氧化物超导体的高压热处理及烧结

 本文利用高温高压条件，对氧化物超导体进行了不同条件的高压热处理及烧结。发现对SrLa(CuAg)₁O_x在空气中烧结的超导体，适当条件的高压热处理，使T_c从20 K提高到28 K。在Y₁Ba₂Cu₃O_7-δ系氧化物中，无论是对空气中烧结的已超导样品的高压热处理，或者直接在高压下烧结样品，均未使T_c得到提高，但是，经过高压烧结，超导样品的硬度显著提高，密度也有所增加，并且改变了超导相的晶格常数。     

高压下制备高密度的Bi系超导体

 首次直接用高压烧结而不必进行任何退火后处理获得超导Bi_0.8Pb_0.2SrCa₂O_x陶瓷样品。此样品是由名义组分为Bi_0.8Pb_0.2SrCa·Cu₂O_x的超导体在1.0 GPa、500 ℃处理2 h后制得的，其零电阻转变温度为97 K。以前未见报导过如此好的结果。高压下烧结的Bi_0.8Pb_0.2SrCa·Cu₂O_x超导体的密度可达6.23 g/cm³（约为理论密度的95%），这比常规法制得的样品的密度（大约4~5 g/cm³）高得多。     

铜氧化合物超导体Pr1-xCaxBa2Cu3O7-δ的高压合成

 高温高压下合成出了新块材超导体Pr_1-xCa_xBa₂Cu₃O_7-δ（0.4≤x≤0.6）,高压合成使Ca在Pr-123相中的溶解度有很大提高,从而有效地补偿了Pr对CuO₂面空穴的填充而使样品超导。     

压力对HgBa2Ca2Cu3O8+y超导转变温度的影响

 利用低温超高压装置，测量了Hg系样品HgBa₂Ca₂Cu₃O_8+y（Hg-1223）超导转变温度T_c在压力作用下的增强效应。压力最高达7.8 GPa，超导起始转变温度常压下为130 K，加压到5.4 GPa时获得最高温度为140 K。在5.4 GPa以下获得dT_c/dp为1.85 K/GPa。用压力作用下氧原子位置的改变使载流子浓度提高和CuO₂面间的耦合作用来解释高温超导的压力效应。     

基于组分Ca2Pr2Cu5O10三元固溶区的范围和晶体结构

 利用X射线衍射分析和Rietveld结构精修方法研究了基于组分Ca₂Pr₂Cu₅O₁₀三元固溶区的范围和晶体结构。根据X射线分析结果和相消失法，为了保持Ca_2+xPr_2-xCu₅O₁₀结构单位晶胞中恒定的氧含量和电价平衡，基于组分Ca₂Pr₂Cu₅O₁₀的三元固溶区的范围可确定为：在Ca_2+xPr_2-xCu₅O₁₀固溶区的富Ca区（Ca/Pr>1），其表达式仍可以表示为一般的化学计量式Ca_2+xPr_2-xCu₅O₁₀（x=0.4, 0.2, 0）；在富Pr区（Ca/Pr<1），其表达式为Ca_1.7Pr_2.2·Cu₅O₁₀、Ca_1.4Pr_2.4Cu₅O₁₀，即：Ca_2+xPr_2-xCu₅O₁₀固溶区范围从Ca_2.4Pr_1.6Cu₅O₁₀至Ca_1.4Pr_2.4·Cu₅O₁₀。研究表明：基于组分Ca₂Pr₂Cu₅O₁₀的三元固溶体晶体结构可以看作是正交晶系NaCuO₂型亚晶胞的无公度相；Ca_2.4Pr_1.6Cu₅O₁₀亚晶胞的点阵常数为a₀=0.282 46(7) nm，b₀=0.636 93(1) nm，c₀=1.067 94(1) nm；其正交超结构的点阵常数为a=5a₀，b=b₀，c=5c₀。Ca_2.4Pr_1.6Cu₅O₁₀的结构也可以用单斜超晶胞来表述，该单斜超晶胞的空间群为P2₁/c，Z=4，a=5a₀，b=b₀，c=c₀/sin β，β=104.79°或136.60°，V=5a₀b₀c₀。在基于组分Ca₂Pr₂Cu₅O₁₀的三元固溶体晶体结构中，Ca和Pr之间在一定范围内是可以相互替代的，而且它们在结构中的占位是无序的。     

固相线下Pr1-xBa2-yCax+yCu3O7±δ体系的相关系和晶体结构

 利用X射线衍射分析和Rietveld结构精修的方法研究了固相线下Pr_1-xBa_2-yCa_x+yCu₃O_7±δ体系的相关系和晶体结构。研究表明：950 ℃、空气条件下，Pr_1-xBa_2-yCa_x+yCu₃O_7±δ（y=0）体系系列样品总是含有两相：Pr123（PrBa₂Cu₃O_7±δ）和BaCuO₂；而且，随Ca含量的增加，杂相BaCuO₂的含量也随之增加。Pr_1-xBa_2-yCa_x+yCu₃O_7±δ（x=0）体系y=0.1、0.2、0.3、0.4的样品都只含有Pr123单相；y=0.5的样品含有3相：Pr123、Ca₂CuO₃和Pr₄Ca₁₀Cu₂₄O₄₁。另外，Pr_1-xBa_2-yCa_x+yCu₃O_7±δ（x=0）系列样品（y=0.1、0.2、0.3、0.4）中Pr123相还发生了正交-四方晶系的结构转化。y=0.1和0.2的样品为正交晶系，晶胞参数分别为a=0.390 33(4)和0.389 36(1) nm，b=0.389 53(4)和0.387 73(1) nm，c=1.173 38(4) 和1.168 20(2) nm；y=0.3和0.4的样品为四方晶系，晶胞参数分别为a=b=0.387 80(0) 和0.387 67(0) nm，c=1.166 00(2) 和1.165 20(2) nm。在Pr123结构中，Ba离子可以被Pr离子代替，但不能被Ca离子所代替。Ca离子占据部分Pr离子的位置。在Pr_1-xBa_2-yCa_x+yCu₃O_7±δ（x=0）体系中Pr、Ba和Ca离子间的相互替代中，离子半径比化学性质起到了更大的作用。在950 ℃、空气条件下，Ca离子在Pr_1-xBa_2-yCa_x+yCu₃O_7±δ体系中的最大溶解量是当x=0时，y≤0.4。     

Cu-系高温超导系列

 简要介绍了利用高温高压合成发现的超导系列CuBa₂Ca_n-1Cu_nO_2n+2+δ，即Cu-12n（三元数法）或Cu-12(n-1)n（四元数法），简称Cu-系。比较了Cu-系和其它主要高温超导体的结构和主要性质，指出了Cu-系和YBa₂Cu₃O₆的同构关系，以及Cu-系具有的良好的高温高场特性。Cu-系超导体因仅含碱土和铜的氧化物，化学组成简单、经济、安全，具有潜在的应用开发前景。     

压力对TlPbxBaCa3Cu3O超导电性的影响

 在零电阻温度T_c0达到120 K的TlBaCa₃Cu₃O_y超导体中，掺入不同含量的Pb后，超导电性受到抑制，点阵常数减小。加压时，T_c先随压力p的增加而上升，样品1（x_Pb=0.05）的dT_c0/dp=1.7 K/GPa，样品2（x_Pb=0.5）的dT_c0/dp=2.2 K/GPa。T_c的峰值随Pb的增加而减小。加压时，对由于Pb的加入引起的Cu—O层的精细结构变化起到调制作用。     

高温超导体低温、高压下的物理性质的研究

 在3~20 GPa压力范围内，测量了含氧量较低的YBa₂Cu₃O_7-δ（δ=0.46）单晶压力增强效应（dT_c/dp=4.9KGPa^-1）；YBa₂Cu₃O₇（T_c0=90 K）单晶在压力下临界电流密度随压力变化；外磁场H=30 kO_e时，T_c与磁场、压力关系；压力达16.5 GPa下，Bi₂Sr₂CaCu₂O_x单晶T_c(p)关系（dT_c/dp=-0.4 KGPa^-1）。发现Y系高温超导体的温度压力导数dT_c/dp与T_c0中间呈dT_c/dp=b-mT_c0线性关系（b、m为常数）。结合压力下Y系超导体结构相变和含氧量对T_c影响，分析这类超导体T_c有很强的正压力效应的原因。把实验结果同几种超导电性微观理论模型进行了分析和比较。     

高温高压合成顶角氧掺杂型高温超导系列

 简要介绍了(Sr,Ca)_n+1Cu_nO_2n(Cl,O)₂,即三元数法的Cl-“02n”（或四元数Cl-02(n-1)n）超导系列,简称Cl-系超导体。和通常的高温超导体不同,Cl-系的电荷库由卤化物构成,而非通常的氧化物。即Cl-系的导电区和电荷库由顶角氯连接,根据这一结构特点用“顶角氧”掺杂,高压合成了T_c≈80 K的(Sr,Ca)₃Cu₂O₄(Cl,O)₂超导体。该化合物具有和全氧化物高温超导体可比的超导性能,表明高温高压合成全新的卤氧超导新材料的潜力,以及开发卤氧化物高温超导体群的研究前景。     

三重兼并费米子拓扑半金属MoP高压超导相的低压截获

碳化钨结构的三重兼并费米子拓扑半金属MoP高压下发生超导转变，然而，高压超导相能否在低压甚至常压截获尚不清楚.本文对两个密封于金刚石对顶砧中的MoP高压超导相在室温条件下进行快速卸压，通过测量低温电阻对温度的依赖关系，验证能否在低压甚至常压截获高压超导相.实验结果表明：初始压力61.5 GPa的样品#1,超导电性可截获的最低压力为44.2 GPa;初始压力95.0 GPa的样品#2,超导电性可截获的最低压力为3.5 GPa.     

第二类拓扑外尔半金属WP2高压超导相的常压截获

第二类拓扑外尔半金属WP₂高压下发生超导转变,然而,高压超导相能否在常压截获还没有相关报道.本文对密封于金刚石对顶砧中的WP₂高压超导相在室温条件下快速卸压至不同压强,原位测量电阻-温度曲线,验证能否在常压截获高压超导相.研究表明：WP₂的高压超导相可以在常压截获,从而提供了一个通过角分辨光电子能谱和扫描隧道谱探寻马约拉纳零能模的拓扑超导体材料平台.     

Superconductivity of lanthanum and cerium at high pressures

A theoretical model involving the presence of an inner 4f shell is developed here to explain the pressure dependence of the superconducting transition temperature of lanthanum and the presence of a superconducting high pressure phase of cerium. In this model, only the conduction electrons are assumed to participate in the superconductivity mechanism, while the 4f electrons tend to inhibit superconductivity. As the 4f character decreases with increasing pressure in lanthanum, it is possible to account for the anomalous pressure dependence of the superconducting transition temperature by assuming a valency a little less than three at normal pressure. The anomalous behaviour of cerium, magnetic at room temperature and normal pressure and superconducting above 50 kbar, is also discussed here.     

Hole formation and charge transfer in Y1−xCaxSr2Cu2GaO7, a new oxide superconductor

The technique of transmission electron energy loss spectrometry (EELS) in parallel detection has been utilized to analyze the fine structures associated with the O K and Cu L₃ absorption edges of recently discovered Y_1−xCa_xSr₂Cu₂GaO₇ (0.1 ≤ x ≤ 0.4) series of compounds, which exhibit superconductivity when annealed under high-pressure oxygen atmosphere. We find subtle but significant changes in EELS spectral features of nonsuperconducting and superconducting specimens of these compounds. A broad pre-edge feature is observed below the O K absorption edge, at about 528.2 eV which emerges with Ca-doping alone. Evidence is presented for another O K secondary pre-edge feature, at about 1.1 eV below the first O K pre-edge feature in only those specimens which have undergone high-pressure oxygen treatment (i.e. superconducting specimens). We interpret the O K pre-edge feature as due to formation of holes on oxygen sites and propose that the first broad pre-edge feature (at 528.2 eV) is associated with holes on oxygen sites other than the CuO₂ planes, which are responsible for normal conductivity. The second smaller pre-edge feature (at 527.1 eV) is most probably associated with holes in the CuO₂ planes, which are associated with superconductivity in this system. The presence of a secondary smaller O K pre-edge feature in the superconducting specimens appears to correlate well with a broad shoulder on the high energy side of the Cu L₃ edge. The results are interpreted in terms of oxidation of CuO₂ planes through charge transfer between copper and oxygen in the CuO₂ planes, i.e. covalent mixing of O and Cu orbitals at the Fermi level as a result of high oxygen pressure annealing.     

Evolution of superconductivity and charge order in pressurized RbV3Sb5

The kagome metals AV₃Sb₅(A=K,Rb,Cs)under ambient pressure exhibit an unusual charge order,from which superconductivity emerges.In this work,by applying hydrostatic pressure using a liquid pressure medium and carrying out electrical resistance measurements for RbV₃Sb₅,we find that the charge order becomes suppressed under a modest pressure pc(1.4 GPa3Sb₅.Our findings point to qualitatively similar temperature-pressure phase diagrams in KV₃Sb₅ and RbV₃Sb₅,{and suggest a close link}between the second superconducting dome and the high-pressure resistance anomalies.     

Predicted superconductive properties of lithium under pressure

A superconducting state of lithium has not been found at ambient pressure, but the present theoretical work shows that high values of the critical temperature, T(c), may be expected for some high-pressure phases. Ab initio electronic structure calculations are used to calculate the electron-phonon coupling in a "rigid-muffin-tin approximation," and estimates using McMillan's formula suggest that under increasing pressure T(c) in fcc-Li may reach 50--70 K before transitions occur to the rhombohedral (hR1-Li) and subsequently to the cI16-Li phase near 40 GPa. In cI16-Li T(c) may reach a maximum in the range 60--80 K.     

压力诱导酌拓扑绝缘体化合物Bi2Te3常压相的超导性

通过高压电阻测量,发现了拓扑绝缘体化合物BizTe3压力诱导的超导性,在3-6GPa的压力范围内,超导临界温度L约为3K．高压下原位同步辐射的结果证明这个超导相来源于常压相结构．通过霍尔效应的测量,发现超导的Bi2Te3样品的载流子为P型．对高压同步辐射结果Reitveld精修得到的晶格参数和原子位置,并以此进行第一性...     

Evidence for a magnetic collapse in the epsilon phase of solid oxygen

Solid oxygen is the only elementary molecular magnet. Under the very high pressure of 96 GPa oxygen transforms into a metal and a superconductor. Theory predicts a nonmagnetic state occurring before the transition into the superconducting xi phase. Nevertheless, until now there was no direct evidence of a magnetic collapse in high-pressure oxygen. For the first time direct information is provided on magnetic properties of the epsilon phase, which is sandwiched between the antiferromagnetic delta phase and the superconducting xi phase. We used magnetic neutron diffraction. The data show that the long-range magnetic order disappears at the delta-epsilon transition. The magnetic collapse occurs at P approximately equal to 8 GPa, far below the pressure of the insulator-metal (superconductor) transition. The collapse is preceded by a decrease in temperature of transition towards the long-range magnetically ordered state (T(LRO)) in the delta phase, at P = 7.6 GPa.     

First-Principles Prediction of High-Pressure Phase of CaC6

The lattice dynamics of rhombohedral GaG6 is studied as a function orpressure to probe Its high pressure phase with low superconducting transition temperature using the density functional liner-response theory. The pressureinduced phase transition in CaC6 is attributable to the softening transverse acoustic （TA） phonon mode at the zone boundary X （0.5, 0.0, 0.5） point. The high pressure phase is then explored by performing fully structural optimization in the supercell which accommodates the atomic displacements corresponding to the eigenvectors of the unstable mode of TA（X）. The high-pressure phase is predicted to be a monoclinic unit cell with space group P21/m.     

Electrostatic tuning of the hole density in NdBa2Cu3O(7-delta) films and its effect on the Hall response

We have used the ferroelectric field effect in heterostructures based on superconducting NdBa2-Cu(3)O(7-delta) and ferroelectric Pb(Zr0.2Ti0.8)O3 to electrostatically modulate in a reversible and nonvolatile fashion the hole carrier density of the superconducting layer. Reversing the ferroelectric polarization induces a constant relative change in the resistivity and Hall constant of 9% and 6%, respectively, at all temperatures above the superconducting transition. The cotangent of the Hall angle displays a T2 dependence with a slope that increases as the carrier density is reduced.