Effect of pressure and of doping on superconductivity in the double-mercury-layer cuprate Hg2 (N = 2). A quantitative analysis based on indirect-exchange pairing

Polaron effects on superexchange interaction: Isotope shifts of T N , T c , and T* in layered copper oxides

“…It is known that the superexchange parameters very strongly depend on the distance between the interacting ions. This mechanism explains well the increase in T C under the action of an external pressure on a high-T C crystal [16,17]. The question naturally arises in this case of what occurs with the lattice parameters upon changing some isotopes for other ones.…”

“…This can be most simply demonstrated with the example of the isotope shift of the Neel temperature in related high-T C compounds. As was already indicated in [1], the Kugel and Khomskii's correction gives a correct sign of the isotope shift of the Neel temperature (T N ) in La 2 CuO 4 upon replacing 16 O with 18 O; however, even the most overrated estimates give a value that is six times lower than the experimental one.…”

“…Substituting here (in electronvolts) ∆ Cu−O = 1.5 [13] and standard values hω a = 0.05, E a = 0.4 [10,14], and using the relationship ∆T N /T N ≈ ∆J/J characteristic for layered cuprates (see [1]), we find that the Neel temperature should decrease by 0.2% upon replacing 16 O with 18 O in YBa 2 Cu 3 O 6.383 . According to measurements in La 2 CuO 4 [15], the shift ≃ 0.6%.…”

“…It is known that the observation of an isotope shift of the superconducting transition temperature (T C ) was of crucial importance in ascertaining the phonon mechanism of pairing in conventional (low-temperature) superconductors. The fact that an isotope effect exists in hightemperature superconductors built up of copperoxygen planes has long been beyond question; however, the relative smallness of the coefficient (for example, α ∼ 0.056 for YBa 2 Cu 4 O 8 upon replacing 16 O with 18 O, instead of the standard value 0.5) and the specific features of its behavior in other compounds upon changing the number of holes in copper-oxygen planes do not fit the Bardeen-Cooper-Schrieffer (BCS) scenario. At the same time, the majority of authors of articles related to the isotope effect (see the recent review [1]) correctly point out that, nevertheless, phonon modes, in some mysterious way, affect the superconducting transition temperature.…”

“…In this context, we believe that the facts of observing an isotope effect for the characteristic temperature of the pseudogap state of underdoped cuprates (so-called pseudogap onset temperature T * ) gain great importance. Thus, according to [2], the isotope exponent upon replacing 16 O with 18 O corresponding to T * is = 0.061. It was natural that this fact suggested a common origin of α TC and α T * [2].…”