How electron stripes yield ferroelectricity
A novel type of ferroelectricity, called “electronic ferroelectricity” – with an essential connection to charge ordering – is strongly suggested by new micrographic evidence. Conventional theory of solids holds that ferroelectricity in general originates from atomic structural polarizations – a familiar example is the notable off-center shift in the perovskite BaTiO3. Yet our new work appearing in Physical Review Letters (LY10555) reveals that ferroelectric LuFe2O4 has a curious ground state distinguished by electron stripes. We discovered that these electron stripes manifest a frustrated charge density wave with a remarkable ferroelectric polarization. This 3-dimensional charge ordering state, occurring at a low temperature of about 20K, was directly revealed for the first time by our in-situ transmission electron microscopy (TEM). A remarkable series of richly varied structural phenomena were also recorded as we lowered the temperature from 300K to 20K. The clear micrographic results have enabled us to detect new details about spontaneous polarization.
P.S. fig.1 (a) Electron diffraction image showing the weak satellite spots from charge stripe order. (b) Model for charge stripes and ferroelectric polarization.
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Orange reflection from a three-dimensional photonic crystal in the
scales of the weevil Pachyrrhynchus congestus pavonius (Curculionidae)
Welch,Victoria/Lousse,Virginie/Deparis,Olivier/Parker,Andrew/Vigneron, Phys Rev E.
The three-dimensional structure which causes the colouration of the
tropical weevil \textit{Pachyrrhynchus congestus pavonius} was studied,
using a combination of electron microscopy, optical spectroscopy and
numerical modelling. The orange scales which cover the coloured rings
on the animal's body were opened, to display the structure responsible
for the colouration. This structure is a three-dimensional photonic
polycrystal, each grain of which showing a face-centred cubic symmetry.
The measured lattice parameter and the observed filling fraction of
this structure explains the dominant reflected wavelength in the reddish
orange. The long-range disorder introduced by the grain boundaries
explain
the paradoxical observation that the reflectance, although generated by
a photonic-crystal, is insensitive to changes in the viewing angle.
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Turning electrons around : Echo as a measure for reversibility
An echo, a phenomenon known for sound (acoustic) waves, can be
observed with electronic waves in atoms. While a sound echo
results from the reflection of a sound wave at a hard object,
the echo of an electronic wave is a reflection induced by a rapid change
of an external field. With such tricks we can reverse the motion of the
electron leading to the recurrence of the initial wave. Using a technique
similar to the spin echo (the echo of the nuclear spins of molecules) we
demonstrate in this paper the echoes of electronic waves in atoms by
reversing the arrow of time. Like sound waves, electron waves may be
damped (decohere) during the propagation due to interactions with the
environment and the intensity of echoes is reduced. Measurements of echoes
can be thus used to quantify how much information initially stored in
atoms survives during the time propagation and can be retrieved.
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X-ray standing wave detects atoms taking tiny steps.
When you pull a string of a guiter to generate a musical note, a standing wave pattern is generated on the string. This is a mechanical wave. We generate standing waves of X-rays, which are electromagnetic waves, in an artificially made layered structure (multilayer) - like those used in the read head in your computer. Standing waves of X-rays can detect an impurity atom taking tiny steps (~ 0.2 nanometer) in such a multilayer. We cause such minute atomic movements (of Fe) in a Pt/C multilayer containing Fe impurity by shooting an energetic ion beam. We track the movements of Fe using an X-ray standing wave as we shoot more and more ions to the multilayer, where Fe atoms are driven out of C layers and captured in the Pt layers. This converts a nonmagnetic material into a ferromagnetic material by forming FePt magnetic nanoparticles. The method holds promise for future Terabit magnetic storage devices as nanometer sized ferromagnetic dots can be created in a nonmagnetic medium using a focused ion beam.
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