28 research outputs found
Stability of metallic stripes in the extended one-band Hubbard model
Based on an unrestricted Gutzwiller approximation (GA) we investigate the
stripe orientation and periodicity in an extended one-band Hubbard model. A
negative ratio between next-nearest and nearest neighbor hopping t'/t, as
appropriate for cuprates, favors partially filled (metallic) stripes for both
vertical and diagonal configurations. At around optimal doping diagonal
stripes, site centered (SC) and bond centered (BC) vertical stripes become
degenerate suggesting strong lateral and orientational fluctuations. We find
that within the GA the resulting phase diagram is in agreement with experiment
whereas it is not in the Hartree-Fock approximation due to a strong
overestimation of the stripe filling. Results are in agreement with previous
calculations within the three-band Hubbard model but with the role of SC and BC
stripes interchanged.Comment: 10 pages, 8 figure
Hidden Order in the Cuprates
We propose that the enigmatic pseudogap phase of cuprate superconductors is
characterized by a hidden broken symmetry of d(x^2-y^2)-type. The transition to
this state is rounded by disorder, but in the limit that the disorder is made
sufficiently small, the pseudogap crossover should reveal itself to be such a
transition. The ordered state breaks time-reversal, translational, and
rotational symmetries, but it is invariant under the combination of any two. We
discuss these ideas in the context of ten specific experimental properties of
the cuprates, and make several predictions, including the existence of an
as-yet undetected metal-metal transition under the superconducting dome.Comment: 12 pages of RevTeX, 9 eps figure
Phylogenetic and morphotaxonomic revision of Ramichloridium and allied genera
The phylogeny of the genera Periconiella, Ramichloridium,
Rhinocladiella and Veronaea was explored by means of partial
sequences of the 28S (LSU) rRNA gene and the ITS region (ITS1, 5.8S rDNA and
ITS2). Based on the LSU sequence data, ramichloridium-like species segregate
into eight distinct clusters. These include the Capnodiales
(Mycosphaerellaceae and Teratosphaeriaceae), the
Chaetothyriales (Herpotrichiellaceae), the Pleosporales, and
five ascomycete clades with uncertain affinities. The type species of
Ramichloridium, R. apiculatum, together with R. musae,
R. biverticillatum, R. cerophilum, R. verrucosum, R. pini, and three new
species isolated from Strelitzia, Musa and forest soil,
respectively, reside in the Capnodiales clade. The human-pathogenic
species R. mackenziei and R. basitonum, together with R.
fasciculatum and R. anceps, cluster with Rhinocladiella
(type species: Rh. atrovirens, Herpotrichiellaceae,
Chaetothyriales), and are allocated to this genus. Veronaea
botryosa, the type species of the genus Veronaea, also resides
in the Chaetothyriales clade, whereas Veronaea simplex
clusters as a sister taxon to the Venturiaceae (Pleosporales), and is
placed in a new genus, Veronaeopsis. Ramichloridium
obovoideum clusters with Carpoligna pleurothecii (anamorph:
Pleurothecium sp., Chaetosphaeriales), and a new combination
is proposed in Pleurothecium. Other ramichloridium-like clades
include R. subulatum and R. epichloës (incertae sedis,
Sordariomycetes), for which a new genus, Radulidium is
erected. Ramichloridium schulzeri and its varieties are placed in a
new genus, Myrmecridium (incertae sedis, Sordariomycetes).
The genus Pseudovirgaria (incertae sedis) is introduced to
accommodate ramichloridium-like isolates occurring on various species of rust
fungi. A veronaea-like isolate from Bertia moriformis with
phylogenetic affinity to the Annulatascaceae (Sordariomycetidae) is
placed in a new genus, Rhodoveronaea. Besides
Ramichloridium, Periconiella is also polyphyletic.
Thysanorea is introduced to accommodate Periconiella papuana
(Herpotrichiellaceae), which is unrelated to the type species, P.
velutina (Mycosphaerellaceae)
Opportunistic, human-pathogenic species in the Herpotrichiellaceae are phenotypically similar to saprobic or phytopathogenic species in the Venturiaceae
Although morphologically similar, species of Cladophialophora
(Herpotrichiellaceae) were shown to be phylogenetically distinct from
Pseudocladosporium (Venturiaceae), which was revealed to be
synonymous with the older genus, Fusicladium. Other than being
associated with human disorders, species of Cladophialophora were
found to also be phytopathogenic, or to occur as saprobes on organic material,
or in water, fruit juices, or sports drinks, along with species of
Exophiala. Caproventuria and Metacoleroa were
confirmed to be synonyms of Venturia, which has Fusicladium
(= Pseudocladosporium) anamorphs. Apiosporina, based on
A. collinsii, clustered basal to the Venturia clade, and
appears to represent a further synonym. Several species with a
pseudocladosporium-like morphology in vitro represent a sister clade
to the Venturia clade, and are unrelated to Polyscytalum.
These taxa are newly described in Fusicladium, which is
morphologically close to Anungitea, a heterogeneous genus with
unknown phylogenetic affinity. In contrast to the
Herpotrichiellaceae, which were shown to produce numerous
synanamorphs in culture, species of the Venturiaceae were
morphologically and phylogenetically more uniform. Several new species and new
combinations were introduced in Cladophialophora, Cyphellophora
(Herpotrichiellaceae), Exophiala, Fusicladium, Venturia
(Venturiaceae), and Cylindrosympodium (incertae
sedis)
Phylogeny of rock-inhabiting fungi related to Dothideomycetes
The class Dothideomycetes (along with Eurotiomycetes)
includes numerous rock-inhabiting fungi (RIF), a group of ascomycetes that
tolerates surprisingly well harsh conditions prevailing on rock surfaces.
Despite their convergent morphology and physiology, RIF are phylogenetically
highly diverse in Dothideomycetes. However, the positions of main
groups of RIF in this class remain unclear due to the lack of a strong
phylogenetic framework. Moreover, connections between rock-dwelling habit and
other lifestyles found in Dothideomycetes such as plant pathogens,
saprobes and lichen-forming fungi are still unexplored. Based on multigene
phylogenetic analyses, we report that RIF belong to Capnodiales
(particularly to the family Teratosphaeriaceae s.l.),
Dothideales, Pleosporales, and Myriangiales, as
well as some uncharacterised groups with affinities to
Dothideomycetes. Moreover, one lineage consisting exclusively of RIF
proved to be closely related to Arthoniomycetes, the sister class of
Dothideomycetes. The broad phylogenetic amplitude of RIF in
Dothideomycetes suggests that total species richness in this class
remains underestimated. Composition of some RIF-rich lineages suggests that
rock surfaces are reservoirs for plant-associated fungi or saprobes, although
other data also agree with rocks as a primary substrate for ancient fungal
lineages. According to the current sampling, long distance dispersal seems to
be common for RIF. Dothideomycetes lineages comprising lichens also
include RIF, suggesting a possible link between rock-dwelling habit and
lichenisation
Homopolar Pulse Welding For Offshore Deep Water Pipelines
Homopolar pulse welding (HPW) is a one-shot resistance welding process being investigated as a method to join API SL carbon steel line pipe. Homopolar pulse welding utilizes the high current, low voltage electrical pulse produced by a homopolar generator to rapidly resistance heat the interface between abutting pipe ends, producing a full circumference resistance forge weld requiring no filler metal in under three seconds. A five year joint industry program is sponsoring HPW research with the goal of developing the process for deep water offshore pipeline construction utilizing the J-lay method. The first two years of the program have concentrated on weld parameter optimization by producing, testing, and evaluating welds in various grades, wall thicknesses, types and compositions of 3 inch nominal (3.S inch OD) diameter API SL carbon steel pipe. Mechanical properties of the welds and parent metal were evaluated by tensile testing, impact testing, and hardness traverse testing according to guidelines and criteria established by the industrial sponsors. Homopolar pulse welding has demonstrated the capability to produce industrially acceptable full circumference welds in carbon steel line pipe via a rapid, one-shot process. Future work will concentrate on developing the process for commercial field installation, with the program's goal being the demonstration of a prototype system for producing HPW welds in 12 inch diameter pipe in a J-lay configuration.Center for Electromechanic
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Homopolar Pulsed Welding of API Line Pipe
Homopolar pulse welding is a welding process that is being developed to rapidly join API line pipe. This process has particular potential for application in offshore pipeline construction utilizing the J lay method. The weld joint produced has high strength and a narrow heat affected zone. There is a thin brittle zone at the weld interface, resulting in low impact toughness. The weld line is characterized by a very thin >light band>, believed to be a ferrite rich zone produced by a combination of localized melting and mechanical upset. Current developmental research is progressing to eliminate this brittle zone and improve weld geometryCenter for Electromechanic