1,323 research outputs found
Hydrodynamic Modes for Granular Gases
The eigenfunctions and eigenvalues of the linearized Boltzmann equation for
inelastic hard spheres (d=3) or disks (d=2) corresponding to d+2 hydrodynamic
modes, are calculated in the long wavelength limit for a granular gas. The
transport coefficients are identified and found to agree with those from the
Chapman-Enskog solution. The dominance of hydrodynamic modes at long times and
long wavelengths is studied via an exactly solvable kinetic model. A
collisional continuum is bounded away from the hydrodynamic spectrum, assuring
a hydrodynamic description at long times. The bound is closely related to the
power law decay of the velocity distribution in the reference homogeneous
cooling state
Critical Behavior of a Heavy Particle in a Granular Fluid
Behavior analogous to a second order phase transition is observed for the
homogeneous cooling state of a heavy impurity particle in a granular fluid. The
order parameter is the ratio of impurity mean square velocity to that
of the fluid, with a conjugate field proportional to the mass ratio. A
parameter , measuring the fluid cooling rate relative to the
impurity--fluid collision rate, is the analogue of the inverse temperature. For
the fluid is ``normal'' with at , as in the case of a
system with elastic collisions. For an ``ordered'' state with occurs at , representing an extreme breakdown of equipartition.
Critical slowing and qualitative changes in the velocity distribution function
for the impurity particle near the transition are notedComment: 4 pages (4 figures included
Gaussian Kinetic Model for Granular Gases
A kinetic model for the Boltzmann equation is proposed and explored as a
practical means to investigate the properties of a dilute granular gas. It is
shown that all spatially homogeneous initial distributions approach a universal
"homogeneous cooling solution" after a few collisions. The homogeneous cooling
solution (HCS) is studied in some detail and the exact solution is compared
with known results for the hard sphere Boltzmann equation. It is shown that all
qualitative features of the HCS, including the nature of over population at
large velocities, are reproduced semi-quantitatively by the kinetic model. It
is also shown that all the transport coefficients are in excellent agreement
with those from the Boltzmann equation. Also, the model is specialized to one
having a velocity independent collision frequency and the resulting HCS and
transport coefficients are compared to known results for the Maxwell Model. The
potential of the model for the study of more complex spatially inhomogeneous
states is discussed.Comment: to be submitted to Phys. Rev.
Continuous Charge Modulated Diagonal Phase in Manganites
We present a novel ground state that explain the continuous modulated charge
diagonal order recently observed in manganese oxides, at hole densities
larger than one half. In this diagonal phase the charge is modulated with a
predominant Fourier component inversely proportional to . Magnetically
this state consist of antiferromagnetic coupled zig-zag chains. For a wide
range of relevant physical parameters as electron-phonon coupling,
antiferromagnetic interaction between Mn ions and on-site Coulomb repulsion,
the diagonal phase is the ground state of the system. The diagonal phase is
favored by the modulation of the hopping amplitude along the zig-zag chains,
and it is stabilized with respect to the one dimensional straight chain by the
electron phonon coupling. For realistic estimation of the physical parameters,
the diagonal modulation of the electron density is only a small fraction of the
average charge, a modulation much smaller than the obtained by distributing
Mn and Mn ions. We discuss also the spin and orbital structure
properties of this new diagonal phase.Comment: 4 pages, 4 figures include
Recommended from our members
Motivations for workplace democratization : a case study of airline mechanics.
Hydrodynamic modes, Green-Kubo relations, and velocity correlations in dilute granular gases
It is shown that the hydrodynamic modes of a dilute granular gas of inelastic
hard spheres can be identified, and calculated in the long wavelength limit.
Assuming they dominate at long times, formal expressions for the Navier-Stokes
transport coefficients are derived. They can be expressed in a form that
generalizes the Green-Kubo relations for molecular systems, and it is shown
that they can also be evaluated by means of -particle simulation methods.
The form of the hydrodynamic modes to zeroth order in the gradients is used to
detect the presence of inherent velocity correlations in the homogeneous
cooling state, even in the low density limit. They manifest themselves in the
fluctuations of the total energy of the system. The theoretical predictions are
shown to be in agreement with molecular dynamics simulations. Relevant related
questions deserving further attention are pointed out
Electronic Phase Separation in Manganite/Insulator Interfaces
By using a realist microscopic model, we study the electric and magnetic
properties of the interface between a half metallic manganite and an insulator.
We find that the lack of carriers at the interface debilitates the double
exchange mechanism, weakening the ferromagnetic coupling between the Mn ions.
In this situation the ferromagnetic order of the Mn spins near the interface is
unstable against antiferromagnetic CE correlations, and a separation between
ferromagnetic/metallic and antiferromagnetic/insulator phases at the interfaces
can occur. We obtain that the insertion of extra layers of undoped manganite at
the interface introduces extra carriers which reinforce the double exchange
mechanism and suppress antiferromagnetic instabilities.Comment: 8 pages, 7 figures include
Linear Response for Granular Fluids
The linear response of an isolated, homogeneous granular fluid to small
spatial perturbations is studied by methods of non-equilibrium statistical
mechanics. The long wavelength linear hydrodynamic equations are obtained, with
formally exact expressions for the susceptibilities and transport coefficients.
The latter are given in equivalent Einstein-Helfand and Green-Kubo forms. The
context of these results and their contrast with corresponding results for
normal fluids are discussed.Comment: Submitted to PR
Canted phase in double quantum dots
We perform a Hartree-Fock calculation in order to describe the ground state
of a vertical double quantum dot in the absence of magnetic fields parallel to
the growth direction. Intra- and interdot exchange interactions determine the
singlet or triplet character of the system as the tunneling is tuned. At finite
Zeeman splittings due to in-plane magnetic fields, we observe the continuous
quantum phase transition from ferromagnetic to symmetric phase through a canted
antiferromagnetic state. The latter is obtained even at zero Zeeman energy for
an odd electron number.Comment: 5 pages, 3 figure
Phase Diagram and Incommensurate Phases in Undoped Manganites
We study the existence of incommensurate phases in the phase diagram of the
two orbital double exchange model coupled with Jahn-Teller phonons and with
superexchange interactions. In agreement with experimental results, we find
that undoped manganites ( being some rare earth element) show
temperature induced commensurate-incommensurate phase transitions. In the
incommensurate phase the magnetic wave vector varies with temperature. The
incommensurate phase arises from the competition between the short range
antiferromagnetic superexchange interaction and the long range ferromagnetic
double exchange interaction
- …