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 Select Performance Enhancement of AlGaN/GaN MIS-HEMTs Realized via Supercritical Nitridation Technology Meihua Liu , Zhangwei Huang , Kuanchang Chang , Xinnan Lin , Lei Li , and Yufeng Jin Chin. Phys. Lett.    2020, 37 (9): 097101 .   DOI: 10.1088/0256-307X/37/9/097101 Abstract   HTML   PDF (909KB) This paper proposes a method of repairing interface defects by supercritical nitridation technology, in order to suppress the threshold voltage shift of AlGaN/GaN metal-insulator-semiconductor high-electron-mobility transistors (MIS-HEMTs). We find that supercritical NH$_{3}$ fluid has the characteristics of both liquid NH$_{3}$ and gaseous NH$_{3}$ simultaneously, i.e., high penetration and high solubility, which penetrate the packaging of MIS-HEMTs. In addition, NH$_{2}^{-}$ produced via the auto coupling ionization of NH$_{3}$ has strong nucleophilic ability, and is able to fill nitrogen vacancies near the GaN surface created by high temperature processes. After supercritical fluid treatment, the threshold voltage shift is reduced from 1 V to 0 V, and the interface trap density is reduced by two orders of magnitude. The results show that the threshold voltage shift of MIS-HEMTs can be effectively suppressed by means of supercritical nitridation technology.
 Select Chalcogenide Perovskite YScS$_{3}$ as a Potential p-Type Transparent Conducting Material Han Zhang, Chen Ming, Ke Yang, Hao Zeng, Shengbai Zhang, and Yi-Yang Sun Chin. Phys. Lett.    2020, 37 (9): 097201 .   DOI: 10.1088/0256-307X/37/9/097201 Abstract   HTML   PDF (2691KB) Transparent conducting materials (TCMs) have been widely used in optoelectronic applications such as touchscreens, flat panel displays and thin film solar cells. These applications of TCMs are currently dominated by n-type doped oxides. High-performance p-type TCMs are still lacking due to their low hole mobility or p-type doping bottleneck, which impedes efficient device design and novel applications such as transparent electronics. Here, based on first-principles calculations, we propose chalcogenide perovskite YScS$_{3}$ as a promising p-type TCM. According to our calculations, its optical absorption onset is above 3 eV, which allows transparency to visible light. Its hole conductivity effective mass is 0.48$m_{0}$, which is among the smallest in p-type TCMs, suggesting enhanced hole mobility. It could be doped to p-type by group-II elements on cation sites, all of which yield shallow acceptors. Combining these properties, YScS$_{3}$ holds great promise to enhancing the performance of p-type TCMs toward their n-type counterparts.
 Select Model Hamiltonian for the Quantum Anomalous Hall State in Iron-Halogenide Qian Sui, Jiaxin Zhang, Suhua Jin, Yunyouyou Xia, and Gang Li Chin. Phys. Lett.    2020, 37 (9): 097301 .   DOI: 10.1088/0256-307X/37/9/097301 Abstract   HTML   PDF (1918KB) We examine quantum anomalous Hall (QAH) insulators with intrinsic magnetism displaying quantized Hall conductance at zero magnetic fields. The spin-momentum locking of the topological edge stats promises QAH insulators with great potential in device applications in the field of spintronics. Here, we generalize Haldane's model on the honeycomb lattice to a more realistic two-orbital case without the artificial real-space complex hopping. Instead, we introduce an intraorbital coupling, stemming directly from the local spin-orbit coupling (SOC). Our $d_{xy}/d_{x^{2}-y^{2}}$ model may be viewed as a generalization of the bismuthene $p_{x}/p_{y}$-model for correlated $d$-orbitals. It promises a large SOC gap, featuring a high operating temperature. This two-orbital model nicely explains the low-energy excitation and the topology of two-dimensional ferromagnetic iron-halogenides. Furthermore, we find that electronic correlations can drive the QAH states to a $c=0$ phase, in which every band carries a nonzero Chern number. Our work not only provides a realistic QAH model, but also generalizes the nontrivial band topology to correlated orbitals, which demonstrates an exciting topological phase transition driven by Coulomb repulsions. Both the model and the material candidates provide excellent platforms for future study of the interplay between electronic correlations and nontrivial band topology.
 Select A New Quasi-One-Dimensional Ternary Molybdenum Pnictide Rb$_{2}$Mo$_{3}$As$_{3}$ with Superconducting Transition at 10.5 K Kang Zhao, Qing-Ge Mu, Bin-Bin Ruan, Meng-Hu Zhou, Qing-Song Yang, Tong Liu, Bo-Jin Pan, Shuai Zhang, Gen-Fu Chen, and Zhi-An Ren Chin. Phys. Lett.    2020, 37 (9): 097401 .   DOI: 10.1088/0256-307X/37/9/097401 Abstract   HTML   PDF (1351KB) We report superconductivity in a new ternary molybdenum pnictide Rb$_{2}$Mo$_{3}$As$_{3}$, synthesized via the solid state reaction method. Powder x-ray diffraction analysis reveals a hexagonal crystal structure with space group $P\bar{6}m2$ (No. 187), and the refined lattice parameters are $a = 10.431(5)$ Å, $c = 4.460(4)$ Å. SEM images show rod-like grains with good ductility, confirming a quasi-one-dimensional (Q1D) structure. Electrical resistivity and dc magnetic susceptibility characterizations exhibit superconductivity with an onset of $T_{\rm c}=10.5$ K. The upper critical field of Rb$_{2}$Mo$_{3}$As$_{3}$ is estimated to be 28.2 T at zero temperature, providing an evidence of possible unconventional superconductivity. Our recent discovery of MoAs-based superconductors above 10 K provides a unique platform for the study of exotic superconductivity in $4d$ electron systems with Q1D crystal structures.
 Select Tunable Superconductivity in 2H-NbSe$_{2}$ via $\boldsymbol In~Situ$ Li Intercalation Kaiyao Zhou, Jun Deng, Liwei Guo, and Jiangang Guo Chin. Phys. Lett.    2020, 37 (9): 097402 .   DOI: 10.1088/0256-307X/37/9/097402 Abstract   HTML   PDF (1354KB) Using the newly-developed solid ionic gating technique, we measure the electrical transport property of a thin-flake NbSe$_{2}$ superconductor ($T_{\rm c} = 6.67$ K) under continuous Li intercalation and electron doping. It is found that the charge-density-wave transition is suppressed, while at the same time a carrier density, decreasing from $7\times 10^{14}$ cm$^{-2}$ to $2\times 10^{14}$ cm$^{-2}$ also occurs. This tunable capability in relation to carrier density is 70%, which is 5 times larger than that found using the liquid ionic gating method [Phys. Rev. Lett. 117 (2016) 106801]. Meanwhile, we find that the scattering type of conduction electrons transits to the $s$–$d$ process, which may be caused by the change of the occupied states of 4$d$-electrons in Nb under the condition of Li intercalation. Simultaneously, we observe a certain decrement of electron-phonon coupling (EPC), based on the electron-phonon scattering model, in the high temperature range. Based on data gathered from in situ measurements, we construct a full phase diagram of carrier density, EPC and $T_{\rm c}$ in the intercalated NbSe$_{2}$ sample, and qualitatively explain the variation of $T_{\rm c}$ within the BCS framework. It is our opinion that the in situ solid ionic gating method provides a direct route to describing the relationship between carrier density and superconductivity, which is helpful in promoting a clearer understanding of electronic phase competition in transition metal dichalcogenides.
 Select Pressure-Dependent Point-Contact Spectroscopy of Superconducting PbTaSe$_2$ Single Crystals Hai Zi, Ling-Xiao Zhao, Xing-Yuan Hou, Lei Shan, Zhian Ren, Gen-Fu Chen, and Cong Ren Chin. Phys. Lett.    2020, 37 (9): 097403 .   DOI: 10.1088/0256-307X/37/9/097403 Abstract   HTML   PDF (5795KB) We develop an experimental tool to investigate the order parameter of superconductors by combining point-contact spectroscopy measurement with high-pressure technique. It is demonstrated for the first time that planar point-contact spectroscopy measurement on noncentrosymmetric superconducting PbTaSe$_2$ single crystals is systematically subjected to hydrostatic pressures up to 12.1 kbar. Under such a high pressure, the normal-state contact resistance is sensitive to the applied pressure, reflecting the underlying variation of contact transparency upon pressures. In a superconducting state, the pressure dependence of the energy gap $\varDelta_0$ and the critical temperature $T_{\rm c}$ for gap opening/closing are extracted based on a generalized Blond–Tinkham–Klapwijk model. The gap ratio $2\varDelta_0/k_{_{\rm B}}T_{\rm c}$ indicates a crossover from weak coupling to strong coupling in electron pairing strength upon pressure for PbTaSe$_2$. Our experimental results show the accessibility and validity of high-pressure point-contact spectroscopy, offering rich information about high-pressure superconductivity.
 Select Mott Transition and Superconductivity in Quantum Spin Liquid Candidate NaYbSe$_{2}$ Ya-Ting Jia, Chun-Sheng Gong, Yi-Xuan Liu, Jian-Fa Zhao, Cheng Dong, Guang-Yang Dai, Xiao-Dong Li, He-Chang Lei, Run-Ze Yu, Guang-Ming Zhang, and Chang-Qing Jin Chin. Phys. Lett.    2020, 37 (9): 097404 .   DOI: 10.1088/0256-307X/37/9/097404 Abstract   HTML   PDF (1103KB) The Mott transition is one of the fundamental issues in condensed matter physics, especially in the system with antiferromagnetic long-range order. However, such a transition is rare in quantum spin liquid (QSL) systems without long-range order. Here we report the experimental pressure-induced insulator to metal transition followed by the emergence of superconductivity in the QSL candidate NaYbSe$_{2}$ with a triangular lattice of 4$f$ Yb$^{3+}$ ions. Detail analysis of transport properties in metallic state shows an evolution from non-Fermi liquid to Fermi liquid behavior when approaching the vicinity of superconductivity. An irreversible structure phase transition occurs around 11 GPa, which is revealed by the x-ray diffraction. These results shed light on the Mott transition in the QSL systems.
 Select Phase-Gradient Metasurfaces Based on Local Fabry–Pérot Resonances Yanyan Cao, Bocheng Yu, Yangyang Fu, Lei Gao, and Yadong Xu Chin. Phys. Lett.    2020, 37 (9): 097801 .   DOI: 10.1088/0256-307X/37/9/097801 Abstract   HTML   PDF (965KB) In this work, we present a new mechanism for designing phase-gradient metasurfaces (PGMs) to control an electromagnetic wavefront with high efficiency. Specifically, we design a transmission-type PGM, formed by a periodic subwavelength metallic slit array filled with identical dielectrics of different heights. It is found that when Fabry–Pérot (FP) resonances occur locally inside the dielectric regions, in addition to the common phenomenon of complete transmission, the transmitted phase differences between two adjacent slits are exactly the same, being a nonzero constant. These local FP resonances ensure total phase shift across a supercell, fully covering a range of 0 to $2\pi$, satisfying the design requirements of PGMs. Further research reveals that, due to local FP resonances, there is a one-to-one correspondence between the phase difference and the permittivity of the filled dielectric. A similar approach can be extended to the reflection-type case and other wavefront transformations, creating new opportunities for wave manipulation.
 Select Ultrafast Quasiparticle Dynamics and Electron-Phonon Coupling in (Li$_{0.84}$Fe$_{0.16}$)OHFe$_{0.98}$Se Qiong Wu, Huaxue Zhou, Yanling Wu, Lili Hu, Shunli Ni, Yichao Tian, Fei Sun, Fang Zhou, Xiaoli Dong, Zhongxian Zhao, and Jimin Zhao Chin. Phys. Lett.    2020, 37 (9): 097802 .   DOI: 10.1088/0256-307X/37/9/097802 Abstract   HTML   PDF (3320KB) Distinctive superconducting behaviors between bulk and monolayer FeSe make it challenging to obtain a unified picture of all FeSe-based superconductors. We investigate the ultrafast quasiparticle (QP) dynamics of an intercalated superconductor (Li$_{1-x}$Fe$_{x}$)OHFe$_{1-y}$Se, which is a bulk crystal but shares a similar electronic structure with single-layer FeSe on SrTiO$_{3}$. We obtain the electron-phonon coupling (EPC) constant $\lambda_{{A}_{\rm 1g}}$ ($0.22 \pm 0.04$), which well bridges that of bulk FeSe crystal and single-layer FeSe on SrTiO$_{3}$. Significantly, we find that such a positive correlation between $\lambda_{{A}_{\rm 1g}}$ and superconducting $T_{\rm c}$ holds among all known FeSe-based superconductors, even in line with reported FeAs-based superconductors. Our observation indicates possible universal role of EPC in the superconductivity of all known categories of iron-based superconductors, which is a critical step towards achieving a unified superconducting mechanism for all iron-based superconductors.
 Select de Haas–van Alphen Quantum Oscillations in BaSn$_{3}$ Superconductor with Multiple Dirac Fermions Gaoning Zhang, Xianbiao Shi, Xiaolei Liu, Wei Xia, Hao Su, Leiming Chen, Xia Wang, Na Yu, Zhiqiang Zou, Weiwei Zhao, and Yanfeng Guo Chin. Phys. Lett.    2020, 37 (8): 087101 .   DOI: 10.1088/0256-307X/37/8/087101 Abstract   HTML   PDF (2436KB) Characterization of Fermi surface of the BaSn$_{3}$ superconductor ($T_{\rm c} \sim 4.4$ K) by de Haas–van Alphen (dHvA) effect measurement reveals its non-trivial topological properties. Analysis of non-zero Berry phase is supported by the ab initio calculations, which reveals a type-II Dirac point setting and tilting along the high symmetric $K$–$H$ line of the Brillouin zone, about 0.13 eV above the Fermi level, and other two type-I Dirac points on the high symmetric $\varGamma$–$A$ direction, but slightly far below the Fermi level. The results demonstrate BaSn$_{3}$ as an excellent example hosting multiple Dirac fermions and an outstanding platform for studying the interplay between nontrivial topological states and superconductivity.
 Select Experimental Observation of Electronic Structures of Kagome Metal YCr$_{6}$Ge$_{6}$ Pengdong Wang, Yihao Wang, Bo Zhang, Yuliang Li, Sheng Wang, Yunbo Wu, Hongen Zhu, Yi Liu, Guobin Zhang, Dayong Liu, Yimin Xiong, and Zhe Sun Chin. Phys. Lett.    2020, 37 (8): 087102 .   DOI: 10.1088/0256-307X/37/8/087102 Abstract   HTML   PDF (1217KB) Using angle-resolved photoemission spectroscopy, we study electronic structures of a Kagome metal YCr$_{6}$Ge$_{6}$. Band dispersions along $k_{z}$ direction are significant, suggesting a remarkable interlayer coupling between neighboring Kagome planes. Comparing ARPES data with first-principles calculations, we find a moderate electron correlation in this material, since band calculations must be compressed in the energy scale to reach an excellent agreement between experimental data and theoretical calculations. Moreover, as indicated by band calculations, there is a flat band in the vicinity of the Fermi level at the $\varGamma$–$M$–$K$ plane in the momentum space, which could be responsible for the unusual transport behavior in YCr$_{6}$Ge$_{6}$.
 Select Type-II Dirac Semimetal State in a Superconductor Tantalum Carbide Zhihai Cui, Yuting Qian, Wei Zhang, Hongming Weng, and Zhong Fang Chin. Phys. Lett.    2020, 37 (8): 087103 .   DOI: 10.1088/0256-307X/37/8/087103 Abstract   HTML   PDF (1021KB) The exploration of topological Dirac semimetals with intrinsic superconductivity can be a most plausible way to discover topological superconductors. We propose that type-II Dirac semimetal states exist in the band structure of TaC, a well-known s-wave superconductor, by using the first-principles calculations and the ${\boldsymbol{k} \cdot {\boldsymbol p}}$ effective model. The tilted gapless Dirac cones, which are composed of Ta $d$ and C $p$ orbitals and are protected by $C_{4v}$ symmetry, are found to be below the Fermi level. The bands from Ta $d$ orbitals are greatly coupled with the acoustic modes around the zone boundary, indicating their significant contribution to the superconductivity. The relatively high transition temperature $\sim$10.5 K is estimated to be consistent with the experimental data. To bring the type-II Dirac points close to chemical potential, hole doping is needed. This seems to decrease the transition temperature a lot, making the realization of topological superconductivity impossible.
 Select Predicting the Potential Performance in P-Type SnS Crystals via Utilizing the Weighted Mobility and Quality Factor Wenke He , Bingchao Qin , and Li-Dong Zhao Chin. Phys. Lett.    2020, 37 (8): 087104 .   DOI: 10.1088/0256-307X/37/8/087104 Abstract   HTML   PDF (774KB) The figure of merit $ZT$ is the direct embodiment of thermoelectric performance for a given material. However, as an indicator of performance improvement, the only $ZT$ value is not good enough to identify its outstanding inherent properties, which are highly sought in thermoelectric community. Here, we utilize one powerful parameter to reveal the outstanding properties of a given material. The weighted mobility is used to estimate the carrier transports of p-type SnS crystals, including the differences in doping level, carrier scattering and electronic band structure. We analyze the difference in carrier scattering mechanism for different crystal forms with the same doping level, then evaluate and confirm the temperature-dependent evolution of electronic band structures in SnS. Finally, we calculate the quality factor $B$ based on the weighted mobility, and establish the relationship between $ZT$ and $B$ to further predict the potential performance in p-type SnS crystals with low cost and earth abundance, which can be realized through taking advantage of the inherent material property, thus improving $B$ factor to achieve optimal thermoelectric level.
 Select Symmetry-Assisted Protection and Compensation of Hidden Spin Polarization in Centrosymmetric Systems Yingjie Zhang, Pengfei Liu, Hongyi Sun, Shixuan Zhao, Hu Xu, and Qihang Liu Chin. Phys. Lett.    2020, 37 (8): 087105 .   DOI: 10.1088/0256-307X/37/8/087105 Abstract   HTML   PDF (3117KB) It was recently noted that in certain nonmagnetic centrosymmetric compounds, spin–orbit interactions couple each local sector that lacks inversion symmetry, leading to visible spin polarization effects in the real space, dubbed “hidden spin polarization (HSP)”. However, observable spin polarization of a given local sector suffers interference from its inversion partner, impeding material realization and potential applications of HSP. Starting from a single-orbital tight-binding model, we propose a nontrivial way to obtain strong sector-projected spin texture through the vanishing hybridization between inversion partners protected by nonsymmorphic symmetry. The HSP effect is generally compensated by inversion partners near the ${\varGamma}$ point but immune from the hopping effect around the boundary of the Brillouin zone. We further summarize 17 layer groups that support such symmetry-assisted HSP and identify hundreds of quasi-2D materials from the existing databases by first-principle calculations, among which a group of rare-earth compounds LnIO (Ln = Pr, Nd, Ho, Tm, and Lu) serves as great candidates showing strong Rashba- and Dresselhaus-type HSP. Our findings expand the material pool for potential spintronic applications and shed light on controlling HSP properties for emergent quantum phenomena.
 Select Structure-Spin-Transport Anomaly in Quasi-One-Dimensional Ba$_{9}$Fe$_{3}$Te$_{15}$ under High Pressure Jun Zhang, Mei-Ling Jin, Xiang Li, Xian-Cheng Wang, Jian-Fa Zhao, Ying Liu, Lei Duan, Wen-Min Li, Li-Peng Cao, Bi-Juan Chen, Li-Juan Wang, Fei Sun, Yong-Gang Wang, Liu-Xiang Yang, Yu-Ming Xiao, Zheng Deng, Shao-Min Feng, Chang-Qing Jin, and Jin-Long Zhu Chin. Phys. Lett.    2020, 37 (8): 087106 .   DOI: 10.1088/0256-307X/37/8/087106 Abstract   HTML   PDF (2065KB) Recently, a series of novel compounds Ba$_{3}$MX$_{5}$ (M =  Fe, Ti, V; X = Se, Te) with hexagonal crystal structures composed of quasi-1-dimensional (1D) magnetic chains has been synthesized by our research team using high-pressure and high-temperature methods. The initial hexagonal phases persist to the maximum achievable pressure, while spin configurations and magnetic interactions may change dramatically as a result of considerable reductions in interchain separations upon pressurization. These compounds therefore offer unique possibilities for studying the evolution of intrinsic electronic structures in quasi-1D magnetic systems. Here we present a systematic investigation of Ba$_{9}$Fe$_{3}$Te$_{15}$, in which the interchain separations between trimerized 1D chains ($\sim$10.2 Å) can be effectively modulated by external high pressure. The crystal structure especially along the 1D chains exhibits an abnormal expansion at $\sim$5 GPa, which accompanies trimerization entangled anomalous mixed-high-low spin transition. An insulator-metal transition has been observed under high pressure as a result of charge-transfer gap closing. Pressure-induced superconductivity emerges at 26 GPa, where the charge-transfer gap fully closes, 3D electronic configuration forms and local spin fully collapses.
 Select Magnetization Reversal of Single-Molecular Magnets by a Spin-Polarized Current Chao Yang, Zheng-Chuan Wang, and Gang Su Chin. Phys. Lett.    2020, 37 (8): 087201 .   DOI: 10.1088/0256-307X/37/8/087201 Abstract   HTML   PDF (775KB) We study the magnetization reversal of single-molecular magnets by a spin-polarized current in the framework of the spinor Boltzmann equation. Because of the spin–orbit coupling, the spin-polarized current will impose a non-zero spin transfer torque on the single-molecular magnets, which will induce the magnetization switching of the latter. Via the $s$–$d$ exchange interaction between the conducting electrons and single-molecular magnets, we can investigate the magnetization dynamics of single-molecular magnets. We demonstrate the dynamics of the magnetization based on the spin diffusion equation and the Heisenberg-like equation. The results show that when the current is large enough, the magnetization of the single-molecular magnets can be reversed. We also calculate the critical current density required for the magnetization reversal under different anisotropy and external magnetic fields, which is helpful for the corresponding experimental design.
 Select Anisotropy Properties of Mn$_{2}$P Single Crystals with Antiferromagnetic Transition Shi-Hang Na, Wei Wu, and Jian-Lin Luo Chin. Phys. Lett.    2020, 37 (8): 087301 .   DOI: 10.1088/0256-307X/37/8/087301 Abstract   HTML   PDF (1064KB) Single crystals of hexagonal structure Mn$_{2}$P are synthesized by Sn flux for the first time. Transport and magnetic properties have been performed on the single crystals, which is an antiferromagnet with Neel temperature 103 K. Obvious anisotropy of resistivity is observed below the Neel temperature, which is manifested by metallic behavior with a current along the $c$-axis and semiconducting behavior with a current along the $a$-axis. The negative slope of temperature-dependent resistivity is observed above the Neel temperature in both $a$ and $c$ directions. Strong anisotropy of magnetic susceptibility is also evident from the magnetization measurements. A weak metamagnetic transition is observed only in $a$-axis plane at high magnetic field near 50–60 K compared to the $c$-axis. We believe these strong anisotropies of magnetic and transport properties are due to the anisotropy of spin arrangement. Mn$_{2}$P could be a candidate for exploration of possible superconductivity due to the low spin state.
 Select Zirconium Aided Epitaxial Growth of In$_{x}$Se$_{y}$ on InP(111) Substrates Cheng Zheng, Dapeng Zhao, Xinqiang Cai, Wantong Huang, Fanqi Meng, Qinghua Zhang, Lin Tang, Xiaopeng Hu, Lin Gu, Shuai-Hua Ji, Xi Chen Chin. Phys. Lett.    2020, 37 (8): 087401 .   DOI: 10.1088/0256-307X/37/8/087401 Abstract   HTML   PDF (1147KB) Layered material indium selenide (In$_{x}$Se$_{y}$) is a promising candidate for building next-generation electronic and photonic devices. We report a zirconium aided MBE growth of this van der Waals material. When co-depositing zirconium and selenium onto an indium phosphide substrate with a substrate temperature of 400℃ at a constant zirconium flux rate of 0.01 ML/min, the polymorphic In$_{x}$Se$_{y}$ layer emerges on top of the insulating ZrSe$_{2}$ layer. Different archetypes, such as InSe, $\alpha$-In$_{2}$Se$_{3}$ and $\beta$-In$_{2}$Se$_{3}$, are found in the In$_{x}$Se$_{y}$ layers. A negative magnetoresistance of 40% at 2 K under 9 T magnetic field is observed. Such an In$_{x}$Se$_{y}$/ZrSe$_{2}$ heterostructure with good lattice-matching may serve as a candidate for device applications.
 Select Electronic Evolution from the Parent Mott Insulator to a Superconductor in Lightly Hole-Doped Bi$_2$Sr$_2$CaCu$_2$O$_{8+\delta}$ Qiang Gao, Lin Zhao, Cheng Hu, Hongtao Yan, Hao Chen, Yongqing Cai, Cong Li, Ping Ai, Jing Liu, Jianwei Huang, Hongtao Rong, Chunyao Song, Chaohui Yin, Qingyan Wang, Yuan Huang, Guo-Dong Liu, Zu-Yan Xu, and Xing-Jiang Zhou Chin. Phys. Lett.    2020, 37 (8): 087402 .   DOI: 10.1088/0256-307X/37/8/087402 Abstract   HTML   PDF (3827KB) High temperature superconductivity in cuprates is realized by doping the Mott insulator with charge carriers. A central issue is how such an insulating state can evolve into a conducting or superconducting state when charge carriers are introduced. Here, by in situ vacuum annealing and Rb deposition on the Bi$_2$Sr$_2$Ca$_{0.6}$Dy$_{0.4}$Cu$_2$O$_{8+\delta}$ (Bi2212) sample surface to push its doping level continuously from deeply underdoped ($T_{\rm c}=25$ K, doping level $p\sim0.066$) to the near-zero doping parent Mott insulator, angle-resolved photoemission spectroscopy measurements are carried out to observe the detailed electronic structure evolution in the lightly hole-doped region for the first time. Our results indicate that the chemical potential lies at about l eV above the charge transfer band for the parent state at zero doping, which is quite close to the upper Hubbard band. With increasing hole doping, the chemical potential moves continuously towards the charge transfer band and the band structure evolution exhibits a rigid band shift-like behavior. When the chemical potential approaches the charge transfer band at a doping level of $\sim$0.05, the nodal spectral weight near the Fermi level increases, followed by the emergence of the coherent quasiparticle peak and the insulator–superconductor transition. Our observations provide key insights in understanding the insulator–superconductor transition in doping the parent cuprate compound and for establishing related theories.
 Select An Origin of Dzyaloshinskii–Moriya Interaction at Graphene-Ferromagnet Interfaces Due to the Intralayer RKKY/BR Interaction Jin Yang, Jian Li, Liangzhong Lin, and Jia-Ji Zhu Chin. Phys. Lett.    2020, 37 (8): 087501 .   DOI: 10.1088/0256-307X/37/8/087501 Abstract   HTML   PDF (729KB) We present a theory of both the itinerant carrier-mediated RKKY interaction and the virtual excitations-mediated Bloembergen–Rowland (BR) interaction between magnetic moments in graphene induced by proximity effect with a ferromagnetic film. It is shown that the RKKY/BR interaction consists of the Heisenberg, Ising, and Dzyaloshinskii–Moriya (DM) terms. In the case of the nearest distance, we estimate the DM term from the RKKY/BR interaction is about $0.13$ meV for the graphene/Co interface, which is consistent with the experimental result of DM interaction $0.16\pm0.05$ meV. Our calculations indicate that the intralayer RKKY/BR interaction may be a possible physical origin of the DM interaction in the graphene-ferromagnet interface. This work provides a new perspective to comprehend the DM interaction in graphene/ferromagnet systems.
 Select A Nonlinear Theoretical Model of Magnetization and Magnetostriction for Ferromagnetic Materials under Applied Stress and Magnetic Fields Pengpeng Shi Chin. Phys. Lett.    2020, 37 (8): 087502 .   DOI: 10.1088/0256-307X/37/8/087502 Abstract   HTML   PDF (723KB) A thermodynamic and micro-statistical model is proposed to explain the magnetization and magnetostriction mechanisms for isotropic ferromagnetic materials. Here a nonlinear magnetostrictive expression enhances the characterization of the nonlinear magnetic-mechanical effect, and the Brillouin function makes it possible to describe the relationship between the equivalent field and magnetization for various types of materials. Through detailed comparisons with the recent models of Wu et al. [Appl. Phys. Lett. 115 (2019) 162406] and Daniel [Eur. Phys. J.: Appl. Phys. 83 (2018) 30904], it is confirmed that the proposed model can provide greater physical insight and a more accurate description of the complex magnetostriction and magnetization behaviors, especially the complex nonlinearity of stress effects.
 Select Mode Structures and Damping of Quantized Spin Waves in Ferromagnetic Nanowires Qingwei Fu, Yong Li, Lina Chen, Fusheng Ma, Haotian Li, Yongbing Xu, Bo Liu, Ronghua Liu, and Youwei Du Chin. Phys. Lett.    2020, 37 (8): 087503 .   DOI: 10.1088/0256-307X/37/8/087503 Abstract   HTML   PDF (760KB) Magnonic devices based on spin waves are considered as a new generation of energy-efficient and high-speed devices for storage and processing of information. Here we experimentally demonstrate that three distinct dominated magneto-dynamic modes are excited simultaneously and coexist in a transversely magnetized ferromagnetic wire by the ferromagnetic resonance (FMR) technique. Besides the uniform FMR mode, the spin-wave well mode, the backward volume magnetostatic spin-wave mode, and the perpendicular standing spin-wave mode are experimentally observed and further confirmed with more detailed spatial profiles by micromagnetic simulation. Furthermore, our experimental approach can also access and reveal damping coefficients of these spin-wave modes, which provides essential information for development of magnonic devices in the future.
 Select Unexpectedly Strong Diamagnetism of Self-Assembled Aromatic Peptides Haijun Yang, Zixin Wang, Liuhua Mu, Yongshun Song, Jun Hu, Feng Zhang, and Haiping Fang Chin. Phys. Lett.    2020, 37 (8): 087504 .   DOI: 10.1088/0256-307X/37/8/087504 Abstract   HTML   PDF (471KB) There is a considerable amount of work that shows the biomagnetism of organic components without ferromagnetic components at the molecular level, but it is of great challenge to cover the giant gap of biomagnetism between their experimental and theoretical results. Here we show that the diamagnetism of aromatic peptides is greatly enhanced for about 11 times by self-assembling, reaching two orders of magnitude higher than the mass susceptibility of pure water. The self-assembly of aromatic rings in the peptide molecules plays the key role in such a strong diamagnetism.
 Select Enhancement of Curie Temperature under Built-in Electric Field in Multi-Functional Janus Vanadium Dichalcogenides Shilei Ji , Hong Wu , Shuang Zhou , Wei Niu , Lujun Wei , Xing-Ao Li , Feng Li, and Yong Pu Chin. Phys. Lett.    2020, 37 (8): 087505 .   DOI: 10.1088/0256-307X/37/8/087505 Abstract   HTML   PDF (3633KB) Functionalized two-dimensional materials with multiferroicity are highly desired to be next-generation electronic devices. Here we theoretically predict a family of Janus vanadium dichalcogenides VXX' (X/X' = S, Se, Te) monolayers with multiferroic properties, combing ferromagnetism, ferroelasticity and piezoelectricity. Due to the unpaired electrons on the V atom, the Janus VXX' monolayers have intrinsic long-range ferromagnetic orders. Particularly, the Curie temperature of 1T-VSeTe monolayer is up to 100 K, which is greatly higher than 2D 1T-VSe$_{2}$ and 1T-VTe$_{2}$. Furthermore, the six Janus VXX' monolayers have similar crater-like ferroelastic switching curves. Compared to black phosphorus, 2H-VSSe monolayer has the similar ferroelastic switching signal and 4 times lower energy barrier. In addition, the out-of-plane piezoelectricity induced by the structure asymmetry in the vertical direction gives the 2H-VXX' monolayers the potential to be piezoelectric materials. It is found that a built-in electric field in the vertical direction due to the different electronegativity values of chalcogen atoms induces the changes of electronic structures, which leads to the appearance of three different types of band gaps in the three H-phase structures. Recently, the experimental growth of the Janus MoSSe monolayers and the electrochemical exfoliation of ferromagnetic monolayered VSe$_{2}$ make the Janus VXX' monolayers possibly fabricated in experiments.
 Select Large Photoluminescence Enhancement by an Out-of-Plane Magnetic Field in Exfoliated WS$_2$ Flakes Sibai Sun, Jianchen Dang, Xin Xie, Yang Yu, Longlong Yang, Shan Xiao, Shiyao Wu, Kai Peng, Feilong Song, Yunuan Wang, Jingnan Yang, Chenjiang Qian, Zhanchun Zuo, and Xiulai Xu Chin. Phys. Lett.    2020, 37 (8): 087801 .   DOI: 10.1088/0256-307X/37/8/087801 Abstract   HTML   PDF (864KB) We report an out-of-plane magnetic field induced large photoluminescence enhancement in WS$_2$ flakes at $4$ K, in contrast to the photoluminescence enhancement provided by an in-plane field in general. Two mechanisms for the enhancement are proposed. One is a larger overlap of the electron and hole caused by the magnetic field induced confinement. The other is that the energy difference between $\varLambda$ and $K$ valleys is reduced by magnetic field, and thus enhancing the corresponding indirect-transition trions. Meanwhile, the Landé $g$ factor of the trion is measured to be $-0.8$, whose absolute value is much smaller than normal exciton, which is around $|-4|$. A model for the trion $g$ factor is presented, confirming that the smaller absolute value of the Landé $g$ factor is a behavior of this $\varLambda$–$K$ trion. By extending the valley space, we believe this work provides a further understanding of the valleytronics in monolayer transition metal dichalcogenides.
 Select Effect of Dopant Concentration in a Base Layer on Photocurrent–Voltage Characteristics of Photovoltaic Power Converters Wen-Xue Huo, Ming-Long Zhao, Xian-Sheng Tang, Li-Li Han, Zhen Deng, Yang Jiang, Wen-Xin Wang, Hong Chen, Chun-Hua Du, and Hai-Qiang Jia Chin. Phys. Lett.    2020, 37 (8): 087802 .   DOI: 10.1088/0256-307X/37/8/087802 Abstract   HTML   PDF (570KB) It is known that the p–n junction of an absorption region is a crucial part for power conversion efficiency of photovoltaic power converters. We fabricate four samples with different dopant concentrations in base layers. The dependences of power conversion efficiency and fill factor on input power are displayed by photocurrent–voltage measurement. Photoluminescence characteristics under open circuit and connected circuit conditions are also studied. It is found that the status of p–n junction matching is the critical factor in affecting the power conversion efficiency. In addition, series resistance of photovoltaic power converters impairs the efficiency especially at high input powers. Both the key factors need to be considered to obtain high efficiency, and this work provides promising guidance on designing photovoltaic power converters.
 Select A Simple Time-Resolved Optical Measurement of Diffusion Transport Dynamics of Photoexcited Carriers and Its Demonstration in Intrinsic GaAs Films Yongyong You , Tianran Jiang , and Tianshu Lai Chin. Phys. Lett.    2020, 37 (8): 087803 .   DOI: 10.1088/0256-307X/37/8/087803 Abstract   HTML   PDF (745KB) We develop a tightly focused pump-probe absorption technique to study diffusion dynamics of photoexcited carriers. It has many advantages including the simple setup and operations, higher detection sensitivity, an analytic descriptive model and fast data samplings. Diffusion dynamics are measured twice, separately using two different-sized probe spots, instead of many time-delayed diffusion profiles of a carrier pocket measured using spatially probe-spot scanning. An analytic model is derived to describe diffusion dynamics. Diffusion dynamics in GaAs are measured to demonstrate the feasibility of this technique. The diffusion coefficient is obtained and agrees well with the reported experimental and theoretical results.
 Select Giant-Capacitance-Induced Wide Quantum Hall Plateaus in Graphene on LaAlO$_{3}$/SrTiO$_{3}$ Heterostructures Ran Tao, Lin Li, Li-Jun Zhu, Yue-Dong Yan, Lin-Hai Guo, Xiao-Dong Fan, and Chang-Gan Zeng Chin. Phys. Lett.    2020, 37 (7): 077301 .   DOI: 10.1088/0256-307X/37/7/077301 Abstract   HTML   PDF (1807KB) Hybrid structures of two distinct materials provide an excellent opportunity to optimize functionalities. We report the realization of wide quantum Hall plateaus in graphene field-effect devices on the LaAlO$_{3}$/SrTiO$_{3}$ heterostructures. Well-defined quantized Hall resistance plateaus at filling factors $v=\pm2$ can be obtained over wide ranges of the magnetic field and gate voltage, e.g., extending from 2 T to a maximum available magnetic field of 9 T. By using a simple band diagram model, it is revealed that these wide plateaus arise from the ultra-large capacitance of the ultra-thin LAO layer acting as the dielectric layer. This is distinctly different from the case of epitaxial graphene on SiC substrates, where the realization of giant Hall plateaus relies on the charge transfer between the graphene layer and interface states in SiC. Our results offer an alternative route towards optimizing the quantum Hall performance of graphene, which may find its applications in the further development of quantum resistance metrology.
 Select Band Alignment at the Al$_{2}$O$_{3}/\beta$-Ga$_{2}$O$_{3}$ Interface with CHF$_{3}$ Treatment Hao Liu , Wen-Jun Liu, Yi-Fan Xiao , Chao-Chao Liu , Xiao-Han Wu , and Shi-Jin Ding  Chin. Phys. Lett.    2020, 37 (7): 077302 .   DOI: 10.1088/0256-307X/37/7/077302 Abstract   HTML   PDF (648KB) The energy band alignment at the atomic layer deposited Al$_{2}$O$_{3}/\beta$-Ga$_{2}$O$_{3}$ interface with CHF$_{3}$ treatment was characterized by x-ray photoelectron spectroscopy and secondary ion mass spectrometry (SIMS). With additional CHF$_{3}$ plasma treatment, the conduction band offset increases from 1.95${\pm}$0.1 eV to 2.32${\pm}$0.1 eV; and the valence band offset decreases from 0.21${\pm}$0.1 eV to $-$0.16${\pm}$0.1 eV. As a result, the energy band alignment changes from type I to type II. This energy band alignment transition could be attributed to the downshift of the core-level of Ga $3d$, resulting from the Ga–F bond formation in the F-rich interfacial layer, which is confirmed by the SIMS results.
 Select Unusual Anomalous Hall Effect in a Co$_{2}$MnSi/MnGa/Pt Trilayer Shan Li, Jun Lu, Lian-Jun Wen, Dong Pan, Hai-Long Wang, Da-Hai Wei, and Jian-Hua Zhao Chin. Phys. Lett.    2020, 37 (7): 077303 .   DOI: 10.1088/0256-307X/37/7/077303 Abstract   HTML   PDF (791KB) An ultra-thin Co$_{2}$MnSi(0.5 nm)/MnGa(1.5 nm) bilayer capped with Pt (5 nm) has been successfully grown by molecular-beam epitaxy. It is a potential candidate of synthetic antiferromagnets due to antiferromagnetic coupling between Co$_{2}$MnSi and MnGa, which is a promising skyrmion-racetrack-memory medium without skyrmion Hall effect after capping with a Pt layer. Unusual humps in transverse Hall resistance loops are clearly observed in the temperature range from 260 to 400 K. This anomaly is generally attributed to topological Hall effect, but other than that, we prove that non-uniform rotation of magnetic moments in the bilayer with magnetic field sweeping is also a possible mechanism contributed to the unusual hump.
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