Equation of state of CeCu2Ge2 at cryogenic temperature
Introduction
There has been a growing interest in the strongly correlated electron systems [1], [2] and among them are the rare-earth compounds possessing the ThCr2Si2-type structure [3]. Recent high-pressure studies have revealed some salient features in the superconducting behavior of these compounds. In the case of CeCu2Si2, application of pressure enhances the superconducting transition temperature Tc from the ambient-pressure value of ∼0.7 K and there occurs a maximum in Tc at ∼3 GPa [4], [5], [6], [7]. Likewise, CeCu2Ge2, after undergoing pressure-induced superconductivity at about 8 GPa with Tc∼0.6 K, shows also a Tc maximum around 16 GPa [8], [9], [10].
The enhancement of Tc has been attributed to pressure-induced topological change in the band structure on the one hand [6], [7] and valence change of Ce on the other [8], [10]. The former interpretation envisages that application of pressure on CeCu2Si2 causes the crystal-field split sub-bands broaden and interfere, leading to a topological change [6], [7]. In the latter case it was shown that an intermediate valence state is introduced when the Kondo temperature becomes of the order of the crystal field splitting [8], [10]. Such a state corresponds to the peak of the residual resistivity ρ0 and to the drop of the coefficient A in the T-power relationship of resistivity in CeCu2Ge2 [8], [10]. Very recently, a theory [11] was put forth by Miyake et al., who phenomenologically interpreted the coincidence of peaks of Tc and ρ0 in CeCu2Ge2 and ascribed them to a rapid valence change of Ce ion, i.e. the mean number of 4f-electrons per Ce ion. In a subsequent paper [12] these authors showed that Tc is enhanced around which the valence change occurs when the f-level is tuned with pressure relative to the Fermi-level.
This paper reports an experimental result which provides us with information on the valence state of CeCu2Ge2 at high pressure and low temperature. No such information was available at the time this experiment was started. Techniques feasible for the study of the valence state of a given substance are X-ray lattice constant and X-ray absorption measurements [13]. While X-ray absorption technique may be disturbed when it is tried at high pressure, with the absorption of the high-pressure wall material for example, X-ray lattice constant measurement can be performed under pressure with relative ease. In fact, pressure-induced valence changes in the rare-earth compounds have been extensively studied by the X-ray lattice constant technique [14]. The technique has become provocative owing to a combination of synchrotron X-ray source and imaging plate detector [15]. Along this line, we have carried out powder X-ray diffraction experiment to measure the equation of state of CeCu2Ge2 at a temperature (actually 10 K) as close as possible to the superconducting temperature (around 2 K) of this compound [7], [8], [9], [10].
Section snippets
Experiment
Polycrystalline sample of CeCu2Ge2 from the same ingot that had exhibited pressure-induced superconductivity [9] was used. X-ray powder diffraction at ambient conditions showed the lattice constants to be and in agreement with the literature [16]. The powdered sample was pressurized with a gasketed diamond-anvil cell driven by a He gas. The culet of the anvil was 600 μm across. The gasket was made of a 60 μm thick stainless steel with a 180 μm diameter sample
Results
Fig. 1 shows evolution of X-ray diffraction patterns of CeCu2Ge2 taken at high pressure. At a pressure of 1.1 GPa, the cooling from room temperature to 10 K does not cause any substantial peak broadening although the pressure-transmitting medium should be frozen upon cooling. Rather, the pressure increase at 10 K gives rise to a slight increase of the peak width and also mergence of some peaks. No new peak appears, however, indicating the persistence of the initial ThCr2Si2-type structure
Discussion
Generally, any anomalous volume contraction along the isothermal equation of state can be caused by at least two origins. These are first-order structural phase transition and valence change. The pressure-driven valence change associated with volume contraction has been widely observed in compounds of the rare-earth elements [14]. No structural phase transition accompanies the valence change [14]. Since, as seen from Fig. 1, no substantial change occurs in the X-ray diffraction patterns between
Conclusions
The present study on the equation of state of CeCu2Ge2 at 10 K has reproducibly documented an anomalous volume contraction at around 15 GPa. No crystallographic phase transition accompanies this volume contraction. A pressure-induced valence change of Ce is likely to be responsible for the volume anomaly. The observation along with considerations are for the valence-change model for the origin of the pressure-induced Tc maximum in CeCu2Ge2.
Acknowledgements
We thank Professor K. Miyake for valuable discussions. The help of K. Wasa and T. Yuasa with the experiment is acknowledged. The study at SPring-8 was performed under 1999B0327-Nd-np and 2000A0083-CD-np. This work was supported in part by the Grant-in-Aid for COE Research (10CE2004) from the Ministry of Education, Sciences, Sports and Culture, Japan.
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Present address: Graduate School of Science, Kyoto University, Sakyo-ku, Kyoto 606-8052, Japan.