143 research outputs found
Optical spectra of the heavy fermion uniaxial ferromagnet UGe
We report a detailed study of UGe single crystals using infrared
reflectivity and spectroscopic ellipsometry. The optical conductivity suggests
the presence of a low frequency interband transition and a narrow free-carrier
response with strong frequency dependence of the scattering rate and effective
mass. We observe sharp changes in the low frequency mass and scattering rate
below the upper ferromagnetic transition . The characteristic
changes are exhibited most strongly at an energy scale of around 12 meV (100
cm). They recover their unrenormalized value above and for 40 meV. In contrast no sign of an anomaly is seen at the lower transition
temperature of unknown nature 30 K, observed in transport and
thermodynamic experiments. In the ferromagnetic state we find signatures of a
strong coupling to the longitudinal magnetic excitations that have been
proposed to mediate unconventional superconductivity in this compound
Valence instability of cerium under pressure in the Kondo-like perovskite LaCeSrMnO
Effect of hydrostatic pressure and magnetic field on electrical resistance of
the Kondo-like perovskite manganese oxide,
LaCeSrMnO with a ferrimagnetic ground state, have
been investigated up to 2.1 GPa and 9 T. In this compound, the Mn-moments
undergo double exchange mediated ferromagnetic ordering at
280 K and there is a resistance maximum, at about 130 K which is
correlated with an antiferromagnetic ordering of {\it cerium} with respect to
the Mn-sublattice moments. Under pressure, the shifts to lower
temperature at a rate of d/d = -162 K/GPa and disappears at a
critical pressure 0.9 GPa. Further, the coefficient, of
term due to Kondo scattering decreases linearly with increase of
pressure showing an inflection point in the vicinity of . These
results suggest that {\it cerium} undergoes a transition from Ce state
to Ce/Ce mixed valence state under pressure. In contrast to
pressure effect, the applied magnetic field shifts to higher
temperature presumably due to enhanced ferromagnetic Mn moments.Comment: to be published in Phys. Rev. B (rapid commun
High-pressure study of non-Fermi liquid and spin-glass-like behavior in CeRhSn
We present measurements of the temperature dependence of electrical
resistivity of CeRhSn up to ~ 27 kbar. At low temperatures, the electrical
resistivity varies linearly with temperature for all pressures, indicating
non-Fermi liquid behavior. Below a temperature Tf ~ 6 K, the electrical
resistivity deviates from a linear dependence. We found that the
low-temperature feature centered at T = Tf shows a pressure dependence dTf/dP ~
30 mK/kbar which is typical of canonical spin glasses. This interplay between
spin-glass-like and non-Fermi liquid behavior was observed in both CeRhSn and a
Ce0.9La0.1RhSn alloy.Comment: 5 pages, 3 figures, accepted for publication to Journal of Physics:
Condensed Matte
Competition between hidden order and antiferromagnetism in URu_2Si_2 under uniaxial stress studied by neutron scattering
We have performed elastic neutron scattering experiments under uniaxial
stress sigma applied along the tetragonal [100], [110] and [001] directions for
the heavy electron compound URu2Si2. We found that antiferromagnetic (AF) order
with large moment is developed with sigma along the [100] and [110] directions.
If the order is assumed to be homogeneous, the staggered ordered moment mu_o
continuously increases from 0.02 mu_B (sigma=0) to 0.22 mu_B (0.25 GPa). The
rate of increase partial mu_o/partial sigma is ~ 1.0 mu_B/GPa, which is four
times larger than that for the hydrostatic pressure (partial mu_o/partial P sim
0.25 mu_B/GPa). Above 0.25 GPa, mu_o shows a tendency to saturate, similar to
the hydrostatic pressure behavior. For sigma||[001], mu_o shows only a slight
increase to 0.028 mu_B (sigma = 0.46 GPa) with a rate of ~ 0.02 mu_B/GPa,
indicating that the development of the AF state highly depends on the direction
of sigma. We have also found a clear hysteresis loop in the isothermal
mu_o(sigma) curve obtained for sigma||[110] under the zero-stress-cooled
condition at 1.4 K. This strongly suggests that the sigma-induced AF phase is
metastable, and separated from the "hidden order" phase by a first-order phase
transition. We discuss these experimental results on the basis of crystalline
strain effects and elastic energy calculations, and show that the c/a ratio
plays a key role in the competition between these two phases.Comment: 9 pages, 7 figures, to appear in Physical Review
Pressure dependence of the magnetization in the ferromagnetic superconductor UGe_2
The recent discovery that superconductivity occurs in several clean itinerant
ferromagnets close to low temperature magnetic instabilities naturally invites
an interpretation based on a proximity to quantum criticality. Here we report
measurements of the pressure dependence of the low temperature magnetisation in
one of these materials, UGe_2. Our results show that both of the magnetic
transitions observed in this material as a function of pressure are first order
transitions and do not therefore correspond to quantum critical points. Further
we find that the known pressure dependence of the superconducting transition is
not reflected in the pressure dependence of the static susceptibility. This
demonstrates that the spectrum of excitations giving superconductivity is not
that normally associated with a proximity to quantum criticality in weak
itinerant ferromagnets. In contrast our data suggest that instead the pairing
spectrum might be related to a sharp spike in the electronic density of states
that also drives one of the magnetic transitions.Comment: to appear in Phys. Rev. Let
The Magnetic Phase Diagram and the Pressure and Field Dependence of the Fermi Surface in UGe
The ac susceptibility and de Haas-van Alphen (dHvA) effect in UGe are
measured at pressures {\it P} up to 17.7 kbar for the magnetic field {\it B}
parallel to the {\it a} axis, which is the easy axis of magnetization. Two
anomalies are observed at {\it B}({\it P}) and {\it B}({\it P}) ({\it
B} {\it B} at any {\it P}), and the {\it P}-{\it B} phase diagram
is presented. The Fermi surface and quasiparticle mass are found to vary
smoothly with pressure up to 17.7 kbar unless the phase boundary {\it
B}({\it P}) is crossed. The observed dHvA frequencies may be grouped into
three according to their pressure dependences, which are largely positive,
nearly constant or negative. It is suggested that the quasiparticle mass
moderately increases as the boundary {\it B}({\it P}) is approached. DHvA
effect measurements are also performed across the boundary at 16.8 kbar.Comment: to be published in Phys. Rev.
Coupled CDW and SDW Fluctuations as an Origin of Anomalous Properties of Ferromagnetic Superconductor UGe_2
It is shown that anomalous properties of UGe_2 can be understood in a unified
way on the basis of a single assumption that the superconductivity is mediated
by the coupled SDW and CDW fluctuations induced by the imperfect nesting of the
Fermi surface with majority spins at T=T_x(P) deep in the ferromagnetic phase.
Excess growth of uniform magnetization is shown to develop in the temperature
range T<T_x(P) as a mode-coupling effect of coupled growth of SDW and CDW
orderings, which has been observed by two different types of experiments. The
coupled CDW and SDW fluctuations are shown to be essentially ferromagnetic spin
fluctuations which induce a spin-triplet p-wave attraction. These fluctuations
consist of two modes, spin and charge fluctuations with large momentum transfer
of the nesting vector. An anomalous temperature dependence of the upper
critical field H_c2(T) such as crossing of H_c2(T) at P=11.4 kbar and P=13.5
kbar, can be understood by the strong-coupling-superconductivity formalism.
Temperature dependence of the lattice specific heat including a large shoulder
near T_x is also explained quite well as an effect of a kind of Kohn anomaly
associated with coupled SDW-CDW transition.Comment: (12 pages, 10 eps figures) submitted to J. Phys. Soc. Jp
Effects of La substitution on superconducting state of CeCoIn5
We report effects of La substitution on superconducting state of heavy
fermion superconductor CeCoIn5, as seen in transport and magnetization
measurements. As opposed to the case of conventional superconductors, pair
breaking by nonmagnetic La results in depression of Tc and indicates strong gap
anisotropy. Upper critical field Hc2 values decrease with increased La
concentration, but the critical field anisotropy, gamma=Hc2(a)/Hc2(c), does not
change in the Ce_{1-x}La_xCoIn5 (x=0-0.15). The electronic system is in the
clean limit for all values of x.Comment: Submitted to Phys. Rev.
Theory of Ferromagnetic Superconductivity
It is argued that the pairing symmetry realized in a ferromagnetic
superconductor UGe must be a non-unitary triplet pairing. This particular
state is free from the Pauli limitation and can survive under a huge internal
molecular filed. To check our identification we examine its basic properties
and several experiments are proposed. In particular, the external field is used
to raise by controlling the internal spontaneous dipole field.Comment: 4 pages, no figure
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