246 research outputs found
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
Theory of antiferroelectric phase transitions
At variance with structural ferroic phase transitions which give rise to
macroscopic tensors coupled to macroscopic fields, criteria defining
antiferroelectric (AFE) phase transitions are still under discussion due to the
absence of specific symmetry properties characterizing their existence. They
are recognized by the proximity of a ferroelectric (FE) phase induced under
applied electric field, with a double hysteresis loop relating the induced
polarization to the electric field and a typical anomaly of the dielectric
permittivity. Here, we show that there exist indeed symmetry criteria defining
AFE transitions. They relate the local symmetry of the polar crystallographic
sites emerging at an AFE phase transition with the macroscopic symmetry of the
AFE phase. The dielectric properties of AFE transitions are deduced from a
Landau theoretical model in which ferroelectric and ferrielectric phases are
shown to stabilize as the result of specific symmetry-allowed couplings of the
AFE order- parameter with the field-induced polarization.Comment: 7 pages, 5 figures, 1 tabl
Order-Parameter Symmetries of Domain Walls in Ferroelectrics and Ferroelastics
The symmetry of boundaries between ferroelectric, ferroelastic and antiphase
domains is a key element for a theoretical understanding of their properties.
Here, we derive this symmetry from their organic relation to the symmetry of
the primary transition order parameters. The domain wall symmetries are shown
to coincide with directions of the order-parameter n-dimensional vector space,
corresponding to sum of the vectors associated with adjacent domain states.
This property is illustrated by the determination of the domain wall maximal
symmetries in BaTiO3, LaAlO3, SrTiO3 and Gd2(MoO4)3. Besides, the domain
pattern in YMnO3 is interpreted as resulting from an annihilation-creation
process, the annihilation of the antiphase domain walls creating six
ferroelectric domain walls merging at a single point.Comment: 5 pages, 3 figure
First-order multi-k phase transitions and magnetoelectric effects in multiferroic Co3TeO6
A theoretical description of the sequence of magnetic phases in Co3TeO6 is
presented. The strongly first-order character of the transition to the
commensurate multiferroic ground state, induced by coupled order parameters
corresponding to different wavevectors, is related to a large magnetoelastic
effect with an exchange energy critically sensitive to the interatomic spacing.
The monoclinic magnetic symmetry C2' of the multiferroic phase permits
spontaneous polarization and magnetization as well as the linear
magnetoelectric effect. The existence of weakly ferromagnetic domains is
verified experimentally by second harmonic generation measurements
Pressure Induced Quantum Phase Transitions
A quantum critical point is approached by applying pressure in a number of
magnetic metals. The observed dependence of Tc on pressure necessarily means
that the magnetic energy is coupled to the lattice. A first order phase
transition occurs if this coupling exceeds a critical value: this is inevitable
if diverges as Tc approaches zero. It is argued that this is the cause of the
first order transition that is observed in many systems. Using Landau theory we
obtain expressions for the boundaries of the region where phase separation
occurs that agree well with experiments done on MnSi and other materials. The
theory can be used to obtain very approximate values for the temperature and
pressure at the tricritical point in terms of quantities measured at ambient
pressure and the measured values of along the second order line. The values of
the tricritical temperature for various materials obtained from Landau theory
are too low but it is shown that the predicted values will rise if the effects
of fluctuations are included.Comment: 12 pages including figure
General two-order-parameter Ginzburg-Landau model with quadratic and quartic interactions
Ginzburg-Landau model with two order parameters appears in many
condensed-matter problems. However, even for scalar order parameters, the most
general U(1)-symmetric Landau potential with all quadratic and quartic terms
contains 13 independent coefficients and cannot be minimized with
straightforward algebra. Here, we develop a geometric approach that circumvents
this computational difficulty and allows one to study properties of the model
without knowing the exact position of the minimum. In particular, we find the
number of minima of the potential, classify explicit symmetries possible in
this model, establish conditions when and how these symmetries are
spontaneously broken, and explicitly describe the phase diagram.Comment: 36 pages, 7 figures; v2: added additional clarifications and a
discussion on how this method differs from the MIB-approac
New critical behavior in unconventional ferromagnetic superconductors
New critical behavior in unconventional superconductors and superfluids is
established and described by the Wilson-Fisher renormalization-group method.
For certain ordering symmetries a new type of fluctuation-driven first order
phase transitions at finite and zero temperature are predicted. The results can
be applied to a wide class of ferromagnetic superconducting and superfluid
systems, in particular, to itinerant ferromagnets as UGe2 and URhGe.Comment: 12 pages, 6 fig
Editorial
Ce numéro du bulletin de la SABIX est consacré à la collection d’instruments scientifiques anciens de la bibliothèque de l’Ecole. On y trouvera la description de certains des objets conservés ainsi que l’analyse de leur fonctionnement. La présentation de ces objets est l’occasion de souligner que l’étude des caractéristiques des instruments anciens, et la répétition des expériences pour lesquelles ils ont été fabriqués, nous renseigne sur la stratégie expérimentale des savants qui les ont con..
Microstructure from ferroelastic transitions using strain pseudospin clock models in two and three dimensions: a local mean-field analysis
We show how microstructure can arise in first-order ferroelastic structural
transitions, in two and three spatial dimensions, through a local meanfield
approximation of their pseudospin hamiltonians, that include anisotropic
elastic interactions. Such transitions have symmetry-selected physical strains
as their -component order parameters, with Landau free energies that
have a single zero-strain 'austenite' minimum at high temperatures, and
spontaneous-strain 'martensite' minima of structural variants at low
temperatures. In a reduced description, the strains at Landau minima induce
temperature-dependent, clock-like hamiltonians, with
-component strain-pseudospin vectors pointing to
discrete values (including zero). We study elastic texturing in five such
first-order structural transitions through a local meanfield approximation of
their pseudospin hamiltonians, that include the powerlaw interactions. As a
prototype, we consider the two-variant square/rectangle transition, with a
one-component, pseudospin taking values of , as in a
generalized Blume-Capel model. We then consider transitions with two-component
() pseudospins: the equilateral to centred-rectangle ();
the square to oblique polygon (); the triangle to oblique ()
transitions; and finally the 3D cubic to tetragonal transition (). The
local meanfield solutions in 2D and 3D yield oriented domain-walls patterns as
from continuous-variable strain dynamics, showing the discrete-variable models
capture the essential ferroelastic texturings. Other related hamiltonians
illustrate that structural-transitions in materials science can be the source
of interesting spin models in statistical mechanics.Comment: 15 pages, 9 figure
- …