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。

Study on Raman Spectra of MgB2 Superconducting Film at Different Temperatures

MgB₂ superconducting film, as the alloy superconductor with the highest superconducting transition temperature so far, has a broad application prospect in the field of electronics because of its simple structure, long coherent length, no weak connection between grain boundaries, high upper critical field, short electron-phonon scattering time and so on. Raman spectroscopy is an effective method to study the electron-phonon interaction and superconducting band. Moreover, Raman spectroscopy has been used to study the electron-phonon characteristics and superconducting band structure of MgB₂. Research shows that sample quality, grain size and test conditions greatly influence the peak position and shape of the Raman peak of MgB₂. The change of Raman spectrum with temperature is also a research priority. However, the temperature range of MgB₂ variable temperature Raman spectrum is relatively small, which is limited to 83 K to room temperature or the region near the transition temperature. In this work, the Raman spectra of MgB₂ film in a large temperature range are studied. The polycrystalline MgB₂ film was prepared on (0001) SiC substrate via hybrid physical-chemical vapor deposition with grain size ~300 nm and superconducting transition temperature 39.3 K. The Raman spectra of MgB₂, from 20 to 1 200 cm-1, were measured and studied in the temperature region from 10 to 293 K. The Raman spectra show that Raman peaks related to MgB₂ appear at ~620 cm-1 in high-frequency region and at ~80 and ~110 cm-1 in low-frequency region. The frequency of the two Raman peaks in the low-frequency region corresponds to the width of the superconducting energy gap, indicating the dual-gap characteristics of MgB₂. Considering the Raman activity of the four phonon modes in MgB₂, the Raman peak at ~620 cm-1 in high-frequency region is contributed by the E_2g vibration mode. And as temperature decreases, no obvious peak position shift is observed. Nevertheless, the FWHM of the Raman peak decreases with temperature. Furthermore, the FWHM is 380.7 cm-1 at 293 K, and 155.7 cm-1 at 10 K. Analysis shows that the non-harmonic effect caused by the nonlinear coupling between E_2g phonon and electronic system may be the main reason for the linear decrease of the FWHM.