Investigation of charge and superconducting correlations in cuprates using nonlinear conductivity

Razvijen je sustav za beskontaktno mjerenje nelinearne vodljivosti u radiofrekventnom području, na velikim snagama. Mjeren je treći harmonik vodljivosti na tri uzorka visoko-temperaturnih kupratnih supravodiča (YBa2Cu3O6.6 (YBCO), La2−0.125Sr0.125Cu O4 (LSCO), La2−xEu0.2SrxCuO4 (LESCO)) u području iznad supravodljivog prijelaza, u kojem se kod nekih spojeva javlja prugasto uređenje. Mjeren je i odgovor nedopiranog pniktida BaFe2As2, s temperaturom nematskog prijelaza na Tnem=138 K. Usporedbom mjerenih rezultata s predviđanjima kvalitativnog modela po uzoru na klasični opis vala gustoće naboja ustanovljena je linearna temperaturna ovisnost parametra prugastog uređenja u YBCO-u i LESCO-u. U uzorku LESCO-a uz statičke pruge izmjeren je i oštar maksimum sličan signalu pniktida na Tnem, koji upućuje na intrinzično topljenje pruga, odnosno lom orijentacijske simetrije neovisan o strukturnom prijelazu. Mjerenja na LSCO-u nisu otkrila postojanje prugastog uređenja.An apparatus for measuring nonlinear conductivity in the radio-frequency range at high powers has been devised. Nonlinear conductivity was measured on three samples (YBa2Cu3O6.6 (YBCO), La2−0.125Sr0.125CuO4 (LSCO), La2−xEu0.2SrxCu O4 (LESCO)) above the superconducting transition, in the range where striped order occurs. Nonlinear response of an undoped ironarsenide sample, known to undergo an electronic nematic phase transition at Tnem=138 K was also measured. Comparing the measured results and qualitative predictions obtained from a classical model, we were able to determine the dependence of the stripe order parameter on temperature in YBCO and LESCO. Measurements on LESCO have also revealed the presence of an intermediate state, marked by a sharp peak, similar to the one measured in the iron arsenide at Tnem. It probably corresponds to stripe melting, i.e. an orientational symmetry breaking state independent of structure. Nonlinear conductivity measurements on LSCO revealed no stripe order phase