709 research outputs found
Dielectric confinement of excitons in type-I and type-II semiconductor nanorods
We theoretically study the effect of the dielectric environment on the
exciton ground state of CdSe and CdTe/CdSe/CdTe nanorods. We show that
insulating environments enhance the exciton recombination rate and blueshift
the emission peak by tens of meV. These effects are particularly pronounced for
type-II nanorods. In these structures, the dielectric confinement may even
modify the spatial distribution of electron and hole charges. A critical
electric field is required to separate electrons from holes, whose value
increases with the insulating strength of the surroundings.Comment: Journal of Physics: Condensed Matter (in press
Two-dimensional Bloch electrons under strong magnetic modulation
The band structure of a high-mobility two-dimensional electron gas patterned with a square lattice of holes (antidots) is studied theoretically under the influence of a magnetic modulation consisting of perpendicular magnetic flux tubes with the same period and nonzero net flux per unit cell. The magnetic field pierces the system through the patterned holes only, so that the coupling with the electrons is purely quantum mechanical. The model takes implicitly into account the coupling between the different Bloch bands. The flux-dependent energy structure exhibits a Hofstadter butterfly-type spectrum. Such a structure is repeated indefinitely without distortion with a period of one magnetic flux quantum through a lattice hole. Rectangular deviations from the square lattice are also studied. It is found that the number and width of the magnetic gaps decrease, and even disappear for large antidot filling fractions
Magnetic modulation of the tunneling between defects states in antidot superlattices
We show theoretically that the tunnelling between properly designed defects in periodic antidot lattices can be strongly modulated by applied magnetic fields. Further, transport channels made up of linear arrangements of tunnel-coupled defects can accommodate Aharonov-Bohm cages, suggesting a magnetic control of the transport through the system. Evidence supporting an unusual robustness of the caging effect against electron-electron interactions is also provided
Delocalized image surface states in defects free SiO2 hollow nanospheres
Delocalized image surface states in free-standing hollow silica nanospheres populated with one or
two electrons or an exciton are theoretically predicted for a wide range of internal radii and shell
thicknesses. The driving force building up these surface states is the image self-polarization
potential originating from the dielectric mismatch between the nanoshell and the surrounding air.
The surface states are localized in a spherical crown beyond the nanoshell border. The transition
from volume to surface state will then have to overcome the spatial confining potential barrier of the
nanoshell. Owing to the different spatial confining barriers of electrons and holes in the silica
nanoshell, electron but no hole density can be concentrated in surface distributions. The
self-polarization potential looks like a double well potential, each well located just beyond the
nanoshell border, with the internal well deeper than the external one, so that an excess carrier is
attracted more strongly by the inner interface. This leads the electron density of a surface state to be
located mainly in the internal surface of the hollow nanosphere. The shorter the inner nanoshell
radius is, the stronger the binding of the excess electron to the surface will be. The volume/surface
ground state phase diagrams of the one-electron, two-electron, and exciton systems have been
calculated. All three diagrams are quite similar, thus revealing the monoelectronic character of the
driving force for the transition from volume to surface state
Dielectric Confinement Enables Molecular Coupling in Stacked Colloidal Nanoplatelets
We show theoretically that carriers confined in semiconductor colloidal nanoplatelets (NPLs) sense the presence of neighbor, cofacially stacked NPLs in their energy spectrum. When approaching identical NPLs, the otherwise degenerate energy levels red-shift and split, forming (for large stacks) minibands that are several millielectronvolts in width. Unlike in epitaxial structures, the molecular behavior does not result from quantum tunneling but from changes in the dielectric confinement. The associated excitonic absorption spectrum shows a rich structure of bright and dark states, whose optical activity and multiplicity can be understood from reflection symmetry and Coulomb tunneling. We predict spectroscopic signatures that should confirm the formation of molecular states, whose practical realization would pave the way for the development of nanocrystal chemistry based on NPLs
Coupled donors in quantum dots: quantum size and dielectric mismatch effects
Spatial and dielectric confinement modulations of the spontaneous emission rates, transition energies, and charge-density distributions of a singly ionized double donor system (D2+) in a spherical quantum dot are calculated within the framework of the effective-mass envelope function approximation. Dipole moments, energy splittings, transition moments, electron-density distributions, and spontaneous emission rates involving bonding and antibonding lowest-lying molecular states are addressed for different dielectric environments, quantum dot radii, and relative locations of the coupled impurities inside the dot. The results indicate that the donor molecule behaves as heteropolar when the spatial confinement breaks the inversion symmetry, which is paralleled by a strong reduction in the excited-state radiative lifetime. Dielectric confinement, acting on a larger length scale than spatial confinement, may recover the bulklike homopolar character when the dot is embedded in a low dielectric constant medium. In the weak spatial confinement regime, dielectric effects can increase the corresponding bulk radiative lifetimes significantly and simultaneously modulate the charge-density distributio
The topological magnetoelectric effect in semiconductor nanostructures: quantum wells, wires, dots and rings
Electrostatic charges placed near the interface between ordinary and
topological insulators induce magnetic fields, through the so-called
topological magnetoelectric effect. Here, we present a numerical implementation
of the associated Maxwell equations. The resulting model is simple, fast and
quantitatively as accurate as the image charge method, but with the advantage
of providing easy access to elaborate geometries when pursuing specific
effects. The model is used to study how magnetoelectric fields are influenced
by the dimensions and the shape of the most common semiconductor
nanostructures: quantum wells, quantum wires, quantum dots and quantum rings.
Point-like charges give rise to magnetic fields of the order of mT, whose sign
and spatial orientation is governed by the geometry of the nanostructure and
the location of the charge. The results are rationalized in terms of the Hall
currents induced on the surface, which constitute a simple yet valid framework
for the deterministic design of magnetoelectric fields.Comment: 9 pages, 9 figure
Generalized method of image dyons for quasi-two dimensional slabs with ordinary-topological insulator interfaces
Electrostatic charges near the interface bewteen topological (TI) and
ordinary (OI) insulators induce magnetic fields in the medium that can be
described through the so-called method of image dyons (electric charge -
magnetic monopole pairs), the magnetoelectric extension of the method of image
charges in classical electrostatics. Here, we provide the expressions for the
image dyons and ensuing magnetoelectric potentials in a system comprised by two
planar-parallel OI-TI interfaces conforming a finite-width slab. The obtained
formulae extend earlier work in that they account for all different
combinations of materials forming the slab and its surroundings, including
asymmetric systems, as well as all possible combinations of external
magnetization orientations on the interfaces. The equations are susceptible of
implementation in simple computational codes, to be solved recurrently, in
order to model magnetoelectric fields in topological quantum wells, thin films,
or layers of two-dimensional materials. We exemplify this by calculating the
magnetic fields induced by a point charge in nanometer-thick quantum wells, by
means of a Mathematica code made available in repositories.Comment: 9 pages, 7 figure
Generalized method of image dyons for quasi-two dimensional slabs with ordinary - topological insulator interfaces
Electrostatic charges near the interface between topological (TI) and ordinary (OI) insulators induce magnetic fields in the medium that can be described through the so-called method of image dyons (electric charge - magnetic monopole pairs), the magnetoelectric extension of the method of image charges in classical electrostatics. Here, we provide the expressions for the image dyons and ensuing magnetoelectric potentials in a system comprised by two planar-parallel OI-TI interfaces conforming a finite-width slab. The obtained formulae extend earlier work in that they account for all different combinations of materials forming the slab and its surroundings, including asymmetric systems, as well as all possible combinations of external magnetization orientations on the interfaces. The equations are susceptible of implementation in simple computational codes, to be solved recurrently, in order to model magnetoelectric fields in topological quantum wells, thin films, or layers of two-dimensional materials. We exemplify this by calculating the magnetic fields induced by a point charge in nanometer-thick quantum wells, by means of a Mathematica code made available in repositories.Funding for open access charge: CRUE-Universitat Jaume
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