arXiv subjects

J. Planelles

Publications and source records attributed to J. Planelles.

11 recordsLinked to original sources

arXiv2023

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.

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arXiv2015

Edge states in dichalcogenide nanoribbons and triangular quantum dots

The electronic structure of monolayer MoS$_2$ nanoribbons and quantum dots have been investigated by means of an effective $\mathbf{k}\cdot \mathbf{p}$ two-band model. Both systems with borders exhibit states spatially localized on the edges and with energies lying in the band gap. We show that the conduction and valence band curvatures determine the presence/absence of these states whose origin has been related to the marginal topological properties of the MoS$_2$ single-valley Hamiltonian.

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arXiv2014

Anisotropy of spin-orbit induced electron spin relaxation in [001] and [111] grown GaAs quantum dots

We report a systematic study of the spin relaxation anisotropy between single electron Zeeman sublevels in cuboidal GaAs quantum dots (QDs). The QDs are subject to an in-plane magnetic field. As the field orientation varies, the relaxation rate oscillates periodically, showing ``magic'' angles where the relaxation rate is suppressed by several orders of magnitude. This behavior is found in QDs with different shapes, heights, crystallographic orientations and external fields. The origin of these angles can be traced back to the symmetries of the spin admixing terms of the Hamiltonian. In [001] grown QDs, the suppression angles are different for Rashba and Dresselhaus spin-orbit terms. By contrast, in [111] grown QDs they are the same, which should facilitate a thorough suppression of spin-orbit induced relaxation. Our results evidence that cubic Dresselhaus terms play a critical role in determining the spin relaxation anisotropy even in quasi-2D QDs.

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arXiv2011

Effect of interface alloying and band-alignment on the Auger recombination of heteronanocrystals

We report a numerical study of the effect of interface alloying and band-alignment on the Auger recombination processes of core/shell nanocrystals. Numerical calculations are carried out using a two-band Kane Hamiltonian. Smooth interfaces are found to suppress Auger recombination, the strength of the suppression being very sensitive to the core size. The use of type-II structures constitutes an additional source of suppression, especially when the shell confines electrons rather than holes. We show that "magic" sizes leading to negligible Auger recombination [Cragg and Efros, Nano Letters 10 (2010) 313] should be easier to realize experimentally in nanocrystals with extended interface alloying and wide band gap.

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arXiv2008

Isospin phases of vertically coupled double quantum rings under the influence of perpendicular magnetic fields

Vertically coupled double quantum rings submitted to a perpendicular magnetic field $B$ are addressed within the local spin-density functional theory. We describe the structure of quantum ring molecules containing up to 40 electrons considering different inter-ring distances and intensities of the applied magnetic field. When the rings are quantum mechanically strongly coupled, only bonding states are occupied and the addition spectrum of the artificial molecules resembles that of a single quantum ring, with some small differences appearing as an effect of the magnetic field. Despite the latter has the tendency to flatten the spectra, in the strong coupling limit some clear peaks are still found even when $B\neq 0$ that can be interpretated from the single-particle energy levels analogously as at zero applied field, namely in terms of closed-shell and Hund's-rule configurations. Increasing the inter-ring distance, the occupation of the first antibonding orbitals washes out such structures and the addition spectra become flatter and irregular. In the weak coupling regime, numerous isospin oscillations are found as a function of $B$.

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arXiv2007

Electron states in quantum rings with defects under axial or in-plane magnetic fields

A comprehensive study of anisotropic quantum rings, QRs, subject to axial and in-plane magnetic field, both aligned and transverse to the anisotropy direction, is carried out. Elliptical QRs for a wide range of eccentricity values and also perfectly circular QRs including one or more barriers disturbing persistent QR current are considered. These models mimic anisotropic geometry deformations and mass diffusion occuring in the QR fabrication process. Symmetry considerations and simplified analytical models supply physical insight into the obtained numerical results. Our study demonstrates that, except for unusual extremely large eccentricities, QR geometry deformations only appreciably influence a few low-lying states, while the effect of barriers disturbing the QR persistent current is stronger and affects all studied states to a similar extent. We also show that the response of the electron states to in-plane magnetic fields provides accurate information on the structural anisotropy.

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arXiv2006

Electronic states of laterally coupled quantum rings

The conduction band electron states of laterally-coupled semiconductor quantum rings are studied within the frame of the effective mass envelope function theory. We consider the effect of axial and in-plane magnetic fields for several inter-ring distances, and find strong changes in the energy spectrum depending on the coupling regime. Our results indicate that the magnetic response accurately monitors the quantum ring molecule dissociation process. Moreover, the anisotropic response of the electron states to in-plane magnetic fields provides information on the orientation of the quantum ring molecule.

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arXiv2006

Electronic structure of few-electron concentric double quantum rings

The ground state structure of few-electron concentric double quantum rings is investigated within the local spin density approximation. Signatures of inter-ring coupling in the addition energy spectrum are identified and discussed. We show that the electronic configurations in these structures can be greatly modulated by the inter-ring distance: At short and long distances the low-lying electron states localize in the inner and outer rings, respectively, and the energy structure is essentially that of an isolated single quantum ring. However, at intermediate distances the electron states localized in the inner and the outer ring become quasi-degenerate and a rather entangled, strongly-correlated system is formed.

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arXiv2006

Trapping electrons in semiconductor air bubbles: A theoretical approach

The role of image charges in nanoporous semiconductor materials is investigated within the framework of the effective mass and envelope function approximations. We show that nanometric air bubbles in these materials can act as electron-trapping centers. This trapping capability originates from a deep stabilizing self-polarization potential well induced by the air - semiconductor dielectric mismatch which can surpass the electroaffinity barrier. The trapping strength is a function of the pore size and the bulk parameters of the matrix material. A {\it trapping parameter} characteristic for each semiconductor material is defined. This parameter provides a simple way to ascertain the maximum pore size in a given material which is able to induce self-trapping of excess electrons.

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arXiv2005

Magnetic field dependence of hole levels in self-assembled InAs quantum dots

Recent magneto-transport experiments of holes in InGaAs quantum dots [D. Reuter, P. Kailuweit, A.D. Wieck, U. Zeitler, O. Wibbelhoff, C. Meier, A. Lorke, and J.C. Maan, Phys. Rev. Lett. 94, 026808 (2005)] are interpreted by employing a multi-band kp Hamiltonian, which considers the interaction between heavy hole and light hole subbands explicitely. No need of invoking an incomplete energy shell filling is required within this model. The crucial role we ascribe to the heavy hole-light hole interaction is further supported by one-band local-spin-density functional calculations, which show that Coulomb interactions do not induce any incomplete hole shell filling and therefore cannot account for the experimental magnetic field dispersion.

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arXiv2005

Aharonov-Bohm effect for pedestrian

When a magnetic field pierces a multiple-connected quantum system, the corresponding wavefunction is altered although no net Lorentz force acts upon its carriers. This is the so called Aharonov-Bohm effect. The most simple multiply-connected quantum system is a quantum ring QR. Nowadays it is possible to obtain QRs in the nanoscopic range providing spectroscopic data vs. and applied external magnetic field. We describe here the most significant quantum effects induced by the magnetic field in a QR by means of simple quantum mechanical models.

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