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“Elusive s-f intrasite interactions and double exchange in solids: ferromagnetic versus nonmagnetic ground state”. Nikolaev AV, Michel KH, Journal of experimental and theoretical physics 109, 286 (2009). http://doi.org/10.1134/S1063776109080147
Abstract: From the theory of many-electron states in atoms, we know that there exists a strong Coulomb repulsion, which results in the electronic term structure of atoms and is responsible for Hunds rules. By expanding the Coulomb on-site repulsion into a multipolar series, we derive this interaction and show that it is also present in solids as a correlation effect, which means that the interaction requires a multideterminant version of the Hartree-Fock method. Of particular interest is the case where this interaction couples states of localized ( f) and delocalized ( s) electrons. We show that the interaction is bilinear in the creation/annihilation operators for localized electrons and bilinear in the operators for conduction electrons. To study the coupling, we consider a simple model in the framework of an effective limited configuration interaction method with one localized f-electron and one itinerant s-electron per crystal site. The on-site multipole interaction between the f- and s-electrons is explicitly taken into account. It is shown that depending on the low-lying excitation spectrum imposed by the crystal electric field, the model can lead not only to ferromagnetism but also to a nonmagnetic state. The model is relevant for solids with localized and itinerant electron states.
Keywords: A1 Journal article; Condensed Matter Theory (CMT)
Impact Factor: 1.196
Times cited: 3
DOI: 10.1134/S1063776109080147
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“Boron nitride mono layer : a strain-tunable nanosensor”. Neek-Amal M, Beheshtian J, Sadeghi A, Michel KH, Peeters FM, The journal of physical chemistry: C : nanomaterials and interfaces 117, 13261 (2013). http://doi.org/10.1021/jp402122c
Abstract: The influence of triaxial in-plane strain on the electronic properties of a hexagonal boron-nitride sheet is investigated using density functional theory. Different from graphene, the triaxial strain localizes the molecular orbitals of the boron-nitride flake in its center depending on the direction of the applied strain. The proposed technique for localizing the molecular orbitals that are close to the Fermi level in the center of boron nitride flakes can be used to actualize engineered nanosensors, for instance, to selectively detect gas molecules. We show that the central part of the strained flake adsorbs polar molecules more strongly as compared with an unstrained sheet.
Keywords: A1 Journal article; Engineering sciences. Technology; Condensed Matter Theory (CMT)
Impact Factor: 4.536
Times cited: 38
DOI: 10.1021/jp402122c
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“Orientation fluctuations, diffuse scattering and orientational order in solid C60”. Michel KH, Copley JRD World Scientific, Singapore, page 381 (1996).
Keywords: H3 Book chapter; Condensed Matter Theory (CMT)
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“The isotope effect in hydrogen-bonded systems”. Bussmann-Holder A, Michel KH, International journal of modern physics: B: condensed matter physics, statistical physics, applied physics
T2 –, 1st International Conference on New Theories, Discoveries, and, Applications of Superconductors and Related Materials (New3SC-1), FEB 19-24, 19 12, 3406 (1998). http://doi.org/10.1142/S021797929800274X
Abstract: The giant isotope effect on the ferro- and antiferroelectric transition temperature upon deuteration of hydrogen-bonded systems is well known experimentally since various decades. Yet, theoretically only recently a microscopic understanding of this effect has been achieved which, specifically, took into account the geometry of the O ... H ... O bond. The implications of this modeling are multiple as numerous hydrogen-bonded organic systems show the same effects as ferro- and antiferroelectrics, i.e., cooperative proton tunneling at a well-defined temperature.
Keywords: A1 Journal article; Condensed Matter Theory (CMT)
Impact Factor: 0.736
DOI: 10.1142/S021797929800274X
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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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“Theory of crystal structures of polymerized C60-fullerite and fullerides AC60, A=K, Rb, Cs”. Michel KH, Nikolaev AV, Verberck B s.l. (2001).
Keywords: H1 Book chapter; Condensed Matter Theory (CMT)
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“Polarizability induced cooperative proton ordering, coexistence of order/disorder and displacive dynamics and isotope effects in hydrogen-bonded systems”. Bussmann-Holder A, Dalal N, Michel KH, The journal of physics and chemistry of solids
T2 –, Williamsburg Workshop on Ferroelectrics 99, JAN 31-FEB 03, 1999, WILLIAMSBURG, VIRGINIA 61, 271 (2000). http://doi.org/10.1016/S0022-3697(99)00292-9
Abstract: Despite the general belief that hydrogen-bonded ferro- and antiferroelectrics undergo a pure order/disorder transition at the structural instability, new NMR data and a new theoretical concept yield convincing evidence that a pronounced displacive component is present in these systems, which modifies substantially the temperature dependencies of the tunnel and lattice mode frequencies. The experiments and their interpretation are presented. (C) 1999 Elsevier Science Ltd. All rights reserved.
Keywords: A1 Journal article; Condensed Matter Theory (CMT)
Impact Factor: 2.059
Times cited: 8
DOI: 10.1016/S0022-3697(99)00292-9
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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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“Bond geometry and phase transition mechanism of H-bonded ferroelectricity”. Bussmann-Holder A, Michel KH, Physical review letters 80, 2173 (1998). http://doi.org/10.1103/PhysRevLett.80.2173
Keywords: A1 Journal article; Condensed Matter Theory (CMT)
Impact Factor: 8.462
Times cited: 81
DOI: 10.1103/PhysRevLett.80.2173
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“Charge transfer and polymer phases in AC60 (A=K, Rb, Cs) fullerides”. Nikolaev AV, Prassides K, Michel KH, The journal of chemical physics 108, 4912 (1998). http://doi.org/10.1063/1.475900
Keywords: A1 Journal article; Condensed Matter Theory (CMT)
Impact Factor: 2.965
Times cited: 14
DOI: 10.1063/1.475900
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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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“Crystal structures of polymerized fullerides AC60, A=K, Rb, Cs, and alkali-mediated interactions”. Verberck B, Michel KH, Nikolaev AV, The journal of chemical physics 116, 10462 (2002). http://doi.org/10.1063/1.1475745
Abstract: Starting from a model of rigid interacting C-60 polymer chains on an orthorhombic lattice, we study the mutual orientation of the chains and the stability of the crystalline structures Pmnn and I2/m. We take into account (i) van der Waals interactions and electric quadrupole interactions between C-60 monomers on different chains as well as (ii) interactions of the monomers with the surrounding alkali atoms. The direct interactions (i) always lead to an antiferrorotational structure Pmnn with alternate orientation of the C-60 chains in planes (001). The interactions (ii) with the alkalis consist of two parts: translation-rotation (TR) coupling where the orientations of the chains interact with displacements of the alkalis, and quadrupolar electronic polarizability (ep) coupling, where the electric quadrupoles on the C-60 monomers interact with induced quadrupoles due to excited electronic d-states of the alkalis. Both interactions (ii) lead to an effective orientation-orientation interaction between the C-60 chains and always favor the ferrorotational structure I2/m, where C-60 chains have a same orientation. The structures Pmnn for KC60 and I2/m for Rb- and CsC60 are the result of a competition between the direct interaction (i) and the alkali-mediated interactions (ii). In Rb- and CsC60 the latter are found to be dominant, the preponderant role being played by the quadrupolar electronic polarizability of the alkali ions. (C) 2002 American Institute of Physics.
Keywords: A1 Journal article; Condensed Matter Theory (CMT)
Impact Factor: 2.965
Times cited: 12
DOI: 10.1063/1.1475745
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“Electronic structure and electric quadrupoles of a polymerized chain in solid AC60”. Nikolaev AV, Michel KH, Solid state communications 117, 739 (2001). http://doi.org/10.1016/S0038-1098(01)00017-5
Keywords: A1 Journal article; Condensed Matter Theory (CMT)
Impact Factor: 1.554
Times cited: 10
DOI: 10.1016/S0038-1098(01)00017-5
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“Many electron- and hole terms of molecular ions C60n\pm”. Nikolaev AV, Michel KH, Coupling In Chemistry And Physics 44, 305 (2003). http://doi.org/10.1016/S0065-3276(03)44019-7
Keywords: A1 Journal article; Condensed Matter Theory (CMT)
Impact Factor: 1.196
Times cited: 1
DOI: 10.1016/S0065-3276(03)44019-7
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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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“Molecular structure and orientational ordering in solid C60”. Michel KH, The journal of chemical physics 97, 5155 (1992). http://doi.org/10.1063/1.463813
Abstract: A microscopic theory, which describes the orientational dynamics of C60 molecules in the face-centered cubic phase of C60-fullerite, is formulated or the case of a complex molecular structure. Interaction centers which comprise atoms, double bonds, and single bonds as molecular constituents contribute to the intermolecular potential. Orientation dependent physical properties are described in terms of symmetry-adapted rotator functions. It is found that a same set of rotator functions is sufficient even in the case of a complex molecular structure. Phase transition temperatures are discussed for various models of molecular structure.
Keywords: A1 Journal article; Condensed Matter Theory (CMT)
Impact Factor: 2.952
Times cited: 20
DOI: 10.1063/1.463813
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“Nanotube field of C60 and C70 molecules in carbon nanotubes”. Verberck B, Michel KH, International journal of quantum chemistry 107, 2294 (2007). http://doi.org/10.1002/qua.21309
Keywords: A1 Journal article; Condensed Matter Theory (CMT)
Impact Factor: 2.92
Times cited: 6
DOI: 10.1002/qua.21309
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“Superexchange and electron correlations in alkali fullerides AC60, A=K, Rb, Cs”. Nikolaev AV, Michel KH, The journal of chemical physics 122, 064310 (2005). http://doi.org/10.1063/1.1844491
Abstract: Superexchange interactions in alkali fullerides AC(60) are derived for C-60 molecular ions separated by interstitial alkali-metal ions. We use a multiconfiguration approach which comprises the lowest molecular orbital states of the C-60 molecule and the excited s and d states of the alkali-metal atom A. Interactions are described by the valence bond (Heitler-London) method for a complex (C-60 – A – C-60) – with two valence electrons. The electronic charge transfer between the alkali-metal atom and a neighboring C-60 molecule is not complete. The occupation probability of excited d and s states of the alkali atom is not negligible. In correspondence with the relative positions of the C-60 molecules and A atoms in the polymer crystal, we consider 180degrees and 90degrees (angle) superexchange pathways. For the former case the ground state is found to be a spin singlet separated from a triplet at similar to20 K. For T < 20 K there appear strong spin correlations for the 180degrees superexchange pathway. The results are related to spin lattice relaxation experiments on CsC60 in the polymerized and in the quenched cubic phase. (C) 2005 American Institute of Physics.
Keywords: A1 Journal article; Condensed Matter Theory (CMT)
Impact Factor: 2.965
Times cited: 11
DOI: 10.1063/1.1844491
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“Superposition of quantum and classical rotational motions in Sc2C2@C84 fullerite”. Michel KH, Verberck B, Hulman M, Kuzmany H, Krause M, The journal of chemical physics 126, 064304 (2007). http://doi.org/10.1063/1.2434175
Keywords: A1 Journal article; Condensed Matter Theory (CMT)
Impact Factor: 2.965
Times cited: 14
DOI: 10.1063/1.2434175
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“Theory of distinct crystal structures of polymerized fullerides AC60, A=K, Rb, Cs: the specific role of alkalis”. Michel KH, Nikolaev AV, Physical review letters 85, 3197 (2000). http://doi.org/10.1103/PhysRevLett.85.3197
Keywords: A1 Journal article; Condensed Matter Theory (CMT)
Impact Factor: 8.462
Times cited: 16
DOI: 10.1103/PhysRevLett.85.3197
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“Piezoelectricity in two-dimensional materials : comparative study between lattice dynamics and ab initio calculations”. Michel KH, Çakir D, Sevik C, Peeters FM, Physical review B 95, 125415 (2017). http://doi.org/10.1103/PHYSREVB.95.125415
Abstract: The elastic constant C-11 and piezoelectric stress constant e(1),(11) of two-dimensional (2D) dielectric materials comprising h-BN, 2H-MoS2, and other transition-metal dichalcogenides and dioxides are calculated using lattice dynamical theory. The results are compared with corresponding quantities obtained with ab initio calculations. We identify the difference between clamped-ion and relaxed-ion contributions with the dependence on inner strains which are due to the relative displacements of the ions in the unit cell. Lattice dynamics allows us to express the inner-strain contributions in terms of microscopic quantities such as effective ionic charges and optoacoustical couplings, which allows us to clarify differences in the piezoelectric behavior between h-BN and MoS2. Trends in the different microscopic quantities as functions of atomic composition are discussed.
Keywords: A1 Journal article; Condensed Matter Theory (CMT)
Impact Factor: 3.836
Times cited: 29
DOI: 10.1103/PHYSREVB.95.125415
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“Sound waves and flexural mode dynamics in two-dimensional crystals”. Michel KH, Scuracchio P, Peeters FM, Physical review B 96, 094302 (2017). http://doi.org/10.1103/PHYSREVB.96.094302
Abstract: Starting from a Hamiltonian with anharmonic coupling between in-plane acoustic displacements and outof-plane (flexural) modes, we derived coupled equations of motion for in-plane displacements correlations and flexural mode density fluctuations. Linear response theory and time-dependent thermal Green's functions techniques are applied in order to obtain different response functions. As external perturbations we allow for stresses and thermal heat sources. The displacement correlations are described by a Dyson equation where the flexural density distribution enters as an additional perturbation. The flexural density distribution satisfies a kinetic equation where the in-plane lattice displacements act as a perturbation. In the hydrodynamic limit this system of coupled equations is at the basis of a unified description of elastic and thermal phenomena, such as isothermal versus adiabatic sound motion and thermal conductivity versus second sound. The general theory is formulated in view of application to graphene, two-dimensional h-BN, and 2H-transition metal dichalcogenides and oxides.
Keywords: A1 Journal article; Condensed Matter Theory (CMT)
Impact Factor: 3.836
Times cited: 6
DOI: 10.1103/PHYSREVB.96.094302
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“Ab initio approach to superexchange interactions in alkali doped fullerides AC60”. Nikolaev AV, Michel KH, AIP conference proceedings
T2 –, 18th International Winterschool/Euroconference on Electronic Properties, of Novel Materials, MAR 06-JUN 13, 2004, Kirchberg, AUSTRIA , 393 (2004). http://doi.org/10.1063/1.1812115
Abstract: The superexchange interactions between the fullerenes arise as a result of the electron transfer from the C-60 molecule to the alkali atom and back. We present a scheme, which is a configuration interaction approach based on the valence bond (Heitler-London) method. The effect of superexchange is described together with chemical bonding by constructing and solving a secular equation, rather than by using a perturbation treatment. We have considered 180degrees and 90degrees superexchange for the C-60 Cs-C-60 pathways. The calculations account for unusual electronic properties of polymer orthorhombic and quenched cubic phases of CsC60: two lines in nuclear magnetic resonance experiments, the development of a spin-singlet ground state and a decrease of magnetic susceptibility as T-->0.
Keywords: P1 Proceeding; Condensed Matter Theory (CMT)
DOI: 10.1063/1.1812115
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“Coupling of orientational and translational modes in solid C60 and C70”. Lamoen D, Michel KH, Phase transitions 67, 789 (1999). http://doi.org/10.1080/01411599908224511
Keywords: A1 Journal article; Condensed Matter Theory (CMT)
Impact Factor: 1.06
Times cited: 2
DOI: 10.1080/01411599908224511
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“Molecular terms and optical transitions of C60n+/- molecular ions”. Nikolaev AV, Michel KH, AIP conference proceedings
T2 –, 16th International Winterschool on Electronic Properties of Novel, Materials, MAR 02-09, 2002, KIRCHBERG, AUSTRIA , 417 (2002)
Abstract: We have studied the molecular energy terms of the hole configurations (h(u)(+))(m), m=2,3,4,5 of C-60(m+) cations and the electronic configurations (t(1u))(n) n=2,3,4, as well as (t(1u))(n-1)t(1g) of the C-60(n-) anions. The lowest terms (within an energy span of 0.03 eV) for C-60(2+) are three triplets T-3(1g), (3)G(g), T-3(2g) and for C-60(3+) are three quartets T-4(1u), (4)G(u), T-4(2u), which favor Jahn-Teller distortions. For the ground state of C-60(2-) we find a triplet T-3(1g) in agreement with Hund's rules. Our method takes into account intramolecular direct and exchange multipolar Coulomb interactions.
Keywords: P1 Proceeding; Condensed Matter Theory (CMT)
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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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“Orientational disorder and order in C60-fullerite and in MC60-alkali metal fullerides”. Nikolaev AV, Michel KH, Copley JRD Plenum Press, New York, page 183 (1999).
Keywords: H3 Book chapter; Condensed Matter Theory (CMT)
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“Proton tunneling and nonlinear polarizability effects in hydrogen-bonded ferroelectrics”. Bussmann-Holder A, Michel KH, AIP conference proceedings
T2 –, 5th Williamsburg Workshop on 1st-Principles Calculations for, Ferroelectrics, FEB 01-04, 1998, WILLIAMSBURG, VA , 202 (1998)
Abstract: Hydrogen-bonded ferroelectrics are modelled by a coupled spin/nonlinear lattice (polarizability) interaction Hamiltonian, where specifically the geometry of the hydrogen bond is included. The model leads to a structural phase transition and describes correctly the isotope effect due to the substitution H/D in hydrogen-bonded systems in terms of bond length changes.
Keywords: P1 Proceeding; 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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“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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