Session L55: Metal-Insulator Transitions and Doping, 2DEGs, and Electric-Field Effects at Oxide Interfaces

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Metal-insulator transition compounds, namely materials that depending on the environment present either metallic or insulating characteristics - and phase transition correlated materials, more generally - are of wide scientific and technological interest, yet many open questions still remain in our understanding: is the transition from a metallic to an insulating state driven by the electronic or the lattice degrees of freedom? What sets the relevant length scales of the transition? What are the relevant features we can use to design new MIT materials? In this talk, I will provide a theoretical and experimental discussion of new results that have helped us address this question. I will show that the transition in the rare-earth nickelates RNiO3 with R a rare-earth, and Ca2RuO4 materials and their thin films and heterostructures is driven by the electron-lattice coupling, rather than the electronic energy. We will use a simple model to understand the way this is affected by dimensionality and strain[1,2]. Then, I will discuss how a heterostructure can be used as a 1D model system to understand the length scales involved in the transition [3]. Finally, I will show how machine learning tools can be used to guide new understanding of this class of systems, and introduce our materials library, machine learning model and new relevant physical features identified using this model [4].

[1] ‘Disentangling lattice and electronic contributions to the metal–insulator transition from bulk vs. layer confined RNiO3’, PNAS, 2019, 116 (29)
[2] 'The Electron-Lattice Coupling Driving the Metal-Insulator Transition: Energy Landscapes of of Ca2RuO4, RNiO3 and their Heterostructures', In Preparation, A.B. Georgescu and A.J. Millis
[3] ‘Length scales of interfacial coupling between metal and insulator phases in oxides’, Nature Materials, Vol,19, 2020
[4] 'A Database and Machine Learning Model to Identify Thermally Driven Metal-Insulator Transition Compounds', https://arxiv.org/abs/2010.13306   

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Understanding the mechanisms that control the metal to insulator transition (MIT) in correlated electron systems is one of the major challenges in condensed matter physics. Moreover, remarkably little is known about the characteristic lengths scale over which a metallic or insulating region can be established and the physics that sets this length scale. In this work, we use experimental and theoretical methods to design and study superlattices of two distinct rare earth nickelate oxides SmNiO₃ and NdNiO₃ that in bulk form show a MIT at two very different temperatures (400 K and 200 K, respectively). We find that, depending on the superlattice periodicity, these new complex oxide superlattices display different MIT behavior than that of the "bulk" materials. We show that the length scale of the metal-insulator transition in the superlattices is set not by the length scale of the propagation of structural motifs across the two materials, which ab-initio calculations and STEM analysis suggest is minimal, but rather by the balance between the energy cost of the boundary between a metal and an insulator and the energy gain of the bulk phases (Domínguez, C., Georgescu, A.B., Mundet, B. et al. Nat. Mater. 19, 1182–1187 (2020)).   

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Intrinsic polar metals are rare, especially in oxides, because free electrons screen electric fields in a metal and eliminate the internal dipoles that are needed to break inversion symmetry. The discovery of LiOsO₃ [Nat. Mater., 12, 1024 (2013)], a metal that transforms from a centrosymmetric R-3c structure to a polar R3c structure at 140 K, has stimulated an active search for new polar metals. In our recent study [Commun. Mater., 1, 1 (2020)], we use first-principles high-throughput calculations to predict a new polar metal BiPbTi₂O₆ and demonstrate 180^o electric-field switching of its polar displacements in its thin film form. Utilizing lone-pair electrons and different valences of Bi and Pb, we find that ordered BiPbTi₂O₆ can crystallize in three polar and conducting structures, each of which can be transformed to another via pressure or strain. In a heterostructure of layered BiPbTi₂O₆ and PbTiO₃, a strong interfacial coupling enables electric fields to first switch PbTiO₃ polarization and subsequently drive a 180^o change of BiPbTi₂O₆ polar displacements. Our work demonstrates the power of high-throughput screening in designing new functional materials and in particular predicts a new electrically switchable polar metal.   

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Electronic correlation effects generate many fascinating properties in 3d transition metal oxides which hold great promise for future novel electronic functionalities. The Mott insulator to metal transitions as a function of band width or band filling may e.g. be harnessed in a Mott transistor. To utilize the metal-insulator transitions in devices, the understanding and control of these effects has yet to be improved.
It has been demonstrated that thin LaTiO₃ films are a promising channel material in future Mottronic devices [1]. Here we study the electronic structure across the band-filling induced phase transition in LaTiO₃ using angle-resolved photoemission spectroscopy as well as complementary transport experiments. The LaTiO₃ films are tuned by excess oxygen doping across the line of the band-filling controlled Mott transition, which allows us to derive the correlation between the band filling and the band mass.

[1] P. Scheiderer, M.Schmitt, J. Gabel, M. Zapf, M. Stübinger, P.Schütz, L. Dudy, C. Schlueter, T.-L. Lee, M.Sing, and R. Claessen, Adv. Mater. 30, 1706708 (2018)   

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We have studied ferroelectric tunnel junctions with La_0.7Sr_0.3MnO₃ (LSMO) bottom electrode, a ferroelectric (FE) BaTiO₃ barrier and La₅SrCu₆O₁₅ (LSCO) top electrode. Here, migration of oxygen atoms is triggered by the large strain modulations building up at the junction interfaces combined with the strong electric field produced at moderate applied voltages. We show that this mechanism can stabilize a superconducting phase at the interface between a ferroelectric and a non-superconducting cuprate. As probed by tunneling conductance, such phase is characterized by a 22 meV superconducting gap and a critical temperature T_C as high as 70 K, as well as by spectral features specific of d-wave superconductors such as YBa₂Cu₃O₇. Furthermore, we show that ferroelectric switching modulates the superconducting gap, which implies the that coupling between ferroelectric polarization and the ionization of oxygen vacancies controls the interfacial phase doping. This ferroionic route appears as promising new strategy to explore phase diagrams at doping levels far beyond the ones reached with field effect or ionic liquid doping.   

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Domain walls that form between tetragonal domains in SrTiO₃ (STO) at low temperatures have been shown to host a number of interesting phenomenon including polar order¹, magnetism², and preferred conductivity³. They also contribute to unexpected transport behaviors in the two-dimensional carrier gas (2DCG) formed at the interface of STO surfaces and interfaces. Here we discuss our recent findings of a finite transverse resistance in zero field, in the 2DCG between amorphous aluminum oxide (AlO_x) and (001) and (111) oriented STO substrates. The effect first arises at temperatures ~70 K but is most prominent around ~40 K, the temperature at which the domain walls become increasingly polar. We argue that the preferential transport of current along polar domain walls can explain the finite transverse resistance. Surprisingly, the effect is stronger in (111) oriented devices than in (001) oriented devices. This difference is correlated with a difference in the dielectric constant between the (001) and (111) oriented substrates at low temperatures.

[1] Scott et al., PRL 109, 187601 (2012)
[2] Christensen et al., Nat. Phys. 15, 269 (2019).
[3] Kalisky et al., Nat. Mat. 12, 1091 (2013).   

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Dislocations often occur in thin films with large misfit strain as a result of strain energy accumulation and can drastically change the physical properties. Here the structure and dislocations in heterostructures with large misfit strain are investigated on atomic scale. When grown on SrTiO₃ (001), the dislocations in both the monolithic BaTiO₃ thin film and its superlattices with SrIrO₃ appear above a critical thickness around 6 nm. The edge component of the dislocations is seen in both cases with the Burgers vector of a<100>. However compared to monolithic BaTiO3 the dislocation density is slightly lower in BaTiO₃/SrIrO₃ superlattices. In this superlattice, when considering the SrTiO₃ lattice constant as the reference, BaTiO₃ has a larger misfit strain comparing with SrIrO₃. It is found that in these two cases, the dislocation formation is only affected by the critical thickness of the film with a larger lattice misfit. It is interesting that a strong octahedral tilt/rotation mismatch at BaTiO₃/SrIrO₃ interface, does not contribute to the creation of dislocations. Our findings show that it is possible to control the position of dislocations, an important step towards defect engineering.   

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A point that is still unclear relating to the two-dimensional electron gas (2DEG) observed at the interfaces between a polar perovskite thin film and a non-polar perovskite substrate, is the character of the insulator to metal transition defining its onset. It is relevant, since, in addition to film thickness, the transition can in principle be induced by an electric across the film for a thickness close to critical, and its being continuous or discontinuous can have important implications. Both kinds of switching have been proposed. Although octahedral tilts have been discussed in systems like LAO/STO, their effect on the 2DEG onset has not been discussed. We propose a theory of 2DEG formation at a polar-nonpolar interface which includes coupling to tilts. We find that they change the character of the transition, in addition to altering the critical value of thickness or electric field. Five distinct transition scenarios are obtained as possible, depending only on the energetics of the tilts, their coupling with the polar mode, and the polar discontinuity, and irrespective of whether the transition occurs via thickness or electric field. We will present first-principles results constraining the picture arising.   

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As a prototypical Mott insulator with ferromagnetic ordering, YTiO₃ (YTO) is of great interest in the study of strong electron correlation effects and orbital ordering. Here we report the first molecular beam epitaxy (MBE) growth of YTO films, combined with theoretical and experimental characterization of the electronic structure and charge transport properties. The obstacles of YTO MBE growth are discussed and potential routes to overcome them are proposed. DC transport and Seebeck measurements on thin films and bulk single crystals identify p-type transport with thermally activated transport behavior, with an activation energy of ~ 0.17 eV in thin films, consistent with the energy barrier for small hole polaron migration from hybrid density functional theory (DFT) calculations. Hard X-ray photoelectron spectroscopy measurements (HAXPES) show the lower Hubbard band (LHB) at 1.1 eV below the Fermi level, whereas a Mott-Hubbard bandgap of ~1.5 eV is determined from photoluminescence (PL) measurements. These findings provide critical insight into the electronic band structure of YTO and related materials.   

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In the two-dimensional conducting gasses formed at the interfaces of SrTiO₃ (STO) based devices, an electric field applied via a back gate voltage (Vg) is known to significantly modulate the density of carriers, the strength of spin-orbit interactions, and disorder of the system. If Vg is applied through the STO substrate, however, gating effects are modulated by the unusual dielectric properties of the STO. Here we report temperature and gate voltage dependent transport measurements on AlO_x/STO interface devices. We find that initially increasing Vg at low temperatures results in an increase in the magnitude of the Hall coefficient RH, but subsequently decreasing Vg reduces RH. This is completely counter to what one would expect if the majority carriers in the interface gas were electrons, as is the case with this system. Additionally, there is a significant deviation of low temperature behavior upon reducing gate voltage after ramping. Measurements on (001) oriented STO devices show some agreement to the predictions of Raslan et al¹, but measurements on (111) oriented devices show completely unexpected behavior.

[1] Raslan et al., Phys. Rev. B 95, 054106 (2017)   

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A polar conductor with Rashba-type spin-orbit coupling is a potential material platform for exotic quantum transport and spintronic functionalities [1,2]. One of their inherent properties is the nonreciprocal transport, where the magnetoresistance becomes inequivalent between the rightward and leftward current directions, due to breaking of both spatial inversion and time reversal symmetries. Such a rectification effect reflecting polar symmetry has been studied at the interface or bulk polar semiconductor [3,4]. Here we have report nonreciprocal transport in polar superconductivity achieved in gated SrTiO₃. We found the gigantic enhancement in the nonlinear resistance in the amplitude and phase fluctuation regions [5]. Also, we discuss possible origins of nonreciprocity in the 2D Rashba superconductor, such as paraconductivity with a parity mixing in the Cooper pairs and rectified vortex motions.
[1] E. Lesne et al., Nat. Mater. 15, 1261–1266 (2016).
[2] R. Ohshima et al., Nat. Mater. 16, 609–614 (2017).
[3] P. He et al., Phys. Rev. Lett. 120, 266802 (2018).
[4] T. Ideue et al., Nat. Phys. 13, 578–584 (2017).
[5] Y. M. Itahashi et al., Sci. Adv. 6, eaay9120 (2020).