950 research outputs found
Staggered Pairing Phenomenology for UPd_2Al_3 and UNi_2Al_3
We apply the staggered-pairing Ginzburg-Landau phenomenology to describe
superconductivity in UPd_2Al_3 and UNi_2Al_3. The phenomenology was applied
successfully to UPt_3 so it explains why these materials have qualitatively
different superconducting phase diagrams although they have the same
point-group symmetry. UPd_2Al_3 and UNi_2Al_3 have a two-component
superconducting order parameter transforming as an H-point irreducible
representation of the space group. Staggered superconductivity can induce
charge-density waves characterized by new Bragg peaks suggesting experimental
tests of the phenomenology.Comment: 4 pages, REVTeX, 2 Postscript figure
Phase Diagram of the Two-Channel Kondo Lattice
The phase diagram of the two-channel Kondo lattice model is examined with a
Quantum Monte Carlo simulation in the limit of infinite dimensions.
Commensurate (and incommensurate) antiferromagnetic and superconducting states
are found. The antiferromagnetic transition is very weak and continuous;
whereas the superconducting transition is discontinuous to an odd-frequency
channel-singlet and spin-singlet pairing state.Comment: 5 pages, LaTeX and 4 PS figures (see also cond-mat/9609146 and
cond-mat/9605109
Effect of c(2x2)-CO overlayer on the phonons of Cu(001): a first principles study
We have examined the effect of a c(2x2) overlayer of CO on the surface
phonons of the substrate, Cu(001), by applying the density functional
perturbation theory with both the local (LDA) and the generalized-gradient
(GGA) density approximations, through the Hedin-Lundqvist and the
Perdew-Burke-Ernzerhof functionals, respectively. Our results (GGA) trace the
Rayleigh wave softening detected by helium atom scattering (HAS) experiments to
changes in the force constants between the substrate surface atoms brought
about by CO chemisorption, resolving an ongoing debate on the subject. The
calculated surface phonon dispersion curves document the changes in the
polarization of some modes and show those of the modes originally along the
direction of the clean surface Brillouin zone (SBZ) which are
back-folded along the direction of the chemisorbed SBZ, to be
particularly consequential. The vertical and shear horizontal section of
in the SBZ of the clean surface, for example, is back-folded as a
longitudinal-vertical mode, indicating thereby that predicted a long
time back along for the clean surface may be indirectly
assessed at upon CO adsorption by standard planar scattering
techniques. These findings further suggest that some of the energy losses
detected by HAS along , which were associated to multiphonon
excitations of the adlayer frustrated translation mode, may actually correspond
to the back-folded substrate surface modes
On the Puzzle of Odd-Frequency Superconductivity
Since the first theoretical proposal by Berezinskii, an odd-frequency
superconductivity has encountered the fundamental problems on its thermodynamic
stability and rigidity of a homogenous state accompanied by unphysical Meissner
effect. Recently, Solenov {\it et al}. [Phys. Rev. B {\bf 79} (2009) 132502.]
have asserted that the path-integral formulation gets rid of the difficulties
leading to a stable homogenous phase with an ordinary Meissner effect. Here, we
show that it is crucial to choose the appropriate saddle-point solution that
minimizes the effective free energy, which was assumed {\it implicitly} in the
work by Solenov and co-workers. We exhibit the path-integral framework for the
odd-frequency superconductivity with general type of pairings, including an
argument on the retarded functions via the analytic continuation to the real
axis.Comment: 6 pages, in JPSJ forma
Encouraging versatile thinking in algebra using the computer
In this article we formulate and analyse some of the obstacles to understanding the notion of a variable, and the use and meaning of algebraic notation, and report empirical evidence to support the hypothesis that an approach using the computer will be more successful in overcoming these obstacles. The computer approach is formulated within a wider framework ofversatile thinking in which global, holistic processing complements local, sequential processing. This is done through a combination of programming in BASIC, physical activities which simulate computer storage and manipulation of variables, and specific software which evaluates expressions in standard mathematical notation. The software is designed to enable the user to explore examples and non-examples of a concept, in this case equivalent and non-equivalent expressions. We call such a piece of software ageneric organizer because if offers examples and non-examples which may be seen not just in specific terms, but as typical, or generic, examples of the algebraic processes, assisting the pupil in the difficult task of abstracting the more general concept which they represent. Empirical evidence from several related studies shows that such an approach significantly improves the understanding of higher order concepts in algebra, and that any initial loss in manipulative facility through lack of practice is more than made up at a later stage
Composition, structure and stability of RuO_2(110) as a function of oxygen pressure
Using density-functional theory (DFT) we calculate the Gibbs free energy to
determine the lowest-energy structure of a RuO_2(110) surface in thermodynamic
equilibrium with an oxygen-rich environment. The traditionally assumed
stoichiometric termination is only found to be favorable at low oxygen chemical
potentials, i.e. low pressures and/or high temperatures. At realistic O
pressure, the surface is predicted to contain additional terminal O atoms.
Although this O excess defines a so-called polar surface, we show that the
prevalent ionic model, that dismisses such terminations on electrostatic
grounds, is of little validity for RuO_2(110). Together with analogous results
obtained previously at the (0001) surface of corundum-structured oxides, these
findings on (110) rutile indicate that the stability of non-stoichiometric
terminations is a more general phenomenon on transition metal oxide surfaces.Comment: 12 pages including 5 figures. Submitted to Phys. Rev. B. Related
publications can be found at http://www.fhi-berlin.mpg.de/th/paper.htm
Possible Odd-Frequency Superconductivity in Strong-Coupling Electron-Phonon Systems
A possibility of the odd-frequency pairing in the strong-coupling
electron-phonon systems is discussed. Using the Holstein-Hubbard model, we
demonstrate that the anomalously soft Einstein mode with the frequency
( is the order of the renormalized
bandwidth) mediates the s-wave odd-frequency triplet pairing against the
ordinary even-frequency singlet pairing. It is necessary for the emergence of
the odd-frequency pairing that the pairing interaction is strongly retarded as
well as the strong coupling, since the pairing interaction for the
odd-frequency pairing is effective only in the diagonal scattering channel,
with
. Namely, the odd-frequency
superconductivity is realized in the opposite limit of the original BCS theory.
The Ginzburg-Landau analysis in the strong-coupling region shows that the
specific-heat discontinuity and the slope of the temperature dependence of the
superfluid density can be quite small as compared with the BCS values,
depending on the ratio of the transition temperature and .Comment: 6 pages, 7 figures, submitted to J. Phys. Soc. Jp
Functional Characterization of Promoter Variants of the Adiponectin Gene Complemented by Epidemiological Data
# These authors contributed equally to the work. *Corresponding author
Microscopic theory of quadrupolar ordering in TmTe
We have calculated the crystal electric field of TmTe (T>T_Q) and have
obtained that the ground state of a Tm 4f hole is the doublet in
agreement with Mossbauer experiments. We study the quadrupole interactions
arising from quantum transitions of 4f holes of Tm. An effective attraction is
found at the L point of the Brillouin zone, . Assuming that the
quadrupolar condensation involves a single arm of we show that
there are two variants for quadrupole ordering which are described by the space
groups C2/c and C2/m. The Landau free energy is derived in mean-field theory.
The phase transition is of second order. The corresponding quadrupole order
parameters are combinations of and components. The obtained
domain structure is in agreement with observations from neutron diffraction
studies for TmTe. Calculated lattice distortions are found to be different for
the two variants of quadrupole ordering. We suggest to measure lattice
displacements in order to discriminate between those two structures.Comment: 10 pages, 2 figures, 5 tables; accepted by PR
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