  Superconductivity was recently observed in the iron-arsenic-based compounds
with a superconducting transition temperature (Tc) as high as 56K [1-7],
naturally raising comparisons with the high Tc copper oxides. The copper oxides
have layered crystal structures with quasi-two-dimensional electronic
properties, which led to speculations that reduced dimensionality (that is,
extreme anisotropy) is a necessary prerequisite for superconductivity at
temperatures above 40 K [8,9]. Early work on the iron-arsenic compounds seemed
to support this view [7,10]. Here we report measurements of the electrical
resistivity in single crystals of (Ba,K)Fe2As2 in a magnetic field up to 60 T.
We find that the superconducting properties are in fact quite isotropic, being
rather independent of the direction of the applied magnetic fields at low
temperature. Such behaviour is strikingly different from all previously-known
layered superconductors [9,11], and indicates that reduced dimensionality in
these compounds is not a prerequisite for high-temperature superconductivity.
We suggest that this situation arises because of the underlying electronic
structure of the iron-arsenide compounds, which appears to be much more three
dimensional than that of the copper oxides. Extrapolations of low-field
single-crystal data incorrectly suggest a high anisotropy and a greatly
exaggerated zero-temperature upper critical field.
