Bulletin of the American Physical Society
APS March Meeting 2015
Volume 60, Number 1
Monday–Friday, March 2–6, 2015; San Antonio, Texas
Session M47: Invited Session: Insights into Ferroelectric and Multiferroic Matrials from Computational Methods |
Hide Abstracts |
Sponsoring Units: DCOMP DCMP Chair: Gabriel Kotliar, Rutgers University Room: 217B |
Wednesday, March 4, 2015 11:15AM - 11:51AM |
M47.00001: Computing the properties of ferroelectrics and magnetoelectrics in applied electric fields Invited Speaker: David Vanderbilt The technology for computing properties of insulators in a finite electric field $\cal E$, based on the coupling of $\cal E$ to the Berry-phase polarization $P$, has been available for over a decade and is currently implemented into several standard code packages. I will give an overview of recent developments in the extension of these methods and their applications to studies of ferroelectrics and multiferroics. I will first discuss the extension to allow calculations at fixed electric displacement field $D$, emphasizing its advantages for calculations on superlattice and ultrathin capacitor geometries. I will also discuss the qualitative differences, as evidenced by their distinct electric equations of state ($P\;vs.\;\cal E$, $P\;vs.\;D$, or $D\;vs.\;\cal E$), for ordinary ferroelectrics, improper ferroelectrics, and ``hyperferroelectrics.'' The latter constitute a new class of proper ferroelectrics that polarize even when the depolarization field is unscreened, i.e., even at fixed displacement field $D$. I will then turn to magnetoelectric effects, which can be computed by studying the change in magnetization as an electric field is applied. A particularly subtle component is the one that comes from the change of orbital magnetization. This is found to have an isotropic component, the so-called ``axion coupling,'' that takes the form of an integral of a Chern-Simons three-form over the three-dimensional BZ, as well as anisotropic components that can be expressed in a more conventional Kubo-Greenwood form. I will end with some comments on current and future challenges. [Preview Abstract] |
Wednesday, March 4, 2015 11:51AM - 12:27PM |
M47.00002: The quantum nature of skyrmions and half-skyrmions in Cu2OSeO3 Invited Speaker: Jeroen van den Brink The Skyrme-particle, the skyrmion, was introduced over half a century ago in the context of dense nuclear matter. But with skyrmions being mathematical objects--special types of topological solitons--they can emerge in much broader contexts. Recently skyrmions were observed in helimagnets, forming nanoscale spin-textures. Extending over length scales much larger than the interatomic spacing, they behave as large, classical objects, yet deep inside they are of quantum nature. Penetrating into their microscopic roots requires a multi-scale approach, spanning the full quantum to classical domain. We performed such an approach for the first time in the skyrmionic Mott insulator Cu$_2$OSeO$_3$. We show that its magnetic building blocks are strongly fluctuating Cu4 tetrahedra, spawning a continuum theory that culminates in 51 nm large skyrmions, in striking agreement with experiment [1]. Another consequence is the presence of two distinct types of modes: a low-energy manifold that includes a gapless Goldstone mode and a set of weakly dispersive high-energy magnons [2]. Using high-field electron spin resonance with a terahertz free-electron laser and pulsed magnetic fields up to 64 T we identified these modes [3], corroborating the presence of fluctuating Cu4 tetrahedra. We also show that the emerging electric polarization \textbf{P} is governed by quadrupolar spin contributions from symmetry inequivalent bonds and calculate the induced \textbf{P} in different crystallographic directions as a function of the orientation of an applied magnetic field, which are confirmed by experiment [2]. One so far untested prediction that ensues is the temperature-dependent decay of skyrmions into half-skyrmions. \\[4pt] [1] Nature Comm. 5, 5376 (2014)\\[0pt] [2] PRB 90, 140404(R) (2014)\\[0pt] [3] PRL 113, 157205 (2014) [Preview Abstract] |
Wednesday, March 4, 2015 12:27PM - 1:03PM |
M47.00003: Electric polarization of Sr$_{0.5}$Ba$_{0.5}$MnO$_{3}$: a multiferroic Mott insulator Invited Speaker: Reza Nourafkan Multiferroics, materials which display simultaneous magnetic and ferroelectric orders, are interesting both for their rich physics and for their promising practical applications. The search for multiferroic materials with strong-magnetoelectric coupling is challenging and requires an understanding of how the magnetic order, or more specifically the correlations, influence the electric polarization and vice versa. A calculations of the electric polarization in the paramagnetic (PM) insulating phase of multiferroics is essential to address this mutual influence. Ab inito calculations of the electric polarization are based on the modern theory of polarization, which is a single-electron theory. Thus, a correlation driven insulating state is beyond the scope of this approach. Here we show that combining correlated band structure calculations (DFT+DMFT) with a formula for the electric polarization of interacting insulators, expressed in terms of the full Green and vertex functions, allows for the first time to reliably calculate the polarization in the PM phase. We focus on the Mott insulator Sr$_{0.5}$Ba$_{0.5}$MnO$_{3}$, in which both magnetic and ferroelectric instabilities are related to the Mn ions. We predict a ferroelectric polarization of $\simeq 16.5~\mu C/cm^2$ in the high temperature paramagnetic phase and recover the measured value of $\simeq 13.3~\mu C/cm^2$ in the low temperature antiferromagnetic phase. Our calculations reveal that the the driving force behind the ferroelectric distortion comes from the tendency of Mn $e_g$ states to establish a stronger covalency with the surrounding oxygens. This covalency is reduced by correlations, in particular by Hund coupling. On the other hand, the half-filled Mn $t_{2g}$ orbitals give rise to the magnetic ordering which decreases the ionic displacement, hence its contribution to the polarization. For fixed ionic displacement, the magnetic order also slightly decreases the electronic contribution to the electric polarization by partially polarizing the Mn eg orbitals through Hund coupling. Despite this last effect, which would lead to positive magneto-electric coupling, the combination of the two effects gives a negative magneto-electric coupling. [Preview Abstract] |
Wednesday, March 4, 2015 1:03PM - 1:39PM |
M47.00004: Mott Multiferroics and Ferroelectric Metals from Dynamical Mean-Field Theory combined with Density-Functional Theory Invited Speaker: Massimo Capone Multiferroic materials, in which ferroelectricity and long-range magnetic ordering coexist, are natural candidates for applications. In this perspective, the most promising compounds are those in which the two phenomena do not simply coexist, but they influence each other through a magnetoelectric coupling. We present different applications of Density Functional Theory combined with Dynamical Mean-Field Theory in which electron-electron correlation effects are crucial in the stabilization of multiferroic behavior and in the magnetoelectric coupling. Within this wide family we can distinguish different cases. In Sr$_{0.5}$Ba$_{0.5}$MnO$_3$ the multiferroic behavior is associated with a Mott insulating state in which the Mn half-filled t$_{2g}$ orbitals are responsible of the magnetic properties and the value of the polarization is strongly affected by the magnetic state [1]. LiOsO$_3$ shares the same electronic configuration with half-filled Os t$_{2g}$ orbitals. Despite this configuration enhances the effect of electron-electron interactions, the material remains metallic and represents a peculiar ferroelectric metal [2]. We propose however how to turn this non-magnetic polar metal into a multiferroic through the design of a superlattice, which increases the degree of correlation, leading to Mott localization of the Os orbitals [3]. In completely different systems, such as organic crystals like (TMTTF)$_2$-X, strong correlations can lead to multiferroicity in organic crystals such as (TMTTF)$_2$-X, where charge ordering promotes a polarization which is favored by an antiferromagnetic ordering [4]. We finally discuss how strong correlations can play a major role away from half-filling when the Hund's coupling is sizable in compounds with a nominal valence of, e.g., two electrons in the three t$_{2g}$ orbitals. Such ``Hund's metals'' are correlated despite being far from Mott localization. This physical regime can be a fertile ground to obtain other ferroelectric metals. \\[4pt] [1] G. Giovannetti, S. Kumar, C. Ortix, M. Capone, and J. van den Brink Phys. Rev. Lett. 109, 107601(2012)\\[0pt] [2] G. Giovannetti and M. Capone, Phys. Rev. B 90, 195113 (2014)\\[0pt] [3] G. Giovannetti, D. Puggioni, M. Capone and J. M. Rondinelli, in preparation\\[0pt] [4] G. Giovannetti, S. Kumar, J.-P. Pouget, and M. Capone, Phys. Rev. B 85, 205146 (2012); G. Giovannetti, R. Nourafkan, G. Kotliar and M. Capone, arXiv:1405.1528 [Preview Abstract] |
Wednesday, March 4, 2015 1:39PM - 2:15PM |
M47.00005: Electric field effects in transition metal oxides, their surfaces and heterostructures Invited Speaker: Karsten Held Modern computational tools such as density functional theory and its merger with dynamical mean field theory are nowadays inevitable for the modeling and understanding of oxides, their heterostructures and surfaces. In this talk, I will concentrate on the impact of electric fields, how they affect the physical properties and how to make use of them. Substantial internal electric fields are created at polar surfaces, and even for an isopolar-interface the electronic reconstruction can lead to a charge transfer and hence a dipole field [1]. Such internal fields can be employed to efficiently separate electrons and holes in a oxide solar cell [2]. Even if the polar dipole field is compensated by a surface reconstruction, a local surface potential remains, and makes SrTiO$_3$ (110) the arguably simplest 2 dimensional electron gas (2DEG) [3]. External electric fields, on the other hand, can trigger ``gigantic'' responses, since correlated oxides are prone to small perturbations. For example, a field effect Mott transistor can be realized in a few layers of SrVO$_3$ with ideal on-off (metal-insulator) switching properties [4]; and interfacing a ferroelectric, BaTiO$_3$, plus a 2DEG with large spin-orbit coupling, BaOsO$_3$, allows for a giant switchable Rashba effect.\\[4pt] [1] J. E. Kleibeuker et al. Phys. Rev. Lett. (2014).\\[0pt] [2] E. Assmann et al., Phys. Rev. Lett. 110, 078701 (2013).\\[0pt] [3] Z. Wang et al, Proc. Natl. Acad. Sci. 111, 3933 (2014).\\[0pt] [4] Z. Zhong et al., arXiv:1312.5989. [Preview Abstract] |
Follow Us |
Engage
Become an APS Member |
My APS
Renew Membership |
Information for |
About APSThe American Physical Society (APS) is a non-profit membership organization working to advance the knowledge of physics. |
© 2024 American Physical Society
| All rights reserved | Terms of Use
| Contact Us
Headquarters
1 Physics Ellipse, College Park, MD 20740-3844
(301) 209-3200
Editorial Office
100 Motor Pkwy, Suite 110, Hauppauge, NY 11788
(631) 591-4000
Office of Public Affairs
529 14th St NW, Suite 1050, Washington, D.C. 20045-2001
(202) 662-8700