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SrTiO3 thin films were used as a model system to study the effects of strain and epitaxial constraint on structural phase transitions of perovskite films. The basic phenomena revealed will apply to a variety of important structural transitions including the ferroelectric transition. Highly strained SrTiO3 films were grown on different substrates, providing both compressive and tensile strain. The measured strain-temperature phase diagram is qualitatively consistent with theory, however the increase in the phase transition temperature is much larger than predicted. Due to the epitaxial strain and substrate clamping, the SrTiO3 lattice is tetragonal at all temperatures. The phase transitions involve only changes in internal symmetry. The low temperature phase under tensile strain has a unique structure with orthorhombic Cmcm space group but a tetragonal lattice, an interesting consequence of epitaxial constraint.An important area of research in recent years has been to understand exactly why the properties of epitaxial films differ from related bulk materials, and to learn how to use these differences to engineer desirable properties. Effects due to strain in films are often thought of as analogous to those of high pressure experiments. The main differences are: (1) The stress in films is typically biaxial rather than hydrostatic or uniaxial. (2) Films are subject to the additional constraint from the substrate. (3) Films can have much larger strains than is usually achievable in pressure cells. (4) Films are easier to work with for many experiments and applications. Ferroelectricity in strained perovskite films has been a particularly noteworthy topic of study. For example, recent experiments have shown strain-induced ferroelectricity in SrTiO 3 (STO) films, and huge changes in the ferroelectric transition temperature T C in both SrTiO 3 and BaTiO 3 (BTO) films under strain [1,2]. Other notable results include large changes in the metal-insulator transition temperature of RNiO 3 films under strain [3] and reports of increasing the transition temperature of La 2−x Sr x CuO 4 under compressive strain [4,5,6,7].In this letter, we present a systematic investigation of the structural phase transition in epitaxial STO films with varying degrees of both compressive and tensile strain. This transition is not ferroelectric, though it has been described by the same theory as used for the ferroelectric transition in STO [8] and referenced in the works above. We construct a strain-temperature phase diagram for this transition with several data points over a wide range of strain. The observed enhancement of the structural phase transition temperature T s , is much greater than predicted by theory, though in many aspects the theoretical predictions are qualitatively correct. We also note that in some cases the film structures have unique symmetry not possible in a free bulk system. We chose the anti-ferrodistortive structural phase transition in STO as a model system for phase transitions in epitaxial films. The principal adv...

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Inelastic neutron scattering measurements of the relaxor ferroelectric PbMg 1/3 Nb 2/3 O3 (PMN) in the temperature range 490 K

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While benefit–cost analysis (BCA) is now a permanent part of the regulatory process in the United States, and many other countries around the world as well as the European Union have adopted it or are moving toward it, there have been few empirical attempts to assess either whether its use improves regulations or how BCA interacts with the political environment. We use a unique US database of the costs and benefits of 109 economically significant regulations issued between 2000 and 2009 to examine whether the amount of information provided in the BCA or political factors surrounding the regulation better correlate with the net benefits of the regulation. We find that there is little correlation between the information provided by the analysis and the net benefits. However, we find that regulations that receive few public comments and are not issued at the end of an administration, have the highest net benefits. These are the regulations that are the least politically salient. This interaction between the political environment and the economic performance of a regulation has been under‐examined and deserves further study.

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An inelastic neutron scattering study of the lattice dynamics of the martensite phase of the ferromagnetic shape memory alloy, It has been over 50 years since Peierls pointed out that in a one-dimensional metal where the electrons are coupled to the underlying lattice, the lattice is unstable at low temperature (1) . The ground state of this system has a charge density wave (CDW) where the electron density is modulated with a wavelength related to the Fermi wave vector, k F and characterized by a gap in the single-particle excitation spectra (2) . In order to maintain charge neutrality, the lattice will also be spatially modulated with wave-vector 2k F .CDW ground states were initially observed in a number of low dimensional solids because a large electronic susceptibility develops as a result of nesting of various flat pieces of the Fermi surface. This wave-vector is usually incommensurate with the lattice.The complex quantity that represents the lattice distortion accompanying the CDW may be taken as the order parameter of the phase transitions and has an amplitude and phase.The fluctuations in these quantities are called, respectively, amplitudons and phason.Because the energy of the system does not depend upon the phase of the order parameter it is considered the symmetry breaking Goldstone mode and is gapless. These excitations have been extensively studied over the last 30 years. The amplitude mode has been