Although line defects such as grain boundaries (GBs) and wrinkles are unavoidable in graphene, difficulties in identification preclude studying their impact on electronic and mechanical properties. As previous methods focus on a single type of line defect, simultaneous measurements of both GBs and wrinkles with detailed structural information have not been reported. Here, we introduce effective visualization of both line defects by controlled gold deposition. Upon depositing gold on graphene, single lines and double lines of gold nanoparticles (NPs) are formed along GBs and wrinkles, respectively. Moreover, it is possible to analyze whether a GB is stitched or overlapped, whether a wrinkle is standing or folded, and the width of the standing collapsed wrinkle. Theoretical calculations show that the characteristic morphology of gold NPs is due to distinct binding energies of line defects, which are correlated to disrupting diffusion of NPs. Our approach could be further exploited to investigate the defect structures of other two-dimensional materials.

A subtle balance between competing interactions in iron-based superconductors (FeSCs) can be tipped by additional interfacial interactions in a heterostructure, often inducing exotic phases with unprecedented properties. Particularly when the proximity-coupled layer is magnetically active, rich phase diagrams are expected in FeSCs, but this has not been explored yet. Here, using high-accuracy 75As and 51V nuclear magnetic resonance measurements, we investigate an electronic phase that emerges in the FeAs layer below T 0 ~ 155 K of Sr2VO3FeAs, a naturally assembled heterostructure of an FeSC and a Mott-insulating vanadium oxide. We find that frustration of the otherwise dominant Fe stripe and V Neel fluctuations via interfacial coupling induces a charge/orbital order in the FeAs layers, without either static magnetism or broken C 4 symmetry, while suppressing the Neel antiferromagnetism in the SrVO3 layers. These findings demonstrate that the magnetic proximity coupling stabilizes a hidden order in FeSCs, which may also apply to other strongly correlated heterostructures.

Strain control is one of the most promising avenues to search for new emergent phenomena in transition-metal-oxide films. Here, we investigate the strain-induced changes of electronic structures in strongly correlated LaNiO3 (LNO) films, using angle-resolved photoemission spectroscopy and the dynamical mean-field theory. The strongly renormalized eg-orbital bands are systematically rearranged by misfit strain to change its fermiology. As tensile strain increases, the hole pocket centered at the A point elongates along the kz-axis and seems to become open, thus changing Fermi-surface (FS) topology from three- to quasi-two-dimensional. Concomitantly, the FS shape becomes flattened to enhance FS nesting. A FS superstructure with Q1 = (1/2,1/2,1/2) appears in all LNO films, while a tensile-strained LNO film has an additional Q2 = (1/4,1/4,1/4) modulation, indicating that some instabilities are present in metallic LNO films. Charge disproportionation and spin-density-wave fluctuations observed in other nickelates might be their most probable origins.

We investigated electronic-structure changes of tensile-strained ultrathin LNO films from 10 to 1 unit cells (UC) using angle-resolved photoemission spectroscopy (ARPES). We found that there is a critical thickness t c between 4 and 3 UC below which Ni e g electrons are confined in two-dimensional space. Furthermore, the Fermi surfaces (FSs) of LNO films below t c consist of two orthogonal pairs of one-dimensional (1D) straight parallel lines. Such a feature is not accidental as observed in constant-energy surfaces at all binding energies, which is not explained by first-principle calculations or the dynamical mean-field theory. The ARPES spectra also show anomalous spectral behaviors, such as no quasiparticle peak at Fermi momentum but fast band dispersion comparable to bare-band one, which is typical in a 1D system. As its possible origin, we propose 1D FS nesting, which also accounts for FS superstructures observed in ARPES.

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