Defect formation in the intermediate layers of YBa2Cu3O7-d superconductors depending on oxygen сontents
In YBa2Cu3O7-d  superconductors with 0≤δ<0.6 the defect formation in the intermediate layers depending on oxygen content was revealed under γ-irradiation with doses 8-250 kGy. In YBa2Cu3O7 the subthreshold creation of Ba, Cu1, O1 defects, which give rise to lowering of the an...
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Chuiko Institute of Surface Chemistry National Academy of Sciences of Ukraine
2002
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Surface| _version_ | 1869291189047394304 |
|---|---|
| author | Adonkin, V. T. Gorelov, B. M. Ogenko, V. M. Melnikov, V. S. Kevdina, I. B. Shantorovich, V. P. |
| author_facet | Adonkin, V. T. Gorelov, B. M. Ogenko, V. M. Melnikov, V. S. Kevdina, I. B. Shantorovich, V. P. |
| author_institution_txt_mv | [
{
"author": "V. T. Adonkin",
"institution": "Інститут хімії поверхні НАН України"
},
{
"author": "B. M. Gorelov",
"institution": "Інститут хімії поверхні НАН України"
},
{
"author": "V. M. Ogenko",
"institution": "Інститут хімії поверхні НАН України"
},
{
"author": "V. S. Melnikov",
"institution": "Institute of Geochemistry, Minerology and Oreformation"
},
{
"author": "I. B. Kevdina",
"institution": "Institute of Chemical Physics"
},
{
"author": "V. P. Shantorovich",
"institution": "Institute of Chemical Physics"
}
] |
| author_sort | Adonkin, V. T. |
| baseUrl_str | |
| collection | OJS |
| datestamp_date | 2018-11-27T09:42:19Z |
| description | In YBa2Cu3O7-d  superconductors with 0≤δ<0.6 the defect formation in the intermediate layers depending on oxygen content was revealed under γ-irradiation with doses 8-250 kGy. In YBa2Cu3O7 the subthreshold creation of Ba, Cu1, O1 defects, which give rise to lowering of the annihilation rate of positrons and lattice parameter c, disorder of Cu-O chains, dominates. The defect formation is weakened with δ growth. In YBa2Cu3O6.7 superconductor the subthreshold pushing out of atoms occurs at low temperatures and in O1→O5 transitions, while in compounds with δ>0.3 the subthreshold defect formation in the intermediate layers is not observed. The subthreshold defect formation is related to the Coulomb ejection of atoms from the lattice sites in the field of the weakly damped low‑frequency collective excitations enhanced by irradiation in YBa2Cu3O7-d superconductors with δ≤0.3. |
| first_indexed | 2025-07-22T19:30:14Z |
| format | Article |
| fulltext |
177
DEFECT FORMATION IN THE INTERMEDIATE
LAYERS OF YBa2Cu3O7-d SUPERCONDUCTORS DEPENDING
ON OXYGEN СONTENTS
V.T. Adonkin1, B.M. Gorelov1, V.M. Ogenko1, V.S. Melnikov2,
I.B. Kevdina3, and V.P. Shantorovich3
1Institute of Surface Chemistry, National Academy of Sciences,
03164 Kyiv, Gen. Naumov Str. 17, UKRAINE
2Institute of Geochemistry, Minerology and Oreformation, 03164 Kyiv, UKRAINE
3Institute of Chemical Physics, 117977 Moscow, RUSSIA
Abstract
In YBa2Cu3O7-d superconductors with 0≤δ<0.6 the defect formation in the
intermediate layers depending on oxygen content was revealed under γ-irradiation with doses
8-250 kGy. In YBa2Cu3O7 the subthreshold creation of Ba, Cu1, O1 defects, which give rise
to lowering of the annihilation rate of positrons and lattice parameter c, disorder of Cu-O
chains, dominates. The defect formation is weakened with δ growth. In YBa2Cu3O6.7
superconductor the subthreshold pushing out of atoms occurs at low temperatures and in
O1→O5 transitions, while in compounds with δ>0.3 the subthreshold defect formation in the
intermediate layers is not observed. The subthreshold defect formation is related to the
Coulomb ejection of atoms from the lattice sites in the field of the weakly damped
low-frequency collective excitations enhanced by irradiation in YBa2Cu3O7-d superconductors
with δ≤0.3.
Introduction
The YBa2Cu3O7-d compounds are superconductors in the 0≤ δ<0.6 region of oxygen
content, whose critical temperature Tc step like arises when δ →0 [1]. The abrupt increase of
Tc to ~90K takes place in the narrow interval of oxygen content 0.25≤δ≤0.3 that is related to a
switching on of the plasmon mechanism of high temperature superconductivity [2]. If this is
true, the YBa2Cu3O7-δ superconductors with δ≤0.25-0.3 are differed from compounds with
δ>0.3 by a presence of the low-frequency collective excitations or the acoustic plasmons. The
propagation of the weakly damped excitations can give rise to the low-energy subthreshold
defect formation in the Ba-O and Cu1-O intermediate layers when an energy of atomic
displacement from lattice sites is Es«Ed and a time of displacement τs»τd, (Ed , τd are the
threshold energy and time of atomic displacement for the impact defect formation due to
elastic collisions with particles having the overthreshold energy). The subthreshold creation of
defects [3] is caused by pushing out atoms from lattice sites to the distance, which excludes
the recombination of defects, during the time when the antinodes of the plasmon charge
density is located on the atom, i.e. τs≤T (T is the period of vibrations) or the frequency of
excitations is:
Ωq= qu ≤ ωD (1),
where q, u are the wave vector and velocity of excitations, ωD is the Debye frequency.
Therefore, if to excite the weakly damped plasmons, for instance under γ-irradiation, in the
superconductors of YBa2Cu3O7-δ with δ≤0.25-0.3 the defect formation can be enhanced due to
the subthreshold creation of defects in the field of low-frequency plasmons, in contrast to
superconductors with δ>0.3, where such plasmons are improbable and the impact atomic
178
displacement takes place. Note that the other type of subthreshold creation of defects occurs
during the relaxation on atoms of the strongly localized electronic excitations whose lifetime
is τ≥1/ωD [4]. In YBa2Cu3O7-δ compounds such states may be formed when the band spectrum
is changed as δ→1. However, in compounds with 0≤δ≤0.6, where the number of free carriers
is high, the existence of the charged local excitations is improbable (the polaron states were
probably observed in the semiconductor of YBa2Cu3O6.3 [5], though coexistence of the spin
excitations and band electrons is possible [6-8].
In present work the influence of oxygen content on the defect formation in the
intermediate layers of YBa2Cu3O7-δ superconductors with 0≤δ≤0.55 was investigated under
irradiation with low doses of γ-quanta. Samples of three compositions with δ=0 and 0.55
which corresponded to the high temperature superconductor, where the low-frequency
plasmons may be realized, and the superconductor, where the existence of such plasmons is
improbable, as well as samples of the intermediate composition with δ=0.3 were studied. To
excite the weakly damped plasmons, samples were irradiated with low doses of γ-quanta,
which create a low number of radiation defects Nd, but enhance a rise of the number of
plasmons, since the screening of perturbations in the charge density δZei (δZ is the effective
charge of perturbations, i is the defect number) in the vicinity of defects leads to the rise of the
spectral density of plasmon states [9]:
),(Im)(1),( 1
,
w
p
w e qq -å-=
qi
iVTS (2),
where ε(q,ω) is the dielectric permeability, Vi(q)=4πδZei
2/q2, ω is the frequency, and the
number of plasmons N(ω)~∫S(ω,T)f(ω,T)dω (f(ω,T) is Bose function). The rise of N(ω)
increases the number of defects nd, created due to the subthreshold mechanism, and nd»Nd in
the interval of low doses. It was used the dose interval 8≤D≤250 kGy, where D«Dn =4 MGy,
since when D≥Dn the irradiation creates the high number of radiation defects in YBa2Cu3O7
which results in the reduction of Tc [10]. The defect formation was studied using the lifetime
positron spectroscopy which admits to evaluate the number of defects in Cu1-O and Ba-O
layers, since positrons annihilate in the intermediate layers of YBa2Cu3O7-δ compounds [11],
X-ray diffraction was used to control the occupancy of O1, O5 oxygen positions in the basal
plane.
Samples and experimental details
The objects of investigations were X-ray single phase polycrystalline samples of
YBa2Cu3O7-δ with density 5.5 g/cm3. The YBa2Cu3O7 compound was prepared from a mixture
of Y2O3, BaCO3 and CuO powders by solid-phase synthesis [3]. The nonstoichiometric
compounds with δ>0 were obtained by annealing samples with δ=0 in vacuum. The amount
of oxygen was determined using Q-1500 derivatograph and X-ray structural analysis. Samples
for investigations were prepared from the same polycrystalline block.
The samples were singly irradiated by γ-quanta from 60Co source with doses 8, 30,
100, 250 kGy at room and nitrogen temperatures Tr.
The positron lifetime annihilation spectra were measured using “ORTEC”
spectrometer with full width at half maximum of resolution function 220 ps at room
temperature. More 106 counts were accumulated for each spectrum. The 22Na positron source
with activity 20 μCi was mounted between two samples with sizes 10´10´1.5 mm3. The
annihilation spectra were analyzed using “POSITRONFIT” program with correction of the
positron annihilation in source material and the resolution function parameters. The positron
annihilation rate λf and capture rate υ were determined from expressions:
λf = I1/τ1 +I2/τ2 , υ= I2(1/τ1 –1/τ2), (3),
179
where I1, I2 and τ2, τ1 are the intensities and positron lifetimes in the quasi-free and bound
states.
X-ray structural analysis was done on DRON-2 and ADP-1 diffractometers. The
parameter η= (CO1-CO5)/(CO1+CO5), where CO1, CO5 are the concentrations of oxygen
atoms in O1 and O5 positions, was determined from ratio of structure amplitudes of the 102
and 012 reflections [12]. The measurements of annihilation and lattice parameters were
carried out on the same samples.
Experimental results and discussions
The lifetimes of positron and the corresponding intensities in YBa2Cu3O7-δ samples
prior and after γ-irradiation are listed in the Table.
Table. Parameters of positron annihilation, concentration and radius of vacancy clusters.
Samples Tr K D,
kGy
τ1, ps I1, % τ2, ps I2, % N+, 10-15
cm-3
r+, Å
YBa2Cu3O7
YBa2Cu3O6,7
YBa2Cu3O6.45
300
77
300
77
300
77
0
8
30
100
250
0
8
30
100
250
0
8
30
100
250
0
8
30
100
250
0
8
30
100
250
0
8
30
100
250
170±2
165±4
176±2
184±2
179±3
170±2
163±7
170±4
186±3
175±3
180±3
176±5
180±3
184±5
173±7
180±3
183±3
187±3
185±3
179±5
189±2
202±5
210±3
179±5
191±2
189±2
204±5
198±7
189±7
197±6
84±2
73±3
89±2
93±2
92±2
84±2
53±3
66±3
86±2
80±2
86±2
80±4
84±2
89±4
80±5
86±2
91±2
91±2
90±3
85±4
77±5
88±5
94±2
62±7
71±5
77±5
90±5
85±7
78±7
85±6
331±27
286±19
410±21
492±41
443±35
331±27
271±19
290±17
386±29
340±27
341±24
290±29
323±30
363±39
291±30
341±24
397±37
363±43
336±49
347±29
300±30
365±44
440±51
268±25
295±32
300±30
366±53
327±45
315±25
343±38
16±2
27±3
11±2
7±2
8±2
16±2
47±3
34±3
14±2
20±2
14±2
20±4
16±2
11±4
20±5
14±2
9±2
9±3
10±3
15±4
23±5
12±5
6±2
38±7
30±5
23±5
9±5
15±7
22±7
15±6
13
20
9.5
5.7
6.4
13
37
26
11
16
8.6
12
9.2
6.6
12
8.6
5.7
5.3
6.1
9.3
12
5.8
3.9
20
15
12
4.9
7.5
12
7.7
2.8
2.6
3.0
3.2
3.1
2.8
2.6
2.5
2.8
2.8
3.5
3.1
3.3
3.6
3.1
3.5
3.8
3.6
3.4
3.5
3.1
3.4
3.3
2.8
2.9
3.1
3.4
3.2
3.1
3.3
180
In the samples with various oxygen content the effect of equal irradiation doses is
distinctly manifested in the behavior of the rates of positron annihilation and capture (Fig.1).
In initial samples λf(δ) smooth reduces as the number of atoms lowers in the intermediate
layers. In YBa2Cu3O7 high temperature superconductors γ-irradiation lowers the λf , and the
changes in λf are enhanced with decreasing irradiation temperature. After irradiation the
smooth reduction in λf(δ) is transformed in the non-monotone behavior of λf in the irradiated
samples, where in the interval 0≤δ<0.3 the λf arises with reduction of oxygen content while at
δ≥0.3 the behavior of λf is not changed. The capture rate slightly depends on D and Tr when
δ=0.3.
Fig. 1. Dependencies of annihilation rate and capture rate of positrons on oxygen contents in
YBa2Cu3O7-δ before (•) and after irradiation at 300 (a, c) and 77 K (b, d) with doses
8 (▼), 30 (○), 100 (□) and 250 (+) kGy.
In compounds with δ=0 and 0.55 at Tr=300 K the changes in υ are similar, while as
temperature lowers these are enhanced in the first system and are weakened in the second
181
system. Thus, the principal irradiation effect is manifested in YBa2Cu3O7 high temperature
superconductors, where λf lowers under irradiation and the variations of λf and υ are enhanced
with decreasing temperature. With growth of δ the changes in λf and υ weaken and in
YBa2Cu3O7-δ superconductors with δ≥0.3 these have the random character and decrease as T
lowers.
In YBa2Cu3O7 high temperature superconductors the annihilation rate non
monotonously reduces with D growth (fig. 2). After irradiation at Tr=300 K the maximum
change is Δλf (D)=λf(0)-λf(D)≈0.21 ns-1, while at Tr=77 K λf slightly arises with D growth.
Such behavior of λf(D) does not correlate with smooth rise of the number of radiation defects
as D arises.
Fig. 2. The rates of annihilation (a) and capture (b) of positrons in YBa2Cu3O7 as function of
irradiation doses at temperatures 300 (1) and 77 K (2). The insert shows the critical
temperature as function of annihilation rate in YBa2Cu3O7 after γ-irradiation (1) and
after oxygen desorption from the intermediate layers (2).
Note that the effects of irradiation and desorption of oxygen atoms from the Cu1-O
layers on λf are analogous in YBa2Cu3O7. The maximum of Δλf(D) equals the change in λf(δ)
caused by removal 0f ~2.5·1021 cm-3
atoms when δ increases from 0 to 0.46. However, in
YBa2Cu3O7 samples the irradiation does not vary an amount of oxygen, since the curves of
mass loss due to oxygen desorption are identical in the initial and irradiated samples. Besides,
the drop in λf caused by irradiation does not influence on Tc and temperature dependence of
182
resistance in YBa2Cu3O7 samples, while the analogous drop in λf due to removal of O1 atoms
leads to the drop in Tc more than 40 K [1] (insert in Fig. 2, a).
The capture rate of positrons by defects is non-monotonously changed with D growth
and has the maximum at D=8 kGy. The dependence of υ(D) does not correlate with λf(D) and
a smooth rise of the number of radiation defects as D increases (Fig. 2, b).
In YBa2Cu3O7-δ superconductors with δ≥0.3 under irradiation the changes in λf are
essentially less than in the system with δ=0 and have the random character. The λf value may
exceed the initial magnitude (Fig. 3, a, c). In samples with δ=0.3 the λf(D) and υ(D)
dependencies are similar, while in the system with δ=0.55 their behavior are practically the
same and do not correlate with smooth rise of the number of radiation defects as D increases.
Note that in YBa2Cu3O6.45 the weakening of variations in λf and υ with temperature is more
noticeable (Fig. 3, b, d).
Fig. 3. The rates of annihilation and capture of positrons in YBa2Cu3O7-δ with δ=0.3 (a, b) and
δ=0.55 (c, d) as function of irradiation doses at temperatures 300 (1) and 77 K (2).
The behavior of η and c parameters of the crystal lattice in YBa2Cu3O7-δ
superconductors under irradiation is shown in Fig. 4. The a and b lattice parameters are not
varied after irradiation. In YBa2Cu3O7 the reduction in λf is usually accompanied by a
decrease of c parameter and the drop in η which indicates the disorder of Cu1-O chains. In the
basal plane the transitions of ~5·1020 and 1.3·1021 cm-3 oxygen atoms from O1 in O5 sites take
place at Tr= 300 and 77 K, and the number of transitions does not practically depends on D
(Fig. 4, a, b). In YBa2Cu3O7 a similarity of the η(D) and λf(D) dependencies, a rise of the
number of O1→O5 transitions and the changes in λf, υ with decreasing temperature indicate
the common mechanism of defect formation and oxygen transitions in the intermediate layers.
Besides, the decrease of c parameter shows that the probable defects are the Y, Ba, Cu
183
vacancies, since the exit of cations in interstices may weaken the Coulomb repulsion among
layers and gives rise to compression of the lattice.
In YBa2Cu3O6.7 samples the behavior of c(D) and λf(D) dependencies does not
correlate. The parameter ή drops when D≥8 kGy and its behavior does not practically depend
on Tc (Fig. 4, c, d). The drop in η corresponds to O1→O5 transitions of ~7.1·1020 cm-3 oxygen
atoms. Note that at Tr=77 K the λf(D) and η(D) dependencies are similar. This may indicate
the common mechanism of their behavior.
In YBa2Cu3O6.45 compound the c(D) and λf(D) dependencies do not correlate, namely,
the annihilation rate lowers when the c increases and vice versa. The parameter η arises with
increasing D, and the number of O5→O1 transitions does not depend on Tr (Fig. 4, e, f). Such
behavior of η may be related to the impact mechanism of transitions of oxygen atoms in
Cu1-O layers, as well as the correlation of c(D), λf(D), υ(D) dependencies may be caused by
the common impact defect formation in the intermediate and CuO2 layers.
Fig. 4. The lattice parameters c and η in YBa2Cu3O7-δ superconductors with δ=0 (a, b),
δ=0.3 (c, d) and δ=0.55 (e, f) at irradiation temperatures 300 (1) and 77 K (2).
Note that γ-irradiation does not change an oxygen content in the nonstoichiometric
compounds, since upon heating the curves of mass variation due to oxygen desorption and
adsorption are identical in the irradiated and unirradiated samples.
Thus, in YBa2Cu3O7-δ compounds the effect of γ-irradiation with low doses on the
parameters of positron annihilation and the crystal lattice depends on the oxygen content. In
YBa2Cu3O7 high temperature superconductors the irradiation stimulates the non-monotone
reduction in λf, growth of υ and O1→O5 transitions, which are enhanced with decreasing
temperature, do not correlate with the number of radiation defects and apparently caused by
184
the common mechanism of defect formation in the intermediate layers. In YBa2Cu3O6.45
superconductors the changes in λf, υ and c have the random character and weaken as
temperature lowers. Their behavior does not correlate with increasing order of the Cu-O
chains as D rises, which does not depend on temperature that can be related to a predominant
role of defect formation in the cuprate layers. In YBa2Cu3O6.7 compound the irradiation effect
has the intermediate character and is manifested in the slight random variations of λf and υ
which do not practically depend on temperature and behavior of the c parameter, though a
similarity of the η(D) and λf(D) dependencies at Tr=77 K may be caused by the common
mechanism of defect formation and transitions of O1 atoms in the intermediate layers.
In the case of oxygen thermodesorption the reduction in λf is caused by the drop in
electron density and the non-involvement of the core electrons of O1 atoms in annihilation.
Under irradiation the drop in λf may be explained by a predominant creation of the Ba and
Cu1 defects. The annihilation rate is related to the charge density of electrons n-(r ) and
positron n+(r ) by [13]:
)]([)()(3
2
2
rrrr --+ò= nnnd
e
crro
f e
p
l , (4),
where )()()( ,
,
*
, rrr k
k
k l
l
len YY= å- , )()()( * rrr +++ YY=å n
n
nn , )(, rk lY , )(r+Yn are the
electron and positron wave functions, k≤kF , kF is the Fermi quasi-momentum of electrons, l
is the band number, n is the positron number, c, ro are the light speed and classical radius of
electron, ε[n-(r )] is the enhancement factor. The electron density in intermediate layers
includes the density of core electrons ρcore of Cu1, Ba, O1, O5, O4 atoms, the electron density
in the B2 band ρB2 formed by pd-orbitals of the Cu1, O1, O4, Cu2 atoms, in which carriers
enter as the number of O1 atoms arises, and the vacant band B3 ρB3 built by dp-orbitals of the
Cu1, O4, O1 atoms [14-16]. Hence, one can write [3]:
)( 32
22
BBcoreoof crrcr rmreperpl ++»= (5),
where the enhancement factor is given by [17]:
32/522/3 1667.04466.001.2414.21512.01)( ssssss rrrrrr ++-++=e (6)
and ε=2.3 for 1)4/3( 3/1 == prsr ( 24106.1 ´=r cm-3).
After desorption of O1 atoms to a value δ=0.55 the reduction in λf corresponds to a
density change 22104´»Dr cm-3, which exceeds the density of free carriers
21
321 105´»++ BBB rrr [1] (ρB1 is the density of carriers in the B1 band formed by
dp-orbitals of Cu2, O2, O3 atoms in the cuprate layers, which do not participate in
annihilation), i.e. 321 BBB rrrr ++>D and 32 BB rrr +>D . Since 2Br » 3Br , we have
corerr D»D and
coreoof crcr reprepl D»D=D 22 , (7),
hence the reduction in λf is caused by the exit of core electrons of O1 atoms from the
annihilation process.
In YBa2Cu3O7 under irradiation at Tr=300 and 77 K the changes in λf correspond to
the density drop 22102.1 ×»Dr and 2.5· 2210 cm-3 that is also caused by the reduction in corer .
Since the oxygen content in the intermediate layers is constant, the defects whose electrons do
not participate in annihilation are the vacancies and interstitial atoms of Ba and Cu1.
Occupying interstices, the positive charge of cations repels positrons, decreases the overlap of
)(, rk lY , )(r+Yn and prevents to the annihilation of core electrons with positrons that
reduces ρ. Besides, the Ba, Cu1 atoms may be repelled in interstices of the Cu2-O and Y
185
layers where positrons do not penetrate. In this case, the ρ is reduced due to the formation of
vacancies in the intermediate layers.
The creation of Cu1 and Ba vacancies is in agreement with a decrease of the c
parameter and an absence of the reduction in Tc, whose behavior is sensitive to an amount of
oxygen defects or oxygen. An appearance of the cation defects essentially lowers the electron
density in the intermediate layers, however the hole concentration in the cuprate layers
responsible for the high temperature superconductivity and behavior of Tc may be unchanged.
The exit of O4 and O1 atoms in interstices of the intermediate layers may reduce the
overlap of )(, rk lY and )(r+Yn wave functions and λf magnitude. However, if the defects
have a negative charge, the noninvolvement of core electrons in annihilation is improbable.
The oxygen transitions from O1 in O5 sites does not apparently affect on λf, since oxygen
atoms remain in the annihilation volume. The exit of a high amount of oxygen atoms in
interstices of cuprate layers is improbable because the Tc is unchanged.
The number of defects in the intermediate layers nd≈Δρ/ne can be estimated assuming
that the average number of core electrons of Ba and Cu1 atoms, which does not participate in
the annihilation, is ne≈45. Then nd equals 2.7·1020 and 5.6·1020 cm-3 after irradiation at 300
and 77 K. The obtained values of nd are probably understated, since the deep core electrons of
defects are not involved in the annihilation.
It should be noted that the reduction in λf may be related to the accumulation of a high
number of defects in the thin surface layer of YBa2Cu3O7 crystals which is manifested in the
blocking of diffusive transitions of the metal atoms after γ-irradiation [18] and may lower the
atomic concentration and electron density in the annihilation volume. If the diffusion length
4104.1 ×»» + bDL t Å ( »+= -1
2211 )//( rIIb tt 180-200 ps is the bulk lifetime of positrons)
essentially exceeds a thickness of the surface layer, the major part of positrons annihilate in
the crystal bulk, where the number of Ba, Cu atoms and ρ are lower than in the initial
samples.
The non-monotone change of the amount of Cu1 and Ba defects as D arises and the
enhancement of defect formation with lowering Tr indicates the non-impact mechanism of
defect formation. Thus, if the number of radiation defects is proportional to D [19] and in the
interval 8- 250 kGy Nd rises in about 31 times, the nd non-monotonously arises at Tr=300 K or
when Tr=77 K that is constant in the range D≤100 kGy and slightly lowers as D increases.
Besides, the non-impact mechanism of defect creation is manifested in O1→O5 transitions,
since their quantity does not depend on D and sharply rises with lowering temperature.
The number of defects nd formed by doses D≤250 kGy is less or exceeds the minimum
content of radiation defects m
dN which give rise to the reduction in Tc. The magnitude of
m
dN can be estimated assuming that the concentrations of radiation defects formed under
γ-irradiation with dose Dm=4 kGy [10] and fast neutrons with fluency 1810)75( ×-=mф cm-2
[20] are equal. The number of defects formed under irradiation by fast neutrons is
Nd= daфNE su )( (8),
where ])/[(2)( 2
dannan EMMEMME +=u , ,nM aM are the masses of neutron and atom,
nE is the neutron energy, aN is the number of atoms in 1 cm-3, ds is the collision
cross-section. If the mean energy of neutrons nE =2 MeV, 24=dE and 20 eV for Cu1 and Ba
atoms [21, 22], ds =2×10-24 cm2 [23], the minimum content of Cu1, Ba defects is
m
dN =(3.4-4.7)×1020 cm-3. Then in the interval 8-250 kGy the doses are differed in
Dm/D=500-16 times, while the concentrations of defects in /m
dN nd=1.2-1.7 and 0.6-0.8 times
at Tr=300 and 77 K. Hence, at Tr=77 K the number of defects nd created under γ-irradiation
186
with doses D«Dm exceeds the amount of defects m
dN formed under irradiation with
Dm=4 MGy owing to the impact displacement of atoms. The formation of a high number of
defects nd>Nd under irradiation with low doses shows that the low-energy subthreshold
mechanism of defect formation occurs in the intermediate layers. Besides, when the defect
content in the intermediate layers is nd> m
dN , an absence of the reduction in Tc may indicate
the dominant role of cuprate layers in the mechanism of high temperature superconductivity.
The single defects of Ba, Cu, O1 created under irradiation do not apparently capture
the positrons, since the dependencies of υ(D) and λf(D) do not correlate. When 271≤τ2≤492 ps
the vacancy clusters are by positron traps [24]. The capture rate is related to the concentration
N+ and radius of vacancy clusters +r by relation [25]
υ= ++
¥
++ =ò rDdRRRPD
r
pp 4)(4 N+ (9),
where N+= ò
¥
+
r
dRRP ,)( )(RP+ is the distribution function of defects in radius, r is the
minimum radius when positrons are captured by traps, +D is the diffusion coefficient of
positrons and in approximation of the spherical symmetry the trap radius is [26]
2/1222 )]/2/()1([ hmUyxr +=+ (10),
here m is the electron mass, h is Planck’s constant, U is the depth of the potential well of
defects, x=1/y{λfτ2/[(1+y2)-1]} and x= π- arctan1/y. Assuming that 2=U eV [27],
+D =0.1 cm2/s [28] in the initial samples we can get N+=13·1015 cm-3, +r =2.8 Å (see the
Table), i.e. the traps are vacancy clusters consisting of about 7 point vacancies. Note, in spite
of estimated values of the cluster parameters their variations register the transport and
transformation of defects enhanced by irradiation in the crystal bulk. From the Table it is
seen, when D≤30 kGy the behavior of υ(D) is caused by growth of N+ and weak reduction in
+r that can be ascribed to association of Ba, Cu1, O5, O1 single vacancies in the clusters. In
the range D>30 kGy the behavior of υ(D) is caused by lowering N+ when +r is constant or
rises. This may be evidence that the transformations of clusters, such as the partial
dissociation, recombination on the surface or association with single defects take place.
Besides, the changes in +r and N+ indicate the enhanced defect transport under irradiation.
Thus, in the intermediate layer of YBa2Cu3O7 under γ-irradiation the subthreshold
formation of Ba, Cu1, O1 defects dominates. The defect formation is accompanied by the
transformation of vacancy clusters. Both processes occur in the normal and superconducting
states and are enhanced as T lowers.
The subthreshold ejection of atoms from lattice sites in the Cu1-O, Ba-O layers and
the defect transport are possible in the modulated field of weakly damped collective
excitations. The Ba and Cu cations are pushed out into interstices in the field of charge
antinodes of moving holes (heavy h-holes) in the B2 band, which are predominantly localized
at oxygen atoms, whose p-orbitals yield the major contribution to the hybrid pd-orbitals of the
band. The O1→O5 transitions are probably caused by the mutual repulsion of charge
antinodes on the adjacent O1 atoms that leads to the similarity of λf(D) and η(D)
dependencies. The number of defects does not depend on the dose, since the total subsystem
of h-holes is excited under irradiation with any dose and takes part in the defect formation.
In the B2 band the weakly damped plasmons may propagate along the c axis. Since in
YBa2Cu3O7-δ the energy of plasma vibrations along the c axis is 10-40 meV [29], the
condition of the subthreshold defect formation Ώq≤ωD (ћωD≈0.05 eV [30]) is correct. Besides,
when the plasmons are propagated along the c axis the screening of the subthreshold ejection
187
of atoms by the carriers (light l-holes) in the B1 band may be weakened. Note that the energy
of plasma vibrations in the plane of cuprate layers is ћΩl≈1.3-2.8 eV [31], therefore the
plasma frequency is Ωl »ωD and the subthreshold defect formation is improbable with
participation of the l-carriers.
The enhancement of subthreshold defect formation and transformation of vacancy
clusters with lowering temperature may be ascribed to a rise of the number of weakly damped
plasmons and a weakening of the plasmon damping. The increase of N(ω,T) is improbably,
since the spectral density of plasmon states [9]
S(ω,T)≈ ú
û
ù
ê
ë
é
-
)2/(2
arctan
2 TTsh
pl
v
vp (11),
where )1(
*
h
l
hpl n
Tm
W
p
v +» , ,hW hn are the band width and concentration of h-carriers, *
lm is
the effective mass of l-carriers, reduces with lowering T. Note that at superconducting
transition the spectrum of the weakly damped plasmons may broaden due to particles with
energy ћΩ<2Δ (Δ is the superconducting gap). Besides, as T lowers the Landau damping of
excitations is weakened due to a decrease of the thermal diffusion of Fermi distribution of l
and h-carriers, which may promote the transport of defects and suppression of recombination,
the defect formation and transformation of vacancy clusters.
In YBa2Cu3O6.7 superconductors under irradiation the changes in λf correspond
to the reduction in the electron density, which do not exceed the density of free carriers and
are accompanied by transitions of 7·1020 cm-3 oxygen atoms from O1 in O5 sites that indicates
an essential weakening of the defect formation in the intermediate layers in contrast to
YBa2Cu3O7 system. In addition, the parameters of vacancy clusters are slightly varied. In
initial samples N+≈8.6·1015 cm-3 and +r ≈3.5 Å, after irradiation the N+ and +r are changed in
1.4-1.6 and 1.1 times (see the Table), which shows the weakening of defect transport. The
random character of the changes in λf indicates an absence of the dominant subthreshold
defect formation in the intermediate layers, though the run of η(D) and the correlation with
λf(D) at Tr=77 K may be ascribed to the exhibition of the subthreshold ejection of atoms at
low temperatures and in the O1→O5 transitions. Therefore the behavior of λf(D) and υ(D) is
caused by the impact and subthreshold defect formations, the radiation-stimulated
redistribution of defects among the cuprate and intermediate layers.
In YBa2Cu3O6.45 superconductors the random character of the changes in λf(D),
υ(D) and c(D) indicates an absence of the subthreshold defect formation in the intermediate
layers caused by the weakly damped excitations. The dependence of O5→O1 transitions from
D shows the impact mechanism of defect formation in the intermediate layers, which does not
depend on temperature. In addition, with lowering temperature the transformations of vacancy
clusters are weakened. In initial samples N+≈1.2·1016 cm-3, +r ≈3.1 Å, after irradiation at
T=300 K the maximum changes in N+ and +r are 3.5 and 1.2 times, respectively, while when
Tr=77 K the N+ and +r are varied in 2.4 and 1.1 times. The behavior of N+ and +r indicates
that the diffusion of defects weakens as T lowers. The random character of defect distribution
among the cuprate and intermediate layers under irradiation apparently gives rise to the
random variations in λf(D), υ(D) and c(D), whose reduction with temperature may be caused
by weakening of the defect diffusion. The changes in λf correspond to variations of the
electron density caused by a decrease or increase of the c parameter, which show that the
formed defects are the metal and oxygen atoms. Note that in YBa2Cu3O6.45 at Tr=300 K an
enhancement of defect formation may be caused by relaxation of the localized electron states
which are created in the B2 band in the cuprate layers when δ arises and the band is destroyed.
In contrast of B2 the B1 band and Fermi distribution of l-carriers are slightly changed when
188
δ≤0.6 [15], therefore the subthreshold defect formation with participation of the l-carriers is
improbable.
Conclusions
Thus, in YBa2Cu3O7-δ superconductors the enhancement of defect formation is
observed in the intermediate layers when oxygen content arises in the interval 0≤δ<0.3 caused
by the additional subthreshold formation of defects. The subthreshold defect formation may
be realized in the field of collective excitations which are propagated along the c axis and
testify to the appearance of low-frequency collective excitations in YBa2Cu3O7-δ high
temperature superconductors when δ<0.3. In YBa2Cu3O7-δ with δ≥0.3 the impact mechanism
of defect formation is dominated.
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YBa2Cu3O7
|
| id | oai:ojs.pkp.sfu.ca:article-96 |
| institution | Surface |
| keywords_txt_mv | keywords |
| language | English |
| last_indexed | 2025-07-22T19:30:14Z |
| publishDate | 2002 |
| publisher | Chuiko Institute of Surface Chemistry National Academy of Sciences of Ukraine |
| record_format | ojs |
| resource_txt_mv | surfacezbircomua/3b/0c10ecaae581d462adb5962a31e4943b.pdf |
| spelling | oai:ojs.pkp.sfu.ca:article-962018-11-27T09:42:19Z Defect formation in the intermediate layers of YBa2Cu3O7-d superconductors depending on oxygen сontents Defect formation in the intermediate layers of YBa2Cu3O7-d superconductors depending on oxygen сontents Defect formation in the intermediate layers of YBa2Cu3O7-d superconductors depending on oxygen сontents Adonkin, V. T. Gorelov, B. M. Ogenko, V. M. Melnikov, V. S. Kevdina, I. B. Shantorovich, V. P. In YBa2Cu3O7-d &nbsp;superconductors with 0≤δ&lt;0.6 the defect formation in the intermediate layers depending on oxygen content was revealed under γ-irradiation with doses 8-250 kGy. In YBa2Cu3O7 the subthreshold creation of Ba, Cu1, O1 defects, which give rise to lowering of the annihilation rate of positrons and lattice parameter c, disorder of Cu-O chains, dominates. The defect formation is weakened with δ growth. In YBa2Cu3O6.7 superconductor the subthreshold pushing out of atoms occurs at low temperatures and in O1→O5 transitions, while in compounds with δ&gt;0.3 the subthreshold defect formation in the intermediate layers is not observed. The subthreshold defect formation is related to the Coulomb ejection of atoms from the lattice sites in the field of the weakly damped low‑frequency collective excitations enhanced by irradiation in YBa2Cu3O7-d superconductors with δ≤0.3. In YBa2Cu3O7-d &nbsp;superconductors with 0≤δ&lt;0.6 the defect formation in the intermediate layers depending on oxygen content was revealed under γ-irradiation with doses 8-250 kGy. In YBa2Cu3O7 the subthreshold creation of Ba, Cu1, O1 defects, which give rise to lowering of the annihilation rate of positrons and lattice parameter c, disorder of Cu-O chains, dominates. The defect formation is weakened with δ growth. In YBa2Cu3O6.7 superconductor the subthreshold pushing out of atoms occurs at low temperatures and in O1→O5 transitions, while in compounds with δ&gt;0.3 the subthreshold defect formation in the intermediate layers is not observed. The subthreshold defect formation is related to the Coulomb ejection of atoms from the lattice sites in the field of the weakly damped low‑frequency collective excitations enhanced by irradiation in YBa2Cu3O7-d superconductors with δ≤0.3. In YBa2Cu3O7-d &nbsp;superconductors with 0≤δ&lt;0.6 the defect formation in the intermediate layers depending on oxygen content was revealed under γ-irradiation with doses 8-250 kGy. In YBa2Cu3O7 the subthreshold creation of Ba, Cu1, O1 defects, which give rise to lowering of the annihilation rate of positrons and lattice parameter c, disorder of Cu-O chains, dominates. The defect formation is weakened with δ growth. In YBa2Cu3O6.7 superconductor the subthreshold pushing out of atoms occurs at low temperatures and in O1→O5 transitions, while in compounds with δ&gt;0.3 the subthreshold defect formation in the intermediate layers is not observed. The subthreshold defect formation is related to the Coulomb ejection of atoms from the lattice sites in the field of the weakly damped low‑frequency collective excitations enhanced by irradiation in YBa2Cu3O7-d superconductors with δ≤0.3. Chuiko Institute of Surface Chemistry National Academy of Sciences of Ukraine 2002-06-12 Article Article application/pdf https://surfacezbir.com.ua/index.php/surface/article/view/96 Surface; No. 7-8 (2002): Chemistry, Physics and Technology of Surface; 177-189 Поверхность; № 7-8 (2002): Химия, физика и технология поверхности; 177-189 Поверхня; № 7-8 (2002): Хімія, фізика та технологія поверхні; 177-189 3154-8091 3154-8083 en https://surfacezbir.com.ua/index.php/surface/article/view/96/95 Авторське право (c) 2002 V.T. Adonkin, B.M. Gorelov, V.M. Ogenko, V.S. Melnikov, I.B. Kevdina, V.P. Shantorovich |
| spellingShingle | Adonkin, V. T. Gorelov, B. M. Ogenko, V. M. Melnikov, V. S. Kevdina, I. B. Shantorovich, V. P. Defect formation in the intermediate layers of YBa2Cu3O7-d superconductors depending on oxygen сontents |
| title | Defect formation in the intermediate layers of YBa2Cu3O7-d superconductors depending on oxygen сontents |
| title_alt | Defect formation in the intermediate layers of YBa2Cu3O7-d superconductors depending on oxygen сontents Defect formation in the intermediate layers of YBa2Cu3O7-d superconductors depending on oxygen сontents |
| title_full | Defect formation in the intermediate layers of YBa2Cu3O7-d superconductors depending on oxygen сontents |
| title_fullStr | Defect formation in the intermediate layers of YBa2Cu3O7-d superconductors depending on oxygen сontents |
| title_full_unstemmed | Defect formation in the intermediate layers of YBa2Cu3O7-d superconductors depending on oxygen сontents |
| title_short | Defect formation in the intermediate layers of YBa2Cu3O7-d superconductors depending on oxygen сontents |
| title_sort | defect formation in the intermediate layers of yba2cu3o7-d superconductors depending on oxygen сontents |
| url | https://surfacezbir.com.ua/index.php/surface/article/view/96 |
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