Two dimensional electron gases (2DEG) have been the arena of many fascinating physics. Recently a new realm was uncovered with the discovery of 2DEG in d-electron transition metal oxides based on SrTiO3 and LaAlO3 heterostructures . As a result of their narrow bands derived from d-orbitals, the electronic states realized in transition metal oxides are intimately linked to magnetism and superconductivity. However, although the quantum size effect of electrons at oxide interfaces may provide interface-driven novel electron phases or functionality, direct experimental data showing such phenomena are limited. One of the key question is how an interface electric field reconstructs the d-electron bands into two-dimensional (2D) subbands, where the coupling between spin and orbital part of electron’s wavefunction induced by an inversion-breaking field is essential [2,3]. We recently demonstrated an unusual type of spin texture (manifestation of spin orbit magnetic field pattern in momentum space) called the “cubic” Rashba effect in an electron gas at SrTiO3 surface using a field-effect transistor (FET) structure (Fig. 1) . Our oxide FETs are unique in that a strict quantum-mechanical two-dimensionality may be reached; i.e., only a few (and possibly single) quantum wells are populated due to extraordinary low carrier density achieved in our extremely high-quality FETs, which facilitates unambiguous determination of the 2DEG state at the interface.
Low temperature magnetotransport was studied in two-dimensional electron gas (2DEG) formed in SrTiO3 channel FETs. Uniqueness of our experimental techniques lies in 1) Gating via parylene insulator which enables continuous modulation of 2DEG carrier density in the range 1011–1012cm-2, which is exceptionally small for an oxide; 2) Shubnikov-de Haas (SdH) and antilocalization analysis using magnetoresistance data obtained for high and low magnetic fields, respectively.
Gate tuning of the two-dimensional electron gas in SrTiO3 transistors realized unprecedentedly low carrier density (nHall=3.7–60×1011cm-2), which, for the first time, lead to the simultaneous observation of the Shubnikov-de Haas oscillation at high fields and an antilocalization magnetoresistance at low fields. The latter gives an estimation of the spin-splitting induced by the interface electric field (the Rashba effect).
Interestingly, two different contributions have been detected in the SdH effect; (i) major oscillation showing large oscillation amplitude which damps only at T>2K (Fig. 2 top); (ii) secondary oscillation appearing at large B which shows rapid damping as T is increased to 900mK (Fig. 2 bottom). By the temperature dependence of the main oscillation we find a cyclotron mass m*=0.71 (at n=7×1011cm-2), which is significantly small compared to cyclotron masses previously measured in bulk or 2D SrTiO3. The nature of secondary oscillation is currently unclear, but indicates existence of a separate heavy-electron band even at such a dilute carrier density.
Our oxide-FETs are already showing evidence of very unique filling of 2D quantum well states; light mass of m*=0.71 has never been observed via transport measurement in other SrTiO3 systems, where the value between m*=1–1.5 had been reported via the SdH measurement. Thus, the electronic structure of SrTiO3 surface electrons at ultralow carrier density explored by our gating technique could provide new insight into the quantum state of two-dimensional d-electron systems.