Doping-induced ferroelectric phase transition in strontium titanate: Observation of birefringence and coherent phonons under ultraviolet illumination

Abstract

The doping-induced ferroelectric phase transition in Ca-doped SrTiO 3 is investigated by observing the birefringence and coherent phonons. The structural phase-transition temperature is determined by the birefringence measurement. Coherent phonons of the soft modes are studied by using ultrafast polarization spectroscopy. Two phonon modes are observed to be softened toward the ferroelectric phase-transition point at 28 K and other two phonon modes are observed to be softened toward the structural phase-transition point at 180 K.Another structural deformation at 25 K is found below the ferroelectric phase-transition temperature. From the temperature dependence of phonon frequencies under an ultraviolet ͑UV͒ illumination, a shift of the ferroelectric phase-transition point toward the lower temperature side is found. A decrease in phonon frequencies after the UV illumination is also found.

Photo-induced Effect in Quantum Paraelectric Materials Studied by Transient Birefringence Measurement

“…In Ca-doped SrTiO 3 , judging from the rising temperature of the transient birefringence amplitude, the ferroelectric phase transition temperature is shifted toward the lower temperature side under the UV illumination. This is consistent with the dielectric measurement (Yamada & Tanaka, 2008) and the coherent phonon experiment (Koyama et al, 2010). The doped Ca ions behave as permanent dipoles, and ferroelectric clusters are formed around the Ca dipoles with the high polarizability of the host crystal.…”

“…The thickness of the samples is 0.2 mm. The structural phase-transition temperature, T C1 =180K, of the Ca-doped SrTiO 3 was obtained from the temperature dependence of the birefringence (Koyama et al, 2010), and the ferroelectric phase-transition temperature, T C2 = 28K, was determined by the measurement of dielectric constants (Yamada & Tanaka, 2008).…”

Light induced non-volatile switching of superconductivity in single layer FeSe on SrTiO3 substrate

“…Exposing freshly cleaved SrTiO 3 (001) surface to intense extreme ultraviolet (EUV) light can lead to the formation of surface two-dimensional electron gas (2DEG) 29–31 , possibly originating from the creation of either oxygen vacancies or surface reconstructions. Additionally, due to the strong coupling between photocarriers and the lattice in SrTiO 3 , UV irradiations were also found to excite soft phonons/polarons 32,33 and cause persistent phonon softening at low temperatures 34 . In FeSe/SrTiO 3 heterostructures, many of these effects can potentially impact the superconducting behaviors in FeSe through the interface, such as generating charge transfer doping 11–13 , modulating interface electron–phonon coupling 15,16 , and producing interface lattice distortions 14,35–39 .…”

“…As discussed above, SrTiO 3 undergoes a quantum paraelectric phase transition at low temperatures 6,21–24 , where quantum fluctuations associated with zero-point energy prevent the onset of long-range ferroelectric order. In this phase, photoexcited electrons can quadratically couple to the T 1u soft mode (relative displacement between the Ti ion and the oxygen octahedra) and directly impact the quantum fluctuations 32–34,45 . In particular, the polarons formed from photocarriers and phonons can serve as effective charge trap to suppress electron–hole recombination and generate local dipole moments 44 , leading to the large photocapacitance observed in our experiments.…”

Doping-Induced Ferroelectric Phase Transition and Ultraviolet-Illumination Effect in a Quantum Paraelectric Material Studied by Coherent Phonon Spectroscopy

“…The doping-induced ferroelectric phase transition in Ca-doped SrTiO 3 is investigated by observing the birefringence and coherent phonons [25]. In the birefringence measurement, the structural and the ferroelectric phase-transition temperatures are examined.…”