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The recent emergence of two-dimensional layered materials - in particular the transition metal dichalcogenides - provides a new laboratory for exploring the internal quantum degrees of freedom of electrons and their potential for new electronics. These degrees of freedom are the real electron spin, the layer pseudospin, and the valley pseudospin. New methods for the quantum control of the spin and these pseudospins arise from the existence of Berry phase-related physical properties and strong spin-orbit coupling. The former leads to the versatile control of the valley pseudospin, whereas the latter gives rise to an interplay between the spin and the pseudospins. Here, we provide a brief review of both theoretical and experimental advances in this field. [ABSTRACT FROM AUTHOR]

Fe1+yTe1−xSex is characterized by its complex magnetic phase diagram and highly orbital-dependent band renormalization. Despite this, the behavior of nematicity and nematic fluctuations, especially for high tellurium concentrations, remains largely unknown. Here we present a study of both B1g and B2g nematic fluctuations in Fe1+yTe1−xSex (0 ≤ x ≤ 0.53) using the technique of elastoresistivity measurement. We discovered that the nematic fluctuations in two symmetry channels are closely linked to the corresponding spin fluctuations, confirming the intertwined nature of these two degrees of freedom. We also revealed an unusual temperature dependence of the nematic susceptibility, which we attributed to a loss of coherence of the dxy orbital. Our results highlight the importance of orbital differentiation on the nematic properties of iron-based materials. Despite a significant amount of research, there are still many unknowns about the underlying mechanisms of the superconductivity in Fe-based superconductors, in particular the roles of magnetism and nematicity. Here, the authors investigate the compositional dependence of the nematic susceptibility in Fe1+yTe1−xSex and the interplay between nematic and spin fluctuations, as well as the effect of orbital differentiation on nematic instability. [ABSTRACT FROM AUTHOR]