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Register a new userCharge ordering at the interface in (LaMnO$_3$)$_{2n}$/(SrMnO$_3$)$_n$ superlattices as the origin of their insulating state
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We have performed ab initio calculations within the LDA+U method in the multilayered system (LaMnO$_3$)$_{2n}$ / (SrMnO$_3$)$_n$. Our results suggest a charge-ordered state that alternates Mn$^{3+}$ and Mn$^{4+}$ cations in a checkerboard in-plane pattern is developed at the interfacial layer, leading to a gap opening. Such an interfacial charge-ordered situation would be the energetically favored reconstruction between LaMnO$_3$ and SrMnO$_3$. This helps understanding the insulating behavior observed experimentally in these multilayers at intermediate values of $n$, whose origin is known to be due to some interfacial mechanism.
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Artificially fabricated 3$d$/5$d$ superlattices (SLs) involve both strong electron correlation and spin-orbit coupling in one material by means of interfacial 3$d$-5$d$ coupling, whose mechanism remains mostly unexplored. In this work we investigated the mechanism of interfacial coupling in LaMnO$_3$/SrIrO$_3$ SLs by several spectroscopic approaches. Hard x-ray absorption, magnetic circular dichroism and photoemission spectra evidence the systematic change of the Ir ferromagnetism and the electronic structure with the change of the SL repetition period. First-principles calculations further reveal the mechanism of the SL-period dependence of the interfacial electronic structure and the local properties of the Ir moments, confirming that the formation of Ir-Mn molecular orbital is responsible for the interfacial coupling effects. The SL-period dependence of the ratio between spin and orbital components of the Ir magnetic moments can be attributed to the realignment of electron spin during the formation of the interfacial molecular orbital. Our results clarify the nature of interfacial coupling in this prototypical 3$d$/5$d$ SL system and the conclusion will shed light on the study of other strongly correlated and spin-orbit coupled oxide hetero-interfaces.
Motivated by recent experiments, we use the $+U$ extension of the generalized gradient approximation to density functional theory to study superlattices composed of alternating layers of LaNiO$_3$ and LaMnO$_3$. For comparison we also study a rocksalt ((111) double perovskite) structure and bulk LaNiO$_3$ and LaMnO$_3$. A Wannier function analysis indicates that band parameters are transferable from bulk to superlattice situations with the exception of the transition metal d-level energy, which has a contribution from the change in d-shell occupancy. The charge transfer from Mn to Ni is found to be moderate in the superlattice, indicating metallic behavior, in contrast to the insulating behavior found in recent experiments, while the rocksalt structure is found to be insulating with a large Mn-Ni charge transfer. We suggest a high density of cation antisite defects may account for the insulating behavior experimentally observed in short-period superlattices.
The modulation of charge density and spin order in (LaMnO$_3$)$_{2n}$/(SrMnO$_3$)$_n$ ($n$=1-4) superlattices is studied via Monte Carlo simulations of the double-exchange model. G-type antiferromagnetic barriers in the SrMnO$_{3}$ regions with low charge density are found to separate ferromagnetic LaMnO$_{3}$ layers with high charge density. The recently experimentally observed metal-insulator transition with increasing $n$ is reproduced in our studies, and $n=3$ is found to be the critical value.
Localization of electrons in the two-dimensional electron gas at the LaAlO$_3$/SrTiO$_3$ interface is investigated by varying the channel thickness in order to establish the nature of the conducting channel. Layers of SrTiO$_3$ were grown on NdGaO$_3$ (110) substrates and capped with LaAlO$_3$. When the SrTiO$_3$ thickness is $leq 6$ unit cells, most electrons at the interface are localized, but when the number of SrTiO$_3$ layers is 8-16, the free carrier density approaches $3.3 times 10^{14}$ cm$^{-2}$, the value corresponding to charge transfer of 0.5 electron per unit cell at the interface. The number of delocalized electrons decreases again when the SrTiO$_3$ thickness is $geq 20$ unit cells. The $sim{4}$ nm conducting channel is therefore located significantly below the interface. The results are explained in terms of Anderson localization and the position of the mobility edge with respect to the Fermi level.
The paradigm of electrons interacting with a periodic lattice potential is central to solid-state physics. Semiconductor heterostructures and ultracold neutral atomic lattices capture many of the essential properties of 1D electronic systems. However, fully one-dimensional superlattices are highly challenging to fabricate in the solid state due to the inherently small length scales involved. Conductive atomic-force microscope (c-AFM) lithography has recently been demonstrated to create ballistic few-mode electron waveguides with highly quantized conductance and strongly attractive electron-electron interactions. Here we show that artificial Kronig-Penney-like superlattice potentials can be imposed on such waveguides, introducing a new superlattice spacing that can be made comparable to the mean separation between electrons. The imposed superlattice potential fractures the electronic subbands into a manifold of new subbands with magnetically-tunable fractional conductance (in units of $e^2/h$). The lowest $G=2e^2/h$ plateau, associated with ballistic transport of spin-singlet electron pairs, is stable against de-pairing up to the highest magnetic fields explored ($|B|=16$ T). A 1D model of the system suggests that an engineered spin-orbit interaction in the superlattice contributes to the enhanced pairing observed in the devices. These findings represent an important advance in the ability to design new families of quantum materials with emergent properties, and mark a milestone in the development of a solid-state 1D quantum simulation platform.
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