11,621 research outputs found
Amperean pairing and the pseudogap phase of cuprate superconductors
The enigmatic pseudogap phase in underdoped cuprate high T_c superconductors
has long been recognized as a central puzzle of the T_c problem. Recent data
show that the pseudogap is likely a distinct phase, characterized by a medium
range and quasi-static charge ordering. However, the origin of the ordering
wavevector and the mechanism of the charge order is unknown. At the same time,
earlier data show that precursive superconducting fluctuations are also
associated with this phase. We propose that the pseudogap phase is a novel
pairing state where electrons on the same side of the Fermi surface are paired,
in strong contrast with conventional BCS theory which pair electrons on
opposite sides of the Fermi surface. In this state the Cooper pair carries a
net momentum and belong to a general class called pair density wave (PDW). The
microscopic pairing mechanism comes from a gauge theory formulation of the
resonating valence bond (RVB) picture, where electrons traveling in the same
direction feel an attractive force in analogy with Ampere's effects in
electromagnetism. We call this Amperean pairing. Charge order automatically
appears as a subsidiary order parameter. Our theory gives a prediction of the
ordering wavevector which is in good agreement with experiment. Furthermore,
the quasiparticle spectrum from our model explains many of the unusual features
reported in photoemission experiments. The Fermi arc and the unusual way the
tip of the arc terminates also come out naturally. We also discuss how the
onset of the Kerr effect in this state can be accommodated. Finally, we propose
an experiment which can directly test the notion of Amperean pairing.Comment: (v4) added phase diagram, Appendix A on the incompatibility of CDW
model, and more discussion of low-temperature properties; (v3) expanded
supplementary section, added figures and discussion on Fermi arc; (v2) added
references, improved figures, corrected typo in Eq.(4
Staggered-flux normal state in the weakly doped t-J model
A normal (non-superconducting) ground state of the t-J model may be
variationally approximated by a Gutzwiller-projected wave function. Within this
approximation, at small hole doping near half-filling, the normal state favors
staggered-flux ordering. Such a staggered-flux state may occur in vortex cores
of underdoped high-temperature cuprate superconductors. From comparing the
energies of the staggered-flux state and of the superconducting state, we
numerically obtain the condensation energy. Extracting the superfluid density
directly from the projected superconducting wave function, we can also estimate
the coherence length at zero temperature.Comment: 5 pages, 4 figure
A Proposal to Use Neutron Scattering to Measure Scalar Spin Chirality Fluctuations in Kagome Lattices
In the theory of quantum spin liquids, gauge fluctuations are emergent
excitations at low energy. The gauge magnetic field is proportional to the
scalar spin chirality, S1.(S2xS3). It is therefore highly desirable to measure
the fluctuation spectrum of the scalar spin chirality. We show that in the
Kagome lattice with a Dzyaloshinskii-Moriya term, the fluctuation in Sz which
is readily measured by neutron scattering contains a piece which is
proportional to the chirality fluctuation.Comment: 8 Pages, 2 Figure
Emergent orbitals in the cluster Mott insulator on a breathing Kagome lattice
Motivated by the recent developments on cluster Mott insulating materials
such as the cluster magnet LiZnMoO, we consider the strong
plaquette charge ordered regime of the extended Hubbard model on a breathing
Kagome lattice and reveal the properties of the cluster Mottness. The plaquette
charge order arises from the inter-site charge interaction and the collective
motion of three localized electrons on the hexagon plaquettes. This model leads
naturally to a reduction of the local moments by 2/3 as observed in
LiZnMoO. Furthermore, at low temperatures each hexagon plaquette
contains an extra orbital-like degree of freedom in addition to the remaining
spin 1/2. We explore the consequence of this emergent orbital degree of
freedom. We point out the interaction between the local moments is naturally
described by a Kugel-Khomskii spin-orbital model. We develop a parton approach
and suggest a spin liquid ground state with spinon Fermi surfaces for this
model. We further predict an emergent orbital order when the system is under a
strong magnetic field. Various experimental consequences for
LiZnMoO are discussed, including an argument that the charge
ordering much be short ranged if the charge per Mo is slightly off
stoichiometry.Comment: 12 pages, 13 figure
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