442 research outputs found
Application of Finite Strain Landau Theory To High Pressure Phase Transitions
In this paper we explain how to set up what is in fact the only possible
consistent construction scheme for a Landau theory of high pressure phase
transitions that systematically allows to take into account elastic
nonlinearities. We also show how to incorporate available information on the
pressure dependence of elastic constants taken from experiment or simulation.
We apply our new theory to the example of the high pressure cubic-tetragonal
phase transition in Strontium Titanate, a model perovskite that has played a
central role in the development of the theory of structural phase transitions.
Armed with pressure dependent elastic constants calculated by density
functional theory, we give a both qualitatively as well as quantitatively
satisfying description of recent high precision experimental data. Our
nonlinear theory also allows to predict a number of additional elastic
transition anomalies that are accessible to experiment.Comment: submitted to Phys. Rev. Let
Induced ferroelectric phases in TbMn_2O_5
The magnetostructural transitions and magnetoelectric effects reported in
TbMn2O5 are described theoretically and shown to correspond to two essentially
different mechanisms for the induced ferroelectricity. The incommensurate and
commensurate phases observed between 38 and 24 K exhibit a hybrid pseudoproper
ferroelectric nature resulting from an effective bilinear coupling of the
polarization with the antiferromagnetic order parameter. This explains the high
sensitivity of the dielectric properties of the material under applied magnetic
field. Below 24 K the incommensurate phase shows a standard improper
ferroelectric character induced by the coupling of two distinct magnetic order
parameters. The complex dielectric behavior observed in the material reflects
the crossover from one to the other transition regime. The temperature
dependences of the pertinent physical quantities are worked out, and previous
theoretical models are discussed
Cloning and Functional Analysis of three Cold Regulated <em>CBF</em> Genes in the Overwintering Crucifer <em>Boechera stricta</em>
Cloning and Functional Analysis of three Cold Regulated <em>CBF</em> Genes in the Overwintering Crucifer <em>Boechera stricta</em>
In this research, we isolated three CBF (C-repeat-Binding Factors) genes from two genotypes of Boechera stricta with contrasting freezing tolerance and characterized their structure and expression patterns in response to cold treatment. An amino acid sequence comparison revealed that the CBF genes in B. stricta showed high conservation in the AP2 domain and PKKP/RAGR motif like other cold adaptable Brassicaceae. The pairwise sequence alignment of the CBF genes isolated from two genotypes of B. stricta showed non-synonymous mutations in CBF 2 and 3. Gene expression analysis demonstrated that CBF genes in B. stricta have expression patterns similar to CBFs in A. thaliana in response to cold treatment, while differential expression at the molecular level in CBF and COR genes was presented between two genotypes of B. stricta. Our results suggest that signal transduction of three CBF genes can be one of the central pathways in the development of freezing tolerance in B. stricta
Finite strain Landau theory of high pressure phase transformations
The properties of materials near structural phase transitions are often
successfully described in the framework of Landau theory. While the focus is
usually on phase transitions, which are induced by temperature changes
approaching a critical temperature T-c, here we will discuss structural phase
transformations driven by high hydrostatic pressure, as they are of major
importance for understanding processes in the interior of the earth. Since at
very high pressures the deformations of a material are generally very large,
one needs to apply a fully nonlinear description taking physical as well as
geometrical nonlinearities (finite strains) into account. In particular it is
necessary to retune conventional Landau theory to describe such phase
transitions. In Troster et al (2002 Phys. Rev. Lett. 88 55503) we constructed a
Landau-type free energy based on an order parameter part, an order
parameter-(finite) strain coupling and a nonlinear elastic term. This model
provides an excellent and efficient framework for the systematic study of phase
transformations for a wide range of materials up to ultrahigh pressures
The nonlinear anomalous lattice elasticity associated with the high-pressure phase transition in spodumene: A high precission static compression study
The high-pressure behavior of the lattice elasticity of spodumene, LiAlSi2O6,
was studied by static compression in a diamond-anvil cell up to 9.3 GPa.
Investigations by means of single-crystal XRD and Raman spectroscopy within the
hydrostatic limits of the pressure medium focus on the pressure ranges around
similar to 3.2 and similar to 7.7 GPa, which have been reported previously to
comprise two independent structural phase transitions. While our measurements
confirm the well-established first-order C2/c-P2(1)/c transformation at 3.19
GPa (with 1.2% volume discontinuity and a hysteresis between 0.02 and 0.06
GPa), both unit-cell dimensions and the spectral changes observed in
high-pressure Raman spectra give no evidence for structural changes related to
a second phase transition. Monoclinic lattice parameters and unit-cell volumes
at in total 59 different pressure points have been used to re-calculate the
lattice-related properties of spontaneous strain, volume strain, and the bulk
moduli as a function of pressure across the transition. A modified Landau free
energy expansion in terms of a one component order parameter has been developed
and tested against these experimentally determined data. The Landau solution
provides a much better reproduction of the observed anomalies than any
equation-of-state fit to data sets truncated below and above P (tr), thus
giving Landau parameters of K (0) = 138.3(2) GPa, K' = 7.46(5), lambda (V) =
33.6(2) GPa, a = 0.486(3), b = -29.4(6) GPa and c = 551(11) GPa
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