arXiv subjects

Chris Ablitt

Publications and source records attributed to Chris Ablitt.

3 recordsLinked to original sources

arXiv2020

From first- to second-order phase transitions in hybrid improper ferroelectrics through entropy stabilisation

Hybrid improper ferroelectrics (HIFs) have been intensely studied over the last few years to gain understanding of their temperature induced phase transitions and ferroelectric switching pathways. Here we report a switching from first- to second-order phase transition pathway for topical HIFs $Ca_{3-x}Sr_xTi_2O_7$, which is driven by the differing entropies of the phases that we identify as being associated with the dynamic motion of octahedral tilts and rotations. A greater understanding of the transition pathways in this class of layered perovskites, which host many physical properties that are coupled to specific symmetries and octahedral rotation and tilt distortions -- such as superconductivity, negative thermal expansion, fast ion conductivity, ferroelectricity, among others -- is a crucial step in creating novel functional materials by design.

cond-mat.mtrl-sci↗

arXiv2019

Fast Neural Network Approach for Direct Covariant Forces Prediction in Complex Multi-Element Extended Systems

Neural network force field (NNFF) is a method for performing regression on atomic structure-force relationships, bypassing expensive quantum mechanics calculation which prevents the execution of long ab-initio quality molecular dynamics simulations. However, most NNFF methods for complex multi-element atomic systems indirectly predict atomic force vectors by exploiting just atomic structure rotation-invariant features and the network-feature spatial derivatives which are computationally expensive. We develop a staggered NNFF architecture exploiting both rotation-invariant and covariant features separately to directly predict atomic force vectors without using spatial derivatives, thereby reducing expensive structural feature calculation by ~180-480x. This acceleration enables us to develop NNFF which directly predicts atomic forces in complex ternary and quaternary-element extended systems comprised of long polymer chains, amorphous oxide, and surface chemical reactions. The staggered rotation-invariant-covariant architecture described here can also directly predict complex covariant vector outputs from local physical structures in domains beyond computational material science.

physics.comp-ph↗

arXiv2018

A Corkscrew Model for Highly Coupled Anisotropic Compliance in Ruddlesden-Popper Oxides with Frozen Octahedral Rotations

A "corkscrew" mechanism, that couples changes in the in-plane rotation angle to strains along the layering axis, has been proposed previously to explain increased compliance in certain Ruddlesden-Popper phases that facilitates uniaxial negative thermal expansion over a wide temperature range. Following the procedure developed to study many simple, auxetic geometries, in the present study we derive the elastic compliances predicted by this corkscrew mechanism assuming that the four shortest metal-anion bonds remain stiff and changes in bond angle are modelled by a harmonic angle potential. We subsequently analyse the limitations of this model and show that it may be extended to An+1BnO3n+1 Ruddlesden-Popper oxide phases of general layer thickness n.

cond-mat.mtrl-sci↗