Determination of excitonic size with sub-nanometer precision via excitonic Aharonov-Bohm effect in type-II quantum dots

Abstract

The Aharonov-Bohm (AB) effect is typically discussed for a quantum charged particle moving along a trajectory enclosing a magnetic flux. There, however, exists a possibility of the AB effect associated with an overall neutral quasi-particle that possesses a radial electric dipole moment (e.g., an exciton in quantum ring or cylindrical type-II quantum dot (QD)). Excitons in type-II QDs are particularly interesting, due to relatively larger spatial separation of charged particles. We present results of magneto-photoluminescence (magneto-PL) studies on stacked submonolayer type-II ZnTe/ZnSe QDs. The AB phase reveals itself in magneto-PL of type-II QDs since, due to the cylindrical symmetry, the exciton ground state initially has a zero orbital angular momentum, which changes to higher values with increasing magnetic field. This transition of the angular momentum to a non-zero value with increasing magnetic field is observed in two ways: (i) in the changes of the exciton ground state energy and (ii) in the quenching of the excitonic PL intensity due to PL selection rules. In the figure we show effects of the applied magnetic field on the integrated intensity of the PL as a function of the magnetic field. The broad peak at ~1.42 T is assigned to the AB transition. To explain the observations, we first point out that single electron density calculations show that the electron, in the absence of strain, will be located either above or below the dot and, therefore, no AB signature is expected. In our case, the stacked character of the systems ensures that the electron's wave-function is "pushed" to the side of the dot due to electron-electron interaction, independent of stress, whereas cylindrical geometry nicely defines the ring-like trajectory for an electron. We thus explain the results as a motion of an electron around an entire stack of QDs, one of which is occupied by a hole (see inset in the figure). In addition we shall also show how the excitonic AB effect can be used to measure size of type-II excitons with sub-nanometer precision and discuss the role of the built-in electric field. Finally, we will show some very recent results of temperature dependent magneto-PL, which we use to investigate quantum phase coherence, without 'worrying about contacts' and show that decoherence is perfectly explained by 1-D ballistic theories. Light refreshments will be served.

Enhancement and narrowing of the Aharonov-Bohm oscillations due to built-in electric field in stacked type-II ZnTe/ZnSe quantum dots: Spectral analysis

“…This allowed us to determine the size of the type-II exciton with sub-nanometer precisions. 27 However, in sample A, B AB does not change significantly, so the radius of electronic orbit is the same across the spectrum; we conclude from such an observation that only one QD stack is present in sample A. We also note that the FWHM of each of the AB peaks (~0.12 to 0.17 T, shown in inset of Fig.…”

“…For example, the lateral excitonic radius is determined with sub-nanometer precisions using the knowledge of variation in the AB transition magnetic field, B AB , as a function of energy of emitted photons. 27 We present results of such an analysis for the B AB as well as the magnitudes of the AB oscillations from differently grown samples; we use these dependencies to discern the role of tellurium in formation of QDs and determine the spatial distribution of excitonic wavefunctions.…”

Decoherence in semiconductor nanostructures with type-II band alignment: All-optical measurements using Aharonov-Bohm excitons

“…We also assume here that the ballistic regime applies, so that . Indeed, as stated above, the lateral electron trajectory radius, R QD , of the dots in all of our samples, is between 15 and 30 nm [34,37], resulting in the electron path around the QD stacks, 2πR QD , between 100 and 200 nm. Thus, for further analyses we assume,…”

“…The stacks are formed due to a strong vertical correlation between the quantum dot containing layers [31,32]. The lateral separation between the QD stacks is much larger [33] than the size of the carrier orbit [34], allowing for the assumption that carriers orbit each stack independently. There are no magnetic impurities expected in these samples due to the nature of the growth procedure.…”

Wide emission-tunable CdTeSe/ZnSe/ZnS core–shell quantum dots and their conjugation with E. coli O-157

“…The phenomenon has been observed in various bulk crystals under laser irradiation [40]. Furthermore, strong electron-LO phonon interaction can create new surface defects and destroy essential radiative transitions through competing with radiative transition by extremely short non-radiative lifetime [41].…”