  The influence of external pressure on the charge-density-wave (CDW) ground
state of the quasi-one-dimensional two-leg ladder compound
Sr$_{10}$Ca$_{4}$Cu$_{24}$O$_{41}$ has been studied by optical reflectivity
measurements as a function of temperature (10 - 300~K) and pressure $P$ (0.3 -
4.3~GPa) over the spectral range 580 - 6000 cm$^1$. With increasing pressure
the CDW transition temperature $T_{CDW}$ decreases with the linear pressure
coefficient $\approx$-70~K/GPa, and above $\approx$3~GPa the CDW phase is
suppressed at all temperatures. This behavior is similar to that in compounds
Sr$_{14-x}$Ca$_x$Cu$_{24}$O$_{41}$ with increasing Ca content $x$ at ambient
pressure, with the simple scaling $x \approx 3\cdot P(GPa)$. The size of the
CDW gap decreases with increasing pressure, whereas the dimensionality of the
high-temperature insulating phase in Sr$_{10}$Ca$_{4}$Cu$_{24}$O$_{41}$ within
the ladder plane is hardly affected by external pressure.
