84 research outputs found
From quantum criticality to enhanced thermopower in strongly correlated layered cobalt oxide
We report on susceptibility measurements in the strongly correlated layered
cobalt oxide [BiBa0.66K0.36O2]CoO2, which demonstrate the existence of a
magnetic quantum critical point (QCP) governing the electronic properties. The
investigated low frequency susceptibility displays a scaling behavior with both
the temperature T and the magnetic field B ranging from the high-T non-Fermi
liquid down to the low-T Fermi liquid. Whereas the inferred scaling form can be
discussed within the standard framework of the quantum critical phenomena, the
determined critical exponents suggest an unconventional magnetic QCP of a
potentially generic type. Accordingly, these quantum critical fluctuations
account for the anomalous logarithmic temperature dependence of the
thermopower. This result allows us to conjecture that quantum criticality can
be an efficient source of enhanced thermopower
Universality and Critical Behavior at the Mott transition
We report conductivity measurements of Cr-doped V2O3 using a variable
pressure technique. The critical behavior of the conductivity near the
Mott-insulator to metal critical endpoint is investigated in detail as a
function of pressure and temperature. The critical exponents are determined, as
well as the scaling function associated with the equation of state. The
universal properties of a liquid-gas transition are found. This is potentially
a generic description of the Mott critical endpoint in correlated electron
materials.Comment: 3 figure
Cluster Dynamical Mean Field analysis of the Mott transition
We investigate the Mott transition using a cluster extension of dynamical
mean field theory (DMFT). In the absence of frustration we find no evidence for
a finite temperature Mott transition. Instead, in a frustrated model, we
observe signatures of a finite temperature Mott critical point in agreement
with experimental studies of kappa-organics and with single site DMFT. As the
Mott transition is approached, a clear momentum dependence of the electron
lifetime develops on the Fermi surface with the formation of cold regions along
the diagonal direction of the Brillouin zone. Furthermore the variation of the
effective mass is no longer equal to the inverse of the quasi particle residue,
as in DMFT, and is reduced approaching the Mott transition.Comment: 4 page
Magnetoresistance scaling in the layered cobaltate Ca3Co4O9
We investigate the low temperature magnetic field dependences of both the
resistivity and the magnetization in the misfit cobaltate Ca3Co4O9 from 60 K
down to 2 K. The measured negative magnetoresistance reveals a scaling behavior
with the magnetization which demonstrates a spin dependent diffusion mechanism.
This scaling is also found to be consistent with a shadowed metalliclike
conduction over the whole temperature range. By explaining the observed
transport crossover, this result shed a new light on the nature of the
elementary excitations relevant to the transport
Unconventional Hall effect in oriented CaCoO thin films
Transport properties of the good thermoelectric misfit oxide
CaCoO are examined. In-plane resistivity and Hall resistance
measurements were made on epitaxial thin films which were grown on {\it c}-cut
sapphire substrates using the pulsed laser deposition technique. Interpretation
of the in-plane transport experiments relates the substrate-induced strain in
the resulting film to single crystals under very high pressure ( 5.5 GPa)
consistent with a key role of strong electronic correlation. They are confirmed
by the measured high temperature maxima in both resistivity and Hall
resistance. While hole-like charge carriers are inferred from the Hall effect
measurements over the whole investigated temperature range, the Hall resistance
reveals a non monotonic behavior at low temperatures that could be interpreted
with an anomalous contribution. The resulting unconventional temperature
dependence of the Hall resistance seems thus to combine high temperature
strongly correlated features above 340 K and anomalous Hall effect at low
temperature, below 100 K.Comment: Submitted to Physical Review B (2005
Strain induced pressure effect in pulsed laser deposited thin films of the strongly correlated oxide V2O3
V2O3 thin films about 10 nm thick were grown on Al2O3 (0001) by pulsed laser
deposition. The XRD analysis is in agreement with R-3c space group. Some of
them exhibit the metal / insulator transition characteristic of V2O3 bulk
material and others samples exhibit a metallic behavior. For the latter, the
XPS analysis indicates an oxidation state of +III for vanadium. There is no
metal / insulator transition around 150 K in this sample and a strongly
correlated Fermi liquid rho = AT2 behavior of the resistivity at low
temperature is observed, with a value of A of 1.2 10-4 ohm cm, 3 times larger
than the bulk value at 25 kbar
Strongly correlated properties of the thermoelectric cobalt oxide Ca3Co4O9
We have performed both in-plane resistivity, Hall effect and specific heat
measurements on the thermoelectric cobalt oxide CaCoO. Four
distinct transport regimes are found as a function of temperature,
corresponding to a low temperature insulating one up to 63 K,
a strongly correlated Fermi liquid up to 140 K, with
and , followed
by an incoherent metal with and a high temperature insulator above
T510 K . Specific heat Sommerfeld coefficient
mJ/(mol.K) confirms a rather large value of the electronic effective mass
and fulfils the Kadowaki-Woods ratio 10 . Resistivity measurements under pressure reveal a
decrease of the Fermi liquid transport coefficient A with an increase of
as a function of pressure while the product remains constant and
of order . Both thermodynamic and transport properties suggest a strong
renormalization of the quasiparticles coherence scale of order that seems
to govern also thermopower.Comment: 5 pages, 6 figures, accepted for publication in Physical Review
Thickness-dependence of the electronic properties in V2O3 thin films
High quality vanadium sesquioxide V2O3 films (170-1100 {\AA}) were grown
using the pulsed laser deposition technique on (0001)-oriented sapphire
substrates, and the effects of film thickness on the lattice strain and
electronic properties were examined. X-ray diffraction indicates that there is
an in-plane compressive lattice parameter (a), close to -3.5% with respect to
the substrate and an out-of-plane tensile lattice parameter (c) . The thin film
samples display metallic character between 2-300 K, and no metal-to-insulator
transition is observed. At low temperature, the V2O3 films behave as a strongly
correlated metal, and the resistivity (\rho) follows the equation \rho =\rho_0
+ A T^2, where A is the transport coefficient in a Fermi liquid. Typical values
of A have been calculated to be 0.14 \mu\Omega cm K^{-2}, which is in agreement
with the coefficient reported for V2O3 single crystals under high pressure.
Moreover, a strong temperature-dependence of the Hall resistance confirms the
electronic correlations of these V2O3 thin films samples.Comment: 4 pages, 4 figure
Mott transition and transport crossovers in the organic compound
We have performed in-plane transport measurements on the two-dimensional
organic salt -(BEDT-TTF)Cu[N(CN)]Cl. A variable (gas)
pressure technique allows for a detailed study of the changes in conductivity
through the insulator-to-metal transition. We identify four different transport
regimes as a function of pressure and temperature (corresponding to insulating,
semi-conducting, ''bad metal'', and strongly correlated Fermi liquid
behaviours). Marked hysteresis is found in the transition region, which
displays complex physics that we attribute to strong spatial inhomogeneities.
Away from the critical region, good agreement is found with a dynamical
mean-field calculation of transport properties using the numerical
renormalization group technique.Comment: 4 pages, 6 figure
Substitution Effect by Deuterated Donors on Superconductivity in -(BEDT-TTF)Cu[N(CN)]Br
We investigate the superconductivity in the deuterated BEDT-TTF molecular
substitution system
-[(h8-BEDT-TTF)(d8-BEDT-TTF)]Cu[N(CN)]Br, where h8
and d8 denote fully hydrogenated and deuterated molecules, respectively.
Systematic and wide range ( = 0 -- 1) substitution can control chemical
pressure finely near the Mott boundary, which results in the modification of
the superconductivity. After cooling slowly, the increase of
observed up to 0.1 is evidently caused by the chemical pressure
effect. Neither reduction of nor suppression of
superconducting volume fraction is found below 0.5. This demonstrates
that the effect of disorder by substitution is negligible in the present
system. With further increase of , both and superconducting
volume fraction start to decrease toward the values in = 1.Comment: J. Phys. Soc. Jp
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