84 research outputs found
Magnetic Excitations in NpCoGa5
We report the results of inelastic neutron scattering experiments on
NpCoGa, an isostructural analogue of the PuCoGa superconductor. Two
energy scales characterize the magnetic response in the antiferromagnetic
phase. One is related to a non-dispersive excitation between two crystal field
levels. The other at lower energies corresponds to dispersive fluctuations
emanating from the magnetic zone center. The fluctuations persist in the
paramagnetic phase also, although weaker in intensity. This supports the
possibility that magnetic fluctuations are present in PuCoGa, where
unconventional d-wave superconductivity is achieved in the absence of magnetic
order.Comment: 4 pages, 5 figure
Mean-field model for magnetic orders in NpTGa5 with T=Co, Ni or Rh
Characteristics of magnetic transitions in NpTGa with T=Co, Ni, Rh are
explained in a unified way with use of a crystalline electric field (CEF) model
of localized 5 electrons. The model takes a CEF doublet and a singlet as
local states, and includes dipolar and quadrupolar intersite interactions in
the mean-field theory. Diverse ordering phenomena are derived depending on the
magnitude of interaction parameters, which qualitatively reproduce the
experimentally observed magnetic behaviors in NpTGa. The quadrupole degrees
of freedom are essential to the diverse magnetic orders. It is argued that
NpRhGa is close to a multicritical point where quadrupoles and dipoles with
different directions are competing to order.Comment: 17 pages, 10 figure
Possible mechanism of superconductivity in PuCoGa5 probed by self-irradiation damage
Measurements of the electrical resistivity of a polycrystalline PuCoGa5 sample reveal significant modifications
of the superconducting properties as a function of time, due to the increase of defects and impurities
resulting from self-irradiation damage. More than four years of aging were necessary to detect a deviation from
linearity in the time dependence of the critical temperature. The observed behavior is understood in the
framework of the Eliashberg theory, confirming the ¿dirty¿ d-wave character which was already suggested by
nuclear magnetic resonance. We show that experimental data accumulated so far can be well reproduced by
assuming a phononic mechanism for superconductivity, with reasonable values of the electron-phonon coupling
and Coulomb pseudopotential. Further experiments are then required to assess the role of spin fluctuations
in stabilizing the superconducting state in this compound.JRC.E.6-Actinides researc
Structural, electronic, and magnetic characteristics of Np_2Co_(17)
A previously unknown neptunium-transition-metal binary compound Np_2Co_(17) has been synthesized and characterized by means of powder x-ray diffraction, ^(237)Np Mössbauer spectroscopy, superconducting-quantum-interference-device magnetometry, and x-ray magnetic circular dichroism (XMCD). The compound crystallizes in a Th_2Ni_(17)-type hexagonal structure with room-temperature lattice parameters α=8.3107(1) Å and c=8.1058(1) Å. Magnetization curves indicate the occurrence of ferromagnetic order below T_C>350 K. Mössbauer spectra suggest a Np^(3+) oxidation state and give an ordered moment of μ_(Np)=1.57(4) μ_B and μ_(Np)=1.63(4) μ_B for the Np atoms located, respectively, at the 2b and 2d crystallographic positions of the P6_3/mmc space group. Combining these values with a sum-rule analysis of the XMCD spectra measured at the neptunium M_(4,5) absorption edges, one obtains the spin and orbital contributions to the site-averaged Np moment [μ_S=−1.88(9) μ_B, μ_L=3.48(9) μ_B]. The ratio between the expectation value of the magnetic-dipole moment and the spin magnetic moment (m_(md)/μS=+1.36) is positive as predicted for localized 5f electrons and lies between the values calculated in intermediate-coupling (IC) and jj approximations. The expectation value of the angular part of the spin-orbit-interaction operator is in excellent agreement with the IC estimate. The ordered moment averaged over the four inequivalent Co sites, as obtained from the saturation value of the magnetization, is μ_(Co)≃1.6 μ_B. The experimental results are discussed against the predictions of first-principles electronic-structure calculations based on the spin-polarized local-spin-density approximation plus the Hubbard interaction
Orbital-based Scenario for Magnetic Structure of Neptunium Compounds
In order to understand a crucial role of orbital degree of freedom in the
magnetic structure of recently synthesized neptunium compounds NpTGa_5 (T=Fe,
Co, and Ni), we propose to discuss the magnetic phase of an effective
two-orbital model, which has been constructed based on a j-j coupling scheme to
explain the magnetic structure of uranium compounds UTGa_5. By analyzing the
model with the use of numerical technique such as exact diagonalization, we
obtain the phase diagram including several kinds of magnetic states. An
orbital-based scenario is discussed to understand the change in the magnetic
structure among C-, A-, and G-type antiferromagnetic phases, experimentally
observed in NpFeGa_5, NpCoGa_5, and NpNiGa_5.Comment: 18 pages, 8 figures, to appear in New Journal of Physic
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