AUTHOR=Gupta Ritu , Maisuradze Alexander , Zhigadlo Nikolai D. , Luetkens Hubertus , Amato Alex , Khasanov Rustem 

TITLE=Self-Consistent Two-Gap Approach in Studying Multi-Band Superconductivity of NdFeAsO0.65F0.35

JOURNAL=Frontiers in Physics

VOLUME=8

YEAR=2020

URL=https://www.frontiersin.org/journals/physics/articles/10.3389/fphy.2020.00002

DOI=10.3389/fphy.2020.00002

ISSN=2296-424X

ABSTRACT=<p>High quality single crystals of NdFeAsO<sub>0.65</sub>F<sub>0.35</sub> (the superconducting transition temperature <italic>T</italic><sub>c</sub> ≃ 30.6 K) were studied in zero-field (ZF) and transverse-field (TF) muon-spin rotation/relaxation (μSR) experiments. An upturn in muon-spin depolarization rate at <italic>T</italic> ≲ 3 K was observed in ZF-μSR measurements and it was associated with the onset of ordering of Nd electronic moments. Measurements of the magnetic field penetration depth (λ) were performed in the TF geometry. By applying the external magnetic field <italic>B</italic><sub>ex</sub> parallel to the crystallographic <italic>c</italic>-axis (<italic>B</italic><sub>ex</sub>||<italic>c</italic>) and parallel to the <italic>ab</italic>-plane (<italic>B</italic><sub>ex</sub>||<italic>ab</italic>), the temperature dependencies of the in-plane component (<inline-formula><mml:math id="M1" xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:msubsup><mml:mrow><mml:mo>λ</mml:mo></mml:mrow><mml:mrow><mml:mi>a</mml:mi><mml:mi>b</mml:mi></mml:mrow><mml:mrow><mml:mo>-</mml:mo><mml:mn>2</mml:mn></mml:mrow></mml:msubsup></mml:math></inline-formula>) and the combination of the in-plane and the out of plane components (<inline-formula><mml:math id="M2" xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:msubsup><mml:mrow><mml:mo>λ</mml:mo></mml:mrow><mml:mrow><mml:mi>a</mml:mi><mml:mi>b</mml:mi><mml:mo>,</mml:mo><mml:mi>c</mml:mi></mml:mrow><mml:mrow><mml:mo>-</mml:mo><mml:mn>2</mml:mn></mml:mrow></mml:msubsup></mml:math></inline-formula>) of the superfluid density were determined, respectively. The out-of-plane superfluid density component (<inline-formula><mml:math id="M3" xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:msubsup><mml:mrow><mml:mo>λ</mml:mo></mml:mrow><mml:mrow><mml:mi>c</mml:mi></mml:mrow><mml:mrow><mml:mo>-</mml:mo><mml:mn>2</mml:mn></mml:mrow></mml:msubsup></mml:math></inline-formula>) was further obtained by combining the results of <italic>B</italic><sub>ex</sub>||<italic>c</italic> and <italic>B</italic><sub>ex</sub>||<italic>ab</italic> set of experiments. The temperature dependencies of <inline-formula><mml:math id="M4" xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:msubsup><mml:mrow><mml:mo>λ</mml:mo></mml:mrow><mml:mrow><mml:mi>a</mml:mi><mml:mi>b</mml:mi></mml:mrow><mml:mrow><mml:mo>-</mml:mo><mml:mn>2</mml:mn></mml:mrow></mml:msubsup></mml:math></inline-formula>, <inline-formula><mml:math id="M5" xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:msubsup><mml:mrow><mml:mo>λ</mml:mo></mml:mrow><mml:mrow><mml:mi>a</mml:mi><mml:mi>b</mml:mi><mml:mo>,</mml:mo><mml:mi>c</mml:mi></mml:mrow><mml:mrow><mml:mo>-</mml:mo><mml:mn>2</mml:mn></mml:mrow></mml:msubsup></mml:math></inline-formula>, and <inline-formula><mml:math id="M6" xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:msubsup><mml:mrow><mml:mo>λ</mml:mo></mml:mrow><mml:mrow><mml:mi>c</mml:mi></mml:mrow><mml:mrow><mml:mo>-</mml:mo><mml:mn>2</mml:mn></mml:mrow></mml:msubsup></mml:math></inline-formula> were analyzed within the framework of a self-consistent two-gap model despite of using the traditional α-model. Interband coupling was taken into account, instead of assuming it to be zero as it stated in the α-model. A relatively small value of the interband coupling constant Λ<sub>12</sub> ≃ 0.01 was obtained, thus indicating that the energy bands in NdFeAsO<sub>0.65</sub>F<sub>0.35</sub> are only weakly coupled. In spite of their small magnitude, the coupling between the bands leads to the single value of the superconducting transition temperature <italic>T</italic><sub>c</sub>. The penetration depth anisotropy γ<sub>λ</sub> = λ<sub><italic>c</italic></sub>/λ<sub><italic>ab</italic></sub> was found to increase upon cooling, consistent with most of Fe-based superconductors, and their behavior is attributed to the multi-band nature of superconductivity in NdFeAsO<sub>0.65</sub>F<sub>0.35</sub>.</p>