Magnetic moment inversion at giant flux jump: dynamical property of critical state in type-II superconductors.

Experimental evidence of tremendous magnetic moment dynamical inversion, from metastable trapping state to the state with essentially the same moment oriented in the opposite direction, appearing during giant flux jump connected to thermomagnetic avalanche process in superconducting YBa2Cu3O7-δ single crystal, is presented. Magnetization inversion takes place in the system, without thermal contact between sample and sample holder, with a tremendous stored energy once the avalanche process is completed in quasi-adiabatic conditions. A model of magnetic moment inversion, caused by the jump between two metastable states of superconductor with the same energy storage, is presented and discussed in terms of the critical state with peculiar evolution of the critical-current spatial distribution. Importantly, knowledge of conditions of the appearance of such a phenomenon is crucial for applications of bulk superconductors as "permanent" magnets, for example, in superconducting levitation devices, etc.

Dimensions and spring constants of rectangular AFM cantilevers determined from resonance measurements.

We present a reliable numerical method that computes the spring constants of a rectangular Atomic Force Microscopy (AFM) cantilever when a small number of experimental resonance frequencies are known. By using a collection of multivariate polynomial regressions followed by an algorithm that solves a set of non-linear equations, the method finds first the values of the cantilever geometric parameters associated with the given set of resonances. The results are then used to determine the normal, longitudinal, and transversal spring constants through a second collection of regressions. Both data collections were built with finite element analysis of realistic rectangular AFM cantilevers. Experiments performed on commercial cantilevers were used to test the numerical method. The computed geometric parameters were confirmed with scanning electron microscopy, demonstrating that the method is viable, self-consistent, and accurate.

Biased four-point probe resistance.

The implications of switching the current polarity in a four-point probe resistance measurement are presented. We demonstrate that, during the inversion of the applied current, any change in the voltage V produced by a continuous drop of the sample temperature T will induce a bias in the temperature-dependent DC resistance. The analytical expression for the bias is deduced and written in terms of the variations of the measured voltages with respect to T and by the variations of T with respect to time t. Experimental data measured on a superconducting Nb thin film confirm that the bias of the normal-state resistance monotonically increases with the cooling rate dT/dt while keeping fixed dV/dT; on the other hand, the bias increases with dV/dT, reaching values up to 13% with respect to the unbiased resistance obtained at room temperature.