纳米层状碳/氮化物(MAX相）的电子-声子耦合及超导电性机制研究

Nano-laminated transition metal carbides and nitrides (the so-called MAX phases) showed many novel physical properties, such as the interesting superconductivity. However, the mechanisms of electron-phonon coupling are not clear, as well as its relationship with the variety of composition and layered crystal structure of MAX phases. The present developments could not provide useful guides to discover new MAX phases with superconductivity. The present project will firstly perform large scale first-principles calculations to precisely calculate the electronic band structure, phonon dispersion curve, electron-phonon coupling coefficient, and to predict critical transition temperature of superconductivity. It is our ambition to succeed in the great challenges to understand the mechanisms of superconductivity of MAX phases and also to predict new members of MAX-phase superconductor. Based on the theoretical simulations, high quality experimental samples will be processed and used to measure their resistivity, heat capacity and Raman spectroscopy at low temperature ranges, which are the proper methods to study the electron-phonon coupling coefficients and to determine the critical transition temperature of metallic superconductors. The experimental results will provide reference data to evaluate the theoretical predictions. The present project will disclose the intrinsic relationship between chemical composition, layered crystal structure and electron-phonon coupling in MAX phases. It will also establish fundamental method to optimize electronic transport properties by tailoring the two factors. We expect that the obtained results will be universally useful to understand and predict superconductivity for new materials with similar nano-laminated microstructures.

过渡金属纳米层状碳/氮化物（MAX相）陶瓷显示了新奇的超导性质，但其与成分和纳米层状晶体结构的内在关联以及材料中电子-声子耦合机制尚不清楚，现有的结果也无法指导发现新的MAX相超导体系。本项目拟首先采用大规模第一性原理并行模拟方法，高效、精确地计算MAX相材料的电子能带结构、声子色散关系、电子-声子耦合系数并预测超导临界转变温度，解决MAX相陶瓷超导电性基理研究面临的重要挑战，探索MAX相超导新材料。在理论预测的基础上，制备高质量（纯相、致密、晶粒尺寸较大）的实验样品，合理采用低温电阻率、比热和Raman谱等实验测量手段，研究候选MAX相超导材料的电子-声子耦合机制及超导临界转变温度，并验证理论预测的结论。本项目的结果将揭示成分和层状晶体结构对MAX相中电子-声子耦合的影响规律，为优化电输运特性提供基础理论方法，同时将对理解和预测具有类似纳米层状微观结构的新材料也有普适的借鉴意义。

过渡金属纳米层状碳/氮化物陶瓷由于其独特的晶体结构，显示出新奇的物理特性，例如超导电性质和磁学性能及热膨胀可调控性等，但这些性能与成分和纳米层状晶体结构的内在关联以及材料中电子-电子、电子-声子和声子-声子耦合机制尚待揭示，以用于解释导电、导热和磁性机理并指导发现新的超导电性、高导热性过渡金属碳/氮化物。本项目采用大规模第一性原理并行模拟方法，高效、精确地计算了多种过渡金属碳化物/氮化物陶瓷材料的电子能带结构、声子色散关系、电子-声子耦合系数、电子-电子自旋耦合、声子-声子散射系数、热膨胀系数、缺陷产生及迁移机制及其对力学和热学性能的影响等，预测了部分材料的超导临界转变温度、磁距和声子非简谐性行为，为过渡金属碳化物陶瓷的超导电性、高导热性、磁距和热膨胀系数可调控性提供了机理解释。在理论预测的基础上，制备出高质量（纯相、致密、晶粒尺寸较大）的实验样品，合理采用低温电阻率、比热和Raman 谱等实验测量手段，研究了候选过渡金属碳/氮化物陶瓷材料的电子-声子、声子-声子耦合机制及超导临界转变温度等，并部分验证了理论预测的结论。本项目的结果揭示了成分和层状晶体结构对过渡金属碳/氮化物陶瓷中电子-电子自旋耦合、电子-声子耦合、声子-声子散射的影响规律，为优化电输运和热输运特性提供了基础理论指导，同时对理解和预测具有类似纳米层状微观结构的新材料及其性能也有普适的借鉴意义。