254 research outputs found
Quantum Phase Transitions
We give a general introduction to quantum phase transitions in
strongly-correlated electron systems. These transitions which occur at zero
temperature when a non-thermal parameter like pressure, chemical
composition or magnetic field is tuned to a critical value are characterized by
a dynamic exponent related to the energy and length scales and
. Simple arguments based on an expansion to first order in the effective
interaction allow to define an upper-critical dimension (where
and is the spatial dimension) below which mean-field description is
no longer valid. We emphasize the role of pertubative renormalization group
(RG) approaches and self-consistent renormalized spin fluctuation (SCR-SF)
theories to understand the quantum-classical crossover in the vicinity of the
quantum critical point with generalization to the Kondo effect in heavy-fermion
systems. Finally we quote some recent inelastic neutron scattering experiments
performed on heavy-fermions which lead to unusual scaling law in
for the dynamical spin susceptibility revealing critical local modes beyond the
itinerant magnetism scheme and mention new attempts to describe this local
quantum critical point.Comment: 13 pages, 4 figure
Supersymmetric Approach to Heavy-Fermion Systems
We present a new supersymmetric approach to the Kondo lattice model in order
to describe simultaneously the quasiparticle excitations and the low-energy
magnetic fluctuations in heavy-Fermion systems. This approach mixes the
fermionic and the bosonic representation of the spin following the standard
rules of superalgebra. Our results show the formation of a bosonic band within
the hybridization gap reflecting the spin collective modes. The density of
states at the Fermi level is strongly renormalized while the Fermi surface sum
rule includes states. The dynamical susceptibility is made of a Fermi
liquid superimposed on a localized magnetism contribution.Comment: 5 pages, 2 figure
Critical phenomena near the antiferromagnetic quantum critical point of Heavy-Fermions
We present a study of the critical phenomena around the quantum critical
point in heavy-fermion systems. In the framework of the S=1/2 Kondo lattice
model, we introduce an extended decoupling scheme of the Kondo interaction
which allows one to treat the spin fluctuations and the Kondo effect on an
equal footing. The calculations, developed in a self-consistent one-loop
approximation, lead to the formation of a damped collective mode with a dynamic
exponent z=2 in the case of an antiferromagnetic instability. The system
displays a quantum-classical crossover at finite temperature depending how the
energy of the mode, on the scale of the magnetic correlation length, compares
to k_B T. The low temperature behavior, in the different regimes separated by
the crossover temperatures, is then discussed for both 2- and 3-dimensional
systems.Comment: 24 pages, 5 figures, added reference
Towards Sustainability Assessment of the Built Environment: A Classification of the Existing Challenges
The application of sustainability assessment in a decision context is associated with various challenges that explain why the transition to action-oriented knowledge still needs to be fulfilled. Therefore, this paper aims to explore the associated challenges in sustainability assessment in the decision context of the built environment. Several publications are reviewed to provide a systemic understanding of the associated complexities. The challenges in sustainability assessment in the built environment are categorized at different levels, from understanding to measurement and implementation. The challenges are further categorized into definition, context, interpretation, data, measurement methods, uncertainties, indicators and indices, results, coordination, conflicts, and action-oriented knowledge. Moreover, according to the nature of each challenge, they are classified into epistemological, methodological, and procedural challenges. The novelty of this review is that it reviews and reports almost all fragmentedly reported challenges in sustainability assessment of the built environment in the literature within a holistic framework that provides a clear understanding of the state of the art and second discusses them within an integrated framework (the Sustainability Assessment Network) including the position of active-role players to resolve them, including strategists, scientist, and stakeholders
Theoretical analysis of the transmission phase shift of a quantum dot in the presence of Kondo correlations
We study the effects of Kondo correlations on the transmission phase shift of
a quantum dot coupled to two leads in comparison with the experimental
determinations made by Aharonov-Bohm (AB) quantum interferometry. We propose
here a theoretical interpretation of these results based on scattering theory
combined with Bethe ansatz calculations. We show that there is a factor of 2
difference between the phase of the S-matrix responsible for the shift in the
AB oscillations, and the one controlling the conductance. Quantitative
agreement is obtained with experimental results for two different values of the
coupling to the leads.Comment: 4 pages, 4 figures, accepted for publication in Physical Review
Letter
On the correct continuum limit of the functional-integral representation for the four-slave-boson approach to the Hubbard model: Paramagnetic phase
The Hubbard model with finite on-site repulsion U is studied via the
functional-integral formulation of the four-slave-boson approach by Kotliar and
Ruckenstein. It is shown that a correct treatment of the continuum imaginary
time limit (which is required by the very definition of the functional
integral) modifies the free energy when fluctuation (1/N) corrections beyond
mean-field are considered. Our analysis requires us to suitably interpret the
Kotliar and Ruckenstein choice for the bosonic hopping operator and to abandon
the commonly used normal-ordering prescription, in order to obtain meaningful
fluctuation corrections. In this way we recover the exact solution at U=0 not
only at the mean-field level but also at the next order in 1/N. In addition, we
consider alternative choices for the bosonic hopping operator and test them
numerically for a simple two-site model for which the exact solution is readily
available for any U. We also discuss how the 1/N expansion can be formally
generalized to the four-slave-boson approach, and provide a simplified
prescription to obtain the additional terms in the free energy which result at
the order 1/N from the correct continuum limit.Comment: Changes: Printing problems (due to non-standard macros) have been
removed, 44 page
Intermediate coupling fixed point study in the overscreened regime of generalized multichannel SU(N) Kondo models
We study a generalized multichannel single-impurity Kondo model, in which the
impurity spin is described by a representation of the SU(N) group which
combines bosonic and fermionic degrees of freedom. The impurity spin states are
described by Abrikosov pseudofermions, and we make use of a method initiated by
Popov and Fedotov which allows a proper handling of the fermionic constraint.
The partition function is derived within a path integral approach. We use
renormalization group techniques to calculate the scaling function
perturbatively in powers of the Kondo coupling constant, which is justified in
the weak coupling limit. The truncated expansion is valid in the overscreened
(Nozieres-Blandin) regime, for an arbitrary SU(N) group and any value of the
parameters characterizing the impurity spin representation. The intermediate
coupling fixed point is identified. We derive the temperature dependence of
various physical quantities at low T, controlled by a unique critical exponent,
and show that the physics of the system in the overscreened regime governed by
the intermediate coupling fixed point is characterized by a non-Fermi liquid
behavior. Our results are in accordance with those obtained by other methods,
as Bethe ansatz and boundary conformal field theory, in the case of various
impurity spin symmetries. We establish in a unified way that the Kondo models
in which the impurity spin is described successively by a fundamental,
symmetric, antisymmetric and mixed symmetry representation yield all the same
low-energy physics in the overscreened regime. Possible generalizations of the
analysis we present to the case of arbitrary impurity spin representations of
SU(N) are also discussed.Comment: 21 pages, 7 figures, REVTeX; final version accepted for publicatio
Platinum-free photoelectrochromic devices working with copper-based electrolytes for ultrastable smart windows
Photoelectrochromic systems are devices designed for large-scale manufacturing of smart windows, capable of changing their transmittance according to external environmental conditions. This communication proposes the replacement of the two most critical photoelectrochemical device components studied so far, namely the counter electrode and the redox mediator. Regarding the first, graphene nanoplatelets are used to replace platinum, maintaining both its optical and electrocatalytic properties, and at the same time reducing the device cost. Secondly, a copper-based redox pair was chosen to solve the corrosion problems typically encountered with the iodine-based mediator. The combination of the above components led to devices with high performance (coloration speeds in the order of seconds, with a maximum contrast ratio of 10.4 : 1), as well as the achievement of a long-term stability record (over 400 days) for these photoelectrochromic systems
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