742 research outputs found
Possibility of valence-fluctuation mediated superconductivity in Cd-doped CeIrIn probed by In-NQR
We report on a pressure-induced evolution of exotic superconductivity and
spin correlations in CeIr(InCd) by means of
In-Nuclear-Quadrupole-Resonance (NQR) studies. Measurements of an NQR spectrum
and nuclear-spin-lattice-relaxation rate have revealed that
antiferromagnetism induced by the Cd-doping emerges locally around Cd dopants,
but superconductivity is suddenly induced at = 0.7 and 0.9 K at 2.34 and
2.75 GPa, respectively. The unique superconducting characteristics with a large
fraction of the residual density of state at the Fermi level that increases
with differ from those for anisotropic superconductivity mediated by
antiferromagnetic correlations. By incorporating the pressure dependence of the
NQR frequency pointing to the valence change of Ce, we suggest that
unconventional superconductivity in the CeIr(InCd) system may
be mediated by valence fluctuations.Comment: Accepted for publication in Physical Review Letter
Evidence for unconventional superconducting fluctuations in heavy-fermion compound CeNi2Ge2
We present evidence for unconventional superconducting fluctuations in a
heavy-fermion compound CeNiGe. The temperature dependence of the
Ge nuclear-spin-lattice-relaxation rate indicates the
development of magnetic correlations and the formation of a Fermi-liquid state
at temperatures lower than K, where is constant. The
resistance and measured on an as-grown sample decrease below K and K, respectively; these
are indicative of the onset of superconductivity. However, after annealing the
sample to improve its quality, these superconducting signatures disappear.
These results are consistent with the emergence of unconventional
superconducting fluctuations in close proximity to a quantum critical point
from the superconducting to the normal phase in CeNiGe.Comment: 4pages,5figures,to appear in J. Phys. Soc. Jp
A Detailed Monte-Carlo Simulation for the Belle TOF System
We have developed a detailed Monte Carlo simulation program for the Belle TOF
system. Based on GEANT simulation, it takes account of all physics processes in
the TOF scintillation counters and readout electronics. The simulation
reproduces very well the performance of the Belle TOF system, including the
dE/dx response, the time walk effect, the time resolution, and the hit
efficiency due to beam background. In this report, we will describe the Belle
TOF simulation program in detail.Comment: To be submitted to NI
Origin of Tc Enhancement Induced by Doping Yttrium and Hydrogen into LaFeAsO-based Superconductors: 57Fe, 75As, 139La, and 1H-NMR Studies
We report our extensive 57Fe-, 75As-, 139La-, and 1H-NMR studies of
La_{0.8}Y_{0.2}FeAsO_{1-y} (La_{0.8}Y_{0.2}1111) and
LaFeAsO_{1-y}H_{x}(La1111H), where doping yttrium (Y) and hydrogen (H) into
optimally doped LaFeAsO_{1-y} (La1111(OPT)) increases T_c=28 K to 34 and 32 K,
respectively. In the superconducting (SC) state, the measurements of
nuclear-spin lattice-relaxation rate 1/T_1 have revealed in terms of a multiple
fully gapped s_\pm-wave model that the SC gap and T_c in La_{0.8}Y_{0.2}1111
become larger than those in La1111(OPT) without any change in doping level. In
La1111H, the SC gap and T_c also increase slightly even though a decrease in
carrier density and some disorders are significantly introduced. As a
consequence, we suggest that the optimization of both the structural parameters
and the carrier doping level to fill up the bands is crucial for increasing T_c
among these La1111-based compounds through the optimization of the Fermi
surface topology.Comment: 4 pages, 4 figures, 1 table, to be published in J. Phys. Soc. Jpn,
Vol. 79, No. 1
Evolution of an Unconventional Superconducting State inside the Antiferromagnetic Phase of CeNiGe under Pressure: a Ge-Nuclear-Quadrupole-Resonance Study
We report a Ge nuclear-quadrupole-resonance (NQR) study on novel
evolution of unconventional superconductivity in antiferromagnetic (AFM)
CeNiGe. The measurements of the Ge-NQR spectrum and the nuclear
spin-lattice relaxation rate () have revealed that the unconventional
superconductivity evolves inside a commensurate AFM phase around the pressure
() where N\'{e}el temperature exhibits its maximum at 8.5 K. The
superconducting transition temperature has been found to be
enhanced with increasing , before reaching the quantum critical
point at which the AFM order collapses. Above , the AFM structure
transits from an incommensurate spin-density-wave order to a commensurate AFM
order at K, accompanied by a longitudinal spin-density fluctuation.
With regard to heavy-fermion compounds, these novel phenomena have hitherto
never been reported in the - phase diagram.Comment: 4 pages, 5 figure
Evolution of Hall coefficient in two-dimensional heavy fermion CeCoIn
We report on the pressure dependence of the Hall coefficient in
quasi-2D heavy fermion CeCoIn. At ambient pressure, below a temperature
associated with the emergence of non-Fermi liquid properties, is
anomalously enhanced. We found that the restoration of the Fermi liquid state
with applied pressure leads to a gradual suppression of this dramatic
enhancement. Moreover, the enhancement in was found to be confined to an
intermediate temperature window, where inelastic electron-electron scattering
is dominant. Our results strongly support the presence of cold and hot spots on
the Fermi surface probably due to anisotropic scattering by antiferromagnetic
fluctuations, which may also prove relevant for the debate on the anomalous
normal-state properties of high- cuprates.Comment: 9 pages, 5 fiqures, to be published in J. Phys. Soc. Jp
Antiferromagnetism and Superconductivity in CeRhIn
We discuss recent results on the heavy fermion superconductor CeRhIn
which presents ideal conditions to study the strong coupling between the
suppression of antiferromagnetic order and the appearance of unconventional
superconductivity. The appearance of superconductivity as function of pressure
is strongly connected to the suppression of the magnetic order. Under magnetic
field, the re-entrance of magnetic order inside the superconducting state shows
that antiferromagnetism nucleates in the vortex cores. The suppression of
antiferromagnetism in CeRhIn by Sn doping is compared to that under
hydrostatic pressure.Comment: 6 pages, 8 figures, to be published in Proc. Int. Conf. Heavy
Electrons (ICHE2010) J. Phys. Soc. Jpn. 80 (2011
New Universality Class of Quantum Criticality in Ce- and Yb-based Heavy Fermions
A new universality class of quantum criticality emerging in itinerant
electron systems with strong local electron correlations is discussed. The
quantum criticality of a Ce- or Yb-valence transition gives us a unified
explanation for unconventional criticality commonly observed in heavy fermion
metals such as YbRh2Si2 and \beta-YbAlB4, YbCu5-xAlx, and CeIrIn5. The key
origin is due to the locality of the critical valence fluctuation mode emerging
near the quantum critical end point of the first-order valence transition,
which is caused by strong electron correlations for f electrons. Wider
relevance of this new criticality and important future measurements to uncover
its origin are also discussed.Comment: 20 pages, 4 figure
75As NQR/NMR Studies on Oxygen-deficient Iron-based Oxypnictide Superconductors LaFeAsO_{1-y} (y=0,0.25,0.4) and NdFeAsO_{0.6}
We report 75As-NQR/NMR studies on the oxygen-deficient iron(Fe)-based
oxypnictide superconductors LaFeAsO_{0.6} (T_c=28 K) along with the results on
LaFeAsO, LaFeAsO_{0.75}(T_c=20 K) and NdFeAsO_{0.6}(T_c=53 K). Nuclear
spin-lattice relaxation rate 1/T_1 of 75As NQR at zero field on LaFeAsO_{0.6}
has revealed a T^3 dependence below T_c upon cooling without the coherence peak
just below T_c, evidencing the unconventional superconducting state with the
line-node gap. We have found an intimate relationship between the nuclear
quadrupole frequencyof 75As and T_c for four samples used in this study. It
implies microscopically that the local configuration of Fe and As atoms is
significantly related to the T_c of the Fe-oxypnictide superconductors, namely,
the T_c can be enhanced up to 50 K when the local configuration of Fe and As
atoms is optimal, in which the band structure may be also optimized through the
variation of hybridization between As 4p orbitals and Fe 3d orbitals.Comment: 4 pages, 5 figures, Accepted for publication in J. Phys. Soc. Jpn.,
vol.77, No.
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