Low-temperature specific heat for off- and near-stoichiometric UAsSe

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Abstract

Two UAsSe single crystals with slight variations of the As/Se content ratio have been investigated. The development of the low-T upturn in the resistivity is accompanied by smooth decrease of the Curie temperature. The sample with TC=116.9K and almost invisible upturn could be regarded as a near-stoichiometric, slightly disordered one, whereas the specimen with TC=101.5K appears to be an off-stoichiometric, more strongly disordered one. Clear differences between both electronic and lattice contributions to the specific heat for off- and near-stoichiometric UAsSe crystals have also been found. We ascribe the smaller low-T specific heat for the off-stoichiometric sample in comparison to the near-stoichiometric one to a weaker hybridization between 5f and conduction-electron states accompanied by the relative shift of 5f band with regard to EF.

Introduction

Uranium pnictochalcogenides UXY, where X=P, As, Sb and Y=S, Se, Te, order ferromagnetically along the c axis. All UXY are recognized as metallic tetragonal compounds whose 5f electrons, according to early magnetic and photoemission data, are believed to be quite well localized [1], [2]. Polarized neutron diffraction measurements for UAsSe also indicate a picture of localized 5f electrons [3]. Unexpectedly, the results of optical and magneto-optical Kerr spectra measurements suggest a narrow 5f band at the Fermi energy, EF, rather than a localized 5f state in UAsSe [4], [5]. Oppeneer et al. [6] have investigated the magneto-optical Kerr spectra of UAsSe using first-principles energy-band calculations. They have obtained, within a band-like description of the 5f electrons, good agreement with the measured optical spectra and find that the uranium 5f states in UAsSe exhibit at least partially itinerant 5f-electron behavior. They conclude that the 5f electrons are delocalized in the ab-plane, but localized along the c-axis. Recent measurements of the angle-resolved photoelectron spectroscopy (ARPES) strongly hint at itinerancy of 5f electrons even in the ferromagnetically ordered state [7], [8]. Further on, the latter experiment shows that the f and d bands are more strongly renormalized than the p bands.

UAsSe crystallizes in the tetragonal PbFCl crystal structure [9] (one crystallographic position of uranium ions) and undergoes a ferromagnetic phase transition around 110 K. UAsSe, which belongs to so-called hard ferromagnets with coercive force close 0.8 T for Hc, is the most frequently investigated system among all uranium pnictochalcogenides. This is because of its unusual low-T transport properties, especially the upturn in the resistivity far below the ferromagnetic transition. The origin of this upturn is very intriguing but not yet clarified.

The electrical resistivity, ρ(T), of UAsSe is strongly sample dependent. The measurements showed that the Curie temperature, TC, as well as the low-T upturn in ρ(T) are very sensitive to small variations of the As/Se content ratio [10]. The latter quantity seems to be responsible for the possible disorder in the anionic sublattice. Although there is not yet unquestionable evidence for it, the X-ray as well as the neutron-diffraction experiments, clearly hint at such a possibility [11].

In spite of many papers concerning transport properties of UAsSe [10], [12], [13], there is only one that deals with its specific heat, C(T), [14]. This 20-year-old result was obtained on a powder sample pressed into a pellet together with a silver powder. An enhanced Sommerfeld coefficient, γ=41mJ/K2mol, was found and since then is often considered as an indication of the tendency towards itinerancy of the 5f electrons in UAsSe.

Very recently we have briefly reported on the sample dependent low-T specific heat of UAsSe [15]. These systematic measurements have been done using high-quality single crystals. The γ(TC) dependence showed a minimum close to TC=111K. The observed decrease of γ upon increasing TC up to 111 K was attributed to a reduction of the concentration of dynamical scattering centers. The subsequent rise of γ(TC) for TC>111 K signals an additional contribution. Its origin will be discussed in this paper.

Section snippets

Experimental

The uranium arsenoselenide single crystals were grown by the chemical vapor transport method. Metallic U as well as As and Se in the desired molar ratio was sealed in an evacuated silica tube together with bromine as transport agent. About 3–5 mg of Br2 per cubic centimeter of the ampoule volume was used. At the first step, with a gradual increase of temperature from 400 to 950 °C, the elements reacted giving a powder product. Next, the UAsSe substrate was homogenized for a few days. Finally, the

Results and discussion

The origin of the upturn in the resistivity of UAsSe upon cooling far below the ferromagnetic transition is still unclear. The size of the low-T upturn can be quantified, e.g. by the RR=R(4.2K)/Rmin ratio, where Rmin is the minimum resistance in the ferromagnetic state.

Fig. 1 displays the ab-plane R(T)/R(300 K) dependencies for the single crystals #102 and #117 in different magnetic fields. The very small magnetoresistivity, far below the ferromagnetic transition in fields up to 13.5 T, points at

Acknowledgements

We thank A.J. Arko and P.M. Oppeneer for valuable discussions. T. Cichorek acknowledges the Alexander von Humboldt Foundation for a Research Fellowship. This work was supported by the Polish Committee for Scientific Research, Grant no. KBN-2 P03B 062 18; 2000–2001.

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