  We have developed a low temperature, high-resolution microwave surface
impedance probe that is able to operate in high static magnetic fields. Surface
impedance is measured by cavity perturbation of dielectric resonators, with
sufficient sensitivity to resolve the microwave absorption of sub-mm-sized
superconducting samples. The resonators are constructed from high permittivity
single-crystal rutile (TiO2) and have quality factors in excess of 10^6.
Resonators with such high performance have traditionally required the use of
superconducting materials, making them incompatible with large magnetic fields
and subject to problems associated with aging and power-dependent response.
Rutile resonators avoid these problems while retaining comparable sensitivity
to surface impedance. Our cylindrical rutile resonators have a hollow bore and
are excited in TE_01(n-d) modes, providing homogeneous microwave fields at the
center of the resonator where the sample is positioned. Using a sapphire
hot-finger technique, measurements can be made at sample temperatures in the
range 1.1 K to 200 K, while the probe itself remains immersed in a liquid
helium bath at 4.2 K. The novel apparatus described in this article is an
extremely robust and versatile system for microwave spectroscopy, integrating
several important features into a single system. These include: operation at
high magnetic fields; multiple measurement frequencies between 2.64 GHz and
14.0 GHz in a single resonator; excellent frequency stability, with typical
drifts < 1 Hz per hour; the ability to withdraw the sample from the resonator
for background calibration; and a small pot of liquid helium separate from the
external bath that provides a sample base temperature of 1.1 K.
