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Anomalous thermal expansion in the metallic phase of SmS under high pressure
ARTICLE IN PRESS
Physica B 359–361 (2005) 148–150 www.elsevier.com/locate/physb
Anomalous thermal expansion in the metallic phase of SmS under high pressure K. Iwasaa,, T. Tokuyamab, M. Kohgib, N.K. Satoc, N. Moˆrid a
Department of Physics, Tohoku University, Aramaki-aza-aoba, Aoba-ku, Sendai 980-8578, Japan b Department of Physics, Tokyo Metropolitan University, Hachioji 192-0397, Japan c Department of Physics, Nagoya University, Nagoya 464-8602, Japan d Department of Physics, Satitama University, Saitama 338-8570, Japan
Abstract SmS exhibits a pressure-induced phase transition at 0.6 GPa from a semiconducting state to a rather metallic state accompanied with a change of Sm valence and volume compression. Using the X-ray diffraction technique under high pressures, we found local minima of the lattice constant of SmS in the metallic phase up to near 2 GPa. The pressure region of the volume minima coincides with that of the low-temperature increase and the humps of electrical resistivity. We succeeded in reproducing the volume minima by a phenomenological model of a Schottky-type behavior due to electronic gap suppressed by pressure. r 2005 Elsevier B.V. All rights reserved. PACS: 61.10.Nz; 65.40.De; 71.28.+d; 71.30.+h Keywords: SmS; Pressure-induced semiconductor–metal transition; Thermal expansion
SmS has been a typical system showing valence instability and undergoes a pressure-induced firstorder phase transition [1,2]. At ambient pressure, the Sm ion is considered to be divalent [3] and a semiconductive transport property appears [4,5]. The resistivity decreases with applying pressure and a semiconductor–metal transition occurs at Corresponding
author. Tel.: +81 22 217 6486; fax: +81 22 217 6489. E-mail address: [email protected] (K. Iwasa).
Pc ¼ 0:6 GPa [4,5], which is accompanied by sudden volume compression with keeping the NaCl-type cubic crystal structure due to Sm-ion valence change [1,6]. The temperature dependence of the electrical resistivity shows a rather complicated behavior of anomalous humps or increase below 20 K between 0.7 and 2 GPa. This fact suggests that SmS in not a simple metal and has a lower energy gap even in the metallic phase. Because the valence state of Sm ions and carrier state are sensitive to the applied pressure as seen in
0921-4526/$ - see front matter r 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.physb.2005.01.018
ARTICLE IN PRESS K. Iwasa et al. / Physica B 359– 361 (2005) 148–150
the volume compression, we investigate a thermal property of crystal lattice under high pressure using X-ray diffraction technique. X-ray diffraction experiments were performed at BL-1B of Photon Factory in High Energy Accelerator Organization, Japan as well as at the conventional X-ray source in Tokyo Metropolitan University. Single-crystal samples of SmS were mounted in diamond-anvil cells together with NaCl crystals and ruby powders as pressure markers. Pressure transmitting media were Daphne oil or 4:1 solution of methanol and ethanol. Sample temperatures were controlled by a closed-cycle helium refrigerator. Temperature variation of Bragg-reflection angles was measured under the condition of pressure accuracy of around 0.1 GPa. Fig. 1 shows thermal expansion DL=L of SmS at various pressures. At ambient pressure, the lattice constant varies monotonically against temperature. On the other hand, at 0.6, 1.1 and 1.6 GPa, 0 10-3
SmS
-2
ambient
-4 0.5 GPa (decreasing)
-54
-50 -52
0.6 GPa
-56 -58
1.0 GPa
-60
-54 -56 -58
-62
1.6 GPa
-60
-64
1.8 GPa
-62
-66
2.6 GPa
-64
3.0 GPa
-68 0
100 200 Temperature [K]
∆L / L [× 10-3]
∆ L / L [× 10-3]
-52
-66 300
Fig. 1. Symbols are measured lattice constants relative to that at ambient pressure and at 300 K. The result at 1.6 GPa with the right vertical axis taken by the conventional X-ray diffractometer shifts due to the systematic errors. Solid lines are fitted results.
149
local minima were observed at intermediate temperatures. The data at 0.5 GPa lower than Pc also shows a similar behavior. We confirmed that this measurement was carried out in the metallic phase, since it was obtained after decreasing pressure following the large hysteresis against pressure and the smaller volume was conserved. The local minimum of lattice constant disappears above 1.8 GPa, while we could not measure the lattice constant above 150 K under the condition of constant pressures. The present experimental result is consistent with that measured by the strain gage method for SmS [7] and for Sm0:9 La0:1 S [8]. The pressure region of local minima of lattice constant coincides with that of the low-temperature anomalies of electrical resistivity. The electronic structure in the metallic phase of SmS is considered to be composed of two sharp peaks of density of states above and below the Fermi level, which are originated from the virtual bound state of hybridized 4f and 5d bands [9]. The energy gap corresponding to the energy difference between these two sharp density of states was evaluated as around 7 meV by the point contact spectroscopy at ambient pressure. The resistivity of SmS increases below 20 K with applying pressure from 0.6 to 1.5 GPa. A similar behavior was seen in TmSe0:45 Te0:55 ; and was explained by an exciton state located between 4f and 5d bands [9]. Based on these gap models, we analyze the present results by a Schottky-type thermal expansion coefficient. For the low-lying thermal expansion as observed at ambient pressure, we took into account a phonon density of state described by the Debye approximation and volume-dependent phonon frequencies by the Gru¨neisen assumption [10]. As shown by lines in Fig. 1, we succeeded in reproducing the data well. The evaluated energy gap as a function of pressure is depicted in Fig. 2 which is also consistent with the results of the strain gage measurement [7] as well as the resistivity [5]. We also carried out neutron diffraction experiments as well as X-ray over 2 GPa to find a phase transition suggested from the cusp of the temperature variation of resistivity [5] and the recent specific-heat and 149Sm Mo¨ssbauer effect measure-
ARTICLE IN PRESS K. Iwasa et al. / Physica B 359– 361 (2005) 148–150
150
140
D. Kawana, I. Goncharenko, J.-M. Mignot and A. Ivanov are acknowledged for their experimental assistance.
Pc
Gap energy [K]
120 100 80
References
60
[1] A. Jayaraman, et al., Phys. Rev. Lett. 25 (1970) 1430. [2] M.B. Maple, D. Wohlleben, Phys. Rev. Lett. 27 (1971) 511. [3] J. Ro¨hler, et al., in: P. Wachter, H. Boppart (Eds.), Valence Instabilities, 1982, p. 215. [4] M. Konczykowski, et al., J. Magn. Magn. Mater. 47–48 (1985) 274. [5] M. Ohashi, et al., Rev. High Pressure Sci. Technol. 7 (1998) 611; N. Moˆri, et al., JJAP Series 11 (1999) 129. [6] D.B. McWhan, et al., Phys. Rev. B 18 (1978) 3623. [7] K. Matsubayashi, et al., J. Magn. Magn. Mater. 272–276 (2004) E277. [8] G. Oomi, et al., J. Magn. Magn. Mater. 226–230 (2001) 1182. [9] P. Wachter, in: K.A. Gschneider Jr., L. Eyring, G.H. Lander, G.R. Choppin (Eds.), Handbook on the Physics and Chemistry of Rare Earths, vol. 19, 1994, p. 177. [10] K. Iwasa, et al., J. Phys. Soc. Japan 68 (1999) 881. [11] A. Barla, et al., Phys. Rev. Lett. 92 (2004) 066401.
40 20 0 0.0
0.5
1.0 1.5 Pressure [GPa]
2.0
2.5
Fig. 2. Evaluated gap energy. Lines are guide to eyes.
ments [11]. However, there has been no clear evidence of ordered phase in the diffraction experiments so far. H. Nakao, Y. Wakabayashi, R. Tazaki, H. Sawa, K. Kuwahara, H. Sagayama, M. Nakajima,
Physica B 359–361 (2005) 148–150 www.elsevier.com/locate/physb
Anomalous thermal expansion in the metallic phase of SmS under high pressure K. Iwasaa,, T. Tokuyamab, M. Kohgib, N.K. Satoc, N. Moˆrid a
Department of Physics, Tohoku University, Aramaki-aza-aoba, Aoba-ku, Sendai 980-8578, Japan b Department of Physics, Tokyo Metropolitan University, Hachioji 192-0397, Japan c Department of Physics, Nagoya University, Nagoya 464-8602, Japan d Department of Physics, Satitama University, Saitama 338-8570, Japan
Abstract SmS exhibits a pressure-induced phase transition at 0.6 GPa from a semiconducting state to a rather metallic state accompanied with a change of Sm valence and volume compression. Using the X-ray diffraction technique under high pressures, we found local minima of the lattice constant of SmS in the metallic phase up to near 2 GPa. The pressure region of the volume minima coincides with that of the low-temperature increase and the humps of electrical resistivity. We succeeded in reproducing the volume minima by a phenomenological model of a Schottky-type behavior due to electronic gap suppressed by pressure. r 2005 Elsevier B.V. All rights reserved. PACS: 61.10.Nz; 65.40.De; 71.28.+d; 71.30.+h Keywords: SmS; Pressure-induced semiconductor–metal transition; Thermal expansion
SmS has been a typical system showing valence instability and undergoes a pressure-induced firstorder phase transition [1,2]. At ambient pressure, the Sm ion is considered to be divalent [3] and a semiconductive transport property appears [4,5]. The resistivity decreases with applying pressure and a semiconductor–metal transition occurs at Corresponding
author. Tel.: +81 22 217 6486; fax: +81 22 217 6489. E-mail address: [email protected] (K. Iwasa).
Pc ¼ 0:6 GPa [4,5], which is accompanied by sudden volume compression with keeping the NaCl-type cubic crystal structure due to Sm-ion valence change [1,6]. The temperature dependence of the electrical resistivity shows a rather complicated behavior of anomalous humps or increase below 20 K between 0.7 and 2 GPa. This fact suggests that SmS in not a simple metal and has a lower energy gap even in the metallic phase. Because the valence state of Sm ions and carrier state are sensitive to the applied pressure as seen in
0921-4526/$ - see front matter r 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.physb.2005.01.018
ARTICLE IN PRESS K. Iwasa et al. / Physica B 359– 361 (2005) 148–150
the volume compression, we investigate a thermal property of crystal lattice under high pressure using X-ray diffraction technique. X-ray diffraction experiments were performed at BL-1B of Photon Factory in High Energy Accelerator Organization, Japan as well as at the conventional X-ray source in Tokyo Metropolitan University. Single-crystal samples of SmS were mounted in diamond-anvil cells together with NaCl crystals and ruby powders as pressure markers. Pressure transmitting media were Daphne oil or 4:1 solution of methanol and ethanol. Sample temperatures were controlled by a closed-cycle helium refrigerator. Temperature variation of Bragg-reflection angles was measured under the condition of pressure accuracy of around 0.1 GPa. Fig. 1 shows thermal expansion DL=L of SmS at various pressures. At ambient pressure, the lattice constant varies monotonically against temperature. On the other hand, at 0.6, 1.1 and 1.6 GPa, 0 10-3
SmS
-2
ambient
-4 0.5 GPa (decreasing)
-54
-50 -52
0.6 GPa
-56 -58
1.0 GPa
-60
-54 -56 -58
-62
1.6 GPa
-60
-64
1.8 GPa
-62
-66
2.6 GPa
-64
3.0 GPa
-68 0
100 200 Temperature [K]
∆L / L [× 10-3]
∆ L / L [× 10-3]
-52
-66 300
Fig. 1. Symbols are measured lattice constants relative to that at ambient pressure and at 300 K. The result at 1.6 GPa with the right vertical axis taken by the conventional X-ray diffractometer shifts due to the systematic errors. Solid lines are fitted results.
149
local minima were observed at intermediate temperatures. The data at 0.5 GPa lower than Pc also shows a similar behavior. We confirmed that this measurement was carried out in the metallic phase, since it was obtained after decreasing pressure following the large hysteresis against pressure and the smaller volume was conserved. The local minimum of lattice constant disappears above 1.8 GPa, while we could not measure the lattice constant above 150 K under the condition of constant pressures. The present experimental result is consistent with that measured by the strain gage method for SmS [7] and for Sm0:9 La0:1 S [8]. The pressure region of local minima of lattice constant coincides with that of the low-temperature anomalies of electrical resistivity. The electronic structure in the metallic phase of SmS is considered to be composed of two sharp peaks of density of states above and below the Fermi level, which are originated from the virtual bound state of hybridized 4f and 5d bands [9]. The energy gap corresponding to the energy difference between these two sharp density of states was evaluated as around 7 meV by the point contact spectroscopy at ambient pressure. The resistivity of SmS increases below 20 K with applying pressure from 0.6 to 1.5 GPa. A similar behavior was seen in TmSe0:45 Te0:55 ; and was explained by an exciton state located between 4f and 5d bands [9]. Based on these gap models, we analyze the present results by a Schottky-type thermal expansion coefficient. For the low-lying thermal expansion as observed at ambient pressure, we took into account a phonon density of state described by the Debye approximation and volume-dependent phonon frequencies by the Gru¨neisen assumption [10]. As shown by lines in Fig. 1, we succeeded in reproducing the data well. The evaluated energy gap as a function of pressure is depicted in Fig. 2 which is also consistent with the results of the strain gage measurement [7] as well as the resistivity [5]. We also carried out neutron diffraction experiments as well as X-ray over 2 GPa to find a phase transition suggested from the cusp of the temperature variation of resistivity [5] and the recent specific-heat and 149Sm Mo¨ssbauer effect measure-
ARTICLE IN PRESS K. Iwasa et al. / Physica B 359– 361 (2005) 148–150
150
140
D. Kawana, I. Goncharenko, J.-M. Mignot and A. Ivanov are acknowledged for their experimental assistance.
Pc
Gap energy [K]
120 100 80
References
60
[1] A. Jayaraman, et al., Phys. Rev. Lett. 25 (1970) 1430. [2] M.B. Maple, D. Wohlleben, Phys. Rev. Lett. 27 (1971) 511. [3] J. Ro¨hler, et al., in: P. Wachter, H. Boppart (Eds.), Valence Instabilities, 1982, p. 215. [4] M. Konczykowski, et al., J. Magn. Magn. Mater. 47–48 (1985) 274. [5] M. Ohashi, et al., Rev. High Pressure Sci. Technol. 7 (1998) 611; N. Moˆri, et al., JJAP Series 11 (1999) 129. [6] D.B. McWhan, et al., Phys. Rev. B 18 (1978) 3623. [7] K. Matsubayashi, et al., J. Magn. Magn. Mater. 272–276 (2004) E277. [8] G. Oomi, et al., J. Magn. Magn. Mater. 226–230 (2001) 1182. [9] P. Wachter, in: K.A. Gschneider Jr., L. Eyring, G.H. Lander, G.R. Choppin (Eds.), Handbook on the Physics and Chemistry of Rare Earths, vol. 19, 1994, p. 177. [10] K. Iwasa, et al., J. Phys. Soc. Japan 68 (1999) 881. [11] A. Barla, et al., Phys. Rev. Lett. 92 (2004) 066401.
40 20 0 0.0
0.5
1.0 1.5 Pressure [GPa]
2.0
2.5
Fig. 2. Evaluated gap energy. Lines are guide to eyes.
ments [11]. However, there has been no clear evidence of ordered phase in the diffraction experiments so far. H. Nakao, Y. Wakabayashi, R. Tazaki, H. Sawa, K. Kuwahara, H. Sagayama, M. Nakajima,