Possibility of a s-wave pairing in heavily Zn-doped YBa₂Cu₃O₇₋δ based on magnetic field effect on Andreev reflection spectra
When the dx₂₋y₂ -wave pairing is suppressed by Zn-doping in YBa₂Cu₃O₇₋δ some of the Andreev reflection spectra were found to be similar to the s-wave spectra of conventional superconductors. The energy gap is rather reproducible (2.3–3.0 meV). It is suppressed by low magnetic field (HcPC = 120–270...
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Фізико-технічний інститут низьких температур ім. Б.І. Вєркіна НАН України
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irk-123456789-1178712017-05-28T03:04:19Z Possibility of a s-wave pairing in heavily Zn-doped YBa₂Cu₃O₇₋δ based on magnetic field effect on Andreev reflection spectra Akimenko, A.I. Gudimenko, V.A. Свеpхпpоводимость, в том числе высокотемпеpатуpная When the dx₂₋y₂ -wave pairing is suppressed by Zn-doping in YBa₂Cu₃O₇₋δ some of the Andreev reflection spectra were found to be similar to the s-wave spectra of conventional superconductors. The energy gap is rather reproducible (2.3–3.0 meV). It is suppressed by low magnetic field (HcPC = 120–270 mT) in great contrast to the d-wave spectra (HcPC > 3 T) with the similar order of gap magnitude. We suppose that the s-wave pairing occurs near the Zn impurities. 2008 Article Possibility of a s-wave pairing in heavily Zn-doped YBa₂Cu₃O₇₋δ based on magnetic field effect on Andreev reflection spectra / A.I. Akimenko, V.A. Gudimenko // Физика низких температур. — 2008. — Т. 34, № 11. — С. 1122–1126. — Бібліогр.: 42 назв. — англ. 0132-6414 PACS: 74.72.Bk;74.45.+c;74.20.Rp;74.62.Dh http://dspace.nbuv.gov.ua/handle/123456789/117871 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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DSpace DC |
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English |
| topic |
Свеpхпpоводимость, в том числе высокотемпеpатуpная Свеpхпpоводимость, в том числе высокотемпеpатуpная |
| spellingShingle |
Свеpхпpоводимость, в том числе высокотемпеpатуpная Свеpхпpоводимость, в том числе высокотемпеpатуpная Akimenko, A.I. Gudimenko, V.A. Possibility of a s-wave pairing in heavily Zn-doped YBa₂Cu₃O₇₋δ based on magnetic field effect on Andreev reflection spectra Физика низких температур |
| description |
When the dx₂₋y₂ -wave pairing is suppressed by Zn-doping in YBa₂Cu₃O₇₋δ some of the Andreev reflection
spectra were found to be similar to the s-wave spectra of conventional superconductors. The energy
gap is rather reproducible (2.3–3.0 meV). It is suppressed by low magnetic field (HcPC = 120–270 mT) in
great contrast to the d-wave spectra (HcPC > 3 T) with the similar order of gap magnitude. We suppose that
the s-wave pairing occurs near the Zn impurities. |
| format |
Article |
| author |
Akimenko, A.I. Gudimenko, V.A. |
| author_facet |
Akimenko, A.I. Gudimenko, V.A. |
| author_sort |
Akimenko, A.I. |
| title |
Possibility of a s-wave pairing in heavily Zn-doped YBa₂Cu₃O₇₋δ based on magnetic field effect on Andreev reflection spectra |
| title_short |
Possibility of a s-wave pairing in heavily Zn-doped YBa₂Cu₃O₇₋δ based on magnetic field effect on Andreev reflection spectra |
| title_full |
Possibility of a s-wave pairing in heavily Zn-doped YBa₂Cu₃O₇₋δ based on magnetic field effect on Andreev reflection spectra |
| title_fullStr |
Possibility of a s-wave pairing in heavily Zn-doped YBa₂Cu₃O₇₋δ based on magnetic field effect on Andreev reflection spectra |
| title_full_unstemmed |
Possibility of a s-wave pairing in heavily Zn-doped YBa₂Cu₃O₇₋δ based on magnetic field effect on Andreev reflection spectra |
| title_sort |
possibility of a s-wave pairing in heavily zn-doped yba₂cu₃o₇₋δ based on magnetic field effect on andreev reflection spectra |
| publisher |
Фізико-технічний інститут низьких температур ім. Б.І. Вєркіна НАН України |
| publishDate |
2008 |
| topic_facet |
Свеpхпpоводимость, в том числе высокотемпеpатуpная |
| url |
http://dspace.nbuv.gov.ua/handle/123456789/117871 |
| citation_txt |
Possibility of a s-wave pairing in heavily Zn-doped
YBa₂Cu₃O₇₋δ based on magnetic field effect on Andreev
reflection spectra / A.I. Akimenko, V.A. Gudimenko // Физика низких температур. — 2008. — Т. 34, № 11. — С. 1122–1126. — Бібліогр.: 42 назв. — англ. |
| series |
Физика низких температур |
| work_keys_str_mv |
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| first_indexed |
2025-07-08T12:56:26Z |
| last_indexed |
2025-07-08T12:56:26Z |
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1837083544464130048 |
| fulltext |
Fizika Nizkikh Temperatur, 2008, v. 34, No. 11, p. 1122–1126
Possibility of a s-wave pairing in heavily Zn-doped
YBa2Cu3O7–� based on magnetic field effect on Andreev
reflection spectra
A.I. Akimenko and V.A. Gudimenko
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 May 5, 2008, revised June 8, 2008
When the d
x y2 2
�
-wave pairing is suppressed by Zn-doping in YBa 2Cu 3O7�� some of the Andreev re-
flection spectra were found to be similar to the s-wave spectra of conventional superconductors. The energy
gap is rather reproducible (2.3–3.0 meV). It is suppressed by low magnetic field (Hc
PC = 120–270 mT) in
great contrast to the d-wave spectra (Hc
PC > 3 T) with the similar order of gap magnitude. We suppose that
the s-wave pairing occurs near the Zn impurities.
PACS: 74.72.Bk Y-based cuprates;
74.45.+c Proximity effects; Andreev effect; SN and SNS junctions;
74.20.Rp Pairing symmetries (other than s wave);
74.62.Dh Effects of crystal defects, doping and substitution.
Keywords: Y-based cuprates, Andreev reflection, N–S boundary, pairing symmetries, s wave, d wave, dop-
ing and substitution.
The d
x y2 2
�
-wave pairing is widely recognized as
a dominant mechanism of superconductivity in the
high-temperature superconductors [1]. However, in some
cases an additional subdominant order parameter (OP)
may better explain the experimental results [2–5]. Theory
predicts the appearance of is- or id xy-subdominant OP
near the (110) surface where the d
x y2 2
�
-wave OP is es-
sentially suppressed due to the change of the order param-
eter sign along the quasiparticle trajectory [6–8]. The re-
cent tunneling [9] and Andreev reflection [5] experiments
are in agreement with the is-subdominant OP in
YBa2Cu3O7–� (YBCO). However, the problem is still un-
der debates. We have proposed the new approach here to
clarify the question.
The order parameter can be also changed by doping.
The Andreev reflection (AR) spectra show the possible
transition from d
x y2 2
�
- to s (or d � is)-wave pairing with
the oxygen doping change in Pr 2�xCe xCuO 4 [10,11]. It
is well known that doping by Zn in YBCO decreases the
critical temperature Tc and energy gap [12,13]. The criti-
cal temperature in YBa 2(Cu 1�xZn x) 3O 7�� falls from
about 90 K to about 25 K with the change of x from zero
to 0.075 [14]. That is why the heavily Zn-doped YBCO
with low Tc is perspective to find out another pairing in
YBCO and has been investigated here. We have found the
typical for the s-wave superconductor Andreev reflection
spectra that are very sensitive to low magnetic field in
contrast to the d-wave spectra.
The first problem is how to distinguish the AR spectra
(or the point-contact spectra when there is a barrier at in-
terface) with the different type (d
x y2 2
�
, d xy , s) of OP.
One of the evidence of the d-wave pairing is presence
of the zero-bias conductance peak (ZBCP) in a tunneling
spectrum (except the lobe-direction tunneling and gapless
superconductor) [15]. For the point contacts with direct
conductivity, ZBCP must be absent if the barrier Z at N/S
boundary is zero. However, due to the difference in Fermi
velocity in the point-contact electrodes (a normal metal and
high-temperature superconductor), Z is always more than
zero [16], and ZPCB has been observed in most experiments
[12,13,17–19]. Boundary roughness, defects and impurities
may decrease the intensity of ZBCP essentially [15].
For the conventional s-wave superconductors, the
modified Blonder–Tinkham–Klapwijk (BTK) theory [20]
discribes an experimental point-contact (PC) spectrum
quite well using three fitting parameters: a gap value �, �
© A.I. Akimenko and V.A. Gudimenko, 2008
and relatively small smearing factor �, �/� �� 1. In the
case of the d-wave superconductor it is not usually possi-
ble to do that with the reasonable value of �. It is because
of the strong gap anisotropy and low-angle resolution for
the orifice-like point contacts [21] ( � 90°). However, the
channel-like point contacts with rough walls may have
the angle resolution close to a tunnel junction. Thus, even
for the d-wave superconductor, it is possible to register a
PC spectrum which is similar to the s-wave one. The gap
anisotropy effect reduces in this case, and a small number
of closely lying gaps forms a PC spectrum, especially if
the channel-like point contact in the lobe direction is real-
ized in an experiment. In different point contacts the di-
rection of electron flow is different as a rule, and the gap
values extracted may be essentially different for the same
Tc in the point-contact region. Taking into account the
possible complex configuration of a real point contact
(several conducting spots with different shape), analysis
only of the PC spectrum form is not reliable method to
know the OP type. Nevertheless, the information about
gap distribution for the d-wave superconductor may be
obtained by the proper histogram building [12].
The critical parameters (Tc and H c2) of the d-wave
superconductor was found to be much higher than that
for the conventional s-wave one (for YBa 2Cu 3O 7��
Tc 95 K and H c
100 T ) [22].
Thus, if one will register the PC spectra without ZBCP
with low critical parameters and similar gap values for dif-
ferent PCs, pairing is very possible to be of the s-wave type.
It was found earlier that doping by Zn decreases
Tc without essential change of electron density in
YBa2(Cu1–xZnx) 3O 7�� [23,24]. At x 0.1–0.12, Tc falls
to about 10 K. The distribution of energy gaps also goes
to zero [12]. At x � 0.05, some of the PC spectra look like
expected for the gapless superconductor [25]. Most
likely, the gapless state appears on the part of Fermi sur-
face close to the node lines.
W e h a v e i n v e s t i g a t e d t h e p o l y c r y s t a l
YBa 2(Cu1�xZn x) 3O 7�� sample with the nominal x = 0.075.
The resistivity measurement shows the wide transition
from normal to the superconducting state (from 40 to 10 K).
It is in agreement with Tc 25 K for x = 0.075 obtained in
the sample with the steeper resistive transition in Ref. 12.
High inhomogeneity in the Zn distribution let us getting
the large variety of the PC spectra in the same experiment
to study the magnetic field effect.
The standard modulation method [26] was applied to
measure dI/dV vs V .
In Fig. 1, two kinds of the PC spectra (with low Z) typi-
cal for the heavily Zn-doped YBCO (x � 0.05) are shown.
The first (a) has the gap-related maximum and relatively
narrow ZBCP, the second (b) has a wide maximum around
V = 0. Theory predicts approximately such a form of PC
spectrum for the gap- and gapless-superconductor, re-
spectively [27,28].
The magnetic field of about 3 T affects both observed
peaks essentially while in the Zn-undoped YBCO, such a
field has no any visible effect on our PC spectra (except
the ZBCP). In the case (a), the magnetic field shifts the
gap-related maximum at V � 6 mV to lower energies
like it was found earlier for conventional superconductors
[29,30]. The absence of splitting of ZBCP with the field
was observed earlier in the tunneling and point-contact
experiments too [11,31,32], and one of the possible rea-
sons is that the field is parallel to the N/S interface
[33,34]. Our point contacts were made between the
rod-shape electrodes (like in Ref. 35) and geometrically
the magnetic field was applied parallel to the N/S inter-
face. However, the real situation is difficult to control be-
cause of surface roughness.
In the case of gapless regime (b), the field suppresses
the peak around V = 0 without any essential change of its
energy location and form. Such a behavior is well known
for the conventional gapless superconductors in tunnel-
ing experiments [36].
Thus, it seems that there is no any difference in mag-
netic field effect (except the value of field applied) on the
d
x y2 2
�
-wave Zn-doped superconductor YBCO and the
Possibility of a s-wave pairing in heavily Zn-doped YBa2Cu3O7–� based on magnetic field effect on Andreev reflection
Fizika Nizkikh Temperatur, 2008, v. 34, No. 11 1123
–20 –10 0 10 20
4.5
5.0
5.5
–75 –50 –25 0 25 50 75
2.0
2.5
3.0
0
H (T)
3.3
1.05
2.1
H
a
b
3.3
0.005
H (T)
2.4
0.675
1.5
H
V, mV
dI
/d
V
,m
S
dI
/d
V
,m
S
Fig. 1. Magnetic fields dependences of the d
x y2 2
�
-wave An-
dreev reflection spectra of YBa2(Cu1�xZnx)3O7�� with x = 0.075:
(a) shows the case of gap-related maximum presence (at
V � 6 mV for H = 0). The position of the maximum in zero
field may be different for different point contacts (for instance,
see the left inset in Fig. 3 and Ref. 12). (b) corresponds to the
gapless superconductivity case. The bath temperature T = 4.2 K.
conventional s-wave superconductor assuming that
Zn-doping does not change the pairing mechanism.
We have also registered some spectra (Figs. 2 and 3)
similar to those found in the numerous studies of the con-
ventional s-wave superconductors. They have clear maxi-
mum at low energy without any ZBCP. In Fig. 4, the
symmetrized experimental curves measured at H 0 are
compared with the calculated ones. There is a good agree-
ment in the gap-related region (interval about � 5 mV
around zero bias). The structure at 5–10 mV seen on
the curve (a) and (c) is often observed, but its origin is not
clear yet [37–39]. The modified BTK fitting procedure
[20] gives the similar values of gap � = 2.35–3.0 meV for
different point contacts with the small enough smearing
factor �/� � 0.2. One shuld to note that the gap value
extracted from the point-contact experiment may be differ-
ent if the bulk gap value depend on pressure. The pressure in
the mechanically made point contacts may be rather diffe-
rent, and it may be a reason of the gap value variation found.
The most interesting finding is that all the spectra are
very sensitive to low magnetic field in great contrast to
those shown in Fig. 1. The gap-related maximum goes to
zero bias in a way characteristic to the conventional
s-wave superconductor [29,30]. It is most clear seen for
1124 Fizika Nizkikh Temperatur, 2008, v. 34, No. 11
A.I. Akimenko and V.A. Gudimenko
–20 –15 –10 –5 0 5 10 15 20
140
160
180
200
220
–10 0 10
135
140
145H, mT
0
30
60
90
126
150
225
3000
V, mV
V, mV
dI
/d
V
,m
S
dI
/d
V
,m
S
Fig. 2. The s-wave type Andreev reflection spectra of
YBa2(Cu1�xZnx)3O7�� (x = 0.075) with the magnetic field
change. T = 4.2 K. Hc
PC
225 mT. Inset shows the case with
another character of the background behavior at
H Hc
PC
� 120 mT: H = 0, 30, 60, 120, 225, 450, 600 mT
from upper curve.
–20 –15 –10 –5 0 5 10 15 20
36
38
40
42
–20 –10 0 10
36
38
40
42
–40 –20 0 20 40
V, mV
3.3
0.9
2.1
H, T
1.2
525
H, mT
2.5 mS
V, mV
dI
/d
V
,m
S
dI
/d
V
,m
S
V, mV
Fig. 3. Unusual transformation of the s-wave type spectrum
with the magnetic field. H = 1.2, 105, 150, 180, 225, 270, 600,
900 mT starting from the bottom at V = 0. T = 4.2 K.
Hc
PC
270 mT. Right inset shows the high-magnetic field ef-
fect. Left inset shows two spectra in the enlarged bias range.
The spectrum at H = 525 mT is shifted up for clarity.
–10 –5 0 5 10
0.8
1.0
1.2
–10 –5 0 5 10
1.00
1.01
1.02
1.03
–10 –5 0 5 10
0.96
0.98
1.00
a b c
V, mV
dI
/d
V
V, mVV, mV
Fig. 4. Comparison of the experimental s-wave spectra (dark dots) with the BTK calculations. The experimental curves were previ-
ously symmetrized and normalized on value at V = � 15 mV. The parameters of fitting are as follows: � = 2.35 meV, � = 0.2 meV,
Z = 0.30 (a); � = 3.00 meV, � = 0.4 meV, Z = 0.55 (b); � = 2.40 meV, � = 0.4 meV, Z = 1.15 (c).
curves in Fig. 2. The critical magnetic field for the po-
int-contact region H c
PC corresponds to the case when the
AR spectrum is entirely suppressed, and for the PC spec-
tra of the s-wave type registered here is 120–270 mT. The
spectrum in Fig. 3 demonstrates more complex shape and
behavior with the increasing magnetic field. It has also
the weaker structure at V = � (10–20 mV) that is not so
changeable with magnetic field (see left inset in Fig. 3) as
the low-bias structure (atV � 5 mV). This high-bias struc-
ture is similar to that found in YBa 2(Cu 1�xZn x) 3O 7��
(with x = 0.025) gap-related structure [12]. The low-bias
double-peak structure transforms into the high ZBCP
which then is suppressed in rather high magnetic field in a
way similar to the spectrum shown in Fig. 1,b (gapless re-
gime). This observation shows that there are different
superconducting phases near N/S boundary in this point
contact. The different reaction on the magnetic field ap-
plied may give such an unusual behavior.
We have also noticed that the s-type spectrum is usu-
ally registered after the spectrum with the resonance scat-
tering peak similar to that found in Refs. 2 and 40. Be-
cause our point contacts were made by the successive
mutual shift of electrodes [35], one can suppose that the
s-type superconductivity locates near a Zn-impurity (or a
cluster).
Proximity effect, induced by the d
x y2 2
�
-superconduc-
tor, may be a reason of the s-wave pairing [41] in a normal
electrode, but in Andreev reflection effect the conduc-
tance maximum (or a kink) is connected only with the
maximum gap along the quasiparticle trajectory [42]. The
«proximity gap» is always less than in d
x y2 2
�
-supercon-
ductor.
In summary, we have found that some of the Andreev-re-
flection spectra observed in the heavily Zn-doped
YBa 2Cu 3O 7�� are similar to the conventional s-wave su-
perconductor ones in a shape, gap value reproducibility and
sensitivity to low magnetic field. It confirms the possibility
of the s-wave pairing in YBa 2Cu 3O 7�� if the d-wave pair-
ing is suppressed.
We acknowledge Prof. I.K. Yanson for the fruitful dis-
cussions and Prof. S. Suleimanov for the YBCO samples.
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