We describe here a new process for the synthesis of very high quality 2D Covalent Organic Frameworks (COFs), such a C 2 N and CN carbon nitrides. This process relies on the use of a metallic surface as both a reagent and a support for the coupling of small halogenated building blocks. The conditions of the assembly reaction are chosen so as to leave the inorganic salts by-products on the surface, to further confine the assembly reaction on the surface and increase the quality of the 2D layers. We found that under these conditions, the process directly returns few layers material. The structure/quality of these materials is demonstrated by extensive cross-characterizations at different scales, combining optical microscopy, Scanning Electron Microscopy (SEM)/Transmission Electron Microscopy (TEM) and Energy Dispersive Spectroscopy (EDS). The availability of such very large, high-quality layers of these materials opens interesting perspectives, for example in photochemistry and electronics (intrinsic transport properties, high gap substrate for graphene, etc...).

In this paper, we focus on the thermal transport properties of antiferromagnetic spin chains cuprates. The chain magnetic excitations, the spinons, partake in heat transport at low temperature, but spinon heat transport decays well below room temperature, possibly because of a coupling with phonons. By means of inelastic neutron scattering, we thoroughly study the lattice dynamics of spin chain compounds Sr2CuO3, Ca2CuO3, along with double spin-chain compounds SrCuO2. We come to the conclusion that there are no obvious anomalies in the phonon dispersions, which suggests a weak spinon-phonon coupling regime.

Conjugated polymers have emerged as very active photocatalysts under visible light.Among these materials, nanostructured polypyrrole (PPy) has recently been used as photocatalyst for degradation of environmental pollutants under UV as well as visible light.In the present study, we specifically explored the photocatalytic activity of conducting PPy synthesized by radiolysis, either in water (PPyH2O) or in dichloromethane solvent (PPyDCM), in the absence of any external dopant or template.The successful preparation of both kinds of conducting polymers was confirmed by complementary spectroscopic techniques and morphological characterizations.Besides, the photocatalytic activity of both materials was evaluated in the degradation of phenol as model pollutant in aqueous solution.PPyH2O as well as PPyDCM were found to exhibit remarkably high photocatalytic activity under both UV and visible light.These organic photocatalysts remain very stable after several cycles and thus, easily reusable.Interestingly, PPyDCM clearly appears as the most efficient photocatalyst due to its longer chain length, highly doped nature, lower optical band gap and extended absorption band from the UV to the near infrared region.The present work definitely validates radiation chemistry as an alternative approach to synthesize conducting polymer-based photocatalysts.The obtained results also highlight that the radiosynthesized PPy, especially those prepared in dichloromethane, constitute promising candidates for photocatalytic depollution of water.

Resonant inelastic x-ray scattering (RIXS) is an evolving tool for investigating spin dynamics of strongly correlated materials, which complements inelastic neutron scattering. Both techniques have found that non-spin-conserving (NSC) excitations in quasi-1D isotropic quantum antiferromagnets are confined to the two-spinon phase space. Outside this phase space, only spin-conserving (SC) four-spinon excitations have been detected using O K-edge RIXS. Here, we investigate SrCuO_2 and find four-spinon excitations outside the two-spinon phase space at both O K- and Cu L_3-edges. Using the Kramers-Heisenberg formalism, we demonstrate that the four-spinon excitations arise from both SC and NSC processes at Cu L_3-edge. We show that these new excitations only appear in the second-order terms of the ultra-fast core-hole lifetime expansion and arise from long-range spin fluctuations. These results thus open a new window to the spin dynamics of quantum magnets.

Conjugated polymers have emerged as very active photocatalysts under visible light. Among these materials, nanostructured polypyrrole (PPy) has recently been used as photocatalyst for degradation of environmental pollutants under UV as well as visible light. In the present study, we specifically explored the photocatalytic activity of conducting PPy synthesized by radiolysis, either in water (PPyH2O) or in dichloromethane solvent (PPyDCM), in the absence of any dopant or template. The successful preparation of both kinds of conducting polymers was confirmed by complementary spectroscopic techniques and morphological characterizations. Besides, the photocatalytic activity of both materials was evaluated in the degradation of phenol as model pollutant in aqueous solution. PPyH2O as well as PPyDCM were found to exhibit remarkably high photocatalytic activity under both UV and visible light. These organic photocatalysts remain very stable after several cycles, and thus easily reusable. Interestingly, PPyDCM clearly appears as the most efficient photocatalyst, due to its longer chain length, its doped nature, its lower optical band gap and its large extended absorption band from the UV to the near infrared region. The present work definitely validates radiation chemistry as an alternative approach to synthesize conducting polymer-based photocatalysts. The obtained results also highlight the fact that radiosynthesized PPy, especially those prepared in dichloromethane, constitute promising candidates for water depollution.

Investigations of magnetically ordered phases on the femtosecond timescale have provided significant insights into the influence of charge and lattice degrees of freedom on the magnetic sub-system. However, short-range magnetic correlations occurring in the absence of long-range order, for example in spin-frustrated systems, are inaccessible to many ultrafast techniques. Here, we show how time-resolved resonant inelastic X-ray scattering (trRIXS) is capable of probing such short-ranged magnetic dynamics in a charge-transfer insulator through the detection of a Zhang–Rice singlet exciton. Utilizing trRIXS measurements at the O K -edge, and in combination with model calculations, we probe the short-range spin correlations in the frustrated spin chain material CuGeO 3 following photo-excitation, revealing a strong coupling between the local lattice and spin sub-systems.

New phases with broken discrete Ising symmetries are uncovered in quantum materials with strong electronic correlations. The two-leg ladder cuprate S r 14− x C a x C u 24 O 41 hosts a very rich phase diagram where, upon hole doping, the system exhibits a spin liquid state ending to an intriguing ordered magnetic state at larger C a content. Using polarized neutron diffraction, we report here the existence of short range magnetism in this material for two C a contents, whose origin cannot be ascribed to Cu spins. This magnetism develops exclusively within the two-leg ladders with a diffraction pattern at forbidden Bragg scattering, which is the hallmark of loop current-like magnetism breaking both time-reversal and parity symmetries. Our discovery shows local discrete symmetry breaking in a one dimensional spin liquid system as theoretically predicted. It further suggests that a loop current-like phase could trigger the long range magnetic order reported at larger doping in two-leg ladder cuprates.

The size of CoFe Prussian blue analogue nanoparticles containing either rubidium or cesium cations was controlled by their formation through a nucleation process inside the calibrated pores of an ordered mesoporous silica matrix. The corresponding references were synthesized in powder form using the same ratio between the metal species in the reaction solution. The obtained samples were characterized via infrared (IR) spectroscopy, X-ray diffraction, and magnetometry to understand the differences arising from the size reduction and the nature of the inserted alkali cation. We were able to show that the nature of the cation has a significant influence on the photoswitching properties at the macroscale in the powders. However, on the nanoscale, the influence of the surface contribution becomes more prominent and cooperative effects disappear, resulting in a new role of the alkali cation in the photomagnetic properties. (C) 2019 Academie des sciences. Published by Elsevier Masson SAS. All rights reserved.

A single crystal of CaTiO3 was prepared by the floating method (FZ) method in which a high external electric field (>= 3 kV.cm(-1)) is applied. Such an electric field is a new powerful tool, in crystalline growth, able to create new chemical structures and original new materials with new physical properties. By varying crystal growth velocity and electric field strength, we show that we can control the macroscopic shape of CaTiO3 single crystals, and we can alter the crystalline orientation of domains. Applying an in-situ electric field during crystalline growth is a new tool to design domain modulated-structures.

Laser-induced crystallization in glasses is of great interest because of its significant applications in optics. However, the mechanisms involved are not yet concluded, and this paper is aimed at making progress on this problem. Major aspects of laser-induced crystallization are the nanostructure formation, textured crystallization, and growth dynamics. Lithium niobium silicate glasses were used as a "glass model" to investigate the nanostructure formation and crystallization by femtosecond (fs) laser irradiation at a high repetition rate (100-500 kHz). Three crystallization regimes can be classified with the increase of laser power. In addition, the boundary between neighboring regimes can be adjusted by changing the writing parameters. Regime 1: modifications of the amorphous structure. Regime 2: appearance of textured nanocrystals embedded in amorphous lamellas. Polar axes of nanocrystals are perpendicular to laser polarization direction. Remarkably, this hierarchical micro-/nanostructure can induce an orientation tunable second harmonic generation and form birefringence. Regime 3: crystallization morphology is sensitive to the angle between laser writing and laser polarization direction. With parallel configuration, crystal grains are at the micron scale. With perpendicular configuration, crystal morphologies resemble regime 2, but they are not textured. A crystallization dynamic model is proposed to explain the various nano-/microcrystals formation and their morphology induced by fs laser irradiation. This model allows thinking that it is possible to extend conclusions to other noncongruent glasses. This provides guidelines for manufacturing multifunctional optical devices.

Growth of BaZrO3 crystals at 2700 °C in a mirror furnace and growth prospects at 1350 °C using BaB2O4 flux are reported.

The calcination of (nano) Prussian blue analogues is now a fully controlled and understood route to synthesize Co–Fe spinel oxides.

We present the results of an Angle-Resolved Photoemission Spectroscopy study on the pristine and doped quasi-one dimensional spin chains cuprates: SrCuO2, SrCu0.99.M-0.001.O-2 with (M = Mg2+ or Zn2+), Sr2CuO3 and Sr2Cu(1-X)NixO3 with (x = 0.01 or 0.02), where the dopant is a non-magnetic impurity. Both systems are quantum critical and obey the Tomonaga-Luttinger spin liquid theory. Due to low dimensionality, the separation of the degrees of freedom of the collective excitation modes of spin and charge occurs, resulting in two distinct band dispersions ascribed to spinon and holon quasi-partides, at a binding energy of about 1.2 eV. Several experimental probes show that the finite size effect, in these strongly correlated electron compounds, resulting from chains breaking by the non-magnetic impurities, has a strong impact on the ground state of their quasi-particles excitations. On the other hand, our Angle Resolved Photoemission Spectroscopy data do not show any evolution of the spinon branch upon doping, in terms of energy position at Gamma. We also extract the antiferromagnetic superexchange coupling and inter-site hopping constants J(AF) and t. (C) 2018 Elsevier B.V. All rights reserved.

Author(s): Kuo, CT; Lin, SC; Conti, G; Pi, ST; Moreschini, L; Bostwick, A; Meyer-Ilse, J; Gullikson, E; Kortright, JB; Nemsak, S; Rault, JE; Le Fevre, P; Bertran, F; Santander-Syro, AF; Vartanyants, IA; Pickett, WE; Saint-Martin, R; Taleb-Ibrahimi, A; Fadley, CS | Abstract: © 2018 American Physical Society. A major remaining challenge in the superconducting cuprates is the unambiguous differentiation of the composition and electronic structure of the CuO2 layers and those of the intermediate layers. The large c axis for these materials permits employing soft x-ray (930.3 eV) standing wave (SW) excitation in photoemission that yields atomic layer-by-layer depth resolution of these properties. Applying SW photoemission to Bi2Sr2CaCu2O8+δ yields the depth distribution of atomic composition and the layer-resolved densities of states. We detect significant Ca presence in the SrO layers and oxygen bonding to three different cations. The layer-resolved valence electronic structure is found to be strongly influenced by the atomic supermodulation structure, as determined by comparison to density functional theory calculations, by Ca-Sr intermixing, and by correlation effects associated with the Cu 3d-3d Coulomb interaction, further clarifying the complex interactions in this prototypical cuprate. Measurements of this type for other quasi-two-dimensional materials with large c represent a promising future direction.

We report inelastic neutron scattering measurements of the phonons modes, in the one-dimensional half integer spin chains cuprate SrCuO2. We study the longitudinal and the transverse modes propagating in the direction of the chains, along Q (0 0 L) and Q (2 0 L), respectively. On the other hand, we investigate the effect of substitution by impurities in the corresponding doped compounds, namely, SrCu0.99M0.01O2 with M=Mg or Zn, and La0.01Sr0.99CuO2. Our results evidence a systematic strong spinon-phonon interaction leading to an important decrease of the phonon scattered intensity as well as a decrease of the group velocity of the transverse acoustic modes upon substitution, and a shift of the transverse optical B3u mode in the La-doped SrCuO2, in terms of energy.

The low energy magnetic excitations spectra of the pristine and doped quasi-one-dimensional spin chains cuprates SrCuO2 have been investigated by inelastic neutron scattering. The momentum-integrated magnetic dynamical structure factor yields a constant integrated intensity with regard to energy in the pure compound, while it shows a strong decay, at low energies, in the compounds doped with nonmagnetic impurities, namely, SrCu0.99M0.01O2 (with M = Zn or Mg) and Sr0.99La0.01CuO2 (Cu+ carrying S = 0 created within the chains). These results evidence the opening of a spin pseudogap in the two-spinon continuum of SrCuO2 upon doping, stemming from disruptions of the spin chains by quantum impurities.

The S = 1/2 Heisenberg spin chain compound SrCuO2 doped with different amounts of nickel (Ni), palladium (Pd), zinc (Zn), and cobalt (Co) has been studied by means of Cu nuclear magnetic resonance (NMR). Replacing only a few of the S = 1/2 Cu ions with Ni, Pd, Zn, or Co has a major impact on the magnetic properties of the spin chain system. In the case of Ni, Pd, and Zn an unusual line broadening in the low temperature NMR spectra reveals the existence of an impurity-induced local alternating magnetization (LAM), while strongly decaying spin-lattice relaxation rates T-1(-1) towards low temperatures indicate the opening of spin gaps. A distribution of gap magnitudes is implied by a stretched spin-lattice relaxation and a variation of T-1(-1) within the broad resonance lines. These observations depend strongly on the impurity concentration and therefore can be understood using the model of finite segments of the spin 1/2 antiferromagnetic Heisenberg chain, i. e., pure chain segmentation due to S = 0 impurities. This is surprising for Ni as it was previously assumed to be a magnetic impurity with S = 1 which is screened by the neighboring copper spins. In order to confirm the S = 0 state of the Ni, we performed x-ray absorption spectroscopy (XAS) and compared the measurements to simulated XAS spectra based on multiplet ligand-field theory. Furthermore, Zn doping leads to much smaller effects on both the NMR spectra and the spin-lattice relaxation rates, indicating that Zn avoids occupying Cu sites. For magnetic Co impurities, T-1(-1) does not obey the gaplike decrease, and the low-temperature spectra get very broad. This could be related to an increase of the Neel temperature and is most likely an effect of the impurity spin S (not equal)0.

Muon spin rotation technique is used to study magnetic ordering in ultra-pure samples of SrCu$_{1-x}$Ni$_x$O$_2$, an archetypical $S=1/2$ antiferromagnetic Heisenberg chain system with a small amount of $S=1$ defects. The ordered state in the parent compound is shown to be highly homogeneous, contrary to previous report [M. Matsuda et al., Phys. Rev. B \textbf{55}, R11953 (1997)]. Even minute amount of Ni impurities result in inhomogeneous order and a decrease of the transition temperature. At as little as $0.5$~\% Ni concentration, magnetic ordering is entirely suppressed. The results are compared to previous theoretical studies of weakly coupled spin chains with site-defects.

We study the effect of oxygen vacancies on the electronic structure of the model strongly correlated metal SrVO_3. By means of angle-resolved photoemission (ARPES) synchrotron experiments, we investigate the systematic effect of the UV dose on the measured spectra. We observe the onset of a spurious dose-dependent prominent peak at an energy range were the lower Hubbard band has been previously reported in this compound, raising questions on its previous interpretation. By a careful analysis of the dose dependent effects we succeed in disentangling the contributions coming from the oxygen vacancy states and from the lower Hubbard band. We obtain the intrinsic ARPES spectrum for the zero-vacancy limit, where a clear signal of a lower Hubbard band remains. We support our study by means of state-of-the-art ab initio calculations that include correlation effects and the presence of oxygen vacancies. Our results underscore the relevance of potential spurious states affecting ARPES experiments in correlated metals, which are associated to the ubiquitous oxygen vacancies as extensively reported in the context of a two-dimensional electron gas (2DEG) at the surface of insulating d^0 transition metal oxides.