Circular photogalvanic effect at inter-band excitation in semiconductor quantum wells

We observed a circular photogalvanic effect (CPGE) in GaAs quantum wells at inter-band excitation. The spectral dependence of the CPGE is measured together with that of the polarization degree of the time-resolved photoluminescence. A theoretical model takes into account spin splitting of conduction and valence bands.     

Observation of the surface circular photogalvanic effect in InN films

A sizable spin-dependent photocurrent related to the interband transition in InN films is observed. The surface charge accumulation layer is suggested to be the origin of the circular photogalvanic current, which is consistent with the result of uniaxial strain experiments and the comparison of front and back incidence. The homogeneous photocurrent demonstrates the existence of spin splitting in the InN surface layer, and the structure inversion asymmetry (SIA)-dominant mechanism indicates a great possibility for the manipulation of spin splitting, which would undoubtedly benefit further research and applications of spintronics.     

Magnetic field effects on excited states,charge transport,and electrical polarization in organic semiconductors in spin and orbital regimes

Magnetic field can influence photoluminescence, electroluminescence, photocurrent, injection current, and dielectric constant in organic materials, organic–inorganic hybrids, and nanoparticles at room temperature by re-distributing spin populations, generating emerging phenomena including magneto-photoluminescence, magneto-electroluminescence, magneto-photocurrent, magneto-electrical current, and magneto-dielectrics. These so-called intrinsic magnetic field effects (MFEs) can be observed in linear and non-linear regimes under one-photon and two-photon excitations in both low- and high-orbital materials. On the other hand, spin injection can be realized to influence spin-dependent excited states and electrical conduction via organic/ferromagnetic hybrid interface, leading to extrinsic MFEs. In last decades, MFEs have been serving as a unique experimental tool to reveal spin-dependent processes in excited states, electrical transport, and polarization in light-emitting diodes, solar cells, memories, field-effect transistors, and lasing devices. Very recently, they provide critical understanding on the operating mechanisms in advanced organic optoelectronic materials such as thermally activated delayed fluorescence light-emitting materials, non-fullerene photovoltaic bulk-heterojunctions, and organic–inorganic hybrid perovskites. While MFEs were initially realized by operating spin states in organic semiconducting materials with delocalized π electrons under negligible orbital momentum, recent studies indicate that MFEs can also be achieved under strong orbital momentum and Rashba effect in light emission, photovoltaics, and dielectric polarization. The transition of MFEs from the spin regime to the orbital regime creates new opportunities to versatilely control light-emitting, photovoltaic, lasing, and dielectric properties by using long-range Coulomb and short-range spin–spin interactions between orbitals. This article reviews recent progress on MFEs with the focus on elucidating fundamental mechanisms to control optical, electrical, optoelectronic, and polarization behaviors via spin-dependent excited states, electrical transport, and dielectric polarization. In this article both representative experimental results and mainstream theoretical models are presented to understand MFEs in the spin and orbital regimes for organic materials, nanoparticles, and organic–inorganic hybrids under linear and non-linear excitation regimes with emphasis on underlying spin-dependent processes.     

Circular Photogalvanic Effect in the Weyl Semimetal TaAs

Weyl semimetal(WSM) is expected to be an ideal spintronic material owing to its spin currents carried by the bulk and surface states with spin-momentum locking. The generation of a sizable photocurrent is predicted in non-centrosymmetric WSM arising from the broken inversion symmetry and the linear energy dispersion that is unique to Weyl systems. In our recent measurements, the circular photogalvanic effect(CPGE) is discovered in the TaAs WSM. The CPGE voltage is proportional to the helicity of the incident light, reversing direction if the radiation helicity changes handedness, a periodical oscillation therefore appears following the alteration of light polarization. We herein attribute the CPGE to the asymmetric optical excitation of the Weyl cone, which could result in an asymmetric distribution of photoexcited carriers in momentum space according to an optical spin-related selection rule.     

Effect of electric-field on spin transport and spin current in an organic semiconductor system

We use the two-component drift-diffusion model to study the spin density polarization in an organic semiconductor system under an external electric-field. The spin-dependent electrical-conductivity, the drift spin current and the diffusion spin current in the organic semiconductor are self-consistently derived. It is found that the spin current could be strongly influenced by the spin-dependent electrical-conductivity. When the spin-dependent conductivity varies from 0 to 0.5%, the spin current presents a very pronounced change almost three orders in magnitude. The electric-field could effectively enhance the spin-dependent electrical-conductivity and the spin current. Furthermore, the spin-dependent electrical-conductivity is position sensitive, but its position sensitivity goes down while electric-field is larger than about 1 mV/μm.     

Inversion of spin photocurrent due to resonant transmission

We report on the inversion of spin-dependent photocurrent via interface localized states formed at the interface of an Fe/n-AlGaAs/GaAs quantum well heterostructure by means of an optical spin orientation technique. A careful adjustment of the excitation photon energy, which is determined by a separate analysis of electroluminescence spectra under a spin injection condition, enables us to explore the spin-dependent characteristics of photoelectron transmission from the quantum well into Fe. The bias dependence of the spin-dependent photocurrent shows clear spikelike features at the voltage which is compatible with the formation of the interface localized resonant states in the Schottky depletion layer.     

Spin-dependent balance equations in spintronics

It is commonly known that the hydrodynamic equations can be derived from the Boltzmann equation. In this paper,we derive similar spin-dependent balance equations based on the spinor Boltzmann equation. Besides the usual charge current, heat current, and pressure tensor, we also explore the characteristic spin accumulation and spin current as well as the spin-dependent pressure tensor and heat current in spintronics. The numerical results of these physical quantities are demonstrated using an example of spin-polarized transport through a mesoscopic ferromagnet.     

Pumped Spin-Current in Single Quantum Dot with Spin-Dependent Electron Temperature

Spin-dependent electron temperature effect on the spin pump in a single quantum dot connected to Normal and/or Ferromagnetic leads are investigated with the help of master equation method. Results show that spin heat accumulation breaks the tunneling rates balance at the thermal equilibrium state thus the charge current and the spin current are affected to some extent. Pure spin current can be obtained by adjusting pumping intensity or chemical potential of the lead. Spin heat accumulation of certain material can be detected by measuring the charge current strength in symmetric leads architectures. In practical devices, spin-dependent electron temperature effect is quite significant and our results should be useful in quantum information processing and spin Caloritronics.     

Spin-excited states and rectification in an organic spin rectifier

Spin-excited states in an asymmetric magnetic organic co-oligomer diode are investigated theoretically. The results demonstrate that the structural asymmetry of the co-oligomer is modulated by the spin-excited states, which is embodied in the wave functions of the eigenstates as well as the spin density wave. By calculating the transport property, a robust spin-current rectification concomitant with a charge-current rectification is observed in all spin-excited states. However, the current through the diode is suppressed distinctly by the spin-excited states, while the rectification ratios may be reduced or enhanced depending on the bias and the excited spins. The intrinsic mechanism is analyzed from the spin-dependent trans- mission combined with the change of molecular eigenstates under bias. Finally, the temperature-induced spin excitation is simulated. Significant rectification behavior is obtained even at room temperature.     

基于Pancharatnam-Berry相位和动力学相位调控纵向光子自旋霍尔效应

提出了一种基于Pancharatnam-Berry相位和动力学相位操控纵向光子自旋霍尔效应的方法.理论分析表明:当光场通过一个由Pancharatnam-Berry相位透镜和动力学相位透镜构成的透镜组时,透镜组会存在两个自旋相关的焦点.首先,当左旋和右旋圆偏振光通过微结构相位延迟为π的Pancharatnam-Berry相位透镜时,由于Pancharatnam-Berry相位的自旋相关性,两个圆偏振分量会获得符号相反的Pancharatnam-Berry相位而导致其中一个被聚焦而另一个发散.然后,在Pancharatnam-Berry相位透镜后再插入普通透镜引入动力学相位调制,由于动力学相位是自旋无关,使得这一透镜组,可以在合适的条件下使不同自旋态的光子分别聚焦于纵向上不同焦点处.纵向自旋分裂由两透镜焦距及间距共同决定,因此可以通过改变两个透镜的焦距及其间距获得任意的纵向自旋分裂值.最后,搭建了一套实验装置,所得实验结果与理论结果一致.     

Spin-dependent delay time and Hartman effect in asymmetrical graphene barrier under strain

We study the spin-dependent tunneling time, including group delay and dwell time, in a graphene based asymmetrical barrier with Rashba spin–orbit interaction in the presence of strain, sandwiched between two normal leads. We find that the spin-dependent tunneling time can be efficiently tuned by the barrier width, and the bias voltage. Moreover, for the zigzag direction strain although the oscillation period of the dwell time does not change, the oscillation amplitude increases by increasing the incident electron angle. It is found that for the armchair direction strain unlike the zigzag direction the group delay time at the normal incidence depends on the spin state of electrons and Hartman effect can be observed. In addition, for the armchair direction strain the spin polarization increases with increasing the RSOI strength and the bias voltage. The magnitude and sign of spin polarization can be manipulated by strain. In particular, by applying an external electric field the efficiency of the spin polarization is improved significantly in strained graphene, and a fully spin-polarized current is generated.     

The Andreev tunneling behaviors in the FM/QD/SC system with intradot spin-flip scattering

The resonant behaviors of spin-dependent linear AR conductance, the spin-dependent AR current, the electron occupation number and spin accumulation in the QD are theoretically investigated in the FM/QD/SC system with intradot spin-flip scattering. The novel resonant behaviors of spin-dependent AR conductance versus Fermi energy are revealed, which are rather different from the AR conductance versus the dot's energy level case [Cao et al., Phys. Rev. B 70 (2004) 235341]. It is proved that the split of the resonant peak can be induced by the competition between the coupling strengths to the FM and SC leads, the intradot spin-flip scattering, and the gate voltage. The number, the widths, and the distance of the peaks could be controlled by tuning the relevant parameters. The resonance of AR current can take place only when the energy level of QD lines up with the right lead chemical potential and blows the left lead chemical potential. The magnitude of the resonant AR current depends on the number of resonant levels involved in the Andreev tunneling process. It is also proved that the spin-flip scattering can suppress the spin accumulation effectively, and induce the spin polarization of AR conductance and AR current simultaneously. The results make us understand better the fundamental in this system, and are useful for the design of spintronic devices.     

Spin polarization in directional photoemission from non-magnetic crystals by unpolarized light

A spin-dependent three-step photoemission formalism for non-magnetic crystals is outlined, in which unpolarized Bloch electrons are diffracted at the surface. Assuming no spin dependence of the excitation process in the bulk, appreciable spin polarization features are obtained for W(001) at off-normal emission, in fairly good agreement with the first angle- and energy-resolved measurements of photo-electron spin polarization (using unpolarized light).     

Thermally driven pure spin current through an interacting quantum dot

We study spin-dependent electron transport properties of a thermally driven interacting quantum dot. When an external magnetic field is applied to the quantum dot, the effective transmissions of spin-up and spin-down electrons are separated from each other and have a perfect mirror symmetry with respect to the incident energy at a certain gate voltage. A pure spin current can be induced in the system and modulated by a magnetic field. Under certain magnetic field strengths, a larger pure spin current can be obtained at gate voltages with the values in a range, not just at a specific voltage. These results indicate that the system can be worked as a pure spin current generator.     

Quantum pump in a system with both Rashba and Dresselhaus spin–orbit couplings

We investigate the adiabatic quantum pump phenomena in a semiconductor with Rashba and Dresselhaus spin–orbit couplings (SOCs). Although it is driven by applying spin-independent potentials, the system can pump out spin-dependent currents, i.e., generate nonzero charge and spin currents at the same time. The SOC can modulate both the magnitude and the direction of currents, exhibiting an oscillating behavior. Moreover, it is shown that the spin current has different sensitivities to two types of the SOC. These results provide an alternative method to adjust pumped current and might be helpful for designing spin pumping devices.     

Spin transverse force on spin current in an electric field

As a relativistic quantum mechanical effect, it is shown that the electron field exerts a transverse force on an electron spin 1/2 only if the electron is moving. The spin force, analogue to the Lorentz for an electron charge in a magnetic field, is perpendicular to the electric field and the spin current whose spin polarization is projected along the electric field. This spin-dependent force can be used to understand the Zitterbewegung of the electron wave packet with spin-orbit coupling and is relevant to the generation of the charge Hall effect driven by the spin current in semiconductors.     

Spin-dependent quantum transport through an Aharonov--Bohm structure spin splitter

Using the tight-binding model approximation, this paper investigates theoretically spin-dependent quantum transport through an Aharonov-Bohm （AB） interferometer. An external magnetic field is applied to produce the spinpolarization and spin current. The AB interferometer, acting as a spin splitter, separates the opposite spin polarization current. By adjusting the energy and the direction of the magnetic field, large spin-polarized current can be obtained.     

Anisotropic spin-dependent thermopower and current in ferromagnetic graphene junctions

We study the spin-dependent thermoelectric transport through two-dimensional normal/ferromagnetic/normal/ferromagnetic/normal graphene (NG/FG/NG/FG/NG) junctions. It is found that both charge and spin thermopowers depend on the FGʼs magnetization direction and exhibit an anisotropic behavior. Interestingly, the spin thermopower can be as large as the charge thermopower and even can exceed the latter in magnitude. Moreover, the pure spin thermopower and spin current emerge in this device. The results obtained here suggest a feasible way of enhancing thermospin effects and generating the pure spin current in two-dimensional graphene.     

Spin distributions in the vibrational excitations of closed-shell nuclei

Self-consistent Hartree-Fock and RPA calculations with the Skyrme-type interaction SGII are used for a systematic investigation of the 1⁺ and the triplet 0^?, 1^?, 2^? states in ⁴⁰Ca and ²⁰⁸Pb. Response functions to spin-dependent multipole operators are calculated and the particle-hole structure of the spin-dependent collective states is studied. Collective spin-dependent 0^? and 1^? states above the giant dipole resonance as well as a collective spin-independent 2^? state (twist mode) are identified. Transition spin and current densities are calculated for the collective excitations and found to be useful for the study of these excitation modes.     

Nuclear spins of ionized phosphorus donors in silicon

We demonstrate the coherent control and electrical readout of ionized phosphorus donor nuclear spins in (nat)Si. By combining time-programed optical excitation with coherent electron spin manipulation, we selectively ionize the donors depending on their nuclear spin state, exploiting a spin-dependent recombination process at the Si/SiO(2) interface, and find a nuclear spin coherence time of 18 ms for the ionized donors. The presented technique allows for spectroscopy of ionized-donor nuclear spins and enhances the sensitivity of electron nuclear double resonance to a level of 3000 nuclear spins.