Bernd Buechner (Dresden): Nanoscale inhomogeneities in underdoped pnictide superconductors

Bernd started his talk by introducing the iron pnictides and showing their crystal structures; during his talk he will mainly focus on the 1111 and 111 compounds. He states that these two different families have considerably distinct properties.

Phase diagram of La-1111

Bernd presents the phase diagram of La-1111, showing resistivity data. The spin-density wave state (SDW) is suppressed with doping, together with the orthorhombic phase. A superconducting dome (SC) develops for intermediary doping levels. Showing data of thermal expansion, magnetization, NMR and resistivity, Bernd explains that in these materials there is a strong link between electronic, structural and magnetic degrees of freedom.

Q (Paul Canfield): Are these polycrystals?
A: Yes.

Now he is presenting detailed temperature dependence of the thermal expansion for different doping levels (F doping on the O site). He also presents mu-SR measurements for different doping, comparing to neutron diffraction data. These data show a very sharp boundary between the SDW and the SC states, indicating a first-order transition, he says.

Comparing to the phase diagram of other 1111 compounds, Bernd poses the question: is the coexistence between SDW and SC intrinsic? He explores in details the phase diagram of the Ce-1111, which seems to present a quantum critical point where both SDW and SC transition lines meet at zero temperature.

Q (Piers Coleman): What happens to the Ce moments in the SC state?
A: Do not find evidence of magnetic order inside the SC phase.

Q (Paul Canfield): How do you evaluate the amount of F doping?
A: WDS measurements.

Bernd presents the phase diagram of electron and hole doped Ba-122 compounds, calling attention to the region of coexistence between SC and SDW and to the relationship between magnetic and structural phase transitions.

Nernst effect

Bernd explains that, in one-band metals, the Nernst signal is zero (Sondheimer cancellation), whereas in multi-band metals, it is expected to be very small. In superconductors, the Nernst signal can be large due to vortex flow, and further enhanced due to vortex fluctuation. Bernd recalls data of Nernst effect in the cuprates and their relation to the pseudogap phase.

Now Bernd is presenting Nernst data for the La-1111 pnictides. For undoped samples, they show a strong enhanced signal for T smaller than TN (Neel temperature). Additional thermopower and Hall measurements indicate the partial gapping of the Fermi surface in the SDW state. For optimally doped samples, the Nernst effect is still enhanced in the SC state, although no SDW order is present. For overdoped samples, the signal enhancement is very weak and practically disappears. Bernd relates these results to NMR data for intermediary doping samples, which indicate slowing down of spin fluctuations below 150K. He points out that this sample does not order magnetically, though.

Q (Zlatko Tesanovic): Is there a connection between the slowing down of spin fluctuations and a pseudogap state?
A: No direct evidence for the pseudogap.

Nuclear magnetic and quadrupole resonance

Bernd explains that NMR and NQR can be performed on As, due to its larger nuclear spin. NMR gives the spin susceptibility (static through Knight shift and dynamic through relaxation rate), while NQR gives the local charge distribution.

First, he shows Knight shift data on doped La-1111, from which he can extract the static susceptibility. Its main feature is that it grows with temperature, and this feature is present for various doping levels. He also finds a decrease of the relaxation rate in the normal state at high temperatures. Bernd explains that although the spin susceptibility is decreasing, the slow AFM spin fluctuations lead to the increase of the relaxation rate. He also points out that the non-constant relaxation rate indicates non-Fermi liquid behavior.

The temperature dependence of the relaxation rate in the SC state follows an unusual polynomial dependence, which suggests that disorder is important in order to be able to determine the SC gap symmetry.

Now, Bernd is presenting NQR data for undoped and optimally doped samples, which indicate one set of charge environment in each As site. However, the doping dependence is opposite to the one predicted by LDA. Bernd shows data on the underdoped region, which indicate the presence of two sets of charge environment, he says. The question he poses is: what is the length scale associated to these two coexisting charge environments? He presents more NMR relaxation rate data, which indicate that the coexistence of these electronic states is in the nanoscale, he says. Bernd discusses different possibilities for the nature of these two local coexisting orders, which could be due to charge and orbital order, for example.

Li-111 compounds

Bernd first shows ARPES data on these compounds, which do not indicate nesting features on the Fermi surface. He also points out the small size of one of the hole pockets at the gamma point (center of the Brillouin zone). He shows that, while LDA calculations predict nesting of the bands, the data do not show it. Bernd also points out that his group is now able to perform ARPES below 1K.

Q (Paul Canfield): asks if the compounds are stoichometric.
A: Yes.

Both ARPES and specific heat measurements presented by Bernd show evidence for two SC gaps in these 111 compounds, according to his analysis. He also discusses resistivity data, which show a not so large residual resistivity. Together with NQR data, he argues that his results indicate a very clean sample, specially when compared to other pnictide compounds.

Bernd now presents NQR relaxation rate data on the Li-111 compound. In the SC state, instead of the expected decrease in the relaxation rate, there is a significant increase. Turning on the magnetic field and performing NMR, Bernd finds that the increase in the relaxation rate below Tc disappears, and the usual behavior is recovered. Impurity effects and vortex contributions can be discarded as the cause for this unusual increase, Bernd argues.

He presents further raw NMR data, with the magnetic field along different directions. No change in the Knight shift is seen in the SC state (with H parallel to the a or b directions), and Bernd argues that this is an indication that no singlet pairs are formed below Tc. However, after changing the magnetic field direction, the data show the expected decrease in the Knight shift. Bernd argues that this is an indication that SC singlet pairs are not compatible to these observations.

Q (Ilya Eremin): Other group did not find this behavior in their Knight shift data.
A: They use powder, which is an important difference.

Bernd shows data on the line width of the NMR spectrum, which indicates the presence of spin fluctuations in the normal state, he says.

Discussion

Q (Andriy Nevidomsky): Why changing the field direction leads to decrease in the Knight shift?
A: A singlet component is induced by the magnetic field.

Q (Andrey Chubukov): Are low temperature data available for Li-111?
A: More data necessary, this is a work in progress.

Q (Takagi): Spin susceptibility anisotropy in the normal state?
A: Still more work necessary due to some issues with the surface.