Through a collaboration between CNR-SPIN Naples, the Physics Department of the University of Naples Federico II, and the Swiss Light Source, Paul Scherrer Institut PSI- team, a novel example of a polar two-dimensional metal has been unveiled.
For years, itinerant charge carriers in ferroelectric insulators were believed to completely quench ferroelectricity. Recent breakthroughs, however, demonstrated the existence of a novel class of quasi-two-dimensional polar metals with promising applications in nonvolatile electronics and spintronics.
By combining temperature-dependent magnetotransport measurements, optical second harmonic generation (SHG), resonant photoemission spectroscopy (ResPES), and X-ray absorption spectroscopy (XAS), we investigated a heterostructure consisting of crystalline BaTiO₃ (BTO) films embedded between an amorphous LaAlO3 (a-LAO) layer and a SrTiO3 (STO) single crystal. Our results demonstrate that the system sustains persistent polar displacements in the BTO layer while simultaneously hosting a two-dimensional electron gas (2DEG).
Specifically, temperature-dependent sheet resistance measurements reveal metallic behavior for BTO thicknesses ranging from 3 to 14 unit cells (uc) (with fixed a-LAO thickness of 2.5 nm), and a transition to insulating behavior for thicknesses beyond 20 uc. Simultaneously, SHG measurements on the metallic a-LAO/BTO(14 uc)/STO heterostructure show strong SHG signals, indicative of pronounced polar distortions. These Ti off-center displacements, compatible with ferroelectric BTO stabilized by compressive strain and low electron doping, are further supported by X-ray linear dichroism (XLD) measurements, which confirm the presence of polar order even in the thinnest (3 uc) BTO layers.
To validate the metallic nature of the system, we performed ResPES measurements to identify signatures of 2DEG in the Ti-3d states. The valence band map of a-LAO/BTO(3 uc)/ STO showed resonant enhancement at the Fermi level, confirming 2DEG formation, with features at−3.5 eV indicative of Ti-3d and O-2p hybridization. The observed band dispersions closely resemble those reported for surface states in pure BTO. A control BTO/STO sample measured under the same conditions exhibited no states at the Fermi level.
In summary, we provide clear evidence for the existence of a two-dimensional polar metal in the a-LAO/BTO/STO heterostructure, characterized by strong polar distortions in the BTO layer. These findings are consistent with theoretical predictions of strain-engineered polar metals and open new avenues for tuning switchable polarity in oxide-based 2DEGs. The potential application of polar metals as electrodes to overcome critical thickness limitations in ferroelectric nanocapacitors highlights the promise of this system for future oxide electronics.
The research results have been published in the journal Nano Letters:
"Signatures of Polar Metal Phase in the Quasi-2D Electron System in PLD-Grown Amorphous-Epitaxial Oxide Heterostructures"
Alessia Sambri, Yu Chen, Federico Mazzola, Emiliano Di Gennaro, Andrea Rubano, Martando Rath, Domenico Paparo, Marco Caputo, Alla Chikina, Deepak Kumar, Vladimir N. Strocov, Marco Salluzzo, and Fabio Miletto Granozio
https://doi.org/10.1021/acs.nanolett.5c03409