The Princeton Nanoscale Microscopy Laboratory (PNML)
Located in the basement of Jadwin Hall, PNML is a state-of-the-art laboratory designed for high precision measurements in condensed matter physics. This space houses a host of different instruments including scanning tunneling microscopes (STM) that have been moved from the University of Illinois at Urbana-Champaign, where our group had been from 1998 to the summer of 2005. The Princeton space is designed with three ultra-quiet rooms, which consist of double wall enclosures built around a massive plinth that is floating to isolate the experiments. Such a design leads to substantial reductions of both acoustic noise (because of the double enclosure) and seismic vibration (by floating the massive 35 Ton floors). One of these ultra-quiet rooms also has radio frequency shielding to make it possible to carry out experiments which are sensitive to radio frequency interference. Overall, the combination of acoustic, vibrational, and RF isolation make this an ideal space for a host of experiments in physics, including high resolution microscopy and spectroscopy measurements with STMs. The lab space has been designed in collaboration with Wilson Architects.
Ultra-low Temperature Scanning Tunneling Microscope (ULT-STM)
Time-lapse photos of ULT-STM build
PNML’s newest experimental facility is the ultra-low temperature scanning tunneling microscope (ULTSTM), which is capable of investigating both single crystal samples and gated device samples at UHV pressures and at a base temperature of 10 millikelvin. This well-controlled sample environment enables us to perform extremely high-resolution (~50μeV), spectroscopic studies of the local electronic mechanisms responsible for a myriad of exotic superconducting and topological systems. Within this single, integrated design, a custom-built dilution refrigerator surrounds a cooled UHV tube chamber, which houses our STM, with specially designed electrical connections at low temperatures to connect to an interchangeable STM head module. The UHV tube chamber and surrounding cryostat hang from a two-stage passive vibration isolation system, consisting of a 7-ton floated granite instrumentation table upon a 30-ton concrete plinth platform. This vibration isolation system, in conjunction with the surrounding soundproof and RF-shielded infrastructure, facilitates the exceptionally quiet and stable conditions necessary for our surface-sensitive measurements.
Additionally, the microscope is outfitted with a superconducting vector magnetic, submerged within a 240L liquid He cryostat, capable of supporting 10 days of continuous operation. Our vector magnet is capable of inducing a 1T magnetic field in any direction, or a 9T magnetic field along the microscope’s z-axis, allowing us to perform high-field and directional-dependence studies of magnetism and superconductivity. This microscope is singular in these capabilities, which are necessary and sufficient for directly addressing the goals of this proposal.
The ULTSTM’s versatility stems from its modular design, which serves two primary purposes (see top right figure). First, it allows the microscope head module to move between low-temperature and room-temperature experimental stages at any time without compromising cryogenics or vacuum, maximizing our ability to efficiently identify the proper surface preparation and device fabrication methods before proceeding with high-resolution experiments at low temperatures. Second, and more importantly, it allows us to select from a gamut of sample holder designs and microscope capabilities, and to assemble this ideal experimental apparatus in situ. Microscope modularity enables us to take full advantage of a number of highly developed scanning probe technique, each with its own specific instrumentation tailored for measuring a particular physical quantity. This ability allows us to cater the functionality of our facility on-demand to perform the experiments best suited to each material of interest without risk of sample contamination or degradation.
Dilution Refrigerator Scanning Tunneling Microscope (DR-STM)
This state-of-the-art STM that is one of the few instruments capable of operating at dilution refrigerator temperatures (50 mK) and high magnetic fields (14 T), extending the parameter space of the phenomena to which PNML has access. The system has been designed to allow for the in situ preparation of tips and samples using standard UHV tools, including the ability to heat samples and tips to > 1900C, Ar-ion sputtering, and depositing metals both at room temperature and cold. The instrument has the capability of taking spectroscopic data with exceptional resolution, and topographic data with atomic resolution, throughout the accessible phase space of temperature and magnetic field. We're currently exploiting the expanded phase space to look at the superconducting ground state of the heavy fermion compound CeCoIn5. Future plans include functionalizing the STM tip to detect magnetic signals, and exploiting rf techniques to achieve nanosecond time resolution, permitting the study of single spin dynamics.
Variable Temperature Scanning Tunneling Microscope (VT-STM)
We have constructed two variable temperature STMs capable of performing measurements in ultra-high vacuum (UHV) from room temperature down to 10 K. These system are equipped with both sputtering and evaporation sources for in-situ preparation of ultra-thin films. A great deal of care has gone into the design of this system to ensure thermal compensation, so as to avoid the typical problems associated with thermal drift. This instrument has been specifically designed to be able to track the same area of a sample surface at atomic scales as we vary the temperature. In this system we also have the added versatility by allowing the exchange of STM tips without venting the UHV system. This apparatus, with its ability to perform STM over a wide range of temperatures, is being used extensively in our investigations of correlated electronic states. We plan to continue to use these instruments to study a variety of phase transition phenomena.
Molecular Beam Epitaxy (MBE) instrument
Molecular beam epitaxy (MBE) is a process for growing thin, epitaxial films of a wide variety of materials, ranging from oxides to semiconductors to metals. It allows for controlled growth of 2D materials monolayer by monolayer, producing very clean, ordered samples for scanning tunneling microscopy (STM) experiments. The advantage of our new MBE system (completed December 2016) is that the PNML researchers will be able to precisely control and engineer sample growth while monitoring crystal structure through the diffraction pattern produced when an electron beam is reflected off of the sample. This allows custom engineering of new 2D material systems. The power of STM lies in imaging how electronic wavefunctions vary spatially on a surface with atomic resolution. Combined with the MBE, STM becomes a powerful tool to study topological superconductors and other phenomena like the quantum Hall effect which require very clean 2D films. The MBE’s vacuum chamber attains one of the purest laboratory vacuums achievable.