  We present a multi-wavelength analysis of the four most relaxed clusters in
the South Pole Telescope 2500 deg^2 survey, which lie at 0.55 < z < 0.75. This
study, which utilizes new, deep data from Chandra and Hubble, along with
ground-based spectroscopy from Gemini and Magellan, improves significantly on
previous studies in both depth and angular resolution, allowing us to directly
compare to clusters at z~0. We find that the temperature, density, and entropy
profiles of the intracluster medium (ICM) are very similar among the four
clusters, and share similar shapes to clusters at z~0. Specifically, we find no
evidence for deviations from self similarity in the temperature profile over
the radial range 10kpc < r < 1Mpc, implying that the processes responsible for
preventing runaway cooling over the past >6 Gyr are, at least roughly,
preserving self similarity. We find typical metallicities of ~0.3 Zsun in the
bulk of the ICM, rising to ~0.5 Zsun in the inner ~100 kpc, and reaching ~1
Zsun at r < 10kpc. This central excess is similar in magnitude to what is
observed in the most relaxed clusters at z~0, suggesting that both the global
metallicity and the central excess that we see in cool core clusters at z~0
were in place very early in the cluster lifetime and, specifically, that the
central excess is not due to late-time enrichment by the central galaxy.
Consistent with observations at z~0, we measure a diversity of stellar
populations in the central brightest cluster galaxies of these four clusters,
with star formation rates spanning a factor of ~500, despite the similarity in
cooling time, cooling rate, and central entropy. These data suggest that, while
the details vary dramatically from system to system, runaway cooling has been
broadly regulated in relaxed clusters over the past 6 Gyr.
