Introduction shows the phase diagram for BaFe2As2 to KFe2As2, indicating that the SC gap at optimal doping is isotropic with S+/- state (Ding EPL 2008), KFe2As2 apparently has nodes (and is d-wave)
Where does the gap symmetry change, exactly?
(Canfield) Samples in phase diagram seem to have large compositional uncertainty – will this be true in your study too?
(Shin) We will use single crystals.
The review continues to show that some pnictides have isotropic gap, others not. There is a significant spread in the values of gap relative to Tc, ranging from 1-8 (depending on which FS is selected and which material) . Kuroki suggests that the Fe-pnotogen height is a possible control variable which tunes from S+/- (large pnictogen height, Ba.6K.4Fe2As2) to d-wave when it is small (KFe2As2). An alternative view is that phonon assisted electron scattering between orbitals can explain the S+/- gap symmetry.
Shin then introduced a new VUV Laser-driven ARPES system at the University of Tokyo, where the high energy resolution is achieved by using a high harmonic of 7 eV Nd:YVO4 laser . It is possible to do measurements at temperatures as low as 1.5 K, and with a resolution of~ 0.25 meV (not sacrificing a wide angular range). A novel feature is the possibility of polarization analysis (linear or circular). Resolution is dominated by the laser linewidth. Conventional synchrotron based photoemission currently is greater than 1 meV, and is limited to T>15 K. So the new technique may be transformative to a range of problems.
(Coleman) What about the momentum resolution?
(Shin) It is not improved relative to conventional PES.
(Canfield) What is lowest TC SC that you have measured?
(Shin) K122, but we are now measuring a substance with TC=3 K.
This technique is also the most bulk sensitive form of PES, since the electron energies are less than 100 eV, and the escape depths as large as 100 angstroms. Hard x-ray PES the escape depth is 10 angstroms.
7 eV laser limits the part of the Brillouin zone that can be accessed in the pnictides.
Measured 3 SC compositions (single crystals), study both temperature dependence and by varying polarization can zero in on different symmetry orbitals.
1. Twinned domains in the low T phase related to transition from tetragonal to orthorhombic structures. Topological change in FS in s-polarization: selection rules imply that the electrons are from dzx or dyz orbitals
(Canfield) What is spot size?
Shin: 100 microns
(Canfield) One domain or more?
Shin: Measurement averages over many twins.
ARPES results are in reasonable agreement with LDA calculation by Oguchi (cylindrical above TN, 3-dimensional below)
>> Orbital-selective electronic reconstruction across TN: FS has single component (dxz) below TN. Dyz component is gapped. AF involves orbital ordering.
Now presentation of SC 122 system BaFe2(As.4P.6)2 (optimally doped) Find that FS is quite different for p and s polarization, and each sheet has different polarization., in agreement with band calculations.
(Nevidomski) Is this the same pi,pi pocket that was calculated by Kuroki? Historically, the calculations only show two pockets. Could it result from folding at pi,pi point?
(Shin) The experiments are clear there are 3 Fermi surface pockets
(Chubukov) This will depend on the specific material.
Evidence is presented for a resonance mode at 12 meV (analog to spin resonance from neutrons?)and at 30 meV (a phonon?)
(Chubukov) Is there really a kink at 13 meV? Perhaps it is a smooth crossover?
(Shin) There is a departure from linearity at 13 meV.
The SC gap has previously been measured in ARPES measurements, and they can now measure it for the different parts of the FS using the polarization analysis. One band (hole band #1) has a 2-peak structure – small gap is 3 meV, 2D/kTC~2. Larger gap is found near M-points, and since it survives above the SC transition, it is not related to SC. With proper symmetrization of the data, it is argued that the gap has a BCS-like temperature dependence, vanishing at ~35 K. Other measurements have found a larger gap in this system.
Larger gap peak is possibly related to the 10 meV spin resonance.
(Takagi) Peak B (large gap ) narrows below Tsc – must have some relation to SC?
(Chubukov) Peak B is OK for BCS gap, small gap is too small.
(Shin) Peak B is not SC gap, since it only appears in the occupied states (Peak A is in both)
(Takagi) it will depend on the polarization as to which gap you will observe. In this polarization this is what you see.
FS of all 3 FS sheets is square, but with similar magnitudes (3 meV). No nodes in any, but smallest in pi,0 directions >> anisotropic s-wave?
(Tetsuo) How about the angle dependence of gap?
(Shim) Don’t know
(Chubukov) What is the largest gap?
(Shim) 3.2 meV along pi,pi
Now to 122 system BaFe2(As0.35P0.65)2 where there is more doping and a smaller pnictogen height. Hole FS is larger according to theory. While parent compound has a single sheeted FS, this system has 3 FS sheets.
SC gap is BCS_like (TC=28 K) and the gaps are 3 – 4 meV. Again, not d-wave, prefer anisotropic s-wave. Not much difference in gap structure or magnitude among the 3 FS sheets. All are pinched in along pi,0 direction. Some evidene of coupling between orbitals, so need a more complicated explanation than `simple spin fluctuations’’.
Dr. Shin stresses the important role of orbital order in the SC state. Gap/Tsc is always small compared to BCS ratio, and there is no evidence from him measurements for nodes.
He identifies the following open questions:
1.What is the 10 meV peak?? What exactly is the possible connection to the spin resonance mode found in neutron scattering measurements?
2.What would the spin fluctuation theory look like if orbital ordering were included?
3. Where are the SC nodes in KFe2As2 parent compound?
(Nevidomski) Why does ARPES never seems to observe gap on electron pocket? Must one go to zone boundary (inaccessible to ARPES)?
(Shin) Yes, our photon energy does not allow us to reach this part of the zone.
(Chubukov) Is orbital order seen both above and below Tsc?
(Shin) We don’t have data above Tsc.
(Chubukov) Laser ARPES from China claims that there are more FS sheets in SC state than normal state. What about in your measurements?
(Shin) Only see one sheet is observed.
(Keimer) Low energy kink may not be related to resonance from neutron scattering. In electron doped cuprate system the mode follows the gap, and has the BCS temperature dependence. Here it seems like the kink energy is temperature independent.
(Shin)There may be a weak temperature dependence.
(Nakatsuji) In P doped 122 some measurements indicate nodal structure but not yours.
(Shin) We cannot measure at the endpoints, so direct comparison is not possible.
(Coleman) Is gap node not d-wave but perhaps a different sort of symmetry?
(Chubukov) S+/- is most likely.
(Chubukov)Can you see gaps on both hole and electron FS? What are the relative gaps? Ding claims to see a gap on the electron FS - what about you?
( Buechner) We see a much bigger gap at gamma than this expt does. Perhaps this expt is more bulk sensitive.
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