Unconventional quantum oscillations and evidence of nonparabolic electronic states in quasi-two-dimensional electron system at complex oxide interfaces

The simultaneous occurrence of electric-field controlled superconductivity and spin-orbit interaction makes two-dimensional electron systems (2DES) constructed from perovskite transition metal oxides promising candidates for the next generation of spintronics and quantum computing. It is, however, essential to understand the electronic bands thoroughly and verify the predicted electronic states experimentally in these 2DES to advance technological applications. Here, we present insights into the electronic states of the 2DES at oxide interfaces through comprehensive investigations of Shubnikov–de Haas oscillations in three different systems: EuO/<a:math xmlns:a="http://www.w3.org/1998/Math/MathML"><a:msub><a:mi>KTaO</a:mi><a:mn>3</a:mn></a:msub></a:math>, <b:math xmlns:b="http://www.w3.org/1998/Math/MathML"><b:msub><b:mi>LaAlO</b:mi><b:mn>3</b:mn></b:msub></b:math>/<c:math xmlns:c="http://www.w3.org/1998/Math/MathML"><c:msub><c:mi>SrTiO</c:mi><c:mn>3</c:mn></c:msub></c:math>, and amorphous-<d:math xmlns:d="http://www.w3.org/1998/Math/MathML"><d:msub><d:mi>LaAlO</d:mi><d:mn>3</d:mn></d:msub></d:math>/<e:math xmlns:e="http://www.w3.org/1998/Math/MathML"><e:msub><e:mi>KTaO</e:mi><e:mn>3</e:mn></e:msub></e:math>. To accurately resolve these oscillations, we conducted transport measurements in high magnetic fields up to 60 T and low temperatures down to 100 mK. For 2D confined electrons at these interfaces, we observed a progressive increase of oscillations frequency and cyclotron mass with the magnetic field. We interpret these universal and intriguing findings by considering the existence of nontrivial electronic bands, for which the <f:math xmlns:f="http://www.w3.org/1998/Math/MathML"><f:mrow><f:mi>E</f:mi><f:mo>−</f:mo><f:mi>k</f:mi></f:mrow></f:math> dispersion incorporates both linear and parabolic relations. In addition to providing experimental evidence for nonparabolic electronic states in <g:math xmlns:g="http://www.w3.org/1998/Math/MathML"><g:msub><g:mi>KTaO</g:mi><g:mn>3</g:mn></g:msub></g:math> and <h:math xmlns:h="http://www.w3.org/1998/Math/MathML"><h:msub><h:mi>SrTiO</h:mi><h:mn>3</h:mn></h:msub></h:math> 2DES, the unconventional oscillations presented in this study establish a paradigm for quantum oscillations in 2DES based on perovskite transition metal oxides, where the oscillation frequencies in <i:math xmlns:i="http://www.w3.org/1998/Math/MathML"><i:mrow><i:mn>1</i:mn><i:mo>/</i:mo><i:mi>B</i:mi></i:mrow></i:math> exhibit quadratic dependence on the magnetic field. Published by the American Physical Society 2024