Biomedical subjects

Ph Chomaz

Publications and source records attributed to Ph Chomaz.

7 recordsLinked to original sources

PubMed2004

Quantum calculations of Coulomb reorientation for sub-barrier fusion.

Classical mechanics and time dependent Hartree-Fock (TDHF) calculations of heavy ions collisions are performed to study the rotation of a deformed nucleus in the Coulomb field of its partner. This reorientation is shown to be independent of the charges and relative energy of the partners. It only depends upon the deformations and inertias. TDHF calculations predict an increase by 30% of the induced rotation due to quantum effects while the nuclear contribution seems negligible. This reorientation modifies strongly the fusion cross section around the barrier for light deformed nuclei on heavy collision partners. For such nuclei a hindrance of the sub-barrier fusion is predicted.

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PubMed2004

Exact pairing correlations for one-dimensionally trapped fermions with stochastic mean-field wave functions.

The canonical thermodynamic properties of a one-dimensional system of interacting spin-1/2 fermions with an attractive zero-range pseudopotential are investigated within an exact approach. The density operator is evaluated as the statistical average of dyadics formed from a stochastic mean-field propagation of independent Slater determinants. For a harmonically trapped Fermi gas and for fermions confined in a 1D-like torus, we observe the transition to a quasi-BCS state with Cooper-like momentum correlations and an algebraic long-range order. For a few trapped fermions in a rotating torus, a dominant superfluid component with quantized circulation can be isolated.

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PubMed2003

Influence of the coulomb interaction on the liquid-gas phase transition and nuclear multifragmentation.

The liquid-gas phase transition is analyzed from the topologic properties of the event distribution in the observables space. A multicanonical formalism allows one to directly relate the standard phase transition with neutral particles to the case where the nonsaturating Coulomb interaction is present, and to interpret the Coulomb effect as a deformation of the probability distributions and a rotation of the order parameter. This formalism is applied to a statistical multifragmentation model and consequences for the nuclear multifragmentation phase transitions are drawn.

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PubMed2003

Transient backbending behavior in the Ising model with fixed magnetization.

The physical origin of the backbendings in the equations of state of finite but not necessarily small systems is studied in the Ising model with fixed magnetization (IMFM) by means of the topological properties of the observable distributions and the analysis of the largest cluster with increasing lattice size. Looking at the convexity anomalies of the IMFM thermodynamic potential, it is shown that the order of the transition at the thermodynamic limit can be recognized in finite systems independently of the lattice size. General statistical mechanics arguments and analytical calculations suggest that the backbending in the caloric curve is a transient behavior which should not converge to a plateau in the thermodynamic limit, while the first-order transition (in the Ehrenfest sense) is still signaled by a discontinuity in the magnetization equation of state.

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PubMed2002

Failure of thermodynamics near a phase transition.

In the vicinity of a first-order phase transition, the equation of state might be different when the extensive variable is controlled instead of the intensive one, violating the uniqueness of thermodynamics. A sufficient condition for this nonequivalence to survive at the thermodynamical limit is worked out for classical systems. If energy consists of a kinetic and a potential part, the microcanonical ensemble does not converge towards the canonical ensemble when the kinetic heat capacity is larger than the modulus of the negative interaction heat capacity.

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PubMed2002

Exact stochastic mean-field approach to the fermionic many-body problem.

We investigate a reformulation of the dynamics of interacting fermion systems in terms of a stochastic extension of time-dependent Hartree-Fock equations. From a path-integral representation of the evolution operator, we show that the exact N-body state can be interpreted as a coherent average over Slater determinants evolving in a random mean-field. The imaginary time propagation is also presented and gives a similar scheme which converges to the exact ground state. In addition, the growth of statistical errors is examined to show the stability of this stochastic formulation.

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PubMed2002

Mechanical and chemical spinodal instabilities in finite quantum systems.

Self-consistent quantum approaches are used to study the instabilities of finite nuclear systems. The frequencies of multipole density fluctuations are determined as a function of dilution and temperature for several isotopes. The spinodal region of the phase diagrams is determined, and it appears that instabilities are reduced by finite size effects. The role of surface and volume instabilities is discussed. It is indicated that the important chemical effects associated with mechanical disruption may lead to isospin fractionation.

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