לוח שנה משולב

Amorphous solids, polymers, and disordered lattices show striking qualitative and quantitative similarities of e.g. their specific heat, thermal conductivity, and internal friction at temperatures below 3K This suggests the existence of a mechanism intrinsic to the disordered state of matter that dictates physical properties at low temperatures. The standard model within which this problem is treated is that of tunneling two-level systems (TLSs), introduced long ago by Anderson Halperin and Varma, and Phillips. Yet, key questions such as the nature of the TLSs, the mechanism dictating universality, and the energy scale dictating the range of the universal regime, are not yet understood. We propose here a model of two types of TLSs, (nearly) symmetric, and asymmetric with respect to inversion symmetry. The former interact weakly with the phonon field, yet gap the latter at low energies. Our model explains well the above and other puzzles related to universality, and may prove useful in treating other problems where TLSs play a crucial role, such as 1/f noise, ageing in glasses, and superconducting qubit decoherence.

Recently we could show that dynamic nuclear polarization (DNP) enhancements > 50 could be obtained in aqueous solutions at a magnetic field strength of 9.2 T (corresponding to a 1H NMR frequency of 400 MHz and a electron spin Larmor frequency of 260 GHz)1-3. This could be achieved with a home-build liquid DNP spectrometer, which consists of a gyrotron microwave source, holo-metallodielectric microwave transmission system and microwave bridge and most importantly a double-resonance structure (260 GHz / 400 MHz) to avoid sample heating4-6. To rationalize this surprisingly large enhancements, the individual factors describing the Overhauser DNP enhancement, as the leakage, saturation and coupling factors, have been individual evaluated and experimentally determined. Here we will show experimental results on TEMPOL nitroxide radicals dissolved in a series of liquids and compare the individual factors and the observed DNP enhancements with theoretical predictions from translational diffusion and MD calculations7.