Prof. Hong Ding (IOP) Talk:
Probing Iron based superconductivity by photoelectrons.
(Arpes studies)
Arpes introduction, energy resolution of the Beijing group 0.9 meV; angular resolution 0.2 degrees
Question to be answered: is there universality in Fermi Surface topology and superconducting gap despite different structures in 1111, 122, 11, 111, 122', 21113 compounds (in the parameters where we have superconductors). The Selenium compound 122' doesnt even contain As.
First lets look at the Ba122 phase diagram: one sees clear electron hole assymetry. One can dope in different ways, with electron (Co compounds) or holes (K compounds), large changes in the phase diagram.
Pnictides more homogeneous than cuprates, so ARPES should be better. Arpes resolves all of the five bands, two holes and two electrons around the gamma point. Upon changing kz, a third hole FS appears. One can also resolve the orbital character of the bands, electron has mixed xy. Fitting of LDA with data observes rough similarity, but with some important differences in terms of bandwidth, mass renormalization, etc. Changing layers, Se, influences the mass renormalization effect.
H. Ding's group looked at the FS evolution with doping. At optimal doping, there is good quasinesting of hole and electron hoping. Upon eavily overdoped hole doping, we lose the electron FS, but additional FS emerge at the M point, which is connected to the hole bands at gamma thru the bandwidth.
In electron doping, the hole FS dissapears much fasted, explaining the much shorter superconducting dome in the electron doped side.
Several plots of the superconducting gap are shown, with temperature dependence. Gap of 12 meV, 2 Delta/Tc=7. The observation of a shoulder due to impurity scattering (probably) is shown at low energy. It has sample dependence, maybe hence due to impurity. Same shoulder as in laser arpes.
Plot that showes nodeless SC gap in BaKFeAs (37K), which close at Tc. Gap is smaller on the larger hole FS. Heuristically, one can look at a coskx cosky gap which can come from strong coupling (but not only! - for example, FRG and RG studies can also obtain such a term), and see how it fits quantitatively (it does fit qualitatively, at least in the BaKFeAs) with the data. Comparison with weak coupling is made, pointed out that Spm from weak coupling scenarion. Resonant mode supports the gap with a wavevector 0,pi. A fermi surface kink similar to the one seen in neutron scattering is also observed in arpes.
H. Ding then shows data in the operdoped hole side, shows clear observation of the superconductor. Superconducting gaps are isotropic around the FS (this is a feature that seems to be true in arpes for the pnictides).
A psudogap is reported above Tc, shadowing the AFM region.
Ratio of the gap/Tc remains roughly constant through the hole doping region.
We then move to electron doped, similar gaps. kz dependence of the gap is presented, and fitted to a s_pm (in x,y) + x (cos kx + cos ky)coskz form, which when the magnitudude x becomes large, is gapless. the value fo x is determined to be roughly 1/6.
We now move to the electron doped Co samples, which show sharp coherence peaks, with 2 Delta/Tc= 8. Along FS isotropic gaps. Data on 11 compounds FeTeSe is now shown (Tc=13k) - for this case, there is a surpsise: gap cannot be fitted into a cos kx cos ky form obtained from J1 J2 (but again, this could be theoratically obtains ohersiwe in spin fluctuation too) . One has to include J3 in the spin model to obtain good agreement to data.
The data on A Fe Se is showed. At 20% electron doping, the hole FS is absent. The gap is isotropic around the FS. What is the symmetry (weak coupling would give Tc=0 as there is no hole pocket). Several theoretical scenarios are presented, coming from quasinodelss d, or nodeless pm. At kz=pi, Ding finds a small electron surface which can help identify the gap. On this pocket the gap is smaller, 6.5 ef. The conclusion is that there is a FS in the z=direction at the Z point, which helps differentiate the symmetries. A d-wave would give nodes on this FS (but not on the slectron), but Ding sees nodeless. Claim: nodeless (on the electron FS) gap ruled out
LiFeAs data consistent to the coskx cosky term, nice fit.
The case for 3 classes of high Tc. J1 NN spin spin coupling is the cuprates. J2 NNN is pnictides. J2+J3 is FeTe. The possible magnetic phase is different for all 3. The case is made for local AFM exchange pairing of Iron-based superconductors.
Q: From Heavily doped to underdoped, for optimal doped, what is the evidence for electron pockets ? What is the difference between system with different electron pockets bottoms?
A: LiFeAs, theres a difference in the bottoms
Q: Analysis of gap in FeTeSe using J2 J3. Neutron scattering peaks are 0,pi; pi,0
A. J2 J3 can explain the superconducting data. J2 J3 can give magnetic at pi/2 pi/2, superconducting neutron resonance at 0,pi
Q: Tunneling spectrum can give you smth like a gap when actually the gap is very small.
A: impurity broadening would not shift the gap size.
Q: J1 J2 J3 construction, once the hole pocket becomes small you find Spm not stable.
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