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BaRuO3: a new type of pseudogap systems
Physica C 364±365 (2001) 480±483
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BaRuO3: a new type of pseudogap systems T.W. Noh a,b,*, Y.S. Lee a, J.S. Lee a, K.W. Kim a, J. Yu b, G. Cao c, J.E. Crow c, M.K. Lee d, C.B. Eom d a
b
School of Physics and Research Center for Oxide Electronics, Seoul National University, Seoul 151-747, South Korea School of Physics and Center for Strongly Correlated Materials Research, Seoul National University, Seoul 151-747, South Korea c National High Magnetic Field Laboratory, Florida State University, Tallahassee, FL 32306, USA d Department of Material Science and Engineering, University of Wisconsin-Madison, Madison, WI 53706, USA
Abstract Pseudogap phenomena have been an interesting issue in strongly correlated systems, especially high-Tc cuprates. In 4d transition metal oxides, for the ®rst time, we introduce BaRuO3 systems as a new kind of systems exhibiting the pseudogap phenomena. Compared with high-Tc cuprates, a development of the pseudogap as well as the coherent component is more clear in their optical conductivity spectra. We expect that our results in the pseudogap phenomena of BaRuO3 can provide useful information for understanding the nature of the pseudogap in strongly correlated systems. Ó 2001 Elsevier Science B.V. All rights reserved. PACS: 78.20.)e; 78.30.)j; 78.66.)w Keywords: Pseudogap; BaRuO3 ; Optical conductivity spectra
Pseudogap phenomena have been observed in many strongly correlated electron systems, such as heavy fermion (HF) and high-Tc superconductors (HTSC) [1,2]. Especially, a great deal of interest has been focused on pseudogap phenomena due to its possible connection to the mechanism of high-Tc superconductivity. However, there are still debates on the origin of the pseudogap and the correlation between superconducting gap and pseudogap. In this paper, we present very intriguing results that pseudogap phenomena do occur in BaRuO3 compounds. To our best knowledge, this is the ®rst
* Corresponding author. Address: School of Physics and Center for Strongly Correlated Material Research, Seoul 151747, South Korea. Tel.: +82-2-880-6616; fax: +82-2-875-1222. E-mail address: [email protected] (T.W. Noh).
observation of pseudogap features in 4d transition metal oxides. Compared to the case of HTSC and HF, our optical spectra show more clearly how the electrodynamic responses change with the pseudogap formation. BaRuO3 has two crystallographically dierent layered structures: four-layer hexagonal phase (4H) and nine-layer rhombohedral phase (9R). As shown in Fig. 1, 9R (4H) BaRuO3 has 9 (4)-layer close-packed stacking with strings of the three (two) sharing faces of RuO6 octahedra, which result in the direct Ru±Ru bondings along c-axis. This additional quasi-one-dimensional (1D) contribution can make physical properties of BaRuO3 dierent from those of cubic perovskite ARuO3 (A Sr and Ca) or layered perovskite HTSC [3]. Two kinds of BaRuO3 samples were prepared. 9R BaRuO3 single crystal was prepared by
0921-4534/01/$ - see front matter Ó 2001 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 4 5 3 4 ( 0 1 ) 0 0 8 2 5 - 5
T.W. Noh et al. / Physica C 364±365 (2001) 480±483
481
Fig. 1. Schematic diagrams of the two crystallographic forms of BaRuO3 : (a) 9R phase and (b) 4H phase.
¯ux-melting method [4]. Its dc resistivity curve showed a metal±insulator-like transition around 110 K: i.e. it is metallic above 110 K but insulatorlike below 110 K. 4H BaRuO3 epitaxial ®lm was fabricated on (1 1 1) SrTiO3 substrate by 90° oaxis sputtering technique [5]. This ®lm remains metallic in all the temperature region below 300 K. Near normal incident re¯ectivity spectra, R x, of the ab plane were measured in a wide photon energy region of 5 meV to 30 eV with temperature variation. The Kramers±Kronig (K±K) analysis was used to calculate optical conductivity spectra, r1 x, from the measured R x. The details were described elsewhere [6]. Fig. 2(a) and (b) show T -dependent r1 x of 9R and 4H BaRuO3 , respectively. Both ruthenates exhibit clear pseudogap formations. r1 x at 300 K appears to show a metallic behavior. However, as T decreases, the spectral weight below 500 cm 1 becomes suppressed and the spectral weight near 800 cm 1 increases, forming a gap-like feature. It is unusual that these clear developments occur in the metallic states. When r1 x is integrated up to 1.0 eV, the optical strength remains nearly constant. That is, the optical sum rule is satis®ed. Note that the Drude-like coherent peaks also exist below the pseudogap region. As shown in Fig. 2(b), the coherent component of 4H BaRuO3 becomes narrower and sharper at a lower temperature. In case of 9R BaRuO3 , the development of the coherent component is not clearly shown in
Fig. 2. T-dependent r1 x of (a) 9R and (b) 4H BaRuO3 . The dotted lines are r1 x obtained from the Hagen±Rubens extrapolations. The arrows in (a) and (b) represent the pseudogap positions, xc;9R and xc;4H of 9R and 4H BaRuO3 , respectively.
r1 x. However, its existence can be con®rmed in 1 x [7]. It should be noted that the clear pseudogap formations in r1 x are quite dierent from that of HTSC. In case of HTSC, although the suppression of frequency-dependent scattering rate in ab plane was suggested as an evidence of pseudogap [8], there is no direct evidence of pseudogap formation in r1 x of the ab plane. On the contrary, r1 x along the c-axis exhibits a suppression of the spectral weight in far-IR region, which can be interpreted as a feature of the pseudogap [9]. However, it is in an insulating state. So, our pseudogap formations in the metallic state can be clearly distinguished from that of HTSC. To get further information, we analyzed the T dependent behaviors of the coherent components. Two important electrodynamic quantities are the scattering rate 1=s and the plasma frequency
482
T.W. Noh et al. / Physica C 364±365 (2001) 480±483
p xp 4pn=m . Here, n and m represent the carrier density and the eective mass of the coherent component, respectively. Note that x2p represents the strength of the coherent component in r1 x. Details of obtaining these electrodynamic quantities were described elsewhere [7]. Fig. 3(a) and (b) show the T-dependences of the electrodynamic quantities for 9R and 4H BaRuO3 , respectively. As T decreases, x2p for both ruthernates decreases. The reduction of x2p can be explained by a reduction of n or an enhancement of m . Using the standard free electron model, we can approximately determine n and m from the experimental values of magnetic susceptibility v T
Fig. 3. T-dependent x2p and 1=s of the coherent component in (a) 9R and (b) 4H BaRuO3 . In (a), the dotted line represents an expected T-dependence of 1=s with ®xed carrier density n (400 K). The dot±dashed line is a ®t to the formula, 1=s T exp D =T with D ' 250 K. Note that the measured Tranges are dierent for two samples. Because T of 9R BaRuO3 might position at a higher T than that of 4H BaRuO3 due to its stronger CDW instability, the measurement of 9R BaRuO3 up to a higher-T region was needed to observe the intriguing electrodynamics with the pseudogap formation clearly.
and x2p [7]. From this analysis, it was found that the reduction of x2p is mainly due to the reduction of n. Especially for 9R BaRuO3 , n decreases by a factor of six in the metallic region between 300 and 110 K. It is quite surprising that such a drastic decrease of n can occur in a metallic region. This unusual behavior of n can be explained by strong suppression of 1=s. With the pseudogap formation, 1=s becomes reduced abruptly in both materials. 1=s of 9R BaRuO3 shows a quite strong deviation from the predicted behavior for a typical metal, shown as dotted line in Fig. 3(a). In 4H BaRuO3 , the suppression of 1=s is also clearly observed. According to the Drude model, r x2p s=4p. Therefore, in spite of a reduction of x2p (or n), a rapid reduction of 1=s can induce metallic states in these ruthenates. What is the origin of the pseudogap in BaRuO3 ? To address this intriguing issue, it is worthwhile to note several experimental facts. First, BaRuO3 has the quasi-1D structural characteristic through the face-sharing of RuO6 . Second, it was found recently that BaIrO3 , which is isostructural with 9R BaRuO3 , exhibits a strong charge density wave (CDW) instability [10]. Third, the spectral weight change of 9R BaRuO3 is qualitatively similar to that of CDW systems where the sum rule should be obeyed [11]. Therefore, we propose that the pseudogap phenomena in BaRuO3 should be originated from CDW ¯uctuations. Our observation of pseudogap phenomena in BaRuO3 can provide a new perspective to the HTSC's pseudogap phenomena. Just like in the HTSC case, the pseudogap in BaRuO3 can also be explained in terms of partial suppression of density of states at EF in the k-space and its possible connection to the quantum critical ¯uctuation. More systematic studies on the comparison of two materials could provide some guidelines to understand the origin of the pseudogap and its possible connection to the superconductivity in HTSC. Acknowledgements This work was supported by Ministry of Science and Technology through the Creative Research Initiative program and by the Korea
T.W. Noh et al. / Physica C 364±365 (2001) 480±483
Science and Engineering Foundation through the CSCMR at Seoul National University. References [1] S. Donovan, A. Schwartz, G. Gr uner, Phys. Rev. Lett. 79 (1997) 1401. [2] Ch. Renner et al., Phys. Rev. Lett. 80 (1998) 149. [3] J.T. Rijssenbeek et al., Phys. Rev. B 59 (1999) 4978.
483
[4] M. Shepard, S. McCall, G. Cao, J.E. Crow, J. Appl. Phys. 81 (1997) 4978. [5] M.K. Lee et al., Appl. Phys. Lett. 77 (2000) 364. [6] H.S. Choi et al., Phys. Rev. B 60 (1999) 4646. [7] Y.S. Lee et al., Europhys. Lett. 55 (2001) 280. [8] D.N. Basov et al., Phys. Rev. Lett. 77 (1996) 4090. [9] C.C. Homes, T. Timusk, R. Liang, D.A. Bonn, W.N. Hardy, Phys. Rev. Lett. 71 (1993) 1645. [10] G. Cao et al., Solid State Commun. 113 (2000) 657. [11] G. Gr uner, Density Waves in Solids, Addison-Wesley, Reading, MA, 1994.
www.elsevier.com/locate/physc
BaRuO3: a new type of pseudogap systems T.W. Noh a,b,*, Y.S. Lee a, J.S. Lee a, K.W. Kim a, J. Yu b, G. Cao c, J.E. Crow c, M.K. Lee d, C.B. Eom d a
b
School of Physics and Research Center for Oxide Electronics, Seoul National University, Seoul 151-747, South Korea School of Physics and Center for Strongly Correlated Materials Research, Seoul National University, Seoul 151-747, South Korea c National High Magnetic Field Laboratory, Florida State University, Tallahassee, FL 32306, USA d Department of Material Science and Engineering, University of Wisconsin-Madison, Madison, WI 53706, USA
Abstract Pseudogap phenomena have been an interesting issue in strongly correlated systems, especially high-Tc cuprates. In 4d transition metal oxides, for the ®rst time, we introduce BaRuO3 systems as a new kind of systems exhibiting the pseudogap phenomena. Compared with high-Tc cuprates, a development of the pseudogap as well as the coherent component is more clear in their optical conductivity spectra. We expect that our results in the pseudogap phenomena of BaRuO3 can provide useful information for understanding the nature of the pseudogap in strongly correlated systems. Ó 2001 Elsevier Science B.V. All rights reserved. PACS: 78.20.)e; 78.30.)j; 78.66.)w Keywords: Pseudogap; BaRuO3 ; Optical conductivity spectra
Pseudogap phenomena have been observed in many strongly correlated electron systems, such as heavy fermion (HF) and high-Tc superconductors (HTSC) [1,2]. Especially, a great deal of interest has been focused on pseudogap phenomena due to its possible connection to the mechanism of high-Tc superconductivity. However, there are still debates on the origin of the pseudogap and the correlation between superconducting gap and pseudogap. In this paper, we present very intriguing results that pseudogap phenomena do occur in BaRuO3 compounds. To our best knowledge, this is the ®rst
* Corresponding author. Address: School of Physics and Center for Strongly Correlated Material Research, Seoul 151747, South Korea. Tel.: +82-2-880-6616; fax: +82-2-875-1222. E-mail address: [email protected] (T.W. Noh).
observation of pseudogap features in 4d transition metal oxides. Compared to the case of HTSC and HF, our optical spectra show more clearly how the electrodynamic responses change with the pseudogap formation. BaRuO3 has two crystallographically dierent layered structures: four-layer hexagonal phase (4H) and nine-layer rhombohedral phase (9R). As shown in Fig. 1, 9R (4H) BaRuO3 has 9 (4)-layer close-packed stacking with strings of the three (two) sharing faces of RuO6 octahedra, which result in the direct Ru±Ru bondings along c-axis. This additional quasi-one-dimensional (1D) contribution can make physical properties of BaRuO3 dierent from those of cubic perovskite ARuO3 (A Sr and Ca) or layered perovskite HTSC [3]. Two kinds of BaRuO3 samples were prepared. 9R BaRuO3 single crystal was prepared by
0921-4534/01/$ - see front matter Ó 2001 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 4 5 3 4 ( 0 1 ) 0 0 8 2 5 - 5
T.W. Noh et al. / Physica C 364±365 (2001) 480±483
481
Fig. 1. Schematic diagrams of the two crystallographic forms of BaRuO3 : (a) 9R phase and (b) 4H phase.
¯ux-melting method [4]. Its dc resistivity curve showed a metal±insulator-like transition around 110 K: i.e. it is metallic above 110 K but insulatorlike below 110 K. 4H BaRuO3 epitaxial ®lm was fabricated on (1 1 1) SrTiO3 substrate by 90° oaxis sputtering technique [5]. This ®lm remains metallic in all the temperature region below 300 K. Near normal incident re¯ectivity spectra, R x, of the ab plane were measured in a wide photon energy region of 5 meV to 30 eV with temperature variation. The Kramers±Kronig (K±K) analysis was used to calculate optical conductivity spectra, r1 x, from the measured R x. The details were described elsewhere [6]. Fig. 2(a) and (b) show T -dependent r1 x of 9R and 4H BaRuO3 , respectively. Both ruthenates exhibit clear pseudogap formations. r1 x at 300 K appears to show a metallic behavior. However, as T decreases, the spectral weight below 500 cm 1 becomes suppressed and the spectral weight near 800 cm 1 increases, forming a gap-like feature. It is unusual that these clear developments occur in the metallic states. When r1 x is integrated up to 1.0 eV, the optical strength remains nearly constant. That is, the optical sum rule is satis®ed. Note that the Drude-like coherent peaks also exist below the pseudogap region. As shown in Fig. 2(b), the coherent component of 4H BaRuO3 becomes narrower and sharper at a lower temperature. In case of 9R BaRuO3 , the development of the coherent component is not clearly shown in
Fig. 2. T-dependent r1 x of (a) 9R and (b) 4H BaRuO3 . The dotted lines are r1 x obtained from the Hagen±Rubens extrapolations. The arrows in (a) and (b) represent the pseudogap positions, xc;9R and xc;4H of 9R and 4H BaRuO3 , respectively.
r1 x. However, its existence can be con®rmed in 1 x [7]. It should be noted that the clear pseudogap formations in r1 x are quite dierent from that of HTSC. In case of HTSC, although the suppression of frequency-dependent scattering rate in ab plane was suggested as an evidence of pseudogap [8], there is no direct evidence of pseudogap formation in r1 x of the ab plane. On the contrary, r1 x along the c-axis exhibits a suppression of the spectral weight in far-IR region, which can be interpreted as a feature of the pseudogap [9]. However, it is in an insulating state. So, our pseudogap formations in the metallic state can be clearly distinguished from that of HTSC. To get further information, we analyzed the T dependent behaviors of the coherent components. Two important electrodynamic quantities are the scattering rate 1=s and the plasma frequency
482
T.W. Noh et al. / Physica C 364±365 (2001) 480±483
p xp 4pn=m . Here, n and m represent the carrier density and the eective mass of the coherent component, respectively. Note that x2p represents the strength of the coherent component in r1 x. Details of obtaining these electrodynamic quantities were described elsewhere [7]. Fig. 3(a) and (b) show the T-dependences of the electrodynamic quantities for 9R and 4H BaRuO3 , respectively. As T decreases, x2p for both ruthernates decreases. The reduction of x2p can be explained by a reduction of n or an enhancement of m . Using the standard free electron model, we can approximately determine n and m from the experimental values of magnetic susceptibility v T
Fig. 3. T-dependent x2p and 1=s of the coherent component in (a) 9R and (b) 4H BaRuO3 . In (a), the dotted line represents an expected T-dependence of 1=s with ®xed carrier density n (400 K). The dot±dashed line is a ®t to the formula, 1=s T exp D =T with D ' 250 K. Note that the measured Tranges are dierent for two samples. Because T of 9R BaRuO3 might position at a higher T than that of 4H BaRuO3 due to its stronger CDW instability, the measurement of 9R BaRuO3 up to a higher-T region was needed to observe the intriguing electrodynamics with the pseudogap formation clearly.
and x2p [7]. From this analysis, it was found that the reduction of x2p is mainly due to the reduction of n. Especially for 9R BaRuO3 , n decreases by a factor of six in the metallic region between 300 and 110 K. It is quite surprising that such a drastic decrease of n can occur in a metallic region. This unusual behavior of n can be explained by strong suppression of 1=s. With the pseudogap formation, 1=s becomes reduced abruptly in both materials. 1=s of 9R BaRuO3 shows a quite strong deviation from the predicted behavior for a typical metal, shown as dotted line in Fig. 3(a). In 4H BaRuO3 , the suppression of 1=s is also clearly observed. According to the Drude model, r x2p s=4p. Therefore, in spite of a reduction of x2p (or n), a rapid reduction of 1=s can induce metallic states in these ruthenates. What is the origin of the pseudogap in BaRuO3 ? To address this intriguing issue, it is worthwhile to note several experimental facts. First, BaRuO3 has the quasi-1D structural characteristic through the face-sharing of RuO6 . Second, it was found recently that BaIrO3 , which is isostructural with 9R BaRuO3 , exhibits a strong charge density wave (CDW) instability [10]. Third, the spectral weight change of 9R BaRuO3 is qualitatively similar to that of CDW systems where the sum rule should be obeyed [11]. Therefore, we propose that the pseudogap phenomena in BaRuO3 should be originated from CDW ¯uctuations. Our observation of pseudogap phenomena in BaRuO3 can provide a new perspective to the HTSC's pseudogap phenomena. Just like in the HTSC case, the pseudogap in BaRuO3 can also be explained in terms of partial suppression of density of states at EF in the k-space and its possible connection to the quantum critical ¯uctuation. More systematic studies on the comparison of two materials could provide some guidelines to understand the origin of the pseudogap and its possible connection to the superconductivity in HTSC. Acknowledgements This work was supported by Ministry of Science and Technology through the Creative Research Initiative program and by the Korea
T.W. Noh et al. / Physica C 364±365 (2001) 480±483
Science and Engineering Foundation through the CSCMR at Seoul National University. References [1] S. Donovan, A. Schwartz, G. Gr uner, Phys. Rev. Lett. 79 (1997) 1401. [2] Ch. Renner et al., Phys. Rev. Lett. 80 (1998) 149. [3] J.T. Rijssenbeek et al., Phys. Rev. B 59 (1999) 4978.
483
[4] M. Shepard, S. McCall, G. Cao, J.E. Crow, J. Appl. Phys. 81 (1997) 4978. [5] M.K. Lee et al., Appl. Phys. Lett. 77 (2000) 364. [6] H.S. Choi et al., Phys. Rev. B 60 (1999) 4646. [7] Y.S. Lee et al., Europhys. Lett. 55 (2001) 280. [8] D.N. Basov et al., Phys. Rev. Lett. 77 (1996) 4090. [9] C.C. Homes, T. Timusk, R. Liang, D.A. Bonn, W.N. Hardy, Phys. Rev. Lett. 71 (1993) 1645. [10] G. Cao et al., Solid State Commun. 113 (2000) 657. [11] G. Gr uner, Density Waves in Solids, Addison-Wesley, Reading, MA, 1994.