Abstract
We report NMR experiments on heavily electron-doped Fe-based superconductor in comparison with the results on the parent Fe-based compounds. The typical parent Fe-based compound LaFe(As
1-
x
...′
P
x
′
)O exhibits the re-emergent antiferromagnetic (AFM) order at
x
′ ~ 0.6 (AFM2) separated from the parent AFM order at
x
′ =0 (AFM1). Systematic
31
P-NMR study on Sr
4
Sc
2
O
6
Fe
2
(As
1−
x
P
x
)
2
(SrSc42622), which has local lattice parameters of iron-pnictogen (Fe
Pn
) layer similar to the series of LaFe(As
1−
x
′
/P
x
′
)O, also revealed that the presence of AFM1 order is universal for most of parent Fe-based compounds. In contrast, the static AFM2 order was absent in this series, however, the dynamical low-energy AFM spin fluctuations are enhanced at around
x
~ 0.8, indicating that the onset of the static AFM2 is quite sensitive to the local lattice parameters of Fe
Pn
layer. In order to elucidate the further universality and diversity, we have carried out
77
Se-NMR measurement on Li
x
(NH
3
)
y
Fe
2−
δ
Se
2
(
T
c
= 44 K) in heavily electron-doped regime. Although the spin fluctuations at low energies does not significantly develops upon cooling, the moderate spin fluctuations were extracted at high temperatures from comparison of the temperature (
T
) dependences of Knight shift and nuclear relaxation rate (1/
T
1
T
). We discuss the universality and diversity of the relationship between the
T
c
and the characteristics of the spin fluctuations in the Fe-based compounds from a microscopic point of the NMR measurements.
We report 75As NMR experiments in heavily electron-doped LaFePnO0.75H0.25 (Pn=As1−xSbx and As1−x′Px′) compounds with the maximum Tcmax(∼33.1K), and compare with the previous results in lightly ...electron-doped LaFePn(O,F) compounds. The Tc of this series can be sensitively controlled by the pnictogen height (hPn) through the substitution at Pn site, and the electron doping level through the substitution at the O site with H or F. In heavily electron-doped LaFePnO(La1111) compounds, we found that spin fluctuations at low-energies were moderately suppressed upon cooling in the Sb-substituted high Tc compounds with high hPn (x ≥ 0), although they are completely suppressed in P-substituted non-superconducting compounds (x′ ≥ 0.2) with lower hPn. This feature is largely different from that in the lightly doped La1111 compounds with the well-nested Fermi surfaces, where the spin fluctuations are critically enhanced upon cooling. Here, we present the characteristics of spin fluctuations over wide doping region of La1111-based compounds, and discuss the relationship with the superconductivity.
We report a site selective Cu-NMR study on underdoped Hg-based five-layered high-Tc cuprate HgBa2Ca4CU5O(12+delta) with a Tc = 72 K. Antiferromagnetism (AFM) has been found to take place at T(N) = ...290 K, exhibiting a large antiferromagnetic moment of 0.67-0.69 microB at three inner planes (IP). This value is comparable to the values reported for nondoped cuprates, suggesting that the IP may be in a nearly nondoped regime. Most surprisingly, the AFM order is also detected with M(AFM)(OP) = 0.1 microB even at two outer planes (OP) that are responsible for the onset of superconductivity (SC). The high-Tc SC at Tc = 72 K can uniformly coexist on a microscopic level with the AFM at OP's. This is the first microscopic evidence for the uniform mixed phase of AFM and SC on a single CuO2 plane in a simple environment without any vortex lattice and/or stripe order.
We have investigated a gap structure in the spin-triplet superconductor Sr2RuO4 through the measurement of the 101Ru nuclear spin-lattice relaxation rate (101)(1/T1) down to 0.09 K at zero magnetic ...field. In the superconducting state, 1/T1 in a high-quality sample with T(c) approximately 1.5 K exhibits a sharp decrease without the coherence peak, followed by a T3 behavior down to 0.15 K. This result is in marked contrast to the behavior observed below approximately 0.4 K in samples with lower T(c), where T1T is a constant. This behavior is demonstrated to be not intrinsic. We conclude that the gap structure in Sr2RuO4 is significantly anisotropic, consistent with line-node-like models.
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