Raman Spectra in Ferroelectric SrTi 18 O 3

Abe, Kohji; Yamashita, Keiji; Tomita, Yoshiharu; Shigenari, Takeshi; Wang, Ruiping; Itoh, Mitsuru

doi:10.1080/713716289

Cited by 17 publications

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“…2 show the soft mode behavior expected in a typical displacive-type mechanism of a ferroelectric phase transition. The present results are completely different from the previous ones observed above 5 cm ÿ1 in STO18 [7][8][9][10][11]. The damping constant E u keeps a small value even just above T c .…”

Section: Prl 96 227602 (2006) P H Y S I C a L R E V I E W L E T T E contrasting

confidence: 56%

“…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.…”

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confidence: 99%

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

2006

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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

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Section: Prl 96 227602 (2006) P H Y S I C a L R E V I E W L E T T E contrasting

confidence: 56%

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confidence: 99%

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

2006

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“…6,7 Yamanaka et al also supported the appearance of ferroelectricity by the measurement of optical birefringence. 8 Wang and Itoh determined the temperature T m at which the dielectric constant takes a maximum value to be 30.4(xϪ0.33) 1/2 for SrTi( 16 3 samples with an exchange rate x. 9 The ferroelectric phase is thought to appear below T m .…”

Section: Introductionmentioning

confidence: 99%

“…2 This phenomenon is thought to be a quantum effect; the quantum fluctuation ͑zero-point vibration͒ destroys the ferroelectric long-range order and the phase remains paraelectric down to 0 K. In this quantum paraelectric phase, it is well known that the ferroelectric phase can be induced by applying small disturbances: the application of an electric field, 3 that of uniaxial stress, 4 and the doping of polar impurities Ca 2ϩ . 5 In 1999, Itoh et al found a new kind of disturbance to induce ferroelectricity: the substitution of 18 O for 16 O in STO16. 6 The realization of ferroelectricity in SrTi 18 O 3 ͑STO18͒ was confirmed by the observation of DE hysteresis loop and supported by the measurement of pyroelectricity.…”

Section: Introductionmentioning

confidence: 99%

Dynamical mechanism of the ferroelectric phase transition ofSrTi18O3studied by light scattering

et al. 2003

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A light scattering experiment has been performed on two SrTi( 18 O x 16 O 1Ϫx ) 3 single crystal plates with x ϭ0.86 ͑STO18-86͒ and xϭ0.53 ͑STO18-53͒. Each of the c 44 pure transverse acoustic mode and the antiferrodistortive E g mode splits into two modes below T c ; T c ϭ27Ϯ1 K for STO18-86 and T c ϭ20Ϯ2 K for STO18-53. These splits strongly imply that the crystal symmetry becomes lower than tetragonal one below T c , and that the phase transition really occurs at T c . Moreover, the ferroelectric E u soft mode is found above T c , but its softening is not complete. This fact indicates that the phase transition mechanism of SrTi( 18 O x 16 O 1Ϫx ) 3 is of displacive type ͑the instability of the E u mode causes the ferroelectric phase transition͒, but is not the ideal one. The displacive character is found to become stronger with decreasing exchange rate x in the range xϾ0.33.

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“…Heat anomaly due to ferroelectric phase transition at T c has not been detected at the present time, perhaps because the atom's displacement is too small to produce a detectable heat anomaly. Plausible reasons for the existence of a clear phase transition are that (1) transverse acoustic c 44 mode splits into two modes at T c [27][28][29], (2) Raman spectra indicate ferroelectric soft mode above and below T c [23,30], (3) linear birefringence [26] measured along [001] tetra shows a clear evidence of biaxiality of the crystal below T c , and (4) pressure dependence of T c (Fig. 7) suggests the existence of phase transition (dT c /dp ≈ ∞, at T = 0 K).…”

Section: Resultsmentioning

confidence: 98%

Dielectric Properties of SrTi 18 O 3

et al. 2003

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Oxygen isotope exchange for quantum paraelectric SrTiO 3 induces ferroelectricity whose T c is given by T c = 30.4(x − 0.33) 1/2 for SrTi( 16 O 1−x 18 O x ) 3 . Small frequency dispersion of , large amplitude and bias dependences of ε , polarization above T c for poled samples, and time relaxation of polarization below T c as well as the results of optical measurements by Raman, Brillouin and optical birefringence conclude that properties of oxygen-isotope exchanged strontium titanate resemble to those of Ca-doped samples, i.e., domain state. Difference between oxygen-isotope exchanged and Ca-doped strontium titanate was observed only in the pressure dependence of T c , which may be able to the difference in the random fields in the system.

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Raman Spectra in Ferroelectric SrTi 18 O 3

Cited by 17 publications

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

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

Dynamical mechanism of the ferroelectric phase transition ofSrTi18O3studied by light scattering

Dielectric Properties of SrTi 18 O 3

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