Thermoelectric power studies on Nd1.82−xSrxCe0.18CuOy:x≤0.18
superconductors
Okram G. Singh, B. D. Padalia, Om Prakash, S. K. Agarwal, and A. V. Narlikar
Citation: J. Appl. Phys. 80, 5169 (1996); doi: 10.1063/1.363500
View online: http://dx.doi.org/10.1063/1.363500
View Table of Contents: http://jap.aip.org/resource/1/JAPIAU/v80/i9
Published by the American Institute of Physics.
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Thermoelectric power studies on Nd1.822 x Srx Ce0.18CuOy : x <0.18
superconductors
Okram G. Singha) and B. D. Padalia
Department of Physics, Indian Institute of Technology, Powai, Bombay 400 076, India
Om Prakash
Department of Metallurgical Engineering and Materials Science, Indian Institute of Technology, Powai,
Bombay 400 076, India
S. K. Agarwal and A. V. Narlikarb)
Superconductivity Group, National Physical Laboratory, New Delhi 110012, India
~Received 21 March 1996; accepted for publication 15 July 1996!
Thermoelectric power (S) studies on a Nd1.822x Srx Ce0.18CuOy :x<0.18 superconducting system in
the temperature range 35–250 K are reported here. In the x50.09 sample, synthesized in the
reduced environment, the small magnitude of S is highly metalliclike and its sign is negative, a
characteristic of electron conduction. The sign of S for the x50.18 sample shows a crossover below
75 K from negative to positive, in apparent conflict with electronic conduction. Interestingly, after
oxygenation this sample exhibits a broadened but positive phonon draglike peak. This oxygenated
sample shows overcompensation of the carrier ~electron! concentration. Critical analysis of the data
suggests that Sr doping seemingly causes a competition between electron- and holelike conduction.
The slope dS/dT is, in general, negative suggesting that the main contribution is coming from the
diffusive part. The observed thermopower features seem to fall in line with the theoretical curves of
Durczewski and Ausloos @Z. Phys. B 85, 59 ~1991!; Phys. Rev. B 53, 1762 ~1996!# based on the
inelastic scattering of quasifree electrons by phonons. © 1996 American Institute of Physics.
@S0021-8979~96!06820-X#
I. INTRODUCTION
Thermoelectric power ~TEP! studies are of considerable
interest in the understanding the nature of the charge carriers
and transport mechanism in cuprate superconductors. Despite several TEP studies on various cuprate superconducting
systems,1–4 discrepancies continue to exist.5 Uher et al.1 and
Kaiser2 argue in favor of electron–phonon enhancement in
the temperature dependence of metallic diffusion thermopower. The idea of the presence of two types of carriers
was suggested by Uher et al.1 in the hole-doped
La22x Srx CuO4 superconductor system which exhibits a positive Hall coefficient as well as negative diffusion thermopower. Lopez-Morales et al.,3 however, do not favor such
a separation of carriers into specific negative and positive
entities. Trodahl4 attempted to explain thermopower features
using the usual Fermi-liquid picture. Nevertheless, the temperature dependence of the TEP still remains unresolved.5
According to this scenario, the discovery of an electrondoped R22x Cex CuO42d; R5Nd, Sm, or Pr superconductor
system becomes significant.6,7 These superconductors are
considered to be potential candidates for the crucial test in
formulating theories to understand the mechanism of superconductivity as the majority charge carriers in superconducting cuprates can be either holes or electrons.7 Electrons are
apparently doped into the Nd22x Cex CuOy ~NCCO! system6,8
due primarily to ~i! tetravalent Ce substitution9 for trivalent
Nd and ~ii! creation of oxygen vacancy through reduction.
a!
Present address: Nuclear Science Centre, Aruna Asaf Ali Road, New Delhi
110067, India.
Electronic mail: [email protected]
b!
J. Appl. Phys. 80 (9), 1 November 1996
An increase in x leads to enhancement in conduction electron
density as Ce continues to be in mixed ~31 and 41! valence
states.9,10 Eventually, for x.0.18, T c disappears.6,11
Interestingly, superconductivity gets revived in this system when holes are nominally doped by the substitution of
divalent alkaline earth elements12 for the trivalent Nd. This
result suggests that superconductivity is attained for certain
concentration of charge carriers and the majority of the carriers are electrons in both the parent NCCO and the revived
Nd1.822x Srx Ce0.18CuOy ~NSCCO! superconductor systems.
Our preliminary study showed that the thermopower sign is
negative, consistent with electron doping.12,13 However, it is
not always negative in its sign, thereby posing difficulty in
explaining the very nature of the transport properties in these
materials. In this article, we present the results of our investigations on the NSCCO system, synthesized in different environments.
II. EXPERIMENTS
The Nd1.822x Srx Ce0.18CuOy :<0.18 samples were synthesized by the usual solid state reaction route. The synthesis
and characterization details are given elsewhere.12 An x-ray
diffraction ~XRD! analysis revealed formation of the T 8 -type
structure and the presence of negligible impurities. The dc
four probe resistivity data were obtained in the temperature
range 5–300 K in a liquid helium bath cryostat using a carbon glass temperature sensor. The entire data acquisition system, comprised of a ~Keithley 181! nanovoltmeter, a ~Keithley 224! constant current source, a ~Lakeshore model 82C!
temperature controller, and an indicator, was hooked to a HP
216 system controller.
0021-8979/96/80(9)/5169/6/$10.00
© 1996 American Institute of Physics
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5169
FIG. 1. Normalized resistance R vs T plots for the Nd1.822x Srx Ce0.18CuOy
with ~a! x50, ~b! x50.09, ~c! x50.18 ~all reduced!, and ~d! x50.18 ~oxygenated! samples.
FIG. 2. S vs T plots for the Nd1.822x Srx Ce0.18CuOy with ~a! x50, ~b!
x50.09, ~c! x50.18 ~all reduced!, and ~d! x50.18 ~oxygenated! samples.
TEP was measured by the differential method in the
temperature range 35–250 K using a closed cycle refrigerator ~CCR!.14 The sample, with a uniform surface, was
clamped between two thin copper plates that served as reference samples as well as voltage leads. The voltages were
measured using a Keithley nanovoltmeter and the temperatures were recorded using silicon diode sensors attached to
the plates. The measured thermopower was corrected for the
~Cu! reference material. A Mylar piece wrapping each sensor
ensured good thermal contact and electrical insulation. One
of the copper plates was placed on the cold head of the CCR
and the other one was kept in contact with the clamp that lies
in the open space of the vacuum chamber. While the system
is cooling down, temperature of the cold-head-facing sample
surface is lower than that at the upper face thereby establishing a temperature gradient ~DT, typically ;2 K! across the
sample. The variation in the temperature gradient DT with
the lowering of temperature does not seem to affect the resulting data. The reason is that the TEP goes to zero at R50
~as expected! in all the measurements carried out in our laboratory using the same setup on the higher T c superconductors
such as YBa2Cu3O72d and Bi2Sr2Ca2Cu3Oy and, more important, its TEP features compare exactly with those already
published in the literature.14 The mean of the temperatures
read at the two sensors was chosen as the sample temperature. For such a temperature, the voltage difference (DV)
between the two terminals was noted to find out the TEP
5(DV/DT). In this manner, the data were collected at the
temperature interval of ;0.5 K but the plots were made using a few data points for the sake of clarity. The overall error
in the TEP measurements is ;10%. All these steps were
done automatically as in the dc four-probe method. The sign
of the TEP was assigned using a conventional criterion: if
the absolute TEP of the reference metal is zero, the higher
temperature terminal will be positive potential with respect
to the lower temperature one, provided the TEP of the
sample under consideration is positive.
5170
III. RESULTS
Figure 1 shows the normalized resistance R vs T plot,
where R5R(T)/R(300) for the Nd1.822x Srx Ce0.18CuOy :
x50, 0.09, 0.18 ~all reduced!, and x50.18 ~oxygenated!
samples, designated as NCCO, Sr0.09, Sr0.18 and
Sr0.18~O!, respectively. The NCCO sample ~curve a! shows
superconductivity onset only at 18 K and no R50 down to 5
K. The normal state resistivity exhibits semiconductinglike
behavior. The Sr0.09 sample ~curve b!, on the other hand,
exhibits a T c onset at 20 K and R50 at 12 K. A more
interesting aspect of this sample is exhibition of a normal
state metalliclike behavior. The metallic behavior in the case
of electron-doped polycrystalline cuprates is rare,15,16 indicating the distinct feature of this ~NSCCO! system.12 For the
higher Sr concentration Sr0.18 sample ~curve c! T c onset is
at 18 K and R50 at 8 K. The normal state behavior is
weakly semiconductinglike. The same sample when oxygenated ~curve d! shows only semiconductinglike resistivity behavior.
The thermopower S vs T of the NCCO, Sr0.09, Sr0.18,
and Sr0.18~O! samples is shown in Fig. 2. All the reduced
samples reveal similar features in thermopower curves; the
magnitude of S is small and is comparable to that of noble
metals, electron-doped cuprates, and YBa2Cu3O7 .5,17,18 For
the NCCO and Sr0.09 samples, the sign of S is negative in
the entire temperature range of investigation, consistent with
electron doping. S suddenly increases below about 50 K suggesting possible extrapolation to S50 at 12 K, @the observed
value of T c (R50)# and has a small dip at 55 K. The dip may
be compared with the phonon-drag peak observed in noble
metals and in some superconducting cuprates, often with
positive S values.5,17 Surprisingly, the thermopower of the
NCCO sample with vanishing superconductivity ~T c onset
;18 K! shows an almost similar trend ~cf. Refs. 11 and 12!.
However, the magnitude of S is small, a typical characteristic
J. Appl. Phys., Vol. 80, No. 9, 1 November 1996
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Singh et al.
of metals. This feature is in contrast to the semiconductinglike behavior exhibited by the sample in the R vs T plot @Fig.
1, curve ~c!#.
The Sr0.18 sample exhibits almost linear S(T) from 250
down to 110 K. Below 110 K, S increases faster and offsets
from negative to positive value at 75 K. Whether the value of
S would turn back to zero at T c ;18 K is not clear from these
data ~the lowest temperature T measured being about 35 K!.
On the contrary, after the sample is oxygenated, ~i! the magnitude of S is 6–8 times larger, ~ii! the phonon-drag peak is
broadened, with positive S value, and ~iii! the two offsets,
negative to positive at 110 K and positive to negative at
~extrapolated! 30 K, are exhibited. However, the enhanced
magnitude of S is of the same order as that of good quality
hole superconductors5 albeit the resistivity data @Fig. 1, curve
~d!# indicate semiconductinglike behavior.
samples.19 These, in addition to the unaccounted for but unavoidable impurities, are expected to collectively contribute
to the thermopower for such materials.
Figure 2 depicts the T dependence of S for the various
samples. In the Sr0.09 superconducting sample, there is marginal, although nonlinear, T dependence. The NCCO and
Sr0.18 samples, on the other hand, show negligible T dependence. The Sr0.18~O! sample also exhibits nonlinearity. The
temperature dependence of thermopower of the
Nd1.822x Srx Ce0.18CuOy materials, therefore, is not linear.
However, it may be mentioned that, in general, the S(T)
curves have dS/dT,0 ~in the temperature range of 100–250
K! in agreement with the earlier studies that diffusion is a
major contribution13 to S.
We now consider contributions from the diffusion (S d )
and phonon-drag (S g ) parts in the total thermopower S. This
can be expressed as20
S5S d 1S g 5AT1BT 3
IV. DISCUSSION
or
For a metal, the sign of diffusion thermopower (S d ) is
considered to be indicative of the sign of the majority charge
carriers, and S d is given by17
S d 52 p k T/3e $ ] @ ln s ~ E !# / ] E % E F ,
~1!
2 2
where s (E) is the conductivity at electron energy E, k is
Boltzmann’s constant, e is the electronic charge, and E F is
the Fermi energy. Equation ~1! assumes ~i! a common relaxation time for scattering in both a temperature gradient and
an electric field, ~ii! elastic scattering in pure metals when
temperature T. u D and in alloys when the resistance is
dominated by impurities, and ~iii! no restriction on the shape
of the Fermi surface provided the Fermi energy E F @kT.
S d is a linear function of temperature; its magnitude and
sign depend on how the conductivity s (E) changes with
electron energy at the Fermi surface. Generally, conductivity
is expected to increase with increasing energy of the electrons, thereby, as a rule, negative thermopowers are expected
unless other conditions are imposed. The detailed behavior
of the diffusion thermopower of all metals and alloys therefore depends on17
s ~ E ! 5e 2 /4p 3 \ 2
H FE E
3 ] ln
~ ] E/ ] k ! 2 t ~ k ! dA/¹ k E
GY J
]E
,
EF
(2)
showing that S d is dependent on the Fermi surface through
( ] E/ ] k) 2 and ¹ k E and on the scattering mechanism through
t (k). Here k is the wave vector. Thus, interpretation of diffusion thermopower is pretty difficult in the real physical
situation even for simple metals and alloys. As a result, linearity of the thermopower with temperature is the theoretical
idealization.
The situation in the case of the Nd1.822x Srx Ce0.18CuOy
system, therefore, should obviously be more challenging
than in other high Temperature Superconductors ~HTSCs!5
because the dopants or oxygen nonstoichiometry are lattice
imperfections in the crystallites of the polycrystalline
J. Appl. Phys., Vol. 80, No. 9, 1 November 1996
~3!
S/T5A1BT 2 ,
where A and B are constants. Using this equation one can
extract the diffusive thermopower contribution S d through
the extrapolated intercept of the linear part of the S/T vs T 2
plot. Such a plot also indicates the extent of S g contribution.
For this purpose, the S/T vs T 2 plots for the samples under
study are shown in Fig. 3. As can be seen, deviation from
linear behavior ~i.e., the phonon-drag enhancement! begins
at temperatures ~defined as T d ! 161, 179, 115, and 152 K for
the NCCO, Sr0.09, Sr0.18, and Sr0.18~O! samples, respectively. It is interesting to note that the extent of the phonondrag process is much larger ~179 K! in the maximum T c
superconductor Sr0.09 than in that ~115 K! of the Sr0.18
sample. This is consistent with the general metallic and
semiconductinglike behavior of the materials.5 It is also
worth noting that, in the case of two semiconducting
samples, NCCO and Sr0.18~O!, T d is about 161 and 152 K,
respectively, which can be understood on the basis of their
composition and on synthesis conditions. The NCCO sample
synthesized under reducing conditions with maximum Ce
content possesses a high density electron concentration. On
the other hand, the Sr0.18~O! sample prepared under an oxidizing environment apparently overcompensates the electron
density generated through Ce incorporation, thereby leading
to a high density of hole concentration and thus behaving in
an almost opposite manner to the NCCO sample as far as the
phonon-drag part is concerned ~Fig. 3!. The small although
negative values of S encountered in the case of the Sr0.18~O!
sample beyond the crossover at about 100 K indicates the
dominance of the electron contribution in the diffusive thermopower region.
TEP, particularly that contributed from normal electrons
S that is also the case in the present study, is related to the
circulatory ( k j ) and electronic ( k e ) heat conductivities21
k j / k e 53eSD ~ T ! / p 2 k 2 T,
~4!
where D(T) is the energy per normal carrier relative to the
superconducting carriers. Assuming that the normal carrier
Singh et al.
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5171
FIG. 3. S/T vs T 2 plots for the Nd1.822x Srx Ce0.18CuOy with ~a! x50, ~b!
x50.09, ~c! x50.18 ~all reduced!, and ~d! x50.18 ~oxygenated! samples.
The straight line is drawn to indicate the dominance regime of the diffusion
thermopower.
TEP (S) below T c is similar to just below it, the ratio
k j / k e }S. Using the Wiedmann–Franz law and the electrical
resistivity, Jezowski and Klamut22 obtained only a very
small electronic contribution ~2.5% at ;300 K and 0.1% at
30 K! to the total thermal conductivity. The thermal conductivity is thus clearly dominated by phonons and compares
well with those found in a classical dielectric solid. In other
words, thermal conductivity associated with the sharp peak
in the Nd22x Cex CuOy system is extraordinarily large compared to that in p-type cuprate superconductors.22 This characteristic may also be compared with the TEP dips ~or peaks!
observed in the present samples as these features are found in
the temperature range where thermal conductivity also
peaks. However, the discrepancy lies in the peaks in
conductivity22 and dips in the TEP of the reduced samples
that are exhibited.
Another focal point is the crossover of the TEP from
negative to positive value in the moderate temperature regime where TEP diffusion dominates. In a recent theoretical
approach, model calculations were performed.23 In that
model, the simple free-electron scattering from the spherical
Fermi surface by acoustic phonons alone, without the usual
electron–phonon mass enhancement effect or Umklapp process, was considered. Sign reversal in the TEP was shown to
have resulted from a competitive distribution of charge carriers of the different signs with the carriers being inelasti5172
cally scattered. Further, the electron and phonon spectra are
considered to be on the same footing. Durczewski and
Ausloos23 showed that the TEP bump ~on the positive side!
grows with characteristic Debye energy or with decreasing
electron energy ( e s ), and is always negative for e s .48 K.
These characteristics can be compared with the TEP data of
the present work in Fig. 2, curves ~c! and ~d!. The comparison is more striking in the case of the oxygenated sample
@Fig. 2, curve ~d!#. The TEP for the reduced sample @Fig. 2,
curve ~c!# is however not necessarily very compatible because it increases monotonically with decreasing temperature
down to 35 K.
We also note that the nonlinear S(T) is associated with
the decrease in dip as the Sr concentration (x) increases. The
dip decreases slowly with the introduction of superconductivity and may be correlated to the increased disorder due to
the increased x.24–27 It is therefore tempting to say that the
thermopower of NSCCO is something more than the sum of
diffusion and phonon-drag components as it is in the
Nd22x Cex CuOy system.28
Presumably both electrons and holes take part in the
conductivity of ~NSCCO! materials.12 It, therefore, raises a
question as to how these two kinds of charge carriers are
going to respond to the overall transport and electronic properties? The observation of a negative sign of thermopower is
indicative of electrons as the majority carriers that in turn
corroborates the x-ray spectroscopic ~XAS! data ~cf. Fig. 2
of the Ce L 3 edge of Ref. 10!. However, in some cases, the
thermopower offsets to positive value thereby making it difficult to understand the transport mechanism.1–4 Uher et al.1
suggested the presence of two types of carriers in the
La22x Srx CuO4 superconductor. Wang et al.29 made similar
conclusions in the Nd22x Cex CuOy system. But, LopezMorales et al.3 do not favor the idea of the separation of
carriers into specific negative and positive entities, in line
with the conclusions of Hu et al.28
For the NSCCO system there is, at present, no evidence
of the presence of two bands.10 However, there is a systematic decrease in the unoccupied density of states with oxygen
depletion.10 This tends to agree with better conductivity or
with superconductivity12 which reflects the change in Fermi
level in the samples. Nevertheless, it should be pointed out
that two bands exist30 in Bi2Sr2CaCu2Oy . This is evident
from the photoemission data that identify a band with the
Bi–O characteristic crossing the Fermi level and generating
an electronlike pocket to supplement the CuO2 plane holes.30
A similar model was also suggested for the Tl2Ba2Ca2Cu3Oy
system and was verified.14,31 Precise understanding of the
thermopower of the NSCCO system is poor at this juncture
as it is in the other HTSCs.5 However, there are some questions to be addressed regarding the underlying physics that
may be hidden in material imperfections such as defects or
impurities. In Nd22x Cex CuOy and Nd1.822x Srx Ce0.18CuOy
systems, the local oxygen as well as cationic distributions
were investigated.19,32 We report that, although the ~cationic!
dopants are uniformly distributed, the lattice oxygen is nonuniform locally which is believed to have far-reaching implications on the bulk physical properties of these materials.19,32
The oxygen distribution for the oxygenated samples ~i.e., the
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Singh et al.
nonsuperconductors! is uniform as anticipated. Such samples
do not form a Fermi surface as is evident from the observed
semiconductinglike behavior @Fig. 1, curve ~d!#. The reduced
samples, on the other hand, form Fermi surfaces, as is
strongly suggested by the observation of better conductivity
or superconductivity.6,16,32
At this juncture it would be beneficial to see the thermal
properties in a wider perspective.33–36 Hansel et al.33 reported that the transport entropy of magnetic flux lines in
Nd1.85Ce0.15CuOy at 18 K in 0.5 T is 1.6310214 J/km, in
agreement with the time prediction of Ginzburg–Landau
theory. On the other hand, the Nernst effect of the same thin
film composition in the mixed state was found to arise from
the motion of the vortices in response to thermal force, and it
is anomalously large.34 This led Jiang et al.34 to conclude
that both electrons and holes participate in the electrical
transport of the superconducting phase of Nd22x Cex CuOy .
This is, incidentally, coincidental with what Wang et al.
concluded.29 The recent theoretical propositions for the thermopower of semimetals and semiconductors that consider
the multiband structure and different types of carriers37 apparently describe the observed features of temperature dependence of the TEP in the regime away from the usual low
temperature phonon-drag region of the high (x50) and optimum (x50.09) electron density samples @Fig. 2, curves ~a!
and ~b!# where the majority carriers are doped electrons.
However, this does not seem to hold for the NSCCO system
as a whole.
The
reduced
samples
have
local
oxygen
nonuniformity19,32 that suggests formation of different
shapes or sizes of Fermi surfaces locally. Therefore, the
Fermi surface, being the backbone in understanding the
mechanism of ~super! conductivity or thermopower of such
materials delving into the Fermi surface topology, is expected to be quite revealing.38 However, in doing this, one
should consider the contrasting features of the semiconductinglike behavior in the resistivity data and the metalliclike
small magnitude of thermopower in the oxygenated samples.
In this situation, one may visualize the resistivity and thermopower arising from conduction by metallic and semiconducting bands in parallel,39 with the contribution of the semiconductor band to the total conductivity being small. Its
contribution to the total thermopower is, however, significant
owing to a very large intrinsic semiconductor thermopower.
V. CONCLUSIONS
In summary, we have measured the thermopower S of
the revived n-type Nd1.822x Srx Ce0.18CuOy superconductors
for different x values. In the Sr0.09 sample, the magnitude of
S is highly metalliclike and the sign is negative, consistent
with electron conduction. However, in the Sr0.18 sample, S
offsets to a positive value below 75 K, in apparent conflict
with the electron conduction. Similar behavior is observed in
the case of oxygenated sample. These data show that Sr
codoping is not a simple addition of holes. Based on the
estimation of the extent of diffusion thermopower, oxygenation of the Sr0.18 sample seems to have overcompensated
the electron density in the material, thereby causing it to
behave in a mirror-imagelike fashion compared to the NCCO
J. Appl. Phys., Vol. 80, No. 9, 1 November 1996
sample. The thermopower behavior of the high and optimum
electron density samples seems to qualitatively match the
theoretical curves generated for the thermopower of semimetals and semiconductors by Durczewski and Ausloos. Further, the thermopower sign reversal in the reduced and oxygenated (x50.18) samples may be viewed in light of the
other recent theory by these authors.
ACKNOWLEDGMENTS
Three of the authors ~O.G.S., B.D.P., and O.P.! acknowledge the support of the Department of Science and Technology, New Delhi ~Project No. SBR 24! for this work. Thanks
are also due to Professor S. N. Bhatia for his comments and
Dr. R. Suba for her help.
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