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Raman study of κ-ET2Cu[N(CN)2]Cl at ambient and ∼300 bars pressures
Synthetic Metals 157 (2007) 252–256
Raman study of -ET2Cu[N(CN)2]Cl at ambient and ∼300 bars pressures K.D. Truong, S. Jandl ∗ , M. Poirier D´epartement de physique & Regroupement qu´eb´ecois sur les mat´eriaux de pointe, Universit´e de Sherbrooke, Sherbrooke, Qu´ebec J1K 2R1, Canada Received 11 July 2006; received in revised form 30 January 2007; accepted 28 February 2007 Available online 27 April 2007
Abstract Anomalies in the temperature dependence of the 246 and 258 cm−1 Raman active phonons below the antiferromagnetic phase transition are reported in -ET2 Cu[N(CN)2 ]Cl at ambient pressure and are compared to measurements under ∼300 bars that render the compound superconducting. At ambient pressure, due to the Mott-gap opening and/or to the spin–lattice interaction and exchange-phonon modulation, frequency softenings, enhanced intensities and linewidths narrowings of these modes occur at low temperatures. © 2007 Elsevier B.V. All rights reserved. Keywords: Organic superconductor; Raman phonons; Spin-lattice interaction
1. Introduction The quasi-2D charge transfer compounds -ET2 X with X = Cu(NCS)2 , Cu[N(CN)2 ]Y (Y = Br and Cl), (ET = BEDTTTF (bis-ethylenedithiotetrathiafulvalene)) have been recognized as highly correlated electron systems [1] with interesting magnetic and superconducting phase transitions [2–4]. The -ET2 Cu(NCS)2 , and -ET2 Cu[N(CN)2 ]Br salts become superconductors at Tc = 10.4 and 11.6 K, respectively [5]. However, in spite of structural similarity, the -ET2 Cu[N(CN)2 ]Cl salt remains insulating at low temperatures exhibiting a semiconductor–insulator like transition at T ∼ 50–60 K [6] and an antiferromagnetic insulator phase transition at T ∼ 30 K [2,7]. Nevertheless, under pressure above 200 bars, it undergoes a superconducting transition at 12.8 K [8]. Unconventional metallic (∼100 K), antiferromagnetic insulator (∼30 K) and superconducting transition under pressure (∼12.8 K) appear successively in the phase diagram [9]. With increasing pressure, one observes a pressure-induced finite-temperature first order Mott phase transition between insulating and metallic phases as predicted by dynamical mean-field theory [10–13]. Recently, a large softening of the sound velocity at the Mott
∗
Corresponding author. Tel.: +1 819 821 8000; fax: +1 819 821 8046. E-mail address: [email protected] (S. Jandl).
0379-6779/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.synthmet.2007.02.005
critical point confirms the divergence of the compressibility indicating that such point could play an important role in the origin of the pseudogap [14]. A T* line separates the metallic phase into unconventional metal with large antiferromagnetic spin fluctuation at T > T* and a metallic state with a possibility of fluctuating density wave formation at T < T* [15]. These anomalies at T ∼ 33 K have been observed in several physical properties such as the spin-lattice relaxation rate (T1 T)−1 [6,16] the spin susceptibility and the resistivity that shows a change of the anisotropy [15]. There is also a speculation on the origin of the insulating phase as being due to conformational disorder of the ET ethylene groups [17]. In terms of the total amount of research devoted so far to -ET2 Cu[N(CN)2 ]Cl, most of the optical measurements have been done in the mid-IR region [18–20]. An anomalous IR frequency shift of the specific molecular vibration ν3 (Ag ) mode at 1320–1330 cm−1 below 50–60 K has been associated by T. Sasaki et al. [20] with a change of the electronic states through electron molecular vibration coupling. To our knowledge, no high resolution low frequency Raman active modes of -ET2 Cu[N(CN)2 ]Cl have been studied excepting the ν9 (Ag ) mode around 500 cm−1 in KBr pellets [21]. Raman scattering could be sensitive to magnetic effects [22], and in a previous study of -ET2 Cu[N(CN)2 ]Cl, we have analyzed the Raman C C stretching modes (∼1400–1500 cm−1 ), CH2 deformations modes (∼1420 cm−1 ) and infrared active CN modes (∼2200 cm−1 ) as a function of temperature under pressure [23].
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We have found that the CH2 and CN modes are sensitive to the thermal expansion around 80 K and to a possible superlattice transition around 160 K while the C C stretching Raman modes ν2 (Ag ) are influenced by the formation of the superconducting state [23]. The purpose of this work is to extend our investigation to the temperature dependence below the Mott and antiferromagnetic transitions of typical low frequency modes. We particularly focus on the temperature evolution, at ambient pressure and at 300 bars, of the intensities, frequencies, and linewidths of the 246 and 258 cm−1 modes. For these vibrations, bending of C C and C–S bonds are involved, causing a charge redistribution between C and S atoms and a lowering of the highest ET donor unpaired electron occupied molecular orbital energy [24,25]. Investigation of these vibrations in the vicinity of the Mott critical point and near the antiferromagnetic transition gives an additional hint about the highly correlated electron systems. 2. Experimental We have synthesized high quality -ET2 Cu(N(CN)2 )Cl single crystals by electrochemical methods as described in Ref. [26]. The selected crystals have the shape of slightly distorted hexagons. The surface is shiny with the a and c axes being along the longer and shorter diagonals, respectively [27,28]. They show no surface texture or defects, and look free of internal subgrain structures under microscope observation. No grease was used in the measurements at ambient pressure. For pressure application, (∼300 bars), the single crystal was embedded in vacuum grease and cooled down to 5 K so that it became superconductor below 12.8 K [8]. In the 200–400 cm−1 frequency range, 1 cm−1 resolution spectra were measured in the back-scattering configuration with the incident and scattered polarizations parallel to the a and c axes, respectively. A Labram microscope spectrometer equipped ˚ and a CCD detector were with a He:Ne laser (λ = 6328 A), used. To avoid sample heating, the power of the laser beam was kept below 0.3 mW using a 10× objective. Single crystals were mounted on a Janis Research Supertran cryostat cold finger with a cooling rate varying from 8 K/min, for rapid cooling from 295 to 130 K, and 2 K/min for slow cooling from 130 to 4.2 K. The peak position and width of Raman modes were determined from curve fittings with mixed Gaussian–Lorentzian functions. 3. Results and discussion The room temperature structure of -ET2 Cu(N(CN)2 )Cl 16 )) with four formula units per is orthorhombic (Pnma (D2h cell (Z = 4) [29]. The basic pattern in the -phase consists of alternating layers of ET donor molecules and polymeric Cu(N(CN)2 )Cl− anions parallel to the (a–c) plane. In the particular range of our Raman study, and when dimerization is absent as in the neutral ET compound, one Raman active phonon is observed around 262 cm−1 [30]. When ET forms complex salts by charge transfers to anions, two Raman active phonons are observed around 245 and 261 cm−1 in -ET2 Cu(NCS)2 [31] and around 246 cm−1 and 277 cm−1 in -ET2 Cu[N(CN)2 ]Br
Fig. 1. B2g Raman active phonons of -Et2 Cu[N(CN)2 ]Cl single crystal in the 220–340 cm−1 range at different temperatures from 5 to 100 K.
[32]. Fig. 1 shows the temperature evolution of B2g Raman active phonons in the 220 and 340 cm−1 frequency range for -ET2 Cu(N(CN)2 )Cl. In contrast to the 282 cm−1 mode which hardens at low temperature, softenings of the 246 and 258 cm−1 modes are observed in Fig. 2(a and b). Also affected are their linewidths (reduced by half) and their intensities (increased by a factor 4) at 5 K (Fig. 3 (a and b)). The same single crystal under pressure (∼300 bars), once embedded in vacuum grease, becomes a superconductor below 12.8 K [8]. No softenings, or noticeable intensities changes below 50 K are then observed (Fig. 4). Hence Mott gap and/or magnetic ordering, when present in -ET2 Cu(N(CN)2 )Cl, result in softenings of these two Raman active modes. Such phonon peculiarities have been observed in YBa2 Cu4 O8 and YBa2 Cu3 O6.57 superconductors above Tc [22], and in perovskite like manganites RMnO3 (with R = lanthanides) [33] where antiferromagnetism interacts with selected phonons provoking their softenings. Hartree Fock calculations for ET salts have predicted that antiferromagnetic ordering occurs between the two dimers with ferromagnetic ordering inside the dimers [34]. The dimer model also predicts a paramagnetic metallic-antiferromagnetic insulator first order phase transition as a function of the Coulomb interaction within the dimer. Experimentally, in the case of -ET2 Cu(N(CN)2 )Cl at ambient pressure, antiferromagnetism develops with localized spins on the dimers having a moment of 0.4B /dimer below 27 K [7]. The spin-lattice coupling may be particularly favored by charge transfer occurring between neighboring molecules in the dimers influencing the phonon frequencies, their linewidths and their intensities. Also below TN , splitting of the two softening modes are observed (244–248 cm−1 ; 253–256 cm−1 (Fig. 2)). They may be due to
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Fig. 2. (a) Temperature dependence of -Et2 Cu[N(CN)2 ]Cl 246 cm−1 phonon frequency and its relative frequency shift (inset); (b) temperature dependence of -Et2 Cu[N(CN)2 ]Cl 258 cm−1 phonon frequency and. its relative frequency shift (inset).
nonequivalent couplings between the magnetic fluctuations and the two unit cell dimers. Merino and McKenzie [35] have calculated phonon frequency anomalies due to the strong electronic correlations in these materials. They have considered a Holstein–Hubbard
model where the local charge density is coupled linearly to the phonon amplitude showing that, for frequencies close to half the Coulomb repulsion in presence of strong interaction, phonon frequencies may have a non-monotonic temperature dependence. Their model predicts the largest effects for phonons
Fig. 3. (a) Temperature dependence of -Et2 Cu[N(CN)2 ]Cl 246 cm−1 phonon relative intensity and linewidth (inset); (b) temperature dependence of Et2 Cu[N(CN)2 ]Cl 258 cm−1 phonon relative intensity and linewidth (inset).
K.D. Truong et al. / Synthetic Metals 157 (2007) 252–256
Fig. 4. Raman spectra of -Et2 Cu[N(CN)2 ]Cl single crystal embedded in vaccum grease in the 220–340 cm−1 range at different temperatures from 5 to 100 K.
with frequencies around 1400 cm−1 . Nevertheless, they have underlined that the non-monotonic temperature dependence is expected for a broader range of phonon frequencies than the model would predict. A recent study, of orthorhombic RMnO3 (R = rare earth) Raman active phonons, has established a universal correlation between magnetic ordering in the manganites xz layers and phonon softening below TN [33]. Granado et al. [36] have interpreted such softening in terms of spin–phonon coupling caused by the phonon modulation of the exchange integral. They particularly considered the phonon renormalization below TN as proportional to the spin–spin correlation function Si Sj for the nearest neighbor spins localized at the ith and jth Mn3+ sites. Their calculated phonon frequency softening ω as a function of the magnetization M and (δ2 J/δ2 u), the second derivative of the exchange integral relatively to the normal phonon coordinate, corresponds to ω ∼ (2/mω) (M2 /4μB ) (δ2 J/δ2 u), with m representing the vibrating atom mass, M the magnetization and μB the Bohr magneton. Applying Granado et al. [36] predictions to -ET2 Cu(N(CN)2 )Cl and considering that sulfur is the typical vibrating atom of the ∼250 cm−1 modes and ˚ 2. M = 0.4μB , the associated (δ2 J/δ2 u) would be ∼250 mRy /A In Kornelsen et al. study [18] of the metal–insulator transition in -ET2 Cu[N(CN)2 ]Cl, the infrared active ν3 (Ag ) mode (at 1320–1330 cm−1 ) softens below 50 K with the Mott gap opening. Similarly, Sasaki et al. [20] have attributed the characteristic temperature dependence of the ν3 (Ag ) mode softening in their IR optical conductivity measurements to the change of the electronic states through the electron-molecular vibration coupling. They also have noted that the same vibrational mode, measured by Raman scattering, shows a monotonic temperature
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dependence of the frequency from room temperature down to 10 K. In our study, the two Raman active modes, that seem to soften somehow around 40 K, could be associated with the Mott gap opening. Their softenings could also be associated with the antiferromagnetic transition and even if they start some ten degrees above TN , they could reflect the presence of antiferromagnetic fluctuations as observed in the spin-lattice relaxation rate measurements of Kawamato et al. [6]. Also these phonon softenings that disappear when pressure is applied on the sample, while the fluctuations due to Mott transition persist as shown by Fournier et al. [14], is rather an indication of sensitivity of the two Raman active modes to the antiferromagnetic transition at ambient pressure. The relative close temperatures of the Mott gap and the antiferromagnetic fluctuations as well as our measurements resolution limits do not allow us to discriminate unambiguously the origin of the phonon softenings. In summary, softenings of Raman active phonons reflect the Mott gap opening and/or the presence of localized spins in ET2 Cu(N(CN)2 )Cl as deduced by the magnetic measurements [7]. Nevertheless, in order to properly apply models with localized spins a detailed description of the magnetic interactions and the low frequency phonon eigenvectors are needed. Usually, below 50 K the phonon intensities are almost constant unless some phase transition occurs. The remarkable increase in intensities of the two low frequencies modes below TN (Fig. 3a and b) reflects charge and polarizability renormalizations that affect particularly the normal modes involving C and S atoms vibrations. 4. Conclusion We have measured the temperature dependence of two low frequency phonons around 250 cm−1 in -ET2 Cu[N(CN)2 ]Cl by Raman scattering at ambient pressure and under ∼300 bars. At ambient pressure, opening of a Mott gap followed by an antiferromagnetic transition affects their frequencies, intensities and linewidths. Polarizabilities and frequencies of these particular phonons are renormalized, similarly to strong electron correlated systems such as the manganites for which Raman active phonon–spin modeling has been applied recently in various magnetic regimes [33]. Acknowledgements Support from the National Science and Engineering Research Council of Canada and the Fonds Qu´eb´ecois de la Recherche sur la Nature et les Technologies, is gratefully acknowledged. References [1] M. Lang, J. M¨uller, in: K.H. Bennenmann, J.B. Ketterson (Eds.), The Physic of Superconductors, vol. II, Springer-Verlag, Berlin, 2003. [2] M.A. Tanatar, T. Ishiguro, H. Ito, M. Kubota, G. Saito, Phys. Rev. B 55 (1997) 12529. [3] H. Weiss, M.V. Kartsovnik, W. Biberacher, E. Steep, E. Balthes, A.G.M. Jansen, K. Andres, N.D. Kushch, Phys. Rev. B 59 (1999) 12370. [4] M.A. Tanatar, T. Ishiguro, T. Kondo, G. Saito, Phys. Rev. B 59 (1999) 3841.
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[5] J.M. Williams, A.J. SChultz, U. Geiser, K.D. Carlson, A.M. Kini, H.H. Wang, W.K. Kwok, M.H. Whangbo, J.E. Schirber, Science 252 (1991) 1501. [6] A. Kawamato, K. Miyagawa, Y. Nakazawa, K. Kanoda, Phys. Rev. B 52 (1995) 15522. [7] K. Miyagawa, A. Kawamoto, Y. Nakazawa, K. Kanoda, Phys. Rev. Lett. 75 (1995) 1174. [8] H.H. Wang, et al., Synth. Metals 41–43 (1991) 1983. [9] K. Kanoda, Physica C 282/287 (1997) 299. [10] M. Imada, A. Fujimori, Y. Tokura, Rev. Mod. Phys. 70 (1998) 1039. [11] G. Kotliar, S. Murthy, M.J. Rozenberg, Phys. Rev. Lett. 89 (2002) 046401. [12] S. Onoda, M. Imada, J. Phys. Soc. Jpn. 70 (2001) 3398. [13] J. Merino, R.H. McKenzie, Phys. Rev. B 61 (2000) 7996. [14] D. Fournier, M. Poirier, M. Castonguay, K.D. Truong, Phys. Rev. Lett. 90 (2003) 127002. [15] T. Sasaki, N. Yoneyama, A. Matsuyama, N. Kobayashi, Phys. Rev. B 65 (2002) 060506. [16] P. Limelette, P. Wzietek, S. Florens, A. Georges, T.A. Costi, C. Pasquier, D. J´erome, C. M´ezi`ere, P. Batail, Phys. Rev. Lett. 91 (2003) 016401. [17] H. Posselt, H. Muller, K. Andres, G. Saito, Phys. Rev. B 49 (1994) 15849. [18] K. Kornelsen, J.E. Eldridge, H.H. Wang, H.A. Charlier, J.M. Williams, A.V. Strieby Crouch, D.M. Watkins, Solid State Commun. 81 (1992) 343. [19] J.E. Eldridge, K. Kornelsen, H.H. Wang, J.M. Williams, A.V. Strieby Crouch, D.M. Watkins, Solid State Commun. 79 (1991) 583. [20] T. Sasaki, I. Ito, N. Yoneyama, N. Kobayashi, N. Hanasaki, H. Tajima, T. Ito, Y. Iwasa, Phys. Rev. B 69 (2004) 064508. [21] Y. Lin, J.E. Eldrige, J. Schlurter, H.H. Wang, A.M. Kini, J.M. Williams, Phys. Rev. B 64 (2001) 24506.
[22] A.P. Litvinchuk, C. Thomsen, M. Cardona, Solid. State Commun. 83 (1992) 343. [23] K. D. Truong, S. Jandl, M. Poirier. Synt. Met. 104 (1999) 157. cademic Press Inc., Newyork,;1; 1988), Vol 27. [24] M.E. Kozlov, K.I. Pokhodnia, A.A. Yurchenko, Spectrochem. Acta 45A (N4) (1989) 437. [25] J. Shumway, S. Chattopadhyay, S. Spathy, Phys. Rev. B 53 (1996) 6677. [26] A.M. Kini, U. Geiser, H.H. Wang, K.D. Carlson, J.M. Williams, W.K. Kwok, K.G. Vanderwoort, J.E. Thompson, D.L. Stupka, D. Jung, M.H. Whangbo, Inorg. Chem. 29 (1990) 2555. [27] R.C. Yu, J.M. Williams, H.H. Wang, J.E. Thompson, A.M. Kini, K.D. Carlson, J. Ren, M.H. Whangbo, P.M. Chaikin, Phys. Rev. B 44 (1991) 6932. [28] H.H. Wang, et al., Chem. Mater. 2 (1990) 482. [29] A.J. Schultz, U. Geiser, H.H. Wang, J.M. Williams, L.W. Finger, R.M. Hazen, Physica C 208 (1993) 277. [30] J.E. Eldridge, C.C. Homes, J.M. Williams, A.M. Kini, H.H. Wang, Spectrochim. Acta 51A (1995) 947. [31] S. Sugai, H. Mori, H. Yamochi, G. Saito, Phys. Rev. B47 (1993) 14374. [32] J.E. Eldridge, Y. Xie, H.H. Wang, J.M. Williams, A.M. Kini, J.A. Schlueter, Spectrochim. Acta 52A (1996) 45. [33] J. Laverdi`ere, S. Jandl, A.A. Mukhin, Yu.V Ivanov, V.G. Ivano, M.N. Iliev, Phys. Rev. B 73 (2006) 214301. [34] H. Kino, H. Fukuyama, J. Phys. Soc. Jpn. 65 (1996) 2158. [35] J. Merino, R.H. McKenzie, Phys. Rev. B 62 (2000) 16442. [36] E. Granado, A. Garcia, J.A. Sanjurjo, C. Rettori, I. Torriani, F. Prado, R.D. Sanchez, A. Caneiro, S.B. Oseroff, Phys. Rev. B 60 (1999) 11897.
Raman study of -ET2Cu[N(CN)2]Cl at ambient and ∼300 bars pressures K.D. Truong, S. Jandl ∗ , M. Poirier D´epartement de physique & Regroupement qu´eb´ecois sur les mat´eriaux de pointe, Universit´e de Sherbrooke, Sherbrooke, Qu´ebec J1K 2R1, Canada Received 11 July 2006; received in revised form 30 January 2007; accepted 28 February 2007 Available online 27 April 2007
Abstract Anomalies in the temperature dependence of the 246 and 258 cm−1 Raman active phonons below the antiferromagnetic phase transition are reported in -ET2 Cu[N(CN)2 ]Cl at ambient pressure and are compared to measurements under ∼300 bars that render the compound superconducting. At ambient pressure, due to the Mott-gap opening and/or to the spin–lattice interaction and exchange-phonon modulation, frequency softenings, enhanced intensities and linewidths narrowings of these modes occur at low temperatures. © 2007 Elsevier B.V. All rights reserved. Keywords: Organic superconductor; Raman phonons; Spin-lattice interaction
1. Introduction The quasi-2D charge transfer compounds -ET2 X with X = Cu(NCS)2 , Cu[N(CN)2 ]Y (Y = Br and Cl), (ET = BEDTTTF (bis-ethylenedithiotetrathiafulvalene)) have been recognized as highly correlated electron systems [1] with interesting magnetic and superconducting phase transitions [2–4]. The -ET2 Cu(NCS)2 , and -ET2 Cu[N(CN)2 ]Br salts become superconductors at Tc = 10.4 and 11.6 K, respectively [5]. However, in spite of structural similarity, the -ET2 Cu[N(CN)2 ]Cl salt remains insulating at low temperatures exhibiting a semiconductor–insulator like transition at T ∼ 50–60 K [6] and an antiferromagnetic insulator phase transition at T ∼ 30 K [2,7]. Nevertheless, under pressure above 200 bars, it undergoes a superconducting transition at 12.8 K [8]. Unconventional metallic (∼100 K), antiferromagnetic insulator (∼30 K) and superconducting transition under pressure (∼12.8 K) appear successively in the phase diagram [9]. With increasing pressure, one observes a pressure-induced finite-temperature first order Mott phase transition between insulating and metallic phases as predicted by dynamical mean-field theory [10–13]. Recently, a large softening of the sound velocity at the Mott
∗
Corresponding author. Tel.: +1 819 821 8000; fax: +1 819 821 8046. E-mail address: [email protected] (S. Jandl).
0379-6779/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.synthmet.2007.02.005
critical point confirms the divergence of the compressibility indicating that such point could play an important role in the origin of the pseudogap [14]. A T* line separates the metallic phase into unconventional metal with large antiferromagnetic spin fluctuation at T > T* and a metallic state with a possibility of fluctuating density wave formation at T < T* [15]. These anomalies at T ∼ 33 K have been observed in several physical properties such as the spin-lattice relaxation rate (T1 T)−1 [6,16] the spin susceptibility and the resistivity that shows a change of the anisotropy [15]. There is also a speculation on the origin of the insulating phase as being due to conformational disorder of the ET ethylene groups [17]. In terms of the total amount of research devoted so far to -ET2 Cu[N(CN)2 ]Cl, most of the optical measurements have been done in the mid-IR region [18–20]. An anomalous IR frequency shift of the specific molecular vibration ν3 (Ag ) mode at 1320–1330 cm−1 below 50–60 K has been associated by T. Sasaki et al. [20] with a change of the electronic states through electron molecular vibration coupling. To our knowledge, no high resolution low frequency Raman active modes of -ET2 Cu[N(CN)2 ]Cl have been studied excepting the ν9 (Ag ) mode around 500 cm−1 in KBr pellets [21]. Raman scattering could be sensitive to magnetic effects [22], and in a previous study of -ET2 Cu[N(CN)2 ]Cl, we have analyzed the Raman C C stretching modes (∼1400–1500 cm−1 ), CH2 deformations modes (∼1420 cm−1 ) and infrared active CN modes (∼2200 cm−1 ) as a function of temperature under pressure [23].
K.D. Truong et al. / Synthetic Metals 157 (2007) 252–256
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We have found that the CH2 and CN modes are sensitive to the thermal expansion around 80 K and to a possible superlattice transition around 160 K while the C C stretching Raman modes ν2 (Ag ) are influenced by the formation of the superconducting state [23]. The purpose of this work is to extend our investigation to the temperature dependence below the Mott and antiferromagnetic transitions of typical low frequency modes. We particularly focus on the temperature evolution, at ambient pressure and at 300 bars, of the intensities, frequencies, and linewidths of the 246 and 258 cm−1 modes. For these vibrations, bending of C C and C–S bonds are involved, causing a charge redistribution between C and S atoms and a lowering of the highest ET donor unpaired electron occupied molecular orbital energy [24,25]. Investigation of these vibrations in the vicinity of the Mott critical point and near the antiferromagnetic transition gives an additional hint about the highly correlated electron systems. 2. Experimental We have synthesized high quality -ET2 Cu(N(CN)2 )Cl single crystals by electrochemical methods as described in Ref. [26]. The selected crystals have the shape of slightly distorted hexagons. The surface is shiny with the a and c axes being along the longer and shorter diagonals, respectively [27,28]. They show no surface texture or defects, and look free of internal subgrain structures under microscope observation. No grease was used in the measurements at ambient pressure. For pressure application, (∼300 bars), the single crystal was embedded in vacuum grease and cooled down to 5 K so that it became superconductor below 12.8 K [8]. In the 200–400 cm−1 frequency range, 1 cm−1 resolution spectra were measured in the back-scattering configuration with the incident and scattered polarizations parallel to the a and c axes, respectively. A Labram microscope spectrometer equipped ˚ and a CCD detector were with a He:Ne laser (λ = 6328 A), used. To avoid sample heating, the power of the laser beam was kept below 0.3 mW using a 10× objective. Single crystals were mounted on a Janis Research Supertran cryostat cold finger with a cooling rate varying from 8 K/min, for rapid cooling from 295 to 130 K, and 2 K/min for slow cooling from 130 to 4.2 K. The peak position and width of Raman modes were determined from curve fittings with mixed Gaussian–Lorentzian functions. 3. Results and discussion The room temperature structure of -ET2 Cu(N(CN)2 )Cl 16 )) with four formula units per is orthorhombic (Pnma (D2h cell (Z = 4) [29]. The basic pattern in the -phase consists of alternating layers of ET donor molecules and polymeric Cu(N(CN)2 )Cl− anions parallel to the (a–c) plane. In the particular range of our Raman study, and when dimerization is absent as in the neutral ET compound, one Raman active phonon is observed around 262 cm−1 [30]. When ET forms complex salts by charge transfers to anions, two Raman active phonons are observed around 245 and 261 cm−1 in -ET2 Cu(NCS)2 [31] and around 246 cm−1 and 277 cm−1 in -ET2 Cu[N(CN)2 ]Br
Fig. 1. B2g Raman active phonons of -Et2 Cu[N(CN)2 ]Cl single crystal in the 220–340 cm−1 range at different temperatures from 5 to 100 K.
[32]. Fig. 1 shows the temperature evolution of B2g Raman active phonons in the 220 and 340 cm−1 frequency range for -ET2 Cu(N(CN)2 )Cl. In contrast to the 282 cm−1 mode which hardens at low temperature, softenings of the 246 and 258 cm−1 modes are observed in Fig. 2(a and b). Also affected are their linewidths (reduced by half) and their intensities (increased by a factor 4) at 5 K (Fig. 3 (a and b)). The same single crystal under pressure (∼300 bars), once embedded in vacuum grease, becomes a superconductor below 12.8 K [8]. No softenings, or noticeable intensities changes below 50 K are then observed (Fig. 4). Hence Mott gap and/or magnetic ordering, when present in -ET2 Cu(N(CN)2 )Cl, result in softenings of these two Raman active modes. Such phonon peculiarities have been observed in YBa2 Cu4 O8 and YBa2 Cu3 O6.57 superconductors above Tc [22], and in perovskite like manganites RMnO3 (with R = lanthanides) [33] where antiferromagnetism interacts with selected phonons provoking their softenings. Hartree Fock calculations for ET salts have predicted that antiferromagnetic ordering occurs between the two dimers with ferromagnetic ordering inside the dimers [34]. The dimer model also predicts a paramagnetic metallic-antiferromagnetic insulator first order phase transition as a function of the Coulomb interaction within the dimer. Experimentally, in the case of -ET2 Cu(N(CN)2 )Cl at ambient pressure, antiferromagnetism develops with localized spins on the dimers having a moment of 0.4B /dimer below 27 K [7]. The spin-lattice coupling may be particularly favored by charge transfer occurring between neighboring molecules in the dimers influencing the phonon frequencies, their linewidths and their intensities. Also below TN , splitting of the two softening modes are observed (244–248 cm−1 ; 253–256 cm−1 (Fig. 2)). They may be due to
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Fig. 2. (a) Temperature dependence of -Et2 Cu[N(CN)2 ]Cl 246 cm−1 phonon frequency and its relative frequency shift (inset); (b) temperature dependence of -Et2 Cu[N(CN)2 ]Cl 258 cm−1 phonon frequency and. its relative frequency shift (inset).
nonequivalent couplings between the magnetic fluctuations and the two unit cell dimers. Merino and McKenzie [35] have calculated phonon frequency anomalies due to the strong electronic correlations in these materials. They have considered a Holstein–Hubbard
model where the local charge density is coupled linearly to the phonon amplitude showing that, for frequencies close to half the Coulomb repulsion in presence of strong interaction, phonon frequencies may have a non-monotonic temperature dependence. Their model predicts the largest effects for phonons
Fig. 3. (a) Temperature dependence of -Et2 Cu[N(CN)2 ]Cl 246 cm−1 phonon relative intensity and linewidth (inset); (b) temperature dependence of Et2 Cu[N(CN)2 ]Cl 258 cm−1 phonon relative intensity and linewidth (inset).
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Fig. 4. Raman spectra of -Et2 Cu[N(CN)2 ]Cl single crystal embedded in vaccum grease in the 220–340 cm−1 range at different temperatures from 5 to 100 K.
with frequencies around 1400 cm−1 . Nevertheless, they have underlined that the non-monotonic temperature dependence is expected for a broader range of phonon frequencies than the model would predict. A recent study, of orthorhombic RMnO3 (R = rare earth) Raman active phonons, has established a universal correlation between magnetic ordering in the manganites xz layers and phonon softening below TN [33]. Granado et al. [36] have interpreted such softening in terms of spin–phonon coupling caused by the phonon modulation of the exchange integral. They particularly considered the phonon renormalization below TN as proportional to the spin–spin correlation function Si Sj for the nearest neighbor spins localized at the ith and jth Mn3+ sites. Their calculated phonon frequency softening ω as a function of the magnetization M and (δ2 J/δ2 u), the second derivative of the exchange integral relatively to the normal phonon coordinate, corresponds to ω ∼ (2/mω) (M2 /4μB ) (δ2 J/δ2 u), with m representing the vibrating atom mass, M the magnetization and μB the Bohr magneton. Applying Granado et al. [36] predictions to -ET2 Cu(N(CN)2 )Cl and considering that sulfur is the typical vibrating atom of the ∼250 cm−1 modes and ˚ 2. M = 0.4μB , the associated (δ2 J/δ2 u) would be ∼250 mRy /A In Kornelsen et al. study [18] of the metal–insulator transition in -ET2 Cu[N(CN)2 ]Cl, the infrared active ν3 (Ag ) mode (at 1320–1330 cm−1 ) softens below 50 K with the Mott gap opening. Similarly, Sasaki et al. [20] have attributed the characteristic temperature dependence of the ν3 (Ag ) mode softening in their IR optical conductivity measurements to the change of the electronic states through the electron-molecular vibration coupling. They also have noted that the same vibrational mode, measured by Raman scattering, shows a monotonic temperature
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dependence of the frequency from room temperature down to 10 K. In our study, the two Raman active modes, that seem to soften somehow around 40 K, could be associated with the Mott gap opening. Their softenings could also be associated with the antiferromagnetic transition and even if they start some ten degrees above TN , they could reflect the presence of antiferromagnetic fluctuations as observed in the spin-lattice relaxation rate measurements of Kawamato et al. [6]. Also these phonon softenings that disappear when pressure is applied on the sample, while the fluctuations due to Mott transition persist as shown by Fournier et al. [14], is rather an indication of sensitivity of the two Raman active modes to the antiferromagnetic transition at ambient pressure. The relative close temperatures of the Mott gap and the antiferromagnetic fluctuations as well as our measurements resolution limits do not allow us to discriminate unambiguously the origin of the phonon softenings. In summary, softenings of Raman active phonons reflect the Mott gap opening and/or the presence of localized spins in ET2 Cu(N(CN)2 )Cl as deduced by the magnetic measurements [7]. Nevertheless, in order to properly apply models with localized spins a detailed description of the magnetic interactions and the low frequency phonon eigenvectors are needed. Usually, below 50 K the phonon intensities are almost constant unless some phase transition occurs. The remarkable increase in intensities of the two low frequencies modes below TN (Fig. 3a and b) reflects charge and polarizability renormalizations that affect particularly the normal modes involving C and S atoms vibrations. 4. Conclusion We have measured the temperature dependence of two low frequency phonons around 250 cm−1 in -ET2 Cu[N(CN)2 ]Cl by Raman scattering at ambient pressure and under ∼300 bars. At ambient pressure, opening of a Mott gap followed by an antiferromagnetic transition affects their frequencies, intensities and linewidths. Polarizabilities and frequencies of these particular phonons are renormalized, similarly to strong electron correlated systems such as the manganites for which Raman active phonon–spin modeling has been applied recently in various magnetic regimes [33]. Acknowledgements Support from the National Science and Engineering Research Council of Canada and the Fonds Qu´eb´ecois de la Recherche sur la Nature et les Technologies, is gratefully acknowledged. References [1] M. Lang, J. M¨uller, in: K.H. Bennenmann, J.B. Ketterson (Eds.), The Physic of Superconductors, vol. II, Springer-Verlag, Berlin, 2003. [2] M.A. Tanatar, T. Ishiguro, H. Ito, M. Kubota, G. Saito, Phys. Rev. B 55 (1997) 12529. [3] H. Weiss, M.V. Kartsovnik, W. Biberacher, E. Steep, E. Balthes, A.G.M. Jansen, K. Andres, N.D. Kushch, Phys. Rev. B 59 (1999) 12370. [4] M.A. Tanatar, T. Ishiguro, T. Kondo, G. Saito, Phys. Rev. B 59 (1999) 3841.
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