Advanced Seminar on Condensed Matter Physics
Sommersemester 2017URL zum ICS-Kalender dieses Seminars
KIP SR 01.404
Metallic magnetic calorimeters (MMCs) are low-temperature particle detectors that are currently strongly advancing the state-of-the-art in single particle detection. They are typically operated at temperatures T< 100mK and imposingly combine a fast signal rise time, an excellent energy resolution, a large energy dynamic range, a high quantum efficiency as well as an almost ideal linear detector response. For this reason, single-channel detectors and medium-size MMC arrays appear to be a key technology for a variety of applications requiring high-resolution and wideband energy-dispersive single-particle detectors. Famous examples are the investigation of highly-charged ions, the search for the neutrinoless double beta decay, the investigation of the electron neutrino mass, nuclear safeguards or mass spectrometry. But in spite of this big success, it can be anticipated that future experiments demand for massive multi-channel detector systems to facilitate imaging or position-sensitive measurements or to increase the overall statistics. To realize such systems, microfabrication techniques that allow to reliably manufacture thousands of virtually identical detectors as well as suitable readout techniques need to be established.
In this talk, we will motivate the anticipated need for MMC based multi-channel detector systems by highlighting several MMC based applications. We will then discuss the state-of-the-art in the microfabrication of MMCs which we successfully established during recent years and that allows to manufacture such large-scale detector arrays. Afterwards, we will put special emphasis on the development of SQUID based readout techniques, both for single-channel detectors as well as large-scale arrays.
Graphene is a one‐atom thick two‐dimensional monolayer material with amazing physical properties. Carrier mobilities higher than those of silicon raised great expectations for disruptive carbon‐based electronics. Graphene represents the ideal two‐dimensional electron gas with negligible spin‐orbit coupling (SOC) as well as hyperfine interaction, which are prerequisites for long electron spin lifetimes. Thus graphene is very appealing for applications in spintronics. The essential benchmark for spintronics devices, i.e. long electron spin lifetimes, has been theoretically predicted on the order of 1 ms. Experimental work using spin‐FET (Field Effect Transistor) or nonlocal spin‐valve measurement devices yielded early on spin lifetimes in the ps range. In 2011 we reached at least 2 ns at room temperature. It was concluded that extrinsic effects are responsible for this shortcoming due to imperfect device technology (exfoliation and handling in air; imperfect tunnel barriers for spin injection from ferromagnets into graphene; charged impurities inducing extrinsic SOC fields). Recent improvements of the device concept, e.g., by “flattening” graphene on top of an h‐BN flake, yielded at room temperature an increase of the carrier mobilities from 1.000 to 20.000 cm2/Vs, spin lifetimes of 12 ns and spin diffusion lengths of 31 μm. These results are encouraging, but yet leave room for improvement and alternative concepts. Our findings rule out previous scalings of spin lifetime vs. momentum scattering time or mobility, which favored a D’yakonov‐Perel’ spin scattering mechanism.