Electrons go missing in strange metal phase of high temperature superconductors
Researchers from the UK and three of the EMFL laboratories have uncovered a striking crossover in the carrier density across the so-called ‘strange metal’ phase of overdoped cuprate superconductors. The work builds on a seminal work carried out at LNCMI-Toulouse back in 2016 and sheds important new light on the nature of the so-called ‘strange metal’ phase out of which high-temperature superconductivity emerges. The work is published in Nature Physics.
Lifting the veil of superconductivity
The phase diagram of high temperature superconductors has intrigued condensed matter physicists around the globe for well over three decades but many outstanding issues remain. In this investigation, the research team headed by Tony Carrington from University of Bristol and Nigel Hussey at HFML-FELIX performed measurements of the in-plane Hall resistivity rxy(H) of two cuprate families Tl2Ba2CuO6+d (Tl2201) and Bi2Sr2CuO6+d (Bi2201) concentrating on the limiting low-temperature behaviour that can only be reached at high magnetic fields when the superconductivity is suppressed. Their measurements showed that the number of charge carriers that contribute to the Hall resistivity at low temperatures transitions smoothly from p to 1 + p (where p is the number of doped holes) from the point where the strength of superconductivity is maximised to the edge of the superconducting dome.
Where have all the carriers gone?
In the earlier work from Toulouse on another family of cuprates, the authors concluded that the p to 1 + p transition occurs at a point in the phase diagram where the so-called pseudogap closes. The pseudogap is a phenomenon unique to the high-temperature superconductors that exists even above the superconducting transition and that removes some of the most energetic electrons from the metallic state. Hence, this would seem the most likely source of the loss of carriers found in experiment. Curiously, however, the present study revealed that the reduction in carriers actually occurs before the pseudogap opens.
Another piece of the puzzle?
Rather, this reduction is found to be correlated with the increase in the strength of the anomalous component to the resistivity that varies linearly with temperature and is the key signature of the strange metal. The strength of this linear-in-temperature resistivity also scales with the superconducting transition temperature suggesting an intimate relation between the two. With this new result, a second correlation has now been established between the strength of this linear-in-temperature resistivity and the number of carriers that survive in the normal state. This correlation appears to connect the growth of the strange metal phase to a gradual loss of coherence of the charge carriers as the pseudogap is approached. A key next step is to establish the link between this loss of coherence and the strengthening of superconductivity, a link that appears counter-intuitive at first, but then again, the metallic state of superconducting cuprates is not called strange for nothing!
Evolution of the low-T Hall number nH(0) across the strange metal regime in Tl2201 (red squares) and Bi2201 (blue circles), as determined from Hall resistivity measurements in high magnetic fields . A crossover from nH(0) ~ p to nH(0) ~ 1 + p is found to occur across a wide doping range beyond p*, the doping level at which the pseudogap vanishes. The grey dashed line is a guide to the eye. At low doping in LSCO, nH(0) follows closely the number of doped holes , as indicated by the green diamonds.
Reduced Hall carrier density in the overdoped strange metal regime of cuprate superconductors, Carsten Putzke, Siham Benhabib, Wojciech Tabis, Jake Ayres, Zhaosheng Wang, Liam Malone, Salvatore Licciardello, Jianming Lu, Takeshi Kondo, Tsunehiro Takeuchi,Nigel E. Hussey, John R. Cooper & Antony Carrington, Nature Physics (2021)
DOI 10.1038/ s41567-021-01197-0