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Physica B 378–380 (2006) 726–727 www.elsevier.com/locate/physb
Specific heat measurement in golden-SmS K. Matsubayashia,0003, K. Imuraa, H.S. Suzukib, G. Chena, N.K. Satoa a
Department of Physics, Nagoya University, Nagoya 464-8602, Japan National Institute for Material Science, Tsukuba, Ibaraki 305-0047, Japan
b
Abstract We have measured the specific heat of SmS single crystals under pressure. We report that a g-term in the golden phase can be represented as the sum of intrinsic and extrinsic contributions, and argue that there seems to be a relation between the intrinsic g and a pseudo gapenergy D as gD ¼ const. r 2006 Elsevier B.V. All rights reserved. PACS: 65.40.Ba; 71.28.þd; 71.30.þh Keywords: SmS; Insulator–metal transition; Specific heat
SmS is a prototypical compound that exhibits a firstorder insulator–metal transition under pressure. In the socalled golden-phase induced above a critical pressure PC , experimental results are controversial; the electrical resistivity shows a semiconductor-like temperature dependence [1], while the specific heat indicates a metallic state having an enhanced electronic specific heat coefficient [2]. According to our recent thermal expansion measurements under pressure, an excitation gap opens in the golden-phase [3]. On the other hand, we confirmed the presence of a large gvalue by our heat-capacity measurement [4]. Therefore, the discrepancy remains to be resolved. At this stage, the problem to be addressed is whether the g-term is intrinsic or not. To resolve this question, we prepared single crystals under various conditions and measured the specific heat of those samples under pressure. First, we synthesized a powdered material by reacting high-purity sulfur with samarium in an evacuated quartz ampoule. Then we grew a single crystal in a tungsten crucible by the Bridgman method. The experimental procedure was described elsewhere [5]. We have measured the heat capacity in a helium-3 cryostat by the heat pulse method. The pressure was generated by a beryllium–copper piston-cylinder clamp device with ZrO2 as a piston and a 0003Corresponding author. Tel.: +81 52 789 2889; fax: +81 52 789 2933.
E-mail address: [email protected] (K. Matsubayashi). 0921-4526/$ - see front matter r 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.physb.2006.01.424
piston-backup. To avoid complication arising from the pressure variations of the background specific heat, we adopted AgCl as a pressure transmitting medium. Actually, we experimentally confirmed that the pressure dependence of the specific heat of AgCl is negligibly small. To determine the pressure at low temperature, we measured the superconducting transition temperature of indium by AC magnetic susceptibility. Fig. 1 shows a plot of C=T versus T 2 for several samples prepared under different conditions. The results taken at ambient pressure are illustrated in (a) and those at P00016 kbar (4PC ) are given in (b). As reported previously, we observe the strong sample dependence at low temperature. Since SmS is a nonmagnetic insulator at ambient pressure, the temperature dependence of the specific heat should be simply described by the conventional T 3 -law of the lattice contribution. Therefore, it seems that the anomalous features are of extrinsic origin. As it is known that Sm2 S3 and Sm3 S4 produce a Schottky-type anomaly at low temperature (exceeding 5 J/K mol) [6,7], we speculate that a tiny amount of such secondary phases causes the deviation from the T 3 -law. We pressurized the above samples above PC (see Fig. 1(b)). The drastic change in the magnitude of C(T) reflects the first-order transition at PC . We find that the smaller the anomalies at ambient pressure are, the smaller g-value in the golden phase becomes. This seems to suggest that the
ARTICLE IN PRESS K. Matsubayashi et al. / Physica B 378–380 (2006) 726–727
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Fig. 1. Plot of C=T versus T for several samples at ambient pressure and 6 kbar (4PC ).
anomaly arising from the impurity phases, which are observed at ambient pressure, gives rise to a spurious gT term in the golden phase. However, if we look at the data of sample #10 which is the best among the samples investigated, there exists yet a finite g-value as large as 000130 mJ=K2 mol. Therefore, we conjecture that there are intrinsic and extrinsic contributions in the g-term. In Fig. 2 we show the temperature dependence of the specific heat for sample #10 at constant pressures. The Tlinear dependence is clearly observed at low temperature. We note that the observed g-value of about 80 mJ=K2 mol at 14.9 kbar is smaller than that reported by Bader et al. (g0001145 mJ=K2 mol at 15 kbar). This can be ascribed to the difference in the amount of the spurious phases. The temperature dependence of the specific heat above about 5 K is well described by a Schottky-type anomaly with an excitation energy D, which is consistent with our thermal expansion measurements [3,8]. This indicates that the specific heat in the measured temperature region is represented by the equation, CðTÞ ¼ gT þ bT 3 þ C Sch , where bT 3 includes the lattice contribution and C Sch denotes the Schottky-type temperature dependence. In the inset of Fig. 2, we plot the g-value as a function of pressure, together with 1=D determined from our thermal expansion measurements. Here, we evaluated g by fitting the data to the above equation. We find that g is proportional to 1=D, i.e., the product gD is independent of pressure. The excitation energy D is an intrinsic bulk
1
10 T (K)
Fig. 2. Temperature dependence of specific heat for sample #10 at several pressures. The inset shows the electronic specific heat coefficient g inferred from the experiment as a function of pressure, together with the inverse of excitation energy D.
property [8], so that this relation seems to indicate that g is intrinsic. This may lend support to the conjecture mentioned above. We believe that this deserves a further investigation. In conclusion, we investigated the sample dependence of the specific heat under pressure, and suggested the possibility that the g-term in the golden phase can be the sum of intrinsic and extrinsic contributions. This work was partially supported by a Grant-in-Aid from the Ministry of Education, Culture, Sports, Science and Technology, Japan, and Arai Science and Technology foundation. K. M. is supported by a Grant-in-Aid for JSPS Fellows.
References [1] [2] [3] [4] [5] [6] [7] [8]
F. Lapierre, et al., Solid State Commun. 40 (1981) 347. S.D. Bader, et al., Phys. Rev. B 7 (1973) 4686. K. Matsubayashi, et al., J. Magn. Magn. Mater. 272–276 (2004) e277. K. Matsubayashi, et al., Physica B 329–333 (2003) 484. K. Matsubayashi, et al., Physica B 359–361 (2005) 151. V. Tikhonov, et al., Sov. Phys. Solid State 19 (1977) 175. J. Coey, et al., J. Appl. Phys. 50 (1979) 1923. K. Imura, et al., in this conference.

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