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Register a new userLattice polarization effects on electron-gas charge densities in ionic superlattices
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The atomic-level control achievable in artificially-structured oxide superlattices provides a unique opportunity to explore interface phases of matter including high-density 2D electron gases. Electronic-structure calculations show that the charge distribution of the 2D gas is strongly modulated by electron-phonon interactions with significant ionic polarization. Anharmonic finite-temperature effects must be included to reproduce experiment. Density functional perturbation theory is used to parameterize a simple model introduced to represent these effects and predict temperature dependencies.
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In this study, we carry out density functional theory calculations to elucidate the polarization switching mechanism in charge-order-induced ferroelectrics. Based on the investigations about (SrVO$_3$)$_1$(LaVO$_3$)$_1$ superlattice, we demonstrate that the charge ordering state couples strongly to lattice modes, and charge-transfer induced polarization switching has to be associated with changes of lattice distortions. Based on the type of lattice mode strongly coupled to charge ordering states, we classify the charge ordering materials in two type, namely polyhedral breathing and off-centering displacement types. We further demonstrate that only in off-centering displacement type charger ordering material, the polarization is switchable under an external field. The implications of this theory to experimental observations are also discussed and we successfully explain the different electrical behaviors in LuFe$_2$O$_4$ and Fe$_3$O$_4$. This study aims to provide guidance for searching and designing charge ordering ferroelectrics with switchable polarization.
Electronic polarization and charge transfer effects play a crucial role in thermodynamic, structural and transport properties of room-temperature ionic liquids (RTILs). These non-additive interactions constitute a useful tool for tuning physical chemical behavior of RTILs. Polarization and charge transfer generally decay as temperature increases, although their presence should be expected over an entire condensed state temperature range. For the first time, we use three popular pyridinium-based RTILs to investigate temperature dependence of electronic polarization in RTILs. Atom-centered density matrix propagation molecular dynamics, supplemented by a weak coupling to an external bath, is used to simulate the temperature impact on system properties. We show that, quite surprisingly, non-additivity in the cation-anion interactions changes negligibly between 300 and 900 K, while the average dipole moment increases due to thermal fluctuations of geometries. Our results contribute to the fundamental understanding of electronic effects in the condensed phase of ionic systems and foster progress in physical chemistry and engineering.
Charge transfer in superlattices consisting of SrIrO$_3$ and SrMnO$_3$ is investigated using density functional theory. Despite the nearly identical work function and non-polar interfaces between SrIrO$_3$ and SrMnO$_3$, rather large charge transfer was experimentally reported at the interface between them. Here, we report a microscopic model that captures the mechanism behind this phenomenon, providing a qualitative understanding of the experimental observation. This leads to unique strain dependence of such charge transfer in iridate-manganite superlattices. The predicted behavior is consistently verified by experiment with soft x-ray and optical spectroscopy. Our work thus demonstrates a new route to control electronic states in non-polar oxide heterostructures.
Ferroelectrics that are also ionic conductors offer possibilities for novel applications with high tunability, especially if the same atomic species causes both phenomena. In particular, at temperatures just below the Curie temperature, polarized states may be sustainable as the mobile species is driven in a controlled way over the energy barrier that governs ionic conduction, resulting in unique control of the polarization. This possibility was recently demonstrated in CuInP2S6, a layered ferroelectric ionic conductor in which Cu ions cause both ferroelectricity and ionic conduction. Here, we show that the commonly used approach to calculate the polarization of evolving atomic configurations in ferroelectrics using the modern theory of polarization, namely concerted (synchronous) migration of the displacing ions, is not well suited to describe the polarization evolution as the Cu ions cross the van der Waals gaps. We introduce an asynchronous Cu-migration scheme, which reflects the physical process by which Cu ions migrate, resolves the difficulties, and describes the polarization evolution both for normal ferroelectric switching and for transitions across the van der Waals gaps, providing a single framework to discuss ferroelectric ionic conductors.
In polar insulators where longitudinal and transverse optical phonon modes differ substantially, the electron-phonon coupling affects the energy-band structure primarily through the long-range Frohlich contribution to the Fan term. This diagram has the same structure as the $GW$ self-energy where $W$ originates from the electron part of the screened coulomb interaction. The two can be conveniently combined by combining electron and lattice contributions to the polarizability. Both contributions are nonanalytic at the origin, and diverge as $1/q^2$ so that the predominant contribution comes from a small region around $q{=}0$. Here we adopt a simple estimate for the Frohlich contribution by assuming that the entire phonon part can be attributed to a small volume of $q$ near $q{=}0$. We estimate the magnitude for $mathbf{q}{rightarrow}0$ from a generalized Lyddane-Sachs-Teller relation, and the radius from the inverse of the polaron length scale. The gap correction is shown to agree with Frohlichs simple estimate $-alpha_Pomega_L/2$ of the polaron effect.
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