165 research outputs found
Ultrafast control of material optical properties via the infrared resonant Raman effect
The Raman effect -- inelastic scattering of light by lattice vibrations
(phonons) -- produces an optical response closely tied to a material's crystal
structure. Here we show that resonant optical excitation of IR and Raman
phonons gives rise to a Raman scattering effect that can induce giant shifts to
the refractive index and induce new optical constants that are forbidden in the
equilibrium crystal structure. We complete the description of light-matter
interactions mediated by coupled IR and Raman phonons in crystalline insulators
-- currently the focus of numerous experiments aiming to dynamically control
material properties -- by including a forgotten pathway through the nonlinear
lattice polarizability. Our work expands the toolset for control and
development of new optical technologies by revealing that the absorption of
light within the terahertz gap can enable control of optical properties of
materials over a broad frequency range.Comment: 15 pages, 6 figure
Interface control of emergent ferroic order in Ruddlesden-Popper SrTiO
We have discovered from first-principles an unusual polar state in the low n
SrTiO Ruddlesden-Popper (RP) layered perovskites in which
ferroelectricity is nearly degenerate with antiferroelectricity, a relatively
rare form of ferroic order. We show that epitaxial strain plays a key role in
tuning the "perpendicular coherence length" of the ferroelectric mode, and does
not induce ferroelectricity in these low dimensional RP materials as is well
known to occur in SrTiO. These systems present an opportunity to manipulate
the coherence length of a ferroic distortion in a controlled way, without
disorder or a free surface
Spin-Phonon Interaction in Yttrium Iron Garnet
Spin-phonon interaction is an important channel for spin and energy
relaxation in magnetic insulators. Understanding this interaction is critical
for developing magnetic insulator-based spintronic devices. Quantifying this
interaction in yttrium iron garnet (YIG), one of the most extensively
investigated magnetic insulators, remains challenging because of the large
number of atoms in a unit cell. Here, we report temperature-dependent and
polarization-resolved Raman measurements in a YIG bulk crystal. We first
classify the phonon modes based on their symmetry. We then develop a modified
mean-field theory and define a symmetry-adapted parameter to quantify
spin-phonon interaction in a phonon-mode specific way for the first time in
YIG. Based on this improved mean-field theory, we discover a positive
correlation between the spin-phonon interaction strength and the phonon
frequency.Comment: 12 pages, 4 figures, 1 table; (Supp. Info. 10 pages, 5 figures, 2
tables
The Magnetoelectric Effect in Transition Metal Oxides: Insights and the Rational Design of New Materials from First Principles
The search for materials displaying a large magnetoelectric effect has
occupied researchers for many decades. The rewards could include not only
advanced electronics technologies, but also fundamental insights concerning the
dielectric and magnetic properties of condensed matter. In this article, we
focus on the magnetoelectric effect in transition metal oxides and review the
manner in which first-principles calculations have helped guide the search for
(and increasingly, predicted) new materials and shed light on the microscopic
mechanisms responsible for magnetoelectric phenomena.Comment: 24 pages, 12 figure
Origin of Ferroelectricity in a Family of Polar Oxides: The Dionî—¸Jacobson Phases
Recent work on layered perovskites
has established the group theoretical guidelines under which a combination
of octahedral distortions and cation ordering can break inversion
symmetry, leading to polar structures. The microscopic mechanism of
this form of ferroelectricityî—¸so-called hybrid-improper ferroelectricityî—¸has
been elucidated in two families of layered perovskites: AA′B<sub>2</sub>O<sub>6</sub> double perovskites and Ruddlesden–Popper
phases. In this work, we use symmetry principles, crystal chemical
models, and first-principles calculations to unravel the crystal chemical
origin of ferroelectricity in the Dion–Jacobson phases, and
show that the hybrid improper mechanism can provide a unifying explanation
for the emergence of polar structures in this family of materials.
We link trends in the magnitude of the induced polarizations to changes
in structure and composition and discuss possible phase-transition
scenarios. Our results suggest that the structures of several Dion–Jacobson
phases that have previously been characterized as centrosymmetric
should be re-examined. Our work adds new richness to theories of how
polar structures emerge in layered perovskites
Enhancement of Ionic Transport in Complex Oxides through Soft Lattice Modes and Epitaxial Strain
Lattice
dynamics is increasingly acknowledged as playing an important
role in the ionic transport mechanisms of many oxide ion conductors.
In particular, specific structural distortions–so-called octahedral
rotations–have been suggested as the origin of the enhanced
mobility observed in Ln<sub>2</sub>NiO<sub>4+δ</sub> Ruddlesden–Popper
phases (Ln = La, Pr, Nd), where oxide interstitial diffusion occurs
through an interstitialcy mechanism. In this work, we use theory and
first-principles calculations to unravel and quantify the microscopic
link between soft lattice modes and migration barriers in the Ln<sub>2</sub>NiO<sub>4+δ</sub> family of materials. We show that
the magnitude of the migration barriers can be correlated with the
tendency of each material to undergo an octahedral rotation distortion:
as the tendency of a material to undergo such a distortion increases,
the migration barrier decreases. We then use this insight to formulate
simple design guidelines for further decreasing migration barriers
through epitaxial strain and that connect trends in the ionic transport
properties of the Ruddlesden–Popper phases with the structures
of the parent ANiO<sub>3</sub> perovskites
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