Sensitivity of the phonon in YBCO to superconductivity by infrared studies on epitaxial film

Epitaxial films of YBCO superconductors whose C axes are primarily perpendicular to the film surface, were prepared. The samples, which were treated by means of a gettered annealing technique, were investigated by the X-ray diffraction pattern method (XRD) and infrared (IR) reflectance spectra in the 140–800 cm⁻¹ spectral range. The strengths of the phonon structures are influenced greatly by the superconductivity of the sample, which can be determined by the annealing conditions. The relation between the resulting IR activity of phonons and the superconductivity is clear and the important role of phonons in superconductivity is demonstrated.     

Refractive indices of SrTiO3 in the infrared region

We measured transmittance and reflectance spectra of SrTiO₃ crystals in the infrared region, at 200 and 100 K. These data and our previous 300 and 20 K data are analyzed in order to derive frequency and temperature dependence of the refractive indices n and k. We thus provide detailed n(v) and k(v) data at 300, 200, 100, and 20 K, in the frequency range 25–16000 cm⁻¹.     

Far infrared polarization measurements of MEM(TCNQ)2 at room temperature and 4.2 K

Polarization measurements have been made in the far infrared absorption spectra on single crystals of MEM(TCNQ)₂ at 1.5, 4.2 and 298 K. We demonstrated experimentally that the direction of polarization of phonons at 126 and 156 cm⁻¹ below spin-Peierls transition are parallel to the c-axis. Secondly, we measured the absorption spectra of MEMI powder at 1.5 and 4.2 K and found that there is no absorption in the range from 130 to 170 cm⁻¹. This indicates that there is no intramolecular vibration of MEM in this region. It supports the assignment of 156 cm⁻¹ absorption as phonon. Thirdly, comparing the FIR absorption spectra at 4.2 and 298 K with linearly polarized radiation field parallel to the c-axis, we found that the height of the strong absorption band observed around 86–180 cm⁻¹ region at 4.2 K becomes lower with the raising of temperature to 298 K. We attributed this band to the pinned CDW. The lowest edge of the CDW is 70 cm⁻¹.     

Self-field reduces critical current density in thick YBCO layers

The engineering current density in YBCO coated conductor applications can be improved in two ways. Either the critical current density should be improved or the superconducting films made thicker. Unfortunately, it has often been observed that the average critical current density decreases when the thickness of films increases. Suggested reasons for this behaviour include e.g. two dimensional pinning properties, microcracks and imperfect crystallographic alignment. However, it is often forgotten that the self-field effect unavoidably reduces the critical current density when the thickness of YBCO films increases and thereby total current rises. In this paper, the influence of self-field on the average critical current density is studied computationally as a function of film thickness. The situation is also scrutinized at different external magnetic fields in order to find ways to distinguish self-field effects from problems related to the manufacturing process. For this purpose, critical current measurements in external field perpendicular to the film surface are proposed.     

FIR-spectroscopy on iron-doped YBaCuO ceramics: energy gap,phonons and a band at 500 cm−1

We have measured the reflectance of superconducting iron-doped YBaCuO ceramics in the frequency range from 20 to 10000 cm^–1 for various temperatures above and belowT _c . In these spectra we could directly observe an energy gap. A Kramers-Kronig analysis was performed. We discuss the temperature dependence of two infrared-active phonons and an absorption band located around 500 cm^–1.     

Tuning the absorption band in the THz range of YBCO films patterned by means of HEHI lithography

High energy heavy ion lithography was used for modulating through implanted nanostructures the local structural and electrical properties of high temperature superconducting YBa₂Cu₃O₇ films. The controlled reduction of the critical temperature of irradiated films results in a localization into heavy ion patterned micro-regions of the electrical dissipation, viable in a given temperature range and driven by ion fluence, bias current and applied magnetic field. The measurement of the response of such nanostructured YBCO films to electromagnetic radiation in the infrared band (MIR–FIR region) is presented. It turns out that the ion induced structural modification of both superconducting film and substrate is actually enabling the infrared optical absorption of YBCO, so that the viability of low noise THz detection above the liquid nitrogen temperature is shown.     

Extra-drude contribution to the far infrared conductivity oxygen deficient YBCO films

We measured reflectance spectra R(v) of oxygen deficient Y₁Ba₂Cu₃O_7–x (YBCO) films of different thickness, with critical temperatures between 85 and 87 K. Measurements were made at room temperature between 15 and 25000 cm^–1. The measured R(v) have a maximum below 100 cm^–1 and then decrease for v 0. This low frequency behaviour of R(v) can be accounted for by considering a relevant extra-Drude contribution to the far infrared optical conductivity of YBCO.     

Magnetic levitation of YBa2Cu3Oy single crystals

By using an ordinary electric balance, we are able to measure the magnetic levitation forces acting on a superconducting YBa₂Cu₃O_y (YBCO) single crystal. It is found that when the external magnetic field is parallel to the c-axis of the single crystal, the hysteretic levitation curve is similar to that of a melt-processed YBCO superconducting sample. However, when the external magnetic field is perpendicular to the c-axis of the YBCO single crystal, the levitation forces are too small to be measured by our equipment. Also, we have introduced a simple model with the Bean's approximations to explain the levitation forces. The critical current density derived from this model by fitting with experimental data is quite close to the value obtained from magnetization measurements.     

在金属基片上激光沉积YBCO超导薄膜

介绍了在金属基片上激光沉积缓冲层和ＹＢａ２Ｃｕ３ｏ７－ｘ（ＹＢＣＯ）高温超导薄膜的研究结果。在带ｙｔｔｒｉａ－ｓｔａｂｉｌｉｚｅｄ－ｚｉｒｃｏｎｉａ（ＹＳＺ）缓冲层的ＮｉＣｒ合金基片上,激光原位沉积出ＹＢＣＯ超导薄膜,薄膜的零电阻转变温度度４８Ｋ,７７Ｋ时临界电流密度约为２００Ａ／ｃｍ＾２;缓冲层的取向可以通过选择适当的沉积参数来改善;用扫描隧道电子显微镜对ＹＢＣＯ薄膜的微观结构分析表明：完善的螺     

Measurement of the impedance of very thin superconducting YBaCuO films at 10 GHz with a cavity perturbation method between 4 and 300 K

We have measured the complex film impedance 1/σd (σ conductivity, d film thickness) of three YBaCuO thin films with d = 44, 115, and 168 nm on MgO substrates at 10.2 GHz in the temperature range between 300 and 4 K. Below T_c, the experimental results are discussed in terms of the two-fluid model and the BCS theory. The residual resistance decreases with the film thickness. The thinnest film has a residual surface resistance of 3 · 10^?4 Ω. For this film, the complex microwave conductivity is calculated and compared with the models. Apart from the residual resistance, the measured conductivity is in agreement with the peak caused by the energy gap of the BCS theory. All measurements were performed with a cavity perturbation method which we have to our knowledge applied for the first time to superconducting thin films. The method allows to determine the complex impedance of films with arbitrary thickness. In particular, films with thicknesses small compared to the skin depth δ or the London penetration depth λ can be measured. Therefore, we are able to measure the impedance both in the normal and superconducting state.     

A new approach to formation of the topology of planar structures on the basis of a YBCO high-temperature superconductor

The paper presents a new etching-free technology for the formation of planar superconductor structures based on YBCO films with metal contacts. The required topology of superconducting elements from YBCO film is created by the proposed “preliminary topology mask” method. The ohmic contacts to the superconducting structure are produced by lift-off photolithography. In order to study the capabilities of this technology, measurements of superconducting bridges with a width of 3, 10, and 50 μm and test structures for measuring the contact resistance are performed.     

MgO衬底上YBa2Cu3O7-δ台阶边沿型约瑟夫森结的制备及特性

MgO衬底上的YBa2Cu3O7-δ(YBCO)台阶边沿型约瑟夫森结(台阶结)在高灵敏度高温超导量子干涉器(superconducting quantum interference device,SQUID)等超导器件研制方面具有重要的应用价值和前景.本文对此类YBCO台阶结的制备和特性进行了研究.首先利用离子束刻蚀技术和两步刻蚀法在MgO(100)衬底上制备陡度合适、边沿整齐的台阶,然后利用脉冲激光沉积法在衬底上生长YBCO超导薄膜,进而利用紫外光刻制备出YBCO台阶结.在结样品的电阻-温度转变曲线中,观测到低于超导转变温度时的电阻拖尾现象,与约瑟夫森结的热激活相位滑移理论一致.伏安特性曲线测量表明结的行为符合电阻分路结模型,在超导转变温度TC附近结的约瑟夫森临界电流密度TC随温度T呈现出(TC-T)^2的变化规律,77 K时JC值为1.4×10^5 A/cm^2.利用制备的台阶结,初步制备了YBCO射频高温超导SQUID,器件测试观察到良好的三角波电压调制曲线,温度77 K、频率1 kHz时的磁通噪声为250μΦ0/Hz^1/2.本文结果为进一步利用MgO衬底YBCO台阶结研制高性能的高温超导SQUID等超导器件奠定了基础.     

插入二氧化铈薄膜提高MOD-YBa2Cu3O7-x厚膜超导性能的研究

YBa₂Cu₃O_7-x(YBCO)膜存在“厚度效应”: 随着厚度增加, YBCO薄膜的临界电流密度下降, 尤其是YBCO薄膜的厚度超过1 μm时, 它的临界电流密度急剧下降. 本文在YBCO薄膜之间引入极薄的二氧化铈(CeO₂)薄膜, 成功制备出结构为YBCO/YBCO/CeO₂/YBCO的超导厚膜. 所制备的厚度为2 μm的YBCO膜临界电流密度为1.36 MA/cm² (77 K, 自场), 其性能比相同厚度的纯YBCO膜有了较大幅度的提升. 研究表明CeO₂薄膜起到了传递织构、松弛应力的作用.     

电子束蒸发制备YBCO超导薄膜研究

采用电子束沉积制备YBCO超导薄膜,研究了760℃—840℃的不同退火温度下高温热处理对YBCO薄膜双轴织构、表面形貌及超导性能的影响。超导临界电流密度测试、X射线衍射(XRD)和扫描电镜(SEM)的结果表明,退火温度在在800℃时,YBCO薄膜具有良好的织构和平整致密的表面形貌,在77K自场下的临界电流密度J可达4.2×106/cm2。     

Unusual crystal structure of non-superconducting Y1Ba2Cu3O7−x films on buffered silicon substrates

Superconducting common c-axis oriented YBCO films as well as non-superconducting films epitaxially grown at significantly reduced substrate temperatures were deposited by laser ablation on CeO₂/YSZ buffered silicon substrates. The crystal structure of the non-superconducting films measured by XRD is collapsed from the original orthorhombic one, whereas the chemical composition detected using RBS and EDX remains identical. Especially the oxygen content in the non-superconducting material is as high as in the common 90 K superconductor. Thus, the change in the electrical behaviour is only due to a structural modification. The crystallographic model of a simple cubic YBCO perovskite unit cell does not represent completely our experimental results measured on the non-superconducting YBCO. A modified model based on detailed TEM and XRD measurements and known crystal defects of orthorhombic YBCO is proposed.     

Strongly enhanced flux pinning in the YBa_2Cu_3O_(7-X) films with the co-doping of Ba TiO_3 nanorod and Y_2O_3 nanoparticles at 65 K

YBa2Cu3O7-x(YBCO) films with co-doping BaTiO3(BTO) and Y2O3 nanostructures were prepared by metal organic deposition using trifluoroacetates(TFA-MOD). The properties of the BTO/Y2O3co-doped YBCO films with different excess yttrium have been systematically studied by x-ray diffraction(XRD), Raman spectra, and scanning electron microscope(SEM). The optimized content of yttrium excess in the BTO/Y2O3co-doped YBCO films is 10 mol.%, and the critical current density is as high as ~ 17 mA/cm2(self-field, 65 K) by the magnetic signal. In addition, the Y2Cu2O5 was formed when the content of yttrium excess increases to 24 mol.%, which may result in the deterioration of the superconducting properties and the microstructure. The unique combination of the different types of nanostructures of BTO and Y2O3 in the doped YBCO films, compared with the pure YBCO films and BTO doped YBCO films, enhances the critical current density(JC) not only at the self-magnetic field, but also in the applied magnetic field.     

低能离子对高温超导YBa_2Cu_3O_(7-δ)薄膜的表面改性和机理

低能氩离子束轰击并后退火处理的离子束表面改性,会影响高温超导薄膜的表面结构和超导特性,但是其中的深刻微观机理不清楚.本文通过连续改变离子束轰击时间,系统研究了离子束表面改性对于超导膜结构和临界电流密度的影响.通过扫描电子显微镜、X射线衍射、J_(c-scanning)测试表征样品的结构特性和超导特性,并得出内应变、氧空位缺陷等参量.研究表明,经过表面改性的钇钡铜氧(YBa_2Cu_3O_(7-δ),YBCO)薄膜,随轰击时间增加表面形貌会变得更加均匀致密,a轴晶粒消失,并且临界电流密度有了显著的提高.由化学键收缩配对模型分析得出,临界电流密度的提高与薄膜内应变增大和引发的局部YBCO结构中Cu—O键收缩有关.     

Far-infrared spectroscopy of the high Tc superconductor YBa2Cu3O7-δ

We report measurements of the far-infrared reflectivity of polycrystalline samples of YBa₂Cu₃O_7-δ which have their superconducting transition temperatures in the 90 K range. The measurements performed between 300 K and 10 K reveal temperature-dependent structures attributed to optical phonons in the 150 cm^-1 – 350 cm^-1 range, and excitation of the electrons across the energy gap. Complicated structures make unambiguous fitting with the Mattis-Bardeen theory difficult, but it seems that the energy gap is around 3.5 kT_c for T⪡T_c. Additionally, our results indicate the possibility of a second gap at an energy corresponding to 5.4 kT_c.     

YBa_2Cu_3O_(7-δ)高温超导膜的Raman标定

ＹＢａ２Ｃｕ３Ｏ７－δ高温超导膜的Ｒａｍａｎ标定张鹏翔１，２，Ｈ．－Ｕ．ＨＡＢＥＲＭＥＩＥＲ２，Ｍ．Ｃａｒｄｏｎａ２（１云南工业大学材料科学与工程系昆明云南６５００５１中国）（２Ｍａｘ－Ｐｌａｕｄ－ＩｎｓｔｉｔｕｔｆüｒＦｅｓｔｋｏｒｐｅｒｆｏｒｓｃ...