482 research outputs found
Underscreened Kondo Necklace
It has been suggested recently by Gan, Coleman, and Andrei that studying the
underscreened Kondo problem may help to understand the nature of magnetism in
heavy fermion systems. Motivated by Doniach's work on the S=1/2 Kondo necklace,
we introduce the underscreened Kondo necklace models with S>1/2. The
underscreened Kondo necklace is the simplest lattice model on which the
competition between Kondo spin compensation, and magnetic ordering due to an
RKKY-type interaction can be examined. We used the mean-field approximation to
determine the phase diagram, and found that the low-temperature phase is always
an x-y antiferromagnet. This contention is further supported by the derivation
of the exact form of the effective hamiltonian in the limit of very large Kondo
coupling: it is found to be an antiferromagnetic x-y model for the residual
(S-1/2)-spins. In general, the degree of moment compensation depends on both
the Kondo coupling, and on S.Comment: 15 pages (2 figures upon request from [email protected]), LATEX,
to appear in Modern Physics Letters
Fermi pockets and quantum oscillations of the Hall coefficient in high temperature superconductors
Recent quantum oscillation measurements in high temperature superconductors
in high magnetic fields and low temperatures have ushered in a new era. These
experiments explore the normal state from which superconductivity arises and
provide evidence of a reconstructed Fermi surface consisting of electron and
hole pockets in a regime in which such a possibility was previously considered
to be remote. More specifically, the Hall coefficient has been found to
oscillate according to the Onsager quantization condition, involving only
fundamental constants and the areas of the pockets, but with a sign that is
negative. Here we explain the observations with the theory that the alleged
normal state exhibits a hidden order, the -density wave, which breaks
symmetries signifying time reversal, translation by a lattice spacing, and a
rotation by an angle , while the product of any two symmetry operations
is preserved. The success of our analysis underscores the importance of
spontaneous breaking of symmetries, Fermi surface reconstruction, and
conventional quasiparticles. We primarily focus on the version of the order
that is commensurate with the underlying crystalline lattice, but also touch
upon the consequences if the order were to incommensurate. It is shown that
while commensurate order results in two independent oscillation frequencies as
a function of the inverse of the applied magnetic field, incommensurate order
leads to three independent frequencies. The oscillation amplitudes, however,
are determined by the mobilities of the charge carriers comprising the Fermi
pockets.Comment: Final version with a correction of a minor typo; 6 pages, 2 figure
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