Ismail El Baggari

Atomic lattice disorder in charge-density-wave phases of exfoliated dichalcogenides (1T-TaS2)

Significance Low-dimensional materials, such as 1T-TaS2, permit unique phases that arise through electronic and structural reshaping known, respectively, as charge-density waves and periodic lattice distortions (PLDs). Determining the atomic structure of PLDs is critical toward understanding the origin of these charge-ordered phases and their effect on electronic properties. Here we reveal the microscopic nature of PLDs at cryogenic and room temperature in thin flakes of 1T-TaS2 using atomic resolution scanning transmission electron microscopy. Real-space characterization of the local PLD structure across the phase diagram will enable harnessing of emergent properties of thin transition-metal dichalcogenides. Charge-density waves (CDWs) and their concomitant periodic lattice distortions (PLDs) govern the electronic properties in several layered transition-metal dichalcogenides. In particular, 1T-TaS2 undergoes a metal-to-insulator phase transition as the PLD becomes commensurate with the crystal lattice. Here we directly image PLDs of the nearly commensurate (NC) and commensurate (C) phases in thin, exfoliated 1T-TaS2 using atomic resolution scanning transmission electron microscopy at room and cryogenic temperature. At low temperatures, we observe commensurate PLD superstructures, suggesting ordering of the CDWs both in- and out-of-plane. In addition, we discover stacking transitions in the atomic lattice that occur via one-bond-length shifts. Interestingly, the NC PLDs exist inside both the stacking domains and their boundaries. Transitions in stacking order are expected to create fractional shifts in the CDW between layers and may be another route to manipulate electronic phases in layered dichalcogenides.

Bending and breaking of stripes in a charge ordered manganite

In charge-ordered phases, broken translational symmetry emerges from couplings between charge, spin, lattice, or orbital degrees of freedom, giving rise to remarkable phenomena such as colossal magnetoresistance and metal–insulator transitions. The role of the lattice in charge-ordered states remains particularly enigmatic, soliciting characterization of the microscopic lattice behavior. Here we directly map picometer scale periodic lattice displacements at individual atomic columns in the room temperature charge-ordered manganite Bi0.35Sr0.18Ca0.47MnO3 using aberration-corrected scanning transmission electron microscopy. We measure transverse, displacive lattice modulations of the cations, distinct from existing manganite charge-order models. We reveal locally unidirectional striped domains as small as ~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 periodic lattice displacements near the charge ordering phase transition, we directly visualize the local competition underpinning spatial heterogeneity in a complex oxide.Charge-lattice coupling plays a central role in the exotic behaviors of multiferroic complex oxides, such as manganites, however, obtaining a microscopic picture is challenging. Here, Savitzky et al. map periodic lattice displacement fields at the picometer scale to study local order-disorder competition.

Nature and evolution of incommensurate charge order in manganites visualized with cryogenic scanning transmission electron microscopy

Significance Charge order is a modulation of the electron density and is associated with unconventional phenomena, including colossal magnetoresistance and metal–insulator transitions. Determining how the lattice responds provides insights into the nature and symmetry of the ordered state. Scanning transmission electron microscopy can measure lattice displacements with picometer precision, but its use has been limited to room-temperature phases only. Here, we demonstrate high-resolution imaging at cryogenic temperature and map the nature and evolution of charge order in a manganite. We uncover picometer-scale displacive modulations whose periodicity is strongly locked to the lattice and visualize temperature-dependent phase inhomogeneity in the modulations. These results pave the way to understanding the underlying structure of charge-ordered states and other complex phenomena. 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 charge–lattice order in a manganite using scanning transmission electron microscopy at room temperature and cryogenic temperature (∼93 K). In diffraction, the ordering wave vector changes upon cooling, a behavior typically associated with incommensurate order. However, using real space measurements, we discover that the ordered state forms lattice-locked regions over a few wavelengths interspersed with phase defects and changing periodicity. The cations undergo picometer-scale (∼6 pm to 11 pm) transverse displacements, suggesting that charge–lattice coupling is strong. We further unearth phase inhomogeneity in the periodic lattice displacements at room temperature, and emergent phase coherence at 93 K. Such local phase variations govern the long-range correlations of the charge-ordered state and locally change the periodicity of the modulations, resulting in wave vector shifts in reciprocal space. These atomically resolved observations underscore the importance of lattice coupling and phase inhomogeneity, and provide a microscopic explanation for putative “incommensurate” order in hole-doped oxides.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 (∼ 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 (∼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.

Aberration-Corrected STEM/EELS at Cryogenic Temperatures

Today’s aberration-corrected scanning transmission electron microscopes (STEM) routinely focus highenergy electrons down to a spot smaller than 1Å in diameter to perform scattering experiments that allow us to study the atomic-scale structure of materials and devices. When combined with electron energy loss spectroscopy analysis of the inelastically scattered electrons, these narrow probes can also provide atomic-scale information about the composition and local electronic structure of bulk materials, defects and interfaces [1, 2].

Emergent Phase Coherence of Stripe Order in Manganites Revealed with Cryogenic Scanning Transmission Electron Microscopy

Low temperature phase diagrams of strongly correlated systems reveal complexity and competition between mismatched orders, best exemplified in manganites where the inhomogeneous coexistence of metallic domains with charge-ordered patches results in colossal magnetoresistance [1]. Charge order, or stripes, is a prevalent electronic instability where electrons form periodic patterns, breaking lattice symmetry and competing with other phases. Disorder is thought to govern the emergence of the low temperature striped phases, causing, for instance, a tendency for gradual crossover rather than sharp phase transitions [1]. Real space visualization of the evolution of stripe order promises a deeper understanding of the onset of ordered phases.

Mapping Picometer Scale Periodic Lattice Distortions with Aberration Corrected Scanning Transmission Electron Microscopy

1. Department of Physics, Cornell University, Ithaca, NY 14853, USA. 2. Department of Physics and Astronomy, Rutgers University, Piscataway, NJ 08854, USA. 3. Rutgers Center for Emergent Materials, Rutgers University, Piscataway, NJ 08854, USA. 4. School of Applied & Engineering Physics, Cornell University, Ithaca, NY 14853, USA. 5. Kavli Institute for Nanoscale Science, Cornell University, Ithaca, NY 14853, USA.

Advances in Mapping Periodic Structural Modulations of Atomic Lattices

1. Department of Physics, Cornell University, Ithaca NY 14853, USA. 2. School of Applied and Engineering Physics, Cornell University, Ithaca, NY 14853, USA. 3. School of Electrical and Computer Engineering, Cornell University, Ithaca NY 14853,USA. 4. Department of Physics and Astronomy, Rutgers University, Piscataway NJ 08854, USA. 5. Rutgers Center for Emergent Materials, Rutgers University, Piscataway NJ 08854, USA. 6. Kavli Institute at Cornell, Cornell University, Ithaca NY 14853, USA.