mK温度量子电容测量技术

在量子系统中，宏观量子态的形成会引起载流子态密度和化学势的变化。这些载流子的量子效应所引起的等效电容称为量子电容。通过测量量子电容随磁场、载流子浓度等的变化可以研究量子相变等物理现象。相比于传统的输运测量方法，量子电容测量能够直接探测到不参与导电的载流子，可以用于研究诸如Wigner晶体等绝缘量子态的特性。同时该方法关注体系内局部区域内的载流子特性，其测量结果的量子效应更明显，对量子态的探测灵敏度更高。高精度量子电容的测量结果可以用于研究待测系统中的能级精细结构。逼近量子极限的电容测量可以研究待测体系中带电准例子的产生和湮灭。采用电极-待测材料-电极的三明治结构可以研究几乎绝缘的样品中出现的相变。.mK温度高精度量子电容测量具有极高的难度。样品电容值可能小至数十fF；而量子效应引起的电容变化约为总电容的1%量级；低温设备中的寄生电容远大于样品电容；必须使用mV量级的激励信号。

The formation of quantum phases usually leads to the variation of the chemical potential and the density of states at the Fermi energy. The equivalent capacitance describing these effects is called quantum capacitance, through which we can study quantum phase transitions as a function of changing magnetic field, charge density, etc. Quantum capacitance can detect compressible phases which are pinned by disorder and cannot be seen by conventional transport measurement, such as Wigner crystals and charge density waves. Secondly, the capacitance measurement probes local effects. Therefore, it's more sensitive than transport measurement when searching fragile quantum phases. High precision quantum capacitance measurements can be used to study mini-band structure in target system, or study the creation and annihilation procedure of charged quasiparticles, or study the quantum phase transitions in nearly-insulating materials when using gate-material-gate devices. High precision quantum capacitance measurement is extremely difficult. The quantum effects only cause a few percent variation of the ~10fF sample capacitance, which is several orders smaller than the ~100pF cable capacitance in the cryostat. Also only mV excitation can be used on samples so that the fragile quantum phases are not destroyed.