Superconductivity in self-flux-synthesized single crystalline R_2Pt_3Ge_5(R=La,Ce,Pr)

In order to study the basic superconductivity properties of R_2Pt_3Ge_5, we synthesized the single crystalline samples by the Pt–Ge self-flux method. R_2Pt_3Ge_5(R = La, Ce) were also grown for a systematic study. Zero-resistivity was observed in both the La-and Pr-based samples below the reported superconducting transition temperatures. However, magnetic susceptibility measurements showed low superconductivity volume fractions in both La_2Pt_3Ge_5 and R_2Pt_3Ge_5(less than2%). Ce_2Pt_3Ge_5 did not show any signature of superconductivity. From the specific heat measurements, we did not observe a superconducting transition peak in R_2Pt_3Ge_5, suggesting that it is not a bulk superconductor. The magnetic susceptibility and heat capacity measurements revealed two antiferromagnetic(AFM) orders in R_2Pt_3Ge_5 at T_(N1)= 4.2 K and T_(N2)= 3.5K, as well as a single AFM transition at TN= 3.8 K in Ce_2Pt_3Ge_5.     

Conservation of the particle-hole symmetry in the pseudogap state in optimally-doped Bi2Sr2CuO6+δ superconductor

The pseudogap state is one of the most enigmatic characteristics in the anomalous normal state properties of the high temperature cuprate superconductors. A central issue is to reveal whether there is a symmetry breaking and which symmetries are broken across the pseudogap transition. By performing high resolution laser-based angle-resolved photoemission measurements on the optimally-doped Bi₂Sr_1.6La_0.4CuO_6+δ superconductor, we report the observations of the particle-hole symmetry conservation in both the superconducting state and the pseudogap state along the entire Fermi surface. These results provide key insights in understanding the nature of the pseudogap and its relation with high temperature superconductivity.     

压力下碱金属铁硒基超导体中的现象与物理

在凝聚态物理研究中, 压力作为对物质状态调控的独立变量得到了广泛的应用. 压力对发现物质的新现象、新规律及对其形成机理的理解和对相关理论的验证起到了重要的作用, 尤其在超导电性的研究中取得了巨大的成功. 文章简要的介绍了通过利用压力手段对具有相分离结构的碱金属铁硒基超导体A_xFe_2-ySe₂ (A=K, Rb, Tl/Rb)开展的系列研究所取得的实验结果, 以及其他一些文献中报道的在此方面的主要实验与理论研究工作, 包括压力导致的超导再进入现象和其产生的量子临界机理、其特有的反铁磁绝缘体相在该类超导体实现超导电性中的作用、化学负压力对超导电性的影响、构成该类超导体的反铁磁序与其寄居的超晶格的关系等.     

重费米子超导与竞争序

与其他非常规超导系列相比, 重费米子超导体往往具有丰富多样的竞争序, 超导与各种竞争序相伴而生, 电子配对与反铁磁涨落、铁磁涨落、价态涨落、电四极矩涨落等量子临界涨落密切相关, 扩充了非常规超导的研究内容. 重费米子材料中的f电子往往同时参与超导与各种竞争序的形成, 表现出局域与巡游的二重性. 重费米子二流体理论为理解重费米子超导与竞争序的关系提供了新的思路.     

Enhancement of subgap conductance in a graphene superconductor junction by valley polarization

We theoretically study the differential conductance of a graphene/graphene superconductor junction, where the valley polarization of Dirac electrons is considered in the nonsuperconducting region. It is shown that the subgap conductance will increase monotonically with the valley-polarization strength when the chemical potential μ is near the Dirac point μ≤ 3?(? is the superconducting gap), whereas it will decrease monotonically when μ is far away from the Dirac point, μ≥ 5?.The former case is induced by the specular Andreev reflection while the retro-reflection accounts for the later result. Our findings may shed light on the control of conductance of a graphene superconductor junction by valley polarization.     

准一维Cr基超导体RbCr3As3的超导能隙

RbCr₃As₃是具有[（Cr₃As₃）^-]_∞线性链的准一维超导体,超导转变温度约为6.6 K.对RbCr₃As₃单晶进行了电输运和极低温热输运性质的研究.低温下,拟合了RbCr₃As₃正常态电阻率随温度的变化,发现其满足费米液体行为.通过拟合超导转变温度随磁场的关系,得到RbCr₃As₃单晶的上临界场约为25.6 T.对RbCr₃As₃进行了零场下的极低温热导率测量,得到其剩余线性项为7.5 μW·K^-2·cm^-1,占正常态热导率值的24%.测量不同磁场下RbCr₃As₃的热导率,发现与单带s波超导体相比较,RbCr₃As₃剩余线性项随磁场增加相对较快.这些结果表明RbCr₃As₃单晶很可能是有节点的非常规超导体.     

高压下的铁基超导体:现象与物理

始于2008年的铁基超导体研究续写了高温超导发展史的新篇章.回顾过去十年对铁基超导体的研究,在理论、实验及应用方面都取得了辉煌的成绩,丰富了人们对高温超导电性的认识,为突破高温超导机理研究、最终实现超导材料的人工设计与更广泛的应用奠定了坚实的基础.本文主要介绍了通过高压实验研究手段在铁基超导体的研究中取得的一些重要进展及呈现出的新现象和新物理,例如压致超导现象、压力导致的超导再进入现象、压力对超导转变温度的提升效应、压力研究对铁基超导体超导转变温度的预测、相分离结构对超导电性的影响及反铁磁-超导双临界点的发现等.希望这些高压研究结果与本文报道的其他各类实验与理论研究成果一起,为全面、深入地理解铁基超导体勾画出一幅较为完整的物理图像.     

Surface-induced orbital-selective band reconstruction in kagome superconductor CsV3Sb5

The two-dimensional (2D) kagome superconductor CsV₃Sb₅ has attracted much recent attention due to the coexistence of superconductivity, charge orders, topology and kagome physics, which manifest themselves as distinct electronic structures in both bulk and surface states of the material. An interesting next step is to manipulate the electronic states in this system. Here, we report angle-resolved photoemission spectroscopy (ARPES) evidence for a surface-induced orbital-selective band reconstruction in CsV₃Sb₅. A significant energy shift of the electron-like band around Γ and a moderate energy shift of the hole-like band around M are observed as a function of time. This evolution is reproduced in a much shorter time scale by in-situ annealing of the CsV₃Sb₅ sample. Orbital-resolved density functional theory (DFT) calculations reveal that the momentum-dependent band reconstruction is associated with different orbitals for the bands around Γ and M, and the time-dependent evolution points to the change of sample surface that is likely caused by the formation of Cs vacancies on the surface. Our results indicate the possibility of orbital-selective control of the band structure via surface modification, which may open a new avenue for manipulating exotic phenomena in this material system, including superconductivity.     

基于低温光声光谱技术的YBa2Cu3O7-δ超导转变过程

利用低温光声系统实时采集高Tc超导材料超导转变过程光声信号,是高Tc超导材料超导机理研究的一种新的实验方法。选取YBa₂Cu₃O_7-8超导体作为试样,分别构建常态→超导态（试样降温过程）和超导态→常态（试样升温过程）,得到转变过程光声信号对比图,并依据光声光谱图对试样失超现象作出可能的解释。     

Preparation and superconductivity of the co-doped superconductor Nd2−x−yCayCexCuO4 with T′ structure

The preparation and superconductivity of the co-doped superconductor Nd_2−x−yCa_yCe_xCuO₄ with T′ structure were studied by XRD and superconductivity measurements. The results indicate that the effect of the co-doping of Ca²⁺ and Ce⁴⁺ on T_O and the carrier type is not significant, but the effect on the range of the superconducting compositions in this system is obvious. The study of co-doped superconductors could be important for an understanding of the effect of co-doping on the carrier concentration and to explore new superconductors.     

Study of the magnetic properties of single-crystal Ba0.6K0.4BiO3

The magnetic properties of single-crystal Ba_0.6K_0.4BiO₃ were studied. The results show that this isotropic superconductor (cubic structure with T_c ≈ 32 K) exhibits irreversibility and relaxation properties similar to those observed in the layered, high temperature superconductors. For fields above 0.1 T, an irreversibility line B_irr = B_irr (0) (1 − t)ⁿ with n = 3/2 and B_irr (0) = 20 T is observed. The comparison among several superconducting systems with different anisotropies suggests that the irreversibility line is unlikely to be the melting line for the YBa₂Cu₃O_7−δ.     

Coexistence of superconductivity and antiferromagentic order in Er2O2Bi with anti-ThCr2Si2 structure

We investigated the coexistence of superconductivity and antiferromagnetic order in the compound Er₂O₂Bi with anti-ThCr₂Si₂-type structure through resistivity, magnetization, specific heat measurements and first-principle calculations. The superconducting transition temperature T_c of 1.23 K and antiferromagnetic transition temperature TN of 3 K are observed in the sample with the best nominal composition. The superconducting upper critical field H_c2(0) and electron-phonon coupling constant λ_e−ph in Er₂O₂Bi are similar to those in the previously reported non-magnetic superconductor Y2O2Bi with the same structure, indicating that the superconductivity in Er₂O₂Bi may have the same origin as in Y₂O₂Bi. The first-principle calculations of Er₂O₂Bi show that the Fermi surface is mainly composed of the Bi 6p orbitals both in the paramagnetic and antiferromagnetic state, implying minor effect of the 4f electrons on the Fermi surface. Besides, upon increasing the oxygen incorporation in Er₂O_xBi, Tc increases from 1 to 1.23 K and T_N decreases slightly from 3 K to 2.96 K, revealing that superconductivity and antiferromagnetic order may compete with each other. The Hall effect measurements indicate that hole-type carrier density indeed increases with increasing oxygen content, which may account for the variations of T_c and T_N with different oxygen content.     

Structural,magnetic and electronic properties of the iron–chalcogenide AxFe2-ySe2 (A=K,Cs, Rb,and Tl,etc.) superconductors

The latest discovery of a new iron–chalcogenide superconductor A_xFe_2-ySe₂ (A = K, Cs, Rb, and Tl, etc.) has attracted much attention due to a number of its unique characteristics, such as the possible insulating state of the parent compound, the existence of Fe-vacancy and its ordering, a new form of magnetic structure and its interplay with superconductivity, and the peculiar electronic structures that are distinct from other Fe-based superconductors. In this paper, we present a brief review on the structural, magnetic and electronic properties of this new superconductor, with an emphasis on the electronic structure and superconducting gap. Issues and future perspectives are discussed at the end of the paper.     

BaTi2Bi2O的电子结构与磁性

采用第一性原理方法,对BaTi₂Bi₂O的电子结构和磁性进行计算.非磁性态的计算结果显示:费米能级处的态密度主要来自d_z²,d_x²-y²和d_xy三个轨道,同时费米面也主要有三部分组成,并且将其沿着矢量q₁=(π/a,0,0)和q₂=(0,π/a,0)平移时,第三部分费米面(沿着X-R连线)与第一部分费米面(M-A连线)嵌套明显,计算得出磁化系数χ₀(q)在X点出现峰值,与峰值出现在M点的FeAs基超导体不同.上述磁化率峰值可以诱导产生自旋密度波,使得BaTi₂Bi₂O材料的磁性基态是bi-collinear antiferromagnetism(AF3)与blocked checkerboard antiferromagnetism(AF4)的二度简并态.随着空穴掺杂,χ₀(q)的峰值降低,而电子掺杂则导致峰值变大.当自旋涨落被完全压制时,超导出现,这可以解释为什么超导只出现在空穴掺杂型化合物而非电子掺杂型.     

Nuclear magnetic resonance studies of vortices in high temperature superconductors

The distinct distribution of local magnetic fields due to superconducting vortices can be detected with nuclear magnetic resonance (NMR) and used to investigate vortices and related physical properties of extreme type II superconductivity. This review summarizes work on high temperature superconductors (HTS) including cuprates and pnictide materials. Recent experimental results are presented which reveal the nature of vortex matter and novel electronic states. For example, the NMR spectrum has been found to provide a sharp indication of the vortex melting transition. In the vortex solid a frequency dependent spin–lattice relaxation has been reported in cuprates, including YBa₂Cu₃O_7-x, Bi₂SrCa₂Cu₂O_8+δ, and Tl₂Ba₂CuO_6+δ. These results have initiated a new spectroscopy via Doppler shifted nodal quasiparticles for the investigation of vortices. At very high magnetic fields this approach is a promising method for the study of vortex core excitations. These measurements have been used to quantify an induced spin density wave near the vortex cores in Bi₂SrCa₂Cu₂O_8+δ. Although the cuprates have a different superconducting order parameter than the iron arsenide superconductors there are, nonetheless, some striking similarities between them regarding vortex dynamics and frequency dependent relaxation.     

Observation of superconductivity (Tc = 50 K) in a new tetragonal alkaline-earth cuprate Sr0.8Ba1.2CuO3+δ, synthesised at ambient pressure

The ambient-pressure synthesis of a new tetragonal alkaline-earth superconducting cuprate, Sr_0.8Ba_1.2CuO_3+δ, from a cupro-oxycarbonate is reported. Magnetic-susceptibility measurements show the presence of a superconducting transition 50 K in a post-annealed sample. The crystal structure, refined from time-of-flight powder neutron-diffraction data was found to have an oxygen-deficient La₂CuO₄-type tetragonal T structure (a = 3.8988(3) Å and C = 12.815(3) Å) with oxygen vacancies located within the CuO₂ planes. Ordering of these oxygen vacancies is responsible for the observation of a superlattice in both neutron- and electron-diffraction measurements. An interpretation of the electron-diffraction patterns suggests that the superlattice in Sr_0.8Ba_1.2CuO_3+δ and also in the isostructural superconductor Sr₂CuO_3+δ are of an identical nature.     

Optimal charge inhomogeneity for the d+id-wave superconductivity in the intercalated graphite CaC6

The coexistence of superconductivity and charge inhomogeneity was observed in many cuprate superconductors. The relationship between those two is still controversial. Similarly, in the graphene sheets of the intercalated graphitic superconductor CaC₆, the charge inhomogeneity was also observed. We simulate such a system by constructing the Hubbard model on the honeycomb lattice with charge inhomogeneity imposed by force. Utilizing the finite-temperature determinant quantum Monte Carlo algorithm, we examine the relationship between the superconducting pairing and the charge inhomogeneity. An optimal charge inhomogeneity for the d+id-wave pairing is found. While for other artificial charge inhomogeneities, the d+id-wave pairing is monotonically suppressed. The possible π-phase shift induced by charge inhomogeneity is also examined.     

Hf-N体系的晶体结构预测和性质的第一性原理研究

为了寻找具有优异力学性能的新型超高温陶瓷材料, 结合进化算法和第一性原理, 系统研究了Hf-N二元体系所有稳定存在的化合物及其晶体结构. 除了实验已知的岩盐结构的HfN之外, 本文还找到了Hf₆N(R-3), Hf₃N(P6₃22), Hf₃N₂(R-3m), Hf₅N₆(C2/m)和Hf₃N₄(C2/m)五种新结构, 基于准简谐近似原理计算了这些稳定结构的声子谱以验证其动力学稳定性, 常温甚至更高温度下的吉布斯自由能以验证其高温热力学稳定性. 结果表明, 这些结构是动力学稳定的, 且在1500 K以下都是热力学稳定的. 同时, 本文还列出了在搜索过程中出现的空间对称性较高、能量较低的亚稳态结构, 包括Hf₂N(P4₂/mnm), Hf₄N₃(C2/m), Hf₆N₅(C2/m), Hf₄N₅(I4/m), Hf₃N₄(I-43d)和Hf₃N₄(Pnma). 之后计算了上述所有结构的力学性质(弹性常数、体模量、 剪切模量、 杨氏模量、硬度), 随着N 所占比例的增加, 硬度呈现的整体趋势是先增大后下降, 在Hf₅N₆处取得最大值, 为21 GPa. 其中Hf₃N₂和Hf₄N₅也展现出了较高的硬度, 都为19 GPa. 最后, 计算了这些结构的电子态密度和晶体轨道汉密尔顿分布, 从电子结构的角度分析了力学性能的成因. 研究结果显示, 较强的Hf-N共价键和较低的结构空位率是Hf₅N₆具有优异力学性能的主要原因.     

Influence of alkali metal superoxides on structure,electronic,and optical properties of Be_(12)O_(12) nanocage:Density functional theory study

The effect of alkali metal superoxides M_3O(M = Li,Na,K) on the electronic and optical properties of a Be_(12)O_(12) nanocage was studied by density functional theory(DFT) and time-dependent density functional theory(TD-DFT).The energy gaps(Eg) of all configurations were calculated.Generally,the adsorption of alkali metal superoxides on the Be_(12)O_(12) nanocage causes a decrease of Eg.Electric dipole moment μ,polarizability α,and static first hyperpolarizability β were calculated and it was shown that the adsorption of alkali metal superoxides on Be_(12)O_(12) increases its polarizability.It was found that the absorption of M_3 O on Be_(12)O_(12) nanocluster improves its nonlinear optical properties.The highest first hyperpolarizability(β≈ 214000 a.u.) is obtained in the K_3O–Be_(12)O_(12)nanocluster.The TD-DFT calculations were performed to investigate the origin of the first hyperpolarizabilities and it was shown that a higher first hyperpolarizability belongs to the structure that has a lower transition energy.     

Structurally tuned superconductivity in heavy-fermion CeMIn5 (M=Co, Ir, Rh)

We discuss systematic trends in the high-temperature physical properties of the heavy Fermion superconductors (HFS) CeCoIn₅ (T_c=2.3 K, γ=300 mJ/molK²), CeIrIn₅ (T_c=0.4 K, γ=750 mJ/molK²), and CeRhIn₅ (P_c=16 kbar, T_c=2.1 K, γ=400 mJ/molK²) in terms of crystalline-electrical-field effects(CEF). We suggest the possibility that the interplay between the symmetry of the CEF ground-state (or low-T CEF scheme of levels) and the f–s hybridization could generate spin fluctuations relevant to the superconducting pairing mechanism in these materials. This hypothesis may provide insight into the fact that some crystal structures appear to favor superconductivity. Further, CeMIn₅ (M=Co, Ir, Rh) appear to be structural relatives of the cubic heavy Fermion superconductor CeIn₃, but with much higher T_c's. We argue that structural layering inherent in the tetragonal CeMIn₅ crystal structure determines the magnetic and electronic anisotropy responsible for the enhanced T_c's. We also describe similarities and differences between these compounds and the high-T_c cuprates.