RESUMO
We present an experimental and theoretical study which compares the phonon anomalies and the electronic gap features in the infrared response of the weakly coupled two-leg-ladders in Sr14-xCaxCu24O41(SCCO) with those of the underdoped high-Tcsuperconductor YBa2Cu3O6+x(YBCO) and thereby reveals some surprising analogies. Specifically, we present a phenomenological model that describes the anomalous doping- and temperature-dependence of some of the phonon features in thea-axis response (field along the rungs of the ladders) of SCCO. It assumes that the phonons are coupled to charge oscillations within the ladders. Their changes with decreasing temperature reveal the formation of a crystal (density wave) of hole pairs that are oriented along the rungs. We also discuss the analogy to a similar model that was previously used to explain the phonon anomalies and an electronic plasma mode in thec-axis response (field perpendicular to the CuO2planes) of YBCO. We further confirm that an insulator-like pseudogap develops in thea-axis conductivity of SCCO which closely resembles that in thec-axis conductivity of YBCO. Most surprisingly, we find that thec-axis conductivity (field along the legs of the ladders) of SCCO is strikingly similar to the in-plane one (field parallel to the CuO2planes) of YBCO. Notably, in both cases a dip feature develops in the normal state spectra that is connected with a spectral weight shift toward low frequencies and can thus be associated with precursor superconducting pairing correlations that are lacking macroscopic phase coherence. This SCCO-YBCO analogy indicates that collective degrees of freedom contribute to the low-energy response of underdoped highTccuprates and it even suggests that the charges in the CuO2planes tend to segregate forming quasi-one-dimensional structures similar to the two-leg ladders, as predicted for the stripe-scenario or certain intertwinned states.
RESUMO
Inspired by the standard computed tomography, a new method of 3D X-ray imaging embedded in FIB-SEM microscope is proposed. The unique combination of TEM-like specimen stage enabling in lens STEM detection (referred to as CompuStage), nanomanipulator (referred to as EasyLift) facilitating in-situ sample transfer from bulk sample to TEM-like stage and pixelated in-situ Timepix X-ray detector in Helios G4 FX FIB-SEM system offers an unprecedented workflow. Motivated by common circular CT scan known from microCT world, the object under study is placed on CompuStage rod which enables two possible rotation (in TEM/SEM terminology called tilt) movements - α-tilt - rotation of the CompuStage rod around its axis, and ß-tilt - rotation around axis perpendicular to CompuStage rod. ß-tilt rotation enables a circular movement of the sample while α-tilt sets the correct position of sample with respect to target and detector. Thin metal lamella of suitable material welded to EasyLift manipulator needle is used as an X-ray target. The final target-sample geometry - position, distance - can be fine-tuned using position control of CompuStage and EasyLift and in-situ monitored by SEM. Both sample and target can also be easily prepared in-situ. Radiographs are recorded by Timepix detector with inherent noise-free operation and energy filtration. For the 3D reconstruction standard microCT reconstruction algorithm is used with the procedure adjusted for the format and quality of nanoCT images. The experiments were carried out on Helios G4 FX DualBeam using titanium and tungsten targets and various semiconductor samples. The ultimate resolution of the proposed method in orders of tens of nanometers was achieved both by the possibility of close target to sample positioning and of adjustment of primary beam energy down to low energies reducing the interaction volume in the target. Since the lower energy radiation is well suited for life-science, the method was also tested on several bio-samples using silver target. The silver target, thanks to its massive low energy Lα line, allowed to distinguish subtle structures in the resin embedded stained mouse brain and also to observe and reconstruct canaliculi in the mouse bone (earlier reported by Dierolf et al. 2010, Nature 467 436).