Wei Cryomagnetics Lab

Wei GROUP

Cyromagnetics Lab (MP 078)

These cryogenic equipment and high-field magnets enable us to subject novel materials to extreme thermodynamic conditions, in order to study various quantum phenomena of electrons in solids. For vibration and acoustic isolation, an underground stealth pit has been constructed to house our dewars and magnets. This pit is made of stainless-reinforced concrete, which is density-decoupled from the ground by sand, foam and rubber. Neumatic pistons inside the pit walls enable the cryomagnetics to be riding on air. This stealth infrastructure is important for such ultra-sensitive measurements as scanning tunneling microscopy (STM), which can achieve sub-angstrom image resolution.

Our ³He refrigerator is a custom system made by Janis Cryogenics. It is based on a charcoal sorption-pump design capable of quiescient operation below 300 mK for over 24 hours. The refrigerator has a sliding-seal cryostat for quick insertion into a 9-Tesla superconducting solenoid with a 3-inch bore. The cryostat has two side bores to enable sample manipulation and optical access. For this ³He refrigerator we have successfully developed an STM head capable of in-situ rotation relative to the magnetic field axis.

The left picture above shows the ³He cryostat without its vacuum can and sitting on a rack next to the stealth pit. The 9-Tesla magnet is inside the dewar which is sitting on an airtable suspended just above the pit. The middle picture shows a close-up of the cryostat, with the 3He pot on the bottom and the 4He pot on top. During cooldown, the 3He pot is sorption-pumped by a cold charcoal pot (above 4He pot), with all mechanical pumps turned off, thus allowing quiescent operation at base temperature (< 0.3K) with long hold time (> 24hr).

Our dilution refrigerator (DF) is a Kelvinox 400 system made by Oxford Instruments. It has a high cooling-power ³He/⁴He mixing chamber capable of reaching below 25mK with continuous circulation. To reduce acoustic noise, our main pumps are enclosed in a separate room. The 18-Tesla magnet has a 2-inch bore and is suspended below ground in a sub-basement environment. This DF has been the workhorse for our study of heavy fermion superconductors, using both electrical transport and Andreev reflection spectroscopy. For the latter, we have successfully developed a point-contract spectroscopy (PCS) setup for measurement down to 90mK and up to 12T. To minimize any Joule heating in our transport and PCS measurments, we use a high-sensitivity AC bridge for the resistance and a sub-microsecond pulse setup for the I-V spectroscopy.

The left picture shows our DF and dewar outside the pit. The 18-Tesla magnet is inside the dewar. The cryostat is suspended without its vacuum can and sample tail. The right picture shows the cryostat with the tail (bottom) attached to various cooling stages: from 75mK to 25mK by circulation of 3He gas; from 75mK to 1.5K by condensation of the 3He/4He mixture in a closed line; and from 4.2K to 1.5K through a pumped-4He pot. To ensure that the samples stays at the tail temperature, all the wiring used are also cold-anchored through the cooling stages.

We have constructed a fleet of probes for quick access to 4.2K by dipping into liquid ⁴He storage dewars or superconducting magnets. These probes are routinely used to characterize our samples, in either single-crystal, thin-film or nanostructured forms. These measurements include electrical resistivity, magnetic susceptibility and I-V spectroscopy, the latter using pulsed signals to minimize Joule heating. For acoustic isolation, the ⁴He dewars can be air-suspended inside our Stealth Pit. Mu-metal shrouds can also be used to shield the probes from stray magnetic fields. Several of our dipper probes are also exclusively designed to hold STM heads, in two different orientations, for both zero-field and in-field measurements down to 4.2K. These ⁴He STM's have successfully achieved atomic resolution on cleavable single crystals and imaging of vortex lattice on superconducting samples.

Our PPMS (Physical Properties Measurement System) setup is made by Quantum Design. It is a turnkey system capable of reaching 1.8 K and 14 Tesla. It has several sample probes and stages, complete with both the electronics and software for measuring: resistivity, magnetization, Hall effect, and heat capacity. This instrument is shared within the CMP group and maintained by our group technician.