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.     

Surface states induced by metal atoms at the Si/electrolyte interface

Metal atoms have been chemically deposited on n-Si and p-Si and the obtained deposits have been characterized with Auger electron spectroscopy. The obtained samples have been used as electrodes in acetonitrile electrolyte. The electrochemical studies heve been performed using classical current-voltage, impedance and Schottky-Mott measurements, and also subgap photocurrent spectroscopy of the surface states. It appears that the deposited metal atoms do induce surface states on the silicon surface. These surface states have a weak effect on the flatband potential (i.e. no strong pinning of the Fermi level is observed even for monolayer coverage) but the subgap photoyield is increased by several orders of magnitude. The shape of the quantum yield versus photon energy curve points to surface states widely distributed through the bandgap. These experiments finally confirm the ability of the subgap photocurrent technique to distinguish between the two kinds of optical processes that may occur between the surface states and the semiconductor bands.     

Comment on “direct observation of elementary steps in charge transfer mediated by surface states on TiO2 electrode under illumination” by Liu Chongyang,Chen Yixuan and Li Wenzhao

Photocurrents observed under subbandgap illumination are interpreted as arising from electronic transitions via surface states. However, since the crystal used was sintered, and also reduced at 700°C for 2 h, a larger number of deep traps will be present in the band structure of the crystal. These traps are the origin of the subbandgap photocurrent, and not any possible surface states. A simple calculation shows that any photocurrent from surface states would be much too high, whereas the number of deep traps is of the usual magnitude.     

Orbital mechanism of the circular photogalvanic effect in quantum wells

It is shown that the free-carrier (Drude) absorption of circularly polarized radiation in quantum well structures leads to an electric current flow. The photocurrent reverses its direction upon switching the light helicity. A pure orbital mechanism of such a circular photogalvanic effect is proposed that is based on the interference of different pathways contributing to light absorption. Calculation shows that the magnitude of the helicity-dependent photocurrent in n-doped quantum well structures corresponds to recent experimental observations. The text was submitted by the author in English.     

Terahertz radiation driven chiral edge currents in graphene

We observe photocurrents induced in single-layer graphene samples by illumination of the graphene edges with circularly polarized terahertz radiation at normal incidence. The photocurrent flows along the sample edges and forms a vortex. Its winding direction reverses by switching the light helicity from left to right handed. We demonstrate that the photocurrent stems from the sample edges, which reduce the spatial symmetry and result in an asymmetric scattering of carriers driven by the radiation electric field. The developed theory based on Boltzmann's kinetic equation is in a good agreement with the experiment. We show that the edge photocurrents can be applied for determination of the conductivity type and the momentum scattering time of the charge carriers in the graphene edge vicinity.     

Photofield emission spectroscopy of electronic states near the fermi level of 111 tungsten

The two closely situated electronic bands for the 111 tungsten direction between the Fermi level and the vacuum level have been observed using photofield emission spectroscopy and s-polarized light in the visible range. Measurements with p-polarized light reveal surface states below the Fermi level. Optical transitions have been found from the shoulders of the photocurrent characteristics. The observed excitations are in good agreement with the bulk band structure and the surface density of states of tungsten calculated by theory.     

Ratchet effects in quantum wells with a lateral superlattice

Experimental and theoretical works on the ratchet effects in quantum wells with a lateral superlattice excited by alternating electric fields of terahertz frequency range has been reviewed. We discuss the Seebeck ratchet effect and helicity driven photocurrents and show that the photocurrent generation is based on the combined action of a spatially periodic in-plane potential and a spatially modulated light.     

A new model for spectral sensitization of photoconduction in zinc oxide powder

Seven organic dyes were adsorbed on photoconductive zinc oxide powder and their sensitizing efficiencies were examined. The adsorption of dye induced a change in the dark conductivity, the magnitude of which appeared to represent the sensitizing power of the dye. The initial slope of sensitized photocurrent was found to be an exponential function of σ₀, the pre-exponential factor in the conductivity equation. A model is proposed, which is consistent with all the observations. In this model, the sensitized photoinjection of electrons is assumed to occur from surface states to the bulk, depending upon the population of the surface states. A critical surface level for the spectral sensitization is introduced to account for the observed temperature effect. Surface states located below the critical energy are assumed not to contribute for the sensitization.     

Particle-transport analysis in reversed field pinch helical states

This Letter reports the result of a numerical study of particle transport in self-organized single helicity (SH) and quasi-SH reversed field pinch plasmas. Our code, benchmarked against experimental data, predicts a large improvement in particle transport for SH compared to the standard multiple helicity states. The contribution of neoclassical effects is noted. An estimate of the ambipolar electric field in helical states and in fully stochastic magnetic fields is given.     

Circular photogalvanic effect caused by the transitions between edge and 2D states in a 2D topological insulator

The electron absorption and the edge photocurrent of a 2D topological insulator are studied for transitions between edge states to 2D states. The circular polarized light is found to produce the edge photocurrent, the direction of which is determined by light polarization and edge orientation. It is shown that the edge-state current is found to exceed the 2D current owing to the topological protection of the edge states.     

Indecomposable representations of Poincaré group and quantization of weak gravitational and electromagnetic fields

In this paper a quantization procedure of the electromagnetic and the weak gravitational fields, in an indefinite (but non-negative) metric space of states, is presented. We show that the covariance axiom can only be formulated with non-unitary representations (of the Poincaré group) whose restrictions to the one-particle states are indecomposable. In the gravitational case the corresponding indecomposable representation describes six states of helicity for gravitons. The quantized field appears as six interacting massless fields of helicities 2, ?2,1, ?1,0 and 0. These six values of helicity correspond to the six modes of polarization in the most general metric theory of gravity. For the electromagnetic field one obtains an indecomposable representation of the Poincaré group with helicities 1, ?1 and 0 corresponding to transverse and longitudinal photons.     

飞秒激光改性6H-碳化硅晶体表面光电导增益现象研究

半绝缘6H型碳化硅(6H-SiC)具有高电阻率性质,在可见光照射下进行光电导测量时,通常光生电流很小;然而经过飞秒激光辐照改性之后,发现在可见光波段的光电导有明显的增益.本文利用紫外-可见-近红外吸收谱、X射线光电子能谱和发光光谱测量分析了激光改性之后碳化硅样品的光谱吸收、发射和晶体元素比例变化情况.分析认为碳化硅光电导增益的原因是飞秒激光辐照过程改变了碳化硅表面的硅碳元素的原子浓度比,形成新的物质结构形式,从而导致了表面光电导性能的提高.     

Theory of electron states at random alloy surfaces - surface states on Cu-Ni (111)

We present first principles calculations (based on the KKRCPA) of the angle-resolved photocurrent emitted from the (111) surfaces of single crystals of $\text{Cu}$-Ni random alloys, and compare the results with new experimental data. Surface states close to the Fermi level are observed, even for concentrated alloys, and their behaviour as a function of composition and $\text{k}$_∥ is correlated with features in the bulk spectral density. Calculations for alloys with a non-uniform concentration profile at the surface (surface segregation) are described, and the effect on the surface states is discussed.     

Phase shift analysis of modulated photocurrent: Determination of the energy scale

For the determination of the density of states in the mobility gap of amorphous semiconductors using the phase shift analysis of modulated photocurrent, this paper suggests that making use of the magnitude of the induced photocurrent helps to remove arbitrariness in the energy scale. The working equations for the density of states and the corresponding energy position are expressed in terms of the intensity of the photocurrent. A simulation is made for a specific distribution, to investigate the validity of the procedure. The results show that the profile of the energetic distribution of localized states and the exact energy position of each state are consistent with the original distribution considered.     

无质量中微子螺旋性混合态

自旋为1/2的中微子遵从Majorana方程,其螺旋性为守恒量.利用压缩相干态可以为中微子给出螺旋算符与哈密顿量之共同本征态的显式表达,但却是不可归一化的.可归一化的波包依然保持为螺旋算符的本征态,但不再是能量的本征态.存在传播速度为零的等权混合螺旋态波包.此情形没有经典对应,在无质量玻色子世界中也无对应.若这样的混合螺旋态波包对应无质量费米子的粒子-反粒子对,则提供了一种暗物质的可能构成方案.     

On the Decay Analysis of Electroproduced Higher-Spin Hadrons

We study exclusive electroproduction of two hadrons where one or both of the hadrons is a higherspin resonance whose decay is analysed. Interesting electroproduction experiments of this type are discussed. A complete formal apparatus is given for the decay analysis of spin-J particles produced in this way with polarized or unpolarized electron beams and targets. The cases J = 1, 3/2, 2 and 5/2 are worked out in detail. A conventional helicity frame analysis in terms of s-channel helicity amplitudes is given. Also we rearrange the formalism for use in the Gottfried-Jackson frame with everything given in terms of t-channel helicity amplitudes. The t-channel formalism makes it possible to completely separate the contributions from longitudinal and transverse virtual photon t-channel helicity states when only the laboratory azimuthal angle between the lepton and hadron planes is variable.     

de Sitter QED

Attention is called to the fact that the well-known and straightforward generalization of electrodynamics to de Sitter space is incompatible with conformal invariance. In addition, there is difficulty in reconciling the space of one-photon states in de Sitter QED, for which the field carries no degree of freedom related to helicity, with that of flat space QED in which both signs of the helicity appear. The requirement of conformal invariance leads to the introduction of two vector potentials in de Sitter electrodynamics and resolves the helicity problem. A conformally invariant, indefinite metric quantization is carried out, and the nature of the flat space limit is clarified. Implications for a theory of composite massless particles are discussed, as well as applications to supersymmetry and supergravity.     

Phase transition to the state with nonzero average helicity in dense neutron matter

The possibility of the appearance of the states with a nonzero average helicity in neutron matter is studied in the model with the Skyrme effective interaction. By providing the analysis of the self-consistent equations at zero temperature, it is shown that neutron matter with the Skyrme BSk18 effective force undergoes at high densities a phase transition to the state in which the degeneracy with respect to helicity of neutrons is spontaneously removed.     

Quantum correlations of helicity entangled states in non-inertial frames beyond single mode approximation

A helicity entangled tripartite state is considered in which the degree of entanglement is preserved in non-inertial frames. It is shown that Quantum Entanglement remains observer independent. As another measure of quantum correlation, Quantum Discord has been investigated. It is explicitly shown that acceleration has no effect on the degree of quantum correlation for the bipartite and tripartite helicity entangled states. Geometric Quantum Discord as a Hilbert–Schmidt distance is computed for helicity entangled states. It is shown that living in non-inertial frames does not make any influence on this distance, either. In addition, the analysis has been extended beyond single mode approximation to show that acceleration does not have any impact on the quantum features in the limit beyond the single mode. As an interesting result, while the density matrix depends on the right and left Unruh modes, the Negativity as a measure of Quantum Entanglement remains constant. Also, Quantum Discord does not change beyond single mode approximation.     

Covariant Helicity Amplitude Analysis for J／ψ→γPP

Covariant helicity amplitude analysis for the process of J/ψ→γPP is discussed starting from the Smatrix elements of decay process,we deduce the formulae of helicity coupling amplitudes for two-body decay process,These formulae are used to analyze intermediate resonance states in the process of J/ψ decay to γππ,γKK,γηη′