314 research outputs found
Current induced Spin Torque in a nanomagnet
In a nanomagnet (whose total spin S< 1000), very small polarized currents can
lead to magnetic reversal. Treating on the same footing the transport and
magnetic properties of a nanomagnet connected to magnetic leads via tunneling
barriers, we derive a closed equation for the time evolution of the
magnetization. The interplay between Coulomb blockade phenomena and magnetism
gives some additional structure to the current induced spin torque. In addition
to the possibility of stabilizing uniform spin waves, we find that the system
is highly hysteretic: up to three different magnetic states can be
simultaneously stable in one region of the parameter (magnetic field and bias
voltage) space.Comment: 5 pages, 3 figures to appear in Phys. Rev. Let
Mott physics and spin fluctuations: a unified framework
We present a formalism for strongly correlated electrons systems which
consists in a local approximation of the dynamical three-leg interaction
vertex. This vertex is self-consistently computed with a quantum impurity model
with dynamical interactions in the charge and spin channels, similar to
dynamical mean field theory (DMFT) approaches. The electronic self-energy and
the polarization are both frequency and momentum dependent. The method
interpolates between the spin-fluctuation or GW approximations at weak coupling
and the atomic limit at strong coupling. We apply the formalism to the Hubbard
model on a two-dimensional square lattice and show that as interactions are
increased towards the Mott insulating state, the local vertex acquires a strong
frequency dependence, driving the system to a Mott transition, while at low
enough temperatures the momentum-dependence of the self-energy is enhanced due
to large spin fluctuations. Upon doping, we find a Fermi arc in the
one-particle spectral function, which is one signature of the pseudo-gap state.Comment: 7 pages, 6 figure
Mott physics and spin fluctuations: a functional viewpoint
We present a formalism for strongly correlated systems with fermions coupled
to bosonic modes. We construct the three-particle irreducible functional
by successive Legendre transformations of the free energy of the
system. We derive a closed set of equations for the fermionic and bosonic
self-energies for a given . We then introduce a local
approximation for , which extends the idea of dynamical mean field
theory (DMFT) approaches from two- to three-particle irreducibility. This
approximation entails the locality of the three-leg electron-boson vertex
, which is self-consistently computed using a quantum
impurity model with dynamical charge and spin interactions. This local vertex
is used to construct frequency- and momentum-dependent electronic self-energies
and polarizations. By construction, the method interpolates between the
spin-fluctuation or GW approximations at weak coupling and the atomic limit at
strong coupling. We apply it to the Hubbard model on two-dimensional square and
triangular lattices. We complement the results of Phys.Rev. B 92, 115109 by (i)
showing that, at half-filling, as DMFT, the method describes the Fermi-liquid
metallic state and the Mott insulator, separated by a first-order
interacting-driven Mott transition at low temperatures, (ii) investigating the
influence of frustration and (iii) discussing the influence of the bosonic
decoupling channel.Comment: 29 pages, 14 figure
Enhancement of Local Pairing Correlations in Periodically Driven Mott Insulators
We investigate a model for a Mott insulator in presence of a time-periodic
modulated interaction and a coupling to a thermal reservoir. The combination of
drive and dissipation leads to non-equilibrium steady states with a large
number of doublon excitations, well above the maximum thermal-equilibrium
value. We interpret this effect as an enhancement of local pairing
correlations, providing analytical arguments based on a Floquet Hamiltonian.
Remarkably, this Hamiltonian shows a tendency to develop long-range staggered
superconducting correlations. This suggests the possibility of realizing the
elusive eta-pairing phase in driven-dissipative Mott Insulators.Comment: 6+5 page
Continuous-time auxiliary field Monte Carlo for quantum impurity models
We present a continuous-time Monte Carlo method for quantum impurity models,
which combines a weak-coupling expansion with an auxiliary-field decomposition.
The method is considerably more efficient than Hirsch-Fye and free of time
discretization errors, and is particularly useful as impurity solver in large
cluster dynamical mean field theory (DMFT) calculations.Comment: 6 pages, 5 figure
Reconstructing non-equilibrium regimes of quantum many-body systems from the analytical structure of perturbative expansions
We propose a systematic approach to the non-equilibrium dynamics of strongly
interacting many-body quantum systems, building upon the standard perturbative
expansion in the Coulomb interaction. High order series are derived from the
Keldysh version of determinantal diagrammatic Quantum Monte Carlo, and the
reconstruction beyond the weak coupling regime of physical quantities is
obtained by considering them as analytic functions of a complex-valued
interaction . Our advances rely on two crucial ingredients: i) a conformal
change of variable, based on the approximate location of the singularities of
these functions in the complex -plane; ii) a Bayesian inference technique,
that takes into account additional known non-perturbative relations, in order
to control the amplification of noise occurring at large . This general
methodology is applied to the strongly correlated Anderson quantum impurity
model, and is thoroughly tested both in- and out-of-equilibrium. In the
situation of a finite voltage bias, our method is able to extend previous
studies, by bridging with the regime of unitary conductance, and by dealing
with energy offsets from particle-hole symmetry. We also confirm the existence
of a voltage splitting of the impurity density of states, and find that it is
tied to a non-trivial behavior of the non-equilibrium distribution function.
Beyond impurity problems, our approach could be directly applied to
Hubbard-like models, as well as other types of expansions.Comment: 16 pages, 18 figures, added comparison with Bethe Ansatz, appendix B
and some discussio
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