Magnetic ac susceptibility of YbRh2Si2 at ultra-low temperatures

Recent magnetization measurements on the heavy fermion compound YbRh2Si2 at ultra-low temperatures (ULT) below 1 mK show that the weak antiferromagnetic phase, which forms at B=0 below TN=70 mK, is not the T=0 ground state of the system. Instead, two additional ULT regimes were discovered: a new phase below TA=2.2 mK, where the phase transition is observed as a pronounced magnetization peak, and a second regime below TB=15 mK, where zero-field-cooled / field-cooled measurements begin to show a hysteresis. To clarify the origin of these two new ULT regimes, particularly to seek evidence for possible superconducting states in YbRh2Si2, we measured the magnetic ac susceptibility at B=0. Initial results indicate a decrease in the real part χ' of the ac susceptibility as the sample temperature is lowered beyond TB=15 mK and a pronounced drop in χ' as T drops below the transition temperature TA=2.2 mK. Whether the response is indeed diamagnetic depends on the yet unknown background signal.