Quantum Paraelectric Materials Research Articles

Terahertz pulse induced second harmonic generation and Kerr effect in the quantum paraelectric KTaO3

We present a joint experimental and computational study of second harmonic generation and Kerr effect induced by intense terahertz electric fields in the quantum paraelectric material ${\mathrm{KTaO}}_{3}$. The single-cycle terahertz (THz) electric pulses induce the free induction decay of the polar soft phonon mode and a ferroelectric polarization response that lasts for several picoseconds in both experiment and computation. We observe a much longer-lasting (43-ps) transient Kerr effect that follows the THz excitation. We explain the disparity in the relaxation times between the THz-induced second harmonic generation and the Kerr effect by the presence of the polar nanoregions intrinsic to ${\mathrm{KTaO}}_{3}$. The origin of the slow THz Kerr effect dynamics is not presently understood. We speculate that it may be associated with the rotational motion of polar nanoregions due to the material's proximity to the ferroelectric phase transition.

Ferroelectric, quantum paraelectric, or paraelectric? Calculating the evolution from BaTiO3 to SrTiO3 to KTaO3 using a single-particle quantum mechanical description of the ions

We present an inexpensive first-principles approach for describing quantum paraelectricity that combines density functional theory (DFT) treatment of the electronic subsystem with quantum mechanical treatment of the ions through solution of the single-particle Schr\"odinger equation with the DFT-calculated potential. Using BaTiO$_3$, SrTiO$_3$ and KTaO$_3$ as model systems, we show that the approach can straightforwardly distinguish between ferroelectric, paraelectric and quantum paraelectric materials, based on simple quantities extracted from standard density functional and density functional perturbation theories. We calculate the influence of isotope substitution and strain on the quantum paraelectric behavior, and find that, while complete replacement of oxygen-16 by oxygen-18 has a surprisingly small effect, experimentally accessible strains can induce large changes. Finally, we collect the various choices for the phonon mass that have been introduced in the literature. We identify those that are most physically meaningful by comparing with our results that avoid such a choice through the use of mass-weighted coordinates.

Melting of charge order in the low-temperature state of an electronic ferroelectric-like system

Strong electronic interactions can drive a system into a state with a symmetry breaking. Lattice frustration or competing interactions tend to prevent symmetry breaking, leading to quantum disordered phases. In spin systems frustration can produce a spin liquid state. Frustration of a charge degree of freedom also can result in various exotic states, however, experimental data on these effects is scarce. In this work we demonstrate how in a Mott insulator on a weakly anisotropic triangular lattice a charge ordered state melts on cooling down to low temperatures. Raman scattering spectroscopy finds that kappa-(BEDT-TTF){}_{2}Hg(SCN){}_{2}Cl enters an insulating “dipole solid” state at T=30,{mathrm{K}}, but below T=15,{mathrm{K}} the order melts, while preserving the insulating energy gap. Based on these observations, we suggest a phase diagram relevant to other quantum paraelectric materials.

Development of a Rapid Temperature Scanning System for Pulsed Magnetic Fields and its Applications

An apparatus, which scans temperature in sub-seconds for measuring the temperature dependence of the resistivity, is proposed. This apparatus contains a thermal bath and a sample platform on which a thin-film thermometer and a thin-film heater are sputtered. By thermally isolating the sample platform from surrounding parts, the sample temperature can be scanned at a high sweep rate of 200-300K/s. As a demonstration of the apparatus, the resistivity of a niobium-titanium alloy is measured in a pulsed magnetic field. In addition, as another application of the apparatus, the dielectric polarization measurement of the quantum-paraelectric material SrTiO3 is described.

Photo‐induced effect in quantum paraelectric materials: Transient birefringence measurement

AbstractThe photo‐induced effects in quantum paraelectric materials, strontium titanate SrTiO3 and Ca‐doped SrTiO3, are investigated. The observed birefringence for Cadoped SrTiO3 shows that an optical anisotropy along the [100] axis is generated by the ultraviolet (UV) illumination. The transient birefringence in a pulsed electric field is studied to probe the dynamical property of the polarization under dark and UV illumination.Under the UV illumination, the change of birefringence is reduced, and the dielectric response of relaxation type was observed both in SrTiO3 and in Ca‐doped SrTiO3. It was found that the ferroelectric phase transition temperature is shifted toward the lower temperature side under the UV illumination. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Optically induced lattice distortion and its spatial diffusion in the relaxed excited state of a quantum paraelectric material

AbstractWe have investigated the diffusion dynamics of the optically induced lattice distortion in the quantum paraelectric region of strontium titanate SrTiO3. The time evolution of the lattice distortion, which is accompanied by the polarization in the relaxed excited state, was observed by using the polarization spectroscopy with the pump‐probe technique. The observed signal of the optically induced lattice distortion has a component which decays in the nanosecond region.The decay curve of the lattice distortion depends on the beam diameter. The relaxation time becomes shorter as the beam diameter is decreased. We observed the lattice distortion induced by the separated pump beam from the probe beam. The signal of the lattice distortion appears later as the separation between the pump and probe beams becomes larger. These experimental results suggest that the optically induced lattice distortion diffuses spatially. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Indication of Tunneling State of Hydrogen Atom in Hydrogen-Bonded Material 5-Bromo-9-hydroxyphenalenon Studied by X-ray and Neutron Diffractions

The dielectric material with an isolated hydrogen-bond, 5-bromo-9-hydroxyphenalenon, was studied by means of neutron and X-ray diffractions in the temperature range between 300 and 10 K. Analyses on nuclear and electron density distributions revealed a discrepancy between the two distributions of the hydrogen atom in the hydrogen-bond region, leading to a local electronic dipole moment . Both a least-square structure analysis and the density distribution analysis by the maximum entropy method confirmed that the hydrogen atom occupies two equivalent sites near the center of the hydrogen-bond through the whole temperature range, even at the lowest temperature measured, 10 K. This feature, together with previous results on calorimetric, incoherent neutron scattering and far-infrared spectrum measurements, concludes that the hydrogen atom in the hydrogen-bond is in a tunneling state over the two sites. Consequently, 5-bromo-9-hydroxyphenalenon was experimentally identified as a quantum paraelectric material.

On the perovskite-related materials of high dielectric permittivity with small temperature dependence and low dielectric loss

Dielectric properties of quantum paraelectric materials, SrTiO3, CaTiO3 and (La1/2Na1/2)TiO3, were presented and discussed since they show the titled dielectric properties in the lowest temperature range. In order to show our strategy to obtain the titled materials for microwave resonator applications, dielectric properties of several solid solution systems, SrTiO3-LaAlO3, Sr2TiO4-SrLaAlO4 and BaTiO3-Ba(Mg1/3Ta1/3)O3 were exemplified. Concerning about relaxor ferroelectric materials, it is pointed out that a drastic broadening of ferroelectric phase transitions in the KNbO3-BaTiO3 and BaTiO3-Ba(Fe1/2Ta1/2)O3 solid solution systems is caused by a strong local field fractuation come from the randomly oriented permanent electric dipoles produced by the heterogeneous cationic charge distribution in the solid solution.