As a result of the collaboration of Petar Popčević, a scientist from the Institute of physics, with scientists from Physics Department of the Faculty of Sciences in Zagreb, Technical University in Vienna and Minnesota University, a paper has been published in prestigious journal Quantum materials of the Nature group, in which the appearance of superconductivity in dc electric conductivity of cuprates has been explained by a simple model of superconducting percolation.

Percolative nature of the direct-current paraconductivity in cuprate superconductors

Petar Popčević , Damjan Pelc, Yang Tang, Kristijan Velebit, Zachary Anderson, Vikram Nagarajan, Guichuan Yu , Miroslav Požek, Neven Barišić, and Martin Greven, Quantum Materials 3, 42 (2018).

DOI: 10.1038/s41535-018-0115-2

Despite a few decades’ great efforts of scientists round the world, the understanding of high temperature superconductors belonging to cuprates family still remains a challenge to scientific community. It is particularly important to understand the regime of superconducting “pre-pairing” above the temperature of macroscopic superconducting transition: namely, this regime yields insight both in normal and superconducting state. The separation of superconducting response from complex behavior of normal state represents a particular experimental challenge. Different experiments have therefore yielded contradictory conclusions on the pairing above the transition temperature, and the situation resulted with wide spectrum of possible theoretical explanations.

Through the experimental studies during the last decade, in which the physicists from Zagreb have played the crucial role, it has been established beyond any doubt that the moving charge carriers in cuprates exhibit the characteristics of the conventional Fermi liquid. However, their behavior is in some parts of the phase diagram masked by the temperature dependent (de)localization of charges. The work presented in „Quantum materials“ concentrates on underdoped area of the phase diagram of cuprates (the so called pseudogap area) in which a relatively wide temperature range with constant number of charge carriers exists. Within this regime, the resistivity in normal state obeys very simple quadratic temperature dependence, in agreement with the Fermi liquid theory. That property has been used to unambiguously separate the contribution of normal state in electrical conductivity from the paraconducting contribution corresponding to the appearance of superconductivity. In this way it has been clearly, and in a simple way, demonstrated that the traces of superconductivity are visible only a few tens of degrees above the temperature of superconducting transition, together with almost exponential decrease of paraconductivity intensity with temperature. It has also been shown that the described dependence is universal in cuprates, with the characteristic temperature scale which does not depend on the details of particular compounds. Finally, a simple model of superconducting percolation has been developed, which includes the distribution of local transition temperatures on nano-scale, with areas of locally enhanced transition temperature. By applying the effective medium theory on such system with localized superconducting domains, one obtains the temperature dependence of conductivity in agreement with experiments. The good agreement of model and experiments indicates that there exists intrinsic inhomogeneity of the superconducting state in cuprates. Clear and unambiguous determination of universal superconducting percolations, as well as an additional confirmation of the conventional behavior of the mobile charge carriers in normal state requires the change of the established paradigms in high temperature superconductivity.