### Apr 2020, Volume 15 Issue 2

Cover illustration

• The phase behavior of water is a topic of perpetual interest due to its remarkable anomalous properties and importance to biology, material science, geoscience, nanoscience, etc. It is predicted confined water at interface can exist in large amounts of crystalline or amorphous states. The confined water layers at a hydrophobic/hydrophobic interface were investigated by advanced atomic force microscopy (AFM). The intercalated water molecules present themselves as two phases, l [Detail] ...

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• RESEARCH ARTICLE
He Gao, Shun-Ke Ai, Zhou-Jian Cao, Bing Zhang, Zhen-Yu Zhu, Ang Li, Nai-Bo Zhang, Andreas Bauswein

With a selected sample of neutron star (NS) equations of state (EOSs) that are consistent with the current observations and have a range of maximum masses, we investigate the relations between NS gravitational mass Mg and baryonic mass Mb, and the relations between the maximum NS mass supported through uniform rotation (Mmax) and that of nonrotating NSs (MTOV). We find that for an EOS-independent quadratic, universal transformation formula ($Mb=Mg+A×Mg2$), the best-fit A value is 0.080 for non-rotating NSs, 0.064 for maximally rotating NSs, and 0.073 when NSs with arbitrary rotation are considered. The residual error of the transformation is ~0.1M for non-spin or maximum-spin, but is as large as ~0.2M for all spins. For different EOSs, we find that the parameter A for non-rotating NSs is proportional to $R1.4−1$ (where R1.4 is NS radius for 1.4M in units of km). For a particular EOS, if one adopts the best-fit parameters for different spin periods, the residual error of the transformation is smaller, which is of the order of 0.01 M for the quadratic form and less than 0.01M⊙ for the cubic form ($Mb=Mg+A1×Mg2+A2×Mg3$). We also find a very tight and general correlation between the normalized mass gain due to spin $Δm≡(Mmax⁡−MTOV)/MTOV$ and the spin period normalized to the Keplerian period $P$, i.e., $log⁡10Δm=(−2.74±0.05)log⁡10P+log⁡10(0.20±0.01)$, which is independent of EOS models. These empirical relations are helpful to study NS-NS mergers with a long-lived NS merger product using multi-messenger data. The application of our results to GW170817 is discussed.

• RESEARCH ARTICLE
Bai-Jiong Lin, Xiang-Ru Li, Wo-Liang Yu

This work investigates the detection of binary neutron stars gravitational wave based on convolutional neural network (CNN). To promote the detection performance and efficiency, we proposed a scheme based on wavelet packet (WP) decomposition and CNN. The WP decomposition is a time-frequency method and can enhance the discriminant features between gravitational wave signal and noise before detection. The CNN conducts the gravitational wave detection by learning a function mapping relation from the data under being processed to the space of detection results. This function-mapping-relation style detection scheme can detection efficiency significantly. In this work, instrument effects are considered, and the noise are computed from a power spectral density (PSD) equivalent to the Advanced LIGO design sensitivity. The quantitative evaluations and comparisons with the state-of-art method matched filtering show the excellent performances for BNS gravitational wave detection. On efficiency, the current experiments show that this WP-CNN-based scheme is more than 960 times faster than the matched filtering.

• RESEARCH ARTICLE
Chuan Yue, Peng-Xiong Ma, Qiang Yuan, Yi-Zhong Fan, Zhan-Fang Chen, Ming-Yang Cui, Hao-Ting Dai, Tie-Kuang Dong, Xiaoyuan Huang, Wei Jiang, Shi-Jun Lei, Xiang Li, Cheng-Ming Liu, Hao Liu, Yang Liu, Chuan-Ning Luo, Xu Pan, Wen-Xi Peng, Rui Qiao, Yi-Feng Wei, Li-Bo Wu, Zhi-Hui Xu, Zun-Lei Xu, Guan-Wen Yuan, Jing-Jing Zang, Ya-Peng Zhang, Yong-Jie Zhang, Yun-Long Zhang

Precise measurements of the energy spectra of cosmic rays (CRs) show various kinds of features deviating from single power-laws, which give very interesting and important implications on their origin and propagation. Previous measurements from a few balloon and space experiments indicate the existence of spectral softenings around 10 TV for protons (and probably also for Helium nuclei). Very recently, the DArk Matter Particle Explorer (DAMPE) measurement about the proton spectrum clearly reveals such a softening with a high significance. Here we study the implications of these new measurements, as well as the groundbased indirect measurements, on the origin of CRs. We find that a single component of CRs fails to fit the spectral softening and the air shower experiment data simultaneously. In the framework of multiple components, we discuss two possible scenarios, the multiple source population scenario and the background plus nearby source scenario. Both scenarios give reasonable fits to the wide-band data from TeV to 100 PeV energies. Considering the anisotropy observations, the nearby source model is favored.

• RESEARCH ARTICLE
Xin Li, Peng Wang, Ya-Qiang Wu, Zhen-Hua Liu, Qian-Qian Zhang, Ting-Ting Zhang, Ze-Yan Wang, Yuan-Yuan Liu, Zhao-Ke Zheng, Bai-Biao Huang

In this work, we prepared ZnGeP2 (ZGP) photocatalyst using single flat temperature zone (SFT) method in a vacuum quartz ampoule. The XRD, SEM, EDS, DRS and XPS were used to characterize the crystal structure, morphology, elemental content, optical absorption and band gap structure of ZGP. The results of photocatalytic hydrogen evolution and apparent quantum efficiency show that ZGP is a promising photocatalyst for hydrogen production both under visible and near-infrared light irradiation. In addition, it is also found that adding the common stabilizer H3PO2 and ultrasonic treatment can efficiently improve the photocatalytic activity and stability of ZGP.

• RESEARCH ARTICLE
Thomas Pope, Werner Hofer

The exact form of the kinetic energy functional has remained elusive in orbital-free models of density functional theory (DFT). This has been the main stumbling block for the development of a generalpurpose framework on this basis. Here, we show that on the basis of a two-density model, which represents many-electron systems by mass density and spin density components, we can derive the exact form of such a functional. The exact functional is shown to contain previously suggested functionals to some extent, with the notable exception of the Thomas–Fermi kinetic energy functional.

• RESEARCH ARTICLE
Chang-Yong Zhu, Shi-Han Zheng, Hou-Jian Duan, Ming-Xun Deng, Rui-Qiang Wang

We study the Andreev reflection (AR) at the interface of the topological insulator with hexagonal warping and superconductor junction. Due to the hexagonal warping effect, the double ARs are found in a certain range of the incident angle, where for one incident electron beam, two beams of holes are reflected back. Interestingly, both the beams of holes are reflected as retro-AR on the same side of the normal line of the interface but with different reflection angles, different from the previously reported double AR with one retro-AR and one specular-AR. The double reflections owing to the warping effect show the optics-like property of the Dirac fermion and can stimulate the double reflections of light in anisotropic crystals. In addition, we find that the double ARs are dependent on the hexagonal warping parameter nonmonotonically, and in an intermediate strength the double AR phenomenon is prominent, providing a possibility to explore the warping parameter of topological insulators.

• RESEARCH ARTICLE
Zhi-Yue Zheng, Rui Xu, Kun-Qi Xu, Shi-Li Ye, Fei Pang, Le Lei, Sabir Hussain, Xin-Meng Liu, Wei Ji, Zhi-Hai Cheng

The phase behavior of water is a topic of perpetual interest due to its remarkable anomalous properties and importance to biology, material science, geoscience, nanoscience, etc. It is predicted confined water at interface can exist in large amounts of crystalline or amorphous states. However, the experimental evidence of coexistence of liquid water phases at interface is still insufficient. Here, a special folding few-layers graphene film was elaborate prepared to form a hydrophobic/hydrophobic interface, which can provide a suited platform to study the structure and properties of confined liquid water. The real-space visualization of intercalated water layers phases at the folding interface is obtained using advanced atomic force microscopy (AFM). The folding graphene interface displays complicated internal interfacial characteristics. The intercalated water molecules present themselves as two phases, lowdensity liquid (LDL, solid-like) and high-density liquid (HDL, liquid-like), according to their specific mechanical properties taken in two multifrequency-AFM (MF-AFM) modes. Furthermore, the water molecules structural evolution is demonstrated in a series of continuous MF-AFM measurements. The work preliminary confirms the existence of two liquid phases of water in real space and will inspire further experimental work to deeply understanding their liquid dynamics behavior.

• RESEARCH ARTICLE
Jian-Ping Li, Lian-Zhong Deng, Ye Zheng, Peng-Peng Ding, Tian-Qing Jia, Zhen-Rong Sun, Jian-Rong Qiu, Shi-An Zhang

The spectral phase of the femtosecond laser field is an important parameter that affects the upconversion (UC) luminescence efficiency of dopant lanthanide ions. In this work, we report an experimental study on controlling the UC luminescence efficiency in Sm3+:NaYF4 glass by 800-nm femtosecond laser pulse shaping using spectral phase modulation. The optimal phase control strategy efficiently enhances or suppresses the UC luminescence intensity. Based on the laser-power dependence of the UC luminescence intensity and its comparison with the luminescence spectrum under direct 266-nm femtosecond laser irradiation, we propose herein an excitation model combining non-resonant two-photon absorption with resonance-mediated three-photon absorption to explain the experimental observations.

• RESEARCH ARTICLE
Ji-Zhou Wu, Yu-Qing Li, Wen-Liang Liu, Jie Ma, Lian-Tuan Xiao, Suo-Tang Jia

The light-induced frequency shift (LIFS) of ultracold molecular ro-vibrational levels originates from the strong coupling of the atomic-scattering state and the bound-molecular state. In this paper, we present our experimental determination of the LIFSs of the lowest vibrational levels (ν= 0, 1) in the purely long-range $0g−$ state of ultracold cesium molecules. A high-resolution double photoassociation spectroscopy is developed, which serves as frequency ruler to measure the frequency shifts of the lowest molecular levels for Cs2. The experimental results are qualitatively consistent with the theoretical expectations.

• RESEARCH ARTICLE
Lin Fa, Jiaojiao Tang, Qi Zhang, Minjin Zhang, Yandong Zhang, Meng Liang, Meishan Zhao

We explore the physical phenomenon of acoustic waves induced at the interface between two different anisotropic rock media. Specifically, one medium is a transversely isotropic medium with a vertical axis of symmetry (VTI medium) and the other one is a transversely isotropic medium with a tilt axis of symmetry (TTI medium). By solving the Kelvin–Christoffel equation, an eighth-order polynomial is established for reflection and refraction angles, which is confirmed from Snell’s law. Three types of analytical expressions of the polarization coefficients of the induced waves are obtained corresponding to different incident angle regions. An effective algorithm has been developed for numerical analysis of the polarization coefficients. Applying characteristic anisotropic parameters reported in the literature, the influencing factors on reflection and refraction coefficients are analyzed, e.g., the anisotropy, the tilt-angle of rock-layer, and the incident-angle. The calculated reflection and refraction coefficients have been rechecked for energy conservation.

• RESEARCH ARTICLE
Tong Liu, Zhen-Fei Zheng, Yu Zhang, Yu-Liang Fang, Chui-Ping Yang

We propose a method for transferring quantum entangled states of two photonic cat-state qubits (cqubits) from two microwave cavities to the other two microwave cavities. This proposal is realized by using four microwave cavities coupled to a superconducting flux qutrit. Because of using four cavities with different frequencies, the inter-cavity crosstalk is significantly reduced. Since only one coupler qutrit is used, the circuit resource is minimized. The entanglement transfer is completed with a singlestep operation only, thus this proposal is quite simple. The third energy level of the coupler qutrit is not populated during the state transfer, therefore decoherence from the higher energy level is greatly suppressed. Our numerical simulations show that high-fidelity transfer of two-cqubits entangled states from two transmission line resonators to the other two transmission line resonators is feasible with current circuit QED technology. This proposal is universal and can be applied to accomplish the same task in a wide range of physical systems, such as four microwave or optical cavities, which are coupled to a natural or artificial three-level atom.

• RESEARCH ARTICLE
Jin-Fu Chen, Yu-Han Ma, Chang-Pu Sun

Quantum walk (QW), which is considered as the quantum counterpart of the classical random walk (CRW), is actually the quantum extension of CRW from the single-coin interpretation. The sequential unitary evolution engenders correlation between different steps in QW and leads to a non-binomial position distribution. In this paper, we propose an alternative quantum extension of CRW from the ensemble interpretation, named quantum random walk (QRW), where the walker has many unrelated coins, modeled as two-level systems, initially prepared in the same state. We calculate the walker’s position distribution in QRW for different initial coin states with the coin operator chosen as Hadamard matrix. In one-dimensional case, the walker’s position is the asymmetric binomial distribution. We further demonstrate that in QRW, coherence leads the walker to perform directional movement. For an initially decoherenced coin state, the walker’s position distribution is exactly the same as that of CRW. Moreover, we study QRW in 2D lattice, where the coherence plays a more diversified role in the walker’s position distribution.

• RESEARCH ARTICLE
Lei-Xia Liang, Yan-Yan Zheng, Yuan-Xia Zhang, Mei Zhang

We propose a scheme for error-detected generation of an N-photon cluster state with a quantum dot (QD) embedded in a single-sided optical microcavity (QD-cavity system). The basic structure of this scheme is an error-detected controlled-phase (C-phase) gate on the hybrid electron–photon system. In this scheme, the fidelity of N-photon cluster state generation can be reached unity even if low-Q cavity and cavity leakage are considered. By using error detecting, the generation of an N-photon cluster state can be performed by repeating until success, which also leads to a high success probability, compared with other schemes assisted by the QD-cavity system. The error-detected generation of an N-photon cluster state in the highly controllable way may benefit on the quantum network in the future.