The specific heat of the 2201 BISCO high-Tc superconductor

Physica C 223 (1994) 57-61

ELSEVIER

The specific heat of the 2201 BISCO high-To superconductor M.K. Yu, J.P. Franck * Department of Physics, Universityof Alberta, Edmonton, Alta, Canada T6G 2J1

Received 1 February 1994

Abstract

The specific heat of two samples of the single-plane 2201 bismuth superconductor was measured. No linear term in Cp was observed at low temperatures. The lattice molar specific heat below 14 K exceeds that of the 2221 and 2223 bismuth superconductors considerably. As a consequence no peak in Cp/T 3 is observed in this superconductor, in contrast to other high-To cuprates. The specific-heat anomaly near Tc could not be resolved.

1. Introduction

The specific heat of the single-plane bismuth-based high-To superconductor 2201 is of considerable interest because of comparisons which can be made with the two- and three-plane superconductors 2212 and 2223. Very few results exist at present on the 2201 phase [ 1 ]. In the work presented here we give data in the low temperature range, as well as near the superconducting transition. Some typical differences between the 2201 and the 2212 bismuth superconductors were found in the lattice specific heat.

2. Experimental and results

We prepared and investigated two samples of nominal composition Bi2.1Srl.9CuOy ("sample 1") and Bi2.osPbo.osSrLgCuOy ("sample 2"). The samples were prepared from Bi203, PbO, SrCO3, and CuO of 99.99% purity or better. The ground mixtures were calcined in air at 800°C for 24 h, with two intermediate grindings. Subsequently, the samples were sin* Corresponding author.

tered at 850°C for 52 h in air, and oven-cooled to room temperature. The samples were characterized by X-ray powder diffraction (Cu K a ) , and DC magnetization at low fields. The magnetic susceptibility in the normal state was also obtained. The low-angle X-ray diffraction pattern is shown in Fig. 1. We see that sample 1 (without Pb) shows the characteristic (002) peak of the 2201 phase near 20=7.15 °. A second peak near 20=7.46 ° in this sample corresponds to the non-superconducting phase "Sr2Bi2CuO6" [2]. In sample 2 (containing Pb) this second phase is not seen in the X-ray diffraction pattern. This phase is therefore phase-pure 2201 at the detection level of X-ray analysis ( ~ 5%). This sample shows a larger transition temperature, To= 10.5 K, compared to Tc= 7.0 K for sample 1. The magnetic transition is also sharper; we observe therefore here again, as is found generally in a bismuth-based superconductor, that addition of small amounts of Pb aids in obtaining phase-pure samples. The magnetic transitions, obtained in a SQUID magnetometer, as shown in Fig. 2, where we show the field-cooled (Meissner) transitions in a field of 1 G. The transition temperatures are obtained in the usual

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M.K. Yu, J.P. Franck/Physica C223 (1994) 57-61

58

sured in an adiabatic calorimeter, using the heat pulse method. Samples o f between 2.5 and 3.0 g were used. We investigated the t e m p e r a t u r e range from 2.0 K to 20 K. In Fig. 3 we show the low t e m p e r a t u r e part o f Cp/T as function o f T 2. The specific heat o f both samples is almost identical. We observe an e x t e n d e d linear range, which extrapolates to C~ T= O, a n d at the lowest t e m p e r a t u r e the characteristic upturn usually observed in all high-To superconductors. F r o m a fit to the expression

(12

E

Cp( T) = A T - 2 + TT+ flT 3

I

I

I

5

6

I

I

7 20 (Cu K,~)

8

9

Fig. 1. The X-ray powder diffraction (Cu Kct) pattern at low angles. The region of the (002) of the Bi2Sr2CuO# phase is shown. Upper curve, sample 1; lower curve, sample 2. 0.24

OOOOoo

C" 2

~wvw~gOOIQQ0000

0.18

ii o oo

o

o o

0.12

FC

(2)

we find t h a t A is in the range 6.6 to 7 X 10 -2 K / m o l e , and 7 = 0 + 0 . 7 m J / m o l e K 2, see Table 1. The mole unit throughout is defined as one formula weight. The low-temperature upturn is usually connected with a Schottky a n o m a l y due to i m p u r i t y spins (e.g. Cu 2+ ); its absolute value in these samples is very similar to those found in the 2212 or 2223 BISCO superconductors [ 3 - 5 ] . The a p p a r e n t absence o f the linear term is also in agreement with the results o f most investigations o f the 2212 a n d 2223 phases [ 3 - 5 ], as well as the results o f von Molmir et al. on 2201 [ 1 ]. The absence o f the linear term in C e is therefore a

°

0.2 o

E

1G

(a) o

=e 0 . 0 6 o

v o

o

0 2

t

1

4

;

i

#

6

8

10

12

T (K)

Fig. 2. The field-cooled (Meissner) magnetization in a field of open circles: BizjSrLgCuOr+~ (sample 1), filled circles: BizosPbo.osSrt.gCuOr+6(sample 2).

'~

0.1

% 0 oO

0

20

0

way. The sharper transition a n d higher Tc for s a m p l e 2, containing Pb, are obvious. The magnetic susceptibility in the normal state was m e a s u r e d on sample 1, a n d fitted to the Curie expression

z ( T ) =Zo + C / T .

40

0.2

60 I

~

(b)

o

f

E 0.1

( 1)

We found that the sample was weakly paramagnetic, with a Curie constant o f C = ( 4 . 2 6 + 0 . 0 7 ) × 1 0 -3 e m u K / m o l e . This is very similar to the Curie constant o f Bil.sPbo.2Sr2CaCu2Oy, ( B I S C O 2212), investigated in Ref. [ 4 ]. The specific heat for these two samples was mea-

oOOo 0 0

20

40

60

T2 (K~) Fig. 3. The specific Cp/Tas a function of T 2. (a) Bi2.tSrt.gCuOr_~ (sample 1), ( b ) Biz0sPb0.osSrL9CuO6+~ (sample 2 ). The straightline fit is obtained by a least-squares fit to Eq. (2).

59

M.K. Yu, J.P. Franck / Physica C 223 (1994) 57-61 Table 1 Characteristic data for the specific heat of BISCO 2201 Sample

Tc (K)

A ( 10 -2 J K/mole)

Yo (m J/mole K 2)

00 (K)

Bi2.1SrLgCUOy Bi2.osPbo.osSrL9CttOy

7.0 10.5

6.6+ 1.2 7.0 + 1.2

0.1 _+0.6 0.3 _+0.7

211 _+1 206 +_1

feature that appears to be common to all bismuthbased superconductors. The third term in Eq. (2) refers to the low-temperature part of the lattice specific heat. We obtained the low-temperature Debye temperature 0D from this (Table 1 ); it is found in the range 206 to 211 IC These values are somewhat lower, by about 20%, than the Debye temperatures found in the 2212 compound. In our own work on the 2212 compound [4], we found 0D=265 K, in good agreement with other results on this compound [5]. A lower 0D in the 2201 compound was also reported by yon Moln~r et al. [ 1 ], who quote OD=195 IC It appears, therefore, that the phonon spectrum of 2201 is considerably softer in the low-frequency range than that of the 2212 and the 2223 compound. No neutron-scattering data for 2201 are at present available for comparison. In Fig. 4 we show the specific-heat data in the range of the magnetically obtained transition. We see no anomaly in Cp/T at To, although for sample 2 a very slight change in slope occurs near To. This absence of an observable discontinuity in Cp/T has quite frequently been found in the Bi based superconductors [3,4,6]. The occurrence of an observable ACp/T at T¢ is obviously very sample dependent. An extensive investigation of this was published by Braun et al. [ 7 ]. They found that the size of the discontinuity is strongly dependent on the oxygen content, and tends to be reduced or is absent in overdoped samples. A very sensitive way of analyzing the low-temperature lattice specific heat is provided by a plot of Cp/ T 3 as a function of T; this is shown in Fig. 5 for both samples. A characteristic feature observed in such a plot is a broad maximum in Cp/T 3, as is found in all high-T¢ superconductors [3,4,8,9]. As can be seen from Fig. 15, this feature is absent in the specific heat of the 2201 compound. We have compared in Fig. 5 the specific-heat data on the present 2201 samples with our earlier measurements on a 2212 sample [ 4 ].

0.24 oo

(a)

o

o

~: 016 o

o

o o

o o

0 .--)

o

o o

o o

0.12

o o o o

o o°°

t

oo oo °

0.06

~°°°

ooOO

o o

Tc

o° I

I

I

I

I

I

I

3

4

5

6

7

8

9

10

T (K) 0.48

(b) ~

oOO OOOO(

0.36 ooo o°°

0

oO

0,24

oOO* oo

Oo

o° l '

ooO o ° °

~

0.12

Tc

oOO OOOO

7

8

9

1

12

1

14

T (K) Fig. 4. The specific heat Cp/T as a function of T. The position of the magnetic transition (onset of Meissner signal) is indicated. (a) Bi2.tSrz.9CuO6+a (sample 1); (b) Bi2.osPbo.osSrLgCuO6+~ (sample 2). Note the difference in scale for the two samples.

One sees that above 14 K the molar specific heat for all three samples is almost identical. The peak in Cp/ Ts is evident for the 2212 sample, but for both 2201 samples the specific heat rises considerable above that of 2212 for T < 14 IC The true T s range, which in the 2212 samples does not extend beyond 6 K, is therefore much wider for the 2201 samples and ranges to about 10 K. The occurrence of the peak in Cp/T 3 has found various explanations. A peak in the phonon density of states at roughly five times the peak temperature could account for this, and for the 90 K superconductor YBa2Cu307 such a structure was observed near

60

M.K. Yu, J.P. Franck / Physica C 223 (1994) 57-61

that the anomaly in Cp/T 3 increases for the very highTc superconductor T 1 2 f a 2 B a 2 C u 3 0 . 8 ( T c ~ 125 K). The experimental observations show, however, a rather reduced anomaly in this compound [3,8]. It is therefore not obvious whether this part of the lattice specific heat has any direct connection with the appearance of superconductivity.

o" 4-" "5 E ~2

~ "'....'---.....

....

¸

c~

Io_

0

10

2

4

6

8

10 12 T (K)

16

18

20

Fig. 5. The specific heat Cp/T 3 as a function of T. The upper two curves present the samples of this work; open circles: Bi2.1Srl.9CuO6+6 (sample 1 ), filled circles: Bi2.05Pbo.osSrl.9CuO6+6 (sample 2). The data are compared with those of a sample of Bil.sPbo.2Sr2CaCuOr, (2212 ) obtained as part of the work of ref. [4], shown by crosses.

11 meV in inelastic neuron scattering [ 10,11 ]. For the Bi based superconductors, however, the peak in Cp/T 3 has moved to about 10 K; no corresponding peak was observed in the neutron-scattering data of BISCO 2212 [ 12 ]. Urbach et al. [ 8 ] proposed a connection of the anomaly with the incommensurate superstructure found in the Bi superconductors. This superstructure is also present in the 2201 compound [13,14], so that the absence of the specific-heat anomaly in this compound argues against this explanation. A very thorough theoretical investigation of this phenomenon has been given by de Wette et al. [ 1517 ]. The lattice dynamical calculations of these authors have found low-lying, almost dispersionless optical modes in the phonon spectra of both YBa2Cu307 and T12Ca2Ba2Cu3Olo. In the latter compound this effect is enormously enhanced. No calculations have so far been published for the 2201 superconductor. The experimental results appear to show that the low dispersive modes in 2212 and 2223 develop much more dispersion in the 2201 compound, reaching to lower energies, and therefore a lowered zero temperature Debye temperature. One could speculate that the phonon spectrum associated with the low-lying optical modes is in some way connected with the occurrence of superconductivity, and that the absence of this feature in the 220 l compound is connected with its relatively low transition temperature. The theoretical calculations of de Wette et al. show

3. Conclusion The specific heat of the single-plane 2201 bismuth superconductor does not show a linear term in Cp at low temperatures, in common with the two- and threeplane compounds 2212 and 2223. The lattice specific heat at low temperatures implies considerable softening in the phonon spectrum; this leads to the absence of the commonly found peak in Cp/T 3, and a reduced Debye temperature.

Acknowledgements We gratefully acknowledge interesting conversations with F.W. de Wette. This work was supported by grants from NSERC (Natural Sciences and Engineering Research Council of Canada).

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