A Ceulemans

Publications (65)141.08 Total impact

• Article: A study of the electronic properties of Au nanowires and Au nanoislands on Au(111) surfaces.

  K Schouteden, E Lijnen, D A Muzychenko, A Ceulemans, Liviu F Chibotaru, P Lievens, C Van Haesendonck
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  ABSTRACT: By means of ion bombardment of clean Au(111) films, atomically flat nanoparticles of various shapes and sizes were created, ranging from several tens of nm(2) down to only a few nm(2). Both two-dimensional Au islands as well as one-dimensional Au nanowire-like structures have been investigated by means of low-temperature scanning tunneling microscopy and spectroscopy. We were able to probe their local electronic structure in a broad energy range, which was found to be dominated by pronounced size-dependent confinement effects. Mapping of the local density of states revealed complex standing wave patterns that arise due to interference of scattered Au surface state electrons at the edges of the Au nanoparticles. The observed phenomena could be modeled with high accuracy by theoretical particle-in-a-box calculations based on a variational method that can be applied to '2D boxes' of arbitrary polygonal shape and that we have previously successfully applied to explain the electronic wave patterns on Co islands on Au(111). Our findings support the general validity of this particle-in-a-box model.
  Nanotechnology 10/2009; 20(39):395401. · 3.98 Impact Factor
• Article: Double toroids as model systems for carbon nanotube junctions: through-bond currents

  E. Lijnen, A. Ceulemans, M. V. Diudea, Cs. L. Nagy
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  ABSTRACT: We show that the through-bond currents in a closed molecular network originate from their topologically invariant edge-homologies. When applied to double toroidal structures they give rise to topologically induced currents whose 3D manifestations are highly sensitive to the way both loops are merged together i.e. on the nature of their junction.
  Journal of Mathematical Chemistry 01/2009; 45(2):417-430. · 1.30 Impact Factor
• Source

  Available from: kuleuven.be

  Article: Unique definition of the Zeeman-splitting g tensor of a Kramers doublet.

  L F Chibotaru, A Ceulemans, H Bolvin
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  ABSTRACT: It has been widely accepted in the past that the g tensor describing the Zeeman splitting of a Kramers doublet is not uniquely defined. Here we show that with two basic requirements, (i) the invariance of the Zeeman Hamiltonian under symmetry transformations and (ii) its continuous change with the variations of the parameters of the system (geometry and crystal field), a unique determination of the elements of the g tensor is achieved.
  Physical Review Letters 08/2008; 101(3):033003. · 7.37 Impact Factor
• Article: The permutational symmetry of the icosahedral orbital quintuplet and its implication for vibronic interactions

  E. Lijnen, A. Ceulemans
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  ABSTRACT: The vibronic coupling between the fivefold degenerate orbital representation of the icosahedral point group and vibrational modes which are also fivefold degenerate is characterised by the existence of two equisymmetric but independent channels. It is shown that this coupling multiplicity of the quintuplet Jahn-Teller Hamiltonian can be resolved using the symmetric group of permutations of six elements, S6, of which the group of rotations of an icosahedron is a subgroup. After the application of this symmetry in atomic physics by Judd and Lo (Phys. Rev. Lett., 82 (1999) 3224), this report forms the first molecular application of this extension of the icosahedral symmetry group.
  EPL (Europhysics Letters) 11/2007; 80(6):67006. · 2.17 Impact Factor
• Source

  Available from: ArXiv

  Article: Thermodynamically stable noncomposite vortices in mesoscopic two-gap superconductors

  L F Chibotaru, V. H. Dao, A Ceulemans
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  ABSTRACT: In mesoscopic two-gap superconductors with sizes of the order of the coherence length noncomposite vortices are found to be thermodynamically stable in a large domain of the $T - H$ phase diagram. In these phases the vortex cores of one condensate are spatially separated from the other condensate ones, and their respective distributions can adopt distinct symmetries. The appearance of these vortex phases is caused by a non-negligible effect of the boundary of the sample on the superconducting order parameter and represents therefore a genuine mesoscopic effect. For low values of interband Josephson coupling vortex patterns with $L_1 \neq L_2$ can arise in addition to the phases with $L_1 =L_2$, where $L_1$ and $L_2$ are total vorticities in the two condensates. The calculations show that noncomposite vortices could be observed in thin mesoscopic samples of MgB$_{2}$. Comment: 5 pages, 3 figures, to be published in Europhysics Letters
  04/2007;
• Source

  Available from: kuleuven.be

  Article: Nucleation of superconductivity in a mesoscopic rectangle

  G. Teniers, L. F. Chibotaru, A. Ceulemans, V. V. Moshchalkov
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  ABSTRACT: We have studied the nucleation of superconductivity in a mesoscopic rectangle. We used an analytical gauge transformation for the vector potential A which gives An = 0 for the normal component along the boundary of the rectangle. Consequently, the linearized Ginzburg-Landau equation is reduced to an eigenvalue problem in the basis set of functions obeying the Neumann boundary condition. Through the application of this technique we are able to accurately determine the field-temperature superconducting phase boundary together with the corresponding vortex patterns. A range of aspect ratios for the rectangle has been investigated and compared with a superconducting square (aspect ratio = 1) and with a superconducting line (aspect ratio = ∞). This also allows us to determine the stability of the vortex patterns with an anti-vortex in the centre, which have been predicted for a superconducting square, with respect to the deformation of the square.
  EPL (Europhysics Letters) 01/2007; 63(2):296. · 2.17 Impact Factor
• Article: Homology groups, symmetry representations and polyhedral clusters

  A. Ceulemans, M. Szopa, P. W. Fowler
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  ABSTRACT: Physical properties of cages and clusters obey symmetry rules that are extensions of the celebrated Euler-Poincaré theorem on polyhedra. A connection is established between this result and a fundamental topological relationship in the theory of homology groups. The connection allows us to assign symmetry representations to physically relevant topological invariants. The results are illustrated by a derivation of the symmetries of the low-lying empty orbitals in leapfrog fullerenes.
  EPL (Europhysics Letters) 01/2007; 36(9):645. · 2.17 Impact Factor
• Source

  Available from: Martino Lorenzini

  Article: Vorticity quantum numbers for confined electrons

  L. F. Chibotaru, A. Ceulemans, M. Lorenzini, V. V. Moshchalkov
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  ABSTRACT: The eigenstates of a confined particle in an applied magnetic field are characterized by vorticity quantum numbers, defined in a conventional way as gradient of phase integrals on the contour along the boundary line of the sample. The energy spectrum of the problem can be rationalized in terms of diabatic states corresponding to definite vorticity numbers in the whole range of applied fields. The vorticity number of a given state equals the sum of winding numbers of separate, spatially resolved vortices. The vortex patterns evolution shows some surprising similarities with mesoscopic superconductors.
  EPL (Europhysics Letters) 01/2007; 63(2):159. · 2.17 Impact Factor
• Source

  Available from: kuleuven.be

  Article: Vortices and their relation to ring currents and magnetic moments in nanographenes in high magnetic field.

  S Compernolle, L F Chibotaru, A Ceulemans
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  ABSTRACT: Much attention has been paid to the role of vortices in the magnetic response properties of superconductors, but less so for molecular systems. Here we present a theoretical analysis on nanographenes subject to a strong homogeneous magnetic field. The analysis is based on the simple Hückel-London model, for which we derive the topological definition of vorticity. The results are confirmed by a more elaborate model that includes nonnearest neighbor interaction, the explicit presence of nuclei and all terms due to the magnetic field. We find that due to frontier orbital intersections, large changes in magnetic dipole moments occur. Orbital energy minima and maxima can be related to change of vortex patterns with flux.
  The Journal of Physical Chemistry B 11/2006; 110(39):19340-51. · 3.70 Impact Factor
• Source

  Available from: Steven Compernolle

  Article: Novel type of magnetic response in carbon nanomaterials

  S Compernolle, L F Chibotaru, A Ceulemans
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  ABSTRACT: We report quantum chemistry investigations of the magnetic response of large polycyclic hydrocarbons to a high magnetic field. In strong fields the magnetisation vs. field of nanosize molecules becomes strongly non-monotonic. For nanographenes containing thou-sands of atoms this effect develops at ca. 10 1 –10 2 T. It is related to the motion of vortices in the frontier molecular orbitals and trans-formations of ring current patterns. The described phenomenon is general, in principle observable in all conjugated planar nanosize molecules.
  07/2006;
• Chapter: The epikernel principle

  A. Ceulemans, L. G. Vanquickenborne
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  ABSTRACT: An epikernel is an intermediate subgroup in the decomposition scheme of a given point group. The epikernel principle states that the preferred distortions of Jahn-Teller unstable molecules are directed towards the maximal allowed epikernels of the undistorted parent group. The group theoretical foundations of this principle are explained, and a wide variety of applications in different areas of chemistry is discussed.
  07/2006: pages 125-159;
• Source

  Available from: S. Clima

  Article: [Mo2(CN)11]:5- A detailed description of ligand-field spectra and magnetic properties by first-principles calculations.

  Marc F A Hendrickx, S Clima, L F Chibotaru, A Ceulemans
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  ABSTRACT: An ab initio multiconfigurational approach has been used to calculate the ligand-field spectrum and magnetic properties of the title cyano-bridged dinuclear molybdenum complex. The rather large magnetic coupling parameter J for a single cyano bridge, as derived experimentally for this complex by susceptibility measurements, is confirmed to a high degree of accuracy by our CASPT2 calculations. Its electronic structure is rationalized in terms of spin-spin coupling between the two constituent hexacyano-monomolybdate complexes. An in-depth analysis on the basis of Anderson's kinetic exchange theory provides a qualitative picture of the calculated CASSCF antiferromagnetic ground-state eigenvector in the Mo dimer. Dynamic electron correlations as incorporated into our first-principles calculations by means of the CASPT2 method are essential to obtain quantitative agreement between theory and experiment.
  The Journal of Physical Chemistry A 11/2005; 109(39):8857-64. · 2.95 Impact Factor
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  Available from: lirias.kuleuven.be

  Article: Ginzburg–Landau description of confinement and quantization effects in mesoscopic superconductors

  L. F. Chibotaru, A. Ceulemans, M. Morelle, G. Teniers, C. Carballeira, V. V. Moshchalkov
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  ABSTRACT: An approach to the Ginzburg–Landau problem for superconducting regular polygons is developed making use of an analytical gauge transformation for the vector potential A which gives An = 0 for the normal component along the boundary line of different symmetric polygons. As a result the corresponding linearized Ginzburg–Landau equation reduces to an eigenvalue problem in the basis set of functions obeying Neumann boundary condition. Such basis sets are found analytically for several symmetric structures. The proposed approach allows for accurate calculations of the order parameter distributions at low calculational cost (small basis sets) for moderate applied magnetic fields. This is illustrated by considering the nucleation of superconductivity in squares, equilateral triangles and rectangles, where vortex patterns containing antivortices are obtained on the Tc–H phase boundary. The calculated phase boundaries are compared with the experimental Tc(H) curves measured for squares, triangles, disks, rectangles, and loops. The stability of the symmetry consistent solutions against small deviations from the phase boundary line deep into the superconducting state is investigated by considering the full Ginzburg–Landau functional. It is shown that below the nucleation temperature symmetry-switching or symmetry-breaking phase transitions can take place. The symmetry-breaking phase transition has the same structure as the pseudo-Jahn-Teller instability of high symmetry nuclear configurations in molecules. The existence of these transitions is predicted to be strongly dependent on the size of the samples.
  Journal of Mathematical Physics 09/2005; 46(9):095108-095108-48. · 1.29 Impact Factor
• Article: Frontier orbitals of trivalent cages: (3,6) cages and (4,6) cages.

  S Compernolle, A Ceulemans
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  ABSTRACT: A qualitative molecular-orbital treatment and group-theoretical analysis reveals the nature of the frontier orbitals of (3,6) and (4,6) polyhedral cages, consisting of a hexagonal network with triangular and square defects, respectively. Leapfrog (3,6) cages have two nonbonding filled orbitals. Leapfrog (4,6) cages have a high HOMO-LUMO gap, while nonleapfrog (4,6) cages with octahedral symmetry have a very small HOMO-LUMO gap. The symmetry of the frontier orbitals is determined.
  The Journal of Physical Chemistry A 04/2005; 109(11):2689-97. · 2.95 Impact Factor
• Article: Pseudo Jahn-Teller mechanism for symmetry-breaking phase transitions in vortex molecules

  L. F. Chibotaru, G. Teniers, A. Ceulemans, V. V. Moshchalkov
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  ABSTRACT: The phase transitions from a symmetric superconducting order parameter to a different or broken symmetry phase are investigated for thin mesoscopic superconductors well below the nucleation temperature. By using an effective two-state model the mechanism of these transitions has been revealed and four distinct types of transitions have been found. The symmetry-breaking phase transition has the same structure as the pseudo Jahn-Teller instability of high symmetry nuclear configurations in molecules. This analogy provides an interesting connection between real and vortex molecules. The existence of broken-symmetry phases is predicted to be strongly dependent on the size of the samples.
  Phys. Rev. B. 09/2004; 70(9).
• Article: The tetrakisoctahedral group of the Dyck graph and its molecular realization†

  A. Ceulemans, E. Lijnen, L. J. Ceulemans, P. W. Fowler
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  ABSTRACT: The group of automorphisms of the 32-vertex Dyck graph is identified as the tetrakisoctahedral group, 4 O. This group has 96 elements and conserves orientation on the standard embedding of the Dyck graph on a surface of genus 3, consisting of 12 octagons. An alternative regular map of the Dyck graph on a torus is found, which is made up of 16 hexagons. Orientation on this surface is conserved by another group of 96 elements, 4 Th , which is non-isomorphic to 4 O. The subgroup structures of 4 O and 4 Th are derived, and character tables of 4 O and some of its subgroups are constructed. The symmetry representations of the Dyck graph and its topological dual are determined. Finally a molecular realization of the Dyck graph on the genus-3 ‘Plumber's nightmare’ is proposed, which can be considered as a new type of octagonal carbon network.
  Molecular Physics 06/2004; 102(11-12):1149-1163. · 1.82 Impact Factor
• Article: Assignment of the electronic spectra of [Mo(CN)(8)](4)(-) and [W(CN)(8)](4)(-) by Ab initio calculations.

  M F A Hendrickx, V S Mironov, L F Chibotaru, A Ceulemans
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  ABSTRACT: CASPT2 calculations are performed on the dodecahedral and square antiprismatic isomers of the [Mo(CN)(8)](4)(-) and [W(CN)(8)](4)(-) complexes. The high-energy experimental bands above 40000 cm(-)(1) are assigned to MLCT transitions. The experimental observed trend of the extinction coefficients for the molybdenum and tungsten complex is reproduced by our CASSCF oscillator strengths. All bands below 40000 cm(-)(1) can be ascribed to ligand-field transitions, although small contributions from forbidden MLCT transitions cannot be excluded. In order to account for all experimental bands in the electronic spectrum of these octacyanocomplexes, a dynamic equilibrium in solution between the two isomeric forms must be hypothesized. Spin-orbit coupling effects are found to be more important for the square antiprismatic isomers; in particular, large singlet-triplet mixings are calculated for this isomer of [W(CN)(8)](4)(-). Ligand-field and Racah parameters as well as spin-orbit coupling constants are determined on the basis of the calculated transition energies. The obtained values for these parameters support the recently proposed model for exchange interactions in magnetic clusters and networks containing pentavalent octocyanometalates of molybdenum and tungsten.
  Inorganic Chemistry 06/2004; 43(10):3142-50. · 4.60 Impact Factor
• Source

  Available from: Steven Compernolle

  Article: Eigenstates and transmission coefficients of finite-sized carbon nanotubes

  S Compernolle, L Chibotaru, A Ceulemans
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  ABSTRACT: The tight-binding eigenstates of isolated finite-sized zigzag and armchair nanotubes are obtained analytically using the transfer matrix method. Edge states are encountered for zigzag tubes but not for armchair tubes. Inclusion of curvature leads to a decaying-oscillating behavior of the highest occupied and lowest unoccupied molecular orbital gap in armchair tubes as a function of length. For zigzag tubes the inclusion of curvature induces the conversion of an extended state at nonbonding energy to an edge state. In addition the transmission coefficient of zigzag and armchair tubes with featureless leads is analytically obtained. The existence of a transmission peak at the Fermi level that decays exponentially with nanotube length for zigzag tubes is explained. © 2003 American Institute of Physics.
  The Journal of Chemical Physics 08/2003; 119:2854. · 3.33 Impact Factor
• Article: The electronic structure and spectrum of Mo(CN)8(3-).

  M F A Hendrickx, L F Chibotaru, A Ceulemans
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  ABSTRACT: State of the art CASSCF and CASPT2 calculations have been performed to elucidate the nature of the electronic transitions observed in the experimental spectrum of the octacyanomolybdate(V) cation. Assuming a triangular dodecahedral structure for this complex gives a convincing agreement between theory and experiment. All absorption bands are assigned to low-lying charge-transfer transitions involving excitations from ligand orbitals to 4dx2-y2. The calculated molecular orbitals reveal weak pi interactions between the metal and ligand orbitals, compared to much stronger sigma interactions. This calculated electronic structure substantiates the previous hypothesis concerning the giant spin ground states of magnetic clusters and networks containing Mo(CN)8(3-) as a constituent part.
  Inorganic Chemistry 02/2003; 42(2):590-7. · 4.60 Impact Factor
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  Available from: lirias.kuleuven.be

  Article: Vector potential gauge for superconducting regular polygons

  L.F. Chibotaru, A. Ceulemans, G. Teniers, V. Bruyndoncx, V.V. Moshchalkov
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  ABSTRACT: An approach to the Ginzburg-Landau problem of superconducting polygons is developed, based on the exact fulfillment of superconducting boundary conditions along the boundary of the sample. To this end an analytical gauge transformation for the vector potential A is found which gives A n = 0 for the normal component along the boundary line of an arbitrary regular polygon. The use of the new gauge reduces the Ginzburg-Landau problem of superconducting polygons in external magnetic fields to an eigenvalue problem in a basis set of functions obeying Neumann boundary conditions. The advantages of this approach, especially for low magnetic fields, are illustrated and novel vortex patterns are obtained which can be probed experimentally.
  Physics of Condensed Matter 05/2002; 27(3):341-346. · 1.53 Impact Factor