Hybrid crystals of cuprates and iron-based superconductors

We propose two possible new compounds, Ba_2CuO_2Fe_2As_2and K_2CuO_2Fe_2Se_2, which hybridize the building blocks of two high temperature superconductors, cuprates and iron-based superconductors. These compounds consist of square CuO_2 layers and antifluorite-type Fe_2X_2(X = As, Se) layers separated by Ba/K. The calculations of binding energies and phonon spectra indicate that they are dynamically stable, which ensures that they may be experimentally synthesized. The Fermi surfaces and electronic structures of the two compounds inherit the characteristics of both cuprates and iron-based superconductors. These compounds can be superconductors with intriguing physical properties to help to determine the pairing mechanisms of high Tc superconductivity.     

BaCuS_2: A Superconductor with Moderate Electron-Electron Correlation

We show that the layered-structure BaCuS₂ is a moderately correlated electron system in which the electronic structure of the CuS layer bears a resemblance to those in both cuprates and iron-based superconductors.Theoretical calculations reveal that the in-plane d-p σ^*-bonding bands are isolated near the Fermi level.As the energy separation between the d and p orbitals are much smaller than those in cuprates and iron-based superconductors,BaCuS₂ is expected to be moderately correlated.We suggest that this material is an ideal system to study the competitive/collaborative nature between two distinct superconducting pairing mechanisms,namely the conventional BCS electron-phonon interaction and the electron-electron correlation,which may be helpful to establish the elusive mechanism of unconventional high-temperature superconductivity.     

A density-wave-like transition in the polycrystalline V3Sb2 sample with bilayer kagome lattice

Recently,transition-metal-based kagome metals have aroused much research interest as a novel platform to explore exotic topological quantum phenomena.Here we report on the synthesis,structure,and physical properties of a bilayer kagome lattice compound V₃Sb₂.The polycrystalline V₃Sb₂ samples were synthesized by conventional solid-state-reaction method in a sealed quartz tube at temperatures below 850℃.Measurements of magnetic susceptibility and resistivity revealed consistently a density-wave-like transition at Tdw≈160 K with a large thermal hysteresis,even though some sample-dependent behaviors were observed presumably due to the different preparation conditions.Upon cooling through Tdw,no strong anomaly in lattice parameters and no indication of symmetry lowering were detected in powder x-ray diffraction measurements.This transition can be suppressed completely by applying hydrostatic pressures of about 1.8 GPa,around which no sign of superconductivity was observed down to 1.5 K.Specific-heat measurements revealed a relatively large Sommerfeld coefficientγ=18.5 mJ·mol^-1·K^-2,confirming the metallic ground state with moderate electronic correlations.Density functional theory calculations indicate that V₃Sb₂ shows a non-trivial topological crystalline property.Thus,our study makes V₃Sb₂ a new candidate of metallic kagome compound to study the interplay between density-wave-order,nontrivial band topology,and possible superconductivity.     

钒基笼目超导体

钒基笼目超导体的发现为研究拓扑物态、超导以及几何阻挫等物理特性提供了全新平台.自发现钒基笼目超导体以来,国内很多研究组对其超导、电荷密度波、配对密度波、电子向列相以及时间反演对称性破缺多方面特性进行了研究,并取得了重要进展,引发了极大的关注.这些新进展展示了钒基笼目超导体有极其丰富的基本物理机制.本文简略讨论国内研究组取得的进展及其对理解钒基笼目超导体性质的意义,并介绍目前领域内值得关注的重要问题.     

Collective coordination of multi-agent systems guided by multiple leaders

A neighbour-based coordination scheme is proposed for a multi-agent system with multiple leaders. Under assumptions of the connectivity of the interconnection topology and a simple first-order dynamics model for each mobile agent, the results show that all the agents will flock to the polytope region formed by the leaders.     

A New Superconductor Parent Compound NaMn6Bi5 with Quasi-One-Dimensional Structure and Lower Antiferromagnetic-Like Transition Temperatures

Mn-based superconductors are very rare and their superconductivity has only been reported in three-dimensional Mn P and quasi-one-dimensional KMn₆Bi₅ and Rb Mn₆Bi₅ with[Mn₆Bi₅]^-columns under high pressures.Here we report the synthesis,magnetism,electrical resistivity,and specific heat capacity of the newly discovered quasione-dimensional Na Mn₆Bi₅.Compared with other AMn₆Bi₅ （...     

那棵消失的树——回忆导师张首晟

<正>首晟走了,走得那么的突然。2018年12月6号那天早晨,我正在赶往机场的路上。和许多人一样,看到手机微信里朋友询问的信息,我还十分肯定地以"别开玩笑"作了答复,可是后面的几十分钟如雪片一样飞来的消息让心情沉重起来,接了几个电话,电话这头的我已经渐渐发不出声音,直到飞机要起飞的那刻,晓亮的电子邮件将最     

Low-energy Ce spin excitations in CeFeAsO and CeFeAsO<Subscript>0.84</Subscript>F<Subscript>0.16</Subscript>

We use inelastic neutron scattering to study the low-energy spin excitations of polycrystalline samples of nonsuperconducting CeFeAsO and superconducting CeFeAsO_0.84F_0.16. Two sharp dispersionless modes are found at 0.85 and 1.16 meV in CeFeAsO below the Ce antiferromagnetic (AF) ordering temperature of T _N ^Ce ? 4 K. On warming to above T _N ^Ce ? 4 K, these two modes become one broad dispersionless mode that disappears just above the Fe ordering temperature T _N ^Fe ? 140 K. For superconducting CeFeAsO_0.84F_0.16, where Fe static AF order is suppressed, we find a weakly dispersive mode center at 0.4 meV that may arise from short-range Ce-Ce exchange interactions. Using a Heisenberg model, we simulate powder-averaged Ce spin wave excitations. Our results show that we need both Ce spin wave and crystal electric field excitations to account for the whole spectra of low-energy spin excitations.     

铁基超导中拓扑量子态研究进展

铁基超导体和拓扑量子材料是近年来凝聚态物理两个重要的前沿研究方向.铁基超导体中是否能衍生出非平庸的拓扑现象是一个非常有意义的问题.本文从晶体对称性、布里渊区高对称点附近的有效模型以及自旋轨道耦合相互作用三个方面具体分析了铁基超导的电子结构的基本特点.在此基础上,重点阐述铁基超导的正常态、临近超导的长程有序态以及超导态中非平庸的拓扑量子态是如何衍生的;具体介绍了相关的理论模型以及结果,回顾了相关的实验进展,展望了该领域的发展前景.