Josephson effects and microwave response of HTS thin films

PhysicaC162-164(1989)1545-1546 North-Holland

JOSEPHSON EFFECTS AND MICROWAVE RESPONSE OF HTS THIN FILMS

J.C. GALLOP,W.J. RADCLIFFE,C.D. LANGHAM,R. SOBOLEWSKI*, W. KULA* and P. GIERLOWSKI* National Physical Laboratory, Teddington TWll OLW, UK *Institute of Physics, Polish Academy of Sciences, PL-02668, Warszawa, Poland The response of thin films YBCO and BSCCO to microwave fields has been investigated. Some unexpected features have been observed and an e,xplanationis given in terms of a robust fluxon lattice whose motion is correlated with the applied microwave field.

1. BXF'BRIMBNTAL PROCEDURE The films,

equally spaced values of

8mmx8mm in

thickness 0.7~

area with

were deposited by d.c

average

magnetron

nhf/2e. where h/2e

constant.

single-crystal

differential

substrates.

superconducting the

I-V

When

characteristic

of

visible

critical current i, being defined as

4.2K, up

of

the

bias

current i,

for

which

the

radiation,

a

function of

bias

voltage,

with

applied. Shapiro steps

were

employed,

characteristic without microwave power

differential resistivity of the film could

by a lock-in amplifier technique.

influenced

strongly

applied

microwave

by

that

additional evidence

features the

in

and

to above 4OK. Some features in the I-V

are

be measured as

difference

the

shows

from the lowest temperature

voltage drop across the film exceeded 100nV. The also

conversion

frequency

resistivity without

microwave power

each film was recorded at various temperatures, a the value

Fig.1

corresponding to

applied frequency and

is the voltage to

sputtering (YBCO)lor sprey pyrolysis (BSCCO)'on MgO



f is the

such

resonances of

arise from internal

Josephson current within the

substrate-film

system. For

the

YBCO

film

i, shows

dependence on temperature over has

been

reported

break-point at i,.

a

power

law

behaviour.

evidence for the existence of

Josephson effects in the film comes influence of microwave characteristic when

I-V

The

4.2K shows a clear

followed by two almost ohmic

regions. rather than Strong

a wide range, as

elsewhere3.

characteristic of the film at

linear

a

film is

BSCCO film showed a power

the

.........

a

the

I-V

incident on the film at a resonant frequency the

cavity (8-20~~4 i, is

of

reduced in amplitude

by about 50%.

The 'differential resistivity of

the

measured as a

film was

voltage

, both with and

power

applied.

In

function of without

addition

bias

microwave to

some

self-resonant structure the former showed clear decreases

in

the differential resistivity at

0921-4534/89/$03.50OElsevierSciencePublishersB.V. (North-Holland)

cavity

observed,

.......... I ......... t * I

!

.........

I -

1*

v-

cavity resonator. With *lOmW of microwave power

curves

When

dependence.

microwave induced features were not

from

in

law

resonator measurements were made on this film the

even at 4.2K. 21

placed

linear

similar

........ Pb I

showed a

a.c

I-V

i, on T,-T but

dependence of

the

radiation on the

the

The

.... .. . .........I. . . . . . . . .,. . . . . . . . I :+::t:::t:. r:::::::.: : -L .

.

.

.

.

.

n=,

.

.

.

.

.

.

.

WI

.

.

.

.

.

.

.

.

.

.

.

.

.

“:3

.

m

I.

--

.

loo

Fig.1 Difference in differential resistivity of YBCO film with and without microwave Power at 16GHz. as a function of .

J.C. Gallop et al. /Josephson effects and microwave response

1546 2. DISCUSSION The

and

observation of Shapiro steps

YBCO

film

drop

along

is

spacing. This

film

the basic

shows

junctions

in

film, which

inter-grain junctions all

identical

properties,

in

which

this expression is

condition for

we

there are

need

with

to

only

large

in

this

experiment, unlike

related

a

We propose an alternative model. a close packed

lattice can

exist in

temperatures4. It

a

irregular,

observations,

few

active

It has been

low

rather stable,

that pinning Thus it may

lattice

though

spatially

exists

low

at

At

overall by weak inhomogeneities, or

influence

of

film

the

the current flow, driven by the

force.

This

fractional a

r.f.

the response of

the

irradiated superconductingAl films.

3. CONCLUSION Our

measurements of film

YBCO

to

microwaves

demonstrates

existence of the a.c. Josephson effect in film. which is not detectable with BSCCO.

model

added weight to the

since it is known

flux

the the This

lattice

that fluxon lattices

to

move

Lorentz

fluxon transport gives rise

may be described either in

terms of

a

current

and magnetic field dependent phase change in the order parameter across the link or weak-link can

be

treated as a

the

same

flux-excluding

barrier which breaks down, allowing the passage of fluxons across it when ib>ic.

Thus our

two

by When

edges.

i,>i, the entire fluxon lattice begins across

including

for YBCO. Note that a single Josephson weak-link

films at

i, values the lattice remains stationary,

the

possibly

low

thin YBCO

temperatures in the YBCO film reported here.

pinned

the

'melt' at lower temperatures in BSCCO films than

is also known

fluxon

single

triangular fluxon

forces are weak in these materials. be that

a

values of n in the I-V characteristic of

result gives

that

Shapiro step in

the

Josephson junction. Only integer steps were seen

near

invoke a

junctions in the film, which seems unrealistic.

shown

a

identical with

structure to be completely

number of

model

and

Unless we are to assume a

out.

of

series

parallel connection throughout the would cause the step washed

hf/2e

contradicts the usual model

inter-grain

many

in

The total voltage

surprising.

the

the

to

a

Faraday law voltage V, such that

models

may

represent only

extremes

of

a

continuum, distinguished mainly by the number of active entities required. Further work planned

is

to investigate the differences between

YBCO and BSCCO.

V = -dQ/dt = ov(i)l@, where u is the density of fluxons per unit 1

is

v(i)

area,

the distance between voltage contacts and is

strong

the fluxon velocity, assumed to

function of i,. The

be

application of

the

flux

flow

to

the

1.

published)

a

2.

field oscillations

4. 2592

where n is an integer and

5.

the microwave field. The volt drop film

along

in this synchronised state then becomes V = eOdN/dt = nQOf

the

W Kula et al., Supercon. Sci & Tech. 1 (1988)

3. A Inam et al., Appl. Phys. Lett. 53 (1988) 908

of passage of fluxons across the strip dN/dt=nf, is the frequency of

(to be

145

over a limited voltage range, such that the rate

f

P Gierlowski et al., Phys. Ser. (1989

a

microwave field may then impose synchronisation of

REFERENCES

P L Gammel et al., Phys. Rev. Lett. 59 (1988)

A T Fiory, Phys. Rev. Lett. 27 (1971) 501