ISSN 1063-7761, Journal of Experimental and Theoretical Physics, 2007, Vol. 105, No. 3, pp. 636–641. © Pleiades Publishing, Inc., 2007.
Original Russian Text © M.A. Tarasov, E.A. Stepantsov, M. Naito, A. Tsukada, D. Winkler, A.S. Kalabukhov, M.Yu. Kupriyanov, 2007, published in Zhurnal Éksperimental’no
Fiziki, 2007, Vol. 132, No. 3, pp. 724–730.
The superconducting transition temperature in mag-
nesium diboride (MgB
) is 39 K, which is the highest
value for compounds that do not contain copper oxide
layers. This substance is a classical superconductor
obeying the Bardeen–Cooper–Schrieffer (BCS) theory
with a relatively large coherence length
in the direction of the
plane. The corresponding values in a YBaCuO
superconductor are almost an order of magnitude lower
0.2–0.5 nm and
ference turns out to be an important factor for manufac-
turing homogeneous and reproducible Josephson junc-
The considerable interest in the problem of prepar-
-based Josephson structures is due to the
potentially high values of their characteristic voltage.
The energy gap in these structures is
7.09 mV in the
band . These values of the gap correspond to
characteristic frequencies of 3.5 and 1.3 THz, which are
substantially higher than the frequency limitations of
700 GHz for niobium junctions.
Until recently, MgB
-based Josephson structures
were mainly prepared using methods that did not per-
mit reproducible control of the parameters of junctions
or the regions of weak bond localization. Such struc-
tures include point contacts, junctions at defects emerg-
10 nm in the
1.5–3.0 nm). This dif-
= 2.7 mV in
ing under the action of mechanical stresses (which will
be referred to as break junctions), and structures of a
type of bridge with a varying thickness, in which a
weak spot was produced either directly by ion implan-
tation, or at random defects formed upon a decrease in
the thickness of a part of the initial film [2
In more controllable structures, one electrode was,
as a rule, a low-temperature superconductor (Nb, Al,
NbN) [12–16]. The properties of MgB
junctions, in which AlO
, AlN [18, 19], and MgO
[20, 21] were used as the insulating (I) interlayer, have
been reported relatively recently.
Unfortunately, the parameters of Josephson junc-
tions reported in these publications were far from opti-
mal. This served as a stimulus for searching for differ-
ent methods for preparing MgB
structures for high-frequency application.
We proposed that a better weak link be formed by
growing a MgB
film on a bicrystalline substrate of mag-
nesium oxide oriented in the (111) plane (MgO (111)).
Mismatching in the lattice constants of MgO in the
= 0.298 nm) and of the MgB
film in the same direction (
For this reason, high-quality MgB
be grown on such a bicrystalline substrate and, as a con-
sequence, an artificial grain boundary intersecting the
entire film thickness can emerge. Subsequent annealing
of such structures in oxygen must be accompanied by
= 0.308 nm) is just 3.4%.
epitaxial films can
Superconducting Weak Bonds at Grain Boundaries in MgB
M. A. Tarasov , E. A. Stepantsov , M. Naito
A. S. Kalabukhov
, and M. Yu. Kupriyanov
Institute of Radio Engineering and Electronics, Russian Academy of Sciences, Moscow, 125009 Russia
Institute of Crystallography, Russian Academy of Sciences, Moscow, 119333 Russia
Department of Applied Physics, Tokyo University of Agriculture and Technology, 2-24-16,
Naka-cho, Kogenei, Tokyo, 184-8588 Japan
Department of Microtechnology and Nanoscience, Chalmers University of Technology, Göteborg, 41296 Sweden
Research Institute of Nuclear Physics, Moscow State University, Moscow, 119992 Russia
Received January 5, 2007
, A. Tsukada
, D. Winkler
in width, intersecting the bicrystalline interface, are formed in epitaxial bicrystalline MgB
these substrates. It is found that annealing of bicrystalline samples in oxygen leads to a systematic decrease in
the critical current, an increase in the temperature width of the superconducting transition region, and to an
improvement of the current–voltage (
) characteristic, which becomes close in shape to the
of a Josephson junction. The response of such a junction to radiation at a frequency of 110 GHz with an ampli-
tude attaining 0.5 mV is measured.
PACS numbers: 74.45.+c, 74.50.+r
—The possibility of preparing bicrystalline Josephson junctions and bolometers based on supercon-
on specially prepared bicrystalline MgO substrates is investigated. Microbridges 0.85–6.00
films grown on
JOURNAL OF EXPERIMENTAL AND THEORETICAL PHYSICS
SUPERCONDUCTING WEAK BONDS AT GRAIN BOUNDARIES IN MgB
its intense diffusion along the artificially produced
interface between the films with the formation of an
insulating MgO or B
layer. Boundaries of the
3 nm thick B
layer between MgB
observed with the help of a transmission electron
microscope in . The interface can also be metallic
with the composition MgB
 or amorphous with the
same composition and a thickness of 5–20 nm, as was
observed in . The boundaries of these three types
can be used for preparing Josephson junctions of the
superconductor–insulator–superconductor (SIS) or
superconductor–normal metal–superconductor (SNS)
2. SAMPLE PREPARATION TECHNIQUE
MgO (111) bicrystalline substrates were prepared
by solid-phase splicing . For this purpose, two
MgO single crystals were brought in contact over the
[(110) – 13
] and [(110) + 13
bicrystals and over the [(112) – 13
planes for another type of bicrystals. In both configura-
tions, the (111) planes of the blocks being spliced were
parallel. The sealing was carried out under ultrahigh
vacuum, heating, and compression. Bicrystals were cut
into 0.5-mm-thick substrates parallel to the (111) plane
and polished. In this way, bicrystalline substrates with
the (111) orientation were prepared with an artificial
grain boundary perpendicular to the surface and with
symmetric rotation of the crystal lattices through 13
opposite directions from the boundary. The boundary
formed an angle of 13
type of substrates and with the
type of substrates. Monocrystalline substrates with the
(111) orientation were also prepared for comparison.
films were grown by thermal evaporation in
ultrahigh vacuum of the specially designed original
evaporation unit (residual gas pressure, 5
made in the form of two (pure magnesium and pure
boron) targets using various electron guns. The evapo-
ration rate for each element was monitored using elec-
tron impact emission spectrometry (EIES). The temper-
ature of the substrate was 280
rate was 0.38 nm/s. To compensate for magnesium
losses due to resputtering, the magnesium evaporation
rate relative to boron was three times as high as the
nominal value. Peculiarities of the film growth were
described in detail in .
Heterostructures consisting of a MgO (111) sub-
strate and a MgB
(001) film were analyzed in an
X-pert Philips four-circle X-ray diffractometer. We
chose the (311)-type planes as skew X-rays reflection
planes for the MgO (111) substrate. The (112) reflect-
ing planes were chosen for the films. The results of
measurement of X-ray diffraction curves during rota-
tion of the sample about the normal to the surface
scan) are presented in Fig. 1. It can be seen from
Fig. 1a that the peaks of reflection from the (112)
planes are displaced by 60
] planes for one type of
] and [(112) + 13
axes in the first
axes in the second
C and the film growth
relative to the reflections
from the (311) planes of the substrate in Fig. 1b. This
means that the crystal lattice in MgB
taxially coupled to the lattice of the MgO (111) sub-
direction on the interface plane in the
film is parallel to the
direction of the substrate.
m in length and 6, 4, 1.5, and 0.85
thickness, which intersected the artificial grain bound-
ary, were formed in the MgB
Each bridge was connected with a planar antenna. Fig-
ure 2 shows an optical image of such an integrated
structure with a log-periodic antenna and a bicrystalline
microbridge at the center.
(001) films is epi-
film by dry ionic etching.
3. EXPERIMENTAL RESULTS AND DISCUSSION
The current–voltage (
) characteristics of the sam-
ples measured immediately after their preparation were
characterized by critical currents from 1.5 to 10 mA
and a normal resistance from 20 to 60
annealing cycles were performed for 1 h in an oxygen
atmosphere under a pressure of 800 mbar at tempera-
tures of 100, 200, 300, 450, and 600
films on bicrystalline substrates in oxygen led to a sys-
. Then several
C. Annealing of
0 –100 200–200
of the MgB
(111) crystal, on which the film was grown during their
rotation about the
axis of the film and for the estab-
lished Bragg position of the source–detector system: (a) for
the film plane of the (112) type and (b) for the substrate
plane of the (311) type.
Curves recorded in X-ray diffractometric scanning
(001) film and a substrate made of a MgO
JOURNAL OF EXPERIMENTAL AND THEORETICAL PHYSICS
TARASOV et al.
tematic decrease in the critical current, an expansion of
the temperature range of the transition, and a transfor-
mation of the shape of IV curves.
Figure 3 shows the IV characteristics of the struc-
tures obtained as a result of annealing at 450°C of
0.9-µm-wide bridges prepared on a 〈110〉 bicrystalline
substrate and on a monocrystalline substrate. Measure-
ments were performed in the temperature interval
4.2−30 K. Figure 3c shows the IV characteristics in var-
ious magnetic fields up to 3.5 T at a temperature of
28 K. It can be seen that, as a result of annealing, the IV
curves acquire the shape typical of shunted Josephson
junctions. Upon a decrease in temperature, the IV
curves acquire a hysteresis. Annealing of a 〈112〉 bi-
crystalline sample at 600°C led to complete vanishing
of the critical current and to an increase in the resis-
tance up to 1 kΩ for the broadest (6-µm) junction and
to infinity for the remaining junctions.
Figure 4 shows the temperature dependences of the
critical current for 0.9-µm-thick bridges on bicrystal-
line and monocrystalline substrates after annealing for
1 h at 600°C. It can be seen that the absolute values of
the critical currents and their temperature dependences
are different. This means that oxidation of the artifi-
cially prepared interface in MgB2 on a bicrystalline
substrate indeed occurs at a higher rate and leads to the
Fig. 2. Optical image of an integrated structure containing a
log-periodic antenna and a MgB2 microbridge intersecting
the bicrystalline boundary at the center.
T = 4.2 K
T = 25 K
T = 29.4 K
20 60 –60–20
H = 1 T
Fig. 3. IV characteristics for a MgB2 〈110〉 bicrystalline bridge 0.9 µm in width, annealed at 450°C, which were measured at tem-
peratures of (a) 25–30, (b) 20, 10, 4.2, and (c) 28 K in magnetic fields of 1, 1.5, 2, 2.5, 3, and 3.5 T, as well as (d) IV curves for
annealed monocrystalline bridge of thickness 0.9 µm at 26.5–29.4 K.
JOURNAL OF EXPERIMENTAL AND THEORETICAL PHYSICS Vol. 105 No. 3 2007
SUPERCONDUCTING WEAK BONDS AT GRAIN BOUNDARIES IN MgB2
30 2928 27 262531 24
Fig. 4. Temperature dependences of the critical current of a
0.9-µm-wide bridge on a monocrystalline (1) and bicrystal-
line (2) substrates after annealing.
emergence of a weak spot in the film in which the crit-
ical current is lower than in the remaining parts of the
In Fig. 5, the temperature dependences Ic(T) of the
critical current are compared with the available theoret-
ical dependences calculated for tunnel SIS structures
 and two-barrier SINIS structures  in the case of
tunneling along the c axis. The same figure shows the
experimental data obtained in structures with weak
coupling at natural boundaries in MgB2  and in
break junctions [4, 5, 8] in the structures formed by ion
implantation , etching by a focused ion beam ,
and in tunnel junctions of MgB2/AlOx/MgB2  and
MgB2/AlN/MgB2 [18, 19].
It can be seen that the available experimental data
can be conditionally divided into three groups. The first
group includes the results obtained in , where the
temperature dependence has a positive curvature and
the slope of the curve in the vicinity of the critical tem-
perature is noticeably higher than for the remaining
aggregate of data. In our opinion, this is due to the fact
that the dynamic state in the structures studied in 
was formed due to depinning of Abrikosov vortices
rather than as a result of breaking of the Josephson cou-
The second group of results was obtained in break
junctions [4, 5, 8] and in MgB2/MgO/MgB2 junctions
[20, 21]. These data are characterized by an excessively
large (as compared to theoretical curves) negative cur-
vature of the Ic(T) curves. Such a behavior of the curves
is apparently associated with the nonuniformity of the
superconducting properties over the width of the junc-
tion. In this case, the decrease in temperature must be
accompanied by not only an increase in the modulus of
the order parameter of the electrodes, but also by the
inclusion of an additional critical current transport
channel. The latter circumstance is precisely responsi-
ble for a higher rate of increase of Ic and a decrease in
The third group of data, including the results
obtained in [3, 10, 18, 19], as well as our results, exhib-
its a satisfactory agreement between the experimental
dependences Ic(T) and the shape of the curve calculated
for two-barrier SINIS structures in the limit of small
values of the effective suppression parameter. The neg-
ative curvature of this curve is a consequence of the
two-band nature of the MgB2 compound. It is absent in
analogous structures with one-band superconductors
 as well as MgB2/I/MgB2 tunnel junctions with tun-
neling in the ab plane . The latter circumstance is not
accidental. It was shown in  that even an insignifi-
cant disorientation (exceeding 0.6°) of crystallographic
directions c of contacting MgB2 blocks leads to the
same result of tunneling in the ab plane as tunneling in
the c direction. It should also be noted that the satisfac-
tory agreement with the shape of the curve calculated
for two-barrier SINIS structures does not mean that we
are dealing precisely with the junction formed by a nor-
mal metal sandwiched between two two-band super-
conductors. The same result can be obtained if we have
a tunnel junction with localized states located at the
center of a tunnel layer with an energy uniformly dis-
tributed in the vicinity of the Fermi energy . Com-
parison precisely with the shape of the Ic(T) curve is
necessitated by the following factors. The spatial distri-
butions of the superconducting and normal current
components in the structures considered here, as well
as bicrystalline HTSC junctions, do not coincide as a
0.20.4 0.6 0.81.0
Fig. 5. Comparison of our temperature dependence of criti-
cal current (crosses) with the available theoretical depen-
dences calculated for tunnel SIS structures  (dashed
curve) and two-barrier SINIS structures  (solid curve)
for tunneling along the c axis with the results obtained by
other authors: ? , ? , ? , ? , ? [14, 18, 19], ?
, ? , and ? .
JOURNAL OF EXPERIMENTAL AND THEORETICAL PHYSICS Vol. 105 No. 3 2007
TARASOV et al.
rule. This leads to the emergence of a shunting resis-
tance and suppression of the values of characteristic
voltage , which complicates the comparison of
absolute experimental values of IcRN with the theory
(RN is the resistance of the normal metal).
Thus, we have shown that oxidation along the bic-
rystalline boundary makes it possible to form controlla-
ble weak bonds in MgB2 in the ab plane. We also mea-
sured the voltage response for such junctions irradiated
at a frequency of 110 GHz. Radiation emitted by an
impact avalanche transit-time (IMPATT) diode via a
scalar horn, attenuators, and an optical window with
cold filters was fed to the cold stage of the cryostat. The
samples were placed on the plane surface of an
extended hyperhemispherical sapphire lens. Figure 6
shows the dependences of the response on the bias cur-
rent and voltage. The peaks of the response correspond
to the positions of the differential resistance peaks. Sha-
piro steps were not observed in explicit form, which can
be due to the nonuniform distribution of the current
over the bridge thickness. In a wider range of bias volt-
ages, the response gradually decreases, which may be
due to overheating by the bias current. The amplitude of
the response attained 0.5 mV, which makes it possible
to use junctions of this type as bolometric receivers at a
temperature of about 30 K.
We prepared MgB2 bridges 0.85–6.00 µm in width
on monocrystalline and bicrystalline MgO (111) sub-
strates of two types: 13°/13° 〈110〉 and 13°/13° 〈112〉.
The 100-nm-thick MgB2 films had a critical tempera-
ture of Tc = 34 K. The critical currents of 0.85-µm-thick
samples immediately after their preparation were
1.5−10 mA for normal resistances of 20–60 Ω. Samples
were annealed in oxygen for 1 h at 100, 200, 300, 450,
and 600°C. Annealing of films on bicrystalline sub-
strates led to a systematic decrease in critical current,
completely suppressed its hysteresis, extended the tem-
perature range of the junctions, and improved the shape
of IV characteristics, which resembled in this case the
IV curves of Josephson junctions. Bridges on a mono-
crystalline substrate did not exhibit any noticeable
changes. Thus, we proved that oxidation along the bi-
crystalline boundary makes it possible to form control-
lable weak bonds in MgB2 bridges. The absence of
explicit oscillations in the dependences on the magnetic
film and microwave radiation power may be due to the
nonuniformity of the current distribution of the current
along the bridge boundary. We also measured the volt-
age response in such junctions upon irradiation at a fre-
quency of 110 GHz with an amplitude of up to 0.5 mV,
which enables us to consider the possibility of using
such junctions as bolometric receivers at a temperature
of about 30 K.
This study was supported by VR and SI Swedish
agencies, the Russian Foundation for Basic Research
(project no. 05-02-19650), and the Federal Agency for
Science and Innovations (grant no. 02.513.11.3157).
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Translated by N. Wadhwa