Influence of rotational instability on the polarization structure of SrTiO3

📝 Abstract

The k-space polarization structure and its strain response in SrTiO3 with rotational instability are studied using a combination of first-principles density functional calculations, modern theory of polarization, and analytic Wannier-function formulation. (1) As one outcome of this study, we rigorously prove-both numerically and analytically-that folding effect exists in polarization structure. (2) After eliminating the folding effect, we find that the polarization structure for SrTiO3 with rotational instability is still considerably different from that for non-rotational SrTiO3, revealing that polarization structure is sensitive to structure distortion of oxygen-octahedra rotation and promises to be an effective tool for studying material properties. (3) Furthermore, from polarization structure we determine the microscopic Wannier-function interactions in SrTiO3. These interactions are found to vary significantly with and without oxygen-octahedra rotation.

💡 Analysis

The k-space polarization structure and its strain response in SrTiO3 with rotational instability are studied using a combination of first-principles density functional calculations, modern theory of polarization, and analytic Wannier-function formulation. (1) As one outcome of this study, we rigorously prove-both numerically and analytically-that folding effect exists in polarization structure. (2) After eliminating the folding effect, we find that the polarization structure for SrTiO3 with rotational instability is still considerably different from that for non-rotational SrTiO3, revealing that polarization structure is sensitive to structure distortion of oxygen-octahedra rotation and promises to be an effective tool for studying material properties. (3) Furthermore, from polarization structure we determine the microscopic Wannier-function interactions in SrTiO3. These interactions are found to vary significantly with and without oxygen-octahedra rotation.

📄 Content

arXiv:1009.4689v1 [physics.comp-ph] 23 Sep 2010 Influence of rotational instability on the polarization structure of SrTiO3 Yanpeng Yao and Huaxiang Fu Department of Physics, University of Arkansas, Fayetteville, AR 72701, USA (Dated: November 19, 2018) Abstract The k-space polarization structure and its strain response in SrTiO3 with rotational instability are studied using a combination of first-principles density functional calculations, modern theory of polarization, and analytic Wannier-function formulation. (1) As one outcome of this study, we rigorously prove—both numerically and analytically—that folding effect exists in polarization structure. (2) After eliminating the folding effect, we find that the polarization structure for SrTiO3 with rotational instability is still considerably different from that for non-rotational SrTiO3, re- vealing that polarization structure is sensitive to structure distortion of oxygen-octahedra rotation and promises to be an effective tool for studying material properties. (3) Furthermore, from po- larization structure we determine the microscopic Wannier-function interactions in SrTiO3. These interactions are found to vary significantly with and without oxygen-octahedra rotation. PACS numbers: 77.80.-e, 77.84.-s, 77.22.Ej 1 I. INTRODUCTION Materials with perovskite or perovskite-based structures have attracted wide attention for their varieties of properties in ferroelectricity, magnetism, superconductivity, etc. In these perovskite solids, an important class of structural distortions that deviate them from ideal (cubic) perovskites is the rotation of the oxygen octahedra, which may considerably alter the physical and chemical properties of materials. For instance, rotation of oxygen octahedra in incipient ferroelectric SrTiO3 was found to couple strongly with tetragonal strain, and consequently, affect significantly the structural and dielectric behaviors below 105K.[1, 2] Octahedral rotation also plays a key role in improper ferroelectrics such as YMnO3, where structural instability due to zone-boundary phonon mode couples with elec- tric polarization[3], and this coupling was recently shown to exhibit an interesting absence of critical thickness in improper-ferroelectric ultrathin films.[4] On magnetism, Mazin and Singh have revealed that one main reason responsible for the drastically different magnetic properties in two chemically similar compounds CaRuO3 and SrRuO3 (where CaRuO3 is paramagnetic, but SrRuO3 is ferromagnetic) consists in a larger rotation angle of RuO6 octahedra in CaRuO3.[5] Similarly, Fang and Terakura found that perovskite superconduc- tor Sr2RuO4 could be turned from non-magnetic to ferromagnetic by simply increasing the rotation angle of the RuO6.[6] Recently, Ray and Waghmare have shown that rotation is the predominant structural instability in a series of biferroics (LuCrO3, YCrO3, LaCrO3, BiCrO3, and YCrO3), and is closely correlated with their magnetic properties.[7] Based on the facts that (1) rotation of oxygen octahedra affects various material properties, and (2) the oxygen-octahedra rotation is common, understanding of the physics caused by oxygen rotation is thus important. The modern theory of polarization[8, 9] has become important in quantitatively determin- ing the electric and dielectric properties of materials within first-principles density functional calculations. According to this theory[8, 9], the change of electric polarization is a physi- cally meaningful quantity and the electronic contribution is determined by an integration of the geometric phase of the occupied multi-fold electron Bloch wavefunctions. Based on the modern theory of polarization, Yao and Fu recently introduced the concept of “polar- ization structure”[10] which may provide a new scheme to study the electronic properties of dielectric materials, and they went on to formulate a theory using localized Wannier func- 2 tions to investigate the polarization dispersion structure of ferroelectrics with spontaneous polarization. More specifically, polarization structure describes the electronic phase angle φ as a function of ⃗k⊥point which lies on a ⃗k-plane perpendicular to the direction of the con- sidered polarization component. At each ⃗k⊥, the phase angle is determined by the electron wavefunctions as[8, 9] φ(⃗k⊥) = i M X n=1 Z G∥ 0 d⃗k∥⟨un⃗k| ∂ ∂k∥ |un⃗k⟩, (1) where subscripts ∥and ⊥indicate, respectively, the directions parallel and perpendicular to the direction of the polarization. un⃗k is the Bloch part of electron wave function, and M is the number of occupied bands. As significant as electronic band structure, polarization structure reveals how individual strings of different ⃗k⊥s contribute to the electronic polar- ization, which thus offers microscopic insight on the polarization properties. Polarization structure can be changed by applied electric fields, in a similar spirit as electronic state in band structure can be changed by photoexcitation. More discussion

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