X-ray absorption measurements at the U LIII, Ru K, and Fe K edges are reported for the hidden order (HO) material URu2−xFexSi2 (x=0, 0.05, 0.08, 0.10, 0.12, 0.15, and 0.20) as a function of x and ...temperature T. When Fe is substituted for Ru, the local structure about Fe shrinks slightly and the first neighbor Fe-Si bond length decreases by ≈0.05Å. More importantly excess disorder is observed below 80–100 K (the coherence temperature T*) in plots of the Debye-Waller factor σ2 (σ is the width of the pair distribution function); at low T the data deviate from the usual Einstein or correlated-Debye model plots. This excess disorder is most prominent for the Ru-Si bond, and σ2 actually increases below 80 K. These results suggest a local orthorhombic distortion with B1g-like symmetry that develops below 80–100 K. A model that describes these local distortions is presented, and discussed in terms of other measurements that indicate a breaking of fourfold symmetry at low T. In addition, the square root of the difference between σ2(T) for the Ru-Si pair and a Debye fit to these data serves as an order parameter for this orthorhombic distortion, in the temperature range below 100 K. This quantity is a length related to a−b, the difference between the a and b lattice constants in the orthorhombic phase, and provides a connection between this distortion and T*. X-ray absorption near edge structure (XANES) measurements also show that there are no changes in the edge positions down to 0.1 eV for any edge as a function of x, for T in the HO regime.
Extraordinary electronic phenomena including an Yb valence transition, a change in Fermi surface topology, and suppression of the heavy fermion quantum critical field at a nominal concentration
0.2 ...have been found in the
system. These phenomena have no discernable effect on the unconventional superconductivity and normal-state non-Fermi liquid behaviour that occur over a broad range of
up to
0.8. However, the variation of the coherence temperature
and the superconducting critical temperature
with nominal Yb concentration
for bulk single crystals is much weaker than that of thin films. To determine whether differences in the actual Yb concentration of bulk single crystals and thin film samples might be responsible for these discrepancies, we employed Vegard's law and the spectroscopically determined values of the valences of Ce and Yb as a function of
to determine the actual composition
of bulk single crystals. This analysis is supported by energy-dispersive X-ray spectroscopy, wavelength-dispersive X-ray spectroscopy, and transmission X-ray absorption edge spectroscopy measurements. The actual composition
is found to be about one-third of the nominal concentration
up to
0.5, and resolves the discrepancy between the variation of the physical properties of
single crystals and thin films with Yb concentration.
Recent experiments on the Pr-based filled skutterudite arsenides and antimonides
PrOs
4
Sb
12
,
Pr
(
Os
1
-
x
Ru
x
)
4
Sb
12
,
Pr
1
-
x
Nd
x
Os
4
Sb
12
,
PrFe
4
As
12
,
PrRu
4
As
12
, and
PrOs
4
As
...12
are reviewed. The heavy fermion compound
PrOs
4
Sb
12
exhibits unconventional strong-coupling superconductivity below
T
c
=
1.85
K
that breaks time reversal symmetry, apparently consists of several distinct superconducting phases, and may involve triplet spin pairing of electrons. Studies of the alloy systems
Pr
(
Os
1
-
x
Ru
x
)
4
Sb
12
and
Pr
1
-
x
Nd
x
Os
4
Sb
12
revealed rich
T–
x phase diagrams and a strong suppression of the high field ordered phase and the unconventional superconductivity of
PrOs
4
Sb
12
with Ru substitution. Among the three Pr-based filled skutterudite arsenides,
PrFe
4
As
12
has a ferromagnetic ground state,
PrRu
4
As
12
exhibits conventional BCS superconductivity, and
PrOs
4
As
12
is an antiferromagnet.
The local structure of two skutterudite families - CeM sub(4)As sub(12) (M = Fe, Ru, Os) and Ln Cu sub(3) Ru sub(4) O sub(12)(Ln = La, Pr, and Nd) - have been studied using the extended x-ray ...absorption fine structure (EXAFS) technique with a focus on the lattice vibrations about the rare-earth "rattler atoms" and the extent to which these vibrations can be considered local modes, with the rattler vibrating inside a nearly rigid cage. The pair distances from EXAFS results agree quite well with the average structure obtained from diffraction. The cage structure is formed by the M and As atoms in Ce M sub(4) As sub(12) and by Cu, O, and Ru atoms in Ln Cu sub(3) Ru sub(4) O sub(12). In addition, we show that the As sub(4) or CuO sub(4) rings are relatively rigid units and that their vibrations are anisotropic within these cubic structures, with stiff restoring forces perpendicular to the rings and much weaker restoring forces in directions parallel to the rings.
An introduction to and overview of the contents of this Special Issue are given. 32 classes of superconducting materials are discussed, grouped under the three categories “conventional”, “possibly ...unconventional” and “unconventional” according to the mechanism believed to give rise to superconductivity.
The microscopic mechanism for electron pairing in heavy-fermion superconductors remains a major challenge in quantum materials. Some form of magnetic mediation is widely accepted with spin ...fluctuations as a prime candidate. A novel mechanism, "composite pairing" based on the cooperative two-channel Kondo effect directly involving the f -electron moments, has also been proposed for some heavy-fermion compounds including CeCoIn5. The origin of the spin-resonance peak observed in neutron-scattering measurements on CeCoIn5 is still controversial and the corresponding hump-dip structure in the tunneling conductance is missing. This is in contrast to the cuprate and Fe-based high-temperature superconductors, where both characteristic signatures are observed, indicating spin fluctuations are likely involved in the pairing process. Here, we report results from planar tunneling spectroscopy along three major crystallographic orientations of CeCoIn5 over wide ranges of temperature and magnetic field. The pairing gap opens at Tp ∼ 5 K, well above the bulk Tc = 2.3 K , and its directional dependence is consistent with dx2 − y2 symmetry. With increasing magnetic field, this pairing gap is suppressed as expected but, intriguingly, a gaplike structure emerges smoothly, increasing linearly up to the highest field applied. This field-induced gaplike feature is only observed below Tp. The concomitant appearance of the pairing gap and the field-induced gaplike feature, along with its linear increase with field, indicates that the f-electron local moments are directly involved in the pairing process in CeCoIn 5.
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