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The Influence of Hydrostatic Pressure on Intermediate Europium Valence State in Compound EuCu2Ge2
Available online at www.sciencedirect.com
ScienceDirect Physics Procedia 71 (2015) 308 – 312
18th Conference on Plasma-Surface Interactions, PSI 2015, 5-6 February 2015, Moscow, Russian Federation and the 1st Conference on Plasma and Laser Research and Technologies, PLRT 2015, 18-20 February 2015
The influence of hydrostatic pressure on intermediate europium valence state in compound EuCu2Ge2 A. P. Menushenkova*, A. A. Yaroslatseva,b,c, A. Y. Geondzhiana, P. A. Alekseeva,d, R. V. Chernikove, B. Gaynanova, F. Baudeletf, L. Nataff a
National Research Nuclear University MEPhI (Moscow Engineering Physics Institute), Kashirskoe sh. 31, Moscow 115409, Russia b European XFEL GmbH, Albert-Einstein-Ring 19, 22761 Hamburg, Germany, c I. Institut für Theoretische Physik, Universität Hamburg, Jungiusstraße 9, 20355 Hamburg, Germany, d National Research Centre “Kurchatov Institute”,Kurchatov sq., 123182 Moscow, Russia, e DESY Photon Science,Notkestr., 85, D-22607 Hamburg, Germany, f Synchrotron SOLEIL, l’Orme des Merisiers, 91190 Saint-Aubin, France
Abstract We report the high-pressure XANES study of europium intermediate valence in the rare-earth compound EuСu2Ge2. The hydrostatic pressure-dependence of europium valence was obtained in a wide pressure range (1-30 GPa) at room temperature. The saturation region of the europium valence was found at the pressure higher than 20 GPa, when the valence reaches the value of 2.89. The study compared the mechanisms of external and “chemical” pressure by the Si substitution in Ge site in series EuCu2(SixGe1-x)2. The experimental results were supported by band structure calculations in the framework of DFT, which allowed us to discuss the features of 3d-4f hybridization model for this system. © Published by by Elsevier B.V.B.V. This is an open access article under the CC BY-NC-ND license ©2015 2015The TheAuthors. Authors. Published Elsevier (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the N ational R esearch Nuclear U niversity M E P hI (M oscow E ngineering Peer-review under responsibility of the National Research Nuclear University MEPhI (Moscow Engineering Physics Institute)
P hys ics Institute) .
Keywords: EuCu2Ge2; intermediate valence state; XANES; high pressure
* Corresponding author. Tel.: +7-495-788-56-99 ext. 9020. E-mail address: [email protected]
1875-3892 © 2015 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the National Research Nuclear University MEPhI (Moscow Engineering Physics Institute) doi:10.1016/j.phpro.2015.08.332
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A.P. Menushenkov et al. / Physics Procedia 71 (2015) 308 – 312
Introduction Compounds EuCu2X2 (X =Si, Ge) crystallize in the ThCr2Si2-type structure (tetragonal space group I4/mmm). The compounds of this type are famous for exhibiting a wide range of fascinating phenomena such as valence fluctuations, “heavy-fermion superconductivity”, non-Fermi liquid states, Kondo insulators, etc. Alekseev et al. (2012), Hossain et al. (2004). Due to the high density of states at the Fermi level these compounds are highly sensitive to the impacts like electron/hole doping or pressure, which lead to the modification of their electronic structure and various phase transitions. That is how the superconductivity was recently found in EuFe 2As2 under compression Miclea et al. (2009). The compound EuCu2Ge2 is antiferromagnetic (TN ≈ 15 K) at ambient conditions and at the same time shows the intermediate europium valence state with a value of 2.15.
Eu
Eu Ge
Cu
Si
Cu
Fig. 1. Left: phase diagram of the transition from antiferromagnetic state to strong valence fluctuating state; Right: crystal structure of compounds EuCu2Ge2 and EuCu2Si2.
Recently it has been reported by Alekseev et al. (2012) that silicon substitution in Ge site in series EuCu2(SixGe1leads to a chemical compression and induces the transition from the magnetically ordered state (x < 0.65) to a heavy-fermion state (0.65 < x < 0.85) and then to a nonmagnetic strong valence fluctuating state which is accompanied by the Eu valence increase from 2.15 in EuCu2Ge2 to 2.67 in EuCu2Si2. The suppression of the magnetic ordering may be due to the predominance of the contribution from non-magnetic Eu3+ (4݂ , J = S = 0). Thus, the europium valence might be an indicator of certain electronic and magnetic properties in this system. In this way we decided to investigate the valence in pure parent compound EuCu 2Ge2 under the high hydrostatic pressure and to compare results with the chemical pressure effects, observed in EuCu 2(SixGe1-x)2. x) 2
Experiment The X-ray absorption experiment was conducted at the hard X-ray energy dispersive beamline ODE of synchrotron SOLEIL (France). The high pressure was generated by the diamond anvil cell (DAC) and controlled using the ruby luminescence. The use of single-crystal DAC limited the quality of experimental signal due to the spurious contributions from the Bragg reflections. Finally, the XANES spectra were measured at Eu-L2 absorption edge in a wide pressure range 1-30 GPa. The separation of the spectral components arising from different europium valence states was done by fitting the experimental XANES, corrected for a polynomial background, with combination of Lorentzian (representing the absorption resonance) and arctangent (describing the transitions to the continuum) curves of constrained widths and energy positions. In this approach the amount of Eu ions in each valence state is assumed to be proportional to the area under the corresponding Lorentzian curve or, equivalently, to its weight in the fitting formula. Europium
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valence state at ambient pressure in EuCu2(SixGe1-x)2 was obtained by us early using the XAFS study above the EuL3 absorption edge Alekseev et al. (2014). Results and Discussion The results of the pressure dependence of europium valence are presented on Fig. 1. In the graph we can point out three regions. The first region extends from 1 to 6 GPa, where the valence is slightly changed with increasing pressure. Probably the cause of the slight increase of the valence is the exchange-spin interaction due to the long magnetic order. Note that the europium valence of transition between the first and the second regions (2,3) corresponds to suppression of the magnetic order in the series EuCu 2(SixGe1-x)2. Further the strong magnetic interaction is replaced by pure valence fluctuations and in the second region, europium valence begins to grow rapidly. However, most interesting is the region after 20 GPa where europium valence goes into saturation. The biggest value of europium valence (2,87) obtained under the pressure of 30 GPa in the compound EuCu 2Ge2 is very close to the value of europium valence in the compound EuCu2Si2 at low temperature (4 K) Alekseev et al. (2012), Fukuda et al. (2003). In such a way, the following question arises: does the europium valence in these compounds reach the value +3? As Fig. 2 (left) shows the “chemical” (top horizontal axis) and external (lower horizontal axis) pressure has a nonlinear relation and consequently a different mechanism of influence on europium valence. This fact was also confirmed by the DFT calculation of the electronic structures of the compounds EuCu2Ge2 and EuCu2Si2. At ambient pressure 3d level of Cu in the compound EuCu2Si2 is higher than 3d Cu level in the compound EuCu2Ge2 and with increasing pressure 3d level of Cu goes down from the Fermi level. Thus EuCu2Ge2 under pressure and EuCu2Si2 have different energy position of 3d Cu levels. But in spite of that the saturation for europium valence under high pressure was obtained in the compound EuCu2Si2, which was studied by inelastic neutron scattering Alekseev et al. (2008).
III
II I
Fig.2 Left: L2 absorption edge of Eu in EuCu2Ge2 at room temperature and pressure 10 GPa. Right: The comprehensive diagram of Eu valence behavior in system EuCu2(SixGe1-x)2 vs. temperature, Si concentration and applied pressure, obtained in our X-ray absorption experiments. I – region where the magnetic order persists; II – region of high hybridization; III – saturation region. The area of the greatest response of the electronic systems is marked in red.
A.P. Menushenkov et al. / Physics Procedia 71 (2015) 308 – 312
311
It was expected that the application of external pressure on the order of 2 GPa would lead to a noticeable change in the valence (quantitative evaluation of the relation between valence and pressure up to 5 GPa for EuCu 2Si2 was given in Ref. Alekseev et al. (2007)), which is experimentally manifested in renormalization of the magnetic spectrum. However, external pressure did not produce a significant change in the structure of the magnetic response nor in its temperature dependence. This fact can be explained in the way of compensation of two main reasons for renormalization of the magnetic spectrum. On the one hand, it is a high hybridization of f-electrons with the conduction band states, and on the other, it is an opposite renormalization expected due to the shift of the average valence to 3. Therefore it is possible that the compound EuCu2Si2 has only quantitative difference but similar (with EuCu2Ge2) physical reasons for the saturation of the europium valence under high pressure .
Fig. 3.Density of state for EuCu2Ge2 for different volume of elementary cell (from left to right: cell’s scale =100%, 98 %, 95 %)
Ab initio electronic structure calculations were performed within the density-functional theory (DFT) using a very accurate full-potential linearized augmented plane-wave (FP-LAPW) approach using the EXCITING code Gulans et al. (2014). The Perdew, Burke, Ernzerhof (PBE) gradient corrected local spin density approximation (LSDA-GGA) for the exchange correlation (XC) potential was used. To account for the Coulomb correlation interaction within the Eu-4f shell, the PBE XC potential corrected according to GGA+U method was additionally considered. For Eu, the values of U and J parameters were taken to be 2.7 and 1.1 eV, respectively. The pressure was modeled by the variation of the elementary cell’s volume. Basing on the DFT calculation, we suggest a qualitative model for the formation of an intermediate valance state of Eu due to hybridization of Cu-3d and Eu-4f electronic level. In this model 4f localized level of Eu splits into few components and one of them overlaps with 3d level of Cu under pressure. The saturation region of the europium valence at high pressure due to the saturation of the 4f-3d levels overlap when 4f level reaches the middle of 3d level. According to this model, the value +3 of europium valence in the compound EuCu 2Ge2 is unreachable. 1. Conclusion The pressure dependence of europium valence in EuCu2Ge2 was firstly obtained in a wide range of pressure by using the XANES spectroscopy at beamline ODE of synchrotron SOLEIL (France). It was found that the mechanisms of the “chemical” and external pressure have different ways of the influence on europium valence. However, the saturation of europium valence near the value of 2.9 was obtained for both silicon and germaniumbased compounds. The region of saturation was described in the way of 4f-3d hybridization. The qualitative model based on the DFT calculation predicts a decrease in the average europium valence with further increase of pressure.
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Acknowledgements This work is supported by Russian Science Foundation (grant 14-22-00098).
Reference Alekseev, P.A., Nemkovski, K.S., Mignot, J.-M. et al., 2007. JETP 105 (2007) 14. Alekseev, P.A., Mignot, J.-M., Nemkovski, K.S., et al., 2008. Physica B 403 (2008) 864. Alekseev, P.A., Nemkovski, K.S., Mignot, J.-M. et al., 2012. J. Phys.: Condens. Matter 24, 375601. Alekseev, P.A., Nemkovski, K.S., Kozlenko, D.P., et al., 2014. JETP Lett. 99, 164. Fukuda, Sh., Nakanuma, Y., Sakurai, J., et al., 2003. J. Phys. Soc. Jap. 72(12), 3189. Gulans, A., Kontur, S., Meisenbichler, C., et al., 2014. J. Phys.: Condens. Matter 26, 363202. Hossain, Z., Geibel, C., Senthilkumaran, N. et al. , 2004. Phys. Rev. B. 69, 014422. Miclea, C.F., Nicklas, M., Jeevan, H.S., 2009. Phys. Rev. B 79, 212509.
ScienceDirect Physics Procedia 71 (2015) 308 – 312
18th Conference on Plasma-Surface Interactions, PSI 2015, 5-6 February 2015, Moscow, Russian Federation and the 1st Conference on Plasma and Laser Research and Technologies, PLRT 2015, 18-20 February 2015
The influence of hydrostatic pressure on intermediate europium valence state in compound EuCu2Ge2 A. P. Menushenkova*, A. A. Yaroslatseva,b,c, A. Y. Geondzhiana, P. A. Alekseeva,d, R. V. Chernikove, B. Gaynanova, F. Baudeletf, L. Nataff a
National Research Nuclear University MEPhI (Moscow Engineering Physics Institute), Kashirskoe sh. 31, Moscow 115409, Russia b European XFEL GmbH, Albert-Einstein-Ring 19, 22761 Hamburg, Germany, c I. Institut für Theoretische Physik, Universität Hamburg, Jungiusstraße 9, 20355 Hamburg, Germany, d National Research Centre “Kurchatov Institute”,Kurchatov sq., 123182 Moscow, Russia, e DESY Photon Science,Notkestr., 85, D-22607 Hamburg, Germany, f Synchrotron SOLEIL, l’Orme des Merisiers, 91190 Saint-Aubin, France
Abstract We report the high-pressure XANES study of europium intermediate valence in the rare-earth compound EuСu2Ge2. The hydrostatic pressure-dependence of europium valence was obtained in a wide pressure range (1-30 GPa) at room temperature. The saturation region of the europium valence was found at the pressure higher than 20 GPa, when the valence reaches the value of 2.89. The study compared the mechanisms of external and “chemical” pressure by the Si substitution in Ge site in series EuCu2(SixGe1-x)2. The experimental results were supported by band structure calculations in the framework of DFT, which allowed us to discuss the features of 3d-4f hybridization model for this system. © Published by by Elsevier B.V.B.V. This is an open access article under the CC BY-NC-ND license ©2015 2015The TheAuthors. Authors. Published Elsevier (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the N ational R esearch Nuclear U niversity M E P hI (M oscow E ngineering Peer-review under responsibility of the National Research Nuclear University MEPhI (Moscow Engineering Physics Institute)
P hys ics Institute) .
Keywords: EuCu2Ge2; intermediate valence state; XANES; high pressure
* Corresponding author. Tel.: +7-495-788-56-99 ext. 9020. E-mail address: [email protected]
1875-3892 © 2015 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the National Research Nuclear University MEPhI (Moscow Engineering Physics Institute) doi:10.1016/j.phpro.2015.08.332
309
A.P. Menushenkov et al. / Physics Procedia 71 (2015) 308 – 312
Introduction Compounds EuCu2X2 (X =Si, Ge) crystallize in the ThCr2Si2-type structure (tetragonal space group I4/mmm). The compounds of this type are famous for exhibiting a wide range of fascinating phenomena such as valence fluctuations, “heavy-fermion superconductivity”, non-Fermi liquid states, Kondo insulators, etc. Alekseev et al. (2012), Hossain et al. (2004). Due to the high density of states at the Fermi level these compounds are highly sensitive to the impacts like electron/hole doping or pressure, which lead to the modification of their electronic structure and various phase transitions. That is how the superconductivity was recently found in EuFe 2As2 under compression Miclea et al. (2009). The compound EuCu2Ge2 is antiferromagnetic (TN ≈ 15 K) at ambient conditions and at the same time shows the intermediate europium valence state with a value of 2.15.
Eu
Eu Ge
Cu
Si
Cu
Fig. 1. Left: phase diagram of the transition from antiferromagnetic state to strong valence fluctuating state; Right: crystal structure of compounds EuCu2Ge2 and EuCu2Si2.
Recently it has been reported by Alekseev et al. (2012) that silicon substitution in Ge site in series EuCu2(SixGe1leads to a chemical compression and induces the transition from the magnetically ordered state (x < 0.65) to a heavy-fermion state (0.65 < x < 0.85) and then to a nonmagnetic strong valence fluctuating state which is accompanied by the Eu valence increase from 2.15 in EuCu2Ge2 to 2.67 in EuCu2Si2. The suppression of the magnetic ordering may be due to the predominance of the contribution from non-magnetic Eu3+ (4݂ , J = S = 0). Thus, the europium valence might be an indicator of certain electronic and magnetic properties in this system. In this way we decided to investigate the valence in pure parent compound EuCu 2Ge2 under the high hydrostatic pressure and to compare results with the chemical pressure effects, observed in EuCu 2(SixGe1-x)2. x) 2
Experiment The X-ray absorption experiment was conducted at the hard X-ray energy dispersive beamline ODE of synchrotron SOLEIL (France). The high pressure was generated by the diamond anvil cell (DAC) and controlled using the ruby luminescence. The use of single-crystal DAC limited the quality of experimental signal due to the spurious contributions from the Bragg reflections. Finally, the XANES spectra were measured at Eu-L2 absorption edge in a wide pressure range 1-30 GPa. The separation of the spectral components arising from different europium valence states was done by fitting the experimental XANES, corrected for a polynomial background, with combination of Lorentzian (representing the absorption resonance) and arctangent (describing the transitions to the continuum) curves of constrained widths and energy positions. In this approach the amount of Eu ions in each valence state is assumed to be proportional to the area under the corresponding Lorentzian curve or, equivalently, to its weight in the fitting formula. Europium
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A.P. Menushenkov et al. / Physics Procedia 71 (2015) 308 – 312
valence state at ambient pressure in EuCu2(SixGe1-x)2 was obtained by us early using the XAFS study above the EuL3 absorption edge Alekseev et al. (2014). Results and Discussion The results of the pressure dependence of europium valence are presented on Fig. 1. In the graph we can point out three regions. The first region extends from 1 to 6 GPa, where the valence is slightly changed with increasing pressure. Probably the cause of the slight increase of the valence is the exchange-spin interaction due to the long magnetic order. Note that the europium valence of transition between the first and the second regions (2,3) corresponds to suppression of the magnetic order in the series EuCu 2(SixGe1-x)2. Further the strong magnetic interaction is replaced by pure valence fluctuations and in the second region, europium valence begins to grow rapidly. However, most interesting is the region after 20 GPa where europium valence goes into saturation. The biggest value of europium valence (2,87) obtained under the pressure of 30 GPa in the compound EuCu 2Ge2 is very close to the value of europium valence in the compound EuCu2Si2 at low temperature (4 K) Alekseev et al. (2012), Fukuda et al. (2003). In such a way, the following question arises: does the europium valence in these compounds reach the value +3? As Fig. 2 (left) shows the “chemical” (top horizontal axis) and external (lower horizontal axis) pressure has a nonlinear relation and consequently a different mechanism of influence on europium valence. This fact was also confirmed by the DFT calculation of the electronic structures of the compounds EuCu2Ge2 and EuCu2Si2. At ambient pressure 3d level of Cu in the compound EuCu2Si2 is higher than 3d Cu level in the compound EuCu2Ge2 and with increasing pressure 3d level of Cu goes down from the Fermi level. Thus EuCu2Ge2 under pressure and EuCu2Si2 have different energy position of 3d Cu levels. But in spite of that the saturation for europium valence under high pressure was obtained in the compound EuCu2Si2, which was studied by inelastic neutron scattering Alekseev et al. (2008).
III
II I
Fig.2 Left: L2 absorption edge of Eu in EuCu2Ge2 at room temperature and pressure 10 GPa. Right: The comprehensive diagram of Eu valence behavior in system EuCu2(SixGe1-x)2 vs. temperature, Si concentration and applied pressure, obtained in our X-ray absorption experiments. I – region where the magnetic order persists; II – region of high hybridization; III – saturation region. The area of the greatest response of the electronic systems is marked in red.
A.P. Menushenkov et al. / Physics Procedia 71 (2015) 308 – 312
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It was expected that the application of external pressure on the order of 2 GPa would lead to a noticeable change in the valence (quantitative evaluation of the relation between valence and pressure up to 5 GPa for EuCu 2Si2 was given in Ref. Alekseev et al. (2007)), which is experimentally manifested in renormalization of the magnetic spectrum. However, external pressure did not produce a significant change in the structure of the magnetic response nor in its temperature dependence. This fact can be explained in the way of compensation of two main reasons for renormalization of the magnetic spectrum. On the one hand, it is a high hybridization of f-electrons with the conduction band states, and on the other, it is an opposite renormalization expected due to the shift of the average valence to 3. Therefore it is possible that the compound EuCu2Si2 has only quantitative difference but similar (with EuCu2Ge2) physical reasons for the saturation of the europium valence under high pressure .
Fig. 3.Density of state for EuCu2Ge2 for different volume of elementary cell (from left to right: cell’s scale =100%, 98 %, 95 %)
Ab initio electronic structure calculations were performed within the density-functional theory (DFT) using a very accurate full-potential linearized augmented plane-wave (FP-LAPW) approach using the EXCITING code Gulans et al. (2014). The Perdew, Burke, Ernzerhof (PBE) gradient corrected local spin density approximation (LSDA-GGA) for the exchange correlation (XC) potential was used. To account for the Coulomb correlation interaction within the Eu-4f shell, the PBE XC potential corrected according to GGA+U method was additionally considered. For Eu, the values of U and J parameters were taken to be 2.7 and 1.1 eV, respectively. The pressure was modeled by the variation of the elementary cell’s volume. Basing on the DFT calculation, we suggest a qualitative model for the formation of an intermediate valance state of Eu due to hybridization of Cu-3d and Eu-4f electronic level. In this model 4f localized level of Eu splits into few components and one of them overlaps with 3d level of Cu under pressure. The saturation region of the europium valence at high pressure due to the saturation of the 4f-3d levels overlap when 4f level reaches the middle of 3d level. According to this model, the value +3 of europium valence in the compound EuCu 2Ge2 is unreachable. 1. Conclusion The pressure dependence of europium valence in EuCu2Ge2 was firstly obtained in a wide range of pressure by using the XANES spectroscopy at beamline ODE of synchrotron SOLEIL (France). It was found that the mechanisms of the “chemical” and external pressure have different ways of the influence on europium valence. However, the saturation of europium valence near the value of 2.9 was obtained for both silicon and germaniumbased compounds. The region of saturation was described in the way of 4f-3d hybridization. The qualitative model based on the DFT calculation predicts a decrease in the average europium valence with further increase of pressure.
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A.P. Menushenkov et al. / Physics Procedia 71 (2015) 308 – 312
Acknowledgements This work is supported by Russian Science Foundation (grant 14-22-00098).
Reference Alekseev, P.A., Nemkovski, K.S., Mignot, J.-M. et al., 2007. JETP 105 (2007) 14. Alekseev, P.A., Mignot, J.-M., Nemkovski, K.S., et al., 2008. Physica B 403 (2008) 864. Alekseev, P.A., Nemkovski, K.S., Mignot, J.-M. et al., 2012. J. Phys.: Condens. Matter 24, 375601. Alekseev, P.A., Nemkovski, K.S., Kozlenko, D.P., et al., 2014. JETP Lett. 99, 164. Fukuda, Sh., Nakanuma, Y., Sakurai, J., et al., 2003. J. Phys. Soc. Jap. 72(12), 3189. Gulans, A., Kontur, S., Meisenbichler, C., et al., 2014. J. Phys.: Condens. Matter 26, 363202. Hossain, Z., Geibel, C., Senthilkumaran, N. et al. , 2004. Phys. Rev. B. 69, 014422. Miclea, C.F., Nicklas, M., Jeevan, H.S., 2009. Phys. Rev. B 79, 212509.