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 Srn+1_{n+1}Tin_nO3n+1_{3n+1}

We have discovered from first-principles an unusual polar state in the low n Sr$_{n+1}$Ti$_n$O$_{3n+1}$ 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$_3$. 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 Dion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 ferroelectricityso-called hybrid-improper ferroelectricityhas been elucidated in two families of layered perovskites: AA′B₂O₆ 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 Ln₂NiO_4+δ 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 Ln₂NiO_4+δ 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 ANiO₃ perovskites