Bulletin of the American Physical Society
APS March Meeting 2013
Volume 58, Number 1
Monday–Friday, March 18–22, 2013; Baltimore, Maryland
Session G21: Focus Session: Theories of Electric, Elastic, Magnetic and Cross-coupling Terms in Ferroic Lattices |
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Sponsoring Units: DMP Chair: Philippe Ghosez, Universite de Liege Room: 323 |
Tuesday, March 19, 2013 11:15AM - 11:27AM |
G21.00001: Converse Piezoelectricity Michael Springborg, Bernard Kirtman Piezoelectricity results from a coupling between responses to mechanical and electric perturbations and leads to changes in the polarization due to strain or stress or, alternatively, the occurrence of strain as a function of an applied external, electrostatic field (i.e., converse piezoelectricity). Theoretical studies of those properties for extended systems require accordingly that their dipole moment or polarization can be calculated. However, whereas the definition of the operator for the dipole moment for any finite system is trivial, it is only within the last 2 decades that the expressions for the equivalent operator in the independent-particle approximation for the infinite and periodic system have been presented. Here, we demonstrate that the so called branch dependence of the polarization for the infinite, periodic system is related to physical observables in contrast to what often is assumed. This is related to the finding that converse piezoelectric properties depend both on the surfaces of the samples of interest even for samples with size well above the thermodynamic limit. However, we shall demonstrate that these properties can be calculated without explicitly taking the surfaces into account. Both the foundations and results for real system shall be presented. [Preview Abstract] |
Tuesday, March 19, 2013 11:27AM - 11:39AM |
G21.00002: Are Polarization and Magnetization really Bulk Properties? Raffaello Bianco, Raffaele Resta Microscopic understanding of P and of (orbital) M was achieved only recently; the modern theories express both as BZ integrals. Since k-space is an artificial construct, all bulk properties must be embedded in the ground state density matrix $\rho({\bf r,r'})$, ``nearsighted'' in insulators, independently of the boundary conditions, either periodic (PBCs) or open (OBCs). A basic tenet of the modern theory is that the bulk electron distribution determines P only modulo a ``quantum'': therefore P is not independent of the boundaries. Instead M is not affected by any quantum indeterminacy and an expression in terms of the bulk $\rho$ is not ruled out: we explicitly find such expression. In a finite homogeneous sample, within OBCs, the macroscopic magnetization is cast as a function of the bulk $\rho({\bf r,r'})$. Remarkably, our approach applies even to topological (Chern) insulators, where M explicitly depends on the chemical potential. The boundary currents contribute to M, but even their contribution is ``bulk'' in the above sense; the value of M is robust and cannot be altered by acting on the boundaries only. Instead, P can be varied (by a quantum) by acting on the boundaries only. Simulations performed on a 2d model Hamiltonian within OBCs demonstrate our approach. [Preview Abstract] |
Tuesday, March 19, 2013 11:39AM - 11:51AM |
G21.00003: Noncollinear magnetism and single-ion anisotropy in multiferroic perovskites Eric Bousquet, Carlo Weingart, Nicola Spaldin The link between the crystal distortions of the perovskite structure and the magnetic exchange interaction (J), the single-ion anisotropy (SIA), and the Dzyaloshinsky-Moriya (DM) interaction are investigated by means of density-functional calculations in AFeO$_3$ systems. We explore the effect of the crystal distortions (Antiferrodistortive-AFD and Ferroelectric) as well as the effect of the A-cation chemistry on the 3 magnetic properties, J, SIA and DM. Our analysis shows a never explored but possible switching of the weak ferromagnetism in the $R3c$ phase of BiFeO$_3$ through the competition of the SIA shapes induced by the AFD and the ferroelectric distortions. We also found that-in spite of the d$^5$ electronic configuration of Fe$^{3+}$, the SIA is very large in some structures and is surprisingly strongly sensitive to the chemistry of the A-site cation of the ABO$_3$ perovskite. To clarify the origin of this unexpected effect, we analyze the crystal field splitting by means of Wannier functions. [Preview Abstract] |
Tuesday, March 19, 2013 11:51AM - 12:27PM |
G21.00004: First-principles theory of flexoelectricity Invited Speaker: David Vanderbilt Flexoelectricity is the linear response of polarization to a strain gradient. Because strain gradients break inversion symmetry, flexoelectricity occurs in all insulating crystals. The flexoelectric effect is negligible on conventional length scales, but it can become very strong at the nanoscale where large strain gradients can significantly affect the functional properties of dielectric thin films and superlattices. Previous theories have tended to focus either on the lattice [1-3] or the electronic (i.e., frozen-ion) [4-5] contribution, and have involved some approximations or limitations. Here we develop a general first-principles theory of the flexoelectric tensor, formulated in such a way that the tensor elements can be computed directly in the context of density-functional calculations. Special attention will be paid to several subtleties, including surface contributions, pseudopotential dependence, the calculation of transverse components, fixed $E$ vs.\ fixed $D$ boundary conditions, and a degree of non-uniqueness that is present for some strain gradients. We introduce several practical supercell-based methods for calculating the flexoelectric coefficients from first principles, and demonstrate them by computing the coefficients for a variety of insulating materials.(Work done in collaboration with Jiawang Hong. Supported by ONR N00014-12-1-1035.) \\[6pt][1] A.K. Tagantsev, Phys. Rev. B {\bf 34}, 5883 (1986). \\[0pt][2] R. Maranganti and P. Sharma, Phys. Rev. B {\bf 80},054109 (2009). \\[0pt][3] I. Ponomareva, A.K. Tagantsev, and L. Bellaiche, Phys. Rev. B {\bf 85}, 104101 (2012). \\[0pt][4] R. Resta, Phys. Rev. Lett. {\bf 105}, 127601 (2010). \\[0pt][5] J. Hong and D. Vanderbilt, Phys. Rev. B {\bf 84}, 180101 (2011). [Preview Abstract] |
Tuesday, March 19, 2013 12:27PM - 12:39PM |
G21.00005: First-principles calculations of flexoelectric coefficients Jiawang Hong, David Vanderbilt Flexoelectricity, which is the linear response of polarization to a strain gradient, can have a significant effect on the functional properties of dielectric thin films, superlattices and nanostructures. Despite growing experimental interest, there have been relatively few theoretical studies of flexoelectricity, especially in the context of first-principles calculations. In this talk, we present a complete theory of both the electronic (or ``frozen-ion'')\footnote{J. Hong and D. Vanderbilt, Phys. Rev. B, {\bf 84} 180101(R) (2011).} and lattice contributions to flexoelectricity, and demonstrate a supercell method for calculating the flexoelectric coefficients using first-principles density-functional methods. Results are presented for cubic materials including CsCl and SrTiO$_3$. In order to obtain all the elements of the flexoelectric tensor, transverse as well as longitudinal, we carry out calculations on supercells extended along different orientations (e.g., [110] as well as [100]), taking special care to carry out conversions between objects calculated under fixed E or fixed D electric boundary conditions in different parts of the procedure. In this way, all the elements of both the electronic and lattice contributions to the flexoelectric tensor are determined. [Preview Abstract] |
Tuesday, March 19, 2013 12:39PM - 12:51PM |
G21.00006: Linear Magnetoelectric Effect by Orbital Magnetism Andrea Scaramucci, Eric Bousquet, Michael Fechner, Maxim Mostovoy, Nicola Spaldin The linear magnetoelectric effect is the linear induction of a static magnetization (electric polarization) by an applied static electric (magnetic) field. Using symmetry analysis and ab initio calculations we show that, in addition to mechanisms involving magnetic moments of spins, such an effect can originate from the response of orbital magnetic moment to polar distortions induced by an applied electric field. Considering LiFePO$_4$ as model compound, we show that spin-orbit coupling partially lifts the quenching of 3d orbitals and causes small orbital magnetic moments at the magnetic ions sites. An applied electric field modifies the sizes of these orbital magnetic moments and results in a net magnetization. Furthermore, we discuss the link between this mechanism and the electric field dependence of magnetocrystalline anisotropy. [Preview Abstract] |
Tuesday, March 19, 2013 12:51PM - 1:03PM |
G21.00007: Longitudinal magnetoelectric susceptibility of Cr2O3: First-principles calculations using the converse approach Sai Mu, A. L. Wysocki, K.D. Belashchenko Temperature-dependent longitudinal magnetoelectric (ME) susceptibility of Cr2O3 is calculated as a response of the magnetization M to the applied electric field E. The ionic displacements are found using the calculated force constant matrix and Born effective charges. The exchange parameters are calculated using total energy calculations for different spin configurations on the perturbed lattice, and the magnetization is evaluated using the pair cluster approximation to the quantum spin-3/2 Hamiltonian. When similar approximations are used, the results agree with the inverse approach of Mostovoy et al. [Phys. Rev. Lett. 105, 087202 (2010)]. The electronic contribution is found to be approximately 40{\%} of the ionic contribution and opposite in sign to it. The ME susceptibility is found to depend strongly on the choice of the Hubbard U parameter, increasing as U is increased. On the other hand, the ME response is only weakly depressed by the inclusion of intersite spin correlations within the pair cluster approximation. The methodology developed here can facilitate the search for new materials with desirable ME properties. [Preview Abstract] |
Tuesday, March 19, 2013 1:03PM - 1:15PM |
G21.00008: A theoretical study of the dynamical magnetic charge tensor in crystalline Cr$_2$O$_3$ Meng Ye, David Vanderbilt Magnetoelectric (ME) materials are of fundamental interest and are investigated for their broad potential for technological applications. The search for, and eventually the theoretical design of, materials with large ME couplings present challenging issues. First-principles methods have only recently been developed to calculate the full ME response tensor $\alpha$ including both electronic and ionic (i.e., lattice-mediated) contributions.\footnote{A. Malashevich et al., Phys. Rev. B, {\bf 86}, 094430 (2012).} In several materials, the dominant contribution to the ME response has been shown to be the ionic term $\alpha_{\rm ion}$, which is proportional to both the Born dynamical electric charge $Z^{\rm e}$ and its analogue, the dynamical magnetic charge $Z^{\rm m}$.\footnote{J. \'{I}\~{n}iguez, Phys. Rev. Lett. {\bf 101}, 117201 (2008).} Here we present a theoretical study whose ultimate goal is to understand the mechanisms that would enhance the magnetic charge $Z^{\rm m}$. Using first-principles density-functional methods within a relativistic framework with the inclusion of the spin-orbit interaction, we calculate the atomic magnetic charge tensors $Z^{\rm m}$ for both Cr and O atoms in Cr$_2$O$_3$, and discuss how these contribute to the ME response in this material. [Preview Abstract] |
Tuesday, March 19, 2013 1:15PM - 1:27PM |
G21.00009: Ferroelectricity induced by interatomic magnetic exchange interaction Chungang Duan, Xiangang Wan, Hang-Chen Ding, Sergey Y. Savrasov Multiferroics, where two or more ferroic order parameters coexist, is one of the hottest fields in condensed matter physics and materials science. To search multiferroics, currently most researches are focused on frustrated magnets, which usually have complicated magnetic structure and low magnetic ordering temperature. Here, we argue that actually simple interatomic magnetic exchange interaction already contains a driving force for ferroelectricity, thus providing a new microscopic mechanism for the coexistence and strong coupling between ferroelectricity and magnetism. We demonstrate this mechanism by showing that even the simplest antiferromagnetic (AFM) insulator MnO, can display a magnetically induced ferroelectricity under a biaxial strain. [Preview Abstract] |
Tuesday, March 19, 2013 1:27PM - 1:39PM |
G21.00010: Domain wall fluctuations in ferroelectrics Richard Brierley, Peter Littlewood Ferroelectric domain walls typically have 90- or 180-degree orientations due to the long-range constraints of dipolar and ferroelastic interactions. We calculate the excitation spectrum for deviations from ideal flat walls in these orientations. In the presence of ferroelastic interactions, fluctuations in the polarization orientation must be matched by changes in local strain. The finite acoustic phonon velocity implies a retarded response of the strain fields. This retardation produces a gap as $k\to 0$, limiting the domain wall motion. [Preview Abstract] |
Tuesday, March 19, 2013 1:39PM - 1:51PM |
G21.00011: Theory of Relaxor Ferroelectric Electrocalorics Gian Guzman-Verri, Peter Littlewood Conventional perovskite ferroelectrics are the material of choice in many modern day technologies such as capacitive energy storage devices, infrared sensors, and random access memories. Conventional ferroelectrics, however, have not been exploited in cooling applications mainly because their narrow region of critical fluctuations of polarization results in a small electrocaloric effect (a few miliKelvin per volt). Relaxor ferroelectrics, on the other hand, exhibit a broad region of critical fluctuations which makes them promising candidates for large electrocalorics. In this talk, we present a theoretical study of electrocalorics in relaxor ferroelectrics. We compute isothermal changes in entropy and adiabatic changes in temperature within a model of polarizable unit cells with local short-range forces, dipolar forces, and compositional disorder. [Preview Abstract] |
Tuesday, March 19, 2013 1:51PM - 2:03PM |
G21.00012: ABSTRACT WITHDRAWN |
Tuesday, March 19, 2013 2:03PM - 2:15PM |
G21.00013: Dynamical mean field studies on mid-gap states of SrTiO$_3$ Chungwei Lin, Alexander A. Demkov We study the mid-gap states obtained by photon luminescence of SrTiO$_3$ using dynamical mean field theory. The 2.4 eV peak observed in the SrTiO$_3$ luminescence experiment is attributed to the strong electron-optical phonon coupling when an electron is excited from oxygen 2p bands to titanium 3d bands, and is conventionally modeled by Franck-Condon model which contains only one fermion and one phonon field. Here we extend this model to a realistic lattice described by the tight-binding approximation, using dynamical mean field theory with exact diagonalization solver. We found the main features of Franck-Condon model preserve. The effects of correlation on oxygens will be discussed. [Preview Abstract] |
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