Magneto-optical response of CdSe nanostructures

Abstract

We present theoretical calculations of the Landé g-factors of semiconductor nanostructures using a time-dependent empirical tight-binding method. The eigenenergies and eigenfunctions of the band edge states are calculated as a function of an external magnetic field with the electromagnetic field incorporated into the tight-binding Hamiltonian in a gauge-invariant form. The spin-orbit interaction and magnetic field are treated non-perturbatively. The g-factors are extracted from the energy splitting of the eigenstates induced by the applied magnetic field. Both electron and hole gfactors are investigated for CdSe nanostructures. The size and aspect ratio dependence of g-factors is studied. We observe that the electron g-factors are anisotropic and find that the calculated values agree quantitatively with experimental data. We conclude that the two distinct g-factor values extracted from time resolved Faraday rotation experiments should be assigned to the anisotropic in plane (g ) and out of plane (gz) electron g-factors, rather than to isotropic electron and exciton g-factors. We find that the anisotropy in the electron g-factor depends on the aspect ratio of the nanocrystal. The g-factor anisotropy derived from the wurtzite structure and from the non-unity aspect ratio may cancel each other in some regime. We observe that hole g-factors oscillate as a function of size, due to size dependent mixing between the heavy hole-light hole components of the valence band edge states. Extension to the calculation of exciton g-factors is discussed.

Room-temperature spin coherence in zinc blende CdSe quantum dots studied by time-resolved Faraday ellipticity

“…In comparison, for the CdSe nanocrystal with wurtzite structure, the g-factor in wurtzite CdSe QDs is thought to be isotropic due to the fact that the expected anisotropy from nanocrystal shape can balance the anisotropy from wurtzite crystal structure. Based on the theoretical analysis, the cancellation between shape-induced anisotropy and structure-induced anisotropy is not complete, which results in a multiple of g-factor in the wurtzite CdSe structure [28,29]. However, for the cubic CdSe QDs with quasispherical shape, the g-factor is isotropic and only a single g value is expected, which is consistent with our current experimental results for three different sizes zinc blende CdSe QDs.…”

Magneto-optical properties of trions in non-blinking charged nanocrystals reveal an acoustic phonon bottleneck

“…2h are consistent with oblate NCs, which are dominated by heavy holes. In this case, a g-factor of 0 is predicted when a crystallographic axis is perpendicular to the magnetic field 8 . The blue squares correspond to NCs, which split into four peaks and reveal hole g-factors tightly clustered around a value of 0.75.…”

“…These g-factors are associated with predominately spherical zinc-blende NCs, which places them in the regime of maximal heavy-hole/light-hole mixing where the hole wavefunction becomes approximately isotropic. Finally, the data from three-peaked trions originate from prolate NCs, which are increasingly dominated by light-hole properties and thus should be more dispersed 8 . Overall, our measurements provide a direct route to determining single-charge g-factors, which now enables unambiguous assignment of g-factors extracted from indirect ensemble measurements 6,7 .…”

“…Charged NCs serve as the ideal system for investigating charge-carrier spin properties. To date, such properties have only been indirectly investigated in CdSe NC ensembles 6,7 yielding results that could not be reconciled with theoretical estimates 8 . In a charged NC, both the ground state and the band-edge excited state have an angular momentum determined by a single-charge carrier.…”

Atomistic theory of coherent spin transfer between molecularly bridged quantum dots

“…[1] Potential spintronic and quantum computational applications [2,3] have led to the development of optical techniques such as time-resolved Faraday rotation (TRFR) for the measurement and manipulation of single spins in these systems, [4,5,6] as well as to theoretical methods to describe these properties. [6,7,8,9] With the spin properties of isolated nanocrystals reasonably well understood, attention has now turned to the study of spin in linked nanocrystals. Ouyang and Awschalom observed coherent transfer between 3.4 nm and 7.0 nm diameter colloidal CdSe coupled by 1,4-dithiolbenzene.…”