Abstract
Despite its disordered nature, the single-layered Bi-based cuprate Bi2Sr2CuO6+δ (Bi2201) offersseveral advantages for the study of high-temperature superconductivity. Its simple electronic
structure features a cylindrical, hole-like Fermi surface (FS) centred at (π, π) which evolves with
doping, transitioning from a Mott insulator to beyond the superconducting (SC) dome. Bi2201
cleaves easily along the ab-plane, making it accessible to bulk and surface-sensitive experiments,
and spans the entire doping phase diagram, from the pseudogap regime to the Fermi liquid
(FL) state. These properties, combined with its structural, chemical, and electronic complexity,
make Bi2201 an ideal platform for investigating the effects of disorder and the mechanisms
underpinning high-Tc superconductivity.
This thesis focuses on the role of intrinsic disorder in the strange metal (SM) state of
overdoped (OD) Bi2201, particularly its interplay with residual resistivity (ρ0) and the SC
transition temperature (Tc). The SM phase, marked by a T-linear resistivity persisting over
several decades, serves as a precursor to many of the cuprates’ enigmatic phenomena, including
the pseudogap (PG) phase and superconductivity. Surprisingly, our study Tc was found to be
robust against significant variations in ρ0, challenging predictions of the ‘dirty’ d-wave theory
(extension of BCS framework for disordered d-wave superconductor), which posits that disorder
suppresses the SC gap (∆), Tc, and superfluid density (ρs). These findings necessitate a critical
reassessment of the theory’s applicability to OD cuprates in general and suggest that our
understanding of disorder in these systems is incomplete.
To explore the origins of this deviation, transmission electron microscopy (TEM) was
utilized to investigate the microstructure of Bi2201 crystals, focusing particularly on identifying
extended defects and microcracks across a sample set with significant variations in ρ0. This TEM
analysis revealed a complex and unexpected microstructure to be consistently present in the
Pb-doped samples. Efforts were made to uncover the underlying causes of this microstructure
and establish a correlation with the observed variations in ρ0.
A pulsed-current setup, combined with high magnetic fields, was utilized at the High Field
Magnet Laboratory (HFML) in the Netherlands to further investigate the normal state of
Bi2201 beyond Tc. Intriguingly, suppression of the normal state below Tc revealed a re-entrant
insulating behaviour. To delve deeper into this phenomenon, the effects of short, high-density
current pulses on the low-temperature in-plane resistivity were studied, uncovering a surprising
influence on the re-entrant insulating behaviour. These findings prompted speculation about
the relationship between this behaviour and the microstructure observed in Pb-doped Bi2201
samples.
This thesis also examines the magnetotransport properties of Bi2201 under pulsed magnetic
fields up to 60 T, assessing the applicability of both novel and established scaling methods in
order to uncover the mechanisms which drive its electronic properties. Additionally, efforts were made to extend the doping range of Bi2201 single crystals beyond the SC dome into the FL
regime through high-pressure oxygen and ozone annealing. These investigations aim to provide
a broader understanding of the interplay between disorder, transport properties, and the SM
state, thereby contributing to the development of a theory for an extended quantum critical
regime.
| Date of Award | 9 Dec 2025 |
|---|---|
| Original language | English |
| Awarding Institution |
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| Supervisor | Nigel E Hussey (Supervisor) |