Periodic Reporting for period 2 - CATCH-22 (High temperature superconductivity and the Catch-22 conundrum)
Período documentado: 2021-04-01 hasta 2022-09-30
CATCH-22 sets out to resolve the mystery of the copper-oxide high temperature superconductors. Hailed as one of the major discoveries of the 20th Century in physics, its central mysteries – the pairing mechanism, the origin of the ‘pseudogap’ and the nature of the ‘strange metal’ phase – have remained elusive for over 30 years. The stark failure to explain these phenomena using conventional theories have led to a consensus that the cuprates represent fundamentally new states of matter that require a radically new theoretical framework.
Typically, what scatters electrons in their metallic state also binds them into pairs. In the cuprates, the strong pairing interaction, of as yet unknown origin, manifests itself in the strange metal phase as intense scattering, so strong in fact that it drives the coherent electronic states required for pairing to the edge of incoherence. In other words, what first promotes high temperature superconductivity ultimately destroys it! Within literary circles, such a logical paradox is known as the Catch-22 conundrum.
CATCH-22, the program, comprises three parts. The focus of Part 1 will be the incoherent transport regime in which the fate of electronic states within the strange metal phase will be determined. A dedicated study of decoherence in cuprates is long overdue. CATCH-22 will seek to demystify the strange metal by studying how the metallic response disintegrates across universal bounds, both as a function of temperature and doping (interaction strength), through a combination of momentum-averaged electrical, optical and thermal conductivity studies in high magnetic fields with momentum-resolved photoemission spectroscopy. Part 2 will seek to access the electronic ground state of optimally doped cuprates for the first time, using THz spectroscopy and pulsed high-current measurements on ultra-thin samples in magnetic fields up to 45 T and 90 T respectively. The latter, if successful, will open up a new frontier in the exploration of unconventional superconductors and other correlated systems in which intense THz light and intense magnetic fields combine to access the ‘terra incognita’ of hidden phases. Finally, in Part 3, we will explore the origins of the strange metal at the edge of the superconducting dome and search for manifestations of incoherence in other candidate strange metals in an attempt to establish the paradigm for strange metal behaviour.
Given that the central mysteries are intertwined – the strange metal is a precursor to the pseudogap which in turn leads to superconductivity - CATCH-22 has the potential to bring significant new insight into all three and pave the way, finally, for the development of a coherent explanation of the cuprate phenomenon.
Typically, what scatters electrons in their metallic state also binds them into pairs. In the cuprates, the strong pairing interaction, of as yet unknown origin, manifests itself in the strange metal phase as intense scattering, so strong in fact that it drives the coherent electronic states required for pairing to the edge of incoherence. In other words, what first promotes high temperature superconductivity ultimately destroys it! Within literary circles, such a logical paradox is known as the Catch-22 conundrum.
CATCH-22, the program, comprises three parts. The focus of Part 1 will be the incoherent transport regime in which the fate of electronic states within the strange metal phase will be determined. A dedicated study of decoherence in cuprates is long overdue. CATCH-22 will seek to demystify the strange metal by studying how the metallic response disintegrates across universal bounds, both as a function of temperature and doping (interaction strength), through a combination of momentum-averaged electrical, optical and thermal conductivity studies in high magnetic fields with momentum-resolved photoemission spectroscopy. Part 2 will seek to access the electronic ground state of optimally doped cuprates for the first time, using THz spectroscopy and pulsed high-current measurements on ultra-thin samples in magnetic fields up to 45 T and 90 T respectively. The latter, if successful, will open up a new frontier in the exploration of unconventional superconductors and other correlated systems in which intense THz light and intense magnetic fields combine to access the ‘terra incognita’ of hidden phases. Finally, in Part 3, we will explore the origins of the strange metal at the edge of the superconducting dome and search for manifestations of incoherence in other candidate strange metals in an attempt to establish the paradigm for strange metal behaviour.
Given that the central mysteries are intertwined – the strange metal is a precursor to the pseudogap which in turn leads to superconductivity - CATCH-22 has the potential to bring significant new insight into all three and pave the way, finally, for the development of a coherent explanation of the cuprate phenomenon.
In spite of the ongoing pandemic, progress has been made within all 3 of the work packages (WPs) within the original CATCH-22 programme (highlighted above) whose structure has, by and large, remained the same. Almost all the envisaged personnel positions have been filled or are about to be filled. Only one important change has been made. This has come about following two key discoveries by our team and collaborators, to be published in Nature and Nature Physics in the coming months, and the development of an exciting new line of enquiry that actually goes further than the original CATCH-22 idea in proposing a paradigm shift in our understanding of high temperature superconductivity in the cuprates.
The original plan for WP1B was to search for different signatures of incoherent charge transport in overdoped cuprates through combined measurements of the magneto-transport, Hall effect and optical conductivity. At the beginning of the programme, Prof. Hussey (PI) and Dr. Ayres (PDRA) duly completed a large collaborative study of the Hall effect in overdoped cuprates [Putzke et al., Nature Physics AOP (2021)] revealing that the striking crossover from p (the number of doped holes) to 1 + p (the total carrier density) previously thought to occur across p* - the critical doping level associated with the end of the pseudogap phase – actually occurs beyond p* and across the strange metal regime. Dr. Ayres’ subsequent magnetoresistance study revealed clear signatures of incoherent transport in the strange metal regime [Ayres, Berben et al., Nature, in press (2021)]. From these combined studies, a picture has emerged that the transport properties of overdoped cuprates are determined not by one charge sector, but by two – one coherent, the other incoherent.
The original plan for WP1B was to search for different signatures of incoherent charge transport in overdoped cuprates through combined measurements of the magneto-transport, Hall effect and optical conductivity. At the beginning of the programme, Prof. Hussey (PI) and Dr. Ayres (PDRA) duly completed a large collaborative study of the Hall effect in overdoped cuprates [Putzke et al., Nature Physics AOP (2021)] revealing that the striking crossover from p (the number of doped holes) to 1 + p (the total carrier density) previously thought to occur across p* - the critical doping level associated with the end of the pseudogap phase – actually occurs beyond p* and across the strange metal regime. Dr. Ayres’ subsequent magnetoresistance study revealed clear signatures of incoherent transport in the strange metal regime [Ayres, Berben et al., Nature, in press (2021)]. From these combined studies, a picture has emerged that the transport properties of overdoped cuprates are determined not by one charge sector, but by two – one coherent, the other incoherent.
Given the presence of two sectors, the most obvious question to address is which of these sectors is responsible for high temperature superconductivity? Conventional wisdom would assume that it is the coherent sector, as stipulated by BCS theory. Our initial examination of this question, however, has revealed a possible new paradigm, namely that the superfluid condensate in overdoped cuprates emerges from that sector which exhibits signatures of incoherence in the normal, i.e. non-superconducting state [Culo, Duffy, et a., SciPost (submitted) (2021)]. This striking postulate, if proved correct, would not only confirm the CATCH-22 scenario, it would also represent a paradigm shift in our understanding of the origin of high-temperature superconductivity. Hence, a new programme has been launched within the CATCH-22 team to test this postulate with quantitative measurements of the superfluid density in the (Bi,Pb)2(Sr,La)2CuO6 family of overdoped cuprates.