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Paper published in the prestigeous journal of the National Academy of Sciences (PNAS) resulting from the research performed within the UKF project "New electronic states driven by frustration in layered materials" (Tutiš, Forro, Barišić i Smontara)
New insight into the cuprate superconductors’ phase diagram

Universal sheet resistance and revised phase diagram of the cuprate high-temperature superconductors

Neven Barišić, Mun K. Chan, Yuan Li, Guichuan Yu, Xudong Zhao, Martin Dressel, Ana Smontara, and Martin Greven

Proceedings of the National Academy of Sciences (PNAS) 110, 12235-12240 (2013)


One of the most investigated subjects in the field of condensed matter physics is the high-temperature superconductivity. It was discovered in the family of cooper-oxides quarter of a century ago, and although the transition temperature over time reached surprisingly high values (~150 K), the understanding of this phenomenon is still lacking. In the meantime it has been stated countless many times that the key for understanding lies in the enigmatic metallic state from which superconductivity evolves upon cooling. This state is characterized by remarkable physical properties, such as the pseudogap phenomenon and a linear temperature dependence of the resistivity. There exist well over 50 cuprate superconductors with different crystal structures, disorder effects, and competing phases. The enormous amount of experiment work has been plagued by non-universal, materials-specific results, which has obfuscated the true nature of these materials. For the theoretical condensed matter physics community, the study of the strange metallic state has been an unprecedented challenge as well, with contributions made by more than half a dozen Nobel Laureates.

New findings of Neven Barišić and his collaborators for the metallic state of the cuprates are both remarkably simple and profound. They studied the high-quality crystals of Hg1201, which is arguably the most desirable cuprate superconductor for experimental work due to its high-symmetry crystal structure and high superconducting transition temperature.

The authors demonstrated that, in addition to the well-known linear resistive regime at higher temperature Hg1201 exhibits a Fermi-liquid-like quadratic temperature regime at lower temperatures. In other words, experiments showed that once the pseudogap has removed quasiparticle spectral weight from the anti-nodal parts of the Fermi surface, the superconducting state actually emerges from a state that exhibits, in effect, conventional metallic behaviour along the nodes.

Motivated by their resistivity measurements for Hg1201, Neven Barišić and collaborators, furthermore, combined their data with published results for three structurally more complex cuprates, being able to obtain the universal, quantitative resistivity per copper-oxygen plane (sheet resistance) throughout most of the temperature-doping phase diagram (see Figure below).
Finally, the authors demonstrated that the doping dependence of the sheet resistance in both the linear and the quadratic temperature regimes is remarkably simple, which lead them to propose a counter-intuitive scenario. More specifically even outside of the pseudogap regime (both at high temperature and high hole concentrations) only the near-nodal states contribute to the planar transport. Conversely, it appears that deep inside the pseudogap regime, close to the antiferromagnetic-insulating state at zero doping, the cuprates may in effect be nodal Fermi liquids. These insights imply the need for a dramatic paradigm shift for the phase diagram of the cuprates.

These new insights already have inspired exciting collaborative work with the group of Prof. Dirk van Der Marel published separately in PNAS (Proc. Natl. Acad. Sci. USA 113 (2013), 15; 5774-5778) with Neven Barišić as one of principal authors. In the latter, optical conductivity measurements revealed a Fermi-liquid-like quadratic frequency dependence and temperature-frequency scaling in Hg1201 samples in which the pure ρ∝T2 behaviour was demonstrated from dc transport. This provides an independent experimental confirmation of the original findings of Neven Barišić et al. Published in the UKF supported work.

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