91福利

I will discuss recent magnetotransport data on an underdoped high-temperature superconductor. To assist with the discussion I will first describe how one goes about mapping the Fermi surface of quasi-two-dimensional materials using high magnetic field measurements, focussing particularly on the technique of angle-dependent magnetoresistance. This will be illustrated using the results of earlier experiments on an organic superconductor, for which a full determination of the Fermi surface was possible. I then will contrast this with the more challenging measurements performed on YBa2Cu3O6+x and explain what conclusions can be drawn in this case.

Solar Magnetism known for its 11-year cyclicity is driven by the shear, internal differential rotation in the solar convection zone, and the breakage of mirror-symmetry of turbulent convective motion (kinetic and magnetic helicity). The origin of helicity comes from background turbulent dynamo action as well as large-scale tilt of bipolar magnetic flux tubes, the building material for sunspots. We carry out quantitative computations of the amounts of helicity in bipolar solar active regions produced by tilting the flux tubes and compare with those produced by the global dynamo. We obtain that these amounts are comparable by the order of magnitude though opposite in sign. We conclude that in smaller active regions the dynamo-born helicity dominate its tilt-related part. The amount of helicity residing in the solar photosphere (that can be observed from available magnetographic data) can be considered as the remainder from the amounts of helicity produced and those leaving the Sun with CMEs and solar wind.
We give observational interpretation and support of the results.