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See detailDipole-induced vortex ratchets in superconducting films with arrays of micromagnets
de Souza Silva, C. C.; Silhanek, Alejandro ULg; Van de Vondel, J. et al

in Physical Review Letters (2007), 98(11),

We investigate the transport properties of superconducting films with periodic arrays of in-plane magnetized micromagnets. Two different magnetic textures are studied: a square array of magnetic bars and ... [more ▼]

We investigate the transport properties of superconducting films with periodic arrays of in-plane magnetized micromagnets. Two different magnetic textures are studied: a square array of magnetic bars and a close-packed array of triangular microrings. As confirmed by magnetic force microscopy imaging, the magnetic state of both systems can be adjusted to produce arrays of almost pointlike magnetic dipoles. By carrying out transport measurements with ac drive, we observed experimentally a recently predicted ratchet effect induced by the interaction between superconducting vortices and the magnetic dipoles. Moreover, we find that these magnetic textures produce vortex-antivortex patterns, which have a crucial role in the transport properties of this hybrid system. [less ▲]

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See detailDisplacement echoes: Classical decay and quantum freeze
Petitjean, Cyril ULg; Bevilaqua, Diego V.; Heller, Eric J. et al

in PHYSICAL REVIEW LETTERS (2007), 98(16), 164101-4

Motivated by neutron scattering experiments, we investigate the decay of the fidelity with which a wave packet is reconstructed by a perfect time-reversal operation performed after a phase-space ... [more ▼]

Motivated by neutron scattering experiments, we investigate the decay of the fidelity with which a wave packet is reconstructed by a perfect time-reversal operation performed after a phase-space displacement. In the semiclassical limit, we show that the decay rate is generically given by the Lyapunov exponent of the classical dynamics. For small displacements, we additionally show that, following a short-time Lyapunov decay, the decay freezes well above the ergodic value because of quantum effects. Our analytical results are corroborated by numerical simulations. [less ▲]

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See detailResonance-assisted decay of nondispersive wave packets
Wimberger, Sandro; Schlagheck, Peter ULg; Eltschka, Christopher et al

in Physical Review Letters (2006), 97(4),

We present a quantitative semiclassical theory for the decay of nondispersive electronic wave packets in driven, ionizing Rydberg systems. Statistically robust quantities are extracted combining resonance ... [more ▼]

We present a quantitative semiclassical theory for the decay of nondispersive electronic wave packets in driven, ionizing Rydberg systems. Statistically robust quantities are extracted combining resonance-assisted tunneling with subsequent transport across chaotic phase space and a final ionization step. [less ▲]

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See detailDirect experimental evidence for atomic tunneling of europium in crystalline Eu8Ga16Ge30
Hermann, Raphaël ULg; Keppens, Veerle; Bonville, Pierre et al

in Physical Review Letters (2006), 97(1),

Mossbauer-effect and microwave absorption experimental evidence unambiguously demonstrates the presence of slow, similar to 450 MHz, tunneling of magnetic europium between four equivalent sites in ... [more ▼]

Mossbauer-effect and microwave absorption experimental evidence unambiguously demonstrates the presence of slow, similar to 450 MHz, tunneling of magnetic europium between four equivalent sites in Eu8Ga16Ge30, a stoichiometric clathrate. Remarkably, six of the eight europium atoms, or 11% of the constituents in this solid, tunnel between these four sites separated by 0.55 angstrom. The off centering of the atoms or ions in crystalline clathrates appears to be a promising route for producing Rabi oscillators in solid-state materials. [less ▲]

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See detailQuantum reversibility and echoes in interacting systems
Petitjean, Cyril ULg; Jacquod, Ph

in PHYSICAL REVIEW LETTERS (2006), 97(12), 124103-4

In echo experiments, imperfect time-reversal operations are performed on a subset of the total number of degrees of freedom. To capture the physics of these experiments, we introduce a partial fidelity M ... [more ▼]

In echo experiments, imperfect time-reversal operations are performed on a subset of the total number of degrees of freedom. To capture the physics of these experiments, we introduce a partial fidelity M(B)(t), the Boltzmann echo, where only part of the system's degrees of freedom can be time reversed. We present a semiclassical calculation of M(B)(t). We show that, as the time-reversal operation is performed more and more accurately, the decay rate of M(B)(t) saturates at a value given by the decoherence rate of the controlled degrees of freedom due to their coupling to uncontrolled ones. We connect these results with NMR spin echo experiments. [less ▲]

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See detailLyapunov generation of entanglement and the correspondence principle
Petitjean, Cyril ULg; Jacquod, Ph

in PHYSICAL REVIEW LETTERS (2006), 97(19), 194103-4

We show how a classically vanishing interaction generates entanglement between two initially nonentangled particles, without affecting their classical dynamics. For chaotic dynamics, the rate of ... [more ▼]

We show how a classically vanishing interaction generates entanglement between two initially nonentangled particles, without affecting their classical dynamics. For chaotic dynamics, the rate of entanglement is shown to saturate at the Lyapunov exponent of the classical dynamics as the interaction strength increases. In the saturation regime, the one-particle Wigner function follows classical dynamics better and better as one goes deeper and deeper in the semiclassical limit. This demonstrates that quantum-classical correspondence at the microscopic level does not require coupling to a large number of external degrees of freedom. [less ▲]

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See detailIrreversible dynamics of the phase boundary in U(Ru(0.96)Rh(0.04))(2)Si(2) and implications for ordering
Silhanek, Alejandro ULg; Jaime, M.; Harrison, N. et al

in Physical Review Letters (2006), 96(13),

We report measurements and analysis of the specific heat and magnetocaloric effect-induced temperature changes at the phase boundary into the single magnetic field-induced phase (phase II) of U(Ru(0.96)Rh ... [more ▼]

We report measurements and analysis of the specific heat and magnetocaloric effect-induced temperature changes at the phase boundary into the single magnetic field-induced phase (phase II) of U(Ru(0.96)Rh(0.04))(2)Si(2), which yield irreversible properties similar to those at the valence transition of Yb(1-x)Y(x)InCu(4). To explain these similarities, we propose a bootstrap mechanism by which lattice parameter changes caused by an electric quadrupolar order parameter within phase II become coupled to the 5f-electron hybridization, giving rise to a valence change at the transition. [less ▲]

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See detailNonlocal magnetic field-tuned quantum criticality in cubic CeIn3-xSnx (x=0.25)
Silhanek, Alejandro ULg; Ebihara, Takao; Harrison, N. et al

in Physical Review Letters (2006), 96(20),

We show that antiferromagnetism in lightly (approximate to 8%) Sn-doped CeIn3 terminates at a critical field mu H-0(c)=42 +/- 2 T. Electrical transport and thermodynamic measurements reveal the effective ... [more ▼]

We show that antiferromagnetism in lightly (approximate to 8%) Sn-doped CeIn3 terminates at a critical field mu H-0(c)=42 +/- 2 T. Electrical transport and thermodynamic measurements reveal the effective mass m(*) not to diverge, suggesting that cubic CeIn3 is representative of a critical spin-density wave (SDW) scenario, unlike the local quantum critical points reported in anisotropic systems such as CeCu6-xAux and YbRh2Si2-xGex. The existence of a maximum in m(*) at a lower field mu H-0(x)=30 +/- 1 T may be interpreted as a field-induced crossover from local moment to SDW behavior as the Neel temperature falls below the Fermi temperature. [less ▲]

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See detailUnusual behavior of the ferroelectric polarization in PbTiO3/SrTiO3 superlattices
Dawber, M.; Lichtensteiger, C.; Cantoni, M. et al

in Physical Review Letters (2005), 95

Artificial PbTiO3/SrTiO3 superlattices were constructed using off-axis rf magnetron sputtering. X-ray diffraction and piezoelectric atomic force microscopy were used to study the evolution of the ... [more ▼]

Artificial PbTiO3/SrTiO3 superlattices were constructed using off-axis rf magnetron sputtering. X-ray diffraction and piezoelectric atomic force microscopy were used to study the evolution of the ferroelectric polarization as the ratio of PbTiO3 to SrTiO3 was changed. For PbTiO3 layer thicknesses larger than the 3-unit cell SrTiO3 thickness used in the structure, the polarization is found to be reduced as the PbTiO3 thickness is decreased. This observation confirms the primary role of the depolarization field in the polarization reduction in thin films. For the samples with ratios of PbTiO3 to SrTiO3 of less than one, a surprising recovery of ferroelectricity that cannot be explained by electrostatic considerations was observed. [less ▲]

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See detailDynamics of a bouncing dimer
Dorbolo, Stéphane ULg; Volfson, D.; Tsimring, L. et al

in Physical Review Letters (2005), 95(4),

We investigate the dynamics of a dimer bouncing on a vertically oscillated plate. The dimer, composed of two spheres rigidly connected by a light rod, exhibits several modes depending on initial and ... [more ▼]

We investigate the dynamics of a dimer bouncing on a vertically oscillated plate. The dimer, composed of two spheres rigidly connected by a light rod, exhibits several modes depending on initial and driving conditions. The first excited mode has a novel horizontal drift in which one end of the dimer stays on the plate during most of the cycle, while the other end bounces in phase with the plate. The speed and direction of the drift depend on the aspect ratio of the dimer. We employ event-driven simulations based on a detailed treatment of frictional interactions between the dimer and the plate in order to elucidate the nature of the transport mechanism in the drift mode. [less ▲]

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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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