Abstract:The interplay and coexistence of superconducting, magnetic and structural order parameters in NaFe1−xCoxAs has been studied using SQUID magnetometry, muon-spin rotation and synchrotron x-ray powder diffraction. Substituting Fe by Co weakens the ordered magnetic state through both a suppression of TN and a reduction in the size of the ordered moment. Upon further substitution of Fe by Co the high sensitivity of the muon as a local magnetic probe reveals a magnetically disordered phase, in which the size of the … Show more
“…When compared to the phase diagram previously known in NaFe 1− x A x As ( A = Co, Cu, or Rh) 23, 24, 39, 40 , a difference is the seeming mutual repulsion of the SDW and bulk superconductivity near x c ~ 0.03, similar to that reported in LaFeAsO 1− x F x 41 and NaFe 1− x Co x As 42 . However, our data are not sufficiently dense to conclude whether the SDW and SC phases coexist in the narrow doping range near x = 0.03 or completely repel each other.Fig.…”
Strong interplay of spin and charge/orbital degrees of freedom is the fundamental characteristic of the iron-based superconductors (FeSCs), which leads to the emergence of a nematic state as a rule in the vicinity of the antiferromagnetic state. Despite intense debate for many years, however, whether nematicity is driven by spin or orbital fluctuations remains unsettled. Here, by use of transport, magnetization, and 75As nuclear magnetic resonance (NMR) measurements, we show a striking transformation of the relationship between nematicity and spin fluctuations (SFs) in Na1−xLixFeAs; For x ≤ 0.02, the nematic transition promotes SFs. In contrast, for x ≥ 0.03, the system undergoes a non-magnetic phase transition at a temperature T0 into a distinct nematic state that suppresses SFs. Such a drastic change of the spin fluctuation spectrum associated with nematicity by small doping is highly unusual, and provides insights into the origin and nature of nematicity in FeSCs.
“…When compared to the phase diagram previously known in NaFe 1− x A x As ( A = Co, Cu, or Rh) 23, 24, 39, 40 , a difference is the seeming mutual repulsion of the SDW and bulk superconductivity near x c ~ 0.03, similar to that reported in LaFeAsO 1− x F x 41 and NaFe 1− x Co x As 42 . However, our data are not sufficiently dense to conclude whether the SDW and SC phases coexist in the narrow doping range near x = 0.03 or completely repel each other.Fig.…”
Strong interplay of spin and charge/orbital degrees of freedom is the fundamental characteristic of the iron-based superconductors (FeSCs), which leads to the emergence of a nematic state as a rule in the vicinity of the antiferromagnetic state. Despite intense debate for many years, however, whether nematicity is driven by spin or orbital fluctuations remains unsettled. Here, by use of transport, magnetization, and 75As nuclear magnetic resonance (NMR) measurements, we show a striking transformation of the relationship between nematicity and spin fluctuations (SFs) in Na1−xLixFeAs; For x ≤ 0.02, the nematic transition promotes SFs. In contrast, for x ≥ 0.03, the system undergoes a non-magnetic phase transition at a temperature T0 into a distinct nematic state that suppresses SFs. Such a drastic change of the spin fluctuation spectrum associated with nematicity by small doping is highly unusual, and provides insights into the origin and nature of nematicity in FeSCs.
“…The structural transition temperature T s and magnetic transition temperature T N determined from the resistivity measurement are similar to those reported in the other literature 21,25 . The T c is obtained from the specific heat measurement and it is set to zero for those that exhibit no superconducting jump despite the fact that the resistivity goes to zero in some samples 27 . Therefore, the superconducting dome plot in Fig.…”
Section: The Eliashberg Theory and Beyondmentioning
We study the normal-state and superconducting properties of NaFe1−xCoxAs system by specific heat measurements. Both the normal-state Sommerfeld coefficient and superconducting condensation energy are strongly suppressed in the underdoped and heavily overdoped samples. The low-temperature electronic specific heat can be well fitted by either an one-gap or a two-gap BCStype function for all the superconducting samples. The ratio γN T 2 c /H 2 c (0) can nicely associate the neutron spin resonance as the bosons in the standard Eliashberg model. However, the value of ∆C/TcγN near optimal doping is larger than the maximum value the model can obtain. Our results suggest that the high-Tc superconductivity in the Fe-based superconductors may be understood within the framework of boson-exchange mechanism but significant modification may be needed to account for the finite-temperature properties.
“…In stoichiometric NaFeAs, the superconducting phase coexists with an antiferromagnetic (AFM) phase which is stabilized by the delocalized Fe d-band electrons [9,10]. This AFM phase is also preceded by a structural transition from a tetragonal to orthorhombic structure [9], similar to "1111" and "122" systems [11].…”
The upper critical field of NaFe 1−x Co x As was measured from x = 0 to x = 0.08, with the magnetic field applied parallel (H c2 ) and normal (H ⊥ c2 ) to the planes. The data were fitted using one-band and two-band models. The orbital and paramagnetic components of the upper critical field were extracted for H c2 , which we find to be strongly dominated by paramagnetic pair breaking. In the parent compound the paramagnetic limit is equal to the value expected by BCS theory. However, substitution of Fe by Co leads to an enhancement above the BCS limit by a factor of approximately 1.6. In the overdoped region, we observe a significant convex curvature in H ⊥ c2 at low temperatures, which we attribute to the two-band nature of the superconductivity and the asymmetry between the two bands. Additionally, we discuss the behavior of critical field anisotropy, coherence length ξ , and the penetration depth λ.
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