38 39 D I A M O N D L I G H T S O U R C E A N N U A L R E V I E W 2 0 2 2 / 2 3 D I A M O N D L I G H T S O U R C E A N N U A L R E V I E W 2 0 2 2 / 2 3 MagneticMaterials Group Beamline I21 a, The experimental geometry of XAS and RIXS experiment. b, Ni L 3 XAS of the parent NdNiO 2 film. c,d, Integrated quasi-elastic peak intensity as a function of momentum transfer along the (±H, 0) direction by excitation at the Ni 3d resonance (A peak in b). e, The resonant energy profile of the charge-order at (+0.333, 0). f, Integrated quasi-elastic peaks as a function of momentum transfer along the (H, 0) direction by probing the Ni 3d–Nd 5d hybridized state (A’ peak in b) and Nd 4f states at the Nd M 5 edge. g,h, RIXS maps on layered nickelate at 20 K at H =−0.35 r.l.u. with σ (g) or π incident X-ray polarisation (h). Charge-ordered phase in layered nickelates Quantum Materials – Hard Condensed Matter – Electronic Properties – Physics – Materials Science – Superconductors Unconventional high-temperature superconductors are at the core of quantum materials and advanced technological applications. Scientists have been trying to synthesise new superconductors with ever-higher superconducting critical temperatures and to unlock the mechanism underlying the zero-resistivity behaviour. Recently a new family of superconductors with layered nickelate-oxide structure was successfully obtained. The latest resonant X-ray spectroscopic studies reveal them to be very close cousins to copper-oxide superconductors (cuprates) and highlight the importance of the strong electronic Coulomb interaction, the quasi-two-dimensional electronic structure of theMO 2 (M=Cu,Ni) buildingblock, andthestrongantiferromagnetic magnetic excitations. All these suggest we may have found the right recipe for producing a high-temperature superconductor. However, significant challenges are to be urgently solved. For instance, so far, superconductivity only exists in thin films of layered nickelates, and less than a dozen research groups could successfully synthesise them, prompting a rather unstable state of the material. Moreover, scientists are still striving to understand why their superconducting critical temperature is about an order of magnitude lower than that of cuprates (T c ~ 130 K ). In cuprates, doped electron or hole charges, besides forming Cooper pairs in the superconducting state, may segregate into regions with periodically variable density , i.e. forming charge-ordered phase. So, naturally, we may wonder whether such charge-ordered phases exist in their close cousin, layered nickelates. An international team of researchers addressed this question using Diamond’s I21-RIXS beamline. Resonant Inelastic X-ray Scattering (RIXS) is one of the few techniques capable of probing the charge-ordered state. In particular, owing to the sub-100 nm probing depth, RIXS is particularly suited for the nm-thick nickelate films. Moreover, as I21 is equipped with high energy resolution and high photon flux, it is an ideal facility for this type of study. The research team found the charge-ordered states exist in the parent layered nickelate NdNiO 2. They show strong resonances for both the Ni 3 d and Nd 5 d states, illustrating a coupled electronic structure between the two. The charge-ordered state also shows a clear temperature dependence, a hallmark of an ordered state with an electronic origin. The continuous tunability of the RIXS spectrometer was then used to show that the charge-ordered state has a non-negligible L -dependence, hinting that it is a three-dimensional object. Despite the differences to cuprates, the existence of the charge-ordered state demonstrates that the layered nickelates are remarkably similar to cuprate superconductors. It is clear that the combination of the MO 2 (M=Cu, Ni) building block, an active dx 2 -y 2 orbital near Fermi level, and spin S=1/2 antiferromagnetic (AFM) correlation, are crucial for achieving unconventional superconductivity. Understanding the common collective excitations of the AFM correlation and the charge-order may provide essential knowledge to eventually unfold the superconducting pairing mechanism. Many materials in condensedmatter physics exhibit remarkable properties such as zero-resistivity, colossal magneto-resistance and the magneto-optical Kerr effect. These novel properties lie in the competition of electronic and magnetic interactionsunder theangstromandnanometre scale. Aspectroscopy technique such as RIXS is imperative to understand and eventuallymake newer materials with richer functionality, due to its remarkable sensitivity. Related publication: Tam, CC. et al. Charge density waves in infinite-layer NdNiO2 nickelates. Nature Materials 21 , 1116–1120 (2022). DOI: 10.1038/s41563-022-01330-1 Funding acknowledgement: Diamond Light Source and the University of Bristol under joint doctoral studentship no. STU0372. NSFC (grant nos. 11774044, 52072059 and 11822411) SPRP-B of CAS (grant no. XDB25000000) NSF of Beijing (grant no. JQ19002) Corresponding authors: Ke-Jin Zhou, Diamond Light Source
[email protected] Ferroelectric topological structures Quantum Materials – Hard Condensed Matter – Electronic Properties – Physics – Materials Science – Multiferroics Although ferromagnetism and ferroelectricity are similar effects, they were thought to be fundamentally different. However, theoreticians suggested that we might see that the two phenomena are surprisingly similar if we could get a close enough look. In ferromagnetism, the Dzyaloshinskii–Moriya interaction (DMi) gives rise to effects such as skyrmions, which are potentially very useful for next-generation electronic devices (spintronics). If an analogous mechanism were present in ferroelectric materials, it would offer intriguing possibilities for future applications. Ferromagnets and ferroelectrics are both ferroic systems characterised by an ordering parameter (magnetisation and polarisation, respectively), aggregating at macroscopic levels into domains that can be switched by applying an external respective field. Under particular conditions, the ferromagnetic order parameter can assemble into complex topological patterns that are currently the subject of intense research as they hold substantial promise for improved devices. According to textbooks, such complex structures should never occur in ferroelectrics. Nevertheless, the dielectric polarisation in a carefully grown single crystal ultrathin film heterostructure consisting of a ferroelectric layer of lead titanate (PbTiO 3 ) sandwiched between strontium ruthenate (SrRuO 3 ) layers assembles into a two-dimensional array of electric dipole vortices. A team of researchers from the University of Warwick used cutting-edge imaging techniques to carefully analyse this pattern. They determined that the polarisation of the PbTiO 3 layer is also ordered along the third dimension, yielding a corkscrew-like pattern that is either helical or cycloidal and modulated in two orthogonal directions. Such a spacially ordered polarisation has not previously been observed in ferroelectric materials. The data support a model that proposes these complex polarisation patterns are driven by a mechanism similar to the DMi in magnetic materials. To fully understand the coupling mechanisms, the team needed to study both the intra- and inter-layer structures across the atomic and meso (tens of nanometer) length scales. To ensure generality in the results, non-destructive and spatially averaged data such as that gleaned from X-ray diffraction was required. They used Diamond’s I16 beamline to perform high-resolution X-ray diffraction. The high resolution and high flux coupled with area detectors enabled them to study periodicities in reciprocal space with necessarily high resolution. The X-ray data showed periodicities in two orthogonal directions that are aligned with the orthorhombic crystal symmetries. The X-ray data are in complete agreement with the more myopic cross-sectional Transmission Electron Microscopy (TEM) and show that the observed topology indeed extends throughout the crystal. In the studied sample, the topologies arise from small tilts of the atomic positions within the crystal, which induce both a small electrical polarisation and strain. The observed ferroic topologies arise as the system attempts to minimise the internal energy. The observation of an electrical equivalent of the DMi shows that complex topologies of the polarisation are now possible. By changing the material properties, this interaction strength can be tweaked, driving new, ever more complex structures that can be stabilised. The plethora of technologies based on ferromagnetic spin textures shows what may be possible in these electrical equivalents. Related publication: Rusu, D. et al. Ferroelectric incommensurate spin crystals. Nature 602 , 240–244 (2022). DOI 10.1038/s41586-021-04260-1 Funding acknowledgement: EPSRC (UK) through grants no. EP/P031544/1 and EP/P025803/1 Corresponding authors: Prof Marin Alexe, University of Warwick,
[email protected] Prof Thomas P. Hase, University of Warwick,
[email protected] MagneticMaterials Group Beamline I16 Reciprocal Space Volume reconstructed from a series of 2D images taken during a rocking curve of the (002) substrate Bragg peak in DySrO 3 (left). Projection onto (Qx – Qy) wave vectors, giving a Reciprocal Space Map (RSM) around the same reflection (middle-top). The derived model of the Dielectric polarization pattern in the lead titanate (PTO) showing polarisation curling assembled in periodic vortices and a second modulation consisted of a cycloidal twist of electric dipoles along the axis of the polar vortice s (right). Red colour indicates a downward polarization direction, blue -upwards, and yellow -horizontal.