329 research outputs found
Superconducting pairing and density-wave instabilities in quasi-one-dimensional conductors
Using a renormalization group approach, we determine the phase diagram of an
extended quasi-one-dimensional electron gas model that includes interchain
hopping, nesting deviations and both intrachain and interchain repulsive
interactions. d-wave superconductivity, which dominates over the
spin-density-wave (SDW) phase at large nesting deviations, becomes unstable to
the benefit of a triplet -wave phase for a weak repulsive interchain
backscattering term , despite the persistence of dominant SDW
correlations in the normal state. Antiferromagnetism becomes unstable against
the formation of a charge-density-wave state when exceeds some
critical value. While these features persist when both Umklapp processes and
interchain forward scattering () are taken into account, the effect
of alone is found to frustrate nearest-neighbor interchain - and
-wave pairing and instead favor next-nearest-neighbor interchain singlet or
triplet pairing. We argue that the close proximity of SDW and
charge-density-wave phases, singlet d-wave and triplet -wave superconducting
phases in the theoretical phase diagram provides a possible explanation for
recent puzzling experimental findings in the Bechgaard salts, including the
coexistence of SDW and charge-density-wave phases and the possibility of a
triplet pairing in the superconducting phase.Comment: 19 pages, 13 figure
CDW Ordering in Stripe Phase of Underdoped Cuprates
The in-plane resistivity and out-of-plane resistivity of non-superconducting
RBCO (R = Y, Tm) and Fe-doped Bi2212 single crystals are discussed. The
comparison of electrical transport properties of the cuprates and quasi-one
dimensional (1D) (TMTSF)2PF6 organic conductor suggests that RBCO and Bi2212
exhibit 1D transport properties, and the step rise at low temperatures in the
resistivities of the cuprates and quasi-1D organic conductor is due to
charge-density-wave ordering. We discuss also phonon-electron interactions in
cuprates at low temperatures.Comment: 10 pages including 4 figure
Superconductivity and Antiferromagnetism in Quasi-one-dimensional Organic Conductors
We review the current understanding of superconductivity in the
quasi-one-dimensional organic conductors of the Bechgaard and Fabre salt
families. We discuss the interplay between superconductivity,
antiferromagnetism, and charge-density-wave fluctuations. The connection to
recent experimental observations supporting unconventional pairing and the
possibility of a triplet-spin order parameter for the superconducting phase is
also presented.Comment: (v1) 30 pages, 13 figures; Review article for the 20th anniversary of
high-Tc superconductivity, to appear in J. Low Temp. Phys. (v2) 1 Ref. adde
Variations spatiotemporelles des compartiments autotrophes et hétérotrophes de la boucle microbienne dans les lacs du sud du Québec
Les variations de la biomasse des compartiments autotrophes et hétérotrophes de la boucle microbienne ont été examinées dans neuf lacs de deux régions du sud du Québec. Six lacs étaient situés dans les Laurentides et trois lacs dans les Cantons de l'Est. Ils se différenciaient en fonction de leur statut trophique, de la géologie du bassin versant, et de la physicochimie des eaux. Dans chaque lac, l'échantillonnage a été réalisé à trois profondeurs correspondant aux strates épi-, méta- et hypolimnétiques et à cinq dates au cours de l'été (1990-1992). Les biomasses moyennes de picoplancton autotrophe (PPA : 16-80 µg·C·L-1), de picoplancton hétérotrophe (PPH : 97-647 µg·C·L-1), de nanoplancton autotrophe (NPA : 7-37 µg·C·L-1) et de nanoplancton hétérotrophe (NPH : 9-29 µg·C·L-1) notées dans les lacs du sud du Québec le long d'un gradient trophique de PT variant de 7 à 73 µg·L-1 étaient du même ordre de grandeur que celles rapportées pour d'autres écosystèmes d'eau douce au Canada.La majeure partie de la variation dans les biomasses des compartiments microbiens étaient reliée aux variations inter-lacs mais il existaient aussi des sources de variation significatives au sein des lacs, soit au niveau spatial entre les strates limnétiques ou au niveau temporel entre les dates d'échantillonnage durant l'été. Toutefois, les patrons de variation spatiotemporelle intra-lac variaient d'un lac à l'autre. Les variations inter-lacs de la biomasse des quatre compartiments microbiens ont été mises en relation avec les changements dans les concentrations de phosphore total (PT) et dans les densités de macrozooplancton. Le niveau trophique des lacs, exprimés en PT, et l'abondance du macrozooplancton avaient un effet sur les biomasses des compartiments picoplanctoniques. La biomasse de PPA et PPH décroissait dans les lacs eutrophes des Cantons de l'Est ayant des concentrations de PT supérieures à 20 µg·L-1 et dans les lacs ayant de fortes densités de macrozooplancton ou un faible rapport micro-macrozooplancton. Notre étude n'a pas mis clairement en évidence les effets du niveau trophique ou du macrozoopancton sur les compartiments nanoplanctoniques (NPA, NPH).The biomass of autotrophic and heterotrophic microbial compartments were measured in nine lakes in two regions of southern Quebec. Six lakes were located in the Laurentides while three lakes were situated in the Eastern Townships. They varied in trophic status, watershed geology, and water chemistry. Each lake was sampled at three depths, corresponding to the epi-, meta-, and hypolimnion strata, and at five dates over the summer season (1991-1992). The mean biomass of autotrophic picoplankton (PPA: 16-80 µg·C·L-1), heterotrophic picoplankton (PPH: 97-647 µg·C·L-1), autotrophic nanoplankton (NPA: 7-37 µg·C·L-1), and heterotrophic nanoplankton (NPH: 9-29 µg·C·L-1) found in Quebec lakes over a PT range of trophy varying from 7 to 73 µg·L-1 were of the same order as those reported in other freshwater environments in Canada.Most of the variation in the biomass of the four microbial compartments was related to among-lake variation. Some variation was also due to within-lake spatial variation among limnetic strata or temporal variation over the summer season. However, within-lake patterns of variation patterns were different among lakes. Among-lake variations in the biomass of the four microbial compartments were examined in relation to changes in total phosphorus concentrations (PT) and macrozooplankton densities between lakes. Both lake trophy, expressed by PT, and macrozooplankton abundance influence the biomass of picoplankton compartments. The biomass of PPA and PPH decline in the most eutrophic lakes of the Eastern Townships where PT > 20 µg·L-1. PPA and PPH biomass were also reduced in lakes with the highest density of macrozooplankton or with low micro-/macrozooplankton ratio. Our study did not clearly detected the effects of lake trophy or macrozooplankton on nanoplanktonic compartments
Renormalization Group calculations with k|| dependent couplings in a ladder
We calculate the phase diagram of a ladder system, with a Hubbard interaction
and an interchain coupling . We use a Renormalization Group method, in
a one loop expansion, introducing an original method to include
dependence of couplings. We also classify the order parameters corresponding to
ladder instabilities. We obtain different results, depending on whether we
include dependence or not. When we do so, we observe a region with
large antiferromagnetic fluctuations, in the vicinity of small ,
followed by a superconducting region with a simultaneous divergence of the Spin
Density Waves channel. We also investigate the effect of a non local backward
interchain scattering : we observe, on one hand, the suppression of singlet
superconductivity and of Spin Density Waves, and, on the other hand, the
increase of Charge Density Waves and, for some values of , of triplet
superconductivity. Our results eventually show that is an influential
variable in the Renormalization Group flow, for this kind of systems.Comment: 20 pages, 19 figures, accepted in Phys. Rev. B 71 v. 2
Triplet superconducting pairing and density-wave instabilities in organic conductors
Using a renormalization group approach, we determine the phase diagram of an
extended quasi-one-dimensional electron gas model that includes interchain
hopping, nesting deviations and both intrachain and interchain repulsive
interactions. We find a close proximity of spin-density- and
charge-density-wave phases, singlet d-wave and triplet f-wave superconducting
phases. There is a striking correspondence between our results and recent
puzzling experimental findings in the Bechgaard salts, including the
coexistence of spin-density-wave and charge-density-wave phases and the
possibility of a triplet pairing in the superconducting phase.Comment: 4 pages, 5 eps figure
A Renormalization group approach for highly anisotropic 2D Fermion systems: application to coupled Hubbard chains
I apply a two-step density-matrix renormalization group method to the
anisotropic two-dimensional Hubbard model. As a prelude to this study, I
compare the numerical results to the exact one for the tight-binding model. I
find a ground-state energy which agrees with the exact value up to four digits
for systems as large as . I then apply the method to the
interacting case. I find that for strong Hubbard interaction, the ground-state
is dominated by magnetic correlations.
These correlations are robust even in the presence of strong frustration.
Interchain pair tunneling is negligible in the singlet and triplet channels and
it is not enhanced by frustration. For weak Hubbard couplings, interchain
non-local singlet pair tunneling is enhanced and magnetic correlations are
strongly reduced. This suggests a possible superconductive ground state.Comment: 8 pages, 11 figures, expanded version of cond-mat/060856
Towards a consistent picture for quasi-1D organic superconductors
The electrical resistivity of the quasi-1D organic superconductor (TMTSF)2PF6
was recently measured at low temperature from the critical pressure needed to
suppress the spin-density-wave state up to a pressure where superconductivity
has almost disappeared. This data revealed a direct correlation between the
onset of superconductivity at Tc and the strength of a non-Fermi-liquid linear
term in the normal-state resistivity, going as r(T) = r0 + AT + BT2 at low
temperature, so that A goes to 0 as Tc goes to 0. Here we show that the
contribution of low-frequency antiferromagnetic fluctuations to the
spin-lattice relaxation rate is also correlated with this non-Fermi-liquid term
AT in the resistivity. These correlations suggest that anomalous scattering and
pairing have a common origin, both rooted in the low-frequency
antiferromagnetic fluctuations measured by NMR. A similar situation may also
prevail in the recently-discovered iron-pnictide superconductors.Comment: ISCOM'09 proceedings to be published in Physica
Superconductivity and antiferromagnetism in quasi-one-dimensional organic conductors (Review Article)
We review the current understanding of superconductivity in the quasi-one-dimensional organic
conductors of the Bechgaard and Fabre salt families. We discuss the interplay between superconductivity,
antiferromagnetism, and charge-density-wave fluctuations. The connection to recent
experimental observations supporting unconventional pairing and the possibility of a
triplet–spin order parameter for the superconducting phase is also presented
Larkin-Ovchinnikov-Fulde-Ferrell state in quasi-one-dimensional superconductors
The properties of a quasi-one-dimensional (quasi-1D) superconductor with {\it
an open Fermi surface} are expected to be unusual in a magnetic field. On the
one hand, the quasi-1D structure of the Fermi surface strongly favors the
formation of a non-uniform state (Larkin-Ovchinnikov-Fulde-Ferrell (LOFF)
state) in the presence of a magnetic field acting on the electron spins. On the
other hand, a magnetic field acting on an open Fermi surface induces a
dimensional crossover by confining the electronic wave-functions wave-functions
along the chains of highest conductivity, which results in a divergence of the
orbital critical field and in a stabilization at low temperature of a cascade
of superconducting phases separated by first order transistions. In this paper,
we study the phase diagram as a function of the anisotropy. We discuss in
details the experimental situation in the quasi-1D organic conductors of the
Bechgaard salts family and argue that they appear as good candidates for the
observation of the LOFF state, provided that their anisotropy is large enough.
Recent experiments on the organic quasi-1D superconductor (TMTSF)ClO
are in agreement with the results obtained in this paper and could be
interpreted as a signature of a high-field superconducting phase. We also point
out the possibility to observe a LOFF state in some quasi-2D organic
superconductors.Comment: 24 pages+17 figures (upon request), RevTex, ORSAY-LPS-24109
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