D. Haskel

Where do the doped holes go in La2−xSrxCuO4? A close look by XAFS

Polarized x-ray absorption fine structure (XAFS) measurements at the La and Sr sites in La2−xSrxCuO4 (0.075≲x≲0.35) indicate that doped holes introduced with Sr are not uniformly distributed in the CuO2 planes but reside in impurity states with the majority of charge located on CuO6 octahedra coupled to the Sr dopants by an apical oxygen (denoted as Sr octahedra). A model based on doped hole-induced Jahn Teller (JT) distortions of these Sr octahedra indicates the impurity states are overlapping singlet and triplet ones resulting from pairings of intrinsic and extrinsic holes. The more mobile singlet pairs reside in the CuO2 planes, are bound by more than 0.1 eV and could Bose-condense into a superconducting state. The predominant appearance of (mostly) out-of plane triplet states at high x intimately related to the 2D-3D crossover in transport properties and could relate to the observed loss of high Tc in the overdoped regime, as confinement of carriers to the CuO2 planes is reduced.

Nanoscopic fluctuational dynamic model of anomalous temperature and concentration dependences in the Sn Mössbauer isomer shift in Ag-Sn alloys

A nanoscopic fluctuational dynamic model is suggested for observed anomalous reversible changes of the M?ssbauer isomer shift (ARCOMIS) found recently in Ag-Sn alloys not affected by long ageing. The model considers fluctuation-induced nanoscale large transient atomic displacements (LTADs) and material disordering. They cause electron localization, taking place simultaneously and permanently in the nanometre vicinities of M?ssbauer impurity atoms in the entire alloy. This causes an additional enhancement in the electron charge density |?f(0)|2 at the M?ssbauer nuclei, leading to the ARCOMIS at temperature Ts. These phenomena are generated by persistent sequences of nanoscale short-lived (picosecond) large energy fluctuations (SLEFs) of atomic particles of the peak thermal energy 0p>?E>>kT, which occur simultaneously and constantly in the material. The model leads to the following results. (i)?A SLEF-mediated dynamic mechanism for the ARCOMIS occurring at relatively low temperatures Ts is proposed. (ii)?The temperature Ts(Ag-Sn)?500?K is calculated, in good agreement with observations. (iii)?The narrow temperature interval ?T 3% is suggested. (vi)?An explanation is offered for the absence of the ARCOMIS in SnO2 material. (vii)?Similar results are obtained for the Au-Sn alloys not affected by long ageing, in good agreement with tentative experimental data.

Anomalous concentration dependence of the Mössbauer isomer shift of Ag-Sn alloys

The position of the Mossbauer resonant absorption line of was measured at various temperatures and concentrations of substitutional Sn impurities in the -phase of the Ag-Sn alloy. Whereas 1.0 and 2.0 at.% Sn alloys showed the expected temperature dependence of the line shift (LS), 4.0 and 8.0 at.% Sn alloys showed several anomalous features. After aging at room temperature (RT) for several months, these alloys showed hysteresis of the LS from RT up to about 500 K. At about 520 K, the unusual slope transforms to its expected value but continues with an increased isomer shift (IS). Further thermal cycles resulted in the expected slope of the LS with no measurable hysteresis, except for the transition region around 520 K above which the IS increase persisted. The experiments indicate that after thermal cycling the high-concentration alloys are in an intermediate state below about 500 K, which becomes metastable at RT as shown by the changes with aging. The transition to the increased-IS state at , however, is independent of the thermal history. A discussion of the observed phenomena is presented, which is based on thermally induced changes in the Sn-Sn distribution (short-range order) that preserve the long-range order of the alloy.