Y. M. Liu

Symmetry constraints and the electronic structures of a quantum dot with thirteen electrons

The symmetry constraints imposed on the quantum states of a dot with 13 electrons is investigated. Based on this study, the favorable structures (FSs) of each state is identified. Numerical calculations are performed to inspect the role played by the FSs. It is found that if a first state has a remarkably competitive FS, this FS will be pursued and the state would be crystal-like and have a specific core-ring structure associated with the FS. The magic numbers are found to be closely related to the FSs.

Effects of light on quantum phases and topological properties of two-dimensional Metal-organic frameworks

Periodically driven nontrivial quantum states open another door to engineer topological phases in solid systems by light. Here we show, based on the Floquet-Bloch theory, that the on-resonant linearly and circularly polarized infrared light brings in the exotic Floquet quantum spin Hall state and half-metal in two-dimensional Metal-organic frameworks (2D MOFs) because of the unbroken and broken time-reversal symmetry, respectively. We also observe that the off-resonant light triggers topological quantum phase transitions and induces semimetals with pseudospin-1 Dirac-Weyl fermions via the photon-dressed topological band structures of 2D MOFs. This work paves a way to design light-controlled spintronics and optoelectronics based on 2D MOFs.

Generalized Hamiltonian for a graphene subjected to arbitrary in-plane strains

The interplay between the linear elastic deformation up to 20% and the unique electronic properties of graphene nanostructures offers an attractive prospect to manipulate their properties by strain. Here we review the recent progress on the electronic response of graphene to the in-plane strains, including the strain-modulated electronic structure and the strain-modulated spin, valley and superconducting transports. A generalized Hamiltonian for a graphene was constructed subjected to arbitrary in-plane strains. The Hamiltonian is helpful to design and optimize the graphene-based nano-electromechanical systems (NEMS).

Probing Strain-Tunable Guided Modes in Graphene Waveguide by Photon-Assisted Tunneling

Strain effect on guided modes and electron transmission through the strained graphene waveguide with oscillating potential are explored theoretically. It is found that the guided mode in the waveguide can be controlled by zigzag or armchair direction strain. The transmission spectrum obtained displays a sharply asymmetric tunneling peak, where the energy difference between the tunneling peak and guided mode is equal to the photon induced by the oscillating potential. In view of the quantitative relationship, we propose the photon-assisted tunneling to probe the strain-tunable guided modes.

Symmetry background of the fractional Aharonov-Bohm oscillation and an oscillation in dipole-transitions of narrow quantum rings with a few electrons

The low-lying spectra of two-dimensional 3-electron and 4-electron narrow rings have been studied. It has been revealed that these spectra are simply dominated by a few favorable internal states. These internal states are free from the symmetry constraint and are nodeless in their wave functions (except the nodes when a pair of electrons overlap). The symmetry background of the fractional Aharonov-Bohm oscillation has been revealed. A very strong oscillation in the low-energy limit of the photon energy absorbed by the ground state against the magnetic field

Topological Insulator GMR Straintronics for Low-Power Strain Sensors

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Tunable electron wave filter and Goos–Hänchen shift in asymmetric graphene double magnetic barrier structures

is found. The amplitude of oscillation is a few mega-electron-volts and is about ten times larger than the one of ground-state energy; the period is the same as the Aharonov-Bohm oscillation. The qualitative features found in this paper hold also for

Guided modes and quantum Goos–Hänchen shift in graphene waveguide: Influence of a velocity barrier

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Resonant tunneling and enhanced Goos–Hänchen shift in a graphene double velocity barrier structure

-electron rings.

Spin-dependent Goos–Hänchen shift and spin beam splitter in gate-controllable ferromagnetic graphene

A quantum spin Hall insulator, i.e., topological insulator (TI), is a natural candidate for low-power electronics and spintronics because of its intrinsic dissipationless feature. Recent density functional theory and scanning tunneling spectroscopy experiments show that the mechanical strain allows dynamic, continuous, and reversible modulations of the topological surface states within the topological phase and hence opens prospects for TI straintronics. Here, we combine the mechanical strain and the giant magnetoresistance (GMR) of a ferromagnet-TI (FM-TI) junction to construct a novel TI GMR straintronics device. Such a FM-strained-FM-TI junction permits several energy spectral ranges for 100% GMR and a robust strain-controllable magnetic switch. Beyond the 100% GMR energy range, we observe a strain-modulated oscillating GMR, which is an alternative hallmark of the Fabry-Pérot quantum interference of Dirac surface states. These strain-sensitive GMR responses indicate that FM-strained-FM-TI junctions are very favorable for practical applications for low-power nanoscale strain sensors.