Flexible luminescent metal−organic frameworks (MOFs) sensitive to physical and chemical variation demonstrate potential application in sensing, but are commonly limited to ones incorporating specialized chromophores, so that the design, preparation, and reversible well-defined crystallographic transitions of such materials remain a great challenge. Here, an easily prepared flexible luminescent silver-chalcogenolate clusterbased metal−organic framework (SCC-MOF), Ag 10 bpy, exceptionally distinguishes chloromethanes CH 2 Cl 2 , CHCl 3 , and CCl 4 by different fluorescent responsive signals, which originated from its crystalline dipole moment orientation relative to chromophores. Moreover, "glass-to-fluid" transition of the distinctly responsive species in the crystalline pore is demonstrated to modulate striking chromism with scarce segmented emission among MOFs, resulting from subtle dynamic solvation of chromophores. Retention of single crystallinity enables elucidation of the relationship between distinct fluorescent response and complicated structural variation at the molecular level. This new type of flexible emissive SCC-MOF is potentially useful for the development of versatile sensing and switching fluorescent materials.
The first known magnetic mineral, magnetite, has unusual properties, which have fascinated mankind for centuries; it undergoes the Verwey transition around 120 K with an abrupt change in structure and electrical conductivity. The mechanism of the Verwey transition, however, remains contentious. Here we use resonant inelastic X-ray scattering over a wide temperature range across the Verwey transition to identify and separate out the magnetic excitations derived from nominal Fe2+ and Fe3+ states. Comparison of the experimental results with crystal-field multiplet calculations shows that the spin–orbital dd excitons of the Fe2+ sites arise from a tetragonal Jahn-Teller active polaronic distortion of the Fe2+O6 octahedra. These low-energy excitations, which get weakened for temperatures above 350 K but persist at least up to 550 K, are distinct from optical excitations and are best explained as magnetic polarons.
The magnetic correlations within the cuprates have undergone intense scrutiny as part of efforts to understand high temperature superconductivity. We explore the evolution of the magnetic correlations along the nodal direction of the Brillouin zone in La2−xSrxCuO4, spanning the doping phase diagram from the anti-ferromagnetic Mott insulator at x = 0 to the metallic phase at x = 0.26. Magnetic excitations along this direction are found to be systematically softened and broadened with doping, at a higher rate than the excitations along the anti-nodal direction. This phenomenology is discussed in terms of the nature of the magnetism in the doped cuprates. Survival of the high energy magnetic excitations, even in the overdoped regime, indicates that these excitations are marginal to pairing, while the influence of the low energy excitations remains ambiguous.
We carried out temperature-dependent (20 -550 K) measurements of resonant inelastic X-ray scattering on LaCoO3 to investigate the evolution of its electronic structure across the spin-state crossover. In combination with charge-transfer multiplet calculations, we accurately quantified the renomalized crystal-field excitation energies and spin-state populations. We show that the screening of the on-site Coulomb interaction of 3d electrons is orbital selective and coupled to the spin-state crossover in LaCoO3. The results establish that the gradual spin-state crossover is associated with a relative change of Coulomb energy versus bandwidth, leading to a Mott-type insulator-to-metal transition.PACS numbers: 75.30. Wx, 71.70.Ch, 78.70.En The orbital degree of freedom of an electron characterizes the shape of the electron cloud and its wave function. It plays an essential role in the physics of phase transitions in solids via the coupling of charge, spin and lattice degrees of freedom, even in the presence of strong Coulomb interactions, for example, as in Mott insulators. The spatial redistribution of the electron cloud as a function of an external parameter such as temperature often manifests as co-operative phenomena leading to a metal-insulator transition [1], orbital ordering [2, 3], nematic transition [4,5], spin-state transition [6-10], etc. These results in exotic properties like superconductivity, quantum criticality, colossal magnetoresistance, etc. As the Coulomb interaction is a key to Mott physics [11][12][13][14], one fundamental question in correlated electron systems with orbital degrees of freedom is: how do the Coulomb correlations change dynamically through the rearrangement of the electronic distribution? This is usually beyond the scope of even multi-orbital model Hamiltonians in which the Coulomb interaction parameters are considered inflexible. An important theoretical advance in this direction is the role of orbital selective screening [15]. The effective Coulomb interaction for t 2g electrons was shown to be significantly reduced due to efficient e g electron screening, providing an improved understanding of LaMO 3 (M = 3d transition metals from Ti to Cu) series of perovskite oxides. This concept of the screened on-site Coulomb interaction has been developed recently using the constrained random-phase-approximation technique [16][17][18].In this Letter, we exploited resonant inelastic X-ray scattering (RIXS) to investigate spin-orbital excitations in LaCoO 3 and to measure its spin-state populations and the renormalized crystal-field excitations across the spinstate transition. an ideal candidate to examine the role of orbital selective screening of Coulomb interactions as a function of temperature. We found that the spin-state crossover is driven by the thermal excitation of high-spin (HS) states and accompanied by the reduction in effective Coulomb energy and an increase of covalency, culminating in an effective Coulomb-energy-vs-bandwidth type insulator-tometal transition.arXiv:1708.044...
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