The diversity of electronic properties of matter continues to grow, for instance recently to include so-called topological insulators and a wide variety of two-dimensional states in layered materials like graphene. Careful control of temperature and magnetic field is standard and essential in their study, but another important thermodynamic variable, mechanical strain, is usually neglected. Strain can have enormous impact on what is measured, but generally samples are simply mounted or wired up with either no strain or random uncontrolled built-in strain. At the same time, it is rare that sensitive and powerful optical techniques are deployed at lower temperatures and in combination with other techniques such as electrical transport. The instrument to be developed in this project will address both of these issues, combining careful control of strain and free-space optics with more conventional measurement capabilities at temperatures down to below 0.1 K, using a helium dilution refrigerator. The team of PIs includes world-leading expertise in optical probes, application of strain using piezo transducers, and low temperature techniques. After completion and testing, the instrument will be operated as a cost center, and maintained with other facilities under the Education and Research program of the Clean Energy Institute at the University of Washington. It will be a key facility for research supported on multiple other grants, including a new NSF MRSEC at UW having a focus on phase transitions in two-dimensional materials. It will thus enhance training of students at UW in a wide range of materials physics and techniques. Although dilution refrigerators are standard facilities in physical science laboratories worldwide, this will be the first used for condensed matter studies in the US Pacific Northwest, filling an important gap and fostering quantum materials activity in the region.
The unique instrument to be developed under this MRI grant will combine real-time control of strain in the sample with a flexible confocal optical system with dc and ac (up to microwave) electrical transport capability at dilution refrigerator temperatures (below 100 mK) and high magnetic fields (14 T). This will enable a wide range of studies of regimes and phenomena in quantum materials that have not been accessible before, for example inducing and studying phase transitions between trivial and topological insulators, or symmetry breaking in exotic superconductors. Samples with dimensions down to the micron scale will be mounted on a custom low-temperature stage which allows three-axis positioning and ability to apply uniaxial strain. A silicon-chip cassette system with mechanical/piezoelectric drive and strain gauge technology is to be developed, properly attacking a number of challenges including thermal expansion of the piezo stacks, the need to quantify the strain, spatial constraints inside the magnet bore, maximizing optical field size with minimal distortion and aberration, vibration isolation and heat sinking, and heating due to internal friction and light absorption. Free-space optics will allow scanning, control of polarization, coherent optical spectroscopy with multiple beams, and ultrafast optical spectroscopy and photocurrent measurements. The stage will include electrical, microwave and fiber-optic access. Close attention will also be paid to minimize electrical, magnetic, vibrational and optical noise sources, to achieve high precision and resolution in all parameters and electron temperatures below 100 mK.
