Download Josephson properties of basal-plane

Josephson properties of basal-plane-faced tilt boundaries in YBa2Cu3O72d
thin films
B. H. Moeckly and R. A. Buhrman
School of Applied and Engineering Physics, Cornell University, Ithaca, New York 14853-2501
~Received 29 August 1994; accepted for publication 7 October 1994!
The Josephson properties of 90° basal-plane-faced tilt boundaries formed between c-axis and a-axis
normal grains in YBa2Cu3O72d thin films are reported. These boundaries have a low conductance
which results in underdamped junction behavior. The junction capacitance and kinetic inductance
both scale directly with junction critical current and conductance. The results emphasize the
inhomogeneous and filamentary nature of the superconducting properties of cuprate grain
boundaries. © 1994 American Institute of Physics.
Considerable effort has been directed toward understanding the weak link properties of high-angle grain boundaries in the cuprate superconductors, and, in the thin film
case, toward the utilization of such weak links as Josephsonlike elements in quantum device applications. This latter effort has been largely concerned with the study of
YBa2Cu3O72d ~YBCO! tilt boundaries ~TBs! created by the
union of two crystalline grains whose in-plane a – b orientations differ by a large ~.15°! amount.1 Such TBs are generally found by electron microscopy to be clean, abrupt, and
free of secondary phases.2 The current–voltage ~I – V! characteristics of these high-angle TBs have more or less the
character of a Josephson element nonideally shunted by a
linear resistor. Notable features of TBs are the much lower
than ideal value of the characteristic voltage I c R n and the
widely observed scaling behavior of I c R n ;J ac , with
reported3,4 values of a ranging from 0.5 to 0.7 depending on
the type or method of formation of the TB. ~Here, I c is the
critical current, J c is the average critical current density, and
R n is the normal-state resistance of the TB.!
The response of in-plane TBs to microwave radiation
and magnetic field H has indicated that they are filamentary
with respect to their superconducting properties.5– 8 The
boundaries can be modeled6 as possessing small areas of
material on each side which have a high degree of basalplane oxygen order and low vacancy density, thereby providing strong superconducting contact. These contact areas are
adjoined by material with lower oxygen content and/or
higher disorder, giving rise to both normal conducting paths,
most likely via localized states,9 and capacitive connections
across the TB. The areas of oxygen disorder are created by
the distressing ease by which the basal-plane oxygen ion can
diffuse through YBCO and by the concomitant ability of
very weak atomic forces to create vacancy aggregates.6,10
Here, we report on studies of an alternative type of TB
Josephson junction consisting of a 90° basal-plane-faced
boundary which is formed by growing one YBCO grain so
that its c axis lies in the plane of the substrate, while its
neighboring grains ~compromising the bulk of the film! grow
with their c axes normal to the substrate. The behavior of
these ‘‘a – c’’ TBs is quantitatively different from that of our
naturally occurring in-plane TBs: The scaling behavior of
I c R n with J c is substantially weaker, and the TB resistance is
considerably higher at a given I c . The resultant under3126
Appl. Phys. Lett. 65 (24), 12 December 1994
damped Josephson junction behavior provides new insights
into the electrodynamics of cuprate TBs.
To produce such TBs we adopted the simple expedient
of lowering the temperature of an MgO substrate in a laser
ablation YBCO growth process to the point at which a fraction ~;1%–5%! of the grains nucleating on the substrate
have an a- or b-axis-normal orientation within the predominantly c-axis-normal film. Since these a-axis grains are, typically, .2 mm long in the in-plane a- ~or b-! axis direction,
the film can be easily patterned so that an ‘‘a grain’’ completely bisects a 1- or 2-mm-wide microbridge. Perhaps because no epitaxial seed layers are necessary in this bruteforce approach, we find by electron microscopy that the
resultant a – c 90° TBs are abrupt and cleanly bounded on
one side by a basal plane of the a-axis-normal grain.2
The bulk of the transition of microbridges containing
a-grain inclusions typically occurs near 88 K, but there are
often two steps in the transition to zero resistance which
occurs at lower T, <75 K. We attribute these to the separated
transitions of the two TBs formed by the presence of the
a-grain inclusion. The transition of the bulk of the a grain is
generally not distinctly evident. We conclude that the a
grains contain a sufficient density of stacking faults and other
extended defects such that the conductivity of the bulk of the
grain is considerably higher than is observed in less-defected
single crystals.11,12
As shown in Fig. 1, the I – V characteristics of these
samples exhibit Josephson behavior in the underdamped or
nearly underdamped regime13,14 which is attributable to the
weaker of the two a – c TBs. The Josephson character of the
TB is supported by a strong Shapiro step response to microwave radiation. Unlike in-plane TBs, the specific resistance
of these a – c TBs tends to be quite high, ranging from
531028 to 1025 V cm2 for the devices examined. Despite
these high resistances and the low values of J c , which range
from 102 to 105 A/cm2, the 4.2-K values of I c R n of the contacts are fairly high, of order 1 mV. While the I c R n of these
a – c TBs does scale roughly with J c , the mean variation as
illustrated by Fig. 2~a! is quite weak, a'0.3.
The hysteresis that is generally seen in the lowtemperature I – V characteristics can be used to establish the
specific shunt capacitance C/A of the junction.13,14 ~A is the
junction area.! C/A is highly variable for our devices and is
correlated with the variation in the junction R n A and/or J c as
0003-6951/94/65(24)/3126/3/$6.00
© 1994 American Institute of Physics
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FIG. 1. Current–voltage characteristics of two YBCO microbridges, each of
which is bisected by an a-axis-normal grain. In ~a! I c R n 5712 mV; in ~b!
I c R n 5620 mV. The inset in ~a! illustrates the current path, and the inset in
~b! shows the response to 25-GHz microwave radiation.
shown in Fig. 2~b!. This variation in C/A by more than an
order of magnitude, while J c charges by less than a factor of
100, clearly indicates that the a – c contact is not a typical
tunnel barrier, while the high R n A and underdamped behavior also rules out the possibility of a uniform superconductornormal-superconductor junction.
Additional information can be obtained from the very
distinct structure that is often seen in their I – V characteristics, examples of which are shown in Fig. 3. This structure
has all the hallmarks of the ‘‘beating mode’’ behavior15–18 of
an underdamped two-junction SQUID in the regime
bL 5L s I c /F 0 >1. ~Here, L s is the SQUID inductance, and F0
is the superconducting flux quantum.! In this situation an
additional solution of the SQUID equations exists which describes a behavior distinctly different from that given by the
standard zero-voltage supercurrent solution and the solution
for bias currents above the maximum critical current of the
device where the phases of the two Josephson elements increase or ‘‘free-run’’ in phase. In the intermediate, beatingmode ~BM! regime, the phase of one junction is approximately stationary, while that of the other junction precesses
by some multiple of 2p. The junctions periodically reverse
their roles in a process that allows one or more flux quanta
nF 0 to enter the SQUID loop through one junction and subsequently exit through the other. An observable consequence
of this BM state is an additional branch in the I – V characteristic between the two usual SQUID states. For symmetrically fed SQUIDs with two identical junctions, a bias H is
required to access the BM regime. For an asymmetrically fed
Appl. Phys. Lett., Vol. 65, No. 24, 12 December 1994
FIG. 2. ~a! The variation of a – c boundary I c R n with critical current density
J c . ~b! The variation of junction capacitance ~open circles! as determined
from the I – V hysteresis at 4.2 K with R n A, and the variation of the minimum junction inductance ~filled circles! necessary to produce the observed
SQUID-like beating-mode behavior.
device, or for one with junctions having substantially different critical currents, this solution can be obtained without a
field bias, although the behavior is strongly varied by a small
H, sufficient to change the enclosed flux by F0/2.
This BM behavior is often very closely mimicked by the
behavior of our a – c TBs as can be seen by comparing Fig.
3 with the predictions of Refs. 15–17. In some instances we
require a small H in order to observe the apparent beating
mode; in other cases the resonance occurs at zero H but can
be readily removed by application of a small field. These
results strongly suggest that the a – c TBs, which are only
1–2 mm wide, can often be effectively modeled as consisting
of two ~or more! very small, localized underdamped Josephson junctions connected by a substantial inductance to effectively form a SQUID-like structure. The calculations15–17 indicate that the BM behavior requires that bL *3/2 and
1&bc &pbL . This condition puts a lower bound on the effective inductance of the a – c contacts that ranges from
10211 to 1028 H, depending upon the TB J c , or R n A, as also
shown in Fig. 2~b!. We note that the positions of the BM
branches on the a-grain I – V curves do roughly correspond
to the LC resonance value predicted from our estimated values of capacitance and inductance.
If the areas of superconducting contact at the TB can be
modeled as filaments of dimensions less than lL , then the
kinetic inductance ~L k ! of these filaments can easily supply
the necessary L s for the beating-mode behavior. The onedimensional Ginzburg–Landau model of a uniform narrow
B. H. Moeckly and R. A. Buhrman
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FIG. 3. Examples of a – c TB I – Vs exhibiting SQUID-like BM behavior
which is seen as an intermediate branch of the I – V located between the
zero-voltage and high-voltage behavior.
superconducting filament of length l gives a kinetic
inductance19
L k5
S
D
2
F0
~ l/ j ! 3
11 ,
2 p I c 3 A3 ~ l/ j ! 2 23 p 2
where j is the superconducting coherence length. Given the
extremely short values of j for YBCO in either the a – b or
c-axis directions, a mean l of less than 5 nm at each contact
is sufficient to provide the requisite L k . If the actual tilt
boundary contact is a high transparency tunnel barrier, then
I c of the filaments leading up to the barrier will be greater
than that of the contact, and a somewhat longer, but still
quite short, effective length would be required. The fact that
the BM behavior is observed over a range of TB I c s varying
by more than a factor of 100 strongly supports an explanation in which the effective SQUID inductance scales at least
roughly with the TB I c . We additionally observe ‘‘subharmonic’’ ~half-integral! Shapiro steps on our a-axis-grain TB
I – Vs in the presence of a magnetic field, consistent with
high-inductance contacts at the grain boundary.7
We have previously shown6,10 that weak electromigration ~EM! forces can readily displace basal-plane oxygen
ions and thereby affect the superconducting properties of
both uniform YBCO thin films and in-plane TBs. The a – c
TBs are also extremely easily affected by room-temperature
EM, and increases in I c by over a factor of 20 have been
accomplished. EM can also lead to decreases in R n by up to
eight times, and overall increases in I c are easily produced.
The persistence of BM behavior in the I – Vs after such
3128
Appl. Phys. Lett., Vol. 65, No. 24, 12 December 1994
changes provides substantial evidence for the picture of the
EM process as one in which the biased oxygen diffusion acts
to add or subtract filamentary contacts.
There is little in the a – c electromagnetic behavior that
distinguishes them from in-plane TBs. We have also observed BM behavior in the latter structures, albeit not always
so clearly or so commonly. The 1/f noise character of the
a – c TBs is nearly identical, when properly scaled, to that of
in-plane TBs.20 We conclude that the transport properties of
both types of TBs are established largely by the inhomogeneous oxygen disorder in the material immediately adjacent
to the boundaries. The higher R n A and generally lower J c of
the a – c boundaries also suggest that the transport across the
boundary occurs only at sparsely distributed locations, possibly at places where extended defects at the basal plane face
of the a-grain provide normal and superconducting pathways
that effectively short the ideal a – c contact. This is simply an
extension of the argument that the low resistivity of the
a-axis grain itself indicates that the intragrain transport in the
c-axis direction proceeds predominately via extended defects
in the thin film grain and not by true c-axis conduction.
This research was supported by the Office of Naval Research ~N0014-89-J-1692! and the Naval Research Laboratory ~N0014-93-K-2001!. Additional support was provided
by the National Science Foundation through use of the National Nanofabrication Facility.
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B. H. Moeckly and R. A. Buhrman
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