Radio-Frequency Method for Detecting Superconductivity Under High Pressure

We introduce a contactless technique for probing superconductivity, metal-insulator transitions, and magnetic ordering in micron-sized samples under extreme pressure. Utilizing a multistage Lenz lens system, directly sputtered onto diamond anvils, we realize a radio-frequency (RF, 50 kHz - 200 MHz) transformer with a sample, of 50-100 microns in diameter, as its core. This configuration enables efficient transfer and focusing of an electromagnetic field within the diamond anvil cell's chamber. Consequently, the transmitted RF signal exhibits high sensitivity to variations in the sample's surface conductivity and magnetic permeability. We validate this method by determining the critical temperatures ($T_{\text{c}}$) of known superconductors, including NbTi, MgB$_2$, Hg-1223, BSCCO, and REBCO in various magnetic fields, as well as the magnetic ordering temperatures of Gd and Tb, and the metal-insulator transition in VO$_2$. Notably, we apply this technique to the (La,Ce)H$_{10-12}$ superhydride at a pressure of about 150 GPa. The observed superconducting transition, at 215-220 K, is noticeably higher than the $T_{\text{c}}$ determined via traditional electrical-resistance measurements (200-205 K), demonstrating the method's enhanced sensitivity. Moreover, we show how multiple repetitions of the RF experiment with the La-Ce superhydride make it possible to detect the increase in $T_{\text{c}}$ over time up to 260-270 K. This finding opens a pathway towards reaching a critical $T_{\text{c}}$ above 0$^\circ$C in the La-based superhydrides.