Evidence of a s-wave subdominant order parameter in YBa₂Cu₃O₇₋δ from break-junction tunneling spectra
The tunneling spectra of YBa₂Cu₃O₇₋δ break-junctions have been investigated for the tunneling direction close to the node one. The behavior of the zero-bias conductance peak (ZBCP) and Josephson current have been studied with temperature and magnetic field. The observed deep splitting of ZBCP which...
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| Цитувати: | Evidence of a s-wave subdominant order parameter in YBa₂Cu₃O₇₋δ from break-junction tunneling spectra / A.I. Akimenko, F. Bobba, F. Giubileo, V.A. Gudimenko, S. Piano, A.M. Cucolo // Физика низких температур. — 2010. — Т. 36, № 2. — С. 212-216. — Бібліогр.: 30 назв. — англ. |
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irk-123456789-1168972017-05-19T03:03:25Z Evidence of a s-wave subdominant order parameter in YBa₂Cu₃O₇₋δ from break-junction tunneling spectra Akimenko, A.I. Bobba, F. Giubileo, F. Gudimenko, V.A. Piano, S. Cucolo, A.M. Свеpхпpоводимость, в том числе высокотемпеpатуpная The tunneling spectra of YBa₂Cu₃O₇₋δ break-junctions have been investigated for the tunneling direction close to the node one. The behavior of the zero-bias conductance peak (ZBCP) and Josephson current have been studied with temperature and magnetic field. The observed deep splitting of ZBCP which starts at TS<20–30 K is in agreement with the theory for the dx²–y²± is order parameter [Y. Tanuma, Y. Tanaka, and S. Kashiwaya, Phys. Rev. B64, 214519 (2001)]. We also observed that a low (0.04 T) magnetic field significantly depresses such splitting. The 1/T temperature dependence of maximum Josephson current that goes to saturation at T<TS also confirms the mixed order parameter formation. 2010 Article Evidence of a s-wave subdominant order parameter in YBa₂Cu₃O₇₋δ from break-junction tunneling spectra / A.I. Akimenko, F. Bobba, F. Giubileo, V.A. Gudimenko, S. Piano, A.M. Cucolo // Физика низких температур. — 2010. — Т. 36, № 2. — С. 212-216. — Бібліогр.: 30 назв. — англ. 0132-6414 PACS: 74.72.–h, 74.20.Rp, 74.50.+r. http://dspace.nbuv.gov.ua/handle/123456789/116897 en Физика низких температур Фізико-технічний інститут низьких температур ім. Б.І. Вєркіна НАН України |
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Digital Library of Periodicals of National Academy of Sciences of Ukraine |
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Свеpхпpоводимость, в том числе высокотемпеpатуpная Свеpхпpоводимость, в том числе высокотемпеpатуpная |
| spellingShingle |
Свеpхпpоводимость, в том числе высокотемпеpатуpная Свеpхпpоводимость, в том числе высокотемпеpатуpная Akimenko, A.I. Bobba, F. Giubileo, F. Gudimenko, V.A. Piano, S. Cucolo, A.M. Evidence of a s-wave subdominant order parameter in YBa₂Cu₃O₇₋δ from break-junction tunneling spectra Физика низких температур |
| description |
The tunneling spectra of YBa₂Cu₃O₇₋δ break-junctions have been investigated for the tunneling direction close to the node one. The behavior of the zero-bias conductance peak (ZBCP) and Josephson current have been studied with temperature and magnetic field. The observed deep splitting of ZBCP which starts at TS<20–30 K is in agreement with the theory for the dx²–y²± is order parameter [Y. Tanuma, Y. Tanaka, and S. Kashiwaya, Phys. Rev. B64, 214519 (2001)]. We also observed that a low (0.04 T) magnetic field significantly depresses such splitting. The 1/T temperature dependence of maximum Josephson current that goes to saturation at T<TS also confirms the mixed order parameter formation. |
| format |
Article |
| author |
Akimenko, A.I. Bobba, F. Giubileo, F. Gudimenko, V.A. Piano, S. Cucolo, A.M. |
| author_facet |
Akimenko, A.I. Bobba, F. Giubileo, F. Gudimenko, V.A. Piano, S. Cucolo, A.M. |
| author_sort |
Akimenko, A.I. |
| title |
Evidence of a s-wave subdominant order parameter in YBa₂Cu₃O₇₋δ from break-junction tunneling spectra |
| title_short |
Evidence of a s-wave subdominant order parameter in YBa₂Cu₃O₇₋δ from break-junction tunneling spectra |
| title_full |
Evidence of a s-wave subdominant order parameter in YBa₂Cu₃O₇₋δ from break-junction tunneling spectra |
| title_fullStr |
Evidence of a s-wave subdominant order parameter in YBa₂Cu₃O₇₋δ from break-junction tunneling spectra |
| title_full_unstemmed |
Evidence of a s-wave subdominant order parameter in YBa₂Cu₃O₇₋δ from break-junction tunneling spectra |
| title_sort |
evidence of a s-wave subdominant order parameter in yba₂cu₃o₇₋δ from break-junction tunneling spectra |
| publisher |
Фізико-технічний інститут низьких температур ім. Б.І. Вєркіна НАН України |
| publishDate |
2010 |
| topic_facet |
Свеpхпpоводимость, в том числе высокотемпеpатуpная |
| url |
http://dspace.nbuv.gov.ua/handle/123456789/116897 |
| citation_txt |
Evidence of a s-wave subdominant order parameter in YBa₂Cu₃O₇₋δ from break-junction tunneling spectra / A.I. Akimenko, F. Bobba, F. Giubileo, V.A. Gudimenko, S. Piano, A.M. Cucolo // Физика низких температур. — 2010. — Т. 36, № 2. — С. 212-216. — Бібліогр.: 30 назв. — англ. |
| series |
Физика низких температур |
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2025-07-08T11:16:55Z |
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© A.I. Akimenko, F. Bobba, F. Giubileo, V.A. Gudimenko, S. Piano, and A.M. Cucolo, 2010
Fizika Nizkikh Temperatur, 2010, v. 36, No. 2, p. 212–216
Evidence of a s-wave subdominant order parameter in
YBa2Cu3O7–δ from break-junction tunneling spectra
A.I. Akimenko1,2, F. Bobba1, F. Giubileo1, V.A. Gudimenko2,
S. Piano1, and A.M. Cucolo1
1 Physics Department, CNR-Supermat Laboratory, University of Salerno, Via S. Allende; 84081 Baronissi, Italy
2
B. Verkin Institute for Low Temperature Physics and Engineering of the National Academy of Sciences of Ukraine
47 Lenin Ave., Kharkov 61103, Ukraine
E-mail: akimenko@ilt.kharkov.ua
Received September 15, 2009
The tunneling spectra of YBa2Cu3O7–δ break-junctions have been investigated for the tunneling direction
close to the node one. The behavior of the zero-bias conductance peak (ZBCP) and Josephson current have been
studied with temperature and magnetic field. The observed deep splitting of ZBCP which starts at ST < 20–30 K
is in agreement with the theory for the dx2
–y2 ± is order parameter [Y. Tanuma, Y. Tanaka, and S. Kashiwaya,
Phys. Rev. B64, 214519 (2001)]. We also observed that a low (0.04 T) magnetic field significantly depresses
such splitting. The 1 / T temperature dependence of maximum Josephson current that goes to saturation at
ST T< also confirms the mixed order parameter formation.
PACS: 74.72.–h Cuprate superconductors;
74.20.Rp Pairing symmetries (other than s-wave);
74.50.+r Tunneling phenomena; Josephson effects.
Keywords: tunneling spectra, Josephson current, zero-bias conductance peak.
For the d -wave superconductors, theory predicts spe-
cific quasiparticle bound states (Andreev bound states)
near scattering structures such as surfaces, interfaces and
other defects [1]. In these areas an order parameter (OP)
may change significantly and subdominant OP may ap-
pear leading to a mixed OP (such as 2 2x yd is− ± or
2 2 xyx yd id− ± ) with the spontaneous breaking of time re-
versal symmetry (BTRS) [2–5].
Andreev bound states manifest themselves in different
tunneling spectra as a zero-bias conductance peak (ZBCP)
in agreement with the theory for the 2 2x yd − -wave pairing
[1]. In the case of breaking of time reversal symmetry (due
to magnetic field or subdominant OP), splitting of ZBCP
was predicted [5–7] and also observed in several experi-
ments [8–12]. Theory shows the different kind of splitting
for the is and xyid subdominant OP [5,13]. This question
has not been studied in tunneling experiments up to now.
The Josephson critical current may also give informa-
tion about subdominant OP presence. Its temperature de-
pendence is predicted to saturate at temperatures T < ST
( ST is the critical temperature for subdominant OP) [14].
It is interesting to note that some theories predict split-
ting of ZBCP without any subdominant OP [15] and even
BTRS [16].
To solve the problem, we have investigated the S – I – S
Josephson junctions. The break-junction method was ap-
plied to a thin film and the tunneling spectra with the deep
splitting of ZBCP at temperatures up to 20–30 K were reg-
istered. Indeed this method is very appropriate when deal-
ing with high cT superconductors since exposition of fresh
and clean surfaces is achieved. The analysis of the tem-
perature and magnetic field dependencies says in favor of
the is-subdominant order parameter presence. The maxi-
mum Josephson current also saturates at T < ST .
The tunnel junctions were produced by applying the
modified break-junction technique [17] to highly biepi-
taxial c -axis oriented YBa 2 Cu 3O 7 δ− thin films (thick-
ness ≈ 200 nm), dc sputtered on (001) SrTiO 3 sub-
strates [18]. Electrical transport characterization showed
critical temperatures cT ( = 0)ρ = 91 K and Δ cT < 1 K.
To determine the lateral lattice alignment between the
films and the substrates the x-ray pole figure analysis was
Evidence of a s-wave subdominant order parameter in YBa2Cu3O7–δ from break-junction tunneling spectra
Fizika Nizkikh Temperatur, 2010, v. 36, No. 2 213
used [19]. The stripe-like samples (with the [110]-direction
long side) were glued to a metallic bending plate by the
epoxy glue. A special epoxy cover over the whole sample
was applied to make the construction stable with the time
and temperature change (more details see in Refs. 17 and
19). As a result we were able to investigate a single break-
junction in about a weak time, with the only small change
in its resistance in the temperature range 4.2–120 K [20].
To maximize the tunnel current along the node direction a
straight groove was scratched in the central part of the
covered sample (perpendicular to [110] direction), where
the bending is maximum. By bending with a differential
screw at low temperature, it is possible to crack the sub-
strate together with the film along the groove and smoothly
adjust the junction resistance by gently approaching the
two cracked electrodes by a micrometer screw. The optical
microscope study showed that the fracture direction can
deviate from the straight line only about 10°. The break-
junction method we used may also give a flat fracture
surface [17]. Thus, we could get the high quality tunnel
junctions.
In Fig. 1 we show the low-bias tunneling spectrum
/dI dV vs V of the YBCO break-junction at T = 10 K
measured by standard modulation technique. One can ob-
serve the simultaneous presence of two peak structures.
Indeed, a well developed, narrow peak (with the width
JW ≈ ± 1 mV) centered at zero energy (see also Fig. 3)
appears superimposed to a less pronounced, wider double-
peak structure W Z ≈ ± 2.5 mV. In addition to these, the
wide gap-related maxima (or the bound states with nonzero
energy [1]) around ± 15 mV are observed that shifts to-
wards lower biases for increasing temperature and disap-
pears at T → cT [20]. The similar peak structure with
ZBCP (without splitting) and gap-related peak were also
found in Ref. 21 for the close to [110] direction tunneling
in the N – I – S ramp-edge junctions.
The narrow peak centered at V = 0 is mostly due to the
Josephson direct current though it corresponds to a
smeared current step at V = 0. The more the junction resis-
tance the less its relative intensity. However, the most de-
cisive argument in favor of the Josephson tunneling is the
magnetic field dependence of the conductance at V = 0
shown in inset of Fig. 1. A similar oscillating behavior was
also found for the Josephson critical current in junctions
with the nonuniform current-density distribution [22]. In
our case, the nonuniform current can be due to small devia-
tions from the planar configuration of the junction. We do
not have a satisfactory explanation for the finite slope of
the Josephson critical current. The similar current step with
the finite conductance at V = 0 was earlier observed in the
YBCO grain boundary junctions [23] as well as in the
YBCO and Nb break-junctions [19,24]. Around junction
cT , thermal and external fluctuations can induce the non-
zero resistance since the Josephson coupling energy JE =
/ 2chI e= is comparable with the thermal energy Bk T .
However, at least for our low resistance junctions ( =NR
= 20–100 Ω) at liquid helium temperature JE was greater
than Bk T by a factor of 20.
The double-peak structure with the width ZW looks
like the expected Andreev bound states structure for the
case of subdominant OP presence [1]. The background
conductance underlying the Josephson peak as inferred
from the low temperature data is showed by thin dashed
line in Fig. 1. This procedure is often used in the literature
when dealing with high cT superconductors or if it is not
possible to separate different effects. Indeed, due to the
extremely high value of the critical field 2cH for these
compounds, it is not possible to observe experimentally the
«intrinsic» condensate normal state at low temperatures,
which knowledge is needed to normalize the superconduct-
ing conductance data at low temperatures. In our case we
cannot depress the Josephson current by magnetic field
without essential change of the double-peak structure. This
effect will be discussed below in detail.
In Fig. 2 we show the temperature evolution of the
structures of Fig. 1. One can see that the underlying double
peak, observed at low temperatures, disappears with the
temperature raise between 20 and 30 K transforming into
the single wide peak. Such splitting of ZBCP with deep
minimum for decreasing temperatures is only predicted
for the 2 2x yd is− ± order parameter (left inset of Fig. 2)
[5,13]. The experimental temperature dependence is si-
milar to that calculated in Ref. 5 (right inset in Fig. 2). We
should note that for the S – I – S junctions investigated
Fig. 1. Tunneling spectrum ( /dI dV vs V ) of YBCO break-
junction at T =10 K (solid line). The dashed line around V = 0
drawn by hand. The Josephson peak (with the width JW ≈
≈ ± 1 mV) superimposes on the double peak structure ( ZW ≈
≈ ± 2.5 mV). Sδ shows the position (from V = 0) of the peak in
the double peak structure. To understand the relative intensity of
the peaks see Fig. 4. Inset: magnetic field dependence of the zero-
bias conductance. The external field was applied parallel to the c-
axis direction. The dependence is similar for the opposite
directions of the field applied.
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A.I. Akimenko, F. Bobba, F. Giubileo, V.A. Gudimenko, S. Piano, and A.M. Cucolo
214 Fizika Nizkikh Temperatur, 2010, v. 36, No. 2
here, the relative intensity of extremums must be more
than that for the N – I – S junctions calculated in Ref. 5
[25]. Nevertheless, the alternative 2 2 xyx yd id− ± order
parameter will not give so deep splitting that are found in
our experiments.
We have also found that in relatively low magnetic field
≈ 0.04 T the depth of the minima around Josephson peak
and distance between peaks 2 Sδ essentially decrease (see
Fig. 3). It is reasonable because such magnetic field may
effect as a strong depairing factor on the s -wave pairing.
On the other hand, magnetic field can effect on the An-
dreev bound states shifting their energies to the higher val-
ues (with proper increase of Sδ ) due to Doppler effect [8].
This effect was observed earlier. It seems difficult to dis-
tinguish these two opposite effects if the splitting is small
(smeared due to roughness of the junction interface, see for
instance [1]). Nevertheless, looking carefully on the results
in Ref. 11, one can find the systematic decrease of Sδ with
the low magnetic field increase too.
Thus, the comparison of both temperature and magnetic
field dependencies of our data with the most accredited
theories, appears in favor of the is-subdominant OP from
tunneling measurements. The similar, in our opinion, but
not so evident conclusion was done in Ref. 26 after analy-
sis of the Andreev reflection point-contact spectra.
The maximum strength of the subdominant s-wave pair-
ing from our measurements is /S cT T ≈ 0.24 much higher
than /S cT T ≈ 0.10 earlier reported [8]. Theory [4] predicts
/S cT T = 0.16.
In addition to this, when the is -wave (or xyid -wave)
subdominant pairing realizes, theory predicts the saturation
of the maximum Josephson current at ST T< due to the
decrease of the density of Andreev bounds states at Fermi
level which transfer the Josephson current [14]. A similar
behavior is observed in our experiments as reports in
Fig. 3. To infer these data, we have integrated the Joseph-
son conductance peak taking into account the inferred
background (see the curve at T = 13 K). The temperature
dependence of such current demonstrates clear satura-
tion at ST < 20–30 K. We have also observed the close to
1/T dependence in large temperature range in agreement
with the experimental results for the ZBCP intensity in
S – I – S junction in Refs. 27, 28 and the theory for the
node direction tunneling in the junctions with the same
order parameter orientation in both electrodes [29]. Such
junctions are most probably realized in our experiments.
In summary, the specific form of the tunneling spec-
trum with the deep splitting, the predicted temperature
Fig. 2. Temperature dependence of normalized tunnel conductance
( )T Vσ at low temperatures .T Full line — 13 K, dotted line —
20 K, dashed line — 30 K. Normalizations of Tσ = /dI dV is
done for T = 13 K at V = 7 mV. The thin dashed line is the
inferred background conductance (see text). Left inset: com-
parison of calculated tunneling conductance Tσ 0( / )eV Δ of
N – I – S junction for the node direction tunneling and for the
dx2
–y2 ± is and dx2
–y2 ± idxy order parameters [5]. Bath temperature
/ cT T = 0.05. /S cT T = 0.2. Right inset: calculated N – I – S
junction tunneling conductance σ T 0( / )eV Δ for the dx2
–y2 ± is
state for the node direction tunneling [5]. / cT T = 0.05, 0.10,
0.12, 0.13 starting from bottom. /S cT T = 0.2.
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Evidence of a s-wave subdominant order parameter in YBa2Cu3O7–δ from break-junction tunneling spectra
Fizika Nizkikh Temperatur, 2010, v. 36, No. 2 215
behavior of the splitted ZBCP and of the Josephson current
have been observed in YBCO break-junctions giving the
clear evidence for the mixed symmetry 2 2x yd is− ± of
order parameter near the (110) surface in contrast to the
2 2 xyx yd id− ± OP. The deduced strength of the is-
subdominant OP is rather high leading to the transition into
the 2 2x yd is− ± states at 20 < ST < 30.
A recent finding of s-wave pairing in the heavily Zn-
doped YBCO [30] says also in favor of is-subdominant OP
in the undoped YBCO.
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Fig. 4. Temperature dependence of tunneling spectrum /dI dV vs V . The curves at T > 13 K have been successively shifted along the
voltage bias (with a 5 mV step) and conductance axes (along the thin solid line). The dashed line is the inferred background under
Josephson peak. Inset: temperature dependence of the Josephson current J NI R normalized to its value at T = 13 K, where NR is the
normal state resistance at V = 100 mV. The current corresponds to a square of the shaded area like shown for the curve at T = 13 K.
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2020
1313
VVoltage, mVoltage, mV
1/T1/T, K, K
––11
II RR (T)(T)JJ NN
II RRJJ NN
1313
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