3,680 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
Electronic structure and Fermi surface tolopogy of NaCoO
We construct an effective Hamiltonian for the motion of T2g highly correlated
states in NaxCoO2. We solve exactly a multiband model in a CoO6 cluster with
electronic occupation corresponding to a nominal Co valence of either +3 or +4.
Using the ensuing ground states, we calculate the effective O mediated hopping
t=0.10 eV between many-body T2g states, and estimate the direct hopping t'~0.04
eV. The trigonal splitting 3D=0.315 eV is taken from recent quantum chemistry
calculations. The resulting effective Hamiltonian is solved using a generalized
slave-boson mean-field approximation. The results show a significant band
renormalization and a Fermi surface topology that agrees with experiment, in
contrast to predictions using the local-density approximation.Comment: 4 pages, 2 figure
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
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.
On finite strain micromorphic elastoplasticity
AbstractIn the micromorphic continuum theory of Eringen, it was proposed that microstructure of materials could be represented in a continuum framework using a micro-deformation tensor governing micro-element deformation, in addition to the deformation gradient governing macro-element deformation. The paper formulates finite strain micromorphic elastoplasticity based on micromorphic continuum mechanics in the sense of Eringen. Multiplicative decomposition into elastic and plastic parts of the deformation gradient and micro-deformation are assumed, and the ClausiusâDuhem inequality is formulated in the intermediate configuration BÂŻ to analyze what stresses, elastic deformation measures, and plastic deformation rates are used/defined in the constitutive equations. The resulting forms of plastic and internal state variable evolution equations can be viewed as phenomenological at their various scales (i.e., micro-continuum and macro-continuum). The phenomenology of inelastic mechanical material response at the various scales can be different, but for demonstration purposes, J2 flow plasticity is assumed for each of three levels of plastic evolution equations identified, with different stress, internal state variables, and material parameters. All evolution equations and a semi-implicit time integration scheme are formulated in the intermediate configuration for future coupled Lagrangian finite element implementation. A simpler two-dimensional model for anti-plane shear kinematics is formulated to demonstrate more clearly how such model equations simplify for future finite element implementation
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]
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
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
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