In the hole-doped cuprates, a small number of carriers suppresses antiferromagnetism and induces superconductivity. In the electron-doped cuprates, on the other hand, superconductivity appears only ...in a narrow window of high-doped Ce concentration after reduction annealing, and strong antiferromagnetic correlation persists in the superconducting phase. Recently, Pr(1.3-x)La0.7Ce(x)CuO4 (PLCCO) bulk single crystals annealed by a protect annealing method showed a high critical temperature of around 27 K for small Ce content down to 0.05. Here, by angle-resolved photoemission spectroscopy measurements of PLCCO crystals, we observed a sharp quasi-particle peak on the entire Fermi surface without signature of an antiferromagnetic pseudogap unlike all the previous work, indicating a dramatic reduction of antiferromagnetic correlation length and/or of magnetic moments. The superconducting state was found to extend over a wide electron concentration range. The present results fundamentally challenge the long-standing picture on the electronic structure in the electron-doped regime.
We have studied the effect of RE substitution on the structure and the local atomic disorder in REO
0.5
F
0.5
BiS
2
(RE = rare-earth) to understand their correlation with the bulk superconductivity ...in these materials. The mean RE size, affecting the chemical pressure, has been varied in two series namely Ce
1−
x
Nd
x
O
0.5
F
0.5
BiS
2
and Nd
1−
y
Sm
y
O
0.5
F
0.5
BiS
2
. The lattice parameters evolve anomalously, showing an anisotropic shrinkage (elongation) of the
c
-axis (
a
-axis) to an isotropic expansion of the lattice with increasing mean RE size. The Bi L
3
-edge extended X-ray absorption fine structure (EXAFS) measurements are performed to investigate local displacements in the BiS
2
lattice, revealing a large disorder and a sharp boundary between the Ce-containing and Sm-containing series with a distinct local structure. The results suggest that the bulk superconductivity in REO
0.5
F
0.5
BiS
2
is correlated with anomalous atomic displacements in the Bi-S1 network, likely to be a combined effect of active Bi 6s electronic states and a possible Jahn-Teller-like instability of the Bi 6p
x
/6p
y
electrons.
We have studied the effect of RE substitution on the structure and the local atomic disorder in REO
0.5
F
0.5
BiS
2
(RE = rare-earth) to understand their correlation with the bulk superconductivity in these materials.
Temperature-dependent Fe K-edge extended X-ray absorption fine structure (EXAFS) and Fe Kβ X-ray emission spectroscopy (XES) measurements are used to study the local structure and local Fe magnetic ...moment in the self-doped CaKFe4As4 superconductor. The local Fe–As and Fe–Fe bond lengths show thermal expansion similar to the one found in other iron-based superconductors. The arsenic height from the Fe–Fe plane changes anomalously with a sharp decrease in the vicinity of the superconducting transition temperature. The Fe Kβ XES reveals that Fe in CaKFe4As4 is in the low spin state, and the local magnetic moment decreases sharply while the sample is cooled across the transition temperature. The results indicate importance of the magnetoelastic coupling in the superconductivity with a key role of interlayer atomic correlations in these materials.
We have combined the extended X-ray absorption fine structure (EXAFS) and X-ray emission spectroscopy (XES) to investigate the local structure and the local iron magnetic moments of (Li
1−
x
Fe
x
...)OHFeSe (
x
∼0.2) superconductors. The local structure, studied by Fe K-edge EXAFS measurements, is found to be inhomogeneous that is characterized by different Fe-Se bond lengths. The inhomogeneous phase exhibits a peculiar temperature dependence with lattice anomalies in the local structural parameters at the critical temperature
T
c
(36 K) and at the spin density wave (SDW) transition temperature
T
N
(130 K). Fe Kβ XES shows iron to be in a low spin state with the local Fe magnetic moment evolving anomalously as a function of temperature. Apart from a quantitative measurement of the local structure of (Li
1−
x
Fe
x
)OHFeSe, providing direct evidence of nanoscale inhomogeneity, the results provide further evidence of the vital role that the coupled electronic, lattice and magnetic degrees of freedom play in the iron-based superconductors.
Anomalous local structure and magnetic moment of an inhomogeneous FeSe based superconductor indicating importance of the structure and magnetic correlations in these materials.
We consider the superstructures, which can be formed in spinels containing on B sites the transition-metal ions with partially filled t(2g) levels. We show that, when such systems are close to the ...itinerant state (e.g., have an insulator-metal transition), there may appear in them an orbitally driven Peierls state. We explain by this mechanism the very unusual superstructures observed in CuIr2S4 (octamers) and MgTi2O4 (chiral superstructures) and suggest that a similar phenomenon should be observed in NaTiO2 and possibly in some other systems.
Ta2NiS5 is supposed to be a simple semiconductor in which excitonic instability of Ta2NiS5 is suppressed due to its large band gap. However, the Ni 2p core-level photoemission of Ta2NiS5 exhibits a ...satellite indicating Ni 3d orbitals are mixed into its conduction band as expected in an excitonic insulator. The valence band does not show dispersion flattening and spectral sharpening which are fingerprints of an excitonic insulator. Instead, Ni 3p − 3d resonant photoemission indicates Mottness of the Ni 3 d electron in Ta 2 NiS 5 with negative charge-transfer energy. The present results show that Ta2NiS5 can be viewed as a valence bond insulator with a band gap exceeding the exciton binding energy.
The local structure of the filled tetragonal tungsten bronze (TTB) niobate Ba3Nb5−xTixO15 (x = 0, 0.1, 0.7, 1.0), showing a metal-insulator transition with Ti substitution, has been studied by Nb ...K-edge extended X-ray absorption fine structure (EXAFS) measurements as a function of temperature. The Ti substitution has been found to have a substantial effect on the local structure, that remains largely temperature independent in the studied temperature range of 80–400 K. The Nb-O bonds distribution shows an increased octahedral distortion induced by Ti substitution, while Nb-Ba distances are marginally affected. The Nb-O bonds are stiffer in the Ti substituted samples, which is revealed by the temperature dependent mean square relative displacements (MSRDs). Furthermore, there is an overall increase in the configurational disorder while the system with Nb 4d electrons turns insulating. The results underline a clear relationship between the local structure and the electronic transport properties suggesting that the metal-insulator transition and possible thermoelectric properties of TTB structured niobates can be tuned by disorder.
We report on a photoemission study of Ta2NiSe5 that has a quasi-one-dimensional structure and an insulating ground state. Ni 2p core-level spectra show that the Ni 3d subshell is partially occupied ...and the Ni 3d states are heavily hybridized with the Se 4p states. In angle-resolved photoemission spectra, the valence-band top is found to be extremely flat, indicating that the ground state can be viewed as an excitonic insulator state between the Ni 3d-Se 4p hole and the Ta 5d electron. We argue that the high atomic polarizability of Se plays an important role to stabilize the excitonic state.
Isovalent substitution of S by Se in LaOBiS2−xSex has a substantial effect on its electronic structure and thermoelectric properties. To investigate the possible role of BiS2 structural instability, ...we have studied the local structure of LaOBiS2−xSex () using temperature dependent Bi L3-edge extended x-ray absorption fine structure measurements. The results reveal that the local structure of the two compounds is significantly different. The BiS2 sub-lattice is largely distorted in LaOBiS2 (x = 0.0), with two in-plane Bi-S1 distances separated by 0.4 Å instead LaOBiSSe (x = 1.0) showing much smaller local disorder with two in-plane Bi-Se distances in the plane being separated by 0.2 Å. Temperature dependent study shows that the two Bi-S1 distances are characterized by different bond strength in LaOBiS2 (x = 0.0) while it is similar for the Bi-Se distances in LaOBiSSe (x = 1.0). The out of plane Bi-S2 bond is harder in LaOBiSSe indicating that the structural instability of BiS2 layer has large effect on the out-of-plane atomic correlations. The results suggest that the local structure of LaOBiS2−xSex is an important factor to describe differing electronic and thermal transport of the two compounds.
We have studied the local structure of LaO0.5F0.5BiS2−xSex by Bi L1-edge extended x-ray absorption fine structure (EXAFS). We find a significant effect of Se substitution on the local atomic ...correlations with a gradual elongation of average in-plane Bi-S bondlength. The associated mean square relative displacement, measuring average local distortions in the BiS2 plane, hardly shows any change for small Se substitution, but decreases significantly for x 0.6. The Se substitution appears to suppress the local distortions within the BiS2 plane that may optimize in-plane orbital hybridization and hence the superconductivity. The results suggest that the local structure of the BiS2-layer is one of the key ingredients to control the physical properties of the BiS2-based dichalcogenides.
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