575 research outputs found
Effects of coupling between octahedral tilting and polar modes on the phase diagram of PbZr1-xTixO3 (PZT)
The results are presented of anelastic and dielectric spectroscopy
measurements on large grain ceramic PZT with compositions near the two
morphotropic phase boundaries (MPBs) that the ferroelectric (FE) rhombohedral
phase has with the Zr-rich antiferroelectric and Ti-rich FE tetragonal phases.
These results are discussed together with similar data from previous series of
samples, and reveal new features of the phase diagram of PZT, mainly connected
with octahedral tilting and its coupling with the polar modes. Additional
evidence is provided of what we interpret as the onset of the tilt instability,
when is initially frustrated by lattice disorder, and the long range order is
achieved at lower temperature. Its temperature T_IT(x) prosecutes the long
range tilt instability line T_T(x) up to T_C, when T_T. It is proposed that the
difficulty of seeing the expected 1/2 modulations in diffraction
experiments is due to the large correlation volume associated with that type of
tilt fluctuations combined with strong lattice disorder. It is shown that the
lines of the tilt instabilities tend to be attracted and merge with those of
polar instabilities. Not only T_IT bends toward T_C and then merges with it,
but in our series of samples the temperature T_MPB of the dielectric and
anelastic maxima at the rhombohedral/tetragonal MPB does not cross T_T, but
deviates remaining parallel or possibly merging with T_T. These features,
together with a similar one in NBT-BT, are discussed in terms of cooperative
coupling between tilt and FE instabilities, which may trigger a common phase
transition. An analogy is found with recent simulations of the tilt and FE
transitions in multiferroic BiFeO3. An abrupt change is found in the shape of
the anelastic anomaly at T_T when x passes from 0.465 to 0.48, possibly
indicative of a rhombohedral/monoclinic boundary.Comment: 15 pages, 11 figure
Piezoelectric softening in ferroelectrics: ferroelectric versus antiferroelectric PbZrTiO
The traditional derivation of the elastic anomalies associated with
ferroelectric (FE) phase transitions in the framework of the Landau theory is
combined with the piezoelectric constitutive relations instead of being
explicitly carried out with a definite expression of the FE part of the free
energy. In this manner it is shown that the softening within the FE phase is of
electrostrictive and hence piezoelectric origin. Such a piezoelectric softening
may be canceled by the better known piezoelectric stiffening, when the
piezoelectric charges formed during the vibration are accompanied by the
depolarization field, as for example in Brillouin scattering experiments. As
experimental validation, we present new measurements on Zr-rich PZT, where the
FE phase transforms into antiferroelectric on cooling or doping with La, and a
comparison of existing measurements made on FE PZT with low frequency and
Brillouin scattering experiments
Effects of aging and annealing on the polar and antiferrodistortive components of the antiferroelectric transition in PZT
The antipolar and antiferrodistortive (AFD) components of the
antiferroelectric (AFE) transition in PbZr1-xTixO3 (x=0.054) can occur
separately and with different kinetics, depending on the sample history, and
are accompanied by elastic softening and stiffening, respectively. Together
with the softening that accompanies octahedral tilting in the fraction of phase
that is not yet transformed into AFE, they give rise to a variety of shapes of
the curves of the elastic compliance versus temperature. All such anomalies
found in samples with x=0.046 and 0.054, in addition to those already studied
at x=0.050, can be fitted consistently with a phenomenological model based on
the simple hypothesis that each of the polar and AFD transitions produces a
step in the elastic modulus, whose position in temperature and width reflect
the progress of each transition. The slowing of the kinetics of the
transformations is correlated with the formation of defect structures during
aging in the ferroelectric or AFE state, which are also responsible for a
progressive softening of the lattice with time and thermal cycling, until
annealing at high temperature recovers the initial conditions
Correlation between molecular orbitals and doping dependence of the electrical conductivity in electron-doped Metal-Phthalocyanine compounds
We have performed a comparative study of the electronic properties of six
different electron-doped metal phthalocyanine (MPc) compounds (ZnPc, CuPc,
NiPc, CoPc, FePc, and MnPc), in which the electron density is controlled by
means of potassium intercalation. In spite of the complexity of these systems,
we find that the nature of the underlying molecular orbitals produce observable
effects in the doping dependence of the electrical conductivity of the
materials. For all the MPc's in which the added electrons are expected to
occupy orbitals centered on the ligands (ZnPc, CuPc, and NiPc), the doping
dependence of the conductivity has an essentially identical shape. This shape
is different from that observed in MPc materials in which electrons are also
added to orbitals centered on the metal atom (CoPc, FePc, and MnPc). The
observed relation between the macroscopic electronic properties of the MPc
compounds and the properties of the molecular orbitals of the constituent
molecules, clearly indicates the richness of the alkali-doped
metal-phthalocyanines as a model class of compounds for the investigation of
the electronic properties of molecular systems
Tuning the electronic transport properties of graphene through functionalisation with fluorine
Engineering the electronic properties of graphene has triggered great
interest for potential applications in electronics and opto-electronics. Here
we demonstrate the possibility to tune the electronic transport properties of
graphene monolayers and multilayers by functionalisation with fluorine. We show
that by adjusting the fluorine content different electronic transport regimes
can be accessed. For monolayer samples, with increasing the fluorine content,
we observe a transition from electronic transport through Mott variable range
hopping in two dimensions to Efros - Shklovskii variable range hopping.
Multilayer fluorinated graphene with high concentration of fluorine show
two-dimensional Mott variable range hopping transport, whereas CF0.28
multilayer flakes have a band gap of 0.25eV and exhibit thermally activated
transport. Our experimental findings demonstrate that the ability to control
the degree of functionalisation of graphene is instrumental to engineer
different electronic properties in graphene materials.Comment: 6 pages, 5 figure
Direct observation of a gate tunable band-gap in electrical transport in ABC-trilayer graphene
Few layer graphene systems such as Bernal stacked bilayer and rhombohedral
(ABC-) stacked trilayer offer the unique possibility to open an electric field
tunable energy gap. To date, this energy gap has been experimentally confirmed
in optical spectroscopy. Here we report the first direct observation of the
electric field tunable energy gap in electronic transport experiments on doubly
gated suspended ABC-trilayer graphene. From a systematic study of the
non-linearities in current \textit{versus} voltage characteristics and the
temperature dependence of the conductivity we demonstrate that thermally
activated transport over the energy-gap dominates the electrical response of
these transistors. The estimated values for energy gap from the temperature
dependence and from the current voltage characteristics follow the
theoretically expected electric field dependence with critical exponent .
These experiments indicate that high quality few-layer graphene are suitable
candidates for exploring novel tunable THz light sources and detectors.Comment: Nano Letters, 2015 just accepted, DOI: 10.1021/acs.nanolett.5b0077
Double-gated graphene-based devices
We discuss transport through double gated single and few layer graphene
devices. This kind of device configuration has been used to investigate the
modulation of the energy band structure through the application of an external
perpendicular electric field, a unique property of few layer graphene systems.
Here we discuss technological details that are important for the fabrication of
top gated structures, based on electron-gun evaporation of SiO. We perform
a statistical study that demonstrates how --contrary to expectations-- the
breakdown field of electron-gun evaporated thin SiO films is comparable to
that of thermally grown oxide layers. We find that a high breakdown field can
be achieved in evaporated SiO only if the oxide deposition is directly
followed by the metallization of the top electrodes, without exposure to air of
the SiO layer.Comment: Replaced with revised version. To appear on New Journal of Physic
- …