Dynamical Properties of Ferroelectric Perovskites (Ba,Sr)TiO3 and Pb(Zr,Ti)O3 Systems from First Principles

A first-principles-based effective Hamiltonian scheme which incorporates coupling between ferroelectric (FE) and antiferrodistortive (AFD) motions is applied to Pb(Zr,Ti)O3 alloys. It validates the existence of two modes of E symmetry (rather than the single E(1TO) soft mode) in the 50-75 cm-1 range for temperatures smaller than 200 K and for compositions falling within the Rhombohedral R3c phase. Coupling between long-range-ordered FE and AFD motions is shown to be the cause of the additional mode and more insight into its nature is provided. This scheme is further used to reveal a field-induced anticrossing involving FE and AFD degrees of freedom for Ti composition of 45%. Molecular dynamics (MD) simulations, across the morphotropic phase boundary (MPB) of disordered Pb(Zr,Ti)O3 solid solutions at 10 K, confirms the existence of similar additional modes in the the monoclinic Cc and tetragonal I4cm phases. Lifting of degeneracy of E modes in the Cc phase each giving A\u27 + A\u27\u27 modes is seen in accordance with group theory predictions. In particular, a compositional-induced anticrossing occurring within the bridging Cc state is revealed, and the difference in frequency between A\u27 and A\u27\u27 modes in the Cc state is linked to a quantity introduced here and termed the monoclinic depth. Analytical models are further developed to reproduce and better understand characteristics of these modes across the MPB. Furthermore, a Fermi resonance (FR) emerging from the nonlinear coupling between ferroelectric motions and tiltings of oxygen octahedra is exposed. This FR manifests itself as the doubling of a nominally single FE mode in a purely FE phase, when the resonant frequency of the FE mode is close to the first overtone of the tiltings. It is shown, through the use of an analytical model (that captures the essence of the effect), that the FR is the result of a nonlinear coupling that is proportional to the spontaneous polarization of the material. MD simulations incorporating a first-principles-based effective Hamiltonian scheme consisting of FE and strain degrees of freedom, are conducted on (Ba,Sr)TiO3 (BST) bulks and epitaxially strained SrTiO3 (STO) thick films at finite temperature. The appearance of a central mode (CM) is confirmed and splitting of soft mode (SM) into out-of-plane and in-plane modes is predicted for strained STO films and two CM\u27s are predicted for Ba0.5Sr0.5TiO3 in FE phase. Symmetries of modes in FE phases originating from the soft-mode are discussed. Electrostriction energy is shown to be governing the Curie temperature Tc and determine the type of FE phase transition each system undergoes. The comparatively large electrostriction energy in BST systems is also pointed to be behind the emergence of the CM in PE phase of them. Moreover, MD simulation are performed of BST bulks and epitaxially strained STO thick films to obtain dielectric tunability &tau(E), as a function of electric field applied along the polarization. Landau-Devoshire theroy based fittings are shown to inaccurately describe &tau(E) in the low-field regime and the presence of strong CM in this regime is claimed to be the cause of this discrepancy in these systems

Spectral signatures of thermal spin disorder and excess Mn in half-metallic NiMnSb

Effects of thermal spin disorder and excess Mn on the electronic spectrum of half-metallic NiMnSb are studied using first-principles calculations. Temperature-dependent spin disorder, introduced within the vector disordered local moment model, causes the valence band at the $\Gamma$ point to broaden and shift upwards, crossing the Fermi level and thereby closing the half-metallic gap above room temperature. The spectroscopic signatures of excess Mn on the Ni, Sb, and empty sites (Mn$_\mathrm{Ni}$, Mn$_\mathrm{Sb}$, and Mn$_\mathrm{E}$) are analyzed. Mn$_\mathrm{Ni}$ is spectroscopically invisible. The relatively weak coupling of Mn$_\mathrm{Sb}$ and Mn$_\mathrm{E}$ spins to the host strongly deviates from the Heisenberg model, and the spin of Mn$_\mathrm{E}$ is canted in the ground state. While the half-metallic gap is preserved in the collinear ground state of Mn$_\mathrm{Sb}$, thermal spin disorder of the weakly coupled Mn$_\mathrm{Sb}$ spins destroys it at low temperatures. This property of Mn$_\mathrm{Sb}$ may be the source of the observed low-temperature transport anomalies.Comment: 5 pages, 7 figures, updated version with minor revisions and an additional figure, accepted in Phys. Rev. B (Rapid Communication

Stationary phase expression of the arginine biosynthetic operon argCBH in Escherichia coli

BACKGROUND: Arginine biosynthesis in Escherichia coli is elevated in response to nutrient limitation, stress or arginine restriction. Though control of the pathway in response to arginine limitation is largely modulated by the ArgR repressor, other factors may be involved in increased stationary phase and stress expression. RESULTS: In this study, we report that expression of the argCBH operon is induced in stationary phase cultures and is reduced in strains possessing a mutation in rpoS, which encodes an alternative sigma factor. Using strains carrying defined argR, and rpoS mutations, we evaluated the relative contributions of these two regulators to the expression of argH using operon-lacZ fusions. While ArgR was the main factor responsible for modulating expression of argCBH, RpoS was also required for full expression of this biosynthetic operon at low arginine concentrations (below 60 μM L-arginine), a level at which growth of an arginine auxotroph was limited by arginine. When the argCBH operon was fully de-repressed (arginine limited), levels of expression were only one third of those observed in ΔargR mutants, indicating that the argCBH operon is partially repressed by ArgR even in the absence of arginine. In addition, argCBH expression was 30-fold higher in ΔargR mutants relative to levels found in wild type, fully-repressed strains, and this expression was independent of RpoS. CONCLUSION: The results of this study indicate that both derepression and positive control by RpoS are required for full control of arginine biosynthesis in stationary phase cultures of E. coli

Elastic excitations in BaTiO_{3} single crystals and ceramics: Mobile domain boundaries and polar nanoregions observed by resonant ultrasonic spectroscopy

The dynamic properties of elastic domain walls in BaTiO3 were investigated using resonance ultrasonic spectroscopy (RUS). The sequence of phase transitions is characterized by minima in the temperature dependence of RUS resonance frequencies and changes in Q factors (resonance damping). Damping is related to the friction of mobile twin boundaries (90° ferroelectric walls) and distorted polar nanoregions (PNRs) in the cubic phase. Damping is largest in the tetragonal phase of ceramic materials but very low in single crystals. Damping is also small in the low-temperature phases of the ceramic sample and slightly increases with decreasing temperature in the single crystal. The phase angle between the real and imaginary part of the dynamic response function changes drastically in the cubic and tetragonal phases and remains constant in the orthorhombic phase. Other phases show a moderate dependence of the phase angle on temperature showing systematic changes of twin microstructures. Mobile twin boundaries (or sections of twin boundaries such as kinks inside twin walls) contribute strongly to the energy dissipation of the forced oscillation while the reduction in effective modulus due to relaxing twin domains is weak. Single crystals and ceramics show strong precursor softening in the cubic phase related to polar nanoregions (PNRs). The effective modulus decreases when the transition point of the cubic-tetragonal transformation is approached from above. The precursor softening follows temperature dependence very similar to recent results from Brillouin scattering. Between the Burns temperature (≈586 K) and Tc at 405 K, we found a good fit of the squared RUS frequency [∼Δ (C11−C12)] to a Vogel–Fulcher process with an activation energy of ∼0.2 eV. Finally, some first-principles-based effective Hamiltonian computations were carried out in BaTiO3 single domains to explain some of these observations in terms of the dynamics of the soft mode and central mode

Elastic excitations in BaTiO_{3} single crystals and ceramics: Mobile domain boundaries and polar nanoregions observed by resonant ultrasonic spectroscopy

The dynamic properties of elastic domain walls in BaTiO3 were investigated using resonance ultrasonic spectroscopy (RUS). The sequence of phase transitions is characterized by minima in the temperature dependence of RUS resonance frequencies and changes in Q factors (resonance damping). Damping is related to the friction of mobile twin boundaries (90° ferroelectric walls) and distorted polar nanoregions (PNRs) in the cubic phase. Damping is largest in the tetragonal phase of ceramic materials but very low in single crystals. Damping is also small in the low-temperature phases of the ceramic sample and slightly increases with decreasing temperature in the single crystal. The phase angle between the real and imaginary part of the dynamic response function changes drastically in the cubic and tetragonal phases and remains constant in the orthorhombic phase. Other phases show a moderate dependence of the phase angle on temperature showing systematic changes of twin microstructures. Mobile twin boundaries (or sections of twin boundaries such as kinks inside twin walls) contribute strongly to the energy dissipation of the forced oscillation while the reduction in effective modulus due to relaxing twin domains is weak. Single crystals and ceramics show strong precursor softening in the cubic phase related to polar nanoregions (PNRs). The effective modulus decreases when the transition point of the cubic-tetragonal transformation is approached from above. The precursor softening follows temperature dependence very similar to recent results from Brillouin scattering. Between the Burns temperature (≈586 K) and Tc at 405 K, we found a good fit of the squared RUS frequency [∼Δ (C11−C12)] to a Vogel–Fulcher process with an activation energy of ∼0.2 eV. Finally, some first-principles-based effective Hamiltonian computations were carried out in BaTiO3 single domains to explain some of these observations in terms of the dynamics of the soft mode and central mode

Expression of in WT, Δ, Δand ΔΔstrains on LB plates containing X-Gal

<p><b>Copyright information:</b></p><p>Taken from "Stationary phase expression of the arginine biosynthetic operon in "</p><p>BMC Microbiology 2006;6():14-14.</p><p>Published online 22 Feb 2006</p><p>PMCID:PMC1413537.</p><p>Copyright © 2006 Weerasinghe et al; licensee BioMed Central Ltd.</p