Unusual elastic behaviour of normal state UPt3

PHYSICA

Physica B 194-196 (1994) 2079-2080 North-Holland

U n u s u a l elastic behaviour of notomal state UPt3 D. Maurer and N. Lingg lnstitut f~r Experimentalphysik, Freie Universit~it Berlin, Amimallee 14,1000 BERLIN 33, FRG V. Mtiller Institut f ~ Physik, Universit~t Augsburg, Memminger Str. 6,8900 AUGSBURG, FRG We report on extensive ultrasonic investigations of the heavy fermion compound UPt3 in the normalconducting state. In particular, careful measurements of the temperature dependent ultrasonic attenuation and sound velocity were performed near 5 K , where long-range antiferromagnetic order of very small magnetic moments -has been claimed to occur. Anomalous hysteresis effects in the ultrasonic properties were observed, which point to a by far more complex relationship between lattice instabilities and the formation of the heavy fermion state than usually expected. The coexistence of antiferromagnetism and superconductivity found in some heavy fermion systems [1] have caused widespread research activity and gives rise to the question whether or not this is a common feature of these systems. In the particular case of UPt 3 the complex superconducting phase diagram has been studied extensively [2] while the antiferromagnetic transition at about 5 K identified by neutron scattering [3] has received considerably less attention. This is mainly attributed to the fact that physical quantities like specific heat, magnetic susceptibility and others are hardly affected if at all by this transition. H o w e v e r , since ultrasonic methods are very sensitive tools to trace out phase transitions, we investigated the elastic behaviour in the vicinity of 5 K by means of ultrasound. We got early hints from the temperature dependent attenuation, which shows a weak anomaly near 5 K. After subtraction of a background and enlarging as shown in Fig.i, we obtain a steplike behaviour, which due to its smoothness is not very reminiscent ,of a phase transition.

of two different transverse sound modes in the vicinity of 5 K , where the attenuation anomaly is found, is shown. Each of these modes behaves well, i.e. the velocity-change Av with temperature is smooth and monotonous and this is what we normally found for all modes including longitudinal sound waves. No singularities in [~v/v](T) can be resolved down to the order of 10 -5 or less. I00

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Hoping that we would get some more information from sound velocity measurements, we studied the elastic behaviour by means of differently polarized sound waves. In Fig.2 the temperature dependence

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0921-4526/94/$07.00 © 1994 - Elsevier Science B.V. All rights reserved S S D I 0921-4526(93)1672-9

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2080 What also found, however, is a hysteretic behaviour between 4K and about 8K. This is observed only after "rapid" cooling from room temperature at cooling rates of = 1000 K/h which are still not extremely high. In order to clarify what happens during cooling the samples from rc×~m temperature, we measured the temperature dependent sound velocity of each mode for quite different cooling rates and we obtained some rather unexpected results. As an example Fig.3 shows a selection of different cooling runs, which we obtained from transverse sound related to the elastic stiffness coefficient Caa and for cooling rates between about 3 and 300 K/h. T h o u g h all sound velocity curves differ markedly above T = 15 K, they nevertheless fit together below this temperature. In all cases shown the sound velocity behaves normal down to about 20 K. Under slow cooling conditions, however, below this temperature the system becomes unstable and reacts rather sensitive to external perturbations - probably' due to temperature fluctuations. This is reflected by drastic changes of the sound velocity over quite narrow temperature intervals. Moreover, during slow cooling one could often see sudden erratic changes. According to Fig.3 it is obvious that all cooling curves tend to meet at the latest near 15 K. Below this temperature, however, only minor d e v i a t i o n s if any' are o b s e r v e d . It must be emphasized that similiar features could be observed in different samples ( Tc : 460 mK; 500 mK ) and for each elastic mode investigated!

According to symmetry' the stiffness coefficient Ca4 can be investigated by transverse sound generated not only perpendicular but also parallel to the hexagonal c-axis. This was done in order to check whether the anomalous behaviour is influenced by thermal expansion, which in UPt 3 is of opposite sign for both directions below 40 K. Since the hysteretic behaviour found is qualitatively the same thermal expansion effects seem to be of minor importance. All transverse as well as the longitudinal sound waves propagated perpendicular to the hexagonal axis indicate a strong hardening when approaching 15 K from above. However, compressional waves generated along the e-axis reflect quite the opposite as shown in Fig.4. This mode related to the elastic stiffness C33 is the only one, which becomes softer when passing the critical temperature range. 3890

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References 1386

I. F.Steglich,U.Ahlheim,C.DBredI.C.GeibeI,A.Grauel,Miang,(2,.Spam,

A.Krimmell,A.Loidl,W.Assmus;J.Magn.Magn.Mater. 1080992-), 5

2. V.Mialler, ChRoth, D.Maurer, E.W.ScheidkKLilders,E.Bucher, HE. 1384

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BOmmel:Phy~Rev.Lett...e4~(1987), 1224 K.Hasselbach,kTaillefer,J.Flouquet;Phys.Rev.Lett_65(1989), 93 G.Bruls,D. Weber, B. WolEP.Thalmeier, B.LUthi,A.deVisser, a.Menovsky:

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