Quartz crystal resonators exhibiting extremely high Q-factors at cryogenic temperatures

5 MHz bulk acoustic wave resonators had already been immersed in cryostats in order to reach quality factors greater than tens of millions at 4 K. The cryostat has been substituted by a cryogenerator, which is easier to use. The first overtones of the three vibration modes of 10 MHz SC-cut electrodeless resonators have then been investigated, leading to extremely high Q- factors of about 130 x 10(6) at 31 MHz, measured at 3.3 K on an unexpected mode.