Perfect Softening of the Ferroelectric Mode in the Isotope-Exchanged Strontium Titanate ofSrTiO318Studied by Light Scattering

Abstract

The E u mode of the isotope-induced ferroelectric strontium titanate SrTi 18 O 3 shows a perfect softening at the ferroelectric phase transition temperature T c , where the frequency of the underdamped mode approaches completely to zero within the instrumental resolution. The spectra of the Raman inactive soft E u mode have been successfully observed owing to local symmetry breaking and by long-term accumulation of the spectral intensity with a high resolution technique. The mechanism of the phase transition is concluded to be an ideal displacive-type accompanied with perfect softening of the Slater-type polar E u mode. The difference between the soft mode behavior of The perovskite oxide strontium titanate SrTiO 3 has been considered to belong to the typical quantum paraelectrics, because it keeps paraelectricity until T 0 K by the quantum fluctuation that might depress the occurrence of the ferroelectric order [1]. Indeed, at low temperatures below 4 K, the dielectric constant increases considerably but remains finite even near 0 K without any divergent behavior. The dynamics of the ferroelectric soft mode in the quantum paraelectrics also shows no freezing phenomenon, staying with the finite phonon frequency until 0 K according to the Lyddane-Sachs-Teller relationship [2,3]. Since the effect of quantum fluctuation was first suggested in Ref.[1], many intensive studies have been accumulated to elucidate the dynamics of the quantum fluctuation and also the soft mode behavior related to the quantum fluctuation in SrTiO 3 and related materials. There are two types of soft modes in the low frequency region of SrTiO 3 , as listed in the phonon correlation table [4]: the ferroelectric modes E u , A 2u [2 -4] described above and the A 1g , E g modes related to the structural phase transition [5].Recently, the ferroelectric phase transition in SrTiO 3 was discovered at 23 K by exchange of 16 O in SrTiO 3 to its isotope 18 O [6]. Since then, the phase transition mechanism of the isotope-exchanged strontium titanate SrTi 18 O 3 (hereafter abbreviated as STO18 and the ordinary one as STO16) has attracted the intense interest of many researchers in order to elucidate its phase transition mechanism related to the quantum fluctuations. The lowest energy phonon mode in STO18 has been searched for and analyzed to elucidate the soft mode dynamics near T c by many authors [7][8][9][10][11]. However, any clear softening behavior has not been reported: By Raman scattering studies, the observation of the Raman inactive soft E u mode has been tried, but the assignment of certain spectral feature to the E u mode has not been convincing and unambiguous. Inelastic neutron scattering has revealed the temperature dependence of the soft mode of

Quantum Effects of Phase Transitions in Ferroelectric Crystals—Experiments and Models

“…24-32cm -1 [11], and the latter is consistent with 13 cm -1 [20]. Our theory supports the complete softening of the phonon mode observed in the Raman spectrum [20].…”

Quantum critical paraelectrics and the Casimir effect in time

“…We are particularly grateful to H. Chamati and to N.S. Tonchev for pointing us to an error in the numerical analysis of equation (41) in an earlier version of this paper. We also acknowledge financial support from the National Science Foundation nsf-dmr 0645461 (L.Palova), NSF-NIRT-ECS-0608842 (P. Chandra) and the Department of Energy, grant DE-FE02-00ER45790 (P. Coleman).…”

Temperature‐dependent Raman scattering study of multiferroic MnWO4

“…As the temperature is lowered from 180 K, these modes shift to lower wavenumbers unlike the conventional phonons. This anomalous behavior could be related to a phase transition at the magnetic or electronic transition temperature, [15,16] a structural phase transition, [17] contraction of the lattice. [18] However, in MnWO 4 , the magnetic and electronic transition temperatures are well below 180 K. In addition to the phonon softening, we have observed that one phonon mode (mode 5) disappeared above 180 K. Therefore, the phonon softening in our sample could be related with microscopic structured change.…”