Abstract
We study the collapse of two-dimensional polymers, via an O($n$) model on the
square lattice that allows for dilution, bending rigidity and short-range
monomer attractions. This model contains two candidates for the theta point,
$\Theta_BN$ and $\Theta_DS$, both exactly solvable. The relative
stability of these points, and the question of which one describes the
`generic' theta point, have been the source of a long-standing debate.
Moreover, the analytically predicted exponents of $\Theta_BN$ have never
been convincingly observed in numerical simulations.
In the present paper, we shed a new light on this confusing situation. We
show in particular that the continuum limit of $\Theta_BN$ is an unusual
conformal field theory, made in fact of a simple dense polymer decorated with
non-compact degrees of freedom. This implies in particular that the
critical exponents take continuous rather than discrete values, and that
corrections to scaling lead to an unusual integral form. Furthermore, discrete
states may emerge from the continuum, but the latter are only
normalizable---and hence observable---for appropriate values of the model's
parameters. We check these findings numerically. We also probe the non-compact
degrees of freedom in various ways, and establish that they are related to
fluctuations of the density of monomers. Finally, we construct a field
theoretic model of the vicinity of $\Theta_BN$ and examine the flow along
the multicritical line between $\Theta_BN$ and $\Theta_DS$.
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