QLC Young Colloquium

The 11th QLC young colloquium

Date & Time : Friday, October 6, 2023. 13:30~14:30
Speakers:Two winners of the QLC2023 Young Researcher Award ,
     Shunsuke NISHIMURA(University of Tokyo)
Place:online using “Zoom”

*If you wish to join this seminar, please register at this site .
*Zoom meeting ID information will be sent by the day of this colloquium to those who registered.

Speaker: Masahiro NARITSUKARIKEN)
Title: Chiral crystalline superconductivity in the monolayer NbSe2 twisted on graphene
 Broken symmetry in a superconductor can lead to unusual phenomena such as mixed-parity Cooper pairing in the non-centrosymmetric materials [1]. Although the signature of broken symmetry should, in principle, manifest itself in the electronic state, its spectroscopic search has been challenging due to the lack of a suitable platform for measuring it. Here, we found that the superconducting monolayer NbSe2 grown on graphene by molecular beam epitaxy is naturally twisted to be chiral, breaking both inversion and reflection symmetries. Such an atomically thin twisted superconductor provides an opportunity to investigate the impact of chirality on superconductivity by means of spectroscopic-imaging scanning tunneling microscopy. We have succeeded in imaging the interference patterns of Bogoliubov quasiparticles with wave vectors incompatible with any combinations of NbSe2– and graphene- principal lattice vectors. This result indicates that superconductivity in this chiral structure can be twisted in a way that is incommensurate with the atomic potential landscape.

[1] Y. Yanase and S. Fujimoto, Non-Centrosymmetric Superconductors, eds. E. Bauer and M. Sigrist (Springer) Chapter 6 (2012).

Speaker: Shunsuke NISHIMURA(University of Tokyo)
Title: Wide-field quantitative magnetic imaging of superconducting vortices using perfectly aligned quantum sensors
 Superconducting quantum vortices, crucial for understanding superconductivity properties, have attracted significant attention. The quantization of vortex flux reveals these properties, leading to the proposal that half-quantization is possible in p-wave superconductors with unconventional pairing. Accurate probes for assessing flux density distribution around these vortices are critical for studying a myriad of superconductors. Though numerous techniques aim to measure these vortices, only a handful can measure local flux densities effectively.
 Recent innovations have highlighted the potential of Nitrogen-Vacancy(NV) centers in diamonds for magnetic field sensing. An NV center comprises a structure formed by nitrogen impurities and a vacancy. They host two electrons, behaving as a spin-1 quantum system, which can be optically observable. Observing the Zeeman effect of this spin allows for magnetic field sensing. Remarkably, NV centers maintain long coherence time even at room temperatures and provide a precise measurement of magnetic fields. They are thus viewed as potential rivals to superconducting quantum interference devices (SQUIDs) and are considered apt for scanning probe microscopy (SPM).
 For techniques using NV center, an innovative method has been recently established to optically capture the signal from NV centers in a wide field of view at once. [1]. This method involves combining an ensemble of NV centers (where NV centers are densely formed, typically at 10,000/μm2, to the extent that they cannot be optically distinguished) on a diamond substrate surface with an image sensor, enabling spatial signal capture. This approach combines high throughput with the potential for application in extreme conditions, such as ultra-high pressures [2].
 In this research, our focus is on this wide-field imaging to quantitatively measure the stray field of quantum vortices. While efforts have been made to image the stray magnetic fields of superconducting quantum vortices [3, 4], achieving magnetic accuracy comparable to SPM [5] has proven challenging. This challenge arises primarily when measurements of superconductors are conducted in low magnetic fields. Here, the inhomogeneity of the sensor’s strain parameter [6] combined with signal overlap from a diamond sensor ensemble with four NV axes makes extracting the field component perpendicular to the superconductors’ surface nearly impossible. To address these issues, we utilize a perfectly aligned NV ensemble sensor [7] and implementing an analysis that eliminates sensor inhomogeneities resulting from strain distribution, complemented by reference measurements in a zero magnetic field. Consequently, we report a quantitative wide-field magnetic imaging of superconducting vortices in a thin film of a typical high-Tc superconductor. In this presentation, we will discuss the operating principles of NV centers as sensors, along with detailed results and future prospects.

[1] S. C. Scholten, et al., J. Appl. Phys. 130, 150902 (2021).
[2] S. Hsieh, et al., Science 366, 1349 (2019).
[3] Y. Schlussel, et al., Phys. Rev. Appl. 10, 034032 (2018).
[4] S. E. Lillie, et al., Nano Letters 20, 1855 (2020).
[5] L. Thiel, et al., Nat. Nanotechnol. 11, 677 (2016).
[6] L. Rondin, et al., Rep. Prog. Phys. 77, 056503 (2014).
[7] T. Tsuji, et al., Diam. Relat. Mater. 123, 108840 (2022).

Committee Chair:Hiroki WADATI(University of Hyogo)