190 research outputs found
Incommensurate Charge and Spin Fluctuations in d-wave Superconductors
We show analytic results for the irreducible charge and spin
susceptibilities, , where is the momentum
transfer between the nodes in d-wave superconductors. Using the BCS theory and
a circular Fermi surface, we find that the singular behavior of the irreducible
charge susceptibility leads to the dynamic incommensurate charge collective
modes. The peaks in the charge structure factor occur at a set of wave vectors
which form an ellipse around and in
momentum space with momentum dependent spectral weight. It is also found that,
due to the non-singular irreducible spin susceptibility, an extremely strong
interaction via random phase approximation is required to support the magnetic
peaks near . Under certain conditions, the peaks in the magnetic
structure factor occur near and .Comment: 5 pages, 3 figure
Dynamical charge susceptibility in layered cuprates: the influence of screened inter-site Coulomb repulsion
The analytical expression for dynamical charge susceptibility in layered
cuprates has been derived in the frame of singlet-correlated band model beyond
random-phase-approximation (RPA) scheme. Our calculations performed near
optimal doping regime show that there is a peak in real part of the charge
susceptibility at {\bf Q} = (, ) at strong
enough inter-site Coulomb repulsion. Together with the strong maximum in the Im
at 15 meV it confirms the formation of low-energetic
plasmons or charge fluctuations. This provides a jsutification that these
excitations are important and together with a spin flcutuations can contribute
to the Cooper pairing in layered cuprates. Analysing the charge susceptibilitiy
with respect to an instability we obtain a new plasmon branch, , along the Brillouin Zone. In particular, we have found that it goes to
zero near {\bf Q}
Pseudogap behavior of nuclear spin relaxation in high Tc superconductors in terms of phase separation
We analyze anew experiments on the NMR in cuprates and find an important
information on their phase separation and its stripe character hidden in the
dependence of on degree of doping. In a broad class of materials
is the sum of two terms: the temperature independent one
attributed to ``incommensurate'' stripes that occur at external doping, and an
``universal'' temperature dependent term ascribed to moving metallic and AF
sub-phases. We argue that the frustrated first order phase transition in a
broad temperature interval bears a dynamical character.Comment: 5 pages, 3 figures; some comments and references added; accepted for
publication in JETP Letter
Microscopic theory of weak pseudogap behavior in the underdoped cuprate superconductors I: General theory and quasiparticle properties
We derive in detail a novel solution of the spin fermion model which is valid
in the quasi-static limit pi T<<omega_sf, found in the intermediate
(pseudoscaling) regime of the magnetic phase diagram of cuprate
superconductors, and use it to obtain results for the temperature and doping
dependence of the single particle spectral density, the electron-spin
fluctuation vertex function, and the low frequency dynamical spin
susceptibility. The resulting strong anisotropy of the spectral density and the
vertex function lead to the qualitatively different behavior of_hot_ (around
k=(pi,0)) and_cold_ (around k=(pi/2,pi/2)) quasiparticles seen in ARPES
experiments. We find that the broad high energy features found in ARPES
measurements of the spectral density of the underdoped cuprate superconductors
are determined by strong antiferromagnetic (AF) correlations and incoherent
precursor effects of an SDW state, with reduced renormalized effective coupling
constant. The electron spin-fluctuation vertex function, i.e. the effective
interaction of low energy quasiparticles and spin degrees of freedom, is found
to be strongly anisotropic and enhanced for hot quasiparticles; the
corresponding charge-fluctuation vertex is considerably diminished. We thus
demonstrate that, once established, strong AF correlations act to reduce
substantially the effective electron-phonon coupling constant in cuprate
superconductors.Comment: REVTEX with EPS figures, uses multicol.sty, epsfig,sty, psfig.st
Doping Dependence of the Pseudogap in La(2-x)Sr(x)CuO(4)
We report the results of Raman scattering experiments on single crystals of
La(2-x)Sr(x)CuO(4) that span the range from underdoped (x = 0.10) to overdoped
(x =0.22). The spectra are consistent with the existence of a strong
anisotropic quasiparticle interaction that results in a normal state depletion
of spectral weight from regions of the Fermi surface located near the zone
axes. The strength of the interaction decreases rapidly with increasing hole
concentration and the spectral evidence for the pseudogap vanishes when the
optimum doping level is reached. The results suggest that the pseudogap and
superconducting gap arise from different mechanisms.Comment: 7 pages, 6 eps figures, added new sections, figures, reference
Cytogerontology since 1881: A reappraisal of August Weismann and a review of modern progress
Cytogerontology, the science of cellular ageing, originated in 1881 with the prediction by August Weismann that the somatic cells of higher animals have limited division potential. Weismann's prediction was derived by considering the role of natural selection in regulating the duration of an organism's life. For various reasons, Weismann's ideas on ageing fell into neglect following his death in 1914, and cytogerontology has only reappeared as a major research area following the demonstration by Hayflick and Moorhead in the early 1960s that diploid human fibroblasts are restricted to a finite number of divisions in vitro.
In this review we give a detailed account of Weismann's theory, and we reveal that his ideas were both more extensive in their scope and more pertinent to current research than is generally recognised. We also appraise the progress which has been made over the past hundred years in investigating the causes of ageing, with particular emphasis being given to (i) the evolution of ageing, and (ii) ageing at the cellular level. We critically assess the current state of knowledge in these areas and recommend a series of points as primary targets for future research
Renormalized mean-field theory of the neutron scattering in cuprate superconductors
The magnetic excitation spectrum of the t-t'-J-model is studied in mean-field
theory and compared to inelastic neutron-scattering (INS) experiments on YBCO
and BSCCO superconductors. Within the slave-particle formulation the dynamical
spin response is calculated from a renormalized Fermi liquid with an effective
interaction ~J in the magnetic particle--hole channel. We obtain the so-called
41meV resonance at wave vector (pi,pi) as a collective spin-1 excitation in the
d-wave superconducting state. It appears sharp (undamped), if the underlying
Fermi surface is hole-like with a sufficient next-nearest-neighbor hopping
t'<0. The double-layer structure of YBCO or BSCCO is not important for the
resonance to form. The resonance energy \omega_{res} and spectral weight at
optimal doping come out comparable to experiment. The observed qualitative
behavior of \omega_{res} with hole filling is reproduced in the underdoped as
well as overdoped regime. A second, much broader peak becomes visible in the
magnetic excitation spectrum if the 2D wave-vector is integrated over. It is
caused by excitations across the maximum gap, and in contrast to the resonance
its energy is almost independent of doping. At energies above or below
\omega_{res} the commensurate resonance splits into incommensurate peaks,
located off (pi,pi). Below \omega_{res} the intensity pattern is of `parallel'
type and the dispersion relation of incommensurate peaks has a negative
curvature. This is in accordance with recent INS experiments on YBCO.Comment: 17pp including 14 figure
REM sleep behavior disorder was associated with Parkinsonβs disease: a community-based study
Bax Function in the Absence of Mitochondria in the Primitive Protozoan Giardia lamblia
Bax-induced permeabilization of the mitochondrial outer membrane and release of cytochrome c are key events in apoptosis. Although Bax can compromise mitochondria in primitive unicellular organisms that lack a classical apoptotic machinery, it is still unclear if Bax alone is sufficient for this, or whether additional mitochondrial components are required. The protozoan parasite Giardia lamblia is one of the earliest branching eukaryotes and harbors highly degenerated mitochondrial remnant organelles (mitosomes) that lack a genome. Here we tested whether human Bax expressed in Giardia can be used to ablate mitosomes. We demonstrate that these organelles are neither targeted, nor compromised, by Bax. However, specialized compartments of the regulated secretory pathway are completely ablated by Bax. As a consequence, maturing cyst wall proteins that are sorted into these organelles are released into the cytoplasm, causing a developmental arrest and cell death. Interestingly, this ectopic cargo release is dependent on the carboxy-terminal 22 amino acids of Bax, and can be prevented by the Bax-inhibiting peptide Ku70. A C-terminally truncated Bax variant still localizes to secretory organelles, but is unable to permeabilize these membranes, uncoupling membrane targeting and cargo release. Even though mitosomes are too diverged to be recognized by Bax, off-target membrane permeabilization appears to be conserved and leads to cell death completely independently of mitochondria
Bid Regulates the Pathogenesis of Neurotropic Reovirus
Reovirus infection leads to apoptosis in both cultured cells and the murine central nervous system (CNS). NF-ΞΊB-driven transcription of proapoptotic cellular genes is required for the effector phase of the apoptotic response. Although both extrinsic death-receptor signaling pathways and intrinsic pathways involving mitochondrial injury are implicated in reovirus-induced apoptosis, mechanisms by which either of these pathways are activated and their relationship to NF-ΞΊB signaling following reovirus infection are unknown. The proapoptotic Bcl-2 family member, Bid, is activated by proteolytic cleavage following reovirus infection. To understand how reovirus integrates host signaling circuits to induce apoptosis, we examined proapoptotic signaling following infection of Bid-deficient cells. Although reovirus growth was not affected by the absence of Bid, cells lacking Bid failed to undergo apoptosis. Furthermore, we found that NF-ΞΊB activation is required for Bid cleavage and subsequent proapoptotic signaling. To examine the functional significance of Bid-dependent apoptosis in reovirus disease, we monitored fatal encephalitis caused by reovirus in the presence and absence of Bid. Survival of Bid-deficient mice was significantly enhanced in comparison to wild-type mice following either peroral or intracranial inoculation of reovirus. Decreased reovirus virulence in Bid-null mice was accompanied by a reduction in viral yield. These findings define a role for NF-ΞΊB-dependent cleavage of Bid in the cell death program initiated by viral infection and link Bid to viral virulence
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