THE SCREENING EFFECT OF PHONON MODES IN HIGH-Tc OXIDE SUPERCONDUCTOR

This paper presents three different kinds of infrared (IR) spectra of oxide materials. The first one is the IR reflection spectra of Ti₂Ba₂Ca₂Cu₃O₁₀ superconducting film at the incident angles 7°, 12° and 30°. The second one is the IR spectra of nonsuperconducting ceramic materials YBa₂Cu₃O₆ and PrBa₂Cu₃O₇. The third one is the IR spectra of superconductor YBa₂Cu₃O_7-δ and of its film with the c-axis perpendicular to the film plane, as well as the changes in the spectra of these materials after annealing at different temperatures. It is found that in the spectra of high-T_c oxide superconductor, only the phonon modes along c-axis can be observed while the modes in a-b plane are absent. But in the spectra of nonsuperconductors the modes in a-b plane can be observed, particularly in the absorption spectra of the semiconductor YBa₂Cu₃O₆, all eleven allowed IR active modes have been detected. We suggest that the above experimental phenomena may result from the fact that the phonon modes vibrating in the a-b plane of the high-T_c oxide superconducting materials are almost screened by the free carriers which can move only in a-b plane.     

Dispersion of neutron spin resonance mode in Ba0.67K0.33Fe2As2

We report an inelastic neutron scattering investigation on the spin resonance mode in the optimally hole-doped iron-based superconductor Ba_0.67K_0.33Fe₂As₂ with T_c=38.2 K. Although the resonance is nearly two-dimensional with peak energy E_R≈14 meV, it splits into two incommensurate peaks along the longitudinal direction ([H, 0, 0]) and shows an upward dispersion persisting to 26 meV. Such dispersion breaks through the limit of total superconducting gaps ∆_tot=|∆_k|+|∆_k+Q|(about 11-17 meV) on nested Fermi surfaces measured by high resolution angle resolved photoemission spectroscopy (ARPES). These results cannot be fully understood by the magnetic exciton scenario under s^±-pairing symmetry of superconductivity, and suggest that the spin resonance may not be restricted by the superconducting gaps in the multi-band systems.     

MgB2超导薄膜的变温拉曼光谱研究

MgB₂作为迄今为止超导转变温度最高的合金超导体,由于其具有结构简单、相干长度长、晶界间不存在弱连接、上临界场很高、电-声散射时间短等特点,MgB₂超导薄膜在电子学领域有着广阔的的应用前景。拉曼光谱是研究电-声子相互作用和超导能带的一种有效方法,且已广泛用于分析MgB₂材料的电子、声子特征以及超导体能带结构,研究表明,样品质量、晶粒尺寸以及测试条件对MgB₂拉曼峰的峰位和峰形影响很大,其中拉曼光谱随温度的变化也是一个研究重点,但目前关于MgB₂变温拉曼光谱的研究,测试的温度范围相对较小,局限在83 K到室温区域或是转变温度附近。研究了大范围温度区间内MgB₂薄膜的拉曼光谱变化,采用混合物理化学沉积法在(0001)SiC衬底上制备了MgB₂多晶薄膜,薄膜的晶粒尺寸约为300 nm,超导转变温度为39.3 K,对其在10～293 K之间的拉曼光谱进行了测试,测量的波数范围为20～1 200 cm-1。变温拉曼光谱的测试结果显示,在高频620 cm-1附近以及低频80和110 cm-1附近存在MgB₂的拉曼峰。经分析,低频区域出现的两个拉曼峰的频率与超导能隙宽度相对应,表明MgB₂的双能隙特性。考虑到MgB₂中四种声子模式的拉曼活性,高频620 cm-1附近的拉曼峰应是由E_2g振动模所贡献的,且随着测试温度的降低,该拉曼峰的峰位未发生明显的偏移,但半高宽显著变小,从293 K时的380.7 cm-1减小到10 K时的155 .7 cm-1,分析表明E_2g声子与电子系统的非线性耦合所引起的非简谐效应可能是拉曼峰半高宽线性变小的主要原因。     

Magnetic transition behavior of perovskite manganites Nd_(0.5)Sr_(0.3)Ca_(0.2)MnO_3 polycrystalline

A polycrystalline sample Nd_(0.5)Sr_(0.3)Ca_(0.2)MnO_3 is prepared by the conventional solid state reaction method. The structure and magnetic properties are investigated with x-ray diffraction(XRD) patterns, a superconducting quantum interference device(SQUID), and electron spin resonance(ESR). The sample is in single phase with the space group Pbnm symmetry. With the decrease of temperature, Nd_(0.5)Sr_(0.3)Ca_(0.2)MnO_3 undergoes three magnetic transitions: ferromagnetic transition at TC≈ 210 K, charge-ordering at TCO≈ 175 K, and antiferromagnetic transition at TN= 155 K. In addition, the activation energy Ea ≈ 52.78 me V can be extracted by curve fitting.     

Neutron scattering studies on heavy-fermion superconductors

We review recent neutron scattering studies on uranium-based heavy-fermion superconductors. The coupling between magnetic and superconducting order parameters was observed in UPt₃, UPd₂Al₃, URu₂Si₂, and UNi₂Al₃. In UPd₂Al₃, the superconducting gap appears in the spin excitation spectra. These results are indicative of the strong interplay between magnetism and superconductivity. We also report the unusual behaviors of the weak antiferromagnetic ordering, the long-range magnetic correlation in UPt₃ at ultra-low temperatures, and the pressure-induced magnetic transition from the weak (0.02μ_B/U) to a high moment state (0.4μ_B/U) at 1.5 GPa in URu₂Si₂.     

Non-resonant microwave absorption and critical current density in YBa2Cu3Ox films prepared by laser ablation

High-quality YBa₂Cu₃O_x superconducting thin films have been prepared on (100)MgO substrates by the laser ablation technique, and characterized by non-resonant microwave absorption (NRMA) with a field modulation, magnetization and four-probe measurements. The pinning properties derived from the NRMA spectra for various YBa₂Cu₃O_x films are correlated with the critical current densities T_c. A relatively high-J_c film prepared from YBa₂Cu₃O_x composition target using a Nd:YAG laser shows a derivative spectrum and a large hysteresis in the NRMA spectra. The highest-J_c film prepared from an YBa₂Cu₄O_x composition (YBa₂Cu₃O_x+CuO) target using an ArF excimer laser does not show a derivative NRMA spectrum, suggesting a strong pinning property.     

Cu1-xHxZr2(PO4)3中Cu2+的EPR谱和局域结构研究

本文采用Cu²⁺斜方对称电子顺磁共振（EPR）参量的高阶微扰公式计算了晶体Cu_1-xH_xZr₂（PO₄）₃中Cu²⁺的EPR参量（g因子和超精细结构常数A因子）．计算结果表明,晶体Cu_1-xH_xZr₂（PO₄）₃中[CuO₆]^10-基团的Cu-O键长分别为R_||≈0.241 nm,R_⊥≈0.215 nm,平面键角τ≈80.1°;由于对称性降低,中心金属离子基态²A_1g（θ）和²A_1g（ε）有一定程度混合,混合系数α≈0.995．所得EPR谱图的理论计算值与实验数据符合得很好．     

基于磁圆二向色谱的单层MoS_2激子能量和线宽温度依赖特性

以二硫化钼(MoS_2)为代表的过渡金属硫属化物属于二维层状材料,样品可以薄至单层.单层MoS_2是一种直接带隙半导体,在纳米逻辑器件、高速光电探测、纳米激光等领域具有广阔的应用前景.在实际应用中,温度是影响半导体材料能带结构和性质的主要因素之一.因此研究单层二维材料能带的温度依赖特性对理解其物理机理以及开展器件应用具有重要的意义.目前,在广泛采用的测量单层MoS_2反射谱的研究中,激子峰往往叠加在一个很强的光谱背底上,难以准确分辨激子的峰位和线宽.基于自行搭建的显微磁圆二向色谱系统,研究了单层MoS_2在65—300 K温度范围内的反射谱和磁圆二向色谱,结果表明磁圆二向色谱在研究单层材料激子能量和线宽方面具有明显的优势.通过分析变温的磁圆二向色谱,得到了不同温度下的A,B激子的跃迁能量和线宽.通过对激子能量和线宽的温度依赖关系进行拟合,进一步讨论了声子散射对激子线宽的影响.     

Epitaxial Bi2Sr2CuOy thin films as p-type transparent conductors

Development of p-type transparent conducting thin films is tireless due to the trade-off issue between optical transparency and conductivity. The rarely concerned low normal state resistance makes Bi-based superconducting cuprates the potential hole-type transparent conductors, which have been realized in Bi₂Sr₂CaCu₂O_y thin films. In this study, epitaxial superconducting Bi₂Sr₂CuO_y and Bi₂Sr_1.8Nd_0.2CuO_y thin films with superior normal state conductivity are proposed as p-type transparent conductors. It is found that the Bi₂Sr_1.8Nd_0.2CuO_y thin film with thickness 15 nm shows an average visible transmittance of 65% and room-temperature sheet resistance of 650 Ω/sq. The results further demonstrate that Bi-based cuprate superconductors can be regarded as potential p-type transparent conductors for future optoelectronic applications.     

MgB2超导膜的厚度与其Jc(5K,0T)的关系

通过混合物理化学气相沉积法 (hybrid physical-chemical vapor deposition, HPCVD), 在(000l) SiC 衬底上制得一系列从10 nm到8 μm的MgB₂超导膜样品, 并对它们的形貌、超导转变温度T_c 和临界电流密度J_c与膜厚度的关系进行了研究. 观察到T_c随膜厚度增加上升到最大值后, 尽管膜继续增厚, 但T_c值保持近乎平稳, 而J_c则先随膜厚度增加上升到最高值后, 继而则随膜的厚度的增加而下降. MgB₂膜的T_c(0)和T_c(onset)值与膜厚的关系基本一致, T_c(0)在膜厚为230 nm处达到最大值T_c(0)=41.4 K, 而J_c(5K,0T)在膜厚为100 nm时达到最大值, J_c (5 K, 0 T)=2.3×10⁸A·cm^-2, 这也说明了我们能用HPCVD方法制备出高质量干净MgB₂超导膜. 本文研究的超导膜厚度变化跨度非常大, 从10 nm级的超薄膜到100 nm级的薄膜, 再到几微米的厚膜, 如此T_c和J_c对膜厚度变化的依赖就有了较完整、成体系的研究. 并且本文的工作对MgB₂超导薄膜制备的厚度选取具有实际应用意义. 关键词： 2超导膜')" href="#">MgB₂超导膜 混合物理化学气相沉积法 厚度 临界电流密度     

基于LED光源的非相干宽带腔增强吸收光谱技术探测HONO和NO2

介绍了基于近紫外发光二极管LED (中心波长约372 nm,半高宽13 nm) 光源的非相干宽带腔增强吸收光谱技术,同时用于探测痕量气体HONO和NO₂. LED出射光经准直后耦合进入长度为70 cm,由两块高反射率镜片组成的高精密光学腔内. 分别测量了氮气消光谱和氦气消光谱,通过两者瑞利散射截面的差异而引起光谱强度的变化来标定镜片反射率.在360—390 nm反演波段内,镜片反射率在390 nm处最大且为0.99962, 对应测量NO₂/HONO混合物时的最大光程约1.71 km,并利用最小二乘拟合反演出了 HONO和NO₂的浓度值.当光谱采集时间为1000 s时, HONO和NO₂的探测灵敏度(1σ) 分别为0.6 ppbv和1.9 ppbv.实验结果表明,该技术为实现大气痕量气体的高灵敏度在线监测提供了另一种可能的途径.     

Observation of pseudogap in SnSe2 atomic layers grown on graphite

Superconducting metal dichalcogenides (MDCs) present several similarities to the other layered superconductors like cuprates. The superconductivity in atomically thin MDCs has been demonstrated by recent experiments, however, the investigation of the superconductivity intertwined with other orders are scarce. Investigating the pseudogap in atomic layers of MDCs may help to understand the superconducting mechanism for these true two-dimensional (2D) superconducting systems. Herein we report a pseudogap opening in the tunneling spectra of thin layers of SnSe₂ epitaxially grown on highly oriented pyrolytic graphite (HOPG) with scanning tunneling microscopy/spectroscopy (STM/STS). A significant V-shaped pseudogap was observed to open near the Fermi level (E_F) in the STS. And at elevated temperatures, the gap gradually evolves to a shallow dip. Our experimental observations provide direct evidence of a pseudogap state in the electron-doped SnSe₂ atomic layers on the HOPG surface, which may stimulate further exploration of the mechanism of superconductivity at 2D limit in MDCs.     

MgO(111)衬底MgB2 超薄膜的制备和性质研究

利用混合物理化学气相沉积法(HPCVD)在MgO(111)衬底上制备了干净的MgB₂超导超薄膜. 在背景气体压强, 载气氢气流量以及沉积时间一定的情况下, 改变B₂H₆的流量, 制备得到不同厚度系列的MgB₂超导薄膜样品, 并测量了其超导转变温度 T_c, 临界电流密度J_c等临界参量. 该系列超导薄膜沿c轴外延生长, 表面具有良好的连接性, 且有很高的超导转变温度T_c(0) ≈ 35-38 K和很小的剩余电阻率ρ(42 K) ≈ 1.8-20.3 μΩ·cm^-1. 随着膜厚的减小而减小, 临界温度变低, 而剩余电阻率变大. 其中20 nm的样品在零磁场, 5K时的临界电流密度J_c ≈ 2.3×10⁷ A/cm². 表明了利用HPCVD在MgO(111)衬底上制备的MgB₂超薄膜有很好的性能, 预示了其在超导电子器件中广阔的应用前景. 关键词： MgO(111)衬底 2超薄膜')" href="#">MgB₂超薄膜 混合物理化学气相沉积     

Microscopic measurements in La-NQR of the ternary carbide superconductor LaNiC2

¹³⁹La-NQR measurements have been carried out in the ternary carbide superconductor LaNiC₂. The nuclear quadrupole frequency and the asymmetry parameter of ¹³⁹La in LaNiC₂ were estimated to be about 1.9 MHz and 0, respectively. In the normal state, the nuclear spin relaxation rate (1/T₁) in the ¹³⁹La NQR signal was proportional to temperature (T) in zero external field above the superconducting transition temperature (T_c) or in an external field larger than the superconducting critical field, which means the system is in the Fermi-liquid state. In the superconducting state, on the other hand, 1/T₁ decreases no more linearly with T, but decreases rapidly exponentially as exp (−Δ/k_BT) at low T with an appreciable enhancement just below T_c. The value of the superconducting energy gap, 2Δ, was estimated to be 3.34k_BT_c, compared with 3.52k_BT_c of the BCS-value. This result strongly suggests that the superconductivity in LaNiC₂ is of a conventional BCS type.     

STUDIES ON INFRARED AND PHOTOLUMINESCENCE PROPERTIES OF CdIn2O4 FILMS DEPOSITED BY RF REACTIVE SPUTTERING

Transparent and conductive CdIn₂O₄(CIO) thin films were prepared successfully by rf reactive sputtering from a Cd-In alloy target in an Ar+O₂ atmosphere. The measurement of transmission and reflection spectra, at wavelengths between 0.2 and 6.0μm, and photoluminescence spectrum of the films are reported. The optical properties of the films are analyzed and discussed in detail. Two reasonable methods for calculating the effective mass of the free carrier in the films are presented.     

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.     

Commensurate and incommensurate spin fluctuations in YBa2Cu3O6+y

We present an interpretation of the recent neutron data on the commensurate and incommensurate spin fluctuations found in YBa₂Cu₃O_6+y based on a special configuration of the electronic dispersion and intervention from the d_x²−y²-wave superconducting phase. The observed switch-over between the commensurate and incommensurate fluctuation spectra at the change of frequency or temperature is naturally accounted within this scenario.     

Pair-Breaking Critical Current Density of Two-Band Superconductor MgB2

Temperature dependence of tbe pair-breaking critical current density of MgB2, jd（T）, is studied using a two-band Ginzburg-Landau theory. The results are shown to be in good agreement with experimental data for the superconducting magnesium diboride MgB2.     

POSITRON LIFETIME STUDIES OF Y1-xCaxBa2Cu3O7-δ AND Y1-xCaxBa2Cu3-xFexO7-δ

In this paper it is reported that the measurement of the bulk positron lifetime as a function of substitution content x in the temperature range from 70 to 220K was performed in two high-T_c superconducting systems, Y_1-xCa_xBa₂Cu_3-xFe_xO_7-δ and Y_1-xCa_xBa₂Cu₃O_7-δ. It was found that τ_B of both systems decreases significantly with x and the temperature dependence of τ_B is very weak in normal state. In lower temperature region (Tc), a dramatic x-dependent temperature variation of τ_B was observed in the Ca-substituted system: from a decrease of τ_B with decreasing T to an increase of τ_B. With increasing x, the temperature dependence of τ_B remains weak in the Ca- and Fe- substituted systems. Compared with the experimental data of positron lifetime in other substituted systems and the calculation of the positron density distribution, the authors suggest that positron bulk lifetime spectra behaviour can be interpreted by the physical model based on the transfer of electron density between the CuO₂ planes and Cu-O chains. Therefore, the study of positron lifetime spectra provides a useful means to detect the local charge density and to study the correlation between the electronic structure and the high-T_c superconductivity.     

The effects of substrate temperature on the superconducting properties of Tl2Ca2Ba2Cu3O10 films sputter-deposited from stoichiometric oxide targets

The annealing characteristics and the superconducting properties of Tl₂Ca₂Ba₂Cu₃O₁₀ thin films sputter-deposited onto yttrium- stabilized ZrO₂ substrate at up to 500°C from two stoichiometric oxide targets are reported. The films deposited at 400–500°C were found to require a lower post-annealing temperature than the films deposited at lower temperatures to attain the highest T_c superconducting state, due to a more pronounced Ba diffusion toward the substrate as indicated by their secondary ion mass spectrometry depth profiles. The highest T_c achieved tends to degrade with increasing substrate temperatures, a zero resistance T_c of 121 and ≈90 K, respectively, being observed for the films deposited at -ambient temperature and at 500°C. The formation of the highest T_c phase (Tl₂Ca₂Ba₂Cu₃O₁₀) generally is associated with a sheet type of crystal growth morphology with smooth and aligned surfaces which can be obtained only from the films capable of sustaining prolonged annealing at 900°C. Annealing at lower temperatures (≈860°C) results in the formation of rod or sphere type of morphologies with rough and randomly oriented crystals and the lower T_c phases such as Tl₂Ca₁Ba₂Cu₂O₈.