Research

Dilution refrigerator is a convenient tool to reach ultra-low temperature environment of<10mK , utilizing the ³He-⁴He phase separation. For electronic transport measurements, electron temperature should be the one to use when comparing to the theories. However, due to the decreasing electron-phonon interaction at ultra-low temperature, the electron temperature is usually higher than the refrigerator temperature typically measured with a resistor thermometer. In a completely untreated refrigerator, we have achieved a<10mK refrigerator temperature and a ~150mk electron temperature.

Filtering is an important method to cool down the electrons through cutting off the electromagnetic interference which is coupled to leads (see Low temperature filtering section). Heat sources including ground loop and vibrations have to be avoided as well. Moreover, the thermal link between sample and cold finger is another factor to affect the electron temperature. We finally achieved a 12mK electron temperature measurement environment and an electron temperature thermometry above 20mK. The lowest refrigerator temperature with all the filters and leads is 6mK.

Rotating sample holder can provide extra degree of freedom under a perpendicular magnetic field. This technique has been widely applied in studies of transport measurements. We have adapted the sample rotation system to a measurement probe of Leiden CF-CS81-600, which could provide a total magnetic field up to 10 T. The sample holder can continuously rotate at 200 mK, with an angle resolution of 0.1°. After the angle is settled, the minimum temperature of the sample holder is the same as the environment temperature. The base temperature to base temperature sample switch time is 10 hours.This system is presented as an application example by Attocube AG.

Hydrostatic pressure provides a method to control the lattice and band structure of materials. This technique has been applied in studies of pressure-induced quantum state, superconductivity transition and topological phase transition. We have adapted the hydrostatic pressure system to a measurement probe of Leiden CF-CS81-600. The base temperature to base temperature sample switch time is 10 hours.

The characterization of thermal properties is very useful for the investigation of new materials. We have developed devices capable of probing the in-plane thermal properties of thin samples in the order of microns. Using AC measurement technique, we can achieve temperature sensitivity as high as 0.01%.

We have realized a home-made setup to measure thermal conductance at ultra-low temperatures. Superconductor wires, Teflon and Kapton are used to reduce thermal leaks from the environment.

When widespread electromagnetic signal is transmitted to our samples through leads, it might heat up the sample at ultra-low temperatures if there is no filtering. We have used three different home-made low temperature filters in our dilution refrigerator (RC filters, Silver-Epoxy filter and Thermocoax filter, respectively as is shown in the picture). According to the test results from a vector network analyzer, Silver-Epoxy filter and Thermocoax filter can cut the radiation mainly from MHz to GHz, while RC filter cut EMI range mainly from KHz to MHz. In our fridge, these filters are connected in series, with another set of room temperature filtering.