5d-oxide based two-dimensional electron system in high magnetic fields
In collaboration with the National University of Singapore (NUS) and the Laboratoire National des Champs Magnétiques Intenses (LNCMI) in Toulouse, researchers from HFML-FELIX have investigated electronic subbands properties of the quasi-two-dimensional electron gas (2DEG) at a fairly new 5d-oxide heterointerface, LaAlO3/KTaO3, via quantum oscillations in high magnetic fields. The high-mobility carriers at the interface were induced by implementing an ionic-liquid gating process at room temperature. In addition to the detailed information about the band structure of KTaO3-2DEG, this study paves a way to create a high-mobility 2DEG based on other 5d oxides for which constructing an epitaxial heterointerface is almost impossible.
Discovered in 2004, the 2DEG at the interface between two insulators, LaAlO3 and SrTiO3, has developed into an intense research topic in fundamental condensed matter physics with some promising application perspectives. Exceptional findings, such as superconductivity, magnetism, large Rashba spin-orbit interaction, and aperiodic quantum oscillations, in this system call for investigations on other related systems. KTaO3, for which the conduction band is made up of Ta:5d orbital instead of 3d orbital in SrTiO3 is indeed such a system with improved specific properties. For example, KTaO3-based 2DEGs exhibit a higher spin-orbit interaction, weaker electron correlation, higher mobility of electrons, and a higher critical temperature of superconductivity.
To investigate these novel systems experimentally, we have performed quantum transport on LaAlO3/KTaO3 heterostructures in high magnetic fields (continuous fields up to 30 T and pulsed fields up to 55 T) and at temperatures down to 80 mK. These extreme conditions are essential to resolve the small quantum oscillations associated with narrowly spaced energy levels. The oscillations in the magnetoresistance measured in different field orientations reveal confinement of the majority of electrons within a few atomic layers of KTaO3 from the interface. The oscillations of multiple frequencies and their temperature dependence evidence the occupancy of electrons in small-energy spaced light (m* = 0.2 me) and heavy (m* = 0.5 me) subbands originated from the Ta:t2g orbital. Overall, the inferred subbands properties are in good agreement with the angle-resolved photoemission spectroscopy and the theoretical calculations.
Additional observation of magnetoresistance fluctuations in the low-density regime, which most likely originated from the quantum interference of electrons, demonstrates a prospect of oxide-based quantum electronic devices and a platform to investigate the quantum coherence transport in a correlated two-dimensional electron system.
Figure: Quantum oscillations in the magnetoresistance of an ionic-liquid gated a-LaAlO3/KTaO3 device measured down to 80 mK. The top inset includes an optical microscope image of a Hall bar patterned sample, a circuit diagram used for the ionic-liquid gating at room temperature and a schematic of gating process.
Electronic subbands in the a−LaAlO3/KTaO3 interface revealed by quantum oscillations in high magnetic fields,Km Rubi, Shengwei Zeng, Femke Bangma, Michel Goiran, A. Ariando, Walter Escoffier, and Uli Zeitler, Physical Review Research 3, 033234 (2021).
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