Origin of Terahertz Soft-Mode Nonlinearities in Ferroelectric Perovskites

Soft modes are intimately linked to structural instabilities and are key for the understanding of phase transitions. The soft modes in ferroelectrics, for example, map directly the polar order parameter of a crystal lattice. Driving these modes into the nonlinear, frequency-changing regime with inte...

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Autores principales:	Shovon Pal, Nives Strkalj, Chia-Jung Yang, Mads C. Weber, Morgan Trassin, Michael Woerner, Manfred Fiebig
Formato:	article
Lenguaje:	EN
Publicado:	American Physical Society 2021
Materias:	Physics QC1-999
Acceso en línea:	https://doaj.org/article/e66ce97dc5a245e7853daceea7095f97
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spelling	oai:doaj.org-article:e66ce97dc5a245e7853daceea7095f972021-12-02T18:27:09ZOrigin of Terahertz Soft-Mode Nonlinearities in Ferroelectric Perovskites10.1103/PhysRevX.11.0210232160-3308https://doaj.org/article/e66ce97dc5a245e7853daceea7095f972021-04-01T00:00:00Zhttp://doi.org/10.1103/PhysRevX.11.021023http://doi.org/10.1103/PhysRevX.11.021023https://doaj.org/toc/2160-3308Soft modes are intimately linked to structural instabilities and are key for the understanding of phase transitions. The soft modes in ferroelectrics, for example, map directly the polar order parameter of a crystal lattice. Driving these modes into the nonlinear, frequency-changing regime with intense terahertz (THz) light fields is an efficient way to alter the lattice and, with it, the physical properties. However, recent studies show that the THz electric-field amplitudes triggering a nonlinear soft-mode response are surprisingly low, which raises the question on the microscopic picture behind the origin of this nonlinear response. Here, we use linear and two-dimensional terahertz (2D THz) spectroscopy to unravel the origin of the soft-mode nonlinearities in a strain-engineered epitaxial ferroelectric SrTiO_{3} thin film. We find that the linear dielectric function of this mode is quantitatively incompatible with pure ionic or pure electronic motions. Instead, 2D THz spectroscopy reveals a pronounced coupling of electronic and ionic-displacement dipoles. Hence, the soft mode is a hybrid mode of lattice (ionic) motions and electronic interband transitions. We confirm this conclusion with model calculations based on a simplified pseudopotential concept of the electronic band structure. It reveals that the entire THz nonlinearity is caused by the off-resonant nonlinear response of the electronic interband transitions of the lattice-electronic hybrid mode. With this work, we provide fundamental insights into the microscopic processes that govern the softness that any material assumes near a ferroic phase transition. This knowledge will allow us to gain an efficient all-optical control over the associated large nonlinear effects.Shovon PalNives StrkaljChia-Jung YangMads C. WeberMorgan TrassinMichael WoernerManfred FiebigAmerican Physical SocietyarticlePhysicsQC1-999ENPhysical Review X, Vol 11, Iss 2, p 021023 (2021)
institution	DOAJ
collection	DOAJ
language	EN
topic	Physics QC1-999
spellingShingle	Physics QC1-999 Shovon Pal Nives Strkalj Chia-Jung Yang Mads C. Weber Morgan Trassin Michael Woerner Manfred Fiebig Origin of Terahertz Soft-Mode Nonlinearities in Ferroelectric Perovskites
description	Soft modes are intimately linked to structural instabilities and are key for the understanding of phase transitions. The soft modes in ferroelectrics, for example, map directly the polar order parameter of a crystal lattice. Driving these modes into the nonlinear, frequency-changing regime with intense terahertz (THz) light fields is an efficient way to alter the lattice and, with it, the physical properties. However, recent studies show that the THz electric-field amplitudes triggering a nonlinear soft-mode response are surprisingly low, which raises the question on the microscopic picture behind the origin of this nonlinear response. Here, we use linear and two-dimensional terahertz (2D THz) spectroscopy to unravel the origin of the soft-mode nonlinearities in a strain-engineered epitaxial ferroelectric SrTiO_{3} thin film. We find that the linear dielectric function of this mode is quantitatively incompatible with pure ionic or pure electronic motions. Instead, 2D THz spectroscopy reveals a pronounced coupling of electronic and ionic-displacement dipoles. Hence, the soft mode is a hybrid mode of lattice (ionic) motions and electronic interband transitions. We confirm this conclusion with model calculations based on a simplified pseudopotential concept of the electronic band structure. It reveals that the entire THz nonlinearity is caused by the off-resonant nonlinear response of the electronic interband transitions of the lattice-electronic hybrid mode. With this work, we provide fundamental insights into the microscopic processes that govern the softness that any material assumes near a ferroic phase transition. This knowledge will allow us to gain an efficient all-optical control over the associated large nonlinear effects.
format	article
author	Shovon Pal Nives Strkalj Chia-Jung Yang Mads C. Weber Morgan Trassin Michael Woerner Manfred Fiebig
author_facet	Shovon Pal Nives Strkalj Chia-Jung Yang Mads C. Weber Morgan Trassin Michael Woerner Manfred Fiebig
author_sort	Shovon Pal
title	Origin of Terahertz Soft-Mode Nonlinearities in Ferroelectric Perovskites
title_short	Origin of Terahertz Soft-Mode Nonlinearities in Ferroelectric Perovskites
title_full	Origin of Terahertz Soft-Mode Nonlinearities in Ferroelectric Perovskites
title_fullStr	Origin of Terahertz Soft-Mode Nonlinearities in Ferroelectric Perovskites
title_full_unstemmed	Origin of Terahertz Soft-Mode Nonlinearities in Ferroelectric Perovskites
title_sort	origin of terahertz soft-mode nonlinearities in ferroelectric perovskites
publisher	American Physical Society
publishDate	2021
url	https://doaj.org/article/e66ce97dc5a245e7853daceea7095f97
work_keys_str_mv	AT shovonpal originofterahertzsoftmodenonlinearitiesinferroelectricperovskites AT nivesstrkalj originofterahertzsoftmodenonlinearitiesinferroelectricperovskites AT chiajungyang originofterahertzsoftmodenonlinearitiesinferroelectricperovskites AT madscweber originofterahertzsoftmodenonlinearitiesinferroelectricperovskites AT morgantrassin originofterahertzsoftmodenonlinearitiesinferroelectricperovskites AT michaelwoerner originofterahertzsoftmodenonlinearitiesinferroelectricperovskites AT manfredfiebig originofterahertzsoftmodenonlinearitiesinferroelectricperovskites
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Origin of Terahertz Soft-Mode Nonlinearities in Ferroelectric Perovskites

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