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Temperature and pressure dependences of Sm valence in intermediate valence compound SmB6
Physica B xxx (xxxx) xxx–xxx
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Temperature and pressure dependences of Sm valence in intermediate valence compound SmB6 ⁎
N. Emia, T. Mitoa, , N. Kawamurab, M. Mizumakib, N. Ishimatsuc, G. Pristášd, T. Kagayamae, K. Shimizue, Y. Osanaif, F. Igag a
Department of Material Science, University of Hyogo, Hyogo 678-1297, Japan Japan Synchrotron Radiation Research Institute (JASRI), SPring-8, Hyogo 679-5198, Japan c Department of Science, Hiroshima University, Hiroshima 739-8526, Japan d Institute of Experimental Physics, Slovak Academy of Science, Koise 04001, Slovakia e Department of Engineering Science, Osaka University, Osaka 560-8531, Japan f Faculty of Science, Ibaraki University, Ibaraki 310-8512, Japan g Department of Science and Engineering, Ibaraki University, Ibaraki 310-8512, Japan b
A R T I C L E I N F O
A BS T RAC T
MSC: 00-01 99-00
We report the results of the X-ray absorption spectroscopy (XAS) on the intermediate valence compound SmB6. The XAS measurements were performed near the nonmagnetic-magnetic phase boundary. Mean Sm valence vSm was estimated from absorption spectra, and we found that vSm near the boundary (P ≥ 10 GPa and T ∼ 12 K ) is far below a trivalent state with magnetic characteristics. Although the result is markedly different from the cases of pressure induced magnetic orders in Yb and Ce compounds, it is likely that the large deviation from the trivalent state seems to be common in some Sm compounds which possess electronic configuration between 4f 5 and 4f 6 with multi 4f electrons.
Keywords: SmB6 Intermediate valence Kondo insulator XAS High pressure
1. Introduction The measurement of the valence of lanthanide ions is one of the most informative methods to evaluate the degree of localization in f electron systems: for example, in the case of Ce, Sm and Yb compounds, the trivalent state of the lanthanide ions corresponds to the strong localization of the f electrons, while an increase in a divalent component (a tetravalent component for Ce) indicates the delocalization. We report the measurement of X-ray absorption spectroscopy (XAS) on the prototypical intermediate valence compound SmB6 to investigate the pressure and temperature dependences of mean Sm valence. SmB6, having the Sm valence of ∼2.6 at room temperature and ambient pressure, shows a semiconducting property with a narrow gap of 50 − 100 K [1,2], whose origin is probably intimately related to effectively temperature dependent hybridization between conduction and f electrons [3]. The ground state of SmB6 drastically changes with pressure, namely the insulating gap collapses at Pc = 10 GPa [4] and simultaneously a magnetically ordered phase appears below ∼12 K [5]. The pressure dependence of the Sm valence in SmB6 has been previously studied at ambient pressure using XAS [6,7]. In this study,
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we have carried out the XAS measurement in the low temperature region where actually the magnetic order occurs, and the results are compared to pressure induced magnetic orders in Yb and Ce compounds as well as in other Sm compounds. 2. Experimental details Single crystalline samples of SmB6 were grown by a floating-zone method using an image furnace with four xenon lamps [8]. The appearance of the magnetically ordered phase in the sample under high pressure and low temperature (P ≥ 10 GPa and T ≤ 12 K ) was confirmed by the resistivity measurement using a single crystalline sample from the same batch. The XAS measurements near the Sm L 3-edge (6.72 keV) were performed at the beamline BL39XU of SPring8, Japan [9]. The XAS spectra were recorded in the transmission mode using ionization chambers. For high pressure measurements, the sample was loaded in a diamond anvil cell (DAC) filled with a mixture of 4:1 methanol:ethanol as a pressure-transmitting medium. Nanopolycrystalline diamond anvils were used to avoid glitches in the XAS spectra [10]. All pressures were applied at room temperature,
Corresponding author. E-mail address: [email protected] (T. Mito).
http://dx.doi.org/10.1016/j.physb.2017.09.053 Received 10 July 2017; Received in revised form 12 September 2017; Accepted 14 September 2017 0921-4526/ © 2017 Elsevier B.V. All rights reserved.
Please cite this article as: Emi, N., Physica B (2017), http://dx.doi.org/10.1016/j.physb.2017.09.053
Physica B xxx (xxxx) xxx–xxx
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Fig. 2. (a) Temperature dependence of ρ at 10.0 GPa measured using a sample taken from the same batch with that used for the XAS measurement. The arrow indicates magnetic ordering temperature. (b) Temperature dependence of vSm of SmB6 at 6.4, 10.3, and 12.1 GPa. The inset shows an expanded view of the data at 10.3 and 12.1 GPa below 20 K.
Fig. 1. Sm L3 -edge absorption spectra of SmB6 at (a) T = 300 K , P = 0.7 GPa , (b) T = 300 K , P = 10.3 GPa , (c) T = 3.9 K , P = 0.6 GPa , and (d) T = 3.3 K , P = 10.4 GPa . The opened circles represent experimental data. The dotted, broken, and solid lines indicate Sm divalent component, trivalent component, and a superposition of these components, respectively. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article).
phase boundary, although the authors observed changes in the pressure dependence of them. A striking feature in the present study is that vSm at the magnetic phase boundary is ∼2.75, far below the magnetic trivalent state. This is markedly different from the cases of pressure induced magnetic orders in Yb and Ce compounds. For example, in YbNi2Ge2 [12], YbPd2Si2 [13], YbAlCu [14], and YbCu2Si2 [12], Yb valence vYb is vYb ≥ 2.9 at critical pressure. The proximity of vYb to 3 implies the strong localization of 4f electrons. Also, in YbRh2Si2 which is thought to locate in the vicinity of nonmagnetic-magnetic criticality at ambient pressure [15,16], vYb is larger than 2.9 as well [17]. For Ce compounds, although there are only a few reports of the valence measurement under pressure so far, for example CePd2Si2 and CeCu2Ge2 are driven through the magneticsuperconducting transition by pressure, and their vCe remains from 3.0 to 3.05 over the superconducting region [18,19]. Thus we have found no serious exception in the Ce and Yb compounds in terms of the proximity to the trivalent state at the magnetic instability. On the other hand, as shown in Fig. 3 , the large deviation of vSm from 3 near the magnetic phase boundary is also observed in other Sm compounds. SmS exhibits the pressure induced magnetic order from the intermediated valence state (the so called “gold phase”) at Pc ∼ 2 GPa [22], and vSm at Pc [20,23] is comparable to that of SmB6. SmOs4Sb12, which is the weak ferromagnet with TC = 3 K , also shows vSm ∼ 2.8 [21]. The peculiar behavior of vSm near the magnetic instability is common in some Sm base compounds, and therefore it
and the data were acquired as the pressure cell was heated from the lowest temperature ∼3 K . The calibration of pressure was performed at each temperature of the measurement using the fluorescence from ruby chips mounted with the sample inside the DAC. 3. Results and discussion Fig. 1(a)-(d) show Sm L 3-edge absorption spectra of SmB6 at different pressures or temperatures. The main peak at 6.722 keV and the shoulder like structure at 6.715 keV correspond to trivalent and divalent components of Sm ion, respectively. As temperature decreases at an almost constant low pressure (from Fig. 1(a) to (c)), the intensity of the main peak is gradually weakened and simultaneously the shoulder like structure is relatively enhanced. On the other hand, the opposite phenomenon is observed when pressure is increased at an almost constant temperature (Fig. 1(a) to (b), or Fig. 1(c) to (d)). These indicate that the Sm valence decreases toward the divalent state at low temperatures and pressures. The Sm valence vSm was estimated from the relative intensities of the divalent and trivalent components in the XAS spectra. Each component was modeled by the sum of Lorentz functions and an arctangent function representing the continuum excitations. The obtained value of vSm is shown in each figure of Fig. 1 as well as the results of fitting. In Fig. 2(a), we show the temperature dependence of resistivity ρ at P = 10.0 GPa . Our sample shows a drop in ρ just below 12 K and above 10 GPa, indicating the appearance of magnetic ordering. The phenomenon as well as the transition temperature and pressure are in good agreement with the previous report [4]. The temperature dependence of vSm at representative pressures is shown in Fig. 2(b). Here, we can regard pressures cramped in the pressure cell to be almost constant for T < 70 K . As temperature increases in the low pressure region, vSm increases with a downward curvature, while the temperature dependence tends to be suppressed at high pressures above 10 GPa (see also Fig. 1(b) and (d)). The vSm − T curves at all measured pressures are more monotonic and smoother over the whole temperature range than shown in a previous report [6]. For P ≥ 10 GPa , we find no anomaly in the vSm − T curves at magnetic ordering temperature (=12 K) within experimental accuracy, indicating that the onset of the magnetic order hardly involves any change in vSm . This may be consistent with the recent measurement of synchrotron powder X-ray diffraction [11]: the crystalline structure remains cubic in the ordered phase, and both lattice constant and Sm-B bond length do not show discontinuity at the
Fig. 3. Temperature dependences of vSm for SmS (2.3 GPa) [20] and for SmB6 (present data). We plotted the data near critical pressure of the nonmagnetic-magnetic transition for these compounds. The data for SmOs4Sb12 (ambient pressure) [21] which is the weak ferromagnet are also plotted. See text for details.
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dx.doi.org/10.1016/S0304-8853(97)00493-9. [9] N. Kawamura, N. Ishimatsu, H. Maruyama, X-ray magnetic spectroscopy at high pressure: performance of SPring-8 BL39XU, J. Synchrotron Radiat. 16 (2009) 730–736. http://dx.doi.org/10.1107/S0909049509034700. [10] N. Ishimatsu, K. Matsumoto, H. Maruyama, N. Kawamura, M. Mizumaki, H. Sumiya, T. Irifune, Glitch-free X-ray absorption spectrum under high pressure obtained using nano-polycrystalline diamond anvils, J. Synchrotron Radiat. 19 (2012) 768–772. http://dx.doi.org/10.1107/S0909049512026088. [11] P. Parisiades, M. Bremholm, M. Mezouar, High-pressure structural anomalies and electronic transitions in the topological kondo insulator SmB6, Europhys. Lett. 110 (2015) 66002. http://dx.doi.org/10.1209/0295-5075/110/66002. [12] H. Yamaoka, I. Jarrige, N. Tsujii, J.-F. Lin, N. Hiraoka, H. Ishii, K.-D. Tsuei, Temperature and pressure-induced valence transitions in YbNi2Ge2 and YbPd2Si2, Phys. Rev. B 82 (2010) 035111. http://dx.doi.org/10.1103/PhysRevB.82.035111. [13] A. Fernandez-Pañella, V. Balédent, D. Braithwaite, L. Paolasini, R. Verbeni, G. Lapertot, J.-P. Rueff, Valence instability of YbCu2Si2 through its magnetic quantum critical point, Phys. Rev. B 86 (2012) 125104. http://dx.doi.org/10.1103/ PhysRevB.86.125104. [14] H. Yamaoka, N. Tsujii, Y. Utsumi, H. Sato, I. Jarrige, Y. Yamamoto, J.-F. Lin, N. Hiraoka, H. Ishii, K.-D. Tsuei, J. Mizuki, Valence transitions in the heavyfermion compound YbCuAl as a function of temperature and pressure, Phys. Rev. B 87 (2013) 205120. http://dx.doi.org/10.1103/PhysRevB.87.205120. [15] O. Trovarelli, C. Geibel, S. Mederle, C. Langhammer, F. Grosche, P. Gegenwart, M. Lang, G. Sparn, F. Steglich, YbRh2Si2: pronounced non-fermi-liquid effects above a low-lying magnetic phase transition, Phys. Rev. Lett. 85 (2000) 626–629. http://dx.doi.org/10.1103/PhysRevLett.85.626. [16] P. Gegenwart, J. Custers, C. Geibel, K. Neumaier, T. Tayama, K. Tenya, O. Trovarelli, F. Steglich, Magnetic-field induced quantum critical point in YbRh2Si2, Phys. Rev. Lett. 89 (2002) 056402. http://dx.doi.org/10.1103/ PhysRevLett.89.056402. [17] H. Nakai, T. Ebihara, S. Tsutsui, M. Mizumaki, N. Kawamura, S. Michimura, T. Inami, T. Nakamura, A. Kondo, K. Kindo, Y. Matsuda, Temperature and magnetic field dependent Yb valence in YbRh2Si2 observed by X-ray absorption spectroscopy, J. Phys. Soc. Jpn. 82 (2013) 124712. http://dx.doi.org/10.7566/ JPSJ.82.124712. [18] H. Yamaoka, Y. Zekko, A. Kotani, I. Jarrige, N. Tsujii, J.-F. Lin, J. Mizuki, H. Abe, H. Kitazawa, N. Hiraoka, H. Ishii, K.-D. Tsuei, Electronic transitions in CePd2Si2 studied by resonant x-ray emission spectroscopy at high pressures and low temperatures, Phys. Rev. B 86 (2012) 235131. http://dx.doi.org/10.1103/ PhysRevB.86.235131. [19] H. Yamaoka, Y. Ikeda, I. Jarrige, N. Tsujii, Y. Zekko, Y. Yamamoto, J. Mizuki, J.F. Lin, N. Hiraoka, H. Ishii, K.-D. Tsuei, T. Kobayashi, F. Honda, Y. Ōnuki, Role of valence fluctuations in the superconductivity of Ce122 compounds, Phys. Rev. Lett. 113 (2014) 086403. http://dx.doi.org/10.1103/PhysRevLett.113.086403. [20] P. Deen, D. Braithwaite, N. Kernavanois, L. Paolasini, S. Raymond, A. Barla, G. Lapertot, J. Sanchez, Structural and electronic transitions in the low-temperature, high-pressure phase of SmS, Phys. Rev. B 71 (2005) 245118. http:// dx.doi.org/10.1103/PhysRevB.71.245118. [21] M. Mizumaki, S. Tsutsui, H. Tanida, T. Uruga, D. Kikuchi, H. Sugawara, H. Sato, The mixed valence states in the unconventional heavy fermion compound SmOs4Sb12, J. Phys. Soc. Jpn. 76 (2007) 053706. http://dx.doi.org/10.1143/ JPSJ.76.053706. [22] A. Barla, J. Sanchez, Y. Haga, G. Lapertot, B. Doyle, O. Leupold, R. Rüffer, M. AbdElmeguid, R. Lengsdorf, J. Flouquet, Pressure-induced magnetic order in golden SmS, Phys. Rev. Lett. 92 (2004) 066401. http://dx.doi.org/10.1103/ PhysRevLett.92.066401. [23] E. Annese, A. Barla, C. Dallera, G. Lapertot, J.-P. Sanchez, G. Vankó, Divalent-totrivalent transition of Sm in SmS: implications for the high-pressure magnetically ordered state, Phys. Rev. B 73 (2006) 140509. http://dx.doi.org/10.1103/ PhysRevB.73.140409. [24] M. Dzero, K. Sun, V. Galitski, P. Coleman, Topological kondo insulators, Phys. Rev. Lett. 104 (2010) 106408. http://dx.doi.org/10.1103/PhysRevLett.104.106408.
may not be characteristic of only SmB6 which has attracted as a candidate of topological Kondo insulator [24]. 4. Summary We have carried out XAS measurements on the intermediate compound SmB6 to estimate mean Sm valence near the phase boundary of the pressure induced nonmagnetic-magnetic transition. The Sm valence vSm increases with increasing pressure and temperature, however its temperature dependence is strongly suppressed at high pressures above 10 GPa. We found that vSm near the phase boundary is far below the trivalent state, which is markedly different from the cases of Yb and Ce compounds. The peculiar behavior of vSm seems to be common in some Sm compounds which possess electronic configuration between 4f 5 and 4f 6 with multi 4f electrons. Acknowledgment The authors are grateful to H. Sumiya and T. Irifune for providing the nano-polycrystalline diamond anvils. This work was supported by JSPS KAKENHI (Grant No. 24540349). The synchrotron radiation experiments were performed at the BL39XU of SPring-8 with the approval of the Japan Synchrotron Radiation Research Institute (JASRI) (Proposal Nos. 2014A1233, 2014B1564, and 2014B2041). One of the authors (N. E.) would like to acknowledge the support from the Motizuki Fund of Yukawa Memorial Foundation. References [1] T. Kasuya, K. Takegahara, T. Fujita, T. Tanaka, E. Bannai, Valence fluctuating state in SmB6, J. Phys. 40 (1979) C5308–C5313. http://dx.doi.org/10.1051/jphyscol:19795107. [2] M. Takigawa, H. Yasuoka, Y. Kitaoka, T. Tanaka, H. Nozaki, Y. Ishizawa, NMR study of a valence fluctuating compound SmB6, J. Phys. Soc. Jpn. 50 (1981) 2525–2532. http://dx.doi.org/10.1143/JPSJ.50.2525. [3] A. Yanase, H. Harima, Band calculations on YbB12, SmB6 and CeNiSn, Prog. Theor. Phys. Suppl. 108 (1992) 19–25. http://dx.doi.org/10.1143/PTPS.108.19. [4] J. Derr, G. Knebel, D. Braithwaite, B. Salce, J. Flouquet, K. Flachbart, S. Gabáni, N. Shitsevalova, From unconventional insulating behavior towards conventional magnetism in the intermediate-valence compound SmB6, Phys. Rev. B 77 (2008) 193107. http://dx.doi.org/10.1103/PhysRevB.77.193107. [5] A. Barla, J. Derr, J. Sanchez, B. Salce, G. Lapertot, B. Doyle, R. Rüffer, R. Lengsdorf, M. Abd-Elmeguid, J. Flouquet, High-pressure ground state of SmB6: electronic conduction and long range magnetic order, Phys. Rev. Lett. 94 (2005) 166401. http://dx.doi.org/10.1103/PhysRevLett.94.166401. [6] M. Mizumaki, S. Tsutsui, F. Iga, Temperature dependence of Sm valence in SmB6 studied by X-ray absorption spectroscopy, J. Phys.: Conf. Ser. 176 (2009) 012034. http://dx.doi.org/10.1088/1742-6596/176/1/012034. [7] N. Butch, J. Paglione, P. Chow, Y. Xiao, C. Marianetti, C. Booth, J. Jeffries, Pressure-resistant intermediate valence in the kondo insulator SmB6, Phys. Rev. Lett. 116 (2016) 156401. http://dx.doi.org/10.1103/PhysRevLett.116.156401. [8] F. Iga, N. Shimizu, T. Takabatake, Single crystal growth and physical properties of kondo insulator YbB12, J. Magn. Magn. Mater. 177–181 (1989) 337–338. http://
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Contents lists available at ScienceDirect
Physica B journal homepage: www.elsevier.com/locate/physb
Temperature and pressure dependences of Sm valence in intermediate valence compound SmB6 ⁎
N. Emia, T. Mitoa, , N. Kawamurab, M. Mizumakib, N. Ishimatsuc, G. Pristášd, T. Kagayamae, K. Shimizue, Y. Osanaif, F. Igag a
Department of Material Science, University of Hyogo, Hyogo 678-1297, Japan Japan Synchrotron Radiation Research Institute (JASRI), SPring-8, Hyogo 679-5198, Japan c Department of Science, Hiroshima University, Hiroshima 739-8526, Japan d Institute of Experimental Physics, Slovak Academy of Science, Koise 04001, Slovakia e Department of Engineering Science, Osaka University, Osaka 560-8531, Japan f Faculty of Science, Ibaraki University, Ibaraki 310-8512, Japan g Department of Science and Engineering, Ibaraki University, Ibaraki 310-8512, Japan b
A R T I C L E I N F O
A BS T RAC T
MSC: 00-01 99-00
We report the results of the X-ray absorption spectroscopy (XAS) on the intermediate valence compound SmB6. The XAS measurements were performed near the nonmagnetic-magnetic phase boundary. Mean Sm valence vSm was estimated from absorption spectra, and we found that vSm near the boundary (P ≥ 10 GPa and T ∼ 12 K ) is far below a trivalent state with magnetic characteristics. Although the result is markedly different from the cases of pressure induced magnetic orders in Yb and Ce compounds, it is likely that the large deviation from the trivalent state seems to be common in some Sm compounds which possess electronic configuration between 4f 5 and 4f 6 with multi 4f electrons.
Keywords: SmB6 Intermediate valence Kondo insulator XAS High pressure
1. Introduction The measurement of the valence of lanthanide ions is one of the most informative methods to evaluate the degree of localization in f electron systems: for example, in the case of Ce, Sm and Yb compounds, the trivalent state of the lanthanide ions corresponds to the strong localization of the f electrons, while an increase in a divalent component (a tetravalent component for Ce) indicates the delocalization. We report the measurement of X-ray absorption spectroscopy (XAS) on the prototypical intermediate valence compound SmB6 to investigate the pressure and temperature dependences of mean Sm valence. SmB6, having the Sm valence of ∼2.6 at room temperature and ambient pressure, shows a semiconducting property with a narrow gap of 50 − 100 K [1,2], whose origin is probably intimately related to effectively temperature dependent hybridization between conduction and f electrons [3]. The ground state of SmB6 drastically changes with pressure, namely the insulating gap collapses at Pc = 10 GPa [4] and simultaneously a magnetically ordered phase appears below ∼12 K [5]. The pressure dependence of the Sm valence in SmB6 has been previously studied at ambient pressure using XAS [6,7]. In this study,
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we have carried out the XAS measurement in the low temperature region where actually the magnetic order occurs, and the results are compared to pressure induced magnetic orders in Yb and Ce compounds as well as in other Sm compounds. 2. Experimental details Single crystalline samples of SmB6 were grown by a floating-zone method using an image furnace with four xenon lamps [8]. The appearance of the magnetically ordered phase in the sample under high pressure and low temperature (P ≥ 10 GPa and T ≤ 12 K ) was confirmed by the resistivity measurement using a single crystalline sample from the same batch. The XAS measurements near the Sm L 3-edge (6.72 keV) were performed at the beamline BL39XU of SPring8, Japan [9]. The XAS spectra were recorded in the transmission mode using ionization chambers. For high pressure measurements, the sample was loaded in a diamond anvil cell (DAC) filled with a mixture of 4:1 methanol:ethanol as a pressure-transmitting medium. Nanopolycrystalline diamond anvils were used to avoid glitches in the XAS spectra [10]. All pressures were applied at room temperature,
Corresponding author. E-mail address: [email protected] (T. Mito).
http://dx.doi.org/10.1016/j.physb.2017.09.053 Received 10 July 2017; Received in revised form 12 September 2017; Accepted 14 September 2017 0921-4526/ © 2017 Elsevier B.V. All rights reserved.
Please cite this article as: Emi, N., Physica B (2017), http://dx.doi.org/10.1016/j.physb.2017.09.053
Physica B xxx (xxxx) xxx–xxx
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Fig. 2. (a) Temperature dependence of ρ at 10.0 GPa measured using a sample taken from the same batch with that used for the XAS measurement. The arrow indicates magnetic ordering temperature. (b) Temperature dependence of vSm of SmB6 at 6.4, 10.3, and 12.1 GPa. The inset shows an expanded view of the data at 10.3 and 12.1 GPa below 20 K.
Fig. 1. Sm L3 -edge absorption spectra of SmB6 at (a) T = 300 K , P = 0.7 GPa , (b) T = 300 K , P = 10.3 GPa , (c) T = 3.9 K , P = 0.6 GPa , and (d) T = 3.3 K , P = 10.4 GPa . The opened circles represent experimental data. The dotted, broken, and solid lines indicate Sm divalent component, trivalent component, and a superposition of these components, respectively. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article).
phase boundary, although the authors observed changes in the pressure dependence of them. A striking feature in the present study is that vSm at the magnetic phase boundary is ∼2.75, far below the magnetic trivalent state. This is markedly different from the cases of pressure induced magnetic orders in Yb and Ce compounds. For example, in YbNi2Ge2 [12], YbPd2Si2 [13], YbAlCu [14], and YbCu2Si2 [12], Yb valence vYb is vYb ≥ 2.9 at critical pressure. The proximity of vYb to 3 implies the strong localization of 4f electrons. Also, in YbRh2Si2 which is thought to locate in the vicinity of nonmagnetic-magnetic criticality at ambient pressure [15,16], vYb is larger than 2.9 as well [17]. For Ce compounds, although there are only a few reports of the valence measurement under pressure so far, for example CePd2Si2 and CeCu2Ge2 are driven through the magneticsuperconducting transition by pressure, and their vCe remains from 3.0 to 3.05 over the superconducting region [18,19]. Thus we have found no serious exception in the Ce and Yb compounds in terms of the proximity to the trivalent state at the magnetic instability. On the other hand, as shown in Fig. 3 , the large deviation of vSm from 3 near the magnetic phase boundary is also observed in other Sm compounds. SmS exhibits the pressure induced magnetic order from the intermediated valence state (the so called “gold phase”) at Pc ∼ 2 GPa [22], and vSm at Pc [20,23] is comparable to that of SmB6. SmOs4Sb12, which is the weak ferromagnet with TC = 3 K , also shows vSm ∼ 2.8 [21]. The peculiar behavior of vSm near the magnetic instability is common in some Sm base compounds, and therefore it
and the data were acquired as the pressure cell was heated from the lowest temperature ∼3 K . The calibration of pressure was performed at each temperature of the measurement using the fluorescence from ruby chips mounted with the sample inside the DAC. 3. Results and discussion Fig. 1(a)-(d) show Sm L 3-edge absorption spectra of SmB6 at different pressures or temperatures. The main peak at 6.722 keV and the shoulder like structure at 6.715 keV correspond to trivalent and divalent components of Sm ion, respectively. As temperature decreases at an almost constant low pressure (from Fig. 1(a) to (c)), the intensity of the main peak is gradually weakened and simultaneously the shoulder like structure is relatively enhanced. On the other hand, the opposite phenomenon is observed when pressure is increased at an almost constant temperature (Fig. 1(a) to (b), or Fig. 1(c) to (d)). These indicate that the Sm valence decreases toward the divalent state at low temperatures and pressures. The Sm valence vSm was estimated from the relative intensities of the divalent and trivalent components in the XAS spectra. Each component was modeled by the sum of Lorentz functions and an arctangent function representing the continuum excitations. The obtained value of vSm is shown in each figure of Fig. 1 as well as the results of fitting. In Fig. 2(a), we show the temperature dependence of resistivity ρ at P = 10.0 GPa . Our sample shows a drop in ρ just below 12 K and above 10 GPa, indicating the appearance of magnetic ordering. The phenomenon as well as the transition temperature and pressure are in good agreement with the previous report [4]. The temperature dependence of vSm at representative pressures is shown in Fig. 2(b). Here, we can regard pressures cramped in the pressure cell to be almost constant for T < 70 K . As temperature increases in the low pressure region, vSm increases with a downward curvature, while the temperature dependence tends to be suppressed at high pressures above 10 GPa (see also Fig. 1(b) and (d)). The vSm − T curves at all measured pressures are more monotonic and smoother over the whole temperature range than shown in a previous report [6]. For P ≥ 10 GPa , we find no anomaly in the vSm − T curves at magnetic ordering temperature (=12 K) within experimental accuracy, indicating that the onset of the magnetic order hardly involves any change in vSm . This may be consistent with the recent measurement of synchrotron powder X-ray diffraction [11]: the crystalline structure remains cubic in the ordered phase, and both lattice constant and Sm-B bond length do not show discontinuity at the
Fig. 3. Temperature dependences of vSm for SmS (2.3 GPa) [20] and for SmB6 (present data). We plotted the data near critical pressure of the nonmagnetic-magnetic transition for these compounds. The data for SmOs4Sb12 (ambient pressure) [21] which is the weak ferromagnet are also plotted. See text for details.
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dx.doi.org/10.1016/S0304-8853(97)00493-9. [9] N. Kawamura, N. Ishimatsu, H. Maruyama, X-ray magnetic spectroscopy at high pressure: performance of SPring-8 BL39XU, J. Synchrotron Radiat. 16 (2009) 730–736. http://dx.doi.org/10.1107/S0909049509034700. [10] N. Ishimatsu, K. Matsumoto, H. Maruyama, N. Kawamura, M. Mizumaki, H. Sumiya, T. Irifune, Glitch-free X-ray absorption spectrum under high pressure obtained using nano-polycrystalline diamond anvils, J. Synchrotron Radiat. 19 (2012) 768–772. http://dx.doi.org/10.1107/S0909049512026088. [11] P. Parisiades, M. Bremholm, M. Mezouar, High-pressure structural anomalies and electronic transitions in the topological kondo insulator SmB6, Europhys. Lett. 110 (2015) 66002. http://dx.doi.org/10.1209/0295-5075/110/66002. [12] H. Yamaoka, I. Jarrige, N. Tsujii, J.-F. Lin, N. Hiraoka, H. Ishii, K.-D. Tsuei, Temperature and pressure-induced valence transitions in YbNi2Ge2 and YbPd2Si2, Phys. Rev. B 82 (2010) 035111. http://dx.doi.org/10.1103/PhysRevB.82.035111. [13] A. Fernandez-Pañella, V. Balédent, D. Braithwaite, L. Paolasini, R. Verbeni, G. Lapertot, J.-P. Rueff, Valence instability of YbCu2Si2 through its magnetic quantum critical point, Phys. Rev. B 86 (2012) 125104. http://dx.doi.org/10.1103/ PhysRevB.86.125104. [14] H. Yamaoka, N. Tsujii, Y. Utsumi, H. Sato, I. Jarrige, Y. Yamamoto, J.-F. Lin, N. Hiraoka, H. Ishii, K.-D. Tsuei, J. Mizuki, Valence transitions in the heavyfermion compound YbCuAl as a function of temperature and pressure, Phys. Rev. B 87 (2013) 205120. http://dx.doi.org/10.1103/PhysRevB.87.205120. [15] O. Trovarelli, C. Geibel, S. Mederle, C. Langhammer, F. Grosche, P. Gegenwart, M. Lang, G. Sparn, F. Steglich, YbRh2Si2: pronounced non-fermi-liquid effects above a low-lying magnetic phase transition, Phys. Rev. Lett. 85 (2000) 626–629. http://dx.doi.org/10.1103/PhysRevLett.85.626. [16] P. Gegenwart, J. Custers, C. Geibel, K. Neumaier, T. Tayama, K. Tenya, O. Trovarelli, F. Steglich, Magnetic-field induced quantum critical point in YbRh2Si2, Phys. Rev. Lett. 89 (2002) 056402. http://dx.doi.org/10.1103/ PhysRevLett.89.056402. [17] H. Nakai, T. Ebihara, S. Tsutsui, M. Mizumaki, N. Kawamura, S. Michimura, T. Inami, T. Nakamura, A. Kondo, K. Kindo, Y. Matsuda, Temperature and magnetic field dependent Yb valence in YbRh2Si2 observed by X-ray absorption spectroscopy, J. Phys. Soc. Jpn. 82 (2013) 124712. http://dx.doi.org/10.7566/ JPSJ.82.124712. [18] H. Yamaoka, Y. Zekko, A. Kotani, I. Jarrige, N. Tsujii, J.-F. Lin, J. Mizuki, H. Abe, H. Kitazawa, N. Hiraoka, H. Ishii, K.-D. Tsuei, Electronic transitions in CePd2Si2 studied by resonant x-ray emission spectroscopy at high pressures and low temperatures, Phys. Rev. B 86 (2012) 235131. http://dx.doi.org/10.1103/ PhysRevB.86.235131. [19] H. Yamaoka, Y. Ikeda, I. Jarrige, N. Tsujii, Y. Zekko, Y. Yamamoto, J. Mizuki, J.F. Lin, N. Hiraoka, H. Ishii, K.-D. Tsuei, T. Kobayashi, F. Honda, Y. Ōnuki, Role of valence fluctuations in the superconductivity of Ce122 compounds, Phys. Rev. Lett. 113 (2014) 086403. http://dx.doi.org/10.1103/PhysRevLett.113.086403. [20] P. Deen, D. Braithwaite, N. Kernavanois, L. Paolasini, S. Raymond, A. Barla, G. Lapertot, J. Sanchez, Structural and electronic transitions in the low-temperature, high-pressure phase of SmS, Phys. Rev. B 71 (2005) 245118. http:// dx.doi.org/10.1103/PhysRevB.71.245118. [21] M. Mizumaki, S. Tsutsui, H. Tanida, T. Uruga, D. Kikuchi, H. Sugawara, H. Sato, The mixed valence states in the unconventional heavy fermion compound SmOs4Sb12, J. Phys. Soc. Jpn. 76 (2007) 053706. http://dx.doi.org/10.1143/ JPSJ.76.053706. [22] A. Barla, J. Sanchez, Y. Haga, G. Lapertot, B. Doyle, O. Leupold, R. Rüffer, M. AbdElmeguid, R. Lengsdorf, J. Flouquet, Pressure-induced magnetic order in golden SmS, Phys. Rev. Lett. 92 (2004) 066401. http://dx.doi.org/10.1103/ PhysRevLett.92.066401. [23] E. Annese, A. Barla, C. Dallera, G. Lapertot, J.-P. Sanchez, G. Vankó, Divalent-totrivalent transition of Sm in SmS: implications for the high-pressure magnetically ordered state, Phys. Rev. B 73 (2006) 140509. http://dx.doi.org/10.1103/ PhysRevB.73.140409. [24] M. Dzero, K. Sun, V. Galitski, P. Coleman, Topological kondo insulators, Phys. Rev. Lett. 104 (2010) 106408. http://dx.doi.org/10.1103/PhysRevLett.104.106408.
may not be characteristic of only SmB6 which has attracted as a candidate of topological Kondo insulator [24]. 4. Summary We have carried out XAS measurements on the intermediate compound SmB6 to estimate mean Sm valence near the phase boundary of the pressure induced nonmagnetic-magnetic transition. The Sm valence vSm increases with increasing pressure and temperature, however its temperature dependence is strongly suppressed at high pressures above 10 GPa. We found that vSm near the phase boundary is far below the trivalent state, which is markedly different from the cases of Yb and Ce compounds. The peculiar behavior of vSm seems to be common in some Sm compounds which possess electronic configuration between 4f 5 and 4f 6 with multi 4f electrons. Acknowledgment The authors are grateful to H. Sumiya and T. Irifune for providing the nano-polycrystalline diamond anvils. This work was supported by JSPS KAKENHI (Grant No. 24540349). The synchrotron radiation experiments were performed at the BL39XU of SPring-8 with the approval of the Japan Synchrotron Radiation Research Institute (JASRI) (Proposal Nos. 2014A1233, 2014B1564, and 2014B2041). One of the authors (N. E.) would like to acknowledge the support from the Motizuki Fund of Yukawa Memorial Foundation. References [1] T. Kasuya, K. Takegahara, T. Fujita, T. Tanaka, E. Bannai, Valence fluctuating state in SmB6, J. Phys. 40 (1979) C5308–C5313. http://dx.doi.org/10.1051/jphyscol:19795107. [2] M. Takigawa, H. Yasuoka, Y. Kitaoka, T. Tanaka, H. Nozaki, Y. Ishizawa, NMR study of a valence fluctuating compound SmB6, J. Phys. Soc. Jpn. 50 (1981) 2525–2532. http://dx.doi.org/10.1143/JPSJ.50.2525. [3] A. Yanase, H. Harima, Band calculations on YbB12, SmB6 and CeNiSn, Prog. Theor. Phys. Suppl. 108 (1992) 19–25. http://dx.doi.org/10.1143/PTPS.108.19. [4] J. Derr, G. Knebel, D. Braithwaite, B. Salce, J. Flouquet, K. Flachbart, S. Gabáni, N. Shitsevalova, From unconventional insulating behavior towards conventional magnetism in the intermediate-valence compound SmB6, Phys. Rev. B 77 (2008) 193107. http://dx.doi.org/10.1103/PhysRevB.77.193107. [5] A. Barla, J. Derr, J. Sanchez, B. Salce, G. Lapertot, B. Doyle, R. Rüffer, R. Lengsdorf, M. Abd-Elmeguid, J. Flouquet, High-pressure ground state of SmB6: electronic conduction and long range magnetic order, Phys. Rev. Lett. 94 (2005) 166401. http://dx.doi.org/10.1103/PhysRevLett.94.166401. [6] M. Mizumaki, S. Tsutsui, F. Iga, Temperature dependence of Sm valence in SmB6 studied by X-ray absorption spectroscopy, J. Phys.: Conf. Ser. 176 (2009) 012034. http://dx.doi.org/10.1088/1742-6596/176/1/012034. [7] N. Butch, J. Paglione, P. Chow, Y. Xiao, C. Marianetti, C. Booth, J. Jeffries, Pressure-resistant intermediate valence in the kondo insulator SmB6, Phys. Rev. Lett. 116 (2016) 156401. http://dx.doi.org/10.1103/PhysRevLett.116.156401. [8] F. Iga, N. Shimizu, T. Takabatake, Single crystal growth and physical properties of kondo insulator YbB12, J. Magn. Magn. Mater. 177–181 (1989) 337–338. http://
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