Nuclear-order-induced quantum criticality and heavy-fermion superconductivity at ultra-low temperatures in YbRh$_2$Si$_2$

Abstract

The tetragonal heavy-fermion metal YbRh2Si2 orders antiferromagnetically at TN = 70 mK and exhibits an unconventional quantum critical point (QCP) of Kondo-destroying type at BN = 60 mT, for the magnetic field applied within the basal (a, b) plane. Ultra-low-temperature magnetization and heat-capacity measurements at very low fields indicate that the 4f -electronic antiferromagnetic (AF) order is strongly suppressed by a nuclear-dominated hybrid order ('A-phase') at TA ≤ 2.3 mK, such that a QCP becomes established at B ≈ 0 [1]. This enables the onset of heavy-fermion superconductivity (Tc = 2 mK) which appears to be suppressed by the primary AF order at elevated temperatures. Measurements of the Meissner effect reveal bulk superconductivity, with Tc decreasing under applied field to Tc < 1 mK at B > 20 mT. The observation of a weak but distinct superconducting shielding signal at a temperature as high as 10 mK suggests the formation of insulated random islands with emergent A-phase order and superconductivity. Upon cooling, the shielding signal increases almost linearly in temperature, indicating a growth of the islands which eventually percolate at T ≈ 6.5 mK, as inferred from recent electrical-resistivity results by [2]. These authors also report on two superconducting regions, the first one below 4 mT, with the highest Tc occurring at B = 0, and a second 'dome-shaped' regime with much lower Tc at 4 mT < B < 60 mT.