604 



THE BELL SYSTEM TECHNICAL JOURNAL, MAY 1956 



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TEMPERATURE IN DEGREES KELVIN 



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Fig. 26 — Illustration of cross over phenomenon for germanium samples con- 

 taining gallium. Sample 314 contains 9 X 10^* cm"^ gallium and sample 302 con- 

 tains 3 X 10'^ cm"^ Samples 316 and 301 are the corresponding samples to which 

 lithium has been added. 



ments were not carried to high temperatures.* In addition the value 

 of the Hall coefficient was carefully checked at each temperature to see 



if it had changed. Since the reciprocal of the Hall coefficient measures 

 the carrier density any reduction in its value would have implied loss of 

 compensation, or precipitation of lithium. 



Over the measured points no appreciable variation of Hall coefficient 

 was noted. Fortunately, the pairing relaxation time is quite small (less 

 than a second) at the high temperatures involved so that it wasn't 

 necessaiy to hold the samples at these temperatures for long periods in 

 order to achieve pairing equilibrium. The times involved were too short 

 for the occurrence of phase equilibrium characterized by precipitation. 



The above discussion points up some of the care that must be taken 

 to obtain reliable measurements. Another factor which enters the pic- 

 ture is the possible existence of a precipitate in the lithium doped bridge. 



* In boron-doped germanium the cross-over was actually observed — no extra- 

 polation having been necessary, because the temperature of intersection was suffi- 

 ciently low. 



