1.Institute of Modern Physics, Chinese Academy of Sciences, Lanzhou 730000, China 2.School of Nuclear Science and Technology, University of Chinese Academy of Sciences, Beijing 100049, China
Abstract:YBa2Cu3O7?δ (YBCO) high temperature superconductor materials have many promising applications in energy, transportation and so on. Nonetheless, the application of YBCO in high magnetic field was limited because of low critical current. One-dimensional latent tracks produced by swift heavy ions irradiation can be effective pinning centers, thus enhancing superconductivity in external field. YBCO high temperature superconducting films were irradiated with 1.9 GeV Ta ions at room temperature and vacuum condition. Structure damages in irradiated samples were characterized by transmission electron microscopy (TEM). Continuous amorphous latent tracks, with diameter from 5 nm to 15 nm, throughout the whole superconducting layer can be observed from TEM images. Physical property measurement system (PPMS) was used to measure superconducting properties of samples before and after irradiation. When irradiated at optimal fluence of 8 × 1010 ions/cm2, critical current reaches its maximum value and pinning force was twice of unirradiated sample, while critical temperature almost unchanged. The analysis of experimental results shows that latent tracks produced by swift heavy ions irradiation can enhance in-field current-carrying capability, without decreasing critical temperature. In the power-law regime ${J_c} \propto {B^{ - \alpha }}$ values of ɑ decreased with the increasing of fluence, indicating a weaker magnetic field dependence of critical current. ɑ reaches its lowest value 0.375 when irradiated at a fluence of 5.0 × 1011 ions/cm2, corresponding to a lowest in-field Jc. This result may be a combination of increasing pinning centers and decreasing superconductor volumes that work together. Normalized pinning force fp = Fp/Fp,max of sample irradiated with different fluence as a function of magnetic field h = H/Hmax was analyzed using Higuchi model. Fitting results show that planar defects are main source of pinning when h > 1, independent of irradiation. Whereas, dominate pinning centers shifting from surface pinning to isotropic normal point pinning with increasing fluence when h < 1. Given that latent tracks produced by Ta ions irradiation act as strong anisotropic pinning centers, the reason of the dominate pinning centers change with increasing fluence remains to be further studied. Keywords:YBCO high temperature superconducting films/ swift heavy ions irradiation/ pinning
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3.1.快重离子辐照引起的超导参数变化
我们对辐照前后YBCO薄膜的临界转变温度、磁场下临界电流密度等进行研究. 当YBCO材料处于超导态时具有抗磁性, 而交流磁化率的实部与超导抗磁转变有关, 故可用交流磁化率来判断超导转变是否发生. 利用PPMS测量不同注量下交流磁化率随温度的变化情况, 将抗磁转变起始温度定义为超导转变温度Ton. 如图1所示, 未辐照样品Ton为87.6 K, 当Ta离子的辐照注量低于8.0 × 1010 ions/cm2时, Ton变化量?Ton仅为0.5 K. 随着注量的进一步增大, Ton呈现大幅度降低的趋势, 当注量达到最大值1.6 × 1012 ions/cm2时, Ton已由最初的87.6 K降至75.4 K, 且转变宽度明显增加. 图 1 不同注量下YBCO高温超导薄膜的交流磁化率随温度的变化 Figure1. AC susceptibility versus temperature of YBCO high temperature superconductor films irradiated with different fluences.
式中: a、b为样品垂直磁场方向的平面尺寸, 且b > a; V为样品超导体积, 其计算公式为$V = a \cdot b \cdot c$, 其中c为超导层厚度, 本式中取300 nm; ?M为同一磁场下磁滞回线曲线之差. 计算结果如图2(a)所示, 随着辐照注量的增加, 临界电流密度大体呈先增加后降低的趋势. 利用指数关系${J_c} \propto {B^{ - \alpha }}$对双对数坐标下的Jc-B曲线进行拟合, 结果标注在图2(a)中. 随着辐照注量的增加, 指数α不断减小, 说明快重离子辐照在薄膜中引入了强钉扎中心, 导致Jc对磁场的依赖关系逐渐减弱. 值得注意的是, 当注量达到5 × 1011 ions/cm2, α值已经降低至0.375, 然而其Jc反而低于未辐照样品, 这可能是钉扎中心增加和超导体积减小共同作用的结果. 图 2 (a) 77 K下经Ta离子辐照后临界电流密度随磁场的变化情况; (b) 77 K下钉扎力随注量的变化情况 Figure2. (a) Field dependence of critical current density at 77 K after Ta ions irradiation; (b) The variance of pinning force at 77 K with increasing fluence.
晶体缺陷与磁通涡旋之间的相互作用可阻止磁通线的运动, 从而提高Jc, 该作用力即钉扎力. 临界电流密度的大小与磁通钉扎密切相关. 为进一步了解不同磁场下的磁通钉扎信息, 对钉扎力Fp进行计算, 其计算公式为${F_{\rm{p}}} = {J_c} \times B$. 辐照前后样品钉扎力计算结果如图2(b)所示, 当注量为8.0 × 1010 ions/cm2时, 钉扎力显著提升, 且在磁场强度为0.70 T时出现峰值, 该磁场下钉扎力高达0.30 MN/m3, 约为未辐照样品(0.146 MN/m3)的两倍. 为研究快重离子辐照对YBCO薄膜中磁通钉扎类型的影响情况, 对图2(b)中的钉扎力曲线进行处理, 得到归一化钉扎力fp = Fp/Fp,max与约化场h = H/Hmax之间的关系, 其中Fp,max为钉扎力的峰值, Hmax为最大钉扎力所对应的磁场值, 归一化结果如图3所示. 结合Higuchi钉扎力标度函数对归一化钉扎力曲线进行拟合[31,32], 拟合公式如表1所示. 据图3所示的拟合结果可知, 当h > 1时, 所有的归一化钉扎力密度曲线都更符合面钉扎方式, 说明快重离子辐照对高场下的钉扎行为影响较弱; 当约化磁场h < 1时, 原始样品的约化钉扎力曲线同样与面钉扎所对应的函数曲线对应, 说明在原始样品中面缺陷钉扎一直占主导地位. 然而, 随着辐照注量的增加, 低场下钉扎类型逐渐从面钉扎转变为各向同性的非超导相的点缺陷钉扎. 图 3 辐照前后YBCO薄膜的约化钉扎力曲线及拟合结果 Figure3. Scaling behavior of normalized pinning force curves for YBCO films before and after irradiation and fit curves.
为进一步研究快重离子与YBCO超导薄膜的相互作用机制, 我们采用SRIM (stopping and range of ions in matter)程序对1.9 GeV的Ta离子在YBCO高温超导材料不同穿透深度下的能量损失进行模拟计算[33], 结果如图4所示. 图 4 Ta离子在YBCO超导材料中的电子能损与核能损值 Figure4. The nuclear and electronic energy loss in YBCO superconductor under Ta ions irradiation.
从图4中可以看到, Ta离子在YBCO超导材料中的能量损失方式有两种, 一种是与靶原子的核外电子发生非弹性碰撞使靶原子电离和激发而损失能量, 也被称为电子能损(electronic energy loss), 一种是直接与靶原子核发生弹性碰撞, 称为核能损(nuclear energy loss). 能量为1.9 GeV的Ta离子在YBCO高温超导薄膜中的电子能损值(Se = 39.4 keV/nm)与核能损值(Sn = 0.04 keV/nm)相差约3个数量级, 故在本实验中Ta离子主要以电子能损的方式将能量传递给YBCO薄膜. 1.9 GeV的Ta离子在YBCO超导体中的射程约为53 μm, 远大于薄膜厚度, 入射离子将停留在蓝宝石衬底中, 故可忽略超导层中的离子注入效应. 快重离子在YBCO超导材料中产生连续潜径迹的阈值为35 keV/nm[34], 由计算结果可知, 1.9 GeV的Ta离子在YBCO薄膜中的电子能损值大于阈值, 故可在材料中产生连续的潜径迹. 采用聚焦离子束系统对辐照注量为1.0 × 1011 ions/cm2的样品进行切割加工, 制备剖面样品, 并在场发射透射电子显微镜下进行观察, 结果见图5(a)和(b). 低倍TEM图谱如图5(a)所示, 箭头标注的方向为Ta离子的入射方向. 由图可知, 1.9 GeV的Ta离子在超导层中产生了分布大致均匀的一维连续潜径迹. 在如图5(b)所示高分辨模式下, 潜径迹周围晶格形貌保存完好, 但其内部的晶体结构被完全破坏, 呈现非晶态, 说明快重离子辐照导致局部原子位移. 潜径迹直径分布在5—15 nm之间. 图 5 注量为1.0 × 1011 ions/cm2、能量为1.9 GeV的Ta离子辐照YBCO超导薄膜TEM图 (a) 低倍TEM图像; (b) 潜径迹的高分辨率TEM图像 Figure5. TEM images of 1.9 GeV Ta ions-irradiated YBCO high temperature superconductor film with the fluence of 1.0 × 1011 ions/cm2: (a) Low-resolution TEM image; (b) high-resolution TEM image of latent track.
若潜径迹的平均尺寸为10 nm, 超导性能急剧退化所对应的注量为5.0 × 1011 ions/cm2, 计算可知, 在该注量下潜径迹占比约为39.3 %, 说明YBCO超导薄膜的结晶部分仍大量保留, 故不能用非晶化潜径迹的相互交叠来解释该辐照注量下超导性能的退化. 快重离子辐照除了在材料中产生非晶化潜径迹之外, 由于潜径迹与其附近的完整晶格之间存在密度差, 在界面处还会形成应力区[35]. 应力区的原子形变和潜径迹都是室温下稳定存在的缺陷. 已有研究结果表明, 潜径迹周围的应力区面积约为潜径迹的3倍左右[36,37], 若以此为依据进行计算, 则当辐照注量为5.0 × 1011 ions/cm2时应力区已经发生了严重的重叠, 故认为该注量下超导性能的急剧恶化与重离子辐照产生的应力区有更直接的关系. 为此, 采用激光共聚焦拉曼光谱仪对样品中的晶格结构变化和应力进行进一步分析. 拉曼光谱是一种无损检测技术, 可对物质的分子结构、应力等进行分析. 采用拉曼光谱对辐照前后的YBCO超导薄膜进行表征, 测试结果如图6所示. 正交结构的YBCO高温超导材料中主要有4种拉曼激活模式, 分别为Ba-Ag、Cu(2)-Ag、O(2, 3)-B1g、O(4)-Ag. O(4)-Ag拉曼峰与材料中的氧含量有着密切的联系[38], 经验公式为: $\delta = 13.58 - $$0.027\nu $, 其中ν为O(4)-Ag峰对应的频率. 未辐照样品的O(4)-Ag振动频率为500 cm–1, 则δ = 0.08, 故推测原样的化学式为YBa2Cu3O6.92. 图 6 (a) 能量为1.9 GeV的Ta离子辐照前后YBCO高温超导薄膜的显微拉曼光谱; (b) O(2, 3)-B1g和O(4)-Ag峰的峰位随着注量的变化情况 Figure6. (a) Micro-Raman spectra of YBCO high temperature superconductor films between pristine sample and samples irradiated with 1.9 GeV Ta ions; (b) variations in the position of O(2, 3)-B1g and O(4)-Ag peaks with different fluences.