Phases of Polonium via Density Functional Theory

in Physical Review Letters (2010), 104

Dynamics of the negative thermal expansion in tellurium based liquid alloys

in Physical Review Letters (2009), 103(24), 2459014-24590142459014

Negative thermal expansion (NTE) in tellurium based liquid alloys (GeTe 6 and GeTe 12) is analyzed through the atomic vibrational properties. Using neutron inelastic scattering, we show that the ... [more ▼]

Negative thermal expansion (NTE) in tellurium based liquid alloys (GeTe 6 and GeTe 12) is analyzed through the atomic vibrational properties. Using neutron inelastic scattering, we show that the structural evolution resulting in the NTE is due to a gain of vibrational entropy that cancels out the Peierls distortion. In the NTE temperature range, these competing effects give rise to noticeable changes in the vibrational density of states spectra. Additional first principles molecular dynamics simulations emphasize the role of the temperature dependance of the Ge atomic environment in this mechanism. For comparison, we extended our study to Ge 2Sb 2Te 5 and Ge 1Sb 2Te 4 phase-change materials. [less ▲]

Engineering Multiferroism in CaMnO3

in Physical Review Letters (2009), 102(11),

Structural instabilities of CaMnO3 are investigated from first principles. We point out that, on top of a strong antiferrodistortive instability responsible for its orthorhombic ground state, the cubic ... [more ▼]

Structural instabilities of CaMnO3 are investigated from first principles. We point out that, on top of a strong antiferrodistortive instability responsible for its orthorhombic ground state, the cubic perovskite structure of CaMnO3 also exhibits a weak ferroelectric instability. Although ferroelectricity is suppressed by antiferrodistortive motions, we show that it can be favored using strain or chemical engineering in order to make CaMnO3 multiferroic. We finally highlight that the ferroelectric instability of CaMnO3 is Mn-dominated. This illustrates that, contrary to common belief, ferroelectricity and magnetism are not necessarily exclusive but can be driven by the same cation. [less ▲]

Operational Families of Entanglement Classes for Symmetric N-Qubit States

in Physical Review Letters (2009), 103

We solve the entanglement classification under stochastic local operations and classical communication (SLOCC) for all multipartite symmetric states in the general N-qubit case. For this purpose, we ... [more ▼]

We solve the entanglement classification under stochastic local operations and classical communication (SLOCC) for all multipartite symmetric states in the general N-qubit case. For this purpose, we introduce 2 parameters playing a crucial role, namely, the diversity degree and the degeneracy configuration of a symmetric state. Those parameters give rise to a simple method of identifying operational families of SLOCC entanglement classes of all symmetric N-qubit states, where the number of families grows as the partition function of the number of qubits. [less ▲]

Thermodynamics of Intragenic Nucleosome Ordering

in Physical Review Letters (2009), 103(18), 118103

The nucleosome ordering observed in vivo along yeast genes is described by a thermodynamical model of nonuniform fluid of 1D hard rods confined by two excluding energy barriers at gene extremities. For ... [more ▼]

The nucleosome ordering observed in vivo along yeast genes is described by a thermodynamical model of nonuniform fluid of 1D hard rods confined by two excluding energy barriers at gene extremities. For interbarrier distances L≲1.5  kbp, nucleosomes equilibrate into a crystal-like configuration with a nucleosome repeat length (NRL) L/n∼165  bp, where n is the number of regularly positioned nucleosomes. We also observe “bistable” genes with a fuzzy chromatin resulting from a statistical mixing of two crystal states, one with an expanded chromatin (NRL ∼L/n) and the other with a compact one (NRL ∼L/(n+1)). By means of single nucleosome switching, bistable genes may drastically alter their expression level as suggested by their higher transcriptional plasticity. These results enlighten the role of the intragenic chromatin on gene expression regulation. [less ▲]

Chaotic bouncing of a droplet on a soap film

in Physical Review Letters (2009), 102

We examine the complex dynamics arising when a water droplet bounces on a horizontal soap film suspended on a vertically oscillating circular frame. A variety of simple and complex periodic bouncing ... [more ▼]

We examine the complex dynamics arising when a water droplet bounces on a horizontal soap film suspended on a vertically oscillating circular frame. A variety of simple and complex periodic bouncing states are observed, in addition to multiperiodicity and period-doubling transitions to chaos. The system is simply and accurately modeled by a single ordinary differential equation, the numerical solution of which captures all the essential features of the observed behavior. Iterative maps and bifurcation diagrams indicate that the system exhibits all the features of a classic low-dimensional chaotic oscillator. [less ▲]

Coherent backscattering of Bose-Einstein condensates in two-dimensional disorder potentials

in Physical Review Letters (2008), 101(2), 020603

We study quantum transport of an interacting Bose-Einstein condensate in a two-dimensional disorder potential. In the limit of a vanishing atom-atom interaction, a sharp cone in the angle-resolved density ... [more ▼]

We study quantum transport of an interacting Bose-Einstein condensate in a two-dimensional disorder potential. In the limit of a vanishing atom-atom interaction, a sharp cone in the angle-resolved density of the scattered matter wave is observed, arising from constructive interference between amplitudes propagating along reversed scattering paths. Weak interaction transforms this coherent backscattering peak into a pronounced dip, indicating destructive instead of constructive interference. We reproduce this result, obtained from the numerical integration of the Gross-Pitaevskii equation, by a diagrammatic theory of weak localization in the presence of nonlinearity. [less ▲]

Swarming and swirling in self-propelled polar granular rods

in Physical Review Letters (2008), 100

Using experiments with anisotropic vibrated rods and quasi-2D numerical simulations, we show that shape plays an important role in the collective dynamics of self-propelled (SP) particles. We demonstrate ... [more ▼]

Using experiments with anisotropic vibrated rods and quasi-2D numerical simulations, we show that shape plays an important role in the collective dynamics of self-propelled (SP) particles. We demonstrate that SP rods exhibit local ordering, aggregation at the side walls, and clustering absent in round SP particles. Furthermore, we find that at sufficiently strong excitation SP rods engage in a persistent swirling motion in which the velocity is strongly correlated with particle orientation. [less ▲]

Dynamics of a bouncing droplet onto a vertically vibrated interface

in Physical Review Letters (2008), 100(16),

Low viscosity (< 100 cSt) silicon oil droplets are placed on a high viscosity (1000 cSt) oil bath that vibrates vertically. The viscosity difference ensures that the droplet is more deformed than the bath ... [more ▼]

Low viscosity (< 100 cSt) silicon oil droplets are placed on a high viscosity (1000 cSt) oil bath that vibrates vertically. The viscosity difference ensures that the droplet is more deformed than the bath interface. Droplets bounce periodically on the bath when the acceleration of its sinusoidal motion is larger than a threshold value. The threshold is minimum for a particular frequency of excitation: droplet and bath motions are in resonance. The bouncing droplet has been modeled by considering the deformation of the droplet and the lubrication force exerted by the air layer between the droplet and the bath. Threshold values are predicted and found to be in good agreement with our measurements. [less ▲]

Tetrahedral clustering in molten lithium under pressure

in Physical Review Letters (2008), 101(7), 0757034

A series of electronic and structural transitions are predicted in molten lithium from first principles. A new phase with tetrahedral local order characteristic of sp 3 bonded materials and poor ... [more ▼]

A series of electronic and structural transitions are predicted in molten lithium from first principles. A new phase with tetrahedral local order characteristic of sp 3 bonded materials and poor electrical conductivity is found at pressures above 150 GPa and temperatures as high as 1000 K. Despite the lack of covalent bonding, weakly bound tetrahedral clusters with finite lifetimes are predicted to exist. The stabilization of this phase in lithium involves a unique mechanism of strong electron localization in interstitial regions and interactions among core electrons. The calculations provide evidence for anomalous melting above 20 GPa, with a melting temperature decreasing below 300 K, and point towards the existence of novel low-symmetry crystalline phases. [less ▲]