SPECIFIC HEATS OF SUBMONOLAYER 4He FILMS ADSORBED ON GRAFOIL COATED WITH MONOLAYER ARGON

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SPECIFIC HEATS OF SUBMONOLAYER 4He FILMS

ADSORBED ON GRAFOIL COATED WITH

MONOLAYER ARGON

C. Koutsogeorgis, G. Daunt

To cite this version:

JOURNAL DE PHYSIQUE

Colloque C6, suppliment au no

8, Tome 39, aoiit 1978,

page

_C6-308

S P E C I F I C HEATS OF SUBMONOLAYER 4 ~ e F I L M S ADSORBED ON G R A F O I L COATED W I T H MONOLAYER ARGON

t

C.M. Koutsogeorgis and G. Daunt

Physics Department, C i t y CoZZege o f New York, New York, N.Y. 10031, U.S.A. *Stevens I n s t i t u t e o f TechnoZogy, Hoboken, N.J. 07030, U.S.A.

Rdsum6.- La chaleur spdcifique, C, de '~e

adsorb6 sur du grafoil recouvert partiellement d'une cou-

che monoatomique d'argon a dtd mesurde dans la gamme de tempdrature de

0.4

K

3 4 K

pour des couches

partielles de x

= 0.200, 0 . 2 7 7 , 0 . 3 6 6 , 0 . 4 3 7 , 0 . 4 5 7 , 0.502

et

0.544

(03 x

= 1

indique que la couche

recouvre complstement le grafoil). Les rdsultats ddmontrent que la variable CINk a une valuer maxi-

mum entre

2.5

et

3 . 2

pour la gamme de tempdratures de

1.95

K

1 2 . 5 4 K .

Pour les temperatures

1

pro-

ximitd de

3.5

K, C/Nk tombe vers une valeur entre

0.85

et

1.0

qui est plus ou moins indlpendante de

la tempdrature. Les diffdrences entre les rdsultats expdrimentaux et la thdorie seront brisvement

discutdes.

Abstract.- Measurements of the specific heat, C, of 'He adsorbed on grafoil coated with an argon mo-

nolayer were made in the temperature range

0.4

K to

4

K for fractional coverages x

= 0 . 2 0 0 , 0 . 2 7 7 , 0 . 3 6 6 , 0.437, 0 . 4 5 7 , 0.502

and

0.544

(x

= 1

corresponds to a completed monolayer). The resulting

curves showed marked peaks, reaching values of C/Nk of from

2.5

to

3 . 2

at temperatures in the range

1.95

K to

2.54

K. Near

3.5

K, C/Nk falls to approximately temperature independent values between

0.85

and

1.0.

A brief discussion is given for differences between the experimental results and

theory.

THE EXPERIMENTAL DATA.- Specific heat measurements

the results are shown in figure

1 .

which plots C/Nk

of 'He submonolayer films adsorbed on Grafoil coa-

ted wlth an Argon monolayer were made i'n the tempe-

rature range

0.4

K to

4

K using an adiabatic calori-

meter containing

57.82

g of grafoil. To determine

the surface area or the grafoil, an adsorption iso-

therm using argon at

77

K was carried out. The mono-

layer coverage, Vm was found to be

5.02

cm3 (STP) /g

and the surf

ace area,

1,

17.09

m2lg.

To determine the '~e monolayer coverage on the

grafoil coated with an argon monolayer, an adsorp-

tion i'sotherm of 'He was measured at

4.2 K.

The mo-

nolayer coverage was found to be

13.0

millimoles for

the whole substrate, yielding an areal density, d,

of

0.079

adatoms/i2, a value which is in agreement

with those obtained experimentally by previous wor-

kers

/ 1 , 2 / .

The temperatures were measured using a germa-

nium resistance thermometer and the temperature

readings, as well as the readings for the heat input

duri'ng the adiabatic heating cycle and for the tem-

perature drift before and after each heating cycle,

were automatically recorded on magnetic tape, so

- - TEMPERATURE IKl

that, with an appropriate program, the specific heat

Fig.

1

:Specific heat per 4 ~ e

adatom, C/Nk, as a

data were obtained by computerization.

function of temperature,

T

for various coverages, x,

equal to

:

V

for

x

= 0.200 ;

+

for x

= 0.277 ;

Measurements of the specific heat, C, were made

Ofor x

= 0.366 ;

O f o r x

= 0.437 ;

O f o r x

= 0.457

at the following 'He fractional coverages, x

= 0.200,

0 . 2 7 7 , 0 . 3 6 6 , 0 . 4 3 7 , 0 . 4 5 7 , 0 . 5 0 2 , a n d 0 * 5 4 4 ( x = I

asafunctionof

T . E x c e p t f o r t h e d a t a a t x = 0 . 4 5 7

corresponds to a completed monolayer) and most

which do not extend above T

= 1.6

K, all the measu-

r e d c u r v e s showed pronounced p e a k s , which f e l l a t t e m p e r a t u r e s , _Tmax i n t h e r a n g e 1.95 K f o r t h e l o - w e s t c o v e r a g e of x = 0.200 t o 2.54 K f o r t h e h i g h e s t

c o v e r a g e of x = 0.544.

The r e s u l t s a r e s u p p o r t e d by s i m i l a r d a t a ob- t a i n e d by C r a r y and V i l c h e s / 3 / f o r t h e same system. The Tmax v a l u e s however, a s w e l l a s t h e a b s o l u t e magnitudes of t h e s p e c i f i c h e a t p e a k s , a r e UIUch g r e a t e r t h a n t h o s e f o r 'He f i l m s o f s i m i l a r covera- g e on b a r e g r a f o i l 141.

DISCUSSION.- I f one makes t h e g e n e r a l i n t e r p r e t a t i o n t h a t Tmax i n d i c a t e s t h e t r a n s i t i o n from a two-dimen- s i o n a l (2-D) g a s t o a condensed p h a s e , t h e n o n e m s t conclude t h a t by c o a t i n g t h e g r a f o i l s u b s t r a t e w i t h a monolayer of a r g o n , t h e c o n d e n s a t i o n t e m p e r a t u r e f o r any g i v e n 'He c o v e r a g e i s markedly i n c r e a s e d .

I n t h e t e m p e r a t u r e r a n g e above T t h e va- max

'

l u e s of [ c / ~ k ) - i ] / d c a l c u l a t e d from our d a t a ( h e r e d i s t h e a r e a l d e n s i t y ) do n o t f o l l o w t h e u n i v e r s a l c u r v e v e r s u s t e m p e r a t u r e c a l c u l a t e d by Siddon and S c h i c k / ~ / i o r a 2-D i n t e r a c t i n g g a s adsorbed on a non- i n t e r a c t i n g s u b s t r a t e . The a p p a r e n t ' ~ e - ~ ~ e i n t e r - a c t i o n s t h e r e f o r e must b e c o n s i d e r e d t o b e s u b s t r a t e dependent It c a n b e shown, f o l l o w i n g a n argument b a s e d on t h e Quantum Theory of Corresponding S t a t e s

161

t h a t agreement w i t h e x p e r i m e n t w i l l r e s u l t f o r a n i n c r e a s e i n t h e s t r e n g t h of t h e a t t r a c t i v e p o t e n t i a l r e l a t i v e t o t h a t of t h e r e p u l s i v e of 12 %. F u r t h e r - more, t h e e x p e r i m e n t a l d a t a of C r a r y e t a l . / 7 / f o r 4 ~ e on n e o n - c o a t e d G r a f o i l c a n b e e x p l a i n e d by a r e l a t i v e d e c r e a s e i n t h e a t t r a c t i v e p o t e n t i a l o f 4 %

ACKNOWLEDGEMENTS.- T h i s work was s u p p o r t e d by a Grant from t h e N a t i o n a l S c i e n c e Foundation. W e a r e i n d e b t e d t o P r o f . W. M i l l e r o f CUNY f o r h i s i n t e - r e s t and s u p p o r t .

R e f e r e n c e s

/ I / L e r n e r , E. and Daunt, J . G . , J. Low Temp. Phys. 10 (1973) 299

-

/ 2 / C a r n e i r o , K . , E l l e n s o n , W.D., P a s s e l l , L . , McTague, J . P . and Taub, H., Phys. Rev. L e t t e r s 37 (1976) 1695

-

/ 3 / C r a r y , S.B. and V i l c h e s , O.E., Phys. Rev. L e t - t e r s

38

(1977) 973

/ 4 / B r e t z , M . , Dash, J . G . , H i c k e r n e l l , D.C.,

McLean, E.O. and V i l c h e s , O.E., Phys. Rev. A ,

8

(1973) 1589

/5/ Siddon, R.L. and S c h i c k , M., Phys. Rev. A ,

2

(1974) 907

/ 6 / Nosanow, L.H., P a r i s h , L.J. and P i n s k i , F . J . , Phys. Rev. B

1

(1975) 191