Superlattices and Microstructures, Vol. 21, No. 3, 1997

Effect of annealing on thermal and electrical transport in
Nd1.85 Ce0.15 CuO4
M. Ikebe, H. Fujishiro, M. Yagi
Faculty for Engineering, Iwate University, Morioka 020, JapanT. FukaseInstitute for Materials Research, Tohoku University, Sendai 980, Japan(Received 8 July 1996)
A quenching process followed by annealing in Ar atmosphere was confirmed to stabilize
superconductivity in sintered Nd1.85 Ce0.15 CuO4 crystals. The electrical resistivity and thethermal conductivity were examined at each stage of the heat treatments. The phonon
scattering mechanisms are analyzed in a systematic way.
c 1997 Academic Press Limited
Key words: high Tc cuprates, stabilization of superconductivity, phonon scattering, electronscattering.

1. Introduction
0Nd2−x Cex CuO4 (NCCO) with T -phase crystal structure belongs to a rather peculiar class of high Tc
cuprates because the doped charge carriers are of electron type in contrast to hole type carriers in the majorityof cuprates [1, 2]. Superconductivity is realized for a small range of Ce concentration 0.15 ≤ x ≤ 0.18 inNCCO only if these crystals are properly annealed in deoxidizing atmosphere. We previously studied the Ceconcentration dependence of the thermal conductivity of NCCO for ‘anneal’ specimen [3], where furnacecooled sintered (‘as-sinter’) samples were heat-treated in Ar at 910 ◦ C. These ‘anneal’ samples, however, did
not stably show superconductivity even for x = 0.15. We have found that addition of quenching processreliably realize superconductivity in Nd1.85 Ce0.15 CuO4 . In this paper, we try to fix the best heat-treatment
conditions for superconductivity in sintered Nd1.85 Ce0.15 CuO4 crystals. The electrical resistivity and thethermal conductivity were measured for typical samples in order to get the information on the origin ofsuperconductivity in this system.
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2. Experimental procedure
Nd2−x Cex CuO4 samples were prepared from stoichiometric mixtures of Nd2 O3 , CeO2 and CuO raw powders. The mixtures were calcined twice at 900 ◦ C for 21 hours in air. The calcined mixtures were pressed intopellets and sintered at 1100 ◦ C for 18 hours in air and then furnace-cooled (‘as-sinter’ sample). One groupof ‘as-sinter’ samples were annealed in flowing Ar gas for 18 hours at 910 ◦ C (‘anneal’ sample). Anothergroup of ‘as-sinter’ samples were again kept 1100 ◦ C in air for 2 hours and quenched into liquid N2 (‘quench’0749–6036/97/030357 + 06 $25.00/0

sm960400

c 1997 Academic Press Limited

358Superlattices and Microstructures, Vol. 21, No. 3, 199799
as-sinter
quenchq+anneal 880°C
q+anneal 910°C
q+anneal 950°C
q+anneal 990°C

6B
Resistivity (mΩ cm)

Resistivity (mΩ cm)

A30300100200Temperature (K)630301020Temperature (K)Fig. 1. A, The electrical resistivity ρ(T ) as a function of temperature T for Nd1.85 Ce0.15 CuO4 samples subjected to several heat
treatments. B, ρ versus T near Tc in an extended scale.
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sample). The ‘quench’ samples, of which superconductivity had already been confirmed, were annealed for
18 hours in flowing Ar gas at various temperatures and then furnace cooled (‘q + anneal’ sample). The x-ray
diffraction analyses confirmed that all samples belonged to the single T 0 -phase. The densities of the present
sintered materials which were measured by Archimedian method were higher than 93% of the ideal density.
The size of the grains were observed by a scanning electron microscope. The thermal conductivity κ(T )was measured by a steady state heat flow method between 15 K and 150 K. A Gifford-McMahon (GM)cycle helium refrigerator was used as a cryostat and the measurement was made fully automatically [4]. The
electrical resistance was measured by a usual four-terminal method.
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3. Experimental results
In this study on sintered Nd1.85 Ce0.15 CuO4 crystals, the superconductive transition was not observed above
10 K for samples in ‘as-sinter’ state nor ‘anneal’ state at annealing temperature TA = 910 ◦ C. The transition
first appeared in ‘quench’ state at relatively low Tcend ≈ 13 K, where Tcend means the zero resistivity transition
point. By annealing the ‘quench’ sample in Ar, Tcend was found to systematically increase with increasing
annealing temperature TA up to 990 ◦ C. The Tc enhancement almost saturated around TA ≈ 970 ◦ C. Fig. 1Ashows the electrical resistivity ρ(T ) of ‘as-sinter’, ‘quench’ and the typical ‘q + anneal’ Nd1.85 Ce0.15 CuO4
as a function of temperature T . Fig. 1B shows ρ(T ) versus T near Tc in an extended scale. The annealing
process resulted in also the reduction of ρ(T ) relative to ‘quench’ sample. It is to be noticed, however, thatthe absolute values of ρ(T ) of ‘as-sinter’ sample are the smallest among all of the studied Nd1.85 Ce0.15 CuO4
samples. The overall temperature dependence of every Nd1.85 Ce0.15 CuO4 sample was semiconductive, i.e.
ρ(T ) increased with decreasing temperature. Figure 2 shows the transition temperatures Tcend , Tcmid , Tconset
and the resistivity ratio ρ(30 K)/ρ(300 K) as a function of TA , Tcend and Tcmid increased with increasing TAup to 970 ◦ C while Tconset remained relatively constant.
Figure 3 shows the temperature dependence of the thermal conductivity κ(T ) of ‘as-sinter’, ‘quench’ andtypical ‘q + anneal’ Nd1.85 Ce0.15 CuO4 samples. κ(T ) increased with increasing temperature and exhibiteda maximum around 25 K. It then gradually decreased with further increase of temperatures. The peak isconsidered to come from phonon–phonon Umklapp processes and the behavior of κ(T ) is consistent with thatof other reports on Nd2−x Cex CuO4 [5–7]. κ(T ) of ‘as-sinter’ sample was reduced by the quenching process.
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Superlattices and Microstructures, Vol. 21, No. 3, 1997

359

30

3
ρ30/ρ300

OnsetMidEnd2

ρ30/ρ300Tc(K)252015880

900920940
960
Annealing temperature (°C)

980

1Fig. 2. Transition temperature Tc and ratio of resistivity (5) at 30 K to at 300 K as a function of annealing temperature TA .
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The Ar annealing process on ‘quench’ sample enhanced κ, resulting in higher κ values of ‘q +anneal’ samples
above those of ‘as-sinter’ sample.

4. Discussion
Accompanied with the appearance of superconductivity, the electrical resistivity of ‘quench’ sample increased in comparison with ‘as-sinter’ one. This means the increase of carrier scattering centers such as
oxygen vacancies through the quenching process. On the other hand, the introduction of oxygen vacancies
should increase the electron density of NCCO. The observed enhance of ρ(T ) means that the increase ofelectron scattering resultantly overwhelm that of electron density. Here we notice two important points forthe appearance of superconductivity in this system: (i) introduction of oxygen vacancy, (ii) enhancement ofelectron scattering.
Accompanied with the reduction of ρ(T ), the superconducting transition temperature Tc is enhanced
through the annealing process in Ar followed by slow cooling down of the sample temperature. One reasonableexplanation for this enhancement of Tc may be attributed to the reduction of the effect of localization causedby the reduction of scattering centers. It is to be noticed, however, that ρ(T ) of ‘q + anneal’ sample withthe highest Tc is still larger than that of ‘as-sinter’ one. This fact seems to suggest another key point that acertain unidentified scattering mechanism besides due to crystalline defects may be at work in superconducting
NCCO.
In order to elucidate how the above mentioned key points are reflected in the thermal transport, we analyze our results of the thermal conductivity κ(T ) based on the relaxation time approximation. Since theWiedemann–Franz law indicates that the electronic contribution κe is very small (∼ 1% of the total κ at100 K), we assume that the thermal conduction is entirely due to phonons in this system. Then κ(T ) is360Superlattices and Microstructures, Vol. 21, No. 3, 1997

Thermal conductivity (mW cm–1 K–1)

400
as-sinter
quenchq+anneal 880°C
q+anneal 950°C

300

200100050100150

Temperature (K)Fig. 3. The thermal conductivity κ(T ) as a function of T for Nd1.85 Ce0.15 CuO4 at typical stages of heat treatment. The lines represent
the calculated fitting curves for respective samples.
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expressed as [8, 9].

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κph =3dn RT 3 v 2
M23D

Z 2D /T0x 4 ex
τph dx,(ex − 1)2(1)where d is the mass density, n(= 7) the numbers of atoms in the formula, M the molar weight, R the gas
constant, v(= 5200 m s−1 ) the measured sound velocity, 2D (= 450 K) the Debye temperature and x is thereduced phonon frequency. The phonon relaxation time τph is assumed to follow Matthiessen’s rule,−1= τb−1 + τp−1 + τU−1 + τe−1 = lb /v + P T 4 x 4 + U x exp(−2D /aT ) + E T x.
τph(2)

Here τb is the scattering time due to grain boundaries, lb the mean value of grain size of the sample and P, Uand E refer to the strength of phonon scattering by point defects (τp−1 ), other phonons (τU−1 ) and charge carriers(τe−1 ), respectively. The function U x exp(−2D /aT ), which is a standard form for Umklapp processes [10]at relatively low temperatures, was found to result in the best fit for all the samples under common U and a(= 1.8) values.
The calculated curves for κ(T ) based on eqns (1) and (2) are also presented in Fig. 3. The parameter values
used and determined in the fitting process are summarized in Table 1. In Table 1 we notice that the reduction inκ of ‘quench’ sample is caused by both the appearance of scattering by carriers (E term) and the enhancementof scattering by point defects (P term) presumably due to increase of oxygen vacancies. The enhancementin κ of ‘q + anneal’ samples originates from the reduction of scattering by point defects in addition to the‘disappearance’ of phonon scattering by carriers.
A peculiar and puzzling point in the κph analyses given above is the appearance of phonon scattering due tocharge carriers in superconducting ‘quench’ sample. The appearance of E term was observed not only for thepresent sintered NCCO but also for a single crystal specimen. Figure 4 presents the detailed fitting curves forthe present ‘quench’ sample as well as our fitting curves for in-plane κ(T ) of a heat-treated superconductive
single crystal of NCCO (x = 0.145) due to Ogasawara et al. [7]. It is clearly demonstrated in the figure that.

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Superlattices and Microstructures, Vol. 21, No. 3, 1997

361

Table 1: Fitting parameters determined by κ(T ) analyses for various treated samples.

τb−1 (s−1 )lb (µm)U (s−1 )P(K−4 s−1 )E(K−1 s−1 )Tc (K)‘as-sinter’‘quench’‘q + anneal’(880 ◦ C)(950 ◦ C)

single crystal [7]as-grownannealed

3.6 × 10815
6.9 × 1011
5770—

3.6 × 10815
6.9 × 1011
650
1.1 × 10713

3.6 × 10815
6.9 × 1011
433018

1.4 × 10836
7.6 × 1011
6950—

3.6 × 10815
6.9 × 1011
433
020.

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300

400

Thermal conductivity (mW cm–1 K–1)

1.4 × 10836
7.6 × 1011
708
1.8 × 107
18

as-grown single crystal(x=0.145)
heat-treated single crystal(x=0.145)
quench sample(x=0.15)

300

200

200

100
100

050

100

0150

Temperature (K)Fig. 4. Fitting curves of κ(T ) for ‘quench’ sample (4), for an as-grown non-superconductive Nd1.85 Ce0.15 CuO4 single crystal ( )and for a heat-treated superconductive single crystal (2). Dotted lines without E T x term cannot reproduce the κ(T ) data of heat-treatedsuperconductive Nd1.85 Ce0.15 CuO4 crystals.
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E term linear in frequency x is necessary for both samples to get a reasonable fitting. The more puzzling
result is, however, the ‘disappearance’ of E term in ‘q + anneal’ samples. As for ‘q + anneal’ samples, it
is noteworthy that κ(T ) values are larger in these samples than those of ‘as-sinter’ one and the point defectscattering is weaker from the viewpoint of the thermal transport. In contrast, ρ(T ) of ‘as-sinter’ sample issmaller than the best superconductive ‘q + anneal’ sample suggesting weaker electron scattering in ‘as-sinter’
sample. It seems probable that a certain electron scattering mechanism other than crystalline defect scattering
may be operative in superconducting NCCO samples.

5. Summary
Superconductivity is stabilized by adding the quenching process to sintered Nd1.85 Ce0.15 CuO4 crystals.

362Superlattices and Microstructures, Vol. 21, No. 3, 1997

Introduction of oxygen vacancies seems to play a key role for the stabilization of superconductivity and,possibly, for the destabilization of the antiferromagnetic order.
Annealing process in Ar atmosphere on quenched (‘quench’) samples reduces phonon scattering bypoint defects. This is consistent with the observed reduction of the electrical resistivity ρ(T ) of annealed
Nd1.85 Ce0.15 CuO4 after the quenching process. The enhancement of Tc in these ‘q + anneal’ samples isconsistent with the reduction of the competing effect of electron localization.
From the analyses of the thermal conductivity κ(T ), the point defect density is smaller in superconductive‘q + anneal’ sample than non-superconductive as-sintered (‘as-sinter’) sample, while ρ(T ) values of ‘q +anneal’ samples are larger than those of ‘as-sinter’ one. Some additional scattering mechanism such as due
to anti-paramagnons may be operative in superconducting samples.
A characteristic scattering mechanism proportional to linear in phonon frequency ω has been confirmed in‘quench’ sample but it has disappeared through the annealing process. The origin of this scattering mechanism
is not clear at present stage but it seems to have some important connection with superconductivity.

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