Spin-orbit coupling and anisotropy of spin splitting in quantum dots

Kramers Degeneracy and Spin Inversion in a Lateral Quantum Dot

We show that the axial symmetry of the Bychkov–Rashba interaction can be exploited to produce electron spin-flip in a circular quantum dot, without lifting the time reversal symmetry. In order to elucidate this effect, we consider ballistic electron transmission through a two-dimensional circular billiard coupled to two one-dimensional electrodes. Using the tight-binding approximation, we derive the scattering matrix and the effective Hamiltonian for the considered system. Within this approach, we found the conditions for the optimal realization of this effect in the transport properties of the quantum dot. Numerical analysis of the system, extended to the case of two-dimensional electrodes, confirms our findings. The relatively strong quantization of the quantum dot can make this effect robust against the temperature effects.

Anisotropic Pauli Spin-Blockade Effect and Spin-Orbit Interaction Field in an InAs Nanowire Double Quantum Dot

We report on experimental detection of the spin-orbit interaction field in an InAs nanowire double quantum dot device. In the spin blockade regime, leakage current through the double quantum dot is measured and is used to extract the effects of spin-orbit interaction and hyperfine interaction on spin state mixing. At finite magnetic fields, the leakage current arising from the hyperfine interaction can be suppressed, and the spin-orbit interaction dominates spin state mixing. We observe dependence of the leakage current on the applied magnetic field direction and determine the direction of the spin-orbit interaction field. We show that the spin-orbit field lies in a direction perpendicular to the nanowire axis but with a pronounced off-substrate-plane angle. The results are expected to have an important implication in employing InAs nanowires to construct spin-orbit qubits and topological quantum devices.

Anisotropy and Suppression of Spin-Orbit Interaction in a GaAs Double Quantum Dot

The spin-flip tunneling rates are measured in GaAs-based double quantum dots by time-resolved charge detection. Such processes occur in the Pauli spin blockade regime with two electrons occupying the double quantum dot. Ways are presented for tuning the spin-flip tunneling rate, which on the one hand gives access to measuring the Rashba and Dresselhaus spin-orbit coefficients. On the other hand, they make it possible to turn on and off the effect of spin-orbit interaction with a high on/off ratio. The tuning is accomplished by choosing the alignment of the tunneling direction with respect to the crystallographic axes, as well as by choosing the orientation of the external magnetic field with respect to the spin-orbit magnetic field. Spin lifetimes of 10 s are achieved at a tunneling rate close to 1 kHz.

Coupling-barrier and non-parabolicity effects on the conduction electron cyclotron effective mass and Landé [Formula: see text] factor in GaAs double quantum wells

We have performed a theoretical study of the effects of the non-parabolicity and coupling barrier in between GaAs quantum wells on the conduction electron cyclotron effective mass and Landé [Formula: see text] factor under the action of a growth-direction applied magnetic field. Numerical calculations are performed within the effective mass approximation and taking into account the non-parabolicity effects for the conduction-band electrons, by means of the Ogg-McCombe effective Hamiltonian. The system consists of two GaAs quantum wells connected by a Ga(1 - x)Al(x)As barrier and surrounded by Ga(1 - y)Al(y)As material. We have found that both the [Formula: see text] factor and the cyclotron effective mass are sensitive to the coupling strength, that is the height and width of the barrier in between the GaAs quantum wells. This behavior is similar for every Landé [Formula: see text] factor and the cyclotron effective mass calculated for different Landau levels. It is noticeable that the splitting between the [Formula: see text] and [Formula: see text] cyclotron effective mass increases with the central barrier width and the growth-direction applied magnetic field. As in a single quantum well, we found that the electron Landé [Formula: see text] factor increases with the growth-direction applied magnetic field, comparing quite well with the experimental reports, and that the magnetic field plays an important role in decoupling the quantum wells of the system. Additionally, we have studied the electron cyclotron effective mass and Landé g factor as functions of the Landau levels, depending on the non-parabolicity. From this result one can infer that their population must be taken into account for a complete study of the band parameters as has been proposed in previous works. The present theoretical results are in very good agreement with previous experimental reports in the limiting geometry of a single quantum well.

Large anisotropy of the spin-orbit interaction in a single InAs self-assembled quantum dot

The anisotropy of the spin-orbit interaction (SOI) is studied for a single uncapped InAs self-assembled quantum dot holding just a few electrons. The SOI energy is evaluated from anticrossing or SOI-induced hybridization between the ground and excited states with opposite spins. The magnetic angular dependence of the SOI energy falls on an absolute cosine function for azimuthal rotation, and a cosinelike function for tilting rotation. Furthermore, the SOI energy is quenched for a specific magnetic field vector. The angular dependence of SOI is found to compare well with calculation of Rashba SOI in a two-dimensional harmonic potential.

Quantum confinement and magnetic-field effects on the electron g factor in GaAs-(Ga, Al)As cylindrical quantum dots

We have performed a theoretical study of the quantum confinement (geometrical and barrier potential confinements) and axis-parallel applied magnetic-field effects on the conduction-electron effective Landé g factor in GaAs-(Ga, Al)As cylindrical quantum dots. Numerical calculations of the g factor are performed by using the Ogg-McCombe effective Hamiltonian-which includes non-parabolicity and anisotropy effects-for the conduction-band electrons. The quantum dot is assumed to consist of a finite-length cylinder of GaAs surrounded by a Ga(1-x)Al(x)As barrier. Theoretical results are given as functions of the Al concentration in the Ga(1-x)Al(x)As barrier, radius, lengths and applied magnetic fields. We have studied the competition between the quantum confinement and applied magnetic field, finding that in this type of heterostructure the geometrical confinement and Al concentration determine the behavior of the electron effective Landé [Formula: see text] factor, as compared to the effect of the applied magnetic field. Present theoretical results are in good agreement with experimental reports in the limiting geometry of a quantum well, and with previous theoretical findings in the limiting case of a quantum well wire.

Coherent three-level mixing in an electronic quantum dot

We observe magnetic-field-induced level mixing and quantum superposition phenomena between three approaching single-particle states in a quantum dot probed via the ground state of an adjacent quantum dot by single-electron resonant tunneling. The mixing is attributed to anisotropy and anharmonicity in realistic dot confining potentials. The pronounced anticrossing and transfer of strengths (both enhancement and suppression) between resonances can be understood with a simple coherent level mixing model. Superposition can lead to the formation of a dark state by complete cancellation of an otherwise strong resonance, an effect resembling coherent population trapping in a three-level-system of quantum and atom optics.

Giant spin splitting through surface alloying

The long-range ordered surface alloy Bi/Ag(111) is found to exhibit a giant spin splitting of its surface electronic structure due to spin-orbit coupling, as is determined by angle-resolved photoelectron spectroscopy. First-principles electronic structure calculations fully confirm the experimental findings. The effect is brought about by a strong in-plane gradient of the crystal potential in the surface layer, in interplay with the structural asymmetry due to the surface-potential barrier. As a result, the spin polarization of the surface states is considerably rotated out of the surface plane.

Giant anisotropy of Zeeman splitting of quantum confined acceptors in Si/Ge

Shallow acceptor levels in Si/Ge/Si quantum well heterostructures are characterized by resonant-tunneling spectroscopy in the presence of high magnetic fields. In a perpendicular magnetic field we observe a linear Zeeman splitting of the acceptor levels. In an in-plane field, on the other hand, the Zeeman splitting is strongly suppressed. This anisotropic Zeeman splitting is shown to be a consequence of the huge light-hole--heavy-hole splitting caused by a large biaxial strain and a strong quantum confinement in the Ge quantum well.