Spin gapless armchair graphene nanoribbons under magnetic field and uniaxial strain

Using Green＇s function method, we investigate the spin transport properties of armchair graphene nanoribbons （AG- NRs） under magnetic field and uniaxial strain. Our results show that it is very difficult to transform narrow AGNRs directly from semiconductor to spin gapless semiconductors （SGS） by applying magnetic fields. However, as a uniaxial strain is exerted on the nanoribbons, the AGNRs can transform to SGS by a small magnetic field. The combination mode be- tween magnetic field and uniaxial strain displays a nonmonotonic arch-pattern relationship. In addition, we find that the combination mode is associated with the widths of nanoribbons, which exhibits group behaviors.     

Excellent thermoelectric performance predicted in Sb2Te with natural superlattice structure

Using first-principles calculations combined with the Boltzmann transport theory, we explore the thermoelectric properties of natural superlattice (SL) structure Sb₂Te. The results show that n-type Sb₂Te possesses larger Seebeck coefficient of 249.59 (318.87) μV/K than p-type Sb₂Te of 219.85 (210.38) μV/K and low lattice thermal conductivity of 1.25 (0.21) W/mK along the in-plane (out-of-plane) direction at 300 K. The excellent electron transport performance is mainly attributed to steeper density of state around the bottom of conduction band. The ultralow lattice thermal conductivity of Sb₂Te is mainly caused by low phonon group velocity and strong anharmonicity. Further analysis shows that the decrease of group velocity comes from flatter dispersion curves which are contributed by the Brillouin-zone folding. The strong anharmonicity is mainly due to the presence of lone-pair electrons in Sb₂Te. Combining such a high Seebeck coefficient with the low lattice thermal conductivity, maximum n-type thermoelectric figure of merit (ZT) of 1.46 and 1.38 could be achieved along the in-plane and out-of-plane directions at room temperature, which is higher than the reported values of Sb₂Te₃. The findings presented here provide insight into the transport property of Sb₂Te and highlight potential applications of thermoelectric materials at room temperature.     

第一性原理研究单层Ge2X4S2(X=P,As)的热电性能

单层Ge₂X₄S₂(X=P, As)是最近预测的一种二维层状材料,它们不仅拥有高的光吸收系数,同时还有较高的载流子迁移率,这意味着它们在光电和热电领域可能有较好的应用前景.本文通过第一性原理和玻尔兹曼输运理论系统地研究了这两种材料的热电性质.结果表明,单层Ge₂P₄S₂和Ge₂As₄S₂在室温下展现较低的晶格热导率,沿armchair方向分别为3.93 W·m^-1·K^-1和3.19 W·m^-1·K^-1, zigzag方向分别为4.38 W·m^-1·K^-1和3.79 W·m^-1·K^-1,这主要是由低的声子群速度、大的格林艾森参数以及小的声子弛豫时间造成的.基于HSE06泛函计算出的能带结构表明单层Ge₂     

离子辐照对磷烯热导率的影响及其机制分析

通过离子辐照产生缺陷,可以非常有效地调控磷烯诸多物理性质.本文应用分子动力学方法模拟离子辐照磷烯的过程,给出了缺陷的形成概率与入射离子能量、离子种类以及离子入射角度之间的关系,并且应用非平衡态分子动力学计算辐照后磷烯热导率的变化.以缺陷形成概率为切入点,系统地研究了辐照离子的能量、辐照剂量、离子的种类以及离子的入射角度对磷烯热导率的影响.应用晶格动力学方法研究了空位缺陷对磷烯声子参与率的影响,并计算了声子局域模式的空间分布.基于量子微扰和键弛豫理论,指出空位缺陷明显降低磷烯热导率的最重要物理机制是空位缺陷附近的低配位原子对声子强烈散射.本文研究可为缺陷工程调控磷烯的热输运性质提供理论参考.     

Spin-polarized transport in graphene nanoribbon superlattices

By the Green’s function method,we investigate spin transport properties of a zigzag graphene nanoribbon superlattice(ZGNS) under a ferromagnetic insulator and edge effect.The exchange splitting induced by the ferromagnetic insulator eliminates the spin degeneracy,which leads to spin-polarized transport in structure.Spin-dependent minibands and minigaps are exhibited in the conductance profile near the Fermi energy.The location and width of the miniband are associated with the geometry of the ZGNS.In the optimal structure,the spin-up and spin-down minibands can be separated completely near the Fermi energy.Therefore,a wide,perfect spin polarization with clear stepwise pattern is observed,i.e.,the perfect spin-polarized transport can be tuned from spin up to spin down by varying the electron energy.     

嵌入线型缺陷的石墨纳米带的热输运性质

采用非平衡格林函数方法研究了嵌入有限长、半无限长、 无限长线型缺陷的锯齿型石墨纳米带 (ZGNR)的热输运性质.结果表明, 缺陷类型和缺陷长度对ZGNR的热导有重要影响. 当嵌入的线型缺陷长度相同时, 包含t5t7线型缺陷的石墨纳米带比包含Stone-Wales线型缺陷的条带热导低. 对于嵌入有限长、同种缺陷的ZGNR, 其热导随线型缺陷的长度增加而降低, 但是当线型缺陷很长时, 其热导对缺陷长度的变化不再敏感.通过比较嵌入有限长、半无限长、无限长线型缺陷的ZGNR, 我们发现嵌入无限长缺陷的条带比嵌入半无限长缺陷的条带热导高, 而后者比嵌入有限长线型缺陷的条带热导高. 这主要是因为在这几种结构中声子传输方向的散射界面数不同所导致的. 散射界面越多, 对应的热导就越低. 通过分析透射曲线和声子局域态密度图, 解释了这些热输运现象. 这些研究结果表明线型缺陷能够有效地调控石墨纳米带的热输运性质. 关键词： 石墨烯 线型缺陷 热导     

L型石墨纳米结的热输运

利用非平衡格林函数方法研究了由半无限长扶手椅型和锯齿型边界石墨纳米带连接而成的L型石墨纳米结的热输运性质.结果表明,L型石墨纳米结的热导依赖于L型石墨纳米结的夹角和石墨纳米带的宽度.在L型石墨纳米结的夹角从30°增加到90°再增加到150°过程中,其热导显著增大.夹角为90°的L型石墨纳米结的热导随着扶手椅型纳米带宽度增加时,在低温区热导随着宽度的增大而降低,在高温区热导随宽度的增大而升高.对于夹角为150°的L型石墨纳米结,其热导无论是在低温区还是在高温区都随着锯齿型纳米带宽度的增加而降低.利用声子透射谱对这些热输运现象进行了合理的解释.研究结果阐明了不同L型石墨纳米结中的热输运机理,为设计基于石墨纳米结的热输运器件提供了重要的物理模型和理论依据. 关键词： 石墨纳米结 热输运 热导     

堆叠石墨片对锯齿型石墨纳米带电子输运的影响

采用格林函数方法研究了堆叠石墨片对锯齿型石墨纳米带电子输运性质的影响,计算了两种不同堆叠方式下锯齿型石墨纳米带的电导.研究发现,由于堆叠石墨片与石墨纳米带的耦合作用,锯齿型石墨纳米带的电导谱出现了电导谷.在远离费米能处,两种堆叠方式下的电导谷位置相近甚至重合;而在费米能附近,两种堆叠方式下的电导谷存在差异.此外,讨论了堆叠石墨片的几何尺寸对锯齿型石墨纳米带电子输运的影响.结果显示,随石墨片几何尺寸的增大,锯齿型石墨纳米带在两种堆叠方式下远离费米能处的电导谷逐渐向费米能方向移动,同时其费米能附近的电导谷在两种堆叠方式下的差异随石墨片尺寸的增大变得更为明显.研究结果表明,堆叠石墨片能够有效地调制锯齿型石墨纳米带的电子输运性质.