QLC Young Colloquium

The 12th QLC young colloquium

Date & Time : Monday, February 19, 2024. 13:30~14:30
Speaker: Two winners of the 5th QLC meeting Young Researcher Award
     Manato FUJIMOTO(Harvard University / University of Tokyo )
     Ryunosuke TAKAHASHI(University of Hyogo)
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: Manato FUJIMOTO(Harvard University / University of Tokyo )
Title: First Landau level in moiré system at zero magnetic field
 Chern insulators, which are the lattice analogues of the quantum Hall states, provide a promising platform to explore fractional Chern insulators with partial band fillings, offering a pathway to manipulate non-Abelian excitations. Moiré materials, such as twisted bilayer graphene and twisted transition metal dichalcogenides, are ideal platform to realize fractional Chern insulators because they can be tuned to host narrow Chern bands with “vortexable” single particle wavefunctions, generalizing those of the lowest Landau level. Fractional Chern insulators were recently observed at zero magnetic field in twisted MoTe2.[1,2] A natural next step is non-Abelian states, such as the Moore-Read Pfaffian state, which appear at half filling of the first Landau level.[3]  In this work, we provide a sharp definition of what first Landau level quantum geometry means when there is no external magnetic field, or magnetic translation symmetry. We find moiré graphene systems, alternative twisted Bernal graphite and strained Bernal graphene, that realize this geometry, and compare with trial wavefunctions that have a manifest first Landau level character.

[1] Zeng, Y., Xia, Z., Kang, K. et al., Nature 622, 69–73 (2023).
[2] Cai, J., Anderson, E., Wang, C. et al., Nature 622, 63–68 (2023).
[3] Moore, G., and N. Read, Nucl. Phys. B360, 362 (1991).

Speaker: Ryunosuke TAKAHASHI(University of Hyogo)
Title: All-optical switching in NiCo2O4 thin films using ultra-short pulse laser
 Laser-induced magnetization switching, called all-optical switching (AOS) since it does not use an external field such as a magnetic field, is very important for applications in next-generation magnetic devices and for understanding the interaction between light and electron spins.
 AOS has been observed in many alloys and magnetic multilayers such as GdFeCo and Co/Pt multilayers but rarely in chemically stable magnetic oxides.
 In this study, we have observed AOS and non-equilibrium magnetic dynamics in a room-temperature ferrimagnetic NiCo2O4 thin film irradiated with ultrashort pulses. This material shows perpendicular magnetic anisotropy and the Curie temperature exceeds 400 K at a film thickness of 30 nm. A pump-probe time-resolved magneto-optical Kerr effect microscope was fabricated using the ultrashort pulse laser PHAROS with a pulse width of 200 fs as a light source, and ultrafast demagnetization of 0.4 ps was observed in NiCo2O4 thin films with linearly polarized pulses. We also studied AOS in NiCo2O4 thin films by observing the magnetic domains after ultrashort laser pulse irradiation with linear polarization. The ring-shaped AOS was observed around the multi domains, which existed randomly in the upper and lower directions of the easy-axis direction of magnetization. The ring-shaped AOS disappeared when the area of the multidomain was too large. From these observations, it is considered that the AOS region could exist stably due to the expansion of the equilibrium domain size caused by the decrease in saturation magnetization due to the increase in temperature. AOS in NiCo2O4 thin films can be realized by controlling the equilibrium domain size by adjusting the saturation magnetization and coercive field, we are currently exploring the realization of AOS at room temperature by controlling the saturation magnetization by changing the film thickness. We are also exploring pulse frequency dependence to investigate the thermal effects of laser irradiation.

Committee Chair:Hiroki WADATI(University of Hyogo)