h−ErMnO3 absorbance, reflectivity, and emissivity in the terahertz to mid-infrared from 2 to 1700 K: Carrier screening, Fröhlich resonance, small polarons, and bipolarons
| Autor: | José Antonio Alonso, Néstor E. Massa, Leire del Campo, Aurélien Canizares, Karsten Holldack, Paula Kayser, Vinh Ta Phuoc |
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| Rok vydání: | 2020 |
| Předmět: |
Physics
Bipolaron Phase transition Condensed matter physics Phonon 02 engineering and technology 021001 nanoscience & nanotechnology Polaron 01 natural sciences Optical conductivity Negative thermal expansion Lattice (order) Molecular vibration 0103 physical sciences Condensed Matter::Strongly Correlated Electrons 010306 general physics 0210 nano-technology |
| Zdroj: | Physical Review B. 102 |
| ISSN: | 2469-9969 2469-9950 |
| DOI: | 10.1103/physrevb.102.134305 |
| Popis: | We report the temperature-dependent THz to mid-infrared response of hexagonal-${\mathrm{ErMnO}}_{3}$ using absorption, reflectivity, and emissivity techniques from 2 to 1700 K. At low temperatures, lowest frequency vibrational modes, which extend up the lock-in ferroelectric temperature, coexist with paramagnon excitations and are associated with well-defined crystal-field rare-earth pure magnetic replicas in an intriguing phonon-magnetic convergence enhancing the multiferroic character of $h\text{\ensuremath{-}}{\mathrm{ErMnO}}_{3}$. Increasing the temperature, a number of vibrational bands close to the space group predicted undergo profile broadening and softening. In particular, a distinctive set of bands in the $288--329\text{\ensuremath{-}}\mathrm{c}{\mathrm{m}}^{\ensuremath{-}1}$ (300 K) range has a component whose profile is carrier screened becoming nearly fully blurred in the intermediate phase between \ensuremath{\sim}830 and \ensuremath{\sim}1500 K. Below ${T}_{C}\ensuremath{\sim}830$ K this asymmetric band, having one component still partially screened, further splits as spin phonon interaction and the tripling of the unit cell take place revealing at ${T}_{\mathrm{N}}\ensuremath{\sim}79$ K a delicate balance of long- and short-range interactions. Ambient Raman scattering brings up evidence of a Fr\"ohlich resonance due to Coulomb interactions between carriers and the macroscopic field linked to the corresponding longitudinal optical-phonon mode. We found it is dynamically correlated to the hexagonal $c$-axis negative thermal expansion. Quantitative analyses of the mid-infrared (MIR) optical conductivity show that it also plays a role in small polarons and mediates in high-temperature bipolarones. Bipolaron profiles at high temperatures change as the sample opacity increases when at \ensuremath{\sim}900 K straight stripes turn curly toward complex vortex-antivortex domain patterns in the paraelectric phase. At still higher temperatures a low-frequency Drude contribution is triggered by electron hopping signaling an insulator-metal phase transition at \ensuremath{\sim}1600 K while the MIR response suggests coexistence between single small polarons and bipolarons. On closing, we draw a parallel with improper ferroelectrics sustaining a lattice incommensurate intermediate phase and unit-cell tripling. We argue that in the $h\text{\ensuremath{-}}R{\mathrm{MnO}}_{3}$ $(R=\text{rare earth}, \mathrm{Y})$ family of compounds the intermediate phase be considered incommensurate with onset at ${T}_{\mathrm{INC}}\ensuremath{\sim}1500$ K and ferroelectric lock-in at ${T}_{C}\ensuremath{\sim}830$ K delimiting this regime in $h\text{\ensuremath{-}}{\mathrm{ErMnO}}_{3}$. |
| Databáze: | OpenAIRE |
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