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Skyrmions emerging from complex order offer superior stability and functionality for data processing and storage, but creating such objects is intrinsically difficult. Homochiral electric and antiferromagnetic structures at the domain walls of a room-temperature multiferroic are now observed.

The universality of the strange metal phase in many quantum materials is often attributed to the presence of a quantum critical point (QCP), a zero-temperature phase transition ruled by quantum fluctuations. In cuprates, where superconductivity hinders direct QCP observation, indirect evidence comes from the identification of fluctuations compatible with the strange metal phase. Here we show that the recently discovered charge density fluctuations (CDF) possess the right properties to be associated to a quantum phase transition. Using resonant x-ray scattering, we studied the CDF in two families of cuprate superconductors across a wide doping range (up to p  = 0.22). At p * ≈ 0.19, the putative QCP, the CDF intensity peaks, and the characteristic energy Δ is minimum, marking a wedge-shaped region in the phase diagram indicative of a quantum critical behavior, albeit with anomalies. These findings strengthen the role of charge order in explaining strange metal phenomenology and provide insights into high-temperature superconductivity. It has been suggested that the strange metal phase in cuprates stems from a quantum critical point slightly above optimal doping. By resonant x-ray scattering in two cuprate families in a wide doping range, Arpaia et al. show that charge density fluctuations could be associated with this quantum critical point.

Cooperative effects determine the spin-state bistability of spin-crossover molecules (SCMs). Herein, the ultimate scale limit at which cooperative spin switching becomes effective is investigated in a complex [Fe(H 2 B(pz) 2 ) 2 (bipy)] deposited on a highly oriented pyrolytic graphite surface, using x-ray absorption spectroscopy. This system exhibits a complete thermal- and light-induced spin transition at thicknesses ranging from submonolayers to multilayers. On increasing the coverage from 0.35(4) to 10(1) monolayers, the width of the temperature-induced spin transition curve narrows significantly, evidencing the buildup of cooperative effects. While the molecules at the submonolayers exhibit an apparent anticooperative behavior, the multilayers starting from a double-layer exhibit a distinctly cooperative spin switching, with a free-molecule-like behavior indicated at around a monolayer. These observations will serve as useful guidelines in designing SCM-based devices. Spin-crossover molecules offer a potential route towards molecular spintronics, but retaining the bistability of the spin state upon surface deposition is challenging. Here, the authors study the spin-crossover behaviours of an Fe(II) complex deposited on graphite, determining the scale limit at which cooperative spin switching becomes effective.

Charge order is now accepted as an integral constituent of cuprate high-temperature superconductors, one that is intimately related to other electronic instabilities including antiferromagnetism and superconductivity1–11. Unlike conventional Peierls density waves, the charge correlations in cuprates have been predicted to display a rich momentum space topology depending on the underlying fermiology12–18. However, charge order has only been observed along the high-symmetry Cu–O bond directions. Here, using resonant soft X-ray scattering, we investigate the evolution of the full momentum space topology of charge correlations in T′-(Nd,Pr)2CuO4 as a function of electron doping. We report that, when the parent Mott insulator is doped, charge correlations first emerge with full rotational symmetry in momentum space, indicating glassy charge density modulation in real space possibly seeded by local defects. At higher doping levels, the orientation of charge correlations is locked to the Cu–O bond directions, restoring a more conventional long-ranged bidirectional charge order. Through charge susceptibility calculations, we reproduce the evolution in topology of charge correlations across the antiferromagnetic phase boundary and propose a revised phase diagram of T′-Ln2CuO4 with a superconducting region extending toward the Mott limit.A new form of charge ordering is observed in a cuprate superconductor. At low doping, a fully rotationally symmetric ordering appears before becoming locked to the Cu–O bond directions at high doping. The link between charge correlations and fermiology give a perspective on the phase diagram.

Controlling the correlations and electronic reconstruction at the interface of transition metal oxide heterostructures provides a new pathway for tuning their unique physical properties. Here, we investigate the effects of interfacial nonstoichiometry and vertical phase separation on the magnetic properties and proximity-induced magnetism of epitaxial La 0.7 Sr 0.3 MnO 3 (LSMO)/SrTiO 3 (001) oxide heterostructures. We also reinvestigate the recently observed inverse hysteresis behavior reported for this system, which we find emanates from the remanent field of the superconducting solenoid and not from antiferromagnetic intra-layer exchange coupling in low coercivity LSMO thin films. Combined atomically resolved electron energy loss spectroscopy, element-specific X-ray magnetic circular dichroism, and interface-sensitive polarized soft X-ray resonant magnetic reflectivity show the formation of a Mn 3+ -enriched interfacial LSMO layer, of a Ti 3+ -derived magnetic interface layer coupled ferromagnetically to La 0.7 Sr 0.3 MnO 3 , together with a small density of O-vacancies at the interface. These results not only advance the understanding of the magnetism and spin structure of correlated oxide interfaces but also hold promise for practical applications, especially in devices where the performance relies on the control and influence of spin polarization currents by the interfacial spin structure. Controlling stoichiometry and electronic reconstruction at the interface of transition metal oxide heterostructures offers a new way to tune their unique properties, leading to behaviors distinct from the bulk components. Using atomically resolved scanning transmission electron microscopy, element specific x-ray resonant magnetic reflectivity and x-ray magnetic circular dichroism, we have demonstrated that the presence of local oxygen deficiency across the interface and charge transfer to the empty conduction band of SrTiO 3 at the interface are the primary drivers for the modified interfacial magnetism in the manganite thin films and Ti 3+ induced ferromagnetism at the La 0.7 Sr 0.3 MnO 3 /SrTiO 3 interface.

An amendment to this paper has been published and can be accessed via a link at the top of the paper.

Magnetic ground states with peculiar spin textures, such as magnetic skyrmions and multifunctional domains are of enormous interest for the fundamental physics governing their origin as well as potential applications in emerging technologies. Of particular interest are multiferroics, where sophisticated interactions between electric and magnetic phenomena can be used to tailor several functionalities. We report the direct observation of a magnetic field induced long-wavelength spin spiral modulation in the chiral compound Ba3TaFe3Si2O14, which emerges out of a helical ground state, and is hallmarked by the onset of a unique chirality-dependent contribution to the bulk electric polarization. The periodicity of the field-induced modulation, several hundreds of nm depending on the field value, is comparable to the length scales of mesoscopic topological defects such as skyrmions, merons, and solitons. The phase transition and observed threshold behavior are consistent with a phenomenology based on the allowed Lifshitz invariants for the chiral symmetry of langasite, which intriguingly contain all the essential ingredients for the realization of topologically stable antiferromagnetic skyrmions. Our findings open up new directions to explore topological correlations of antiferromagnetic spintronic systems based on non-collinear magnetic systems with additional ferroic functionalities.

Besides superconductivity, copper-oxide high-temperature superconductors are susceptible to other types of ordering. We used scanning tunneling microscopy and resonant elastic x-ray scattering measurements to establish the formation of charge ordering in the high-temperature superconductor Bi2Sr2CaCu2O8+x. Depending on the hole concentration, the charge ordering in this system occurs with the same period as those found in Y-based or La-based cuprates and displays the analogous competition with superconductivity. These results indicate the similarity of charge organization competing with superconductivity across different families of cuprates. We observed this charge ordering to leave a distinct electron-hole asymmetric signature (and a broad resonance centered at +20 milli–electron volts) in spectroscopic measurements, indicating that it is likely related to the organization of holes in a doped Mott insulator.

Controlling the correlations and electronic reconstruction at the interface of transition metal oxide heterostructures provides a new pathway for tuning their unique physical properties. Here, we investigate the effects of interfacial nonstoichiometry and vertical phase separation on the magnetic properties and proximity-induced magnetism of epitaxial La Sr MnO (LSMO)/SrTiO (001) oxide heterostructures. We also reinvestigate the recently observed inverse hysteresis behavior reported for this system, which we find emanates from the remanent field of the superconducting solenoid and not from antiferromagnetic intra-layer exchange coupling in low coercivity LSMO thin films. Combined atomically resolved electron energy loss spectroscopy, element-specific X-ray magnetic circular dichroism, and interface-sensitive polarized soft X-ray resonant magnetic reflectivity show the formation of a Mn -enriched interfacial LSMO layer, of a Ti -derived magnetic interface layer coupled ferromagnetically to La Sr MnO , together with a small density of O-vacancies at the interface. These results not only advance the understanding of the magnetism and spin structure of correlated oxide interfaces but also hold promise for practical applications, especially in devices where the performance relies on the control and influence of spin polarization currents by the interfacial spin structure.

The quasi-two-dimensional van der Waals magnet Fe\\(_{5-\\delta}\\)GeTe\\(_2\\) has emerged as a promising platform for electronic and spintronic functionalities at room temperature, owing to its large ferromagnetic ordering temperature \\(T_{\\text{C}}\\) \\(\\sim\\) 315 K. Interestingly, by cobalt (Co) substitution of iron in F5GT, \\(i.e.\\) \\(({\\text{Fe}}_{1-x}{\\text{Co}}_x)_{5-\\delta}{\\text{GeTe}}_2\\) (Co-F5GT), not only can its magnetic transition temperature be further enhanced, but the magnetic and structural ground states can also be tuned. Specifically, an antiferromagnetic (AFM) order is induced beyond the Co doping level \\(x \\ge 0.4\\). Here, we investigate the magnetic properties of a Co-F5GT single crystal at \\(x = 0.45(1)\\), by utilizing the element specific, resonant magnetic x-ray scattering technique. Our study reveals an A-type, Ising-like AFM ground state, with a transition temperature \\(T_{\\text{N}}\\) \\(\\sim\\) 340 K. In addition, our work unveils an important contribution from Co magnetic moments to the magnetic order. The application of the in-plane magnetic fields gradually polarize the spin moments along the field direction, but without inducing incommensurate spin texture(s).