UPPER CRITICAL FIELD IN Y0.8Ln0.2Ba2Cu3O7-z, Ln = Dy, Er and Tm SUPERCONDUCTORS

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UPPER CRITICAL FIELD IN Y0.8Ln0.2Ba2Cu3O7-z,

Ln = Dy, Er and Tm SUPERCONDUCTORS

U. De, S. Kalavathi, T. Radhakrishnan, G. Subba Rao

To cite this version:

JOURNAL DE PHYSIQUE

Colloque C8, Suppldment au no 12, Tome 49, ddcembre 1988

UPPER CRITICAL FIELD IN Y0.8Ln0.2Ba2Cu307-z, Ln

=

Dy,

Er

and

Tm

SUPERCONDUCTORS

U. De, S . Kalavathi, T. S. Kadhakrishnan and G . V. Subba Rao

Materials Science Division, Indira Gandhi Centre for Atomic Research, Kalpakkam 603102, India

Abstract. - Resistively measured upper critical field, Hc2 (T)

,

for fields up to 120 kOe are reported for the new superconductors Y o . ~ L n o . ~ B a ~ C u 3 0 7 ~ , , Ln = Dy, Er and Tm. A concave curvature is observed for all the Hc2 (T) vs.

T

graphs at lower fields. Origin of the curvature from the anisotropy of Hcg and alternatively from critical fluctuations are discussed.

Discovery of superconductivity in Y1Ba2Cu307-, or YBCO type compounds, was soon followed by the finding that the upper critical field, Hc2 (T)

,

a t a tem- perature,

T,

in these compounds is about an order of magnitude higher [l]. The present work reports 4-

probe resistive measurements of H,2 in single phase Yo.~Lno.~Ba2Cu~O,-, for Ln = Dy, Er and Tm. In addition, the common feature of the broadening of the superconducting transition in presence of a mag- netic field, observed [2, 31 also for YBCO, is investi- gated.

The samples were prepared [4] as oxygenated sin- tered pellets XRD showed the 123-phase [I, 41 and clear orthorhombic splitting. Impurity phases were much less than 5 %

.

Electrical leads were either sil- ver pasted into finely drilled holes or soldered t o Cu or Cu-over-Au film-strips, vacuum evaporated onto the samples. The sample current (typically 1 mA) was perpendicular to the applied magnetic field, H. The sample was inside a tubular He-exchange-gas chamber, designed to be inserted into a superconducting magnet and allow sample temperatures upto 300 K. An agree- ment of the resistivity data taken during heating and cooling was ensured in all cases, as shown in figure 1

of [5]. A carbon glass thermometer, used for H

>

0, was calibrated in-situ at H = 0 against Ge and P t thermometers.

Resistance,

R,

us. temperature, T, graphs with Has a variable parameter (Fig. I), show that the effect of H

on R is negligible between the on-set ( R = R,) point and 100 K. Hence as a preliminary experiment, it was

measured a t 300 K for one of the samples, Ln = Tm. It gave dR/dH =-0.08 mQ/T for 8

T

<

H

<

12 T . This confirms the negative magnetoresistance reported earlier [6] for YBCO for the same transverse geom- etry, although the magnitude measured by them up t o 200 K was smaller. Sensitiveness of these transport properties t o factors like 0-stoichiometry and granular structure and hence to the sample preparation details might explain the difference in magnitude. In fact it is

Fig. 1. - Superconducting transitions and the estimate (in- set) of the upper critical field, Hc2 (T)

,

from these graphs defining T a t 0.50 R,.

not clear whether this unusual negative sign is due to any such effect or due t o any new and unusual mecha- nism [8]. The field is seen to affect the transition least in the R = R, region and most in the

R

= 0 region. This shows up in the Hc2 (T) vs. T graphs (Fig. 2)

also. The 0.75 Rn graph has the largest - (dHc2

/

dT)

,

while this value is least for the 0.25

Rn

graph.

TEMPERATURE (K)

Fig. 2. - Hc2 (T) vs. T graph defining T at 0.75 Rn (erect triangles), 0.50 R, (squares) and 0.25 R, (inverted triangles) points on the R - T graphs (not shown).

'prof. Subba Rao is with the Materials Science Research Centre, Indian Institute of Technology, Madras 600 036, India.

C8 - 2168 _{JOURNAL DE PHYSIQUE}

In addition, each of the HC2 — T graphs shows a

concave curvature [7] near the (Tc,0) point in contrast

t o a linear behaviour expected from WHH and other theories of HC2- Hence for estimating H*2 (0) , the

or-bital critical field at 0 K, the linear portion value of (difcs / dT) and the corresponding extrapolated Tc of

table I are used. These high values of the slope and

H*i (0), calculated in table I, are comparable to those

Table I. - Estimate from the O.bORn graph of HC2 vs.

T, of (dJ?c2/dT) over the linear portion (Hi,H2). An extrapolation of this portion back to -ffc2=o gave

T = Te, and hence H*2 (0) = -0.693Te (difc2 / dT).

Ln Dy Er T m ( f l i . f l i ) i n T (0, 8) (10, 12) (8, 11.5) (8, 11) (dHc2/dT) i n T / K - 1 . 7 - 4 . 0 - 1 . 8 - 3 . 7 Te i n K 85.2 85.8 H*c2 (0) i n T 106 220

reported [2, 3] for YBCO. Similar Tc and (dHC2 / dT)

have been reported [8] for YBCO and full substitution of Y by Dy, Er and Tm, but without details on whether the slope varies at lower H. Still it can be concluded t h a t partial or full substitution of Y by magnetic ions did not affect HC2 to any substantial degree. This is

now understood to be due to a very short coherence length.

The near parallelism and A T proportional t o AH between the transition graphs are observable for H > 8 T. It results in the linear HC2 vs. T dependence

ob-served for such higher fields (Fig. 2). Anisotropy of HC2

[1] and the fact that our sample consists of differently oriented grains can explain the characteristic broad-ening of the transitions at lower fields, and hence the curvature of HC2 vs. T graph. The broadening is due

to more and more unfavourably oriented grains becom-ing normal as if is increased. Beyond 8 T no further broadening occurs, implying t h a t only the favourably oriented grains contribute t o the superconductivity in this region.

An alternative mechanism involving critical fluctua-tions [9] has been used by Oh et al. [1] to explain the above-mentioned broadening of the resistive transition in YBCO single crystals and highly oriented films. In our polycrystalline samples the measured effect is due to grains at different orientations to H. Still on fitting our limited Hc2 (T) vs. t = (Tc - T) / Tc data

t o the same type of expression

the value v = 0 . 7 ± 0 . 1 and L (0) = ( 1 . 7 ± 0.7) A were obtained. Since this magnitude of L (0) is not unlikely

for an average of coherence, the implication of the other result lengths [1] is now considered. The the-oretical mean field value of v is 1/2, and v > 1/2 is taken as an indication of critical fluctuations. Math-ematically 1v > 1 in the above equation is a man-ifestation of the observed concave curvature of the #c2 — T graphs. Such an explanation, not involving differently oriented grains, is essential in case of sin-gle crystals showing such curvature [1]. But it cannot explain the observations [7] of such curvatures for Nb-Ti and NbaSn low-Tc superconductors, in which the

critical regions should be much narrower [9]. More experiments on single crystals, checked t o be free of sample inhomogeneities, and further theoretical con-siderations are needed to identify the real mechanism or mechanisms giving rise to the now widely observed concave curvaure of Hc2 — T graphs near Tc.

Note added during proof correction.

The above type of fit has recently been interpreted from on interesting Flux Creep Model by M. Tinkham

in Phys. Rev. Lett. 6 1 (1988) 1658.

[1] Geballe, T H. and Hulm, J. K., Science 239 (1988) 367.

[2] Laborde, O., Tholence, J. L., Lejay, P., Sulpice, A., Tournier, R., Capponi, J. J., Michel, C. and Provost, J., Solid State Commun. 6 3 (1987) 877. [3] van Bentum, P. J. M., van Kempen, H., van

de Leemput, L. E. C , Perenboom, J. A. A. J., Schreurs, L. W. M. and Teunissen, P. A. A., Phys.

Rev. B 36 (1987) 5279.

[4] Subba Rao, G. V., Varadaraju, U. V., Vi-jayashree, R., Padmanavan, K., Balakrishnan, R., Mary, T. A., Raju, N. P., Srinivasan, R., De, U., Janaki, J. and Radhakrishnan, T. S., Physica B

148 (1987) 237.

[5] De, U., Janaki, J., Rao, G. V. N., Raghunathan, V. S. and Radhakrishnan, T. S., Mater. 6 (1988) 331.

[6] Oussena, M., Senoussi, S., Collin, G., Broto, J. M., Rakoto, H., Askenazy, S. and Ousset, J. C ,

Phys. rev. B 36 (1987) 4014.

[7] De, U. and Radhakrishnan, T. S., Jpn J. Appl.

Phys. 26-3 (1987) 925 and references therein.

[8] Orlando, T. P., Delin, K. A., Foner, S., McNiff, Jr., E. J., Tarascon, J. M., Greene, L. H., McK-innon, W. R. and Hull, G. W., Phys. Rev. B 36 (1987) 2394.

Also Noel, H., Gougeon, P., Padiou, J., Levet, J. c , Potel, M., Laborde, O. and Monceau, P.,

Solid State Commun. 6 3 (1987) 915.