Nanoscale Physics
Thermo-Effect at Transition Surface Electron to Surface Polaron over Superfluid Helium Film on Structured Substrate. 
  V.A. Nikolaenko, S. S. Sokolov
B.Verkin Institute for Low Temperature Physics and Engineering of NAS of Ukraine, Prospekt Nauky, 47, Kharkov - 61103, Ukraine
E-mail: nikolaenko@ilt.kharkov.ua

The exchange interaction of electrons with a neutral matter is study now. The surface electrons on helium (SEs) can use as a research tool. SE mobility along is limited by interaction with the helium atoms in vapor and with ripplons. At large electric field and/or at enough vapor density the SE forms a surface polaron (SP) [1]. The large mass of the complex leads to drop its mobility caused hydrodynamic.   A mono-crystalline Si substrate 1 cm2  in square by 0.3 mm in thickness with pores of 2 μm is used for study transition. The substrate is covered the superfluid helium film ~ 20 nm.    The Sommer-Tanner measurement transport method is applied.
  
The mobility of SE in gas region is μg = 8·e / (3·π·ħ·σs·ng·γ) and the mobility of SP is μsp = 2·π·ħ·α5/2/ (η·m1/2·e2·Eeff3) (here σs is the electron scattering section on helium atom at density ng,  γ-1 is the effective Bohr’s radius of SE, α and η is the surface tension and the viscosity of liquid helium accordingly. The value Eeff  was ~ 7·105 V/m. In this work (left figure), the σ vs T smoothly decreases from value of ~ 10-8 sim at 1.75 K to 5.3·10-9 sim at 2.03K and at T = 2.04 K take a place SE-SP transition with drop of σ on 2 orders. The monitoring T vs t, (right) shows a temperature dip at transition.    From the energy balance of the SE-SP transition energy and of the helium heat capacity change, as  e·ΔV·N=CHe·m·ΔT, the value ΔT- dip is ~ 5 mK about ~ 10 s in time. Here ΔV~102 V is the SE-SP potential [2]; N ~ 1010  is number  electrons on substrate with developed square 25cm2; CHe  and m are the specific heat capacity (~1 J/g·K) and mass of He film (~2.5·10-5 g) accordingly. Notice, the magnitude of C the Cu (chamber and cell) or Si (substrate) is 5 – 7 orders lower helium one.     The effect can be related to effects like the electro-caloric or the magneto-caloric one.
1.	Monarkha Yu.P. Self-trapped electron surface states above a helium film //Phys. Nizk. Temp. – 1975, - 1, -P. 526–534 (in Russian). 
2.	Kovdrya Yu.Z., Mende F.F., Nikolaenko V.A. High-frequency conductivity of electrons above helium film in the regime of localization on substrate potential inhomogeneities// Phys. Nizk. Temp.,- 1984 -.10 (11), -P. 1129–1140 (in Russian).