Takuma Okunishi

Resonating Hartree–Fock Approach for Electrons Confined in Two Dimentional Square Quantum Dots

We studied the applicability of the resonating unrestricted Hartree–Fock (res-UHF) approach for configuration interaction (CI) by considering the electronic structure of two electrons confined in a two-dimensional (2D) square quantum dot (SQD). The UHF solutions for the SQD are nonorthogonal mutually but are appropriate for the basis functions for the res-UHF CI calculation because they represent the conceivable electron-spin configurations rationally. Consequently, the res-UHF CI using these UHF non-orthogonal solutions has an advantage for narrowing down the number of employed Slater determinants, although at the expense of the orthogonality among the Slater bases.

Ground state instability in spin polarization for electrons confined in two-dimensional square quantum dots

We present a theoretical study of the ground state electronic structure and the spin polarization for four electrons confined in two-dimensional (2D) square quantum dots (SQDs). We employ standard mean field theory (MFT) approaches using the unrestricted Hartree–Fock (UHF) and density functional theory calculations. The resonant UHF configuration interaction (res-UHF CI) calculation was also performed in order to incorporate the electron correlation more intuitively. The MFT ground state is expected to be spin-polarized when SQDs have a small confinement length L or aspect ratio δ= Lx/Ly = 1, in agreement with Hunds rule. In contrast, the spin-unpolarized ground state singlet is expected in all in other SQDs. Thus, the MFT calculations produce the anti-Hund state, where the spin-density wave forms having the zero of the total spin, even though the SQD has the point group symmetry D4h. However, the res-UHF CI calculation restores the geometrical symmetry in the resulting ground state when the Coulomb interaction is strengthened. Nevertheless, the res-UHF CI ground state maintains the zero total spin. Thus, ground state instability is expected in the spin-polarization of the SQD system, which eventually violates Hunds rule in accordance with the Coulomb interaction.

Theoretical study of the time-dependent phenomenon of photon-assisted tunneling through a charged quantum dot

We have studied numerically the time-dependent photon-assisted tunneling (TD-PAT) process for electrons confined in quantum dots (QDs) by employing the finite difference method under the scheme of the TD-density functional theory (DFT). We have found the quasi-dark state (quasi-DS), where the injected electron remains in the QD. By varying the barrier thickness, we have calculated the TD profile of the electron density in a QD, and found the optimal geometry of the lozenge QD. We have also discussed how the charged QD modulates the PAT process.

Time-dependent resonant UHF CI approach for the photo-induced dynamics of the multi-electron system confined in 2D QD

We extend the static multi-reference description (resonant UHF) to the dynamic system in order to include the correlation effect over time, and simplify the TD Schrodinger equation (TD-CI) into a time-developed rate equation where the TD external field Ĥ′(t) is then incorporated directly in the Hamiltonian without any approximations. We apply this TD-CI method to the two-electron ground state of a 2D quantum dot (QD) under photon injection and study the resulting two-electron Rabi oscillation.