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Register a new userShell structure and electron-electron interaction in self-assembled InAs quantum dots
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Using far-infrared spectroscopy, we investigate the excitations of self-organized InAs quantum dots as a function of the electron number per dot, 1<n<6, which is monitored in situ by capacitance spectroscopy. Whereas the well-known two-mode spectrum is observed when the lowest s - states are filled, we find a rich excitation spectrum for n=3, which reflects the importance of electron-electron interaction in the present, strongly non-parabolic confining potential. From capacitance spectroscopy we find that the electronic shell structure in our dots gives rise to a distinct pattern in the charging energies which strongly deviates from the monotonic behavior of the Coulomb blockade found in mesoscopic or metallic structures.
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We investigate the electronic structure of the InAs/InP quantum dots using an atomistic pseudopotential method and compare them to those of the InAs/GaAs QDs. We show that even though the InAs/InP and InAs/GaAs dots have the same dot material, their electronic structure differ significantly in certain aspects, especially for holes: (i) The hole levels have a much larger energy spacing in the InAs/InP dots than in the InAs/GaAs dots of corresponding size. (ii) Furthermore, in contrast with the InAs/GaAs dots, where the sizeable hole $p$, $d$ intra-shell level splitting smashes the energy level shell structure, the InAs/InP QDs have a well defined energy level shell structure with small $p$, $d$ level splitting, for holes. (iii) The fundamental exciton energies of the InAs/InP dots are calculated to be around 0.8 eV ($sim$ 1.55 $mu$m), about 200 meV lower than those of typical InAs/GaAs QDs, mainly due to the smaller lattice mismatch in the InAs/InP dots. (iii) The widths of the exciton $P$ shell and $D$ shell are much narrower in the InAs/InP dots than in the InAs/GaAs dots. (iv) The InAs/GaAs and InAs/InP dots have a reversed light polarization anisotropy along the [100] and [1$bar{1}$0] directions.
We show here the existence of the indirect coupling of electron and magnetic or nuclear ion spins in self-assembled quantum dots mediated by electron-electron interactions. With a single localized spin placed in the center of the dot, only the spins of electrons occupying the zero angular momentum states couple directly to the localized spin. We show that when the electron-electron interactions are included, the electrons occupying finite angular momentum orbitals interact with the localized spin. This effective interaction is obtained using exact diagonalization of the microscopic Hamiltonian as a function of the number of electronic shells, shell spacing, and anisotropy of the electron-Mn exchange interaction. The effective interaction can be engineered to be either ferromagnetic or antiferromagnetic by tuning the parameters of the quantum dot.
Measuring single-electron charge is one of the most fundamental quantum technologies. Charge sensing, which is an ingredient for the measurement of single spins or single photons, has been already developed for semiconductor gate-defined quantum dots, leading to intensive studies on the physics and the applications of single-electron charge, single-electron spin and photon-electron quantum interface. However, the technology has not yet been realized for self-assembled quantum dots despite their fascinating quantum transport phenomena and outstanding optical functionalities. In this paper, we report charge sensing experiments in self-assembled quantum dots. We choose two adjacent dots, and fabricate source and drain electrodes on each dot, in which either dot works as a charge sensor for the other target dot. The sensor dot current significantly changes when the number of electrons in the target dot changes by one, demonstrating single-electron charge sensing. We have also demonstrated real-time detection of single-electron tunnelling events. This charge sensing technique will be an important step towards combining efficient electrical readout of single-electron with intriguing quantum transport physics or advanced optical and photonic technologies developed for self-assembled quantum dots.
We have fabricated superconductor-quantum dot-superconductor (SC-QD-SC) junctions by using SC aluminum electrodes with narrow gaps laterally contacting a single self-assembled InAs QD. The fabricated junctions exhibited clear Coulomb staircases and Coulomb oscillations at 40 mK. Furthermore, clear suppression in conductance was observed for the source-drain voltage $|V_{rm SD}| < 2Delta/e$, where $Delta$ is the SC energy gap of Al. The absence of Josephson current that flows through QDs is due to the strong Coulomb interaction and non-negligible thermal fluctuation in our measurement system.
A detailed study of the $g$-factor anisotropy of electrons and holes in InAs/In$_{0.53}$Al$_{0.24}$Ga$_{0.23}$As self-assembled quantum dots emitting in the telecom spectral range of $1.5-1.6$ $mu$m (around 0.8 eV photon energy) is performed by time-resolved pump-probe ellipticity technique using a superconducting vector magnet. All components of the $g$-factor tensors are measured, including their spread in the quantum dot (QD) ensemble. Surprisingly, the electron $g$ factor shows a large anisotropy changing from $g_{mathrm{e},x}= -1.63$ to $g_{mathrm{e},z}= -2.52$ between directions perpendicular and parallel to the dot growth axis, respectively, at an energy of 0.82 eV. The hole $g$-factor anisotropy at this energy is even stronger: $|g_{text{h},x}|= 0.64$ and $|g_{text{h},z}|= 2.29$. On the other hand, the in-plane anisotropies of electron and hole $g$ factors are small. The pronounced out-of-plane anisotropy is also observed for the spread of the $g$ factors, determined from the spin dephasing time. The hole longitudinal $g$ factors are described with a theoretical model that allows us to estimate the QD parameters. We find that the QD height-to-diameter ratio increases while the indium composition decreases with increasing QD emission energy.
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