Microstructuring YbRh2Si2 for resistance and noise measurements down to ultra-low temperatures

Abstract

The discovery of superconductivity in the quantum critical Kondo-lattice system YbRh _2 Si _2 at an extremely low temperature of 2 mK has inspired efforts to perform high-resolution electrical resistivity measurements down to this temperature range in highly conductive materials. Here we show that control over the sample geometry by microstructuring using focused-ion-beam techniques allows to reach ultra-low temperatures and increase signal-to-noise ratios (SNRs) tenfold, without adverse effects to sample quality. In five experiments we show four-terminal sensing resistance and magnetoresistance measurements which exhibit sharp phase transitions at the Néel temperature, and Shubnikov–de-Haas (SdH) oscillations between 13 T and 18 T where we identified a new SdH frequency of 0.39 kT. The increased SNR allowed resistance fluctuation (noise) spectroscopy that would not be possible for bulk crystals, and confirmed intrinsic $1/f$ -type fluctuations. Under controlled strain, two thin microstructured samples exhibited a large increase of $T_\mathrm{N}$ from 67 mK up to 188 mK while still showing clear signatures of the phase transition and SdH oscillations. Superconducting quantum interference device-based thermal noise spectroscopy measurements in a nuclear demagnetization refrigerator down to 0.95 mK, show a sharp superconducting transition at $T_\mathrm{c} = 1.2$ mK. These experiments demonstrate microstructuring as a powerful tool to investigate the resistance and the noise spectrum of highly conductive correlated metals over wide temperature ranges.

Keywords

• strongly correlated electron systems
• heavy fermions
• electrical and thermal conduction in crystalline metals and alloys
• fluctuation phenomena
• random processes
• noise