Electron-beam irradiation has been used to induce controllable variations in the properties of YBa 2 Cu 3 O 7Ϫx biepitaxial grain-boundary Josephson junctions. A correlation between the transport properties and the microstructure was obtained by determining the ratio of a barrier thickness to the dielectric constant of the junctions with different barriers. These results give evidence of the role of the oxygen content and the dielectric constant of the interface region in transport phenomena. The experiment also demonstrates frequency tunability in a resonant soliton oscillator. ͓S0163-1829͑98͒51322-2

A Di Chiara

B Nadgorny

E Sarnelli

F Carillo

F Lombardi

F Tafuri

M Gurvitch

S Shokhor

CiteSeerX

RAPID COMMUNICATIONS
PHYSICAL REVIEW B 1 JUNE 1998-IIVOLUME 57, NUMBER 22Barrier properties in YBa 2Cu3O72x grain-boundary Josephson junctions using electron-beam
irradiation
F. Tafuri
Dipartimento di Ingegneria dell’ Informazione, Seconda Universita´ di Napoli, Aversa (CE), Italy
and INFM, Dipartimento di Scienze Fisiche, Universita´ di Napoli ‘‘Federico II,’’ Napoli, Italy
B. Nadgorny,* S. Shokhor, and M. Gurvitch
Department of Physics, State University of New York at Stony Brook, Stony Brook, New York 11794
F. Lombardi, F. Carillo, and A. Di Chiara
INFM, Dipartimento di Scienze Fisiche, Universita´ di Napoli ‘‘Federico II,’’ Napoli, Italy
E. Sarnelli
Instituto di Cibernetica del CNR, Via Toiano 6 I-80072, Arco Felice (NA), Italy
~Received 19 March 1998!
Electron-beam irradiation has been used to induce controllable variations in the properties of YBa2Cu3O72x
biepitaxial grain-boundary Josephson junctions. A correlation between the transport properties and the micro-
structure was obtained by determining the ratio of a barrier thickness to the dielectric constant of the junctions
with different barriers. These results give evidence of the role of the oxygen content and the dielectric constant
of the interface region in transport phenomena. The experiment also demonstrates frequency tunability in a
resonant soliton oscillator.@S0163-1829~98!51322-2#an
or
e
ic
pa
en
e
e
h
o
flu
io
o
n
al
a
f
m
h
n
th
n-
ep
se
m
o
of
rs
en
her
cts
of
on
i-
the
he
ge
l as
he
se
and
ns,
ier.
by
a-
the
ine
the
rier
ial
the
ionThe correlation between grain-boundary interfaces
the Josephson effect gained considerable interest since
discovery of the high-critical-temperature superconduct
~HTS’s!.1 The properties of grain boundaries~GB’s! are
critical for several applications, especially in order to d
velop Josephson junctions for superconducting electron2
and for fundamental experiments on the symmetry of the
wave function in HTS’s.3 Some correspondence betwe
transport regimes and the GB microstructures has also b
established. Nevertheless there is no conclusive evidenc
the actual role of grain boundaries in the conduction mec
nisms, due to either the concomitance of different effects
their structural complexity. Their presence seems to in
ence the properties of the adjacent superconducting reg
due to the oxygen exchange, which occurs in the vicinity
the grain boundaries. The barrier region, therefore, exte
into the electrodes, i.e., into the regions that are nomin
superconducting. In terms of transport properties a gr
boundary has been represented as a filament structure o
perconducting channels separated by insulating or nor
metal layers4 or as a continuous insulating barrier with a hig
density of localized states.5 More recently studies have bee
focused on the effects of intrinsic faceting combined with
role of the symmetry of the order parameter.6 In the classical
scenario it would be desirable to evaluate the actual role
the proximity effect7,8 on quasiparticle and Cooper pair co
duction.
In this paper we present a measurement of the GB Jos
son junctions whose properties are modified by focu
electron-beam irradiation. The nature of local order para
eter suppression resulting from electron irradiation
YBa2Cu3O72x ~YBCO! thin films and single crystals is fairly
well understood.9,10 The effect is due to the generation
displacement defects which act as strong scattering cente
the Cu-O planes. In this work they are primarily oxyg570163-1829/98/57~22!/14076~4!/$15.00d
the
s
-
s
ir
en
of
a-
r
-
ns
f
ds
ly
in
su-
al
e
of
h-
d
-
f
in
efects; at higher irradiation energy~.300 keV! Cu defects
may be created as well. In our system the situation is furt
complicated by the presence of a grain boundary which a
as a sink for the migration of defects, affecting the kinetics
their accumulation.10 Our biepitaxial YBa2Cu3O72x junc-
tions employ an MgO film oriented along the~110! direction
as a seed layer to modify the crystal orientation of YBCO
a ~110! oriented SrTiO3 substrate. YBCO grows predom
nantly along the~103! direction on SrTiO3 substrate and
~001! on the MgO layer, respectively,11 resulting in highly
reproducible structures. We focused an electron beam of
scanning electron microscope~SEM! Philips CM-12 with an
energy of 120 keV on the grain-boundary region of t
junction.12,13Electron irradiation changes the current-volta
(I -V) characteristics, and presumably the barrier as wel
the microstructure of the grain boundary by modifying t
oxygen content in the vicinity of such interfaces. The
changes can be controlled by varying the electron dose
partially restored by isothermal annealing of the junctio
thus offering a unique opportunity to characterize the barr
The presence of resonance steps in theI -V characteristics
and their voltage shift induced by irradiation and modified
a subsequent annealing give important additional inform
tion on the grain boundary structure. From the position of
resonance steps it is possible to self-consistently determ
the ratio of the barrier thicknesst to the relative dielectric
constant« r as well as the surface resistanceRs .
14–16 This
represents a way to correlate the microstructure and
transport properties, thus giving direct evidence that a bar
can be controllably adjusteda posteriori.
Details of the fabrication procedure for the biepitax
junction, their Josephson and normal-state properties, and
effects of irradiation on~001! and ~103! YBCO films have
been given elsewhere.11,12,13The electron beam with a 5 nm
nominal spot size was focused on the grain-boundary regR14 076 © 1998 The American Physical Society
a
at
’
fit
.
of
ce-
RAPID COMMUNICATIONS
57 R14 077BARRIER PROPERTIES IN YBa2Cu3O72x GRAIN- . . .FIG. 1. ~a! Typical I vs V curves of the junc-
tion biep No. 36 before irradiation are shown as
function of temperature. In the inset the curve
T510 K, obtained by subtracting the ‘‘ohmic’
current V/RN from the total currentI , is com-
pared with the expected Lorentzian. From this
we estimated QS,N50.8 and RS5140 mV.
~b! Typical I vs V curves of the junction biep No
36 after irradiation are shown as a function
temperature. In the inset, using the same pro
dure as in~a! for the data atT58.5 K, we ob-
tainedQS,N50.27 andRS5240 mV.n
f
.
e
o
in
es
0
-
ti-
t
ent
ifi-and scanned once all the way across the microbridge~along
the grain boundary! with a linear fluence of 1.531016
e/cm.12,14 In order to study the time variation of junctio
parameters, the samples were measured after a series o
thermal annealings in the He atmosphere of the cryostat
The typicalI -V curves of the biepitaxial junctions befor
irradiation are shown at different temperatures in Fig. 1~a!.
At voltages on the order of 1024 V steps can be clearly
identified. These resonance steps have been frequently
served before in HTS GB Josephson junctions15,16 and asso-
ciated with the propagation of the electromagnetic waves
junction that acts as a long parallel plate resonator~Fiske
modes!. The ac Josephson effect can cause self-induced r
nance whenf n5nc/2L52eVn /h, wheren is the order of the
resonance at a frequencyf n andL is the junction length. The
Swihart velocity is given byc5c0(t/« rd)
1/2, wherec0 is the
free space speed of light andd5t12lL , respectively. The
London penetration depthlL at T50 K has been taken 14iso-
b-
a
o-
nm in agreement with the experimental observations.15 Sub-
tracting the ‘‘ohmic’’ currentV/RN ~RN is the normal-state
resistance! from the total currentI , a typical resonance pro
file is obtained@inset of Fig. 1~a!#. By fitting this curve with
the expected Lorentzian, an estimate of the total lossQ in the
system can be extracted.Q consists mainly of losses from
the quasiparticle conductance (;1/Qqp), the surface resis-
tance (;1/QS,N) and radiation (;1/Qrad); the latter has
practically no contribution in our geometry, so 1/Qexp
51/Qqp11/QS,N . SinceQqp can be evaluated, we can es
mate QS,N and hence the surface resistanceRS,N5pm0(t
12lL)/QS,N . For the data atT510 K we estimated for the
first-order resistanceQS,N50.8 and RS5140 mV. Figure
1~b! shows theI -V curves of the same junction at differen
temperatures after irradiation. In this case the critical curr
decreases approximately by a factor of 8 atT59 K, while the
maximum working temperature of the junctionTc decreases
approximately from 73 to 45 K. We also observed a sign
e
n
ra-
g
RAPID COMMUNICATIONS
R14 078 57F. TAFURI et al.FIG. 2. I vs V curves, measured atT58 K,
before irradiation~open triangles!, after irradia-
tion ~full triangles!, after the first ~12 min at
330 °C! ~full squares! and the second~24 h at
room temperature! ~open circles! annealings, re-
spectively. In the inset~a! the ‘‘ohmic’’ current
V/RN is subtracted from the total current. In th
inset~b! the dependence of the principal junctio
parameters, normalized to the values before ir
diation, is shown as a function of the annealin
time t. The values before irradiation~T54.2 K!
areI C544 mA, TC573 K, RN520 V, t/« r50.18
Å, andRS5140 mV, respectively.ke
-
.
a
af
re
pr
k
m
te
av
lin
t
th
h
a
th
a
re
n
se
i-
k-
ta
u
a
ion
r re-
50
the
rent
t
v-
K.
r in
hson
ng a
a-
and
to
ary
re,
n-
ions
of
to
and
the
the
ve
r-
the
lly
becant increase ofRN from 20 to 65 V. More importantly,
irradiation induces a shift in the position of the first Fis
resonance from 240 to 150mV. This corresponds to a de
crease of the Swihart velocity from 2.43106 to 1.53106
m/sec and thet/« r value from 0.18 to 0.07 Å, respectively
lL has been assumed weakly dependent on irradiation
aging effects according to experimental evidence.15,17 Using
the same procedure as in Fig. 1~a! for the data atT58.5 K in
the inset of Fig. 1~b! we obtainedQS,N50.27 andRS5240
mV. Hence the surface resistance noticeably increases
irradiation.
The original properties of the junctions can be partly
stored by annealing, as shown in Fig. 2. In the inset~a! of
Fig. 2 the resonance profiles and their voltage shifts are
sented. Annealing tends to shift the position of the Fis
steps to higher voltages, thus increasing the ratio oft/« r . In
the inset~b! the dependence of the principal junction para
eters, normalized to the values before irradiation, is plot
as a function of the annealing time. All the parameters h
been measured atT54.2 K. I C , TC , and t/« r increased
in the course of annealing, whileRS andRN decreased, with
the most rapid change observed in the first hour of annea
The simultaneous change of the junction parameters
well as their dependence on the annealing time suggests
the effects due to the electron irradiation, being linked to
junction configuration, can reveal basic information on t
grain-boundary microstructure. Within a framework of
simple tunnelling theory, a transmission coefficientT expo-
nentially depends on the barrier thicknesst, and on the wave
vector within the barrier. An increase oft and the barrier
height would explain a dramatic decrease ofI C and increase
of RN . The experimental evidence, that the changes ofI C
andRN induced by irradiation and their dependencies on
annealing time are different, suggests that Cooper pairs
quasiparticle conduction processes are governed by diffe
effective barriers. This situation can occur, for instance, i
defect-assisted tunnelling.7,5 On the other hand, the decrea
of the t/« r ratio after irradiation would indicate that the d
electric constant« r increases faster than the barrier thic
ness. This effect could be explained by assuming a cer
type of barrier, that depends on the dielectric constant, snd
ter
-
e-
e
-
d
e
g.
as
hat
e
e
e
nd
nt
a
in
ch
as a semiconductorlike barrier.7 Consequently, bothI C and
RN would be linked to« r .
Additional evidence of the possibility of this type of
barrier is given by the same experiment on a similar junct
where an irradiation dose of 231021 e/cm2 was used. In this
case an electron beam was scanned within a rectangula
gion containing the grain boundary, about 1mm in width.
The junction resistanceR before and after irradiation is
shown in Fig. 3 as a function of temperatureT. The resis-
tanceR after irradiation increases at temperatures below
K as in semiconducting contacts.R after irradiation atT54.2
K is 103 times higher than the normal-state resistance of
Josephson junction before irradiation. The Josephson cur
observed before irradiation@inset ~a! of Fig. 3# was com-
pletely suppressed. However, after a 6 month annealing a
room temperature the junction exhibited metalliclike beha
ior of R in the entire temperature range from 300 to 4.2
For temperatures lower than 10 K, the nonlinear behavio
I vs V characteristics at low voltages, visible in the inset~b!
Fig. 3, is associated with the reappearance of the Josep
current. These results can also be explained by assumi
barrier with semiconductorlike properties. A uniform irradi
tion enhances the oxygen exchange between the barrier
the adjacent regions, providing more favorable conditions
observe the carrier deficiency close to the grain-bound
region due to disordered or deficient oxygen.
Oxygen exchange induced by irradiation would, therefo
modify both the barrier structure, affecting its dielectric co
stant and transparency, and the superconducting reg
close to the interface. The fact that the original properties
the GB junction do not completely recover allows us
speculate that the defects migrate to the grain boundary
get trapped in the adjacent region.
Finally the observed dependence of the/« r ratio on irra-
diation and annealing time has important implications for
fundamental issues of the Josephson effect, providing
tunability of the phase velocity for the electromagnetic wa
and vortex propagation in the YBCO GB junctions. In pa
ticular our experiment shows that it is possible to tune
frequency of a resonant soliton oscillator, which is usua
fixed once the structure is fabricated. Moreover, it can
at
v-
-
RAPID COMMUNICATIONS
57 R14 079BARRIER PROPERTIES IN YBa2Cu3O72x GRAIN- . . .FIG. 3. ResistanceR of junction biep No. 37
is shown as a function of temperatureT before
irradiation ~open squares! and after high dose ir-
radiation~full circles!. In the inset~a! the I vs V
characteristic before irradiation is reported
T54.2 K, exhibiting a typical Josephson beha
ior. In the inset~b ! I vs V characteristics of biep
No. 37 after a 6 month annealing at room tem
perature are shown as a function ofT. The junc-
tion exhibited metalliclike behavior ofR in the
entire temperature range from 300 to 4.2 K.a
e
on
a
p
tiv
e
u
ay
ro
e of
o-
en
n-
r-
p-
ofdone locally and even only within a selected part of the b
rier, thus providing a versatile tool to introduce inhomog
neities and to modify the boundary conditions for solit
propagation.14
In summary, we have demonstrated that electron-be
irradiation induces a shift in the position of the Fiske ste
and changes the ratio of the barrier thickness to the rela
dielectric constant. After a uniform ‘‘blanket’’ irradiation th
junction resistance increases at lower temperatures. Altho
the possibility of ad-wave nature of the HTS electrodes m
influence some aspects of our interpretation, we argue fr-
-
m
s
e
gh
m
the consistency of our results that we have given evidenc
modification of the barrier and the grain-boundary micr
structure. This is likely to be due to the change in the oxyg
content in the vicinity of the junction and the dielectric co
stant of the grain boundary.
The authors would like to thank Professor Antonio Ba
one for valuable comments. This work was partially su
ported by the project PRA-INFM ‘‘HTS Devices.’’ Many
thanks to Maria Chiara for her help during the preparation
this paper.s.
e,
-
i,
.
.
D.
.
*Present address: Naval Research Laboratory, Washington, D
20375.
1P. Chaudari, J. Mannhart, D. Dimos, C. C. Tsuei, C. C. Chi, M
M. Oprysko, and M. Scheuermann, Phys. Rev. Lett.60, 1653
~1988!; 61, 219 ~1988!.
2I. Braginski, in New Superconducting Electronics, edited by H.
Weinstock and R. Walstson~Kluver Academic, Dordrecht,
1993!, p. 89; S. Shokhor, B. Nadgorny, M. Gurvitch, V. Se-
menov, Yu. Polyakov, K. K. Likharev, S. H. Hou, and J. M.
Phillips, Appl. Phys. Lett.67, 2869~1995!.
3C. C. Tsuei, J. R. Kirtley, C. C. Chi, L. S. Yu-Jahnes, A. Gupta
T. Shaw, J. Z. Sun, and M. B. Ketchen, Phys. Rev. Lett.73, 593
~1994!; J. R. Kirtley et al. Nature~London! 373, 225 ~1995!; P.
Chaudary and S.-Y. Lin, Phys. Rev. Lett.72, 1084~1994!.
4B. H. Moeckly, D. K. Lathorp, and R. A. Buhrman, Phys. Rev. B
47, 400 ~1993!.
5R. Gross and B. Mayer, Physica C180, 235 ~1991!.
6J. Mannhart, H. Hilgenkamp, B. Mayer, Ch. Gerber, J. R. Kirtley
K. A. Moler, and M. Sigrist, Phys. Rev. Lett.77, 2782~1996!,
and references therein.
7E. L. Wolf, Principles of Electron Tunneling Spectroscopy~Clar-
endon, New York, 1985!.
8M. Yu. Kupryanov and V. F. Luckichev, Zh. Eksp. Teor. Fiz.94,
139 ~1988! @Sov. Phys. JETP67, 1163~1988!#; F. Tafuri, Phys.
Rev. B56, 91 ~1997!, and references therein..C.
.
,
,
9J. Giapinzakis, D. M. Ginsberg, M. A. Kirk, and S. Ockers, Phy
Rev. B 50, 15 967 ~1994!; A. Legris, F. Rullier-Albenque, E.
Radeva, and P. Lejay, J. Phys. I3, 1605~1993!; E. M. Jackson,
B. D. Weaver, G. P. Summers, P. Shapiro, and E. A. Burk
Phys. Rev. Lett.74, 3033~1995!.
10S. Tolpygo, J.-Y. Lin, M. Gurvitch, S. Y. Hou, and J. M. Phillips,
Phys. Rev. B53, 12 462~1996!.
11A. Di Chiara, F. Lombardi, F. Miletto Granozio, U. Scotti di
Uccio, F. Tafuri, and M. Valentino, IEEE Trans. Appl. Super
cond.7, 3327~1997!.
12F. Tafuri, S. Shokhor, B. Nadgorny, M. Gurvitch, F. Lombard
and A. Di Chiara, Appl. Phys. Lett.71, 125 ~1997!.
13S. K. Tolpygo, S. Shokhor, B. Nadgorny, A. Bourdillon, J. Y
Lin, S. H. Hou, J. M. Phillips, and M. Gurvitch, Appl. Phys.
Lett. 63, 1696~1993!.
14A. Barone and G. Paterno´, Physics and Applications of the Jo-
sephson Effect~Wiley, New York, 1982!.
15D. Winkler, Y. M. Zhang, P. A. Nilsson, E. A. Stepanstov, and T
Claeson, Phys. Rev. Lett.72, 1260~1994!.
16J. Mannhart, R. Gross, K. Hipler, R. P. Huebner, C. C. Tsuei,
Dimos, and P. Chaudari, Science245, 839 ~1989!.
17S. K. Tolpygo, S. Shokhor, B. Nadgorny, J. Y. Lin, S. H. Hou, J
M. Phillips, and M. Gurvitch, IEEE Trans. Appl. Supercond.5,
2521 ~1995!; A. Hoffmann, I. Schuller, A. Gilabert, M. G.
Medici, F. Schmidl, and P. Seidel, Appl. Phys. Lett.70, 2461
~1997!.


Barrier properties in YBa 2 Cu 3 O 7؊x grain-boundary Josephson junctions using electron-beam irradiation

http://citeseerx.ist.psu.edu/viewdoc/download;jsessionid=0C03593DB299551572710E8F7D94C86E?doi=10.1.1.1045.1040&rep=rep1&type=pdf

Barrier properties in YBa 2 Cu 3 O 7؊x grain-boundary Josephson junctions using electron-beam irradiation

Abstract

Similar works

Full text

Available Versions

CiteSeerX