Specific heat, magnetic susceptibility and superconductivity of YBa2Cu3O7−δ doped with iron

Physica C 152 (1988) 495-504 North-Holland, Amsterdam

S P E C I F I C HEAT, M A G N E T I C S U S C E P T I B I L I T Y A N D S U P E R C O N D U C T I V I T Y O F YBa2Cu3OT-6 D O P E D W I T H I R O N A. J U N O D , A. B E Z I N G E , D. ECKERT, T. G R A F and J. M U L L E R Dbpartement de Physique de la Matibre Condenske, Universitb de Genbve, CH-1211 Genbve 4, Switzerland

Received 22 April 1988 Revised manuscript received 11 May 1988

YBa2(Cu~ _xFe,)307_6 with x=0, 1%, 2% and 4% was prepared by the citrate pyrolysis method. The samples were characterized by X-ray diffraction, micrographs, electron microprobe, a.c. susceptibility near the critical temperature To, field cooling Meissner effect, d.c. susceptibility from 100 to 250 K and specific heat from I to 300 K, These measurements confirm that iron does not segregate. The localized moment is close to 4.0/ta/Fe-atom and the constant component of the susceptibility increases from 283 to 530 × 10 -6 cm3/mol-f.u. Calorimetric data indicate that for x>~2% the superconducting phase is no longer homogeneous. The low-temperature linear term in the specific heat reflects a growing nonsuperconducting fraction. The apparent contradiction between a high Meissner effect at 4 K and the absence of any definite specific heat jump at Tc in the sample with x=4% is understood as an effect of the short coherence length in the presence of repulsive defects. Using the latter tetragonal sample as a normal reference compound, the shape of the specific heat difference Cs- Cn near Tc for the undoped sample (calorimetric transition width: 1.3 K, AC/Tc = 49 mJ/( K2 mol ) ) is discussed.

1. Introduction M a e n o et al. [ 1,2 ] have shown that small a m o u n t s o f Fe, Co and G a substituted for Cu in the 92 K-sup e r c o n d u c t o r YBa2Cu307_6 ( " Y B C O " ) induce an o r t h o r h o m b i c to tetragonal structural phase transformation. The superconducting properties do not show any discontinuity across the phase boundary. This b e h a v i o u r contrasts with the absence o f superconductivity in the oxygen depleted tetragonal phase o b t a i n e d by quenching or by v a c u u m annealing. It was pointed out that the F e - d o p e d tetragonal phase is rich in oxygen (>_-6.8 oxygen per formula unit [1,3,4]). I f bulk superconductivity is confirmed in the tetragonal phase, the consequences are i m p o r t a n t both from a theoretical and from a technological p o i n t o f view. The absence o f Cu(1 ) - 0 ( 4 ) chains in the tetragonal phase implies that the supercurrent flows p r e d o m i n a n t l y in the Cu ( 2 ) - O ( 2 ) - O ( 3 ) semiplanar structure elements in u n d o p e d YBCO. On the other hand, the presence o f twinning in o r t h o r h o m bic polycrystals adds a high density o f scattering centers that should be a v o i d e d in a superconductor

characterized by a very short coherence length if useful critical current densities are a i m e d at, especially in view o f p r e l i m i n a r y reports indicating a - b plane anisotropy [5 ]. So far, informations on the superconducting volume fraction in F e - d o p e d samples have been obtained only by a.c. susceptibility techniques on p o w d e r e d samples. An overestimate due to d y n a m i c shielding cannot be excluded. We present here the results o f true bulk measurements: field cooling Meissner effect and specific heat j u m p at To, together with a careful characterization by X-ray diffraction, micrographs, electron microprobe, a.c. susceptibility near the critical t e m p e r a t u r e T~, d.c. susceptibility from 100 to 250 K and specific heat from 1 to 300 K.

2. Sample preparation The homogeneous dissolution o f 1% F e / C u (about 0.05 g Fe203 for 15 g starting p r o d u c t s ) is not a trivial p r o b l e m and we t u r n e d to wet chemical methods in o r d e r to ensure mixing at the molecular level. The

0 9 2 1 - 4 5 3 4 / 8 8 / $ 0 3 . 5 0 © Elsevier Science Publishers B.V. ( N o r t h - H o l l a n d Physics Publishing D i v i s i o n )

496

A. J u n o d et a / / ?;pec(/ic heat q f YBa: (('u,.

,ki',)¢O .,

limits and that Fe indeed substitutes for Cu inside the grains. Similar studies in the Y - B a - C u - O system have also shown that the homogeneity d o m a i n of the 1-2-3 phase, if it exists, is extremely narrow

citrate pyrolysis technique suggested by Blank el al. [6] followed by o p t i m i z e d heat t r e a t m e n t s [7] appeared suitable. The starting products are CuO ( > 9 9 % ) , Y203 (99.999%), BaCO3 ( > 9 9 % , main i m p u r i t y 0.3% Sr), all from Fluka A.G., and Fe20~ (99.9%) from Ventron. The oxides and carbonates are dissolved into hot nitric acid. After mixing, citric acid is a d d e d and the solution is neutralized with a m m o n i a . It is then e v a p o r a t e d on a hot plate until spontaneous c o m b u s t i o n occurs. The resulting submicron p o w d e r is then deposited onto Pt foils, dried in vacuo at 200°C, and fired in oxygen, raising progressively the t e m p e r a t u r e to 950°C. After a 14 hour calcination, the powders are cold pressed into flat cylinders, 0.5 g each, and sintered in flowing oxygen for 17 hours at 980°C. The final oxygen uptake occurs during the slow cooldown ( - 50 ° C / h ) to room temperature. The macroscopic density obtained from mass, d i a m e t e r and thickness m e a s u r e m e n t s is 88 to 95% o f the X-ray theoretical density (6.38 g / c m 3 ) .

The samples were e x a m i n e d by X-ray powder diffraction at room temperature, using the G u i n i c r m e t h o d and Cu-Kc~ radiation. The YBCO powders were mixed with Si standard. All lines could be indexed in the o r t h o r h o m b i c s y m m e t r y for the samples with 0, 1% and 2% F e / C u , and in the tetragonal s y m m e t r y for the sample with 4% F e / C u . The parameters are given in table It. A smooth variation o f all p a r a m e t e r s versus Fe concentration occurs as expected in a homogeneous solid solution. Micrographs under polarized light (fig. 1) show twinning in all grains when x~< 2% and complete absence o f twinning when x = 4 % , thus confirming Xray data. This does not exclude twinning at a submicroscopic level.

3. Sample characterization

4. Experimental

The final metal concentrations were checked by E D X analysis. A C a m b r i d g e Stereoscan 360 SEM microscope fitted with a Tracor N o r t h e r n Z-2 att a c h m e n t was used. In the absence of suitable standards, systematic errors m a y affect the absolute concentrations, but relative variations o f the metal p r o p o r t i o n s are significant. The d a t a o f table I are averages over several locations from the center to the edge of the samples, the grains were always larger than the active area. F r o m these data, we conclude that the 1-2-3 stoichiometry is preserved within error

The a.c. susceptibility o f the bulk samples was measured at 73 Hz and 0.01 Oe rms in helium gas. The heating curve from 5 to 100 K was recorded al the rate of 15 K / h . The transition m i d p o i n t s and widths are given in table Ill. A S Q U I D m a g n e t o m e t e r was used for Meissner effect m e a s u r e m e n t s [9 ]. The constant applied field (19 G ) was calibrated using Au and Pb standards. Only field cooling experiments are reported here. We expect that the applied field quenches most internal Josephson junctions, thus avoiding an overestimate

[81.

Table 1 Results of the EDX (energy dispersive X-ray ) analyses; the total metal concentration is normalized to 100%. The errors indicate statistical variations between different locations. The systematic errors cannot be assessed owing to the lack of a suitable standard. Sample code

x (%)

Fe metal (%)

Cu metal (%)

Ba metal (%)

Y metal (%)

Cu + Fe metal (%)

Ideal J449 J452 J450 J451

a 0 I 2 4

x/2 0.09_+0.05 0.48-+0.01 1.03_+0.11 1.90_+0.15

50- (x/2) 49.68+_0.01 49.00_+0.18 48.49_+0.15 47.35_+0.10

33.33 32.43+0.01 32.65+0.05 32.80-+0.05 32.72_+0.13

16.67 17.81 +0.06 17.87+0.16 17.68+0.10 18.02+0.10

50 49.77 49.48 49.52 49.25

,4. Junod et al. / Specific heat o f YBa2 (Cu l_ ~Fe~)sO7_~

497

Table II Lattice parameters of the YBa2 ( Cu ~ ,Fe ~) 307 _ ~ samples. Sample code

Fe C--u

a (A)

b (A)

c (A)

V (A 3)

(%) J449 J452 J450 J451

0 1 2 4

2(b-a) b+a

(%) 3.8179(7) 3.8246(6) 3.8341(8) 3.8626(6)

3.8868(8) 3.8849(7) 3.8808(7) 3.8626(6)

of the superconducting volume fraction, but is low enough to avoid freezing the metastable Shubnikov phase by flux pinning. The Meissner flux expulsion ratio f w a s evaluated using an effective sample volume given by m / d where m is the mass and d is 6.38 g/cm 3. Demagnetizing field effects were altogether neglected in view of the favourable aspect ratio. The field was applied parallel to the disk diameter. The d.c. susceptibility was measured from 100 to 250 K in the same apparatus. A correction for saturable moments using 0.5 and 2 T data was applied in all cases. These impurity moments, expressed as equivalent ferromagnetic Fe concentrations, are 2.3 ppm ( x = 0 ) , 0.3 ppm ( x = 1%), 0.8 ppm ( x = 2 % ) and <0.8 ppm (x--4% Fe/Cu). Clearly iron is not ferromagnetic at this dilution level and hence the precipitation of large iron clusters in the samples is most unlikely. The low temperature specific heat measurements (1-20 K) were performed with a microcomputercontrolled thermal relaxation method, on samples weighing 80 mg. The addenda heat capacity was measured separately and subtracted. The uncertainty on the results is estimated to be less than 5%. From 30 to 300 K we used a computerized version of the adiabatic, constant heating method [10]. A high reproducibility ( < 1 % ) was obtained using samples with the same mass (0.5 g), same geometry and identical heating rates in order to cancel uncertainties due to addenda heat capacity and temperature gradients. The accuracy is estimated to be 5%. Platinum resistance thermometry was used for all experiments above 25 K.

11.676(2) 11.683(2) 11.681(2) 11.685(4)

173.26 173.59 173.81 174.34

1.79

1.56 1.21 0

5. Results and discussion

5. I. Magnetic properties The d.c. susceptibility in the normal state is shown in fig. 2. The regularly spaced curves indicate that the Fe content has been controlled down to low dopant levels. These data are analyzed according to the formula Z(T) =Zo + c / T , where Zo is the sum of the core, Pauli, van Vleck and (small) Landau-Peierls terms, and c is the Curie constant. As seen from the linearity of the z T versus Tplot in fig. 3, it is not necessary to include a CurieWeiss temperature. The parameters tabulated in table III are obtained by a fit including all data from 100 to 250 K. Before'converting the Curie constants into effective moments per Fe atom, we subtracted the background Curie constant obtained in the ironfree sample, which is equivalent to 0.9% Cu2+/ Cut°tal; it was assumed to be due to an impurity such as BaCuO2. The remaining effective moment, 4.07 ~B/Fe, is very slightly reduced ( - 3 % ) in the tetragonal phase with respect to the orthorhombic one. The moment is clearly smaller than expected for Fe 2÷ (5.4 /tn) or Fe 3÷ (5.9 ~ta). The difference reflects the partial quenching of the orbital moment by the crystal field. The constant component Zo increases monotonically with the Fe content ( ~ 2 × 10-3 emu/mol-Fe). This may be possibly due to the van Vleck type orbital susceptibility of the iron atoms; but the order of magnitude (about 30 times the value of paramagnetic iron) would be very large. More likely it reflects an increasing Stoner enhancement coupled with a variation of the density of states (DOS) at the

498

4. Junod et al. /Specilic heal (~f YBa: (('u~ ,Fe,) <0 ,, Fermi level. We found in a previous work [ 10] that T,. does not depend on the DOS. The magnetic data presented here interpolate or continue smoothly the published data taken at other Fe concentrations [ 4,11,12 ]. 5.2. Critical temperature and Meissner effect

The critical temperature determined by a.c. diamagnetism decreases very slowly in the orthorhombic domain (table III). Reproducibility tests were performed, using samples of various shapes made from the same precursor pyrolyzed powder. The transition midpoint is reproducible in the orthorhombic region. The variations observed in the tetragonal samples are indicative of shielding. Breaking the latter samples into pieces depresses T~, by ~ 10 K, powdering by ~ 20 K. Similar tests on a sample with x = 1% do not affect T~. The Meissner field cooling curve was flat below 80 K for pure YBa2CusOv samples prepared by the same technique [7,10]. In the present study, data were collected only at a few selected temperatures (table IlI). Again, the Meissner fraction ,f is not significantly different at 60 K and at 4 K in the orthorhombic doped samples. On the other hand, the Meissner transition is considerably smeared in the tetragonal phase ( x = 4 % ) . This confirms that the high T,. observed by a.c. susceptibility is due to shielding, 5.3. Spec(lTc heat

Fig. I. Microstructural examination under polarized light of YBCO samples doped with (a) x=l%, (b) x=2%, and (c) x=4% Fe/Cu. Twinning patterns are observed in the first two. The samples are etched in a solution of CH~COOH+ C2HsOH + H20 ("Alacet").

The specific heat from 1 to 300 K is shown in fig. 4. The normalization assumes 13 atoms per formula unit. The peaks near 235 K are of instrumental origin [10]. The undoped sample exhibited instabilities (oscillations of the heating rate without net effect on the specific heat) in the 230-330 K range, supposedly related to rearrangements of the twin patterns. The atomic specific heats are very close to each other over the whole temperature range. The high temperature values are identical for 0~
0 0 0 0 1 1 1 2 2 2 2 4 4 4 4

J452a J452b J452c

J450a J450b J450c J450d

J451a J451b J451c J451d

(%)

Fe CH

J449b J449c J449d J449h

Sample code

77.3 78.0 77.3 68.3*

89.3 89.6 89.1 88.4

91.2 90.7 91.2

14 13 10 11

2.0 1.7 1.3 1.6

1.0 1.3

1.8 1.9 1.8 1.3

(K)

(K)

91.4 91.4 90.6 91.5

ATe

Tc mJ

(~0)

( ~ 16)

48

49

\,1~

(

A C / Tc ~

14% ( 6 0 K ) 31% (40 K) 39% (4 K)

47%(60K) 48% (40-4 K)

66%(60 K) 61%(60-4K)

66%(60 K)

(%)

f

530

404

319

283

(10-6cm3~ \ mol ,/

Zo

246

134

72

10

(.10-3 cm3 K ' \ mol

c

3.96

4.07

4.07

(,UB)

Pert

Table III Summary of the physical measurements. T~, superconducting transition midpoint, a.c. diamagnetism measurement on bulk samples (for calorimetric determinations, see fig. 6 ). The value with an asterisk was measured on a sample broken into pieces. ATe, 10-90% transition width. AC, specific heat jump at T~; f, fraction of Meissner flux expulsion, field cooling in 19 G; Zo, temperature-independent component of the susceptibility; c, Curie constant (the Curie-Weiss temperature is set to zero); P~ff, localized moment per Fe atom (see text).

e~

?

I

g~

.4. Junod el al. / Spec(fic heal of YBa, ( ( u /

500

250

.

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__

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---'~ . . . . . . . . J

_ _

loo

i

t

__

150

200

250

2 o

TIKI

~f,//

Fig. 2. Normal-state susceptibility X per gram-atom ( 1/ 13 tool ) versus t e m p e r a t u r e T for 0 - 4 % F e / C u d o p i n g . T h e c o n s t a n t c o m p o n e n t Xo is i n d i c a t e d by d a s h e d lines. T h e m a g n e t i c fields used are 5 a n d 20 k G .

. J-"

/ / / / --/" ./ / // / /" /

o

i

0

/"

./" f~

/ / " - -

-'" .....

/ //

/"

//

\ /

obtained by vacuum annealing, which has a higher atomic heat below 200 K [13], in agreement with the changes in the generalized phonon density of states observed by inelastic neutron scattering [14,151. The low temperature specific heat is shown in fig. 5. This part of the curve is exceedingly sensitive to traces of impurities such as BaCuO2 [ 16 ]. The upturns at low temperatures look like Schottky tails at first glance, but do not follow a T - 2 power law, and there is no direct proportion between their amplitude and the Fe content. It is not easy either to de30

q

l

~

....

~ .....

i

x=Z,,% ~ ~ ~

~

lo

__

00

i

J

50

100

. _ _

t

1

150

200

250

T[K] Fig. 3. Normal-state susceptibility per gram-atom m u l t i p l i e d b ) the temperature x T versus temperature T f o r 0 - 4 % F e / C u doping. T h e e x t r a p o l a t e d o r d i n a t e gives the C u r i e t e r m a n d the slope gives the c o n s t a n t t e r m Xo.

00

50

100

150

200

250

~0C

T [K]

Fig. 4. Specific heat p e r g r a m - a t o m ( ' versus t e m p e r a t u r e } ' u p to r o o m t e m p e r a t u r e . N o t e the scale offset for e a c h Fe d o p i n g .

fine unambiguously a linear contribution 7* 7". The specific heat of the undoped sample may be fitted within 1.3% rms in the 1-17 K range by a polynomial C=AT

: + 7 * T-F ozT 3 + ,BT -" .

where A=28.1 mJ K / m o l , 7"=10.6 mJ/K-" m o i l c~=0.265 m J / K 4 tool), i.e. 0 ( 0 ) = 4 5 7 K, and fl= 0.75 # J / ( K ° tool ). The linear term is about twice as large as the lowest ones published to date [ 17,18 ]. Using the last two terms of this fit as a reference laltice baseline, the excess electronic specific heat AC due to doping and impurities can be separated out (see fig. 5, inset). The linear term 7", estimated from this plot in the 7-15 K range, increases with the Fe content at a rate of the order of 10 m J / ( K 2 mol) per percent Fe/Cu. It is interesting to compare it with the rate that would be expected from an increasing number of normal-state regions localized around Fe magnetic ions. Assuming 7 ~ 3 3 m J / K -~ mol) in the undoped sample [ 10], and inferring from Meissner flux expulsion that no more than 61% of the x = 4 % sample can be in the normal state, we expect a dependence of about 5 m J / ( K 2 mol) per percent Fe/ Cu or lower. There seems to be a discrepancy by a

A. Junod et aL /Specific heat of YBa, (Cul_ xFeQ3Oz_6 25

I

I

- - 6o

'

'

I

/l

Y Be 2 (CUl-x Fex) 307-5 .~

,,o°°o.

'

0

~ll

'

~

r 10

5

N,"

o

501

TIK]

~ 5 ~

0

~ Q

0

~

I 15

zxza _ o 8 • t . h zx ~ 8 ~ e

8 +++++4~" . , ~ ~ , - o~--

@ c

I 100

+..¢,,r" ÷++,,~"

I 200

b

x - 1%

c

x=2%

d

x= 4%

J 300

400

T 2 [K 2]

Fig. 5. Specificheat per gram-atom divided by the temperature C/Tversus T 2 below 20 K for 0-4% Fe/Cu doping. Inset: specificheat C of the same samplesafter subtraction of the lattice contribution Cj = ctT 3+ fiT 5 of the iron-freesample.

factor of two. It can be easily resolved however if one remembers that the susceptibility, and hence possibly the density of states at the Fermi level, has also increased by a factor of two in the sample with x = 4% (we have shown earlier that the non-Pauli contributions to the magnetic susceptibility nearly compensate in YBCO [ 10 ] ). Thus it is not unreasonable to assume that the low-temperature linear term associated with Fe additions represents the contribution of normal electrons. 57Fe M6ssbauer spectra have shown that the iron atoms order magnetically at low temperature [ 19 ] (for x = 0.01, Tmag ~,~3 K depending on the iron site ). No sharp transition appears in the specific heat in this temperature range, but some features of the excess specific heat (fig. 5, inset) may reflect a progressive ordering. 5.4. Specific heat j u m p at Tc

The specific heat of the undoped sample is shown in fig. 6a. The calorimetric transition midpoint is in good agreement with the a.c. susceptibility determination. If one defines the 0-100% transition width as the difference between the temperatures where the C / T data start deviating from their linear behaviour

observed just below and just above To then the total width is 2.3 K. The width between the calorimetric 10% and 90% points is 1.3 K. The idealized specific heat jump is found by a short (10%) linear extrapolation and amounts to 49 m J / K 2 mol). Due to the sharpness of the transition, this figure is considered to be highly reliable; somewhat larger jumps however have been determined in samples with broader transitions [10,20]. Note that two independent measurements with different cooling schemes are included in fig. 6a, both using a heating rate of about 0.2 K/min. The transition region for the doped samples is shown in fig. 6b. The jump for the x = 1% samples coincides within experimental uncertainty with the jump for the undoped sample; the transition interval is still extremely narrow (1.3 K, 10-90%; 2.5 K, 0100%). Superconductivity is therefore completely unaffected by impurities in the dilute limit, an argument that excludes p-wave pairing. As one approaches the orthorhombic-to-tetragonal limit, the behaviour is radically different from that expected from Tc versus x "ptiase diagrams"; instead of a shift of a well-defined specific heat jump, one observes a severe smearing of the transition when x = 2% and a complete disappearance when x = 4 % . We carefully

502

,,t..lunod et al. I Spectyic heal of" YBa: (('u~ ,k2",) O- .

91 3 K

&

lOL

~d

I

102

QI

l~"~'

1K

i

t"

93,6K

.,100

~-g2

¢

x=O

'

810

910

-

I00

-

1110

120

t

(b)

q

T{KI

106

/

x~1%

#

•

~4

_~

102

~

I 8o

~,t:

,

I

k 9o

C/TI s

i

"",,~"~

I _ i ~oo

J 11o

~

I 12o

T[K)

Fig. 6. Specific heat per gram-atom divided by the temperature in the vicinity of the superconducting transition 7~.. (a) Iron-free sample (different symbols mark two independent measurements). The solid line indicates the extrapolation to obtain the idealized j u m p AC/T~. (b) Doped samples with 1-4% Fe/Cu. Inset: C divided by 7 "L3 (mJ K 2s g_at-.]) for the tetragonal sample, showing the absence of any bulk transition at the onset of diamagnetism.

looked at the region below 77 K, where the diamagnetic transition is seen, by plotting C / T " , 1 < n < 2 , versus T in order to flatten out the background lattice contribution (e.g. fig. 6b, inset). There is no transition within experimental scatter. In view of the smearing of the Meissner transition, this result is not unexpected, although pinning might broaden the field cooling Meissner transition. Keeping in mind the fact that the heat treatments used consistently in our preparation technique have produced the sharpest bulk transitions for orthorhombic samples, these results cast some doubt on the existence of a homogeneous superconducting

phase in tetragonal Fe-doped samples. The structural homogeneity itself has been questioned; a microtwinned pseudo-tetragonal phase [3] can yield an overall tetragonal symmetry, without locally destroying the chain structure, A high degree of disorder must be assumed to account for the sluggish onset of bulk superconductivity. On the other hand, the superconducting order parameter cannot be spatially homogeneous in the presence of repulsive defects if the coherence length # is short. ~ is a measure of the size of a perturbation over which the order parameter can "jump". In classical superconductors. is larger than the thickness d of the barriers introduced by doping at usual levels, and a bulk transition occurs at a temperature defined by average microscopic properties. In the high-T,, oxide YBCO where the order of magnitude of ~ is 10 A, small clusters of defects can efficiently suppress the order parameter (especially in the one- or two-dimensional case) and a distribution of critical temperatures is expected, depending on local physical variables. A crossover between the two regimes ( ~ d ) apparently takes place between 1 and 2% Fe/Cu, i.e. not at the structural phase limit, but within the orthorhombic phase as shown by the behaviour of the calorimetric transition (fig. 6a,b ). This phenomenon is not limited to high-T<, oxides. It has been found in this laboratory that it is extremely difficult to obtain sharp calorimetric transitions at Tc in the doped Chevrel-phase superconductors Pb~_~Ga,Mo6Ss, which are also characterized by a short coherence length [21 ].

5.5. Specific heat d~[lerence C , - C , ( T ) . The free energy difference between the superconducting and the normal state, and its derivatives the entropy and the specific heat difference, are of utmost importance for the understanding of the superconducting state since their shape can be in principle calculated by means of the Eliashberg equations if the anisotropic spectral coupling function is given. Conversely, much insight into the coupling mechanism can be gained if the precise shape of C~- Cn (T) is known. Since it is not possible to quench high-temperature superconductivity by available magnetic fields, we discuss the difference between the orthorhombic samples and the tetragonal doped sample, which does

A. Junod et aL / Specific heat ofYBa2(Cul_xFe~)30z ~

not show any sharp transition, and which has practically the same specific heat above Tc, as seen in fig. 6. This is clearly only a first approach and we shall emphasize its limitations. The point-to-point differences in C~ T are shown in fig. 7. The weak-coupling BCS behaviour (adjusted at To) is shown for comparison. Three main differences are seen: the measured peak is more A-like, the sign change occurs at a higher reduced temperature and the net entropy difference is not zero, but strongly negative. None of these points can be assigned to the progressive onset of superconductivity in the subtracted x = 4 % compound below 70 K. Strong coupling will shift the sign change to higher reduced temperatures but will not affect the entropy balance. The appropriate explanation is that the specific heat of the x = 4 % compound (say, above 70 K) does not represent adequately the normal-state behaviour of the ironfree sample. The symmetry change can modify the lattice contribution of the tetragonal sample (in this case the equality of the specific heats in the 95-120 K range for all values of x would be fortuitious) or else a minor phase transition occurs exactly at Tc in the orthorhombic samples, as suggested by anomalies reported in the orthorhombic strain [22 ] and in the velocity of sound [23]. In the latter case, the

*0.2

?/~/ 0% Fe/Cu

0

r

I

D~ -0.2

--

-0 4

.

,~q~"" -

~"I

~'~ ,,..~ ~ 1% ge/Cu I

I

/l

~

I÷04 ÷02

/¢I

0

J

+02

0

-02

02

1

I

215

510

....I

715

04

I

100

I

125

150

T[K]

Fig. 7. Specific heat difference AC between the orthorhombic samples ( x = 0 - 2 % Fe/Cu) and the tetragona] sample ( x = 4 % Fe/Cu). The full scale (0.4 J / K g-at)) represents about 4% of the total specific heat at I00 K. The BCS behaviour adjusted at Tc is shown as a dashed line.

503

continuity of the slope of the lattice contribution at T~ is no longer requested. These remarks show that modelling the phonon spectrum [ 24 ] or doping the sample [ 20 ] to provide normal-state data is a fundamentally delicate procedure.

6. Conclusion

Micrographs, X-ray, EDX and susceptibility measurements have shown that Fe in YBa2 (Cut _xFex)307_a samples prepared by the citrate pyrolysis route is homogeneously dissolved. Superconductivity in the macroscopically tetragonal phase ( x = 4 % ) cannot be attributed to traces of a second phase as evidenced by the high Meissner fraction ( ~ 40% at 4 K). Either the superconducting orthorhombic phase is macroscopically present, but undetectable by X-ray diffraction techniques because of twinning on a very small scale, or the tetragonal phase is the superconducting one. In any case, the new information given by the present measurements is that bulk superconductivity occurs at much lower temperatures than suggested by simple resistivity measurements, and that it occurs gradually in a disordered phase. Very sharp calorimetric transitions are observed in the samples with 0 and 1% Fe/Cu only, i.e. well inside the orthorhombic domain. Beyond this limit, the system is still homogeneous from a crystallographic point of view, at least macroscopically, but is in a mixed phase below the resistive Tc as far as superconducting properties are concerned. This is evidenced by the suppression of the specific heat jump, by the significant increase of the linear term x*T at low temperatures and by the broadened and incomplete Meissner transition. The answer to the question raised in the Introduction, is the tetragonal phase really a bulk superconductor, needs a more elaborate answer than yes or no. At 4 K, it is; just below the resistive transition, it is not. By analogy with the Shubnikov phase, the description of the superconducting phase diagram in the T - x plane requires two boundary lines xc2(T) (or equivalently Tc2(x) ) and xcl (T) (or Tct ( x ) ) . Below Tcl (x), the samples are in a homogeneous superconducting state. Our measurements are compatible with Tc~ (x) fall-

504

A. .lunod el al. / Sped[ic heat o f YBa : (CtO ,kk~,) ~0- ,~

ing to zero at or very close to the orthorhombic to tetragonal phase transition. Between T~.2(x) and T~.~(x), the samples are in a zero-resistance state, but the volume fraction of the superconducting phase grows gradually from zero at T = T,.2(x) to a maximum value at T = T~.~( x ) . T h e curve Tc2(x) falls off at a much slower rate than T,,~ (x) and reaches zero only near x = 0 . 1 6 [4]. A volume transition in the thermodynamic sense occurs only when T~.~(x) = T~.2(x), and we found that this occurs only when x = 0 and 1% Fe/Cu where T~ ~ 91 K. This limit has no relation with the crystallographic phase limit which occurs at a higher Fe concentration, x ~ 3%; by analogy with the condition for type-II superconductivity, we anticipate that the homogeneity limit occurs when the coherence length equals a characteristic length scale of the defects due to Fe doping. Our interpretation is that superconductivity in the mixed phase region is local@ suppressed by the presence of iron; since iron does not order, this suppression has a statistical character, i.e. regions with a lower local iron concentration become superconducting at higher temperatures than regions with a higher local concentration, hence the gradual increase of the superconducting volume betwen T~.2and Td. We claim that this is not due to a macroscopic inhomogeneity of the samples. The mixed phase state in the tetragonal Fe-rich sample is considered to be a consequence of the short coherence length in the presence of a spatial distribution of repulsive defects, i.e. an intrinsic mechanism.

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