Collapse of metallicity and high- T c superconductivity in the high-pressure phase of FeSe 0.89 S 0.11

We investigate the high-pressure phase of the iron-based superconductor FeSe0.89S0.11 using transport and tunnel diode oscillator studies using diamond anvil cells. We construct detailed pressure-temperature phase diagrams that indicate that the superconducting critical temperature is strongly enhanced by more than a factor of four towards 40 K above 4 GPa. The resistivity data reveal signatures of a fan-like structure of non-Fermi liquid behaviour which could indicate the existence of a putative quantum critical point buried underneath the superconducting dome around 4.3 GPa. With further increasing the pressure, the zero-field electrical resistivity develops a non-metallic temperature dependence and the superconducting transition broadens significantly. Eventually, the system fails to reach a fully zero-resistance state, and the finite resistance at low temperatures becomes strongly current-dependent. Our results suggest that the high-pressure, high-Tc phase of iron chalcogenides is very fragile and sensitive to uniaxial effects of the pressure medium, cell design and sample thickness. This high-pressure region could be understood assuming a real-space phase separation caused by nearly concomitant electronic and structural instabilities

Unveiling the quasiparticle behaviour in the pressure-induced high-Tc phase of an iron-chalcogenide superconductor

Superconductivity of iron chalocogenides is strongly enhanced under applied pressure yet its underlying pairing mechanism remains elusive. Here, we present a quantum oscillations study up to 45 T in the high-Tc phase of tetragonal FeSe0.82S0.18 up to 22 kbar. Under applied pressure, the quasi-two dimensional multi- band Fermi surface expands and the effective masses remain large, whereas the superconductivity displays a three-fold enhancement. Comparing with chemical pressure tuning of FeSe1−xSx, the Fermi surface enlarges in a similar manner but the effective masses and Tc are suppressed. These differences may be attributed to the changes in the density of states influenced by the chalcogen height, which could promote stronger spin fluctuations pairing under pressure. Furthermore, our study also reveals unusual scattering and broadening of superconducting transitions in the high-pressure phase, indicating the presence of a complex pairing mechanism

Unconventional localization of electrons inside of a nematic electronic phase

The magnetotransport behavior inside the nematic phase of bulk FeSe reveals unusual multiband effects that cannot be reconciled with a simple two-band approximation proposed by surface-sensitive spectroscopic probes. In order to understand the role played by the multiband electronic structure and the degree of two-dimensionality, we have investigated the electronic properties of exfoliated flakes of FeSe by reducing their thickness. Based on magnetotransport and Hall resistivity measurements, we assess the mobility spectrum that suggests an unusual asymmetry between the mobilities of the electrons and holes, with the electron carriers becoming localized inside the nematic phase. Quantum oscillations in magnetic fields up to 38 T indicate the presence of a hole-like quasiparticle with a lighter effective mass and a quantum scattering time three times shorter, as compared with bulk FeSe. The observed localization of negative charge carriers by reducing dimensionality can be driven by orbitally dependent correlation effects, enhanced interband spin fluctuations, or a Lifshitz-like transition, which affect mainly the electron bands. The electronic localization leads to a fragile two-dimensional superconductivity in thin flakes of FeSe, in contrast to the two-dimensional high-Tc induced with electron doping via dosing or using a suitable interface.</p

Unconventional localization of electrons inside of a nematic electronic phase

The magnetotransport behaviour inside the nematic phase of bulk FeSe reveals unusual multiband effects that cannot be reconciled with a simple two-band approximation proposed by surface-sensitive spectroscopic probes. In order to understand the role played by the multiband electronic structure and the degree of two-dimensionality we have investigated the electronic properties of exfoliated flakes of FeSe by reducing their thickness. Based on magnetotransport and Hall resistivity measurements, we assess the mobility spectrum that suggests an unusual asymmetry between the mobilities of the electrons and holes with the electron carriers becoming localized inside the nematic phase. Quantum oscillations in magnetic fields up to 38 T indicate the presence of a hole-like quasiparticle with a lighter effective mass and a quantum scattering time three times shorter, as compared with bulk FeSe. The observed localization of negative charge carriers by reducing dimensionality can be driven by orbitally-dependent correlation effects, enhanced interband spin-fluctuations or a Lifshitz-like transition which affect mainly the electron bands. The electronic localization leads to a fragile two-dimensional superconductivity in thin flakes of FeSe, in contrast to the two-dimensional high-Tc induced with electron doping via dosing or using a suitable interface.Comment: 22 pages, 14 figure

Robust superconductivity and fragile magnetism induced by the strong Cu impurity scattering in the high-pressure phase of FeSe : Superconductivity tuned by pressure in Fe1-xCuxSe

The data were created while performing transport measurements on different single crystals of FexCu1-xSe (x~0.0025(4)) under applied pressure using a piston cylinder cell fitted in 16T PPMS. Measurements were performed as a function of temperature, magnetic field at different applied pressures. The data are standard ASCII files and they are part of a publication with the same title: Robust superconductivity and fragile magnetism induced by the strong Cu impurity scattering in the high-pressure phase of FeSe, by by Z. Zajicek, S. J. Singh, and A. I. Coldea to appear in Physical Review Research in 2022. The data is accompanied by the resulting Figures in PDF format which belong to the original publication

High pressure studies of iron-based superconductors

Unconventional superconductors are promising systems that can be used in practical applications, due to their large critical temperature, critical current densities and upper critical fields. This thesis investigates the electronic and superconducting behaviour of a new family of unconventional iron-chalcogenide superconductors tuned by applied hydrostatic pressure. I will present transport measurements, at low temperatures and high magnetic fields of different single crystals of the FeSe family, to understand their complex electronic phase diagrams under pressure. The parent compound shows an unexpected fourfold increase in superconductivity under pressure, whereas as a monolayer on a substrate it displays superconductivity above the liquid nitrogen temperature. Furthermore, bulk FeSe has an unusual electronic nematic phase, but applied pressure stabilises an additional magnetic order which manifests as an upturn in resistivity. Firstly, I report the effect of impurity scattering on the electronic behaviour induced by copper substitution in the conducting iron planes. This substitution is strongly disruptive to the nematic and superconducting phases which are quickly suppressed. The enhanced impurity scattering leads to a marked increase in resistivity and strongly reduced charge carrier mobilities, as determined from magnetotransport measurements. As the amount of impurities increases, both the suppression of critical temperatures and the temperature dependence of the upper critical field are consistent with a superconductor having a sign changing order parameter. In a subsequent study of Fe1−xCuxSe under pressure I find that, even in the presence of a small amount of Cu impurity, the high-Tc high pressure superconducting phase is robust. Surprisingly, no signatures of the pressure stabilised magnetic order are found in zero field, only in strong magnetic fields. In the next chapters, I investigate the phase diagrams of systems with isoelectronic substitution of sulphur for selenium outside of the iron planes. The first system, FeSe0.82S0.18, explores the electronic structure in the absence of nematicity, which displays a threefold enhancement in the superconducting transition temperature in the high pressure phase. Quantum oscillations in high magnetic fields and simulations probe the expansion and topology of the large Fermi surface under pressure. Despite the enhanced critical temperature under pressure, the quasiparticle effective masses are almost pressure independent. Notably, the quantum oscillations of a different system, FeSe0.96Se0.04, only display small frequencies in the high pressure phase. I find its phase diagram shows no signature of magnetic order outside of the nematic phase in zero field, but upturns in resistivity are observed inside the nematic phase in zero field and at high pressures in strong magnetic fields. Overall, the high pressure phase of all systems investigated display an enhanced superconducting phase, and highlight the sensitivity of the magnetic phase to chemical substitution and magnetic fields

Unveiling the quasiparticle behaviour in the pressure-induced high-Tc phase of an iron-chalcogenide superconductor

This data set corresponds to the generated data for the publication entitled with the same name to appear in npj Quantum Materials in 2024. The data were collected while measuring resistivity in high magnetic fields and under applied pressure for a novel iron-based superconductor, FeSe1-xSx (x~0.18). Most of the data are ASCII files related to the figures provided (often (x,y) format)

Unconventional localization of electrons inside of a nematic electronic phase: FeSe thin flakes magnetotransport

These are magnetotransport data on devices made of thin flakes of FeSe. The data were collected in a cryostat in magnetic fields as a function of temperature and at fixed temperatures the magnetic field was ramped to the maximum value both in the CFAS lab in Oxford and at the HMFL in Nijmegen. The magnetotransport data were collected using Hall bar geometries or a spider geometry. The data contain mainly ASCII files and the PDF figures are provided. This work is part of the publication "Unconventional localization of electrons inside of a nematic electronic phase" which will appear in PNAS 2022

Collapse of metallicity and high-Tc superconductivity in the high-pressure phase of FeSe0.89S0.11

We investigate the high-pressure phase of the iron-based superconductor FeSe0.89S0.11 using transport and tunnel diode oscillator studies using diamond anvil cells. We construct detailed pressuretemperature phase diagrams that indicate that the superconducting critical temperature is strongly enhanced by more than a factor of four towards 40K above 4GPa. The resistivity data reveal signatures of a fan-like structure of non-Fermi liquid behaviour which could indicate the existence of a putative quantum critical point buried underneath the superconducting dome around 4.3GPa. With further increasing the pressure, the zero-field electrical resistivity develops a non-metallic temperature dependence and the superconducting transition broadens significantly. Eventually, the system fails to reach a fully zero-resistance state, and the finite resistance at low temperatures becomes strongly current-dependent. Our results suggest that the high-pressure, high-Tc phase of iron chalcogenides is very fragile and sensitive to uniaxial effects of the pressure medium, cell design and sample thickness. These high-pressure region could be understood assuming a real-space phase separation caused by nearly concomitant electronic and structural instabilities