Gapped fermionic excitations in twodimensional antiferromagnets
and superconductors
Upon developing a theory of a quantum phase transition, one normally only considers the
low energy excitations which become gapless and critically damped at the quantum
critical point. So e.g. across a magnetic phase transition, as discussed
under the corresponding category, the quantum field theory is expressed only in terms of the S=1 exciton which condenses at the transition.
The papers below show that gapped excitations can also have interesting and nontrivial
critical behavior across such a transition. We consider the problem of a single hole
injected into an antiferromagnet moving across a transition from a Neel state to a
confining, spin gap, Mott insulator. In both these phases, the single hole spectral
function has an infinitely sharp quasiparticle pole at the bottom of the hole band. These
papers show that the residue of this pole vanishes as we approach the magnetic quantum
critical point from either side; right at the critical point there is a dissipative
continuum of gapped, singlehole excitations which are characterized by a new anomalous dimension.
PAPERS

Hole motion in a quantum Neel state, S. Sachdev,
Physical Review B 39, 12232 (1989).

Spin orthogonality catastrophe
in twodimensional antiferromagnets and superconductors, S. Sachdev, M. Troyer, and M. Vojta, Physical Review Letters 86, 2617 (2001); condmat/0011232.

Static hole in a critical antiferromagnet:
fieldtheoretic renormalization group,
S. Sachdev, Physica C 357, 78
(2001);
condmat/0011233.

Quantum impurity in a magnetic environment, S. Sachdev, Proceedings
of the conference on Field Theory and Statistical Mechanics, Rome, June 2002 in honor of G. JonaLasinio,
Journal of Statistical Physics 115, 47 (2004); condmat/0304171.
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