297 research outputs found
Anisotropic Local Correlations and Dynamics in a Relaxor Ferroelectric
Relaxor ferroelectrics have been a focus of intense attention due to their
anomalous dielectric characteristics, diffuse phase transitions, and strong
piezoelectricity. Understanding the structure and dynamics of relaxors has been
one of the long-standing challenges in solid-state physics, with the current
model of polar nanoregions in a non-polar matrix providing only a qualitative
description of the relaxor phase transitions. In this paper, we investigate the
local structure and dynamics in 75%PbMgNbO-25%PbTiO
(PMN-PT) using molecular dynamics simulations and the dynamic pair distribution
function technique. We show for the first time that relaxor transitions can be
described by local order parameters. We find that structurally, the relaxor
phase is characterized by the presence of highly anisotropic correlations
between the local cation displacements. These correlations resemble the
hydrogen bond network in water. Our findings contradict the current polar
nanoregion model; instead, we suggest a new model of a homogeneous random
network of anisotropically coupled dipoles.Comment: We combine our manuscript and supplementary information in one file.
5 pages and 3 figures in main text. 3 pages and 3 figures in supplementary
informatio
Magnetoelectric control of topological phases in graphene
Topological antiferromagnetic (AFM) spintronics is an emerging field of research, which involves the topological electronic states coupled to the AFM order parameter known as the Néel vector. The control of these states is envisioned through manipulation of the Néel vector by spin-orbit torques driven by electric currents. Here we propose a different approach favorable for low-power AFM spintronics, where the control of the topological states in a two-dimensional material, such as graphene, is performed via the proximity effect by the voltage induced switching of the Néel vector in an adjacent magnetoelectric AFM insulator, such as chromia. Mediated by the symmetry protected boundary magnetization and the induced Rashba-type spin-orbit coupling at the interface between graphene and chromia, the emergent topological phases in graphene can be controlled by the Néel vector. Using density functional theory and tight-binding Hamiltonian approaches, we model a graphene/Cr2O3 (0001) interface and demonstrate nontrivial band gap openings in the graphene Dirac bands asymmetric between the K and K′ valleys. This gives rise to an unconventional quantum anomalous Hall effect (QAHE) with a quantized value of 2e^2/h and an additional steplike feature at a value close to e^2/2h, and the emergence of the spin-polarized valley Hall effect (VHE). Furthermore, depending on the Néel vector orientation, we predict the appearance and transformation of different topological phases in graphene across the 180° AFM domain wall, involving the QAHE, the valley-polarized QAHE, and the quantum VHE, and the emergence of the chiral edge states along the domain wall. These topological properties are controlled by voltage through magnetoelectric switching of the AFM insulator with no need for spin-orbit torques
Magnetoelectric control of topological phases in graphene
Topological antiferromagnetic (AFM) spintronics is an emerging field of research, which involves the topological electronic states coupled to the AFM order parameter known as the Néel vector. The control of these states is envisioned through manipulation of the Néel vector by spin-orbit torques driven by electric currents. Here we propose a different approach favorable for low-power AFM spintronics, where the control of the topological states in a two-dimensional material, such as graphene, is performed via the proximity effect by the voltage induced switching of the Néel vector in an adjacent magnetoelectric AFM insulator, such as chromia. Mediated by the symmetry protected boundary magnetization and the induced Rashba-type spin-orbit coupling at the interface between graphene and chromia, the emergent topological phases in graphene can be controlled by the Néel vector. Using density functional theory and tight-binding Hamiltonian approaches, we model a graphene/Cr2O3 (0001) interface and demonstrate nontrivial band gap openings in the graphene Dirac bands asymmetric between the K and K′ valleys. This gives rise to an unconventional quantum anomalous Hall effect (QAHE) with a quantized value of 2e^2/h and an additional steplike feature at a value close to e^2/2h, and the emergence of the spin-polarized valley Hall effect (VHE). Furthermore, depending on the Néel vector orientation, we predict the appearance and transformation of different topological phases in graphene across the 180° AFM domain wall, involving the QAHE, the valley-polarized QAHE, and the quantum VHE, and the emergence of the chiral edge states along the domain wall. These topological properties are controlled by voltage through magnetoelectric switching of the AFM insulator with no need for spin-orbit torques
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