The search for oxide materials with physical properties similar to the cuprate high Tc superconductors, but based on alternative transition metals such as nickel, has grown and evolved over time [1][2][3][4][5][6][7][8][9][10]. The recent discovery of superconductivity in doped infinite-layer nickelates RNiO2 (R = rare-earth element) [11,12] further strengthens these efforts. With a crystal structure similar to the infinite-layer cupratestransition metal oxide layers separated by a rare-earth spacer layerformal valence counting suggests that these materials have monovalent Ni 1+ cations with the same 3d electron count as Cu 2+ in the cuprates. Here, we use x-ray spectroscopy in concert with density functional theory to show that the electronic structure of RNiO2 (R = La, Nd), while similar to the cuprates, includes significant distinctions. Unlike cuprates with insulating spacer layers between the CuO2 planes, the rare-earth spacer layer in the infinite-layer nickelate supports a weaklyinteracting three-dimensional 5d metallic state. This three-dimensional metallic state hybridizes with a quasi-two-dimensional, strongly correlated state with 3dx 2 -y 2 symmetry in the NiO2 layers. Thus, the infinite-layer nickelate can be regarded as a sibling of the rare earth intermetallics [13-15], well-known for heavy Fermion behavior, where the NiO2 correlated layers play an analogous role to the 4f states in rare-earth heavy Fermion compounds. This unique Kondo-or Anderson-lattice-like "oxide-intermetallic" replaces the Mott insulator as the reference state from which superconductivity emerges upon doping.While the mechanism of superconductivity in the cuprates remains a subject of intense research, early on it was suggested that the conditions required for realizing high Tc superconductivity are rooted in the physics of a two-dimensional electron system subject to strong local repulsion [16,17]. This describes the Mott (charge-transfer) insulators in the stoichiometric parent compounds, characterized by spin ½ Heisenberg antiferromagnetism, from which superconductivity emerges upon doping. A long-standing question regards whether these "cuprate-Mott" conditions can be realized in other oxides; and extensive efforts to synthesize and engineer nickel oxides (nickelates) have promised such a realization [1-10]. Infinite-layer NdNiO2 became the first such nickelate superconductor following the recent discovery of superconductivity in Srdoped samples [11]. The undoped parent compound, produced by removing the apical oxygen atoms from the perovskite nickelate NdNiO3 using a metal hydride-based soft chemistry reduction process [10,[18][19][20], appears to be a close sibling of the cuprates-it is isostructural to the infinitelayer cuprates with monovalent Ni 1+ cations and possesses the same 3d 9 electron count as that of Cu 2+ cations in undoped cuprates. Yet, as we will reveal, the electronic structure of the undoped RNiO2 (R = La and Nd) remains distinct from the Mott, or charge-transfer, compounds of undoped cuprates, and even...

The stability of the Li-electrolyte interface is critical to the practical applications of Li metal anodes. Correspondingly, we developed a high-quality nanodiamond protection layer to reinforce the native solid-electrolyte interphase on Li metal. A double-layer film design was proposed to enhance the defect tolerance of the artificial interface, improving the macroscopic uniformity of the Li-ion flux; the exceptional mechanical property of a modulus of >200 GPa can be realized, which effectively arrested dendrite propagation, resulting in controlled Li deposition and significantly improved cycling efficiency.

The discovery of superconductivity in infinite-layer nickelates brings us tantalizingly close to a material class that mirrors the cuprate superconductors. We measured the magnetic excitations in these nickelates using resonant inelastic x-ray scattering at the Ni L3-edge. Undoped NdNiO2 possesses a branch of dispersive excitations with a bandwidth of approximately 200 milli–electron volts, which is reminiscent of the spin wave of strongly coupled, antiferromagnetically aligned spins on a square lattice. The substantial damping of these modes indicates the importance of coupling to rare-earth itinerant electrons. Upon doping, the spectral weight and energy decrease slightly, whereas the modes become overdamped. Our results highlight the role of Mottness in infinite-layer nickelates.

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Copper oxide high-T C superconductors possess a number of exotic orders that coexist with or are proximal to superconductivity. Quantum fluctuations associated with these orders may account for the unusual characteristics of the normal state, and possibly affect the superconductivity 1-4 . Yet, spectroscopic evidence for such quantum fluctuations remains elusive. Here, we use resonant inelastic X-ray scattering to reveal spectroscopic evidence of fluctuations associated with a charge order 5-14 in nearly optimally doped Bi 2 Sr 2 CaCu 2 O 8+δ . In the superconducting state, while the quasielastic charge order signal decreases with temperature, the interplay between charge order fluctuations and bond-stretching phonons in the form of a Fano-like interference increases, an observation that is incompatible with expectations for competing orders. Invoking general principles, we argue that this behaviour reflects the properties of a dissipative system near an orderdisorder quantum critical point, where the dissipation varies with the opening of the pseudogap and superconducting gap at low temperatures, leading to the proliferation of quantum critical fluctuations, which melt charge order.Charge order (CO), which is ubiquitous in hole-doped cuprates [5][6][7][8][9][10][11][12][13][14] , is accompanied by a negligible lattice deformation (approximately 0.1 pm, ref. 15 ); however, signatures of valence electron density modulations due to CO can be detected by resonant inelastic X-ray scattering (RIXS) at the Cu L edge. RIXS resolves both the quasistatic and dynamical inelastic signals 8,16 , as highlighted in Fig. 1a, particularly the intensity of the inelastic branch of excitations below 0.1 eV. These excitations possess an energy scale similar to that of bond-stretching phonons, which exhibit anomalous softening and broadening in certain portions of reciprocal space, observed using inelastic neutron scattering and non-resonant inelastic X-ray scattering 17,18 . These behaviours have suggested a coupling with CO 17,18 and possibly some form of charge collective mode 19 . However, while neutron and non-resonant X-ray scattering measure the phonon self-energy (meaning the dynamical structure factor), RIXS largely reflects the electron-phonon coupling itself and its interplay with charge excitations 16,20 . With superb momentum resolution, RIXS at the Cu L edge has already revealed two distinct anomalies associated with CO excitations due to a Fano-like interference effect 16 , as shown in Fig. 1b: (1) an apparent softening of the RIXS phonon at the CO wave-vector (Q CO ), and (2) creation of a 'funnel'-like spectral weight emanating from Q CO with a

Group-IV color centers in diamond have garnered great interest for their potential as optically active solid-state spin qubits. Future utilization of such emitters requires the development of precise site-controlled emitter generation techniques that are compatible with high-quality nanophotonic devices. This task is more challenging for color centers with large group-IV impurity atoms, which are otherwise promising because of their predicted long spin coherence times without a dilution refrigerator. For example, when applied to the negatively charged tin-vacancy (SnV − ) center, conventional 1 arXiv:1910.14165v1 [cond-mat.mes-hall] 30 Oct 2019 site-controlled color center generation methods either damage the diamond surface or yield bulk spectra with unexplained features. Here we demonstrate a novel method to generate site-controlled SnV − centers with clean bulk spectra. We shallowly implant Sn ions through a thin implantation mask and subsequently grow a layer of diamond via chemical vapor deposition. This method can be extended to other color centers and integrated with quantum nanophotonic device fabrication.Keywords diamond color centers, tin-vacancy center, CVD growth, ion implantation Group-IV color centers in diamond have emerged as promising candidates for optically active, solid-state spin qubits. 1-4 These color centers are comprised of a split vacancy in the diamond lattice and an interstitial group-IV atom. The inversion symmetry of this structure provides group-IV color centers beneficial properties such as insensitivity to electric field fluctuations to first order and high Debye-Waller factors. 5 These color centers also possess long-lived electron spins that can be harnessed as quantum memories. 6-8 All of these characteristics make group-IV color centers well suited to interface optical photons in nanophotonic platforms for applications in quantum networks.An outstanding challenge in implementing these color centers in scalable applications is their generation. The two most common methods of group-IV color center generation are ion implantation and synthesis. Ion implantation facilitates site-controlled generation of color centers by using either a mask 9,10 or focused ion beam (FIB). 11,12 However, the quality of ion-implanted emitters is often degraded by the large amount of damage introduced during implantation. 4 Synthesis techniques such as high-pressure high-temperature (HPHT) growth and chemical vapor deposition (CVD) growth often yield higher quality, more stable emitters with lower inhomogeneous broadening than ion implantation. [13][14][15][16] Unfortunately, synthesis techniques do not enable site-controlled generation. A better color center generation method is severely lacking.

Integrating solid-state quantum emitters with photonic circuits is essential for realizing large-scale quantum photonic processors. Negatively charged tin-vacancy (SnV − ) centers in diamond have emerged as promising candidates for quantum emitters because of their excellent optical and spin properties, including narrow-linewidth emission and long spin coherence times. SnV − centers need to be incorporated in optical waveguides for efficient onchip routing of the photons they generate. However, such integration has yet to be realized. In this Letter, we demonstrate the coupling of SnV − centers to a nanophotonic waveguide. We realize this device by leveraging our recently developed shallow ion implantation and growth method for the generation of high-quality SnV − centers and the advanced quasi-isotropic diamond fabrication technique. We confirm the compatibility and robustness of these techniques through successful coupling of narrow-linewidth SnV − centers (as narrow as 36 ± 2 MHz) to the diamond waveguide. Furthermore, we investigate the stability of waveguide-coupled SnV − centers under resonant excitation. Our results are an important step toward SnV − -based on-chip spin-photon interfaces, single-photon nonlinearity, and photon-mediated spin interactions.

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