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## COVER ARTICLE

2020, 69 (3): 036501. doi: 10.7498/aps.69.20191906
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## EDITOR'S SUGGESTION

2020, 69 (3): 037201. doi: 10.7498/aps.69.20191623
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$\sim$4 GHz, which is obviously larger than the unhomogeneous Doppler broadening of $\sim$500 MHz for a macro atomic vapor cell. At the same time, the detuning frequency spectrum of total noise in the two atomic vapor cells is studied. In the macro atomic vapor cell, the total noise intensity strongly relies on the detuning frequency of the laser with respect to the atomic resonance transition. In the micron-scaled vapor cell, due to the strong homogeneous broadening, the center of the detuning frequency spectrum of the total noise is observed to dip. Finally, a simplified physical model is established to compute the broadening of the micron-scaled vapor cell. The homogeneous broadening of atoms is explained experimentally and theoretically in the micron-scaled vapor cell.">In this paper, the spin dynamics and broadening mechanism of cesium vapor in cells without buffer gas is investigated by means of spin noise spectroscopy. In a macro atomic vapor cell, the lineshape of detuning frequency spectrum of spin relaxation rate is of Gaussian distribution. For a micron-scaled vapor cell with strong spatial locality, the lineshape of detuning frequency spectrum of spin relaxation rate is of Lorentzian distribution. The parameter dependence of detuning frequency spectrum of spin relaxation rate, such as temperature, is studied quantitatively. The detuning frequency spectrum of the spin relaxation rate is measured experimentally to be broadened by $\sim$4 GHz, which is obviously larger than the unhomogeneous Doppler broadening of $\sim$500 MHz for a macro atomic vapor cell. At the same time, the detuning frequency spectrum of total noise in the two atomic vapor cells is studied. In the macro atomic vapor cell, the total noise intensity strongly relies on the detuning frequency of the laser with respect to the atomic resonance transition. In the micron-scaled vapor cell, due to the strong homogeneous broadening, the center of the detuning frequency spectrum of the total noise is observed to dip. Finally, a simplified physical model is established to compute the broadening of the micron-scaled vapor cell. The homogeneous broadening of atoms is explained experimentally and theoretically in the micron-scaled vapor cell.

## EDITOR'S SUGGESTION

2020, 69 (3): 038701. doi: 10.7498/aps.69.20191607
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α-synuclein (α-syn) is a key protein involved in Parkinson’s disease. There have been many researches about α-syn in recent years. It was suggested that the aggregation of α-syn may induce the lipid membranes to disrupted, which is related to the pathology of neurodegenerative diseases. Thus the studying of the dynamics of α-syn on membranes, especially in the presence of high-concentration protein, is important for understanding its function and its role in the pathology. In this study, we use LipoFRET, a single molecule method based on the principle of energy transfer between the donor labeled on the biomolecule and the quenchers encapsulated in the liposome. The quenchers encapsulated in liposomes attenuate the fluorescence attached to membrane proteins near the membrane, or penetrating in the membranes. If interesting site of membrane protein can be labeled, the LipoFRET could probe positional changes of a single membrane protein in the direction normal to the membrane. In the research of α-syn by LipoFRET, some interesting results can be obtained with different concentrations of protein. On the one hand, with the increase of concentration of α-syn in solution, the centre domain of α-syn can leave the surface of the lipid bilayer and enter into the aqueous solution. However, this domain of α-syn is located around the membrane surface at low concentration. On the other hand, the N-terminus of α-syn with three main positions at low concentration of protein, maintains three but different positions in the membrane at high concentration, where each position is closer to or above the outer surface of liposome. The above phenomena suggeste that the interaction between α-syn and membranes might be weakened with the increase of concentration of protein. At the same time, with single molecule fluorescence imaging, we also observe the promoted dissociation rates for individual fluorophore labeled α-syn from liposomes with high concentration of unlabeled proteins in solution. The result is consistent with the result of our single-molecule experiment with LipoFRET. Along with the results from LipoFRET, it could be indicated that there is a competition process where each α-syn could be occupied by the other one at high protein concentration, which leads to the dissociation. The concentration-dependent dissociation may be the property that regulates the aggregation of α-syn in vivo, which is one of the important factors that influence the pathology of the neurodegenerative diseases.

## EDITOR'S SUGGESTION

2020, 69 (3): 030601. doi: 10.7498/aps.69.20191525
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Optical scatterometry, as a fast, low-cost, and non-contact measurement instrument, is widely used in the profile characterization of nanostructure in the semiconductor manufacturing industry. In general, it involves two procedures, i.e. the forward optical modeling of sub-wavelength nanostructures and the reconstruction of structural profiles from the measured signatures. Here, the general term signature means the scattered light information from the diffractive grating structure, which can be in the form of reflectance, ellipsometric angles, Stokes vector elements, or Mueller matrix elements. The profile reconstruction process is an inverse problem with the objective of optimizing a set of floating profile parameters (e.g., critical dimension, sidewall angle, and height) whose theoretical signatures can best match the measured ones through regression analysis or library search. During solving the inverse problem, the refractive index and distinction coefficient of the material of nanostructure are assumed to be constants and they are generally fixed. This assumption is valid for most of the materials in semiconductor industry, but not for certain materials that are very photosensitive. That is, the optical constants of photosensitive materials may vary with the illumination time of the incident light beam in spectroscopic ellipsometer, and the error caused by the variation of optical constants propagates to the final extracted results of structural profiles, which should not be neglected, especially for high precision and accuracy metrology.Experiments performed on SiO2 and polymethyl methacrylate (PMMA) thin films are conducted and demonstrate that the extracted geometric parameters and optical constants of SiO2 film do not change with illumination time increasing, while the twenty groups of values of extracted refractive index n and distinction coefficient k of PMMA resist film vary obviously, and the difference between the extracted maximum and minimum film thickness has reached 40.5 nm, which to some extent illustrates that the above assumption is not valid for PMMA resist, so that the incident light beam of spectroscopic ellipsometer has a great influence on the extracted film thickness. Further, simulations based on a three-dimensional PMMA grating also indicate that the error of optical constant has considerably transferred to the extracted profile parameters. This finding is of significance for improving the accuracy of nanostructure characterization in optical scatterometry.

## EDITOR'S SUGGESTION

2020, 69 (3): 034205. doi: 10.7498/aps.69.20191515
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Tunable diode laser absorption spectroscopy (TDLAS) is a widely used technology for measuring absorption spectrum. However, the measurement efficiency of TDLAS is greatly limited by the narrow tuning range of conventional tunable laser diode. Exploiting a wideband, narrow linewidth tuning laser source, hyperspectral absorption spectroscopy possesses the ability to provide the overall absorption information over a continuous waveband in a single scan, which would significantly improve the data volume and diagnostic capability of TDLAS. With profound and strong absorption lines of water and carbon dioxide, the 2 μm waveband is an ideal candidate for water and carbon dioxide related absorption spectrum. An absorption line recognition threshold of 0.07 nm is derived for the absorption spectrum measurement of water around 2 μm through theoretical analysis. Utilizing the wideband emission spectrum of Tm-doped fiber, a wideband tunable, narrow linewidth fiber laser operating at 2 μm is built by combining a tunable FP filter with a fiber saturable absorber. The tunable FP filter is responsible for the wavelength control of the laser system, with which a 60 nm wideband tuning range from 1840 nm to 1900 nm is achieved. With a section of Tm-Ho codoped fiber as the fiber saturable absorber which is used for linewidth compression, a static linewidth of 0.05 nm is attained. This wideband tunable, narrow linewidth fiber laser is tested for the hyperspectral absorption spectrum measurement of water around 2 μm. Drived with a 0–10 V triangle wave at a repetition rate of 50 Hz, the output spectrum of the laser spans over a wavelength range of about 30 nm from 1856 nm to 1886 nm. The laser beam propagates about 50 cm through an open air, and then enters into the detectors for direct measurement. The 35 absorption lines of water are recognized after processing the data. Within the 1870–1880 nm range, comparisons with the theoretical absorption spectra at different laser linewidths, derived from the HITRAN2012 absorption database, show that the measured data cannot effectively distinguish two absorption lines adjacent to the strong absorption line at 1873 nm and 1877 nm. And, the measured results can be best fitted to a laser linewidth of about 0.08 nm, demonstrating that in the dynamic scanning process, the linewidth of the laser is expanded beyond the absorption line recognition threshold. Thus, when operating in a fast wideband scanning mode, the laser system should further compress its linewidth.

## EDITOR'S SUGGESTION

2020, 69 (3): 034701. doi: 10.7498/aps.69.20191011
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Bluff bodies possess many engineering applications. The flow past a bluff body is a classical issue in fluid mechanics. In most of previous studies, the role of streamwise vorticity is mainly stressed in three-dimensional wake flow, such as in the physical origin of streamwise vortices in the mode A, and the complete suppression of alternatively shedding Kármán vortices under the effect of geometric disturbances introduced in the bluff body. However, through the careful investigation of two examples above, the vertical vorticity actually plays a key role. Furthermore, there is a physical phenomenon, special relationship among dominant vorticity components with specific signs or vorticity sign law, occurring in the wake of a bluff body with geometric disturbances. In the present paper, through direct numerical simulations at low Reynolds numbers, such a phenomenon is summarized in two kinds of cylinders, i.e. basic straight cylinder and geometrically disturbed cylinder. Two typical cross-sections are examined, including the circular and square sections. Three sub-regimes (front surfaces, shear layers and wake) are mainly investigated. The numerical results show that two generalized vorticity sign laws exist in the wake of a bluff body. For example, the first sign law shows that the sign of streamwise vorticity is always the same as that of vertical vorticity in the upper shear layer, but opposite in the lower shear layer. The second sign law shows that the sign combination of three components of vorticity is always negative in the shear layers and wake. As for the physical mechanism of sign laws appearing in the present two kinds of cylinders, the main difference between the small perturbance which induces the natural three-dimensional instability and the geometric disturbance leads to the evolution of generated surface vorticity under the effect of inertial forces. These generalized sign laws have been already verified theoretically in the previous work recently. Moreover, sign laws also indicate that the different vortex-shedding patterns in the wake of different bluff bodies are inherently identical from the point of vorticity sign. Considering the physical fact that the wall is the only source of new vorticity in the present vortex dynamics, the theoretical results also indicate that the Π-type vortex with specific groups of vorticity components is a basic vortex pattern generated on the walls, once the three-dimensional wake first appears in the wake at low Reynolds number.

## EDITOR'S SUGGESTION

2020, 69 (3): 038501. doi: 10.7498/aps.69.20191263
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In recent years, metal halide perovskite materials, owing to their excellent photoelectric properties including high photoluminescence quantum yield, high color purity, tunable band gap, etc., have been regarded as new-generation lighting sources and are widely used to fabricate perovskite light-emitting diodes (PeLEDs). Though great progresses have been made in recent years, neither the efficiency nor stability has not yet reached the requirements of commercialization. Thus, further improvement is needed. In this work, a small organic molecule material, namely 4,4'-cyclohexylidenebis[N,N-bis(p-tolyl)aniline] (TAPC) with a wide bandgap and a good hole transport ability, is used as an exciton blocking layer by utilizing the spin-coating method to improve the stability and efficiency of PeLEDs. Highly efficient and stable CsPbBr3 PeLEDs are finally realized. The physical mechanism related to the improved electroluminescence performance is investigated thoroughly. Firstly, the stepped energy level alignment is formed, since the highest occupied molecular orbital energy level (HOMO) of TAPC is located between the HOMO of (3,4-ethylenedioxythiophene):poly(p-styrene sulfonate) (PEDOT: PSS) and the valence band of CsPbBr3, which is beneficial to hole injection and transport. Meanwhile, the lowest unoccupied molecular orbital level of TAPC is high enough to prevent electrons from leaking into the anode effectively and confine electrons and excitons well in the emitting layer. Secondly, the introduction of the TAPC layer can avoid the direct contact between the perovskite light emitting layer and the strong acidic layer of PEDOT:PSS, thereby eliminating the related excitons quenching, which can further increase the radiative recombination.

## INVITED REVIEW

2020, 69 (2): 020301. doi: 10.7498/aps.69.20191627
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In recent years, by introducing topological invariants into condensed matter systems, new phases of mater are revealed. Of these new phases, the topological insulator, topological semimetal and topological superconductor are the most important. They are called topological materials due to nontrivial topological parameters. Topological superconductors hold Majorana zero modes at the edges, satisfying non-abelian statistics, which makes them major candidate for realizing topological quantum computation. Besides exploring intrinsic topological superconductor, a promising way to realize topological superconductor is to induce superconductivity into other kinds of topological materials. Up to now, experimentalists have developed some techniques, such as gating, doping, high pressure, interface effect and hard point contact to introduce superconductivity into various topological materials, and also they have studied the topological properties of the induced superconductivity. In this review, we summarize the representative researches on intrinsic topological superconductor candidates and induced superconductivities in topological insulators and semimetals. The advantages and disadvantages of different techniques are discussed. Besides, the potential evidences of topological superconductors are analyzed. In the end, the outlook of this actively pursued research field is given.

## EDITOR'S SUGGESTION

2020, 69 (2): 027501. doi: 10.7498/aps.69.20191655
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The conventional magnetic ordering models, exchange interaction, super-exchange (SE) interaction and double exchange (DE) interaction models relating to the valence electron structure in the materials, were proposed about in or before the 1950's, the time when there was little experimental evidence. Since the 1970's, more and more experimental results for the valence electron states have been reported. These experimental results suggested that the conventional magnetic ordering models need improving. i) Many experimental results, including the electron energy-loss spectra (EELS), X-ray absorption spectra (XAS), and X-ray photoelectron spectra (XPS), indicate that there are O anions in addition to O2– anions in oxides, and that the percentage of O anions may reach 30% or more. This suggests that the SE model and DE model both need to improving, in which all oxygen anions are assumed to be O2– anions. ii) Several experimental results, including gamma radiation diffraction, XAS and magnetic circular dichroism spectra (XMCD), suggest that part of 4s electrons enter into 3d orbits and transit into the 3d electrons in the process of forming metals from free atoms. The effect of the orbital magnetic moment on the magnetic moment of a bulk metal is far smaller than the spin magnetic moments. These provide the evidence of exploring the relation between magnetic moment and electrical resistivity of the magnetic metal. iii) Using density function theory (DFT) to fit physical properties yields plenty of results for many materials, but there exist serious difficulties for magnetic materials. This is due to magnetic ordering energy is included in the exchange correlation energy, which has been find no phenomenological expression so far, and has to be fitted using various models in DFT calculation. These investigations provide an opportunity to improve magnetic ordering models. Therefore, our group proposed three models of magnetic ordering in typical magnetic materials, they including an O 2p itinerant electron model for magnetic oxides (IEO model), a new itinerant electron model for magnetic metal (IEM model), and a Weiss electron pair (WEP) model for the origin of magnetic ordering energy. Replacing the SE model and DE model with the IEO model, the magnetic structures of Co, Ni, Cu doped spinel ferrites as well as Cr and Ti doped spinel ferrites can be explained. The dependence of the magnetic moment on the Sr content in perovskite manganites La1–xSrxMnO3 can also be explained, for which there have been many ongoing disputes about the cation distributions. With the IEM model, we can explain qualitatively the relation of the magnetic moment with the resitivity for each of Fe, Co, Ni, Cu metals, and fit the curves of the resistivity of NiCu alloy versus test temperature and the Cu doped level. With the WEP model, we can explain why Fe, Co, Ni metal, NiCu alloys, Fe3O4 and La0.7Sr0.3MnO3 oxides have different Curie temperature values. The new itinerant electron model is different from the classical model in the following three elementary characteristics. First, the s electrons in free 3d transition metal atoms are divided into two parts when they form a metal or alloy. One part of these s electrons enter into the d orbits and change into the d electrons. and the other part of those electrons are the free electrons which are no longer called the s electrons. Second, only the d electrons occupying the outer orbit of an ion core in a metal or alloy may form itinerant electrons with a certain probability, while the remaining d electrons are local electrons. Third, whether in a magnetic metal or in a magnetic oxide, the transition of the itinerant electrons is the spin-dependent transition below the Curie temperature, and the transition probability decreases with test temperature increasing. The transition of the itinerant electrons turns into the spin-independent transition when the temperature is above the Curie temperature. In this paper, first, we introduce several typical experimental results of the valence electron states. Then, we present the new magnetic ordering models proposed by our group and analyze the elementary differences between the new models and the conventional models. Finally, we point out the challenge to the future work.

## EDITOR'S SUGGESTION

2020, 69 (2): 026801. doi: 10.7498/aps.69.20190756
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