Hideo has started by an outline which divided the talk on two parts:
I) Exploration of the new SC : basically 122 systems without Fe [ATM_2P_2 and TM being Rh, Pd, Ir] and
II) quasiparticle interference in the superconducting FeSeTe using STM/STS spectroscopy: evidence for the s+- symmetry of the order parameter
Then he moves to the part I. The first materials to look are BaRh2P2 and CaPd2P2 and BaIr2P2 and SrIr2As2 (remark by Canfield that these are most expensive chemicals!), all of them show low-Tc superconductivity. Piers: How do you know that these systems are unrelated to Fe-based compounds, Answer: there is an indication from LDA: the Fermi surface (FS) [he then has shown for comparison the FSs of BaIr2P2 and BaFe2As2 - indeed they are different]
Next he continues with the crystal structure of Ca(La,Sr)Pt_3P, newly synthesized systems. These are either I4mm (polar) or P4/nmm (non-polar) crystal structures. Tc is about 1.5 K, 6.6, and 8.5 K, respectively.
Looking at the transport (resistivity) and thermodynamics (specific heat) the estimated Sommerfeld coeff. gamma \approx 12.7 mJ/mol K^2, and the ratio of \Delta C /\gamma T_c \sim 2.5. The latter number may indicate strongly coupled SC (remarks: or disorder -Andrey, or multiband effect - Raphael).
Again new systems, different possible pnictide materias TMP and TMAs, where TM again stands for one of the 3d, 4d, or even 5d (W) elements. Examples are RuAs and RuP (all in MnP structure): 3D network of distorted MnP6 octahedra. Transport for RuP - Metal-Insulator transition at 250 origin is not clear but definitely not nesting as FS has no sign of it, RuAs - obvious anomaly at 270 K but what kind of the order is not clear. Both (RuAs and RuP) can be doped by Rh (instead of Ru). In RuRhAs series the anomaly ("the order") is suppressed at 25% Rh doping. No SC yet observed down to 1.5K. Here Takagi finished with an optimistic remark that there is still hope to find it at lower temperatures!
In RuP doped by Rh a suppression of the order occurs at 50% and no sign of SC as well is found. Cheong raised an issue of the critical doping, Kapitulnik noticed saturation at low T, some discussion on the chemistry aspects [size of the crystals and so on], blogger has not followed all the details.
Ok some theory consideration for the next set of systems: the idea is to look on the non-centrosymmetric crystals with 4d or 5d elements. In such a case (like in a well-known example CePt_3Si) there is a hope to find some exotic SC with a mixture of s- and p-wave symmetries. This is allowed due to lack of inversion symmetry. Overall Hideo reported 9 new superconductors with POSSIBLY non-centrosymmetric ATMSi_3 structure. Examples discussed are BaPdSi_3 and BaPtSi3 with an emphasis on the specific heat data. Unfortunately, looks very much like standard BCS superconductors. Another two examples mentioned are Rh2Ga9 (1.9K)and Ir2Ga9 (2.2 K).
NOW the second topic (II) - iron-based superconductor Fe(Se,Te) and QPI in the magnetic field. The idea starts from a earlier work in cuprates by Hanaguri et al. in 2009 and is based on the assumption that in a d-wave superconductor the constant energy maps of the conductance should be in direct correspondence to the LDOS of system at the same energies. Namely, the impurities, resposnible for the QPI, induce the scattering between the banana-like shapes of the LDOS which appear in a d-wave symmetry of the superconducting gap (He means here cos kx- cos ky function of the gap). Then the lobes of the banana will have a large density of states and the scattering between them dominates the QPI maps at all energies lower than Delta_0 [maximum of the gap]. Overall 9 wavevectors can be identified.
In addition, remember that these are Bogolyubov quasiparticles - the coherence factors originating from the u-v transformations are involved. As a result, QPI induced by magnetic and non-magnetic impurities looks quite different. Now if on top of this, the SC order parameter has some specific symmetry, the QPI pattern due to magnetic and also non-magnetic impurities will look quite unique which allows to identify the order by means of FT-STM. The most efficient way is also to apply the magnetic field which generates vortices which then act as magnetic scatters. By substracting the QPI maps made with and without magnetic field you can identify at which momenta the scattering is suppressed or enhanced which tells you about the symmetry of the sc gap.
In cuprates one finds two types of the scattering wave vectors either with suppressed or with enhanced intensity in the magnetic field. Looking at the constant energy maps of the LDOS for d-wave superconductor one observes the effect of coherence factors and then confirm the coskx-cosky function to be the symmetry of the SC gap in cuprates,
Now Takagi shows the result for the iron-based superconductors. He gives short introduction to s+- symmetry of the SC order and theoretical arguments based on the spin fluctuation theories why it may occur in iron-based superconductors. Basically as most of us know for the FS given in pnictides with pockets at Gamma and (pi,0) and (0,pi) points of the BZ the gap is constant at each of the pocket but with opposite phase, i.e. opposite signs at the pockets separated by the (0,pi) or (pi,0) momenta.
In the QPI one expects to see several wave vectors: (pi,0) and (0,pi) which change sign of the SC order parameter as well as diagonal scattering and intraband low-q scattering where no change of sign occurs. Now the experimental data comes: Fe(SeTe), Tc~14.5. We look at the conductance maps and the first thing one finds is that the sc gap is only ~2meV [Piers has asked about why it is particularly small, the answer blogger did not understand] which closes (from the coherence peak ) at around 11K. Another important remark is that there are some extra features (dip-hump) at around 4meV at both positive (+4meV) and negative (-4meV) voltages but with an asymmetric shape. Some speculations are made that these are most likely the many-body effects.
Then Takagi shows the conductance maps which should refer to the QPI. The wave vectors are clearly identified (first at zero magnetic field), they roughly can be associated with wavevectors of the scattering between the FSs in ferropnictides (here actually in FeSeTe). By applying the magnetic field one notices that at those q which have opposite sign for the s+- the intensity is indeed enhanced while at all others [diagonal and due to intraband scattering] it is suppressed. The results are consistent with the s+- wave symmetry.
Summary: magnetic field dependence of the QPI is consistent with s+- symmetry but not consistent with any other symmetries (d, p). issues for the future direct observation of impurity states in STM, antisymmetric shape of the dI/dV curves. Few minutes Takagi spent on he discussion that it is important to understand why s+- is not that sensitive to disorder, the blogger, however, thinks that this is not an issue, as in the s+- only the interband (large Q, thus small) scattering is bad for superconductivity while intraband (low q, thus strong) scattering which should be the largest does not produce a significant effect on superconductivity.
Zlatko: 1) as compared to cuprates the scattering wave vectors are commensurate in FPs
which would be indistinguishable with regard to the Bragg peaks. Do you have a different doping level
Reply: in 11 compounds there is a magic doping whether SC is bulk, so the answer is no
Blogger: did you observe vortices Reply: Now here we do not know where they are located as compared to cuprates, thus we did not know where we could see them
Andrey remarks on the interband versus intraband impurity scattering which blogger already mentioned above.
Lu was wondering whether the materials shown in the first part are indeed non-centrosymmetric, the answer was that despite the expectations the were not. The second Lu's question was about the origin of the transition in RuAs which Takagi in the reply attributed to the orbital order though without going into the details of it.
Piers and few others [blogger was disrupted by some discussion on the non-related topic which happened next to him, thus missed the names] were curious on what is the particular effect of the magnetic field in iron-based superconductors is [as there were no signs of vortices]. The overall opinion at the end was that even in the case there are no well defined signs of vortices the magnetic field still enhances the scattering induced by magnetic impurities, therefore the argumentations holds.
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