Novel Metal-Insulator Transition at the SmTiO 3 /SrTiO 3 Interface

PRL 118, 236803 (2017) week ending 9 JUNE 2017 PHYSICAL REVIEW LETTERS Novel Metal-Insulator Transition at the SmTiO 3 =SrTiO 3 Interface Kaveh Ahadi and Susanne Stemmer * Materials Department, University of California, Santa Barbara, California 93106-5050, USA (Received 2 March 2017; published 9 June 2017) We report on a metal-insulator transition (MIT) that is observed in an electron system at the SmTiO 3 =SrTiO 3 interface. This MIT is characterized by an abrupt transition at a critical temperature, below which the resistance changes by more than an order of magnitude. The temperature of the transition systematically depends on the carrier density, which is tuned from ∼1 × 10 14 to 3 × 10 14 cm −2 by changing the SmTiO 3 thickness. An analysis of the transport properties shows non-Fermi-liquid behavior and mass enhancement as the carrier density is lowered. We compare the MIT characteristics with those of known MITs in other material systems and show that they are distinctly different in several aspects. We tentatively conclude that both long-range Coulomb interactions and the fixed charge at the polar interface are likely to play a role in this MIT. The strong dependence on the carrier density makes this MIT of interest for field- tunable devices. DOI: 10.1103/PhysRevLett.118.236803 Metal-to-insulator transitions (MITs) that are caused by strong electron correlations are the source of some of the most interesting phenomena in condensed matter physics [1–4]. They can occur either at very high carrier densities, where on-site Coulomb interactions are strong (“strongly correlated systems”), or at ultralow densities, where long- range Coulomb interactions remain unscreened. The cor- responding insulating states are the Mott insulator and the Wigner crystal, respectively. Real materials are often significantly removed from either one of these ideal states and contain additional, complex interactions. Furthermore, disorder, which even without correlations can cause locali- zation (“Anderson insulator”) [5], can play a central role [6,7]. Transition metal oxides with metallic densities (∼one electron/primitive unit cell) have been central to the study of Mott insulators [2], whereas low-dimensional, low- density (sheet densities on the order of 10 11 cm −2 and below) semiconductors are employed in the search for a Wigner crystal in the solid state [8]. Electron systems at oxide interfaces offer access to a parameter space that is not easily available in the systems mentioned above. Unlike doped or alloyed bulk transition metal oxides, electrostatic doping (for example, by a polar discontinuity [9–11]) avoids dopant atoms, a source of disorder. Unlike two-dimensional electron gases in semi- conductors, electron masses are high, which promotes localization. Spin-orbit coupling via Rashba fields can also be strong, which may enhance interactions [12]. There are, however, significant challenges. In particular, low carrier mobilities result in high sheet resistances. If these exceed the Mott-Ioffe-Regel limit or quantum resistance (h=e 2 ∼ 25 kΩ=□), systems are insulating at most temperatures. This insulating state preempts corre- lation-induced MITs or other phenomena that may emerge out of an itinerant correlated electron system. MITs near the Mott-Ioffe-Regel limit have been observed in electron systems at LaAlO 3 =SrTiO 3 [13] and SmTiO 3 =SrTiO 3 interfaces [14]. Here, we study SmTiO 3 =SrTiO 3 interfaces that have sheet resistances well below the quantum limit. The polar discontinuity at such interfaces corresponds to a fixed positive charge of 3 × 10 14 cm −2 at the interface (Fig. 1), which can be compensated by free (mobile) carriers in the SrTiO 3 [15]. Mobile carrier densities of 3 × 10 14 cm −2 per interface are observed for a wide range of sample geom- etries [11,16]. When SmTiO 3 layers are reduced to a few unit cells (u.c.), the sheet carrier densities are reduced below this value and depend systematically on the SmTiO 3 thickness [14]. Given carrier densities that are not close to either one of the two limiting cases discussed above, conventional wisdom would suggest that a correlation- driven MIT is unlikely to occur. Contrary to this expect- ation, we show that an abrupt MIT occurs in this interfacial electron system as the temperature is lowered. A transition temperature that scales systematically with the carrier density and other characteristics point to the importance of electron correlations. Epitaxial SrTiO 3 layers (thicknesses of 20, 60, and 80 nm) were grown using molecular beam epitaxy on (001) ðLa 0.3 Sr 0.7 ÞðAl 0.65 Ta 0.35 ÞO 3 single crystals. Following SrTiO 3 growth, epitaxial SmTiO 3 layers with thicknesses of 3, 5, 7, and 20 u.c., respectively, were deposited in situ (all thicknesses refer to the pseudocubic u.c. parameter of SmTiO 3 ∼3.91 A). Details of the growth have been reported elsewhere [19,20]. Electron beam evaporation through a shadow mask was used to deposit Au=Ti (400=40 nm) contacts for Hall and sheet resistance (R s ) measurements using square van der Pauw structures. Temperature- (T-) dependent electrical measurements were carried out using a Quantum Design physical property measurement system. © 2017 American Physical Society