Muonium quantum diffusion and spin dynamics in solid xenon

We present measurements of the transverse (T ₂ ^–1 ) and longitudinal (T ₁ ^–1 ) spin relaxation rates of muonium (Mu) atoms in solid natural xenon (n-Xe) as well as pure¹³⁶Xe (which has no nuclear moments). The temperature dependences ofT ₂ ^–1 andT ₁ ^–1 in natural Xe belowT 115 K demonstrate the quantum character of Mu diffusion governed by one-phonon interactions. Taking into account both the polaron effect (PE) and the effect of fluctuational preparation of the barrier (FPB) makes it possible to consistently describe Mu diffusion in Xe. Mu spin relaxation in¹³⁶Xe at high temperatures is not due to nuclear hyperfine (NHF) interactions.     

Nonequilibrium spin transport through a diluted magnetic semiconductor quantum dot system with noncollinear magnetization

The spin-dependent transport through a diluted magnetic semiconductor quantum dot (QD) which is coupled via magnetic tunnel junctions to two ferromagnetic leads is studied theoretically. A noncollinear system is considered, where the QD is magnetized at an arbitrary angle with respect to the leads’ magnetization. The tunneling current is calculated in the coherent regime via the Keldysh nonequilibrium Green’s function (NEGF) formalism, incorporating the electron–electron interaction in the QD. We provide the first analytical solution for the Green’s function of the noncollinear DMS quantum dot system, solved via the equation of motion method under Hartree–Fock approximation. The transport characteristics (charge and spin currents, and tunnel magnetoresistance (TMR)) are evaluated for different voltage regimes. The interplay between spin-dependent tunneling and single-charge effects results in three distinct voltage regimes in the spin and charge current characteristics. The voltage range in which the QD is singly occupied corresponds to the maximum spin current and greatest sensitivity of the spin current to the QD magnetization orientation. The QD device also shows transport features suitable for sensor applications, i.e., a large charge current coupled with a high TMR ratio.     

Effects of impurity on fidelity of quantum state transfer via spin channels

By adopting the concept of fidelity, we investigated efficiency of quantum state transfer with the XX chain as the quantum channel. Different from the previous works, we concentrated on effects of spin and magnetic impurity on fidelity of quantum state transfer. Our results revealed that the spin impurity cannot prevent one from implementing perfect transfer of an arbitrary one-qubit pure state across the spin channel, however, the presence of magnetic impurity or both spin and magnetic impurities may destroy the otherwise perfect spin channels.     

Observation of hysteretic transport due to dynamic nuclear spin polarization in a GaAs lateral double quantum dot

We report a new transport feature in a GaAs lateral double quantum dot that emerges for magnetic-field sweeps and shows hysteresis due to dynamic nuclear spin polarization (DNP). This DNP signal appears in the Coulomb blockade regime by virtue of the finite interdot tunnel coupling and originates from the crossing between ground levels of the spin triplet and singlet extensively used for nuclear spin manipulations in pulsed-gate experiments. The magnetic-field dependence of the current level is suggestive of unbalanced DNP between the two dots, which opens up the possibility of controlling electron and nuclear spin states via dc transport.     

本征GaAs量子阱中电子自旋扩散输运的时-空分辨吸收光谱研究

发展了一种时-空分辨圆偏振光抽运-探测光谱及其理论,并用于本征GaAs量子阱中电子自旋扩散输运的实验研究.获得室温下本征GaAs量子阱中的“自旋双极扩散系数”为D_as=37.5±15 cm²/s.此结果比用自旋光栅法测量到的掺杂GaAs量子阱中电子自旋扩散系数小.解释为是由于“空穴库仑拖曳”效应减慢了电子自旋波包的扩散输运. 关键词： 时-空分辨抽运-探测光谱 电子自旋扩散 GaAs量子阱     

Plasma diffusion and transport in a magnetic duct filter

A cathodic arc plasma source equipped with a curved magnetic duct to filter macro-particles was used to study plasma diffusion and transport in the duct. We determine the optimal duct bias, at which the magnetic duct produces the maximum plasma output, for titanium cathodic arc plasma at 50, 100 and 150 A arc current, and we investigate the parametric effects of the arc current and guiding magnetic field on the optimal duct bias. The optimal bias decreases as the guiding magnetic field increases from 0.01 to 0.04T and is almost independent of the guiding magnetic field when the magnetic field strength ranges from 0.04 to 0.06T, the upper limit for our equipment. The optimal duct bias at 0.04T guiding magnetic field decreases with increasing arc current.     

Fingerprints of the magnetic polaron in nonequilibrium electron transport through a quantum wire coupled to a ferromagnetic spin chain

We study nonequilibrium quantum transport through a mesoscopic wire coupled via local exchange to a ferromagnetic spin chain. Using the Keldysh formalism in the self-consistent Born approximation, we identify fingerprints of the magnetic polaron state formed by hybridization of electronic and magnon states. Because of its low decoherence rate, we find coherent transport signals. Both elastic and inelastic peaks of the differential conductance are discussed as a function of external magnetic fields, the polarization of the leads, and the electronic level spacing of the wire.     

Conclusive and perfect quantum state transfer with a single spin chain

We have investigated the quantum state transfer in randomly coupled spin chains. By using local memories storing the information and dividing the process into transfer and decoding, conclusive transfer is spontaneously achieved with just one single spin chain. Especially, we can decode information from memories without adding extra spin chain. Compared with Time-reversed protocol, the average decoding time is much less in our scheme.     

Logical XOR gate response in a quantum interferometer: A spin dependent transport

We examine spin dependent transport in a quantum interferometer composed of magnetic atomic sites based on transfer matrix formalism. The interferometer, threaded by a magnetic flux ϕ, is symmetrically attached to two semi-infinite one-dimensional (1D) non-magnetic electrodes, namely, source and drain. A simple tight-binding model is used to describe the bridge system, and, here we address numerically the conductance-energy and current-voltage characteristics as functions of the interferometer-to-electrode coupling strength, magnetic flux and the orientation of local the magnetic moments associated with each atomic site. Quite interestingly it is observed that, for ϕ = ϕ ₀/2 (ϕ ₀ = ch/e, the elementary flux-quantum) a logical XOR gate like response is observed, depending on the orientation of the local magnetic moments associated with the magnetic atoms in the upper and lower arms of the interferometer, and it can be changed by an externally applied gate magnetic field. This aspect may be utilized in designing a spin based electronic logic gate.     

Hysteresis, spin transitions, and magnetic ordering in the fractional quantum Hall effect

We have studied the development of metastable properties associated with a nearly spin-degenerate two-dimensional electron system. Application of large hydrostatic pressure significantly reduces the g-factor experienced by electrons in GaAs/AlGaAs heterostructure, and various fractional quantum Hall effect (FQHE) states are found to undergo transition to a spin-unpolarized ground state. In case of even numerator FQHE states, the spin transitions are accompanied by hysteresis and nonlinearity in the magnetotransport. These results strongly support a recent theory of quantum Hall magnetism in which competition between spin-polarized and spin-unpolarized ground states leads to an ordered phase that exhibits ferromagnetic correlation.     

Bloch electrons in a magnetic field: Topology of quantum states and transport

Quantum states and Hall conductances of electrons in n-type heterojunctions and holes in p-type heterojunctions in a field of a lateral superlattice and a perpendicular magnetic field were studied. It is shown that the energy spectrum of magnetic subbands in a periodic potential without inversion center is not symmetric about the reversal of the quasi-momentum sign. The properties of wave functions and the related topological invariants determining the Hall conductance were examined. The method of calculating the magnetic Bloch states of holes was developed on the basis of the Luttinger Hamiltonian, allowing the spin and spin-orbit interactions to be taken into account in this problem. The Hall conductance quantization law was determined for 2D holes in a periodic superlattice potential.     

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.     

Optical measurement and control of spin diffusion in n-doped GaAs quantum wells

Transient spin gratings are used to study spin diffusion in lightly n-doped GaAs quantum wells at low temperatures. The spin grating is shown to form in the excess electrons from doping, providing spin relaxation and transport properties of the carriers most relevant to spintronic applications. We demonstrate that spin diffusion of the these carriers is accelerated by increasing the density or energy of the optically excited carriers. These results can be used to better understand and even control spin transport in experiments that optically excite spin-polarized carriers.