Contactless Measurements of Low-Temperature Metal-to-Insulator Transitions at Microwave Frequencies

We have developed an apparatus to make sensitive measurements of metal-to-insulator transitions in condensed matter systems. The technique uses a high-Q resonator that operates at microwave frequencies and it does not require electrical contact to the sample under investigation. The sample and resonator are cooled to low temperatures using a two-stage closed-cycle cryocooler. The sample temperature can be controlled independently of the temperature of the resonator and the cryocooler's base temperature. Reflection coefficient measurements made using a low-cost vector network analyzer (VNA) allow us to track the quality factor of the loaded resonator as a function of the sample temperature. We will discuss the design and construction of the apparatus. Preliminary measurements of the so-called Verwey transition at 120 K in a single-crystal sample of Fe₃O₄ (magnetite) will be presented. We will also discuss an observed metal-to-insulator transition near 30 K in a sample of electron-doped silicon.