_k ‘. __ __ 43 PHYSICA ti ELSEVIER Physica C 282-287 (1997) 2059-2060

Observation of the vortex melting line in YBa2Cu307 by specific heat measurements in very high magnetic field : H 5 23 Tesla.

C. Marcenat a, R. Calemczuk a, A. Erb b, E. Walker b and A. Junod b

a Departement e d Recherche Fondamentale sur la Matiere Condensee / SPSMS / LCP C.E.A./ Grenoble, 17 rue des Martyrs, 38054 Grenoble cedex 9, FRANCE

b Departement de physique de la matiere condensee, Universite de Geneve, 24 quai E. Ansermet, CH-1211 Geneve 4, Switzerland

We have performed specific heat (C) measurements using a very sensitive adiabatic calorimeter on a twinned, high-purity single crystal (18mg) of YBa2Cu307 (referred thereafter as YBCO). In the same crystal, for 5T 2 B I 16T, very well defined peaks in C have been reported at the vortex lattice melting transition occuring at a characteristic temperature Tm[ 11. We tracked this line B(Tm) up to 21.5 Tesla and, finally, no anomaly was detectable in 23 Tesla, suggesting the existence of an ending critical point for a characteristic magnetic field B*, 21ST I B* 5 23T. In these very strong magnetic fields, the shape of the anomaly in C associated with the freezing of the vortex-liquid state becomes strongly dependent on the experimental conditions.

1. INTRODUCTION

The observation of sharp anomalies in specific heat [1,2], in resistance [3,4], and in magnetization [5,6] at the freezing of the vortex state has attracted a lot of interest and it is generally believed that these features are associated with a 1st order melting transition of the flux lattice. However, no consensus exists, yet, on the exact nature of this transition. The widths of the resistance anomalies are much smaller than those for specific heat or magnetization jumps. This is probably a consequence of a percolative phenomena, the resistance not being a bulk thermodynamic quantity. Moreover, the magnitude of the magnetization jumps in Bi-2212 was showed to be correlated with the irreversibility displayed by the vortex solid and they may have an artefactual origin [6]. And, in the particular crystal of YBCO used for this study, thermodynamically incompatible specific heat steps and magnetization jumps are observed below 6T (B//c) [ 11.

Central to this paper is the demonstration that the magnetic field dependent specific heat across the melting line in YBCO in a strong magnetic field is very sensitive to the experimental conditions (magnetic noise). This implies that specific heat experiments are more affected by irreversibility problems than generally believed.

2. EXPERIMENTAL

The characteristics and the details of preparation of the single crystal used in this work are described

O92t-4534/97/$17.00 8 Elsevier Science B.V. All rights resewed. PI1 SO921-4534(97)01127-l

in Ref [l]. Briefly, the crystal was grown in a non- contaminating BaZr03 crucible and was annealed in high pressure oxygen (100 bar) at low temperature (300°C) during 140 hours. Such an oxygenation treatment results in a dramatic suppression of the concentration of pinning centers and then the so- called magnetic fishtail effect is absent (71,.

Specific heat measurements were carried out in a continuous heating adiabatic calorimeter. As an extension of the experiments performed in Geneva [I] between OT and 16T, magnetic fields between 16T and 23T were applied parallel to the c-axis in a resistive helicoidal coil at the L.C.M.I. (High Magnetic Field Laboratory) in Grenoble. To verify the surprising results that we obtained under these conditions (see Figures 1 and 2), subsequent measurements were carried out in a superconducting coil between 16T and 18T, which is the limit of the magnet in our laboratory.

2. RESULTS

The behavior of the specific heat across the melting line is illustrated in Fig. 1 where we plotted the difference AC=[C(B)-C(O)] as a function of the temperature. The very well defined peaks observed in lower fields are suddenly replaced by very large (~1% of Ctotal) and steep steps (width=300mK). The steps are located exactly on the linear extrapolation of the line Bm(T) determined below 16T. Finally, no anomaly was detectable in 23 Tesla, suggesting the

2060 C. Marcenat et al./Physica C 282-287 (1997) 2059-2060

6

0 60 65 70 75

VW

Figure l.Field dependence of the specific heat of YBCO after subtraction of the zero field data AC=C(B)-C(O), in the vicinity of the irreversibility line. All these data were obtained in a resistive coil in LCMI-Grenoble and the curves have been arbitrarily shifted verticaly for clarity. The heating rate was v = 12 mK/s.

existence of an ending critical point for a characteristic magnetic field B*, 21.5T I B* 5 23T. A multicritical point in the magnetic phase diagram for the mixed state has been already observed in YBCO by resistive measurements [8] but at a much lower field B*=lOT. In principle the position of this critical point may shift to lower fields with increasing disorder. In Bi-2212, Zeldov et al. [5] also observed by local magnetic measurements the existence of a well-defined critical point beyond which no discontinuous transition is detectable. The most surprising result is that these steps in C were very dependent on the heating rate (see Figure 2). Subsequent measurements showed that this dependence is not intrinsic since it was absent when the magnetic field is produced by a superconducting magnet. One possibility is that we obtained a disordered vortex solid state (vortex-glass) by quenching the crystal in the presence of magnetic noise (a few Gauss at 50Hz for B=20T) unavoidable in a resistive coil. In this case, we would observe heat releases from this metastable state. A more plausible explanation is that these heat releases

B=18 T ;

55 60 65 70 75 T(K)

Figure 2.The behavior of the anomaly in the specific heat detected at the freezing of the vortex-liquid state depends strongly on the magnetic noise. In the superconducting magnet, AC is independent of the heating rate (full circles) in contrast to the results obtained in the resistive coil. (+) v = 5 mWs and (o)v= 12mK/s

are induced by dissipation into the sample

(P=l.t.oX”Hac*W*). An estimation with X”- 0.1 and with a magnitude of Hat as above mentioned gives the right order of magnitude needed to explain the results. But, this would probably imply also that the melting line and the irreversibility line are one.

Further systematic studies are needed to clarify this complex situation.

REFERENCES

1. A. Junod et al, submitted to Physica C; and M. Roulin et al. in this conference.

2. A. &hilling et al., Nature, 382 (1996) 791 3. H. Safar et al., Phys. Rev. Lett. 69 (1992) 824 4. J.A. Fendrich et al., Phys. Rev. Lett. 74 (1995) 1210 5. E. Zeldov et al., Nature, 375 (1995) 373 6. D.E. Farrel et al., Phys. Rev. B53 (1996) 11807 7 A. Erb et al., J. Low Temp. Phys.105 (1996) 1023; also this conference and reference therein. 8. H. Safar et al., Phys. Rev. Lett. 70 (1993) 3800