Enhanced microwave absorption, larger than that in the normal state, is observed in fine grains of type-II superconductors (MgB2 and K3C60) for magnetic fields as small as a few % of the upper critical field. The effect is predicted by the theory of vortex motion in type-II superconductors, however its direct observation has been elusive due to skin-depth limitations; conventional microwave absorption studies employ larger samples where the microwave magnetic field exclusion significantly lowers the absorption. We show that the enhancement is observable in grains smaller than the penetration depth. A quantitative analysis on K3C60 in the framework of the Coffey–Clem (CC) theory explains well the temperature dependence of the microwave absorption and also allows to determine the vortex pinning force constant.

The spin relaxation time in solids is determined by several competing energy scales and processes, and distinct methods are called for to analyze the various regimes. We present a stochastic model for the spin dynamics in solids which is equivalent to solving the spin Boltzmann equation and takes the relevant processes into account on an equal footing. The calculations reveal yet unknown parts of the spin-relaxation phase diagram, where strong reversible spin dephasing occurs in addition to spin relaxation. Spin-relaxation times are obtained for this regime by introducing the numerical Loschmidt echo. This allows us to construct a generic approximate formula for the spin-relaxation time, τ s , for the entire phase diagram, involving the quasiparticle scattering rate, , spin-orbit coupling strength, L, and a magnetic term, Z due to the Zeeman effect. The generic expression reads ash/τ s ≈ L 2 /(2 + L 2 + 2 Z).

Microwave reflectance probed photoconductivity (or µ-PCD) measurement represents a contactless and non-invasive method to characterize impurity content in semiconductors. Major drawbacks of the method include a difficult separation of reflectance due to dielectric and conduction effects and that the µ-PCD signal is prohibitively weak for highly conducting samples. Both of these limitations could be tackled with the use of microwave resonators due to the well-known sensitivity of resonator parameters to minute changes in the material properties combined with a null measurement. A general misconception is that time resolution of resonator measurements is limited beyond their bandwidth by the readout electronics response time. While it is true for conventional resonator measurements, such as those employing a frequency sweep, we present a time-resolved resonator parameter readout method which overcomes these limitations and allows measurement of complex material parameters and to enhance µ-PCD signals with the ultimate time resolution limit being the resonator time constant. This is achieved by detecting the transient response of microwave resonators on the timescale of a few 100 ns during the µ-PCD decay signal. The method employs a high-stability oscillator working with a fixed frequency which results in a stable and highly accurate measurement.

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The entropy change during spin relaxation for a realistic model system is studied, whose spin dynamics can be handled with the Boltzmann equation. The time evolution of the von Neumann entropy is monitored during the process and is compared with the recently introduced concept of the Loschmidt echo envelope. The time evolution of the two quantities is remarkably similar which helps to distinguish reversible and irreversible changes to the ensemble spin state. The method is also demonstrated for a toy model of nuclear magnetic resonance, where the usual π spin echo is performed numerically, and the echo envelope also follows the time evolution of the von Neumann entropy. The numerical approach highlights the utility of the entropy concept in analyzing various processes which occur during spin relaxation.