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Register a new userMagnetothermopower of $delta$-doped LaTiO$_3$/SrTiO$_3$ interfaces in the Kondo regime
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Measurements of magneto-thermopower (S(H, T)) of interfacial delta doped LaTiO$_3$/SrTiO$_3$ (LTO/STO) heterostructure by an iso-structural antiferromagnetic perovskite LaCrO$_3$ are reported. The thermoelectric power of the pure LTO/STO interface at 300 K is $approx$ 118 $mu$V/K, but increases dramatically on $delta$-doping. The observed linear temperature dependence of S(T) over the temperature range 100 K to 300 K is in agreement with the theory of diffusion thermopower of a two-dimensional electron gas. The S(T) displays a distinct enhancement in the temperature range (T $<$ 100 K) where the sheet resistance shows a Kondo-type minimum. We attributed this maximum in S(T) to Kondo scattering of conduction electron by localized impurity spins at the interface. The suppression of S by a magnetic field, and the isotropic nature of the suppression in out-of-plane and in-plane field geometries further strengthen the Kondo model based interpretation of S(H, T).
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In pursuit of creating cuprate-like electronic and orbital structures, artificial heterostructures based on LaNiO$_3$ have inspired a wealth of exciting experimental and theoretical results. However, to date there is a very limited experimental understanding of the electronic and orbital states emerging after interfacial charge-transfer and their connections to the modified band structure at the interface. Towards this goal, we have synthesized a prototypical superlattice composed of correlated metal LaNiO$_3$ and doped Mott insulator LaTiO$_{3+delta}$, and investigated its electronic structure by resonant X-ray absorption spectroscopy combined with X-ray photoemission spectroscopy, electrical transport and theory calculations. The heterostructure exhibits interfacial charge-transfer from Ti to Ni sites giving rise to an insulating ground state with orbital polarization and $e_textrm{g}$ orbital band splitting. Our findings demonstrate how the control over charge at the interface can be effectively used to create exotic electronic, orbital and spin states.
We study a model SrTiO$_3$ interface in which conduction $t_{2g}$ electrons couple to the ferroelectric (FE) phonon mode. We treat the FE mode within a self-consistent phonon theory that captures its quantum critical behavior, and show that proximity to the quantum critical point leads to universal tails in the electron density of the form $n(z) sim (lambda+z)^{-2}$, where $lambda sim T^{2-d/mathfrak{z}}$, with $d=3$ the dimensionality and $mathfrak{z}=1$ the dynamical critical exponent. Implications for the metal-insulator transition at low electron density are discussed.
We report the effect of oxygen pressure during growth ($P_{O_{2}}$) on the electronic and magnetic properties of PrAlO$_3$ films grown on $rm TiO_{2}$-terminated SrTiO$_3$ substrates. Resistivity measurements show an increase in the sheet resistance as $P_{O_{2}}$ is increased. The temperature dependence of the sheet resistance at low temperatures is consistent with Kondo theory for $P_{O_{2}} ge 10^{-5}$ torr. Hall effect data exhibit a complex temperature dependence that suggests a compensated carrier density. We observe behavior consistent with two different types of carriers at interfaces grown at $P_{O_{2}} ge 10^{-4}$ torr. For these interfaces, we measured a moderate positive magnetoresistance (MR) due to a strong spin-orbit (SO) interaction at low magnetic fields that evolves into a larger negative MR at high fields. Positive high MR values are associated with samples where a fraction of carriers are derived from oxygen vacancies. Analysis of the MR data permitted the extraction of the SO interaction critical field ( e.g. $ H_{SO}=$1.25 T for $P_{O_{2}}=10^{-5}$ torr). The weak anti-localization effect due to a strong SO interaction becomes smaller for higher $P_{O_{2}}$ grown samples, where MR values are dominated by the Kondo effect, particularly at high magnetic fields.
Almost all oxide two-dimensional electron gases are formed in SrTiO$_3$-based heterostructures and the study of non-SrTiO$_3$ systems is extremely rare. Here, we report the realization of a two-dimensional electron gas in a CaTiO$_3$-based heterostructure, CaTiO$_3$/LaTiO$_3$, grown epitaxially layer-by-layer on a NdGaO$_3$ (110) substrate via pulsed laser deposition. The high quality of the crystal and electronic structures are characterized by in-situ reflection high-energy electron diffraction, X-ray diffraction, and X-ray photoemission spectroscopy. Measurement of electrical transport validates the formation of a two-dimensional electron gas in the CaTiO$_3$/LaTiO$_3$ superlattice. It is revealed the room-temperature carrier mobility in CaTiO$_3$/LaTiO$_3$ is nearly 3 times higher than in CaTiO$_3$/YTiO$_3$, demonstrating the effect of TiO$_6$ octahedral tilts and rotations on carrier mobility of two-dimensional electron gases. Due to doped CaTiO$_3$ being an A-site polar metal, our results provide a new route to design novel A-site two-dimensional polar metals.
Multiple experiments have observed a sharp transition in the band structure of LaAlO$_3$/SrTiO$_3$ (001) interfaces as a function of applied gate voltage. This Lifshitz transition, between a single occupied band at low electron density and multiple occupied bands at high density, is remarkable for its abruptness. In this work, we propose a mechanism by which such a transition might happen. We show via numerical modeling that the simultaneous coupling of the dielectric polarization to the interfacial strain (electrostrictive coupling) and strain gradient (flexoelectric coupling) generates a thin polarized layer whose direction reverses at a critical density. The Lifshitz transition occurs concomitantly with the polarization reversal and is first-order at $T=0$. A secondary Lifshitz transition, in which electrons spread out into semiclassical tails, occurs at a higher density.
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