Carrier-induced lattice distortion (signature of strong electron–lattice interaction) in oxypnictide superconductors is found by a local structural study using extended X-ray absorption fine ...structure (EXAFS). Local lattice instability (polaron formation) is detected as an anomalous upturn of the mean-square relative displacement for the nearest neighbor distances (Fe–As) in LaFeAsO1−xFx (x=0.07). Intimate correlation between local lattice mode and superconductivity is revealed. The results suggest that strong electron–lattice interaction is present as a common ingredient in the microscopic mechanism of superconducting transition.
75As and 139La NMR studies on LaFeAs(O1-xFx) Ishida, K.; Nakai, Y.; Kitagawa, S. ...
Physica. B, Condensed matter,
10/2009, Letnik:
404, Številka:
19
Journal Article, Conference Proceeding
Recenzirano
We report experimental results of 75As and 139La nuclear magnetic resonance (NMR) in the layered oxypnictide system LaFeAs(O1-xFx) (x=0.0, 0.04, 0.07 and 0.11) where superconductivity occurs in x ...greater than 0.04. In the undoped LaFeAsO, 1/T1 of 139La exhibits a distinct peak at TN~142K below which the La NMR spectra become broadened due to the internal magnetic field attributed to an antiferromagnetic (AFM) ordering. In the x=0.04 sample, 1/T1T of 75As exhibits a Curie–Weiss temperature dependence down to 30K, suggesting the development of AFM spin fluctuations with decreasing temperature. In the x=0.11 sample, in contrast, pseudogap behavior is observed in 1/T1T at the 75As site with a gap value of ΔPG~175K. The spin dynamics vary markedly with F-doping, which is ascribed to the change of the Fermi-surface structure by the electron-doping. As for the superconducting properties for x=0.04, 0.07 and 0.11, 1/T1 of 75As in all compounds does not exhibit a coherence peak just below Tc and follows a T3 dependence at low temperatures. These results seemingly suggests that unconventional superconductivity with zero gap along lines, whereas the lack of the residual density of states at the low temperatures is incompatible with the presence of the line-nodes. We discuss similarity and difference between LaFeAs(O1-xFx) and cuprates, and also discuss the relationship between spin dynamics and superconductivity on the basis of F-doping dependence of Tc and 1/T1.
75As-nuclear magnetic relaxation rate (1/T1) was measured under pressure in LaFeAsO1−xFx (x=0.08 and 0.14). For the x=0.14 doped samples, 1/T1T=constant is observed at ambient pressure just above Tc, ...and application of pressure enhances the value, similar to the behavior of Tc. 1/T1T=constant originates from the Korringa relation, and an increase in DOS due to pressure application leads to the enhancement of Tc. In the case of x=0.08, 1/T1T=constant is observed in a wide T range above Tc, and the value is enhanced compared to the case of x=0.14. The enhancement is attributable to antiferromagnetic spin fluctuation rather than an increase in DOS.
We have been investigating the effect of (CoFe
2
O
4
)
x
and (ZnFe
2
O
4
)
x
(
x
is the wt%) nano-sized ferrites on the superconducting (SC) properties of the high-
T
c
SC compound GdBa
2
Cu
3
O
7-δ
...(Gd123) prepared by the conventional solid-state reaction technique. In this work, the magnetic hysteresis for pure Gd123 samples and samples added with nano-sized ferrites was measured using a superconducting quantum interference device (SQUID) magnetometer using a dc field up to 30,000 Oe. The magnetic
J
c
was estimated using the extended Bean model. While a clear increase in
J
c
values with the addition of (CoFe
2
O
4
)
x
nano-sized ferrites is observed, a less clear positive effect on the
J
c
values is observed with the addition of (ZnFe
2
O
4
)
x
nano-sized ferrites. These results indicate an improvement of grain connectivity, decrease of voids between the grains, and decrease of porosity in Gd123 compound resulting from CoFe
2
O
4
addition consistent with the mechanical properties measured before. (CoFe
2
O
4
)
x
nano-sized ferrite addition has thus resulted in an improvement in the SC properties of Gd123 high-
T
c
SC compound making it more convenient for applications.
The relationship between antiferromagnetic (AF) fluctuation and superconductivity was investigated in the La1111 series, LaFeAsO
1−
x
F
x
(
x
=0.05, 0.08, and 0.14) by examining nuclear relaxation ...rates (1/
T
1
) at both ambient pressure and 3.0 GPa. Although the AF fluctuation is enhanced by applying pressure in the underdoped regime (
x
=0.05, and 0.08), the increase in critical transition temperature (
T
c
) is small, whereas
T
c
increases remarkably in the overdoped regime (
x
=0.14) where the AF fluctuation is absent, suggesting that high
T
c
above 40 K originates not from the AF fluctuation but from the density of states at the electron pocket.
We have been investigating the effect of (CoFe{sub 2}O{sub 4}){sub x} and (ZnFe{sub 2}O{sub 4}){sub x} (x is the wt%) nano-sized ferrites on the superconducting (SC) properties of the high-T{sub c} ...SC compound GdBa{sub 2}Cu{sub 3}O{sub 7-δ} (Gd123) prepared by the conventional solid-state reaction technique. In this work, the magnetic hysteresis for pure Gd123 samples and samples added with nano-sized ferrites was measured using a superconducting quantum interference device (SQUID) magnetometer using a dc field up to 30,000 Oe. The magnetic J{sub c} was estimated using the extended Bean model. While a clear increase in J{sub c} values with the addition of (CoFe{sub 2}O{sub 4}){sub x} nano-sized ferrites is observed, a less clear positive effect on the J{sub c} values is observed with the addition of (ZnFe{sub 2}O{sub 4}){sub x} nano-sized ferrites. These results indicate an improvement of grain connectivity, decrease of voids between the grains, and decrease of porosity in Gd123 compound resulting from CoFe{sub 2}O{sub 4} addition consistent with the mechanical properties measured before. (CoFe{sub 2}O{sub 4}){sub x} nano-sized ferrite addition has thus resulted in an improvement in the SC properties of Gd123 high-T{sub c} SC compound making it more convenient for applications.
Carrier-induced lattice distortion (signature of strong electron-lattice interaction) in oxypnictide superconductors is found by a local structural study using extended x-ray absorption fine ...structure (EXAFS). Local lattice instability (polaron formation) is detected as an anomalous upturn of the mean-square relative displacement for the nearest neighbor distances (Fe-As) in LaFeAsO1-xFx (x 0.07), implying a local lattice distortion that develops below 70 K and disappears at the superconducting critical temperature. Comparing the results with the lattice response in cuprate superconductors, intimate correlation between local lattice mode and superconductivity is revealed. The results suggest that strong electron-lattice interaction is present as a common ingredient in the microscopic mechanism of s uperconducting transition.
CuCr2S4 shows a p-type metallic conductivity and ferromagnetism with Curie temperatures Tc of 375K, and exhibits magnetic anomaly at 84K (=Tm). The partial Ge substitution for Cu in CuCr2S4 greatly ...reduces Tc. Moreover Ge-substitution gives metal-nonmetal transition at in Cu1-xGexCr2S4. These results suggest that Cr 3d band filling control can be expected to give functional magnetoelectric transport properties in CuCr2S4.
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