Surface Pyroelectricity in Cubic SrTiO₃

Abstract

material gives rise to a range of interesting properties such as superconductivity, [1] high electron mobility, [2,3] ferromagnetism [4,5] and 2D electron gases. [6] In the multitude of the properties exhibited by SrTiO 3 , pyroelectricity, ferroelectricity, and piezoelectricity are markedly absent as these properties are symmetry prohibited in centrosymmetric crystal lattices. The polar phase required for exhibiting these properties can, however, be induced artificially by modification of the lattice. Large biaxial strain, [7] light pulses, [8,9] and elemental substitutions [10] can convert SrTiO 3 into a ferroelectric material, whereas strain gradients can induce a polarization via the flexoelectric effect [11,12] or oxygen vacancies migration. [13] Electron diffraction measurements furthermore show that the top TiO 2 surface layer of SrTiO 3 undergoes surface relaxation and oxygen ions move outward from the surface relative to the titanium ions, leading to a polarization of this layer. [14][15][16] This is supported by shell model and density functional theory calculations. However, the calculations also predict that the SrO layers underneath display a polarization opposite to that of the TiO 2 surface layer, [17][18][19] leaving the question of the predicted net polarization and a possible pyroelectricity at the surface wide open.Symmetry-imposed restrictions on the number of available pyroelectric and piezoelectric materials remain a major limitation as 22 out of 32 crystallographic material classes exhibit neither pyroelectricity nor piezoelectricity. Yet, by breaking the lattice symmetry it is possible to circumvent this limitation. Here, using a unique technique for measuring transient currents upon rapid heating, direct experimental evidence is provided that despite the fact that bulk SrTiO 3 is not pyroelectric, the (100) surface of TiO 2 -terminated SrTiO 3 is intrinsically pyroelectric at room temperature. The pyroelectric layer is found to be ≈1 nm thick and, surprisingly, its polarization is comparable with that of strongly polar materials such as BaTiO 3 . The pyroelectric effect can be tuned ON/OFF by the formation or removal of a nanometric SiO 2 layer. Using density functional theory, the pyroelectricity is found to be a result of polar surface relaxation, which can be suppressed by varying the lattice symmetry breaking using a SiO 2 capping layer. The observation of pyroelectricity emerging at the SrTiO 3 surface also implies that it is intrinsically piezoelectric. These findings may pave the way for observing and tailoring piezo-and pyroelectricity in any material through appropriate breaking of symmetry at surfaces and artificial nanostructures such as heterointerfaces and superlattices. PyroelectricitySrTiO 3 has been the object of immense attention for over half a century owing to its multifunctional nature and popularity as a template for epitaxial growth of artificial nanostructures such as heterostructures, superlattices, and vertically aligned nanostructures. It is a classic example whe...

Pyroelectricity and field-induced spin-flop in (4-(Aminomethyl)pyridinium) 2 MnCl 4 · 2H 2 O

“…The bulk photovoltaic effect can also exist in SrTiO 3 , which is caused by flexoelectricity [26]. Pyroelectricity in 1 is similar that found in SrTiO 3 [29], which is not caused by the ferroelectricity or spontaneous polarization. The space group of a single crystal of 1 is Pı, which is centrosymmetric symmetry.…”

Piezoelectric and pyroelectric effects induced by interface polar symmetry

“…10 This symmetry breaking is associated with a variety of emergent functionalities such as piezoelectric, pyroelectric and bulk photovoltaic effects for any materials such as centrosymmetric SrTiO 3 and TiO 2 . [11][12][13][14][15] Despite its universal nature, the real application of this intriguing flexoelectric effect is hampered by its rather small effective coefficients and a complicated setup for inducing large strain gradients. Thus, an alternative would be highly desirable for developing or tuning applications based on induced symmetry breaking.…”

“…With careful designs, the interface polar effects can also work concurrently with bulk effects arising from inherent 7,8 or externally induced polarity by, e.g. strain gradients [10][11][12][13][14][15] , to achieve an enhanced piezo-/pyroelectric coefficients or even new effects. (Extended Data Fig.…”

Coherent Acoustic Interferometry During the Photo-Driven Oxygen Evolution Reaction Associates Strain Fields with the Reactive Oxygen Intermediate

“…32 Two mechanisms exist for the electron-phonon case in particular: through the deformation potential to form an acoustic polaron, 63 and through the piezoelectric effect to form a piezoelectric polaron. 64 Normally, piezoelectricity is symmetry forbidden in room temperature STO because it is centrosymmetric, but the recent discovery of a surface piezoelectric effect induced by interfacial electric fields 65 and surface relaxation 66 indicates that such an effect is present, possibly to a large extent, at the STO/water interface. It is also worth noting that such a surface piezoelectric effect has been associated with perovskite octahedra rotation, 67 which could reduce the effects of strain on OER activity.…”