Trilayer nickelates, which exhibit a high degree of orbital polarization combined with an electron count (d 8.67 ) corresponding to overdoped cuprates, have been identified as a promising candidate platform for achieving high-Tc superconductivity. One such material, La4Ni3O8, undergoes a semiconductor-insulator transition at ~105 K, which was recently shown to arise from the formation of charge stripes. However, an outstanding issue has been the origin of an anomaly in the magnetic susceptibility at the transition and whether it signifies formation of spin stripes akin to single layer nickelates. Here we report single crystal neutron diffraction measurements (both polarized and unpolarized) that establish that the ground state is indeed magnetic. The ordering is modeled as antiferromagnetic spin stripes that are commensurate with the charge stripes, the magnetic ordering occurring in individual trilayers that are essentially uncorrelated along the crystallographic c-axis. Comparison of the charge and spin stripe order parameters reveals that, in contrast to single-layer nickelates such as La2-xSrxNiO4 as well as related quasi-2D oxides including manganites, cobaltates, and cuprates, these orders uniquely appear simultaneously, thus demonstrating a stronger coupling between spin and charge than in these related low-dimensional correlated oxides. Main text:There has been intense interest in stripe phases due to the interplay of charge, spin and lattice degrees of freedom as well as their relevance to high-temperature superconductivity in cuprates [1][2][3][4][5][6][7][8][9]. Uncovering cuprate-like superconductivity in oxides containing transition metals other than copper remains a daunting challenge [10], and in this regard R4Ni3O8 (R=La, Pr, or Nd) compounds have emerged as potential candidates [11][12][13]. These layered materials possess structures that resemble the n=3 Ruddlesden-Popper phase (Rn+1NinO3n+1) [14], but they differ in that all apical oxygens are absent, resulting in trilayers of NiO2 planes in which all Ni ions possess square-planar coordination of oxygen anions. The electron count (3d 8.67 ) coincides with the over-doped regime of cuprates [12,15]. Recent work indicates that these nickelates possess a low-spin state of Ni, large orbital polarization of the eg states with predominantly 2 − 2 orbital character near the Fermi energy, and significant O 2p-Ni 3d hybridization, all of which are considered to be important ingredients for superconductivity in the high-Tc cuprates [12]. Thus R4Ni3O8 compounds (particularly Pr4Ni3O8 which is metallic in its ground state [12]) are more similar to the superconducting cuprates than previously studied nickelates with octahedral coordination, such as La2-xSrxNiO4 (LSNO) [16][17][18] and LaNiO3-based heterostructures [19].Unlike metallic Pr4Ni3O8, La4Ni3O8 undergoes a semiconductor-insulator transition upon cooling through ∼105 K [11,13,[20][21][22][23][24][25][26][27][28], and we have recently shown that the insulating state is characterized by the formation ...
Epitaxial SrRuO3 and CaRuO3 films were grown under an excess flux of elemental ruthenium in an adsorption-controlled regime by molecular-beam epitaxy (MBE), where the excess volatile RuOx (x = 2 or 3) desorbs from the growth front leaving behind a single-phase film. By growing in this regime, we were able to achieve SrRuO3 and CaRuO3 films with residual resistivity ratios (ρ300 K/ρ4 K) of 76 and 75, respectively. A combined phase stability diagram based on the thermodynamics of MBE (TOMBE) growth, termed a TOMBE diagram, is employed to provide improved guidance for the growth of complex materials by MBE.
Combining multiple fast image acquisitions to mitigate scan noise and drift artifacts has proven essential for picometer precision, quantitative analysis of atomic resolution scanning transmission electron microscopy (STEM) data. For very low signal-to-noise ratio (SNR) image stacks - frequently required for undistorted imaging at liquid nitrogen temperatures - image registration is particularly delicate, and standard approaches may either fail, or produce subtly specious reconstructed lattice images. We present an approach which effectively registers and averages image stacks which are challenging due to their low-SNR and propensity for unit cell misalignments. Registering all possible image pairs in a multi-image stack leads to significant information surplus. In combination with a simple physical picture of stage drift, this enables identification of incorrect image registrations, and determination of the optimal image shifts from the complete set of relative shifts. We demonstrate the effectiveness of our approach on experimental, cryogenic STEM datasets, highlighting subtle artifacts endemic to low-SNR lattice images and how they can be avoided. High-SNR average images with information transfer out to 0.72 Å are achieved at 300 kV and with the sample cooled to near liquid nitrogen temperature.
Superconductivity is among the most fascinating and well-studied quantum states of matter. Despite over 100 years of research, a detailed understanding of how features of the normal-state electronic structure determine superconducting properties has remained elusive. For instance, the ability to deterministically enhance the superconducting transition temperature by design, rather than by serendipity, has been a long sought-after goal in condensed matter physics and materials science, but achieving this objective may require new tools, techniques and approaches. Here, we report the transmutation of a normal metal into a superconductor through the application of epitaxial strain. We demonstrate that synthesizing RuO2 thin films on (110)-oriented TiO2 substrates enhances the density of states near the Fermi level, which stabilizes superconductivity under strain, and suggests that a promising strategy to create new transition-metal superconductors is to apply judiciously chosen anisotropic strains that redistribute carriers within the low-energy manifold of d orbitals.
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