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Register a new userCarrier density independent scattering rate in SrTiO3-based electron liquids
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We examine the carrier density dependence of the scattering rate in two- and three-dimensional electron liquids in SrTiO3 in the regime where it scales with T^n (T is the temperature and n <= 2) in the cases when it is varied by electrostatic control and chemical doping, respectively. It is shown that the scattering rate is independent of the carrier density. This is contrary to the expectations from Landau Fermi liquid theory, where the scattering rate scales inversely with the Fermi energy (E_F). We discuss that the behavior is very similar to systems traditionally identified as non-Fermi liquids (n < 2). This includes the cuprates and other transition metal oxide perovskites, where strikingly similar density-independent scattering rates have been observed. The results indicate that the applicability of Fermi liquid theory should be questioned for a much broader range of correlated materials and point to the need for a unified theory.
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We investigate correlation physics in high-density, two-dimensional electron liquids that reside in narrow SrTiO3 quantum wells. The quantum wells are remotely doped via an interfacial polar discontinuity and the three-dimensional (3D) carrier density is modulated by changing the width of the quantum well. It is shown that even at 3D densities well below one electron per site, short-range Coulomb interactions become apparent in transport, and an insulating state emerges at a critical density. We also discuss the role of disorder in the insulating state.
We discuss the Seebeck coefficient and the Hall mobility of electrons confined in narrow SrTiO3 quantum wells as a function of the three-dimensional carrier density and temperature. The quantum wells contain a fixed sheet carrier density of ~ 7x10^14 cm^-2 and their thickness is varied. At high temperatures, both properties exhibit apparent Fermi liquid behavior. In particular, the Seebeck coefficient increases nearly linearly with temperature (T) when phonon drag contributions are minimized, while the mobility decreases proportional to T^2. Furthermore, the Seebeck coefficient scales inversely with the Fermi energy (decreasing quantum well thickness). In contrast, the transport scattering rate is independent of the Fermi energy, which is inconsistent with a Fermi liquid. At low temperatures, the Seebeck coefficient deviates from the linear temperature dependence for those electron liquids that exhibit a correlation-induced pseudogap, indicating a change in the energy dependence of the scattering rate. The implications for describing transport in strongly correlated materials are discussed.
Two-dimensional electron systems found at the interface of SrTiO3-based oxide heterostructures often display anisotropic electric transport whose origin is currently under debate. To characterize transport along specific crystallographic directions, we developed a hard-mask patterning routine based on an amorphous CeO2 template layer. The technique allows preparing well-defined microbridges by conventional ultraviolet photolithography which, in comparison to standard techniques such as ion- or wet-chemical etching, does not induce any degradation of interfacial conductance. The patterning scheme is described in details and the successful production of microbridges based on amorphous Al2O3-SrTiO3 heterostructures is demonstrated. Significant anisotropic transport is observed for T < 30 K which is mainly related to impurity/defect scattering of charge carriers in these heterostructures.
Carrier density and disorder are two crucial parameters that control the properties of correlated two-dimensional electron systems. In order to disentangle their individual contributions to quantum phenomena, independent tuning of these two parameters is required. Here, by utilizing a hybrid liquid/solid electric dual-gate geometry acting on the conducting LaAlO3/SrTiO3 heterointerface, we obtain an additional degree of freedom to strongly modify the electron confinement profile and thus the strength of interfacial scattering, independent from the carrier density. A dual-gate controlled nonlinear Hall effect is a direct manifestation of this profile, which can be quantitatively understood by a Poisson-Schrodinger subband model. In particular, the large nonlinear dielectric response of SrTiO3 enables a very wide range of tunable density and disorder, far beyond that for conventional semiconductors. Our study provides a broad framework for understanding various reported phenomena at the LaAlO3/SrTiO3 interface.
Magnetotransport and superconducting properties are investigated for uniformly La-doped SrTiO3 films and GdTiO3/SrTiO3 heterostructures, respectively. GdTiO3/SrTiO3 interfaces exhibit a high-density two-dimensional electron gas on the SrTiO3-side of the interface, while for the SrTiO3 films carriers are provided by the dopant atoms. Both types of samples exhibit ferromagnetism at low temperatures, as evidenced by a hysteresis in the magnetoresistance. For the uniformly doped SrTiO3 films, the Curie temperature is found to increase with doping and to coexist with superconductivity for carrier concentrations on the high-density side of the superconducting dome. The Curie temperature of the GdTiO3/SrTiO3 heterostructures scales with the thickness of the SrTiO3 quantum well. The results are used to construct a stability diagram for the ferromagnetic and superconducting phases of SrTiO3.
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