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Multi-frequency ESR in EuFe2As2
Masami Ikeda,Masayuki Hagiwara,Tatsuya Kobayashi,Wataru Hirata,Shigeki Miyasaka,Setsuko Tajima 한국물리학회 2013 THE JOURNAL OF THE KOREAN PHYSICAL SOCIETY Vol.62 No.12
We have performed multi-frequency electron spin resonance (ESR) measurements on EuFe2As2,which is one of the parent compounds of Fe-based superconductors. At TSDW=190 K, this compoundshows a spin-density-wave (SDW) transition. At temperatures above TSDW, the relaxationof Eu2+ spins takes place via the coupling of conduction electrons, resulting in a Korringa-type increasein the linewidth. At temperatures below TSDW, however, this compound shows an anisotropictemperature-independent linewidth except at temperatures near the long-range ordering temperature(19 K) of Eu2+ spins, giving evidence for a gap opening at TSDW. This change of physicalproperties indicates a change in the exchange interaction between localized Eu2+ spins and conductionelectrons in the FeAs layers. We evaluate the exchange constant, which was not reported inprevious X-band ESR studies, and compare it with the value for another Eu metallic compound.
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Takayoshi Kouchi,Kyohei Yoshinaga,Tomoya Asano,Mitsuharu Yashima,Hidekazu Mukuda,Sotaro Nishioka,Tsuyoshi Kawashima,Shigeki Miyasaka,Akira Iyo 한국물리학회 2023 새물리 Vol.73 No.12
We performed isotope <SUP>121,123</SUP>Sb-NMR studies to investigate the role of nematic fluctuations for superconductivity in Sb-substituted iron-based compounds LaFe(As<SUB>1-x</SUB>Sb<SUB>x</SUB>)O<SUB>1-y</SUB>F<SUB>y</SUB>. The temperature(T) dependence of nuclear spin relaxation rate(1=T₁) of <SUP>121,123</SUP>Sb were measured in the parent compound LaFe(As<SUB>0.6</SUB>Sb<SUB>0.4</SUB>)O(F0Sb40) and the lightly electron doped LaFe(As<SUB>0.7</SUB>Sb<SUB>0.3</SUB>)O<SUB>0.9</SUB>F<SUB>0.1</SUB>(F10Sb30). In the F0Sb40, the <SUP>121,123</SUP>(1=T₁) were enhanced upon cooling toward structural transition temperature (T<SUB>s</SUB> ~ 135 K) and show a peak at T<SUB>s</SUB>. The isotopic ratio (<SUP>123</SUP>T₁<SUP>-1</SUP> /<SUP>121</SUP>T₁<SUP>-1</SUP> ) reveals that the electric quadrupole relaxation originated from the dynamics of electric field gradient at Sb site is enhanced toward Ts. It is due to the presence of critically enhanced nematic fluctuations of the stripe-type order of Fe-3d<SUB>xz</SUB> (or 3d<SUB>yz</SUB>) orbitals. In the F10Sb30(T<SUB>c</SUB> ~ 20 K), the nematic fluctuations were significantly suppressed, but it remains a small enhancement below 80 K while keeping the C₄ symmetry. These results suggested that the orbital degrees of freedom remain even in the tetragonal structure of the lightly electron doped superconducting phase (SC1) of La1111 compounds. Here, we discuss the contribution of nematic fluctuations in Fe-pnictide superconductivity by mean of the electric quadrupole moment of isotope <SUP>121,123</SUP>Sb nuclei.
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