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Instruments / Technical Area	Publication Details	Published
Theoretical Physics	FeSe and the missing electron pocket problem Luke C. Rhodes , Matthias Eschrig , Timur K. Kim , Matthew D. Watson Frontiers In Physics 10 DOI: 10.3389/fphy.2022.859017 Open Access Show Abstract ▼ Abstract: The nature and origin of electronic nematicity remains a significant challenge in our understanding of the iron-based superconductors. This is particularly evident in the iron chalcogenide, FeSe, where it is currently unclear how the experimentally determined Fermi surface near the M point evolves from having two electron pockets in the tetragonal state, to exhibiting just a single electron pocket in the nematic state. This has posed a major theoretical challenge, which has become known as the missing electron pocket problem of FeSe, and is of central importance if we wish to uncover the secrets behind nematicity and superconductivity in the wider iron-based superconductors. Here, we review the recent experimental work uncovering this nematic Fermi surface of FeSe from both ARPES and STM measurements, as well as current theoretical attempts to explain this missing electron pocket of FeSe, with a particular focus on the emerging importance of incorporating the dxy orbital into theoretical descriptions of the nematic state. Furthermore, we will discuss the consequence this missing electron pocket has on the theoretical understanding of superconductivity in this system and present several remaining open questions and avenues for future research.	May 2022
	k z selective scattering within quasiparticle interference measurements of FeSe Luke Rhodes , Matthew Watson , Timur K. Kim , Matthias Eschrig Physical Review Letters 123 DOI: 10.1103/PhysRevLett.123.216404 Show Abstract ▼ Abstract: Quasiparticle interference (QPI) provides a wealth of information relating to the electronic structure of a material. However, it is often assumed that this information is constrained to two-dimensional electronic states. We show that this is not necessarily the case. For FeSe, a system dominated by surface defects, we show that it is actually all electronic states with negligible group velocity in the z axis that are contained within the experimental data. By using a three-dimensional tight-binding model of FeSe, fit to photoemission measurements, we directly reproduce the experimental QPI scattering dispersion, within a T -matrix formalism, by including both k z = 0 and k z = π electronic states. This result unifies both tunnelling based and photoemission based experiments on FeSe and highlights the importance of k z within surface sensitive measurements of QPI.	Nov 2019
I05-ARPES	Scaling of the superconducting gap with orbital character in FeSe Luke C. Rhodes , Matthew D. Watson , Amir A. Haghighirad , Daniil Evtushynsky , Matthias Eschrig , Timur K. Kim Physical Review B 98 DOI: 10.1103/PhysRevB.98.180503 Diamond Proposal Number(s): [17532] Open Access Show Abstract ▼ Abstract: We use high-resolution angle-resolved photoemission spectroscopy to map the three-dimensional momentum dependence of the superconducting gap in FeSe. We find that on both the hole and electron Fermi surfaces, the magnitude of the gap follows the distribution of d y z orbital weight. Furthermore, we theoretically determine the momentum dependence of the superconducting gap by solving the linearized gap equation using a tight-binding model which quantitatively describes both the experimental band dispersions and orbital characters. By considering a Fermi surface only including one electron pocket, as observed spectroscopically, we obtain excellent agreement with the experimental gap structure. Our finding of a scaling between the superconducting gap and the d y z orbital weight supports the interpretation of superconductivity mediated by spin fluctuations in FeSe.	Nov 2018
I05-ARPES	Strongly enhanced temperature dependence of the chemical potential in FeSe L. C. Rhodes , Matthew Watson , A. A. Haghighirad , M. Eschrig , T. K. Kim Physical Review B 95 DOI: 10.1103/PhysRevB.95.195111 Diamond Proposal Number(s): [12799, 14454] Open Access Show Abstract ▼ Abstract: Employing a 10-orbital tight-binding model, we present a set of hopping parameters fitted directly to our latest high-resolution angle-resolved photoemission spectroscopy (ARPES) data for the high-temperature tetragonal phase of FeSe. Using these parameters, we predict a large 10 meV shift of the chemical potential as a function of temperature. To confirm this large temperature dependence, we performed ARPES experiments on FeSe and observed a ∼25 meV rigid shift to the chemical potential between 100 and 300 K. This strong shift has important implications for theoretical models of superconductivity and of nematic order in FeSe materials.	May 2017
I05-ARPES	Evidence for unidirectional nematic bond ordering in FeSe Matthew Watson , Timur Kim , Luke Rhodes , Matthias Eschrig , Moritz Hoesch , Amir Haghighirad , Amalia Coldea Physical Review B 94, 201107 DOI: 10.1103/PhysRevB.94.201107 Diamond Proposal Number(s): [10203, 11804, 12799] Open Access Show Abstract ▼ Abstract: The lifting of d xz -d yz orbital degeneracy is often considered a hallmark of the nematic phase of Fe-based superconductors, including FeSe, but its origin is not yet understood. Here we report a high resolution Angle-Resolved Photoemission Spectroscopy study of single crystals of FeSe, accounting for the photon-energy dependence and making a detailed analysis of the temperature dependence. We find that the hole pocket undergoes a fourfold-symmetry-breaking distortion in the nematic phase below 90~K, but in contrast the changes to the electron pockets do not require fourfold symmetry-breaking. Instead, there is an additional separation of the existing d xy and d xz/yz bands - which themselves are not split within resolution. These observations lead us to propose a new scenario of "unidirectional nematic bond ordering" to describe the low-temperature electronic structure of FeSe, supported by a good agreement with 10-orbital tight binding model calculations.	Nov 2016

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