33 research outputs found
Yb-Yb correlations and crystal-field effects in the Kondo insulator YbB12 and its solid solutions
We have studied the effect of Lu substitution on the spin dynamics of the
Kondo insulator YbB12 to clarify the origin of the spin-gap response previously
observed at low temperature in this material. Inelastic neutron spectra have
been measured in Yb1-xLuxB12 compounds for four Lu concentrations x = 0, 0.25,
0.90 and 1.0. The data indicate that the disruption of coherence on the Yb
sublattice primarily affects the narrow peak structure occurring near 15-20 meV
in pure YbB12, whereas the spin gap and the broad magnetic signal around 38 meV
remain almost unaffected. It is inferred that the latter features reflect
mainly local, single-site processes, and may be reminiscent of the inelastic
magnetic response reported for mixed-valence intermetallic compounds. On the
other hand, the lower component at 15 meV is most likely due to dynamic
short-range magnetic correlations. The crystal-field splitting in YbB12
estimated from the Er3+ transitions measured in a Yb0.9Er0.1B12 sample, has the
same order of magnitude as other relevant energy scales of the system and is
thus likely to play a role in the form of the magnetic spectral response.Comment: 16 pages in pdf format, 9 figures. v. 2: coauthor list updated; extra
details given in section 3.2 (pp. 6-7); one reference added; fig. 5 axis
label change
Two band gap field-dependent thermal conductivity of
The thermal conductivity of the new superconductor was
studied as a function of the temperature and a magnetic field. No anomaly in
the thermal conductivity is observed around the superconducting
transition in absence or presence of magnetic fields up to 14 Tesla; upon that
field the superconductivity of persisted. The thermal conductivity in
zero-field shows a -linear increase up to 50K. The thermal conductivity is
found to increase with increasing field at high fields. We interpret the
findings as if there are two subsystems of quasiparticles with different
field-dependent characters in a two ( and )-band superconductor reacting
differently with the vortex structure. The unusual enhancement of at low temperature but higher than a () critical field
is interpreted as a result of the overlap of the low energy states outside the
vortex cores in the -band.Comment: 6 pages,3 figure
Studies on X-ray Thomson Scattering from Antiferroquadrupolar Order in TmTe
We study Thomson scattering from the antiferroquadrupole ordering phase in
TmTe. On the basis of the group theoretical treatment, we classify the
selection rules of the scattering intensity governed by the orientation of the
scattering vector G. Then, numerical verification is performed by invoking the
ground states which are deduced from a J=7/2 multiplet model. The obtained
intensity varies drastically depending on the magnitude and direction of G. We
also calculate the scattering intensities under the applied field for H//(001)
and (110). Their results behave differently when the orientation of G is
changed, which is ascribed to the difference of their primary order parameters;
O_{2}^{0} and O_{2}^{2} for H // (001) and (110), respectively. We make
critical comparisons between our results for TmTe and the experimental ones for
CeB_6. First, we assert that the intensities expected from TmTe at several
forbidden Bragg spots are sufficient enough to be experimentally detected.
Second, their intensities at (7/2,1/2,1/2) differ significantly and may be
attributed to the difference of the order parametersbetween the \Gamma_3-type
(O_{2}^{2} and O_{2}^{0}) and \Gamma_5-type (O_{yz}, O_{zx}, and O_{xy})
components, respectively.Comment: 18 pages, 3 figures, to be published in J. Phys. Soc. Jp
Dynamical Effective Medium Theory for Quantum Spins and Multipoles
A dynamical effective medium theory is presented for quantum spins and higher
multipoles such as quadrupole moments. The theory is a generalization of the
spherical model approximation for the Ising model, and is accurate up to
O(1/z_n) where z_n is the number of interacting neighbors. The polarization
function is optimized under the condition that it be diagonal in site indices.
With use of auxiliary fields and path integrals, the theory is flexibly applied
to quantum spins and higher multipoles with many interacting neighbors. A
Kondo-type screening of each spin is proposed for systems with extreme quantum
fluctuations but without conduction electrons.Comment: 16 pages, 3 Postscript figure
Collective magnetic excitations in mixed-valence Sm0.83Y0.17S
The magnetic spectral response of black-phase mixed-valence Sm0.83Y0.17S has
been measured by inelastic neutron scattering on a single crystal. Two magnetic
peaks are observed in the energy range of the Sm2+ spin-orbit transition (25-40
meV). Both of them exhibit significant dispersion along the three main symmetry
directions, reminiscent of the spin-orbit exciton branch found in pure divalent
SmS. The results can be reproduced by a simple phenomenological model
accounting for the existence of sizeable Sm-Sm exchange interactions, and a
microscopic mechanism is proposed on the basis of the "local-bound-state"
theory developed previously for SmB6.Comment: 6 pages in pdf format, 3 figures, submitted to Phys. Rev.
Microscopic theory of quadrupolar ordering in TmTe
We have calculated the crystal electric field of TmTe (T>T_Q) and have
obtained that the ground state of a Tm 4f hole is the doublet in
agreement with Mossbauer experiments. We study the quadrupole interactions
arising from quantum transitions of 4f holes of Tm. An effective attraction is
found at the L point of the Brillouin zone, . Assuming that the
quadrupolar condensation involves a single arm of we show that
there are two variants for quadrupole ordering which are described by the space
groups C2/c and C2/m. The Landau free energy is derived in mean-field theory.
The phase transition is of second order. The corresponding quadrupole order
parameters are combinations of and components. The obtained
domain structure is in agreement with observations from neutron diffraction
studies for TmTe. Calculated lattice distortions are found to be different for
the two variants of quadrupole ordering. We suggest to measure lattice
displacements in order to discriminate between those two structures.Comment: 10 pages, 2 figures, 5 tables; accepted by PR
Dramatic impact of intermediate-valence impurity on induced magnetism in singlet ground state system PrNi
We studied the influence of La (nonmagnetic) and Ce (intermediate valence) impurities on the ferromagnetic ordering temperature Tc of compounds RE1-x(La,Ce)xNi (RE = Pr, Gd). The ordering temperature Tc was measured in a wide range of impurity concentration. Crystal electric field (CEF) splitting for the PrNi and Pr0.25Ce0.75Ni was defined by neutron spectroscopy measurements. The strong difference in the influence of La and Ce on Tc of PrNi and GdNi based systems has been obtained. The introduction of a spin-fluctuating intermediate valence impurity in PrNi leads to a dramatic increase in the area of the ordered state on the phase diagram, due to an increase in Tc, compared to the impact of La-impurity over the entire concentration range. The dependence of Tc on the Ce concentration for the Pr1-xCexNi was calculated within the framework of the model of microscopic states. The essential modification of the CEF splitting by the Ce impurity appears to be the decisive factor affecting Tc wherein the Ce ion carries zero static magnetic moments similar to the La case. The latter explains the identical dependencies of the ordering temperature on the impurity concentration observed in systems with a long-range order of local magnetic moments Gd1-xLaxNi and Gd1-xCexNi
Phonon density-of-states in the new superconductor MgB
We provide here experimental data on the phonon density-of-states of MgB2
obtained by the inelastic neutron scattering technique. The measurements were performed
for the natural boron-based magnesium diboride with use of a time-of-flight neutron
spectrometer. Several phonon bands were observed in the phonon spectrum at energies of
about 33, 55, 82 and 99 meV. We show that the cut-off energy of the density-of-states
occurs at around 105 meV which is much higher than expected so far from heat-capacity
data and partially explains the high Tc value observed for MgB2. The characteristic phonon
energies are indicative of an intermediate coupling regime in this compound. We conclude
that a much needed neutron experiment aimed at the study of the isotopic effect in the
phonon density-of-states of MgB2 is conceivable