302 research outputs found
Ferroelectric properties of charge-ordered alpha-(BEDT-TTF)2I3
A detailed investigation of the out-of-plane electrical properties of
charge-ordered alpha-(BEDT-TTF)2I3 provides clear evidence for
ferroelectricity. Similar to multiferroic alpha-(BEDT-TTF)2Cu[N(CN)2]Cl, the
polar order in this material is ascribed to the occurrence of bond- and
site-centered charge order. Dielectric response typical for relaxor
ferroelectricity is found deep in the charge-ordered state. We suggest an
explanation in terms of the existence of polar and nonpolar stacks of the
organic molecules in this material, preventing long-range ferroelectricity. The
results are discussed in relation to the formation or absence of electronic
polar order in related charge-transfer salts.Comment: 8 pages, 4 figures. Revised version as accepted for publication in
Phys. Rev.
Dielectric behavior of Copper Tantalum Oxide
A thorough investigation of the dielectric properties of Cu2Ta4O12, a
material crystallizing in a pseudo-cubic, perovskite-derived structure is
presented. We measured the dielectric constant and conductivity of single
crystals in an exceptionally broad frequency range up to GHz frequencies and at
temperatures from 25 - 500 K. The detected dielectric constant is unusually
high (reaching values up to 105) and almost constant in a broad frequency and
temperature range. Cu2Ta4O12 possesses a crystal structure similar to
CaCu3Ti4O12, the compound for which such an unusually high dielectric constant
was first observed. An analysis of the results using a simple equivalent
circuit and measurements with different types of contact revealed that
extrinsic interfacial polarization effects, derived from surface barrier
capacitors are the origin of the observed giant dielectric constants. The
intrinsic properties of Cu2Ta4O12 are characterized by a (still relatively
high) dielectric constant in the order of 100 and by charge transport via
hopping conduction of Anderson-localized charge carriers.Comment: 18 pages, 6 figures, submitted to Jouranl of Physical Chemestr
Dielectric responses of the layered cobalt oxysulfide Sr_2Cu_2CoO_2S_2 with CoO_2 square-planes
We have studied the dielectric responses of the layered cobalt oxysulfide
SrCuCoOS with the CoO square-planes. With decreasing
temperature below the N\'eel temperature, the resistivity increases like a
semiconductor, and the thermopower decreases like a metal. The dielectric
constant is highly dependent on temperature, and the dielectric relaxation is
systematically changed with temperature, which is strongly correlated to the
magnetic states. These behaviors suggest that carriers distributed
homogeneously in the paramagnetic state at high temperatures are expelled from
the antiferromagnetically ordered spin domain below the N\'eel temperature.Comment: 3 pages, 4 eps figures, to be published in J. Appl. Phy
Propylene Carbonate Reexamined: Mode-Coupling Scaling without Factorisation ?
The dynamic susceptibility of propylene carbonate in the moderately viscous
regime above is reinvestigated by incoherent neutron and
depolarised light scattering, and compared to dielectric loss and solvation
response. Depending on the strength of relaxation, a more or less
extended scaling regime is found. Mode-coupling fits yield consistently
and K, although different positions of the
susceptibility minimum indicate that not all observables have reached the
universal asymptotics
Electronic and optical properties of LiBC
LiBC, a semiconducting ternary borocarbide constituted of the lightest
elements only, has been synthesized and characterized by x-ray powder
diffraction, dielectric spectroscopy, and conductivity measurements. Utilizing
an infrared microscope the phonon spectrum has been investigated in single
crystals. The in-plane B-C stretching mode has been detected at 150 meV,
noticeably higher than in AlB2, a non-superconducting isostructural analog of
MgB2. It is this stretching mode, which reveals a strong electron-phonon
coupling in MgB2, driving it into a superconducting state below 40 K, and is
believed to mediate predicted high-temperature superconductivity in hole-doped
LiBC [H. Rosner, A. Kitaigorodsky, and W. E. Pickett, Phys. Rev. Lett. 88,
127001 (2002)].Comment: 4 pages, 4 figure
Electrode Polarization Effects in Broadband Dielectric Spectroscopy
In the present work, we provide broadband dielectric spectra showing strong
electrode polarization effects for various materials, belonging to very
different material classes. This includes both ionic and electronic conductors
as, e.g., salt solutions, ionic liquids, human blood, and
colossal-dielectric-constant materials. These data are intended to provide a
broad data base enabling a critical test of the validity of phenomenological
and microscopic models for electrode polarization. In the present work, the
results are analyzed using a simple phenomenological equivalent-circuit
description, involving a distributed parallel RC circuit element for the
modeling of the weakly conducting regions close to the electrodes. Excellent
fits of the experimental data are achieved in this way, demonstrating the
universal applicability of this approach. In the investigated ionically
conducting materials, we find the universal appearance of a second dispersion
region due to electrode polarization, which is only revealed if measuring down
to sufficiently low frequencies. This indicates the presence of a second
charge-transport process in ionic conductors with blocking electrodes.Comment: 9 pages, 6 figures, experimental data are provided in electronic form
(see "Data Conservancy"
Test of mode coupling theory for a supercooled liquid of diatomic molecules. II. q-dependent orientational correlators
Using molecular dynamics computer simulations we study the dynamics of a
molecular liquid by means of a general class of time-dependent correlators
S_{ll'}^m(q,t) which explicitly involve translational (TDOF) and orientational
degrees of freedom (ODOF). The system is composed of rigid, linear molecules
with Lennard- Jones interactions. The q-dependence of the static correlators
S_{ll'}^m(q) strongly depend on l, l' and m. The time dependent correlators are
calculated for l=l'. A thorough test of the predictions of mode coupling theory
(MCT) is performed for S_{ll}^m(q,t) and its self part S_{ll}^{(s)m}(q,t), for
l=1,..,6. We find a clear signature for the existence of a single temperature
T_c, at which the dynamics changes significantly. The first scaling law of MCT,
which involves the critical correlator G(t), holds for l>=2, but no critical
law is observed. Since this is true for the same exponent parameter lambda as
obtained for the TDOF, we obtain a consistent description of both, the TDOF and
ODOF, with the exception of l=1. This different behavior for l \ne 1 and l=1
can also be seen from the corresponding susceptibilities
(chi'')_{ll}^m(q,omega) which exhibit a minimum at about the same frequency
omega_{min} for all q and all l \ne 1, in contrast to (chi'')_{11}^m(q,omega)
for which omega'_{min} approx 10 omega_{min} . The asymptotic regime, for which
the first scaling law holds, shrinks with increasing l. The second scaling law
of MCT (time-temperature superposition principle) is reasonably fulfilled for l
\ne 1 but not for l=1. Furthermore we show that the q- and (l,m)-dependence of
the self part approximately factorizes, i.e. S_{ll}^{(s)m}(q,t) \cong
C_l^{(s)}(t) F_s(q,t) for all m.Comment: 11 pages of RevTex, 16 figure
Hertz-to-infrared electrodynamics of single-crystalline barium-lead hexaferrite Ba1-xPbxFe12O19
Broadband electrodynamic response of single-crystalline lead-substituted barium hexaferrite Ba1-xPbxFe12O19 is studied at temperatures from 5 to 300 K in the range from 1 Hz to 240 THz that includes radio, sub-terahertz, terahertz and infrared frequencies and altogether spans over 14 frequency decades. Discovered phenomena include relaxational radio-frequency dynamics of domains and domain walls, temperature-unstable terahertz excitations connected with electric dipoles induced by off-center displacements in the ab-plane of the lead ions, narrow terahertz excitations associated with electronic transitions between the fine-structure components of the Fe2+ground state, dielectric gigahertz resonances presumably of magneto-electric origin and polar lattice vibrations
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
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
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