LX12358

Pendulum in Fermi liquid

When a pendulum is immersed in a liquid, its oscillation frequency is
generally reduced because of the flow of the liquid around the bob.
This is not necessarily the case in a Fermi liquid. In ordinary
liquids atoms or molecules continuously collide with each other. This
leads to standard hydrodynamic description of the fluid, which is
essentially similar as in water. A Fermi liquid is different because
at low temperatures the bob of the pendulum excites quasiparticles
which are ballistic, i.e. they fly off like a ball hit by a bat. In
spite of being ballistic, the quasiparticles interact with each
other, as argued by Lev Landau in 1957. We show that for attractive
interactions, the frequency of the pendulum is increased by immersing
it in a the Fermi liquid. Such a case is realized in isotopic
mixtures of helium (3He & 4He). Experiments made in this system show
evidence of the frequency increase. This "Landau force" is a new
application of Landau's theory, which is one of the central paradigms
of condensed matter physics.

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LY12754

A phonodiode for detection of ultrashort acoustic pulses

We describe an all-electrical method for the detection of ultrashort acoustic pulses using a semiconductor device. The device could be named a ‘phonodiode’ in analogy with a photodiode used for detecting light. Such devices could find applications in high speed acoustically-driven switching of electronic circuits, direct conversion of terahertz sound to terahertz electrical signals, and heterodyne mixing of terahertz acoustical and electrical signals.

There has in recent years been a rapid expansion in the area of phononics, which is concerned with the physics and applications of sub-terahertz coherent phonons (the quanta of vibration, or acoustic, energy). This has been driven by development of femtosecond laser-based techniques for generating picosecond-duration acoustic phonon pulses. The traditional method of detecting the acoustic signals is to use an optical probe. We demonstrate high-speed electrical detection of the acoustic pulses using a metal-semiconductor contact (Schottky diode). We show that the device detects, with good sensitivity and temporal resolution, the acoustic pulses generated by femtosecond laser excitation of a metal film. A phonodiode, used in conjunction with an electrical method of generation of coherent phonon pulses, e.g. a saser, could make an integrated all electrical system for manipulations with coherent sub-terahertz phonon pulses.