Conditions Extrêmes et Matériaux : Haute Température et Irradiation

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N.Ortega, A.Kumar, O.Maslova, Yu.I.Yuzyuk, J.F.Scott, R.S.Katiyar, 'Effect of periodicity and composition in artificial BaTiO3/(Ba,Sr)TiO3 superlattices', Phys. Rev. B 83 144108 (2011) doi:10.1103/physrevb.83.144108

We have prepared two series of BaTiO3/SrTiO3 (BT/ST) superlattices (SLs). In the first set, the modulation period ( ) was varied from 136 to 16 A° , whereas in the second set ST was replaced with Ba(1−x)SrxTiO3 (BST) (0 < x < 1) with constant periodicity of ∼ 136 A° . A total stack height of 10 000 A° (1 μm) was maintained for both systems. The compositional changes give large spectral changes at x = 40% where the lowest E mode in BT/BST SL becomes underdamped. X-ray-diffraction patterns and surface topography of the modulated period revealed a Stranski-Krastanov growthmechanism. The upward frequency shift of the E(1TO) in SLs with decrease in or Ba concentration in the BST layer was attributed to the in-plane compressive stress induced by the lattice mismatch of the constituent layers. [N.B., here the notation E(1TO) or A1(2TO) simply designates the vibrational mode of E or A1 symmetry, with 1TO or 2TO designating the transverse optical branch that is lowest, 1, or second lowest, 2, in energy.] In the low-frequency region, the folded acoustic phonon doublets were observed. Since the low-frequency out-of-plane dielectric constant is directly related to the E(1TO) soft mode frequency, strain engineering in artificial SLs allowed us to modify their dielectric properties by varying either the Ba/Sr in BST sublayer concentration or the modulation periodicity in the superlattices.