6,428 research outputs found
Properties of electrons near a Van Hove singularity
The Fermi surface of most hole-doped cuprates is close to a Van Hove
singularity at the M point. A two-dimensional electronic system, whose Fermi
surface is close to a Van Hove singularity shows a variety of weak coupling
instabilities. It is a convenient model to study the interplay between
antiferromagnetism and anisotropic superconductivity. The renormalization group
approach is reviewed with emphasis on the underlying physical processes.
General properties of the phase diagram and possible deformations of the Fermi
surface due to the Van Hove proximity are described.Comment: Proceedings of SNS-01 to appear in the Journal of Physics and
Chemistry of Solids, SNS-0
Variational approach to the Hubbard model in a C60 cluster
5 págs.; 4 figs.; 3 tabs.We propose a computational scheme for the Hubbard model in the C60 cluster in which the interaction with the Fermi sea of charges added to the neutral molecule is switched on sequentially. This is applied to the calculation of the balance of charging energies, within a low-energy truncation of the space of states which produces moderate errors for an intermediate range of the interaction strength. ©1996 American Physical SocietyPeer Reviewe
Phase diagram of carbon nanotube ropes
14 págs.; 9 figs.; 1 tab. ; PACS number(s): 71.20.Tx, 71.10.Pm, 74.50.1rThe zero-temperature phase diagram of carbon nanotube ropes is studied using a computational framework that incorporates the renormalization of intratube interactions and the effect of intertube Coulomb screening. This allows us to undertake a systematic analysis as a function of the number of metallic nanotubes in the rope and the effective strength of the phonon-mediated interaction. We find that there is in general a weak-coupling regime of the interactions, corresponding to Luttinger-liquid behavior in thin ropes and to superconducting behavior in sufficiently thick ropes. Furthermore, we show the existence of exotic phases in the strong-coupling regime, characterized by the appearance of charge instabilities that depend on the helicity and the doping level of the nanotubes in the rope. Our approach allows for the simultaneous analysis of the scaling of the Cooper-pair tunneling amplitude between metallic nanotubes, making it possible to discern the crossover from purely one-dimensional physics to the setting of three-dimensional Cooper-pair coherence. We provide then good estimates of the superconducting transition temperature and discuss the connection of our results with recent experiments. ©2004 American Physical SocietyJ. V. A. acknowledges support from S. Moukouri and
stimulating discussions with J. Allen. J. G. acknowledges
financial support from the Ministerio de Ciencia y TecnologĂa
Grant No. BFM2003-05317.Peer Reviewe
Electronic instabilities in 3D arrays of small-diameter (3,3) carbon nanotubes
We investigate the electronic instabilities of the small-diameter (3,3)
carbon nanotubes by studying the low-energy perturbations of the normal
Luttinger liquid regime. The bosonization approach is adopted to deal exactly
with the interactions in the forward-scattering channels, while renormalization
group methods are used to analyze the low-energy instabilities. In this
respect, we take into account the competition between the effective e-e
interaction mediated by phonons and the Coulomb interaction in backscattering
and Umklapp channels. Moreover, we apply our analysis to relevant experimental
conditions where the nanotubes are assembled into large three-dimensional
arrays, which leads to an efficient screening of the Coulomb potential at small
momentum-transfer. We find that the destabilization of the normal metallic
behavior takes place through the onset of critical behavior in some of the two
charge stiffnesses that characterize the Luttinger liquid state. From a
physical point of view, this results in either a divergent compressibility or a
vanishing renormalized velocity for current excitations at the point of the
transition. We observe anyhow that this kind of critical behavior occurs
without the development of any appreciable sign of superconducting
correlations.Comment: 10 pages, 12 figure
Integration of biophysical connectivity in the spatial optimization of coastal ecosystem services
Ecological connectivity in coastal oceanic waters is mediated by dispersion
of the early life stages of marine organisms and conditions the structure of
biological communities and the provision of ecosystem services. Integrated
management strategies aimed at ensuring long-term service provision to society
do not currently consider the importance of dispersal and larval connectivity.
A spatial optimization model is introduced to maximise the potential provision
of ecosystem services in coastal areas by accounting for the role of dispersal
and larval connectivity. The approach combines a validated coastal circulation
model that reproduces realistic patterns of larval transport along the coast,
which ultimately conditions the biological connectivity and productivity of an
area, with additional spatial layers describing potential ecosystem services.
The spatial optimization exercise was tested along the coast of Central Chile,
a highly productive area dominated by the Humboldt Current. Results show it is
unnecessary to relocate existing management areas, as increasing no-take areas
by 10% could maximise ecosystem service provision, while improving the spatial
representativeness of protected areas and minimizing social conflicts. The
location of protected areas was underrepresented in some sections of the study
domain, principally due to the restriction of the model to rocky subtidal
habitats. Future model developments should encompass the diversity of coastal
ecosystems and human activities to inform integrative spatial management.
Nevertheless, the spatial optimization model is innovative not only for its
integrated ecosystem perspective, but also because it demonstrates that it is
possible to incorporate time-varying biophysical connectivity within the
optimization problem, thereby linking the dynamics of exploited populations
produced by the spatial management regime.Comment: 30 pages, 5 figures, 2 tables; 1 graphical abstract. In this version:
numbering of figures corrected, updated figure 2, typos corrected and
references fixe
Superconducting and pseudogap phases from scaling near a Van Hove singularity
We study the quantum corrections to the Fermi energy of a two-dimensional
electron system, showing that it is attracted towards the Van Hove singularity
for a certain range of doping levels. The scaling of the Fermi level allows to
cure the infrared singularities left in the BCS channel after renormalization
of the leading logarithm near the divergent density of states. A phase of
d-wave superconductivity arises beyond the point of optimal doping
corresponding to the peak of the superconducting instability. For lower doping
levels, the condensation of particle-hole pairs due to the nesting of the
saddle points takes over, leading to the opening of a gap for quasiparticles in
the neighborhood of the singular points.Comment: 4 pages, 6 Postscript figures, the physical discussion of the results
has been clarifie
Deformation of the Fermi surface in the extended Hubbard model
The deformation of the Fermi surface induced by Coulomb interactions is
investigated in the t-t'-Hubbard model. The interplay of the local U and
extended V interactions is analyzed. It is found that exchange interactions V
enhance small anisotropies producing deformations of the Fermi surface which
break the point group symmetry of the square lattice at the Van Hove filling.
This Pomeranchuck instability competes with ferromagnetism and is suppressed at
a critical value of U(V). The interaction V renormalizes the t' parameter to
smaller values what favours nesting. It also induces changes on the topology of
the Fermi surface which can go from hole to electron-like what may explain
recent ARPES experiments.Comment: 5 pages, 4 ps figure
Fermi surface renormalization in Hubbard ladders
We derive the one-loop renormalization equations for the shift in the
Fermi-wavevectors for one-dimensional interacting models with four Fermi-points
(two left and two right movers) and two Fermi velocities v_1 and v_2. We find
the shift to be proportional to (v_1-v_2)U^2, where U is the Hubbard-U. Our
results apply to the Hubbard ladder and to the t_1-t_2 Hubbard model. The
Fermi-sea with fewer particles tends to empty. The stability of a saddle point
due to shifts of the Fermi-energy and the shift of the Fermi-wavevector at the
Mott-Hubbard transition are discussed.Comment: 5 pages, 4 Postscript figure
Kinematics of electrons near a Van Hove singularity
A two dimensional electronic system, where the Fermi surface is close to a
Van Hove singularity, shows a variety of weak coupling instabilities, and it is
a convenient model to study the interplay between antiferromagnetism and
anisotropic superconductivity. We present a detailed analysis of the kinematics
of the electron scattering in this model. The similitudes, and differences,
between a standard Renormalization Group approach and previous work based on
parquet summations of log divergences are analyzed, with emphasis on the
underlying physical processes. General properties of the phase diagram are
discussed.Comment: 5 pages, 3 postscript figure
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