187 research outputs found
Electronic structure and electric-field gradients analysis in
Electric field gradients (EFG's) were calculated for the compound at
both and sites. The calculations were performed within
the density functional theory (DFT) using the augmented plane waves plus local
orbital (APW+lo) method employing the so-called LDA+U scheme. The
compound were treated as nonmagnetic, ferromagnetic, and antiferromagnetic
cases. Our result shows that the calculated EFG's are dominated at the
site by the Ce-4f states. An approximately linear relation is
intuited between the main component of the EFG's and total density of states
(DOS) at Fermi level. The EFG's from our LDA+U calculations are in better
agreement with experiment than previous EFG results, where appropriate
correlations had not been taken into account among 4f-electrons. Our result
indicates that correlations among 4f-electrons play an important role in this
compound and must be taken into account
Density of States and NMR Relaxation Rate in Anisotropic Superconductivity with Intersecting Line Nodes
We show that the density of states in an anisotropic superconductor with
intersecting line nodes in the gap function is proportional to for , where is the maximum value of
the gap function and is constant, while it is proportional to if
the line nodes do not intersect. As a result, a logarithmic correction appears
in the temperature dependence of the NMR relaxation rate and the specific heat,
which can be observed experimentally. By comparing with those for the heavy
fermion superconductors, we can obtain information about the symmetry of the
gap function.Comment: 7 pages, 4 PostScript Figures, LaTeX, to appear in J. Phys. Soc. Jp
Unconventional Superconductivity in Heavy Fermion Systems
We review the studies on the emergent phases of superconductvity and
magnetism in the -electron derived heavy-fermion (HF) systems by means of
the nuclear-quadrupole-resonance (NQR) under pressure. These studies have
unraveled a rich variety of the phenomena in the ground state of HF systems. In
this article, we highlight the novel phase diagrams of magnetism and
unconventional superconductivity (SC) in CeCuSi, HF antiferromagnets
CeRhIn, and CeIn. A new light is shed on the difference and common
features on the interplay between magnetism and SC on the magnetic criticality.Comment: 15 pages, 13 figures, to appear in J. Phys. Soc. JPN, 74, No.1
(2005), special issue "Kondo Effect- 40 Years after the Discovery
Incommensurate spin fluctuations in hole-overdoped superconductor KFe2As2
A neutron scattering study of heavily hole-overdoped superconducting
KFeAs revealed a well-defined low-energy incommensurate spin
fluctuation at [),0] with = 0.16. The incommensurate
structure differs from the previously observed commensurate peaks in
electron-doped FeAs ( = Ba, Ca, or Sr) at low energies. The
direction of the peak splitting is perpendicular to that observed in Fe(Te,Se)
or in Ba(Fe,Co)As at high energies. A band structure calculation
suggests interband scattering between bands around the and X points as
an origin of this incommensurate peak. The perpendicular direction of the peak
splitting can be understood within the framework of multiorbital band
structure. The results suggest that spin fluctuation is more robust in
hole-doped than in electron-doped samples, which can be responsible for the
appearance of superconductivity in the heavily hole-doped samples.Comment: 4 pages, 4 figure
Pressure-induced unconventional superconductivity in the heavy-fermion antiferromagnet CeIn3: An 115In-NQR study under pressure
We report on the pressure-induced unconventional superconductivity in the
heavy-fermion antiferromagnet CeIn3 by means of nuclear-quadrupole-resonance
(NQR) studies conducted under a high pressure. The temperature and pressure
dependences of the NQR spectra have revealed a first-order quantum-phase
transition (QPT) from an AFM to PM at a critical pressure Pc=2.46 GPa. Despite
the lack of an AFM quantum critical point in the P-T phase diagram, we
highlight the fact that the unconventional SC occurs in both phases of the AFM
and PM. The nuclear spin-lattice relaxation rate 1/T1 in the AFM phase have
provided evidence for the uniformly coexisting AFM+SC phase. In the HF-PM phase
where AFM fluctuations are not developed, 1/T1 decreases without the coherence
peak just below Tc, followed by a power-law like T dependence that indicates an
unconventional SC with a line-node gap. Remarkably, Tc has a peak around Pc in
the HF-PM phase as well as in the AFM phase. In other words, an SC dome exists
with a maximum value of Tc = 230 mK around Pc, indicating that the origin of
the pressure-induced HF SC in CeIn3 is not relevant to AFM spin fluctuations
but to the emergence of the first-order QPT in CeIn3. When the AFM critical
temperature is suppressed at the termination point of the first-order QPT, Pc =
2.46 GPa, the diverging AFM spin-density fluctuations emerge at the critical
point from the AFM to PM. The results with CeIn3 leading to a new type of
quantum criticality deserve further theoretical investigations
Spin Fluctuation Induced Superconductivity Controlled by Orbital Fluctuation
A microscopic Hamiltonian reflecting the correct symmetry of -orbitals is
proposed to discuss superconductivity in heavy fermion systems. In the
orbitally degenerate region in which not only spin fluctuations but also
orbital fluctuations develop considerably, cancellation between spin and
orbital fluctuations destabilizes -wave superconductivity.
Entering the non-degenerate region by increasing the crystalline electric
field, -wave superconductivity mediated by antiferromagnetic
spin fluctuations emerges out of the suppression of orbital fluctuations. We
argue that the present scenario can be applied to recently discovered
superconductors CeTIn (T=Ir, Rh, and Co).Comment: 4 pages, 3 figure
Quasiparticle spin susceptibility in heavy-fermion superconductors : An NMR study compared with specific heat results
Quasi-particle spin susceptibility () for various heavy-fermion
(HF) superconductors are discussed on the basis of the experimental results of
electronic specific heat (), NMR Knight shift () and NMR
relaxation rate () within the framework of the Fermi liquid model for a
Kramers doublet crystal electric field (CEF) ground state.
is calculated from the enhanced Sommerfeld coefficient and
from the quasi-particle Korringa relation
via the relation of
where is the hyperfine
coupling constant, the Abogadoro's number and the Bohr magneton.
For the even-parity (spin-singlet) superconductors CeCuSi, CeCoIn
and UPdAl, the fractional decrease in the Knight shift, , below the superconducting transition temperature () is due to
the decrease of the spin susceptibility of heavy quasi-particle estimated
consistently from and . This result
allows us to conclude that the heavy quasi-particles form the spin-singlet
Cooper pairs in CeCuSi, CeCoIn and UPdAl. On the other
hand, no reduction in the Knight shift is observed in UPt and
UNiAl, nevertheless the estimated values of and
are large enough to be probed experimentally. The odd-parity
superconductivity is therefore concluded in these compounds. The NMR result
provides a convincing way to classify the HF superconductors into either even-
or odd- parity paring together with the identification for the gap structure,
as long as the system has Kramers degeneracy.Comment: 11 pages, 3 tables, 5 figures, RevTex4(LaTex2e
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