Effective Conductivity of a Cloudy Ionosphere - Hydromagnetic Waves in the Magnetosphere and the Ionosphere - page 307

T=4 ° K

h ν

Spot mask
(positive or negative)

H 0

Magnetic field
H = 0~30 kGs

- - -

+ + +

Si sample

Electric field
~1V/cm + -

Fig. 10.4. A schematic drawing of the modeling experiment to clarify the influ-
ence of random inhomogeneities on the effective conductivity of anisotropic media.
Plates of crystal Si at the liquid helium temperature were placed into crossed elec-
tric ( E ≈ 1V / cm) and magnetic (0-30 kGs) fields. The samples were illuminated
through either positive (the background transparency is less than the spot trans-
parency) or negative ( in window (spot) is less than the background transparency)
spot masks

fields [7]. Essentially the idea was to create, in a structurally homogeneous
semiconductor, a stochastic distribution of current carriers by an illumination
through special masks. In those regions of the semiconductor that have been
illuminated, non-equilibrium carriers were created.
Non-uniform illumination of homogeneous semiconductors was used to
create an inhomogeneous concentration of carriers n ( r ) and in that way to
model the transport processes in an inhomogeneous ionosphere. One possible
way is to illuminate a thin semiconductor plate through a special masking
film in which different sectors have different transparencies. Then n ( r )inthe
plate is inversely related to the local density in the mask. Plates of crys-
tal silicon placed in liquid He were chosen as the study object. At helium
temperatures, concentrations of free carriers are totally determined by photo
excitation. By using varying masks, it was possible to change the variability
of inhomogeneity, the sizes, the forms and the distances between inhomo-
geneities in a sample. A schematic drawing of the experiment is shown in
Figure 10.4.
The measurements were taken on thin (500 µm) disk-shaped plates with
an aperture in the center. The outer diameter of the plate was 10 mm and the
inner 4 mm. A potential difference was applied to side surfaces of large and
small radii. The magnetic field was perpendicular to the plane of the plate
(see Fig. 10.5). This sample shape has been chosen in order to simulate the
situation of a closed Hall current. In this case, the Hall current flows around
the disk.