Two-Dimensional van der Waals Materials

Two-dimensional (2D) van der Waals (vdW) materials and heterostructures have opened unprecedented opportunities to explore new physics and device concepts. Especially interesting are recently discovered 2D ferroic vdW materials, providing new functionalities associated with switchable spontaneous electric or magnetic polarizations in these materials.  Using theoretical modeling based on density functional theory (DFT) and model Hamiltonian approaches, we investigate electronic and magnetic properties of these heterostructures, including spin- and valley-dependent transport and quantum phenomena.  

One direction along these lines explores spin-dependent electronic transport in vdW magnetic tunnel junctions (MTJs).  For MTJs composed of Fe₃GeTe₂ ferromagnetic electrodes and a graphene or hexagonal boron nitride (h-BN) spacer layer, we find that the junction resistance changes by thousands of percent when the magnetization of the electrodes is switched from parallel to antiparallel.

A different type of MTJs exploits a spin filtering effect using CrI₃ as a tunnel barrier. CrI₃ is a magnetic semiconductor exhibiting transitions between ferromagnetic and antiferromagnetic orderings under the influence of an applied magnetic field. We explore spin-dependent transport in tunnel junctions utilizing a CrI₃ tunnel barrier and find nearly 100% spin polarization of the tunneling current for a ferromagnetically-ordered CrI₃ and TMR of about 5,000% associated with a change of magnetic ordering in CrI₃.

Another direction involves valleytronics – an emerging field of research which employs energy valleys in the band structure of 2D electronic materials to encode information. We exploit spin-valley locking in 2D materials with a large spin-orbit coupling and electric field reversible valley spin polarization, such as germanene, stanene, aand 1T′-transition metal dichalcogenide, to realize a valley spin valve. The valley spin polarization in these materials can be switched by an external electric field, which enables functionalities of a valley spin polarizer or a valley spin analyzer. When placed in series, they constitute a valley spin valve – a device whose conductance state is ON or OFF depending on the relative valley spin polarization of the polarizer and the analyzer.

Finally, we propose exploiting 2D ferroelectrics, such as In₂X₃ (X = S, Se, Te), as functional tunneling barrier materials. Due to the weak coupling between the atomic layers in these materials, the relative dipole alignment between them can be controlled by applied voltage. This allows transitions between ferroelectric and antiferroelectric orderings to realize a 2D antiferroelectric tunnel junction. Using quantum-mechanical modeling of the electronic transport, we explore in-plane and out-of-plane tunneling across In₂S₃ van der Waals bilayers, and predict giant tunneling electroresistance (TER) effects and multiple non-volatile resistance states driven by ferroelectric-antiferroelectric order transitions. 

References

1. J. Ding, D.-F. Shao, M. Li, L.-W. Wen, and E. Y. Tsymbal, “A two-dimensional antiferroelectric tunnel junction,” Physical Review Letters 126, 057601  (2021). 
2. Y. Su, X. Li, M. Zhu, J. Zhang, L. You, and E. Y. Tsymbal, “Van der Waals multiferroic tunnel junctions,” Nano Letters 21, 175–181 (2021).
3. L. L. Tao and Evgeny Y. Tsymbal, Topical Review: “Perspectives of spin-textured ferroelectrics,” Journal of Physics D: Applied Physics 54, 113001 (2021).
4. Lingling Tao and Evgeny Y. Tsymbal, “Insulator-to-conductor transition controlled by the Rashba-Zeeman effect,” npj Computational Materials 6, 172 (2020).
5. L. L. Tao, A. Naeemi, and E. Y. Tsymbal, “Valley-spin logic gates,” Physical Review Applied 13, 054043 (2020).
6. D. Li, X. Huang, Z. Xiao, H. Chen, L. Zhang, Y. Hao, J. Song, D.-F. Shao, E. Y. Tsymbal, Y. Lu, and X. Hong, “Polar coupling enabled nonlinear optical filtering at MoS₂/ferroelectric heterointerfaces,” Nature Communications 11, 1422 (2020).
7. F. Ibrahim, A. Hallal, D. S. Lerma, X. Waintal, E. Y. Tsymbal, and M. Chshiev, “Unveiling multiferroic proximity effect in graphene,” 2D Materials 7, 015020 (2020).
8. L. L. Tao and Evgeny Y. Tsymbal, “Two-dimensional spin-valley locking spin valve,” Physical Review B – Rapid Communications 100, 161110(R) (2019).
9. X. L. Li, J.-T. Lü, J. Zhang, Y. R. Su, and E. Y. Tsymbal, “Spin-dependent transport in van der Waals magnetic tunnel junctions with Fe₃GeTe₂ electrodes,” Nano Letters 19, 5133-5139 (2019).
10. P. Sharma, F.-X. Xiang, D.-F. Shao, D. Zhang, E. Y. Tsymbal, A. R. Hamilton, and J. Seidel, “A room temperature ferroelectric semimetal,” Science Advances 5, eaax5080 (2019).
11. Tula R. Paudel and Evgeny Y. Tsymbal, “Spin filtering in CrI3 tunnel junctions,” ACS Applied Materials & Interfaces 11, 15781–15787 (2019).
12. H. Takenaka, S. Sandhoefner, A. A. Kovalev, and E. Y. Tsymbal, “Magnetoelectric control of topological phases in graphene,” Physical Review B 100, 125156 (2019); Editor’s Suggestion.