由於具有半導體元件所缺乏的獨特性質，超導元件（例如約瑟夫森結或超導量子干涉儀）成為許多先進科技儀器(如量子電腦…等)中重要的元件之一。若能控制超導體的超導性，則有機會可以開發出超導元件的新應用。近期所發現的二維材料可以透過外在控制或元件設計，達到改變電子能帶結構以及其特性。這導致超導二維材料（如二硒化鈮）對基礎物理研究和各種元件應用都產生了影響。在本論文中，我們針對少層數二硒化鈮的基本性質進行研究，並透過外加應力的方式改變其超導性。在測量樣品微分電阻時，我們發現在樣品回到普通態之前，出現了一個不尋常的小平台。除此之外，我們也觀察到在沒有外加磁場的情況下，超導臨界電流會異常的大。這兩樣反常的發現都對外加磁場非常敏感。另外一方面，在二硒化鈮及波紋結構的六方氮化硼所形成的異質結構中，超導臨界溫度有了很直接的變化。在測量微分電阻時，由該異質結構所造成超導的不均勻也進一步導致電壓出現變化和臨界值的分裂。我們的研究成果能用來發展新的超導元件及應用。

Superconducting devices, such as Josephson junction or SQUID, are import components to build up today’s technology owing to the unique characteristics not available in standard semiconductor devices. Controlling the superconductivity of superconductors can expand the applications of superconducting devices. Recently, the discovery of superconductivity in van der Waals materials, such as NbSe2, bears consequences for both fundamental physics and device applications due to the capability to engineer their electronic band structure and properties via a large variety of means. Here, we studied the fundamental transport properties and the strain modulation in few-layer NbSe2. In the differential resistance measurement, there are small plateaus before transiting to normal state. An unconventional critical current is also observed at zero magnetic field. Both unusual properties are sensitive to the magnetic field. In addition, we observed a direct variation of superconducting critical temperature in the NbSe2/hBN corrugation heterostructure. The inhomogeneity of superconductivity caused by the hBN corrugation leads to the voltage response and transition splitting while measuring the differential resistance. Our results may develop new applications in superconducting electronic devices and quantum metrology.

1. H. Onnes. Commun. Phys. Lab. Univ. Leiden. Suppl. 29 (Nov. 1911).
2. W. Meissner, R. Ochsenfeld, Ein neuer effekt bei eintritt der supraleitfähigkeit. Naturwissenschaften 21, 787-788 (1933).
3. J. Rjabinin, L. Shubnikow, Magnetic properties and critical currents of supra-conducting alloys. Nature 135, 581-582 (1935).
4. A. Tsen et al., Nature of the quantum metal in a two-dimensional crystalline superconductor. Nature Physics 12, 208-212 (2016).
5. A. Benyamini et al., Fragility of the dissipationless state in clean two-dimensional superconductors. Nature Physics 15, 947-953 (2019).
6. F. Miao, S.-J. Liang, B. Cheng, Straintronics with van der Waals materials. npj Quantum Materials 6, 1-4 (2021).
7. S.-C. Ho et al., Hall effects in artificially corrugated bilayer graphene without breaking time-reversal symmetry. Nature Electronics 4, 116-125 (2021).
8. C. Chen et al., Strain-Controlled Superconductivity in Few-Layer NbSe2. ACS Applied Materials & Interfaces 12, 38744-38750 (2020).
9. A. Wieteska et al., Uniaxial Strain Tuning of Superconductivity in 2H-NbSe2. arXiv preprint arXiv:1903.05253, (2019).
10. F. Pizzocchero et al., The hot pick-up technique for batch assembly of van der Waals heterostructures. Nature communications 7, 1-10 (2016).
11. S. De Graaf et al., Charge quantum interference device. Nature Physics 14, 590-594 (2018).
12. O. Astafiev et al., Coherent quantum phase slip. Nature 484, 355-358 (2012).
13. J. Baez, Kosterlitz–Thouless Transition. https://johncarlosbaez.wordpress.com/
2016/10/07/kosterlitz-thouless-transition/amp/?fbclid=IwAR3RcVSyv-zlqcVNA
IwHIjD3Rct4N55KkmxIM7uUN-k7UwdVLx701H68ZG4
14. M. Beasley, J. Mooij, T. Orlando, Possibility of vortex-antivortex pair dissociation in two-dimensional superconductors. Physical Review Letters 42, 1165 (1979).
15. R. Frindt, Superconductivity in ultrathin nb se 2 layers. Physical Review Letters 28, 299 (1972).
16. X. Xi et al., Ising pairing in superconducting NbSe 2 atomic layers. Nature Physics 12, 139-143 (2016).
17. Q. Li et al., Interlayer coupling effect in high-T c superconductors probed by YBa 2 Cu 3 O 7− x/PrBa 2 Cu 3 O 7− x superlattices. Physical review letters 64, 3086 (1990).
18. T. Schneider, Z. Gedik, S. Ciraci, Transition temperature of superconductor-insulator superlattices. EPL (Europhysics Letters) 14, 261 (1991).
19. K. Medvedyeva. (2003). Characteristic properties of two-dimensional superconductors close to the phase transition in zero magnetic field. (Doctoral thesis Department of Physics, Umeå University, Umeå, Sweden). Retrived from http://umu.diva-portal.org/smash/record.jsf?pid=diva2%3A139987&dswid=5167
20. Y. Cao et al., Unconventional superconductivity in magic-angle graphene superlattices. Nature 556, 43-50 (2018).
21. J. M. Park, Y. Cao, K. Watanabe, T. Taniguchi, P. Jarillo-Herrero, Tunable strongly coupled superconductivity in magic-angle twisted trilayer graphene. Nature 590, 249-255 (2021).
22. A. Devarakonda et al., Clean 2D superconductivity in a bulk van der Waals superlattice. Science 370, 231-236 (2020).
23. Y. Qin, C. L. Vicente, J. Yoon, Magnetically induced metallic phase in superconducting tantalum films. Physical Review B 73, 100505 (2006).
24. Y. Seo, Y. Qin, C. Vicente, K. Choi, J. Yoon, Origin of nonlinear transport across the magnetically induced superconductor-metal-insulator transition in two dimensions. Physical review letters 97, 057005 (2006).
25. Y. Li, C. L. Vicente, J. Yoon, Transport phase diagram for superconducting thin films of tantalum with homogeneous disorder. Physical Review B 81, 020505 (2010).
26. A. Hamill et al., Two-fold symmetric superconductivity in few-layer NbSe 2. Nature Physics, 1-6 (2021).
27. Y. Cao et al., Correlated insulator behaviour at half-filling in magic-angle graphene superlattices. Nature 556, 80-84 (2018).