Excess conduction of YBaCuO point contacts between 100 and 200 K

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by  L. F. Rybal'chenko

Excess conduction of YBaCuO point contacts between 100 and 200 K
L. F. Rybal’chenko, I. K. Yanson, N. L. Bobrov, V. V. Fisun, M. A. Obolenskii*,
A. V. Bondarenko*, Yu. D. Tret’yakov**, A. R. Kaul’**, and I. E. Graboi**
Physicotechnical Institute of Low Temperatures,
Academy of Sciences of the Ukrainian SSR, Kharkov,
A. M. Gorky State University, Kharkov*,
and M. V. Lomonosov State University, Moscow**
Email address: [email protected]
7 (Dated: January 24, 2017; Published (Fiz. Nizk. Temp., 16, 58 (1990)); (Sov. J. Low Temp. Phys., 16, 30 (1990))
1
YBaCuO − Ag pressure point contacts with direct conduction are investigated. The excess
0
2 (relativetothenormalstate)conductivitymainlycausedbyfluctuationalpairingofelectronsabove
n Tc ismeasuredinthetemperatureinterval100-200K. Thesuperconductivityabove120K isfound
a to be of the two-dimensional type. The obtained preliminary results indicate the presence of small
J amount of an unknown phase with Tc(cid:48) >∼200 K in YBaCuO.
1
2
PACSnumbers: 74.45.+c,73.40.-c,74.78.-w,74.20.Mn,74.40.+k.-n,74.70.Ad,74.72.-h,74.72.Bk

]
n
o Fluctuationalelectronpairingabovethesuperconduct- nal being recorded, which considerably complicates the
c
ing transition temperature T is known to lead to an determination of the type of superconducting fluctua-
- c
r additional contribution to electrical conductivity, whose tions within stoichiometry regions.
p
u fluctuational dependence on temperature can be used to In order to minimize the effect caused by sample
s determine the geometrical dimensionality of the object
. heterogeneity, we measured the excess conductivity in
t
a under investigation or of an individual subsystem re- YBa Cu O byusingpressurepointscontacts(PC)of
2 3 7−δ
m sponsible for superconductivity [1]. In view of the small
the S-N type (N=Ag) with the metal type conductivity,
- chargecarrierconcentrationinnewhigh-temperaturesu-
d thus restricting the region under investigation to submi-
n perconductors (HTS), the fluctuational correction σ(cid:48) = cron size. It turned out that in many cases supercon-
o [σ(T)−σ ]toconductivity(σ(T)istheconductivitybe-
N ducting effects are observed up to 220-230 K, and in the
c
ing measured and σ the conductivity corresponding to
[ N temperatureintervalfromabout120to180K theexcess

1 the normal state) is considerably larger than for tradi- conductivity obeys the law typical of two-dimensional
v tional superconductors, which causes, for example, no- systems. The noticeable effect of the magnetic field on
5 ticeable”rounding”effectsintheresistancedependences
excess conductivity at T > 2T suggests that there ex-
3 c
R(T) above T .
0 c ist small amounts of unidentified superconducting phase
06 The measurements of fluctuational corrections to the with Tc >∼200 K.
. conductivity σ(T) and to the magnetic susceptibility Point contacts were prepared by pressing a sharpened
1
0 χ(T) made in a number of works [2–4] not only for ce- N (Ag) electrode to a freshly fractured surface of the
7
ramic, but also monocrystalline YBa Cu O samples YBa Cu O sample. In some cases, the sample was
1 2 3 7−δ 2 3 7−δ
: did not provide an unambiguous information about the intheformofaceramicpelletpreparedaccordingtocry-
v
dimensionality of the superconducting subsystem. For ochemical technique [5], and in other cases it was in the
i
X example, the authors of Ref. [2] reported about three- form of a single crystal of size ∼1×1×0.2 mm3 grown
r dimensional fluctuations observed in this compound at bythesolution-melttechnique. Inbothcases,theexper-
a
τ > 0.1(τ = T/T − 1), while in Ref. [3] these fluc- imental results were close. The design of the device used
c
tuations are regarded as two-dimensional. The reason for PC studies was such that the point of contact could
behind such contradictory conclusions apparently lies in be moved over the fractured surface during one measur-
insufficient homogeneity of investigated samples includ- ing cycle with the help of a mechanical drive fixed to
ing monocrystalline ones. Due to the presence of a large the cryostat from outside. As was pointed out earlier
numberofweakbondsbetweenindividualstructuralele- [6], such a technique allowed us to determine individual
ments(granules,grains,andsubcrystallites)inasample, regions on fresh fractures where the critical parameters
these bonds make a considerable contribution to the sig- attain high values (T = 90−95 K). The excess con-
c
2
10−4. The error in the measurement of temperature
above 100 K was ∼ 0.5 K. It should be noted that
when the temperature decreases below T , the intensity
c
of this minimum increases considerably due to the An-
dreevelectronreflectioninitiatedbytheemergenceofan
energygapinthesuperconductingelectrode. Inthiscase,
the principal minimum acquires an additional structure
in the form of weak gap minima at ±eV =∆(cid:39)40 meV
and a narrow minimum at V =0, whose nature will not
be discussed here.
The typical values of the differential resistance for the
investigated contacts at eV (cid:29) ∆ were R ∼ 10 Ω.
N
Using the Maxwell formula for estimating the diame-
ter d of the point contact constriction and neglect-
pc
ing the contribution of the silver bank to R , we ob-
N
tain d =ρ /2R ∼500 ˚A, considering that the
pc YBaCuO N
mean values of the resistivity ρ of the ceramic
YBaCuO
under consideration are close to 10−4 Ω·cm. The PC
characteristicsaremainlydeterminedbythecontactcon-
striction (which is small for our samples), and hence we
can assume that the effect of intercrystallite weak cou-
plingontheexcessconductivityisreducedtoaminimum
in our experiments.
The value of the excess conductivity σ(cid:48)(T) approxi-
FIG. 1: Dependence of the differential resistance on voltage matelycorrespondstothedepthofthecentralminimum
(curves1-10)andIVC(curve1’)fortheYBaCuO−Agpoint (nearV =0)onthedV/dI curves(seeFig. 1). Theexact
contactatdifferenttemperaturesT,K: 20(1,1’),40(2),80(3), values of σ(cid:48)(T) were obtained by using another method.
90(4),100(5),125(6),155(7),220(8),180(9)and230(10). The At first, the R(0)(T) dependence (the differential resis-
dot-and-dash line is the expected IVC in the N-state. D
tance of PC at V = 0) was registered in a wide temper-
ature range (Fig. 2). Then this dependence was approx-
imated in the high-temperature region by the straight
ductivity above 100 K was observed in the present work
line R (T) which is supposed to reflect the background
B
just in such regions. It should be emphasized that the
changeintheresistanceofthebulkconductor. Theabso-
excess conductivity is manifested only when there is a
lutevalueoftheexcessconductivityateachtemperature
considerable excess current I on the current-voltage
exc value was determined by using the formula
characteristics (IVC) of the corresponding contacts (Fig.
1 1
1,curve1’). Thisisanindicationofthepredominantfill- Σ(cid:48)(T)= −
ing of the PC region with a superconducting phase and R(0)(T) RB(T)
D
of the absence of barrier layers with a low transparency
According to Ref. [1], the fluctuational conductivity
at the contact between electrodes.
intraditionalsuperconductorsisσ (T)=Aτ−K,where
AL
The excess conductivity of point contacts caused by K=1/2;1;3/2;and2,respectively,forthree-,two-,one-,
superconductivity was manifested in the form of a broad andzero-dimensionalobjectsrespectively,τ =T/T −1,
c
symmetric minimum near V = 0 on the voltage de- and A is a temperature-independent coefficient. It was
pendences of differential resistance R (V) ∝ dV/dI(V) shown by Maki [7] and Thompson [8] that the forma-
D
above 100 K (see Fig. 1). The differential resistance tionoffluctuationalpairsinthepurelimitisalsorespon-
of a point contact was determined from the first har- sible for a considerable increase in the conductivity of
monic of a weak modulating signal V (V) ∝ R (V) of unpaired electrons. However, the Maki-Thompson con-
1 D
frequency487Hzinabridgecircuitwithacurrentsource. tribution can be disregarded in materials with a short
The relative error of measurements was not higher than electron mean free path (like HTS). Figure 3 shows in
3
FIG.2: Temperaturedependenceofthedifferentialresistance
R(0) forYBaCuO−Ag pointcontactatV =0(thedot-and-
D
dash line is the approximation of R(0)(T) at T >200 K, i.e.,
D
R (T).
B
logarithmic coordinates the results of measurements of
the excess conductivity σ(cid:48)(T) of several point contacts,
FIG. 3: The normalized excess conductivity σ(cid:48)(T) at V = 0
which is normalized to σ (at 100 K), as a function of
B forseveralYBaCuO−Ag pointcontactsasafunctionofre-
the reduced temperature τ (Tc = 90 − 95 K was de- duced temperature in logarithmic coordinates (the dot-and-
termined from the midpoint of a sharp change R(0)(t) dash lines show the expected slope of these dependences for
D
as, for example, in Fig. 2). It was taken into ac- three- and two-dimensional fluctuations; different symbols
countthatσ(cid:48)(T)/σ (100K)=Σ(cid:48)(T)/Σ (100K),where correspond to different contacts).
B B
Σ (100 K)=1/R (100 K).
B B
In spite of the considerable spread in the values of the
ratio σ(cid:48)(T)/σ (100 K) for different contacts, which can e2/16 h¯d. Having determined the value of A/σB(100 K)
B
from Fig. 3 and assuming that d = 12 ˚A, we can cal-
beattributedtovariationsofthechemicalcompositionof
theS-electrodeintheregionofPCconstriction,theσ(cid:48)(T) culate the resistivity of the PC region which is found
to be ρ (100 K) ∼ (0.3−3)·10−4 Ω·cm. This value
dependence above 120 K approximately obeys a power B
is in reasonable agreement with the data on bulk mate-
law with the exponent k = 1. As the temperature ap-
rial. Strictlyspeaking,ananalysisofexperimentalresults
proachesT ∼2T ,thislawisviolated,butitisimpossible
c
to determine the behavior of σ(cid:48)(T) in this temperature for layered superconducting compounds like YBaCuO
should be carried out by using the Lawrence-Doniach
region. With lowering temperature, the 2D-dependence
model [9]. However, far away from T (T (cid:29) T ) this
isreplacedbythe3D-modewhichismanifestedonlyina c c
model leads to an expression which does not differ much
narrow temperature interval (10-15 K). With a further
fromσ(cid:48) (T),whichconfirmsthecorrectnessofourcon-
decrease in temperature, the power law is modified, in AL
clusions.
particular, due to an increase in the contribution from
strong (critical) fluctuations in the immediate vicinity of We also analyzed the effect of a magnetic field on the
T , which are not analyzed in this work. excess conductivity of the investigated point contacts
c
Thus, an analysis of Fig. 3 leads to the conclusion (Fig. 4). A comparison of the dV/dI characteristics
thatweaksuperconductingfluctuationsinYBa Cu 0 in different fields and at different temperatures shows
2 3 7−δ
far from T are two-dimensional as should be expected that the maximum field-induced relative changes in the
c
in view of the anisotropic structure of this substance. depth of the central minimum (which directly correlates
In the Aslamazov-Larkin formula for 2D-systems, A = with the excess conductivity) occur far from T , i.e. at
c
4
FIG. 5: Gap-like minima on the dV/dI dependences for
YBaCuO −Ag point contact at different values of T, K:
90(1), 96(2), 104(3), 120(4), 130(5), 142(6), 150(7), 160(8),
166(9) and 170(10) (the contact resistance R(0)(96 K)(cid:39)4 Ω
D
at T =96 K and V =0).
ruledoutthatasmallamountofanunknownphasewith
Tc >∼ 220 K, which makes a certain contribution to the
paraconductivity observed above 100 K, gets in the re-
gionofconstrictionintheYBaCuO-basedpointcontacts
studied in the present work.
In connection with the last assumption, it is also in-
teresting to analyze the YBaCuO characteristics of PC,
obtained on a monocrystalline dV/dI(V) sample (Fig.
FIG.4: TheeffectofthemagneticfieldonthedV/dI charac-
5). As in the prevous cases, a considerable excess con-
teristicsofYBaCuO−Ag pointcontact: H =0,T =150K,
R(0) = 6.8 Ω (1); H = 43 kOe, T = 150 K (2); H = 0, duction is observed, but this case is distinguished by the
D
T = 180 K (3); H = 49 kOe, T = 180 K (4); H = 0, emergence of broad gas-like minima on the dV/dI de-
T = 220 K (5); H = 47 kOe, T = 220 K (6); and H = 0, pendences near ±eV (cid:39) 100 meV. The position of these
T =230 K, R(0) =7.3 Ω(7). minima on the energy axis does not depend on temper-
D
ature. We can assume that the constriction region of
the point contact presented in Fig. 5 contains a larger
T ∼220K , which does not agree with fluctuational the- amount of the unknown phase mentioned above or that
oryofparaconductivitydevelopedbyHikamiandLarkin this phase is closer to the center of the contact than in
for layered superconductors [10]. As a matter of fact, the cases considered earlier. Then the values of energy
according to Ref. [10] relatively weak fields (of the or- corresponding to the minima on the dV/dI dependences
der of several tens kilooersteds) cannot considerably af- determine the value of the energy gap in the unknown
fect σ(cid:48)(T) away from T , and this effect becomes weaker phase. The absence of the temperature dependence of
c
with increasing temperature. On the contrary, a sharp the positions of these minima on the energy axis can be
increase in the magnetoresistance can take place only related to the specific nature of this phase as, for exam-
in the immediate proximity of T . Indeed, the experi- ple, in the Kulik theory [11] where the situation when
c
ments carried out by Freitas et al. [2] revealed that the the gap of the pairs does not depend on temperature is
positive magnetoresistance is ∆ρ/ρ ∼ 0.1−0.2% in the analyzed.
field 40 kOe at τ = 0.2. Consequently, it cannot be Thus,theindicationsofthepresenceofsmallamounts
5
of a new unidentified phase with Tc(cid:48) >∼ 200 K were ob- the quasi-two-dimensional nature of fluctuational super-
tained in the present work along with the observation of conductivity in YBaCuO.
[1] L. G. Aslamazov and A. I. Larkin, Fiz. Tverd. Tela brov, V.M. Kirgner, Yu. D. Tret’yakov, A.R. Kaul’, I.E.
(Leningrad) 10, 1104 (1968) [Sov. Phys. Solid State 10, Graboi, Fiz. Nizk. Temp. 14, 732 (1988) [Sov. J Low
875 (1968)]. Temp. Phys. 14, 402 (1988)].
[2] P. P. Freitas, C.C. Tsuei, and T. S. Flasket, Phys. Rev. [7] K. Maki, Progr. Theor. Phys. 40, 193 (1968).
B36. 833 (1987). [8] R. S. Thompson, Phys. Rev. Bl, 327 (1970).
[3] M.AusloosandCh.Laurent,Phys.Rev.B37,611(1988). [9] W. E. Lawrence and S. Doniach, in: Proc. of 12-th
[4] Y. Matsuda, T. Hirai, and S. Komiyama, Solid State Int. Conf. on Low-Temp. Phys., Kyoto, LT-12 (ed. by
Camnun. 68, 103 (1988). Kanda), Keigaku, Tokyo (1971), pp. 361-362.
[5] A.R.Kaul’, I.E.Graboi, andYu.D.Tret’yakov, in: Su- [10] S. Hikami and A. I. Larkin, Mod. Phys. Lett. 2, 693
perconductivity.InvestigationofHigh-T MetaloxideSu- (1988).
c
perconductors, Issue 1 [in Russian], Izd. IAE, Moscow [11] I.O. Kulik, Fiz. Nizk. Temp. 14, 209 (1988)[Sov.J. Low
(1987). Temp. Phys. 14, 116 (1988)]
[6] I.K. Yanson, L. F. Rybal’chenko, V. V. Fison, N.L. Bo-
6
Paraconductivity of YBaCuO point contacts above
100 K
I. K. Yanson, L. F. Rybalchenko, N. L. Bobrov, and
V. V. Fisun
Institute for Low Temperature Physics and
Engineering, Kharkov, Fiz. Nizk. Temp. 16, 661
(May 1990)
1 Soviet - West-German Symposium on NTSC
Due to the low charge-carrier concentration in the new
high-T superconductors, the fluctuating pairing of electrons
c
at temperatures above T is much stronger than in
c
conventional superconductors. In this study we observed
excess conductivity of point contacts between
YBa Cu O and Ag at temperature above T . Contacts
2 3 7−y c
having a resistance R 15<Ω were made by the shear
N
method inside the cryostat. At T <T (T =92−94 K)
c c
they were characterized by a large excess current, which
indicates that the constriction region at the S-electrode side
consisted mainly of the superconducting 1-2-3 phase and
there were no apppreciable barrier layers at the electrode
interface. The excess conductivity of such contacts appeared
as a wide minimum on the dV/dI(V) dependences at
temperaturesaboveT upto200-220K. Inthistemperature
c
range we calculated the excess conductivity at V =0 and
σ(cid:48) =1/R −1/R , where R is the contact resistance at a
T N T
given temperature and R is the normal state resistance.
N
Plotting these results in logarithmic coordinates
σ(cid:48)/σ −τ(τ =T/T −1), we find that in the interval
N c
120-180 K the excess conductivity can, with good accuracy,
be respresented by the power law σ(cid:48)/σ∝τ−1 characteristic
of the two-dimensional systems. The noticeable effect of the
magnetic field on the excess conductivity at T >2T enables
c
one to postulate the presence of small quantities of an
unknown superconductive phase with T >200 K.
c