in the presence of Rashba spin-orbit coupling
within the Bernevig-Hughes-Zhang model, for both a single isolated edge and for
a finite width ribbon. In the single-edge case, we analytically quantify the
energy dependence of the spin orientation, which turns out to be weak for a
realistic HgTe quantum well. Nevertheless, finite size effects combined with
Rashba spin-orbit coupling result in two avoided crossings in the energy
dispersions, where the spin orientation variation of the edge states is very
significantly increased for realistic parameters. Finally, our analytical
results are found to compare well to a numerical tight-binding regularization
of the model.Comment: 21 pages (main text 11 pages), 11 figure
Mesoscopic behavior of the transmission phase through confined correlated electronic systems
We investigate the effect of electronic correlations on the transmission
phase of quantum coherent scatterers, considering quantum dots in the Coulomb
blockade regime connected to two single-channel leads. We focus on transmission
zeros and the associated \pi-phase lapses that have been observed in
interferometric experiments. We numerically explore two types of models for
quantum dots: (i) lattice models with up to eight sites, and (ii) resonant
level models with up to six levels. We identify different regimes of parameters
where the presence of electronic correlations is responsible for the increase
or the decrease of the number of transmission zeros vs. electrochemical
potential on the dot. However, we show that interaction effects cannot
reproduce the universal behavior of alternating resonances and phase lapses,
experimentally observed in many-electron Coulomb blockaded dots. Our numerical
results strongly suggest that the main experimentally observed features are
captured by the theory for chaotic ballistic dots of Molina et al., [Phys. Rev.
Lett. 108, 076803 (2012)] incorporating one-particle wave-function correlations
but ignoring many-particle electronic correlations.Comment: 17 pages, 14 figure
Dynamics of coherence, localization and excitation transfer in disordered nanorings
Self-assembled supramolecular aggregates are excellent candidates for the
design of efficient excitation transport devices. Both artificially prepared
and natural photosynthetic aggregates in plants and bacteria present an
important degree of disorder that is supposed to hinder excitation transport.
Besides, molecular excitations couple to nuclear motion affecting excitation
transport in a variety of ways. We present an exhaustive study of exciton
dynamics in disordered nanorings with long-range interactions under the
influence of a phonon bath and take the LH2 system of purple bacteria as a
model. Nuclear motion is explicitly taken into account by employing the Davydov
ansatz description of the polaron and quantum dynamics are obtained using a
time-dependent variational method. We reveal an optimal exciton-phonon coupling
that suppresses disorder-induced localization and facilitate excitation
de-trapping. This excitation transfer enhancement, mediated by environmental
phonons, is attributed to energy relaxation toward extended, low-energy
excitons provided by the precise LH2 geometry with anti-parallel dipoles and
long-range interactions. An analysis of localization and spectral statistics is
followed by dynamical measures of coherence and localization, transfer
efficiency and superradiance. Linear absorption, 2D photon-echo spectra and
diffusion measures of the exciton are examined to monitor the diffusive
behavior as a function of the strengths of disorder and exciton-phonon
coupling.Comment: 18 pages, 13 figure
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