Correlation effects in the iron pnictides

One of the central questions about the iron pnictides concerns the extent to which their electrons are strongly correlated. Here we address this issue through the phenomenology of the charge transport and dynamics, single-electron excitation spectrum, and magnetic ordering and dynamics. We outline the evidence that the parent compounds, while metallic, have electron interactions that are sufficiently strong to produce incipient Mott physics. In other words, in terms of the strength of electron correlations compared to the kinetic energy, the iron pnictides are closer to intermediately-coupled systems lying at the boundary between itinerancy and localization, such as V2O3 or Se-doped NiS2, rather than to simple antiferromagnetic metals like Cr. This level of electronic correlations produces a new small parameter for controlled theoretical analyses, namely the fraction of the single-electron spectral weight that lies in the coherent part of the excitation spectrum. Using this expansion parameter, we construct the effective low-energy Hamiltonian and discuss its implications for the magnetic order and magnetic quantum criticality. Finally, this approach sharpens the notion of magnetic frustration for such a metallic system, and brings about a multiband matrix t-J1-J2 model for the carrier-doped iron pnictides.Comment: 14 pages, 2 figures, discussions on several points expanded, published in the Focus Issue on Iron-Based Superconductor

Research on the Outlook and Solutions in the Development of Marathon Series in China

Today, Marathon Series rise gradually and grow fast, and their development brings undeniable influences, even can drive the development of a city in every aspect, playing an incredible catalytic role in the regional development. The author deeply explored the status and laws of the development of Marathon Series in our country through literature material laws, field investigation and expert interview survey and believes: Marathon Series can even stimulate enthusiasm among common people in running a marathon, can enhance communication and connection between cities, can better promote balanced development between each territory and plays a proactive role in the establishment of Marathon brand; for the future development of Marathon Series in our country, we should vigorously implement “Internet plus” strategies adapt to the era, prepare cooperation programs for the race well, make a plan as sufficiently as possible, actively carry out innovation and realize sustainable development, with a view to contributing fresh strength to the profound and long-lasting development of marathon business and even sport industry in our country as well as the Healthy China 2030 initiative

Is It Always Better to Be a Big Fish in a Little Pond?

We analyzed archival data of 4,005 students' actual exam scores during their high school in which they had been streamed into highversus low-ability classes. Results show that being in the high-ability classes can be either academically positive or negative, depending on the nature of the particular comparison