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Upper Pseudogap Phase: Magnetic Characterizations
It is proposed that the upper pseudogap phase (UPP) observed in the high-Tc
cuprates correspond to the formation of spin singlet pairing under the bosonic
resonating-valence-bond (RVB) description. We present a series of evidence in
support of such a scenario based on the calculated magnetic properties
including uniform spin susceptibility, spin-lattice and spin-echo relaxation
rates, which consistently show that strong spin correlations start to develop
upon entering the UPP, being enhanced around the momentum (\pi, \pi) while
suppressed around (0, 0). The phase diagram in the parameter space of doping
concentration, temperature, and external magnetic field, is obtained based on
the the bosonic RVB theory. In particular, the competition between the Zeeman
splitting and singlet pairing determines a simple relation between the
"critical" magnetic field, H_{PG}, and characteristic temperature scale, T0, of
the UPP. We also discuss the magnetic behavior in the lower pseudogap phase at
a temperature Tv lower than T0, which is characterized by the formation of
Cooper pair amplitude where the low-lying spin fluctuations get suppressed at
both (0, 0) and (\pi, \pi). Properties of the UPP involving charge channels
will be also briefly discussed.Comment: 11 pages, 5 figures, final version to appear in PR
Fermi-liquid ground state in n-type copper-oxide superconductor Pr0.91Ce0.09LaCuO4-y
We report nuclear magnetic resonance studies on the low-doped n-type
copper-oxide Pr_{0.91}LaCe_{0.09}CuO_{4-y} (T_c=24 K) in the superconducting
state and in the normal state uncovered by the application of a strong magnetic
field. We find that when the superconductivity is removed, the underlying
ground state is the Fermi liquid state. This result is at variance with that
inferred from previous thermal conductivity measurement and contrast with that
in p-type copper-oxides with a similar doping level where high-T_c
superconductivity sets in within the pseudogap phase. The data in the
superconducting state are consistent with the line-nodes gap model.Comment: version to appear in Phys. Rev. Let
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