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See detailExperimental study of granular compaction dynamics at different scales: Grain mobility, hexagonal domains, and packing fraction
Lumay, Geoffroy ULg; Vandewalle, Nicolas ULg

in Physical Review Letters (2005), 95(2), 28002

We present an original experimental study of the compaction dynamics for two-dimensional granular systems. Compaction dynamics is measured at three different scales: the macroscopic scale through the ... [more ▼]

We present an original experimental study of the compaction dynamics for two-dimensional granular systems. Compaction dynamics is measured at three different scales: the macroscopic scale through the normalized packing fraction rho, the mesoscopic scale through the normalized fraction phi of hexagonal domains in the system, and the microscopic scale through the grain mobility mu. Moreover, the hexagonal domains are found to obey a growth process dominated by the displacement of domain boundaries. A global picture of compaction dynamics relevant at each scale is proposed. [less ▲]

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See detailNonlinear resonant transport of Bose-Einstein condensates
Paul, T.; Richter, K.; Schlagheck, Peter ULg

in Physical Review Letters (2005), 94(2),

The coherent flow of a Bose-Einstein condensate through a quantum dot in a magnetic waveguide is studied. By the numerical integration of the time-dependent Gross-Pitaevskii equation in the presence of a ... [more ▼]

The coherent flow of a Bose-Einstein condensate through a quantum dot in a magnetic waveguide is studied. By the numerical integration of the time-dependent Gross-Pitaevskii equation in the presence of a source term, we simulate the propagation process of the condensate through a double barrier potential in the waveguide. We find that resonant transport is suppressed in interaction-induced regimes of bistability, where multiple scattering states exist at the same chemical potential and the same incident current. We demonstrate, however, that a temporal control of the external potential can be used to circumvent this limitation and to obtain enhanced transmission near the resonance on experimentally realistic time scales. [less ▲]

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See detailResonance- and chaos-assisted tunneling in mixed regular-chaotic systems
Eltschka, C.; Schlagheck, Peter ULg

in Physical Review Letters (2005), 94(1), 014101

We present evidence that nonlinear resonances govern the tunneling process between symmetry-related islands of regular motion in mixed regular-chaotic systems. In a similar way as for near-integrable ... [more ▼]

We present evidence that nonlinear resonances govern the tunneling process between symmetry-related islands of regular motion in mixed regular-chaotic systems. In a similar way as for near-integrable tunneling, such resonances induce couplings between regular states within the islands and states that are supported by the chaotic sea. On the basis of this mechanism, we derive a semiclassical expression for the average tunneling rate, which yields good agreement in comparison with the exact quantum tunneling rates calculated for the kicked rotor and the kicked Harper. [less ▲]

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See detailFerroelectricity and tetragonality in ultrathin PbTiO3 films
Lichtensteiger, C.; Triscone, J. M.; Junquera, J. et al

in Physical Review Letters (2005), 94

The evolution of tetragonality with thickness has been probed in epitaxial c-axis oriented PbTiO3 films with thicknesses ranging from 500 down to 24 Angstrom. High resolution x ray pointed out a ... [more ▼]

The evolution of tetragonality with thickness has been probed in epitaxial c-axis oriented PbTiO3 films with thicknesses ranging from 500 down to 24 Angstrom. High resolution x ray pointed out a systematic decrease of the c-axis lattice parameter with decreasing film thickness below 200 Angstrom. Using a first-principles model Hamiltonian approach, the decrease in tetragonality is related to a reduction of the polarization attributed to the presence of a residual unscreened depolarizing field. It is shown that films below 50 Angstrom display a significantly reduced polarization but still remain ferroelectric. [less ▲]

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See detailFrom DNA sequence analysis to modeling replication in the human genome
Brodie of Brodie, E. B.; Nicolay, Samuel ULg; Touchon, M. et al

in Physical Review Letters (2005), 94

We explore the large-scale behavior of nucleotide compositional strand asymmetries along hum chromosomes. As we observe for 7 of 9 origins of replication experimentally identified so far, the (TA GC) skew ... [more ▼]

We explore the large-scale behavior of nucleotide compositional strand asymmetries along hum chromosomes. As we observe for 7 of 9 origins of replication experimentally identified so far, the (TA GC) skew displays rather sharp upward jumps, with a linear decreasing profile in between two successi jumps. We present a model of replication with well positioned replication origins and random terminatio that accounts for the observed characteristic serrated skew profiles. We succeed in identifying 287 pairs putative adjacent replication origins with an origin spacing 1­2 Mbp that are likely to correspond replication foci observed in interphase nuclei and recognized as stable structures that persist througho subsequent cell generations. [less ▲]

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See detailQuantum critical 5f electrons avoid singularities in U(Ru,Rh)(2)Si-2
Silhanek, Alejandro ULg; Harrison, N.; Batista, C. D. et al

in Physical Review Letters (2005), 95(2),

We present specific heat measurements of 4% Rh-doped URu2Si2 at magnetic fields around the proposed metamagnetic transition field H(m)similar to 34 T, revealing striking similarities to the isotructural ... [more ▼]

We present specific heat measurements of 4% Rh-doped URu2Si2 at magnetic fields around the proposed metamagnetic transition field H(m)similar to 34 T, revealing striking similarities to the isotructural Ce analog CeRu2Si2 for H > H-m. This suggests that strongly renormalized hybridized-band models apply equally well to both systems. The vanishing bandwidths as H -> H-m are consistent with a quantum-critical point close to H-m. The existence of a phase transition into an ordered phase in the vicinity of H-m for 4% Rh-doped URu2Si2, but not for CeRu2Si2, is consistent with a stronger superexchange in the case of the U 5f system. Irreversible processes at the transition indicate a strong coupling of the 5f orbitals to the lattice, most suggestive of electric quadrupolar order. [less ▲]

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See detailGrowth of carbon nanotubes on metal nanoparticles: a microscopic mechanism from ab initio molecular dynamics simulations
Raty, Jean-Yves ULg; Gygi, F.; Galli, G.

in Physical Review Letters (2005), 95(9), 0961031-09610340961034

We report on ab initio molecular dynamics simulations of the early stages of single-walled carbon nanotube (SWCNT) growth on metal nanoparticles. Our results show that a sp2 bonded cap is formed on an ... [more ▼]

We report on ab initio molecular dynamics simulations of the early stages of single-walled carbon nanotube (SWCNT) growth on metal nanoparticles. Our results show that a sp2 bonded cap is formed on an iron catalyst, following the diffusion of C atoms from hydrocarbon precursors on the nanoparticle surface. The weak adhesion between the cap and iron enables the graphene sheet to "float" on the curved surface, as additional C atoms covalently bonded to the catalyst "hold" the tube walls. Hence the SWCNT grows capped. At the nanoscale, we did not observe any tendency of C atoms to penetrate inside the catalyst, consistent with total energy calculations showing that alloying of Fe and C is very unlikely for 1 nm particles. Root growth was observed on Fe but not on Au, consistent with experiment [less ▲]

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See detailTemperature-induced density anomaly in Te-rich liquid Germanium tellurides: p versus sp 3 bonding?
Bichara, C.; Johnson, M.; Raty, Jean-Yves ULg

in Physical Review Letters (2005), 95(26), 2678011-26780142678014

The density anomaly of liquid Ge 0.15Te 0.85 measured between 633 and 733 K is investigated with ab initio molecular dynamics calculations at four temperatures and at the corresponding experimental ... [more ▼]

The density anomaly of liquid Ge 0.15Te 0.85 measured between 633 and 733 K is investigated with ab initio molecular dynamics calculations at four temperatures and at the corresponding experimental densities. For box sizes ranging from 56 to 112 atoms, an 8 k-points sampling of the Brillouin zone is necessary to obtain reliable results. Contrary to other Ge chalcogenides, no sp 3 hybridization of the Ge bonding is observed. As a consequence, the negative thermal expansion of the liquid is not related to a tetrahedral bonding as in the case of water or silica. We show that it results from the symmetry recovery of the local environment of Ge atoms that is distorted at low temperature by a Peierls-like mechanism acting in the liquid state in the same way as in the parent solid phases [less ▲]

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See detailOn the importance of an M2 depopulating channel for a KrII metastable state
Biémont, Emile ULg; Derkatch, A.; Lundin, P. et al

in Physical Review Letters (2004), 93(6),

An experimental investigation of the radiative lifetime of the metastable 4s(2)4p(4)(P-3)4d D-4(7/2) level in Kr II shows an unusual situation regarding the importance of an M2 depopulation channel. While ... [more ▼]

An experimental investigation of the radiative lifetime of the metastable 4s(2)4p(4)(P-3)4d D-4(7/2) level in Kr II shows an unusual situation regarding the importance of an M2 depopulation channel. While the first order M1 and E2 channels are expected to contribute in a dominant way to the decay, the experimental result, obtained using a laser probing technique on a stored ion beam, tau=0.57+/-0.03 s, is far too short to be due to these channels according to our relativistic multiconfiguration Dirac-Fock calculation. Only if second order contributions to the decay branches (including essentially the M2 contribution) are taken into account in the calculations could the unexpected short lifetime be explained. [less ▲]

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See detailFirst-principles study of the electro-optic effect in ferroelectric oxides
Veithen, M.; Gonze, X.; Ghosez, Philippe ULg

in Physical Review Letters (2004), 93

We first present a method to compute the electro-optic tensor from first principles, explicitly taking into account the electronic, ionic and piezoelectric contributions. We then study the nonlinear optic ... [more ▼]

We first present a method to compute the electro-optic tensor from first principles, explicitly taking into account the electronic, ionic and piezoelectric contributions. We then study the nonlinear optic behavior of three paradigmatic ferroelectric oxides. Our calculations reveal the dominant contribution of the soft mode to the electro-optic coefficients in LiNbO3 and BaTiO3 and its minor role in PbTiO3. We identify the coupling between the electric field and the polar atomic displacements along the B-O chains as the origin of the large electro-optic response in perovskite ABO(3) compounds. [less ▲]

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See detailFunctional modes of proteins are among the most robust
Nicolay, Samuel ULg; Sanejouand, Y. H.

in Physical Review Letters (2004), 96

It is shown that a small subset of modes which are likely to be involved in protein functional motions of large amplitude can be determined by retaining the most robust normal modes obtained using ... [more ▼]

It is shown that a small subset of modes which are likely to be involved in protein functional motions of large amplitude can be determined by retaining the most robust normal modes obtained using different protein models. This result should prove helpful in the context of several applications proposed recently, like for solving difficult molecular replacement problems or for fitting atomic structures into low- resolution electron density maps. It may also pave the way for the development of methods allowing us to predict such motions accurately. [less ▲]

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See detailLow frequency rythms in human DNA sequences: A key to the organization of gene location and orientation?
Nicolay, Samuel ULg; Argoul, F.; Touchon, M. et al

in Physical Review Letters (2004), 93(10), 108101

We explore large-scale nucleotide compositional fluctuations of the human genome using multi- resolution techniques. Analysis of the GC content and of the AT and GC skews reveals the existence of rhythms ... [more ▼]

We explore large-scale nucleotide compositional fluctuations of the human genome using multi- resolution techniques. Analysis of the GC content and of the AT and GC skews reveals the existence of rhythms with two main periods of 110 20 kb and 400 50 kb that enlighten a remarkable cooperative gene organization. We show that the observed nonlinear oscillations are likely to display all the characteristic features of chaotic strange attractors which suggests a very attractive deterministic picture: gene orientation and location, in relation with the structure and dynamics of chromatin, might be governed by a low-dimensional nonlinear dynamical system. [less ▲]

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See detailEinstein oscillators in thallium filled antimony skutterudites
Hermann, Raphaël ULg; Jin, R. J.; Schweika, W. et al

in Physical Review Letters (2003), 90(13),

Inelastic neutron scattering measurements on several Tl filled skutterudites (Tl0.5Co3.5Fe0.5Sb12, Tl0.8Co3FeSb12, and Tl0.8Co4Sb11Sn) all show a sharp peak in the vibrational density of states at 57 ... [more ▼]

Inelastic neutron scattering measurements on several Tl filled skutterudites (Tl0.5Co3.5Fe0.5Sb12, Tl0.8Co3FeSb12, and Tl0.8Co4Sb11Sn) all show a sharp peak in the vibrational density of states at 57 +/- 2 K, which is absent in the unfilled skutterudite CoSb3. Heat capacity measurements on Tl0.8Co4Sb11Sn as compared to CoSb3 are consistent with the presence of a localized vibrational mode associated with the "rattling" thallium atoms in this filled skutterudite compound. Both results are well described by a localized Einstein mode model with an Einstein temperature Theta(E) of 53 +/- 1 K. These data provide perhaps the clearest example of local mode behavior in a concentrated metallic system. [less ▲]

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See detailQuantum confinement and fullerenelike surface reconstructions in nanodiamonds
Raty, Jean-Yves ULg; Galli, G.; Bostedt, C. et al

in Physical Review Letters (2003), 90(3), 0374011-03740140374014

We present x-ray absorption and emission experiments and ab initio calculations showing that the size of carbon diamond must be reduced to at least 2 nm, in order to observe an increase of its optical gap ... [more ▼]

We present x-ray absorption and emission experiments and ab initio calculations showing that the size of carbon diamond must be reduced to at least 2 nm, in order to observe an increase of its optical gap, at variance with Si and Ge where quantum confinement effects persist up to 6-7 nm. In addition, our calculations show that the surface of nanodiamond particles larger than sime 1 nm reconstructs in a fullerenelike manner, giving rise to a new family of carbon clusters: bucky diamonds. Signatures of these surface reconstructions are compatible with pre-edge features observed in measured absorption spectra [less ▲]

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See detailBenchmarks for the forward observables at RHIC, the Tevatron-run II, and the LHC
Cudell, Jean-René ULg; Ezhela, Vladimir V.; Gauron, P. et al

in Physical Review Letters (2002), 89(20), 201801

We present predictions on the total cross sections and on the ratio of the real part to the imaginary part of the elastic amplitude (rho parameter) for present and future pp and pbar p colliders, and on ... [more ▼]

We present predictions on the total cross sections and on the ratio of the real part to the imaginary part of the elastic amplitude (rho parameter) for present and future pp and pbar p colliders, and on total cross sections for gamma p -> hadrons at cosmic-ray energies and for gamma gamma-> hadrons up to sqrt{s}=1 TeV. These predictions are based on an extensive study of possible analytic parametrisations invoking the biggest hadronic dataset available at t=0. The uncertainties on total cross sections, including the systematic errors due to contradictory data points from FNAL, can reach 1.9% at RHIC, 3.1% at the Tevatron, and 4.8% at the LHC, whereas those on the rho parameter are respectively 5.4%, 5.2%, and 5.4%. [less ▲]

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See detailComment on "Strong vortex liquid correlation from multiterminal measurements on untwinned YBa2Cu3O7-delta single crystals"
Maiorov, B.; Silhanek, Alejandro ULg; de la Cruz, F. et al

in Physical Review Letters (2002), 88(13),

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See detailAdsorption of selenium wires in silicalite-1 zeolite: a first order transition in a microporous system
Bichara, C.; Raty, Jean-Yves ULg; Pellenq, RJ-M

in Physical Review Letters (2002), 89(1), 016101-1-016101-016101-4016101-016101-4

A tight binding grand canonical Monte Carlo simulation of the adsorption of selenium in silicalite-1 zeolite is presented. The calculated adsorption-desorption isotherms exhibit characteristic features of ... [more ▼]

A tight binding grand canonical Monte Carlo simulation of the adsorption of selenium in silicalite-1 zeolite is presented. The calculated adsorption-desorption isotherms exhibit characteristic features of a first order transition, unexpected for adsorption in a microporous system with pore size of the order of 0.5 to 0.6 nm. We analyze this behavior as a result of the favored twofold coordinated chain structure of selenium that grows inside the complex three-dimensional microchannel network of silicalite. This analysis is confirmed by simpler calculations of a lattice gas-type model [less ▲]

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See detailTime-resolved laser spectroscopy of multiply ionized atoms: Natural radiative lifetimes in CeIV
Zhang, Z. G.; Svanberg, S.; Quinet, Pascal ULg et al

in Physical Review Letters (2001), 87(27),

Radiative lifetimes have been measured for two excited levels of Ce IV using the time-resolved laser-induced fluorescence technique. Ce3+ ions were produced in a laser-induced plasma. In the measurements ... [more ▼]

Radiative lifetimes have been measured for two excited levels of Ce IV using the time-resolved laser-induced fluorescence technique. Ce3+ ions were produced in a laser-induced plasma. In the measurements, a suitable magnetic field was applied to reduce the recombination between electrons and the ions and thus the background light from the recombination, and special care was exercised to avoid flight-out-of-view effects on the lifetime measurements for the high-velocity ions. The experimental lifetime results, tau = 30(2) ns for the level 49 737 cm(-1) and tau = 30(3) ns for the level 52 226 cm(-1), were compared with relativistic Hartree-Fock calculations (tau = 30.5 and 30.0 ns) indicating a particularly excellent agreement. [less ▲]

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See detailAvalanches of Popping Bubbles in Collapsing Foams
Vandewalle, Nicolas ULg; Lentz, J. F.; Dorbolo, Stéphane ULg et al

in Physical Review Letters (2001), 86(1), 179-82

We report acoustic measurements of popping bubbles during the collapsing of aqueous foams. The sound pattern is analyzed using classical methods of statistical physics. It is found that membrane rupture ... [more ▼]

We report acoustic measurements of popping bubbles during the collapsing of aqueous foams. The sound pattern is analyzed using classical methods of statistical physics. It is found that membrane rupture concerns a wide variety of situations: small and large membranes at the air/foam interface. Avalanches of popping bubbles are put into evidence. Time durations in between successive pops seem to be distributed on a universal power law. [less ▲]

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See detailResonance-assisted tunneling in near-integrable systems
Brodier, O.; Schlagheck, Peter ULg; Ullmo, D.

in Physical Review Letters (2001), 87(6), 064101

Dynamical tunneling between symmetry related invariant tori is studied in the near-integrable regime. Using the kicked Harper model as an illustration, we show that the exponential decay of the wave ... [more ▼]

Dynamical tunneling between symmetry related invariant tori is studied in the near-integrable regime. Using the kicked Harper model as an illustration, we show that the exponential decay of the wave functions in the classically forbidden region is modified due to coupling processes that are mediated by classical resonances. This mechanism leads to a substantial deviation of the splitting between quasidegenerate eigenvalues from the purely exponential decrease with 1/(h) over bar obtained for the integrable system. A simple semiclassical framework, which takes into account the effect of the resonance substructure on the invariant tori, allows one to quantitatively reproduce the behavior of the eigenvalue splittings. [less ▲]

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