8,633 research outputs found
Iron substitution in NdCoAsO: crystal structure and magnetic phase diagram
The effects of replacing small amounts of Co with Fe in NdCoAsO are reported.
Polycrystalline materials with compositions NdCo1-xFexAsO (x = 0.05, 0.10,
0.15, and 0.20) are studied and the results compared to previous reports for
NdCoAsO. Rietveld analysis of powder x-ray diffraction data shows that as Fe
replaces Co on the transition metal (T) site, the T-As distance increases, and
the As tetrahedra surrounding the T-site become more regular. Electrical
resistivity and magnetization measurements indicate that the three magnetic
phase transitions in NdCoAsO are suppressed as Co is replaced by Fe, and these
transitions are not observed above 1.8 K for x = 0.20. Based on these results,
the magnetic phase diagram for the Co-rich side of the NdCoAsO-NdFeAsO system
is constructed.Comment: Accepted for publication in Physical Review B, revised text and
figures, 5 pages, 5 figure
Anisotropic thermal expansion of Fe1.06Te and FeTe0.5Se0.5 single crystals
Heat capacity and anisotropic thermal expansion was measured for Fe1.06Te and
FeTe0.5Se0.5 single crystals. Previously reported phase transitions are clearly
seen in both measurements. In both cases the thermal expansion is anisotropic.
The uniaxial pressure derivatives of the superconducting transition temperature
in FeTe0.5Se0.5 inferred from the Ehrenfest relation have opposite signs for
in-plane and c-axis pressures. Whereas the Gruneisen parameters for both
materials are similar and only weakly temperature-dependent above ~ 80 K, at
low temperatures (in the magnetically ordered phase) the magnetic contribution
to the Gruneisen parameter in Fe1.06Te is significantly larger than electron
and phonon contributions combined
Bulk Superconductivity at 14 K in Single Crystals of Fe1+yTexSe1-x
Resistivity, magnetic susceptibility and heat capacity measurements are
reported for single crystals of Fe1+yTexSe1-x grown via a modified Bridgeman
method with 0 < y < 0.15, and x= 1, 0.9, 0.75, 0. 67, 0.55 and 0.5. Although
resistivity measurements show traces of superconductivity near 14 K for all x
except x=1, only crystals grown with compositions near x=0.5 exhibit bulk
superconductivity. The appearance of bulk superconductivity correlates with a
reduction in the magnitude of the magnetic susceptibility at room temperature
and smaller values of y, the concentration of Fe in the Fe(2) site.Comment: Submitted to Phys. Rev.
Unusual Phase Transitions and Magnetoelastic Coupling in TlFe1.6Se2 Single Crystals
Structural, magnetic, electrical transport, and heat capacity data are
reported for single crystals of TlFe1.6Se2. This compound crystallizes in a
tetragonal structure similar to the ThCr2Si2 structure, but with vacancies in
the Fe layer. The vacancies can be ordered or disordered depending on
temperature and thermal history. If the vacancies are ordered, the basal plane
lattice constant increases from a to \sqrt{5}a. Antiferromagnetic order with
the Fe spins along the c-axis occurs below T_N ~ 430K as shown by single
crystal neutron diffraction and the magnetic structure is reported. In
addition, for the vacancy ordered crystal, two other phase transitions are
found at T_1 ~ 140K, and T_2 ~ 100K. The phase transitions at T_1 and T_2 are
evident in heat capacity, magnetic susceptibility, resistivity data, a and c
lattice parameters, and in the unusual temperature dependence of the magnetic
order parameter determined from neutron scattering. The phase transitions at
T_1 and T_2 result in significant changes in the magnetic moment per iron, with
1.72(6)\mu_B observed at 300K, 2.07(9)\mu_B at 140\,K, 1.90(9)\,\mu_B at
115\,K, and 1.31(8)\mu_B for 5\,K if the same "block checkerboard" magnetic
structure is used at all temperatures. The phase transitions appear to be
driven by small changes in the c lattice constant, large magnetoelastic
coupling, and the localization of carriers with decreasing temperature.Comment: Accepted for publication in Physical Review
Superconductivity at 22 K in Co-doped BaFe2As2 Crystals
Here we report bulk superconductivity in BaFe1.8Co0.2As2 single crystals
below Tc = 22 K, as demonstrated by resistivity, magnetic susceptibility, and
specific heat data. Hall data indicate that the dominant carriers are
electrons, as expected from simple chemical reasoning. This is the first
example of superconductivity induced by electron doping in this family of
materials. In contrast to the cuprates, the BaFe2As2 system appears to tolerate
considerable disorder in the FeAs planes. First principles calculations for
BaFe1.8Co0.2As2 indicate the inter-band scattering due to Co is weak.Comment: 9 pages, 3 figure
Many-body system with a four-parameter family of point interactions in one dimension
We consider a four-parameter family of point interactions in one dimension.
This family is a generalization of the usual -function potential. We
examine a system consisting of many particles of equal masses that are
interacting pairwise through such a generalized point interaction. We follow
McGuire who obtained exact solutions for the system when the interaction is the
-function potential. We find exact bound states with the four-parameter
family. For the scattering problem, however, we have not been so successful.
This is because, as we point out, the condition of no diffraction that is
crucial in McGuire's method is not satisfied except when the four-parameter
family is essentially reduced to the -function potential.Comment: 8 pages, 4 figure
Magnetic Phase Transitions in NdCoAsO
Magnetization measurements reveal that NdCoAsO undergoes three magnetic phase
transitions below room temperature. The crystal and magnetic structures of
NdCoAsO have been determined by powder neutron diffraction, and the effects of
the phase transitions on physical properties are reported. Near 69 K a
ferromagnetic state emerges with a small saturation moment of about 0.2 Bohr
magnetons, likely on Co atoms. At 14 K the material enters an antiferromagnetic
state with propagation vector (0 0 1/2) and small ordered moments (~0.4 Bohr
magnetons) on Co and Nd. Near 3.5 K a third transition is observed, and
corresponds to the antiferromagnetic ordering, with the same propagation
vector, of larger moments on Nd reaching 1.30(2) Bohr magnetons at 1.4 K. In
addition, transport properties and heat capacity results are presented, and
show anomalies at all three phase transitions.Comment: Some minor changes made, and lower temperature neutron diffraction
results are included. Accepted for publication in Physical Review
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