579 research outputs found
Quantum critical point and superconducting dome in the pressure phase diagram of o-TaS3
We measure the electrical resistance of o-TaS3 between 1K and 300K under
pressures up to 20GPa. We observe a gradual decrease of the charge density wave
transition temperature with increasing pressure P following a mean-field
quantum fluctuation power law with a quantum critical point at a pressure Pc =
11.5GPa. Around the quantum critical point we observe a superconducting dome
with a maximum superconducting transition temperature Tc = 3.1K. Such dome is
similar to superconducting domes around other types of order suggesting that
the QCP is directly responsible for the enhancement of superconductivity
through a universal mechanism still not well understood.Comment: To be published in PRB as a Rapid Communication. 11 pages with 4
figure
Electrical resistivity of the Ti4O7 Magneli phase under high pressure
We have measured resistivity as a function of temperature and pressure of
Ti4O7 twinned crystals using different contact configurations. Pressures over
4kbar depress the localization of bipolarons and allow the study of the
electrical conduction of the bipolaronic phase down to low temperatures. For
pressures P > 40 kbar the bipolaron formation transition is suppressed and a
nearly pressure independent behavior is obtained for the resistivity. We
observed an anisotropic conduction. When current is injected parallel to the
principal axis, a metallic conduction with interacting carrier effects is
predominant. A superconducting state was not obtained down to 1.2 K, although
evidences of the proximity of a quantum critical point were noticed. While when
current is injected non-parallel to the crystal's principal axis, we obtained a
logarithmic divergence of the resistivity at low temperatures. For this case,
our results for the high pressure regime can be interpreted in the framework of
interacting carriers (polarons or bipolarons) scattered by Two Level Systems.Comment: 9 Revtex pages, 12 EPS figures included, submitted to The European
Physical Journal B. Contact author: C. Acha (e-mail address: [email protected]
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
Belousov-Zhabotinsky type reactions: the non-linear behavior of chemical systems
Chemical oscillators are open systems characterized by periodic variations of some reaction species concentration due to complex physico-chemical phenomena that may cause bistability, rise of limit cycle attractors, birth of spiral waves and Turing patterns and finally deterministic chaos. Specifically, the Belousov-Zhabotinsky reaction is a noteworthy example of non-linear behavior of chemical systems occurring in homogenous media. This reaction can take place in several variants and may offer an overview on chemical oscillators, owing to its simplicity of mathematical handling and several more complex deriving phenomena. This work provides an overview of Belousov-Zhabotinsky-type reactions, focusing on modeling under different operating conditions, from the most simple to the most widely applicable models presented during the years. In particular, the stability of simplified models as a function of bifurcation parameters is studied as causes of several complex behaviors. Rise of waves and fronts is mathematically explained as well as birth and evolution issues of the chaotic ODEs system describing the Györgyi-Field model of the Belousov-Zhabotinsky reaction. This review provides not only the general information about oscillatory reactions, but also provides the mathematical solutions in order to be used in future biochemical reactions and reactor designs
Evidence for the coexistence of Dirac and massive carriers in a-(BEDT-TTF)2I3 under hydrostatic pressure
Transport measurements were performed on the organic layered compound \aI3
under hydrostatic pressure. The carrier types, densities and mobilities are
determined from the magneto-conductance of \aI3 . While evidence of
high-mobility massless Dirac carriers has already been given, we report here,
their coexistence with low-mobility massive holes. This coexistence seems
robust as it has been found up to the highest studied pressure. Our results are
in agreement with recent DFT calculations of the band structure of this system
under hydrostatic pressure. A comparison with graphene Dirac carriers has also
been done.Comment: 5 pages 5 figure
Absence of a structural transition up to 40 Gpa in MgB2 and the relevance of magnesium non-stoichiometry
We report measurements on MgB2 up to ~40GPa. Increasing pressure yields a
monotonous decrease of the lattice parameters and of the c/a ratio, but no
structural transition down to parameters smaller than those of AlB2. The
transition superconducting temperature also decreases with temperature in a
sample dependent way. The results are explained by an increase of the filling
of the 2D pxy bands with pressure, the Mg stoichiometry determining the
starting position of the Fermi level. Our measurements indicate that these hole
bands are the relevant ones for superconductivity.Comment: submitted March 9th 2001, PRB accepte
Aperiodic quantum oscillations of particle-hole asymmetric Dirac cones
We report experimental measurements and theoretical analysis of Shubnikov-de
Haas (SdH) oscillations in a Dirac cone system: the a-(BEDT-TTF)2I3 organic
metal under hydrostatic pressure. The measured SdH oscillations reveal
anomalies at high magnetic fields B where the 1/B oscillations periodicity is
lost above 7 T. We interpret these unusual results within a theoretical model
that takes into account intrinsic distortions of the a-(BEDT-TTF)2I3 Dirac
cones such as a parabolic particle-hole asymmetric correction. Others possible
causes, such as a cone tilting or a Zeeman effect, are carefully ruled out. The
observations are consistent among a-(BEDT-TTF)2I3 samples with different Fermi
levels
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
Phonon assisted dynamical Coulomb blockade in a thin suspended graphite sheet
The differential conductance in a suspended few layered graphene sample is
fou nd to exhibit a series of quasi-periodic sharp dips as a function of bias
at l ow temperature. We show that they can be understood within a simple model
of dyn amical Coulomb blockade where energy exchanges take place between the
charge carriers transmitted trough the sample and a dissipative electromagnetic
envir onment with a resonant phonon mode strongly coupled to the electrons
High pressure effects in fluorinated HgBa2Ca2Cu3O(8+d)
We have measured the pressure sensitivity of Tc in fluorinated
HgBa2Ca2Cu3O(8+d) (Hg-1223) ceramic samples with different F contents, applying
pressures up to 30 GPa. We obtained that Tc increases with increasing pressure,
reaching different maximum values, depending on the F doping level, and
decreases for a further increase of pressure. A new high Tc record (166 K +/- 1
K) was achieved by applying pressure (23 GPa) in a fluorinated Hg-1223 sample
near the optimum doping level. Our results show that all our samples are at the
optimal doping, and that fluorine incorporation decreases the crystallographic
-parameter concomitantly increasing the maximum attainable Tc. This effect
reveals that the compression of the axes is one of the keys that controls
the Tc of high temperature superconductors.Comment: 4 pages, 4 figures, submitted to Phys. Rev.
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