78 research outputs found
Anomalous enhancement of tetragonality in PbTiO3 induced by negative pressure
Using a first-principles approach based on density-functional theory, we find
that a large tetragonal strain can be induced in PbTiO3 by application of a
negative hydrostatic pressure. The structural parameters and the dielectric and
dynamical properties are found to change abruptly near a crossover pressure,
displaying a ``kinky'' behavior suggestive of proximity to a phase transition.
Analogous calculations for BaTiO3 show that the same effect is also present
there, but at much higher negative pressure. We investigate this unexpected
behavior of PbTiO3 and discuss an interpretation involving a phenomenological
description in terms of a reduced set of relevant degrees of freedom.Comment: 9 pages, with 9 postscript figures embedded. Uses REVTEX and epsf
macros. Also available at
http://www.physics.rutgers.edu/~dhv/preprints/st_pbti/index.htm
A ferroelectric memristor
Memristors are continuously tunable resistors that emulate synapses.
Conceptualized in the 1970s, they traditionally operate by voltage-induced
displacements of matter, but the mechanism remains controversial. Purely
electronic memristors have recently emerged based on well-established physical
phenomena with albeit modest resistance changes. Here we demonstrate that
voltage-controlled domain configurations in ferroelectric tunnel barriers yield
memristive behaviour with resistance variations exceeding two orders of
magnitude and a 10 ns operation speed. Using models of ferroelectric-domain
nucleation and growth we explain the quasi-continuous resistance variations and
derive a simple analytical expression for the memristive effect. Our results
suggest new opportunities for ferroelectrics as the hardware basis of future
neuromorphic computational architectures
Geometric frustration in compositionally modulated ferroelectrics
Geometric frustration is a broad phenomenon that results from an intrinsic
incompatibility between some fundamental interactions and the underlying
lattice geometry1-7. Geometric frustration gives rise to new fundamental
phenomena and is known to yield intriguing effects, such as the formation of
exotic states like spin ice, spin liquids and spin glasses1-7. It has also led
to interesting findings of fractional charge quantization and magnetic
monopoles5,6. Geometric frustration related mechanisms have been proposed to
understand the origins of relaxor behavior in some multiferroics, colossal
magnetocapacitive coupling and unusual and novel mechanisms of high Tc
superconductivity1-5. Although geometric frustration has been particularly well
studied in magnetic systems in the last 20 years or so, its manifestation in
the important class formed by ferroelectric materials (that are compounds
exhibiting electric rather than magnetic dipoles) is basically unknown. Here,
we show, via the use of a first-principles-based technique, that
compositionally graded ferroelectrics possess the characteristic "fingerprints"
associated with geometric frustration. These systems have a highly degenerate
energy surface and exhibit original critical phenomena. They further reveal
exotic orderings with novel stripe phases involving complex spatial
organization. These stripes display spiral states, topological defects and
curvature. Compositionally graded ferroelectrics can thus be considered as the
"missing" link that brings ferroelectrics into the broad category of materials
able to exhibit geometric frustration. Our ab-initio calculations allow a deep
microscopic insight into this novel geometrically frustrated system.Comment: 14 pages, 5 Figures;
http://www.nature.com/nature/journal/v470/n7335/full/nature09752.htm
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