Unconventional superconductivity and other previously unknown phases of matter exist in the vicinity of a quantum critical point (QCP): a continuous phase change of matter at absolute zero. Intensive theoretical and experimental investigations on itinerant systems have shown that metallic ferromagnets tend to develop via either a first-order phase transition or through the formation of intermediate superconducting or inhomogeneous magnetic phases. Here, through precision low-temperature measurements, we show that the Grüneisen ratio of the heavy fermion metallic ferromagnet YbNi(4)(P(0.92)As(0.08))(2) diverges upon cooling to T = 0, indicating a ferromagnetic QCP. Our observation that this kind of instability, which is forbidden in d-electron metals, occurs in a heavy fermion system will have a large impact on the studies of quantum critical materials.

We report the observation of a bulk charge modulation in La1.88Sr0.12CuO4 (LSCO) with a characteristic in-plane wave-vector of (0.236, ±δ), with δ=0.011 r.l.u. The transverse shift of the ordering wave-vector indicates the presence of rotated charge-stripe ordering, demonstrating that the charge ordering is not pinned to the Cu-O bond direction. On cooling through the superconducting transition, we find an abrupt change in the growth of the charge correlations and a suppression of the charge order parameter indicating competition between the two orderings. Orthorhombic LSCO thus helps bridge the apparent disparities between the behavior previously observed in the tetragonal "214" cuprates and the orthorhombic yttrium and bismuth-based cuprates and thus lends strong support to the idea that there is a common motif to charge order in all cuprate families.

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...