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

The discovery of superconductivity in the quantum critical Kondo-lattice system YbRh₂Si₂ 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 T̀N 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 T̀c = 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.