2DEGs at Oxide Interfaces

The discovery of a two dimensional electron gas (2DEG) at the interface between two insulating oxides LaAlO3 and SrTiO3 has attracted significant interest due to possible applications in all-oxide electronic devices. Stimulated by this discovery density-functional calculations are performed to understand properties of the 2DEG and explore new functionalities which may be realized using the 2DEGs. In particular we have predicted that the 2DEG confinement can be explained by the SrTiO3 conduction band bending and the formation of metal induced gap states in the band gap of SrTiO3. We have explored all-oxide heterostructures incorporating ferroelectric constituents. We found that the polar discontinuity in these heterostructures may lead to the formation of a 2DEG switchable between two conduction states as controlled by ferroelectric polarization orientation. We have also proposed a method to achieve a spin-polarized 2DEG by employing a ferromagnetic insulator, as one of the constituents in the oxide heterostructure.

We are also involved in collaboration with experimentalists at University of Wisconsin-Madison to elucidate factors controlling properties of 2DEGs. Our collaborative research has discovered that the formation of 2DEGs at complex oxide interfaces is affected by electronic correlations which make the interface conducting or insulting depending on their strength. We have also discovered that the critical thickness of the LaAlO3 layer and carrier concentration of the 2DEG can be tailored by epitaxial strain through electric polarization induced in SrTiO3.  These new functionalities may be interesting for device applications of the 2DEGs.

 

References:

  1. C. W. Bark, P. Sharma, Y. Wang, S.H. Beak, S. Lee, S. Ryu, C. M. Folkman, T. R. Paudel, A. Kumar, S.V. Kalinin, A. Sokolov, E. Y. Tsymbal, M. S. Rzchowski, A. Gruverman, C.B. Eom, “Switchable induced polarization in LaAlO3/SrTiO3 heterostructures,” Nano Letters  12, 1765–1771 (2012).
  2. N. Pavlenko, T. Kopp, E. Y. Tsymbal, G. A. Sawatzky, and J. Mannhart, “Magnetism and superconductivity at LAO/STO-interfaces: the role of Ti 3d interface electrons,” Physical Review B – Rapid Communications 85, 020407(R) (2012).
  3. J. D. Burton and E. Y. Tsymbal, “Highly spin-polarized conducting state at the interface between non-magnetic band insulators: LaAlO3/FeS2 (001),” Physical Review Letters 107, 166601 (2011).
  4. H. W. Jang, D. A. Felker, C. W. Bark, Y. Wang, M. K. Niranjan, C. T. Nelson, Y. Zhang, D. Su, C. M. Folkman, S. H. Baek, S. Lee, K. Janicka, Y. hu, X. Q. Pan, D. D. Fong, E. Y. Tsymbal, M. S. Rzchowski, and C. B. Eom, "Metallic and insulating oxide interfaces controlled by electronic correlations," Science 331, 886 (2011). Featured at University of Nebraska News Releases and Lincoln Journal Star.
  5. C. W. Bark, D. A. Felker, Y. Wang, Y, Zhang, H. W. Jang, C. M. Folkman, J. W. Park, S. H. Baek, X. Q. Pan, E. Y. Tsymbal, M. S. Rzchowski, and C. B. Eom, “Tailoring a two-dimensional electron gas at the LaAlO3/SrTiO3 (001) interface by epitaxial strain,” Proceedings of National Academy of Sciences 108, 4720 (2011).
  6. M. K. Niranjan, Y. Wang, S. S. Jaswal, and E. Y. Tsymbal, "Prediction of a switchable two-dimensional electron gas at ferroelectric oxide interfaces," Phys. Rev. Lett. 103, 016804 (2009).
  7. Y. Wang, M. K. Niranjan, S. S. Jaswal, and E. Y. Tsymbal, "First-Principles Studies of a Two-Dimensional Electron Gas at the Interface in Ferroelectric Oxide Heterostructures," Phys. Rev. B 80, 165130 (2009).
  8. K. Janicka, J. P. Velev, and E. Y. Tsymbal, "Quantum nature of two-dimensional electron gas confinement at LaAlO3/SrTiO3 interfaces," Phys. Rev. Lett. 102, 106803 (2009).
  9. J. D. Burton, J. P. Velev and E. Y. Tsymbal, "Oxide tunnel junctions supporting a two-dimensional electron gas," Phys. Rev. B 80, 115408 (2009).
  10. Y. Wang, M. K. Niranjan, J. D. Burton, J. M. An, K. D. Belashchenko, and E. Y. Tsymbal, "Prediction of a spin-polarized two-dimensional electron gas at the LaAlO3/EuO(001) interface," Phys. Rev. B 79, 212408 (2009).
  11. K. Janicka, J. P. Velev, and E. Y. Tsymbal, "Magnetism of LaAlO3/SrTiO3 superlattices," J. Appl. Phys.  103, 07B508 (2008).

Atomic structure of the LaAlO3/SrTiO3 interface
Atomic structure of the LaAlO3/SrTiO3 interface.

 

Effect of ferroelectric polarization on 2DEG at the oxide interface
Effect of ferroelectric polarization on 2DEG at the oxide interface.

 

Spin-polarized 2DEG in EuO/LaO/EuO system
Spin-polarized 2DEG in EuO/LaO/EuO system.

 

Strain effect on 2DEG at LaAlO3/SrTiO3 interface.
Strain effect on 2DEG at LaAlO3/SrTiO3 interface.