Effect of epitaxial strain on the spontaneous polarization of thin film ferroelectrics

Epitaxial strain can substantially enhance the spontaneous polarizations and Curie temperatures of ferroelectric thin films compared to the corresponding bulk materials. In this Letter we use first principles calculations to calculate the effect of epitaxial strain on the spontaneous polarization of the ferroelectrics , , and , and the multiferroic material . We show that the epitaxial strain dependence of the polarization varies considerably for the different systems, and in some cases is, in fact, very small. We discuss possible reasons for this different behavior and show that the effect of epitaxial strain can easily be understood in terms of the piezoelectric and elastic constants of the unstrained materials. Our results provide a computational tool for the quantitative prediction of strain behavior in ferroelectric thin films.     

Comment on the analysis of angle-dependent X-ray magnetic circular dichroism in systems with reduced dimensionality

The magnetic dipole term T appearing in the X-ray magnetic circular dichroism (XMCD) spin sum rule can be eliminated from the analysis within the spin sum rule by angle-dependent XMCD spectroscopy if the effects of spin–orbit coupling are small so that T_x+T_y+T_z≈0. It is shown by the ab initio electron theory for the extreme case of a low-dimensional system, i.e., for a free-standing monatomic Co wire that this relation is strongly violated, indicating that the determination of T by the angle-dependent XMCD is possibly not very reliable for low-dimensional magnetic systems.     

X-ray emission spectra of vanadium atoms in a new series of (Cu,V)-based high-Tc superconductors

Non-resonant V L_2,3 and OKα X-ray emission spectra of a new series of (Cu,V)Sr₂Ca_n−1Cu_nO_y (n=3-7) superconductors are presented. We have found that oxygen atoms surround V-atoms in the given compounds, form a (VO₄)³⁻ tetrahedrons and have a pentavalent state. This induces holes due to the substitution of divalent copper by pentavalent vanadium ions providing a hole-doping mechanism of superconductivity in this series of compounds. A spectral estimation of the oxygen concentration shows that the oxygen content under high-pressure/high-temperature synthesis conditions is not changed considerably from the start to the final product.     

X-ray microprobe of orbital alignment in strong-field ionized atoms

We have developed a synchrotron-based, time-resolved x-ray microprobe to investigate optical strong-field processes at intermediate intensities (10(14) - 10(15) W/cm2). This quantum-state specific probe has enabled the direct observation of orbital alignment in the residual ion produced by strong-field ionization of krypton atoms via resonant, polarized x-ray absorption. We found strong alignment to persist for a period long compared to the spin-orbit coupling time scale (6.2 fs). The observed degree of alignment can be explained by models that incorporate spin-orbit coupling. The methodology is applicable to a wide range of problems.     

Electronic Structure and Crystalline Coherence in Fe/Si Multilayers

Soft x-ray fluorescence spectroscopy has been used to examine the electronic structure of deeply buried silicide thin films that arise in Fe/Si multilayers. These systems exhibit antiferromagnetic (AF) coupling of the Fe layers, despite their lack of a noble metal spacer layer found in most GMR materials. Also, the degree of coupling is very dependent on preparation conditions, especially spacer layer thickness and growth temperature. The valence band spectra are quite different for films with different spacerlayer thickness yet are very similar for films grown at different growth temperatures. The latter result is surprising since AF coupling is strongly dependent on growth temperature. Combining near-edge x-ray absorption with the fluorescence data demonstrates that the local bonding structure in the silicide spacer layer in epitaxial films which exhibit AF coupling are metallic. These results indicate the equal roles of crystalline coherence and electronic structure in determining the magnetic properties of these systems.     

Resonant inelastic scattering in dilute magnetic semiconductors by soft X-ray fluorescence spectroscopy

1-x Mn_xS (where 0.05<x<1) were measured as the energy of the exciting radiation was tuned across the S and Mn-L_2,3 absorption edges of these compounds. Strong resonance peaks in Mn-L emission spectra and the systematic appearance of new spectral features in S-L emission spectra were observed. Partial substitution of Zn by a magnetic Mn ion results in strong hybridization of the Mn 3d orbitals with the sp band of the host semiconductor. A detailed study of resonant inelastic scattering in the vicinity of the S and Mn-L_2,3 absorption edges of these DMS is presented. Received: 6 March 1997