22 research outputs found
New Superconducting and Magnetic Phases Emerge on the Verge of Antiferromagnetism in CeIn
We report the discovery of new superconducting and novel magnetic phases in
CeIn on the verge of antiferromagnetism (AFM) under pressure () through
the In-nuclear quadrupole resonance (NQR) measurements. We have found a
-induced phase separation of AFM and paramagnetism (PM) without any trace
for a quantum phase transition in CeIn. A new type of superconductivity
(SC) was found in GPa to coexist with AFM that is magnetically
separated from PM where the heavy fermion SC takes place. We propose that the
magnetic excitations such as spin-density fluctuations induced by the
first-order magnetic phase transition might mediate attractive interaction to
form Cooper pairs.Comment: 4 pages, 4 EPS figures, submitted to J. Phys. Soc. Jp
Meta-orbital Transition in Heavy-fermion Systems: Analysis by Dynamical Mean Field Theory and Self-consistent Renormalization Theory of Orbital Fluctuations
We investigate a two-orbital Anderson lattice model with Ising orbital
intersite exchange interactions by means of dynamical mean field theory
combined with the static mean field approximation of the intersite orbital
interactions. Focusing on Ce-based heavy-fermion compounds, we examine the
orbital crossover between the two orbital states, when the total f-electron
number per site n_f is n_f ~ 1. We show that a "meta-orbital" transition, at
which the occupancy of the two orbitals changes steeply, occurs when the
hybridization between the ground-state f-electron orbital and conduction
electrons are smaller than that between the excited f-electron orbital and
conduction electrons. Near the meta-orbital critical end point, the orbital
fluctuations are enhanced, and couple with the charge fluctuations. A critical
theory of the meta-orbital fluctuations is also developed by applying the
self-consistent renormalization theory of itinerant electron magnetism to the
orbital fluctuations. The critical end point, first-order transition and
crossover are described within Gaussian approximations of orbital fluctuations.
We discuss the relevance of our results to CeAl2, CeCu2Si2, CeCu2Ge2 and the
related compounds, which all have low-lying crystalline-electric-field excited
states.Comment: 11 pages, 6 figures, J. Phys. Soc. Jpn. 79, (2010) 11471
Signatures of valence fluctuations in CeCu2Si2 under high pressure
Simultaneous resistivity and a.c.-specific heat measurements have been
performed under pressure on single crystalline CeCu2Si2 to over 6 GPa in a
hydrostatic helium pressure medium. A series of anomalies were observed around
the pressure coinciding with a maximum in the superconducting critical
temperature, . These anomalies can be linked with an abrupt change
of the Ce valence, and suggest a second quantum critical point at a pressure
GPa, where critical valence fluctuations provide the
superconducting pairing mechanism, as opposed to spin fluctuations at ambient
pressure. Such a valence instability, and associated superconductivity, is
predicted by an extended Anderson lattice model with Coulomb repulsion between
the conduction and f-electrons. We explain the T-linear resistivity found at
in this picture, while other anomalies found around can be
qualitatively understood using the same model.Comment: Submitted to Phys. Rev.
Mass Enhancement in an Intermediate-Valent Regime of Heavy-Fermion Systems
We study the mechanism of the mass enhancement in an intermediate-valent
regime of heavy-fermion materials. We find that the crossovers between the
Kondo, intermediate valent, and almost empty f-electron regimes become sharp
with the Coulomb interaction between the conduction and f electrons. In the
intermediate-valent regime, we find a substantial mass enhancement, which is
not expected in previous theories. Our theory may be relevant to the observed
nonmonotonic variation in the effective mass under pressure in CeCu2Si2 and the
mass enhancement in the intermediate-valent compounds alpha-YbAlB4 and
beta-YbAlB4.Comment: 4 pages, 4 figure
Gutzwiller Method for an Extended Periodic Anderson Model with the c-f Coulomb Interaction
We study an extended periodic Anderson model with the Coulomb interaction Ucf
between conduction and f electrons by the Gutzwiller method. The crossovers
between the Kondo, intermediate-valence, and almost empty f-electron regimes
become sharper with Ucf, and for a sufficiently large Ucf, become first-order
phase transitions. In the Kondo regime, a large enhancement in the effective
mass occurs as in the ordinary periodic Anderson model without Ucf. In
addition, we find that a large mass enhancement also occurs in the
intermediate-valence regime by the effect of Ucf.Comment: 9 pages, 7 figure
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
Multiband Superconductivity in Heavy Fermion Compound CePt3Si without Inversion Symmetry: An NMR Study on a High-Quality Single Crystal
We report on novel superconducting characteristics of the heavy fermion (HF)
superconductor CePt3Si without inversion symmetry through 195Pt-NMR study on a
single crystal with T_c= 0.46 K that is lower than T_c= 0.75 K for
polycrystals. We show that the intrinsic superconducting characteristics
inherent to CePt3Si can be understood in terms of the unconventional
strong-coupling state with a line-node gap below T_c= 0.46 K. The mystery about
the sample dependence of T_c is explained by the fact that more or less
polycrystals and single crystals inevitably contain some disordered domains,
which exhibit a conventional BCS s-wave superconductivity (SC) below 0.8 K. In
contrast, the Neel temperature T_N= 2.2 K is present regardless of the quality
of samples, revealing that the Fermi surface responsible for SC differ from
that for the antiferromagnetic order. These unusual characteristics of CePt3Si
can be also described by a multiband model; in the homogeneous domains, the
coherent HF bands are responsible for the unconventional SC, whereas in the
disordered domains the conduction bands existing commonly in LaPt3Si may be
responsible for the conventional s-wave SC. We remark that some impurity
scatterings in the disordered domains break up the 4f-electrons-derived
coherent bands but not others. In this context, the small peak in 1/T_1 just
below T_c reported in the previous paper (Yogi et al, 2004) is not due to a
two-component order parameter composed of spin-singlet and spin-triplet Cooper
pairing states, but due to the contamination of the disorder domains which are
in the s-wave SC state.Comment: 10 pages, 9 figures, Accepted for publication in J. Phys. Soc. Jpn.,
vol.78, No.1 (2009
Water quality assessment of highly polluted rivers in a semi-arid Mediterranean zone Oued Fez and Sebou River (Morocco)
Oued Fez (one of the Sebou River tributaries - Morocco) allowed us to study and quantify the effect of the lack of wastewater treatment on surface water quality in semi-arid hydrological context. The analysis is based on field data collected from June 2009 to December 2011. Concentration and load patterns of nitrogen, phosphorus and chromium (used in the processing of leather) are compared in stable hydrological conditions during low flow and high flow periods in an eight-location sampling network. The Oued Fez and the Sebou River are characterised by severe pollution downstream from the city of Fez, particularly TN (mainly NH4 and N-org), TP (mainly P-part) and TCr. The most polluted sites are those directly under the influence of domestic and industrial waste water inputs, particularly tannery effluents. Obviously, the concentrations measured at these locations are above all environmental quality standards. Pollutant loads are very heavy in the Sebou River and can contaminate the river course for kilometres. Moreover, as the water of the Sebou River is used for the irrigation of vegetables, serious problems of public health could arise. A better understanding of contaminant dynamics and self-purifying processes in these rivers will help implement actions and steps aimed at improving water quality in the Sebou River, which is the primary water supply source in Morocco and is used for agricultural and industrials purposes as well as for drinking water