16,621 research outputs found
Gap anisotropy and universal pairing scale in a spin fluctuation model for cuprates
We consider the evolution of d-wave pairing, mediated by nearly critical spin
fluctuations, with the coupling strength. We show that the onset temperature
for pairing, T*, smoothly evolves between weak and strong coupling, passing
through a broad maximum at intermediate coupling. At strong coupling, T* is of
order the magnetic exchange energy J. We argue that for all couplings, pairing
is confined to the vicinity of the Fermi surface. We also find that thermal
spin fluctuations only modestly reduce T*, even at criticality, but they
substantially smooth the gap anisotropy. The latter evolves with coupling,
being the largest at weak coupling.Comment: 5 pages, 4 figure
Odd Parity and Line Nodes in Heavy Fermion Superconductors
Group theory arguments have demonstrated that a general odd parity order
parameter cannot have line nodes in the presence of spin-orbit coupling. In
this paper, it is shown that these arguments do not hold on the
zone face of a hexagonal close packed lattice. In particular, three of the six
odd parity representations vanish identically on this face. This has potential
relevance to the heavy fermion superconductor .Comment: 5 pages, revte
The Temperature Evolution of the Spectral Peak in High Temperature Superconductors
Recent photoemission data in the high temperature cuprate superconductor
Bi2212 have been interpreted in terms of a sharp spectral peak with a
temperature independent lifetime, whose weight strongly decreases upon heating.
By a detailed analysis of the data, we are able to extract the temperature
dependence of the electron self-energy, and demonstrate that this intepretation
is misleading. Rather, the spectral peak loses its integrity above Tc due to a
large reduction in the electron lifetime.Comment: 5 pages, revtex, 4 encapsulated postscript figure
Protein kinase C modulates the activity of a cloned gamma-aminobutyric acid transporter expressed in Xenopus oocytes via regulated subcellular redistribution of the transporter
We report that activators and inhibitors of protein kinase C (PKC) and protein phosphatases regulate the activity of a cloned rat brain gamma- aminobutyric acid (GABA) transporter (GAT1) expressed in Xenopus oocytes. Four compounds known to activate PKC increased GABA uptake 2- 3.5-fold over basal control levels. Inhibition of PKC by bisindolylmaleimide reduced basal GABA uptake 80% and blocked the phorbol 12-myristate 13-acetate (PMA)-induced stimulation of transport. Okadaic acid, a protein phosphatase inhibitor, stimulated transport 2.5- fold; a 4-fold increase in GABA uptake occurred when oocytes were treated with cyclosporin A, a specific inhibitor of protein phosphatase 2B. Modulation resulted in changes to Vmax but not to Km and was influenced by the functional expression level of the transporter protein; as expression level increased, the ability to up-regulate transporter activity decreased. Down-regulation of transporter activity was independent of expression level. Modulation did not occur through phosphorylation of the three consensus PKC sites predicted by the primary protein sequence since their removal had no effect on the susceptibility of the transporter to modulation by PMA or bisindolylmaleimide. Subcellular fractionation of oocyte membranes demonstrated that under basal level conditions, the majority of GAT1 was targeted to a cytoplasmic compartment corresponding to the trans- Golgi or low density vesicles. Stimulation of PKC with PMA resulted in a translocation of transporters from this compartment to the plasma membrane. At higher expression levels of GAT1 protein, a larger portion of GAT1 was found on the plasma membrane during basal level conditions and treatment with bisindolylmaleimide resulted in removal of these transporters from the plasma membrane. At expression levels demonstrated to be resistant to modulation by PMA, PMA-treatment still resulted in translocation of transporters from the cytoplasm to the plasma membrane. Thus, the inability of PMA to increase uptake at high expression of the GAT1 protein is due to saturation at a step subsequent to translocation. These findings 1) demonstrate the presence of a novel regulated secretory pathway in oocytes and 2) suggest a modulatory mechanism for neurotransmitter transporters that could have significant effects upon synaptic function
Hot Jupiters in binary star systems
Radial velocity surveys find Jupiter mass planets with semi-major axes a less
than 0.1 AU around ~1% of solar-type stars; counting planets with as large
as 5 AU, the fraction of stars having planets reaches ~ 10% {Marcy,Butler}. An
examination of the distribution of semi-major axes shows that there is a clear
excess of planets with orbital periods around 3 or 4 days, corresponding to
a~0.03$ AU, with a sharp cutoff at shorter periods (see Figure 1). It is
believed that Jupiter mass planets form at large distances from their parent
stars; some fraction then migrate in to produce the short period objects. We
argue that a significant fraction of the `hot Jupiters' (a<0.1 AU) may arise in
binary star systems in which the orbit of the binary is highly inclined to the
orbit of the planet. Mutual torques between the two orbits drive down the
minimum separation or periapse r_p between the planet and its host star (the
Kozai mechanism). This periapse collapse is halted when tidal friction on the
planet circularizes the orbit faster than Kozai torque can excite it. The same
friction then circularizes the planet orbit, producing hot Jupiters with the
peak of the semimajor axis distribution lying around 3 days. For the observed
distributions of binary separation, eccentricity and mass ratio, roughly 2.5%
of planets with initial semimajor axis a_p ~ 5au will migrate to within 0.1au
of their parent star. Kozai migration could account for 10% or more of the
observed hot Jupiters.Comment: accepted to ApJ main journal, added one figure and expanded
discussion
Chain configurations in light nuclei
The model of nuclear matter built from alpha-particles is proposed. The
strong deformed shape for doubly even N=Z nuclides from carbon to magnesium has
been determined according to this model. In this paper we undertake very simple
approach, which assumes the existence of low lying chain configurations.Comment: 6 pages, 5 figure
Temperature-dependent quantum pair potentials and their application to dense partially ionized hydrogen plasmas
Extending our previous work \cite{filinov-etal.jpa03ik} we present a detailed
discussion of accuracy and practical applications of finite-temperature
pseudopotentials for two-component Coulomb systems. Different pseudopotentials
are discussed: i) the diagonal Kelbg potential, ii) the off-diagonal Kelbg
potential iii) the {\em improved} diagonal Kelbg potential, iv) an effective
potential obtained with the Feynman-Kleinert variational principle v) the
``exact'' quantum pair potential derived from the two-particle density matrix.
For the {\em improved} diagonal Kelbg potential a simple temperature dependent
fit is derived which accurately reproduces the ``exact'' pair potential in the
whole temperature range. The derived pseudopotentials are then used in path
integral Monte Carlo (PIMC) and molecular dynamics (MD) simulations to obtain
thermodynamical properties of strongly coupled hydrogen. It is demonstrated
that classical MD simulations with spin-dependent interaction potentials for
the electrons allow for an accurate description of the internal energy of
hydrogen in the difficult regime of partial ionization down to the temperatures
of about K. Finally, we point out an interesting relation between the
quantum potentials and effective potentials used in density functional theory.Comment: 18 pages, 11 figure
Spin-memory loss at Co/Ru interfaces
We have determined the spin-memory-loss parameter, , by
measuring the transmission of spin-triplet and spin-singlet Cooper pairs across
Co/Ru interfaces in Josephson junctions and by Current-Perpendicular-to-Plane
Giant Magnetoresistance (CPP-GMR) techniques. The probability of spin-memory
loss at the Co/Ru interface is . From the CPP-MR, we
obtain that is in good agreement with
obtained from spin-triplet transmission. For
spin-singlet transmission, we have that is
different from that obtained from CPP-GMR and spin-triplet transmission. The
source of this difference is not understood.Comment: 9 pages, 9 figure
Differential coupling of G protein alpha subunits to seven-helix receptors expressed in Xenopus oocytes
Xenopus oocytes were used to examine the coupling of the serotonin 1c (5HT1c) and thyrotropin-releasing hormone (TRH) receptors to both endogenous and heterologously expressed G protein alpha subunits. Expression of either G protein-coupled receptor resulted in agonist- induced, Ca(2+)-activated Cl- currents that were measured using a two- electrode voltage clamp. 5HT-induced Cl- currents were reduced 80% by incubating the injected oocytes with pertussis toxin (PTX) and inhibited 50-65% by injection of antisense oligonucleotides to the PTX- sensitive Go alpha subunit. TRH-induced Cl- currents were reduced only 20% by PTX treatment but were inhibited 60% by injection of antisense oligonucleotides to the PTX-insensitive Gq alpha subunit. Injection of antisense oligonucleotides to a novel Xenopus phospholipase C-beta inhibited the 5HT1c (and Go)-induced Cl- current with little effect on the TRH (and Gq)-induced current. These results suggest that receptor- activated Go and Gq interact with different effectors, most likely different isoforms of phospholipase C-beta. Co-expression of each receptor with seven different mammalian G protein alpha subunit cRNAs (Goa, Gob, Gq, G11, Gs, Golf, and Gt) was also examined. Co-expression of either receptor with the first four of these G alpha subunits resulted in a maximum 4-6-fold increase in Cl- currents; the increase depended on the amount of G alpha subunit cRNA injected. This increase was blocked by PTX for G alpha oa and G alpha ob co-expression but not for G alpha q or G alpha 11 co-expression. Co-expression of either receptor with Gs, Golf, or Gt had no effect on Ca(2+)-activated Cl- currents; furthermore, co-expression with Gs or Golf also failed to reveal 5HT- or TRH-induced changes in adenylyl cyclase as assessed by activation of the cystic fibrosis transmembrane conductance regulator Cl- channel. These results indicate that in oocytes, the 5HT1c and TRH receptors do the following: 1) preferentially couple to PTX-sensitive (Go) and PTX-insensitive (Gq) G proteins and that these G proteins act on different effectors, 2) couple within the same cell type to several different heterologously expressed G protein alpha subunits to activate the oocyte's endogenous Cl- current, and 3) fail to couple to G protein alpha subunits that activate cAMP or phosphodiesterase
The modulated spin liquid: a new paradigm for URuSi
We argue that near a Kondo breakdown critical point, a spin liquid with
spatial modulations can form. Unlike its uniform counterpart, we find that this
occurs via a second order phase transition. The amount of entropy quenched when
ordering is of the same magnitude as for an antiferromagnet. Moreover, the two
states are competitive, and at low temperatures are separated by a first order
phase transition. The modulated spin liquid we find breaks symmetry, as
recently seen in the hidden order phase of URuSi. Based on this, we
suggest that the modulated spin liquid is a viable candidate for this unique
phase of matter.Comment: 4 pages, 2 figure
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