Interaction-driven spin precession in quantum-dot spin valves

We analyze spin-dependent transport through spin valves composed of an interacting quantum dot coupled to two ferromagnetic leads. The spin on the quantum dot and the linear conductance as a function of the relative angle theta of the leads' magnetization directions is derived to lowest order in the dot-lead coupling strength. Because of the applied bias voltage spin accumulates on the quantum dot, which for finite charging energy experiences a torque, resulting in spin precession. The latter leads to a nontrivial, interaction-dependent, theta dependence of the conductance. In particular, we find that the spin-valve effect is reduced for all theta not equal pi.     

Detection of single spin decoherence in a quantum dot via charge currents

We consider a quantum dot attached to leads in the Coulomb blockade regime that has a spin 1 / 2 ground state. We show that, by applying an ESR field to the dot spin, the stationary current in the sequential tunneling regime exhibits a new resonance peak whose linewidth is determined by the single spin decoherence time T2. The Rabi oscillations of the dot spin are shown to induce coherent current oscillations from which T2 can be deduced in the time domain. We describe a spin inverter which can be used to pump current through a double dot via spin flips generated by ESR.     

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.     

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.     

Optical tailoring of electron spin coherence in quantum dots

We address the precession of an ensemble of electron spins, each confined in a (In, Ga)As/GaAs self-assembled quantum dot. The quantum dot inhomogeneity is directly reflected in the precession of the optically oriented electron spins about an external magnetic field, which is subject to fast dephasing on a nanoseconds time scale. Proper periodic laser excitation allows synchronization of the electron spin precessions with the excitation cycle. The experimental conditions can be tailored such that eventually all (about a million) electron spins that are excited by the laser precess with a single frequency. In this regime the ensemble can be exploited during the single electron spin coherence times being in the microseconds range.     

Spin-flip mesoscopic transport through a carbon nanotube quantum dot system

The pure spin transport in an entire metallic single-wall carbon-nanotube (SWCN) interacting quantum dot (QD) system is investigated by using non-equilibrium Green's function (NEGF) technique. The novel spin current performance introduced by one constant and one rotating magnetic fields shows the unique four-fold degenerate electron shell structure which exists the SWCN QD sensitively. Spin transport properties can be designed by tuning the orbital and Zeeman configuration in the central resonant region, which are greatly influenced by the Coulomb interaction and the magnetic fields.     

Quantum spin and charge pumping through double quantum dots with ferromagnetic leads

The pumping of electrons through double quantum dots (DQDs) attached to ferromagnetic leads have been theoretically investigated by using the nonequilibrium Green?s function method. It is found that an oscillating electric field applied to the quantum dot may give rise to the pumped charge and spin currents. In the case that both leads are ferromagnet, a pure spin current can be generated in the antiparallel magnetization configuration, where no net charge current exists. The possibility of manipulating the pumped spin current is explored by tuning the dot level and the ac field. By making use of various tunings, the magnitude and direction of the pumped spin current can be well controlled. For the case that only one lead is ferromagnetic, both of the charge and spin currents can be pumped and flow in opposite directions on the average. The control of the magnitude and direction of the pumped charge and spin currents is also discussed by means of the magnetic flux threading through the DQD ring.     

Nonlocal Andreev reflection with Rashba spin-orbital interaction in a triple-quantum-dot ring

We theoretically studied the nonlocal Andreev reflection with Rashba spin-orbital interaction in a triple-quantumdot(QD) ring,which is introduced as Rashba spin-orbital interaction to act locally on one component quantum dot.It is found that the electronic current and spin current are sensitive to the systematic parameters.The interdot spin-flip term does not play a leading role in causing electronic and spin currents.Otherwise the spin precessing term leads to shift of the peaks of the the spin-up and spin-down electronic currents in different directions and results in the spin current.Moreover,the spin-orbital interaction suppresses the nonlocal Andreev reflection,so we cannot obtain the pure spin current.     

Probe‐assisted spin manipulation in one‐dimensional quantum dots

We study a spin structure that arises in a one‐dimensional quantum dot with zero total spin under the action of a charged tip of a scanning probe microscope in the presence of a weak magnetic field. The evolution of spin structure with changing the probe position is traced to show that the movable probe can be an effective tool to manipulate the spin. The spin structures are formed when the probe is located in certain regions along the dot due to Coulomb interaction of electrons as they are redistributed between the two sections in which the quantum dot is divided by the potential barrier created by the probe. There are two main states: spin‐polarized and non‐polarized ones. The transition between them is accompanied by a spin precession governed by the Rashba spin–orbit interaction induced by the electric field of the probe. In the transition region the spin density changes strongly while charge distribution remains nearly unchanged. (© 2015 WILEY‐VCH Verlag GmbH &Co. KGaA, Weinheim)     

Electronic spin precession in graphene-based superlattice with periodical gate and magnetic modulation

The spin precession in graphene superlattice with periodically modulated electrostatic field and efficient exchange field is investigated theoretically. It is found that the efficient exchange field can induce a spin precession, which is different from the case of the Rashba spin–orbit interaction. The spin precession is complete isoamplitude for normal incidence. For inclined incidence, the precession disappears when the effective exchange field is set into a certain range. It is also found periodical electrostatic field can revive the disappeared precession, but electronic transport is suppressed, which leads to some dips in the conductance spectrum.     

相互作用量子点系统中的场致自旋电流

从理论上研究了相互作用量子点在外部旋转磁场下的非平衡自旋输运性质，研究结果表明，量子点中的相干自旋振荡可以导致自旋电流的产生，当计入库仑关联相互作用后，近藤共振效应受外部进动磁场的影响很强，特别是当磁场的进动频率与塞曼能移满足共振条件时，每个自旋近藤峰就会劈裂为两个自旋共振峰的叠加，在低温强耦合区，这种近藤型共隧穿过程对自旋电流带来重要贡献。     

Zeeman energy and spin relaxation in a one-electron quantum dot

We have measured the relaxation time, T1, of the spin of a single electron confined in a semiconductor quantum dot (a proposed quantum bit). In a magnetic field, applied parallel to the two-dimensional electron gas in which the quantum dot is defined, Zeeman splitting of the orbital states is directly observed by measurements of electron transport through the dot. By applying short voltage pulses, we can populate the excited spin state with one electron and monitor relaxation of the spin. We find a lower bound on T1 of 50 micros at 7.5 T, only limited by our signal-to-noise ratio. A continuous measurement of the charge on the dot has no observable effect on the spin relaxation.     

Spin current through a quantum dot in the presence of an oscillating magnetic field

Nonequilibrium spin transport through an interacting quantum dot is analyzed. The coherent spin oscillations in the dot provide a generating source for spin current. In the interacting regime, the Kondo effect is influenced in a significant way by the presence of the processing magnetic field. In particular, when the precession frequency is tuned to resonance between spin-up and spin-down states of the dot, Kondo singularity for each spin splits into a superposition of two resonance peaks. The Kondo-type cotunneling contribution is manifested by a large enhancement of the pumped spin current in the strong coupling low temperature regime.     

Coherent precession of magnetization in the superfluid 3He A-phase

We report the first observation of coherent precession of magnetization in superfluid 3He A-like phase (CP-A) in aerogel. The coherent precession in bulk 3He A-phase is unstable due to the positive feedback of spin supercurrent to the gradient of phase of precession. It was predicted that the homogeneous precession will be stable if the orbital momentum of the 3He A-phase can be oriented along the magnetic field. We have succeeded to prepare this configuration by emerging 3He in uniaxially deformed anisotropic aerogel. The dissipation rate of coherent precession states in aerogel is much larger than that in bulk 3He B-phase. We propose a mechanism of this dissipation.     

Novel behavior of bond-centered muonium in heavily doped n-type silicon: Curie-like spin susceptibility and charge screening

Bond-centered muonium ( Mu(0)(BC)) has been observed in very heavily doped n-type Si:P. It exhibits a Curie-like electronic spin susceptibility which leads to a giant negative shift in the muon spin precession frequency. At high dopant levels, the Mu(0)(BC) hyperfine parameters, deduced from a model involving spin exchange with free carriers, are significantly reduced from those in intrinsic Si. This indicates that the spin density distribution for Mu(0)(BC) in metallic Si:P is altered significantly by charge screening effects, likely a general phenomenon for deep impurities in materials with high carrier concentrations.     

Carrier tunneling in asymmetric coupled quantum dots

Exciton spin relaxation at low temperatures in InAlAs–InGaAs asymmetric double quantum dots embedded in AlGaAs layers has been investigated as a function of the barrier thickness by the time-resolved photoluminescence measurements. With decreasing the thickness of the AlGaAs layer between the dots, the spin relaxation time change from 3 ns to less than 500 ps. The reduction in the spin relaxation time was considered to originate from the spin-flip tunneling between the ground state in InAlAs dot and the excited states in InGaAs dot, and the resultant tunneling leads to the spin depolarization of the ground state in InGaAs dot.     

Enhancing the coherence of a spin qubit by operating it as a feedback loop that controls its nuclear spin bath

In many realizations of electron spin qubits the dominant source of decoherence is the fluctuating nuclear spin bath of the host material. The slowness of this bath lends itself to a promising mitigation strategy where the nuclear spin bath is prepared in a narrowed state with suppressed fluctuations. Here, this approach is realized for a two-electron spin qubit in a GaAs double quantum dot and a nearly tenfold increase in the inhomogeneous dephasing time T?* is demonstrated. Between subsequent measurements, the bath is prepared by using the qubit as a feedback loop that first measures its nuclear environment by coherent precession, and then polarizes it depending on the final state. This procedure results in a stable fixed point at a nonzero polarization gradient between the two dots, which enables fast universal qubit control.     

Tunable spin loading and T1 of a silicon spin qubit measured by single-shot readout

We demonstrate single-shot readout of a silicon quantum dot spin qubit, and we measure the spin relaxation time T1. We show that the rate of spin loading can be tuned by an order of magnitude by changing the amplitude of a pulsed-gate voltage, and the fraction of spin-up electrons loaded can also be controlled. This tunability arises because electron spins can be loaded through an orbital excited state. Using a theory that includes excited states of the dot and energy-dependent tunneling, we find that a global fit to the loading rate and spin-up fraction is in good agreement with the data.     

Internal waves and synchronized precession in a cold vapor

Exchange in a Boltzmann gas of bosons with several internal states leads to collective transport of internal polarization. The internal dynamics can be understood as Larmor precession in the presence of a torque induced by atoms on each other via exchange coupling. A generalized Bloch equation that includes interatomic exchange effects as well as orbital motion in the gas is derived and used to interpret a recent experiment by Lewandowski et al. as an excitation of a collective wave of internal state polarization. It is shown that exchange leads to formation of domains in which precession frequencies are synchronized.     

X-ray detected magnetic resonance of YIG thin films in the nonlinear regime of spin waves

We discuss the information content of element/edge resolved X-ray detected magnetic resonance (XDMR) experiments carried out on yttrium iron garnet (YIG) thin films. Starting with a phenomenological approach, it is shown that the photoionisation of deep atomic core levels by circularly polarized X-rays can be used to probe the precession dynamics of spin or orbital magnetization components in empty final states of proper symmetry. Crude estimates of the opening angle of the uniform precession mode were tentatively deduced from the ratio of the XDMR and XMCD absorption cross-sections either at the iron or yttrium absorbing sites. The implications of the most recent experimental results collected at the ESRF are analyzed, keeping in mind that: (i) the Fe K-edge XDMR signal is largely dominated by the precession of orbital magnetization components at the tetrahedral iron sites; (ii) the Y L-edges XDMR signal essentially describes the precession of induced spin magnetization involving the 4d states of yttrium. In the magnetostatic regime, we produce clear experimental evidence of collective excitations of orbital magnetization waves, especially under high pumping power. Several coupling mechanisms could explain our observations, starting with pseudo-dipolar interactions in ferromagnetic systems. In ferrimagnetic systems in which orbital degeneracy and orbital ordering make the excitation of orbitons possible, one may envisage additional modes of excitation or relaxation of orbital magnetization waves. This interpretation looks fully consistent with the results of band structure calculations carried out recently on YIG with fully relativistic LMTO-LSDA methods.