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1. Spin Filtering with Insulating Altermagnets

   https://doi.org/10.1021/acs.nanolett.4c05672  

   Samanta, Kartik; Shao, Ding-Fu; Tsymbal, Evgeny Y (February 2025, Nano Letters)

   Altermagnetic (AM) materials have recently attracted significant interest due to their non-relativistic momentum-dependent spin splitting of their electronic band structure which may be useful for antiferromagnetic (AFM) spintronics. So far, however, most research studies have been focused on conducting properties of AM metals and semiconductors, while functional properties of AM insulators have remained largely unexplored. Here, we propose employing AM insulators (AMIs) as efficient spin-filter materials. By analyzing the complex band structure of rutile-type altermagnets MF2 (M = Fe, Co, Ni), we demonstrate that the evanescent states in these AMIs exhibit spin- and momentum-dependent decay rates resulting in momentum-dependent spin polarization of the tunneling current. Using a model of spin-filter tunneling across a spin-dependent potential barrier, we estimate the TMR effect in spin-filter magnetic tunnel junctions (SF-MTJs) that include two magnetically decoupled MF2 (001) barrier layers. We predict a sizable spin-filter TMR ratio of about 150-170% in SF-MTJs based on AMIs CoF2 and NiF2 if the Fermi energy is tuned to be close to the valence band maximum. Our results demonstrate that AMIs provide a viable alternative to conventional spin-filter materials, potentially advancing the development of next-generation AFM spintronic devices. 

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2. Topological Nanomaterials

   https://doi.org/doi.org/10.1038/s41578-019-0113-4  

   Liu, Pengzi; Williams, James; Cha, Judy (January 2019, Nature reviews. Materials)

   The past decade has witnessed the emergence of a new frontier in condensed matter physics: topological materials with an electronic band structure belonging to a different topological class from that of ordinary insulators and metals. This non-trivial band topology gives rise to robust, spin-polarized electronic states with linear energy–momentum dispersion at the edge or surface of the materials. For topological materials to be useful in electronic devices, precise control and accurate detection of the topological states must be achieved in nanostructures, which can enhance the topological states because of their large surface-to-volume ratios. In this Review, we discuss notable synthesis and electron transport results of topological nanomaterials, from topological insulator nanoribbons and plates to topological crystalline insulator nanowires and Weyl and Dirac semimetal nanobelts. We also survey superconductivity in topological nanowires, a nanostructure platform that might enable the controlled creation of Majorana bound states for robust quantum computations. Two material systems that can host Majorana bound states are compared: spin–orbit coupled semiconducting nanowires and topological insulating nanowires, a focus of this Review. Finally, we consider the materials and measurement challenges that must be overcome before topological nanomaterials can be used in next-generation electronic devices. 

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3. Three-dimensional Topological Insulator Tunnel Diodes

   Fluckey, Stephen P.; Tiwari, Sabyasachi; Hinkle, Christopher L.; Vandenberghe, William G. (January 2022, Physical review)

   Topological insulators open many avenues for designing future electronic devices. Using the Bardeen transfer Hamiltonian method, we calculate the current density of electron tunneling between two slabs of Bi2Se3. 3D TI tunnel diode current-voltage characteristics are calculated for different doping concentrations, tunnel barrier height and thickness, and 3D TI bandgap. The difference in the Fermi levels of the slabs determines the peak and trough voltages. The tunnel barrier width and height affect the magnitude of the current without affecting the shape of the current-voltage characteristics. The bandgap of the 3D TI determines the magnitude of the tunnel current, albeit at a lesser rate than the tunnel barrier potential, thus the device characteristics are robust under changing TI material. The high peak-to-trough ratio of 3D TI tunnel diodes, the controllabilty of the trough current location, and the simple construction provide advantages over other NDR devices. 

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4. Andreev bound states in magnetic topological insulator Josephson junctions

   https://doi.org/10.1016/j.physb.2025.417296  

   Zadorozhnyi, Andrei; Rivlis, Raz; Dahnovsky, Yuri (September 2025, Physica B: Condensed Matter)

   We study Andreev bound states in the presence of a magnetic moment in a ferromagnetic topological insulator in superconductor/magnetic topological insulator/superconductor Josephson junctions. We analytically find zero energy states for out-of-plane and in-plane directions of the magnetic moment. In the case of the out-of-plane magnetic moment, the energy is independent of the scattering angle. If both magnetic and nonmagnetic scattering mechanisms are considered, the zero energy state requires the scattering angle to the electrode to be zero as in the case of Majorana fermions. In the presence of an in-plane magnetic moment, the energy band always exhibits a nonvanishing gap if the magnetic moment has a nonzero component, i.e., there are no zero energy states. Here we assume that the electrons tunnel in the direction. If the magnetic moment is aligned with the tunneling direction, the zero energy states always exist and are independent of the scattering angle. Contrary to the Majorana fermion case, the phase shift between two superconductor electrodes is not. This phase difference depends on the system parameters such as the Fermi velocity, the barrier potential magnitude, the exchange coupling between localized and delocalized electrons, and the component of the magnetic moment. We find an anomalous Josepheson current when the magnetic moment has a component in the direction, where the current is nonzero despite. This is due to the violation of time reversal and chiral symmetries in the Josepheson junction. This leads to the observation of the Josephson Diode effect as well. For large scattering magnitudes, we find that the transmission coefficient approaches one at larger barrier magnitudes. This is the main reason why in superconductor/magnetic topological insulator/superconductor Josephson junctions critical current is much higher than in superconductor/normal metal/superconductor junctions. This effect is similar in origin to Klein Tunneling for relativistic Dirac electrons. In the case of nonmagnetic and out-of-plane magnetic scatterings, the current vanishes when the barrier amplitudes are approximately equal and large. This effect cannot be explained by the relativistic nature of the Dirac equation and is specific to the model. We also study temperature dependencies for in- and out-of-plane magnetic moments. We find that current at high temperatures is significantly smaller than at low temperatures. The current approaches a constant value at low temperatures, at approximately. This value depends on the other system parameters. The existence of new zero energy states in magnetic topological insulators in superconductor/magnetic topological insulator/superconductor Josephson junctions opens new opportunities in quantum computing because of the presence of the additional symmetry with respect to the scattering angle. 

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5. Spin–momentum locking induced non-local voltage in topological insulator nanowire

   https://doi.org/10.1039/d0nr06590k  

   Chen, Jen-Ru; Tse, Pok Lam; Krivorotov, Ilya N.; Lu, Jia G. (November 2020, Nanoscale)

   null (Ed.)

   The momentum and spin of charge carriers in the topological insulators are constrained to be perpendicular to each other due to the strong spin–orbit coupling. We have investigated this unique spin–momentum locking property in Sb 2 Te 3 topological insulator nanowires by injecting spin-polarized electrons through magnetic tunnel junction electrodes. Non-local voltage measurements exhibit an asymmetry with respect to the magnetic field applied perpendicular to the nanowire channel, which is remarkably different from that of a non-local measurement in a channel that lacks spin–momentum locking. In stark contrast to conventional non-local spin valves, simultaneous reversal of magnetic moments of all magnetic contacts to the Sb 2 Te 3 nanowire alters the non-local voltage. This unusual asymmetry is a clear signature of the spin–momentum locking in the Sb 2 Te 3 nanowire surface states. 

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