172 research outputs found
Anisotropic Charge Modulation in Ladder Planes of Sr_14-xCa_xCu_24O_41
The charge response of the ladders in Sr_14-xCa_xCu_24O_41 is characterized
by dc resistivity, low frequency dielectric and optical spectroscopy in all
three crystallographic directions. The collective charge-density wave screened
mode is observed in the direction of the rungs for x=0, 3 and 6, in addition to
the mode along the legs. For x=8 and 9, the charge-density-wave response along
the rungs fully vanishes, while the one along the legs persists. The transport
perpendicular to the planes is always dominated by hopping.Comment: 4 pages, 3 figures, submitted to PRB R
The angular dependent magnetoresistance in alpha-(BEDT-TTF)_2KHg(SCN)_4
In spite of extensive experimental studies of the angular dependent
magnetoresistance (ADMR) of the low temperature phase (LTP) of
alpha-(BEDT-TTF)_2KHg(SCN)_4 about a decade ago, the nature of LTP remains
elusive. Here we present a new study of ADMR of LTP in alpha-(ET)_2 salts
assuming that LTP is unconventional charge density wave (UCDW). In the presence
of magnetic field the quasiparticle spectrum in UCDW is quantized, which gives
rise to striking ADMR in UCDW. The present model appears to account for many
existing ADMR data of alpha-(BEDT-TTF)_2KHg(SCN)_4 remarkably well.Comment: 6 pages, 4 figure
Unconventional charge density wave in the organic conductor alpha-(BEDT-TTF)_2KHg(SCN)_4
The low temperature phase (LTP) of alpha-(BEDT-TTF)_2KHg(SCN)_4 salt is known
for its surprising angular dependent magnetoresistance (ADMR), which has been
studied intensively in the last decade. However, the nature of the LTP has not
been understood until now. Here we analyse theoretically ADMR in unconventional
(or nodal) charge density wave (UCDW). In magnetic field the quasiparticle
spectrum in UCDW is quantized, which gives rise to spectacular ADMR. The
present model accounts for many striking features of ADMR data in
alpha-(BEDT-TTF)_2KHg(SCN)_4.Comment: 5 pages, 6 figure
Anisotropic charge dynamics in the quantum spin-liquid candidate -(BEDT-TTF)Cu(CN)
We have in detail characterized the anisotropic charge response of the dimer
Mott insulator -(BEDT-TTF)\-Cu(CN) by dc conductivity, Hall
effect and dielectric spectroscopy. At room temperature the Hall coefficient is
positive and close to the value expected from stoichiometry; the temperature
behavior follows the dc resistivity . Within the planes the dc
conductivity is well described by variable-range hopping in two dimensions;
this model, however, fails for the out-of-plane direction. An unusually broad
in-plane dielectric relaxation is detected below about 60 K; it slows down much
faster than the dc conductivity following an Arrhenius law. At around 17 K we
can identify a pronounced dielectric anomaly concomitantly with anomalous
features in the mean relaxation time and spectral broadening. The out-of-plane
relaxation, on the other hand, shows a much weaker dielectric anomaly; it
closely follows the temperature behavior of the respective dc resistivity. At
lower temperatures, the dielectric constant becomes smaller both within and
perpendicular to the planes; also the relaxation levels off. The observed
behavior bears features of relaxor-like ferroelectricity. Because
heterogeneities impede its long-range development, only a weak tunneling-like
dynamics persists at low temperatures. We suggest that the random potential and
domain structure gradually emerge due to the coupling to the anion network.Comment: 14 pages, 13 figure
Suppression of the charge-density-wave state in Sr_14Cu_24O_41 by calcium doping
The charge response in the spin chain/ladder compound Sr_14-xCa_xCu_24O_41 is
characterized by DC resistivity, low-frequency dielectric spectroscopy and
optical spectroscopy. We identify a phase transition below which a
charge-density wave (CDW) develops in the ladder arrays. Calcium doping
suppresses this phase with the transition temperature decreasing from 210 K for
x=0 to 10 K for x=9, and the CDW gap from 130 meV down to 3 meV, respectively.
This suppression is due to the worsened nesting originating from the increase
of the inter-ladder tight-binding hopping integrals, as well as from disorder
introduced at the Sr sites. These results altogether speak in favor of
two-dimensional superconductivity under pressure.Comment: 4 pages, 4 figures, accepted for publication in PR
Charge-density wave formation in Sr_{14}Cu_{24}O_{41}
The electrodynamic response of the spin-ladder compound
SrCaCuO () has been studied from
radiofrequencies up to the infrared. At temperatures below 250 K a pronounced
absorption peak appears around 12 cm in SrCuO for
the radiation polarized along the chains/ladders ().
In addition a strongly temperature dependent dielectric relaxation is observed
in the kHz - MHz range. We explain this behavior by a charge density wave which
develops in the ladders sub-system and produces a mode pinned at 12 cm.
With increasing Ca doping the mode shifts up in frequency and eventually
disappears for because the dimensionality of the system crosses over from
one to two dimensions, giving way to the superconducting ground state under
pressure.Comment: One name added to author list 4 pages, 2 figures, email:
[email protected]
Origin of Low-Energy Excitations in Charge-Ordered Manganites
The low-energy excitations in the charge-ordered phase of polycrystalline
La0.25Ca0.75MnO3 are explored by frequency-domain terahertz spectroscopy. In
the frequency range from 4 cm^-1 to 700 cm^-1 (energies 0.4 meV to 90 meV) and
at temperatures down to 5 K, we do not detect any feature that can be
associated with the collective response of the spatially modulated charge
continuum. In the antiferromagnetically ordered phase, broad absorption bands
appear in the conductivity and permittivity spectra around 30 cm^-1 and 100
cm^-1 which are assigned to former acoustic phonons optically activated due to
a fourfold superstructure in the crystal lattice. Our results indicate that
characteristic energies of collective excitations of the charge-ordered phase
in La0:25Ca0:75MnO3, if any, lie below 1 meV. At our lowest frequencies of only
few wavenumbers a strong relaxation is observed above 100 K connected to the
formation of the charge-ordered state.Comment: 5 pages, 3 figure
Influence of doping on the Hall coefficient in Sr_{14-x}Ca_xCu_24O_41
We present Hall-effect measurements of two-leg ladder compounds
Sr_{14-x}Ca_xCu_24O_41 (0 <= x <= 11.5) with the aim to determine the number of
carriers participating in dc transport. Distribution of holes between the
ladder and chain subsystems is one of the crucial questions important for
understanding the physics of these compounds. Our Hall effect and resistivity
measurements show typical semiconducting behavior for x < 11.5. However, for
x=11.5, the results are completely different, and the Hall coefficient and
resistivity behavior is qualitatively similar to that of high temperature
copper-oxide superconductors. We have determined the effective number of
carriers at room temperature and compared it to the number of holes in the
ladders obtained by other experimental techniques. We propose that going from
x=0 to x=11.5 less than 1 hole per formula unit is added to the ladders and is
responsible for a pronounced change in resistivity with Ca doping.Comment: 8 pages, 7 figures, revised versio
The Hall effect in the organic conductor TTF-TCNQ: Choice of geometry for accurate measurements of highly anisotropic system
We have measured the Hall effect on recently synthesized single crystals of
quasi-one-dimensional organic conductor TTF-TCNQ, a well known charge transfer
complex that has two kinds of conductive stacks: the donor (TTF) and the
acceptor (TCNQ) chains. The measurements were performed in the temperature
interval 30 K < T < 300 K and for several different magnetic field and current
directions through the crystal. By applying the equivalent isotropic sample
(EIS) approach, we have demonstrated the importance of the choice of optimal
geometry for accurate Hall effect measurements. Our results show, contrary to
past belief, that the Hall coefficient does not depend on the geometry of
measurements and that the Hall coefficient value is around zero in high
temperature region (T > 150 K), implying that there is no dominance of either
TTF or TCNQ chain. At lower temperatures, our measurements clearly prove that
all three phase transitions of TTF-TCNQ could be identified from Hall effect
measurements.Comment: Revised version; 7 pages, 5 figure
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