265 research outputs found
Raman scattering study of electron-doped PrCaFeAs superconductors
Temperature-dependent polarized Raman spectra of electron-doped
superconducting PrCaFeAs () single crystals
are reported. All four allowed by symmetry even-parity phonons are identified.
Phonon mode of B symmetry at 222 cm, which is associated with the
c-axis motion of Fe ions, is found to exhibit an anomalous frequency hardening
at low temperatures, that signals non-vanishing electron-phonon coupling in the
superconducting state and implies that the superconducting gap magnitude
meV.Comment: 4 pages, 3 figure
Phonon-mediated tuning of instabilities in the Hubbard model at half-filling
We obtain the phase diagram of the half-filled two-dimensional Hubbard model
on a square lattice in the presence of Einstein phonons. We find that the
interplay between the instantaneous electron-electron repulsion and
electron-phonon interaction leads to new phases. In particular, a
d-wave superconducting phase emerges when both anisotropic phonons
and repulsive Hubbard interaction are present. For large electron-phonon
couplings, charge-density-wave and s-wave superconducting regions also appear
in the phase diagram, and the widths of these regions are strongly dependent on
the phonon frequency, indicating that retardation effects play an important
role. Since at half-filling the Fermi surface is nested, spin-density-wave is
recovered when the repulsive interaction dominates. We employ a functional
multiscale renormalization-group method that includes both electron-electron
and electron-phonon interactions, and take retardation effects fully into
account.Comment: 8 pages, 5 figure
A Field-Induced Re-Entrant Novel Phase and A Ferroelectric-Magnetic Order Coupling in HoMnO3
A re-entrant novel phase has been observed in the hexagonal ferroelectric
HoMnO3 in the presence of magnetic fields, in the temperature ranges defined by
the plateau of the dielectric constant anomaly. The dielectric plateau evolves
with fields from a narrow sharp dielectric peak at the Mn-spin rotation
transition at 32.8 K in zero magnetic field. Such a field-induced dielectric
plateau anomaly appears both in the temperature sweep at a constant field and
in the field sweep at a constant temperature without detectable hysteresis.
This is attributed to the indirect coupling between the ferroelectric and
antiferromagnetic orders, arising from an antiferromagnetic domain wall effect,
where the magnetic order parameter of the Mn subsystem has to change sign
across the ferroelectric domain wall in the compound, that influences the
ferroelectric domains via a local magnetostrictive effect
Raman scattering study of (KSr)FeAs ( = 0.0, 0.4)
Polarized Raman spectra of non-superconducting SrFeAs and
superconducting KSrFeAs ( K) are reported.
All four phonon modes (A + B + 2E) allowed by symmetry, are
found and identified. Shell model gives reasonable description of the spectra.
No detectable anomalies are observed near the tetragonal-to-orthorhombic
transition in SrFeAs or the superconducting transition in
KSrFeAs.Comment: 4 pages, 4 figures, 2 table
Thermoelectric properties of Zn_5Sb_4In_(2-δ)(δ=0.15)
The polymorphic intermetallic compound Zn_5Sb_4In_(2−δ) (δ = 0.15(3)) shows promising thermoelectric properties at low temperatures, approaching a figure of merit ZT of 0.3 at 300 K. However, thermopower and electrical resistivity changes discontinuously at around 220 K. Measurement of the specific heat locates the previously unknown temperature of the order-disorder phase transition at around 180 K. Investigation of the charge carrier concentration and mobility by Hall measurements and infrared reflection spectroscopy indicate a mixed conduction behavior and the activation of charge carriers at temperatures above 220 K. Zn_5Sb_4In_(2−δ) has a low thermal stability, and at temperatures above 470 K samples decompose into a mixture of Zn, InSb, and Zn_4Sb_3
Mutual maintenance of di- and triploid Pelophylax esculentus hybrids in R-E systems: results fro
Background: Interspecies animal hybrids can employ clonal or hemiclonal reproduction modes where one or all
parental genomes are transmitted to the progeny without recombination. Nevertheless, some interspecies
hybrids retain strong connection with the parental species needed for successful reproduction. Appearance of
polyploid hybrid animals may play an important role in the substitution of parental species and in the
speciation process.
Results: To establish the mechanisms that enable parental species, diploid and polyploid hybrids coexist we
have performed artificial crossing experiments of water frogs of Pelophylax esculentus complex. We identified
tadpole karyotypes and oocyte genome composition in all females involved in the crossings. The majority of
diploid and triploid hybrid frogs produced oocytes with 13 bivalents leading to haploid gametes with the
same genome as parental species hybrids usually coexist with. After fertilization of such gametes only diploid
animals appeared. Oocytes with 26 bivalents produced by some diploid hybrid frogs lead to diploid gametes,
which give rise to triploid hybrids after fertilization. In gonads of all diploid and triploid hybrid tadpoles we
found DAPI-positive micronuclei (nucleus-like bodies) involved in selective genome elimination. Hybrid male
and female individuals produced tadpoles with variable karyotype and ploidy even in one crossing owing to
gametes with various genome composition.
Conclusions: We propose a model of diploid and triploid hybrid frog reproduction in R-E population systems.
Triploid Pelophylax esculentus hybrids can transmit genome of parental species they coexist with by producing
haploid gametes with the same genome composition. Triploid hybrids cannot produce triploid individuals
after crossings with each other and depend on diploid hybrid females producing diploid eggs. In contrast to
other population systems, the majority of diploid and triploid hybrid females unexpectedly produced gametes
with the same genome as parental species hybrids coexist with
Spectroscopy of infrared-active phonons in high-temperature superconductors
For a large variety of superconducting materials both experimental and theoretical lattice dynamical studies have been performed to date. The assignment of the observed infrared- and Raman-active phonon modes to the particular lattice eigenmodes is generally accepted. We will concentrate here upon the analysis of the changes of the infrared-phonon parameters (frequency and linewidth) upon entering the superconducting state which, as will be shown, may provide information on the magnitude of the superconductivity-related gap and its dependence on the superconducting transition temperature Tc
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