Abstract-Infrared, terahertz, and microwave spectroscopy of the soft and central modes in Pb(Mg1/3Nb2/3)O3

D. Nuzhnyy, J. Petzelt, V. Bovtun, M. Kempa, S. Kamba, J. Hlinka, B. Hehlen

https://128.84.21.199/abs/1712.06493

From the new infrared (IR) reflectivity and time-domain terahertz (THz) spectra combined with available high-frequency dielectric data above the MHz range in a broad temperature range of 10-900 K, a full picture of the soft and central mode behavior in the classical relaxor ferroelectric Pb(Mg1/3Nb2/3)O3 (PMN) is suggested. A detailed comparison is given with the recent hyper-Raman spectroscopy data (Phys. Rev. Lett. 117, 155501 (2016)), and also with other available experiments based on inelastic light and neutron scattering. The closest agreement is with the hyper-Raman data, both techniques yield the same number of soft-mode components and the same high-temperature softening towards the temperature T* ~ 400 K. In addition to evaluation of the IR-THz data using fitting with standard factorized form of the dielectric function, we performed a successful fitting of the same data using the effective medium approach (EMA), originally based on the assumption that the mesoscopic structure of PMN consists of randomly oriented uniaxially anisotropic polar nanodomains (PNDs) with somewhat harder TO polar modes in the direction along the local PND dipole (Phys. Rev. Lett. 96, 027601 (2006)). Evaluation using the Bruggeman EMA modelling has been successfully applied in the entire investigated temperature range. These results suggest that the response perpendicular to the local dipole moment, at high temperatures induced by random fields rather than PNDs, undergoes a classical softening from high temperatures with permittivity obeying the Curie-Weiss law, eps_per = C/(T-Tc), C = 1.7 x 10^5 K and Tc = 380 K. Below the Burns temperature ~620 K, a GHz relaxation ascribed to flipping of the PNDs emerges from the soft mode response, slows down and broadens, remaining quite strong towards the cryogenic temperatures, where it can be assigned to fluctuations of the PND boundaries.

Abstract-Infrared, terahertz, and microwave spectroscopy of the soft and central modes in Pb ( M g 1 / 3 N b 2 / 3 ) O 3

D. Nuzhnyy, J. Petzelt, V. Bovtun, M. Kempa, S. Kamba, J. Hlinka,  B. Hehlen

https://journals.aps.org/prb/abstract/10.1103/PhysRevB.96.174113

From the new infrared (IR) reflectivity and time-domain terahertz spectra combined with available high-frequency dielectric data above the megahertz range in a broad temperature range of 10 to 900 K, a full picture of the soft- and central-mode behavior in the classical relaxor ferroelectric $\mathrm{Pb}\left(M{g}_{1/3}N{b}_{2/3}\right)O_3$ (PMN) is suggested. A detailed comparison is made with the recent hyper-Raman spectroscopy data [Hehlen et al., Phys. Rev. Lett. 117, 155501 (2016)] and also with other available experiments based on inelastic light and neutron scattering. It is revealed that each type of experiment provides slightly different data. The closest agreement is with the hyper-Raman data: both techniques yield the same number of soft-mode components and the same high-temperature softening towards the temperature $T^{\ast }\approx 400K$. In addition to evaluation of the IR-terahertz data using fitting with the standard factorized form of the dielectric function, we performed a successful fitting of the same data using the effective medium approach (EMA), originally based on the assumption that the mesoscopic structure of PMN consists of randomly oriented uniaxially anisotropic polar nanodomains (PNDs) with somewhat harder transverse optical polar modes in the direction along the local PND dipole [Hlinka et al., Phys. Rev. Lett. 96, 027601 (2006)]. Evaluation using Bruggeman EMA modeling has been successfully applied in the entire investigated temperature range. These results suggest that the response perpendicular to the local dipole moment, at high temperatures induced by random fields rather than PNDs, undergoes a classical softening from high temperatures with permittivity obeying the Curie-Weiss law, ${\varepsilon }_{\perp }=C/(T-T_C)$, where $C=1.7\times {10}^{5}K$ and $T_C=380K$, whereas the response parallel to it shows no softening. Below the Burns temperature, $\sim 620K$, a gigahertz relaxation ascribed to flipping of the PNDs emerges from the soft-mode response, slows down, and broadens, remaining quite strong towards the cryogenic temperatures, where it can be assigned to fluctuations of the PND boundaries.

• 
• 
• 
• 
• 
• 
• 

Abstract-Systematic study of terahertz response of SrTiO3 based heterostructures: Influence of strain, temperature, and electric field

Phys. Rev. B 89, 214116 – Published 30 June 2014

V. Skoromets, C. Kadlec, J. Drahokoupil, J. Schubert, J. Hlinka, and P. Kužel

https://journals.aps.org/prb/abstract/10.1103/PhysRevB.89.214116

Epitaxial heterostructures consisting of a variable number of SrTiO3/DyScO3 bilayers deposited on DyScO3substrates were investigated using time-domain terahertz spectroscopy down to helium temperatures. Interdigitated electrodes deposited on top of the structures allowed probing of the terahertz response upon an applied electric field. The phase transition into a ferroelectric state is observed in SrTiO3 films in all samples close to room temperature (between 250 and 310 K) due to in-plane epitaxial strain induced by the substrate and intercalated layers of DyScO3. Evolution of the dielectric spectra with temperature and external electric field is described by a general model which involves a damped harmonic oscillator (soft mode) coupled to a Debye relaxation (central mode). Both modes are connected with the soft mode eigenvector, as recently shown by molecular dynamics simulations, and they reflect a strong anharmonicity of the soft-mode potential. At high temperatures the soft-mode frequency variation drives all the changes observed in the spectra with temperature and applied field. At low temperatures, deep in the ferroelectric phase, the soft mode significantly hardens and loses its importance for the terahertz dynamics; the central mode becomes stronger and it almost completely determines the shape of the measured spectra. The observed variation of phase transition temperature and of the dielectric response among the structures is ascribed to a partial epitaxial strain relaxation confirmed also by x-ray diffraction.

DOI: http://dx.doi.org/10.1103/PhysRevB.89.214116

• 
• 
• 
• 
• 
• 
•