102 research outputs found
On self-protecting singlets in cuprate superconductors
The basal area (Cu-Cu grid) of the cuprate superconductors not only tends to
shrink on hole doping, as expected from single electron quantum chemistry, but
exhibits also an electronically incompressible "hump'' around optimum doping
n_opt = 0.16. The hump collapses near critical doping n_crit = 0.19. We analyze
the origin of the hump in terms of a classical liquid of interacting
incompressible particles in a container with antiferromagnetic walls. Oxygen
holes interacting with the wall form singlets, protect themselves against other
holes by an incompressible "spin fence'', and thus interact also with the
lattice. Occupation of the CuO_2 lattice with holes must therefore follow a
non-double-occupant constraint also for the oxygen cage enclosing the copper
hole. Closest packing of self-protecting singlets is found to occur around
critical doping; closest packing of paired self-protecting singlets around
optimum doping. These singlet-states are bosonic, but are not magnetic
polarons.Comment: reviewed version, 7 pages, 10 figure
Plane dimpling and Cu 4s hybridization in YBa_2Cu_3O_x
Oxygen doping dimples the CuO_2 planes of YBa_2Cu_3O_{6.8-6.92} by displacing
copper normal to the planes and further towards Ba. The correlated oxygen
displacements, however, are constrained to the in-plane axes. This displacement
pattern is discussed in terms of doping dependent Cu 4s - 3d hybridizations.Comment: 2 pages, submitted to Physica B (Proc. LT22
Nonlinear Doping of Cuprate Superconductors -- The case of Bi_2Sr_{2-x}La_xCuO_{6+\delta}
We analyze the hole doping mechanism in Bi_2Sr_{2-x}La_xCuO_{6+\delta}
(BSLCO). The singular optimum around x=0.35 is found to be connected with a
feedback between the doped CuO_2 layers and its dopant reactant
[La^{3+}/Bi^{3+}-O_\delta] locking the number of doped holes preferentially on
to the universal optimum n_opt=0.16.Comment: Typos correcte
Intraplanar couplings in the CuO_2 lattice of cuprate superconductors
We have investigated the doping dependencies of the basal areas in
single-layer high-T_c cuprates La_{2-x}Sr_xCuO_4 and HgBa_2CuO_x, as well as in
two-layer Y_{1-y}Ca_yBa_2Cu_3O_x and HgBa_2CaCu_2O_x. The basal areas not only
tend to shrink on hole doping, as expected from single electron quantum
chemistry, but exhibit also a "bulge'' around optimum doping. We attribute the
"bulge'' to the effects of the strongly correlated quantum liquid on the CuO_2
lattice, rendering it nearly incompressible around optimum doping, but highly
compressible in the weakly overdoped regime. Inhomogenous doping cannot account
for this anomaly in the electronic compressibility of the CuO_2 lattice.Comment: 2 pages, 2 figures, to appear in Physica C (Proc. M2S-HTSC-VII
The dimpling in the CuO_2 planes of YBa_2Cu_3O_x (x=6.806-6.984, T=20-300 K) measured by yttrium EXAFS
The dimpling of the CuO_2 planes (spacing between the O2,3 and Cu2 layers) in
YBa_2Cu_3O_x has been measured as a function of oxygen concentration and
temperature by yttrium x-ray extended-fine-structure spectroscopy (EXAFS). The
relative variations of the dimpling with doping (x=6.806-6.984) and temperature
(20-300 K) are weak (within 0.05 AA), and arise mainly from displacements of
the Cu2 atoms off the O2,3 plane towards Ba. The dimpling appears to be
connected with the transition from the underdoped to the overdoped regimes at
x=6.95, and with a characteristic temperature in the normal state, T*=150 K.Comment: 6 pages, 2 ps figs, LaTEX, Elsevier Elsart styl
Metal-insulator crossover in the Boson-Fermion model in infinite dimensions
The Boson-Fermion model, describing a mixture of tightly bound electron pairs
and quasi-free electrons hybridized with each other via a charge exchange term,
is studied in the limit of infinite dimensions, using the Non-Crossing
Approximation within the Dynamical Mean Field Theory. It is shown that a
metal-insulator crossover, driven by strong pair fluctuations, takes place as
the temperature is lowered. It manifests itself in the opening of a pseudogap
in the electron density of states, accompanied by a corresponding effect in the
optical and dc conductivity.Comment: 4 pages, 3 figures, to be published in Phys. Rev. Let
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