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“Molecular structure, crystal field and orientational order in solid C60”. Lamoen D, Michel KH s.l., page 183 (1994).
Keywords: H1 Book chapter; Condensed Matter Theory (CMT)
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“Translation-rotation coupling, phase transitions and elastic phenomena in orientationally disordered crystals”. Lynden-Bell RM, Michel KH, Reviews of modern physics 66, 721 (1994). http://doi.org/10.1103/RevModPhys.66.721
Keywords: A1 Journal article; Condensed Matter Theory (CMT)
Impact Factor: 29.604
Times cited: 128
DOI: 10.1103/RevModPhys.66.721
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“Crystal field, orientational order, and lattice contraction in solid C60”. Lamoen D, Michel KH, The journal of chemical physics 101, 1435 (1994). http://doi.org/10.1063/1.467768
Abstract: A model of the intermolecular potential in solid C-60, which is based on Born-Mayer repulsions, van der Waals attractions, and electrostatic multipoles, is presented. The potential is expanded in terms of multipolar rotator functions. The orientation-orientation interaction and the crystal field are calculated. The orientational phase transition to the Pa3 phase is studied with the methods of statistical mechanics. The discontinuity of the order parameter at the transition and the temperature evolution of the order parameter are calculated. The lattice contraction at the phase transition is evaluated. The influence of the lattice contraction on the crystal field and on the orientational order is studied.
Keywords: A1 Journal article; Condensed Matter Theory (CMT)
Impact Factor: 2.952
Times cited: 39
DOI: 10.1063/1.467768
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“Dynamics of a quadrupolar glass”. Bostoen C, Michel KH, Physical review : B : condensed matter and materials physics 43, 4415 (1991)
Keywords: A1 Journal article; Condensed Matter Theory (CMT)
Impact Factor: 3.736
Times cited: 33
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“Theory of phonon dispersions and piezoelectricity in multilayers of hexagonal boron-nitride”. Michel KH, Verberck B, Physica status solidi: B: basic research 248, 2720 (2011). http://doi.org/10.1002/pssb.201100084
Abstract: Taking into account covalent, Coulomb and van der Waals interactions, we construct the dynamical matrix and calculate the phonon dispersion relations for multilayer crystals of hexagonal boron-nitride. Coulomb interactions account for a strong overbending of optical phonons. Applying and extending Born's long-wave theory to the case of multilayer crystals, we calculate the piezoelectric stress constant equation image as a function of the number of layers equation image. In agreement with group theory, we find that equation image for equation image even; for an uneven number equation image of layers we obtain equation image, i.e. the piezoelectric constant decreases as equation image.
Keywords: A1 Journal article; Condensed Matter Theory (CMT)
Impact Factor: 1.674
Times cited: 13
DOI: 10.1002/pssb.201100084
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“Anisotropic packing and one-dimensional fluctuations of C60 molecules in carbon nanotubes”. Michel KH, Verberck B, Nikolaev AV, Physical review letters 95, 185506 (2005). http://doi.org/10.1103/PhysRevLett.95.185506
Abstract: The confinement of a C-60 molecule encapsulated in a cylindrical nanotube depends on the tube radius. In small tubes with radius R-T less than or similar to 7 A, a fivefold axis of the molecule coincides with the tube axis. The interaction between C-60 molecules in the nanotube is then described by a O-2-rotor model on a 1D liquid chain with coupling between orientational and displacive correlations. This coupling leads to chain contraction. The structure factor of the 1D liquid is derived. In tubes with a larger radius the molecular centers of mass are displaced off the tube axis. The distinction of two groups of peapods with on- and off-axis molecules suggests an explanation of the apparent splitting of A(g) modes of C-60 in nanotubes measured by resonant Raman scattering.
Keywords: A1 Journal article; Condensed Matter Theory (CMT)
Impact Factor: 8.462
Times cited: 40
DOI: 10.1103/PhysRevLett.95.185506
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“Mercator maps of orientations of a C60 molecule in single-walled nanotubes with distinct radii”. Michel KH, Verberck B, Nikolaev A, AIP conference proceedings 786, 69 (2005)
Abstract: We study the confinement of a C-60 molecule encapsulated in a cylindrical nanotube as a function of the tube radius. Drawing the Mercator maps of the potential, we find two distinct molecular orientations; for tubes with small radii, R-T less than or similar to 7 angstrom, a fivefold axis of the molecule coincides with the tube long axis, for larger radii, R-T less than or similar to 8 angstrom, a threefold axis of the molecule coincides with the tube long axis. These different orientations are caused by the relative importance of the repulsive and the attractive parts of the van der Waals potentials of the molecule with the tube wall for small and large tubes respectively. Experimental evidence is provided by the apparent splitting of A(g) modes of the C-60 molecule in resonant Raman scattering.
Keywords: A1 Journal article; Condensed Matter Theory (CMT)
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“High resolution neutron and X-ray diffraction studies as a function of temperature and electric field of the ferroelectric phase transition of RDP”. Mattauch S, Heger G, Michel KH, Crystal research and technology 39, 1027 (2004). http://doi.org/10.1002/crat.200410289
Abstract: Neutron and high resolution X-ray diffraction investigations on perfect single crystals of RbH2PO4 (RDP), a hydrogen bonded ferroelectric of KDP type are reported. The results of crystal structure analysis from diffraction data, below and above the paraelectric – ferroelectric phase transition, support a disorder – order character Of [PO4H2](-)-groups. The tetragonal symmetry of the paraelectric phase with the double well potential of the hydrogen atoms obtained by diffraction, results simply from a time-space average of orthorhombic symmetry. According to the group – subgroup relation between the tetragonal space group 142d and the orthorhombic Fdd2 a short range order of ferroelectric clusters in the tetragonal phase is observed. With decreasing temperature the ferroelectric clusters increase and the long range interaction between their local polarisation vectors leads to the formation of lamellar ferroelectric domains with alternating polarisation directions at T-C = 147 K. From the high resolution X-ray data it is concluded that below T-C the ferroelastic strain in the (a,b)-plane leads to micro-angle grain boundaries at the domain walls. The tilt angle is enhanced by an applied electric field parallel to the ferroelectric axis. The resulting dislocations at the domain walls persist in the paraelectric phase leading to a memory effect for the arrangement of twin lamellae. With increased electric field the phase transition temperature T-C is decreased.
Keywords: A1 Journal article; Condensed Matter Theory (CMT)
Impact Factor: 1
Times cited: 12
DOI: 10.1002/crat.200410289
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“Orientational charge density waves and the metal-insulator transition in polymerized KC60”. Verberck B, Nikolaev AV, Michel KH, AIP conference proceedings 723, 339 (2004)
Abstract: Polymerized KC60 undergoes a structural phase transition accompanied by a metal-insulator transition around 50 K. To explain the structural aspect, a mechanism involving small orientational deviations of the valence electron density on every C-60 monomer orientational charge density waves (OCDWs) – has already been proposed earlier. In the present work, we address the metal-insulator transition using the OCDW concept. We are inspired by the analogy between a polymer chain exhibiting an OCDW and a linear atomic chain undergoing a static lattice deformation doubling the unit cell: such a deformation implies a band gap at the zone boundary, yielding an insulating state (Peierls instability). Within our view, a similar mechanism occurs in polymerized KC60; the OCDW plays the role of the lattice deformation. We present tight-binding band structure calculations and conclude that the metal-insulator transition can indeed be explained using OCDWs, but that the threedimensionality of the crystal plays an unexpected key role.
Keywords: A1 Journal article; Condensed Matter Theory (CMT)
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“Symmetry lowering at the structural phase transitions in NpO2 and UO2”. Nikolaev AV, Michel KH, Physical review : B : condensed matter and materials physics 68, 054112 (2003). http://doi.org/10.1103/PhysRevB.68.054112
Abstract: The structural phase transitions with electric-quadrupole long-range order in NpO2 (Fm (3) over barm-->Pn (3) over barm) and UO2 (Fm (3) over barm-->Pa (3) over bar) are analyzed from a group theoretical point of view. In both cases, the symmetry lowering involves three quadrupolar components belonging to the irreducible representation T-2g (Gamma(5)) of O-h and condensing in a triple-q structure at the X point of the Brillouin zone. The Pa (3) over bar structure is close to Pn (3) over barm, but allows for oxygen displacements. The Pa (3) over bar ordering leads to an effective electrostatic attraction between electronic quadrupoles while the Pn (3) over barm ordering results in a repulsion between them. It is concluded that the Pn (3) over barm structure can be stabilized only through some additional process such as strengthening of the chemical bonding between Np and O. We also derive the relevant structure-factor amplitudes for Pn (3) over barm and Pa (3) over bar, and the effect of domains on resonant x-ray scattering experiments.
Keywords: A1 Journal article; Condensed Matter Theory (CMT)
Impact Factor: 3.836
Times cited: 15
DOI: 10.1103/PhysRevB.68.054112
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“Theoretical model for the structural phase transition at the metal-insulator transition in polymerized KC60”. Verberck B, Nikolaev AV, Michel KH, Physical Review B 66, 165425 (2002). http://doi.org/10.1103/PhysRevB.66.165425
Abstract: The recently discovered structural transition in polymerized KC60 at about 50 K results in a doubling of the unit cell volume and accompanies the metal-insulator transition. Here we show that the ((a) over right arrow+(c) over right arrow,(b) over right arrow,(a) over right arrow-(c) over right arrow) superstructure results from small orientational charge density waves along the polymer chains and concomitant displacements of the surrounding K+ ions. The effect is specific for the space group Pmnn of KC60 and is absent in RbC60 and CsC60 (space group I2/m). The mechanism is relevant for the metal-insulator transition.
Keywords: A1 Journal article; Condensed Matter Theory (CMT)
Impact Factor: 3.836
Times cited: 10
DOI: 10.1103/PhysRevB.66.165425
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“Microscopic theory of orientational disorder and the orientational phase transition in solid C60”. Michel KH, Copley JRD, Neumann DA, Physical review letters 68, 2929 (1992). http://doi.org/10.1103/PhysRevLett.68.2929
Abstract: We have developed a microscopic theory which describes the orientational dynamics of C60 molecules in the face-centered-cubic phase of C60 fullerite. The molecular interaction potential and the crystal-field potential are formulated in terms of symmetry-adapted rotator functions. The phase transition to the Pa3BAR structure is driven by an active multipolar mode of T2g symmetry belonging to the l = 10 manifold. The Birman criterion is satisfied. The transition is found to be of first order.
Keywords: A1 Journal article; Condensed Matter Theory (CMT)
Impact Factor: 7.512
Times cited: 94
DOI: 10.1103/PhysRevLett.68.2929
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“Theory of rigid-plane phonon modes in layered crystals”. Michel KH, Verberck B, Physical review : B : condensed matter and materials physics 85, 094303 (2012). http://doi.org/10.1103/PhysRevB.85.094303
Abstract: The lattice dynamics of low-frequency rigid-plane modes in metallic (graphene multilayers, GML) and in insulating (hexagonal boron-nitride multilayers, BNML) layered crystals is investigated. The frequencies of shearing and compression (stretching) modes depend on the layer number N and are presented in the form of fan diagrams. The results for GML and BNML are very similar. In both cases, only the interactions (van der Waals and Coulomb) between nearest-neighbor planes are effective, while the interactions between more distant planes are screened. A comparison with recent Raman scattering results on low-frequency shear modes in GML [Tan et al., Nat. Mater., in press, doi: 10.1038/nmat3245, (2012)] is made. Relations with the low-lying rigid-plane phonon dispersions in the bulk materials are established. Master curves, which connect the fan diagram frequencies for any given N, are derived. Static and dynamic thermal correlation functions for rigid-layer shear and compression modes are calculated. The results might be of use for the interpretation of friction force experiments on multilayer crystals.
Keywords: A1 Journal article; Condensed Matter Theory (CMT)
Impact Factor: 3.836
Times cited: 38
DOI: 10.1103/PhysRevB.85.094303
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