Project

muon spin rotation/relaxation; dilution refrigerator; high magnetic fields; unconventional superconductors; quantum magnetism; strongly correlated electrons; magnetic semiconductors; Superconductivity; Magnetism; Muon Spin Rotation/Realaxation; Low Temperatures

Lead
Lay summary
The response of a physical system to external stimuli (such as pressure, magnetic fields,..) is a powerful tool for studying the intrinsic properties of matter. For example an externally applied magnetic field directly couples to electronic charges and magnetic moments of the constituents and can be used to dramatically alter the physical properties of matter and to drive the material into different (and possibly exotic) states, thus providing unique insight into the material's ground states.A large fraction of the worldwide contemporary condensed matter research is focused on the study of strongly correlated materials with novel electronic properties and exhibiting complex phase diagrams.Extraordinary properties emerge near so-called quantum phase transitions (or quantum critical points) appearing in systems with competing interactions. The occurrence of quantum phase transitions can be tuned by external parameters as pressure and/or magnetic fields.Many complementary techniques can be applied to unravel the fundamental mechanisms of these processes, but among them a key role is devoted to the so called microscopic techniques. Muon Spin Rotation/Relaxation makes use of positive polarized muons which implanted in the sample are extremely sensitive local probes. For instance ?SR is the ideal tool to trace down spin fluctuations and weak magnetic order on local scale at the vicinity of quantum critical points. The last years have seen a surge of studies making use of the muon-spin spectroscopy techniques (?SR) and Switzerland, hosting the most intense muon source worldwide (S?S at PSI), is a leading player of the game.The proposed custom designed dilution fridge will be used on the new High-Field ?SR instrument at S?S and will address these experimental constraints. The very-low temperature capability on the High-Field ?SR instrument will greatly enhance the ability at the Paul Scherrer Institute to investigate strongly correlated systems and is of high interest to many researchers in Switzerland and abroad.The possibility for performing local measurements at very high magnetic fields (up to 10 Tesla) and simultaneously at very low temperatures (to the mK range) would enable entirely new research possibilities. Examples are studies of the phase diagram of oxyde and unconventional superconductor systems, investigations of coupling of magnetic and superconducting order, Bose-Einstein condensation in magnetic systems, vortex matter or magnetic field Induced quantum phase transitions.

Direct link to Lay Summary

Last update: 21.02.2013

Number	Title	Start	Funding scheme

We propose the purchase of a custom designed dilution refrigerator for the new high field muSR instrument. This novel instrument will be located at the Swiss Muon Source at the Paul Scherrer Institute. Similar to all the other instruments of our facility it is open, free of charge, to scientific visitors from Switzerland and abroad, who have submitted a proposal and obtained approval from the user committee.Extensive cryogenic know-how is available at PSI. This will ensure that the dilution refrigerator and the instrument will be well maintained. They will be available for experiments with the support of in-house instrument scientists. We expect that the dilution refrigerator and the high-field instrument will be used for at least 20 years.The high-field instrument will generate fields (transverse and longitudinal to the muon spin polarization) up to 10T. The dilution refrigerator will allow temperatures as low as 25mK. The entire instrument including the cryostat has been specifically designed based on a detailed simulation of a muon spin rotation experiment in a high magnetic field and low temperature environment. To obtain minimum impact on the muon trajectory and highest possible decay positron detection efficiency, the magnet is very compact and has horizontal warm bore. Equipping the magnet with a dilution refrigerator is crucial for the experiments included in this proposal. The dilution refrigerator is inserted horizontally in one of the two ports. It has its own vacuum vessel with dimensions (e.g. outer radius) optimized to allow for an efficient detection of the decay positrons. The positrons are detected by a segmented detection system surrounding the sample chamber. The system has been specially developed at PSI to fulfill all the requirements for a high field, low temperature measurement (high time resolution, small detector radius) [1,2]. The instrument with the dilution fridge will be available at a dedicated surface muon beam line an PSI.The proposed instrument with dilution refrigerator will allow new experiments in a wide range of scientific fields. These include quantum and frustrated magnetism, unconventional and high-temperature superconductors (including the recently discovered iron-based superconductors), multiferroic materials, magnetic and superconducting organic materials and magnetic semiconductors This proposal includes a few specific projects in these fields, that illustrate the usefulness and importance of this instrument.[1] K. Sedlak et al. Physica B (2009), in press: http://dx.doi.org/10.1016/j.physb.2008.11.215.[2] A. Stoykov et al., to be published in Nucl. Instruments and Methods A (2009).