MoTe2/Nb约瑟夫森结低温输运性质的研究

RESEARCH ON LOW TEMPERATURE TRANSPORT PROPERTY OF MoTe2/Nb JOSEPHSON JUNCTION

耿浩

11849053

硕士

材料科学与工程

何洪涛

2020-05-28

2020-07-15

哈尔滨工业大学

深圳

近期提出的拓扑超导体是区别于传统超导体的一种新的量子物质态。拓扑超导体边缘态有受拓扑保护的能量为零的准粒子激发，这种准粒子与高能物理领域中的马约拉纳费米子具有类似的性质，具有奇异的非阿贝尔统计特性，有望应用于拓扑量子计算。因此，对拓扑超导的研究成为目前凝聚态物理最前沿的研究领域之一。最新的研究还指明人们可以通过制备超导体/拓扑量子材料异质结构，把超导电性引入拓扑量子体系，从而实现人们孜孜以求的拓扑超导态。在本论文中，我们较先开展了对MoTe2体系超导邻近效应及约瑟夫森效应的研究。我们利用电子束光刻技术制备了超导体（Nb）－外尔半金属（MoTe2）－超导体（Nb）约瑟夫森结, 并在低温变磁场系统下测量了该约瑟夫森结的电输运特性。包括：1.电阻变温曲线，2.伏安特性曲线，3.零磁场下不同温度的微分电阻谱（dV/dI-V），4.温度为1.7 K时不同磁场的微分电阻谱。在电阻变温曲线中，我们观察到了两个超导相变转变温度，分别为Tc1=9 K，Tc2≈3.2 K。并且在零磁场不同温度的微分电阻谱中，我们发现当温度T＞Tc2≈3.2 K时，微分电阻谱是连续且平坦的曲线。当温度T＜Tc2≈3.2 K时，微分电阻谱在电压为±ΔL以及±ΔS处出现了微分电阻峰（|ΔL|＞|ΔS|），且当电压在|V|＜ΔL以及|V|＜ΔS范围内时，微分电阻下降明显。经过分析，我们认为以上现象是由超导邻近效应以及安德烈夫反射所致。超导电极Nb通过超导邻近效应，诱导MoTe2的表面态及体态产生eΔL和eΔS的超导能隙。通过BCS理论对不同温度下ΔL和ΔS的拟合，我们得到了温度为0 K时的表面态和体态的超导带隙分别为0.41 meV，0.11 meV。当温度T＜2.7 K后，微分电阻谱在零偏压附近还出现了非常陡峭的深谷。对器件的伏安特性测量，发现该器件在低温下表现出类似于约瑟夫森超流的现象。这些现象表明，在低温下（T<2.7 K）我们在Nb-MoTe2-Nb约瑟夫森结中观察到了直流约瑟夫森效应。以上研究将为我们继续探索拓扑超导及马约拉纳费米子等前沿物理打下坚实的基础。

Recently, the topological superconductor was proposed, as a new state of quantum matter which is distinctively different from conventional superconductors. The topological superconductor hosts mysterious Majorana fermions, which obey non-abelian statistics and are thus of great importance in the topological quantum computation. Therefore, the study of topological superconductors is now one of the hottest topics in condensed matter physics and material science. It’s also predicted that such a topological superconducting state can be realized in heterostructures composed of s-wave superconductor and topological material by combining the superconductivity and strong spin-orbit interaction.In this dissertation, we have performed systematic study of the superconducting proximity effect and Josephson effect in type-II Weyl semimetal MoTe2. We have fabricated Nb-MoTe2-Nb Josephson junction by electron beam lithography and then measured the transport properties of it under low temperature and magnetic field conditions. As temperature decreases, the device resistance drops sharply at the superconducting transition temperature of Nb (~ 9 K). But at 3.2 K, another rapid decrease of resistance is observed. Above 3.2 K, the differential resistance curve is smooth and flat, without any feature. But when the temperature is below 3.2 K, there is obvious decrease of the differential resistance within the voltage range |V |<ΔL. Besides this, the differential resistance drops more rapidly when |V |<ΔS, where ΔS<ΔL. It is believed that these features indicate the emergence of Andreev reflection and the superconducting proximity effect has induced superconducting gaps eΔL and eΔS in the surface and bulk states of MoTe2, respectively. We have also used the BCS theory to fit the obtained ΔL & ΔS at different temperatures. From the fitting, the induce zero-temperature superconducting gaps for the surface and bulk states are 0. 41 meV and 0.11 meV, respectively.Furthermore, with temperatures decreasing below 2.7 K, the differential resistance spectrum exhibits another very sharp dip around zero bias. The measured current-voltage curve also resembles the supercurrent behavior generally observed in Josephson junctions. All these lead us to believe that we have observed the DC Josephson effect in our Nb-MoTe2-Nb device at low temperatures (T< 2.7 K). Our work will pave the way for future study of possible topological superconductivity or Majorana fermions in Weyl semimetals.

拓扑超导体 约瑟夫森结 安德烈夫反射 外尔半金属

topological superconductor Josephson junction Andreev reflection Weyl semimetal

中文

联合培养

南方科技大学

耿浩. MoTe2/Nb约瑟夫森结低温输运性质的研究[D]. 深圳. 哈尔滨工业大学,2020.