Paper MN-TuP8
MEMS-Based, High-Resolution Nanocalorimeter for Characterizing Phase Transitions in Samples in the Sub-Microgram Range

Tuesday, October 31, 2017, 6:30 pm, Room Central Hall

High quality thermodynamic measurements are among the essential tools to investigate fundamental properties of materials. Novel materials are often only available in minute amount when first synthesised. Hence, it is of paramount importance to develop thermodynamic measurement techniques for small-sized samples. However, studies involving samples of sub-μg in masses are extremely difficult to perform. Conventional calorimeters are marked by the large heat capacity of the calorimeter cell (addenda), thus, not suitable for measuring sub-μg samples. Rapid development in the MEMS industry has led to the creation of nanocalorimeters [1]. Typically, these devices are constructed on bulk micromachined membranes, whose contribution to the device addenda is vanishingly small.

We have developed a MEMS-based, truly differential nanocalorimetry platform, which is capable of performing specific heat measurements on sub-μg of high-quality, single crystalline samples in a wide temperature range between 0.3 K – 400 K [2,3]. It operates according to a refined ac steady-state method, leading to both ultrahigh resolution (better than several in 10^-5) and superior absolute accuracy (1 – 2%) [2]. The calorimeter consists of two 150-nm-thick SiN_x windows, one serving as the sample cell while the other being the reference cell. Each calorimeter cell contains a GeAu thermometer, a titanium ac heater, and an offset heater. The annealed GeAu thermometer shows an almost temperature-independent relative sensitivity |dlnR/dlnT| ~ 1, covering the whole temperature span of interest.

We demonstrate the capability of our nanocalorimeter through measurements of high purity gallium samples with masses in the range of several hundred nanograms to a few micrograms. α-Ga, the stable polymorph of solid gallium, melts at 302.9 K. Upon cooling, significant supercooling occurs and it may solidify into the metastable β-phase. Compared with α-Ga, β-Ga retains significantly enhanced electron-phonon coupling, leading to an elevated superconducting transition temperature above 6 K. We show that the melting and solidification transition of gallium can be monitored in-situ on our nanocalorimeter utilizing the offset heater, and present specific heat data of both phases across the full temperature range. The high-resolution of our calorimetry scheme also allows in-depth characterization of the superconducting transition of β-Ga and the deduction of a number of important superconducting parameters.

[1] E. A. Olson et al., J. Microelectromech. Syst. 12 (2003) 355 – 364.

[2] S. Tagliati et al., Rev. Sci. Instrum. 83 (2012) 055107.

[3] Z. Diao et al., Phys. Rev. B 93 (2016) 014509.