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Ferroelectric-like metallic state in electron doped BaTiO3.

Fujioka J, Doi A, Okuyama D, Morikawa D, Arima T, Okada KN, Kaneko Y, Fukuda T, Uchiyama H, Ishikawa D, Baron AQ, Kato K, Takata M, Tokura Y - Sci Rep (2015)

Bottom Line: We report that a ferroelectric-like metallic state with reduced anisotropy of polarization is created by the doping of conduction electrons into BaTiO3, on the bases of x-ray/electron diffraction and infrared spectroscopic experiments.The crystal structure is heterogeneous in nanometer-scale, as enabled by the reduced polarization anisotropy.The enhanced infrared intensity of soft phonon along with the resistivity reduction suggests the presence of unusual electron-phonon coupling, which may be responsible for the emergent ferroelectric structure compatible with metallic state.

View Article: PubMed Central - PubMed

Affiliation: Department of Applied Physics and Quantum-Phase Electronics Center (QPEC), University of Tokyo, Hongo, Tokyo 113-8656, Japan.

ABSTRACT
We report that a ferroelectric-like metallic state with reduced anisotropy of polarization is created by the doping of conduction electrons into BaTiO3, on the bases of x-ray/electron diffraction and infrared spectroscopic experiments. The crystal structure is heterogeneous in nanometer-scale, as enabled by the reduced polarization anisotropy. The enhanced infrared intensity of soft phonon along with the resistivity reduction suggests the presence of unusual electron-phonon coupling, which may be responsible for the emergent ferroelectric structure compatible with metallic state.

No MeSH data available.


(a) Optical conductivity spectra at 360 K. The closed triangle, open triangle and closed square mark the polarization relaxation mode, Slater mode and Last mode, respectively. The thin blue, green and dashed lines denote the contribution from the Drude response, the damped harmonic oscillators coupled to Debye mode, and fitted spectra, respectively. (b) The optical conductivity spectra at various temperatures. The closed (open) circles mark the dc-conductivity at 10 K (360 K) deduced from the resistivity measurements. (c) Temperature evolution of the spectral intensity of soft phonon band. (d) The loss function Im[1/ε] spectra at 360 K. The diamond denotes the energy of longitudinal optical (LO) phonon.
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f4: (a) Optical conductivity spectra at 360 K. The closed triangle, open triangle and closed square mark the polarization relaxation mode, Slater mode and Last mode, respectively. The thin blue, green and dashed lines denote the contribution from the Drude response, the damped harmonic oscillators coupled to Debye mode, and fitted spectra, respectively. (b) The optical conductivity spectra at various temperatures. The closed (open) circles mark the dc-conductivity at 10 K (360 K) deduced from the resistivity measurements. (c) Temperature evolution of the spectral intensity of soft phonon band. (d) The loss function Im[1/ε] spectra at 360 K. The diamond denotes the energy of longitudinal optical (LO) phonon.

Mentions: To clarify the entanglement between conduction electrons and electric polarization, we have investigated the charge-lattice coupled dynamics using the infrared spectroscopy. Figure 4(a) is a magnified view of the optical conductivity spectra below 0.04 eV in the paraelectric phase. Two broad peaks and one sharp peak are identified around 7, 15 and 23 meV, respectively. Referring to the infrared phonon modes for BaTiO32425, the sharp peak at 23 meV is assigned to the phonon governed by the Ba motion (Last mode), while the broad peaks around 7 and 15 meV can be the polarization relaxation mode (central mode) or Slater mode (soft phonon). Figure 4(b) shows the temperature dependence of optical conductivity spectra. The spectral intensity is significantly enhanced as temperature decreases, while the typical energy scale of the peaks barely changes. Below 100 K, the two peaks combine into, nearly, a single peak. Henceforth, we do not distinguish these two modes and regard them as a soft phonon. In order to quantify the temperature dependence of its spectral intensity, we fitted the spectra with the model of a damped harmonic oscillator coupled to a Debye mode (Supplement Information 3). Figure 4(c) shows the temperature evolution of spectral intensity for the soft phonon. The spectral intensity monotonically increases with decreasing temperature in the whole temperature range. Specifically, it is steeply enhanced below TS2 and nearly doubles at the lowest temperature. Moreover, the width of soft mode remains as large as 20 meV even at 10 K. The overdamped character and temperature-insensitive energy of the soft phonon well below the transition temperature as well as its enhanced spectral intensity are rarely observed in conventional ferroelectrics. Indeed, in pristine BaTiO3, the soft phonon usually exhibits a hardening below the ferroelectric transition temperature while reducing its overdamped character in accord with the growth of ferroelectric order2627. This suggests that the delocalized conduction electrons play an important role in the polarization dynamics.


Ferroelectric-like metallic state in electron doped BaTiO3.

Fujioka J, Doi A, Okuyama D, Morikawa D, Arima T, Okada KN, Kaneko Y, Fukuda T, Uchiyama H, Ishikawa D, Baron AQ, Kato K, Takata M, Tokura Y - Sci Rep (2015)

(a) Optical conductivity spectra at 360 K. The closed triangle, open triangle and closed square mark the polarization relaxation mode, Slater mode and Last mode, respectively. The thin blue, green and dashed lines denote the contribution from the Drude response, the damped harmonic oscillators coupled to Debye mode, and fitted spectra, respectively. (b) The optical conductivity spectra at various temperatures. The closed (open) circles mark the dc-conductivity at 10 K (360 K) deduced from the resistivity measurements. (c) Temperature evolution of the spectral intensity of soft phonon band. (d) The loss function Im[1/ε] spectra at 360 K. The diamond denotes the energy of longitudinal optical (LO) phonon.
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC4542543&req=5

f4: (a) Optical conductivity spectra at 360 K. The closed triangle, open triangle and closed square mark the polarization relaxation mode, Slater mode and Last mode, respectively. The thin blue, green and dashed lines denote the contribution from the Drude response, the damped harmonic oscillators coupled to Debye mode, and fitted spectra, respectively. (b) The optical conductivity spectra at various temperatures. The closed (open) circles mark the dc-conductivity at 10 K (360 K) deduced from the resistivity measurements. (c) Temperature evolution of the spectral intensity of soft phonon band. (d) The loss function Im[1/ε] spectra at 360 K. The diamond denotes the energy of longitudinal optical (LO) phonon.
Mentions: To clarify the entanglement between conduction electrons and electric polarization, we have investigated the charge-lattice coupled dynamics using the infrared spectroscopy. Figure 4(a) is a magnified view of the optical conductivity spectra below 0.04 eV in the paraelectric phase. Two broad peaks and one sharp peak are identified around 7, 15 and 23 meV, respectively. Referring to the infrared phonon modes for BaTiO32425, the sharp peak at 23 meV is assigned to the phonon governed by the Ba motion (Last mode), while the broad peaks around 7 and 15 meV can be the polarization relaxation mode (central mode) or Slater mode (soft phonon). Figure 4(b) shows the temperature dependence of optical conductivity spectra. The spectral intensity is significantly enhanced as temperature decreases, while the typical energy scale of the peaks barely changes. Below 100 K, the two peaks combine into, nearly, a single peak. Henceforth, we do not distinguish these two modes and regard them as a soft phonon. In order to quantify the temperature dependence of its spectral intensity, we fitted the spectra with the model of a damped harmonic oscillator coupled to a Debye mode (Supplement Information 3). Figure 4(c) shows the temperature evolution of spectral intensity for the soft phonon. The spectral intensity monotonically increases with decreasing temperature in the whole temperature range. Specifically, it is steeply enhanced below TS2 and nearly doubles at the lowest temperature. Moreover, the width of soft mode remains as large as 20 meV even at 10 K. The overdamped character and temperature-insensitive energy of the soft phonon well below the transition temperature as well as its enhanced spectral intensity are rarely observed in conventional ferroelectrics. Indeed, in pristine BaTiO3, the soft phonon usually exhibits a hardening below the ferroelectric transition temperature while reducing its overdamped character in accord with the growth of ferroelectric order2627. This suggests that the delocalized conduction electrons play an important role in the polarization dynamics.

Bottom Line: We report that a ferroelectric-like metallic state with reduced anisotropy of polarization is created by the doping of conduction electrons into BaTiO3, on the bases of x-ray/electron diffraction and infrared spectroscopic experiments.The crystal structure is heterogeneous in nanometer-scale, as enabled by the reduced polarization anisotropy.The enhanced infrared intensity of soft phonon along with the resistivity reduction suggests the presence of unusual electron-phonon coupling, which may be responsible for the emergent ferroelectric structure compatible with metallic state.

View Article: PubMed Central - PubMed

Affiliation: Department of Applied Physics and Quantum-Phase Electronics Center (QPEC), University of Tokyo, Hongo, Tokyo 113-8656, Japan.

ABSTRACT
We report that a ferroelectric-like metallic state with reduced anisotropy of polarization is created by the doping of conduction electrons into BaTiO3, on the bases of x-ray/electron diffraction and infrared spectroscopic experiments. The crystal structure is heterogeneous in nanometer-scale, as enabled by the reduced polarization anisotropy. The enhanced infrared intensity of soft phonon along with the resistivity reduction suggests the presence of unusual electron-phonon coupling, which may be responsible for the emergent ferroelectric structure compatible with metallic state.

No MeSH data available.