252 research outputs found
Doping driven Small-to-Large Fermi surface transition and d-wave superconductivity in a two-dimenional Kondo lattice
We study the two-dimensional Kondo lattice model with an additional
Heisenberg exchange between localized spins. In a first step we use mean-field
theory with two order parameters. The first order parameter is a complex
pairing amplitude between conduction electrons and localized spins which
describes condensation of Kondo (or Zhang-Rice) singlets. A nonvanishing value
implies that the localized spins contribute to the Fermi surface volume. The
second order parameter describes singlet-pairing between the localized spins
and competes with the Kondo-pairing order parameter. Reduction of the carrier
density in the conduction band reduces the energy gain due to the formation of
the large Fermi surface and induces a phase transition to a state with strong
singlet correlations between the localized spins and a Fermi surface which
comprises only the conduction electrons. The model thus shows a doping-driven
change of its Fermi surface volume. At intermediate doping and low temperature
there is a phase where both order parameters coexist, which has a gapped large
Fermi surface and d-wave superconductivity. The theory thus qualitatively
reproduces the phase diagram of cuprate superconductors. In the second part of
the paper we show how the two phases with different Fermi surface volume emerge
in a strong coupling theory applicable in limit of large Kondo exchange. The
large-Fermi-surface phase corresponds to a `vacuum' of localized Kondo singlets
with uniform phase and the quasiparticles are spin-1/2 charge fluctuations
around this fully paired state. In the small-Fermi-surface phase the
quasiparticles correspond to propagating Kondo-singlets or triplets whereby the
phase of a given Kondo-singlet corresponds to its momentum. In this picture a
phase transition occurs for low filling of the conduction band as well.Comment: Revtex file, 17 pages, 14 eps-figure
Ordered magnetic and quadrupolar states under hydrostatic pressure in orthorhombic PrCu2
We report magnetic susceptibility and electrical resistivity measurements on
single-crystalline PrCu2 under hydrostatic pressure, up to 2 GPa, which
pressure range covers the pressure-induced Van Vleck
paramagnet-to-antiferromagnet transition at 1.2 GPa. The measured anisotropy in
the susceptibility shows that in the pressure-induced magnetic state the
ordered 4f-moments lie in the ac-plane. We propose that remarkable pressure
effects on the susceptibility and resistivity are due to changes in the
quadrupolar state of O22 and/or O20 under pressure. We present a simple
analysis in terms of the singlet-singlet model.Comment: 14 pages, 9 figures submitted to Phys. Rev.
High-Field de Haas-van Alphen Effect in non-centrosymmetric CeCoGe3 and LaCoGe3
We report on de Haas-van Alphen effect measurements in the
non-centrosymmetric systems CeCoGe3 and LaCoGe3 in magnetic field up to 28
Tesla. In both compounds, two new high frequencies were observed in high
fields. The frequencies were not detected in previous lower field measurements.
The frequencies do not originate from magnetic breakdown, and, therefore, are
likely to be intrinsic features of the compounds. In CeCoGe3, the corresponding
effective masses are strongly enhanced, being of the order of 30 bare electron
masses.Comment: 3 pages, 4 figures, to be published in Proc. Int. Conf. Heavy
Electrons (ICHE2010) J. Phys. Soc. Jpn. 80 (2011
Novel phase diagram for antiferromagnetism and superconductivity in pressure-induced heavy-fermion superconductor CeRhIn probed by In-NQR
We present a novel phase diagram for the antiferromagnetism and
superconductivity in CeRhIn probed by In-NQR studies under pressure
(). The quasi-2D character of antiferromagnetic spin fluctuations in the
paramagnetic state at = 0 evolves into a 3D character because of the
suppression of antiferromagnetic order for 1.36 GPa (QCP:
antiferromagnetic quantum critical point). Nuclear-spin-lattice-relaxation rate
measurements revealed that the superconducting order occurs in the
range 1.36 -- 1.84 GPa, with maximum 0.9 K around
1.36 GPa.Comment: 5 pages, 5 figures, submitted to PR
Thermoelectric response near a quantum critical point: the case of CeCoIn5
We present a study of thermoelectric coefficients in CeCoIn_5 down to 0.1 K
and up to 16 T in order to probe the thermoelectric signatures of quantum
criticality. In the vicinity of the field-induced quantum critical point, the
Nernst coefficient nu exhibits a dramatic enhancement without saturation down
to lowest measured temperature. The dimensionless ratio of Seebeck coefficient
to electronic specific heat shows a minimum at a temperature close to threshold
of the quasiparticle formation. Close to T_c(H), in the vortex-liquid state,
the Nernst coefficient behaves anomalously in puzzling contrast with other
superconductors and standard vortex dynamics.Comment: 4 pages, 4 figures,final published versio
Anomalous quasiparticle transport in the superconducting state of CeCoIn5
We report on a study of thermal Hall conductivity k_xy in the superconducting
state of CeCoIn_5. The scaling relation and the density of states of the
delocalized quasiparticles, both obtained from k_xy, are consistent with d-wave
superconducting symmetry. The onset of superconductivity is accompanied by a
steep increase in the thermal Hall angle, pointing to a striking enhancement in
the quasiparticle mean free path. This enhancement is drastically suppressed in
a very weak magnetic field. These results highlight that CeCoIn_5 is unique
among superconductors. A small Fermi energy, a large superconducting gap, a
short coherence length, and a long mean free path all indicate that CeCoIn_5 is
clearly in the superclean regime (E_F/Delta<<l/xi), in which peculiar vortex
state is expected.Comment: 5 pages, 5 figure
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