Logo
ScienceStack
Table of Contents
Fetching ...
Sign In

Strain-driven spin mixing and dark-exciton recombination in a neutral Ni2+ doped quantum dot

K. E. Polczynska, S. Karouaz, W. Pacuski, L. Besombes

Abstract

We investigate the optical properties of neutral excitons in CdTe/ZnTe quantum dots containing a single Ni2+ ion. We show that the photoluminescence spectra provide a direct spectroscopic signature of strain induced mixing of the Ni2+ spin states. A misalignment between the principal axis of the local strain tensor and the quantum dot growth direction reorients the spin quantization axis of the magnetic ion, reducing the hole Ni2+ exchange interaction at low magnetic field and giving rise to photoluminescence replicas around the partially linearly polarized bright-exciton transitions. A longitudinal magnetic field restores the circularly polarized optical selection rules, allowing the three spin projections S_z = 0, +-1 of the Ni2+ ion to be spectrally resolved. Dark exciton emission appears on the low energy side of the spectra and is dominated at low field by transitions involving spin flips of the magnetic ion. An effective spin Hamiltonian including strain orientation and valence band mixing reproduces the magnetic field evolution of both bright and dark exciton spectra. These results highlight the key role of the local strain environment in determining the spin exciton coupling of transition metal dopants in semiconductor quantum dots.

Strain-driven spin mixing and dark-exciton recombination in a neutral Ni2+ doped quantum dot

Abstract

We investigate the optical properties of neutral excitons in CdTe/ZnTe quantum dots containing a single Ni2+ ion. We show that the photoluminescence spectra provide a direct spectroscopic signature of strain induced mixing of the Ni2+ spin states. A misalignment between the principal axis of the local strain tensor and the quantum dot growth direction reorients the spin quantization axis of the magnetic ion, reducing the hole Ni2+ exchange interaction at low magnetic field and giving rise to photoluminescence replicas around the partially linearly polarized bright-exciton transitions. A longitudinal magnetic field restores the circularly polarized optical selection rules, allowing the three spin projections S_z = 0, +-1 of the Ni2+ ion to be spectrally resolved. Dark exciton emission appears on the low energy side of the spectra and is dominated at low field by transitions involving spin flips of the magnetic ion. An effective spin Hamiltonian including strain orientation and valence band mixing reproduces the magnetic field evolution of both bright and dark exciton spectra. These results highlight the key role of the local strain environment in determining the spin exciton coupling of transition metal dopants in semiconductor quantum dots.
Paper Structure (16 sections, 7 equations, 5 figures, 2 tables)

This paper contains 16 sections, 7 equations, 5 figures, 2 tables.

Table of Contents

  1. Introduction
  2. Optical probing of neutral Ni$^{2+}$-doped quantum dots
  3. Sample and experiments
  4. Photo-luminescence of X-Ni$^{2+}$
  5. Magneto-optical properties of X-Ni$^{2+}$
  6. Bright excitons in a Ni$^{2+}$-doped quantum dot
  7. Model of X-Ni$^{2+}$
  8. Longitudinal magnetic field dependence of X-Ni$^{2+}$
  9. Calculated magnetic field dependence of X-Ni$^{2+}$ and dark excitons recombination
  10. Calculated B$_z$ dependence of the PL of X-Ni$^{2+}$
  11. PL of dark excitons in a Ni$^{2+}$-doped QD
  12. Identification of the dark exciton lines
  13. Evolution of dark excitons at low B$_z$
  14. Dark excitons under large B$_z$, anticrossings
  15. Conclusion
  16. ...and 1 more sections

Figures (5)

  • Figure 1: (a) PL spectra of the exciton in a Ni$^{2+}$-doped QD at B$_z$=0T for an excitation on an excited state at E = 2129.3 meV. (b) PL excitation spectra of the neutral exciton. (c) Intensity map of the linearly polarized PL of the exciton at B$_z$=0 T. (d) Linearly polarized PL spectra recorded along two orthogonal directions.
  • Figure 2: Circularly polarized PL intensity map of the longitudinal magnetic field (B$_z$) dependence of the bright (X$_b$) and dark (X$_d$) excitons in Ni$^{2+}$-doped CdTe/ZnTe QD. The fan-like structure of the dark exciton transitions reflects recombination processes involving changes in the Ni$^{2+}$ spin levels.
  • Figure 3: (a) Detailed magnetic-field dependence of the dark-exciton PL transition energies in a Ni$^{2+}$-doped QD. (b) Circularly polarized PL spectra of the neutral exciton in a Ni$^{2+}$-doped QD at $B_z = 9$ T. (c) Excitation-power dependence of the exciton PL intensity in $\sigma^+$ polarization at $B_z = 9$ T
  • Figure 4: (a) Calculated PL intensity map of the magnetic-field dependence of a neutral exciton in a Ni$^{2+}$-doped QD, obtained using the parameters listed in Table \ref{['TableParX']}. The insets show details of dark-bright excitons anti-crossings. (b) Calculated linearly polarized PL intensity map at $B_z = 0$ T. The intensity maps are displayed on a logarithmic scale. The effective temperature is set to $T_{\mathrm{eff}} = 10$ K, and the emission lines are broadened with a Lorentzian profile of full width at half maximum (FWHM) of 50 $\mu$eV. Zero energy corresponds to an exciton with no exchange interaction.
  • Figure 5: Energy-level scheme at zero magnetic field and under a large $B_z$ for the initial (X–Ni$^{2+}$) and final (Ni$^{2+}$) states of a neutral Ni$^{2+}$-doped QD in the presence of strain misorientation. The dominant recombination pathways for the bright and dark excitons are shown in panels (a) and (b), respectively. "i" denotes an optical transition involving a change in the Ni$^{2+}$ level of i.