Friday, 10th August,
Hai Hu Wen (Nanjing University, China)
Materials and pairing mechanism in iron pnictides/chalcogenides:
what we have learned and have still to learn.
Blogged by Leni Bascones and Piers Coleman
Hai Hu will start presenting the FeAs and FeSe based materials and structures. He talks about the discovery of superconductivity at 26 K in doped LaOFeAs by Hosono's group. He calls attention to the large resisitivity of this material and the resistivity anomaly which was observed and that was later known to be an spin density wave transition. He notes that many of the parent materials had been synthesized some time before but were not studied in detailed. Quickly after the discovery of superconductity above 50 K was found in related materials of the same family, so called 1111. Later on superconductivity was discovered in other families, like 122 or 111, what also contained FeAs layers, and in 11 famlily with simple structure based in FeSe layers. Nowdays there are seven related families showing superconducting. Superconductivity is also found in Ni-compounds.
In most cases parent materials are non-superconducting. Superconductivity can be induced by electron or hole doping, by non-dopant substitution or by the application of pressure. Parent materials show an antiferromagnetic instability . It was early proposed that antiferromagnetism was due to Fermi nesting. Later on a different interpretation in terms of local moments was put forward. Magnetic order is called stripe or co-linear and show antiferromagnetic order along one direction and ferromagnetic along the other direction. Through the phase diagram, plotted as a function of pressure, the superconducting critical temperature shows a dome shape. A similar phase diagram is found when doping with electrons or holes. Antiferromagnetism disappears and superconductivity shows up.
Hai Hu compares the phase diagram of iron superconductors with that of cuprates. Cuprates are single band antiferromagnetic Mott insulators . with an exchange constant of order 150 meV while pnictides are multi-band metallic materials with an estimated second nearest neighbor exchange constant of order 50 meV.
He focuses on the transition towards antiferromagnetism and presents experimental results which show that at temperatures slightly larger than the Neel temperature there is also an structural transition. The gap in temperatures between the structural and Neel temperatures depends on the family. He says that there are two main proposals to explain the structural transitions one based in spin nematicity and the other one in orbital ordering. Andrey Chubukov points that so far there are no microscopic calculations to support the orbital ordering proposal. Structural details like As height or Fe-As.Fe angle seem to determine the optimal superconductivity. Theoretical input on why this could happen was given by Kuroki et al in 2009.
Hai-Hu wonders whether nematicity is important for superconductivy or not and whether it is just a consequence of magnetic or orbital fluctuations. He talks about transport measurements in detwinned samples by I. R. Fisher group which showed resistivity anisotropy with conductivity larger in the antiferromagnetic direction. Anisotropy was observed even above the structural transition and it has been discussed in terms of nematicity. Rafael Fernandes points out that because strain is applied to detwin the samples, above the structural transition one cannot talk about symmetry breaking. Other experiments showing anisotropic features where published by Davis's and L. Green's group. Recent experiments in Matsuda's group suggest that the nematicity starts at temperatures much higher than the structural transition.
Hai-Hu suggests to try all kinds all of possibilities to make new materials. In particular he suggests to try systems with FeB and FeGe layers. So far only Tc ~5K have been achived in these compoung but he hopes that Tc could become larger. To try new possibilities for materials lead to the observation of superconductivity in hole doped 1111 compounds, oxygen free 1111 materials and in more complex FeAs based materials. More recently superconductivity was observed in KFe2Se2 this compound is heavily electron doped. Some related compounds show Fe vacancies and are antiferromagnetic insulators. This family is called 245. Two different superconducting phases have been found in this family as a function of pressure. Q: Rafael Fernandes asks whether superconductivity in this materials is a bulk property . A:Hai Hu notes that the superconducting volume in not very large. Q: Andrey Chubukov asks whether there is phase separation A: yes it seems so. There are current efforts to avoid Fe vacancies.
He concludes the talk saying that there is still some space to be explored for making new superconductors.
Q: About Sr4V2O6Fe2As2 at high magnetic fields, whether a 2D vortex pancakes has been observed. A: Not directly.
Q: Why is iron special? A: Because of a possible balance between correlations and Drude weight.
Hai-Hu believes that many other superconductors without Cu or Fe will be found
Q: About the mechanism of superconductivity. A: Phonons seem not to mediate superconductivity, which seems related .to antiferromagnetic or orbital fluctuations.
Some other questions were asked but I could not hear too well the participants from my place.
Part II
Well, we're off to the races again! After a short break, Hai Hu is begining his second lecture. He mentions that in his opinion, the iron based superconductors are intermediate coupling in character -
lying between the two extremes of spin fluctuation theory and the RVB theory of superconductivity. He writes down the BCS formula, noting that over regions where the interaction is strongly repulsive, the gap likes to have different signs. Since the strongest interaction is betwen the electron and hole pockets, this lead to the proposal(by Igor Mazin and others), that the order parameter must have s+- symmetry, changing sign between the electron and hole pockets, but without a node crossing the Fermi surfaces. Proving that this is the nature of the pairing proves to be difficult, he says, and we don't have much direct evidence for the change of sign of the OP at this stage, though we have lots of evidence for a fully gapped Fermi surface.
Measurement by ARPES, shows that there are in fact two gaps, a feature of multiband superconductivity. (I was not sure which part has the largest gap - the electrons or the holes? )
Using Hall probes, it becomes possible to measure the superfluid density. There is an interesting temperature dependence, which can be fit to a two-component form, in which rho_a and rho_b both have BCS temperature dependence, with gaps
Deltaa= 1.6 meV, Delta_b = 9.2 meV
rho = x rho_a + (1-x) rho_b
with a fit value of x =
Now the Hall constant is also consistent with multi-band physics. The strong temperature dependnece of the Hall constant is said to be a consequence of the multi-band character. This is contraversial, and some interpret this as a strong correlation effect, as a result of temperature dependent Z. I prefer to keep the interpretation simple, he says.
Now Hai Hu turns to NMR measurements. He notes the relationship
1/T_1T ~ Sum A(q)^2 chi''(q,omega)/omega at \omega
at omega = 0. The upturn in 1/(T1 T) is consistent with antiferromagnetic spin fluctuations.
Now RPA spin fluctuation theory is predicted to have a sharp peak below Tc - a resonant peak that is indeed observed in experiments. This is consistent with s+-. A second measurement is using Fourier transformed STM measurements, carried out by Haneguri et al. The magnetic field enhances sign-changing scattering, and this is what is observed in Fe(Se,Te). Unfortunately so far, we have been unable to reproduce this scattering.
Q: what about sc loop experiments with half flux by Tsui et al?
HHW: Unfortunately, this also has not been reproduced and I am very sceptical.
Now he turns to the debate on gap structure. There is a table -
ARPES - sees isotropic gap
SUPERFLUID density - most time shows powerlaw dependence, evidence of nodal gap.
NMR low T 1/ T1T shows power law, consistent with nodal gap or the s+- under impurity scattering
THERMODYNAMICS : also shows specific heat consistent with nodal gap
Now the spin fluctuation models the iron pnictides are found to give rise to gap anisotropy, but ARPES has not see it.
Q: Isn't this apples and oranges, doesn't a gap anisotropy exist in the 111
A: yes - (but blogger didn't understand the details of the answer...)
Linear dependence of the superfluid density (from penetration depth) is consistent with line-nodes in some materials, but with full gap on other materials. Now many samples have lambda ~ (T)^n, with n =2 . Not consistent with ARPES data.
Thermal conductivity in Ba(Fe(1-x)Ni_xAs)_2, for current along the c-axis, there is evidence for a node in this direction. Now in Ba(FeAs1-xPx)_2, there is evidence from four fold oscllation in the thermal conductivity, that there may be nodal lines in two directions (see figure below). Yet Zhang et al on the same material have data consistent with a different gap assignment.
Hai Hu also sees a four-fold oscillation of the C/T in a field in Fe-Se. (see Nature Comm, 1, 157 (2010). This has been phenomenologically fitted by Vorontosv and Vekhter, PRL 105, 187003 (2010) and Chubukov and Eremin (reference not gotten).
Continuing the discussion about gap anisotropy, in the sf theory, intrapocket scattering will enhance gap symmetry. This remains a very contraversial issue and many groups are working hard on this topic.
Quantum Critical Point: Much circumstantial evidence for a hidden quantum critical point.
(1) Entropy S(T) ~ T ln T
(2) Resistivity rho(T)= rho_0 + AT^n, where n is found to go down to n=1 at x = 0.4, but rises towards n=2 either side.
(3) Penetration depth seen to have a divergence.
How strong is the pairing: do we have a pairing group? In the K-doped BaFe2As2, the jump in C/T is about 100mJ/mol/K^2. This is consistent with delta C/C_n ~ 2, like strong coupling. Canfield gorup has found C/Tc ~ T_c^2, which has been interpreted by different groups in different ways.
(impurities + s+- one group, quantum criticality another group).
STM -
Hai Hu talks about STM in BaK Fe2As2. His group has seen a dip-hump structure in the STM measurements. Giving an Eliashberg style interpretation, they can read-off
Delta = 7.5meV
Omega = 14meV
Delt + Omega = 21.5 meV
Hai Hu compares with the classic electron-phonon example, Rowell et al, and others used d^2I/dV^2 to read off the phonon spectrum. This is the classic example from lead. HHW compares the two -
On the left hand side, dI/dV, x=1 corresponds to omega = gap + resonant frequency. Now shows results for the irons.
Now compares with Eliashberg simulation, puts in a strong mode at 14.5meV, for inter-pocket scattering. Comparing the blue simulation with the red expt, the two look qualitatively similar.
Similar calculations for cuprates,
To convince you that the 14meV mode is not phonons, we have done this on more than one material - now on NaFeCoAs- still see a wiggle feature similar to the 122. Gap energy 5meV, mode energy 7-8 meV, consistent with the neutron resonance feature. Putting them all together, neutron resonance energy versus STM wiggle data, find that
mode energy/ Tc ~ 4.5
Emerging Challenges
(1) Why does superconductivity survive with enormous impurity centers. ON site doping induces stron impurity scattering, but superconductivity survives. 25% doping still gives superconductivity.
This is not consistent with an Abrikosov-Gorkov theory of splusminus. A glaring inconsistency!
Remark: Pressure effect and doping effect give comparable Tcs, even though one adds disorder, one does.
Q: What happens if you dope zinc onto the Fe site.
HHW: No effect, but maybe on overdoped side.
Further data - Co doped Ba122 has an enormous gamma, but Tc is not effected.
Further data, Cu and Mn doping on the Fe site, exactly smae Tc reduction.
Rafael - one of the difficulties is that we don't know how to model the detailed impurity potentials.
HHW - still very hard to understand - one should still get large momentum transfer.
Further data - STM shows that Co doping does not affect the DOS very much. One would expect an ingap state forming from cobalt, but not seen. Small scattering potential or small momentum transfer scattering? (The blogger is very sceptical about all of this - it all sounds like adding epicycles!)
(2) KyFe2-ySe2
arXiv1012.5164
Absence of hole pocket - yet still pairing. The absence of the pocket is different to DFT, and it is not seen in ARPES. My group found that these samples are not homogenious, a dip in the field dependent magnetization. Signs of phase separation. Big debate in china
(1) I think the sample is inhomgenious. Second phase K2Fe4Se5
(2) Many disagree.
But I think now we know that the sample is in homogenious.
Just recently STM data - white area is 245, only the grey area is the superconducting phase - islands? Perhaps consistent with small sc fraction.
But with this sample, K0.8Fe1.6 Se2 - Keimers group sees a resonance - which they suggest corresponds to coupling between 0,pi and pi,0 electron pockets. This experiment claims to reconcile the basic picture based on the AF SF mediated pairing.
Q: Why is the sc volume so small still see this feature in the neutron scattering. ?
(3) Drude Weight. Slide from Basov. If you want high Tc, need strong correlation, but mobile carriers.
Concluding remarks.
s+- model - support from many experiments. Multiband widely observed.
(2) In most vases, the gap is nodeless. While gap anisotropy should ne an event with high probbablity, whcih so far disagrees with ARPES>
(3) The weak coupling picture with the Splus minus can account for some results, but it is challenged by some recent expts - the pair breaking effect induced by nm disorder is weak and the missing of hole pockets in KyFe2-ySe2.
(4) QCP, electronic nematicity, orbital physics have been observed in some systems. It needs more efforts to resolve how they relate to superconductivity.
Q: Is there any evidence for superconducting fluctuations.
HHW: They are weak. No Nernst effect like in the cuprates.
Q: Given you have multi-band, you can have diversity of band structure, and then you probably don't need a node to fit the data.
HHW: At low T, the small gap will still dominate, but we don't go to low enough temperature.
Materials and pairing mechanism in iron pnictides/chalcogenides:
what we have learned and have still to learn.
Blogged by Leni Bascones and Piers Coleman
Hai Hu will start presenting the FeAs and FeSe based materials and structures. He talks about the discovery of superconductivity at 26 K in doped LaOFeAs by Hosono's group. He calls attention to the large resisitivity of this material and the resistivity anomaly which was observed and that was later known to be an spin density wave transition. He notes that many of the parent materials had been synthesized some time before but were not studied in detailed. Quickly after the discovery of superconductity above 50 K was found in related materials of the same family, so called 1111. Later on superconductivity was discovered in other families, like 122 or 111, what also contained FeAs layers, and in 11 famlily with simple structure based in FeSe layers. Nowdays there are seven related families showing superconducting. Superconductivity is also found in Ni-compounds.
In most cases parent materials are non-superconducting. Superconductivity can be induced by electron or hole doping, by non-dopant substitution or by the application of pressure. Parent materials show an antiferromagnetic instability . It was early proposed that antiferromagnetism was due to Fermi nesting. Later on a different interpretation in terms of local moments was put forward. Magnetic order is called stripe or co-linear and show antiferromagnetic order along one direction and ferromagnetic along the other direction. Through the phase diagram, plotted as a function of pressure, the superconducting critical temperature shows a dome shape. A similar phase diagram is found when doping with electrons or holes. Antiferromagnetism disappears and superconductivity shows up.
Hai Hu compares the phase diagram of iron superconductors with that of cuprates. Cuprates are single band antiferromagnetic Mott insulators . with an exchange constant of order 150 meV while pnictides are multi-band metallic materials with an estimated second nearest neighbor exchange constant of order 50 meV.
He focuses on the transition towards antiferromagnetism and presents experimental results which show that at temperatures slightly larger than the Neel temperature there is also an structural transition. The gap in temperatures between the structural and Neel temperatures depends on the family. He says that there are two main proposals to explain the structural transitions one based in spin nematicity and the other one in orbital ordering. Andrey Chubukov points that so far there are no microscopic calculations to support the orbital ordering proposal. Structural details like As height or Fe-As.Fe angle seem to determine the optimal superconductivity. Theoretical input on why this could happen was given by Kuroki et al in 2009.
Hai-Hu wonders whether nematicity is important for superconductivy or not and whether it is just a consequence of magnetic or orbital fluctuations. He talks about transport measurements in detwinned samples by I. R. Fisher group which showed resistivity anisotropy with conductivity larger in the antiferromagnetic direction. Anisotropy was observed even above the structural transition and it has been discussed in terms of nematicity. Rafael Fernandes points out that because strain is applied to detwin the samples, above the structural transition one cannot talk about symmetry breaking. Other experiments showing anisotropic features where published by Davis's and L. Green's group. Recent experiments in Matsuda's group suggest that the nematicity starts at temperatures much higher than the structural transition.
Hai-Hu suggests to try all kinds all of possibilities to make new materials. In particular he suggests to try systems with FeB and FeGe layers. So far only Tc ~5K have been achived in these compoung but he hopes that Tc could become larger. To try new possibilities for materials lead to the observation of superconductivity in hole doped 1111 compounds, oxygen free 1111 materials and in more complex FeAs based materials. More recently superconductivity was observed in KFe2Se2 this compound is heavily electron doped. Some related compounds show Fe vacancies and are antiferromagnetic insulators. This family is called 245. Two different superconducting phases have been found in this family as a function of pressure. Q: Rafael Fernandes asks whether superconductivity in this materials is a bulk property . A:Hai Hu notes that the superconducting volume in not very large. Q: Andrey Chubukov asks whether there is phase separation A: yes it seems so. There are current efforts to avoid Fe vacancies.
He concludes the talk saying that there is still some space to be explored for making new superconductors.
Q: About Sr4V2O6Fe2As2 at high magnetic fields, whether a 2D vortex pancakes has been observed. A: Not directly.
Q: Why is iron special? A: Because of a possible balance between correlations and Drude weight.
Hai-Hu believes that many other superconductors without Cu or Fe will be found
Q: About the mechanism of superconductivity. A: Phonons seem not to mediate superconductivity, which seems related .to antiferromagnetic or orbital fluctuations.
Some other questions were asked but I could not hear too well the participants from my place.
Part II
Well, we're off to the races again! After a short break, Hai Hu is begining his second lecture. He mentions that in his opinion, the iron based superconductors are intermediate coupling in character -
lying between the two extremes of spin fluctuation theory and the RVB theory of superconductivity. He writes down the BCS formula, noting that over regions where the interaction is strongly repulsive, the gap likes to have different signs. Since the strongest interaction is betwen the electron and hole pockets, this lead to the proposal(by Igor Mazin and others), that the order parameter must have s+- symmetry, changing sign between the electron and hole pockets, but without a node crossing the Fermi surfaces. Proving that this is the nature of the pairing proves to be difficult, he says, and we don't have much direct evidence for the change of sign of the OP at this stage, though we have lots of evidence for a fully gapped Fermi surface.
Measurement by ARPES, shows that there are in fact two gaps, a feature of multiband superconductivity. (I was not sure which part has the largest gap - the electrons or the holes? )
Using Hall probes, it becomes possible to measure the superfluid density. There is an interesting temperature dependence, which can be fit to a two-component form, in which rho_a and rho_b both have BCS temperature dependence, with gaps
Deltaa= 1.6 meV, Delta_b = 9.2 meV
rho = x rho_a + (1-x) rho_b
with a fit value of x =
Now the Hall constant is also consistent with multi-band physics. The strong temperature dependnece of the Hall constant is said to be a consequence of the multi-band character. This is contraversial, and some interpret this as a strong correlation effect, as a result of temperature dependent Z. I prefer to keep the interpretation simple, he says.
Now Hai Hu turns to NMR measurements. He notes the relationship
1/T_1T ~ Sum A(q)^2 chi''(q,omega)/omega at \omega
at omega = 0. The upturn in 1/(T1 T) is consistent with antiferromagnetic spin fluctuations.
Now RPA spin fluctuation theory is predicted to have a sharp peak below Tc - a resonant peak that is indeed observed in experiments. This is consistent with s+-. A second measurement is using Fourier transformed STM measurements, carried out by Haneguri et al. The magnetic field enhances sign-changing scattering, and this is what is observed in Fe(Se,Te). Unfortunately so far, we have been unable to reproduce this scattering.
Q: what about sc loop experiments with half flux by Tsui et al?
HHW: Unfortunately, this also has not been reproduced and I am very sceptical.
Now he turns to the debate on gap structure. There is a table -
ARPES - sees isotropic gap
SUPERFLUID density - most time shows powerlaw dependence, evidence of nodal gap.
NMR low T 1/ T1T shows power law, consistent with nodal gap or the s+- under impurity scattering
THERMODYNAMICS : also shows specific heat consistent with nodal gap
Now the spin fluctuation models the iron pnictides are found to give rise to gap anisotropy, but ARPES has not see it.
Q: Isn't this apples and oranges, doesn't a gap anisotropy exist in the 111
A: yes - (but blogger didn't understand the details of the answer...)
Linear dependence of the superfluid density (from penetration depth) is consistent with line-nodes in some materials, but with full gap on other materials. Now many samples have lambda ~ (T)^n, with n =2 . Not consistent with ARPES data.
Thermal conductivity in Ba(Fe(1-x)Ni_xAs)_2, for current along the c-axis, there is evidence for a node in this direction. Now in Ba(FeAs1-xPx)_2, there is evidence from four fold oscllation in the thermal conductivity, that there may be nodal lines in two directions (see figure below). Yet Zhang et al on the same material have data consistent with a different gap assignment.
Hai Hu also sees a four-fold oscillation of the C/T in a field in Fe-Se. (see Nature Comm, 1, 157 (2010). This has been phenomenologically fitted by Vorontosv and Vekhter, PRL 105, 187003 (2010) and Chubukov and Eremin (reference not gotten).
Continuing the discussion about gap anisotropy, in the sf theory, intrapocket scattering will enhance gap symmetry. This remains a very contraversial issue and many groups are working hard on this topic.
Quantum Critical Point: Much circumstantial evidence for a hidden quantum critical point.
(1) Entropy S(T) ~ T ln T
(2) Resistivity rho(T)= rho_0 + AT^n, where n is found to go down to n=1 at x = 0.4, but rises towards n=2 either side.
(3) Penetration depth seen to have a divergence.
How strong is the pairing: do we have a pairing group? In the K-doped BaFe2As2, the jump in C/T is about 100mJ/mol/K^2. This is consistent with delta C/C_n ~ 2, like strong coupling. Canfield gorup has found C/Tc ~ T_c^2, which has been interpreted by different groups in different ways.
(impurities + s+- one group, quantum criticality another group).
STM -
Hai Hu talks about STM in BaK Fe2As2. His group has seen a dip-hump structure in the STM measurements. Giving an Eliashberg style interpretation, they can read-off
Delta = 7.5meV
Omega = 14meV
Delt + Omega = 21.5 meV
Hai Hu compares with the classic electron-phonon example, Rowell et al, and others used d^2I/dV^2 to read off the phonon spectrum. This is the classic example from lead. HHW compares the two -
On the left hand side, dI/dV, x=1 corresponds to omega = gap + resonant frequency. Now shows results for the irons.
Now compares with Eliashberg simulation, puts in a strong mode at 14.5meV, for inter-pocket scattering. Comparing the blue simulation with the red expt, the two look qualitatively similar.
Similar calculations for cuprates,
To convince you that the 14meV mode is not phonons, we have done this on more than one material - now on NaFeCoAs- still see a wiggle feature similar to the 122. Gap energy 5meV, mode energy 7-8 meV, consistent with the neutron resonance feature. Putting them all together, neutron resonance energy versus STM wiggle data, find that
mode energy/ Tc ~ 4.5
Emerging Challenges
(1) Why does superconductivity survive with enormous impurity centers. ON site doping induces stron impurity scattering, but superconductivity survives. 25% doping still gives superconductivity.
This is not consistent with an Abrikosov-Gorkov theory of splusminus. A glaring inconsistency!
Remark: Pressure effect and doping effect give comparable Tcs, even though one adds disorder, one does.
Q: What happens if you dope zinc onto the Fe site.
HHW: No effect, but maybe on overdoped side.
Further data - Co doped Ba122 has an enormous gamma, but Tc is not effected.
Further data, Cu and Mn doping on the Fe site, exactly smae Tc reduction.
Rafael - one of the difficulties is that we don't know how to model the detailed impurity potentials.
HHW - still very hard to understand - one should still get large momentum transfer.
Further data - STM shows that Co doping does not affect the DOS very much. One would expect an ingap state forming from cobalt, but not seen. Small scattering potential or small momentum transfer scattering? (The blogger is very sceptical about all of this - it all sounds like adding epicycles!)
(2) KyFe2-ySe2
arXiv1012.5164
Absence of hole pocket - yet still pairing. The absence of the pocket is different to DFT, and it is not seen in ARPES. My group found that these samples are not homogenious, a dip in the field dependent magnetization. Signs of phase separation. Big debate in china
(1) I think the sample is inhomgenious. Second phase K2Fe4Se5
(2) Many disagree.
But I think now we know that the sample is in homogenious.
Just recently STM data - white area is 245, only the grey area is the superconducting phase - islands? Perhaps consistent with small sc fraction.
But with this sample, K0.8Fe1.6 Se2 - Keimers group sees a resonance - which they suggest corresponds to coupling between 0,pi and pi,0 electron pockets. This experiment claims to reconcile the basic picture based on the AF SF mediated pairing.
Q: Why is the sc volume so small still see this feature in the neutron scattering. ?
(3) Drude Weight. Slide from Basov. If you want high Tc, need strong correlation, but mobile carriers.
Concluding remarks.
s+- model - support from many experiments. Multiband widely observed.
(2) In most vases, the gap is nodeless. While gap anisotropy should ne an event with high probbablity, whcih so far disagrees with ARPES>
(3) The weak coupling picture with the Splus minus can account for some results, but it is challenged by some recent expts - the pair breaking effect induced by nm disorder is weak and the missing of hole pockets in KyFe2-ySe2.
(4) QCP, electronic nematicity, orbital physics have been observed in some systems. It needs more efforts to resolve how they relate to superconductivity.
Q: Is there any evidence for superconducting fluctuations.
HHW: They are weak. No Nernst effect like in the cuprates.
Q: Given you have multi-band, you can have diversity of band structure, and then you probably don't need a node to fit the data.
HHW: At low T, the small gap will still dominate, but we don't go to low enough temperature.
From the pnictides, can we learn anything about the cuprates? Or are these rather unrelated phenomena that coincidentally both show superconductivity up to high temperatures?
ReplyDelete(multiband vs. single band; d- vs. possibly s±-orbital pairing; probably different glue if any; coexistence with antiferromagnetism)
Can a relative sign of the gaps in two bands of pnictides significantly influence the coherence factors entering the expressions for ultrasound attenuation and nuclear spin relaxation rates? And can thus the relative sign be evaluated from such experimental data?
ReplyDelete