2,215 research outputs found
Effective gravity and OSp(N,4) invariant matter
We re-examine the OSp(N,4) invariant interacting model of massless chiral and
gauge superfields, whose superconformal invariance was instrumental, both in
proving the all-order no-renormalization of the mass and chiral
self-interaction lagrangians, and in determining the linear superfield
renormalization needed. We show that the renormalization of the gravitational
action modifies only the cosmological term, without affecting higher-order
tensors. This could explain why the effect of the cosmological constant is
shadowed by the effects of newtonian gravity.Comment: 12 pages, LaTeX, 4 figures, PACS: 04.65.+e, substantial revisions, to
appear in Phys. Rev.
Influence of dimensionality on superconductivity in carbon nanotubes
We investigate the electronic instabilities in carbon nanotubes (CNs),
looking for the break-down of the one dimensional Luttinger liquid regime due
to the strong screening of the long-range part of the Coulomb repulsion. We
show that such a breakdown is realized both in ultra-small single wall CNs and
multi wall CNs, while a purely electronic mechanism could explain the
superconductivity (SC) observed recently in ultra-small (diameter ) single wall CNs () and entirely end-bonded multi-walled
ones (). We show that both the doping and the screening of
long-range part of the electron-electron repulsion, needed to allow the SC
phase, are related to the intrinsically 3D nature of the environment where the
CNs operate.Comment: 5 pages, 3 figures, PACS: 71.10.Pm,74.50.+r,71.20.Tx, to appear in J.
Phys. Cond. Ma
Transport through a double barrier in Large Radius Carbon Nanotubes in the presence of a transverse magnetic field
We discuss the Luttinger Liquid behaviour of Large Radius Carbon Nanotube
e.g. the Multi Wall ones (MWNT), under the action of a transverse magnetic
field . Our results imply a reduction with in the value of the
critical exponent, , for the tunneling density of states, which
is in agreement with that observed in transport experiments. Then, the problem
of the transport through a Quantum Dot formed by two intramolecular tunneling
barriers along the MWNT, weakly coupled to Tomonaga-Luttinger liquids is
studied, including the action of a strong transverse magnetic field . {We
predict the presence of some peaks in the conductance G versus , related to
the magnetic flux quantization in the ballistic regime (at a very low
temperature, ) and also at higher , where the Luttinger behaviour
dominates}. The temperature dependence of the maximum of the
conductance peak according to the Sequential Tunneling follows a power law,
with linearly dependent on the critical
exponent, , strongly reduced by .Comment: 8 pages, 3 figures, PACS numbers: 05.60.Gg, 71.10.Pm, 73.63.-b,
71.20.Tx, 72.80.R
Large N Effects and Renormalization of the Long-Range Coulomb Interaction in Carbon Nanotubes
We develop a dimensional regularization approach to deal with the low-energy
effects of the long-range Coulomb interaction in 1D electron systems. The
method allows us to avoid the infrared singularities arising from the
long-range Coulomb interaction at D = 1, providing at the same time insight
about the fixed-points of the theory. We show that the effect of increasing the
number N of subbands at the Fermi level is opposite to that of approaching the
bare Coulomb interaction in the limit D --> 1. Then, we devise a double scaling
limit, in which the large N effects are able to tame the singularities due to
the long-range interaction. Thus, regular expressions can be obtained for all
observables right at D = 1, bearing also a dependence o the doping level of the
system. Our results imply a variation with N in the value of the exponent for
the tunneling density of states, which is in fair agreement with that observed
in different transport experiments involving carbon nanotubes. As the doping
level is increased in nanotubes of large radius and multi-walled nanotubes, we
predict a significant reduction of order N^{-1/2} in the critical exponent of
the tunneling density of states.Comment: 16 pages, 5 figures, PACS codes: 73.40, 11.10.
Electronic screening and correlated superconductivity in carbon nanotubes
A theoretical analysis of the superconductivity observed recently in Carbon
nanotubes is proposed. We argue that ultra-small (diameter )
single wall carbon nanotubes (with transition temperature )
and entirely end-bonded multi-walled ones () can superconduct
by an electronic mechanism, basically the same in both cases. By a Luttinger
liquid -like approach, one finds enhanced superconducting correlations due to
the strong screening of the long-range part of the Coulomb repulsion. Based on
this finding, we perform a detailed analysis on the resulting
Hubbard-like model, and calculate transition temperatures of the same order
of magnitude as the measured ones.Comment: 6 pages, 1 figure, PACS: 71.10.Pm,74.50.+r,71.20.Tx, to appear in
Phys. Rev.
Crossover from Luttinger liquid to Coulomb blockade regime in carbon nanotubes
We develop a theoretical approach to the low-energy properties of 1D electron
systems aimed to encompass the mixed features of Luttinger liquid and Coulomb
blockade behavior observed in the crossover between the two regimes. For this
aim we extend the Luttinger liquid description by incorporating the effects of
a discrete single-particle spectrum. The intermediate regime is characterized
by a power-law behavior of the conductance, but with an exponent oscillating
with the gate voltage, in agreement with recent experimental observations. Our
construction also accounts naturally for the existence of a crossover in the
zero-bias conductance, mediating between two temperature ranges where the
power-law behavior is preserved but with different exponent.Comment: 5 pages, 3 figure
Single Wall Nanotubes: Atomic Like Behaviour and Microscopic Approach
Recent experiments about the low temperature behaviour of a Single Wall
Carbon Nanotube (SWCNT) showed typical Coulomb Blockade (CB) peaks in the zero
bias conductance and allowed us to investigate the energy levels of interacting
electrons. Other experiments confirmed the theoretical prediction about the
crucial role which the long range nature of the Coulomb interaction plays in
the correlated electronic transport through a SWCNT with two intramolecular
tunneling barriers. In order to investigate the effects on low dimensional
electron systems due to the range of electron electron repulsion, we introduce
a model for the interaction which interpolates well between short and long
range regimes. Our results could be compared with experimental data obtained in
SWCNTs and with those obtained for an ideal vertical Quantum Dot (QD).
For a better understanding of some experimental results we also discuss how
defects and doping can break some symmetries of the bandstructure of a SWCNT.Comment: 8 pages, 4 figure
The geometry of N=4 twisted string
We compare N=2 string and N=4 topological string within the framework of the
sigma model approach. Being classically equivalent on a flat background, the
theories are shown to lead to different geometries when put in a curved space.
In contrast to the well studied Kaehler geometry characterising the former
case, in the latter case a manifold has to admit a covariantly constant
holomorphic two-form in order to support an N=4 twisted supersymmetry. This
restricts the holonomy group to be a subgroup of SU(1,1) and leads to a
Ricci--flat manifold. We speculate that, the N=4 topological formalism is an
appropriate framework to smooth down ultraviolet divergences intrinsic to the
N=2 theory.Comment: 20 pages, LaTe
Suppression of electron-electron repulsion and superconductivity in Ultra Small Carbon Nanotubes
Recently, ultra-small-diameter Single Wall Nano Tubes with diameter of have been produced and many unusual properties were observed, such as
superconductivity, leading to a transition temperature , much
larger than that observed in the bundles of larger diameter tubes.
By a comparison between two different approaches, we discuss the issue
whether a superconducting behavior in these carbon nanotubes can arise by a
purely electronic mechanism. The first approach is based on the Luttinger Model
while the second one, which emphasizes the role of the lattice and short range
interaction, is developed starting from the Hubbard Hamiltonian. By using the
latter model we predict a transition temperature of the same order of magnitude
as the measured one.Comment: 7 pages, 3 figures, to appear in J. Phys.-Cond. Ma
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