Fractionalization: superconductivity and stable hc/e vortices
Doping the quantum paramagnets with deconfined
spinon excitations leads to states with unusual properties.
Apart from a possible Wigner crystalline state at very low doping, the
ground state can be a superconductor.
The S=1/2 spinons are expected to survive as neutral excitations in the
superconductor, but such excitations are no longer exotic: the familiar Bogoliubov
excitations of a BCS superconductor also have S=1/2 and can be considered neutral.
However, the presence of a parent insulating state with spin-charge separation
does lead to some exotic properties in the superconductor.
It was argued in paper
3 (and reviewed in paper 4) that such a superconductor should support stable
hc/e vortices in the underdoped region. There has been renewed interest
in these issues in recent years: the work of Senthil and Fisher
(T. Senthil and Matthew P. A. Fisher Physical Review B 62, 7850 (2000)) has made the connection to deconfined insulating states, as described
by a Z2 gauge theory, especially clear.
The online version of paper 4 contains
a brief discussion of the relationship of the recent work to the earlier
ideas. These authors have also proposed an interesting flux-memory effect which is intimately related
to the effects leading to stable hc/e vortices, as discussed in the on-line version of paper 5.
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Large N expansion for frustrated and doped quantum
antiferromagnets, S. Sachdev and N. Read, International Journal of
Modern Physics B 5, 219 (1991); cond-mat/0402109.
Superconducting, metallic, and insulating phases in a
model of CuO2 layers, J. Ye and S. Sachdev, Physical Review
B 44, 10173 (1991).
Stable hc/e vortices in a gauge theory of superconductivity
in strongly correlated electronic systems, S. Sachdev, Physical Review
B 45, 389 (1992).
Stable hc/e vortices in
superconductors with spin-charge separation, S. Sachdev, International
Journal of Modern Physics B 6, 509 (1992).
Quantum criticality: competing
ground states in low dimensions, S. Sachdev, Science 288, 475
Fractionalized Fermi liquids, T. Senthil, S. Sachdev, and M. Vojta, Physical Review Letters 90, 216403 (2003); cond-mat/0209144.
Understanding correlated electron systems by a classification of Mott insulators, S. Sachdev, Annals of Physics 303, 226 (2003); cond-mat/0211027.