13 research outputs found
Gauge Invariance and Spinon-Dopon Confinement in the t-J Model: implications for Fermi Surface Reconstruction in the Cuprates
We discuss the application of the two-band spin-dopon representation of the
t-J model to address the issue of the Fermi surface reconstruction observed in
the cuprates. We show that the electron no double occupancy (NDO) constraint
plays a key role in this formulation. In particular, the auxiliary lattice spin
and itinerant dopon degrees of freedom of the spin-dopon formulation of the t-J
model are shown to be confined in the emergent U(1) gauge theory generated by
the NDO constraint. This constraint is enforced by the requirement of an
infinitely large spin-dopon coupling. As a result, the t-J model is equivalent
to a Kondo-Heisenberg lattice model of itinerant dopons and localized lattice
spins at infinite Kondo coupling at all dopings. We show that mean-field
treatment of the large vs small Fermi surface crossing in the cuprates which
leaves out the NDO constraint, leads to inconsistencies and it is automatically
excluded form the t - J model framework
Low-energy effective representation of the Gutzwiller-projected BCS Hamiltonian close to half filling
We investigate analytically a connection between the t-J model and the
strongly correlated Bardeen-Cooper-Schrieffer (BCS) Hamiltonian, with the
effect of strong electron correlations accounted by the Gutzwiller projection.
We show that in the immediate vicinity of half filling the projected 2D BCS
Hamiltonian with strong pairing develops an antiferromagnetically (AF) ordered
ground state. This result explicitly demonstrates that antiferromagnetism in
this model appears as a natural consequence of the strong Coulomb repulsion in
a low doped regime. At moderate doping the ground state of the
Gutzwiller-projected BCS Hamiltonian becomes qualitatively similar to
Anderson's resonating valence bond state which is known to fit nicely the
properties of the t-J model in this regime. These two properties taken together
indicate that the projected BCS Hamiltonian captures the essential low-energy
physics of the t-J model in the whole underdoped region
Resonating Valence Bond Theory of Superconductivity for Dopant Carriers: Application to the Cobaltates
Within the -- model Hamiltonian we present a RVB mean field theory
directly in terms of dopant particles. We apply this theory to
and show that the
resulting phase diagram versus doping is in qualitative agreement with
the experimental results
Doped carrier formulation of the t-J model: the projection constraint and the effective Kondo-Heisenberg lattice representation
We show that the recently proposed doped carrier Hamiltonian formulation of
the t-J model should be complemented with the constraint that projects out the
unphysical states. With this new important ingredient, the previously used and
seemingly different spin-fermion representations of the t-J model are shown to
be gauge related to each other. This new constraint can be treated in a
controlled way close to half-filling suggesting that the doped carrier
representation provides an appropriate theoretical framework to address the t-J
model in this region. This constraint also suggests that the t-J model can be
mapped onto a Kondo-Heisenberg lattice model. Such a mapping highlights
important physical similarities between the quasi two-dimensional heavy
fermions and the high-T superconductors. Finally we discuss the physical
implications of our model representation relating in particular the small
versus large Fermi surface crossover to the closure of the lattice spin gap.Comment: corrected and enlarged versio
On representation of the t-J model via spin-charge variables
We show that the t-J Hamiltonian is not in general reduced to H(S,f), where S
and f stand for independent ([S,f]=0) SU(2) (spin) generators and spinless
fermionic (hole) field, respectively. The proof is based upon an identification
of the Hubbard operators with the generators of the su(2|1) superalgebra in the
degenerate fundamental representation and ensuing SU(2|1) path integral
representation of the partition function.Comment: 15 pages, latex, no figure
The breakdown of the mean-field description of the Nagaoka phase
We discuss the relevance of the improved mean-field slave-fermion theory to
describe the Nagaoka () limit of the Hubbard model. In this theory
the crucial on-site constraint of no double electron occupancy is taken into
account rigorously prior to the mean-field approximation. At one-loop
approximation the effective mean-field action shows a long-range ferromagnetic
order over the whole doping range. This indicates that the slave-fermion
mean-field theory does not constitute an appropriate framework to describe the
physics of the Nagaoka phase. We discuss the drawbacks of this mean-field
theory and present some results on the derivation of a low-energy effective
spin action to describe the Nagaoka phase beyond the mean-field approximation.Comment: 10 pages, revised and extended version. To be published in Phys. Rev.