157 research outputs found
Dependence of transport coefficients of Yb(RhCo)Si intermetallics on temperature and cobalt concentration
Dependence of transport coefficients of the Yb(RhCo)Si
series of alloys on temperature and cobalt concentration is explained by an
asymmetric Anderson model which takes into account the exchange scattering of
conduction electrons on ytterbium ions and the splitting of 4-states by the
crystalline electric field (CEF). The substitution of rhodium by cobalt is
described as an increase of chemical pressure which reduces the exchange
coupling and the CEF splitting. The scaling analysis and numerical NCA solution
of the model show that the effective degeneracy of the 4-state at a given
temperature depends on the relative magnitude of the Kondo scale and the CEF
splitting. Thus, we find that dependence of the thermopower, , on
temperature and cobalt concentration can be understood as an interplay of
quantum fluctuations, driven by the Kondo effect, and thermal fluctuations,
which favor a uniform occupation of the CEF states. The theoretical model
captures all the qualitative features of the experimental data and it explains
the evolution of the shape of with the increase of cobalt concentration.Comment: 8 pages, 4 figure
Pr magnetism and its interplay with the Fe spin density wave in PrFeAsO
We have studied the magnetism of the Pr3+ ions in PrFeAsO_1-xF_x (x = 0;
0.15) and its interaction with the Fe magnetic order (for x = 0). Specific heat
data confirm the presence of a first excited crystal electric field (CEF) level
around 3.5 meV in the undoped compound PrFeAsO. This finding is in agreement
with recent neutron scattering experiments. The doped compound is found to have
a much lower first CEF splitting of about 2.0 meV. The Pr ordering in PrFeAsO
gives rise to large anomalies in the specific heat and the thermal expansion
coefficient. In addition, a field-induced transition is found at low
temperatures that is most pronounced for the magnetostriction coefficient. This
transition, which is absent in the doped compound, is attributed to a reversal
of the Fe spin canting as the antiferromagnetic Pr order is destroyed by the
external magnetic field.Comment: 8 pages, 6 figure
The superconducting gaps in LiFeAs: Joint study of specific heat and ARPES
We present specific heat, c_P, and ARPES data on single crystals of the
stoichiometric superconductor LiFeAs. A pronounced anomaly is found in c_P at
the superconducting transition. The electronic contribution can be described by
two s-type energy gaps with magnitudes of approximately Delta1 = 1.2 meV and
Delta2 = 2.6 meV and a normal-state gamma coefficient of 10 mJ/mol K^2. All
these values are in remarkable agreement with ARPES results.Comment: 4 pages, 3 figure
Thermopower and thermal conductivity in the Weyl semimetal NbP
The Weyl semimetal NbP exhibits an extremely large magnetoresistance (MR) and
an ultra-high mobility. The large MR originates from a combination of the
nearly perfect compensation between electron- and hole-type charge carriers and
the high mobility, which is relevant to the topological band structure. In this
work we report on temperature- and field-dependent thermopower and thermal
conductivity experiments on NbP. Additionally, we carried out complementary
heat capacity, magnetization, and electrical resistivity measurements. We found
a giant adiabatic magnetothermopower with a maximum of 800 V/K at 50 K in
a field of 9 T. Such large effects have been observed rarely in bulk materials.
We suggest that the origin of this effect might be related to the high
charge-carrier mobility. We further observe pronounced quantum oscillations in
both thermal conductivity and thermopower. The obtained frequencies compare
well with our heat capacity and magnetization data.Comment: 6 pages, 3 figure
Evidence for a Kondo destroying quantum critical point in YbRh2Si2
The heavy-fermion metal YbRhSi is a weak antiferromagnet below
K. Application of a low magnetic field T () is sufficient to continuously suppress the antiferromagnetic (AF) order.
Below K, the Sommerfeld coefficient of the electronic specific
heat exhibits a logarithmic divergence. At K, (), while the electrical resistivity
(: residual resistivity). Upon
extrapolating finite- data of transport and thermodynamic quantities to , one observes (i) a vanishing of the "Fermi surface crossover" scale
, (ii) an abrupt jump of the initial Hall coefficient and
(iii) a violation of the Wiedemann Franz law at , the field-induced
quantum critical point (QCP). These observations are interpreted as evidence of
a critical destruction of the heavy quasiparticles, i.e., propagating Kondo
singlets, at the QCP of this material.Comment: 20 pages, 8 figures, SCES 201
Interplay between Kondo suppression and Lifshitz transitions in YbRhSi at high magnetic fields
We investigate the magnetic field dependent thermopower, thermal
conductivity, resistivity and Hall effect in the heavy fermion metal YbRh2Si2.
In contrast to reports on thermodynamic measurements, we find in total three
transitions at high fields, rather than a single one at 10 T. Using the Mott
formula together with renormalized band calculations, we identify Lifshitz
transitions as their origin. The predictions of the calculations show that all
experimental results rely on an interplay of a smooth suppression of the Kondo
effect and the spin splitting of the flat hybridized bands.Comment: 5 pages, 4 figure
Magnetism and superconductivity driven by identical 4 states in a heavy-fermion metal
The apparently inimical relationship between magnetism and superconductivity
has come under increasing scrutiny in a wide range of material classes, where
the free energy landscape conspires to bring them in close proximity to each
other. This is particularly the case when these phases microscopically
interpenetrate, though the manner in which this can be accomplished remains to
be fully comprehended. Here, we present combined measurements of elastic
neutron scattering, magnetotransport, and heat capacity on a prototypical heavy
fermion system, in which antiferromagnetism and superconductivity are observed.
Monitoring the response of these states to the presence of the other, as well
as to external thermal and magnetic perturbations, points to the possibility
that they emerge from different parts of the Fermi surface. This enables a
single 4 state to be both localized and itinerant, thus accounting for the
coexistence of magnetism and superconductivity.Comment: 4 pages, 4 figure
Interplay between unconventional superconductivity and heavy-fermion quantum criticality: CeCuSi versus YbRhSi
In this paper the low-temperature properties of two isostructural canonical
heavy-fermion compounds are contrasted with regards to the interplay between
antiferromagnetic (AF) quantum criticality and superconductivity. For
CeCuSi, fully-gapped d-wave superconductivity forms in the vicinity of
an itinerant three-dimensional heavy-fermion spin-density-wave (SDW) quantum
critical point (QCP). Inelastic neutron scattering results highlight that both
quantum critical SDW fluctuations as well as Mott-type fluctuations of local
magnetic moments contribute to the formation of Cooper pairs in CeCuSi.
In YbRhSi, superconductivity appears to be suppressed at
mK by AF order ( = 70 mK). Ultra-low temperature measurements reveal a
hybrid order between nuclear and 4f-electronic spins, which is dominated by the
Yb-derived nuclear spins, to develop at slightly above 2 mK. The hybrid
order turns out to strongly compete with the primary 4f-electronic order and to
push the material towards its QCP. Apparently, this paves the way for
heavy-fermion superconductivity to form at = 2 mK. Like the pressure -
induced QCP in CeRhIn, the magnetic field - induced one in YbRhSi
is of the local Kondo-destroying variety which corresponds to a Mott-type
transition at zero temperature. Therefore, these materials form the link
between the large family of about fifty low- unconventional heavy - fermion
superconductors and other families of unconventional superconductors with
higher s, notably the doped Mott insulators of the cuprates, organic
charge-transfer salts and some of the Fe-based superconductors. Our study
suggests that heavy-fermion superconductivity near an AF QCP is a robust
phenomenon.Comment: 30 pages, 7 Figures, Accepted for publication in Philosophical
Magazin
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