Fine structure of an exciton coupled to a single ${\mathrm{Fe}}^{2+}$ ion in a CdSe/ZnSe quantum dot

Fine structure of an exciton coupled to a single ${Fe}^{2 +}$ ion in a CdSe/ZnSe quantum dot

T. Smoleński, T. Kazimierczuk, M. Goryca, W. Pacuski, and P. Kossacki

Phys. Rev. B 96, 155411 – Published 5 October 2017

Abstract

We present a polarization-resolved photoluminescence study of the exchange interaction effects in a prototype system consisting of an individual ${Fe}^{2 +}$ ion and a single neutral exciton confined in a CdSe/ZnSe quantum dot. A maximal possible number of eight fully linearly polarized lines in the bright exciton emission spectrum is observed, evidencing complete degeneracy lifting in the investigated system. We discuss the conditions required for such a scenario to take place: anisotropy of the electron-hole interaction and the zero-field splitting of the ${Fe}^{2 +}$ ion spin states. Neglecting either of these components is shown to restore partial degeneracy of the transitions, making the excitonic spectrum similar to those previously reported for all other systems of quantum dots with single magnetic dopants.

Received 21 February 2017

DOI:https://doi.org/10.1103/PhysRevB.96.155411

Physics Subject Headings (PhySH)

Dopants Excitons Impurities Luminescence Magnetic anisotropy Magneto-optical effect Semiconductors Spintronics

0-dimensional systems Doped semiconductors Quantum dots

Fluorescence spectroscopy Molecular beam epitaxy Optical microscopy

Condensed Matter, Materials & Applied Physics

Authors & Affiliations

T. Smoleński^*, T. Kazimierczuk, M. Goryca, W. Pacuski, and P. Kossacki

Institute of Experimental Physics, Faculty of Physics, University of Warsaw, ul. Pasteura 5, 02-093 Warsaw, Poland

^*Tomasz.Smolenski@fuw.edu.pl

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Vol. 96, Iss. 15 — 15 October 2017

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Figure 1
PL spectra of the neutral exciton in two different CdSe/ZnSe QDs containing single ${Fe}^{2 +}$ ions. The spectra were measured without polarization resolution. (a) QD1 with negligible $δ_{Fe}$ splitting of the two ${Fe}^{2 +}$ ground states. (b) QD2 exhibiting large $δ_{Fe}$ splitting as well as a strong anisotropic part of the electron-hole exchange interaction. Note that the spectral resolution of the QD1 spectrum is approximately 1.5 times lower than in the case of QD2, which is due to the shorter length of the monochromator used. The approximate values of both resolutions are schematically marked by horizontal bars displayed on the left side of each spectrum.
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Figure 2
The scheme illustrating the energies of both initial and final states of the bright exciton $X_{b}$ recombination in a QD with a single ${Fe}^{2 +}$ ion (note that level spacing is not to scale). Numbers in parentheses denote the degeneracy of the energy levels. For the sake of clarity, we omitted the higher-energy states of the ${Fe}^{2 +}$ ion with $| S_{z} | < 2$ that are not populated at helium temperatures.
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Figure 3
(a) Color-scale map presenting the $X$ PL spectra measured for QD2 using different orientations of detected linear polarization. The polarization angle $φ$ is measured with respect to the [100] crystallographic direction. For all spectra the continuous background was subtracted. Additionally, the subsequent spectra were corrected for a small-intensity drift of the whole QD emission as well as residual spectral diffusion ( $< 0.03$ meV) using the reference unpolarized PL signal originating from the trion recombination in the same QD. (b) Theoretical simulation of the experimental data from (a) within the frame of the spin Hamiltonian model described in the text. The exchange energies used for the fit yield $Δ_{s p - d} = 0.53$ meV, $δ_{1} = 0.39$ meV, and $δ_{Fe} = 0.23$ meV. (c) The $X$ PL spectra detected in two orthogonal linear polarizations $π_{∥}$ (black) and $π_{⊥}$ [red (gray)] oriented along the principal axes $φ = θ$ and $φ = θ + 90^{\circ}$ of the electron-hole exchange interaction anisotropy (solid lines represent the model calculations). (d) Cartoon with definitions of the angles used in the discussion.
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Figure 4
(a) Color-scale map presenting the magnetic field dependence of the neutral exciton PL spectrum in QD2 (the field was applied in the Faraday geometry, i.e., along the growth axis of the sample). The data were measured in the $σ^{-}$ polarization of detection. (b) Corresponding simulation of the $X$ magneto-PL performed within the frame of the spin Hamiltonian $H$ from Eq. (1) extended with a phenomenological diamagnetic shift term with $γ = 0.70 μ eV / T^{2}$ as well as the ion and the exciton Zeeman terms with the corresponding $g$ factors equal to $g_{Fe} = 2.2$ and $g_{X} = 1.7$ , respectively. The values of all the other model parameters are the same as in the zero-field fit from Fig. 3.
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Physical Review B

covering condensed matter and materials physics

Fine structure of an exciton coupled to a single ${Fe}^{2 +}$ ion in a CdSe/ZnSe quantum dot

T. Smoleński, T. Kazimierczuk, M. Goryca, W. Pacuski, and P. Kossacki

Phys. Rev. B 96, 155411 – Published 5 October 2017

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Physical Review B

covering condensed matter and materials physics

Fine structure of an exciton coupled to a single Fe2+ ion in a CdSe/ZnSe quantum dot

T. Smoleński, T. Kazimierczuk, M. Goryca, W. Pacuski, and P. Kossacki

Phys. Rev. B 96, 155411 – Published 5 October 2017

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Fine structure of an exciton coupled to a single ${Fe}^{2 +}$ ion in a CdSe/ZnSe quantum dot