Charge fluctuation of the superconducting molecular crystals

Charge fluctuation of the superconducting molecular crystals

ARTICLE IN PRESS Physica B 405 (2010) S237–S239 Contents lists available at ScienceDirect Physica B journal homepage: www.elsevier.com/locate/physb ...

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ARTICLE IN PRESS Physica B 405 (2010) S237–S239

Contents lists available at ScienceDirect

Physica B journal homepage: www.elsevier.com/locate/physb

Charge fluctuation of the superconducting molecular crystals T. Yamamoto a,, Y. Nakazawa a, R. Kato b, K. Yakushi c, H. Akutsu d, A.S. Akustu d, H. Yamamoto b, A. Kawamoto e, S.S. Turner f, P. Day g a

Graduate School of Science, Osaka University, Toyonaka, Osaka 560-0043, Japan RIKEN, Wako, Saitama 351-0198, Japan c Institute for Molecular Science, Okazaki, Aichi 444-8581, Japan d School of Science and Graduate School of Material Sciences, University of Hyogo, Kamigouri, Hyogo 678-1297, Japan e Graduate School and Faculty of Sciences, Hokkaido University, Sapporo, Hokkaido 060-0810, Japan f Department of Chemistry, Warwick University, Gibbet Hill Road, Coventry CV4 7AL, UK g Department of Chemistry, University College London, 20 Gordon Street, London WC1H 0AJ, UK b

a r t i c l e in fo

Keywords: Organic superconductors b00 -type ET salts Vibrational spectra Charge fluctuation Inter-site coulomb interaction

abstract In recent years, concern has been raised about the charge fluctuation of the superconducting transition in the loosely dimerized molecular conductors. Not only the observation of the charge fluctuation is of considerably important but also the understanding of the mechanism of the fluctuation. We have observed degree of charge fluctuation of several b00 -type ET salts. The b00 -type ET salt is one of the best model compounds because the direction of the largest inter-site Coulomb interaction is perpendicular to that of the largest transfer integral. This structural property allows us to examine the role of inter-site Coulomb interaction from the viewpoint of the inter-molecular distance. The difference in the molecular charges between the charge rich site and the charge poor sites, Dr, is correlated with the conducting behavior; the superconducting materials have the small but finite Dr, whereas Dr of the insulating (metallic) materials is large (almost zero). After the analysis of the configuration in the intermolecular distances, we have found that the degree of fluctuation, Dr, is attributed to the number of the most stable charge distribution(s), NS, and the number of the energy levels of the allowed charge distribution, NA. The superconducting materials belong to the condition of NS Z 2 and NA Z 2. Indeed, this condition contributes to the fluctuation of the molecular charges. & 2009 Elsevier B.V. All rights reserved.

1. Introduction In the field of the molecular conductors, research efforts have been dedicated towards the study of the antiferromagnetic– superconducting phase transition of the k-type ET salts, whose two-dimensional layer consists of dimerized ET molecules. On the other hand, the insulator–superconducting transition and the metal–superconducting transition for the non-dimerized or loosely dimerized ET salts have not been studied so far. According to the theoretical study done by Merino and Mackenzie, they proposed that a charge fluctuation can contribute to the pairing mechanism in the loosely dimerized molecular conductors [1]. The short cut to study the charge fluctuation experimentally is to observe the charge fluctuation directly rather than to observe the charge ordered (CO) state. In addition to the observation of the charge fluctuation, our efforts should be directed to obtain the mechanism of the charge fluctuation. To satisfy above aims, the selection of the

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E-mail address: [email protected] (T. Yamamoto). 0921-4526/$ - see front matter & 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.physb.2009.11.059

good model compound as well as the selection of the methodology for observing the fluctuation is required. The vibrational spectroscopy is one of the most powerful methods to observe the charge fluctuation, because the time-averaged molecular charges are monitored from the frequencies of the charge sensitive modes. The development (or reduction) of the charge ordered state is also monitored using the e-mv coupling mode. This methodology can be applied to most of the ET-containing molecular conductors [2]. The b00 -type ET salts is one of the best model compounds because of the following reasons. As for the most of the b00 -type ET salts, the direction of the largest inter-site Coulomb interaction, VS, is perpendicular to that of the largest transfer integral, tC [3]. When both directions are not perpendicular to each other, the distribution of site charges required from VS is often compete with the distribution required from tC. On the other hand, most of the b00 -type ET salts are free from such problem. Therefore, the distributions of site charges can be examined from the viewpoint of the reduction the inter-site Coulomb interaction. Furthermore, the b00 -type ET salts have a rich variety in the conducting properties and several superconductors are available [4–11]. The other advantage of using the b00 -type ET salts is a rich variety in the inter-molecular distances. As presented in Table 1,

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Table 1 NA, NS, Dr and configurations of the inter-molecular distance. Materials

Group I, insulator (ET)4M(CN)4H2O, M =Ni, Pd (ET)6Cl44H2O

Configuration

NA

NS

Dr

Fig. 1

Ref.

[LsL0 s0 ]

4

1

0.3

I-1, I-2

[4,5]

Column Aa: [LL0 s] Column Ba: [LsL0 ]

3

1

0.25

[LsL0 s] [LL0 sL0 ]

3 3

[6,7]

The frequencies of the two kinds of the CQC stretching modes, n2 and n27 modes, have an almost linear relationship with the molecular charge, r [2]. When the molecular charges are not uniform, the n2 and n27 modes exhibit either splitting or broadening. Then, the difference in the molecular charges between the charge-rich site and the charge-poor site, Dr, is obtained from the frequencies of the n2 (and n27) modes corresponding to the charge rich and poor sites.

3. Results and discussions

b

Group II (ET)4Pt(CN)4H2O (ET)4[(H3O)Ga(ox)3]sol, sol =PhNO2, pyridine, CH2Cl2 Group III, metal (ET)2X(diiodoacethylene), X= Br, Cl (ET)2AuBr2

0

2 2

0.15 0.13  0.05

[ss ]

1

2

0.03

[Ls]

1

2

0

II-1 II-2  II-4

III-1, III-2 III-3

[4] [8,9]

[10] [11]

The notations in Fig. 1 are also shown in the right column. a

Repeat unit contains two stacking columns. In the temperature-Dr, phase diagram, Group II is located around the boundaries among the insulator, superconducting, and metallic phases [14]. b

b00 -type ET salts give several kinds of configurations in the intermolecular distances. In this table, s, L, s0 and L0 denote the shortest, the longest, the secondary shortest and the secondary longest distances, respectively. Then, the number of the allowed charge distributions, the most stable charge distribution(s), etc. depend on the configurations of the inter-molecular distance. Owing to the variety in the inter-molecular distance, we can examine which configuration contributes to the charge fluctuation, the CO state or the homogeneous charge distribution. Therefore, the degree of fluctuation as well as the conducting properties can be examined from the viewpoint of the inter-molecular distances. In this symposium, we will present the correlation between the degree of charge fluctuation and conducting properties based on the recent works [12–14]. We have obtained the degree of charge fluctuation using the vibrational spectra of several 3/4filled b00 -type ET salts. The degree of fluctuation in one of the 2/3filled salts is obtained from the pressure dependence of the vibrational spectra. Table 1 presents the parts of the materials, which we will give comments on the degree of fluctuations. The configurations of the inter-molecular distances along the stacking direction are also shown. Based on the analysis of the intermolecular distances, we will discuss what dictates the degree of fluctuation and the conducting behavior.

2. Experiments and analyses Raman spectra were obtained using spectrometer combined with a microscope. The polarized reflectance spectra were observed with FT-IR spectrometer equipped with a microscope. The IR-conductivity spectra were obtained after Kramers–Kronig transformation from the polarized reflectance spectra. The spectra of the 3/4-filled salts were observed at the ambient pressure. Raman spectra of one of the 2/3-filled salt, (ET)3Cl22H2O, were observed under hydrostatic pressures by using the sapphire anvil cell. Hereafter, we designate this material as (ET)6Cl44H2O because the six molecules are participate in the repeat unit. The samples were cooled using a helium-flow type cryostat.

Fig. 1 shows the vibrational spectra of several 3/4-filled salts.

Dr of the metallic material is almost zero and Dr in the CO state has large value. Interestingly, Dr of the superconductor has small but finite value. This trend is also observed in the pressure dependence of the vibrational spectra of (ET)6Cl44H2O, which is shown in the bottom panel of Fig. 1. According to the temperature dependence of the electrical resistivity under the hydrostatic pressure, the insulating phase is suppressed, and the superconducting state has emerged [15]. With increasing the pressure further, only the metallic phase remains [15]. These observations indicate that the conducting behavior is mapped with Dr, in other words, the conducting behavior is correlated with the degree of fluctuation of the site charges. Let us examine the degree of fluctuation from the viewpoint of the inter-molecular distance. In our previous works, we have shown that the distributions of the site charges depend on the configuration of the inter-molecular distance along the stacking direction [12,13]. As for the 2/3-filled salts, two holes are accommodated into three molecules. When the repeat unit consists of two short distances and one long distance, [s s L], only one distribution, [s P s R L R], is stable, where R and P denote the charge rich and poor molecules, respectively [12,13]. The other two distributions are not allowed because these distributions include charge rich pair with a short distance. Therefore, the ground state is the CO state. On the other hand, when the repeat unit consists of one short distance and two long distances, [L L s], the number of the most stable distributions is two; [L R L R s P] and [L R L P s R]. The other distribution, [L P L R s R], is not allowed because of charge rich pair with a short distance. As far as no additional structural change, the distribution can take both two stable distributions. Therefore, the site charges exhibits the frustration. [12,13]. Since the space is too limited to show all the distributions in Table 1, we show the summaries obtained from the analysis of the inter-molecular distance. We have proposed that the degree of fluctuation and the conducting properties depend on the following two factors; one is the number of the most stable distributions, NS, and the other is the number of the energy levels of the allowed distributions, NA. The b00 -type ET salts are roughly classified into three groups: Group I: NS =1 and NA: arbitrary Group II: NS Z2 and NA Z2 Group III: NS Z2 and NA = 1 At first, the 3/4-filled salts are examined. The ground state of Group I is the CO state and Dr is large because only one distribution is the most stable. As for Group III, a charge cannot be localized at a molecule unless there is any additional interaction; e–ph interaction, molecular deformation, anion–cation interaction, etc. Then, the ground state is the metallic state whose Dr is (almost) zero. It is interesting to examine the molecular charges in Group II. Owing to NS Z2, the molecular charge does not take the CO state. When the energy levels of allowed distributions are close to each other, these

ARTICLE IN PRESS T. Yamamoto et al. / Physica B 405 (2010) S237–S239

Wavenumber (cm-1) 1400

1450

1500 ν2

1550 ν2

I-1

I-2 II-1 ν27

ν27

 () (arb. unit)

I () (arb. units)

II-1

II-2

II-3

S239

distributions can participate in the time-averaged distribution. However, homogeneous site charges cannot be produced because of NA Z2. The most stable (the second stable) distribution has a large contribution to the time-averaged distribution as compared with the second stable (the third stable) distribution. As a result, Dr is the small but finite value. Therefore, the Group II takes neither the CO state nor the homogeneous site charges. Next, the pressure dependence for (ET)6Cl44H2O is examined. This compound belongs to Group I at ambient pressure, but the energy levels of allowed distributions approach to each other with increasing hydrostatic pressure, so that this material is changed into Groups II and III. Our analysis can be applied to the other ambient pressure superconductor, (ET)2(SF5CH2CF2SO3). Since the repeat unit consists of four ET molecules, this compound belongs to Group II. Indeed, Dr is small but finite value [16,17]. Based on the analysis above, we have found that the superconductor belongs to Group II. This result suggests that the charge fluctuation can contribute to the pairing mechanism for the superconducting transition of the b00 -type ET salts. This result also answers the question why the repeat unit of the superconductor contains a large number of molecules.

4. Conclusions The conducting behavior of the b00 -type ET salts is mapped with the degree of charge fluctuation, Dr. The degree of fluctuation is determined from the two factors; the number of the most stable charge distribution and the number of the energy levels of the allowed charge distribution. As for the superconductor, these factors can contribute to the enhancement of the charge fluctuation.

II-4

III-1 III-2

ν2 References

III-3

ν27

ν2

ν2

I () (arb. units)

0.1 GPa

1.3 GPa

2.0 GPa

1400

ν2

1450 Raman shifts

1500

1550

(cm-1)

Fig. 1. (Upper panel) Charge sensitive n2 and n27 modes of 3/4-filled b00 -type ET salts at the ambient pressure. The notations, I-1, II-2, etc. correspond to those in Table 1. (Bottom panel) Pressure dependence of the n2 mode for (ET)6Cl44H2O. The sample temperatures are above the liquid-helium temperature, 5–10 K.

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