873 research outputs found
Dynamic response functions for the Holstein-Hubbard model
We present results on the dynamical correlation functions of the
particle-hole symmetric Holstein-Hubbard model at zero temperature, calculated
using the dynamical mean field theory which is solved by the numerical
renormalization group method. We clarify the competing influences of the
electron-electron and electron-phonon interactions particularity at the
different metal to insulator transitions. The Coulomb repulsion is found to
dominate the behaviour in large parts of the metallic regime. By suppressing
charge fluctuations, it effectively decouples electrons from phonons. The
phonon propagator shows a characteristic softening near the metal to
bipolaronic transition but there is very little softening on the approach to
the Mott transition.Comment: 13 pages, 19 figure
La enseñanza de "Fuerza y Movimiento" como cambio conceptual
Research has shown that many students hold alternative conceptions about motion and the factors which influence it and that an important component of these conceptions are epistemological commitments, one example of which is a cause-effect relationship. The article describes a series of microcomputer programs designed to facilitate conceptual change from an impetus-type view to a Newtonian view. A significant feature of these programs is the explicit focus given to the nature of the relationship between cause and effect and its role in the conceptual change process
Kondo resonance narrowing in d- and f-electron systems
By developing a simple scaling theory for the effect of Hund's interactions
on the Kondo effect, we show how an exponential narrowing of the Kondo
resonance develops in magnetic ions with large Hund's interaction. Our theory
predicts an exponential reduction of the Kondo temperature with spin S of the
Hund's coupled moment, a little-known effect first observed in d-electron
alloys in the 1960's, and more recently encountered in numerical calculations
on multi-band Hubbard models with Hund's interactions. We discuss the
consequences of Kondo resonance narrowing for the Mott transition in d-band
materials, particularly iron pnictides, and the narrow ESR linewidth recently
observed in ferromagnetically correlated f-electron materials.Comment: 4 pages, 3 figure
Enseñanza para un cambio conceptual : ejemplos de fuerza y de movimiento
In this article the possibility of learning as a conceptual change and the way of teaching appropriate for this is discussed. This teaching has to have certain characteristics and ensure that the debate in the classroom is centered on the explicit ideas of the children, the status of which must be discussed and negotiated. To achieve this, the curriculum must give importance to the justification of ideas, the debate must be metacognitive and the role of the profesor must be more active and diversified. One example of this way of teaching is presented in the article
Generalized Schrieffer-Wolff Formalism for Dissipative Systems
We present a formalized perturbation theory for Markovian open systems in the
language of a generalized Schrieffer-Wolff (SW) transformation. A non-unitary
rotation decouples the unper- turbed steady states from all fast degrees of
freedom, in order to obtain an effective Liouvillian, that reproduces the exact
low excitation spectrum of the system. The transformation is derived in a
constructive way, yielding a perturbative expansion of the effective Liouville
operator. The presented formalism realizes an adiabatic elimination of fast
degrees of freedom to arbitrary orders in the perturbation. We exemplarily
employ the SW formalism to two generic open systems and discuss general
properties of the different orders of the perturbation.Comment: 11 pages, 1 figur
Comment on "Projective Quantum Monte Carlo Method for the Anderson Impurity Model and its Application to Dynamical Mean Field Theory"
A comment about importance of Anderson's orthogonality catastrophe for
projective Quantum Monte Carlo methods.Comment: Replaced by final versio
The Strong Coupling Fixed-Point Revisited
In recent work we have shown that the Fermi liquid aspects of the strong
coupling fixed point of the s-d and Anderson models can brought out more
clearly by interpreting the fixed point as a renormalized Anderson model,
characterized by a renormalized level , resonance width,
, and interaction , and a simple prescription for their
calculation was given using the numerical renormalization group (NRG). These
three parameters completely specify a renormalized perturbation theory (RPT)
which leads to exact expressions for the low temperature behaviour. Using a
combination of the two techniques, NRG to determine ,
, and , and then substituting these in the RPT
expressions gives a very efficient and accurate way of calculating the low
temperature behaviour of the impurity as it avoids the necessity of subtracting
out the conduction electron component. Here we extend this approach to an
Anderson model in a magnetic field, so that , ,
and become dependent on the magnetic field. The de-renormalization
of the renormalized quasiparticles can then be followed as the magnetic field
strength is increased. Using these running coupling constants in a RPT
calculation we derive an expression for the low temperature conductivity for
arbitrary magnetic field strength.Comment: Contribution to JPSJ volume commemorating the 40th anniversary of the
publication of Kondo's original pape
Spin-Valley Kondo Effect in Multi-electron Silicon Quantum Dots
We study the spin-valley Kondo effect of a silicon quantum dot occupied by electrons, with up to four. We show that the Kondo
resonance appears in the Coulomb blockade regimes, but not
in the one, in contrast to the spin-1/2 Kondo effect, which
only occurs at odd. Assuming large orbital level spacings, the
energy states of the dot can be simply characterized by fourfold spin-valley
degrees of freedom. The density of states (DOS) is obtained as a function of
temperature and applied magnetic field using a finite-U equation-of-motion
approach. The structure in the DOS can be detected in transport experiments.
The Kondo resonance is split by the Zeeman splitting and valley splitting for
double- and triple-electron Si dots, in a similar fashion to single-electron
ones. The peak structure and splitting patterns are much richer for the
spin-valley Kondo effect than for the pure spin Kondo effect.Comment: 8 pages, 4 figures, in PRB format. This paper is a sequel to the
paper published in Phys. Rev. B 75, 195345 (2007
Spin splitting and Kondo effect in quantum dots coupled to noncollinear ferromagnetic leads
We study the Kondo effect in a quantum dot coupled to two noncollinear
ferromagnetic leads. First, we study the spin splitting
of an energy level
in the quantum dot by tunnel couplings to the ferromagnetic leads, using the
Poor man's scaling method. The spin splitting takes place in an intermediate
direction between magnetic moments in the two leads. , where is the spin
polarization in the leads, is the angle between the magnetic moments,
and is an asymmetric factor of tunnel barriers (). Hence the spin
splitting is always maximal in the parallel alignment of two ferromagnets
() and minimal in the antiparallel alignment (). Second,
we calculate the Kondo temperature . The scaling calculation
yields an analytical expression of as a function of
and , , when .
is a decreasing function with respect to
. When is
relevant, we evaluate using the
slave-boson mean-field theory. The Kondo resonance is split into two by finite
, which results in the spin accumulation in the quantum dot and
suppression of the Kondo effect.Comment: 11 pages, 8 figures, revised versio
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