2,936 research outputs found
Pressure effects in the triangular layered cobaltites NaxCoO2
We have measured transport properties as a function of temperature and
pressure up to 30GPa in the NaxCoO2 system. For the x=0.5 sample the transition
temperature at 53K increases with pressure, while paradoxically the sample
passes from an insulating to a metallic ground state. A similar transition is
observed in the x=0.31 sample under pressure. Compression on the x=0.75 sample
transforms the sample from a metallic to an insulating state. We discuss our
results in terms of interactions between band structure effects and Na+ order.Comment: 18 pages, 5 figure
Mott-Hubbard quantum criticality in paramagnetic CMR pyrochlores
We present a correlated {\it ab initio} description of the paramagnetic phase
of TlMnO, employing a combined local density approximation (LDA)
with multiorbital dynamical mean field theory (DMFT) treatment. We show that
the insulating state observed in this colossal magnetoresistance (CMR)
pyrochlore is determined by strong Mn intra- and inter-orbital local
electron-electron interactions. Hybridization effects are reinforced by the
correlation-induced spectral weight transfer. Our result coincides with optical
conductivity measurements, whose low energy features are remarkably accounted
for by our theory. Based on this agreement, we study the disorder-driven
insulator-metal transition of doped compounds, showing the proximity of
TlMnO to quantum phase transitions, in agreement with recent
measurements.Comment: 4 pages, 4 figure
Pressure control of conducting channels in singlewall nanotube networks
We measure electrical transport on networks of single wall nanotube ropes as
a function of temperature T, voltage V and pressure up to 22GPa. We observe
Luttinger liquid (LL) behavior, a conductance ~T^alpha and a dynamic
conductance ~V^alpha. With pressure conductance increases while alpha
decreases, enabling us to test the theoretical prediction for LL on the alpha
dependence of the T and V independent coefficient of the tunneling conductance,
and to obtain the high frequency cut-off of LL modes. The possible transition
to a fermi liquid at alpha -> 0 is unattainable, as nanotubes collapse to an
insulating state at high pressures.Comment: 13 pages, 4 figure
Strong enhancement of superconductivity at high pressures within the charge-density-wave states of 2H-TaS 2 and 2H-TaSe 2
We present measurements of the superconducting and charge density wave
critical temperatures (Tc and TCDW) as a function of pressure in the transition
metal dichalchogenides 2H-TaSe2 and 2H-TaS2. Resistance and susceptibility
measurements show that Tc increases from temperatures below 1 K up to 8.5 K at
9.5 GPa in 2H-TaS2 and 8.2 K at 23 GPa in 2H-TaSe2. We observe a kink in the
pressure dependence of TCDW at about 4 GPa that we attribute to the lock-in
transition from incommensurate CDW to commensurate CDW. Above this pressure,
the commensurate TCDW slowly decreases coexisting with superconductivity within
our full pressure range.Comment: Published in Phys. Rev B 93, 184512 (2016
Novel Phases of Planar Fermionic Systems
We discuss a {\em family} of planar (two-dimensional) systems with the
following phase strucure: a Fermi liquid, which goes by a second order
transition (with non classical exponent even in mean-field) to an intermediate,
inhomogeneous state (with nonstandard ordering momentum) , which in turn goes
by a first order transition to a state with canonical order parameter. We
analyze two examples: (i) a superconductor in a parallel magnetic field (which
was discussed independently by Bulaevskii)for which the inhomogeneous state is
obtained for where is the critical temperature (in Kelvin) of the superconductor
without a field and is measured in Tesla, and (ii) spinless (or, as is
explained, spin polarized) fermions near half-filling where a similar, sizeable
window (which grows in size with anisotropy) exists for the intermediate CDW
phase at an ordering momentum different from . We discuss the
experimental conditions for realizing and observing these phases and the
Renormalization Group approach to the transitions.Comment: ([email protected],[email protected]) 29 p Latex 4 figs
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Estudio experimental de procesos de calentamiento y enfriamiento. Resultados e implicaciones didĂĄcticas
A detailed experimental study on different processes of heating and cooling allows to deduce that temperature variation, in these processes, in the course of time is not always the same, but it depends, to a great extent, on how the process is carried out. So to confuse the T-Q relations (always lineal) wirth T-t relations (dependent on the process) is a mistake that is necessary to avoid in our classes
Interlayer Magnetic Frustration in Quasi-stoichiometric Li1-xNi1+xO2
Susceptibility, high-field magnetization and submillimeter wave electron spin
resonance measurements of layered quasi-stoichiometric Li1-xNi1+xO2 are
reported and compared to isomorphic NaNiO2. A new mechanism of magnetic
frustration induced by the excess Ni ions always present in the Li layers is
proposed. We finally comment on the possible realization of an orbital liquid
state in this controversial compound.Comment: 4 pages, 5 figures, submitted to Phys.Rev.B, Rapid Com
New Superhard Phases for 3D C60-based Fullerites
We have explored new possible phases of 3D C60-based fullerites using
semiempirical potentials and ab-initio density functional methods. We have
found three closely related structures - two body centered orthorhombic and one
body centered cubic - having 52, 56 and 60 tetracoordinated atoms per molecule.
These 3D polymers result in semiconductors with bulk moduli near 300 GPa, and
shear moduli around 240 GPa, which make them good candidates for new low
density superhard materials.Comment: To be published in Physical Review Letter
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