Microwave-Resonance-Induced Resistivity: Evidence of Ultrahot Surface-State Electrons on LiquidHe3

Abstract

Measurements of the dc resistivity of surface-state electrons on liquid helium exposed to microwave radiation are reported. It is shown that the resonant microwave excitation of surface-state electrons is accompanied by a strong increase in their resistivity, which is opposite to the result expected from the previously used two-level model. We show that even a very small fraction of electrons excited to the first excited state and decaying back due to vapor-atom scattering strongly heat the electron system, causing a population of higher subbands. The calculated resistivity change is in good agreement with the observed data.

Photoresonance and conductivity of surface electrons on liquid He3

“…Under the resonance condition the effective electron temperature T e can be much higher than the ambient temperature T . Such decay heating of SEs appears already at very low excitation rates affecting electron conductivity [10] and the MW resonance linewidth [11]. The important point is that in the vapor atom scattering regime, SE conductivity at the MW resonance steadily decreases with power, and there is no the sign change of this effect at high excitation powers similar to that reported previously [7].…”

“…K, the electron collision frequency has a typical resonance structure with a maximum positioned at E^= 93 V/cm. Thus, under the condition, the MW resonance excitation leads to a decrease in SE conductivity, as expected for the vapor atom scattering regime [10]. In this case, the increase in the collision rate is caused by decay heating of SEs and their occupation of higher surface levels, where the inter-level collision rate is high.…”

“…It is remarkable that the outcome of the competition of these two factors strongly depends on the holding electric field. For typical holding fields used in our experiments ( E^~100 V/cm), the first factor dominates and the effective collision frequency n ( ) a increases with W without an observable sign of a decrease [10]. Nevertheless, our new calculations conducted for weak holding fields ( .…”

“…For the vapor atom scattering regime, it is proven that even a very small fraction of SE excited by the MW from the first to the second surface level can substantially heat the entire electron system [10,18]. This happens because the decay rate of electrons occupying the second surface level t This leads to a strong increase in electron temperature even at very low excitation rates r 12 21 3 10 t~-.…”

“…Recent conductivity measurements for SEs exposed to resonant MW radiation above liquid 3 He in the vapor atom scattering regime [10] indicate that heating of the electron system is the key occurrence of a MW resonance experiment with SEs. This heating occurs because of electron decay from the excited Rydberg level to the ground level induced by interaction with vapor atoms.…”