P. Canfield (Ames): What we have learned from BaFeTMAs studies: empirical rule to tell theory

Lecture Notes

Paul starts his talk by a very short introduction to the discoveries of iron-based superconductors including the works by Kamihara et al. in 1111 compound, and Rotter et al. in 122 system.
To him the most important discovery in iron-based superconductors is that you can introduce superconductivity by disorder and not only by doping.

Next slides are sort of philosophical discussion where an important part of it is an argument that one has to think prior of doing measurements.

On the next slide Paul introduced the tool how the single crystalline samples were grown out of flux (slow cooling of a melt in a self flux - blogger cannot believe he is able to write these words). Easy and difficult examples, among them CeSb. Few more slides on the beauty of growing the ternary compounds, like RCu2Ge2 can be grown out of ternary in the same way. Similar growth techniques have been applied to the AFe2As2 families of compounds.

Now the top statement: "Within 14 hours of hearing about superconductivity in K doped BaFe2As2 compounds they had grown first single crystals out of a Sc-rich quaternary melt! Beautiful pictures of the single crystals on the millimeter paper.

A substitution of K for Ba or Sr in the 122 materials is difficult BUT they can be grown out of a FeAs melt. By shifting from one transition metal to another one can change the solvent. Using the elemental analysis one is able to determine the composition of each batch used.

Generally a lot of information can be place into and extracted from phase diagram. At the same time it is often difficult out of a single measurement techniques and the more experimental methods that provide consistent information is needed and the likely they are to be accurate.

Next Paul moves to the transport: Important point on the analysis of the resistivity, it can be used to learn about the superconducting state especially rho (H,T) in a field. One has , however, remember about the effect of the local moments. rho_0 increases with increased disorder. Resistivity also helps to identify multiple transitions, sensitive to the changes of the FS (identification of the density wave transition. - double hump)

Now next slides: Magnetic susceptibility: used for determining Curie-Weiss or antiferromagnetic transition , maximum in chi only gives rough estimate to it. It is better to take d\chi/dt which has often the sam temp dep. as the specific heat. BTW d\rho/dT shows similar temp dependence near T_N. But life can be complicated especially in case of multiple transitions. Example is given for the cascade of the transitions between 6 and 5 K in HoNi2B2C.

We are back to FeAs systems: combined structural and AFM transitions, nicely seen from susceptibility and resistivity data. Neutron and X-ray scattering data: clear separation between stuctural and the magnetic in BaFeCoAs systems. Remarkable, one also finds a competion of AF and SC in the coexistence region, even the re-entrant behavior. Theoretical explanation by Fernandes and Schmalian in favor of s+- superconductivity [PRB 81 (2010)]. Comment by Andrey follows to the understanding in the audience that the re-entrant bahavior we do not see on the slide, Paul has to update his slide.

Again "philosophical" break:

to experimentalists: for the phase diagram, carefully state what is your criterion?,
to theorists: many data have to be taken with a skepticism.

Bak to iron-based superconductors: Ni and Cu doping suppresses the upper transition similar to Co, whereas Ni stabilizes SC; Cu substitution does not show SC for T>2K. Next comes the phase diagrams BaFeNiAs and BaFeCuAs, the latter shows very weak SC. What is with Cu, is it a poison? Answer: No, adding Cu to the existing BaFeCoAs composition does not affect SC actually. Instead crucial is to ave the right doping range! Examples: For Co doping, e =x, Ni doping e =2x and for Cu e=3x. This tells us that there is a doping (e) which supports superconductivity. Thus suppression of AF/ST helps to uncover SC and no scaling of the SC on the underdoped side of the phase diagram.

Now comes a comparison of Rh-doped and Co-doped samples: one again finds identical phase diagrams [change of lattice parameters is not sensitive for SC, Piers this is a remark goes directly to your post] . Phase diagram shows that there is a nice scaling of SC with doping (roughly linear behavior)

How do we understand the scaling Tc vs e? answers come from TEP (thermoelectric power) and Hall coefficient measurements. Observe a dramatic change in TEP over the whole temp. range measured as X from 0.02 to 0.024, the low temp. Hall coefficient changes for the same x values. The same results for Cu except the change are at e~0.025. This is consistent with the idea that there is a change in the band structure or Fermi surface of Bafe2As2. And indeed you see this also in ARPES (refers to Kaminski's data), change of the size of the hole pocket [Gamma pocket disappears].

Similar phase diagrams with pressure: doping and pressure changes the structural transition in the same way and reveal the same dome of SC. Example: BaFeRuAs the changes in the unit cell dimensions and volume are remarkably similar for Ru and Rh. On the other hand the phase diagrams are very different. Now again Ru doping resemble to tuning with pressure The right change of the c-axis parameter may catch the salient physics.

Effect of pressure can increase Tc dramatically on the underdoped side but does little on the increase of Tc on the overdoped side. Main role of pressure is to get rid of structural transition and to reveal the dome of superconductivity.

Summary:

1) structural and AFM transitions are suppressed in a similar manner by many TM elements and scales roughly with x

2) there is a region of e which supports sc

3) the sc dome scales will with e doping on the overdoped thetragonal phase

4) the onset of the sc dome on the underdoped depends on how quickly the suppression of AF occurs.

Open questions


-Cu affects Tc 3 times faster than anything else. Cu3+ is difficult to imagine, what is this: band filling, scattering, ... ?

- the dramatic change in TEP with Co dopin into BAFeAs is remarkable, comparable to Yb-based materials, what is the difference between them?

Questuins:
blogger: what about the data which says that there is a magic correspondance of Tc and magic angle of the tetrahedra [see the post]. Reply: it seems that we do not find this relation and I doubt it is correct.

Chubukov asks about the interrelation of magnetism and Cu and whether the statement from the talk might be that superconductivity exists independent on magnetism it simply depends on the killing of the AF order. Reply: to be more precise it is more exactly to say that it reduces the size of distortion and/or ordered moment or change fluctuation spectrum.

Buechner slightly disagrees that disorder does nothing to SC [see F. Hammerath et al., PRB 81, 140504 (2010) ] Reply: indeed this is an important remark