- Email: [email protected]
Magnetic and lattice excitations in intermediate-valence EuCu2Si2
ARTICLE IN PRESS
Physica B 403 (2008) 864–865 www.elsevier.com/locate/physb
Magnetic and lattice excitations in intermediate-valence EuCu2Si2 Pavel A. Alekseeva, Jean-Michel Mignotb, Kirill S. Nemkovskia, Elena V. Nefeodovaa,, Vladimir N. Lazukova, Denis Yu. Karpunina, Robert I. Bewleyc, Alexandr V. Gribanovd a RRC ‘‘Kurchatov Intitute’’, Kurchatov sq., 123182 Moscow, Russia Laboratoire Le´on Brillouin, CEA-CNRS, CEA/Saclay, 91191 Gif sur Yvette, France c Rutherford Appleton Laboratory, Chilton, Didcot, Oxon OX11 0QX, UK d Department of Chemistry, Moscow State University, Leninskie Gory, 119899 Moscow, Russia b
Abstract The magnetic and phonon excitation spectra in the intermediate-valence compound EuCu2Si2 have been studied by inelastic neutron scattering in a wide temperature range and under pressures up to 20 kbar. r 2007 Elsevier B.V. All rights reserved. Keywords: Intermediate valence; Neutron scattering; Magnetic and Phonon excitation
EuCu2Si2 is an intermetallic intermediate-valence (IV) system with a strong dependence of the Eu valence on temperature. It was found in a recent inelastic neutron scattering (INS) experiment that EuCu2Si2 exhibits an unusual magnetic excitation spectrum [1]. This spectrum consists of a double peak at about 35 meV at low temperature, with a quasielastic component appearing for T4100 K. The observed structure cannot be explained in terms of crystal-field effects on the Eu ions and most likely results from hybridization processes associated with the valence instability. Indications for a possible influence of the IV regime on the phonon spectrum in EuCu2Si2 have been reported in Ref. [2] based on a bulk modulus anomaly. In this paper, we present a study of (i) the magnetic response of EuCu2Si2 under external pressure, (ii) the influence of the valence instability on phonon excitations in this compound. The hydrostatic pressure shifts the Eu valence further towards the trivalent state [3]. It could be anticipated that the anomalous magnetic spectral shape might transform to a more conventional pattern defined by the spin-orbit (SO) splitting (46 meV for Eu3+) and crystal-field effects.
Corresponding author. Tel.: +7 499 196 7662; fax: +7 499 196 5973
E-mail address: [email protected] (E.V. Nefeodova). 0921-4526/$ - see front matter r 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.physb.2007.10.049
The neutron scattering measurements were performed on the time-of-flight spectrometer HET (ISIS, RAL) operated at the incident neutron energy Ei ¼ 100 meV. Powder samples of EuCu2Si2 (prepared using low-absorption 153 Eu isotope) and LaCu2Si2 (serving as a reference compound) were measured in the temperature range T ¼ 6–250 K. Pressures up to 20 kbar were generated in a McWhan-type pressure cell. For subtracting the background the signal from the empty pressure cell was measured at T ¼ 100 and 230 K, as well as the ambientpressure signal at T ¼ 230 K with the sample inside the pressure cell. Surprisingly, a pressure on the order of 20 kbar (or the increase of the Eu valence) does not produce a significant change in the structure of the magnetic response nor in its temperature dependence. In particular, no shift was observed in the magnetic response towards the energy of the Eu3+ SO transition (46 meV). The temperature dependences of the two magnetic excitation energies at P ¼ 0 and 20 kbar are presented in Fig. 1. It is seen that the application of pressure results in a minor increase in the energy of the higher magnetic excitation (M2) at higher temperature, and in a larger decrease in that of the lower magnetic excitation (M1). The splitting between the two magnetic peaks is therefore increased, and this is the main experimental observation. It can be traced back to the
ARTICLE IN PRESS P.A. Alekseev et al. / Physica B 403 (2008) 864–865
EuCu2Si2
Energy (meV)
40
M2
30 M1 P=20 kbar P=0 20
M1
M2 P=20 kbar P=0 0
100 200 Temperature (K)
Fig. 1. Energies of the two magnetic excitation in EuCu2Si2 at P ¼ 0 and 20 kbar as a function of temperature.
S(Q, E) (meV-1)
0
20
40
60
1
This work was supported by RFBR grant no. 05-0216426. References [1] [2] [3] [4] [5]
0 S(Q, E) (meV-1)
intensity ratio of the two peaks near 16 meV. The observed temperature behavior is not a direct consequence of the lattice expansion and could be indicative of a coupling between the valence fluctuations and the lattice vibrations. In summary, the magnetic response under pressure and the phonon spectrum in EuCu2Si2 have been studied by INS in a wide temperature range. It can be suggested that the higher magnetic excitation is the renormalized SO transition by analogy with Ce-based systems [4]. The lower magnetic excitation seems to be exciton-like one similarly Sm-based compounds [5]. A renormalization of some phonon energies caused by the valence instability has also been observed.
LaCu2Si2 T = 45 K T = 250 K
EuCu2Si2 T=6K T = 250 K
1
0 0
20 40 Energy transfer (meV)
60
Fig. 2. INS spectra for LaCu2Si2 (a) and EuCu2Si2 (b) measured at 6 and 250 K for an incident energy Ei ¼ 100 meV and a scattering angle /2YS ¼ 1141. Spectra have been corrected for the detailed-balance factor.
enhancement of the hybridization resulting from the application of pressure. The phonon spectra of EuCu2Si2 as well as those for the isostructural compound LaCu2Si2 are shown in Fig. 2. They display distinct structures: a double peak near 16 meV and a high energy component near 38 meV. Increasing temperature does not cause substantial changes in the LaCu2Si2 spectrum (Fig. 2a). On the contrary, the EuCu2Si2 spectrum shifts slightly towards the lower energies (2 meV) with increasing temperature (Fig. 2b), which correspond to the enhancement of the valence instability. Moreover, a change appears to occur in the
865
P.A. Alekseev, et al., JETP 105 (2007) 14. R. Mock, et al., J. Magn. Magn. Magn. 47&48 (1985) 312. J. Ro¨hler, et al., Phys. Rev. B 49 (1982) 65. V.N. Lazukov, et al., Physica B 359–361 (2005) 245. P.A. Alekseev, et al., Physica B 281&282 (2000) 34.
Physica B 403 (2008) 864–865 www.elsevier.com/locate/physb
Magnetic and lattice excitations in intermediate-valence EuCu2Si2 Pavel A. Alekseeva, Jean-Michel Mignotb, Kirill S. Nemkovskia, Elena V. Nefeodovaa,, Vladimir N. Lazukova, Denis Yu. Karpunina, Robert I. Bewleyc, Alexandr V. Gribanovd a RRC ‘‘Kurchatov Intitute’’, Kurchatov sq., 123182 Moscow, Russia Laboratoire Le´on Brillouin, CEA-CNRS, CEA/Saclay, 91191 Gif sur Yvette, France c Rutherford Appleton Laboratory, Chilton, Didcot, Oxon OX11 0QX, UK d Department of Chemistry, Moscow State University, Leninskie Gory, 119899 Moscow, Russia b
Abstract The magnetic and phonon excitation spectra in the intermediate-valence compound EuCu2Si2 have been studied by inelastic neutron scattering in a wide temperature range and under pressures up to 20 kbar. r 2007 Elsevier B.V. All rights reserved. Keywords: Intermediate valence; Neutron scattering; Magnetic and Phonon excitation
EuCu2Si2 is an intermetallic intermediate-valence (IV) system with a strong dependence of the Eu valence on temperature. It was found in a recent inelastic neutron scattering (INS) experiment that EuCu2Si2 exhibits an unusual magnetic excitation spectrum [1]. This spectrum consists of a double peak at about 35 meV at low temperature, with a quasielastic component appearing for T4100 K. The observed structure cannot be explained in terms of crystal-field effects on the Eu ions and most likely results from hybridization processes associated with the valence instability. Indications for a possible influence of the IV regime on the phonon spectrum in EuCu2Si2 have been reported in Ref. [2] based on a bulk modulus anomaly. In this paper, we present a study of (i) the magnetic response of EuCu2Si2 under external pressure, (ii) the influence of the valence instability on phonon excitations in this compound. The hydrostatic pressure shifts the Eu valence further towards the trivalent state [3]. It could be anticipated that the anomalous magnetic spectral shape might transform to a more conventional pattern defined by the spin-orbit (SO) splitting (46 meV for Eu3+) and crystal-field effects.
Corresponding author. Tel.: +7 499 196 7662; fax: +7 499 196 5973
E-mail address: [email protected] (E.V. Nefeodova). 0921-4526/$ - see front matter r 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.physb.2007.10.049
The neutron scattering measurements were performed on the time-of-flight spectrometer HET (ISIS, RAL) operated at the incident neutron energy Ei ¼ 100 meV. Powder samples of EuCu2Si2 (prepared using low-absorption 153 Eu isotope) and LaCu2Si2 (serving as a reference compound) were measured in the temperature range T ¼ 6–250 K. Pressures up to 20 kbar were generated in a McWhan-type pressure cell. For subtracting the background the signal from the empty pressure cell was measured at T ¼ 100 and 230 K, as well as the ambientpressure signal at T ¼ 230 K with the sample inside the pressure cell. Surprisingly, a pressure on the order of 20 kbar (or the increase of the Eu valence) does not produce a significant change in the structure of the magnetic response nor in its temperature dependence. In particular, no shift was observed in the magnetic response towards the energy of the Eu3+ SO transition (46 meV). The temperature dependences of the two magnetic excitation energies at P ¼ 0 and 20 kbar are presented in Fig. 1. It is seen that the application of pressure results in a minor increase in the energy of the higher magnetic excitation (M2) at higher temperature, and in a larger decrease in that of the lower magnetic excitation (M1). The splitting between the two magnetic peaks is therefore increased, and this is the main experimental observation. It can be traced back to the
ARTICLE IN PRESS P.A. Alekseev et al. / Physica B 403 (2008) 864–865
EuCu2Si2
Energy (meV)
40
M2
30 M1 P=20 kbar P=0 20
M1
M2 P=20 kbar P=0 0
100 200 Temperature (K)
Fig. 1. Energies of the two magnetic excitation in EuCu2Si2 at P ¼ 0 and 20 kbar as a function of temperature.
S(Q, E) (meV-1)
0
20
40
60
1
This work was supported by RFBR grant no. 05-0216426. References [1] [2] [3] [4] [5]
0 S(Q, E) (meV-1)
intensity ratio of the two peaks near 16 meV. The observed temperature behavior is not a direct consequence of the lattice expansion and could be indicative of a coupling between the valence fluctuations and the lattice vibrations. In summary, the magnetic response under pressure and the phonon spectrum in EuCu2Si2 have been studied by INS in a wide temperature range. It can be suggested that the higher magnetic excitation is the renormalized SO transition by analogy with Ce-based systems [4]. The lower magnetic excitation seems to be exciton-like one similarly Sm-based compounds [5]. A renormalization of some phonon energies caused by the valence instability has also been observed.
LaCu2Si2 T = 45 K T = 250 K
EuCu2Si2 T=6K T = 250 K
1
0 0
20 40 Energy transfer (meV)
60
Fig. 2. INS spectra for LaCu2Si2 (a) and EuCu2Si2 (b) measured at 6 and 250 K for an incident energy Ei ¼ 100 meV and a scattering angle /2YS ¼ 1141. Spectra have been corrected for the detailed-balance factor.
enhancement of the hybridization resulting from the application of pressure. The phonon spectra of EuCu2Si2 as well as those for the isostructural compound LaCu2Si2 are shown in Fig. 2. They display distinct structures: a double peak near 16 meV and a high energy component near 38 meV. Increasing temperature does not cause substantial changes in the LaCu2Si2 spectrum (Fig. 2a). On the contrary, the EuCu2Si2 spectrum shifts slightly towards the lower energies (2 meV) with increasing temperature (Fig. 2b), which correspond to the enhancement of the valence instability. Moreover, a change appears to occur in the
865
P.A. Alekseev, et al., JETP 105 (2007) 14. R. Mock, et al., J. Magn. Magn. Magn. 47&48 (1985) 312. J. Ro¨hler, et al., Phys. Rev. B 49 (1982) 65. V.N. Lazukov, et al., Physica B 359–361 (2005) 245. P.A. Alekseev, et al., Physica B 281&282 (2000) 34.