526 research outputs found
Magnetic scaling in cuprate superconductors
We determine the magnetic phase diagram for the YBaCuO and
LaSrCuO systems from various NMR experiments. We discuss the
possible interpretation of NMR and neutron scattering experiments in these
systems in terms of both the non-linear -model of nearly localized
spins and a nearly antiferromagnetic Fermi liquid description of magnetically
coupled quasiparticles. We show for both the 2:1:4 and 1:2:3 systems that bulk
properties, such as the spin susceptibiltiy, and probes at the
antiferromagnetic wavevector , such as , the
spin relaxation time, both display a crossover at a temperature , which
increases linearly with decreasing hole concentration, from a non-universal
regime to a scaling regime characterized by spin pseudogap behavior. We
pursue the consequences of the ansatz that corresponds to a fixed
value of the antiferromagnetic correlation length, , and show how this
enables one to extract the magnitude and temperature dependence of from
measurements of alone. We show that like , the temperature
which marks a crossover at low temperatures from the scaling regime to a
quantum disordered regime, exhibits the same dependence on doping for the 2:1:4
and 1:2:3 systems, and so arrive at a unified description of magnetic behavior
in the cuprates, in which the determining factor is the planar hole
concentration. We apply our quantitative results for YBaCuO to the
recent neutron scattering experiments of Fong {\em et al}, and show that the
spin excitation near measured by them corresponds to a spin gap
excitation, which is overdamped in the normal state, but becomes visible in the
superconducting state.Comment: 18 pages, RevTex, 18 figures are available upon request; submitted to
Phys. Rev.
Effect of Anisotropic Reactivity on the Rate of Diffusion-Controlled Reactions: Comparative Analysis of the Models of Patches and Hemispheres
AbstractA comparative analysis of two models of anisotropic reactivity in bimolecular diffusion-controlled reaction kinetics is presented. One is the conventional model of reactive patches (MRP), where the surface of a molecule is assumed to be reactive over a certain region (circular patch) with the rest of the surface being inert. Another one is the model of reactive hemispheres (MRH), assuming that a molecule is reactive within a certain distance from a point on its surface. The accuracy of the known and newly derived simple analytical expressions for the reaction rate is tested by comparison with the simulation results obtained by the original Brownian dynamics method. These formulas prove to be quite accurate in the practically important limit of strong anisotropy corresponding to small size of the reactive patches or hemispheres. Numerical calculations confirm earlier predictions that the MRP rates are much smaller than the MRH rates for the same radii of the reactive regions, especially in the case where both reacting molecules are anisotropic
Unconventional properties of superconducting cuprates
We present an explanation of the unusual peak/dip/hump features observed in
photoemission experiments on Bi2212 at . We argue that these
features arise from the interaction of the fermionic quasi-particles with
overdamped spin fluctuations. We show that the strong spin-fermion interaction
combined with the feedback effect on the spin damping due to superconductivity
yields a Fermi-liquid form of the fermionic spectral function for where is the maximum value of the superconducting gap, and a
non-Fermi-liquid form for . In the Fermi-liquid regime,
the spectral function displays a quasiparticle peak at
; in the non-Fermi-liquid regime it possesses a broad
maximum (hump) at . In between the two regimes, the
spectral function has a dip at . We argue that our
theory also explains the tunneling data for the superconducting density of
states.Comment: 4 pages, RevTeX, 4 eps figures embedded in the tex
Quantum Protectorates in the Cuprate Superconductors
Following the identification of the pairing state, the major challenge in
understanding the cuprate superconductors has been determining the evolution
with doping and temperature of their anomalous normal state behavior. Key to
this understanding is the experimentally determined magnetic phase diagram for
the cuprates, which provides information on the protected magnetic properties
of the normal state, generic behavior that is reliably the same one system to
the next, regardless of details. I discuss the constraints this places on
candidate quantum protectorates, and the status of microscopic model
calculations for a protectorate consistent with these constraints, the nearly
antiferromagnetic Fermi liquid.Comment: Invited paper to be published in Physica C as part of the proceedings
of M2S-HTSC-VI, Houston, Feb.200
Quantitative expression of the spin gap via bosonization for a dimerized spin-1/2 chain
Using results on the mass gap in the sine-Gordon model combined with the
exact amplitudes in the bosonized representation of the Heisenberg spin-1/2
chain and one-loop renormalization group, we derive a quantitative expression
for the gap in a dimerized spin-1/2 chain. This expression is shown to be in
good agreement with recent numerical estimates when a marginally irrelevant
perturbation is taken into account.Comment: 5 pages, 2 EPS figures, uses svjour.cl
- …