Infrared reflectivity spectra of cubic SrMnO 3 ceramics reveal 18% stiffening of the lowest-frequency phonon below the antiferromagnetic phase transition occurring at T N = 233 K. Such a large temperature change of the polar phonon frequency is extraordinary and we attribute it to an exceptionally strong spin-phonon coupling in this material. This is consistent with our prediction from first-principles calculations. Moreover, polar phonons become Raman active below T N , although their activation is forbidden by symmetry in the P m3m space group. This gives evidence that the cubic P m3m symmetry is locally broken below T N due to a strong magnetoelectric coupling. Multiphonon and multimagnon scattering is also observed in Raman spectra. Microwave and THz permittivity is strongly influenced by hopping electronic conductivity, which is caused by small nonstoichiometry of the sample. Thermoelectric measurements show room-temperature concentration of free carriers n e = 3.6 × 10 20 cm −3 and the sample composition Sr 2+ Mn 4+ 0.98 Mn 3+ 0.02 O 2− 2.99 . The conductivity exhibits very unusual temperature behavior: THz conductivity increases on cooling, while the static conductivity markedly decreases on cooling. We attribute this to different conductivity of the ceramic grains and grain boundaries.

Crystal of GaV 4 S 8 , a multiferroic system hosting a Néel-type skyrmion lattice phase, has been investigated by polarized Raman and IR spectroscopy above and below the ferroelectric phase transition. Counts of the observed IR and Raman-active modes belonging to distinct irreducible representations agree quite well with group-theory predictions. Phonon spectra are assigned and interpreted with the aid of ab initio calculations of the phonon spectra in the ferroelectric phase. Results allow appreciation of phonon frequencies of the modes involved in Jahn-Teller distortion and their contribution to the spontaneous polarization. DOI: 10.1103/PhysRevB.94.060104 The ternary chalcogenide GaV 4 S 8 (GVS) has recently attracted considerable attention due to its multiferroic properties. First of all, the low-temperature zero-field phase of GVS is simultaneously ferroelectric and ferromagnetic up to about T FM ≈ 5 K. Its magnetization corresponds to S = 1/2 per formula unit and its electric polarization is also appreciable (P s ≈ 1 μC cm −2 ). Moreover, clear experimental evidence has been found for a sizable magnetoelectric coupling [1]. Secondly, GVS exhibits an interesting magnetic cycloidal order between T FM and T N = 13 K, which can be transformed by a moderate magnetic field (of about 40 mT) into a Néel-type skyrmion lattice (SkL) phase [2]. As predicted in the seminal works of Bogdanov et al. [3][4][5] the important prerequisite for the appearance of this very unusual spin texture (Néel-type SkL) is the polar macroscopic symmetry of the underlying crystal lattice. In GVS, the suitable C 3v symmetry emerges together with the ferroelectric polarization below the JahnTeller phase transition T JT = 42 K [6,7]. This unusual phase transition was shown to be driven by a strong electron-phonon coupling between the unpaired electron in the highest-energy, orbitally degenerate electronic state of the V 4 cluster and two corresponding Jahn-Teller-active zone-center modes of the noncentrosymmetric paraelectric structure of GVS [8].The aim of this Rapid Communication is to explore the role of the zone-center phonon modes in the phase transition using Raman and IR spectroscopy and the ab initio approach. In particular, we have (i) determined the spectrum of zonecenter phonon modes of GVS, (ii) identified two phonon modes primarily involved in the phase transition, and (iii) related these findings to the Jahn-Teller phase transition and dielectric properties of GVS.GVS is a lacunar spinel, i.e., it can be presented as a spinel structure lacking every second Ga atom. It can be viewed as a face-centered cubic (fcc) lattice formed of * hlinka@fzu.cz tetrahedral GaS 4 and cubane V 4 S 4 clusters [see Fig. 1(a)]. The paraelectric phase of GaV 4 S 8 has a noncentrosymmetric cubic (T d ) symmetry [6]. The hybridization of the valence states of the vanadium metal ions within the V 4 S 4 magnetic building blocks leads to one unpaired localized electron occupying a triply degenerate t 2 cluster orbital with an overall spin 1/2. The Ga...

The combination of strain and electrostatic engineering in epitaxial heterostructures of ferroelectric oxides offers many possibilities for inducing new phases, complex polar topologies, and enhanced electrical properties. However, the dominant effect of substrate clamping can also limit the electromechanical response and often leaves electrostatics to play a secondary role. Releasing the mechanical constraint imposed by the substrate can not only dramatically alter the balance between elastic and electrostatic forces, enabling them to compete on par with each other, but also activates new mechanical degrees of freedom, such as the macroscopic curvature of the heterostructure. In this work, an electrostatically driven transition from a predominantly out‐of‐plane polarized to an in‐plane polarized state is observed when a PbTiO3/SrTiO3 superlattice with a SrRuO3 bottom electrode is released from its substrate. In turn, this polarization rotation modifies the lattice parameter mismatch between the superlattice and the thin SrRuO3 layer, causing the heterostructure to curl up into microtubes. Through a combination of synchrotron‐based scanning X‐ray diffraction imaging, Raman scattering, piezoresponse force microscopy, and scanning transmission electron microscopy, the crystalline structure and domain patterns of the curved superlattices are investigated, revealing a strong anisotropy in the domain structure and a complex mechanism for strain accommodation.