E l e c t r i c i t y D i v i s i o n N a t i o n a l Bureau of S t a n d a r d s G a i t h e r s b u r g , MD 20899 USA A b s t r a c tThe quantum H a l l e f f e c t is being used t o monitor t h e U.S. l e g a l u n i t of r e s i s t a n c e QNBS. Measurements have been made on a r e g u l a r b a s i s s i n c e August 1983. T h i s paper w i l l d i s c u s s t h e h i s t o r y , t h e improvement, and t h e r e s u l t s of t h e s e measurements. I n t r o d u c t i o n I n t h e i n t e g r a l quantum H a l l e f f e c t [I],t h e H a l l r e s i s t a n c e RH(^) of t h e i t h i n t e g e r p l a t e a u of a f u l l y q u a n t i z e d two-dimensional e l e c t r o n gas is g i v e n by:Here V H ( i ) is t h e H a l l v o l t a g e of t h e i t h p l a t e a u , I is, t h e c u r r e n t t h r o u g h t h e sample and RH i s t h e q u a n t i z e d H a l l r e s i s t a n c e , e q u a l t o h / e 2 ' % 25,812.8 R where h is t h e Planck c o n s t a n t and e is t h e elementary c h a r g e . For p r e c i s i o n measurements, t h e quantum i n t e g e r i i s u s u a l l y chosen t o be e i t h e r 2 or 4 . S i n c e t h e q u a n t i z e d H a l l r e s i s t a n c e of a p l a t e a u is an i n v a r i a n t of n a t u r e , i t can be used t o determine t h e time dependence of an a r t i f a c t -b a s e d n a t i o n a l u n i t of r e s i s t a n c e . One such u n i t i s t h e U.S. Legal Ohm, RNBS, which i s d e f i n e d i n terms of t h e mean r e s i s t a n c e o f f i v e one-ohm, T h o m a s -t y p e , wire-wound r e s i s t o r s maintained i n an o i l b a t h a t t h e N a t i o n a l Bureau of S t a n d a r d s i n G a i t h e r s b u r g . Measurements of RH(^) i n terms of RNBs have been c a r r i e d o u t on a r e g u l a r b a s i s s i n c e A u g u s t 1 9 8 3 , y i e l d i n g a r e l i a b l e v a l u e of dQNBS/dt. I t i s t h e purpose of t h i s paper t o d i s c u s s t h e h i s t o r y , improvement, and t h e r e s u l t s of t h e s e measurements. Summary T h r e e GaAsIAlGaAs h e t e r o s t r u c t u r e samples have been used. They were grown by molecular beam e p i t a x y and have 6,453.2 0 ( i = 4 ) s t e p s a t 6 T . T h e i r z e r o magnetic f i e l d m o b i l i t i e s a r e about 1 0 0 , 0 0 0 cm2/(Vs) a t 4 . 2 K . Two d i f f e r e n t H a l l probe s e t s a r e used f o r each of t h e t h r e e samples. The quantum H a l l p l a t e a u s a r e f l a t t o w i t h i n li-0.01 ppm over a magnetic f i e l d range t h a t i s 2% of t h e c e n t r a l f i e l d v a l u e s when t h e d e v i c e s a r e cooled t o 1.2 K . The t e m p e r a t u r e [ 2 ] and c u r r e n t [31 dependence c o r r e c t i o n s f o r t h e s e samples have been i n t e n s i v e l y s t u d i e d . The l a r g e s t t e m p e r a t u r e dependence c o r r e c t i o n e v e r r e q u i r e d has been (0.026 f 0.002) ppm f o r one p a r t i c u l a r H a l l v o l t a g e measurement a t 1 . 2 K . The t y p i c a l c o r r e c t i o n i s ( 0 . 0 0 0 i-0 . 0 0 2 ) ppm....
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