2,024 research outputs found
A eubacterial origin for the human tRNA nucleotidyltransferase?
tRNA CCA-termini are generated and maintained by tRNA nucleotidyltransferases. Together with poly(A) polymerases and other enzymes they belong to the nucleotidyltransferase superfamily. However, sequence alignments within this family do not allow to distinguish between CCA-adding enzymes and poly(A) polymerases. Furthermore, due to the lack of sequence information about animal CCA-adding enzymes, identification of corresponding animal genes was not possible so far. Therefore, we looked for the human homolog using the baker's yeast tRNA nucleotidyltransferase as a query sequence in a BLAST search. This revealed that the human gene transcript CGI-47, (\#AF151805) deposited in GenBank is likely to encode such an enzyme. To identify the nature of this protein, the cDNA of the transcript was cloned and the recombinant protein biochemically characterized, indicating that CGI-47 encodes a bona fide CCA-adding enzyme and not a poly(A) polymerase. This confirmed animal CCA-adding enzyme allowed us to identify putative homologs from other animals. Calculation of a neighbor-joining tree, using an alignment of several CCA-adding enzymes, revealed that the animal enzymes resemble more eubacterial ones than eukaryotic plant and fungal tRNA nucleotidyltransferases, suggesting that the animal nuclear cca genes might have been derived from the endosymbiotic progenitor of mitochondria and are therefore of eubacterial origin
Quantum Effects and Broken Symmetries in Frustrated Antiferromagnets
We investigate the interplay between frustration and zero-point quantum
fluctuations in the ground state of the triangular and Heisenberg
antiferromagnets, using finite-size spin-wave theory, exact diagonalization,
and quantum Monte Carlo methods. In the triangular Heisenberg antiferromagnet,
by performing a systematic size-scaling analysis, we have obtained strong
evidences for a gapless spectrum and a finite value of the thermodynamic order
parameter, thus confirming the existence of long-range N\'eel order.The good
agreement between the finite-size spin-wave results and the exact and quantum
Monte Carlo data also supports the reliability of the spin-wave expansion to
describe both the ground state and the low-energy spin excitations of the
triangular Heisenberg antiferromagnet. In the Heisenberg model, our
results indicate the opening of a finite gap in the thermodynamic excitation
spectrum at , marking the melting of the antiferromagnetic
N\'eel order and the onset of a non-magnetic ground state. In order to
characterize the nature of the latter quantum-disordered phase we have computed
the susceptibilities for the most important crystal symmetry breaking
operators. In the ordered phase the effectiveness of the spin-wave theory in
reproducing the low-energy excitation spectrum suggests that the uniform spin
susceptibility of the model is very close to the linear spin-wave prediction.Comment: Review article, 44 pages, 18 figures. See also PRL 87, 097201 (2001
Origin of time reversal symmetry breaking in Y(1-y)Ca(y)Ba(2)Cu(3)O(7-x)
We have studied the Zero Bias Conductance Peak (ZBCP) of the tunneling
conductance measured on (1,1,0) oriented Y(1-y)Ca(y)Ba(2)Cu(3)O(7-x) thin films
as a function of doping and of magnetic field. A spontaneous (zero field) split
of the ZBCP was observed only in overdoped samples (either by O or by Ca). The
magnitude of this split was found to be linear in doping. All samples exhibited
a magnetic field splitting, also strongly doping dependent. The field
susceptibility chi=d(delta)/dH diverges at the point at which spontaneous ZBCP
splitting occurs, its inverse value, chi^(-1), following a linear doping
dependence on both the underdoped and overdoped sides. We discuss these results
in terms of recent theoretical models of Time Reversal Symmetry Breaking
(TRSB).Comment: 5 figure
Phase transition between d-wave and anisotropic s-wave gaps in high temperature oxides superconductors
We study models for superconductivity with two interactions: due to
antiferromagnetic(AF) fluctuations and due to phonons, in a weak coupling
approach to the high temperature superconductivity. The nature of the two
interactions are considerably different; is positive and sharply peaked
at (,) while is negative and peaked at () due to
weak phonon screening. We numerically find (a) weak BCS attraction is enough to
have high critical temperature if a van Hove anomaly is at work, (b) (AF)
is important to give d-wave superconductivity, (c) the gap order parameter
is constant(s-wave) at extremely overdope region and it
changes to anisotropic s-wave as doping is reduced, (d) there exists a first
order phase transition between d-wave and anisotropic s-wave gaps. These
results are qualitatively in agreement with preceding works; they should be
modified in the strongly underdope region by the presence of antiferromagnetic
fluctuations and ensuing AF pseudogap.Comment: 4 pages in RevTex (double column), 4 figure
Coexistence of a triplet nodal order-parameter and a singlet order-parameter at the interfaces of ferromagnet-superconductor Co/CoO/In junctions
We present differential conductance measurements of Cobalt / Cobalt-Oxide /
Indium planar junctions, 500nm x 500nm in size. The junctions span a wide range
of barriers, from very low to a tunnel barrier. The characteristic conductance
of all the junctions show a V-shape structure at low bias instead of the
U-shape characteristic of a s-wave order parameter. The bias of the conductance
peaks is, for all junctions, larger than the gap of indium. Both properties
exclude pure s-wave pairing. The data is well fitted by a model that assumes
the coexistence of s-wave singlet and equal spin p-wave triplet fluids. We find
that the values of the s-wave and p-wave gaps follow the BCS temperature
dependance and that the amplitude of the s-wave fluid increases with the
barrier strength.Comment: 5 pages, Accepted to Phys. Rev.
Kinetic energy change with doping upon superfluid condensation in high temperature superconductors
In conventional BCS superconductors, the electronic kinetic energy increases
upon superfluid condensation (the change DEkin is positive). Here we show that
in the high critical temperature superconductor Bi-2212, DEkin crosses over
from a fully compatible conventional BCS behavior (DEkin>0) to an
unconventional behavior (DEkin<0) as the free carrier density decreases. If a
single mechanism is responsible for superconductivity across the whole phase
diagram of high critical temperature superconductors, this mechanism should
allow for a smooth transition between such two regimes around optimal doping.Comment: 3 pages, 2 figure
Andreev reflection in Au/La_{2-x}Sr_{x}CuO_{4} point-contact junctions: separation between pseudogap and phase-coherence gap
We made point-contact measurements with Au tips on La_{2-x}Sr_{x}CuO_{4}
samples with 0.08 < x < 0.20 to investigate the relationship between
superconducting gap and pseudogap. We obtained junctions whose conductance
curves presented typical Andreev reflection features at all temperatures from
4.2 K up to T_c^A close to the bulk T_c. Their fit with the
BTK-Tanaka-Kashiwaya model gives good results if a (s+d)-wave gap symmetry is
used. The doping dependence of the low temperature dominant isotropic gap
component Delta_{s} follows very well the T_{c} vs. x curve. These results
support the separation between the superconducting (Andreev) gap and the
pseudogap measured by angle-resolved photoemission spectroscopy (ARPES) and
tunneling.Comment: 4 pages, 5 eps figures, 1 table. SNS 2001 Conferenc
Andreev reflection spectroscopy of the heavy-fermion superconductor CeCoIn along three different crystallographic orientations
Andreev reflection spectroscopy has been performed on the heavy-fermion
superconductor (HFS) CeCoIn single crystals along three different
crystallographic orientations, (001), (110), and (100), using Au tips as
counter-electrodes. Dynamic conductance spectra are reproducible over wide
temperature ranges and consistent with each other, ensuring the spectroscopic
nature. Features common to all directions are: i) asymmetric behaviors of the
background conductance, which we attribute to the emerging coherent
heavy-fermion liquid; ii) energy scales (~1 meV) for conductance enhancement
due to Andreev reflection; iii) magnitudes of enhanced zero-bias conductance
(10 - 13 %). These values are an order of magnitude smaller than the predicted
value by the Blonder-Tinkham-Klapwijk (BTK) theory, but comparable to those for
other HFSs. Using the d-wave BTK model, we obtain an energy gap of ~ 460 ueV.
However, it is found that extended BTK models considering the mismatch in Fermi
surface parameters do not account for our data completely, which we attribute
to the shift of spectral weight to low energy as well as to the suppressed
Andreev reflection. A qualitative comparison of the conductance spectra with
calculated curves shows a consistency with d-symmetry, providing
the first spectroscopic evidence for the order parameter symmetry and resolving
the controversy over the location of the line nodes.Comment: invited talk submitted to the 8th M2S conference to be held in
Dresden Germany, July 9-14, 2006, 4 pages, 3 figure
Determination of the critical current density in the d-wave superconductor YBCO under applied magnetic fields by nodal tunneling
We have studied nodal tunneling into YBa2Cu3O7-x (YBCO) films under magnetic
fields. The films' orientation was such that the CuO2 planes were perpendicular
to the surface with the a and b axis at 450 form the normal. The magnetic field
was applied parallel to the surface and perpendicular to the CuO2 planes. The
Zero Bias Conductance Peak (ZBCP) characteristic of nodal tunneling splits
under the effect of surface currents produced by the applied fields. Measuring
this splitting under different field conditions, zero field cooled and field
cooled, reveals that these currents have different origins. By comparing the
field cooled ZBCP splitting to that taken in decreasing fields we deduce a
value of the Bean critical current superfluid velocity, and calculate a Bean
critical current density of up to 3*10^7 A/cm2 at low temperatures. This
tunneling method for the determination of critical currents under magnetic
fields has serious advantages over the conventional one, as it avoids having to
make high current contacts to the sample.Comment: 8 pages, 2 figure
Quantitative Simulation of the Superconducting Proximity Effect
A numerical method is developed to calculate the transition temperature of
double or multi-layers consisting of films of super- and normal conductors. The
approach is based on a dynamic interpretation of Gorkov's linear gap equation
and is very flexible. The mean free path of the different metals, transmission
through the interface, ratio of specular reflection to diffusive scattering at
the surfaces, and fraction of diffusive scattering at the interface can be
included. Furthermore it is possible to vary the mean free path and the BCS
interaction NV in the vicinity of the interface. The numerical results show
that the normalized initial slope of an SN double layer is independent of
almost all film parameters except the ratio of the density of states. There are
only very few experimental investigations of this initial slope and they
consist of Pb/Nn double layers (Nn stands for a normal metal). Surprisingly the
coefficient of the initial slope in these experiments is of the order or less
than 2 while the (weak coupling) theory predicts a value of about 4.5. This
discrepancy has not been recognized in the past. The autor suggests that it is
due to strong coupling behavior of Pb in the double layers. The strong coupling
gap equation is evaluated in the thin film limit and yields the value of 1.6
for the coefficient. This agrees much better with the few experimental results
that are available.
PACS: 74.45.+r, 74.62.-c, 74.20.F
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