75As nuclear magnetic resonance study of antiferromagnetic fluctuations in the normal state of LiFeAs
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- These conflicting results raise an important question about the significance of AFM fluctuations and the placement of LiFeAs in the general Fe-As superconductor phase diagram.
- This is in line with findings in the “122” compounds22 but in striking contrast to studies of the “1111” compounds, where the calculated and measured Q’s agree well for the optimized As z position.
- Color online a Experimental solid red line and calculated dotted black line 7Li NMR spectra at 300 K and magnetic field 4.7 T ref LiCl =77.7247 MHz of LiFeAs polycrystalline sample.
- The authors begin with the determination of the spin part of the 75As NMR Knight shift from the temperature dependence of the high-frequency singularity of the 75As central transition Fig. 1 c .
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Frequently Asked Questions (14)
Q2. How is the Korringa factor reduced in cuprates?
For instance, in cuprates,25 a prototypical example of a system where AFM fluctuations are important, is reduced by a factor of 15, compared to the noninteracting electron scenario with transferred hyperfine coupling.
Q3. What is the reason for the slowing down of AFM fluctuations?
Below 40 K the authors detect a slight enhancement in T1T −1 followed by a sharp decrease below Tc. However, since T1T −1 does not follow the PG-like behavior seen in Ks, the authors conclude that AFM fluctuations are present already above 40 K, which is the reason for almost temperature-independent T1T −1 above Tc. Enhancement and divergent behavior of T1T −1 due to the slowing down of AFM fluctuations has been reported for underdoped “122” superconductors.23
Q4. What is the correct reference for the on-site coupling?
In this case, the correct reference valid for the on-site coupling is 0=1 and the enhancement of T1−1 in LiFeAs is reduced to a factor of 10 5 speaking for weaker AFM fluctuations.
Q5. What is the effect of the Knight shift on the T11 measurements?
The presence of a PG in the uniform spin susceptibility measured by the 75As Knight shift is overshadowed by AFM fluctuations in the T1−1 measurements.
Q6. What is the symmetry of the line shape?
Over the entire temperature range the line shape remains characteristic for an axially symmetric EFG tensor, in accordance with the 75As site symmetry 4mm, indicating the absence of a structural phase transition, as encountered in the undoped “1111” and “122” members of the Fe-As superconductors family.
Q7. How much T11 is enhanced in LiFeAs?
the enhancement of T1−1 in LiFeAs for a factor as large as 40 20 at low temperatures suggests strong AFM fluctuations, as recently predicted by quantum chemical calculations.
Q8. What is the structure of the so-called “111” member of the Fe-As?
LiFeAs, the so-called “111” member of the Fe-As superconductors, has been reported13 to undergo a transition to the SC state at Tc=18 K without additional doping and apparent AFM ordering or accompanying structural phase transition.
Q9. What was the basis set for the semicore state?
As basis set Li /2s2p3d+3s3p , Fe 3s3p /4s4p3d+5s5p , and As 3s3p /4s4p3d+5s5p were chosen for semicore /valence+polarization states.
Q10. What is the Korringa factor for noninteracting spins?
For noninteracting spins, q , n has no strong singularities in the q space, and can be taken out of the summation integrals in Eq. 2 .
Q11. What is the orbital contribution of the 75As?
For the75As orbital contribution the authors assume Korb=0.15%, which leads to Ks T→0 =0 inset of Fig. 3 a in accordance with the spinsinglet Cooper pairing.
Q12. How did you extract the Q of 7Li?
From the 7Li NMR linewidth 90 kHz, the authors conclude that 7Li has a very small Q. In order to extract7Li Q the authors performed an echodecay measurement.
Q13. What is the quadrupole amplitude of the 7Li NMR line?
The 7Li echo amplitude clearly shows characteristic quadrupole oscillations as a function of interpulse delay in the two-pulse − − − −echo experiment Fig. 2 b .21 Oscillations with the period tQ=59 s yield Q=2 / tQ 34 kHz.
Q14. What is the effect of the aFM on LiFeAs?
Regardless of this uncertainty, the analysis above demonstrates the enhancement of T1−1 at low temperatures with respect to noninteracting electron limits in both scenaria considered above, and demonstrates the strength of AFM fluctuations in LiFeAs.