Nuclear spin nanomagnet in an optically excited quantum dot

Linearly polarized light tuned slightly below the optical transition of the negatively charged exciton (trion) in a single quantum dot causes the spontaneous nuclear spin polarization (self-polarization) at a level close to 100%. The effective magnetic field of spin-polarized nuclei shifts the optical transition energy close to resonance with photon energy. The resonantly enhanced Overhauser effect sustains the stability of the nuclear self-polarization even in the absence of spin polarization of the quantum dot electron. As a result the optically selected single quantum dot represents a tiny magnet with the ferromagnetic ordering of nuclear spins-the nuclear spin nanomagnet.     

Electron spin dynamics in a self-assembled semiconductor quantum dot: the limit of low magnetic fields

Using the trion as an optical probe, we uncover novel electron spin dynamics in CdSe/ZnSe Stranski-Krastanov quantum dots. The longitudinal spin lifetime obeys an inverse power law associated with recharging processes in the dot ensemble. No hint at spin-orbit mediated spin relaxation is found. At very weak magnetic fields (< 50 mT), electron spin dynamics related to the hyperfine interaction with the lattice nuclei is uncovered. A strong Knight field gives rise to nuclear ordering and formation of dynamical polarization on a 100-micros time scale under continuous electron spin pumping. The associated spin transients are temperature robust and can be observed up to 100 K.     

On the possibility of the dynamic self-polarization of nuclear spins in a quantum dot

The possibility of self-polarization of nuclear spins predicted by M.I. D’yakonov and V.I. Perel’ (JETP Lett. 16, 398 (1972)) has been investigated in the case of the electric current passing through a single quantum dot. The mechanisms of nuclear spin relaxation in the quantum dot leading to the polarization and depolarization of the nuclei are discussed. To make the nuclear polarization possible, it has been proposed to increase the nuclear polarization rate via the interaction of an electron localized in the quantum dot with electromagnetic oscillations in an electric circuit, whose proper frequency is tuned to a resonance with the Zeeman splitting of an electron level in the quantum dot.     

Dynamics of quantum dot nuclear spin polarization controlled by a single electron

We present measurements of the buildup and decay of nuclear spin polarization in a single semiconductor quantum dot. Our experiment shows that we polarize the nuclei in a few milliseconds, while their decay dynamics depends drastically on external parameters. We show that a single electron can very efficiently depolarize nuclear spins in milliseconds whereas in the absence of the electron the nuclear spin lifetime is on the scale of seconds. This lifetime is further enhanced by 1-2 orders of magnitude by quenching the nonsecular nuclear dipole-dipole interactions with a magnetic field of 1 mT.     

Electron spin-echo envelope modulation at S-band. 3: Analysis of nuclear quadrupole interaction effects

In the present paper the nuclear modulation of electron spin echo signals at S-band is investigated in the case of interacting nuclei with a quadrupole moment high enough to cause nuclear quadrupole couplings not negligible with respect to the nuclear Zeeman and dipolar hyperfine couplings. Both the two-pulse and three-pulse electron spin echo envelope modulation (ESEEM) due to²⁷Al and¹⁴N are simulated at different values of the nuclear quadrupole coupling by numerical diagonalization of the nuclear Hamiltonians. The behavior of their amplitude and periods is discussed on the basis of the ratios between the strengths of the nuclear quadrupole interaction and the nuclear Zeeman and the dipolar hyperfine interactions. The interpretation of their trends in terms of the eigenfunctions and eigenvectors of the nuclear Hamiltonians is carried out by using analytical equations obtained by perturbation approaches. First order perturbation treatments for integer and half-integer nuclear spin quantum numbers are developed when the nuclear quadrupole coupling is the main interaction. A discussion on the limits of the interpretation based on the perturbation approach is also given by comparing the magnitude Fourier transform of the patterns calculated by exact diagonalization and analytical equations.     

Spin-dependent transport through quantum dot with inelastic interactions

Using the Keldysh nonequilibrium Green function method, we theoretically investigate the electron transport properties of a quantum dot coupled to two ferromagnetic electrodes, with inelastic electron-phonon interaction and spin flip scattering present in the quantum dot. It is found that the electron-phonon interaction reduces the current, induces new satellite polaronic peaks in the differential conductance spectrum, and at the same time leads to oscillatory tunneling magnetoresistance effect. Spin flip scattering suppresses the zero-bias conductance peak and splits it into two, with different behaviors for parallel and anti-parallel magnetic configuration of the two electrodes. Consequently, a negative tunneling magnetoresistance effect may occur in the resonant tunneling region, with increasing spin flip scattering rate.     

Spin hedgehog and collective magnetic quadrupole moments induced by parity and time invariance violating interaction

P- and T-odd interaction produces a spin distribution with the spins directed along the radius in spherically symmetric quantum systems. Due to the spin-dependent strong interaction this spin hedgehog contributes to P- and T-odd effects in neutron scattering, nuclear moments, etc. In deformed nuclei the P- and T-odd interaction produces a collective magnetic quadrupole moment. This moment induces P- and T -violating effects in atoms and molecules.     

Electron spin decoherence in quantum dots due to interaction with nuclei

We study the decoherence of a single electron spin in an isolated quantum dot induced by hyperfine interaction with nuclei. The decay is caused by the spatial variation of the electron wave function within the dot, leading to a nonuniform hyperfine coupling A. We evaluate the spin correlation function and find that the decay is not exponential but rather power (inverse logarithm) lawlike. For polarized nuclei we find an exact solution and show that the precession amplitude and the decay behavior can be tuned by the magnetic field. The decay time is given by (planck)N/A, where N is the number of nuclei inside the dot, and the amplitude of precession decays to a finite value. We show that there is a striking difference between the decoherence time for a single dot and the dephasing time for an ensemble of dots.     

Nuclear spin switch in semiconductor quantum dots

We show that by illuminating an InGaAs/GaAs self-assembled quantum dot with circularly polarized light, the nuclei of atoms constituting the dot can be driven into a bistable regime, in which either a thresholdlike enhancement or reduction of the local nuclear field by up to 3 T can be generated by varying the pumping intensity. The excitation power threshold for such a nuclear spin "switch" is found to depend on both the external magnetic and electric fields. The switch is shown to arise from the strong feedback of the nuclear spin polarization on the dynamics of the spin transfer from electrons to the nuclei of the dot.     

Electron–Nuclear Spin Dynamics in Semiconductor QDs

This work presents an overview of investigations of the nuclear spin dynamics in nanostructures with negatively charged InGaAs/GaAs quantum dots characterized by strong quadrupole splitting of nuclear spin sublevels. The main method of the investigations is the experimental measurements and the theoretical analysis of the photoluminescence polarization as a function of the transverse magnetic field (effect Hanle). The dependence of the Hanle curve profile on the temporal protocol of optical excitation is examined. Experimental data are analyzed using an original approach based on separate consideration of behavior of the longitudinal and transverse components of the nuclear polarization. The rise and decay times of each component of the nuclear polarization and their dependence on transverse magnetic field strength are determined. To study the role of the Knight field in the dynamic of nuclear polarization, a weak additional magnetic field parallel to the optical axis is used. We have found that, only taking into account the nuclear spin fluctuations, we can accurately describe the measured Hanle curves and evaluate the parameters of the electron–nuclear spin system in the studied quantum dots. A new effect of the resonant optical pumping of nuclear spin polarization in an ensemble of the singly charged (In,Ga)As/GaAs quantum dots subjected to a transverse magnetic field is discussed. Nuclear spin resonances for all isotopes in the quantum dots are detected in that way. In particular, transitions between the states split off from the ±1/2 doublets by the nuclear quadrupole interaction are identified.     

准一维半导体量子点中电偶极自旋共振的物理机理

半导体量子点中的电子自旋具有较长相干时间以及可扩展性的特点, 在近十几年来引起了人们的广泛兴趣. 人们常常利用电子自旋共振技术来对单个自旋进行操纵. 这样不但需要一个静磁场来使电子产生赛曼劈裂, 同时还需要一个与之垂直的局域振荡磁场. 但是, 在实验上产生足够强且具有固定频率的局域磁场是比较困难的. 后来人们发现, 局域的振荡电场也可以操纵单个电子自旋, 也就是所谓的电偶极自旋共振. 众所周知, 自旋只有自旋磁矩, 不会与电场有任何直接的相互作用. 所以, 电偶极自旋共振的发生必须依赖于某些媒质. 这些媒质包括：量子点材料中的自旋轨道耦合作用, 量子点中的局域磁场梯度, 以及量子点中电子自旋与核自旋的超精细相互作用. 这些媒质能诱导出自旋与电场之间间接的相互作用, 从而外电场操纵单个电子自旋得以实现. 本文总结归纳了目前半导体量子点系统中发生电偶极自旋共振的三种主要物理机理.     

Spectrum of an electron spin coupled to an unpolarized bath of nuclear spins

The main source of decoherence for an electron spin confined to a quantum dot is the hyperfine interaction with nuclear spins. To analyze this process theoretically we diagonalize the central spin Hamiltonian in the high magnetic B-field limit. Then we project the eigenstates onto an unpolarized state of the nuclear bath and find that the resulting density of states has Gaussian tails. The level spacing of the nuclear sublevels is exponentially small in the middle of each of the two electron Zeeman levels but increases superexponentially away from the center. This suggests to select states from the wings of the distribution when the system is projected on a single eigenstate by a measurement to reduce the noise of the nuclear spin bath. This theory is valid when the external magnetic field is larger than a typical Overhauser field at high nuclear spin temperature.     

Decoherence in solid-state qubits

The interaction of solid-state qubits with environmental degrees of freedom strongly affects the qubit dynamics, and leads to decoherence. In quantum information processing with solid-state qubits, decoherence significantly limits the performances of such devices. Therefore, it is necessary to fully understand the mechanisms that lead to decoherence. In this review, we discuss how decoherence affects two of the most successful realizations of solid-state qubits, namely, spin qubits and superconducting qubits. In the former, the qubit is encoded in the spin 1/2 of the electron, and it is implemented by confining the electron spin in a semiconductor quantum dot. Superconducting devices show quantum behaviour at low temperatures, and the qubit is encoded in the two lowest energy levels of a superconducting circuit. The electron spin in a quantum dot has two main decoherence channels, a (Markovian) phonon-assisted relaxation channel, due to the presence of a spin–orbit interaction, and a (non-Markovian) spin bath constituted by the spins of the nuclei in the quantum dot that interact with the electron spin via the hyperfine interaction. In a superconducting qubit, decoherence takes place as a result of fluctuations in the control parameters, such as bias currents, applied flux and bias voltages, and via losses in the dissipative circuit elements.     

Transport spectroscopy of an impurity spin in a carbon nanotube double quantum dot

We make use of spin selection rules to investigate the electron spin system of a carbon nanotube double quantum dot. Measurements of the electron transport as a function of the magnetic field and energy detuning between the quantum dots reveal an intricate pattern of the spin state evolution. We demonstrate that the complete set of measurements can be understood by taking into account the interplay between spin-orbit interaction and a single impurity spin coupled to the double dot. The detection and tunability of this coupling are important for quantum manipulation in carbon nanotubes.     

Coherent spin precession of electrons and excitons in charge tunable InP quantum dots

Coherent spin precession of electrons and excitons is observed in charge tunable InP quantum dots under the transverse magnetic field by means of time-resolved Kerr rotation. In a quantum dot doped by one electron, spin precession of the doped electron in the quantum dot starts out of phase with spin precession of the doped electrons in a GaAs substrate just after a trion is formed and persists for more than 2 ns even after the trion recombines. Simultaneously spin precession of a trion (hole) starts. Observation of spin precession of both a doped electron and a trion (hole) confirms creating coherent superposition of an electron and a trion as the initialization process of spin of doped electrons in quantum dots. In a neutral quantum dot, the exciton spin precession starts out of phase with spin precession of the doped electrons in a GaAs substrate and the precession frequency does not converge to 0 at the zero field limit. It contains the electron–hole exchange interaction and corresponds to the splitting between bright and dark excitons under the transverse magnetic field.     

Subsecond spin relaxation times in quantum dots at zero applied magnetic field due to a strong electron-nuclear interaction

A key to ultralong electron spin memory in quantum dots (QDs) at zero magnetic field is the polarization of the nuclei, such that the electron spin is stabilized along the average nuclear magnetic field. We demonstrate that spin-polarized electrons in n-doped (In,Ga)As/GaAs QDs align the nuclear field via the hyperfine interaction. A feedback onto the electrons occurs, leading to stabilization of their polarization due to formation of a nuclear spin polaron [I. A. Merkulov, Phys. Solid State 40, 930 (1998)]. Spin depolarization of both systems is consequently greatly reduced, and spin memory of the coupled electron-nuclear spin system is retained over 0.3 sec at temperature of 2 K.     

Transport through a single discrete level for non-collinear magnetic polarizations of the electron reservoirs

Spin-polarized transport through a non-interacting single-level quantum dot coupled to two ferromagnetic leads with non-collinear magnetizations is analyzed theoretically by the non-equilibrium Green function technique. It is shown that spin of an electron that has tunnelled on the dot precesses around an effective exchange field before it leaves the dot. The exchange field originates from interaction between the electron and external electrodes. Electric current, tunnel magnetoresistance, and average value of spin accumulated on the dot are calculated for an arbitrary angle between magnetic moments of the electrodes.     

Nuclear and ion spins in semiconductor nanostructures

Spin interactions are studied between conduction band electrons in GaAs heterostructures and local moments, specifically the spins of constituent lattice nuclei and of partially filled electronic shells of impurity atoms. Nuclear spin polarizations are addressed through the contact hyperfine interaction resulting in the development of a method for high-field optically detected nuclear magnetic resonance sensitive to 10⁸ nuclei. This interaction is then used to generate nuclear spin polarization profiles within a single parabolic quantum well; the position of these nanometer-scale sheets of polarized nuclei can be shifted along the growth direction using an externally applied electric field. In order to directly investigate ion spin dynamics, doped GaMnAs quantum wells are fabricated in the regime of very low Mn concentrations. Measurements of coherent electron spin dynamics show an antiferromagnetic exchange between s-like conduction band electrons and electrons localized in the d-shell of the Mn impurities, which varies as a function of well width.     

Electron transport in a double quantum dot governed by a nuclear magnetic field

We investigate theoretically electron transfer in a double dot in a situation where spin blockade is lifted by nuclear magnetic field: this has been recently achieved in experiment [F. Koppens, Science 309, 1346 (2005)]. We show that for a given realization of nuclear magnetic field spin blockade can be restored by tuning external magnetic field; this may be useful for quantum manipulation of the device.     

Effect of a magnetic impurity on the optical properties of a spherical ZnSe quantum dot

We consider the electron and hole states in a semiconductor ZnSe spherical quantum dot, in the center of which a magnetic impurity atom of manganese is located. In calculations the quantum dot is approximated by a spherical rectangular well with a finite depth. Within the framework of perturbation theory, the effect of exchange spin interaction of an electron and a hole with a magnetic impurity on the band structure of the system is considered. The optical spectrum of the system for different polarizations of the incident light is studied also.