|TITLE||Direct Imaging of Two Dimensional Electron Gas by Electron Holography|
Atomically controlled interfaces of complex oxides provide new opportunities for materials design and synthesis. They have been the origin of a wide variety of new physical phenomena and properties, arising primarily from the natural quantum confinement of electrons at these interfaces, involving a strong correlation between the electronic and atomic structure. One notable example is the electronic reconstruction of the interface between insulating perovskite oxides that leads to the formation of an interfacial two-dimensional electron gas (2DEG). The 2DEG is known to be formed from the occupied 3d-orbitals of cations within a few nanometers of the interface and often involve an interplay of electronic states with distinct orbital character and symmetry. Different 2DEGs and the related properties are expected by the orbital-selective quantum confinement which is strongly correlated with the crystallographic orientation.
Prof. Sang Ho OH of Department of Energy Science demonstrated that 2DEGs at oxide interfaces can be spatially mapped at subnanometer resolution using in-line electron holography and illustrated the power of this method by looking at the 2DEGs formed at (001) and (111) oriented LaAlO3/SrTiO3 interfaces and showing distinctly different spatial extent and charge density profiles across them. Prof. Sang Ho Oh and his Ph.D. student, Dr. Kyung Song now at KIMS, have successfully calibrated all variables affecting the 2DEG distribution, for example, the sample thickness, the mean inner potential and permittivity (ε), and thereby extracted intrinsic properties of 2DEG. Especially, taking account of the nonlinearity of the permittivity of oxide with electric field is essential, as the presence of the 2DEG leads to strong electric fields near the interface where the 2DEG is confined. The field-dependent permittivity has been calculated via analytical approach based on Landau theory and also directly through DFT calculation. These results provide the first direct evidence of the control of 2DEG properties through the interface orbital configuration and reveal the unprecedented capability of in-line holography to probe oxide heterostructures.
According to Prof. OH, the electron holography technique developed in the present work will play an important role in development of future oxide-based electronic devices as it is a unique tool bridging various emergent properties arising from quantized electrons at interfaces, such as ferromagnetism, superconductivity and metal-insulator transition, with the function and performance of devices.
The work has been conducted through international collaboration with Prof. Chang-Beom EOM, Prof. Christoph KOCH, Prof. Mark RZCHOWSKI and Prof. Evgeny TSYMBAL, Prof. Young-Min KIM and Prof. Si-Young CHOI and published recently in the March issue of Nature Nanotechnology. A companion paper has been published back to back in Nature Materials, demonstrating the formation of two-dimensional hole gas (2DHG) at the p-type LaAlO3/SrTiO3 interface. The work has been supported by National Research Foundation (NRF) of Korea and AFOSR Asian Office of Aerospace Research and Development (AOARD).
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