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 Select Continuously Doping Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8+\delta}$ into Electron-Doped Superconductor by CaH$_{2}$ Annealing Method Jin Zhao, Yu-Lin Gan, Guang Yang, Yi-Gui Zhong, Cen-Yao Tang, Fa-Zhi Yang, Giao Ngoc Phan, Qiang-Tao Sui, Zhong Liu, Gang Li, Xiang-Gang Qiu, Qing-Hua Zhang, Jie Shen, Tian Qian, Li Lu, Lei Yan, Gen-Da Gu, and Hong Ding Chin. Phys. Lett.    2022, 39 (7): 077403 .   DOI: 10.1088/0256-307X/39/7/077403 Abstract   HTML   PDF (1360KB) As a typical hole-doped cuprate superconductor, Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8+\delta}$(Bi2212) carrier doping is mostly determined by its oxygen content. Traditional doping methods can regulate its doping level within the range of hole doping. Here we report the first application of CaH$_{2}$ annealing method in regulating the doping level of Bi2212. By continuously controlling the anneal time, a series of differently doped samples can be obtained. The combined experimental results of x-ray diffraction, scanning transmission electron microscopy, resistance and Hall measurements demonstrate that the CaH$_{2}$ induced topochemical reaction can effectively change the oxygen content of Bi2212 within a very wide range, even switching from hole doping to electron doping. We also found evidence of a low-$T_{\rm c}$ superconducting phase in the electron doping side.
 Select Novel and Self-Consistency Analysis of the QCD Running Coupling $\alpha_{\rm s}(Q)$ in Both the Perturbative and Nonperturbative Domains Qing Yu, Hua Zhou, Xu-Dong Huang, Jian-Ming Shen, and Xing-Gang Wu Chin. Phys. Lett.    2022, 39 (7): 071201 .   DOI: 10.1088/0256-307X/39/7/071201 Abstract   HTML   PDF (548KB) The quantum chromodynamics (QCD) coupling $\alpha_{\rm s}$ is the most important parameter for achieving precise QCD predictions. By using the well measured effective coupling $\alpha^{g_1}_{\rm s}(Q)$ defined from the Bjorken sum rules as a basis, we suggest a novel self-consistency way to fix the $\alpha_{\rm s}$ at all scales: The QCD light-front holographic model is adopted for its infrared behavior, and the fixed-order pQCD prediction under the principle of maximum conformality (PMC) is used for its high-energy behavior. Using the PMC scheme-and-scale independent perturbative series, and by transforming it into the one under the physical V scheme, we observe that a precise $\alpha_{\rm s}$ running behavior in both the perturbative and nonperturbative domains with a smooth transition from small to large scales can be achieved.
 Select Partially Diffusive Helium-Silica Compound under High Pressure Cong Liu, Junjie Wang, Xin Deng, Xiaomeng Wang, Chris J. Pickard, Ravit Helled, Zhongqing Wu, Hui-Tian Wang, Dingyu Xing, and Jian Sun Chin. Phys. Lett.    2022, 39 (7): 076101 .   DOI: 10.1088/0256-307X/39/7/076101 Abstract   HTML   PDF (2586KB) Helium is the second most abundant element in the universe, and together with silica, they are important components of giant planets. Exploring the reactivity and state of helium and silica under high pressure is crucial for understanding of the evolution and internal structure of giant planets. Here, using first-principles calculations and crystal structure predictions, we identify four stable phases of a helium-silica compound with seven/eight-coordinated silicon atoms at pressure of 600–4000 GPa, corresponding to the interior condition of the outer planets in the solar system. The density of HeSiO$_{2}$ agrees with current structure models of the planets. This helium-silica compound exhibits a superionic-like helium diffusive state under the high-pressure and high-temperature conditions along the isentropes of Saturn, a metallic fluid state in Jupiter, and a solid state in the deep interiors of Uranus and Neptune. These results show that helium may affect the erosion of the rocky core in giant planets and may help to form a diluted core region, which not only highlight the reactivity of helium under high pressure but also provide evidence helpful for building more sophisticated interior models of giant planets.
 Select Quantum Spin Liquid Phase in the Shastry–Sutherland Model Detected by an Improved Level Spectroscopic Method Ling Wang, Yalei Zhang, and Anders W. Sandvik Chin. Phys. Lett.    2022, 39 (7): 077502 .   DOI: 10.1088/0256-307X/39/7/077502 Abstract   HTML   PDF (841KB) We study the spin-$1/2$ two-dimensional Shastry–Sutherland spin model by exact diagonalization of clusters with periodic boundary conditions, developing an improved level spectroscopic technique using energy gaps between states with different quantum numbers. The crossing points of some of the relative (composite) gaps have much weaker finite-size drifts than the normally used gaps defined only with respect to the ground state, thus allowing precise determination of quantum critical points even with small clusters. Our results support the picture of a spin liquid phase intervening between the well-known plaquette-singlet and antiferromagnetic ground states, with phase boundaries in almost perfect agreement with a recent density matrix renormalization group study, where much larger cylindrical lattices were used [J. Yang et al., Phys. Rev. B 105, L060409 (2022)]. The method of using composite low-energy gaps to reduce scaling corrections has potentially broad applications in numerical studies of quantum critical phenomena.
 Select Coexistence of Quasi-two-dimensional Superconductivity and Tunable Kondo Lattice in a van der Waals Superconductor Shiwei Shen, Tian Qin, Jingjing Gao, Chenhaoping Wen, Jinghui Wang, Wei Wang, Jun Li, Xuan Luo, Wenjian Lu, Yuping Sun, and Shichao Yan Chin. Phys. Lett.    2022, 39 (7): 077401 .   DOI: 10.1088/0256-307X/39/7/077401 Abstract   HTML   PDF (4084KB) Realization of Kondo lattice in superconducting van der Waals materials not only provides a unique opportunity for tuning the Kondo lattice behavior by electrical gating or intercalation, but also is helpful for further understanding the heavy fermion superconductivity. Here we report a low-temperature and vector-magnetic-field scanning tunneling microscopy and spectroscopy study on a superconducting compound (4Hb-TaS$_{2})$ with alternate stacking of 1T-TaS$_{2}$ and 1H-TaS$_{2}$ layers. We observe the quasi-two-dimensional superconductivity in the 1H-TaS$_{2}$ layer with anisotropic response to the in-plane and out-of-plane magnetic fields. In the 1T-TaS$_{2}$ layer, we detect the Kondo resonance peak that results from the Kondo screening of the unpaired electrons in the Star-of-David clusters. We also find that the intensity of the Kondo resonance peak is sensitive to its relative position with the Fermi level, and it can be significantly enhanced when it is further shifted towards the Fermi level by evaporating Pb atoms onto the 1T-TaS$_{2}$ surface. Our results not only are important for fully understanding the electronic properties of 4Hb-TaS$_{2}$, but also pave the way for creating tunable Kondo lattice in the superconducting van der Waals materials.
 Select Resistance Anomaly and Linear Magnetoresistance in Thin Flakes of Itinerant Ferromagnet Fe$_{3}$GeTe$_{2}$ Honglei Feng, Yong Li, Youguo Shi, Hong-Yi Xie, Yongqing Li, and Yang Xu Chin. Phys. Lett.    2022, 39 (7): 077501 .   DOI: 10.1088/0256-307X/39/7/077501 Abstract   HTML   PDF (1679KB) Research interests in recent years have expanded into quantum materials that display novel magnetism incorporating strong correlations, topological effects, and dimensional crossovers. Fe$_{3}$GeTe$_{2}$ represents such a two-dimensional van der Waals platform exhibiting itinerant ferromagnetism with many intriguing properties. Up to date, most electronic transport studies on Fe$_{3}$GeTe$_{2}$ have been limited to its anomalous Hall responses while the longitudinal counterpart (such as magnetoresistance) remains largely unexplored. Here, we report a few unusual transport behaviors on thin flakes of Fe$_{3}$GeTe$_{2}$. Upon cooling to the base temperature, the sample develops a resistivity upturn that shows a crossover from a marginally $-\ln T$ to a ${-}{T}^{1/2}$ dependence, followed by a lower-temperature deviation. Moreover, we observe a negative and non-saturating linear magnetoresistance when the magnetization is parallel or antiparallel to the external magnetic field. The slope of the linear magnetoresistance also shows a nonmonotonic temperature dependence. We deduce an anomalous contribution to the magnetoresistance at low temperatures with a scaling function proportional ${-HT}^{1/2}$, as well as a temperature-independent linear term. Possible mechanisms that could account for our observations are discussed.
 Select Pressure-Driven Ne-Bearing Polynitrides with Ultrahigh Energy Density Lulu Liu, Shoutao Zhang, and Haijun Zhang Chin. Phys. Lett.    2022, 39 (5): 056102 .   DOI: 10.1088/0256-307X/39/5/056102 Abstract   HTML   PDF (3604KB) Neon (Ne) can reveal the evolution of planets, and nitrogen (N) is the most abundant element in the Earth's atmosphere. Considering the inertness of neon, whether nitrogen and neon can react has aroused great interest in condensed matter physics and space science. Here, we identify three new Ne–N compounds (i.e., NeN$_6$, NeN$_{10}$, and NeN$_{22}$) under pressure by first-principles calculations. We find that inserting Ne into N$_2$ substantially decreases the polymeric pressure of the nitrogen and promotes the formation of abundant polynitrogen structures. Especially, NeN$_{22}$ acquires a duplex host-guest structure, in which guest atoms (Ne and N$_2$ dimers) are trapped inside the crystalline host N$_{20}$ cages. Importantly, both NeN$_{10}$ and NeN$_{22}$ not only are dynamically and mechanically stable but also have a high thermal stability up to 500 K under ambient pressure. Moreover, ultra-high energy densities are obtained in NeN$_{10}$ (11.1 kJ/g), NeN$_{22}$ (11.5 kJ/g), tetragonal t-N$_{22}$ (11.6 kJ/g), and t-N$_{20}$ (12.0 kJ/g) produced from NeN$_{22}$, which are more than twice the value of trinitrotoluene (TNT). Meanwhile, their explosive performance is superior to that of TNT. Therefore, NeN$_{10}$, NeN$_{22}$, t-N$_{22}$, and t-N$_{20}$ are promising green high-energy-density materials. This work promotes the study of neon-nitrogen compounds with superior properties and potential applications.
 Select Unusual Band Splitting and Superconducting Gap Evolution with Sulfur Substitution in FeSe Yuanyuan Yang, Qisi Wang, Shaofeng Duan, Hongliang Wo, Chaozhi Huang, Shichong Wang, Lingxiao Gu, Dong Qian, Jun Zhao, and Wentao Zhang Chin. Phys. Lett.    2022, 39 (5): 057302 .   DOI: 10.1088/0256-307X/39/5/057302 Abstract   HTML   PDF (1243KB) High-resolution angle-resolved photoemission measurements were taken on FeSe$_{1-x}$S$_x$ ($x$ = 0, 0.04, and 0.08) superconductors. With an ultrahigh energy resolution of 0.4 meV, unusual two hole bands near the Brillouin-zone center, which was possibly a result of additional symmetry breaking, were identified in all the sulfur-substituted samples. In addition, in both of the hole bands highly anisotropic superconducting gaps with resolution limited nodes were evidenced. We find that the larger superconducting gap on the outer hole band is reduced linearly to the nematic transition temperature while the gap on the inner hole is nearly S-substitution independent. Our observations strongly suggest that the superconducting gap increases with enhanced nematicity although the superconducting transition temperature is not only governed by the pairing strength, demonstrating strong constraints on theories in the FeSe family.