  Quantum antiferromagnets have proven to be some of the cleanest realizations
available for theoretical, numerical, and experimental studies of quantum
fluctuation effects. At finite temperatures, however, the additional effects of
thermal fluctuations in the restricted phase space of a low-dimensional system
have received much less attention, particularly the situation in frustrated
quantum magnets, where the excitations may be complex collective (bound or even
fractionalized) modes. We investigate this problem by studying the
thermodynamic properties of the frustrated two-leg S=1/2 spin ladder, with
particular emphasis on the fully frustrated case. We present numerical results
for the magnetic specific heat and susceptibility, obtained from exact
diagonalization and quantum Monte Carlo studies, which we show can be rendered
free of the sign problem even in a strongly frustrated system and which allow
us to reach unprecedented sizes of L=200 ladder rungs. We find that frustration
effects cause an unconventional evolution of the thermodynamic response across
the full parameter regime of the model. However, close to the first-order
transition they cause a highly anomalous reduction in temperature scales with
no concomitant changes in the gap; the specific heat shows a very narrow peak
at very low energies and the susceptibility rises abruptly at extremely low
temperatures. Unusually, the two quantities have different gaps over an
extended region of the parameter space. We demonstrate that these results
reflect the presence of large numbers of multi-particle bound-state
excitations, whose energies fall below the one-triplon gap in the transition
region.
