Recently, CuO has been proposed as a potential multiferroic material with high transition temperature. Competing models based on spin current and ionic displacements are invoked to explain ferroelectricity in CuO. The theoretical model based on ionic displacement predicted very small displacement (∼10−5 Å) along the b axis. Experimentally detecting displacements of such a small amplitude in a particular direction is extremely challenging. Through our detailed angle resolved polarized Raman spectroscopy study on single crystal of CuO, we have validated the theoretical study and provided direct evidence of displacement along the b axis. Our study provides important contribution in the high temperature multiferroic compounds and showed for the first time, the use of the polarized Raman scattering in detecting ionic displacements at the femtometer scale.
The so-called ferroaxial transition characterized by a rotational structural distortion that breaks a mirror symmetry has gained growing interest in terms of a new class of ferroic state in crystalline materials. RbFe(MoO 4 ) 2 , which belongs to glaseritetype compounds, X (□;1) Y (□;2) [M(TO 4 ) 2 ], is one of the most representative materials showing a ferroaxial transition, i.e., ferroaxial materials. Considering a variety of glaserite-type compounds, we expect that they provide a good arena for ferroaxial materials. In this work, we explored new ferroaxial materials by formula-based screening using a regular expression search and the symmetry detection algorithm. As a result, we found that a glaserite-type compound, K 2 Zr(PO 4 ) 2 , is one of the promising candidates for ferroaxial materials. Experimentally, we demonstrate that K 2 Zr(PO 4 ) 2 shows a ferroaxial transition at about 700 K, which is well explained by ab initio phonon calculations. The ferroaxial nature of K 2 Zr(PO 4 ) 2 is further confirmed by the observation of its domain structures using a linear electrogyration effect, i.e., optical rotation in proportion to an applied electric field. Our work provides an effective approach to exploring ferroaxial materials.
Circular dichroism observed by resonant x-ray diffraction at the Cu L 3 edge was investigated in a chiral antiferromagnet, Pb(TiO)Cu 4 (PO 4 ) 4 , which shows magnetic quadrupole order (T N = 7 K). At temperatures above T N , space-group-forbidden reflection 100 is observed due to the anisotropic tensor of susceptibility (ATS) scattering. With decreasing temperature, the reflection intensity shows substantial circular dichroism below T N . We found one-to-one correspondence between the sign of the circular dichroism and that of the sample's crystallographic chirality. In addition, the reflection intensity depends on the product of poling magnetic and electric fields. The circular dichroism observed in this study is interpreted as the interference between the ATS and the magnetic scatterings. This finding shows that the interference scattering probes both the chirality and the order parameter of the magnetic quadrupole order in this chiral antiferromagnet.
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