## Volume 38 Issue 10

#### Highlighted Articles

Editors Suggestion

#### Realization of High-Fidelity Controlled-Phase Gates in Extensible Superconducting Qubits Design with a Tunable Coupler

Yangsen Ye, Sirui Cao, Yulin Wu, Xiawei Chen, Qingling Zhu, Shaowei Li, Fusheng Chen, Ming Gong, Chen Zha, He-Liang Huang, Youwei Zhao, Shiyu Wang, Shaojun Guo, Haoran Qian, Futian Liang, Jin Lin, Yu Xu, Cheng Guo, Lihua Sun, Na Li, Hui Deng, Xiaobo Zhu, and Jian-Wei Pan
Chin. Phys. Lett., 2021, 38 (10): 100301   PDF ( 349 )   HTML ( 494 )
Editors Suggestion

#### Prediction of an $\varOmega_{bbb}\varOmega_{bbb}$ Dibaryon in the Extended One-Boson Exchange Model

Ming-Zhu Liu and Li-Sheng Geng
Chin. Phys. Lett., 2021, 38 (10): 101201   PDF ( 116 )   HTML ( 217 )
Editors Suggestion

#### Effect of Oxide Content of Graphene Oxide Membrane on Remarkable Adsorption for Calcium Ions

Jie Jiang, Long Yan, and Haiping Fang
Chin. Phys. Lett., 2021, 38 (10): 106801   PDF ( 105 )   HTML ( 109 )
Editors Suggestion

#### Charge Density Wave and Electron-Phonon Interaction in Epitaxial Monolayer NbSe$_{2}$ Films

Xuedong Xie, Dongjing Lin, Li Zhu, Qiyuan Li, Junyu Zong, Wang Chen, Qinghao Meng, Qichao Tian, Shao-Chun Li, Xiaoxiang Xi, Can Wang, and Yi Zhang
Chin. Phys. Lett., 2021, 38 (10): 107101   PDF ( 400 )   HTML ( 702 )
Express Letter

#### High $T_{\rm c}$ Superconductivity in Heavy Rare Earth Hydrides

Hao Song, Zihan Zhang, Tian Cui, Chris J. Pickard, Vladimir Z. Kresin, and Defang Duan
Chin. Phys. Lett., 2021, 38 (10): 107401   PDF ( 901 )   HTML ( 744 )
Editors Suggestion

#### The 4$f$-Hybridization Strength in Ce$_m$$M$$_n$In$_{3m+2n}$ Heavy-Fermion Compounds Studied by Angle-Resolved Photoemission Spectroscopy

Jiao-Jiao Song, Yang Luo, Chen Zhang, Qi-Yi Wu, Tomasz Durakiewicz, Yasmine Sassa, Oscar Tjernberg, Martin Månsson, Magnus H. Berntsen, Yin-Zou Zhao, Hao Liu, Shuang-Xing Zhu, Zi-Teng Liu, Fan-Ying Wu, Shu-Yu Liu, Eric D. Bauer, Ján Rusz, Peter M. Oppeneer, Ya-Hua Yuan, Yu-Xia Duan, and Jian-Qiao Meng
Chin. Phys. Lett., 2021, 38 (10): 107402   PDF ( 187 )   HTML ( 234 )
Editors Suggestion

#### Photoemission Spectroscopic Evidence of Multiple Dirac Cones in Superconducting BaSn$_3$

Zhe Huang, Xianbiao Shi, Gaoning Zhang, Zhengtai Liu, Soohyun Cho, Zhicheng Jiang, Zhonghao Liu, Jishan Liu, Yichen Yang, Wei Xia, Weiwei Zhao, Yanfeng Guo, and Dawei Shen
Chin. Phys. Lett., 2021, 38 (10): 107403   PDF ( 91 )   HTML ( 157 )
Editors Suggestion

#### Detection of Magnetic Gap in Topological Surface States of MnBi$_{2}$Te$_{4}$

Hao-Ran Ji, Yan-Zhao Liu, He Wang, Jia-Wei Luo, Jia-Heng Li, Hao Li, Yang Wu, Yong Xu, and Jian Wang
Chin. Phys. Lett., 2021, 38 (10): 107404   PDF ( 369 )   HTML ( 342 )
Editors Suggestion

#### Updated Progresses in Perovskite Solar Cells

Zihan Qu, Fei Ma, Yang Zhao, Xinbo Chu, Shiqi Yu, and Jingbi You
Chin. Phys. Lett., 2021, 38 (10): 107801   PDF ( 420 )   HTML ( 245 )

#### Current Issue

GENERAL
 Select Realization of High-Fidelity Controlled-Phase Gates in Extensible Superconducting Qubits Design with a Tunable Coupler Yangsen Ye, Sirui Cao, Yulin Wu, Xiawei Chen, Qingling Zhu, Shaowei Li, Fusheng Chen, Ming Gong, Chen Zha, He-Liang Huang, Youwei Zhao, Shiyu Wang, Shaojun Guo, Haoran Qian, Futian Liang, Jin Lin, Yu Xu, Cheng Guo, Lihua Sun, Na Li, Hui Deng, Xiaobo Zhu, and Jian-Wei Pan Chin. Phys. Lett. 2021, 38 (10): 100301 .   DOI: 10.1088/0256-307X/38/10/100301 Abstract   PDF(pc) (1325KB) ( 349 ) PDF(mobile)(1458KB) ( 37 ) HTML ( 494 )  High-fidelity two-qubit gates are essential for the realization of large-scale quantum computation and simulation. Tunable coupler design is used to reduce the problem of parasitic coupling and frequency crowding in many-qubit systems and thus thought to be advantageous. Here we design an extensible 5-qubit system in which center transmon qubit can couple to every four near-neighboring qubits via a capacitive tunable coupler and experimentally demonstrate high-fidelity controlled-phase (CZ) gate by manipulating central qubit and one near-neighboring qubit. Speckle purity benchmarking and cross entropy benchmarking are used to assess the purity fidelity and the fidelity of the CZ gate. The average purity fidelity of the CZ gate is 99.69$\pm 0.04$% and the average fidelity of the CZ gate is 99.65$\pm 0.04$%, which means that the control error is about 0.04%. Our work is helpful for resolving many challenges in implementation of large-scale quantum systems.
THE PHYSICS OF ELEMENTARY PARTICLES AND FIELDS
 Select Prediction of an $\varOmega_{bbb}\varOmega_{bbb}$ Dibaryon in the Extended One-Boson Exchange Model Ming-Zhu Liu and Li-Sheng Geng Chin. Phys. Lett. 2021, 38 (10): 101201 .   DOI: 10.1088/0256-307X/38/10/101201 Abstract   PDF(pc) (812KB) ( 116 ) PDF(mobile)(1395KB) ( 9 ) HTML ( 217 )  Since Yukawa proposed that the pion is responsible for mediating the nucleon-nucleon interaction, meson exchanges have been widely used in understanding hadron-hadron interactions. The most studied mesons are the $\sigma$, $\pi$, $\rho$, and $\omega$, while other heavier mesons are often argued to be less relevant because they lead to short range interactions. However, whether the range of interactions is short or long should be judged with respect to the size of the system studied. We propose that one charmonium exchange is responsible for the formation of the $\varOmega_{ccc}\varOmega_{ccc}$ dibaryon, recently predicted by lattice QCD simulations. The same approach can be extended to the strangeness and bottom sectors, leading to the prediction on the existence of $\varOmega\varOmega$ and $\varOmega_{bbb}\varOmega_{bbb}$ dibaryons, while the former is consistent with the existing lattice QCD results, the latter remains to checked. In addition, we show that the Coulomb interaction may break up the $\varOmega_{ccc}\varOmega_{ccc}$ pair but not the $\varOmega_{bbb}\varOmega_{bbb}$ and $\varOmega\varOmega$ dibaryons.
ATOMIC AND MOLECULAR PHYSICS
 Select Production of $^{87}$Rb Bose–Einstein Condensate in an Asymmetric Crossed Optical Dipole Trap Zhu Ma, Chengyin Han, Xunda Jiang, Ruihuan Fang, Yuxiang Qiu, Minhua Zhao, Jiahao Huang, Bo Lu, and Chaohong Lee Chin. Phys. Lett. 2021, 38 (10): 103701 .   DOI: 10.1088/0256-307X/38/10/103701 Abstract   PDF(pc) (1175KB) ( 120 ) PDF(mobile)(1277KB) ( 5 ) HTML ( 145 )  We report the production of $^{87}$Rb Bose–Einstein condensate in an asymmetric crossed optical dipole trap (ACODT) without the need of an additional dimple laser. In our experiment, the ACODT is formed by two laser beams with different radii to achieve efficient capture and rapid evaporation of laser cooled atoms. Compared to the cooling procedure in a magnetic trap, the atoms are firstly laser cooled and then directly loaded into an ACODT without the pre-evaporative cooling process. In order to determine the optimal parameters for evaporation cooling, we optimize the power ratio of the two beams and the evaporation time to maximize the final atom number left in the ACODT. By loading about $6\times10^{5}$ laser cooled atoms in the ACODT, we obtain a pure Bose–Einstein condensate with about $1.4\times10^{4}$ atoms after 19 s evaporation. Additionally, we demonstrate that the fringe-type noises in optical density distributions can be reduced via principal component analysis, which correspondingly improves the reliability of temperature measurement.
CONDENSED MATTER: STRUCTURE, MECHANICAL AND THERMAL PROPERTIES
 Select Observation of the Pinning-Induced Crystal-Hexatic-Glass Transition in Two-Dimensional Colloidal Suspensions Xiaoyan Sun, Huaguang Wang, Hao Feng, Zexin Zhang, and Yuqiang Ma Chin. Phys. Lett. 2021, 38 (10): 106101 .   DOI: 10.1088/0256-307X/38/10/106101 Abstract   PDF(pc) (1745KB) ( 82 ) PDF(mobile)(2032KB) ( 5 ) HTML ( 89 )  Identification of the glass formation process in various conditions is of importance for fundamental understanding of the mechanism of glass transitions as well as for developments and applications of glassy materials. We investigate the role of pinning in driving the transformation of crystal into glass in two-dimensional colloidal suspensions of monodisperse microspheres. The pinning is produced by immobilizing a fraction of microspheres on the substrate of sample cells where the mobile microspheres sediment. Structurally, the crystal-hexatic-glass transition occurs with increasing the number fraction of pinning $\rho_{\rm pinning}$, and the orientational correlation exhibits a change from quasi-long-range to short-range order at $\rho_{\rm pinning} = 0.02$. Interestingly, the dynamics shows a non-monotonic change with increasing the fraction of pinning. This is due to the competition between the disorder that enhances the dynamics and the pinning that hinders the particle motions. Our work highlights the important role of the pinning on the colloidal glass transition, which not only provides a new strategy to prevent crystallization forming glass, but also is helpful for understanding of the vitrification in colloidal systems.
 Select Effect of Oxide Content of Graphene Oxide Membrane on Remarkable Adsorption for Calcium Ions Jie Jiang, Long Yan, and Haiping Fang Chin. Phys. Lett. 2021, 38 (10): 106801 .   DOI: 10.1088/0256-307X/38/10/106801 Abstract   PDF(pc) (1138KB) ( 105 ) PDF(mobile)(1579KB) ( 3 ) HTML ( 109 )  Graphene oxide membranes (GOMs), as one of the most promising novel materials, have gained great interest in the field of adsorption. However, the oxygen content of graphene oxide is directly related to its adsorption properties, such as suspension stability, adsorption capacity, and reusability of GOMs. Here, a series of reduced GOMs with oxygen content from 28% to 12% were conveniently prepared by the thermally reduced and the corresponding interlayer spacing of these membranes changed from 8.0 Å to 3.7 Å. These prepared GOMs have remarkable Ca$^{2+}$ adsorption capacity, which increases with the oxygen content or interlayer spacing of GOMs. Importantly, the max adsorption capacity of the mass ratio between adsorbed Ca$^{2+}$ and pristine GOMs can reach up to 0.481 g/g, which is about one order of magnitude higher than the adsorption capacity of activated sludge, magnetic Fe$_{3}$O$_{4}$, functionalized silica, zeolite molecular sieve, and other reported previously. Moreover, GOMs show excellent stability and the Ca$^{2+}$ can be easily desorbed by water, so that the GOMs can be reused. Our previous theoretical analysis suggests that this remarkable adsorption is attributable to the strong interactions between Ca$^{2+}$ and GO sheets, including the ion-$\pi$ interactions between Ca$^{2+}$ and aromatic graphitic rings as well as the electrostatic interaction between Ca$^{2+}$ and oxygen-containing groups.
CONDENSED MATTER: ELECTRONIC STRUCTURE, ELECTRICAL, MAGNETIC, AND OPTICAL PROPERTIES
 Select Charge Density Wave and Electron-Phonon Interaction in Epitaxial Monolayer NbSe$_{2}$ Films Xuedong Xie, Dongjing Lin, Li Zhu, Qiyuan Li, Junyu Zong, Wang Chen, Qinghao Meng, Qichao Tian, Shao-Chun Li, Xiaoxiang Xi, Can Wang, and Yi Zhang Chin. Phys. Lett. 2021, 38 (10): 107101 .   DOI: 10.1088/0256-307X/38/10/107101 Abstract   PDF(pc) (7500KB) ( 400 ) PDF(mobile)(7964KB) ( 45 ) HTML ( 702 )  Understanding the interplay between superconductivity and charge-density wave (CDW) in NbSe$_{2}$ is vital for both fundamental physics and future device applications. Here, combining scanning tunneling microscopy, angle-resolved photoemission spectroscopy and Raman spectroscopy, we study the CDW phase in the monolayer NbSe$_{2}$ films grown on various substrates of bilayer graphene (BLG), SrTiO$_{3}$(111), and Al$_{2}$O$_{3}$(0001). It is found that the two stable CDW states of monolayer NbSe$_{2}$ can coexist on NbSe$_{2}$/BLG surface at liquid-nitrogen temperature. For the NbSe$_{2}$/SrTiO$_{3}$(111) sample, the unidirectional CDW regions own the kinks at $\pm 41$ meV and a wider gap at 4.2 K. It is revealed that the charge transfer from the substrates to the grown films will influence the configurations of the Fermi surface, and induce a 130 meV lift-up of the Fermi level with a shrink of the Fermi pockets in NbSe$_{2}$/SrTiO$_{3}$(111) compared with the NbSe$_{2}$/BLG. Combining the temperature-dependent Raman experiments, we suggest that the electron-phonon coupling in monolayer NbSe$_{2}$ dominates its CDW phase transition.
 Select High $T_{\rm c}$ Superconductivity in Heavy Rare Earth Hydrides Hao Song, Zihan Zhang, Tian Cui, Chris J. Pickard, Vladimir Z. Kresin, and Defang Duan Chin. Phys. Lett. 2021, 38 (10): 107401 .   DOI: 10.1088/0256-307X/38/10/107401 Abstract   PDF(pc) (3378KB) ( 901 ) PDF(mobile)(6825KB) ( 80 ) HTML ( 744 )  Sulfur and lanthanum hydrides under compression display superconducting states with high observed critical temperatures. It has been recently demonstrated that carbonaceous sulfur hydride displays room temperature superconductivity. However, this phenomenon has been observed only at very high pressure. Here, we theoretically search for superconductors with very high critical temperatures, but at much lower pressures. We describe two of such sodalite-type clathrate hydrides, YbH$_{6}$ and LuH$_{6}$. These hydrides are metastable and are predicted to superconduct with $T_{\rm c} \sim 145$ K at 70 GPa and $T_{\rm c} \sim 273$ K at 100 GPa, respectively. This striking result is a consequence of the strong interrelationship between the $f$ states present at the Fermi level, structural stability, and the final $T_{\rm c}$ value. For example, TmH$_{6}$, with unfilled 4$f$ orbitals, is stable at 50 GPa, but has a relatively low value of $T_{\rm c}$ of 25 K. The YbH$_{6}$ and LuH$_{6}$ compounds, with their filled $f$-shells, exhibit prominent phonon “softening”, which leads to a strong electron-phonon coupling, and as a result, an increase in $T_{\rm c}$.
 Select The 4$f$-Hybridization Strength in Ce$_m$$M$$_n$In$_{3m+2n}$ Heavy-Fermion Compounds Studied by Angle-Resolved Photoemission Spectroscopy Jiao-Jiao Song, Yang Luo, Chen Zhang, Qi-Yi Wu, Tomasz Durakiewicz, Yasmine Sassa, Oscar Tjernberg, Martin Månsson, Magnus H. Berntsen, Yin-Zou Zhao, Hao Liu, Shuang-Xing Zhu, Zi-Teng Liu, Fan-Ying Wu, Shu-Yu Liu, Eric D. Bauer, Ján Rusz, Peter M. Oppeneer, Ya-Hua Yuan, Yu-Xia Duan, and Jian-Qiao Meng Chin. Phys. Lett. 2021, 38 (10): 107402 .   DOI: 10.1088/0256-307X/38/10/107402 Abstract   PDF(pc) (794KB) ( 187 ) PDF(mobile)(886KB) ( 11 ) HTML ( 234 )  We systemically investigate the nature of Ce 4$f$ electrons in structurally layered heavy-fermion compounds Ce$_m$$M$$_n$In$_{3m+2n}$ (with $M$ = Co, Rh, Ir, and Pt, $m=1$, 2, $n=0$–2), at low temperature using on-resonance angle-resolved photoemission spectroscopy. Three heavy quasiparticle bands $f^0$, $f^1_{7/2}$ and $f^1_{5/2}$, are observed in all compounds, whereas their intensities and energy locations vary greatly with materials. The strong $f^0$ states imply that the localized electron behavior dominates the Ce 4$f$ states. The Ce 4$f$ electrons are partially hybridized with the conduction electrons, making them have the dual nature of localization and itinerancy. Our quantitative comparison reveals that the $f^1_{5/2}$–$f^0$ intensity ratio is more suitable to reflect the $4f$-state hybridization strength.
 Select Photoemission Spectroscopic Evidence of Multiple Dirac Cones in Superconducting BaSn$_3$ Zhe Huang, Xianbiao Shi, Gaoning Zhang, Zhengtai Liu, Soohyun Cho, Zhicheng Jiang, Zhonghao Liu, Jishan Liu, Yichen Yang, Wei Xia, Weiwei Zhao, Yanfeng Guo, and Dawei Shen Chin. Phys. Lett. 2021, 38 (10): 107403 .   DOI: 10.1088/0256-307X/38/10/107403 Abstract   PDF(pc) (9195KB) ( 91 ) PDF(mobile)(9540KB) ( 3 ) HTML ( 157 )  Signatures of topological superconductivity (TSC) in superconducting materials with topological nontrivial states prompt intensive researches recently. Utilizing high-resolution angle-resolved photoemission spectroscopy and first-principles calculations, we demonstrate multiple Dirac fermions and surface states in superconductor BaSn$_3$ with a critical transition temperature of about 4.4 K. We predict and then unveil the existence of two pairs of type-I topological Dirac fermions residing on the rotational axis. Type-II Dirac fermions protected by screw axis are confirmed in the same compound. Further calculation for the spin helical texture of the observed surface states originating from the Dirac fermions gives an opportunity for realization of TSC in one single material. Hosting multiple Dirac fermions and topological surface states, the intrinsic superconductor BaSn$_3$ is expected to be a new platform for further investigation of topological quantum materials as well as TSC.
 Select Detection of Magnetic Gap in Topological Surface States of MnBi$_{2}$Te$_{4}$ Hao-Ran Ji, Yan-Zhao Liu, He Wang, Jia-Wei Luo, Jia-Heng Li, Hao Li, Yang Wu, Yong Xu, and Jian Wang Chin. Phys. Lett. 2021, 38 (10): 107404 .   DOI: 10.1088/0256-307X/38/10/107404 Abstract   PDF(pc) (3789KB) ( 369 ) PDF(mobile)(6051KB) ( 34 ) HTML ( 342 )  Recently, intrinsic antiferromagnetic topological insulator MnBi$_{2}$Te$_{4}$ has drawn intense research interest and leads to plenty of significant progress in physics and materials science by hosting quantum anomalous Hall effect, axion insulator state, and other quantum phases. An essential ingredient to realize these quantum states is the magnetic gap in the topological surface states induced by the out-of-plane ferromagnetism on the surface of MnBi$_{2}$Te$_{4}$. However, the experimental observations of the surface gap remain controversial. Here, we report the observation of the surface gap via the point contact tunneling spectroscopy. In agreement with theoretical calculations, the gap size is around 50 meV, which vanishes as the sample becomes paramagnetic with increasing temperature. The magnetoresistance hysteresis is detected through the point contact junction on the sample surface with an out-of-plane magnetic field, substantiating the surface ferromagnetism. Furthermore, the non-zero transport spin polarization coming from the ferromagnetism is determined by the point contact Andreev reflection spectroscopy. Combining these results, the magnetism-induced gap in topological surface states of MnBi$_{2}$Te$_{4}$ is revealed.
 Select Updated Progresses in Perovskite Solar Cells Zihan Qu, Fei Ma, Yang Zhao, Xinbo Chu, Shiqi Yu, and Jingbi You Chin. Phys. Lett. 2021, 38 (10): 107801 .   DOI: 10.1088/0256-307X/38/10/107801 Abstract   PDF(pc) (6140KB) ( 420 ) PDF(mobile)(6245KB) ( 45 ) HTML ( 245 )  In the last decade, perovskite solar cells (PSCs) have greatly drawn researchers' attention, with the power conversion efficiency surging from 3.8% to 25.5%. PSCs possess the merits of low cost, simple fabrication process and high performance, which could be one of the most promising photovoltaic technologies in the future. In this review, we focus on the summary of the updated progresses in single junction PSCs including efficiency, stability and large area module. Then, the important progresses in tandem solar cells are briefly discussed. A prospect into the future of the field is also included.