• Session Info

Interaction induced magnetism in 2D semiconductor moiré superlattices
Many-body interactions between carriers lie at the heart of correlated physics. The ability to tune such interactions would open the possibility to access and control complex electronic phase diagrams on demand. Recently, moiré superlattices formed by two-dimensional materials have emerged as a promising platform for quantum engineering such phenomena. In this talk, I will present a systematic study of the emergent magnetic interactions (both antiferromagnetic and ferromagnetic) in strongly correlated transition metal
dichalcogenides moiré superlattices. I will show that the combination of doping, electric field, and optical excitation provide dynamic controls of the rich many-body Hamiltonian of moiré quantum matter.

• Session Info

Spin-Orbit Torque and Spin Pumping in van der Waals Heterostructures of Magnetic Two-Dimensional Materials

The bilayer heterostructures composed of an ultrathin ferromagnetic metal (FM) and a nonmagnetic material hosting strong spin-orbit coupling (SOC) are a principal resource for spin-orbit torque (SOT) [1] and spin-to-charge conversion [2] effects in next generation spintronics. The key to understand these effect is current-driven nonequilibrium spin density [3]. For example, it generates SOT when it is noncollinear to the direction of local magnetization and it can arise due to variety of microscopic mechanisms, including the spin Hall effect, spin-orbit proximity effect and different interfacial scattering mechanisms. The recently discovered two-dimensional (2D) magnetic materials offer new avenue for highly efficient and gate- or disorder-tunable SOT in van der Waals (vdW) heterostructures composed of few monolayers of atomically thin materials where the spin Hall effect from the bulk is absent. Using first-principles quantum transport calculations, which combine nonequilibrium Green functions with noncollinear density functional theory [1], we predicted [4] that injecting unpolarized charge current parallel to the interface of bilayer-CrI3/monolayer-TaSe2 vdW heterostructure will induce SOT-driven dynamics of magnetization on the first monolayer of CrI3 that is in direct contact with metallic transition metal dichalcogenide (TMD) TaSe2 and SO-proximitized by it. By combining calculated complex angular dependence of SOT with the Landau-Lifshitz-Gilbert equation for classical dynamics of magnetization, we find that this can reverse the direction of magnetization on the first monolayer to become parallel to that of the second monolayer, thereby converting bilayer CrI3 from antiferromagnet to ferromagnet as the signature of nonequilibrium phase transition. Such transition can be detected by passing vertical current, and it is of potentially great interest to magnetic memory applications since it does not require any external magnetic field. Another vdW heterostructure exhibiting SOT is doubly proximitized graphene, which is neither magnetic nor hosts SOC in its isolated form, but proximity induced magnetic moments will exhibit SOT in Cr2Ge2Te6/graphene/WS2 vdW heterostructure which can be tuned by two orders of magnitude via the gate voltage [5].

Finally, we predict [6] that SO-proximitized 2D magnets, pushed out of equilibrium by microwave absorption which leads to steadily precessing magnetization, will pump spin and charge currents exhibiting high harmonics of the microwave frequency with cutoff order ~20, in contrast to two decades old “standard model” [7] of spin pumping at a single frequency.

References
[1] B. K. Nikolic, K. Dolui, M. Petrovic, P. Plechác, T. Markussen, and K. Stokbro, in W. Andreoni and S. Yip (eds.),
Handbook of Materials Modeling (Springer, Chan, 2018); arXiv:1801.05793.
[2] F. Mahfouzi, N. Nagaosa, and B. K. Nikolic, Phys. Rev. B 90, 115432 (2014).
[3] P.-H. Chang, T. Markussen, S. Smidstrup, K. Stokbro, and B. K. Nikolic, Phys. Rev. B 92, 201406(R) (2015).
[4] K. Dolui, M. D. Petrovic, K. Zollner, P. Plechác, J. Fabian, and B. K. Nikolic, Nano Lett. 20, 2288 (2020).
[5] K. Zollner, M. D. Petrovic, K. Dolui, P. Plechác, B. K. Nikolic, and J. Fabian, Phys. Rev. Res. 2, 043057 (2020).
[6] J. Varela Manjarres and B. K. Nikolic, https://arxiv.org/abs/2112.14685.
[7] Y. Tserkovnyak, A. Brataas, G. E. W. Bauer, and B. I. Halperin, Rev. Mod. Phys. 77, 1375 (2005).

• Session Info

Dual magnetism and magnetic fluctuations in a magnetic Weyl semimetal Co₃Sn₂S₂

Co₃Sn₂S₂ has drawn much attention from multiple aspects, including a magnetic Weyl semimetal candidate, a quantum anomalous Hall effect testbed, and a non-Bravais Kagome-lattice magnet. All these interesting phenomena critically require a comprehensive understanding on the magnetic structure of Co₃Sn₂S₂, which is, however, heatedly debated in literature and remains elusive. Here, we report our experimental efforts using femto-second laser based optical techniques to address the magnetism and fluctuations in this system. Using electric quadrupole second harmonic generation (EQ SHG), we show the presence of two magnetic phases – one onset at TC,1 ~ 175K for the Ising-type ferromagnetic order for the out-of-plane spin component, and the other onset at TC,2 ~ ~ 125K for the XY-type antiferromagnetic order for the in-plane spin component. Using magneto-optical spectroscopy, together with anomalous Hall effect measurement, we show significant fluctuations at TC,1 ~ 175K which disappears at lower temperatures.

• Session Info

Harnessing Spin in a-Sn

Dirac semimetals are a recently discovered topological phase of quantum matter. α -Sn is unique among the Dirac semimetals because it is a single-element material and is therefore relatively easy to grow. Further, it can be transformed into other topological phases, such as a topological insulator or a Weyl semimetal, under strains or external fields. I will discuss our recent experimental work on α -Sn thin films along four directions:
(1) Growth of α -Sn thin films by a CMOS-compatible sputtering technique.
(2) Large damping enhancement in a ferromagnetic thin film due to the presence of topological surface states in an adjacent α -Sn thin film [1].
(3) Current-induced magnetization switching via topological surface states in an α -Sn/Ag/CoFeB trilayer [2].
(4) Spin-momentum-locking driven large magnetoresistance in α -Sn thin films that scales linearly with both magnetic and electric fields [3]. These results indicate that topological Dirac semimetal α -Sn holds exciting promise of application in next-generation electronics and quantum technologies.

References:
[1] “Large damping enhancement in Dirac-semimetal – ferromagnetic-metal layered structures caused by topological surface states,” Jinjun Ding, Chuanpu Liu, Yuejie Zhang, Vijaysankar Kalappattil, Rui Yu, Uppalaiah Erugu, Jinke Tang, Haifeng Ding, Hua Chen, and Mingzhong Wu, Advanced Functional Materials, 2008411 (2021). DOI: 10.1002/adfm.202008411
[2] “Switching of a magnet by spin-orbit torque from a topological Dirac semimetal,” Jinjun Ding, Chuanpu Liu, Vijaysankar Kalappattil, Yuejie Zhang, Oleksandr Mosendz, Uppalaiah Erugu, Rui Yu, Jifa Tian, August DeMann, Stuart B. Field, Xiaofei Yang, Haifeng Ding, Jinke Tang, Bruce Terris, Albert Fert, Hua Chen, and Mingzhong Wu, Advanced Materials 33, 2005909 (2021). https://doi.org/10.1002/adma.202005909
[3] “Large magneto-electric resistance in the topological Dirac semimetal α-Sn,” Yuejie Zhang, Vijaysankar Kalappattil, Chuanpu Liu, M. Mehraeen, Steven S.-L. Zhang, Jinjun Ding, Uppalaiah Erugu, Zhijie Chen, Jifa Tian, Kai Liu, Jinke Tang, and Mingzhong Wu, Science Advances 8, eabo0052 (2022). DOI: 10.1126/sciadv.abo0052

• Session Info

Spin currents and topology in magnetic heterostructures

An ability to control spin currents is important for probing many spin-related phenomena in the field of spintronics, and for designing logic and memory devices with low dissipation. Spin-orbit torque is an important example in which spin current is used to control magnetization dynamics. In this talk, I will discuss the interplay between topology and spin flows that can arise in various types of magnetic systems, e.g., in magnetic insulators, magnetic multilayers, and van der Waals magnets. In particular, I will discuss spin Hall effect of magnons and its possible observation with NV centers, I will discuss spin superfluid transport in van der Waals magnets, and I will discuss absorption of spin currents at interfaces concentrating on interplay between intrinsic and extrinsic contributions and interfacial effects which can have implications for spin-orbit torques.

• Session Info

Hybrid topological superconductors: Toward an optimal platform for topological quantum computation

Topology has emerged as a new paradigm of classifying electronic materials over the past decade, and a series of topological materials including topological insulators (TIs) and topological semimetals (TSMs) have been predicted and subsequently identified experimentally. On the other hand, another group of topological materials, the topological superconductors (TSCs), have not enjoyed as much progress due to various materials challenges. Unlike TIs and TSMs, whose topological characters can be reasonably well identified from band structure calculations, there does not exist such a general formalism for identifying a TSC. As a work-around, it was shown in 2008 by Fu and Kane that proximity effect between an s-wave superconductor and a TI can be used to implement TSC that can host the Majorana zero mode, which is the building block for topological quantum computation. Despite the conceptual simplicity of this proximity-based approach, its implementation - typically based on elemental superconductors such as Al, Pb, and Nb - suffers from undesirable interfacial problems with the chalcogenide TIs, and this has been a major roadblock to the proximity-based TSC. However, over the past decade, my group has demonstrated that with well-maneuvered thin film engineering, it is possible to get around many of the seemingly intractable materials problems in a variety of topological quantum matters. Here, I will present our ongoing endeavors, at the forefront of thin film engineering, to overcome these obstacles to a proximity-based TSC, with a few surprises found along the way.

• Session Info

Observation of the Layer Hall Effect in Topological Axion Antiferromagnet MnBi₂Te₄

While ferromagnets have been known and exploited for millennia, antiferromagnets were only discovered in the 1930s. The elusive nature indicates antiferromagnets’ unique properties: At large scale, due to the absence of global magnetization, antiferromagnets may appear to behave like any non-magnetic material; At the microscopic level, however, the opposite alignment of spins forms a rich internal structure. In topological antiferromagnets, such an internal structure leads to a new possibility, where topology and Berry phase can acquire distinct spatial textures. We study this exciting possibility in an antiferromagnetic Axion insulator, even-layered MnBi₂Te₄ flakes. We report the observation of a new type of Hall effect, the layer Hall effect, where electrons from the top and bottom layers spontaneously deflect in opposite directions.