Unconventional isotope effects in the high-temperature cuprate superconductors

Abstract

We review various isotope effects in the high-Tc cuprate superconductors to assess the role of the electron-phonon interaction in the basic physics of these materials. Of particular interest are the unconventional isotope effects on the supercarrier mass, on the charge-stripe formation temperature, on the pseudogap formation temperature, on the EPR linewidth, on the spin-glass freezing temperature, and on the antiferromagnetic ordering temperature. The observed unconventional isotope effects strongly suggest that lattice vibrations play an important role in the microscopic pairing mechanism of high-temperature superconductivity.

Unconventional superconductivity

“…There are several collections of experimental reviews of the cuprates, e. g. devoted to High Temperature Superconductors, here are also specialized experimental reviews, or even multiple reviews, on just one measurement technique or property in the cuprates (e. g. SQUID devices: Koelle et al [297], phase sensitive detection of the pairing symmetry: Van Harlingen [298], Andreev reflection spectroscopy: Deutscher [299], scanning tunneling microscopy (STM) and spectroscopy (STS): Fischer et al [300], photoemission measurements: Damascelli et al [301], pseudogap behavior: Timusk and Statt [40] and Rice et al [39] and Norman, Pines and Kallin [302], and isotope effect: Zhao et al [303] and Franck [304].…”

“…While it is clear that electron phonon coupling alone is not sufficient to explain the high Tc's in the cuprates (references: R. Heid et al [352], Bohnen et al [353], Giustino, Cohen and Louie [354]), as we will discuss there are models that suggest that such coupling can enhance the pairing correlations from another dominant interaction, e. g. spin fluctuations. For reviews of the isotope effect in cuprates, see J. Franck [304] and Zhao, Keller and Conder [303].…”

Pairing interactions and pairing mechanism in high-temperature copper oxide superconductors

“…On the other hand, there is overwhelming evidence that electron-phonon coupling is very strong in the cuprate superconductors [5][6][7][8][9][10][11][12][13][14][15][16][17][18][19][20] . In particular, various unconventional oxygen-isotope effects Zhao and his coworkers have observed since 1994 clearly indicate that the electron-phonon interactions are so strong that polarons/bipolarons are formed in doped cuprates [5][6][7][8][9]11,12,[14][15][16][17]19 and manganites 21,22 , in agreement with a theory of high-temperature superconductivity 23 and the original motivation for the discovery of high-temperature superconductivity 1 .…”

Evidence for Polaron Formation in High-Temperature Superconducting Cuprates: Experiment and Theory

“…It was found that for all cuprate HTS families the OIE exponent α O shows a generic trend: In the underdoped regime, α O is large (even exceeding the BCS value of α O = 0.5) and becomes vanishingly small close to optimal doping [1][2][3][4]. An example of this generic behavior of α O as a function of T c , together with theoretical calculations using an extended version of the above model, is shown in Fig.…”

“…Early and more recent isotope effect experiments on the superconducting transition temperature T c in high-temperature superconducting cuprates (HTS) have shown that the isotope effect is strongly doping dependent in these systems and especially strongly enhanced as compared to the BCS value at the borderline to the antiferromagnetic state [1][2][3][4]. Even though these observations do not necessarily support phonon mediated superconductivity in the classical sense, further isotope effects on other quantities suggest that lattice involvement is essential to the physics of HTS.…”