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1. Exchange coupling in Bi2Se3/EuSe heterostructures and evidence of interfacial antiferromagnetic order formation

   https://doi.org/10.1103/PhysRevB.108.195308  

   Wang, Ying; Lauter, Valeria; Maximova, Olga; Konakanchi, Shiva T; Upadhyaya, Pramey; Keum, Jong; Ambaye, Haile; Wang, Jiashu; Zhukovskyi, Maksym; Orlova, Tatyana A; et al (November 2023, Physical Review B)

   Spatial confinement of electronic topological surface states (TSSs) in topological insulators poses a formidable challenge because TSSs are protected by time-reversal symmetry. In previous works formation of a gap in the electronic spectrum of TSSs has been successfully demonstrated in topological insulator/magnetic material heterostructures, where ferromagnetic exchange interactions locally lift the time-reversal symmetry. Here we report experimental evidence of exchange interaction between a topological insulator Bi2Se3 and a magnetic insulator EuSe. Spin-polarized neutron reflectometry reveals a reduction of the in-plane magnetic susceptibility within a 2 nm interfacial layer of EuSe, and the combination of superconducting quantum interference device (SQUID) magnetometry and Hall measurements points to the formation of an interfacial layer with a suppressed net magnetic moment. This suppressed magnetization survives up to temperatures five times higher than the Néel temperature of EuSe. Its origin is attributed to the formation of an interfacial antiferromagnetic state. Abrupt resistance changes observed in high magnetic fields are consistent with antiferromagnetic domain reconstruction affecting transport in a TSS via exchange coupling. The high-temperature local control of TSSs with zero net magnetization unlocks new opportunities for the design of electronic, spintronic, and quantum computation devices, ranging from quantization of Hall conductance in zero fields to spatial localization of non-Abelian excitations in superconducting topological qubits. 

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2. Thermoelectric transport contribution from topological surface states vs 2D-electron gas in 10 nm Bi ₂ Se ₃

   https://doi.org/10.1063/5.0100991  

   Nimmagadda, Lakshmi Amulya; Bai, Yang; Rajagopal, Manjunath C.; Suresh Babu, Soorya; Eckstein, James N.; Sinha, Sanjiv (October 2022, Journal of Applied Physics)

   Topological surface states (TSSs) coexist with a rapidly formed two-dimensional electron gas (2DEG) at the surface of Bi 2 Se 3 . While this complex band structure has been widely studied for its interactions between the two states in terms of electrical conductivity and carrier density, the resulting thermopower has not been investigated as thoroughly. Here, we report measurements of the temperature dependent Seebeck coefficient ( S) and electrical conductivity ( σ) on an undoped 10 nm thin Bi 2 Se 3 film over the temperature range of 100–300 K to find an overall metal-like behavior. The measured S is consistent with the theory when assuming that both the TSS and the 2DEG contribute to thermoelectric transport. Our analysis further shows that the coefficient corresponds to a Fermi level situated well above the conduction band minima of the 2DEG, resulting in comparable contributions from the TSS and the 2DEG. The thermoelectric power factor ( S 2 σ) at 300 K increases by 10%–30% over the bulk. This work provides insights into understanding and enhancing thermoelectric phenomena in topological insulators. 

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3. ZrTe2/CrTe2: an epitaxial van der Waals platform for spintronics

   https://doi.org/10.1038/s41467-022-30738-1  

   Ou, Yongxi; Yanez, Wilson; Xiao, Run; Stanley, Max; Ghosh, Supriya; Zheng, Boyang; Jiang, Wei; Huang, Yu-Sheng; Pillsbury, Timothy; Richardella, Anthony; et al (May 2022, Nature Communications)

   Abstract The rapid discovery of two-dimensional (2D) van der Waals (vdW) quantum materials has led to heterostructures that integrate diverse quantum functionalities such as topological phases, magnetism, and superconductivity. In this context, the epitaxial synthesis of vdW heterostructures with well-controlled interfaces is an attractive route towards wafer-scale platforms for systematically exploring fundamental properties and fashioning proof-of-concept devices. Here, we use molecular beam epitaxy to synthesize a vdW heterostructure that interfaces two material systems of contemporary interest: a 2D ferromagnet (1T-CrTe₂) and a topological semimetal (ZrTe₂). We find that one unit-cell (u.c.) thick 1T-CrTe₂grown epitaxially on ZrTe₂is a 2D ferromagnet with a clear anomalous Hall effect. In thicker samples (12 u.c. thick CrTe₂), the anomalous Hall effect has characteristics that may arise from real-space Berry curvature. Finally, in ultrathin CrTe₂(3 u.c. thickness), we demonstrate current-driven magnetization switching in a full vdW topological semimetal/2D ferromagnet heterostructure device. 

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4. Thermally generated spin current in the topological insulator Bi ₂ Se ₃

   https://doi.org/10.1126/sciadv.adi4540  

   Jain, Rakshit; Stanley, Max; Bose, Arnab; Richardella, Anthony R.; Zhang, Xiyue S.; Pillsbury, Timothy; Muller, David A.; Samarth, Nitin; Ralph, Daniel C. (December 2023, Science Advances)

   We present measurements of thermally generated transverse spin currents in the topological insulator Bi₂Se₃, thereby completing measurements of interconversions among the full triad of thermal gradients, charge currents, and spin currents. We accomplish this by comparing the spin Nernst magneto-thermopower to the spin Hall magnetoresistance for bilayers of Bi₂Se₃/CoFeB. We find that Bi₂Se₃does generate substantial thermally driven spin currents. A lower bound for the ratio of spin current density to thermal gradient is $\frac{J_s}{{\mathbf{\nabla }}_{\mathit{x}}\mathit{T}}$= (4.9 ± 0.9) × 10⁶ $\left(\frac{\hslash }{2e}\right)\frac{\mathbf{A}{\mathbf{m}}^{-2}}{\mathbf{K}\text{μ}{\mathbf{m}}^{-1}}$, and a lower bound for the magnitude of the spin Nernst ratio is −0.61 ± 0.11. The spin Nernst ratio for Bi₂Se₃is the largest among all materials measured to date, two to three times larger compared to previous measurements for the heavy metals Pt and W. Strong thermally generated spin currents in Bi₂Se₃can be understood via Mott relations to be due to an overall large spin Hall conductivity and its dependence on electron energy. 

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5. Electrical switching of the edge current chirality in quantum anomalous Hall insulators

   https://doi.org/10.1038/s41563-023-01694-y  

   Yuan, Wei; Zhou, Ling-Jie; Yang, Kaijie; Zhao, Yi-Fan; Zhang, Ruoxi; Yan, Zijie; Zhuo, Deyi; Mei, Ruobing; Wang, Yang; Yi, Hemian; et al (January 2024, Nature Materials)

   A quantum anomalous Hall (QAH) insulator is a topological phase in which the interior is insulating but electrical current flows along the edges of the sample in either a clockwise or counterclockwise direction, as dictated by the spontaneous magnetization orientation. Such a chiral edge current eliminates any backscattering, giving rise to quantized Hall resistance and zero longitudinal resistance. Here we fabricate mesoscopic QAH sandwich Hall bar devices and succeed in switching the edge current chirality through thermally assisted spin–orbit torque (SOT). The well-quantized QAH states before and after SOT switching with opposite edge current chiralities are demonstrated through four- and three-terminal measurements. We show that the SOT responsible for magnetization switching can be generated by both surface and bulk carriers. Our results further our understanding of the interplay between magnetism and topological states and usher in an easy and instantaneous method to manipulate the QAH state. 

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