Final Report Summary - FESUME (Iron-based superconductivity: Fermi Surface and superconducting gap anisotropy)
EU funded research project “Iron-based superconductivity: Fermi Surface and superconducting gap anisotropy” (FESUME) was an experimental study to elucidate the mechanism that leads to the high temperature superconductivity in the recently discovered iron-pnictide superconductors.
The discovery in 2008 of iron-based superconductivity at high temperature marked an exciting turning point in the study of high temperature superconductivity (HTS). Compared with the cuprates, their metallic ground state, relative malleability and electronic isotropy, unexpectedly large upper critical field and strong native vortex pinning make them useful for candidates for applications. However, from the present point of view, the most important aspect of these materials is that they present a new route to investigate the mechanism which cause high temperature superconductivity: giving new insight into iron based and the older copper based materials. It is widely believed that the unique topology of the Fermi surface of the iron pnictide superconductors is a crucial ingredient. For this reason knowing the fine details of the Fermi surface (FS) topology and its tendency towards instabilities is a fundamental step for understanding the origin of superconductivity in these compounds.
Within this project, we proposed a series of experiments to gain an understanding of the relationship between the band-structure and the superconducting properties in the iron based superconductors. In particular, the project has been mainly focused on the recently discovered P-doped arsenides BaFe2(As1-xPx)2 and LiFeAs/LiFeP. These are interesting systems where this problem can be addressed in a controlled and systematic way since their magnetic and superconducting properties evolve substantially as a function of As/P cross-substitution. These are particularly clean because systems the substitution of As by the isovalent ion P dramatically changes their electronic properties without changing the electron-hole balance and without inducing appreciable scattering. The series of the 122 pnictides BaFe2(As1-xPx)2 goes from antiferromagnetism (x=0) to superconductivity and then to a non superconducting paramagnetic state (x=1). There is substantial evidence for a quantum critical point at the x=0.3 where the superconducting transition is maximal. The 111 family of iron pnictides is unique because both LiFeAs and its counterpart LiFeP are nonmagnetic and superconduct with Tc of around 18 K and 5 K, respectively.
To perform a comprehensive study of fundamental electronic properties of the P-doped arsenides, in both normal and superconducting state, we have used three versatile and powerful experimental techniques, de Haas van Alphen (dHvA), London penetration depth and specific heat measurements that are important and well established methods to measure the Fermi surface and the superconducting gap structure. The experiments have been carried out at temperatures down to the milikelvin regime and under very high magnetic fields up to 80 T using the facilities available in Bristol as well as through measurements in international high field facilities such as the LNCMI in Toulouse (France), the HLD in Dresden (Germany) and the NHMFL in Tallahassee (USA). The experiments were performed in collaboration with A. Carrington, C. Putzke (Bristol), A. Coldea, M.Watson (Oxford) and local contacts at the high field facilities. Samples were provided by the group of Y. Matsuda (Kyoto).
In the 111 iron pnictide superconductors we studied the electronic properties in the normal state of the two end members of the series LiFeP and LiFeAs through measurements of the dHvA effect. By following the angular dependence of the dHvA frequencies, we obtained a detailed picture of the FS topology. In both compounds, the observed FS orbits agree well with predictions by density functional theory calculations. The electron and hole FS have a similar size being close to satisfy the nesting condition. This is an strong indication which favours the hypothesis that nesting between electron and hole pocket is an important condition for superconductivity to appear in the iron based superconductors. From measurements of the temperature dependence of the dHvA oscillations, we determined the mass enhancements in the different FS orbits. The effective masses measured in LiFeAs (Tc=18K), are larger than in LiFeP (Tc=5K), suggesting that superconductivity in these compounds is driven by the same interaction responsible for the mass renormalization. In LiFeP, the mass enhancement on the different orbits is observed to be less uniform than in LiFeAs which strongly indicates that pairing interaction is induced by k-dependent spin fluctuations. This is also likely related to origin of the different nodal and nodeless behaviour found, respectively, in the superconducting gap structure of these compounds. This contribution has been published in Physical Review Letters [PRL 108, 047002 (2012)] and reviewed in ’The 2011 Highlights Issue’ of MagnetLab Reports by the NHMFL (USA).
The goal of the studies in BaFe2(As1-xPx)2 was to elucidate the nature of the changes in the electronic structure as the putative quantum critical point (QCP) at x=0.3 was approached. To this end a detailed study of the dHvA effect as a function of doping x was undertaken along with a complementary study of specific heat. The specific heat measures the interaction induced quasiparticle mass enhancements integrated across all the Fermi surface. On the other hand the dHvA effect gives information about specific quasiparticle orbits and therefore is momentum resolved. Our dHvA study revealed a substantial logarithmic-type enhancement of the effective mass in the electron pocket when approaching the QCP which is consistent with the results from heat capacity measurements. A quantitative comparison between the mass enhancements obtained from heat capacity, dHvA studies and previous penetration depth measurements has shown that the mass enhancements resulting from the different studies are in great quantitative agreement. This is surprising because these techniques are sensitive to correlations over different part of Fermi surface and, thus, evidences that the mass is uniformly enhanced on all Fermi surface sheets. This work has given the first thermodynamic evidence for the existence of a quantum critical point (QCP) at x=0.3 which affects the majority of the Fermi surface by enhancing the quasiparticle mass. This is also the point where Tc reaches the maximum value and strongly indicates that quantum critical fluctuations drive the high temperature superconductivity in this system by mass renormalization. This work has been recently published in Physical Review Letters [PRL 110, 257002 (2013)].
During the project, the superconducting properties of other pnictide superconductors have been investigated. Systematic studies of the heat capacity in annealed samples of LaFePO were made under oxygen and argon atmospheres. The high quality single crystals of LaFePO used for this study were grown during a two month stay at the University of Stanford. A substantial change in the superconducting properties were found as a function of the annealing conditions, but the insensitivity of the change to the annealing atmosphere indicates that changes in doping is not the primary cause. Rather it indicates that the superconductivity is modified by disorder which is consistent with the nodal gap structure found previously in this compound. This work is being continued after the end of the project to include a detailed study of the dHvA effect as a function of the annealing conditions which will give more definitive evidence as to the mechanism.
Summarizing, the work performed during this project has allowed to significantly advance in the ultimate goal of this research: the better understanding of the high temperature superconductivity in the iron-based superconductors. It is seems clear now that mass renormalization is responsible for the enhancement of the superconducting critical temperature in iron-based superconductors which is, in some systems, accompanied by the presence of a quantum critical instabilities. We expect that our results will impact other studies in these and other related materials and provide for new fresh input in the everlasting effort of obtaining new materials for future technologies based on superconductors.
The discovery in 2008 of iron-based superconductivity at high temperature marked an exciting turning point in the study of high temperature superconductivity (HTS). Compared with the cuprates, their metallic ground state, relative malleability and electronic isotropy, unexpectedly large upper critical field and strong native vortex pinning make them useful for candidates for applications. However, from the present point of view, the most important aspect of these materials is that they present a new route to investigate the mechanism which cause high temperature superconductivity: giving new insight into iron based and the older copper based materials. It is widely believed that the unique topology of the Fermi surface of the iron pnictide superconductors is a crucial ingredient. For this reason knowing the fine details of the Fermi surface (FS) topology and its tendency towards instabilities is a fundamental step for understanding the origin of superconductivity in these compounds.
Within this project, we proposed a series of experiments to gain an understanding of the relationship between the band-structure and the superconducting properties in the iron based superconductors. In particular, the project has been mainly focused on the recently discovered P-doped arsenides BaFe2(As1-xPx)2 and LiFeAs/LiFeP. These are interesting systems where this problem can be addressed in a controlled and systematic way since their magnetic and superconducting properties evolve substantially as a function of As/P cross-substitution. These are particularly clean because systems the substitution of As by the isovalent ion P dramatically changes their electronic properties without changing the electron-hole balance and without inducing appreciable scattering. The series of the 122 pnictides BaFe2(As1-xPx)2 goes from antiferromagnetism (x=0) to superconductivity and then to a non superconducting paramagnetic state (x=1). There is substantial evidence for a quantum critical point at the x=0.3 where the superconducting transition is maximal. The 111 family of iron pnictides is unique because both LiFeAs and its counterpart LiFeP are nonmagnetic and superconduct with Tc of around 18 K and 5 K, respectively.
To perform a comprehensive study of fundamental electronic properties of the P-doped arsenides, in both normal and superconducting state, we have used three versatile and powerful experimental techniques, de Haas van Alphen (dHvA), London penetration depth and specific heat measurements that are important and well established methods to measure the Fermi surface and the superconducting gap structure. The experiments have been carried out at temperatures down to the milikelvin regime and under very high magnetic fields up to 80 T using the facilities available in Bristol as well as through measurements in international high field facilities such as the LNCMI in Toulouse (France), the HLD in Dresden (Germany) and the NHMFL in Tallahassee (USA). The experiments were performed in collaboration with A. Carrington, C. Putzke (Bristol), A. Coldea, M.Watson (Oxford) and local contacts at the high field facilities. Samples were provided by the group of Y. Matsuda (Kyoto).
In the 111 iron pnictide superconductors we studied the electronic properties in the normal state of the two end members of the series LiFeP and LiFeAs through measurements of the dHvA effect. By following the angular dependence of the dHvA frequencies, we obtained a detailed picture of the FS topology. In both compounds, the observed FS orbits agree well with predictions by density functional theory calculations. The electron and hole FS have a similar size being close to satisfy the nesting condition. This is an strong indication which favours the hypothesis that nesting between electron and hole pocket is an important condition for superconductivity to appear in the iron based superconductors. From measurements of the temperature dependence of the dHvA oscillations, we determined the mass enhancements in the different FS orbits. The effective masses measured in LiFeAs (Tc=18K), are larger than in LiFeP (Tc=5K), suggesting that superconductivity in these compounds is driven by the same interaction responsible for the mass renormalization. In LiFeP, the mass enhancement on the different orbits is observed to be less uniform than in LiFeAs which strongly indicates that pairing interaction is induced by k-dependent spin fluctuations. This is also likely related to origin of the different nodal and nodeless behaviour found, respectively, in the superconducting gap structure of these compounds. This contribution has been published in Physical Review Letters [PRL 108, 047002 (2012)] and reviewed in ’The 2011 Highlights Issue’ of MagnetLab Reports by the NHMFL (USA).
The goal of the studies in BaFe2(As1-xPx)2 was to elucidate the nature of the changes in the electronic structure as the putative quantum critical point (QCP) at x=0.3 was approached. To this end a detailed study of the dHvA effect as a function of doping x was undertaken along with a complementary study of specific heat. The specific heat measures the interaction induced quasiparticle mass enhancements integrated across all the Fermi surface. On the other hand the dHvA effect gives information about specific quasiparticle orbits and therefore is momentum resolved. Our dHvA study revealed a substantial logarithmic-type enhancement of the effective mass in the electron pocket when approaching the QCP which is consistent with the results from heat capacity measurements. A quantitative comparison between the mass enhancements obtained from heat capacity, dHvA studies and previous penetration depth measurements has shown that the mass enhancements resulting from the different studies are in great quantitative agreement. This is surprising because these techniques are sensitive to correlations over different part of Fermi surface and, thus, evidences that the mass is uniformly enhanced on all Fermi surface sheets. This work has given the first thermodynamic evidence for the existence of a quantum critical point (QCP) at x=0.3 which affects the majority of the Fermi surface by enhancing the quasiparticle mass. This is also the point where Tc reaches the maximum value and strongly indicates that quantum critical fluctuations drive the high temperature superconductivity in this system by mass renormalization. This work has been recently published in Physical Review Letters [PRL 110, 257002 (2013)].
During the project, the superconducting properties of other pnictide superconductors have been investigated. Systematic studies of the heat capacity in annealed samples of LaFePO were made under oxygen and argon atmospheres. The high quality single crystals of LaFePO used for this study were grown during a two month stay at the University of Stanford. A substantial change in the superconducting properties were found as a function of the annealing conditions, but the insensitivity of the change to the annealing atmosphere indicates that changes in doping is not the primary cause. Rather it indicates that the superconductivity is modified by disorder which is consistent with the nodal gap structure found previously in this compound. This work is being continued after the end of the project to include a detailed study of the dHvA effect as a function of the annealing conditions which will give more definitive evidence as to the mechanism.
Summarizing, the work performed during this project has allowed to significantly advance in the ultimate goal of this research: the better understanding of the high temperature superconductivity in the iron-based superconductors. It is seems clear now that mass renormalization is responsible for the enhancement of the superconducting critical temperature in iron-based superconductors which is, in some systems, accompanied by the presence of a quantum critical instabilities. We expect that our results will impact other studies in these and other related materials and provide for new fresh input in the everlasting effort of obtaining new materials for future technologies based on superconductors.