Emergence of superconductivity in the canonical heavy-electron metal YbRh ₂ Si ₂

Abstract

One-sentence summary: We demonstrate that heavy-electron superconductivity develops in YbRh 2 Si 2 due to the weakening of its antiferromagnetism by the ordering of nuclear spins, providing evidence that quantum criticality is a robust mechanism for unconventional superconductivity.We report magnetic and calorimetric measurements down to T = 1 mK on the canonical heavy-electron metal YbRh 2 Si 2 . The data reveal the development of nuclear antiferromagnetic order slightly above 2 mK. The latter weakens the primary electronic antiferromagnetism, thereby paving the way for heavy-electron superconductivity below T c = 2 mK. Our results demonstrate that superconductivity driven by quantum criticality is a general phenomenon.Unconventional (i.e., non-phonon mediated) superconductivity, which has been attracting much interest since the early 1980s, is often observed at the border of antiferromagnetic (AF) order [1].As exemplified by heavy-electron (or heavy-fermion) metals, the suppression of the AF order opens up a wide parameter regime where the physics is controlled by an underlying quantum 1 arXiv:1707.03006v1 [cond-mat.str-el] 10 Jul 2017 critical point (QCP) [2,3]. A central question, then, concerns the interplay between quantum criticality and unconventional superconductivity in strongly correlated electron systems such as heavy-electron metals. In many of the latter superconductivity turns out to develop near such a QCP [2][3][4]. However, the absence of superconductivity in the prototypical quantum critical material YbRh 2 Si 2 (Ref. [5]) has raised the question as to whether the presence of an AF QCP necessarily gives rise to the occurrence of superconductivity. Because YbRh 2 Si 2 exists in the form of high-quality single crystals, it is meaningful to address this issue at very low temperatures without seriously encountering the limitations posed by disorder. We have therefore used this heavy-electron compound to carry out the first study on quantum critical metals at ultra-low temperatures.YbRh 2 Si 2 exhibits AF order below a Néel temperature T AF = 70 mK. A small magnetic field of B = 60 mT, when applied within the basal plane of the tetragonal structure, continuously suppresses the magnetic order and induces a QCP, presumably of unconventional nature [6,7].Electrical resistivity measurements down to 10 mK have failed to show any indications for superconductivity [5]. Recognizing that a critical field of 60 mT is unlikely to sustain even heavyelectron superconductivity with a T c of less than 10 mK, a different means of suppressing the antiferromagnetism is needed to eventually reveal any potential superconductivity at its border.We take advantage of the early recognition that hyperfine coupling to nuclear spins can considerably influence the electronic spin properties near a quantum phase transition [8]. Furthermore, measurements on PrCu 2 and related compounds have demonstrated a large coupling between the electronic and nuclear spins in rare-earth-based intermetallics at temperatures as high as 50 mK...

Magnetism of europium under extreme pressures

“…This validates recent x-ray absorption experiments showing that Eu remains nearly divalent to at least 87 GPa [26] and extends these findings beyond the pressure regime where superconductivity first emerges at low temperature. The persistence of local moments across magnetic-superconducting phase transitions has been observed in superconductors with possible unconventional pairing, including Ce [14] as well as CeCu 2 Si 2 , YbRh 2 Si 2 , CeRhIn 5 , and most recently CeNiAsO [55][56][57][58][59][60][61]. In these systems, as in Eu above 80 GPa, the proximity of the superconducting state to a magnetically ordered phase raises the possibility that spin fluctuations may be important in the pairing interaction.…”

Two types of superconducting domes in unconventional superconductors

“…CePt 2 In 7 Superconductivity has an extended dome, as in many pnictides, with an AFM QCP intercepting the lower part of the SC dome [192]. YbRh 2 Si 2 AFM QCP and NFL coincide, but the SC state vanishes before the QCP/ NFL point [193]. UCoGe SC dome, FM QCP, and NFL state (n→1) coincide [61].…”

Magnetic field dependence of the neutron spin resonance inCeB6

“…Of particular interest are inelastic neutron scattering (INS) results obtained on CeCoIn 5 , where a sharp resonance peak was observed within the superconducting phase [5,[11][12][13]. At first glance similar peaks were found in the antiferromagnetic (AFM) superconductor UPd 2 Al 3 [14,15], as well as in the normal state of the heavy-fermion metal YbRh 2 Si 2 [16], where superconductivity was recently discovered below ∼ 2 mK [17]. Another striking example of a resonant mode is given by the well known nonsuperconducting heavyfermion antiferromagnet CeB 6 [18,19].…”