564 research outputs found
Magnetic phase transitions in SmCoAsO
Magnetization, x-ray diffraction and specific-heat measurements reveal that
SmCoAsO undergoes three magnetic phase transitions. A ferromagnetic transition
attributed to the Co ions, emerges at TC=57 K with a small saturation moment of
0.15muB/Co. Reorientation of the Co moment to an antiferromagnetic state is
obtained at TN2=45 K. The relative high paramagnetic effective moment Peff=1.57
MuB/Co indicates an itinerant ferromagnetic state of the Co sublattice. The
third magnetic transition at TN1=5 K is observed clearly in the specific-heat
study only. Both magnetic and 57Fe Mossbauer studies show that substitution of
small quantities of Fe for Co was unsuccessful.Comment: 10pages text+Figures: comments welcome ([email protected]
Crystal growth, structural studies and superconducting properties of beta-pyrochlore KOs2O6
Single crystals of KOs2O6 have been grown in a sealed quartz ampoule.
Detailed single crystal X-ray diffraction studies at room temperature show
Bragg peaks that violate Fd-3m symmetry. With a comparative structure
refinement the structure is identified as non-centrosymmetric (F-43m). Compared
to the ideal beta-pyrochlore lattice (Fd-3m), both Os tetrahedral and O
octahedral network exhibit breathing mode like volume changes accompanied by
strong anisotropic character of the K channels. The crystals show metallic
conductivity and a sharp transition to the superconducting state at Tc = 9.65
K. Superconducting properties have been investigated by magnetization
measurements performed in a temperature range from 2 to 12 K and in magnetic
fields from 0 to 60 kOe. The temperature dependence of the upper critical field
Hc2(T) has been determined and the initial slope (dHc2/dT)Tc = -33.3 kOe/K has
been obtained near Tc. The upper critical field at zero temperature was
estimated to be Hc2(0) \cong 230 kOe, which is a value close to the Pauli
paramagnetic limiting field Hp(0)\cong 250 kOe. Then, the Ginzburg-Landau (GL)
coherence length xi GL(0) \approx 3.8 nm was calculated, and the Maki parameter
alpha \approx \sqrt 2 was obtained, suggesting the possibility that KOs2O6
might behave unconventionally at low temperatures and high magnetic fields
Phonon Dynamics and Multipolar Isomorphic Transition in beta-pyrochlore KOs2O6
We investigate with a microscopic model anharmonic K-cation oscillation
observed by neutron experiments in beta-pyrochlore superconductor KOs2O6, which
also shows a mysterious first-order structural transition at Tp=7.5 K. We have
identified a set of microscopic model parameters that successfully reproduce
the observed temperature dependence and the superconducting transition
temperature. Considering changes in the parameters at Tp, we can explain
puzzling experimental results about electron-phonon coupling and neutron data.
Our analysis demonstrates that the first-order transition is multipolar
transition driven by the octupolar component of K-cation oscillations. The
octupole moment does not change the symmetry and is characteristic to
noncentrosymmetric K-cation potential.Comment: 5 pages, 4 figures, submitted to J. Phys. Soc. Jp
Local spin and charge properties of beta-Ag0.33V2O5 studied by 51V NMR
Local spin and charge properties were studied on beta-Ag0.33V2O5, a
pressure-induced superconductor, at ambient pressure using 51V-NMR and
zero-field-resonance (ZFR) techniques. Three inequivalent Vi sites (i=1, 2, and
3) were identified from 51V-NMR spectra and the principal axes of the
electric-field-gradient (EFG) tensor were determined in a metallic phase and
the following charge-ordering phase. We found from the EFG analysis that the V1
sites are in a similar local environment to the V3 sites. This was also
observed in ZFR spectra as pairs of signals closely located with each other.
These results are well explained by a charge-sharing model where a 3d1 electron
is shared within a rung in both V1-V3 and V2-V2 two-leg ladders.Comment: 12pages, 16figure
Magnetic, thermodynamic, and electrical transport properties of the noncentrosymmetric B20 germanides MnGe and CoGe
We present magnetization, specific heat, resistivity, and Hall effect
measurements on the cubic B20 phase of MnGe and CoGe and compare to
measurements of isostructural FeGe and electronic structure calculations. In
MnGe, we observe a transition to a magnetic state at K as identified
by a sharp peak in the ac magnetic susceptibility, as well as second phase
transition at lower temperature that becomes apparent only at finite magnetic
field. We discover two phase transitions in the specific heat at temperatures
much below the Curie temperature one of which we associate with changes to the
magnetic structure. A magnetic field reduces the temperature of this transition
which corresponds closely to the sharp peak observed in the ac susceptibility
at fields above 5 kOe. The second of these transitions is not affected by the
application of field and has no signature in the magnetic properties or our
crystal structure parameters. Transport measurements indicate that MnGe is
metal with a negative magnetoresistance similar to that seen in isostructural
FeGe and MnSi. Hall effect measurements reveal a carrier concentration of about
0.5 carriers per formula unit also similar to that found in FeGe and MnSi. CoGe
is shown to be a low carrier density metal with a very small, nearly
temperature independent diamagnetic susceptibility.Comment: 16 pages 23 figure
Superconductivity suppression of Ba0.5K0.5Fe2-2xM2xAs2 single crystals by substitution of transition-metal (M = Mn, Ru, Co, Ni, Cu, and Zn)
We investigated the doping effects of magnetic and nonmagnetic impurities on
the single-crystalline p-type Ba0.5K0.5Fe2-2xM2xAs2 (M = Mn, Ru, Co, Ni, Cu and
Zn) superconductors. The superconductivity indicates robustly against impurity
of Ru, while weakly against the impurities of Mn, Co, Ni, Cu, and Zn. However,
the present Tc suppression rate of both magnetic and nonmagnetic impurities
remains much lower than what was expected for the s\pm-wave model. The
temperature dependence of resistivity data is observed an obvious low-T upturn
for the crystals doped with high-level impurity, which is due to the occurrence
of localization. Thus, the relatively weak Tc suppression effect from Mn, Co,
Ni, Cu, and Zn are considered as a result of localization rather than
pair-breaking effect in s\pm-wave model.Comment: 8 pages, 9 figures, to be published in Phys. Rev.
A Possible Phase Transition in beta-pyrochlore Compounds
We investigate a lattice of interacting anharmonic oscillators by using a
mean field theory and exact diagonalization. We construct an effective
five-state hopping model with intersite repulsions as a model for
beta-pyrochlore AOs_2O_6(A=K, Rb or Cs). We obtain the first order phase
transition line from large to small oscillation amplitude phases as temperature
decreases. We also discuss the possibility of a phase with local electric
polarizations. Our theory can explain the origin of the mysterious first order
transition in KOs_2O_6.Comment: 4 pages, 4 figures, submitted to J. Phys. Soc. Jp
- …