Defect-induced magnetism in II-VI quantum dots

The identification of physical parameters that lead to magnetism in apparent nonmagnetic semiconductor systems has continuously challenged our community. In particular, for quantum-dot systems, they are key factors that contribute to their magneto-optical properties. We report, experimentally, optical evidences of induced nanomagnetism in nonmagnetic CdSe quantum dots and assess theoretically the role played by charged and uncharged vacancies. The analysis of these effects rests upon both the chemical and strain environments where the quantum dots are embedded. The interplay of spin-orbit interaction with built-in axial strains has been demonstrated to be a key factor for the magnetic moment alignment. This has been achieved in this paper by emulating the electronic structure at atomistic levels, considering various defect configurations and taking into account both the quantum dot composition and the influence of the host lattice.

Figure 1

(a) TEM images of two monolayers of CdSe quantum dots taken from Ref. [19]. (b) Color map of the lattice constant in the growth direction estimated from high-resolution TEM imaging. (c) and (d) micro-PL spectra of three different quantum dots at various fields for circular polarized detection. Unpolarized emission spectra have been also added for certain fields. (e) Corresponding Zeeman splitting as a function of the magnetic field.

Figure 2

(a) Measured time resolved integrated intensity at $B=0$ and linear polarized excitation, for ${\sigma }^{+}$ and ${\sigma }^{-}$ circular polarized emissions, denoted by blue and red open circles, respectively. Solid curves indicate the emulated evolution of the spin density for each spin polarization. (c) Emulated degree of circular polarization as a function of time and ${\omega }_Z$.

Figure 3

Structures and isosurfaces of the spin density $[\mathrm{\Delta }\rho ={\rho }_{\uparrow }\left(\mathbf{r}\right)-{\rho }_{\downarrow }\left(\mathbf{r}\right)]$ for neutral $\mathrm{CdSe}$ QDs immersed in a $\mathrm{ZnSe}$ matrix with vacancies. The isosurfaces for $\mathrm{\Delta }\rho =-0.014\mathrm{e}/{\AA }^3$ are depicted in red. (a) $\mathrm{Cd}$-in: $\mathrm{Cd}$ vacancy inside the QD; (b) $\mathrm{Cd}$-edge: $\mathrm{Cd}$ vacancy at the edge of the QD; (c) $\mathrm{Zn}$-matrix: $\mathrm{Zn}$ vacancy created at the host material; (d) $\mathrm{Se}$-in: $\mathrm{Se}$ vacancy inside the QD, which does not present magnetic moment.

Figure 4

Calculated $E_F$ for the $\mathrm{Cd}$, $\mathrm{Se}$, and $\mathrm{Zn}$ monovacancies in a $\mathrm{CdSe}$ QD immersed in a $\mathrm{ZnSe}$ host. Charge states corresponding to $q=-1,0,+1$ were considered [see Eq. (3)]. The values for the local magnetic moments were indicated for each configuration.

Figure 5

DFT calculations: (a) atoms in a pristine QD and (b) the same atoms surrounding a $\mathrm{Cd}$ vacancy, where the atomic displacements are depicted by (blue) arrows. Panel (c) presents the evolution of $E_F$ with the strain in pristine $\mathrm{CdSe}$. Panel (d) shows a phase diagram, demonstrating the lowest energy defect as a function of ${\mu }_{\text{Cd}}$ and ${\mu }_{\text{Se}}$.

Figure 6

$E_F$ as a function of the electronic chemical potential ${\mu }_E$ for $q=-1,0,+1$ [see Eq. (3)].

Figure 7

Magnetic anisotropy energy and magnetic moment (inset) as function of strain. Compressive and tensile strain in the hydrostatic direction does not change the MAE and the magnetic moment significantly, whereas the major variation was obtained by using the tensile strain for the axial quantization.

Figure 8

Defect levels introduced by a neutral $\mathrm{Cd}$ monovacancy in $\mathrm{CdSe}$ systems. (i) defect levels in CdSe bulk. (ii) exchange splitting and (iii) structural anisotropy splittings happening in $\mathrm{CdSe}$/$\mathrm{ZnSe}$ QDs.