66 research outputs found
Dynamics of relaxor ferroelectrics
We study a dynamic model of relaxor ferroelectrics based on the spherical
random-bond---random-field model and the Langevin equations of motion. The
solution to these equations is obtained in the long-time limit where the system
reaches an equilibrium state in the presence of random local electric fields.
The complex dynamic linear and third-order nonlinear susceptibilities
and , respectively, are calculated as
functions of frequency and temperature. In analogy with the static case, the
dynamic model predicts a narrow frequency dependent peak in ,
which mimics a transition into a glass-like state.Comment: 15 pages, Revtex plus 5 eps figure
Dynamics of relaxor ferroelectrics
We study a dynamic model of relaxor ferroelectrics based on the spherical
random-bond---random-field model and the Langevin equations of motion. The
solution to these equations is obtained in the long-time limit where the system
reaches an equilibrium state in the presence of random local electric fields.
The complex dynamic linear and third-order nonlinear susceptibilities
and , respectively, are calculated as
functions of frequency and temperature. In analogy with the static case, the
dynamic model predicts a narrow frequency dependent peak in ,
which mimics a transition into a glass-like state.Comment: 15 pages, Revtex plus 5 eps figure
Materials with Colossal Dielectric Constant: Do They Exist?
Experimental evidence is provided that colossal dielectric constants, epsilon
>= 1000, sometimes reported to exist in a broad temperature range, can often be
explained by Maxwell-Wagner type contributions of depletion layers at the
interface between sample and contacts, or at grain boundaries. We demonstrate
this on a variety of different materials. We speculate that the largest
intrinsic dielectric constant observed so far in non-ferroelectric materials is
of order 100.Comment: 3 figure
Tuning a sign of magnetoelectric coupling in paramagnetic NH2(CH3)2Al1-xCrx(SO4)*6H2O crystals by metal ion substitution
Hybrid organometallic systems offer a wide range of functionalities,
including magnetoelectric interactions. However, the ability to design
on-demand ME coupling remains challenging despite a variety of host-guest
configurations and ME phases coexistence possibilities. Here, we report the
effect of metal-ion substitution on the magnetic and electric properties in the
paramagnetic ferroelectric DMAAS crystals. Doing so we are able to induce and
even tune a sign of the ME interactions in the paramagnetic ferroelectric
state. Both studied samples with 6.5% and 20% of Cr become paramagnetic,
contrary to the initial diamagnetic compound. Due to the isomorphous
substitution with Cr the ferroelectric phase transition temperature increases
nonlinearly, with the shift being larger for the sample with Cr content of
6.5%. A magnetic field applied along the polar c axis increases
ferroelectricity for this sample and shifts Tc to higher values, while inverse
effects are observed for sample containing 20% of Cr. The ME coupling
coefficient of 1.7ns/m found for a crystal with 20% of Cr is among the highest
reported up to now. The observed sign change of ME coupling coefficient with a
small change in Cr content paves the way for ME coupling engineering.Comment: 7 pages, 6 figures. New Organometallic Magnetoelectric and
Ferroelectric compoun
Ground State of Relaxor Ferroelectric
High energy x-ray diffraction measurements on Pb(ZnNb)O
(PZN) single crystals show that the system does not have a rhombohedral
symmetry at room temperature as previously believed. The new phase (X) in the
bulk of the crystal gives Bragg peaks similar to that of a nearly cubic lattice
with a slight tetragonal distortion. The Bragg profile remains sharp with no
evidence of size broadening due to the polar micro crystals (MC). However, in
our preliminary studies of the skin, we have found the expected rhombohedral
(R) phase as a surface state. On the other hand, studies on an electric-field
poled PZN single crystal clearly indicate a rhombohedral phase at room
temperature.Comment: 11 pages with 3 figure
Sr2V3O9 and Ba2V3O9: quasi one-dimensional spin-systems with an anomalous low temperature susceptibility
The magnetic behaviour of the low-dimensional Vanadium-oxides Sr2V3O9 and
Ba2V3O9 was investigated by means of magnetic susceptibility and specific heat
measurements. In both compounds, the results can be very well described by an
S=1/2 Heisenberg antiferromagnetic chain with an intrachain exchange of J = 82
K and J = 94 K in Sr2V3O9 and Ba2V3O9, respectively. In Sr2V3O9,
antiferromagnetic ordering at T_N = 5.3 K indicate a weak interchain exchange
of the order of J_perp ~ 2 K. In contrast, no evidence for magnetic order was
found in Ba2V3O9 down to 0.5 K, pointing to an even smaller interchain
coupling. In both compounds, we observe a pronounced Curie-like increase of the
susceptibility below 30 K, which we tentatively attribute to a staggered field
effect induced by the applied magnetic field. Results of LDA calculations
support the quasi one-dimensional character and indicate that in Sr2V3O9, the
magnetic chain is perpendicular to the structural one with the magnetic
exchange being transferred through VO4 tetrahedra.Comment: Submitted to Phy. Rev.
Colossal dielectric constants in transition-metal oxides
Many transition-metal oxides show very large ("colossal") magnitudes of the
dielectric constant and thus have immense potential for applications in modern
microelectronics and for the development of new capacitance-based
energy-storage devices. In the present work, we thoroughly discuss the
mechanisms that can lead to colossal values of the dielectric constant,
especially emphasising effects generated by external and internal interfaces,
including electronic phase separation. In addition, we provide a detailed
overview and discussion of the dielectric properties of CaCu3Ti4O12 and related
systems, which is today's most investigated material with colossal dielectric
constant. Also a variety of further transition-metal oxides with large
dielectric constants are treated in detail, among them the system La2-xSrxNiO4
where electronic phase separation may play a role in the generation of a
colossal dielectric constant.Comment: 31 pages, 18 figures, submitted to Eur. Phys. J. for publication in
the Special Topics volume "Cooperative Phenomena in Solids: Metal-Insulator
Transitions and Ordering of Microscopic Degrees of Freedom
Dielectric signature of charge order in lanthanum nickelates
Three charge-ordering lanthanum nickelates La2-xAxNiO4, substituted with
specific amounts of A = Sr, Ca, and Ba to achieve commensurate charge order,
are investigated using broadband dielectric spectroscopy up to GHz frequencies.
The transition temperatures of the samples are characterized by additional
specific heat and magnetic susceptibility measurements. We find colossal
magnitudes of the dielectric constant for all three compounds and strong
relaxation features, which partly are of Maxwell-Wagner type arising from
electrode polarization. Quite unexpectedly, the temperature-dependent colossal
dielectric constants of these materials exhibit distinct anomalies at the
charge-order transitions.Comment: 7 pages, 6 figure
Tuning a sign of magnetoelectric coupling in paramagnetic NH2(CH3)(2)Al1-xCrx(SO4)(2) x 6H(2)O crystals by metal ion substitution
Hybrid organometallic systems offer a wide range of functionalities, including magnetoelectric (ME) interactions. However, the ability to design on-demand ME coupling remains challenging despite a variety of host-guest configurations and ME phases coexistence possibilities. Here, we report the effect of metal-ion substitution on the magnetic and electric properties in the paramagnetic ferroelectric NH2(CH3)(2)Al1-xCrx(SO4)(2) x 6H(2)O. Doing so we are able to induce and even tune a sign of the ME interactions, in the paramagnetic ferroelectric (FE) state. Both studied samples with x = 0.065 and x = 0.2 become paramagnetic, contrary to the initial diamagnetic compound. Due to the isomorphous substitution with Cr the ferroelectric phase transition temperature (T-c) increases nonlinearly, with the shift being larger for the 6.5% of Cr. A magnetic field applied along the polar c axis increases ferroelectricity for the x = 0.065 sample and shifts Tc to higher values, while inverse effects are observed for x = 0.2. The ME coupling coefficient alpha(ME) = 1.7 ns/m found for a crystal with Cr content of x = 0.2 is among the highest reported up to now. The observed sign change of aME with a small change in Cr content paves the way for ME coupling engineering
Dielectric properties and dynamical conductivity of LaTiO3: From dc to optical frequencies
We provide a complete and detailed characterization of the
temperature-dependent response to ac electrical fields of LaTiO3, a
Mott-Hubbard insulator close to the metal-insulator transition. We present
combined dc, broadband dielectric, mm-wave, and infrared spectra of ac
conductivity and dielectric constant, covering an overall frequency range of 17
decades. The dc and dielectric measurements reveal information on the
semiconducting charge-transport properties of LaTiO3, indicating the importance
of Anderson localization, and on the dielectric response due to ionic
polarization. In the infrared region, the temperature dependence of the phonon
modes gives strong hints for a structural phase transition at the magnetic
ordering temperature. In addition, a gap-like electronic excitation following
the phonon region is analyzed in detail. We compare the results to the
soft-edge behavior of the optical spectra characteristic for Mott-Hubbard
insulators. Overall a consistent picture of the charge-transport mechanisms in
LaTiO3 emerges.Comment: 11 pages, 8 figures, 1 tabl
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