1,094 research outputs found
RVB Contribution to Superconductivity in
We view as electronically equivalent to (non-staggered) graphite
( layer) that has undergone a zero gap semiconductor to a superconductor
phase transition by a large c-axis (chemical) pressure due to layers.
Further, like the \ppi bonded planar organic molecules, graphite is an old
resonating valence bond (RVB) system. The RVB's are the `preexisting cooper
pairs' in the `parental' zero gap semiconducting (graphite) sheets that
manifests themselves as a superconducting ground state of the transformed
metal. Some consequences are pointed out.Comment: 4 pages, 2 figure, RevTex. Based on a talk given at the Institute
Seminar Week, IMSc, Madras (12-16, Feb. 2001
The connection between superconducting phase correlations and spin excitations in YBaCuO: A magnetic field study
One of the most striking universal properties of the
high-transition-temperature (high-) superconductors is that they are all
derived from the hole-doping of their insulating antiferromagnetic (AF) parent
compounds. From the outset, the intimate relationship between magnetism and
superconductivity in these copper-oxides has intrigued researchers. Evidence
for this link comes from neutron scattering experiments that show the
unambiguous presence of short-range AF correlations (excitations) in cuprate
superconductors. Even so, the role of such excitations in the pairing mechanism
and superconductivity is still a subject of controversy. For
YBaCuO, where controls the hole-doping level, the most
prominent feature in the magnetic excitations spectra is the ``resonance''.
Here we show that for underdoped YBaCuO, where and
are below the optimal values, modest magnetic fields suppress the resonance
significantly, much more so for fields approximately perpendicular rather than
parallel to the CuO planes. Our results indicate that the resonance
measures pairing and phase coherence, suggesting that magnetism plays an
important role in the superconductivity of cuprates. The persistence of a field
effect above favors mechanisms with preformed pairs in the normal state
of underdoped cuprates.Comment: 12 pages, 4 figures, Nature (in press
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
Beta decay of r-process waiting-point nuclei in a self-consistent approach
Beta-decay rates for spherical neutron-rich r-process waiting-point nuclei
are calculated within a fully self-consistent Quasiparticle Random-Phase
Approximation, formulated in the Hartree-Fock-Bogolyubov canonical
single-particle basis. The same Skyrme force is used everywhere in the
calculation except in the proton-neutron particle-particle channel, where a
finite-range force is consistently employed. In all but the heaviest nuclei,
the resulting half-lives are usually shorter by factors of 2 to 5 than those of
calculations that ignore the proton-neutron particle-particle interaction. The
shorter half-lives alter predictions for the abundance distribution of
r-process elements and for the time it takes to synthesize them.Comment: 14 pages RevTex, 10 eps figures, submitted to Phys. Rev.
Limits on different Majoron decay modes of Mo and Se for neutrinoless double beta decays in the NEMO-3 experiment
The NEMO-3 tracking detector is located in the Fr\'ejus Underground
Laboratory. It was designed to study double beta decay in a number of different
isotopes. Presented here are the experimental half-life limits on the double
beta decay process for the isotopes Mo and Se for different
Majoron emission modes and limits on the effective neutrino-Majoron coupling
constants. In particular, new limits on "ordinary" Majoron (spectral index 1)
decay of Mo ( y) and Se ( y) have been obtained. Corresponding bounds on the
Majoron-neutrino coupling constant are
and .Comment: 23 pages includind 4 figures, to be published in Nuclear Physics
Beta-decay in odd-A and even-even proton-rich Kr isotopes
Beta-decay properties of proton-rich odd-A and even-even Krypton isotopes are
studied in the framework of a deformed selfconsistent Hartree-Fock calculation
with density-dependent Skyrme forces, including pairing correlations between
like nucleons in BCS approximation. Residual spin-isospin interactions are
consistently included in the particle-hole and particle-particle channels and
treated in Quasiparticle Random Phase Approximation. The similarities and
differences in the treatment of even-even and odd-A nuclei are stressed.
Comparison to available experimental information is done for Gamow-Teller
strength distributions, summed strengths, and half-lives. The dependence of
these observables on deformation is particularly emphasized in a search for
signatures of the shape of the parent nucleus.Comment: 29 pages, 16 figure
Measurement of double beta decay of 100Mo to excited states in the NEMO 3 experiment
The double beta decay of 100Mo to the 0^+_1 and 2^+_1 excited states of 100Ru
is studied using the NEMO 3 data. After the analysis of 8024 h of data the
half-life for the two-neutrino double beta decay of 100Mo to the excited 0^+_1
state is measured to be T^(2nu)_1/2 = [5.7^{+1.3}_{-0.9}(stat)+/-0.8(syst)]x
10^20 y. The signal-to-background ratio is equal to 3. Information about energy
and angular distributions of emitted electrons is also obtained. No evidence
for neutrinoless double beta decay to the excited 0^+_1 state has been found.
The corresponding half-life limit is T^(0nu)_1/2(0^+ --> 0^+_1) > 8.9 x 10^22 y
(at 90% C.L.).
The search for the double beta decay to the 2^+_1 excited state has allowed
the determination of limits on the half-life for the two neutrino mode
T^(2nu)_1/2(0^+ --> 2^+_1) > 1.1 x 10^21 y (at 90% C.L.) and for the
neutrinoless mode T^(0nu)_1/2(0^+ --> 2^+_1) > 1.6 x 10^23 y (at 90% C.L.).Comment: 23 pages, 7 figures, 4 tables, submitted to Nucl. Phy
Probing New Physics Models of Neutrinoless Double Beta Decay with SuperNEMO
The possibility to probe new physics scenarios of light Majorana neutrino
exchange and right-handed currents at the planned next generation neutrinoless
double beta decay experiment SuperNEMO is discussed. Its ability to study
different isotopes and track the outgoing electrons provides the means to
discriminate different underlying mechanisms for the neutrinoless double beta
decay by measuring the decay half-life and the electron angular and energy
distributions.Comment: 17 pages, 14 figures, to be published in E.P.J.
Conformational adaptation of Asian macaque TRIMCyp directs lineage specific antiviral activity
TRIMCyps are anti-retroviral proteins that have arisen independently in New World and Old World primates. All TRIMCyps comprise a CypA domain fused to the tripartite domains of TRIM5α but they have distinct lentiviral specificities, conferring HIV-1 restriction in New World owl monkeys and HIV-2 restriction in Old World rhesus macaques. Here we provide evidence that Asian macaque TRIMCyps have acquired changes that switch restriction specificity between different lentiviral lineages, resulting in species-specific alleles that target different viruses. Structural, thermodynamic and viral restriction analysis suggests that a single mutation in the Cyp domain, R69H, occurred early in macaque TRIMCyp evolution, expanding restriction specificity to the lentiviral lineages found in African green monkeys, sooty mangabeys and chimpanzees. Subsequent mutations have enhanced restriction to particular viruses but at the cost of broad specificity. We reveal how specificity is altered by a scaffold mutation, E143K, that modifies surface electrostatics and propagates conformational changes into the active site. Our results suggest that lentiviruses may have been important pathogens in Asian macaques despite the fact that there are no reported lentiviral infections in current macaque populations
- …