Several angle resolved photoemission spectroscopy (ARPES) studies reveal a poorly nested Fermi surface of LiFeAs, far away from a spin density wave instability, and clear-cut superconducting gap anisotropies. On the other hand a very different, more
nested Fermi surface and dissimilar gap anisotropies have been obtained from quasiparticle interference (QPI) data, which were interpreted as arising from intraband scattering within hole-like bands. Here we show that this ARPES-QPI paradox is completely resolved by interband scattering between the hole-like bands. The resolution follows from an excellent agreement between experimental quasiparticle scattering data and T-matrix QPI calculations (based on experimental band structure data), which allows disentangling interband and intraband scattering processes.
The Nernst effect has recently proven a sensitive probe for detecting unusual normal state properties of unconventional superconductors. In particular, it may sensitively detect Fermi surface reconstructions which are connected to a charge or spin de
nsity wave (SDW) ordered state, and even fluctuating forms of such a state. Here we summarize recent results for the Nernst effect of the iron pnictide superconductor $rm LaO_{1-x}F_xFeAs$, whose ground state evolves upon doping from an itinerant SDW to a superconducting state, and the cuprate superconductor $rm La_{1.8-x}Eu_{0.2}Sr_xCuO_4$ which exhibits static stripe order as a ground state competing with the superconductivity. In $rm LaO_{1-x}F_xFeAs$, the SDW order leads to a huge Nernst response, which allows to detect even fluctuating SDW precursors at superconducting doping levels where long range SDW order is suppressed. This is in contrast to the impact of stripe order on the normal state Nernst effect in $rm La_{1.8-x}Eu_{0.2}Sr_xCuO_4$. Here, though signatures of the stripe order are detectable in the temperature dependence of the Nernst coefficient, its overall temperature dependence is very similar to that of $rm La_{2-x}Sr_xCuO_4$, where stripe order is absent. The anomalies which are induced by the stripe order are very subtle and the enhancement of the Nernst response due to static stripe order in $rm La_{1.8-x}Eu_{0.2}Sr_xCuO_4$ as compared to that of the pseudogap phase in $rm La_{2-x}Sr_xCuO_4$, if any, is very small.
We have investigated the thermal conductivity kappa_mag of high-purity single crystals of the spin chain compound Sr2CuO3 which is considered an excellent realization of the one-dimensional spin-1/2 antiferromagnetic Heisenberg model. We find that th
e spinon heat conductivity kappa_mag is strongly enhanced as compared to previous results obtained on samples with lower chemical purity. The analysis of kappa_mag allows to compute the spinon mean free path l_mag as a function of temperature. At low-temperature we find l_magsim0.5mum, corresponding to more than 1200 chain unit cells. Upon increasing the temperature, the mean free path decreases strongly and approaches an exponential decay ~1/T*exp(T*/T) which is characteristic for umklapp processes with the energy scale k_B T*. Based on Matthiesens rule we decompose l_mag into a temperature-independent spinon-defect scattering length l0 and a temperature dependent spinon-phonon scattering length l_sp(T). By comparing l_mag(T) of Sr2CuO3 with that of SrCuO2, we show that the spin-phonon interaction, as expressed by l_sp is practically the same in both systems. The comparison of the empirically derived l_sp with model calculations for the spin-phonon interaction of the one-dimensional spin-1/2 XY model yields reasonable agreement with the experimental data.
We study the impact of a weak bond disorder on the spinon heat transport in the S=1/2 antiferromagnetic (AFM) Heisenberg chain material Sr_{1-x}Ca_xCuO_2. We observe a drastic suppression in the magnetic heat conductivity kappa_mag even at tiny disor
der levels (i.e., Ca-doping levels), in stark contrast to previous findings for kappa_mag of S=1/2 two-dimensional square lattice and two-leg spin-ladder systems, where a similar bond disorder has no effect on kappa_mag. Hence, our results underpin the exceptional role of integrability of the S=1/2 AFM Heisenberg chain model and suggest that the bond disorder effectively destroys the ballistic nature of its heat transport. We further show that the suppression of kappa_mag is captured by an effective spinon-impurity scattering length, which exhibits the same doping dependence as the long-distance exponential decay length of the spin-spin correlation as determined by density-matrix renormalization group calculations.
We present the first comprehensive derivation of the intrinsic electronic phase diagram of the iron-oxypnictide superconductors in the normal state based on the analysis of the electrical resistivity $rho$ of both LaFeAsO$_{1-x}$F$_x$ and SmFeAsO$_{1
-x}$F$_x$ for a wide range of doping. Our data give clear-cut evidence for unusual normal state properties in these new materials. In particular, the emergence of superconductivity at low doping levels is accompanied by distinct anomalous transport behavior in $rho$ of the normal state which is reminiscent of the spin density wave (SDW) signature in the parent material. At higher doping levels $rho$ of LaFeAsO$_{1-x}$F$_x$ shows a clear transition from this pseudogap-like behavior to Fermi liquid-like behavior, mimicking the phase diagram of the cuprates. Moreover, our data reveal a correlation between the strength of the anomalous features and the stability of the superconducting phase. The pseudogap-like features become stronger in SmFeAsO$_{1-x}$F$_x$ where superconductivity is enhanced and vanish when superconductivity is reduced in the doping region with Fermi liquid-like behavior.
