Specific heat of the spin density wave state in Ce(Ru0.85Rh0.15)2Si2under pressure

J. Tang*, T. Matsumoto, H. Abe, A. Matsushita

National Research Institute for Metals, 1-2-1, Sengun, Tsukuba, Ibaraki 305, Japan

Received 17 November 1998; accepted 3 December 1998 by H. Kamimura

Abstract

We have investigated the pressure effect on the magnetic state of the heavy-fermion compound Ce(Ru0.85Rh0.15)2Si2 in whichthe spin density wave (SDW) is formed below 5.5 K at ambient pressure. The study was carried out by means of measurementsof the specific heat up to 1.6 GPa between 0.7 and 10 K for a polycrystalline sample. In the low pressure region below 0.6 GPa,the Neel temperatureTN and the Sommerfeld coefficientg show a slight decrease from 5.5 to 4.9 K and from 310 to 290 mJ/K2 mol, respectively. At higher pressure the peak in the specific heat becomes broad butTN remains above 5 K, whileg rapidlydecreases. As a possible origin for the observed pressure dependence ofTN andg across 0.6 GPa, we propose the change in theSDW state at higher pressure.q 1999 Elsevier Science Ltd. All rights reserved.

Keywords:D. Heat conduction; D. Heavy fermions; D. Kondo effects; E. Strain, high pressure

1. Introduction

The heavy-fermion compound systemCe(Ru12xRhx)2Si2 with the ThCr2Si2 structure attractsmuch attention because of its unusual magnetic andsuperconducting properties. The ground state ofCeRu2Si2 does not exhibit long-range order andshows heavy-fermion behavior. In the Ru rich region�0:05 , x , 0:3�; the magnetic structure has beenshown [1] to be a sinusoidal spin density wave(SDW) formed by nesting of the Fermi surface. Forintermediate Rh content, non-Fermi liquid behaviorhas been reported [2]. In the Rh rich region, thecompound becomes a localized antiferromagnetwhoseTN and moment increase withx. A recent inves-tigation [3] of the effect of pressure on the terminalantiferromagnetic compound CeRh2Si2 revealed the

disappearance ofTN and the appearance of supercon-ductivity above 0.9 GPa. Therefore it is exciting toinvestigate the effect of pressure on the magneticphases of Ce(Ru12xRhx)2Si2. Among the magneticphases mentioned earlier, the SDW is believed todepend sensitively on the electronic structure. Inorder to clarify how the SDW state behaves for themodification of electronic structure under pressure,we have studied the pressure dependence of the speci-fic heat of Ce(Ru0.85Rh0.15)2Si2 which has the maxi-mum ordering temperature of SDW at ambientcondition.

2. Experimental

A polycrystalline Ce(Ru0.85Rh0.15)2Si2 sample wasprepared by Ar arc-melting a stoichiometric mixtureof pure elements under continuous titanium gettering.The sample was sealed in a quartz ampule and

Solid State Communications 109 (1999) 445–448

0038-1098/99/$ - see front matterq 1999 Elsevier Science Ltd. All rights reserved.PII: S0038-1098(98)00602-4

PERGAMON

* Corresponding author. Tel:1 81 298 59 2728; fax:1 81 29859 2701; e-mail: tang@nrim.go.jp

annealed at 8008C for 195 h. In the X-ray powderdiffraction experiment, we did not observe any otherphase except the ThCr2Si2 structure. The specific heatmeasurement was carried out with a quasi-adiabaticmethod up to 1.6 GPa between 0.7 and 10 K. A Be–Cu piston and cylinder clamp were used to generate

and retain pressure which was applied at roomtemperature. AgCl was used as a pressure medium.

3. Results and discussion

Fig. 1 shows the temperature dependence of thespecific heat of polycrystalline Ce(Ru0.85Rh0.15)2Si2at various pressures. The specific heat (C) vs.temperature (T) curve measured at 0.05 GPa showsa peak at about 5.3 K corresponding to the antiferro-magnetic ordering. With increasing pressure the peakin the specific heat atTN is rapidly suppressed. Above1 GPa, we can observe broad humps aroundTN whichare very similar to a behavior of the specific heatmeasured in Ce(Ru0.75Rh0.25)2Si2 [4] in which TN ofSDW was decided by a small knee in theC vs. Tcurve. From this result we can obtain the pressuredependence of the Ne`el temperatureTN as indicatedin Fig. 2. TheTN atP� 0 which was determined fromthe specific heat measurement using a standardsample holder is also shown. In the low pressureregion TN shifts slightly to lower temperatures from5.5 K at 0 GPa to 4.9 K at 0.6 GPa, while at higherpressureTN remains above 5 K.

In Fig. 3 the relationship betweenC/T and T isshown as a function of pressure for the

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Fig. 1. Temperature dependence of the specific heat of polycrystal-line Ce(Ru0.85Rh0.15)2Si2 measured at various pressures.

Fig. 2. Pressure dependence of the Ne`el temperatureTN for poly-crystalline Ce(Ru0.85Rh0.15)2Si2.

Fig. 3. C/T vs. T for polycrystalline Ce(Ru0.85Rh0.15)2Si2 at variouspressures.

Ce(Ru0.85Rh0.15)2Si2 sample. Above 1.0 GPa, theC/Tvalues show a weak temperature dependence below5 K. This phenomenon observed at higher pressures isprobably because of the overlapping of the suppres-sion of the peak in theC–T curve and the enhance-ment of the density of states near the partiallyremaining Fermi surface with heavy mass belowTN.The pressure dependence ofg obtained from theC/Tvs. T2 plot is shown in Fig. 4. The decrease ofg issmall for pressures below 0.6 GPa, but with furtherincreasing pressure the decrease takes place at a muchfaster rate. In general, the Kondo temperature,TK ofthe heavy-fermion compound is believed to increaseat high pressure. Thus, it is reasonable that theg (P),being proportional toT21

K decreases with increasingpressure.

The behavior ofTN under pressure in this systemseems to be unusual. Volume changes frequently havethe major influence on the magnetic–nonmagnetictransition. In fact, there are many reports in whichTN can be tuned to a Doniach phase diagram underpressure; the magnetic transition temperature by anoccurrence of the long range order shows the

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Fig. 4. Pressure dependence of the Sommerfeld coefficientg for polycrystalline Ce(Ru0.85Rh0.15)2Si2.

Fig. 5. Pressure dependence ofn for polycrystallineCe(Ru0.85Rh0.15)2Si2. The data at ambient condition with openmarks is replotted from the previous work [6].

maximum or decreases continuously to zero withincreasing pressure.

For the SDW of Ce7Ni3 the ordering temperature isreported to be suppressed continuously with increas-ing pressure [5]. The SDW state is sensitive to theshape of the Fermi surface which should be easilymodified under pressure. When a metallic systemhas a complex Fermi surface, it seems to be difficultto infer the simple behavior ofTN of the SDW in thewhole range of pressure. The change in the shape ofthe Fermi surface should combine with the modifica-tion of the SDW wave vector which varies not only inmagnitude but also in direction.

From the logC vs. logT plots of the present data,the specific heat was analyzed asC � ATn belowTN,corresponding to an anisotropic gap opening at theFermi surface. The pressure dependence ofn isshown by solid circles in Fig. 5 with the value(denoted byW) at P � 0 reported previously [6].The n value decreases and approaches rapidly about1 above 0.6 GPa. In the previous work on the relatedcompounds [6], then values of 1.4 and 1.1 are givenfor Ce(Ru12xRhx)2Si2 (x � 0.1,0.15) and(Ce0.87La0.13)Ru2Si2, respectively. Also the SDWwave vector is shown to be along thec-axis in theRh doped system and along thea-axis in the La

doped. From these results it is implied that the changeof the electronic state under pressure results in thechange of the SDW wave vector which explains thesignificant pressure dependence ofTN, g or n acrossthe pressure of 0.6 GPa. In order to clarify the spec-ulation, it is necessary to carry out further single crys-tal experiments at high pressure, such as the neutrondiffraction and magneto-resistance.

References

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