Polar Catastrophe Model Research Articles

Understanding interfacial conductance in non-stoichimetric Ca x Ta y O3−δ /SrTiO3

In search of novel conducting oxide heterointerfaces, we previously uncovered an distinctive quasi two-dimensional electron gas (q-2DEG) type behaviour in non-stoichimetric CaxTayO3-δ/SrTiO3 heterostructure. However, the underlying mechanism remained enigmatic. In this study, we delve into the intricate interplay of growth conditions, stoichiometry, and transport properties of these heterostructures. Using (Ca0.5TaO3)2 Target and the pulsed laser deposition technique, we grow the epitaxial thin films while systematically varying growth parameters, inculding laser energy density, oxygen pressures, and post-deposition annealing. Structural analysis unveiled a notable presence of oxygen vacancies in the as-grown films, while annealed samples exhibited an oxygen surplus. Building upon these findings, our comprehensive charge transport measurements revealed that while oxygen vacancies do contribute to conductivity, the polar catastrophe model takes precedence as the primary source of interfacial conductance in these heterostructures. This study provides valueable insights into the behavior of these innovative heterostructures, paving the way for future advancements in the field.

Suppression of conductivity by 1 uc buffer layer at LAO/STO interface

The conducting properties of oxide heterointerface can be significantly affected by buffer layers, gate voltage, capping layer, etc.; sometimes, a monolayer is enough to show a drastic change in these properties. Herein, we report that the insertion of one unit-cell (uc) of CaMnO3 material as a buffer layer at conducting LaAlO3/SrTiO3 interface turns it into an insulator. The presence of MnO2-layer in CaMnO3 uc at the interface blocks the charge transfer to SrTiO3 which makes the interface conducting as explained by the polar catastrophe model. The study may help to resolve the puzzle of the origin of conductivity at oxide heterostructure interfaces.

Magnetotransport in Graphene/Pb0.24Sn0.76Te Heterostructures: Finding a Way to Avoid Catastrophe.

While heterostructures are ubiquitous tools enabling new physics and device functionalities, the palette of available materials has never been richer. Combinations of two emerging material classes, two-dimensional materials and topological materials, are particularly promising because of the wide range of possible permutations that are easily accessible. Individually, both graphene and Pb1-xSnxTe (PST) are widely investigated for spintronic applications because graphene's high carrier mobility and PST's topologically protected surface states are attractive platforms for spin transport. Here, we combine monolayer graphene with PST and demonstrate a hybrid system with properties enhanced relative to the constituent parts. Using magnetotransport measurements, we find carrier mobilities up to 20 000 cm2/(V s) and a magnetoresistance approaching 100%, greater than either material prior to stacking. We also establish that there are two distinct transport channels and determine a lower bound on the spin relaxation time of 4.5 ps. The results can be explained using the polar catastrophe model, whereby a high mobility interface state results from a reconfiguration of charge due to a polar/nonpolar interface interaction. Our results suggest that proximity induced interface states with hybrid properties can be added to the still growing list of behaviors in these materials.

Tuning the magnetic anisotropy of La0.67Sr0.33MnO3 by CaTiO3 spacer layer on the platform of SrTiO3

The control of perpendicular magnetic anisotropy(PMA) of complex oxides on the widely used SrTiO3 (STO) or STO buffered silicon substrates is of technology importance in the development of integrated multifunctional oxide spintronic devices. Perovskite manganite shows great potential as magnetic layer due to its rich physical properites and spin polarized character. However, when it is directly grown on the STO or STO buffered silicon substrates, the magnetization vector lies in the in-plane direction of the film, hindering its applications such as integrated spin orbital torque devices. In this work, we manipulate the PMA of the La0.67Sr0.33MO3 (LSMO) film by regulating its lattice deformation via CaTiO3 (CTO) spacer layer on the platform of STO. High crystal quality of the heterostructures with atomic sharp interfaces and elongation of LSMO lattice along c-axis were characterized. The vibration sample magnetometer (VSM) measurement indicates that the magnetic anisotropy of LMSO can be effectively tuned from in-plane to out-of-plane by inserting CTO between STO and LSMO. The improvement of PMA in LSMO films is also dependent on the thickness of the CTO spacer layer. Furthermore, the M-T curves imply that there are different magnetic phases in the LSMO films, including the appearance of antiferromagnetic phase. The analysis by electron energy loss spectroscopy (EELS) in scanning transmission electron microscopy mode found that the valence state of Mn near the CTO/LSMO interface is lower than the LSMO bulk phase, which could explained by a polar catastrophe model. This work provides a promising avenue to tailoring PMA of perovskite manganite and paves the way to realize integrated oxide spintronic devices on STO and STO buffered silicon platform.

Quantum physical reality of polar-nonpolar oxide heterostructures

Conducting interfaces between polar and nonpolar insulating oxides, e.g., ${\mathrm{LaAlO}}_{3}/{\mathrm{SrTiO}}_{3}$, have generated interest for both fundamental physics and oxide-electronics applications. Current understanding is based on an amalgamation of a classical electrostatic model (polar catastrophe model) that was originally derived for semi-infinite solids and quantum density-functional-theory (DFT) results on ultrathin films. Here we report comprehensive DFT calculations that unveil a very different purely quantum physical reality. We show that, for ultrathin polar films, the interfacial dipole does not control the electrostatic potential in the polar film---the surface and interface play equal roles, and the absence or presence of centrosymmetry in the physical LAO film results in different, purely quantum mechanisms for the generation of a conducting interface, neither involving physical-charged transfer. Predictions are made that can be tested and can guide technology development.

LaScO3/SrTiO3: A conducting polar heterointerface of two 3d band insulating perovskites

This work reports a quasi-two-dimensional electron gas (q-2DEG) system at the interface of two wideband-gap insulators, (TiO2-terminated) SrTiO3 and LaScO3, with a minimum thickness of 4-unit cell (uc). The highly crystalline and abrupt heterointerface is confirmed with high-resolution electron microscopy. The mixed Ti4+ and Ti3+ valence states (for 4 uc of LaScO3) obtained from the x-ray photoelectron spectroscopy study suggest an intrinsic electronic reconstruction at the interface, leading to a metallic nature. This origin is well supported by density functional theory calculations that reveal an emergence of 3.3 states/eV/spin at the Fermi level for 4 uc in accordance with the polar catastrophe model. The study offers one more perovskite heterostructure, like LaAlO3/SrTiO3, for unraveling the q-2DEG phenomena toward a clear mechanism and futuristic applications.

Formation of two-dimensional small polarons at the conducting LaAlO3/SrTiO3 interface

A long-lasting puzzle of the conducting $\mathrm{LaAl}{\mathrm{O}}_{3}/\mathrm{SrTi}{\mathrm{O}}_{3}$ (LAO/STO) interface is that the excess electron density counted in the transport measurements is much less than 0.5 electrons per unit cell as predicted by the polar catastrophe model. In this study, via first-principles calculations, we show that the excess electrons at the LAO/STO interface favor the formation of small polarons. These electrons interact strongly with the interfacial Ti ions, forming localized midgap states, which make an insignificant contribution to the conductivity. We also find that the interaction between the neighboring spin-polarized small polarons is weak and does not lead to long-range magnetic ordering. Compared with bulk STO, the formation of the polarons is more favorable at the LAO/STO interface, which is ascribed to the reduced symmetry of the crystal field and the increased lattice distortion. Our results suggest that a large number of the excess electrons at the LAO/STO interface are localized in the form of small polarons, which can partially explain the unexpected fewer electrons in the transport measurements, and also shed light on understanding various properties of other complex perovskite oxide interfaces.

Coexistence of quasi-two dimensional electron and hole gas in a single tier Ca0.5TaO3/SrTiO3 oxide heterostructure

Quasi-two-dimensional electron gas has been realized at the polar-nonpolar interface of several insulating oxide heterostructures. However, its hole counterpart remains elusive. In an attempt to find a novel system that exhibits quasi-two-dimensional hole gas (q-2DHG) at the heterointerface, we adopt to materials search, first based on phenomenology followed by a comprehensive set of calculations based on first-principles density functional theory. Our studies show the epitaxial growth of cubic Ca0.5TaO3 on TiO2 terminated substrate display (q-2DHG). The hole gas emanates from the O 2p orbitals of the TiO2 layers of the substrate. On the other hand, an electron gas is formed at the (001) TaO2 top surface, thereby representing the heterostructure as a coupled quantum well system. The partial filling of the Ta 5dt2g conduction band indicates electron reconstruction, in agreement with the polar catastrophe model. Besides, a critical thickness of three monolayers is deduced from the calculations for the formation of q-2DHG in the Ca0.5TaO3/SrTiO3 heterostructure, which is consistent with the model prediction based on the modern theory of polarization. With both cubic systems, Ca0.5TaO3 and SrTiO3, having a similar underlying symmetry and minimal lattice mismatch, epitaxial growth with an abrupt interface can be well anticipated. Such a single-tier oxide heterostructure composed of separated confined hole-electron subsystems is expected to provide a platform to unravel exciting physics and also for functional devices related to oxide electronics.

Appearance and disappearance of ferromagnetism in ultrathin LaMnO3 on SrTiO3 substrate: A viewpoint from first principles

The intrinsic magnetic state (ferromagnetic or antiferromagnetic) of ultra-thin LaMnO$_3$ films on the mostly used SrTiO$_3$ substrate is a long-existing question under debate. Either strain effect or non-stoichiometry was argued to be responsible for the experimental ferromagnetism. In a recent experiment [Science \textbf{349}, 716 (2015)], one more mechanism, namely the self-doping due to polar discontinuity, was argued to be the driving force of ferromagnetism beyond the critical thickness. Here systematic first-principles calculations have been performed to check these mechanisms in ultra-thin LaMnO$_3$ films as well as superlattices. Starting from the very precise descriptions of both LaMnO$_3$ and SrTiO$_3$, it is found that the compressive strain is the dominant force for the appearance of ferromagnetism, while the open surface with oxygen vacancies leads to the suppression of ferromagnetism. Within LaMnO$_3$ layers, the charge reconstructions involve many competitive factors and certainly go beyond the intuitive polar catastrophe model established for LaAlO$_3$/SrTiO$_3$ heterostructures. Our study not only explains the long-term puzzle regarding the magnetism of ultra-thin LaMnO$_3$ films, but also shed light on how to overcome the notorious magnetic dead layer in ultra-thin manganites.

Universality of electron mobility in LaAlO3/SrTiO3 and bulk SrTiO3

Metallic LaAlO3/SrTiO3 (LAO/STO) interfaces attract enormous attention, but the relationship between the electron mobility and the sheet electron density, ns, is poorly understood. Here, we derive a simple expression for the three-dimensional electron density near the interface, n3D, as a function of ns and find that the mobility for LAO/STO-based interfaces depends on n3D in the same way as it does for bulk doped STO. It is known that undoped bulk STO is strongly compensated with N≃5×1018 cm−3 background donors and acceptors. In intentionally doped bulk STO with a concentration of electrons n3D<N, background impurities determine the electron scattering. Thus, when n3D<N, it is natural to see in LAO/STO the same mobility as in the bulk. On the other hand, in the bulk samples with n3D>N, the mobility collapses because scattering happens on n3D intentionally introduced donors. For LAO/STO, the polar catastrophe which provides electrons is not supposed to provide an equal number of random donors and thus the mobility should be larger. The fact that the mobility is still the same implies that for the LAO/STO, the polar catastrophe model should be revisited.

Local Enhancement of Polarization at PbTiO3/BiFeO3 Interfaces Mediated by Charge Transfer.

Ferroelectrics hold promise for sensors, transducers, and telecommunications. With the demand of electronic devices scaling down, they take the form of nanoscale films. However, the polarizations in ultrathin ferroelectric films are usually reduced dramatically due to the depolarization field caused by incomplete charge screening at interfaces, hampering the integrations of ferroelectrics into electric devices. Here, we design and fabricate a ferroelectric/multiferroic PbTiO3/BiFeO3 system, which exhibits discontinuities in both chemical valence and ferroelectric polarization across the interface. Aberration-corrected scanning transmission electron microscopic study reveals an 8% elongation of out-of-plane lattice spacing associated with 104%, 107%, and 39% increments of δTi, δO1, and δO2 in the PbTiO3 layer near the head-to-tail polarized interface, suggesting an over ∼70% enhancement of polarization compared with that of bulk PbTiO3. Besides that in PbTiO3, polarization in the BiFeO3 is also remarkably enhanced. Electron energy loss spectrum and X-ray photoelectron spectroscopy investigations demonstrate the oxygen vacancy accumulation as well as the transfer of Fe3+ to Fe2+ at the interface. On the basis of the polar catastrophe model, FeO2/PbO interface is determined. First-principles calculation manifests that the oxygen vacancy at the interface plays a predominate role in inducing the local polarization enhancement. We propose a charge transfer mechanism that leads to the remarkable polarization increment at the PbTiO3/BiFeO3 interface. This study may facilitate the development of nanoscale ferroelectric devices by tailing the coupling of charge and lattice in oxide heteroepitaxy.

Charge fractionalization in oxide heterostructures: A field-theoretical model

LaAlO3/SrTiO3 heterostructure with polar and non-polar constituents has been shown to exhibit interface metallic conductivity due to fractional charge transfer to the interface. The interface reconstruction by electron redistribution along the (001) orientation, in which half of an electron is transferred per two-dimensional unit cell to the adjacent planes, resulting in a net transfer of half of the charge to both the interface and topmost atomic planes, has been ascribed to a polar discontinuity at the interface in the polar catastrophe model. This avoids the divergence of the electrostatic potential, as the number of layers are increased, producing an oscillatory electric field and finite potential. Akin to the description of charge fractionalization in quasi–one-dimensional polyacetylene by the field-theoretic Jackiw-Rebbi model with fermions interacting with a topologically non-trivial background field, we show an analogous connection between the polar catastrophe model and the Bell-Rajaraman model, where the charge fractionalization occurs in the soliton free sector as an end effect.

Two-dimensional electron gas at thePbTiO3/SrTiO3interface: Anab initiostudy

In the polar catastrophe scenario, polar discontinuity accounts for the driving force of the formation of a two-dimensional electron gas (2DEG) at the interface between polar and non-polar insulators. In this paper, we substitute the usual, non-ferroelectric, polar material with a ferroelectric thin film and use the ferroelectric polarization as the source for polar discontinuity. We use ab initio simulations to systematically investigate the stability, formation and properties of the two-dimensional free-carrier gases formed in PbTiO$_3$/SrTiO$_3$ heterostructures under realistic mechanical and electrical boundary conditions. Above a critical thickness, the ferroelectric layers can be stabilized in the out-of-plane monodomain configuration due to the electrostatic screening provided by the free-carriers. Our simulations also predict that the system can be switched between three stable configurations (polarization up, down or zero), allowing the non-volatile manipulation of the free charge density and sign at the interface. Furthermore, the link between ferroelectric polarization and free charge density demonstrated by our analysis constitutes compelling support for the polar catastrophe model that is used to rationalize the formation of 2DEG at oxide interfaces.

Metal-to-insulator transition inLaAl1−xCrxO3/SrTiO3oxide heterostructures guided by electronic reconstruction

Despite continuous efforts, a conceptual perception on the origin of quasi-two-dimensional electron gas (q-2DEG) in oxide heterostructures remains far from any generalization. Along this perspective, a systematic study identifying systems that exhibit heterointerface conductivity (${\mathrm{LaAlO}}_{3}{/\mathrm{SrTiO}}_{3}$) and that do not (${\mathrm{LaCrO}}_{3}{/\mathrm{SrTiO}}_{3}$) is undertaken. High quality ${\mathrm{LaAl}}_{1\ensuremath{-}x}{\mathrm{Cr}}_{x}{\mathrm{O}}_{3}$ ($0\ensuremath{\le}x\ensuremath{\le}1$) films were fabricated on ${\mathrm{TiO}}_{2}$ terminated ${\mathrm{SrTiO}}_{3}$ (001) substrates using a pulsed laser deposition technique and its growth was monitored by reflection high energy electron diffraction (RHEED). The transport and photoemission spectroscopy experiments reveal that the the origin of heterointerface q-2DEG is associated with ${\mathrm{Ti}}^{3+}$ states and that it ceases to exist with increasing Cr ion concentration in the overlayer films. Following experimental evidences of the Cr concentration dependent metal-insulator electronic phase transition at the heterointerface, explanation is sought within the realms of a polar catastrophe model. Based on our transport and spectroscopy measurements, we propose that an electronic reconstruction drives the formation of q-2DEG for pristine LAO/STO and Al rich samples due to the formation of ${\mathrm{Ti}}^{3+}{/\mathrm{Ti}}^{4+}$ mixed valent state, while for heavily doped Cr samples, the required electron count necessary to solve the polar catastrophe instability are trapped in the ${\mathrm{LaAl}}_{1\ensuremath{-}x}{\mathrm{Cr}}_{x}{\mathrm{O}}_{3}$ overlayers in the Cr $3d$ states. These trapped electrons in the overlayers are manifested in the form of ${\mathrm{Cr}}^{2+}$ ions.

Tuning the carrier density of LaAlO3/SrTiO3 interfaces by capping La1−xSrxMnO3

We present a systematical study on the electronic transport properties of the insulating LaAlO3 (3 unit cells)/SrTiO3 interfaces capping with thin layers of La1−xSrxMnO3, whose formal polarization is continually tuned by Sr doping. When the Sr doping is lower than 2/3, the LaAlO3/SrTiO3 interfaces show metallic behaviors. The carrier mobility is almost independent on the Sr doping for metallic interface, indicating that the capping layer does not change the density of the oxygen vacancies and the interface intermixing. However, the sheet carrier densities monotonically decrease as increasing Sr doping, which is ascribed to the decrease of the La1−xSrxMnO3 formal polarization. These results strongly support the intrinsic mechanism of the polar catastrophe model and provide a new approach to tailor the interface states of complex oxide heterostructures.

Tunable conductivity threshold at polar oxide interfaces

The physical mechanisms responsible for the formation of a two-dimensional electron gas at the interface between insulating SrTiO(3) and LaAlO(3) have remained a contentious subject since its discovery in 2004. Opinion is divided between an intrinsic mechanism involving the build-up of an internal electric potential due to the polar discontinuity at the interface between SrTiO(3) and LaAlO(3), and extrinsic mechanisms attributed to structural imperfections. Here we show that interface conductivity is also exhibited when the LaAlO(3) layer is diluted with SrTiO(3), and that the threshold thickness required to show conductivity scales inversely with the fraction of LaAlO(3) in this solid solution, and thereby also with the layer's formal polarization. These results can be best described in terms of the intrinsic polar-catastrophe model, hence providing the most compelling evidence, to date, in favour of this mechanism.

Electronic charges and electric potential at LaAlO3/SrTiO3interfaces studied by core-level photoemission spectroscopy

We studied LaAlO${}_{3}$/SrTiO${}_{3}$ interfaces for varying LaAlO${}_{3}$ thickness by core-level photoemission spectroscopy. In Ti $2p$ spectra for conducting ``$n$-type'' interfaces, Ti${}^{3+}$ signals appeared, which were absent for insulating ``$p$-type'' interfaces. The intensity of the Ti${}^{3+}$ signals was significantly higher than that expected for the transport carrier densities, indicating that part of the carriers at the interfaces are localized. The Ti${}^{3+}$ signals increased with LaAlO${}_{3}$ thickness, but started well below the critical thickness of 4 unit cells for metallic transport. Core-level shifts with LaAlO${}_{3}$ thickness were much smaller than predicted by the polar catastrophe model. We attribute these observations to surface defects and/or adsorbates providing charges to the interface even below the critical thickness.