244 research outputs found
Quasi-Ballistic Electron Transport in Random Superlattices
We theoretically study electron transport in disordered, quantum-well based,
semiconductor superlattices with structural short-range correlations. Our
system consists of equal width square barriers and quantum wells with two
different thicknesses. The two kinds of quantum wells are randomly distributed
along the growth direction. Structural correlations are introduced by adding
the constraint that one of the wells always appears in pairs. We show that such
correlated disordered superlattices exhibit a strong enhancement of their dc
conductance as compared to usual random ones, giving rise to quasi-ballistic
electron transport. Our predictions can be used to demonstrate experimentally
that structural correlations inhibit the localization effects of disorder. We
specifically describe the way superlattices should be built and experiments
should be carried out for that purpose.Comment: REVTeX 3.0, 7 pages, 4 figures on request from FD-A
([email protected]). Submitted to Physical Review B. Preprint
MA/UC3M/12/199
Coherent phenomena in semiconductors
A review of coherent phenomena in photoexcited semiconductors is presented.
In particular, two classes of phenomena are considered: On the one hand the
role played by optically-induced phase coherence in the ultrafast spectroscopy
of semiconductors; On the other hand the Coulomb-induced effects on the
coherent optical response of low-dimensional structures.
All the phenomena discussed in the paper are analyzed in terms of a
theoretical framework based on the density-matrix formalism. Due to its
generality, this quantum-kinetic approach allows a realistic description of
coherent as well as incoherent, i.e. phase-breaking, processes, thus providing
quantitative information on the coupled ---coherent vs. incoherent--- carrier
dynamics in photoexcited semiconductors.
The primary goal of the paper is to discuss the concept of quantum-mechanical
phase coherence as well as its relevance and implications on semiconductor
physics and technology. In particular, we will discuss the dominant role played
by optically induced phase coherence on the process of carrier photogeneration
and relaxation in bulk systems. We will then review typical field-induced
coherent phenomena in semiconductor superlattices such as Bloch oscillations
and Wannier-Stark localization. Finally, we will discuss the dominant role
played by Coulomb correlation on the linear and non-linear optical spectra of
realistic quantum-wire structures.Comment: Topical review in Semiconductor Science and Technology (in press)
(Some of the figures are not available in electronic form
Simulational studies of epitaxial semiconductor superlattices: Quantum dynamical phenomena in ac and dc electric fields
Using high-accuracy numerical methods we investigate the dynamics of independent electrons in both ideal and realistic superlattices subject to arbitrary ac and/or dc electric fields. For a variety of superlattice potentials, optically excited initial wave packets, and combinations of ac and dc electric fields, we numerically solve the time-dependent Schrodinger equation. In the case of ideal periodic superlattice potentials, we investigate a long list of dynamical phenomena involving multiple miniband transitions and time-dependent electric fields. These include acceleration effects associated with interminiband transitions in strong fields, Zener resonances between minibands, dynamic localization with ac fields, increased single-miniband transport with an auxiliary resonant ac field, and enhanced or suppressed interminiband probability exchange using an auxiliary ac field. For all of the cases studied, the resulting time-dependent wave function is analyzed by projecting the data onto convenient orthonormal bases. This allows a detailed comparison with approximate analytic treatments;In an effort to explain the rapid decay of experimentally measured Bloch oscillation (BO) signals we incorporate a one-dimensional representation of interface roughness (IR) into our superlattice potential. We show that as a result of IR, the electron dynamics can be characterized in terms of many discrete, incommensurate frequencies near the Bloch frequency. The interference effects associated with these frequencies cause a substantial decrease in amplitude of the signal after several Bloch periods. We suggest that this is an important source of coherence loss in BO signals at low temperature and low carrier density. We also propose an experimental method that should significantly reduce the effects of IR by exciting electrons to only a single layer of the superlattice. This is accomplished by doping the central GaAs layer with a very small amount (\u3c1%) of In, thus reducing the energy gap for this layer. Thus, a laser excitation pulse tuned somewhat below the nominal electron-hole excitation energy, will only excite a few Wannier-Stark eigenstates associated with this In-doped layer. Our numerical simulations show that the THz signal from electrons optically excited using this novel procedure is nearly free from all inhomogeneous broadening associated with IR
Excess resistivity in graphene superlattices caused by umklapp electron-electron scattering
Umklapp processes play a fundamental role as the only intrinsic mechanism
that allows electrons to transfer momentum to the crystal lattice and,
therefore, provide a finite electrical resistance in pure metals. However,
umklapp scattering has proven to be elusive in experiment as it is easily
obscured by other dissipation mechanisms. Here we show that electron-electron
umklapp scattering dominates the transport properties of
graphene-on-boron-nitride superlattices over a wide range of temperatures and
carrier densities. The umklapp processes cause giant excess resistivity that
rapidly increases with increasing the superlattice period and are responsible
for deterioration of the room-temperature mobility by more than an order of
magnitude as compared to standard, non-superlattice graphene devices. The
umklapp scattering exhibits a quadratic temperature dependence accompanied by a
pronounced electron-hole asymmetry with the effect being much stronger for
holes rather than electrons. Aside from fundamental interest, our results have
direct implications for design of possible electronic devices based on
heterostructures featuring superlattices
Dissipative Chaos in Semiconductor Superlattices
We consider the motion of ballistic electrons in a miniband of a
semiconductor superlattice (SSL) under the influence of an external,
time-periodic electric field. We use the semi-classical balance-equation
approach which incorporates elastic and inelastic scattering (as dissipation)
and the self-consistent field generated by the electron motion. The coupling of
electrons in the miniband to the self-consistent field produces a cooperative
nonlinear oscillatory mode which, when interacting with the oscillatory
external field and the intrinsic Bloch-type oscillatory mode, can lead to
complicated dynamics, including dissipative chaos. For a range of values of the
dissipation parameters we determine the regions in the amplitude-frequency
plane of the external field in which chaos can occur. Our results suggest that
for terahertz external fields of the amplitudes achieved by present-day free
electron lasers, chaos may be observable in SSLs. We clarify the nature of this
novel nonlinear dynamics in the superlattice-external field system by exploring
analogies to the Dicke model of an ensemble of two-level atoms coupled with a
resonant cavity field and to Josephson junctions.Comment: 33 pages, 8 figure
Metal-semiconductor (semimetal) superlattices on a graphite sheet with vacancies
It has been found that periodically closely spaced vacancies on a graphite
sheet cause a significant rearrange-ment of its electronic spectrum: metallic
waveguides with a high density of states near the Fermi level are formed along
the vacancy lines. In the direction perpendicular to these lines, the spectrum
exhibits a semimetal or semiconductor character with a gap where a vacancy
miniband is degenerated into impurity levels.Comment: 4 pages, 3 figure
Tunable Correlated Chern Insulator and Ferromagnetism in Trilayer Graphene/Boron Nitride Moir\'e Superlattice
Studies on two-dimensional electron systems in a strong magnetic field first
revealed the quantum Hall (QH) effect, a topological state of matter featuring
a finite Chern number (C) and chiral edge states. Haldane later theorized that
Chern insulators with integer QH effects could appear in lattice models with
complex hopping parameters even at zero magnetic field. The ABC-trilayer
graphene/hexagonal boron nitride (TLG/hBN) moir\'e superlattice provides an
attractive platform to explore Chern insulators because it features nearly flat
moir\'e minibands with a valley-dependent electrically tunable Chern number.
Here we report the experimental observation of a correlated Chern insulator in
a TLG/hBN moir\'e superlattice. We show that reversing the direction of the
applied vertical electric field switches TLG/hBN's moir\'e minibands between
zero and finite Chern numbers, as revealed by dramatic changes in
magneto-transport behavior. For topological hole minibands tuned to have a
finite Chern number, we focus on 1/4 filling, corresponding to one hole per
moir\'e unit cell. The Hall resistance is well quantized at h/2e2, i.e. C = 2,
for |B| > 0.4 T. The correlated Chern insulator is ferromagnetic, exhibiting
significant magnetic hysteresis and a large anomalous Hall signal at zero
magnetic field. Our discovery of a C = 2 Chern insulator at zero magnetic field
should open up exciting opportunities for discovering novel correlated
topological states, possibly with novel topological excitations, in nearly flat
and topologically nontrivial moir\'e minibands.Comment: 16 pages, 4 figures, and 2 extended figure
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