FI G. 3.10
N(E)
71.
co m p en sated o r n o t, is of A n d e rs o n ty p e . T h e im p o rta n c e of c o rre la tio n v is - a - v is d is o rd e r m u s t be d e te rm in e d b y e x p e rim e n t.
In ch ap ter 4, we d iscu ss in som e d e ta il the e x p e rim e n ta l evid en ce of the m e ta l-n o n m e ta l tra n s itio n and how the re s u lts r e la te to the id eas e x p re s s e d in th is c h a p te r. P e rh a p s the m o s t im p o rta n t point to note h e re is th a t th e o ry is in a flu id s itu atio n and even the s im p le s t m a te ria ls e x liib it p ro p e rtie s th a t a re not fu lly u n d ersto o d .
C h a p te r 4
R E V IE W O F E X P E R IM E N T A L D A T A O N T H E M E T A L - N O N M E T A L T R A N S IT IO N
73. 4 . 1 In tro d u c tio n
In th is c h ap ter w e s u rv e y the re s u lts of e x p e rim e n ts p e rta in in g to the m e ta l-n o n m e ta l tra n s itio n . W e b eg in w ith a re s u m e of
c o n d u ctio n p ro c es s es illu s tr a tin g the e ^ (i = 1 - 3) m e ch a n is m s o u tlin e d in ch ap ter 3 and re v ie w the evid en ce fo r the exis te n ce of a m in im u m m e ta llic c o n d u c tiv ity . B rin k m a n -R ic e en h an cem en t effects a re e xa m in ed using the m e a s u re m e n ts of e le c tro n spin s u s c e p tib ility and e le c tro n ic s p e c ific h e a t. O th e r e x p e rim e n ts p e rtin e n t to the m e ta llic tra n s itio n a re also catalo g u ed . A d e ta ile d account of
n u c le a r m a g n etic reso n an ce e x p e rim e n ts in Si :P fo llo w s . F in a lly , c o n s id e ra tio n is m ade of the p o s s ib ility of c re a tin g a m e ta l-n o n m e ta l tra n s itio n th ro u g h the use of a la r g e m a g n etic fie ld and we discuss th e a s s o c ia te d phenom enon o f 'm a g n e tic fre e z e o u t'.
4 .2 R e s is tiv ity and H a ll E ffe c t
T h e th re e a c tiv a tio n e n e rg ie s e ^ ( i = 1 - 3 ) a r e re a d ily v is ib le 72
in the w o rk of M o tt and D a v is on co m p en sated Ge:Sb as shown in
fig u re (4. l). A s expected, in lo w d e n sity sam p les c o n d u c tiv ity is v ia
e x c ita tio n to the conduction band w ith e n erg y e ^ a t h ig h e r
te m p e ra tu re s w h ils t a t lo w te m p e ra tu re s hopping w ith e n erg y e ^ ta k es o v e r. A t s u ffic ie n tly lo w te m p e ra tu re s v a r ia b le ran g e hopping
73
o c c u rs as o b s erv e d by A lle n and A d k in s and A lle n , W a llis and 74
A d k in s . In the in te rm e d ia te re g io n of c o n c e n tra tio n , e ^ conduction is a p p are n t and fin a lly , a m e ta l- lik e te m p e ra tu re 17 -3
independent r e s is tiv it y is o b tain ed fo r N > ( '^ 1 0 cm in G etSb).
2 5 X 10' JZ 7*0 xIO' M X 10 He r£ 0 0-1 02 0-4