NSLS-II Seminar

"Experimental Observation of Optical Aharonov-Bohm Effect in Stacked ZnTe/ZnSe Type-II Quantum Dots"

Presented by Igor L. Kuskovsky, Department of Physics, Queens College of CUNY

Wednesday, March 17, 2010, 10:00 am — Bldg. 703, Large Conference Room

The Aharonov-Bohm (AB) effect is typically discussed for a quantum charged particle moving along a closed trajectory in a magnetic field. There, however, exists a possibility of the AB effect associated with an overall neutral quasi-particle that possesses a radial electric dipole moment, such as excitons in cylindrical type-II quantum dots (QDs). The AB phase reveals itself in photoluminescence (PL) properties 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 influences the optical properties in two ways: (i) the ground state energy will oscillate as the orbital angular momentum states cross and (ii) the PL intensity will change from strong (bright exciton with zero angular momentum) to weak (dark excitons with non-zero angular momentum) with increasing magnetic field.
We present experimental studies on type-II magneto-excitons in stacked ZnTe/ZnSe type-II QDs. Results show strong AB oscillations in both the energy and intensity of the PL from the same structure. This is the only system for which the oscillations in both energy and intensity have been reported. In addition, we find that the AB oscillations are remarkably robust against temperature, with the AB signature visible up to 180 K. We believe this to be the highest temperature at which the AB effect, and therefore quantum coherence, has been observed in semiconductor ring-like structures.
To explain the observations, we evoke an idea of the QD stacking, which 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. In our model, an electron moves around an entire stack of QDs, one of which is occupied by a hole.

Hosted by: Hanffei Yan

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