  Dynamical properties of the impurity spin-$\frac12$ in 2D and quasi-2D
Heisenberg antiferromagnets (AFs) at $T\ge0$ are discussed. The specific case
of an impurity coupled symmetrically to two neighboring host spins is
considered. The specific feature of this problem is that the defect is
degenerate (frustrated) being located in zero molecular field. It is shown that
this problem can be described by spin-boson model without tunneling term and
with a more complex interaction. We demonstrate that the effect of the host
system on the defect is completely described by the spectral function. It is
found within the spin-wave approximation that for not too small $\omega$ the
spectral function is proportional to $\omega^2/J^3$, where $J$ is the exchange
constant between the host spins. The defect dynamical susceptibility is derived
using Abrikosov's pseudofermion technique and diagrammatic expansion. The
calculations are performed within the fourth order of the dimensionless
coupling parameter $f$. It is found that transverse impurity susceptibility
$\chi_\perp(\omega)$ has a Lorenz peak with the width proportional to
$f^4J(T/J)^3$ which disappears at T=0, and a non-resonant term. The later term
diverges logarithmically as $\omega,T \to 0$. The static susceptibility
$\chi(0)$ has the free-spin-like contribution $1/(4T)$, and a logarithmic
correction proportional to $f^2\ln(J/T)$. The influence of finite concentration
of the defects $n$ on the low-temperature properties of AF is also
investigated. A logarithmic correction to spin-wave velocity of the form
$nf^4\ln|J/\omega|$ and an anomalous damping of spin waves proportional to
$nf^4|\omega|$ are obtained.
