Session X53: Multiferroics, Magnetoelectrics, Spin-Electric Coupling, and Ferroelectrics -5

Live

We apply a microscopic theory of polarization and magnetization to crystalline insulators at zero temperature, and consider the orbital electronic contribution of the linear response to non-uniform and non-static electromagnetic fields. These microscopic fields can be decomposed into “site” polarization and magnetization fields, from which “site multipole moments” can be constructed. Macroscopic polarization and magnetization fields follow, and we identify the relevant contributions to them. Within the independent particle and frozen-ion approximations, a description of optical activity and related magneto-optical phenomena then immediately follows; both optical rotary dispersion and circular dichroism can be described with this strategy. Earlier expressions describing the magnetoelectric effect are recovered as the zero frequency limit of our more general equations. Since our site quantities are introduced with the use of Wannier functions, the site multipole moments and their macroscopic analogs are generally gauge dependent. However, the resulting macroscopic charge and current densities, together with the optical effects to which they lead, are gauge invariant, as would be physically expected.   

Live

Spin spirals (SS) are a special case of non-collinear magnetic ordering, where the magnetic-moment direction rotates along an axis. Such materials have potential applications in spintronics and information technology. The presence of SS ordering may reduce the symmetry of the electron density and induce a spontaneous electrical polarization (ferroelectricity, FE) without atomic displacements. Research on SS-FE coupling has been on multilayered/bulk materials and artificial metal-atom chains, but not on monolayer (ML) two-dimensional (2D) materials. Using density functional theory, we demonstrate that SS ordering, found in bulk FeOCl, also exists in ML FeOCl whose synthesis was recently reported. The propagation wavelength and energetic stability of the SS can be tuned by electronic doping and uniaxial strain. Relative to the FM state, the spin-spiral state’s bandgap increases in both bulk and ML FeOCl by ~0.6 eV, enabling bandgap engineering through magnetism manipulation. Moreover, the SS induces out-of-plane FE in ML FeOCl, switchable via the spiral’s chirality. Finally, forming a heterostructure, e.g., with graphene or boron nitride, is another way to modulate and tune the SS ordering.   

Live

Strong coupling of light and matter has been a fruitful research topic in condensed matter physics in the last years. Inside an optical cavity, some effects of strong coupling can appear even "in the dark", i.e. without any external drive but using only the fluctuations of the eigenmodes of the cavity. Our project aims to modify physical properties of functional materials using this phenomenon.

A recent theoretical work by Y. Ashida et al. (arXiv:2003.13695) suggests that an infrared cavity could enhance the phase transition temperature of a ferroelectric material, as well as induce ferroelectricity in "quantum paraelectric" materials such as SrTiO₃. It proposes a plane-parallel cavity with two metallic mirrors in direct contact with the functional material layer. This geometry is identical to that of a parallel-plate capacitor. The predicted magnitude of the effect depends on the plasma frequency of the metal layers.

We will present dielectric measurements performed on SrTiO₃ single-crystals as well as on ferroelectric (Pb,Sr)TiO₃ epitaxial thin films using electrodes with different plasma frequencies, and discuss the relevance of our results in the context of light-matter coupling.   

Live

Domain walls are a trade-off between the energy cost of the wall and the energy gain for domain formation. They break translational symmetry and can exhibit fascination properties absent in the bulk parent structure. In the pure ferroelastic CaTiO3 the order parameter is the strain defined by the oxygen octahedral tilts in the unit cell. At the domain walls, one octahedral tilt goes to zero, allowing the central Ti cation to off-center, creating polarity. We first map the polar domain walls using low energy electron microscopy (LEEM). Then, using electron energy loss spectroscopy in the LEEM we reveal band gap narrowing of up to 0.5 eV in the domain walls. The narrowing may be due to the intrinsically different symmetry [1] and/or the presence of oxygen vacancies [2] in the walls. The intrinsic modification of the electronic structure within the wall is compared with density functional theory calculations and opens up perspectives for engineering of domain wall functionality.

[1] G.F. Nataf et al., Phys. Rev. Mater. 1, 074410 (2017)
[2] L. He, D. Vanderbilt, Phys. Rev. B 68, 1341031 (2003)   

Live

Due to their large number of properties, such as switchable polarization, piezoelectricity or optical absorption, ferroelectrics are particularly promising in order to develop the next generation of electronic microdevices. Among these ferroelectrics, lead zirconium titanate (PZT) has been widely investigated for its high electromechanical properties and more recently for its light-induced deformation¹, commonly named photostriction, combining simultaneously bulk photovoltaic and converse piezoelectric effects². Although this phenomenon was discovered in multiple ferroelectrics materials these last decades³, the photostriction is not fully understood. In this work, the interplay between the electric polarization and the photoinduced strain is investigated in PZT epitaxial-based cantilevers. For this purpose, the resonance frequencies of the cantilevers and their deflection were studied under UV illumination, through electrical measurements and optical profilometry respectively. The results have shown that the sign of both the photoinduced shift of resonance frequency and photoinduced deflection strongly depend on the polarization state in the ferroelectric thin film.
¹S. M.,doi: 10.1002/aelm.201800709.
²C. P.,doi: 10.1103/PhysRevLett.116.247401.
³B. K.,doi: 10.1063/1.4905505.   

Live

Ferroelectricity with out-of-plane polarization has so far been found in several two-dimensional (2D) materials, including monolayers comprising three to five planes of atoms (MoTe₂, In₂Se₃). Here, we explore creating the first single-atom-thick monolayer ferroelectric using hexagonal boron nitride. We performed density-functional-theory calculations to explore inducing ferroelectricity through incorporation of isovalent substitutional impurities that are larger than the host atoms. This disparity in bond lengths causes a buckling of the h-BN, either up or down, which amounts to a dipole with two equivalent orientations. We tested several impurities and optimized the centrosymmetric and distorted structures. Using these structures, we then determined the magnitude of the induced dipole and the switching energy barrier for dipole inversion. The effects of strain and vertical heterostructures with graphene were further explored. We are exploring how dense the impurities can be placed within the h-BN while maintaining sufficiently localized distortions and dipoles. We expect that this work will help foster new ways to include functionality in layered 2D-material-based applications.   

Live

Recently, interest in low-dimensional ferroelectric materials has grown rapidly across multiple scientific and engineering disciplines. Here, a serial group-VI oxyhalides MO_3-nX_2n is studied based on density functional theory calculations. For n=1, the dioxydihalides display quasi 2D properties with weak vdW interaction between layers. Each layer is predicted to present noncollinear ferrielectricity, induced by competing ferroelectric and antiferroelectric softmodes [1]. This intrinsic noncollinearity of dipoles generates unique physical properties, such as Z₂×Z₂ topological domains and atomic-scale dipole vortices [1]. For n=2, the dioxytetrahalides display quasi 1D characteristics. The robust ferroelectric distortion within each chain and weak vdW coupling between chains make them candidates for applications as high-density non-volatile memories [2]. Our investigations should open the door to a new branch of low-dimensional materials in the pursuit of intrinsically strong noncollinear ferrielectricity and high-performance functional materials.

[1] L.-F. Lin, et al., Phys. Rev. Lett. 123, 067601 (2019).
[2] L.-F. Lin, et al., Phys. Rev. Materials. 3, 111401(R) (2019).   

Live

The ground state of multiferrioc BiFeO₃ has rhombohedral structure with space group R3c. Despite decades of research its band gap remains a subject of debate. Reported band gaps vary over a wide range of 2.5 – 3.1 eV, the reasons for which are yet unclear. Using hybrid density functional theory calculations, we find a band gap of 3.4 eV for R3c BiFeO₃, which is slightly larger than a recent reported band gap of 3.1 eV for BiFeO₃ single crystals based on optical absorption and photoluminescence measurements. We also computed the effects of the ferroelectric displacement and the antiferrodistortive rotations of the FeO₆ octahedra on the band edge positions and the band gap, going from the ferroelectric ground-state R3c structure, to a nonpolar R-3c structure containing only the octahedral rotations, and to cubic Pm-3m structure. We find the band gap decreases from 3.4 eV for the R3c, to 2.9 eV for the R-3c, and to 1.6 eV for the cubic BiFeO₃, by significant lowering of the conduction band and slight raising of the valence band. We also find that the Bi 6s-O 2p coupling notably increases with the ferroelectric displacement, not with the octahedral rotations. Our results explain the large variation of the reported band gaps between single crystals and partially relaxed thin films.   

Live

Ta₂NiSe₅, an excitonic insulator candidate, exists in a novel broken symmetry phase below 327K. Strong electron-phonon interaction possibly due to an excitonic instability leads to a structural transition in this material into a ferro-rotational ordering of dipoles. Such an ordering, which preserves both spatial inversion and time-reversal symmetries, has been difficult to measure due to low coupling to dipolar fields. The second harmonic (SHG) signal arising from higher-order couplings with the optical field for such a system remains low. The effect of such an order on the optoelectronic transport properties of the system is investigated in our work. We use circular photogalvanic effect (CPGE), and by eliminating any lower-order contributions using symmetries of the system, show that ferro-rotational order can be directly probed. A phenomenological model will be discussed to understand the CPGE response along with the implications of the ferro-rotational order.   

Live

CuO is unique among insulating transition-metal monoxides in possessing monoclinic crystal structure and complex magnetic phases. It received renewed attention in the past decade for multiferroicity^a in its spiral antiferromagnetic phase (213 K < T < 230 K) and quasi-one-dimensional magnetism at higher temperatures.^b We will discuss the temperature and electric field dependent thermal conductivity (κ) measured on high-quality single crystals^c along the principal crystallographic axes. Modest electric fields (up to ~5 kV/cm) were applied along the [010] direction, transverse to the heat flow. A remarkably strong reduction of κ (up to 60% near room temperature) is associated with charge flow from a very small density of charge carriers, correlating with the sample conductivity. Possible mechanisms for this effect will be discussed.

^a T. Kimura et al., Nat. Mater. 7, 291 (2008).
^b H. Jacobsen et al., Phys. Rev. B 97, 144401 (2018).
^c A. Rebello, Z. C. M. Winter, S. Viall, and J. J. Neumeier, Phys. Rev. B 88, 094420 (2013).   

Live

Remote control of magnetic order can be achieved through opto-magnetic effects, such as the inverse Faraday and inverse Cotton-Mouton effects. Here, we theoretically describe phonon analogs of these effects, in which coherently excited infrared-active phonons replace photons in the scattering process, which is mediated through spin-phonon coupling. We compare the strengths of the opto- and phono-magnetic effects in antiferromagnetic NiO, in which we find them to generate comparable magnitudes of effective magnetic fields acting on the spins. We further investigate the phonon inverse Faraday effect in paramagnetic CeCl₃, in which extraordinarily strong spin-phonon coupling has been observed. We predict that giant effective magnetic fields of up to 100 T that align the paramagnetic spins can possibly be generated through coherent phonon excitation with experimentally achievable pulse strengths. Our results show that phono-magnetic effects emerge as promising tools for ultrafast spin control at terahertz frequencies.   

Not Participating

Hexagonal LuFe₂O₄ and LuFeO₃ based systems have recently come under the limelight of scientific interest and curiosity following the observation of near room temperature multiferroic and magnetoelectric phenomena in their superlattices.¹ The origin of these phenomena lies within the hole-doped LuFe₂O₄ layer^2,3, which is contingent on various microscopic factors, such as geometric frustration, spin-orbit coupling and charge ordering. These factors vary in degrees with the variation in the doping concentration in the system and this in turn is expected to contribute to the manifestation of interesting quantum phenomena. We, therefore, have studied the bulk LuFe₂O₄ under the influence of electron-hole doping to gain further insight into the interesting quantum phenomena in these systems.

1. Mundy, J. A. et al. Nature 537, 523 (2016).
2. Das, H. et al. Nat. Commun. 5, 2998 (2014).
3. Fan, S., Das, H. et al. Nat. Commun. 11, 5582 (2020)   

Live

Over the last decade, Fe₂Mo₃O₈ has attracted intensive attention due to its giant magnetoelectricity effect which is a consequence of the field dependent magnetic ground state (antiferromagnetic versus ferrimagnetic). However, the microscopic mechanism of magnetoelectricity is currently highly debated. To shed new light in this direction, we have investigated the electronic and magnetic properties of Fe₂Mo₃O₈ by element-selective X-ray absorption spectroscopy (XAS) and X-ray magnetic circular dichroism spectroscopy (XMCD). Our measurements show that the iron ions in both octahedral and tetrahedral sites are in the Fe²⁺ configuration, which agrees with first principle calculations. XMCD measurements at Fe-L_2,3 and Mo-L_2,3 edges respectively reveal large magnetic moments at the Fe²⁺ sites and no net-moment on the Mo sites. Full-multiplet calculations of XMCD and crystal field theory show the existence of large spin (~3.5m_B) and orbital (~1m_B) moments for Fe²⁺ ions at both sites and in both magnetic phases, with the total magnetic moment larger on the octahedral site. The existence of large orbital moments implies the importance of the interplay between the spin-orbit coupling, the structural distortions, and the magnetic field to the microscopic mechanism of magnetoelectricity.