Bending and Breaking of Stripes in a Charge-Ordered Manganite

In complex electronic materials, coupling between electrons and the atomic lattice gives rise to remarkable phenomena, including colossal magnetoresistance and metal-insulator transitions. Charge-ordered phases are a prototypical manifestation of charge-lattice coupling, in which the atomic lattice undergoes periodic lattice displacements (PLDs). Here we directly map the picometer scale PLDs at individual atomic columns in the room temperature charge-ordered manganite Bi$_{0.35}$Sr$_{0.18}$Ca$_{0.47}$MnO$_3$ using aberration corrected scanning transmission electron microscopy (STEM). We measure transverse, displacive lattice modulations of the cations, distinct from existing manganite charge-order models. We reveal locally unidirectional striped PLD domains as small as $\sim$5 nm, despite apparent bidirectionality over larger length scales. Further, we observe a direct link between disorder in one lattice modulation, in the form of dislocations and shear deformations, and nascent order in the perpendicular modulation. By examining the defects and symmetries of PLDs near the charge-ordering phase transition, we directly visualize the local competition underpinning spatial heterogeneity in a complex oxide.Comment: Main text: 20 pages, 4 figures. Supplemental Information: 27 pages, 14 figure

Commensurate Stripes and Phase Coherence in Manganites Revealed with Cryogenic Scanning Transmission Electron Microscopy

Incommensurate charge order in hole-doped oxides is intertwined with exotic phenomena such as colossal magnetoresistance, high-temperature superconductivity, and electronic nematicity. Here, we map at atomic resolution the nature of incommensurate order in a manganite using scanning transmission electron microscopy at room temperature and cryogenic temperature ($\sim$ 93K). In diffraction, the ordering wavevector changes upon cooling, a behavior typically associated with incommensurate order. However, using real space measurements, we discover that the underlying ordered state is lattice-commensurate at both temperatures. The cations undergo picometer-scale ($\sim$6-11 pm) transverse displacements, which suggests that charge-lattice coupling is strong and hence favors lattice-locked modulations. We further unearth phase inhomogeneity in the periodic lattice displacements at room temperature, and emergent phase coherence at 93K. Such local phase variations not only govern the long range correlations of the charge-ordered state, but also results in apparent shifts in the ordering wavevector. These atomically-resolved observations underscore the importance of lattice coupling and provide a microscopic explanation for putative "incommensurate" order in hole-doped oxides

Charge order textures induced by non-linear lattice coupling in a half-doped manganite

The self-organization of strongly interacting electrons into superlattice structures underlies the properties of many quantum materials. How these electrons arrange within the superlattice dictates what symmetries are broken and what ground states are stabilized. Here we show that cryogenic scanning transmission electron microscopy enables direct mapping of local symmetries and order at the intra-unit-cell level in the model charge-ordered system Nd$_{1/2}$Sr$_{1/2}$MnO$_{3}$. In addition to imaging the prototypical site-centered charge order, we discover the nanoscale coexistence of an exotic intermediate state which mixes site and bond order and breaks inversion symmetry. We further show that nonlinear coupling of distinct lattice modes controls the selection between competing ground states. The results demonstrate the importance of lattice coupling for understanding and manipulating the character of electronic self-organization and highlight a novel method for probing local order in a broad range of strongly correlated systems

Antiferromagnetic metal phase in an electron-doped rare-earth nickelate

Long viewed as passive elements, antiferromagnetic materials have emerged as promising candidates for spintronic devices due to their insensitivity to external fields and potential for high-speed switching. Recent work exploiting spin and orbital effects has identified ways to electrically control and probe the spins in metallic antiferromagnets, especially in noncollinear or noncentrosymmetric spin structures. The rare earth nickelate NdNiO3 is known to be a noncollinear antiferromagnet where the onset of antiferromagnetic ordering is concomitant with a transition to an insulating state. Here, we find that for low electron doping, the magnetic order on the nickel site is preserved while electronically a new metallic phase is induced. We show that this metallic phase has a Fermi surface that is mostly gapped by an electronic reconstruction driven by the bond disproportionation. Furthermore, we demonstrate the ability to write to and read from the spin structure via a large zero-field planar Hall effect. Our results expand the already rich phase diagram of the rare-earth nickelates and may enable spintronics applications in this family of correlated oxides.Comment: 25 pages, 4 figure

The Future of the Correlated Electron Problem

The understanding of material systems with strong electron-electron interactions is the central problem in modern condensed matter physics. Despite this, the essential physics of many of these materials is still not understood and we have no overall perspective on their properties. Moreover, we have very little ability to make predictions in this class of systems. In this manuscript we share our personal views of what the major open problems are in correlated electron systems and we discuss some possible routes to make progress in this rich and fascinating field. This manuscript is the result of the vigorous discussions and deliberations that took place at Johns Hopkins University during a three-day workshop January 27, 28, and 29, 2020 that brought together six senior scientists and 46 more junior scientists. Our hope, is that the topics we have presented will provide inspiration for others working in this field and motivation for the idea that significant progress can be made on very hard problems if we focus our collective energies.Comment: 55 pages, 19 figure

Atomic-Scale Visualizations of Charge Order Phenomena

219 pagesIn materials where electrons strongly interact with other degrees of freedom, novel electronic patterns and properties emerge. One of the most fascinating manifestations is charge ordering, whereby electrons form superstructures which break the translational symmetry of the atomic lattice. Charge order is under intense scrutiny due to its relationship to unconventional superconductivity, the colossal magnetoresistance effect, metal-insulator transitions, and phase transitions in general. Often, these electronic states have complex spatial variations at the nanoscale and subtle microscopic details which are difficult to ascertain. The lattice response in these phases– how atoms move upon entering the charge-ordered state– is deeply entwined with the electronic behavior but has not been visualized at the atomic scale. In this thesis, we apply scanning transmission electron microscopy (STEM) to map the lattice behavior in charge-ordered materials. We develop a methodology to extract tiny atomic shifts, on the picometer scale, and reveal the underlying ground states of charge ordering in various systems. Many exotic electronic phases, including charge ordering, typically emerge at low temperatures, yet STEM measurements have been limited to room temperature due to stringent stage stability requirements. We demonstrate for the first time cryogenic STEM with sub-Angstrom resolution and picometer precision which enables novel studies of low temperature phenomena. We map topological defects and elastic deformations of stripes in a manganite material near its transition temperature and visualize emergent coherence upon cooling. These measurements also determine the nature of temperature-dependent incommensurate charge order in which the wavevector is an irrational fraction of the reciprocal lattice. We find that incommensuration reflects phase disorder and that locally charge ordering is commensurate with the lattice. Next we address the microscopic nature of charge and orbital order in a half-doped manganite, an ordered phase that occurs below 150 K and requires cryogenic STEM. Both site-centered (stripe) and bond-centered (bi-stripe, Zener polaron) orders have been proposed in the half-doped compounds. They differ by the charge and orbital arrangement within the superlattice and are expected to behave differently because they have distinct symmetries. By measuring picoscale periodic lattice distortions using cryogenic STEM, we find two distinct ground states coexisting over tens of nanometers. The first is consistent with site-centered order. The second represents bi-stripes which, unlike the proposed Zener polaron, are not purely bond-centered. Instead, the bi-stripes are intermediate between bond- and site-centered order and break inversion symmetry. We extend these microscopy techniques to the layered material TaTe2 which contains a complex stacking of distorted, trimerized tantalum clusters. This complicates interpretation of the modulated state since we cannot measure lattice displacements as was done for the manganites; the staggered arrangement of atoms in this material blurs structural information due to the projection nature of STEM. We develop new tools to extract structural information from contrast modulation in the atomic resolution STEM image and visualize an additional, orthogonal trimerization involving Ta sites and subtle distortions of Te sites at low temperature. Coupled with density functional theory calculations and image simulations, this approach overcomes limitations of projection imaging and opens the door for atomic-scale visualization of complex stacking order in a variety of layered systems. Together, these atomic-scale measurements and methodologies solve fundamental problems about the nature of electronic orders and their fluctuations. More broadly, the successful demonstration and application of low temperature STEM provides unprecedented access to exotic electronic phases