STUDY OF PHONON TRANSMISSION AT THE SOLID AND LIQUID INTERFACE OF ^3He

设计并制作了在35atm ^3He熔化压力下工作的声波池。在声波电池中，氧化锌用作换能器。为了提高^3He的传输效率，在换能器和^3He之间溅射双层增透涂层。首先，我们观察到在400MHz频率下，零声在^3He液体中传播到约2 mK的温度。这是第一次在400MHz的高频率上观察到零声。在2mK温度下，声量子具有比热激发能更高的能量，因此在准粒子碰撞中会发生声量子的吸收。根据朗道-费米液体理论，这种状态下的吸收与频率的平方成正比，与温度无关。从我们的实验结果来看，在绝对零度下，吸收是有限的。估计可以观察到零声吸收的量子极限。我们观察了声音在固体中的传播，以此来测试我们的系统。在低功率状态下，声信号通过固体的传播随输入声功率的增加而增加。当声功率增加时，发生非线性吸收，声信号通过固体^4He传播达到饱和。这一事实证实了我们的系统有足够的灵敏度和功率来观察声音通过固体He的传播。在我们的电池中，我们尝试在传感器之间制造固体和液体界面，但很难控制和稳定界面。目前还没有成功地观测到在界面处传播或反射的声信号。

The sonic cell was designed and made, which bore under the ^3He melting pressure of 35 atm. ZnO is used as the transducer in the sonic cell. In order to increase transmission efficiency into ^3He, double layred anti-reflection coating is sputtered between the transducer and ^3He.First, we observed the propagation of zero sound in liquid ^3He down to temperature of about 2 mK at a frequency of 400MHz. This is the first observation of zero sound at the high frequency of 400MHz. In the 2mK temperature regime, the sound quanta has higher energy than thermal excitation energy so that the absorption of sound quanta takes place in the collisions of quasiparticles. According to Landau Ferimi liquid theory, the absorption in this regime is proportional to the square of frequency and independent of temperature. From our experimental result, the absorption is seen to remain finite at absolute zero temperature. The quantum limit of zero sound absorption is estimated to be observed.We observed sound propagation in solid ^4He as a test of our system. The sound signal propagating through solid ^4He increased inearly with input sound power in the low power regime. when the sound power was increased, nonlinear absorption occurred and the sound signal propagating through solid ^4He was saturated. This fact confirmed that ur system had enough sensitivity and power to observe sound propaation through solid He.We have tried to make solid and liquid interface between the transducer in our cell but it was hard to control and stabilize the interface. It has not been succeed to observe the sund signal which ropagates through or is reflected at the interface.