Lowering of ground state induced by core-shell structure in strontium titanate (original) (raw)
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Core-Shell structure induces new ground states in strontium titanate
HAL (Le Centre pour la Communication Scientifique Directe), 2017
Hyper-Raman scattering (HRS) is a non-linear spectroscopy sensitive to all polar excitations, in particular the soft modes inactive in Raman. HRS has been applied to nano-ceramics of strontium titanate (SrTiO 3) of controlled grain size. Contrary to infrared absorption which sees an average medium, the vibrational responses of the core and the shell(s) are split in HRS, allowing thereby probing the structural and dielectric properties of the two subsystems. The resulting structural model successfully reproduces the effective dielectric permittivity measured by dielectric experiments [1]. These results confirm the strong, but still under-exploited, potentialities of HRS for the investigation of polar materials [2]. We also demonstrate that a new ground state can be obtained by tailoring the core-shell structure of the particles. High energy X-rays at synchrotron combined to neutron diffraction and HRS revealed a lowering of the ferroelastic ground state towards a new antiferrodistortive phase, accompanied with strong shift of the critical temperature [3,4]. This new phase is discussed within the Landau theory, and the crucial competition between particle shape anisotropy, surface tension, and shear strain is analyzed as well. This shows that controlling the core-shell structure provides an easy way to stabilize new phases that cannot exist in bulk material, just like film deposition on a substrate.
An electrical field-induced structural effect in strontium titanate at room temperature
Applied Physics A, 2005
An electric field-induced reversible structural distortion at room temperature is observed at an as-cut singlecrystalline SrTiO 3 [001] wafer. The structural changes have been characterised in situ by means of X-ray diffraction. It is concluded that the effect requires a distortion of the cubic crystal structure, as it was present only at the rough unpolished side of the wafer. The appearance of the phenomenon was found to depend on the direction of the electric field. Structural changes, depending on the electric field strength, were completely reversible. Regarding the possible electromechanical coupling phenomena, the piezoelectric and electrostrictive effects have to be considered. Cubic SrTiO 3 belongs to the centrosymmetric point group m3m; hence, no piezoelectric effect is present, whereas second-order effects like electrostriction may occur. Peculiarities arising as a result of the special constraints are discussed.
2020
The properties of quantum materials are commonly tuned using experimental variables such as pressure, magnetic field and doping. Here we explore a different approach: irreversible, plastic deformation of single crystals. We show for the superconductor SrTiO$_3$ that compressive plastic deformation induces low-dimensional superconductivity significantly above the superconducting transition temperature ($T_c$) of undeformed samples, with evidence of superconducting correlations at temperatures two orders of magnitude above the bulk TcT_cTc. The superconductivity enhancement is correlated with the appearance of self-organized dislocation structures, as revealed by diffuse neutron and X-ray scattering. We also observe signatures of deformation-induced quantum-critical ferroelectric fluctuations and inhomogeneous ferroelectric order via Raman scattering. These results suggest that the strain surrounding the self-organized dislocation structures induces local ferroelectricity and quantum-cr...
Observation of a sample-dependent 37 K anomaly on the lattice parameters of strontium titanate
Europhysics Letters (epl), 2003
The lattice parameters of tetragonal strontium titanate are determined to about 1 part in 10 6 with penetrating X-rays. While no peculiarity is detected in a bulk sample, a distinct anomaly at 37 K is observed in an optically polished thin platelet derived from the same single crystal. This suggests that the anomaly is related to a high density of dislocations. A possible mechanism is that dislocations stabilize transverse antiphase boundaries which undergo a ferroelectric transition near 37 K. If so, this transition stresses the boundaries which then strain the bulk leading to the observed anomaly.
Tailoring strain in the SrTiO 3 compound by low-energy He + irradiation
EPL (Europhysics Letters), 2010
The ability to generate a change of the lattice parameter in a near-surface layer of a controllable thickness by ion implantation of strontium titanate is reported here using low energy He + ions. The induced strain follows a distribution within a typical near-surface layer of 200 nm as obtained from structural analysis. Due to clamping effect from the underlying layer, only perpendicular expansion is observed. Maximum distortions up to 5-7% are obtained with no evidence of amorphisation at fluences of 10 16 He + ions/cm 2 and ion energies in the range 10-30 keV. Accepted for publication in Europhysics Letter
SrTiO3—Glimpses of an Inexhaustible Source of Novel Solid State Phenomena
Condensed Matter
The purpose of this selective review is primarily to demonstrate the large versatility of the insulating quantum paraelectric perovskite SrTiO3 explained in “Introduction” part, and “Routes of SrTiO3 toward ferroelectricity and other collective states” part. Apart from ferroelectricity under various boundary conditions, it exhibits regular electronic and superconductivity via doping or external fields and is capable of displaying diverse coupled states. “Magnetoelectric multiglass (Sr,Mn)TiO3” part, deals with mesoscopic physics of the solid solution SrTiO3:Mn2+. It is at the origin of both polar and spin cluster glass forming and is altogether a novel multiferroic system. Independent transitions at different glass temperatures, power law dynamic criticality, divergent third-order susceptibilities, and higher order magneto-electric interactions are convincing fingerprints.
Systematic ab initio study of the phase diagram of epitaxially strained SrTiO[sub 3]
Journal of Applied Physics, 2006
We use density-functional theory with the local-density approximation to study the structural and ferroelectric properties of SrTiO3 under misfit strains. Both the antiferrodistortive (AFD) and ferroelectric (FE) instabilities are considered by calculating all the phases predicted by Pertsev et al. [Phys. Rev. B 61, R 825 (2000)] based on the phenomenological Landau theory. The rotation of the oxygen octahedra and the movement of the atoms are fully relaxed within the constraint of a fixed in-plane lattice constant. We find a rich misfit strain-induced phase transition sequence which is in overall agreement with the prediction by Pertsev et al. and is obtained only when the AFD distortion is taken into account. Nevertheless, the calculated locations of the phase boundaries are different from the prediction by Pertsev et al. We also find that compressive misfit strains induce ferroelectricity in the tetragonal low temperature phase only whilst tensile strains induce ferroelectricity in the orthorhombic phases only. The calculated FE polarization for both the tetragonal and orthorhombic phases increases monotonically with the magnitude of the strains. The AFD rotation angle of the oxygen octahedra in the tetragonal phase increases dramatically as the misfit strain goes from the tensile to compressive strain region whilst it decreases slightly in the orthorhombic (FO4) phase. This reveals why the polarization in the epitaxially strained SrTiO3 would be larger when the tensile strain is applied, since the AFD distortion is found to reduce the FE instability and even to completely suppress it in the small strain region. Finally, our analysis of the average polar distortion and the charge density distribution suggests that both the Ti-O and Sr-O layers contribute significantly to the FE polarization.
Interaction between quantum paraelectricity and ferroelasticity in SrTiO 3
Journal of Physics: Condensed Matter, 2005
The dielectric susceptibility of SrTiO 3 is measured as a function of temperature between room temperature and 32 K. These data show an anomaly at approximately 105 K, which is associated with the cubic-tetragonal ferroelastic phase transition. The form of this anomaly is shown to be consistent with biquadratic coupling between the ferroelectric and ferroelastic modes. The quantum paraelectric state of SrTiO 3 is studied in the framework of a quantum mechanical Landau potential. The saturation temperature is determined from published measurements of the dielectric susceptibility at very low temperatures. When the coupling to the ferroelastic mode is accounted for, the saturation temperature obtained from these measurements (θ S = 20(1) K) is the same as that seen in phase diagrams for the ferroelectric transition, such as T C versus chemical dopant or O isotope content.
arXiv (Cornell University), 2022
We demonstrate an approach for calculating temperature-dependent quantum and anharmonic effects with beyond density-functional theory accuracy. By combining machine-learned potentials and the stochastic self-consistent harmonic approximation, we investigate the cubic to tetragonal transition in strontium titanate and show that the paraelectric phase is stabilized by anharmonic quantum fluctuations. We find that a quantitative understanding of the quantum paraelectric behavior requires a higher-level treatment of electronic correlation effects via the random phase approximation. This approach enables detailed studies of emergent properties in strongly anharmonic materials beyond density-functional theory.