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Temperature dependence of the electronic structure of Sr14Cu24O41 studied by resonant inelastic X-ray scattering
Physica C 470 (2010) S145–S146
Contents lists available at ScienceDirect
Physica C journal homepage: www.elsevier.com/locate/physc
Temperature dependence of the electronic structure of Sr14Cu24O41 studied by resonant inelastic X-ray scattering M. Yoshida a,b,*, K. Ishii b, K. Ikeuchi c, I. Jarrige b, Y. Murakami b,c, J. Mizuki b, K. Tsutsui b, T. Tohyama d, S. Maekawa e, K. Kudo e,f, Y. Koike f, Y. Endoh b,g a
Department of Physics, Tohoku University, Sendai 980-8578, Japan Synchrotron Radiation Research Center, Japan Atomic Energy Agency, Hyogo 679-5148, Japan Institute of Materials Structure Science, Tsukuba 305-0801, Japan d Yukawa Institute for Theoretical Physics, Kyoto University, Kyoto 606-8502, Japan e Institute for Materials Research, Tohoku University, Sendai 980-8577, Japan f Department of Applied Physics, Tohoku University, Sendai 980-8579, Japan g International Institute for Advanced Studies, Kizu, Kyoto 619-0025, Japan b c
a r t i c l e
i n f o
Article history: Accepted 9 November 2009 Available online 13 November 2009 Keywords: Resonant inelastic X-ray scattering Sr14Cu24O41 Two-leg ladder
a b s t r a c t We report a resonant inelastic X-ray scattering (RIXS) study of charge excitations in the two-leg ladder Sr14Cu24O41. RIXS spectra at 1–5 eV are found to be dependent on temperature. An intraband excitation of the ladder, which appears as a continuum intensity below the Mott gap, decreases in intensity with temperature. Because the intraband excitation is related to the dynamics of doped holes in the ladder, its decrease of the intraband excitation is attributed to the reduction of the mobile holes in the ladder at low temperature. Ó 2009 Elsevier B.V. All rights reserved.
1. Introduction Spin ladder systems have attracted much interest owing to a theoretical prediction about the occurrence of superconductivity [1]. A representative material system of the two-leg ladder is Sr14xCaxCu24O41 for which superconductivity is realized at x > 10 by applying high pressure [2,3]. In fact, Sr14xCaxCu24O41 is a composite crystal in which Cu2O3 two-leg ladders and CuO2 chains coexist with a different periodicity and the two-leg ladder is responsible for the conductivity. The parent Sr14Cu24O41 exhibits semiconductive behavior, even though the ladder contains already some holes [4]. Recently, a resonant soft X-ray diffraction study identified the ground state of the ladder in Sr14Cu24O41 as a charge ordered [5]. Characteristic features of a charge density wave (CDW) were also observed in lowfrequency dielectric spectroscopy [6]. Unlike a conventional CDW by the Peierls instability, the lattice distortion is missing or very small in the charge-ordered state of Sr14Cu24O41. A purely electronic, many-body interaction is proposed as a possible origin for the charge order [7]. Associated with the development of the
* Corresponding author. Address: Department of Physics, Tohoku University, Sendai 980-8578, Japan. E-mail address: [email protected] (M. Yoshida). 0921-4534/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.physc.2009.11.082
charge order, optical conductivity changes with temperature in wide energy range up to a few eV [8]. These results indicate that the strong electron correlation, which is much higher than the energy scale of temperature, governs the electronic states of Sr14Cu24O41. While the optical conductivity gives fruitful information of the zero momentum transfer, resonant inelastic X-ray scattering (RIXS) is a powerful experimental method to investigate electronic excitation at finite momentum transfer. RIXS in the hard X-ray regime has been developed in the last decade by utilizing the brilliant synchrotron radiation X-rays. In addition to the momentum resolution, RIXS has another advantage of element selectivity, that is, electronic excitations related to the Cu orbitals are enhanced by tuning the incident photon energy to the Cu K-edge. RIXS works on (La,Sr,Ca)14Cu24O41 at room temperature have already been reported by some of the present authors [9] and others [10,11]. Here we briefly summarize the results here. An interband excitation across the Mott gap excitation, more precisely an excitation from the Zhang–Rice singlet band to the upper Hubbard band, appears at 2–4 eV and its dispersion relation is folded at the Brillouin zone boundary of the ladder. Simultaneously, an intraband excitation in the Zhang–Rice band is observed as a continuum intensity around 1 eV. The intensity of the intraband excitation is proportional to the hole concentration of the ladder. Therefore the RIXS spectra below 4 eV is dominated by the ladder component, while the chain is silent in this energy region. Here we report
S146
M. Yoshida et al. / Physica C 470 (2010) S145–S146
400K 10K
q=(π,π) 15
q=(π,0)
10
q=(0,π)
5
elastic
Intensity (~cps@100mA)
20
q=(0,0) 0
-3
-2
-1
0
1
2
3
4
5
Energy loss (eV) Fig. 1. RIXS spectra at high-symmetric momenta measured at 400 and 10 K (open circles and filled circles, respectively). For clarity, each spectrum is offset vertically by 5 in intensity.
the temperature dependence of the electronic structure of Sr14Cu24O41 using RIXS. Charge excitation spectra significantly change with temperature and we discuss the relation with the charge order. 2. Experimental details The RIXS experiments were performed at BL11XU at SPring-8. A Si(4 0 0) channel-cut monochromator and a spherically bent Ge(733) analyzer were used for the monochromatization of the incident and scattered beams, respectively. A total energy resolution of about 400 meV was estimated from the full width at half maximum of the elastic peak. A single crystal of Sr14Cu24O41 was grown by the traveling-solvent floating-zone method [12]. The surface normal to the b-axis was irradiated by X-rays and the crystal was mounted so that the bc-plane was almost parallel to the scattering plane. Instead of the absolute momentum transfers using the Miller indices ~ ¼ ðH; 13:5; LÞ, we represent the momentum transfer reduced in Q the first Brillouin zone of the single two-leg ladder as ~ q ¼ ðqrung ; qleg Þ. The incident X-ray energy was tuned to 8993 eV where the excitations around a few eV were resonantly enhanced [9]. The spectra were measured at both 400 K and 10 K. 3. Results and discussion The temperature dependence of the RIXS spectra of Sr14Cu24O41 is shown in Fig. 1. Differences between the 400 and 10 K spectra are apparent. In contrast, RIXS spectra of La2xSrxCuO4 change very little with temperature [13]. At the zone center ~ q ¼ ð0; 0Þ, the two spectra intersect each other at 2 eV and the spectral weight transfers from the interband excitation to the intraband excitation with decreasing temperature. This temperature dependence is in good agreement with the c-axis (along the leg) optical conductivity
[8]. It is naturally comprehensible because the charge dynamics of the zero momentum transfer is measured with both experimental methods. Focusing on the intraband excitation around 1.0–1.5 eV, the RIXS intensity decreases at low temperature. This is observed irrespective of the momentum. One might suspect that the decrease in intensity comes from the tail of the elastic peak which originates from the quasi-elastic background. However the intensity of the elastic tail in the anti-Stokes side is almost unchanged below 1 eV. Therefore the variation in intensity around 1.0–1.5 eV comes from an intrinsic change of the electronic structure. We attribute the decrease of the intraband excitation to the reduction of the mobile holes in the ladder. One possibility to account for the reduction of the mobile holes is localization by the charge order. Temperature dependence of the electric resistivity shows a kink around 150–200 K [14] which roughly coincides with the transition temperature of the charge order [5,6]. If this is the case, an interaction of the order of the eV, namely, the strong electron correlation, might cause the localization of the holes and the associated charge order (or hole crystallization [5]), and hence it results in the temperature dependence of the charge excitation spectra in a wide energy range. On the other hand, a NMR study claims that the hole number in the ladder itself decreases at low temperature due to the back transfer of the holes from the ladder to the chain [15], which is the other possibility of the reduction of the mobile holes in the ladder. In this case, reconstruction of the electronic structure upon hole doping extends to a few eV, as does in high-Tc cuprates [16,13]. 4. Summary We have measured the electronic excitations in Sr14Cu24O41 using RIXS at the Cu K-edge. The spectra are dependent on temperature across a wide energy range, which indicates that the electron correlation is important for the electronic structure of Sr14Cu24O41. The intraband excitation of the ladder decreases in intensity with temperature. We ascribe this trend to the reduction of the concentration mobile holes in the ladder at low temperature. Acknowledgments This work was performed under the inter-university cooperative research program of the Institute of Materials Research, Tohoku University and supported by the Grant-in-Aid for Scientific Research on Priority Areas ‘‘Invention of anomalous quantum materials” from MEXT. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16]
E. Dagotto et al., Science 271 (1996) 618. M. Uehara et al., J. Phys. Soc. Jpn. 65 (1996) 2764. K.M. Kojima et al., J. Electron Spectrosc. Relat. Phenom. 117–118 (2001) 237. M. Kato et al., Physica C 258 (1996) 284. P. Abbamonte et al., Nature 431 (2004) 1078. T. Vuletic´ et al., Phys. Rev. Lett. 90 (2003) 257002. K. Wohlfeld et al., Phys. Rev. B 75 (2007) 180501. H. Eisaki et al., Phys. C Supercond. 341–348 (2000) 363. K. Ishii et al., Phys. Rev. B 76 (2007) 045124. L. Wray et al., Phys. Rev. B 76 (2007) 100507. A. Higashiya et al., New J. Phys. 10 (2008) 053033. K. Kudo et al., J. Phys. Soc. Jpn. 70 (2001) 437. Y.-J. Kim et al., Phys. Rev. B 70 (2004) 094524. K. Kudo et al., Physica B 284–288 (2000) 651. Y. Piskunov et al., Phys. Rev. B 72 (2005) 064512. S. Uchida et al., Phys. Rev. B 43 (1991) 7942.
Contents lists available at ScienceDirect
Physica C journal homepage: www.elsevier.com/locate/physc
Temperature dependence of the electronic structure of Sr14Cu24O41 studied by resonant inelastic X-ray scattering M. Yoshida a,b,*, K. Ishii b, K. Ikeuchi c, I. Jarrige b, Y. Murakami b,c, J. Mizuki b, K. Tsutsui b, T. Tohyama d, S. Maekawa e, K. Kudo e,f, Y. Koike f, Y. Endoh b,g a
Department of Physics, Tohoku University, Sendai 980-8578, Japan Synchrotron Radiation Research Center, Japan Atomic Energy Agency, Hyogo 679-5148, Japan Institute of Materials Structure Science, Tsukuba 305-0801, Japan d Yukawa Institute for Theoretical Physics, Kyoto University, Kyoto 606-8502, Japan e Institute for Materials Research, Tohoku University, Sendai 980-8577, Japan f Department of Applied Physics, Tohoku University, Sendai 980-8579, Japan g International Institute for Advanced Studies, Kizu, Kyoto 619-0025, Japan b c
a r t i c l e
i n f o
Article history: Accepted 9 November 2009 Available online 13 November 2009 Keywords: Resonant inelastic X-ray scattering Sr14Cu24O41 Two-leg ladder
a b s t r a c t We report a resonant inelastic X-ray scattering (RIXS) study of charge excitations in the two-leg ladder Sr14Cu24O41. RIXS spectra at 1–5 eV are found to be dependent on temperature. An intraband excitation of the ladder, which appears as a continuum intensity below the Mott gap, decreases in intensity with temperature. Because the intraband excitation is related to the dynamics of doped holes in the ladder, its decrease of the intraband excitation is attributed to the reduction of the mobile holes in the ladder at low temperature. Ó 2009 Elsevier B.V. All rights reserved.
1. Introduction Spin ladder systems have attracted much interest owing to a theoretical prediction about the occurrence of superconductivity [1]. A representative material system of the two-leg ladder is Sr14xCaxCu24O41 for which superconductivity is realized at x > 10 by applying high pressure [2,3]. In fact, Sr14xCaxCu24O41 is a composite crystal in which Cu2O3 two-leg ladders and CuO2 chains coexist with a different periodicity and the two-leg ladder is responsible for the conductivity. The parent Sr14Cu24O41 exhibits semiconductive behavior, even though the ladder contains already some holes [4]. Recently, a resonant soft X-ray diffraction study identified the ground state of the ladder in Sr14Cu24O41 as a charge ordered [5]. Characteristic features of a charge density wave (CDW) were also observed in lowfrequency dielectric spectroscopy [6]. Unlike a conventional CDW by the Peierls instability, the lattice distortion is missing or very small in the charge-ordered state of Sr14Cu24O41. A purely electronic, many-body interaction is proposed as a possible origin for the charge order [7]. Associated with the development of the
* Corresponding author. Address: Department of Physics, Tohoku University, Sendai 980-8578, Japan. E-mail address: [email protected] (M. Yoshida). 0921-4534/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.physc.2009.11.082
charge order, optical conductivity changes with temperature in wide energy range up to a few eV [8]. These results indicate that the strong electron correlation, which is much higher than the energy scale of temperature, governs the electronic states of Sr14Cu24O41. While the optical conductivity gives fruitful information of the zero momentum transfer, resonant inelastic X-ray scattering (RIXS) is a powerful experimental method to investigate electronic excitation at finite momentum transfer. RIXS in the hard X-ray regime has been developed in the last decade by utilizing the brilliant synchrotron radiation X-rays. In addition to the momentum resolution, RIXS has another advantage of element selectivity, that is, electronic excitations related to the Cu orbitals are enhanced by tuning the incident photon energy to the Cu K-edge. RIXS works on (La,Sr,Ca)14Cu24O41 at room temperature have already been reported by some of the present authors [9] and others [10,11]. Here we briefly summarize the results here. An interband excitation across the Mott gap excitation, more precisely an excitation from the Zhang–Rice singlet band to the upper Hubbard band, appears at 2–4 eV and its dispersion relation is folded at the Brillouin zone boundary of the ladder. Simultaneously, an intraband excitation in the Zhang–Rice band is observed as a continuum intensity around 1 eV. The intensity of the intraband excitation is proportional to the hole concentration of the ladder. Therefore the RIXS spectra below 4 eV is dominated by the ladder component, while the chain is silent in this energy region. Here we report
S146
M. Yoshida et al. / Physica C 470 (2010) S145–S146
400K 10K
q=(π,π) 15
q=(π,0)
10
q=(0,π)
5
elastic
Intensity (~cps@100mA)
20
q=(0,0) 0
-3
-2
-1
0
1
2
3
4
5
Energy loss (eV) Fig. 1. RIXS spectra at high-symmetric momenta measured at 400 and 10 K (open circles and filled circles, respectively). For clarity, each spectrum is offset vertically by 5 in intensity.
the temperature dependence of the electronic structure of Sr14Cu24O41 using RIXS. Charge excitation spectra significantly change with temperature and we discuss the relation with the charge order. 2. Experimental details The RIXS experiments were performed at BL11XU at SPring-8. A Si(4 0 0) channel-cut monochromator and a spherically bent Ge(733) analyzer were used for the monochromatization of the incident and scattered beams, respectively. A total energy resolution of about 400 meV was estimated from the full width at half maximum of the elastic peak. A single crystal of Sr14Cu24O41 was grown by the traveling-solvent floating-zone method [12]. The surface normal to the b-axis was irradiated by X-rays and the crystal was mounted so that the bc-plane was almost parallel to the scattering plane. Instead of the absolute momentum transfers using the Miller indices ~ ¼ ðH; 13:5; LÞ, we represent the momentum transfer reduced in Q the first Brillouin zone of the single two-leg ladder as ~ q ¼ ðqrung ; qleg Þ. The incident X-ray energy was tuned to 8993 eV where the excitations around a few eV were resonantly enhanced [9]. The spectra were measured at both 400 K and 10 K. 3. Results and discussion The temperature dependence of the RIXS spectra of Sr14Cu24O41 is shown in Fig. 1. Differences between the 400 and 10 K spectra are apparent. In contrast, RIXS spectra of La2xSrxCuO4 change very little with temperature [13]. At the zone center ~ q ¼ ð0; 0Þ, the two spectra intersect each other at 2 eV and the spectral weight transfers from the interband excitation to the intraband excitation with decreasing temperature. This temperature dependence is in good agreement with the c-axis (along the leg) optical conductivity
[8]. It is naturally comprehensible because the charge dynamics of the zero momentum transfer is measured with both experimental methods. Focusing on the intraband excitation around 1.0–1.5 eV, the RIXS intensity decreases at low temperature. This is observed irrespective of the momentum. One might suspect that the decrease in intensity comes from the tail of the elastic peak which originates from the quasi-elastic background. However the intensity of the elastic tail in the anti-Stokes side is almost unchanged below 1 eV. Therefore the variation in intensity around 1.0–1.5 eV comes from an intrinsic change of the electronic structure. We attribute the decrease of the intraband excitation to the reduction of the mobile holes in the ladder. One possibility to account for the reduction of the mobile holes is localization by the charge order. Temperature dependence of the electric resistivity shows a kink around 150–200 K [14] which roughly coincides with the transition temperature of the charge order [5,6]. If this is the case, an interaction of the order of the eV, namely, the strong electron correlation, might cause the localization of the holes and the associated charge order (or hole crystallization [5]), and hence it results in the temperature dependence of the charge excitation spectra in a wide energy range. On the other hand, a NMR study claims that the hole number in the ladder itself decreases at low temperature due to the back transfer of the holes from the ladder to the chain [15], which is the other possibility of the reduction of the mobile holes in the ladder. In this case, reconstruction of the electronic structure upon hole doping extends to a few eV, as does in high-Tc cuprates [16,13]. 4. Summary We have measured the electronic excitations in Sr14Cu24O41 using RIXS at the Cu K-edge. The spectra are dependent on temperature across a wide energy range, which indicates that the electron correlation is important for the electronic structure of Sr14Cu24O41. The intraband excitation of the ladder decreases in intensity with temperature. We ascribe this trend to the reduction of the concentration mobile holes in the ladder at low temperature. Acknowledgments This work was performed under the inter-university cooperative research program of the Institute of Materials Research, Tohoku University and supported by the Grant-in-Aid for Scientific Research on Priority Areas ‘‘Invention of anomalous quantum materials” from MEXT. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16]
E. Dagotto et al., Science 271 (1996) 618. M. Uehara et al., J. Phys. Soc. Jpn. 65 (1996) 2764. K.M. Kojima et al., J. Electron Spectrosc. Relat. Phenom. 117–118 (2001) 237. M. Kato et al., Physica C 258 (1996) 284. P. Abbamonte et al., Nature 431 (2004) 1078. T. Vuletic´ et al., Phys. Rev. Lett. 90 (2003) 257002. K. Wohlfeld et al., Phys. Rev. B 75 (2007) 180501. H. Eisaki et al., Phys. C Supercond. 341–348 (2000) 363. K. Ishii et al., Phys. Rev. B 76 (2007) 045124. L. Wray et al., Phys. Rev. B 76 (2007) 100507. A. Higashiya et al., New J. Phys. 10 (2008) 053033. K. Kudo et al., J. Phys. Soc. Jpn. 70 (2001) 437. Y.-J. Kim et al., Phys. Rev. B 70 (2004) 094524. K. Kudo et al., Physica B 284–288 (2000) 651. Y. Piskunov et al., Phys. Rev. B 72 (2005) 064512. S. Uchida et al., Phys. Rev. B 43 (1991) 7942.