The electronic specific heat of YBa2(Cu1−yNiy)3O7 from 2K to 300K

Physiea C 235-240

(1994) 1771-1772

PHYSICA

North-Holland

The Electronic Specific Heat of YBa2(CUl-yNiy)307 from 2K to 300K K. A. Mirza, J. W. Loram and J. R. Cooper. IRC in Superconductivity, University of Cambridge, Madingley Road, Cambridge CB3 0HE Using a high resolution differential technique we have investigated from 2K and 300K the electromc and magnetic contributions to the specific heat of YBa2(CUl.yNiy)307 for y=0, .025, 05, .1 and .15. The initial fractional decreases per Ni in Tc and the specific heat jump at Tc are -3.2 and -1 i respectively, factors 4 and 2 smaller than for Zn doping. From susceptibility and entropy we obtain a Ni spin -112 and negligible change in normal state density of states with Ni doping. The specific heats of the. 1 and 15 Ni samples are influenced by a broad anomaly at around 20K with a small sharp peak at 17 K probably due to an impurity phase (Y2BaCuO5). 1. I N T R O D U C T I O N It is widely accepted that divalent Ni and Zn substitute predominantly for Cu on the CuO2 planes. The relatively weak suppression of Tc by (magnetic) Ni and the strong suppression by (nonmagnetic) Zn doping (Fig I) are therefore both surprising and significant. We present here the results of specific heat and magnetic susceptibility measurements on a series of ceramm YBa2(Culy N i y ) 3 0 7 samples with Ni content y=0, 0.025, 0 05, 0 1 and 0.15 synthesized by conventional solid state reaction by S. M. Kilcoyne. From X-ray analysis the samples with y<0 05 are single phase but the 0.1 and 0.15 samples show slgmficant levels of Y2BaCuO5 and BaCuO2 impurity phases

3 5 and 4.1 (10 -4 emu/mole), effective moments 1.98, 1.66, 2.15 and 2.10 I.tB/NI and paramagnetlc Curie temperatures 0= -9.8, 2 8, 23.9 and 25 7K for the 2.5, 5, 10 and 15% Ni samples respectively The values of )~0 are close to that for pure YBCO73 0.10 -4 emu/mole and the moments are consistent with a spm -1/2 to 1 tor the N1 (I 7- 2 4 laB). Using a high resolution differential techmquc [11 we have measured the dlfferent.e m spemfic heat between each N~ doped sample and a 7% Zn doped YBCO 7 reference, the raw data ~°tty,T)~/l°tref(T) reflecting the ddference m electromc terms plus the difference in phonon term,, between sample and reference {.v=C/T) A pre~mu,, ,nvcstlgatmn [2] ot the YBa2(Cu l-yZny)307 system showed that the phonon and normal state electronic specific heats are almost independent of Zn content, and that for 7%

2.

RESULTS The normal state magnetic susceptibihty exhibits a substantml Curie-Weiss term increasing systematically with Ni doping. Fitting the results to the exp-ession Z=;t0+C/(T-0) yields ~0 = 3 2, 3.8, 51- '

,

,

i

,

,

,

,

,

,

,

,

,

,

,

'

6

~

'

1

'

'

'

1

'

~

-160 ~ I

"~--~-~, Zn ,

,

0

,

" " - "x ,

I

,

, ' , ~ ' 1 - ~

8

'

0

0

-ex--------~-------cx 4 20

x

4

i

E

F" to

1

100

4 3b,",, ' z.

'

| ,

i

-2

,_.~-~-Jn

12

t6

0

40

80

T(K)

120

1o0

200

Y (%)

Fig 2 7tm(y,T)@°tref~T) showing spec~hc heat data relative to a 7% Zn doped Y B C O 7 reference The electronic tcrm ¥(y,T) fm 0<_)_<005 is glvcn b) adding Yrcf=l 6 m J/g-at K 2 to the raw data

Fig I Tc from 7 (squares) and 9 (circles) and specific heat jump 8~Tc) (triangles) for Nl doped YBCL, 7 compmed with values for Zn doped YBCOT. 0921-4534/94/$07

SSDI

00 © 1994 - ELsevmr Science

0921-45q4(94)014';2-3

13 V

MI nghts

rescrvcd

1772

KA Mirza et al /Ph.vsica C 235-240 (1994) 1771-1772

Zn doping superconductivity is almost entirely suppressed. Thus 7% Zn doped YBCO 7 is an almost ideal reference which approximates closely to "nonsupert.onducting" YBCO7. The results for the Ni doped series are shown Fig 2. As for Zn doping [2[ the raw data is dominated by the difference in electronic/magnetic terms between sample and reference with only minimal changes in the phonon specific heat. Neglecting residual differences in phonon specific heat, which are in fact negligible for 0
matched by the increased entropy associated with ~/LT In this range of y the behaviour is similar to that of Zn doping [2], ~,LT being dominated by normal excitations due to pair breaking. Forl0% and 15% Ni the very large entropy associated with the broad peak at 20K persists above To. The excess entropy AS plotted in Fig 4 correlates closely with the Curie constant C in the susceptibility for all samples, and is consistent with a Ni spin-l/2 for y<0.05 and between 1/2 and 1 for y>.l. 3.

CONCLUSIONS In view of the X-ray analyses described above we can only be confident that our results reflect intrinsic behaviour for y_<0.05. In this region T c and the step height 8)'(Tc) decrease with Ni doping factors 4 and 2 less rapidly than for Zn doping. An upper limit J=l/2 is found for the NI spin from entropy and susceptibility. The broad peak at 20K and sharp anomaly at 17K for y=0 1 and 0.15 probably result from an impurity phase, possibly Y2BaCuO5 [3,4]. Since this makes a major contribution to the excess entropy AS above Tc and AS correlates with the Curie constadt C, values of the spin and effective moment per Ni derived from these quantities are overestimates for these two samples.

_

06

4. REFERENCES 1 J W Loram, J Phys El6, 367 (1983) 2 J. W" Loram, K A. Mirza and P. A Freeman, Phystca CI71, 243 (1990) 3 A Junod, m "Physical Properties of Hlgh Temperature Super._'Jnductors Vol 2", ed D. M Gmsberg, World Sclenufic, Singapore (1989) 4. N. E. Philhps, R. A. Fisher, I. E. Gordon, Prog. Low Temp. ~'hys. 13, 267 (1992) 03

....

I ....

J''

, r.- I . . . .

03

,! I~/I, I .... I ' ' '[i I .... II I '1 I ' h ~ 0

&o 04

.J=l/2

o2 ~

-.....

-

-10,

r.~ ,

Oq

o2 t

o

0 0

50

100

150

200

250

300

T(K) Fig 3 The clectromc entropy S lbr Iql doptd YBCO7

5

I

m

I

i

i

t

I

10 15 y%Nt

i

i

i

i

20

Fig 4 The exce~,s entropy AS (squares) and Curie constant C (triangies) Values of AS and C for a N1 spin 1/2 are shown bv bloken hnes