Searcharxiv⌕ Search

arXiv subjects

Josep Planelles

Publications and source records attributed to Josep Planelles.

At least 19 recordsLinked to original sources
arXiv2026

Excitonic structure in CsPbBr$_3$ nanocubes, nanorods and nanoplatelets: the effect of dimensionality

We present a theoretical study comparing the excitonic ground state properties of CsPbBr$_3$ nanocrystals with different dimensionality: nanorods (quasi-1D), nanoplatelets (quasi-2D) and nanocubes (quasi-3D). All three systems are described on equal footing, by means of a general variational effective mass model, which captures the influence of quantum confinement, dielectric confinement, electron-hole correlations and polaronic effects (within a Haken model). The strongly confined directions squeeze the exciton (X) wavefunction and enhance Coulomb attractions along the weakly confined directions. This stimulates superradiance, thus making radiative recombination rates speed up from cubes to platelets and to rods, in line with recent experiments. The anisotropic local field factor is a secondary, yet non-negligible, mechanism further enhancing radiative rates. X binding energies are also determined primarily by the directions of strong confinenement, which is also consistent with experiments. Weakly confined directions become however influential for small aspect ratios. Dielectric confinement plays a major role in determining the binding energies, and less so in the interparticle-distances. For all dimensionalities, the biexciton (XX) geometry is that of a distorted tetrahedron, rather than squared or linear distributions that would result in Coulomb-governed 2D and 1D structures.

cond-mat.mes-hall↗
arXiv2025

Electronic structure of InP/ZnSe quantum dots: effect of tetrahedral shape, valence band coupling and excitonic interactions

The energy levels and optical transitions of tetrahedral core/shell InP/ZnSe quantum dots (QDs) are investigated by means of multi-band k$\cdot$p theory. Despite the $\overline{T}_d$ symmetry relaxing spherical selection rules, the near-band-edge excitonic spectrum is reminiscent of that obtained for spherical nanocrystals. Exceptions appear in large (red-emitting) QDs, where transitions violating the (quasi-)angular momentum selection rule ($\Delta L=0,\pm 2$) are observed, and the ground state does not become dark ($P_{3/2}$-like). Valence band coupling is important in determining the symmetry, degeneracy and energy of hole states, with split-off holes playing a greater role than in CdSe QDs. The ($1S_e$-like) electron ground state exhibits moderate delocalization into the ZnSe shell. The confinement regime is then strong even for thick shells, which results in Coulomb interactions being mostly perturbative. Electrons remain largely localized in the InP core even in negative trions, despite electron-electron repulsions. At the same time, the asymmetry between Coulomb attractions and repulsions leads to negative (positive) trions being bound (antibound) by tens of meV. The biexciton binding energy switches from positive to negative, depending on the QD size.

cond-mat.mes-hall↗
arXiv2025

Internal dynamics and dielectric screening of confined multiexciton states

Recent experimental and computational studies suggest that biexcitons (BX) confined in large CsPbBr$_3$ nanocrystals experience reduced dielectric screening as compared to excitons (X) and trions (X$^*$). Here we provide a physical rationale to explain such a behavior. A characteristic frequency is introduced, which describes the internal dynamics of an exciton within the excitonic complex. By means of effective mass--variational Quantum Monte Carlo simulations, we show that, in large nanocrystals, the frequency is similar for X and X$^*$, but smaller for BX. Because the frequencies exceed that of the bulk longitudinal optical phonon, this leads to a reduced dielectric constant for BX, which is in contrast with the behavior of strongly confined nanocrystals.

cond-mat.mes-hall↗
arXiv2024

Binding energy of polaronic trions and biexcitons in CsPbBr$_3$ nanocrystals

The effect of polaron formation on the ground state of excitons, trions and biexcitons confined in CsPbBr$_3$ nanocrystals is studied in the framework of effective mass Hamiltonians, using a Haken-like (Bajaj) potential for carrier-phonon coupling. The binding energy of trions agrees well with that observed in experiments, with position-dependent dielectric screening playing a significant role. For biexcitons, however, neither polaronic effects, nor dielectric confinement, nor electronic correlations -- here accounted for with a variational Quantum Monte Carlo method -- suffice to explain the large binding energies reported by single nanocrystal spectroscopy experiments. This result reinforces the hypothesis that biexcitons polarize the perovskite lattice differently from excitons and trions.

cond-mat.mes-hall↗
arXiv2023

Excitons in layered metal halide perovskites: an effective mass description of polaronic, dielectric and quantum confinement effects

A theoretical model for excitons confined in layered metal halide perovskites is presented. The model accounts for polaronic effects, dielectric and quantum confinement by means of effective mass theory, image charges and Haken potentials. We use it to describe the band edge exciton of MAPbI$_3$ structures surrounded by organic ligands. It is shown that the quasi-2D quantum and dielectric confinement of layered perovskites squeezes the exciton radius, and this in turn enhances polaronic effects as compared to 3D structures. The strong polaronic effects boost the binding energies and radiative recombination probabilities, which allows one to match experimental data in related systems. The thickness dependence of Coulomb polarization and self-energy potentials is in fair agreement with sophisticated atomistic models.

cond-mat.mes-hall↗
arXiv2023

Two Biexciton Types Coexisting in Coupled Quantum Dot Molecules

Coupled colloidal quantum dot molecules are an emerging class of nanomaterials, introducing new degrees of freedom for designing quantum dot-based technologies. The properties of multiply excited states in these materials are crucial to their performance as quantum light emitters but cannot be fully resolved by existing spectroscopic techniques. Here we study the characteristics of biexcitonic species, which represent a rich landscape of different configurations, such as segregated and localized biexciton states. To this end, we introduce an extension of Heralded Spectroscopy to resolve different biexciton species in the prototypical CdSe/CdS coupled quantum dot dimer system. We uncover the coexistence and interplay of two distinct biexciton species: A fast-decaying, strongly-interacting biexciton species, analogous to biexcitons in single quantum dots, and a long-lived, weakly-interacting species corresponding to two nearly-independent excitons separated to the two sides of the coupled quantum dot pair. The two biexciton types are consistent with numerical simulations, assigning the strongly-interacting species to two excitons localized at one side of the quantum dot molecule and the weakly-interacting species to excitons segregated to the two quantum dot molecule sides. This deeper understanding of multiply excited states in coupled quantum dot molecules can support the rational design of tunable single- or multiple-photon quantum emitters.

cond-mat.mes-hall↗
arXiv2022

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.

cond-mat.mes-hall↗
arXiv2022

Axion Electrodynamics in Topological Insulators for beginners

Lecture-notes: introducing and discussing basics on axion electrodynamics with application in the calculation of the images generated by an electric (or a magnetic) charge located close to an interface ordinary insulator-topological insulator

cond-mat.mes-hall↗
arXiv2021

Comparison between trion and exciton electronic properties in CdSe and PbS nanoplatelets

The optoelectronic properties of metal chalcogenide colloidal nanoplatelets are often interpreted in terms of excitonic states. However, recent spectroscopic experiments evidence the presence of trion states, enabled by the slow Auger recombination in these structures. We analyze how the presence of an additional charge in trions modifies the emission energy and oscillator strength as compared to neutral excitons. These properties are very sensitive to dielectric confinement and electronic correlations, which we describe accurately using image-charge and variational Quantum Monte Carlo methods in effective mass Hamiltonians. We observe that the giant oscillator strength of neutral excitons is largely suppressedin trions. Both negative and positive trions are redshifted with respect to the exciton, and their emission energy increases with increasing dielectric mismatch between the platelet and its surroundings, which is a consequence of the self-energy potential. Our results are consistent with experiments in the literature, and assess on the validity of previous theoretical approximations.

cond-mat.mes-hall↗
arXiv2020

A simple variational quantum Monte Carlo-effective mass approach for excitons and trions in quantum dots

A computational model is presented to calculate the ground state energy of neutral and charged excitons confined in semiconductor quantum dots. The model is based on the variational Quantum Monte Carlo method and effective mass Hamiltonians. Through an iterative Newton-Rhapson process, minimizing the local energy, and (optional) parallelization of random walkers, fast and accurate estimates of both confinement and Coulomb binding energies can be obtained in standard desktop computers. To illustrate the reach of the model, we provide Fortran programs and illustrative calculations for colloidal CdSe nanoplatelets with large lateral dimensions and dielectric confinement, where electronic correlations are strong. The results compare well with exact variational calculations and largely outperform configuration interaction calculations in computational efficiency.

cond-mat.mes-hall↗
arXiv2020

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 redshift and split, forming (for large stacks) minibands of several meV 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 which should confirm the formation of molecular states, whose practical realization would pave the way to the development of nanocrystal chemistry based on NPLs.

cond-mat.mes-hall↗
arXiv2018

Tuning intraband and interband transition rates via excitonic correlation in low-dimensional semiconductors

We show that electron-hole correlation can be used to tune interband and intraband optical transition rates in semiconductor nanostructures with at least one weakly confined direction. The valence-to-conduction band transition rate can be enhanced by a factor $(L/a_B)^{N}$ -- with $L$ the length of the weakly confined direction, $a_B$ the exciton Bohr radius and $N$ the dimensionality of the nanostructure -- while the rate of intraband and inter-valence-band transitions can be slowed down by the inverse factor, $(a_B/L)^{N}$. Adding a hitherto underexplored degree of freedom to engineer excitonic transition rates, this size dependence is of interest for various opto-electronic applications. It also offers an interpretation of the superlinear volume scaling of two-photon absorption (TPA) cross-section recently reported for CdSe nanoplatelets, thus laying foundations to obtain unprecedented TPA cross sections, well above those of conventional two-photon absorbers. Further, our concept explains the background of the validity of the universal continuum absorption approach for the determination of particle concentrations via the intrinsic absorption. Potential applications of our approach include low excitation intensity confocal two-photon imaging, two-photon autocorrelation and cross correlation with much higher sensitivity and unprecedented temporal resolution as well as TPA based optical stabilization and optimizing of inter-subband transition rates in quantum cascade lasers.

cond-mat.mes-hall↗
arXiv2017

Magnetic field dependence of edge states in MoS$_2$ quantum dots

We study the electronic structure of monolayer MoS$_2$ quantum dots subject to a perpendicular magnetic field. The coupling between conduction and valence band gives rise to mid-gap topological states which localize near the dot edge. These edge states are analogous to those of 1D quantum rings. We show they present a large, Zeeman-like, linear splitting with the magnetic field, anticross with the delocalized Fock-Darwin-like states of the dot, give rise to Aharonov-Bohm-like oscillations of the conduction (valence) band low-lying states in the K (K') valley, and modify the strong field Landau levels limit form of the energy spectrum.

cond-mat.mes-hall↗
arXiv2017

Excitons in core-only, core-shell and core-crown CdSe nanoplatelets: interplay between in-plane electron-hole correlation, spatial and dielectric confinement

Using semi-analytical models we calculate the energy, effective Bohr radius and radiative lifetime of neutral excitons confined in CdSe colloidal nanoplatelets (NPLs). The excitonic properties are largely governed by the electron-hole in-plane correlation, which in NPLs is enhanced by the quasi-two-dimensional motion and the dielectric mismatch with the organic environment. In NPLs with lateral size $L \gtrsim 20$ nm the exciton behavior is essentially that in a quantum well, with superradiance leading to exciton lifetimes of 1 ps or less, only limited by the NPL area. However, for $L < 20$ nm excitons enter an intermediate confinement regime, hence departing from the quantum well behavior. In heterostructured NPLs, different response is observed for core/shell and core/crown configurations. In the former, the strong vertical confinement limits separation of electrons and holes even for type-II band alignment. The exciton behavior is then similar to that in core-only NPL, albeit with weakened dielectric effects. In the latter, charge separation is also inefficient if band alignment is quasi-type-II (e.g. in CdSe/CdS), because electron-hole interaction drives both carriers into the core. However, it becomes very efficient for type-II alignment, for which we predict exciton lifetimes reaching $μs$.

cond-mat.mes-hall↗
arXiv2017

Simple correlated wave-functions for excitons in 0D, quasi-1D and quasi-2D quantum dots

We propose correlated yet extremely simple single-parameter-dependent wave-functions with a Slater-type correlation factor, to describe excitons in 0D, quasi-1D and quasi-2D semiconductor quantum dots. We provide closed-form formulas for the wave-function normalization factor, electron/hole single-particle density and the expectation value of the kinetic energy. We additionally supply fast integration procedures for the Coulomb interaction in the presence of dielectric mismatch with the surrounding medium for nanoplatelets (quasi-2D systems), and for the bare-Coulomb integral in long nanorods (quasi-1D systems).

cond-mat.mes-hall↗
arXiv2016

Electronic Origin of Linearly Polarized Emission in CdSe/CdS Dot-in-Rod Heterostructures

Seeded CdSe/CdS nanorods exhibit intense polarized emission along the rod main axis. The degree of linear polarization cannot be explained by dielectric effects alone, an additional electronic contribution is present whose nature has not been settled up to date. Using multi-band k.p theory, we analyse the potential influence of several factors affecting excitonic emission and show that shear strain is the main electronic mechanism promoting linear polarization. It favors energetically light hole excitons over heavy hole ones, via deformation potential, and makes their radiative recombination faster via piezoelectricity. Implications of this mechanism are that linear emission can be enhanced by growing long but thin CdS shells around large, prolate CdSe cores, which indeed supports and rationalizes recent experimental findings. Together with the well-known dielectric effects, these results pave the way for controlled degree of linear polarization in dot-in-rods through dedicated structural design.

cond-mat.mes-hall↗
arXiv2015

Theory of electrons, holes and excitons in GaAs polytype quantum dots

Single and multi-band (Burt-Foreman) k.p Hamiltonians for GaAs crystal phase quantum dots are developed and used to assess ongoing experimental activity on the role of such factors as quantum confinement, spontaneous polarization, valence band mixing and exciton Coulomb interaction. Spontaneous polarization is found to be a dominating term. Together with the control of dot thickness [Vainorious Nano Lett. 15, 2652 (2015)] it enables wide exciton wavelength and lifetime tunability. Several new phenomena are predicted for small diameter dots [Loitsch et al. Adv. Mater. 27, 2195 (2015)], including non-heavy hole ground state, strong hole spin admixture and a type-II to type-I exciton transition, which can be used to improve the absorption strength and reduce the radiative lifetime of GaAs polytypes.

cond-mat.mes-hall↗