Results 1 - 10 of 88
Results 1 - 10 of 88. Search took: 0.016 seconds
|Sort by: date | relevance|
[en] Electron energy-loss spectroscopy has been used to investigate the loss-function between 0 and 70 eV of single-crystalline Sr14-xCaxCu24O41, composed of two-leg Cu2O3 ladders and edge-sharing CuO2 chains, with various compositions. We found significant excitations in the low-energy range which are different for momentum transfer q parallel to the a- and c-axes. Comparison with reflectivity data from literature shows a good agreement with our data. Also the dispersions of a charge-carrier plasmon like in Bi2Sr2CaCu2O8 was obeserved for Sr3Ca11Cu24O41.
[en] Sr2IrO4 is the main example of a spin-orbit assisted Mott insulator. In the family of iridates, Sr2IrO4 has also been postulated as a candidate to emulate the physics of the parent compound of the high-temperature superconductors cuprates, where the doping effect in the insulator to metal transition is still not well understood. In this work, we classify different predominant defects in Sr2IrO4, with low temperature STM/S. We probe the spatial structure symmetry of these defects. From the tunneling spectra, we identify the energy of the upper and lower Jeff = 1/2 Hubbard bands, the Mott gap, and the variation of the electronic structure due to defects. A charge transfer-like behavior for the defect caused ingap states is observed. Our measurements provide detailed results about the defect effects to the electronic properties of Sr2IrO4, which can be important for further understanding of the doping effect in iridates and the insulator to metal transition in Mott insulators.
[en] Magnetic Resonance Imaging (MRI) is one of the most powerful non-invasive techniques in medical diagnostics that provides spatial and functional information. The sensitivity of NMR, imaging or spectroscopy, is strongly limited by the low Boltzmann polarization that can be reached at room temperature and any achievable field strength. A way out is the use of the Dynamic Nuclear Polarization (DNP) scheme, used for the operation of polarized solid targets in numerous nuclear and particle physics experiments since the early 1960s. Several imaging applications will benefit from the strong DNP-signal, e.g., available from hyperpolarized 13C-labeled contrast agents. The input and present status of DNP-hyperpolarization in this new emerging field is presented.
[en] We present 63,65Cu Nuclear Magnetic Resonance (NMR) measurements on undoped SrCuO2 and Ca doped Sr0.9Ca0.1CuO2 single crystals. The crystal structure contains one dimensional CuO2 double chains that are magnetically decoupled. The system orders magnetically only below 1.5 K. Nevertheless, the Cu NMR spectra broaden already at temperatures below 100 K and show an anomalous two peak structure at low temperatures. For the Ca doped sample, this broadening is reduced. The reason for this unusual broadening is not known, but a similar broadening has been reported for the single chain compound Sr2CuO3. The spin lattice relaxation rate, 1/T1, is temperature independent. This is typical for spin chains and has also been reported for Sr2CuO3. Surprisingly, non-magnetic Ca induces a gap like behaviour in 1/T1
[en] We have studied the interplay of magnetism and superconductivity in LaFeAsO1-xFx with 0≤x ≤0.125. For low doping with x≤0.04, our data confirm a moderate suppression of both the structural transition and the antiferromagnetic spin density wave formation. For x≥0.05, both anomalies are completely suppressed and superconductivity is observed. Remarkably, the temperature dependence of the normal state susceptibility well above TC is almost independent of doping, i.e. both the absolute value and the slope are nearly unchanged compared to the undoped case. This implies at least local antiferromagnetic interactions which barely depend on hole doping although the ground state changes entirely from an orthorhombic antiferromagnetic poor metal (x≤0.04) to a tetragonal superconductor (x≥0.05). These surprising results are discussed in terms of (i) - pseudogap formation, (ii) - antiferromagnetic correlations, and (iii) - preformed bipolarons which might be relevant to the pairing mechanism.
[en] We present Nernst-effect investigations on LaFeAsO1-xFx. In the parent compound the formation of a SDW state leads to a huge enhancement of the Nernst coefficient at T< TN. Despite the absence of SDW order at underdoped superconducting doping levels, a similar anomalous behavior is also observed (with smaller magnitude), which is suggestive of a spin-fluctuation enhanced Nernst-effect. Interestingly, at optimal doping level the Nernst coefficient is only weakly temperature dependent and appears more conventional.
[en] Electronic transport properties of the new unconventional superconductor LiFeAs and Co doped LiFeAs have been studied. Unlike in other iron arsenide superconductors this undoped compound exhibits no spin density wave but superconducting properties. Resistivity measurements show this transition at T=17 K. We have studied the magnetoresistance of LiFeAs and extracted the Hc2(T) phase diagram. In contrast to other Fe-As based compounds we find that in LiFe1-xCoxAs electron doping suppresses superconductivity.
[en] The layered crystalline structure is a characteristic shared by most iron-based superconductors. A very interesting question is how the superconductivity evolves if the dimensionality is further reduced. Iron-chalcogen-based structures are extremely flexible and allow the exploration of dimensionality effects on the physical properties since iron atoms can form low-dimensional structures including chains (e.g. BaFe2S4) or ladders (BaFe2S3). BaFe2S3 and BaFe2Se3 are antiferromagnetic Mott insulators and have been reported to become metallic under pressure. The metallic phase presents superconductivity at low temperatures. In this work, we focus on the synthesis of single crystals of BaFe2S3. We present a comprehensive characterization together with the resistivity and the magnetization. Above the Neel temperature (TN ∝ 120 K), the magnetization decreases upon cooling. This non-Curie-Weiss behavior is similar to what is found in the parent compounds of the iron-based superconductors. The resistivity presents an insulating-like behavior with a first kink at T ∝ 200 K and a second at the Neel temperature.