Rotating cavity for high-field angle-dependent microwave spectroscopy of low-dimensional conductors and magnets

The cavity perturbation technique is an extremely powerful method for measuring the electrodynamic response of a material in the millimeter- and submillimeter spectral range (10GHz–1THz), particularly in the case of high-field/frequency magnetic resonance spectroscopy. However, the application of such techniques within the limited space of a high-field magnet presents significant technical challenges. We describe a 7.62mm×7.62mm (diameter×length) rotating cylindrical cavity which overcomes these problems. The cylinder is mounted transverse to the bore of the magnet, coupling is achieved through the sidewalls of the cavity, and the end plate is then rotated (by means of an external drive) instead of the body of the cavity itself. Therefore, rotation does not affect the cylindrical geometry, or the mechanical connections to the incoming waveguides. The TE011 mode frequency of the cavity is 51.863GHz, with the possibility to work on higher-order modes to frequencies of order 350GHz. Neither the quality factor (∼22000 for the fundamental mode) or the coupling to the cavity are significantly affected for full 360° of rotation. The rotation mechanism provides excellent angle resolution (<0.1°), and is compact enough to enable measurements in the high-field (up to 45T) magnets at the National High Magnetic Field Laboratory. Two-axis rotation capabilities are also possible in conjunction with split-pair magnet configurations. We present examples of angle-dependent measurements which illustrate the unique capabilities of this rotating cavity, including: high-field angle-dependent measurements of an unusual form of cyclotron resonance in anisotropic organic conductors; and angle-dependent high-frequency single-crystal electron paramagnetic resonance measurements in single-molecule magnets.