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....

Abstrucl-Results of a new realization of the ohm and farad using the NBS calculable capacitor and associated apparatus are reported. The results show that both the NBS representation of the ohm and the NBS representation of the farad are changing with time, cNBS at the rate of-0.054 ppmlyear and FNss at the rate of 0.010 ppm/year. The realization of the ohm is of particular significance at this time because of its role in assigning an SI value to the quantized Hall resistance. The estimated uncertainty of the ohm realization is 0.022 ppm (l o) while the estimated uncertainty of the farad realization is 0.014 ppm (l a) .

Abstruct-Results from NBS experiments to realize the ohm and the watt, to determine the proton gyromagnetic ratio by the low field method, to determine the time dependence of the NBS representation of the ohm using the quantum Hall effect, and to maintain the NBS representation of the volt using the Josephson effect, are appropriately combined to obtain an accurate value of the fine-structure constant and of the quantized Hall resistance in SI units, and values in SI units of the Josephson frequency-to-voltage quotient, Planck constant, and elementary charge.

No abstract