Magnetic study of an overdoped “1223” system: TlBa0.8Sr1.2Ca2Cu3O9−δ single crystals

PIIYSICA ELSEVIER

Physica C 244 (1995) 341-348

Magnetic study of an overdoped "1223" system: T1Bao.8Srl.zCa2Cu309_ 8 single crystals A. Wahl *, A. Maignan, V. Hardy, J. Provost, Ch. Simon, B. Raveau Laboratoire CRISMA T-CNRS URA 1318-1SMRa-Universit~ de Caen, Boulevard du Mar~chal Juin, 14050 Caen Cedex, France Received 30 November 1994; revised manuscript received 1 February 1995

Abstract The influence of the doping state upon the magnetic properties of overdoped superconducting T1Bao.sSrj.2CazCu309_8 single crystals has been investigated. The temperature dependence of the in-plane London penetration depth, of the irreversibility line and of the so-called fishtail effect for different annealing treatments in Ar-H2 and in oxygen pressure is fundamentally different from that observed for other overdoped cuprates and especially from thallium bilayer cuprates. This remarkable difference of behavior between thallium monolayer and bilayer overdoped cuprates is interpreted in terms of crystallographic structure and hole-reservoir effects.

1. Introduction Numerous investigations have been performed on the layered thallium cuprateg on account on their very high critical temperatures covering a wide range up to 130 K. Their high sensitivity to the nature of the annealing atmosphere [ 1-3 ] has shown that the hole carrier density is an important factor for the optimization of the critical temperatures of these materials. Although the influence of the oxygen non-stoichiometry on Tc is well established, its effects upon the reversible and irreversible superconducting properties are so far not well understood. A systematic study of polycrystalline samples of the cuprate TIBa2_xSrxCazCu309_ ~ (TI1223 Ba/Sr) [4] has shown the importance of various annealing treatments for the optimization of the critical temperature of this phase. In order to gain better understanding of the superconductivity in this oxide, single crystals of the 1223 phase have been grown whose Tc * Corresponding author. 0921-4534/95/$09.50 © 1995 Elsevier Science B.V. All rights reserved SSDI0921-4534(95)00072-0

variation upon annealing treatments (from 95 K to 115 K) is similar to that of the ceramics. We report herein on the investigation of the doping dependence of the reversible part of magnetization, the irreversibility lines and the "fishtail" effects of overdoped single crystals of composition TlBao.8Sr~.2Ca2Cu309_ ,~.

2. Experimental Single crystals with the 1223 structure were grown from a mixture of powders with a nominal composition T1BaSrCa2Cu309. The stoichiometric mixture was prepared from the oxides SrCuO2, T1203, CuO, BaO2 and CaO. The powder was ground in an agate mortar, placed in an alumina crucible and sealed in a silica ampoule. The sealed tube was then heated in a vertical furnace whose temperature gradient was chosen according to thermal treatments described elsewhere [ 5 ]. Three crystals were selected for X-ray diffraction and magnetic measurements. Their Weissenberg pat-

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A. Wahl et al. /Physica C 244 (1995) 341-348

terns correspond to the reflections of a unique crystal with cell parameters a = 3 . 8 5 A and c = 15.55 A. For single crystals synthesized under the same conditions, i.e. exhibiting a very similar oxygen content, one knows that the barium cuprate T1Ba2Ca2Cu309_ a exhibits a c parameter of 15.88 A [6] whereas the partially substituted cuprate T1Ba2/3Sr4/3Ca2Cu309_ a has a c parameter of 15.52 ,~ [5]. If one assumes a linear decrease of c for the T1Ba2_xSrxCa2Cu309_~ series when x increases [4], the value c = 15.55 ,& of our selected crystals suggests that they are Sr rich and present the same Ba/Sr = 0.66 ratio. Thus, their formula can be written as TIBao.sSrl.2Ca2Cu309 _ a. The recent study of the substitution of Sr for Ba in thallium "1223" monolayer ceramics [4] has shown that the critical temperature of the optimized superconductor does not strongly depend on the Ba/Sr ratio but rather on the annealing treatment, and it was found that the maximum Tc was = 120 K for the optimized samples whatever the Ba/Sr ratio was, indicating that the oxygen non-stoichiometry is the most important factor for optimization of the To's. In order to stress the influence of the variation of T~ on the superconducting magnetic properties, two asgrown single crystals (AG) have been submitted to opposite thermal annealing treatments. The first one (labelled an-O) was annealed under an oxygen pressure of 90 bar for 6 h at 300°C in order to obtain a full oxygen content and the second one (labelled an-AH) in a H2-Ar flow at 360°C for 30 min to decrease the oxygen content. The xi(T) curves, registered with an AC field of 10 G, for the as-grown sample (AG) and the annealed ones (an-O and an-AH) are reported in Fig. 1; the x~(T) curve of the pure Ba 1223 phase [6] is also presented. The internal susceptibility Xi of the crystals was calculated from the relation Xi=Xext/ ( 1 - ~TXext) where Xext and r/correspond to the measured external susceptibility and the demagnetizing factor, respectively. From the dimensions of the crystals (Table 1 ), r/ values were obtained considering the inscribed ellipsoid. A corrected value ofxi was deduced probing the nearly perfect shielding of the crystals. We have considered that the extrapolation to zero of the tangent at the midpoint of the transition gives reliable estimates of T~; one obtains for the single crystals: T~ = 95 K for the an-O crystal; Tc = 98 K for the asgrown one (AG) and T~ = 115 K for the an-AH crystal. Note that the an-AH crystal exhibits a c parameter of

89

90

100

119

/ ,'

I. ,

"/ /

/' I

0 i

120

//'\\

\ an-~ AG

/C

°4 i

/

,

I

r -06

~ i

i

I 5

I 80

,__

90

J

L 100

110

120

T(K) Fig. 1. Internal susceptibility xi(T) measurements for T1-1223 Ba from Ref. [61 and T1-1223 Ba/Sr single crystals (an-O, an-AH, AG) under 10 G.

15.58 .&, similar to that of the as-grown crystal ( 15.55 ). Annealing the specimens under an oxygen pressure decreases Tc as shown for the an-O crystal, whereas the opposite is observed for an Ar-H2 annealing (see the an-AH crystal). This variation of Tc suggests that the samples lie on the overdoped side of the parabolic curve of T~ versus hole concentration found for several HTSC's. In connection with this, the small difference in T~ between the AG and the an-O sample shows that both crystals exhibit a similar hole concentration. Of course, further annealing treatments have been tried in both cases (the oxygen and the reducing atmosphere) but were found unsuccessful in inducing further decrease or increase in T~ without a degradation of the superconducting volume. Finally, the observed difference of T~'s obtained through the different annealing Table 1 Sizes and demagnetizing factors ("O) along the c-axis of the studied crystals Reference

Size ( ixm3)

r/

AG As-grown an-O Oxygenated an-AH Reduced

760 × 610 X 280

0.57

520 X 455 X 280

0.48

690 x440 x 280

0.51

A. Wahlet al. / Physica C 244 (1995)341-348 treatments (ATe = 20 K) is consistent with the previous studies on the polycrystalline phase where a maximum of Tc of 120 K and a minimum of 93 K was found for T1Ba2_xSrxCazCu309_n, as mentioned above. For each sample, magnetization measurements were performed using a Quantum Design SQUID magnetometer (5.5 T). The sample holder used in the SQUID was rigid (20 cm long) and guided by two O rings (at better than 1 mm) ensuring a maximum deviation of 0.3 ° between the magnetic field and the sample holder axis. The crystal was directly glued on the holder in the geometry B IIc-axis and the quality of the alignment (external faces perpendicular to the holder axis) was controlled to be less than 1°. Thus, the total misalignment of the c-axis permits one to assert the validity of the magnetization data for the geometry B IIc-axis.

t

,,

, .........

. . . . . . . . .

500

o

: AG

•

:

*

" GR--O

gn

.o

A~

400

%

C3

500 0

200 ~

E0

[a,

100

o*

wb

0

*"

E

i o,6

o.7

o.a

o.9

~o

Fig. 2. Temperaturedependenceof the in-planeLondonpenetration depth A~,(t= T/T~) for T1-1223BaSr (an-O, an-AHand AG). The field noted Hw is the field of the second peak on the decreasing branchof the loop.

3. R e s u l t s

3.1. In-plane London penetration depth (l~ab), upper critical field (He2) and in-plane coherence length ( ~b) The reversible part of the hysteresis loops, i.e. above the irreversibility line, provides a region in the (H, T) plane where basic superconducting parameters can be extracted. In the intermediate field region He] << H << H~z, the London model predicts a linear dependence of M upon In(H) although minor corrections may be added as discussed by Hao and Clem [7]. Recently, the latter analysis" has been used to extract basic superconducting parameters of two thalliumbased compounds, Tlo.sPbo.sSr2Ca2Cu309 and TIzBa2Ca2Cu30]o [ 8]; the results demonstrate that in the range of magnetic field 0-5.5 T, the linear regime of M(H) (Abrikosov formula) was never reached. Besides, the thermodynamic fluctuations in HTSC can significantly distort the mean-field behaviour. But, it is also expected that the further the temperature decreases away from T¢, the smaller are the effects of those fluctuations and they can be neglected. Thus, far away from Tc and for He1 << H << Hc2 we have used the London model expression (mean field behavior) given by -4~M=

600

343

8,trASh(T) In -

-

,

(1)

where fl= 1.16; Aab and H~2 are, respectively, the inplane London penetration depth and the upper critical

field. Moreover using the Werthamer-HelfandHohenberg (WHH) [9] and Ginzburg-Landau theories we have

(dnc2~

¢,o

He2(0) = - 0 . 6 9 Tc~---~)Tc -- 2,rrSCa2b(0) .

(2)

The reversible part of the magnetization has been recorded for the three samples an-O, an-AH and AG at different temperatures and superconducting parameters have been extracted fitting the data with Eq. ( 1 ). The temperature dependence of the in-plane London penetration depth, Aab(T), is presented in Fig. 2. For each sample, these experimental Aab(T) data fit very well with the two-fluid model according to the equation Aab(T) - [1

~°~(0) ( T/411/2 " -!,~) ]

(3)

Whatever the values of Tc for each sample, the )tab(0) values determined from this fitting procedure are the same within the experimental uncertainty. This remarkable feature will be discussed in the following. Finally, Table 2 lists the values of Aab(0), He2(0) and ~:~b(0) derived from this analysis of the three T1Bao.sSr].2Ca2Cu309_ ~ single crystals showing different T¢'s. The value of A~b(0) agrees with values already reported for thallium cuprates (see for instance

344

A. Wahl et al. / Physica C 244 (1995) 341-348

Table 2 Basic superconductingparameters

800

Reference

A~t,(0) (nm)

H~2(0) (T)

~.l,(O) (nm)

T¢ (K)

AG an-O an-AH

164+6 166_+6 166-+6

90+5 90_+5 105-+5

1.91 _+0.05 1.91 +0.05 1.77_+0.05

98 95 115

70K

600

--- - :

: AG : on AH an-O

400 /

\

200

(_9 0

Refs. [ 8 ] and [ 10 ] ). Uemura et al. [ 11 ] have reported (IxSR) measurements of A,b(0) in HTSC's where a universal linear relation between T~ and A~b(0) -2 has been found in the underdoped region. In overdoped samples, however, T~ shows a saturation for a value of A , b ( 0 ) - z which depends on the multiplicities of the CuO2 planes. The T~ dependence of A,b(0) observed in this study for the T1-1223 B a / S r phase does not follow the predictions of the IxSR model of Uemura for overdoped systems. One can compare our results to the situation observed for overdoped TI2Ba2CuO 6 a and (La~ _~S&)2CUO4 crystals [ 12] which show a decrease of T~ in the overdoped domain without any large reduction of A~b(0) 2. A similar conclusion was obtained in (Lal _~S&) 2CUO4 single crystals [ 13 ] where A,b(0) varies in the underdoped region much more rapidly than in the overdoped one. The values of ~:,,b(0) significantly differ from an-AH to AG or to an-O single crystals. This fact may be a consequence of the T~'s difference as previously suggested [ 14]. Assuming that the BCS theory is reliable in our case, one can consider that the superconducting gap A is proportional to T~ and also inversely proportional to sc. This is in agreement with the fact that the highest T~ (an-AH sample) corresponds to the lowest sc,,b(0) value. 3.2. Irreversibility lines and fishtail effects

200 -400

/ /

\

/ \

I/'

60O

-800

/ / \ .

~

/

L

L

0

10000

Fig. 3. Half hysteresis loops for the three single crystals (an-O, anAH and AG) registered at 70 K. to depend strongly on the nature of the compound (structure and cationic composition). The Hsp lines of the three T1-1223 Ba/Sr single crystals are reported in Fig. 4 in a reduced temperature scale• The line of the T1-1223 pure barium has been added for comparison. It can be noticed that the Hsp lines of the an-O and AG single crystals are superimposed while the an-AH one is located below them.

t -"

i\"\

2,}L tO

"

Z2

'

,

,

Irreversible properties have been investigated by recording half hysteresis loops M ( H ) at different temperatures for the three crystals. Typical M ( H ) curves registered at 70 K are reported in Fig. 3 exhibiting the so-called fishtail shape• This feature corresponds to the double-peak structure of the hysteresis loop, i.e. the rise of a second peak on both branches at a field, noted Hsp, above the complete penetration. The location of this characteristic field, which was evidenced for single crystals of different thallium phases [ 15 ], was shown

20OO0

n (c)

\

I

-\

~

i

71

722J

'-%'*

Se

-:~

-:£~

86

0.7

] q

00

0 i

02

03

8 .t

05

08

09

t Fig. 4. H~plines vs t = T/Tc of the T1-1223Ba/Sr phase submitted to different thermal treatments (an-O, an-AHand AG). The line of TI1223 Ba has been added for comparison.

345

A. Wahl et al. / Physica C 244 (1995) 341-348

Several hypotheses have been proposed to interpret the differences in the Hsp location observed for different phases of thallium-based single crystals. More recently, the fishtail effect was assigned to a modification in the nature of the vortex state [ 15-17 ] and the characteristic field, Hsp, is assumed to be connected to intrinsic superconducting parameters of each compound. In such a case, one possible relevant model is a fluctuationinduced Josephson decoupling transition [ 15 ] similar to the one initially proposed by Deutscher and Kapitulnik [ 18]. According to this model, this crossover field can be written [ t x o n = A f ( t) y - 2]

with T t = ~c' where y is the electronic anisotropy,

d,@)+xq A

~

4"rr/XOA3b(0)kT~

and (1 f(t)

-

__/4)3/2 t

for this Josephson decoupling model. Here, dc is the repeat distance of the structure. If one identifies the/-/so values to the crossover fields of the above formula, y values can be derived from the Hsp lines of our three T! (Ba/Sr) single crystals and are presented in Table 3. The used values of Aab(0) and Tc are those collected in Table 2. The dc parameter is deduced from the XRD study to be 15.55 ~,. Generally, one might expect the electronic anisotropy (y) to increase with oxygen deficiency as previ-

ously reported in underdoped YBCO [19]. But surprisingly, the 3' values derived for an-O, an-AH and AG crystals of the T1-1223 Ba/Sr phase are identical as determined by utilizing this decoupling model (Table 3). Thus, the difference of location in the Hsp lines between the an-O, an-AH and AG samples exclusively follows from the variation of Tc and not from the different values of basic parameters such as A,b(0) or Y. In Table 3 is also reported for comparison the 3/value of T1-1223 Ba determined through the same analysis [ 15]. One observes that the anisotropy of the T1-1223 Ba/Sr cuprate is lower than that of the T1-1223 Ba cuprate. One of the most important characteristics of the HTSC's is the low-lying irreversibility line (IL) which separates the field-temperature diagram between reversible and irreversible magnetic behavior. It should be noted that the low-To superconductors also exhibit irreversibility lines. From the study of several single crystals of different thallium phases [ 15 ], the location of this boundary in the (H, t = T / T ~ ) plane has been correlated to intrinsic parameters. The IL's for T1-1223 Ba and T1-1223 Ba/Sr (an-O, an-AH and AG) are reported in Fig. 5. This figure demonstrates, in a reduced scale of temperature, identical irreversibility lines for the T!-1223 Ba/Sr single crystals, independently of the thermal treatment. This result is in agreement with those obtained for the polycrystalline

~-\

50000

~-~"

40000

¢ ,

(0 : "J .soooo:-~: ~L0

Table 3 y (electronicanisotropy) values in the frameworkof a Josephson decouplingmodel;fromRef. [ 15]

\

'

i ~~

,\

*

y

AG

29

an-O

28

an-AH T1-1223Ba [ 15]

29 33

-~--

~ .....

' ~

On--O

:

q]

" \", "'X'"

2oooob

t ] ".

10000 k

.

E

.,~..

....

k 05

.....

',,

k

Reference

,

T! 0.6

0.7

!223

J "* ~-~"" Bo 0.8

~*

t q

J

99

+ Irreversibilitylinesof the TI- 1223 Ba/St phaseas-grownand submitted to differentthermal treatments. The line of T1-1223Ba has beenaddedfor comparison.

Fig. 5,

346

A. Wahl et al. /Physica C 244 (1995) 341-348

cu½

TI-1223 Ba/Sr phase [20] and shows that these thallium 1223 cuprates differ significantly from other overdoped cuprates.

BaSr0

4. Discussion

The above type of magnetic study as a function of the oxygen content has not previously been performed to date in monolayer thallium cuprates. Although very few data are available for bilayer thallium compounds, a comparison can be made. Contrary to the T1 monolayer cuprates, the TI bilayer cuprates (T1-2212 and T12223) exhibit a significant variation of their irreversibility lines for a small variation of their critical temperature [ 15 ], i.e. the IL's are steepened by increasing the oxygen content. This fact is also observed in Bi-2212 [ 16] and T1-2201 [21 ] where the variation of Tc's is much more important (ATe > 20 K). Clearly, the TI monolayer cuprates exhibit a remarkable difference from the TI bilayer cuprates. In the quasi-two-dimensional HTSC's, the strength of the coupling along the c-axis direction is known to affect the irreversible behavior [22]. Within such a model, H~rris supposed to directly depend on the c-axis resistivity (pc). This suggests that Pc should not depend on the thermal treatments in the T1-1223 Ba/Sr cuprates, in contrast to the T1 or Bi bilayer cuprates [23,24] for which a dependence of Pc versus oxygen content has been evidenced. Then, the comparison of the experimental results from Refs. [16,21,23,24] shows that there exists a relation between Pc and the irreversibility lines. In order to interpret this different effect of the oxygen non-stoichiometry between T1 monolayer and TI bilayer cuprates, we have concentrated our discussion on two points which are in fact connected: the oxygen environment of the [T10]~ layer and the modification of its electronic structure through a hole-reservoir effect. Firstly, one may explain the difference between TI monolayer and bilayer cuprates by considering the different stacking of atomic layers along the c-axis. The neutron-diffraction studies of TI monolayer cuprates [5,25] have clearly shown that the structure of the [TIO]~ monolayers (Fig. 6(a)) consists of distorted TIO4 tetrahedra, and that oxygen non-stoichiometry appears only on the oxygen atoms of the "TIO" planes

~ , ~

Ba.SrO

/ (a)

Cu02

\ ', i/// //.~,~

" "":-"

TIO

~~(~) (b)

BaSrO

Ba.Sr 0

CuO2

Fig. 6. (a) Tetrahedral oxygen environment in the thallium monolayer cuprates. (b) Octahedral oxygen environment in the thallium bilayer cuprates. The split positions in the TI-O layers are drawn (dotted lines).

(empty circles), the apical oxygen sites of the tetrahedra (hatched circles) being untouched. Consequently, the chain " C u - O - T I - O - C u " along the c-axis and hence the coupling are not affected by the oxygen non-stoichiometry. This is not the case for the T1 bilayer cuprates, whose structural studies [ 26-28 ] show a distorted octrahedral coordination for thallium. In the latter bilayers (Fig. 6(b)), oxygen non-stoichiometry takes place in the basal plane of the T106 octahedra (empty circles) so that the sequence " C u - O - T 1 - O TI-O-Cu" along the c-axis, and consequently the coupling, are affected by the oxygen non-stoichiometry in spite of the fact that the apical oxygens of the CuO5 pyramids (hatched circles) remain untouched.

A. Wahl et al. / Physica C 244 (1995) 341-348

Secondly, the modification of the electronic structure in an a n n e a l i n g process is different in the thallium m o n o l a y e r and bilayer cuprates. This deals with the fact that in the TI bilayer cuprates, the hole-reservoir effect of the [ T1202] ~olayers is very important, according to an internal redox m e c h a n i s m formulated T i m + Cut~= T1m - 8 + Cu H+ 8 as previously reported by several authors [ 2 9 - 3 1 ] . For the thallium m o n o l a y e r cuprates, that exhibit a large C u ( I I I ) content according to the stoichiometric formula, this charge reservoir is not evidenced for experimentally [ 3 2 - 3 4 ] . In the case of the double thallium layers, one can assume that a modification of the o x y g e n content m a y affect this charge transfer b e t w e e n [Ti202]oo and [ C u O ] ~ layers leading to a modification o f the coupling along the stacking direction and then of their electronic anisotropy. (This m e c h a n i s m would be the same for the Bi c o m p o u n d s . ) In contrast, as no charge transfer occurs in the thallium monolayers, a variation of the o x y g e n content would not change the coupling and hence the electronic anisotropy.

5. Conclusion Magnetic m e a s u r e m e n t s on TIBao.8Sr~.2Ca2Cu3O 9 - ~ (T1-1223 B a / S r ) single crystals have been performed for the first time in order to investigate the influence of overdoping on reversible and irreversible magnetic behaviors of this c o m p o u n d . These results clearly demonstrate that the in-plane penetration depth and the irreversibility-line locations in the TI monolayer cuprates are u n c h a n g e d w h e n the o x y g e n stoichiometry is modified. The study of the fishtail effect allows the calculation of the electronic anisotropy to be made s h o w i n g that the anisotropy also does not vary with the o x y g e n stoichiometry. This spectacular difference from the overdoped bilayer cuprates is discussed in terms of a f u n d a m e n t a l structural difference of the thallium oxygen layers and the absence of hole-reservoir effects.

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