65 research outputs found
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
First-order structural transition in the magnetically ordered phase of Fe1.13Te
Specific heat, resistivity, magnetic susceptibility, linear thermal expansion
(LTE), and high-resolution synchrotron X-ray powder diffraction investigations
of single crystals Fe1+yTe (0.06 < y < 0.15) reveal a splitting of a single,
first-order transition for y 0.12. Most
strikingly, all measurements on identical samples Fe1.13Te consistently
indicate that, upon cooling, the magnetic transition at T_N precedes the
first-order structural transition at a lower temperature T_s. The structural
transition in turn coincides with a change in the character of the magnetic
structure. The LTE measurements along the crystallographic c-axis displays a
small distortion close to T_N due to a lattice striction as a consequence of
magnetic ordering, and a much larger change at T_s. The lattice symmetry
changes, however, only below T_s as indicated by powder X-ray diffraction. This
behavior is in stark contrast to the sequence in which the phase transitions
occur in Fe pnictides.Comment: 6 page
Second harmonic generation on incommensurate structures: The case of multiferroic MnWO4
A comprehensive analysis of optical second harmonic generation (SHG) on an
incommensurate (IC) magnetically ordered state is presented using multiferroic
MnWO4 as model compound. Two fundamentally different SHG contributions coupling
to the primary IC magnetic order or to secondary commensurate projections of
the IC state, respectively, are distinguished. Whereas the latter can be
described within the formalism of the 122 commensurate magnetic point groups
the former involves a breakdown of the conventional macroscopic symmetry
analysis because of its sensitivity to the lower symmetry of the local
environment in a crystal lattice. Our analysis thus foreshadows the fusion of
the hitherto disjunct fields of nonlinear optics and IC order in
condensed-matter systems
Symmetry and magnetically driven ferroelectricity in rare-earth manganites RMnO3 (R=Gd, Tb, Dy)
This work investigates the magnetically driven ferroelectricity in
orthorhombic manganites RMnO3 (R=Gd, Dy or Tb) from the point of view of the
symmetry. The method adopted generalizes the one used to characterize the polar
properties of displacive modulated structures to the case of an irreducible
magnetic order parameter. The symmetry conditions for magnetically induced
ferroelectricity are established and the Landau-Devonshire free energy
functionals derived from general symmetry considerations. The ferroelectric
polarisation observed in DyMnO3 and TbMnO3 at zero magnetic field is explained
in terms of the symmetry of a reducible magnetic order parameter. The
polarisation rotation induced in these compounds by external magnetic fields
and the stabilization of a ferroelectric phase in GdMnO3 are accounted for by a
mechanism in which magnetization and polarization are secondary order
parameters that are not directly coupled but compete with each other through
their coupling to competing primary modulated order parameters.Comment: Article submitted to Physical Review B, 39 page
Landau model for the phase diagrams of the orthorhombic rare-earth manganites RMnO3
The present work aims to describe, within a single phenomenological approach,
the specific sequence of phase transitions observed in the rare-earth
manganites RMnO3 at zero magnetic field. It is shown that a single integrated
description of the temperature versus composition phase diagrams of these
compounds and related solid solutions can be obtained within the scope of
Landau theory by adopting the so called type-II description of the modulated
phases.Comment: 37 pages, 7 figures, 4 table
Quasi-molecular and atomic phases of dense solid hydrogen
The high-pressure phases of solid hydrogen are of fundamental interest and
relevant to the interior of giant planets; however, knowledge of these phases
is far from complete. Particle swarm optimization (PSO) techniques were applied
to a structural search, yielding hitherto unexpected high-pressure phases of
solid hydrogen at pressures up to 5 TPa. An exotic quasi-molecular mC24
structure (space group C2/c, stable at 0.47-0.59 TPa) with two types of
intramolecular bonds was predicted, providing a deeper understanding of
molecular dissociation in solid hydrogen, which has been a mystery for decades.
We further predicted the existence of two atomic phases: (i) the oC12 structure
(space group Cmcm, stable at > 2.1 TPa), consisting of planar H3 clusters, and
(ii) the cI16 structure, previously observed in lithium and sodium, stable
above 3.5 TPa upon consideration of the zero-point energy. This work clearly
revised the known zero-temperature and high-pressure (>0.47 TPa) phase diagram
for solid hydrogen and has implications for the constituent structures of giant
planets.Comment: accepted in The Journal of Physical Chemistr
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