Intertwined density waves in a metallic nickelate

Discovery of a long-ranged charge order with 1/4 Ge1-dimerization in an antiferromagnetic Kagome metal

Exotic quantum states arise from the interplay of various degrees of freedom such as charge, spin, orbital, and lattice. Recently, a short-ranged charge order (CO) was discovered deep inside the antiferromagnetic phase of Kagome magnet FeGe, exhibiting close relationships with magnetism. Despite extensive investigations, the CO mechanism remains controversial, mainly because the short-ranged behavior hinders precise identification of CO superstructure. Here, combining multiple experimental techniques, we report the observation of a long-ranged CO in high-quality FeGe samples, which is accompanied with a first-order structural transition. With these high-quality samples, the distorted 2 × 2 × 2 CO superstructure is characterized by a strong dimerization along the c-axis of 1/4 of Ge1-sites in Fe3Ge layers, and in response to that, the 2 × 2 in-plane charge modulations are induced. Moreover, we show that the previously reported short-ranged CO might be related to large occupational disorders at Ge1-site, which upsets the equilibrium of the CO state and the ideal 1 × 1 × 1 structure with very close energies, inducing nanoscale coexistence of these two phases. Our study provides important clues for further understanding the CO properties in FeGe and helps to identify the CO mechanism.

Record-High Superconducting Transition Temperature in a Ti1-xMnx Alloy with the Rich Magnetic Element Mn

It is well-known that magnetic moments are very harmful to superconductivity. A typical example is the element Mn, whose compounds usually exhibit strong magnetism. Thus, it is very difficult to achieve superconductivity in materials containing Mn. Here, we report enhanced superconductivity with a superconducting transition temperature (Tc) up to a record-high value of about 26 K in a beta-phase Ti1-xMnx alloy containing the rich magnetic element Mn under high pressures. This is contrary to the intuition that magnetic moments always suppress superconductivity. Under high pressures, we also found that in the middle-pressure regime, the Pauli limit of the upper critical field is surpassed. The synchrotron X-ray diffraction data show an unchanged beta-phase with a continuous contraction of the cell volume, which is well-supported by the first-principles calculations. Although the theoretical results based on electron-phonon coupling can interpret the Tc value in a certain pressure region, the monotonic enhancement of superconductivity by pressure cannot seek support from the theory. Our results show a surprising enhancement of superconductivity in the Ti1-xMnx alloy with a considerable Mn content.

Superconductivity in pressurized trilayer La4Ni3O10-δ single crystals

The pursuit of discovering new high-temperature superconductors that diverge from the copper-based model1-3 has profound implications for explaining mechanisms behind superconductivity and may also enable new applications4-8. Here our investigation shows that the application of pressure effectively suppresses the spin-charge order in trilayer nickelate La4Ni3O10-δ single crystals, leading to the emergence of superconductivity with a maximum critical temperature (Tc) of around 30 K at 69.0 GPa. The d.c. susceptibility measurements confirm a substantial diamagnetic response below Tc, indicating the presence of bulk superconductivity with a volume fraction exceeding 80%. In the normal state, we observe a strange metal behaviour, characterized by a linear temperature-dependent resistance extending up to 300 K. Furthermore, the layer-dependent superconductivity observed hints at a unique interlayer coupling mechanism specific to nickelates, setting them apart from cuprates in this regard. Our findings provide crucial insights into the fundamental mechanisms underpinning superconductivity, while also introducing a new material platform to explore the intricate interplay between the spin-charge order, flat band structures, interlayer coupling, strange metal behaviour and high-temperature superconductivity.

Polymorphism in the Ruddlesden-Popper Nickelate La3Ni2O7: Discovery of a Hidden Phase with Distinctive Layer Stacking

We report the discovery of a novel form of Ruddlesden-Popper (RP) nickelate that stands as the first example of long-range, coherent polymorphism in this class of inorganic solids. Rather than the well-known, uniform stacking of perovskite blocks ubiquitously found in RP phases, this newly discovered polymorph of the bilayer RP phase La3Ni2O7 adopts a novel stacking sequence in which single-layer and trilayer blocks of NiO6 octahedra alternate in a "1313" sequence. Crystals of this new polymorph are described in space group Cmmm, although we note evidence for a competing Imam variant. Transport measurements at ambient pressure reveal metallic character with evidence of a charge density wave transition with an onset at T ≈ 134 K. The discovery of such polymorphism could reverberate to the expansive range of science and applications that rely on RP materials, particularly the recently reported signatures of superconductivity in bilayer La3Ni2O7 with Tc as high as 80 K above 14 GPa.

Discovery of charge density wave in a kagome lattice antiferromagnet

A hallmark of strongly correlated quantum materials is the rich phase diagram resulting from competing and intertwined phases with nearly degenerate ground-state energies^1,2. A well-known example is the copper oxides, in which a charge density wave (CDW) is ordered well above and strongly coupled to the magnetic order to form spin-charge-separated stripes that compete with superconductivity^1,2. Recently, such rich phase diagrams have also been shown in correlated topological materials. In 2D kagome lattice metals consisting of corner-sharing triangles, the geometry of the lattice can produce flat bands with localized electrons^3,4, non-trivial topology^5-7, chiral magnetic order^8,9, superconductivity and CDW order^10-15. Although CDW has been found in weakly electron-correlated non-magnetic AV₃Sb₅ (A = K, Rb, Cs)^10-15, it has not yet been observed in correlated magnetic-ordered kagome lattice metals^4,16-21. Here we report the discovery of CDW in the antiferromagnetic (AFM) ordered phase of kagome lattice FeGe (refs. ^16-19). The CDW in FeGe occurs at wavevectors identical to that of AV₃Sb₅ (refs. ^10-15), enhances the AFM ordered moment and induces an emergent anomalous Hall effect^22,23. Our findings suggest that CDW in FeGe arises from the combination of electron-correlations-driven AFM order and van Hove singularities (vHSs)-driven instability possibly associated with a chiral flux phase^24-28, in stark contrast to strongly correlated copper oxides^1,2 and nickelates^29-31, in which the CDW precedes or accompanies the magnetic order.

Review on quasi-2D square planar nickelates

Quasi-2D square planar nickelates exhibit key ingredients of high-T_c superconducting cuprates. Whether bulk samples are superconducting remains an open question, single crystals are ideal platforms for addressing such fundamental questions.