The role of lattice dynamics in ferroelectric switching

in Nature Communications (2022)

Direct observation of ferroelectricity in two-dimensional MoS2

in npj 2d materials and applications (2022)

Giant voltage amplification from electrostatically induced incipient ferroelectric states

in Nature Materials (2022)

Origin of negative electrocaloric effect in Pnma-type antiferroelectric perovskites

in Physical Review. B (2022)

Quantification of the Electromechanical Measurements by Piezoresponse Force Microscopy

in Advanced Materials (2022)

Origin of nonlinear magnetoelectric response in rare-earth orthoferrite perovskite oxides

in Physical Review. B (2022), 105

Archetypal Soft-Mode-Driven Antipolar Transition in Francisite Cu3Bi(SeO3)(2)O2Cl

in PHYSICAL REVIEW LETTERS (2020), 124(9), 097603-6

Model materials are precious test cases for elementary theories and provide building blocks for the understanding of more complex cases. Here, we describe the lattice dynamics of the structural phase ... [more ▼]

Model materials are precious test cases for elementary theories and provide building blocks for the understanding of more complex cases. Here, we describe the lattice dynamics of the structural phase transition in francisite Cu3Bi(SeO3)(2)O2Cl at 115 K and show that it provides a rare archetype of a transition driven by a soft antipolar phonon mode. In the high-symmetry phase at high temperatures, the soft mode is found at (0,0,0.5) at the Brillouin zone boundary and is measured by inelastic x-ray scattering and thermal diffuse scattering. In the low-symmetry phase, this soft-mode is folded back onto the center of the Brillouin zone as a result of the doubling of the unit cell, and appears as a fully symmetric mode that can be tracked by Raman spectroscopy. On both sides of the transition, the mode energy squared follows a linear behavior over a large temperature range. First-principles calculations reveal that, surprisingly, the flat phonon band calculated for the high-symmetry phase seems incompatible with the displacive character found experimentally. We discuss this unusual behavior in the context of an ideal Kittel model of an antiferroelectric transition. [less ▲]

Vibrational properties of LaNiO3 films in the ultrathin regime

in APL MATERIALS (2020), 8(6),

Collective rotations and tilts of oxygen polyhedra play a crucial role in the physical properties of complex oxides such as magnetism and conductivity. Such rotations can be tuned by preparing thin films ... [more ▼]

Collective rotations and tilts of oxygen polyhedra play a crucial role in the physical properties of complex oxides such as magnetism and conductivity. Such rotations can be tuned by preparing thin films in which dimensionality, strain, and interface effects come into play. However, little is known of the tilt and rotational distortions in films a few unit cells thick including the question of if coherent tilt patterns survive at all in this ultrathin limit. Here, a series of films of perovskite LaNiO3 is studied and it is shown that the phonon mode related to oxygen octahedral tilts can be followed by Raman spectroscopy down to a film thickness of three pseudocubic perovskite unit cells (similar to 1.2 nm). To push the limits of resolution to the ultrathin regime, a statistical analysis method is introduced to separate the Raman signals of the film and substrate. Most interestingly, these analyses reveal a pronounced hardening of the tilt vibrational mode in the thinnest films. A comparison between the experimental results, first principles simulations of the atomic structure, and the standing wave model, which accounts for size effects on the phononic properties, reveals that in the ultrathin regime, the Raman spectra are a hybrid entity of both the bulk and surface phononic behavior. These results showcase Raman spectroscopy as a powerful tool to probe the behavior of perovskite films down to the ultrathin limit. [less ▲]

Ultralow Voltage Manipulation of Ferromagnetism

in Advanced Science (2020)

Manipulating magnetoelectric energy landscape in multiferroics

in Nature Communications (2020)