50 research outputs found
Dynamical Symmetry Enlargement Versus Spin-Charge Decoupling in the One-Dimensional SU(4) Hubbard Model
We investigate dynamical symmetry enlargement in the half-filled SU(4)
Hubbard chain using non-perturbative renormalization group and Quantum Monte
Carlo techniques. A spectral gap is shown to open for arbitrary Coulombic
repulsion . At weak coupling, , a SO(8) symmetry between
charge and spin-orbital excitations is found to be dynamically enlarged at low
energy. At strong coupling, , the charge degrees of freedom
dynamically decouple and the resulting effective theory in the spin-orbital
sector is that of the SO(6) antiferromagnetic Heisenberg model. Both regimes
exhibit spin-Peierls order. However, although spin-orbital excitations are
in the SO(6) regime they are in the SO(8) one. The
cross-over between these regimes is discussed.Comment: 4 pages, 2 figure
Trionic and quartetting phases in one-dimensional multicomponent ultracold fermions
We investigate the possible formation of a molecular condensate, which might
be, for instance, the analogue of the alpha condensate of nuclear physics, in
the context of multicomponent cold atoms fermionic systems. A simple
paradigmatic model of N-component fermions with contact interactions loaded
into a one-dimensional optical lattice is studied by means of low-energy and
numerical approaches. For attractive interaction, a quasi-long-range molecular
superfluid phase, formed from bound-states made of N fermions, emerges at low
density. We show that trionic and quartetting phases, respectively for N=3,4,
extend in a large domain of the phase diagram and are robust against small
symmetry-breaking perturbations.Comment: Contribution to the SOTANCP 2008 worksho
André Vincent : de l’ingénieur en organisation à la comptabilité nationale
Pendant la Deuxième Guerre mondiale,
à l’Institut de conjoncture, l’ingénieuréconomiste André Vincent travaille aux côtés
d’Alfred Sauvy sur le progrès technique et la
comptabilité nationale. Les travaux sur la
« nébuleuse modernisatrice » formée dans
le sillage de Jean Fourastié font l’impasse
sur la carrière d’ingénieur de Vincent et
peinent Ă expliquer pourquoi il s’intĂ©resse Ă
la productivité, jusqu’à devenir un des
spécialistes français de la question. Qui est
André Vincent ? Comment cet ingénieur
diplômé des Arts et Métiers, à l’origine
spécialisé dans le planning et le calcul de
prix de revient, devient-il progressivement
économiste ainsi que l’un des pères de la
comptabilité nationale française ? Comment
la rationalité de l’ingénieur se met au service
de la mesure et de l’évaluation du progrès
technique ?Titre du volume : Les ingénieurs qui lisent les bilans. Savoirs techniques et gestionnaires au prisme de la comptabilité (années 1850-1950
Heat transport of clean spin-ladders coupled to phonons: Umklapp scattering and drag
We study the low-temperature heat transport in clean two-leg spin ladder
compounds coupled to three-dimensional phonons. We argue that the very large
heat conductivities observed in such systems can be traced back to the
existence of approximate symmetries and corresponding weakly violated
conservation laws of the effective (gapful) low--energy model, namely
pseudo-momenta. Depending on the ratios of spin gaps and Debye energy and on
the temperature, the magnetic contribution to the heat conductivity can be
positive or negative, and exhibit an activated or anti-activated behavior. In
most regimes, the magnetic heat conductivity is dominated by the spin-phonon
drag: the excitations of the two subsystems have almost the same drift
velocity, and this allows for an estimate of the ratio of the magnetic and
phononic contributions to the heat conductivity.Comment: revised version, 8 pages, 3 figures, added appendi
Jean Fourastié, « un professeur pur » ?
À la fin des années trente, sa réputation
grandissante d’expert en assurances et en
comptabilité ouvre toutes grandes les portes
du Conservatoire national des arts et métiers
(Cnam) à Jean Fourastié. Il y enseignera
pendant 34 ans dans un amphithéâtre qui
porte dorénavant son nom. Au cours de cette
longue carrière, fort de son rôle fondamental
dans l’élaboration et la mise en œuvre d’une
politique modernisatrice après la deuxième
guerre mondiale comme du succès de ses
nombreux essais brillants, Jean Fourastié
donne Ă comprendre aux Ă©tudiants, dans
l’esprit de Jean-Baptiste Say, les mécanismes
Ă©conomiques de long terme dont la
productivité constitue le cœur
Nonequilibrium Current in the One Dimensional Hubbard Model at Half-Filling
Nonlinear transport in the one dimensional Hubbard model at half-filling
under a finite bias voltage is investigated by the adaptive time-dependent
density matrix renormalization group method. For repulsive on-site interaction,
dielectric breakdown of the Mott insulating ground state to a current-carrying
nonequilibrium steady state is clearly observed when the voltage exceeds the
charge gap. It is found that by increasing the voltage further the
current-voltage characteristics are scaled only by the charge gap and the
scaling curve exhibits almost linear dependence on the voltage whose slope is
suppressed by the electron correlation. In the case of attractive interaction
the linear conductance is the perfect one which agrees with the
prediction by the Luttinger liquid theory.Comment: 4 pages, 7 figure
Molecular superfluid phase in systems of one-dimensional multicomponent fermionic cold atoms
We study a simple model of N-component fermions with contact interactions
which describes fermionic atoms with N=2F+1 hyperfine states loaded into a
one-dimensional optical lattice. We show by means of analytical and numerical
approaches that, for attractive interaction, a quasi-long-range molecular
superfluid phase emerges at low density. In such a phase, the pairing
instability is strongly suppressed and the leading instability is formed from
bound-states made of N fermions. At small density, the molecular superfluid
phase is generic and exists for a wide range of attractive contact interactions
without an SU(N) symmetry between the hyperfine states.Comment: published versio
Effect of Hund coupling in the one-dimensional SU(4) Hubbard model
The one-dimensional SU(4) Hubbard model perturbed by Hund coupling is
studied, away from half-filling, by means of renormalization group and
bosonization methods. A spectral gap is always present in the spin-orbital
sector irrespective of the magnitude of the Coulomb repulsion. We further
distinguish between two qualitatively different regimes. At small Hund
coupling, we find that the symmetry of the system is dynamically enlarged to
SU(4) at low energy with the result of {\it coherent} spin-orbital excitations.
When the charge sector is not gapped, a superconducting instability is shown to
exist. At large Hund coupling, the symmetry is no longer enlarged to SU(4) and
the excitations in the spin sector become {\it incoherent}. Furthermore, the
superconductivity can be suppressed in favor of the conventional charge density
wave state.Comment: 10 pages, 1 figur
The embedding method beyond the single-channel case: Two-mode and Hubbard chains
We investigate the relationship between persistent currents in multi-channel
rings containing an embedded scatterer and the conductance through the same
scatterer attached to leads. The case of two uncoupled channels corresponds to
a Hubbard chain, for which the one-dimensional embedding method is readily
generalized. Various tests are carried out to validate this new procedure, and
the conductance of short one-dimensional Hubbard chains attached to perfect
leads is computed for different system sizes and interaction strengths. In the
case of two coupled channels the conductance can be obtained from a statistical
analysis of the persistent current or by reducing the multi-channel scattering
problem to several single-channel setups.Comment: 14 pages, 13 figures, submitted for publicatio
Effect of symmetry breaking perturbations in the one-dimensional SU(4) spin-orbital model
We study the effect of symmetry breaking perturbations in the one-dimensional
SU(4) spin-orbital model. We allow the exchange in spin () and orbital
() channel to be different and thus reduce the symmetry to SU(2)
SU(2). A magnetic field along the direction is also applied. Using
the formalism developped by Azaria et al we extend their analysis of the
isotropic , h=0 case and obtain the low-energy effective theory near
the SU(4) point in the asymmetric case. An accurate analysis of the
renormalization group flow is presented with a particular emphasis on the
effect of the anisotropy. In zero magnetic field, we retrieve the same
qualitative low-energy physics than in the isotropic case. In particular, the
massless behavior found on the line extends in a large
anisotropic region. We discover though that the anisotropy plays its trick in
allowing non trivial scaling behaviors of the physical quantities. When a
magnetic field is present the effect of the anisotropy is striking. In addition
to the usual commensurate-incommensurate phase transition that occurs in the
spin sector of the theory, we find that the field may induce a second
transition of the KT type in the remaining degrees of freedom to which it does
not couple directly. In this sector, we find that the effective theory is that
of an SO(4) Gross-Neveu model with an h-dependent coupling that may change its
sign as h varies.Comment: 14 pages, 5 Figs, added referenc