SIMES Publications

Abstract

Although charge density waves (CDWs) are omnipresent in cuprate high-temperature superconductors, they occur at significantly different wave vectors, confounding efforts to understand their formation mechanism. Here, we use resonant inelastic x-ray scattering to investigate the doping- and temperature-dependent CDW evolution in La_2−xBa_xCuO₄ (x=0.115–0.155). We discover that the CDW develops in two stages with decreasing temperature. A precursor CDW with a quasicommensurate wave vector emerges first at high temperature. This doping-independent precursor CDW correlation originates from the CDW phase mode coupled with a phonon and “seeds” the low-temperature CDW with a strongly doping-dependent wave vector. Our observation reveals the precursor CDW and its phase mode as the building blocks of the highly intertwined electronic ground state in the cuprates.

Abstract

The progress in integration of nanodiamond with photonic devices is analyzed in the light of quantum optical applications. Nanodiamonds host a variety of optically active defects, called color centers, which provide rich ground for photonic engineering. Theoretical introduction describing light and matter interaction between optical modes and a quantum emitter is presented, including the role of the Debye–Waller factor typical of color center emission. The synthesis of diamond nanoparticles is discussed in an overview of methods leading to experimentally realized hybrid platforms of nanodiamond with gallium phosphide, silicon dioxide, and silicon carbide. The trade-offs in the substrate index of refraction values are reviewed in the context of the achieved strength of light and matter interaction. Thereby, the recent results on the growth of color center-rich nanodiamond on prefabricated silicon carbide microdisk resonators are presented. These hybrid devices achieve up to fivefold enhancement of the diamond color-center light emission and can be employed in integrated quantum photonics.

Abstract

High-temperature copper oxide superconductors consist of stacked CuO₂ planes, with electronic band structures and magnetic excitations that are primarily two-dimensional^1,2, but with superconducting coherence that is three-dimensional. This dichotomy highlights the importance of out-of-plane charge dynamics, which has been found to be incoherent in the normal state^3,4 within the limited range of momenta accessible by optics. Here we use resonant inelastic X-ray scattering t High-temperature copper oxide superconductors consist of stacked CuO₂ planes, with electronic band structures and magnetic excitations that are primarily two-dimensional^1,2, but with superconducting coherence that is three-dimensional. This dichotomy highlights the importance of out-of-plane charge dynamics, which has been found to be incoherent in the normal state^3,4 within the limited range of momenta accessible by optics. Here we use resonant inelastic X-ray scattering to explore the charge dynamics across all three dimensions of the Brillouin zone. Polarization analysis of recently discovered collective excitations (modes) in electron-doped copper oxides^5,6,7 reveals their charge origin, that is, without mixing with magnetic components^5,6,7. The excitations disperse along both the in-plane and out-of-plane directions, revealing its three-dimensional nature. The periodicity of the out-of-plane dispersion corresponds to the distance between neighbouring CuO₂ planes rather than to the crystallographic c-axis lattice constant, suggesting that the interplane Coulomb interaction is responsible for the coherent out-of-plane charge dynamics. The observed properties are hallmarks of the long-sought ‘acoustic plasmon’, which is a branch of distinct charge collective modes predicted for layered systems^8,9,10,11,12 and argued to play a substantial part in mediating high-temperature superconductivity^10,11,12.

Abstract

Using Cu-L₃ edge resonant inelastic x-ray scattering (RIXS) we measured the dispersion and damping of spin excitations (magnons and paramagnons) in the high-T_c superconductor (Bi,Pb)₂(Sr,La)₂CuO_6+δ (Bi2201), for a large doping range across the phase diagram (0.03≲p≲0.21). Selected measurements with full polarization analysis unambiguously demonstrate the spin-flip character of these excitations, even in the overdoped sample. We find that the undamped frequencies increase slightly with doping for all accessible momenta, while the damping grows rapidly, faster in the (0,0)→(0.5,0.5) nodal direction than in the (0,0)→(0.5,0) antinodal direction. We compare the experimental results to numerically exact determinant quantum Monte Carlo (DQMC) calculations that provide the spin dynamical structure factor S(Q,ω) of the three-band Hubbard model. The theory reproduces well the momentum and doping dependence of the dispersions and spectral weights of magnetic excitations. These results provide compelling evidence that paramagnons, although increasingly damped, persist across the superconducting dome of the cuprate phase diagram; this implies that long-range antiferromagnetic correlations are quickly washed away, while short-range magnetic interactions are little affected by doping.

Abstract

Mechanical stimuli can modify the energy landscape of chemical reactions and enable reaction pathways, offering a synthetic strategy that complements conventional chemistry^1,2,3. These mechanochemical mechanisms have been studied extensively in one-dimensional polymers under tensile stress^4,5,6,7,8,9 using ring-opening¹⁰ and reorganization¹¹, polymer unzipping^6,12 and disulfide reduction^13,14 as model reactions. In these systems, the pulling force stretches chemical bonds, initiating the reaction. Additionally, it has been shown that forces orthogonal to the chemical bonds can alter the rate of bond dissociation¹⁵. However, these bond activation mechanisms have not been possible under isotropic, compressive stress (that is, hydrostatic pressure). Here we show that mechanochemistry through isotropic compression is possible by molecularly engineering structures that can translate macroscopic isotropic stress into molecular-level anisotropic strain. We engineer molecules with mechanically heterogeneous components—a compressible (‘soft’) mechanophore and incompressible (‘hard’) ligands. In these ‘molecular anvils’, isotropic stress leads to relative motions of the rigid ligands, anisotropically deforming the compressible mechanophore and activating bonds. Conversely, rigid ligands in steric contact impede relative motion, blocking reactivity. We combine experiments and computations to demonstrate hydrostatic-pressure-driven redox reactions in metal–organic chalcogenides that incorporate molecular elements that have heterogeneous compressibility^16,17,18,19, in which bending of bond angles or shearing of adjacent chains activates the metal–chalcogen bonds, leading to the formation of the elemental metal. These results reveal an unexplored reaction mechanism and suggest possible strategies for high-specificity mechanosynthesis.