A neutron crystal-field study of the pseudogap in the underdoped high-Tc superconductor HoBa2Cu418O8

Evidence for strong lattice effects as revealed from huge unconventional oxygen isotope effects on the pseudogap temperature in La2−xSrxCuO4

“…The values of the pseudogap temperature T * as well as the corresponding values of T c as measured for all 16 are summarized in Fig. 4a and Table I 19 ) are also displayed in Fig.…”

“…These are discussed as precursors to the paired state, spin den-sity formation, charge density formation, onset of stripe formation and other exotic scenarios 21 . Here we define the pseudogap T * as the temperature where lattice effects are apparent in terms of deviations of the local structure from the average one (this temperature is also denoted as charge-stripe ordering temperature in the literature 14 16 O samples were obtained in parallel via oxygen isotope exchange in closed quartz glass tubes under controlled gas pressure slightly above 1 bar at 800 • C during 72 hours with subsequent cooling. The oxygen-isotope enrichment was determined from the weight changes and was about 90% of 18 O for all samples.…”

“…Five absorption spectra were collected at the same temperature in an Oxford Instruments OptistatPT cryostat and their mean value taken in order to increase the signal to noise ratio. Figure 2 shows the normalized Cu K-edge XANES spectra for the 16 O and 18 O La 1.91 Sr 0.09 CuO 4 samples at T = 220 K as a representative for the entire series. The spectra show the typical absorption features of the cuprates with a square planar geometry where the main features are denoted as A 1 , A 2 , B 1 , and B 2 with peak intesities a 1 , a 2 , b 1 , and b 2 .…”

“…The effect of the oxygen-isotope substitution on the XANES spectra on the distribution of local Cu-site conformations is hardly seen directly in the raw XANES spectra shown in Fig. 2, but can be extracted from the slightly different XANES peak intensities a 1 and b 1 of the 16 O and 18 O samples 14 . Deviations from the square plane geometry of the CuO 4 coordination plaquette can be quantified by the XANES peak ratio 14…”

“…In Fig. 3 the XANES intensity ratio R is plotted for 16 O and 18 O samples of La 2−x Sr x CuO 4 (x = 0.9 and x = 0.15) as a function of temperature. From the temperature dependence of R, the pseudogap temperature T * is determined by the interception of the horizontal line above T * and to the linear slope below T * as shown in Fig.…”