933 research outputs found
Quantum Topological Excitations: from the Sawtooth Lattice to the Heisenberg Chain
The recently elucidated structure of the delafossite YCuO reveals a
Cu-O network with nearly independent chains having different
interactions between the spins. Motivated by this result, we study the
chain for various ratios of the base-base and
base-vertex interactions. By exact diagonalization and extrapolation, we show
that the elementary excitation spectrum, which (within numerical error) is the
same for total spins and 1, has a gap only in the interval
. The gap is dispersionless
for , but has increasing -dependence as moves away from unity, related to the instability of dimers in
the ground state.Comment: 4 pages, 6 figures (revtex twocolumn
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
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]
Tomonaga-Luttinger liquids and Coulomb blockade in multiwall carbon nanotubes under pressure
We report that the conductance of macroscopic multiwall nanotube (MWNT)
bundles under pressure shows power laws in temperature and voltage, as
corresponding to a network of bulk-bulk connected Tomonaga-Luttinger Liquids
(LL). Contrary to individual MWNT, where the observed power laws are attributed
to Coulomb blockade, the measured ratio for the end and bulk obtained
exponents, ~2.4, can only be accounted for by LL theory. At temperatures
characteristic of interband separation, it increases due to thermal population
of the conducting sheets unoccupied bands.Comment: 16 pages, 3 Figures, .pdf. Accepted in Phys. Rev. Let
Band-structure trend in hole-doped cuprates and correlation with Tcmax
By calculation and analysis of the bare conduction bands in a large number of
hole-doped high-temperature superconductors, we have identified the energy of
the so-called axial-orbital as the essential, material-dependent parameter. It
is uniquely related to the range of the intra-layer hopping. It controls the Cu
4s-character, influences the perpendicular hopping, and correlates with the
observed Tc at optimal doping. We explain its dependence on chemical
composition and structure, and present a generic tight-binding model.Comment: 5 pages, Latex, 5 eps figure
Superconducting and normal phases of FeSe single crystals at high pressure
We report on the synthesis of superconducting single crystals of FeSe, and
their characterization by X-ray diffraction, magnetization and resistivity. We
have performed ac susceptibility measurements under high pressure in a
hydrostatic liquid argon medium up to 14 GPa and we find that TC increases up
to 33-36 K in all samples, but with slightly different pressure dependences on
different samples. Above 12 GPa no traces of superconductivity are found in any
sample. We have also performed a room temperature high pressure X-ray
diffraction study up to 12 GPa on a powder sample, and we find that between 8.5
GPa and 12 GPa, the tetragonal PbO structure undergoes a structural transition
to a hexagonal structure. This transition results in a volume decrease of about
16%, and is accompanied by the appearance of an intermediate, probably
orthorhombic phase
Observation of the Decay Λ0b→Λ+cτ−¯ν
The first observation of the semileptonic b-baryon decay Λb0→Λc+τ-ν¯τ, with a significance of 6.1σ, is reported using a data sample corresponding to 3 fb-1 of integrated luminosity, collected by the LHCb experiment at center-of-mass energies of 7 and 8 TeV at the LHC. The τ- lepton is reconstructed in the hadronic decay to three charged pions. The ratio K=B(Λb0→Λc+τ-ν¯τ)/B(Λb0→Λc+π-π+π-) is measured to be 2.46±0.27±0.40, where the first uncertainty is statistical and the second systematic. The branching fraction B(Λb0→Λc+τ-ν¯τ)=(1.50±0.16±0.25±0.23)% is obtained, where the third uncertainty is from the external branching fraction of the normalization channel Λb0→Λc+π-π+π-. The ratio of semileptonic branching fractions R(Λc+)B(Λb0→Λc+τ-ν¯τ)/B(Λb0→Λc+μ-ν¯μ) is derived to be 0.242±0.026±0.040±0.059, where the external branching fraction uncertainty from the channel Λb0→Λc+μ-ν¯μ contributes to the last term. This result is in agreement with the standard model prediction
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