Model for the Low-Temperature Lattice Anomaly in SrTiO3and KTaO3

Abstract

nent in the electronic wave function outside a solid surface. This feature is unique to INS among the electron spectroscopies used to determine surface electronic structure.Because the effective neutralization energy of the ion is ~ 2 eV less than the free-space value, 5 we can estimate the distance s m where neutralization occurs. Equating &E n = 2 eV to the principal interaction term, namely the image interaction (3.6 eV A)/s m , we obtain s m~ 1.8 A as the distance of the ion from the image plane. This simple image interaction has been shown to hold with good accuracy to 1.5 A. 11 From the equation s m =(l/a)ln(A/av) given earlier we obtain As m = (l/a)\n(v 2 /v^) as the difference in neutralization distances for ions of velocities v Y and v 2 . The parameter a that specifies the rate of exponential falloff of R t (s) is taken as 2X, where exp(-As) is the rate of decay outside the surface of a p wave function at E -£ vac ~ -7 eV, the principal determinant of where neutralization occurs. From A = (2mE i /n 2 ) 1/2 with E^l eV we obtain a =2.8 A" 1 . This estimate agrees closely with the variation of charge density outside the Si surface calculated by Appelbaum and Hamann. 12 Adding the 2-eV image acceleration to the experimental ion energies we obtain ^2/^t = (f|) 1/2 = l 0 35 and s m~ (1/2.8) xln(1.35)~0.1 A. Hence the relative change in the Hg d orbital components that we see in the U(E) functions of Fig. 2 occurs over a distance increment of ~0.1 A at an ion-surface separation of ~2 A.The observed changes in the Hg d orbital intensity have only relative significance. The absolute magnitude cannot be determined because we do not know a priori either the electron escape probability function or the factor R 2 used in the Strontium titanate and potassium tantalate are incipient ferroelectrics. Their static dielectric constant follows a Curie-Weiss law down to about one-curve method.We conclude that ion-neutralization spectroscopy using our new one-curve debroadening procedure yields information about the relative variation of wave-function components outside a surface, We believe this to be a significant addition to the capabilities of electron spectroscopies because we expect it to help us understand the intensity differences of orbital resonances in chemisorption systems as observed by INS and UPS. This, in turn, should yield information about the wave-function character of particular orbital resonances.We acknowledge with thanks helpful discussions with J. A. Appelbaum and J. E. Rowe, as well as the technical assistance of Philip Petrie. 1 H. D. Hagstrum, Phys. Rev. 150, 495 (1966). 2 J. A. Appelbaum and D. R. Hamann, Phys. Rev. B 12, 5590 (1975).The low-temperature anomaly in the lattice dynamics of SrTi0 3 and KTa0 3 is described as resulting from an anharmonic coupling between an optical and an acoustic branch. The numerical calculations performed for oo TO at the center of the Brillouin zone give an excellent fit to the available data from 4 to 300 K. It is shown that the ferroelectric phase transition i...

First-principles study of ferroelectric and antiferrodistortive instabilities in tetragonalSrTiO3

“…[15][16][17][18][19][20][21] Recently, efforts have focused on the socalled ''quantum paraelectric state'' in which quantum fluctuations of the atomic positions suppress the FE transition and lead to a stabilized paraelectric state. [17][18][19][20]22,23 This hypothesis has received dramatic support from a recent experiment showing that isotopically exchanged SrTi 18 O 3 appears to become ferroelectric at 23 K, 24 suggesting that normal SrTi 16 O 3 must be very close indeed to the ferroelectric threshold.…”

Quantum paraelectricity and subsequent disappearance of bond alternation of molecule caused by proton dynamics in squaric acid crystal

“…From a viewpoint of dielectrics, the C\h-distorted H2SQ molecule produces a large polarizability, but cannot show long-range ordering due to the quantum fluctuation of the proton-skeleton coupled motion. Therefore, the presently observed phenomenon may be interpreted in terms of the concept of quantum paraelectricity, which was originally introduced to interpret the anomalous dielectric properties for SrTiC^ and KTaC>3 [19,20] near 0 K. A similar proton-skeleton coupled motion may also play some role in the T-P phase diagrams for the hydrogen-bonded ionic crystals of KDP families [5,6], in which the PC>4 3~ tetrahedrons show temperature-dependent C2-type distortions [21], and large dielectric constants near 0 K in their pressureinduced PE phase [5].…”

Effect of magnetic field on the capacitance of the La0.5Ca0.5MnO3‐based heterojunction

“…The emergence of a maximal capacitance is a feature that has been observed in other manganite junctions before 8, 9, and can be ascribed to SrTiO 3 . As reported, the dielectric properties of SrTiO 3 undergo an abnormal variation below ∼60 K 11, 12. It is interesting to note the reduction of the maximal capacitance in LCMO/LMO/STON, which could be a signature of an enhanced built‐in potential in depletion layer.…”

“…This behavior appears accompanying the dielectric anomaly of SrTiO 3 , and could be a signature of dielectric correlation between electric dipole moments. It has been suggested that the antiferroelectricity or quantum paraelectricity occurs in SrTiO 3 below ∼60 K 11, 12. The evolution of the frequency dependency of the capacitance with dc biases deserves special attention.…”