Heat capacity of the spin-Peierls compound CuGeO3

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Physica B 219&220 (1996) 110-111

Heat capacity of the spin-Peierls compound CuGeO3 S. Sahling a'b, G. Rem6nyi a, J.C. Lasjaunias a'*, N. Hegman a'c, G. Dhalenne d, A. Revcolevschi d aCRTBT-CNRS, BP 166, 38042 Grenoble-Cedex 9, France blnstitut fiir Tiefiemperaturphysik, TU Dresden, D-01062, Germany Clnstitute of Nuclear Research, 4001 Debreeen, P.O. Box 51, Hungary aLab. de Chimie des Solides, Univ. Paris-Sud, 91405-Orsay, France

Abstract We report on heat capacity measurements of the spin-Peierls compound CuGeO3 under magnetic fields between 0 and 22T. We have determined the magnetic phase diagram between the three different phases (U, M, D) of this system, unknown up to now for H > 14T. We have studied the second-order phase transition for H = const, at the crossing of D - U and M - U phases, and we have defined the different contributions (lattice, magnetic excitations) to the specific heat and studied their variation under H.

1. Experimental We report on a series of measurements of the heat capacity of the spin-Peierls inorganic compound CuGeO3 [1], under magnetic fields up to H - - 2 2 T , performed in CRTBT and LCMI Grenoble. Experiments were done on a single crystal (m = 0.45 g), with the field oriented along either the c or the b axis. These high-field experiments are delicate due to the noise induced by the resistive Bitter-type magnet and the magneto-resistance of thermometers. We have used film deposited Au-Ge thermometers, characterized by a weak magneto-resistance (less than 20% for correction of T for T > 6 K in field H = 21 T), a technique developed in CRTBT [2]. Heat capacity data are obtained with a transient heat pulse technique, with relative increments of temperature of the order of 5% or less.

2. Results and discussion Two typical specific heat data C p ( T ) results are reported in Fig. 1, either with zero field or under a field of * Corresponding author.

21T applied along the b axis. (Note that the magnetic chains direction is parallel to the c axis.) For H = 0, the transition from the D (dimerized) phase towards the U (uniform) phase appears very clearly as a typical )~shape, asymmetric, anomaly at Tc = 14.2 K, with a specific heat jump of AC = 3.0-3.3 J/mol K (1 tool = 184.1 g). Above To, we have analyzed Cp as the sum of a magnonlike contribution and phonoL contributions. For this uniform antiferromagnetic (S = ½) quasi-lD magnetic structure, we suppose [4, 5] that the magnetic specific heat varies as 7 T, up to T < 20 K. So we have analyzed Cp = y T + f i T 3

for the uniform U phase, with 7 = 0.80 mJ/gK z (or 147 mJ/molK z) and fl = 0.00139 mJ/gK 4 (or 0.256 m J/molK 4) corresponding to a Debye temperature of 6)0 = 336 K, correspo,ading to r = 5 atoms per formula unit. Under a field of 21 T, H applied along the b axis, the transition from modulated (M) to uniform (U) phase remains sharp, typically second-order with Tc = 9.5 K. The jump AC is considerably reduced to 0.8 J/mol K in comparison to zero field. For H parallel to c, transition occurs at Tc = 9.9 K, with AC = 1.0 J/mol K.

0921-4526/96/$15.00 © 1996 Elsevier Science B.V. All rights reserved SSDI 0 9 2 1 - 4 5 2 6 ( 9 5 ) 0 0 6 6 6 - 4

S. Sahling et al. / Physica B 219&220 (1996) 110-111

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Fig. 3. Specific heat jump at the transition between D-U (H < 13T) and M-U (13 < H < 22T) phases. Two kinds of points represent error bars.

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T (K) Fig. 2. Magnetic phase diagram obtained from thermodynamic measurements, between uniform (U), dimerized (D), and modulated (M) phases, for H parallel to the c axis. We have determined in this way the whole phase boundary by sweeping the temperature between T = 5 and 20 K at fixed field up to H = 22 T applied parallel to c: the transition temperature is reported in Fig. 2, and the specific heat j u m p at T~ in Fig. 3. We find a correlation between the two diagrams: T~ decreases rather rapidly down to 10.5 K when H increases up to 12.5-13 T, which is the upper limit for the existence of D phase. F o r fields above 15T, the temperature of transition between the M and U phases stabilizes around 10 K. The j u m p AC considerably decreases for H increasing up to 13T, to a value which is about three times smaller than for H = 0, for the transition D - U at H > 13 T. The amplitude of j u m p AC can be correlated to the discontinuity of the

longitudinal elastic constants under field H parallel to b or ¢ using Ehrenfest relations valid for second-order transitions. A similar decrease by a factor of three between H = 0 and 20T is also observed in acoustic measurements [6]. We have determined for each field the coefficients 7 and/~ of the magnon and phonons contributions, respectively. We have observed a decrease of 7 for H increasing up to 12T and an increase of/~ (corresponding to a softening of the lattice). We have also studied the variation of the magnetic contribution Cm(T) below To: in both the M and D phases, the variation can be analyzed as an exponentially decreasing function, for T well below Tc(T < 8 K), corresponding to opening of a gap in the magnetic excitations spectrum, even in the M phase [7]. C 2 2

References [1] M. Hase, J. Terasaki and K. Uchinokura, Phys. Rev. Lett. 70 (1993) 3651; M. Hase, J. Terasaki, K. Uchinokura et al., Phys. Rev. B 48 (1993) 9616. [2] S. Sahling, J.C. Lasjaunias, P. Monceau and A. Revcolevschi, Solid State Commun. 92 (1994) 423. 1-3] O. B6thoux, R. Brusetti, J.C. Lasjaunias and S. Sahling, Cryogenics, to appear. [4] J.C. Bonner and M.E. Fisher, Phys. Rev. A 135 (1964) 640. 1-5] M. Takahashi, Progr. Theor. Phys. 50 (1973) 1519. [6] M. Saint-Paul, G. Rem6nyi, N. Hegman, P. Monceau, G. Dhalenne and A. Revcolevschi, submitted. [7] G. Remenyi, S. Sahling, J.C. Lasjaunias, N. Hegman, G. Dhalenne and A. Revcolevschi, to be published.