Coherent d-wave superconducting gap in underdoped La2-xSrxCuO4 by angle-resolved photoemission spectroscopy

We present angle-resolved photoemission spectroscopy data on moderately underdoped La1.855Sr0.145CuO4 at temperatures below and above the superconducting transition temperature. Unlike previous studies of this material, we observe sharp spectral peaks along the entire underlying Fermi surface in the superconducting state. These peaks trace out an energy gap that follows a simple d-wave form, with a maximum superconducting gap of 14 meV. Our results are consistent with a single gap picture for the cuprates. Furthermore our data on the even more underdoped sample La1.895Sr0.105CuO4 also show sharp spectral peaks, even at the antinode, with a maximum superconducting gap of 26 meV.     

一些高Tc超导体的弹性性质

 本文用超声相比较方法测定了高T_c超导体La_1.85Sr_0.15CuO₄，La₂CuO₄和YBa₂Cu₃O₇的纵波和横波声速，进而导出了它们的纵向模量、切变模量、杨氏模量、泊松比、德拜温度及定体比热。在La_1.85Sr_0.15CuO₄样品上，还进行了压力实验，发现所有弹性模量都是随压强增加而增加。定体比热c_V和泊松比σ在高压下则略有下降。德拜温度是随压强增加而增加的。     

对靶溅射技术制备的La2/3Ca1/3MnO3/YBa2Cu4O8/La2/3Ca1/3MnO3薄膜中Tc的降低

本文用对靶溅射技术制备了La2/3Ca1/3MnO3/YBa2Cu4O8/La2/3Ca1/3MnO3薄膜.与YBCO单层薄膜相比,由于超导/铁磁系统中的磁性邻近效应,三层薄膜表现出较低的超导转变温度.薄膜的R～T测量曲线显示出超磁阻(CMR)效应和超导转变,预示着超导和铁磁特性共存于LCMO/YBCO/LCMO三文治结构.     

Evidence for coexistence of the superconducting gap and the pseudogap in Bi-2212 from intrinsic tunneling spectroscopy

We present intrinsic tunneling spectroscopy measurements on small Bi2Sr2CaCu2O8+x mesas. The tunnel conductance curves show both sharp peaks at the superconducting gap voltage and broad humps representing the c-axis pseudogap. The superconducting gap vanishes at Tc, while the pseudogap exists both above and below Tc. Our observation implies that the superconducting and pseudogaps represent different coexisting phenomena.     

Phase diagram of the electron-doped La2-xCexCuO4 cuprate superconductor from Andreev bound states at grain boundary junctions

We use quasiparticle tunneling across La2-xCexCuO4 grain boundary junctions to probe the superconducting state and its disappearance with increasing temperature and magnetic field. A zero bias conductance peak due to zero energy surface Andreev bound states is a clear signature of the phase coherence of the superconducting state. Hence, such a peak must disappear at or below the upper critical field Bc2(T). For La2-xCexCuO4 this approach sets a lower bound for Bc2(0) approximately 25 T which is substantially higher than reported previously. The method of probing the superconducting state via Andreev bound states should also be applicable to other cuprate superconductors.     

Inhomogeneous magnetic-field response of YBa2Cu3Oy and La2-xSrxCuO4 persisting above the bulk superconducting transition temperature

We report that in YBa2Cu3Oy and La2-xSrxCuO4 there is a spatially inhomogeneous response to the magnetic field for temperatures T extending well above the bulk-superconducting transition temperature Tc. An inhomogeneous magnetic response is observed above Tc even in ortho-II YBa2Cu3O6.50, which has highly ordered doping. The degree of the field inhomogeneity above Tc tracks the hole-doping dependences of both Tc and the density of the superconducting carriers below Tc, and therefore is apparently coupled to superconductivity.     

高温超导GdBa_2Cu_3O_(7-δ)薄膜2×4阵列微桥单元及多元器件特性研究

在高 Tc超导 Gd Ba2 Cu3O7-δ薄膜上 ,采用光刻技术制成 2× 4阵列微桥单元及多元器件 ,对器件性能在液氮温度下进行了系统的观测。实验表明 ,Gd Ba2 Cu3O7-δ薄膜超导性能优异 ,稳定性很好 ,桥区尺寸较小的微桥呈现与双晶晶界结相似的 I- V特性 ,可能具有弱连接行为。而多单元串接微桥器件出现的多台阶式的特性 ,可望在红外探测计量及高频方面获得重要应用。     

Charge order, metallic behavior, and superconductivity in La2-xBaxCuO4 with x=1/8

The ab-plane optical properties of a cleaved single crystal of La2-xBaxCuO4 for x=1/8 (Tc approximately =2.4 K) have been measured over a wide frequency and temperature range. The low-frequency conductivity is Drude-like and shows a metallic response with decreasing temperature. However, below approximately =60 K, corresponding to the onset of charge-stripe order, there is a rapid loss of spectral weight below about 40 meV. The behavior is quite different from that typically associated with the pseudogap in the normal state of the cuprates. Instead, the gapping of the normal-state single-particle excitations looks surprisingly similar to that observed in superconducting La2-xSrxCuO4, including the presence of a residual Drude peak with reduced weight.     

Superconductivity at 44.4 K achieved by intercalating EMIM~+ into FeSe

Superconductivity with transition temperature Tc above 40 K was observed in protonated Fe Se(Hy-Fe Se) previously with the ionic liquid EMIM-BF4 used in the electrochemical process. However, the real superconducting phase is not clear until now. And detailed structural, magnetization, and electrical transport measurements are lacking. By using similar protonating technique on Fe Se single crystals, we obtain superconducting samples with Tc above 40 K. We show that the obtained superconducting phase is not Hy-Fe Se but actually an organic-ion(C_6H_(11)N_2~+ referred to as EMIM~+)-intercalated phase(EMIM)x Fe Se. By using x-ray diffraction technique, two sets of index peaks corresponding to different c-axis lattice constants are detected in the obtained samples, which belong to the newly formed phase of intercalated(EMIM)_x Fe Se and the residual Fe Se, respectively. The superconductivity of(EMIM)x Fe Se with Tc of 44.4 K is confirmed by resistivity and magnetic susceptibility measurements. Temperature dependence of resistivity with different applied magnetic fields reveals that the upper critical field H_(c2) is quite high, while the irreversibility field Hirris suppressed quickly with increasing temperature till about 20 K. This indicates that the resultant compound has a high anisotropy with a large spacing between the Fe Se layers.     

Interfacially controlled transient photoinduced superconductivity

We report on a large transient photoinduced enhancement of the superconducting critical temperature (DeltaTc = 23 K) in epitaxial YBa2Cu3O(6.7)/La(0.7)Ca(0.3)MnO3 bilayers upon visible light illumination. The effect relaxes with a characteristic time of 100 s at low temperatures, which is 4 orders of magnitude faster than the persistent photoconductivity or persistent photoinduced superconductivity previously found in single high-Tc superconducting films. This result is discussed in terms of light induced charge transfer through the interface similar to what happens in semiconductor junctions.     

d+s波超导体磁掺杂转变

研究了Anderson掺杂极限Δs/Δd《1的情况下,d+s波对称下的超导态。此模型包括哈密顿量中类似BSC项和自恰平均场似下的Anderson掺杂。随着掺杂中心数的增加或比率Δs/Δd的减小,可推出从低能下在费米能级附近具有双峰的态到N(0)≈(Δs/Δd)2态的转变。如果掺杂共振的能量为最小能量标度,则转变不连续。     

固相反应法制备高温超导材料YBCO实验的研究

介绍了固相反应法制备高温超导材料钇钡铜氧（YBCO）的实验方法,并研究了YBCO样品制备过程中不同烧结时间对样品的超导转变温度Tc及超导转变宽度ΔTc的影响。给出了实验过程中合适的烧结温度和烧结时间的参考值。     

Magnetic-field dependence of the maximum supercurrent of La2-xCexCuO4-y interferometers: evidence for a predominant dx2-y2 superconducting order parameter

We performed a phase-sensitive test of the symmetry of the superconducting order parameter of the electron doped cuprate La(2-x)Ce(x)CuO(4-y) using a superconducting quantum interferometer with spatially distributed Josephson junctions. The studies were made on a thin film grown on a SrTiO3 tetracrystal substrate. The superconducting transition temperature was about 29 K which indicates that the sample is close to optimal doping. The magnetic field dependence of the critical current gives strong evidence for a predominant dx(2)(-y(2)) order parameter symmetry of the sample measured. It also gives upper limits for the s-wave component in a mixed order parameter of the type s+idx(2)(-y(2)).     

Direct relation between the low-energy spin excitations and superconductivity of overdoped high-Tc superconductors

The dynamic spin susceptibility, chi(")(omega), has been measured over the energy range of 2相似文献   

正常金属/超导/正常金属双垒隧道结中的量子相干输运

考虑到量子相干效应和界面散射效应 ,利用 L ambert理论模型 ,计算正常金属 /绝缘层 /超导 /绝缘层 /正常金属双垒隧道结中的准粒子输运系数和隧道谱。研究表明 :( 1)所有的准粒子输运系数和电导谱在超导能隙之上都随能量作周期性振荡 ,其振荡周期依赖于超导层的厚度 ;( 2 )在超导能隙之上 Andreev反射系数随能量呈现周期性消失现象 ;( 3)在绝缘层势垒强度取很大的隧道极限下 ,超导层中会形成一系列的准粒子束缚态 ,其位置由量子化条件决定 ;( 4)界面散射效应不仅能压低各子能隙电导峰 ,还能使子能隙电导峰劈裂为两个峰。     

Manifestation of new interference effects in a superconductor-ferromagnet spin valve

Superconductor-ferromagnet (S/F) spin valve effect theories based on the S/F proximity phenomenon assume that the superconducting transition temperature Tc of F1/F2/S or F1/S/F2 trilayers for parallel magnetizations of the F1 and F2 layers (T(c)(P)) are smaller than for the antiparallel orientations (T(c)(AP)). Here, we report for CoOx/Fe1/Cu/Fe2/In multilayers with varying Fe2-layer thickness the sign-changing oscillating behavior of the spin valve effect ΔT(c) = T(c)(AP) - T(c)(P). We observe the full direct effect with T(c)(AP) > T(c)(P) for Fe2-layer thickness d(Fe2) < 1 nm and the full inverse (T(c)(AP) < T(c((P)) effect for d(Fe2) ≥ 1 nm. Interference of Cooper pair wave functions reflected from both surfaces of the Fe2 layer appear as the most probable reason for the observed behavior of ΔT(c).     

Discriminating the superconducting gap from the pseudogap in Bi(2)Sr(2)CaCu(2)O(8+delta) by interlayer tunneling spectroscopy

Tunneling spectroscopy using a very thin stack of intrinsic Josephson junctions has revealed that the superconducting gap is definitely different from the pseudogap in the Bi(2)Sr(2)CaCu(2)O(8+delta) system. In the underdoped region, the conductance peak arising from the superconducting gap is independently observed in the dI/dV-V curve and its position is much lower than that of the pseudogap. Near the optimum doping level and in the overdoped region, both peaks are located in close proximity. These findings are in conflict with a previous understanding of the pseudogap.     

原位法MgB_2-B_4C复相超导体合成及相含量调控研究

以B4C和Mg为原料合成的MgB2-B4C复相超导体具有高的临界电流密度(Jc)和高的超导转变温度(Tc),是一种有潜力的实用MgB2超导材料,其成相机理对复相MgB2超导体的相含量调控和磁通钉扎研究具有重要意义。结合经典烧结理论,研究了B4C-Mg真空固相烧结制备MgB2-B4C复相超导体的超导相形成和晶粒生长过程,给出了B4C-Mg的金斯特林格扩散模型和MgB2晶粒生长过程。通过选择B4C原料粒径,MgB2-B4C复相超导体超导相体积相含量在18%-88%范围可控。相含量88%的MgB2-B4C复相超导体临界转变温度达33.5K,转变宽度1.5K。10 K环境6T外场下电流密度可以达到1×104A/cm2,表明MgB2-B4C复相超导体具有良好的磁通钉扎行为。     

Spin dynamics in iron-based layered superconductor (La0.87Ca0.13)FePO revealed by 31P and 139La NMR studies

We report 31P and 139La NMR studies of (La0.87Ca0.13)FePO, which is a family member of the recently discovered superconductor LaFeAs(O1-xFx). In the normal state, Knight shift and nuclear spin-lattice relaxation rate divided by T (1/T1T) show that a Fermi-liquid state with moderate ferromagnetic fluctuations emerges below 30 K. From 1/T1T of 31P and 139La, a quasi-two- dimensional electronic structure is suggested, in which the FeP layer is more conductive than the LaO layer. In the superconducting (SC) state, although a clear Meissner signal was observed, 1/T1T increases below Tc, in contrast to a decrease of 1/T1T due to the opening of a SC gap, suggesting that novel low-energy spin dynamics develop in the SC state.     

高Tc超导GdBa_2Cu_3O_(7-δ)薄膜双晶晶界结及双晶结光探测器

在晶界夹角分别为 2 4°,3 2°的 YSZ双晶基片上 ,制备了高 Tc Gd Ba2 Cu3O7-δ双晶超导薄膜 ,采用光刻技术在晶界上刻出了两种不同尺寸的双晶晶界结 ,在液氮温度下观测了结的直流 I-V特性 ,用 1 0 GHz微波辐照双晶结结区 ,观察到了结临界电流的压缩和 Shapiro台阶 ,表明双晶结具有约瑟夫逊弱连接行为。用上述双晶结进行光探测 ,用波长为 0 .63 2 8μm的 He-Ne激光器辐照结区 ,系统的观测了其光响应特性 ,结果如下 :噪声等效功率 NEP =1 .9× 1 0 - 1 3W,归一化探测率 D*=5.3× 1 0 9cm Hz12 W- 1 ,响应率 Rv =4 .2× 1 0 7V/W,响应时间 τ<6.3 5× 1 0 - 7s