38 research outputs found
The Dominant Role of Critical Valence Fluctuations on High Superconductivity in Heavy Fermions
Despite almost 40 years of research, the origin of heavy-fermion
superconductivity is still strongly debated. Especially, the pressure-induced
enhancement of superconductivity in CeCuSi away from the magnetic
breakdown is not sufficiently taken into consideration. As recently reported in
CeCuSi and several related compounds, optimal superconductivity occurs
at the pressure of a valence crossover, which arises from a virtual critical
end point at negative temperature . In this context, we did a
meticulous analysis of a vast set of top-quality high-pressure electrical
resistivity data of several Ce-based heavy fermion compounds. The key novelty
is the salient correlation between the superconducting transition temperature
and the valence instability parameter , which is in
line with theory of enhanced valence fluctuations. Moreover, it is found that,
in the pressure region of superconductivity, electrical resistivity is governed
by the valence crossover, which most often manifests in scaling behavior. We
develop the new idea that the optimum superconducting of a given
sample is mainly controlled by the compound's and limited by
non-magnetic disorder. In this regard, the present study provides compelling
evidence for the crucial role of critical valence fluctuations in the formation
of Cooper pairs in Ce-based heavy fermion superconductors besides the
contribution of spin fluctuations near magnetic quantum critical points, and
corroborates a plausible superconducting mechanism in strongly correlated
electron systems in general.Comment: Supplementary Material follows after the bibliograph
Upper critical field of CeCoIn5
We present a detailed analysis of the upper critical field for CeCoIn5 under
high pressure. We show that, consistently with other measurements, this system
shows a decoupling between maximum of the superconducting transition
temperature Tc and maximum pairing strength. This puts forward CeCoIn5 as an
important paradigm for this class of unconventional, strongly correlated
superconductors.Comment: 15 pages, 5 figures, author version, published in New J. Phy
Evidence for Coexistence of Bulk Superconductivity and Itinerant Antiferromagnetism in the Heavy Fermion System CeCo(InCd)
In the generic phase diagram of heavy fermion systems, tuning an external
parameter such as hydrostatic or chemical pressure modifies the superconducting
transition temperature. The superconducting phase forms a dome in the
temperature-tuning parameter phase diagram, which is associated with a maximum
of the superconducting pairing interaction. Proximity to antiferromagnetism
suggests a relation between the disappearance of antiferromagnetic order and
superconductivity. We combine muon spin rotation, neutron scattering, and x-ray
absorption spectroscopy techniques to gain access to the magnetic and
electronic structure of CeCo(InCd) at different time scales.
Different magnetic structures are obtained that indicate a magnetic order of
itinerant character, coexisting with bulk superconductivity. The suppression of
the antiferromagnetic order appears to be driven by a modification of the
bandwidth/carrier concentration, implying that the electronic structure and
consequently the interplay of superconductivity and magnetism is strongly
affected by hydrostatic and chemical pressure.Comment: Article + Supplementary information 33 pages, 13 figure
The Dominant Role of Critical Valence Fluctuations on High Tc Superconductivity in Heavy Fermions
Despite almost 40 years of research, the origin of heavy-fermion superconductivity is still strongly debated. Especially, the pressure-induced enhancement of superconductivity in CeCu2Si2 away from the magnetic breakdown is not sufficiently taken into consideration. As recently reported in CeCu2Si2 and several related compounds, optimal superconductivity occurs at the pressure of a valence crossover, which arises from a virtual critical end point at negative temperature Tcr. In this context, we did a meticulous analysis of a vast set of top-quality high-pressure electrical resistivity data of several Ce-based heavy fermion compounds. The key novelty is the salient correlation between the superconducting transition temperature Tc and the valence instability parameter Tcr, which is in line with theory of enhanced valence fluctuations. Moreover, it is found that, in the pressure region of superconductivity, electrical resistivity is governed by the valence crossover, which most often manifests in scaling behavior. We develop the new idea that the optimum superconducting Tc of a given sample is mainly controlled by the compound’s Tcr and limited by non-magnetic disorder. In this regard, the present study provides compelling evidence for the crucial role of critical valence fluctuations in the formation of Cooper pairs in Ce-based heavy fermion superconductors besides the contribution of spin fluctuations near magnetic quantum critical points, and corroborates a plausible superconducting mechanism in strongly correlated electron systems in general
Field-induced compensation of magnetic exchange as the origin of superconductivity above \texorpdfstring{40\,T}{40~T} in \texorpdfstring{\UTe}{UTe2}
The potential spin-triplet heavy-fermion superconductor \UTe exhibits
signatures of multiple distinct superconducting phases. For field aligned along
the axis, a metamagnetic transition occurs at \HmT. It
is associated with magnetic fluctuations that may be beneficial for the
field-enhanced superconductivity surviving up to \Hm. Once the field is tilted
away from the towards the axis, a reentrant superconducting phase
emerges just above \Hm. In order to better understand this remarkably
field-resistant superconducting phase, we conducted magnetic-torque and
magnetotransport measurements in pulsed magnetic fields. We determine the
record-breaking upper critical field of \HcT and its
evolution with angle. Furthermore, the normal-state Hall effect experiences a
drastic suppression indicative of a reduced band polarization above \Hm in the
angular range around caused by a partial compensation between the
applied field and an exchange field. This promotes the Jaccarino-Peter effect
as a possible mechanism for the reentrant superconductivity above \Hm.Comment: Main text: 27 pages, 4 figure, supplement: 10 pages, 5 figure
Thermodynamic signatures of short-range magnetic correlations in UTe
The normal-state out of which unconventional superconductivity in UTe
emerges is studied in detail using a variety of thermodynamic and transport
probes. Clear evidence for a broad Schottky-like anomaly with roughly R ln 2
entropy around K is observed in all measured quantities.
Comparison with high magnetic field transport data allows the construction of
an phase diagram resembling that of the ferromagnetic
superconductor URhGe. The low field electronic Gr\"uneisen parameter of
and that of the metamagnetic transition at T are comparable
pointing to a common origin of both phenomena. Enhanced Wilson and Korringa
ratios suggests that the existence of short range ferromagnetic fluctuations
cannot be ruled out
Organic shell wrapped silicon nanowires as an energy storage material
International audienceSilicon nanowires were first produced by lithography or CVD for electronics, sensing and optical applications. Independently, silicon has emerged as highly promising in lithium-ion battery anodes because of its absorbing 10 times more lithium than the standard carbon anodes. Silicon in battery anodes is submitted to intense mechanical constraints due to lithiation-delithiation, that only very small crystals can handle. Silicon nanowires then appeared as particularly efficient as they can withstand such constraints and maintain battery cycling over several hundreds of cycles. However, silicon nanowires grown as thin films do not fit as material for lithium-ion batteries, neither in terms of mass produced nor in terms of production cost: even a coin cell contains several milligrams of anode material, while silicon nanowires are grown at μg/cm by CVD.We recently patented [1] a new technology of silicon nanowire synthesis designed for mass production as a powder. The nanowires are grown in a glass or steel reactor at medium temperature (430°C) from metal nanoparticles deposited on an inert support, and from an air-stable organosilane as the silicon source. Table salt (NaCl) is usually used as a support that can be conveniently removed by washing with water after growth. Growth on salt also avoids handling silicon nanowires as a dry powder, preventing risk of inhaling nanoparticles. The synthesis yields silicon nanowires in gram scale, with a yield of 70-80%. Tests of the pure silicon nanowires in lithium-metal batteries show an excellent capacity retention over 1000 cycles