2,944 research outputs found
Pressure effects in PrT2B2C (T = Co, Ni, Pt): Applied and chemical pressure
High-pressure electrical resistivity, r(T), measurements on intermetallic
Pr(Co, Ni, Pt)2B2C compounds were performed down to 2K. At room pressure the
r(T) in a-b direction curves for the non superconducting Pr(Co, Ni)2B2C
compounds exhibit magnetic correlations at about 10 and 4 K, respectively. At
low temperatures, PrCo2B2C shows a large spin-dependent electron scattering in
comparison to PrNi2B2C. Under applied pressure the magnetic scattering tends to
be suppressed more effectively in PrCo2B2C than in PrNi2 B2C. The low
temperature behavior of r(T,P) for PrNi2B2C and PrCo2B2C suggests a spin
fluctuations mechanism. In the other hand PrPt2B2C compound shows
superconductivity at about 6 K and under pressure its superconducting
transition temperature tends to be degraded at a rate dTc/dP = -0.34 K/GPa, as
expected in compounds with transition metals. The experimental results in Co,
Ni and Pt based compounds are analyzed from the point of view of the external
and chemical internal pressure effects
Direct observation of the influence of the As-Fe-As angle on the Tc of superconducting SmFeAsOF
The electrical resistivity, crystalline structure and electronic properties
calculated from the experimentally measured atomic positions of the compound
SmFeAsOF have been studied up to pressures ~20GPa. The
correlation between the pressure dependence of the superconducting transition
temperature (Tc) and crystallographic parameters on the same sample shows
clearly that a regular FeAs tetrahedron maximizes Tc, through
optimization of carrier transfer to the FeAs planes as indicated by the
evolution of the electronic band structures.Comment: 15pages, 4 figure
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.
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
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
Kondo Screening and Magnetic Ordering in Frustrated UNi4B
UNi4B exhibits unusual properties and, in particular, a unique
antiferromagnetic arrangement involving only 2/3 of the U sites. Based on the
low temperature behavior of this compound, we propose that the remaining 1/3 U
sites are nonmagnetic due to the Kondo effect. We derive a model in which the
coexistence of magnetic and nonmagnetic U sites is the consequence of the
competition between frustration of the crystallographic structure and
instability of the 5f moments.Comment: 4 pages, 2 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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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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