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We report an investigation of the combined structural and electronic properties of the bronze Na 1.5 ( PO 2 ) 4 ( WO 3 ) 20 . Its low-dimensional structure and possible large reconstruction of the Fermi surface due to charge density wave instability make this bulk material a natural superlattice with a reduced number of carriers and Fermi energy. Signatures of multilayered two-dimensional (2D) electron weak localization are consequently reported, with an enhanced influence of quantum oscillations. A crossover between these two antagonistic entities, previously observed only in genuine low-dimensional materials and devices, is shown to occur in a bulk crystal due to its hidden 2D nature., A.P. would like to thank Thierry Klein and Andrea Gauzzi for valuable remarks on the competition between weak localization and quantum oscillation conditions which motivated our analysis. Financial support by the ANR Projects No. ANR-18-CE92-0014 and No. ANR-11-BS04- 0004 is gratefully acknowledged. Work in Spain was supported by MICIU (PGC2018-096955-B-C44 and PGC2018- 093863-B-C22), MINECO through the Severo Ochoa (SEV2015-0496) and Maria de Maeztu (MDM-2017-0767) Programs, and the Generalitat de Catalunya (2017SGR1506 and 2017SGR1289). E.D.-N. was supported by the project NanoCent-Nanomaterials center for advanced applications, Project No. CZ.02.1.01/0.0/0.0/15_003/0000485, financed by the ERDF.

Model materials are precious test cases for elementary theories and provide building blocks for the understanding of more complex cases. Here, we describe the lattice dynamics of the structural phase transition in francisite Cu3Bi(SeO3)(2)O2Cl at 115 K and show that it provides a rare archetype of a transition driven by a soft antipolar phonon mode. In the high-symmetry phase at high temperatures, the soft mode is found at (0,0,0.5) at the Brillouin zone boundary and is measured by inelastic x-ray scattering and thermal diffuse scattering. In the low-symmetry phase, this soft-mode is folded back onto the center of the Brillouin zone as a result of the doubling of the unit cell, and appears as a fully symmetric mode that can be tracked by Raman spectroscopy. On both sides of the transition, the mode energy squared follows a linear behavior over a large temperature range. First-principles calculations reveal that, surprisingly, the flat phonon band calculated for the high-symmetry phase seems incompatible with the displacive character found experimentally. We discuss this unusual behavior in the context of an ideal Kittel model of an antiferroelectric transition.

International audience

International audience; The anharmonic lattice dynamics of MgO has been studied at ambient conditions and at high temperatures by infrared spectroscopy and inelastic X-rays scattering measurements combined with density functional perturbation theory calculations. The agreement between the measured phonon energies and widths with ab-initio calculated values provides a direct and pertinent test of the validity of advanced theoretical methods. Long observed anharmonic features in the infrared reflectivity find a clear explanation in terms of well-defined multi-phonons scattering processes and lattice dynamics peculiarities, also responsible of a significant and sharp reduction of the longitudinal optical phonon lifetime at critical finite wave vectors. Our work highlights the importance of multi-phonons scattering processes on collective dynamics and related materials properties.

Model materials are precious test cases for elementary theories and provide building blocks for the understanding of more complex cases. Here, we describe the lattice dynamics of the structural phase transition in francisite Cu3Bi(SeO3)2O2Cl at 115 K and show that it provides a rare archetype of a transition driven by a soft antipolar phonon mode. In the high-symmetry phase at hightemperatures, the soft mode is found at (0,0,0.5) at the Brillouin zone boundary and is measured by inelastic X-ray scattering and thermal diffuse scattering. In the low-symmetry phase, this softmode is folded back onto the center of the Brillouin zone as a result of the doubling of the unit cell, and appears as a fully symmetric mode that can be tracked by Raman spectroscopy. On both sides of the transition, the mode energy squared follows a linear behaviour over a large temperature range. First-principles calculations reveal that, surprisingly, the flat phonon band calculated for the high-symmetry phase seems incompatible with the displacive character found experimentally. We discuss this unusual behavior in the context of an ideal Kittel model of an antiferroelectric transition., Comment: 6 pages, 5 Figures, supplemental material included

We report a persistent low-energy phonon broadening around $q_{B} \sim 0.28$ r.l.u. along the Cu-O bond direction in the high-$T_c$ cuprate Bi$_2$Sr$_2$CaCu$_2$O$_{8+\delta}$ (Bi-2212). We show that such broadening exists both inside and outside the conventional charge density wave (CDW) phase, via temperature dependent measurements in both underdoped and heavily overdoped samples. Combining inelastic hard x-ray scattering, diffuse scattering, angle-resolved photoemission spectroscopy, and resonant soft x-ray scattering at the Cu $L_3$-edge, we exclude the presence of a CDW in the heavily overdoped Bi-2212 similar to that observed in the underdoped systems. Finally, we discuss the origin of such anisotropic low-energy phonon broadening, and its potential precursory role to the CDW phase in the underdoped region., Comment: 11 pages, 9 figures

We report a persistent low-energy phonon broadening around $q_{B} \sim 0.28$ r.l.u. along the Cu-O bond direction in the high-$T_c$ cuprate Bi$_2$Sr$_2$CaCu$_2$O$_{8+��}$ (Bi-2212). We show that such broadening exists both inside and outside the conventional charge density wave (CDW) phase, via temperature dependent measurements in both underdoped and heavily overdoped samples. Combining inelastic hard x-ray scattering, diffuse scattering, angle-resolved photoemission spectroscopy, and resonant soft x-ray scattering at the Cu $L_3$-edge, we exclude the presence of a CDW in the heavily overdoped Bi-2212 similar to that observed in the underdoped systems. Finally, we discuss the origin of such anisotropic low-energy phonon broadening, and its potential precursory role to the CDW phase in the underdoped region., 11 pages, 9 figures

The lattice dynamics of α-quartz has been studied in great details by combining inelastic X-ray scattering (IXS) from single- and polycrystalline samples, 3D mapping of thermal diffuse scattering (TDS) and ab initio calculations. Pronounced features in TDS patterns have been identified and the origin of first peak in vibrational density of states is unambiguously revealed

National audience

The temperature dependence of the phonon spectrum in the superconducting transition metal dichalcogenide 2H-NbS$_2$ is measured by diffuse and inelastic x-ray scattering. A deep, wide and strongly temperature dependent softening, of the two lowest energy longitudinal phonons bands, appears along the $\mathrm{\Gamma M}$ symmetry line in reciprocal space. In sharp contrast to the iso-electronic compounds 2H-NbSe$_2$, the soft phonons energies are finite, even at very low temperature, and no charge density wave instability occurs, in disagreement with harmonic ab-initio calculations. We show that 2H-NbS$_2$ is at the verge of the charge density wave transition and its occurrence is only suppressed by the large anharmonic effects. Moreover, the anharmonicity and the electron phonon coupling both show a strong in-plane anisotropy., Comment: 6 pages, 5 figures, accepted for publication in Physical Review B

The lattice dynamics of low-dimensional BaVS3 is reported across the hexagonal-orthorhombic phase transition occurring at T-S = 250 K using a combination of the thermal diffuse scattering of x rays, inelastic x-ray scattering, and density functional theory calculations. We observed the occurrence of strongly temperature-dependent diffuse scattering upon approaching the transition, centered at the F points associated with weak Bragg reflections with odd L. Inelastic scattering experiments related this observation to the condensation of a low-lying overdamped optical phonon at the Brillouin zone center, evidenced by significant variations of quasielastic scattering. These results unravel the dynamical origin of the zigzag distortion of the V chains at T-S, which is probably a prerequisite of the Peierls anomaly driving the metal-insulator transition at T-MI = 70 K.

Controversy in the description/identification of so-called intermediate phase(s) in PbHfO3, stable in the range similar to 420-480 K, has existed for a few decades. A synchrotron diffraction experiment on a partially detwinned crystal allowed the structure to be solved in the superspace group Imma(00 gamma)s00 (No. 74.2). In contrast to some previously published reports, in the pure compound only one distinct phase was observed between Pbam PbZrO3-like antiferroelectric and Pm3m paraelectric phases. The modulation vector depends only slightly on temperature. The major structure modulation is associated with the displacement of lead ions, which is accompanied by a smaller amplitude modulation for the surrounding O atoms and tilting of HfO6 octahedra. Tilting of the octahedra results in a doubling of the unit cell compared with the parent structure.

We report a type of structural phase transitions in dielectric materials: the triggered incommensurate (IC) transition. We demonstrate evidence for such a transition in the perovskite antiferroelectric PbHfO3 by means of single-crystal x-ray diffraction, diffuse, and inelastic-scattering experiments, which we interpret using the Landau theory of phase transitions. This transition is not driven by an IC soft mode, as is the case for the majority of IC dielectrics, but by the soft mode associated with a different, coexisting order parameter related to antiferrodistortive (AFD) tilts of the oxygen octahedra. When cooling from the high-temperature cubic phase these two order parameters are established simultaneously and discontinuously at T-IC approximate to 468 K. Two lattice instabilities are present in the cubic phase: AFD instability with critical temperature T-0 approximate to 441 K and ferroelectric instability with T-0 approximate to 408 K. The IC instability is absent. The analysis of the transition mechanism within the Landau theory of coupled order parameters indicates that the transition is of triggered character, conditioned upon the attractive biquadratic coupling between the IC modulation and the AFD octahedral tilts, and is driven by the AFD soft mode.

Nanomaterials 11(8), 1945 (1-18) (2021). doi:10.3390/nano11081945, Neutrons can be an instrument or an object in many fields of research. Major efforts all over the world are devoted to improving the intensity of neutron sources and the efficiency of neutron delivery for experimental installations. In this context, neutron reflectors play a key role because they allow significant improvement of both economy and efficiency. For slow neutrons, Detonation NanoDiamond (DND) powders provide exceptionally good reflecting performance due to the combination of enhanced coherent scattering and low neutron absorption. The enhancement is at maximum when the nanoparticle diameter is close to the neutron wavelength. Therefore, the mean nanoparticle diameter and the diameter distribution are important. In addition, DNDs show clustering, which increases their effective diameters. Here, we report on how breaking agglomerates affects clustering of DNDs and the overall reflector performance. We characterize DNDs using small-angle neutron scattering, X-ray diffraction, scanning and transmission electron microscopy, neutron activation analysis, dynamical light scattering, infra-red light spectroscopy, and others. Based on the results of these tests, we discuss the calculated size distribution of DNDs, the absolute cross-section of neutron scattering, the neutron albedo, and the neutron intensity gain for neutron traps with DND walls., Published by MDPI, Basel

Journal of applied crystallography 54(1), 287 - 294 (2021). doi:10.1107/S1600576720016039, The complete elastic stiffness tensor of thiourea has been determined fromthermal diffuse scattering (TDS) using high-energy photons (100 keV).Comparison with earlier data confirms a very good agreement of the tensorcoefficients. In contrast with established methods to obtain elastic stiffnesscoefficients (e.g.Brillouin spectroscopy, inelastic X-ray or neutron scattering,ultrasound spectroscopy), their determination from TDS is faster, does notrequire large samples or intricate sample preparation, and is applicable toopaque crystals. Using high-energy photons extends the applicability of theTDS-based approach to organic compounds which would suffer from radiationdamage at lower photon energies., Published by Wiley-Blackwell, [S.l.]