291 research outputs found
Spin Gap Fixed Points in the Double Chain Problem
Applying the bosonization procedure to weakly coupled Hubbard chains we
discuss the fixed points of the renormalization group flow where all spin
excitations are gapful and a singlet pairing becomes the dominant instability.Comment: 15 pages, TeX, C Version 3.
Electromagnetic Response of a Superconductor: Effect of Order Parameter Collective Modes
Effects of order parameter collective modes on electromagnetic response are
studied for a clean spin-triplet superconductor with orbital
symmetry, which has been proposed as a candidate pairing symmetry for
SrRuO. It is shown that the superconductor has
characteristic massive collective modes analogous to the clapping mode in the
A-phase of superfluid He. We discuss the contribution from the collective
modes to ultrasound attenuation and electromagnetic absorption. We show that in
the electromagnetic absorption spectrum the clapping mode gives rise to a
resonance peak well below the pair breaking frequency, while the ultrasound
attenuation is hardly influenced by the collective excitations.Comment: 4 pages RevTex, 1 eps figur
A crib-shaped triplet pairing gap function for an orthogonal pair of quasi-one dimensional Fermi surfaces in SrRuO
The competition between spin-triplet and singlet pairings is studied
theoretically for the tight-binding - bands in SrRuO,
which arise from two sets of quasi-one dimensional Fermi surfaces. Using
multiband FLEX approximation, where we incorporate an anisotropy in the spin
fluctuations as suggested from experiments, we show that (i) the triplet can
dominate over the singlet (which turns out to be extended s), and (ii) the
triplet gap function optimized in the Eliashberg equation has an unusual, very
non-sinusoidal form, whose time-reversal-broken combination exhibits a
crib-shaped amplitude with dips.Comment: 5 pages, RevTeX, to appear in Phys.Rev.B (Rapid Communications
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Soft topographic map for clustering and classification of bacteria
In this work a new method for clustering and building a
topographic representation of a bacteria taxonomy is presented. The method is based on the analysis of stable parts of the genome, the so-called “housekeeping genes”. The proposed method generates topographic maps of the bacteria taxonomy, where relations among different
type strains can be visually inspected and verified. Two well known DNA alignement algorithms are applied to the genomic sequences. Topographic maps are optimized to represent the similarity among the sequences according to their evolutionary distances. The experimental analysis is carried out on 147 type strains of the Gammaprotebacteria
class by means of the 16S rRNA housekeeping gene. Complete sequences of the gene have been retrieved from the NCBI public database. In the experimental tests the maps show clusters of homologous type strains and present some singular cases potentially due to incorrect classification
or erroneous annotations in the database
Magnetic Phase Diagram of Ca2-xSrxRuO4 Governed by Structural Distortions
We constructed, by the first-principles calculations, a magnetic phase
diagram of SrRuO in the space spanned by structural distortions. Our
phase diagram can qualitatively explain the experimental one for
CaSrRuO. We found that the rotation and the tilting of RuO
octahedron are responsible for the ferro- and antiferro-magnetism,
respectively, while the flattening of RuO is the key factor to stabilize
those magnetic ground states. Our results imply that the magnetic and the
structural instabilities in SrRuO are closely correlated cooperatively
rather than competitively.Comment: 3 figures; accepted by PRB as rapid communicatio
Transport and the Order Parameter of Superconducting SrRuO
Recent experiments make it appear more likely that the order parameter of the
unconventional superconductor SrRuO has a spin-triplet -wave
symmetry. We study ultrasonic absorption and thermal conductivity of
superconducting SrRuO and fit to the recent data for various -wave
candidates. It is shown that only -wave symmetry can account
qualitatively for the transport data.Comment: 4 pages, 2 figures, references added and update
Jahn-Teller polarons and their superconductivity in a molecular conductor
We present a theoretical study of a possibility of superconductivity in a
three dimensional molecular conductor in which the interaction between
electrons in doubly degenerate molecular orbitals and an {\em intra}molecular
vibration mode is large enough to lead to the formation of
Jahn-Teller small polarons. We argue that the effective polaron-polaron
interaction can be attractive for material parameters realizable in molecular
conductors. This interaction is the source of superconductivity in our model.
On analyzing superconducting instability in the weak and strong coupling
regimes of this attractive interaction, we find that superconducting transition
temperatures up to 100 K are achievable in molecular conductors within this
mechanism. We also find, for two particles per molecular site, a novel Mott
insulating state in which a polaron singlet occupies one of the doubly
degenerate orbitals on each site. Relevance of this study in the search for new
molecular superconductors is pointed out.Comment: Submitted to Phys. Rev.
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