共查询到20条相似文献，搜索用时 125 毫秒
1.
G Blumberg M Kang MV Klein K Kadowaki C Kendziora 《Science (New York, N.Y.)》1997,278(5342):1427-1432
Electronic Raman scattering from high- and low-energy excitations was studied as a function of temperature, extent of hole doping, and energy of the incident photons in Bi2Sr2CaCu2O8+/-delta superconductors. For underdoped superconductors, short-range antiferromagnetic (AF) correlations were found to persist with hole doping, and doped single holes were found to be incoherent in the AF environment. Above the superconducting (SC) transition temperature Tc, the system exhibited a sharp Raman resonance of B1g symmetry and energy of 75 millielectron-volts and a pseudogap for electron-hole excitations below 75 millielectron-volts, a manifestation of a partially coherent state forming from doped incoherent quasi particles. The occupancy of the coherent state increases with cooling until phase ordering at Tc produces a global SC state. 相似文献
2.
Laurent P Rodriguez J Wilms J Cadolle Bel M Pottschmidt K Grinberg V 《Science (New York, N.Y.)》2011,332(6028):438-439
Because of their inherently high flux allowing the detection of clear signals, black hole x-ray binaries are interesting candidates for polarization studies, even if no polarization signals have been observed from them before. Such measurements would provide further detailed insight into these sources' emission mechanisms. We measured the polarization of the gamma-ray emission from the black hole binary system Cygnus X-1 with the International Gamma-Ray Astrophysics Laboratory Imager on Board the Integral Satellite (INTEGRAL/IBIS) telescope. Spectral modeling of the data reveals two emission mechanisms: The 250- to 400-keV (kilo-electron volt) data are consistent with emission dominated by Compton scattering on thermal electrons and are weakly polarized. The second spectral component seen in the 400-keV to 2-MeV band is by contrast strongly polarized, revealing that the MeV emission is probably related to the jet first detected in the radio band. 相似文献
3.
Abbamonte P Venema L Rusydi A Sawatzky GA Logvenov G Bozovic I 《Science (New York, N.Y.)》2002,297(5581):581-584
An unresolved issue concerning cuprate superconductors is whether the distribution of carriers in the CuO2 plane is uniform or inhomogeneous. Because the carriers comprise a small fraction of the total charge density and may be rapidly fluctuating, modulations are difficult to detect directly. We demonstrate that in anomalous x-ray scattering at the oxygen K edge of the cuprates, the contribution of carriers to the scattering amplitude is selectively magnified 82 times. This enhances diffraction from the doped holes by more than 10(3), permitting direct structural analysis of the superconducting ground state. Scattering from thin films of La2CuO4+delta (superconducting transition temperature = 39 K) at temperature = 50 +/- 5 kelvin on the reciprocal space intervals (0,0,0.21) --> (0,0,1.21) and (0,0,0.6) --> (0.3,0,0.6) shows a rounding of the carrier density near the substrate suggestive of a depletion zone or similar effect. The structure factor for off-specular scattering was less than 3 x 10(-7) electrons, suggesting an absence of in-plane hole ordering in this material. 相似文献
4.
Studies on the long-term degradation of organic light-emitting devices (OLEDs) based on tris(8-hydroxyquinoline) aluminum (AlQ3), the most widely used electroluminescent molecule, reveal that injection of holes in AlQ3 is the main cause of device degradation. The transport of holes into AlQ3 caused a decrease in its fluorescence quantum efficiency, thus showing that cationic AlQ3 species are unstable and that their degradation products are fluorescence quenchers. These findings explain the success of different approaches to stabilizing OLEDs, such as doping of the hole transport layer, introducing a buffer layer at the hole-injecting contact, and using mixed emitting layers of hole and electron transporting molecules. 相似文献
5.
Wang Y Ono S Onose Y Gu G Ando Y Tokura Y Uchida S Ong NP 《Science (New York, N.Y.)》2003,299(5603):86-89
We have determined the upper critical field Hc2 as a function of hole concentration in bismuth-based cuprates by measuring the voltage induced by vortex flow in a driving temperature gradient (the Nernst effect), in magnetic fields up to 45 tesla. We found that Hc2 decreased steeply as doping increased, in both single and bilayer cuprates. This relationship implies that the Cooper pairing potential displays a trend opposite to that of the superfluid density versus doping. The coherence length of the pairs xi(0) closely tracks the gap measured by photoemission. We discuss implications for understanding the doping dependence of the critical temperature Tc0. 相似文献
6.
The magnon pairing mechanism is derived to explain the high-temperature superconductivity of both the La2-xSrxCu(1)O(4) and Y(1)Ba(2)Cu(3)O(7) systems. Critical features include (i) a one- or two-dimensional lattice of linear Cu-O-Cu bonds that contribute to large antiferromagnetic (superexchange) coupling of the Cu(II)(d(9)) orbitals; (ii) holes in the oxygen ppi bands [rather than Cu(III)(d(8))] leading to high mobility hole conduction; and (iii) strong ferromagnetic coupling between oxygen ppi holes and adjacent Cu(II)(d(9)) electrons. The ferromagnetic coupling of the conduction electrons with copper d spins induces the attractive interaction responsible for the superconductivity, leading to triplet-coupled pairs called "tripgems." The disordered Heisenberg lattice of antiferromagnetically coupled copper d spins serves a role analogous to the phonons in a conventional system. This leads to a maximum transition temperature of about 200 K. For La(1.85)Sr(0.15)Cu(1)O(4), the energy gap is in excellent agreement with experiment. For Y(1)Ba(2)Cu(3)O(7), we find that both the CuO sheets and the CuO chains can contribute to the supercurrent. 相似文献
7.
Bourges P Sidis Y Fong HF Regnault LP Bossy J Ivanov A Keimer B 《Science (New York, N.Y.)》2000,288(5469):1234-1237
A comprehensive inelastic neutron scattering study of magnetic excitations in the near optimally doped high-temperature superconductor YBa(2)Cu(3)O(6.85) is presented. The spin correlations in the normal state are commensurate with the crystal lattice, and the intensity is peaked around the wave vector characterizing the antiferromagnetic state of the insulating precursor, YBa(2)Cu(3)O(6). Profound modifications of the spin excitation spectrum appear abruptly below the superconducting transition temperature T(c), where a commensurate resonant mode and a set of weaker incommensurate peaks develop. The data are consistent with models that are based on an underlying two-dimensional Fermi surface, predicting a continuous, downward dispersion relation connecting the resonant mode and the incommensurate excitations. The magnetic incommensurability in the YBa(2)Cu(3)O(6+)(x) system is thus not simply related to that of another high-temperature superconductor, La(2-)(x)Sr(x)CuO(4), where incommensurate peaks persist well above T(c). The temperature-dependent incommensurability is difficult to reconcile with interpretations based on charge stripe formation in YBa(2)Cu(3)O(6+x) near optimum doping. 相似文献
8.
Crawford MK Farneth WE McCarronn EM Harlow RL Moudden AH 《Science (New York, N.Y.)》1990,250(4986):1390-1394
The oxygen isotope effect on the superconducting transition temperature (alpha(o)) varies as a function of x in La2-xSrxCuO(4) and La2-xBaxCuO(4), with the maximum alpha(o) values (alpha(o) >/= 0.5) found for x near 0.12. This unusual x dependence implies that the isotope effect is influenced by proximity to the Abma --> P4(2)/ncm structural phase transition in these systems. Synchrotron x-ray difaction measurements reveal little change in lattice parameters or orthorhombicity due to isotope exchange in strontium-doped materials where alpha(o) > 0.5, eliminating static structural distortion as a cause of the large isotope effects. The anomalous behavior of alpha(o) in both strontium- and barium-doped materials, in combination with the previously discovered Abma --> P4(2)/ncm structural phase-transition in La(1.88)B(0.12)CuO(4), suggests that an electronic contribution to the lattice instability is present and maximizes at approximately 1/8 hole per copper atom. These observations indicate a dose connection between hole doping of the Cu-O sheets, tilting instabilities of the CuO(6) octahedra, and superconductivity in La(2)CuO(4)-based superconductors. 相似文献
9.
N Webb D Cseh E Lenc O Godet D Barret S Corbel S Farrell R Fender N Gehrels I Heywood 《Science (New York, N.Y.)》2012,337(6094):554-556
Relativistic jets are streams of plasma moving at appreciable fractions of the speed of light. They have been observed from stellar-mass black holes (~3 to 20 solar masses, M(⊙)) as well as supermassive black holes (~10(6) to 10(9) M(⊙)) found in the centers of most galaxies. Jets should also be produced by intermediate-mass black holes (~10(2) to 10(5) M(⊙)), although evidence for this third class of black hole has, until recently, been weak. We report the detection of transient radio emission at the location of the intermediate-mass black hole candidate ESO 243-49 HLX-1, which is consistent with a discrete jet ejection event. These observations also allow us to refine the mass estimate of the black hole to be between ~9 × 10(3) M(⊙) and ~9 × 10(4) M(⊙). 相似文献
10.
以聚丁（对甲苯硫酸）－２，４－己炔－１，６－二醇酯（ＰＤＡ－ＴＳ）为例，探讨了含共轭三键的聚合物的掺杂和导电特性的研究，对其微观掺杂机制，在比较现有的掺杂模型的基础上，提出了新的掺杂模型。 相似文献
11.
Kliewer J Berndt R Chulkov EV Silkin VM Echenique PM Crampin S 《Science (New York, N.Y.)》2000,288(5470):1399-1402
A long-standing discrepancy between experimental and theoretical values for the lifetimes of holes in the surface-state electron bands on noble metal surfaces is resolved; previous determinations of both are found to have been in error. The ability of the scanning tunneling microscope to verify surface quality before taking spectroscopic measurements is used to remove the effects of defect scattering on experimental lifetimes, found to have been a significant contribution to prior determinations. A theoretical treatment of inelastic electron-electron scattering is developed that explicitly includes intraband transitions within the surface state band. In our model, two-dimensional decay channels dominate the electron-electron interactions that contribute to the hole decay and are screened by the electron states of the underlying three-dimensional electron system. 相似文献
12.
2011 年5耀10 月在内蒙古呼伦贝尔市海拉尔东山,对达乌尔黄鼠的12 个未哺乳达乌尔黄鼠洞穴进行挖
掘和测量结果显示,达乌尔黄鼠洞穴有临时洞夏季洞”和居住洞冬眠洞两种洞穴的洞口朝向一般为西北或东
北方向临时洞的结构较简单多数只有一个洞口曰冬眠洞的结构较复杂冬眠洞的洞口光滑洞口宽度一般为6,9
cm洞口近处的洞道斜向下方,然后急转向下近乎垂直接着斜行一段入巢洞穴深度多为90,100 cm洞穴总长为
110,126 cm分叉较少洞道中部的巢穴形状扁平。巢穴中絮有碎草,塑料,泡沫等。巢穴附近有厕所,大多数达乌尔
黄鼠洞穴有盲洞,盲洞位于离洞口30,60 cm 处 相似文献
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14.
Lee P Gao Y Sheu HS Petricek V Restori R Coppens P Darovskikh A Phillips JC Sleight AW Subramanian MA 《Science (New York, N.Y.)》1989,244(4900):62-63
The distribution of the bismuth atoms over the cation sites in the 2212 Bi-Sr-Ca-Cu-O superconductor has been determined by anomalous scattering synchrotron crystallography. The analysis of reflection pairs measured at wavelengths of 0.9243 and 0.9600 angstrom shows a delocalization of the bismuth atoms over the calcium and strontium sites. The "mixed" plane between the CuO(2) layers contains 6.0(1.4) percent bismuth (where the number in brackets represents the statistical standard deviation derived from the least-squares refinement of the data), and a much smaller amount of strontium than often assumed. The strontium deficiency is charge-compensated by the creation of electron holes in the CuO(2) layer. The result supports the view that neither extra oxygen nor overlap of the bismuth 6p and copper 3d bands is needed to account for the holes, which are an essential feature of the superconductivity mechanism. 相似文献
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16.
以沙柳和高密度聚乙烯（HDPE）为主要原料，通过沙柳碎料和高密度聚乙烯均匀混合的方式，采用热压工艺制备沙柳/HDPE复合碎料板，使用驻波管法研究背腔深度0~50 mm、钻孔深度0~18 mm、钻孔直径0~5 mm和孔排列形式（圆形、正方形、米字形和无规则）对沙柳/HDPE复合碎料板在低频区域200~2 000 Hz范围内声学性能的影响机制。结果表明:1)随背腔深度增加，沙柳/HDPE碎料板与空气层组成的板/腔结构平均吸声系数和降噪系数均呈增大变化趋势，最小吸声系数频率由400 Hz降至315 Hz，吸声系数趋于稳定频率向低频区域移动；2)随钻孔深度增加，沙柳/HDPE复合碎料板平均吸声系数和降噪系数呈增大变化趋势，最小吸声系数频率向低频区域移动，吸声系数趋于稳定频率向低频区域移动；3)复合碎料板平均吸声系数和降噪系数随直径增加而增大，最小吸声系数频率向低频区域移动；4)孔排列形式对平均吸声系数和降噪系数性能影响不明显，正方形排列综合声学性能优于米字形、无规则和圆形排列。通过改变孔形态或调整背腔深度，可提高沙柳/HDPE复合碎料板吸声性能和调整吸声频段。 相似文献
17.
孔洞对木梁弯曲应变分布影响的试验研究 总被引：2，自引：2，他引：0
为了研究孔洞对木梁弯曲应变分布的影响,采用数字图像相关法进行了无疵木梁和含孔洞木梁的四点弯曲试验,分析了3种不同的孔洞位置(孔洞位于中心、受压区和受拉区)对木梁弯曲应变分布的影响,探讨了中性轴位置的偏移规律。结果表明:在极限载荷时,3种不同孔洞位置的木梁,其压应变区域均大于拉应变区域,其中孔洞位于受压区时木梁的压应变区域最大,孔洞位于中心时次之,孔洞位于受拉区时最小。并且在整个加载过程中,中性轴会随着载荷的增大向木梁下缘偏移,孔洞位于受压区时中性轴的偏移距离最大,孔洞位于中心时次之,孔洞位于受拉区时最小。在此基础上,依据平截面假定和弹塑性理论的Hoffman屈服准则,初步分析了含孔洞木梁弯曲应变分布和中性轴偏移规律的机理。研究结果为进一步从理论上定量推导含孔洞木梁的弯曲应变和应力计算公式提供了依据。 相似文献
18.
Xu G Aeppli G Bisher ME Broholm C DiTusa JF Frost CD Ito T Oka K Paul RL Takagi H Treacy MM 《Science (New York, N.Y.)》2000,289(5478):419-422
Magnetic neutron scattering provides evidence for nucleation of antiferromagnetic droplets around impurities in a doped nickel oxide-based quantum magnet. The undoped parent compound contains a spin liquid with a cooperative singlet ground state and a gap in the magnetic excitation spectrum. Calcium doping creates excitations below the gap with an incommensurate structure factor. We show that weakly interacting antiferromagnetic droplets with a central phase shift of pi and a size controlled by the correlation length of the quantum liquid can account for the data. The experiment provides a quantitative impression of the magnetic polarization cloud associated with holes in a doped transition metal oxide. 相似文献
19.
在青海省大通东峡林区的白桦林中,用钻孔法取汁,4月中旬开始溢出,5月中下旬终止,产汁量周期变化过程用修正的Logistic曲线方程模拟,效果理想(R=0.986)。一天中白天产汁量占60％～62％,夜里不超过40％,日产汁高峰出现在12:00～16:00时区。产汁量与树木胸径大小没有直接关系,采汁方向不影响采汁量,桦汁中含11种氨基酸,19种微量元素,还有糖类和少量维生素。 相似文献
20.
[目的]探讨3种能源微藻细胞数量和藻液光密度之间的相关性,验证以光密度法测定细胞生长密度的可行性,为能源微藻的生长和生理生化研究提供简便方法与参考依据.[方法]选取三角褐指藻Phaeod1actylum tricornutum、亚心形扁藻Platymonas subcordiformis和海洋小球藻Chlorella vulgaris 3种能源微藻为生物材料,以添加f/2营养液的人工海水为培养介质,在植物培养箱中利用三角瓶进行一次性培养试验.利用显微镜计数法观测微藻的细胞密度,测定各藻液在680 nm处的光密度值,并进行线性拟合获取回归方程,比较两种测定方法所得的藻细胞密度值.[结果]3种能源微藻的细胞密度与藻液光密度有显著的线性正相关,其回归方程分别为:三角褐指藻细胞密度(104个/mL)=1405OD680-59.745,R2=0.9942;亚心形扁藻细胞密度(104个/mL)=216.94 OD680+0.9743,R2=0.9947;海洋小球藻细胞密度(104个/mL )=2170.1 OD680+ 18.569,R2=0.9927.在各微藻生长第5、7和9d中,利用显微镜计数法所得细胞数目和光密度法换算所得细胞数目没有明显差别.[结论]光密度法是一种可测定微藻细胞生长的快速、简便、有效的方法,可广泛应用于能源微藻的生长、生理生化研究. 相似文献