241 research outputs found
Spin Dynamics in the Normal State of High T_c Superconductors
We summarize our recent theoretical studies on the spin dynamics in the
normal state of the metallic cuprates. The contrasting wave vector dependence
of the dynamical spin structure factor in LaSrCuO and
YBaCuO systems are attributed to the differences in the fermiology, in
conjunction with strong Coulomb correlations. These effects are found to
account also for the anomalous temperature and frequency dependence of . We conclude that the low energy spin dynamics of the metallic
cuprates are described in terms of correlated quasiparticles with a Luttinger
Fermi surface and a non-zero antiferromagnetic exchange interaction.Comment: 30 pages, REVTE
Quantum Criticality and the Kondo Lattice
Quantum phase transitions (QPTs) arise as a result of competing interactions
in a quantum many-body system. Kondo lattice models, containing a lattice of
localized magnetic moments and a band of conduction electrons, naturally
feature such competing interactions. A Ruderman-Kittel-Kasuya-Yosida (RKKY)
exchange interaction among the local moments promotes magnetic ordering.
However, a Kondo exchange interaction between the local moments and conduction
electrons favors the Kondo-screened singlet ground state. This chapter
summarizes the basic physics of QPTs in antiferromagnetic Kondo lattice
systems. Two types of quantum critical points (QCPs) are considered.
Spin-density-wave quantum criticality occurs at a conventional type of QCP,
which invokes only the fluctuations of the antiferromagnetic order parameter.
Local quantum criticality describes a new type of QCP, which goes beyond the
Landau paradigm and involves a breakdown of the Kondo effect. This critical
Kondo breakdown effect leads to non-Fermi liquid electronic excitations, which
are part of the critical excitation spectrum and are in addition to the
fluctuations of the magnetic order parameter. Across such a QCP, there is a
sudden collapse of the Fermi surface from large to small. I close with a brief
summary of relevant experiments, and outline a number of outstanding issues,
including the global phase diagram.Comment: 27 pages, 6 figures; Chapter of the book "Understanding Quantum Phase
Transitions", ed. Lincoln D. Carr (CRC Press/Taylor & Francis, Boca Raton,
2010
Topological defects of N\'eel order and Kondo singlet formation for Kondo-Heisenberg model on a honeycomb lattice
Heavy fermion systems represent a prototypical setting to study magnetic
quantum phase transitions. A particular focus has been on the physics of Kondo
destruction, which captures quantum criticality beyond the Landau framework of
order-parameter fluctuations. In this context, we study the spin one-half
Kondo-Heisenberg model on a honeycomb lattice at half filling. The problem is
approached from the Kondo destroyed, antiferromagnetically ordered insulating
phase. We describe the local moments in terms of a coarse grained quantum
non-linear sigma model, and show that the skyrmion defects of the
antiferromagnetic order parameter host a number of competing order parameters.
In addition to the spin Peierls, charge and current density wave order
parameters, we identify for the first time Kondo singlets as the competing
orders of the antiferromagnetism. We show that the antiferromagnetism and
various competing singlet orders can be related to each other via generalized
chiral transformations of the underlying fermions. We also show that the
conduction electrons acquire a Berry phase through their coupling to the
hedgehog configurations of the N\'eel order, which cancels the Berry phase of
the local moments. Our results demonstrate the competition between the
Kondo-singlet formation and spin-Peierls order when the antiferromagnetic order
is suppressed, thereby shedding new light on the global phase diagram of heavy
fermion systems at zero temperature.Comment: 14 pages, 4 figure
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