Part $2 - Superconductivity 1: LTS - Special superconductors, etc.

P r e s s u r e - i n d u c e d s u p e r c o n d u c t i v i t y o f iodani l

Tatsuya Yokota ~ Nao Takeshita', Katsuya Shimizu ~ Kiichi Amaya', Akifumi Onodera ~ Ichimin Shirotani b

and Shoichi Endo ~

"Faculty of Engineering Science, Osaka University, Toyonaka, Osaka 560, Japan

bMuroran Institute of Technology, Muroran 050, Japan

CResearch Center for Ultra Science, Osaka University, Toyonaka, Osaka 560, Japan

The electrical resistance and the crystal structure of an organic molecular crystal of iodanil C614Oz are studied. The

crystal is found to become metallic and to transform to amorphous slate under pressures above 30GPa. At temperatures

around 2K, the onset of superconductivity is observed under pressure of 52GPa.

1.INTRODUCTION

The crystal structure of iodanil (C~I402) is

monoclinic and belongs to space group P2~/C with two

molecules in a unit cell. The crystal is an insulator at

ambient pressure but expected to become a metal under

pressure. Electrical resistance of iodanil was measured

at room temperature by Onodera el alJ ~) and the

resistivity of 5• was observed at the

estimated highest pressure of 25GPa. In the present

experiments, we measured temperature dependence of

the resistance as well as the X-ray diffraction under

pressure so as to study the metallic state in detail.

2.RESULTS

The iodanil crystal used in the experiments was

prepared by the reaction of bromanil with potassium

iodide and recrystallized from ethyl acetate. The

pressures are generated by a clamp type diamond-anvil

cell made of nonmagnetic hardened BeCu alloy. The

sample and the nonmagnetic stainless gasket was

insulated by AlzO 3 fine powder mixed with epoxy. The

app!ied pressure was determined by ruby fluorescence at

liquid nitrogen temperature. The sample temperature is

Czechoslovak Journal of Physics, Vol. 46 (1996), Suppl. $2

supposed to be given by tile Ge thermometer on tile

pressure cell which are cooled down to 90mK by

~He/4He dilution refrigerator.

10 lo

10 8

106

o 10 4

102

I I I I I I I

I0danil

�9 Loading

o Unloading

10o0 J , , , I , , 10 20 3 0 4 0

.P (OPa)

Figurel Pressure dependence of electrical resistance of

iodanil at room temperature.

817

Figure 1 shows the pressure dependence of the

resistance at room temperature. The resistance decreases

rapidly with increasing pressure. The decrease of the

resistance by several order of magnitude is observed at

the pressure range between several GPa and 35GPa.

There are no abrupt change suggesting a insulator-metal

transition.

Figure 2 shows the temperature dependence of the

resistance. We observed that the sign of dR/dT changes

from negative to positive at pressure around 30GPa and

iodanil behaves like metallic at higher pressures. The

resistance decreases with decreasing temperature in

proportional to temperature above 50K and gradually

approaches to the residual values at liquid He

temperature region.

1.21

1.1

1

0.9

| ! i i i I i i

Iodanil

36GPa

I ! I I ! I I

0 20 40 60 T(K)

Figure 2 Temperature dependence of electrical resistance of

iodanil tot fixed pressures around 30GPa.

We tried X-ray analysis of iodanil under pressure

and found that Ihe diffraction peaks observed at low |

pressures shifts continuously and becomes weak with

increasing pressure. Finally, at 30GPa, all the peaks

disappears, suggesting the transformation to amorphous

stale. This anmrphous stale is found to be quenched by

reducing pressures.

Figure 3 shows the resistance as a function of '

818

temperature at 52GPa. The small drop of the resistance

at 2K is considered to be the onset of superconductivity.

The finite resistance appearing at temperature below

0.5K depends on the pressure as well as sampling. This

residual resistance may come from the crystal

imperfections caused by the uniaxial stress.

160

o ~" 140

120

0

i I I I

/ Iodanil P ~ 5 2 G P a

J 1 , , ! , I , I

2 4

r(K)

Figure 3 Temperature dependence of electrical resistance of

iodanil in pressure of 52GPa.

3.CONCLUSIONS

Electrical resistance measurements of a simple

organic molecular crystal of iodanil are performed down

to low temperatures of 90mK and under high pressure

up to 52GPa. At about 30GPa, the resistance shows

metallic temperature dependence and also the crystal

transform to alnorphous stale. The onset of

superconductivity is observed at 2K in pressure of

52GPa. The study of the X-ray diffraction in the

measured highest pressure and the detection of the

Meissner effect are now going on.

REFERENCES

~)l.Shirotani, A.Onodera, Y.Kamura, H.Inokuchi and

N.Kawai,J.Solid Stale Chem.18(1976)235.

2)A.Onodera, I .Shirotani , H. lnokuchi and

N.Kawai,Chenl.Phys.Lett.25(1974)296.

Czech. J. Phys. 46 (1996), Suppl. S2