David C. Lee

© The Author 2017.The observation of replica bands in single-unit-cell FeSe on SrTiO3 by angle-resolved photoemission spectroscopy has led to the conjecture that the coupling between FeSe electrons and the STO phonons are responsible for the enhancement of T c over other FeSe-based superconductors. However the recent observation of a similar superconducting gap in single-unit-cell FeSe/STO raised the question of whether a similar mechanism applies. Here we report the ARPES study of the electronic structure of FeSe/STO. Similar to the results in FeSe/STO, clear replica bands are observed. We also present a comparative study of STO and STO bare surfaces, and observe similar replica bands separated by approximately the same energy, indicating this coupling is a generic feature of the STO surfaces and interfaces. Our findings suggest that the large superconducting gaps observed in FeSe films grown on different STO surface terminations are likely enhanced by a common mechanism.

© 2016 Macmillan Publishers Limited.Layered transition metal dichalcogenides are ideal systems for exploring the effects of dimensionality on correlated electronic phases such as charge density wave order and superconductivity. In bulk NbSe 2 a CDW sets in at T CDW = 33 K and superconductivity sets in at T c = 7.2 K. Below T c these electronic states coexist but their microscopic formation mechanisms remain controversial. Here we present an electronic characterization study of a single two-dimensional layer of NbSe 2 by means of low-temperature scanning tunnelling microscopy/spectroscopy, angle-resolved photoemission spectroscopy, and electrical transport measurements. We demonstrate that 3 × 3 CDW order in NbSe 2 remains intact in two dimensions. Superconductivity also still remains in the 2D limit, but its onset temperature is depressed to 1.9 K. Our STS measurements at 5 K reveal a CDW gap of= 4 meV at the Fermi energy, which is accessible by means of STS owing to the removal of bands crossing the Fermi level for a single layer. Our observations are consistent with the simplified electronic structure of single-layer NbSe 2, thus providing insight into CDW formation and superconductivity in this model strongly correlated system.

A distinction between knowledge and belief is set out and justified at the end of Book V of Plato’s Republic. The justification is intended to establish the claim of the philosophers to rule in an ideal state. I set out the argument and explain why considerable disagreement remains about the nature of the distinction and the assumptions on which it rests. I discuss the main options for interpreting the justification, briefly assessing their strengths and weaknesses. I conclude with comments on recent developments, and by drawing attention to a neglected aspect of Plato’s distinction.

There are two different approaches to the Hippias Major. The first emphasises its conformity to a pattern, with the aim of uncovering a single argumentative structure common to several ‘Socratic’ dialogues. The second approach emphasises elements specific to the Hippias Major, including dramatic features such as character, with the aim of finding the best reading of the dialogue taken individually. I make use of the second approach to show that a careful reading of the dialogue by itself does not support the widely held view that Socrates’ interlocutor, the sophist Hippias, misunderstands the central question. I propose a new basis for the Socrates-Hippias exchange which gives an important role to the notion of explanation, and I show that this reading enables us to detect and analyse an argument presented against the position advocated by Hippias. I make a connection between the interpretation defended here and some broader issues about our understanding of the development of Plato’s philosophy.