Advanced Seminar on Condensed Matter Physics
KIP SR 01.404
Although there is cast iron evidence for a massive neutrino, obtained through the observation of neutrino oscillations, the absolute neutrino mass scale is yet unknown. Different approaches are pursued to fix this parameter, which is of importance for particle physics and cosmology. In contrast to the investigations of neutrinoless double beta decay and cosmological observations, single beta decay experiments allow for a direct, model-independent access to the neutrino mass scale. The next generation large-scale tritium beta decay experiment KATRIN (Karlsruhe Tritium Neutrino experiment) is designed to determine the anti-electron neutrino mass with a sensitivity of 200 meV/c2 (90% C.L.). It is currently being assembled at the Karlsruhe Institute of Technology, with a planned start of tritium operation in 2016. The seminar covers an overview of the different approaches to the absolute neutrino mass scale, focusing on the KATRIN experiment. An introduction to the working principle of KATRIN as well as an insight into the current experimental status, focusing on the recently commissioned spectrometer and detector section, will be given.
Nuclear spin clocks are the most accurate clocks (absolute scale) today and therefore very useful for probing physics beyond the Standard Model. Typically two spin species arrangements are used in a comagnetometer setup, in order to get ridge of magnetic field changes caused by the environment. In our experiments we use the two noble gas isotopes 3He and 129Xe. We measure the sinussodial varing (about 10 Hz) magnetization of the free induction decay of our polarized spin species within a very small and a very homogeneous magnetic field (400 nT) with low noise DC-SQUIDS. Depending on the setup, a new hypothetical interaction, sometimes called a fifth force, will cause tiny frequency shift in our setup. After presenting the principles and features of our comagnetometer, I will report on two searches where we are able to set new limits on hypothetical interactions. First we performed a direct search for axion like interactions. Axions are strong candidates for dark matter and therefore in the focus of interest. With a second setup we search for a Lorentz invariance violating preferred frame interactions in a Huges-Drever like experiment.
In my talk, I will describe the design and performance of a series of fast, precise current sensing noise thermometers. The thermometers have been fabricated with a range of resistances from 1.290 Ω down to 0.2 m Ω. This results in either a thermometer that has been optimised for speed, taking advantage of the improvements in superconducting quantum interference device noise and bandwidth, or a thermometer optimised for ultra-low temperature measurement, minimising the system noise temperature. With a single temperature calibration point, we show that noise thermometers can be used for accurate measurements over a wide range of temperatures below 4 K. Comparisons with a melting curve thermometer, a calibrated germanium thermometer and a pulsed platinum nuclear magnetic resonance thermometer are presented. For the 1.290 Ω resistance we measure a 1 % precision in just 100 ms, and have shown this to be independent of temperature.
Skyrmions are topologically stable spin textures with the spins pointing in all directions wrapping up a sphere. They emerge in chiral-magnets and arrange in a hexagonal lattice with typical lattice constants of several tens of nm. A detailed understanding of the collective spin excitations and damping mechanisms in these materials  is of great interest if one thinks about the application of the hexagonally ordered magnetic Skyrmion-lattice phase in spintronics and magnonics. We have used cryogenic broadband GHz spectroscopy based on a coplanar waveguide (CPW) and a vector network analyzer  to explore the magnetization dynamics across the entire magnetic phase diagrams of a metallic (MnSi), a semiconducting (Fe0.8Co0.2Si) and an insulating chiral magnet (Cu2OSeO3). The CPW excites and simultaneously probes the spin excitations in the helimagnets . For the metallic, semiconducting, and insulating chiral magnets the spin excitations occur at different GHz frequencies depending on the material and applied magnetic field. Still we provide a unified quantitative account of their field dependent resonance frequencies across the whole magnetic phase diagram. The universal behavior of these excitations sets the stage for purpose-designed applications based on the resonant response of chiral magnets with tailored electric conductivity offering an unprecedented freedom for integration with electronics.
Financial support by the DFG via SFB/TRR80 and the German excellence cluster “Nanosystems Initiative Munich (NIM)” is acknowledged.
 Y. Onose et al., PRL, 109, 037603 (2012)
 S. S. Kalarickal et al., JAP, 99, 093909 (2006)
 T. Schwarze et al., unpublished