RSi(R=Co,Fe)单晶低温下的电、磁学性能研究

ELECTRICAL AND MAGNETIC PROPERTIES OF RSi(R=Co,Fe) SINGLE CRYSTAL AT LOW TEMPERATURE

江林辉

11849291

硕士

材料工程

梅佳伟

2020-05-29

2020-05-29

哈尔滨工业大学

深圳

外尔半金属是研究外尔费米子的重要材料，其在磁学和电学性能上具有不同寻常的行为表现。单硅化合物（CoSi、FeSi、RhSi、MnSi等）—一类外尔半金属材料，随着技术的进步正在被人们更清晰地认识。通过ARPES观察CoSi的能带验证了其具有的多种外尔费米子，给学术界带来了新的研究热潮。FeSi观测到的外尔费米子声子能谱和其具有的三维拓扑绝缘体特性引起人们的关注。我们使用助熔剂方法来生长所需研究的单晶。我们分别使用助熔剂Ga和Te来生长CoSi单晶样品；使用Ga助熔剂生长FeSi单晶样品。我们对生长的单晶样品进行X射线衍射、扫描电镜观察和能谱分析等基本材料表征，用以确保生长的CoSi单晶和FeSi单晶样品质量，结果表明CoSi单晶和FeSi单晶的质量符合研究需求。我们使用MPMS磁学测量系统对CoSi单晶和FeSi单晶进行磁学性能表征，结果显示不同助熔剂生长的CoSi单晶在外加磁场下表现不同的行为。Ga助熔剂生长的CoSi单晶在6-300 K表现为顺磁性，而在6 K处磁性急剧的减小是由于单晶中Ga的残留。Te助熔剂生长的CoSi单晶在2-300 K均表现为抗磁性，低温下存在局域磁矩。FeSi单晶从高温到低温表现出反铁磁到顺磁的转变。我们还使用低温强磁场系统PPMS对CoSi单晶和FeSi单晶进行电学性能表征。Ga助熔剂生长的CoSi单晶的电阻与温度的曲线在6 K时的电阻突然下降，其原因是来自Ga的超导，与磁性测量现象一致。相比而言，Te助熔剂生长的CoSi单晶，未在低温出现显著的电学变化。两种不同助熔剂生长的CoSi单晶都表现出负磁阻现象，表明负磁阻的出现是CoSi的本征特性。低温条件下，由于体系出现微弱磁性破坏时间反演对称性，导致弱反局域化的消失，CoSi表现为正磁阻。30 K之后的负磁阻出现的原因归结于CoSi中多种费米子在磁场的作用下表现出的弱反局域化。FeSi单晶的电阻与温度曲线在29 K以下表现出电阻下降，预示表面态的电子逐渐占据主导。在低温下FeSi单晶的表面态与其各向异性磁阻的关系尚需进一步研究。以上这些结果丰富了对CoSi和FeSi单晶的研究，有助于进一步理解单硅化合物中多种费米子的物理内涵。

Weyl semimetal is an important material for researching Weyl fermions, and it has unusual behaviors in magnetic and electrical properties. Transition-metal monosilicides (CoSi, FeSi, RhSi, MnSi, etc.)-a class of Weyl semimetals, are being recognized more clearly with the advancement of technology. The band structure of CoSi observed by ARPES proves that it has many kinds of Weyl fermions, which leading to a new research fever to academia. The Weyl fermions phonon energy spectrum of FeSi and its three-dimensional topological insulator properties have attracted people's attention.We used Ga and Te as flux to grow CoSi single crystal samples, and Ga flux to grow FeSi single crystal samples. Using X-ray diffraction, scanning electron microscope observation and energy spectrum analysis to characterize the single crystal samples, showed that the high quality of CoSi and FeSi, which demand for magnetization and electric transport measurements.We used the magnetic property measurement system (MPMS) to characterize the magnetic properties of CoSi single crystals and FeSi single crystals. The results showed that CoSi single crystals grown with different fluxes behaved differently under an applied magnetic field. The CoSi single crystal grown with Ga flux exhibits paramagnetism at 6-300 K, while the sharp decrease in magnetic properties at 6 K is due to the Ga residue in the single crystal. The CoSi single crystal grown with Te flux shows diamagnetism at 2-300 K, and there occur a local magnetic moment at low temperature. The FeSi single crystal exhibits a transition from antiferromagnetic to paramagnetic with decrease of temperature.We also used a physical property measurement system (PPMS) with strong magnetic field and low-temperature to characterize the electrical properties of CoSi single crystals and FeSi single crystals. The resistance of CoSi single crystal grown by Ga flux suddenly drops at 6 K, which is consistent with the phenomenon of magnetic measurement, we attribute it to the superconductivity of Ga. In contrast, there is no significant electrical change at low temperature for the CoSi single crystal grown by Te flux. CoSi single crystals grown with two different fluxes both exhibit negative magnetoresistance, indicating it is an intrinsic characteristic of CoSi. Due to the weak magnetic appears at low temperature, the time inversion symmetry of the system is destructed, leading to the disappearance of weak anti-localization (WAL). Therefore, CoSi exhibits positive magnetoresistance. The reason of negative magnetoresistance above 30 K is attributed to the weak anti-localization of many fermions in CoSi under the magnetic field. The RT curve of FeSi single crystal shows a decrease in resistance below 29 K, indicating the transport is gradually dominated by surface state. The relationship between the surface state of FeSi single crystal and its anisotropic magnetoresistance at low temperature needs further study. These results enrich the study of CoSi and FeSi single crystals, and help to further understand the physical connotation of various fermions in single silicon compounds.

外尔半金属 单硅化合物 负磁阻 弱反局域化 拓扑绝缘体

Weyl semimetal monosilicides negative magnetoresistance weak anti-localization topological insulator

中文

联合培养

南方科技大学

江林辉. RSi(R=Co,Fe)单晶低温下的电、磁学性能研究[D]. 深圳. 哈尔滨工业大学,2020.